Coinage metal complexes with bridging hybrid phosphine–NHC ligands: synthesis of di- and tetra-nuclear complexes

Abstract

A series of P–NHC-type hybrid ligands containing both PR₂ and N-heterocyclic carbene (NHC) donors on meta-bis-substituted phenylene backbones, L^Cy, L^tBu and L^Ph (R = Cy, tBu, Ph, respectively), was accessed through a modular synthesis from a common precursor, and their coordination chemistry with coinage metals was explored and compared. Metallation of L^Ph·n(HBr) (n = 1, 2) with Ag₂O gave the pseudo-cubane [Ag₄Br₄(L^Ph)₂], isostructural to [Ag₄Br₄(L^R)₂] (R = Cy, tBu) (T. Simler, P. Braunstein and A. A. Danopoulos, Angew. Chem., Int. Ed., 2015, 54, 13691), whereas metallation of L^R·HBF₄ (R = Ph, tBu) led to the dinuclear complexes [Ag₂(L^R)₂](BF₄)₂ which, in the solid state, feature heteroleptic Ag centres and a ‘head-to-tail’ (HT) arrangement of the bridging ligands. In solution, interconversion with the homoleptic ‘head-to-head’ (HH) isomers is facilitated by ligand fluxionality. ‘Head-to-tail’ [Cu₂Br₂(L^R)₂] (R = Cy, tBu) dinuclear complexes were obtained from L^R·HBr and [Cu₅(Mes)₅], Mes = 2,4,6-trimethylphenyl, which also feature bridging ligands and heteroleptic Cu centres. Although the various ligands L^R led to structurally analogous complexes for R = Cy, tBu and Ph, the rates of dynamic processes occurring in solution are dependent on R, with faster rates for R = Ph. Transmetallation of both NHC and P donor groups from [Ag₄Br₄(L^tBu)₂] to Au^I by reaction with [AuCl(THT)] (THT = tetrahydrothiophene) led to L^tBu transfer and to the dinuclear complex [Au₂Cl₂L^tBu] with one L^tBu ligand bridging the two Au centres. Except for the silver pseudo-cubanes, all other complexes do not exhibit metallophilic interactions.

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Introduction

Phosphine and NHC donors are often compared because they readily coordinate to metal centres and display bonding analogies and tuneable stereo-electronic properties.¹ However, despite the fact that both are considered as strong σ-donors, emerging evidence reveals subtly different σ-donating and π-accepting properties, diversifying across the periodic table.² This can lead to transition metal complexes with beneficial catalytic properties, e.g. finely controlled lability and metal electronic tuning, stability of the catalytically active species etc. The complementary roles of both types of donors participating in the same metal coordination sphere may enhance synergism,³ although counter examples have been described.⁴ The beneficial synergism may be enhanced if the hetero donors are part of a hybrid ligand. This background justifies synthetic efforts towards the design of new phosphine-functionalised NHC (P–NHC) complexes,⁵ with reported high activities in C–C coupling reactions (Pd^II, Ir^I),⁶ amination of aryl chlorides (Pd^II)⁷ and transfer hydrogenation of ketones (Ru^II).⁸

Among the P–NHC-type ligands, bidentate hybrid ligands with direct P–N bond,⁹ flexible alkyl,^{6a–c,f,8,10} or more rigid and tuneable aryl spacer between the donors, 1a and 1b, respectively, have attracted most attention (Fig. 1);^6d,7,11 in particular, we and others have been interested in the meta-bis-substituted phenylene framework 1c–1d as potential precursor to non-symmetrical PCC^NHC ‘pincer’ complexes.¹² Relevant PC^NHCP pincer and P₂(C^NHC)₂ macrocyclic ligands 2^6f,9c,13 and 3,^5a respectively, have been described.

Fig. 1 Bidentate P-NHC-type (1a–1d), tridentate PC^NHCP (2) and macrocyclic P₂(C^NHC)₂ (3) hybrid ligands reported.

The coordination chemistry of P–NHC-type ligands has mainly been focussed on late transition metals; the few structurally characterized examples¹⁴ incorporating Ag^I,^{9a,c,10g,11a,12a,c,e,15} or Cu^I are depicted in Fig. 2.^{9c,10f,12e,13b,15c} This relative scarcity is surprising, considering the interest for air stable group 11 NHC complexes. Silver P–NHC complexes are usually obtained by the reaction of the corresponding imidazolium salts with Ag₂O,¹⁶ or by initial formation of the free carbene ligand followed by coordination to Ag^I.^9c,15c In addition to their structural diversity, they have proved to be efficient NHC transfer reagents to metals,¹⁶ such as Ru^II,^15a,17 Rh^I,^11a,18 Pd^II,^6f,10g,13a and Au^I;^9a,15c,19 but in rare cases the transmetallation did not proceed neatly.^10g,12c

Fig. 2 Structurally characterised P–NHC-type silver(I) and copper(I) complexes reported in the literature.

Interestingly, P–NHC-type copper(I) complexes are accessible by transmetallation from the corresponding Ag^I complexes,^9a,12e,15c and by other methodologies e.g. the coordination of the pre-formed free carbene to a labile Cu^I precursor,^{9c,10f,13b,15c} or the reaction of the imidazolium salt with precursors featuring a coordinated base (e.g. copper(I) acetate,^13b mesitylcopper(I) [Cu₅(Mes)₅]^12e and [CuN(SiMe₃)₂]).^9c

Lastly, P–NHC gold(I) complexes are scarce (Fig. 3) but arouse increasing interest due to their attractive photophysical properties and the occurrence of metallophilic interactions in their structures.^{9a,c,e,15c,19}

Fig. 3 Gold(I) complexes with P–NHC-type ligands reported in the literature (all since 2013).

Extending our previous work on P-based NHC hybrid ligands,^12a,c,e herein we report an efficient and modular access to the ^RP–NHC-type (^RP = PCy₂, PtBu₂ or PPh₂) ligands (see 1d in Fig. 1) and their coinage metal complexes.

Results and discussion

Ligand synthesis

A synthetic strategy for the synthesis of phosphine imidazolium precursors employing silane (SiHMeCl₂ or SiHCl₃) reduction²⁰ of readily available phosphoryl-imidazolium salts has ample literature precedence,^{6b,10a,15b,21} including attempted preparation of precursors of similar topology to those described below.^12c This methodology requires the use of excess silane reductants and forcing conditions, usually leading to moderate yields. Therefore, an alternative, wider scope synthetic strategy was developed, that is easily adaptable to different phosphine substituents (Scheme 1).

Scheme 1 Introduction of phosphine moieties to obtain hybrid P-NHC-type ligands. The synthesis of L^Cy·2HBr and L^tBu·2HBr has been reported in a previous communication.^12e

Starting from the imidazolium–bromobenzyl derivative A, the air-stable phosphonium–imidazolium salts L^Cy·2HBr and L^tBu·2HBr were obtained by quaternisation of dicyclohexyl- and di-tert-butylphosphine in acetonitrile,^12e and converted to the corresponding phosphine-imidazolium salts L^Cy·HBr and L^tBu·HBr by treatment with NEt₃. Successful single deprotonation was confirmed in the ¹H-NMR spectra by the disappearance of the deshielded signal due to the acidic P–H proton (¹J_P–H ≈ 480–490 Hz). Singlets at δ 5.8 and 32.0 ppm for L^Cy·HBr and L^tBu·HBr, respectively, were observed in the ³¹P{¹H}-NMR spectra. Due to the relative air-sensitivity of the trialkyl-phosphine products, borane-protection of the phosphine in L^Cy·HBr was carried out and yielded L^Cy·HBr·BH₃ as an air-stable crystalline solid, the structure of which is shown in Fig. 4 (left).

Fig. 4 The molecular structures of the borane-protected L^Cy·HBr·BH₃ (left) and of L^Ph·HBr (right) with thermal ellipsoids at 40% probability. For clarity, only one disordered nBu chain in L^Cy·HBr·BH₃ is displayed and H atoms have been omitted, except for the imidazolium and borane moieties. Selected bond distances (Å) and angles [°] for L^Cy·HBr·BH₃: B1–P1 1.914(3), C1–N1 1.342(3), C1–N2 1.318(3), C1H⋯Br1 3.511(2); N1–C1–N2 108.6(2), P1–C14–C12 117.9(2); for L^Ph·HBr: C20–N1 1.334(3), C20–N2 1.321(3), C20H⋯Br1 3.598(3); N1–C20–N2 108.7(2), P1–C13–C14 111.5(2).

When an analogous synthetic route was applied to L^Ph·HBr, it failed in the step of the direct quaternisation of diphenylphosphine by A owing to the lower nucleophilicity of the former. To circumvent the problem, lithium diphenylphosphide (LiPPh₂), generated in situ, was reacted with A at low temperature (Scheme 1). Formation of L^Ph·HBr was confirmed by a phosphorus resonance at δ −8.5 ppm. In the different L^R precursors, the imidazolium backbone protons usually gave rise in the ¹H-NMR spectra to apparent triplets (overlapping dd, ³J_HH ≈ ⁴J_HH ≈ 1.6–1.8 Hz), and the NCHN signal was observed in the range δ 11.17–11.46 ppm.

In the structure of the moderately air-stable L^Ph·HBr (Fig. 4, right) the imidazolium and central aryl ring planes form an angle of 13.4° (vs. 22.6° for L^Cy·HBr·BH₃). Other bond distances and angles are unremarkable. H-bonding interactions in the solid state were evidenced by a close contact between the NCHN proton and the bromide anion, in addition to the high directionality of the C–H⋯Br⁻ interaction.²² Anion metathesis of L^Ph·HBr and L^tBu·HBr with excess NaBF₄ resulted in the corresponding L^Ph·HBF₄ and L^tBu·HBF₄ salts (see Experimental section). In their ¹H-NMR spectra, the signal of the NCHN proton appeared shifted upfield (δ 9.05 and 9.18 ppm, respectively),²³ consistent with weaker hydrogen bonding compared to the bromide salts.

Formation of the free carbenes

The free carbenes L^Cy, L^tBu and L^Ph were obtained by the double deprotonation of the corresponding phosphonium–imidazolium L^R·2HBr or the single deprotonation of phosphine–imidazolium L^R·HBr salts with stoichiometric amounts of KN(SiMe₃)₂ (Scheme 1). The free carbenes were obtained in high yields (79–90%) as very air sensitive, pentane soluble, dark green oils that turned red on standing for ca. 30 min at room temperature. The reason for such colour change is still unclear but probably linked to thermal and/or photochemical instability, however, the products of decomposition were not identified. Despite the difficulties associated with the long-term storage and handling of the isolated L^Cy, L^tBu and L^Ph, unequivocal spectroscopic evidence for their identity and purity was obtained. Deprotonation and carbene formation was evidenced by the disappearance of the imidazolium signal in the ¹H-NMR spectra of the oils and the observation of the NCN carbene resonance at δ 215.9–216.2 ppm.²⁴ Due to the difficult handling of L^R, the synthesis of the coinage metal complexes described below is based on reactions with the imidazolium salt precursors L^R·n(HBr) (n = 1, 2).

Synthesis and structure of silver complexes

The availability of L^R·HBr opened the way for a comparative study of the coordination chemistry of L^R as a function of R. Treatment of L^Ph·HBr with 1 mole equiv. of Ag₂O in acetonitrile, in the presence of 4 Å molecular sieves, afforded [Ag₄Br₄(L^Ph)₂] in low yields (<50%) after recrystallization from CH₂Cl₂/Et₂O (Scheme 2, route (a)). Upon formation of the silver complex, the disappearance of the signal due to the acidic imidazolium proton and the downfield shift of the broad singlet at δ 3.2 ppm in the ¹H-NMR and ³¹P-NMR spectrum, respectively, confirmed NHC formation and coordination of the P atom. The absence of P–Ag couplings (¹⁰⁷Ag 51.8% and ¹⁰⁹Ag 48.2%, both I = 1/2) can be rationalised by a dynamic behaviour involving rapid P–Ag bond breaking/formation on the ³¹P-NMR timescale.^12e In the ¹³C-NMR spectrum, the coordinated C^NHC was detected as a broad singlet (Δν_1/2 = 12 Hz) at δ 186.5 ppm, in the typical range for NHC–Ag complexes.²⁵ The absence of ¹³C–^107/109Ag coupling has been reported in related NHC–AgX clusters,^12a,c,23,26 and points towards dynamic behaviour in solution²⁷ and a high lability of the NHC–Ag bond.^16,28

Scheme 2 Synthesis of the silver complex [Ag₄Br₄(L^Ph)₂] (yields based on L^Ph).

The structure of [Ag₄Br₄(L^Ph)₂] (Fig. 5) corresponds to a distorted Ag₄Br₄ cubane cluster with alternating vertices of the cube being occupied by Ag and Br atoms. The two bridging L^Ph-κP,κC^NHC ligands each span the Ag⋯Ag diagonal of two parallel Ag₂Br₂ faces of the cube, forming 9-membered dimetallocycles, as observed with a closely related phosphinite–NHC ligand^12a,c and in the structures of [Ag₄Br₄(L^R)₂] (R = Cy, tBu) recently reported.^12e All bromides are capping three Ag centres. The Ag⋯Ag separations (3.300(1) Å and 3.400(1) Å) are shorter than the sum of the van der Waals radii for Ag (3.44 Å),²⁹ implying weak d¹⁰–d¹⁰ interactions.³⁰ Related [Ag₄(halide)₄L₂] cubane structures have been described with L = phosphine ligands,³¹ and recently obtained with bidentate ligands incorporating NHC donors (bis-NHC^23,26,32 or P–NHC-type^10g,12a,c,e ligands). Containing non-symmetrical ligands, the observed molecular structure is chiral due to the lack of an improper axis of rotation (see Fig. 6); however, the two enantiomers are present in the asymmetric unit (related by the inversion centre of P1̄).

Fig. 5 The molecular structure of [Ag₄Br₄(L^Ph)₂] with thermal ellipsoids at 40% probability. For clarity, H atoms are omitted and only the ipso carbons of the phenyl substituents in the lower ligand are shown. Selected metrical data are given in Table 1.

Fig. 6 Schematic representation of the two enantiomers of [Ag₄Br₄(L^Ph)₂] in the crystallographic unit cell.

Comparison of [Ag₄Br₄(L^Ph)₂] with the previously reported structures of [Ag₄Br₄(L^R)₂] (R = Cy, tBu)^12e reveals that the substituents on the phosphorus have little influence on the adopted motif or the metrical data. For example, with L^Ph and L^Cy, the Ag–C^NHC and Ag–P bond distances are comparable, while Ag–P is marginally longer in [Ag₄Br₄(L^tBu)₂] (difference <0.040 Å). A more notable difference is in the Ag⋯Ag separation in each bridged face of the pseudocubane (mean Ag⋯Ag ca. 3.350 Å for [Ag₄Br₄(L^Ph)₂], 3.188 Å for [Ag₄Br₄(L^Cy)₂] and 3.089 Å for [Ag₄Br₄(L^tBu)₂]), leading to complexes with increased distortion from the idealised cubane geometry, which may be ascribed to intramolecular repulsions of the bulkier P-substituents.³³ Comparative metrical data for the different silver complexes are provided in Table 1.

Table 1 Selected interatomic distances (Å) and angles [°] for the Ag(I) complexes [Ag₄Br₄(L^R)₂] and [Ag₂(L^R)₂](BF₄)₂

	[Ag4Br4(L^tBu)2]^a	[Ag4Br4(L^Cy)2]^a	[Ag4Br4(L^Ph)2]	[Ag2(L^Ph)2](BF4)2·2CH2Cl2	[Ag2(L^tBu)2](BF4)2·CH2Cl2 ^b
a Data taken from ref. 12e. b There are two dinuclear complexes exhibiting similar metrical data in the asymmetric unit, the second set of values refers to the other molecule.
Ag1⋯Ag2	3.101(1)	3.188(1)	3.400(1)	5.361(1)	5.508(1)/5.762(1)
Ag3⋯Ag4	3.076(1)	3.188(1)	3.300(1)
Ag1⋯Ag3	3.821(1)	3.721(1)	3.761(1)
Ag1⋯Ag4	3.712(1)	3.687(1)	3.562 (1)
Ag1–Br1	2.688(1)	2.880(1)	2.949(1)
Ag1–Br2	3.006(1)	2.812(1)	2.748(1)
Ag1–Br4	2.895(1)	2.708(1)	2.721(1)
Ag2–P1	2.422(1)	2.402(1)	2.407(1)	Ag1–P2 2.386(2)	Ag1–P2 2.376(2)/2.357(2)
Ag4–P2	2.425(1)	2.391(1)	2.388(1)	Ag2–P1 2.376(2)	Ag2–P1 2.375(2)/2.395(2)
Ag1–C1	2.148(4)	2.135(3)	2.147(5)	Ag1–C1 2.113(6)	Ag1–C1 2.106(6)/2.094(6)
Ag3–C27	2.123(4)	2.144(4)	2.127(5)	Ag2–C27 2.108(6)	Ag2–C23 2.109(7)/2.113(7)
N1–C1–N2	103.7(4)	103.9(3)	103.7(5)	104.6(1)	104.1(5)/104.7(5)
N3–C27–N4	103.2(3)	103.3(3)	103.1(5)	105.1(5)	103.1(6)/102.5(6)

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In view of the similarity between [Ag₄Br₄(L^Ph)₂] and [Ag₄Br₄(L^R)₂] (R = Cy, tBu), the latter having been obtained from the corresponding phosphonium–imidazolium salts, we reasoned that better yields of [Ag₄Br₄(L^Ph)₂] should also be attainable by the reaction of L^Ph·2HBr with one mole equiv. Ag₂O. Indeed, the reaction of L^Ph·2HBr with Ag₂O afforded the expected cubane complex in very good yields (>80%). It is worth noticing that the method of choice for the preparation of L^Ph·2HBr consisted of protonation of L^Ph·HBr by dry HBr, generated in situ by methanolysis of an exactly stoichiometric amount of SiMe₃Br in dichloromethane under oxygen-free, controlled conditions (Scheme 2, route (b)). We also noted that the reaction of L^Ph·HBr with 0.5 mole equiv. Ag₂O in acetonitrile resulted in the formation of another silver complex featuring ¹H NMR resonances distinct from [Ag₄Br₄(L^Ph)₂], the structure of which remains elusive to date.

The crucial role of halides in the formation of the cubane structures described above raised the question of the possible reaction outcome under halide-free conditions. The reaction of L^Ph·HBF₄ with 0.5 mole equiv. of Ag₂O in acetonitrile led to the complex [Ag₂(L^Ph)₂](BF₄)₂ (Scheme 3). Examination of its ¹H and ³¹P{¹H} NMR spectra revealed an equilibrium involving two isomers in solution. Notably, dissolution in CD₃CN gave rise, in the ³¹P{¹H} NMR spectrum, to two sets of two doublets (total 8 lines) observed at δ 21.3 (two doublets, ¹J_{P–¹⁰⁷Ag} ≈ 500 Hz, ¹J_{P–¹⁰⁹Ag} ≈ 580 Hz) and 11.2 ppm (two doublets, ¹J_{P–¹⁰⁷Ag} ≈ 475 Hz, ¹J_{P–¹⁰⁹Ag} ≈ 550 Hz) in a 1 : 1.1 ratio, respectively. Evaporation of the solvent and re-dissolution in CD₂Cl₂ led to a similar set of peaks but in a ca. 4 : 1 ratio, respectively. The reversibility of this procedure confirmed the solvent-dependency of the equilibrium. Due to limited solubility in CD₃CN, the ¹³C{¹H}-NMR spectrum was recorded in CD₂Cl₂, where only the signals for the major isomer were clearly visible. In order to gain more insight into the structures of these two isomers, crystallisations from either CH₂Cl₂ or CH₃CN solutions were attempted. Products corresponding to [Ag₂(L^Ph)₂](BF₄)₂·(solvent)_x were obtained from both solvents, which crystallized in different space groups as ‘head-to-tail’ (HT) (heteroleptic) isomers with respect to the mutual arrangement of the ligands. However, the molecular structure of the products (Fig. 7, left and Fig. S1 in ESI‡) revealed the same atom connectivity and very similar metrical data, indicating that only one and the same isomer crystallised (with a possible shift of the equilibrium between ‘head-to-head’ (HH) (homoleptic) and HT isomers upon crystallisation).

Scheme 3 Synthesis of the silver complexes [Ag₂(L^R)₂](BF₄)₂ (R = Ph, tBu) and ‘head-to-tail’ (HT)/‘head-to-head’ (HH) isomerisation in solution (see text). Yields are based on L^R.

Fig. 7 Structure of the dication in [Ag₂(L^Ph)₂](BF₄)₂·2CH₂Cl₂ (left) and of one of the two dications in [Ag₂(L^tBu)₂](BF₄)₂·CH₂Cl₂ (right), with thermal ellipsoids at 40% probability. Anions, H atoms and crystallisation solvent are omitted for clarity. C atoms for the Ph, tBu and nBu groups are depicted as spheres of arbitrary radii (only one C atom is displayed for these groups in the lower ligands). Selected metrical data are given in Table 1. The structure of the [Ag₂(L^Ph)₂](BF₄)₂ complex, obtained by crystallisation from CH₃CN, is presented in the ESI.‡

In the structure of [Ag₂(L^Ph)₂](BF₄)₂·2CH₂Cl₂ (Fig. 7, left), the two L^Ph ligands bridge two Ag metal centres (Ag1⋯Ag2 5.361(1) Å) in a ‘head-to-tail’ arrangement. The C^NHC–Ag–P angles slightly deviate from linearity (C1–Ag1–P2 172.2(2)° and C27–Ag2–P1 172.7(2)°) and the two NHC rings are not parallel, their mean planes forming an angle of 12.8°. Such an arrangement has already been observed in other P–NHC-type silver complexes;^10g,12c,15b the linear coordination geometry is also encountered in bis-NHC silver complexes with non-coordinating anions.^23,34 The Ag–C^NHC bond distances follow trends observed for related complexes,^25a being slightly longer in the NHC silver–halide clusters (mean ca. 2.137 Å)¹⁴ than in the complexes with non-coordinating anions (mean ca. 2.111 Å).¹⁴

In order to gain insight into the solution behaviour of [Ag₂(L^Ph)₂](BF₄)₂, the corresponding [Ag₂(L^tBu)₂](BF₄)₂ was similarly prepared (Scheme 3). In this case too, ¹H- and ³¹P{¹H}-NMR analysis in CD₂Cl₂ revealed the presence of two isomers, in a 1 : 2 ratio, the nature of which could be determined by perusal of the ¹³C{¹H}-NMR spectrum. Spectra of sufficient quality were obtained by acquisition with a cryogenically cooled probe head. A complex pattern (10 lines in total) in the region δ 180–177 ppm, corresponding to the C^NHC–Ag signals was successfully simulated, revealing two different C^NHC–Ag environments associated with the different isomers (Fig. 8): the two doublets centred at δ 178.8 ppm (¹J_{C–¹⁰⁷Ag} = 183 Hz, ¹J_{C–¹⁰⁹Ag} = 212 Hz) were attributed to an isomer with homoleptic Ag^I centres and symmetrical NHC–Ag–NHC coordination (HH isomer), while two doublets of doublets at δ 178.5 ppm (¹J_{C–¹⁰⁷Ag} = 190 Hz, ¹J_{C–¹⁰⁹Ag} = 219 Hz, ²J_P–Ag–C = 62 Hz) were assigned to the second and major isomer, with heteroleptic NHC–Ag–P connectivity (HT isomer). Further indication of the nature of the former isomer was obtained from the observation in ¹³C-NMR of ‘virtual’ triplets of the X_nAA′X′_n (X = X′ = C, A = A′ = P) spin system involving the carbon atoms directly bound to phosphorus, resulting from a strong ²J_PAgP coupling between trans-coordinated P donors.³⁵ Interestingly, for all [Ag₂(L^R)₂](BF₄)₂ (R = Ph, tBu) complexes, the ¹H-NMR signals for the NHC backbone protons were detected as apparent triplets, likely due to ⁴J_HAg and ³J_HH coupling constants falling in the same range.³⁶

Fig. 8 Details of the ¹³C{¹H}-NMR spectrum of [Ag₂(L^tBu)₂](BF₄)₂ in the δ 177–180 ppm region (D) and simulated spectra (TOPSPIN-DAISY module) of the homoleptic (HH) NHC–Ag–NHC isomer (A), the heteroleptic (HT) NHC–Ag–P isomer (B) and both the HH and HT isomers in a 1 : 2 ratio (C). Spectrum recorded in CD₂Cl₂ at 125.77 MHz and 298 K with a cryogenically cooled probe head; accumulation of 1024 scans.

An X-ray diffraction study of [Ag₂(L^tBu)₂](BF₄)₂ also revealed a ‘head-to-tail’ coordination of the bidentate ligand (Fig. 7, right), with two crystallographically independent but very similar dinuclear complexes in the unit cell (Table 1). The bond distances and angles in [Ag₂(L^R)₂]²⁺ for R = Ph and tBu are very close or within experimental error, showing that the nature of the P donor group has only little influence on the solid state structure.

Interestingly, Hofmann and co-workers recently reported the formation of P–NHC-type ‘head-to-head’ and ‘head-to-tail’ copper(I) complexes.^10f Depending on the nature of NHC wingtip, either the homoleptic or the heteroleptic isomer was isolated. Mutual ‘trans-coordination’ of the NHC and P donors, electronically disfavoured,³⁷ was rationalised by minimisation of the steric repulsion in the ‘head-to-head’ complex. Yet for these complexes, no ‘head-to-head’/‘head-to-tail’ isomerisation was detected in different NMR solvents.

Synthesis and structure of dinuclear copper(I) complexes

We have already reported the synthesis of tetranuclear, ladder-type P–NHC-type Cu^I complexes by transmetallation from [Ag₄Br₄(L^R)₂] or by reaction of the phosphonium–imidazolium L^R·2HBr salts with mesitylcopper(I) [Cu₅(Mes)₅],^12e which has been used before to form Cu^I NHC complexes from imidazolium salts.³⁸ The coordination chemistry of the L^R ligands with Cu^I was further investigated by using the monoprotic pro-ligands L^R·HBr.

Reaction of L^R·HBr (R = Ph, tBu, Cy) with [Cu₅(Mes)₅] resulted in the formation of the corresponding [Cu₂Br₂(L^R)₂] complexes in good yields (Scheme 4). Completion of the reaction was evidenced by ¹H NMR spectroscopy (i.e. disappearance of the imidazolium NCHN signal). For all three Cu^I complexes, the ³¹P{¹H}-NMR spectra revealed a singlet assignable to the coordinated P donor, only slightly shifted from the position observed in the starting L^R·HBr. In the ¹³C{¹H}-NMR spectra, the Cu^I–C^NHC resonance was detected in the region δ 183–186 ppm, typical for Cu^I–NHCs.^25b The C^NHC signal was observed as a doublet (²J_PC ≈ 46–47 Hz) for the dialkyl phosphine derivatives or as a broad signal for [Cu₂Br₂(L^Ph)₂], possibly due to a different rate of fluxionality of the C^NHC–Cu bonds in these two complexes. In the ¹H-NMR spectrum of [Cu₂Br₂(L^tBu)₂], the line-shape of the signals for the methylene protons was field-dependent, pointing towards a dynamic process in solution.

Scheme 4 Synthesis of the dinuclear copper(I) complexes [Cu₂Br₂(L^R)₂]. Yields are based on L^R.

The structures of [Cu₂Br₂(L^Cy)₂] and [Cu₂Br₂(L^tBu)₂]·2CH₂Cl₂ were determined crystallographically and are depicted in Fig. 9. Both complexes crystallised as dimers with two L^R ligands bridging the two copper centres, reminiscent of the coordination behaviour of the ligands in [Ag₂(L^R)₂](BF₄)₂. Both structures present a ‘head-to-tail’ arrangement for the NHC and P donors. The three-coordinate Cu centres adopt a distorted planar T-shaped coordination geometry, the third donor being a bromide. The Cu–C^NHC distances, from 1.938(6) to 1.960(6) Å, and the Cu–P bond lengths lie within the range reported for related complexes.^10f,39 The large separation between the two Cu^I centres (from 6.836(1) to 7.138(1) Å) can be traced to the large 1,3-phenylene spacer linking the NHC and phosphine donors.

Fig. 9 The molecular structures of [Cu₂Br₂(L^Cy)₂] (left) and [Cu₂Br₂(L^tBu)₂]·2CH₂Cl₂ (right) with thermal ellipsoids at 40% probability. In [Cu₂Br₂(L^Cy)₂] only one Cy carbon and one disordered position for the nBu chain are shown for clarity. C atoms for the nBu, Cy and tBu groups are depicted as spheres of arbitrary radii. H atoms and crystallisation solvents have been removed for clarity. Selected metrical data are given in Table 2.

In order to study further the dynamic behaviour of the Cu^I complexes in solution, we undertook a variable temperature (VT) ¹H-NMR study of [Cu₂Br₂(L^tBu)₂] in CD₂Cl₂ prompted by its relatively simple line-shape compared to the L^Ph and L^Cy analogues (Fig. 10). At room temperature, very broad signals were observed at 600 MHz for the various protons, suggesting possible coalescence. Upon cooling to −41 °C, two sharp doublets at δ 1.43 and 0.86 ppm (9 H each) assignable to the tBu groups on P indicated a static structure (H_D). At this temperature, the signal of the methylene protons (H_C) was split into two complex multiplets, due to the geminal ²J_HH and ²J_PH coupling in an ABX (A = B = H, X = P) spin system. Interestingly, the NCH₂ protons (H_B) of the NHC wingtip also appeared as diastereotopic. The backbone H_A proton, closer to the aryl spacer, gives rise to a doublet at this temperature owing to ³J_HH coupling. For comparison, at 35 °C, one broad singlet (18 H) was assignable to the tBu groups on P and a doublet was observed for the methylene protons (H_C) in accordance with a relatively fast exchange of their positions on the NMR time scale. The spectral characteristics at lower temperature are in agreement with the solid-state structure being retained in solution. The dynamic behaviour at higher temperatures may have diverse origins, i.e. conformational changes in the dimeric structure involving flipping of the phenylene linker and/or the reversible formation of ‘head-to-head’ coordinated dimers by ligand (hemi)lability. The activation barrier corresponding to the fluxional behaviour of the tBu groups was found to be ΔG^‡ = 56.5 ± 1.0 kJ mol⁻¹. Based on the current data there is no preference for any of the above explanations. The latter hypothesis is however less likely since only one singlet is observed in the ³¹P{¹H}-NMR spectrum at room temperature. Recent work involving ligands with NHC and P donors held together by a CH₂ linker ascribed stereo-isomerisations at the Cu centre to fluxionality.^10f

Fig. 10 VT ¹H NMR (600 MHz) spectra of [Cu₂Br₂(L^tBu)₂] in CD₂Cl₂ at temperatures from −41 to +35 °C.

In contrast, the reaction of [Cu₅(Mes)₅] with the phosphonium–imidazolium L^R·2HBr, or the transmetallation of the corresponding [Ag₄Br₄(L^R)₂] cubanes with 4 mole equiv. of [CuBr·SMe₂] (R = Cy, tBu) gave rise to the tetranuclear clusters [Cu₄Br₄(L^R)₂].^12e Metrical data regarding the di- and tetranuclear Cu^I complexes are reported in Table 2.

Table 2 Selected interatomic distances (Å) and angles [°] for the copper complexes [Cu₂Br₂(L^R)₂] (R = Cy, tBu) and comparison with [Cu₄Br₄(L^Cy)₂]^a

	[Cu2Br2(L^tBu)2]	[Cu2Br2(L^Cy)2]^b	[Cu4Br4(L^Cy)2]^a
a Data taken from ref. 12e. b There are two dinuclear complexes exhibiting similar metrical data in the asymmetric unit, the second set of values refers to the other molecule.
Cu1⋯Cu2	6.899(1)	7.138(1)/6.836(1)	2.790(1)
Cu1–Br1	2.497(1)	2.453(1)/2.493(1)	2.442(1)
Cu2–Br2	2.483(1)	2.438(1)/2.493(1)	2.503(1)
Cu1–P2	2.231(2)	2.222(2)/2.230(2)
Cu2–P1	2.237(2)	2.217(2)/2.230(2)	2.211(2)
Cu1–C1	1.960(6)	1.952(6)/1.947(6)	1.903(5)
Cu2–C23/C27	1.938(6)	1.943(6)/1.947(6)
P2–Cu1–Br1	114.8(1)	109.7(1)/142.4(2)
P2–Cu1–C1	145.1(2)	137.9(2)/107.5(1)
C1–Cu1–Br1	100.0(2)	112.2(2)/110.1(2)
∑angles around Cu1	359.9	359.8/360.0
∑angles around Cu2	360.0	360.0/360.0

---

The longer Cu–C^NHC and Cu–P bond distances in the [Cu₂Br₂(L^R)₂] complexes (mean distances ca. 1.948 and 2.228 Å, respectively) in comparison to the [Cu₄Br₄(L^Cy)₂] cluster (1.903(5) Å and 2.211(2) Å) probably originate from the competition between mutually trans strong P and NHC σ-donors.

Synthesis and structure of a dinuclear gold(I) complex

Since transmetallation of the silver cubane [Ag₄Br₄(L^tBu)₂] with Cu^I always led to tetranuclear complexes,^12e we wondered what would happen with Au^I. Reaction of [Ag₄Br₄(L^tBu)₂] with 4 mole equiv. of [AuCl(THT)] led to the homodinuclear gold complex [Au₂Cl₂L^tBu] (Scheme 5). ¹³C{¹H}-NMR spectral analysis supported the NHC transmetallation as a downfield singlet was detected at δ 170.3 ppm, in a range typical for Au^I–C^NHC functionalities.⁴⁰ A singlet at δ 79.0 ppm in the ³¹P{¹H}-NMR spectrum also confirmed concomitant phosphine transfer to gold. However, a minor peak was observed at δ 80.1 ppm and ascribed to analogous complexes originating from partial halide scrambling (Cl/Br); this was also supported by elemental analysis (cf. Experimental section).

Scheme 5 Transmetallation from [Ag₄Br₄(L^tBu)₂]^12e to obtain the dinuclear [Au₂Cl₂L^tBu] complex.

The structure of [Au₂Cl₂L^tBu] (Fig. 11) revealed an approximate linear coordination of the Au^I centres (P–Au–Cl: 177.7(1)° and C^NHC–Au–Cl: 176.4(2)°), common for NHC gold(I) complexes. The Au–C^NHC (1.985(5) Å) and Au–P (2.239(1) Å) bond distances are in the expected range.^19,40 Contrary to a recent report by Roesky and co-workers on related P–NHC-type gold(I) complexes (Fig. 3) obtained by transmetallation from the silver analogues,¹⁹ no intra- or inter-molecular Au–Au interactions were observed in the solid state for [Au₂Cl₂L^tBu].

Fig. 11 The molecular structure of [Au₂Cl₂L^tBu] with thermal ellipsoids at 40% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles [°]: Au1–Cl1 2.323(1), Au2–Cl2 2.325(1), Au1–C1 1.985(5), Au2–P1 2.239(1); C1–Au1–Cl1 176.39(15), P1–Au2–Cl2 177.69(5), N1–C1–N2 104.3(4).

Attempts to synthesise heterobimetallic silver–gold complexes proved unsuccessful, as the reaction of [Ag₄Br₄(L^tBu)₂] with 2 mole equiv. of [AuCl(THT)] led to a mixture of products containing [Au₂Cl₂L^tBu].

Conclusion

The rational synthesis of a range of hybrid P–NHC-type (pro-)ligands with systematically varied substitution at P, provided insight into their coordination chemistry with coinage metals. The main features observed can be summarised as follows: (i) in all cases studied, the ligands bridge two metal centres, irrespective of the type of phosphine donor; (ii) in the presence of Br⁻, all silver complexes isolated adopt structures based on the [Ag₄Br₄(L^R)₂] motif comprising a distorted Ag₄Br₄ cubane core, bridging L^R ligands and weak metallophilic interactions; (iii) in the presence of the non-coordinating BF₄⁻, [Ag₂(L^R)₂](BF₄)₂ complexes were obtained with bridging ‘head-to-tail’ ligand arrangement in the solid state and ‘head-to-tail’/‘head-to-head’ isomerisation in solution; (iv) the nature of the R substituent on the P end does not impact the structures of the Ag complexes characterised, but seems to influence the rates of dynamic processes in solution, presumably due to competition of electronic and steric factors of the P donor. The relative lability of the two types of donor ends in P–NHC-type hybrid ligands has been inferred from the nature of products obtained from the reaction of [Ag₄Br₄(L^R)₂] with [Ir(COD)(μ-Cl)]₂;^12e (v) dinuclear [Cu₂Br₂(L^R)₂] complexes with bridging ligands were easily accessible from L^R·HBr and [Cu₅(Mes)₅] and are also non-rigid in solution; (vi) transmetallation of [Ag₄Br₄(L^R)₂] with [AuCl(THT)] results in transfer of both donor groups of the hybrid P–NHC-type ligands, leading to the dinuclear [Au₂Cl₂L^tBu] complex.

Guided by the synthesis of non-symmetrical (pro)ligands and through the understanding of their emerging coordination chemistry, ligand alterations may be targeted to favour chelating and/or pincer rather than bridging coordination behaviour. In addition, the pre-organized tethering of the two types of strong σ-donors on the same skeleton (as on L^R) will provide insight into the donor competition behaviour that may lead to (hemi)labile or stable complexes with catalytic potential.^12e

Experimental section

General methods

All air- and moisture-sensitive manipulations were performed under dry argon atmosphere using standard Schlenk techniques. THF and Et₂O were dried by refluxing over sodium/benzophenone ketyl and distilled under an argon atmosphere prior use. Methanol and ethanol were refluxed over CaH₂, distilled under an argon atmosphere and stored over 3 Å molecular sieves. Other solvents (pentane, CH₂Cl₂, toluene and acetonitrile) were dried by passing through columns of activated alumina and subsequently purged with argon. C₆D₆ and toluene-d₈ were distilled over KH; other deuterated solvents were dried over 4 Å (CD₂Cl₂ and CDCl₃) or 3 Å (CD₃OD) molecular sieves, degassed by freeze–pump–thaw cycles, and stored under argon. Mesityl copper(I)⁴¹ and [AuCl(THT)]⁴² were prepared according to literature methods and all other chemicals were obtained from commercial sources and used without further purification. The synthesis of 1-(3-(bromomethyl)phenyl)-3-butyl-1H-imidazol-3-ium bromide (A), L^Cy·2HBr, L^tBu·2HBr, [Ag₄Br₄(L^Cy)₂], [Ag₄Br₄(L^tBu)₂] and [Cu₄Br₄(L^Cy)₂] has already been reported in a recent communication.^12e

NMR spectra were recorded on Bruker spectrometers (AVANCE I – 300 MHz, AVANCE III – 400 MHz, AVANCE III – 600 MHz or AVANCE I – 500 MHz equipped with a cryogenic probe). Downfield shifts are reported in ppm as positive and referenced using signals of the residual protio solvent (¹H), the solvent (¹³C) or externally (³¹P, ¹¹B). All NMR spectra were measured at 298 K, unless otherwise specified. The multiplicity of the signals is indicated as s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sext = sextet, m = multiplet and br = broad. Unequivocal determination of ⁿJ_PC coupling constants in ambiguous cases was carried out by recording the ¹³C{¹H}-NMR spectra on two different field spectrometers. Assignments (Fig. 12) were determined either on the basis of unambiguous chemical shifts, coupling patterns and ¹³C-DEPT experiments or 2D correlations (¹H–¹H COSY, ¹H–¹³C HSQC, ¹H–¹³C HMBC). Spin-simulation was carried out using the DAISY module of the Topspin 2.1 software (BRUKER). Elemental analyses were performed by the “Service de microanalyses”, Université de Strasbourg. Electrospray mass spectra (ESI-MS) were recorded on a microTOF (Bruker Daltonics, Bremen, Germany) instrument using nitrogen as drying agent and nebulizing gas.

Fig. 12 Atom numbering used for the assignment of the NMR resonances.

Synthesis of 3-butyl-1-(3-((dicyclohexylphosphino)methyl)phenyl)-1H-imidazol-3-ium bromide (L^Cy·HBr). To a solution of L^Cy·2HBr (5.51 g, 9.63 mmol) in degassed methanol (15 mL) was added under argon a solution of NEt₃ (6.5 mL, 4.88 g, 48 mmol) in methanol (5 mL). After the resulting solution was stirred at r.t. for 1 h, all the volatiles were evaporated under reduced pressure. The oily residue was redissolved in CH₂Cl₂ and the solution was extracted three times with degassed water to remove the triethylammonium salt. The organic phase was dried over anhydrous MgSO₄ and concentrated under reduced pressure. Addition of a mixture of Et₂O and pentane precipitated L^Cy·HBr as a white powder that was isolated by filtration and dried under vacuum. Yield: 4.20 g (8.55 mmol), 89%. Anal. Calcd for C₂₆H₄₀BrN₂P (491.49): C, 63.54; H, 8.20; N, 5.70. Found: C, 63.04; H, 8.07; N, 5.64. ¹H NMR (500.13 MHz, CD₂Cl₂): δ 11.19 (t, ⁴J_HH = 1.6 Hz, 1H, CH_imid. H2), 7.67 (d, ³J_HH = 7.8 Hz, 1H, CH_arom. H7/H9), 7.64 (t, ³J_HH = ⁴J_HH = 1.7 Hz, 1H, CH_imid. H4/H5), 7.60 (br s, 1H, CH_arom. H11), 7.57 (t, ³J_HH = ⁴J_HH = 1.7 Hz, 1H, CH_imid. H5/H4), 7.47 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 7.42 (d, ³J_HH = 7.8 Hz, 1H, CH_arom. H9/H7), 4.57 (t, ³J_HH = 7.4 Hz, 2H, NCH₂), 2.90 (br s, 2H, CH₂P), 1.98 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.81–1.63 (m, 10H, Cy), 1.63–1.54 (m, 2H, Cy), 1.44 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 1.31–1.06 (m, 10H, Cy), 0.99 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 144.8 (d, ²J_PC = 10.0 Hz, C_arom. C10), 136.7 (CH_imid. C2), 134.9 (C_arom. C6), 131.5 (d, J_PC = 7.1 Hz, CH_arom.), 130.6 (CH_arom.), 122.9 (CH_arom.), 122.4 (d, J_PC = 8.2 Hz, CH_arom), 120.7 (CH_arom.), 119.4 (CH_arom.), 50.4 (NCH₂), 33.9 (d, ¹J_PC = 14.9 Hz, CH_Cy), 32.6 (NCH₂CH₂), 30.2 (d, J_PC = 13.1 Hz, CH_{2 Cy}), 29.7 (d, J_PC = 9.1 Hz, CH_{2 Cy}), 29.3 (d, ¹J_PC = 21.7 Hz, CH₂P), 27.64 (d, J_PC = 10.8 Hz, CH_{2 Cy}), 27.56 (d, J_PC = 8.3 Hz, CH_{2 Cy}), 26.8 (s, CH_{2 Cy}), 19.8 (NCH₂CH₂CH₂), 13.7 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 5.8.

Synthesis of 3-butyl-1-(3-((dicyclohexylphosphino)methyl)phenyl)-1H-imidazol-3-ium bromide borane adduct (L^Cy·HBr·BH₃). To a suspension of L^Cy·HBr (0.50 g, 1.0 mmol) in THF precooled at −10 °C was added dropwise BH₃·SMe₂ (0.55 mL of a 2.0 M THF solution, 1.1 mmol). The reaction mixture was allowed to reach r.t. and stirred for 2 h. All volatiles were evaporated under reduced pressure and the resulting white powder was washed with Et₂O and dried under vacuum. Yield: 0.50 g (0.99 mmol), 99%. Single crystals suitable for X-ray diffraction were obtained by slow diffusion of Et₂O in a CH₂Cl₂ solution of L^Cy·HBr·BH₃. Anal. Calcd for C₂₆H₄₃BBrN₂P (505.33): C, 61.80; H, 8.58; N, 5.54. Found: C, 61.50; H, 8.50; N, 5.52. ¹H NMR (300.13 MHz, CDCl₃): δ 11.42 (t, ⁴J_HH = 1.7 Hz, 1H, CH_imid. H2), 7.91 (dm, ³J_HH = 8.0 Hz, 1H, CH_arom. H7/H9), 7.80 (q, ⁴J_HH = ⁴J_PH = 1.8 Hz, 1H, CH_arom. H11), 7.69 (t, ³J_HH = ⁴J_HH = 1.8 Hz, 1H, CH_imid. H4/H5), 7.52 (t, ³J_HH = 7.9 Hz, 1H, CH_arom. H8), 7.37 (t, ³J_HH = ⁴J_HH = 1.8 Hz, 1H, CH_imid. H5/H4), 7.36 (br overlapping d, ³J_HH ≈ 8.0 Hz, 1H, CH_arom. H9/H7), 4.59 (t, ³J_HH = 7.4 Hz, 2H, NCH₂), 3.13 (d, ²J_PH = 12.1 Hz, 2H, CH₂P), 1.99 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.94–1.63 (m, 12H, Cy), 1.46 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 1.40–1.12 (m, 10H, Cy), 1.00 (t, ³J_HH = 7.3 Hz, 3H, CH₃), 0.41 (br d, ¹J_BH ≈ 90–100 Hz, 3H, BH₃). ¹³C{¹H} NMR (75.48 MHz, CDCl₃): δ 137.0 (d, J_PC = 3.5 Hz), 136.4, 134.3 (d, J_PC = 2.4 Hz), 131.5 (d, J_PC = 4.5 Hz), 130.7, 123.2 (d, J_PC = 3.6 Hz), 122.6, 120.52, 120.48, 50.4 (NCH₂), 32.4 (NCH₂CH₂), 32.1 (d, ¹J_PC = 30.9 Hz, CH_Cy), 27.4 (d, ¹J_PC = 27.1 Hz, CH₂P), 27.1–26.9 (m, 3 CH_{2 Cy}), 26.8 (d, J_PC = 1.4 Hz, CH_{2 Cy}), 26.0 (br s, CH_{2 Cy}), 19.6 (NCH₂CH₂CH₂), 13.6 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CDCl₃): δ 28.7 (br s).

Synthesis of 3-butyl-1-(3-((di-tert-butylphosphino)methyl)phenyl)-1H-imidazol-3-ium bromide (L^tBu·HBr). To a solution of L^tBu·2HBr (1.32 g, 2.54 mmol) in degassed methanol (15 mL) was added under argon a solution of NEt₃ (2 mL, 1.5 g, 15 mmol) in methanol (5 mL). After the resulting solution was stirred at r.t. for 1 h, all the volatiles were evaporated under reduced pressure. The oily residue was redissolved in CH₂Cl₂ and the solution was extracted three times with degassed water to remove the triethylammonium salt. The organic phase was dried over anhydrous MgSO₄ and concentrated under reduced pressure. Addition of a mixture of Et₂O and pentane precipitated L^tBu·HBr as a white powder that was isolated by filtration and dried under vacuum. Yield: 0.72 g (1.64 mmol), 65%. Anal. Calcd for C₂₂H₃₆BrN₂P (439.41): C, 60.13; H, 8.26; N, 6.38. Found: C, 59.90; H, 8.23; N, 6.87. ¹H NMR (500.13 MHz, CD₂Cl₂): δ 11.17 (t, ⁴J_HH = 1.6 Hz, 1H, CH_imid. H2), 7.71 (s, 1H, CH_arom. H11), 7.66 (t, ³J_HH = ⁴J_HH = 1.8 Hz, 1H, CH_imid. H4/H5), 7.64 (d, ³J_HH = 8.2 Hz, 1H, CH_arom. H7/H9), 7.61 (t, ³J_HH = ⁴J_HH = 1.7 Hz, 1H, CH_imid. H5/H4), 7.52 (d, ³J_HH = 7.8 Hz, 1H, CH_arom. H9/H7), 7.46 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 4.57 (t, ³J_HH = 7.2 Hz, 2H, NCH₂), 2.94 (d, ²J_PH = 2.6 Hz, 2H, CH₂P), 1.97 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.43 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 1.13 (d, ³J_PH = 11.0 Hz, 18H, C(CH₃)₃), 0.98 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 146.0 (d, ²J_PC = 13.7 Hz, C_arom. C10), 136.6 (CH_imid. C2), 134.8 (C_arom. C6), 131.8 (d, J_PC = 8.7 Hz, CH_arom.), 130.5 (CH_arom.), 123.0 (CH_arom.), 122.7 (d, J_PC = 9.7 Hz, CH_arom.), 120.8 (CH_arom.), 119.2 (CH_arom.), 50.4 (NCH₂), 32.6 (NCH₂CH₂), 32.2 (d, ¹J_PC = 22.2 Hz, C(CH₃)₃), 29.9 (d, ²J_PC = 13.3 Hz, C(CH₃)₃), 28.7 (d, ¹J_PC = 25.2 Hz, CH₂P), 19.8 (NCH₂CH₂CH₂), 13.7 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 32.0.

Synthesis of 3-butyl-1-(3-((di-tert-butylphosphino)methyl)phenyl)-1H-imidazol-3-ium tetrafluoroborate (L^tBu·HBF₄) by anion metathesis. A solution of L^tBu·HBr (0.55 g, 1.25 mmol) and NaBF₄ (2.75 g, 25 mmol) in degassed ethanol was stirred overnight and evaporated to dryness. The oily residue was redissolved in CH₂Cl₂ and the solution was extracted three times with degassed water. The organic phase was dried over anhydrous MgSO₄ and concentrated under reduced pressure to afford a low melting point solid that was directly used in the next step. Yield: 0.40 g (0.90 mmol), 73%. ¹H NMR (400.13 MHz, CD₂Cl₂): δ 9.18 (s, 1H, CH_imid. H2), 7.63–7.55 (m, 3H, CH_imid. H4/H5 + CH_arom. H7/H9 + CH_arom. H11), 7.49 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 7.47 (t, ³J_HH = ⁴J_HH = 1.8 Hz, 1H, CH_imid. H5/H4), 7.40 (d, ³J_HH = 7.8 Hz, 1H, CH_arom. H9/H7), 4.37 (t, ³J_HH = 7.5 Hz, 2H, NCH₂), 2.95 (d, ²J_PH = 2.7 Hz, 2H, CH₂P), 1.95 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.45 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 1.15 (d, ³J_PH = 11.0 Hz, 18H, C(CH₃)₃), 1.01 (t, ³J_HH = 7.3 Hz, 3H, CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 37.4. ¹¹B NMR (128.38 MHz, CD₂Cl₂): δ −1.0 (quint, ¹J_BF = 1.3 Hz).

Synthesis of 3-butyl-1-(3-((diphenylphosphino)methyl)phenyl)-1H-imidazol-3-ium bromide (L^Ph·HBr). To a solution of diphenylphosphine (0.59 g, 3.17 mmol) in THF (10 mL) cooled at −78 °C was added dropwise a solution of nBuLi in hexane (1.9 mL of a 1.6 M hexane solution, 3.04 mmol). The resulting orange solution was stirred at −78 °C for 45 min and added via cannula to a suspension of A (1.03 g, 2.76 mmol) (see Scheme 1) in THF (5 mL). The resulting mixture was allowed to reach r.t. and was stirred for 3 h. After removing all the volatiles under reduced pressure, the remaining orange oil was redissolved in CH₂Cl₂ and the solution was extracted three times with degassed water. The organic phase was dried over anhydrous MgSO₄ and evaporated to dryness under reduced pressure leading to a sticky powder. Stirring overnight with pentane afforded L^Ph·HBr as an off-white powder, which was isolated by filtration and dried under vacuum. Yield: 0.97 g (2.02 mmol), 73%. Single crystals suitable for X-ray diffraction were obtained by slow diffusion of pentane in a CH₂Cl₂ solution of L^Ph·HBr. Anal. Calcd for C₂₆H₂₈BrN₂P (479.40): C, 65.14; H, 5.89; N, 5.84. Found: C, 65.33; H, 5.92; N, 6.00. ¹H NMR (400.13 MHz, CD₂Cl₂): δ 11.28 (t, ⁴J_HH = 1.6 Hz, 1H, CH_imid. H2), 7.66 (br d, ³J_HH = 8.0 Hz, 1H, CH_arom. H7/H9), 7.46–7.32 (m, 13H, 2 CH_imid. + CH_arom. H8 + 10 CH_PPh), 7.29 (q, ⁴J_HH = ⁴J_PH = 1.8 Hz, 1H, CH_arom. H11), 7.21 (d, ³J_HH = 7.6 Hz, 1H, CH_arom. H9/H7), 4.54 (t, ³J_HH = 7.4 Hz, 2H, NCH₂), 3.54 (s, 2H, CH₂P), 1.96 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.43 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 1.00 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 141.1 (d, ²J_PC = 8.0 Hz, C_arom. C10), 137.8 (d, ¹J_PC = 15.2 Hz, C_PPh), 136.6 (CH_imid. C2), 134.7 (d, ⁴J_PC = 1.8 Hz, C_arom. C6), 133.3 (d, J_PC = 18.8 Hz, CH_PPh), 131.3 (d, ³J_PC = 6.2 Hz, CH_arom. C9), 130.6 (d, ⁴J_PC = 1.6 Hz, CH_arom. C8), 129.3 (CH_PPh), 128.9 (d, J_PC = 6.6 Hz, CH_PPh), 122.9 (CH_imid. C4), 122.5 (d, ³J_PC = 6.1 Hz, CH_arom. C11), 120.5 (CH_imid. C5), 119.7 (d, ⁵J_PC = 2.6 Hz, CH_arom. C7), 50.4 (NCH₂), 35.8 (d, ¹J_PC = 17.1 Hz, CH₂P), 32.5 (NCH₂CH₂), 19.8 (NCH₂CH₂CH₂), 13.6 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ −8.5.

Synthesis of 3-butyl-1-(3-((diphenylphosphonio)methyl)phenyl)-1H-imidazol-3-ium (L^Ph·2HBr). To a solution of L^Ph·HBr (0.35 g, 0.73 mmol) in CH₂Cl₂ (10 mL) was added methanol (0.1 mL, 79 mg, 2.5 mmol) and, dropwise, bromotrimethylsilane (0.11 mL, 128 mg, 0.84 mmol). After 2 h of stirring at room temperature, the solution was concentrated to ca. 2 mL under reduced pressure. Addition of Et₂O precipitated L^Ph·2HBr as an off-white powder that was collected by filtration and dried under vacuum. Yield: 0.40 g (0.72 mmol), 98%. ¹H NMR (500.13 MHz, CDCl₃): δ 11.19 (t, ⁴J_HH = 1.7 Hz, 1H, CH_imid. H2), 10.89 (br d, ¹J_PH ≈ 500 Hz, 1H, PH), 8.75 (t, ³J_HH = ⁴J_HH = 1.8 Hz, 1H, CH_imid. H5), 8.63 (br s, 1H, CH_arom. H11), 8.27 (dd, J_PH = 14.0 Hz, ³J_HH = 7.5 Hz, 4H, CH_PPh), 7.76 (dt, ³J_HH = 8.3 Hz, ⁴J_HH = 2.1 Hz, 1H, CH_arom. H7), 7.67–7.61 (m, 3H, CH_arom. H9 + 2 CH_PPh), 7.54 (dt, ³J_HH = 7.7 Hz, J_PH = 3.2 Hz, 4H, CH_PPh), 7.52 (overlapping t, ³J_HH = ⁴J_HH = 1.7 Hz, 1H, CH_imid. H4), 6.90 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 5.01 (d, ²J_PH = 16.1 Hz, 2H, CH₂P), 4.41 (t, ³J_HH = 7.4 Hz, 2H, NCH₂), 1.97 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.40 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 0.96 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CDCl₃): δ 135.5 (CH_imid. C2), 135.0 (d, ⁴J_PC = 2.6 Hz, CH_PPh), 134.7 (d, ⁴J_PC = 3.5 Hz, C_arom. C6), 134.0 (d, J_PC = 10.7 Hz, CH_PPh), 132.3 (d, ³J_PC = 6.5 Hz, CH_arom. C9), 131.5 (d, ²J_PC = 6.4 Hz, C_arom. C10), 131.0 (d, ⁴J_PC = 2.6 Hz, CH_arom. C8), 130.3 (d, J_PC = 13.0 Hz, CH_PPh), 123.5 (d, ³J_PC = 5.6 Hz, CH_arom. C11), 122.7 (CH_imid. C4), 122.2 (CH_imid. C5), 121.2 (d, ⁵J_PC = 3.6 Hz, CH_arom. C7), 116.3 (d, ¹J_PC ≈ 80 Hz, C_PPh), 50.3 (NCH₂), 32.2 (NCH₂CH₂), 28.0 (d, ¹J_PC = 44.0 Hz, CH₂P), 19.6 (NCH₂CH₂CH₂), 13.6 (CH₃). ³¹P NMR (161.98 MHz, CDCl₃): δ 0.1 (br d, ¹J_PH ≈ 490 Hz, PH).

Synthesis of 3-butyl-1-(3-((diphenylphosphino)methyl)phenyl)-1H-imidazol-3-ium tetrafluoroborate (L^Ph·HBF₄) by anion metathesis. A solution of L^Ph·HBr (1.60 g, 3.34 mmol) and NaBF₄ (7.33 g, 66.8 mmol) in degassed ethanol was stirred overnight and evaporated to dryness. The oily residue was redissolved in CH₂Cl₂ and the solution was extracted three times with degassed water. The organic phase was dried over anhydrous MgSO₄ and concentrated under reduced pressure, leading to a cream-coloured oil. Stirring overnight with Et₂O afforded L^Ph·HBF₄ as an off-white powder, which was isolated by filtration and dried under vacuum. Yield: 1.49 g (3.06 mmol), 92%. ¹H NMR (500.13 MHz, CDCl₃): δ 9.05 (t, ⁴J_HH = 1.7 Hz, 1H, CH_imid. H2), 7.46–7.32 (m, 14H, 2 CH_imid. + 2 CH_arom. + 10 CH_PPh), 7.27 (dm, ³J_HH = 7.1 Hz, 1H, CH_arom. H7/H9), 7.11 (q, ⁴J_HH = ⁴J_PH = 1.8 Hz, 1H, CH_arom. H11), 4.34 (t, ³J_HH = 7.5 Hz, 2H, NCH₂), 3.54 (s, 2H, CH₂P), 1.92 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.43 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 1.00 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ³¹P{¹H} NMR (161.98 MHz, CDCl₃): δ −8.3.

General procedure for the synthesis of free carbene L^R

To a suspension of L^R·HBr (resp. L^R·2HBr) in diethyl ether (10 mL), precooled at −78 °C, was added a solution of 1.1 equiv. (resp. 2.1 equiv.) of KN(SiMe₃)₂ in diethyl ether (10 mL). The resulting suspension was allowed to reach room temperature and stirred for 1 h, giving a suspension of KBr in a yellow solution. Removal of the volatiles under reduced pressure, extraction of the resulting oil with pentane (25 mL) filtration and evaporation of the solvent gave L^R as a dark coloured oil.

Synthesis of 3-butyl-1-(3-((dicyclohexylphosphino)methyl)phenyl)-imidazol-2-ylidene (L^Cy). Following the general procedure, L^Cy was synthesised from L^Cy·HBr (0.20 g, 0.41 mmol) and KN(SiMe₃)₂ (0.088 g, 0.44 mmol). Yield: 0.15 g (0.37 mmol), 90%. The oil turned green over a period of 1 h even when stored under inert atmosphere, however spectroscopic data remained unchanged. ¹H NMR (400.13 MHz, C₆D₆): δ 8.20 (s, 1H, CH_arom. H11), 7.67 (d, ³J_HH = 7.8 Hz, 1H, CH_arom. H7/H9), 7.22 (d, ³J_HH = 7.6 Hz, 1H, CH_arom. H9/H7), 7.15 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 7.06 (d, ³J_HH = 1.6 Hz, 1H, CH_imid. H4/H5), 6.44 (br s, 1H, CH_imid. H5/H4), 3.87 (t, ³J_HH = 7.2 Hz, 2H, NCH₂), 2.74 (s, 2H, CH₂P), 1.84–1.75 (m, 2H, CH_Cy), 1.74–1.47 (m, 10H, CH_{2 Cy}), 1.60 (quint, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂), 1.28–1.04 (m, 10H, CH_{2 Cy}), 1.18 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 0.78 (t, ³J_HH = 7.3 Hz, 3H, CH₃). ¹³C{¹H} NMR (100.62 MHz, C₆D₆): δ 216.2 (C_NHC), 143.1, 142.2 (d, J_PC = 9.0 Hz), 129.2, 127.1 (d, J_PC = 7.6 Hz), 122.3 (d, J_PC = 7.4 Hz), 119.7, 118.0, 116.9, 51.2 (NCH₂), 34.0 (d, ¹J_PC = 16.6 Hz, CH_Cy), 33.8 (NCH₂CH₂), 30.3 (d, J_PC = 13.2 Hz, CH_{2 Cy}), 29.81 (d, ¹J_PC = 21.5 Hz, CH₂P), 29.79 (d, J_PC = 10.0 Hz, CH_{2 Cy}), 27.7 (d, J_PC = 9.7 Hz, CH_{2 Cy}), 27.6 (d, J_PC = 7.6 Hz, CH_{2 Cy}), 26.9 (CH_{2 Cy}), 20.0 (NCH₂CH₂CH₂), 13.9 (CH₃). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ 0.0.

Synthesis of 3-butyl-1-(3-((di-tert-butylphosphino)methyl)phenyl)-imidazol-2-ylidene (L^tBu). Following the general procedure, L^tBu was synthesised from L^tBu·2HBr (0.53 g, 1.02 mmol) and KN(SiMe₃)₂ (0.42 g, 2.12 mmol). Yield: 0.29 g (0.81 mmol), 79% (dark brown oil). ¹H NMR (400.13 MHz, toluene-d₈): δ 8.09 (q, ⁴J_HH = ⁴J_PH = 1.7 Hz, 1H, CH_arom. H11), 7.58 (d, ³J_HH = 7.9 Hz, 1H, CH_arom. H7/H9), 7.23 (d, ³J_HH = 7.8 Hz, 1H, CH_arom. H9/H7), 7.11 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 7.09 (d, ³J_HH = 1.7 Hz, 1H, CH_imid. H4/H5), 6.52 (d, ³J_HH = 1.6 Hz, 1H, CH_imid. H5/H4), 3.85 (t, ³J_HH = 7.3 Hz, 2H, NCH₂), 2.71 (d, ²J_PH = 2.3 Hz, 2H, CH₂P), 1.60 (quint, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂), 1.19 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 1.02 (d, ³J_PH = 10.7 Hz, 18H, C(CH₃)₃), 0.79 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (100.62 MHz, toluene-d₈): δ 215.9 (C_NHC), 143.3 (d, J_PC = 13.0 Hz), 142.9, 129.0, 127.3 (d, J_PC = 9.7 Hz), 122.3 (d, J_PC = 8.6 Hz), 119.7, 117.9, 116.9, 51.2 (NCH₂), 34.0 (NCH₂CH₂), 31.9 (d, ¹J_PC = 24.3 Hz, C(CH₃)₃), 30.0 (d, ²J_PC = 13.6 Hz, C(CH₃)₃), 29.1 (d, ¹J_PC = 25.8 Hz, CH₂P), 20.2 (NCH₂CH₂CH₂), 13.9 (CH₃). ³¹P{¹H} NMR (161.98 MHz, toluene-d₈): δ 33.1.

Synthesis of 3-butyl-1-(3-((diphenylphosphino)methyl)phenyl)-imidazol-2-ylidene (L^Ph). Following the general procedure, L^Ph was synthesised from L^Ph·HBr (0.057 g, 0.12 mmol) and KN(SiMe₃)₂ (0.026 g, 0.13 mmol). Yield: 0.040 g (0.10 mmol), 84% (dark-green oil). ¹H NMR (300.17 MHz, C₆D₆): δ 7.78 (dm, ³J_HH = 7.9 Hz, 1H, CH_arom. H7/H9), 7.72 (q, ⁴J_HH = ⁴J_PH = 1.7 Hz, 1H, CH_arom. H11), 7.39–7.30 (m, 4H, CH_PPh), 7.07–7.01 (m, 6H, CH_PPh), 7.01 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 6.89 (d, ³J_HH = 1.7 Hz, 1H, CH_imid. H4/H5), 6.85 (dm, ³J_HH = 7.8 Hz, 1H, CH_arom. H9/H7), 6.44 (d, ³J_HH = 1.7 Hz, 1H, CH_imid. H5/H4), 3.86 (t, ³J_HH = 7.2 Hz, 2H, NCH₂), 3.24 (s, 2H, CH₂P), 1.60 (quint, ³J_HH = 7.3 Hz, 2H, NCH₂CH₂), 1.18 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 0.79 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (75.49 MHz, C₆D₆): δ 216.1 (C_NHC), 142.9, 139.09 (d, J_PC = 16.5 Hz, C_PPh), 139.05 (d, J_PC = 8.2 Hz), 133.4 (d, J_PC = 18.6 Hz, CH_PPh), 129.2 (d, J_PC = 1.6 Hz), 128.8 (CH_PPh), 128.6 (d, J_PC = 6.4 Hz, CH_PPh), 127.0 (d, J_PC = 6.8 Hz), 122.0 (d, J_PC = 6.7 Hz), 119.7, 118.8 (d, J_PC = 2.7 Hz), 116.9, 51.2 (NCH₂), 36.3 (d, ¹J_PC = 16.8 Hz, CH₂P), 33.8 (NCH₂CH₂), 20.0 (NCH₂CH₂CH₂), 13.9 (CH₃). ³¹P{¹H} NMR (161.98 MHz, C₆D₆): δ −9.9.

Synthesis of the tetranuclear silver cluster [Ag₂(μ₃-Br)₂(μ-PPh₂-NHC,κP,κC^NHC)]₂ ([Ag₄Br₄(L^Ph)₂]) (route (b)). L^Ph·2HBr (0.40 g, 0.72 mmol) and Ag₂O (0.185 g, 0.80 mmol) were charged in a Schlenk flask along with molecular sieves 4 Å. Degassed acetonitrile (20 mL) was added and the mixture was stirred for 2 days at 40 °C under exclusion of light. After evaporation of the solvent under reduced pressure, the remaining slurry was extracted twice with CH₂Cl₂, and the resulting solution was filtered over Celite® and concentrated to ca. 1 mL. Complex [Ag₄Br₄(L^Ph)₂] was precipitated by addition of Et₂O. The white powder was collected by filtration and dried under vacuum. Yield: 0.46 g (0.30 mmol), 83% based on the ligand. Single crystals suitable for X-ray diffraction were obtained by slow diffusion of Et₂O in a CH₂Cl₂ solution of the complex. Anal. Calcd for C₅₂H₅₄Ag₄Br₄N₄P₂ (1548.05): C, 40.34; H, 3.52; N, 3.62. Found: C, 40.23; H, 3.61; N, 3.45. ¹H NMR (500.13 MHz, CD₂Cl₂): δ 8.55 (q, ⁴J_HH = ⁴J_PH = 1.8 Hz, 1H, CH_arom. H11), 7.79–7.74 (m, 4H, CH_PPh), 7.48–7.38 (m, 6H, CH_PPh), 7.24 (d, ³J_HH = 1.7 Hz, 1H, CH_imid. H4/H5), 7.18 (dm, ³J_HH = 7.8 Hz, 1H, CH_arom. H7), 7.08 (d, ³J_HH = 1.7 Hz, 1H, CH_imid. H5/H4), 7.07 (t, ³J_HH = 7.8 Hz, 1H, CH_arom. H8), 6.34 (br d, ³J_HH = 7.7 Hz, 1H, CH_arom. H9), 4.27 (t, ³J_HH = 7.4 Hz, 2H, NCH₂), 3.69 (d, ²J_PH = 7.8 Hz, 2H, CH₂P), 1.84 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.41 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 0.94 (t, ³J_HH = 7.5 Hz, 3H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 186.5 (br s, Δν_1/2 = 12 Hz, C_NHC), 141.4 (d, J_PC = 3.3 Hz, C_arom. C6/C10), 136.9 (d, J_PC = 2.4 Hz, C_arom. C10/C6), 134.0 (d, J_PC = 16.3 Hz, CH_PPh), 132.8 (d, ¹J_PC = 21.1 Hz, C_PPh), 130.8 (d, J_PC = 1.6 Hz, CH_PPh), 129.5 (d, ³J_PC = 4.8 Hz, CH_arom. C9), 129.0 (d, J_PC = 9.6 Hz, CH_PPh), 128.8 (d, ⁴J_PC = 2.8 Hz, CH_arom. C8), 126.8 (d, ³J_PC = 5.2 Hz, CH_arom. C11), 121.4 (d, ⁵J_PC = 3.5 Hz, CH_arom. C7), 121.2 (CH_imid. C4/C5), 121.1 (CH_imid. C5/C4), 52.3 (NCH₂), 35.5 (d, ¹J_PC = 8.8 Hz, CH₂P), 34.0 (NCH₂CH₂), 20.2 (NCH₂CH₂CH₂), 14.0 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 3.0 (br s). MS (ESI+): m/z (%) 1540.68 (<1) [M + H]⁺, 1248.96 (100) [M − 2Ag − Br + 2H]⁺i.e. [C₅₂H₅₆Ag₂Br₃N₄P₂]⁺ with the corresponding isotopic pattern.

General procedure for the synthesis of silver(I) complexes [Ag₂(L^R)₂](BF₄)₂

L^R·HBF₄ and Ag₂O (0.55 equiv.) were charged in a Schlenk flask along with molecular sieves 4 Å. Degassed acetonitrile (15 mL) was added and the mixture was stirred for 2 days at 40 °C under exclusion of light. After evaporation of the solvent under reduced pressure, the remaining slurry was extracted twice with CH₂Cl₂ and the resulting solution was filtered over Celite® and concentrated to ca. 1 mL. The complex [Ag₂(L^R)₂](BF₄)₂ was precipitated with diethyl ether. The white powder was collected by filtration and dried under vacuum.

Synthesis of [Ag(μ-PPh₂-NHC,κP,κC^NHC)]₂(BF₄)₂ ([Ag₂(L^Ph)₂](BF₄)₂). Following the general procedure, [Ag₂(L^Ph)₂](BF₄)₂ was synthesised from L^Ph·HBF₄ (0.12 g, 0.25 mmol) and Ag₂O (0.032 g, 0.14 mmol). Yield: 0.13 g (0.11 mmol), 85% based on the ligand. Single crystals suitable for X-ray diffraction were obtained by slow diffusion of Et₂O in a CH₂Cl₂ or CH₃CN solution of the complex. Anal. Calcd for C₅₂H₅₄Ag₂B₂F₈N₄P₂·0.6CH₂Cl₂ (1237.28): C, 51.06; H, 4.50; N, 4.53. Found: C, 50.98; H, 4.42; N, 4.72. Examination of the ¹H and ³¹P{¹H} NMR spectra revealed the presence of the “head-to-tail” and “head-to-head” isomers in a ca. 4 : 1 HT/HH ratio in CD₂Cl₂ (see text) and 1 : 1.1 in CD₃CN. ¹H NMR (400.13 MHz, CD₂Cl₂): δ 8.00 (br q, ⁴J_HH = ⁴J_PH = 1.6 Hz, 0.8H, CH_arom. H11), 7.64 (t, ³J_HH = ⁴J_HAg = 1.7 Hz, 0.8H, CH_imid.), 7.63–7.22 (m, 11.6H, CH_arom.), 7.17 (t, ³J_HH = ⁴J_HAg = 1.7 Hz, 0.8H, CH_imid.), 7.09 (t, ³J_HH = 7.9 Hz, 0.2H, CH_arom. H8), 7.04 (t, ³J_HH = 7.8 Hz, 0.8H, CH_arom. H8), 6.92 (s, 0.2H, CH_arom.), 6.60 (d, ³J_HH = 7.7 Hz, CH_arom. H7/H9), 4.30 (t, ³J_HH = 7.3 Hz, 1.6H, NCH₂), 4.20 (t, ³J_HH = 7.3 Hz, 0.4H, NCH₂), 3.93 (AB part of an ABX spin system with A = B = H and X = P, J = 10.5, 4.6 Hz, 1.6H, CH₂P), 3.89–3.82 (br s, 0.4H, CH₂P), 1.91 (quint, ³J_HH = 7.4 Hz, 0.4H, NCH₂CH₂), 1.82 (quint, ³J_HH = 7.4 Hz, 1.6H, NCH₂CH₂), 1.40 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 1.03 (t, ³J_HH = 7.3 Hz, 0.6H, CH₃), 0.96 (t, ³J_HH = 7.4 Hz, 2.4H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ C_NHC peak not observed, 140.7 (C_arom.), 137.6 (C_arom.), 133.6 (dd, J_PC = 14.6 Hz, J_AgC = 2.3 Hz, CH_PPh), 132.1 (d, J_PC = 1.8 Hz, CH_PPh), 130.4 (d, J = 5.4 Hz, CH_arom.), 129.9 (d, J = 2.9 Hz, CH_arom.), 129.8 (d, J_PC = 10.4 Hz, CH_PPh), 129.1 (dd, ¹J_PC = 36.7 Hz, ²J_AgC = 4.4 Hz, C_PPh), 126.3 (d, J = 8.4 Hz, CH_arom.), 122.8 (d, J = 5.5 Hz, CH_arom.), 122.7 (d, J = 4.0 Hz, CH_arom.), 122.2 (d, J = 6.3 Hz, CH_arom.), 52.3 (NCH₂), 34.4 (d, ¹J_PC = 17.1 Hz, CH₂P), 33.9 (NCH₂CH₂), 19.9 (NCH₂CH₂CH₂), 13.9 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 25.2 (two doublets, ¹J_{P–¹⁰⁷Ag} = 503 Hz, ¹J_{P–¹⁰⁹Ag} = 581 Hz, integrating for 0.8P), 17.6 (two doublets, ¹J_{P–¹⁰⁷Ag} ≈ 515 Hz, ¹J_{P–¹⁰⁹Ag} ≈ 595 Hz, integrating for 0.2P). ¹¹B NMR (128.38 MHz, CD₂Cl₂): δ −0.9 (quint, ¹J_BF = 1.5 Hz).

¹H NMR (400.13 MHz, CD₃CN): δ 7.67–7.25 (m, 13.6H, CH_arom.), 7.14–6.88 (br s, 2H, CH_arom.), 6.82–6.60 (br s, 0.4H, CH_arom.), 5.45 (s, 0.4H, residual CH₂Cl₂), 4.20 (t, ³J_HH = 7.3 Hz, 2H, NCH₂), 3.70 (br s, 2H, CH₂P), 1.91–1.69 (br s, 2H, NCH₂CH₂), 1.37 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 0.98 (t, ³J_HH = 7.3 Hz, 3H, CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₃CN): δ 21.3 (two doublets, ¹J_{P–¹⁰⁷Ag} ≈ 500 Hz, ¹J_{P–¹⁰⁹Ag} ≈ 580 Hz, integrating for 1.0P), 11.2 (two doublets, ¹J_{P–¹⁰⁷Ag} ≈ 475 Hz, ¹J_{P–¹⁰⁹Ag} ≈ 550 Hz, integrating for 1.1P).

Synthesis of [Ag(μ-PtBu₂-NHC,κP,κC^NHC)]₂(BF₄)₂ ([Ag₂(L^tBu)₂](BF₄)₂). Following the general procedure, [Ag₂(L^tBu)₂](BF₄)₂ was synthesised from L^tBu·HBF₄ (0.40 g, 0.90 mmol) and Ag₂O (0.12 g, 0.50 mmol). Yield: 0.34 g (0.31 mmol), 68% based on the ligand. Single crystals suitable for X-ray diffraction were obtained by slow diffusion of toluene in a CH₂Cl₂ solution of the complex. Anal. Calcd for C₄₄H₇₀Ag₂B₂F₈N₄P₂ (1106.36): C, 47.77; H, 6.38; N, 5.06. Found: C, 47.88; H, 6.20; N, 5.02. Examination of the ¹H and ³¹P{¹H} NMR spectra revealed the presence of the “head-to-head” and “head-to-tail” isomers in a HH/HT ratio of ca. 1 : 2 in CD₂Cl₂ (see text). ¹H NMR (400.13 MHz, CD₂Cl₂): 7.74 (br s, 0.7H, CH_arom. H11), 7.66 (t, ³J_HH = ⁴J_HAg = 1.7 Hz, 0.7H, CH_imid.), 7.58 (t, ³J_HH = ⁴J_HAg = 1.8 Hz, 0.3H, CH_imid.), 7.50 (br s, 0.3H, CH_arom. H11), 7.40 (br d, ³J_HH = 7.8 Hz, 0.3H, CH_arom. H7/H9), 7.35 (br d, ³J_HH = 7.9 Hz, 0.7H, CH_arom. H7/H9), 7.31 (t, ³J_HH = ⁴J_HAg = 1.8 Hz, 0.3H, CH_imid.), 7.31–7.26 (overlapping m, 0.7H, CH_arom. H9/H7), 7.28 (t, ³J_HH = ⁴J_HAg = 1.7 Hz, 0.7H, CH_imid.), 7.23–7.19 (overlapping m, 0.3H, CH_arom. H9/H7), 7.23 (t, ³J_HH = 7.8 Hz, 0.7H, CH_arom. H8), 7.14 (t, ³J_HH = 7.8 Hz, 0.3H, CH_arom. H8), 4.25 (t, ³J_HH = 7.3 Hz, 0.6H, NCH₂), 4.18 (t, ³J_HH = 7.3 Hz, 1.4H, NCH₂), 3.32 (AB part of an ABX spin system with A = B = H and X = P, J = 10.1 Hz, 4.2 Hz, 1.4H, CH₂P), 3.27 (AB part of an ABX spin system with A = B = H and X = P, J = 9.0 Hz, 3.7 Hz, 0.6H, CH₂P), 1.96 (quint, ³J_HH = 7.5 Hz, 0.6H, NCH₂CH₂), 1.86 (quint, ³J_HH = 7.5 Hz, 1.4H, NCH₂CH₂), 1.46 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 1.40 (virtual t, |³J_PCH + ⁵J_PH| = 14.9 Hz, 6H, C(CH₃)₃), 1.29 (d, ³J_PH = 14.7 Hz, 12H, C(CH₃)₃), 1.04 (t, ³J_HH = 7.3 Hz, 1H, CH₃), 1.01 (t, ³J_HH = 7.4 Hz, 2H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 178.8 (two doublets, ¹J_{C–¹⁰⁷Ag} = 183 Hz, ¹J_{C–¹⁰⁹Ag} = 212 Hz, minor C_NHC), 178.5 (two dd, ¹J_{C–¹⁰⁷Ag} = 190 Hz, ¹J_{C–¹⁰⁹Ag} = 219 Hz, ²J_P–Ag–C = 62 Hz, major C_NHC), 141.3 (minor C_arom.), 141.1 (major C_arom.), 140.6 (minor C_arom.), 140.2 (major C_arom.), 131.1 (d, J_PC = 5.7 Hz, major CH_arom.), 130.7 (minor CH_imid.), 130.3 (major CH_imid.), 129.1 (virtual t, |ⁿJ_PC + ⁿ⁺²J_PAgPC| = 8.2 Hz, minor CH_arom.), 125.9 (d, J_PC = 9.6 Hz, major CH_arom.), 125.2 (virtual t, |ⁿJ_PC + ⁿ⁺²J_PAgPC| = 6.0 Hz, minor CH_arom.), 123.0 (d, J_PC = 5.8 Hz, major CH_arom.), 122.9 (d, J_PC = 5.8 Hz, minor CH_arom.), 122.8 (d, J = 5.2 Hz, minor CH_arom.), 122.5 (minor CH_imid.), 122.3 (major CH_imid.), 122.1 (d, J = 5.3 Hz, major CH_arom.), 52.7 (minor NCH₂), 52.6 (major NCH₂), 35.3 (doublet of virtual t, |¹J_PC + ³J_PAgPC| = 11.9 Hz, ²J_CAg = 3.9 Hz, minor C(CH₃)₃), 34.8 (dd, ¹J_PC = 12.2 Hz, ²J_CAg = 3.7 Hz, major C(CH₃)₃), 34.3 (major NCH₂CH₂), 33.9 (minor NCH₂CH₂), 29.95–29.75 (m, C(CH₃)₃), 27.0 (d, ¹J_PC = 12.9 Hz, major CH₂P), 26.4 (virtual t, |¹J_PC + ³J_PAgPC| = 14.0 Hz, minor CH₂P), 20.3 (minor NCH₂CH₂CH₂), 20.1 (major NCH₂CH₂CH₂), 13.84 (major CH₃), 13.81 (minor CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 73.6 (two doublets, ¹J_{P–¹⁰⁷Ag} = 460 Hz, ¹J_{P–¹⁰⁹Ag} = 530 Hz, integrating for 1P), 72.4 (two doublets, ¹J_{P–¹⁰⁷Ag} = 477 Hz, ¹J_{P–¹⁰⁹Ag} = 552 Hz, integrating for 2P).

General procedure for the synthesis of copper(I) complexes [Cu₂Br₂(L^R)₂]

To a suspension of L^R·HBr in THF (5 mL) was added a solution of mesityl copper(I) (1.0 equiv. based on Cu) in THF (5 mL) at room temperature. The resulting clear solution was stirred at room temperature for 24 h. After evaporation of the solvent under reduced pressure, the solid residue was washed with diethyl ether and redissolved in CH₂Cl₂. Addition of pentane precipitated [Cu₂Br₂(L^R)₂] as a white powder that was collected and dried under vacuum.

Synthesis of [CuBr(μ-PCy₂-NHC,κP,κC^NHC)]₂ [Cu₂Br₂(L^Cy)₂]. Following the general procedure, [Cu₂Br₂(L^Cy)₂] was synthesised from L^Cy·HBr (0.19 g, 0.38 mmol) and mesityl copper(I) (0.073 g, 0.40 mmol). Yield: 0.20 g (0.18 mmol), 95% based on the ligand. Single crystals suitable for X-ray diffraction were obtained by slow vapour diffusion of Et₂O in a THF solution of the complex. Anal. Calcd for C₅₂H₇₈Br₂Cu₂N₄P₂ (1108.05): C, 56.37; H, 7.10; N, 5.06. Found: C, 56.52; H, 7.31; N, 4.92. ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.77 (d, ³J_HH = 7.5 Hz, 1H, CH_arom. H7), 7.26 (t, ³J_HH = 7.6 Hz, 1H, CH_arom. H8), 7.17 (s, 1H, CH_arom. H11), 7.09 (d, ³J_HH = 7.5 Hz, 1H, CH_arom. H9), 6.91 (s, 1H, CH_imid. H4), 6.70 (s, 1H, CH_imid. H5), 4.20 (t, ³J_HH = 7.4 Hz, 2H, NCH₂), 2.78 (d, ²J_PH = 6.5 Hz, 2H, CH₂P), 1.97–1.53 (m, 12H, Cy), 1.80 (quint, ³J_HH = 7.6 Hz, 2H, NCH₂CH₂), 1.50–1.12 (m, 10H, Cy), 1.41 (sext, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂CH₂), 0.96 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 185.1 (d, ²J_PC = 47.0 Hz, C_NHC), 140.7 (C_arom. C6), 139.0 (C_arom. C10), 129.6 (CH_arom. C8), 129.4 (d, ³J_PC = 3.1 Hz, CH_arom. C9), 123.7 (d, ³J_PC = 4.5 Hz, CH_arom. C11), 122.5 (CH_arom. C7), 120.9 (CH_imid. C4), 119.3 (CH_imid. C5), 51.4 (NCH₂), 34.3 (NCH₂CH₂), 34.0 (d, ¹J_PC = 14.6 Hz, CH_Cy), 29.5 (d, ³J_PC = 2.5 Hz, CH_{2 Cy}), 29.2 (d, ³J_PC = 4.0 Hz, CH_{2 Cy}), 28.9 (d, ¹J_PC = 8.6 Hz, CH₂P), 27.6 (d, ²J_PC = 11.8 Hz, CH_{2 Cy}), 27.3 (d, ²J_PC = 10.2 Hz, CH_{2 Cy}), 26.5 (CH_{2 Cy}), 20.4 (NCH₂CH₂CH₂), 14.0 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 7.4.

Synthesis of [CuBr(μ-PtBu₂-NHC,κP,κC^NHC)]₂ ([Cu₂Br₂(L^tBu)₂]). Following the general procedure, [Cu₂Br₂(L^tBu)₂] was synthesised from L^tBu·HBr (0.18 g, 0.41 mmol) and mesityl copper(I) (0.078 g, 0.43 mmol). Yield: 0.18 g (0.17 mmol), 85% based on the ligand. Single crystals suitable for X-ray diffraction were obtained by slow diffusion of toluene in a CH₂Cl₂ solution of the complex. Anal. Calcd for C₄₄H₇₀Br₂Cu₂N₄P₂ (1003.90): C, 52.64; H, 7.03; N, 5.58. Found: C, 52.61; H, 7.22; N, 5.67. ¹H NMR (400.13 MHz, CD₂Cl₂): δ 8.12 (d, ³J_HH = 7.3 Hz, 1H, CH_arom. H7), 7.44 (s, 1H, CH_arom. H11), 7.17 (t, ³J_HH = 7.6 Hz, 1H, CH_arom. H8), 7.08 (d, ³J_HH = 7.5 Hz, 1H, CH_arom. H9), 6.93 (d, ³J_HH = 1.4 Hz, 1H, CH_imid. H4), 6.67 (s, 1H, CH_imid. H5), 4.27 (br s, 2H, NCH₂), 2.98 (d, ²J_PH = 7.4 Hz, 2H, CH₂P), 2.01 (quint, ³J_HH = 7.6 Hz, 2H, NCH₂CH₂), 1.50 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 1.56–0.98 (br s, 18H, C(CH₃)₃), 1.01 (t, ³J_HH = 7.3 Hz, 3H, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 185.2 (d, ²J_PC = 46.0 Hz, C_NHC), 140.7 (C_arom. C6), 140.3 (C_arom. C10), 130.5 (d, J_PC = 3.2 Hz, CH_arom. C9), 129.6 (CH_arom. C8), 123.5 (CH_arom. C7), 122.8 (CH_arom. C11), 121.2 (CH_imid. C4), 119.3 (CH_imid. C5), 51.5 (NCH₂), 34.7 (br d, ¹J_PC = 7.0 Hz, C(CH₃)₃), 34.1 (NCH₂CH₂), 29.8 (br s, C(CH₃)₃), 27.9 (d, ¹J_PC = 5.9 Hz, CH₂P), 20.5 (NCH₂CH₂CH₂), 14.0 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 32.2.

Synthesis of [CuBr(μ-PPh₂-NHC,κP,κC^NHC)]₂ ([Cu₂Br₂(L^Ph)₂]). Following the general procedure, [Cu₂Br₂(L^Ph)₂] was synthesised from L^Ph·HBr (0.13 g, 0.27 mmol) and mesityl copper(I) (0.057 g, 0.31 mmol). Yield: 0.13 g (0.12 mmol), 90% based on the ligand. Anal. Calcd for C₅₂H₅₄Br₂Cu₂N₄P₂ (1083.86): C, 57.62; H, 5.02; N, 5.17. Found: C, 55.66; H, 4.86; N, 4.92. Better elemental analyses and single crystals suitable for X-ray diffraction studies could not be obtained despite several attempts. ¹H NMR (300.13 MHz, CD₂Cl₂): δ 7.61 (m, 4H, CH_PPh), 7.50–7.30 (m, 8H, CH_imid. + CH_arom. + 6 CH_PPh), 7.03 (br s, 1H, CH_arom.), 6.96 (d, ³J_HH = 1.6 Hz, 1H, CH_imid.), 6.72 (t, ³J_HH = 7.5 Hz, 1H, CH_arom.), 6.44 (d, ³J_HH = 7.3 Hz, 1H, CH_arom.), 4.13 (t, ³J_HH = 7.4 Hz, 2H, NCH₂), 3.24 (br s, 2H, CH₂P), 1.65 (quint, ³J_HH = 7.5 Hz, 2H, NCH₂CH₂), 1.24 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 0.85 (t, ³J_HH = 7.3 Hz, CH₃). ¹³C{¹H} NMR (125.77 MHz, CD₂Cl₂): δ 183.7 (br s, C_NHC), 140.4 (C_arom.), 137.1 (C_arom.), 133.7 (d, J_PC = 13.8 Hz, CH_PPh), 133.4 (d, ¹J_PC = 25.4 Hz, C_PPh), 130.4 (CH_PPh), 129.4 (CH_arom.), 129.1 (br s, CH_arom.), 128.9 (d, J_PC = 8.5 Hz, CH_PPh), 124.1 (br s, CH_arom.), 121.2 (CH_arom.), 120.9 (CH_arom.), 119.4 (br s, CH_arom.), 51.5 (NCH₂), 34.7 (d, ¹J_PC = 11.5 Hz, CH₂P), 34.0 (NCH₂CH₂), 20.2 (NCH₂CH₂CH₂), 13.9 (CH₃). ³¹P{¹H} NMR (121.49 MHz, CD₂Cl₂): δ −9.8.

Synthesis of [Au₂Cl₂(μ-P(tBu)₂-NHC,κP,κC^NHC)] ([Au₂Cl₂L^tBu]). To a solution of [Ag₄Br₄(L^tBu)₂] (0.076 g, 0.052 mmol) in CH₂Cl₂ (5 mL) was added a solution of [AuCl(THT)] (4 equiv., 0.066 g, 0.21 mmol) in CH₂Cl₂ (2 mL) under protection against light. A white precipitate appeared instantaneously and the resulting suspension was stirred overnight. Filtration through Celite® and evaporation of the solvent afforded [Au₂Cl₂L^tBu] as a white powder. Yield: 0.083 g (0.10 mmol), 97%. Single crystals suitable for X-ray diffraction were obtained by slow vapour diffusion of Et₂O in a CH₂Cl₂ solution of the complex. Anal. Calcd for C₂₂H₃₅Au₂BrCl₂N₂P (823.34): C, 32.09; H, 4.28; N, 3.40. Found: C, 31.25; H, 4.13; N, 3.24. These experimental values fit better with the formula C₂₂H₃₅Au₂Br_0.5Cl_1.5N₂P (845.57): C, 31.25; H, 4.17; N, 3.31, corresponding to partial halide exchange between AgBr and AuCl. ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.89 (s, 1H, CH_arom. H11), 7.74 (d, ³J_HH = 1.6 Hz, 1H, CH_imid.), 7.73 (d, ³J_HH = 7.9 Hz, 1H, CH_arom. H7/H9), 7.55 (d, ³J_HH = 7.9 Hz, 1H, CH_arom. H9/H7), 7.47 (t, 1H, ³J_HH = 7.9 Hz, 1H, CH_arom. H8), 7.18 (d, ³J_HH = 1.9 Hz, 1H, CH_imid.), 4.27 (t, ³J_HH = 7.3 Hz, 2H, NCH₂), 3.37 (d, ²J_PH = 11.0 Hz, 2H, CH₂P), 1.91 (quint, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂), 1.42 (sext, ³J_HH = 7.4 Hz, 2H, NCH₂CH₂CH₂), 1.39 (d, ³J_PH = 15.1 Hz, 18H, C(CH₃)₃), 0.99 (t, ³J_HH = 7.4 Hz, 3H, CH₃). ¹³C{¹H} NMR (100.62 MHz, CD₂Cl₂): δ 170.3 (C_NHC), 140.0 (C_arom.), 138.6 (C_arom.), 131.3 (d, J_PC = 5.8 Hz, CH_arom.), 130.1 (d, J_PC = 1.6 Hz, CH_arom.), 127.5 (d, J_PC = 7.0 Hz, CH_arom.), 124.4 (CH_arom.), 122.8 (CH_arom.), 121.4 (CH_arom.), 52.1 (NCH₂), 36.7 (d, ¹J_PC = 25.4 Hz, C(CH₃)₃), 33.3 (NCH₂CH₂), 29.9 (d, ²J_PC = 4.8 Hz, C(CH₃)₃), 27.8 (d, ¹J_PC = 25.5 Hz, CH₂P), 20.1 (NCH₂CH₂CH₂), 13.8 (CH₃). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 80.1 (minor) and 79.0 (major), due to the presence of chlorido and bromido derivatives, consistent with the elemental analysis data. HRMS (ESI⁺): m/z calcd for C₂₂H₃₅Au₂ClN₂P 787.1552, found 787.1547.

X-ray crystallography

Suitable crystals for the X-ray analysis of all compounds were obtained as described above. Summary of the crystal data, data collection and refinement are given in Table S1 (see ESI‡).

Data sets for L^Ph·HBr, L^Cy·HBr·BH₃, [Ag₄Br₄(L^Ph)₂], [Ag₂(L^Ph)₂](BF₄)₂·2CH₂Cl₂, [Cu₂Br₂(L^Cy)₂] and [Au₂Cl₂L^tBu] were collected at 173(2) K on a Bruker APEX-II CCD Duo diffractometer (graphite-monochromated Mo-Kα radiation, λ = 0.71073 Å). Data sets for [Ag₂(L^Ph)₂](BF₄)₂, [Ag₂(L^tBu)₂](BF₄)₂·CH₂Cl₂ and [Cu₂Br₂(L^tBu)₂]·2CH₂Cl₂ were collected at 173(2) K on a Kappa CCD diffractometer (graphite-monochromated Mo-Kα radiation, λ = 0.71073 Å). Specific comments for each data set are given below. The cell parameters were determined using DENZO⁴³ (Kappa) or APEX2⁴⁴ (APEX-II) softwares. The structures were solved by direct methods using the program SHELXS-97 (compounds L^Cy·HBr·BH₃, L^Ph·HBr, [Cu₂Br₂(L^Cy)₂] and [Cu₂Br₂(L^tBu)₂]·2CH₂Cl₂) or SHELXS-2013 (complexes [Ag₄Br₄(L^Ph)₂], [Ag₂(L^Ph)₂](BF₄)₂·2CH₂Cl₂, [Ag₂(L^Ph)₂](BF₄)₂, [Ag₂(L^tBu)₂](BF₄)₂·CH₂Cl₂ and [Au₂Cl₂L^tBu]).⁴⁵ The refinement and all further calculations were carried out using SHELXL-97 (compound L^Ph·HBr, [Cu₂Br₂(L^Cy)₂] and [Cu₂Br₂(L^tBu)₂]·2CH₂Cl₂) or SHELXL-2013 (all other compounds).^45b The H-atoms were introduced into the geometrically calculated positions (SHELXL-97 or SHELXL-2013 procedures) unless stated otherwise and refined riding on the corresponding parent atoms. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F².

The following special comments apply to the models of the structures:

L^Cy·HBr·BH₃: the alkyl atoms C5, C6 and C7 are disordered on two positions.

L^Ph·HBr: A SQUEEZE procedure⁴⁶ was applied and the residual electron density was assigned to one half disordered molecule of CH₂Cl₂.

[Ag₄Br₄(L^Ph)₂]: A SQUEEZE procedure⁴⁶ was applied and the residual electron density was assigned to one disordered molecule of ether.

[Ag₂(L^Ph)₂](BF₄)₂: the alkyl atoms C31, C32 and C33 are disordered on two positions. A SQUEEZE procedure⁴⁶ was applied and the residual electron density was assigned to two disordered molecules of acetonitrile. The structure of this complex can be found in the ESI.‡

[Ag₂(L^tBu)₂](BF₄)₂·CH₂Cl₂: thermal motions affect the alkyl chains on the ligands. The carbons atoms C49, C50 and C73 are disordered on two positions. The carbon atom C48 is also disordered on two positions but C48 and C48B have been imposed at the same position to avoid short-contacts between the H-atoms and subsequent alerts in the Checkcif. A SQUEEZE procedure⁴⁶ was applied and the residual electron density was assigned to one and a half disordered molecules of CH₂Cl₂.

[Cu₂Br₂(L^Cy)₂]: The asymmetric unit contains one and a half molecules of the complex. The alkyl atoms C6 and C7 are disordered on two positions.

[Cu₂Br₂(L^tBu)₂]·2CH₂Cl₂: The space group is chiral (P2₁) and the value of Flack parameter is −0.008(9). A SQUEEZE procedure⁴⁶ was applied and the residual electron density was assigned to one disordered molecule of toluene.

Acknowledgements

The USIAS, CNRS, Unistra, Région Alsace and Communauté Urbaine de Strasbourg are acknowledged for the award of fellowships and a Gutenberg Excellence Chair (2010–11) to AAD. We thank the CNRS and the MESR (Paris) for funding and for a PhD grant to TS. We are grateful to Corinne Bailly and Dr Lydia Karmazin (Service de Radiocristallographie, Unistra) for solving crystal structures, and to Maurice Coppe and Dr Bruno Vincent (Service de Résonance Magnétique Nucléaire, Unistra) for assistance in the NMR studies. We thank Johnson Matthey PLC for a generous loan of gold precursors.

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Footnotes

† Dedicated to the memory of Prof. Peter Hofmann, a dear colleague and friend who made major research advancements and contributed much to the promotion of chemistry.

‡ Electronic supplementary information (ESI) available: X-ray structure of [Ag₂(L^Ph)₂](BF₄)₂ and crystallographic summary table. CCDC 1445698–1445706. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt00275g

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