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

A series of P – NHC-type hybrid ligands containing both PR 2 and N-heterocyclic carbene (NHC) donors on meta -bis-substituted phenylene backbones, L Cy , L t Bu and L Ph (R = Cy, t Bu, 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 Ag 2 O gave the pseudo-cubane [Ag 4 Br 4 ( L Ph ) 2 ], isostructural to [Ag 4 Br 4 ( L R ) 2 ] (R = Cy, t Bu) (T. Simler, P. Braunstein and A. A. Danopoulos, Angew. Chem., Int. Ed. , 2015, 54 , 13691), whereas metallation of L R ·HBF 4 (R = Ph, t Bu) led to the dinuclear complexes [Ag 2 ( L R ) 2 ](BF 4 ) 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 ﬂ uxionality. ‘ Head-to-tail ’ [Cu 2 Br 2 ( L R ) 2 ] (R = Cy, t Bu) dinuclear complexes were obtained from L R ·HBr and [Cu 5 (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, t Bu 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 [Ag 4 Br 4 ( L t Bu ) 2 ] to Au I by reaction with [AuCl(THT)] (THT = tetrahydrothiophene) led to L t Bu transfer and to the dinuclear complex [Au 2 Cl 2 L t Bu ] with one L t Bu ligand bridging the two Au centres. Except for the silver pseudo-cubanes, all other complexes do not exhibit metallophilic interactions.


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 NHC P pincer and P 2 (C NHC ) 2 macrocyclic ligands 2 6f,9c,13 and 3, 5a respectively, have been described.
The coordination chemistry of P-NHC-type ligands has mainly been focussed on late transition metals; the few structurally characterized examples 14 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 † 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 [Ag 2 (L Ph ) 2 ](BF 4 ) 2 and crystallographic summary table. CCDC 1445698-1445706. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt00275g to the R P-NHC-type ( R P = PCy 2 , PtBu 2 or PPh 2 ) 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 (SiHMeCl 2 or SiHCl 3 ) reduction 20 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).
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 NEt 3 . Successful single deprotonation was confirmed in the 1 H-NMR spectra by the disappearance of the deshielded signal due to the acidic P-H proton ( 1 J 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 31 P { 1 H}-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·BH 3 as an air-stable crystalline solid, the structure of which is shown in Fig. 4 (left).
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 (LiPPh 2 ), 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 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 rise in the 1 H-NMR spectra to apparent triplets (overlapping dd, 3 J HH ≈ 4 J 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·BH 3 ). 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 NaBF 4 resulted in the corresponding L Ph ·HBF 4 and L tBu ·HBF 4 salts (see Experimental section). In their 1 H-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 phosphoniumimidazolium L R ·2HBr or the single deprotonation of phosphine-imidazolium L R ·HBr salts with stoichiometric amounts of KN(SiMe 3 ) 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 longterm 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 1 H-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).
Scheme 2 Synthesis of the silver complex [Ag 4 Br 4 (L Ph ) 2 ] (yields based on L Ph ). 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 1 H-NMR and 31 P-NMR spectrum, respectively, confirmed NHC formation and coordination of the P atom. The absence of P-Ag couplings ( 107 Ag 51.8% and 109 Ag 48.2%, both I = 1/2) can be rationalised by a dynamic behaviour involving rapid P-Ag bond breaking/formation on the 31 P-NMR timescale. 12e In the 13 C-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 13 C-107/109 Ag coupling has been reported in related NHC-AgX clusters, 12a,c,23,26 and points towards dynamic behaviour in solution 27 and a high lability of the NHC-Ag bond. 16,28 The structure of [Ag 4 Br 4 (L Ph ) 2 ] (Fig. 5) corresponds to a distorted Ag 4 Br 4 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 Ag 2 Br 2 faces of the cube, forming 9-membered dimetallocycles, as observed with a closely related phosphinite-NHC ligand 12a,c and in the structures of [Ag 4 Br 4 (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 d 10 -d 10 interactions. 30 Related [Ag 4 (halide) 4 L 2 ] 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-NHCtype 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 P1).
Comparison of [Ag 4 Br 4 (L Ph ) 2 ] with the previously reported structures of [Ag 4 Br 4 (L R ) 2 ] (R = Cy, tBu) 12e reveals that the sub-stituents 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 [Ag 4 Br 4 (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 [Ag 4 Br 4 (L Ph ) 2 ], 3.188 Å for [Ag 4 Br 4 (L Cy ) 2 ] and 3.089 Å for [Ag 4 Br 4 (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.
In view of the similarity between [Ag 4 Br 4 (L Ph ) 2 ] and [Ag 4 Br 4 (L R ) 2 ] (R = Cy, tBu), the latter having been obtained from the corresponding phosphonium-imidazolium salts, we reasoned that better yields of [Ag 4 Br 4 (L Ph ) 2 ] should also be attainable by the reaction of L Ph ·2HBr with one mole equiv. Ag 2 O. Indeed, the reaction of L Ph ·2HBr with Ag 2 O 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 SiMe 3 Br 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. Ag 2 O in acetonitrile resulted in the formation of another silver complex featuring 1 H NMR resonances distinct from [Ag 4 Br 4 (L Ph ) 2 ], the structure of which remains elusive to date.  Table 1. 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 ·HBF 4 with 0.5 mole equiv. of Ag 2 O in acetonitrile led to the complex [Ag 2 (L Ph ) 2 ](BF 4 ) 2 (Scheme 3). Examination of its 1 H and 31 P{ 1 H} NMR spectra revealed an equilibrium involving two isomers in solution. Notably, dissolution in CD 3 CN gave rise, in the 31 P{ 1 H} NMR spectrum, to two sets of two doublets (total 8 lines) observed at δ 21.3 (two doublets, 1 J P-107 Ag ≈ 500 Hz, 1 J P-109 Ag ≈ 580 Hz) and 11.2 ppm (two doublets, 1 J P-107 Ag ≈ 475 Hz, 1 J P-109 Ag ≈ 550 Hz) in a 1 : 1.1 ratio, respectively. Evaporation of the solvent and re-dissolution in CD 2 Cl 2 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 CD 3 CN, the 13 C{ 1 H}-NMR spectrum was recorded in CD 2 Cl 2 , 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 CH 2 Cl 2 or CH 3 CN solutions were attempted. Products corresponding to [Ag 2 (L Ph ) 2 ] (BF 4 ) 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 con-nectivity 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).
In the structure of [Ag 2 (L Ph ) 2 ](BF 4 ) 2 ·2CH 2 Cl 2 ( 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 [Ag 2 (L Ph ) 2 ](BF 4 ) 2 , the corresponding [Ag 2 (L tBu ) 2 ](BF 4 ) 2 was similarly prepared (Scheme 3). In this case too, 1 H-and 31 P{ 1 H}-NMR analysis in CD 2 Cl 2 revealed the presence of two isomers, in a 1 : 2 ratio, the nature of which could be determined by perusal of the 13 C{ 1 H}-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  (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)   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 ( 1 J C-107 Ag = 183 Hz, 1 J C-109 Ag = 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 ( 1 J C-107 Ag = 190 Hz, 1 J C-109 Ag = 219 Hz, 2 J 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 13 C-NMR of 'virtual' triplets of the X n AA′X′ n (X = X′ = C, A = A′ = P) spin system involving the carbon atoms directly bound to phosphorus, resulting from a strong 2 J PAgP coupling between trans-coordinated P donors. 35 Interestingly, for all [Ag 2 (L R ) 2 ](BF 4 ) 2 (R = Ph, tBu) complexes, the 1 H-NMR signals for the NHC backbone protons were detected as apparent triplets, likely due to 4 J HAg and 3 J HH coupling constants falling in the same range. 36 An X-ray diffraction study of [Ag 2 (L tBu ) 2 ](BF 4 ) 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 [Ag 2 (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' Fig. 8 Details of the 13 C{ 1 H}-NMR spectrum of [Ag 2 (L tBu ) 2 ](BF 4 ) 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 CD 2 Cl 2 at 125.77 MHz and 298 K with a cryogenically cooled probe head; accumulation of 1024 scans. Fig. 7 Structure of the dication in [Ag 2 (L Ph ) 2 ](BF 4 ) 2 ·2CH 2 Cl 2 (left) and of one of the two dications in [Ag 2 (L tBu ) 2 ](BF 4 ) 2 ·CH 2 Cl 2 (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 [Ag 2 (L Ph ) 2 ](BF 4 ) 2 complex, obtained by crystallisation from CH 3 CN, is presented in the ESI. ‡ 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, laddertype P-NHC-type Cu I complexes by transmetallation from [Ag 4 Br 4 (L R ) 2 ] or by reaction of the phosphonium-imidazolium L R ·2HBr salts with mesitylcopper(I) [Cu 5 (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 proligands L R ·HBr.
Reaction of L R ·HBr (R = Ph, tBu, Cy) with [Cu 5 (Mes) 5 ] resulted in the formation of the corresponding [Cu 2 Br 2 (L R ) 2 ] complexes in good yields (Scheme 4). Completion of the reaction was evidenced by 1 H NMR spectroscopy (i.e. disappearance of the imidazolium NCHN signal). For all three Cu I complexes, the 31 P{ 1 H}-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 13 C{ 1 H}-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 ( 2 J PC ≈ 46-47 Hz) for the dialkyl phosphine derivatives or as a broad signal for [Cu 2 Br 2 (L Ph ) 2 ], possibly due to a different rate of fluxionality of the C NHC -Cu bonds in these two complexes. In the 1 H-NMR spectrum of [Cu 2 Br 2 (L tBu ) 2 ], the line-shape of the signals for the methylene protons was field-dependent, pointing towards a dynamic process in solution.
The structures of [Cu 2 Br 2 (L Cy ) 2 ] and [Cu 2 Br 2 (L tBu ) 2 ]·2CH 2 Cl 2 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 [Ag 2 (L R ) 2 ](BF 4 ) 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.
In order to study further the dynamic behaviour of the Cu I complexes in solution, we undertook a variable temperature (VT) 1 H-NMR study of [Cu 2 Br 2 (L tBu ) 2 ] in CD 2 Cl 2 prompted by its relatively simple line-shape compared to the L Ph and L Cy Scheme 4 Synthesis of the dinuclear copper(I) complexes [Cu 2 Br 2 (L R ) 2 ]. Yields are based on L R . Fig. 9 The molecular structures of [Cu 2 Br 2 (L Cy ) 2 ] (left) and [Cu 2 Br 2 (L tBu ) 2 ]·2CH 2 Cl 2 (right) with thermal ellipsoids at 40% probability. In [Cu 2 Br 2 (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. 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 2 J HH and 2 J PH coupling in an ABX (A = B = H, X = P) spin system. Interestingly, the NCH 2 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 3 J 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 31 P{ 1 H}-NMR spectrum at room temperature. Recent work involving ligands with NHC and P donors held together by a CH 2 linker ascribed stereo-isomerisations at the Cu centre to fluxionality. 10f In contrast, the reaction of [Cu 5 (Mes) 5 ] with the phosphonium-imidazolium L R ·2HBr, or the transmetallation of the corresponding [Ag 4 Br 4 (L R ) 2 ] cubanes with 4 mole equiv. of [CuBr·SMe 2 ] (R = Cy, tBu) gave rise to the tetranuclear clusters [Cu 4 Br 4 (L R ) 2 ]. 12e Metrical data regarding the di-and tetranuclear Cu I complexes are reported in Table 2.
The longer Cu-C NHC and Cu-P bond distances in the [Cu 2 Br 2 (L R ) 2 ] complexes (mean distances ca. 1.948 and 2.228 Å, respectively) in comparison to the [Cu 4 Br 4 (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.   40 A singlet at δ 79.0 ppm in the 31 P{ 1 H}-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). The structure of [Au 2 Cl 2 L tBu ] (Fig. 11) revealed an approximate linear coordination of the Au I centres (P-Au-Cl: 177.7(1)°a nd 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 [Au 2 Cl 2 L 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 [Ag 4 Br 4 (L R ) 2 ] motif comprising a distorted Ag 4 Br 4 cubane core, bridging L R ligands and weak metallophilic interactions; (iii) in the presence of the non-coordinating BF 4 − , [Ag 2 (L R ) 2 ](BF 4 ) 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 [ 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

General methods
All air-and moisture-sensitive manipulations were performed under dry argon atmosphere using standard Schlenk techniques. THF and Et 2 O were dried by refluxing over sodium/ benzophenone ketyl and distilled under an argon atmosphere prior use. Methanol and ethanol were refluxed over CaH 2 , distilled under an argon atmosphere and stored over 3 Å molecular sieves. Other solvents ( pentane, CH 2 Cl 2 , toluene and acetonitrile) were dried by passing through columns of activated alumina and subsequently purged with argon. C 6 D 6 and toluene-d 8 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 ( 1 H), the solvent ( 13 C) or externally ( 31 P, 11 B). 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 n J PC coupling constants in ambiguous cases was carried out by recording the 13 C{ 1 H}-NMR spectra on two different field spectrometers. Assignments (Fig. 12) were determined either on the basis of unambiguous chemical shifts, coupling patterns and 13 C-DEPT experiments or 2D correlations ( 1 H-1 H COSY, 1 H-13 C HSQC, 1 H-13 C 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.
The following special comments apply to the models of the structures: L Cy ·HBr·BH 3 : the alkyl atoms C5, C6 and C7 are disordered on two positions. L Ph ·HBr: A SQUEEZE procedure 46 was applied and the residual electron density was assigned to one half disordered molecule of CH 2 Cl 2 .
[Ag 4 Br 4 (L Ph ) 2 ]: A SQUEEZE procedure 46 was applied and the residual electron density was assigned to one disordered molecule of ether.
[Ag 2 (L Ph ) 2 ](BF 4 ) 2 : the alkyl atoms C31, C32 and C33 are disordered on two positions. A SQUEEZE procedure 46 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. ‡ [Ag 2 (L tBu ) 2 ](BF 4 ) 2 ·CH 2 Cl 2 : 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 procedure 46 was applied and the residual electron density was assigned to one and a half disordered molecules of CH 2 Cl 2 .
[Cu 2 Br 2 (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.
[Cu 2 Br 2 (L tBu ) 2 ]·2CH 2 Cl 2 : The space group is chiral (P2 1 ) and the value of Flack parameter is −0.008 (9). A SQUEEZE procedure 46 was applied and the residual electron density was assigned to one disordered molecule of toluene.