Synthesis and characterisation of bulky guanidines and phosphaguanidines: precursors for low oxidation state metallacycles

Abstract

Reactions of alkali metal amides or phosphides with the bulky carbodiimide, ArN=C=NAr (Ar = C₆H₃Prⁱ₂-2,6), followed by aqueous work-ups, have yielded several guanidines, ArNC(NR₂)N(H)Ar (R = cyclohexyl (GisoH) or Prⁱ (PrisoH); NR₂ = cis-NC₅H₈Me₂-2,6 (PipisoH)), a bifunctional guanidine, {ArNCN(H)Ar}₂{μ-N(C₂H₄)₂N} (Pip(GisoH)₂), and two phosphaguanidines, ArNC(PR₂)N(H)Ar (R = cyclohexyl (CyP-GisoH) or Ph (PhP-GisoH)). A very bulky guanidine, ArNC{N(Ar)SiMe₃}N(H)Ar (ArSi-Giso), and an aryl coupled bifunctional guanidine, {ArN(H)C(NPrⁱ₂)NC₆H₂Prⁱ₂-2,6-}₂ (PrisoH)₂, have been prepared by other routes. All compounds have been crystallographically characterised and shown to exist in a number of isomeric forms in the solid state. These appear to be largely retained in solution. The deprotonation of GisoH with BuⁿLi in either hexane or THF led to crystallographically characterised dimeric and monomeric complexes respectively, viz. [Li{Li(κ²-N,N′-Giso)₂}] and [Li(THF)(η¹-N,η³-Ar-Giso)]. Deprotonation of PrisoH and Pip(GisoH)₂ with K[N(SiMe₃)₂] gave the unsolvated polymer, [{K(η¹-N,η⁶-Ar-Priso)}_∞], and the solvated complex, [{K(THF)₂}{Pip(Giso)₂}{K(THF)₃}], respectively.

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Introduction

The coordination chemistry of anionic amidinate ([RNC(R)NR]⁻, R = H, alkyl, aryl etc.) and guanidinate ([RNC(NR₂)NR]⁻) ligands has been extensively studied, giving rise to numerous complexes incorporating metals from across the periodic table.¹ In these, the ligands have displayed an impressive array of coordination modes which depend upon the nature and bulk of the substituents (R), and the metal involved. This structural diversity is one of the main factors that have led to such complexes finding many applications in catalysis,^2–4 materials science⁵ and synthesis,¹ to name but a few.

Recent developments in this area have concentrated on the use of very bulky amidinates to stabilise low nuclearity s- and p-block metal complexes which show significant potential as, for example, lactide polymerisation catalysts.² Of most note here is the Piso⁻ ligand, [ArNC(Bu^t)NAr]⁻, which incorporates sterically demanding 2,6-diisopropylphenyl (Ar) substituents at its N-centres and a tert-butyl group on the backbone carbon. The spatial profile and ligating abilities of this ligand have been likened to those of β-diketiminates, the most commonly utilised examples of which also possess N–Ar substituents, e.g. [(ArNCR)₂CH]⁻ (R = Me or Bu^t).⁶ Although complexes of β-diketiminates are widely used in catalytic processes, they are perhaps more notable for their capacity to kinetically stabilise complexes containing low oxidation state metal centres. A salient illustration of this is the synthesis and structural characterisation of the homologous series of monomeric, N,N′-chelated group 13 metal(I) complexes, [:M{(ArNCMe)₂CH}] (M = Al, Ga, In or Tl),⁷ which have shown remarkable further chemistry.

In contrast to β-diketiminates, bulky amidinates (e.g. Piso⁻) had rarely been employed in the preparation of low oxidation state metal complexes. In 2005, we began to address this paucity with the preparation of the group 13 metal(I) complexes [:M(Piso)] (M = In or Tl).⁸ However, unlike their β-diketiminate counterparts, [:M{(ArNCMe)₂CH}], the Piso⁻ ligand in these complexes is localised and chelates the metal centre in an η¹-N,η³-arene-fashion. In addition, the analogous Ga^I and Al^I complexes could not be stabilised. These results suggested that related, but bulkier ligands would need to be accessed to enforce N,N′-chelation and allow stabilisation of lighter group 13 metal(I) centres. To this end, the very large guanidinate ligand, [ArNC(NCy₂)NAr]⁻ (Giso⁻; Cy = cyclohexyl), was developed and used in the syntheses of the remarkably stable monomeric four-membered heterocycles, [:M(κ²-N,N′-Giso)] (M = Ga or In; N.B. the Al(I) heterocycle has not yet been accessed),⁹ the coordination chemistry of which was later explored.¹⁰ In addition to the increased steric bulk of Giso⁻ over Piso⁻, the greater stabilising ability of the guanidinate can be attributed to the fact that it is a more N-electron rich donor than the amidinate, a result of it possessing a zwitterionic resonance form containing two negatively charged N-donor centres, viz. [Cy₂N⁺=C(N⁻Ar)₂].

Over the last three years we have extended our application of Giso⁻, and a range of other Ar-substituted guanidinate and phosphaguanidinate ([ArNC(PR₂)NAr]⁻)¹¹ ligands, to the stabilisation of heterocyclic complexes containing low oxidation state metal centres from all blocks of the periodic table (e.g. Mg(I),¹²Ge(I),¹³As(I),¹⁴ various d-block metal(I)¹⁵ and f-block metal(II)¹⁶ species) with considerable success. Moreover, we have used these ligands in the synthesis of a variety of gallyl–metal complexes, including examples exhibiting unprecedented Ga–Zn¹⁷ and Ga–Sn¹⁸ bonds. In all these studies, the ligands have been prepared by the deprotonation of neutral guanidines or phosphaguanidines with alkali metal reagents. Although some preliminary details of the synthesis of the neutral ligand precursors have been previously been described by us,^9–16 it seemed that a full report of the preparation and characterisation of these compounds would aid other researchers seeking to harness their unique properties for their own purposes. The value of this is highlighted by the fact that prior to our involvement in this field, only one guanidine bearing 2,6-diisopropylphenyl substituents at its N-centres, viz.ArNC{N(H)Ar}₂, had appeared in the literature.¹⁹ Here, we report on the synthesis, structures and properties of eight N–Ar substituted guanidines and phosphaguanidines, and some of their alkali metal derivatives.

Results and discussion

(i) Synthesis of bulky guanidines and phosphaguanidines

A number of synthetic routes are known for the preparation of guanidines.¹ One of the most versatile of these involves the addition of metallated amides to carbodiimides (RN=C=NR), followed by aqueous work-up. Here, this route has been employed to synthesise the guanidines GisoH (1), PrisoH (2), PipisoH (3), as well as the bifunctional guanidine, Pip(GisoH)₂ (4), in high to quantitative yields (Scheme 1). In all preparations, THF was used as the solvent and the initial addition reactions were carried out at either ambient temperature and/or under reflux conditions.

Scheme 1 Reagents and conditions: (i) ArN=C=NAr, THF, 20 °C or reflux; (ii) H₂O.

It appears that this route does have steric and electronic limitations, as the attempted addition of some amides to the carbodiimide (ArN=C=NAr) were not successful. For example, lithiated cis-2,6-dimethylpiperidine adds to the carbodiimide to give compound 3, whereas lithiated 2,2,6,6-tetramethylpiperidine does not react with ArN=C=NAr in THF at reflux. Moreover, M[N(SiMe₃)₂] (M = Li, Na or K) do not react with ArN=C=NAr under similar conditions, though these reagents are known to add to smaller carbodiimides at room temperature.²⁰

Although considerably less sterically demanding than some of the amide precursors mentioned above, lithium carbazolyl did not react with ArN=C=NAr in THF at reflux, and only carbazole and the carbodiimide were recovered after work-up. This lack of reactivity probably derives from the lower nucleophilicity of the aromatic carbozyl anion, relative to the bulkier amides used in the preparation of 1–3.

Interest in the coordination chemistry of phosphaguanidinates, [RNC(PR′₂)NR]⁻, has recently begun to escalate.^1d,21 One of the main reasons behind this is that the phosphino group of these ligands is pyramidal, unlike the planar amino substituent of guanidinates. Therefore, the zwitterionic resonance form of these ligands, [R′₂P⁺=C(N⁻R)₂], does not play a significant role in their chemistry. As a result, phosphaguanidinates are coordinatively versatile, and in many of their complexes the phosphino group acts a P-lone pair donor.^1d,21 Despite this emerging importance, there had been no reports of N–Ar substituted phosphaguanidinates or phosphaguanidines in the literature. We have reversed this situation with the synthesis of CyP-GisoH, 5, and PhP-GisoH, 6, via the addition of the relevant lithium phosphide to ArN=C=NAr (Scheme 1). Aqueous work-ups of these compounds were performed under an inert atmosphere to prevent oxidation of the phosphorus atom. However, we have found that the products can be handled in moist air as solids or in solution without significant oxidation occurring, as judged by ³¹P NMR spectroscopy. It is noteworthy that the addition of phosphines to smaller carbodiimides to form phosphaguanidines, in the presence of catalytic amounts of s-block amide or alkyl bases, has recently been reported.^22,23

Although the addition of metal amides to ArN=C=NAr is a versatile route to bulky guanidine compounds, its limitations centre on the bulk of the reacting amide complex (as mentioned above). Because of this, a different approach was used to synthesise the exceedingly bulky guanidine, ArSi–GisoH 7 (Scheme 2). This involved lithiation of the known guanidine, ArNC{N(H)Ar}₂, the product of which was subsequently quenched with Me₃SiCl in THF at reflux to give 7 in good yield.

Scheme 2 Reagents and conditions: (i) BuⁿLi, THF; (ii) Me₃SiCl, THF reflux.

One further bifunctional guanidine has been prepared in this study via a route not involving carbodiimide addition. Though this synthesis was originally not intended, it is moderately yielding, reproducible and thus is included here. In an attempt to form a Mn(II) complex of Priso⁻, K[Priso] was reacted with commercially available MnI₂ in THF. This, instead led to the isolation of the aryl-coupled guanidine, (PrisoH)₂8, in a 30% yield (Scheme 3) without aqueous work-up. When the reaction was repeated with a pure sample of [MnI₂(THF)₃], compound 8 was not obtained. Presumably, the commercially sourced MnI₂ initially employed, was contaminated with significant amounts of higher oxidation state manganese species. It is believed that the reaction of the impure MnI₂ with K[Priso] led to the oxidative coupling of two Priso⁻ anions through aryl para-positions on each. This seems reasonable in light of the fact that we have recently shown that Priso⁻ can coordinate the Rh(COD) fragment (COD = 1,5-cyclooctadiene) solely through one aryl substituent in a η⁵-cyclohexadienyl fashion.¹⁵ A Mn(>II)-Priso complex in which the ligand exhibits this cyclohexadienyl binding mode can easily be envisaged as an intermediate in the oxidative coupling that gave 8. The possibility that 8 was alternatively formed via the oxidative coupling of two Priso⁻ anions by a diiodine contaminant in the impure sample of MnI₂ was examined and discounted.

Scheme 3 Reagents and conditions: (i) K[N(SiMe₃)₂], THF; (ii) MnI₂, THF.

(ii) Structural and spectroscopic properties of prepared compounds

The crystal structures of all compounds 1–8 have been determined (see Fig. 1–6 for the molecular structures of 1, 4–8; those of 2 and 3 can be found in ESI†). The compounds display solid state structures comparable to those of previously characterised guanidines and phosphaguanidines.^1,21 Each of the guanidines, 1–3, possesses a close to planar backbone amino (–NR₂) fragment which in no case is co-planar with the CN₃ core of the molecule. Therefore, any interaction of the amino N-lone pair within the π-system of the largely localised guanidine CN₃ backbone must be limited. It is noteworthy that the –NR₂ fragments of 4 are significantly more distorted from planar than those of the monofunctional guanidines. Similarly, the two phosphaguanidines display distorted pyramidal phosphorus centres, the lone pairs of which are directional and therefore cannot be involved with the π-system of their localised CN₂P cores. The bond lengths and angles within these core fragments (see Table 1) are consistent with these descriptions.

Fig. 1 Molecular structure of 1 (25% thermal ellipsoids are shown; hydrogen atoms, except H(1), omitted for sake of clarity).

Fig. 2 Molecular structure of 4 (25% thermal ellipsoids are shown; hydrogen atoms, except H(1), omitted for sake of clarity). Symmetry operation:′−x + 1, −y + 1, −z.

Fig. 3 Molecular structure of 5 (25% thermal ellipsoids are shown; hydrogen atoms, except H(2), omitted for sake of clarity).

Fig. 4 Molecular structure of 6 (25% thermal ellipsoids are shown; hydrogen atoms, except H(1), omitted for sake of clarity).

Fig. 5 Molecular structure of 7 (25% thermal ellipsoids are shown; hydrogen atoms, except H(3), omitted for sake of clarity). Selected bond lengths (Å) and angles (°): Si(1)–N(1) 1.7762(16), N(1)–C(1) 1.410(2), C(1)–N(2) 1.285(2), C(1)–N(3) 1.383(2); N(2)–C(1)–N(3) 130.87(17), N(2)–C(1)–N(1) 116.38(16), N(3)–C(1)–N(1) 112.73(16), C(1)–N(1)–C(5) 119.99(15), C(1)–N(1)–Si(1) 119.94(12), C(5)–N(1)–Si(1) 119.81(12).

Fig. 6 Molecular structure of 8 (25% thermal ellipsoids are shown; hydrogen atoms, except H(1) and H(6), omitted for sake of clarity). Selected bond lengths (Å) and angles (°): N(1)–C(1) 1.391(3), C(1)–N(3) 1.290(4), C(1)–N(2) 1.378(4), N(4)–C(44) 1.290(3), N(5)–C(44) 1.383(4), N(6)–C(44) 1.387(4); N(3)–C(1)–N(2) 121.1(2), N(3)–C(1)–N(1) 122.2(3), N(2)–C(1)–N(1) 116.7(3), C(1)–N(2)–C(17) 120.2(2), C(1)–N(2)–C(14) 119.8(2), C(17)–N(2)–C(14) 115.5(2), C(44)–N(5)–C(45) 119.5(2), C(44)–N(5)–C(48) 119.9(2), C(45)–N(5)–C(48) 116.0(2), N(4)–C(44)–N(5) 120.8(3), N(4)–C(44)–N(6) 122.0(3), N(5)–C(44)–N(6) 117.3(2).

Table 1 Selected bond lengths (Å) and angles (°) for 1–6 (E = N or P)

	1	2	3	4	5	6
ArN=C	1.290(2)	1.2911(16)	1.285(2)	1.287(2)	1.2909(19)	1.311(2)
ArN–C	1.384(3)	1.3910(16)	1.394(2)	1.373(2)	1.375(2)	1.346(2)
C–ER2	1.388(2)	1.3807(16)	1.385(2)	1.398(2)	1.8708(17)	1.8798(18)
ArN–C=N	121.26(17)	122.02(11)	124.10(17)	124.67(15)	123.26(14)	121.51(16)
R2E–C=N	121.66(18)	120.57(11)	119.91(16)	119.84(15)	121.76(11)	119.98(13)
R2E–C–N	117.08(17)	117.42(10)	115.99(16)	115.48(14)	114.96(11)	118.51(13)
∑ angles about E	353.3	357.0	353.5	341.9	302.3	304.4

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Several different isomeric forms of the compounds have been identified in this study. To allow comparisons with related amidines, the backbone unit (R₂N or R₂P) has been defined as the lower priority in determining the stereo-configuration of the compounds (see refs. 1d and 1e for a description of the four isomeric and tautomeric forms of amidines, viz. Z-anti, Z-syn, E-anti and E-syn). The guanidines, 1–3 (see Fig. 1 for the structure of 1), and the phosphaguanidine, 5 (Fig. 3), exist in the Z-anti-form which is common for guanidines but not for uncoordinated phosphaguanidines which normally occur in the solid state in their E-syn-form.^1d,21 Indeed, this is the isomer adopted by the phosphaguanidine, 6, in the solid state (Fig. 4). In contrast, the extremely bulky guanidine, 7 (Fig. 5), crystallises in the rarely observed Z-syn-form, probably because of steric buttressing of its aryl groups by the larger N(Ar)SiMe₃ substituent. It is of note that the Z-syn-isomer of amidines with very bulky backbone C-substituents have been previously reported, e.g.(tript)C{N(H)R}(NR) (tript = 9-triptycenyl, R = Cy or Prⁱ).²⁴ Both the bifunctional amidines, 4 and 8 (Fig. 2 and 6, respectively), exist in the solid state as Z-anti-,Z-anti-isomers, as has been previously documented for bifunctional amidines.¹

Often, amidines and guanidines will be present in solution in more than one of their four possible isomeric forms. This can lead to complicated NMR spectra for such compounds. However, the guanidines and phosphaguanidines, 1–6, display relatively simple ¹H and ¹³C{¹H} NMR spectra, which are suggestive of only one, or predominantly one, isomer occurring in solution. These spectra imply that each compound has two chemically inequivalent Ar substituents, and that both alkyl or aryl groups on the backbone –ER₂ (E = N or P) groups are equivalent. If the compounds retain their solid state isomeric forms in solution, which seems likely, the latter observation requires their –ER₂groups to partially rotate on the NMR timescale, thus leading to the compounds possessing averaged mirror planes incorporating their ECN₂ fragments.

Although the isomeric forms adopted by the guanidines, 1–4, in solution cannot be certain without two-dimensional NMR experiments, some insight into the solution conformations of the phosphaguanidines, 5 and 6, can be gained from their ¹H NMR spectra. That for 5 shows only one isomer, the NH resonance of which exists as a doublet (³J_PH = 14.1 Hz; ³¹P{¹H} NMR: δ−2.9 ppm). The spectrum of 6 reveals the compound to exist as two isomers in solution in an approximately 90 : 10 ratio. The NH resonance of the major isomer (³¹P{¹H} NMR: δ−18.5 ppm) is a singlet, while that for the minor isomer (³¹P{¹H} NMR: δ−13.3 ppm) is a doublet (³J_PH = 18.2 Hz). In an excellent paper on phosphaguanidinate solution behaviour, Coles et al. have shown that isomer interconversion can readily occur by one or more of a number of possible pathways.^21e Importantly, they also showed that the closely related phosphaguanidine, Cy₂PC{N(H)Prⁱ}(NPrⁱ), is present in solution in both its E-syn- (major) and Z-anti- (minor) forms (14 : 1 ratio at 298 K). The NH resonance of the E-syn-form shows no coupling to the P-centre, while that of the minor Z-anti-isomer does (³J_PH = 14.5 Hz). Accordingly, we conclude that compound 5 exists solely as its Z-anti-form in solution (as in the solid state), whereas the major solution state isomeric form of 6 is E-syn (as in the solid state), and the minor form is Z-anti.

Many of the signals in the ¹H NMR spectrum of ArSi-GisoH, 7, are very broad and suggest one or more dynamic processes are occurring in solution. Despite efforts, the spectrum could not be resolved, and thus we could not shed light on the nature of the dynamic behaviour. One possibility, however, is that it involves a restricted rotation of the Ar and/or SiMe₃ groups about the N–C or N–Si bonds of 7. In this respect, it should be noted that similar solution dynamic behaviour has been observed for the closely related compound, ArNC{N(H)Ar}₂, an exhaustive variable-temperature NMR study of which showed this behaviour to be derived from restricted rotation of its three Ar groups.¹⁹ Another possibility for 7 is that there is a fluxional interconversion between two or more isomers of the compound, which is occurring at close to the NMR timescale. This seems less likely, however, when the imposing sterics of the compound are taken into account.

Little information could be gained from the solution NMR spectra of the bifunctional guanidine, 8. These are very complicated and point towards more than one isomer existing in solution. For example, several overlapping N–H resonances were seen in its ¹H NMR spectrum, where only one would be expected if it retained its solid state Z-anti-, Z-anti-isomeric form in solution. As a result, the spectra proved difficult to assign.

(iii) Metallation of bulky guanidines and phosphaguanidines

The guanidines and phosphaguanidines prepared here (with the exception of ArSi-Giso 7), can be easily deprotonated by standard metallation procedures. The reactions of these ligands with one equivalent of BuⁿLi or K[N(SiMe₃)₂] proceed rapidly and near quantitatively in common solvents such as hexane, toluene, THF or diethyl ether at ambient temperature or below. The solvent and metal involved in the reaction can have a striking bearing on the nuclearity of the formed complex, and the conformation adopted by the guanidinate or phosphaguanidinate ligand. This is important as it can influence the product obtained from, for example, further salt metathesis reactions of these alkali metal complexes with other metal halides. In this study, we have structurally and spectroscopically characterised four lithium or potassium salts of the ligands prepared above.

The lithiation of GisoH, 1, with BuⁿLi in hexane led to the solvent free dimeric complex, 9, whilst in THF the monomeric solvated complex, 10, was formed (Scheme 4). In contrast, metallation of PrisoH, 2, with K[N(SiMe₃)₂] in toluene afforded the polymeric, solvent free complex, 11, whereas metallation of Pip(GisoH)₂ with the same reagent in THF gave the solvated complex, 12 (Scheme 4). The NMR spectroscopic data for 9–11 are more symmetrical than their solid state structures (vide infra) would suggest and imply that fluxional processes are occurring in solution that are rapid on the NMR timescale. This is not uncommon for alkali-metal amidinates and guanidinates,¹ and therefore no efforts were made to investigate these dynamic behaviours by variable temperature NMR studies. Once crystallised from the reaction mixture, compound 12 has negligible solubility in normal deuterated solvents (including D₈-THF) and therefore no meaningful NMR spectroscopic data could be obtained for this compound.

Scheme 4 Reagents and conditions: (i) BuⁿLi, hexane (Cy = cyclohexyl); (ii) BuⁿLi, THF; (iii) K[N(SiMe₃)₂], toluene; (iv) K[N(SiMe₃)₂], THF.

The molecular structure of 9 is depicted in Fig. 7 and shows it to be dimeric with two different lithium coordination environments. Li(1) is coordinated by two chelating Giso⁻ ligands that have largely localised N(1)–C(1)–N(2) fragments. The Li(1)–N bond lengths of 2.072(2) Å (to N(2) and N(2)′) and 2.240(5) Å (to N(1) and N(1)′), although different, lie within the normal range for amidinate and guanidinate N–Li interactions.²⁵ The two more distant N-atoms (N(1) and N(1)′) also coordinate the bent two-coordinate Li(2) centre with short interactions (1.954(4) Å). The coordination sphere of the both Li atoms is completed by agostic interactions to ligand hydrogen atoms; Li(1) has two such interactions (both ca. 2.23 Å), whereas Li(2) has four (from ca. 2.03 Å to ca. 2.27 Å). When these close contacts are taken into account, both Li-centres can be thought of as having heavily distorted octahedral geometries. A survey of the Cambridge Crystallographic Database revealed two similar dimeric lithium amidinates, [Li{κ²-N,N′-(SiMe₃)NC(R)N(SiMe₃)}₂{Li(OEt₂)}] (R = C₆H₅CF₃-4 or C₆H₅F-2),²⁶ though the non-chelated Li centre of both is further coordinated by an ether molecule.

Fig. 7 Molecular structure of 9 (25% thermal ellipsoids are shown; hydrogen atoms omitted for sake of clarity). Selected bond lengths (Å) and angles (°): N(1)–C(1) 1.394(3), N(2)–C(1) 1.323(3), N(3)–C(1) 1.409(3), N(1)–Li(2) 1.954(4), N(1)–Li(1) 2.240(5), N(2)–Li(1) 2.072(2); N(2)–C(1)–N(1) 114.3(3), N(2)–Li(1)–N(1) 63.78(12), N(2)′–Li(1)–N(1) 119.6(2), N(1)′–Li(2)–N(1) 121.7(4). Symmetry operation:′−x, y, −z + 1/2.

The molecular structure of monomeric 10 is shown in Fig. 8. The localised guanidinate ligand is acting as an amide that coordinates the Li atom in an η¹-fashion through N(2). In addition, there is an approximately η³-interaction of the Li-centre with the Ar-substituent of N(1). The coordination sphere on the Li(1) is completed by one THF molecule. A similar coordination mode (but minus the coordinated THF) has been reported for the thallium(I) complex, [Tl(η¹-N,η³-Ar-Giso)].⁸

Fig. 8 Molecular structure of 10 (25% thermal ellipsoids are shown; hydrogen atoms omitted for sake of clarity). Selected bond lengths (Å) and angles (°): N(1)–C(1) 1.3149(16), C(1)–N(2) 1.3587(16), C(1)–N(3) 1.4092(16), Li(1)–N(2) 1.943(3), Li(1)–C(2) 2.290(3), Li(1)–C(3) 2.458(3), Li(1)–C(7) 2.591(3), O(1)–Li(1) 1.889(3); N(1)–C(1)–N(2) 121.56(11), N(1)–C(1)–N(3) 117.46(11), N(2)–C(1)–N(3) 120.98(11), C(1)–N(3)–C(32) 117.05(10), C(1)–N(3)–C(26) 120.82(10), C(32)–N(3)–C(26) 115.51(10), C(1)–N(2)–Li(1) 117.59(11).

Like the structure of 10, the guanidinate moieties of the potassium complexes, 11 and 12 (Fig. 9 and 10, respectively), adopt the Z-anti-configuration but with more localised coordinated NCN fragments. In addition, the arene-K interactions in both are close to η⁶-, as opposed to the η³-Ar-Li coordination seen in 10. In 11, this leads to a one-dimensional polymeric structure in which one Ar-group of each ligand bridges two K-centres. Compound 12 is monomeric, and in addition to arene and N-attachments, one K-centre is coordinated by two THF molecules, while the other is ligated by three. All the distances to the K-centres in both complexes are in the normal range.²⁵

Fig. 9 Molecular structure of 11 (25% thermal ellipsoids are shown; hydrogen atoms omitted for sake of clarity). Selected bond lengths (Å) and angles (°): K(1)–N(1) 2.755(3), K(1)–Ar centroid 3.077(1), K(1)′–Ar centroid 2.945(1), C(1)–N(2) 1.329(5), C(1)–N(3) 1.402(5), N(1)–C(1) 1.340(5); N(2)–C(1)–N(1) 121.7(3), N(2)–C(1)–N(3) 115.1(3), N(1)–C(1)–N(3) 123.2(3), C(1)–N(3)–C(26) 122.0(3), C(1)–N(3)–C(29) 121.5(3), C(26)–N(3)–C(29) 114.7(3), C(1)–N(1)–K(1) 128.2(2). Symmetry operation:′x− 1/2, −y + 1/2, −z.

Fig. 10 Molecular structure of 12 (25% thermal ellipsoids are shown; hydrogen atoms and isopropyl groups omitted for sake of clarity). Selected bond lengths (Å) and angles (°): K(1)–O(1) 2.681(3), K(1)–O(2) 2.698(3), K(1)–O(5) 2.780(3), K(1)–N(1) 2.823(2), K(2)–O(3) 2.646(3), K(2)–O(4) 2.710(3), K(2)–N(5) 2.735(3), K(1)–Ar centroid 3.007(1), K(2)–Ar centroid 2.915(1), N(1)–C(1) 1.328(4), N(2)–C(1) 1.321(4), N(3)–C(1) 1.437(4), N(4)–C(30) 1.426(4), N(5)–C(30) 1.336(4), N(6)–C(30) 1.322(4); N(2)–C(1)–N(1) 124.6(3), N(2)–C(1)–N(3) 114.8(3), N(1)–C(1)–N(3) 120.6(2), C(1)–N(1)–K(1) 123.88(18), C(30)–N(5)–K(2) 129.42(18), N(6)–C(30)–N(5) 123.6(3), N(6)–C(30)–N(4) 115.2(2), N(5)–C(30)–N(4) 121.2(2).

Conclusion

In conclusion, the synthesis and characterisation of a variety of guanidine, bifunctional guanidine and phosphaguanidine compounds, all bearing 2,6-diisopropylphenyl N-substituents, have been described. In the solid state, the Z-anti-isomeric form is observed for all guanidines, except in one extremely bulky example, ArSi-GisoH 7. The sterics of this necessitate it occurring as the rarely observed Z-syn-isomer. Of the phosphaguanidinates, the bulkier example, CyP-GisoH 5, crystallises in the Z-anti-form, while PhP-GisoH, 6, adopts the E-syn-conformation. In solution, most of the described compounds appear to retain their stereochemistry, though in some cases isomer mixtures were observed. Several of the prepared compounds have been deprotonated with alkali metal reagents and the resulting salts crystallographically characterised. In the case of the deprotonation of GisoH 1 with BuⁿLi, the nuclearity and guanidinate coordination mode displayed by the formed complexes are dependent upon the reaction solvent employed. We are currently systematically exploring the use of bulky guanidinates and phosphaguanidinates, prepared from the neutral compounds 1–8, for the stabilisation of low oxidation metallacycles incorporating metals from all blocks of the periodic table.

Experimental

General considerations

All manipulations were performed under an inert atmosphere (dinitrogen or argon) using Schlenk or glove box techniques. Aqueous organic work-ups were carried out in air, except those for the phosphaguanidines, 5 and 6. Melting points were determined in sealed capillaries under a dinitrogen atmosphere, except those for the guanidines, 1–4, which were determined in open capillaries. Reaction solvents were dried over potassium or Na/K alloy prior to use, except dichloromethane and chloroform which were used as received. Mass spectra were recorded at the EPSRC National Mass Spectrometric Service, Swansea University. Microanalyses were obtained from either Medac Ltd or Campbell Microanalytical, Ottago. IR spectra were recorded using a Nicolet 510 FT-IR spectrometer as Nujol mulls between NaCl plates. ¹H and ¹³C{¹H} NMR spectra were recorded on either Bruker DXP400, Bruker DPX300, Jeol Eclipse 300 or Bruker WM250 spectrometers and were referenced to the resonances of the solvent used. ³¹P{¹H} NMR spectra were recorded on a Jeol Eclipse 300 spectrometer and were referenced to external 85% H₃PO₄. Cy₂NH, Prⁱ₂NH, cis-2,6-dimethylpiperidine and piperazine were obtained commercially, dried over molecular sieves, and distilled under dinitrogen prior to use. K[N(SiMe₃)₂] was prepared by treating (SiMe₃)₂NH with KH in toluene at 20 °C. ArN=C=NAr²⁷ and ArNC{N(H)Ar}₂¹⁹ were synthesised according to literature procedures. All other reagents were obtained from commercial sources and used as received.

Preparation of GisoH 1

BuⁿLi (5.33 cm³ of a 1.6 M solution in hexanes, 8.52 mmol) was added to a solution of Cy₂NH (1.58 g, 1.73 cm³, 8.69 mmol) in THF (40 cm³) at 20 °C over 5 min and the resultant solution stirred for 1 h. ArN=C=NAr (3.00 g, 8.27 mmol) was then added, the suspension stirred for 15 min, followed by heating at reflux for 1.5 h (or alternatively stirred at room temperature for 4 h). All volatiles were removed under reduced pressure and diethyl ether (40 cm³) and H₂O (10 cm³) added to the residue. The mixture was stirred for 30 min to give two clear solution phases. The organic phase was separated and the aqueous layer was extracted with CH₂Cl₂ (3 × 30 cm³). The combined organic phases were dried (MgSO₄), filtered, and volatiles evaporated from the filtrate under vacuum. The oily residue solidified upon standing to give 1 as colourless crystals (yield 4.40 g, 98%). The product can be recrystallised from hot hexane (yield 80%); mp 140–141 °C. ¹H NMR (300 MHz, 298 K, CDCl₃): δ 0.91 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.08–0.90 (m, 8 H, CH₂), 1.21 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.36 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.38 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.47–1.70 (m, 8 H, CH₂), 2.05 (m, 4 H, CH₂CHN), 2.97 (tt, J = 11.7, 3.3 Hz, 2 H, CHN), 3.22 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 3.32 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 4.95 (s, 1 H, NH), 6.89–7.17 (m, 6 H, ArH); ¹H NMR (250 MHz, 298 K, C₆D₆): δ 0.96 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.12–1.32 (m, 6 H, CH₂), 1.44 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.50–1.68 (m, 2 H, CH₂), 1.54 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.75–1.92 (m, 8 H, CH₂), 2.21–2.44 (m, 4 H, CH₂), 3.23 (tt, J = 11.7, 3.3 Hz, 2 H, CHN), 3.60 (sept, J = 6.8 Hz, 4 H, CH(CH₃)₂), 5.32 (s, 1 H, NH), 7.06–7.44 (m, 6 H, ArH); ¹³C{¹H} NMR (75.5 MHz, 298 K, CDCl₃): δ 21.6 (CH(CH₃)₂), 22.5 (CH(CH₃)₂), 24.9 (CH₂), 26.0 (CH(CH₃)₂), 26.1 (CH(CH₃)₂), 27.1 (CH(CH₃)₂), 28.6 (CH(CH₃)₂), 29.0 (CH₂), 32.6 (CH₂), 58.0 (HCN), 121.6, 122.8, 123.5, 126.9, 135.9, 140.0, 145.5, 145.6, (ArC), 148.0 (CN₃), ¹³C{¹H} NMR (75.5 MHz, 298 K, C₆D₆): δ 21.7 (CH(CH₃)₂), 22.3 (CH(CH₃)₂), 25.2 (CH₂), 26.2 (CH(CH₃)₂), 27.3 (CH(CH₃)₂), 28.7 (CH(CH₃)₂), 29.3 (CH(CH₃)₂), 32.9 (CH₂), 39.8 (CH₂), 58.2 (HCN), 122.6, 123.3, 123.7, 127.1, 136.1, 139.9, 145.5, 145.6 (ArC), 148.5 (CN₃); IR (Nujol): ν/cm⁻¹ = 3384 (m), 1614 (s), 1583 (s), 1259 (m), 1163 (m), 1110 (m), 1072 (m), 986 (m), 954 (w), 894 (m), 799 (m), 761 (m), 700 (w); MS/APCI: m/z (%) = 544.7 (MH⁺, 100).

Preparation of PrisoH 2

A procedure analogous to that used to prepare 1 was employed, but using Prⁱ₂NH (colourless crystals: crude yield 99%; ca. 90% after recrystallisation); mp 144–145 °C; ¹H NMR (250 MHz, 298 K, CDCl₃): δ 0.89 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.12 (overlapping d, J = 6.8 Hz, 18 H, CH(CH₃)₂), 1.20 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.37 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 3.17 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 3.25 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 3.49 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 4.80 (s, 1 H, NH), 6.80–7.18 (m, 6 H, ArH); ¹H NMR (400 MHz, 298 K, C₆D₆): δ 0.98 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.36 (overlapping d, J = 6.8 Hz, 18 H, CH(CH₃)₂), 1.51 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.54 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 3.68 (sept, J = 6.8 Hz, 6 H, CH(CH₃)₂), 5.25 (s, 1 H, NH), 7.09–7.39 (m, 6 H, ArH), ¹³C{¹H} NMR (75.5 MHz, 298 K, CDCl₃): δ 21.6 (CH(CH₃)₂), 21.9 (NCH(CH₃)₂), 22.7 (CH(CH₃)₂), 24.7 (CH(CH₃)₂), 25.7 (CH(CH₃)₂), 28.2 (CH(CH₃)₂), 28.8 (CH(CH₃)₂), 47.7 (HCN), 121.7, 122.7, 123.6, 126.9, 135.4, 139.8, 145.3, 145.8 (ArC), 148.3 (CN₃); ¹³C{¹H} NMR (100.6 MHz, 298 K, C₆D₆): δ 22.2 (CH(CH₃)₂), 22.5 (NCH(CH₃)₂), 23.1 (CH(CH₃)₂), 25.5 (CH(CH₃)₂), 26.0 (CH(CH₃)₂), 28.8 (CH(CH₃)₂), 29.7 (CH(CH₃)₂), 48.3 (HCN), 123.2, 123.8, 124.3, 127.7, 136.1, 140.3, 145.9, 146.2 (ArC), 149.1 (CN₃); IR (Nujol): ν/cm⁻¹ = 3364 (m), 1608 (s), 1580 (s), 1303 (m), 1245 (m), 1184 (m), 1154 (m), 1109 (m), 1046 (m), 1002 (m), 932 (m), 828 (m), 798 (m), 767 (m), 714 (m); MS/APCI: m/z (%) = 464.4 (MH⁺, 100).

Preparation of PipisoH 3

A procedure analogous to that used to prepare 1 was employed, but using cis-2,6-dimethylpiperidine (colourless crystals: crude yield 98%; ca. 88% after recrystallisation); mp 128–130 °C. ¹H NMR (400 MHz, 298 K, CDCl₃): δ 0.89 (d, J≈ 6.1 Hz, 6 H, NCH(CH₃)), 1.11 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.16 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.18–1.76 (m, 6 H, CH₂), 1.26 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 3.12 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 3.16 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 3.70 (m_c, 2 H, NCH(CH₃)), 4.85 (s, 1 H, NH), 6.88–7.18 (m, 6 H, ArH); ¹H NMR (400 MHz, 298 K, C₆D₆): δ 1.01 (d, J≈ 6.0 Hz, 6 H, NCH(CH₃)), 1.28–1.73 (m, 6 H, CH₂), 1.42 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.49 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.54 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 1.56 (d, J = 6.8 Hz, 6 H, CH(CH₃)₂), 3.55 (sept, J = 6.8 Hz, 4 H, CH(CH₃)₂), 4.11 (m_c, 2 H, NCH(CH₃)), 5.34 (s, 1 H, NH), 7.11–7.39 (m, 6 H, ArH); ¹³C{¹H} NMR (75.5 MHz, 298 K, CDCl₃): δ 14.4 (CH₂), 20.8 (NCH(CH₃)), 21.5 (CH(CH₃)₂), 22.9 (CH(CH₃)₂), 24.1 (CH(CH₃)₂), 25.6 (CH(CH₃)₂), 28.2 (CH(CH₃)₂), 28.9 (CH(CH₃)₂), 30.0 (CH₂), 48.3 (HCN), 121.8, 122.6, 123.5, 126.7, 135.1, 139.4, 145.3, 145.6 (ArC), 149.8 (CN₃); ¹³C{¹H} NMR (100.6 MHz, 298 K, C₆D₆): δ 15.0 (CH₂), 21.4 (NCH(CH₃)), 22.2 (CH(CH₃)₂), 23.4 (CH(CH₃)₂), 25.0 (CH(CH₃)₂), 25.9 (CH(CH₃)₂), 28.9 (CH(CH₃)₂), 29.8 (CH(CH₃)₂), 30.7 (CH₂), 49.0 (HCN), 123.3, 123.7, 124.3, 127.6, 135.9, 140.0, 145.8, 146.1 (ArC), 150.7 (CN₃); IR (Nujol): ν/cm⁻¹ = 3378 (m), 1616 (s), 1579 (s), 1303 (m), 1258 (m), 1183 (m), 1145 (m), 1169 (m), 1079 (m), 1023 (m), 934 (m), 803 (m), 765 (m), 755 (m); MS/APCI: m/z (%) = 476.4 (MH⁺, 100).

Preparation of Pip(GisoH)₂4

BuⁿLi (5.00 cm³ of a 1.6 M solution in hexanes, 8.00 mmol) was added to a solution of piperazine (0.339 g, 3.94 mmol) in THF (40 cm³) at 20 °C over 5 min and the resultant solution stirred for 1 h. ArN=C=NAr (2.93 g, 8.08 mmol) was then added and the mixture stirred for 30 min, before being heated at reflux for 2 h. After cooling to ambient temperature, water (ca. 3 cm³) was added and volatiles removed under reduced pressure. More water (ca. 30 cm³) and CH₂Cl₂ (60 cm³) were then added to the residue and the mixture vigorously stirred until two clear solution phases were formed. The organic phase was separated and the aqueous layer was extracted with CH₂Cl₂ (3 × 30 cm³). The combined organic phases were dried (MgSO₄), filtered and volatiles removed from the filtrate under reduced pressure. The residue was recrystallised from CHCl₃ at –30 °C to give 4 as colourless crystals (yield: 1.88 g, 75%); mp 196–198 °C; ¹H NMR (400 MHz, 298 K, CDCl₃): δ 0.89 (br d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.00 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.15 (br d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.23 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 2.89 (br s, 8 H NCH₂), 3.04 (m_c of overlapping sept., J = 6.8 Hz, 8 H, CH(CH₃)₂), 4.93 (s, 2 H, NH), 6.90–7.16 (m, 12 H, ArH); ¹³C{¹H} NMR (100.6 MHz, 298 K, CDCl₃): δ 22.8 (CH(CH₃)₂), 23.3 (CH(CH₃)₂), 24.5 (CH(CH₃)₂), 25.7 (CH(CH₃)₂), 28.8 (CH(CH₃)₂), 28.9 (CH(CH₃)₂), 47.3 (NCH₂), 123.1, 123.4, 124.3, 127.3, 134.1, 140.1, 144.4, 145.2 (ArC), 150.8 (N₃C); IR (Nujol): ν/cm⁻¹ = 3391 (m), 1623 (s), 1585 (m), 1261 (m), 1196 (m), 1145 (m), 1109 (m), 1041 (m), 988 (m), 935 (m), 840 (m), 799 (m), 759 (m); MS/APCI: m/z (%) = 811.4 (MH⁺, 100).

Preparation of CyP-GisoH 5

BuⁿLi (4.00 cm³ of a 1.6 M solution in hexanes, 6.40 mmol) was added to a solution of Cy₂PH (1.27 g, 6.40 mmol) in THF (20 cm³) at 0 °C over 5 min. The resultant solution was stirred for 1 h at room temperature. A solution of ArN=C=NAr (2.54 g, 6.28 mmol) in THF (15 cm³) was then added to the mixture which was subsequently heated at reflux for 1.5 h. After cooling, degassed water (1 cm³) was added, the mixture vigorously stirred for 1 h, and all volatiles removed under reduced pressure. The residue was extracted with warm hexane (2 × 50 cm³). The extract was dried over MgSO₄, then filtered and concentrated to ca. 15 cm³. Slow cooling of the filtrate to −30 °C overnight yielded colourless crystals of 5 (yield: 2.85 g, 81%); mp 150–152 °C. ¹H NMR (400 MHz, 298 K, CDCl₃): δ 0.81 (d, J = 6.7 Hz, 6 H, CH(CH₃)₂), 1.09 (d, J = 6.7 Hz, 6 H, CH(CH₃)₂), 1.10–1.25 (m, 8 H, CH₂), 1.23 (d, J = 6.7 Hz, 6 H, CH(CH₃)₂), 1.25 (d, J = 6.7 Hz, 6 H, CH(CH₃)₂), 1.58–2.04 (m, 14 H, CHP and CH₂), 3.00 (sept., J = 6.7 Hz, 2 H, CH(CH₃)₂), 3.18 (sept., J = 6.7 Hz, 2 H, CH(CH₃)₂), 5.44 (d, J_PH = 14.1 Hz, 1 H, NH), 6.92–7.18 (m, 6 H, ArH); ¹³C{¹H} NMR (100.6 MHz, 298 K, CDCl₃): δ 22.3 (CH₂), 22.4 (CH₂), 25.2 (CH(CH₃)₂), 26.1 (CH(CH₃)₂), 27.0 (CH(CH₃)₂), 27.8 (CH(CH₃)₂), 27.9 (CH(CH₃)₂), 28.0 (CH(CH₃)₂), 28.1 (CH₂), 28.8 (CH₂), 29.1 (CH₂), 29.1 (CH₂), 32.2 (d, J = 20 Hz, CH₂), 33.7 (d, J = 13.2 Hz, CH₂), 123.3, 123.4, 123.5, 128.4, 133.9, 139.0, 145.2, 147.2 (ArC), 160.1 (d, J = 13.1 Hz, backbone PCN₂); ³¹P{¹H} NMR (121 MHz, 298 K, C₆D₆): δ–2.9 (s); IR (Nujol): ν/cm⁻¹ = 3354 (NH), 1620 (m), 1592 (m), 1568 (s), 1324 (m), 1259 (s), 1173 (m), 1109 (m), 1043 (m), 934 (m), 884 (m), 852 (m), 799 (s), 756 (s); MS/EI: m/z (%) = 560.4 (M⁺, 4), 517.4 (M⁺− C₃H₇, 100). Accurate mass (EI), m/z: calc. for M⁺: 560.4254, found: 560.4251.

Preparation of PhP-GisoH 6

BuⁿLi (2.80 cm³ of a 1.6 M solution in hexanes, 4.52 mmol) was added to a solution of Ph₂PH (0.85 g, 4.56 mmol) in THF (10 cm³) at −70 °C over 5 min then warmed to room temperature and stirred for 2 h. To the resultant red solution was added ArN=C=NAr (1.61 g, 4.43 mmol) in THF (10 cm³) at −70 °C. The mixture was subsequently heated under reflux for 1.5 h. It was then cooled to room temperature and ca. 0.3 cm³ degassed H₂O was added with stirring. Volatiles were removed in vacuo and the residue extracted into diethyl ether (80 cm³) and filtered. The filtrate was concentrated and stored at –30 °C to give colourless blocks of 6 (yield: 1.66 g, 68%); mp 160–162 °C. ¹H NMR (400 MHz, 298 K, CDCl₃): δ 0.83 (4 × overlapping d, J = 6.8 Hz, 24 H, CH₃), 2.78 (sept, J = 6.8 Hz, 2 H, CH), 3.15 (sept, J = 6.8 Hz, 2 H, CH), 5.60 (s, 1 H, NH), 6.85–7.51 (m, 16 H, Ar–H); ¹³C{¹H} NMR (100.6 MHz, 298 K, CDCl₃): δ 21.9, 22.1, 24.4, 25.4 (CH(CH₃)₂), 28.5, 28.8 (CH(CH₃)₂), 122.2, 122.8, 123.0, 123.1, 127.6, 128.0, 128.4, 129.1, 129.7, 137.4, 138.7, 145.9, 146.1, 146.4 (ArC), 155.6 (J = 16.1 Hz, PCN₂); ³¹P{¹H} NMR (121 MHz, 298 K, CDCl₃,): δ−18.5; MS/APCI, m/z (%): 549 (M⁺, 100); IR (Nujol): ν/cm⁻¹ = 1607 (s), 1579 (s), 1434 (m), 1258 (s), 1185 (m), 1099 (m), 742 (m), 693 (m); C₃₇H₄₅N₂P requires: C 80.99%, H 8.27%, N 5.10%, found: C 80.84%, H 8.38%, N 5.25%.

Preparation of ArSi-GisoH 7

BuⁿLi (1.64 cm³ of a 1.6 M solution in hexanes, 2.62 mmol) was added to a solution of ArNC{N(H)Ar}₂ (1.35 g, 2.50 mmol) in THF (15 cm³) at room temperature over 5 min. The solution was then stirred for 1 h. Me₃SiCl (0.36 g, 2.85 mmol) was added at room temperature and the mixture subsequently heated at reflux for 2.5 h. All volatiles were removed under reduced pressure and the residue was extracted into warm hexane (60 cm³). The solution was concentrated under reduced pressure to ca. 12 cm³ and cooled to 4 °C to obtain colourless crystals of 7 (yield 0.96 g, 63%); mp 257–258 °C. ¹H NMR (400 MHz, 298 K, CDCl₃): δ−0.3 (v br s, 9 H, Si(CH₃)₃), 0.92–0.64 (m, 12 H, CH(CH₃)₂), 1.14 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.17 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 2.81 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 3.54–3.14 (m, 4 H, CH(CH₃)₂), 5.59 (s, 1 H, NH), 7.28–6.54 (m, 9 H, ArH); ¹³C{¹H} NMR (75.5 MHz, 298 K, CDCl₃): only resonances of one aryl substituent are resolved. Others, as well as those for the SiMe₃ group, are too broad to be detected. δ 23.2, 26.0, 28.2, (CH(CH₃)₂ and CH(CH₃)₂), 122.4, 126.6, 134.2, 145.5 (ArC), 146.8 (CN₃); IR (Nujol): ν/cm⁻¹= 3378 (m), 1618 (s), 1578 (s), 1378 (m), 1246 (m), 1223 (m), 1107 (m), 1007 (m), 970 (m), 843 (m), 822 (m), 752 (m); MS/EI: m/z (%) = 611 (M⁺, 14), 596 (M⁺− CH₃, 5), 568 (M⁺− C₃H₇, 15), 539 (M⁺− SiMe₃, 15), 496 (M⁺− SiMe₃− C₃H₆, 18).

Preparation of (PrisoH)₂8

A solution of K[N(SiMe₃)₂] (0.65 g, 3.24 mmol) in THF (10 cm³) was added to PrisoH 2 (1.50 g, 3.24 mmol) in THF (10 cm³) at 20 °C and the mixture stirred for 1 h. A solution of impure MnI₂ (Aldrich Chemical Company, 1.00 g, 3.24 mmol) in THF (20 cm³) was then added at −78 °C and the reaction mixture slowly warmed to room temperature overnight. All volatiles were removed in vacuo and the residue was extracted with hexane (40 cm³). Filtration, concentration and slow cooling overnight to −30 °C yielded colourless crystals of 8 (yield 0.44 g, 30%); mp 223–225 °C. ¹H NMR (400 MHz, 298 K, C₆D₆): δ 0.81–1.01 (m of overlapping br d, 18 H, CH(CH₃)₂), 1.05–1.47 (m of overlapping broad d, 54 H, CH(CH₃)₂), 3.11–3.49 (m of overlapping br sept, 12 H, CH(CH₃)₂), 4.74–4.91 (m, 2 H, NH), 6.88–7.29 (m, 10 H, ArH); ¹³C{¹H} NMR (100.6 MHz, 298 K, C₆D₆): δ 21.83, 21.87, 22.70, 22.75, 24.73, 25.69, 28.12, 28.28, 28.81, 28.94, 31.63 (CH(CH₃)₂ and CH(CH₃)₂), 47.60 (CHN), 121.32, 121.41, 121.89, 122.68, 123.55, 123.61, 139.69, 139.78, 140.12, 145.59, 145.78 (ArC), 148.24, 148.33 (CN₃), NB: more than one isomer present. Only major resonances reported with tentative assignments; IR (Nujol): ν/cm⁻¹ = 3364 (m), 1611 (s), 1589 (s), 1376 (s), 1342 (m), 1261 (m), 1184 (m), 1111 (m), 999 (m), 870 (m), 802 (m), 761 (m), 715 (m); MS/EI: m/z (%) = 924.7 (M⁺, 14), 881.7 (M⁺–C₃H₇, 52). Accurate mass (EI), m/z: calc. for M⁺: 924.7691, found: 924.7688; CHN: C₆₂H₉₆N₆ requires: C 80.46%, H 10.45%, N 9.08%; found: C 79.77%, H 10.99%, N 9.31%.

Preparation of [Li{Li(Giso)₂}] 9

BuⁿLi (0.70 cm³ of a 1.6 M solution in hexanes, 1.12 mmol) was added over 5 min to a solution of GisoH 1 (0.58 g, 1.07 mmol) in hexane (20 cm³) at 20 °C. The resultant solution was stirred for 30 min then concentrated under reduced pressure to ca. 8 cm³. It was then stored at 4 °C overnight to afford colourless crystals of 9 (yield: 0.42 g, 71%); mp 190–192 °C (melts with slow decomposition); ¹H NMR (300 MHz, 298 K, C₆D₆): δ 0.85–1.17 (m, 16 H, CH₂), 1.18 (d, J = 6.8 Hz, 24 H, CH(CH₃)₂), 1.49 (d, J = 6.8 Hz, 24 H, CH(CH₃)₂), 1.78–1.48 (m, 16 H, CH₂), 2.03 (m, 8 H, CH₂CHN), 3.35 (br t, J≈ 11 Hz, 4 H, CHN), 3.57 (br sept, J = 6.8 Hz, 8 H, CH(CH₃)₂), 7.26–6.94 (m, 12 H, ArH); ¹³C{¹H} NMR (75.5 MHz, 298 K, C₆D₆): δ 23.7, 25.0, 26.9, 27.9, 28.7, (CH₂), CH(CH₃), CH(CH₃)), 35.1 (CH₂), 58.6 (HCN), 120.8 (ArC), 124.0 (ArC), 141.3 (ArC), 150.2 (br, ArC), 160.6 (v br, CN₃); ⁷Li NMR (155.5 MHz, 298 K, C₆D₆): δ 2.6 (s); IR (Nujol): ν/cm⁻¹ = 1612 (s), 1583 (s), 1236 (s), 1156 (m), 1110 (m), 1027 (m), 933 (m), 895 (m), 792 (m), 748 (m); MS/EI: m/z (%) = 543.7 (GisoH⁺, 5), 500 (GisoH⁺− C₃H₆, 62).

Preparation of [Li(THF)(Giso)] 10

BuⁿLi (2.00 cm³ of a 1.6 M solution in hexanes, 3.20 mmol) was added over 5 min to a solution of GisoH 1 (1.71 g, 3.14 mmol) in THF (20 cm³) at 0 °C. The solution was then stirred for 1 h and volatiles removed under reduced pressure. Hexane (15 cm³) was added to the residue and the resultant solution concentrated to ca. 6 cm³. This was filtered and cooled to −30 °C to yield large colourless crystals of 10. Concentration of the supernatant solution at room temperature yielded another crop of 10 (yield 1.52 g; 78%); mp 208–210 °C. ¹H NMR (400 MHz, 298 K, C₆D₆): δ 0.97 (m_c, 4 H, THF-CH₂), 1.27 (br m_c, 12 H, CH(CH₃)₂), 1.30–1.45 (m, 6 H, CH₂), 1.63 (d, ³J_HH = 6.8 Hz, 12 H, CH(CH₃)₂), 1.71 (m_c, 2 H, CH₂), 1.93 (m_c, 4 H, CH₂), 2.06 (m_c, 4 H, CH₂), 2.52 (m_c, 4 H, CH₂), 2.63 (m_c, 4 H, THF-OCH₂), 3.38 (m_c, 2 H, NCH), 3.85 (sept, J = 6.8 Hz, 4 H, CH(CH₃)₂), 6.98 (t, J = 7.5 Hz, 2 H, p-ArH), 7.26 (d, J = 7.5 Hz, 4 H, m-ArH); ¹³C{¹H} NMR (100.6 MHz, 298 K, C₆D₆): δ 22.8 (CH(CH₃)₂), 23.4 (CH₂), 25.3 (CH₂), 26.3 (CH₂, THF), 27.1 (CH(CH₃)₂), 32.3 (CH₂), 56.5 (NCH), 66.7 (OCH₂), 118.8, 122.2, 140.1, 149.7 (ArC), 156.6 (backbone, CN₃); ⁷Li NMR (116.8 MHz, 298 K, C₆D₆): δ 1.64 (s); IR (Nujol): ν/cm⁻¹ = 3378 (m), 1618 (s), 1578 (m), 1378 (m), 1246 (s), 1228 (m), 1198 (m), 1107 (m), 1008 (m), 970 (m), 843 (m), 822 (m), 752 (m); MS/EI: m/z (%) = 543.7 (GisoH⁺, 100).

Preparation of [{K(Priso)}_∞] 11

Toluene (30 cm³) was added to a mixture of PrisoH 2 (1.02 g, 2.20 mmol) and K[N(SiMe₃)₂] (0.45 g, 2.26 mmol) and the resultant suspension stirred vigorously for 4 h at room temperature. All volatiles were removed under reduced pressure and the residue washed with hexane (15 cm³). Recrystallisation from a toluene solution at −30 °C yielded colourless crystals of 11 (yield 0.95 g, 86%); mp >300 °C. ¹H NMR (400 MHz, 298 K, C₆D₆): δ 1.08 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.43 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 1.51 (d, J = 6.8 Hz, 12 H, CH(CH₃)₂), 3.41 (sept, J = 6.8 Hz, 2 H, CH(CH₃)₂), 3.68 (two overlapping sept, J = 6.8 Hz, 4 H, CH(CH₃)₂), 6.76–7.18 (m, 6 H, ArH); ¹³C{¹H} NMR (100.6 MHz, 298 K, C₆D₆): δ 22.7 (NCH(CH₃)₂), 24.0 (CH(CH₃)₂), 24.2 (CH(CH₃)₂), 27.4 (CH(CH₃)₂), 47.0 (HCN), 117.3, 122.5, 141.1, 145.5 (ArC), 153.2 (CN₃); IR (Nujol): ν/cm⁻¹ = 1613 (s), 1584 (m), 1261 (m), 1152 (m), 1098 (m), 933 (m), 779 (m); MS/EI: m/z (%) = 501.3 (M⁺, 3), 420.3 (M⁺− K − C₃H₆, 100). Accurate mass (EI), m/z: calc. for M⁺: 501.3480, found: 501.3484.

Preparation of [{K(THF)₂}{Pip(Giso)₂}{K(THF)₃}] 12

A solution of K[N(SiMe₃)₂] (0.336 g, 1.68 mmol) in THF (15 cm³) was added to a solution of Pip(GisoH)₂4 (0.65 g, 0.801 mmol) in THF (25 cm³) at 20 °C. The resultant mixture was stirred for 1 h, concentrated to ca. 15 cm³ and then cooled to −30 °C to afford colourless crystals of 12 (yield 0.46 g, 52%); mp >300 °C; IR (Nujol): ν/cm⁻¹ = 1620 (s), 1584 (m), 1238 (m), 1195 (m), 1050 (m), 987 (m), 840 (m), 799 (m), 758 (m), 736 (m); MS/APCI: m/z (%) = 811.4 (Pip(GisoH)₂H⁺, 100). N.B. The very low solubility of 12 in common deuterated solvents precluded the acquisition of meaningful NMR data.

X-Ray crystallography

Crystals of 1–12 suitable for X-ray structural determination were mounted in silicone oil. Crystallographic measurements were made using a Nonius Kappa CCD diffractometer. The structures were solved by direct methods and refined on F² by full-matrix least squares (SHELX97)²⁸ using all unique data. Hydrogen atoms have been included in calculated positions (riding model) for all structures, with the exception of the methyl hydrogens of C(21) and C(24) in the structure of 11 which were not included in the refinement. Two crystallographically independent molecules were refined in the asymmetric units of the crystal structures of 3 and 7. No significant geometric differences were found between the two molecules in each structure and therefore only the metrical parameters for one molecule from each structure are reported here. The Flack parameter for the crystal structure of compound 11 is 0.01(6). Crystal data, details of data collections and refinement are given in Table 2.

Table 2 Crystal data for compounds 1–12

Compound	1	2	3	4·2CHCl3	5	6
Empirical formula	C37H57N3	C31H49N3	C32H49N3	C56H80Cl6N6	C37H57N2P	C37H45N2P
M r	543.86	463.73	475.74	1049.96	560.82	548.72
T/K	123(2)	150(2)	150(2)	150(2)	150(2)	150(2)
Crystal system	Monoclinic	Orthorhombic	Monoclinic	Monoclinic	Monoclinic	Triclinic
Space group	P21/n	Pbca	P21/c	P21/c	P21/n	P1̄
a/Å	12.265(3)	18.397(4)	19.236(4)	13.042(3)	10.960(2)	10.847(2)
b/Å	17.424(4)	15.542(3)	16.327(3)	12.030(2)	26.459(5)	10.942(2)
c/Å	15.775(3)	20.168(4)	19.536(4)	18.989(4)	12.934(3)	14.156(3)
α/°	90	90	90	90	90	96.42(3)
β/°	90.43(3)	90	106.01(3)	91.97(3)	112.61(3)	101.60(3)
γ/°	90	90	90	90	90	102.50(3)
V/Å^3	3371.2(12)	5767(2)	5898(2)	2977.7(10)	3462.6(12)	1585.4(6)
Z	4	8	8	2	4	2
D c/Mg m^−3	1.072	1.068	1.072	1.171	1.076	1.149
μ(Mo-Kα)/mm^−1	0.062	0.062	0.062	0.328	0.105	0.114
F(000)	1200	2048	2096	1120	1232	592
No. reflections collected	38 325	19 664	21 080	10 581	14 712	10 001
No. independent reflns	7339	5343	11498	5520	7517	5420
R int	0.1167	0.0382	0.0500	0.0272	0.0299	0.0306
Final R1 (I > 2σ(I))	0.0629	0.0435	0.0603	0.0465	0.0479	0.0464
Final wR2 (all data)	0.1584	0.1069	0.1527	0.1147	0.1227	0.1159

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Compound	7	8·hexane	9	10	11	12·2THF
Empirical formula	C40H61N3Si	C68H110N6	C74H112Li2N6	C41H64LiN3O	C31H48KN3	C82H132K2N6O7
M r	612.01	1011.62	1099.58	621.89	501.82	1392.14
T/K	123(2)	150(2)	123(2)	150(2)	123(2)	150(2)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Orthorhombic	Monoclinic
Space group	P21/c	P21/c	C2/c	P21/c	P212121	P21/n
a/Å	35.510(7)	15.467(3)	20.707(4)	18.807(4)	11.462(2)	24.500(5)
b/Å	9.9877(2)	22.956(5)	12.031(2)	11.783(2)	11.997(2)	16.473(3)
c/Å	21.966(4)	19.774(4)	26.802(5)	18.773(4)	21.319(4)	20.494(4)
α/°	90	90	90	90	90	90
β/°	104.59(3)	111.87(3)	104.33(3)	113.20(3)	90	93.48(3)
γ/°	90	90	90	90	90	90
V/Å^3	7456(3)	6516(2)	6469(2)	3823.8(13)	2931.6(10)	8256(3)
Z	8	4	4	4	4	4
D c/Mg m^−3	1.090	1.031	1.129	1.080	1.137	1.120
μ(Mo-Kα)/mm^−1	0.093	0.059	0.064	0.063	0.204	0.168
F(000)	2688	2240	2416	1368	1096	3040
No. reflections collected	58 137	16 169	18 157	24 208	22 486	27 961
No. independent reflns	16 069	11 404	5651	8282	5060	14 497
R int	(0.0810)	(0.0423)	(0.1428)	(0.0304)	(0.1036)	(0.0408)
Final R1 (I > 2σ(I))	0.0546	0.0755	0.0669	0.0501	0.0737	0.0713
Final wR2 (all data)	0.1380	0.1828	0.1301	0.1236	0.1371	0.1858

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Acknowledgements

We gratefully acknowledge financial support from the Australian Research Council (fellowships for C. J. and A. S.), the Leverhulme Trust (fellowship for A. S.), the Erasmus scheme of the European Union (travel grant for K. L.), the Royal Society (fellowship for G. J.), and the US Air Force Asian Office of Aerospace Research and Development. Thanks also go to the EPSRC Mass Spectrometry Service, Swansea.

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Footnote

† Electronic supplementary information (ESI) available: ORTEP diagrams for 2 and 3. Crystallographic data (excluding structure factors) for the structures of 1–12. CCDC reference numbers 704662 (1), 704663 (2), 704664 (3), 704665 (4·2CHCl₃), 704666 (5), 704667 (6), 704668 (7), 699384 (8·hexane), 704669 (9), 704670 (10), 704671 (11), 704672 (12·2THF). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b809120j

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