Coordinative versatility in main group complexes of C-2,6-terphenyl substituted amidinates
Graphical abstract
Lithium and aluminium amidinate complexes have been prepared from two new C-terphenyl amidines. The solid-state structures of these complexes show significant congestion about the metal centres, which leads to different ligand binding modes and lability of coordinated solvents.
Introduction
The amidinate, [R1NC{R2}NR3]−, is a ligand set closely related to the β-diketiminate. Amidinate complexes of s-, p-, d- and f-block metals have been extensively studied [1]. Amidinate complexes have been employed as single-source precursors to nitride containing materials [2] and as auxiliary ligands in olefin polymerization precatalysts [3], [4]. Careful tuning of the steric bulk at the N- and C-substituents of the amidinate has led to an array of coordination modes being reported [1]. Notably, the introduction of a sterically demanding C- or N-substituents can prevent the amidinate functioning as a bridging ligand, by inducing a more acute ligand bite angle, thereby enforcing an N,N'-chelating motif [5], [6], [7], [8]. C-2,6-terphenyl substituted amidines were first reported by Arnold [9], and later by Clyburne [10], as a means of introducing steric bulk above and below the plane of the amidinate donor set. C-triptycenyl substituted amidines have also been reported [11]. Lithium complexes of these bulky C-substituted amidinates typically display monodentate binding of the metal (Fig. 1) [9], [11], [12]. Whilst the bidentate N,N'-chelation mode is typically observed for larger metals [9], [10], [12], [13].
Here we report two new C-2,6-terphenyl substituted amidine ligands and explore their coordination chemistries with main group metals.
Section snippets
General considerations
Unless otherwise stated, all manipulations were performed using conventional Schlenk or glovebox techniques under an atmosphere of high purity argon in flame-dried glassware. Diethyl ether, THF, toluene, n-pentane and n-hexane were dried over sodium wire and purged with nitrogen prior to distillation from sodium benzophenone ketyl. Benzene-d6 (C6D6) was dried over sodium and freeze-pump-thaw degassed prior to use. Infrared spectra were recorded as Nujol mulls using sodium chloride plates on a
Results and discussion
Bulky C-2,6-terphenyl amidines, N,N′-dicyclohexyl-2,6-di(4-tolyl)benzamidine (L1H) and N,N′-dicyclohexyl-2,6-dimesitylbenzamidine (L2H), were prepared similarly to the C-2,6-terphenyl amidines reported by Arnold [9]. Lithium terphenyl was prepared in situ by treating a diethyl ether solution of terphenyl iodide with n-butyllithium. This was then treated with N,N'-dicyclohexylcarbodiimide followed by hydrolysis and recrystallization from hexane (Scheme 1).
The IR spectra of L1H and L2H are in
Conclusion
In summary, we have successfully prepared two new C-2,6-terphenyl substituted amidines. Solvated lithium complexes derived from these amidines were prepared. The less bulky amidinate coordinates the lithium by a N,N′-chelating mode, whilst the larger amidinate coordinates the lithium in a monodentate fashion. The latter also undergoes partial desolvation in toluene to afford a dimeric lithium amidinate species which features an unprecedented coordination motif. Mono(amidinato)aluminium
Acknowledgement
The authors would like to thank the Australian Research Council (DP110104759) for financial support of this research.
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