CommunicationHeteroleptic lead and aluminium complexes ligated by a bulky non-symmetrical triazenide
Graphical abstract
Introduction
Bidentate monoanionic N,N′-donor ligands are widely deployed in coordination chemistry because these chelating ligands are able to stabilize and shield reactive sites [1,2]. Of this ligand class the most widely employed are the β-diketiminates (Fig. 1) [[3], [4], [5], [6]]. However, several recent studies have shown these ligands may not behave as simple spectators, specifically there are several examples of this ligand set displaying redox non-innocence [7]. Amidinate ligands (Fig. 1) are widely utilized alternatives [[8], [9], [10], [11], [12]]. A frequently overlooked ligand class, isoelectronic to formamidinates, are the triazenides (Fig. 1) [13]. As demonstrated by Niemeyer and others, a notable feature of this ligand subclass is the ease of which sterically demanding substituents can be added [[14], [15], [16], [17], [18]]. There has been renewed interest in developing sterically demanding N,N′-ligands to stabilize reactive complexes by kinetically preventing ligand redistributions [[19], [20], [21], [22], [23]]. Notable examples are the development of amidinate [[24], [25], [26]], guanidinate [27] and β-diketiminates [28,29] bearing N-2,6-dibenzhydrylphenyl- substituents.
We are interested in the stabilization of reactive low coordinate main group metal complexes by sterically demanding neutral and anionic ligands [[30], [31], [32], [33], [34]]. Several heteroleptic lead complexes featuring amidinate [35,36], iminoanilide [37] and β-diketiminate [[38], [39], [40], [41], [42], [43], [44], [45], [46]] ligands have been reported. By contrast there is only a single example of a heteroleptic lead triazenide complex, [(AdN3Si(SiMe3)3)Pb{Si(SiMe3)3}] [47]. Recent attempts by Fox and Johnson to prepare a heteroleptic amido lead triazenide complex, [(Dipp2N3)Pb{N(SiMe3)2}], by treating the parent triazene with an equimolar amount of [Pb{N(SiMe3)2}2] afforded the bis(triazenide) complex, Pb(N3Dipp2)2, as the only isolable product [48]. There are few heteroleptic triazenide complexes of the lighter Group 14 metals [33,49,50]. In each instance, a triazenide bearing bulky N-substituents is deployed, which likely hinders the formation of bis(triazenide) species.
In this contribution we prepare a new bulky non-symmetrically substituted triazene. We demonstrate its size by isolating a heteroleptic amido lead triazenide complex. Subsequently, we attempt to quantify the steric bulk of this triazenide using readily prepared model aluminium complexes.
Section snippets
Results and discussion
Niemeyer has reported the large-scale preparation of triazenes bearing bulky substituents by reacting lithiated aryls with aryl azides followed by hydrolysis [14]. Utilizing an analogous synthetic route, we have prepared a new bulky non-symmetrically substituted triazene, DmpN3(H)Dipp∗ (1) in excellent yield (94%) on an 8.6 mmol scale (Scheme 1). This incorporates a Dmp (2,6-dimesitylphenyl-) at one nitrogen terminus and a Dipp∗ (2,6-dibenzhydryl-4-methylphenyl-) at the other. The 1H and 13C{1
Conclusions
In summary, we have prepared and structurally characterized, with the help of a very bulky aryl-substituted triazenide ligand, a stable heteroleptic amido lead complex. The steric bulk introduced by this ligand was further quantified using metrics taken from the solid-state structures of model dimethyl aluminium complexes.
General information
All manipulations were performed using conventional Schlenk or glovebox techniques under an atmosphere of high purity argon in flame-dried glassware. Diethyl ether, THF 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-thaw degassed prior to use. Infrared spectra were recorded as Nujol mulls using sodium chloride plates on a Nicolet Avatar 320 FTIR spectrophotometer. Spectra
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the Australian Research Council (DP110104759) for financial support of this research and the Australian Government for funding postgraduate scholarships. The authors would like to thank Dr. Matthew R. Gyton for insightful comments and for the generous gift of Dipp∗2N3H.
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