Bulky guanidinato and amidinato zinc complexes and their comparative stabilities

Abstract

The preparation of a series of amidinato and guanidinato zinc halide complexes incorporating ligands of varying steric bulk is described, and their thermal stabilities compared. Salt elimination reactions between [M(Giso)] (M = K or Li; Giso = [(ArN)₂CNCy₂]⁻, Ar = 2,6-diisopropylphenyl, Cy = cyclohexyl) and ZnX₂ (X = I or Br) have yielded the monomeric complexes [(Giso)ZnI] and [(Giso)Zn(μ-Br)₂Li(OEt₂)₂]. Both have been crystallographically characterised and the former shown to slowly decompose in solution at ambient temperature to give the carbodiimide, ArNCNAr. In contrast, reactions between alkali metal complexes of a less bulky guanidinate, [M(Priso)] (Priso = [(ArN)₂CNPrⁱ₂]⁻) and ZnX₂ have yielded [(IZn)₂(μ-NPrⁱ₂){μ-N,N′-(NAr)₂CH}] and [(Priso)Zn(μ-Br)₂Li(OEt₂)₂]. The latter decomposes in solution at ambient temperature, generating ArNCNAr, which was also produced in the preparation of the former. Analogies are drawn between the decomposition of [(Priso)Zn(μ-Br)₂Li(OEt₂)₂] and the carbonic anhydrase catalysed dehydration of bicarbonate. Two bulky amidinato zinc complexes, [{(Piso)Zn(μ-Br)}₂] and [Zn(Piso)₂] (Piso = [(ArN)₂CBu^t]⁻) have been prepared, structurally characterised and shown to be markedly more thermally stable than the zinc guanidinate compounds. Attempts to reduce several of the zinc(II) halide complexes to dimeric zinc(I) compounds were so far unsuccessful, in all cases leading to the deposition of zinc metal.