Bulky guanidinato and amidinato zinc complexes and their comparative stabilities

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)(2)CNCy(2)](-), Ar = 2,6-diisopropylphenyl, Cy = cyclohexyl) and ZnX(2) (X = I or Br) have yielded the monomeric complexes [(Giso)ZnI] and [(Giso)Zn(mu-Br)(2)Li(OEt(2))(2)]. Both have been crystallographically characterised and the former shown to slowly decompose in solution at ambient temperature to give the carbodiimide, ArN=C=NAr. In contrast, reactions between alkali metal complexes of a less bulky guanidinate, [M(Priso)] (Priso = [(ArN)(2)CNPr(2)(i)](-)) and ZnX(2) have yielded [(IZn)(2)(mu-NPr(2)(i)){mu-N,N'-(NAr)(2)CH}] and [(Priso)Zn(mu-Br)(2)Li(OEt(2))(2)]. The latter decomposes in solution at ambient temperature, generating ArN C NAr, which was also produced in the preparation of the former. Analogies are drawn between the decomposition of [(Priso)Zn(mu-Br)(2)Li(OEt(2))(2)] and the carbonic anhydrase catalysed dehydration of bicarbonate. Two bulky amidinato zinc complexes, [{(Piso)Zn(mu-Br)}(2)] and [Zn(Piso)(2)] (Piso = [(ArN)(2)CBu(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.