Organozinc complexes with Monoanionic, Chelating Phenolates or [2]-Pyridylmethanol : Molecular Structure of [Zn(CH2SiMe3){OCH2(2-Py)}]4

The influence of the chelate ring size and of the presence of sterically demanding groups in the chelating ligand on the degree of association of organozinc ortho-substituted phenolates and a-substituted 2-pyridylmethanolates has been studied. Organozinc derivatives were made of phenols with one or two potentially ortho-chelating (dimethy1amino)methyl groups and of the a-substituted 2-pyridylmethanols, 2- (2-pyridyl)propan-2-01,2,4-dimethyl-3-(2-pyridyl)pentan3-01, diphenyl-2-pyridylmethanol, l-phenyl-l-(2-pyridyl)ethanol, and 2,2-dimethyl-l-(2-pyidyl)propanol. The CHzNMepsubstituted phenolate compound la, containing a six-membered chelate ring via Zn-N coordination is dimeric, whereas compounds 2b and 2b’, having two CHzNMez substituents in the phenolate ligand, exist in benzene as a monomer-dimer equilibrium. The alkylzinc 2-pyridylmethanolate complexes 3c and 3c’, in which intramolecular Zn-N coordination leads to a five-membered chelate ring, are trimeric in solution. Surprisingly, 3c is a tetramer in the solid state. Introduction of substituents in the five-membered chelate ring of these 2-pyridylmethanolates affects the stability of the aggregates in solution. Complexes 3d and 343, with small substituents on the a-carbon atoms of the pyridylcarbinolate fragment, exist in solution as equilibria of dimers and trimers, whereas compounds 3e, 3f, and 3h, with more sterically demanding a-substituents, are concentration independent dimers in solution. The molecular structure of tetrameric 3c in the solid state contains a puckered eight-membered Zn404 ring in which the zinc atoms are coordinatively saturated by four intramolecularly coordinating pyridyl fragments. Crystals of [Zn(CH&iMe3){OCH2(2-Py))l4, 3c, are monoclinic, space group E1/c with unit-cell dimensions a = 12.625(1) A, b = 17.815(1) A, c = 12.299(1) A, @ = 111.23(1)’, and final R = 0.028.