Transition-metal Schiff-base complexes as ligands in tin chemistry. Part 5. Structural studies of intimate ion-paired heterobimetallic complexes of tin(IV) and nickel, copper or zinc with 3-methoxysalicylaldimine ligands

Abstract

1 : 1 Adducts have been formed between SnMe₂(NCS)₂ and SnBuⁿ₂(NCS)₂ with [Ni(3MeO-salpd)]·H₂O [3MeO-H₂salpd =N,N′-bis(3-methoxysalicylidene)propane-1.3-diamine] and [Ni(3MeO-salbn)]·H₂O [3MeO-H₂salbn =N,N′-bis(3-methoxysalicylidene)butane-1,4-diamine] and are best represented as the intimate ion-paired adducts [SnR₂]²⁺·[NiL(NCS)₂]^2–[L = Schiff-base ligand; R = Me or Buⁿ] in each of which the tin atom sits in the plane of, and is held by donor bonds from, the four Schiff-base oxygen atoms. A crystal-structure determination of [SnMe₂]²⁺·[Ni(3MeO-salpd)(NCS)₂]^2– revealed a tin co-ordination geometry which can be described as skew trapezoidal bipyramidal. The large unoccupied volume of the tin co-ordination sphere results in tin readily increasing its co-ordination number to seven, as demonstrated crystallographically in [SnMe₂·dmf]²⁺·[Ni(3MeO-salpd)(NCS)₂]²⁺(dmf = dimethyl-formamide), which contains tin in a pentagonal-bipyramidal co-ordination environment with trans methyl groups and five donor oxygen atoms in the equatorial plane. X-Ray structural studies of both [SnBuⁿ₂]²⁺·[Ni(3MeO-salpd)(NCS)₂]^2– and [SnBuⁿ₂]²⁺·[Ni(3MeO-salbn)(NCS)₂]^2– revealed the presence of molecular association resulting from weak intermolecular thiocyanate contacts, thus providing each tin with pseudo-pentagonal-bipyramidal co-ordination geometry. These latter adducts readily formed tin donor bonds with dmf. The 1:1 adduct formed between SnPh₂(NCS)₂ and [Ni(3MeO-salpd)]·H₂O was shown, from an X-ray crystallographic study, to be the intimate ion-paired heterobimetallic complex [SnPh₂(NCS)]⁺·[Ni(3MeO-salpd)(NCS)(MeCN)]^– in which tin has pentagonal-bipyramidal geometry with trans phenyl groups. Infrared and tin-119 Mössbauer spectroscopic data clearly demonstrate that a similar pentagonal-bipyramidal co-ordination geometry exists about tin in the complexes [SnR₂(NCS)]⁺·[ML(NCS)]^–·nH₂O (R = Me or Buⁿ; M = Cu or Zn; n= 0, 1 or 2).