title: Strong Trans Influence Methoxymethyl Ligand in B(12) Cobaloxime and Imine/Oxime Model Complexes: Structural, Spectroscopic, and Molecular Mechanics Investigations.
abstract: The CH(2)OCH(3) ligand has a large trans influence comparable to that of bulky alkyl groups but lacks the complication of marked steric effects. However, crystals of model complexes with this ligand have proved difficult to obtain. The crystal structures of the cobaloximes reported here, Me(3)BzmCo(DH)(2)CH(2)OCH(3).0.6CH(3)OH (1) and 4-MepyCo(DH)(2)CH(2)OCH(3) (2) [DH = monoanion of dimethylglyoxime, Me(3)Bzm = 1,5,6-trimethylbenzimidazole, and 4-Mepy = 4-methylpyridine], triple the number of cobaloxime structures with CH(2)OCH(3). Also, these are the first structures in this class of models with an N-donor planar heterocyclic axial donor ligand, L. The planes of the Me(3)Bzm and 4-Mepy ligands are almost perpendicular to the respective planes of the four equatorial DH nitrogen donors and bisect the (O(-))N-Co-N(OH) angles. The Co-N(L) bond distances average 2.096(2) Å, confirming the strong trans influence of CH(2)OCH(3). Geometry optimization via molecular mechanics using MacroModel 5.0 and an AMBER-type force field was applied to both cobaloxime-type and imine/oxime-type B(12) models. In our initial work with some imine/oxime models, the calculated structures did not compare well to the solid-state structures. Therefore, adjustments to the force field were evaluated. The major adjustment that improved the fit of the computed and experimental structures was an approximately 10% reduction of the van der Waals (vdw) parameters for both the N donors and the C(sp(2)) atoms linked to the N donors; this adjustment may reflect the electron-withdrawing effect of the metal center. Analysis of (1)H-(13)C coupling constants of Me(3)Bzm lends support to the concept that the atoms in the ligands were modified, but only slightly, by the metal center. To reproduce the dependence of geometric features on the trans influence, different force field parameters for L-Co bonds were used for compounds with weak (Cl) and those with strong (CH(2)OCH(3)) trans influence ligands. These small modifications allowed us to model the structural features of both classes of models well.
authors.affiliation: Department of Chemical and Biosystem Sciences and Technologies, University of Siena, Pian dei Mantellini 44, I-53100 Siena, Italy, and Department of Chemistry, Emory University, Atlanta, Georgia 30322.
authors.forename: Renzo
authors.initials: R
authors.lastname: Cini
authors.forename: Scott J.
authors.initials: SJ
authors.lastname: Moore
authors.forename: Luigi G.
authors.initials: LG
authors.lastname: Marzilli