Dinuclear (d(3)-d(3)) Diolate Complexes of Molybdenum and Tungsten. 3.(1) Bridging and Chelating Isomers of M(2)(NMe(2))(2)(O approximately approximately CHMe approximately approximately O)(2), Where O approximately approximately CHMe approximately approximately O Is the Dianion of 2,2'-Ethylidenebis(4,6-di-tert-butylphenol). Kinetic versus Thermodynamic Considerations.

Abstract

Hydrocarbon solutions of Mo(2)(NMe(2))(6) and 2,2'-ethylidenebis(4,6-di-tert-butylphenol), HO approximately approximately CHMe approximately approximately OH (2 equiv), react to give a mixture of two isomers, A and B, of formula Mo(2)(NMe(2))(2)(O approximately approximately CHMe approximately approximately O)(2). Both A and B are shown to contain the bridging &mgr;-O approximately approximately CHMe approximately approximately O ligands. They are diastereomers differing with respect to the positioning of the CHMe moiety as a result of ring closure with elimination of HNMe(2). Compounds A and B are thermally persistent at 100 degrees C in the solid state and in toluene, but compound A isomerizes in the presence of pyridine to the thermodynamically favored chelate isomer Mo(2)(NMe(2))(2)(eta(2)-O approximately approximately CHMe approximately approximately O)(2), C. Compound C is related to the previously characterized isomer Mo(2)(NMe(2))(2)(eta(2)-O approximately approximately CH(2) approximately approximately O)(2), where the methylene proton that is distal to the Mo-Mo triple bond is replaced by a Me group. Compound B cannot rearrange to this isomer without Mo-O bond dissociation. W(2)(NMe(2))(6) and HO approximately approximately CHMe approximately approximately OH (2 equiv) react to give W(2)(NMe(2))(2)(eta(2)-O approximately approximately CHMe approximately approximately O)(2), D, which is an analogue of C. The three molybdenum isomers A-C and tungsten complex D have been structurally characterized by single crystal X-ray studies. The results of this work are discussed in terms of the earlier work involving the related 2,2'-methylenebis(6-tert-butyl-4-methylphenoxides) and reveal that the initial ring closure reaction determines the stereochemistry of the subsequent substitution by the biphenoxide at the dinuclear center. Crystal data: for A.PhMe at -168 degrees C, a = 16.585(5) Å, b = 19.480(6) Å, c = 10.960(3) Å, alpha = 98.39(2) degrees, beta = 103.19(1) degrees, gamma = 91.44(2) degrees, space group P&onemacr;; for B.2PhH at -168 degrees C, a = 15.167(3) Å, b = 18.210(3) Å, c = 13.964(3) Å, alpha = 92.81(1) degrees, beta = 100.40(1) degrees, gamma = 71.16(1) degrees, space group P&onemacr;; for C.1.5Et(2)O at -168 degrees C, a = 16.483(3) Å, b = 16.817(3) Å, c = 14.305(2) Å, alpha = 106.13(1) degrees, beta = 108.80(1) degrees, gamma = 71.70(1) degrees, space group P&onemacr;; and for D at -168 degrees C, a = 14.638(2) Å, b = 21.188(4) Å, c = 10.273(2) Å, alpha = 94.80(1) degrees, beta = 95.05(1) degrees, gamma = 100.85(1) degrees, space group P&onemacr;.