Synthesis and structural characterization of dinuclear and trinuclear chromium complexes and dinuclear tungsten complexes

Abstract

Dichromium(II) compounds are unique among metal-metal quadruply bonded systems in the range of metal-metal distances they display and in the tendency of the metal atoms in neutral molecules to bind axial ligands. X-ray crystallographic studies have been made of twelve compounds of general formula Cr(,2)(O(,2)CR)(,4)L(,2) where L = pyridine or substituted pyridine, R = C(CH(,3))(,3), CH(,3), H, CClH(,2), CF(,2)H, to determine the response of the Cr-Cr quadruple bond length to (a) the donor ability of axial ligands and (b) the inductive character of the R groups. The results are sufficient to indicate a general relationship between the donor ability of the bridging ligands, and of the axial ligands with the lengths of the quadruple Cr-Cr bond and the Cr-L(,ax) bond. The Cr-Cr distance, however, is sensitive to a variety of additional factors due to the broad, relatively shallow potential function for the Cr-Cr quadruple bond interaction. The preparation and structural characterization of two compounds of general formula Cr(,3)((mu)(,3)-O)(O(,2)CCF(,2)H)(,6)L(,3)(.)solv is reported with L = pyridine, solv = (C(,2)H(,5))(,2)O and L = 4-cyanopyridine, solv = toluene. These are the first compounds containing a planar-triangular Cr(,3)O moiety in a neutral rather than cationic Cr(,3)O(O(,2)CR)(,6)L(,3) unit. An X-ray crystallographic study has been made on the quadruply bonded dinuclear compound Cr(,2){(C(,6)H(,5))NC(O)C(CH(,3))}(,4). The results, together with those on two Cr(,2){ArNC(O)R}(,4)-type compounds, show that the variations in Cr-Cr distance can be accounted for solely by postulating linear dependence on the torsion angle of the Ar group. This angle serves as a measure of the bridging ligand inductive effect caused by conjugation of the ligand (pi) system with the amido (pi) system. In these three molecules axial ligand effects are not present. X-ray crystallographic studies have been made on two Cr(,2)(O(,2)CR)(,4)(C(,4)H(,8)O)(,2)-type compounds, R = CH(,3) and H. The results were used to calculate the Cr-Cr quadruple bond length of Cr(,2)(O(,2)CR)(,4)-type compound in the absence of axial interaction. Reduction of tungsten(IV) tetrachloride with sodium amalgam in the presence of carboxylic acid salt allows the isolation of carboxylato ditungsten(II) compounds. X-ray crystallographic studies have been made of three of these compounds with R = C(,6)H(,5), 4-(OCH(,3))C(,6)H(,4), and 2,4,6-(CH(,3))(,3)C(,6)H(,2). The electronic absorption spectra of these compounds were measured in THF and toluene solutions and assignments are proposed.