Rearrangements and catalytic activity of selected transition metal carbonyl complexes

Abstract

The cis-M(CO)4 (13CO)PR3 [double harpoon] trans-M(CO)4(13CO)PR3 CM = Cr, R = Et; M = W, R = OMe, Me, Et, i-Pr) isomerization reactions have been studied kinetically in cyclohexane-d12 by 13C NMR spectroscopy. These isomerization reactions were shown to proceed via an intramolecular process based on the lack of 13CO incorporation into the M(CO)5PR3 derivatives or formation of M(CO)5(13CO) under the reaction conditions of ligand rearrangement. The cis [double harpoon] trans CO scrambling process in the tungsten pentacarbonyl derivatives displays a dependence on the steric nature of the phosphorus ligand, with the rate decreasing PMe3 > Pet3 > Pi-Pr3. The origin of this steric effect is seen in the activation parameters (ΔH = 9.2 kcal-mol^-1 and ΔS = -54.9 e.u.) which are consistent with a trigonal twist type mechanism involving little metal-ligand bond weakening. On the other hand activation parameters for carbon monoxide ligand isomerization in Cr(CO)4(13CO)Pet3 (ΔH = 26.6 kcal-mol^-1 and ΔS = 1.80 e.u.) are consistent with considerable metal-ligand bond breaking in the transition-state with little reorganization taking place. The cis trans isomerization of HCr(CO)4(13CO)- has been studied kinetically. Activation parameters (ΔH = 18.0 kcal and ΔS = 14.0 e.u.) determined by D NMR spectroscopy suggest rearrangement via a pseudo-pentacoordinate intermediate or transition state. The homogeneously catalyzed production of alkyl formates from alcohols and carbon monoxide by transition metal carbonyl complexes in the presence of alkali metal alkoxides is reported. A significant enhancement of catalytic activity of this reaction by group 6 metal carbonyl/alkoxides catalysts over the alkali metal alkoxides catalyzed process is due to the dual role of the metal carbonyl to provide a more electrophilic source of carbon monoxide and to promote the removal of trace water impurities through water gas shift chemistry...