Mechanism of the interconversions between C- and N-bound transition metal alpha-cyanocarbanions.

Abstract

The first presentation of the intra- and intermolecular mechanisms of the C-N interconversions of transition metal alpha-cyanocarbanions is described. A pair of N- and C-bound isomers of isonitrile complex Ru+Cp(NCCH-SO2Ph)(PPh3)(CN-t-Bu) (1) and RuCp[CH(CN)SO2Ph](PPh3)(CN-t-Bu) (2) was synthesized for the mechanistic studies on the N-to-C isomerizations. Structural characterization by X-ray diffractions of 1 and 2 indicated their typical zwitterionic and alpha-metalated structures. The kinetic studies on the irreversible isomerization of 1 to 2 in benzene-d6 at 333-348 K were carried out using 1H NMR spectroscopy, affording the first-order rate constants k1 and the activation parameters DeltaH = 107 +/- 2 kJ.mol-1 and DeltaS = -22 +/- 5 J.K-1.mol-1. The almost identical values of k1 were obtained upon similar treatment of 1 with 4 equiv of external ligands such as PPh3, CH3CN, and t-BuNC at 333 K, indicating that the N-to-C isomerization proceeds in an intramolecular manner without dissociation of a ligand. As a model system for the C-to-N isomerization, the irreversible transformation of RuCp[CH(CN)SO2Ph](PPh3)2 (3) to Ru+Cp(NCCH-SO2Ph)(PPh3)2 (4) was investigated under various reaction conditions. The reaction of 3 at room temperature in THF affords the coordination dimers (RRu*,SC*,RRu*,SC*)-{RuCp[CH(CN)SO2Ph](PPh3)}2 (5) stereoselectively, and its distorted mu2-C,N-bound structure was determined by X-ray analysis. The reaction profiles for the isomerization of 3 includes the generation- and temperature-dependent decays of dimeric species 5 and its diastereomer 6, which strongly suggests that the intra- and intermolecular pathways are included in the C-to-N isomerization. The intramolecular process of the C-to-N isomerization of 3 has been confirmed by the kinetic studies on the isomerization of 3 with excess amount of PPh3 in benzene-d6 at 333-348 K which afford the first-order kinetics with the activation parameters of DeltaH = 121 +/- 1 kJ.mol-1 and DeltaS = 42 +/- 4 J.K-1.mol-1. Treatment of 5 with PPh3 in boiling benzene gives rise to the quantitative formation of N-bound complex 4. The controlled kinetic experiments on the cleavage of 5 with PPh3 have concluded that the cleavage of 5 with PPh3 proceeds via simultaneous C-Ru and N-Ru bond scissions, indicating the temperature-dependent participation of intermolecular process in the C-to-N isomerization of 3.