CHEMICAL AND ELECTROCHEMICAL OXIDATION OF GROUP-6 CYCLOPENTADIENYLMETAL HYDRIDES - 1ST ESTIMATES OF 17-ELECTRON METAL HYDRIDE CATION RADICAL THERMODYNAMIC ACIDITIES AND THEIR DECOMPOSITION TO 17-ELECTRON NEUTRAL RADICALS

The oxidation chemistry of the organometallic hydride complexes (η5-C5H5)M(CO)3H (M = Cr, 1; Mo, 2; W, 3), (η5-C5Me5)Mo(CO)3H (4), and (η5-C5H5)W(CO)2(PMe3)H (5) has been investigated in acetonitrile solution by electrochemical and other methods. The thermodynamic acidities of the cation radicals of 1-5 have been estimated by the use of a thermochemical cycle based on the oxidation potentials of 1-5 and their conjugate bases (anions) as well as the solution pKa values of 1-5. The metal hydride cation radical pKa estimates fall in the range -10 to +5, which makes these complexes the most acidic hydrides for which pKa values have been determined. Coulometric measurements show that 1-5 undergo overall two-electron oxidations. For compounds 2-5, cationic acetonitrile complexes were isolated in good yields after two-electron preparative electrolyses. Chemical oxidation of 5 gives a 1:1 mixture of dihydride (η5-C5H5)W(CO)2(PMe3)H2+ (13) and (η5-C5H5)W-(CO)2(PMe3)(NCMe)+ (10), the latter as a mixture of cis and trans isomers in the thermodynamic 95:5 ratio. Hydride abstraction from 5 using a substituted trityl cation also yields 10 but with a cis:trans ratio of 10:90. Addition of a catalytic amount of cobaltocene causes the conversion of trans-10 into the thermodynamic isomer mixture without ligand dissociation. The products generated upon oxidation of the hydrides are proposed to be formed by initial deprotonation of the metal hydride cation radicals. Subsequent nucleophilic attack by acetonitrile followed by a one-electron oxidation of 19-electron adducts then generates the products. The electrocatalytic isomerization of trans-10 demonstrates that the 19-electron radical derived from reduction of 10 has a finite lifetime and thus supports the notion that 19-electron species are competent intermediates, not only transition states, in ligand substitution processes of 17-electron organometallic radicals. © 1990, American Chemical Society. All rights reserved.