Nucleophilic activation of coordinated carbon monoxide. 3. Hydroxide and methoxide reactions with the trinuclear clusters M3(CO)12 (M = Fe, Ru, or Os). Implications with regard to catalysis of the water gas shift reaction

Abstract

Reported are quantitative investigations of the reactions of the triangular clusters M/sub 3/(CO)/sub 12/ (M = Fe, Ru, or Os) with methoxide ion in solution. In methanol under a CO atmosphere, both the osmium and ruthenium species form stable 1:1 methoxycarbonyl adducts (M/sub 3/(CO)/sub 12/ + NaOCH/sub 3/ in equilibrium (M/sub 3/(CO)/sub 11/(CO/sub 2/CH/sub 3/))Na); however, for the triiron analogue this adduct undergoes fragmentation to give Fe(CO)/sub 4/(CO/sub 2/CH/sub 3/)/sup -/. Initial adduct formation in each case occurs with an equilibrium constant of about 10/sup 3/ M/sup -1/. In mixed tetrahydrofuran/methanol solutions, K/sub eq/ for Ru/sub 3/(CO)/sub 11/(CO/sub 2/CH/sub 3/)/sup -/ is much larger, an indication of the greater activity of NaOCH/sub 3/ in the less protic solvent. Notably, in such solvent mixtures, the presence of excess methoxide also led to the formation of 2:1 adducts. Rates of adduct formation were examined by using stopped-flow kinetics techniques, and it was shown that in methanol the second-order rate constants (25/sup 0/C) are 11.3 x 10/sup 3/, 2.1 x 10/sup 3/, and 0.6 x 10/sup 3/ M/sup -1/ s/sup -1/ for Fe/sub 3/(CO)/sub 12/, Ru/sub 3/(CO)/sub 12/, and Os/sub 3/(CO)/sub 12/, respectively. Rates were much higher in the mixed THF(tetrahydro-furan)/CH/sub 3/OH solutions;more » for example, k/sub 1/ (25/sup 0/C) for Ru/sub 3/(CO)/sub 12/ is 2.0 x 10/sup 5/ M/sup -1/ s/sup -1/ in 90/10 THF/CH/sub 3/OH (v/v). Monosubstitution of the ruthenium cluster with (CH/sub 3/O)/sub 3/P markedly reduced the reactivity toward the anionic nucleophile. The reaction of the triruthenium species with hydroxide (Ru/sub 3/(CO)/sub 12/ + OH/sup -/ in equilibrium Ru/sub 3/(CO)/sub 11/(CO/sub 2/H)/sup -/ ..-->.. HRu/sub 3/(CO)/sub 11//sup -/ + CO/sub 2/) was also investigated. Analysis of the reaction kinetics leads to the conclusion that formation of the initial hydroxycarbonyl adduct is somewhat less favorable and is slower than the analogous reaction of methoxide.« less