ALKYL SUBSTITUENT EFFECTS IN THE REDOX THERMOCHEMISTRY OF COORDINATION-COMPOUNDS - OXIDATION AND REDUCTION ENERGETICS FOR RUTHENIUM TRIS(BETA-DIKETONATE) COMPLEXES IN SOLUTION AND THE GAS-PHASE

Alkyl substituent effects in gas-phase and solution redox thermochemis try have been studied for a series of ruthenium coordination complexes . The gas-phase free energies of ionization (Delta G(i) degrees) and e lectron attachment (Delta G(a) degrees) are compared to electrochemica l oxidation and reduction half-wave potentials (E(1/2)) for six ruthen ium tris-(beta-diketonate) complexes (RuL(3), where L = CH(COR)(2)(-) and R = methyl, ethyl, n-propyl, n-butyl, isopropyl, and tert-butyl). Values for Delta G(i) degrees and Delta G(a) degrees were determined f rom electron-transfer equilibrium measurements by using Fourier transf orm ion cyclotron resonance mass spectrometry. Substituted benzenes an d metallocenes were used as reference compounds. Cyclic voltammetry wa s used to determine E(1/2) values, which were obtained in N,N-dimethyl formamide and measured relative to the ferrocene/ferrocenium couple. S ubstitution of methyl with larger substituents results in cathodic shi fts in both oxidation and reduction potentials, and the solution data correlate well with the sums of Taft alkyl substituent parameters (sig ma(I)). Gas-phase cations and anions are stabilized relative to the ne utral by larger alkyl substituents, rendering Delta G(i) degrees less endoergic and Delta G(a) degrees more exoergic as the alkyl group size increases. The trends for solution and gas-phase reduction of the neu tral ruthenium(III) complex are therefore reversed. Estimates for the differential solvation free energies for one-electron oxidation (Delta Delta G(solv)degrees(0/+)) and reduction (Delta Delta G(solv)degrees( 0/-)) are obtained by combining the Delta G(i) degrees and Delta G(a) degrees data with electrochemical E(1/2) data. Values of Delta Delta G (solv)degrees(0/-) are less exoergic for complexes with larger alkyl s ubstituents and range from -48 +/- 5 kcal mol(-1) (R = methyl) to -36 +/- 5 kcal mol(-1) (R tert-butyl), and these changes in Delta Delta G( solv)degrees(0/-) are identified as the cause of the reversal in the t rends for gas-phase and solution reduction free energies. In contrast, Delta Delta G(solv)degrees(0/-) values all fall in the range -19 to - 16 kcal mol(-1) and show no correlation with the size of the alkyl sub stituents. Analysis of the gas-phase data by a model of substituent ef fects based on polarizability and inductive contributions of a group s hows that alkyl inductive effects are small in these complexes and tha t relative stabilities of gas-phase ions are primarily due to differen ces in polarizability of the alkyl substituents R on the ligands. A si milar analysis for R = CF3 complexes suggests that the substantial eff ect of that group on gas-phase ionization and electron-attachment ener gies is almost entirely due to a large inductive effect.