ENTHALPIES OF REACTION OF CP-ASTERISK-RU(CH3CN)3+O3SCF3-(CP-ASTERISK = ETA-5-C5ME5) WITH ARENES - SOLUTION THERMOCHEMICAL STUDY OF ARENE BINDING TO THE CP-ASTERISK-RU+ FRAGMENT

The enthalpies of reaction of Cp*Ru(CH3CN)3OTf (Cp* = eta5-C5(CH3)5, OTf = O3SCF3) with a series of arenes have been measured by solution calorimetry. The results of this study are used to establish the following relative stability scale leading to the formation of Cp*Ru(arene)OTf complexes in THF (in kcal/mol): naphthalene, -1.7 +/- 0.1; benzene, -3.4 +/- 0.1; biphenyl, -3.6 +/- 0.1; toluene, -4.3 +/- 0.2; p-xylene, -4.6 +/- 0.2; (trimethylsilyl)benzene, -5.0 +/- 0.2; anisole, -5.4 +/- 0.2; mesitylene, -5.5 +/- 0.1; indole, -7.3 +/-0.2; NN-dimethylaniline, -7.5 +/- 0.1; p-bis(dimethylamino)benzene, -8.3 +/- 0.2. The solid-state structure of one of these arene complexes, Cp*Ru[C6H5(Si(CH3)3)]OTf, was determined by a low-temperature data collection X-ray crystallographic study. This investigation illustrates the presence of steric interactions between the phenyl TMS methyl groups and the Cp* methyl group and explains the small measured enthalpy of reaction. The enthalpies of arene substitution from Cp*Ru(CH3CN)3OTf span some 6.6 kcal/mol and vary as a function of the electron-donating ability of the arene. Factors affecting the Ru-arene bond energy are discussed, and thermodynamic comparisons with other organometallic systems are presented.