PNNL

Abstract

A free energy map interconnecting nine tungsten complexes has been determined based on chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies (BDFEs) are 59.3(3) kcal mol-1 for CpW(CO)2(IMes)H and 59(1) kcal mol-1 for the dihydride [CpW(CO)2(IMes)(H)2]+. For the radical cation, [CpW(CO)2(IMes)H]•+, W-H bond homolysis to generate the 16-electron cation [CpW(CO)2(IMes)]+ is followed by MeCN uptake, with free energies for these steps being 51(1) kcal mol-1 and -16.9(5) kcal mol-1 respectively. Based on these two steps, the free energy change for conversion of [CpW(CO)2(IMes)H]•+ to [CpW(CO)2(IMes)(MeCN)]+ in MeCN is 34(1) kcal mol-1. The pKa of CpW(CO)2(IMes)H in MeCN was determined to be 31.9(1), significantly higher than the 26.6 reported for the related phosphine complex, CpW(CO)2(PMe3)H. This difference is attributed to the electron donor strength of IMes far exceeding that of PMe3. The pKa values for [CpW(CO)2(IMes)H]•+ and [CpW(CO)2(IMes)(H)2]+ were determined to be 6.3(5) and 6.3(8), much closer to the pKa values reported for the PMe3 analogs. The free energy of hydride abstraction from CpW(CO)2(IMes)H is 74(1) kcal mol-1, and the resultant [CpW(CO)2(IMes)]+ cation is significantly stabilized by binding MeCN to form [CpW(CO)2(IMes)(MeCN)]+, giving a hydride donor ability of 57(1) kcal mol-1. Electrochemical oxidation of [CpW(CO)2(IMes)]- shows a fully reversible wave (E° = -1.65 V vs. Cp2Fe+/0 in MeCN), and CpW(CO)2(IMes)H is reversibly oxidized (E° = -0.13(3) V) at high scan rates (800 V s-1). High pressure NMR experiments provide an estimate of ?G° = 10.3(4) kcal mol-1 for the displacement of MeCN by H2 for [CpW(CO)2(IMes)(MeCN)]+ to give [CpW(CO)2(IMes)(H)2]+.