Structural and Spectroscopic Characterization of 17- and 18-Electron Piano-Stool Complexes of Chromium. Thermochemical Analyses of Weak Cr–H Bonds

The 17-electron radical CpCr(CO)2(IMe)• (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized by the reaction of IMe with [CpCr(CO)3]2, and characterized by single crystal X-ray diffraction and by electron paramagnetic resonance (EPR), IR, and variable temperature 1H NMR spectroscopy. The metal-centered radical is monomeric under all conditions and exhibits Curie paramagnetic behavior in solution. An electrochemically reversible reduction to 18-electron CpCr(CO)2(IMe)− takes place at E1/2 = −1.89(1) V vs Cp2Fe+•/0 in MeCN, and was accomplished chemically with KC8 in tetrahydrofuran (THF). The salts K+(18-crown-6)[CpCr(CO)2(IMe)]−·1/2THF and K+[CpCr(CO)2(IMe)]−·3/4THF were crystallographically characterized. Monomeric ion pairs are found in the former, whereas the latter has a polymeric structure because of a network of K···O(CO) interactions. Protonation of K+(18-crown-6)[CpCr(CO)2(IMe)]−·1/2THF gives the hydride CpCr(CO)2(IMe)H, which could not be isolated, but was characterized in solution; a pKa of 27.2(4) was determined in MeCN. A thermochemical analysis provides the Cr–H bond dissociation free energy (BDFE) for CpCr(CO)2(IMe)H in MeCN solution as 47.3(6) kcal mol–1. This value is exceptionally low for a transition metal hydride, and implies that the reaction 2 [Cr–H] → 2 [Cr•] + H2 is exergonic (ΔG = −9.0(8) kcal mol–1). This analysis explains the experimental observation that generated solutions of the hydride produce CpCr(CO)2(IMe)• (typically on the time scale of days). By contrast, CpCr(CO)2(PCy3)H has a higher Cr–H BDFE (52.9(4) kcal mol–1), is more stable with respect to H2 loss, and is isolable.