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 <i>E</i>_1/2 = −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 p<i>K</i>_a 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 (Δ<i>G</i> = −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