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 H 2 loss, and is isolable.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry