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.