A Pair of Remarkably Stable Mononuclear Chromium(III) and Chromium(IV) Hydrides

摘要

The coordination and organometallic chemistry of chromium(IV) is much less developed than that of chromium in the oxidation states +II, +III, and +IV. This can be traced back to the high oxidation potential of chromium(IV) complexes and their pronounced tendency to undergo valence disproportionation and reductive elimination reactions. Therefore, it is not surprising that only very few reports have appeared on chromium (IV) and chromium(III) hydrides, which are suggested to be thermodynamically unstable and quite reactive compounds. In fact, CrH_4 remains elusive and its tetrahedral ~3A_2 ground-state structure was calculated to lie 38.5 kcal mol~(-1) higher in energy than that of the dihydrogen complex isomer [CrH_2(η~2-H_2)] (~5A_1) at the CCSD/TZP level of theory. Similarly, [Cr~(IV)H(O)(OH)], which was generated upon photolysis of matrix-isolated [CrO_2(η~2-H_2], was calculated to rearrange exothermally to its tautomer [Cr~(II)(OH)_2] (ΔE = -22.4 kcal mol~(-1), B3LYP/6-311 + + G(d, p)), and pyramidal CrH_3 (~4A_1) is 20.5 kcal-mol~(-1) less stable than [CrH(η~2-H_2)] (~6A_1) at the B3PW91/6-311 + + G(d, p)/SDD level of theory. To date, mononuclear chromium (III) hydrides are unknown and only two chromium (IV) hydrides, [CrH_4(dmpe)_2] (dmpe = Me_2PCH_2CH_2PMe_2)~(18) and the ansa-chromocene derivative [CrH{C_2Me_4(η~5-C_5H_4)(η~2-C_5H_3B(C_6F_5)_3}(CO)]~[9] have been isolated and structurally characterized. We report herein the synthesis, characterization, and theoretical calculations of mononuclear chromium(III) and chromium(IV) hydrides stabilized by a triamidoamine ligand.