Ring-borylated 15-electron and 17-electron ansa-chromocene complexes, their physical properties and molecular structures

A detailed study of the thermal decomposition of the zwitterionic, ring-borylated ansa-chromocene hydrido carbonyl complex [Cr(CO)HMe(4)C(2)(C(5)H(4))[C(5)H(3)B(C(6)F(5))(3)]] (2) is described. This complex is formed in the reaction between [Cr(CO)Me(4)C(2)(C(5)H(4))(2)] (1) and B(C(6)F(5))(3) in toluene at -78 degrees C. Above -25 degrees C, 2 decomposes to a 50:50 mixture of the low-spin, 17e Cr(III) complexes [Cr(CO)Me(4)C(2)(C(5)H(4))[C(5)H(3)B(C(6)F(5))(3)]] (3b) and [Cr(CO)Me(4)C(2)(C(5)H(4))(2)][HB(C(6)F(5))(3)] (4). Carbon monoxide elimination from 3 b generates high-spin, 15 e [CrMe(4)C(2)(C(5)H(4))[C(5)H(3)B(C(6)F(5))(3)]] (3a), which coordinates two other electron-donating ligands, such as xylyl isocyanide, PMe3, and PPh(2)Me to form the low-spin, 17 e electron complexes 3c, 3d, and 3e, respectively. High-spin, 15 e [CrMe(4)C(2)(C(5)H(4))(2)][HB(C(6)F(5))(3)] (5) is generated by heating 3 b in toluene at 100 degrees C and periodically removing the evolved CO. Efforts to isolate more than a few X-ray quality crystals of 5 were thwarted by its tendency to form an insoluble precipitate (6) with the same molecular formula. Heating the solution of 5 at 120 degrees C results in its partial conversion (ca. 28 %) to 3a, thereby allowing the formation of 3a in yields as high as 74 % from the reaction between 1 and B(C(6)F(5))(3). The X-ray crystal structures of 3 b-e and 5 are described. Cyclic voltammetry measurements on 3 a-e reveal a dramatic reduction in the redox potentials of the complexes relative to their non-borylated analogues. DFT calculations show that this is due primarily to electrostatic stabilization of the oxidized species by the negatively charged borylate group. EPR and 19F NMR spectroscopy allow 3a to be distinguished from its Lewis base adducts 3 b-e and reveal the relative affinities of different Lewis bases for the chromium.