Electron-donating properties of boratabenzene ligands

The reaction of (C5H5B-R)Cp*ZrMe2 (Cp* = eta(5)-C5Me5; R = NMe2 (4), OEt (5), Ph (6)) with B(C6F5)(3) affords zwitterionic complexes of the type [(C5H5B-R)Cp*ZrMe] [MeB(C6F5)(3)] (R = NMe2 (7), OEt (8), Ph (9)). The molecular structures of 7 and 9 were determined by single-crystal X-ray diffraction studies, and they were found to be similar to those observed for standard group 4 metallocenes. The boratabenzene ligand in 7 more closely resembles an eta(5)-pentadienyl fragment than in 9, where it is eta(6)-bound. Variable-temperature H-1 NMR spectroscopy shows that ion-pair dissociation/recombination processes occur in solution. Data over large temperature ranges were obtained by the combination of line shape and spin saturation transfer techniques (100 degrees C for 4 and 5, 65 degrees C for 4). The rates of these processes give insight into how the orbital overlap between boron and the exocyclic group affects the rates of elementary reactions at the metal. The Delta H-double dagger values for ion-pair dissociation/recombination were found to increase with decreasing donor strength of the substituent: 12.2(2), 12.6(1), and 17.6(3) kcal/mol for 7, 8, and 9, respectively. Exchange reactions between 9 and 4 reveal that 7 is formed exclusively, indicating that the aminoboratabenzene ligand can better accommodate the increased positive charge on the metal center. The dependence of the carbonyl stretching frequency on the extent of metal back-bonding in complexes of the type (C5H5B-R)Cp*Zr(CO)(2) (R = NMe2 (10), OEt (11), Me (12), Ph (13)) and (C5H5B-R)(2)Zr(CO)(2) (R = NMe2 (14), OEt (15), Me (16), Ph (17)) can also be used to gauge the electron density at Zr. Complexes 10-17 were prepared by reductive carbonylation of the corresponding dichlorides. The measured reduction potentials of the dichlorides, (C5H5B-R)(2)ZrCl2, show that it is progressively more difficult to reduce the metal center as the donor strength of the boratabenzene ligand increases. The dynamic NMR, IR, and electrochemical data are consistent with the notion that the donor properties in [C5H5B-R] ligands decrease in the order R = NMe2 > OEt approximate to Me > Ph.