Electron-donating properties of boratabenzene ligands
The reaction of (C{sub 5}H{sub 5}B-R)Cp*ZrMe{sub 2} (Cp* = {eta}{sup 5}-C{sub 5}Me{sub 5}; R - NMe{sub 2} (4), OEt (5), Ph (6)) with B(C{sub 6}F{sub 5}){sub 3} affords zwitterionic complexes of the type [(C{sub 5}H{sub 5}B-R)Cp*ZrMe][MeB(C{sub 6}F{sub 5}){sub 3}] (R - NMe{sub 2} (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){sup 5}-pentadienyl fragment than in 9, where it is {eta}{sup 6}-bound. Variable-temperature {sup 1}H 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 C for 4 and 5, 65 C for 6). 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{sub {double{underscore}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 (C{sub 5}H{sub 5}B-R)Cp*Zr(CO){sub 2} (R = NMe{sub 2}(10), OEt (11), Me (12), Ph (13)) and (C{sub 5}H{sub 5}B-R){sub 2}Zr(CO){sub 2} (R = NMe{sub 2} (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, (C{sub 5}H{sub 5}B-R){sub 2}ZrCl{sub 2}, 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 [C{sub 5}H{sub 5}B-R] ligands decrease in the order R = NMe{sub 2} > OEt {approx} Me > Ph.
- Research Organization:
- Univ. of Rochester, NY (US)
- Sponsoring Organization:
- USDOE; Petroleum Research Fund
- OSTI ID:
- 20017363
- Journal Information:
- Journal of the American Chemical Society, Vol. 122, Issue 7; Other Information: PBD: 23 Feb 2000; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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