The reaction of (C5H5B−R)Cp*ZrMe2 (Cp* = η5-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 η5-pentadienyl fragment than in 9, where it is η6-bound.
Variable-temperature 1H 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 ΔH
⧧ 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)2Zr(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)2ZrCl2, 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 ≈ Me > Ph.
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