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Notes: Metal π Complexes of Benzene Derivatives, 50. - Arylboranes as Sandwich Ligands: Preparation of [(iPrO)2B(η6-Ph)]2M and [Mes2B(η6-Ph)]2M (M = V, Cr). Redox Properties and EPR Study of M(d5) Neutral Complexes and Boron-Centered Radical AnionsBis(η6-arene)metal complexes bearing peripheral boryl substituents have been prepared by means of metal-atom ligand-vapor cocondensation leading to [(iPrO)2B-η6-C6H5]2Cr (7) and [(iPrO)2B-η6-C6H5]2V (8) as well as by lithiation and subsequent reaction with Mes2BF with formation of [Mes2B-η6-C6H5]2Cr (11) and [Mes2B-η6-C6H5]2V (12). According to the NMR spectra, rotation about the η-C-BR2 bonds in 7 and 11 is free at 25°C, restrictions setting in at -85°C. The neutral radicals 8• and 12• have been studied by EPR spectroscopy: despite of the profound difference in the electron-accepting properties of the groups (iPrO)2B- and Mes2B, the hyperfine coupling constants a(51V) differ by 5% only; the extent of perturbation of the electronic structure manifests itself more clearly in the g tensor, which is tetragonal for 8 but orthorhombic for 12, the degeneracy of the LUMO's e1g being raised in the latter. Cyclovoltammetry on 11 exhibits reversible oxidation as well as reduction processes. Reduction which, according to EPR, is ligand-centered, occurs in two steps which are separated by the redox splitting ΔE1/2 = 570 mV. This value, which is a measure of electronic communication between the two Mes2B groups, lies between those of 1,4-bis(dimesitylboryl)benzene (δE1/2 = 690 mV) and 4,4′-bis(dimesitylboryl)biphenyl (ΔE1/2 = 270 mV). The EPR spectrum of the radical anion 11-• reveals hyperfine coupling to one boron nucleus of a magnitude very similar to that of the free ligand radical anion Mes2BC6H5-•. Therefore, reduction of 11 is ligand-centered, and an intramolecular electron exchange between the boron centers is slow on the EPR time scale. As inferred from the observation of the hyperfine interactions a(53Cr) and a(1H), oxidation of 11 is metal-centered; relative to parent bis(η6-benzene)chromium (9), the two Mes2B groups cause an anodic shift of 290 mV for the couple 11+/0. The radical cations 11+• are prone to protodeborylation, in this aspect resembling the respective silyl derivatives.

Notes: Metal π Complexes of Benzene Derivatives, 49. - Halfsandwich Complexes of Trimesitylborane Mes3B: Synthesis and Structure of Mes2[B(η6-Mes)Cr(CO)3], MesB[η6-MesCr(CO)3]2, and B[η6-MesCr(CO)3]3. Redox Behavior and Questions of Intramolecular InteractionReactions of trimesitylborane (15) with hexacarbonylchromium (16), under varying conditions of stoichiometry and duration, afford the halfsandwich complexes 15[Cr(CO)3] = 17, 15[Cr(CO)3]2 = 18 and 15[Cr(CO)3]3 = 19, which have been characterized by X-ray structure analysis. As for the free ligand 15, the propeller shape of the complexes 17-19 induces chirality; the respective unit cells contain both enantiomers. The steric demand of the Cr(CO)3 fragments causes significant structural changes of the Mes3B unit: in 17 and 18 the bond lengths B-C are increased and the C-B-C bond angles in the reference plane ER1, which is spanned by the three carbon atoms bonded to boron, deviate from 120°; the largest differences was observed for 18. Coordination of Cr(CO)3 fragments to 15 leads to increased dihedral angles between the reference plane ER1 and the mesityl planes; the values of 50.1° for 15 and 61.8° for 19 are representative. Because of the lower symmetry within 17 and 18, the dihedral angles differ; a maximum of 71.1°, relative to the reference plane, is assumed by the noncoordinated ring of 18. The main objective of the study of 17-19 relates to the question of intermetallic communication between moieties separated by sp2-hybridized boron. According to IR data, interaction between the Cr(CO)3 units appears to be minimal. Cyclovo-Itammetry is more revealing: boron-centered reduction, which occurs at -1.94 V for 15, involves anodic shifts E1/2 (0/-) of + 0.24 ± 0.04 V per Cr(CO)3 unit for 17, 18 and 19. This trend is surprising since with increasing degree of coordination the dihedral angles also increase and, therefore, conjugation between the B(2pz) orbital and the mesitylene π systems decreases. Consequently, the redox shifts reflect competition between conjugative and inductive effects, the latter exceeding the former. Subsequent reduction to the dianions 172- -192- is quasi-reversible at -50°C. Chromium-centered oxidation in the +1 V region yields CV waves that fail to reveal resolved redox splitting δE1/2 between subsequent redox steps. However, based on the current ip(0/-) of one electron reduction, the peak currents for the oxidations of 17, 18 and 19 represent one-, two- and three-electron processes, respectively. Although these waves deviate from ideal reversibility, a gradual shift to more positive potentials and an increase in peak separation is discernible. From these features, the value δE1/2 ≤ 70 mV for subsequent oxidation processes at 18 and 19 may be derived as a crude estimate, attesting to weak interaction between the Cr(CO)3 moieties. The radical anions 15-•, 17-•, 18-• and 19-• were generated electrochemically and studied by means of EPR spectroscopy. The hyperfine coupling constants a(11B) increase in the order 17-• 〈 18-• ≤ 15-• 〈 19-•, which again demonstrates the action of stereoelectronic effects. Proton hyperfine coupling is resolved only for the radical anion 15-• of the free ligand. This implies that for the complex radical anions 17-• -19-•, due to the larger angles between the B(2pz) orbital and the z axes of the mesitylene π systems, conjugation Blarr;mesitylene is diminished. The UV/Vis spectra of 17-19 exhibit MLCT bands, which, relative to (η6-C6H6)Cr(CO)3 (λ = 317 nm), show large bathochrome shifts [λ (17) = 458 nm]. The additional shifts effected by introducing a second and third Cr(CO)3 unit are small however [λ (18) = 491 nm, λ (19) = 516 nm]. The energies ΔEop of the MLCT transitions may be compared to the differences ΔEcv = E1/2 (+/0, metal-centered) - E1/2(0/-, ligand-centered), the quantity δE = ΔEop - ΔEcv representing χout the outer-sphere reorganisation energy. For 17-19, the value δE = 0.18 ± 0.1 eV is thus obtained. Interestingly, for p-Me2NC6H4BMes2 δE = 0.29 eV has been reported, suggesting a similarity between a Me2N substituent and a Cr(CO)3 fragment bound to tris(aryl)borane.