Metall‐π‐Komplexe von Benzolderivaten, 49. Halbsandwichkomplexe des Trimesitylborans Mes3B: Darstellung und Struktur von Mes2B[(η6‐Mes)Cr(CO)3], MesB[(η6‐Mes)Cr(CO)3]2 und B[(η6‐Mes)Cr(CO)3]3. Redoxverhalten und Fragen der intramolekularen Wechselwirkung
Authors: Elschenbroich Christoph , Kühlkamp Peter , Behrendt Andreas , Harms Klaus 
Published: July 2007

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 Interaction

Reactions 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.