Molybdenum Complexes with Linked Cycloheptatrienyl−Phenolate Ligands

Publisher Website
Google Scholar
Abstract
The tropylium salt (2-hydroxy-3,5-dimethylphenyl)cycloheptatrienylium tetrafluoroborate (3) has been synthesized in three steps from THP-protected 2,4-dimethylphenol and tropylium tetrafluoroborate, (C7H7)BF4. In CH2Cl2 solution, the unexpected formation of tricyclic 2,4-dimethylbenzo[b]cyclohepta[d]furanylium tetrafluoroborate (5) has been observed, which must have formed from 3 by loss of H2. 5 was characterized by an X-ray crystal structure determination and could independently be synthesized by treatment of 3 with NaHCO3 to give 2,4-dimethylbenzo[b]cyclohepta[d]furan (4) followed by acidification with HBF4·Et2O. The arene transfer reaction of 3 with [(η-p-xylene)Mo(CO)3] furnished the cycloheptatrienyl complex [(HOC6H2Me2-η7-C7H6)Mo(CO)3]BF4 (6), which could be converted into the chiral chelate complexes (9a, R = Ph; 9b, R = c-C6H11; 9c, i-Pr) by subsequent treatment with NaI, PR3, and NaH. The linked cycloheptatienyl−phenolate ligand in 9a could be protonated at the coordinated oxygen atom employing HBF4·Et2O to yield (10). In 10, the appended phenol group is coordinated in a hemilabile fashion, which allowed the introduction of 2,6-dimethylphenyl isocyanide and CO and formation of complexes [(HOC6H2Me2-η7-C7H6)Mo(CO)(PPh3)L]BF4 (11, L = XyNC; 12, L = CO). On thermal reaction of [(HOC6H2Me2-η7-C7H6)Mo(CO)2I] (7) with dppe, the addition of the diphosphine was observed together with the simultaneous formation of the molydenum−oxygen bond to yield (13), in which the cycloheptatrienyl ring has reduced its hapticity from seven to three. The pseudooctahedral complex 13 exhibits an interesting fluxional behavior in solution, which has been studied by means of variable-temperature 31P and 1H NMR spectroscopy. A η3 → η7 hapticity reversion could be achieved by UV irradiation of a solution of 13 in THF to give the electron-rich complex (14). 14 was readily oxidized with ferrocenium hexafluorophosphate, and the resulting paramagnetic monocationic complex 15 has been studied by means of ESR spectroscopy and X-ray diffraction. In addition, the X-ray crystal structures of complexes 9a, 10 ·2CH2Cl2, 12, and 13 are reported.