Molybdenum complexes with linked cycloheptatrienyl-phenolate ligands.

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, (C(7)H(7))BF(4). In CH(2)Cl(2) 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 H(2). 5 was characterized by an X-ray crystal structure determination and could independently be synthesized by treatment of 3 with NaHCO(3) to give 2,4-dimethylbenzo[b]cyclohepta[d]furan (4) followed by acidification with HBF(4).Et(2)O. The arene transfer reaction of 3 with [(eta-p-xylene)Mo(CO)(3)] furnished the cycloheptatrienyl complex [(HOC(6)H(2)Me(2)-eta(7)-C(7)H(6))Mo(CO)(3)]BF(4) (6), which could be converted into the chiral chelate complexes [(OC6H2Me2-eta7-C7H6)Mo(CO)(PR3)](9a, R = Ph; 9b, R = c-C(6)H(11); 9c, i-Pr) by subsequent treatment with NaI, PR(3), and NaH. The linked cycloheptatienyl-phenolate ligand in 9a could be protonated at the coordinated oxygen atom employing HBF(4).Et(2)O to yield [(HOC6H2Me2-eta7-C7H6)Mo(CO)(PPh3)]-BF4 (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 [(HOC(6)H(2)Me(2)-eta(7)-C(7)H(6))Mo(CO)(PPh(3))L]BF(4) (11, L = XyNC; 12, L = CO). On thermal reaction of [(HOC(6)H(2)Me(2)-eta(7)-C(7)H(6))Mo(CO)(2)I] (7) with dppe, the addition of the diphosphine was observed together with the simultaneous formation of the molydenum-oxygen bond to yield [(OC6H2Me2-eta3-C7H6)Mo(CO)2(dppe)] (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 (31)P and (1)H NMR spectroscopy. A eta(3) --> eta(7) hapticity reversion could be achieved by UV irradiation of a solution of 13 in THF to give the electron-rich complex [(OC6H2Me2-eta7-C7H6)Mo(dppe)] (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, 10.2CH(2)Cl(2), 12, and 13 are reported.