Molybdenum Complexes with Sterically Demanding Cycloheptatrienyl Ligands

Synthesis and reactivity of functionalized cycloheptatrienyl-cyclopentadienyl sandwich complexes

This feature article provides an overview of the synthesis and reactivity of functionalized cycloheptatrienyl-cyclopentadienyl transition metal sandwich complexes of the type [(eta(7)-C(7)H(7))M(eta(5)-C(5)H(5))] (M = group 4, 5 or 6 metal), which can be used as building blocks for the preparation of metallopolymers and polymetallic complexes. Emphasis is placed on 16-electron group 4 complexes (M = Ti, Zr, Hf) and their reactivity towards sigma-donor/pi-acceptor ligands, which indicates that these complexes bear a close resemblance to Lewis acidic M(+IV) complexes. Based on theoretical calculations, this behavior can be mainly attributed to the strong and appreciably covalent metal-cycloheptatrienyl interaction with the cycloheptatrienyl ring acting more as a -3 ligand than as a +1 ligand in these mixed ring complexes.

Mono- and dinuclear molybdenum complexes with sterically demanding cycloheptatrienyl ligands

Sterically demanding cycloheptatrienylium (tropylium) salts of the type (1,3,5-C7H4R3)BF4 [R = t-Bu, (3a)BF4; R = SiMe3, (3b)BF4] have been prepared from the corresponding 1,3,5-trisubstituted benzene derivatives 1 by ring expansion with diazomethane followed by hydride abstraction with triphenylcarbenium tetrafluoroborate, (Ph3C)BF4. Complexation can be achieved by arene exchange and Mo(CO)3 group transfer employing [(eta6-p-xylene)Mo(CO)3] (4) to yield the cationic complexes (5)BF4. In refluxing mesitylene, [(eta7-C7H4t-Bu3)Mo(CO)3]BF4, (5a)BF4, undergoes CO substitution to furnish the mesitylene sandwich complex (6a)BF4. A cyclic voltammetric study reveals that this complex exhibits a reversible one-electron oxidation to the dicationic 17e complex 6a2+, which can also be accessed by chemical oxidation with AgBF4. On the contrary, the reduction of 6a+ is irreversible and does not yield a stable 19e complex 6a. To study the fate of the reduced 19e form, (5a)BF4 was treated with Na2Hg to diastereoselectively afford the C-C coupled bicycloheptatriene complex 7a. Paramagnetic, dinuclear complexes of the type [(eta7-C7H4R3)Mo(mu-Cl)3Mo(eta7-C7H4R3)] (8) have been obtained from the reaction of (5)BF4 with Me3SiCl. These can be regarded as mixed-valence Mo(0)/Mo(+I) compounds with a metal-metal bond order of 0.5. Cyclic voltammetric studies reveal that both complexes 8a and 8b undergo reversible one-electron oxidation as well as reduction. Treatment with one equivalent of ferrocenium hexafluorophosphate leads to removal of the unpaired electron and formation of the diamagnetic complexes (8)PF6. Theoretical DFT calculations have been carried out to further elucidate the bonding in these systems. In addition, the X-ray crystal structures of (5b)BF4, (6a)BF4 x CH2Cl2, (6a)(BF4)2 x (acetone)2, 7a x CH2Cl2, 8a x 0.5C6H14, and (8a)PF6 x Et2O are reported.

Molybdenum complexes with linked cycloheptatrienyl-phenolate ligands

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.