Treatment of IrCp*(CO)2 (1) with C6F5I in benzene under reflux gives the oxidative addition product IrCp*(C6F5)(CO)I (2). Treatment of 2 with PMe3 gives IrCp*(C6F5)(PMe3)I (3), which, on treatment with AgO3SCF3 in the presence of traces of water, affords the cationic complex [IrCp*(C6F5)(PMe3)(OH2)]O3SCF3 (4). Treatment of 4 with 1,8-bis(dimethylamino)naphthalene (Proton Sponge) affords the hydride complex IrCp*(C6F5)(PMe3)H (5), which reacts with n-BuLi to give the tetrafluorobenzyne complex IrCp*(η2-C6F4)(PMe3) (6) in high yield. Similarly, treatment of RhCp*(CO)2 (7) with C6F5I in toluene at 80 °C affords a pentafluorophenyl complex of rhodium, RhCp*(C6F5)(CO)I (8). Treatment of 8 with PMe3 at room temperature affords RhCp*(C6F5)(PMe3)I (9), which reacts with NaBH4 to give RhCp*(C6F5)(PMe3)H (10). Treatment of 10 with n-BuLi gives the rhodium tetrafluorobenzyne complex RhCp*(η2-C6F4)(PMe3) (11) in high yield. Treatment of IrCp*(CO)2 with 1-fluoro-2-iodobenzene in toluene under reflux gives the oxidative addition product IrCp*(2-C6FH4)(CO)I (12), which is converted to IrCp*(2-C6FH4)(PMe3)I (13) by treatment with PMe3. Treatment of 13 with NaBH4 gives IrCp*(2-C6FH4)(PMe3)H (14), which, on treatment with n-BuLi, is converted to a mixture of the nonfluorinated benzyne complex IrCp*(η2-C6H4)(PMe3) (15) and the phenyl butyl complex IrCp*(C6H5)(n-Bu)(PMe3) (16). Treatment of Ir(C5Me4Et)(CO)2 with 2,3,4,5-tetrafluorobenzoyl chloride at 110 °C under reflux gives the oxidative addition product Ir(C5Me4Et)(2,3,4,5-C6F4H)(CO)Cl (17), which on treatment with PMe3 gives Ir(C5Me4Et)(C6F4H)(PMe3)Cl (18). Using analogous methodology this was converted to the unsymmetrical trifluorobenzyne complex Ir(C5Me4Et)(η2-3,4,5-C6F3H)(PMe3) (21), in which the benzyne ligand is shown to be stereochemically rigid on the NMR time scale, allowing a minimum value for the rotational barrier about the iridium−benzyne bond to be estimated at 20.3 kcal mol-1. The crystal structures of complexes 2, 3, 9, 11−15, 17, and 18 are reported and discussed, and the “through-space” nature of some coupling constants between 31P, 1H, and 19F is confirmed using heteronuclear Overhauser enhancement (HOESY) NMR spectroscopy.

Experimental and computational evidence points to unimolecular transformation of terminal alkynes on the title Rh(I) metal fragments. Lack of isotopic scrambling in double-crossover experiments is inconsistent with a previously proposed bimolecular pathway. Focusing on a unimolecular manifold, alkyne binding to the metal forms the Rh(I) alkyne π-complex 2, which isomerizes to the Rh(III) hydrido-(alkynyl) species 4, ultimately leading to Rh(I) vinylidene product 5. In making alkyne-free precursors, use of heterocyclic ligand (i-Pr) 2 PIm′ (1b, Im′ ) 1-methyl-4-tert-butylimidazol-2-yl) led to species 8 with a labile P,N chelate, whereas a geometrically similar o-tolyl ligand suffered metalation at the methyl group and was unsuitable for alkyne transformation studies. Kinetic studies comparing 1b and (i-Pr) 2 PPh (1c) allowed determination of rate constants for the alkyne binding event and conversion of 2 to 5 (the latter, k 2-5 , being 9.6 times faster for 1b). Based on a scan of the two-dimensional reaction surface, combined density functional/molecular mechanics calculations predict that η 2 -(C,H) alkyne complex 3 is in a fast equilibrium with the lower energy hydrido(alkynyl) complex 4, and neither species is expected to be present at observable concentrations. Eyring model estimates of the rate constants from these computational data predict the available experimental values in this work to within a factor of 2 and the ratio of the rate constants k 2-5 for 1b and 1c to within 10%. The calculations also agree with the qualitative observation that reaction rates are faster for both ligands 1b and 1c than for (i-Pr) 3 P and predict that reactions using triphenylphosphine will be faster than those with (i-Pr) 3 P. NMR coupling constants, particularly 1 J CC values, were used to evaluate bonding and back-bonding in isotopomers of 2a-c and 5a-c derived from H 13 C 13 CH.

The synthesis, characterization, and reactivity of the new water-soluble ansa-molybdocene catalyst [{C2Me4(C5H4)2}Mo(OH)(OH2)][OTs] (3) and the related hydroxo-bridged dimer [{C2Me4(C5H4)2}Mo(μ-OH)]2[OTs]2 (5) are described. The effect of the ethylene bridge on the metallocene structure was evaluated by comparing the crystal structures of {C2Me4(C5H4)2}MoH2 (2) and 5 to those of the non-ansa analogues. The ethylene bridge changed the bite angles of the metallocene fragment by only a few degrees in both ansa structures. To probe the electronic consequences of the tetramethylethylene bridge, the {C2Me4(C5H4)2}Mo(CO)H (4) complex was prepared. On the basis of the ν(C⋮O) stretching frequencies, the ansa ligand C2Me4(C5H4)2 was found to be electron-withdrawing relative to two η5-C5H5 ligands. The reactivity of 3 in nitrile hydration, phosphate ester hydrolysis, and carboxylic acid ester hydrolysis was explored, and the rate constants for these transformations were compared to rate constants obtained using the Cp2Mo(OH)(OH2)+ and Cp‘2Mo(OH)(OH2)+ catalysts. In all cases, the Cp2Mo(OH)(OH2)+ catalyst, having intermediate electron density, had the largest rate constants. The reactivity trends for the three catalysts are explained by the relative electrophilicities of the Mo centers. If electron-donating cyclopentadienyl ligands are employed, the reactivity of the bound substrate is decreased relative to Cp and the rate is decreased. Conversely, if electron-withdrawing Cp cyclopentadienyl ligands are employed, the reactivity of the bound hydroxo nucleophile is decreased and the rate is decreased. In the case of the Cp2Mo(OH)(OH2)+ complex, these two opposing trends converge, and optimal activity is observed.