Experimental and computational studies of the isomerization reactions of bidentate phosphine ligands in triosmium clusters: Kinetics of the rearrangements from bridged to chelated isomers and X-ray structures of the clusters Os_3(CO)_(10)(dppbz), 1,1-Os_3(CO) _(10)(dppbzF_4), HOs_3(CO)_9[μ-1,2- PhP(C_6H_4

摘要

The diphosphine ligand 1,2-bis(diphenylphosphino)benzene (dppbz) reacts with the activated cluster 1,2-Os_3(CO)_(10)(MeCN)_2 (1) at room temperature to furnish a mixture of the triosmium clusters 1,2-Os_3(CO)_(10)(dppbz) (2) and 1,1-Os_3(CO) _(10)(dppbz) (3), along with a trace amount of the hydride cluster HOs_3(CO)_9[μ-1,2-PhP(C_6H_4- η~1)C_6H_4PPh_2] (4). The dppbz-bridged cluster 2 forms as the kinetically controlled product and irreversibly transforms to the corresponding chelated isomer 3 at ambient temperature. The disposition of the dppbz ligand in 2 and 3 has been established by X-ray crystallography and 31P NMR spectroscopy, and the kinetics for the conversion 2 → 3 have been followed by UV-vis spectroscopy in toluene over the temperature range 318-343 K. The calculated activation parameters (ΔH~1 = 21.6(3) kcal/mol; ΔS~1 = -11(1) eu) and lack of CO inhibition support an intramolecular isomerization mechanism that involves the simultaneous migration of phosphine and CO groups about the cluster polyhedron. The reaction between 1 and the fluorinated diphosphine ligand 1,2-bis(diphenylphosphino)tetrafluorobenzene (dppbzF _4) was examined under similar reaction conditions and was found to afford the chelated cluster 1,1-Os_3(CO)_(10)(dppbzF _4) (6) as the sole observable product. The absence of the expected bridged isomer 1,2-Os_3(CO)_(10)(dppbzF_4) (5) suggests that the dppbzF4 ligand destabilizes 5, thus accounting for the rapid isomerization of 5 to 6. Near-UV irradiation of clusters 3 and 6 leads to CO loss and ortho metalation of an ancillary aryl group. The resulting hydride clusters 4 and HOs_3(CO)_9[μ-1,2-PhP(C _6H_4-η~1)C_6F_4PPh _2] (7) have been isolated and fully characterized by spectroscopic and X-ray diffraction analyses. Both 4 and 7 react with added CO under mild conditions to regenerate 3 and 6, respectively, in quantitative yield. The rearrangements of bridged to chelated diphosphine complexes in this genre of decacarbonyl clusters have been investigated by DFT calculations. The computational results support a concerted process, involving the scrambling of equatorial CO and phosphine groups via a classical merry-go-round exchange scheme. The barriers computed for this mechanism agree well with those that have been measured, and steric compression within the bridged diphosphine groups of the reactants has been calculated to reduce the barrier heights for the rearrangement.