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 Os₃(CO)₁₀(dppbz), 1,1-Os₃(CO)₁₀(dppbzF₄), HOs₃(CO)₉[μ-1,2-PhP(C₆H₄-η¹)C₆H₄PPh₂], and HOs₃(CO)₉[μ-1,2-PhP(C₆H

Abstract

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 6 H 4-η 1)C 6 H 4 PPh 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 31 P NMR spectroscopy, and the kinetics for the conversion 2 f 3 have been followed by UV-vis spectroscopy in toluene over the temperature range 318-343 K. The calculated activation parameters (ΔH q = 21.6(3) kcal/mol; ΔS q =-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 dppbzF 4 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 6 H 4-η 1)C 6 F 4 PPh 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.

New osmium cluster compounds containing the heterocyclic ligand 2,3-bis-(diphenylphosphino)quinoxaline (dppq): Ligand isomerization and crystal structures of dppq, the isomeric clusters Os3(CO)10(dppq), and HOs3(CO)9[μ-2,3-PhP(η1-C6H4)(Ph2P)quinoxaline]

“…The DFT calculations on the isomeric Os 3 (CO) 10 (dppq) clusters confirm the greater stability of dppq-chelated species vis-à-vis the bridging isomer 2b. Here 2c is computed to have a 4.8 kcal/mol lower free energy than 2b, which translates to a K eq > 3300 in favor of 2c [34]; this finding is in keeping with our recent calculations on a series of diphosphinesubstituted Os 3 (CO) 10 (PeP) clusters [7].…”

“…Mechanistically speaking, a non-dissociative, unimolecular process involving the migration of phosphorus and CO groups about the cluster polyhedron is in keeping with the kinetic data and the Eyring activation parameters. DFT calculations on the rearrangement of bridged-to-chelated isomers have verified a merry-go-round exchange of one phosphine and two CO groups [7]. Here the concerted permutation of the three migrating ligands parallels the exchange scheme originally postulated by Cotton over forty years ago for CO migration about the CoeCo vectors in Co 4 (CO) 12 [8].…”

“…This perturbation serves as the driving force for the isomerization of 2b to 2c (vide supra), paralleling the isomerization behavior of the diphosphine ligands depicted in Scheme 1. Moreover, the theoretical underpinning for the isomerization of the PeP ligand in Os(CO) 10 (PeP) clusters has been thoroughly examined by us as a function of the diphosphine ligand [7]. The B3LYP optimized structure of 2b is depicted in Fig.…”

“…Numerous reports now exist on the non-dissociative, bridge-tochelate isomerization of the ancillary diphosphine in Os(CO) 10 (PeP) clusters [2e5, 7,32]. Naturally, this led us to examine the thermal stability of 2b in order to establish the influence of the dppq ligand on Fig.…”

“…When heated under CO, 2b isomerizes cleanly to 2c and this confirms that the latter cluster is the thermodynamically more stable form of Os(CO) 10 (dppq). The addition of CO suppresses the formation of the hydride cluster 3, but CO addition has been shown not to affect the equilibrium for the bridged-to-chelated ligand isomerization in other decacarbonyl clusters Os 3 (CO) 10 (PeP) examined by our group [2e5, 7,32]. Scheme 2 illustrates this isomerization and the fate of the unsaturated intermediate, Os 3 (CO) 9 (dppq) (not shown), that forms from decarbonylation of 2c and which functions as the precursor to the hydride cluster 3.…”