Reaction of 6-Methyl-2,2′-bipyridine with 1,2-Os3(CO)10(MeCN)2: Syntheses, Reductive Elimination/Ligand Displacement Kinetics, and X-ray Diffraction Structures of the Isomeric Clusters HOs3(CO)9(μ2-N2C11H9) and H2Os3(CO)8(μ3-N2C11H8)

Abstract

Treatment of Os3(CO)10(MeCN)2 (1) with the heterocyclic ligand 6-methyl-2,2′-bipyridine (6-Me-2,2′-bpy) at room temperature leads to the formation of the isomeric hydride-bridged clusters HOs3(CO)9(μ2-CH2N2C10H7) (2) and HOs3(CO)9(μ2-N2C11H9) (3). The cyclometalation of the ancillary 6-Me group in 2 and the ortho metalation of the nonsubstituted pyridyl ring in 3 have been confirmed by spectroscopic and crystallographic methods. Thermolysis of 2 leads to the formation of 3 and the dihydride cluster H2Os3(CO)8(μ3-N2C11H8) (4); the latter cluster, whose structure has been crystallographically determined, derives from a formal loss of CO and C−H bond activation of the methylene moiety in 2. Heating 2 in the presence of ligand-trapping agents proceeds with the release of the 6-Me-2,2′-bpy ligand and formation of Os3(CO)9L3 [where L = CO, P(OMe)3]. The kinetics for the reaction between 2 and added ligand have been investigated by UV−vis and NMR spectroscopies and found to be first-order in starting cluster and independent of the incoming ligand. Parallel kinetic experiments employing the deuterated cluster DOs3(CO)9(μ2-CD2N2C10H7) (2-d3), which was prepared from cluster 1 and 6-Me-d3-2,2′-bpy, confirm the existence of a primary kinetic isotope effect (KIE) of 1.78 at 323 K. The KIE data and the calculated activation parameters [ΔH⧧ = 21.7(4) kcal/mol; ΔS⧧ = −13(1) eu] are strongly suggestive of a reaction scheme involving a rate-limiting reductive coupling of the bridging hydride ligand and cyclometalated alkyl moiety in 2 to furnish a putative sigma complex containing an intact methyl group bound to the Os3 cluster, prior to the generation of the unsaturated cluster Os3(CO)9(μ-N2C11H10). Thermolysis of 3 in the presence of added P(OMe)3 does not furnish free 6-Me-2,2′-bpy but proceeds by a ligand-induced displacement of the methyl-substituted pyridyl ring and formation of the cluster compound HOs3(CO)9[P(OMe)3](μ2-N2C11H9) (5). The kinetics for the reaction between 3 and P(OMe)3 have been studied over the temperature range 333−356 K, and on the basis of the observed activation parameters [ΔH⧧ = 13.0(3) kcal/mol; ΔS⧧ = −30(1) eu] and the first-order dependence on the cluster and ligand, an associative process that involves P(OMe)3 ligand attack on the cluster and release of the methyl-substituted pyridyl ring in the rate-limiting step is proposed.