Generation of Coordinative Unsaturation at Osmium via Ring-Opening Equilibration of a 2-Pyridonato Chelate Complex

Ring opening of a strained chelate complex of osmium has been investigated as a means to transient coordinative unsaturation under mild conditions in normally kinetically inert, third-row, platinum-group metals. cis-L4Os(H)(NC5H4X) (L = PMe3; X = O (1), NH (2), S (3)) were prepared by treatment of fac-L3Os(H)(η 2-CH2PMe2) with 2-(HX)C5H4N. An X-ray structure determination establishes 1 to be the κ N tautomer with the keto group pointed away from the hydride and toward the PMe3 ligand trans to hydride. NMR spectra support the κ N structure in solution. Which nitrogen is coordinated in 2 is not established, but NMR supports the κ S tautomer of 3 in solution. At 80 °C in C6D6, 1 incorporates L‘ (P(CD3)3) in a first-order reaction (t 1/2 ≈ 14 min) stereospecifically at site b (trans to hydride) with ΔH ⧧ = 26.9 ± 1.4 kcal/mol and ΔS ⧧ = 3.8 ± 4.4 eu for the substitution. Complex 2 at 80 °C also exchanges with L‘ only in site b, but more slowly than 1 (t 1/2 ≈ 3.5 h). Sublimation of 1 at 55 °C under vacuum yields mer-L3Os(H)(NC5H4O-κ 2 N,O) (mer-4), believed to have mer-4-(HON) geometry (H trans to nitrogen). In toluene at reflux 1 loses L, forming fac-4. Heating mer-4 in benzene 3 h at 110 °C forms fac-4. At 22 °C, L‘ adds to fac-4 with t 1/2 ≈ 3 h, generating pure (L a ‘)(L a )(L b )(L c )Os(H)(NC5H4O-κ N), which then undergoes the much slower exchange with L‘ typical of 1 specifically forming (L a ‘)(L a )(L b ‘)(L c )Os(H)(NC5H4O-κ N). Reaction of mer-4 with excess L‘ in ca. 2 h generates trans-L3 L‘Os(H)(NC5H4O) then trans-L‘4Os(H)(NC5H4O) over 10 h, and finally cis-L‘4Os(H)(NC5H4O-κ N) over 10 days. A scheme is presented that correlates all of the above transformations. Several reactions of fac-4 were examined. fac-4 and CO2 (10 atm, benzene-d 6, 80 °C, 1 h) afforded fac-L3Os(κ 1-O2CH)(κ 2-NC5H4O), 5. Heating pure 5 in C6D6 (110 °C, 2 h) regenerated 4. Reaction of fac-4 with D2 gas (0.84 atm, C6D6, 80 °C) caused formation of Os-D with no other changes. fac-4 in neat 1-hexene (80 °C, 17 h) gave fac-L3Os(n-hexyl)(κ 2-NC5H4O) (6) quantitatively, while fac-4 under 1 atm of ethylene (C6D6, 80 °C, 36 h) gave fac-L3Os(ethyl)(κ 2-NC5H4O) (7) quantitatively. Heating 6 in C6D6 at 80 °C for 48 h gave fac-4 back again with a mixture of hexene isomers with a 2:1 terminal:internal alkene ratio. Heating 7 (C6D6, 80 °C) also regenerated 4 and ethylene. Heating at 80 °C for 7 days of a sample containing 1-hexene in C6D6 solvent and 5 mol % of fac-4 under 1 atm of hydrogen