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)(N C5H4X) (L = PMe3; X = O (1), NH (2), S (3)) were prepared by treatment of f ac-L3Os(H)(eta(2)-CH2PMe2) with 2-(HX)C5H4N. An X-ray structure determinati on establishes 1 to be the kappa N tautomer with the keto group pointed awa y from the hydride and toward the PMe3 ligand trans to hydride. NMR spectra support the kappa N structure in solution. Which nitrogen is coordinated i n 2 is not established, but NMR supports the KS tautomer of 3 in solution. At 80 degrees C in C6D6, 1 incorporates L' (P(CD3)(3)) in a first-order rea ction (t(1/2) approximate to 14 min) stereospecifically at site b (trans to hydride) with Delta H-double dagger = 26.9 +/- 1.4 kcal/mol and Delta S-do uble dagger = 3.8 +/- 4.4 eu for the substitution. Complex 2 at 80 degrees C also exchanges with L' only in site b, but more slowly than 1 (t(1/2) app roximate to 3.5 h). Sublimation of 1 at 55 degrees C under vacuum yields me r-L3Os(H)(NC5H4O-kappa(2)N, O) (mer-4), believed to have mer-4- (HON) geome try (H trans to nitrogen). In toluene at reflux 1 loses L, forming fac-4. H eating mer-4 in benzene 3 h at 110 degrees C forms fac-4. At 22 degrees C, L' adds to fac-4 with t(1/2) approximate to 3 h, generating pure (L-a')(L-a )(L-b)(L-c)Os(H)(NC5H4O-kappa N), which then undergoes the much slower exch ange with L' typical of 1 specifically forming (L-a')(L-a)(L-b')(L-c)Os(H)( NC5H4O-kappa N). Reaction of mer-4 with excess L' in ca. 2 h generates tran s-L3L'Os(H)(NC5H4O) then trans-L'Os-4(H)(NC5H4O) over 10 h, and finally cis -L'Os-4(H)(NC5H4O-kappa N) over 10 days. A scheme is presented that correla tes all of the above transformations. Several reactions of fac-4 were exami ned. fac-4 and CO2 (10 atm, benzene-d(6), 80 degrees C, 1 h) afforded fac-L 3Os(kappa(1)O(2)CH)-(kappa(2)-NC5H4O), 5. Heating pure 5 in C6D6 (110 degre es C, 2 h) regenerated 4. Reaction of fac-4 with D-2 gas (0.84 atm, C6D6, 8 0 degrees C) caused formation of Os-D with no other changes. fac-4 in neat 1-hexene (80 degrees C, 17 h) gave fac-L3Os(n-hexyl)(K-2-NC5H4O) (6) quanti tatively, while fac-4 under 1 atm of ethylene (C6D6, 80 degrees C, 36 h) ga ve fac-L3Os(ethyl)(kappa(2)-NC5H4O) (7) quantitatively. Heating 6 in C6D6 a t 80 degrees C for 48 h gave fac-4 back again with a mixture of hexene isom ers with a 2:1 terminal:internal alkene ratio. Heating 7 (C6D6, 80 degrees C) also regenerated 4 and ethylene. Heating at 80 degrees C for 7 days of a sample containing 1-hexene in C6D6 solvent and 5 mol % of fac-4 under 1 at m of hydrogen gas quantitatively gave hexane, while 4 remained unchanged. T his is a substantial increase in reactivity over L4Os(H)(2) through the use of the strained chelate.