Half-Sandwich Guanidinate-Osmium(II) Complexes: Synthesis and Application in the Selective Dehydration of Aldoximes

The novel guanidinate-osmium(II) complexes [OsCl{kappa(2)-(N, N')-C(NR)(NiPr) NHiPr}(eta(6)-p-cymene)] [R = Ph (3a), 4-C6H4F (3b), 4-C6H4Cl (3c), 4-C6H4CF3 (3d), 3-C6H4CF3 (3e), 3,5C(6)H(3)(CF3)(2) (3f), 4-C6H4CN (3g), 4-C6H4Me (3h), 3-C6H4Me (3i), 2-C6H4Me (3j), 4-C(6)H(4)tBu (3k), 2,6-C(6)H(3)iPr(2) (3l), 2,4,6-C6H2Me3 (3m)] have been synthesized in high yields (70-88 %) by treatment of THF solutions of the dimeric precursor [{OsCl(mu-Cl)(eta(6)-p-cymene)}(2)] (1) with 4 equivalents of the corresponding guanidine (iPrHN) 2C= NR (2a-m) at room temperature. The easily separable guanidinium chloride salts [(iPrHN)(2)C(NHR)]Cl (4a-m) were also formed in these reactions. The structures of 3a, 3d, and 3h were unequivocally confirmed by X-ray diffraction methods. Complexes 3a-m proved to be active in the catalytic dehydration of aldoximes. The best results were obtained with [OsCl{kappa(2)-(N, N')-C(N-4-C6H4CF3)(NiPr) NHiPr}(eta(6)-p-cymene)] (3d; 5 mol-%), which, in acetonitrile at 80 degrees C, was able to convert selectively a large variety of aromatic, heteroaromatic, alpha,beta-unsaturated, and aliphatic aldoximes into the corresponding nitriles in high yields and short reaction times.