SYNTHESIS, STRUCTURE AND REACTIONS OF A dihapto-FORMALDEHYDE COMPLEX, Os(η2-CH2O)(CO)2(PPh3)2

Article abstract of DOI:10.1016/S0022-328X(00)81212-7

Os(η²-CH2O)(CO)2(PPh3)2 results from direct reaction of formaldehyde with the zerovalent complex, Os(CO)2(PPh3)3.The structure of Os(η²-CH2O)(CO)2(PPh3)2 was determined by X-ray crystallography.The crystals are triclinic, space group P<*>, a 18.739(2), b 11.157(1), c 9.986(1) Angstroem, α 116.70(1), β 93.20(1), γ 107.93(1) deg, V 1727.69 Angstroem³, Z = 2, D_m 1.55(2), D_c 1.57 g cm^-3.Refinement of atomic parameters was by full-matrix least-squares methods, employing anisotropic thermal parameters for all non-hydrogen atoms except for the carbon atomsof the phenyl rings.The formaldehyde hydrogen atoms were located from difference electron density maps, other hydrogens were included in calculated positions.Final residuals were R = 0.047 and R_w = 0.061 for 3508 unique observed reflections measured on an automatic diffractometer.The complex itself is monomeric, although interstices in the crystal lattice are occupied by hydrogen-bonded water dimers which fulfil a purely space-filling role.The osmium is bonded to two mutually trans triphenylphosphines, two carbonyls, and the η²-formaldehyde, in an arrangement which is best described as distorted octahedral.The geometry of the coordinated formaldehyde is characterised by Os-O 2.039(7), Os-C 2.186(8) and C-O 1.584 (11) Angstroem.The Os-P bonds are equivalent at 2.372(2) and 2.378(2) Angstroem but the Os-CO bond trans to the formaldehyde carbon 1.931(7) Angstroem is longer than that trans to the formaldehyde oxygen 1.907(7) Angstroem.Os(η²-CH2O)(CO)2(PPh3)3 has proved to be a useful synthetic precursor for stable formyl, hydroxymethyl, methoxymethyl, and halomethyl (CH2X, X = Cl, Br, I) complexes.The compoundsOs(CHO)H(CO)2(PPh3)2, Os(CH2OH)H(CO)2(PPh3)2, Os(CH2OMe)Cl(CO)2(PPh3)2 and Os(CH2Cl)Cl(CO)2(PPh3)2 are illustrative of the many compounds which have been characterised.A general synthetic route to neutral formyl osmium complexes, Os(CHO)X(CO)2(PPh3)2 (X = halide or alkyl) has been developed from reaction of the cations ⁺ with BH4^-.Acetaldehyde also reacts with Os(CO)2(PPh3)3 forming Os(η¹-CCH3)H(CO)2(PPh3)2.No reaction was observed with benzaldehyde, and trichloroacetaldehyde affords the cation, ⁺.