X-ray structural studies of I. low valence gold complexes, II. transition metal complexes containing terminal organoimido ligands

Abstract

Essentially this thesis presents the X-ray crystallographic results of eleven crystal structures, comprised of seven low valence gold, one amido (NHR) transition metal and three imido (NR) transition metal complexes. The first section of the thesis deals with the gold structures. These range from simple linear two-fold coordinate gold(I) to a novel high nuclear gold complex where pseudo octahedral coordination can be defined for some of the gold atoms. In contrast with similar gold(I)-thiolate systems the crystal structure of [PPh4][Au(SPh)2] is a further example of the marked tendency for gold(I) complexes to exhibit linear two-fold coordination. However, this structure is only the second to be reported of a gold(I) atom coordinated to two thiolate ligands. Two non-equivalent, symmetric, anions for the complex [Au(I)2][…OPPh2(CH2)Ph2PO…H…] show notably dissimilar Au-I bond lengths. The lower corresponds to the sum of the gold and iodine covalent radii. No obvious reason for the existence of the longer Au-I bond length could be found. The polymeric cation shows very short O…O separations and although the linking protons could not be found they are expected to be symmetrically located between the oxygen atoms. Essentially the complex, Au2(dppe)Cl2 (where dppe = 1,2-bis(diphenylphosphino)ethane), shows a linear Au(I) coordination geometry. However, each gold atom is further associated with a short Au…Au interaction so that they can be defined as having pseudo "T-shaped" coordination geometries. The Au…Au interactions are compared with those existing in analogous structures. To add to the very limited array of reported higher coordinate gold(I) structures (three- and four-coordinate) three, four-coordinate, gold(I)-phosphine structures have been determined. Two involve the chelating dppe ligand, and contain the cation, [Au(dppe)2]+, the third involves the slightly longer chelate ligand, dppp (dppp = 1,3-bis(diphenylphosphino)propane), and contains the cation, [Au(dppp)2]+. Due to the restricted bite angle of the chelate ligands, all three complexes exhibit distorted tetrahedral coordination geometries. Comparisons are made between the Au-P bond lengths of the three structures, and these are compared with Au-P bond lengths in other structures containing at least one Au(I)-P linkage. The final gold structure to be reported on is that of the novel, high nuclear gold complex, Au18(μ3-S)8(dppe)6, and although the coordination of each gold atom may be said to be linear, binding either to two sulphur or one sulphur and one phosphorus donor ligand, the many, short, Au…Au interactions preclude a definitive assignment. As the neutrality or otherwise of this cluster is in some doubt, further complications arise. Thus, it is not possible to define the Au…Au interactions as being either passive or active, the latter involving formal Au-Au bonding, (i.e. gold oxidation states <1). The second section of the thesis deals with four structures exhibiting transition metal to nitrogen multiple bonds. Three of these involve terminal organoimido ligands (NR) which are triply bound to either tungsten ({WC1(NPh)(OEt)2(μ-OEt)}2 and {W2(NPh)2(μ-O)(pinacol)2(pinacol-H)2}pinacol-H2 = 2,3-dimethyl-2, 3-butandiol) or tantalum ({TaCl2(NtBu)(μ-OEt)(NH2tBu)}2) atoms. For two of these complexes the imido function exhibits a strictly linear geometry, M-N-C > 170°, but those for the complex, {W(NPh)(μ-O)(pinacol)(pinacol-H)2}, show small deviations from this ideal geometry. The exact reason(s) for these deviations cannot be ascertained, but the more likely are: a competitive influence exerted by the terminal alkoxide ligands for the π-bonding orbitals of tungsten, and the distorted coordination geometry about each metal centre. The amido complex {W2(NHPh)2(μ-pinacol)(pinacol)4}, was obtained together with one of the above imido complexes, {W2(NPh)2(μ-O)(pinacol)2(pinacol-H)2}, form the reaction of W(NPh)cl4, with the diol, pinacol-H2. As such, even though the title for this section is, "Transition Metal Complexes Containing Terminal Organoimido Ligands", it was thought justifiable to include the structural results to the amido as well as the imido complex. The W⇋N π-bonding of the amido ligand is comparable with that of the terminal W⇋O alkoxide π-bonding within the amido and imido complexes. Values of the cell parameters for the complexes studied are tabulated in the appropriate chapters and are not presented here.