Late transition metal oxo complexes. The use of polyoxometalate as a stabilizing ligand

The use of polyoxometalate (POM) as stabilizing ligands for the synthesis and isolation of late-transition metal oxo (LTMO) complexes is addressed. Metal-oxo species, in particular terminal metal-oxo (O2-) complexes of the late-transition-metal elements have long been thought to exist as transient intermediates in systems ranging from Cu oxidase enzymes to the surfaces of noble metal oxidation catalysts. However, despite decades of speculation and attempted synthesis, no terminal metal-oxo complexes of any element to the right of Ru in the periodic table had been reported prior to 2004 with the exception of the d4 (mesityl)3IrV-oxo complex of G. Wilkinson and co-workers. A current bonding paradigm argues that terminal metal oxo groups are stabilized at metal centers with no more than four d electrons.;This dissertation reports several isolated and fully characterized molecular terminal oxo complexes of the group 10 and 11 elements with the use of polytungstate ligand environments. Polytungstates, which share many structural and reactivity features in common with the metal oxides of broad importance in catalytic technologies (TiO2, CeO2, others), are both good sigma-donating and pi-accepting ligands that may facilitate stabilization and isolation of terminal late transition metal-oxo units. A total of four structural types of LTMO complexes are presented in this study: (1) M(O)(OH2){A-PW 9}2 with one bridging octahedral metal unit between two [A-alpha-PW 9O34]9- ligands (M = Pt and Au); (2) M(O)(OH)W(O)(OH 2){A-PW9}2 with two linkages between {A-PW 9} units, a metal and a tungsten atom (M = Pd); (3) M(O)(OH2)(O=WOH 2)2{A-PW9}2 with the terminal M = O group incorporated in a clam shell-like monovacant polytungstate ligand formed by the fusion of two {A-PW9} units by two tungsten atoms (M = Pd and Au); and (4) (O=MOH2)2W(O)(OH2){A-PW 9}2 (M = Pd), which represents the unique example having two terminal M=O groups coordinated in one molecule. All these molecular LTMO complexes have been carefully studied by geometric and electronic structure methods, including single crystal X-ray diffraction, neutron diffraction, extended X-ray absorption fine structure methods, 31P and 17O NMR spectroscopy, and other chemical and physicochemical methods.;Importantly, the counterion effect in the controlled speciation of the late transition metal substituted polytungstates is discussed. By strongly interacting with the polyanion unit, Cs+ countercations can prevent the hydrolytic decomposition of the tri-metal sandwich structure in solution. As a result, the formation of conventional d8 Pd(II)-substituted polytungstates and unprecedented terminal Pd=O complexes can be controlled.;Reactivity studies are conducted on M(O)(OH2)(O=WOH2 )2{A-PW9}2 (M = Pd and Au), a structure that is quite stable in both aqueous and organic solution. The stoichiometric oxo transfer from terminal M=O to other substrates and subsequent reoxidization of the deoxygenated form by air are confirmed by spectroscopy methods. Furthermore, the deoxygenated product, [PdII(O=WOH2)2(A-alpha-PW 9O34)2]8-, can be isolated as crystalline plates, and X-ray diffraction confirms the existence of a four-coordinate square-planar Pd(II) center. These results and subsequent catalytic oxidation studies strongly suggest the involvement of terminal M=O species in noble metal-based homogenous and heterogenous catalysts in O2-based green organic oxidations.