Development of bifunctional catalysts containing hydride-relay ligands for CO2 hydrogenation
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The research described in this thesis focuses on the synthesis of bifunctional catalysts for hydrogenation of CO2. These catalysts contain redox active phenanthridinium and benzimidazolium groups capable of mediating hydride transfer incorporated within a transition metal phosphine complex. These redox active groups, called “hydride relays,” are NADH-type organic hydride donors that can reversibly accept or donate hydride ions and transfer them to substrates, like CO2. A series of bifunctional ligands, LPhenH, LPhen+, LBI+ and LPhenBI+, were synthesized in which the targeted hydride relays are tethered to a phosphine donor via an ethylene linker so that they can be combined with transition metal ions that have complementary proton acceptor abilities. Ni(II) complexes proved to be too sterically hindered to bind these ligands when used with polydentate phosphine ligands. However, [Ni(LPhenBI+)Br3], a Ni(II) species containing bromide ligands, was synthesized. A crystal structure was obtained for [Ni(LPhenBI+)Br3]: monoclinic, P21/c, a = 18.3574(13), b = 15.8651(8), c = 20.1961(12) Å, β = 106.102 (7)°, R = 0.0709, and RW = 0.1150. Pd(II) complexes using redox active ligands in conjunction with ancillary polydentate phosphine ligands like PP2 [PhP(CH2CH2PPh2)2], and PNHP [HN(CH2CH2PPh2)2] were synthesized. Crystal structures were obtained for the complexes containing phenanthridinium relays, [Pd(PP2)(LPhenH)][BF4]2•(CH3CN)3.25 and [Pd(PP2)(LPhen+)][BF4]3•(CH3CN)3. For [Pd(PP2)(LPhenH)][BF4]2•(CH3CN)3.25: monoclinic, P21/n, a = 11.2927(4), b = 41.743(1), c = 12.9751(4) Å, β = 90.251(3)°, R = 0.0772, and RW = 0.1483 and for [Pd(PP2)(LPhen+)][BF4]3•(CH3CN)3: monoclinic, P21/c, a = 21.822(2), b = 13.632(1), c = 22.575(2) Å, β = 91.116(8)°, R = 0.0632, and RW = 0.1349. Pd0 complexes were also synthesized. A crystal structure was obtained for the complex [Pd0(PP2)(LPhenH)]: triclinic, P-1, a = 11.5406(6), b = 12.6451(6), c = 18.3167(7) Å, α = 79.558(4)°, β = 80.469(4)°, γ = 78.061(4)°, R = 0.0680, and RW = 0.1176. These complexes were also characterized using 1H and 31P NMR spectroscopy. These studies show that the hydride relays can be successfully incorporated in close proximity to the transition metal of tetraphosphine complexes.