Computational and characterization studies of homogeneous and heterogeneous tris(beta-diketonato) complexes with catalytic applications

[摘要] English: Nine [Mn(β-diketonato)3] [β-diketonato = dipivaloymethanato (dpm), 1; acetylacetonato (acac),2; benzoylacetonato (ba), 3; dibenzoylmethanato (dbm), 4; trifluoroacetylacetonato (tfaa), 5;thenoyltrifluoroacetonato (tfth), 6; trifluorofuroylacetonato (tffu), 7; trifluorobenzoylacetonato(tfba), 8 and hexaflouroacetylacetonato, 9] complexes were synthesized by adapted methods fromliterature. An attempt was made to graft [Mn(β-diketonato)3] complexes 2-9 onto two dimensional(2-D) Si-wafer supports (S5-S12) via a silane linker. The X-ray photoelectron spectroscopy (XPS)Mn:F atomic ratio results of fluorine containing [Mn(β-diketonato)3] complexes grafted ontoamino-functionalized Si-wafer (S9-S12) gave an indicaton that the [Mn(β-diketonato)3] complexesdecomposed during the grafting process. Studying Mn2O3 on the hydroxylated Si-wafer surfaceby the use of XPS, it was confirmed that the Mn 2p photoelectron lines observed for S5-S12 consistmostly of Mn2O3 that has no interaction with the silane linker. Eight [Mn(β-diketonato)3]complexes 1-8 was wet impregnated onto various three dimensional (3-D) solid supports (S12-S28) to form [Mn(β-diketonato)3] model catalysts. Additionally a selection of [M(acac)3]complexes ([Co(acac)3], 12; [Rh(acac)3], 13, and [Ir(acac)3], 14) were grafted onto twodimensional (2-D) Si-wafer supports (S29-S31). While [M(acac)3] complexes ([Cr(acac)3], 10;[Fe(acac)3], 11; [Co(acac)3], 12; [Rh(acac)3], 13, and [Ir(acac)3], 14) were wet impregnated ontovarious three dimensional (3-D) solid supports (forming S32-S36). The model 2-D and 3-Dcatalysts were studied and characterized by the means of XPS, TGA and computational chemistrycalculations. Selected model 2-D (S5, S7, S8, S12 and S29-S31) and Mn(acac)3 immobilized ontoSiO2 (3-D catalysts, S14 as is and heat treated at 100, 130, 250 and 350 °C) were tested for thecatalytic self-solvating reaction between ethanol and hexamethylenediisocyanate (HDI) to formhexamethylenediurethane (HDU) to mimic the industrial production of polyurethane. The model2-D [Mn(β-diketonato)3] (S5, S7, S8 and S12) catalysts showed that as the total groupelectronegativity, 3(R + R'), increase a general decrease in turnover frequency (TOF) wasobserved. The model 2-D [M(acac)3] catalysts (S29-S31) showed that with an increase in themetal centre's Pauling electronegativity a general increase in TOF was observed. The catalytictest on the Mn(acac)3 immobilized onto SiO2 showed that the sample heat treated at 100°C beforehand has the highest TOF, which is most probably due to the loss of only one �?diketonato ligandmaking it more active. The chromium(0) Fischer carbene complexes (Cr-FCCs) ([Cr(CO)4(PPh3)=C(OEt)(Fu)], C1;[Cr(CO)5=C(NHCy)(Fu)], C2; [Cr(CO)5=C(NHCy)(Th)], C3 and [Cr(CO)5=C(OEt)(ThTh)], C4)study was in collaboration with the research group of Dr. M Landman at the University of Pretoria.The four Cr(0) FCCs, were obtained and characterized by electrochemistry, XPS andcomputational chemistry calculations in this study. Penta-carbonyl Cr-FCCs[Cr(CO)5=C(OEt)(Fu)], C5, and [Cr(CO)5=C(OEt)(Th)], C6, were also supplied and anchoredonto an amino-functionalized Si-wafer to create an immobilized Cr-FCCs C7 and C8. XPS resultsshowed that the Cr(CO)5=C(OEt)(Fu)], C5, and [Cr(CO)5=C(OEt)(Th)], C6, was successfullyanchored onto an amino-functionalized Si-wafer (C7 and C8).The electrochemical study of C1-4 showed that the oxidation potential (Epa) is influenced by thevaried substituents on different sites of the Cr-FCCs. The oxidation order of the Cr-FCCs is:[Cr(CO)5=C(OEt)(ThTh)], C4 > [Cr(CO)5=C(NHCy)(Th)], C3 > [Cr(CO)5=C(NHCy)(Fu)], C2 >[Cr(CO)4(PPh3)=C(OEt)(Fu)], C1. The reduction followed the same trend except for the[Cr(CO)4(PPh3)=C(OEt)(Fu)], C1, complex that is reduced at a higher potential than theaminocarbene complexes, [Cr(CO)5=C(OEt)(ThTh)], C4 > [Cr(CO)4(PPh3)=C(OEt)(Fu)], C1 >[Cr(CO)5=C(NHCy)(Th)], C3 > [Cr(CO)5=C(NHCy)(Fu)], C2. The electronic energies of thedifferent conformations obtained by the computational study showed that certain conformationsare preferred over others. The computational results were in good agreement with experimentalcharacterization method results. Computational study of HOMO and LUMO orbitals of the Cr-FCCs confirmed that the oxidation process is metal based and the reduction is based across thecarbene ligand. The correlation of the oxidation potential (Epa) of the Cr0/Cr+1 redox couple andreduction potential (Epc) of the ligand based reduction with the HOMO energy (EHOMO) and LUMOenergy (ELUMO) respectively, showed that with decreasing molecular orbital energy (HOMO andLUMO) an increase to more positive Epa and Epc potentials are obtained.

[发布机构] University of the Free State