Mthembeni M. Zulu

Reinvestigation of the visible absorption bands of the 2,2′-bipyrimidine complexes W(CO)4(bpym) and (μ-bpym)[M(CO)4]2 (M=Mo, W) with resonance Raman spectroscopy; the emission spectrum of (μ-bpym)[Mo(CO)4]2

Abstract The character of the two lowest energy transitions of W(CO) 4 (bpym) and (μ-bpym)[M(CO) 4 ] 2 (M=Mo, W) were established with resonance Raman spectroscopy. According to these spectra the two bands belong to MLCT transitions to different π* orbitals of the bpym ligand. Contrary to expectations it is not the first (lowest energy) but the second and more intense electronic transition which, according to the resonance Raman spectra, is directed to the lowest lying π* orbital (b 2u *, LUMO) of these complexes. This interpretation explains the different band intensities and the untypically low g values of the ESR signals of corresponding anion radicals. Excitation of (μ-bpym)[Mo(CO) 4 ] 2 in CH 2 Cl 2 at 400 nm produced a weak emission with an onset at 700 nm. According to the excitation spectrum, this emission originates from the lowest MLCT-excited state of the complex.

Synthesis and crystal structures of [Cu2gm(Ph2P)2PY}3] [P6]2 and [Cu2 {μ-(Ph2P)2PY}2(η2-bipy)2][PF6]2 [(Ph2P)2PY = 2,6-bis(diphenylphosphino)pyridine; bipy = 2,2′-bipyridine]

Abstract Treatment of [Cu(MeCN)4]PF6 in acetonitrile with 2,6-bis(diphenylphosphino)pyridine [(Ph2P)2py] afforded a colourless crystalline material characterised as [Cu2gm-(Ph2P)2PY3][PF612. An X-ray diffraction study confirmed that the three ligands bridge the two copper atoms bonding solely through the phosphorus atoms, each copper being trigonally coordinated; a cavity thus exists in the centre of the dinuclear dication bounded by the nitrogen donor atoms, the Cu… Cu distance and the diameter of the cavity being 4.744(2) and ca 4.9 A, respectively. Measurements of the solution emission spectra of [Cu2{μ-(Ph2P)2PY}3] [PF6]2 in the presence and absence of Cu+ and Ag+ ions suggest that these ions are trapped in the cavity on their addition to an acetonitrile solution of the complex. Addition of (Ph2P)2PY to [Cu(η2-bipy)(MeCN)2]PF6 (bipy = 2,2′-bipyridyl) in acetone afforded a yellow crystalline solid characterised as [Cu2{μ-(Ph2P)2py}2(η2-bipy)2][PF6]2. A crystal structure determination revealed that the two copper atoms are bridged by the two (Ph2P)2PY ligands bonding solely through the phosphorus atoms, with the irregular tetrahedral coordination at each copper being completed by the nitrogen atoms of the chelating bipyridyl ligands. The Cu… Cu distance is 6.806(2) A.

Coordination-induced switch between the singly occupied and the lowest unoccupied molecular orbitals in two methylviologen-derived chromophores

The aromatic heterocycles 2,7-diazapyrene (dap) and 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (4-bptz) each exhibit two very close-lying lowest unoccupied molecular orbitals (MOs) with very different character viz. a methylviologen-like MO (b1u) and a quinonoid (dap) or tetrazine-localized MO (4-bptz) of au symmetry. Calculations (HMO semiempirical NDO type ab initio) suggest that the orbital crossing between b1u and au and thus the character of the singly occupied MO in one-electron reduced compounds can be effected by removal of electron density from the potentially coordinating pyridyl nitrogen centres. We have explored this suggestion experimentally by coordination of electrophiles either alkyl cation R+ or neutral complex fragments W(CO)5, to both pyridine N centres. Experimental data from UV–VIS–NIR and especially EPR spectroelectrochemistry show that coordination of two neutral W(CO)5 fragments to dap is sufficient to cause a change from singly occupied au(in dap˙–) to singly occupied b1u in {dap[W(CO)5]2}˙–. In contrast the 4-bptz˙– radical system requires coordination of two electrophiles R+(R = CH3, C2H5) in order to move b1u below au while the ditungsten(0) radical anion complex is still a 2Au species.