Oxo-bridged heterobinuclear units of the type Zr(IV)OCo(II) covalently anchored on the pore surface of mesoporous silica SBA-15 have been synthesized with high selectivity. The unit exhibits a visible light absorbing metal-to-metal charge-transfer absorption (MMCT) extending to about 550 nm. The oxo-bridged structure of the binuclear moiety is manifested by spectral blue-shifts of the optical Co(II) spin−orbit bands due to reduced π-electron donating ability of the bridging oxygen caused by the electron-withdrawing Zr center. EXAFS measurements of the Zr and Co K-edges and curve fitting analysis revealed a Zr to Co distance of 3.4 Å. The coordination geometry of the Zr and Co metal centers in monometallic Zr and Co-SBA-15 samples is closely preserved in the ZrOCo unit. Illumination of the MMCT absorption at 420 nm and shorter wavelengths resulted in the reduction of CO 2 to gas phase CO and HCO 2 − , the latter adsorbed on the silica pore surface. The branching between carbon monoxide and formate was found to be determined by the fate of the sacrificial donor (triethyl-or diethylamine), namely proton transfer versus H atom transfer to CO 2 interacting with the transient Zr(III) center. The ZrOCo(II) unit on a silica surface constitutes the first example of an all-inorganic heterobinuclear unit for the photoinduced splitting of CO 2 to free CO. Moreover, transient Co(III) formed upon MMCT excitation should possess sufficient oxidation potential for driving a catalyst for water oxidation, thereby opening up opportunities for replacing the sacrificial donor by water as electron source.
The phosphine-free "second-generation" Blechert/Hoveyda-Grubbs catalyst Ru(dC(H)C 6 H 4 -o-O-i-Pr)(H 2 IMes)Cl 2 (H 2 IMes ) 4,5-dihydro-1,3-dimesitylimidazol-2-ylidene) and Piers catalyst [Ru(dCHPCy 3 )-(H 2 IMes)Cl 2 ]BF 4 (Cy ) cyclo-C 6 H 11 ) for olefin metathesis effected cross-metathesis (CM) of vinyl chloride and 1,2-dichloroethene with several unhindered terminal and internal alkenes in up to 95% yield (5 mol % catalyst). In most cases, 1,2-dichloroethene was more successful than vinyl chloride. Ring-opening CM of cyclooctene provided greater yields than CM: with vinyl chloride, 93% yield; with 1,2dichloroethene, >95%. Other common Ru-based catalysts failed to effect CM under similar conditions, but instead underwent rapid decomposition. The dimeric ruthenium-monochloromethylidene complex [Ru(dCHCl)(H 2 IMes)Cl(µ-Cl)] 2 was isolated as a thermally unstable intermediate. CM reactions with 1,2-dibromoethene afforded 22% CM product in the best case; halide exchange with the catalyst was significant. CM reactions involving vinyl fluoride typically led to <3 turnovers, but the dimeric intermediate [Ru(dCHF)(H 2 IMes)Cl(µ-Cl)] 2 was sufficiently long-lived to be characterized by single-crystal X-ray diffraction. Ring-opening CM of cyclooctene with vinyl fluoride (55% yield) was more favorable than CM.
Analysis of extended X-ray absorption fine structure (EXAFS) measurements of binuclear TiIVOMnII visible light charge-transfer chromophores anchored on silica nanopore surfaces reveals an oxo-bridged structure of a Ti and a Mn center. For TiMn-SBA-15 samples with 1 mol % for each metal, Ti and Mn K-edge EXAFS curve fitting indicates a common TiMn distance of 3.3 Å (Ti edge: 3.36 ± 0.05 Å; Mn edge: 3.25 ± 0.07 Å) and a bond angle of 111°. The first sphere coordination of the two metal centers, a distorted tetrahedral for Ti and a pseudo octahedral for Mn, is largely preserved upon formation of the linkage of the binuclear unit. Increasing the Ti loading from 1 to 3% does not introduce titania clusters, whereas loading of Mn beyond 2% leads to some Mn oxide cluster formation. The binuclear unit retains its structural integrity upon prolonged exposure to air or heating at high temperature (350 °C) in the presence of oxygen. The oxidation state increase of the Mn center upon calcination is accompanied by a shortening of the oxo bridge. The results provide the first detailed structural information on the TiIVOMnII MMCT unit, which is a promising candidate as a visible light charge-transfer chromophore for driving multielectron catalysts for artificial photosynthesis.
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