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The effect of core geometry constraints of hydrogenase H-cluster analogues on reduction chemistry have been explored by a combination of structural, electrochemical and IR spectroelectrochemical (IR-SEC) studies. A series of phosphido-bridged diiron hexacarbonyl complexes, Fe2(µ2-PPh2(CH2) x PPh2)(CO)6, x = 2 (2P) and 4 (4P) and previously reported with x = 3 (3P) and the unlinked bis-diphenylphosphido (DP) analogues were investigated. The X-ray structures of the neutral complexes demonstrate the effect of the linking group on the Fe2P2 core geometry with P–Fe–Fe–P torsion angles of 95 (2P), 101 (3P), 108 (4P) and 109° (DP) and a twisting of the Fe(CO)3 fragments from an eclipsed geometry (2P , 3P and DP) for 4P. For all four compounds the primary reduction process involves two close-spaced one-electron reactions (E1 and E2) with a systematic trend to more negative reduction potentials with a shorter link between the bridging phosphorus atoms. This reflects the greater constraint that the bridging group places on the adoption of a planar 2Fe2P geometry. The sensitivity of the core geometry is greater for E2 than E1 and this impacts the stability of the monoanion with respect to disproportion (K disp(298 K) = 0.02 (2P), 2.4 (3P) and 3540 (4P and DP)). 4P has a stable dianion and gives reversible cyclic voltammetry at 298 K and is quasi-reversible at 253 K, whereas the response of 2P is irreversible at 298 K, with two distinct daughter products, but becomes quasi-reversible at 253 K. IR-SEC measurements enabled elucidation of the spectra and time evolution of the reduction products. These results are consistent with a bimolecular reaction giving a distinct reduced product modelled as a dimeric, 4Fe species. The sensitivity of the reduction chemistry of the bridged diiron compounds underpins their utility as catalytic proton reduction catalysts and the systematic trends delineated in this investigation provide the framework for charting the path of their redox-coupled chemical reactions. Integration of IR spectroelectrochemical and electrochemical simulation of the series of phosphido-bridged diiron hexacarbonyl complexes, Fe2(µ2-PPh2(CH2) x PPh2)(CO)6, x = 2 (2P) and (4P) and previously reported x = 3 (3P) and bis-diphenylphosphido (DP) analogues have allowed delineation of the impact of constraints on the 2Fe2P core geometry on the reduction chemistry. The sensitivity of the reduction chemistry of the bridged diiron compounds underpins their utility as catalytic proton reduction catalysts and the systematic trends delineated in this investigation provide the framework for charting the path of their redox-coupled chemical reactions. [ABSTRACT FROM AUTHOR]

Analysis of the products formed in the reaction of the environmental free radical oxidant NO[sub 3] [sup •] with permethylated uridine 1 and thymidine 2 in solution revealed highly complex reaction pathways following initial NO[sub 3] [sup •] induced oxidative electron transfer at the pyrimidine ring. Product formation was found to depend not only on the nature of the nucleobase, but also on the presence of other free radical oxidants, namely NO[sub 2] [sup •] . In the reaction of 1 with NO[sub 3] [sup •] , which was generated through CAN photolysis, apart from formation of the highly oxidized nucleoside derivative 4 as the major product, cleavage of the C-N glycosidic bond did also occur, resulting in formation of ribolactone 5 and the free nucleobase 6. The suggested mechanism involves in situ generation of NO[sub 2] [sup •] during the course of the reaction, which promotes conversion of the initially formed radical cation 7 to 4 in an autocatalytic fashion. When the reaction of NO[sub 2] [sup •] with O[sub 3] was used to generate NO[sub 3] [sup •] , the initially formed radical cation 7 in the reaction with permethylated uridine 1 is rapidly trapped by NO[sub 2] [sup •] to give 5-nitrouridine 18 in a radical mediated vinylic substitution reaction. In contrast to this, under similar conditions in the reaction involving thymidine 2 the highly oxidized products 20 and 21 are obtained as major compounds, which result from addition to the C5-C6 double bond. No direct reaction between NO[sub 3] [sup •] and the carbohydrate moiety in 1 and 2 was found. Also, no reaction occurred between the nucleosides and mixtures of NO[sub 2] [sup •] /N[sub 2] O[sub 4] and O[sub 3] /O[sub 2] , respectively. Product studies showed that the reaction of the night-time atmospheric free-radical oxidant NO[sub 3] [sup •] with uridine 1 and thymidine 2 is very complex and is influenced by the presence of other free radical species, specifically NO[sub 2] [sup •] . Depending on the conditions, the reaction can either lead to cleavage of the glycosidic bond, addition to the C=C double bond, or substitution of R at the nucleobase. [ABSTRACT FROM AUTHOR]

The effect of core geometry constraints of hydrogenase H-cluster analogues on reduction chemistry have been explored by a combination of structural, electrochemical and IR spectroelectrochemical (IR-SEC) studies. A series of phosphido-bridged diiron hexacarbonyl complexes, Fe2(µ2-PPh2(CH2) x PPh2)(CO)6, x = 2 (2P) and 4 (4P) and previously reported with x = 3 (3P) and the unlinked bis-diphenylphosphido (DP) analogues were investigated. The X-ray structures of the neutral complexes demonstrate the effect of the linking group on the Fe2P2 core geometry with P–Fe–Fe–P torsion angles of 95 (2P), 101 (3P), 108 (4P) and 109° (DP) and a twisting of the Fe(CO)3 fragments from an eclipsed geometry (2P , 3P and DP) for 4P. For all four compounds the primary reduction process involves two close-spaced one-electron reactions (E1 and E2) with a systematic trend to more negative reduction potentials with a shorter link between the bridging phosphorus atoms. This reflects the greater constraint that the bridging group places on the adoption of a planar 2Fe2P geometry. The sensitivity of the core geometry is greater for E2 than E1 and this impacts the stability of the monoanion with respect to disproportion (K disp(298 K) = 0.02 (2P), 2.4 (3P) and 3540 (4P and DP)). 4P has a stable dianion and gives reversible cyclic voltammetry at 298 K and is quasi-reversible at 253 K, whereas the response of 2P is irreversible at 298 K, with two distinct daughter products, but becomes quasi-reversible at 253 K. IR-SEC measurements enabled elucidation of the spectra and time evolution of the reduction products. These results are consistent with a bimolecular reaction giving a distinct reduced product modelled as a dimeric, 4Fe species. The sensitivity of the reduction chemistry of the bridged diiron compounds underpins their utility as catalytic proton reduction catalysts and the systematic trends delineated in this investigation provide the framework for charting the path of their redox-coupled chemical reactions. Integration of IR spectroelectrochemical and electrochemical simulation of the series of phosphido-bridged diiron hexacarbonyl complexes, Fe2(µ2-PPh2(CH2) x PPh2)(CO)6, x = 2 (2P) and (4P) and previously reported x = 3 (3P) and bis-diphenylphosphido (DP) analogues have allowed delineation of the impact of constraints on the 2Fe2P core geometry on the reduction chemistry. The sensitivity of the reduction chemistry of the bridged diiron compounds underpins their utility as catalytic proton reduction catalysts and the systematic trends delineated in this investigation provide the framework for charting the path of their redox-coupled chemical reactions. [ABSTRACT FROM AUTHOR]