Bridgehead isomer effects in bis(phosphido)-bridged diiron hexacarbonyl proton reduction electrocatalysts

Abstract

The influence of the substitution, orientation and structure of the phosphido bridges in [Fe(CO)(μ-PR)] electrocatalysts of proton reduction has been studied. The isomers e,a-[Fe(CO){μ-P(Ar)H}] (1a(Ar): Ar = Ph, 2'-methoxy-1,1'-binaphthyl (bn')), e,e-[Fe(CO){μ-P(Ar)H}] (1b(Ar): Ar = Ph, bn') were isolated from reactions of iron pentacarbonyl and the corresponding primary phosphine, syntheses that also afforded the phosphinidene-capped tri-iron clusters, [Fe(CO)(μ-CO)(μ-Pbn')] (2) and [Fe(CO)(μ-PAr)] (3(Ar), Ar = Ph, bn'). A ferrocenyl (Fc)-substituted dimer [Fe(CO){μ:μ'-1,2-(P(CHFc)CH)CH}] (4), in which the two phosphido bridges are linked by an o-xylyl group, was also prepared. The molecular structures of complexes 1a(Ph), 1b(Ph), 1b(bn'), 2 and 4 were established by X-ray crystallography. All complexes have been examined as electrocatalysts for proton reduction using p-toluene sulfonic acid in tetrahydrofuran. Cyclic voltammograms of the dimers with acid exhibit two catalysis waves for proton reduction. The first wave, which appears at the potential of the primary reduction, reaches maximum current (turnover) at moderate acid concentrations and is rapidly overtaken by the second wave, which appears at more negative potential. Both of these reductive waves show an initial first order dependence on acid. The electrochemistry and electrocatalyses of the [Fe(CO)(μ-PR)] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms.

Rational Design of Fe2(μ‐PR2)2(L)6 Coordination Compounds Featuring Tailored Potential Inversion

“…In general, the ΔΔG values calculated for these models suggest that the presence of a chelating linker favours potential inversion (ΔΔG < 0 were obtained for 16, 17 and 19 with BP86-D3), in line with the experimental observation that the complex Fe 2 (μ, μ-PPh(CH 2 ) 3 PPh)(CO) 6 , which is equivalent to 17 but for the substitution of Ph with Me, is also 2-electron reduced in non-polar solvent (THF) [39] (unlike, for example, 5; see above, ref [32] and Table S2).…”

“…The redox behaviour of compound 5 in THF was experimentally investigated in the context of hydrogenase biomimetics, [32] showing that it undergoes two successive monoelectronic reductions. Computation of the first and second reduction potentials (see Table S2) in THF indicates a normal ordering of potential, with a ΔΔG = 11.7 kcal/mol (508 mV), in agreement with experimental observations.…”

“…[31] Although similar to Fe 2 S 2 biomimicry, Fe 2 P 2 complexes show a wellestablished reversible redox behaviour entailing two-electron transfer (excluding very few cases). [32,33] The reversibility of the potential inversion phenomenon makes the Fe 2 P 2 framework particularly interesting in the view of developing a synthetic electron-bifurcating device. In particular, it was reported that the elongation of the intermetallic bond and planarization of the M 2 P 2 structure observed upon reduction of M 2 (μ-PR 2 ) 2 L 6 complexes is related to potential inversion.…”

“…To evaluate the relationships between the stereo-electronic features of Fe 2 (μ-PR 2 ) 2 (L) 6 systems and their redox properties, we have used DFT to investigate a set of 26 coordination compounds with different R groups and L ligands. The few compounds already characterized experimentally [32,[34][35][36][37] allowed us to validate the computational protocol, whereas the "virtual" molecules allowed us to thoroughly evaluate and rationalize the effects of structural modification on the redox properties of Fe 2 (μ-PR 2 ) 2 (L) 6 systems. The "molecular recipes" obtained from our study are expected to address and inform the synthesis of compounds with tailored potential inversion, which is a crucial step for the design of electron bifurcating synthetic molecular devices.…”

“…The choice is motivated mainly for two reasons: the resemblance of the Fe 2 L 2 core with the iron clusters found in metalloenzymes which might carry out electron bifurcation, and the large amount of experimental data already available, which allow a critical assessment of the results and conclusions derived from quantum chemical calculations . Although similar to Fe 2 S 2 biomimicry, Fe 2 P 2 complexes show a well‐established reversible redox behaviour entailing two‐electron transfer (excluding very few cases) . The reversibility of the potential inversion phenomenon makes the Fe 2 P 2 framework particularly interesting in the view of developing a synthetic electron‐bifurcating device.…”