Energetics of Concerted Two-Electron Transfer and Metal−Metal Bond Cleavage in Phosphido-Bridged Molybdenum and Tungsten Carbonyl Complexes

Abstract

The kinetics and thermodynamics of concerted two-electron transfer and metal-metal bond cleavage in the binuclear phosphido-bridged complexes [M 2 (µ-PPh 2 ) 2 (CO) 8 ] 0/2-[M ) Mo (1 0/2-), W(2 0/2-)] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF 6 /acetone. The reductions of 1°and 2°a re accompanied by an increase of 1.08 Å in M-M distance and expansion and contraction by 29°of the M-P-M and P-M-P angles, respectively, within an intact M 2 (µ-PPh 2 ) 2 unit. The one-electron electrode potentials of these systems are highly inverted: ∆E°′ ) E 2 °′ -E 1 °′ ) +0.17 V for 1 0/2and +0.18 V for 2 0/2-, and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first: k sh,2 /k sh,1 ) 0.1 for Mo and 0.018 for W. Results are consistent with progressive cleavage of the metal-metal bond in two one-electron steps, of which the second is rate-limiting, because it is accompanied by a larger part of the structural change. EHMO calculations reveal that the redox-active orbital is a metal-metal antibonding (σ*) orbital with substantial bridging-ligand character that decreases markedly in energy on passing from the metal-metal bonded M(I) 2 state to nonbonded M(0) 2 . Despite this feature, electron-transfer thermodynamics and kinetics are not significantly metal-dependent. Rather, comparisons with structurally similar sulfido-bridged complexes reveal that electron-transfer energetics are influenced more extensively by the bridging ligand, with more positive potentials and larger electron-transfer rates observed for RSversus R 2 Pbridged species.

A Silicon‐Heteroaromatic System as Photosensitizer for Light‐Driven Hydrogen Production by Hydrogenase Mimics

“…[32] Additionally, Glass and co-workers reported a related [2Fe2S(Sn)] complex. [61][62][63][64][65] The structural change can either be concerned with one of the electron transfers [32,52,[56][57][58][59] or appears as the intervening step of an ECE process. [48] We assume that the large angle (170.54°) between the planes generated by C1-Si1-C2 and S1-C1C2-S2 is an indicator for an effective photoelectron transfer between the photosensitizer and the diiron center.…”

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

“…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 . It is worth noting that, differently from natural bifurcating centres known so far (such as flavins), here protonation is not needed to observe or modulate potential inversion.…”

“…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. [34][35][36][37][38][39][40][41] It is worth noting that, differently from natural bifurcating centres known so far (such as flavins), here protonation is not needed to observe or modulate potential inversion. The latter is, in fact, only due to structural rearrangements.…”

Rethinking Life and Predicting Its Origin

“…) is an appealing coupling (or free energy converting) mechanism to the origin of life because of its general physicochemical nature (43), being accessible to wide-ranging molecules under different conditions (44). And indeed, researchers have shown that certain molybdenum and tungsten organometallic complexes attain inverted potentials (49,50). This goes to show how the right geological landscape and suitable redox gradients could have serendipitously created chemical disequilibria in the early stages of Earth.…”