A Trigold(I) Ketenylidene Cation

Valence Delocalization and Metal-Metal Bonding in Carbon-Bridged Mixed-Valence Iron Complexes

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe2 (μ-CCO)2 ]+ complex with two carbon bridges (4) is S=1 / 2 ${{ 1/2 }}$ , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant. In contrast, one-electron reduction of the [Fe2 (μ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S=7 / 2 ${ 7/2 }$ ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm-1 . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.

Partial Deoxygenative CO Homocoupling by a Diiron Complex

One route to address climate change is converting carbon dioxide to synthetic carbon-neutral fuels. Whereas carbon dioxide to CO conversion has precedent in homo- and heterogeneous catalysis, deoxygenative coupling of CO to products with C-C bonds-as in liquid fuels-remains challenging. Here, we report coupling of two CO molecules by a diiron complex. Reduction of Fe2 (CO)2 L (2), where L2- is a bis(β-diketiminate) cyclophane, gives [K(THF)5 ][Fe2 (CO)2 L] (3), which undergoes silylation to Fe2 (CO)(COSiMe3 )L (4). Subsequent C-OSiMe3 bond cleavage and C=C bond formation occurs upon reduction of 4, yielding Fe2 (μ-CCO)L. CO derived ligands in this series mediate weak exchange interactions with the ketenylidene affording the smallest J value, with changes to local metal ion spin states and coupling schemes (ferro- vs. antiferromagnetism) based on DFT calculations, Mössbauer and EPR spectroscopy. Finally, reaction of 5 with KEt3 BH or methanol releases the C2 O2- ligand with retention of the diiron core.

Crystal structure of tris-(2-di-cyclo-hexyl-phosphino-2',6'-dimeth-oxy-1,1'-biphenyl-κP)-μ-oxoethenyl-idene-triangulo-trigold(I) bis-(tri-fluoro-methane-sulfon-yl)imide

The title ketenyl-idene, [Au3(C2O)(C26H35O2P)3](C2F6NO4S2), was obtained upon exposure of [2-(di-cyclo-hexyl-phosphino)-2',6'-dimeth-oxy-1,1'-biphen-yl]gold(I) bis-(tri-fluoro-methane-sulfon-yl)imide to acetic anhydride at elevated temperature. The ketenyl-idene bridge caps the tri-gold cluster. The title compound has provided crystals that upon analysis represent the first tri-gold ketenyl-idene with atomic distances indicative of bonding inter-action between the gold atoms.