Monitoring the Formation and Reactivity of Organometallic Alkane and Fluoroalkane Complexes with Silanes and Xe Using Time-Resolved X-ray Absorption Fine Structure Spectroscopy

Abstract

Complexes with weakly coordinating ligands are often formed in chemical reactions and can play key roles in determining the reactivity, particularly in catalytic reactions. Using time-resolved X-ray absorption fine structure (XAFS) spectroscopy in combination with time-resolved IR (TRIR) spectroscopy and tungsten hexacarbonyl, W­(CO)6, we are able to structurally characterize the formation of an organometallic alkane complex, determine the W–C distances, and monitor the reactivity with silane to form an organometallic silane complex. Experiments in perfluorosolvents doped with xenon afford initially the corresponding solvated complex, which is sufficiently reactive in the presence of Xe that we can then observe the coordination of Xe to the metal center, providing a unique insight into the metal–xenon bonding. These results offer a step toward elucidating the structure, bonding, and chemical reactivity of transient species by X-ray absorption spectroscopy, which has sensitivity to small structural changes. The XAFS results indicate that the bond lengths of metal–alkane (W–H–C) bond in W­(CO)5(heptane) as 3.07 (±0.06) Å, which is longer than the calculated W–C (2.86 Å) for binding of the primary C–H, but shorter than the calculated W–C (3.12 Å) for the secondary C–H. A statistical average of the calculated W–C alkane bond lengths is 3.02 Å, and comparison of this value indicates that the value derived from the XAFS measurements is averaged over coordination of all C–H bonds consistent with alkane chain walking. Photolysis of W­(CO)6 in the presence of HSiBu3 allows the conversion of W­(CO)5(heptane) to W­(CO)5(HSiBu3) with an estimated W–Si distance of 3.20 (±0.03) Å. Time-resolved TRIR and XAFS experiments following photolysis of W­(CO)6 in perfluoromethylcyclohexane (PFMCH) allows the characterization of W­(CO)5(PFMCH) with a W–F distance of 2.65 (±0.06) Å, and doping PFMCH with Xe allows the characterization of W­(CO)5Xe with a W–Xe bond length of 3.10 (±0.02) Å.

A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane

“… 9 In solution such complexes have only been observed using low-temperature in situ NMR spectroscopy (lifetimes of minutes), 10 or on very short time scales (lifetimes of microseconds to seconds) using time-resolved infrared (TRIR) 11 or XAFS techniques. 12 These analyses are necessarily coupled with the generation of a vacant site on the metal center using ligand photoejection or protonation of a metal–alkyl bond. Using these methodologies, σ-alkane complexes from methane to dodecane have been generated.…”

Isolating reactive metal-based species in Metal–Organic Frameworks – viable strategies and opportunities

“…Characterising such reactive intermediates in situ is challenging due to their short solution lifetimes 5,6 and typically requires kinetic trapping via low temperature matrix isolation strategies, 7,8 and/or studied by laser-pulsed time-resolved spectroscopic techniques that allow rapid interrogation of the chemistry. [9][10][11] One approach to enhance the stability of a reactive species, to facilitate structural characterisation, is via encapsulation within the cavity of a host. [12][13][14] Examples of molecular capsules as hosts to trap reactive entities are well-established, dating back to Cram and co-workers who, for example, demonstrated the isolation of cyclobutadiene within a carcerand.…”

Reversible Encapsulation of Xenon and CH2Cl2 in a Solid‐State Molecular Organometallic Framework (Guest@SMOM)

“…[10] This supports the structural reorganization associated with the reactive metal center and also allows reversible access for gases and small organic compounds,albeit in what is essentially an on-porous material. [1] That s-alkane complexes are unstable and transient in solution, even at low temperature, [11] demonstrates the stabilizing effect of the noncovalent anion microenvironment. While these systems also promote catalysis (e.g.…”