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.
Highlights
The formation of W(CO)5(heptane) and reaction with HSiBu3 has been previously monitored by fast time-‐resolved IR spectroscopy (TRIR),22 a useful technique for characterising reaction intermediates and monitoring reaction kinetics of metal carbonyls.23 These experiemnts served as the pre-‐cursor to the pump-‐probe X-‐ray absorption spectroscopy (XAS) experiments conducted at the Advanced Photon Source, 11-‐ID-‐D to collect W LIII X-‐ray absorption fine structure (XAFS) on these tungsten complexes
It is more than 40 years since the first reported evidence for the interaction of M(CO)5 (M = Cr, Mo and W) with noble gases and CH4 in matrices at cryogenic temperatures.1 These pioneering low temperature spectroscopic experiments provided the first strong circumstantial evidence through infrared and UV/visible spectroscopy for the existence of organometallic alkane complexes, which are widely known as s -‐complexes, and in the coordination of noble gases which are some the most weakly coordinating ligands
We use time-‐resolved X-‐ray absorption fine structure (XAFS) spectroscopy to examine the formation and reactivity of organometallic alkane and noble gas complexes and monitor the reactivity of metal alkane with silanes to form complexes since these have been extensively studied by other methods and they they are key to many catalytic transformations
Summary
The formation of W(CO)5(heptane) and reaction with HSiBu3 has been previously monitored by fast time-‐resolved IR spectroscopy (TRIR),22 a useful technique for characterising reaction intermediates and monitoring reaction kinetics of metal carbonyls.23 These experiemnts served as the pre-‐cursor to the pump-‐probe XAS experiments conducted at the Advanced Photon Source, 11-‐ID-‐D to collect W LIII XAFS on these tungsten complexes. The photolysis reactions were monitored by pump-‐ probe difference X-‐ray absorption spectroscopy (XAS) at the W LIII absorption edge, taking spectra collected after the laser pulse and subtracting the spectral data averaged 15 μs before laser pulse.
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