Formation and Reactions of Tetracarbonyl Intermediates of the Fischer Carbene Complex (CO)₅WC(OMe)Ph. A Laser Flash Photolysis Study Using Time-Resolved Infrared and UV/Vis Spectroscopy

Abstract

The results of nanosecond laser flash photolysis (XeCl excimer irradiation at 308 nm) of the Fischer carbene complex [(CO) 5 WdC(OMe)Ph] (C Z ) in di-n-butyl ether and n-hexane are reported. Spectrokinetical detection of the intermediates was based on transient absorptions simulaneously recorded in the UV/vis and the IR spectral regions. The primary photoproducts are the unstable isomer C E produced with a quantum yield of about 30% and a tetracarbonyl complex I-S, probably of structure [(CO) 4 (S)WdC(OMe)Ph], where S is a solvent molecule. The quantum yield of CO photoelimination was estimated as 1.7%. The C E isomer was found to return to the stable C Z isomer within a few microseconds, confirming previous results of McGarvey and co-workers. The tetracarbonyl species reacts with cosolutes present in the solution such as N 2 , H 2 O, acetonitrile, and methyl trans-crotonate to form secondary complexes of cis-tetracarbonyl complex structure [(CO) 4 (L)WdC(OMe)Ph] (I-L). The tetracarbonyl complexes react readily with CO. For I-N 2 in n-hexane the rate constant is 3.5 × 10 8 M -1 s -1 . The rate constants of the corresponding reactions with acetonitrile (MeCN) and the olefin are larger by a factor of 2. In the presence of CO the complexes I-MeCN and I-olefin revert to C Z in a dissociative reaction mechanism. The rate constants for the elimination of MeCN or olefin are 5 and 10 5 s -1 , respectively. The I-olefin complex irreversibly decomposes with a rate constant of 8 × 10 3 s -1 .

Direct Observation of Metal Ketenes Formed by Photoexcitation of a Fischer Carbene using Ultrafast Infrared Spectroscopy

“…Additional bands are apparent in Figure A that can be assigned to previously reported side reactions. Peaks at 1930, 1953, and 2070 cm –1 are a result of anti → syn isomerization of the methoxy substituent in A . ,, Similarly, bands at 1899 and 2044 cm –1 are assigned to a CO-loss product. These side reactions have been reported previously. , Changes in the intensities of these bands on the picosecond time scale are attributed to spectral overlap with the absorptions of the metal ketenes and bleaches of the parent complex, all of which exhibit dynamics on the picosecond time scale.…”

“…Recent work by Fernández, Cossío, Sierra, and co-workers led to the proposal of a mechanism for the photocarbonylation reaction involving initial excitation into a triplet state which, in the presence of coordinating solvents, undergoes a solvent-induced spin crossover to yield a metal ketene in which the solvent coordinates to the metal center to stabilize the vacant site. − A new photochemical reaction was also experimentally uncovered through which tungsten-based complexes, in addition to their chromium counterparts, were able to undergo a 1,2-dyotropic rearrangement to yield imine products, thus demonstrating that tungsten-based Fischer carbenes can indeed be photoreactive . Photochemical investigations into both chromium and tungsten complexes demonstrated that UV excitation also leads to anti → syn isomerization of the methoxy substituent as one of the primary photochemical processes, with CO loss also occurring to a lesser extent. , Despite these new insights into the photochemistry of group 6 Fischer carbenes, the key photocarbonylation mechanism has, until now, not been directly characterized by experiments.…”

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“…Some reaction chemistry of the tetracarbonyl intermediates is described. 76 . 77 [WCl(CO)(Bu t CCH)(NCMe)-(PPh 3 ) 2 ] 2 -(SnCl 6 ), prepared via an observable [WCl(SnCl 3 )(CO)(Bu t CCH) 2 (NCMe)] intermediate, is the first example of a structurally characterised tungsten(II) complex containing a terminal alkynyl ligand.…”

Synthesis and Reactivity of Metallocarbene-Containing Polymers

“…This value was in good agreement with the measured change in M n for a postirradiated sample of P1 (Figure S11). Given that it is unusual for tungsten carbenes to release CO, , we sought to further validate this observation. Rhodium complex 5 , which exhibits a colorimetric response to CO binding, , was selected to confirm CO release from our polymers using UV–vis spectroscopy (Figure B; Figures S12–19).…”

“…59 In contrast to the behavior of chromium Fischer carbenes, 60,61 CO extrusion from tungsten carbenes is rare. 62 Agostic interactions between tungsten and alkoxy substituents, however, have been reported to facilitate expulsion of CO, 63 and we envisioned that analogous stabilization within P1 could enable CO release.…”