A matrix isolation and flash photolysis study of the cyclization reactions of chromium amino carbenes

The photochemistry of chromium amino carbene complexes have been studied using IR and UV–vis spectroscopy in matrices at 12 K and using UV–vis spectroscopy following laser flash photolysis in solution at room temperature (r.t.). The complexes studied can be divided into two classes. The Class 1 complexes are (CO)5Cr[C(NMe2)Me], (CO)5Cr[C(NBz2)Me], (CO)5Cr[C(NMe2)Ph] which are known to have a low or no photo-reactivity with imines to form β-lactams. Class 2 complexes are (CO)5Cr[C(NMe2)H], (CO)5Cr[C(NBz2)H], (CO)5Cr[C(NH2)Me] and these complexes are known to undergo efficient photochemical reactions with imines to form β-lactams. The proposed active intermediate in the reaction to form β-lactams is a metal ketene complex. For both classes of complexes no ketene complex was observed upon irradiation in either the cryogenic matrix or the solution studies. The only photochemical process which was observed in all experiments was CO loss and the major product for all complexes was assigned to be cis-(CO)4Cr[(N(R2)R′]. This species behaved differently for the two classes of complexes. In the matrix isolation experiments on Class 2 complexes this species reacted with N2, ethene or CO present in the matrix cage, whereas for Class 1 complexes it did not. In the flash photolysis studies on Class 1 complexes the rate of reaction of this transient species with CO showed only a moderate dependence on the nature of the solvent. The rate of reaction for the same process measured for the transient species formed from Class 2 complexes showed a significantly greater dependence on the solvent. We propose that for Class 1 complexes the vacant site formed by photo-dissociation of a cis-CO ligand is blocked, whereas for the Class 2 complexes it is not. Therefore, in the latter case the site is open to reaction with active species in the cryogenic matrices and the solvent acts as a ‘token’ ligand in the solution studies. The difference in the reactivity of cis-(CO)4Cr[C(NR2)R′] for the two classes of complex may aid the explanation of the differences in their synthetic photochemistry.