Structure Determination of Triplet Diphenylcarbenes by in Situ X-ray Crystallographic Analysis

Crystalline-state photoreactions of the following diphenyldiazomethanes were investigated by in
situ X-ray crystallography, spectroscopy, and theoretical calculations:  bis(2,4,6-trichlorophenyl)diazomethane
(1-N2), bis(2,4,6-tribromophenyl)diazomethane (2-N2), bis(2,6-dibromo-4-methylphenyl)diazomethane (3-N2), bis(2,6-dibromo-4-tert-butylphenyl)diazomethane (4-N2), (2,4,6-tribromophenyl)-(2,6-dimethyl-4-tert-butylphenyl)diazomethane (5-N2), bis(4-bromophenyl)diazomethane(6-N2), and diazofluorene (7-N2). Crystal
structures of photoinduced triplet diphenylcarbenes (DPCs) of 1, 2, and 4 were determined. We found
remarkable differences between their structural information obtained in the crystalline state and that
previously obtained spectroscopically in a glass matrix. Although the triplet DPCs of 1, 2, and 4 have
significantly different stabilities in solution, only subtle differences in their structural parameters, except for
their C(:)−Ar bond lengths, are observed. It is noteworthy that the average bond length of C(:)−Ar for 4
(1.374 Å) is considerably shorter than those for 31 and 32 (1.430 and 1.428 Å, respectively), provided that
the two C(:)−Ar bonds being compared were chemically equivalent. The most likely explanations for the
small and large differences in bond lengths in 1, 2, and 4 may be derived from the packing effect. The
packing patterns of 1 and 2 are identical, but that of 4 is totally different from those of 1 and 2. Moreover,
these results are interpreted as indicating that triplet DPCs undergo relaxation upon softening of the
environments. Theoretical calculations indicate that the potential energy surface of triplet DPCs in terms of
the carbene angle is extremely flat and changes in the angles have little effect on the energies. Triplet
DPCs with a sterically congested carbene center are trapped in a structure dictated by the precursor structure
in a rigid matrix, even if this is not the thermodynamically most stable geometry, but undergo geometrical
relaxation upon softening the matrix to relieve steric compression. ESR studies indicate that the interplanar
angles are more flexible than the bond angles.