Large Gas-Solid Structural Differences in Complexes of Haloacetonitriles with Boron Trifluoride

Phillips, J. A.; Halfen, J. A.; Wrass, J. P.; Knutson, C. C.; Cramer, C. J.
Inorg. Chem. 2006, 45, 722.

The structural properties of the singly halogenated derivatives of CH₃CN-BF3 (X-CH2CN-BF₃: X = F, Cl, Br, I) have been investigated via single crystal x-ray crystallography, solid-state infrared spectroscopy, and correlated electronic-structure theory. Taken together, these data illustrate large differences between the gas-phase and solid-state structures of these systems. Calculated gas-phase structures (B3PW91/aug-cc-pVTZ) of FCH₂CN-BF₃, ClCH₂CN-BF₃, and BrCH₂CN-BF₃ indicate that the B-N dative bonds in these systems are quite weak, with distances of 2.422 ang, 274 ang, and 2.341 ang, respectively. However, these distances, as well as other calculated structural parameters and normal-mode vibrational frequencies indicate that the dative interactions do become slightly stronger in proceeding from F- to BrCH₂CN-BF₃. In contrast, solid-state structures for FCH₂CN- BF₃, ClCH₂CN-BF₃, and ICH₂CN-BF₃ from X-ray crystallography all have B-N distances that are quite short, about 1.65 ang. Thus, the B-N distances of the F- and Cl-containing derivatives contract by over 0.7 ang upon crystallization. Large shifts in the vibrational modes involving motions of the BF₃ sub-unit parallel these structural changes. An X-ray crystal structure could not be determined for BrCH₂CN-BF₃ (s), but the solid-state IR spectrum is consistent with those obtained previously for related complexes and suggests that the solid-state structure resembles those of the others, and in turn, implicates a large gas-solid structural difference for this species as well.

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