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

Abstract

The structural properties of the singly halogenated derivatives of CH(3)CN-BF(3) (X-CH(2)CN-BF(3): 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 FCH(2)CN-BF(3), ClCH(2)CN-BF(3), and BrCH(2)CN-BF(3) indicate that the B-N dative bonds in these systems are quite weak, with distances of 2.422, 2.374, and 2.341 A, 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 Br-CH(2)CN-BF(3). In contrast, solid-state structures for FCH(2)CN-BF(3), ClCH(2)CN-BF(3), and ICH(2)CN-BF(3) from X-ray crystallography all have B-N distances that are quite short, about 1.65 A. Thus, the B-N distances of the F- and Cl-containing derivatives contract by over 0.7 A upon crystallization. Large shifts in the vibrational modes involving motions of the BF(3) subunit parallel these structural changes. An X-ray crystal structure could not be determined for BrCH(2)CN-BF(3)(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.

Boron and other Triel Lewis Acid Centers: From Hypovalency to Hypervalency

“…The latter explanations on the boron acidity for BH 3 and boron trihalides are in line with recent calculations for the BF 3 -NCH [9] and BH 3 -NCH [10] complexes in which stronger Lewis acid properties of BH 3 were observed, because the binding energy for the latter complex is À19.9 kcal mol À1 , whereas for the former complex it is À6.3 kcal mol À1 [MP2/aug-cc-pVTZ level, basis set superposition error (BSSE) correction not included]. [12,13] The importance of the frontier orbital interactions in small complexes such as BH 3 -NH 3 and BH 3 -CO was analyzed earlier. [11] Other complexes of boron trihalides and BH 3 were also analyzed both theoretically and experimentally.…”

“…[14,15] The interaction of the sp 2 -hybridized boron center with Lewis bases often leads to rehybridization of the boron center, as the Lewis base disrupts the p bond in the B trigonal reactant and leads to sp 3 hybridization of the product. [13] The hydridic character of the hydrogen atoms of the ÀBH 3 group attached to the Lewis base centers, that is, at which the boron is tetravalent, was analyzed recently in numerous structures. For example, amine systems, such as the MP2/aug-cc-pVTZ calculations were performed on triel trifluorides, ZF 3 (Z = B, Al, Ga, In, Tl), and their complexes with N 2 and HCN species, which are acting as Lewis bases.…”

“…[11] Other complexes of boron trihalides and BH 3 were also analyzed both theoretically and experimentally. [12,13] The importance of the frontier orbital interactions in small complexes such as BH 3 -NH 3 and BH 3 -CO was analyzed earlier. [14,15] The interaction of the sp 2 -hybridized boron center with Lewis bases often leads to rehybridization of the boron center, as the Lewis base disrupts the p bond in the B trigonal reactant and leads to sp 3 hybridization of the product.…”

“…One can mention X-ray diffraction measurements supported by solid-state infrared spectroscopy and correlated electronic-structure theory for haloacetonitrile-BF 3 complexes. [13] The hydridic character of the hydrogen atoms of the ÀBH 3 group attached to the Lewis base centers, that is, at which the boron is tetravalent, was analyzed recently in numerous structures. [18] Frustrated Lewis pairs, which are inter-and intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C 6 F 5 ) 2 moieties, were analyzed, and the possibility of tetravalency of the B centers was considered.…”

Infrared spectrum of CH3CN–HCl in solid neon, and modeling matrix effects in CH3CN–HCl and H3N–HCl

“…Equilibrium geometries were obtained within the C3v point group, and all gas-phase optimizations were performed using the "opt=tight" option with an "ultrafine" integration grid, while standard convergence criteria were employed for optimizations in dielectric media. Seeking a prediction of the gas-phase H-Cl stretching frequency for CH3CN-HCl, we initially attempted a validation study based on the prediction of the frequency of free HCl, akin to our previous studies of Lewis acid-base systems [30,31]. In this process, we explored the performance of 18 different density functionals as well as MP2, and found that anharmonic frequency predictions were essential to obtain any reasonable agreement with the experimental frequency [32].…”