Monochlorogallane:  Physical Properties and Structure of the Gaseous Molecule H₂Ga(μ-Cl)₂GaH₂ As Determined by Vibrational, Electron Diffraction, and ab Initio Studies

Abstract

Monochlorogallane, synthesized by the metathesis of gallium(III) chloride with an excess of trimethylsilane at ca. 250 K, has been characterized by chemical analysis, by its IR, Raman, and 1H NMR spectra, and by the products of its reaction with trimethylamine. The vibrational spectra of the vapor species isolated in solid Ar, N2, or CH4 matrixes at ca. 12 K imply the presence of only one species, viz. the dimer with an equilibrium structure conforming to D2h symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements augmented by the results of ab initio molecular orbital calculations. An equilibrium structure with D2h symmetry has been assumed in the analysis of the electron diffraction pattern. However, as the molecule has a very low frequency Ga(mu-Cl)2Ga ring-puckering mode, a dynamic model was used to describe it with the aid of a set of pseudoconformers spaced at even intervals (deltadelta = 5 degrees, deltamax, = 20 delta) around the ring-puckering angle delta and Boltzmann-weighted according to a quartic potential V(delta) = V4delta4 + V2delta2. The differences in bond distances and angles between the different pseudoconformers were constrained to the values derived from the ab initio calculations employing second-order Moller-Plesset (MP2) methods (with all the electrons included in the correlation calculations) and a 6-311G(d) basis set. The results for the weighted average of the principal distances (ralpha) and angles (<alpha) (with estimated 1sigma uncertainties) from the combined GED/ab initio study are r(Ga-Cl) 2.350(2) A, r(Ga-H) 1.523(20) A, <Cl-Ga-Cl 89.7(5)degrees, <H-Ga-H 135.1 degrees, V4 3.0 x 10(-6) kcal mol(-1) deg(-4), and V2 6.0 x 10(-4) kcal mol(-1) deg(-2).

Convenient Access to Gallium(I) Cations through Hydrogen Elimination from Cationic Gallium(III) Hydrides

“…The crystal structure (Figure 1) showed some residual electron density close to the gallium center in the approximate region where hydride substituents would be expected. However, the failure to detect a gallium hydride signal in the 1 H NMR spectrum or an absorption around 2000 cm −1 (ν Ga−H ) in the IR spectrum, 19 followed by the detection of a signal in the 71 Ga NMR spectrum at −703 ppm, which is in the typical region for gallium(I) compounds, 2 convinced us that we had indeed isolated a gallium(I) species. As this finding was supported by preliminary DFT calculations, which indicated that the Hydrosilanes have long been known to convert chlorogallanes into hydrogallanes at or below room temperature, as according to eq 2.…”

“…The corresponding data for (H 2 GaCl) 2 are 5.46 ppm and 2021 cm −1 . 19 Solutions of compound 4 in C 6 D 6 slowly eliminate H 2 at room temperature, and after several hours the 71 Ga NMR spectrum shows a weak peak at −645 ppm (w 1/2 = 1100 Hz), indicating the presence of Ga(I) in an asymmetric environment. A microcrystalline solid of a compound tentatively identified as GaN(SO 2 CF 3 ) 2 , 5, was obtained after leaving a hexane/C 6 D 6 solution of 4 standing at room temperature for 4 weeks.…”

“…In order to test this hypothesis, the reaction was performed as above (see eq 1) followed by the addition of excess Et 3 SiH, which gave the target compound in 48% yield in the form of colorless crystals. A variation of this procedure, the addition of the silylium salt to in-situ generated (H 2 GaCl) 2 19 in the presence of excess Et 3 SiH, avoids the presence of an intermediate and likely highly reactive [Cl 2 Ga][CHB 11 Cl 11 ], leading to an overall cleaner product in 42% yield. It is not clear at this point if the silylium ion abstracts a chloride or hydride ion from (H 2 GaCl) 2 (Scheme 1).…”

“…The mother liquor contained numerous compounds derived from the decomposition of the [B(C 6 F 5 ) 4 ] − anion as shown by 19 F NMR spectroscopy (see Figure S15). As the silylium salt [Et 3 Si][B(C 6 F 5 ) 4 ] was not isolated in this synthesis, the triphenylmethane side product was found to be coordinated to the gallium(I) cation due to its higher basicity compared to fluorobenzene.…”

Structure of 4-(Dimethylamino)benzonitrile Using Gas Electron Diffraction: A New Lease of Life for the Only Gas Electron Diffractometer in the U.K.

“…The University of York GED instrument was formerly housed at the University of Reading and was initially commissioned in the late 1960s. During the 1990s and into the first decade of the new millennium, Aarset, Rice, and Page oversaw a particularly prolific period of operation − in which many gas-phase structures of organometallics and small organic and main-group molecules were characterized using the instrument, and collaborative work was carried out with other electron diffractionists including Rankin and Hedberg . The instrument was acquired by the University of York in 2015, and this article details the recommissioning of the instrument and describes the contemporary GED experiment.…”