Heather E. Robertson

The molecular structure of tetra-tert-butyldiphosphine: an extremely distorted, sterically crowded molecule

The molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction. Ab initio methods were employed to gain a greater understanding of the structural preferences of this molecule in the gas phase, and to determine the intrinsic P-P bond energy, using recently described methods. Although the P-P bond is relatively long [GED 226.4(8) pm; X-ray 223.4(1) pm] and the dissociation energy is computed to be correspondingly small (150.6 kJ mol(-1)), the intrinsic energy of this bond (258.2 kJ mol(-1)) is normal for a diphosphine. The gaseous data were refined using the new Edinburgh structure refinement program ed@ed, which is described in detail. The molecular structure of gaseous P(2)Bu(t)(4) is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C(2) symmetry. The P-P-C angles returned from the gas electron diffraction refinement are 118.8(6) and 98.9(6) degrees, a difference of 20 degrees, whilst the C-P-C angle is 110.3(8) degrees. The corresponding parameters in the crystal are 120.9(1), 99.5(1) and 109.5(1) degrees. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.

Structures and Aggregation of the Methylamine−Borane Molecules, MenH3−nN·BH3 (n = 1−3), Studied by X-ray Diffraction, Gas-Phase Electron Diffraction, and Quantum Chemical Calculations

The structures of the molecules methylamine-borane, MeH(2)N.BH(3), and dimethylamine-borane, Me(2)HN.BH(3), have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations. The crystal structures have also been determined for methylamine-, dimethylamine-, and trimethylamine-borane, Me(n)H(3-n)N.BH(3) (n = 1-3); these are noteworthy for what they reveal about the intermolecular interactions and, particularly, the N-H...H-B dihydrogen bonding in the cases where n = 1 or 2. Hence, structures are now known for all the members of the ammonia- and amine-borane series Me(n)H(3-n)N.BH(3) (n = 0-3) in both the gas and solid phases. The structural variations and energetics of formation of the gaseous adducts are discussed in relation to the basicity of the Me(n)H(3-n)N fragment. The relative importance of secondary interactions in the solid adducts with n = 0-3 has been assessed by the semi-classical density sums (SCDS-PIXEL) approach.

Structures of the radical P[N(SiMe3)2](NPri2), its dimer, cation and chloro derivativeElectronic supplementary information (ESI) available: The final Cartesian coordinates of the computed structures of the diphosphine 2 and the corresponding radical 3, a selected list of distances and associated amplitudes of vibrations and the correlation matrix from the electron diffraction least-squares refinements. See http://www.rsc.org/suppdata/dt/b4/b402926g/

Treatment of PCl[N(SiMe3)2](NPri2) (1) with potassium-graphite in thf afforded the colourless, crystalline diphosphine [P[N(SiMe3)2](NPri2)]2 (2) in good yield. Sublimation of 2 in vacuo yielded the yellow phosphinyl radical P[N(SiMe3)2](NPri2) (3), which upon cooling reverted to 2; the latter in C6D6 at 298 K was a mixture of rac and meso diastereoisomers. The yellow, crystalline phosphenium salt [P[N(SiMe3)2](NPri2)][AlCl4] (4) was obtained from 1 and 1/2Al2Cl6 in CH2Cl2. By single-crystal X-ray diffraction (XRD) the structures of the known compound 1 and of 2 and 4 were determined. The structure of the radical 3, formed by the thermal homolytic dissociation of the diphosphine 2, was determined in the gas phase by electron diffraction (GED), utilising data from UMP2/6-31+G*ab initio calculations. The model of the molecule in the GED structure analysis was described by a set of internal coordinates and an initial set of Cartesian coordinates from ab initio calculations, facilitating the structure analysis. The experimental data were found to be consistent with the presence of a single conformer of the radical in the gas phase. The computed standard homolytic dissociation enthalpy of the P-P bond in the corresponding diphosphine 2, corrected for BSSE, 54 kJ mol(-1), is substantially reduced compared to the dissociation enthalpy of tetramethyldiphosphine by the reorganisation energies of the fragments that form upon dissociation. The intrinsic energy content of the P-P bond in the diphosphine 2 was estimated to be 286 kJ mol(-1), in agreement with the results of previous work on a series of crowded diphosphines.

The molecular structures of the gaseous dimeric molecules Me2Ga(µ-H)2GaMe2 and Me2Ga(µ-Cl)2GaMe2 as determined by electron diffraction

The structures of gaseous dimethylgallane and dimethylgallium chloride have been determined by electron diffraction. The results indicate that the predominant vapour species at low pressures and temperatures of 290–350 K are dimeric molecules with diborane-like structures, Me2Ga(µ-X)2GaMe2(where X = H or Cl), with heavy-atom skeletons conforming to D2h symmetry. Salient structural parameters in the ra structures are: (i) for [Me2GaH]2, r(Ga ⋯ Ga) 261.0(0.5), r(Ga–C) 195.4(0.4), and r(Ga–Hb) 170.8(1.4) pm; Ga–Hb–Ga 99.6(1.4) and C–Ga–C 123.2(1.5)°; (ii) for [Me2GaCl]2, r(Ga ⋯ Ga) 330.3(1.9), r(Ga–C) 194.6(0.3), and r(Ga–Clb) 237.8(0.4) pm; Ga–Clb–Ga 88.0(0.9) and C–Ga–C 132.1 (2.7)°(‘b’ denotes a bridging atom). Dimethylgallane thus represents the first gallium hydride containing a Ga(µ-H)2Ga bridging unit to be characterised structurally; it is notable for the shortness of the Ga[graphic omitted]Ga distance. The two molecules invite structural comparisons with related systems like [Me2EH]2(E = B or Al), [Me2AlCl]2, and Ga2Cl6.

Determination of the molecular structure of tris(trimethylsilyl)phosphine in the gas phase by electron diffraction, supported by molecular mechanics calculations

Abstract The molecular structure of tris(trimethylsilyl)phosphine in the gas phase has been determined by electron diffraction. Results of three refinements are reported. In the first refinement, overall C 3 symmetry and local C 3 symmetry for the SiMe 3 groups were assumed, but these groups were allowed to tilt away from one another so that their C 3 local axes no longer coincided with the PSi bonds. The major geometrical parameters were r a (PSi) 225.9 (1), r (SiC) 188.2(1) pm, ∠ SiPSi 105.1(2) and ∠ CSiC 107.9(1)°. The tilt angle was 5.9(3)°, and the SiMe 3 groups were twisted 17.2(2)° away from the staggered position, at which the overall molecular symmetry was C 3v ; the PSiC angles were therefore 106.8, 109.3 and 116.7°. In the other two refinements the restriction of C 3 symmetry within the SiMe 3 groups was removed, and the differences between SiC distances and between CSiC angles were first fixed at values calculated by molecular mechanics (MM2), and then these new restrictions were partially relaxed. Neither of these refinements fitted the experimental data quite as well as the first one.

The molecular structures of pentaborane(9) and pentaborane(11) in the gas phase as determined by electron diffraction

Abstract The structure of gaseous arachno-B5H11 has been redetermined by electron diffraction and shown to be similar to that found in the solid state at low temperature (93 K) except that the inner basal interatomic distance B(3)B(4) appears to be somewhat shorter in the gas phase. The data are consistent with the presence of asymmetric B(2)H(2,3)B(3) and B(5)H(4,5)B(4) bridges with the two halves of each bridge differing in length by ca. 12 pm. The unique endo/face-capping H atom attached to the apical B(1) atom has not been located with high precision, but the best fit to the data is obtained for an asymmetric structure with the distances B(2) … H(1)endo and B(5) … H(1)endo differing by 31 pm. For comparison, the structure of nido-B5H9 has also been redetermined by electron diffraction. The interatomic distances are in excellent agreement with those previously obtained from microwave data. The directly-bonded BH(bridge) distances reveal an unusually large amplitude of vibration of the bridging H atoms but it was not possible to establish whether this was a real effect or whether the structure has a lower symmetry than that expected.

An electron diffraction, ab initio and vibrational spectroscopic study of 1,2-di-tert-butyldisilane

Abstract The molecular structure of 1,2-di-tert-butyldisilane has been accurately determined by gas-phase electron diffraction (GED) and ab initio calculations. These techniques show that the large majority of molecules at room temperature have the anti conformation with overall symmetry C2, and vibrational spectra confirm this conclusion. Infrared spectra of the gas and liquid phases, and Raman spectra of the liquid and solid phases, have been recorded for (CH3)3CSiH2SiH2C(CH3)3 and (CH3)3CSiD2SiD2C(CH3)3. The most striking feature of this structure (ra) is a relatively large deviation of the SiSiC angle from the parent tetrahedral angle 109.5° (113.7(3)°, GED; 114.4°, SCF 6-31 G ∗ as calculated for the anti form). That the SiSi bond length does not show any substantial deviation from its usual value (234.8(3) pm, GED; 236.8 pm, SCF 6-31 G ∗ computed for the anti form) is also substantiated by the value of the SiSi valence force constant (169 N m−1) given by normal coordinate analysis. The t-butyl groups are tilted so that the SiC bonds (GED ( SCF 6-31 G ∗ ): 190.1(1) (191.9) pm) do not coincide with the local C3 axes of the C(CH3)3 groups in which the CC bond length is 154.1(1) (GED); 154.0 ( SCF 6-31 G ∗ ) pm. The conformations along all the single bonds are more or less staggered.

New chemistry of 1,2-closo-P2B10H10 and 1,2-closo-As2B10H10; in silico and gas electron diffraction structures, and new metalladiphospha- and metalladiarsaboranes

The molecular structures of 1,2-closo-P(2)B(10)H(10) (1) and 1,2-closo-As(2)B(10)H(10) (2) have been determined by gas electron diffraction and the results obtained compared with those from computation at the MP2/6-31G** level of theory. The level of agreement is good for 2 (root-mean-square [rms] misfit for As and B atoms 0.0297 Å) and very good for 1 (rms misfit for P and B atoms 0.0082 Å). In comparing the structures of 1 and 2 with that of 1,2-closo-C(2)B(10)H(12) (I) it is evident that expansion of the polyhedron from I to 1 to 2 is restricted only to the heteroatom vertices and the B(6) face to which these are bound. Following deboronation (at B3) and subsequent metallation, compounds 1 and 2 have been converted into the new metalladiheteroboranes 3-(η-C(9)H(7))-3,1,2-closo-CoAs(2)B(9)H(9) (4), 3-(η-C(10)H(14))-3,1,2-closo-RuAs(2)B(9)H(9) (5), 3-(η-C(5)H(5))-3,1,2-closo-CoP(2)B(9)H(9) (6), 3-(η-C(9)H(7))-3,1,2-closo-CoP(2)B(9)H(9) (7) and 3-(η-C(10)H(14))-3,1,2-closo-RuP(2)B(9)H(9) (8), the last three constituting the first examples of metalladiphosphaboranes. Together with the known compound 3-(η-C(5)H(5))-3,1,2-closo-CoAs(2)B(9)H(9) (3), compounds 4-8 have been analysed by NMR spectroscopy and (except for 8) single-crystal X-ray diffraction. The (11)B NMR spectra of analogous pairs of metalladiphosphaborane and metalladiarsaborane (6 and 3, 7 and 4, 8 and 5) reveal a consistently narrower (9-10 ppm) chemical shift range for the metalladiarsaboranes, the combined result of a deshielding of the lowest frequency resonance (B6) and an increased shielding of the highest frequency resonance (B8) via an antipodal effect. In crystallographic studies, compounds 3 and 5B (one of two crystallographically-independent molecules) suffer As/B disorder, but in both cases it was possible to refine distinct, ordered, components of the disorder, the first time this has been reported for metalladiarsaboranes. Moreover, whilst the Cp compounds 6 and 3 are disordered, their indenyl analogues 7 and 4 are either ordered or significantly less disordered, a consequence of both the reduced symmetry of an indenyl ligand compared to a Cp ligand and the preference of the former for a distinct conformation relative to the cage heteroatoms. Unexpectedly, whilst this conformation in the cobaltadiphosphaborane 7 is cis-staggered (similar to that previously established for the analogous cobaltadicarborane), in the cobaltadiarsaborane 4 the conformation is close to cis-eclipsed.

A conformational and vibrational study of CF3COSCH2CH3

The molecular structure and conformational properties of S-ethyl trifluorothioacetate, CF(3)COSCH(2)CH(3), were determined in the gas phase by electron diffraction and vibrational spectroscopy (IR and Raman). The experimental investigations were supplemented by ab initio (Moller Plesset of second order) and density functional theory quantum chemical calculations at different levels of theory. Both experimental and theoretical methods reveal two structures with C(s) (anti, anti) and C(1) (anti, gauche) symmetries, although there are disagreements about which is more stable. The electron diffraction intensities are best interpreted with a mixture of 51(3)% anti, anti and 49(3)% anti, gauche conformers. This conformational preference was studied using the total energy scheme and the natural bond orbital scheme. In addition, the infrared spectra of CF(3)COSCH(2)CH(3) are reported for the gas, liquid and solid phases as well as the Raman spectrum of the liquid. Using calculated frequencies as a guide, evidence for both C(s) and C(1) structures is obtained in the IR spectra. Harmonic vibrational frequencies and scaled force fields have been calculated for both conformers.

Monochlorogallane: synthesis, properties, and structure of the dimer H2Ga(µ-Cl)2GaH2 in the gas phase as determined by electron diffraction

Monochlorogallane, synthesised by metathesis involving gallium(III) chloride and trimethylsilane, has been characterised by its spectroscopic and chemical properties; electron diffraction has established the structure of the dimer H2Ga(µ-Cl)2GaH2, the predominant vapour species at low pressure.