Paul T. Brain

Synthesis and characterisation of pseudocloso iridium and ruthenium diphenyl carbaboranes. Molecular structures of 1,2-Ph2-3-(η-C6H6)-3,1,2-pseudocloso-RuC2B9H9 and 1,2-Ph2-3-(cym)-3,1,2-pseudocloso-RuC2B9H9(cym =p-cymene) and individual gauge for localised orbitals calculations on carbametallaboranes

The syntheses and characterisations of 1,2-Ph2-3-(η-C5Me5)-3,1,2-pseudocloso-IrC2B9H91, 1,2-Ph2-3-(η-C6H6)-3,1,2-pseudocloso-RuC2B9H92, 1,2-Ph2-3-(cym)-3,1,2-pseudocloso-RuC2B9H9(cym =p-cymene)3 and 1,2-Ph2-3-(η-C6Me6)-3,1,2-pseudocloso-RuC2B9H94 are reported from the reactions between Tl2[7,8-Ph2-nido-7,8-C2B9H9] and either [{IrCl2(C5Me5)}2] or the appropriate [{RuCl2(arene)}2] species. By 11B NMR spectroscopy all these compounds have pseudocloso geometries, in which the C(1)⋯ C(2) connectivity is broken and an approximately square M(3)C(1)B(6)C(2) face is generated. Crystallographic studies on 2 and 3 confirm this suggestion. It is suggested that distortion in these species arises from steric crowding between the Cphenyl substituents, forced to adopt conformations with high θ values by the presence of the η-bonded substituent at M(3). Individual gauge for localised orbitals calculations, reported for the first time on transition-metal heteroboranes, reasonably reproduce the (previously assigned)11B NMR chemical shifts of 3-(η-C5Me5)-3,1,2-closo-RhC2B9H11, and allow a tentative assignment of those of 1,2-Ph2-3-(η-C5Me5)-3,1,2-pseudocloso-RhC2B9H9.

Tetraborane(10), B4H10: structures in gaseous and crystalline phases

Supplementing gas-phase electron-diffraction data with restraints derived from a graded series of ab initio calculations makes possible refinement of all geometrical parameters and amplitudes of vibration. By avoiding the need to fix some parameters, this technique yields structures which are more completely refined and thus have more reliable standard deviations than procedures used previously. It has been applied to the gas-phase structure of the arachno boron hydride tetraborane(10), B4H10. Salient structural parameters (rα0 structure) were found to be: r[B(1)–B(2)] 186.6(2), r[B(1)–B(3)] 173.7(5), r[B(1)–H(1,2)] 123.0(15), r[B(2)–H(1,2)] 141.7(8), r[B(1)–H(1)] 119.8(8), r[B(2)–H(2)]endo] 121.0(8) and r[B(2)–H(2)exo] 120.5(8) pm; butterfly angle 117.2(4)°. The crystal structure was also redetermined at 100 K. All gas-phase, crystallographic and ab initio structural parameters were found to be in good agreement.

The molecular structures of pentaborane(11), B5H11, and hexaborane(12), B6H12, in the gas phase as determined by electron diffraction and ab initio calculations

Abstract The electron-diffraction patterns of gaseous arachno -B 5 H 11 and arachno -B 6 H 12 have been reanalysed. Inclusion of ab initio (MP2/6-31G*) computed bond-length differences in the refinements afforded new optimum geometries with improved R factors ( R G = 0.053 and 0.057, respectively) compared with the structures resported previously. In contrast to the latter, the new geometries cna be employed to calculate 11 B NMR chemical shifts (DZ//GED level) which are in good agreement with the experimental NMR data.

Differences Between Gas‐Phase and Solid‐State Molecular Structures of the Simplest Phosphonium Ylide, Me3P=CH2

Clearly different from local C3 symmetric is the heavy-atom core of Me3 P=CH2 , the simplest phosphonium ylide. The geometry obtained by reanalysis of gas-electron-diffraction data from 1977 is now consistent with theoretical calculations, but different from the molecular structure in the solid state. The picture shows the structure of Me3 P=CH2 in the gas phase (a) and in the crystal (c) together with the calculated transition state (b) (viewed along the P=C bond).

Molecular structure of gaseous 1,7-dichloro-1,7-dicarba-closo-dodecaborane(12), 1,7-Cl2-1,7-C2B10H10, as studied by electron diffraction and ab lnitio calculations

Gas-phase electron diffraction (GED) data for 1,7-dichloro-1,7-dicarba-closo-dodecaborane(12). 1,7-Cl2-1,7-C2B10H10, were fitted by a structure possessing overall C2v, symmetry. Not all of the independent parameters could be refined, and some differences between C–B and B–B bond lengths were fixed at values calculated ab initio.Assumption of local C5v symmetry for the CB5 pentagonal pyramids led to the same GED fit (RG= 0.065) as with a model in which the B5 rings adjacent to the carbon atoms were not quite regular, as found by ab initio optimisations [HF/6-31G* and MP2(fc)/6-31G*]. Nearest-neighbour C–B and B–B separations do not deviate significantly from normal values, and distortion of the cage from regular icosahedral symmetry is small. The C–Cl bond length, 175.9(9) pm, [MP2/6-31G* 174.9 pm], is shorter than in C(sp3)–Cl systems and longer than in C(sp2)–Cl systems. The geometrical parameters calculated at the MP2(fc)/6-31G* level, as well as 11B NMR chemical shifts, calculated by the individual gauge for localised orbitals method, are in satisfactory agreement with the experimental observations.

Gallaborane, H2Ga(µ-H)2BH2: synthesis, properties, and structure of the gaseous molecule as determined by electron diffraction

The novel hydride gallaborane, GaBH6, synthesised by metathesis involving monochlorogallane and lithium tetrahydridoborate, has been characterised by its spectroscopic and chemical properties; electron diffraction confirms that the gaseous molecule has a diborane-like structure, H2Ga(µ-H)2BH2.

2,4-Ethanotetraborane derivatives. 3.[1] determination of the molecular structure of 2,4-(t-butylethano)tetraborane(10), 2,4-(ButCHCH2)B4H8, in the gas phase by electron diffraction

Abstract The molecular structure of 2,4-( t -butylethano)tetraborane(10), 2,4-(Bu t CHCH 2 )B 4 H 8 , in the gas phase has been determined by analysis of electron diffraction data, restrained by MP 2 6–31 G ∗ ab initio calculations. The effect of the t -butyl group is mainly to twist the C-C bond of the C 2 B 4 ‘basket’ by 6.6(14)°, so that the local symmetry of the C 2 B 4 fragment is reduced from C 2v to C 2 . The concomitant distortion of the B 4 H 8 group from C 2v local symmetry is insignificant. Ab initio calculations at the same level are also reported for related compounds in which the basket ‘handle’ is trans -Bu t CHCHBu t , Me 3 SiCHCH 2 , trans -Me 3 SiCHCHSiMe 3 , cyclo -C 5 H 8 , cis -MeCHCHMe and Me 2 CCMe 2 , and for all possible compounds with one or two terminal hydrogen atoms of the C 2 H 4 B 4 H 8 basket replaced by ethyl groups.

The molecular structure of thiazole, determined by the combined analysis of gas-phase electron diffraction (GED) data and rotational constants and by ab initio calculations

The molecular structure of thiazole has been determined by the joint analysis of data obtained from gas-phase electron diffraction (GED), microwave (MW) spectroscopy and abinitio molecular orbital calculations. The combined approach, making use of the SARACEN method, has led to a very precise structure in which all independent geometric parameters are well defined, and the quoted uncertainties reflect the true accuracy of the measurements. The ground-state average (≡rα° or rz) structural parameters obtained in the combined study of thiazole were; r[S(1)–C(2)]=172.37(11) pm, r[S(1)–C(5)]=171.38(13) pm, r[C(2)–N(3)]=131.0(2) pm, r[C(4)–C(5)] pm=136.90(19) pm, r[N(3)–C(4)]=137.2(2) pm, r[C(2)–H(6)]=109.8(4) pm, r[C(4)–H(7)]=109.9(4) pm, r[C(5)–H(8)]=109.7(4) pm, C(2)–S(1)–C(5)=89.41(4)°, S(1)–C(2)–N(3)=115.16(6)°, S(1)–C(5)–C(4)=109.52(8)°, C(2)–N(3)–C(4)=109.97(9)°, N(3)–C(4)–C(5)=115.95(11)°, S(1)–C(2)–H(6)=120.7(2)°, C(5)–C(4)–H(7)=125.0(4)°, S(1)–C(5)–H(8)=121.7(2)°.

The molecular structures of gaseous tetrakis(dimethylamino)-diboron, B2(NMe2)4, and tetrakis(methoxy)diboron, B2(OMe)4, as determined by electron diffraction

The structures of gaseous B2(NMe2)4 and B2(OMe)4 have been determined by electron diffraction. The results indicate that the amine adopts the expected fully staggered conformation with D2 symmetry whilst, surprisingly, the methoxy derivative has not a planar heavy-atom skeleton as thought previously, but a partially staggered conformation. Salient structural parameters (ra) are (i) for B2(NMe2)4, r(B–B) 176.2(1.1) and r(B–N) 140.8(3) pm; NBBN 90.0(1.1), NBN 124.0(5) and BBNC 19.7(1.1)°; and (ii) for B2(OMe)4, r(B–B) 172.0(6) and r(B–O) 136.9(3) pm; OBBO 49.5(1.2), OBO 119.9(4) and BBOC 12.1(2.7) and 21.9(1.9)°(clockwise and anticlockwise out of the BBO plane).

Molecular structure of trimethylamine–gallane, Me3N·GaH3: ab initio calculations, gas-phase electron diffraction and single-crystal X-ray diffraction studies†

The structure of the gallane adduct Me3N·GaH3 has been investigated by ab initio quantum chemical calculations. The results of gas-phase electron-diffraction (GED) measurements, together with earlier microwave measurements, have been reanalysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rαo structure) were found to be: r(Ga–H) 151.1(13), r(Ga–N) 213.4(4), r(N–C) 147.6(3), r(C–H) 108.4(4) pm; H–Ga–N 99.3(8) and Ga–N–C 108.8(2)°. Unlike the corresponding alane derivative, the adduct is monomeric in the crystalline phase with dimensions very close to those of the gaseous molecule, as confirmed by a redetermination of the structure of a single crystal at 150 K.