BllHI4in aqueous sodium hydroxide reacts with oxygen to give OBI1H12-, the first example of an oxaborane species where oxygen is part of the boron-hydrogen cluster.
Improved synthesis of OB11H12" by oxydation of B11H14" with aqueous NaOH is described. Thus, [NEt4] is recovered with moderate yield (24%). Using NBu4Br (instead of NEt4Br) for the metathesis reaction results in a much lower yield. AM1 calculations were performed with the 12-vertex 28-electron nido-oxaborane anion OB11H12"· A good agreement was obtained for the bond lengths and geometry of this OBnH-12" anion and of closely related isoelectronic clusters. From the calculated charge densities performed on the anion the negative charge is not located on the oxygen atom, but instead dissipated through the skeleton, thus making the heteroatom more electropositive. Similar calculations were also performed with the unknown isoelectronic nido 12-vertex 28-electron XB11H12 species (X=CH 2 ", NH", F) and with the icosahedral closo 12-vertex 26-electron species YB11H11 (Y=NH, O, F + ). In every case, the oxygen-derivative is predicted to be the most stable, the closo OBnHn being even more stable than the known NB11H12. Experimental Computational detailsThe structures and heats of formation were obtained from the AM1 semi-empirical SCF-MO method at the restricted Hartree-Fock (RHF) level [10a]. Heats of formation AHf were calculated at OK and were obtained [10b] from the total energy by substracting the electronic energies and adding the experimental heats of formation of the atoms, i.e. for a molecule : 247 Brought to you by | Purdue University Libraries Authenticated Download Date | 5/30/15 8:41 AM Vol. 20, No. 4, 1997 New Results About the Oxaborane Anion OB/; Ηj 2 ΔΗΓ 1 =Ε ™ο1_ ΣΕ Α +ΣΔ Α A A The ions can be handled by the same formula, the total-energy term taking the charge into account. The AM1 calculations were carried out using the AMPAC [11] package. All geometric parameters were obtained without symmetry constraints by a derivative optimization method, based on the Davidon-Fletcher-Powell (DFP) algorithm [12,13]. While the basic AM1 parameters were used, the additional unique Gaussian type function in the core-core repulsion term (CRT) [10a] for boron-hydrogen, boron-oxygen, boron-nitrogen and boron-fluorine bonds [14] were also included in the calculation. The object of the additional gaussian expression was to improve the character of the potential energy function between two atoms at short distances [14]. Spectroscopic characterizationsNMR spectra were recorded at 300K on a Bruker DRX500 spectrometer operating at 500 MHz for proton, using CDCI3 as a solvent, TMS as a reference. 11 Β spectra were collected at 160.45 MHz with a 2 H internal lock using CH2CI2/CD2CI2 (1:1) as a solvent and BF3.Et20 as a reference. Two-dimensional 11 B-11 B spectra were recorded using the standard Bruker Library program for sensitive heteronuclei (cosydc). The following acquisition parameters were applied : 1s for relaxation delay, SW= 4500Hz in both domains. F2 domain : TD=1024 (16 transients) with no zero-filling, F1 domain : 512 with one level zero-filling, and a final resolution in both domains of 4.4Hz. Exponential multipl...
B 3 H 7 XH 3 and H 3 BXH 3 (X ) N, P, and As) have been studied as donor-acceptor complex type at the G2-(MP2) level of theory. Both single-and double-bridged structures of B 3 H 7 Lewis acid are taken into account. Although the double-bridged structure is energetically favored in the isolated state, the coordination prefers the single-bridged one. The monoborane adducts adopt the staggered arrangement with C 3V symmetry. The energetic analysis by natural bond orbital theory shows that the decrease of hyperconjugative contribution upon complexation in the B 3 H 7 moiety has, as consequence, a loss of symmetry for B 3 H 7 XH 3 (X) P and As) complexes. In the two series, the coordination is the result of two opposing interactions. The first one is stabilizing, developed between a′ symmetry fragment molecular orbitals, and is mainly of 3MO-4e nature. The second one is destabilizing and developed between occupied molecular orbitals (2MO-4e) of donor and acceptor fragments having a′′ symmetry. Furthermore, in the case of BH 3 complexes, the complexation is made along the C 3 axis that joined the two fragments, whereas for B 3 H 7 , it is mainly centered on the nonequivalent boron atom, but the two other ones are also weakly involved in the coordination through a suitable p orbital even if the donor fragment is not on the three boron triangle center.
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