ab initio calculations
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Dodecamethyl-2,3,5,6,7,8-hexa-sila-lλ3,4λ3-diphosphabicyclo[2.2.2]oc-tane (1) and its arsenic analogue 2 are readily accessible in 69 and 73% yield, respectively, by the cyclocondensation reaction of 1,2-dichloro-1,1,2,2-tetrame-thyldisilane (5) with the lithium pnictides [LiEH2(dme)] (E = P (6), As(7); dme = 1,2-dimethoxyethane). The reactions proceed via 1,4-diphosphaoctamethyltetrasi-lacyclohexane (8) and its arsenic analogue 9, respectively, which were isolated and structurally characterized by X-ray crystallography. The molecular structures of 1 and 2, which are isotypic, were also established by single-crystal X-ray analysis: they possess D3 point symmetry with the expected Si-E bond lengths (E = P, As) but unusually long Si-Si bonds. The latter are 0.02-0.03 Å longer than those in 8 and 9, mainly due to through-bond interactions (TB) between donating n orbitals of the E atoms and the σ* acceptor orbitals of the Si-Si bond. The first expanded analogues of 1, namely, 12 and 14, with hexamethyltrisilane and dodecamethyl-hexasilane chains bridging the two phosphorus atoms, were synthesized in a onepot cyclocondensation reaction of the corresponding 1,3- and 1,6-dichloro-oligosilanes 11 and 13, respectively, with [LiPH2(dme)]6. Ab initio calculations on the parent compounds 1a, 12a, and the second-row analogue 1,4-diazabicyclo-[2.2.2]octane (B) were carried out in order to analyze the different coupling constants and magnitudes of intramolecular interactions (through-space/through-bond coupling). TS and TB coupling in B were found to be about two times stronger than in the congener 1a, due to the compactness of the N2C6 skeleton and the greater extent of s, p hybridization at nitrogen. Evidence for TB interactions in 1 was obtained by photoelectron spectroscopy and by comparison of the two first vertical ionization potentials with calculated values for 1a. The best agreement with experimental data was achieved when 1a was calculated at the MP2 level. Compound 1a preferentially adopts D3 point symmetry; the higher-symmetry D3h form possesses one imaginary frequency and is slightly less stable (0.46 kcal mol-1 at HF/6-31 G*//HF/6-31 G* and 1.58 kcal mol-1 at MP2/ 6-31 G*//HF/6-31 G* level), suggesting that this structure corresponds to a transition state on the potential energy surface. The structures corresponding to the global minimum of B and 12a have D3h and C3h symmetry, respectively. At the HF/6-31 G*//HF/6-31 G* level the D3h form of 12a is 17.61 kcal mol-1 less stable than the C3h minimum.
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