THROUGH-BOND INTERACTIONS IN SILICON-PHOSPHORUS AND SILICON-ARSENIC COMPOUNDS - A FACILE SYNTHESIS OF AMBDA(3),4-LAMBDA(3)-DIPHOSPHABICYLO[2.2.2]OCTANE, ITS ARSENIC ANALOG, AND RELATED-COMPOUNDS

Dodecamethyl-2,3,5,6,7,8-hexasila-1 lambda(3),4 lambda(3)-diphosphabic yclo[2.2.2]octane (1) and its arsenic analogue 2 are readily accessibl e in 69 and 73% yield, respectively, by the cyclocondensation reaction of 1.2-dichloro-1,1,2,2-tetramethyldisilane (5) with the lithium pnic tides [LiEH2(dme)] (E = P (6), As(7); dme = 1,2-dimethoxyethane). The reactions proceed via 1,4-diphosphaoctamethyltetrasilacyclohexane (8) and its arsenic analogue 9, respectively, which were isolated and stru cturally characterized by X-ray crystallography. The molecular structu res of 1 and 2, which are isotypic. were also established by single-cr ystal X-ray analysis: they possess D-3 point symmetry with the expecte d SI-E bond lengths (E = P, As) but unusually long Si-Si bonds. The la tter are 0.02-0.03 Angstrom longer than those in 8 and 9: mainly due t o through-bond interactions (TB) between donating n orbitals of the E atoms and the sigma acceptor orbitals of the Si-Si bond. The first ex panded analogues of 1, namely, 12 and 14, with hexamethyltrisilane and dodecamethylhexasilane chains bridging the two phosphorus atoms, were synthesized in a one-pot cyclocondensation reaction of the correspond ing 1,3- and 1,6-dichlorooligosilanes 11 and 13, respectively, with [L iPH2(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 car ried out in order to analyze the different coupling constants and magn itudes of intramolecular interactions (through-space/through-bond coup ling). TS and TB coupling in a were found to be about two times strong er than in the congener 1 a, due to the compactness of the N2C6 skelet on and the greater extent of s,P hybridization at nitrogen. Evidence f or TB interactions in 1 was obtained by photoelectron spectroscopy and by comparison of the two first vertical ionization potentials with ca lculated values for ta. The best agreement with experimental data was achieved when 1 a was calculated at the MP2 level. Compound la prefere ntially adopts D-3 point symmetry; the higher-symmetry D-3h form posse sses one imaginary frequency and is slightly less stable (0.46 kcal mo l(-1) at HF/6-31G// HF/6-31G* and 1.58 kcal mol(-1) at MP 2/ 6-31G*// HF/6-31G level), suggesting that this structure corresponds to a tran sition state on the potential energy surface. The structures correspon ding to the global minimum of a and 12a have D-3h and C-3h symmetry, r espectively. At the PH/G-31G//HF/6-31G* level the D-3h form of 12a is 17.61 kcal mol(-1) less stable than the C-3h minimum.