Through‐Bond Interactions in Silicon–Phosphorus and Silicon–Arsenic Compounds: A Facile Synthesis of Dodecamethyl‐2,3,5,6,7,8‐hexasila‐1λ 3 ,4λ 3 ‐diphosphabicyclo[2.2.2]octane, Its Arsenic Analogue, and Related Compounds

Dodecamethyl‐2,3,5,6,7,8‐hexa‐sila‐lλ 3 ,4λ 3 ‐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 [LiEH 2 (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 D 3 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 [LiPH 2 (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 N 2 C 6 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 D 3 point symmetry; the higher‐symmetry D 3h 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 D 3h and C 3h symmetry, respectively. At the HF/6–31 G * //HF/6–31 G * level the D 3h form of 12a is 17.61 kcal mol −1 less stable than the C 3h minimum.