Through‐Bond Interactions in Silicon–Phosphorus and Silicon–Arsenic Compounds: A Facile Synthesis of Dodecamethyl‐2,3,5,6,7,8‐hexasila‐1λ3,4λ3‐diphosphabicyclo[2.2.2]octane, Its Arsenic Analogue, and Related Compounds
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E-Article
Title:
Through‐Bond Interactions in Silicon–Phosphorus and Silicon–Arsenic Compounds: A Facile Synthesis of Dodecamethyl‐2,3,5,6,7,8‐hexasila‐1λ3,4λ3‐diphosphabicyclo[2.2.2]octane, Its Arsenic Analogue, and Related Compounds
Description:
<jats:title>Abstract</jats:title><jats:p>Dodecamethyl‐2,3,5,6,7,8‐hexa‐sila‐lλ<jats:sup>3</jats:sup>,4λ<jats:sup>3</jats:sup>‐diphosphabicyclo[2.2.2]oc‐tane (<jats:bold>1</jats:bold>) and its arsenic analogue <jats:bold>2</jats:bold> are readily accessible in 69 and 73% yield, respectively, by the cyclocondensation reaction of 1,2‐dichloro‐1,1,2,2‐tetrame‐thyldisilane (<jats:bold>5</jats:bold>) with the lithium pnictides [LiEH<jats:sub>2</jats:sub>(dme)] (E = P (<jats:bold>6</jats:bold>), As(<jats:bold>7</jats:bold>); dme = 1,2‐dimethoxyethane). The reactions proceed via 1,4‐diphosphaoctamethyltetrasi‐lacyclohexane (<jats:bold>8</jats:bold>) and its arsenic analogue <jats:bold>9</jats:bold>, respectively, which were isolated and structurally characterized by X‐ray crystallography. The molecular structures of <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold>, which are isotypic, were also established by single‐crystal X‐ray analysis: they possess <jats:italic>D</jats:italic><jats:sub>3</jats:sub> 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 <jats:bold>8</jats:bold> and <jats:bold>9</jats:bold>, mainly due to through‐bond interactions (TB) between donating n orbitals of the E atoms and the σ<jats:sup>*</jats:sup> acceptor orbitals of the Si–Si bond. The first expanded analogues of <jats:bold>1</jats:bold>, namely, <jats:bold>12</jats:bold> and <jats:bold>14</jats:bold>, 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 <jats:bold>11</jats:bold> and <jats:bold>13</jats:bold>, respectively, with [LiPH<jats:sub>2</jats:sub>(dme)]<jats:bold>6</jats:bold>. Ab initio calculations on the parent compounds <jats:bold>1a, 12a</jats:bold>, and the second‐row analogue 1,4‐diazabicyclo‐[2.2.2]octane (<jats:bold>B</jats:bold>) 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 <jats:bold>B</jats:bold> were found to be about two times stronger than in the congener <jats:bold>1a</jats:bold>, due to the compactness of the N<jats:sub>2</jats:sub>C<jats:sub>6</jats:sub> skeleton and the greater extent of s, p hybridization at nitrogen. Evidence for TB interactions in <jats:bold>1</jats:bold> was obtained by photoelectron spectroscopy and by comparison of the two first vertical ionization potentials with calculated values for <jats:bold>1a</jats:bold>. The best agreement with experimental data was achieved when <jats:bold>1a</jats:bold> was calculated at the MP2 level. Compound <jats:bold>1a</jats:bold> preferentially adopts <jats:italic>D</jats:italic><jats:sub>3</jats:sub> point symmetry; the higher‐symmetry <jats:italic>D</jats:italic><jats:sub>3h</jats:sub> form possesses one imaginary frequency and is slightly less stable (0.46 kcal mol<jats:sup>−1</jats:sup> at HF/6–31 G<jats:sup>*</jats:sup>//HF/6–31 G<jats:sup>*</jats:sup> and 1.58 kcal mol<jats:sup>−1</jats:sup> at MP2/ 6–31 G<jats:sup>*</jats:sup>//HF/6–31 G<jats:sup>*</jats:sup> level), suggesting that this structure corresponds to a transition state on the potential energy surface. The structures corresponding to the global minimum of <jats:bold>B</jats:bold> and <jats:bold>12a</jats:bold> have <jats:italic>D</jats:italic><jats:sub>3h</jats:sub> and <jats:italic>C</jats:italic><jats:sub>3h</jats:sub> symmetry, respectively. At the HF/6–31 G<jats:sup>*</jats:sup>//HF/6–31 G<jats:sup>*</jats:sup> level the <jats:italic>D</jats:italic><jats:sub>3h</jats:sub> form of <jats:bold>12a</jats:bold> is 17.61 kcal mol<jats:sup>−1</jats:sup> less stable than the <jats:italic>C</jats:italic><jats:sub>3h</jats:sub> minimum.</jats:p>