Characterization of the Sterically Encumbered Terphenyl-Substituted Species 2,6-Trip<sub>2</sub>H<sub>3</sub>C<sub>6</sub>S̈n−Sn(Me)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>-2,6-Trip<sub>2</sub>, an Unsymmetric, Group 14 Element, Methylmethylene, Valence Isomer of an Alkene, Its Related Lithium Derivative 2,6-Trip<sub>2</sub>H<sub>3</sub>C<sub>6</sub>(Me)<sub>2</sub>Sn−Sn(Li)(Me)C<sub>6</sub>H<sub>3</sub>-2,6-Trip<sub>2</sub>, and the Monomer Sn(<i>t</i>-Bu)C<sub>6</sub>H<sub>3</sub>-2,6-Trip<sub>2</sub> (Trip = C<sub>6</sub>H<sub>2</sub>-2,4,6-<i>i</i>-Pr<sub>3</sub>)
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The reaction of the recently reported sterically encumbered terphenyl tin(II) halide species Sn(Cl)C6H3-2,6-Trip2 (Trip = C6H2-2,4,6-i-Pr3), 1, with 1 equiv of MeLi or MeMgBr afforded 2,6-Trip2H3C6S̈n−Sn(Me)2C6H3-2,6-Trip2, 2, which is the first stable group 14 element methylmethylene (i.e., CH3CH) analogue of ethylene (H2CCH2). Reaction of 1 with 1.5 equiv of MeLi yielded the stannylstannate species 2,6-Trip2H3C6(Me)2Sn−Sn(Li)(Me)C6H3-2,6-Trip2, 3, whereas reaction of 1 with 1 equiv of t-BuLi gave the heteroleptic stannanediyl monomer Sn(t-Bu)C6H3-2,6-Trip2 (4). The compounds 2−4 were characterized by 1H, 13C (7Li, 3 only), and 119Sn NMR spectroscopy in solution and by UV−vis spectroscopy. The X-ray crystal structures of 2−4 were also determined. The formation of the stannylstannanediyl 2 instead of the expected symmetrical, valence isomer “distannene” form {Sn(Me)C6H3-2,6-Trip2}2, 6, is explained through the ready formation of LiSn(Me)2C6H3-2,6-Trip2, 5, which reacts rapidly with 1 to produce 2 which can then react with a further equivalent of MeLi to give 3. The stability of singly bonded 2 in relation to the formally doubly bonded 6 was rationalized on the basis of the difference in the strength of their tin−tin bonds. In contrast to the methyl derivatives, the reaction of 1 with t-BuLi proceeded smoothly to give the monomeric compound 4. Apparently, the formation of a t-Bu analogue of 5 was prevented by the more crowding t-Bu group. Compound 2 is also the first example of a stable molecule with bonding between a two-coordinate, bivalent tin and four-coordinate tetravalent tin. Both compounds 2 and 3 display large J 119Sn−119Sn couplings between their tin nuclei and the tin−tin bond lengths in 2 (2.8909(2) Å) and 3 (2.8508(4) Å) are relatively normal despite the presence of the sterically crowding terphenyl substituents.
