Germanium and Tin Analogues of Alkynes and Their Reduction Products

The reduction of terphenylgermanium(II) or terphenyltin(II) chlorides with alkali metals was investigated. Treatment of Ar'GeCl or ArGeCl (Ar' = C(6)H(3)-2,6-Dipp(2), Dipp = C(6)H(3)-2,6-Pr(i)(2); Ar = C(6)H(3)-2,6-Trip(2), Trip = C(6)H(2)-2,4,6-Pr(i)(3)) with lithium, sodium, or potassium afforded the neutral alkyne analogues Ar'GeGeAr', 1, ArGeGeAr, 2, the singly reduced radical species NaArGeGeAr, 3, or KAr'GeGeAr', 4, or the doubly reduced compounds Li(2)Ar'GeGeAr', 5, Na(2)ArGeGeAr, 6, or K(2)ArGeGeAr, 7. Similarly, reduction of Ar'SnCl or ArSnCl afforded the neutral Ar'SnSnAr', 8, or ArSnSnAr, 9, the radical anions [(THF)(3)Na[rSnSnAr]], 10, [K(THF)(6)][Ar'SnSnAr'], 11, [K(THF)(6)][ArSnSnAr], 12, [K(18-crown-6)(THF)(2)] [ArSnSnAr], 13, or the doubly reduced Na(2)ArSnSnAr, 14, K(2)Ar'SnSnAr', 15, or K(2)ArSnSnAr, 16. The compounds were characterized by UV-vis, (1)H and (13)C NMR or EPR spectroscopy. The X-ray crystal structures of all compounds were determined except those of 2 and 9. The neutral 1 and 8 displayed planar, trans-bent CMMC (M = Ge and Sn) cores with M-M-C angles of 128.67(8) and 125.24(7) degrees, respectively. The M-M bond lengths, 2.2850(6) and 2.6675(4)A, indicated considerable multiple character and a bond order approaching two. Single and double reduction of the neutral species resulted in the narrowing of the M-M-C angles by ca. 12-32 degrees and changes in the Ge-Ge and Sn-Sn bond lengths. One-electron reduction afforded a slight (ca. 0.03-0.05A) lengthening of the Ge-Ge bonds in the case of germanium species 3 and 4 and a greater lengthening (ca. 0.13-0.15A) for the Sn-Sn bonds in the tin compounds 10-13. The addition of another electron yielded salts of the formal dianions [Ar'MMAr'](2)(-) and [ArMMAr](2)(-) which are isoelectronic to the corresponding doubly bonded, neutral arsenic and antimony derivatives. All the dianion salts were obtained as contact ion triples with two alkali metal cations complexed between aryl rings. The Ge-Ge bonds in the dianions of 5-7 were longer, whereas the Sn-Sn distances in the dianions in 14, 15, and 16 were shorter than those in the monoanions. Unusually, the Li(2)Ar'GeGeAr' salt, 5, displayed a longer Ge-Ge bond (by ca. 0.06A) than those of its Na(+) or K(+) analogue salts which was attributed to the greater polarizing power of Li(+). It was concluded that the M-M bond lengths in 3-7 and 10-16 are dependent on several factors that include M-M-C angle, Coulombic repulsion, alkali metal cation size, and the character of the molecular energy levels. The M-M bonding in the neutral compounds was accounted for in terms of a second-order Jahn-Teller mixing of sigma- and a pi-orbital which afforded bond orders near two for the neutral compounds, 1, 2, 8, and 9. Calculations on MeMMMe (M = Ge or Sn) model species showed that the LUMO corresponded to an orbital that had n(+) lone pair character. The slight Ge-Ge bond length increase upon one-electron reduction is consistent with these results, and the further bond lengthening upon double reduction is consistent with increased Coulombic repulsion. The greater Sn-Sn bond length increase seen for one-electron reduction of the tin species is probably due to the increased p-character of orbitals comprising the Sn-Sn sigma-bond when the Sn-Sn-C angle is decreased by ca. 30 degrees. Upon further reduction, the slight decrease in the Sn-Sn bond is probably a result of the reduced importance of Coulombic repulsion due to the larger size of tin and a widening of the Sn-Sn-C angles which may shorten the Sn-Sn sigma-bond.