Exceptionally Long (> = 29 A) CC Bonding Interactions in pi-[TCNE]_2~(2-) Dimers: Two-Electron Four-Center Cation-Mediated CC Bonding Interactions Involving pi~* Electrons

摘要

Three groups of singlet ground state [TCNE]_2~(2-) (TCNE = tetracyanoethylene) dimers with characterstic intradimer CC separations (r) and dihedral angles (d) [i.e., group S_t (r approx 1.6 A; d = 180 deg), L_t (r approx 3.5 A; d = 180 deg), and L_c (r approx 2.9 A; d = approx 0 deg); notation: S/L: short/long bond length; subscript t/c: trans/cis, respectively] are experimentally characterized. The S_t group is comprised of sigme-dimers of [TCNE]~(·-) and octacyanobutanediide, [C_4(CN)_8]~(2-), which have a typical, albeit long, sp~3-sp~3 sigma bond (r approx 1.6 A) between each [TCNE]~(·-) that are either eclipsed with r approx 2.9 A (L_c) and the nitriles bend away from the nominal TCNE plane away from the nominal TCNE plane away from the center of the dimer by 5.0 deg (approx sp~(2.17)) or are noneclipsed with r approx 3.5 A (L_t) and the nitriles bend toward the center of the dimer by 1.9 deg (approx 2.9~(2.06)). Ab initio computations on isolated dimers were used to study the formation and stability of these exceptionally long CC (> = 2.9 A) bonding interactions as well as the process of pi-[TNCE]_2~(2-) dimer formation for the L_c and L_t groups. The results of these computational studies show that the ground-state potential curve is that of a closed-shell/openh-shell singlet, depending on the distance. The short S_t group (r approx 1.6 A) of dimers in this surface are true minimum-energy structures; however, the L_t and L_c groups are unstable, although two different nonphysical minima are found when imposing a double occupancy of the orbitals. These minima are metastable relative to dissociation into the isolated [TCNE]~(·-) units. Consequently, the existence of dimer dianions in crystals is due to cation ···[TCNE]~- interactions, which provide the electrostatic stabilization necessary to overcome the intradimer electrostaic repulsion. This cation-mediated pi~*-pi~* [TCNE]~-··· [TCNE]~- interaction complies with Pauling's definition of a chemical bond. This bonding interaction involves the pi~* orbitals of each fragment, and arise from the overlap of the b_2g SOMO on each of the two [TCNE]~(·-) to form a filled b_2a [TCNE]_2~(2-) orbital. Although a pi dimer typically forms, if the fragments are close enough a sigma dimer can form. Due to the presence of cation-mediated intradimer CC bonding interactions the L_c group of pi-[TCNE]_2~(2-) dimers exhibits experimentally observable v_CN IR absorptions at 2191 (+-) 2 (m), 2173 (+-) 3 (s), and 2162 (+-) 3 cm~(-1) (s) and v_CC at 1364 (+-) 3 cm~(-1) (s) as well as a new UV-Vis feature in the range of 15000 to 18 200 cm~(-1) (549 to 667 nm) and averaging 16825 (+-) 1180 cm~(-1) (594 nm) assigned to the predicted new intradimer ~1A_1g -> ~1B_1u transition and is purple on reflected light. Upon cooling to 77 K in 2-methyl tetrahydrofuran, this new band occurs at 18940 cm~(-1) (528 nm) for {[Et_4N]~+}_2[TCNE]_2~(2-), and the yellow solution turns deep red. Group L_t is characterized by v_CN absorptions at 2215 (+-)2, 2197 (+-) 3, and 2180 (+-) 4 cm~(-1) and v_CC at 1209 (+-) 9 cm~(-1) (w), while group S_T and v_CN bands at 2215 (+-) 4, 2157 (+-)3, and 2107 (+-) 4 cm~(-1) and v_CC at 1385 (+-) 2 cm~(-1) (vs).