A New Conformation With an Extraordinarily Long, 3.04 angstrom Two-Electron, Six-Center Bond Observed for the -[TCNE](2)(2-) Dimer in [NMe4](2)[TCNE](2) (TCNE=Tetracyanoethylene)

作者：Graham Adora G; Mota Fernando; Shurdha Endrit; Rheingold Arnold L; Novoa Juan J^*; Miller Joel S

来源：Chemistry - A European Journal, 2015, 21(38): 13240-13245.

DOI：10.1002/chem.201501963

• ab initio calculations
• bond energy
• cyanocarbon
• electronic structure
• long multicenter bond

[NMe4](2)[TCNE](2) (TCNE=tetracyanoethenide) formed from the reaction of TCNE and (NMe4)CN in MeCN has (CN) IR absorptions at 2195, 2191, 2172, and 2156cm(-1) and a (CC) absorption at 1383cm(-1) that are characteristic of reduced TCNE. The TCNEs have an average central CC distance of 1.423 angstrom that is also characteristic of reduced TCNE. The reduced TCNE forms a previously unknown non-eclipsed, centrosymmetric -[TCNE](2)(2-) dimer with nominal C-2 symmetry, 12 sub van der Waals interatomic contacts <3.3 angstrom, a central intradimer separation of 3.039(3)angstrom, and comparable intradimer CN distances of 3.050(3) and 2.984(3)angstrom. The two pairs of central CC atoms form a ?CCCC of 112.6 degrees that is substantially greater than the 0 degrees observed for the eclipsed D-2h -[TCNE](2)(2-) dimer possessing a two-electron, four-center (2e(-)/4c) bond with two CC components from a molecular orbital (MO) analysis. A MO study combining CAS(2,2)/MRMP2/cc-pVTZ and atoms-in-molecules (AIM) calculations indicates that the non-eclipsed, C-2 -[TCNE](2)(2-) dimer exhibits a new type of a long, intradimer bond involving one strong CC and two weak CN components, that is, a 2e(-)/6c bond. The C-2 -[TCNE](2)(2-) conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J/k(B)=1000K (700cm(-1); 86.8meV; 2.00kcalmol(-1); H=-2JS(a)S(b)); at the CAS(2,2)/MBMP2 level the triplet is computed to be 9.0kcalmol(-1) higher in energy than the closed-shell singlet ground state. The results from CAS(2,2)/NEVPT2/cc-pVTZ calculations indicate that the C-2 and D-2h conformers have two different local metastable minima with the C-2 conformer being 1.3kcalmol(-1) less stable. The different natures of the C-2 and D-2h conformers are also noted from the results of valence bond (VB) qualitative diagram that shows a 10e(-)/6c bond with one CC and two CN bonding components for the C-2 conformer as compared to the 6e(-)/4c bond for the D-2h conformer with two CC bonding components.

• 出版日期2015-9-14