title: A New Conformation With an Extraordinarily Long, 3.04 Å Two-Electron, Six-Center Bond Observed for the π-[TCNE]2 (2-) Dimer in [NMe4 ]2 [TCNE]2 (TCNE=Tetracyanoethylene).
abstract: [NMe4 ]2 [TCNE]2 (TCNE=tetracyanoethenide) formed from the reaction of TCNE and (NMe4 )CN in MeCN has νCN IR absorptions at 2195, 2191, 2172, and 2156 cm(-1) and a νCC absorption at 1383 cm(-1) that are characteristic of reduced TCNE. The TCNEs have an average central CC distance of 1.423 Å that is also characteristic of reduced TCNE. The reduced TCNE forms a previously unknown non-eclipsed, centrosymmetric π-[TCNE]2 (2-) dimer with nominal C2 symmetry, 12 sub van der Waals interatomic contacts <3.3 Å, a central intradimer separation of 3.039(3) Å, and comparable intradimer C⋅⋅⋅N distances of 3.050(3) and 2.984(3) Å. The two pairs of central C⋅⋅⋅C atoms form a ∢C-C⋅⋅⋅C-C of 112.6° that is substantially greater than the 0° observed for the eclipsed D2h π-[TCNE]2 (2-) dimer possessing a two-electron, four-center (2e(-) /4c) bond with two C⋅⋅⋅C 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, C2 π-[TCNE]2 (2-) dimer exhibits a new type of a long, intradimer bond involving one strong C⋅⋅⋅C and two weak C⋅⋅⋅N components, that is, a 2e(-) /6c bond. The C2 π-[TCNE]2 (2-) conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J/kB =1000 K (700 cm(-1) ; 86.8 meV; 2.00 kcal mol(-1) ; H=-2 JSa ⋅Sb ); at the CAS(2,2)/MBMP2 level the triplet is computed to be 9.0 kcal mol(-1) higher in energy than the closed-shell singlet ground state. The results from CAS(2,2)/NEVPT2/cc-pVTZ calculations indicate that the C2 and D2h conformers have two different local metastable minima with the C2 conformer being 1.3 kcal mol(-1) less stable. The different natures of the C2 and D2h conformers are also noted from the results of valence bond (VB) qualitative diagram that shows a 10e(-) /6c bond with one C⋅⋅⋅C and two C⋅⋅⋅N bonding components for the C2 conformer as compared to the 6e(-) /4c bond for the D2h conformer with two C⋅⋅⋅C bonding components.
authors.affiliation: Department of Chemistry, University of Utah, Salt Lake City, UT 84112-0850 (USA).
authors.forename: Adora G
authors.initials: AG
authors.lastname: Graham
authors.affiliation: Departamento de Química Física & IQTCUB, Facultad de Química, Universidad de Barcelona, Av. Diagonal, 645, 08028-Barcelona (Spain).
authors.forename: Fernando
authors.initials: F
authors.lastname: Mota
authors.affiliation: Department of Chemistry, University of Utah, Salt Lake City, UT 84112-0850 (USA).
authors.forename: Endrit
authors.initials: E
authors.lastname: Shurdha
authors.affiliation: Department of Chemistry and Biochemistry, University of California, San Diego, Urey Hall 5128,code 0358, 9500 Gilman Drive, La Jolla, CA 92093-0358 (USA).
authors.forename: Arnold L
authors.initials: AL
authors.lastname: Rheingold
authors.affiliation: Departamento de Química Física & IQTCUB, Facultad de Química, Universidad de Barcelona, Av. Diagonal, 645, 08028-Barcelona (Spain). juan.novoa@ub.edu.
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Novoa
authors.affiliation: Department of Chemistry, University of Utah, Salt Lake City, UT 84112-0850 (USA). jsmiller@chem.utah.edu.
authors.forename: Joel S
authors.initials: JS
authors.lastname: Miller
keywords: ab initio calculations; bond energy; cyanocarbon; electronic structure; long multicenter bond