A New Conformation With an Extraordinarily Long, 3.04 Å Two‐Electron, Six‐Center Bond Observed for the π‐[TCNE]₂²⁻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.

Cation Dependence of the Dimerization Enthalpy for A2[tetracyanoethylene]2 (A=NMe4, Mepy, NEt4) Possessing a Long, Multicenter Bond

“…[NMe 4 ] 2 [TCNE] 2 , [Mepy] 2 [TCNE] 2 , and [NEt 4 ] 2 [TCNE] 2 were synthesized as previously reported. 2‐Methyltetrahydrofuran (MeTHF) was purchased from Alfa Aesar, distilled over sodium benzophenone under nitrogen, and placed over 3 Å molecular sieves for ≥24 h and stored in an oxygen free environment .…”

“…However, these values could not be quantitatively determined by EPR measurements due to apparent temperature‐dependent hysteresis . Thus, a reinvestigation of these thermodynamic parameters was initiated and reported herein for [NEt 4 ] 2 [TCNE] 2 and [Mepy] 2 [TCNE] 2 (Mepy= N ‐methylpyridinium, H 3 CNC 5 H 5 + ) possessing D 2 h π–[TCNE] 2 2− , and compared to [NMe 4 ] 2 [TCNE] 2 possessing C 2 π–[TCNE] 2 2− in the solid state. …”

“…Recently, however, a new, slipped, C 2 conformation of the π–[TCNE] 2 2− dimer with a 3.04 Å, 2e − /6c C−C (4 C+2 N atoms contributing) bond has been reported for [NMe 4 ] 2 [TCNE] 2 , Figure b . The C 2 π–[TCNE] 2 2− conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J / k B =1000 K (700 cm −1 ; 86.8 meV; 2.00 kcal mol −1 ; H =−2 J S a ⋅S b ) above the ground state, and it is computed to be 1.3 kcal mol −1 less stable than the D 2 h conformation …”

“… Structure (50 % electron density ORTEP) showing the intradimer bonding components and HOMO for D 2 h π–[TCNE] 2 2− possessing a 2.9 Å, 2e − /4c C−C bond (a), and C 2 π–[TCNE] 2 2− possessing a 3.04 Å 2e − /6c C−C (4 C+2 N atoms) bond (b) (adapted from Ref. ).…”

Solid‐State 13C NMR Evidence for Long Multicenter Intradimer Bonding in Zwitterion‐like Structures

“…[10,13,14] Additionally,t here are thirty reported cases of the p-[TCNE] 2 2À dimer with various cations havinga verage intra-dimers eparations of 2.895 AE 0.047 . [10] Other examples of long bonds include 2e À /6c bonds in [NMe 4 ] 2 [TCNE] 2 , [15] 2e À /8c bondsi n[ tetracyanopyrazene] 2 2À , [16] 2e À /10c bonds in [tetracyanobenzoquinone] 2 2À , [17] and 2e À /5c bonds in a, b and d- [TTF···TCNE]. [12] Both p-[TTF] 2 2 + [9] and p-[TCNE] 2 2À [11,12,18] have been studied by av ariety of methods including solid-state NMR (SSNMR).…”