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Exceptionally Long (≥2.9 Å) CC Bonding Interactions in π‐[TCNE] 2 2− Dimers: Two‐Electron Four‐Center Cation‐Mediated CC Bonding Interactions Involving π* Electrons
Author(s) -
Del Sesto Rico E.,
Miller Joel S.,
Lafuente Pilar,
Novoa Juan J.
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20021104)8:21<4894::aid-chem4894>3.0.co;2-2
Subject(s) - tetracyanoethylene , dimer , crystallography , chemistry , dihedral angle , singlet state , stereochemistry , center (category theory) , ab initio , moiety , molecule , hydrogen bond , excited state , physics , photochemistry , atomic physics , organic chemistry
Three groups of singlet ground state [TCNE] 2 2− (TCNE=tetracyanoethylene) dimers with characteristic intradimer CC separations ( r ) and dihedral angles ( d ) [i.e., group S t ( r ∼1.6 Å; d =180°), L t ( r ∼3.5 Å; d =180°), and L c ( r ∼2.9 Å; d =∼0°); notation: S/L: short/long bond length; subscript t/c: trans / cis , respectively] are experimentally characterized. The S t group is comprised of σ‐dimers of [TCNE] .− and octacyanobutanediide, [C 4 (CN) 8 ] 2− , which have a typical, albeit long, sp 3 –sp 3 σ bond ( r ∼1.6 Å) between each [TCNE] .− moiety and characteristic ν CN , ν CC , and δ CCN IR absorptions. The L groups are structurally characterized as π‐dimers of [TCNE] .− that are either eclipsed with r ∼2.9 Å (L c ) and the nitriles bend away from the nominal TCNE plane away from the center of the dimer by 5.0° (∼sp 2.17 ) or are noneclipsed with r ∼3.5 Å (L t ) and the nitriles bend toward the center of the dimer by 1.9° (∼sp 2.06 ). Ab initio computations on isolated dimers were used to study the formation and stability of these exceptionally long CC (≥2.9 Å) bonding interactions as well as the process of π‐[TCNE] 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/open‐shell singlet, depending on the distance. The short S t group ( r ∼1.6 Å) 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 electrostatic repulsion. This cation‐mediated π*–π* [TCNE] − ⋅⋅⋅[TCNE] − interaction complies with Pauling's definition of a chemical bond. This bonding interaction involves the π* orbitals of each fragment, and arise from the overlap of the b 2g SOMO on each of the two [TCNE] .− s to form a filled b 2u [TCNE] 2 2− orbital. Although a π dimer typically forms, if the fragments are close enough a σ dimer can form. Due to the presence of cation‐mediated intradimer CC bonding interactions the L c group of π‐[TCNE] 2 2− dimers exhibits experimentally observable ν CN IR absorptions at 2191±2 (m), 2173±3 (s), and 2162±3 cm −1 (s) and ν CC at 1364±3 cm −1 (s) as well as a new UV‐Vis feature in the range of 15 000 to 18 200 cm −1 (549 to 667 nm) and averaging 16 825±1180 cm −1 (594 nm) assigned to the predicted new intradimer 1 A 1g → 1 B 1u transition and is purple on reflected light. Upon cooling to 77 K in 2‐methyl tetrahydrofuran, this new band occurs at 18 940 cm −1 (528 nm) for {[Et 4 N] + } 2 [TCNE] 2 2− , and the yellow solution turns deep red. Group L t is characterized by ν CN absorptions at 2215±2, 2197±3, and 2180±4 cm −1 and ν CC at 1209±9 cm −1 (w), while group S T has ν CN bands at 2215±4, 2157±3, and 2107±4 cm −1 and ν CC at 1385±1 cm −1 (vs).