Solid‐State 13C NMR Evidence for Long Multicenter Intra‐dimer Bonding in Zwitterion‐like π‐[TTFδ+•••TCNEδ‐] (TTF = tetrathiafulvalene; TCNE = tetracyanoethylene)

The principal values of the (13) C chemical shift tensor for the beta and delta polymorphs of pi-[TTFTCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) have been analyzed to understand the abnormally long intra-dimer bonding of singlet pi-[TTF(delta+) TCNE(delta-) ]. These structures possess 12 intradimer contacts <3.40 A, with the shortest intra pi-[TTFTCNE] separations involving 2-center (2c) C-S and 3c C-C-C orbital overlap contributions between the [TTF](delta+) and [TCNE](delta-) . This solid-state NMR study compares the [TTFTCNE] (13) C tensor data against previously reported pi-[TTF]2 (2+) and pi-[TCNE]2 (2-) homo-dimers to determine how the tensor principal values change as a function of electronic structure for both TTF and TCNE moieties. In the beta and delta phases of [TTFTCNE], the TCNE ethylenic (13) C shift tensors predict TCNE oxidation states of -0.46 and -0.73, respectively. The TTF sites are less similar to benchmark (13) C data with the beta-phase differing primarily in the ethylenic pi-electrons. The delta form differs significantly from the homo-dimer data at all principal values at both the ethylenic and CH sites, indicating changes to both the pi-electrons and sigma-bonds. In both hetero-dimer phases, the NMR changes supports long bond formation at nitrile and CH sites not observed in homo-dimers.