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

The principal values of the ¹³ C chemical shift tensor for the β and δ polymorphs of π-[TTF⋅⋅⋅TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) have been analyzed to understand the abnormally long intra-dimer bonding of singlet π-[TTF^δ+ ⋅⋅⋅TCNE^δ- ]. These structures possess 12 intradimer contacts <3.40 Å, with the shortest intra π-[TTF⋅⋅⋅TCNE] separations involving 2-center (2c) C-S and 3c C-C-C orbital overlap contributions between the [TTF]^δ+ and [TCNE]^δ- . This solid-state NMR study compares the [TTF⋅⋅⋅TCNE] ¹³ C tensor data against previously reported π-[TTF]₂ ²⁺ and π-[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 β and δ phases of [TTF⋅⋅⋅TCNE], 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 β-phase differing primarily in the ethylenic π-electrons. The δ form differs significantly from the homo-dimer data at all principal values at both the ethylenic and CH sites, indicating changes to both the π-electrons and σ-bonds. In both hetero-dimer phases, the NMR changes supports long bond formation at nitrile and CH sites not observed in homo-dimers.