Stabilization of isolated mixed-valence trimers in a novel nickel dithiolene complex with CF2 substituents

The preparation of the novel paramagnetic nickel dithiolene complex Ni(F2pdt)2-* (F2pdt2-: 2,2-difluoro-1,3-propanediyldithioethylene-1,2-dithiolate) and its X-ray crystal structure as n-Bu4N+ salt are described. (n-Bu4N)Ni(F2pdt)2] (2) crystallizes in the orthorhombic system, space group Pna2(1) with a = 21.379(4) A, b = 8.9702(18) A, and c = 18.527(4) A. The radical anions are isolated from each other by the bulky n-Bu4N+ cations and exhibit a Curie-type magnetic behavior. Two reversible redox waves corresponding to the redox couples Ni(F2pdt)2(2-/-*) and Ni(F2pdt)2(-*/0) are observed at -0.55 and 0.30 V vs SCE, illustrating the electron withdrawing effect of the CF2 substituents. As a consequence, (TTF)3(BF4)2 oxidation of the radical anion does not afford the neutral Ni(F2pdt)2(0) but a TTF salt formulated as TTF](3)Ni(F2pdt)2]3CH2Cl2]. It crystallizes in the triclinic system, space group P1 with a = 12.330(3) A, b = 12.726(3) A, c =15.706(3) A, alpha = 91.10(3), beta = 110.78(3), and gamma = 116.01(3). Donor and acceptor moieties are organized into (TTF)3(2+) and Ni(F2pdt)2]3(2-) trimers whose dicationic and dianionic charges have been inferred from the intramolecular bond lengths evolution and the singlet-triplet magnetic behavior. These trimers arrange orthogonally to each other into chess-board-like slabs, characterized by a segregation of the CF2 fragments and further stabilized by weak C-H.F interactions. Extended Hückel calculations show that only the nickel dithiolene complex trimer actually contributes to the magnetic susceptibility.