Spin-Crossover 2D Metal-Organic Frameworks with a Redox-Active Ligand: [Fe(ttf-adpy)(2)M(CN)(4)]center dot nH(2)O (ttf-adpy=4-Tetrathiafulvalenylcarboxamidopyridine; M-II = Ni, Pd, Pt)

A new ttf (tetrathiofulvalene) ligand containing an amidopyridine moiety was synthesized and characterized. The electrochemical study of the 4-tetrathiofulvalenylcarboxamidopyridine (ttf-adpy) ligand showed two reversible oxidation processes at E-I'(1/2) = 0.08 V/Fc(+)-Fc and E-II'(1/2) = 0.26 V/Fc(+)-Fc. The crystal structure of [(ttf-adpyH)(2)Pt(CN)(4)] (1) Was solved at 293 K, where 1 displays the triclinic space group P (1) over bar. The ttf-ddpyH(+) molecule is planar, and the bond lengths within the ttf core are in the usual range for neutral ttf moieties. The ttf-adpyH(+) molecules and the [Pt(CN)(4)](2-) anions organize in a three-dimensional network by means of hydrogen bonds and short S center dot center dot center dot S contacts. In the network, the ttf-adpy molecules order following the pattern -D-A-D-A- (D = donor, pyridine; A = acceptor, ttf moieties). Reaction of the Fe(BF4)(2)center dot 6H(2)O salt, ttf-adpy ligand and the [M(CN)(4)](2-) coordinating counter-ions led to the 2D Hofmann-like polymers {Fe(ttf-adPY)(2)[M(CN)(4)]}center dot nH(2)O [M = Ni-II; n = 1.5 (2), 0 (5); Pd-II; n = 2 (3), 0 (6); Pt-II n = 2 (4), 0 (7)]. A 2D structure for 2-7 was proposed on the basis of analytical data. Compounds 24 present. a complete thermally induced two-step spin transition. It is observed in the temperature interval from 100 to 200 K for 2; whereas for 3 and 4 it occurred in the interval of 200-250 K. The spin-transition properties of 2-4 were found to be strongly dependent on the water content, as non-hydrated materials 5-7 are paramagnetic in the studied temperature range (5-350 K). The thermodynamic parameters associated with the spin transition estimated from DSC measurements are: Delta H-1 = 5.7 kJ mol(-1) (3), 6.2 kJ mol(-1) (4). Delta H-2 = 6.5 kJ mol(-1) (3), 7.2 kJ mol(-1) (4) and Delta S-1 24 JK(-1) mol(-1) (3), 26.6 JK(-1) mol(-1) (4), Delta S-2 = 30.4 JK(-1) mol(-1) (3) and 32.8 JK(-1) mol(-1)(4).