Spin-Crossover 2D Metal–Organic Frameworks with a Redox-Active Ligand: [Fe(ttf-adpy)2M(CN)4]·nH2O (ttf-adpy = 4-Tetrathiafulvalenylcarboxamidopyridine; MII = Ni, Pd, Pt)

Abstract

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)₂Pt(CN)₄] (1) was solved at 293 K, where 1 displays the triclinic space group P. The ttf–adpyH⁺ 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···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)₂·6H₂O salt, ttf–adpy ligand and the [M(CN)₄]^2– coordinating counterions led to the 2D Hofmann-like polymers {Fe(ttf–adpy)₂[M(CN)₄]}·nH₂O [M = Ni^II; n = 1.5 (2), 0 (5); Pd^II; n = 2 (3), 0 (6); Pt^IIn = 2 (4), 0 (7)]. A 2D structure for 2–7 was proposed on the basis of analytical data. Compounds 2–4 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 nonhydrated materials 5–7 are paramagnetic in the studied temperature range (5–350 K). The thermodynamic parameters associated with the spin transition estimated from DSCmeasurements are: ΔH₁ = 5.7 kJ mol^–1 (3), 6.2 kJ mol^–1 (4),ΔH₂ = 6.5 kJ mol^–1 (3), 7.2 kJ mol^–1 (4) and ΔS₁ = 24 J K^–1 mol^–1 (3), 26.6 J K^–1 mol^–1 (4), ΔS₂ = 30.4 J K^–1 mol^–1 (3) and32.8 J K^–1 mol^–1(4).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)