Spin‐Crossover 2D Metal–Organic Frameworks with a Redox‐Active Ligand: [Fe(ttf‐adpy)₂M(CN)₄]·<i>n</i>H₂O (ttf‐adpy = 4‐Tetrathiafulvalenylcarboxamidopyridine; M^II = Ni, Pd, Pt)

Abstract

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) 2 Pt(CN) 4 ] ( 1 ) was solved at 293 K, where 1 displays the triclinic space group P $\bar {1}$ . 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(BF 4 ) 2 · 6H 2 O 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 ]} · n H 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 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: Δ H 1 = 5.7 kJ mol –1 ( 3 ), 6.2 kJ mol –1 ( 4 ),Δ H 2 = 6.5 kJ mol –1 ( 3 ), 7.2 kJ mol –1 ( 4 ) and Δ S 1 = 24 J K –1 mol –1 ( 3 ), 26.6 J K –1 mol –1 ( 4 ), Δ S 2 = 30.4 J K –1 mol –1 ( 3 ) and32.8 J K –1 mol –1 ( 4 ).(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2009)