Spin crossover, reversible redox, and supramolecular interactions in 3d complexes of 4-(4-pyridyl)-2,5-dipyrazyl-pyridine

Abstract

A new terpyridine-inspired terdentate ligand, 4-(4-pyridyl)-2,5-dipyrazyl-pyridine (py-pzpypz), featuring three "spare" nitrogen donors "out the back", has been used to synthesize five bis-ligand complexes, [M II (py-pzpypz) 2 ]X 2, where M = Mn with X = ClO 4, or M = Fe, Co, Ni, and Zn with X = BF 4. In contrast, when M = Cu II, regardless of the M:L ratio employed, 1:1 M:L products were obtained: for X = BF 4 a 1D chain {[Cu II (py-pzpypz)(DMF) 2 ](BF 4) 2 } n, and for X = Cl a monometallic complex [Cu(py-pzpypz)Cl 2 ]. All seven complexes were structurally characterized, confirming the expected N 6 coordination of the M II centers in all cases except Cu II. Notably, a Jahn-Teller elongation is observed in the Co II complex, consistent with it being low spin at 100 K. The Cu II 1D chain complex has an N 4 O 2 coordination sphere as in this case the "spare" pyridine donor out the back of the py-pzpypz ligand bridges to the next Cu II center in the chain, hence providing both a terdentate site and a monodentate pyridine to the next Cu II center, and the coordination sphere is completed by weak axial coordination by two DMF solvent molecules. The Cu II center in the monometallic complex has an N 3 Cl 2 square pyramidal coordination sphere. In all cases, the noncoordinating, "spare", pyrazine nitrogen atoms are involved in interesting intermolecular interactions, including N Pz -π interactions and nonclassical C-H···N Pz hydrogen bonding. The Fe II complex is low spin as expected. Two polymorphs of the Co II complex were obtained, both of which showed gradual spin crossover, with a room temperature T 1/2. Two reversible redox processes are observed for [Co II (py-pzpypz) 2 ](BF 4) 2, with E m (M I /M II) = -0.63 V and E m (M II /M III) = +0.37 V, and a quasireversible redox process for [Fe II (py-pzpypz) 2 ](BF 4) 2, with E m (M II /M III) = +1.26 V, versus 0.01 M AgNO 3 /Ag in MeCN. These potentials are shifted to significantly higher potentials (by ∼0.45 V) than the literature values for the corresponding Fe II and Co II complexes of the equivalent all-pyridine ligand, consistent with replacement of the two pyridine rings by two pyrazine rings significantly stabilizing the lower oxidation states.