Spin Crossover, Reversible Redox, and Supramolecular Interactions in 3d Complexes of...

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]X2, where M = Mn with X = ClO4, or M = Fe, Co, Ni, and Zn with X = BF4. In contrast, when M = Cu(II), regardless of the M:L ratio employed, 1:1 M:L products were obtained: for X = BF4 a 1D chain {[Cu(II)(py-pzpypz)(DMF)2](BF4)2}n, and for X = Cl a monometallic complex [Cu(py-pzpypz)Cl2]. All seven complexes were structurally characterized, confirming the expected N6 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 N4O2 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 N3Cl2 square pyramidal coordination sphere. In all cases, the noncoordinating, "spare", pyrazine nitrogen atoms are involved in interesting intermolecular interactions, including NPz-π interactions and nonclassical C-H···NPz 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 T1/2. Two reversible redox processes are observed for [Co(II)(py-pzpypz)2](BF4)2, with Em(M(I)/M(II)) = -0.63 V and Em(M(II)/M(III)) = +0.37 V, and a quasireversible redox process for [Fe(II)(py-pzpypz)2](BF4)2, with Em(M(II)/M(III)) = +1.26 V, versus 0.01 M AgNO3/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.