Spin Crossover, Reversible Redox, and Supramolecular Interactions in 3d Complexes of 4‑(4-Pyridyl)-2,5-dipyrazyl-pyridine

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,
[MII(py-pzpypz)2]­X2,
where M = Mn with X = ClO4, or M = Fe, Co, Ni, and Zn with
X = BF4. In contrast, when M = CuII, regardless
of the M:L ratio employed, 1:1 M:L products were obtained: for X =
BF4 a 1D chain {[CuII(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 MII centers in all cases except CuII. Notably, a Jahn–Teller
elongation is observed in the CoII complex, consistent
with it being low spin at 100 K. The CuII 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 CuII center
in the chain, hence providing both a terdentate site and a monodentate
pyridine to the next CuII center, and the coordination
sphere is completed by weak axial coordination by two DMF solvent
molecules. The CuII 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 FeII complex is low spin as expected. Two polymorphs of the CoII complex were obtained, both of which showed gradual spin
crossover, with a room temperature T1/2. Two reversible redox processes are observed for [CoII(py-pzpypz)2]­(BF4)2, with Em(MI/MII) = −0.63 V and Em(MII/MIII) = +0.37 V, and a quasireversible redox process
for [FeII(py-pzpypz)2]­(BF4)2, with Em(MII/MIII) = +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
FeII and CoII 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.