Incorporation of Pyrazine and Bipyridine Linkers with High-Spin Fe(II) and Co(II) in a Metal–Organic Framework

A series of isoreticular
metal–organic frameworks (MOFs) of the formula M­(BDC)­(L) (M
= Fe­(II) or Co­(II), BDC = 1,4-benzenedicarboxylate, L = pyrazine (pyz)
or 4,4′-bipyridine (bipy)) has been synthesized and characterized
by N2 gas uptake measurements, single crystal and powder
X-ray diffraction, magnetometry, X-ray absorption spectroscopy, and
Mössbauer spectroscopy. These studies indicate the formation
of a permanently porous solid with high-spin Fe­(II) and Co­(II) centers
that are weakly coupled, consistent with first-principles density
functional theory calculations. This family of materials represents
unusual examples of paramagnetic metal centers coordinated by linkers
capable of mediating magnetic or electronic coupling in a porous framework.
While only weak interactions are observed, the rigid 3D framework
of the MOF dramatically impacts the properties of these materials
when compared with close structural analogues.