Tuning of Adsorption and Magnetic Properties in a Series of Self-Templated Isostructural Ni(II) Metal−Organic Frameworks

An
isomorphous series of five metal–organic frameworks of
formula [Ni4(O2CR)­(OH)2­(4-pyc)5] [4-pyc = 4-pyridinecarboxylate or isonicotinate; R = C6H5 (1), 4-OMe-C6H4 (2), 2,6-(OMe)2-C6H3 (3), 3,5-(OMe)2-C6H3 (4), 3,4,5-(OMe)3-C6H2 (5)] were obtained by solvothermal reactions. These
compounds display a three-dimensional framework where the nickel atoms
are coordinated to the hydroxyde anions and two different organic
ligands: isonicotinate and phenylcarboxylate. Both hydroxyde (μ3-OH) and phenylcarboxylate (μ3-1κO,2κO,3κO′)
ligands are coordinated to nickel atoms of the same secondary building
unit (SBU). The SBU consists of four edge-sharing NiA6 octahedra
(A = O, N). The isonicotinate ligands, however, act as linkers between
SBUs displaying three different coordination modes: μ-1κN,2κO; μ3-1κN,2κO,3κO′;
and μ4-1κN,2:3κ2O,4κO′. Nitrogen
adsorption measurements were done to obtain textural parameters of
these microporous networks. Micropore size distributions indicate
cylindrical pores with diameters of approximately 0.80 nm. The values
of Brunauer–Emmett–Teller surface areas (SBET) obtained are in the range of 382–488 m2/g, and the micropore volumes are between 0.13 cm3/g and 0.19 cm3/g. Both parameters are influenced by the
substitution grade and position of the methoxy groups of the phenylcarboxylate
ligand. The magnetic properties, which also depend on the arylcarboxylate
ligands, vary from compound 1 (with only antiferromagnetic
interactions) to compound 5, which shows a spin glass
behavior (Tg = 15 K).