Effecting structural diversity in a series of Co(II)-organic frameworks by the interplay between rigidity of a dicarboxylate and flexibility of bis(tridentate) spanning ligands

In a one-pot self-assembly reaction of Co(OAc)(2)center dot 4H(2)O, thiophene-2,5-dicarboxylic acid (H(2)tdc) and four different bis(tridentate) polypyridyl spanning ligands under ambient conditions, a series of structurally diverse metal-organic frameworks has been synthesised and characterized by single crystal X-ray diffraction: {[Co-2(tdc)(2)(tpbn)(H2O)(2)]center dot solvent}(n)(solvent = 2H(2)O,1; solvent = 2CH(3)OH, 2H(2)O,1a), {[Co-2(tdc)(2)(tphn)]center dot solvent}(n)(solvent = H2O,2; solvent = CH3OH, 2.5H(2)O,2a), {[Co-2(tdc)(2)(tpchn)(H2O)(2)]center dot solvent}(n)(solvent = 5H(2)O,3; solvent = C2H5OH, 2H(2)O,3a), and {[Co-2(tdc)(2)(tpxn)]center dot solvent}(n)(solvent = 6H(2)O,4; when no solvent,4a), where tpbn (N,N',N '',N'"-tetrakis(2-pyridylmethyl)-1,4-diaminobutane), tphn = N,N',N '',N'"-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, tpchn = N,N '-(cyclohexane-1,4-diylbis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine) and tpxn =N,N'-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine). There is a profound effect of the nature of spacer between the alkyl nitrogens in the spanning ligands (flexiblevs. semiflexible) on the molecular structures of1a-4a. The notable differences are (a) the binding mode of the tridentate part of polypyridyl ligands to the Co(ii) center is facial in1a,3aand4abut meridional in2a, (b) the Co(ii) centers in1a-3aare hexacoordinated (with a coordinated water in1aand3a) but are pentacoordinated in4a, and (c) the binding mode of tdc linker is bis(monodentate) in1a,3aand4abut chelated in one end and monodentate in the other end in2a. Thus, the overall framework structure of1a,2a,3aand4aiscis-decalin type 2D polymer, ladder-shaped 1D polymer, hexagonal 2D net andcis-decalin type 2D polymer, respectively. Their thermal stabilities have been established by thermogravimetric analysis (TGA). The presence of an unsaturated metal center in4has provided us an opportunity for its use as an efficient Lewis acid catalyst for the Knoevenagel condensation reaction of malononitrile with various aldehydes (100% conversion in 60 minutes with 2 mol% catalyst in methanol).