The self-assembly of [Nb(IV)(CN)(8)](4-) with different 3d metal centers in an aqueous solution and an excess of pyrazole resulted in the formation of four 3D isostructural compounds {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)].4H(2)O}(n), where M(II) = Mn, Fe, Co, and Ni for 1-4, respectively. All four assemblies crystallize in the same I4(1)/a space group and show identical cyanido-bridged structures decorated with pyrazole molecules coordinated to M(II) centers. All four compounds show also long-range magnetic ordering below 24, 8, 6, and 13 K, respectively. A thorough analysis of the structural and magnetic data utilizing the molecular field model has allowed for an estimation of the values of coupling constants J(M-Nb) attributed to the one type of M(II)-NC-Nb(IV) linkage existing in 1-4. The J(M-Nb) values increase monotonically from -6.8 for 1 through -3.1 for 2 and +3.5 for 3, to +8.1 cm(-1) for 4 and are strongly correlated with the number of unpaired electrons on the M(II) metal center. Average orbital contributions to the total exchange coupling constants J(M-Nb) have also been identified and calculated: antiferromagnetic J(AF) = -21.6 cm(-1) originating from the d(xy), d(xz), and d(yz) orbitals of M(II) and ferromagnetic J(F) = +15.4 cm(-1) originating from d(z(2)) and d(x(2)-y(2)) orbitals of M(II). Antiferromagnetic interaction is successively weakened in the 1-4 row with each additional electron on the t(2g) level, which results in a change of the sign of J(M-Nb) and the nature of long-range magnetic ordering from ferrimagnetic in 1 and 2 to ferromagnetic in 3 and 4.

Switchable cobalt coordination polymers exhibit valence tautomeric or spin crossover transitions upon application of an external stimulus such as heating or cooling.-One-and two-dimensional switchable cobalt coordination polymers are reviewed.-New directions in the field oriented towards materials applications include incorporating switchable cobalt coordination polymers into nano-and microparticles and grafting the particles onto surfaces.

The combination of the divergent bis-pyridyl linking ligands 1,2-bis(4-pyridyl)ethane (1,2-bpe), 4,4'-trans-azopyridine (azpy), and 1,3-bis(4-pyridyl)propane (1,3-bpp) with cobalt and 3,5-di-tert-butyldioxolene (3,5-dbdiox) ligands has afforded the complexes [Co(3,5-dbdiox)2(1,2-bpe)]∞ (1), [Co(3,5-dbdiox)2(azpy)]∞ (2), [trans-Co(3,5-dbdiox)2(1,3-bpp)]∞ (3a), and [cis-Co(3,5-dbdiox)2(1,3-bpp)]∞ (3b). All species are 1D coordination polymers that crystallize as solvated forms; the geometric isomers 3a,b cocrystallize. Complexes 1, 2, and 3a exhibit around the Co centers a trans disposition of the N-donor atoms from the pyridyl linkers, while an unusual cis disposition is evident in 3b. Single-crystal X-ray structural analysis at 100 or 130 K of solvated forms of these complexes indicates that all complexes possess the {Co(III)(3,5-dbcat)(3,5-dbsq)} (3,5-dbcat = 3,5-di-tert-butylcatecholate; 3,5-dbsq = 3,5-di-tert-butylsemiquinonate) charge distribution at the temperature of data collection. Variable-temperature magnetic susceptibility studies reveal that 1, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O (3 = 3a·3b) all exhibit thermally induced valence tautomeric (VT) transitions above 200 K. Multiple heating and cooling cycles indicate that in some cases the behavior is strongly dependent on desolvation processes. Most notably, further desolvation of 1·1.5MeCN·2H2O above 340 K affords χmT values that suggest unusual ferromagnetic coupling in the {hs-Co(II)(3,5-dbsq)2} valence tautomer. Compound 3·MeCN·H2O exhibits a two-step VT transition that may be ascribed to the presence of the cis and trans geometric isomers. Compounds 1, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O all also exhibit a single photoinduced VT transition, comparable to those generally observed for nonpolymeric cobalt-dioxolene complexes.

The combination of the divergent tetrakis-pyridyl linking ligand 1,2,4,5-tetra(4pyridyl)benzene (tpb) with cobalt and 3,5-di-tert-butyldioxolene (3,5-dbdiox) ligands has afforded the complexes [Co(3,5-dbdiox)2(tpb)]∞ (1) and [Co(3,5-dbdiox)2(tpb)0.5]∞ (2). Both species are 1D coordination polymers that crystallize as the hydrated compounds 1•3H2O and 2•9H2O. While the tpb in complex 1 is a 2-connecting linker, affording a conventional 1D chain, the tpb in 2 is a 4-connecting linker, giving rise to a ribbon topology. Single crystal X-ray 2 structural analysis indicates that 1•3H2O and 2•9H2O both possess the {Co III (3,5-dbcat)(3,5dbsq)} (3,5-dbcat = 3,5-di-tert-butylcatecholate; 3,5-dbsq = 3,5-di-tert-butyl-semiquinonate) charge distribution at 100 or 130 K, respectively. Variable temperature magnetic susceptibility studies of partially dehydrated 1•H2O indicate a thermally induced valence tautomeric (VT) transition above 300 K. 19 obtained are instead 1D coordination polymers with chain (1) and ribbon (2) topologies. To our knowledge complex 1 is the first example of tpb acting as 2-connecting ligand, while complex 2 is a rare example of a 4-connecting tbp with a staggered conformation of the pyridyl groups. Magnetochemical, structural and spectroscopic studies suggest that compound 1•H2O can be formulated as {ls-Co III (3,5-dbcat)(3,5-dbsq)} below 300 K, with an incomplete thermallyinduced VT transition to a {hs-Co II (3,5-dbsq)2} species commencing above 300 K. ASSOCIATED CONTENT Supporting information. The Supporting Information is available free of charge on the ACS Publication website. Additional structural figures, thermogravimetric analysis data and infrared spectra (PDF). Accession Codes CCDC 1522817 and 1522818 contain the supplementary crystallographic data for 1•3H2O and 2•9H2O, respectively. These data can be obtained free of charge via

The cationic one-dimensional coordination polymers [{Co(pyterpy)2}{M(hfac)2}] 2+ (M = Co(II) or Zn(II); pyterpy = 4'-(4-pyridyl)-2,2':6',2''-terpyridine; hfac-= hexafluoroacetyloacetonate) feature alternating {Co(pyterpy)2} and {M(hfac)2} units linked through heteroditopic pyterpy ligands. Cocrystallization with [M(hfac)3]-(M = Co(II) or Zn(II)) counteranions has afforded the compounds: [{Co(pyterpy)2}{Co(hfac)2}][Co(hfac) 3 ] 2 (1) and [{Co(pyterpy)2}{Co0.7Zn0.3(hfac)2}][Zn(hfac) 3 ] 2 (2). Variable temperature crystallographic and magnetic studies indicate low spin cobalt(II) ions in the {Co(pyterpy)2} components of the coordination polymers up to room temperature; magnetic studies suggest the onset of a thermallyinduced spin crossover at these centers above room temperature.