Infinite network involving two hydrogen-bonded tetrameric water rings (S₄ and D_2h) in a (4,4)-square grid crystal host

A proton migration across a short strong hydrogen bond can be triggered by spin crossover of a remote Fe ²⁺ cation, with the onset of a photoinduced activation energy barrier for proton motion at low temperatures.

Four new high-spin cobalt(II) complexes of formulae [Co₂(bta)(4,4′-bpy)₂(H₂O)₂]_n (1), {Hbpe[Co(Hbta)(bpe)(H₂O)₂]}_n (2), {[Co(H₂bta)(azpy)(H₂O)₂]·azpy}_n (3) and {[Co₂(bta)(bpa)₂(H₂O)₄]·8H₂O}_n (4) with H₄bta = 1,2,4,5-benzenetetracarboxylic acid, 4,4′-bpy = 4,4′-bipyridine, bpe = trans-1,2-bis(4-pyridyl)ethene, azpy = 4,4′-azobispyridine and bpa = trans-1,2-bis(4-pyridyl)ethane, have been prepared and characterized by single crystal X-ray diffraction. Compounds 1–4 exhibit two-dimensional networks where the fully (1 and 4) or partially (2 and 3) deprotonated tetracarboxylic ligand connects two (2 and 3) or four (1 and 4) cobalt(II) ions through two trans-carboxylate (1–3) or all the carboxylate groups (4) whereas the co-ligands 4,4′-bpy (1), bpe (2), azpy (3) and bpa (4) act as two-fold connectors in trans- (bpe and azpy) and gauche- (bpa) conformations. The cobalt(II) ions in 1 are five-coordinated with two nitrogen atoms from two 4,4′-bpy ligands in the axial positions and a water molecule and two carboxylate-oxygens building the trigonal plane. Six-coordinated cobalt(II) ions occur in 2–4 with two pyridyl-nitrogen atoms, two carboxylate-oxygens and two trans-coordinated water molecules in a somewhat distorted octahedral surrounding. The magnetic properties of 1–4 have been investigated in the temperature range 1.9–300 K. Compound 1 shows a weak antiferromagnetic interaction between the five-coordinated cobalt(II) ions (0.13 ≤ |J| ≤ 0.28 cm⁻¹, Ĥ = −JŜ₁·Ŝ₂), the exchange pathway involved being the double carboxylate bridges in the syn–syn conformation [cobalt–cobalt separation] of 3.6465(9) Å. As the magnetic interactions between the cobalt(II) ions through the phenyl ring of the bta group in 2–4 and those across the extended bpe (2), azpy (3) and bpa (4) bridges are negligible, the strong decrease of χ_MT upon cooling observed for 2–4 is mainly due to the depopulation of the higher Kramer doublets of the six-coordinated cobalt(II) ions.

Scheme of the different phase transitions of (( R )-(−)-3-hydroxyquinuclidium)[FeCl ₄ ].