Heat capacities of the spin quantum number S = 1/2 two-dimensional Heisenberg antiferromagnet bis(2-amino-5-chloropyridinium) tetrabromocuprate(II) [(5CAP)2CuBr4] crystal and its nonmagnetic analogue, the bis(2-amino-5-chloropyridinium) tetrabromozincate(II) [(5CAP)2ZnBr4] crystal, were measured by adiabatic calorimetry. For the (5CAP)2ZnBr4 crystal, single-crystal X-ray diffraction was also performed. The (5CAP)2ZnBr4 crystal belongs to the orthorhombic space group Pbca, with a = 16.074(2) Å, b = 7.688(2) Å, c = 30.538(6) Å, and Z = 8. In the (5CAP)2CuBr4 crystal, an antiferromagnetic phase transition occurred at T N = 5.08 K, and a thermal anomaly arising from the short-range order characteristic of two-dimensional magnetic substances was found above T N; the heat capacity of the (5CAP)2ZnBr4 crystal showed no thermal anomaly. The enthalpy and entropy gains due to the magnetic transition were estimated to be 49.3 J mol-1 and 5.65 J K-1 mol-1, respectively. The value of the entropy gain coincides well with the R ln 2 (5.76 J K-1 mol-1, R stands for the gas constant) expected for S = 1/2 spin systems. The thermal anomaly observed above T N is well accounted for in terms of the S = 1/2 two-dimensional antiferromagneic Heisenberg model of a square lattice with J/k B = −4.3 K. Spin wave analysis of the magnetic heat capacities below T N suggests that the (5CAP)2CuBr4 crystal is in a three-dimensional antiferromagnetic state, which is realized by a weak interlayer magnetic interaction between the two-dimensional layers.

Ruthenium(II)-pterin complexes were prepared using tetradentate and tripodal tris(2-pyridylmethyl)amine (TPA) and tris(5-methyl-2-pyridylmethyl)amine (5-Me3-TPA) as auxiliary ligands together with 2-(N,N-dimethyl)-6,7-dimethylpterin (Hdmdmp) and 6,7-dimethylpterin (Hdmp) as pterin derivatives for ligands. Characterization was made by spectroscopic methods, X-ray crystallography, and electrochemical measurements. The pterin ligands coordinated to the ruthenium centers as monoanionic bidentate ligands via the 4-oxygen of the pyrimidinone moiety and the 5-nitrogen of the pyrazine parts. The striking feature is that the coordinated dmp- ligand exhibits a quinonoid structure rather than a deprotonated biopterin structure, showing a short C-N bond length for the 2-amino group. Those complexes exhibit reversible two-step protonation for both pterin derivatives coordinated to the ruthenium centers to give a drastic spectral change in the UV-vis spectroscopy. Doubly protonated Ru(II)-pterin complexes were stabilized by pi-back-bonding interaction and exhibited clear and reversible proton-coupled electron transfer (PCET) to give ruthenium-coordinated neutral monohydropterin radicals as intermediates of PCET processes. Those ESR spectra indicate that the unpaired electron delocalizes onto the PCET region (N5-C6-C7-N8) of the pyrazine moiety.

Heat capacities of the organic free radical ferromagnet, 4-(4-chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-oxyl (CATMP) have been measured in the temperature range between 0.1 and 311 K. A ferromagnetic phase transition was observed at 0.28 K and a heat capacity hump was found above the transition temperature. This hump is due to the short-range order characteristic of low-dimensional magnetic spin systems. The enthalpy and entropy gains due to the ferromagnetic phase transition were estimated to be 4.06 J mol-1 and 5.71 J K-1 mol-1, respectively. This entropy agrees well with the theoretical value Rln 2 ( = 5.76 J K-1 mol-1) expected for the magnetic entropy of a spin system with the spin quantum number S = 1/2, where R is the gas constant. The heat capacity hump above the transition temperature is well accounted for in terms of the S = 1/2 two-dimensional ferromagnetic Heisenberg model of square lattice with the intralayer exchange interaction J/kB = 0.42 K, where the spin Hamiltonian H = -2JSi·Sj is adopted and kB stands for the Boltzmann constant. The analysis of the magnetic heat capacities below the transition temperature on the basis of the spin wave theory clarified that the magnetic system settles down in three-dimensional ferromagnetic order below the transition temperature with the interlayer exchange interaction J′/kB = 0.024 K.