Various zwitterionic carboxylate ligands form Mn^II coordination polymers with carboxylate and pseudo-halide bridges mixed in different ways. With mono-zwitterions, [Mn(N₃)₂(L¹)(H₂O)]_n and [Mn(N₃)₂(L²)]_n contains 1D chains with (μ-1,1-N₃)(μ-1,3-OCO) and (μ-1,1-N₃)₂(μ-1,3-OCO) bridges, respectively. With di-zwitterions, [Mn(N₃)(L³)]_n(ClO₄)_n forms a 2D network based on chains with (μ-1,1-N₃)(μ-1,3-OCO)₂ bridges, whereas [Mn₂(X)₄(L⁴)]_n·2nH₂O (X = N₃ or NCO) are interpenetrating 3D networks based on chains with (μ-1,1-X)₂(μ-1,3-OCO) bridges. With a tri-zwitterion, [Mn₇(N₃)₁₄(OH)₂(L⁵)(H₂O)₆]_n·4nH₂O is a 2D network based on a novel chain in which μ-1,1-N₃- and μ-1,3-OCO-bridged trinuclear motifs and μ₃-OH- and μ-1,1-N₃-bridged tetranuclear motifs are linked by μ-1,3-N₃ and μ₃-1,1,3-N₃. All compounds exhibit antiferromagnetic interactions, the magnitude of which depends upon the nature and mixing mode of the bridges, and follow the trend (μ-N₃)(μ-OCO) < (μ-N₃)(μ-OCO)₂ < (μ-N₃)₂(μ-OCO) < (μ-NCO)₂(μ-OCO).

Two azide-bridged coordination polymers, [Cu(mptz)(N₃)₂] (1) and [Mn(mptz)(N₃)₂H₂O] (2), (mptz = N-methyl-4-pyridinium tetrazolate), were synthesized and crystallographically and magnetically characterized. Compound 1 is a neutral 1D coordination polymer in which the Cu^II ions, which are in a square pyramidal surrounding, are linked by double end-on azide bridges in an alternating basal–basal and basal–apical fashion. Compound 2 exhibits a 3D Mn^II–azide coordination framework with the (10,3)-b net topology, in which the double end-on–azide-bridged dimanganese units are interlinked by single end-to-end azide bridges. The magnetic analyses indicated that 1 behaves magnetically as a quasi-dimer with an S = 1 ground state due to a strong ferromagnetic interaction through the basal–basal azide bridges and a very weak antiferromagnetic coupling through the basal–apical azide bridges. Compound 2 exhibits 3D antiferromagnetic behaviors with T_N = 24.0 K, where the end-to-end and end-on azide bridges mediate the antiferromagnetic and ferromagnetic interactions, respectively.

Three novel coordination polymers with azide and a bifunctional zwitterionic ligand bearing carboxylate and tetrazolate as bridging groups, [M(L)(N₃)]⋅x H₂O [L=1-(carboxylatomethyl)-4-(5-tetrazolato)pyridinium, M=Cu (1, x=2), Ni (2, x=1), and Co (3, x=1)], have been synthesized and characterized by X-ray crystallography and magnetic measurements. The compounds consist of two-dimensional coordination layers in which uniform anionic chains with the unprecedented tricomponent (μ-azide)(μ-tetrazolate)(μ-carboxylate) bridges are cross-linked by cationic 1-methylenepyridinium spacers. The tricomponent bridges induce ferromagnetic interactions in all the compounds. Furthermore, this isostructural series of ferromagnetic-chain-based compounds has allowed us to observe distinct bulk properties that are dependent upon the natures of the different spin carriers: with the isotropic Cu^II ion, 1 exhibits a paramagnetic phase of the ferromagnetic chains without long-range magnetic order above 2 K; with the weakly anisotropic Ni^II ions, 2 displays antiferromagnetic ordering and field-induced metamagnetism without slow dynamic relaxation; and with Co^II, which has strong magnetic anisotropy due to first-order spin-orbital coupling, 3 exhibits magnetic hysteresis and slow magnetization dynamics typical of single-chain magnets.

Two manganese(II) isocyanate complexes with different flexible zwitterionic dicarboxylate ligands, [Mn₂(bcpp)(NCO)₄]_n (1; bcpp=1,3-bis(N-carboxylatomethyl-4-pyridinio)propane) and [Mn₂(bcp)(NCO)₄]_n (2; bcp=bis(N-carboxylatomethyl)-4,4′-bipyridinium, have been synthesized and characterized by X-ray crystallography and magnetic measurements. Both compounds consist of two-dimensional coordination layers in which uniform anionic chains with mixed (NCO)₂(COO) triple bridges are cross-linked by flexible cationic 4,4′-trimethylenedipyridinium spacers. Magnetic studies revealed antiferromagnetic interactions through the triple bridges (J=−8.0 cm⁻¹ (1) and J=−8.6 cm⁻¹ (2)), which are stronger than those in the isoelectronic analogue (N₃)₂(COO). To complement the experimental data, periodic and finite-cluster DFT and CASPT2 calculations were performed on the dimeric units of the (NCO)₂(COO) and (N₃)₂(COO) mixed-bridged systems to support the Heisenberg picture and stress the relative efficiency of the magnetic couplers. It was found that the isocyanate ligand plays a greater role in the conveyance of antiferromagnetic behavior than the azide counterpart, and that both pseudohalide bridges function cooperatively with the carboxylate group.

Herein we present a systematic study of the structures and magnetic properties of six coordination compounds with mixed azide and zwitterionic carboxylate ligands, [M(N₃)₂(2-mpc)] (2-mpc=N-methylpyridinium-2-carboxylate; M=Co for 1 and Mn for 2), [M(N₃)₂(4-mpc)] (4-mpc=N-methylpyridinium-4-carboxylate; M=Co for 3 and Mn for 4), [Co₃(N₃)₆(3-mpc)₂(CH₃OH)₂] (5), and [Mn₃(N₃)₆(3-mpc)₂] (6; 3-mpc=N-methylpyridinium-3-carboxylate). Compounds 1–3 consist of one-dimensional uniform chains with (μ-EO-N₃)₂(μ-COO) triple bridges (EO=end-on); 5 is also a chain compound but with alternating [(μ-EO-N₃)₂(μ-COO)] triple and [(EO-N₃)₂] double bridges; Compound 4 contains two-dimensional layers with alternating [(μ-EO-N₃)₂(μ-COO)] triple, [(μ-EO-N₃)(μ-COO)] double, and (EE-N₃) single bridges (EE=end-to-end); 6 is a layer compound in which chains similar to those in 5 are cross-linked by a μ₃-1,1,3-N₃ azido group. Magnetically, the three Co^II compounds (1, 3, and 5) all exhibit intrachain ferromagnetic interactions but show distinct bulk properties: 1 displays relaxation dynamics at very low temperature, 3 is an antiferromagnet with field-induced metamagnetism due to weak antiferromagnetic interchain interactions, and 5 behaves as a noninnocent single-chain magnet influenced by weak antiferromagnetic interchain interactions. The magnetic differences can be related to the interchain interactions through π–π stacking influenced by different substitution positions in the ligands and/or different magnitudes of intrachain coupling. All of the Mn^II compounds show overall intrachain/intralayer antiferromagnetic interactions. Compound 2 shows the usual one-dimensional antiferromagnetism, whereas 4 and 6 exhibit different weak ferromagnetism due to spin canting below 13.8 and 4.6 K, respectively.

Two Cu^II complexes with the new zwitterionic ligand 1-carboxymethylpyridinio-3-benzoate (L), [Cu₄L₄(OH)₂](ClO₄)₂·8H₂O (1) and [Cu₄L₄(OH)₂]Cl₂·8H₂O (2), have been synthesized and characterized crystallographically and magnetically. The two compounds contain isostructural chains, in which the anionic basic copper(II) carboxylate clusters [Cu₄(μ₃-OH)₂(RCOO)₈]^2– are quadruply interlinked by cationic organic spacers. Magnetic analyses using a tetranuclear model indicate that antiferromagnetic interactions between neighboring copper(II) ions are operative through the double hydroxo bridges and the mixed hydroxo/carboxylato bridges.

The structure and magnetic properties of double azido-bridged Cu^II binuclear complex 1 with the chelating chiral ligand (S,S)-2,2′-isopropylidenebis(4-phenyl-2-oxazoline) were analyzed by combining experimental and theoretical techniques. The Cu^II ions adopt a square pyramidal geometry with different degrees of distortion, whereas the two end-on azido bridges disposed at the equatorial positions exhibit significantly different Cu–N–Cu angles, 110.6 and 97.3°, which are respectively smaller and larger than the critical 108° value distinguishing the ferromagnetic and antiferromagnetic regimes. The asymmetry in 1 arises from the use of the bulky asymmetric ligand, giving rise to two different magnetic pathways between the Cu^II ions. The magnetic pathway along the large Cu–N–Cu angle value dominates over the small one, resulting in a net antiferromagnetic behavior with J = –78.6 cm^–1. On the basis of wavefunction calculations, we investigate the exchange interactions in synthetic compound 1 and fictitious analogs 2 and 3 holding either two large (i.e., 110.6°) or two small (i.e., 97.3°) Cu–N–Cu angles. The calculated exchange interaction in 1(–104 cm^–1) is in relatively good agreement with the experimental value and corresponds precisely to the average between the antiferromagnetic value in 2 (–218 cm^–1) and the ferromagnetic one in 3 (21 cm^–1). The significant enhancement in the antiferromagnetic contribution accompanying the expansion of one of the Cu–N–Cu bridging angles is undoubtedly the driving force for the observed antiferromagnetic behavior in 1. The control of the local metal environments allowed us to monitor the exchange coupling interactions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The crystal structures and magnetic properties of two new Co^II molecular magnets, [Co(N₃)₂(btzb)] (1) and [Co(N₃)₂(btze)₂] (2), are described and discussed (btzb=1,4-bis(tetrazol-1-yl)butane and btze=1,4-bis(tetrazol-1-yl)ethane). In the materials, (4,4) layers with μ-1,3-azide bridges are cross-linked by the monolayered btzb bridging ligands or spaced by bilayered btze terminal ligands to give a 3D (1) or 2D (2) coordination network with significantly different interlayer separations (10.6 vs. 15.2 Å). The observation that the layers in 1 and 2 are almost identical have not only allowed us to determine how the interlayer separation imposes its influences on their magnetic behavior, but also helps us understand the complex magnetic behavior of each structure. In the high-temperature range (>25 K), almost identical magnetic behaviors, typical of 2D antiferromagnetic systems, are observed for 1 and 2. At low temperature they exhibit unusual and different behaviors that combine spin canting (weak ferromagnetism), metamagnetism, and stepped hysteresis. It has been found that the interlayer separation has little influence on the ordering temperature (23 vs. 22 K), but imposes very-strong influence on the metamagnetic critical field (6500 vs. 450 Oe), the coercivity (7500 vs. 650 Oe), and the hysteresis-step size. It may also play an adjusting role in determining the canting angle. Taking into account the strong anisotropy of the systems and the interlayer dipolar interactions, we have reasonably interpreted the unusual metamagnetic and hysteresis behaviors and the differences between 1 and 2. In particularly, the stepped hysteresis loops have been explained by two weak ferromagnetic states.