Azido has been one of the well used bridging ligands in the construction of magnetic molecule materials, which has stimulated considerable interest in magnetochemistry. This tutorial review is concentrated on the study of azido metal compounds with their magnetochemistry and consists of eight main sections. The first section offers an introduction to azido complexes and some important results obtained from magneto–structural correlation in azido complexes which will be referred to in the following sections. The next six sections give overviews of azido-mediated ferromagnets, ferrimagnets, canted (weak) ferromagnets, antiferromagnets, single molecule magnets (SMMs) and single chain magnets (SCMs), and metamagnets. The last section is the conclusion and gives a perspective of azido-mediated magnets.

Azido-mediated systems showing different magnetic behaviors

Framework-structured weak ferromagnets are new rising stars in molecule-based magnetic materials. The framework structures are powerful carriers for long-range ordering of spins. And weak ferromagnetism due to spin canting is an effective approach for magnets because of its frequent occurrence and desired spontaneous magnetization as long as the canting angle γ is large enough. In this critical review, we provide an overview of the various framework-structured weak ferromagnets based on different grades of ligands (from mono-atom to three-atom-like ligands). Particular emphasis is given to the relationships between structural features and the properties, rational employment of the ligands, and weak ferromagnetic strategies for molecule-based magnets with exciting properties and applications (273 references).

Framework-structured weak ferromagnets

The synthesis of transition metal spin-clusters has become an important sub-discipline of coordination chemistry, especially since the discovery of single-molecule magnets (SMMs) in the early nineties. In this context, complexes with high spin-numbers and large anisotropy are particularly desired. The use of the azide ligand to act as a bridge and magnetic coupler within cages of paramagnetic ions has become increasingly common in this field, especially during the last five years, mainly because of the coordination versatility of this ligand, which is capable of bridging several metals in a variety of coordination modes, and its ability to induce ferromagnetic interactions. Several high-spin 3d metal aggregates have been prepared in this manner, a significant number of which behave as SMMs. This review covers the rapid progress made with this relatively recent synthetic approach by describing the structures and summarising the magnetic properties of the systems prepared in this manner. Some of the trends identified could serve as a privileged starting point for the further development of this promising area. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Azide as a Bridging Ligand and Magnetic Coupler in Transition Metal Clusters

A series of 2D and one 3D transition-metal−azido coordination polymers with pyridine carboxylate N-oxide, isonicotinate N-oxide (INO), and nicotinate N-oxide (NNO) as the coligands were synthesized by a hydrothermal method and structurally and magnetically characterized. These complexes have the formulas [M(L)(N3)(H2O)]n (L = INO and M = Mn, 1·Mn; Co, 2·Co; Ni, 3·Ni and L = NNO and M = Mn, 4·Mn; Co, 5·Co; Ni, 6·Ni) and [Cu(L)(N3)(H2O)0.5]n (L = INO, 7·Cu; NNO, 8·Cu). All complexes consist of end-on azido and syn-syn carboxylato mixed-bridged M−N3/COO chains that are further linked by the pyridine N-oxide group of the INO or NNO to give rise to 2D layered structures for 1·Mn, 2·Co, 3·Ni, 4·Mn, 5·Co, 6·Ni, and 8·Cu and a 3D framework for 7·Cu. The high-temperature magnetic susceptibilities are dominated by low-dimensional behavior while long-range ordering sets in at low temperatures. The Mn complexes are antiferromagnets, and the others are metamagnets. In addition, 5·Co exhibits slow magnetic relaxation b...

Coordination polymers with end-on azido and pyridine carboxylate N-oxide bridges displaying long-range magnetic ordering with low dimensional character.

The bridging azido ligand as a central “player” in high-nuclearity 3d-metal cluster chemistry

Two new one-dimensional single azide-bridged metal(II) compounds [[M(5-methylpyrazole)4(N3)]n](ClO4)n(H2O)n [M = Co (1a), Ni (2a)] were prepared by treating an M(II) ion with stoichiometric amount of sodium azide in the presence of four equivalents of the 3(5)-methylpyrazole ligand. The isostructural compounds 1a and 2a crystallize in the monoclinic space group P2(1)/n. The azide bridging ligands have a unique end-to-end coordination mode that brings two neighboring metal centers into a cis-position with respect to the azide unit to form single end-to-end azide-bridged cobalt(II) and nickel(II) chains. The two neighboring metal atoms at inversion centers adopt octahedral environments with four equatorial 3(5)-methylpyrazole ligands and two axial azide bridges. Two adjacent equatorial least-squares planes form dihedral angles of 60.5 degrees and 60.6 degrees for Co and Ni, respectively. In addition, the metal-azide-metal units form large M-N3-M torsion angles, which are magnetically important geometrical parameters, of 71.6 degrees for M=Co and 75.7 degrees for M=Ni. It should also be noted that the M-N-N angles associated with end-to-end azide group, another magnetically important structural parameter, fall into the experimentally observed range of 120-140 degrees as 128.3(3) and 147.8(3) degrees for cobalt species and 128.4(2) and 146.1(3) degrees for nickel species; these values deviate from the theoretical value of around 164 degrees at which the incidental orthogonality is achieved under the torsion angle of 0 degrees. The compounds 1a and 2a have unique magnetic properties of ferromagnetism, zero-field splitting, and spin canting. The MO calculations indicate that the quasiorthogonality between the magnetic orbitals of metal ions and the p atomic orbitals of the bridging azide is possible in the observed structures and leads to the ferromagnetism. The spin canting related to the perturbation of ferromagnetism arises from the magnetic anisotropy and antisymmetric interactions judged by the structural parameters of the zero-field splitting and the tilted MN4 planes in a chain. The enhancement of magnetic interactions was accomplished by dehydrating the chain compounds to afford two soft magnets with critical temperature T(C) and coercive field of 2 K and 35 G for 1b and 2.3 K and 20 G for 2b, respectively.

http://www.wiley-vch.de/contents/jc_2111/2001/f3244_s.pdf

Yaung Soo Kim

Papers

Unusual ferromagnetic couplings in single end-to-end azide-bridged cobalt(II) and nickel(II) chain systems.