Ferromagnetic End-to-End Azides and Tetrahedral Cobalt: Unusual Findings in the M-BPA-Pseudohalide₂ System

Abstract

Two novel unusual compounds [Co(NCO) 2 (bpa)] (1) and [Cu(N 3 ) 2 (bpa)] (2) with cyanate and azide pseudohalides are shown in this work. These compounds have been characterized by X-ray diffraction measurements, Fourier transform-infrared (FT-IR) and UV-visible spectroscopies, thermogravimetric studies, electron spin resonance (ESR) spectroscopy, and magnetic measurements. Compound 1 shows a one-dimensional (1D) structure with the cobalt atoms bridged by bpa ligands and with the metal atoms showing an unusual tetrahedral coordination sphere. Compound 2 shows a two-dimensional (2D) structure, where the copper(II) ions are bridged by double 1,3-azido ligands to form chains that are joined through single bpa bridges to give the global 2D structure. Magnetic measurements show predominance of spin-orbit coupling for the cobalt compound (1), while ferromagnetic interactions, unusual for the 1,3-azido bridging, are observed in compound 2. This compound represents the first example of copper-double (μ 1,3 -azido)-bridged chain exhibiting ferromagnetic interactions through this kind of bridging. ' INTRODUCTIONThe design and synthesis of molecular-based magnetic materials with ferromagnetic coupling continue to be a challenge in magnetochemistry. 1 Azide and cyanate pseudohalides have focused the attention in this field due to their ability as magnetic, even ferromagnetic, exchange couplers. As we know, azide as a short bridging ligand that can provide rich coordination modes and efficient pathways of magnetic exchange, and hence can affect the spin ground state of a molecule, has been widely used in magnetochemistry. 2 This ion can act as a bridging ligand either in the end-to-end (μ-1,3-N 3 ) or in the end-on (μ-1,1-N 3 ) mode to give polynuclear complexes from dinuclear to three-dimensional (3D) systems. It is the general norm that the μ-1,3-coordination mode gives rise to antiferromagnetism, while μ-1,1-coordination results in ferromagnetic 3 interactions between the metal centers. From the large number of complexes with azido ligands magnetically characterized in the last decades, 2,4 examples exhibiting ferromagnetic coupling for the end-to-end type of bridging are really scarce. 5 The characterized compounds with the cyanate ligand are very scarce in comparison with those of the azide ligand. 6 This ion shows a lower versatility and clearly prefers the end-on N-cyanate bridging mode, which has been shown to give ferromagnetic type interactions. It is therefore interesting to obtain more examples concerning this bridging ligand.On the other hand, the 4-4 0 -type bpa ligand has been chosen because of its recognized ability to interconnect metal ions and increase the dimensionality of the systems where it participates. 7

Ligand-Directed Assembly of Azide-Bridged Tetranuclear, One-Dimensional and Two-Dimensional Cu(II) Complexes: Synthesis, Crystal Structure and Magnetic Properties

“…Among the four Cu–N azide coordination bonds only one strong Cu–N bond is present, and the other three Cu–N bonds are involved in axial coordination. Thus, the interchain magnetic coupling should be very weak, either ferromagnetic ,,, or antiferromagnetic. , The weak interchain and interlayer magnetic interactions preclude the occurrence of long-range magnetic ordering in complex 3 , which is verified by the field-cooled magnetization (FCM) measurement (Figure S8, SI).…”

End-to-End Azido-Bridged Lanthanide Chain Complexes (Dy, Er, Gd, and Y) with a Pentadentate Schiff-Base [N3O2] Ligand: Synthesis, Structure, and Magnetism

“…Like other pseudohalides (CN – , NCS – ), the azide anion is often used as a bridging ligand in the design of polynuclear transition-metal complexes . Spin coupling mediated by azide bridging ligands depends strongly on the coordination mode: it is predominantly ferromagnetic for the end-on mode and antiferromagnetic for the end-to-end mode. − A plethora of azide metal complexes was reported, mainly for transition metals, including clusters, chains, and extended 2D and 3D magnetic structures. − By contrast, lanthanide azide complexes are much less common; in fact, only a limited number of Ln azide complexes are presently known . Recently, tetranuclear lanthanide (Gd, Dy) azide complexes have been characterized structurally and magnetically, in which Ln ions are bridged by azide ligands in the end-to-end mode resulting in a weak Ln-Ln exchange interaction…”