Syntheses, Structural, Magnetic, and Electron Paramagnetic Resonance Studies of Monobridged Cyanide and Azide Dinuclear Copper(II) Complexes: Antiferromagnetic Superexchange Interactions

Abstract

The reactions of Cu(ClO4)2 with NaCN and the ditopic ligands m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*) yield [Cu2(μ-CN)(μ-Lm)2](ClO4)3 (1) and [Cu2(μ-CN)(μ-Lm*)2](ClO4)3 (3). In both, the cyanide ligand is linearly bridged (μ-1,2) leading to a separation of the two copper(II) ions of ca. 5 Å. The geometry around copper(II) in these complexes is distorted trigonal bipyramidal with the cyanide group in an equatorial position. The reaction of [Cu2(μ-F)(μ-Lm)2](ClO4)3 and (CH3)3SiN3 yields [Cu2(μ-N3)(μ-Lm)2](ClO4)3 (2), where the azide adopts end-on (μ-1,1) coordination with a Cu-N-Cu angle of 138.0° and a distorted square pyramidal geometry about the copper(II) ions. Similar chemistry in the more sterically hindered Lm* system yielded only the coordination polymer [Cu2(μ-Lm*)(μ-N3)2(N3)2]. Attempts to prepare a dinuclear complex with a bridging iodide yield the copper(I) complex [Cu5(μ-I4)(μ-Lm*)2]I3. The complexes 1 and 3 show strong antiferromagnetic coupling, -J = 135 and 161 cm(-1), respectively. Electron paramagnetic resonance (EPR) studies coupled with density functional theory (DFT) calculations show that the exchange interaction is transmitted through the dz(2) and the bridging ligand s and px orbitals. High field EPR studies confirmed the dz(2) ground state of the copper(II) ions. Single-crystal high-field EPR has been able to definitively show that the signs of D and E are positive. The zero-field splitting is dominated by the anisotropic exchange interactions. Complex 2 has -J = 223 cm(-1) and DFT calculations indicate a predominantly d(x(2)-y(2)) ground state.

Magnetic resonance studies of copper (II) sorbitol complex, in solution, reveal a supramolecular structure compatible to the crystal structure

“…Figure a shows the effect of the temperature on the signals bellow 0 ppm. This strong chemical shift displacement is also observed in organometallic structures with paramagnetic shift effect . Figure b shows the ratio between the areas of signals bellow 0 ppm (assigned to [Cu 2+ ‐Sorb] complex) and the areas of the signal between 3.6 to 4 ppm (sorbitol).…”

“…This figure shows the presence of several small peaks in very high and low chemical shift values, outside of the normal spectral window of approximately 10 ppm. This wide range of chemical shifts is a well‐known paramagnetic shift effect observed in organometallic compounds with paramagnetic ions . These shielding and deshielding effect depend on the orientation and distance of the hydrogen atoms to the local magnetic field produced by the unpaired electron of the copper ions.…”

A computational study of structural and magnetic properties of bi- and trinuclear Cu(II) complexes with extremely long Cu---Cu distances

“…between the corresponding singly-occupied molecular orbitals (so-called SOMOs), determines the type of coupling -ferromagnetic when the ground electronic state corresponds to the high-spin configuration with parallel orientation of electronic spins and antiferromagnetic when the ground electronic state is characterized by the low total spin with antiparallel orientation of the electronic spins [1,2]. Nowadays, the most prevalent and well studied organometallic magnetic systems are dinuclear copper(II) complexes with different types of linkers between the ligand-coordinated magnetic centers (-Hal-, [6,7], -OH-, [4,6,8,9] -OR-, [10][11][12][13] CN-, [14] -N^N-, [8,9,15] -N^O- [16,17] etc.). These complexes can demonstrate a variety of coupling types from strongly antiferomagnetic (exchange coupling constant J CuCu is strongly negative up to -1000 cm -1 ) [4][5][6] to moderately ferromagnetic (J CuCu is positive, typically of order 100 cm -1 ) [10,18].…”

Synthesis, Characterization and Crystal Structures of Mixed-Ligand Complexes of [Cu(bipy)₂N₃(ClO₄)] and [Cu(5,5’-DiMebipy)₂(N₃)](ClO₄)

“…The coordination chemistry of ambidentate ligands possessing two different atoms such as SCN -, NO2 -, N3 -, Cl -etc capable of coordinating to metal ions has been a subject of interest to both synthetic and theoretical coordination chemists for a long time [11][12][13][14][15][16][17].…”