Exchange-Coupled Cobalt(II) Dimer with Unusual Magnetic Circular Dichroism Saturation Behavior

Abstract

The exchange-coupled complex [Co(2)(mu-H(2)O)(mu-OAc)(2)(OAc)(2)(tmen)(2)] (OAc = CH(3)COO(-) acetato; tmen = N,N,N',N'-tetramethylenediamine) has been studied by magnetic circular dichroism (MCD) spectroscopy and magnetization measurements. A peculiar behavior of the MCD spectra was observed with a change in the magnetic field. The intensity of particular lines initially increases, then decreases with an increase in the magnetic field strength, disappears with a further field increase, and appears again with the opposite sign. This behavior was explained by the overlap of the electronic transitions at different but near wavelengths. The obtained MCD magnetization curves, together with magnetization obtained by a SQUID magnetometer, are well reproduced using the Hamiltonian, which takes into account an orbital magnetic moment.

Synthesis, Magnetic Properties, and Phosphoesterase Activity of Dinuclear Cobalt(II) Complexes

“…The κ value for some of the complexes approaches the free ion value (∼0.93), and a similar observation is made for the [Co 2 (bhmp)­(CH 3 COO) 2 ]­(BPh 4 ) and [Co 2 (bomp)­(CH 3 COO) 2 ]­(BPh 4 ) complexes . The values of λ are smaller than that reported for the free ion (λ o = −180 cm –1 ), but in the range reported for other cobalt­(II) complexes; the difference is ascribed to covalency effects. ,,,− The ZFS parameter D is typical for six coordinate cobalt­(II) complexes and ranges from +95 to +146 cm –1 . ,,,− The positive values for the axial splitting parameter Δ (205–651 cm –1 ) suggest that the axis is elongated and that the orbital singlet is lowest in energy. − The magnitude of J ( H = - J ex S 1 · S 2 ) for [Co 2 (CO 2 EtH 2 L1 )­(CH 3 COO) 2 ]­(PF 6 ), [Co 2 (CO 2 Et L2 )­(CH 3 COO) 2 ]­(PF 6 ), [Co 2 (CH 3 L2 )­(CH 3 COO) 2 ]­(PF 6 ), and [Co 2 (NO 2 L2 )­(CH 3 COO) 2 ]­(PF 6 ) ( J = −0.66, −0.10, −0.67, and +0.42 cm –1 (0.5 T), respectively) suggests that the intramolecular coupling between the two cobalt­(II) sites is extremely weak and is similar to those determined for [Co 2 (bhmp)­(CH 3 COO) 2 ]­(BPh 4 ) and [Co 2 (bomp)­(CH 3 COO) 2 ]­(BPh 4 ) ( J = −0.33 to −0.70 cm –1 ), complexes with a similar ligand μ-phenoxo-bis­(μ-acetato) core . For the [Co 2 (Br L2 )­(CH 3 COO) 2 ]­(PF 6 ) complex ( J = +3.13 cm –1 ), the situation is less clear.…”

Magnetic and structural properties of dinuclear singly bridged-phenoxido metal(ii) complexes

“…In general, bridged-phenoxido dinuclear metal(II) complexes, which are derived from ligands with pendant pyridyl and/or pyridyl derivative arms (Chart 1) exhibit weak to moderate antiferromagnetic coupling. 8,37,53,54,[70][71][72][73] Copper(II) complexes in which a singly bridged phenoxido group is the only bridge exhibit very weak antiferromagnetic coupling as this was the case in complex 4 ( J = −0.79 cm −1 ). This has been observed in a number of related complexes such as [Cu 2 (µ-L Me -O)Cl 2 ]ClO 4 ( J = 0 cm −1 ), 53 Table 1 DFT-calculated net Mulliken spin densities (ρ), expected values <S 2 >, and isotropic exchange parameters ( J) from high-spin (HS) and broken symmetry spin (BS) states of the dinuclear molecular fragments based on X-ray structures of 1-4 weak ferromagnetic coupling was also reported in some related systems: [Co 2 (µ-L1 NO2 -O)(µ-OAc) 2 ]PF 6 ( J = +3.09 cm −1 ) and [Co 2 (µ-L Br -O)(µ-OAc) 2 ]PF 6 ( J = +0.78 cm −1 ) (see Chart 1).…”

“…75 In dicobalt(II) compounds, several factors were addressed to account for this weak interactions. These include the Co(II)-Co(II) and Co-O( phenoxido) bond distances as well as Co-O-Co bond angle, 2,71,74,76,77 but unlike coupled dicopper and dinickel(II) complexes, [78][79][80] the magneto-structural relationship for dicobalt(II) is not well resolved. The results of dicobalt(II) and dicopper(II) compounds clearly indicate that the µ-phenoxido bridge is a poor mediator for exchange magnetic interaction in this series of complexes.…”

“…Similar magnetic trends were reported with other 3d metal ions such as in[Ni 2 (µ-L Me -O)(µ-OAc) 2 ]BF 4 •2MeOH ( J = −2.3 cm −1 ) and [Mn 2 (µ-L′ Me -O)(µ-OAc) 2 ]ClO 4 ( J = −4.3 cm −1 ) (see Chart 1). 2,8,[70][71][72] Interestingly, nickel(II) complexes 2 and 3 revealed relatively high to moderate antiferromagnetic coupling with J values of −39.0 and −30.2 cm −1 , respectively. As indicated above, this was attributed to the efficient overlap between the Ni(II) magnetic orbitals dx 2 −y 2 and d z 2 and the oxygen 2p orbitals of the phenolate oxygen atom and to an increase of the Ni-O-Ni bond angle.…”

Modeling Spin Interactions in a Triangular Cobalt(II) Complex with Triaminoguanidine Ligand Framework: Synthesis, Structure, and Magnetic Properties

“…Polynuclear complexes of paramagnetic metal ions are continuously attracting considerable attention, not only due to their intriguing structures and magnetic properties but also due to their potential application in functional molecule-based materials. − The discovery of single-molecule magnets (SMMs) and their potential applications in high-density information storage has immensely stimulated the investigation of metal complexes in search of SMMs with high blocking temperature and slow magnetization relaxation. , This has particularly led to the discovery of a series of cobalt-based oligonuclear complexes exhibiting SMM behavior. , Besides this high-nuclearity approach recent years have shown that even mononuclear metal complexes provide a basis for systems exhibiting slow magnetic relaxation. − Also within this new class of single-ion magnets (SIMs) cobalt­(II) high-spin complexes with their S = 3/2 half-integer spin state attracted increasing interest in the past few years. − Toward the investigation of the electronic structure of such cobalt­(II)-based systems various spectroscopic methods have been employed such as multifrequency, high-field ESR, and variable-temperature, variable-field magnetic circular dichroism (MCD). − In this context the magnetic properties of polynuclear cobalt­(II) complexes have also been of general interest, and along this line numerous dinuclear, − trinuclear, − …”