Exchange Coupling in Homo- and Heterodinuclear Complexes CuIIM [M = Cr(III), Mn(III), Mn(II), Fe(III), Co(III), Co(II), Ni(II), Cu(II), Zn(II)]. Synthesis, Structures, and Spectroscopic Properties

Abstract

A series of heterodinuclear complexes CunM, where M = Cr(III) (1), Mn(III) (2), Mn(II) (3), Fe(III) (4), Co(II) (5), Co(IlI) (6), Ni(II) (7), Cu(II) (8), and Zn(II) (9), containing the oximato dianion (Dopn* 12~) as bridging ligand and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as one of the two capping ligands have been synthesized by using the [Cu(DopnH)]+ cation (H2Dopn = 3,9-dimethyl-4,8-diazaundeca-3,8-diene-2,10-dione dioxime) as a ligand for the different ML"+ centers. The compounds have been characterized on the basis of IR, electronic, and EPR spectroscopy and variable-temperature (2-295 K) magnetic susceptibility measurements. The dinuclear complexes are quasi-isostructural with the copper(H) ion in a distorted square pyramidal environment, CuN40, and the M ion, except for that in 8, is six-coordinate with the MN3O3 or MN3O2CI coordination sphere. For M = Cu (8), the coordination geometry of M with the CuN302 chromophore is also square pyramidal. The crystal and molecular structures of the compounds [(Dopn)CuI1(OH2)CrIII(OCH3)L](C104)2,H20 (1) and [(Dopn)Cuu(u-CH3COO)-MnnIL] (C104)r2H20 (2) have been established by X-ray diffraction. 1 crystallizes in the monoclinic system space group P2i/n, with cell constants a = 13.096(3) A, b= 17.933(4) A, c= 15.994(3) A, /3= 113.49(3)°, V= 3444.9(13) A3 4, and Z = 4. The structure consists of oximato-bridged CunCrIn dications and noncoordinated perchlorate anions, with a Cu-• -Cr distance of 3.86 A. The crystal data for 2 are as follows: orthorhombic, space group Z>212121, a = 12.275(4) A,b = 14.171(9) A, c = 19.780(3) A, V -3441(2) A3, Z = 4. The structure consists of a six-coordinate Mn(III) center, MnN3C>3, and the copper(II) center has an N40 donor set. An acetate group bridges the manganese and copper ions with a Cu-• -Mn separation of 3.54 A. A low-quality X-ray structure determination for the CunFein complex is also reported. Analysis of the susceptibility data yields a strong antiferromagnetic interaction (27 = -596 cm-1) between adjacent Cu(II) ions in 8, showing once again that bridging oximes are good mediators for exchange interactions. The strength of the effective antiferromagnetic interaction decreases with increasing number of unpaired electrons in this series, 8>7>5>4~3.Moderately strong ferromagnetic interactions have been observed for CunMnIU (2) (27 = +109 cm-1) and CunCrin (1) (27 = +37 cm-1). A qualitative rationale has been provided for the difference in magnetic behaviors. The X-band EPR spectra (3-77 K) have been measured to establish the ground states of the dinuclear complexes. Well-resolved S = 2 EPR spectra for different heterometal systems have been observed.

Asymmetry and Magnetism in Bis(oximato)-Bridged Heterobimetallic Compounds: A Computational Approach

“…Calculated exchange-coupling constants (J) for the model complexes (Table 2) are in excellent agreement with the experimental ones reported by Birkelbach et al [28] The computational strategy used to extract the coupling constant can therefore also reproduce these parameters for compounds in which the different number of electrons on each metal complicates the evaluation of the energy for the HS and BS states.…”

“…The relative magnitudes were qualitatively discussed by various authors [28,36] on the basis of the ideas of van Vleck [44] and Anderson. [45,46] They suggest that the exchange interaction in a binuclear compound with magnetic centers A and B can be separated in partial contributions that involve different pairs of orbitals bearing unpaired electrons on both magnetic centers [Eq.…”

“…The model also suggests that the global exchangecoupling interaction for the dimers with M Cr III and M Mn III should be ferromagnetic, in good agreement with both the experimental data and our calculated J values. A more detailed discussion of the application of this model to cases with an intermediate situation is described in references [28] and [36].…”

“…the relation between the magnetic properties and spin topologies of these compounds were studied experimentally, [28,36] and magneto ± structural correlations [30] were found for the most important structural parameters.…”

Ferromagnetic exchange in a twisted, oxime-bridged [MnIII2] dimer

“…16 The bands centred at 1633 cm -1 may be tentatively attributed to the (C=N) oxime stretching mode, as observed elsewhere. 17 Attempts at assigning the (N-O) stretching modes in Napth-saoH 2 and 1 were severely hampered by the significant spectral overlap in the spectral range and so should be considered here. 18 A full report on these findings is given in the ESI.…”