Structural and Magnetic Properties of Mn^III and Cu^II Tetranuclear Azido Polyoxometalate Complexes: Multifrequency High‐Field EPR Spectroscopy of Cu₄ Clusters with S=1 and S=2 Ground States

Abstract

Two new azido-bridged polyoxometalate compounds were synthesized in acetonitrile/methanol media and their molecular structures have been determined by X-ray crystallography. The [[(gamma-SiW10O36)Mn2(OH)2(N3)(0.5)(H2O)(0.5)]2(mu-1,3-N3)](10-) (1 a) tetranuclear Mn(III) complex, in which an end-to-end N3- ligand acts as a linker between two [(gamma-SiW10O36)Mn2(OH)2]4- units, represents the first manganese-azido polyoxometalate. The magnetic properties have been studied considering the spin Hamiltonian H = -J1(S1S2+S1*S2*)-J2(S1S1*), showing that antiferromagnetic interactions between the paramagnetic centers (g = 1.98) occur both through the di-(mu-OH) bridge (J1 = -25.5 cm(-1)) and the mu-1,3-azido bridge (J2 = -19.6 cm(-1)). The [(gamma-SiW10O36)2Cu4(mu-1,1,1-N3)2(mu-1,1-N3)2]12- (2 a) tetranuclear Cu(II) complex consists of two [gamma-SiW10O36Cu2(N3)2]6- subunits connected through the two mu-1,1,1-azido ligands, the four paramagnetic centers forming a lozenge. The magnetic susceptibility data have been fitted. This reveals ferromagnetic interactions between the four Cu(II) centers, leading to an S=2 ground state (H = -J1(S1S2+S1*S2*)-J2(S2S2*), J1 = +294.5 cm(-1), J2 = +1.6 cm(-1), g = 2.085). The ferromagnetic coupling between the Cu(II) centers in each subunit is the strongest ever observed either in a polyoxometalate compound or in a diazido-bridged Cu(II) complex. Considering complex 2 a and the previously reported basal-basal di-(mu-1,1-N3)-bridged Cu(II) complexes in which the metallic centers are not connected by other magnetically coupling ligands, the linear correlation J1 = 2639.5-24.95*theta(av) between the theta(av) bridging angle and the J1 coupling parameter has been proposed. The electronic structure of complex 2 a has also been investigated by using multifrequency high-field electron paramagnetic resonance (HF-EPR) spectroscopy between 95 and 285 GHz. The spin Hamiltonian parameters of the S = 2 ground state (D = -0.135(2) cm(-1), E = -0.003(2) cm(-1), g(x) = 2.290(5), g(y) = 2.135(10), g(z) = 2.158(5)) as well as of the first excited spin state S = 1 (D = -0.960(4) cm(-1), E = -0.080(5) cm(-1), g(x) = 2.042(5), g(y) = 2.335(5), g(z) = 2.095(5)) have been determined, since the energy gap between these two spin states is very small (1.6 cm(-1)).

Robust Cluster Building Unit: Icosanuclear Heteropolyoxocopperate Templated by Carbonate

“…[25] Figure6as hows the powder EPR spectrumo f1 (200 K), which at first view suggests almost axial symmetry.However,itisprobably aspectrum that encompasses many individually different Cu II centers, mutually accompanied by an et of strongm agnetic interactions. Figure 6b shows the single-crystal EPR spectrum of 1 with the crystal in an arbitrary orientation with respectt ot he magnetic field at 200 K. Possible hyperfine structurei sn ot resolved in either the powder or single-crystals pectrum,a lso as ar esult of strong magnetic interactions between the individually different Cu II centers.…”

Effect of Cyanato, Azido, Carboxylato, and Carbonato Ligands on the Formation of Cobalt(II) Polyoxometalates: Characterization, Magnetic, and Electrochemical Studies of Multinuclear Cobalt Clusters

“…[8] Recently, insertion into paramagnetic polyoxometalate matrices of exogenous ligands and especially that of the azido anion [9] has shown that such polydentate ligands can also allow the assembly of magnetic polyoxometalate subunits, [10] but in addition could impose ferromagnetic interactions within the cluster. [11,12] Co II polyoxometalate compounds containing from two to nine interacting paramagnetic centers have been reported.…”

Efficient Oxidative Alkyne Homocoupling Catalyzed by a Monomeric Dicopper‐Substituted Silicotungstate

“…[8] This reaction mechanism has generally been accepted, although some detailed mechanistic work is still necessary. [4][5][6][7][8] Herein we report that the dicopper-substituted g-Keggin silicotungstate TBA 4 [g-H 2 SiW 10 O 36 Cu 2 (m-1,1-N 3 ) 2 ] (I, Figure 1; TBA = tetra-n-butylammonium) [9] is an effective homogeneous catalyst for the oxidative homocoupling of various kinds of structurally diverse alkynes [Eq. [4][5][6][7][8] Herein we report that the dicopper-substituted g-Keggin silicotungstate TBA 4 [g-H 2 SiW 10 O 36 Cu 2 (m-1,1-N 3 ) 2 ] (I, Figure 1; TBA = tetra-n-butylammonium) [9] is an effective homogeneous catalyst for the oxidative homocoupling of various kinds of structurally diverse alkynes [Eq.…”

“…[10] The reaction proceeded efficiently even under 1 atm of air, although a longer reaction time was required (Table 1, entry 2). [9] respectively (Table 1, entries 3-5), [11] while non-polar 1,2dichloroethane and toluene were found to be poor solvents (Table 1, entries 6 and 7, respectively). Blue and green spheres show the copper and nitrogen atoms, respectively.…”