Chain Compounds Based on Paddle-wheel Copper(II) Carboxylate Bearing Four Nitroxide Radicals

Chain compounds of paddle-wheel Cu2-clusters of 3-carboxy-2,2,5,5-tetramethyl1-pyrrolidinyloxy (Hcaproxy) and 4-carboxy-2,2,6,6-tetramethylpiperidinyloxy (Hcatempo) and N,N’-bidentate ligands (L = 4,4′-bipyridine (4,4′-bpy), 1,2-bis(4-pyridyl)ethane (bpe), trans-1,2-bis(4-pyridyl)ethylene (bpel), 4,4′-dipyridyl disulfide (pds), 1,4-diazabicyclo[2.2.2]octane (dabco), and pyrazine (pyz)), [Cu2(caproxy)4(L)]n, and [Cu2(catempo)4(L)]n, were synthesized and characterized by elemental analysis, infrared and UV-vis spectra and temperature dependence of magnetic susceptibilities (4.5–300 K). The crystal structures of [Cu2(caproxy)4(pds)]n, [Cu2(catempo)4(4,4-bpy)]n, and [Cu2(catempo)4(bpe)]n revealed zigzag or linear chains consisting of alternate arrangement of the dinuclear cluster bearing four nitroxide radicals and N,N’-bidentate ligand. Temperature dependence of magnetic susceptibilities showed a considerable antiferromagnetic interaction between the two copper(II) ions within the dinuclear cluster, and weak antiferromagnetic interaction between the dinuclear clusters and/or the radical and dinuclear cluster.


Synthesis of Chain Compounds
The parent dinuclear copper(II) carboxylates, [Cu 2 (caproxy)  (2), were synthesized, according to the previously reported method [18].Reaction of the parent dinuclear copper(II) carboxylate and linking ligand formed coordination polymers with a one-dimensional chain structure: [Cu 2 (caproxy) (12), and [Cu 2 (catempo) 4 (dabco)] n (13).Elemental analysis data of the isolated compounds are in accordance with the formulation of these coordination polymers.Many trials to prepare the chain adduct of catempo system with pyrazine were unsuccessful, resulting in recovery of the parent complex, which was confirmed by elemental analysis and infrared spectra.

Infrared Spectra of Chain Compounds
Infrared spectra of the chain compound 3 are compared with those of the parent dinuclear copper(II) complex 1 in Figure 3 as a representative example.Infrared spectral data of all of the present compounds are listed in Tables 1 and 2. The present compounds showed two COO stretching bands at 1609-1635 and 1398-1423 cm −1 , with the difference in energy characteristic of syn-syn-bridging carboxylate as similar to the parent dinuclear copper(II) complexes, showing the preserved structure of the paddle-wheel-like dinuclear core in the chain compounds [29].The NO stretching band due to the nitroxide moiety was observed at 1361-1365 cm −1 , as similar to the parent dinuclear copper(II) complexes, showing the presence of the nitroxide radicals.The medium band at around 1009-1074 cm −1 can be assigned as the pyridine ring-breathing modes, suggesting the presence of the pyridine-containing linking ligands in the compounds 3-6 and 9-12 [30].It is known that the shift of the pyridine ring vibrational band (ν (CC/CN)) to the higher energy side is indicative of the coordination of the pyridine nitrogen atom to metal ion [31][32][33].The corresponding vibration of free 4,4 -bpy, bpe, bpel, and pds could be observed as a strong band at 1597, 1596, 1596, and 1574 cm −1 , respectively.These vibration bands tend to be shifted to a higher energy region in most of the chain compounds, suggesting the coordination of the linking ligand to the dinuclear cluster.In the infrared spectra of 7 and 13, two medium bands corresponding to the skeletal modes of dabco molecule [34] can be found, suggesting the presence of dabco ligand in these compounds.(9), bpe (10), bpel (11), pds (12), and dabco (13).

Electronic Spectra of Chain Compounds
The absorption bands that were observed in the electronic spectra of compounds 1-13 are listed in Tables 3 and 4. The diffused reflectance spectra of all the compounds (Figure 4) can be characterized as three main absorption bands, as previously reported for copper(II) acetate type cluster systems [5,7].The ligand to metal charge transfer is responsible for the high intensity bands in the UV region.The shoulder-like absorption band at near-UV region can be assumed as a characteristic band of copper(II) acetate type clusters [5,7].Furthermore, all broad bands spanned in the visible and NIR regions until around 1000 nm are typically interpreted as d-d transitions [35].Although these main absorption bands appear at almost the same region, the d-d transition bands of the chain compounds have more clear shoulder band at lower energy side compared with those of the parent complexes, reflecting the coordination of the N,N'-bidentate ligand in the chain compounds.The comparatively strong σ-electron donation of the axial nitrogen donor than the oxygen donor of the aqua ligand of the parent compounds may explain this difference [35].Methanol solutions of these complexes show an intense UV-band with λ max at 205-225 nm (ε ≈ 2820-12030 mol −1 dm 3 cm −1 ), which might be ascribed to ligand-to-metal charge transfer (LMCT).The broad absorption band at Vis-NIR region with λ max centered around 700 nm (ε ≈ 110-340 mol −1 dm 3 cm −1 ) can be assigned to the characteristic copper(II) d-d transitions [35].Despite those transition bands, the expected absorption for the copper acetate type clusters may be superimposed with the nearest LMCT band.We occasionally detected a vague shouldering band at around 370 nm.broad absorption band at Vis-NIR region with λmax centered around 700 nm (ε ≈ 110-340 mol −1 dm 3 cm −1 ) can be assigned to the characteristic copper(II) d-d transitions [35].Despite those transition bands, the expected absorption for the copper acetate type clusters may be superimposed with the nearest LMCT band.We occasionally detected a vague shouldering band at around 370 nm.

Crystal Structures of Chain Compounds
Single crystals suitable for X-ray diffraction work were obtained for the parent dinuclear complex [Cu2(catempo)4(H2O)2] (2).In this study, we obtained better data with good quality crystals, as compared with the previously reported data [18].Crystal data and details concerning data collection are given in Table 5. Selected bond lengths and angles are listed in Table 6.The crystal and molecular structure of 2 is almost the same as the reported one [18], as shown in Figure 5.The molecule has a copper acetate type dinuclear core with four catempo -carboxylato-bridges,

Crystal Structures of Chain Compounds
Single crystals suitable for X-ray diffraction work were obtained for the parent dinuclear complex [Cu 2 (catempo) 4 (H 2 O) 2 ] (2).In this study, we obtained better data with good quality crystals, as compared with the previously reported data [18].Crystal data and details concerning data collection are given in Table 5. Selected bond lengths and angles are listed in Table 6.The crystal and molecular structure of 2 is almost the same as the reported one [18], as shown in Figure 5.The molecule has a copper acetate type dinuclear core with four catempo -carboxylato-bridges, which has a crystallographic inversion center at the mid-point of the Cu 2 core.The axial position of each copper atom is occupied by the water molecule, forming an elongated square-pyramidal geometry.The copper atoms lie on the basal O 4 plane toward the axial nitrogen atom by 0.18 Å.The Cu•••Cu' distance is 2.6018(4) Å.This feature is normal as the copper acetate type clusters [3][4][5][6][7].The N1-O3 and N2-O6 bond distances of the catempo -moieties are 1.2825 (17) and 1.2879(15) Å, respectively, which is comparable to that of tempo radical (1.283(9) Å) [36], showing a normal distance as a free nitroxide radical.Most of the chain compounds were obtained as microcrystalline precipitates, which are not suitable for single-crystal X-ray analysis.We barely isolated small crystals of 6, 9, and 10 after many trials to grow single crystals.Although the qualities of these crystals were not good enough to obtain the precise crystal structures, we could see the chain structures with alternating arrangement of the dinuclear carboxylate cluster and the linking ligand.The chain adduct of Cu 2 (caproxy) 4 system with pds ligand, [Cu 2 (caproxy) 4 (pds)] n (6), crystallizes in the triclinic system.A perspective view of the crystal structure of compound 6 is shown in Figure 6.The chain molecule consists of alternating linking of dinuclear Cu 2 (caproxy)  6) and 2.165(6) Å, respectively, to form a distorted square pyramidal geometry.This feature is originated from the Jahn-Teller distortion of copper(II) ion and has been similarly observed in several related chain compounds of copper(II) carboxylate groups [10,12,19,21,22].The copper atom lies on the basal O 4 plane toward the axial nitrogen atom by 0.20 Å for Cu1 and Cu2.As our expectation, the pds ligand being a bidentate ligand resulted in the formation of one-dimensional coordination polymer, although the chain is achiral.In view of the fact that the Cu1 -Cu1-N5, C39-S1-S2, S1-S2-C44, and N6-Cu2-Cu2" are 174.38 (17), 105.0(3), 105.3(3), and 176.81 (19) • , respectively, it is clear that the structural twisting and bending are escorted by the pds ligand, and thus, this complex has a zig-zag chain feature.The S-S bond distance that is found in the present complex is 2.027(4) Å.Additionally, the C-S-S-C torsion angle of this pds complex is 94.55 • and within the range of torsion angles (78.0-96.The chain adduct of Cu 2 (catempo) 4 system with dpe ligand, [Cu 2 (catempo) 4 (dpe)] n (10), crystallizes in the tetragonal system.Selected good crystals were subjected to the X-ray crystal structure analysis.Nonetheless, these crystals easily decomposed and provided only poor crystallographic data.A perspective view of the molecular structure of 10 is depicted in Figure 10.The two copper atoms are equatorially linked by the four bridging carboxylate radicals (catempo -), because of the crystallographic symmetry and the pyridine rings of bpe are axially coordinated.The molecular structure of 10 consists of alternating linking of dinuclear Cu 2 (catempo) 4  In the crystal, water molecules are included and formed hydrogen-bondings with nitrogen atom of bpe ligand, as shown in Figure 11.
From the crystal structures, it was found out that the present linking ligands can work as a connecter to assemble the copper-acetate type dinuclear clusters with four radical spins.The pds linking ligand was expected to form a chiral chain for constructing chiral magnetic materials.However, we could not find out the way to isolate such species in the present study.Further studies are needed to obtain a chiral chain compound.

Magnetic Data of One-Dimensional Coordination Polymers.
The magnetic properties for the chain compounds 6 and 9 are displayed in Figure 12, as the temperature variations of effective magnetic moment (μeff) and susceptibility (χM) as representative examples.The present complexes exhibit room-temperature magnetic moments at around 4 μB per mole of the dinuclear unit, as shown in Table 7.In general, the observed values of effective magnetic moments tend to be a little lower than the theoretical value.The calculated spin-only value is 4.24 μB for non-interacting six 1/2 spin that was sourced from the dinuclear copper core with four nitroxide radicals.When cooling, the magnetic moments steadily decrease from 300 to ca. 15 K, and then abruptly diminish to a value of approximately 3-3.5 μB per mole of the dinuclear unit at 4.5 K. Overall, each chain adduct shows a similar pattern of magnetic moment decreasing In the crystal, water molecules are included and formed hydrogen-bondings with nitrogen atom of bpe ligand, as shown in Figure 11.In the crystal, water molecules are included and formed hydrogen-bondings with nitrogen atom of bpe ligand, as shown in Figure 11.
From the crystal structures, it was found out that the present linking ligands can work as a connecter to assemble the copper-acetate type dinuclear clusters with four radical spins.The pds linking ligand was expected to form a chiral chain for constructing chiral magnetic materials.However, we could not find out the way to isolate such species in the present study.Further studies are needed to obtain a chiral chain compound.

Magnetic Data of One-Dimensional Coordination Polymers.
The magnetic properties for the chain compounds 6 and 9 are displayed in Figure 12, as the temperature variations of effective magnetic moment (μeff) and susceptibility (χM) as representative examples.The present complexes exhibit room-temperature magnetic moments at around 4 μB per mole of the dinuclear unit, as shown in Table 7.In general, the observed values of effective magnetic moments tend to be a little lower than the theoretical value.The calculated spin-only value is 4.24 μB for non-interacting six 1/2 spin that was sourced from the dinuclear copper core with four nitroxide radicals.When cooling, the magnetic moments steadily decrease from 300 to ca. 15 K, and then abruptly diminish to a value of approximately 3-3.5 μB per mole of the dinuclear unit at 4.5 K. Overall, each chain adduct shows a similar pattern of magnetic moment decreasing with lowering of temperature, suggesting that the magnetic behavior is antiferromagnetic as a From the crystal structures, it was found out that the present linking ligands can work as a connecter to assemble the copper-acetate type dinuclear clusters with four radical spins.The pds linking ligand was expected to form a chiral chain for constructing chiral magnetic materials.However, we could not find out the way to isolate such species in the present study.Further studies are needed to obtain a chiral chain compound.

Magnetic Data of One-Dimensional Coordination Polymers.
The magnetic properties for the chain compounds 6 and 9 are displayed in Figure 12, as the temperature variations of effective magnetic moment (µ eff ) and susceptibility (χ M ) as representative examples.The present complexes exhibit room-temperature magnetic moments at around 4 µ B per mole of the dinuclear unit, as shown in Table 7.In general, the observed values of effective magnetic moments tend to be a little lower than the theoretical value.The calculated spin-only value is 4.24 µ B for non-interacting six 1/2 spin that was sourced from the dinuclear copper core with four nitroxide radicals.When cooling, the magnetic moments steadily decrease from 300 to ca. 15 K, and then abruptly diminish to a value of approximately 3-3.5 µ B per mole of the dinuclear unit at 4.5 K. Overall, each chain adduct shows a similar pattern of magnetic moment decreasing with lowering of temperature, suggesting that the magnetic behavior is antiferromagnetic as a whole.Therefore, the magnetic property of present complexes was described by the equation (Equation ( 1)) based on a chain model consisting of a dinuclear copper(II) unit and four radicals, in which J is the exchange integral for the two copper(II) ions, θ is the Weiss temperature to account for the interaction between the radical and copper(II) ion, radical-radical, and/or the dinuclear cluster-dinuclear cluster interaction, and the other symbols have their usual meanings [14,18].The best fitting parameters are shown in Table 7.The J values of the chain adducts are in the range of −125-−245 cm −1 for the caproxy and catempo systems, confirming that the copper(II) ions are coupled antiferromagnetically.These coupling constants are comparable to those of the parent complexes as well as the copper(II) carboxylate clusters [3][4][5][6][7].The interpretation of small negative θ values can be deduced for the radical and copper(II) ion, radical-radical, and/or the dinuclear cluster-dinuclear cluster interaction.In the various complexes of paddle-wheel type dinuclear copper(II) carboxylates with axially-coordinated nitroxide radicals [14,25,26], the copper atoms are normally found to weakly interact with the radicals in antiferromagnetic fashion.The interaction between dinuclear clusters via the linking N,N'-bidentate ligands is weak probably because of the long distance between the dinuclear clusters.whole.Therefore, the magnetic property of present complexes was described by the equation (Equation ( 1)) based on a chain model consisting of a dinuclear copper(II) unit and four radicals, in which J is the exchange integral for the two copper(II) ions, θ is the Weiss temperature to account for the interaction between the radical and copper(II) ion, radical-radical, and/or the dinuclear cluster-dinuclear cluster interaction, and the other symbols have their usual meanings [14,18].The best fitting parameters are shown in Table 7.The J values of the chain adducts are in the range of −125-−245 cm −1 for the caproxy and catempo systems, confirming that the copper(II) ions are coupled antiferromagnetically.These coupling constants are comparable to those of the parent complexes as well as the copper(II) carboxylate clusters [3][4][5][6][7].The interpretation of small negative θ values can be deduced for the radical and copper(II) ion, radical-radical, and/or the dinuclear cluster-dinuclear cluster interaction.In the various complexes of paddle-wheel type dinuclear copper(II) carboxylates with axially-coordinated nitroxide radicals [14,25,26], the copper atoms are normally found to weakly interact with the radicals in antiferromagnetic fashion.The interaction between dinuclear clusters via the linking N,N'-bidentate ligands is weak probably because of the long distance between the dinuclear clusters.Sarma et al. have reported chain systems for the paddle wheel unit of [Cu2(o-NO2-C6H4COO)4(BPNO)2] with two kinds of spacer ligands (4,4′-bipyridyl-N,N'-dioxide and 2,2′-bipyridyl-N,N'-dioxide) and pointed out no discernible interaction between the units [39].

Table 7 .
Magnetic data of the chain compounds.