Copper(II) Carboxylates with 2,3,4-Trimethoxybenzoate and 2,4,6-Trimethoxybenzoate: Dinuclear Cu(II) Cluster and µ -Aqua-Bridged Cu(II) Chain Molecule

Copper(II) Carboxylates with 2,3,4-Trimethoxybenzoate and 2,4,6-Trimethoxybenzoate: Dinuclear Cu(II) Cluster and µ -Aqua-Bridged Cu(II) Chain Molecule. Abstract: Copper(II) complexes with 2,3,4-trimethoxybenzoic acid (H234-tmbz) and 2,4,6-trimethoxyb enzoic acid (H246-tmbz), [Cu 2 (234-tmbz) 4 (H 2 O) 2 ] ( 6 ) and [Cu(246-tmbz) 2 ( µ -H 2 O) 2 (H 2 O) 2 ] n ( 7 ), were synthesized and characterized by elemental analysis, infrared and UV-vis spectra and temperature dependence of magnetic susceptibilities (1.9–300 K). The X-ray crystal structures revealed that the former 6 is a dinuclear cluster having syn-syn -bridged Cu 2 ( µ -234-tmbz) 4 core with Cu ··· Cu separation of 2.6009(7) Å, while the latter 7 is a µ -aqua-bridged chain molecule consisting of Cu(246-tmb) 2 ( µ H 2 O) 2 (H 2 O) 2 units with Cu ··· Cu separation of 4.1420(5) Å. Temperature dependence of magnetic susceptibilities showed that an antiferromagnetic interaction with 2 J = − 272 cm − 1 for 6 and a weak antiferromagnetic interaction with J = − 0.21 cm − 1 for 7 , between the two copper(II) ions. The adsorption isotherm of 6 showed Types I behavior having a 125.4 m 2 g − 1 of speciﬁc surface area.


Infrared Spectra of Copper(II) Carboxylates
In the infrared spectra of the complex 6, antisymmetric and symmetric stretching bands for COO − group were observed at 1602 and 1468 cm −1 with the energy difference of νas(COO) and νs(COO) of 134 cm −1 , which is similar to those observed for dinuclear copper(II) carboxylates with syn-syn mode of carboxylato bridges [15,23]. On the other hand, the complex 7 exhibited two COO stretching bands at 1608 and 1414 cm −1 with the greater separation of ∆ν of 194 cm −1 , which is characteristic of monodentate coordination of carboxylate ligands [15,23], and consistent with the crystal structure, as described in Section 2.4. The strong band at 3441 cm −1 in 6 can be assigned to OH stretching band of coordinated or crystallization water molecules [23]. The four medium bands at around 3634-3120 cm −1 in 7 also can be assigned as OH stretching bands, suggesting the presence of bridging water molecules as well as coordinated and crystal water molecules in the compound [24,25]. It is known that the frequency shift of the ν(OH) bands to the lower energy side is indicative of the hydrogen bonded state of the water molecules [26]. The stretching vibrations of the CH3 of methoxy groups appeared at 2941 and 2839 cm −1 in 6 and 2948 and 2840 cm −1 in 7, respectively, confirming the presence of the methoxy groups of 2,3,4-trimethoxybenzoate and 2,4,6-trimethoxybenzoate ligands, respectively [26].

Electronic Spectra of Copper(II) Carboxylates.
The diffused reflectance spectra of the present complexes are shown in Figure 3. The spectra of 6 show a broad band at around 246 and 286 nm, which can be assigned to ligand-to-metal charge transfer bands in the UV-region, a shoulder band at around 360 nm, which can be a distinctive CT band characteristic of copper acetate type dinuclear clusters [3,7,11], and a broad band assignable to d-d transitions at around 706 nm. The d-d band of 6 is located at higher energy side compared with that of 7. Moreover, the typical broad asymmetric band with a shoulder at around 1000 nm is in harmony with the distorted square pyramidal coordination of copper(II) [27] as found in the crystal structure of 6. On the other hand, the spectra of 7 can be characterized as four absorption bands, being a little different from those previously reported for copper acetate type clusters, lacking a

Infrared Spectra of Copper(II) Carboxylates
In the infrared spectra of the complex 6, antisymmetric and symmetric stretching bands for COO − group were observed at 1602 and 1468 cm −1 with the energy difference of ν as (COO) and ν s (COO) of 134 cm −1 , which is similar to those observed for dinuclear copper(II) carboxylates with syn-syn mode of carboxylato bridges [15,23]. On the other hand, the complex 7 exhibited two COO stretching bands at 1608 and 1414 cm −1 with the greater separation of ∆ν of 194 cm −1 , which is characteristic of monodentate coordination of carboxylate ligands [15,23], and consistent with the crystal structure, as described in Section 2.4. The strong band at 3441 cm −1 in 6 can be assigned to OH stretching band of coordinated or crystallization water molecules [23]. The four medium bands at around 3634-3120 cm −1 in 7 also can be assigned as OH stretching bands, suggesting the presence of bridging water molecules as well as coordinated and crystal water molecules in the compound [24,25]. It is known that the frequency shift of the ν(OH) bands to the lower energy side is indicative of the hydrogen bonded state of the water molecules [26]. The stretching vibrations of the CH 3 of methoxy groups appeared at 2941 and 2839 cm −1 in 6 and 2948 and 2840 cm −1 in 7, respectively, confirming the presence of the methoxy groups of 2,3,4-trimethoxybenzoate and 2,4,6-trimethoxybenzoate ligands, respectively [26].

Electronic Spectra of Copper(II) Carboxylates
The diffused reflectance spectra of the present complexes are shown in Figure 3. The spectra of 6 show a broad band at around 246 and 286 nm, which can be assigned to ligandto-metal charge transfer bands in the UV-region, a shoulder band at around 360 nm, which can be a distinctive CT band characteristic of copper acetate type dinuclear clusters [3,7,11], and a broad band assignable to d-d transitions at around 706 nm. The d-d band of 6 is located at higher energy side compared with that of 7. Moreover, the typical broad asymmetric band with a shoulder at around 1000 nm is in harmony with the distorted square pyramidal coordination of copper(II) [27] as found in the crystal structure of 6. On the other hand, the spectra of 7 can be characterized as four absorption bands, being a little different from those previously reported for copper acetate type clusters, lacking a distinctive shoulder-like absorption at near-UV region. The absorption bands at 212, 254, and 310 nm can be assigned to ligand to metal charge transfer bands, which are responsible for the high intensity bands in the UV region. Furthermore, a broad band at 750 nm spanned in the visible and NIR regions until around 1200 nm is typically interpreted as d-d transitions of the elongated octahedral copper(II) [27], which is observed for the crystal structure of 7 in Section 2.4.
Magnetochemistry 2021, 7, x FOR PEER REVIEW 4 of 14 distinctive shoulder-like absorption at near-UV region. The absorption bands at 212, 254, and 310 nm can be assigned to ligand to metal charge transfer bands, which are responsible for the high intensity bands in the UV region. Furthermore, a broad band at 750 nm spanned in the visible and NIR regions until around 1200 nm is typically interpreted as d-d transitions of the elongated octahedral copper(II) [27], which is observed for the crystal structure of 7 in Section 2.4.

Crystal Structures of Copper(II) Carboxylates
Single crystals were obtained by recrystallization from methanol for complexes 6 and 7. Crystal data and details concerning data collection are given in Table 1. Selected bond lengths and angles are listed in Table 2. Both of the presented complexes crystallized in the monoclinic lattice. As for 6, the crystal contains coordinating methanol molecules with a formula [Cu2(234-tmbz)4(CH3OH)2] (6′), slightly different from 6. A perspective view of the molecular structure of 6′ is shown in Figure 4. The asymmetric unit consists of half of a [Cu2(234-tmbz)4(CH3OH)2] molecule with the crystallographic inversion center at the midpoint of the Cu2 core. The molecule has a copper acetate type dinuclear core with four syn-syn carboxylate-bridges and the structure is similar to that of the one reported for [Cu2(345-tmbz)4(CH3OH)2] [16]. The copper atom is coordinated by four carboxylate oxygen atoms of 234-tmbz − with the Cu1-O distances of 1.9504(18)-1.9827 (17) Å and an apical oxygen atom of methanol with the Cu1-O11 distance of 2.1309(19) Å to form a distorted square pyramidal geometry. The apical methanol molecules came from the recrystallization solvent. The copper atom lies on the basal O4 plane toward the apical oxygen atom by 0.178 Å. The Cu···Cu' distance is 2.6009(7) Å, which is normal as found in copper(II) acetate type dinuclear clusters [4][5][6][7][8][9][10][11]. This feature is originated from the pseudo Jahn-Teller distortion of copper(II) ion and has been similarly observed in copper(II) acetate type clusters. The relationship between the Cu···Cu distance and apical coordination was recently elucidated [28,29]. The coordination of apical ligand weakens the Cu-Cu interaction which becomes longer upon the apical coordination and the distortion of the Cu atom from the planar arrangement can be understood to be due to electrostatic attraction between the Cu(II) and apical ligand's dipole moment, reflecting trans influence of apical ligand [29]. The benzoate moieties are not planar as like the re-

Crystal Structures of Copper(II) Carboxylates
Single crystals were obtained by recrystallization from methanol for complexes 6 and 7. Crystal data and details concerning data collection are given in Table 1. Selected bond lengths and angles are listed in Table 2. Both of the presented complexes crystallized in the monoclinic lattice. As for 6, the crystal contains coordinating methanol molecules with a formula [Cu 2 (234-tmbz) 4 (CH 3 OH) 2 ] (6 ), slightly different from 6. A perspective view of the molecular structure of 6 is shown in Figure 4. The asymmetric unit consists of half of a [Cu 2 (234-tmbz) 4 (CH 3 OH) 2 ] molecule with the crystallographic inversion center at the midpoint of the Cu 2 core. The molecule has a copper acetate type dinuclear core with four syn-syn carboxylate-bridges and the structure is similar to that of the one reported for [Cu 2 (345-tmbz) 4 (CH 3 OH) 2 ] [16]. The copper atom is coordinated by four carboxylate oxygen atoms of 234-tmbz − with the Cu1-O distances of 1.9504(18)-1.9827(17) Å and an apical oxygen atom of methanol with the Cu1-O11 distance of 2.1309(19) Å to form a distorted square pyramidal geometry. The apical methanol molecules came from the recrystallization solvent. The copper atom lies on the basal O 4 plane toward the apical oxygen atom by 0.178 Å. The Cu···Cu' distance is 2.6009(7) Å, which is normal as found in copper(II) acetate type dinuclear clusters [4][5][6][7][8][9][10][11]. This feature is originated from the pseudo Jahn-Teller distortion of copper(II) ion and has been similarly observed in copper(II) acetate type clusters. The relationship between the Cu···Cu distance and apical coordination was recently elucidated [28,29]. The coordination of apical ligand weakens the Cu-Cu interaction which becomes longer upon the apical coordination and the distortion of the Cu atom from the planar arrangement can be understood to be due to electrostatic attraction between the Cu(II) and apical ligand's dipole moment, reflecting trans influence of apical ligand [29]. The benzoate moieties are not planar as like the related dinuclear copper(II) benzoate analogues [8,30,31]. The dihedral angle (φ bend ) between the O1-C7-O2 plane of the carboxylato bridge and the Cu1-O1···O2-Cu1 plane and the dihedral angle (φ rot ) between the O1-C7-O2 plane and the benzoate C1-C2-C3-C4-C5-C6 ring are 1.9(3) • and 38.3(2) • , respectively. The φ bend and φ rot angles for the O6-C17-O7, Cu1-O6···O7-Cu1 , and C11-C12-C13-C14-C15-C16 planes are 5.9(4) • and 30.3(3) • , respectively. The distortion from the planar arrangement may be due to the packing effect in the crystal. As shown in Figure 5, the dinuclear molecules are loosely bound to the adjacent dinuclear molecules by the hydrogen bonds between the apical methanol molecules and the carboxylatooxygen atoms of the neighbor dinuclear molecules [O11···O2(x, 1 + y, z) 2.819 Å]. In the crystal, the hydrogen-bonded array of the dinuclear molecules are related by the crystallographic C 2 axis to the neighboring the hydrogen-bonded array of the dinuclear molecules. A perspective view of the molecular structure of 7 is depicted in Figure 6

Magnetic Properties of Copper(II) Carboxylates
The magnetic data for complex 6 is displayed in Figure 8 as the temperature variation of effective magnetic moment (μM) and magnetic susceptibility (χM) per dinuclear unit. The effective magnetic moment of 6 at 300 K is 1.53 µB per Cu atom, which is lower than the spin-only value of 1.73 µB for a magnetically isolated S = 1/2 spin with g = 2.0. The magnetic moment gradually decreases with a lowering of temperature, reaching the value of 0.11 µB at 1.9 K, suggesting an antiferromagnetic interaction between the copper(II) ions. The magnetic data were analyzed by the molecular field approximation (Equation (1)

Magnetic Properties of Copper(II) Carboxylates
The magnetic data for complex 6 is displayed in Figure 8 as the temperature variation of effective magnetic moment (µ M ) and magnetic susceptibility (χ M ) per dinuclear unit. The effective magnetic moment of 6 at 300 K is 1.53 µ B per Cu atom, which is lower than the spin-only value of 1.73 µ B for a magnetically isolated S = 1/2 spin with g = 2.0. The magnetic moment gradually decreases with a lowering of temperature, reaching the value of 0.11 µ B at 1.9 K, suggesting an antiferromagnetic interaction between the copper(II) ions. The magnetic data were analyzed by the molecular field approximation (Equation (1) [32]), for the Bleaney-Bowers Equation (2) [2] based on the Heisenberg model, H = −2JS 1 •S 2 , taking account of magnetic interaction between the neighboring dinuclear units as zJ (z = number of interacting neighbors), where g is g value, J is an exchange coupling constant for the two copper(II) ions within the cluster, p = the fraction of mononuclear copper(II) impurity, and Nα is the temperatureindependent paramagnetism, which was set to be 60 × 10 −6 cm 3 mol −1 for each copper(II) ion [14]. The best-fitting parameters are g = 2.16, 2J = −272 cm −1 , p = 0.0070, and zJ = −5 cm −1 as shown in Figure 8. The 2J value is comparable to those found in dinuclear copper(II) benzoate and its derivatives (2J = −250-−350 cm −1 ) [8,12,14,16,18,30,31,33]. It is known that there is a dependence of the magnetic coupling within the dinuclear cluster on the apical ligand species [34,35]. Considering for the apical ligand H 2 O for 6, the magnetic coupling within the dinuclear cluster of 6 can be regarded as relatively weak among the copper(II) benzoate analogues. The relatively weak antiferromagnetic interaction may be attributed to the bending of the benzoate moieties with the larger φ bend angles of 5.9(4) and 1.9(3) • in 6 , which induces a poor overlap between the magnetic orbital and the 2p x orbital of the benzoate oxygen atom, causing a suppression of the spin-exchange interaction via the benzoate-bridge [8,18]. The magnetic data for the complex 7 are shown in Figure 9 as the temperature variation of effective magnetic moment (µ A ) and magnetic susceptibility (χ A ) per mononuclear unit. The effective magnetic moment of 7 at 300 K is 1.91 µ B per the mononuclear unit. When cooling, the magnetic moment keeps constant until 20 K and steadily decreases from 20 to ca. 5 K, and then diminishes to a value of 1.69 µ B at 1.9 K, suggesting a weak antiferromagnetic interaction between the adjacent copper(II) ions. The crystal structure of 7 showed that the complex is essentially polynuclear copper(II) with an elongated octahedral geometry, where the axial µ-aqua-bonds with the distance of 2.3018(12) Å can be considered to intervene with the adjacent magnetic orbitals in the chain molecule. Therefore, magnetic interaction between the adjacent copper(II) ions was analyzed by the Bonner-Fisher equation (3) for an isolated Heisenberg 1D chain, χ A = (Ng 2 µ B 2 /kT)(0.25 + 0.14995x + 0.30094x 2 )/(1.0 + 1.9862x + 0.68854x 2 + 6.0626x 3 ) + Nα in which x = |J|/kT and J is the exchange integral for the two copper(II) ions, and the other symbols have their usual meanings [36]. The best fitting parameters are g = 2.179 (2) and lie in the equatorial plane involving the benzoate-and aqua-oxygen donors, and thus the superexchange interaction via the axial aqua-oxygen might be negligible. However, the superexchange interaction via the Cu-O-C-O···H-O-H···O-C-O-Cu is possible, because of the hydrogen bonding between the non-coordinating oxygen atom of the monodentate benzoate ligand and the axial aqua-oxygen atom, resulting in the weak antiferromagnetic interaction.

Adsorption Properties of Copper(II) Carboxylate
We measured the adsorption property of 6 for N2 to see if complex 6 has a porous structure or not. Intriguingly, the adsorption isotherm of N2 at 77 K showed an adsorption property with the Type I behavior having a 125.4 m 2 g −1 of specific surface area estimated from Langmuir plot as shown in Figure 10, meaning the existence of a uniform micropore in 6. A t-plot analysis of the N2 adsorption isotherm suggested a diameter of micropore to be 0.76 nm. If we refer to the crystal structure of 6′, it seems to have almost no voids in the crystal. However, very narrow voids faced each other by the benzoate rings can be found in the crystal structure as shown in Figure 11. Similar narrow voids were found in the chain compound of dinuclear rhodium(II) benzoate with pyrazine, which is known as a porous material with the Type I adsorption isotherm for N2 gas [41,42].

Adsorption Properties of Copper(II) Carboxylate
We measured the adsorption property of 6 for N 2 to see if complex 6 has a porous structure or not. Intriguingly, the adsorption isotherm of N 2 at 77 K showed an adsorption property with the Type I behavior having a 125.4 m 2 g −1 of specific surface area estimated from Langmuir plot as shown in Figure 10, meaning the existence of a uniform micropore in 6. A t-plot analysis of the N 2 adsorption isotherm suggested a diameter of micropore to be 0.76 nm. If we refer to the crystal structure of 6 , it seems to have almost no voids in the crystal. However, very narrow voids faced each other by the benzoate rings can be found in the crystal structure as shown in Figure 11. Similar narrow voids were found in the chain compound of dinuclear rhodium(II) benzoate with pyrazine, which is known as a porous material with the Type I adsorption isotherm for N 2 gas [41,42]. micropore in 6. A t-plot analysis of the N2 adsorption isotherm suggested a diameter of micropore to be 0.76 nm. If we refer to the crystal structure of 6′, it seems to have almost no voids in the crystal. However, very narrow voids faced each other by the benzoate rings can be found in the crystal structure as shown in Figure 11. Similar narrow voids were found in the chain compound of dinuclear rhodium(II) benzoate with pyrazine, which is known as a porous material with the Type I adsorption isotherm for N2 gas [41,42].

Materials and Methods
All the chemicals were commercial products and were used as supplied.

Materials and Methods
All the chemicals were commercial products and were used as supplied. Synthesis of [Cu 2 (234-tmbz) 4

Conclusions
In this study, two new copper(II) carboxylates 6 and 7 were prepared by a reaction of copper(II) nitrate with 2,3,4-trimethoxybenzoic acid or 2,4,6-trimethoxybenzoic acid. The X-ray crystal structure analysis revealed that 6 is the syn-syn-µ-carboxylato-bridged dinuclear copper(II) cluster with an antiferromagnetic interaction and with Type I N 2adsorption behavior having a 125.4 m 2 g −1 of specific surface area, while 7 is not dinuclear cluster, but the µ-aqua-bridged copper(II) chain molecule, where the magnetic interaction via the µ-aqua bridge was found to be weak and antiferromagnetic. In the cases of 3,4,5trimethoxybenzoic acid, 2,3,4-trimethoxybenzoic acid, and even more bulky 3,4,5-tri-Obenzylgalic acid, dinuclear copper(II) clusters were formed. Thus far, only in the case of 2,4,6-trimethoxybenzoic acid, the polynuclear copper(II) chain molecule was found for the first time here. Considering these results, the 2,4,6-trimethoxy groups of the benzoate ring should invoke a large rotation to the OCO plane because of the steric hindrance with the 2and 6-methoxy groups of the benzoate rings to form the Cu(II) chain compound.