Coordination Modes of Para-Substituted Benzoates Towards Divalent Copper Centers in the Presence of Diimines

Abstract

The coordination modes of several para-substituted benzoates towards a copper(II) center is investigated in the presence of α-diimines. The coordination environment of the metal ion also includes nitrogen atoms from 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy) and, occasionally, oxygen atoms from coordinated water, ethanol molecules or nitrate ions. The compounds are investigated by visible and infrared spectroscopy and by single crystal X-ray diffraction. Although the reaction scheme involved equimolar amounts of the reactants, compounds with metal-to-benzoate-to-diimine ratios of 1:2:1, 1:1:2 and 1:1:1 are realized, being either neutral or cationic in nature and either mono- or dinuclear. The better coordinating ability of nitrate relative to perchlorate is verified, as well as the subtle role of the para-substituent on the coordination mode of the benzoate and consequently on the overall structure of the compounds formed.

1. Introduction

The energetic stability and the structural versatility of divalent copper have elevated it to the status of one of the most studied ions in inorganic chemistry. Coupled with the possession of a single lone electron in its valence d orbital set, the above properties render Cu²⁺ coordination compounds as targets for the formation of a wide variety of both local and overall structures. The presence of ligands possessing more than one lone pair of electrons, either on the same or on different atoms of the ligand, may propagate the formation of oligomers or polymers in the form of 1D up to 3D frameworks [1,2]. The electron distribution within such structures gives rise to either localized or dispersed electron populations leading, in turn, to either unique or tunable electric, magnetic and optical properties. Such properties have made copper compounds suitable candidates for practically every application related to modern life requirements. Copper compounds have also found applications as catalysts for the oxidation of alkanes to alkyl hydroperoxides and alcohols to ketones [3], biomimicking processes [4,5], power sources in solid propellants [6], and ferromagnetic or antiferromagnetic materials [7,8]. Copper(II) clusters especially have been studied by both experimental and computational means in an effort to unravel magnetostructural correlations [9]. Furthermore, the availability of the metal as univalent promotes its participation in a variety of redox reactions [10]. The availability of lone pairs on both oxygen atoms in a carboxylate ion provides the background for the adoption of several coordination modes of the COO⁻ moiety to metal centers, among which the bridging of neighboring metal ions is not uncommon [11,12]. This behavior gives rise to the formation of dimer and oligomer compounds where short metal–metal interactions occur, while solvent molecules may also participate in the formation of the chromophore [12]. Of course, exogenous factors like the reaction medium, the ligand substituents and the nature of the counterions play a role in determining the overall structure of metal benzoates [13,14,15,16].

In the present study, we focused on p-Br, p-I, p-CN and p-NO₂-substituted benzoic acids, and their coordination towards Cu(II) was investigated in the presence of bipyridine and phenanthroline. The corresponding benzoates are symbolized as p-Y where Y is the substituent at the para- position of the benzoic acid ring. The compounds synthesized could be grouped in accordance with the metal-to-acid-to-diimine ratio as neutral 1:2:1 and cationic 1:1:2 or 1:1:1, the latter being candidates for the formation of dimmers or oligomers. In the process, some of the compounds were crystallographically characterized as [Cu(p-CN)₂(phen)], 1, [Cu(p-I)₂(bipy)(OH₂)], 2, [Cu(p-I)₂(phen)(OH₂)], 3, [Cu(p-Br)(phen)₂]ClO₄, 4, [Cu(p-Br)(phen)(OH₂)(ONO₂)], 5, [Cu(p-NO₂)(phen)(OH₂)(ONO₂)], 6, [Cu₂(p-Bu^t)₂(phen)₂(CH₃OH)₂](ClO₄)₂, 7 and [Cu₂(p-I)₃(phen)₂]ClO₄, 8, as well as the no-diamine-containing complex [Cu₂(p-Br)₄(CH₃OH)₂], 9.

2. Experimental

2.1. X-Ray Crystallographic Data Details

Single crystals of complexes 1–9 suitable for crystal structure analysis were obtained by slow evaporation of their mother liquids at room temperature. They were mounted at room temperature on a Bruker Kappa APEX2 diffractometer equipped with a Triumph monochromator using Mo Kα (λ = 0.71073 Å, source operating at 50 kV and 30 mA) radiation. Unit cell dimensions were determined and refined using the angular settings of at least 111 high-intensity reflections (>10σ(I)) in the range 11 < 2θ < 36°. Intensity data were recorded using φ- and ω-scans. All crystals presented no decay during the data collection. The frames collected for each crystal were integrated with the Bruker SAINT Software package version 1.0 [17], using a narrow-frame algorithm. Data were corrected for absorption using the numerical method (SADABS) based on crystal dimensions [18]. The structure was solved using the SUPERFLIP package [19], incorporated in Crystals. Data refinement (full-matrix least-squares methods on F²) and all subsequent calculations were carried out using the Crystals version 14.61 build 7809 program package [20]. All non-hydrogen non-disordered atoms were refined anisotropically. For the disordered atoms, their occupation factors under fixed isotropic thermal parameters were first detected. Afterwards, all were refined with fixed occupation factors, isotropically in the case of compound 4 (solvent water molecules) and anisotropically in the case of compound 8 (solvent ethanol molecules).

Hydrogen atoms riding on non-disordered parent atoms were located from different Fourier maps and refined at idealized positions riding on the parent atoms with isotropic displacement parameters Uiso(H) = 1.2Ueq(C) or 1.5Ueq (methyl and -OH hydrogens) and at distances C–H 0.95 Å and O–H 0.82 Å. All methyl and OH hydrogen atoms were allowed to rotate. Hydrogen atoms riding on disordered oxygen atoms of water and ethanol solvent molecules were positioned geometrically to fulfill hydrogen bonding demands. The remaining methyl–methylene hydrogen atoms were positioned geometrically to their parent atoms. Illustrations with 50% ellipsoids probability were drawn by CAMERON [21]. Essential crystallographic data for complexes 1–9 are presented in

Table 1. (a). Crystal data and crystallographic details for complexes 1–5, (b). Crystal data and crystallographic details for complexes 6–9.

(a)
Compound	1	2	3	4	5
Chemical formula	C28H16CuN4O4	C24H18CuI2N2O5	C26H18CuI2N2O5	C31H22BrClCuN4O7	C19H14BrCuN3O6
Chemical formula moiety	C28H16CuN4O4	C24H18CuI2N2O5	C26H18CuI2N2O5	C31H20BrCuN4O2, ClO4, H2O	C19H14BrCuN3O6
Crystal system	Triclinic	Monoclinic	Triclinic	Triclinic	Monoclinic
Space group	Pī	P21/c	Pī	Pī	P21/c
a (Å)	6.4984 (4)	13.237 (5)	8.042 (5)	11.274 (3)	8.9881 (7)
b (Å)	9.5836 (6)	16.140 (6)	10.770 (7)	16.300 (4)	14.1521 (10)
c (Å)	19.5348 (12)	12.003 (5)	15.511 (10)	17.379 (4)	14.9097 (10)
α (°)	92.924 (3)	90	99.063 (17)	84.414 (6)	90
β (°)	94.241 (3)	103.63 (1)	94.715 (16)	83.927 (6)	97.996 (3)
γ (°)	106.537 (2)	90	108.488 (16)	76.660 (6)	90
V (Å3)	1159.76 (13)	2492.3 (16)	1245.6 (13)	3081.4 (12)	1878.1 (2)
Z	2	4	2	4	4
Refinement
R[F2 > 2σ(F2)]	0.035	0.055	0.058	0.052	0.031
wR(F2)	0.071	0.074	0.114	0.075	0.057
S	1.00	1.00	1.00	1.00	1.00
Reflections used	3849	3743	3990	7660	2686
No. of parameters	334	307	333	809	271
No. of restraints	1		4
Δρmax, Δρmin (eÅ−3)	0.52, −0.39	1.12, −0.80	1.33, −1.42	0.80, −0.54	0.36, −0.32
(b)
Compound	6	7	8	9
Chemical formula	C19H14CuN4O8	C48H50Cl2Cu2N4O14	C46H31ClCu2I3N4O10.50	C30H24Br4Cu2O10
Chemical formula moiety	C19H14CuN4O8	C48H50Cu2N4O6, 2(ClO4)	2 (C45H28Cu2I3N4O6), 2(ClO4), C2H6O	C30H24Br4Cu2O10
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
Space group	Pī	Pī	Pī	Pī
a (Å)	7.5991 (16)	10.698 (3)	11.295 (6)	6.5871 (14)
b (Å)	9.195 (2)	12.332 (4)	13.485 (7)	10.966 (3)
c (Å)	14.004 (3)	20.899 (8)	16.700 (9)	12.635 (3)
α (°)	89.094 (7)	98.004 (11)	74.63 (2)	90.03 (1)
β (°)	75.143 (6)	99.061 (10)	87.904 (19)	103.85 (1)
γ (°)	87.963 (7)	110.58 (1)	73.551 (19)	99.931 (10)
V (Å3)	945.2 (4)	2491.9 (15)	2350 (2)	872.1 (4)
Z	2	2	2	1
Refinement
R[F2 > 2σ(F2)]	0.047	0.048	0.043	0.043
wR(F2)	0.108	0.111	0.056	0.055
S	1.00	1.00	1.00	1.00
Reflections used	3229	6883	6186	2429
No. of parameters	289	631	613	208
No. of restraints			3
Δρmax, Δρmin (eÅ−3)	0.83, −0.70	0.39, −1.34	0.91, −0.67	0.76, −0.52

(a), (b); the complete data set is available as Supplementary Materials.

2.2. Synthesis of the Complexes

In a typical synthesis reaction, 1 mmol of the acid was dissolved in 10 mL methanol, and to the solution, 1.2 mL of a 1 M NaOH in methanol was added. The solution was stirred at room temperature for 30 min. In a separate beaker, 1 mmol of a Cu²⁺ salt and 1 mmol of phenanthroline were added to 10 mL of methanol, and the mixture was stirred at room temperature for 30 min. Then, the deprotonated acid solution was streamed into the metal–diimine solution, and 20 mL of dichloromethane was added to facilitate precipitation of the inorganic salt produced. After stirring for 30 min at room temperature, the mixture was filtered and left to evaporate at room temperature, providing a microcrystalline or crystalline solid. Yield for all the synthesized complexes varies for 69 to 77% relative to the metal ion.

3. Results and Discussion

The reaction between Cu(II) salts and carboxylates in the presence of diimines should not be considered a simple and straightforward interaction since the high Lewis acidity of the metal ion, the formation of chelate rings and the generally shallow-potential-energy surfaces of the resulting compounds may give rise to a wide variety of local and overall structures. This is verified by the structural characterization of a couple of compounds isolated as by-products of the reaction scheme applied in the study: an oxo bridged hexamer of Cu(II) centers which has been described previously [22], and a typical dimer in the form of a paddle wheel formed by the p-bromo benzoate (compound 9).

3.1. Infrared Spectroscopy

In compounds 5 and 6 and their bipyridine counterparts, the infrared spectra revealed the presence of mono-coordinated nitrate anions, in the form of a couple of strong sharp bands around 730 and 770 cm⁻¹ and a very strong one at 1300 cm⁻¹. The absence of intense bands at higher frequencies attributable to the N=O bond indicates that these nitrates are not chelating or bridging [23]. In a number of perchlorate compounds synthesized for comparison, the characteristic intense broad band of the non-coordinating perchlorate ion is present at 1080 cm⁻¹, serving as an indication of the cationic nature of the compounds synthesized (compounds 4, 6 and 8). These observations indicate that the stoichiometry of the compounds synthesized may not follow the one predicted by the simple molar ratio of the reactants used. Water molecule coordination has been verified for compounds 2, 3, 5 and 6, based on the splitting of the otherwise broad band in the 3200–3800 cm⁻¹ region as well as the appearance of weak bands in the region 1640–1660 cm⁻¹ [24]. Only minor changes were observed in the spectra of the phenanthroline compounds relative to the ones with bipyridine, indicating similar local and overall structures for the two series of compounds.

3.2. UV–Visible Spectroscopy

The spectra recorded present the typical intense band in the region below 300 nm, which is dominated by intense and narrow π-π* bands accompanied by less intense and broader n-π* bands and charge transfer bands as evinced by analysis of the spectra line into Gaussian-type components. This is expected in view of the lone electron being located in a mainly copper d orbital and its presence in the frontier molecular orbital zone, making it a potential donor and acceptor of charge transfer electron excitations. Recording of spectra in dichloromethane revealed a minor shift of the main envelope edges relative to the spectra recorded in methanol without any profound effect on the overall spectra line. This is considered as a result of a minor shift of the underlying n-π* or CT bands. The bipyridine complexes reveal a broad band with a non-resolved maximum in the region of 240–270 nm and a shoulder around 295 nm, whereas the phenanthroline analogues present two well-separated maxima around 240 nm and 270 nm with a shoulder near 300 nm.

An analogous spectra line analysis was carried out on some of the low energy bands in the region above 650 nm. These bands are typical of forbidden excitations; therefore, 1.0 mM solutions were prepared for their recording. The analysis indicated the presence of at least four components. The compounds studied are by no means ideal octahedral or even square pyramidal; therefore, their overall symmetry is reduced, and more than the ideally predicted d-d excitations are going to be observed. In our analysis on several compounds, more than three broad low energy bands were identified in the region.

3.3. Description of the Structures

In general when para-substituted benzoic acids react with Cu(II) ions in the presence of diimines, the formed complexes present a variety of coordination modes (concerning the acids anions) as well as a plethora of coordination geometries. The complexes found to be either mononuclear or binuclear and the coordination geometries could be square planar, square pyramidal and trigonal bipyramidal. The coordination modes of the acids anions can be simple monodentate, bridging monodentate and bridging bidentate. The coordinated diimines follow the classic bidentate chelate coordination mode. Methanol, water and nitrate ligands were always found monodentately coordinated.

More specifically, Complex 1 crystallizes in the triclinic Pī space group with two molecules in the unit cell. The formed complex is neutral and mononuclear with formula [Cu(C₁₂H₈N₂)(C₈H₄O₂N)₂]. Cu(II) is coordinated to one phen ligand and two p-CN deprotonated ligands. The coordination mode of both p-CN can be better described as monodentate as one carboxylic oxygen from each acid anion is close to Cu(II), while the second is far distanced and its interaction with the metal ion cannot be considered as bonding. Selected interatomic distances of the complex can be found in

Table 2. Interatomic distances (Å) and bond angles (°) of complex 1.

Cu1—O1	1.9294 (18)	O1—Cu1—O3	96.94 (8)
Cu1—O3	1.9535 (17)	O1—Cu1—N1	91.60 (8)
Cu1—N1	2.0050 (19)	O3—Cu1—N1	169.08 (8)
Cu1—N2	2.0107 (19)	O1—Cu1—N2	164.12 (8)
Cu1—O2	2.791 (3)	O3—Cu1—N2	91.27 (8)
Cu1—O4	2.585 (2)	N1—Cu1—N2	82.11 (8)

. Figure 1 presents the plot of the molecular structure of 1. The coordination geometry can be described as distorted square planar. The mean coordination plane formed from O1, O3, N1, and N2 contains the Cu(II) ion with a deviation of 0.054 Å (nearly equal with the error).

Complex 2 crystallizes in the monoclinic P2₁/c space group with four molecules in the unit cell. The asymmetric unit comprises one mononuclear neutral complex with formula [Cu(C₁₀H₈N₂)(C₇H₄O₂I)₂(H₂O)]. Cu(II) ion has a coordination number of five and a chromophore CuO₃N₂. The nitrogen atoms come from the bipy ligand. Two oxygen atoms belong to the deprotonated p-I ligands, and the third oxygen comes from a ligated water molecule. Figure 2 presents the molecular geometry of complex 2, and

Table 3. Interatomic distances (Å) and bond angles (°) of complex 2.

Cu1—O1	1.964 (3)	O1—Cu1—O3	99.09 (13)
Cu1—O3	2.208 (3)	O1—Cu1—O5	93.61 (13)
Cu1—O5	1.959 (3)	O3—Cu1—O5	91.47 (13)
Cu1—N1	2.007 (4)	O1—Cu1—N1	161.25 (15)
Cu1—N2	1.997 (4)	O3—Cu1—N1	99.05 (14)
		O5—Cu1—N1	90.63 (13)
		O1—Cu1—N2	91.15 (15)
		O3—Cu1—N2	96.16 (14)
		O5—Cu1—N2	170.27 (14)
		N1—Cu1—N2	82.22 (16)

contains selected interatomic distances and angles of the complex. The geometry around the five-coordinated Cu(II) ion was confirmed via the trigonality index τ₅ = (φ₁–φ₂)/60°, where φ₁ and φ₂ are the largest angles in the coordination sphere; the value τ₅ = 0 denotes a perfect square pyramid; the value τ₅ = 1 denotes a perfect trigonal bipyramid [25]. As τ₅ = 0.15, the coordination geometry can be described as distorted square pyramidal where N1, N2, O1, O5 form the basal plane and O3 lies on the apical position.

Intramolecular hydrogen bonding interactions arise from the water hydrogen atoms to the non-coordinated carboxylic oxygens stabilizing the complex.

Complex 3 crystallizes in the triclinic Pī space group with multiplicity of two for the asymmetric unit. The asymmetric unit contains a complex very similar to complex 2 but coordinated to phen instead of bipy.

Table 4. Interatomic distances (Å) and bond angles (°) of compound 3.

Cu1—O1	1.953 (5)	O1—Cu1—O3	98.8 (2)
Cu1—O3	2.299 (5)	O1—Cu1—O5	95.2 (2)
Cu1—O5	1.994 (5)	O3—Cu1—O5	91.5 (2)
Cu1—N1	2.025 (6)	O1—Cu1—N1	166.9 (2)
Cu1—N2	2.016 (6)	O3—Cu1—N1	92.3 (2)
		O5—Cu1—N1	91.4 (2)
		O1—Cu1—N2	89.3 (2)
		O3—Cu1—N2	102.7 (2)
		O5—Cu1—N2	164.3 (2)
		N1—Cu1—N2	81.5 (2)

contains selected interatomic distances and angles of complex 3, while Figure 3 presents its molecular geometry.

The trigonality index value τ₅ = 0.04 indicates a distorted square pyramidal geometry around the metal ion with O1, O5, N1 and N2 forming the basal plane and O5 on the apical position.

Complex 4 crystallizes in the triclinic Pī space group with a multiplicity of the asymmetric unit of two. The asymmetric unit contains two monocationic Cu(II) complexes, two perchlorate counter ions and two solvate water molecules disordered over four equal positions. Each complex cation contains one Cu(II) coordinated to two phen ligands and one p-Br deprotonated ligand. Both complexes are formulated as [Cu(C₁₂H₈N₂)₂(C₇H₄O₂Br)]·ClO₄·H₂O. Figure 4a,b present the molecular geometry of 4a and 4b complexes present in the asymmetric unit of 4.

Table 5. Interatomic distances (Å) and bond angles (°) for both complexes 4a and 4b present in the asymmetric unit of complex 4.

4a		4b
Cu1—O1	1.979 (3)	Cu2—O3	1.948 (3)
Cu1—O2	2.819 (3)	Cu2—O4	2.821 (3)
Cu1—N1	2.067 (4)	Cu2—N5	2.040 (4)
Cu1—N2	1.983 (3)	Cu2—N6	2.021 (3)
Cu1—N3	1.983 (3)	Cu2—N7	2.010 (3)
Cu1—N4	2.235 (3)	Cu2—N8	2.201 (3)
O1—Cu1—N1	165.41 (13)	O3—Cu2—N5	156.55 (12)
O1—Cu1—N2	93.66 (14)	O3—Cu2—N6	89.65 (13)
N1—Cu1—N2	80.80 (15)	N5—Cu2—N6	80.34 (14)
O1—Cu1—N3	91.02 (13)	O3—Cu2—N7	93.46 (12)
N1—Cu1—N3	93.85 (14)	N5—Cu2—N7	95.68 (14)
N2—Cu1—N3	174.20 (15)	N6—Cu2—N7	175.71 (15)
O1—Cu1—N4	97.01 (13)	O3—Cu2—N8	99.65 (12)
N1—Cu1—N4	97.43 (13)	N5—Cu2—N8	103.29 (13)
N2—Cu1—N4	104.22 (13)	N6—Cu2—N8	103.68 (13)
N3—Cu1—N4	78.56 (13)	N7—Cu2—N8	78.72 (13)

contains selected interatomic distances and angles of the two complexes.

In both complexes, Cu(II) is five-coordinated with four nitrogen atoms (two from each phen ligand) and one oxygen atom from the monodentate p-Br ligand. The value of the trigonality index τ₅ was 0.16 for the complex containing Cu1 and 0.32 for the complex containing Cu2. For the complex of Cu1, the coordination geometry can be better described as distorted square pyramidal. For the Cu2 containing complex, the τ₅ value indicates a highly distorted geometry which can be described either as square pyramidal—with N8 on the apical position—or better as distorted trigonal bipyramidal with N6 and N7 atoms on the axial positions.

Hydrogen bonding and π-π stacking interactions throughout the cell are shown in Figure 4c. Neighboring complexes have their phenanthroline ligands parallel due to symmetry reasons and at distances of 3.36 Å indicating π-π stacking interactions, while all hydrogen atoms of the disordered solvate water molecules interact with the oxygen atoms of the perchlorate anions. All these interactions form a stable network and a rigid crystal lattice.

Compound 5 crystallizes in the monoclinic P2₁/c space group with four complexes in the unit cell. The asymmetric unit contains one mononuclear neutral Cu(II) complex with formula [Cu(C₁₂H₈N₂)(C₇H₄O₂Br)(NO₃)(H₂O)].

Table 6. Selected interatomic distances (Å) and bond angles (°) of complex 5.

Cu1—O1	1.947 (2)	O1—Cu1—O3	95.48 (10)
Cu1—O3	1.960 (2)	O1—Cu1—O4	90.32 (10)
Cu1—O4	2.376 (3)	O3—Cu1—O4	94.77 (10)
Cu1—N1	1.990 (3)	O1—Cu1—N1	172.93 (11)
Cu1—N2	2.005 (3)	O3—Cu1—N1	91.12 (11)
		O4—Cu1—N1	91.65 (10)
		O1—Cu1—N2	90.44 (10)
		O3—Cu1—N2	164.15 (11)
		O4—Cu1—N2	99.89 (10)
		N1—Cu1—N2	82.53 (11)

contains selected interatomic distances and angles of complex 5, while Figure 5a presents a plot of its structure.

Cu(II) cation is five-coordinated with two nitrogen atoms from a phen bidentate chelate ligand and three oxygen atoms, the first coming from one monodentate p-Br deprotonated carboxylate ligand, the second from a monodentate nitrate anion and the third from a ligated water molecule. The value of the trigonality index τ₅ = 0.16 indicates a square planar coordination geometry with O4 occupying the apical position.

Inter- and intramolecular hydrogen bonding interactions from the hydrogen atoms of the ligated water molecule to the non-coordinated carboxylic oxygen O2 and the free nitrate oxygen atom O4i from a neighboring complex create a zig zag chain parallel to c crystallographic axis, forming a 1D lattice and giving extra stability to the structure formed, as shown in Figure 5b.

Complex 6 has a similar structure as the previously described complex 5. The crystal is formed in the triclinic Pī space group, and the difference from complex 5 is that the substituted benzoate acid is p-NO₂. Interatomic distances and bond angles can found summarized in

Table 7. Selected interatomic distances (Å) and bond angles (°) of complex 6.

Cu1—O1	1.948 (3)	O1—Cu1—O5	94.82 (13)
Cu1—O5	1.954 (3)	O1—Cu1—O6	100.16 (13)
Cu1—O6	2.319 (3)	O5—Cu1—O6	91.08 (14)
Cu1—N1	2.018 (3)	O1—Cu1—N1	170.41 (14)
Cu1—N2	2.010 (3)	O5—Cu1—N1	91.26 (14)
		O6—Cu1—N1	87.10 (14)
		O1—Cu1—N2	90.02 (13)
		O5—Cu1—N2	165.43 (13)
		O6—Cu1—N2	101.60 (14)
		N1—Cu1—N2	82.34 (14)

. The coordination geometry can be described just as in the case of complex 5 and the trigonality index calculated as τ₅ = 0.08. Consequently, the structure can be described as distorted square pyramidal having O6 on the apical position. As in the case of complex 5, hydrogen bonding interactions give rigidity to the structure as they form an 1D lattice with chains parallel to a crystallographic axis, keeping O5 and its connected hydrogen atoms close to O2 and O7ⁱ from a neighboring complex as shown in Figure 6b.

Complex 7 crystallizes in the triclinic Pī space group with Z = 2. The asymmetric unit contains one binuclear bicationic Cu(II) complex with formula [Cu₂(C₁₂H₈N₂)₂(C₁₁H₁₃O₂)₂(CH₃OH)₂]²⁺ and two perchlorate counter anions. The ligands p-Bu^t are both deprotonated and act as bidentate agents with each carboxylate oxygen coordinated to one copper(II) ion, thus bridging them through the carboxylate groups. Each phen ligand is coordinated in a bidentate chelate mode to one metal ion. Finally, the total coordination number of five for each Cu(II) is completed with an oxygen atom coming from a ligated methanol molecule. Interatomic distances and bond angles of the complex are shown in

Table 8. Selected interatomic distances (Å) and bond angles (°) of compound 7.

Cu1—Cu2	3.0263 (10)
Cu1—O1	1.925 (3)	Cu2—O2	1.925 (3)
Cu1—O3	1.917 (3)	Cu2—O4	1.961 (3)
Cu1—O6	2.188 (3)	Cu2—O5	2.272 (3)
Cu1—N1	2.030 (3)	Cu2—N3	2.010 (3)
Cu1—N2	2.014 (3)	Cu2—N4	2.015 (3)
O1—Cu1—O3	94.20 (12)	O2—Cu2—O4	93.92 (12)
O1—Cu1—O6	91.84 (12)	O2—Cu2—O5	101.27 (14)
O3—Cu1—O6	92.62 (12)	O4—Cu2—O5	88.20 (14)
O1—Cu1—N1	90.97 (13)	O2—Cu2—N3	91.73 (13)
O3—Cu1—N1	165.99 (13)	O4—Cu2—N3	173.14 (13)
O6—Cu1—N1	100.23 (13)	O5—Cu2—N3	94.52 (14)
O1—Cu1—N2	172.40 (13)	O2—Cu2—N4	165.91 (14)
O3—Cu1—N2	91.42 (13)	O4—Cu2—N4	91.93 (13)
O6—Cu1—N2	93.01 (13)	O5—Cu2—N4	91.71 (14)
N1—Cu1—N2	82.42 (14)	N3—Cu2—N4	81.71 (14)

, and Figure 7 presents the structure of 7.

As the distance Cu1—Cu2 of 3.0263 (10) Å cannot be considered as bonding, the coordination environment of each five-coordinated Cu(II) was defined using the trigonality index τ₅ value. For Cu1, τ₅ = 0.11, and for Cu2, τ₅ = 0.12. Consequently, both the metal ions present a distorted square pyramidal coordination geometry with the methanol oxygen atoms occupying the apical positions of both pyramids.

Hydrogen bonding interactions arise from the methanolic hydroxide hydrogen atoms to the O9 and O11 perchlorate oxygen atoms and are shown in Figure 7. The phenanthroline ligands’ mean planes are also almost parallel, forming an angle of 6.67°, giving rise to π-π stacking interactions between them with a centroid to centroid distance of 3.78 Å. Both types of interactions give extra stability to the structure formed.

Complex 8 crystallizes in the triclinic Pī space group. In the asymmetric unit, there is one binuclear monocationic copper(II) complex, as well as one perchlorate counter anion and half of a solvate ethanol molecule. The multiplicity of the asymmetric unit to form the unit cell is 2. The complex is formed from two Cu(II) cations, three p-I deprotonated ligands and two ligated phen molecules with formula [Cu₂(C₁₂H₈N₂)₂(C₇H₄O₂I)₃]⁺.

From the three p-I ligands, two of them are coordinated in a bidentate mode bridging the two metal ions through both the terminal oxygen atoms of the carboxylate groups. The third one is monodentately coordinated from the O5 oxygen atom which is bonded as a single atom bridge to both the metal ions. Thus, both Cu(II) cations are five-coordinated. Figure 8 presents the structure of 8, and

Table 9. Selected interatomic distances (Å) and bond angles (°) of complex 8.

Cu1—O1	1.944 (3)	Cu2—O2	1.940 (3)
Cu1—O3	1.927 (3)	Cu2—O4	2.144 (3)
Cu1—O5	2.260 (3)	Cu2—O5	2.036 (3)
Cu1—N1	2.008 (4)	Cu2—O6	2.710 (3)
Cu1—N2	2.025 (3)	Cu2—N3	2.049 (4)
O1—Cu1—O3	95.31 (14)	O2—Cu2—O4	91.51 (14)
O1—Cu1—O5	93.24 (12)	O2—Cu2—O5	97.87 (13)
O3—Cu1—O5	97.40 (14)	O4—Cu2—O5	97.74 (13)
O1—Cu1—N1	160.93 (13)	O2—Cu2—N3	89.68 (14)
O3—Cu1—N1	89.52 (14)	O4—Cu2—N3	113.92 (14)
O5—Cu1—N1	104.44 (13)	O5—Cu2—N3	147.28 (14)
O1—Cu1—N2	90.02 (13)	O2—Cu2—N4	170.33 (14)
O3—Cu1—N2	165.78 (13)	O4—Cu2—N4	89.43 (14)
O5—Cu1—N2	95.45 (13)	O5—Cu2—N4	91.53 (14)
N1—Cu1—N2	81.37 (14)	N3—Cu2—N4	81.15 (14)

contains the interatomic distances and bond angles of the complex. Trigonality index τ₅ value for the coordination environment of Cu1 is equal to 0.08, suggesting that the geometry around Cu1 is slightly distorted square pyramidal with O5 occupying the apical position. For Cu2, the τ₅ value is 0.38. This value indicates an intermediate geometry between square pyramidal and trigonal bipyramidal. We prefer to describe the coordination geometry around Cu2 as distorted trigonal bipyramidal with a basal plane formed from N3, O4 and O5 atoms, while on the axial positions lie N4 and O2 atoms. The hydroxide hydrogen atom of the disordered ethanol solvate molecule interacts with O3, and the hydrogen bond formed accumulates structure stability.

In an erroneous setup, the deprotonated acid was transferred to a Cu(II) solution to which no diimine had been added. The resulting compound was, of course, entirely different from the rest and typical of the paddle-wheel Cu₂(benzoate)₄(solvent)₂ compound where the coordinated solvent molecules give rise to a local square pyramidal copper geometry. The molecular structure of complex 9 formulated as [Cu₂(C₇H₄O₂Br)₄(CH₃OH)₂] is shown in Figure 9a, and selected interatomic distances and bond angles are summarized in

Table 10. Selected interatomic distances (Å) and bond angles (°) of complex 9.

Cu1—O4i	1.955 (3)	O4i—Cu1—O2i	90.56 (16)
Cu1—O2i	1.942 (3)	O4i—Cu1—Cu1i	86.64 (10)
Cu1—Cu1i	2.5864 (12)	O2i—Cu1—Cu1i	85.70 (10)
Cu1—O1	1.933 (3)	O4i—Cu1—O1	89.43 (16)
Cu1—O3	1.954 (3)	O2i—Cu1—O1	168.92 (13)
Cu1—O5	2.145 (3)	Cu1i—Cu1—O1	83.24 (10)
		O4i—Cu1—O3	168.72 (13)
		O2i—Cu1—O3	89.22 (15)
		Cu1i—Cu1—O3	82.10 (10)
		O1—Cu1—O3	88.64 (15)
		O4i—Cu1—O5	94.72 (14)
		O2i—Cu1—O5	95.47 (13)

. Complex 9 crystallized in the triclinic system with space group Pī. It is a dinuclear centrosymmetric Cu(II) complex presenting the well-established paddle-wheel structural pattern. The four deprotonated p-Br ligands are bidentate-forming syn–syn μ_1,3–bridges between the copper atoms that are lying at a Cu…Cu distance of 2.5864(12) Å, which is typical for such complexes. The Cu(II) ions are five-coordinated, and their CuO₅ coordination sphere can be described as presenting a slightly distorted square pyramidal geometry, as suggested by the value of the trigonality index τ₅ = (168.92(13)°−168.72(13)°)/60° = 0.003. The four oxygen atoms O1, O2ⁱ, O3 and O4ⁱ of the p-Br ligands form the basal plane of the pyramid and O5 from the methanol ligand occupies the apical position. The structure gains stability from the formed intermolecular hydrogen bonds keeping the methanolic hydroxide hydrogen close to O1ii from a neighboring complex and finally forming chains parallel to a crystallographic axis and a rigid 1D lattice, as shown in Figure 9b.

3.4. Overall Results

The para-substituted benzoic acids formed the backbone for the introduction of the Hammett equation in describing their reactivity, although extensions of its use have been applied in several other groups of compounds [26]. In our case, the reaction considered is that of the acid ionization, a larger rate constant for which would, under the conditions of the experiments, provide a larger initial amount of the anion available for coordination. Provided that the coordination reaction mechanism is identical for all the studied acids, a more positive σ- Hammett constant would result in an elevated participation of the benzoate in the final product. This is apparent in the case of compound 1, where two para-cyano benzoates are coordinated to the metal center. An analogous result was expected in the case of the para-nitro-substituted acid; however, in the presence of nitrate ions, which act antagonistically to the benzoates, monodentate coordination of the nitrate anion leads to the formation of metal-to-acid 1:1 compounds. In similar cases, in the presence of the less-donating perchlorate ions, the coordination sphere of the metal is completed with solvent molecules or by additional diimine ligands, owing to their chelate effect (compounds 4 and 8). Our observations appear to be in line with the values of the σ- Hammett parameters of the benzoic acid substituents, implying that the reaction originating at the ρ- parameter remains practically constant. However, the versatility of the copper cation and the benzoate anions make it possible to realize more complex structures than expected. In fact, compound 7 was synthesized in an effort to utilize the negative Hammett value of alkyl groups and the isolation of typical [Cu(benzoate)(diimine)]⁺ units with chelating benzoate. However, although the stoichiometry of the product was the expected one, the compound crystallized as a dimer with bridging carboxylates.

4. Materials and Measurements

The copper salts, the benzoic acids (ACROS) and the diimines (Merck) were of the best quality provided. No precaution was taken to completely dry the solvents used. FT ATR spectra of the compounds were recorded on a Thermo Scientific™ Nicolet™ iS20 FTIR spectrometer. A Jasco V-750 UV-Vis spectrometer was used for the recording of the electronic excitation spectra; 0.1 mM solutions were used and placed in 1 cm quartz cuvettes.

5. Conclusions

The versatility of the coordinating function of para-benzoates is evident in the structures of the compounds studied, along with the well-documented Lewis acid strength of divalent copper. The Hammett parameter of the benzoate substituent plays an important role in determining the overall stoichiometry of the compounds; however, the nitrate anion, when present, acts antagonistically to the benzoates suppressing their participation in the final product. The reaction mechanism for the synthesis of the heteroleptic compounds is not a simple one; however, it is apparently the same for all the benzoate ligands studied.