Marijana
Jurić
,
Krešimir
Molčanov
*,
Dijana
Žilić
and
Biserka
Kojić-Prodić
Ruđer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia. E-mail: kmolcano@irb.hr
First published on 22nd June 2016
A series of six novel mononuclear, binuclear and linear one-dimensional (1D) compounds of copper(II) with chloranilic acid (3,6-dichloro-2,5-dihydroxybenzoquinone, H2CA) is prepared and a design strategy for the preparation of such complexes is discussed. Four described compounds are linear 1D coordination polymers [Cu(CA)]n, whereas another two involve a binuclear and a mononuclear, Cu2(CA)3 and Cu(CA)2, core unit. A linear polymer incorporating bulky aromatic imidazole has been synthesized as a result of investigation of the influence of pH on the reaction mixture. Two coordination modes of the chloranilate dianion are observed. The bridging (bis)bidentate mode generates linear 1D polymeric species. Among these one reveals square-pyramidal coordination of Cu2+, whereas the three polymers contain Cu2+ in an octahedral arrangement. However, the combination of both, terminal bidentate (ortho-quinone) and bridging (bis)bidentate modes of coordination produces a binuclear complex anion, which comprises a square-pyramidal coordination of Cu2+ complex anions forming a supramolecular honeycomb-like network encapsulating 4,4′-bipyridine cations. When the chloranilate dianion coordinates the Cu2+ atom only in a terminal bidentate mode, a mononuclear complex with an octahedral environment of the metal centre is formed. The presence of the bulky ancillary ligand imidazole produces an unprecedented packing involving chiral (racemic) and achiral (meso-compound) coordination polymers in the same crystal. Electron spin resonance spectroscopy of polycrystalline samples determined g-tensor parameters of copper(II) ions in different coordination geometries and revealed weak exchange interactions (|J| < 1 cm−1) in linear metal-complex polymers and dimeric species.
Substituted 2,5-dihydroxybenzoquinonates (2,5-DHQs), such as 3,6-dichloro-2,5-dihydroxybenzoquinone (chloranilic acid, H2CA) are among the most versatile ligands for construction of the coordination polymers with different topologies.11–20 They are prone to form a variety of coordination modes19–24 and to mediate electronic effects between paramagnetic metal ions via their four oxygen donors and π-electron systems, respectively.25 Usually 2,5-dihydroxyquinones are bridging (bis)bidentate ligands, but sometimes may also act as bidentate + bridging monodentate ligands,26 with the possibility to generate polymeric complexes. Mononuclear complexes with chloranilic acid acting as a terminal bidentate ligand are known as well.26,27
Actually, 2,5-DHQs can be regarded as “extended” (i.e. sterically bulkier) version of the oxalate.26 Conjugated bonds of the quinoid ring and presence of two acidic hydroxy-groups make the electronic structure of 2,5-DHQs26,27 uniquely malleable depending on ionisation, molecular environment, and degree of electron delocalisation characteristics; it can exist as an ortho- or para-quinoid structure, or a dianion-like structure with two separated delocalised systems (Scheme 1). Quinones are also weak oxidants, and their proton-accepting properties can be enhanced by electronegative substituents, thus enabling the charge transfer between a metal centre and the quinoid ring; the quinone is then reduced into a semiquinone radical.28,29 Their potential in design of magnetic, charge-transfer and spin-crossover materials has been little studied so far.
Scheme 1 Dissociation of chloranilic acid and schematic representation of its binding to copper(II) ions investigated in this work. |
The rational design and optimisation of synthesis of the coordination polymers based on chloranilic acid and first-row transition metals is the goal of our research. The use of bridging mode of CA ligand should be in favour of formation of linear Cu(II) polymeric chains due to its ability to mediate electronic effects between paramagnetic metal ions, even at large Cu⋯Cu separation distance (exceeding 7 Å). The size of the metal cation is an important parameter: larger cations (for example: Cr3+, Mn2+, Fe3+) can better accommodate large multidentate ligands than smaller ones (Cu2+, Zn2+). Therefore, Cu2+ complexes with CA and bidentate N-donor ligands are almost exclusively mononuclear,26,27 while larger cations can accommodate zig–zag 1D polymers.22,30 Linear 1D-polymers of Cu2+ are obtained if additional bidentate ligands were replaced by solvent molecules such as water31 and methanol13 acting as monodentate ancillary ligands. An appropriate bridging (bis)monodentate ligand, such as pyrazine, can link the chains into a 2D network.13
Fig. 1 ORTEP-3 (ref. 33) drawings of the Cu–chloranilate complex units: [Cu(CA)2(H2O)2]2− (1), [Cu2(CA)3(H2O)2]2− (2), [Cu(CA)(CH3CN)]n, (3), [Cu(CA)(H2O)2]n (4), [Cu(CA)(EtOH)2]n (5), a C2-symmetric chain A and achiral centrosymmetric chain B of [Cu(CA)(im)2]n (6A and 6B, respectively). Displacement ellipsoids are drawn at the probability of 50% and hydrogen atoms are shown as spheres of arbitrary radii. |
The compound 2 is the complex salt of a dinuclear anion with 4,4′-bpy cation and crystal water molecules (4,4′-H2bpy)[Cu2(CA)3(H2O)2]·2H2O. Its asymmetric unit is a half of the dianion (Fig. 1) and a half of the cation (Fig. S2†), both having a Ci symmetry. The coordination of copper atom is a distorted square-pyramid with a water molecule at the apical position (Table S1†).
In the polymer 3, [Cu(CA)(CH3CN)]n (Fig. 1), copper coordination is also a distorted square-pyramid (Table S1†), where its apical position is occupied by an acetonitrile molecule. This is the first [Cu(CA)]n coordination polymer with pentacoordinated copper(II) ion; so far pentacoordination of Cu2+ was observed in mononuclear species26,27 and three dinuclear complexes containing bridging CA ligand.34–36
The analogous compounds 4, 5 and 6 consist of linear 1D chains with chemical compositions of [Cu(CA)(H2O)2]n, [Cu(CA)(EtOH)2]n and [Cu(CA)(im)2]n, respectively (Fig. 1). The Cu atom and chloranilate ring have crystallographic symmetry Ci in 4 and C2 in 5. In all these complexes, copper coordination is a distorted octahedron (Table S1†): the metal is coordinated by two chloranilates in the equatorial position (in a trans arrangement) and two water/ethanol/imidazole molecules occupying two apical positions. Previously prepared and investigated compound {[Cu(CA)(H2O)2]·H2O}n (ref. 31) contains the same polymeric unit as our compound 4, however, it is a monohydrate, while we report the structure without a crystal water.
In 6 exist two symmetry-independent chains (designated as A and B; Fig. 1), both parallel to [010] direction. The chain A is C2-symmetric, with twofold axis passing through Cu atoms and midpoints of chloranilate rings. It is therefore chiral, with both R and S enantiomers (Scheme 2) present due to the centrosymmetric space group. The chain B is centrosymmetric, with Cu atoms and chloranilate rings having a crystallographic symmetry Ci. It can be regarded as a meso-compound, and differs from its diastereomer A in orientation of imidazole ligands, as seen in Fig. 2 and Scheme 2. This is a rare example of co-crystallization of a chiral and a meso-molecule.37
Fig. 2 Overlay of two symmetry-independent chains in [Cu(CA)(im)2]n (6). Chiral C2-symmetric chain A is red, while the achiral meso-chain B is blue. |
In four prepared polymers chloranilate dianions act as bridging (bis)bidentate ligands, and have dianion-like geometry with two delocalized systems separated by two single C–C bonds (Scheme 1 and Table 1). In compounds 1 and 2, two chloranilates act as terminal bidentate ligands and have an ortho-quinone-like geometry, while the central one in 2 acts as a bridging (bis)bidentate ligand and has dianion-like characteristics (Table 1).26,27 So far, only a few compounds are known that comprise two different coordination modes of chloranilic acid,15,17,24,38–40 which could be easily distinguished by IR spectra.
1 | 2-Terminal | 2-Bridging | ||||||
---|---|---|---|---|---|---|---|---|
Single | C1–O1 | 1.283(3) | Single | C4–O3 | 1.286(3) | Delocalised | C1–O1 | 1.261(3) |
Single | C2–O2 | 1.278(3) | Single | C5–O5 | 1.278(3) | Delocalised | C3–O2 | 1.261(3) |
Double | C4–O3 | 1.228(3) | Double | C7–O5 | 1.238(3) | Delocalised | C1–C2 | 1.391(3) |
Double | C5–O4 | 1.224(3) | Double | C8–O6 | 1.229(3) | Delocalised | C2–C3 | 1.398(3) |
Single | C1–C2 | 1.524(4) | Single | C4–C5 | 1.506(3) | Single | C1–C3i | 1.552(3) |
Double | C2–C3 | 1.370(4) | Double | C4–C9 | 1.375(3) | |||
Single | C3–C4 | 1.410(4) | Double | C5–C6 | 1.379(3) | |||
Single | C4–C5 | 1.554(4) | Single | C6–C7 | 1.419(3) | |||
Single | C5–C6 | 1.424(4) | Single | C7–C8 | 1.554(3) | |||
Double | C6–C1 | 1.367(4) | Single | C8–C9 | 1.424(3) |
3 | 4 | 5 | ||||
---|---|---|---|---|---|---|
Delocalised | C1–O1 | 1.269(4) | Delocalised | C1–O1 | 1.252(9) | 1.254(4) |
Delocalised | C2–O2 | 1.257(4) | Delocalised | C3–O2 | 1.273(8) | 1.259(4) |
Delocalised | C4–O3 | 1.264(4) | Delocalised | C1–C2 | 1.404(10) | 1.383(4) |
Delocalised | C5–O4 | 1.265(4) | Delocalised | C2–C3 | 1.372(9) | 1.382(5) |
Single | C1–C2 | 1.527(5) | Single | C1–C3i | 1.533(9) | 1.547(4) |
Delocalised | C2–C3 | 1.397(4) | ||||
Delocalised | C3–C4 | 1.382(4) | ||||
Single | C4–C5 | 1.524(5) | ||||
Delocalised | C5–C6 | 1.396(4) | ||||
Delocalised | C6–C1 | 1.376(4) |
6a | 6b | |||
---|---|---|---|---|
Delocalised | C1–O1 | 1.248(2) | C1–O1 | 1.257(2) |
Delocalised | C3–O2 | 1.256(2) | C3–O2 | 1.244(2) |
Delocalised | C1–C2 | 1.401(2) | C1–C2 | 1.392(2) |
Delocalised | C2–C3 | 1.392(2) | C2–C3 | 1.407(2) |
Single | C3–C3ii | 1.539(3) | C1–C3i | 1.536(3) |
Generally, the positions of the symmetric and asymmetric carboxylate stretching vibrations in the IR spectrum are indicative of the binding mode of the ligand. Selected absorption bands ascribed to the vibrations of the bidentate26,27 and bis(bidentate)11,41–43 chloranilate anion for compounds 1–6 are given in Table 2. In terminal bidentate ligand, single and double C–O bonds can be distinguished,26,27 with their respective stretching vibrations close to the neutral chloranilic acid.44,45 In the case of bridging (bis)bidentate ligand, whose structure is similar to that of the free dianion, only one type of the stretching vibration of the C–O bond is present in the spectrum, consistent with bond length about 1.26 Å (in between single and double bond, Table 1). The spectra of 1 and 2 demonstrate some differences (Table 2); in compound 1 chloranilate anion has bidentate coordination mode so stretching vibrations of ν(C–O) and ν(CO) could be observed, while in compound 2, having bidentate and bridging bis(bidentate) chloranilate anions, ν(C–O), ν(CO) and ν(C⋯O). In compounds 3–6 chloranilate anion is bridging and only [ν(C⋯O)] is found (Table 2). In the spectra of compounds 1, 2 and 6 other absorption bands of significant intensities correspond to different vibrations of the imidazole and 4,4′-bipyridine molecules.46
Compound | ν(C⋯O) | ν(CO) | ν(C–O) | ν(C–Cl) | C–Cl wagging |
---|---|---|---|---|---|
1 | — | 1640, 1597 | 1340 | 851 | 569 |
2 | 1508, 1366 | 1637, 1609 | 1337 | 866, 847 | 573 |
3 | 1490, 1370 | — | — | 866 | 579 |
4 | 1507, 1380 | — | — | 856 | 579 |
5 | 1486, 1368 | — | — | 866 | 580 |
6 | 1512, 1370 | — | — | 851 | 576 |
D–H⋯A | D–H/Å | H⋯A/Å | D⋯A/Å | D–H⋯A/° | Symm. op. on A |
---|---|---|---|---|---|
1 | |||||
N1–H1⋯O1 | 0.86 | 2.04 | 2.867(3) | 161 | x, y, z |
N2–H2⋯O5 | 0.86 | 1.96 | 2.810(3) | 172 | 1 − x, −y, 2 − z |
O5–H5A⋯O4 | 0.92(4) | 1.90(4) | 2.794(3) | 162(4) | x, −1 + y, z |
O5–H5B⋯O3 | 0.92(3) | 1.86(3) | 2.692(3) | 149(3) | −x, 1 − y, 1 − z |
O5–H5B⋯O4 | 0.92(3) | 2.58(3) | 3.325(3) | 139(3) | −x, 1 − y, 1 − z |
2 | |||||
N1–H1⋯O8 | 0.86 | 1.95 | 2.724(4) | 148 | 1 − x, 1 − y, 2 − z |
O7–H7A⋯O5 | 0.94(3) | 1.89(3) | 2.811(3) | 165(3) | −x, 1 − y, 1 − z |
O7–H7B⋯O5 | 0.92(5) | 1.90(3) | 2.784(3) | 161(4) | 1 + x, y, z |
O8–H8A⋯O3 | 0.94(4) | 2.34(4) | 3.228(3) | 157(4) | x, y, z |
O8–H8A⋯O7 | 0.94(4) | 2.42(4) | 2.948(3) | 116(4) | x, y, z |
O8–H8B⋯Cl3 | 0.92(4) | 2.79(5) | 3.518(3) | 136(4) | −x, 1 − y, 2 − z |
O8–H8B⋯O6 | 0.92(4) | 2.21(3) | 2.965(3) | 138(4) | −x, 1 − y, 2 − z |
3 | |||||
C8–H8A⋯O3 | 0.94(4) | 2.73(6) | 3.367(7) | 130(7) | 1 − x, −y, 1 − z |
C8–H8A⋯O4 | 0.94(4) | 2.63(5) | 3.119(6) | 113(3) | −1 + x, y, z |
4 | |||||
O3–H3A⋯Cl1 | 0.95(10) | 2.61(10) | 3.492(7) | 155(7) | −x, −1/2 + y, 3/2 − z |
O3–H3B⋯O1 | 0.95(9) | 2.55(10) | 3.107(9) | 118(6) | x, −1 + y, z |
5 | |||||
O3–H3⋯O2 | 0.81(4) | 2.15(4) | 2.940(4) | 167(7) | x, −y, 1/2 + z |
O3–H3⋯Cl1 | 0.81(4) | 2.91(7) | 3.395(4) | 121(5) | x, −y, 1/2 + z |
6 | |||||
N2A–H2A⋯O2B | 0.86 | 2.10 | 2.833(2) | 143 | 1/2 + x, 3/2 − y, 1/2 + z |
N2B–H2B⋯O2A | 0.86 | 2.16 | 2.879(3) | 141 | x, 1 + y, z |
N2B–H2B⋯Cl1A | 0.86 | 2.90 | 3.503(3) | 128 | 1 − x, 1 − y, −z |
C4A–H4A⋯O1A | 0.93 | 2.48 | 2.970(3) | 113 | 1 − x, y, 1/2 − z |
C4B–H4B⋯O1B | 0.93 | 2.43 | 2.905(3) | 111 | x, y, z |
C6A–H6A⋯O2A | 0.93 | 2.63 | 3.321(3) | 130 | 1 − x, 1 + y, 1/2 − z |
C6B–H6B⋯O1B | 0.93 | 2.63 | 3.051(3) | 175 | 1/2 − x, 3/2 − y, −z |
Compound | Cg(1)⋯Cg(2) | Cg(1)a⋯Cg(2)/Å | α b/° | β c | Cg(1)···plane[Cg(2)]/Å | Offsetd/Å | Symm. op. on Cg(2) |
---|---|---|---|---|---|---|---|
a Cg = centre of gravity of the aromatic ring. b α = angle between planes of two interacting rings. c β = angle between Cg⋯Cg line and normal to the plane of the first interacting ring. d Offset can be calculated only for the strictly parallel rings (α = 0.00°). For slightly inclined rings (α ≤ 5°) an approximate value is given. | |||||||
1 | C1 → C6⋯C1 → C6 | 3.6365(17) | 0.04(13) | 24.2 | 3.3173(11) | 1.490 | −x, 1 − y, 1 − z |
N1 → C9⋯N1 → C9 | 3.635(2) | 0.0(2) | 26.0 | 3.2668(15) | 1.594 | 1 − x, −y, 2 − z | |
2 | Cu1 → O2⋯Cu1 → O4 | 3.6526(11) | 6.49(8) | 22.82 | 3.2181(7) | — | −x, 2 − y, 1 − z |
Cu1 → O1⋯C4 → C9 | 3.6244(10) | 2.59(8) | 26.91 | 3.2777(7) | 1.64d | −x, 2 − y, 1 − z | |
Cu1 → O4⋯Cu1 → O4 | 4.0643(10) | 0 | 33.74 | 3.3798(7) | 2.257 | −x, 2 − y, 1 − z | |
C3 → C3i⋯C4 → C9 | 3.5689(11) | 0.82(8) | 24.09 | 3.2370(7) | 1.46d | 1 + x, y, z | |
C4 → C9⋯Cu1 → O2 | 3.6244(10) | 2.59(8) | 25.27 | 3.2320(7) | 1.55d | −x, 2 − y, 1 − z | |
C4 → C9⋯C3 → C3i | 3.5690(11) | 0.82(8) | 24.91 | 3.2582(7) | 1.50d | −1 + x, y, z | |
3 | Cu1 → O2⋯Cu1 → O4ii | 3.6885(17) | 6.55(13) | 26.27 | 3.1566(12) | — | −x, 1 − y, 1 − z |
Cu1 → O2⋯C1 → C6 | 3.7498(18) | 2.60(14) | 31.57 | 3.2735(12) | 1.63d | 1 − x, 1 − y, 1 − z | |
C1 → C6⋯C1 → C6 | 3.6670(19) | 0.00 | 26.91 | 3.2700(13) | 1.660 | 1 − x, 1 − y, 1 − z | |
6 | N1A → C6A⋯C1B → C3B | 4.0151(15) | 2.24(15) | 33.3 | 3.2684(10) | 2.22d | 1 − x, y, 1/2 − z |
Packing of 2 reveals a 3D network realised through seven symmetry-independent hydrogen bonds (Table 3). Complex anions [Cu2(CA)3(H2O)2]2− and uncoordinated water molecules form a porous honeycomb-like structure (Fig. 4a) which is further stabilised by π-stacking of the anions in the direction [100] (Fig. 4b, Table 4). Cations of 4,4′-bpy fill in the voids (Fig. 4a) and they are linked into water–anion network by N–H⋯O hydrogen bonds (Table 3). The rationale behind use of 4,4′-bpy was that it may occupy apical positions in the Cu coordination sphere, linking the linear polymers into the layers, similar to the previously known pyrazine complex, [Cu(CA)(pyz)]n.13
However, basicity of 4,4′-bpy is too high, so it gets protonated in the presence of moderately strong chloranilic acid (its pKa values being 0.76 and 3.08, respectively47), therefore it is present in the compound in the cationic state. Attempts of moderating pH of the aqueous medium by use of a weak, non-complexing base such as ammonia, yielded no single crystals suitable for X-ray measurement, and the obtained samples were too inhomogeneous and impure to allow any meaningful analysis. Due to the presence of the 4,4′-bpy cations, the infinite neutral [Cu(CA)]n chains (as present in studied compounds 3–6) did not form; instead, discrete dianionic complexes, balancing the charge of the 4,4′-bpy cations, were obtained. However, it is worth nothing that the anions [Cu(CA)2(H2O)2]2− and [Cu2(CA)3(H2O)2]2− are actually fragments of the polymeric chains; thus, we may speculate that the [Cu(CA)]n polymer is generated by addition of similar discrete [Cu(CA)2] or [Cu2(CA)3] units. In the case of 1 and 2 growth of chains was probably terminated due to the presence of the cations.
Infinite chains of 3 extend in the direction [100] (Fig. 5a); the square-pyramidal coordination of Cu centres allows close contact between planar delocalised quinoid and chelate rings, so two chains stack into pairs by π-interactions (Fig. 5b, Table 4). An electron delocalization occurring in the five-membered Cu-chelate rings make them susceptible to interactions with other π-systems in the unit cell;48–52 in fact, it appears that they are quite common in metal–organic chelate complexes.26,27,48–52 There are also C–H⋯O hydrogen bonds which link the pairs of chains into layers parallel to (011) (Fig. 5a, Table 3).
In crystal packing of 4 and 5 linear 1D coordination polymers are extended in the directions [100] (Fig. 6 and 7). Since octahedral coordination of Cu centres prevents close contact between π-systems, there are no stacking interactions and crystal packing is realised through O–H⋯O and O–H⋯Cl hydrogen bonding. Due to steric similarities and existence of inter-chain hydrogen bonding (there are two symmetry-independent hydrogen bonds in both compounds, Table 3), the comparison of their crystal packing (Fig. 6 and 7) reveals inter chain hydrogen bonds in 4 generate a 3D hydrogen-bonding network, while in 5 there are hydrogen-bonded layers. The previously published hydrate of 4, {[Cu(CA) (H2O)2]·H2O}n,31 also forms a 3D network; its packing includes uncoordinated water molecule linking the chains, rather than direct hydrogen bonding between the chains.
Fig. 6 Crystal packing of 4 showing hydrogen bonding O–H⋯Cl and O–H⋯O (dashed lines) between the polymeric chains: (a) viewed in the direction [010], (b) in the direction [100]. |
Fig. 7 Crystal packing of 5 showing inter chain (chains running parallel to [100]) hydrogen bonding O–H⋯O and O–H⋯Cl (dashed lines) viewed in the direction (a) approximately [0 −2 1] and (b) [100]. |
Crystal packing of 6 is generally similar to packing of 4 and 5: polymeric chains are hydrogen bonded into a 3D network (Fig. 8, Table 3). Imidazole ligands of A chains are proton donors towards chloranilate oxygens of B chains and vice versa, forming layers parallel to (110); additionally the layers are stabilised by π-interactions between imidazole ligands (Fig. 8, Table 4). A 3D network is achieved by linking the layers through N–H⋯O hydrogen bonds (Table 3).
The simulations were carried out by the EasySpin software.53 To simulate spectra, the following form of spin-Hamiltonian was assumed:
H = μBB·g·S | (1) |
In eqn (1), B is magnetic field, S is electron spin operator and g is g-tensor. Hyperfine splitting tensor A, due to interaction between electron S = 1/2 and nuclear spin I = 3/2 in copper ions, is approximated to be zero. The simulated spectra, shown in Fig. 9, were obtained using the parameters presented in Table 5. The same parameters were used for the simulation of the spectra at low and room temperatures by taking into consideration only line-broadening effect (ESR lines were broader at higher temperatures). Line shapes used in simulation were always Lorentzian. The simulations reproduce well the experimentally observed spectra for compounds 1, 2 and 5, as could be seen in Fig. 9. The additional lines observed in spectra of compounds 3, 4 and 6 are probably connected with presence of trace impurity (i.e. electron in the vicinity of 14N and 1H nuclei).
Compound | 1 | 2 | 3 | 4 | 5 | 6 |
g x | 2.058 | 2.07 | 2.070 | 2.039 | 2.08 | 2.035 |
g y | 2.103 | 2.07 | 2.078 | 2.092 | 2.08 | 2.143 |
g z | 2.360 | 2.33 | 2.40 | 2.30 | 2.36 | 2.278 |
R | 0.175 | 0 | 0.02 | 0.25 | 0 | 0.8 |
|J| (cm−1) | 0.03 | 0.27 | 0.52 | 0.66 | 0.10 | 0.13 |
Observed Lorentzian lineshapes and absence of hyperfine interaction for copper(II) ions point to the presence of exchange interactions in the investigated systems. The values of exchange interaction parameters, |J|, could be approximately calculated using linewidth analysis and the method of moments:54,55
Γexp ∼ γ(Γd)2/ωexch | (2) |
Here, ωexch ∼ J[S(S + 1)]1/2 and γ is gyromagnetic ratio, Γexp is the experimental linewidth (Γ = FWHM/(2 × 1.18)) while Γd is the dipolar linewidth due to the contribution of the nearest copper(II) ions.56,57 If we consider all copper ions in the sphere of radius 15 Å, the values of the dipolar linewidths are Γd ∼ 5–25 mT. Taking into consideration experimental linewidths (values at 80 K used in the simulations) Γexp ∼ 0.7–2.6 mT, exchange interaction parameters, |J|, were calculated and presented in Table 5. The values calculated in this way have approximate accuracy, but from Table 5 it could be seen that weak exchange interaction are found for all compounds (|J| < 1 cm−1).
The obtained axial g-tensors with gz > gx = gx reveal that the ground state of unpaired electron is the dx2−y2 orbital, for compounds 2 and 5. Other compounds show “rhombic” spectra, exhibiting three different g-values. For complexes of this type, a parameter R is the indication of the predominance of the dx2−y2 or dz2 orbital in the ground state.58 The systems where gz > gy > gx, R parameter is defined as:
R = (gy − gx)/(gz − gy) | (3) |
For R > 1, the ground state is predominantly dz2, whereas for R < 1, the ground state is predominantly dx2−y2. The obtained R parameters are shown in Table 5, indicating that the greater contribution to the ground state arises from dx2−y2 orbital. This is in agreement with the crystal structure of the compounds. Both types of observed coordination of copper atoms (a square pyramid in compounds 2 and 3 and elongated octahedron in compounds 1, 4, 5 and 6) give so called normal spectra of Cu2+ ions (gz > gy ≈ gx > 2.0023).58
The distance between chloranilate-bridged Cu2+ atoms within a single molecule range between 7.6 and 8.1 Å, while the inter chain distances of the Cu2+ atoms are considerably shorter than intrachain ones (Table 6). However, relative orientations of dx2−y2 orbitals of interacting Cu2+ atoms are important. In 1, 2, 3 and 5, the dx2−y2 orbital is parallel to the chloranilate plane, while in 4 and 6 it lies in the plane defined by Cu–O2(CA) and Cu–Laxial vectors, approximately normal to the chloranilate plane9,10 (Table 1). However, in all studied compounds, orientations of dx2−y2 orbitals in neighbouring molecules are not favourable for orbital overlap.57 Therefore, weak exchange interaction is observed inside [Cu(CA)]n chains.
Compound | No. of close contacts | Close intrachain distance/Å | Close interchain distance/Å |
---|---|---|---|
1 | 2 | — | 8.747 |
2 | 1 | 7.735 | 3.877 |
3 | 1 | 7.578 | 3.937 |
4 | 2 | 8.087 | 4.832 |
5 | 1 | 7.650 | 5.199 |
6 | 2 | 8.107 | 6.929 |
In coordination polymers, copper(II) usually has a distorted octahedral coordination, and the compound 3 is a rare example of Cu–chloranilate coordination polymer with pentacoordinated Cu2+. Also, compound 2 is a rare example of a complex with two different modes of coordination of chloranilate, a terminal bidentate and a bridging (bis)bidentate ones. Changes of electronic structure of the 2,5-dihydroxyquinonate ring upon various coordination modes affects significantly geometry of the chloranilate moiety.
To the best of our knowledge, so far no crystal structure comprising both a racemate and a meso-compound has been described. The compound 6 is probably the first such example.
ESR spectroscopy gave the g-tensors for Cu(II) ions. The spectra simulations reveal dx2−y2 orbital as the ground state of unpaired electron in copper ions for compounds 2 and 5, whereas for other four compounds, due to “rhombic” spectra, the greater contribution to the ground state arises from dx2−y2 orbital. These results are in agreement with cooper coordination, which is square pyramid in the compounds 2 and 3, and elongated octahedron in other compounds. Furthermore, ESR linewidth analysis revealed the presence of weak exchange interactions (|J| < 1 cm−1) between the copper ions in the investigated complexes, which is consistent with specific crystal packing and also with the values found in similar compounds.9,10
IR data (KBr, cm−1): 3434 (m, br), 3153 (w), 2923 (w), 2852(w), 1657 (w), 1640 (s), 1597 (m), 1535 (s), 1499 (m), 1462 (vs), 1350 (w), 1340 (m), 1269 (w), 1203 (w), 1177 (sh), 1102 (w), 1059 (m), 1003 (w), 944 (w), 917 (w), 851 (m), 825 (w), 781 (w), 722 (w), 669 (w), 622 (w), 611 (w), 569 (m), 541 (w), 515 (w), 446 (w).
IR data (KBr, cm−1): 3424 (m, br), 3130 (w), 3084 (w), 2922 (w), 2852(w), 1637 (m), 1609 (m), 1588 (w), 1545 (s), 1508 (vs), 1366 (vs), 1337 (m), 1266 (w), 1242 (w), 1203 (w), 1131 (w), 1107 (w), 1036 (w), 1001 (w), 866 (m), 847 (m), 797 (m), 710 (w), 626 (w), 599 (w), 573 (m), 527 (w), 461 (w), 414 (w).
IR data (KBr, cm−1): 2923 (w), 2852 (w), 1707 (w), 1620 (w), 1490 (vs), 1370 (vs), 1314 (m), 1285 (w), 1092 (sh), 1011 (w), 866 (s), 635 (w), 579 (m), 537 (m), 517 (sh), 469 (w).
IR data (KBr, cm−1): 3432 (br, m), 2922 (w), 2858 (m), 1709 (w), 1617 (m), 1507 (vs), 1380 (s), 1290 (w), 1256 (w), 1006 (w), 959 (w), 856 (m), 805 (w), 698 (w), 603 (w), 579 (m), 481 (w), 401 (w).
IR data (KBr, cm−1): 3451 (m, br), 2922 (w), 2852 (w), 1705 (w), 1620 (w), 1486 (vs), 1368 (vs), 1314 (m), 1093 (w), 1051 (sh), 1011 (w), 866 (s), 635 (w), 580 (m), 540 (m), 516 (w), 468 (w).
IR data (KBr, cm−1): 2924 (w), 2853 (w), 1627 (s), 1512 (vs), 1437 (w), 1370 (m), 1328 (w), 1293 (w), 1257 (w), 1204 (w), 1179 (w), 1132 (w), 1101 (w), 1070 (m), 1004 (w), 994 (w), 948 (w), 919 (w), 851 (m), 826 (w), 768 (w), 672 (w), 656 (w), 618 (w), 600 (w), 576 (m), 541 (w), 517 (w), 465 (w).
Molecular geometry calculations were performed by PLATON,62 and molecular graphics were prepared using ORTEP-3,33 and CCDC-Mercury.63 Crystallographic and refinement data for the structures reported in this paper are shown in Table 7.
Compound | 1 | 2 | 3 | 4 | 5 | 6 |
Empirical formula | C18H14Cl4CuN4O10 | C28H18Cl6Cu2N2O16 | C8H3Cl2CuNO4 | C6H4Cl2CuO6 | C10H12Cl2CuO6 | C12H8Cl2CuN4O4 |
Formula wt/g mol−1 | 651.68 | 978.24 | 311.55 | 306.53 | 362.64 | 406.67 |
Crystal dimensions/mm | 0.15 × 0.12 × 0.10 | 0.12 × 0.10 × 0.05 | 0.09 × 0.07 × 0.04 | 0.10 × 0.05 × 0.05 | 0.20 × 0.16 × 0.12 | 0.20 × 0.04 × 0.04 |
Space group | P | P | P21/c | P21/c | C2/c | C2/c |
a/Å | 8.2635(10) | 9.3314(5) | 7.5779(3) | 8.0866(6) | 17.8431(10) | 25.3992(8) |
b/Å | 8.7473(8) | 9.4880(4) | 8.2261(4) | 4.8324(4) | 7.6498(4) | 8.1066(2) |
c/Å | 9.1791(7) | 10.3517(6) | 16.7585(8) | 12.9148(9) | 10.3003(5) | 14.2791(4) |
α/° | 70.332(8) | 80.069(4) | 90 | 90 | 90 | 90 |
β/° | 71.570(9) | 89.845(4) | 94.337(4) | 108.388(8) | 107.903(6) | 96.491(3) |
γ/° | 67.319(10) | 66.524(5) | 90 | 90 | 90 | 90 |
Z | 1 | 1 | 4 | 2 | 4 | 8 |
V/Å3 | 563.05(10) | 825.83(7) | 1041.68(8) | 478.91(6) | 1337.87(12) | 2921.24(14) |
D calc/g cm−3 | 1.922 | 1.967 | 1.987 | 2.126 | 1.800 | 1.849 |
μ/mm−1 | 6.356 | 6.795 | 7.703 | 8.484 | 6.183 | 5.735 |
θ range/° | 5.24–75.99 | 4.35–75.85 | 5.29–75.69 | 3.61–75.81 | 6.35–75.75 | 3.50–75.92 |
T/K | 293(2) | 293(2) | 293(2) | 293(2) | 293(2) | 293(2) |
Radiation wavelength | 1.54179 (CuKα) | 1.54179 (CuKα) | 1.54179 (CuKα) | 1.54179 (CuKα) | 1.54179 (CuKα) | 1.54179 (CuKα) |
Diffractometer type | Xcalibur Nova | Xcalibur Nova | Xcalibur Nova | Xcalibur Nova | Xcalibur Nova | Xcalibur Nova |
Range of h, k, l | −9 < h < 10 | −11 < h < 11 | −9 < h < 9 | −10 < h < 10 | −22 < h < 21 | −31 < h < 29 |
−9 < k < 10 | −8 < k < 12 | −10 < k < 6 | −6 < k < 6 | −9 < k < 8 | −10 < k < 6 | |
−7 < l < 11 | −12 < l < 12 | −21 < l < 20 | −16 < l < 10 | −12 < l < 8 | −17 < l < 17 | |
Reflections collected | 4503 | 6802 | 4736 | 4605 | 2868 | 6948 |
Independent reflections | 2278 | 3371 | 2146 | 1047 | 1367 | 2994 |
Observed reflections (I ≥ 2σ) | 2094 | 2993 | 1888 | 960 | 1056 | 2373 |
Absorption correction | Multi-scan | Multi-scan | Multi-scan | Multi-scan | Multi-scan | Multi-scan |
R int | 0.0400 | 0.0224 | 0.0361 | 0.0439 | 0.0370 | 0.0275 |
R(F) | 0.0429 | 0.0356 | 0.0444 | 0.0730 | 0.0479 | 0.0353 |
R w(F2) | 0.1270 | 0.1053 | 0.1292 | 0.1913 | 0.1452 | 0.1075 |
Goodness of fit | 1.077 | 1.048 | 1.060 | 1.144 | 1.146 | 1.028 |
H atom treatment | Mixed | Mixed | Free | Free | Mixed | Riding |
No. of parameters | 177 | 260 | 157 | 79 | 88 | 210 |
No. of restraints | 3 | 6 | 9 | 3 | 1 | 0 |
Δρmax, Δρmin (eÅ−3) | 0.528; −0.751 | 0.476; −0.502 | 0.490; −0.801 | 0.561; −0.592 | 0.452; −0.708 | 0.376; −0.508 |
Footnote |
† Electronic supplementary information (ESI) available: CIF files, geometric parameters of copper(II) coordination spheres in compounds 1–6 (Table S1), views of the imidazolium and 4,4′-bipyridinium cations (Fig. S1 and S2). CCDC 1433294–1433297, 1473407 and 1473408. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra13809h |
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