IR and electronic spectra
The IR spectra (range 4000–200 cm−1) of complexes 1 and 2 display absorption bands at 1592 and 1602 cm−1, respectively, which are assigned to the CN stretching frequencies of the coordinated ligand; for the free ligand this band is observed at 1658 cm−1. The shifts of this band towards lower wave numbers upon coordination are indicative of the binding of the azomethine nitrogen atom to the metal centre.25 The IR spectrum of the free hydrazone molecule contains a strong CO absorption band at 1651–1659 cm−1. For the coordination compounds, this band is not present; instead, a new CO absorption peak appears at 1198 and 1177 cm−1 for complexes 1 and 2, respectively, which clearly suggests that HL undergoes deprotonation to L− upon coordination. The coordination of the anion L− to the copper(II) ion is substantiated further by prominent bands observed at 445 and 370 cm−1 for 1, and at 437 and 378 cm−1 for 2, which can be attributed to the νCuN and νCuO vibrations, respectively. A strong band is observed at 2133 cm−1 for 1, which characterises the presence of bridging SCN− anions. The two bands observed at 310 and 297 cm−1 for 2 are due to vibrations of the CuCl bonds.26 Strong, well-resolved, sharp absorption bands are found in the region 1495–1063 cm−1 for 1, and 1488–1130 cm−1 for 2, which are both assigned to coordinated pyridine rings.25
The electronic spectral data for both coordination compounds, recorded in HPLC grade acetonitrile, are in good agreement with their geometry. The UV absorption bands observed in the range of 217 and 281 nm for 1, and 223 and 298 nm for 2 are due to π–π* transition within the hydrazine ligand. The UV absorption band observed at 385 for 1, and 370 nm for 2, is ascribed to the ligand–metal charge-transfer transition (LMCT) between the hydrazone ligand and cop per(II).27, 28 The visible region of the spectrum for 1 displays a single broad band between 525 and 640 nm. These spectral features are consistent with the five-coordinate geometry of 1. Typically, copper(II) coordination compounds with a square-pyramidal or distorted square-pyramidal geometry exhibit a band in the range 550–660 nm, whereas trigonal-bipyramidal complexes usually show a maximum at a λ value greater than 800 nm, which is associated with a high-energy shoulder.29
EPR spectroscopy and temperature-dependant magnetic susceptibility measurements
The EPR spectra of polycrystalline samples of 1 recorded at room temperature and 100 K are characterised by a slight rhombicity and three g values (gx=2.174, gy=2.074, gz=2.050 at RT, and gx=2.172, gy=2.078, gz=2.050 at 100 K) with the order gx>gy>gz>ge.30 The experimental and simulated spectrum at room temperature are shown in Figure 5 (for experimental and simulated spectrum at 100 K, see Figure S1 in the Supporting Information). No resonances below 2500 and above 3500 G are detected. In these situations, the ground state can be described as a linear combination of d and d orbitals,31 and the parameter R=(gy−gz)/(gx−gy) is indicative of the predominance of d or d orbital (if R>1, the greater contribution to the ground state arises from d; if R <1, the greater contribution to the ground state arises from d orbital).30 The R values for 1 (0.24 at RT and 0.30 at 100 K) confirm the distorted square-pyramidal arrangement, for which a ground state based on the d orbital is expected.30 The χM T product (molar magnetic susceptibility: χM) of 1 remains practically constant at approximately 0.422–0.427 cm3 mol−1 K from 300 down to 4.5 K and only then decreases very slightly down to 0.404 cm3 mol−1 K at 2 K (see Figure 6), indicating the presence of very weak antiferromagnetic interactions. Given the thiocyanato-bridged, one-dimensional arrangement of the copper(II) ions in the structure of 1, the experimental susceptibility data were fitted to the Bonner and Fisher regular chain model,32 providing a good simulation (solid line in Figure 6) for the best-fit parameters: g=2.134(1) and J/kB=−0.14(1) K (or J=0.10(1) cm−1). The g value is in good agreement with the EPR data, while the very weak antiferromagnetic coupling can be satisfactorily correlated with the structural parameters of the thiocyanate bridge in 1. Its end-to-end coordination mode, occupying an equatorial position at one copper(II) ion and an apical position at the adjacent one, with a long axial CuS bond length of 2.745(1) Å, can only result in a very weak exchange coupling, given the absence of spin density at the apical position of the copper(II) ion in a square-pyramidal environment. Indeed, similarly weak exchange couplings, with |J| values ranging up to 0.1 cm−1 have been reported for structurally characterised, single thiocyanato-bridged copper(II) complexes, for which the arrangement of the donor atoms around the metal centre is square pyramidal.14b, 33 The low efficiency of the thiocyanato ligand to mediate magnetic interactions through the out-of-plane pathway contrasts with that observed when it binds to the equatorial positions of two adjacent copper(II) ions.34
Figure 5. X-band EPR spectrum of a polycrystalline sample of complex 1 at RT: a) experimental and b) simulated spectrum.
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Figure 6. A plot of χM T versus T for complexes 1 (•) and 2 (○), where χM is the molar magnetic susceptibility. Only data below 200 K are shown. Lines represent the best fits to the adequate model (see text).
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The solid-state polymeric structure of 1 collapses in acetonitrile, dimethyl sulfoxide (DMSO) or dimethylformamide (DMF), where mononuclear units are formed. The solution EPR spectra in DMSO and DMF are characterised by an axial symmetry with the unpaired electron in the d orbital (Figure 7; for the simulated spectra, see Figures S2 and S3 in the Supporting Information). The parameters are comparable in all of the three solvents, and these similarities can be explained by the breaking of the weak Cu⋅⋅⋅S coordination bond and the formation of discrete [CuL(NCS)] species (acetonitrile: g||=2.242, A||= 171×10−4 cm−1, g⊥=2.054, A⊥=15×10−4 cm−1; DMSO: g||=2.244, A||=170×10−4 cm−1, g⊥=2.052, A⊥=14×10−4 cm−1; and DMF: g||=2.240, A||=171×10−4 cm−1 g⊥=2.056, A⊥=14×10−4 cm−1). The formation of tetragonal mononuclear copper(II) species after the dissolution of the crystalline solid in coordinating organic solvents has been observed for other polymeric NCS-bridged copper(II) compounds.35
Figure 7. Anisotropic X-band EPR spectra recorded at 100 K of polycrystalline samples of 1 dissolved in a) DMSO and b) DMF.
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The powder EPR spectrum of a polycrystalline sample of 2 shows two resonances around 2950 and 3200 G (Figure 8 a). No variation with temperature and no other absorption bands, other than the two at around 2950 and 3200 G, were observed. The values measured are 2.214 for g|| and 2.039 for g⊥ at room temperature, and 2.215 for g|| and 2.042 for g⊥ at 100 K in dichloromethane/toluene (50:50 v/v) (Figure 8; for the simulated spectra, see Figures S4 and S5 in the Supporting Information). These values are again consistent with the square-pyramidal geometry of the copper(II) ions in 2 and a d ground state.30 The experimental behaviour (axial spectrum and no spectral change as a function of the temperature) is in agreement with the data reported in the literature for similar compounds.36 The forbidden triplet–singlet transition (ΔMS=±2), often detected at half-field for dinuclear copper(II) complexes, is not observed. The χM T product of 2 shows a very smooth decrease from approximately 0.85 cm3 mol−1 K at 300 K down to 0.810 cm3 mol−1 K at 35 K, which is attributable to the temperature-independent paramagnetism (TIP) of the copper(II) ions (Figure 5). A more abrupt decrease then sets in to reach 0.375 cm3 mol−1 K at 2 K, clearly indicating the presence of a weak anti-ferromagnetic intramolecular interaction. The experimental data were satisfactorily fitted (solid line in Figure 6) to the expression given in Eq. (1)37 derived for an exchange-coupled pair of S=1/2 spins.((1))
Figure 8. X-band EPR spectra of polycrystalline samples of 2 dissolved in dichloromethane/toluene (50:50 v/v) at a) RT and b) 100 K.
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Adding a fixed TIP value of 1.2×10−4 cm3 mol−1 K resulted in the best-fit parameters g=2.10(1) and J/kB=−3.9(1) K (or J=2.7(1) cm−1). The very strong intensity of the EPR signal is in agreement with the presence of such a weak magnetic interaction and in line with conclusions from a number of magneto–structural correlation studies with dinuclear dichlorido-bridged copper(II) complexes described in the literature.36c, d, g, 38 Indeed, the exchange-coupling constant J is expected to depend on the value of the CuClCu bridging angle (α), as well as on the bond length of the axial (longer) CuCl bond (R), although the different types of arrangement of the two copper polyhedra can also have a great influence on the magnetic behaviour of such complexes.38b–e For a square pyramid sharing one base-to-apex edge, but with parallel basal planes, such as compound 2, extended Hückel calculations show that the magnetic interaction occurs through a π* interaction between the copper d and the chloride p orbitals, and that the extent of the magnetic coupling depends on small structural deviations from the ideal square arrangement of the Cu2Cl2 core.38e A theoretical correlation between the magnetic coupling and both parameters (α and R) shows that for small α values and relatively short R values (2: α=85.4°, R=2.746 Å; Table 2) the magnetic coupling should be very weak;36d in particular, for a value of θ/R lower than 32.6° Å−1 or higher than 34.8° Å−1 (31.1° Å−1 for 2) the exchange interaction is anti-ferromagnetic,38b, c which is in agreement with our evaluation of the exchange-coupling constant J here.
The behaviour of 2 in solution depends on the solvent used (Figure 9). In a non-coordinating solvent (e.g., CH2Cl2, CHCl3, toluene) or in a mixture of non-coordinating solvent (e.g., 50:50 v/v CH2Cl2/toluene), the structure is retained (see Figure 8 b), as observed for other similar compounds.39 On the contrary, in a coordinating solvent (e.g., DMSO or DMF), the longest CuCl bond is broken and mononuclear copper(II) species are formed; these observations are in agreement with what is found in many other similar complexes.36c, e, 40 The EPR spectrum in DMSO shows an axial symmetry and d ground state (Figure 9 a; for the simulated spectrum, see Figure S6 in the Supporting Information). The spectral parameters (g||=2.251, A||=168×10−4 cm−1, g⊥=2.052, A⊥=14×10−4 cm−1), though slightly different from those measured for 1 in DMSO, are consistent with the presence of a chloride rather than a thiocyanato ligand in the fourth equatorial position of the copper ion. Furthermore, the equatorial donor set (Npyr, Nimine, , Cl−) is confirmed by the A|| value (168×10−4 cm−1), which is smaller than that of ([, Nimine, , Cl−] given by the ligand 2-((E)-(2-hydroxyethylimino)methyl)-4-bromophenol (A||=176.5×10−4 cm−1);36f however, it is noteworthy that the weak axial coordination of DMSO molecules is also possible. The EPR spectrum in DMF (Figure 9 c; see also Figure S8 in the Supporting Information) is characterised by g⊥>g||∼ge (g⊥=2.219, g||=2.016); this order can be explained by considering a d ground state and a trigonal-bipyramidal geometry.30 It is plausible that DMF inserts into the first coordination sphere of 2, forming a penta-coordinated species. Interestingly, in DMSO/DMF (50:50 v/v), both the octahedral [CuLCl(DMSO)2] and the trigonal-bipyramidal species [CuLCl(DMF)] are present (Figure 9 b). The binding of a solvent molecule, like DMF, to copper to give a trigonal-bipyramidal complex is observed with the tetradentate calixarene capped by a tris(2-aminoethyl)amine (tren) unit.41 Remarkably, competitive binding experiments have also demonstrated that, for a coordinating solvent, the preference order for an exchangeable metal site is DMF>ethanol>acetonitrile.41 The super-hyperfine structure visible in the parallel region of the EPR spectrum (Figure 9 c), which can be attributed to the interaction of the copper(II) unpaired electron with two 14N (I=1) equivalent nuclei, belonging to the pyridine ring and imino groups. The AN value of 15 ×10−4 cm−1 is in good agreement with those reported in the literature.42 The weak resonances observable in DMF can be attributed to the minor species [CuLCl(DMF)2] with two axially coordinated DMF molecules (Figure S7 in the Supporting Information); the presence of more than one species in an organic solvent, one originating from the dissociation of the dinuclear core, and another originating from their solvation, has already been demonstrated for other di-μ-chloro copper(II) complexes.36e
Figure 9. Anisotropic X-band EPR spectra recorded at 100 K of polycrystalline samples of 2 dissolved in a) DMSO, b) DMSO/DMF (50:50 v/v) and c) DMF. The small graduated line indicates the position of the absorptions due to the super-hyperfine coupling with 14N nuclei and the asterisks (*) indicate the parallel resonances of the minor species [CuLCl].
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