CuII2, CuII4 and CuII6 complexes with 3-(2-pyridyl)pyrazolate. Structure, magnetism and core interconversion

Polyhedron 171 (2019) 365–373

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Review

CuII2, CuII4 and CuII6 complexes with 3-(2-pyridyl)pyrazolate. Structure, magnetism and core interconversion Arup Kumar Das a, Anindita De b,1, Prerna Yadav a, Francesc Lloret c, Rabindranath Mukherjee b,⇑ a

Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 246, India Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India c `nica/Instituto de Ciencia Molecular (ICMOL), Universitat de València, Polígono de la Coma, s/n, 46980-Paterna, València, Spain Departament de Química, Inorga b

a r t i c l e

i n f o

Article history: Received 7 June 2019 Accepted 13 July 2019 Available online 26 July 2019 Keywords: CuII complexes: dimer, tetramer and hexamer 3-(2-Pyridyl)pyrazolate as a terminal and bridging ligand Crystal structures Magnetism

a b s t r a c t Reactions of stoichiometric amounts of L1(
) (HL1 = 3-(2-pyridyl)pyrazole) with [Cu(H2O)6](ClO4)2, with 0 or without PhCO
2 , in MeOH or N,N -dimethylformamide (dmf), led to the isolation of three copper(II) complexes of varying nuclearity, [CuII2(L1)2(ClO4)2(MeOH)2] (1), [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) and [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3). Structural analysis reveals two centrosymmetric four-coordinate {CuII(L1)(ClO4)(MeOH)} units are dipyrazolate-bridged in 1, giving rise to a square-pyramidal (SP; s = 0.13) coordination to the CuII ion. In 2, two centrosymmetric four-coordinate dipyrazolate-bridged {CuII2(l-L1)2(MeOH)2}2+ units in two layers are held by two syn–anti PhCO
2 bridges, giving rise to CuII centres with a distorted SP geometry (s = 0.28) geometry. In 3, a three-coordinate dipyrazolate-bridged {CuII2(l-L1)2}2+ unit is held between two {CuII2(L1)2(ClO4)(dmf)2}}2+ units in two different layers by two syn–anti bridging PhCO
2 ligands in a centrosymmetric manner, generating two SP (s = 0.03 and 0.28) and a square-planar geometry. In 2, two successive layers are held by two and in 3 by one PhCO
2 bridge(s). Systematic core-interconversion studies as a function of PhCO
2 , solvent (MeOH, dmf) and concentration have been done and the observed results have been rationalized. Variable-temperature magnetic studies (1.9–300 K) for powdered samples of 1–3 revealed strong antiferromagnetic coupling between the two copper(II) centres in the {CuII2(l-pyrazolato)2}2+ units. For 2 and 3, additional magnetic interactions through PhCO
2 bridge(s) between two layers have also been realized. The magnitude of the coupling is discussed in terms of the structural parameters. Ó 2019 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Synthesis of the complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. [CuII2(L1)2(ClO4)2(MeOH)2] (1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Core interconversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Conversion of 1 to 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Conversion of 1 to 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Conversion of 2 to 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4. Conversion of 1 and 2 to 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Physical measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Magnetic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. Crystallographic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Fax: +91 512 259 7436. 1

E-mail address: [email protected] (R. Mukherjee). Current address: Department of Chemistry & Biochemistry, School of Basic Sciences and Research, Sharda University, Greater Noida 201301, UP, India.

https://doi.org/10.1016/j.poly.2019.07.023 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

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3.

Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. The complexes and their general characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Crystal structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. [CuII2(L1)2(ClO4)2(MeOH)2] (1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Core interconversion studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Absorption and EPR spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction In the general area of metallo-supramolecular chemistry with multidentate 3-(2-pyridyl)pyrazole (HL1)-derived ligands [1–3], coordination-driven self-assembly [4] has produced many examples of synthetically challenging coordination cages [5] of appealing structural and topological novelty, with interesting guest-binding [1c] and catalytic properties [1c,2]. HL1 and HL1derived ligands have also been used as simple chelators to many metal ions producing mononuclear as well as isolated two/three coordination units in a given complex [3d,6], dinuclear mixedligand complexes with ancillary mono-carboxylate ligands [7a], dinuclear [7b,c], mono- and dinuclear with naphthyl spacer [7d] and dinuclear mixed-ligand complexes [7e], molecular cleft [8], polyoxometallates consisting of {M(HL1)} cations [9], coordination polymers [3d,10], including ancillary mono- and di-carboxylate ligands [7a], and mono-/bi-/poly-metallic organometallic molecules [11]. Investigation of the magnetic [12,13] and luminescent properties [6,7a,d,e], catalytic activity [14] of organometallic molecules, and potential medical application [15] of complexes of this class of ligands continue to be of current interest. The investigation of the coordination chemistry of 3-(2-pyridyl) pyrazole-based ligands (Fig. 1) has been a part of our research activity [16,17]. We have paid our attention to spin-equilibria phe-

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nomena in FeIIN6 complexes with L3 and L4 [16a,b], ethyl acetate hydrolysis by {CuII2(l-OH)2}2+ (the CuII-coordinated hydroxide ion acts as a nucleophile) with L2 [16c] and magneto-structural studies on acetate-bridged CoII2 (Ref. [17a]), and dipyrazolate-bridged NiII2 (Ref. [17b]) and CuII2 (Ref. [17b]) systems, supported by L1(
), L2, L3 and L5. For the synthesis of high-nuclearity coordination complexes of a labile copper(II) ion with L1(
), one must control the metal–ligand interactions1b due to the stereoelectronic preference [3d,6a] of the CuII ion (square-based geometry). To the best of our knowledge, only three dinuclear complexes, [CuII2(L1)2(OClO3)2(MeCN)2] [7b], [CuII2(L1)2(ONO2)2(H2O)2] [12b] and [CuII2(l-L1)2(HL1)2](NO3)0.5 (4,40 -dibromobiphenyl)H2O [7c], and four discrete tetranuclear complexes, [CuII4 (l-L1)6(S)2](PF6)22dmf (S = dmf or MeOH), [CuII4(l-L1)6(ONO2)2] and [CuII4(l-L1)6(L1)2], have so far been structurally-characterized [12,18]. We reasoned whether or not planar {CuII(l-L1)2CuII}2+ units could be brought together by additional carboxylate-bridging to extend the nuclearity in a controllable manner to tetra-, hexa- complexes etc. From this perspective, in the present work we have prepared and structurally characterized the new dinuclear [CuII2(L1)2(ClO4)2(MeOH)2] (1), tetranuclear [CuII4(L1)4 (O2CPh)2(MeOH)4](ClO4)22H2O (2) and hexanuclear [CuII6(L1)6 (O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3) complexes with L1(
) (i) to pinpoint the factors responsible for the successful synthesis of CuII complexes of varying nuclearity, (ii) to establish the underlying structure-directing property of the {CuII(l-L1)2CuII}2+ unit, triggered by solvent, complex concentration and benzoate-bridge, for differentiating the formation of a given nuclearity over others and (iii) to study their magneto-structural properties in a systematic manner. 2. Experimental 2.1. General All reagents were obtained from commercial sources and used as received. 3-(2-Pyridyl)pyrazole (HL1) was synthesized following a reported procedure [19]. The metal salt [CuII(H2O)6](ClO4)2 was prepared from the reaction between CuCO3 and a mixture of water and 70% HClO4 (1:1, v/v). 2.2. Synthesis of the complexes

Fig. 1. Ligands of pertinence to this work.

2.2.1. [CuII2(L1)2(ClO4)2(MeOH)2] (1) To a solution of HL1 (0.050 g, 0.35 mmol) in MeOH (4 mL), Et3N (0.035 g, 0.35 mmol) was added dropwise and the resulting solution was stirred for 30 min. To this, solid [CuII(H2O)6](ClO4)2 (0.13 g, 0.35 mmol) was added and stirred. After 3 h of stirring, the clear blue solution that resulted was kept for slow evaporation in air. After 3 days, a blue crystalline solid precipitated. Recrystal-

A.K. Das et al. / Polyhedron 171 (2019) 365–373

lization was achieved by vapour-diffusion of Et2O into a solution of the complex in MeOH. Yield: 0.082 g (77%). Anal. Calc. for C18H20Cl2Cu2N6O10 (1; fw 678.38): C, 31.86; H, 2.97; N, 12.39. Found: C, 31.66; H, 2.88; N, 12.52%. IR (KBr, cm
1, selected peaks): 3340 m (OAH), 1100 (split) and 630 m(ClO
4 ). Absorption spectra [kmax, nm (e, M
1 cm
1)] (in MeOH): 323 (9970), 658 (605); (in dmf) 307 (6080), 653 (250). 2.2.2. [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) Solid HL1 (0.2 g, 1.38 mmol) was dissolved in MeOH (8 mL) and Et3N (0.14 g, 1.38 mmol) was added dropwise and the resulting solution was stirred for 30 min. To this, a solution of [CuII(H2O)6] (ClO4)2 (0.51 g, 1.38 mmol) in MeOH (6 mL) and solid NaO2CPh (0.10 g, 0.69 mmol) were added simultaneously dropwise. The colour of the reaction mixture changed from light blue to intense blue and finally to greenish blue. After stirring for 3 h, the reaction mixture was filtered and kept for slow evaporation in air. After 3– 4 days, a blue crystalline solid was obtained. Recrystallization was achieved from MeOH by vapour-diffusion of Et2O. Yield: 0.042 g (79%). Anal. Calc. for C50H54Cl2Cu4N12O18 (2; fw 1436.10): C, 41.83; H, 3.79; N, 11.70. Found: C, 41.70; H, 3.70; N, 12.00%. IR (KBr, cm
1, selected peaks): 3452 m(OAH), 1618, 1405 m(PhCOO
),
1 1110 and 634 m(ClO
cm
1)] 4 ). Absorption spectra [kmax, nm (e, M (in MeOH): 343 (16 930), 624 (340); (in dmf): 305 (7500), 622 (315). 2.2.3. [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3) To a solution of HL1 (0.1 g, 0.70 mmol) in N,N0 -dimethylformamide (dmf) (2.5 mL), Et3N (0.07 g, 0.70 mmol) was added dropwise and the resulting solution was stirred for 30 min. Solid [CuII(H2O)6](ClO4)2 (0.26 g, 0.70 mmol) and NaO2CPh (0.033 g, 0.23 mmol) were then added pinch by pinch. The colour of the reaction mixture changed from light blue to intense blue to finally to greenish blue. After stirring for 3 h, the blue solution was kept for Et2O diffusion. After 6–7 days a blue crystalline solid was obtained. Recrystallization was achieved by dmf/Et2O vapour diffusion. Yield: 0.18 g (78%). Anal. Calc. for C80H92Cl4Cu6N24O28 (3; fw 2360.81): C, 40.70; H, 3.93; N, 14.24. Found: C, 40.76; H, 3.85; N, 14.50%. IR (KBr, cm
1, selected peaks): 3435 m(OAH), 1609, 1455 m(O2CPh), 1100 (split) and 634 m(ClO
4 ). Absorption spectrum [kmax, nm (e, M
1 cm
1)] (in dmf): 306 (7500), 656 (406). 2.3. Core interconversions 2.3.1. Conversion of 1 to 2 To a solution of 1 (0.05 g, 0.073 mmol) in MeOH (4 mL), NaO2CPh (0.01 g, 0.07 mmol) dissolved in MeOH (1 mL) was added dropwise and the resulting solution was stirred at 298 K. After 3 h, the reaction mixture was filtered and the filtrate was kept for slow evaporation in the air. After 2–3 days, a blue crystalline solid was obtained. Recrystallization was achieved by MeOH/Et2O vapour-diffusion. Yield: 0.04 g (76%). Elemental analysis data of the isolated product are very similar to those obtained from the direct synthesis. Cell parameters of the single-crystals obtained from this procedure are identical to those of 2. 2.3.2. Conversion of 1 to 3 Complex 1 (0.07 g, 0.102 mmol) was dissolved in dmf (3 mL) and stirred for 10 min. A solution of NaO2CPh (0.01 g, 0.07 mmol) in dmf (1 mL) was added dropwise and the resulting solution was stirred at 298 K. After 3 h, the clear solution thus obtained was kept for vapour-diffusion with Et2O. After 6–7 days, a blue crystalline solid was obtained. Recrystallization was achieved by dmf/Et2O vapour-diffusion. Yield: 0.058 g (63%). Elemental analysis data of the isolated solid are very similar to those obtained from

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the direct synthesis. Cell parameters of the single-crystals thus obtained are identical with those of 3. 2.3.3. Conversion of 2 to 3 A solid sample of 2 (0.042 g, 0.03 mmol) was dissolved in dmf (4 mL) and stirred. After 2 h, the clear solution thus obtained was kept for vapour-diffusion with Et2O. After 5–6 days, a blue crystalline solid precipitated. Recrystallization was achieved by dmf/ Et2O vapour-diffusion. Yield: 0.02 g (54%). Elemental analysis data of the isolated product are very similar to those obtained from the direct synthesis. Cell parameters of the single-crystals thus obtained are identical with that of 3. 2.3.4. Conversion of 1 and 2 to 3 Complex 1 reacts with NaO2CPh in MeOH at 298 K under stirring for 3 h affording 2. Moreover, 1 reacts with NaO2CPh in dmf at 298 K under stirring for 3 h resulting in the isolation of 3. Notably, at 298 K the transformation of 2 to 3 is solvent-dependent. However, when a dilute solution (0.65 mM) of 2 was recrystallized from dmf/Et2O, the microcrystalline solid of 3 was obtained. Elemental analysis data of the isolated products are very similar to those obtained from the direct synthesis. Cell parameters of the single-crystals thus obtained are identical with those of 2 or 3. Caution! Perchlorate salts of compounds containing organic ligands are potentially explosive! 2.4. Physical measurements Elemental analyses were obtained using a Thermo Quest EA 1110 CHNS-O, Italy. Spectroscopic measurements were made using the following instruments: IR (KBr, 4000–500 cm
1), Perkin Elmer FT-IR; electronic, Perkin Elmer Lambda 950 and Agilent 8453 diode-array spectrophotometers. X-band EPR spectra were recorded using a Bruker EMX 1444 EPR spectrometer, operating at 9.455 GHz. The EPR spectra were calibrated with diphenylpicrylhydrazyl, DPPH (g = 2.0037). Spectra were treated using the Bruker WINEPR software. 2.4.1. Magnetic measurements Magnetic susceptibility measurements (València, Spain) on a powder form of the single-crystals of 1–3 were carried out with a Superconducting Quantum Interference Design (SQUID) magnetometer in the temperature range 1.9–300 K, under an applied magnetic field of 0.01 Tesla for T < 50 K in order to avoid saturation effects and 1 Tesla for T > 50 K. 2.4.2. Crystallographic studies Single-crystals of suitable dimensions were used for the data collection. Diffraction intensities were collected on a Bruker SMART APEX CCD diffractometer, with graphite-monochromated Mo Ka (k = 0.71073 Å) radiation at 100(2) K. For data reduction, the ‘Bruker Saint Plus’ program was used. Data were corrected for Lorentz and polarization effects; an empirical absorption correction (SADABS) was applied. The structures were solved by SHELXT and refined with SHELXL-2016 [20], as incorporated in the OLEX2 1.2-alpha crystallographic package [21]. The positions of the hydrogen atoms were calculated by assuming ideal geometries, but not refined. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares procedures on F2. For 3, the Cl1 atom of one of the perchlorate anions is highly disordered and we successfully modelled it by the PART command as Cl1 and Cl1A, with occupancies of 0.75 and 0.25, respectively. Full details of the crystal structure refinements have been deposited with the Cambridge Crystallographic Data Centre (CCDC). A brief summary of the data collection and refinement is provided

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Table 1 Data collection and structure refinement parameters for [CuII2(L1)2(ClO4)2(MeOH)2] (1), [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) and [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3).

Formula Formula weight Crystal colour, habit T (K) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z DCalc (g cm
3) l (mm
1) Reflections measured Unique reflections/Rint Reflections used I > 2r(I)] R1a wR2b [I > 2r(I)] R1a wR2b (all data) GOF on F2 a b

1

2

3

C18H20Cl2Cu2N6O10 678.38 blue, block 100(2) triclinic P
ı

C50H54Cl2Cu4N12O18 1436.10 blue, prism 100(2) monoclinic P21/n

C80H92Cl4Cu6N24O28 2360.82 blue, block 100(2) triclinic P
ı

7.8165(4) 10.0685(5) 15.9372(8) 82.487(4) 79.984(4) 74.618(4) 1186.17(11) 2 1.899 2.087 7482 4504/0.0255 3981 R1 = 0.0365a wR2 = 0.0906b R1 = 0.0415a wR2 = 0.0951b 1.041

13.2305(4) 10.5043(3) 21.1541(7) 90 103.821(3) 90 2854.82(16) 2 1.671 1.647 11 426 6231/0.0279 5246 R1 = 0.0418a wR2 = 0.0929b R1 = 0.0529a wR2 = 0.0987b 1.051

10.5844(4) 14.5488(7) 17.0437(9) 107.177(4) 100.700(4) 103.757(4) 2341.1(2) 1 1.674 1.544 16 734 10 225/0.0251 8975 R1 = 0.0381a wR2 = 0.0873b R1 = 0.0445a wR2 = 0.0915b 1.033

R1 = R||Fo|
|Fc||/R|Fo|. wR2 = {R[w(|Fo|2
|Fc|2)2]/R[w(|Fo|2)2]}1/2.

Table 2 Selected bond lengths (Å) and bond angles (°) for [CuII2(L1)2(ClO4)2(MeOH)2] (1). Cu1AN1 Cu1AN2 Cu1AN3 Cu1AO9 Cu1AO1 Cu1  Cu1*

*

2.023(2) 1.962(2) 1.959(2) 2.0235(18) 2.3907(19) 3.933(6)

N2ACu1AN3 N1ACu1AN3 N1ACu1AN2 N3ACu1AO9 N2ACu1AO9 N1ACu1AO9 N3ACu1AO1 N2ACu1AO1 N1ACu1AO1 O9ACu1AO1

Table 4 Selected bond lengths (Å) and bond angles (°) for [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4] (ClO4)22dmf2H2O (3). Cu1AN1 Cu1AN2 Cu1AN6 Cu1AO1 Cu1AO2 Cu2AN3 Cu2AN4 Cu2AN5 Cu2AO3 Cu3AN7 Cu3AN8 Cu3AN9 Cu3AO4 Cu1  Cu2 Cu2  Cu3 Cu3  Cu3*

97.81(9) 175.59(8) 81.74(9) 91.06(8) 167.83(8) 88.77(8) 94.33(8) 99.81(8) 90.06(7) 87.74(7)

Symmetry operator: 11
x,
y, 2
z; 21
x, 2
y, 1
z.

2.037(2) 1.973(2) 1.962(2) 1.967(18) 2.309(18) 1.938(2) 2.003(2) 1.974(2) 1.977(17) 2.010(2) 1.951(2) 1.951(2) 1.935(17) 3.924(7) 3.267(6) 3.896(7)

Table 3 Selected bond lengths (Å) and bond angles (°) for [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2). Cu1AN6 Cu1AN1 Cu1AN2 Cu1AO8 Cu1AO7 Cu2AN4 Cu2AN5 Cu2AN3 Cu2AO9 Cu2AO6 Cu1  Cu2 Cu1  Cu2*

*

1.955(2) 2.030(2) 1.965(2) 1.981(2) 2.313(2) 2.029(2) 1.974(2) 1.957(2) 1.979(2) 2.280(2) 3.929(7) 3.377(5)

Symmetry operator: 1
x, 1
y, 1
z.

O8ACu1AO7 O8ACu1AN6 O8ACu1AN1 O8ACu1AN2 O7ACu1AN6 O7ACu1AN1 O7ACu1AN2 N6ACu1AN1 N6ACu1AN2 N1ACu1AN2 O9ACu2AO6 O9ACu2AN4 O9ACu2AN5 O9ACu2AN3 O6ACu2AN4 O6ACu2AN5 O6ACu2AN3 N4ACu2AN5 N4ACu2AN3 N5ACu2AN3

90.10(9) 91.67(9) 90.34(9) 159.66(9) 90.93(9) 85.77(9) 107.40(9) 176.15(10) 98.17(10) 80.95(10) 92.73(8) 91.51(9) 161.53(9) 90.06(9) 88.59(9) 103.85(9) 90.60(9) 80.99(10) 178.26(10) 97.72(10)

*

O1ACu1AO2 O1ACu1AN2 O1ACu1AN1 N6ACu1AO1 N6ACu1AO2 N6ACu1AN2 N6ACu1AN1 N2ACu1AO2 N2ACu1AN1 N1ACu1AO2 O3ACu2AN4 N5ACu2AO3 N5ACu2AN4 N3ACu2AO3 N3ACu2AN5 N3ACu2AN4 O4ACu3AN8 O4ACu3AN7 O4ACu3AN9 N8ACu3AN7 N8ACu3AN9 N9ACu3AN7

93.35(7) 170.89(8) 90.79(8) 90.16(8) 99.93(8) 97.57(9) 169.08(8) 90.08(8) 80.71(9) 90.87(8) 89.86(8) 159.79(8) 81.47(9) 91.82(8) 97.86(9) 176.56(8) 166.03(8) 90.06(8) 91.29(8) 81.46(8) 98.35(8) 173.71(8)

Symmetry operator: 1
x, 1
y, 1
z.

in Table 1 respectively).

(CCDC

numbers

1909207–1909209

for

1–3,

3. Results and discussion 3.1. The complexes and their general characterization The complexes were prepared in a straightforward manner. The reaction between HL1, Et3N and [CuII(H2O)6](ClO4)2 in MeOH (mole ratio 1:1:1, respectively), followed by vapour-diffusion of Et2O into a MeOH solution of the microcrystalline solid, led to isolation of blue crystals of [CuII2(L1)2(ClO4)2(MeOH)2] (1). The same methodology as

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369

Fig. 2. Perspective view of [CuII2(L1)2(ClO4)2(MeOH)2] (1). Only donor atoms are labelled and the hydrogen atoms are omitted for clarity.

Fig. 4. Perspective view of [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3). Only donor atoms are labelled and the hydrogen atoms are omitted for clarity.

Fig. 3. Perspective view of [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2). Only donor atoms are labelled and the hydrogen atoms are omitted for clarity.

for the synthesis of 1 with additional NaO2CPh (mole ratio 2:2:2:1, respectively), followed by vapour-diffusion of Et2O into a solution in MeOH of the microcrystalline solid afforded the isolation of greenish-blue crystals of [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2). Following the procedure for 2, but instead of MeOH this time the solvent used was dmf (mole ratio 3:3:3:1, respectively) and crystallization of the crude solid by dmf-Et2O vapour-diffusion led to the isolation of greenish-blue crystals of [CuII6(L1)6(O2CPh)2(ClO4)2 (dmf)4] (ClO4)22dmf2H2O (3). Complexes 1–3 exhibit IR bands (Fig. S1, ESI) assignable to bridging benzoate (m(CO
2 )) [22], coordinated dmf (m(C@O))/MeOH (m(OH)) and m(ClO
4 ). Elemental analyses conform to the compositions of 1–3 and are confirmed by single-crystal X-ray diffraction studies (see below). 3.2. Crystal structures To determine the molecular structures of the copper(II) complexes, X-ray crystallographic analysis was carried out. Selected inter-atomic distances and angles are listed in Tables 2–4.

3.2.1. [CuII2(L1)2(ClO4)2(MeOH)2] (1) The asymmetric unit contains two crystallographically-independent molecules of 1. Both molecules have essentially identical coordination geometries and the corresponding bond lengths and bond angles are very similar. A perspective view of one of the molecules of 1 is shown in Fig. 2 and selected inter-atomic distances and angles for such a molecule are listed in Table 2. Structural analysis reveals that 1 has a centrosymmetric {CuII2(l-L1)2}2+ bridging unit. The Cu1 ion is coordinated by a pyridine (N1) and two pyrazole (N2 and N3) nitrogen atoms. An oxygen atom from MeOH (O9) satisfies the four-coordination. A perchlorate oxygen atom interacts weakly with the Cu1 centre with an elongated distance, Cu1AO5 = 2.3907(19) Å. The basal plane for each copper(II) centre consists of two pyrazolate N atoms of L1(
), a a pyridine N atom and a methanol O atom. Each Cu1 and Cu2 centre has a square-pyramidal geometry with a CuN3O2 coordination environment. This leads to a distorted square-pyramidal geometry around each copper(II) ion. This is further confirmed by the Addison parameter s = 0.13 (for a perfect square-pyramidal geometry s = 0 and for a trigonal–bipyramidal geometry s = 1) [23]. The Cu  Cu separation is 3.933(6) Å. The structure of 1, consisting of the {CuII2(l-L1)2}2+ unit, is very similar to that reported in the literature [7a–c,12b]. 3.2.2. [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) A perspective view of 2 is shown in Fig. 3. Selected inter-atomic distances and angles are listed in Table 3. Here two parallel dinuclear {CuII2(l-L1)2}2+ units are linked by two benzoate bridges. Each of the dinuclear units is bridged by the pyrazolate ion of L1(
) and sits on a crystallographically-imposed inversion centre. Hence only half of the dimeric unit is unique and other half is symmetry-related; also the asymmetric unit of such a crystal contains one H2O molecule, as the solvent of crystallization. Both of the copper(II) centres,

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A.K. Das et al. / Polyhedron 171 (2019) 365–373

Scheme 1. Schematic representation of the synthesis and transformation of CuII2, CuII4 and CuII6 cores.

Cu1 and Cu2, are coordinated by two pyrazole nitrogen atoms (N2 and N6 for Cu1, and N3 and N5 for Cu2) and a pyridine nitrogen atom (N1 for Cu1 and N4 for Cu2). A benzoate oxygen atom (O8 for Cu1 and O9 for Cu2) completed the four-coordination of Cu1 and Cu2. Moreover, an oxygen donor from MeOH provides axial coordination of Cu1 (O7) and Cu2 (O6). The basal plane for each copper(II) centre consists of two pyrazolate N atoms of L1(
), a pyridine N atom and a benzoate O atom. Two dinucleardipyrazolate-bridging units {CuII2(l-L1)2}2+ are held by two benzoate bridges and such a metal–ligand coordination environment generates a unique metallacycle. Each Cu1 and Cu2 centre has a square-pyramidal geometry with a CuN3O2 coordination environment. The Addison’s structural index parameter (s) values are 0.28 for both Cu1 and Cu2 centres, implying a distorted square-pyramidal geometry [23]. The Cu  Cu separations are 3.929(7) (horizontal plane; Cu1  Cu2) and 3.377 (5) (vertical plane; Cu1  Cu2*) Å. The structure of 2, consisting of two {CuII2(l-L1)2}2+ units held by two benzoate bridges leading to a {CuII4(L1)4(O2CPh)2}2+ core, is unique. The reported tetranuclear complexes have a grid-like ‘CuII4(L1)6}2+’ core [12,18].

atom from a perchlorate anion interacts weakly with the Cu2 centre with an elongated Cu2AO6 distance of 2.513 Å. This leads to a distorted square-pyramidal geometry around the Cu1 and Cu2 centres, but a square-planar geometry around the Cu3 centre. Notably, the Cu1 atom is coordinated by a pyridine (N1) and a two pyrazoles (N2 and N6) nitrogen atoms, and two oxygen atoms from dmf (O1 and O2), forming five-coordination at the Cu1 centre. The Addison parameters are s = 0.03 for Cu1 and 0.28 for Cu2 [23]. The Cu  Cu separations are 3.924(7) (Cu1  Cu2), 3.267(6) (Cu2  Cu3) and 3.896(7) Å (Cu3  Cu3*). The structure of 3, consisting of three {CuII2(l-L1)2}2+ units held by two benzoate bridges leading to a {CuII6(L1)4(O2CPh)2]}2+ core, is unique and unprecedented. Three dinuclear dipyrazolate-bridging {CuII2(l-L1)2}2+ units in consecutive layers are held by two centrosymmetric benzoate bridges between the first and second layers and between the second and third layers, and such a metal–ligand coordination arrangement generates a molecular staircase.

3.2.3. [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3) A perspective view of 3 is shown in Fig. 4. Selected inter-atomic distances and angles are listed in Table 4. In 3, three parallel dinuclear {CuII2(l-L1)2}2+ units are linked by two benzoate bridges. The dication sits on a crystallographically-imposed inversion centre and hence only half of the dimeric {CuII2(l-L1)2}2+ unit is unique and the other half is symmetry-related; also the asymmetric unit of such a crystal contains one molecule of dmf and one H2O molecule as the solvent of crystallization. The Cu2 and Cu3 centres are coordinated by two pyrazole nitrogen atoms (N3 and N5 for Cu2, and N8 and N9 for Cu3) and a pyridine nitrogen atom (N4 for Cu2 and N7 for Cu3). Additionally, a benzoate oxygen atom (O3 for Cu2 and O4 for Cu3) satisfies the four-coordination of Cu2 and Cu3. An oxygen

Exhaustive core interconversion studies have been carried out to explore the substitutional flexibility of the dimeric unit(s) present in 1, 2 and 3, given the widely varied reaction conditions. A schematic representation of the core-formation and core-transformation methodology between the CuII2, CuII4 and CuII6 cores is given in Scheme 1. Interestingly, 1 reacts with NaO2CPh in MeOH to afford formation of 2. Crystallization of 2 from dilute dmf solutions results in the isolation of 3. Moreover, 1 reacts with NaO2CPh in dmf to afford 3. Transformation of 1 to 2 with the addition of PhCO
2 in MeOH is a definitive case of bridging-ligand-assisted self-assembly of two {CuII2(l-pyrazolate)2}2+ units in two different layers, leading to the increase in nuclearity from two to four. Here two {CuII2(l-pyrazo-

3.3. Core interconversion studies

A.K. Das et al. / Polyhedron 171 (2019) 365–373

371

ramer with a coordinatively unsaturated {CuII2(l-pyrazolate)2}2+ unit with labile site(s). Addition of a second equivalent of PhCO
2 provides coordination to the labile sites and also self-assembly with another {CuII2(l-pyrazolate)2}2+ unit in the next layer, leading to the formation of the hexamer 3. We believe that the strongly coordinating dmf solvent assists in the generation of the {CuII2(l-pyrazolate)2}2+ unit with labile sites. Solvent- as well as concentration- and counteranion- dependent transformation of tetramer 2 to hexamer 3 is thus understandable. The concentration-dependent formation of 3 from 1 and 2 suggests that increasing dilution favours the formation of a larger proportion of the smaller assembly – the dimeric unit {CuII2(l-pyrazolate)2}2+ – for entropic reasons. On the contrary, comparatively high concentrations favour the larger assembly (formation of hexamer 3 from 1), despite the unfavourable entropic factors associated with its formation. The unfavourable electrostatic factor is compensated by coordination of bridging PhCO
2 group(s) and ClO
4 anion(s). A similar observation was encountered by Ward and coworkers [3e]. It should be mentioned here that although transformations of 1 to 2, 1 to 3 and 2 to 3 are achievable (Scheme 1), the reverse transformations of 3 to 2, 3 to 1 and 2 to 1 are not feasible. 3.4. Absorption and EPR spectra Absorption spectral properties of 1 and 2 were investigated both in MeOH and dmf. Complex 3 is not appreciably soluble in MeOH; it is reasonably soluble in dmf. Complexes 1–3 display a crystal-field transition in the range 600–700 nm and a ligand-tometal charge-transfer (LMCT) transition at 300 nm (Fig. S2, ESI). EPR spectra of powdered samples of 1–3 recorded at 300 K display (Fig. S3, ESI) an isotropic signal at g = 2.12, 2.10 and 2.16, respectively. Complex 3 exhibits a half-field signal at g  4.3 [24], as well. As the temperature is lowered down to 120 K, for 1 and 3 anisotropy develops and in each case a half-field signal at g  4.3 is clearly observable. The signal at g  4.3 implies the presence of dimeric unit(s) in 1–3. 3.5. Magnetic properties

Fig. 5. vM vs T and vMT vs T plots for (a) [CuII2(L1)2(ClO4)2(MeOH)2] (1) and (b) [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) and (c) [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4] (ClO4)22dmf2H2O (3). The solid lines correspond to the best theoretical-fit.

late)2}2+ units are held by two PhCO
2 bridges in a centrosymmetric manner. The synthesis of the hexamer 3 from dilute solutions of the dimer 1 in dmf with concomitant addition of one equivalent of II PhCO
2 can be rationalized,if we invoke that initially two {Cu2 (lpyrazolate)2}2+ units are held by one PhCO
bridge, generating a tet2

In order to evaluate the type and extent of the magnetic interactions between the CuII ions, variable-temperature (1.9–300 K) magnetic susceptibility measurements were performed on powder forms of the single-crystals of 1–3. Fig. 5 displays the change in vMT values with temperature, vM being the corrected molar magnetic susceptibility per two (a), four (b) or six (c) CuII ions. For 1–3, the effective magnetic moment values per copper (leff/Cu) at 300 K are 1.48, 1.34 and 1.51 lB, respectively, which are smaller than that expected for magnetically isolated CuII ion (ca. 1.75 lB). In each case, the vMT values continuously decrease upon cooling and attain almost a zero value below 60 K. This type of magnetic behaviour indicates that the CuII ions in 1–3 are antiferromagnetically coupled. Given the dimeric nature of 1, its experimental vM values can be described by using the modified Bleaney-Bowers equation, based on the Heisenberg spin Hamiltonian, H =
JS1S2 (Eq. (1)), allowing for the presence of a monomeric copper(II) impurity behaving as a Curie paramagnet, q is the mole fraction of such an impurity; the other symbols have their usual meaning. Two different coupling constant values, J and j, are present for 2 and 3 (see Scheme 2) and their magnetic susceptibilities can be described through Hamiltonians (2) and (3). The least-squares best-fit parameters using Eq. (1) are: J =
252 cm
1, g = 2.15 and q = 0.008 for 1. Matrix diagonalization techniques using Hamiltonians (2) and (3).

vM ¼

  Nb2 g 2 ð1
qÞ q þ ½3 þ expð
J=kTÞ 4 kT

ð1Þ

372

A.K. Das et al. / Polyhedron 171 (2019) 365–373

Scheme 2. Schematic presentation of the orbital interactions for magnetic-exchange coupling.

Fig. 6. Depiction of the magnetic-exchange pathways for (a) 2 and (b) 3.

^ ¼
Jð^S1 ^S2 þ ^S2 ^S3 þ ^S3 ^S4 þ ^S1 ^S4 Þ
jð^S1 ^S3 þ ^S2 ^S4 Þ H

ð2Þ

^ ¼
Jð^S1 ^S2 þ ^S3 ^S4 þ ^S5 ^S6 Þ
jð^S2 ^S3 þ ^S4 ^S5 Þ H þ gbHð^S1 þ ^S2 þ ^S3 þ ^S4 þ ^S5 þ ^S6 Þ

ð3Þ

through the VPMAG program [25] led to the following set of the best-fit parameters: J =
241.1 cm
1, j =
73 cm
1, g = 2.14 and q = 0.001 for 2; J =
230.4 cm
1, j =
20.3 cm
1, g = 2.12 and q = 0.005 for 3. The calculated g values are very close to those observed from the EPR spectra of the complexes (see Fig. S3, ESI). The J values obtained for 1–3 are comparable to those reported in the literature for closely-related systems [12,13]. The pathway for magnetic-exchange is expected to be propagated through the bridging pyrazolate ligands [12,13,17b,26]. From the molecular structures of 1–3 (Figs. 2–4), we can conclude that the unpaired electron in each CuII centre is clearly described by the dx2
y2 magnetic orbital, which is coplanar with the pyrazolate skeleton. The expected orbital interactions between the two CuII centres in 1 are through only pyrazolate-bridge and for (2 and 3) through the pyrazolate- and l2-1,3-benzoato-bridges [see (a) of Scheme 2 (for 2) and see (a) and (b) of Scheme 2 (for 3)]. The significant overlap between these magnetic orbitals accounts for the strong antiferromagnetic coupling observed. The J value in the plane is
252 cm
1 for 1,
241 cm
1 for 2 and
230 cm
1 for 3. The s values are 0.13 for 1, 0.28 for the two CuII centres in 2, and 0.03 (Cu1) and 0.28 (Cu2) for 3. Since the bidentate planar N,N-pyridine-pyrazolyl unit L1(
) holds two copper(II) ions in the same plane and the benzoate bridges two copper(II) centres in two adjacent layers in a syn–syn fashion, the overlap between the magnetic-orbitals of the two CuII ions is expected to lead to an effective antiferromagnetic coupling

(Scheme 2b). The Addison parameter s is crucial for the magnitude of
J [12b]. For the square-pyramidal geometry (s = 0), the dx2
y2 magnetic-orbitals most effectively overlap. This overlap diminishes when the geometry of the CuII centre distorts towards trigonal–bipyramidal (s = 1). The observed s values are 0.13 in 1, 0.28 for the two CuII centres in 2, and 0.03 and 0.28 for the two five-coordinate CuII centres (Cu1 and Cu2) in 3. The
J (antiferromagnetic coupling) values follow the trend 1 > 2 > 3. Given the structural results, it is understandable why 1 has the strongest antiferromagnetic coupling. The greater antiferromagnetic coupling in 2 (both CuII centres are fivecoordinate) compared to 3 (Cu1 and Cu2 are five-coordinate, but Cu3 is four coordinate) could be due to the CuII coordination environment. The
j value of 2 is
73 cm
1 and that of 3 is
20 cm
1. A comparison of the molecular structures of 2 and 3 reveals two l2-1,3-benzoato-bridges in 2 and one such a bridge in 3. This must be the reason for the observed trend. In the reported tetranuclear complex, the
j value is more negative (more strongly antiferromagnetically coupled) than that of 2 and 3. Given the presence of {CuII2(l-pyrazolate)2}2+ bridges in the reported example [12a] compared to the l2-1,3-benzoato-bridges present in 2 and 3, the trend is understandable (Fig. 6). 4. Conclusion The coordination ability of L1(
) has been investigated and three new multinuclear copper(II) complexes, [CuII2(L1)2(ClO4) (MeOH)4] (1), [CuII4(L1)4(O2CPh)2(MeOH)4](ClO4)22H2O (2) and [CuII6(L1)6(O2CPh)2(ClO4)2(dmf)4](ClO4)22dmf2H2O (3), have been synthesized and characterized by X-ray crystallography. To the best of our knowledge, the tetramer 2 and the hexamer 3, supported by L1(
) and PhCO
2 bridge(s), are reported for the first time. Systematic core-interconversion studies have been done and the observed results rationalized. Factors, such as coordination by the counteranion and benzoate bridges, solvent, concentration, responsible for the formation of 1–3 have been identified. For 1–3, variable-temperature magnetic measurements reveal the presence of antiferromagnetic coupling within the {CuII2(l-pyrazolate)2}2+ unit(s) and in 2 and 3 inter-dimer antiferromagnetic coupling mediated by PhCO
2 has also been realized. The observed coupling has been rationalized by considering metal–ligand orbital interactions. Using L1(
), we have been able to synthesize/self assemble coordination clusters with even numbers of CuII centres. Currently our efforts are focussed on synthesizing coordination clusters with odd numbers of CuII centres,

A.K. Das et al. / Polyhedron 171 (2019) 365–373

such as tricopper(II). The outcome of which will be the subject matter of a future publication.

[7]

Acknowledgements This work is supported by a J. C. Bose fellowship from the Department of Science & Technology (DST), Government of India. R. M. sincerely thanks the DST for this fellowship. A. K. Das gratefully acknowledges the award of the SRF by the Council of Scientific & Industrial Research, Government of India. We sincerely acknowledge Dr. Arunava Sengupta for his help in X-ray structure determination. Appendix A. Supplementary data CCDC 1909207–1909209 contains the supplementary crystallographic data for 1–3, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2019.07.023. References [1] (a) M.D. Ward, J.A. McCleverty, J.C. Jeffery, Coord. Chem. Rev. 222 (2001) 251– 272; (b) Z.R. Bell, J.A. McCleverty, M.D. Ward, Aust. J. Chem. 56 (2003) 665–670; (c) M.D. Ward, C.A. Hunter, N.H. Williams, Acc. Chem. Res. 51 (2018) 2073– 2082. [2] W. Cullen, M.C. Misuraca, C.A. Hunter, N.H. Williams, M.D. Ward, Nat. Chem. 8 (2016) 231–236. [3] (a) R.L. Paul, Z.R. Bell, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Proc. Natl. Acad. Sci. USA 99 (2002) 4883–4888; (b) Z.R. Bell, L.P. Harding, M.D. Ward, Chem. Commun. (2003) 2432–2433; (c) R.L. Paul, S.P. Argent, J.C. Jeffery, L.P. Harding, J.M. Lynam, M.D. Ward, Dalton Trans. (2004) 3453–3458; (d) S.P. Argent, H. Adams, T. Riis-Johannessen, J.C. Jeffery, L.P. Harding, W. Clegg, R.W. Harrington, M.D. Ward, Dalton Trans. (2006) 4996–5013; (e) W. Cullen, C.A. Hunter, M.D. Ward, Inorg. Chem. 54 (2015) 2626–2637; (f) A.J. Metherell, M.D. Ward, Chem. Sci. 7 (2016) 910–915. [4] R. Chakrabarty, P.S. Mukherjee, P.J. Stang, Chem. Rev. 111 (2011) 6810–6918. [5] H. Amouri, C. Desmarets, J. Moussa, Chem. Rev. 112 (2012) 2015–2041. [6] (a) J.S. Fleming, K.L.V. Mann, S.M. Couchman, J.C. Jeffery, J.A. McCleverty, M.D. Ward, J. Chem. Soc., Dalton Trans. (1998) 2047–2052; (b) K.L.V. Mann, J.C. Jeffery, J.A. McCleverty, M.D. Ward, J. Chem. Soc., Dalton Trans. (1998) 3029–3035; (c) A. Morsali, A. Ramazani, M. Babaee, F. Jamali, F. Gouranlou, H. Arjmandfar, A. Yanovsky, J. Coord. Chem. 56 (2003) 455–461; (d) X.-S. Shi, C.-S. Liu, J.-R. Li, Y. Guo, J.-N. Zhou, X.-H. Bu, J. Mol. Struct. 754 (2005) 71–76; (e) R.-Q. Zou, C.-S. Liu, Z. Huang, T.-L. Hu, X.-H. Bu, Cryst. Growth Des. 6 (2006) 99–108; (f) C.-S. Liu, J.-R. Li, R.-Q. Zou, J.-N. Zhou, X.-S. Shi, J.-J. Wang, X.-H. Bu, J. Mol. Struct. 843 (2007) 66–77; (g) S.-Y. Chang, J.-L. Chen, Y. Chi, Y.-M. Cheng, G.-H. Lee, C.-M. Jiang, P.-T. Chou,

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