Low pH constructed Co(II) and Ni(II) 1D coordination polymers based on C_α-substituted analogues of zoledronic acid: structural characterization, and spectroscopic and magnetic properties

Abstract

Three novel coordination compounds based on α,α-disubstituted analogues of zoledronic acid with a cyclopropane (cpp) or cyclobutane (cbt) ring on the C_α carbon, isomorphous [Co(H₂cppZol)(H₂O)]·H₂O (1a), [Ni(H₂cppZol)(H₂O)]·H₂O (1b) and [Co(H₂cbtZol)(H₂O)]·H₂O (2a), were synthesized under hydrothermal conditions at low pH. Single-crystal X-ray diffraction analysis revealed that all the compounds had a 1D double zig–zag chain architecture with an 8 + 8 ring motif formed by alternately arranged symmetrical (–O–P–O–)₂ bridges linking equivalent octahedral metal centres. Both the ligand coordination mode and chain architecture displayed by 1a, 1b and 2a are unique among 1D [M(H₂L)(H₂O)_x]·yH₂O coordination polymers based on nitrogen-containing bisphosphonates reported so far. All the compounds exhibit similar decomposition pathways upon heating with thermal stabilities decreasing in the order 1b > 1a > 2a. The IR spectra revealed that lattice water release above 227, 178 and 97 °C, respectively, does not change the chain architecture leaving them intact up to ca. 320, 280 and 240 °C. Magnetic behaviour investigations indicated that 1a, 2a and 1b exhibit weak alternating antiferromagnetic–ferromagnetic exchange interactions propagated between the magnetic centres through double (–O–P–O–)₂ bridges. The boundary between antiferro- and ferromagnetic couplings for the Co–O⋯O–Co angle in 1a and 2a was estimated to be ca. 80°. This value is also applicable for recently reported [M₃(HL)₂(H₂O)₆]·6H₂O (M = Co, Ni) complexes based on α,α-disubstituted analogues of zoledronic acid and can be used to the explain magnetic behaviour of 1b.

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1. Introduction

Organophosphonic acids are a class of ligands widely known for their excellent affinity for metal ions. The propensity to coordinate and bridge various metal centres resulting from multiple O-donor atoms occurring in tetrahedral phosphonic/phosphonate group(s) allows them to form coordination compounds with diverse supramolecular architectures¹ and promising applications in the field of catalysis,² gas storage,³ biotechnology,⁴ photoluminescence⁵ or magnetism.⁶ Therefore, metal-organophosphonate solids belong to an important and extensively developed class of functional materials.

Among a wide range of organophosphonate ligands, special attention has been concentrated on the family of H₂O₃P–C(R₁)(R₂)–PO₃H₂ bisphosphonic acids (BPs), which due to presence of geminal phosphonic groups attached to the carbon atom can capture a large number of metal ions simultaneously. Moreover, using various synthesis conditions and/or introducing donor-rich functional groups in R₁ and/or R₂ positions allows them to control both nuclearity and dimensionality of metal-bisphosphonate materials.⁷ This can be primarily realized by controlling the pH-dependent protonation state of phosphonate and other functional groups of ligand. In addition, other factors such as the nature and geometry of selected metal centre, steric strain imposed by R₁/R₂ substituents as well as an introduction of auxiliary ligands have significant impact on the architecture of desired compounds.⁸ To achieve pure phase materials, hydrothermal technique is commonly used in the synthesis of BPs-based coordination compounds. Therefore, careful determination of appropriate reaction conditions such as: metal-to-ligand stoichiometric ratio, temperature or reaction time also play a significant role.

To explore and expand the area of metal-BPs coordination compounds we have focused our research on new derivatives of zoledronic acid, H₄Zol (Scheme 1). This compound is a member of N-containing bisphosphonates family (NBPs), currently considered to be the most potent agent in the treatment of bone diseases⁹ and successfully exploited as multidentate ligand suitable for a synthesis of various metal complexes.¹⁰ With the aim to determine how structural modifications of H₄Zol imply topologies and architectures of coordination compounds formed with d-block metal ions, we synthesized a series of its derivatives with sterically demanding substituents on C_α atom, such as methyl groups (H₄dmtZol), a cyclopropane (H₄cppZol), cyclobutane (H₄cbtZol) or cyclopentane (H₄cptZol) ring.¹¹ All the ligands were structurally characterized^12,13 and examined for their ability to interact with Co(II) and Ni(II) under different pH conditions. Our recent studies have demonstrated that at pH ca. 4.0–5.0, both cations form one-dimensional coordination polymers with a general formula M₃(HL)₂(H₂O)₆·6H₂O (M = Co(II), Ni(II), HL = HdmtZol³⁻, M = Co(II), HL = HcppZol³⁻, HcbtZol³⁻)^12–14 or M₃(HL)₂(H₂O)₄·2H₂O (M = Co(II), HL = HcptZol³⁻),¹³ featuring alternately arranged [M2₂(HL)(H₂O)₂]₂ entities and {M1O₆} octahedrons with crystallographically and spectroscopically distinct M2 and M1 centres. In contrast, less sterically hindered HZol³⁻ ligand tends to form under similar conditions 2D and 3D coordination networks.¹⁵

Scheme 1 Zoledronic acid and its α,α-disubstituted analogues.

In this article we show that carrying the reaction at lower pH affords diverse systems architecture. By reacting Co(II) or Ni(II) chlorides with H₄cppZol (1) or H₄cbtZol (2) (Scheme 1) under hydrothermal conditions at 2.5–3.5 pH range we synthesized a series of 1D coordination polymers: isomorphous [Co(H₂cppZol)(H₂O)]·H₂O (1a), [Ni(H₂cppZol)(H₂O)]·H₂O (1b) and [Co(H₂cbtZol)(H₂O)]·H₂O (2a).

The search in the Cambridge Structural Database (CSD)¹⁶ has revealed almost twenty crystal structures of similar 1D [M(H₂L)(H₂O)_x]·yH₂O (x = 0–1, y = 0–3) coordination polymers based on various NBPs. (Table S1, ESI†). Among them, d-block metal complexes of the type [M(H₂L)]·xH₂O (x = 0–3; M = Cd(II), Zn(II), Mn(II), Co(II), Cu(II), H₂L²⁻ = 2-(4-pyridinyl)-1-hydroxyl-1,1-ethylidenediphosphonate;¹⁷ M = Mn(II), Cd(II), H₂L²⁻ = risedronate;¹⁸ M = Mn(II), Cu(II), H₂L²⁻ = pamidronate;¹⁹ M = Mn(II), H₂L²⁻ = alendronate,²⁰ zoledronate²¹) and [M(H₂L)(H₂O)] (M = Fe(II), Co(II), Mn(II), H₂L²⁻ = risedronate²²) have been found to reveal the ladder chain architecture. Commonly, in this type of structures, phosphonate μ₂-O bridges link two equivalent metal centres into a four-membered (MO)₂ dinuclear unit, whereas phosphonate O–P–O bridges connect adjacent dinuclear units to form eight-membered (M–O–P–O)₂ rings resulting in double 1D infinite chain. It is to note, that formation of such 4 + 8 rings motif along the coordination chain is favoured regardless of varying geometries and nature of metal centres^17–22 as well as subtle differences in ligands coordination modes (Fig. S1a–d, ESI†).²² The double zig–zag chain architecture is characteristic for Ca(II) complexes. The four-membered ring motif generated by μ₂-O bridges between metal centres is propagated along double chains displayed by seven-coordinate [Ca(H₂Pam)(H₂O)]·H₂O and [Ca(H₂Zol)(H₂O)] (Fig. S2a, ESI†)²³. On the other hand, six-coordinate [Ca(H₂Ale)]·H₂O complex represents the structure, in which a combination of phosphonate O–P–O and μ₂-O bridges between metal centres leads to 6 + 6 rings motif propagated along the coordination chain (Fig. S2b, ESI†).²⁴

Herein, we report the structures of 1a, 1b and 2a determined by single-crystal X-ray diffraction and their detailed characterization with the use of TG-DTA, DSC, FT-IR, NIR-UV/Vis techniques and magnetic susceptibility measurements. Interestingly, both ligands coordination mode and chains architecture of the reported compounds are unique among NBPs-based [M(H₂L)(H₂O)_x]·yH₂O coordination polymers reported so far.

2. Experimental

2.1 Materials

All commercially available reagents and solvents were used as received without further purification. 1-Hydroxy-2-[1-(1H-imidazol-1-yl)cyclopropyl]ethylidene-1,1-diphosphonic acid (H₄cppZol) and 1-hydroxy-2-[1-(1H-imidazol-1-yl)cyclobutyl] ethylidene-1,1-diphosphonic acid (H₄cbtZol) were synthesized according to previously described procedure.¹¹ Details of synthesis and purity confirmation of both ligands are reported in ref. 12 and 13.

2.2 Synthesis of Co(H₂cppZol)(H₂O)·H₂O (1a)

H₄cppZol (0.10 mmol, 0.0298 g) and CoCl₂·6H₂O (0.05 mmol, 0.0119 g) were dissolved in water (5 ml). Then, the pH of the reaction mixture was adjusted to 2.5 by 1 M NaOH. The obtained clear and pink solution was sealed in a glass tube and heated at 100 °C for 72 h. After the reaction system had been cooled slowly to room temperature, pink crystals of 1a were isolated, washed with distilled water and dried at room temperature. Crystals were collected as a monophasic material based on the powder XRD (Fig. S3, ESI†). Yield 16.4 mg (41.6% based on H₄cppZol). Elemental anal. found (calc.) for C₇H₁₄CoN₂O₉P₂ (%): C, 21.70 (21.49); H, 3.31 (3.61); N, 7.09 (7.16). IR (ATR, cm⁻¹): 3605 br & w, 3356 br & w, 3145 w, 3043 br & w, 2936 br & w, 1659 w, 1620 w, 1582 w, 1534 w, 1461 w, 1424 w, 1398 w, 1348 w, 1216 w, 1196 w, 1172 w, 1161 w, 1120 m, 1071 s, 1048 s, 1020 s, 1001 s, 977 m, 946 s, 901 m, 888 s, 874 s, 845 s, 804 m, 791 m, 770 m, 700 m, 655 s, 631 m, 614 s, 560 s, 544 s, 528 s, 488 s, 465 s, 449 s, 410 s.

2.3 Synthesis of Ni(H₂cppZol)(H₂O)·H₂O (1b)

H₄cppZol (0.10 mmol, 0.0298 g) and NiCl₂·6H₂O (0.07 mmol, 0.0167 g) were dissolved in water (5 ml). Then, the pH of the reaction mixture was adjusted to 2.9 by 1 M NaOH. The obtained clear and green solution was sealed in a glass tube and heated at 100 °C for 72 h. After the reaction system had been cooled slowly to room temperature, green crystals of 1b were isolated, washed with distilled water and dried at room temperature. Crystals were collected as a monophasic material based on the powder XRD (Fig. S4, ESI†). Yield 11.6 mg (29.5% based on H₄cppZol). Elemental anal. found (calc.) for C₇H₁₄NiN₂O₉P₂ (%): C, 21.66 (21.51); H, 3.40 (3.62); N, 7.01 (7.17). IR (ATR, cm⁻¹): 3606 br & w, 3350 br & w, 3146 w, 3042 br & w, 2933 br & w, 1644 w, 1620 w, 1582 w, 1533 w, 1460 w, 1426 w, 1401 w, 1347 w, 1218 w, 1195 w, 1175 w, 1162 w, 1125 m, 1070 s, 1048 s, 1020 s, 1000 s, 945 s, 890 s, 849 s, 808 m, 793 m, 772 m, 701 m, 654 s, 631 m, 615 s, 546 s, 529 s, 488 s, 468 s, 450 s, 415 s.

2.4 Synthesis of Co(H₂cbtZol)(H₂O)·H₂O (2a)

H₄cbtZol (0.10 mmol, 0.0312 g) and CoCl₂·6H₂O (0.10 mmol, 0.0238 g) were dissolved in water (5 ml). Then, the pH of the reaction mixture was adjusted to 3.5 by 1 M NaOH. The obtained clear and pink solution was sealed in a glass tube and heated at 100 °C for 72 h. After the reaction system had been cooled slowly to room temperature, pink crystals of 2a were isolated, washed with distilled water and dried at room temperature. Crystals were collected as a monophasic material based on the powder XRD (Fig. S5, ESI†). Yield 21.3 mg (52.2% based on H₄cbtZol). Elemental anal. found (calc.) for C₈H₁₆CoN₂O₉P₂ (%): C, 23.74 (23.71); H, 3.61 (3.99); N, 6.80 (6.92). IR (ATR, cm⁻¹): 3600 br & w, 3526 br & w, 3360 br & w, 3176 w, 3151 w, 3119 w, 3057 br & w, 3033 br & w, 3001 br & w, 2966 br & w, 1653 w, 1609 w, 1584 w, 1534 w, 1480 w, 1465 w, 1443 w, 1386 w, 1354 w, 1319 w, 1276 w, 1230 w, 1203 w, 1179 m, 1128 w, 1089 s, 1063 s, 1038 s, 1013 s, 986 s, 948 s, 872 s, 837 s, 786 m, 771 m, 741 m, 655 s, 629 m, 604 s, 537 s, 511 s, 467 s, 439 s.

2.5 X-ray crystallography

Crystallographic measurements for 1a, 1b and 2a were performed on κ-geometry diffractometer Agilent Technologies Xcalibur R with a Ruby CCD camera and with graphite monochromatized MoKα radiation at 110, 100 and 80 K, respectively, using an Oxford Cryosystems cooler. Data collection, cell refinement, data reduction and analysis as well as analytical absorption correction were carried out with CrysAlisPro.²⁵ The crystal structures of 1b and 2a were solved with direct methods using SHELXS-97 (ref. 26) and refined on F² by a full-matrix least squares technique with SHELXL-2014 (ref. 27) with anisotropic thermal parameters for all the ordered non-H atoms. As complex 1a is isomorphous with 1b, the refinement was started by using the coordinates of heavy (non-H) atoms taken from 1b and further refined as described above.

H atoms in 1a, 1b and 2a were found in difference Fourier maps and were refined isotropically. In the final refinement cycles, all C-bound H atoms, as well as N-bound H atoms in 1a and 1b, were repositioned in their calculated positions and refined using a rigid model, with C–H = 0.95–0.99 Å, N–H = 0.88 Å, and with U_iso(H) = 1.2U_eq(C, N). The H atoms of hydroxyl group in 1a, 1b and 2a, phosphonate group in 2a and all attached to water molecules were refined with the O–H bond lengths restrained to 0.840(2) Å and with U_iso(H) = 1.5U_eq(O). Then the AFIX 147 and AFIX 3 riding models were used to constrain phosphonate H atoms in 1a, 1b and 2a. Additionally, the H⋯H distances in the water molecules were restrained to 1.360(2) or 1.380(2) Å in 1a, 1b or 2a, respectively. N-bound H atom in 2a was refined with the N–H distance restrained to 0.880(2) Å and with U_iso(H) = 1.2U_eq(N). Then, it was constrained to ride on its parent N atom (AFIX 3 instruction).

Crystallographic data and structure refinement parameters are summarized in Table 1 and the crystallographic information files (CIFs) deposited at the Cambridge Crystallographic Data Centre (CCDC no. 1940766–1940768) and provided as ESI.† All figures were made using the DIAMOND program.²⁸

Table 1 Crystal data and structure refinement for compounds 1a, 1b and 2a

Compound	1a	1b	2a
CCDC no.	1940766	1940767	1940768
Chemical formula	C7H14CoN2O9P2	C7H14N2NiO9P2	C8H16CoN2O9P2
M r	391.07	390.85	405.10
Crystal system	Triclinic	Triclinic	Monoclinic
Space group	P1̄	P1̄	C2/c
a (Å)	7.848(3)	7.816(3)	13.666(4)
b (Å)	7.886(3)	7.849(3)	8.447(2)
c (Å)	10.880(5)	10.849(4)	22.247(5)
α (°)	90.97(3)	91.03(3)
β (°)	93.60(3)	93.26(3)	94.45(3)
γ (°)	114.95(3)	114.92(3)
V (Å^3)	608.6(5)	602.0(4)	2560.4(11)
T (K)	110	100	80
Z	2	2	8
D c (g cm^−3)	2.134	2.156	2.102
μ (mm^−1)	1.73	1.93	1.65
Crystal size (mm)	0.12 × 0.04 × 0.04	0.12 × 0.01 × 0.01	0.27 × 0.06 × 0.02
Diffractometer	Agilent Technologies, Xcalibur R, Ruby	Agilent Technologies, Xcalibur R, Ruby	Agilent Technologies, Xcalibur R, Ruby
Absorption correction	Analytical	Analytical	Analytical
T min, Tmax	0.855, 0.941	0.873, 0.973	0.796, 0.969
Radiation type	Mo Kα	Mo Kα	Mo Kα
Wavelength (Å)	0.71073	0.71073	0.71073
F(000)	398	400	1656
Refl. measured	16 719	5948	9127
Refl. independent	5098	3636	3029
Refl. observed [I > 2σ(I)]	3840	1815	2510
No. of parameters	192	192	200
No. of restrains	7	133	9
R int	0.038	0.062	0.027
(sin θ/λ)max (Å^−1)	0.679	0.605	0.680
R[F^2> 2σ(F^2)]	0.033	0.042	0.032
wR(F^2)	0.064	0.060	0.075
S	0.93	0.70	1.04
Δρmax, Δρmin (e Å^−3)	0.51, −0.59	0.84, −0.75	0.49, −0.57

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2.6 Powder X-ray diffraction analysis

PXRD data were collected on a Bruker D8-Advance diffractometer equipped with a VÅNTEC-1 detector (λ_CuKα = 1.5418 Å). The equipment was operated at 30 kV and 40 mA, and data were collected at room temperature in the range of 2θ = 5–50°.

2.7 ATR FT-IR measurements

The ATR FT-IR (ATR is attenuated total reflectance) spectra were recorded on a Bruker Vertex 70v Fourier transform IR spectrometer equipped with a diamond ATR cell. The spectral data were collected at room temperature over the range 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹. Instrument control and initial data processing were performed using OPUS software (v. 7.0 Bruker Optics, Ettlingen, Germany).

2.8 Thermal analysis

Thermogravimetric analysis (TG-DTA) was carried out using a Setaram SETSYS 16/18 analyzer, operating under a nitrogen atmosphere, with a heating rate of 5 K min⁻¹ in the range 303–1273 K (sample mass of 10.3, 8.6 and 11.0 mg for 1a, 1b and 2a, respectively). Differential Scanning Calorimetry (DSC) measurements were performed on a Setaram DSC 92 instrument. Samples (3.2–7.0 mg) were contained in alumina pans in the presence of air as the furnace atmosphere. Measurements were performed from ambient temperature up to 219, 314, 387, 457 and 600 °C for 1a, 284, 352, 529 and 600 °C for 1b and 175, 329, 525 and 600 °C for 2a with a heating rate of 5 K min⁻¹.

2.9 NIR-UV/Vis-spectroscopic studies

The NIR-UV/Vis diffuse-reflectance electronic spectra of 1a, 1b, 2a and ligands 1 and 2 were measured on a Cary 500 Scan spectrophotometer over the range 5000–50 000 cm⁻¹ with measure step of 10 cm⁻¹ at room temperature. The spectra were recorded with identical parameters as a white reference sample. To obtain the band positions of components of d–d bands in the spectra of cobalt(II) (1a, 2a) and nickel(II) (1b) complexes the variable digital method²⁹ was used with the following parameters: step = 20 cm⁻¹, α = 400 and N = 40 (1a); step = 20 cm⁻¹, α = 400 and N = 30 (2a); and step = 20 cm⁻¹, α = 500, N = 40 (1b).

2.10 Magnetic measurements

Magnetic susceptibility in the temperature range from 1.7 to 300 K in a field of 100 mT (1a and 2a) or 500 mT (1b) and magnetization up to 5 T were measured with a Quantum Design SQUID magnetometer. Diamagnetic corrections (−202 × 10⁻⁶, −212 × 10⁻⁶ and −202 × 10⁻⁶ emu mol⁻¹ for 1a, 2a and 1b, respectively) were calculated using Pascal's constants. The exchange Hamiltonian in the form H = −JS₁S₂ was used consistently throughout the paper. All values of magnetization and magnetic susceptibility are reported per mole of metal.

3. Results and discussion

3.1 Syntheses of 1a, 1b and 2a

Compounds 1a, 1b and 2a were synthesized through hydrothermal reaction at 100 °C for 72 h. However, despite their similarity, obtaining pure phase, the best yield and the highest crystals quality required each reaction to proceed under specific conditions.

Compounds 1a and 1b were synthesized with the use of H₄cppZol and CoCl₂·6H₂O or NiCl₂·6H₂O salts. The highest yields and qualities of crystalline products were achieved in a narrow pH range 2.4–2.6 for 1a and 2.8–3.0 for 1b. The decrease in pH below the optimum value resulted in reduction of yield without product deterioration. The increase of pH up to ca. 3.5 lead to increase of the reaction yield accompanied by significant deterioration of 1a and 1b while in the pH range 3.5–4.0 minor amount of previously reported Co₃(HcppZol)₂(H₂O)₆·6H₂O¹² or unrecognized green amorphous solid were observed, respectively. Regardless of the reaction pH, the excess of ligand was desirable. It was found that optimum H₄cppZol : Co(II)/Ni(II) molar ratios to obtain pure phases of 1a and 1b are 2–1.5 : 1 and 1.5 : 1, respectively. The use of stoichiometric amount of reagents or an excess of metal salt resulted in deterioration of crystals quality and reduction of reaction yield even if the most appropriate pH was reached.

The most favourable pH to obtain optimal crystallinity and reaction yield of 2a is ca. 3.0–4.0. Additionally, both stoichiometric amount of reagents or an excess of ligand (H₄cbtZol : Co(II) molar ratio 1.5–1 : 1) can be used. At pH below 3.0, the decrease of yield without negative impact on product quality was observed while a mixture of 2a and unidentified, pink, amorphous impurities were obtained above pH 4.0.

The most appropriate for the synthesis of 1a, 1b and 2a are Co(II) and Ni(II) chloride salts. Additionally, 2a can be obtained when cobalt(II) chloride is replaced by sulphate salt. Attempts to prepare 1a, 1b and 2a by using Co(II)/Ni(II) acetates were unsuccessful.

3.2 Crystal structures of 1a, 1b and 2a

The isomorphous compounds 1a, 1b crystallize in triclinic P1̄, and 2a in monoclinic C2/c space group as one-dimensional coordination polymers. Table 1 summarizes all crystal data and structure refinement parameters. Selected geometrical parameters and proposed hydrogen bonds are collected in Tables S2–S4 (ESI†).

The asymmetric units of 1a, 1b and 2a consist of one M1 centre (M1 = Co(II) in 1a and 2a or Ni(II) in 1b), one coordinated H₂L²⁻ ligand (H₂L²⁻ = H₂cppZol²⁻ in 1a and 1b or H₂cbtZol²⁻ in 2a), one coordinated and one lattice water molecule. In all the compounds, the central atom adopts slightly distorted octahedral geometry with equatorial sites occupied by O2, O7 O4^i/iii and O1W atoms and axial sites filled by O6, and O1^ii/iv atoms (Fig. 1 and S6, ESI†). The average equatorial and axial M1–O distances calculated based on data from Table S3 (ESI†), of 2.118 and 2.059 Å in 1a, 2.097 and 2.048 Å in 1b and 2.152 and 2.054 Å in 2a, result in tetragonality distortion T index defined as T = R_int/R_out (R_int and R_out are the average in-plane and out-of-plane M–O (M = Co, Ni) distances³⁰) of 1.03, 1.02 and 1.05, respectively, thus indicating slight compression of octahedrons along the axial O2–M1–O4^i/iii bond.

Fig. 1 The building unit and the atom-numbering scheme of (a) the representative compound 1a and (b) 2a. Lattice water molecules are omitted for clarity. Displacement ellipsoids for non-H atoms (except Co1) are drawn at 80% probability level. (c) Coordination environments of Co1 centre in 1a and 2a. Symmetry codes are given in Table S3 (ESI†).

In all 1a, 1b and 2a, coordinated ligands contain protonated imidazole N atom, one fully deprotonated and one monoprotonated phosphonate group yielding the overall charge −2. The values of N1–C2–C1–O7 dihedral angles of 165.1(2)°, 165.3(4)° and −172.7(2)°, respectively, (Table S2, ESI†) clearly demonstrate the same ap conformation of coordinated H₂cppZol²⁻/H₂cbtZol²⁻ ligands, recently demonstrated to be rare among NBP's and their anions within coordination compounds and salts.³¹ In such arrangement, each ligand exhibits a preference for chelating-bridging mode donating five oxygen atoms to coordinate three M1 (M1 = Co(II)/Ni(II)) ions. The phosphonate O2, O6 and hydroxyl O7 atoms coordinate one M1 centre in a tridentate fashion whereas phosphonate O1 and O4 are involved in monodentate binding of two symmetry related M1^ii/iv and M1^i/iii. The O5 and protonated O3 atoms remain uncoordinated (Fig. S7, ESI†). As a consequence, the pair of M1 and M1^i/iii ions is connected through O4–P2–O6 phosphonate bridges from two equivalent ligands to form dinuclear [M1(H₂L)(H₂O)]₂ unit with puckered, eight-membered (M1–O–P–O)₂ ring and M1⋯M1^i/iii distance within the ring of 4.850(3) Å in 1a, 4.797(3) Å in 1b and 4.822(2) Å in 2a. The O1–P1–O2 bridges extend dinuclear units into infinite zig–zag double chain running along [110] crystallographic direction in 1a and 1b or along b axis in 2a. As a result another eight-membered ring with M1⋯M1^ii/iv distance of 4.841(3) Å in 1a and 1b and 4.859(2) Å in 2a is formed (Fig. S8, ESI†). The O7–H7⋯O6^i/iii intermolecular hydrogen bond provides additional stabilization of coordination chain (Fig. 2a, S9a and S10a, ESI†). The chains are extended into 2D hydrogen bonded layer by means of N2–H2N⋯O5^vii/x interaction (Fig. 2b, S9b and S10b, ESI†) and further connected into 3D supramolecular network through O1W–H1W⋯O6^ix/xii and O1W–H2W⋯O2^ix/xii hydrogen bonds between coordinated water molecule and phosphonate oxygen atoms with formation of R₂²(6) ring motif. Additional stabilization of the supramolecular architecture in 1a and 1b is provided by O2W–H3W⋯O5^ix and O2W–H4W⋯O1ⁱⁱ interactions involving O2W lattice water molecules as hydrogen donors (Fig. 2c, S9c and S10c, ESI†).

Fig. 2 The packing diagram of the representative 1a. (a) 1D polymeric chain running along [110] crystallographic direction; (b) chains organized into 2D layer by N–H⋯O hydrogen bonds are shown as blue dashed lines; (c) 3D supramolecular network formed through connection of adjacent layers by O–H⋯O hydrogen bonds (orange dashed lines) between coordinated and lattice water molecules and phosphonate O-atoms. All H atoms not involved in the creation of hydrogen bonds and lattice water molecules (pictures a and b) are omitted for clarity. Symmetry codes are given in Table S4 (ESI†).

Both ligands binding mode and resulting 8 + 8 rings motif propagated along 1D coordination chains are unique among structures of 1D [M(H₂L)(H₂O)_x]·yH₂O (x = 0–1, y = 0–3) coordination polymers based on NBP ligands reported so far (Table S1, ESI†). Somewhat similar behaviour is only noted for the [Zn(H₂Ris)] complex.³² However, the fulfilment of both metal and ligand requirements to achieve optimal geometry and energetics leads to tetrahedral geometry maintained by Zn(II) and hydroxyl group of H₂Ris²⁻ remaining uncoordinated (Table S1, ESI†). As a result, eight-membered (Zn–O–P–O)₂ dinuclear units being common with 1a, 1b and 2a are extended through twelve-membered rings into polymeric ladder-type structure (Fig. S11, ESI†).

3.3 IR characterization of 1a, 1b and 2a

The FT-IR spectra of the compounds 1a, 1b and 2a are presented in Fig. S12–S15 (ESI†). The bands assignment (Table S5, ESI†) is based on detailed analysis of the IR spectra of H₄dmtZol, H₄cppZol, H₄cbtZol, H₄cptZol ligands supported by DFT calculations (ref. 31, Fig. S16, S17 and Table S6, ESI†) and tentative assignments for Co(II)/Ni(II) coordination polymers reported previously.^12–14

The spectra of 1a, 1b and 2a exhibit many common features manifested in location and intensity of characteristic bands assigned to ligand skeleton and functional groups. Additionally, the range 3600–3200 cm⁻¹ is dominated by broad features associated with ν(O–H), ν(PO–H) and ν(N–H) stretching vibrations, consistent with presence of extensive hydrogen bonding involving water molecules (coordinated and lattice), phosphonate and imidazole groups (Table S4, ESI†). A week sharp band at ca. 3150 cm⁻¹ is contributed by ν(C–H) vibrations of imidazole ring. The symmetric and asymmetric C–H stretching vibrations of cyclopropane/cyclobutane CH₂ groups give rise to a set of weak bands in the range 3050–2950 cm⁻¹. The band at ca. 1650 cm⁻¹ corresponds to H–O–H bending vibrations of coordinated and lattice water molecules. The stretching vibrations of imidazole C=C, N=C and N–C bonds contribute weak bands observed at ca. 1580 and 1530 cm⁻¹. The region gathering most of the bending vibrations attributed to imidazole, cyclopropane/cyclobutane rings as well as hydroxyl C–O–H is located between ca. 1500 and 1150 cm⁻¹. Herein, noticeable differences in the spectra are associated with specific deformation vibrations of C_α substituents.

The phosphonate groups stretching and bending vibrations absorb mainly below 1350 cm⁻¹. Their detailed analysis supported by DFT calculations (see ref. 31 and Table S6, ESI†) have indicated that ν(P–O) vibrations are usually strongly mixed with δ(P–O–H) and stretching/deformation vibrations of imidazole ring and aliphatic substituents, thus making unambiguous interpretation of experimental IR spectra difficult. The bands near 1350 cm⁻¹ are attributed to δ(P–O–H) vibrations. The ν(P–O) modes of P2 pendant O atom give rise to bands at ca. 1120 and 1030 cm⁻¹. The bands at ca. 945 and 840 cm⁻¹ can be considered as contributed by ν(P–O) vibrations involving Co(II)/Ni(II)-coordinated phosphonate O atoms. The ν(P–OH) vibrations of P1 group contribute bands at lower 790–770 cm⁻¹ wavenumbers. The vibrations contributed mostly by stretching of the C–P bond also appear below 800 cm⁻¹. The δ(O–P–O) deformation vibrations of phosphonate groups are observed around 500 cm⁻¹.

3.4 Thermal stabilities of 1a, 1b and 2a

Thermal decomposition of 1a, 1b and 2a was investigated on crystalline samples by means of TG-DTA and DSC experiments conducted from room temperature up to 1000/600 °C under nitrogen and air atmosphere, respectively. Fig. 3 compiles TG curves in the temperature range 30–600 °C. Full set of experimental data is collected in Fig. S18–S23 (ESI†).

Fig. 3 TG curves recorded for 1a, 1b and 2a.

The studied compounds exhibit similar decomposition pathways with thermal stabilities decreasing in the order 1b > 1a > 2a. The first step, taking place in the temperature ranges 236–271 °C (1b), 190–218 °C (1a) and 121–169 °C (2a), corresponds to release of lattice water molecule accompanied by weight loss of 4.62, 4.54, 5.18% (calcd 4.60, 4.61, 4.45%) with dehydration enthalpies of 9.98, 9.12, and 30.7 kJ mol⁻¹, respectively. The IR spectra of samples heated up to temperatures at which this process is completed clearly indicate that removal of lattice water has no negative impact on polymeric chains architecture (Fig. S24–S26, ESI†). On further heating up to 600 °C, compounds 1a and 2a behave similarly losing a total 24.4 and 27.9% weight in three distinct exothermic stages with maximum rates of reaction (T_max) of 303, 367, 430 (1a) and 257, 436, 508 °C (2a). For 1b, only two exothermic processes accompanied by a total weight loss of 15.5% with T_max of 340 and 470 °C occur. These processes correspond to collapse of coordination chains by breaking M–O bonds, initiated by release of coordinated water molecule, then followed by elimination of C_α-substituents and subsequent release of imidazole rings (Fig. S27–S29, ESI†). However, the IR spectrum shows that in 1b the last process is not completed even at 600 °C (Fig. S30, ESI†).

The last stage of thermal decomposition starts above ca. 700 °C and is characterized by intense 31.5% (1a), 37.7% (1b) and 30.3% (2a) weight losses that can be attributed to combustion of organic parts of ligands. The resultant decomposition products constitute 39.5, 41.1 and 36.7% of initial samples masses and contain likely Co₂(P₂O₇) (1a, 2a) or Ni₂(P₂O₇) (1b) (Fig. S31, ESI†). It is to note, that in contrast to previously analysed Co₃(HL)₂(H₂O)₆·6H₂O^12,13 and Co₃(HL)₂(H₂O)₄·2H₂O,¹³ the products of 1a and 2a thermolysis are almost free from other residues (calcd 37.3, 36.03%). A support for such conclusion is provided by IR spectra revealing the absence of a weak band at ca. 1300 cm⁻¹ (Fig. S32 and S33, ESI†).

3.5 NIR-UV/Vis spectroscopic characterization of 1a, 1b and 2a

In all 1a, 1b and 2a the donor oxygen atoms form the chromophore (M = Co(II)/Ni(II)) of pseudo-octahedral geometry compressed along the O4–M–O2 bond (Fig. 1 and S6, ESI†). It is to note, that the magnitude of compression increases with the increase of C_α substituent size (T = 1.03, 1.02 for 1a and 1b, T = 1.05 for 2a).

In the solid-state diffuse-reflectance electronic spectra of 1a and 2a (Fig. 4), the absorption in the 5000 cm⁻¹ to 30 000 cm⁻¹ spectral range is attributed to the d–d transitions being characteristic for high-spin Co(II) ion (d⁷ configuration). The spectra are almost identical with a lot of slightly marked maxima of the broad absorption in the 18 000–22 000 cm⁻¹ range, thus confirming that crystal-field symmetry is lower than O_h. Consistent with the increase of tetragonal compression of the octahedron (2a > 1a), the maxima are better resolved in the spectrum of 2a.

Fig. 4 The NIR-Vis-UV diffuse-reflectance electronic spectra of 1a, 2a and H₄cppZol, H₄cbtZol ligands (Scheme 1) obtained at room temperature.

In the compressed tetragonal crystal field of D₄ symmetry, degenerate ⁴T_1g(⁴F, O_h) level splits into ⁴A₂ + ⁴E components where the ⁴A₂ is the ground state.^33–35 The next quartet ⁴T_2g((⁴F), O_h) level consists of ⁴E + ⁴B₂ states, while the ⁴A_2g(⁴F, O_h) state is nondegenerate and transforms as ⁴B₁.^34,36 Likewise, the crystal field of D₄ symmetry causes splitting of the last quartet ⁴T_1g((⁴P), O_h) level into ⁴E and ⁴A₂ states. As the absorption in 13 500–25 000 cm⁻¹ spectral range is composed of multiple unresolved bands (Fig. 4), the spectra were deconvoluted with the use of variable digital filtering method²⁹ in order to refine band splitting and obtain energies of its components (Fig. S34, ESI†). The obtained pairs of bands with maxima at 6830 cm⁻¹, 8250 cm⁻¹ (1a) and 6950 cm⁻¹, 8050 cm⁻¹ (2a) are generated by spin-allowed quartet–quartet ⁴A₂ → 4E and ⁴A₂ → ⁴B₂ transitions (⁴E and ⁴B₂ states from ⁴T_2g(⁴F) in O_h).³⁷ However, the energy values 6830 cm⁻¹ (1a) and 6950 cm⁻¹ (2a) obtained for the ⁴A₂ → ⁴E transition are quite ambiguous because of presence of strong ligand bands in the region up to 7000 cm⁻¹. The spin and symmetry forbidden transitions become possible by splitting of the ²E_g(²G, O_h) state into ²A₁ + ²B₁. Under filtration process the pairs of bands at 10 210 cm⁻¹/12 510 cm⁻¹ (1a) and 10 330 cm⁻¹/12 430 cm⁻¹ (2a) clearly appeared and can be assigned to the ⁴A₂ → ²A₁ and ⁴A₂ → ²B₁ transitions, respectively. The well seen bands found at 14 010 cm⁻¹ (1a) and 13 890 cm⁻¹ (2a) originate from the spin-allowed quartet–quartet ⁴A₂ → ⁴B₁ transition and are less intense as compared to other spin-allowed transitions. This assignment is drawn based on Wildner research explaining appearance of bands at 13 350 cm⁻¹ and 15 200 cm⁻¹ in the polarized (β polarization) electronic spectra of tetragonally distorted Co²⁺ octahedrons in NaCo₂(SeO₃)₂OH³⁷ and kieserite-type CoSO₄·H₂O and CoSeO₄·H₂O salts.³⁸ The same approach has also been applied to explain the origin of similar bands in the electronic spectrum of [Co₃(HcptZol)₂(H₂O)₄]·2H₂O.¹³

From the ²G term, the ²T_1g(²G) and ²T_2g(²G) states split into pairs of ²A₂ + ²E and ²B₂ + ²E levels. The energies of these transitions are rather close to energy of quartet ⁴B₁ state, although they are situated between the bands correlated with quartet–quartet ⁴A₂ → ⁴B₁(⁴F) and ⁴A₂ → ⁴E(⁴P) transitions. In the spectrum of 1a, one band arises at 16 930 cm⁻¹, while for 2a analogous band is splitting into two components with positions at 16 330 cm⁻¹ and 17 170 cm⁻¹ (Table 2, Fig. 4 and S34, ESI†). The successive, well seen bands with maxima at 18850cm⁻¹ (1a) and 18 650 cm⁻¹ (2a) are correlated with spin-allowed transition to ⁴E state being one of the two components of parental ⁴T_1g((⁴P), O_h) level. The second component, ⁴A₂(⁴T_1g, ⁴P) state, is expected higher. The energies of ⁴A₂ → ⁴E and ⁴A₂ → ⁴A₂(⁴P) transitions correspond to 18 850 cm⁻¹/21 650 cm⁻¹ for 1a and 18 650 cm⁻¹/21 490 cm⁻¹ for 2a (Table 2, Fig. S34, ESI†). Indeed, a detailed filtration analysis of 19 000–21 000 cm⁻¹ region has revealed the presence of extra bands between these pairs of bands, which can be correlated with the combined spin- and symmetry forbidden transitions. The ²T_1g(²P) state involves the ²A₂ and ²E levels, while the ²A_1g state from the ²G term is nondegenerate and keeps its symbol (²A₁). We tentatively assigned the energy of ⁴A₂ → ²A₂(²T_1g, ²P) transition as 20 310 cm⁻¹ for 1a and 20 350 cm⁻¹ for 2a. Neither corresponding band has been found in the spectra for the ⁴A₂ → ²A₁(²A_1g, ²G) or ⁴A₂ → ²E(²T_1g, ²P) transitions (Fig. S34, ESI†).

Table 2 Band assignments, experimental and theoretical energies (cm⁻¹) of components of spin-allowed and spin-forbidden d–d transitions for 1a, 1b and 2a^a

Cobalt(II) complexes (d^7 configuration)								Nickel(II) complex (d^8 configuration)
Assignment	1a				2a			Assignment	1b
O h	D 4	Spect.	Filter	Calcd	Spect.	Filter	Calcd	O h	D 4	Spect.	Filter	Calcd.
a Spect. – the energy found form the experimental spectra; filter – the energy found from the filtering process (filter parameters: (1a) step = 20 cm^−1, α = 400 and N = 40; (2a) step = 20 cm^−1, α = 400 and N = 30; (1b) step = 20 cm^−1, α = 300 and N = 30); calcd – calculated energy; − not observed; the spin-allowed transitions are marked in bold.
^4T1g(^4F)	^4A2	0	—	0	0	—	0	^3 A 2g ( ^3 F)	^3 B 1	—	—	0
	^4E		—	2001	—	—	1966
^4 T 2g ( ^4 F)	^4 E	6830	6830	6734	6950	6950	6629	^3 T 2g ( ^3 F)	^3 B 2	7000	6990	6590
	4B 2	8250	8250	8226	8050	8050	8190		^3 E(1)	8500	8370	8310
^2Eg(^2G)	2A1	—	10 210	11 309	—	10 330	11 275
Ligand		11 300	11 350	—	11 300	11 330	—	Ligand		11 230	11 430	—
	^2B1	—	12 510	12 444	—	12 430	12 266	^3 T 1g ( ^3 F)	^3 A 2	13 040	12 890	12 860
^4 A 2g ( ^4 F)	^4 B 1	14 200	14 010	14 059	14 020	13 890	13 908		^3 E(2)	13 040	12 890	12 955
^2T1g(^2G)	^2A2	—	—	15 539	—		15 419
^2T2g(^2G)	^2B2	16 950	16 930	16 852	16 780	16 330	16 623	^1Eg (^1D)	^1A1	14 660	14 770	14 470
^2T2g(^2G)	^2E	—	—	17 440	—	17 170	17 326		^1B1	14 660	14 770	14 570
^2T1g(^2G)	^2E	—	—	17 924	—		17 743	^1A1g(^1G)	1	20 600	20 490	20 860
^4 T 1g ( ^4 P)	^4 E	19 150	18 850	18 685	18 880	18 650	18 620	^1T2g(^1D)	^1B2	—	—	22 250
^2T1g(^2P)	^2A2	20 510	20 310	20 461	20 410	20 350	20 310		^1E	23 100	23 190	23 250
^2A1g(^2G)	^2A1	—	—	21 280			21 048
^4 T 1g ( ^4 P)	^4 A 2	21 310	21 650	21 741	21 340	21 490	21 520	^3 T 1g ( ^3 P)	^3 A 2	24 200	24 290	24 230
^2T1g(^2P)	^2E2	—	—	21 760	—	—	21 616		^3 E(3)	24 200	24 290	24 320
^2T2g(^2G)	^2E	—	—	24 189	—	—	24 562
^2T1g(^2G)	^2E	24 500	24 830	24 811	24 750	24 810	24 710	^1T1g(^1G)	^1A2	—	25 170	25 215
^2T2g(^2G)	^2B2	—	—	24 902	—	—	26 035		^1E	—	—	27 310
^2T1g(^2G)	^2A2	26 220	26 270	26 226	—	26 370	26 331

---

Further, in the region between 22 000 cm⁻¹ and the charge transfer absorption edge, a weak spectral feature at 30 000 cm⁻¹ is present (Fig. 4) that can only be attributed to the spin-forbidden transitions. An assignment of feature attributed to the components of remaining spin-forbidden ⁴A₂ → ²E + ²B₂(²T_2g,²G, O_h) and ⁴A₂ → ²E + ²A₂(²T_1g, ²G, O_h) transitions is ambiguous. Finally, weak bands observed at 24 830 cm⁻¹/26 270 cm⁻¹ in the spectrum of 1a and at 24 810 cm⁻¹/26 370 cm⁻¹ in the spectrum of 2a are caused by spin-forbidden transitions from the ground state to higher doublet ²T_1g(²G, O_h) state.

The diffuse-reflectance spectrum of 1b shows three main regions of absorption: 5800–10 000 cm⁻¹, 10 500–17 000 cm⁻¹ and 18 800–28 700 cm⁻¹, which are characteristic for the three spin-allowed: I-st (³A_2g → ³T_2g(³F, O_h)), II-nd (³A_2g → ³T_1g(³F, O_h)) and III-rd (³A_2g → ³T_1g(³P, O_h)) d–d transitions for nickel(II) ion of d⁸ configuration in a pseudooctahedral crystal field (Fig. 5). These broad bands are slightly asymmetric and exhibit pairs of maxima at ca. 7000 cm⁻¹ and 8500 cm⁻¹ (I-st d–d), 11 230 cm⁻¹ and 13 040 cm⁻¹ (II-nd d–d) as well as 20 600 cm⁻¹ and 24 200 cm⁻¹ (III-rd d–d). Moreover, the spectrum presents sharp feature at 14 660 cm⁻¹ as well as a shoulder on the low energetic branch of III-r d d–d at ca. 20 600 cm⁻¹.

Fig. 5 The NIR-Vis-UV diffuse-reflectance electronic spectra of 1b and related H₄cppZol ligand (Scheme 1) obtained at room temperature.

Generally, the bands corresponding to d–d transitions are red-shifted with respect to related bands in the spectrum of recently reported [Ni₃(HdmtZol)₂(H₂O)₆]·6H₂O.¹⁴ This indicates that energies of d–d transitions are lower. Indeed, smaller tetragonal compression around Ni(II) (T = 1.02) leads to noticeably lower asymmetry of d–d bands compared with those recorded for Ni(II) complex based on HdmtZol³⁻ ligand (T = 1.04).

The filtering process has revealed splitting of I-st and III-rd d–d bands into two components (Fig. S35, ESI†), which justifies an analysis based on the model assuming slightly compressed tetragonal D₄ symmetry with ³B₁ state as a ground term.^33,39 The tetragonal crystal filed causes splitting of each spin allowed d–d band as follows: ³T_2g(³F, O_h) state splits into ³B₂ and ³E(1), ³T_1g(³F, O_h) splits into ³E(2) + ³A₂ states and third state ³T_1g(³P, O_h) consists of ³E(3) and ³A₂ levels. According to the energy level diagram,^33,39 the spin and symmetry–forbidden transitions also become possible by splitting of the ¹E_g and ¹T_2g states (both from ¹D term, O_h) into ¹B₁ + ¹A₁ and ¹B₂ + ¹E levels, respectively. The ¹A_1g state from ¹G term is nondegenerate and stays the same (¹A₁), while successive ¹T_1g state from the ¹G term consists of ¹E and ¹A₂ levels.

The ³B₁ → ³B₂ and ³B₁ → ³E(1) transitions with energies found under filtration process to be ca. 6990 cm⁻¹ and 8370 cm⁻¹ are assigned as components of the I-st d–d band. These experimental energies stay in good agreement with the calculated ones (Table 2). The calculated energies obtained for ³B₁ → ³A₂ and ³B₁ → ³E(2) transitions (II-nd d–d) equal 12 860 and 12 955 cm⁻¹. However, due to a very small difference in energy values of ca. ∼100 cm⁻¹, even the filter does not revel splitting but only a shoulder effect. The sharp feature at 14 770 cm⁻¹ arises from ¹E_g in O_h and is composed of ¹A₁ + ¹B₁ in D₄. The splitting of this band corresponding to spin-forbidden transition is very difficult to observe in diffuse-reflectance spectrum, however, is confirmed by calculations giving the values of energies of 14 470 cm⁻¹ and 14 570 cm⁻¹ for ¹A₁ and ¹B₁ states, respectively. The experimental energy of subsequent spin-forbidden transition, ³B₁ → ¹A₁(¹A_1g, ¹G) at 20 490 cm⁻¹, corresponds well with calculated value of 20 860 cm⁻¹. The ³B₁ → ¹B₂ + ¹E transitions, being the components of ¹T_2g(¹D, O_h) state, are usually situated between the II-nd and III-rd d–d bands. A single band is found in filtering process at 23 190 cm⁻¹ with low energetic shoulder at 22 230 cm⁻¹, while calculated energies of these states are 22 250 cm⁻¹ and 23 250 cm⁻¹. The III-rd d–d band observed with maximum at 24 290 cm⁻¹ seems to remain unsplitted. Its position is confirmed by close values of calculated energies of 24 230 cm⁻¹ and 24 320 cm⁻¹ obtained for ³B₁ → ³A₂ and ³B₁ → ³E(3) transitions. Finally, the bands of higher energetic transitions of forbidden nature, namely, ³B₁ → ¹A₂(¹T_1g(G), O_h) and ³B₁ → ¹E(¹T_1g(G), O_h) are expected over 25 000 cm⁻¹. Their energies were calculated as 25 215 cm⁻¹ and 27 310 cm⁻¹, however, they are not observed in the spectrum even as an effect of filtration (Fig. S35, ESI†).

The observed and calculated energy levels alongside with corresponding assignments are collected in Table 2. The agreement between the observed and calculated values is relatively satisfying. For cobalt(II) compounds (1a and 2a), the tetragonal crystal field parameters D_q, D_t and D_s as well as the Racah parameters B and C were calculated using the energies of quartet–quartet transitions. The following sets were obtained: D_q = 583 cm⁻¹, D_t = −168 cm⁻¹, D_s = 568 cm⁻¹, B = 890 cm⁻¹, C/B = 4.12 (1a) and D_q = 572 cm⁻¹, D_t = −177 cm⁻¹, D_s = 537 cm⁻¹, B = 888 cm⁻¹, C/B = 4.08 (2a). The Dt is dependent on the splitting of the first quartet state of the octahedral ⁴T_2g(⁴F) and its negative sing calculated for both 1a and 2a is in agreement with the compressed geometry of {CoO₆} polyhedrons in NaCo₂(SeO₃)(OH)³⁷ and [Co₃(HcptZol)₂(H2O)₄] coordination unit.¹³ For nickel(II) complex 1b, the values of D_q, D_s, D_t and Racah parameter B were calculated using the energies of six-triplet states. The following values were obtained: D_q = 659 cm⁻¹ (exp. energy 699 cm⁻¹ for ³B₁ → ³B₂ transition, Table 2), D_s = 307 cm⁻¹, D_t = −196 cm⁻¹ and B = 932 cm⁻¹.

3.6 Magnetic properties of 1a, 1b and 2a

The magnetic susceptibilities of 1a, 2a, and 1b show Curie–Weiss behaviour above ca. 50 K with the Weiss constants θ = −12.4, −8.3, and −2.1 K, and magnetic moments of 5.32 (2.75), 5.23 (2.70), and 3.34 (2.30) μ_B, respectively. The expected values for g factors corresponding to the respective moments for free Co(II) (1a, 2a) or Ni(II) (1b) ions are given in brackets. Full results of magnetic measurements and calculations are presented in Fig. S36–S39 (ESI†). The magnetic behaviours of 1a and 2a are very similar, therefore Fig. 6 presents the temperature dependence of the effective magnetic moments (μ_eff) only for 1a and 1b. The magnetic susceptibilities of both compounds increase monotonically as the temperature is decreased up to 1.7 K. Upon cooling, the effective magnetic moment of Co(II) complex 1a slightly decreases from 300 to 100 K and then decreases faster, reaching a value of 4.1 μ_B at ∼2 K. An effective moment of the nickel complex 1b slowly decreases below room temperature, reaches a shallow minimum at 55 K, then passes through a pronounced maximum at 5 K and drops sharply below this temperature.

Fig. 6 Effective magnetic moment of 1a at B = 100 mT (triangles) and 1b at B = 500 mT (diamonds). The solid lines represent the theoretical fits (see the text).

The crystals structures of all the compounds comprise zig–zag chains, in which metal centres are alternately linked by double, symmetrical (–O1–P1–O2–)₂ and (–O4–P2–O6–)₂ bridges (Fig. S8†). Therefore, the magnetic susceptibility of 1a, 2a and 1b was analyzed in the framework of an alternating-exchange spin-chain.

Using the Hamiltonian:

H = –J₁∑S_2iS_2i+1 − J₂∑S_2i+1S_2i+2 (1)

Cortés⁴⁰ has derived the exact expression for magnetic susceptibility of J₁J₂ alternating chain with Heisenberg-type exchange between the classical spins:

χ_alt = Nβ²g²S(S + 1)/3k_BT·F_alt (2)

F_alt = (1 + u₁ + u₂ + u₁u₂)/(1 − u₁u₂) (3)

where u_i = coth[J_iS(S + 1)/k_BT] − k_BT/J_iS(S + 1) and N, β, g, and k_B have their usual meanings.

If the antiferromagnetic coupling is weak or its character is ferromagnetic, the influence of zero-field splitting (zfs) must be taken into account in an analysis of the magnetic behaviour at low temperatures. The bulk susceptibility of 1a, 2a and 1b was calculated in the full temperature range by the combination of zfs and the alternating chain^41–43 as:

χ = χ_zfsF_alt + TIP. (4)

According to Boca,⁴⁴ the zfs contribution can be represented by χ_zfs = (χ_‖ + 2χ_⊥)/3 where:

χ_‖ = (Nβ²g_‖²/4k_BT)(1 + 9d²) / (1 + d²), (5a)

χ_⊥ = (Nβ²g_⊥²/k_BT) [1 + 3k_BT/4D·(1 – d²)]/(1 + d²), (5b)

for the cobalt compounds (1a, 2a) or:

χ_‖ = (Nβ²g_‖²/k_BT)2d/(1 + 2d), (6a)

χ_⊥ = (Nβ²g_⊥²/k_BT)2k_BT/D·(1 – d)/(1 + 2d) (6b)

for the nickel compound (1b), D is the axial zfs parameter and d = e^−D/k_BT. In order to avoid overparametrization, isotropic g values were assumed.

The best agreement between theory and experiment was obtained for J₁ = −0.72, −1.07, −2.47 cm⁻¹, J₂ = 0.33, 0.42, 0.67 cm⁻¹, D = 58.2, 51.4, −1.86 cm⁻¹, g = 2.69, 2.67, 2.25, and R (defined as Σ[(χT)_exp – (χT)_calc]²/Σ[(χT)_exp]²) equal to 6.2 × 10⁻⁵, 5.6 × 10⁻⁵, 7.8 × 10⁻⁶, for 1a, 2a and 1b, respectively. During the calculations, the temperature independent paramagnetism (TIP) was fixed at 60 × 10⁻⁶ emu per Co. Good fit of calculated and experimental values of magnetic susceptibility of 1b at temperatures above 100 K could be achieved with a TIP value of 201 × 10⁻⁶ emu per Ni. The susceptibility curves resulting from the above parameters (depicted as μ_eff(T) and χ⁻¹(T)) are shown in Fig. 6 and S36–S38 (ESI†). Since it is not possible to calculate field dependent magnetization within the above model, we have tried to ignore weak exchange interactions, treating the system as a set of isolated Co(II) ions and leaving the rest of the parameters unchanged. The M(B) dependences calculated for various external fields are drawn in Fig. S39 (ESI†) as a solid line and fit the experimental data pretty well.

The magnetic susceptibility of cobalt compounds calculated in the zero magnetic field using formulas 2–5 does not depend on the sign of the D parameter. In strong magnetic fields (above ∼0.5 T) a significant difference between the magnetization calculated for D and −D is observed (Fig. S39, ESI†) with a clear indication of positive values. This corresponds to the compressed tetragonal bipyramid around the cobalt centres (Fig. 1) and is consistent with the results of the analysis of the electronic spectra of 1a and 2a. It is also possible to fit the temperature dependence of magnetic susceptibility using both positive and negative values of parameter D for the nickel compound (1b). However, in this case the set of selected parameters with negative D (which again corresponds to the compressed bipyramid) better describes the behaviour of magnetization as a function of the magnetic field (Fig S39, ESI†).

Although coordination compounds containing various metal centres connected by –O–P–O– bridges are quite common and the magnetic properties of many of them have been investigated,^{15b,17b,18,20,22,45–49} a question still arises about the nature of magnetic exchange couplings through –O–P–O– bridges. Correlations of exchange integrals with M–O–P–O dihedral angles (related to deviation of the phosphonate group from the mean M–O⋯O–M plane)^45,46 or with the angle between the O–P–O plane and the M–O axis,⁴⁷ supported by the DFT calculations,^46,48a suggest that the most important structural parameter controlling the exchange coupling is the M–O⋯O–M dihedral angle over the O–P–O bridge. The antiferromagnetic coupling decreases with the increase of the M–O⋯O–M angle and the coupling can become ferromagnetic in the case of larger angles.^46,48,49 The J values calculated for 1a, 2a and 2b can readily be rationalized in terms of magnetostructural correlations outlined above. For both 1a and 2a, the boundary between ferro- and antiferromagnetic couplings estimated by this method for Co–O⋯O–Co angle is ca. 80°, and as seen in Fig. S40 and Table S7 (ESI†), the same observation seems to be applicable for 1D Co₃(HL)₂(H₂O)₆·6H₂O polymers featuring chains of linear symmetric Co2 Co1 Co2 trimers (HL = HdmtZol³⁻, HcppZol³⁻, HcbtZol³⁻, Scheme 1).^12,13 Moreover, the same value can also be used to explain magnetic behaviour of 1b and recently reported Ni₃(HdmtZol)₂(H₂O)₆·6H₂O complex.¹⁴

4. Conclusions

Three Co(II) and Ni(II) 1D coordination polymers based on zoledronic acid analogues with a cyclopropane (cpp) or cyclobutane (cbt) ring on the C_α carbon, namely [Co(H₂cppZol)(H₂O)]·H₂O (1a), [Ni(H₂cppZol)(H₂O)]·H₂O (1b) and [Co(H₂cbtZol)(H₂O)]·H₂O (2a), were synthesized under hydrothermal conditions and characterized by means of single-crystal X-ray diffraction, spectroscopic and thermal techniques. The studied compounds exhibit 1D double zig–zag chains architecture with the 8 + 8 rings motif formed by alternately arranged symmetrical (−O–P–O–)₂ bridges linking equivalent octahedral Co(II)/Ni(II) centres.

All the compounds exhibit similar decomposition pathways upon heating with thermal stabilities decreasing in the order 1b > 1a > 2a. The IR spectra revealed that lattice water release does not change chains architecture leaving them intact up to ca. 320, 280 and 240 °C, respectively. A good correlation between calculated energies of d–d transitions and experimental NIR-UV/VIS spectra was obtained with the assumption that metal centres of 1a, 1b and 2a adopt pseudo-octahedral compressed geometry.

Variable temperature magnetic susceptibility measurements analyzed in the framework of an alternating-exchange spin-chain revealed the occurrence of weak alternating antiferromagnetic–ferromagnetic exchange interactions propagated between the magnetic centres along double (–O–P–O–)₂ bridges with J₁ = −0.72, −1.07, −2.47 cm⁻¹ and J₂ = 0.33, 0.42, 0.67 cm⁻¹ for 1a, 1b and 2a respectively. The boundary between antiferro- and ferromagnetic couplings for Co–O⋯O–Co angle was established to be ca. 80°. This value can also be used to explain magnetic behaviour of 1b and is applicable for recently reported M₃(HL)₂(H₂O)₆·6H₂O (M = Co, Ni) complexes based on α,α-disubstituted analogues of zoledronic acid.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Department of Chemistry of Wroclaw University of Science and Technology is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1940766–1940768. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9ra05673d

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