Two 3d-4f Heterometallic Coordination Polymers of Macrocyclic Oxamide with Benzotriazole-5-carboxylate Co-ligand: Syntheses, Crystal Structures and Magnetic Properties

Two heterometallic 3d–4f coordination polymers, [Gd(CuL)2(Hbtca)(btca)(H2O)]·2H2O (1) and [Er(CuL)2(Hbtca)(btca)(H2O)]·H2O·CH3OH (2) (CuL, H2L = 2,3‐dioxo‐5,6,14,15‐dibenzo‐1,4,8,12‐tetraazacyclo‐pentadeca‐7,13‐dien; H2btca = benzotriazole‐5‐carboxylic acid) were synthesized by solvothermal methods and characterized by single‐crystal X‐ray diffraction. Complexes 1 and 2 exhibit a double‐strand meso‐helical chain structures formed by [LnIIICuII2] (LnIII = Gd, Er) units by oxamide and benzotriazole‐5‐carboxylate bridges. They are isomorphic except that one free water molecule of 1 is replaced by a methanol molecule. All 1D chains are further interlinked by hydrogen bonds resulting in a 3D supramolecular architecture. The magnetic properties of the compound 1 and 2 are also discussed.

Macrocyclic oxamide complex; Crystal structure; Magnetic properties; 3d‐4f Heterometallic coordination polymers

The research on the synthesis and characterization of 3d–4f heterometallic coordination polymers has attracted much attention not only because of their structural diversity, but also due to their potential applications in magnetism, luminescence, adsorption, catalysis, etc.[1] , [2] , [3] , [4] , [5] , [6] , [7] , [8] , [9] , [10] , [11] , [12] , [13] , [14] Up to date, large volumes of works on 3d‐4f coordination polymers have been reported, and most of the previously reported 3d‐4f coordination polymers were synthesized by using one‐pot synthetic approach. Adopting this synthetic approach, the ligands usually contain both N‐ and O‐donors because the nitrogen and oxygen atoms can coordinate to 3d and 4f metal ions, respectively, according to the hard‐soft acid based theory.[14] , [15] , [16] , [17] , [18] , [19] , [20] , [21] , [22] , [23] , [24] , [25] In this paper, we chose mononuclear macrocyclic oxamide complex CuL (see Scheme [NaN] a) and benzotriazole‐5‐carboxylate as co‐ligands to react with gadolinium acetate and erbium nitrate. Because CuL and benzotriazole‐5‐carboxylate both contain O‐donors, which prefer to bond to LnIII ions. Especially, benzotriazole‐5‐carboxylate has five coordination atoms and different bonding modes, which makes it easy to design and construct extended frameworks.[26] , [27] , [28] , [29] , [30] , [31] , [32] In our previous work, although some 3d‐3d′ heterometallic coordination polymers based on macrocyclic oxamide and aromatic multicarboxylate have been obtained,[33] , [34] , [35] , [36] , [37] , [38] those containing 3d‐4f have been prepared rarely.

On the other hand, the field of metal complex‐based magnetic materials containing 3d‐4f ions has received particular attention.[39] , [40] , [41] , [42] , [43] , [44] , [45] , [46] . To date, many complexes containing 3d‐4f ions, which show long‐range magnetic ordering or slow relaxation (SMMs and SCMs), have been reported.[47] , [48] , [49] , [50] Although the large volumes of works on 3d‐4f coordination polymers have been reported, the theoretical models to reproduce the magnetic susceptibility of such systems have been made rarely except 3d‐Gd systems.[51] , [52] , [53] Thus it is essential for further development of 3d‐4f magnetochemistry to obtain accurate information on the 3d‐4f magnetic interactions.

Herein, we report the syntheses, structures, and magnetic properties of the two complexes [Gd(CuL)2(Hbtca)(btca)(H2O)]·2H2O (1) and [Er(CuL)2(Hbtca)(btca)(H2O)]·H2O·CH3OH (2).

Single‐crystal X‐ray analyses reveal that compounds 1 and 2 are isostructural (except one free water of 1 replaced by a methanol molecule) and exhibit a double‐strand meso‐helical chain structure constructed from trinuclear [LnIIICuII2] units. Compounds 1 and 2 crystallize in the triclinic space group P1. As shown in Figure [NaN] a and Figure S1 (Supporting Information), the fundamental building unit of the crystal structure for 1 and 2 is composed of [Ln(CuL)2(Hbtca)(btca)(H2O)] [Ln = Gd (1), Er (2)]. The LnIII ion is nine‐coordinate and surrounded by four oxygen atoms from two macrocyclic oxamide groups, two oxygen atoms from btca2–, two oxygen atoms from Hbtca–, and one oxygen atom from coordination water molecule. The Ln–O distances are in the range of 2.368(3)–2.558(3) for 1 and 2.3129(19)–2.549(2) Å for 2, whereas the O–Ln–O bond angles are in the range of 52.30(8)–145.14(9) for 1 and 53.14(6)–150.82(6)° for 2.

The coordination arrangement of the LnIII ion [Ln = Gd (1), Er (2)] is a distorted one capped square antiprism. In complexes 1 and 2, the external CuII ion is five‐coordinate by four nitrogen atoms from the macrocyclic oxamide group, one nitrogen from btca2– or Hbtca–, and the coordination arrangement of the CuII ion is distorted square pyramidal with τ values of 0.018–0.028 for 1 and 0.026–0.035 for 2 calculated from τ = (β – α)/60.[54] The basal positions are occupied by four nitrogen atoms from the macrocyclic oxamide group, and the Cu–N bond lengths range from 1.995(3) to 2.043(3) Å for 1 and 1.998(2) to 2.046(2) Å for 2. The apical positions are occupied by a nitrogen atom (N9A from Hbtca– for Cu1, N12B from btca2–) to form weak Cu–N bonds with lengths of 2.412(3)–2.418(3) Å for 1 and 2.398(2)–2.416(3) Å for 2. Cu1, Cu2, and Ln1 are interlinked through the macrocyclic oxamide ligand to form a trinuclear Cu2Ln unit. Adjacent trinuclear units are connected by two Hbtca– and two btca2– bridging ligands to form a double‐strand meso‐helical chain, as shown in Figure [NaN] b and Figure S2 (Supporting Information). In the 1D structure, Hbtca– and btca2– both act as a bidentate connector to bridge one CuII ion and one LnIII ion (Scheme [NaN] b and c), and the relationship of the adjacent Hbtca‐ or btca2– with respect to each other is anti. Furthermore, the neighboring chains are linked together with O–H···N (uncoordinated nitrogen atoms from btca2– with the hydrogen atoms on the coordinated waters from neighboring chains), N–H···O (the oxygen atom on the free water with the hydrogen atom on the uncoordinated NH group from Hbtca– of a neighboring chain), and O–H···O (the coordinated oxygen atoms on a COO– group from btca2– with the hydrogen atoms on the free waters) intermolecular hydrogen bonding to form a 3D framework (Figure [NaN] c and Figure S3, Supporting Information). The hydrogen bonding system in 2 is similar to that of 1 except the coordinated oxygen atoms on COO– groups from Hbtca– with the hydrogen atoms of the free methanol molecule. The data of hydrogen bonds of complexes 1 and 2 are listed in Table [NaN] .

Hydrogen bond lengths /Å and bond angles /° for 1 and 2

Under the same reaction conditions such as solvent, pH, time, and temperature, coordination polymers 1 and 2 were obtained by using benzotriazole‐5‐carboxylate and macrocyclic oxamide as potential bridging ligands to react with gadolinium acetate and erbium nitrate. The experimental powder X‐ray diffraction patterns match well with the corresponding simulated ones obtained from the single‐crystal data (Figure [NaN] ), indicative that 1 and 2 both have good phase purity. The elemental analyses of 1 and 2 are also consistent with the results of the structural analyses. The IR spectra of 1 and 2 show strong peaks in the region 1630–1610 cm–1 and 1570–1550 cm–1, which are assigned to ν(C=O) and ν(C=N) vibrations, respectively.[55] The absorptions at 1680–1720 cm–1 were not found, indicating complete deprotonation of the carboxyl groups. The broad absorptions centered at 3423 cm–1 for 1 and 3411 cm–1 for 2 are characteristic of the hydroxyl from H2O and NH from Hbtca–.[55]

The magnetization measurements for complexes 1 and 2 were carried out under 1 kOe.

The value χMT = 8.79 cm3·K·mol–1 at 300 K for powder sample 1 is slight larger than the expected value (8.63 cm3·K·mol–1) for the uncoupled GdIII (8S7/2, g = 2) and two CuII ions (S = 1/2, g = 2). On lowering the temperature, χMT decreases continuously and reaches 8.04 cm3·K·mol–1 at 26 K (Figure [NaN] ) which can be contributed to the antiferromagnetic interaction present in the compound 1; and then increases to a value of 9.05 cm3·K·mol–1 at 2 K, indicative there is a ferromagnetic interaction.[56] In order to further investigate the magnetic behavior of 1, the filed dependent magnetization for 1 was measured at 2 K. The Mmol/Nβ vs. H value exhibits a fast increase for low fields, and then gradually increases to 9.81 Nβ at 60 kOe (insert of Figure [NaN] ). The value 9.81 Nβ is slight larger than the saturation value 9 Nβ, which can prove that there is ferromagnetic interaction in 1.

The value χMT = 12.08 cm3·K·mol–1 at 300 K for powder sample 2 is slight lower than the calculated value of 12.19 cm3·K·mol–1 expected for one independent ErIII (4I15/2, g = 6/5) and two independent CuII ions (S = 1/2, g = 2). On lowering the temperature, χMT decreases continuously and reaches 5.64 cm3·K·mol–1 at 2 K (Figure [NaN] ). Complex 2 is actual a 1D Cu2Er entity formed by trianuclear units [Er(CuL)2(Hbtca)(btca)(H2O)] linked by the Hbtca– or btca2–, but the magnetic interactions for Cu···Er through the Hbtca– or btca2– bridges between the adjacent trianuclear units [Er(CuL)2(Hbtca)(btca)(H2O)] can be neglected because of the larger separation of Cu···Ln (about 8.7–10.6 Å). Thus, the coupling topology deduced from the crystal structure has to be considered as the Cu2Er trianuclear unit. In order to roughly quantitatively estimate the magnetic interaction parameters between CuII and ErIII ions, an approximate model to reproduce the magnetic susceptibility was made because of the large anisotropy of the ErIII ion.[57] , [58] Firstly, the total magnetic susceptibility of 2 can be calculated by one ErIII plus two CuII ions (χtotal = 2χCu + χEr, χCu = (Nβ2gCu2) / 4kT; (gCu = 2)].

For ErIII ion, the crystal field perturbation is larger than spin‐orbit coupling, so, on lowering the temperature, χMT values decrease mainly due to the thermal depopulation of the lower Stark sublevels.[58] Based on the above analysis, the magnetic analysis was carried out by using the spin Hamiltonian: Ĥ = ΔĴz2, and the magnetic susceptibility of χEr may be expressed as in Equation.

χEr=Nβ24kT⋅3625⋅BBCC

CC=exp⁡(−225x)+exp⁡(−169x)+exp⁡(−121x)+exp⁡(−81x)+exp⁡(−49x)+exp⁡(−25x)+exp⁡(−9x)+exp⁡(−x)BB=225exp⁡(−225x)+169exp⁡(−169x)+121exp⁡(−121x)+81exp⁡(−81x)+49exp⁡(−49x)+25exp⁡(−25x)+9exp⁡(−9x)+exp⁡(−x)

x = Δ/2.78T

Lastly, so far as magnetic interactions between ErIII and CuII ions are concerned, a correction for a molecular field was used (χM = χtotal/[1–χtotal(2zj′/Ng2β2)]).

The least‐squares fit to the data (26–300 K) leads to Δ = 0.58 cm–1, g = 1.75, zj′ = 0.08 cm–1, and the agreement factor defined as R = ∑[(χM)cal–(χM)obsd]2 / ∑(χM)obsd2 is 1.93 × 10–3. The point below 26 K can not be reproduced with this model, possibly due to the spin‐orbit coupling interaction. The positive zj′ suggests a very weak ferromagnetic interaction between the ErIII and CuII ions in 2. In order to probe dynamic magnetic behaviors of 2, alternating current magnetic measurements were performed under a 3.5 Oe ac fields, the results show that frequency dependence of the in‐ (χM′) and out‐of phase (χM′′) signal is not observed for 2 (Figure S4, Supporting Information).

Two double‐strand meso‐helical chainlike polymers consisting of Cu2Ln unit (for 1: Ln = Gd; for 2: Ln = Er) were synthesized with macrocyclic oxamide and benzotriazole‐5‐carboxylate co‐ligands under same solvothermal reaction conditions. In both 1 and 2, O–H···N, N–H···O, and O–H···O intermolecular hydrogen bonds link the chainlike polymers to form a 3D superamolecular architecture. The magnetic analyses show that ferromagnetic interaction are present in 1 and 2, and the anisotropy of ErIII ion plays a important role in magnetic behaviors of 2.

Material and Synthesis: All the starting reagents were of A. R. grade and were used as purchased. The complex ligand CuL was prepared as described elsewhere.[59]

Synthesis of [Gd(CuL)2(HBtca)(Btca)(H2O)]·2H2O (1): A mixture of Gd(OAc)3·6H2O(0.05 mmol, 16.8 mg), CuL (0.1 mmol, 19.6 mg), H2Btca (0.1 mmol, 16.3 mg), H2O (10 mL) and CH3OH (4 mL) was stirred for 20 min at room temperature, and the pH value of the solution was adjusted to about 7–8 with triethylamine. After stirring, the mixture was transferred to an 18 mL Teflon‐lined reactor, and heated at 150 °C for 72 h. The reaction system was cooled to room temperature during 36 h, and dark green crystals for 1 were isolated in 56 % yield. C52H45Cu2GdN14O11: calcd. C 47.05; H 3.39; N 14.48 %; found: C 47.06; H 3.40; N 14.50 %. IR (KBr, selected bands): ν̃ = 3414s (br), 1620 s, 1554 s, 1505 m, 1481 m, 1433 w, 1403 s, 1342 m, 1276 w, 1034 w, 907 w, 847 w, 781 m, 756 m cm–1.

Synthesis of [Er(CuL)2(HBtca)(Btca)(H2O)]·H2O·CH3OH (2): The synthetic method was similar to that for 1 by using Er(NO3)3·6H2O(0.05 mmol, 22.5 mg) instead of Gd(OAc)3·6H2O, and dark green crystals for 2 were isolated in 58 % yield. C53H47Cu2ErN14O11: calcd. C 47.10; H 3.48; N 14.51 %; found: C 47.11; H 3.49; N 14.52 %. IR (KBr, selected bands): ν̃ = 3432 s (br), 1620 s, 1554 s, 1530 m, 1493 m, 1439 w, 1403 s, 1336 m, 1270 w, 1209 w, 1046 w, 913 w, 847 w, 781 m, 756 m.

Physical Measurements: Elemental analyses (C, H, N) were determined with a Perkin‐Elmer 240 Elemental analyzer. IR spectra were recorded as KBr discs with a Shimadzu IR‐408 infrared spectrophotometer in the 4000–600 cm–1 range. XRPD spectra for the power were recorded with a Model D/MAX‐2550V, Rigaku, Japan. Variable‐temperature magnetic susceptibilities of single crystals were measured with an MPMS‐7 SQUID magnetometer. Diamagnetic corrections were made with Pascal's constants for all the constituent atoms.

X‐ray Crystallography: Single crystal X‐ray diffraction analyses of 1 were carried out with a SuperNova, Dual, Cu at zero, AtlasS2 diffractometer equipped with a mirror monochromated Cu‐Kα radiation (λ = 1.54184 Å). The crystal was kept at 173.00(10) K during data collection. Using Olex2,[60] the structure was solved with the ShelXT[61] structure solution program using Intrinsic Phasing and refined with the ShelXL[62] refinement package using Least Squares minimization. Single crystal X‐ray diffraction analyses of 2 were carried out with a Bruker Smart Apex II CCD diffractometer equipped with a graphite monochromated Mo‐Kα radiation (λ = 0.71073 Å) by using φ/ω scan technique at 173(2) K. Using Olex2,[60] the structure was solved with the ShelXS[63] structure solution program using Direct Methods and refined with the ShelXL[62] refinement package using Least Squares minimization. The crystallographic data and selected bond lengths and angles for 1 and 2 are listed in Table [NaN] , Table [NaN] , and Table [NaN] .

Crystal data and structure refinement for complexes 1 and 2

1 R1 = Σ||Fo|–|Fc||/Σ|Fo|.

2 wR2 = {Σ[w(Fo2–Fc2)2]/Σ[w(Fo)2]}1/2.

Selected bond lengths /Å and angles /° for 1

Selected bond lengths /Å and angles /° for 2

Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers CCDC‐1485652 and CCDC‐1485653. (Fax: +44‐1223‐336‐033; E‐Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)

Supporting Information (see footnote on the first page of this article): additional images of the structures and details of the magnetic measurements.

This work was supported by the National Natural Science Foundation of China (No. 20771083) and the Program for Innovative Research Team in University of Tianjin (TD12–5038) and Tianjin Normal University (No. 52XC1102).

Supporting information for this article is available on the WWW under https://doi.org/10.1002/zaac.201600377 or from the author.

Graph: The coordinated modes of macrocyclic oxamide complex ligands and benzotriazole‐5‐carboxylate ( M = Cu, Ln = Gd, Er).

Graph: (a) Portion of the crystal structure of 1 showing coordinate environments of Gd III and Cu II ions; all hydrogen atoms are omitted for clarity (symmetry code: A 1– x , 1– y , 1– z ; B 2– x , – y , – z ). (b) View of the self‐assembled double‐strand meso ‐helical chain constructed by [Gd(CuL) 2 (Hbtca)(btca)(H 2 O)]. (c) View of the self‐assembled 3D framework through hydrogen bond interactions in 1.

Graph: Experimental (black) and simulated (red) PXRD spectra of complexes 1 and 2.

Graph: Plot of χMT vs. T for complex 1. Inset: Plots of M vs. H for 1 at 2 K.

Graph: χM (O) vs. T and χMT (Δ) vs. T plots for complex 2.

Graph: Supporting Information