Structural variation from heterometallic heptanuclear or heptanuclear to cubane clusters based on 2-hydroxy-3-ethoxy-benzaldehyde: effects of pH and temperature

Abstract

Five new Hheb complexes, HN(C₂H₅)₃·[M₄Na₃(heb)₆(μ₃-N₃)₆], (M = Ni (1), and Co (2), [Co₄(heb)₄(μ₃-OCH₃)₄(μ₁-HOCH₃)₄]·(H₂O)₂ (3), [M₇(heb)₆(μ₃-OCH₃)₆]·(ClO₄)₂ (M = Ni (4), and Co (5), Hheb = 2-hydroxy-3-ethoxy-benzaldehyde), were synthesized by reaction of hexahydrate perchlorate salt, Hheb, and NaN₃ under different temperature and pH conditions. Careful investigation of the effect of the reaction temperature and pH resulted in a series of compounds with different compositions and nuclearities. The diverse compounds obtained illustrate the marked sensitivity of the structural chemistry of Co- or Ni-Hheb ligand-like systems to synthesis conditions. Complexes 1 and 2, which are heterometallic heptanuclear anion [M₄Na₃(heb)₆(N₃)₆]⁻ clusters, are formed at a pH of 5.5 and at room temperature. At a pH of 7.5 and at room temperature, a neutral molecular cubane cluster, namely 3, is formed with a lower nuclearity. Further increase of the reaction temperature to 140 °C at the same pH resulted in formation of two heptanuclear cation [M₇(heb)₆(μ₃-OCH₃)₆]²⁺ clusters, 4 and 5. The results show that the pH and reaction temperatures play a key role in the structural control of the self-assembly process. Interestingly, heterometallic heptanuclear anion [M₄Na₃(heb)₆(N₃)₆]⁻ clusters have never been reported for the family of μ₃-N₃⁻ or μ₃-O-bridged heptanuclear clusters. The magnetic properties of 1–5 were investigated and are discussed in detail.

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1. Introduction

The rational design and synthesis of polynuclear complexes have undergone tremendous development because they have fascinating structures and functional applications as optical, electronic, catalytic, fluorescent, and magnetic materials.^1–4 Particular interest has focused on the development of single molecule magnets (SMM).⁵ An effective and facile approach for the synthesis of such complexes is still dependent on suitable choice of well-designed organic ligands as bridges or terminal groups with metal ions or metal clusters as nodes. Although systematic, accurate prediction and total design of crystal structure are not yet possible, efforts have been made to understand and control certain reaction parameters such as the metal-to-ligand molar ratio, the ligand denticity, the type of metal ions, the organic guest molecules, the presence of solvent molecules, the pH of the solution, the counter-ions, and the reaction temperatures that play crucial roles in the structure formation processes.^2e,f,6,7 For example, Li et al. reported that pH has a crucial role in structural formation of 3,5-pyrazoledicarboxylic acid systems.^6c,d Jacobson et al. reported that reaction temperature and pH influence the coordination modes of the 1,4-benzenedicarboxylate ligand.^6e In addition, our investigation of systems involving 2-hydroxy-benzaldehyde ramification (HL) ligands or (1H-benzimidazol-2-yl)-methanol ramification ligands illustrated that steric hindrance of the ligands has a crucial role in structure formation.^2f,3f As part of our ongoing interest in understanding the effects of temperature and pH on the assembly of clusters, we have recently succeeded in selective synthesis of five clusters by controlling reaction temperature and pH. Details of the synthesis and characterization of these complexes with multidentate ligand 2-hydroxy-3-ethoxy-benzaldehyde (Hheb, Scheme 1) are described in this paper.

Scheme 1 Coordination mode of the ligand.

This ligand has three oxygen atoms from the phenoxo, aldehyde and C₂H₅O⁻ groups, and has potential as use as a linker for construction of interesting coordination polymers with abundant hydrogen bonds.^2f,5a,8 It is worth noting that the ligand 2-hydroxy-3-methoxy-benzaldehyde has been reported to possess four potential coordination modes: μ₅:η²:η²:η¹,⁹ μ₂:η¹:η¹,^5a,10 μ₄:η¹:η²:η¹,^2f,11 and μ₆:η³:η²:η¹,¹² whereas its analogue, 2-hydroxy-3-ethoxy-benzaldehyde (Hheb) has been reported to possess only three coordination modes, μ₂:η¹:η¹,¹³ μ₄:η¹:η²:η¹,^1e and μ₅:η²:η²:η¹.¹⁴ As a result, it was decided to investigate the coordination mode of the multidentate Hheb ligand with the objective of constructing new clusters by controlling temperature and pH during the preparation of these compounds.

2. Experimental

Materials and instrumentation

All chemicals were commercially available and used as received without further purification. Elemental analyses (CHN) were performed using an Elemental Vario-EL CHN elemental analyzer. FT-IR spectra were recorded from KBr pellets in the range of 4000–400 cm⁻¹ on a Bio-Rad FTS-7 spectrometer. The X-ray crystal structures were determined by single-crystal X-ray diffraction using the SHELXL crystallographic software for molecular structures. The PXRDs of 1–5 were determined by Rigaku D/max 2500v/pc. Magnetization measurements were carried out with a Quantum Design MPMS-XL7 SQUID to 50 000 Oe for 1–5 ((HN(C₂H₅)₃·[M₄Na₃(heb)₆(μ₃-N₃)₆] with M = Ni (1) and Co (2), [Co₄(heb)₄(μ₃-OCH₃)₄(μ₁-HOCH₃)₄]·(H₂O)₂ (3), [M₇(heb)₆(μ₃-OCH₃)₆]·(ClO₄)₂ with M = Ni (4) and Co (5)). It must be mentioned that the χ_MT vs. T curves for all complexes of interest (except 3) cannot be fitted by the Magpack method.

Syntheses

HN(C₂H₅)₃·[Ni₄Na₃(heb)₆(N₃)₆] (1). A mixture of Ni(ClO₄)₂·6H₂O (0.36 g, 1 mmol), NaN₃ (0.065, 1 mmol), Hheb (0.166 g, 1 mmol), acetonitrile (4 mL), and methanol (4 mL) with a pH adjusted to 5.5 by addition of triethylamine was stirred for 30 min at room temperature. The resulting solution was left at room temperature and green crystals of 1 were obtained after 30 min. The green crystals of 1 were collected by filtration, washed with methanol and dried in air. Phase pure crystals of 1 were obtained by manual separation (yield: 145 mg, ca. 52.72% based on Hheb ligand). Anal. calc. for 1: C₆₀H₇₀N₁₉Na₃Ni₄O₁₈ (M_r = 1649.08), calc.: C, 43.69; H, 4.28; N, 16.13%; found: C, 43.57; H, 4.38; N, 16.19%. IR data for 1 (KBr, cm⁻¹, Fig. S1†): 3439 w, 2978 w, 2087 s, 1618 s, 1550 m, 1442 s, 1346 m, 1216 s,1102 m, 1006 w, 897 m, 747 m, 659 w, 577 w.

HN(C₂H₅)₃·[Co₄Na₃(heb)₆(N₃)₆] (2). Complex 2 was prepared in a similar way to 1, except that Ni(ClO₄)₂·6H₂O was replaced by Co(ClO₄)₂·6H₂O. Red crystals of 2 were collected by filtration, washed with methanol and dried in air. Phase pure crystals of 2 were obtained by manual separation (yield: 142 mg, ca. 51.60% based on Hheb ligand). Anal. calc. For 2: C₆₀H₇₀N₁₉Na₃Co₄O₁₈ (M_r = 1650.04), calc.: C, 43.67; H, 4.28; N, 16.12%; found: C, 43.56; H, 4.41; N, 16.20%. IR data for 2 (KBr, cm⁻¹, Fig. S1†) 3432 w, 2930 w, 2093 s, 1624 s, 1542 m, 1462 m, 1340 m, 1210 s, 1094 m, 1006 w, 904 m, 747 m, 645 w, 557 w.

[Co₄(heb)₄(μ₃-OCH₃)₄(μ₁-HOCH₃)₄]·(H₂O)₂ (3). Complex 3 was prepared in a similar way to 2, except that a pH of 7.5 was used instead of 5.5. Red crystals of 3, which were obtained after 3 days, were collected by filtration, washed with methanol and dried in air. Phase pure crystals of 3 were obtained by manual separation (yield: 136 mg, ca. 45.92% based on Hheb ligand). Anal. calc. For 3: C₄₄H₆₈Co₄O₂₂ (M_r = 1184.70), calc.: C, 44.61; H, 5.70%; found: C, 44.53; H, 5.79%. IR data for 3 (KBr, cm⁻¹, Fig. S1†): 3433 m, 2814 w, 1632 s, 1536 m, 1428 m, 1332 w, 1238 m, 1204 s, 1053 m, 891 w, 741 m, 645 w, 565 w, 455 m.

[Ni₇(heb)₆(μ₃-OCH₃)₆]·(ClO₄)₂ (4). Complex 4 was prepared in a similar way to 1, except that the mixture was poured into a Teflon-lined autoclave (15 mL) and then heated at 140 °C for 5 days. Green crystals of 4 were collected by filtration, washed with methanol and dried in air. Phase pure crystals of 4 were obtained by manual separation (yield: 178 mg, ca. 69.72% based on Ni ions). Anal. calc. For 4: C₆₀H₇₂Cl₂Ni₇O₃₂ (M_r = 1786.91), calc.: C, 40.33; H, 4.06%; found: C, 40.23; H, 4.14%. IR data for 1 (KBr, cm⁻¹, Fig. S1†): 3439 w, 3304 m, 2930 m, 1618 s, 1550 w, 1468 s, 1326 m, 1210 s, 1088 s, 891 w, 836 w, 741 m, 605 m.

[Co₇(heb)₆(μ₃-OCH₃)₆]·(ClO₄)₂ (5). Complex 5 was prepared in a similar way to 4, except that Ni(ClO₄)₂·6H₂O was replaced by Co(ClO₄)₂·6H₂O. Red crystals of 5 were collected by filtration, washed with methanol and dried in air. Phase pure crystals of 5 were obtained by manual separation (yield: 162 mg, ca. 63.40% based on Co ions). Anal. calc. For 5: C₆₀H₇₂Cl₂Co₇O₃₂ (M_r = 1788.59), calc.: C, 40.29; H, 4.06%; found: C, 40.21; H, 4.15%. IR data for 5 (KBr, cm⁻¹, Fig. S1†): 3432 w, 3290 m, 2930 m, 2814 w, 1624 s, 1542 w, 1462 s, 1326 m, 1210 s, 1088 s, 884 m, 823 m, 735 m, 625 w, 577 m.

Caution: perchlorate salts and azide of metal complexes with organic ligands are potentially explosive.

Crystal structure determination

The diffraction data were collected on an Agilent G8910A CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å), using the ω–θ scan mode in the ranges 2.97° ≤ θ ≤ 25.10° (1), 2.88° ≤ θ ≤ 25.01° (2), 2.89° ≤ θ ≤ 25.10° (3), 3.49° ≤ θ ≤ 25.08° (4), and 3.17° ≤ θ ≤ 25.05° (5). Raw frame data were integrated with the SAINT program. The structures were solved by direct methods using SHELXS-97 and refined by full-matrix least-squares on F² using SHELXS-97.¹⁵ An empirical absorption correction was applied with the program SADABS.¹⁵ All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were positioned geometrically and refined as riding. Calculations and graphics were performed with SHELXTL.¹⁵ The anisotropic displacement parameters of atoms C55–C60 in 1 and 2 were restrained to be identical with a standard uncertainty of 0.01 Ų. The structures contain solvent accessible voids of 250 and 235 ų for 1 and 2, respectively. The highest peaks with 1.340 e Å⁻³ of 3 and 1.187 e Å⁻³ of 5 in the residual electron density are located 0.09 Å from atom H6A and 0.75 Å from atom H8B, respectively. The crystallographic details are provided in Table 1. Selected bond distances and angles for 1–5 are listed in Tables S1–S4† and hydrogen bond lengths (Å) and angles (°) in Complexes 1–3 are listed in Tables S5–S7.†

Table 1 Crystallographic data for complexes 1–5

Complexes	1	2	3	4	5
a R1 = Σ‖Fo| − |Fc‖/Σ|Fo|.b wR2 = [Σw(|Fo^2| − |Fc^2|)^2/Σw(|Fo^2|)^2]^1/2.
Formula	C60H70N19Na3Ni4O18	C60H70N19Na3Co4O18	C44H68Co4O22	C60H72Cl2Ni7O32	C60H72Cl2Co7O32
Mr	1649.08	1650.04	1184.70	1786.95	1788.59
Crystal size (mm)	0.25 × 0.10 × 0.06	0.22 × 0.19 × 0.16	0.12 × 0.10 × 0.08	0.18 × 0.16 × 0.15	0.16 × 0.14 × 0.12
Crystal system	Monoclinic	Monoclinic	Tetragonal	Hexagonal	Hexagonal
Space group	P21/n	P21/n	P4/ncc	P3̄	P3̄
a (Å)	13.200 (1)	13.245 (1)	17.0166 (4)	14.649 (1)	14.818 (1)
b (Å)	26.664 (1)	26.530 (1)	17.0166 (4)	14.649 (1)	14.818 (1)
c (Å)	22.120 (1)	22.159 (1)	18.7995 (8)	9.663 (1)	9.391 (1)
α (°)	90	90	90	90	90
β (°)	96.526 (3)	96.648 (3)	90	90	90
γ (°)	90	90	90	120	120
V (Å^3)	7735.1 (5)	7734.0 (4)	5443.7 (3)	1795.7 (2)	1785.8 (3)
F(000)	3412	3396	2464	918	911
Z	4	4	4	1	1
Dc (g cm^−3)	1.417	1.418	1.446	1.652	1.663
μ (mm^−1)	1.051	0.935	1.271	1.956	1.747
θ range (°)	2.97–25.10	2.88–25.01	2.89–25.10	3.49–25.08	3.17–25.05
Ref. meas./indep.	34 163, 13 773	58 550, 13 561	12 303, 2432	3860, 2128	3653, 2111
Obs. ref. [I > 2σ(I)]	8437	9059	1872	1602	1602
Rint	0.0400	0.0565	0.0296	0.0210	0.0296
R1 [I ≥ 2σ (I)]^a	0.0549	0.0716	0.0543	0.0547	0.0566
wR2 (all data)^b	0.1797	0.2229	0.2013	0.1865	0.1657
Goof	1.002	1.070	1.009	1.008	1.008
Δρ (max, min) (e Å^−3)	0.707, −0.459	0.807, −0.605	1.340, −0.601	0.830, −0.577	1.187, −0.677

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3. Results and discussion

Description of the crystal structures

HN(C₂H₅)₃·[M₄Na₃(heb)₆(N₃)₆] (1 and 2). The single-crystal X-ray diffraction analysis reveals that 1 and 2 are homeomorphism complexes and contain a heterometallic heptanuclear anion-type cluster, [M₄Na₃(heb)₆(μ₃-N₃)₆]⁻. Additionally, they represent the first {M₄Na₃} complexes to possess planar hexagonal disc-like structures. As illustrated in Fig. 1, 1 consists of one wheel-like anion heterometallic heptanuclear cluster, [Ni₄Na₃(heb)₆(N₃)₆]⁻, and one counter cation, HN(C₂H₅)₃⁺. It can also be seen that the Ni1 atom located in the center of the structure is bridged by six μ₃-N₃⁻ groups, which are linked by three Ni atoms and three Na atoms on the rim that represents the bottom segment of the formed disc-like structure. The coordination geometry of the central Ni1 ion can be described as a slightly distorted octahedron with bond lengths of 2.107–2.138 Å and bond angles in the range of 79.3 (2)–95.3 (1)° (cis-angles) and 170.9 (1)–171.9 (2)° (trans-angles). On the rim, each Ni atom adopts a NiN₂O₄ coordination configuration resulting from coordination of two heb ligands and two μ₃-N₃⁻ groups. The Ni atoms on the rim also formed distorted octahedral geometries as evidenced by the cis- and trans-angle values ranging between 79.9 (1) and 95.4 (1)°, and 170.7 (14) and 179.2 (1)°, respectively. Coordinated bond lengths around the rim Ni atoms are in the range of 1.992 (3)–2.134 (4) Å. All Na atoms formed distorted trigonal bipyramid geometries with bond lengths of 2.232 (4)–2.469 (4) Å, which fall well within the range of Na–O distances of sodium complexes (Na–O distances are typically in the range of 2.234–2.609 Å).^2a,16 Therefore, heb, a hetero-multinucleating ligand, can coordinate to either a “soft” metal center (Ni²⁺) or a “hard” one (Na⁺). As such, the heb ligand, which displays a μ₂:η¹:η²:η¹ coordination mode, is linked to one nickel and one sodium in the compound of interest (Scheme 1a). Although many heptanuclear clusters have been reported, e.g., {Ni₇} clusters,¹⁷ {Mn₇} cluster,¹⁸ {Zn₇} clusters,^17a,18e,19 {Fe₇} clusters,^18g,20 {Co₇} clusters,^2a,2e,21 and {M^II₆M^III} (M = Co, Ni, Fe),^2f,20a,21e 1 and 2 represent the first high nuclear heterometallic heptanuclear anion clusters to the best of our knowledge (only a very limited number of anion-like clusters have been reported in the literature).²² It is interesting to mention that the counter ion HN(C₂H₅)₃⁺ is connected to the heterometallic heptanuclear anion cluster [M₄Na₃(heb)₆(μ₃-N₃)₆]⁻ through N–H⋯N hydrogen bonds (N19–H19E⋯N6ⁱ, 2.938 (9) Å, symmetry code: (i) x − 1, y, z, Table S5†). All azide ligands in 1 and 2 are linear, herein, the N–N–N bond angles range from 176.3 (6) to 179.1 (8)°. The N–N bond lengths of the azide ligands are distinguishable from those of Ni(II) complexes containing end-on doubly bridging azides.²³ It can also be seen that the heb ligand-based clusters 1 and 2 show that the expansion direction of the heb ligands is different from that observed in other heptanuclear clusters constructed by 2-hydroxy-benzaldehyde ramifications.^2a,f,17c This is most likely because each pair of heb ligands faces each other rather than facing in the same direction as typically found for the 2-hydroxybenzaldehyde-type heptanuclear clusters. This difference may be attributed to the type of rings that can be formed in these clusters. It is well known that in compounds with 2-hydroxy-benzaldehyde ramifications, the phenoxo and aldehyde groups form a six-membered ring with the metal ion, whereas phenoxo and RO-groups form a more open five-membered ring. As a result, a more open five-membered ring would facilitate incorporation and coordination of the Na ion better than in a system comprising a six-membered ring. This finding may open up new perspectives for better design of cluster-type systems.

Fig. 1 The anion structures of [M₄Na₃(heb)₆(N₃)₆]⁻ (M = Ni (1), Co (2)). All H-atoms are omitted for clarity.

[Co₄(heb)₄(μ₃-OCH₃)₄(μ₁-HOCH₃)₄]·(H₂O)₂ (3). The single-crystal X-ray crystallographic analysis reveals that 3 contains a methoxy anion bridged discrete cubane-based cluster of formula [Co₄(heb)₄(μ₃-OCH₃)₄(μ₁-HOCH₃)₄]·(H₂O)₂ (Fig. 2). There is one independent Co(II) ion, assuming a distorted octahedral geometry with cis-angles O–Co–O in the range of 80.5–99.0° and Co–O bonds of 2.034 (3)–2.156 (4) Å from three μ₃-OMe, a dichelating heb, and one MeOH. The adjacent Co⋯Co distances and the Co–O–Co angles are in the range of 3.093–3.193 Å and 94.7–99.5°, respectively. It is worthwhile noting that heb exists as a mono-anion and bonds via a μ₁:η¹:η¹ coordination mode (Scheme 1b). Additionally, intramolecular hydrogen bonds exist between adjacent phenolato oxygen atoms and the proton from the MeOH molecule with distances of 2.764 Å across four of the six faces of the cubane (Table S3†). As a result, these four faces exhibit shorter Co⋯Co distances (3.090 Å) and smaller magnetic exchange Co–O–Co angles (94.7, 95.5°). Thus, 3 displays an approximate S₄ site symmetry, while the exact crystallographic point symmetry is C₁.

Fig. 2 Molecular structure of 3. All H-atoms and water molecules are omitted for clarity.

As shown in Fig. S2,† the different cubes present in the cluster are well separated from each other by the heb ligands and methoxy groups resulting in cent–cent distances of adjacent cubanes of 9.40 Å in the c direction and 12.03 Å in the ab plan of intramolecular hydrogen bonds. Moreover, the nearest intercluster Co⋯Co distances are 9.464 Å in the ab plan and 7.41 Å in the c direction.

[M₇(heb)₆(μ₃-OCH₃)₆]·(ClO₄)₂ (4 and 5). As the single-crystal X-ray diffraction analysis also confirms that 4 and 5 are homeomorphism complexes and contain a heterometallic heptanuclear cation-type cluster, [M₇(heb)₆(μ₃-OCH₃)₆]²⁺, only 4, which belongs to the trigonal space group P3̄, will be discussed in detail. Its unit cell contains one disc-like heptanuclear nickel cluster [Ni₇(heb)₆(μ₃-OCH₃)₆]²⁺ and two counter anions ClO₄⁻, which lie on a site with a threefold symmetry (Fig. 3). In addition, six μ₂-phenoxo groups bridge the six nickel ions on the rim, which are linked to the central nickel ion through six μ₃-OCH₃ bridging groups. These latter groups act as the ‘bottom’ segment in the formed disc-like structure. The coordination geometry of the central nickel ion (Ni2) can be described as a slightly distorted octahedron with bond lengths of 2.077 (3) Å (Ni2–O4) and O–Ni2–O angles in the range of 82.2 (2)–97.8 (2)° (cis-angles), whereas all of the trans-angles have a value of 180.0°. Each nickel ion on the rim (Ni1) adopts a NiO6 configuration as Ni1 is coordinated by four O atoms from two different heb ligands and two O(O4, O4b) atoms from two methoxy groups resulting in the formation of a slightly distorted octahedral geometry (Ni1–O1, 1.990 (4); Ni1–O1a, 1.997 (4); Ni1–O2a, 2.029 (5); Ni1–O4b, 2.045 (4); Ni1–O4; 2.076 (4) and Ni1–O3, 2.345 (4) Å; symmetry code: (a) x − y, x, 2 − z and (b) y − x, − x, 2 + z). The O–Ni1–O angles lie in the range of 72.4 (2)–104.7 (2)° and 151.9 (2)–172.8 (1)° for the cis- and trans-angles, respectively. Additionally, the magnetic exchange angles of Ni–O–Ni are in the range of 96.8 (2)–102.6 (2)°. The assignment of the metal ions on the rim (Ni1) and at the center (Ni2) of the structure was based on the valence sum calculation. The heb anionic ligands, which bridge the peripheral Ni^II centers, display a μ₂:η¹:η²:η¹ coordination mode (Scheme 2c) and lie alternately above and below the {Ni^II} plane. Moreover, the packing mode for 4 and 5 can be described as AAA along the c axis as shown in the packing diagram of 4 (Fig. S3†). The disc-like Ni7 units are well separated from each other by heb ligands, methoxy groups and ClO₄⁻ ions, thus resulting in center distances between adjacent clusters of 14.65 Å and 14.82 Å in the ab plane, and 9.66 and 9.39 Å in the c direction for 4 and 5, respectively (Fig. S3†). On the other hand, the core of the cluster can be described as an almost planar {Ni6} moiety (a ±0.1143 Å deviation from the average plane), which is composed of [Ni1] and its five centrosymmetric equivalent Ni atoms, with the central nickel ion (Ni2) lying on a site with a threefold symmetry. Although the structural motif belongs to the expanding {M7} family,^17–21 it is worth mentioning that only a few heptanuclear clusters have been reported so far.^17–21 More importantly, to the best of our knowledge, 5 is the first example of a 2-hydroxy-3-ethoxy-benzaldehyde cobalt cluster.

Fig. 3 The cation structures of [M₇(heb)₆(μ₃-OCH₃)₆]²⁺ (M = Ni (4), and Co (5)). All H-atoms are omitted for clarity.

Scheme 2 pH and temperature effects on the structural variations from heterometallic heptanuclear or heptanuclear clusters to cubane clusters based on 2-hydroxy-3-ethoxy-benzaldehyde.

Structural and synthetic aspects

Our aim was to investigate the effects of the temperature and pH on self-assembly of supramolecules and clusters. Co(II), Ni(II), and Hheb were selected as starting materials. The synthetic strategy for the Hheb system is depicted in Scheme 2. Complexes 1–3 were obtained under similar conditions with the exception of the pH. At first, we carried out the complex synthesis at a pH of 5.5 and at room temperature, and compounds 1 and 2 with heterometallic heptanuclear anion [M₄Na₃(heb)₆(N₃)₆]⁻ clusters were obtained. When the pH of the reaction was raised to 7.5, complex 3 with a cubane cluster was obtained. It is interesting to note that the anionic group N₃⁻ is not present in 3, unlike as seen in 1 and 2. On the basis of the synthetic conditions used to prepare 1–3, it can be concluded that the pH is most likely responsible for the structural differences observed between 1, 2, and 3. In addition, to affect the deprotonation of the organic ligands used in the formation of the three complexes, an increase in the pH conditions also results in the presence of solvent anionic species such as CH₃O⁻ at higher pH (7.5). As a result, the methanol molecules exist in the form of CH₃O⁻ anionic species, which can be coordinated as μ₃-OCH₃ in 3 at a pH of 7.5. On the other hand, the methanol molecules only function as solvent species in the formation of 1 and 2 when the pH is maintained at 5.5. It should be mentioned that a different single crystal growth time was used for the formation of the three complexes, which could also affect the type of systems obtained. The single crystal growth time was about 30 minutes for 1 and 2, whereas a much longer time was required for 3 (3 days).

In addition to the pH effect, temperature also has an influence on the type of the compounds obtained. Increasing the temperature from room temperature to 140 °C resulted in formation of two heptanuclear cationic [M₇(heb)₆(μ₃-OCH₃)₆]²⁺ clusters 4 and 5 when the pH was adjusted with triethylamine to 7.5. In comparison with 3, which was synthesized at room temperature, the findings regarding 4 and 5 show that the solvothermal temperature also plays an important role in formation of different clusters (e.g., 1–5). According to the above discussion and considering the comparison of 1 and 2 with 3, we can conclude that a lower pH (5.5) is necessary for coordination of azide ions whereas a higher pH (7.5) is needed for the presence of CH₃O⁻ anions in the solution, which can then coordinate as μ₃-OCH₃ species in 3. At the same time, comparing 3 and 4 with 5, we can conclude that lower temperatures (room temperature) and higher temperatures (140 °C) may result in formation of lower and higher nuclear complexes, respectively. This is most likely because of an increase in the degree of condensation of metal polyhedra and the ligand coordination ability when the reaction temperature is increased.^6e,7f,24

Furthermore, it was decided to investigate the influence of the bulky ethyl group on the formation of such self-assembly systems. As such, 2-hydroxy-3-methoxy-benzaldehyde (Hhmb) was used instead of 2-hydroxy-3-ethoxy-benzaldehyde under the same experimental conditions as 1 and 2. The two compounds obtained, which result in formation of two heterometallic [M₂Na₂(μ_1,1,1-N₃)₂(hmb)₄(CH₃CN)₂]·(CH₃CN)₂ (M = Ni (a),²⁵ M = Co (b),²⁶ Fig. S4†) clusters with two face-sharing cubes (each with one vertex missing), were different from those expected (HN(C₂H₅)₃·[M₄Na₃(hmb)₆(N₃)₆]). This is most likely because Hhmb and Hheb present different coordination abilities, resulting presumably from the steric hindrance of the RO groups, even if the phenolic hydroxyl, aldehyde, and RO-groups in the coordination sites of Hhmb and Hheb are similar.

Magnetic properties

The magnetic susceptibilities of 1–5 were measured from crushed single crystalline samples (the phase purities of 1–5 have been checked by PXRD patterns, see Fig. S5†), and variable-temperature direct-current (dc) magnetic susceptibility data were collected for 1–5 in the temperature range of 2–300 K under an applied field of 1000 Oe.

Magnetic properties of 1 and 4

For complexes 1 and 4, spin–orbital coupling of Ni(II) ions gives rise to a value of the χ_MT product of 5.84 cm³ K mol⁻¹ (1) and 9.13 cm³ K mol⁻¹ (4) at room temperature (Fig. 4). This behavior suggests an orbital contribution of the distorted octahedral Ni(II) ions. For 1, this value is higher than the calculated spin-only value of 4.4 cm³ K mol⁻¹ from four non-interaction high-spin Ni(II) ions assuming g = 2.2. The observed value of 1 is also slightly higher than those obtained for [Ni₄(ROH)₄L₄] (H₂L = salicylidene-2-ethanolamine; R = Me or Et) (∼5.1 cm³ K mol⁻¹)²⁷ and [Ni₄(μ₃-OMe) (MeOH)₄L₄] (H₂L is 2-hydroxy-3-methoxybenzaldehyde) (∼5.45 cm³ K mol⁻¹),^5a but smaller than for [HN(C₂H₅)₃]₈·[Ni₄(dchaa)₄(N₃)₄]₂ (dchaa is the anion of 3,5-dichloro-2-hydroxy-benzylaminoacetic acid) (∼6.54 cm³ K mol⁻¹).^22b The χ_MT value of 1 at room temperature lies in the range of other tetranuclear nickel clusters, which have χ_MT values between 5.1 and 6.61 cm³ K mol⁻¹.^5a,22b,28,29 Upon decreasing T, the χ_MT products of 1 and 4 gradually increase to maximum values of 14.67 and 10.13 cm³ K mol⁻¹ at 35 K, and then smoothly fall to 5.7 and 7.47 cm³ K mol⁻¹ at 2 K, respectively. Similar magnetic behaviors were observed for [Ni₄(ROH)₄L₄],²⁷ [Ni₄(μ₃-OMe)(MeOH)₄L₄],^5a [Ni₄(η¹,μ₃-N₃)₄(dbm)₄(EtOH)₄]·2C₇H₈ (Hdbm is dibenzoylmethane),²⁸ and [Ni₇(mmimp)₆(CH₃O)₆]·(X)₂.^17c The sudden decrease of χ_MT is assigned to zero-field splitting in the ground state, Zeeman effects, or intercluster antiferromagnetic interactions at low temperatures.

Fig. 4 Plots of χ_MT and χ_M vs. T measured in a 1000 Oe field for 1 (a) and 4 (b).

The temperature dependence of the reciprocal susceptibility χ_M⁻¹ above 50 K follows the Curie–Weiss law [χ = C/(T − θ)] with Weiss and Curie constants of 39.34 (1) K and 5.18 (1) cm³ K mol⁻¹ for 1 and 5.18 (1) K and 9.01 (1) cm³ K mol⁻¹ for 4, respectively (Fig. S6a and d†). The larger positive Weiss constants suggest an intramolecular ferromagnetic interaction between adjacent Ni(II) ions through the μ₃-N₃ and μ₃-O bridges in 1 and 4. For 1, the observation of a maximum of χ_MT at 35 K is indicative of an S = 4 ground state (with g = 2.25, χ_MT = 0.125g²S_T(S_T + 1) ≈ 12.66 cm³ K mol⁻¹). As such, this pattern is compatible with moderate ferromagnetic coupling.²⁹ Further evidence of ferromagnetic coupling between Ni(II) ions was observed in the variable-field magnetization curves plotted in Fig. S7a and d.† At low fields from 0 to 10 000 Oe, the magnetization of 1 and 4 sharply increases. For 1, above 10 000 Oe, the magnetization slowly increases and saturates at 8.85 Nμ_B when a field of 50 000 Oe is achieved, which is consistent with M = gS_T = 2.22 × 4 = 8.88 Nμ_B. For 4, above 10 000 Oe, it slowly increases but does not saturate at 50 000 Oe. Finally, AC susceptibility measurements were carried out in the 2–10 K range at frequencies of 10 Hz, 100 Hz, 300 Hz, and 997 Hz for 1 and 100, 997 Hz for 4 (Fig. S8a and d†). The results of these measurements confirmed that 1 and 4 do not behave as SMMs, which is corroborated by no out-of-phase ac signals being observed above 2 K.

Magnetic properties of 2, 3, and 5

As seen for 1, the complexes 2, 3, and 5 show that, at room temperature, the spin–orbital coupling of the Co(II) ions gives rise to χ_MT products of 12.53, 9.15, and 31.85 cm³ K mol⁻¹ for 2, 3, and 5, respectively (Fig. 5). For 2 and 3, the obtained values are much higher than the calculated spin-only value of 7.5 cm³ K mol⁻¹ from four non-interacting high-spin Co(II) ions, assuming g = 2.0.^5a,30 On the other hand, 5 shows that the χ_MT value is much higher than the calculated spin-only value (13.1 cm³ K mol⁻¹) of seven high-spin non-interacting Co(II) ions with the assumption that g is equal to 2.0.^21h These findings can be explained by the orbital contribution to the magnetic moment of Co(II). For 2, with decreasing T, χ_MT gradually rises to a local maximum of 20.08 cm³ K mol⁻¹ at 14 K before falling to 8.38 cm³ K mol⁻¹ at 2 K. The characteristic pattern often observed for Co(II) complexes because of a strong orbital contribution, consisting of a significant decrease in the value of χ_MT as the temperature is lowered, is not seen for 2.³¹ This can be explained as follows: (i) the orbital contribution is partially quenched because of the distortions from the typical octahedral symmetry at the Co(II) centers, and (ii) there is an increase in χ_MT below 37 K because of the ferromagnetic exchange interactions between the Co(II) centers. This is consistent with similar magnetic behavior that was observed for a Co₁₂ wheel.³² For 3, the χ_MT value slowly falls off on cooling to a value of 8.5 cm³ K mol⁻¹ at 60 K, after which it rapidly decreases to 4.23 cm³ K mol⁻¹ when a temperature of 2 K is achieved. This pattern most likely indicates the occurrence of a relatively weak antiferromagnetic intra-cluster interaction between the four Co(II) ions. For 5, the χ_MT value decreases gradually and reaches a minimum of 30.05 cm³ K mol⁻¹ at 40 K (Fig. 5). In the range of 300–40 K, the magnetic properties of 5 mainly exhibit single-ion behavior of the Co(II) ion. Below 40 K, it can be suggested that the pattern observed (a slight increase of the χ_MT value up to a maximum of 31.37 cm³ K mol⁻¹ at 14 K, and then a sharp decrease to 9.87 cm³ K mol⁻¹ at 2 K) is the consequence of ferromagnetic coupling between the Co(II) ions offsetting the effect of the spin–orbital coupling, and thus compensating for the decrease in the χ_MT value. This is consistent with similar magnetic behavior observed for [Co₇(bzp)₆(N₃)₉(CH₃O)₃](ClO₄)₂·2H₂O,³³ [Co^II₄Co^III₃(HL)₆(NO₃)₃(H₂O)₃]²⁺ {H₃L = H₂NC(CH₂OH)₃},^21c Co₁₂ wheel,³² and [Co₇(immp)₆(CH₃O)₆](ClO₄)₂ (immp is 2-iminomethyl-6-methoxy-phenolic anion).^2a

Fig. 5 Plots of χ_MT and χ_M vs. T measured in a 1000 Oe field for 2 (a), 3 (b), and 5 (c). The solid lines represent the best fits of data between 300 and 50 K as described in the text for 3.

The temperature dependence of the reciprocal susceptibility χ_M⁻¹ above 50 K follows the Curie–Weiss law [χ = C/(T − θ)] with Weiss and Curie constants of 5.00 (1) K and 12.56 (1) cm³ K mol⁻¹, −4.29 (1) K and 9.33 (1) cm³ K mol⁻¹, and −1.98 K and 32.39 cm³ K mol⁻¹ for 2, 3, and 5, respectively (Fig. S6b, c and e†). Compared with 3 and 5, the larger positive Weiss constant observed for 2 also suggests an intramolecular ferromagnetic interaction between adjacent Co(II) ions through the μ₃-N₃ bridges, whereas the maximum of χ_MT at 14 K is indicative of an S = 6 ground state (with g = 2.00, χ_MT = 0.125g²S_T(S_T +1) ≈ 21 cm³ K mol⁻¹). Similarly, the larger negative Weiss constant observed for 3 compared with 1 and 5 suggests dominant intramolecular antiferromagnetic interactions between adjacent Co(II) ions through the μ₃-O bridges. Finally, the contribution of a spin–orbital interaction discussed earlier has also an effect on the ferromagnetic exchange occurring in 5 (it is diminished). As a result, a smaller negative value of the Weiss constant was obtained. Further evidence of ferromagnetic coupling between Co(II) ions in 2, 3, and 5 was observed in the variable-field magnetization curves plotted in Fig. S7b, c and e.† At low fields from 0 to 10 000 Oe, the magnetization sharply increases, whereas above 10 000 Oe, the magnetization slowly increases but does not saturate at 50 000 Oe for 2 and 3. This is different from what was observed for 5 as above 10 000 Oe the magnetization slowly increases and then saturates to 23.40 Nμ_B at 50 000 Oe, which is consistent with M = gS_T = 2.22 × 21/2 = 23.31 Nμ_B. Furthermore, the AC susceptibility measurements that were carried out in the 2–10 K range at frequencies of 100 Hz and 997 Hz (for 2 and 5), and 10, 100, 300, 600, 997 Hz for 3 (Fig. S8b, c and e†) suggested that 2, 3, and 5 do not behave as SMMs as no out-of-phase ac signals were detected above 2 K.

According to the cubic structure of the cluster, the magnetic exchange between Co(II) ions in the core of Co₄O₄ observed for 3 can be assessed using Van Vleck's equation (eqn (1)). This is based on Kambe's method, which requires the use of the isotropic spin Hamiltonian and where J₁ is the coupling constant between the Co(II) ions.

The best fitting in the temperature range from 300 to 50 K gave g = 2.21 and J₁ = −0.87 (1) cm ⁻¹ with R = 1.9 × 10⁻⁴. The negative coupling constant indicates a relatively weak antiferromagnetic intracube Co(II) interaction, which is different from those observed in several related octahedral Co(II) complexes with similar cuboidal cores.^5a,34

Conclusions

Five new polynuclear clusters have been synthesized by modulating the pH and reaction temperature conditions. The results show that these two factors play a key role in the structural control of the self-assembly process. Additionally, magnetic studies indicate that 1, 2, 4, and 5 display dominant ferromagnetic intracluster interactions, whereas 3 displays an antiferromagnetic interaction between Co(II) ions. Subsequent works will focus on construction of novel polymers using 2-hydroxy-3-ethoxy-benzaldehyde as a basic building unit.

Acknowledgements

This work is financially supported by the National Nature Science Foundation of China (no. 21161006), Program for Excellent Talents in Guangxi Higher Education Institutions (Gui Jiao Ren [2012]41). G.M. thanks the Henry Dreyfus Teacher-Scholar Award for financial support.

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26. Complex b can be prepared in a similar way than a except that Ni(ClO₄)₂·6H₂O was replaced by Co(ClO₄)₂·6H₂O. Red crystals of b were collected by filtration, washed with methanol and dried. Phase pure crystals of 2 were obtained by manual separation (yield: 243 mg, ca. 47.80% based on Hhmb ligand). Crystal data for a heterometallic tetranuclear cluster a: C₄₀H₄₀Co₂N₁₀Na₂O₁₂, M_r = 1016.66 g mol⁻¹, monoclinic, P21/C, a = 13.345 (2), b = 12.088 (2), c = 14.716 (2) Å, β = 106.13 (1)°, V = 2280.6 (5) ų, θ = 28.99°, λ = 0.71073 Å, T = 293 K, μ(Mo Kα) = 0.818 mm⁻¹. 7824 reflections were collected of which 3907 were unique (R_int = 0.1519). The structure was solved by direct methods and refined by full-matrix least squares of F², R₁ = 0.0939(I > 2σ(I)), wR₂ = 0.2689 (all data). Max/min residual electron density 0.838/−0.517 e ų and the structure of (a) see Fig. S4.†.
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Footnote

† Electronic supplementary information (ESI) available: The IR of 1–5. Packing drawing of 3–5. The structures of a and b. The PXRDs of 1–5. The plots of χ_M⁻¹ vs. T of 1–5. The plots of M–H of 1–5. The plots of χ′′ and χ′ vs. T of 1–5. CCDC 982504–982510. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra09687h

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