Rare Nuclearities in Ni(II) Cluster Chemistry: An Unprecedented {Ni₁₂} Nanosized Cage from the Use of N-Naphthalidene-2-Amino-5-Chlorobenzoic Acid

Abstract

The self-assembly reaction between NiI₂, benzoic acid (PhCO₂H) and the Schiff base chelate, N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH₂), in the presence of the organic base triethylamine (NEt₃), has resulted in the isolation and the structural, spectroscopic, and physicochemical characterization of the dodecanuclear [Ni₁₂I₂(OH)₆(O₂CPh)₅(nacb)₅(H₂O)₄(MeCN)₄]I (1) cluster compound in ~30% yield. Complex 1 has a cage-like conformation, comprising twelve distorted, octahedral Ni^II ions that are bridged by five μ₃-OH⁻, one μ-OH⁻, an I⁻ in 55% occupancy, five PhCO₂⁻ groups (under the η¹:η¹:μ, η¹:η²:μ₃ and η²:η²:μ₄ modes), and the naphthoxido and carboxylato O-atoms of five doubly deprotonated nacb²⁻ groups. The overall {Ni₁₂} cluster exhibits a nanosized structure with a diameter of ~2.5 nm and its metallic core can be conveniently described as a series of nine edge- or vertex-sharing {Ni₃} triangular subunits. Complex 1 is the highest nuclearity coordination compound bearing the nacbH₂ chelate, and a rare example of polynuclear Ni^II complex containing coordinating I⁻ ions. Direct current (DC) magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the Ni^II ions, while photophysical studies of 1 in the solid-state showed a cyan-to-green centered emission at 520 nm, upon maximum excitation at 380 nm. The reported results demonstrate the rich coordination chemistry of the deprotonated nacb²⁻ chelate in the presence of Ni^II metal ions, and the ability of this ligand to adopt a variety of different bridging modes, thus fostering the formation of high-nuclearity molecules with rare, nanosized dimensions and interesting physical (i.e., magnetic and optical) properties.

1. Introduction

Polynuclear 3d-metal complexes (or 3d-metal clusters) remain one of the most attractive research fields in the cross-disciplinary areas of chemistry, physics, and materials science [1]. This is mainly due to the ability of these nanosized molecular species to exhibit very interesting magnetic, optical, biological, and catalytic properties, to name just a few [2]. From a structural perspective, the motifs of these cluster compounds often resemble the aesthetically beautiful structures of highly-symmetric inorganic solids, such as cubic and hexagonal structures, perovskites, brucites, and supertetrahedra, due to the presence of multiple bridging oxido and hydroxido groups [3]. In the molecular magnetism arena, ferromagnetically-coupled 3d-systems with a large ground state spin value, S, and an appreciable magnetic anisotropy of the Ising (or easy-axis) type can behave as single-molecule magnets (SMMs) [4]. SMMs exhibit slow magnetization relaxation over and/or through an anisotropy barrier and they represent a molecular or “bottom-up” approach to nanoscale magnetism with potential applications in the fields of information storage, molecular electronics and spintronics [5].

The first and most well-studied family of SMMs is the mixed-valence [Mn^III₈Mn^IV₄O₁₂(O₂CR)₁₆(X)₄], where RCO₂⁻ and X are various carboxylate bridging groups and terminal solvate molecules, respectively [6]. The SMM behavior of these compounds originates from the combined S = 10 spin ground state and the enhanced magnetic anisotropy resulting from the parallel alignment of the Mn^III Jahn-Teller axes. In polynuclear Ni^II cluster chemistry, the number of SMMs is substantially smaller [7] and only a few of them show slow relaxation of magnetization in the absence of an external DC field, as well as magnetization hysteresis, the diagnostic property of a magnet. This is predominately due to the small zero-field splitting parameter, D, that a polynuclear Ni^II complex often exhibits when the Ni^II atoms adopt the favorable octahedral coordination geometry. Exceptional examples of Ni^II cluster-based SMMs are the ferromagnetic [Ni₁₂(chp)₁₂(O₂CMe)₁₂(THF)₆(H₂O)₆] with a ring-like topology [8], and the family of [Ni₄(hmp)₄(ROH)₄Cl₄] complexes with a distorted cubane [Ni₄(OR)₄]⁴⁺ core [9], where chpH and hmpH are the organic chelates chloro-2-hydroxypyridine and 2-hydroxymethylpyridine, respectively, and ROH are various terminally-bound alcohol solvates.

It becomes apparent that the choice of the organic chelating/bridging ligand is of fundamental importance in the self-assembly synthesis of high-nuclearity Ni^II complexes with high-spin values and interesting magnetic dynamics. To this end, we have recently started a research program aiming at the exploration of Schiff base chelates, which are based on the tridentate N-salicylidene-o-aminophenol (saphH₂, Scheme 1) scaffold, in 3d-metal cluster chemistry as a means of obtaining nanosized molecular materials, primarily those with interesting magnetic properties [10]. To increase the coordination and bridging potential of the organic chelate, we initially turned our attention to the tetradentate ligand N-salicylidene-2-amino-5-chlorobenzoic acid (sacbH₂, Scheme 1); this has led to the structurally impressive {Ni₁₈} and {Ni₂₆} clusters [11], and a {Dy₂} SMM with a large energy barrier for magnetization reversal [12]. A reasonable leap forward would be the replacement of the phenyl ring of the N-salicylidene moiety with a naphthalene one. The resulting ligand N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH₂, Scheme 1) shows the following salient features: (a) it is still a tetradentate like sacbH_2, but is undoubtedly more rigid and sterically demanding than sacbH₂, and (b) it includes the naphthalene substituent, a well-known fluorescent group [13], which could open new prospects in the emission properties of Ni^II coordination compounds with O- and N-donor atoms. Both features presage the synthesis of new cluster compounds with potentially interesting magnetic and emission properties. Indeed, the initial employment of nacbH₂ in Ni^II chemistry has afforded a series of {Ni₅} and {Ni₆} clusters with diverse magnetic and optical properties, but with limited bridging affinity for nacb²⁻ [14]. In this work, we have unveiled the bridging capacity of nacb²⁻ in conjunction with ancillary bridging benzoate groups. We herein report an unprecedented {Ni₁₂} cluster compound with the highest nuclearity in metal cluster chemistry of nacbH₂, and one of the rarest nuclearities in Ni^II cluster chemistry.

2. Results and Discussion

2.1. Synthetic Comments

The general reaction system NiX₂/nacbH₂, where X⁻ are various anions with either a strong (i.e., NO₃⁻, β-diketonates and pseudohalides) or weak coordinating (Cl⁻, Br⁻ and ClO₄⁻) ability, has been studied by us [15], and—in almost all the cases—small in nuclearity clusters (i.e., {Ni₅} and {Ni₆}) were isolated and structurally characterized [14]. These results have demonstrated the unpredictability of the nacbH₂ chelate toward the coordination with 3d-metal ions in solution and subsequently the stabilization of different cluster compounds in the solid-state. We have thus decided to introduce to the Ni^II/nacbH₂ system anions with very limited coordinating capacity, such as iodides (I⁻), in conjunction with organic anionic groups with a superior bridging ability, such as benzoates (PhCO₂⁻), in an attempt to harness a flexible ”synthetic blend” which would potentially lead to high-nuclearity Ni^II clusters. Indeed, the reaction between NiI₂, nacbH₂, PhCO₂H, and NEt₃ in a 2:1:1:3 molar ratio, in solvent acetonitrile (MeCN), afforded dark-green crystals of the dodecanuclear cluster compound [Ni₁₂I₂(OH)₆(O₂CPh)₅(nacb)₅(H₂O)₄(MeCN)₄]I (1) in 30% yield. The general formation of 1 is summarized by the following stoichiometric Equation (1).

12 NiI₂ + 5 nacbH₂ + 5 PhCO₂H + 21 NEt₃ + 10 H₂O + 4 MeCN → [Ni₁₂I₂(OH)₆(O₂CPh)₅(nacb)₅(H₂O)₄(MeCN)₄]I + 21 NHEt₃I

(1)

Under the context of chemical reactivity, several synthetic parameters were explored to either increase the yield of the isolated product 1 or alter the nuclearity of the {Ni₁₂} compound and subsequently isolate a new product. In particular, the employment of NiCl₂ or NiBr₂ in place of NiI₂ afforded the already reported (NHEt₃)₂[Ni₆(OH)₂(nacb)₆(H₂O)₄] [14], whereas the replacement of PhCO₂H by other carboxylic acids, such as MeCO₂H or EtCO₂H, led to green-colored microcrystalline products, of which we were unable to determine the crystal structures due to the very small size of the obtained crystallites. The presence of NEt₃ as an external base was also essential for the clean preparation of 1, providing a proton acceptor to facilitate the complete deprotonation of nacbH₂ and PhCO₂H, and fostering the metal-assisted deprotonation of H₂O molecules in solution to the coordinating OH⁻ groups (vide infra). Various similar reactions in other organic solvents (i.e., alcohols (ROH), CH₂Cl₂ and mixtures of MeCN/ROH) and/or external bases (i.e., trimethylamine (NMe₃), tripropylamine (NPr₃), diethylamine (Et₂NH), dimethylamine (Me₂NH) and tetramethylammonium hydroxide (Me₄NOH) yielded amorphous solids that we were unable to recrystallize and further characterize. Finally, it is worth mentioning that the presence of iodide ions, either as bound groups or counterions, or both, in Ni^II coordination chemistry is limited to a handful of previously reported dinuclear or trinuclear compounds [16,17,18,19].

2.2. Description of Structure

A partially labeled representation of the cation of complex 1 is shown in Figure 1. The positively charged cluster cation [Ni₁₂I₂(OH)₆(O₂CPh)₅(nacb)₅(H₂O)₄(MeCN)₄]⁺ is counterbalanced by an I⁻ anion in the crystal lattice of 1. The I⁻ counterion (I7) is closely held with the {Ni₁₂} cluster through H-bonding interactions with three of the bridging OH⁻ groups; these separations are: O1∙∙∙I7 = 3.226(3) Å, O2∙∙∙I7 = 3.425(1) Å and O3∙∙∙I7 = 3.222(3) Å. Selected interatomic distances and angles of 1 are listed in

Table 1. Selected interatomic distances (Å) and angles (°) for complex 1.

Bond	Distances	Bond	Distances	Bond	Angles	Bond	Angles
Ni1–O1	1.994(5)	Ni7–O8	2.017(5)	Ni3–I1–Ni2	78.6(4)	Ni2–O8–Ni6	116.0(2)
Ni1–O26	2.020(6)	Ni7–O9	2.017(5)	Ni3–I1–Ni1	81.6(4)	Ni7–O8–Ni6	99.2(2)
Ni1–O15	2.087(5)	Ni7–O23	2.114(5)	Ni2–I1–Ni1	78.4(4)	Ni8–O9–Ni7	126.9(3)
Ni1–O3	2.151(6)	Ni7–O17	2.128(5)	Ni1–O1–Ni10	96.6(2)	Ni8–O9–Ni11	112.0(3)
Ni1–I2	2.484(2)	Ni7–O12	2.137(5)	Ni1–O1–Ni2	112.8(2)	Ni7–O9–Ni11	102.8(2)
Ni1–I1	2.643(1)	Ni7–O14	2.176(5)	Ni10–O1–Ni2	104.7(2)	Ni9–O10–Ni8	98.8(2)
Ni2–O1	2.005(5)	Ni8–O21	2.005(6)	Ni3–O2–Ni5	113.3(2)	Ni9–O11–Ni8	99.2(2)
Ni2–O8	2.007(5)	Ni8–O9	2.006(5)	Ni3–O2–Ni2	107.2(2)	Ni9–O12–Ni7	113.7(2)
Ni2–O27	2.014(5)	Ni8–O19	2.031(6)	Ni5–O2–Ni2	120.9(2)	Ni10–O14–Ni7	118.6(2)
Ni2–O2	2.057(5)	Ni8–O11	2.113(6)	Ni3–O3–Ni1	103.8(2)	Ni10–O15–Ni1	92.9(2)
Ni2–O16	2.134(5)	Ni8–O10	2.138(6)	Ni3–O4–Ni12	101.5(2)	Ni2–O16–Ni10	95.3(2)
Ni2–I1	2.628(1)	Ni8–I4	2.479(5)	Ni3–O4–Ni4	97.5(2)	Ni7–O17–Ni11	91.2(2)
Ni3–O2	2.001(5)	Ni9–N1	1.988(7)	Ni12–O4–Ni4	99.4(2)	Ni6–O23–Ni7	97.4(2)
Ni3–O4	2.040(6)	Ni9–O10	2.025(6)	Ni3–O5–Ni4	94.9(2)	Ni6–O24–Ni5	100.7(2)
Ni3–O5	2.061(5)	Ni9–O11	2.039(6)	Ni6–O7–Ni5	94.9(2)	Ni3–O29–Ni12	99.6(2)
Ni3–O29	2.063(5)	Ni9–N2	2.092(8)	Ni2–O8–Ni7	119.0(2)	Ni12–O30–Ni4	99.2(2)
Ni3–O3	2.150(5)	Ni9–O12	2.117(5)
Ni3–I1	2.532(1)	Ni9–I5	2.367(9)
Ni4–O31	1.996(6)	Ni10–N3	1.994(6)
Ni4–N9	2.016(7)	Ni10–O1	2.000(5)
Ni4–O4	2.049(5)	Ni10–O15	2.026(5)
Ni4–O30	2.069(6)	Ni10–O25	2.039(6)
Ni4–O5	2.113(5)	Ni10–O14	2.126(5)
Ni4–I3	2.498(5)	Ni10–O16	2.158(5)
Ni5–O2	2.011(5)	Ni11–O20	2.015(6)
Ni5–O24	2.063(5)	Ni11–O18	2.016(6)
Ni5–O28	2.064(6)	Ni11–O9	2.019(6)
Ni5–O6	2.066(5)	Ni11–O22	2.050(6)
Ni5–N8	2.093(7)	Ni11–N5	2.115(9)
Ni5–O7	2.126(5)	Ni11–O17	2.283(5)
Ni6–O24	1.997(5)	Ni12–N6	2.024(7)
Ni6–O23	2.002(5)	Ni12–O30	2.030(6)
Ni6–N4	2.013(6)	Ni12–O4	2.045(6)
Ni6–O8	2.044(5)	Ni12–N7	2.076(8)
Ni6–O13	2.052(5)	Ni12–O29	2.081(5)
Ni6–O7	2.120(5)	Ni12–I6	2.482(4)

. Bond valence sum (BVS) calculations for the inorganic bridging O-atoms with 100% occupancies gave values of: 1.23 (for O1), 1.16 (for O2 and O8), 1.10 (for O4), and 1.18 (for O9), in excellent agreement with their assignment as OH⁻ groups. Oxygen BVS values in the ~1.7–2.0, ~1.0–1.2, and ~0.2–0.4 ranges are indicative of non-, single- and double-protonation, respectively [20].

Complex 1 is a closed cage-like cluster consisting of 12 Ni^II ions that are bridged by five μ₃-OH⁻, one μ-OH⁻, an I⁻ in 55% occupancy, and the naphthoxido and carboxylato O-atoms of five double-deprotonated nacb²⁻ groups. The latter are arranged into three classes (Figure 2), with all of them acting as tridentate chelates to a Ni^II ion, and additionally bridging two (η¹:η¹:η²:η¹:μ₃ mode), three (η²:η¹:η²:η¹:μ₄ mode) and four (η²:η¹:η²:η²:μ₅ mode) metal ions. The variety of binding modes of nacb²⁻ in complex 1 clearly emphasizes the coordination affinity of this Schiff base ligand with Ni^II ions, and its ability to stabilize high-nuclearity 3d-metal clusters with unprecedented structural motifs and nanosized dimensions. To this end, the space-filling plot (Figure 3) shows that 1 has a nearly ”bowl”-shaped conformation with the longest intramolecular C···C distance being ~25 Å, excluding the H atoms. The shortest Ni···Ni distance between neighboring {Ni₁₂} clusters in the crystal is 12.246(2) Å, thus confirming the good separation of the cluster compounds due to the bulky naphthalene substituents of the nacb²⁻ ligands.

Additional bridging about the twelve Ni^II ions is provided by five PhCO₂⁻ groups, which are arranged into three classes; three of them are bridging under the η¹:η¹:μ mode, one is acting as an η¹:η²:μ₃ ligand, and the last one adopts the rare η²:η²:μ₄ mode. Thus, the resulting core is [Ni₁₂(μ₃-OH)₅(μ₃-Ι/H₂O)(μ-OR)₁₅]³⁺ (Figure 4), and peripheral ligation about this core is further provided by four terminally bound MeCN molecules (on Ni5, Ni9, Ni11, and Ni12) and a total of five I⁻/H₂O group combinations (on Ni1, Ni4, Ni8, Ni9, and Ni12). All Ni^II atoms are six-coordinate with distorted octahedral geometries. The metallic core of 1 can be conveniently described as a series of nine edge- or vertex-sharing {Ni₃} triangular subunits (Figure 5), which are held together by μ₃-OH⁻ and μ-OR⁻ groups. An alternative description of the {Ni₁₂} metal arrangement is that of a central {Ni₇} subunit possessing a distorted disk-like topology [Ni(1,2,3,5,6,7,10)], which is attached to a {Ni₃} triangular [Ni(3,4,12)] and a {Ni₄} rhombus-shaped [Ni(7,8,9,11)] subunits by sharing the common Ni3 and Ni7 vertices, respectively. Finally, the nuclearity of complex 1 is the largest reported to date of a metal cluster bearing nacb²⁻ chelate, and it joins a relatively rare family of {Ni₁₂} Werner-type compounds with a cage-like conformation [21,22,23,24].

2.3. Solid-State Magnetic Susceptibility Studies

Variable-temperature (2.0–300 K range), direct-current (DC) magnetic susceptibility measurements were performed on a freshly-prepared microcrystalline solid of 1 under a weak DC field of 0.03 T to avoid saturation effects. The data are shown as χ_MT versus T plot in Figure 6. The value of the χ_ΜT product at 300 K is 10.40 cm³ Kmol⁻¹, slightly lower than the value of 12 cm³ Kmol⁻¹ (calculated with g = 2.0) expected for twelve non-interacting, high-spin Ni^II (S = 1) atoms. Upon cooling, the χ_ΜT product continuously decreases down to a value of 3.06 cm³ Kmol⁻¹ at 2 K. A slightly different curvature of the plot is observed below ~5 K, and this likely due to the onset of zero-field splitting, intermolecular antiferromagnetic interactions between the {Ni₁₂} clusters, and/or Zeeman effects [11,15]. The overall shape of the χ_MT versus T plot is suggestive of the presence of predominant antiferromagnetic exchange interactions between the metal centers, as frequently observed in many high-nuclearity and low-symmetry Ni^II cage-like clusters where many different magnetic exchange pathways are in effect [11]. To this end, a fit of the experimental data to a theoretical model (H = −2J_ijŜ_i·Ŝ_j convention) was not feasible. Undoubtedly, 1 possesses a small ground-state spin value, with the χ_ΜT value at 2 K being consistent with an S ~ 2 ground state (for g = 2). The antiferromagnetic response of the {Ni₁₂} compound can be tentatively assigned to the majority of obtuse Ni–O–Ni bond angles (close to or larger than 100°) and the presence of {Ni₃} triangular subunits, which are prone to spin frustration effects [25]. Magnetization (M) versus field (H) measurements (Figure 6, inset) at 2 K show a continuous increase of M as the field increases, reaching a non-saturated value of 6.0 Nμ_B at 5 T; this is likely due to the presence of low-lying excited states, as reported previously for other high-nuclearity Ni^II complexes [26]. As a result, attempts to fit the reduced magnetization data assuming that only the ground state is populated were very poor.

2.4. Solid-State Emission Studies

The photophysical properties of complex 1 were carried out in the solid-state and at room temperature due to its structural instability in solution. This was confirmed by performing electrospray ionization mass spectrometry (ESI-MS) studies in various solvent media (Figure S1). The optical response of the free-ligand nacbH₂ has been reported by us in a previous work [15]. Briefly, it was shown that nacbH₂ is a promising “antenna” group for the promotion of energy transfer effects. Upon maximum excitation at ~350 nm, nacbH₂ exhibits a strong emission in the visible range with two clear maxima at ~390 and 410 nm, and a weak shoulder at ~480 nm. Complex 1 shows an interesting photophysical response, given its large nuclearity, the presence of many different binding groups, and the possible quenching effects from the coordinating O- and N-atoms. The dodecanuclear compound 1 exhibits a cyan-to-green centered emission at 520 nm, upon maximum excitation at 380 nm (Figure 7). The red-shifted emission of 1 with respect to the free nacbH₂ can be tentatively assigned to the coordination of the deprotonated nacb²⁻ ligands with the metal ions and/or the presence of additional binding groups with emission efficiency, such as benzoates, which could affect the charge transfer process and resulting emission [14,15].

In general, the loss of energy due to vibrations is reduced as a result of the coordination of a ligand to a metal center; this binding enhances the organic ligand’s rigidity [27]. In addition, the usually observed optical quenching effects from the paramagnetic metal ions can be prevented using organic fluorescent groups, such as the naphthalene, anthracene, and phenanthrene substituents [28]. Red-shifted emissions are commonly observed in most fluorescent compounds in the solid-state, likely due to the π–π stacking interactions of the aromatic rings [29]. Due to the structural complexity of 1, as a result of the many different ligands present and the number of metal ions, among other electronic and steric perturbations, an in-depth analysis of the photophysical properties of 1 would be unrealistic.

3. Materials and Methods

3.1. Materials, Physical and Spectroscopic Measurements

All manipulations were performed under aerobic conditions using materials (reagent grade) and solvents as received unless otherwise noted. The Schiff base ligand nacbH₂ was prepared, purified, and characterized as described elsewhere [14,15]. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 Series II Analyzer (Foster City, CA, USA). Infrared spectra were recorded in the solid state on a Bruker’s FT-IR spectrometer (ALPHA’s Platinum ATR single reflection) (Billerica, MA, USA) in the 4000–400 cm⁻¹ range. Excitation and emission spectra were recorded in the solid state at room temperature conditions using a Cary Eclipse spectrofluorometer. The repeatability and reproducibility of the emission was verified by recording the emission spectra of the material three times in two different days using the same scan rate and the same excitation and emission monochromator slits. Variable-temperature magnetic susceptibility studies were performed on a MPMS5 Quantum Design magnetometer equipped with a 5 T magnet and operating in the 2–300 K range. The microcrystalline sample was embedded in solid eicosane to prevent torquing. Diamagnetic corrections were applied to the observed paramagnetic susceptibility using Pascal’s constants [30].

3.2. Synthesis of [Ni₁₂I₂(OH)₆(O₂CPh)₅(nacb)₅(H₂O)₄(MeCN)₄]I (1)

To a stirred, orange suspension of nacbH₂ (0.07 g, 0.20 mmol) in MeCN (20 mL) was added NEt₃ (84 μL, 0.60 mmol). Solids NiI₂ (0.13 g, 0.40 mmol) and PhCO₂H (0.03 g, 0.20 mmol) were added to the resulting yellow solution, and a noticeable color change to a dark-green solution was observed over the period of 1 h, under a continuous magnetic stirring. The final solution was filtered, and the filtrate was carefully layered with Et₂O (40 mL). After 20 days, X-ray quality dark-green plate-like crystals of 1 were formed, and these were collected by filtration, washed with cold MeCN (2 × 3 mL) and Et₂O (2 × 3 mL), and dried in air. The yield was 30% (based on the ligand available). The air-dried solid was found to be slightly hygroscopic and it was satisfactorily analyzed as 1∙3H₂O. Anal. calc. for C₁₃₃H₁₀₇N₉O₃₈Ni₁₂Cl₅I₃ (found values in parentheses): C 43.16% (43.31%), H 2.91% (3.06%), N 3.41% (3.28%). Selected IR data (ATR): ν = 3300 (mb), 1598 (s), 1575 (s), 1535 (s), 1503 (w), 1472 (m), 1450 (m), 1427 (m), 1406 (s), 1382 (w), 1337 (s), 1298 (m), 1249 (m), 1217 (m), 1180 (m), 1156 (m), 1113 (m), 1088 (m), 985 (m), 961 (w), 885 (w), 852 (m), 828 (m), 743 (s), 718 (s), 670 (w), 630 (w), 555 (w), 451 (m).

3.3. Single-Crystal X-ray Crystallography

A suitable single-crystal of complex 1 was selected and mounted on the respective cryoloop using adequate inert oil [31]. Diffraction data were collected on a Bruker X8 Kappa APEX II Charge-Coupled Device (CCD) area-detector diffractometer (Billerica, MA, USA) controlled by the APEX2 software [32] package (Mo Kα graphite-monochromated radiation, λ = 0.71073 Å), and equipped with an Oxford Cryosystems Series 700 cryostream, monitored remotely with the software interface Cryopad [33]. Images were processed with the software SAINT+ [34], and the absorption effects were corrected by the multi-scan method implemented in SADABS [35]. The structure was solved using the algorithm implemented in SHELXT-2014 [36,37], and refined by successive full-matrix least-squares cycles on F² using the latest SHELXL-v.2014 [36,38]. The non-hydrogen atoms of the crystal structure were successfully refined using anisotropic displacement parameters, and H-atoms bonded to carbon of the ligands were placed at their idealized positions using appropriate HFIX instructions in SHELXL. All these atoms were included in subsequent refinement cycles in riding-motion approximation with isotropic thermal displacement parameters (U_iso) fixed at 1.2 or 1.5 × U_eq of the relative atom. The refinement model revealed the presence of two coordinated iodide (I⁻) ions and four coordinated H₂O molecules partially disordered over six coordinative positions with distinct complementary occupancies (I1/O1W = 0.55/0.45; I2/O2W = 0.35/0.65; I3/O3W = 0.30/0.70; I4/O4W = 0.25/0.75; I5/O5W = 0.25/0.75, and I6/O6W = 0.30/0.70). In addition, the non-coordinated I⁻ ion is disordered over two positions with occupancies of 0.65 and 0.35. As a result of the severely disordered structure of 1, the H-atoms of the coordinated water molecules and hydroxido groups were not included in the refined model, but they were considered in the final molecular formula of the compound.

Substantial electron density was found on the data of complex 1, most probably due to additional disordered solvate molecules occupying the spaces originated by the close packing of the cluster compound. Various efforts to properly locate, model, and refine these residues were unsuccessful, and the examination for the total potential solvent area using the software package PLATON [39] clearly confirmed the existence of cavities with potential solvent accessible void volume. Thus, the original data set was treated with the program SQUEEZE [40], which calculates the contribution of the smeared electron density in the lattice voids and adds this to the calculated structure factors from the structural model when refining against the hkl file. The programs used for molecular graphics were MERCURY [41] and DIAMOND [42]. Unit cell parameters, structure solution and refinement details for 1 are summarized in

Table 2. Crystallographic data for complex 1.

Parameter	1
Empirical formula	C133H101N9O35Ni12Cl5I3
FW/g mol−1	3647.69
Temperature/K	150(2)
Crystal type	Green plate
Crystal size/mm3	0.22 × 0.10 × 0.04
Crystal system	Triclinic
Space group	P-1
a/Å	19.422(2)
b/Å	22.654(3)
c/Å	25.321(3)
α/°	115.783(4)
β/°	92.992(5)
γ/°	109.118(5)
Volume/Å3	9443(2)
Z	2
ρcalc/g cm−3	1.283
μ/mm−1	1.786
F(000)	3644
θ range/°	3.65 to 25.03
Radiation	Mo Kα (λ = 0.71073)
Index ranges	−23 ≤ h ≤ 22 −22 ≤ k ≤ 26 −30 ≤ l ≤ 30
Reflections collected	134,933
Independent reflections	32,481 (Rint = 0.0476)
Goodness-of-fit on F2	1.040
Final R indexes [I ≥ 2σ(I)] a,b	R1 = 0.0872 wR2 = 0.2099
Final R indexes [all data]	R1 = 0.1221 wR2 = 0.2361
(Δρ)max,min/e Å−3	1.613 and −1.594

^a R₁ = ∑(||F_o| − |F_c||)/∑|F_o|. ^bwR₂ = [∑[w(F_o² − F_c²)²]/∑[w(F_o²)²]^1/2, w = 1/[σ²(F_o²) + (ap)² + bp], where p = [max(F_o², 0) + 2F_c²]/3.

. Further crystallographic details can be found in the corresponding CIF file provided in the ESI. Crystallographic data (excluding structure factors) for the structure reported in this work have been deposited to the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication number: CCDC-1988904. Copies of the data can be obtained online using https://summary.ccdc.cam.ac.uk/structure-summary-form.

4. Conclusions and Perspectives

In conclusion, we have herein reported the synthesis, and structural and physicochemical characterization, of a new dodecanuclear Ni^II cluster compound with an unprecedented structural motif and nanoscale dimensions, resulted from the successful employment of the nacbH₂/I⁻/PhCO₂⁻ ”ligand blend” in Ni^II chemistry. The N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH₂) ligand also contributed to the observation of unquenched optical emission from the {Ni₁₂} complex 1 in the solid-state, a very unusual phenomenon in high-nuclearity 3d-metal cluster chemistry with N-/O-donor atoms. Complex 1 is by far the highest nuclearity Ni^II cluster compound with coordinating I⁻ ions, also supported by ancillary chelating and bridging organic groups, thus presaging a new synthetic approach to nanoscale molecular materials with interesting structural motifs and physical properties. We are currently investigating the Ni^II/RCO₂⁻/nacbH₂ tertiary system as a means of obtaining higher-nuclearity, nanosized Ni^II clusters with interesting magneto-optical properties.