Rare Nuclearities in Ni(II) Cluster Chemistry: An Unprecedented {Ni 12 } Nanosized Cage from the Use of N -Naphthalidene-2-Amino-5-Chlorobenzoic Acid

: The self-assembly reaction between NiI 2 , benzoic acid (PhCO 2 H) and the Schiff base chelate, N -naphthalidene-2-amino-5-chlorobenzoic acid (nacbH 2 ), in the presence of the organic base triethylamine (NEt 3 ), has resulted in the isolation and the structural, spectroscopic, and physicochemical characterization of the dodecanuclear [Ni 12 I 2 (OH) 6 (O 2 CPh) 5 (nacb) 5 (H 2 O) 4 (MeCN) 4 ]I ( 1 )clustercompoundin~30%yield. Complex 1 hasacage-likeconformation, comprisingtwelvedistorted, octahedral Ni II ions that are bridged by five µ 3 -OH − , one µ -OH − , an I − in 55% occupancy, five PhCO 2 − groups (under the η 1 : η 1 : µ , η 1 : η 2 : µ 3 and η 2 : η 2 : µ 4 modes), and the naphthoxido and carboxylato O-atoms of five doubly deprotonated nacb 2 − groups. The overall {Ni 12 } 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 {Ni 3 } triangular subunits. Complex 1 is the highest nuclearity coordination compound bearing the nacbH 2 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 nacb 2 − chelate in the presence of Ni II metal ions, and the ability of this ligand to adopt a variety of di ﬀ erent bridging modes, thus fostering the formation of high-nuclearity molecules with rare, nanosized dimensions and interesting physical (i.e., magnetic and optical) properties.


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 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 III 8Mn IV 4O12(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 chpΗ 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-oaminophenol (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-2amino-5-chlorobenzoic acid (nacbH2, Scheme 1) shows the following salient features: (a) it is still a tetradentate like sacbH2, 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 nacb 2− [14]. In this work, we have unveiled the bridging capacity of nacb 2− 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.

Synthetic Comments
The general reaction system NiX 2 /nacbH 2 , where X − are various anions with either a strong (i.e., NO 3 − , β-diketonates and pseudohalides) or weak coordinating (Cl − , Br − and ClO 4 − ) ability, has been studied by us [15], and-in almost all the cases-small in nuclearity clusters (i.e., {Ni 5 } and {Ni 6 }) were isolated and structurally characterized [14]. These results have demonstrated the unpredictability of the nacbH 2 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 /nacbH 2 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 (PhCO 2 − ), in an attempt to harness a flexible "synthetic blend" which would potentially lead to high-nuclearity Ni II clusters. Indeed, the reaction between NiI 2 , nacbH 2 , PhCO 2 H, and NEt 3 in a 2 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 {Ni 12 } compound and subsequently isolate a new product. In particular, the employment of NiCl 2 or NiBr 2 in place of NiI 2 afforded the already reported (NHEt 3 ) 2 [Ni 6 (OH) 2 (nacb) 6 (H 2 O) 4 ] [14], whereas the replacement of PhCO 2 H by other carboxylic acids, such as MeCO 2 H or EtCO 2 H, 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 NEt 3 as an external base was also essential for the clean preparation of 1, providing a proton acceptor to facilitate the complete deprotonation of nacbH 2 and PhCO 2 H, and fostering the metal-assisted deprotonation of H 2 O molecules in solution to the coordinating OH − groups (vide infra). Various similar reactions in other organic solvents (i.e., alcohols (ROH), CH 2 Cl 2 and mixtures of MeCN/ROH) and/or external bases (i.e., trimethylamine (NMe 3 ), tripropylamine (NPr 3 ), diethylamine (Et 2 NH), dimethylamine (Me 2 NH) and tetramethylammonium hydroxide (Me 4 NOH) 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].

Description of Structure
A partially labeled representation of the cation of complex 1 is shown in Figure 1      Complex 1 is a closed cage-like cluster consisting of 12 Ni II ions that are bridged by five µ 3 -OH − , one µ-OH − , an I − in 55% occupancy, and the naphthoxido and carboxylato O-atoms of five double-deprotonated nacb 2− 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 (η 1 :η 1 :η 2 :η 1 :µ 3 mode), three (η 2 :η 1 :η 2 :η 1 :µ 4 mode) and four (η 2 :η 1 :η 2 :η 2 :µ 5 mode) metal ions. The variety of binding modes of nacb 2− 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 {Ni 12 } 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 nacb 2− ligands. Complex 1 is a closed cage-like cluster consisting of 12 Ni II ions that are bridged by five μ3-OH − , one μ-ΟΗ − , an I − in 55% occupancy, and the naphthoxido and carboxylato O-atoms of five doubledeprotonated nacb 2− 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 (η 1 :η 1 :η 2 :η 1 :μ3 mode), three (η 2 :η 1 :η 2 :η 1 :μ4 mode) and four (η 2 :η 1 :η 2 :η 2 :μ5 mode) metal ions. The variety of binding modes of nacb 2− 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 nacb 2− 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 η 1 :η 1 :μ mode, one is acting as an η 1 :η 2 :μ3 ligand, and the last one adopts the rare η 2 :η 2 :μ4 mode. Thus, the resulting core is [Ni12(μ3-ΟΗ)5(μ3-Ι/Η2Ο)(μ-ΟR)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 edgeor vertex-sharing {Ni3} triangular subunits ( Figure 5), which are held together by μ3-OH − and μ-OR −  Complex 1 is a closed cage-like cluster consisting of 12 Ni II ions that are bridged by five μ3-OH − , one μ-ΟΗ − , an I − in 55% occupancy, and the naphthoxido and carboxylato O-atoms of five doubledeprotonated nacb 2− 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 (η 1 :η 1 :η 2 :η 1 :μ3 mode), three (η 2 :η 1 :η 2 :η 1 :μ4 mode) and four (η 2 :η 1 :η 2 :η 2 :μ5 mode) metal ions. The variety of binding modes of nacb 2− 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 nacb 2− 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 η 1 :η 1 :μ mode, one is acting as an η 1 :η 2 :μ3 ligand, and the last one adopts the rare η 2 :η 2 :μ4 mode. Thus, the resulting core is [Ni12(μ3-ΟΗ)5(μ3-Ι/Η2Ο)(μ-ΟR)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 edgeor vertex-sharing {Ni3} triangular subunits ( Figure 5), which are held together by μ3-OH − and μ-OR − Additional bridging about the twelve Ni II ions is provided by five PhCO 2 − groups, which are arranged into three classes; three of them are bridging under the η 1 :η 1 :µ mode, one is acting as an η 1 :η 2 :µ 3 ligand, and the last one adopts the rare η 2 :η 2 :µ 4 mode. Thus, the resulting core is [Ni 12 (µ 3 -OH) 5 (µ 3 -I/H 2 O)(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 − /H 2 O 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 {Ni 3 } triangular subunits ( Figure 5), which are held together by µ 3 -OH − and µ-OR − groups. An alternative description of the {Ni 12 } metal arrangement is that of a central {Ni 7 } subunit possessing a distorted disk-like topology [Ni(1,2,3,5,6,7,10)], which is attached to a {Ni 3 } triangular [Ni (3,4,12)] and a {Ni 4 } 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 nacb 2− chelate, and it joins a relatively rare family of {Ni 12 } Werner-type compounds with a cage-like conformation [21][22][23][24].

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 χMT versus T plot in Figure 6. The value of the χΜT product at 300 K is 10.40 cm 3 Kmol −1 , slightly lower than the value of 12 cm 3 Kmol −1 (calculated with g = 2.0) expected for twelve non-interacting, high-spin Ni II (S = 1) atoms. Upon cooling, the χΜT product continuously decreases down to a value of 3.06 cm 3 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 χMT 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 = −2JijŜi·Ŝj convention) was not feasible. Undoubtedly, 1 possesses a small ground-state spin value, with 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 nacb 2− chelate, and it joins a relatively rare family of {Ni12} Werner-type compounds with a cage-like conformation [21][22][23][24].

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 χMT versus T plot in Figure 6. The value of the χΜT product at 300 K is 10.40 cm 3 Kmol −1 , slightly lower than the value of 12 cm 3 Kmol −1 (calculated with g = 2.0) expected for twelve non-interacting, high-spin Ni II (S = 1) atoms. Upon cooling, the χΜT product continuously decreases down to a value of 3.06 cm 3 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 χMT 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 = −2JijŜi·Ŝj convention) was not feasible. Undoubtedly, 1 possesses a small ground-state spin value, with

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 M T versus T plot in Figure 6. The value of the χ M T product at 300 K is 10.40 cm 3 Kmol −1 , slightly lower than the value of 12 cm 3 Kmol −1 (calculated with g = 2.0) expected for twelve non-interacting, high-spin Ni II (S = 1) atoms. Upon cooling, the χ M T product continuously decreases down to a value of 3.06 cm 3 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 {Ni 12 } clusters, and/or Zeeman effects [11,15]. The overall shape of the χ M T 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 ijŜi ·Ŝ j convention) was not feasible. Undoubtedly, 1 possesses a small ground-state spin value, with the χ M T value at 2 K being consistent with an S~2 ground state (for g = 2). The antiferromagnetic response of the {Ni 12 } 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 {Ni 3 } 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. 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 NμB at 5 T; this is likely due to the presence of low-lying excited states, as reported previously for other highnuclearity 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.

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 nacb 2− 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.

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 nacbH 2 has been reported by us in a previous work [15]. Briefly, it was shown that nacbH 2 is a promising "antenna" group for the promotion of energy transfer effects. Upon maximum excitation at~350 nm, nacbH 2 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 nacbH 2 can be tentatively assigned to the coordination of the deprotonated nacb 2− 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.

Materials, Physical and Spectroscopic Measurements
All manipulations were performed under aerobic conditions using materials (reagent grade) and

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 nacbH 2 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].

Synthesis of [Ni 12 I 2 (OH) 6 (O 2 CPh) 5 (nacb) 5 (H 2 O) 4 (MeCN) 4 ]I (1)
To a stirred, orange suspension of nacbH 2 (0.07 g, 0.20 mmol) in MeCN (20 mL) was added NEt 3 (84 µL, 0.60 mmol). Solids NiI 2 (0.13 g, 0.40 mmol) and PhCO 2 H (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 Et 2 O (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 Et 2 O (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 1·3H 2

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 F 2 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. 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. 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.

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 nacbH 2 /I − /PhCO 2 − "ligand blend" in Ni II chemistry. The N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH 2 ) ligand also contributed to the observation of unquenched optical emission from the {Ni 12 } 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 /RCO 2 − /nacbH 2 tertiary system as a means of obtaining higher-nuclearity, nanosized Ni II clusters with interesting magneto-optical properties.
Author Contributions: P.S.P. and K.N.P. conducted the syntheses, crystallization, purification, optimization, conventional characterization and interpretation of the structural and magnetic data of complex 1; L.C.-S. collected single-crystal X-ray diffraction data, solved the structure and performed the complete refinement; V.B. collected, plotted, rationalized and discussed the optical properties of complex 1; A.E. collected, plotted and discussed magnetic and part of the spectroscopic data of the compound; T.C.S. coordinated the research, contributed to the interpretation of the results and wrote the paper based on the reports of his collaborators; All the authors exchanged ideas and comments regarding the explanation of the results and discussed upon the manuscript at all stages. All authors have read and agreed to the published version of the manuscript.