Hafnium(IV) Chemistry with Imide–Dioxime and Catecholate–Oxime Ligands: Unique {Hf5} and Metalloaromatic {Hf6}–Oxo Clusters Exhibiting Fluorescence

Hafnium(IV) molecular species have gained increasing attention due to their numerous applications ranging from high-resolution nanolithography, heterogeneous catalysis, and electronics to the design of molecule-based building blocks in metal–organic frameworks (MOFs), with applications in gas separation, sorption, luminescence sensing, and interim storage of radioactive waste. Despite great potential, their chemistry is relatively underdeveloped. Here, we use strong chelators (2Z-6Z)-piperidine-2,6-dione (H3pidiox) and 2,3-dihydroxybenzaldehyde oxime (H3dihybo) to synthesize the first ever reported pentanuclear {Hf5/H3pidiox} and hexanuclear {Hf6/H3dihybo} clusters (HfOCs). The {Hf6} clusters adopt unique core structures [Hf6IV(μ3-O)2(μ-O)3] with a trigonal-prismatic arrangement of the six hafnium atoms and have been characterized via single-crystal X-ray diffraction analysis, UV–vis spectroscopy in the solid state, NMR, fluorescence spectroscopy, and high-resolution mass spectrometry in solution. One-dimensional (1D) and two-dimensional (2D) 1H NMR and mass spectroscopies reveal the exceptional thermodynamic stability of the HfOCs in solution. Interestingly, the conjunction of the oxime group with the catechol resulted in the remarkable reduction of the clusters’ band gap, below 2.51 eV. Another prominent feature is the occurrence of pronounced metalloaromaticity of the triangular {Hf3} metallic component revealed by its NICSzz scan curve calculated by means of density functional theory (DFT). The NICSzz(1) value of −44.6 ppm is considerably higher than the −29.7 ppm found at the same level of theory for the benzene ring. Finally, we investigated the luminescence properties of the clusters where 1 emits light in the violet region despite the lack of fluorescence of the free H3pidiox ligand, whereas the {Hf6} 3 shifts the violet-emitting light of the H3dihybo to lower energy. DFT calculations show that this fluorescence behavior stems from ligand-centered molecular orbital transitions and that HfIV coordination has a modulating effect on the photophysics of these HfOCs. This work not only represents a significant milestone in the construction of stable low-band-gap multinuclear HfIV clusters with unique structural features and metal-centered aromaticity but also reveals the potential of Hf(IV) molecule-based materials with applications in sensing, catalysis, and electronic devices.


■ INTRODUCTION
Group IV metal oxo clusters (MOCs) are polynuclear compounds exhibiting an inorganic core formed by group IV metals in their highest oxidation state linked by oxygen atoms and stabilized by capping ligands. Although there are many reported applications of group IV MOCs, 1−8 their chemistry is still underdeveloped 9−13 and particularly the hafnium chemistry in marked contrast to the titanium and zirconium. HfOCs have potential applications in high-resolution nanolithography, 14−16 in heterogeneous catalysis, 17 and they have been used as molecule-based building blocks in metal−organic frameworks (MOFs), with applications in gas separation, 18,19 sorption, 20 catalysis, 21−28 luminescence sensing, 29 and interim storage of radioactive waste. 30 In addition, the study of HfOCs is of fundamental importance because they are processable molecular analogues of HfO 2 which have numerous applications including protective surface coatings, 31 metal-oxide-semiconductor field-effect transistors, 32,33 random access memory devices, 34−37 and teeth prosthetics. 38 The band gap energy of HfO 2 ranges from 5.3 to 6.0 eV depending on its different phases and its formation under different experimental conditions. 39 This large band gap is practically prohibitive for the utilization of HfO 2 in photocatalytic applications. The modulation of the band gap can be achieved by employing organic chelators 40−42 leading to visible light absorption by HfOCs.
Recently, our group studied the reaction of MCl 4 (M = Ti IV , Zr IV ) with the organic ligands (2Z,6Z)-piperidine-2,6-dione dioxime (H 3 pidiox) 43,44 and 2,3-dihydroxybenzaldehyde (H 3 dihybo) (Scheme 1). 40 Both ligands are strong binders to hard metals in their highest oxidation state, 45,46 and have been used for the extraction of heavy hard metal ions from seawater. 47−49 The employment of H 3 pidiox and H 3 dihybo led to the formation of polynuclear clusters with unique structural features, allowing the induction of metalloaromaticity and the modulation of the compounds' band gap. The reaction of HfCl 4 with the ligand H 3 dihybo gave a hexanuclear cluster, {Hf 6 }, with two cyclo-{Hf 3 } metallic cores which exhibit metalloaromaticity.
The discovery of the aromatic nature of benzene had a great impact on many fields of science such as organic, industrial, and medicinal chemistry, and life sciences. 50 The extensive investigation of metalloaromaticity that took place in the last few years, led to a better understanding of the concept and exponential growth of examples in the literature 51 due to the discovery of new metalloaromatic species such as metallobenzenes 52 and all-metal clusters [i.e., (Al 4 2− ), 53 (Au 5 Zn + ) 54 ]. Owing to the clusters' metalloaromaticity-induced wide range of applications 50,55−57 such as catalysis, drugs, molecular electronic devices, aerospace engineering, molecular magnets, the revelation of unknown aspects of chemical bonding, and understanding the electronic and surface properties of metal oxides, and mixed-metal clusters, this family of compounds has attracted the attention of numerous research groups. Metalloaromaticity of all-metal cyclic species is due to the formation of σ, π, δ, and φ molecular orbitals (MOs), 50 in marked contrast to the organic aromatic rings where electron delocalization is supported by π MOs only. 50 Manifestation of metalloaromatic behavior has been evaluated using several different criteria that have been developed over the years that are directly related to structural, energetic, magnetic, and electronic characteristics of the reported species. Among them, the magnetic nucleus-independent chemical shift (NICS) criterion appears to be one of the most powerful. 58 Therefore, in this work, we employ the NICS criterion to investigate whether the cyclic trinuclear {Hf 3 } metallic ring cores of the newly synthesized compounds exhibit metalloaromatic behavior.
Based on the above observations, our work is inspired by the prospect of generating new building units for the construction of metal−organic frameworks with potential application in catalysis, sensing, and gas separation. Moreover, the combination of the inherent inertness of Hf species and higher stabilities compared to first-row transition metals along with the highly polarized Hf−X bonds will potentially influence the catalytic, optoelectronic, and luminescence sensing applications. Herein, we report the synthesis, structure, and physicochemical characterization of the pentanuclear HfOC [Hf 5 (μ-OH) 4 (OH 2 ) 4 (μ-η 1 ,η 1 ,η 2 -Hpidiox-O,N,O′) 4 (η 1 ,η 1 ,η 1 -Hpidiox-O,N,O′) 4 ]·KCl·3.25CH 3 OH·16.5H 2 O (1) and of three hexanuclear HfOCs with general formula [Hf 6 (μ 3 -O) 2 (μ-O) 3 (OH 2 ) 6 (μ-η 1 ,η 2 ,η 1 -Hdihybo-O,O′,N) 6 ] (2−4) with the organic chelators H 3 pidiox and H 3 dihybo. HfOC 1 is the first example of a pentanuclear hafnium cluster to be reported. 9 Both the core structure [Hf 6 (μ 3 -O) 2 (μ-O) 3 ] and the trigonalprismatic arrangement of the six hafnium atoms in compounds 2−4 are unique. Moreover, the ligation of H 3 pidiox and H 3 dihybo ligands to Hf IV induces fluorescence in solution at room temperature, rendering these Hf/H 3 pidiox/H 3 dihybo clusters highly promising candidates for applications in sensing, catalysis, and optoelectronics. 59 (2). To a stirred moist methyl alcohol solution (4 mL) were successively added H 3 dihybo (47.8 mg, 0.312 mmol) and HfCl 4 (100.0 mg, 0.312 mmol). The colorless solution of the ligand turned light orange upon addition of HfCl 4 . Then, 1.6 mL of tetrabutylammonium hydroxide, 0.39 M in methyl alcohol (162.0 mg, 0.624 mmol) was added in one portion. The solution was filtered and a light orange filtrate was obtained and was kept at room temperature (∼20°C) for 4−5 days, during which period yellow crystals were formed. We were unable to prepare an analytically pure sample on a preparative scale because the compound is very hygroscopic. Even though single crystals were also obtained in this case, the data were not of publishable quality due to the hygroscopic nature of the single crystal but allowed us to determine the content of the unit cell (see Figure S1) and provide the formula above.
[Hf 6 IV (μ 3 -O) 2 (μ-O) 3 66 (revPBE-D4) was used together with a Slater type basis set of triple zeta quality augmented with an additional polarization function (TZP). The COSMO 67 implicit solvation scheme was also employed with the default parameters for methanol. Due to the presence of heavy elements, the scalar Zero Order Regular Approximated (ZORA) Hamiltonian 68 was applied in the optimization runs. Geometry optimizations were performed on the {Hf 5 } and {Hf 6 } molecular models constrained to the C 2 point group symmetry to allow for Jahn−Teller distortions in the excited state. Vertical excited state energies were computed with the spin− orbit perturbative approach (SOPERT) of the time-dependent 69 density functional (TD-revPBE-D4) and these showed no changes in either transition energy or oscillator strengths relative to the spin-free wavefunctions. We found that this approach affords a reasonable compromise between accuracy and computational performance.
The calculation of the NICS values employed the gauge-including atomic orbital (GIAO) DFT method 70,71 as implemented in the Gaussian09 series of programs 72 employing the PBE0 functional in combination with the 6-31G(d,p) basis set for all nonmetal atoms, E and the Def2-TZVP basis set for Hf atoms (the computational protocol is denoted as GIAO/PBE0/Def2-TZVP(Hf)U6-31G(d,p)-(E)).

■ RESULTS AND DISCUSSION
Synthesis of the HfOCs 1−4. Pentanuclear HfOC/ H 3 pidiox 1 was synthesized via a one-pot three-component reaction (eq 1 and Scheme 2) at room temperature, while the hexanuclear HfOCs/H 3 dihybo 2, 3, and 4 (eq 2 and Scheme 2) were also synthesized in a similar fashion. When KOH was used as a base in the reaction mixture of HfCl 4 with H 3 dihybo, no suitable crystals for X-ray structure analysis of {Hf 6 } were obtained, while the use of n Bu 4 NOH led to hygroscopic {Hf 6 } 2. Finally, the use of Et 3 N and pyridine resulted in the isolation of {Hf 6 } HfOCs 3 and 4, respectively, which were suitable for physicochemical measurements. (2) Crystal Structures. The structure of HfOC 1 contains a pentanuclear hafnium(IV) core supported by four chelate Hpidiox 2− ligands, four chelate-bridging Hpidiox 2− and four μ 2 -OH − groups ( Figure 1A). Interestingly, the {Hf 5 IV } cluster is the only single pentanuclear hafnium(IV) cluster reported so far. 9 The four outer hafnium(IV) atoms adopt a distorted tetrahedral arrangement ( Figure 1B The X-ray crystallographic study of HfOC 3, which crystallizes in a centrosymmetric space group and the unit cells contain a hexanuclear molecular structure ( Figure 3A). The core structure of the hexanuclear {Hf 6 } hafnium(IV) cluster is formed from two [Hf 3 IV (μ 3 -O)] subunits, which are connected by three μ-O 2− bridges with the six hafnium(IV) atoms in a trigonal-prismatic arrangement ( Figure 3B). The {Hf 3 IV } subunits are supported by three chelate-bridging doubly deprotonated Hdihybo 2− ligands ( Figure 3C).
The only hexanuclear HfOCs {Hf 6 } reported thus far that incorporate six metal atoms in three different structural arrangements are shown in Figure S2; more specifically: the octahedral ( Figure S2A), 73 the star-shaped cyclic planar ( Figure S2B), 74 and the cyclic planar ( Figure S2C) 75 arrangements of the six hafnium atoms. Thus, the trigonalprismatic arrangement of the six hafnium atoms found in 3 is unique.
The μ 3 -O 2− is located 0.489(4) Å above the plane defined by the three hafnium(IV) atoms Hf(1)−Hf (2) (4) and 2.217(4) Å, respectively, and are very close to those reported in the literature. 76 The doubly deprotonated catecholate moiety adopts a singly bridging chelate μ-(O,O′,O′) mode of coordination. The d(Hf IV −N ox ) av value of 2.384(6) Å is much higher than the reported mean value of 2.187(8) Å, 77 but in the latter case, the oxime is deprotonated and acts as a μ 2 -η 1 ,η 2 -N,O − bridging ligand. Compound 3 is the first example of a polyoxocatecholate hafnium(IV) compound reported to date.
HfOCs 2 and 4 contain identical {Hf 6 } structural units to cluster 3. Thus, only the structural features of compound 3 are discussed in detail (vide supra). In the case of HfOC 2, its structure was not possible to be finalized due to severe disorder of the tetrabutyl ammonium counterions as can be seen in Figure S1. Nevertheless, the remaining part of the structure is well resolved. Additionally, the X-ray structure analysis of 4 revealed a unit cell with two {Hf 6 } clusters, where one of them was found to accommodate two methoxy terminal ligands and four aqua ligands instead of six aqua ligands ( Figure S3). The structural features of 4's metallic core are identical to those of 3.
Solid-State UV−Vis Spectroscopy. Figure 4 shows          13 C} grHMBC spectrum ( Figure S8)] to a lower field compared with the chemical shifts of the free ligand ( Table 1).
The 1 H and 13 C NMR peaks of the free ligand are also shifted in the hexanuclear HfOCs 3 or 4 due to the ligation of Hdihybo 2− to the Hf IV metal centers. The coordination of the metal ion to the oxime nitrogen atom induces deshielding of C(7) (Scheme 5) for 3 (0.3 ppm, Table 2).

Electrospray Ionization Mass Spectrometry (ESI-MS).
The characterization of the pentanuclear {Hf 5 } cluster 1 in solution was carried out using high-resolution ESI-MS to determine the compound's stability in solution. 79 The ESI-MS studies were performed in methanol in negative ionization mode. The presence of higher-intensity isotopic distribution envelopes is due to the existence of the pentanuclear moiety, resulting from the combination of protons, counterions, and solvent molecules. Occasionally, the in situ induction of partial fragmentation during the course of ion transfer in addition to alteration of the metal's oxidation state is quite common and has been reported frequently in the literature. 80,81 A series of doubly charged distribution envelopes that can be assigned to the intact {Hf 5 } cluster ( Figure S9) can be observed in the higher region of m/z values. In this case, a group of distribution envelopes are clustered within the range of ca. 1100−1300 m/z. More specifically, the central isotopic distribution envelope at 1191.95 m/z can be assigned to the {Hf 5 IV (C 5 H 6 N 3 O 2 ) 8 (OH) 4 K 6 (OH 2 ) 4 } 2− anion which is flanked by isotopic envelopes of the same moiety associated with a different number of solvent molecules with the general formula of {Hf 5 (C 5 H 6 N 3 O 2 ) 8 (OH) 4 K 6 (OH 2 ) x } 2− , where x = 0−12 (Table 3). A representative example of this group of species is the expanded distribution envelope shown in Figure  S10 along with its simulated pattern, which corresponds to the intact {Hf 5 } cluster. In the range of ca. 1500−1600 m/z values, another group of envelopes has been identified and assigned as a singly charged tetrameric cluster, which is due to the partial fragmentation of the {Hf 5 } moiety which takes place during the ionization and ion transfer process 80,81 with general formula {Hf 4 IV L 4 K y (OH 2 ) z (OCH 3 ) 7 } − , where y = 0, 1 and z = 0, 2, 3, or 4 (Table 3).
In a similar manner, the {Hf 6 } cluster preserves its structural features in solution, as identified by a series of singly charged distribution envelopes (Figure 7). The assignment of the observed isotopic distribution envelopes reveals the main structural motif {Hf 3 IV Hf 3 III O 5 (C 7 H 5 NO 3 ) 3 (C 7 H 4 NO 3 ) 3 }-(solv) x associated with different amalgamations of solvent molecules (H 2 O or CH 3 OH) coordinated or associated with the ionized {Hf 6 } cluster, see Table 4 for a detailed assignment.
In a similar fashion, the observation of changes in the metal's oxidation state is quite common as discussed above.
Interestingly, the lower m/z region (ca. 1000−1200 m/z) revealed the presence of the fundamental trimeric building block generated during the ionization process. The singly charged distribution envelopes centered at 1179.80, 1183.80, and 1193.85 m/z values demonstrate the presence of the Hfoxo-centered triangles that can provide crucial information about the formation of these clusters. The identification of the trimeric building blocks indicates the formation of the trimeric clusters prior to their subsequent utilization as building blocks for the construction of the hexanuclear species that can be isolated as single crystals. Similar behavior has been observed in the solution studies conducted for the {Ti 6 } and {Zr 6 } species reported previously by our group. 40 Hf-based oxo-centered molecular triangles, as shown in Figure  S11.
Metalloaromaticity of the Cyclic Trinuclear {Hf 3 } Metallic Ring Cores of HfOC 3 {Hf 6 }. Next, we set out to    50 and to probe the aromaticity/antiaromaticity of the rings, we employed the magnetic criterion, i.e., the nucleus-independent chemical shifts (NICS) by computing the NICS zz scan curves. 82−84 Accordingly, we calculated the NICS zz scan curve of the cyclo-Hf 3 trinuclear metallic core with the PBE0 functional that has been found to perform well in modeling molecular properties of heavy metals. 85 The NICS zz curve is given in Figure 8, while the inset picture depicts the positions of the Bq ghost atoms. Sigma aromaticity arises from σ MOs delocalized in the plane of the ring, while π, δ, and ϕ arise from π, δ, and ϕ MOs delocalized over the ring plane, thus, σ type aromaticity is expressed by NICS zz in the center of the plane [NICS zz (0)], while π, δ, and ϕ type aromaticities are expressed by NICS zz 1 Å above the center of the plane [NICS zz (1)]. Note that the NICS zz scan curve given in Figure 8 was calculated for the structure obtained from the X-ray structural analysis of this system. The NICS zz (1) (Bq located 1 Å above the {Hf 3 } core) is equal to −44.6 ppm indicative of aromaticity. The magnetic aromaticity of the {Hf 3 } core is comparable to that of benzene for which the NICS zz (1) is calculated to be equal to −29.9 ppm at the GIAO/PBE0/6-31G(d,p) level. Also, the {Hf 3 } core is more aromatic compared to a similar system bearing a {Zr 3 } core and exhibiting NICS zz (1) equal to −37.3 ppm. 40 However, the presence of the O ligand capping the {Hf 3 } ring contributes to the NICS zz (1) values and so the aromaticity of the ring should be smaller. In this context, we used the NICS zz (0) values to quantify the aromaticity of the three-member ring {Hf 3 }. Accordingly, NICS zz (0) is equal to −62.7 ppm suggesting strong σ-type metalloaromaticity of the {Hf 3 } ring core. Interestingly, the {Hf 3 } ring core exhibits a stronger metalloaromaticity than the respective {Zr 3 } ring core with a NICS zz (0) value equal to −40.1 ppm at the same level of theory. 36 The smaller size of the {Hf 3 } ring relative to {Zr 3 } ring accounts well for the observed higher metalloaromaticity of the former (the ring radius of the {Hf 3 } ring is 1.996 Å as compared to the ring radius of 2.002 Å in the {Zr 3 }). The perusal of Figure 8 reveals that the NICS zz curve indicates the existence of multiple and alternating aromaticity/antiaromaticity zones. Away from the cyclo-{Hf 3 } metallic cores, at distances of 2−3 Å from the metallic ring centers, there are two antiaromatic zones with NICS zz (R) peak values around 32 ppm (at 2 Å below the ring plane) and around 18.1 ppm (at 2.6 Å above the ring plane). In contrast, at 0.5 near the O capping ligand (which is located 0.488 Å above the ring plane) the NICS zz (1) is extremely high with a value equal to −107.3 ppm.
Photophysical Properties of HfOCs 1 and 3. Taking into consideration the interesting electronic structure and decreased band gap values observed above, we embarked on exploring the luminescence properties of the H 3 dihybo and HfOCs 1 and 3 that are shown below in Table 5. The imide− dioxime organic molecule H 3 pidiox (Scheme 1) does not show any significant light emission. However, the pentanuclear HfOC 1 emits light at 455 nm upon excitation at 362 nm ( Figure 9A). The excitation spectrum of HfOC 1 exhibits two excitation peaks at 362 and 400 nm. Cluster 1 has a low absorption of up to 500 nm ( Figure S6). The intensity of the excitation peaks might be due to the energy transfer from the higher energy transitions of 1. 86 The catecholate−oxime organic molecule H 3 dihybo (Scheme 1), emits light at 417 nm (Table 5) upon excitation at 362 nm.   Upon complexation of the H 3 dihybo ligand to the Hf(IV) centers in the hexanuclear HfOC 3, both the excitation and emission maximum wavelengths are shifted to lower energy, by 60 and 100 nm, respectively ( Figure 9B and Table 5). HfOC 3 emits light in the cyan-green region (505 nm). The theoretical investigation of the luminescence properties of 1 and 3 (vide infra) predicts that the emitted light of these HfOCs is due to ligand−ligand electron transitions. However, ligation of the ligands to the Hf IV centers results in a significant change in intensity and the wavelength of the emitted light. Considering that both ligands are strong chelators for hard metals, these ligands can be used for the extraction of Hf IV by chelation and the above luminescence properties can be employed for sensing Hf IV ions in solutions.
In an effort to further explore the emission behavior of clusters 1 and 3, we investigated the decay time of the excited state. In the case of compound 1 (Figure 10), the detection wavelength selected using the monochromator was 455 nm and the acquisition time was approximately 12 h to ensure a good signal-to-noise ratio as shown. The lifetime exhibits a biexponential decay, where the fast component dominates significantly (∼95%) at around 260 ps. The slow component makes up only 5% and is significantly slower, with a lifetime of just under 3 ns.
In the case of compound 3 (Figure 10), the detection wavelength was selected instead to be 520 nm and the acquisition time was as little as 3 h as it was slightly more emissive than the first sample. This ensured a similar signal-tonoise ratio to compound 1. The lifetime of the hexanuclear cluster 3 also exhibits a bi-exponential decay, where the fast component also dominates ( The absorption spectrum of both systems was calculated at the time-dependent DFT (TD-revPBE-D4/TZP) level. It may be seen that for {Hf 6 } 3 ( Figure 11) the absorption wavelengths have excellent agreement with the experimental values although the absorption intensities are reversed. This may be an indication that in solution there may be a strong interaction with the solvent possibly through hydrogen bonds which cannot be captured by the implicit solvation scheme, and this leads consequently to symmetry breaking.
For {Hf 5 } 1 ( Figure 12) the agreement is not so good, and the bands are uniformly UV-shifted by 20−30 nm and their relative intensities are also reversed. Again, this may indicate relaxation effects and strong interaction with the solvent, inaccuracies in the level of theory, or the presence of protonation equilibria on the part of the hydroxo groups.
As the excitation source of the experiment is 405 nm this coincides with the first major band in {Hf 6 } and the second one in {Hf 5 }. As such the corresponding excited states that originate from those regions of the spectrum were optimized also by the TD-revPBE-D4 methodology.
In the case of {Hf 6 }, the geometries of both the ground and excited states were optimized in global C 2 symmetry meaning that these will have A and B irreducible representations. Its absorption maximum (405 nm) corresponds to the 24th excited states in either symmetry representation, herein labeled as 24( 1 A) and 24( 1 B) and these are strictly degenerate in the ground state geometry. These electronic transitions are ascribed to the molecular orbitals that are mostly centered on the ligand. For this case, it was decided to pursue the relaxation of the 24( 1 A) state.
In the case of {Hf 5 }, the 20( 1 A) and 20( 1 B) states are not strictly degenerate but are still symmetry-related counterparts and are the origin of the first band maximum in the absorption region of 400 nm. The 20( 1 A) root was therefore optimized.   Table 6.
The transition density of {Hf 6 } reflects the transition to and from the ligands' π orbital manifold ( Figure 13). The associated natural transition orbitals for state 24( 1 A) reflect a multi-configurational character with the majority of the transition being NTO1 → NTO2 (Table 6) while the second largest contribution stems from the NTO3 → NTO4 transition ( Figure S14).
In the case of {Hf 5 }, the transitions also exhibit an LLCT character ( Figure 14) yet with a slightly more subtle metal 5d orbital involvement. The overwhelming contributions of the 20( 1 A) state stem from NTO1 → NTO2 (Table 6) with a smaller contribution from NTO3 → NTO4 ( Figure S15). Both calculated emission wavelengths (515 and 443 nm) are in good agreement with the experimentally determined maxima (506 and 455, respectively).

■ CONCLUSIONS
In summary, we have synthesized and characterized physicochemically a series of HfOCs with oxime ligands following a simple one-pot three-component reaction at room temperature. The flexible imide/dioxime ligand H 3 pidiox stabilized a unique pentanuclear {Hf 5 3 ] with a trigonal-prismatic arrangement of the six hafnium atoms. Both the core structure and the trigonal-prismatic arrangement of the six hafnium atoms are unique.
The NICS zz scan curves of the {Hf 6 } system reveal the pronounced metalloaromaticity of the metallic {Hf 3 } ring core. The calculated NICS zz (1) value of −44.6 ppm is higher than that of −37.3 ppm for benzene and its {Zr 3 } ring core analogue   at the same level of theory. The ligation of the catechol oxime ligands to the Hf IV ions, which are in a delocalized co-planar mode, gave rise to a remarkable reduction of the {Hf 6 } clusters' band gap in comparison to HfO 2 . X-ray diffraction studies and 2D { 1 H} NOESY NMR spectra revealed that the ligands of the two planes defined from each of the two {Hf 3 } cores are at a short distance and interact with each other through π-bonds enhancing further the reduction of the observed band gap. In addition, the pentanuclear and hexanuclear clusters formed by the ligation of the H 3 pidiox and H 3 dihybo to Hf IV result in either emergence (in the case of 1) or substantial shifting the ligand's light emission (in the case of 1−3). Theoretical studies revealed that the origin of the luminescence properties observed in {Hf 5 } and {Hf 6 } HfOCs are due to intra-ligand electron transitions. There is a good agreement between the calculated and the experimentally determined emission band maxima. The difference in luminescence activity between the HfOCs and the free ligands might be attributed to the structural features of the former, in which the π orbital manifold of the organic ligands become polarized by their spatial arrangement in the molecules. These findings hint at an alternative strategy to develop new molecule-based materials that are photoactive in the visible region of the electromagnetic spectrum, by employing ligands that are only photoactive with UV light. Furthermore, NMR and ESI-MS studies in solution revealed that the reported HfOCs are thermodynamically stable. The strong chelation of the catecholate−oxime ligand H 3 dihybo to Hf IV , and the remarkable aromaticity of the Hf 3 IV rings induce additional stability to the hexanuclear Hf IV / H 3 dihybo HfOCs. The facile synthesis, thermodynamic stability, and the unique electronic structure that induces unique properties (aromaticity, fluorescence, low band gap values) to the hexanuclear Hf IV /H 3 dihybo HfOCs render them highly promising candidates for the design of novel moleculebased materials with potential applications in chemistry and materials science, such as catalysis, molecular electronic devices, and sensing devices for hafnium. Development of hafnium sensing devices and optoelectronics is currently underway.

■ ASSOCIATED CONTENT Data Availability Statement
The datasets for the stationary points obtained for the ground and excited state geometries are uploaded in the iochem-bd 87 database and accessible free of charge via https://doi.org/10. 19061/iochem-bd-6-147.