A multicentre-bonded [ZnI]8 cluster with cubic aromaticity

Polynuclear zinc clusters [Znx] (x>2) with multicentred Zn–Zn bonds and +1 oxidation state zinc (that is, zinc(I) or ZnI) are to our knowledge unknown in chemistry. Here we report the polyzinc compounds with an unusual cubic [ZnI8(HL)4(L)8]12− (L=tetrazole dianion) cluster core, composed of zinc(I) ions and short Zn–Zn bonds (2.2713(19) Å). The [ZnI8]-bearing compounds possess surprisingly high stability in air and solution. Quantum chemical studies reveal that the eight Zn 4s1 electrons in the [ZnI8] cluster fully occupy four bonding molecular orbitals and leave four antibonding ones entirely empty, leading to an extensive electron delocalization over the cube and significant stabilization. The bonding pattern of the cube represents a class of aromatic system that we refer to as cubic aromaticity, which follows a 6n+2 electron counting rule. Our finding extends the aromaticity concept to cubic metallic systems, and enriches Zn–Zn bonding chemistry. Zinc(I) bimetallic clusters have previously been reported, but they are not stable in air. Here, the authors synthesize octametallic Zinc(I) clusters with multi-centred zinc–zinc bonds and extensive electron delocalization over the cluster, resulting in cubic aromaticity and enhanced stability.

Here we report the synthesis, characterization and theoretical analysis of two Zn I compounds with a rare cubic [Zn I 8 (HL) 4 (L) 8 ] 12 À (L ¼ tetrazole dianion) cluster containing multicentred Zn-Zn bonds. These Zn I 8 -bearing compounds are surprisingly stable in solutions and in air at temperature far above ambient temperature. Quantum chemical studies reveal extensive electron delocalization over the Zn I 8 cube. Our finding extends the aromaticity concept from the all-metal aromaticity [24][25][26][27][28][29][30] and spherical aromaticity obeying 2(n þ 1) 2 rule 31 to cubic metal systems.

Syntheses.
To form LOS Zn-Zn-bonded compounds, we designed a series of reactions for the in situ reduction of Zn II compounds under a solvothermal condition. The same approach was used previously for in situ reduction of Cu II and Fe III to yield LOS ions 32 . Here we report the syntheses of two compounds, Na 2  3 ], NaN 3 and Zn II salts under solvothermal conditions, with the yield of B26 and B15%, respectively. The crystals of 1 can also be produced by using ZnO or other organonitriles, such as 7,7,8,8-tetracyanoquinodimethane, biphenyl-4,4-dicarbonitrile, 1,1,3,3-tetrakis-cyanopropane and so on, although the yields are much lower and the qualities of the crystals are relatively poor (Supplementary Table 1).
Analyses of stability. Both 1 and 2 are stable in air, as revealed by powder X-ray diffraction of the samples exposed to air for at least half a year (Supplementary Figs 1 and 2) and thermogravimetry analyses ( Supplementary Fig. 3). These two kinds of compounds are insoluble in common organic solvents. However, 1 can easily dissolve in water and recrystallize on adding DMF into water, as is supported by crystal structure analyses of the recrystallized samples. It follows that the [Zn I 8 (HL) 4 (L) 8 ] 12 À cluster can stably exist in water, which is further confirmed by the high-resolution electrospray ionization mass spectrometry (ESI-MS) of 1 in water. Series of zinc isotope peaks centred at m/z ¼ 738. 8141  compounds, which survived only in low temperature and were so air sensitive as to burn out spontaneously 4 .
Structural analyses. The molecular structures of 1 and 2 were determined by single-crystal X-ray diffraction, and their nonhydrogen atoms were also identified by X-ray photoelectron spectroscopy (XPS) analyses ( Supplementary Fig. 6). In addition, small molecules or cations within the pores of 1 and 2, such as DMF, Na þ , K þ , H 2 L and H 2 O, were characterized by 1 H and 13 C nuclear magnetic resonance (NMR), ion chromatography ( Supplementary Fig. 7), infrared spectra (IR), elemental analyses and inductively coupled plasma (ICP). While crystallized in different space groups of I4/m for 1 and P-43m for 2, they possess a unique tetragonally distorted Zn I 8 cubic cluster with direct multicentred Zn-Zn bonding, and the Zn I 8 core is supported by twelve tetrazole rings, forming [Zn I 8 (HL) 4 (L) 8 ] 12 À motifs. In addition to the Zn-Zn bonds, the adjacent two Zn I ions are further connected by two N atoms from the tetrazole ring ligands, which orient along the corresponding twelve edges of the Zn I 8 cube. Thus, each Zn I ion coordinates to three N atoms of the tetrazole ligands and three neighbouring Zn I ions in the cube. Different from all previously reported [Zn I 2 ] 2 þ species 11 , where various bulky ligands, for example, C 5 Me 5 , [{(2,6-i-Pr 2 C 6 H 3 )N(Me)C} 2 CH], and [2,6-(2,6-i-Pr 2 C 6 H 3 )C 6 H 3 ], were utilized to protect the Zn I -Zn I bond, compounds 1 and 2 provide the first example of stable Zn I -Zn I bonds supported only by small tetrazole ligands.
In these compounds, eight Zn I ions act as the vertices of the distorted cube, forming Zn I 8 core with D 4h symmetry in 1 and D 2d symmetry in 2. There are twelve Zn I -Zn I bonds in 1 (   Å, where the latter is slightly shorter than the shortest Zn-Zn bonds (2.295 Å) in Zn 2 (F 5 -C 5 Me 4 Et) 2 (ref. 14), and the four Zn I -Zn I bonds of 2.766(2) Å are weaker. In 1, each [Zn I 8 (HL) 4 (L) 8 ] 12 À cluster is linked to eight such clusters as its nearest neighbours by sixteen Na þ ions, forming a three-dimensional metal-organic framework ( Supplementary  Figs 8 and 9). In 2, the C and N atoms in the [Zn I 8 (HL) 4 (L) 8 ] 12 À cluster coordinate to external Zn II , Na þ and K þ ions, generating two types of cages ( Supplementary Fig. 10). The small closed cages and large open cages are alternately arranged, giving a fascinating three-dimensional framework (Fig. 2). Each closed cage consists of six [Zn I 8 (HL) 4 (L) 8 ] 12 À clusters, four Zn II , four Na þ and twelve K þ , while each open cage is constructed by twelve [Zn I 8 (HL) 4 (L) 8 ] 12 À clusters, four Zn II , four Na þ and twenty-four K þ . These different types of ions connected with the [Zn I 8 (HL) 4 (L) 8 ] 12 À cluster significantly influence the ultravioletvisible and luminescence spectra of 1 and 2 ( Supplementary  Fig. 11).
Spectrocopic and theoretical studies. The existence of Zn I -Zn I bonds in 1 and 2 is supported by both experimental and theoretical evidences. From the measured Raman spectra of 1 and 2 ( Supplementary Fig. 12), several low-frequency vibration peaks involving the contribution of Zn-Zn stretching modes are observed between 100 and 400 cm À 1 (refs 18-20), but a clear-cut assignment is difficult due to significant Zn-Zn and Zn-L modes mixing. Geometry optimizations, bond order and electron localization function (ELF) calculations using density functional theory methods also confirm direct Zn-Zn bond. We find that the different Zn-Zn distances in 1 and 2 are related to the varied donation ability of the tetrazole ligand with different outside cations, indicating the tetrazole ligands also play a role in stabilizing the Zn I 8 (Fig. 3).
As Zn I is usually much less stable than Zn II in chemistry, it is remarkable that all the zinc atoms in the Zn 8 core of 1 and 2 exist in a þ 1 oxidation state. The XPS data reveal that the binding energy of Zn ions appears at 1019.5 eV for 1 and 1020.1 eV for 2 ( Supplementary Fig. 6), which is markedly smaller than the reported Zn 2p 3/2 binding energy (1021.8 eV) of Zn II ions 33 . As LOS always has smaller binding energy than its higher one, the large deviation to our experimental values from those of Zn II ions suggests the presence of LOS Zn ions in 1 and 2. By comparison with 1, the Zn II ions coordinated to Br atoms in 2 cause the broadening and blue-shift of the XPS peaks, thus accounting for their difference. We also carried out density functional theory calculations on the model clusters [Na 8 Zn 8 (HL) 4 (NaL) 8 ] n þ by assuming Zn I (n ¼ 4) or Zn II (n ¼ 12). The geometry optimized for the [Zn I 8 ] cluster agrees well with the experimental crystal structure, whereas that of [Zn II 8 ] will lead to extremely long ZnyZn distance and eventually the collapse of [Zn II 8 ] cluster, providing additional credence for Zn I in 1 and 2.
X-ray absorption near-edge structures. We further performed X-ray absorption near-edge structure (XANES) analyses of four samples, including 1 and 2 together with Zn(NO 3 ) 2 Á 6H 2 O and Zn foil as standard control groups (Fig. 4). Although no previous XANES data of Zn I ions are reported, there exists a linear correlation between the Zn K-edge energies of these samples and their formal oxidation states. Our XANES data show that the Zn K-edge energies in 1 and 2 exactly correspond to the þ 1 oxidation state, thus providing an unequivocal evidence for the existence of Zn I ions in these compounds, originated from the reduction of Zn II ions caused by the cleavage of C-C bonds.

Discussion
To elucidate the bonding of the Zn I 8   cube are formed through overlap of the 4s orbitals, with slight mixing from the Zn 4p and Zn 3d orbitals. Among them, four bonding orbitals with a 1g and t 1u symmetry are fully occupied, giving a ground-state electronic configuration of (a 1g ) 2 (t 1u ) 6 , whereas four antibonding orbitals with t 2g and a 2u symmetry are completely empty. The bonding pattern of the eight MOs is comparable to those of Au 3 þ and benzene molecules that have the famous planar sand p-aromaticity 34 and obey Hückel's 4n þ 2 rule (Supplementary Fig. 15). This bonding pattern represents a rare type of aromaticity, which we refer to as cubic aromaticity. Such systems follow a 6n þ 2 electron counting rule or approximately the 2(n þ 1) 2 rule 29 , which might help to search for heteroatomic cubic clusters with apt number of electrons.
This special cubic aromaticity arises from the electron delocalization over the entire Zn I 8 cube. The calculated nucleusindependent chemical shift indices at the cubic-, face-, bondcentre display considerable negative values, which are comparable to those in benzene, also confirming the special aromaticity of the Zn I 8 cube (Supplementary Table 3). Through theoretical analyses of the Zn I 8 cube using analytic Hückel MO model and Kohn-Sham MOs, the estimated resonance energy (4.00 b ZnZn ) appears to be comparable with that of benzene (2.00 b CC ), far larger than 1.00 b ZnZn in reported Zn I 2 compounds with Zn-Zn bonds, where b is the resonance integral. Furthermore, the additional multi-centre bonding among the eight Zn atoms provides extra stabilization than the two-centre two-electron bonding in Zn 2 compounds. As a result, the cubic aromaticity, additional Zn 4p orbital interactions, and the strong metal-ligand interactions are responsible for the robustness of the [Zn I 8 ] core and the unique stability of the octanuclear Zn I compounds.
Our finding of the stable [Zn I 8 ] motif to form self-assembled materials may have practical applications. Particularly, as these materials with specially stabilized Zn I and the high energy density tetrazole rings 35 can undergo explosive combustion above 300°C, they might find applications as propellants and high temperature explosives. The stable [Zn I 8 ] core with electron delocalization might be used to construct robust non-linear optical materials. Insofar as the special stability of the Zn I 8 cube, we anticipate that the cubic aromaticity via s-, p-, d-, f-type atomic orbital overlap might also exist in other cubic metal cluster materials. Remarkably, the colourless polyhedral crystals (1a) were isolated after recrystallization of 1 from a H 2 O/DMF mixture at room temperature, which were determined by single-crystal structure analysis. This result indicates that the [Zn I 8 (HL) 4 (L) 8 ] 12 À cluster remains intact in solvent because Zn I ions in water is very unstable and easily disproportionates into Zn(0) and Zn II . 1a would not be obtained by recrystallization if [Zn I 8 (HL) 4 (L) 8 ] 12 À cluster had undergone the disproportionation reaction to decompose. Unfortunately, effort of exchanging free Na þ and K þ in 1 by other cations such as NH 4 þ , [N(CH 3 ) 4 ] þ , and [N(C 2 H 5 ) 4 ] þ was not successful.
Method B. A mixture of ZnO (24.4 mg, 0.3 mmol), NaN 3 (19.5 mg, 0.3 mmol) and K[C(CN) 3 ] (12.9 mg, 0.1 mmol) in DMF (6 ml) was sealed in a 25-ml Teflon-lined stainless steel autoclave and heated at 160°C for 4 days and then cooled to room temperature at a rate of 1°C per hour to form colourless octahedral crystals (1b), but with low yield.
Method E. A mixture of ZnBr 2 (67.6 mg, 0.3 mmol), NaN 3 (39 mg, 0.6 mmol) and biphenyl-4,4 0 -dicarbonitrile (10.2 mg, 0.05 mmol) in DMF (6 ml) was sealed in a 25-ml Teflon-lined stainless steel autoclave and heated at 160°C for 4 days and then slowly cooled to room temperature at the rate of 1°C per hour. Finally, colourless square-pyramidal crystals (1e) were obtained directly, but with poor quality and low yield as well. X-ray crystallographic analyses. Data were collected on an Oxford SuperNova (TM) CCD Diffractometer with a SuperNova X-ray Source (Mo-Ka). The structure was solved using SHELXS-97 and refined using SHELXL-97 contained in Olex2 programme. All hydrogen atoms were attached to their parent atom in a riding model, using the appropriate command. See Supplementary Data 1 and 2 for the CIF crystallographic information file, and Supplementary Methods for the discussion of data collection and refinement.