**1. Introduction**

Several metallic materials have extensive uses, such as sensors, catalysis, fluids, regulated drug delivery and pigments [1–5]. Integration of certain metals to form polyoxometalates (POM) is a feasible and promising strategy for making new heteropolyoxometalates, which are of great interest due to their abundant structures and conceivable applications in magnetism, catalysis, medicine and electrochemistry [6–15]. Many different elements have been reported as compositions of heteropolyanions [16–18], and POMs containing different elements inspired an enormous amount of new research due to their range of intriguing applications [19–24]. Vanadium is of particular interest since it shows flexible coordination geometries as well as a variety of chemical valence states. During the past years, significant progress has been made in the syntheses of polyoxovanadates by incorporating group 15 elements (AsIII/SbIII, vanadoarsenates [25–38] and vanadoantimonates [39–52]) into the well-known {V18O42} shell. In 2015, Monakhov, Bensch and Kögerler published a milestone review on derivatives of polyoxovanadates [53], in which the syntheses and structures of vanadoarsenates, vanadoantimonates and vanadogermanates were systematically reviewed. In addition, we have focused on preparations of vanadoarsenates [25–30],

**Citation:** Guo, H.-Y.; Qi, H.; Zhang, X.; Cui, X.-B. First Organic–Inorganic Hybrid Compounds Formed by Ge-V-O Clusters and Transition Metal Complexes of Aromatic Organic Ligands. *Molecules* **2022**, *27*, 4424. https://doi.org/10.3390/ molecules27144424

Academic Editor: Santiago Reinoso

Received: 17 June 2022 Accepted: 6 July 2022 Published: 11 July 2022

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vanadoantimonates [40,41] and secondary transition metal substituted As-V-O clusters [54] for years. Here, we further extended our interest in Ge-V-O [55–65] and secondary transition metal substituted Ge-V-O clusters [6,66] based on two considerations. Firstly, the {AsIII 2O5} of the As-V-O cluster is not favorable for forming extended structures because the arsenic center did not have terminal oxygens, which can further interact with other bridging metal centers, whereas the {GeIV 2O7} of the Ge-V-O cluster has two additional terminal oxygens, which can provide opportunities for forming extended structures via metal-oxygen covalent and dative bonds. Secondly, like the As-V-O cluster, some vanadiums of the Ge-V-O cluster can also be substituted by secondary transition metals to yield new organic– inorganic hybrid clusters [54]. Vanadogermanates can significantly expand the area of polyoxovanadate chemistry due to the introduction of a different functionality compared to the As-containing congeners. In 2003, A. J. Jacobson [57], A. Clearfield [60] and Lin [56] respectively reported the preparations of a series of Ge-V-O compounds, and then W. Bensch reported several Ge-V-O compounds in 2006, 2010 and 2013 [59,61,64]. In 2010 and 2014, Yang reported the syntheses of several secondary transition metal-substituted Ge-V-O clusters [6,66]. However, compared with vanadoarsenates, the number of vanadogermanates is still far too small, and especially the secondary transition metal substituted Ge-V-O clusters. It is still a great challenge for chemists to synthesize new vanadogermanates.

We found that all the previously reported Ge-V-O compounds were totally based on aliphatic organic ligands [6,39–52,66], while no Ge-V-O compounds constructed out of aromatic organic ligands were reported. The reason only aliphatic-ligand involving Ge-V clusters were reported can be listed as below: (1) GeO2 is inert in neutral and acidic aqueous solutions; (2) the aqueous solution of the aromatic nitrogen-containing organic ligands is neutral. It is not favorable for the aggregation of Ge-V clusters. Therefore, it is very difficult to prepare aromatic-ligand-containing Ge-V clusters. The first Ge-V clusters were reported in 2003 [60], and no aromatic-ligand-containing Ge-V clusters have been prepared. On the other hand, the introduction of aromatic organic ligands can not only can enrich the structures of this kind of compound but can also ameliorate their polar, electricity, acid and redox properties [67–70]. The introduction of aromatic organic ligands may thereby lead to compounds with more interesting structures, topologies and properties (It is well known that the robustness of almost all MOFs is derived from the aromatic organic ligands [71]). An example: recently, S. K. Das reported an aromatic-ligand-containing polyoxometalate that can be used as an efficient electrocatalyst for water oxidation [72], but the aliphatic analog did not exhibit such an excellent electrocatalytic property. Based on aforementioned points, we then chose phen as the aromatic organic ligand to prepare Ge-V-O compounds. Fortunately, we successfully synthesized [Cd(phen)(en)]2[Cd2(phen)2V12O48Ge8(OH)8(H2O)]·12.5H2O (**1**), [Cd(DETA)]2[Cd(DETA)2]0.5[Cd2(phen)2V12O41Ge8(OH)7(0.5H2O)]·7.5H2O (**2**) and [Cd(en)3]{[Cd(η2-en)2]3[Cd(η2-en)(η2-en)(η2-μ2-en)Cd][Ge6V15O48(H2O)]}·5.5H2O (**3**), of which compounds **1** and **2** are the first Ge-V-O compounds based on aromatic organic ligands. Compound **1** is the first dimer of Ge-V-O compound, of which Ge-V clusters are linked by a double bridge of [Cd(phen)(en)]2+. Compound **2** exhibits a novel 1-D chain structure of which Ge-V-O clusters are fused by [Cd2(DETA)2] 4+ TMCs. Compound **3** is a novel 3-D structure which is constructed out of [Ge6V15O48(H2O)]<sup>12</sup><sup>−</sup> clusters and five different types of TMCs. We also synthesized [Zn2(enMe)3][Zn(enMe)]2[Zn(enMe)2(H2O)]2 [Ge6V15O48(H2O)]·3H2O (**4**) [6] and [Cd(en)2]2{H8[Cd(en)]2Ge8V12O48(H2O)}·6H2O (**5**), which have been reported previously [54]. In addition, the catalytic properties of these five compounds have been investigated.

## **2. Experimental Section**

#### *2.1. Chemicals and Data Analysis*

All the chemicals used were of reagent grade without further purification. C, H, N elemental analyses were carried out on a Perkin-Elmer 2400 CHN elemental analyser (Shanghai, China). Infrared spectra were recorded as KBr pellets on a Perkin-Elmer SPEC-TRUM ONE FTIR spectrophotometer. UV-vis spectra were recorded on a Shimadzu UV- 3100 spectrophotometer. Powder XRD patterns were obtained with a Scintag X1 powder diffractometer system using Cu Kα radiation with a variable divergent slit and a solid-state detector. Electron spin resonance (ESR) spectra were performed on a JEOL JES-FA200 spectrometer(Guangzhou, China) operating in the X-band mode. The g value was calculated by comparison with the spectrum of 1,1-diphenyl-2-picrylhydrazyl (DPPH), whereas the spin concentrations were determined by comparing the recorded spectra with that of an Mn marker and DPPH, using the built-in software of the spectrometer.
