3.2.1. [Cd(phen)(en)]2[Cd2(phen)2V12O40Ge8(OH)8(H2O)]·12.5H2O (**1**)

The asymmetric unit of **1** consists of a di-Cd-substituted Ge-V-O cluster [H8Cd2(phen)2 Ge8V12O48(H2O)]<sup>4</sup><sup>−</sup> (Cd2Ge8V12), two [Cd(phen)(en)]2+ and 12.5 water molecules. As shown in Figure 1, an unusual feature of **1** is that two [Cd(phen)]2+ take the place of the

two VO2+ fragments located at the two opposite positions of {Ge8V14O50} [59], forming Cd2Ge8V12. The two substituted cadmiums each is coordinated by four oxygens from two {Ge2O7} units with Cd-O distances of 2.290(5)–2.366(5) Å, and two nitrogens from a phen ligand with Cd-N distances of 2.353(6)–2.427(7) Å. That is to say, two phen ligands were decorated onto the surface of Cd2Ge8V12. The two phen located at the two sides of Cd2Ge8V12 are not parallel to each other. There is a dihedral angle of 36.116◦ between the two phenanthroline-planes. All the bond distances in **1** are comparable to those of previously reported compounds [6,55–66]. Bond valence sum (BVS) calculations for Ge and V indicate that both Ge and V exist in the +4 oxidation-state (Table S1). BVS calculations were also conducted for the cadmium and oxygen atoms in compound **1** to determine the locations of the hydrogen atoms in compound **1** (see supporting information and discussions in "BVS calculations to determine the locations of hydrogen atoms for compounds **1**–**3**") [75].

**Figure 1.** Ball-and-stick and wire representation of the di-Cd-substituted Ge-V-O cluster (**a**) and the dimer in compound **1** (**b**).

Except for [Cd(phen)]2+, there are two [Cd(phen)(en)]2+. It should be noted that the two [Cd(phen)(en)]2+ are different from each other. Cadmium of [Cd(4)(phen)(en)]2+ of the two is bonded to four nitrogens from a phen and an en with Cd-N distances of 2.250(8)–2.320(7) Å, a terminal oxygen from Cd2Ge8V12 with the Cd-O distance of 2.385(5) Å and a water molecule with the Cd-O distance of 2.583(9) Å, exhibiting a cis-octahedral geometry. Therefore, the cluster acts as a monodentate inorganic ligand coordinating with Cd(4), forming a cluster supported transition metal complex (TMC). Cadmium of [Cd(3)(phen)(en)]2+ of the two receives contributions from four nitrogens from a phen and an en with Cd-N distances of 2.290(7)–2.367(7) Å and two terminal oxygens from two Cd2Ge8V12 with Cd-O distances of 2.284(5)–2.443(5) Å. It should be noted that the two terminal oxygens involving Cd-O bonds are distinct: one is from a {Ge2O7}, but the other comes from a {VO4}. Thus, Cd(3) TMC acts as a bridge linking two Cd2Ge8V12 to construct a novel cluster dimer. It should be noted that there are two Cd(3) TMCs acting as a double bridge linking two Cd2Ge8V12. The dimer further supports two Cd(4) TMCs at the two sides of the dimer. That is to say, Cd(3) TMCs act as bridges joining Cd2Ge8V12, but Cd(4) TMCs terminate the connection of the clusters by the terminating water molecule.

Distances between the central water molecule of the Cd2Ge8V12 and Cd(3) and Cd(4) are 7.886–7.888 Å, and the angle of Cd(3)-O1w-Cd(4) is 109.754(1)◦.

The dimer of clusters was reported by our group in 2002 [76] and very recently [77]; the first one was based on the Mo8V6 cluster, and the second one was based on the V15O36 cluster. However, compound **1** here is the most complex one of the three, which is the first example of dimer of substituted clusters. The other two reported compounds are both based on traditional clusters but not the substituted one.
