Synthesis and characterization of a polyoxovanadate cluster representing a new topology

A new type of mixed-valence polyoxoanionic cluster, [V V13 V IV3 O 42 (Cl)] 8 2 , composed of 14 {VO 5 } square pyramids and 2 {VO 4 } tetrahedral units, hosting a chloride ion has been synthesized and characterized.

shown in Fig. 1.The cage has crystallographic twofold symmetry, but approaches closely to mm2 (C 2v ) symmetry.It may be described as formed by the fusion of 14 {VO 5 } square pyramids and two {VO 4 } tetrahedra.As is common in polyoxovanadate cages, each of the five-coordinate vanadium atoms has a single terminal oxygen atom, with its remaining four oxygens participating in the cage framework.The tetracoordinate vanadiums are bound to two terminal and two framework oxygen atoms, which is a less common situation.
The V-O framework bonds define four distinct types of environments within the cage for the seven independent fivecoordinate vanadium atoms.One of these atoms (V1) is bound to four The negatively charged vanadium oxide shell in 1 hosts a chloride ion at its center.There is no covalent interaction of this ion with the atoms in the vanadium oxide framework, as shown by its average distance of 3.630 A ˚from the vanadium atoms of shell.The encapsulation of chloride and bromide ions has been previously observed in the negatively charged vanadium oxide shells [V 15 O 36 ] 52 and [H 4 V 18 O 42 ] 82 . 8 {V 16  The results of the bond valence sum calculation and manganometric titration of reduced vanadium (V IV sites) indicate the presence of three V IV sites per formula unit which is consistent with the formulation and charge balance requirements of 1.The temperature dependent magnetic measurement results are presented in Fig. 3. 18 The compound exhibits Curie paramagnetism.The data were fitted to the Curie-Weiss law at high temperature: The best fit was C 5 0.96 emu K/(Oe mol), h 5 3.25 K, x TI 5 0.00083 emu/(Oe mol).The effective magnetic moment (m eff ~ffiffiffiffiffiffiffiffiffiffi ffi 8x 0 T p ) at 300 K is 2.79 m B corresponding to three V IV ions.The EPR spectrum of the compound has one resonance line with g 5 1.97.
Thermogravimetric analysis 19 of 1 reveals a three-step weight loss accounting for 38.5% mass of the compound.The first weight loss between 45-130 uC corresponds to the loss of four solvate water molecules.The second weight loss, between 130-330 uC, corresponds to the loss of the eight pyridinium ions.The last step between 330-430 uC corresponds to the removal of the chloride ion.There was no further weight change up to 800 uC.The IR spectrum of the black residue exhibits medium intensity bands at 678, 532, and 450 cm 21 indicating a reduced vanadium oxide phase that has not been further characterized.
In summary, we have discovered a mixed-valence polyoxovanadate cluster representing a new vanadium oxide framework structure.Presence of potentially reactive {VO 4 } units on the cluster surface make it an attractive building block for nanostructured materials 15 derived from these motifs.
This work was partly supported by grants (to M. I. K.) from the American Chemisal Society's Petroleum Research Fund (ACS-PRF#35591-AC5), NSF (CHE-0210354) and IIT Research Institute.
O 40 } core, composed of pyramidal {VO 5 } units and encapsulating a water molecule has been observed in the extended structure chain compound Cs 10.5 [{V 16 O 40 }{Si 4.5 V 1.5 O 10 }]? 3.5H 2 O. 16 Also, a {V 16 O 38 } shell has been observed in a threedimensional open-framework solid, [{Cu(1,2-pn) 2 } 7 {V 16 O 38 -(H 2 O)} 2 ]?4H 2 O, 17 made of {V 16 O 38 (H 2 O)} 72 clusters interlinked by {Cu(1,2-pn) 2 } 2+ groups, and more recently in the discrete cluster {V 16 O 38 (Cl)} 82 . 10Unlike the shell observed in 1, the {V 16 O 38 } shell in these compounds contains 16 {VO 5 } square pyramids that share edges and corners.The topology of the vanadium oxide shell in 1 is totally different and unprecedented.Fig. 2 shows a view of the unit cell contents of the crystals of 1. Charge is balanced by the presence of eight pyridinium ions, two of which are disordered, per cage.There are also four water molecules of crystallization per {V 16 O 42 } unit, which participate in hydrogen bonds to both cluster oxygen atoms (e.g., O7 … O22 [at (x, 1 2 y, z 2 K)] 2.799 A ˚, O18 … O22 [at ( 2 x, K 2 y, 1 2 z)] 2.879 A ˚) and pyridinium N-H groups ions [e.g., O22 … H33(N3) 5 1.86 A ˚].This network of hydrogen bonds may play a role in the insolubility of 1 in water and common organic solvents.

Fig. 3
Fig. 3 Dependencies of the magnetic susceptibility (#), effective magnetic moment (%) and reverse susceptibility (D) of 1 on temperature T. The lines drawn through the data are the fits to the Curie-Weiss law.