pH as a Structure Director in the Synthesis of Copper

This paper deals with the investigation of the reactions of aqueous molybdates in the presence of copper ions under varying pH to obtain new copper molybdates and rationalize the structures formed in terms of the self-assembling nano-building blocks of polyoxomolybdates (POMs). The reactions led to the precipitation of porous copper molybdates: 1D polymeric anions in (NH4)6[{Cu(H2O)4} {Mo36O112(H2O)16}].21H2O, 1, a 3D framework bimetallic oxide in Cu3Mo2O9, 2. Our results reported here clearly suggest that copper complexes formed during the initial stages of the reaction probably dictate the architecture of the final solids. Interestingly the magnetic properties studied on these compounds showed that 1 shows paramagnetic ordering since magnetically active centres i.e. Cu(II) are separated by large diamagnetic centres of {Mo36} cluster anions where as 2 shows the dominance of antiferromagnetic near-neighbor interaction. article History Received: 26 October 2017 Accepted: 23 November 2017

introduction Polyoxomolybdate (POM) is an important class of inorganic materials that is currently attracting considerable attention due to its importance in the area of catalysis, sensors etc 1,4 .The recent discovery of giant molybdenum clusters by Müller and his group 5,8 has evoked remarkable interest to understand the self-assembly processes leading to the formation of POM clusters with varying size and shape with a variety of supramolecular assemblies thereby occurring in the solid state.A significant addition to this complex range of solids is the introduction of transition metal, which provides innumerable possibilities by coordinating linking the POM clusters into multidimensional structures.For construction of a specific architecture with one-, twoand three-dimensional networks such as molecular grids, bricks, ladders, rings, boxes, honeycombs, helicates etc. [9][10][11] , design and selection of suitable metal ions are extremely crucial; it is also necessary to develop a rational synthetic protocol to engineer the structure of the final solid.Soft chemical routes have been adopted by several groups, in particular Zubieta et al [12][13][14][15][16] to crystallize a large number of POMs from aqueous molybdate solution.Almost all groups have mainly employed hydrothermal/ solvothermal reactions to prepare these solids.However, there is very little or no attempt to rationalize the synthetic procedure in terms of the reaction chemistry that is taking place in aqueous solution which dictates the fate of the final structure.The problem is also complicated by the black-box nature of the hydrothermal reaction and the dynamic nature of POM clusters that continuously change to smaller or larger fragments with varying pH, ionic strength and nature of metal ions.
Our group has been investigating the reaction of aqueous molybdates with a number of metal ions under varying pH in the presence of various organic amines that are linear, cyclic, aromatic and long chain surfactants with a view to obtain solids based on self-assembling nano-building blocks of POMs 17- 20 .In this work, we preferred cupric ions as a large number of structurally diverse copper molybdates are isolated from aqueous solution.Also, these can exhibit promising magnetic and catalytic properties.This paper is an attempt to examine the influence of pH on the products formed.Our results reported here clearly suggest that copper complexes formed during the initial stages of the reaction dictate the architecture of the final solids.

Experimental Section Method
All the reagents were purchased from Aldrich and used as such without further purification.The samples prepared in this chapter are synthesized under slow evaporation.(NH 4 ) 6 Mo 7 O 24 .4H 2 O (0.625mmol, 0.7724g) and CuCl 2. 2H 2 O (5mmol, 0.8524 g) were dissolved in 40 mL of distilled water and the pH was adjusted using dilute HCl.When pH was ~2, the compound 1 crystallized out after two weeks while the same solution at pH~6 yielded instantaneously an amorphous phase.This phase on heating at 300°C resulted in a single phasic solid 2.

Characterization
Single crystal diffraction studies were carried out on a Bruker AXS SMART Apex CCD diffractometer with a MoKα (0.71073Å) sealed tube at 28°C.The software SADABS was used for absorption correction and SHELXTL for space group and structure determination and refinements [21][22] .The molybdenum atoms were located first and then remaining atoms were deduced from subsequent difference Fourier syntheses.The hydrogen atoms were located using geometrical constraints.All the atoms except H were refined anisotropically.The least-squares refinement cycles on F 2 were performed until the model converged.Crystal data is provided in table 1.
The infrared spectra were recorded on a Nicolet 5DX spectrophotometer with pressed KBr pellets.TG and DT analyses were carried out using Perkin-Elmer TGA7 and DTA7 system on well ground samples in flowing nitrogen atmosphere with a heating rate of 10°C/min.Room-temperature X-ray powder diffraction data were collected on a Bruker D8 Advance diffractometer using Ni-filtered CuKα radiation.Data were collected with a step size of 0.02° and at count time of 2s per step over the range 2° < 2θ < 60°.The susceptibility measurements as a function of temperature were performed with a SQUID magnetometer.Diamagnetic corrections were estimated using Pascal's constants.2).Each Mo 36 unit contains two Mo 18 subunits built of sixteen edge-sharing and  vertex-sharing MoO 6 octahedra and two pentagonal bipyramids (Figure 3) that are in turn related by an inversion center.Among the 128 oxygen atoms of the cluster anion, 16 oxygens were water molecules as suggested by the bond valence sum calculations (Table 2).The Cu(II) sites exhibit octahedral geometry 1D chains made of Mo36 cluster cores have been reported earlier in few solids [23][24][25] .The 1D chain observed in our case is similar to that of (NH 4 ) The powder X-ray diffraction pattern (Figure 4) of Cu 3 Mo 2 O 9 , 2, is isostructural with the literature reports [27][28] .We did not succeed in growing suitable single crystals of 2 under our reaction condition.In this structure, copper exhibits both octahedral and square pyramidal geometry and molybdenum is tetrahedrally coordinated.The building block of this structure appears to be the occurrence of a trimeric copper cluster [29][30][31] arising from edge sharing of the two copper square pyramids with an octahedron as shown in Figure 5.The trimeric clusters are further linked to {MoO 4 } tetrahedra to form the 3D network (Figure 6).Around both the copper atoms, there is a strong Jahn-Teller distortion with considerable extension of the apical bonds in comparison to the equatorial bonds.

Magnetic Properties
Organic/inorganic hybrid materials provide new opportunities in the realm of molecular magnetism for studying magnetically condensed (i.e.exchangecoupled) systems.Introduction of secondary metal have shown that the resulting structures are influenced by the coordination preferences of the secondary metal.The magnetism of 1 and 2 may be attributed solely due to the presence of Cu 2+ ions (3d 9 , S=1/2).Since Mo 6+ (3d 0 ; S=0) ions do not possess an effective magnetic moment, they do not contribute to the bulk properties.The temperature dependent magnetic susceptibilities of 1 and 2 have been measured in the range 3-300K.All of them follow the Curie-Weiss law at high temperature.
In  7. The inverse susceptibility plot as a function of temperature is linear, following the Curie-Weiss law with C = 4.1 emu mol-1 and θ =-5.9K.This deviation in Curie and Weiss constants can be due to the presence of paramagnetic impurities in the sample.
Cu 3 Mo 2 O 9 , 2, consists of copper trimeric chains.The magnetic susceptibility χ and 1/χ measured in an applied field of 1KOe is shown in figure 8.The thermal variation of χT is shown in figure 9. From 300 to 3 K, χT decreases reaching a minimum at 10 K indicating antiferromagnetic interactions.In the temperature range of 50-300 K, the magnetic susceptibility follows the Curie-Weiss law, χ = C / (Tθ), with the Curie constant of C = 1.7 emu mol-1 and the Weiss temperature of θ = -235K.The negative Weiss temperature suggests the dominance of antiferromagnetic near-neighbor interaction.

{CuO 6 }
with four oxygen donors of water molecules on a basal plane and two bridging oxo groups of the Mo36 clusters at the apical position extending it into 1D infinite chain.The Cu-O distances [~2.513 (1) Ǻ] linking the anion clusters are much larger than Cu-O distances [~2.233 (3) and 2.179(1) Ǻ] on the basal plane.

Table 2 : Bond valence sum calculations of 1. Bond Bond Bond val- atoms Length valence ence sum
The crystal structure analysis of (NH 4 ) 6 [{Cu(H 2 O) 4 } {Mo 36 O 112 (H 2 O) 16 }].21H 2 O, 1 showed the occurrence of 1D covalently bonded chains, built up from the polyoxomolybdate cluster containing thirty six molybdenum atoms linked through copper hydrate complex (Figure