Homo and heterometallic assemblies from lewis acidic and basic metallic precursors 5

Chapter Five Heterometallic Oxalato Complexes as Precursors to Metal Oxides... As mentioned in Section 1.4, the oxalato complexes can be decomposed by thermolysis to give metal oxides o

Chapter Five

Heterometallic Oxalato Complexes as Precursors to

Metal Oxides

acting as a bridging tetradentate ligand [Figure 1-9, (d) and (f)] The complexes

obtained from this ligand often give polymeric network structures As mentioned

in Section 1.4, the oxalato complexes can be decomposed by thermolysis to give metal oxides or metals with elimination of carbon oxides This discussion will pay special attention to the heterometallic and intermetallic oxalato complexes of Cr(III), as an effort to search for the complexes which act as suitable precursors to metal oxides or metals This direction is a current research focus of many research groups.83 Chromium oxide has been used as catalyst,158 materials for secondary lithium batteries159 and solid-oxide fuel cells.160

A literature survey of the known mixed-metal Cr(III)-based oxalato

crystallographically characterised by Atovmyan et al (5.1)161b and M M Bélombé

et al (5.2)162 respectively The compounds were prepared according to the

literature methods.161a, 162 Their synthetic pathways are shown in Equations 5-1

4 BaCrO4 (aq) + 16 H2C2O4 (aq) + 2 BaC2O4 (aq) →

{[BaII6(H2O)17][CrIII(C2O4)3]4} ⋅ 7 H2O 5.2 (s) + 16 H2O (aq) + 12 CO2 (aq)

Equation 5-2

Both the Cr(III) and Mn(II) centres in complex 5.1 as well as the Cr(III)

and Ba(II) centres in complex 5.2 are bridged by the oxalato ligand via its four

oxygen atoms The anionic network in complex 5.1 forms a honeycomb-like

structure (Figure 5-1)76c,82, 161 while complex 5.2 forms a 3D network structure

supported by the oxalato and aqua ligands.162 Part of the polymeric network 5.2 is

shown in Figure 5-2

One of the major differences between 5.1 and 5.2 is that in the former, both

metals are surrounded by nothing but oxalate ligands whereas in the latter, only

the Cr(III) metal is surrounded by oxalates These two materials thus can be used

to compare the relationship between the oxalate coordination and the thermal

stabilities As mentioned in Chapter One, the oxalato ligand in its metal complexes readily decomposes to CO and CO2 while the aqua ligands can be removed upon heating (Section 1.4.1) Therefore it is in principle possible to obtain metal oxide

or alloys via the decomposition of complexes 5.1 and 5.2

O O O O

O

O Mn

O

O

Cr O O

O O

Cr O

O

O O

O O O O

O O O O

Cr Mn

O O

O O O O

O

O Mn

O

O

Cr O O

O O

Cr O O

O O Mn

O O O

O O

O O

Mn

O O O O O O

Mn

O O

Cr O O

O

O CrO

O

O O

O O O O

O

O Cr OO O O

Mn O O

O O

O Cr O

O

O O

O O

O OMn

O

O Cr O

O

O O

O

O O

n

-Figure 5-1: A honeycomb-like structure of the anionic network 5.1 The (n-C4 H 9 ) 4 N cations are omitted for clarity.76c, 82, 161

OOCr

O

H2O

OO

Cr

OO

H2O

OH2Ba

OH2

OCrO

O

OOO

Ba OH2

OH2O

OO

CrOO

O

OO

Figure 5-2: A segment of the polymeric network of 5.2.162

5.2 Results and Discussion

Thermolyses of the oxalato complexes 5.1 and 5.2 were carried out and their

decomposition pathways were studied by TG analysis The decomposition products have been characterised These results are discussed in Section 5.2.1 Attempt to synthesise novel heterometallic Cr(III) oxalato complexes however resulted in the formation of insoluble precipitates A new polymeric complex viz

[KIn(C2O4)2(H2O)4]n 5.3 was isolated in the process of the preparation of an In/Cr

oxalato complex The results will be presented and explained in Section 5.2.2

5.2.1 Thermolysis of {[(n-C4H9)4N][MnIICrIII(C2O4)3]}n 5.1 and

Thermolysis of 5.1 at 500 °C for 10 hours gave a brownish black solid of

Mn1.5Cr1.5O4 spinel,163 which was characterised by powder-XRD, FT-IR and elemental analysis The XRD pattern (Figure 5-3) is in agreement with the XRD pattern of cubic Mn1.5Cr1.5O4 (PDF#00-033-0892) The broad XRD pattern suggests that the product is amorphous

Figure 5-3: Powder XRD pattern of Mn1.5 Cr 1.5 O 4 spinel obtained from thermolysis of 5.1 at

500 °C XRD pattern of cubic Mn 1.5 Cr 1.5 O 4 (PDF# 00-033-0892) is shown in filled square drop line

ligand decomposition The alkyl absorptions at 2971, 2940 and 2880 cm-1 for the

counter cation ([(n-C4H9)4N]+) correspondingly disappeared as well, thus suggesting

its full disintegration Elemental analysis of 5.1 and the thermolysed product provided

information on the composition for both compounds (Calculated for Mn1.5Cr1.5O4 ∙ 5.5

H2O: C = 0%; H = 3.40%; N = 0%; Mn = 25.48%; Cr = 24.12% Found: C = < 0.5%;

H = <0.5%; N = 0%; Mn = 25.84%; Cr = 27.50%) Elemental analysis suggested that

the 1:1 manganese and chromium ratio in 5.1 is maintained in the product.

Table 5-1: IR band assignments for 5.1 before and after thermolysis at 500 ° C

Before thermolysis

/ cm-1

After thermolysis

/ cm-1

Band assignment164

volatile side products The observed weight loss from 300-400 °C (71 %) is closed

to the calculated weight loss (76 %) in the formation of Mn1.5Cr1.5O4 Subsequently, thermolysis was carried out at a slightly higher temperature (500 °C)

to ensure complete conversion A SEM analysis of the resultant particles (Figure

5-5) showed that they are non-uniform with sizes in the region of ca 5 µm

Figure 5-5: SEM micrograph of Mn1.5 Cr 1.5 O 4 spinel obtained from thermolysis of 5.1 at 350

times magnification The insert is a SEM micrograph magnified at 3,500 times

Mn1.5Cr1.5O4 spinel was formulated by Chamberland et al to be valenced, viz MnII[MnIII0.5CrIII1.5]O4.163b It was previously obtained from a

mixed-reduction of two oxides, viz MnIII2O3 and CrIII2O3 (Path b) or mixed-metal oxides

viz MnIIICrIIIO3 or MnIIICrVO4 (Path a & c) under high temperatures (Scheme 1).163 These syntheses require harsh conditions such as high pressure or high temperature The current molecular pathway (Path d) is unique as it provides a lower temperature alternative from a single-molecular bimetallic precursor It takes the advantage of the ligand’s ability to hold the metals in the degradation process There is no requirement for a second substrate, external oxidant or reductant

Scheme 5-1: Different methods for preparation of Mn1.5 Cr 1.5 O 4 163

Other manganese- or chromium-containing by-products were not observed

The degradative pathway proceeded via an internal redox mechanism The TG

experiment conducted under nitrogen flow gave similar result to the TG curve under air flow A single-step decomposition with weight loss of 73 % was observed This result suggested that the conversion readily occurs even in the absence of molecular oxygen It is known that the ammonium cation could be a

source of amine at ca 300 °C.165 The proposed degradative pathways and products are summarised in Equations 5-3, 5-4 and 5-5 All byproducts [C4H8, H2, CO2, CO

and (n-C4H9)3N] are volatile and thus they are easily removed from the products This offers a distinct advantage in the synthesis

It is unusual for a bi- or polymetallic complex to undergo an internal redox

decomposition to generate a mixed-metal composite material without any

metal-containing by-products Complex 5.1 is hence a unique precursor A thermal

treatment of {[BaII6(H2O)17][CrIII(C2O4)3]4} ⋅ 7H2O 5.2 gave a mixture containing

the mixed-metal oxide BaIICrVIO4 This reaction mixture was believed to contain

BaCO3 as a possible side product A similar result was observed by Gleizes166 on

the thermal study of [Ba(H2O)5][Cu(C2O4)2(H2O)], which gave a mixture of

BaCuO2, CuO and BaCO3.166 Previous thermal study of Ba[Co(C2O4)2] ∙ 8H2O

suggested the possibility of formation of BaCO3 in the temperature range of 328

-332 ºC.167 The decomposition of 5.2 involved an oxidation of Cr(III) to Cr(VI), as

facilitated by the redox mechanism given in Equation 5-6 As such, the TG

experiment under nitrogen flow again suggested that molecular oxygen does not

play an active role in the degradation

Ba6(H2O)17(Cr(C2O4)3)4 ⋅ 7 H2O (s) → 4 BaCrO4 (s) + 2 BaCO3 (s) + 18 CO (g) +

4 CO2 (g) + 24 H2O (g) Equation 5-6

A complicated powder X-ray diffraction pattern was obtained from the final products A superimposed experimental XRD pattern and the powder XRD patterns of orthorhombic BaIICrVIO4 (PDF#00-035-0642) and orthorhombic

BaIICO3 (PDF#01-071-2394) is shown in Figure 5-6 It is possible that other products might be present as other unidentified peaks were observed BaIICrVIO4was previously prepared by reacting Ba(II) salts (e.g BaCl2,168 barium bis(2-ethylhexyl)sulphosuccinate169 and Ba(NO3)2170) with Na2CrO4 It has not been prepared from a heterometallic precursor This leads to a challenge to material scientists as BaCrO4 nanomaterials are of current interest.168-170, 171

Figure 5-6: A complicated XRD pattern obtained from the thermolysed product mixture of 5.2 The experimental pattern is shown in solid line

Orthorhombic BaCrO 4 PDF# 00-035-0642 ( -, blue) and Orthorhombic BaCO3 PDF# 01-071-2394 (···, red) are added for comparison

Figure 5-7 shows the TG curve of 5.2 The key step which occurred at

about 300 °C and completed at around 450 °C, generated two metal materials

The weight losses observed from room temperature to ca 300 °C are attributed

to the dehydration of lattice and water hydrate The experimental weight losses under air (28 %) and nitrogen (28 %) flow are similar to each other These values are consistent with the calculated weight (33 %) (Table 5-2) The

decomposition took place at ca 400oC, which suggested a high thermal

stability of 5.2 than that of 5.1 The subsequent experiments on thermal degradation of 5.2 were carried out at 500 °C to ensure complete conversion

Table 5-2: TG analysis of decomposition of complex 5.2

Temp (˚C) Weight Loss (%)

Process

Possible Decomposition Products

Start End Obs

The unchanged Cr/Mn stoichiometric ratio of 1:1 in 5.1 Mn1.5Cr1.5O4indicated the stability of both the substrate and product in this ratio The polymeric 2-dimensional network with a honeycomb-like structure decomposed in the process to give the mixed metal oxide.161b On the other

hand, conversion of 5.2 (Ba:Cr = 1.5:1.0) to BaIICrVIO4 (Ba:Cr = 1:1) required the elimination of excess Ba(II) in the form of BaIICO3 The Ba(II) centres in

5.2 are part of the inter-connected network with Cr(III) It is supported by the

hydrate and oxalato ligands The two metals clearly show different modes of coordination.162 Since the early phase of decomposition was primarily dehydration, it implied that at the end of this dehydration process, the Ba(II) is coordinatively unsaturated and activated Some form of reorganisation would

be needed for the Ba(II) to remain in the supramolecular lattice One possible way is that the Ba(II) could coordinate to other oxalate ligand through its oxygen donor atoms This enhanced Ba-O interactions could set the scene for subsequent decomposition and formation of BaCO3 The competition for oxygen donors between the two heterometals (Cr and Ba) could also explain

why it resulted in a complicated product mixture marked by competing decomposition pathways

5.2.2 Attempted Synthesis of the Heterometallic Cr(III) Oxalato

Complexes

Thermolyses of two known oxalato complexes have been described in

Section 5.2.1 The results highlighted the potential of complex 5.1 as a molecular

precursor to the mixed oxide Mn1.5Cr1.5O4 and the influence of hetero-metal pairs

on the decomposition outcomes They also suggested a dependent relationship between the structures of substrates and the product compositions These findings also highlighted the importance of the study on the thermal behaviours of different types of oxalato complexes With this goal in mind, attempts were made to prepare new heterometallic oxalato complexes Reactions of K3[Cr(C2O4)3] ∙ 3H2O172 with various metal salts gave insoluble solids which precluded their characterisation Recently there are many studies focusing on the preparation of indium compounds from a single-source precursor route.173 These compounds are known to be useful as semiconductors.173 The nanocrystalline indium oxide was previously studied for its use as optoelectronic device.174 Synthesis of an In/Cr oxalato complex was attempted This reaction, however, resulted in a ligand migration reaction and led to the isolation of polymeric compound [KIn(C2O4)2(H2O)4]n 5.3 This complex has been analysed by the X-ray single-

crystal crystallography The formation pathway and its structure are described and

The reaction of K3[Cr(C2O4)3] · 3H2O172 and InCl3 in 1:1 ratio was originally aimed at obtaining a heterobimetallic In/Cr oxalato complex, which could be a molecular source of In/Cr oxide A mixture of white solid and purple solution was observed upon slow mixing of the aqueous solution of the two reagents In order to obtain more information on the white solid, the reaction was repeated and the solution mixture was layered with ethanol The colourless crystal obtained was analysed by single-crystal X-ray diffraction technique and was identified to be [KIn(C2O4)2(H2O)4]n 5.3 The isolation of this unexpected complex suggested that the oxalato ligands are transferred from Cr(III) to In(III)

centres Equation 5-7 was proposed to account for this ligand migration reaction

Complex 5.3 exists as an infinite polymer network, which will be described later

K3[Cr(C2O4)3] · 3 H2O (aq) + 7 H2O (aq) + InCl3 (aq) →

[KIn(C2O4)2(H2O)4] 5.3(s) + [Cr(H2O)6]Cl3 (aq) + K2C2O4 (aq) Equation 5-7

The formation of complex 5.3 was accompanied by the generation of

[Cr(H2O)6]Cl3 and K2C2O4 The hexaaqua ion, [Cr(H2O)6]3+ is a known complex with a regular octahedral geometry and can occur in aqueous solution as violet hydrate, [Cr(H2O)6]Cl3.71d K2C2O4 is also a known metal salt The formation of 5.3,

[Cr(H2O)6]Cl3 and K2C2O4 was driven by the high thermodynamic stabilities of all the three products

The preparations of K/In oxalato complexes have been reported previously 164a, 175 However, there has been no definitive structural report based

on the single-crystal X-ray crystallographic analysis Instead, these materials were