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

Chapter Five Heterometallic Oxalato Complexes as Precursors to Metal Oxides 148 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides Chapter Five Heterometallic Oxalato Complexes as Precursors to Metal Oxides 5.1 Introduction The work described in this chapter is focussed on the complexes bridged by the oxalato ligand. The chemistry and the applications of oxalato complexes have been reviewed in Section 1.4. Different from the bridging bidendate bipyridine, disphosphine and the phosphido ligands described in Chapter Two, Chapter Three and Chapter Four, the oxalato ligand can link two or more metal centres while 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 complexes revealed two suitable complexes, viz {[(n-C4 H9 )4 N][MnIICrIII(C2 O4 )3 ]}n, 5.1161 and {[BaII6(H2O)17][CrIII(C2 O4)3]4 } ⋅ 7H2 O, 5.2, 162 which have been 149 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides 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 (5.1)161a and 5-2 (5.2).162 K3[Cr(C2O4)3] 3H2O (aq) + MnCl2 4H2O (aq) - KCl K[MnCr(C2O4)3](aq) [(n-C4H9)4N]Cl - KCl [(n-C4H9)4N][MnCr(C2O4)3] 5.1 (s) Equation 5-1 BaCrO4 (aq) + 16 H2C2O4 (aq) + BaC2 O4 (aq) → {[BaII6(H2O)17][CrIII(C2O4)3]4} ⋅ 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 150 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides 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 n- O O O O O Cr O O Cr O O O O O O O OO O O O O O Mn O O O Mn O O Mn O O O O O O O O O O O O O O O Cr O O Cr O O O O O Cr O O O O O O O O OO O O O O Mn O O O O Mn O Mn O O O O Mn O O O O O O O O O O O O O O O Cr O O O O Cr O O Cr O O O Cr O O O O O O O O O O OO O O O O O O O Mn O O Mn O O Mn O O O O O O O O O O O O O O Cr O O Cr O O Cr O O O O O O OO O OO O O O Mn O O Mn O O O O O O O O O O O O Figure 5-1: A honeycomb-like structure of the anionic network 5.1. The (n-C4H9)4N cations are omitted for clarity.76c, 82, 161 151 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides O O O O H2O O O O O Cr O O O O O O O O OH2 Ba OH2 OH2 Ba O O Ba O O OH2 H2O Ba O Ba OH2 H2O O H2O O OH2 OH2 O OH OH2 O H2O O O O O O O O O O Cr Cr O O O O OO Cr Ba O O H2O O H2O O O O O O n 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. 152 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides 5.2.1 Thermolysis of {[(n-C4H9)4N][MnIICrIII(C2O4)3]}n 5.1 and {[BaII6(H2O)17][CrIII(C2O4)3]4}⋅7H2O 5.2 Thermolysis of 5.1 at 500 °C for 10 hours gave a brownish black solid of Mn1.5Cr1.5 O4 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 Mn1.5Cr1.5O4 spinel obtained from thermolysis of 5.1 at 500 °C. XRD pattern of cubic Mn1.5Cr1.5O4 (PDF# 00-033-0892) is shown in filled square drop line. A FTIR spectra (KBr) comparison (Table 5-1) revealed that the oxalato ligand peaks at 1628, 1387, 1340, and 804 cm-1 in 5.1 have been weakened (1628 cm-1) or vanished (the rest of the signals) upon thermolysis, suggesting the completion of 153 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides 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 = 2 sigma (I)]: R1 = 0.0192, wR2 = 0.0452. R1 and wR2 (all data): R1 = 0.0209, wR2 = 0.0459. Largest diff. peak and hole, aÅ-3 = 0.617 and -0.260. 177 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides 5.4.3 Preparation of Complex 5.3 A colourless aqueous solution (2ml) of InCl3 (0.21g, 0.93mmol) was slowly added to a purple solution (2ml) of K3[Cr(C2O4)3] ∙ 3H2O (0.45g, 0.93mmol). The solution remained purple in colour and slight white precipitate was observed. It was left to react at room temperature for 15 h without stirring. White solids in purple solution were formed. The white powder (0.17g, 45%) was separated from the purple filtrate. Colourless crystals of [KIn(C2O4)2(H2O)4]n 5.3 were obtained from the filtrate by layering of the solution mixture with ethanol. Analytical data for complex 5.3: (Found: C, 12.06 H, 1.59 In, 26.07 K, 6.95. Anal. Calc. for proposed [KIn(C2O4)2(H2O)4]n: C, 11.94 H, 1.99 In, 28.60 K, 9.72). IR (cm-1 KBr): 3536 (νO-H), 3416 (νO-H), 1651 (νasC=O), 1628 (νC=O), 1357, 1318 (νsC-O + νC-C), 814 (δO-C=O + νM-O), 478 (δO-C=O). 178 [...]... using JEOL JSM -52 00 Scanning Microscope JEOL JFC-1600 auto fine coater was used for Pt target coating Thermolysis of 5. 1 and 5. 2 were carried out by using furnace Carbolite RHF 14/9 at a set temperature of 50 0 °C 5. 4.2 X-Ray Crystallography The powdered samples of Mn1.5Cr1.5O4 and the mixture (BaCrO4 and BaCO3) were analysed by Siemens D50 05 X-ray Diffractometer with CuKα1 radiation (λ = 1 .54 056 Å), operated... as Precursors to Metal Oxides Figure 5- 9: Packing diagram of complex 5. 3 (top) and the enlarged grid (bottom) showing that the K(I) and In(III) metals are linked by the oxalato ligands (Dark green = indium, Blue = potassium, Red = oxygen, Grey = carbon) 170 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides Hydrogen bonds were observed in complex 5. 3 (Figure 5- 10 and Table 54 )... 60 50 40 0 100 200 300 400 50 0 600 700 o Temperature / C Figure 5- 7: TG curve of 5. 2 under flowing air at 20 °C/min 161 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides Table 5- 2: TG analysis of decomposition of complex 5. 2 Temp (˚C) Start End 25 310 Weight Loss (%) Obs Obs Possible Process Calcd Products (N2) (air) 310 23 22 18 dehydration 51 0 28 28 33 decomposition 51 50 ...Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides [(n-C4H9)4N]+ + 1 e- → (n-C4 H9 )3 N + C4 H8 + 1/2 H2 Equation 5- 3 3 [MnCr(C2O4)3]- → 2 Mn1 .5 Cr1 .5 O4 + 10 CO2 + 8 CO + 3e- Equation 5- 4 Overall: 3 [(n-C4H9)4N][MnCr(C2O4)3] (s) → 2 Mn1 .5 Cr1 .5 O4 (s) + 3 (n-C4 H9 )3 N (g) + 3 C4H8 (g) + 10 CO2 (g)+ 8 CO (g) + 3/2 H2 (g) Equation 5- 5 It is unusual for a bi- or polymetallic complex... oxalato chromate in complexes 5. 176c, 161 and 5. 2162 (Section 5. 1) are different from that of the oxalato indate in complex 171 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides 5. 3 Therefore it is necessary to compare these oxalato metallates in order to gain a better understanding on the similarities and differences between them This understanding is important since they... C(1)-C(1)#8 1 .53 5(4) Bond Angles (˚) O(1)#1-In(1)-O(1) 152 .7(8) O(1)#2-In(1)-O(1) 93.2(2) O(1)-In(1)-O(1)#3 93.2(2) O(1)#1-In(1)-O(2) 76.3(6) O(1)#2-In(1)-O(2) 136.3(6) O(1)-In(1)-O(2) 80.8(6) O(1)#3-In(1)-O(2) 71.0 (5) O(1)-In(1)-O(2)#2 71.0 (5) O(2)-In(1)-O(2)#2 1 35. 0 (5) O(1)-In(1)-O(2)#1 76.3(6) O(2)-In(1)-O(2)#1 65. 5(7) O(1)-In(1)-O(2)#3 136.3(6) O(2)-In(1)-O(2)#3 1 35. 0 (5) O(2)#1-K(1)-O(2) 54 .9(6) O(2)#4-K(1)-O(2)... O(2)-In(1)-O(2)#3 1 35. 0 (5) O(2)#1-K(1)-O(2) 54 .9(6) O(2)#4-K(1)-O(2) 142.0(4) O(2)-K(1)-O(2) #5 142.0(4) O(2)-K(1)-O(3)#4 74.0 (5) O(2)-K(1)-O(3) #5 120 .5( 5) O(2)-K(1)-O(3)#1 96.8(6) O(2)#1-K(1)-O(3) 96.8(6) O(2)#4-K(1)-O(3) 120 .5( 5) O(2)-K(1)-O(3) 69.4 (5) O(2) #5- K(1)-O(3) 74.0 (5) O(3)#4-K(1)-O(3) 91.0(1) O(3) #5- K(1)-O(3) 91.0(1) O(3)#1-K(1)-O(3) 164.9(9) O(2)#7-C(1)-O(1) 126.8(2) O(1)-C(1)-C(1)#8 116.8(2)... In(C2O4)1 .5( H2O)3,177 NH4[In(C2O4)2] ∙ 2H2O189 or [In(OH)(C2O4)(H2O)]3 ∙ H2O190 This study has clearly demonstrated the possibility of using heterometallic In(III) oxalato complexes as precursors to mixed oxides of indium 5. 3 Conclusion Heterometallic oxalato complex 5. 1 could serve as a suitable singlemolecule precursor to composite metal oxide Mn1.5Cr1.5O4 The coordinative versatility of the oxalato ligand... bis(2ethylhexyl)sulphosuccinate169 and Ba(NO3 )2 170) with Na2 CrO4 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, 171 159 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides Figure 5- 6: A complicated XRD pattern obtained from the thermolysed product mixture of 5. 2 The experimental... = 626 [R(int) = 0.0270] No of parameters = 49 Good-of-fit = 1.173 R1 and wR2 [I>2 sigma (I)]: R1 = 0.0192, wR2 = 0.0 452 R1 and wR2 (all data): R1 = 0.0209, wR2 = 0.0 459 Largest diff peak and hole, aÅ-3 = 0.617 and -0.260 177 Chapter Five: Heterometallic Oxalato Complexes as Precursors to Metal Oxides 5. 4.3 Preparation of Complex 5. 3 A colourless aqueous solution (2ml) of InCl3 (0.21g, 0.93mmol) was . 156 Figure 5- 5: SEM micrograph of Mn 1 .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 ,50 0 times. Mn 1 .5 Cr 1 .5 O 4 . 300 $C. 1 65 The proposed degradative pathways and products are summarised in Equations 5- 3, 5- 4 and 5- 5. All byproducts [C 4 H 8 , H 2 , CO 2 , CO and (n-C 4 H 9 ) 3 N] are volatile and thus. 71.0 (5) O(1)-In(1)-O(2)#2 71.0 (5) O(2)-In(1)-O(2)#2 1 35. 0 (5) O(1)-In(1)-O(2)#1 76.3(6) O(2)-In(1)-O(2)#1 65. 5(7) O(1)-In(1)-O(2)#3 136.3(6) O(2)-In(1)-O(2)#3 1 35. 0 (5) O(2)#1-K(1)-O(2) 54 .9(6)