Chapter One Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphidoand Oxalato- Ligands Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands Chapter One Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands 1.1 Introduction The synthesis and reactivity of the homo-1 and heterometallic2 transition metal complexes is a research topic that has been widely studied by inorganic chemists. These complexes are particularly interesting because they often show different reactivity compared to the mononuclear complexes. This difference is due to the cooperative effect between the metals within a molecule.3 Such an effect has an implication in both stoichiometric3, and catalytic5 reactions involving the homo- and heterometallic complexes. These homo- and heterometallic complexes are normally prepared from a cationic metal complex (Lewis acid) and a metalloligand (Lewis base). This thesis is aimed at the preparation of the homo- and heterometallic complexes based on three types of selective and representative chemical complexes, namely the homo- and heterometallic based on pincer [PCP] complexes and the heterometallic complexes supported by the phosphido- as well as the oxalato- ligands. An overview of these complexes is given in this chapter in order to provide the background information of the complexes under study. The Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands homo- and heterometallic based on pincer complexes will be introduced in the next section, followed by the homo- and heterometallic complexes bridged by the phosphido- and the oxalato- ligands. The objective and scope of this study will be defined in the final section of the chapter. 1.2 Oligonuclear Homo- and Heterometallic Transition Metal Complexes from the Pincer [ECE] Type Ligands (C = Carbanion, E = Nitrogen, Phosphorus, Sulfur) The pioneering studies by Shaw6 and van Koten7 on the pincer complexes have inspired many research groups to work on this type of complexes. The pincer compounds are well known for their application in catalysis.8 Apart from that, they have been used to prepare the polynuclear homo- and the heterometallic complexes.9 These homo- and heterometallic complexes have various structures and some of them show interesting properties.10 These properties make them potentially useful as catalysts,11 electronic and optical devices,12 metal templates in the construction of macrocyclic rings13 as well as building blocks and crosslinkers for polygons14 and polymers.15 The oligonuclear pincer complexes play an important role in the chemistry of dendritic molecules.16 The dendrimers incorporated with pincer complexes have shown to be useful in catalysis,17 green chemistry,18 material chemistry19 and gas sensing technology.20 However, the research that is specifically focussed on the oligonuclear pincer complexes is rare.9 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands In this section, the oligonuclear pincer complexes of transition metals with different types of ligand are reviewed. The discussion will be focussed on the complexes with cyclometallated pincer ligands (Figure 1-1), where C refers to either an aliphatic (1.1) or an aromatic (1.2) carbanion, while E refers to the donor atoms like N, P and S. Other complexes such as those contain pincer [EC’E] (C’ = carbene), EEE or EE’E (E ≠ E’) type ligands fall outside of the scope of current discussion and will be excluded. ERn M ERn (1.1) ERn L M L ERn (1.2) M = Transition Metals; E = S, n = 1; E = N, P, n = Figure 1-1: Chemical structures of the pincer [ECE] type complexes.8 1.2.1 Oligonuclear Pincer Complexes Obtained by Coordination of Bridging Ligands to Metal Vacant Sites It is known that the halide-ligand trans to the M-C σ bond in a pincer [ECE] complex can be easily replaced by a solvent molecule or a weakly coordinating ligand.21 This active site is used for the coordination of other substrates. The following examples show how a homo- and heterometallic complex can be prepared by using the metal active site. These complexes are Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands prepared by either bridging the two pincer moieties using a bridging ligand or by coordinating a metalloligand to the metal centre. The anionic ligands such as halo,22 hydrido,23 acetato24 and cyano25 ligands are known to serve as both terminal26 and bridging ligands22-25 in the coordination complexes. The complexes with these ligands have been used as catalyst precursors in the organic reactions.27 Some of these complexes have been identified as an intermediate in the chemical reactions.28 It has been known that the oligonuclear pincer complexes are readily formed from the halo, hydrido, acetato and cyano ligands. Some of these examples are given in this section with their significances highlighted. The preparation of dinuclear pincer type complexes was first reported by van Koten and co-workers.29 In this report, a series of the monohalo-bridged dinuclear Pd(II) and Pt(II) pincer complexes based on the tridentate ligand [{o,o’(Me2NCH2)2C6H3}]- were prepared (Scheme 1-1). The chemical structure of complex 1.3a was confirmed by the single crystal X-ray diffraction technique. The heterometallic complex 1.5 was also synthesised. This pioneering work has led to the preparation of other similar compounds.30 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands NMe2 M X Me2N + H2O NMe2 M X' Me2N - H2O Me2 N Me2 N X M M' NMe2 Me2N X' M = M' = Pt; X = Cl (1.3a), Br (1.3b), I (1.3c); X' = BF4 M = M' = Pd; X = Cl (1.4a), Br (1.4b), I (1.4c) ; X' = BF4 M = Pd, M' = Pt; X = Br; X' = BF4 (1.5) Scheme 1-1: Preparation of the monohalo-bridged dinuclear pincer [NCN] complexes of Pd(II) and Pt(II).29 The stability of the monochloro-bridged dinuclear pincer complex vs the mononuclear aqua pincer complex was studied (Scheme 1-2).31 The results suggested that the product selectivity was apparently affected by the different counter anions. Crystallographic and DFT calculation also suggested the existence of hydrogen bonding in the mononuclear aqua complex. The DFT calculation was previously performed on the dinuclear complexes 1.4a, 1.6a, 1.6b and the mononuclear complex 1.7. The results showed a smaller energy difference (∆Ee) between the BF4- complexes (∆Ee = + 11 kcal/mol) and the [B{C6H4(SiMe3)}4]complexes (∆Ee = + 30 kcal/mol). It was believed that the formation enthalpy of AgCl or NaCl played a role in shifting the equilibrium to the mononuclear aqua complex 1.7.31 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands NMe2 Pd OH2 NMe2 1.7 NMe2 Me2N NMe2 (i) BF4 (ii) Pd Cl Pd Cl Pd X' NMe2 Me2N NMe2 X' = BF4 1.4a (acetone only) [B{C6H4(SiMe3)}4 1.6a, [B{C6H4(SiMe2CH2CH2C6F13)}4] 1.6b Scheme 1-2: Synthetic pathways of dinuclear complexes 1.4a, 1.6a, 1.6b and mononuclear complex 1.7. (i) AgBF4, (CH3)2CO/H2O. (ii) M[B{C6H4(SiMe2R)}4] (M = Na, Ag; R = Me, CH2CH2C6F13), (CH3)2CO/H2O or (CH3)2CO.31 The dichloro-bridged homo- and heterometallic Rh(I) complexes were prepared from the aqua complex 1.8.32 The reactions of complex 1.8 with [MCl(COD)]2 (M = Rh, Ir) led to the dichloro-bridged dirhodium complex 1.9a and the mixed metal Rh/Ir complex 1.9b (Scheme 1-3). On the other hand, reaction of complex 1.8 with PdCl2(COD) and CuCl2 gave Rh2M (M = Pd, Cu) trinuclear complexes 1.10a and 1.10b. It was proposed that the two isomers of complex 1.10b existed in the solution, giving rise to two sets of signal in its 1H NMR spectrum. The phosphine complex that is analogous to complex 1.9 was also prepared and analysed by the X-ray crystallography.11h Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands Cl Rh NMe2 OH2 N Cl Me2 1.8 PdCl2(COD) or [MCl(COD)]2 Cl NMe2 Rh Cl N Me2 Cl M CuCl2 Cl NMe2 Cl Rh N Me2 Cl Cl M Me2N Cl Rh N Cl Me2 M = Rh (1.9a), Ir (1.9b) M = Cu (1.10a), Pd(1.10b) Scheme 1-3: Preparation of complexes 1.9 and 1.10 from complex 1.8.32 Studies of the monohydrido-bridged complexes are of particular importance since they have been identified as possible intermediates in the hydride transfer reaction.28c The monohydrido-bridged complexes 1.11a and 1.11b, as well as the dihydrido bridged complexes 1.12a and 1.12b were first prepared by van Koten and co-workers (Figure 1-2).29 The dinuclear pincer [PCP] hydrido Pd(II) and Pt(II) complexes 1.13a and 1.13b were prepared via the reaction of an (CH3)2CO coordinated precursor complex with NaO2CH (Scheme 1-4).33 The monohydrido-bridged heterometallic complex 1.13c was prepared similarly as complex 1.3.29 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands Me N Pt Me2 N H NMe2 H M H NMe2 Pt Me N NMe2 X' X' =BF4 (1.11a), OTf (1.11b) Pt PPh3 PPh3 BF4 M = Pd (1.12a), Pt (1.12b) Figure 1-2: Chemical structures of complexes 1.11 and 1.12.29 PPh2 M Cl PPh2 AgOTf, acetone - AgCl M O PPh2 PPh2 CH3 OTf CH3 NaO2CH, CH3OH Ph2 H Ph2 P P M Ph P M' PPh2 OTf M = M' = Pd (1.13a) M = M' = Pt (1.13b) M = Pd,M' = Pt (1.13c) PPh2 PPh2 Pt H PPh2 + M O PPh2 CH2 OTf CH3 Scheme 1-4: Preparation of the dinuclear complexes 1.13a, 1.13b and 1.13c.33 The dinuclear Pd(II) acetato-bridged complex 1.14 was prepared by Soro and co-workers (Figure 1-3).30 This complex crystallised as ethanol solvate of Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands [Pd2(NCN)2(μ-CH3CO2)]2[Hg2Cl6] with the dipincer cation, [Hg2Cl6]2- and ethanol in 2:1:1 ratio. The two Pd(II) pincer moieties were found to stack on each other with a Pd-Pd separation of 3.069(1) Å. The monomeric cation is stacked on another dinuclear pincer Pd(II) unit with a Pd-Pd distance of 3.253(1) Å, forming a Pd(II) chain [Pd-Pd-Pd angle = 164.7(1)˚]. N Pd N O C N Pd N CH3 BAr'4 O Figure 1-3: Chemical structure of complex 1.14.30 The trinuclear pincer [NCN] carbonato Pd(II) complex 1.15 was isolated by Protasiewicz and co-workers in an attempt to purify the iodo Pd(II) complex 1.16 (Scheme 1-5).34 The X-ray structure of complex 1.15 showed three Pd(II) pincer moieties bridged by a μ3-carbonato ligand with I3- as counteranion. The three pincer moieties in each individual trimer have the same chirality. The molecule has a pseudo-three fold symmetry axis that passes through the carbon of the carbonato linker and perpendicular to the plane of the carbonato ligand bridging the pincer moieties. Complex 1.15 was believed to be formed by a reaction of complex 1.16 with CO2 (from atmosphere) and OH- (from water or base).34 10 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands complexes with aliphatic carbon backbones have not been reported. It is noteworthy that a number of mononuclear pincer complexes which can be used to synthesise these di- and oligonuclear complexes are available.51 Study of the formation of these complexes is significant as it gives a better understanding on the mechanism involved in chemical reactions catalysed by a pincer type complex. An example has been given in scheme 1-6 where the isolation of dinitrogen bridged Ir(I) complex 1.19 provided useful information to understand how a pincer [PCP] Ir(I) catalyst is deactivated in the catalytic dehydrogenation reaction of alkene.38 The following discussion is focussed on another class of compounds, namely the homo- and heterometallic complexes bridged by the phosphido ligands. The chemical properties of a phosphido ligand and its complexes are different from the ligands and their corresponding complexes discussed in the previous section. Therefore, it is important to understand its chemistry so as to gain a clearer view on the similarities and differences between the phosphido ligand and the bridging ligands mentioned earlier, as well as the complexes supported by these ligands. 1.3 Homo- and Heterometallic Complexes Bridged by the Phosphido Ligands Phosphido ligand [(PR2) -] is an important supporting ligand in homo-52 and heterometallic53 complexes. It often serves as a four-electron donor ligand bridging across the homo-54 or heterometals55 (Figure 1-7, compounds 1.30 – 17 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands 1.35). Although the transition metal complexes bridged by the phosphido ligand have been known long ago, their new chemistry continues to reveal throughout the years. There are examples where the phosphido ligand acts as a terminal ligand56 or a capping ligand between three metal centers.57 An example is given in Scheme 1-9. In these complexes, one of the phenyl rings in the phosphido ligand acts as an additional support to the metals (Compounds 1.37 and 1.38).57 Ph2 P SiR3 (OC)3Fe Pt t Ph2 P Pt (p-Tolyl)3P Pt P Ph2 1.31 P(p-Tolyl)3 R2P PPh3 Pt P M Ph2 PPh3 M = Ag 1.33a, Au 1.33b Cl PR2 Ph2 P C6F5 Pd Pd Ph2 P M C6F5 C6F5 Cl R2 P Pd Re(CO)4 H 1.32 Cl Bu (OC)4Re R = OMe 1.30a, Ph 1.30b Cl t P NCtBu t BuCN Bu [NBu4]+ Pt C6F5 Ph2 P Pt P Ph2 Pt Br 2- Ph2 P C6F5 Pt Br C6F5 Cl i R = N Pr2 1.34a, N(Cy)2 1.34b 1.35 Figure 1-7: Complexes supported by the phosphido ligand.52-54 18 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands Ph Ph Pd P C6F5 Ph2 P NCCH3 M Pt P NCCH3 C6F5 Ph2 C6F5 1.36 M = Pd Pt C6F5 - NCCH3 Pd P C6F5 Pt P P Ph2 Ph2 C6F5 1.37 M = Pt C6F5 Pt C6F5 C6F5 Ph2 P Pt Pt P P Ph2 Ph PPh2 Pt C6F5 1.38 Scheme 1-9: Pd(II) and Pt(II) complexes with the phosphido ligands in a non-classical coordination mode.57 1.3.1 Synthesis and Chemical Reactivity of the Complexes Bridged by the Phosphido Ligands The complexes bridged by the phosphido ligands are typically obtained either by an activation of the P-X bond in the ligand precursor PPh2X (X = H,58 Cl54c, 59), the P-C bond of a phosphine ligand60 or the P-P bond61 of the dimeric compound (PPh2)2 in the presence of a metal complex. Scheme 1-10 shows the synthetic pathways of a Mo/Ni (1.39)62 and a Cr/Co (1.40)63 heterometallic complex obtained via a P-C bond cleavage of a phosphine ligand, whereas the chromium complexes (1.41, 1.42 and 1.43) shown in Scheme 1-11 were obtained through a desulfurisation reaction of a thiophosphinito ligand.64 19 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands O C Ni CO Mo CO Mo refluxing C O + THF, days excess PPh2(CH + . CO Ni Ph2P Mo CO CH=CH 2) 1.39 O Ph CO Co CO Cr CO Co CO OC OC OC CO C O O C Ph CO O P Co CO Cr PPh2 OC Co CO OC 1.40 PPh2 Ph2P O Ph O CH 2Cl2, reflux Scheme 1-10: Formation of the phosphido complexes 1.39 and 1.40 via a P-C bond cleavage.62, 63 Cr OC OC + S P R OC CO OC Cr R Cr CO OC CO toluene,110° C OC CO H Cr OC Cr P H3C CH3 CO CO + 1.41 H3C CH3 Cr P Cr + Cr P H3C CO OC CO S Cr Cr P CH3 1.42 H3C CH3 1.43 + [CpCr(CO)2]2 + [Cp4Cr4S4] Scheme 1-11: Formation of the phosphido complexes 1.41, 1.42 and 1.43 via a desulfurisation reaction of a thiophosphinito ligand.64 20 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands The di- and polynuclear complexes supported by the phosphido ligand are resistant to fragmentation due to the strong M-P bond.65 Scheme 1-12 shows a series of reactions between the dinuclear molybdenum complex 1.44 and various reagents. 65b The Mo-P bonds were not affected at the end of the reactions.65b Cy2 P OC OC Mo Mo H 1.45 CO CO CO OC Mo Cy2 P Mo Mo CO OC CO Mo Mo CO HSnPh3 MnCp'(CO)2 CO OC 1.47 CNtBu Cy2 P H 1.44 Mo2Cp2(CO)6 photolysis OC Cy2 P Mo Mo N CO t Bu C H 1.46 OC Cy2 P Mo Mo H OC Mn Cy2 OC P Mo Mo Sn Ph3 CO 1.49 CO CO 1.48 Scheme 1-12: Reactivity of the dinuclear molybdenum complex 1.44 towards various reagents. 65b Strong M-P-M cores allow the complexes to be used as precursors to the homo- and heterometallic complexes.66 Metal(s) can be incorporated into the low 21 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands nuclearity complexes without breaking the M-P-M core, as shown in Scheme 113.66a, 66b Cy2 P OC Fe Cp LiBu Fe Cp CO H Cy2 P Li (CO)CpFe trans 1.50 (PiPr3)AuCl FeCp(CO) OC C Mo Cp Mo(CO)6 Mo Cp (OC) Mo CO OC Cp 1.54 H Cy2 P C photolysis Cp Fe (CO) Cp Mo C H H2 1.53 CO Au PiPr3 cis 1.52a, trans 1.52b Cy2 P Mo Fe Cp 1.51 photolysis Cp Fe H Cy2P Cy2 P OC CO - H2 - CO Mo CO Mo Fe(CO)3 Cp OC Fe (CO)3 1.55 Scheme 1-13: The homo- and heterometallic complexes generated from complexes 1.50 and 1.53.66a, 66b Although the complexes supported by a phosphido-ligand are generally thought to be stable, there were reports on the transformation of phosphido ligand67 or the cleavage of M-P bond.68 As shown in Scheme 1-14, the insertion of sulfur into the Mo-P bonds in complex 1.54 gave dithiol complexes 1.55 and 1.57.67a Another example shown in Scheme 1-15 depicts the formation of Pt/Os heterometallic complexes 1.60 and 1.61 which involves the cleavages of the Pt-P and the Os-P bonds in complex 1.59.68a 22 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands Ph2 P (OC)2W CH2Cl2, reflux, 40 h Mo(CO)4 S8 + 1.54 W S Ph2P S S S S Mo Mo PPh2 S S S S S + W S S + W S S P Ph2 1.56 1.55 S W S Mo S S Ph2 P S + PPh2 S W (CO)3 Mo(CO)5 1.58 1.57 Scheme 1-14: Sulfur insertion into the Mo-P bonds of the phosphido complex 1.54.67a t t Bu Bu P Pt t Bu3P Os H Os C Me C Os Pt Me 1.59 t t 174 oC 2h Me Bu Os H t Bu P H C C C + Os Os Pt t Bu3P 1.60 Bu tBu P Pt Pt Os Os Os Pt Pt P tBu t Bu 1.61 Scheme 1-15: Formation of the Pt/Os complexes 1.60 and 1.61 from the vinylidene Pt/Os complex 1.59, which involves the cleavages of the Pt-P and Os-P bonds.68a The phosphido ligand can be used as a bridging ligand to link two relatively bulky moieties. The mononuclear phosphido complex 1.62 was used as a precursor to the dinuclear pincer type complex 1.63 (Scheme 1-16).69 The heterometallic Pt/Pd phosphido-bridged complex 1.64 has been prepared in a 23 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands similar way by reacting [Pd(C6H4CH2NMe2-2)(CH3CN)2][BF4] with complex 1.62. However, the analytical data showed the product to be the aqua complex 1.65 (Figure 1-8). It was believed that complex 1.65 was formed via a substitution of the CH3CN ligand in complex 1.64 by an aqua ligand. The crystal structure of complex 1.65 showed that the Pt- P bond length [2.439(2) Å] is longer than those observed for the two molecules of the NCN Pt(II) diphenylphosphine complex 1.66 [The bond lengths observed for two molecules in the unit cell are 2.346(2) Å and 2.360(2) Å]. The angle between the two metal coordination planes is 71.1(2)˚. Me2 N Pt (i) Ph Ph Me2 N P Me2N Pt BF4 NMe2 NMe2 Pt 1.63 PPh2 NMe2 Ph 1.62 Ph (ii) Me2 N Pt BF4 P Pd NMe2 H3CCN N Me2 1.64 Scheme 1-16: Preparation of complexes 1.63 and 1.64 from complex 1.62. (i) [Pd(OH2)(NCN)][BF4]. (ii) [Pd(C6H4CH2)NMe2-2)(CH3CN)2][BF4]. 69 24 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands Ph Me2 N Pt Ph NMe2 P Pd NMe2H2O BF4 N Me2 Pt PHPh2 NMe2 1.66 1.65 Figure 1-8: Chemical structures of complexes 1.65 and 1.66.69 1.3.2 Recent Work on the Phosphido-bridged Complexes There are continuing efforts to study the reactivity of the phosphidobridged complexes.70 Ruiz et al have investigated the reactivity of the unsaturated complexes [M2Cp2(µ-COMe)(µ-PR2)2]BF4 [M = W, R = Ph (1.67a); M = Mo, R = Et (1.67b) ].70a Reactions of the complexes with carbonyl, isocyanide, bidentate phosphines and diazoalkane gave rise to a series of compounds as shown in Scheme 1-17. 25 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands OCH3 C M L P P OCH3 C BF4 M + W L t BuNC M = Mo, P = PEt2, L = CO 1.68a, CNtBu 1.68b P CNtBu BF4 W P + NtBu H3CO C C P Mo Mo P t BuN C P = PEt2 1.71 1.70 M = W, P = PPh2, L = CO 1.69a, CNtBu 1.69b BF4 CO or CNtBu N CRR' R2 R2 P P M M C OCH3 N2CRR' BF4 Ph2 N P W P Ph2 W OC R, R' = H, Si(CH3)3 1.72a M = Mo, R = Et 1.67a; M = W, R =Ph 1.67b R, R' = Ph 1.72b Me2P -CH2 -PMe2 OCH3 M C P M BF4 + W P Me2P PMe2 M = Mo, P = PEt2 1.73a M = W, P = PPh2 1.73b Me2P P P W C OCH3 PMe2 P = PPh2 1.74 Ph2 Ph2 P P BF4 + W Me2P W H BF4 PMe2 1.75 Scheme 1-17: Chemical reactivity of the dimolybdenum and ditungsten complexes 1.67a and 1.67b.70a 1.3.3 Summary The study of structures of the homo- and heterometallic complexes supported by the phosphido ligand and their chemical reactivity remains a fascinating research topic in chemistry. Although the complexes supported by phosphido ligand have been known for decades, there are active ongoing efforts to explore the unknown knowledge of these compounds. Some of these examples have been given in Sections 1.3.1 and 1.3.2. The diverse coordination modes of the ligand make it suitable as a support to the homo- and heterometallic complexes, with or without a direct M-M bond. 26 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands 1.4 Homo- and Heterometallic Complexes Bridged by the Oxalato Ligand Two types of oligonuclear homo- and heterometallic complexes have been introduced in Section 1.2 and Section 1.3, namely oligonuclear homo- and heterometallic complexes based on the pincer complexes and the homo- and heterometallic complexes bridged by the phosphido ligand. This section will be focussed on the homo- and heterometallic complexes bridged by the oxalato ligand. Their synthetic pathways, structures and applications are reviewed. 1.4.1 Synthesis, Structures and Applications of the Homo- and Heterometallic Oxalato-bridged Complexes The oxalate anion [(C2O4)2-] has been known as one of the most versatile ligands in coordination chemistry due to its ability to bind to metals in different coordination modes.71 It usually acts as a tetradentate bridging ligand between two metals.72 Various coordination modes of the oxalato ligand are shown in Figure 19.71d M O O M O O O O O O M - O O M O (b) (a) O O M O O (c) O M O O (d) M O O M M O O M M M O O (e) (f) Figure 1-9: Various bonding modes of the oxalato ligand.71d 27 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands The oxalato-complexes can be formed through the reaction of a metal salt with oxalic acid,73 metal oxalate74 or tris(oxalato)metallate.75 In some cases, a cation exchange is needed in order to induce crystallisation of the final products.76 These complexes can also be obtained from a cyanopyrimidine77 or a tricarboxylic acid78 precursor via a hydrothermal reaction. The choice of cations and metals bridged by the oxalato ligand, 73 as well as other auxiliary ligands79 are important as they are known to have an effect on the structures, packing and the properties of the oxalato-complexes. Figure 1-10 shows a decagon ring of 3D network of compounds {X+[Fe2(OH)(C2O4)2Cl2] ∙ 2H2O}n (X+ = EtNH3+ 1.76a, PrNH3+ 1.76b) obtained from FeCl3, H2C2O4 and an ammonium salt (EtNH3Cl or PrNH3Cl),73 while Figure 1-11 shows a network of [MII(2,2’-bpy)3][ClO4][MnIIFeIII(C2O4)3] (MII = Fe, Ru; 2,2’-bpy = 2,2’-bipyridine) 1.77, which was obtained from the octahedral Mn(II) cations and [Fe(C2O4)3]3- followed by an addition of [MII(2,2’bpy)3][ClO4].80 O O O O Fe O O O H O O O Fe Cl O O O O O O O Cl Cl O H O O O O Cl O Fe O O O Fe Fe Fe H Cl O Cl O O H O O O Cl O O OH HO HO Fe Fe OH H O O O Fe Cl O O O O O O H O O Fe O O O Cl O Cl O O O O O O O O O Figure 1-10: A decagon ring of the 3D network of compounds {X+[Fe2(OH)(C2O4)2Cl2] ∙ 2H2O}n (X+ = EtNH3+ 1.76a, PrNH3+ 1.76b).73 28 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands O O O O Mn O O O O Fe O O O O O Mn O O O O O Fe O O O Mn O O O O O O O O O O O O Mn Mn O Fe O O O O O O Fe O O O O O Mn Mn O O Fe O O O O O O O O O Fe O O O O O O O O Fe O O O O O O O O Mn O Mn O O O O O O O O O O Fe O O O O O O O O O Fe O O O O O O O O O O O O O O O O Fe O Mn O Mn O O O O O O O O Fe O Fe O O O O O O O O O O Mn O Fe O O O O O O O O Mn Mn O O O O O O O O O Fe O O O O O Mn O O O O O O O O Fe Fe O O O O O O O O O Mn O O O O O Fe O O O O O O O O Mn Mn O O O O O O O O O Fe O Figure 1-11: Chemical structure of network of 1.77. The [MII(bpy)3]2+ cations and the ClO4- anions in the cavities are not shown.80 The metal complexes containing oxalato ligand are widely studied for their potential application as molecular magnets.81 The reason for choosing oxalato ligand for this purpose is the capability of the ligand to mediate magnetic exchange between the metals.74, 82 Apart from being tested as magnetic materials, the oxalato-complexes were also explored for their application as precursors to metal oxides or metals.83 It has been shown that the oxalato-complexes could decompose easily to give volatile CO2 and CO. So they could serve as a source of metal oxide. For example, thermolysis of Fe(C2O4) ∙ 2H2O 1.78 gave Fe3O4 nanoparticles as primary product at 415˚C with the elimination of carbon oxides (Scheme 1-18).83c 29 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands 210°C FeC2O4 2H2O FeC2O4 - 6H2O 1.78 415°C - 4CO - 2CO2 Fe3O4 Scheme 1-18: Decomposition of 1.78 to give Fe3O4.83c Pure metals can be obtained if a suitable precursor is used and an optimised experimental condition is applied. As shown in Scheme 1-19, the decomposition of isocyano oxalato Cu(I) complexes 1.79a and 1.79b under a suitable condition led to the formation of Cu(I) cyanide which could be further reduced to give elemental copper.83d R R N N O C C O Cu Cu O C O N CN R R R = H 1.79a, tBu 1.79b - CO2 - CO 200 °C - H2O - HCN - 4(8) Cu2(CN)2 ~ 620 °C - (CN)2 Cu (s) Scheme 1-19: Decomposition of the isocyano oxalato Cu(I) complexes 1.79a and 1.79b to give metal copper.83d 30 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands 1.4.2 Summary As described in Section 1.4.1, the oxalato ligand is a useful bridging ligand for the homo- and the heterometallic complexes. Complexes with different structures have been prepared, ranging from discrete molecules to complex networks. The metal complexes containing oxalato ligand have been studied for their applications as magnetic materials as well as precursors to the metal oxides and metals. These examples highlighted the potential use of oxalato-complexes as single source precursors. It is anticipated that many new materials based on the oxalato-complexes will be available in the near future as the chemistry of oxalatocomplexes continues to be an active area of research. 1.5 Objective and Scope The background details of three types of chemical complexes, namely homo- and heterometallic complexes based on the pincer complexes, homo- and heterometallic complexes bridged by the phosphido- as well as oxalato- ligands have been given in Sections 1.2, 1.3 and 1.4. These complexes belong to coordination compounds with different types of ligand, which give rise to different structures. The pincer Pd(II) complexes, Ag/Mn bimetallic clusters bridged by the phosphido ligand and the Cr(III) complexes of oxalato ligand were studied in this work. The aim of this research was to design simple synthetic methodologies as well as to understand the formation pathways and structures of these compounds. The use of Lewis acidic and basic metal complexes, the metalloligands as well as 31 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands the anionic metallates as precursors suggested the diverse strategies that one can approach in the heterometallic syntheses. The remaining thesis is structured as follows: Chapter Two and Chapter Three describe the synthesis and the structural characterisation of mononuclear and dinuclear pincer [PCP] Pd(II) complexes. The use of ESI-MS in the study of the pincer complexes is shown. Chapter Four is focussed on the discussion of the results on the preparation of the four Ag/Mn bimetallic clusters. Their structures and reaction pathways will be covered. Chapter Five is devoted to the presentation of the results on the thermolyses of two known oxalato-complexes and the attempted experiments to prepare the novel heterometallic oxalato-complexes. The conclusion is given in Chapter Six in which the future possibilities are highlighted in the conclusion. This research will provide a clearer picture on the differences and the relationship between the pincer Pd(II) complexes, Ag/Mn bimetallic clusters bridged by the phosphido ligand and the Cr(III) complexes of oxalato ligand together with their advantages. The results of this study could be useful for the researchers who wish to conduct further studies on these complexes. It is particularly worthy for those who wish to enter the field of heterometallic chemistry through these convenient and simple synthetic methodologies. This thesis gives a good account of those ligands and metals available for the preparation of different types of homo- and heterometallic complexes. 32 [...]... oligonuclear homo- and heterometallic complexes have been introduced in Section 1. 2 and Section 1. 3, namely oligonuclear homo- and heterometallic complexes based on the pincer complexes and the homo- and heterometallic complexes bridged by the phosphido ligand This section will be focussed on the homo- and heterometallic complexes bridged by the oxalato ligand Their synthetic pathways, structures and applications...Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands H3C 3+ CH3 N N CO2 from atmosphere 3 N H3C Pd OH- from water or base Pd Pd N N O O C 3 I- O I CH3 N N Pd N 1. 16 1. 15 H3C CH3 N N Pd = Pd N N H3C CH3 Scheme 1- 5: Formation of complex 1. 15 from complex 1. 16.34 The cyano-bridged dinuclear complexes 1. 17 and 1. 18 were synthesised... complexes supported by these ligands 1. 3 Homo- and Heterometallic Complexes Bridged by the Phosphido Ligands Phosphido ligand [(PR2) -] is an important supporting ligand in homo- 52 and heterometallic5 3 complexes It often serves as a four-electron donor ligand bridging across the homo- 54 or heterometals55 (Figure 1- 7, compounds 1. 30 – 17 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes... given in Sections 1. 3 .1 and 1. 3.2 The diverse coordination modes of the ligand make it suitable as a support to the homo- and heterometallic complexes, with or without a direct M-M bond 26 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands 1. 4 Homo- and Heterometallic Complexes Bridged by the Oxalato Ligand Two types of... bond 61 of the dimeric compound (PPh2)2 in the presence of a metal complex Scheme 1- 10 shows the synthetic pathways of a Mo/Ni (1. 39)62 and a Cr/Co (1. 40)63 heterometallic complex obtained via a P-C bond cleavage of a phosphine ligand, whereas the chromium complexes (1. 41, 1. 42 and 1. 43) shown in Scheme 1- 11 were obtained through a desulfurisation reaction of a thiophosphinito ligand.64 19 Chapter One: Homo- ... of M-P bond.68 As shown in Scheme 1- 14, the insertion of sulfur into the Mo-P bonds in complex 1. 54 gave dithiol complexes 1. 55 and 1. 57.67a Another example shown in Scheme 1- 15 depicts the formation of Pt/Os heterometallic complexes 1. 60 and 1. 61 which involves the cleavages of the Pt-P and the Os-P bonds in complex 1. 59.68a 22 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes... tBu t Bu 1. 61 Scheme 1- 15: Formation of the Pt/Os complexes 1. 60 and 1. 61 from the vinylidene Pt/Os complex 1. 59, which involves the cleavages of the Pt-P and Os-P bonds.68a The phosphido ligand can be used as a bridging ligand to link two relatively bulky moieties The mononuclear phosphido complex 1. 62 was used as a precursor to the dinuclear pincer type complex 1. 63 (Scheme 1- 16).69 The heterometallic. .. C12H25 n+ C12H25 CH3 E E O Pt O O n ClO4 N N Pt N N O Pt N E E n ClO4 E - E = dppm, n = 2 (1. 21a) E - E = deprotonated pyrazole, n = 1 (1. 21b) E - E = dppm, n = 2 (1. 20a) E - E = deprotonated pyrazole, n = 1 (1. 20b) E - E = deprotonated imidazole, n = 1 (1. 20c) E - E = deprotonated imidazole, n = 1 (1. 21c) Figure 1- 5: Chemical structures of the dinuclear pincer Pt(II) complexes 1. 20 and 1. 21. 43a Recently... toluene ,11 0° C OC CO H Cr OC Cr P H3C CH3 CO CO + 1. 41 H3C CH3 Cr P Cr + Cr P H3C CO OC CO S Cr Cr P CH3 1. 42 H3C CH3 1. 43 + [CpCr(CO)2]2 + [Cp4Cr4S4] Scheme 1- 11: Formation of the phosphido complexes 1. 41, 1. 42 and 1. 43 via a desulfurisation reaction of a thiophosphinito ligand.64 20 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato-... and 1. 21b) and deprotonated imidazole ligands (1. 20c and 1. 21c) were synthesised by reacting their respective mononuclear chloro -precursors with the bridging ligands, followed by an anion exchange reaction using LiClO4 (Figure 1- 5).43a 13 Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer Complexes, Phosphido- and Oxalato- Ligands n+ H3C O O O O N N Pt N C12H25 . 9 Me 2 N Pt NMe 2 H Pt Me 2 N Me 2 N X' X' =BF 4 ( 1. 11a ), OTf ( 1. 11b ) NMe 2 NMe 2 M BF 4 H H Pt PPh 3 PPh 3 M = Pd ( 1. 12a ), Pt ( 1. 12b ) Figure 1- 2: Chemical structures of complexes 1. 11 and 1. 12. 29 PPh 2 PPh 2 M. I - 3 CO 2 from atmosphere OH - from water or base 1. 15 1. 16 3+ Scheme 1- 5: Formation of complex 1. 15 from complex 1. 16. 34 The cyano-bridged dinuclear complexes 1. 17 and 1. 18 were synthesised. complexes 1. 11a and 1. 11b, as well as the dihydrido bridged complexes 1. 12a and 1. 12b were first prepared by van Koten and co-workers (Figure 1- 2). 29 The dinuclear pincer [PCP] hydrido Pd(II) and