Late Transition Metal Oxo Complexes. The Use of Polyoxometalate as a Stabilizing Ligand Público

Abstract

The use of polyoxometalate (POM) as stabilizing ligands for the synthesis and isolation of late-transition metal oxo (LTMO) complexes is addressed. Metal-oxo species, in particular terminal metal-oxo (O^2-) complexes of the late-transition-metal elements have long been thought to exist as transient intermediates in systems ranging from Cu oxidase enzymes to the surfaces of noble metal oxidation catalysts. However, despite decades of speculation and attempted synthesis, no terminal metal-oxo complexes of any element to the right of Ru in the periodic table had been reported prior to 2004 with the exception of the d⁴ (mesityl)3Ir^V-oxo complex of G. Wilkinson and co-workers. A current bonding paradigm argues that terminal metal oxo groups are stabilized at metal centers with no more than four d electrons.

This dissertation reports several isolated and fully characterized molecular terminal oxo complexes of the group 10 and 11 elements with the use of polytungstate ligand environments. Polytungstates, which share many structural and reactivity features in common with the metal oxides of broad importance in catalytic technologies (TiO2, CeO2, others), are both good -donating and -accepting ligands that may facilitate stabilization and isolation of terminal late transition metal-oxo units. A total of four structural types of LTMO complexes are presented in this study: (1) M(O)(OH2){A-PW9}2 with one bridging octahedral metal unit between two [A-alfa-PW9O34]^9- ligands (M = Pt and Au); (2) M(O)(OH)W(O)(OH2){A-PW9}2 with two linkages between {A-PW9} units, a metal and a tungsten atom (M = Pd); (3) M(O)(OH2)(O=WOH2)2{A-PW9}2 with the terminal M = O group incorporated in a clam shell-like monovacant polytungstate ligand formed by the fusion of two {A-PW9} units by two tungsten atoms (M = Pd and iv Au); and (4) (O=MOH2)2W(O)(OH2){A-PW9}2 (M = Pd), which represents the unique example having two terminal M=O groups coordinated in one molecule. All these molecular LTMO complexes have been carefully studied by geometric and electronic structure methods, including single crystal X-ray diffraction, neutron diffraction, extended X-ray absorption fine structure methods, ³¹P and ¹⁷O NMR spectroscopy, and other chemical and physicochemical methods.

Importantly, the counterion effect in the controlled speciation of the late transition metal substituted polytungstates is discussed. By strongly interacting with the polyanion unit, Cs⁺ countercations can prevent the hydrolytic decomposition of the tri-metal sandwich structure in solution. As a result, the formation of conventional d⁸ Pd(II)-substituted polytungstates and unprecedented terminal Pd=O complexes can be controlled.

Reactivity studies are conducted on M(O)(OH2)(O=WOH2)2{A-PW9}2 (M = Pd and Au), a structure that is quite stable in both aqueous and organic solution. The stoichiometric oxo transfer from terminal M=O to other substrates and subsequent reoxidization of the deoxygenated form by air are confirmed by spectroscopy methods. Furthermore, the deoxygenated product, [Pd^II(O=WOH2)2(A-alfa-PW9O34)2]^8-, can be isolated as crystalline plates, and X-ray diffraction confirms the existence of a four-coordinate square-planar Pd(II) center. These results and subsequent catalytic oxidation studies strongly suggest the involvement of terminal M=O species in noble metal-based homogenous and heterogenous catalysts in O2-based green organic oxidations.

Table of Contents

ABSTRACT ACKNOWLEDGMENT

CHAPTER 1 INTRODUCTION 1.1 TERMINAL TRANSITION METAL OXO UNITS 1.2 SUPPORTED NOBLE METAL CATALYSIS 1.3 BRIDGING NOBLE METAL-OXO SPECIES 1.4 POLYOXOMETALATES - METAL-OXIDE CLUSTERS 1.5 USE OF POLYOXOMETALATES AS STABILIZING LIGAND FOR M=O REFERENCES CHAPTER 2 [MO(OH2)(PW9O34)2] (M = PT OR AU) 2.1 ABSTRACT 2.2 INTRODUCTION 2.3 EXPERIMENTAL SECTION General Methods and Materials Synthesis of K7Na9[PtO(OH2)(PW9O34)2]21.5H2O (1) Synthesis of K15H2[AuO(OH2)(PW9O34)2]25H2O (2) X-ray Crystallographic Studies Neutron Diffraction Studies Electrochemistry X-ray Absorption Spectroscopy Studies ¹⁷O NMR Studies Computational Methods 2.4 RESULTS AND DISCUSSION Synthesis of Terminal Pt-oxo (1) and Au-oxo (2) complexes Purity of Complexes, 1 and 2 X-ray crystal structures of 1 and 2 Neutron Diffraction Studies Optical Spectroscopy Electrochemistry X-ray Absorption Spectroscopy Studies ¹⁷O NMR studies Computational Studies REFERENCES CHAPTER 3 [MO(OH)WO(OH2)(PW9O34)2] (M = PD) 3.1 ABSTRACT 3.2 INTRODUCTION 3.3 EXPERIMENTAL SECTION General Methods and Materials Synthesis of H5.6K7.6Na1.4[Pd0.3{WO(OH2)}0.7(PW9O34)2] Synthesis of K10Na3[PdO(OH)WO(OH2)(PW9O34)2]16H2O (3) Crystallographic Studies Titration Studies Electronic Absorption Studies ¹⁷O NMR Studies X-ray Absorption Studies Electrochemistry Computational Procedures 3.4 RESULTS AND DISCUSSION Synthesis of K10Na3[PdO(OH)WO(OH2)(PW9O34)2]16H2O (3) Crystallographic Studies X-ray Absorption Studies Titration Studies ¹⁷O NMR Studies Electronic Absorption Studies Electrochemistry Pd(IV) oxidation state assignment Computational Results REFERENCES CHAPTER 4 [MO(OH2)(WO(OH2))2(PW9O34)2] (M = PD OR AU) 4.1 ABSTRACT 4.2 INTRODUCTION 4.3 EXPERIMENTAL SECTION General Methods and Materials Synthesis of K8[Pd^IVO(OH2)P2W20O70(OH2)2]22H2O (4) Synthesis of K7H2[Au^IIIO(OH2)P2W20O70(OH2)2]27H2O (5) X-ray Crystallographic Studies X-ray Absorption Spectroscopy Studies Ultra-Low-Temperature Electronic Absorption Spectroscopy pH Dependent UV-vis and ³¹P NMR Titrations Electrochemistry Chemical Titrations ¹⁷O NMR Studies 4.4 RESULTS AND DISCUSSION Synthesis Magnetism of Au LTMO complexes, 2 and 5 X-ray Crystallographic Studies X-ray Absorption Spectroscopy Studies Ultra-Low-Temperature Electronic Absorption Spectroscopy pH Dependent UV-vis and ³¹P NMR Titrations Electrochemistry Chemical Titrations ¹⁷O NMR Studies Atomic Occupancy at the Au Position Au Oxidation State 4.5 CONCLUSIONS REFERENCES CHAPTER 5 [(MO(OH2))2WO(OH2)(PW9O34)2] (M = PD) 5.1 ABSTRACT 5.2 INTRODUCTION 5.3 EXPERIMENTAL SECTION General Methods and Materials Synthesis of Cs3.5K3Na3.5[(O=Pd^IV(OH2))2P2W19O68(OH)2]20H2O (6) Crystallography Studies X-ray Absorption Spectroscopy Reduction of 6 with Na2SO3 and (CH3)2S Computational Studies 5.4 RESULTS AND DISCUSSION Synthesis Thermogravimetric Analysis (TGA) Structural Studies Pd(IV) Oxidation State Assignment Reduction of 6 Computational Studies REFERENCES CHAPTER 6 COUNTERCATION EFFECT 6.1 ABSTRACT 6.2 INTRODUCTION 6.3 EXPERIMENTAL SECTION General Methods and Materials Synthesis of K12[Pd^II 3(PW9O34)2]18H2O (K9) Synthesis of Cs8Na4[Pd^II 3(PW9O34)2]18H2O (CsNa9) Synthesis of Cs9Na5[Pd^II 3(SiW9O34)2]16H2O (10) Synthesis of Cs5K3Na4[Pd^II 2WO(OH2)(SiW9O34)2]16H2O (11) Crystallographic Studies 6.4 RESULTS AND DISCUSSION Synthesis of K9 and CsNa9 Effect of Cs Countercation in the Synthesis, Controlled Speciation of 10 and 11 Crystallographic Studies of K9, CsNa9, 10 and 11 6.5 CONCLUSION REFERENCES CHAPTER 7 REACTIVITY STUDIES 7.1 ABSTRACT 7.2 INTRODUCTION 7.3 EXPERIMENTAL SECTION General Methods and Materials Stoichiometric Oxo Transfer of Au-oxo Complex 5 Stoichiometric Oxo Transfer of Pd-oxo Complex 4 Crystallography Studies of the Deoxygenated Product 12 Reoxygenation of Pd(II) Complex 12 Catalytic Properties of Pd-oxo Complex 4 Comparative Reaction: [Pd^IIBr4]^2- + Ph3P 7.4 RESULTS AND DISCUSSION43 Stoichiometric Oxo Transfer of Au-oxo Complex 5 Stoichiometric Oxo Transfer of Pd-oxo Complex 4 Crystallography Studies of 12 Reoxygenation of Pd(II) Complex 12 Catalytic Oxidation by Pd-oxo Complex 4 Comparative Reactions REFERENCES

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School	Laney Graduate School
Department	Chemistry
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Chemistry, Inorganic
Palabra Clave	Polyoxometalate Late Transition Metal Oxo X-ray Absorption Spectroscopy Neutron Diffraction Polytungstate X-ray Diffraction
Committee Chair / Thesis Advisor	Hill, Craig L, Emory University
Committee Members	Heaven, Michael, Emory University Edmondson, Dale E, Emory University

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