ABSTRACT

The highly nucleophilic bridging sulfide centers in bis(μ-sulfido)tetrakis(triphenylphosphine) diplatinum(II), [Pt₂(µ-S)₂(PPh₃)₄] enables the incorporation of any organic functionality (R) through facile monoalkylation to form cationic complex [Pt₂(µ-S)(µ-SR)(PPh₃)₄]⁺. The organic electrophiles; N,N’-(2-dichloroethyl) piperazine-4-carboxi-amine, N-(2-chloroethyl) morpholine-4-carboxi-amine, and N-(2-chloroethyl)-1-methylpiperazine-4-carboxi-amine derived from isocyanate were synthesised by the reactions of piperazine, morpholine and  methyl piperazine respectively with 2-chloroethyl isocyanate in diethyl ether. This potentially formed highly functionalised organic electrophiles N-(2-chloroethyl) morpholine-4-carboxi-amine, and N-(2-chloroethyl)-1-methylpiperazine-4-carboxi-amine was incorporated into [Pt₂(µ-S)₂(PPh₃)₄] in methanol to yield the corresponding monoalkylated derivatives [Pt₂(μ-S)(μ-SCH₂CH₂NHC(O)N(CH₂CH₂)₂O)(PPh₃)₄]⁺ and [Pt₂(μ-S)(μ-SCH₂CH₂NHC(O)N(CH₂CH₂)₂N CH₃)(PPh₃)₄]⁺. The reaction of [Pt₂(μ-S)₂(PPh₃)₄] with the functionalised dialkylating agent ClCH₂CH₂NHC(O)N(CH₂CH₂)₂NC(O)HNCH₂CH₂Cl proceeded in two stages in a 2:1 mole ratio. The first stage is the monoalkylation of [Pt₂(μ-S)₂(PPh₃)₄] to give the monocation [Pt₂(μ-S)(μ-SCH₂CH₂NHC(O)N(CH₂CH₂)₂NC(O)HNCH₂CH₂Cl)(PPh₃)₄]⁺. The monoalkylated derivative provided the enabling condition for a second intermolecular nucleophilic attack by another molecule of [Pt₂(μ-S)₂(PPh₃)₄] yielding the bridging Pt₄ aggregate spanned by SCH₂CH₂NHC(O)N(CH₂CH₂)₂NC(O)HNCH₂CH₂S. The resulting products was isolated as the   tetraphenyl borate (BPh₄^–) salts and characterized by Electrospray Ionization Mass Spectrometry (ESI-MS), FT-IR, ¹H, ¹³C and ³¹P {H} NMR.

 

 

TABLE OF CONTENTS

Title Page                                                                                                                                i

Approval Page                                                                                                                                    ii

Certification                                                                                                                            iii

Dedication                                                                                                                              iv

Acknowledgements                                                                                                                v

Abstract                                                                                                                                  vi

Table of Contents                                                                                                                   vii

List of Tables                                                                                                                          x

List of Figures                                                                                                                        xi

List of Schemes                                                                                                                      xii

List of General Abbreviations                                                                                                            xiv

List of Chemical Abbreviations                                                                                              xv

 

CHAPTER ONE

1.0       Introduction                                                                                                                1

1.1       Background of  Study                                                                                                            1

1.2       Statement of Problem                                                                                                 4

1.3       Justification of Study                                                                                                 5

1.4       Aims of Study                                                                                                                        5

1.5       Objectives of Study                                                                                                    5

CHAPTER TWO

2.0       Literature Review                                                                                                       6                                               2.1       Synthesis of [Pt₂(μ-S)₂(PPh₃)₄]                                                                                   6

2.2       The Main Geometrical and Electronic Features of {Pt₂(μ-S)₂} Complexes               8

2.3       Protonation and Monoalkylation of [Pt₂(μ-S)₂(PPh₃)₄]                                              10

2.4       Reaction of [Pt₂(μ-S)₂(PPh₃)₄] with α,ω-Dialkylating Agent                                     12

2.5       Spectroscopic Methods for Structural Elucidation                                                     15

2.5.1    Mass Spectrometry                                                                                                     15

2.5.1.1 Soft Ionisation Techniques                                                                                         17

2.7.1.2 Description of Electrospray Ionisation Mass Spectrometry                                       17

2.7.1.3 Electrospray Ionisation Mass Spectrometry-As a Technique in Probing the

Reactivity of [Pt₂(μ- S)₂(PPh₃)₄]                                                                                 20

CHAPTER THREE

3.1       General Reagent Information                                                                                     23

3.2       General Analytical Informatio                                                                                    23

3.3       Synthesis of the Alkylating Agent                                                                             24

3.4       Synthesis of the Alkylated Derivatives of [Pt₂(µ-S)₂(PPh₃)₄]                                    26

CHAPTER FOUR

4.0       Result  and Discussion                                                                                                28

4.1       Synthesis                                                                                                                     28

4.1.1    Synthesis and Characterisation of the Alkylating Agents                                          30

4.1.2    General Procedure                                                                                                      30

4.1.3    Mass Spectrometric and Spectroscopic Characterisation                                           32

4.1.4    IR Spectroscopic of the Alkylating Agents                                                                33

4.2       Synthesis and Spectroscopic Characterisation of the Alkylated Derivatives of

[Pt₂(μ-S)₂(PPh₃)₄]                                                                                                        34

4.2.1    General Procedure                                                                                                      34

4.2.2    Mass Spectrometric and Spectroscopic Characterisation                                           35

4.2.2.1 ³¹P {H} NMR Spectroscopy                                                                                       40

4.3       Intermolecular Bridging Alkylation                                                                            42

4.3.1    IR Spectroscopy                                                                                                         44

4.3.1.1 Comparison of the IR Spectral of [Pt₂(μ-S)₂(PPh₃)₄] and the Complexes                  45

4.4       Conclusion                                                                                                                  46

Reference                                                                                                                                47

Appendixes                                                                                                                             54

 

 

CHAPTER ONE

1.1      Background of Study

 

Investigation of the chemistry of platinum and sulphur has attracted considerable attention in recent years due to the broad applications of the two elements and their compounds, in biological systems¹, applied catalysis^2,3 and to the chemistry of novel molecular systems⁴. Other main areas of application are in the design of homo- and hetero-polynuclear  clusters⁵, fine wires^6,7, jewellery, antitumor drugs⁸, the self-assembly of supramolecular structures, and the photophysical properties of new luminescent and mesogenic phases⁹. Platinum, however has six naturally occurring isotopes, ¹⁹⁰Pt, ¹⁹²Pt, ¹⁹⁴Pt, ¹⁹⁵Pt, ¹⁹⁶Pt and ¹⁹⁸Pt with a maximum oxidation state of +6, the oxidation states of +2 and +4 being the most  stable^10,11 and the rare odd number form of +1 and +3 oxidation states are found in dinuclear Pt-Pt bonded complexes¹².

Sulphur also exhibits an important chemical properties especially as a versatile coordinating ligand which is illustrated by its ability to catenate forming polysulfide ligands (S_n²) with n ranging from 1 to 8. It also has the ability to expand its coordination from terminal groups example ([Mo₂S₁₀]^2-)^13, to μ-sulfido group e.g. [Pt₂(l-S)₂(PPh₃)₄]¹⁴ and to an encapsulated form e.g. [Rh₁₇(S)₂(CO)₃₂]^3-  consisting of a S-Rh-S moiety in the cavity of a rhodium-carbonyl cluster^15,. The coordination chemistry of sulfur ligands has been reviewed and has shown a unique variety of structure in its reactions with most transition metals in different oxidation states¹⁶.

 

 

The outstanding ability of sulphur to bind to heavy metals is not only evidenced by the enormous variety of the metal sulfide minerals found in nature but also by the appearance of platinum group metals in mineral ores different from the naturally occurring ores^17,18. examples are Cooperite (Pt_0.6Pd_0.3Ni_0.1S)^17,18, and Braggite (Pt_0.38Pd_0.50 Ni_0.10S_1.02)¹⁷.

The development of platinum sulfide complexes has received much less attention for many years after the first platinum-sulfur complex, (NH₄)₂[Pt(η₂-S₅)₃], was isolated in 1903¹⁹ . However the main features in the field of platinum(II)sulfur chemistry was established by Chatt and Mingos in 1970, who obtained several complexes of various nuclearities and structures²⁰. Among them,  [Pt₂(μ-S)₂(PMe₂Ph)₄] followed by [Pt₂(μ-S)₂(PPh₃)₄]¹⁴ {bis(μ-sulfido)tetrakis (triphenylphosphine) diplatinum (II)} reported by Ugo et al¹⁴ a year later,  constitutes the first examples  of platinum(II)sulphide complexes containing the  {Pt₂(μ-S)₂} core²¹. The compound is a fine orange powder, insoluble in hydrocarbon solvents and water but sparingly soluble in methanol. It is soluble by reaction with mild alkylating agents, e.g CH₂Cl₂, CH₃Cl which indicates the high nucleophilicity of the sulfide centres.

The exceptional nucleophilicity of the sulfido ligands in {Pt₂(μ-S)₂} core accounts for their ability to act as  potent metalloligands towards a diverse range of metal centres, including main group^21-23and transition metals^23-28, as well as the actinide uranium⁹ and also enhances the development of homo-, hetero- and inter-metallic sulfide complexes²³ (Scheme 1.1). The advancement in the chemistry of [Pt₂(μ-S)₂(PPh₃)₄] and the other sulfide-bridged complexes with the {Pt₂(μ-S)₂} core, as well as the improvement made in their synthesis, structures, and reactivity have been exceptionally reviewed by Fong and Hor,  who have made important contributions to this field²³. However, the overall ability of the sulfido ligands in the {Pt₂(μ-S)₂} core to extend their coordination mode from μ-S to μ₃-S give rise to the behaviour of [Pt₂(μ-S)₂(PPh₃)₄]¹⁴  as building blocks for the synthesis of multimetallic sulfide bridged aggregates.  Scheme 1.0 shows the different formation of multimetallic aggregates^23,25 . It involves the bridging of the two sulfur atoms in a molecule of [Pt₂(μ-S)₂(PPh₃)₄] by a metal fragment.

Scheme 1.0  The formation of different multimetallic aggregates

 

It is a clear fact from literature sources that alkylation of metal–sulfido complexes is potentially a very general means of synthesising  metal–thiolate complexes^29,30. It has continued to be developed as a versatile means of synthesising dinuclear platinum thiolate complexes³¹ resulting in its ability to generate a wide variety of functionalised thiolate ligands at platinum by appropriate choice of alkylating agent and the reaction conditions (Scheme 1.1)_. In some cases it may be possible to obtain thiolate ligands not easily accessible by other methodologies. This methodology has been employed in the synthesis of thiolate ligands containing, for example, fluorinated substituents³²,  semicarbazone,  urea, oxime and other groups³³. The use of a dialkylating agent allows extension of the methodology to generate complexes containing a dithiolate ligand, and it investigation under electrospray mass spectrometry (ESI-MS) conditions indicates that the outcome of dialkylation indeed depends on the strength of the electrophile and the spectral study however provide an effective means of screening the electrophiles and their activities towards [Pt₂(μ-S)₂(PPh₃)₄]³⁴.

Scheme 1.1 The structure of Monoalkylation, Homo-, Hetero- and Bridging dialkylation

containing thiolate and dithiolate ligand

 

1.2     Statement of Problem

            Previous study on alkylation of [Pt₂(μ-S)₂(PPh₃)₄] was narrowed to simple alkyl, aryl and very few functionalised organic electrophiles²⁹. There are still many functionalised organic substrate whose reactivity with [Pt₂(μ-S)₂(PPh₃)₄] are not yet known. The current research on the synthesis and characterisation of  novel functionalised organic electrophiles and their complexes was informed by the interest to explore the other areas of these complexes. The chemistry of this investigation will be of great interest considering the observed variable reactivity of [Pt₂(μ-S)₂(PPh₃)₄] with different electrophiles as these can lead to unexpected reactions like displacement  of the terminal PPh₃ through coordination of donor atoms of the incorporated groups.

 

1.3     Justification of Study

In view of the aforementioned problems, this research  is justified by exploring the other areas of new functionalised organic electrophiles involving derivatives of isocyanate. Isocyanate derivatives of good ligating properties towards metal centers, for examples  Piperazine (HN(X)₂NH), Morpholine (HN(X)₂O) and 1-methylpiperazine (HN(X)₂NCH₃) are carefully incoporated by reacting with 2-Chloroethylisocyanate (Cl(X)₂NCO) (X= CH₂CH₂) into suitable electrophiles for [Pt₂(μ-S)₂(PPh₃)₄].

 

The aim of the study was to synthesize and characterise the alkylated isocyanate derivatives of [Pt₂(μ-S)₂(PPh₃)₄].

 

The specific objectives of the study  were:

1. To synthesize multifunctional electrophiles derived from isocyanate morphline, piperazine, and 1-methylpiperazine.
2. To Study their reactivities towards [Pt₂(μ-S)₂(PPh₃)₄]
3. To Incorporate the alkylating agents in [Pt₂(μ-S)₂(PPh₃)₄] forming monoalkylated and intermolecular dialkylated complexes
4. To characterise the resulting new functionalised electrophiles and their [Pt₂(μ-S)₂(PPh₃)₄] alkylated derivatives by IR, ¹H, ¹³C and ³¹P {H} NMR spectroscopy.

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