Thesis 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 21 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.

Thesis Overview

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 catalysis2,3 and to the chemistry of novel molecular systems4. Other main areas of application are in the design of homo- and hetero-polynuclear clusters5, fine wires6,7, 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 stable10,11 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 (Sn2) 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 cluster15,. 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 ores17,18. examples are Cooperite (Pt0.6Pd0.3Ni0.1S)17,18, and Braggite (Pt0.38Pd0.50 Ni0.10S1.02)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 group21-23and transition metals23-28, 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 aggregates23,25 . 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 complexes29,30. 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.