SYNTHESIS AND CHARACTERISATION OF ALKYLATED ISOCYANATE DERIVATIVES OF [PT2(Μ-S)2(PPH3)4]

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, andN-(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, andN-(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.

CHAPTER ONE

• Introduction

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^2,3andto 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.