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,    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.

CHAPTER ONE

1.0     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  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 (S 2) 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.

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