ABSTRACT
This project work present the alkylating reaction of [Pt2(μ-S)2(PPh3)4] with boronic acid alkylating agents.The reactivity of the metalloligand [Pt2(μ-S)2(PPh3)4] with the boron-functionalized alkylating agents BrCH2(C6H4)B(OR)2 (R = H or C(CH3)2) was investigated by electrospray ionization mass spectrometry (ESI-MS) in real time using the pressurized sample infusion (PSI). The macroscopic reaction of [Pt2(μ-S)2(PPh3)4] with one mole equivalent of alkylating agents BrCH2(C6H4)B{OC(CH3)2}2and BrCH2(C6H4)B(OH)2 gave the dinuclear monocationic µ-sulfide thiolate complexes
[Pt2(µ-S){µ-SCH2(C6H4)B{OC(CH3)2}2}(PPh3)4]+ and [Pt2(µ-S){µ-S+CH2
(C6H4)B(OH)(O–)}(PPh3)4]. The products were isolated as the [PF6]– salts and zwitterion respectively, and fully characterized by ESI-MS, IR, 1H and 31P NMR
spectroscopy and single crystal X-ray structure determinations. The alkylation reaction of BrCH2(C6H4)B{OC(CH3)2}2 with [Pt2(µ-S)2(PPh3)4 + H]+was determined via kinetic analysis by PSI-ESI-MS to be second order consistent with the expected SN2 mechanism for an alkylation reaction. The PSI-ESI-MS microscale synthesis
showed that[Pt2(µ-S)2(PPh3)4]disappeared rapidly with consequent formation of onlymonoalkylated cationic product, [Pt2(µ-S){µ- SCH2(C6H4)B{OC(CH3)2}2}(PPh3)4]+. This was indicated by the immediate appearance of the monoalkylated product peak at m/z 1720.6.The reaction came to completion within 6 minutes after injection and no trace of any other product or dialkylated species. The desk top synthesis observed after further stirring for six hours also show the formation of no other product. The reaction ofBrCH2(C6H4)B(OH)2, with({[Pt2(µ-S)2(PPh3)4] + H}+)within same time interval yielded three monocationic species that were detected by ESI-MS and assignable to the three alkylated products: [Pt2(µ-S){µ-SCH2C6H5)(PPh3)4]+, m/z 1593.4 from the loss of B(OH)2 moiety; a
hemiketal-like species [Pt2(µ-S){µ-SCH2(C6H4)B(OH)(OCH3)}(PPh3)4]+, m/z 1651.5 and [Pt2(µ-S){µ-SCH2(C6H4)OH}(PPh3)4]+, m/z 1609.5. The laboratory scale
synthesis indicated the same products.The masses were identified by comparing the experimental isotope patterns with calculated ones. No peak was observed in the mass spectrum that was attributable to the formation of the expected product [Pt2(µ-
S){µ-SCH2(C6H4)B(OH)2}(PPh3)4]+. The structural determination by X-ray
diffraction showed that the compound formed was a zwitter ion (neutral complex) [Pt2(µ-S){µ-S+CH2(C6H4)B(OH)(O-)}(PPh3)4]. [Pt2(µ-S){µ-S+CH2(C6H4)B(OH)(O-
)}(PPh3)4] is a neutral species and not detectable in ESI-MS. 1H NMR spectra showed a complicated set of resonances in the aromatic region due to the terminal triphenylphosphine ligands and were broadly assigned as such. However, SCH2 hydrogen atoms were easily identified as broad peaks at δ 3.59 ppm and 3.60 ppm for [Pt2(µ-S){µ-SCH2(C6H4)B{OC(CH3)2}2}(PPh3)4]+PF6 and [Pt2(µ-S){µ- S+CH2(C6H4)B(OH)(O-)}(PPh3)4], respectively. The monoalkylated products shows IR and 31P{1H} NMR spectra expected of the complexes. The OH vibration (3336 cm-
1) in 2.1 shifted to 3435 cm-1 in 2.1a. The absorption bands of the B-O bond in 2.2
(1355 cm-1) and 2.1 (1350 cm-1) shifted to 1360 cm-1 and 1367 cm-1 in 2.2a·(PF6) and
2.1a respectively. The 31P{1H} NMR spectra showed nearly superimposed central resonances and clearly separated satellite peaks due to 195Pt coupling. The 1J(PtP) coupling constants showed the differences due to the trans influences of the
substituted and the unsubstituted sulfide centers. The trans influence of the unsubstituted sulfide is greater than the thiolate (substituted) species demonstrated by the coupling constants at (2628 and 3291 Hz) for 2.2a·(PF6) and (2632 and 3272 Hz)
2.1a,respectively.
CHAPTER ONE
1.0 Introduction
1.1 Background of Study
The diverse study on platinum and sulfur element has been possible due to their rich individual chemistries.Their compounds have been extensively studied due to their wide range of applications in both biology and industry1. Platinum was first discovered in 1735 by Don Antonio de Ulloa. It has high melting point and good resistance to corrosion and chemical attack2. Consequence to its resistance to wear and tarnish and its beautiful looks, it is employed in jewellery production3,4. It is also used in laboratory equipment, electrical contacts, catalytic converters, dentistry equipment, electrodes, antioxidation processes, catalysis, biomedical applications and hard disk4,5,6,7, 8-11. Platinum compounds like cisplatin, carboplatin and oxaliplatin are
used in cancer treatments12,13,14. The use of cisplatin in cancer chemotherapy is limited by ototoxicity, emetogenesis effect, neurotoxicity, and nephrotoxicity of the drug15-18. It has been suggested that the toxicity of the drug is as a result of bonding between platinum and protein sulfur atoms19.
Platinum exists in different oxidation states, 0 to +6, due to its vacant d orbitals. The most common oxidation state is +2 including non-even20 with +1 and +3 found in dinuclear Pt-Pt bonded complexes. These properties make platinum form
coordination compounds easily.
Sulfur is commonly used in the manufacturing of important chemical like sulfuric acid. It is also used to refine oil and in processing ores11. It is an essential element in most biochemical processes. Sulfur compounds serve as substrates in biochemical process (serving as an electron acceptor in anaerobic respiration of
sulfate-sulfur eubacteria), fuels (electron donors) and respiratory (oxygen alternative) in metabolism22. Vitamins such as thiamine and biotin, antioxidants like thioredoxin and glutathiones, and myriads of enzymes contain organic sulfur23. Organic sulfur has an anti-neoplastic effect and used in oral and other cancers treatment24.
Sulfur ligands coordinate with most transition metals in different oxidation states25. The chemical properties of sulfur as a versatile coordination ligand is illustrated by its tendency to extend its coordination from terminal groups example ([Mo2S10]2-)26 to μ-sulfido group e.g. [Pt2(µ-S)2(PPh2Py)4]27 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 cluster28. It has the propensity to catenate and give rise to 2- polysulfide ligands (Sn
) with n ranging from 1 to 8. Sulfur ligands coordination
chemistry is widely manifested in the variety of structures it forms with most of the transition metals25. The important roles of metal sulfide compounds are seen in catalysis29, bioinorganic and rich solid-state chemistry 30. The metal-sulfur bonding serves as key part of the active site component in reactivity of the biological macromolecule31-35.
{Pt2S2} chemistry is dated back to 1903 when Hofmann and Hochlen reported a work on isolation of the first platinum-sulfur complex [(NH4)2[Pt(η2-S5)3]36. Platinum sulfido complexes are classified as homometallic sulfido complexes and heterometallic sulfido complexes. The homometallic sulfido complex of platinum was further classified into groups consisting of the platinum atom metal-metal bond bridged by single sulfur, and that in which the two non-bonded platinum atoms are held together by two sulfur ligands. The sulfur atoms, in both complexes have the capability of bonding further to other metals or ligands. Following the development reported by Hofmann and Hochlen in 1903, a metal-metal bond bridged by single sulfur complex [Pt2(µ-S)(CO)2(PPh3)3] was reported by Baird and Wilkinson as a product of the reaction of [Pt(PPh3)3] with COS37. On heating in chloroform, the intermediate [Pt(PPh3)2(COS)] gave an orange air-stable compound which was identified using infra-red spectroscopy and elemental analysis technique38. X-ray crystallography showed that the compound had only one CO ligand and the structure was reported by Skapski and Troughton39.
This material content is developed to serve as a GUIDE for students to conduct academic research
ALKYLATION OF [PT2(µ-S)2(PPH3)4] WITH BORONIC ACID DERIVATIVES BY PRESSURIZED SAMPLE INFUSION ELECTROSPRAY IONIZATION MASS SPECTROMETRY (PSI- ESI-MS) TECHNIQUE
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