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.
TABLE OF CONTENTS
Title Page i
Certification ii
Declaration iii
Dedication iv
Acknowledgement v
Abstract vi
Table of Contents viii
List of Tables x
List of Figures xi
List of Abbreviations xiii
CHAPTER ONE
1.0 Introduction 1
1.1 Background of the Study 1
1.2 Statement of Problem 4
1.3 Justification of Study 5
1.4 Aims and Objectives of the Study 6
CHAPTER TWO
2.0 Literature Review 7
2.1 Brief Summary of [Pt2(μ-S)2(PPh3)4] 7
2.2 Electronic and Molecular Features of [Pt2(μ-S)2(PPh3)4] 8
2.3 Protonation of [Pt2(μ-S)2(PPh3)4] 10
2.4 Role of [Pt2(μ-S)2(PPh3)4] as a Metalloligand 11
2.5 Mono-, Homo- and Heterodi Alkylation reactions of [Pt2(μ-S)2(PPh3)4]13
2.6 Effect of Alkylation on {Pt2(μ-S)2} Geometry 18
2.7 Effect of Leaving Group (Halogens) in Alkylation Reactions 20
2.8 Formation of Inter and Intramolecular Bridging Di-Alkylation
Reactivity of [Pt2(μ-S)2(PPh3)4] 21
2.9 Spectroscopic Methods For Structural Characterization 25
2.9.1Electrospray Ionisation Mass Spectrometry(ESI-MS) 25
2.9.1.1 Application of ESI-MS in Chemical Analysis 29
2.9.1.2 Electrospray Ionization Mass Spectrometry- An Indispensible
Tool for the Preliminary Screening of [Pt2(μ-S)2(PPh3)4] Chemistry 30
viii
CHAPTER THREE
3.0 Experimental 34
3.1 General Reagent Information 34
3.2 General Analytical Information 34
3.3 Synthesis of the Alkylated Derivatives of [Pt2(μ-S)2(PPh3)4] 35
3.3.1 Pre-Synthetic Kinetic Profile of the Reaction of [Pt2(μ-S)2(PPh3)4]
withBrCH2(C6H4)B{OC(CH3)2}2 35
3.3.2 Synthesis of [Pt2(μ-S){μ-CH2(C6H4)B{OC(CH3)2}2}
(PPh3)4](PF6), 2.2a·(PF6) 36
3.3.3 Synthesis of [Pt2(μ-S){μ-S+CH2(C6H4)B(OH)(O-)}(PPh3)4] 2.1a 37
CHAPTER FOUR
4.0 Results and Discussion 38
4.1 Synthesis and Spectroscopic Characterization 39
4.2 X-Ray Crystal Structures 46
4.3 X-Ray Structure Determinations of 2.2a·(PF6) and 2.1a 51
4.4 [Pt2(μ-S){μ-SCH2(C6H4)B{OC(CH3)2}2}(PPh3)4](PF6), 2.2a·(PF6) 54
4.5 [Pt2(μ-S){μ-S+CH2(C6H4)B(OH)(O-)}(PPh3)4], 2.1a 55
Conclusions 58
References 59
Appendix 1: 1H and 31P {1H} NMR of complex 2.1a and 2.29 respectively
Appendix 1 Published Journal of Article of this work (Journal of coordination
chemistry; Topic: Alkylation of [Pt2(μ-S)2(PPh3)4] with boronic acid derivatives;
DOI: 10.1080/00958972.2016.1226503, Publication Date: 19th August, 2016)
ix
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
2
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
polysulfide ligands (Sn
2-) 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
3
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.
S
Pt Pt
Ph3P
Ph3P
PPh3
CO
Figure 1.1: Structure of[Pt(PPh3)2(COS)] formed by the reaction of [Pt(PPh3)3] with
COS.
A related synthesis which uses CS2 instead of COS was also reported40. The reaction
of [Pt(dppe)(CS2)] with [Pt(PPh3)2(C2H 4)] is a typical example of the synthetic
reaction and gives the complex in Figure 1.2.
S
Pt Pt
PPh2
PPh2
PPh3
CS
Figure 1.2: Product for the reaction of [Pt(dppe)(CS2)] with [Pt(PPh3)2(C2H4)]
Chatt and Mingos41 in 1970 reported a related complex [Pt2(PMe2Ph)(μ-S)2]4
– having
two non-bonded platinum atoms held together by two sulfur ligands so-called {Pt2(μ-
S)2}. Ugo et al26 followed almost immediately in a study of the reaction of zerovalent
platinum phosphine complexes with H2S and or elemental sulfur to give di-μ-
4
sulfidotetrakis-(triphenylphosphine) diplatinum(II) [Pt2(μ-S)2(PPh3)4] 1.0. Similar
complexes having different terminal ligands that have been reported also include: 2-
(diphenylphosphino)pyridine [Pt2(μ-S)2(PPh2Py)4]27 1.2, Redox active 1,1’-
bis(diphenyiphosphino) ferrocene [Pt2(μ-S)2(dppf)2]36 1.3, 1,2-bis
(diphenylphosphino)[Pt2(μ-S)2(dppe)2]42 1.4, dimethylphenylphosphane
[Pt2(PMe2Ph)4(μ-S)2]43 1.5 , 1,3-bis(diphenylphosphino) propane [Pt2(μ-S)2(dppp)2]
1.644, [Pt2(μ-S)2(Ptolyl3)2]45 1.7, diphosphines such as (Ph2P(CH2S)nPPh2)2
46 (n = 2,3).
Chiral phosphine such as O-Isopropylidene-2,3-dihydroxy-1,4-
bis(diphenylphosphino)butane (DIOP)47 have also been studied but to a lesser extent.
[Pt2(μ-S)2(PPh3)4] is the most widely studied of the complexes due to its ease of
preparation, from air-stable starting materials, and its tendency to produce crystalline
derivatives which was highlighted in an excellent review by Fong and Hor48.
González-Duarte46 and co-workers also worked on the development of other sulfidebridged
complexes with the {Pt2(μ-S)2} core, as well as the synthesis of its
derivatives, structure, and reactivities. They also synthesized series of di-μ-thiolate
complexes with the {M2(μ-S)2} core (where M = Ni, Pd or Pt),49,50,51 provided the
molecular orbital study of the hinge distortion of the {Pt2(μ-S)2} ring52 and used
chelating diphosphines as terminal ligands53.
1.2 Statement of Problem
Sulfide alkylation chemistry of di-μ-sulfidotetrakis-(triphenylphosphine)
diplatinum(II) [Pt2(μ-S)2(PPh3)4] (1.0) using alkyl, aryl, and functionalised organic
electrophiles46,54 to form thiolate55 ligands has been a subject of researchers interest.
However, no derivatives of [Pt2(μ-S)2(PPh3)4] containing boronic acid electrophiles,
4-bromomethyl phenyl boronic acid pinacolester, BrCH2(C6H4)B{OC(CH3)2}2 and 4-
bromomethylphenylboronic acid, BrCH2(C6H4)B(OH)2 or any metalloid
5
functionalized thiolate ligands has been synthesised using sulfide alkylation. Kinetic
analysis has not been previously applied in the investigation of the synthetic
complexities surrounding the alkylation of {Pt2S2}. Boronic acid derivatives have
been used in the synthesis of bi- and polyaryl compounds via the Suzuki–Miyaura
coupling reactions56-60. To date, no derivatives of 1.0 containing boron or any
metalloid functionalized thiolate ligands have been synthesized using sulfide
alkylation. We present in this report the first experimental kinetic analysis of
alkylation of 1.0, and the first synthesis and characterization of boronic acid
derivatives of 1.0. The isolation and crystallographic identification of the dinuclear
structures incorporating boron thiolate substituents suggests that useful synthetic
precursor groups can be incorporated into 1.0, and in particular open up avenues for
preparing larger multinuclear assemblies on the nanometer scale. Therefore there is a
need to further develop the alkylation chemistry of this system by investigation the
reactivity of other potentially synthetic precursor groups. Detailed investigation of the
reaction kinetics by careful monitoring of the reaction in real time using Pressurized
Sample Infusion Electrospray Ionization Mass Spectrometry (PSI-ESI-MS) has never
been reported.
1.3 Justification of Study
Despite the fact that much work has been reported on 1.0 complex, no derivatives of
1.0 containing boron has been used to generate coordinated functionalized thiolate
ligands (-SR) on 1.0. In view of this, this research work will investigate the
incorporation of new functionalized organic electrophiles of boronic acid derivatives
and monitor the reaction kinetics with the aid of Pressurized Sample Infusion
Electrospray Ionization Mass Spectrometry (PSI-ESI-MS). This work will present the
first study on the monoalkylation chemistry of 1.0 towards organic electrophiles
6
BrCH2(C6H4)B{OC(CH3)2}2 and BrCH2(C6H4)B(OH)2.The chemistry of this system
is of great interest due to the reactivity of 1.0 with different electrophiles as observed
in the ESI-MS, NMR and IR spectroscopic result.
1.4 Aims and Objectives of the Study
The objectives of this study are:
1. To design, synthesize and characterise functionalized monoalkylated
derivatives of 1.0; acquire and analyze the kinetic data of the monoalkylation
reaction between boronic acid derivatives BrCH2(C6H4)B{OC(CH3)2}2 and
BrCH2(C6H4)B(OH)2.
2. Use the data obtained from (1) to incorporate the boronic acid derivatives in
the desktop/laboratory scale synthesis.
3. Characterize the isolated products using conventional spectroscopic technique:
NMR, IR, and X-ray crystallography.
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