Alkylation of [Pt2(µ-S)2(Pph3)4] With Boronic Acid Derivatives by Pressurized Sample Infusion Electrospray Ionization Mass Spectrometry (Psi-Esi-Ms) Technique

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.