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Hybrid P,E Ligands: Synthesis, Coordination Chemistry and Catalysis

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posted on 2021-11-14, 09:00 authored by Allan, Kathryn Mary

This thesis provides an account of research into a family of novel hybrid P,E ligands containing an o-xylene backbone. A methodology for the synthesis of these ligands has been developed, and their coordination behaviour with platinum(II) and platinum(0) precursors has been explored, with particular focus on a phosphinethioether (P,S) ligand of this type. The coordination modes of this P,S ligand with palladium precursors have also been investigated, and the utility of the ligand in a palladium and copper co-catalysed Sonogashira carbon-carbon bond-forming reaction has been evaluated. A range of hybrid P,E ligands of the type o-C₆H₄(CH₂PBut₂)(CH₂E) (E = PR₂, SR, S(O)But, NR₂, SiPh₂H) have been synthesised in two or three steps from the novel substrate, o-C₆H₄(CH₂PBut₂(BH₃)}(CH₂Cl). The initial step involved treatment of the substrate with the appropriate nucleophilic reagent, or preparation of a Grignard reagent from o-C₆H₄{CH₂PBut₂(BH₃)}(CH₂Cl) and reaction with the appropriate electrophile. In most cases, this versatile strategy produced air-stable crystalline ligand precursors. Phosphine deprotection was achieved via one of three methods, dependent upon the properties of the second functional group. The reactivity of three of these ligands — o-C₆H₄{CH₂PBut₂)(CH₂SBut) (14a), o-C₆H₄{CH₂PBut₂){CH₂S(O)But} (16) and o-C₆H₄(CH₂PBut₂)(CH₂NMe₂) (18a) — with Pt(II) and Pt(0) precursor complexes has been investigated. Chelated [PtCl₂(P,E)] complexes were synthesised with P,S ligand 14a and P,N ligand 18a, but attempts to produce the equivalent species with P,S=O ligand 16 were unsuccessful. The X-ray crystal structure of [PtCl₂(P,S)] complex 21 displayed an unexpectedly small ligand bite angle of 86.1°. A series of platinum(II) hydride complexes of the types [PtHL(P,S)₂] and [PtHL(P,S)₂]CH(SO₂CF₃)₂ (L = Cl¯, H¯, NCMe, −CH₂SBut , CO, pta) have been synthesised, where ligand 14a binds in a monodentate fashion through the phosphorus donor atom. This work has demonstrated the hemilability of ligand 14a, via the facile and reversible conversion between [PtH(κ¹P-14a)(κ²P,S-14a)]CH(SO₂CF₃)₂ (26) and [PtH(NCMe)(κ¹P-14a)₂]CH(SO₂CF₃)₂ (28). The X-ray crystal structure of [PtH₂(P,S)₂] complex 25 was used to calculate a cone angle of 180° for the phosphine moiety in ligand 14a. Reaction of P,S ligand 14a and P,S=O ligand 16 with [Pt(alkene)₃] complexes (alkene = ethene, norbornene) gave the chelated [Pt(alkene)(P,E)] complexes 32–35; however, under similar conditions a [Pt(norbornene)(P,N)] complex did not form. A large ligand bite angle of 106.6° was observed in the X-ray crystal structure of [Pt(norbornene)(P,S)] complex 34. Reaction of two equivalents of each of the P,E ligands with [Pt(norbornene)₃] gave the corresponding 14-electron linear complexes [Pt(P,E)₂] (36–38) with the ligands coordinated through the phosphorus donor atoms only. The reactivity of [Pt(norbornene)(P,S)] complex 34 and [Pt(P,S)₂] complex 36 has been investigated, resulting in the complexes [PtH{CH(SO₂CF₃)₂}(P,S)] (39), [Pt(norbornyl)(P,S)] (40), [Pt(ethyne)(P,S)] (41) and [Pt(O₂)(P,S)₂] (42). The reactivity of P,S ligand 14a was investigated with Pd(II) and Pd(0) precursors, resulting in the identification of five coordination modes of this ligand. Monodentate binding was observed in [Pd(P,S)₂] complex 44, and chelation in the [Pd(alkene)(P,S)] complexes 47 (alkene = norbornene) and 48 (alkene = dba). Reaction of ligand 14a with [PdCl₂(NCBut)₂] at raised temperature resulted in S−C bond cleavage and the formation of palladium dimer 43 with bidentate coordination of the ligand through phosphine and bridging thiolate moieties. Reaction of ligand 14a with [Pd(OAc)₂] resulted in C−H activation of the aryl backbone and formation of [Pd(μ-OAc)(P,C)]₂ dimer 46. In the presence of excess [Pd(OAc)₂], palladium hexamer 45 was formed, with a combination of P,C palladacycle and monodentate thioether binding resulting in bridging P,C,S coordination of ligand 14a. The Sonogashira cross-coupling of 4-bromoanisole and phenylethyne was performed with 3 mol% of a pre-catalyst mixture containing P,S ligand 14a, [Pd(OAc)₂] and CuI, resulting in quantitative conversion to 4-(phenylethynyl)anisole in four hours. Two enyne by-products were also identified from the reaction. Variations to the pre-catalyst mixture and catalyst loading indicated there was a significant ligand dependence on the yield and selectivity of the reactions. Mercury drop tests and dynamic light scattering experiments confirmed the presence of palladium nanoparticles in the reaction solution; however, the active catalytic species in these reactions has not been identified.


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Te Herenga Waka—Victoria University of Wellington

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Author Retains Copyright

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Degree Grantor

Te Herenga Waka—Victoria University of Wellington

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Degree Name

Doctor of Philosophy

ANZSRC Type Of Activity code

970103 Expanding Knowledge in the Chemical Sciences

Victoria University of Wellington Item Type

Awarded Doctoral Thesis



Victoria University of Wellington School

School of Chemical and Physical Sciences


Spencer, John; Harvey, Joanne