Hybrid P,E Ligands: Synthesis, Coordination Chemistry and Catalysis
Authors:   , Allan Kathryn Mary 
Published: November 2021

<p>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.</p>