Late Transition Metal Complexes of Pyridyldiphosphines
Authors:   , Vaughan Teresa Florence 
Published: November 2021

<p>This thesis provides an account of research into the properties of pyridyldiphosphines with o-xylene and m-xylene backbones. The coordination behaviour of the o-xylene based ligand with platinum, palladium, silver, rhodium and iridium metal centres has been studied, with an emphasis on whether the presence of the pyridyl rings affects the products formed. Platinum and palladium pincer complexes have been synthesised and the intermediates investigated. The formation of trimetallic complexes with these ligands acting as bridging ligands has also been explored. Two new pyridyldiphosphines, o-C₆H₄(CH₂PPy₂)₂ (3) and m-C₆H₄(CH₂PPy₂)₂ (4), and one known pyridyldiphosphine, PPy₂(CH₂)₃PPy₂ (5), have been synthesised via an improved method. Tris(2-pyridyl)phopshine was reacted with a lithium dispersion to give LiPPy₂, which was then reacted with the appropriate dichloride or dibromide compound to yield the desired ligand. The phosphine selenides of 3 and 4 were synthesised and the ¹J PSe values of 738 and 742 Hz indicated these ligands were less basic than PPh₃. While the ligands themselves were not water-soluble, protonation by a strong acid, such as HCl or H₂C(SO₂CF₃)₃, rendered them soluble in water. A series of [MX₂(PP)] complexes (where M = Pt, X = Cl, I, Me, Et, PP = 3, 5; M = Pd, X = Cl, Me PP = 3, 5) were synthesised. Complexes of 3 displayed dynamic behaviour in solution which was attributed to the backbone of the ligand inverting. When [PtMeCl(PP)] (27) was reacted with NaCH(SO₂CF₃)₂ no evidence for the coordination of the pyridyl nitrogens was observed. The synthesis of a series of unsymetrical [PtMeL(PP)]⁺ complexes enabled the comparison of the cis and trans influences of a range of ligands. The following cis influence series was compiled based on ³¹P NMR data of these complexes: Py ≈ Cl > SEt₂ > PTA > PPh₃. Reaction of 27 with NaCH(SO₂CF₃)₂ and carbon monoxide slowly formed an acyl complex, where the CO had inserted in the Pt–Me bond. The bis-chelated complexes [M(PP)₂] where M = Pt, Pd, and [Ag(PP)₂]⁺ were formed. In these complexes 3 acted as a diphosphine ligand and there was no evidence for any interaction between the pyridyl nitrogen atoms and the metal centre. Reaction of 3 with [Ir(COD)(μ-Cl)]₂ formed [IrCl(PP)(COD)] (42). When the chloride ligand in 42 was abstracted, the pyridyl nitrogens were able to interact with the iridium centre faciliating the isomerisation of the 1,2,5,6-ƞ⁴-COD ligand to a 1-к-4,5,6-ƞ³-C₈H₁₂ ligand. The X-ray crystal structure of [Ir(1-к-4,5,6-ƞ³-C₈H₁₂)(PPN)]BPh₄ (43) confirmed the P,P,N chelation mode of the ligand. In solution, 43 displayed hemilabile behaviour, with the pyridyl nitrogens exchanging at a rate faster than the NMR time scale at room temperature. The coordinated pyridyl nitrogen was able to be displaced by carbon monoxide to form [Ir(1-к-4,5,6-ƞ³-C₈H₁₂)(CO)(PP)]⁺. A series of [PtXY(μ-PP)]₂ complexes, where X = Y = Cl, Me, X = Cl, Y = Me and PP = 4, were formed initially when 4 was reacted with platinum(II) complexes. When heated, the dimers containing methyl ligands eliminated methane to form [PtX(PCP)] pincer complexes, X = Cl (49), Me (51). When the chloride ligand in 49 was abstracted no evidence of pyridyl nitrogen coordination was observed. Protonation of 49 did not yield a water-soluble pincer complex. The [PdCl₂(μ-PP)]₂ complex readily metallated when heated to give the pincer complex [PdCl(PCP)]. Given pyridyl nitrogen atoms are known to be good ligands for “hard” metal centres, the ability of the pyridyl nitrogens in 3 and 4 to coordinate to metal centres was investigated. While complexes with chloride ligands were found to form insoluble products, the synthesis of [(PtMe₂)₃(PP)], from the reaction of either 3 or [PtMe₂(PP)] (17) with dimethyl(hexa-1,5-diene)platinum, proceeded smoothly through a dimetallic intermediate. The same reactivity was observed in the synthesis of [(PtMe₂)₂PtMe(PCP)]. In contrast, the cationic heterotrimetallic complexes [{M(COD)}₂PtMe(PP)]²⁺ and [{M(COD)}₂PtMe(PCP)]²⁺, where M = Rh or Ir, were synthesised without the detection of any intermediates. However, dimetallic complexes were formed as part of a mixture when 17 or 51 was reacted with one equivalent of the appropriate metal complex.</p>