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Reaction of thiols with bimetallic Pd-Pd and Pd-pt complexes containing a P₂N₄ ligand Foo, Serena Jo Ling

Abstract

This thesis describes the synthesis, structure, solution behaviour, and reactivity of bimetallic Pd₂ and PdPt complexes containing a P₂N₄ ligand. Though homobimetallic complexes of the type Pd₂X₂ (μ-PP)₂ are widely known (X = CI, Br, or I; PP is a chelating diphosphine such as dppm (Ph₂PCH₂PPh₂)), few containing a single diphosphine bridge are found in the literature. One such new compound is Pd₂Cl₂ (μ-dmapm) 1, which had been structurally characterized by a former group member, and may be the first example of a bimetallic Pd¹₂ complex supported by a tetradentate ligand. The fluxional behaviour of 1 was studied by variable temperature NMR, while reaction with a chiral reagent supports its stereoselective formation with respect to the coordinated P-atoms. 1 reacts with thiols to yield the thiolate-bridged complexes 2. The proposed mechanism involves the oxidative addition of the thiol across the Pd¹-Pd¹ bond (see Scheme) [formulas not included], and a hydridothiolate intermediate was detected at low temperature. Further reaction with triflic acid yields the thiolate-bridged salts, [Pd₂Cl₂ (μ-SR)(μ-dmapm)]⁺[OTfr]⁻ (R = Me, ⁿPr, ⁿBu, Ph, and Bz), and H₂. The μ-thiolate products were characterized by NMR, microanalysis, UV-vis, and conductivity, while the corresponding chloride salts (R = Et, ⁿPr) have been further characterized by X-ray crystallography. The mixed-metal complex, PdPtCl₂(μ-dmapm), was also synthesized and found to react with thiols; in the presence of triflic acid (HOTf), the thiolate-bridged salts, [PdPtCl₂(μ-SR)(μ-dmapm)]⁺[OTf]⁻ (R = Et, ⁿPr) 3, and H₂ were generated. The proposed mechanism is thought to proceed via the oxidative addition of RSH across the Pd[sup I]-Pt[sup I] bond, in a fashion similar to that for the Pd₂ analogues. The μ-thiolate products were characterized by NMR, microanalysis, UV-vis, and conductivity, and mass spectrometry; [PdPtCl₂ (μ-SEt)(μ-dmapm)]⁺[OTf]⁻ was further characterized by X-ray crystallography. Variable temperature NMR was performed to probe the fluxional processes in 1, 2 (R = ⁿPr), and 3 (R = ⁿPr) in solution; these were identified as involving the NCH₃ groups coordinated to Pd or Pt and non-coordinated at the Pd or Pt ends. Pd₂Cl₂(dmapm) also reacts with SO₂; however, the final products are not well-characterized. Preliminary studies on the reaction Pd₂Cl₂ (μ-dppm)₂ with RSH in the presence of acid were also performed; a reaction does indeed occur, but the identities of the products remain uncertain. In an attempt to synthesize a bimetallic Pd complex without coordinated halides, Pd(hfac)₂ (hfac = [CF₃C(O)CHC(O)CF₃]⁻) was reacted with dmapm; again the product remains poorly characterized. The reactivity of MoRu(CO)₆(μ-dppm)₂ (4) with H₂S and thiols was also restudied, following an earlier, unpublished study by a former group member. Although generation of 4 in high in situ or isolated yield was not accomplished, its reaction with H₂S yields the bridged-sulfide complex 5 and H₂, while reaction with EtSH generates the bridged-thiolate complex 6. Attempts to establish intermediates were non-conclusive.

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