| Summary: | The 2-pyridyiphosphine ligands are analogues of the ligand triphenyiphosphine (PPh₃)
with the phenyl groups being sequentially replaced by 2-pyridyl groups (PN[sub n]; n=1,PN₁=
PPh₂py; n=2, PN₂=PPhpy₂;n=3, PN₃=Ppy₃). Replacement of the phenyl groups with 2-pyridyl
groups adds heteropolydentate characteristics as well as hydrophilic properties to these tertiary
phosphine ligands.
Zero-valent platinum complexes with PN ligands, namely Pt(PN₁)₃ and Pt(PN₃)₄ have
been previously synthesized. In this work, the complex Pt(PN₂)₃ was synthesized and
characterized using ³¹P{¹H} and ¹⁹⁵Pt{¹H} NMR spectroscopy, as well as elemental analysis.
The reaction of Pt(PN₂)₃ with methyl iodide resulted in the formation of trans- Pt(Me)I(PN₂)₂,
similar to the reaction of Pt(PPh₃)₃ with methyl iodide. The tetrakis (PN₂) complex, Pt(PN₂)₄
was observed to be formed in situ, using ³¹P{¹H} NMR spectroscopy, when aCD₂Cl₂ solution
containing a mixture of Pt(PN₂)₃ and PN₂ was cooled to -60° C.
The reaction of dioxygen with solutions of Pt(PN₁)₃ and Pt(PN₂)₃ resulted in the
formation of dioxygen complexes. The isolated compounds were characterized using NMR
(³¹P{¹H} and ¹⁹⁵Pt{¹H}) and infrared spectroscopy, and the structures were found to be side-on
bonded peroxo complexes analogous to the well-characterized triphenylphosphine complex
Pt(O₂)(PPh₃)₂.Both Pt(O₂)(PN₁)₂ and Pt(O₂)(PN₂)₂ were found to react with gaseous HCl to
form the dichioro compounds cis- PtCl₂(PN₁)₂ and cis- PtCl₂ (PN₂)₂,respectively, with
concomitant formation of H₂O₂, similar to the reported reaction of Pt(O₂)(PPh₃)2 with HCl.
Attempts to use the Pt(O₂)(PPh₃)₂/HCl system for the catalytic O₂-oxidation of the
thioether diethyl sulfide, at room temperature, were unsuccessful, although stoichiometric
oxidation occurred via the liberated H₂O₂.
None of the zero-valent platinum 2-pyridyiphosphine complexes was soluble in water.
However, reaction of aqueous suspensions of either Pt(PN₂)₃ or Pt(PN₃)₄ with aqueous HCl
resulted in protonation of the 2-pyridyl moieties of the coordinated phosphine ligands, and hence water-solubilization of the complexes. Metathesis of the chloride ion within the proton containing
products using PF₆- or BPh₄- salts enabled isolation of the 2-pyridinium salts from
water. The chloride salts of the PN₂ and PN₃ 2-pyridinium complexes were synthesized by
reaction of THF solutions of Pt(PN₂)₃ and Pt(PN₃)₄,respectively, with DMA HCl (N,N’-dimethylacetamide hydrochloride). The water-soluble 2-pyridinium PN₂ and PN₃ salts were
characterized by ³¹P{¹H} NMR and infrared spectroscopy as well as by elemental analysis, and
are considered to be Pt(PN₂)₃ 2HX and Pt(PN₃)₃ HX, respectively, where X is Cl, PF₆,or
BPh₄. The Pt(PN₂)₃ 2HCl species in CH₂Cl₂ converts reversibly at lower temperature to trans
PtHCl(PN₂)₂,and in acetone at ambient temperature to cis- PtCl₂(PN₂)₂.
The presence of unprotonated pyridyl groups appears to be necessary for the water
solubilization of Pt(PN₂)₃ and Pt(PN₃)₄: no water-soluble 2-pyridinium complexes were formed
on acidification of an aqueous suspension of Pt(PN₁)₃.This is confirmed by the reaction of
Pt(PN₁)₃ with DMA•HCl which resulted in the formation of the covalent products cis
PtCl₂(PN₁)₂ and trans- PtHCl(PN₁).
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