Water-Soluble Phosphine Capable of Dissolving Elemental Gold: The Missing Link between 1,3,5-Triaza-7-phosphaadamantane (PTA) and Verkade’s Ephemeral Ligand

We herein describe a tricyclic phosphine
with previously unreported
tris­(homoadamantane) cage architecture. That water-soluble,
air- and thermally stable ligand, 1,4,7-triaza-9-phosphatricyclo­[5.3.2.14,9]­tridecane (hereinafter referred to as CAP) exhibits unusual
chemical behavior toward gold and gold compounds: it readily reduces
Au­(III) to Au(0), promotes oxidative dissolution of nanocrystalline
gold(0) with the formation of water-soluble trigonal CAP–Au­(I)
complexes, and displaces cyanide from [Au­(CN)2]− affording triangular [Au­(CAP)3]+ cation. From
the stereochemical point of view, CAP can be regarded as an intermediate
between 1,3,5-triaza-7-phosphaadamantane (PTA) and very unstable aminophosphine
synthesized by Verkade’s group: hexahydro-2a,4a,6a-triaza-6b-phosphacyclopenta­[cd]­pentalene. The chemical properties
of CAP are likely related to its anomalous stereoelectronic profile:
combination of strong electron-donating power (Tolman’s electronic
parameter 2056.8 cm–1) with the low steric demand
(cone angle of 109°). CAP can be considered as macrocyclic counterpart
of PTA with the electron-donating power approaching that of strongest
known phosphine electron donors such as P­(t-Bu)3 and PCy3. Therefore, CAP as sterically undemanding
and electron-rich ligand populates the empty field on the stereoelectronic
map of phosphine ligands: the niche between the classic tertiary phosphines
and the sterically undemanding aminophosphines.