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.

本文报道了一种具有此前未被报道的三(高金刚烷)笼状结构的三环膦（tricyclic phosphine）。该水溶性、耐空气且热稳定的配体1,4,7-三氮杂-9-磷杂三环[5.3.2.1⁴,⁹]十三烷（下文简称CAP）对金及其化合物表现出反常的化学行为：可轻易将三价金（Au(III)）还原为零价金（Au(0)），促进纳米晶零价金发生氧化溶解，生成水溶性三角构型的CAP-一价金（Au(I)）配合物；还能从二氰合金(I)配阴离子[Au(CN)₂]⁻中置换出氰根，得到三角构型的三(CAP)合金(I)阳离子[Au(CAP)₃]⁺。从立体化学视角来看，CAP可被视作1,3,5-三氮杂-7-磷杂金刚烷（PTA）与弗尔卡德（Verkade）课题组合成的极不稳定氨基膦——六氢-2a,4a,6a-三氮杂-6b-磷杂环戊二烯并[cd]戊搭烯——之间的中间体。CAP的化学性质大概率与其反常的立体电子特性密切相关：兼具强给电子能力（托爾曼电子参数（Tolman's electronic parameter）为2056.8 cm⁻¹）与低空间位阻（锥角为109°）。CAP可被认为是PTA的大环类似物，其给电子能力可与目前已知最强的膦给电子体如三叔丁基膦（P(t-Bu)₃）和三环己基膦（PCy₃）相媲美。因此，作为空间位阻小且富电子的配体，CAP填补了膦配体立体电子图谱中的空白区域：位于经典叔膦与低空间位阻氨基膦之间的独特位点。