Argentophilic Interactions in Solution: An EXAFS Study of Silver(I) Nitrene Transfer Catalysts

Dinuclear Silver Complexes in Catalysis

Over the past two decades, there has been a substantial increase in the number of synthetically useful transformations catalyzed by silver. Across the range of silver-catalyzed reactions that have been reported, dinuclear species often emerge as a common feature, either as the (pre-)catalysts themselves or as intermediates during catalysis. This Minireview explores the role of dinuclear silver complexes in homogeneous catalysis, which we hope will aid in the development of improved design principles for silver catalysts.

Dinuclear Silver(I) Nitrate Complexes with Bridging Bisphosphinomethanes: Argentophilicity and Luminescence

Two silver nitrate complexes with bisphosphines were obtained and characterized: [Ag(dcypm)]2(NO3)2 (1; dcypm = bis(dicyclohexylphosphino)methane) and [Ag(dppm)]2(Me2PzH)n(NO3)2 (n = 1, 2a; n = 2, 2b; dppm = bis(diphenylphosphino)methane, Me2PzH = 3,5-dimethylpyrazole). The steric repulsions of bulky cyclohexyl substituents prevent additional ligand coordination to the silver atoms in 1. Compounds obtained feature the bimetallic eight-member cyclic core [AgPCP]2. The intramolecular argenthophilic interaction (d(Ag···Ag) = 2.981 Å) was observed in complex 1. In contrast, the coordination of pyrazole led to the elongation of Ag···Ag distance to 3.218(1) Å in 2a and 3.520 Å in 2b. Complexes 1 and 2a possess phosphorescence both in the solution and solid state. Time-dependent density-functional theory (TD-DFT) calculations demonstrate the origin of their different emission profile. In the case of 1, upon excitation, the electron leaves the Ag–P bonding orbital and locates on the intramolecular Ag···Ag bond (metal-centered character). Complex 2a at room temperature exhibits a phosphorescence originating from the 3(M + LP+N)LPhCT state. At 77 K, the photoluminescence spectrum of complex 2a shows two bands of two different characters: 3(M + LP+N)LPhCT and 3LCPh transitions. The contribution of Ag atoms to the excited state in both complexes 2a and 2b decreased relative to 1 in agreement with the structural changes caused by pyrazole coordination.

Silver-Catalyzed Enantioselective Propargylic C-H Bond Amination through Rational Ligand Design

Asymmetric C-H amination via nitrene transfer is a powerful tool to prepare enantioenriched amine precursors from abundant C-H bonds. Herein, we report a regio- and enantioselective synthesis of γ-alkynyl γ-aminoalcohols via a silver-catalyzed propargylic C-H amination. The protocol was enabled by a new bis(oxazoline) (BOX) ligand designed via a rapid structure-activity relationship (SAR) analysis. The method utilizes accessible carbamate esters bearing γ-propargylic C-H bonds and furnishes versatile products in good yields and excellent enantioselectivity (90-99% ee). The putative Ag-nitrene is proposed to undergo enantiodetermining hydrogen-atom transfer (HAT) during the C-H amination event. Density functional theory calculations shed insight into the origin of enantioselectivity in the HAT step.

Mixed ligand complexes of silver(i) supported by highly fluorinated pyrazolates, and chelating and bridging N-heterocycles

A range of silver complexes featuring all nitrogen coordination environments involving pyrazolates and chelating and non-chelating bidentate N-heterocycles are reported.