An unusual co-crystal [(μ 2 -dcpm)Ag 2 (μ 2 -O 2 CH)(η 2 -NO 3 )] 2 ·[(μ 2 -dcpm) 2 Ag 4 (μ 2 -NO 3 ) 4 ] and its connection to the selective decarboxylation of formic acid in the gas phase.

ESI/MS of an acetonitrile solution containing a mixture of AgNO3  :  bis(dicyclohexylphosphino)methane (dcpm, L) : NaO2 CH in a molar ratio of 2 : 1 : 1 gave an abundant peak due to [LAg2 (O2 CH)]+ and a minor peak assigned as [L2 Ag4 (O2 CH)3 ]+ . When this acetonitrile solution was frozen and layered with diethyl ether and left undisturbed for six days, crystalline material suitable for X-ray crystallography was identified and separated from amorphous solids. Single crystal X-ray diffraction revealed an unusual co-crystal consisting of two discrete tetranuclear silver clusters [(μ2 -dcpm)Ag2 (μ2 -O2 CH)(η2 -NO3 )]2 ·[(μ2 -dcpm)2 Ag4 (μ2 -NO3 )4 ]. While all of the coordinated formates in [LAg2 (O2 CH)]+ and [L2 Ag4 (O2 CH)3 ]+ can be decarboxylated in the gas-phase under conditions of collision induced dissociation, only the hydride [LAg2 (H)]+ thus formed reacts with formic acid via protonation and liberation of H2 to regenerate to formate, thereby closing a catalytic cycle for the selective decomposition of formic acid.