An unusual co-crystal [(μ2-dcpm)Ag2(μ2-O2CH)(η2-NO3)]2·[(μ2-dcpm)2Ag4(μ2-NO3)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) : NaO2CH in a molar ratio of 2 : 1 : 1 gave an abundant peak due to [LAg2(O2CH)]+ and a minor peak assigned as [L2Ag4(O2CH)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-O2CH)(η2-NO3)]2·[(μ2-dcpm)2Ag4(μ2-NO3)4]. While all of the coordinated formates in [LAg2(O2CH)]+ and [L2Ag4(O2CH)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.