Nanoclusters of Dicyanocuprate(I) Anions in Aqueous Solutions: Investigating Cuprophilic Interactions

Abstract

The oligomerization of cyanocuprate(I) ions in aqueous solutions lacking and containing alkali cyanide salts (NaCN/ KCN) has been investigated by infrared, Raman, and low temperature luminescence spectroscopy. In solution metallophilic interactions between Cu(I) metal centers bind the ions together into ∼ 1D chains. The structure and luminescence assignment of these nanoclusters was determined using simple Group Theory and Density Functional Theory calculations. Measurements show that the presence of KCN and NaCN in solution drives oligomerization of CuCN 2 À ions to higher order nanoclusters. Calculations of dimer and trimer clusters with K + and Na + ions indicate that alkali metals favor the formation of linear clusters as opposed to those that are bent. These findings fill an important gap in the understanding of d 10 cyanometallate oligomers given the prevalence of cyanocuprate(I) in metal-organic frameworks.Spontaneous oligomerization of d 10 cyanometallates in solution was first reported for aqueous solutions of Ag(CN) 2 À and Au (CN) 2 À ions. [1][2][3][4] Interest in these systems was driven by the prevalence of Au(I) and Ag(I) metals in photoactive materials. These nanoclusters display strong photoluminescence properties at extremely low concentrations. Measurements revealed that absorbance and emission energies are heavily dependent on the oligomer size, as clearly demonstrated by the red shift of the UV-vis absorption edge within increasing ion concentration. The culprit for the formation of these clusters is ground state aurophilic and argentophilic interactions between metal centers. The strong interaction significantly stabilizes the ion cluster permitting the formation of large clusters of size n > 4. An investigation of the excited states of [Au(CN) 2 À ] n clusters was later performed by Iwamura and others using pico-and femtosecond spectroscopy. [5][6][7][8] In all cases it was concluded that these oligomers exist as loosely bound close-shelled clusters in the ground state but become tightly bound in the open-shelled excited state. For those of n � 3, this results in ground state structures, Scheme 1, that are bent in nature but contract to form a linear AuÀ Au-Au upon photoexcitation.Curiously, despite the importance of this work and its prevalence throughout the literature, there has been no similar investigation of the cyanocuprate(I) analog. Presumably this system behaves in a similar manner to Au(CN) 2 À and Ag(CN) 2 À due to their similar electronic configurations. The need for this investigation comes at a time when researchers are actively seeking cheaper, more abundant alternatives to precious metals such as Pt, Ag, and Au. [9][10][11] To address this discrepancy we have prepared solutions of CuCN 2 À in DI water with and without the alkali cyanide salts KCN and NaCN. It has been demonstrated that coexisting simple cations (i.e Li + , K + , Rb + , Cs + ) are capable of stabilizing larger [Au(CN) 2 À ] n oligomers. [6,8] [a] Dr.