Amphiphilic Long-chain Citric Acid Ethers

In recent years much attention has been paid to the synthesis of metal and metal oxide nanoparticles with tailored size and morphology, and to appropriate functionalization in their periphery [1]. In addition to several types of polymers, long-chain organic molecules play an important role as ligands for nanoparticles to prevent their agglomeration and to provide the necessary functionalization. Long-chain organothiols are suitable ligands for “soft” [2] metal surfaces [3] and long-chain carboxylic acids for “hard” [2] metal and metal oxide surfaces [1a]. With respect to possible applications, there is further demand for sufficient surface coverage of the nanoparticles, for strong particle ligand bonding and for low ligand exchange rates. In this context, we [4] and independently the group of Lee [5] have synthesized functionalized and non-functionalized long-chain tridentate organotrithiols (type I in Fig. 1). It has been demonstrated that this type of ligands feature the smallest ratio between surface coverage and number of binding sites [4, 5b]. Furthermore, they offer excellent kinetic and thermodynamic stability of nanoparticles, which are made of “soft” metals [4]. In this paper, we present the synthesis of functionalized and non-functionalized polydentate long-chain organic acids of type II, which are promising ligands for “hard” metals and metal oxides. Citric acid is a suitable starting material for the preparation of type-II ligands. It already contains