amidinium

Amidinium refers to the amidinium cation, a positively charged species derived from amidines by protonation of the imine-like nitrogen. In an amidine, the core structure is R-C(=NH)-NR', and protonation on the imine nitrogen yields the amidinium ion R-C(=NH2+)-NR'. The positive charge is stabilized by resonance within the C=N–N framework, giving a relatively rigid and planar cation.

Amidinium salts are typically isolated with a range of counterions, such as chloride, tetrafluoroborate, or hexafluorophosphate. The properties of these salts, including solubility and hydrogen-bonding behavior, depend on the substituents on the amidinium framework and the chosen counterion. The amidinium cation is a strong hydrogen-bond donor and acceptor, contributing to characteristic interactions with anions and other hydrogen-bonding partners.

Preparation generally involves protonation of an amidine or exchange from other amidinium precursors under acidic conditions. Because of their pronounced cationic character and resonance stabilization, amidinium ions participate in a variety of supramolecular contexts and material applications.

Applications and relevance include crystal engineering and supramolecular chemistry, where amidinium cations form directional hydrogen-bonded motifs with carboxylates, phosphates, and related anions. These interactions can be used to build hydrogen-bonded frameworks and self-assembled materials. In broader chemical synthesis and medicinal contexts, amidinium groups serve as cationic motifs and are encountered as counterions in salt forms of amine- or amidine-derived compounds.

See also: guanidinium, amidine.