Construction of copper chains with new fluorescent guanidino-functionalized naphthyridine ligands

The three new blue-fluorescent ligands 2,7-bis(tetramethylguanidino)-1,8-naphthyridine (1), 2,7-bis(N,N'-dimethylethylene-guanidino)-1,8-naphthyridine (2) and 2,7-bis(N,N'-diisopropylguanidino)-1,8-naphthyridine (3) are synthesized, and their optical properties (electronic absorption and emission spectroscopy) studied. Reactions of 1 or 2 with [Cu(CH₃CN)₄]BF₄ yield the Cu₄ chain compounds [Cu₄(1)₂](BF₄)₄ (that crystallizes as [Cu₄(1)₂(CH₃CN)₂](BF₄)₄·2CH₂Cl₂) and [Cu₄(2)₂](BF₄)₄. The variations of the optical properties upon coordination are evaluated, and the electronic transitions identified by time-dependent DFT (TD-DFT) calculations. Then the redox properties of the new Cu₄ chain complexes are studied. In the course of these experiments, the new Cu₆ complex [Cu₄(1)₂(CuCl₂)₂]²⁺, in which two CuCl₂^- units coordinate to the Cu₄ chain in [Cu₄(1)₂]⁴⁺, was fully characterized. In addition, the Cu₃ chain complexes [Cu₃(1)₃]³⁺ and [Cu₃(1)₂]³⁺ were isolated as products of redox-induced degradation processes. Finally, we show by quantum chemical calculations that in [M₄(1)₂]⁴⁺ complexes (M = coinage metal), the HOMO changes from a ligand-centered to a metal-centered orbital for replacement of M = Cu by Au.