New tetrazole-based Cu(I) homo- and heteroleptic complexes with various P^P ligands: synthesis, characterization, redox and photophysical properties

Four Cu(I) complexes with general formulas [Cu(N^N)2][BF4] and [(P^P)Cu(N^N)][BF4] were prepared, where N^N stands for 2-(2-tert-butyl-2H-tetrazol-5-yl)pyridine and P^P is a chelating diphosphine, namely bis-(diphenylphosphino)methane (dppm), bis-(diphenylphosphino)ethane (dppe) or bis[2-(diphenylphosphino)phenyl]ether (POP). In an acetonitrile medium, the Electro-Spray Ionization Mass Spectrometry (ESI-MS) determination provided the preliminary evidence for the occurrence of the dppmcontaining complex as a mixture of a cationic mononuclear [Cu(N^N)(dppm)]+ species and a bis-cationic dinuclear [Cu2(N^N)2(dppm)2]2+-type compound. Definitive evidence of peculiar structural features came from X-ray crystallography, which showed both the dppm- and, unexpectedly, the dppe-based heteroleptic compounds to crystallize as diphosphine-bridged Cu(I) dimers, unlike [Cu(N^N)2]+ and [(POP)Cu(N^N)]+ which are mononuclear species. In solutions of non-coordinating solvents, 31P NMR studies at variable temperatures and dilution titrations confirmed that the dppm-based complex undergoes a monomer–dimer dynamic equilibrium, while the dppe-containing complex occurs as the bis-cationic dinuclear species, [Cu2(N^N)2(dppe)2]2+, within a concentration range comprised between 10−2 and 10−4 M. Differences among heteroleptic complexes might be related to the smaller natural bite angle displayed by dppm and dppe phosphine ligands (72° and 85°, respectively), with respect to that reported for POP (102°). The electrochemical features of the new species have been investigated by cyclic voltammetry. Despite the irreversible and complicated redox behaviour, which is typical for copper complexes, the reductions have been attributed to the tetrazole ligand whereas the oxidations are characterized as Cu(I/II) processes with a substantial contribution from the P^P-based ligands in the case of the heteroleptic species. All the four complexes are weakly or not luminescent in CH2Cl2 solution, but heteroleptic complexes are bright green luminophores in a solid matrix, with quantum yields as high as 45% (dppm complex) even at room temperature. This makes them potential candidates as cheap emitting materials for electroluminescent devices.