A new type of homoleptic copper complex using an imine-phosphaalkene based bidentate ligand framework was synthesized and characterized.[1] The ligand, a 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)-functionalized phosphaalkene [DBU-PA], has previously been studied with regards to stabilizing Lewis acid base interactions, as well as formation of a cis-PdCl2 complex, showing its general versatility.[2]

The reaction of a Cu(I) source and DBU-PA at room temperature results in rapid formation of a deeply red colored complex. Despite numerous attempts, crystal growth proved impossible; however, optimizing the structure in gas phase with DFT calculations showed two possible conformations with an energy difference of only 3.6 kcal/mol, suggesting that the complex exists as a mixture in solution.

Mass spectrometry and 31P NMR measurements were performed to prove the formation of the complex. Spectroscopic analysis including a 3D emission/excitation correlation in a frozen MeCN matrix at 77 K showed a clear distinction between LC and MLCT emission states, the latter being of modest strength in the green region of the visible spectrum. Its excited state lifetime of ~ 9 ns is modest, suggesting that despite the considerable bulk of the ligand, typical deactivation of Cu(I) complexes by geometric rearrangement in the excited state can still take place.

Electrochemical studies showed three reductive events, all almost completely chemically irreversible. DFT calculations showed the LUMO to be mostly localized on the phosphaalkene-imine antibonding orbitals, suggesting that reductions take place first on the ligand, destabilizing its bond to the metal center. On the oxidative side, the Cu(I)/Cu(II) couple is semi-reversible at higher scan rate, again showing ligand lability. Further events are fully ligand-based. Overall, ligand non-innocence is a significant contributing factor to the electrochemical properties.

Despite this particular complex exhibiting low stability and unwanted dynamic behavior, it is a promising first entry into a new class of Cu-based dye compounds with potential photosensitizing applications. The Mes substituent of the DBU-PA ligand is on the one hand too bulky to result in a stable complex, while also not resulting in a favorable configuration to result in the spectroscopic properties desirable for a photosensitizer. However, the synthetic route to DBU-PA easily allows replacement of the Mes moiety, making the complex studied herein an excellent starting point to establish a library of related compounds with properties fine-tuned for eventual photocatalytic applications.

[1] S. T. Clausing, D. Morales Salazar, A. Orthaber, Inorganica Chimica Acta 2020, 513, 119958. https://doi.org/10.1016/j.ica.2020.119958
[2] D. Morales Salazar, A. K. Gupta, A. Orthaber, Dalton Trans. 2018, 47, 10404–10409. https://doi.org/10.1039/C8DT01607K