Palladium bis-pincer complexes with controlled rigidity and inter-metal distance

We report a series of redox-active bis(pincer) Pd(ii) complexes in which the redox active units are based on either a diarylamido or a carbazolide framework. Compounds 1 and 2 contain two full diarylamido/bis(pincer) PNP units connected either via an Ar-O-Ar linker (1) or an Ar-Ar bond (2). Compound 3 is a fused bis(pincer) where the two PNP units share an aromatic ring. Compound 4 is built around an indolo[3,2-b]carbazole core in which two NNN pincers share an aromatic ring similarly to 3. These metal complexes all display two reversible oxidation waves with the Delta E values increasing in the order of 1 < 2 < 4 < 3. The same trend in increasing electronic coupling emerges from the analysis of the IV-CT bands in the NIR portion of the optical spectra. The analysis of these compounds was further advanced by data from EPR spectroscopy, X-ray diffractometry, and DFT calculations. It is concluded that the monooxidized cations 2(+)-4(+) belong to Class III on the Robin-Day classification of mixed-valence compounds. Compound 4 possesses enforced near-planarity that enables delocalization of the unpaired electron in 4(+) across a broader conjugated system compared to 3(+).