Heteroleptic palladium(II) complexes of dipyrrin-1,9-dione supported by intramolecular hydrogen bonding

Abstract

The dipyrrin-1,9-dione framework, which is characteristic of the propentdyopent pigments deriving from heme metabolism, coordinates metal ions as monoanionic bidentate donors. The resulting analogs of dipyrrinato complexes undergo reversible ligand-based reductions, thus showcasing the ability of the dipyrrindione scaffold to act as an electron reservoir. Herein we report the synthesis and characterization of three heteroleptic palladium complexes of the redox-active dipyrrindione ligand. Primary amines were chosen as additional ligands so as to assemble complexes of planar geometries with complementary interligand hydrogen-bonding. Full chemical characterization confirms the hydrogen bonding interactions between the primary amine ligands and the acceptor carbonyl groups on the dipyrrolic ligand. The resulting heteroleptic compounds display reversible one-electron reduction events that are centered on the dipyrrindione ligand as revealed by voltammetry and spectroelectrochemistry data. Within these planar Pd(II) complexes, the propentdyopent motif therefore combines reversible ligand-based redox chemistry with interligand hydrogen bonding in the primary coordination sphere of the metal center.

Stabilization of a Triplet Diradical on a Binuclear palladium(II) Dipyrrindione

“…67,68 In this context, we sought to investigate the ability of the dipyrrindione framework to host unpaired spins in a bridged dinuclear system. In our previous investigations of the palladium(II) dipyrrindione complexes featuring primary amine ligands (e.g., [Pd(en)(pdp)] + , Chart 2d), 52 we found that the intramolecular hydrogen bonds are important for complex stability. This observation prompted us to explore the synthesis of a similar diaqua complex.…”

“…Monitoring the reaction progress via optical absorption spectroscopy, we observed the gradual decrease of the single absorption band of Hpdp•MeOH at 280 nm accompanied by the growth of two main bands at 382 and 545 nm, consistent with our previous reports on dipyrrindione complexes. 50,52 In the course of our synthetic manipulations, we observed that exposing a dichloromethane solution of this red complex to an aqueous wash resulted in an immediate color change of the organic phase from red to blue. The absorption spectrum of the blue solution in CH2Cl2 presents a shift in the main bands to 372 and 585 nm (Figure 1, Table S1), clearly indicating the formation of a different species.…”

“…In contrast, the dipyrrin-1,9-dione motif serves as a compact dipyrrolic platform to host ligand-centered radicals. [50][51][52] This bidentate ligand could therefore extend the diverse applications of dipyrrin complexes to include one-electron redox chemistry and a variety of spin states.…”

“…Historically referred to as propentdyopents, the naturally occurring dipyrrindione pigments result from the oxidative metabolism of heme 53,54 and were initially discovered within the urinary pigments collected from hyperbilirubinemia patients. 55 The first dipyrrindione complexes were isolated from the degradation of bilindione complexes; 23,56 however, we recently employed the Hpdp•MeOH adduct for the preparation of homoleptic 50 and heteroleptic 51,52 compounds (Chart 2). Because of the availability of lowlying π* molecular orbitals, these complexes typically undergo one-electron reduction processes localized on the dipyrrolic ligand framework.…”

Dipyrrin based metal complexes: reactivity and catalysis

“…The Suzuki-Miyaura cross-coupling is an important tool in organic chemistry with active research in the development of new palladium catalysts. 135 Surprisingly, while homo-and heteroleptic Pd(II) dpm complexes have been extensively investigated, 33,[136][137][138][139][140][141][142][143][144][145] only one report described the potential of these compounds for C-C bond formation. 140 Mono-and bi-nuclear heteroleptic complexes 64 and 65 (Fig.…”