Janus Pyrrolopyrrole Aza‐dipyrrin: Hydrogen‐Bonded Assemblies and Slow Magnetic Relaxation of the Cobalt(II) Complex in the Solid State

A novel pyrrolopyrrole azadipyrrin (Janus‐PPAD) with Janus duality was synthesized by a Schiff base–forming reaction of diketopyrrolopyrrole. The orthogonal interactions of the hydrogen‐bonding ketopyrrole and metal‐coordinating azadipyrrin moieties in Janus‐PPAD enabled the metal ions to be arranged at regular intervals: zinc(II) and cobalt(II) coordination provided metal‐coordinated Janus‐PPAD dimers, which can subsequently form hydrogen‐bonded one‐dimensional arrays both in solution and in the solid state. The supramolecular assembly of the zinc(II) complex in solution was investigated by 1H NMR spectroscopy based on the isodesmic model, in which a binding constant for the elongation of assemblies is constant. Owing to the tetrahedral coordination, in the solid state, the cobalt(II) complex exhibited a slow magnetic relaxation due to the negative D value of −27.1 cm−1 with an effective relaxation energy barrier Ueff of 38.0 cm−1. The effect of magnetic dilution on the relaxation behavior is discussed. The relaxation mechanism at low temperature was analyzed by considering spin lattice interactions and quantum tunneling effects. The easy‐axis magnetic anisotropy was confirmed, and the relevant wave functions were obtained by ab initio CASSCF calculations. Look both ways after crossing: A pyrrolopyrrole azadipyrrin with a Janus duality (Janus‐PPAD) resulting from the hydrogen‐bonding ketopyrrole and metal‐coordinating azadipyrrin moieties was synthesized by a Schiff base–forming reaction of diketopyrrolopyrrole and azaarylamine. In addition to the self‐complementary assembly, the cobalt complex functions as a single‐molecule magnet in the presence of a static magnetic field.