Redox Ladder of Ni₃ Complexes with Closed‐Shell, Mono‐, and Diradical Triphenylene Units: Molecular Models for Conductive 2D MOFs

Abstract

We report the isolation and characterization of a series of trinickel complexes with 2,3,6,7,10,11‐hexaoxytriphenylene (HOTP), [(Me3TPANi)3(HOTP)](BF4)n (Me3TPA=N,N,N‐tris[(6‐methyl‐2‐pyridyl)methyl]amine) (n=2, 3, 4 for complexes 1, 2, 3). These complexes comprise a redox ladder whereby the HOTP core displays increasingly quinoidal character as its formal oxidation state changes from −4, to −3, and −2 in 1, 2, and 3, respectively. No formal oxidation state changes occur on Ni, allowing the isolation of singlet diradical, monoradical, and closed‐shell configurations for HOTP in 1, 2, and 3, respectively, with a concomitant decrease in the spin coupling strength upon oxidation. Because the three complexes can be considered models of the smallest building blocks of 2D conductive metal‐organic frameworks such as Ni9HOTP4, these results serve as possible inspiration for the construction of extended materials with targeted electric and magnetic properties.

Strong magnetic exchange coupling in a radical-bridged trinuclear nickel complex

“…29 These results also confirm that radical-mediated coupling with amine-based HITP in 1 is much stronger than with catechol-based HOTP in 2 and 3, and stronger too than with closed-shell HITP tritopic bridges. 18,19,24,26 The enhanced coupling for nitrogenous ligands is in line with previous observations in diradicalbridged complexes, 30 and is likely due to the more diffuse nitrogen orbitals of the radical bridge and better orbital energy match with the metal centers. These results provide motivation for using HITP 3−• as a particularly attractive bridging ligand for other magnetic molecules and solids, where uniform strong coupling may be achieved through supramolecular design or crystal packing.…”

“…Notably, magnetic measurements reveal strong Ni–HITP coupling between HITP 3− ˙ and two Ni 2+ spins that persists at room temperature. This interaction is much stronger than the previously reported metal–ligand spin coupling in the oxygen analogs, [(Tp Ph Ni) 3 (HOTP 3− ˙)] ( 2 ) 18 and [(Me 3 TPANi) 3 (HOTP 3− ˙)] ( 3 ) 19 (Me 3 TPA = N , N , N -tris[(6-methyl-2-pyridyl)methyl]amine), and is attributed to strong orbital delocalization between Ni 2+ and the nitrogen atoms on HITP. These results provide inspiration for the rational design of new room-temperature magnetic materials.…”

“…The HITP-centered delocalized spin is further strongly polarized by the Ni spins, an effect commonly observed for π-ligand bridges and radicals. 19,28 The larger asymmetry in the metal-ligand exchange coupling constants in 1 compared with 2 and 3 is likely related to stronger geometric distortion, which results from the more diffuse orbital of the bridgehead atoms and stronger ligand-metal covalent interaction for the nitrogen-based HITP. 6 Additionally, the BS-DFT analysis together with crystallographic and magnetic characterizations allow tentative assignment of the Ni identity.…”

“…This reflects the influence of global symmetry, beside local symmetry, on the nature of exchange coupling between the metal and the radical bridge, as can be observed in the structurally related compounds 2 and 3. 18,19…”

Redox Ladder of Ni3 Complexes with Closed‐Shell, Mono‐, and Diradical Triphenylene Units: Molecular Models for Conductive 2D MOFs

“…Ellipsoids are set at 50 %p robability for elements other than hydrogen. Hydrogen atoms and part of the Me 3 TPA backbones in (b)are omitted for clarity [45].…”