Intertwining of Localized (d) and Delocalized (π) Spins in Magnetically Frustrated Two-Dimensional Metal-Organic Frameworks

Two-dimensional metal-organic frameworks (2D MOFs) are emerging as a new class of multifunctional materials for diversified applications, although magnetic properties have not been widely explored. The metal ions and organic ligands in some of the 2D MOFs are arranged in the well-known Kagome lattice, leading to geometric spin frustration. Hence, such systems could be the potential candidates to exhibit an exotic quantum spin liquid (QSL) state, as was observed in Cu3(HHTP)2 (HHTP = hexahydroxytriphenylene), with no magnetic transition down to 38 mK. Hereto, we have investigated the spin intertwining in a bimetallic 2D MOF system, M3(HHTP)2 (M = Cu/Zn), arising from the localized (d-electron) and delocalized (π-electron) S = 1/2 spins from the Cu(II) ions and the HHTP radicals, respectively. The origin of the spin frustration (down to 5K) was critically examined by varying the metal composition in bimetallic systems, CuxZn3-x(HHTP)2 (x = 1, 1.5, 2), containing both S = 1/2 and S = 0 spins. Additionally, to gain a deeper understanding, we studied the spin interaction in the pristine Zn3(HHTP)2 system containing only S = 0 Zn(II) ions. In view of the quantitative estimate of the localized and delocalized spins, the d-π spin correlation appears essential in understanding the unusual magnetic and/or other physical properties of such hybrid organic-inorganic 2D crystalline solids.

Understanding the Electronic Structure of Magnetic Trinuclear Complexes Based on the Tris-Dioxolene Triphenylene Non-Innocent Bridging Ligand, a Theoretical Study

A complete theoretical analysis using first the simple Hückel model followed by more sophisticated multi-reference calculations on a trinuclear Ni(II) complex (Tp#Ni3 HHTP), bearing the non-innocent bridging ligand HHTP3- , is carried out. The three semiquinone moieties of HHTP3- couple antiferromagnetically and lead to a single unpaired electron localized on one of the moieties. The calculated exchange coupling integrals together with the zero-field parameters allow, when varied within a certain range, reproducing the experimental data. These results are generalized for two similar other trinuclear complexes containing Ni(II) and Cu(II). The electronic structure of HHTP3- turns out to be independent of both the chemical nature and the geometry of the metal ions. We also establish a direct correlation between the geometrical and the electronic structures of the non-innocent ligand that is consistent with the results of calculations. It allows experimentalists to get insight into the magnetic behavior of this type of complexes by an analysis of their X-ray structure.

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

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene hexahydrochloride (HATP·6HCl) and (Tp^PhNi)Cl (Tp^Ph = tris(3,5-diphenyl-1-pyrazolyl)borate) produces the radical-bridged trinickel complex [(Tp^PhNi)₃(HITP)] (HITP^3-˙ = 2,3,6,7,10,11-hexaiminotriphenylene). Magnetic measurements and broken-symmetry density functional theory calculations reveal strong exchange coupling persisting at room temperature between HITP^3-˙ and two of the three Ni²⁺ centers, a rare example of strong radical-mediated magnetic coupling in multimetallic complexes. These results demonstrate the potential of radical-bearing tritopic HITP ligands as building blocks for extended molecule-based magnetic materials.

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

We report the isolation and characterization of a series of trinickel complexes with 2,3,6,7,10,11-hexaoxytriphenylene (HOTP), [(Me₃ TPANi)₃ (HOTP)](BF₄ )_n (Me₃ TPA=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 Ni₉ HOTP₄ , these results serve as possible inspiration for the construction of extended materials with targeted electric and magnetic properties.

A hybrid bioinspired catechol-alloxazine triangular nickel complex stabilizing protons and electrons

A new class of redox-active ligands merging catechol and alloxazine structures is reported. A trimetallic triangular complex is formed upon complexation to nickel.

Two-dimensional triphenylene cored hole-transporting materials for efficient perovskite solar cells

A PSC based on a two-dimensional triphenylene-cored HTM is unveiled that shows an optimal power conversion efficiency (PCE) of 19.4%.