Structural, Thermodynamic, and Transport Properties of the Small-Gap Two-Dimensional Metal–Organic Kagomé Materials Cu₃(hexaiminobenzene)₂ and Ni₃(hexaiminobenzene)₂

Abstract

Metal–organic frameworks (MOFs) provide exceptional chemical tunability and have recently been demonstrated to exhibit electrical conductivity and related functional electronic properties. The kagomé lattice is a fruitful source of novel physical states of matter, including the quantum spin liquid (in insulators) and Dirac fermions (in metals). Small-bandgap kagomé materials have the potential to bridge quantum spin liquid states and exhibit phenomena such as superconductivity but remain exceptionally rare. Here we report a structural, thermodynamic, and transport study of the two-dimensional kagomé metal–organic frameworks Ni3(HIB)2 and Cu3(HIB)2 (HIB = hexaiminobenzene). Magnetization measurements yield Curie constants of 0.989 emu K (mol Ni)−1 Oe–1 and 0.371 emu K (mol Cu)−1 Oe–1, respectively, close to the values expected for ideal S = 1 Ni2+ and S = 1/2 Cu2+. Weiss temperatures of −10.6 and −14.3 K indicate net weak mean field antiferromagnetic interactions between ions. Electrical transport measurements reveal that both materials are semiconducting, with gaps (E g) of 22.2 and 103 meV, respectively. Specific heat measurements reveal a large T-linear contribution γ of 148(4) mJ mol-fu–1 K–2 in Ni3(HIB)2 with only a gradual upturn below ∼5 K and no evidence of a phase transition to an ordered state down to 0.1 K. Cu3(HIB)2 also lacks evidence of a phase transition above 0.1 K, with a substantial, field-dependent, magnetic contribution below ∼5 K. Despite them being superficially in agreement with the expectations of magnetic frustration and spin liquid physics, we ascribe these observations to the stacking faults found from a detailed analysis of synchrotron X-ray diffraction data. At the same time, our results demonstrate that these MOFs exhibit localized magnetism with simultaneous proximity to a metallic state, thus opening up opportunities to explore the connection between the insulating and metallic ground states of kagomé materials in a highly tunable chemical platform.

Cu(I)/Cu(II) Creutz-Taube Mixed-Valence 2D Coordination Polymers

“…Both exhibit paramagnetism above 150 K and large negative Curie temperatures, suggesting antiferromagnetic (AFM) coupling of the Cu ions. 27 This conclusion is also supported by the X-ray magnetic circular dichroism (XMCD) spectra (Figure 3f and S11b), and agrees with our DFT calculations, i.e., the intralayer Cu ions are ferromagnetically (FM) coupled but interlayer AFM as ground states. At the same time, the liquid oxygen spoor was detected around 52 K, suggesting that oxygen had not been entirely eliminated from the samples.…”

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

“…On the one hand, the Cu 3 (HHTP) 2 system has the potential to emerge as an archetypical QSL candidate in the domain of 2D MOFs, and on the other hand, magnetic frustration in 2D MOFs like Cu 3 (HIB) 2 and Ni 3 (HIB) 2 (HIB = hexaiminobenzene) is being assigned to structural disorders . Therefore, our observation on the robust magnetic frustration in bimetallic Cu x Zn 3– x (HHTP) 2 ( x = 1, 1.5, and 2) systems down to 5 K is intriguing and anticipated to be useful for chemically designing a wide variety of possible MOF-based QSL candidates.…”

Organic quantum materials: A review

“…On the other hand, disorders suppress spin ordering in 2D MOFs as in ET salts. Berry et al 234 reported paramagnetic behaviors in Ni 3 (HIB) 2 and Cu 3 (HIB) 2 (HIB is hexaiminobenzene). While no signals of spin freezing were detected in these MOFs with high θ / T order values over 10, which satisfied the frustration condition, the origin of the order absence was attributed to their stacking faults.…”