High Electrical Conductivity in Ni3(2,3,6,7,10,11- hexaiminotriphenylene)2, a Semiconducting Metal−Organic Graphene Analogue

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni in aqueous NH3 solution under aerobic conditions produces Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metal−organic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 and 40 S·cm−1, respectively, both records for MOFs and among the best for any coordination polymer. T (2D) electronic materials are of considerable interest due to their potential applications in future electronics. The most prominent example is graphene, an atomically thin organic 2D material with in-plane πconjugation. Although graphene exhibits exceptional charge mobility and mechanical stability, its use in semiconductorbased devices is limited by its zero bandgap. Dimensional reduction and chemical functionalization can increase the bandgap, rendering graphene semiconducting, but these methods drastically reduce its charge mobility and can introduce numerous defects. This has led to a sustained effort toward identifying 2D materials with intrinsic non-zero bandgaps that could replace conventional semiconductors. Two broad classes of materials have dominated these efforts: layered metal chalcogenides (e.g., MoS2, WSe2) and 2D covalent−organic frameworks (COFs). The former can be deposited as large-area single sheets in a “top-down” approach. They have been shown to perform well in device testing but do not easily lend themselves to chemical functionalization and tunability. In contrast, COFs are prepared by “bottom-up” solution-based synthetic methods and are attractive because they are subject to rational modification. Nevertheless, the electronic properties of COFs are largely inferior to those of metal chalcogenides because the functional groups used to connect their building blocks typically do not allow in-plane conjugation. Bridging the divide between 2D inorganic and organic materials is a recent class of “bottom-up” compounds assembled from multitopic dithiolene and o-semiquinone aromatic organic moieties bridged by square-planar metal ions. These 2D metal−organic networks exhibit non-zero bandgaps and good electrical conductivity, enabled by full charge delocalization in the 2D plane. They can therefore be described as semiconducting metal−organic graphene analogues (s-MOGs). Certain members of this class have also recently been predicted to behave as topological insulators, a realm currently dominated by purely inorganic compounds. Clearly, the synthesis and characterization of new s-MOGs could give rise to important new electronic materials with exotic electronic states and potential applications in the semiconductor device industry. Inspired by the success of dithiolene-based s-MOGs, whose metal linkages mimic classic Class III-delocalized homoleptic Ni(dithiolene)2 complexes, 25,26 we identified Ni(isq)2 (isq = odiiminobenzosemiquinonate) as an attractive target for the construction of a fully charge-delocalized s-MOG. Although first isolated in 1927 from o-phenylenediamine and NiCl2 in ammoniacal water, Ni(isq)2 evaded structural and electronic characterization for quite a long time but is now known to be fully π-conjugated, with its ground state having partial singlet biradical character. Crystalline Ni(isq)2 itself exhibits high mobility in organic field effect transistors and increased conductivity upon doping with I2. 33 Here, we show that twodimensional extension of Ni(isq)2 through the reaction of 2,3,6,7,10,11-hexaaminotriphenylene hexahydrochloride (HATP·6HCl) with ammoniacal NiCl2 produces a new crystalline s-MOG with very high electrical conductivity that is linearly proportional to temperature. Remarkably, the conductivity of the new material vastly exceeds that of previous s-MOGs and other conductive MOFs and is higher than even some of the best organic conductors. Under conditions mimicking the synthesis of Ni(isq)2, HATP·6HCl was treated with an aqueous solution of NiCl2· 6H2O under air, followed by the addition of aqueous NH3 under constant stirring. The reaction mixture was heated to 65 °C and stirred under air for an additional 2 h. This yielded the new material Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylenesemiquinonate) as a bulk black powder and a very dark blue-violet film (see Scheme 1). Upon extensive washing with water in an ultrasonic bath, the charge neutrality of Ni3(HITP)2, and implicitly the formation of monoanionic oReceived: March 18, 2014 Published: April 21, 2014 Communication