Carl K. Brozek

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(2+) 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.

Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5

The metal nodes in metal-organic frameworks (MOFs) are known to act as Lewis acid catalysts, but few reports have explored their ability to mediate reactions that require electron transfer. The unique chemical environments at the nodes should facilitate unusual redox chemistry, but the difficulty in synthesizing MOFs with metal ions in reduced oxidation states has precluded such studies. Herein, we demonstrate that MZn3O(O2C-)6 clusters from Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) serve as hosts for V(2+) and Ti(3+) ions and enable the synthesis of the first MOFs containing these reduced early metal ions, which can be accessed from MOF-5 by postsynthetic ion metathesis (PSIM). Additional MOF-5 analogues featuring Cr(2+), Cr(3+), Mn(2+), and Fe(2+) at the metal nodes can be obtained by similar postsynthetic methods and are reported here for the first time. The inserted metal ions are coordinated within an unusual all-oxygen trigonal ligand field and are accessible to both inner- and outer-sphere oxidants: Cr(2+)- converts into Cr(3+)-substituted MOF-5, while Fe(2+)-MOF-5 activates NO to produce an unusual Fe-nitrosyl complex.

Lattice-Imposed Geometry in Metal-Organic Frameworks: Lacunary Zn4O Clusters in MOF-5 Serve as Tripodal Chelating Ligands for Ni2+

The inorganic clusters in metal–organic frameworks can be used to trap metal ions in coordination geometries that are difficult to achieve in molecular chemistry. We illustrate this concept by using the well-known basic carboxylate clusters in Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) as tripodal chelating ligands that enforce an unusual pseudo-tetrahedral oxygen ligand field around Ni2+. The new Ni-based MOF-5 analogue is characterized by porosity measurements and a suite of electronic structure spectroscopies. Classical ligand field analysis of the Ni2+ ion isolated in MOF-5 classifies the Zn3O(carboxylate)6 “tripodal ligand” as an unusual, stronger field ligand than halides and other oxygen donor ligands. These results may inspire the widespread usage of MOFs as chelating ligands for stabilizing site-isolated metal ions in future reactivity and electronic structure studies.

Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst

Current heterogeneous catalysts lack the fine steric and electronic tuning required for catalyzing the selective dimerization of ethylene to 1-butene, which remains one of the largest industrial processes still catalyzed by homogeneous catalysts. Here, we report that a metal–organic framework catalyzes ethylene dimerization with a combination of activity and selectivity for 1-butene that is premier among heterogeneous catalysts. The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) was leveraged to create a well-defined and site-isolated Ni(II) active site bearing close structural homology to molecular tris-pyrazolylborate complexes. In the presence of ethylene and methylaluminoxane, the material consumes ethylene at a rate of 41,500 mol per mole of Ni per hour with a selectivity for 1-butene of up to 96.2%, exceeding the selectivity reported for the current industrial dimerization process.

Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst.

Current heterogeneous catalysts lack the fine steric and electronic tuning required for catalyzing the selective dimerization of ethylene to 1-butene, which remains one of the largest industrial processes still catalyzed by homogeneous catalysts. Here, we report that a metal–organic framework catalyzes ethylene dimerization with a combination of activity and selectivity for 1-butene that is premier among heterogeneous catalysts. The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) was leveraged to create a well-defined and site-isolated Ni(II) active site bearing close structural homology to molecular tris-pyrazolylborate complexes. In the presence of ethylene and methylaluminoxane, the material consumes ethylene at a rate of 41,500 mol per mole of Ni per hour with a selectivity for 1-butene of up to 96.2%, exceeding the selectivity reported for the current industrial dimerization process.

Ligand Redox Non-innocence in the Stoichiometric Oxidation of Mn2(2,5-dioxidoterephthalate) (Mn-MOF-74)

Unsaturated metal sites within the nodes of metal-organic frameworks (MOFs) can be interrogated by redox reagents common to small molecule chemistry. We show, for the first time, that an analogue of the iconic M2(2,5-dioxidoterephthalate) (M2DOBDC, MOF-74) class of materials can be stoichiometrically oxidized by one electron per metal center. The reaction of Mn2DOBDC with C6H5ICl2 produces the oxidized material Cl2Mn2DOBDC, which retains crystallinity and porosity. Surprisingly, magnetic measurements, X-ray absorption, and infrared spectroscopic data indicate that the Mn ions maintain a formal oxidation state of +2, suggesting instead the oxidation of the DOBDC(4-) ligand to the quinone DOBDC(2-). These results describe the first example of ligand redox non-innocence in a MOF and a rare instance of stoichiometric electron transfer involving the metal nodes. The methods described herein offer a synthetic toolkit that will be of general use for further explorations of the redox reactivity of MOF nodes.

NO Disproportionation at a Mononuclear Site-Isolated Fe2+ Center in Fe2+-MOF-5

The weak-field ligand environments at the metal nodes of metal-organic frameworks (MOFs) mimic the electronic environment of metalloenzyme active sites, but little is known about the reactivity of MOF nodes toward small molecules of biological relevance. Here, we report that the ferrous ions in Fe(2+)-exchanged MOF-5 disproportionate nitric oxide to produce nitrous oxide and a ferric nitrito complex. Although mechanistic studies of N-N bond forming transformations often invoke a hyponitrite species, as in nitric oxide reductase and NOx reduction catalysis, little is known about this intermediate in its monoanionic state. Together with the first report of N-N coupling between NO molecules in a MOF, we present evidence for a species that is consistent with a ferric hyponitrite radical, whose isolation is enabled by the spatial constraints of the MOF matrix.

Solvent‐Dependent Cation Exchange in Metal–Organic Frameworks

We investigated which factors govern the critical steps of cation exchange in metal-organic frameworks by studying the effect of various solvents on the insertion of Ni(2+) into MOF-5 and Co(2+) into MFU-4l. After plotting the extent of cation insertion versus different solvent parameters, trends emerge that offer insight into the exchange processes for both systems. This approach establishes a method for understanding critical aspects of cation exchange in different MOFs and other materials.

Metal–Metal Interactions in C3-Symmetric Diiron Imido Complexes Linked by Phosphinoamide Ligands

The tris(phosphinoamide)-bridged Fe(II)Fe(II) diiron complex Fe(μ-(i)PrNPPh2)3Fe(η(2)-(i)PrNPPh2) (1) can be reduced in the absence or presence of PMe3 to generate the mixed-valence Fe(II)Fe(I) complexes Fe(μ-(i)PrNPPh2)3Fe(PPh2NH(i)Pr) (2) or Fe(μ-(i)PrNPPh2)3Fe(PMe3) (3), respectively. Following a typical oxidative group transfer procedure, treatment of 2 or 3 with organic azides generates the mixed-valent Fe(II)Fe(III) imido complexes Fe((i)PrNPPh2)3Fe≡NR (R = (t)Bu (4), Ad (5), 2,4,6-trimethylphenyl (6)). These complexes represent the first examples of first-row bimetallic complexes featuring both metal-ligand multiple bonds and metal-metal bonds. The reduced complexes 2 and 3 and imido complexes 4-6 have been characterized via X-ray crystallography, Mössbauer spectroscopy, cyclic voltammetry, and SQUID magnetometry, and a theoretical description of the bonding within these diiron complexes has been obtained using computational methods. The effect of the metal-metal interaction on the electronic structure and bonding in diiron imido complexes 4-6 is discussed in the context of similar monometallic iron imido complexes.

Monomeric and Dimeric Disulfide Complexes of Nickel(II)

Elemental sulfur reacts with a bulky bis(phosphine)nickel(0) complex to give a monomeric nickel(II) eta(2)-disulfido complex, oxidation of which results in the elimination of sulfur with dimerization to give an eta(2),eta(2)-disulfidodinickel(II) derivative in which the S-S bond can be reductively cleaved in a redox-reversible fashion.