Benjamin S. Gelfand

Enhancing proton conduction in a metal-organic framework by isomorphous ligand replacement.

Using the concept of isomorphous replacement applied to entire ligands, a C(3)-symmetric trisulfonate ligand was substituted with a C(3)-symmetric tris(hydrogen phosphonate) ligand in a proton conducting metal-organic framework (MOF). The resulting material, PCMOF2½, has its proton conduction raised 1.5 orders of magnitude compared to the parent material, to 2.1 × 10(-2) S cm(-1) at 90% relative humidity and 85 °C, while maintaining the parent MOF structure.

A Water Stable Magnesium MOF That Conducts Protons over 10–2 S cm–1

From the outset of the study of MOFs as proton conductors, both conductivity and hydrolytic robustness of the materials have needed to be improved. Here, we report a layered magnesium carboxyphosphonate framework, PCMOF10, that shows an extremely high proton conductivity value of 3.55 × 10(-2) S·cm(-1) at 70 °C and 95% RH. Moreover, PCMOF10 is water stable owing to strong Mg phosphonate bonding. The 2,5-dicarboxy-1,4-benzenediphosphonic acid (H6L) linker anchors a robust backbone and has hydrogen phosphonate groups that interact with the lattice water to form an efficient proton transfer pathway.

Design of a Humidity-Stable Metal–Organic Framework Using a Phosphonate Monoester Ligand

Phosphonate monoesters are atypical linkers for metal-organic frameworks, but they offer potentially added versatility. In this work, a bulky isopropyl ester is used to direct the topology of a copper(II) network from a dense to an open framework, CALF-30. CALF-30 shows no adsorption of N2 or CH4 however, using CO2 sorption, CALF-30 was found to have a Langmuir surface area of over 300 m(2)/g and to be stable to conditions of 90% relative humidity at 353 K owing to kinetic shielding of the framework by the phosphonate ester.

Tuning Intrinsic and Extrinsic Proton Conduction in Metal–Organic Frameworks by the Lanthanide Contraction

Seven isomorphous lanthanide metal-organic frameworks in the PCMOF-5 family, [Ln(H5L)(H2O)n](H2O) (L = 1,2,4,5-tetrakis(phosphonomethyl)benzene, Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) have been synthesized and characterized. This family contains 1-D water-filled channels lined with free hydrogen phosphonate groups and gives a very low activation energy pathway for proton transfer. The lanthanide contraction was employed to systematically vary the unit cell dimensions and tune the proton conducting pathways. LeBail fitting of the crystalline series shows that the crystallographic a-axis, along the channel, can be varied in increments less than 0.02 Å correspondingly shortening the proton transfer pathway. The proton conductivities for the La and Pr complexes were roughly an order of magnitude higher than other members of the series (10-3 S cm-1 versus 10-4 S cm-1). Single crystal structures of the high and low conducting members of the series (La, Pr for high and Ce for low) affirm the structural similarities extend beyond the unit cell parameters to positions of free acid groups and included water molecules. Scanning electron microscopy reveals marked differences in particle size of the different members of the Ln series owing to lattice strain effects induced by changing the lanthanide. Notably, the high conducting La and Pr complexes have the largest particle sizes. This result contradicts any notion that degradation of the MOF at grain boundaries is enabling the observed conductivity as proton conduction dominated by extrinsic pathways would be enabled by small particles (i.e., the La and Pr complexes would be the worst conductors). Proton conductivity measurements of a ball milled sample of the La complex corroborate this result.

Mediating Order and Modulating Porosity by Controlled Hydrolysis in a Phosphonate Monoester Metal–Organic Framework

A crystalline and permanently porous copper phosphonate monoester framework has been synthesized from a tetraaryl trigonal phosphonate monoester linker. This material has a surface area over 1000 m2  g-1 , as measured by N2 sorption, the highest reported for a phosphonate-based metal-organic framework (MOF). The monoesters result in hydrophobic pore surfaces that give a low heat of adsorption for CO2 and low calculated selectivity for CO2 over N2 and CH4 in binary mixtures. By careful manipulation of synthetic conditions, it is possible to selectively remove some of the monoesters lining the pore to form a hydrogen phosphonate while giving an isomorphous structure. This increases the affinity of the framework for CO2 giving higher ambient uptake, higher heat of adsorption, and much higher calculated selectivity for CO2 over both N2 and CH4 . Formation of the acid groups is noteworthy as complexation with the parent acid gives a different structure.

Achieving Superprotonic Conduction in Metal–Organic Frameworks through Iterative Design Advances

Two complementary design strategies, isomorphous ligand replacement and heterocycle doping, have been applied to iteratively enhance the proton conductivity of a metal-organic framework, β-PCMOF2. The resulting materials, PCMOF21/2(Pz) and PCMOF21/2(Tz) (Pz = 1H-pyrazole, Tz = 1H-1,2,4-triazole), have their proton conduction raised almost 2 orders of magnitude compared to β-PCMOF2. The bulk conductivities of these materials are over 10-1 S cm-1 at 85 °C and 90% relative humidity (RH), while maintaining the parent MOF structure. A solid state synthetic route for doping 1-D channels is also presented.

Reconciling order, stability, and porosity in phosphonate metal–organic frameworks via HF-mediated synthesis

We report two porous Zr phosphonate frameworks. The kinetic product, ZrH2L-A, shows low order and is less porous. An HF-modulated structure, CALF-31, is more ordered and has higher porosity. While not single crystalline, CALF-31 is highly robust and the CO2 heats of adsorption give insights into ordering in phosphonate MOFs.

Dithienophosphole-Based Phosphinamides with Intriguing Self-Assembly Behavior.

Abstract A new, highly adaptable type of phosphinamide‐based hydrogen bonding is representatively demonstrated in π‐conjugated phosphole materials. The rotational flexibility of these intermolecular P=O−H−N hydrogen bonds is demonstrated by X‐ray crystallography and variable‐concentration NMR spectroscopy. In addition to crystalline compounds, phosphinamide hydrogen bonding was successfully introduced into the self‐assembly of soft crystals, liquid crystals, and organogels, thus highlighting the high general value of this type of interaction for the formation of organic soft materials.

Tuning the aggregation-induced enhanced emission behavior and self-assembly of phosphole-lipids

Herein, we report on the synthesis, self-assembly, as well as the photophysical properties of a novel series of P-benzylated phosphole-lipids. In the context of this structure–property study we have systematically altered the number, position, and length of the alkyl chains in the 3-,4-, and 5-position of the benzyl group. Both the self-assembly and photophysical properties of the compounds were found to correlate strongly with the alkyl chain length and chain arrangement of the mesogenic moiety.

Extracting structural trends from systematic variation of phosphonate/phosphonate monoester coordination polymers

Here we report on three new barium-phosphonate coordination polymers of systematically varied ligand features – a linear diphosphonatebis(monoisopropylester), a trigonal planar triphosphonate, and a trigonal planar triphosphonatetris(monoisopropylester). These coordination polymers are compared to a known prototypical pillared layered barium-diphosphonate, based on a linear diphosphonate, and to each other in order to develop systematic structural trends as a result of the varied ligand features. The findings here indicate that in going from linear cores to trigonal planar cores, there is a disruption in barium-phosphonate coordination, resulting in increased hydration on the cation to compensate and fill the barium coordination sphere. Furthermore, addition of the monoisopropyl ester produces significant steric effects, resulting in truncated structures compared to the acid analogues, along with the formation of bilayers featuring the hydrophobic esters on the periphery. Changing of the ligand geometry and the addition of the monoester operate orthogonally and in conjunction if both changes are present.