Redox-Active Boron Clusters

ConspectusIn this Account, we discuss our group's research over the past decade on a class of functionalized boron clusters with tunable chemical and physical properties, with an emphasis on accessing and controlling their redox behavior. These clusters can be thought of as three-dimensional aromatic systems that have distinct redox behavior and photophysical properties compared to their two-dimensional organic counterparts. Specifically, our lab has studied the highly tunable, multielectron redox behavior of B12(OR)12 clusters and applied these molecules in various settings. We first discuss the spectroscopic and electrochemical characterization of B12(OR)12 clusters in various oxidation states, followed by their use as catholytes and/or anolytes in redox flow batteries and chemical dopants in conjugated polymers. Additionally, the high oxidizing potential and visible light-absorbing nature of fluoroaryl-functionalized B12(OR)12 clusters have been leveraged by our group to generate weakly coordinating, photoexcitable species that can promote photooxidation chemistry.We have further translated these solution-phase studies of B12(OR)12 clusters to the solid state by using the precursor [B12(OH)12]2- cluster as a robust building block for hybrid metal oxide materials. Specifically, we have shown that the boron cluster can act as a thermally stable cross-linking material, which enhances electron transport between metal oxide nanoparticles. We applied this structural motif to create TiO2- and WO3-containing materials that showed promising properties as photocatalysts and electroactive materials for supercapacitors. Building on this concept, we later discovered that B12(OCH3)12, the smallest of the B12(OR)12 family, could retain its redox behavior in the solid state, a previously unseen phenomenon. We successfully harnessed this unique behavior for solid-state energy storage by implementing this boron cluster as a cathode-active material in a Li-ion prototype cell device. Recently, our group has also explored how to tune the redox properties of clusters other than B12(OR)12 species by synthesizing a library of vertex-differentiated clusters containing both B-OR and B-halogen groups. Due to the additive qualities of different functional groups on the cluster, these species allow access to a region of electrochemical potentials previously inaccessible by fully substituted closo-dodecaborate alkoxy-based derivatives.Lastly, we discuss our research into smaller-sized redox-active polyhedral boranes (B6- and B10-based cluster cores). Interestingly, these clusters show significantly less redox stability and reversibility than their dodecaborate-based counterparts. While exploring the functionalization of closo-hexaborate to create fully substituted derivates (i.e., [B6R6Hfac]-), we observed unique oxidative decomposition pathways for this cluster system. Consequently, we leveraged this oxidative instability to generate useful alkyl boronate esters via selective chemical oxidation. We further explored a closo-decaborate cluster as a platform to access electrophilic [B10H13]+ species capable of directly borylating arene compounds with unique regioselectivity. Upon chemical oxidation of the arylated decaborate clusters, we successfully synthesized various aryl boronate esters, establishing the generality of the oxidative cluster deconstruction concept.Overall, our work shows that boron clusters are an appealing class of redox-active molecules, and this fundamental and understudied property can be leveraged for constructing novel materials with tunable physical and electrochemical properties, as well as producing unique chemical reagents for small molecule synthesis.

Electrochemical Cycling of Redox-Active Boron Cluster-Based Materials in the Solid State

This work demonstrates the first successful electrochemical cycling of a redox-active boron cluster-based material in the solid state. Specifically, we designed and synthesized an ether-functionalized dodecaborate cluster, B₁₂(OCH₃)₁₂, which is the smallest redox-active building block in the B₁₂(OR)₁₂ family. This species can reversibly access four oxidation states in solution, ranging from a dianion to a radical cation. We show that a chemically isolated and characterized neutral [B₁₂(OCH₃)₁₂]⁰ cluster can be utilized as a cathode active material in a PEO-based rechargeable all-solid-state cell with a lithium metal anode. The cell exhibits an impressive active material utilization close to 95% at C/20 rate, a high Coulombic efficiency of 96%, and reversibility, with only 4% capacity fade after 16 days of cycling. This work represents a conceptual departure in the development of redox-active components for electrochemical storage and serves as an entry point to a broader class of borane-based materials.

Benchmarking the dynamic luminescent properties and UV stability of B18H22-based materials

The dynamic photoluminescent properties, and potential quenching mechanisms, of anti-B18H22, 4,4’-Br2-anti-B18H20, and 4,4’-I2-anti-B18H20 are investigated in solution and polymer films. UV stability studies of the neat powders show no decomposition...

Synthesis and structural properties of a 2D Zn(II) dodecahydroxy-closo-dodecaborate coordination polymer

In this work, we discuss the synthesis and characterization of a 2D coordination polymer composed of a dianionic perhydroxylated boron cluster, [B12(OH)12]2-, coordinated to Zn(II)—the first example of a transition...

Elucidating the Variable-Temperature Mechanical Properties of a Negative Thermal Expansion Metal-Organic Framework

We report the first experimental study into the thermomechanical and viscoelastic properties of a metal-organic framework (MOF) material. Nanoindentations show a decrease in the Young's modulus, consistent with classical molecular dynamics simulations, and hardness of HKUST-1 with increasing temperature over the 25-100 °C range. Variable-temperature dynamic mechanical analysis reveals significant creep behavior, with a reduction of 56% and 88% of the hardness over 10 min at 25 and 100 °C, respectively. This result suggests that, despite the increased density that results from increasing temperature in the negative thermal expansion MOF, the thermally induced softening due to vibrational and entropic contributions plays a more dominant role in dictating the material's temperature-dependent mechanical behavior.