An official website of the United States government
Abstract A series of molecular Mn catalysts featuring aniline groups in the second‐coordination sphere has been developed for electrochemical and photochemical CO2reduction. The arylamine moieties were installed at the 6 position of 2,2’‐bipyridine (bpy) to generate a family of isomers in which the primary amine is located at theortho‐(1‐Mn),meta‐(2‐Mn), orpara‐site (3‐Mn) of the aniline ring. The proximity of the second‐sphere functionality to the active site is a critical factor in determining catalytic performance. Catalyst1‐Mn, possessing the shortest distance between the amine and the active site, significantly outperformed the rest of the series and exhibited a 9‐fold improvement in turnover frequency relative to parent catalyst Mn(bpy)(CO)3Br (901 vs. 102 s−1, respectively) at 150 mV lower overpotential. The electrocatalysts operated with high faradaic efficiencies (≥70 %) for CO evolution using trifluoroethanol as a proton source. Notably, under photocatalytic conditions, a concentration‐dependent shift in product selectivity from CO (at high [catalyst]) to HCO2H (at low [catalyst]) was observed with turnover numbers up to 4760 for formic acid and high selectivities for reduced carbon products.
more »
« less
Electrocatalytic Hydrogen Evolution using a Nickel‐based Calixpyrrole Complex: Controlling the Secondary Coordination Sphere on an Electrode Surface
Abstract Incorporating design elements from homogeneous catalysts to construct well defined active sites on electrode surfaces is a promising approach for developing next generation electrocatalysts for energy conversion reactions. Furthermore, if functionalities that control the electrode microenvironment could be integrated into these active sites it would be particularly appealing. In this context, a square planar nickel calixpyrrole complex,Ni(DPMDA)(DPMDA=2,2′‐((diphenylmethylene)bis(1H‐pyrrole‐5,2‐diyl))bis(methaneylylidene))bis(azaneylylidene))dianiline) with pendant amine groups is reported that forms a heterogeneous hydrogen evolution catalyst using anilinium tetrafluoroborate as the proton source. The supportedNi(DPMDA)catalyst was surprisingly stable and displayed fast reaction kinetics with turnover frequencies (TOF) up to 25,900 s−1or 366,000 s−1 cm−2. Kinetic isotope effect (KIE) studies revealed a KIE of 5.7, and this data, combined with Tafel slope analysis, suggested that a proton‐coupled electron transfer (PCET) process involving the pendant amine groups was rate‐limiting. While evidence of an outer‐sphere reduction of theNi(DPMDA)catalyst was observed, it is hypothesized that the control over the secondary coordination sphere provided by the pendant amines facilitated such high TOFs and enabled the PCET mechanism. The results reported herein provide insight into heterogeneous catalyst design and approaches for controlling the secondary coordination sphere on electrode surfaces.
more »
« less
- Award ID(s):
- 2018414
- PAR ID:
- 10476285
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 29
- Issue:
- 65
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract With the aim of constructing hydrogen‐bonding networks in synthetic complexes, two new ligands derived fromcis,cis‐1,3,5‐triaminocyclohexane (TACH) have been prepared that feature pendant pyrrole or indole rings as outer‐sphere H‐bond donors. The TACH framework offers a facial arrangement of threeN‐donors, thereby mimicking common coordination motifs in the active sites of nonheme Fe and Cu enzymes. X‐ray structural characterization of a series of CuI‐X complexes (X=F, Cl, Br, NCS) revealed that these neutral ligands (H3LR, R=pyrrole or indole) coordinate in the intended facialN3manner, yielding four‐coordinate complexes with idealizedC3symmetry. The N−H units of the outer‐sphere heterocycles form a hydrogen‐bonding cavity around the axial (pseudo)halide ligand, as verified by crystallographic, spectroscopic, and computational analyses. Treatment of H3Lpyrroleand H3Lindolewith divalent transition metal chlorides (MIICl2, M=Fe, Cu, Zn) causes one heterocycle to deprotonate and coordinate to the M(II) center, giving rise to tetradentate ligands with two remaining outer‐sphere H‐bond donors. Further ligand deprotonation is observed upon reaction with Ni(II) and Cu(II) salts with weakly coordinating counteranions. The reported complexes highlight the versatility of TACH‐based ligands with pendant H‐bond donors, as the resulting scaffolds can support multiple protonation states, coordination geometries, and H‐bonding interactions.more » « less
-
Abstract Tuning of redox‐active complexes featuring metals with high coordination numbers by incorporation of secondary redox‐inactive cations has received far less attention than it deserves. Here, appending moderate steric bulk to a tripodal ligand framework has been tested for its influence on secondary‐cation‐driven structural and electrochemical tuning of cerium, a lanthanide that tends to adopt high coordination numbers. Aquasi‐C3‐symmetric cerium(III) complex denoted[Ce]has been prepared that features pendant benzyloxy groups, and this work demonstrates that this species offers a site capable of binding single Na+or Ca2+ions. Electrochemical and UV‐visible spectroscopic studies reveal equilibrium binding affinity of[Ce]for Na+in acetonitrile solvent, contrasting with tight binding of all cations in all other previously studied systems of this type. The modulated cation binding can be attributed to the bulky benzyloxy groups, which impact the thermodynamics of cation binding but do not impede the formation of cerium centers with coordination number 8 upon binding of either Na+or Ca2+. The Ce(IV/III) reduction potential was found to be tunable under the equilibrium binding conditions, highlighting the potentially significant role that controlled structural changes can play in modulating the solution properties of heterobimetallic complexes.more » « less
-
Iron–sulfur clusters play essential roles in biological systems, and thus synthetic [Fe4S4] clusters have been an area of active research. Recent studies have demonstrated that soluble [Fe4S4] clusters can serve as net H atom transfer mediators, improving the activity and selectivity of a homogeneous Mn CO2 reduction catalyst. Here, we demonstrate that incorporating these [Fe4S4] clusters into a coordination polymer enables heterogeneous H atom transfer from an electrode surface to a Mn complex dissolved in solution. A previously reported solution-processable Fe4S4-based coordination polymer was successfully deposited on the surfaces of different electrodes. The coated electrodes serve as H atom transfer mediators to a soluble Mn CO2 reduction catalyst displaying good product selectivity for formic acid. Furthermore, these electrodes are recyclable with a minimal decrease in activity after multiple catalytic cycles. The heterogenization of the mediator also enables the characterization of solution-phase and electrode surface species separately. Surface enhanced infrared absorption spectroscopy (SEIRAS) reveals spectroscopic signatures for an in situ generated active Mn–H species, providing a more complete mechanistic picture for this system. The active species, reaction mechanism, and the protonation sites on the [Fe4S4] clusters were further confirmed by density functional theory calculations. The observed H atom transfer reactivity of these coordination polymer-coated electrodes motivates additional applications of this composite material in reductive H atom transfer electrocatalysis.more » « less
-
Abstract Ammonia (NH3) electrosynthesis gains significant attention as NH3is essentially important for fertilizer production and fuel utilization. However, electrochemical nitrogen reduction reaction (NRR) remains a great challenge because of low activity and poor selectivity. Herein, a new class of atomically dispersed Ni site electrocatalyst is reported, which exhibits the optimal NH3yield of 115 µg cm−2h−1at –0.8 V versus reversible hydrogen electrode (RHE) under neutral conditions. High faradic efficiency of 21 ± 1.9% is achieved at ‐0.2 V versus RHE under alkaline conditions, although the ammonia yield is lower. The Ni sites are stabilized with nitrogen, which is verified by advanced X‐ray absorption spectroscopy and electron microscopy. Density functional theory calculations provide insightful understanding on the possible structure of active sites, relevant reaction pathways, and confirm that the Ni‐N3sites are responsible for the experimentally observed activity and selectivity. Extensive controls strongly suggest that the atomically dispersed NiN3site‐rich catalyst provides more intrinsically active sites than those in N‐doped carbon, instead of possible environmental contamination. This work further indicates that single‐metal site catalysts with optimal nitrogen coordination is very promising for NRR and indeed improves the scaling relationship of transition metals.more » « less
