Imitating Nature: The Effects of Asymmetry and Strain on Copper and Iron Complexes with Schiff Base Ligands

Metalloenzymes are incredibly complex "machines" which catalyze reactions essential for life. Metal ions are found in more than half of all known proteins; particularly, copper and iron are abundant in active sites performing complex reactions. The coordination environment around the metal center itself drives the metal ion to undergo biospecific, highly optimized, and selective reactions, such as the oxidation of methane to methanol, splitting of water, or generation of ammonia. Most of the active sites are asymmetric and some metal ions are forced into strained geometries. These factors facilitate catalysis and electronic tuning. Against this background, aspects of strain and asymmetry were probed in model complexes with iron and copper ions. Utilizing Schiff base ligands as molecules that can be easily manipulated in steric and electronic properties via building block assembly, iron and copper complexes were synthesized.The ferric diiron complexes [(FeL)2(µ-OH)]+, [(FeLNO2)2(µ-OH)]+ and [(FeL)2(µ-O)] (L = 2,2'-(2-Methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)- bis(methylene)- diphenolate and LNO2 = 2,2'-(2-Methyl-2-(pyridin-2-yl)propane-1,3-diyl bis(azanediyl)- bis(methylene)bis(4-nitrophenolate)) show structural and spectroscopic similarities to bridged hydr(oxo)diiron containing active sites of metalloproteins. Unique properties of the ligand create asymmetry of the diiron unit as well as bent Fe-O-Fe structures. The stability of the bio-relevant mixed-valence state was investigated by electrochemical means, as well as by chemical reduction. Unfortunately, the Fe(II)/Fe(III) state is not accessible in the complex presented here. However, a mechanistic pathway of decomposition is suggested and an interesting feature of the ligand used in this study may allow for stabilization and characterization of the mixed-valence state, and even further, it may reveal the role of a unique amino acid residue interaction with the diiron site in rubrerythrin.Two copper(II) complexes (dimeric [Cu(HL2)]2(ClO4)4 and monomeric [Cu(L3)](ClO4)2) with novel, related Schiff base ligands (HL2 = 2-((2-Methyl-2-(pyridin-2-yl)-3-(pyridin-2-ylmethyleneamino)propylimino)- methyl)phenol and L3 = 2-Methyl-2-(pyridin-2-yl)-N1,N3-bis(pyridin-2-ylmethylene)propane-1,3-diamine) were synthesized and characterized. One series of studies revealed a synthesis plan for asymmetric Schiff base complexes through unique properties of the ligand. It was shown that intermediate electronic tuning of the copper(II) complex is possible in the asymmetric complex compared to the fully symmetric analogues.In another study, the aspect of geometrical strain was investigated. The distorted copper(II) complexes [CuL5-gem] and [CuL5-ortho] (L5-gem = Pseudo-gem-N,N'-bissalicylidene-4,15-diamino[2.2]paracyclophane and L5-ortho = Pseudo-ortho-N,N'-bissalicylidene-4,16-diamino[2.2]paracyclophane) which were obtained in this study are reminiscent of blue copper protein centers which allow for fast electron transfer by having geometries between tetrahedral and square planar. Structural and spectroscopic studies show that strain is present in both the solid and solution state. The distortion causes a shift in redox potential of about half a volt compared to square planar analogues. Initial studies revealed that strained complexes with other divalent metal ions are possible to form (e.g. [CoL5-gem] was isolated and characterized). Strained complexes are interesting for catalysis as geometrical strain may enhance reactivity

The critical role of lithium nitrate in the gas evolution of lithium–sulfur batteries

Sulfur–carbon composites are promising next generation cathode materials for high energy density lithium batteries and thus, their discharge and charge properties have been studied with increasing intensity in recent years. While the sulfur-based redox reactions are reasonably well understood, the knowledge of deleterious side reactions in lithium–sulfur batteries is still limited. In particular, the gassing behavior has not yet been investigated, although it is known that lithium metal readily reacts with the commonly used ethereal electrolytes. Herein, we describe, for the first time, gas evolution in operating lithium–sulfur cells with a diglyme-based electrolyte and evaluate the effect of the polysulfide shuttle-suppressing additive LiNO3. The use of the combination of two operando techniques (pressure measurements and online continuous flow differential electrochemical mass spectrometry coupled with infrared spectroscopy) demonstrates that the additive dramatically reduces, but does not completely eliminate gassing. The major increase in pressure occurs during charge, immediately after fresh lithium is deposited, but there are differences in gas generation during cycling depending on the addition of LiNO3. Cells with LiNO3 show evolution of N2 and N2O in addition to CH4 and H2, the latter being the main volatile decomposition products. Collectively, these results provide novel insight into the important function of LiNO3 as a stabilizing additive in lithium–sulfur batteries

Strategies Based on Nitride Materials Chemistry to Stabilize Li Metal Anode

Partial funding for Open Access provided by the UMD Libraries' Open Access Publishing Fund.Lithium metal battery is a promising candidate for high-energy-density energy storage. Unfortunately, the strongly reducing nature of lithium metal has been an outstanding challenge causing poor stability and low coulombic efficiency in lithium batteries. For decades, there are significant research efforts to stabilize lithium metal anode. However, such efforts are greatly impeded by the lack of knowledge about lithium-stable materials chemistry. So far, only a few materials are known to be stable against Li metal. To resolve this outstanding challenge, lithium-stable materials have been uncovered out of chemistry across the periodic table using first-principles calculations based on large materials database. It is found that most oxides, sulfides, and halides, commonly studied as protection materials, are reduced by lithium metal due to the reduction of metal cations. It is discovered that nitride anion chemistry exhibits unique stability against Li metal, which is either thermodynamically intrinsic or a result of stable passivation. The results here establish essential guidelines for selecting, designing, and discovering materials for lithium metal protection, and propose multiple novel strategies of using nitride materials and high nitrogen doping to form stable solid-electrolyte-interphase for lithium metal anode, paving the way for high-energy rechargeable lithium batteries

Redox and acid???base properties of asymmetric non-heme (hydr)oxo-bridged diiron complexes

[(FeL)2(μ-OH)]+, [(FeL)2(μ-O)] and [(FeLNO2)2(μ-OH)]+, were synthesized and characterized. Electrochemical and chemical reduction of [(FeL)2(μ-OH)]BPh4 revealed disproportionation followed by proton transfer, and [(FeL)2(μ-O)] was formed upon exposure to oxygen.</p

Online Continuous Flow Differential Electrochemical Mass Spectrometry with a Realistic Battery Setup for High-Precision, Long-Term Cycling Tests

We describe the benefits of an <i>online</i> continuous flow differential electrochemical mass spectrometry (DEMS) method that allows for realistic battery cycling conditions. We provide a detailed description on the buildup and the role of the different components in the system. Special emphasis is given on the cell design. The retention time and response characteristics of the system are tested with the electrolysis of Li₂O₂. Finally, we show a practical application in which a Li-ion battery is examined. The value of long-term DEMS measurements for the proper evaluation of electrolyte decomposition is demonstrated by an experiment where a Li_1+<i>x</i>Ni_0.5Mn_0.3Co_0.2O₂ (NMC 532)/graphite cell is cycled over 20 charge/discharge cycles