Tuning Supramolecular Selectivity for Hydrosulfide: Linear Free Energy Relationships Reveal Preferential C-H Hydrogen Bond Interactions.

Supramolecular anion receptors can be used to study the molecular recognition properties of the reactive yet biologically critical hydrochalcogenide anions (HCh-). Achieving selectivity for HCh- over the halides is challenging but necessary for not only developing future supramolecular probes for HCh- binding and detection, but also for understanding the fundamental properties that govern these binding and recognition events. Here we demonstrate that linear free energy relationships (LFERs)-including Hammett and Swain-Lupton plots-reveal a clear difference in sensitivity to the polarity of an aryl C-H hydrogen bond (HB) donor for HS- over other HCh- and halides. Analysis using electrostatic potential maps highlights that this difference in sensitivity results from a preference of the aryl C-H HB donor for HS- in this host scaffold. From this study, we demonstrate that LFERs are a powerful tool to gain interpretative insight into motif design for future anion-selective supramolecular receptors and highlight the importance of C-H HB donors for HS- recognition. From our results, we suggest that aryl C-H HB donors should be investigated in the next generation of HS- selective receptors based on the enhanced HS- selectivity over other competing anions in this system.

Analysis of the Puzzling Exchange-Coupling Constants in a Series of Heterobimetallic Complexes.

The exchange-coupling constants (J) in a series of bimetallic complexes with an M2+(µ-OH)Fe3+ core (M = Mn, Fe, Ni, and Cu; series 1), which were reported in a recent study ( Sano et al. Inorg. Chem. 2017 , 56 , 14118 - 14128 ), have been analyzed with the help of density functional theory (DFT) calculations. The experimental J values of series 1 showed the remarkable property that they were virtually independent of metal M. This behavior contrasts with that observed for a related series of complexes with M2+Fe3+ cores reported by Chaudhuri and co-workers ( Biswas et al. Inorg. Chem. 2010 , 49 , 626 - 641 ) (series 2) in which J increases toward the upper end of the series. Broken symmetry DFT calculations for J, which yielded values in good agreement for the MnFe and NiFe complexes of series 1, gave for the CuFe complex a J value that was persistently much larger than that obtained from the experiment. Attempts to bridge the discrepancy by invoking various basis sets and corrections for hydrogen-bonding effects on J were not successful. The J values for series 1 were subsequently analyzed in the context of an exchange pathway model. From this analysis, it emerged that, in addition to the regular 2e-pathways, which contribute antiferromagnetic terms to J, there are also 3e-pathways that contribute ferromagnetic terms and have the propensity to keep J constant along series 1. It is shown that, while DFT evaluates the 2e-pathway terms reliably, this method seriously underestimates the 3e-pathway contributions, resulting in a too high J value for the CuFe complex of series 1. The pathway analysis of series 2 reveals that the 3e-pathway contributions to J are considerably smaller than those in series 1, resulting in J values that increase toward the upper end of the series, in accordance with the experiment.

Expanding reversible chalcogenide binding: supramolecular receptors for the hydroselenide (HSe-) anion.

Synthetic supramolecular receptors have been widely used to study reversible solution binding of anions; however, few systems target highly-reactive species. In particular, the hydrochalcogenide anions hydrosulfide (HS-) and hydroselenide (HSe-) have been largely overlooked despite their critical roles in biological systems. Herein we present the first example of reversible HSe- binding in two distinct synthetic supramolecular receptors, using hydrogen bonds from N-H and aromatic C-H moieties. The arylethynyl bisurea scaffold 1 t Bu achieved a binding affinity of 460 ± 50 M-1 for HSe- in 10% DMSO-d 6/CD3CN, whereas the tripodal-based receptor 2CF3 achieved a binding affinity of 290 ± 50 M-1 in CD3CN. Association constants were also measured for HS-, Cl-, and Br-, and both receptors favored binding of smaller, more basic anions. These studies contribute to a better understanding of chalcogenide hydrogen bonding and provide insights into further development of probes for the reversible binding, and potential quantification, of HSe- and HS-.

Reactive sulfur species (RSS): persulfides, polysulfides, potential, and problems.

Sulfur is a crucial element in biology due to its unique properties and wide range of accessible oxidation states. This reactivity gives rise to the generation of reactive sulfur species (RSS), which have emerged as a diverse class of small molecules and functional groups with important roles in chemical biology and bioinorganic chemistry. In this review, we focus on basic properties of simple RSS, highlight recent insights into the interconnectivity of RSS with reactive nitrogen species, and discuss recent advances in methods for RSS detection and measurement. Finally, we highlight key reactivity considerations that must be taken into account when working with RSS and interpreting the outcomes of labeling methods for RSS and related species.

Modular bimetallic complexes with a sulfonamido-based ligand.

A series of bimetallic complexes prepared with the ligands N,N,N',N'-tetramethylethane-1,2-diamine (TMEDA) and N,N',N''-[2,2',2''-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3-) is described. Four diiron compounds of the formulation (TMEDA)FeII(X)-(µ-OH)-FeIIIMST were prepared, in which the X- ligands are the anions OTf-, Br-, SCN-, or N3-. Additionally, two heterobimetallic compounds of the formulation (TMEDA)MII(OTf)-(µ-OH)-FeIIIMST (MII = CoII or NiII) were synthesized. All these compounds have similar spectroscopic and structural properties. The diiron compounds exhibit perpendicular-mode electron paramagnetic resonance spectra consistent with S = 1/2 spin ground states, which is expected for high-spin FeII and FeIII centres that are antiferromagnetically coupled. The heterobimetallic (TMEDA)NiII(OTf)-(µ-OH)-FeIIIMST complex had a spin state of S = 3/2 that also resulted from antiferromagnetic coupling between the high-spin NiII and FeIII centres. The modularity of this system is further demonstrated by the substitution of the TMEDA ligand with ethylenediamine (en); for this species two equivalents of en coordinate to the FeII centre to form [(en)2FeII-(µ-OH)-FeIIIMST]OTf. These results demonstrate that a modular bimetallic system has been developed in which the key components can be modified.

Modular tripodal receptors for the hydrosulfide (HS-) anion.

Hydrogen sulfide (H2S) is an endogenously-produced gasotransmitter and is predominantly speciated as HS- at physiological pH. Despite this importance, reversible binding of HS- to synthetic receptors remains rare and confined to highly-engineered receptor systems. Here we demonstrate the generality of reversible HS- binding in a family of tren-based receptors.

Models for Unsymmetrical Active Sites in Metalloproteins: Structural, Redox, and Magnetic Properties of Bimetallic Complexes with MII-(µ-OH)-FeIII Cores.

Bimetallic complexes are important sites in metalloproteins but are often difficult to prepare synthetically. We have previously introduced an approach to form discrete bimetallic complexes with MII-(µ-OH)-FeIII (MII = Mn, Fe) cores using the tripodal ligand N,N',N″-[2,2',2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3-). This series is extended to include the rest of the late 3d transition metal ions (MII = Co, Ni, Cu, Zn). All of the bimetallic complexes have similar spectroscopic and structural properties that reflect little change despite varying the MII centers. Magnetic studies performed on the complexes in solution using electron paramagnetic resonance spectroscopy showed that the observed spin states varied incrementally from S = 0 through S = 5/2; these results are consistent with antiferromagnetic coupling between the high-spin MII and FeIII centers. However, the difference in the MII ion occupancy yielded only slight changes in the magnetic exchange coupling strength, and all complexes had J values ranging from +26(4) to +35(3) cm-1.

Terminal NiII-OH/-OH2 complexes in trigonal bipyramidal geometries derived from H2O.

The preparation and characterization of two NiII complexes are described, a terminal NiII-OH complex with the tripodal ligand tris[(N)-tertbutylureaylato)-N-ethyl)]aminato ([H3buea]3-) and a terminal Ni II-OH2 complex with the tripodal ligand N,N',N″-[2,2',2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3-). For both complexes, the source of the -OH and -OH2 ligand is water. The salts K2[NiIIH3buea(OH)] and NMe4[NiIIMST(OH2)] were characterized using perpendicular-mode X-band electronic paramagnetic resonance, Fourier transform infrared, UV-visible spectroscopies, and its electrochemical properties were evaluated using cyclic voltammetry. The solid state structures of these complexes determined by X-ray diffraction methods reveal that they adopt a distorted trigonal bipyramidal geometry, an unusual structure for 5-coordinate NiII complexes. Moreover, the NiII-OH and NiII-OH2 units form intramolecular hydrogen bonding networks with the [H3buea]3- and [MST]3- ligands. The oxidation chemistry of these complexes was explored by treating the high-spin NiII compounds with one-electron oxidants. Species were formed with S = 1/2 spin ground states that are consistent with formation of monomeric NiIII species. While the formation of NiIII-OH complexes cannot be ruled out, the lack of observable O-H vibrations from the putative Ni-OH units suggest the possibility that other high valent Ni species are formed.

Sulfonamido tripods: tuning redox potentials via ligand modifications.

A series of FeII-OH2 complexes were synthesized with ligands based on the tetradentate sulfonamido tripod N,N',N"-[2,2',2"-nitrilotris(ethane-2,1-diyl)]-tris-({R-Ph}-sulfonamido). These complexes differ by the substituent on the aryl rings and were fully characterized, including their molecular structures via X-ray diffraction methods. All the complexes were five-coordinate with trigonal bipyramidal geometry. A linear correlation was observed between the electronic effects of each ligand, given by the Hammett constants of the para-substituents, and the potential of the FeII/FeIII redox couple, which were determined using cyclic voltammetry. It was found that the range of redox potentials for the complexes spanned approximately 160 mV.