Synthesis and reactivity of a tris(carbene) zinc chloride complex.

The [PhB(tBuIm)3]1- ligand has gained increased attention since it was first reported in 2006 due to its ability to stabilize highly reactive first row transition metal complexes. In this work, we investigate the coordination chemistry of this ligand with redox-inert zinc to understand how a zinc metal center behaves in such a strong coordinating environment. The PhB(tBuIm)3ZnCl (1) complex can be formed via deprotonation of [PhB(tBuIm)3][OTf]2 followed by the addition of ZnCl2. Salt metathesis reaction with nucleophilic n-BuLi yields the highly carbon-rich zinc coordination complex PhB(tBuIm)3ZnBu (2) with three carbene atom donors and one carbanion donor. In contrast, reaction of complex 1 with a less nucleophilic polysulfide reagent, [K.18-C-6]2[S4], leads to the formation of a tetrahedral zinc tetrasulfido complex via protonation of one carbene donor to form PhB(tBuIm)2(tBuImH)Zn(κ2-S4) (3).

Thionitrite (SNO- ) and Perthionitrite (SSNO- ) are Simple Synthons for Nitrosylated Iron Sulfur Clusters.

S/N crosstalk species derived from the interconnected reactivity of H2 S and NO facilitate the transport of reactive sulfur and nitrogen species in signaling, transport, and regulatory processes. We report here that simple Fe2+ ions, such as those that are bioavailable in the labile iron pool (LIP), react with thionitrite (SNO- ) and perthionitrite (SSNO- ) to yield the dinitrosyl iron complex [Fe(NO)2 (S5 )]- . In the reaction of FeCl2 with SNO- we were able to isolate the unstable intermediate hydrosulfido mononitrosyl iron complex [FeCl2 (NO)(SH)]- , which was characterized by X-ray crystallography. We also show that [Fe(NO)2 (S5 )]- is a simple synthon for nitrosylated Fe-S clusters via its reduction with PPh3 to yield Roussin's Red Salt ([Fe2 S2 (NO)4 ]2- ), which highlights the role of S/N crosstalk species in the assembly of fundamental Fe-S motifs.

C-H⋯S hydrogen bonding interactions.

The short C-H⋯S contacts found in available structural data for both small molecules and larger biomolecular systems suggest that such contacts are an often overlooked yet important stabilizing interaction. Moreover, many of these short C-H⋯S contacts meet the definition of a hydrogen bonding interaction. Using available structural data from the Cambridge Structural Database (CSD), as well as selected examples from the literature in which important C-H⋯S contacts may have been overlooked, we highlight the generality of C-H⋯S hydrogen bonding as an important stabilizing interaction. To uncover and establish the generality of these interactions, we compare C-H⋯S contacts with other traditional hydrogen bond donors and acceptors as well as investigate how coordination number and metal bonding affect the preferred geometry of interactions in the solid state. This work establishes that the C-H⋯S bond meets the definition of a hydrogen bond and serves as a guide to identify C-H⋯S hydrogen bonds in diverse systems.

Synthesis of Terminal Bis(hydrosulfido) and Disulfido Complexes of Ni(II) from a Geometrically Frustrated Tetrahedral Ni(II) Chloride Complex.

Recent studies have highlighted how reactive sulfur species (RSS) can be regulated and transported by metal-sulfur coordination compounds. We report herein the reactivity of PhB(tBuIm)3NiCl (1) with RSS, including the hydrosulfide anion ([Bu4N][SH]) and a reduced tetrasulfide ([K18-C-6]2[S4]). The strongly donating tris(carbene) ligand in 1 is geometrically constrained to a tetrahedral geometry, and the energetically preferable square planar geometry is not achievable with the [PhB(tBuIm)3]- ligand. Upon reaction of 1 with [Bu4N][SH] and [K18-C-6]2[S4], the square planar complexes PhB(tBuIm)2(tBuImH)Ni(SH)2 (2) and PhB(tBuIm)2(tBuImH)Ni(η2-S2) (3) are formed, respectively, via the protonation of one carbene ligand donor atom. Mechanistic investigation suggest that protonation occurs either from decomposition of 1 during the reaction progress, reactions with advantageous [Bu4N]+/[K18-C-6]+ countercations or from the generation of transient unidentified RSS that facilitate proton transfer reactions.

Hydrosulfide-selective ChemFETs for aqueous H2S/HS- measurement.

We have prepared and characterized hydrosulfide-selective ChemFET devices based on a nitrile butadiene rubber membrane containing tetraoctylammonium nitrate as a chemical recognition element that is applied to commercially available field-effect transistors. The sensors have fast (120 s) reversible responses, selectivity over other biologically relevant thiol-containing species, detection limits of 8 mM, and a detection range from approximately 5 to 500 mM. Sensitivities are shown to be 53 mV per decade at pH 8. Use of this compact, benchtop sensor platform requires little training - only the ability to measure DC voltage, which can be accomplished with a conventional multimeter or a simple analog data acquisition device paired with a personal computer. To the best of our knowledge, this report describes the first example of direct potentiometric measurement of the hydrosulfide ion in water.

Ligand Conjugation Directs the Formation of a 1,3-Dihydropyridinate Regioisomer.

The selective formation of the 1,4-dihydropyridine isomer of NAD(P)H is mirrored by the selective formation of 1,4-dihydropyridinate ligand-metal complexes in synthetic systems. Here we demonstrate that ligand conjugation can be used to promote selective 1,3-dihydropyridinate formation. This represents an advance toward controlling and tuning the selectivity in dihydropyridinate formation chemistry. The reaction of (I2P2-)Al(THF)Cl [1; I2P = bis(imino)pyridine; THF = tetrahydrofuran] with the one-electron oxidant (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) afforded (I2P-)Al(TEMPO)Cl (2), which can be reduced with sodium to the twice-reduced ligand complex (I2P2-)Al(TEMPO) (3). Compounds 2 and 3 serve as precursors for high-yielding and selective routes to an aluminum-supported 1,3-dihydropyridinate complex via the reaction of 2 with 3 equiv of potassium metal or the reaction of 3 with KH.

Hydrosulfide Oxidation at a Molybdenum Tetrasulfido Complex.

Hydrogen sulfide (H2S) is an important biological signaling molecule and one of three established gasotransmitters. Upon oxidation, H2S can form reactive sulfur species (RSS) that play a central role in protein persulfidation. Here we report that a molybdenum tetrasulfide can react directly with hydrosulfide to form polysulfides and oxidize the Mo center. Specifically, [NBu4][TpMoS(S4)] reacts with 2 equiv of [NBu4][SH] to form [NBu4][TpMoS3]. Trapping experiments with BnBr confirm the formation of polysulfides, as evidenced by the direct trapping of Bn2S2. This work demonstrates new reaction pathways for H2S oxidation and RSS generation from metal-bound polysulfides that will increase our understanding of the potential roles that metals play in signaling processes.

Solvent-Dependent Linear Free-Energy Relationship in a Flexible Host-Guest System.

Supramolecular chemistry provides an effective strategy for the molecular recognition of diverse molecules. Significant efforts to design synthetic hosts have enabled the successful binding of many types of guests; however, less is known about how host-guest environments influence binding. Herein, we present a comprehensive study in which we measure the host-guest binding of a bis(arylethynyl phenylurea) host with a chloride guest in eight solvents spanning ET(30) values ranging from nonpolar (40.7 kcal mol-1) to polar (47.4 kcal mol-1). Polar solvents show significantly weaker binding in comparison to nonpolar solvents, and the bulk solvent polarity parameter, ET(30), shows a linear free-energy relationship with respect to the free energy of binding in the host-guest complex. These studies provide a better understanding of how host-guest binding in flexible receptors is governed by their environments and highlight the importance of host reorganization contributions in the free energy of binding. In addition, these studies highlight that preorganization may not be as important as previously thought for weak binding in which enthalpic contributions are smaller versus in polar solvents where solvent effects are magnified.

Syntheses of Square Planar Gallium Complexes and a Proton NMR Correlation Probing Metalloaromaticity.

Syntheses of square planar (SP) coordination complexes of gallium(III) are reported herein. Using the pyridine diimine ligand (PDI), we prepared both (PDI2-)GaH (4) and (PDI2-)GaCl (5), which were spectroscopically and structurally characterized. Reduction of PDI using Na metal afforded "Na2PDI", which reacts with in situ-prepared "GaHCl2" or GaCl3 to afford the SP 4 and 5. The planar geometry of these and previously reported SP Al(III) complexes is attributed to energetic stabilization derived from a ring-current effect, or metalloaromaticity. Typically, aromaticity in metal-containing ring systems can be difficult to characterize or confirm experimentally. An experimental approach employing proton NMR spectroscopy and described here provided an estimate of a downfield chemical shift promoted by a small ring-current associated with metalloaromaticity. Near infrared spectroscopic analyses display ligand-metal charge transfer bands which support the assignment of aromaticity. The SP complexes (PDI2-)AlH (1), (PDI2-)AlCl (2), (PDI2-)AlI (3), 4, and 5 are all discussed in this report, using aromaticity as a model for their electronic structure and reactivity properties.

Progress toward colorimetric and fluorescent detection of carbonyl sulfide.

We report here that a fluorescent benzobisimidazolium salt (TBBI) can be used for the fluorescent and colorimetric detection of carbonyl sulfide (COS) over related heterocumulenes including CO2 and CS2 in wet MeCN. The reaction between TBBI and COS in the presence of fluoride yields a highly fluorescent (λem = 354 nm) and colored product (λmax = 321, 621 nm), that is readily observed by the naked eye. We view these results as a first step toward developing activity-based probes for COS detection.

Delocalization tunable by ligand substitution in [L2Al] n- complexes highlights a mechanism for strong electronic coupling.

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I2P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I2P-)(I2P2-)Al and [(I2P2-)(I2P3-)Al]2- charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I2P-)(I2P2-)Al and [(I2P2-)(I2P3-)Al]2- show intervalence charge transfer (IVCT) transitions in the regions 6850-7740 and 7410-9780 cm-1, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I2P2-)(I2P3-)Al]2- complexes containing -PhNMe2 or -PhF5 substituents. Minor localization of charge in [(I2P2-)(I2P3-)Al]2- was observed when -PhOMe substituents are included.

Organic Electron Delocalization Modulated by Ligand Charge States in [L2M]n- Complexes of Group 13 Ions.

Water-stable organic mixed valence (MV) compounds have been prepared by the reaction of reduced bis(imino)pyridine ligands (I2P) with the trichloride salts of Al, Ga, and In. The coordination of two tridentate ligands to each ion affords octahedral complexes that are accessible with five ligand charge states: [(I2P0)(I2P-)M]2+, [(I2P-)2M]+, (I2P-)(I2P2-)M, [(I2P2-)2M]-, and [(I2P2-)(I2P3-)M]2-, and for M = Al only, [(I2P3-)2M]3-. In solid-state structures, the anionic members of the redox series are stabilized by the intercalation of Na+ cations within the ligands. The MV members of the redox series, (I2P-)(I2P2-)M and [(I2P2-)(I2P3-)M]2-, show characteristic intervalence transitions, in the near-infrared regions between 6800-7400 and 7800-9000 cm-1, respectively. Cyclic voltammetry (CV), NIR spectroscopic, and X-ray structural studies support the assignment of class II for compounds [(I2P2-)(I2P3-)M]2- and class III for M = Al and Ga in (I2P-)(I2P2-)M. All compounds containing a singly reduced I2P- ligand exhibit a sharp, low-energy transition in the 5100-5600 cm-1 region that corresponds to a π*-π* transition. CV studies demonstrate that the electron-transfer events in each of the redox series, Al, Ga, and In, span 2.2, 1.4, and 1.2 V, respectively.

Electrochemical Reduction of N2 to NH3 at Low Potential by a Molecular Aluminum Complex.

Electrochemical generation of ammonia (NH3 ) from nitrogen (N2 ) using renewable electricity is a desirable alternative to current NH3 production methods, which consume roughly 1 % of the world's total energy use. The use of catalysts to manipulate the required electron and proton transfer reactions with low energy input is also a chemical challenge that requires development of fundamental reaction pathways. This work presents an approach to the electrochemical reduction of N2 into NH3 using a coordination complex of aluminum(III), which facilitates NH3 production at -1.16 V vs. SCE. Reactions performed under 15 N2 liberate 15 NH3 . Electron paramagnetic resonance spectroscopic characterization of a reduced intermediate and investigations of product inhibition, which limit the reaction to sub-stoichiometric, are also presented.

Reversible Coordination of H2 by a Distannyne.

The terphenyl tin(II) hydride [AriPr4Sn(µ-H)]2 (1) (AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2) was shown to form an equilibrium with the distannyne AriPr4SnSnAriPr4 (2) and H2 in toluene at 80 °C. The equilibrium constant and Gibbs free energy for the dissociation of H2 are 2.23 × 10-4 ± 4.9% and 5.89 kcal/mol ± 0.68%, respectively, by 1H NMR spectroscopy and 2.33 × 10-4 ± 6.2% and 5.86 kcal/mol ± 0.73%, respectively, by UV-vis spectroscopy, indicating that the hydride 1 is strongly favored. Further heating of 2 at ca. 100 °C afforded the known pentagonal-bipyramidal Sn7 cluster Sn5(SnAriPr4)2 (3). Mechanistic studies show that 3 is formed from distannyne 2, which is generated from 1. The order of the reaction for the conversion of 2 into 3 was found to be zero, and the rate constant is 1.77 × 10-5 M s-1 at 100 °C. Hydride 1 was further characterized by cyclic voltammetry, and its pKa was found to be 18.8(2) via titration with 1,8-diazabicyclo[5.4.0]undec-7-ene. The bond dissociation free energy was estimated to be 51.1 kcal/mol ± 3.4% on the basis of its pKa and reduction potential. Studies with deuterium indicate ready exchange of D2 with the hydrides in 1.

Control of Ligand pKa Values Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex.

Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pKa values can be used in the design of a reaction mechanism for a ligand-based electron- and proton-transfer pathway, where the ligand retains a negative charge and enables dihydrogen evolution. A bis(pyrazolyl)pyridine ligand, iPrPz2P, reacts with 2 equiv of AlCl3 to afford [(iPrPz2P)AlCl2(THF)][AlCl4] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [(iPrHPz2P-)AlCl2] (2), where each of the electron- and proton-transfer events is ligand-centered. Protonation of 2 would formally close a catalytic cycle for dihydrogen production. At -1.26 V versus SCE, in a 0.3 M Bu4NPF6/tetrahydrofuran solution with salicylic acid or (HNEt3)+ as the source of H+, 1 produced dihydrogen electrocatalytically, according to cyclic voltammetry and controlled potential electrolysis experiments. The mechanism for the reaction is most likely two electron-transfer steps followed by two chemical steps based on the available reactivity information. A comparison of this work with our previously reported aluminum complexes of the phenyl-substituted bis(imino)pyridine system (PhI2P) reveals that the pKa values of the N-donor atoms in iPrPz2P are lower, which facilitates reduction before ligand protonation. In contrast, the PhI2P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.

Synthesis and characterization of bis(imino)pyridine complexes of divalent Mg and Zn.

Phenyl-bis(imino)pyridine (PhI2P) complexes, (PhI2P)ZnCl2 (1), (PhI2P−)ZnCl (2) and (PhI2P−)Zn(py)Cl (3) were obtained with the I2P ligand in both the neutral and the one-electron reduced state. In all examples, the metal ion is Zn(II). Metrical parameters obtained from solid state structures of 2 and 3 indicate that the PhI2P− ligand exists as a radical which is supported at the carbon atom of the imino donor, and this electronic state is also apparent in the analogous one-electron reduced ligand Al(III) complex, (PhI2P−)AlCl2 (4), that we prepared for comparison. We were unable to obtain PhI2P Mg complexes, and so the more electron rich methyl-substituted bis(imino)pyridine ligand, MeI2P, was investigated. Reaction of two-electron reduced MeI2P with MgCl2 and Mg(OTf)2 did afford the two-electron reduced ligand complexes [(MeI2P2−)Mg(THF)]2(µ-MgCl2) (5) and (MeI2P2−)Mg(THF)2 (6), respectively (MeI2P = 2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine). Complex 5 crystallizes as a trinuclear Mg complex consisting of two (MeI2P2−)Mg moieties bridged by MgCl2 and the (MeI2P2−) ligand is symmetric across the pyridine ring, but is not planar. In contrast, the (MeI2P2−) ligand in 6 is asymmetric across the pyridine ring and all atoms in the ligand are coplanar. Cyclic voltammetry measurements reveal that in complexes, 1, 4, 5, 6, the I2P0, I2P−, and I2P2− ligand charge states are accessible electrochemically.