Automated iterative Csp3-C bond formation.

Fully automated synthetic chemistry would substantially change the field by providing broad on-demand access to small molecules. However, the reactions that can be run autonomously are still limited. Automating the stereospecific assembly of Csp3-C bonds would expand access to many important types of functional organic molecules1. Previously, methyliminodiacetic acid (MIDA) boronates were used to orchestrate the formation of Csp2-Csp2 bonds and were effective building blocks for automating the synthesis of many small molecules2, but they are incompatible with stereospecific Csp3-Csp2 and Csp3-Csp3 bond-forming reactions3-10. Here we report that hyperconjugative and steric tuning provide a new class of tetramethyl N-methyliminodiacetic acid (TIDA) boronates that are stable to these conditions. Charge density analysis11-13 revealed that redistribution of electron density increases covalency of the N-B bond and thereby attenuates its hydrolysis. Complementary steric shielding of carbonyl π-faces decreases reactivity towards nucleophilic reagents. The unique features of the iminodiacetic acid cage2, which are essential for generalized automated synthesis, are retained by TIDA boronates. This enabled Csp3 boronate building blocks to be assembled using automated synthesis, including the preparation of natural products through automated stereospecific Csp3-Csp2 and Csp3-Csp3 bond formation. These findings will enable increasingly complex Csp3-rich small molecules to be accessed via automated assembly.

Assembly and Disassembly of Supramolecular Hypervalent Iodine Macrocycles via Anion Coordination.

This study explores the dynamic self-assembly and disassembly of hypervalent iodine-based macrocycles (HIMs) guided by secondary bonding interactions. The reversible disassembly and reassembly of HIMs are facilitated through anion binding via the addition of tetrabutylammonium (TBA) salts or removal of the anion by the addition of silver nitrate. The association constants for HIM monomers with TBA(Cl) and TBA(Br) are calculated and show a correlation with the strength of the iodine-anion bond. A unique tetracoordinate hypervalent iodine-based compound was identified as the disassembled monomer. Last, the study reveals the dynamic bonding nature of these macrocycles in solution, allowing for rearrangement and participation in dynamic bonding chemistry.

Exfoliable Transition Metal Chalcogenide Semiconductor NbSe2I2.

As the field of exfoliated van der Waals electronics grows to include complex heterostructures, the variety of available in-plane symmetries and geometries becomes increasingly valuable. In this work, we present an efficient chemical vapor transport synthesis of NbSe2I2 with the triclinic space group P1̅. This material contains Nb-Nb dimers and an in-plane crystallographic angle γ = 61.3°. We show that NbSe2I2 can be exfoliated down to few-layer and monolayer structures and use Raman spectroscopy to test the preservation of the crystal structure of exfoliated thin films. The crystal structure was verified by single-crystal and powder X-ray diffraction methods. Density functional theory calculations show triclinic NbSe2I2 to be a semiconductor with a band gap of around 1 eV, with similar band structure features for bulk and monolayer crystals. The physical properties of NbSe2I2 have been characterized by transport, thermal, optical, and magnetic measurements, demonstrating triclinic NbSe2I2 to be a diamagnetic semiconductor that does not exhibit any phase transformation below room temperature.

Formation of Red Elemental Selenium from Seleniferous Oxyanions: Deoxygenation by a Homogeneous Iron Catalyst.

Seleniferous oxyanions are groundwater contaminants from both anthropogenic and natural sources, while pure amorphous selenium nanoparticles have a variety of industrial applications. Biology can achieve the multicomponent 6 e-/8 H+ reduction of selenate to amorphous selenium using multiple metalloenzymes, like selenate and selenite reductase. Inspired by biology, we developed a new homogeneous system that can generate pure elemental selenium with no caustic waste. The stoichiometric reductions of selenate, selenite, and selenium dioxide with an iron(II) complex produced an iron(III)-oxo and red elemental selenium, the latter of which has been characterized by a variety of spectroscopic techniques. The catalytic reduction of SeO42- and SeO32- directly to amorphous Se and isolated as Se=PPh3 is reported with a turnover number of 12 and 7, respectively.

Insights into the Mechanism of CO2 Electroreduction by Molecular Palladium-Pyridinophane Complexes.

Herein, we report the synthesis, characterization, and electrocatalytic CO2 reduction activity of a series of Pd(II) complexes supported by tetradentate pyridinophane ligands. In particular, we focus on the electrocatalytic CO2 reduction activity of a Pd(II) complex supported by the mixed hard--soft donor ligand 2,11-dithia[3.3](2,6)pyridinophane (N2S2). We also provide spectroscopic evidence of a CO-induced decomposition pathway for the same catalyst, which provides insights into catalyst poisoning for molecular Pd CO2 reduction electrocatalysts.

Quasi-One-Dimensional Transition-Metal Chalcogenide Semiconductor (Nb4Se15I2)I2.

The discovery of new low-dimensional transition-metal chalcogenides is contributing to the already prosperous family of these materials. In this study, needle-shaped single crystals of a quasi-one-dimensional (1D) material, (Nb4Se15I2)I2, were grown by chemical vapor transport, and the structure was solved by single-crystal X-ray diffraction (XRD). The structure has 1D (Nb4Se15I2)n chains along the [101] direction, with two I- ions per formula unit directly bonded to Nb5+. The other two I- ions are loosely coordinated and intercalated between the chains. Individual chains are chiral and stack along the b axis in opposing directions, giving space group P21/c. The phase purity and crystal structure were verified by powder XRD. Density functional theory calculations show (Nb4Se15I2)I2 to be a semiconductor with a direct band gap of around 0.6 eV. Resistivity measurements of bulk crystals and micropatterned devices demonstrate that (Nb4Se15I2)I2 has an activation energy of around 0.1 eV, and no anomaly or transition was seen upon cooling. Low-temperature XRD shows that (Nb4Se15I2)I2 does not undergo a structural phase transformation from room temperature to 8.2 K, unlike related compounds (NbSe4)nI (n = 2, 3, or 3.33), which all exhibit charge-density waves. This compound represents a well-characterized and valence-precise member of a diverse family of anisotropic transition-metal chalcogenides.

Organometallic Fe2(µ-SH)2(CO)4(CN)2 Cluster Allows the Biosynthesis of the [FeFe]-Hydrogenase with Only the HydF Maturase.

The biosynthesis of the active site of the [FeFe]-hydrogenases (HydA1), the H-cluster, is of interest because these enzymes are highly efficient catalysts for the oxidation and production of H2. The biosynthesis of the [2Fe]H subcluster of the H-cluster proceeds from simple precursors, which are processed by three maturases: HydG, HydE, and HydF. Previous studies established that HydG produces an Fe(CO)2(CN) adduct of cysteine, which is the substrate for HydE. In this work, we show that by using the synthetic cluster [Fe2(µ-SH)2(CN)2(CO)4]2- active HydA1 can be biosynthesized without maturases HydG and HydE.

Homoleptic Rhodium Pyridine Complexes for Catalytic Hydrogen Oxidation.

The homoleptic rhodium pyridine complex [Rh(py)4]+ ([1]+) is prepared from simple precursors. Lacking good π-acceptor ligands but being sterically protected, [1]+ reversibly oxidizes to colorless [Rh(py)4(thf)2]2+. This monomeric S = 1/2 Rh(II) complex activates H2 to give [HRh(py)4L]2+, which can also be generated by protonation of [1]+. The Rh(III)-H bond is weak, being susceptible to H atom abstraction as well as deprotonation. These results underpin a novel catalytic system for the oxidation of H2 by ferrocenium.

Computational and Experimental Investigations of the Fe2(µ-S2)/Fe2(µ-S)2 Equilibrium.

Density functional theory (DFT) calculations on Fe2S2(CO)6-2n(PMe3)2n for n = 0, 1, and 2 reveal that the most electron-rich derivatives (n = 2) exist as diferrous disulfides lacking an S-S bond. The thermal interconversion of the FeII2(S)2 and FeI2(S2) valence isomers is symmetry-forbidden. Related electron-rich diiron complexes [Fe2S2(CN)2(CO)4]2- of an uncertain structure are implicated in the biosynthesis of [FeFe]-hydrogenases. Several efforts to synthesize electron-rich derivatives of Fe2(µ-S2)(CO)6 (1) are described. First, salts of iron persulfido cyanides [Fe2(µ-S2)(CO)5(CN)]- and [Fe2(µ-S2)(CN)(CO)4(PPh3)]- were prepared by the reactions of NaN(tms)2 with 1 and Fe2(µ-S2)(CO)5(PPh3), respectively. Alternative approaches to electron-rich diiron disulfides targeted Fe2(µ-S2)(CO)4(diphosphine). Whereas the preparation of Fe2(µ-S2)(CO)4(dppbz) was straightforward, that of Fe2(µ-S2)(CO)4(dppv) required an indirect route involving the oxidation of Fe2(µ-SH)2(CO)4(dppv) (dppbz = C6H4-1,2-(PPh2)2, dppv = cis-C2H2(PPh2)2). DFT calculations indicate that the oxidation of Fe2(µ-SH)2(CO)4(dppv) produces singlet diferrous disulfide Fe2(µ-S)2(CO)4(dppv), which is sufficiently long-lived as to be trapped by ethylene. The reaction of 1 and dppv mainly afforded Fe2(µ-SCH=CHPPh2)(µ-SPPh2)(CO)5, implicating a S-centered reaction.

Modulation of π-stacking modes and photophysical properties of an organic semiconductor through isosteric cocrystallization.

We report on the control of π-stacking modes (herringbone vs slipped-stack) and photophysical properties of 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA), an anthracene-based organic semiconductor (OSC), by isosteric cocrystallization (i.e., the replacement of one functional group in a coformer with another of "similar" electronic structure) with 2,4,6-trihalophenols (3X-ph-OH, where X = Cl, Br, and I). Specifically, BP4VA organizes as slipped-stacks when cocrystallized with 3Cl-ph-OH and 3Br-ph-OH, while cocrystallization with 3I-ph-OH results in a herringbone mode. The photoluminescence and molecular frontier orbital energy levels of BP4VA were effectively modulated by the presence of 3X-ph-OH through cocrystallization. We envisage that the cocrystallization of OSCs with minimal changes in cocrystal formers can provide access to convenient structural and property diversification for advanced single-crystal electronics.

Surprising Condensation Reactions of the Azadithiolate Cofactor.

Azadithiolate, a cofactor found in all [FeFe]-hydrogenases, is shown to undergo acid-catalyzed rearrangement. Fe2 [(SCH2 )2 NH](CO)6 self-condenses to give Fe6 [(SCH2 )3 N]2 (CO)17 . The reaction, which is driven by loss of NH4+ , illustrates the exchange of the amine group. X-ray crystallography reveals that three Fe2 (SR)2 (CO)x butterfly subunits interconnected by the aminotrithiolate [N(CH2 S)3 ]3- . Mechanistic studies reveal that Fe2 [(SCH2 )2 NR](CO)6 participate in a range of amine exchange reactions, enabling new methodologies for modifying the adt cofactor. Ru2 [(SCH2 )2 NH](CO)6 also rearranges, but proceeds further to give derivatives with Ru-alkyl bonds Ru6 [(SCH2 )3 N][(SCH2 )2 NCH2 ]S(CO)17 and [Ru2 [(SCH2 )2 NCH2 ](CO)5 ]2 S.

Reactions of [Fe6C(CO)14(S)]2-: Cluster Growth, Redox, Sulfiding.

Redox reactions, substitutions, and metalations are reported for the iron carbido sulfide [Fe6C(CO)14(S)]2- ([1]2-). Dianion [1]2- oxidized to [Fe6C(CO)16(S)]0 ([2]0) upon treatment with of [Fe(C5H5)2]BF4 or HBF4 (H2 formation) under an atmosphere of CO. Reaction of [2]0 with tBuNC gave [Fe6C(S)(CO)13(tBuNC)5], consisting of Fe5C(CO)13 and [Fe(tBuNC)5]2+ subunits linked by a µ3-S2-. The Fe7CS cluster [Fe7C(CO)17(S)]2- formed upon treatment of (Ph4P)2[1] with Fe(benzylideneacetone)(CO)3. The Fe7 species is an edge-fused cluster with [Fe6C(CO)10(µ-CO)4] and Fe(CO)3 subunits joined by µ3-S and two Fe-Fe bonds. The analogous reaction using Mo(CO)4(norbornadiene) gave [MoFe6C(CO)18(S)]2-. In this cluster, the Mo center is located in the octahedral subunit. Treatment of [1]2- with SO2 afforded [Fe6C(S)(SO2)(CO)13]2-. This cluster features an Fe6C core decorated with µ3-S and µ2-SO2 ligands. These experiments were undertaken in an effort to connect organometallic clusters to FeMoco.

Super- and Ferroelastic Organic Semiconductors for Ultraflexible Single-Crystal Electronics.

Like silicon, single crystals of organic semiconductors are pursued to attain intrinsic charge transport properties. However, they are intolerant to mechanical deformation, impeding their application in flexible electronic devices. Such contradictory properties, namely exceptional molecular ordering and mechanical flexibility, are unified in this work. We found that bis(triisopropylsilylethynyl)pentacene (TIPS-P) crystals can undergo mechanically induced structural transitions to exhibit superelasticity and ferroelasticity. These properties arise from cooperative and correlated molecular displacements and rotations in response to mechanical stress. By utilizing a bending-induced ferroelastic transition of TIPS-P, flexible single-crystal electronic devices were obtained that can tolerate strains (ϵ) of more than 13 % while maintaining the charge carrier mobility of unstrained crystals (µ>0.7 µ0 ). Our work will pave the way for high-performance ultraflexible single-crystal organic electronics for sensors, memories, and robotic applications.

Iron Carbide-Sulfide Carbonyl Clusters.

Described is the preparation of the first iron carbide-sulfides. The cluster [Fe6C(CO)15(SO2)]2- ([2]2-), which is generated quantitatively from [Fe6C(CO)16]2- ([1]2-), was O-methylated to give the sulfinite [2Me]-. Demethoxylation of [2Me]- with BF3 gave the face-capped octahedral cluster Fe6C(CO)15(SO) (3). In solution, 3 spontaneously converted to the sulfide Fe6C(CO)16(S) (4), an edge-fused double cluster with Fe5C and Fe3S subunits. Although 4 undergoes 1e- reduction reversibly, 2e- reduction (or base hydrolysis) of 4 gives closo-[Fe6C(CO)14(S)]2- ([5]2-). The synthetic entries into the Fe6CS x manifold may underpin the preparation of active-site analogues of the FeMoco and FeVco cofactors.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates.

Described are the syntheses of several Ni(µ-SR)2Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS3)- and Ni(NS3)- (PS33- = tris(phenyl-2-thiolato)phosphine, NS33- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C6F5)2. The following new nickel precursor complexes were characterized: PPh4[Ni(NS3)] and the dimeric imino/amino-phosphine complexes [NiCl2(PCH═NAn)]2 and [NiCl2(PCH2NHAn)]2 (P = Ph2PC6H4-2-). The iron(II) reagents include [CpFe(CO)2(thf)]BF4, [Cp*Fe(CO)(MeCN)2]BF4, FeI2(CO)4, FeCl2(diphos)(CO)2, and Fe(pdt)(CO)2(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS3), [Cp(CO)Fe(µ-pdt)Ni(dppbz)]BF4, [( R,R-DIPAMP)Ni(µ-pdt)(H)Fe(CO)3]BArF4, [(PCH═NAn)Ni(µ-pdt)(Cl)Fe(dppbz)(CO)]BF4, [(PCH2NHAn)Ni(µ-pdt)(Cl)Fe(dppbz)(CO)]BF4, [(PCH═NAn)Ni(µ-pdt)(H)Fe(dppbz)(CO)]BF4, [(dppv)(CO)Fe(µ-pdt)]2Ni, {H[(dppv)(CO)Fe(µ-pdt)]2Ni]}BF4, and (C6F5)2Ni(µ-pdt)Fe(CO)2(dppv) (DIPAMP = (CH2P(C6H4-2-OMe)2)2; BArF4- = [B(C6H3-3,5-(CF3)2]4-)) Within the context of Ni-(SR)2-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(µ-pdt)]2Ni}+, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)4 site, as supported by density functional theory calculations.

Product Distribution from Precursor Bite Angle Variation in Multitopic Alkyne Metathesis: Evidence for a Putative Kinetic Bottleneck.

In the dynamic synthesis of covalent organic frameworks and molecular cages, the typical synthetic approach involves heuristic methods of discovery. While this approach has yielded many remarkable products, the ability to predict the structural outcome of subjecting a multitopic precursor to dynamic covalent chemistry (DCC) remains a challenge in the field. The synthesis of covalent organic cages is a prime example of this phenomenon, where precursors designed with the intention of affording a specific product may deviate dramatically when the DCC synthesis is attempted. As such, rational design principles are needed to accelerate discovery in cage synthesis using DCC. Herein, we test the hypothesis that precursor bite angle contributes significantly to the energy landscape and product distribution in multitopic alkyne metathesis (AM). By subjecting a series of precursors with varying bite angles to AM, we experimentally demonstrate that the product distribution, and convergence toward product formation, is strongly dependent on this geometric attribute. Surprisingly, we discovered that precursors with the ideal bite angle (60°) do not afford the most efficient pathway to the product. The systematic study reported here illustrates how seemingly minor adjustments in precursor geometry greatly affect the outcome of DCC systems. This research illustrates the importance of fine-tuning precursor geometric parameters in order to successfully realize desirable targets.

Electron-Rich, Diiron Bis(monothiolato) Carbonyls: C-S Bond Homolysis in a Mixed Valence Diiron Dithiolate.

The synthesis and redox properties are presented for the electron-rich bis(monothiolate)s Fe2(SR)2(CO)2(dppv)2 for R = Me ([1]0), Ph ([2]0), CH2Ph ([3]0). Whereas related derivatives adopt C2-symmetric Fe2(CO)2P4 cores, [1]0-[3]0 have Cs symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [1]0-[3]0 undergo 1e- oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe2(SR)2(CO)2(dppv)2]+. As established crystallographically, [3]+ adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [1]+ and [2]+ and many other [Fe2(SR)2L6]+ derivatives, [3]+ undergoes C-S bond homolysis, affording the diferrous sulfido-thiolate [Fe2(SCH2Ph)(S)(CO)2(dppv)2]+ ([4]+). According to X-ray crystallography, the first coordination spheres of [3]+ and [4]+ are similar, but the Fe-sulfido bonds are short in [4]+. The conversion of [3]+ to [4]+ follows first-order kinetics, with k = 2.3 × 10-6 s-1 (30 °C). When the conversion is conducted in THF, the organic products are toluene and dibenzyl. In the presence of TEMPO, the conversion of [3]+ to [4]+ is accelerated about 10×, the main organic product being TEMPO-CH2Ph. DFT calculations predict that the homolysis of a C-S bond is exergonic for [Fe2(SCH2Ph)2(CO)2(PR3)4]+ but endergonic for the neutral complex as well as less substituted cations. The unsaturated character of [4]+ is indicated by its double carbonylation to give [Fe2(SCH2Ph)(S)(CO)4(dppv)2]+ ([5]+), which adopts a bioctahedral structure.

Cobalt-Catalyzed and Lewis Acid-Assisted Nitrile Hydrogenation to Primary Amines: A Combined Effort.

The selective hydrogenation of nitriles to primary amines using a bench-stable cobalt precatalyst under 4 atm of H2 is reported herein. The catalyst precursor was reduced in situ using NaHBEt3, and the resulting Lewis acid formed, BEt3, was found to be integral to the observed catalysis. Mechanistic insights gleaned from para-hydrogen induced polarization (PHIP) transfer NMR studies revealed that the pairwise hydrogenation of nitriles proceeded through a Co(I/III) redox process.

Conducting Molecular Nanomagnet of DyIII with Partially Charged TCNQ Radicals.

Bifunctional electrically conducting single-molecule magnets are highly promising platforms for non-volatile memory devices and quantum computing applications. The development of these molecular materials, however, has largely been hindered by the lack of straightforward synthetic methods. Herein a facile and modular approach is demonstrated for the realization of bifunctional materials that does not require electrochemical or chemical oxidation to obtain partially charged organic radicals. Magnetic and electrical conductivity studies reveal that the DyIII compound exhibits slow relaxation of the magnetization between 5.0-8.0 K and semiconducting behavior over the range 180-350 K. DC magnetic fields have been found to suppress the quantum tunneling of the magnetization and affect the spin-canted antiferromagnetic interactions.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex.

The radical bridged compound [(Ni(TPMA))2-µ-bmtz•-](BF4)3·3CH3CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 NiII centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm-1 (-2JNi-radSNiSrad). DFT calculations support the existence of strong ferromagnetic exchange.