Reaction Chemistry at Discrete Organometallic Fragments on Black Phosphorus.

Black phosphorus (bP) is a two-dimensional van der Waals material unique in its potential to serve as a support for single-site catalysts due to its similarity to molecular phosphines, ligands quintessential in homogeneous catalysis. However, there is a scarcity of synthetic methods to install single metal centers on the bP lattice. Here, we demonstrate the functionalization of bP nanosheets with molecular Re and Mo complexes. A suite of characterization techniques, including infrared, X-ray photoelectron and X-ray absorption spectroscopy as well as scanning transmission electron microscopy corroborate that the functionalized nanosheets contain a high density of discrete metal centers directly bound to the bP surface. Moreover, the supported metal centers are chemically accessible and can undergo ligand exchange transformations without detaching from the surface. The steric and electronic properties of bP as a ligand are estimated with respect to molecular phosphines. Sterically, bP resembles tri(tolyl)phosphine when monodentate to a metal center, and bis(diphenylphosphino)propane when bidentate, whereas electronically bP is a σ-donor as strong as a trialkyl phosphine. This work is foundational in elucidating the nature of black phosphorus as a ligand and underscores the viability of using bP as a basis for single-site catalysts.

Probing Edge/Support Electronic Cooperativity in Single Edge Fe/Co6Se8 Clusters.

This study provides insights into the electronic structure of an atomically precise Fe/Co6Se8 cluster and the extent of redox cooperativity between the Fe active site and the noninnocent Co6Se8 support. Chemical oxidation studies enable the isolation of two types of oxidized Fe/Co6Se8 clusters, in which the nature of the counterion (I- or OTf-) significantly impacts the structural interactions between Fe and the Co6Se8 unit. Experimental characterization by single crystal X-ray diffraction, 57Fe Mössbauer spectroscopy, and 31P{1H} NMR spectroscopy is complemented by computational analysis. In aggregate, the study reveals that upon oxidation, the charge is shared between the Fe edge site and the Co6Se8 core.

Metal/Support Interactions Regulate Substrate Binding in Fe/Co/Se Cluster Catalysts.

Here, we investigate the stereoelectronic requirements of a family of Fe/Co6Se8 molecular clusters to achieve a Goldilocks regime of substrate affinity for the catalytic coupling of tosyl azide and tert-butyl isocyanide. The reactivity of a catalytically competent iron-nitrenoid intermediate, observed in situ, is explored toward nitrene transfer and hydrogen-atom abstraction. The dual role of isocyanide, which, on the one hand, prevents catalyst degradation but, in large amounts, slows down reactivity, is exposed. The impact of distal changes (the number of neighboring active sites and the identity of supporting ligands) on the substrate affinity, electronic properties, and catalytic activity is investigated. Overall, the study reveals that the dynamic, push-pull interactions between the substrate (tBuNC), active site (Fe), and support (Co6Se8) create a regime where increased substrate activation occurs with facile dissociation.

Metal-Support Interactions in Molecular Single-Site Cluster Catalysts.

This study provides atomistic insights into the interface between a single-site catalyst and a transition metal chalcogenide support and reveals that peak catalytic activity occurs when edge/support redox cooperativity is maximized. A molecular platform MCo6Se8(PEt3)4(L)2 (1-M, M = Cr, Mn, Fe, Co, Cu, and Zn) was designed in which the active site (M)/support (Co6Se8) interactions are interrogated by systematically probing the electronic and structural changes that occur as the identity of the metal varies. All 3d transition metal 1-M clusters display remarkable catalytic activity for coupling tosyl azide and tert-butyl isocyanide, with Mn and Co derivatives showing the fastest turnover in the series. Structural, electronic, and magnetic characterization of the clusters was performed using single crystal X-ray diffraction, 1H and 31P nuclear magnetic resonance spectroscopy, electronic absorption spectroscopy, cyclic voltammetry, and computational methods. Distinct metal/support redox regimes can be accessed in 1-M based on the energy of the edge metal's frontier orbitals with respect to those of the cluster support. As the degree of electronic interaction between the edge and the support increases, a cooperative regime is reached wherein the support can deliver electrons to the catalytic site, increasing the reactivity of key metal-nitrenoid intermediates.

Informed Chemical Classification of Organophosphorus Compounds via Unsupervised Machine Learning of X-ray Absorption Spectroscopy and X-ray Emission Spectroscopy.

We analyze an ensemble of organophosphorus compounds to form an unbiased characterization of the information encoded in their X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectra (VtC-XES). Data-driven emergence of chemical classes via unsupervised machine learning, specifically cluster analysis in the Uniform Manifold Approximation and Projection (UMAP) embedding, finds spectral sensitivity to coordination, oxidation, aromaticity, intramolecular hydrogen bonding, and ligand identity. Subsequently, we implement supervised machine learning via Gaussian process classifiers to identify confidence in predictions that match our initial qualitative assessments of clustering. The results further support the benefit of utilizing unsupervised machine learning as a precursor to supervised machine learning, which we term Unsupervised Validation of Classes (UVC), a result that goes beyond the present case of X-ray spectroscopies.

Multi-active Site Dynamics on a Molecular Cr/Co/Se Cluster Catalyst.

This study uncovers the interconnected reactivity of the three catalytically active sites of an atomically precise nanocluster Cr3(py)3Co6Se8L6 (1(py)3, L = Ph2PNTol-, Ph = phenyl, Tol = 4-tolyl). Catalytic and stoichiometric studies into tosyl azide activation and carbodiimide formation enabled the isolation and crystallographic characterization of key catalytically competent metal-imido intermediates, including the tris(imido) cluster 1(NTs)3, the catalytic resting state 1(NTs)3(CNtBu)3, and the site-differentiated mono(imido) cluster 1(NTs)(CNtBu)2. In the stoichiometric regime, nitrene transfer proceeds via a stepwise mechanism, with the three active sites engaging sequentially to produce carbodiimide. Moreover, the chemical state of neighboring active sites was found to regulate the affinity for substrates of an individual Cr-imido edge site, as revealed by comparative structural analysis and CNtBu binding studies.

Redox-Switchable Allosteric Effects in Molecular Clusters.

We demonstrate that allosteric effects and redox state changes can be harnessed to create a switch that selectively and reversibly regulates the coordination chemistry of a single site on the surface of a molecular cluster. This redox-switchable allostery is employed as a guiding force to assemble the molecular clusters Zn3Co6Se8L'6 (L' = Ph2PN(H)Tol, Ph = phenyl, Tol = 4-tolyl) into materials of predetermined dimensionality (1- or 2-D) and to encode them with emissive properties. This work paves the path to program the assembly and function of inorganic clusters into stimuli-responsive, atomically precise materials.

Hydroalkylation of Alkynes: Functionalization of the Alkenyl Copper Intermediate through Single Electron Transfer Chemistry.

We have developed a method for the stereoselective coupling of terminal alkynes and α-bromo carbonyls to generate functionalized E-alkenes. The coupling is accomplished by merging the closed-shell hydrocupration of alkynes with the open-shell single electron transfer (SET) chemistry of the resulting alkenyl copper intermediate. We demonstrate that the reaction is compatible with various functional groups and can be performed in the presence of aryl bromides, alkyl chlorides, alkyl bromides, esters, nitriles, amides, and a wide range of nitrogen-containing heterocyclic compounds. Mechanistic studies provide evidence for SET oxidation of the alkenyl copper intermediate by an α-bromo ester as the key step that enables the cross coupling.

Tuning the Electronic Structure of Atomically Precise Sn/Co/Se Nanoclusters via Redox Matching of Tin(IV) Surface Sites.

A new strategy is reported to tailor the electronic properties of a superatomic metal chalcogenide cluster by redox matching the cluster core with surface tin(IV) sites. Two ternary clusters (SnR2)3Co6Se8L6 (R = Me, nBu) are synthesized by salt metathesis from the hexalithiated salt [Li2(py)2]3Co6Se8L6 and R2SnCl2. Cyclic and differential-pulse voltammetry studies reveal that the tristannylated clusters feature two new, near-degenerate, electronic states within the highest occupied molecular orbital-lowest unoccupied molecular orbital gap of the Co6Se8 core, which are attributed to the reduction of a surface tin site. Single-crystal X-ray diffraction analysis reveals that no Sn···Se coordination is present in the solid state. The single-crystal X-ray structure of the hexalithiated salt starting material is reported for the tetrahydrofuran (THF) adduct variant [Li2(THF)2]6Co6Se8L6.

Surface Modification of Black Phosphorus with Group 13 Lewis Acids for Ambient Protection and Electronic Tuning.

Herein we introduce a facile, solution-phase protocol to modify the Lewis basic surface of few-layer black phosphorus (bP) and demonstrate its effectiveness at providing ambient stability and tuning of electronic properties. Commercially available group 13 Lewis acids that range in electrophilicity, steric bulk, and Pearson hard/soft-ness are evaluated. The nature of the interaction between the Lewis acids and the bP lattice is investigated using a range of microscopic (optical, atomic force, scanning electron) and spectroscopic (energy dispersive, X-ray photoelectron) methods. Al and Ga halides are most effective at preventing ambient degradation of bP (>84 h for AlBr3 ), and the resulting field-effect transistors show excellent IV characteristics, photocurrent, and current stability, and are significantly p-doped. This protocol, chemically matched to bP and compatible with device fabrication, opens a path for deterministic and persistent tuning of the electronic properties in bP.

Surface Functionalization of Black Phosphorus with Nitrenes: Identification of P=N Bonds by Using Isotopic Labeling.

Surface functionalization of two-dimensional crystals is a key path to tuning their intrinsic physical and chemical properties. However, synthetic protocols and experimental strategies to directly probe chemical bonding in modified surfaces are scarce. Introduced herein is a mild, surface-specific protocol for the surface functionalization of few-layer black phosphorus nanosheets using a family of photolytically generated nitrenes (RN) from the corresponding azides. By embedding spectroscopic tags in the organic backbone, a multitude of characterization techniques are employed to investigate in detail the chemical structure of the modified nanosheets, including vibrational, X-ray photoelectron, solid state 31 P NMR, and UV-vis spectroscopy. To directly probe the functional groups introduced on the surface, R fragments were selected such that in conjunction with vibrational spectroscopy, 15 N-labeling experiments, and DFT methods, diagnostic P=N vibrational modes indicative of iminophosphorane units on the nanosheet surface could be conclusively identified.

Inorganic clusters as metalloligands: ligand effects on the synthesis and properties of ternary nanopropeller clusters.

Redox-active multimetallic platforms with synthetically addressable and hemilabile active sites are attractive synthetic targets for mimicking the reactivity of enzymatic co-factors toward multielectron transformations. To this end, a family of ternary clusters featuring three edge metal sites anchored on a [Co6Se8] multimetallic support via amidophosphine ligands are a promising platform. In this report, we explore how small changes in the stereoelectronic properties of these ligands alter [Co6Se8] metalloligand formation, but also substrate binding affinity and strength of the edge/support interaction in two new ternary clusters, M3Co6Se8L6 (M = Zn, Fe; L(-) = Ph2PN(-)iPr). These clusters are characterized extensively using a range of methods, including single crystal X-ray diffraction, electronic absorption spectroscopy and cyclic voltammetry. Substrate binding studies reveal that Fe3Co6Se8L6 resists coordination of larger ligands like pyridine or tetrahydrofuran, but binds the smaller ligand CNtBu. Additionally, investigations into the synthesis of new [Co6Se8] metalloligands using two aminophosphines, Ph2PN(H)iPr (LH) and iPr2PN(H)iPr, led to the synthesis and characterization of Co6Se8LH6, as well as the smaller clusters Co4Se2(CO)6LH4, Co3Se(µ2-PPh2)(CO)4LH3, and [Co(CO)3(iPr2PN(H)iPr)]2. Cumulatively, this study expands our understanding on the effect of the stereoelectronic properties of aminophosphine ligands in the synthesis of cobalt chalcogenide clusters, and, importantly on modulating the push-pull dynamic between the [Co6Se8] support, the edge metals and incoming coordinating ligands in ternary M3Co6Se8L6 clusters.

Hierarchical nanosheets built from superatomic clusters: properties, exfoliation and single-crystal-to-single-crystal intercalation.

Tuning the properties of atomic crystals in the two-dimensional (2D) limit is synthetically challenging, but critical to unlock their potential in fundamental research and nanotechnology alike. 2D crystals assembled using superatomic blocks could provide a route to encrypt desirable functionality, yet strategies to link the inorganic blocks together in predetermined dimensionality or symmetry are scarce. Here, we describe the synthesis of anisotropic van der Waals crystalline frameworks using the designer superatomic nanocluster Co3(py)3Co6Se8L6 (py = pyridine, L = Ph2PN(Tol)), and ditopic linkers. Post-synthetically, the 3D crystals can be mechanically exfoliated into ultrathin flakes (8 to 60 nm), or intercalated with the redox-active guest tetracyanoethylene in a single-crystal-to-single-crystal transformation. Extensive characterization, including by single crystal X-ray diffraction, reveals how intrinsic features of the nanocluster, such as its structure, chirality, redox-activity and magnetic profile, predetermine key properties of the emerging 2D structures. Within the nanosheets, the strict and unusual stereoselectivity of the nanocluster's Co edges for the low symmetry (α,α,ß) isomer gives rise to in-plane structural anisotropy, while the helically chiral nanoclusters self-organize into alternating Δ- and Λ-homochiral rows. The nanocluster's high-spin Co edges, and its rich redox profile make the nanosheets both magnetically and electrochemically active, as revealed by solid state magnetic and cyclic voltammetry studies. The length and flexibility of the ditopic linker was varied, and found to have a secondary effect on the structure and stacking of the nanosheets within the 3D crystals. With these results we introduce a deterministic and versatile synthetic entry to programmable functionality and symmetry in 2D superatomic crystals.

Atomically Defined Nanopropeller Fe3Co6Se8(Ph2PNTol)6: Functional Model for the Electronic Metal-Support Interaction Effect and High Catalytic Activity for Carbodiimide Formation.

Atomically defined interfaces that maximize the density of active sites and harness the electronic metal-support interaction are desirable to facilitate challenging multielectron transformations, but their synthesis remains a considerable challenge. We report the rational synthesis of the atomically defined metal chalcogenide nanopropeller Fe3Co6Se8L6 (L = Ph2PNTol) featuring three Fe edge sites, and its ensuing catalytic activity for carbodiimide formation. The complex interaction between the Fe edges and Co6Se8 support, including the interplay between oxidation state, substrate coordination, and metal-support interaction, is probed in detail using chemical and electrochemical methods, extensive single crystal X-ray diffraction, and electronic absorption and Mössbauer spectroscopy.

Mechanism and Scope of Phosphinidene Transfer from Dibenzo-7-phosphanorbornadiene Compounds.

Dibenzo-7-phosphanorbornadiene compounds, RPA (A = C14H10 or anthracene), are investigated as phosphinidene sources upon thermally induced (70-90 °C) anthracene elimination. Analysis of substituent effects reveals that π-donating dialkylamide groups are paramount to successful phosphinidene transfer; poorer π-donors give reduced or no transfer. Substituent steric bulk is also implicated in successful transfer. Molecular beam mass spectrometry (MBMS) studies of each derivative reveal dialkylamide derivatives to be promising precursors for further gas-phase spectroscopic studies of phosphinidenes; in particular, we present evidence of direct detection of the dimethylamide derivative, [Me2N═P]. Kinetic investigations of iPr2NPA thermolysis in 1,3-cyclohexadiene and/or benzene-d6 are consistent with a model of unimolecular fragmentation to yield free phosphinidene [iPr2N═P] as a transient reactive intermediate. This conclusion is probed by density functional theory (DFT) calculations, which favored a mechanistic model featuring free singlet aminophosphinidenes. The breadth of phosphinidene acceptors is expanded to unsaturated substrates beyond 1,3-dienes to include olefins and alkynes; this provides a new synthetic route to valuable amino-substituted phosphiranes and phosphirenes, respectively. Stereoselective phosphinidene transfer to olefins is consistent with singlet phosphinidene reactivity by analogy with the Skell hypothesis for singlet carbene addition to olefins.

Building Diatomic and Triatomic Superatom Molecules.

In this study, we have developed a method to create Co6Se8 superatoms in which we program the metal-ligand bonds. We exclusively form the Co6Se8 core under simple reaction conditions with a facile separation of products that contain differential substitution of the core. The combination of Co2(CO)8 and PR3 with excess Se gives the differentially and directionally substituted superatoms, Co6Se8(CO)x(PR3)(6-x). The CO groups on the superatom can be exchanged quantitatively with phosphines and isonitriles. Substitution of the CO allows us to manipulate the type and length of chemical bridge between two redox-active superatomic centers in order to modulate intersuperatomic coupling. Linking two superatoms together allows us to form the simplest superatom molecule: a diatomic molecule. We extend the superatom molecule concept to link three superatoms together in a linear arrangement to form acyclic triatomic molecules. These superatom molecules have a rich electrochemical profile and chart a clear path to a whole family of superatom molecules with new and unusual collective properties.

A Molecular Precursor to Phosphaethyne and Its Application in Synthesis of the Aromatic 1,2,3,4-Phosphatriazolate Anion.

Dibenzo-7-phosphanorbornadiene Ph3PC(H)PA (1, A = C14H10, anthracene) is reported here as a molecular precursor to phosphaethyne (HC≡P), produced together with anthracene and triphenylphosphine. HCP generated by thermolysis of 1 has been observed by molecular beam mass spectrometry, laser-induced fluorescence, microwave spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. In toluene, fragmentation of 1 has been found to proceed with activation parameters of ΔH(⧧) = 25.5 kcal/mol and ΔS(⧧) = -2.43 eu and is accompanied by formation of an orange insoluble precipitate. Results from computational studies of the mechanism of HCP generation are in good agreement with experimental data. This high-temperature method of HCP generation has pointed to new reaction chemistry with azide anion to produce the 1,2,3,4-phosphatriazolate anion, HCPN3(-), for which structural data have been obtained in a single-crystal X-ray diffraction study. Negative-ion photoelectron spectroscopy has shown the adiabatic detachment energy for this anion to be 3.555(10) eV. The aromaticity of HCPN3(-) has been assessed using nucleus-independent chemical shift, quantum theory of atoms in molecules, and natural bond orbital methods.

Conformations of cyclopentasilane stereoisomers control molecular junction conductance.

Here we examine the impact of ring conformation on the charge transport characteristics of cyclic pentasilane structures bound to gold electrodes in single molecule junctions. We investigate the conductance properties of alkylated cyclopentasilane cis and trans stereoisomers substituted in the 1,3-position with methylthiomethyl electrode binding groups using both the scanning tunneling microscope-based break junction technique and density functional theory based ab initio calculations. In contrast with the linear ones, these cyclic silanes yield lower conductance values; calculations reveal that the constrained dihedral geometries occurring within the ring are suboptimal for σ-orbital delocalization, and therefore, conductance. Theoretical calculations reproduce the measured conductance trends for both cis and trans isomers and find several distinct conformations that are likely to form stable molecular junctions at room temperature. Due to the weakened σ-conjugation in the molecule, through-space interactions are found to contribute significantly to the conductance. This manuscript details the vast conformational flexibility in cyclopentasilanes and the tremendous impact it has on controlling conductance.

Assembly and stabilization of {E(cyclo-P3)2} (E = Sn, Pb) as a bridging ligand spanning two triaryloxyniobium units.

Complexes (THF)0-2E[P3Nb(ODipp)3]2 (E = Sn, Pb; Dipp = 2,6-(i)Pr2C6H3) were isolated (>90%) from the salt metathesis of [Na(THF)3][P3Nb(ODipp)3] with E(2+) salts. The reaction of (THF)Sn[P3Nb(ODipp)3]2 with pyridine-N-oxide was investigated as a method to deposit a new SnP6 phase. Additionally, the neutral complex P3Nb(ODipp)2(py)2 (py = pyridine) was prepared from [Na(THF)3][P3Nb(ODipp)3] in the presence of pyridine and salts of coordinating cations (Mg(II), Sn(II), Pb(II), Ge(II), Hg(II) and Ag(I)). P3Nb(ODipp)2(py)2 was found to successfully produce AsP3 upon treatment with AsCl3. The characterization of complexes (THF)0-1Sn[P3Nb(ODipp)3]2, (THF)2Pb[P3Nb(ODipp)3]2 and P3Nb(ODipp)2(py)2, including their solid state structures, is discussed.