Supramolecular Catalyzed Cascade Reduction of Azaarenes Interrogated via Data Science.

Catalysis of multicomponent transformations requires controlled assembly of reactants within the active site. Supramolecular scaffolds possess synthetic microenvironments that enable precise modulation over noncovalent interactions (NCIs) engaged by reactive, encapsulated species. While molecular properties that describe the behavior of single guests in host cavities have been studied extensively, multicomponent transformations remain challenging to design and deploy.

Panoply of P: An Array of Rhenium-Phosphorus Complexes Generated from a Transition Metal Anion.

We expand upon the synthetic utility of anionic rhenium complex Na[(BDI)ReCp] (, BDI = -bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) to generate several rhenium-phosphorus complexes. Complex reacts in a metathetical manner with chlorophosphines PhPCl, NHP-Cl, and OHP-Cl to generate XL-type phosphido complexes , , and , respectively (NHP-Cl = 2-chloro-1,3-dimethyl-1,3,2-diazaphospholidine; OHP-Cl = 2-chloro-1,3,2-dioxaphospholane). Crystallographic and computational investigations of phosphido triad , , and reveal that increasing the electronegativity of the phosphorus substituent (C < < O) results in a shortening and strengthening of the rhenium-phosphorus bond.

Triple Inverse Sandwich versus End-On Diazenido: Bonding Motifs across a Series of Rhenium-Lanthanide and -Actinide Complexes.

While synthesizing a series of rhenium-lanthanide triple inverse sandwich complexes, we unexpectedly uncovered evidence for rare examples of end-on lanthanide dinitrogen coordination for certain heavy lanthanide elements as well as for uranium. We begin our report with the synthesis and characterization of a series of trirhenium triple inverse sandwich complexes with the early lanthanides, Ln[(μ-η:η-Cp)Re(BDI)](THF) (, Ln = La, Ce, Pr, Nd, Sm; Cp = cyclopentadienide, BDI = ,'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate). However, as we moved across the lanthanide series, we ran into an unexpected result for gadolinium in which we structurally characterized two products for gadolinium, namely, (analogous to ) and a diazenido dirhenium double inverse sandwich complex Gd[(μ-η:η-N)Re(η-Cp)(BDI)][(μ-η:η-Cp)Re(BDI)](THF) ().

Spectroscopic and Computational Evidence of Uranium Dihydrogen Complexes.

Dihydrogen complexation, a phenomenon with robust precedent in the transition metal series, is spectroscopically detected for a uranium(III) complex and thereby extended for the first time to the 5f series. The vacant coordination site and low valence of (CHSiMe)U prove to be key to the reversible formation of (CHSiMe)U-H (complex ), and the paramagnetism of the f center facilitates the detection of complex by NMR spectroscopy. Density functional theory calculations reveal that the delocalization of the 5f electron density from (CHSiMe)U onto the side-on dihydrogen ligand is crucial to complex formation, an unusual bonding situation for an actinide acid-base complex.

Tunable Electrochemical Entropy through Solvent Ordering by a Supramolecular Host.

An aqueous electrochemically controlled host-guest encapsulation system demonstrates a large and synthetically tunable redox entropy change. Electrochemical entropy is the basis for thermally regenerative electrochemical cycles (TRECs), which utilize reversible electrochemical processes with large molar entropy changes for thermogalvanic waste-heat harvesting and electrochemical cooling, among other potential applications. A supramolecular host-guest system demonstrates a molar entropy change of 4 times that of the state-of-the-art aqueous TREC electrolyte potassium ferricyanide.

Diagnostic traction and dorsal locking plate stabilization of a fifth and sixth thoracic vertebral fracture/luxation in a golden retriever: Case report.

Traction was used to diagnose instability of a T5-T6 traumatic luxation that was stabilized with locking plates in the laminae and dorsal pedicles. A two-year-old, 27 kg, female spayed golden retriever was presented to a veterinary teaching hospital after being referred for possible mandibular and spinal fractures after being hit by a car. The dog presented non-ambulatory paraparetic with intact pain perception.

Heterobimetallic-Mediated Dinitrogen Functionalization: N-C Bond Formation at Rhenium-Group 9 Diazenido Complexes.

We report the synthesis and characterization of rhenium-group 9 heterobimetallic diazenido species (η-Cp)Re(μ-BDI)(μ-N)M(η-COD) (, M = Ir or Rh, Cp = cyclopentadienide, BDI = ,'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate, COD = 1,5-cyclooctadiene), formed from salt elimination reactions between Na[(η-Cp)Re(BDI)] and [MCl(η-COD)]. Additionally, we find that these same reagents react under an argon atmosphere to instead produce bridging hydride complexes (BDI)Re(μ-η:η-CH)(μ-H)M(η-COD) (-), which undergo rearrangements upon protonation to form the alternative bridging hydrides [(η-Cp)Re(μ-BDI)(μ-H)M(η-COD)][(B(-CH(CF)))] (). Further, we demonstrate the first example of N-C bond formation at a heterobimetallic dinitrogen complex through reactions of and methyl triflate, which produces the alkylated species [(η-Cp)Re(μ-N(Me)N)(μ-BDI)M(η-COD)][OTf] (, OTf = trifluoromethanesulfonate).

Engendering reactivity at group 5-heteroatom multiple bonds π-loading.

In this Perspective, we discuss the strategy of π-loading, , coordination of two or more strongly π-donating ligands to a single metal center, as it applies to promoting reactivity at group 5 transition metal-imido groups. When multiple π-donor ligands compete to interact with the same symmetrically-available metal d orbitals, the energy of the imido-based frontier molecular orbitals increases, leading to amplified imido-based reactivity. This strategy is of particular relevance to group 5 metals, as mono(imido) complexes of these metals tend to be inert at the imido group.

Impact of Host Flexibility on Selectivity in a Supramolecular Host-Catalyzed Enantioselective aza-Darzens Reaction.

A highly enantioselective aza-Darzens reaction (up to 99% ee) catalyzed by an enantiopure supramolecular host has been discovered. To understand the role of host structure on reaction outcome, nine new gallium(III)-based enantiopure supramolecular assemblies were prepared via substitution of the external chiral amide. Despite the distal nature of the substitution in these catalysts, changes in enantioselectivity (61 to 90% ee) in the aziridine product were observed.

Source of Rate Acceleration for Carbocation Cyclization in Biomimetic Supramolecular Cages.

The results of quantum chemical and molecular dynamics calculations reveal that polyanionic gallium-based cages accelerate cyclization reactions of pentadienyl alcohols as a result of substrate cage interactions, preferential binding of reactive conformations of substrate/HO pairs, and increased substrate basicity. However, the increase in basicity dominates. Experimental structure-activity relationship studies in which the metal vertices and overall charge of the cage are varied confirm the model derived calculations.

[3 + 2] Cycloadditions and Retrocycloadditions of Niobium Imido Complexes: An Experimental and Computational Mechanistic Study.

We demonstrate reactivity between a β-diketiminate-supported niobium(III) imido complex and alkyl azides to form niobatetrazene complexes (BDI)Nb(NBu)(RNNNNR) (BDI = ,-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; R = cyclohexyl (), benzyl ()). Intriguingly, niobatetrazene complexes and can be interconverted via addition of an appropriate alkyl azide, likely through a series of concerted [3 + 2] cycloaddition and retrocycloaddition reactions in which π-loaded bis(imido) intermediates are formed. The bis(imido) intermediates were trapped upon addition of alkyl isocyanides to yield five-coordinate bis(imido) complexes (BDI)Nb(NBu)(NCy)(CNR) (R = -butyl (), cyclohexyl ()).

Applications of Low-Valent Transition Metalates: Development of a Reactive Noncarbonyl Rhenium(I) Anion.

Low-valent transition metalates─anionic, electronic-rich organometallic complexes─comprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M-E bond formation, among others. The inherent reactivity of such electron-rich metal centers has necessitated the widespread use of strong backbonding ligands, particularly carbonyls, to aid in the isolation and handling of metalate reagents, albeit sometimes at the expense of partially masking their full reactivity. However, recent synthetic explorations into transition-metalate complexes devoid of archetypic back-bonding ligands have led to the discovery of highly reactive metalates capable of performing a variety of novel chemical transformations.

Computed tomography evaluation of proposed implant corridors in canine thoracic vertebrae.

Objective: Identify acceptable implant corridors in the normal canine thoracic vertebrae (T) from T1 to T9.

Study Design: Retrospective study.

Sample Population: Computed tomographic (CT) studies of normal canine thoracic spines (n = 39).

A Diverse Array of C-C Bonds Formed at a Tantalum Metal Center.

We demonstrate the formation of a diverse array of organic and organometallic products containing newly formed C-C bonds via successive methyl transfers from di-, tri-, and tetramethyl Ta(V) precursors to unsaturated small molecule substrates under mild conditions. The reactions of Ta(V) methyl complexes - [HB(Im)]TaMeX (X = Me, Cl; Im = imidazole, Mes = 2,4,6-trimethylphenyl) with CO led to oxo enolate Ta(V) products, in which the enolate ligands were constructed from Ta-Me groups and two equivalents of CO. Similarly, the reaction of with CNXyl yielded an imido enamine Ta(V) product.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes.

The bis(imido) complexes (BDI)Nb(N Bu) and (BDI)Nb(N Bu)(NAr) (BDI = ,'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; Ar = 2,6-diisopropylphenyl) were shown to engage in 1,2-addition and [2 + 2] cycloaddition reactions with a wide variety of substrates. Reaction of the bis(imido) complexes with dihydrogen, silanes, and boranes yielded hydrido-amido-imido complexes 1,2-addition across Nb-imido π-bonds; some of these complexes were shown to further react insertion of carbon dioxide to give formate-amido-imido products. Similarly, reaction of (BDI)Nb(N Bu) with -butylacetylene yielded an acetylide-amido-imido complex.

σ or π? Bonding interactions in a series of rhenium metallotetrylenes.

Salt metathesis reactions between a low-valent rhenium(i) complex, Na[Re(η5-Cp)(BDI)] (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate), and a series of amidinate-supported tetrylenes of the form ECl[PhC(NtBu)2] (E = Si, Ge, Sn) led to rhenium metallotetrylenes Re(E[PhC(NtBu)2])(η5-Cp)(BDI) (E = Si (1a), Ge (2), Sn (4)) with varying extents of Re-E multiple bonding. Whereas the rhenium-stannylene 4 adopts a σ-metallotetrylene arrangement featuring a Re-E single bond, the rhenium-silylene (1a) and -germylene (2) both engage in π-interactions to form short Re-E multiple bonds. Temperature was found to play a crucial role in reactions between Na[Re(η5-Cp)(BDI)] and SiCl[PhC(NtBu)2], as manipulation of reaction conditions led to isolation of an unusual rhenium-silane, (BDI)Re(μ-η5:η1-C5H4)(SiH[PhC(NtBu)2]) (1b) and a dinitrogen bridged rhenium-silylene, (η5-Cp)(BDI)Re(μ-N2)Si[PhC(NtBu)2] (1c), in addition to 1a.

Chemoselective and Site-Selective Reductions Catalyzed by a Supramolecular Host and a Pyridine-Borane Cofactor.

Supramolecular catalysts emulate the mechanism of enzymes to achieve large rate accelerations and precise selectivity under mild and aqueous conditions. While significant strides have been made in the supramolecular host-promoted synthesis of small molecules, applications of this reactivity to chemoselective and site-selective modification of complex biomolecules remain virtually unexplored. We report here a supramolecular system where coencapsulation of pyridine-borane with a variety of molecules including enones, ketones, aldehydes, oximes, hydrazones, and imines effects efficient reductions under basic aqueous conditions.

An isolated water droplet in the aqueous solution of a supramolecular tetrahedral cage.

Water under nanoconfinement at ambient conditions has exhibited low-dimensional ice formation and liquid-solid phase transitions, but with structural and dynamical signatures that map onto known regions of water's phase diagram. Using terahertz (THz) absorption spectroscopy and ab initio molecular dynamics, we have investigated the ambient water confined in a supramolecular tetrahedral assembly, and determined that a dynamically distinct network of 9 ± 1 water molecules is present within the nanocavity of the host. The low-frequency absorption spectrum and theoretical analysis of the water in the GaL host demonstrate that the structure and dynamics of the encapsulated droplet is distinct from any known phase of water.

Heterogeneous Supramolecular Catalysis through Immobilization of Anionic ML Assemblies on Cationic Polymers.

Although most of the currently developed supramolecular catalysts that emulate enzymatic reactivity with unique selectivity and activity through specific host-guest interactions work under homogeneous conditions, enzymes in nature can operate under heterogeneous conditions as membrane-bound enzymes. In order to develop such a heterogeneous system, an immobilized chiral supramolecular cluster Ga () was introduced into cross-linked polymers with cationic functionalities. These heterogeneous supramolecular catalysts were used in aza-Prins and aza-Cope reactions and successfully applied to continuous-flow reactions.

Electronic Structures of Rhenium(II) β-Diketiminates Probed by EPR Spectroscopy: Direct Comparison of an Acceptor-Free Complex to Its Dinitrogen, Isocyanide, and Carbon Monoxide Adducts.

Electron paramagnetic resonance (EPR) studies of the rhenium(II) complex Re(η-Cp)(BDI) (; BDI = ,'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) have revealed that this species reversibly binds N in solution: flash frozen toluene solutions of disclose entirely different EPR spectra at 10 K when prepared under N versus Ar atmospheres. This observation was additionally verified by the synthesis of stable CO and 2,6-xylylisocyanide (XylNC) adducts of , which display EPR features akin to those observed in the putative N complex. While we found that displays an extremely large value of 3.

Diverse Reactivity of a Rhenium(V) Oxo Imido Complex: [2 + 2] Cycloadditions, Chalcogen Metathesis, Oxygen Atom Transfer, and Protic and Hydridic 1,2-Additions.

We present a wide range of reactivity studies focused on the rhenium(V) oxo imido complex (DippN)(O)Re(BDI) (, Dipp = 2,6-diisopropylphenyl and BDI = ,'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate). This complex, which was previously shown to possess a highly polarized Re oxo moiety, has proven to be a potent nucleophile and a valuable precursor to a variety of rare structural motifs in rhenium coordination complexes. For example, the Re oxo moiety of undergoes [2 + 2] cycloadditions with carbodiimides, isocyanates, carbon dioxide, and isothiocyanates at room temperature.

Facile Activation of Triarylboranes by Rhenium(V) Oxo Imido Complexes.

We report the synthesis and reactivity studies of a pair of rhenium(V) oxo imido complexes. Oxidation of the rhenium(III) terminal oxo ORe(η-DHF)(BDI) (DHF = dihydrofulvalene, BDI = ,'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) with organic azides R-N (R = Bu, 2,6-diisopropylphenyl) yields the title complexes. Computational studies confirm that the rhenium oxo moieties of these complexes are polarized and correspondingly nucleophilic, owing to the preferential π bonding of the imido ligand to the Re center.

Electron acceptors promote proton-hydride tautomerism in low valent rhenium β-diketiminates.

We report a series of β-diketiminate (BDI) complexes in which tautomeric rhenium(iii) hydride and rhenium(i) protio-BDI species readily interconvert between the solid and solution states. This phenomenon, which is only operative when an electron-accepting ligand is bound to rhenium, was investigated by a combination of experimental and computational methods.