Ynone Co-Coordination at a Nickel Borane Complex: An Assessment of Secondary Coordination Sphere Effects.

Secondary coordination sphere ligand effects can be used to direct or organize small molecule substrates at a metal center. Herein, we assess the bifunctional ambiphilic diphosphine, tri-tert-butylboranyldiphosphinoethane (ttbbpe) and its ability to influence stereoselective substrate coordination, while appended to nickel. This report takes a synthetic/computational approach to test the impacts and limitations associated with ligand-directed substrate coordination using [Ni(ttbbpe)(η2:η2-COD)] (COD = 1,5-cyclooctadiene) and ynones (alkynes having an α-carbonyl group at the propargylic position) as model substrates.

A Blueprint for Secondary Coordination Sphere Editing: Approaches Toward Lewis-Acid Assisted Carbon Dioxide Co-Activation.

Carbon dioxide (CO2 ) is a potent greenhouse gas of environmental concern. Seeking to offer a solution to the "CO2 -problem", the chemistry community has turned a focus toward transition metal complexes which can activate, reduce, and convert CO2 into carbon-based products. The design of such systems involves judicious selection of both metal and accompanying donor ligand; in part, these efforts are motivated by biological metalloenzymes that undertake similar transformations. As a design element, metal-ligand cooperativity, which leverages intramolecular interactions between a transition metal and an adjacent secondary ligand site, has been acknowledged as a vitally important component by the CO2 activation community. These systems offer a "push-pull" style of activation where electron density is chaperoned onto CO2 with an accompanying electrophile, such as a Lewis-acid, playing the role of acceptor. This pairing allows for the stabilization of reactive Cx Hy Oz -containing intermediates and can bias CO2 product selectivity. In the laboratory, chemists can test hypotheses and ideas, enabling rationalization of why a given pairing of transition metal/Lewis-acid leads to selective CO2 reduction outcomes. This Concept identifies literature examples and highlights key design properties, allowing interested contributors to design, create, and implement novel systems for productive transformations of a small molecule (CO2 ) with huge potential impact.

An Investigation of Allyl-Substituted Bis(Diphosphine) Iron Complexes: Towards Precursors for Cooperative CO2 Activation.

In developing homogenous catalysts capable of CO2 activation, interaction with a metal center is often imperative. This work provides primary efforts towards the cooperative activation of CO2 using a Lewis acidic secondary coordination sphere (SCS) and iron via a paired theoretical/experimental approach. Specifically, this study reports efforts towards [Fe(diphosphine)2 (N2 )] as a CO2 -coordinated synthon where diphosphine=1,2-bis(di(3-cyclohexylboranyl)propylphosphino)ethane) (P2 BCy 4 ) or its precursor, 1,2-bis(diallylphosphino)ethane (tape). Initial efforts toward the {Fe(0)-N2 } complex were focused on deprotonation reactions of [Fe(diphosphine)2 (H)(NCCH3 )]+ and reduction of [Fe(tape)2 Cl2 ]. In the latter case, a mixture of intramolecularly π-bonded alkene and associated metallacyclic Fe(II)-H species were produced - heating this mixture provided the hydride as the major product. Notably, the interconversion of this pair counters that of related intermolecular reactions between [Fe(depe)2 ] (depe=1,2-bis(diethylphosphino)ethane) and ethylene, where hydride formation occurs subsequent to π-coordination; this has been probed by theoretical calculations. Finally, reactivity of the metallacyclic {Fe(II)-H} complex with CO2 was probed, resulting in a pair of isomeric ferra(II)lactones.

An Azomethine-H-Based Fluorogenic Sensor for Formic Acid.

Formic acid (FA) is an important C1-containing feedstock that serves as a masked source of dihydrogen gas (H2). To encourage the adoption of cleaner (noncarbonaceous) energy sources, FA detection and sensing is thus of considerable interest. Here, we examine the use of a commercially available dye, azomethine-H (Az-H), for FA sensing. Solution studies confirm that FA quenches both the absorbance and the luminescence properties of Az-H. FA was additionally found to attenuate a known Az-H (E)-to-(Z) conformational change, suggesting an Az-H/FA interaction, possibly through hydrogen bonding; this phenomenon was probed using 1H NMR spectroscopy. Moving toward a solid-state sensor, the Az-H probe was incorporated into a gelatin-based matrix. On exposure to FA, the luminescence of this system was found to increase in a FA-dependent manner, attributed to the formation of stable hydrogen-bonded structures, facilitating a (Z)-to-(E) isomerization via imine protonation, allowing for production of the more luminescent (E)-isomer. This fluorogenic signal was used as a FA sensor with an estimated detection limit of ca. 0.4 ppb FA vapor. This work constitutes an important step toward a highly sensitive FA sensor in both the solution and solid state, opening new space for the detection of organic acids in differing chemical environments.

Cooperative bond activations by a tucked-in iron complex.

Herein, we report the first example of a 'tucked-in' iron diphosphine complex, formed through deprotonation of a Cp*-(CH̲3) (Cp* = C5Me5-) group by n-butyllithium. The reactivity of this complex was demonstrated by activation of organic and metal-containing substates, including CO2, benzaldehyde, Br-AuI-PPh3, B(C6F5)3, and HBCy2 (Cy = cyclohexyl).

Nickel Complexes of Allyl and Vinyldiphenylphosphine.

Monodentate phosphine-ligated nickel compounds, e.g., [Ni(PPh3)4] are relevant as active catalysts across a broad range of reactions. This report expands upon the coordination chemistry of this family, offering the reactivity of allyl- and vinyl-substituted diphenylphosphine (PPh2R) with [Ni(COD)2] (COD = 1,5-cyclooctadiene). These reactions provide three-coordinate dinickelacycles that are intermolecularly tethered through adjacent {Ni}-olefin interactions. The ring conformation of such cycles has been studied in the solid-state and using theoretical calculations. Here, a difference in reaction outcome is linked to the presence of an allyl vs vinyl group, where the former is observed to undergo rearrangement, bringing about challenges in clean product isolation.

Reactions of nickel boranyl compounds with pnictogen-carbon triple bonds.

The catalytic conversion of unsaturated small molecules such as nitriles into reduced products is of interest for the production of fine chemicals. In this vein, metal-ligand cooperativity has been leveraged to promote such reactivity, often conferring stability to bound substrate - a balancing act that may offer activation at the cost of turnover efficiency. This report describes the reactivity of a [(diphosphine)Ni] compound with pnictogen carbon triple bonds (R-C[triple bond, length as m-dash]E; E = N, P), where the diphosphine contains two pendant borane groups. For E = N, cooperative nitrile coordination is observed to afford {Ni}2 complexes displaying B-N interactions, whereas for E = P, B-P interactions are absent. This work additionally outlines a structure-activity relationship that uses nitrile dihydroboration as a model reaction to unveil the effect of SCS stabilization, employing [(diphosphine)Ni] where the diphosphine contains 0, 1, or 2 pendant Lewis acid groups.

Secondary Coordination Sphere Alkylation Promotes Cyclometalation.

Diphosphines have taken on a dominant role as supporting ligands in transition-metal chemistry. Here, we describe complexes of the type [Cp*Fe(diphosphine)(X)] (X = Cl, H) where for diphosphine = 1,2-bis(di-allylphosphino)ethane (tape), a Lewis-acidic secondary coordination sphere (SCS) was installed via allyl group hydroboration using dicyclohexylborane (HBCy2). The resulting chloride complex, [Cp*Fe(P2BCy4)(Cl)] (P2BCy4 = 1,2-bis(di(3-cyclohexylboranyl)propylphosphino)ethane), was treated with n-butyllithium (1-10 equiv), resulting incyclometalation at iron. This reactivity is contrasted with [Cp*Fe(dnppe)(Cl)] (dnppe = 1,2-bis(di-n-propylphosphino)ethane), whereby addition of n-butyllithium provides a mixture of products. Overall, cyclometalation is a common elementary transformation in organometallic chemistry; here we describe how this outcome is accessed in response to Lewis acid SCS incorporation.

Cooperative Nitrile Coordination Using Nickel and a Boron-Containing Secondary Coordination Sphere.

Metal-ligand cooperation has emerged as a versatile tool for substrate activation in chemical reactivity. Herein, we provide the synthesis and characterization of a monoboranyl-containing analogue of the ubiquitous bulky diphosphine ligand, 1,2-bis(di-tert-butylphosphino)ethane, whose reactivity has been examined using nickel. Together, the pairing of nickel and boron provides a platform that allows for the cooperative coordination of organonitriles, giving unusual examples of intermolecularly bound dinickelacycles.

A guide to secondary coordination sphere editing.

This tutorial review showcases recent (2015-2021) work describing ligand construction as it relates to the design of secondary coordination spheres (SCSs). Metalloenzymes, for example, utilize SCSs to stabilize reactive substrates, shuttle small molecules, and alter redox properties, promoting functional activity. In the realm of biomimetic chemistry, specific incorporation of SCS residues (e.g., Brønsted or Lewis acid/bases, crown ethers, redox groups etc.) has been shown to be equally critical to function. This contribution illustrates how fundamental advances in organic and inorganic chemistry have been used for the construction of such SCSs. These imaginative contributions have driven exciting findings in many transformations relevant to clean fuel generation, including small molecule (e.g., H+, N2, CO2, NOx, O2) reduction. In most cases, these reactions occur cooperatively, where both metal and ligand are requisite for substrate activation.

Bis(1-bora-4-phosphorinane) ring closure at Cp*M (M = Fe, Co) complexes.

Bis(1-bora-4-phosphorinane) metal complexes have been synthesized using a Cp*M-protecting (M = FeIICl, CoI, Cp* = C5Me5-) strategy and structurally authenticated by NMR spectroscopy and single crystal X-ray diffraction. Synthesis of these scaffolds is highly sensitive to the identity of vinylphosphine precursor. This approach provides a new method for the generation of saturated P,B-containing main-group ring systems.

Competitive gold/nickel transmetalation.

Transmetalation is a key method for the construction of element-element bonds. Here, we disclose the reactivity of [NiII(Ar)(I)(diphosphine)] compounds with arylgold(I) transmetalating agents, which is directly relevant to cross-coupling catalysis. Both aryl-for-iodide and unexpected aryl-for-aryl transmetalation are witnessed. Despite the strong driving force expected for Au-I bond formation, aryl scrambling can occur during transmetalation and may complicate the outcomes of attempted catalytic cross-coupling reactions.

Lewis Acid-Promoted Oxidative Addition at a [Ni0 (diphosphine)2 ] Complex: The Critical Role of a Secondary Coordination Sphere.

Oxidative addition represents a critical elementary step in myriad catalytic transformations. Here, the importance of thoughtful ligand design cannot be overstated. In this work, we report the intermolecular activation of iodobenzene (PhI) at a coordinatively saturated 18-electron [Ni0 (diphosphine)2 ] complex bearing a Lewis acidic secondary coordination sphere. Whereas alkyl-substituted diphosphine complexes of Group 10 are known to be unreactive in such reactions, we show that [Ni0 (P2 BCy 4 )2 ] (P2 BCy 4 =1,2-bis(di(3-dicyclohexylboraneyl)-propylphosphino)ethane) is competent for room-temperature PhI cleavage to give [NiII (P2 BCy 4 )(Ph)(I)]. This difference in oxidative addition reactivity has been scrutinized computationally - an outcome that is borne out in ring-opening to provide the reactive precursor - for [Ni0 (P2 BCy 4 )2 ], a "boron-trapped" 16-electron κ1 -diphosphine Ni(0) complex. Moreover, formation of [NiII (P2 BCy 4 )(Ph)(I)] is inherent to the P2 BCy 4 secondary coordination sphere: treatment of the Lewis adduct, [Ni0 (P2 BCy 4 )2 (DMAP)8 ] with PhI provides [NiII (P2 BCy 4 )2 (DMAP)8 (I)]I via iodine-atom abstraction and not a [NiII (Ph)(I)(diphosphine)] compound - an unusual secondary sphere effect. Finally, the reactivity of [Ni0 (P2 BCy 4 )2 ] with 4-iodopyridine was surveyed, which resulted in a pyridyl-borane linked oligomer. The implications of these outcomes are discussed in the context of designing strongly donating, and yet labile diphosphine ligands for use in a critical bond activation step relevant to catalysis.

Preparation of a borane-appended Co(III) hydride: evidence for metal-ligand cooperativity in O-H bond activation.

Cobalt hydrides are known to mediate a number of important chemical transformations including proton (H+), hydride (H-), and hydrogen-atom (H˙) transfer. Central to the tunability of such frameworks is judicious ligand design, which offers the flexibility to alter fundamental properties relevant to reactivity. Herein, we report the preparation of one such cobalt(III) hydride: [Cp*CoIII(P2BCy4)(H)]BPh4 (Cp* = C5Me5-, P2BCy4 = 1,2-bis(di(3-dicyclohexylborane)propylphosphino)ethane) that is encircled by a boron-based Lewis-acidic secondary coordination sphere. The structure of this species is supported by synchrotron-radiation crystallography, evidencing a terminal Co(III) hydride with four sp2-hybridized boranes that invite Lewis base coordination. To this end, electrochemical reactivity studies performed using [Cp*CoIII(P2BCy4)Cl]+ or an "all-akyl" model, [Cp*CoIII(dnppe)Cl]+ (dnppe = 1,2-bis(di-n-propylphosphino)ethane) with benzoic or 4-pyridylbenzoic acid show divergent responses for protonation of electrochemically-generated Co(I) to give a Co(III) hydride. For [Cp*CoIII(P2BCy4)Cl]+, this process is complex, not only involving protonation, but also engagement of the pendant borane moieties in Lewis acid/base interactions. For protonation by benzoic acid, for example, borane-benzoate contacts are substantiated by variable temperature NMR spectroscopic measurements and theoretical calculations, pointing to a cooperative Co-H/B-O bond forming process. These data are discussed in the context of designing new molecular catalysts for ligand-assisted hydrogen evolution reactivity.

Octaboraneyl [Ni(H)(diphosphine)2]+ Complexes: Exploiting Phosphine Ligand Lability for Hydride Transfer to an [NAD]+ Model.

Ligand design represents a central tenet of synthetic chemistry, wherein simple modification can lead to major differences in reactivity. Herein, we describe the preparation of two bis(diphosphino)nickel(II) hydride complexes that contain eight pendant boranes in their secondary coordination sphere, [Ni(H)(P2BR4)2]+ (R = Cy or Mes; Mes = 2,4,6-trimethylphenyl). Divergent reactivity of the cyclohexyl analogue toward the [NAD]+ model, 3-acetyl-N-benzylpyridinium bromide ([BNAcP]Br), is underscored. While [Ni(H)(P2BCy4)2]+ undergoes rapid hydride transfer, the related species [Ni(H)(dnppe)2]+ [dnppe = 1,2-bis(di-n-propylphosphino)ethane] and adduct [Ni(H)(P2BCy4)2(DMAP)8]+ (DMAP = 4-N,N-dimethylaminopyridine) exhibit no such reactivity. This borane-appended nickel(II) hydride distinguishes itself from its "all-alkyl" cousins and provides future opportunities for the design of [Ni(H)(diphosphine)2]+ reagents for hydride transfer.

Generating Potent C-H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII-H Complex Demonstrates a Promising Strategy.

Highly reactive organometallic species that mediate reductive proton-coupled electron transfer (PCET) reactions are an exciting area for development in catalysis, where a key objective focuses on tuning the reactivity of such species. This work pursues ligand-induced activation of a stable organometallic complex toward PCET reactivity. This is studied via the conversion of a prototypical Cp*FeIII-H species, [FeIII(η5-Cp*)(dppe)H]+ (Cp* = C5Me5-, dppe = 1,2-bis(diphenylphosphino)ethane), to a highly reactive, S = 1/2 ring-protonated endo-Cp*H-Fe relative, triggered by the addition of CO. Our assignment of the latter ring-protonated species contrasts with its previous reported formulation, which instead assigned it as a hypervalent 19-electron hydride, [FeIII(η5-Cp*)(dppe)(CO)H]+. Herein, pulse EPR spectroscopy (1,2H HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [FeI(endo-η4-Cp*H)(dppe)(CO)]+. A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe0(exo-η4-Cp*H)(dppe)(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFEC-H ≈ 29 kcal mol-1 and 25 kcal mol-1, respectively) cf. BDFEFe-H of 56 kcal mol-1 for [FeIII(η5-Cp*)(dppe)H]+. These weak C-H bonds are shown to undergo proton-coupled electron transfer (PCET) to azobenzene to generate diphenylhydrazine and the corresponding closed-shell [FeII(η5-Cp*)(dppe)CO]+ byproduct.

Platinum complexes of a boron-rich diphosphine ligand.

Herein, we describe the preparation, characterization, and reactivity of two PtII bis-hydrocarbyl complexes containing the 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P2BCy4) ligand. These scaffolds are readily accessed from four-fold hydroboration of 1,2-bis(diallylphosphino)ethane PtII precursors. The electrophilcity of such frameworks is showcased by facile coordination of the strong Lewis base, 4-N,N-dimethylaminopyridine (DMAP). Thermolysis reactions of [Pt(P2BCy4)(R)2] (R = CH3 or Ph) show enhanced (and divergent) reactivity when compared to their "all-alkyl" diphosphine counterparts, implicating involvement of the pendant borane groups. This behaviour is attenuated by protection of these units with DMAP.

Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes.

Nitrogen fixation, the six-electron/six-proton reduction of N2, to give NH3, is one of the most challenging and important chemical transformations. Notwithstanding the barriers associated with this reaction, significant progress has been made in developing molecular complexes that reduce N2 into its bioavailable form, NH3. This progress is driven by the dual aims of better understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this review, we highlight both mechanistic understanding of nitrogen fixation that has been developed, as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen fixation increasingly appealing. We begin with a historical discussion of N2 functionalization chemistry that traverses a timeline of events leading up to the discovery of the first bona fide molecular catalyst system and follow with a comprehensive overview of d-block compounds that have been targeted as catalysts up to and including 2019. We end with a summary of lessons learned from this significant research effort and last offer a discussion of key remaining challenges in the field.

Octaboraneyl Complexes of Nickel: Monomers for Redox-Active Coordination Polymers.

Herein, we establish the preparation, characterization, and reactivity of a new diphosphine ligand, 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P2 BCy 4 ), a scaffold that contains four pendant boranes. An entryway into the coordination chemistry of P2 BCy 4 is established by using nickel, providing the octaboraneyl complex [Ni(P2 BCy 4 )2 ]-this species contains a boron-rich secondary coordination sphere that reacts readily with Lewis bases. In the case of 4,4'-bipyridine, an air-sensitive coordination polymer is obtained. Characterization of this material by solid-state NMR and EPR spectroscopy reveals the presence of a charge-transfer polymer, which forms as a function of intramolecular Ni→4,4'-bpy electron transfer (ET), providing an array of oxidized nickel sites and reduced 4,4'-bpy radical anion sites. Notably, the related intermolecular reaction between the model fragments [Ni(dnppe)2 ] (dnppe=1,2-bis(di-n-propylphosphino)ethane) and a bis(boraneyl)-protected 4,4'-bpy, provides no ET. Overall, the P2 BCy 4 fragment provides a unique opportunity for Lewis base activation, in one case allowing for the facile construction of monomers for incorporation into redox-active macromolecules.

Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer.

We report the characterization of an S= 1/2 iron π-complex, [Fe(η6 -IndH)(depe)]+ (Ind=Indenide (C9 H7- ), depe=1,2-bis(diethylphosphino)ethane), which results via C-H elimination from a transient FeIII hydride, [Fe(η3 :η2 -Ind)(depe)H]+ . Owing to weak M-H/C-H bonds, these species appear to undergo proton-coupled electron transfer (PCET) to release H2 through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open-shell π-arene complex to have a BDFEC-H value of ≈50 kcal mol-1 , roughly equal to the BDFEFe-H of its FeIII -H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η5 -Cp/Cp*) compounds, for which terminal FeIII -H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring- versus a metal-bound H-atom by way of continuous-wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.