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.

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.

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.

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.

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.

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.

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.

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.

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.

1,3-N,O-Complexes of late transition metals. Ligands with flexible bonding modes and reaction profiles.

1,3-N,O-Chelating ligands are ubiquitous in nature owing to their occurrence as α-chiral amino acids in metalloproteins. These structural units also display diverse coordination modes, which lend themselves to applications in catalysis as well as novel fundamental stoichiometric reactivity, including the activation of inert bonds. This review comments on recent developments in N,O-ligated late transition metal complexes with an emphasis on preparation, characterization, and reactivity.

Catalytic Functionalization of Styrenyl Epoxides via 2-Nickela(II)oxetanes.

Low-valent nickel is shown to preferentially isomerize mono- or disubstituted epoxides into their corresponding aldehydes. Experiments with tetrasubstituted epoxides demonstrate that these reactions proceed via reactive 2-nickelaoxetane intermediates, and that the oxidative addition step likely occurs with retention of configuration. The monosubstituted aldehyde isomerization products were found to rapidly react with HBpin to form boronate esters. These hydroboration reactions could be performed catalytically.

Toward anti-Markovnikov 1-Alkyne O-Phosphoramidation: Exploiting Metal-Ligand Cooperativity in a 1,3-N,O-Chelated Cp*Ir(III) Complex.

Metal-ligand cooperation between iridium(III) and a 1,3-N,O-chelating phosphoramidate ligand has been used to develop a protocol for the intermolecular O-phosphoramidation of 1-alkynes. This selective C-O bond-forming reaction differs from that of standard amidation reactions, highlighting the ability to control N- or O-functionalization based on judicious choice of N,O-chelating ligand and metal center. Advances toward the development of catalytic anti-Markovnikov O-phosphoramidation using iridium(III), including characterization of rare reactive intermediates that invoke 1,3-bidentate donor ligand hemilability, are disclosed.

Phosphoramidate-Supported Cp*Ir(III) Aminoborane H2 B=NR2 Complexes: Synthesis, Structure, and Solution Dynamics.

Reaction of aminoboranes H2 B=NR2 (R=iPr or Cy) with the cationic Cp*Ir(III) phosphoramidate complex [IrCp*{κ(2) -N,O-Xyl(N)P(O)(OEt)2 }][BAr(F) 4 ] generates the aminoborane complexes [IrCp*(H){κ(1) -N-η(2) -HB-Xyl(N)P(OBHNR2 )(OEt)2 }][BAr(F) 4 ] (R=iPr or Cy) in which coordination of a P=O bond with boron weakens the B=N multiple bond. For these complexes, solution- and solid-state, as well as DFT computational techniques, have been employed to substantiate B-N bond rotation of the coordinated aminoborane.

Exploring Regioselective Bond Cleavage and Cross-Coupling Reactions using a Low-Valent Nickel Complex.

Recently, esters have received much attention as transmetalation partners for cross-coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}2(µ-η(2):η(2)-C6H6)] (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form η(2)-carbonyl complexes. In contrast, acetylthioesters underwent rapid Cacyl-S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross-coupling with phenylboronic acid to yield sulfides. While ethyl trifluoroacetate was also found to form an η(2)-carbonyl complex, phenyl esters were found to predominantly undergo Caryl-O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2-naphthyl acetate was also found to undergo clean Caryl-O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive.

Capturing HBCy2: Using N,O-Chelated Complexes of Rhodium(I) and Iridium(I) for Chemoselective Hydroboration.

1,3-N,O-chelated complexes of Rh(I) and Ir(I) cooperatively and reversibly stabilized the B-H bond of HBCy2 to afford six-membered metallaheterocycles (M=Rh (7) or Ir (8)) having a δ-[M]⋅⋅⋅H-B agostic interaction. Treatment of these Shimoi-type borane adducts 7 or 8 with both an aldehyde and an alkene resulted in chemoselective aldehyde hydroboration and reformation of the 1,3-N,O-chelated starting material. The observed chemoselectivity is inverted from that of free HBCy2 , which is selective for alkene hydroboration.

Oxidation State Dependent Coordination Modes: Accessing an Amidate-Supported Nickel(I) δ-bis(C-H) Agostic Complex.

Amidate-supported two- and three-coordinate NiI complexes were synthesized by reduction of the corresponding NiII precursors. A dramatic change in binding mode is observed upon reduction from NiII to NiI . The NiI derivatives include an unprecedented NiI bis(C-H) agostic complex and a two-coordinate NiI complex.

3-Rhoda-1,2-diazacyclopentanes: a series of novel metallacycle complexes derived from C-N functionalization of ethylene.

Rh-containing metallacycles, [(TPA)Rh(III)(κ(2)-(C,N)-CH2CH2(NR)2-]Cl; TPA = N,N,N,N-tris(2-pyridylmethyl)amine have been accessed through treatment of the Rh(I) ethylene complex, [(TPA)Rh(η(2)-CH2CH2)]Cl ([1]Cl) with substituted diazenes. We show this methodology to be tolerant of electron-deficient azo compounds including azo diesters (RCO2N=NCO2R; R = Et [3]Cl, R = iPr [4]Cl, R = tBu [5]Cl, and R = Bn [6]Cl) and a cyclic azo diamide: 4-phenyl-1,2,4-triazole-3,5-dione (PTAD), [7]Cl. The latter complex features two ortho-fused ring systems and constitutes the first 3-rhoda-1,2-diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N-N coordination followed by insertion of ethylene into a [Rh]-N bond. In terms of reactivity, [3]Cl and [4]Cl successfully undergo ring-opening using p-toluenesulfonic acid, affording the Rh chlorides, [(TPA)Rh(III)(Cl)(κ(1)-(C)-CH2CH2(NCO2R)(NHCO2R)]OTs; [13]OTs and [14]OTs. Deprotection of [5]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end-on coordinated diazene [(TPA)Rh(III)(κ(2)-(C,N)-CH2CH2(NH)2-](+) [16]Cl, a hitherto unreported motif. Treatment of [16]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)Rh(III)(κ(2)-(C,N)-CH2CH2(NAc)2-](+), [17]Cl. Treatment of [1]Cl with AcN=NAc did not give the Rh-N insertion product, but instead the N,O-chelated complex [(TPA)Rh(I)(κ(2)-(O,N)-CH3(CO)(NH)(N=C(CH3)(OCH=CH2))]Cl [23]Cl, presumably through insertion of ethylene into a [Rh]-O bond.

A catalytic collaboration: pairing transition metals and Lewis acids for applications in organic synthesis.

The use of metal catalysts to accelerate an organic transformation has proven indispensable for access to structural motifs having applications across medicinal, polymer, materials chemistry, and more. Most catalytic approaches have cast transition metals in the "leading role"; these players mediate important reactions such as C-C cross coupling and the hydrogenation of unsaturated bonds. These catalysts may require collaboration, featuring Lewis acidic or basic additives to promote a desired reaction outcome. Lewis acids can serve to accelerate reactions by way of substrate stabilization and/or activation, and as such, are valuable in optimizing catalytic transformations. A burgeoning area of chemical research which unifies these concepts has thus sought to develop transition metal complexes having ambiphilic (containing a Lewis basic and acidic unit) ligands. This approach takes advantage of metal-ligand cooperativity to increase the efficiency of a given chemical transformation, leveraging intramolecular interactions between a transition metal and an adjacent secondary ligand site. While this has shown significant potential to facilitate challenging and important transformations, there remains unexplored depth for creativity and future advancement. This Frontier highlights inter- and intramolecular combinations of transition metals and Lewis acids that together, provide a collaborative platform for chemical synthesis.