Octaboraneyl Complexes of Nickel: Monomers for Redox‐Active Coordination Polymers

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.

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.

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.

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(P₂B^Cy₄)(Cl)] (P₂B^Cy₄ = 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.

Facile Synthesis of Uranium Complexes with a Pendant Borane Lewis Acid and 1,2-Insertion of CO into a U-N Bond

In this contribution, we illustrate uranium complexes bearing a pendant borate (i.e. 1 and 2) or a pendant borane (i.e. 3 and 4) moiety via reaction of the highly strained uranacycle I with various 3-coordinate boranes. Complexes 3 and 4 represent the first examples of uranium complexes with a pendant borane Lewis acid. Moreover, complex 3 was capable of activation of CO, delivering a new CO activation mode, and an abnormal CO 1,2-insertion pathway into a U-N bond. The importance of the pendant borane moiety was confirmed by the controlled experiments.

An Adaptive Rhodium Catalyst to Control the Hydrogenation Network of Nitroarenes

An adaptive catalytic system that provides control over the nitroarene hydrogenation network to prepare a wide range of aniline and hydroxylamine derivatives is presented. This system takes advantage of a delicate interplay between a rhodium(III) center and a Lewis acidic borane introduced in the secondary coordination sphere of the metal. The high chemoselectivity of the catalyst in the presence of various potentially vulnerable functional groups and its readiness to be deployed at a preparative scale illustrate its practicality. Mechanistic studies and density functional theory (DFT) methods were used to shed light on the mode of functioning of the catalyst and elucidate the origin of adaptivity. The competition for interaction with boron between a solvent molecule and a substrate was found crucial for adaptivity. When operating in THF, the reduction network stops at the hydroxylamine platform, whereas the reaction can be directed to the aniline platform in toluene.

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

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, NO_x, 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 = Fe, Co, Cp* = C5Me5-) strategy and structurally authenticated by NMR spectroscopy and single crystal X-ray diffraction. Synthesis of...

Competitive gold/nickel transmetalation

Transmetalation is a key method for the construction of element-element bonds. Here, we disclose the reactivity of [Ni^II(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 B^Cy ₄ )₂ ] (P₂ B^Cy ₄ =1,2-bis(di(3-dicyclohexylboraneyl)-propylphosphino)ethane) is competent for room-temperature PhI cleavage to give [Ni^II (P₂ B^Cy 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 B^Cy ₄ )₂ ], a "boron-trapped" 16-electron κ¹ -diphosphine Ni(0) complex. Moreover, formation of [Ni^II (P₂ B^Cy ₄ )(Ph)(I)] is inherent to the P₂ B^Cy ₄ secondary coordination sphere: treatment of the Lewis adduct, [Ni⁰ (P₂ B^Cy ₄ )₂ (DMAP)₈ ] with PhI provides [Ni^II (P₂ B^Cy ₄ )₂ (DMAP)₈ (I)]I via iodine-atom abstraction and not a [Ni^II (Ph)(I)(diphosphine)] compound - an unusual secondary sphere effect. Finally, the reactivity of [Ni⁰ (P₂ B^Cy 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.

Platinum complexes of a boron-rich diphosphine ligand

Herein, we describe the preparation, characterization, and reactivity of two Pt^II bis-hydrocarbyl complexes containing the 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P₂B^Cy₄) ligand.