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

Lewis Acid-Tethered (cAAC)-Copper Complexes: Reactivity for Hydride Transfer and Catalytic CO2 Hydrogenation

We present a series of borane-tethered cyclic (alkyl)(amino)carbene (cAAC)-copper complexes, including a borane-capped Cu(I) hydride. This hydride is unusually hydridic and reacts rapidly with both CO2 and 2,6-dimethylphenol at room temperature. Its reactivity is distinct from variants without a tethered borane, and the underlying principles governing the enhanced hydricity were evaluated experimentally and theoretically. These stoichiometric results were extended to catalytic CO2 hydrogenation, and the borane-tethered (intramolecular) system exhibits ~3-fold enhancement relative to an intermolecular system.

Synthesis and characterization of NiAl-hydride heterometallics: perturbing electron density within Al-H-Ni subunits

Heterometallic hydride complexes are of growing interest due to their potential to contribute to highly active insertion-based catalysis; however, methods to modulate electron density within this class of molecules are underexplored. Addition of ancillary ligands to heterotrimetallic NiAl2H2 species (1) results in the formation of heterobimetallic NiAl-hydride complexes with varying phosphine donors (2-(L)2). Incorporation of sigma donating ancillary ligands of increasing strength led to contractions of the Ni-Al distances correlated to a strengthening of a back donation interaction to the Al-H sigma antibonding orbital, most prominently present in 2-(PMe3)2. Demethylation of the aryl ether from 2-(PMe3)2 provides access to a novel anionic nickel-aluminum complex (3) with a maintained bridged hydride moiety between Ni and Al. Increased negative charge in complex 3 results in an elongation of the Ni-Al interaction. Combined crystallographic, spectroscopic, and computational studies support a 3-center interaction within the Al-H-Ni subunits and were used to map the degree of Ni-H character of the series within the Al-H-Ni bonding continuum.

Cobalt-catalyzed double hydroboration of pyridines

Cobalt(ii) complexes were prepared from a modular phosphinopyridonate platform and applied to the hydroboration of pyridines. The synthetically useful, yet challenging, double hydroboration toward tetrahydropyridine derivatives was successfully performed with high activity and regiocontrol. This new method enabled the direct synthesis of N-heterocyclic allylic boronates from commercial pyridines and pinacolborane (HBpin). One-pot acetylation afforded the bench-stable borylated N-acetyl tetrahydropyridines in good yields. The synthetic utility of this procedure was demonstrated by a gram-scale double hydroboration-acetylation sequence followed by chemical diversification. Mechanistic experiments indicated metal-ligand cooperativity involving ligand-centered C-H activation and the intermediacy of a cobalt(iii) hydride species.

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...

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.