Coordinatively Unsaturated Metallates of Cobalt(II), Nickel(II), and Zinc(II) Guarded by a Rigid and Narrow Void

Both natural enzymatic systems and synthetic porous material catalysts utilize well-defined and uniform channels to dictate reaction selectivities on the basis of size or shape. Mimicry of this design element in homogeneous systems is generally difficult owing to the flexibility inherent in most small molecular species. Herein, we report the synthesis of a tripodal ligand scaffold that orients a narrow and rigid cavity atop accessible metal coordination space. The permanent void is formed through a macrocyclization reaction whereby the 3,5-dihydroxyphenyl arms are covalently linked through methylene bridges. Deprotonative metallation leads to anionic and coordinatively unsaturated complexes of divalent cobalt, nickel, and zinc. An analogous series of trigonal monopyramidal complexes bearing a nonmacrocyclized variant of the tripodal ligand are also reported. Physical characterization of the coordination complexes has been carried out using multiple spectroscopic techniques (NMR, EPR, and UV-vis), cyclic voltammetry, and X-ray diffraction. Complexes of the macrocyclized [L^OCH2O]^3- ligand retain a rigid cavity upon metallation, with this cavity guarding the entrance to the open axial coordination site. Through a combination of spectroscopic and computational studies, it is shown that acetonitrile entry into the void is sterically precluded, disrupting anticipated coordination at the intracavity site.

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.

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.