The thermochemistry of cation−macrocycle interactions in nickel pincer complexes bearing a hemilabile aza-15-crown-5 or aza-18-crown-6 macrocycle is investigated and applied to cation-controlled reversible ligand binding. Cation− crown interactions were examined in a noncoordinating, low polarity solvent (dichloromethane) and a coordinating, polar solvent (acetonitrile). Structural studies provide solid-state information on cation−crown interactions, whereas binding affinity studies in solution provide quantitative thermodynamic information. The different hemilabile ligand coordination modes have vastly different cation binding affinities, with the tridentate pincer coordination mode binding cations more than 100 000 times more strongly than the tetradentate coordination mode with a crown ether oxygen donating to nickel. Dichloromethane enforces strong cation−crown interactions without disrupting the hemilabile ether ligand, whereas acetonitrile disrupts hemilability by displacing the ethers from the nickel center and supports weaker cation−crown interactions. In dichloromethane, lithium binding provides at least 7 kcal•mol −1 of stabilization to assist in ligand binding, and the extent of stabilization can be tuned by the choice of cation. The newfound thermodynamic insight guided the development of in situ switchable ligand binding and release at nickel using cations. The nickel complex binds pentafluorophenylnitrile only upon addition of Li + salts in dichloromethane, and the nitrile ligand is readily released upon sequestration of the Li + with 12-crown-4.

Nickel catalysts supported by diethylamine-or aza-crown ether-containing aminophosphinite (NCOP) pincer ligands catalyze the insertion of benzaldehyde into a C−H bond of acetonitrile. The catalytic activity of neutral (NCOP)Ni(O t Bu) and cationic [(NCOP)Ni(NCCH 3 )] + are starkly different. The neutral tert-butoxide precatalysts are active without any added base and give good yields of product after 24 h, while the cationic precatalysts require a base cocatalyst and still operate much more slowly (120 h in typical runs). A series of in situ spectroscopic studies identified several intermediates, including a nickel cyanoalkoxide complex that was observed in all of the reactions regardless of the choice of precatalyst. Reaction monitoring also revealed that the neutral tert-butoxide precatalysts decompose to form the cationic acetonitrile complex during catalysis; this deactivation involves alkoxide abstraction and can be hastened by the addition of lithium salts. While the deactivated cationic species is inactive under standard base-free conditions, catalysis can be reinitiated by the addition of catalytic amounts of base. ■ INTRODUCTIONOrganonickel complexes orchestrate an array of molecular elaborations, 1−8 but nickel catalysts have lagged behind precious metals in the functionalization of C−H bonds. 6−8 The choice of substrate can be a deciding factor in nickelmediated C−H functionalization: substrates containing a directing group can hold a C−H bond in close proximity to the metal center, 9−13 or substrates containing relatively acidic C−H bonds can facilitate pathways that rely on deprotonation. 14,15 Acetonitrile is an attractive substrate for nickel-catalyzed C− C bond-forming reactions because the sp 3 C−H bond is relatively acidic (pK a = 25 in H 2 O) 16 and the products would contain a valuable nitrile functionality. Direct deprotonation of acetonitrile is often impractical or not tolerated by nearby functional groups, motivating the development of catalytic methods that avoid stoichiometric amounts of strong base.This article describes the insertion of benzaldehyde into a C−H bond of acetonitrile (Scheme 1) catalyzed by asymmetric Ni aminophosphinite (NCOP) pincer complexes with little or no base added. Only two nickel catalysts are found among the handful 17−22 of late-transition-metal catalysts known to carry out this cyanomethylation reaction, 23,24 which produces synthetically useful β-hydroxy nitriles. 25−27 In 2005, Ozerov and co-workers reported a cationic Ni catalyst that required stoichiometric base to couple nitriles and aldehydes. 23 In 2013, Guan and co-workers isolated a C-bound cyanomethyl species (Ni−CH 2 CN) that proved to be a highly active catalyst (turnover number (TON) of up to 82 000) for C−H insertion without added base. 24 The two Ni catalysts were proposed to operate through quite different mechanisms, despite the fact that both are supported by pincer ligands. The cationic, base-promoted catalyst was proposed to react through the Lewis acid mechanism shown in Scheme 2A: nitril...

Outfitting an aza-crown ether with an organotransition-metal pendant provides a mechanism for tuning its supramolecular properties. The binding affinity can be tuned by more than 2 orders of magnitude by changing the identity of the transition metal-center, altering the overall charge of the complex, or engaging in organometallic ligand substitution reactions. High Li + selectivity (up to 29-fold higher affinity in comparison to Na + ), proton-responsive behavior, and ion pair (ditopic) binding capabilities are observed in the metallacrown ethers.

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