Controlled Regulation of the Nitrile Activation of a Peroxocobalt(III) Complex with Redox-Inactive Lewis Acidic Metals

Abstract

Redox-inactive metal ions play vital roles in biological O2 activation and oxidation reactions of various substrates. Recently, we showed a distinct reactivity of a peroxocobalt­(III) complex bearing a tetradentate macrocyclic ligand, [CoIII(TBDAP)­(O2)]+ (1) (TBDAP = N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane), toward nitriles that afforded a series of hydroximato­cobalt­(III) complexes, [CoIII(TBDAP)­(R–C­(NO)­O)]+ (R = Me (3), Et, and Ph). In this study, we report the effects of redox-inactive metal ions on nitrile activation of 1. In the presence of redox-inactive metal ions such as Zn2+, La3+, Lu3+, and Y3+, the reaction does not form the hydroximato­cobalt­(III) complex but instead gives peroxyimidato­cobalt­(III) complexes, [CoIII(TBDAP)­(R–C­(NH)­O2)]2+ (R = Me (2) and Ph (2 Ph )). These new intermediates were characterized by various physicochemical methods including X-ray diffraction analysis. The rates of the formation of 2 are found to correlate with the Lewis acidity of the additive metal ions. Moreover, complex 2 was readily converted to 3 by the addition of a base. In the presence of Al3+, Sc3+, or H+, 1 is converted to [CoIII(TBDAP)­(O2H)­(MeCN)]2+ (4), and further reaction with nitriles did not occur. These results reveal that the reactivity of the peroxocobalt­(III) complex 1 in nitrile activation can be regulated by the redox-inactive metal ions and their Lewis acidity. DFT calculations show that the redox-inactive metal ions stabilize the peroxo character of end-on Co−η1-O2 intermediate through the charge reorganization from a CoII–superoxo to a CoIII–peroxo intermediate. A complete mechanistic model explaining the role of the Lewis acid is presented.

Modulation of a μ-1,2-Peroxo Dicopper(II) Intermediate by Strong Interaction with Alkali Metal Ions

“…8 As was also mentioned, however, the impact of the redox-active metal ion on the metal-peroxo intermediate remains underexplored, and the mechanism of action is poorly understood. 8 In particular, crucial structural information about these adducts between reactive metal− oxygen species and Lewis acids is scarce.…”

“…In molecular systems, strongly Lewis acidic metal ions such as Zn 2+ or Sc 3+ have been used to, inter alia , stabilize unusual oxocobalt­(IV) complexes, induce valence tautomerism in high-valent oxomanganese porphyrinoids for enhancing their reactivity, or tune the redox reactivity of oxometal intermediates by enabling metal ion-coupled electron transfer (MCET) . Just recently, it was shown that reaction between a peroxocobalt­(III) complex and nitriles leads to completely different products in the absence or presence of M n + (M n + = Zn 2+ , La 3+ , Lu 3+ , or Y 3+ ) . As was also mentioned, however, the impact of the redox-active metal ion on the metal-peroxo intermediate remains underexplored, and the mechanism of action is poorly understood .…”

Assembly of metallo-macrocycles through reductive C–C coupling of alkylnitriles by an Mg–Mg-bonded compound

“…to form the keteniminate moieties (CvCvN), 9,[16][17][18] and so on. [19][20][21] These reactions can be mediated by a variety of metal complexes (mostly mononuclear ones). [4][5][6][7][8][9][10][11][14][15][16][17][18][19][20][21] In recent years, the low-valent metal-metal-bonded compounds have been increasingly employed in small molecule activation.…”

“…[19][20][21] These reactions can be mediated by a variety of metal complexes (mostly mononuclear ones). [4][5][6][7][8][9][10][11][14][15][16][17][18][19][20][21] In recent years, the low-valent metal-metal-bonded compounds have been increasingly employed in small molecule activation. [22][23][24][25] Especially, for the Mg-Mg bond, several reaction types are known toward nitriles (and isonitriles).…”

“…Interestingly, several triangular metallo-macrocyclic complexes, [LMg(μ-{(NuC-C(R)C(CH 2 R)-NH})] 3 [K 3 (Solv) 6 ] (2-4; Scheme 1), have been assembled upon a series of reactions, reductive dehydrogenation of α-H, subsequent C (α-C) -C (CN) coupling, and accom-panying 1,3-H shift (to form amido-ene-nitrile moieties that bridge the [LM] units; vide infra). It is noticeable that such a deprotonation-coupled C-C bond formation of nitriles is known only for the small or relatively active acetonitrile and benzylnitrile, 19,[35][36][37][38] while the longer alkyl-chain analogues have not been reported. Thus the formation of products 2-4 (Scheme 1) not only represents a promising approach for constructing metallo-macrocycles but also is a rare example for activation of alkyl nitriles.…”