Electronic structure and reactivity of nickel(i) pincer complexes: their aerobic transformation to peroxo species and site selective C-H oxygenation

• Published in: Chemical science (Cambridge) 2016-01, Vol.7 (6), p.3533-3542
• Main Authors: Rettenmeier, Christoph A, Wadepohl, Hubert, Gade, Lutz H
• Format: Article
• Language: English
• Subjects: Adducts; Autoxidation; Bonding; Decomposition; Electronic structure; Ligands; Methyl alcohol; Transformations
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c5sc04644k

The study is aimed at a deeper understanding of the electronic structure of the T-shaped nickel(i) complex [Lig (iso)Ni] ( ), bearing the iso-PyrrMeBox (bis(oxazolinylmethylidene)pyrrolidinido) pincer ligand, and its CO adduct [Lig (iso)Ni(CO)] ( ) as well as to provide insight into the mechanism of autoxidation of the different nickel peroxo species of this ligand type. CO was found to react reversibly with complex resulting in the corresponding CO adduct . The EPR data as well as the results of DFT modeling revealed significant differences in the electronic structure of and . Reaction of [Lig (iso)Ni] and [Lig (iso)Ni] ( and ) with dioxygen yielded the 1,2-μ-peroxo complexes [Lig(iso)NiO] and which reacted with hydrogen peroxide to give the hydroperoxo complexes [Lig(iso)NiOOH] and . Thermal aerobic decomposition of the peroxo species and in the presence of O led to a C-H activation of the ligand at the benzylic position of the oxazoline ring forming diastereomeric cyclic peroxo complexes and . For the 1,2-μ-peroxo complex the autoxidation of the pincer in the absence of O occurred at the tertiary C-H bond of the Pr-group and led to a selective formation of the terminal hydroxo complex [Lig (iso)NiOH] and the cyclic alkoxy complex in equimolar quantities, while the corresponding cyclic peroxo species was formed along with in the presence of oxygen. Whether or not O-O bond cleavage occurred in the generation of was established upon performing labeling experiments which indicate that the transformation does not involve an initial O-O bond cleaving step. Based on these observations and a series of stoichiometric transformations a tentative proposal for the processes involved in the anaerobic and aerobic decomposition of has been put forward. Finally, the nickel(ii) methyl complex [Lig (iso)NiMe] reacted with O to give the methylperoxo complex [Lig (iso)NiOOMe] which slowly converted to a mixture of near equal amounts of the formato and the hydroxo complexes, [Lig (iso)NiOOCH] and [Lig (iso)NiOH] , along with half an equivalent of methanol. The formato complex itself decomposed at elevated temperatures to CO , dihydrogen as well as the nickel(i) species .