Unexpected Reactivity of a PONNOP ‘Expanded Pincer’ Ligand

Scheerder, Arthur; Lutz, Martin; Broere, Daniël L. J.

doi:10.26434/chemrxiv.12023352

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“…Utilizing a related NDI ligand, Bera and co-workers have recently reported a dicopper(I) complex that is active for catalytic alcohol dehydrogenation. 44 Use of symmetrical diphosphino-substituted naphthyridine ligands has been explored by Broere and co-workers, who obtained dicopper(I) complexes that bind ligands in the bridging position, 86,87 as was observed for the DPFN-supported compounds. In addition, in a manner similar to that the Milstein group explored with related PNP ligands, 88 Broere and co-workers reported deprotonations of the methylene-linked phosphine side-arms, which led to activation of H2 and isolation of a bridging mesityl complex, analogous to our DPFN-based phenyl complex 10.…”

Section: Recent Studies Of Other Naphthyridine-based Systemsmentioning

confidence: 99%

Bimetallics in a Nutshell: Complexes Supported by Chelating Naphthyridine-Based Ligands

et al. 2020

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Conspectus: Bimetallic motifs are a structural feature common to some of the most effective and synthetically useful catalysts known, including in the active sites of many metalloenzymes and on the surfaces of industrially relevant heterogeneous materials. However, the complexity of these systems often hampers detailed studies of their fundamental properties. To glean valuable mechanistic insight into how these catalysts function, this research group has prepared a family of dinucleating 1,8-naphthyridine ligands that bind two first-row transition metals in close proximity, originally designed to help mimic the proposed active site of metal oxide surfaces. Of the various bimetallic combinations examined, dicopper(I) is particularly versatile, as neutral bridging ligands adopt a variety of different binding modes depending on the configuration of frontier orbitals available to interact with the Cu centers. Organodicopper complexes are readily accessible, either through the traditional route of salt metathesis or via the activation of tetraarylborate anions through aryl group abstraction by a dicopper(I) unit. The resulting bridging aryl complexes engage in C-H bond activations, notably with terminal alkynes to afford bridging alkynyl species. The µhydrocarbyl complexes are surprisingly tolerant of water and elevated temperatures. This stability was leveraged to isolate a species that typically represents a fleeting intermediate in Cu-catalyzed azide-alkyne coupling (CuAAC); reaction of a bridging alkynyl complex with an organic azide afforded the first example of a well-defined, symmetrically bridged dicopper triazolide. This complex was shown to be an intermediate during CuAAC, providing support for a proposed bimetallic mechanism. These platforms are not limited to formally low oxidation states; chemical oxidation of the hydrocarbyl complexes cleanly results in formation of mixed-valent Cu I Cu II complexes with varying degrees of distortion in both the bridging moiety and the dicopper core. Higher oxidation states, i.e. dicopper(II), are easily accessed via oxidation of a dicopper(I) compound with air to give a Cu II 2(µ-OH)2 complex. Reduction of this compound with silanes resulted in the unexpected formation of pentametallic copper(I) dihydride clusters or trimetallic monohydride complexes, depending on the nature of the silane. Finally, development of an unsymmetrical naphthyridine ligand with mixed donor side-arms enables selective synthesis of an isostructural series of six heterobimetallic complexes, demonstrating the power of ligand design in the preparation of heterometallic assemblies.

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