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

Abstract

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.

Robust dicopper(I) µ-boryl complexes supported by a dinucleating naphthyridine-based ligand

“…Therefore, the Cu−B bond tends to be substantially more polarised towards the B atom. Natural Localised Molecular Orbital (NLMO) 31 analysis of 2 and 4 reveals σ-donation of the boryl ligand to the empty 4s orbitals on the Cu atoms (as previously observed for [(DPFN)Cu2] n+ complexes) 9 with negligible back-donation from the metals to boron. This 3c-2e bonding scenario is illustrated in Figure 3.…”

“…Previous results from this laboratory established 2,7-bis(fluorodi(2-pyridyl)methyl)-1,8-naphthyridine (DPFN) as an effective platform for stabilisation of bimetallic units. Dicopper(I) [(DPFN)Cu2] + complexes, accessed via the convenient starting material [(DPFN)Cu2(µ-NCMe)][NTf2]2 (1), 9 exhibit remarkable bimetallic electrophilicity and have allowed for the study of several elusive reactive intermediates such as a dicopper(I,II) nitrenoid, 10 pentanuclear metal hydrides, 11 and an unprecedented dicopper(I) bridging triazolide (Scheme 2). [12][13][14] Thus, [(DPFN)Cu2] n+ seemed a promising framework for the synthesis and stabilisation of dicopper boryl units.…”

Selective Anion Binding Drives the Formation of AgI8L6 and AgI12L6 Six-Stranded Helicates

“…Building on the discovery that silver(I) assembles with 2formyl-1,8-naphthyridine (1), 52 a tritopic subcomponent, and anionic templates to form a trigonal prism with disilver vertices, 17 we investigated the use of linear ditopic anilines in place of triangular ones. Initial experiments, involving the mixture of benzidine (2) together with 1, various silver salts, and prospective guests in acetonitrile (Figure 1a), gave in all cases an intractable gel (SI Section 8).…”

Dinickel Active Sites Supported by Redox-Active Ligands

“…Our initial efforts to design binucleating redox-active ligands focused on the use of 1,8-naphthyridine heterocycles, which have been used in other ligands to generate well-defined dinuclear complexes . Upon combining i ‑Pr NDI (1 equiv) and Ni­(COD) 2 (2 equiv) in C 6 H 6 , the dark-brown diamagnetic ( i ‑Pr NDI)­Ni 2 (C 6 H 6 ) complex ( 1 ) is formed in quantitative yield .…”