Binding, Release and Functionalization of Intact Pnictogen Tetrahedra Coordinated to Dicopper Complexes.

The bridging MeCN ligand in the dicopper(I) complexes [(DPFN)Cu2 (µ,η1 : η1 -MeCN)][X]2 (X=weakly coordinating anion, NTf2 (1 a), FAl[OC6 F10 (C6 F5 )]3 (1 b), Al[OC(CF3 )3 ]4 (1 c)) was replaced by white phosphorus (P4 ) or yellow arsenic (As4 ) to yield [(DPFN)Cu2 (µ,η2 : η2 -E4 )][X]2 (E=P (2 a-c), As (3 a-c)). The molecular structures in the solid state reveal novel coordination modes for E4 tetrahedra bonded to coinage metal ions. Experimental data and quantum chemical computations provide information concerning perturbations to the bonding in coordinated E4 tetrahedra. Reactions with N-heterocyclic carbenes (NHCs) led to replacement of the E4 tetrahedra with release of P4 or As4 and formation of [(DPFN)Cu2 (µ,η1 : η1 -Me NHC)][X]2 (4 a,b) or to an opening of one E-E bond leading to an unusual E4 butterfly structural motif in [(DPFN)Cu2 (µ,η1 : η1 -E4 Dipp NHC)][X]2 (E=P (5 a,b), E=As (6)). With a cyclic alkyl amino carbene (Et CAAC), cleavage of two As-As bonds was observed to give two isomers of [(DPFN)Cu2 (µ,η2 : η2 -As4 Et CAAC)][X]2 (7 a,b) with an unusual As4 -triangle+1 unit.

A Dicopper Nitrenoid by Oxidation of a CuICuI Core: Synthesis, Electronic Structure, and Reactivity.

A dicopper nitrenoid complex was prepared by formal oxidative addition of the nitrenoid fragment to a dicopper(I) center by reaction with the iminoiodinane PhINTs (Ts = tosylate). This nitrenoid complex, (DPFN)Cu2(µ-NTs)[NTf2]2 (DPFN = 2,7-bis(fluorodi(2-pyridyl)methyl)-1,8-naphthyridine), is a powerful H atom abstractor that reacts with a range of strong C-H bonds to form a mixed-valence Cu(I)/Cu(II) µ-NHTs amido complex in the first example of a clean H atom transfer to a dicopper nitrenoid core. In line with this reactivity, DFT calculations reveal that the nitrenoid is best described as an iminyl (NR radical anion) complex. The nitrenoid was trapped by the addition of water to form a mixed-donor hydroxo/amido dicopper(II) complex, which was independently obtained by reaction of a Cu2(µ-OH)2 complex with an amine through a protonolysis pathway. This mixed-donor complex is an analogue for the proposed intermediate in copper-catalyzed Chan-Evans-Lam coupling, which proceeds via C-X (X = N or O) bond formation. Treatment of the dicopper(II) mixed donor complex with MgPh2(THF)2 resulted in generation of a mixture that includes both phenol and a previously reported dicopper(I) bridging phenyl complex, illustrating that both reduction of dicopper(II) to dicopper(I) and concomitant C-X bond formation are feasible.

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

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 µ-hydrocarbyl 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 valence CuICuII complexes with varying degrees of distortion in both the bridging moiety and the dicopper core. Higher oxidation states, e.g., dicopper(II), are easily accessed via oxidation of a dicopper(I) compound with air to give a CuII2(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.

A Dicopper Platform that Stabilizes the Formation of Pentanuclear Coinage Metal Hydride Complexes.

Reduction of a dicopper(II) bis(hydroxide) complex with silanes in the presence of external copper or silver cations results in the formation of multinuclear hydride clusters, which were characterized by a variety of NMR spectroscopic experiments and X-ray crystallography. In particular, the pentanuclear complexes adopt an unusual planar "bow tie" configuration. The copper hydride complexes are efficient catalysts for the dehydrogenation of formic acid to H2 and CO2 .

Titanocene-Mediated Dinitrile Coupling: A Divergent Route to Nitrogen-Containing Polycyclic Aromatic Hydrocarbons.

A general synthetic strategy for the construction of large, nitrogen-containing polycyclic aromatic hydrocarbons (PAHs) is reported. The strategy involves two key steps: (1) a titanocene-mediated reductive cyclization of an oligo(dinitrile) precursor to form a PAH appended with di(aza)titanacyclopentadiene functionality; (2) a divergent titanocene transfer reaction, which allows final-step installation of one or more o-quinone, diazole, or pyrazine units into the PAH framework. The new methodology enables rational, late-stage control of HOMO and LUMO energy levels and thus photophysical and electrochemical properties, as revealed by UV/vis and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations. More generally, this contribution presents the first productive use of di(aza)metallacyclopentadiene intermediates in organic synthesis, including the first formal [2 + 2 + 2] reaction to form a pyrazine ring.

Selective Synthesis of a Series of Isostructural MII CuI Heterobimetallic Complexes Spontaneously Assembled by an Unsymmetrical Naphthyridine-Based Ligand.

Metal-metal cooperation is integral to the function of many enzymes and materials, and model complexes hold enormous potential for providing insights into the capabilities of analogous multimetallic cores. However, the selective synthesis of heterobimetallic complexes still presents a significant challenge, especially for systems that hold the metals in close proximity and feature open or reactive coordination sites for both metals. To address this issue, a rigid, naphthyridine-based dinucleating ligand featuring distinct binding environments was synthesized. This ligand enables the selective synthesis of a series of MII CuI bimetallic complexes (M=Mn, Fe, Co, Ni, Cu, Zn), in which each metal center exclusively occupies its preferred binding pocket, from simple chloride salts. The precision of this selectivity is evident from cyclic voltammetry, ESI-MS and anomalous X-ray diffraction measurements.

Isomerism and dynamic behavior of bridging phosphaalkynes bound to a dicopper complex.

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu2(µ-η 1 :η 1 -MeCN)DPFN](NTf2)2, was treated with phosphaalkynes (RC[triple bond, length as m-dash]P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare µ-η 2:η 2 binding coordination modes. X-ray crystallography revealed that these unusual "tilted" structures exist in two isomeric forms (R "up" vs. R "sideways"), depending on the steric profile of the phosphaalkyne's alkyl group (R = Me, Ad, or t Bu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and t Bu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution, and NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu2(µ-η 1:η 1-MeCN)DPFN](NTf2)2 with 2-butyne affords [Cu2(µ-η 2:η 2-(MeC[triple bond, length as m-dash]CMe))DPFN](NTf2)2: its similar ligand geometry demonstrates that the tilted µ-η 2:η 2 binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding.