Unexpected reactivity of a PONNOP 'expanded pincer' ligand

Diborane Reductions of CO2 and CS2 Mediated by Dicopper μ-Boryl Complexes of a Robust Bis(phosphino)-1,8-naphthyridine Ligand

A dinucleating 1,8-naphthyridine ligand featuring fluorene-9,9-diyl-linked phosphino side arms (PNNPFlu) was synthesized and used to obtain the cationic dicopper complexes 2, [(PNNPFlu)Cu2(μ-Ph)][NTf2]; [NTf2] = bis(trifluoromethane)sulfonimide, 6, [(PNNPFlu)Cu2(μ-CCPh)][NTf2], and 3, [(PNNPFlu)Cu2(μ-OtBu)][NTf2]. Complex 3 reacted with diboranes to afford dicopper μ-boryl species (4, with μ-Bcat; cat = catecholate and 5, with μ-Bpin; pin = pinacolate) that are more reactive in C(sp)-H bond activations and toward activations of CO2 and CS2, compared to dicopper μ-boryl complexes supported by a 1,8-naphthyridine-based ligand with di(pyridyl) side arms. The solid-state structures and DFT analysis indicate that the higher reactivities of 4 and 5 relate to changes in the coordination sphere of copper, rather than to perturbations on the Cu-B bonding interactions. Addition of xylyl isocyanide (CNXyl) to 4 gave 7, [(PNNPFlu)Cu2(μ-Bcat)(CNXyl)][NTf2], demonstrating that the lower coordination number at copper is chemically significant. Reactions of 4 and 5 with CO2 yielded the corresponding dicopper borate complexes (8, [(PNNPFlu)Cu2(μ-OBcat)][NTf2]; 9, [(PNNPFlu)Cu2(μ-OBpin)][NTf2]), with 4 demonstrating catalytic reduction in the presence of excess diborane. Related reactions of 4 and 5 with CS2 provided insertion products 10, {[(PNNPFlu)Cu2]2[μ-S2C(Bcat)2]}[NTf2]2, and 11, [(PNNPFlu)Cu2(μ,κ2-S2CBpin)][NTf2], respectively. These products feature Cu-S-C-B linkages analogous to those of proposed CO2 insertion intermediate.

A one-pot reduction route to bimetallic manganese 1,8-naphthyridine complexes

Reaction of the dinucleating ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (MeL) with the MnI and MnII precursors MnBr(CO)5 and MnCl2 resulted in the formation of the monometallic complexes [MnBr(CO)3(MeL)] (1) and [MnCl2(MeL)] (3). In both cases, formation of bimetallic manganese complexes could be achieved by reduction with KC8, yielding the carbonyl-bridged complex [Mn2(CO)6(MeL)] (2) and the helicate complex [Mn2(MeL)2] (4), respectively. EPR results demonstrate that 4 represents a novel, weakly antiferromagnetically coupled homovalent dimer (J = -0.85 cm-1). The two formally Mn0 ions are both high spin (S = 3/2) and exhibit a zero-field splitting of ≈1 cm-1, suggesting reduction of the complex is substantially ligand centered, and may be better described as a MnII complex coupled to two open shell singlet ligands [MnII2(MeL2-)2]. X-ray crystallography, UV-Vis spectroscopy and DFT analysis support this finding.

Utilising a Proton-Responsive 1,8-Naphthyridine Ligand for the Synthesis of Bimetallic Palladium and Platinum Complexes

We present four proton-responsive palladium and platinum complexes, [MCl2 (R PONNHO)] (M=Pd, Pt; R=i Pr, t Bu) synthesised by complexation of PdCl2 or PtCl2 (COD) with the 1,8-naphthyridine ligand R PONNHO. Deprotonation of [MCl2 (tBu PONNHO)] switches ligand coordination from mono- to dinucleating, offering a synthetic pathway to bimetallic PdII and PtII complexes [M2 Cl2 (tBu PONNO)2 ]. Two-electron reduction gives planar MI -MI complexes [M2 (tBu PONNO)2 ] (M=Pd, Pt) containing a metal-metal bond. In contrast to the related nickel system that forms a metallophosphorane [Ni2 (tBu PONNOPONNO)], an unusual phosphinite binding mode is observed in [M2 (tBu PONNO)2 ] containing close phosphinite-naphthyridinone P⋅⋅⋅O interactions, which is investigated spectroscopically, crystallographically and computationally. The presented proton-responsive and structurally-responsive R PONNHO and bimetallic R PONNO complexes offer a novel platform for future explorations of metal-ligand and metal-metal cooperativity with palladium and platinum.

Oxidative Addition and β-Hydride Elimination by a Macrocyclic Dinickel Complex: Observing Bimetallic Elementary Reactions

The dinickel(I) complex Ni2 (tBu PONNOPONNO), featuring a planar macrocyclic diphosphoranide ligand tBu PONNOPONNO, offers a unique architectural platform for observing bimetallic elementary reactions. Oxidative addition reactions of alkyl halides produce dinickel(II) complexes of the type Ni2 (μ-R)(μ-X)(tBu PONNOPONNO). However, when R=Et β-hydride elimination is observed to form a dinickel monohydride, with the rate dependent on the nature of X. DFT studies suggest a new mechanism for bimetallic β-hydride elimination, where the rate dependence arises from the steric pressure imposed by the X group on the opposing trans face of the dinickel macrocycle. This work enhances understanding of bimetallic elementary reactions, particularly β-hydride elimination, which have not been well-explored for dinuclear systems.

Bimetallic Nickel Complexes Supported by a Planar Macrocyclic Diphosphoranide Ligand

Metal-metal cooperativity is emerging as an important strategy in catalysis. This requires appropriate ligand scaffolds that can support two metals in close proximity. Here we report nickel-promoted formation of a dinucleating planar macrocyclic ligand that can support bimetallic dinickel(II) and dinickel(I) complexes. Reaction outcomes can be tuned by variation of the substituents and reaction conditions to favour dinucleating macrocyclic, mononucleating macrocyclic or conventional pincer architectures.

Quantification of the Steric Properties of 1,8-Naphthyridine-Based Ligands in Dinuclear Complexes

Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work, we present an expansion of the buried volume and the G-parameter to quantify the steric properties of 1,8-napthyridine-based dinuclear complexes. Using this methodology, we explored the tunability of the steric properties associated with these ligands and complexes.

Combining metal-metal cooperativity, metal-ligand cooperativity and chemical non-innocence in diiron carbonyl complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)-H bonds by ∼25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

Synthesis of an expanded pincer ligand and its bimetallic coinage metal complexes

An expanded pincer ligand tBu-PONNOP (2,7-bis(di-tert-butylphosphinito)-1,8-naphthyridine) has been synthesised and its coordination to coinage metals has been studied. Bimetallic complexes were produced with metal halide salts of the type [M₂X₂(tBu-PONNOP)] (X = Cl, M = Au, Ag, Cu; X = I, M = Cu) with a varying degree of interaction with the naphthyridyl backbone in the order Au < Ag < Cu. The salts [Ag₂(tBu-PONNOP)₂][BAr^F₄]₂ (Ar^F = 3,5-C₆H₃(CF₃)₂) and [Ag₂(NCMe)₂(tBu-PONNOP)]X₂ (X = BAr^F₄, PF₆) were prepared, which may serve as a source of tBu-PONNOP via transmetallation.

Recent Advances in Chemistry of Unsymmetrical Phosphorus-Based Pincer Nickel Complexes: From Design to Catalytic Applications

Pincer complexes play an important role in organometallic chemistry; in particular, their use as homogeneous catalysts for organic transformations has increased dramatically in recent years. The high catalytic activity of such bis-cyclometallic complexes is associated with the easy tunability of their properties. Moreover, the phosphorus-based unsymmetrical pincers showed higher catalytic activity than the corresponding symmetrical analogues in several catalytic reactions. However, in modern literature, an increasing interest in the development of catalysts based on non-precious metals is observed. For example, nickel, which is an affordable and sustainable analogue of platinum and palladium, known for its low toxicity, has attracted increasing attention in the catalytic chemistry of transition metals in recent years. Thus, this mini-review is devoted to the recent advances in the chemistry of unsymmetrical phosphorus-based pincer nickel complexes, including the ligand design, the synthesis of nickel complexes and their catalytic applications.

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 Cu^ICu^II 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 Cu^II₂(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.