Rhodium(III) and iridium(III) complexes of a NHC-based macrocycle : persistent weak agostic interactions and reactions with dihydrogen

The synthesis and characterization of five-coordinate rhodium(III) and iridium(III) 2,2′-biphenyl complexes [M(CNC-12)(biph)][BArF4] (M = Rh (1a), Ir (1b)), featuring the macrocyclic lutidine- and NHC-based pincer ligand CNC-12 are reported. In the solid state these complexes are notable for the adoption of weak ε-agostic interactions that are characterized by M···H–C contacts of ca. 3.0 Å by X-ray crystallography and ν(CH) bands of reduced wavenumber by ATR IR spectroscopy. Remarkably, these interactions persist on dissolution and were observed at room temperature using NMR spectroscopy (CD2Cl2) and solution-phase IR spectroscopy (CCl4). The associated metrics point toward a stronger M···H–C interaction in the iridium congener, and this conclusion is borne out on interrogation of 1 in silico using DFT-based NBO and QTAIM analyses. Reaction of 1 with dihydrogen resulted in hydrogenolysis of the biaryl and formation of fluxional hydride complexes, whose ground state formulations as [Rh(CNC-12)H2][BArF4] (2a″) and [Ir(CNC-12)H2(H2)][BArF4] (2b‴) are proposed on the basis of inversion recovery and variable-temperature NMR experiments, alongside a computational analysis. Reactions of 1 and 2 with carbon monoxide help support their respective structural properties

Divergent stereoisomers of molybdenum carbonyl complexes of NHC-based pincer ligands

The first molybdenum complexes of widely used NHC-based CNC and C^N^C pincer ligands are described, viz. [Mo(L)(CO)3] (L = 2,6-bis(mesityl-imidazolylidene)pyridine ≡ CNC-Mes, 1; α,α’-(diimidazolylidene-dodecamethylene)lutidine ≡ C^N^C-12, 2). These complexes have been thoroughly characterised in solution and the solid-state, revealing different stereochemical preferences of the tridentate ligands depending on the nature of the scaffold. In the case of flexible C^N^C-12 an uncommon fac-coordination geometry is observed, whilst the complex of rigid CNC-Mes adopts the expected mer-configuration. For the combination of donors associated with the ligands, DFT calculations establish preferential fac-coordination, however, within the CNC (ΔΔG = +63.1 kJ·mol-1) and C^N^C (ΔΔG = +20.0 kJ·mol-1) scaffolds this conformation is significantly destabilised relative to the mer-alternative

Oxidative addition of a mechanically entrapped C(sp)–C(sp) bond to a rhodium(I) pincer complex

By use of a macrocyclic phosphinite pincer ligand and bulky substrate substituents, we demonstrate how the mechanical bond can be leveraged to promote the oxidative addition of an interlocked 1,3‐diyne to a rhodium(I) center. The resulting rhodium(III) bis(alkynyl) product can be trapped out by reaction with carbon monoxide or intercepted through irreversible reaction with dihydrogen, resulting in selective hydrogenolysis of the C–C σ‐bond

Rhodium(III) and iridium(III) pincer complexes of a neopentyl-substituted PNP pincer ligand which feature agostic interactions

The synthesis and characterization of five-coordinate rhodium(III) and iridium(III) complexes of the form [M(PNP-Np)(biph)][BArF4] are described, where PNP-Np is the neopentyl-substituted pincer ligand 2,6-(Np2PCH2)2C5H3N (Np = CH2tBu), biph = 2,2′-biphenyl, and ArF = 3,5-(CF3)2C6H3. These complexes are notable for the adoption of δ-agostic interactions in the solid state, as evidenced by X-ray crystallography (50–150 K) and ATR-IR spectroscopy, but are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the 1H NMR time scale (185–308 K). The strength of the agostic interactions is discussed with reference to the known tert-butyl-substituted analogues [M(PNP-tBu)(biph)][BArF4], probed by reaction with carbon monoxide, and quantified computationally through NBO analysis, from which the conclusion is that 3-center–2-electron bonding increases in the order M = Ir > Rh (cf. 1.5× greater perturbation energy) and pincer ligand = PNP-Np > PNP-tBu (cf. 3.3× greater perturbation energy)

Isolation and structural characterisation of rhodium(III) η2–fluoroarene complexes : experimental verification of predicted regioselectivity

The isolation and solid-state characterisation of complexes featuring partially coordinated benzene, fluorobenzene and all three isomers of difluorobenzene are described. Supported by a DFT analysis, this well-defined homologous series demonstrates the preference for η2-coordination of fluoroarenes via the HC[double bond, length as m-dash]CH sites adjacent to a fluorine substituent

Synthesis and organometallic chemistry of rhodium and iridium complexes of macrocyclic PCP and POCOP pincer ligands

Conferring high thermal stability and supporting a broad range of metal-based reactivity, mer-tridentate “pincer” ligands have become ubiquitous in contemporary organometallic chemistry and transformed homogeneous catalysis. Phosphine-based systems bearing a central aryl donor, derived from meta-xylene (PCP) or resorcinol (POCOP), are archetypical examples and complexes of rhodium and iridium have in particular found successful applications in inert bond activation reactions, with the catalytic dehydrogenation of alkanes most remarkable. Motived by the desire to further our understanding of these processes, the objective of this project was to explore the organometallic chemistry of macrocyclic PCP and POCOP pincer complexes featuring mechanically interlocked hydrocarbon substrates: [2]rotaxane and [2]catenanes. The interwoven topology of these systems was chosen as a means to circumvent problems associated with weak metal hydrocarbon interactions and provide a well-defined platform for interrogating their subsequent activation. The multistep synthesis of macrocyclic POCOP-14’ and PCP-14’ proligands is reported herein, using racemic or asymmetric procedures, respectively. These proligands can be readily metalated and homologous series of MI(CO) and MIIICl2(CO) derivatives (M = Rh, Ir) were isolated and fully characterised in solution and the solid state. The latter were critically evaluated as precursors for the construction of interlocked 1,3-diyne derivatives by Grignard-mediated alkynylation, decarbonylation, and C(sp)–C(sp) bond reductive elimination. Using this strategy, [Rh(POCOP-14)(Ar’C4Ar’)] (Rh-25o, Ar’ = 3,5-tBu2C6H3) was most notably isolated and its dynamic behaviour and reactivity comprehensively studied. This interlocked complex remarkably displays reversible C(sp)–C(sp) bond activation, but under carefully chosen conditions the axle can be reduced all the way to the corresponding 1,4-diarylbut-1-ene

Rhodium(I) pincer complexes of nitrous oxide

The synthesis of two well‐defined rhodium(I) complexes of nitrous oxide (N2O) is reported. These normally elusive adducts are stable in the solid state and persist in solution at ambient temperature, enabling comprehensive structural interrogation by 15N NMR and IR spectroscopy, and single‐crystal X‐ray diffraction. These methods evidence coordination of N2O through the terminal nitrogen atom in a linear fashion and are supplemented by a computational energy decomposition analysis, which provides further insights into the nature of the Rh–N2O interaction. The synthetic exploitation of nitrous oxide (N2O) is an enduring challenge that draws topical interest as a means to remediate the detrimental impact emission of this kinetically stable gas on the environment.1 Whilst the application of homogenous transition‐metal complexes is an attractive prospect, the underpinning inorganic chemistry is conspicuously under‐developed.2 Indeed, the number of discrete transition‐metal complexes of N2O is currently limited to a handful of examples (A–D), of which only two have been structurally characterised in the solid state using X‐ray diffraction (Figure 1).3, 4-7 This paucity is attributed to the extremely poor ligand properties of N2O, conferred by a low dipole moment, weak σ‐donor and π‐acceptor characteristics, and the propensity of these adducts for subsequent N−N or N−O bond cleavage.

Safe Generation and Direct Use of Chlorine Azide in Flow Chemistry – 1,2‐Azidochlorination of Olefins and Access to Triazoles

A safe, fast procedure for the formation of chlorine azide and its trapping by 1,2‐addition reaction on olefins is described. ClN3 was generated in situ from NaN3 and NaOCl in the presence of acetic acid, hosted in an organic phase to avoid decomposition and exposed to various alkenes. A copper catalyzed “Click” reaction then afforded triazoles from the resulting addition products. Telescoping of both reaction was enabled by an in‐line workup and subsequent liquid‐liquid separation

Probing the donor properties of pincer ligands using rhodium carbonyl fragments : an experimental and computational case study

Metal carbonyls are commonly employed probes for quantifying the donor properties of monodentate ligands. With a view to extending this methodology to mer‐tridentate “pincer” ligands, the spectroscopic properties [ν(CO), δ13C, 1JRhC] of rhodium(I) and rhodium(III) carbonyl complexes of the form [Rh(pincer)(CO)][BArF4] and [Rh(pincer)Cl2(CO)][BArF4] have been critically analysed for four pyridyl‐based pincer ligands, with two flanking oxazoline (NNN), phosphine (PNP), or N‐heterocyclic carbene (CNC) donors. Our investigations indicate that the carbonyl bands of the rhodium(I) complexes are the most diagnostic, with frequencies discernibly decreasing in the order NNN > PNP > CNC. To gain deeper insight, a DFT‐based energy decomposition analysis was performed and identified important bonding differences associated with the conformation of the pincer backbone, which clouds straightforward interpretation of the experimental IR data. A correlation between the difference in carbonyl stretching frequencies Δν(CO) and calculated thermodynamics of the RhI/RhIII redox pairs was identified and could prove to be a useful mechanistic tool

Synthesis and structural dynamics of five-coordinate Rh(III) and Ir(III) PNP and PONOP pincer complexes

The synthesis and characterization of a homologous series of five-coordinate rhodium(III) and iridium(III) complexes of the PNP (2,6-(tBu2PCH2)2C5H3N) and PONOP (2,6-(tBu2PO)2C5H3N) pincer ligands are described: [M(PNP)(biph)][BArF4] (M = Rh, 1a; Ir, 1b; biph = 2,2’-biphenyl; ArF = 3,5-(CF3)2C6H3) and [M(PONOP)(biph)][BArF4] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the 1H NMR timescale (ΔG‡ ca. 60 kJmol-1) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (ΔG‡ ca. 10 kJmol-1). Remarkably, the latter is sufficiently facile that it persists in the solid state, leading to temperaturedependent disorder in the associated X-ray crystal structures. Reaction of 1 and 2 with CO occurs for the iridium congeners 1b and 2b, leading to the formation of sterically congested carbonyl derivatives