Synthesis, structure and dynamics of NHC-based palladium macrocycles

A series of macrocyclic CNC pincer pro-ligands based on bis(imidazolium)lutidine salts with octa-, deca- and dodecamethylene spacers have been prepared and their coordination chemistry investigated. Using a Ag2O based transmetallation strategy, cationic palladium(II) chloride complexes [PdCl{CNC–(CH2)n}][BArF4] (n = 8, 10, 12; ArF = 3,5-C6H3(CF3)2) were prepared and fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state, by X-ray crystallography. The smaller macrocyclic complexes (n = 8 and 10) exhibit dynamic behaviour in solution, involving ring flipping of the alkyl spacer across the Pd–Cl bond, which was interrogated by variable temperature NMR spectroscopy. In the solid-state, distorted coordination geometries are observed with the spacer skewed to one side of the Pd–Cl bond. In contrast, a static C2 symmetric structure is observed for the dodecamethylene based macrocycle. For comparison, palladium(II) fluoride analogues [PdF{CNC–(CH2)n}][BArF4] (n = 8, 10, 12) were also prepared and their solution and solid-state structures contrasted with those of the chlorides. Notably, these complexes exhibit very low frequency 19F chemical shifts (ca. −400 ppm) and the presence of C–HF interactions (2hJFC coupling observed by 13C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC–(CH2)8}][BArF4] exceptionally shows C2v time averaged symmetry in solution at room temperature (CD2Cl2, 500 MHz) as a consequence of dual fluxional processes of the pincer backbone and alkyl spacer

Synthesis and reactivity of NHC-based rhodium macrocycles

Using a general synthetic procedure employing readily accessed terminal alkene-functionalized pro-ligands and macrocyclization by ring-closing olefin metathesis, rhodium carbonyl complexes have been prepared that contain lutidine (1a; n = 1) and pyridine (1b; n = 0) derived tridentate CNC macrocycles with dodecamethylene spacers. In solution, 1a shows temperature-invariant time-averaged C2 symmetry by 1H NMR spectroscopy (CD2Cl2, 500 MHz), whereas in the solid-state, two polymorphs can be obtained showing different conformations of the alkyl spacer about the metal–carbonyl bond (asymmetric and symmetric). In contrast, time-averaged motion of alkyl spacer in 1b can be halted by cooling below 225 K (CD2Cl2, 500 MHz), and the complex crystallizes as a dimer with an interesting unsupported Rh···Rh bonding interaction (3.2758(6) Å). Oxidative addition reactions of 1a and 1b, using MeI and PhICl2, have been studied in situ by 1H NMR spectroscopy, although pure Rh(III) adducts can be ultimately isolated only with the pyridine-based macrocyclic ligand. The lutidine backbone of 1a can be deprotonated by addition of K[N(SiMe3)2], and the resulting neutral dearomatized complex (5) has been fully characterized in solution, by variable-temperature 1H NMR spectroscopy, and in the solid state, by X-ray diffraction

Late transition metal complexes of NHC-based macrocycles

N-heterocyclic carbene (NHC) ligands are an important and diverse ligand class, conferring stability and enabling reaction control of coordinated metal centres. Pincer ligand architectures bearing such donors are an increasingly prevalent design motif, combining the strong donor characteristics of carbenes with the favourable thermal stability possible with a mer-tridentate geometry. Macrocyclic variants of NHC-based pincers are of interest as they may impart further stability and reaction control, in addition to serving as potential building blocks for the construction of interlocked, supramolecular systems. Enabling study of the reactivity and structural properties of late transition metal complexes of macrocyclic CNC pincer ligands, the coordination chemistry of a series of pro-ligands of varying ring size (n = 1; m = 8, 10, 12) has been explored. Investigation of the dynamics and structures of palladium(II) halide complexes identified an optimal ring size, which was subsequently employed in the preparation of rhodium-based derivatives (n = 0, 1; m = 12) via two converging synthetic routes. The organometallic chemistry, dynamic behaviour and structures of the resulting rhodium complexes are described. Through investigation of terminal alkyne coupling reactions using a macrocyclic rhodium ethylene complex, potential for the preparation of novel supramolecular assemblies was explored. As a result, a series of complexes bearing mechanically entrapped enynes were isolated and fully characterised in solution and the solid-state

Well-defined coinage metal transfer agents for the synthesis of NHC-based nickel, rhodium and palladium macrocycles

With a view to use as carbene transfer agents, well-defined silver(I) and copper(I) complexes of a macrocyclic NHC-based pincer ligand, bearing a central lutidine donor and a dodecamethylene spacer [CNC–(CH2)12, 1], have been prepared. Although the silver adduct is characterised by X-ray diffraction as a dinuclear species anti-[Ag(μ-1)]22+, variable temperature measurements indicate dynamic structural interchange in solution involving fragmentation into mononuclear [Ag(1)]+ on the NMR time scale. In contrast, a mononuclear structure is evident in both solution and the solid-state for the analogous copper adduct partnered with the weakly coordinating [BArF4]− counter anion. A related copper derivative, bearing instead the more coordinating cuprous bromide dianion [Cu2Br4]2−, is notable for the adoption of an interesting tetranuclear assembly in the solid-state, featuring two cuprophilic interactions and two bridging NHC donors, but is not retained on dissolution. Coinage metal precursors [M(1)]n[BArF4]n (M = Ag, n = 2; M = Cu, n = 1) both act as carbene transfer agents to afford palladium, rhodium and nickel complexes of 1 and the effectiveness of these precursors has been evaluated under equivalent reaction conditions

Terminal alkyne coupling reactions through a ring : effect of ring size on rate and regioselectivity

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC‐based pincer ligands – which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC‐n)(C2H4)][BArF4] (n = 12, 14, 16; ArF = 3,5‐(CF3)2C6H3) – have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr′ (Ar′ = 3,5‐tBu2C6H3) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3‐enynes by C–C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E‐alkenyl complexes derived from HC≡CtBu and gem‐alkenyl complexes derived from HC≡CAr′, and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E‐tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E‐alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar′C≡CC(=CH2)Ar′ is encumbered relative to head‐to‐head coupling of HC≡CAr′ and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC‐Me)(C2H4)][BArF4] and solvent effects

Synthesis and Reactivity of NHC-Based Rhodium Macrocycles

Using a general synthetic procedure employing readily accessed terminal alkene-functionalized pro-ligands and macrocyclization by ring-closing olefin metathesis, rhodium carbonyl complexes have been prepared that contain lutidine (<b>1a</b>; <i>n</i> = 1) and pyridine (<b>1b</b>; <i>n</i> = 0) derived tridentate CNC macrocycles with dodecamethylene spacers. In solution, <b>1a</b> shows temperature-invariant time-averaged <i>C</i>₂ symmetry by ¹H NMR spectroscopy (CD₂Cl₂, 500 MHz), whereas in the solid-state, two polymorphs can be obtained showing different conformations of the alkyl spacer about the metal–carbonyl bond (asymmetric and symmetric). In contrast, time-averaged motion of alkyl spacer in <b>1b</b> can be halted by cooling below 225 K (CD₂Cl₂, 500 MHz), and the complex crystallizes as a dimer with an interesting unsupported Rh···Rh bonding interaction (3.2758(6) Å). Oxidative addition reactions of <b>1a</b> and <b>1b</b>, using MeI and PhICl₂, have been studied in situ by ¹H NMR spectroscopy, although pure Rh­(III) adducts can be ultimately isolated only with the pyridine-based macrocyclic ligand. The lutidine backbone of <b>1a</b> can be deprotonated by addition of K­[N­(SiMe₃)₂], and the resulting neutral dearomatized complex (<b>5</b>) has been fully characterized in solution, by variable-temperature ¹H NMR spectroscopy, and in the solid state, by X-ray diffraction

NHC-based pincer ligands : carbenes with a bite

In this frontier article we overview the emergence and scope of NHC-based CCC and CNC pincer systems, i.e. complexes containing mer-tridentate ligands bearing two NHC donor groups, comment on their effectiveness in applications, and highlight areas for future development and exploitation

Terminal alkyne coupling reactions through a ring : mechanistic insights and regiochemical switching

The mechanism and selectivity of terminal alkyne coupling reactions promoted by Rh(I) complexes of NHC‐based CNC pincer ligands have been investigated. Synthetic and kinetic experiments support E‐ and gem‐enyne formation through a common reaction sequence involving hydrometallation and rate determining C‐C bond reductive elimination. The latter is significantly affected by the ligand topology: employment of a macrocyclic variant enforces exclusive head‐to‐head coupling, contrasting the high selectivity for head‐to‐tail coupling observed for the corresponding acyclic pincer

Guidelines for the training of parliamentary staff

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