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.

The mechanism and selectivity of terminal alkyne coupling reactions promoted by rhodium(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 enforced exclusive head‐to‐head coupling, contrasting the high selectivity for head‐to‐tail coupling observed for the corresponding acyclic pincer ligand.

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}][BAr(F)4] (n = 8, 10, 12; Ar(F) = 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}][BAr(F)4] (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 (19)F chemical shifts (ca. -400 ppm) and the presence of C-H···F interactions ((2h)J(FC) coupling observed by (13)C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC-(CH2)8}][BAr(F)4] exceptionally shows C(2v) 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.

Silver(i) and copper(i) complexes of a macrocyclic NHC-based pincer ligand, bearing a central lutidine donor and a dodecamethylene spacer, have been prepared and evaluated under equivalent conditions in transmetallation reactions.

Using a general synthetic procedure employing readily accessed terminal alkene functionalised pro-ligands and macrocyclisation 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 C 2 symmetry by 1 H NMR spectroscopy (CD 2 Cl 2 , 500 MHz), while 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 (CD 2 Cl 2 , 500 MHz) and the complex crystallises 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 PhICl 2 , have been studied in situ by 1 H NMR spectroscopy, although pure Rh(III) adducts can ultimately only be isolated with the pyridine-based macrocyclic ligand. The lutidine backbone of 1a can be deprotonated by addition of K[N(SiMe 3 ) 2 ] and the resulting neutral dearomatised complex (5) has been fully characterised in solution, by variable temperature 1 H NMR spectroscopy, and in the solid-state, by X-ray diffraction.

(2015) Coordination induced atropisomerism in an NHC-based rhodium macrocycle. Organometallics, 34 (5). pp. 913-917. Permanent WRAP URL:http://wrap.warwick.ac.uk/76675 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or notfor profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher's statement:This document is the Accepted Manuscript version of a Published Work that appeared in final form in Organometallics, copyright © American Chemical Society after peer review and technical editing by the publisher.To access the final edited and published work see: http://dx.doi.org/10.1021/om501292k A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP URL' above for details on accessing the published version and note that access may require a subscription.For more information, please contact the WRAP Team at: wrap@warwick.ac.ukRevised manuscript (om-2014-01292k) Coordination Induced Atropisomerism in a NHC-based Rhodium Macrocycle

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincerl igands-which feature dodecamethylene,t etradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNCn)(C 2 H 4)][BAr F 4 ](n = 12, 14, 16;A r F = 3,5-(CF 3) 2 C 6 H 3)-have been investigated, using the bulky alkynes HCCtBu and HCCAr'(Ar' = 3,5-tBu 2 C 6 H 3)a ss ubstrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives andp roduce rhodium(I) complexes of conjugated 1,3-enynes by CÀCbond reductiveelimination through the annuluso ft he ancillary ligand. The intermediates are formed with orthogonal regioselectivity,w ith E-alkenyl complexes derived fromH C CtBu and gem-alkenyl complexes derived from HCCAr', and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HCCtBu, E-tBuCCCH= CHtBu is produced via direct reductivee limination from the corresponding rhodium(III)a lkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductivee limination of Ar'CCC(= CH 2)Ar'i se ncumbered relative to head-to-head coupling of HCCAr'a nd it is only with the largest macrocyclic ligand studied that the two processesa re competitive.T hese results showcaseh ow macrocyclic ligands can be used to interrogate the mechanism and tune the outcomeo ft erminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C 2 H 4)][BAr F 4 ] and solventeffects. Scheme1.Metal catalysed reactionsoft erminal alkynes. Mes = 2,4,6-Me 3 C 6 H 2 .

The mechanism and selectivity of terminal alkyne coupling reactions promoted by rhodium(I) complexes of NHC-based CNC pincer ligands have been investigated. Synthetic and kinetic experiments support E-and gem-enyne formation through ac ommon reaction sequence involving hydrometallation and rate-determining C À Cb ond reductive elimination. The latter is significantly affected by the ligand topology:E mployment of am acrocyclic variant enforced exclusive head-to-head coupling,contrasting the high selectivity for head-to-tail coupling observed for the corresponding acyclic pincer ligand.