A bench stable formal Cu(iii) N-heterocyclic carbene accessible from simple copper(ii) acetate

Abstract

The first stable formal Cu(iii) NHC and its unusual reactivity with acetate are reported. Several products of this reaction are identified and fully characterised. It reactivity is extensively investigated and additionally explored by means of theoretical, electrochemical and isotope labelling experiments.

An Adaptable N‐Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

“…In the major product 1 , the metal ion is hosted within the macrocyclic cavity and is bound to the two NHC donors in a quasi‐linear geometry (C NHC ‐Cu‐C NHC : 169.6(1)°). The Cu−C NHC bond lengths of 1.939(2) Å and 1.946(2) Å agree with reported values for Cu I −NHC complexes . The two pyridine rings adopt a near‐orthogonal orientation with respect to the plane defined by the two C NHC and the two pyridine N atoms, with interplanar angles of 78.6(1)° and 69.7(1)°, respectively.…”

“…The mononuclear structure of 1 contrasts with the structures of the corresponding Ag I and Au I complexes of the hybrid macrocycle L, and the Cu I complex of a related macrocycle lacking the pyridine N donors , which were shown to exist as dinuclear or even tetranuclear ([L 2 Ag 4 ][PF 6 ] 4 ) complexes in the solid state, with linearly coordinated metal ions bound to NHC or pyridine donors from two different macrocycles . Dinuclear or tetranuclear structural motifs ([L NHC Cu 2 ]I 2 , [L NHC 2 Cu 4 ][PF 6 ] 4 ) have also been observed for copper(I) complexes of macrocyclic tetracarbene ligands. In iron complexes [LFe II (NCMe) 2 ][PF 6 ] 2 and [LFe III (NCMe) 2 ][PF 6 ] 3 , as well as in the nickel complex [LNi II ]Br 2 , the macrocyclic ligand L binds as a typical tetradentate C NHC 2 N py 2 donor with the Fe−N py and Ni−N py bonds along the planes of the pyridine rings; the macrocycles are severely puckered and trans ‐located imidazol‐2‐ylidene rings are twisted with respect to each other.…”

“…To date, numerous examples of Cu I complexes are known, and they usually adopt a linear geometry that is common to d 10 coinage metal ions. In contrast, NHC‐ligated Cu II complexes are much rarer, and a single NHC‐Cu III complex has been authenticated; the latter is based on a rigid tetra(N‐heterocyclic) carbene macrocycle (Figure b) and is the only organocopper(III) complex of a neutral ligand reported to date …”

“…Several computational studies suggested the existence of NHC‐Cu III species during carbene‐halide/aryl reductive elimination and oxidative coupling of carbenes, but such intermediates have so far eluded isolation. The seminal tetra(NHC)‐Cu III complex shown in Figure b was synthesized by a disproportionation reaction starting from the corresponding tetra(imidazolium) salt and Cu 2 (OAc) 4 , and it shows the square planar coordination geometry expected for a low‐spin d 8 ion . However, details on its electronic structure and valence state were not reported.…”

“…Furthermore, in this case, as well as in most Cu III systems reported to date, the drastically different coordination geometry preferences for Cu I versus Cu II and Cu III prevented any reversible redox shuttling between the different oxidation states, because redox changes are usually accompanied by major structural changes. Accordingly, the Cu I complex of Kühn's tetra(NHC) macrocycle, obtained when using CuOAc as the copper source, exhibits a tetranuclear structure composed of two tetra(NHC) ligands and four linearly coordinated Cu I ions …”