Heteroleptic Copper Complexes as Catalysts for the CuAAC Reaction: Counter-Ion Influence in Catalyst Efficiency

Abstract

A series of nine cationic heteroleptic aryl-BIAN-copper(I) (BIAN = bis-iminoacenaphthene) complexes with the general formula [Cu((E-C6H4)2BIAN)(PPh3)2][X] (E = p-Me, p-iPr, o-iPr; X = BF4, OTf, NO3) 1X–3X were synthesized and fully characterized using several analytical techniques, including NMR spectroscopy and single-crystal X-ray diffraction. Except for complexes 2BF4 and 3BF4, which were already reported in our previous works, all remaining complexes are herein described for the first time. Two different strategies were used for the preparation of the complexes: complexes bearing BF4− or OTf− counter-ions (1BF4, 1OTf, 2OTf, and 3OTf) were obtained using the appropriate copper(I) precursors [Cu(NCMe)4][BF4] or [Cu(NCMe)4][OTf], whereas for derivatives 1NO3–3NO3, [Cu(PPh3)2NO3] was used. Their activity as catalysts for the copper azide-alkyne cycloaddition (CuAAC) was assessed alongside other high activity, previously reported Cu(I) complexes. Comparative studies to determine the influence of the counter-ion and of the aryl substituents were performed. All complexes behaved as active catalysts under neat reaction conditions, at 25 °C and in short reaction times without requiring the use of any additive, with complex 2NO3 being the most efficient derivative, along with other NO3−-bearing complexes.

New copper(i) complexes of bulky 5-substituted-2-iminopyrrolyl ligands as catalysts for azide–alkyne cycloaddition

“…The catalytic results obtained with the present system led a TOF value as high as 859 h −1 for the production of 1-benzyl-4-phenyl-1 H -1,2,3-triazole, resulting from the cycloaddition of phenylacetylene with (azidomethyl)benzene, operating at concentrations of catalyst as low as 0.1 mol%. Considering the set of activities recorded for the present catalyst system, we can conclude it is less efficient than the mononuclear copper complexes bearing NHC (maximum TOF = 950 h −1 ) 20 or diimine ligands (maximum TOF = 1920 h −1 ), 22 slightly better than the catalysts bearing phosphine/phosphonite/phosphinite ligands (maximum TOF = 313 h −1 ) 21 and considerably better than the in situ prepared ones (maximum TOF = 27 h −1 ). 7…”

“…19 Some examples of well-defined copper( i ) complexes capable of performing CuAAC reactions have been reported in the literature, in which the respective copper centers were supported by N-heterocyclic carbenes (Chart 1, A , maximum TOF = 950 h −1 ), 20 phosphines/phosphonites/phosphinites (Chart 1, B , maximum TOF = 313 h −1 ), 21 or nitrogen-based diimine ligands (Chart 1, C , maximum TOF = 1920 h −1 ). 22 As a representative example of the development of nitrogen-based ligands toward CuAAC reactions, Tilley and co-workers, isolated a series of dinuclear copper( i ) intermediates using the dinucleating ligand 2,7-bis(fluoro-di(2-pyridyl)-methyl)-1,8-naphthyridine (Chart 1, D ), 23 which supported the propensity of catalytic dinuclear species in CuAAC reactions, with cooperation between the two copper centers. 24 When considering neutral dinuclear copper( i ) complexes, several examples of their preparation and structural characterization are to be noted, the vast majority utilizing monoanionic ligand systems such as amidinates (X = CR) or guanidinates (X = N) (Chart 1, E ).…”

Elucidating the Structural Features of Bis(arylimino)acenaphthene (Aryl-BIAN) Bismuth Complexes: A Combined Single-Crystal X-ray Diffraction and Hirshfeld Analysis Approach

“…Bis(arylimino) acenaphthene (Aryl-BIAN) ligands can enhance activity in certain systems due to their extensive π-system and rigid s-cis diimine unit, which can chelate to a metal center [17]. This ligand type is mainly relevant to late transition metals, such as palladium, nickel, copper, and zinc, for various organic transformations, including hydrogenation [18], ethylene polymerization [19,20], reverse atom transfer radical polymerization (ATRP) of styrene [21], click chemistry [22], selective oxidation of alcohols [23], and formation of cyclic carbonates [24].…”