Elusive Terminal Copper Arylnitrene Intermediates

Abstract

We report herein three new modes of reactivity between arylazides N3Ar with a bulky copper(I) β‐diketiminate. Addition of N3ArX3 (ArX3=2,4,6‐X3C6H2; X=Cl or Me) to [iPr2NN]Cu(NCMe) results in triazenido complexes from azide attack on the β‐diketiminato backbone. Reaction of [iPr2NN]Cu(NCMe) with bulkier azides N3Ar leads to terminal nitrenes [iPr2NN]Cu]=NAr that dimerize via formation of a C−C bond at the arylnitrene p‐position to give the dicopper(II) diketimide 4 (Ar=2,6‐iPr2C6H3) or undergo nitrile insertion to give diazametallocyclobutene 8 (Ar=4‐Ph‐2,6‐iPr2C6H2). Importantly, reactivity studies reveal both 4 and 8 to be “masked” forms of the terminal nitrenes [iPr2NN]Cu=NAr that undergo nitrene group transfer to PMe3, tBuNC, and even into a benzylic sp3 C−H bond of ethylbenzene.

Direct Manipulation of Metal Imido Geometry: Key Principles to Enhance C–H Amination Efficacy

“…7,8 As such, the isoelectronic synthetic analogues, metal nitrenoid complexes (e.g., imido, M(NR 2− ); 4,[9][10][11] iminyl, M( 2 NR − ); [12][13][14][15][16][17][18][19] nitrene adducts, M( 3 NR) [20][21][22][23] ) have received great attention as viable N−group transfer reagents into unactivated C-H bonds. In particular, late, first-row transition metal-based nitrenoid complexes have been extensively studied for direct C−N bond formation (Mn, 24 Fe, 18,19,[25][26][27][28][29][30][31][32][33][34] Co, [35][36][37][38][39][40][41][42][43] Ni, 10,11,44,45 and Cu 23,46,47 ). Among the structurally characterized metal nitrenoid complexes, however, only a few manifest productive C-H amination d...…”

Electronic Structures and Reactivity Profiles of Aryl Nitrenoid-Bridged Dicopper Complexes

“…The clearest picture of the electronic structure for the anionic imidos arises from the combined Cu (L 2,3 , K-edges) and N (K-edge) XAS spectra. While neutral imidos 5 and 6 feature prominent pre-edge absorptions in the Cu K-edge spectra, the anionic imidos (8,9) lack clear experimental pre-edge features (Figure 5b, inset) similar to cuprous standard ( tBu L)Cu I (NCMe) (10). The weak pre-edge features observed in TDDFT calculations can be attributed to addition of an electron to the Cu and N based LUMO in 5/6.…”

“…At lower temperatures (4 K), the hyperfine coupling becomes anisotropic with copper hyperfine coupling values 63 Cu 2 A x = 9.3 MHz, A y = 43.9 MHz, and A z = 9.7 MHz and 14 9) is consistent with a significant contribution of the N 2p x orbital to the now singly occupied redox active molecular orbital (vida infra). Absorptions of near-infrared wavelengths (1000-2000 nm) were detected for both the neutral (5,6) and anionic (8,9) . This largely precludes assignment of any of these NIR bands as intervalence charge transfer .…”

“…The formally mixed valence anionic imidos (7)(8)(9) possess doublet ground states exhibiting well-defined EPR signals between . Solutionphase EPR spectra display isotropic hyperfine coupling (i.e., electronic contribution from the 2s orbital), dominated by Cu character with minimal N contribution and consistent with spectra observed for fully delocalized, Class IIIA mixed valence dicopper complexes 51 -53 in the Robin-Day classification.…”

“…The dicopper imido complexes reported by Warren and co-workers are proposed to be in equilibrium with a mononuclear copper complex and a terminal copper nitrenoid species, to which the latter is attributed for the observed C─H bond amination reactivity (Figure 1). [5][6][7][8] Steric profile tuning of the ancillary ligands for both β-diketiminate and dipyrrin platforms allowed for observation of reductively coupled terminal nitrenoid complexes 8 as well as the isolation of authentic terminal copper nitrene complexes, 9 respectively. The amination reactivity can be contrasted with the partial hydroxylation of methane to methanol by zeolite Cu-ZSM-5, 10 ascribed to a bent, dicupric oxide core (Figure 1).…”