Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

Abstract

A series of copper/nitrosoarene complexes were created that mimic several steps in biomimetic O 2 activation by copper(I). The reaction of the copper(I) complex of N,N,N′,N′tetramethypropylenediamine with a series of para-substituted nitrosobenzene derivatives leads to adducts in which the nitrosoarene (ArNO) is reduced by zero, one, or two electrons, akin to the isovalent species dioxygen, superoxide, and peroxide, respectively. The geometric and electronic structures of these adducts were characterized by means of X-ray diffraction, vibrational analysis, ultraviolet−visible spectroscopy, NMR, electrochemistry, and density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the ArNO moiety: κN for ArNO, mononuclear η 2-NO and dinuclear μ-η 2 :η 1 for ArNO •− , and dinuclear μ-η 2 :η 2 for ArNO 2−. 15 N isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm −1 for κN-ArNO, 1226 cm −1 for η 2-ArNO •− , 1133 cm −1 for dinuclear μ-η 2 :η 1-ArNO •− , and 875 cm −1 for dinuclear μ-η 2 :η 2 for ArNO 2−). The 15 N NMR signal disappears for the ArNO •− species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O 2 analogues. DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion. 47 usually too oxidative to be stable above −60°C.

Generation of a Sulfinamide Species from Facile N–O Bond Cleavage of Nitrosobenzene by a Thiolate-Bridged Diiron Complex

“…In addition, the Mössbauer spectrum of complex 2c (Figure S46) is similar to that of 2a , thereby indicating that both complexes possess a similar electronic structure. The bridging nitrosoarene ligands hence feature the same valence state irrespective of the substituents, a situation different from a series of copper nitrosoarene adducts reported earlier …”

“…The bridging nitrosoarene ligands hence feature the same valence state irrespective of the substituents, a situation different from a series of copper nitrosoarene adducts reported earlier. 15 The infrared (IR) spectrum of 2a shows a diagnostic moderate absorption peak at 1159 cm −1 for the N−O stretch vibration, 12 which shifts to 1146 cm −1 upon 15 N substitution. This value measured for 2a is dramatically lower than that of free PhNO (1506 cm −1 ); 11 therefore, there is considerable electron density populating into the PhNO π* orbital.…”

“…53,54 Furthermore, to verify that the released aniline is derived from the sulfinamide moiety, we next carried out a series of test experiments (Figure 4c). First, we synthesized [Cp*Fe(μη 2 :η 2 -15 N-mpbsa)(μ-η 1 :η 1 -Ph 14 NO)FeCp*][PF 6 ] ( 14 N/ 15 N-3a) by reaction of 2a with Ph 15 NO under similar conditions to the synthetic pathway of 3a. Treatment of 14 N/ 15 N-3a with 4 equiv of BH 3 •THF led to the liberation of Ph 15 NH 2 in 78% yield determined by GC analysis.…”

“…Crystals of 2a suitable for Xray diffraction experiment were grown from saturated CH 2 Cl 2 solution layered by Et 2 O at room temperature. 1 A sample of 15 N-2a was synthesized using an analogous synthetic procedure starting from Ph 15 NO in 75% yield. The 1 H NMR spectrum is similar to that of the unlabeled complex.…”

“…So far, chemists have reported a variety of stable transition-metal complexes bearing PhNO and its derivatives in different binding modes. − Although the bonding pattern of the PhNO ligand can readily be confirmed by X-ray diffraction analysis, it is usually very difficult to unambiguously assign its oxidation state, viz. PhNO 0 , PhNO •– , or PhNO 2– due to its redox noninnocent character. , Among numerous monometallic complexes, PhNO commonly adopts a η 1 - N , η 1 - O , or η 2 - N , O coordination fashion.…”