Chemical Oxidation of a Coordinated PNP-Pincer Ligand Forms Unexpected Re–Nitroxide Complexes with Reversal of Nitride Reactivity

Abstract

Because of the thermodynamic demands of N2 cleavage, N2-derived nitride complexes are often unreactive. The development of multistep N2 functionalization reactions hinges on methods for modulating nitride reactivity with supporting ligands. Here, we describe the reactions of N2-derived Re–nitride complexes, including the first Re nitrides supported by a nitroxide-containing pincer ligand, and unusual examples of Re6+–nitride complexes. The previously reported N2-derived complex (PNP)­Re­(N)­(Cl) (PNP = N­(CH2CH2PtBu2)2) can be oxidized by O atom transfer to the backbone amide to form a novel nitroxide-pincer complex or by 1e– to form a rare S = 1/2 Re6+–nitride complex. The Re–nitrido interaction in a series of Re- and ligand-oxidized complexes is characterized using 15N NMR spectroscopy, IR spectroscopy, and DFT calculations, and shows changes in the Re–N bond order from both ligand- and metal-centered oxidations. Chemical oxidation of the supporting ligand to form a nitroxide-pincer ligand results in subtle electronic changes at Re and a more electron-deficient nitride ligand. Combined ligand- and metal-centered oxidation to form a Re6+–nitroxide complex results in a reversal of reactivity at the nitride ligand from nucleophilic to electrophilic. These systematic electronic structure and reactivity studies demonstrate methods for inducing reactivity in N2-derived nitride complexes.

Facile conversion of ammonia to a nitride in a rhenium system that cleaves dinitrogen

“…S2 † ), confirming that it corresponds to protons bound to N. This 15 N chemical shift is significantly upfield from related nitride complexes (371–393 ppm) and closer to that for the protonated P N P backbone of [( H PNP)Re(N)Cl] + (5) (−336 ppm). 66 All spectroscopic signatures are consistent with the formulation of 3 as the amido complex (PNP)Re(NH 2 )Cl.…”

“…The Re– N H 2 bond in 3 is 0.3 Å longer than the Re N bond in the nitride complex 2 ( Table 1 ). 66 In the supporting ligand, the (P N P)–Re bond is 0.1 Å shorter in 3 than in the Re–nitride complex 2, indicating increased π-bonding from the nitrogen of the pincer ligand in 3 ( Table 1 ). The (P N P)–Re and Re– N H 2 amide bond lengths in 3 are within 0.02 Å of each other with planar coordination of the nitrogen atoms in both cases ( Σ P N P = 360°, Σ N H 2 = 357°).…”

Rhenium‐Mediated Conversion of Dinitrogen and Nitric Oxide to Nitrous Oxide

“…[10] As the N-terminus defines the electrophilic site of nitric oxide, [11] a related platform was chosen to evaluate nitride/NO coupling. To avoid undesired pincer oxidation, [12] we resorted to the robust diphenylamido ligand N(C 6 H 3 -4-CH 3 -P i Pr 2 ) 2 À (PNP À ). Ison and co-workers recently reported the synthesis of the Re V nitride [Re(N)Cl(PNP)] ( 1), yet not by N 2 activation.…”

Rhenium and Osmium PNP Pincer Complexes for Nitrogen Fixation and Nitride Transfer

“…Interestingly, in a very recent study, XXI was oxidized at the ligand backbone with 3chloroperbenzoic acid and subsequently at the metal center to form the nitroxide Re(vi) complex [ReNCl(P O NP tBu )] PF6 . [155] These oxidations now lead to an umpolung of the nitride reactivity, which does form (unstable) phosphoraneiminato complexes with various phosphines, rendering it electrophilic.…”

“…In order to be able to evaluate the success or failure of a reaction, the expected prod- 6) Å in XXI, [155] most likely due to the distorted geometry. The short distance to the backbone nitrogen donor (d(Re1-N1) = 2.0086(18) Å) and the planar coordination of the nitrogen atom (Σ(∡ N1 ) = 352.6°) support the formulation as an amide.…”