Dinitrogen Splitting Coupled to Protonation

Abstract

The coupling of electron- and proton-transfer steps provides a general concept to control the driving force of redox reactions. N splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N-N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo-N -Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N-N bond cleavage is an unusual case of a proton-coupled metal-to-ligand charge transfer process, highlighting the use of proton-responsive ligands for nitrogen fixation.

Dinitrogen‐Molybdenum Complex Induces Dinitrogen Cleavage by One‐Electron Oxidation

“…These results indicate that afivecoordinate Mo I À N 2 complex is ak ey species for N 2 cleavage via aM o II À N = N À Mo II core structure.I na ddition, Schneider and co-worker have reported that N 2 cleavage of aMo III = N = N=Mo III complex into Mo V nitride complex is induced by changing spin multiplicity in its ground state upon protonation to N(CH 2 CH 2 P t Bu 2 ) 2 ,w hich is an ancillary ligand. [21] Inspired by these findings,w ep ostulate that trans-[Mo-(depe) 2 (N 2 ) 2 ]( 1)( depe = Et 2 PCH 2 CH 2 PEt 2 ), which is well known as aC hatt-type complex, also has the potential for promoting N 2 cleavage and is converted into aM o IV nitride complex. This transformation is possible because 1 can undergo one-electron oxidation to generate afive-coordinate Mo I -N 2 structure by dissociation of coordinated N 2 from the Mo II À N = N À Mo II core structure.H erein, we report that oneelectron oxidation of aM o 0 -N 2 complex, supported by bidentate phosphine ligands,leads to formation of an unstable Mo II ÀN=NÀMo II species via af ive-coordinate Mo I -N 2 complex to produce aM o IV nitride complex through dinitrogen cleavage,a ss hown in Scheme 1.…”

“…TheR aman band is attributed to as trongly activated N 2 molecule in ad imeric [(m-N 2 ){Mo(depe) 2 } 2 ] 2+ species (3)asa[N 2 ] 2À ligand, which is lower in frequencythan the analogous band in [{(PNP)ClMo}(N 2 )] [PNP = N(CH 2 CH 2 P t Bu 2 ) 2 ); 1343 cm À1 ]d escribed previously as the N=Nb ond by Schneider and co-workers. [21] Moreover,t he Raman spectrum of 1,when irradiated with a355 nm laser for 1h our, did not change without the CPE, suggesting that the species observed at 1292 cm À1 was generated by CPE. The Raman bands observed in the range of 1450 to 1600 cm À1 are assigned to ac ombination of an ethyl group vibration at 283 cm À1 and an N 2 stretching vibration at 1292 cm À1 for dimeric [(m-N 2 ){Mo(depe) 2 } 2 ] 2+ (3;see Figure S17).…”

Selectivity of tungsten mediated dinitrogen splitting vs. proton reduction

“…In the solid state ( Fig. 3), 9 + is isostructural with the molybdenum analogue, 19 featuring square-pyramidally coordinated tungsten with the nitride ligand in the apical site. Hydrogen bonding of the amine proton with the triate anion is indicated by the short NH .…”

“…19 Notably, immediate formation of the oxidation product 7 + was observed with [H(OEt 2 ) 2 ][BAr F 24 ], even at temperatures down to À75 C. The enhanced stability of 10 OTf suggests an interaction of the immediate protonation product with the tri-ate anion. Contact-ion pair formation is conrmed by 19 F and 1 H DOSY NMR spectroscopy at À35 C. The diffusion coefficient of the triate anion in 10 OTf (D ¼ 2.29 Â 10 À6 cm 2 s À1 ) is in the same range as that of the cation (D ¼ 2.18-2.14 Â 10 À6 cm 2 s À1 ) and signicantly reduced compared to free triic acid (D ¼ 5.11 Â 10 À6 cm 2 s À1 ). We tentatively attribute the solution ionpairing to hydrogen bonding of the triate with the pincer N-H proton, as found in the solid state for 9 OTf (Fig.…”

“…17,18 In contrast, the {p 8 d 4 } complex [(N 2 ) {MoCl(PNP)} 2 ] (4) features stronger N 2 -activation comparable to 2 2+ (Table 1), but is thermally stable. 19 Unexpectedly, splitting of 4 at r.t. was obtained upon protonation of the pincer backbone (Scheme 2), which was rationalized by a protonation induced low-spin {p 8 d 4 } to high-spin {p 10 d 2 } transition that facilitates electron transfer to N 2 .…”