Diazene Chemistry: Metal-Free Models of N₂ Reduction Intermediates

Abstract

Interest in main group chemistry related to the Haber−Bosch process has drawn less attention than that of transition metal species. Herein, we show that the steric demands in (tBuO 2 CN) 2 block initial interaction of B(C 6 F 5 ) 3 with nitrogen and prompt loss of methylpropene and CO 2 to diazene (N 2 H 2 ) borane adduct 1 and the analogous hydrazine (N 2 H 4 ) adduct 2. These species react with basic phosphines to give anions of 3 and 5 containing N 2 H and N 2 H 3 fragments, respectively. While these species are not derived directly from N 2 , they represent metal-free species containing N 2 H n (n = 1−4) fragments, which model plausible intermediates in the reduction of N 2 .

Reactivity of frustrated Lewis pairs with BOC protected diazocarboxylates: FLP capture of diazene

“…The respective P-N, N-N and N-B distances were determined to be 1.656(2) Å, 1.449(2) Å, and 1.547(3) Å. We note that the N-N bond distance in 5 is significantly longer than that seen in (C 6 F 5 ) 3 BNHNHB(C 6 F 5 ) 3 (1.230(3) Å) 22 and similar to that in (C 6 F 5 ) 3 BNH 2 NH 2 B(C 6 F 5 ) 3 (1.462(2) Å). 22 This contrast between 5 and (C 6 F 5 ) 3 BNHNHB(C 6 F 5 ) 3 is consistent with the formal oxidation of the P center and the reduction of the diazene fragment.…”

“…DFT computational work previously showed that B(C 6 F 5 ) 3 activates the tert-butyl group inducing proton transfer to a diazo N-atom affording the adduct E with the release of CH 2 QCMe 2 and CO 2 . 22 We note that this reaction is kinetically 1.2 kcal mol À1 less favourable than in the presence of B(C 6 F 5 ) 3 /TMP FLP (above). Additional DFT calculations [23][24][25][26][27][28][29][30][31][32][33] showed that the addition of phosphine (o-Tol) 3 P to E is À20.1 kcal mol À1 exergonic over a low barrier of only 14.1 kcal mol À1 (via TS4) to form the observed zwitterionic product 4 (Fig.…”

“…While reactions of sterically unencumbered diazo-diesters afford heterocyclic products, 21 the reaction of the sterically encumbered diazo-diester ( t BuO 2 CN) 2 with B(C 6 F 5 ) 3 results in the liberation of methyl-propene and CO 2 affording the diazene-borane adduct (N 2 H 2 )(B(C 6 F 5 ) 3 ) 2 which is slowly converted to the analogous hydrazine adduct. 22 Further, these species react with basic phosphines to give the corresponding anions [(N 2 H)(B(C 6 F 5 ) 3 ) 2 ] − and [(N 2 H 3 )(B(C 6 F 5 ) 3 ) 2 ] − , respectively (Fig. 1).…”

An orbitally adapted push–pull template for N2 activation and reduction to diazene-diide

“…The strong bathochromic shift recorded for the stretching frequency of 7 (1399 vs. 1712 cm −1 for 10 ) and the structural data indeed support the NN 2− depiction. 37,40–42 The reaction of 10 with 2 points to diborane coordination-triggered 2-e – reduction of N 2 , and somewhat recalls the evolution of the N–N bond upon the two-fold protonation of related group 6 N 2 complexes affording hydrazido complexes. 29 The resulting positively charged compounds show N–N bond lengths ranging from 1.30–1.37 Å depending on the trans ligand and H-bonding in the lattice.…”

Borane catalyzed transesterification of tert-butyl esters using α-aryl α-diazoesters

“…14 In other work, we also recently reported the seemingly unrelated reactions in which the diazo diester (NCO 2 t Bu) 2 with B(C 6 F 5 ) 3 effected the degradation of the t -butyl group, affording the diazine product (HNB(C 6 F 5 ) 3 ) 2 . 15 This observation suggested that other t- butyl esters would undergo reaction with diazo esters in the presence of B(C 6 F 5 ) 3 . Herein, we examine the reactions of 3-alkenyl-oxindoles and other unsaturated tert -butyl esters with diazo esters in the presence of B(C 6 F 5 ) 3 .…”