Nitric Oxide Abstracts a Nitrogen Atom from an Osmium Nitrido Complex To Give Nitrous Oxide

Abstract

The chemistry of nitric oxide, NO, is attracting widespread interest, from environmental to biological chemistry. 1,2 As a free radical, NO reacts rapidly with other odd-electron species, such as superoxide and alkyl radicals, and reversibly dimerizes to N 2 O 2 . NO can be oxidized to NO + or to NO 2 -(by formal addition of O •-). Reduction of NO to N 2 O commonly occurs via deoxygenation of N 2 O 2 , often in a metal-mediated process. 3 Reported here is the first example of reduction of NO to N 2 O that occurs by one-step nitrogen-atom transfer.Stirring a benzene solution of the osmium nitrido complex TpOs(N)Cl 2 (1) 4 under ∼0.75 atm of NO for 24 h results in stoichiometric conversion to N 2 O and the osmium nitrosyl complex TpOs(NO)Cl 2 (2) 5 (eq 1; Tp ) HBpz 3 , hydrotris-(pyrazolyl)borate). 6 The osmium stoichiometry was determined by 1 H NMR and the N 2 O was detected by gas-phase IR. The IR spectrum of N 2 O at moderate pressures (∼0.10 Torr), using a low-resolution PE1600 FT-IR, shows broad R and P branches, with maxima at 2214 and 2238 cm -1 and a minimum at 2224 cm -1 (the unobserved Q-branch position; Figure 1). The yield of N 2 O was 90 ( 10%, based on a Beer's law calibration at 2224 cm -1 and subtraction of the residual N 2 O present in the NO reactant (Figure 1, inset). 7 Kinetic data obtained by 1 H NMR in CD 2 Cl 2 under 3 atm of NO (pseudo-first-order conditions) are consistent with first-order decay of 1. Reducing the pressure by a factor of 3 increases the half-life by roughly the same factor. Reaction 1 thus appears to be first order in both 1 and NO, with rate constant k ≈ 3 × 10 -4 M -1 s -1 at 21°C. 8 There are two reasonable types of mechanisms for reaction 1. The nitrosyl complex 2 could be formed by oxygen atom transfer to the nitrido ligand in 1, analogous to the conversion of 1 to 2 by Me 3 NO (eq 2). 9 In reaction 1, the source of oxygen could be from an NO-derived species such as bound or free N 2 O 2 . Alternatively, addition of NO to the nitrido ligand would form the osmium-N 2 O complex, [TpOs(NNO)Cl 2 ] (Scheme 1). Loss of N 2 O would give [TpOsCl 2 ], which would be converted to 2 by the excess NO present. An 15 N-labeling experiment can distinguish the two pathways because the first incorporates the nitrido ligand into the nitrosyl group, while the second converts the nitrido ligand into the terminal nitrogen of the N 2 O. The reaction of TpOs( 15 N)Cl 2 4 with 14 NO supports the second pathway (Scheme 1), as only TpOs( 14 NO)Cl 2 is formed by IR and mass spectroscopies [<10% Os( 15 NO)]. Gas-phase IR spectra of the volatiles from the reaction show both the residual 14 N 14 NO impurity in the NO (unobserved Q branch at 2224 cm -1 ) and a new feature at lower frequency whose R branch overlaps with the P branch of 14 N 14 NO (Figure 1, bottom). The Q branch minimum of the new feature at 2201 cm -1 agrees at this level of resolution with the 2202.5 cm -1 reported for 15 N 14 NO. 10 Direct, rate-limiting attack of NO on the nitrido ligand of 1 is consistent with the apparent...

Synthetic chemistry with nitrous oxide

“…Recently, it was shown that the labile azoxymetallacyclopentene complex can be trapped by reactions with MeO 3 SCF 3 (alkylation of the b-N-atom). 96,97 It is interesting to note that the reverse reaction, the formation of N 2 O from a metal nitride complex and nitric oxide, has been observed for an osmium 101 and a ruthenium 49 nitride complex. In case of Ti, the initial N 2 O adduct is more stable and can be used for further reactions.…”

Terminal Nitrido and Imido Complexes of the Late Transition Metals

“…In general, Os(VI) nitrido complexes are electrophilic (though organometallic nitrides are nucleophillic) [79] and react with various nucleophiles to form products with new bonds to the nitride nitrogen atom (Scheme 8). [80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98] The nitrido group in these Os compounds does not display significant nucleophilicity. In fact, Osnitrido species react with electrophilic acylating reagents via ligand substitution pathways that keep the OsN bond intact.…”

Addition of Carbenes to an Osmium(VI) Nitride Complex

“…Osmium(vi) nitride species containing nitrogen-based ligands, such as cis-and trans- [Os VI (N)(tpy)Cl 2 ] + (tpy = 2,2Ј:6Ј,2ЈЈ-terpyridine), [Os VI (N)(tpm)Cl 2 ] + [tpm = tris(1pyrazolyl)methane], [Os VI (N)(Tp)Cl 2 ] [Tp -= hydrotris(1pyrazolyl)borate], and [Os VI (N)(bpy)Cl 3 ] (bpy = 2,2Ј-bi-pyridine), have been shown to exhibit novel electrophilic properties. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] A variety of reagents such as phosphanes, amines, cyanide, azide, Grignard reagents, arylboranes, amine N-oxides, alkenes, and disubstituted benzenethiols have been reported to add to the nitride ligand. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] These reactions are summarised in Scheme 1.…”