Reduction of Dinitrogen to Ammonia at a Well-Protected Reaction Site in a Molybdenum Triamidoamine Complex

Abstract

We have synthesized a triamidoamine ligand ([(RNCH2CH2)3N]3-) in which R is 3,5-(2,4,6-i-Pr3C6H2)2C6H3 (HexaIsoPropylTerphenyl or HIPT). The reaction between MoCl4(THF)2 and H3[HIPTN3N] in THF followed by 3.1 equiv of LiN(SiMe3)2 led to formation of orange [HIPTN3N]MoCl. Reduction of [HIPTN3N]MoCl with magnesium in THF under dinitrogen led to formation of salts that contain the {[HIPTN3N]Mo(N2)}- ion. The {[HIPTN3N]Mo(N2)}- ion can be oxidized by zinc chloride to give [HIPTN3N]Mo(N2) or protonated to give [HIPTN3N]Mo-N=N-H. Other relevant compounds that have been prepared include {[HIPTN3N]Mo-N=NH2}+, [HIPTN3N]MoN, {[HIPTN3N]Mo=NH}+, and {[HIPTN3N]Mo(NH3)}+. (The anion is usually {B(3,5-(CF3)2C6H3)4}- = {BAr'4}-.) Reduction of [HIPTN3N]Mo(N2) with CoCp2 in the presence of {2,6-lutidinium}BAr'4 in benzene leads to formation of ammonia and {[HIPTN3N]Mo(NH3)}+. Preliminary X-ray studies suggest that the HIPT substituent creates a deep, three-fold symmetric cavity that protects a variety of dinitrogen reduction products against bimolecular decomposition reactions, while at the same time the metal is left relatively open toward reactions near the equatorial amido ligands.

Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center

“…In C 6 D 6 the solubility of ammonia was found to be 0.1 M atm Ϫ1 , and the solubility of 15 ) can be determined readily. In a series of experiments of this type, K eq was established to be Ϸ0.1 (see supporting information for details).…”

“…The catalysts are Mo complexes that contain the [(HIPTNCH 2 CH 2 ) 3 Fig. 1 (15)(16)(17). The [HIPTN 3 N] 3Ϫ ligand was designed to prevent any bimetallic reactions (aside from electron transfer), maximize steric protection of a metal coordination site in a monometallic species, and provide increased solubility of intermediates in nonpolar solvents.…”

Cationic silyldiazenido complexes of the Fe(diphosphine)2(N2) platform: structural and electronic models for an elusive first intermediate in N2fixation

“…13 The Fe-N bond lengths (see Table 1) are appreciably shorter, and N-N bond lengths notably longer, than in 1 (cf. 1.748 (8) 8,15 Collectively these data correlate with an enhanced transfer of electron density from Fe into the NN fragment upon electrophilic attack at the terminal nitrogen (N β ), which results in an increased Fe-N bond order attendant with a reduction in the N-N bond order. Recently, the borane adduct Fe(depe) 2 (NN·B(C 6 F 5 ) 3 ) has been structurally verified, 16 which permits a valuable comparison between electrophiles of differing Lewis acidity upon the extent of electron transfer from Fe to N 2 , using the same parent system 1.…”

Can [M(H)2(H2)(PXP)] Pincer Complexes (M=Fe, Ru, Os; X=N, O, S) Serve as Catalyst Lead Structures for NH3 Synthesis from N2 and H2?

“…A C H T U N G T R E N N U N G isopropylterphenyltriamidoamine) proved to be an active catalyst for catalytic NH 3 synthesis. [10] In this approach N 2 was first coordinated at the molybdenum complex, and subsequently protonated/reduced repeatedly in the presence of acids (proton source) and metallocenes (reducing agent), generating NH 3 . Meanwhile some intermediates were characterized spectroscopically and crystallographically, while important details of the mechanism were elucidated by computational studies.…”