Tin(II) halides and germanium(II) iodide (known two-electron-reducing agents) have been shown to be effective reducing agents in the transformation of nitrogen hydride intermediates derived from N2 into ammonia and hydrazine. Addition of SnX2 (X = Br, Cl), [Et4N] [SnCl3], or GeI2 to a mixture of trans-[Mo(N2)2(PS-diphos)(PPh2Me)2] (PS-diphos = −P(Ph)CH2CH2PPh2 attached to polystyrene-2% divinylbenzene) and HBr in CH2Cl2 (a mixture from which no ammonia is formed) causes substantial amounts of ammonia to be produced. Significant increases in the yield of ammonia were obtained when these same reducing agents were added to a mixture of trans-[Mo-(N2)2(dppe)(PPh2Me)2] (1) (dppe = Ph2PCH2CH2PPh2) and HBr in CH2Cl2: yields (mol/mol of Mo) of NH3, N2H4, and N2 changed from 0.39, 0.44, and 1.39 (100% N atom balance) in the absence of reducing agent to 1.24, 0.13, and 1.22 (99% N atom balance) in the presence of GeI2 (4 mol). This is ca. 300% increase in the yield of ammonia. The reactions of HX (X = Br, Cl) with 1 in THF and CH2Cl2 in the presence of tin(II) species were investigated. Solid 1 reacted with HX (10 mol) to produce [MoX(NNH2)(dppe)(PPh2Me)2]X, which was stable in MeOH but lost a phosphine ligand in both THF and CH2Cl2 solution to form [MoX2(NNH2)(dppe)(PPh2Me)]. Reaction of HX (6 mol) with 1 in THF in the presence of MX2 (M = Sn, Zn) produced the insoluble, crystalline molybdenum(V) hydrazido(2−) complex [MoX3(NNH2)(dppe)] (2) in high yield (X = Br, 82%, g = 2.03; X = Cl, 72%, g = 1.98). The same product was formed in the absence of MX2, but 2 was contaminated with [HPPh2Me]Br. 2 crystallized from the reaction mixture with 1 mol of THF in the monoclinic space group P21/c with a = 11.430(4) Å, b = 20.528(5) Å, c = 14.681(3) Å, β = 107.81(2)°, and Z = 4. [MoBr3(NNH2)(PS-diphos)] (g = 2.01) was synthesized by reacting trans-[Mo(N2)2(PS-diphos)(PPh2Me)2] with HBr and ZnBr2 in THF. 2 (X = Br) disproportionates in the presence of HBr in CH2Cl2 to yield 0.35 mol of NH3, 0.22 mol of N2H4, and 0.60 mol of N2 per 1 mol of 2. Addition of SnCl2 (4 mol) together with HBr in CH2Cl2 caused a doubling of the yields of both ammonia and hydrazine and a commensurate decrease in the yield of N2; the nitrogen atom balance was quantitative in both cases. The reaction of 1 and HX in CH2Cl2 solution produced ammonia and hydrazine after ca. 0.25 h at room temperature; there was no evidence of the formation of 2. Sn(acac)2 replaced one N2 ligand in trans-[Mo-(N2)2(dppe)2] (3) whereas SnX2 oxidized 1 and 3 with the formation of tin metal and small amounts of the corresponding hydrazido(2−) complex. The ability of tin(II) compounds to enhance the yields of ammonia formed from N2 complexes of molybdenum is discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry