Vanadium Complexes of the N(CH₂CH₂S)₃^3-and O(CH₂CH₂S)₂^2-Ligands with Coligands Relevant to Nitrogen Fixation Processes

Abstract

Vanadium(III) and vanadium(V) complexes derived from the tris(2-thiolatoethyl)amine ligand [(NS3)3-] and the bis(2-thiolatoethyl)ether ligand [(OS2)2-] have been synthesized with the aim of investigating the potential of these vanadium sites to bind dinitrogen and activate its reduction. Evidence is presented for the transient existence of (V(NS3)(N2)V(NS3), and a series of mononuclear complexes containing hydrazine, hydrazide, imide, ammine, organic cyanide, and isocyanide ligands has been prepared and the chemistry of these complexes investigated. [V(NS3)O] (1) reacts with an excess of N2H4 to give, probably via the intermediates (V(NS3)(NNH2) (2a) and (V(NS3)(N2)V(NS3) (3), the V(III) adduct [V(NS3)(N2H4)] (4). If 1 is treated with 0.5 mol of N2H4, 0.5 mol of N2 is evolved and green, insoluble [(V(NS3))n] (5) results. Compound 4 is converted by disproportionation to [V(NS3)(NH3)] (6), but 4 does not act as a catalyst for disproportionation of N2H4 nor does it act as a catalyst for its reduction by Zn/HOC6H3Pri2-2,6. Compound 1 reacts with NR1(2)NR2(2) (R1 = H or SiMe3; R2(2) = Me2, MePh, or HPh) to give the hydrazide complexes [V(NS3)(NNR2(2)] (R2(2) = Me2, 2b; R2(2) = MePh, 2c; R2(2) = HPh, 2d), which are not protonated by anhydrous HBr nor are they reduced by Zn/HOC6H3Pri2-2,6. Compound 2b can also be prepared by reaction of [V(NNMe2)(dipp)3] (dipp = OC6H3Pri2-2,6) with NS3H3. N2H4 is displaced quantitatively from 4 by anions to give the salts [NR3(4)][V(NS3)X] (X = Cl, R3 = Et, 7a; X = Cl, R3 = Ph, 7b; X = Br, R3 = Et, 7c; X = N3, R3 = Bu(n), 7d; X = N3, R3 = Et, 7e; X = CN, R3 = Et, 7f). Compound 6 loses NH3 thermally to give 5, which can also be prepared from [VCl3(THF)3] and NS3H3/LiBun. Displacement of NH3 from 6 by ligands L gives the adducts [V(NS3)(L)] (L = MeCN, nu CN 2264 cm-1, 8a; L = ButNC, nu NC 2173 cm-1, 8b; L = C6H11NC, nu NC 2173 cm-1, 8c). Reaction of 4 with N3SiMe3 gives [V(NS3)(NSiMe3)] (9), which is converted to [V(NS3)(NH)] (10) by hydrolysis and to [V(NS3)(NCPh3)] (11) by reaction with ClCPh3. Compound 10 is converted into 1 by [NMe4]OH and to [V(NS3)NLi(THF)2] (12) by LiNPri in THF. A further range of imido complexes [V(NS3)(NR4)] (R4 = C6H4Y-4 where Y = H (13a), OMe (13b), Me (13c), Cl (13d), Br (13e), NO2 (13f); R4 = C6H4Y-3, where Y = OMe (13g); Cl (13h); R4 = C6H3Y2-3,4, where Y = Me (13i); Cl (13j); R4 = C6H11 (13k)) has been prepared by reaction of 1 with R4NCO. The precursor complex [V(OS2)O(dipp)] (14) [OS2(2-) = O(CH2CH2S)2(2-)] has been prepared from [VO(OPri)3], Hdipp, and OS2H2. It reacts with NH2NMe2 to give [V(OS2)(NNMe2)(dipp)] (15) and with N3SiMe3 to give [V(OS2)(NSiMe3)(dipp)] (16). A second oxide precursor, formulated as [V(OS2)1.5O] (17), has also been obtained, and it reacts with SiMe3NHNMe2 to give [V(OS2)(NNMe2)(OSiMe3)] (18). The X-ray crystal structures of the complexes 2b, 2c, 4, 6, 7a, 8a, 9, 10, 13d, 14, 15, 16, and 18 have been determined, and the 51V NMR and other spectroscopic parameters of the complexes are discussed in terms of electronic effects.

Vanadium(IV and V) Complexes ContainingSNO (Dithiocarbonylhydrazone; Thiosemicarbazone) Donor Sets

“…[38Ϫ46] The bond lengths between the vanadium centre and the three ligand functions also compare to those of the vanadium-ONS complex [V(nap-amtph) 2 ], where nap-amtph is the Schiff base formed between o-hydroxynaphthaldehyde and o-aminothiophenol, [47] and to [50] On the other hand, VϪS bonds in V V complexes containing the tetrafunctional trithiolate N(CH 2 CH 2 S Ϫ ) 3 are still shorter (averaging around 2.25 Å [29,30] ) than in the aforementioned complexes with ONS donor sets. The VϪCl bond in 3a [2.341(1) Å ] again compares to that in other chlorovanadium complexes.…”

Theoretical study on the mechanism of catalytic reduction of hydrazine to ammonia mediated by vanadium (III) thiolate complexes

“…In view of the reported inability of the VNS3 [S3 ¼ ðSCH 2 CH 2 Þ 3À 3 ] complex to convert hydrazine to ammonia [16], we have decided to carry out similar calculation for this system as well. The geometrical parameters obtained for both the vanadium systems mediated intermediates follow almost a similar trend (Figs.…”

“…So far, limited synthetic models are known that catalyze the conversion of N 2 H 4 to NH 3 , the late stage of ammonia synthesis [9][10][11][12][13][14][15]. Davies and coworkers reported vanadium complexes of the NðCH 2 CH 2 SÞ 3À 3 and OðCH 2 CH 2 SÞ 2À 2 ligands [16]. They successfully isolated the V(NS3)N 2 H 4 adduct but none of these complexes catalyze the reduction of N 2 H 4 to NH 3 .…”

First Row Transition Metal Mediated Redox Reactions of Hydrazines and Isonitriles

“…Similarly, limited examples of catalytic hydrazine reduction have been reported despite its much weaker N-N single bond (38 kcal/mol vs. 226 kcal/mol). [28][29][30][31][32][33][34][35][36][37][38][39][40] Most of these examples utilize the metals found in the Nase active site, iron, molybdenum, and vanadium. Early systems developed were based primarily on molybdenum, following Schrock's 1991 report of a molybdenum-pentamethylcyclopentadienyl (Cp*) catalyst (Scheme 2).…”

“…34,35 Although not catalytic, similar vanadium complexes developed by the Sanders group featuring a tris(thiolato)amine ligand are also capable of hydrazine reduction to ammonia. 36,37 It is important to note the structural analogy of these thiolate-ligated complexes to the Nase active site, in addition to being functional mimics.…”