Synthesis of Tungsten (1-Pyridinio)imido Complexes:  Facile N−N Bond Cleavage To Form Pyridine from Coordinated Dinitrogen¹

Abstract

Tungsten hydrazido complexes [WCl2(NNH2)(L)(PMe2Ph)2] (L = CO, C2H4) derived from dinitrogen complex [W(N2)2(PMe2Ph)4] reacted with 2,4,6-trimethylpyrylium or 2,6-diethoxycarbonyl-4-phenylpyrylium tetrafluoroborate to give novel (1-pyridinio)imido complexes in high yields, which were spectroscopically and crystallographically characterized. The (2,4,6-trimethylpyridinio)imido complexes [WCl2(NNC5H2Me3)(L)(PMe2Ph)2][BF4] underwent facile and selective N−N bond cleavage upon treatment with KOH or cobaltocene at room temperature to yield 2,4,6-trimethylpyridine in high yields.

Synthesis and Reductive N−N Bond Cleavage of Neutral and Cationic Titanium (1-Pyridinio)imido Complexes

“…In contrast, the N−N bond lengths in complexes 3a − c are 1.363(3) Å, 1.364(3) and 1.361(3) Å, and 1.367(5) Å, respectively, and can be regarded as in the range of N−N single bonds. These values are similar to that found in the (1-pyridinio)imido complex [WCl 2 (NNC 5 H 2 Me 3 -2,4,6)(CO)(PMe 2 Ph) 2 ][OTf] (1.366(10) Å; OTf = OSO 2 CF 3 ), and slightly shorter than those in the related hydrazidium complexes [WCl(NNH 3 )(PMe 3 ) 4 ]Cl 2 (1.396(20) Å), [Cp*MoMe 3 (NNMe 3 )][OTf] (1.426(5) Å),4b and [Cp*TaS(SBu t )(NNMe 2 Bu t )] (1.46(1) Å) . Complexes 3a − c contain a titanium center coordinated by one Cp (or Cp*), two chlorine atoms, and one imido nitrogen atom, making the complexes monomeric.…”

“…Interestingly, reductive N−N bond cleavage was not observed for [Cp*MoMe 3 (NNMe 3 )][OTf] with zinc-amalgam or cobaltocene, and on the basis of this fact it was suggested that hydrazidium complexes (M⋮N−NH 3 + ) are not intermediates of the most facile N−N bond cleavage reaction path of hydrazine and hydrazido ligands 4b. On the other hand, the N−N bond in the cationic W(IV) complex [WCl 2 (NNC 5 H 2 Me 3 -2,4,6)(CO)(PMe 2 Ph) 2 ][OTf] could be reductively cleaved by cobaltocene (vide supra) . Furthermore, it has been demonstrated by Holm and co-workers that N−N bond cleavage easily takes place in the reaction between TsNNC 5 H 5 (Ts = tosyl) and the Mo(IV) complex [MoO(Et 2 dtc) 2 ] (Et 2 dtc = diethyldithiocarbamate) …”

“…In the course of our ongoing investigation into reactivities of dinitrogen complexes of molybdenum and tungsten, we have recently found that the tungsten dinitrogen complex cis -[W(N 2 ) 2 (PMe 2 Ph) 4 ] can be converted into (1-pyridinio)imido complexes such as [WCl 2 (NNC 5 H 2 Me 3 -2,4,6)(CO)(PMe 2 Ph) 2 ][BF 4 ] . Interestingly, the N−N bond of this complex was cleaved to give 2,4,6-collidine under relatively mild conditions.…”

Beyond Ammonia: Nitrogen–Element Bond Forming Reactions with Coordinated Dinitrogen

“…83 which could be further transformed to free pyridine and ammonia by reaction with KOH. 84 The nucleophilicity of the terminal nitrogen atom in [W(N 2 )(NCS)(dppe) 2 ] − was also leveraged to promote N−C bond formation by S N Ar pathways. Fluoroarenes such as p-FC 6 H 4 CO 2 Me that are activated by complexation to Cr(CO) 3 , [(η 5 -C 5 H 5 )Fe], or [(η 5 -C 5 H 5 )Ru] underwent N−C bond formation and the resulting bimetallic complexes were characterized by X-ray diffraction (Scheme 11).…”

“…By contrast, the hydrazido derivative [WX 2 (NNH 2 )­(PMe 2 Ph) 3 ] (X = Cl or Br) underwent further functionalization with phthalaldehyde to generate the corresponding (phthalimidin-2-yl)­imido complexes . In addition, treatment with pyrilium compounds produced (1-pyridinio)­imido complexes which could be further transformed to free pyridine and ammonia by reaction with KOH …”