Facile Synthesis of Alkynyl− and Vinylidene−Niobocene Complexes. Unexpected η¹-Vinylidene−η²-Alkyne Isomerization

Abstract

Treatment of Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (Cl)(L) (1) with Mg(CtCR) 2 in toluene, under appropriate reaction conditions, leads to the alkynyl complexes Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (CtCR)(L) (2: L ) CO, R ) Ph (2a); LThe alkynyl-containing niobocene species 2 can be chemically or electrochemically oxidized to give the corresponding cation-radical alkynyl complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (CtCR)(L)] •+ [BPh 4 ] -(3: L ) CO, R ) Ph (3a); L ) CO, R ) t Bu (3c); L ) PMe 2 Ph, R ) Ph (3d)). These complexes, under different experimental conditions, give rise to the mononuclear vinylidene d 2 niobocene species [Nb(η 5 -C 5 H 4 -SiMe 3 ) 2 (dCdCHR)(L)][BPh 4 ] (4: L ) CO, R ) Ph (4a); L ) CO, R ) t Bu (4c); L ) PMe 2 Ph, R ) Ph (4d)) with a hydrogen atom by abstraction from the solvent or, for 3a, the binuclear divinylidene d 2 niobocene complex [(η 5 -C 5 H 4 SiMe 3 ) 2 (CO)NbdCdC(Ph)(Ph)CdCdNb(CO)(η 5 -C 5 H 4 SiMe 3 ) 2 ][BPh 4 ] 2 (4a′) from a competitive ligand-ligand coupling process. Complexes 4 were also prepared by an alternative procedure in which the corresponding complexes 2 were reacted with HBF 4 . Finally, in solution the CO-containing vinylidene mononuclear complexes 4a and 4c undergo an unexpected isomerization process to give the η 2 -alkyne derivatives [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 (C,C)-HCtCR)(CO)] + (5: R ) Ph (5a); R ) t Bu (5c)). The structure of 5a was determined by single-crystal diffractometry. DFT calculations were carried out on [NbCp 2 (dCdCHCH 3 )(L)] + /[NbCp 2 (HCtCCH 3 )(L)] + (Cp ) η 5 -C 5 H 5 ; L ) CO, PH 3 ; exo, endo) model systems in order to explain the η 1 -vinylidene-η 2 -alkyne rearrangement observed. Calculations have shown that in both carbonyl-niobocene and phosphine-niobocene systems the η 1vinylidene and the η 2 -alkyne complexes are isoenergetic, in marked contrast with the systems previously considered in theoretical studies. The reaction takes place through an intraligand 1,2-hydrogen shift mechanism where η 2 (C,H)-alkyne species are involved. The energy barrier for the isomerization process in the phosphine-containing niobocene systems is almost 10 kcal mol -1 higher than in the analogous process for the carbonyl-containing niobocene system. This increase in activation barrier indicates that the different experimental behavior between 4a, 4c, and 4d has a kinetic rather than a thermodynamic origin. Finally, the interconversion between exo and endo isomers has been studied.

Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne Complexes

“…59 While the addition of 1 equiv of alkyne generated the desired alkyne complex (Cp Si ) 2 M(RCCR)Cl ( 29), treating 28 with 1 equiv of a diyne, such as 1,4-diphenylbuta-1,3-diyne, afforded the related complex Cp′ 2 M(RCCCCR)Cl (30). 60 Although the majority of work in this field has focused on cyclopentadienyl-containing systems such as the ones discussed above, alkyne moieties have been installed via a similar reduction protocol in Ta and Nb complexes featuring a variety of ancillary ligand sets. For example, McConville and co-workers 30 investigated the synthesis of Ta alkyne complexes featuring a pyridine-diamido pincer ligand, BDPP, as shown in Scheme 9.…”

“…86,87 Similar to the case for complexes 52−54, Schrock and co-workers reported 88 the synthesis of a niobium alkyne chloride complex that is also believed to proceed via a reductive elimination step (Scheme 15). The hydrogenolysis of Cp*NbMe 2 Cl 2 afforded the Nb III dimer [Cp*NbCl 2 ] 2 (59), likely via the loss of H 2 from the putative Nb V dihydride Cp*Nb(H) 2 Cl 2 (58); treating complex 59 with 3-hexyne generated the mononuclear Nb alkyne complex Cp*Nb(3hexyne)Cl 2 (60). In contrast to the Schwartz−Herberich or Rothwell systems (52−54), where the low-valent M III intermediate is generated in situ, complex 59 is stable enough to be isolated, similar to the case for complexes 28 (Otero 59 ) and 33 (Wolczanski 61 ).…”

Sandwich and Half‐Sandwich (Imido)niobium Complexes

“…Couplings constants are given in Table 1. (9) Recently, we reported the preparation of first alkynylcontaining niobocene complexes [63] and by a natural extension of this work the alkynyl(imido)niobium complexes [Nb(ϭNtBu)(η 5 (41), tBu (42)], were synthesized. [55] The general synthetic method involved the reaction of dialkynylmagnesium reagents and the corresponding (imido)niobocene halide derivative.…”

“…[55] Protonation of these complexes may occur at the alkynyl and/or the imido moiety as observed previously in niobocene complexes containing only one of these ligands. [53,63] In the reactions of 30 and 37 with CF 3 COOH and HBF 4 , respectively, the protonated compounds were isolated [Equation (13)]. It was found that in these alkynyl-(imido) complexes protonation takes place preferentially at the nitrogen atom of the imido group leading to the formation of the cationic amide species [Nb(NHtBu)(η 5 (50) was isolated by recrystallization from a hexane solution.…”

Pyrrolyl substituted allenylidene complexes of ruthenium

“…We note in this context, that related vinylidene or alkynyl complexes frequently undergo carbon centered coupling reactions after oxidation, although the corresponding radicals are primarily metal centered species. [67][68][69][70][71][72][73][74] In IR spectroscopy, the intense allenylidene band at 1958 cm −1 loses much of its initial intensity while being shifted to 1954 cm −1 (see Fig. 4).…”