Ruthenium−Aminoallenylidene Complexes from Butatrienylidene Intermediates via an Aza-Cope Rearrangement:  Synthetic, Spectroscopic, Electrochemical, Spectroelectrochemical, and Computational Studies

Abstract

Ruthenium-aminoallenylidene complexes trans-[Cl(L 2) 2 RuCCC(NR 2)CH 2 R′] + EF 6-(4af; E) P, Sb, L 2) chelating diphosphine) are accessible from the respective dichloro precursors, NaEF 6 , butadiyne, and an allylic amine in a one-pot procedure. The reactions proceed via the primary butatrienylidene intermediate trans-[Cl(L 2) 2 RudCdCdCdCH 2 ] + and the initial addition products trans-[Cl(L 2) 2 Ru-CtCC(NR 2 R′)dCH 2 ] + via an Aza-Cope type rearrangement. Amine adducts have been isolated for (dimethylamino)-2-pentyne (3f) and 1-methyl-1,2,5,6-tetrahydropyridine (3g). The former cleanly converts to its aminoallenylidene isomer upon warming. All products have been characterized by various spectroscopic techniques, including NMR, IR, and UV/vis spectroscopy and cyclic voltammetry; complex 4b was also characterized by X-ray crystallography. Most notable are the considerable bond length alternations along the unsaturated C 3 ligand and the trigonalplanar nitrogen, indicative of its sp 2 character. Aminoallenylidene complexes of this type are best described as a hybrid between true cumulenic and iminium alkynyl resonance forms, with major contributions of the latter, as is also evident from the high energy barriers for rotation around the iminium type CdN bond. The effect of the electron density on the metal on the spectroscopic and electrochemical properties of the cations in 4 has been probed for the dimethylallylamine-derived complexes trans-[Cl(L 2) 2 RuCCC(NMe 2)C 4 H 7 ] + EF 6-(4a-c), which only differ in the nature of the chelating diphosphine ligand. Aminoallenylidene complexes 4 undergo reversible one-electron oxidation. In contrast, their reduction is irreversible at room temperature but partially reversible at temperatures between 233 and 195 K. The spectroscopic changes accompanying oxidation were monitored by in situ UV/ vis, IR, and EPR techniques. DFT calculations have been performed on the model complexes trans-[Cl(L 2) 2 RudCdCdCdCH 2 ] + and trans-[Cl(L 2) 2 RuC 3 {N(CH 3) 2 }CH 3 ] +. Our results explain the regioselectivity of nucleophilic addition to the proposed butatrienylidene intermediate and the spectroscopic and electrochemical properties of aminoallenylidene complexes 4. Both orbital and steric effects are equally important in the regioselective addition to C 3. The calculations further indicate primarily metal-based oxidation and ligand-based reduction of complexes 4, in accordance with experimental observations. They also let us assign the experimental UV/vis bands and the two main IR absorptions in the 2000-1500 cm-1 region.

(Allenylidene)ruthenium Complexes with Redox‐Active Substituents and Ligands

“…This accounts for the strong Metal-to-Ligand Charge-Transfer (MLCT) character of this band. [4] This assignment also seems to be valid here, since a much weaker absorption at considerably lower energies (907 nm in 2a and at 902 nm in 2b) was observed for the forbidden HOMO Ǟ LUMO transition. With reference to the trans-[Cl(dppm) 2 Ru]-derived complexes, the HOMO should also be dominated by the metal unit and have appreciable MLCT character as well.…”

“…This has been explained in terms of a larger contribution from the alkynyl-type resonance form II as opposed to the genuine cumulenylidene-type resonance form I (Scheme 2) when the metal center is more electron-rich and the substituents at C-γ are less able to support a positive charge adjacent to them. [1,2,4,20,21] In the light of the stability of the ferrocenylmethylene carbenium ion [22] the ferrocenyl substituent may be expected to stabilize the alternative alkynyl resonance form III, as indicated in Scheme 2, in line with the results from IR spectroscopy. We note here that Le Bozec et al arrived at a similar conclusion in their study of [(C 6 Me 6 )-Cl(PMe 3 )RuϭCϭCϭC(Ph)(Fc)] ϩ PF 6 Ϫ , closely related to 3a and 3b.…”

“…[7] Characteristic attributes of allenylidene complexes include: (i) the asymmetric valence band known as the ''allenylidene stretch'', giving rise to highly intense absorption bands in their infrared spectra, (ii) low-field signals for the carbon atoms of the unsaturated ligands in their 13 C NMR spectra, [4,8] and (iii) their optical spectra, which feature two bands in the visible region. The band at lower energy is due to the forbidden HOMO Ǟ LUMO transition and so is only weak, while that at higher energy is much more intense and represents the allowed transition from the lower lying second-highest occupied molecular orbital HOMO-1.…”

“…[1,4,24] The triphenylphosphane-substituted complex 3a was found to exist as two isomers, as shown by the two sets of resonance signals for the carbon atoms of the cumulenylidene ligand. These are observed at δ ϭ 287.0, 184.3, and 168.2 ppm for the major isomer and δ ϭ 282.8, 180.5 and 169.1 ppm for the minor counterpart, and point to hindered rotation of the allenylidene ligand around the RuϭC bond.…”

“…[1,4] By combining electrochemical and spectroscopic methods we have been able to characterize the oxidation as a primarily metal-based process (i.e., the Ru II/III couple) and the reduction as ligandbased, the latter assignment being confirmed by EPR and IR spectroscopy on their reduced forms. The reduction of the diphenyl-substituted complex trans-[Cl(dppe) 2 …”