Formation of η²-Iminoacyls, η³-Azaallyls, and Heterometallacycles during the Rearrangements of Methyl Complexes of Molybdenum and Tungsten Containing Isocyanide Ligands

Abstract

The metalate salts Y+[Cp‘M(CO)2(CNR)]- (Y = Na, K; M = Mo, W; CNR = alkylisocyanide), in the presence of the soft electrophile MeI, experience alkylation at the metal center to afford methyl complexes of composition Cp‘MMe(CO)2(CNR). The kinetic stability of these alkyls toward rearrangement to the η2-iminoacyls Cp‘M(η2-C(NR)Me)(CO)2 or the η3-1-azaallyls Cp‘M(η3-CH2C(H)NR)(CO)2 has been found to depend markedly upon the nature of the metal, the isocyanide R substituent, and the solvent used. With respect to the latter, solvents with efficient Lewis basic capabilities (e.g., pyridine or acetonitrile) favor η2-iminoacyl formation while those which are poor donors (benzene, diethyl ether) preferentially generate the η3-azaallyls. The use of acetone as the solvent in reactions involving the Me−W−CNR complexes has allowed the isolation of metallacyclic structures of composition Cp‘W(η2(C,O)-C(Me)N(R)C(Me)2O)(CO)2, formally as the result of a [3 + 2] cycloaddition of a W−η1-C(NR)Me residue to a molecule of acetone. Two of the newly reported complexes, namely the η3-1-azaallyl CpMo(η3-CH2CHN-t-Bu)(CO)2 (1az) and the heterometallacycle CpW(η2(C,O)-C(Me)N(Me)C(Me)2O)(CO)2 (9cad) have been structurally characterized by X-ray crystallography.

Synthesis, Structure, and Reactions of a (η3-α-silabenzyl)molybdenum Complex: A Synthetic Equivalent of a Coordinatively Unsaturated Silyl Complex

“…The IR spectrum of 4 shows a CN stretching absorption band at 1647 cm –1 . This band is shifted to lower wavenumbers in comparison with those of the ( N -alkyliminoacyl)molybdenum complexes Cp′Mo(CO) 2 {η 2 ( C , N )-C(R′)NR″} (1693–1732 cm –1 ) . These results indicate the existence of a significant contribution of the (amido)(carbene)molybdenum canonical form K in addition to the (alkyl)(imino)molybdenum canonical form J in the bonding mode of 4 illustrated in Scheme .…”

“…The 13 C{ 1 H} NMR spectrum of 4 shows a signal assignable to the iminoacyl carbon at 236.4 ppm, and this chemical shift is almost identical with that for the tungsten analogue D (235.2 ppm) . However, the signal is shifted considerably downfield in comparison with those (189.7–199.2 ppm) of iminoacyl carbons in the ( N -alkyliminoacyl)molybdenum complexes Cp′Mo(CO) 2 {η 2 ( C , N )-C(R′)NR″} (Cp′ = η 5 -C 5 H 5 , η 5 -C 5 H 4 Me, η 5 -C 5 Me 5 ; R′ = Me, Et; R″ = Me, Et, i -Pr, t -Bu) . The IR spectrum of 4 shows a CN stretching absorption band at 1647 cm –1 .…”

η3-Allyl carbonyl complexes of group 6 metals: Structural aspects, isomerism, dynamic behaviour and reactivity

“…Further diversity is introduced by the incorporation of heteroatoms into the allyl ligand. While such compounds remain beyond the scope of the present article, the interested reader will find notable examples of such species in the following references: η 3 -azaallyl [58][59][60][61][62][63][64], η 3 -silaallyl [343][344][345], and silabenzyl [346][347][348][349] complexes, while the related η 3 -bound benzyl species are detailed in [350][351][352][353][354][355][356][357]. The benzyl and related complexes offer an alternative route to coordinative unsaturation, via a relatively facile η 3 → η 1 haptotropic shift.…”

“…Computational studies, however, have challenged this conjecture. [58] The difference in orbital energies concerning 13 and 20 (similarly 15 and 21) are presumably also influenced by a) the endo conformation exclusively adopted by η 3 -azaallyl complexes of this type, [58][59][60][61][62][63][64] as opposed to the exo form of 13 and 15, b) the tert-butyl substituent on the azaallyl ligand, and c) the modified metal-ligand bonding properties as a consequence of the heteroatom incorporated within the allylic backbone. Thus direct comparison of the orbital for the η 3allyl and η 3 -azallyl complexes is here not appropriate.…”

Experimental and DFT Computational Study of β-Me and β-H Elimination Coupled with Proton Transfer: From Amides to Enamides in Cp*2MX (M = La, Ce)

“…The θ angle is less than that expected if it were classified as an aza allylic group in which the θ angle is near 90°, since the bonding occurs between the carbon and nitrogen π-orbitals and the metallocene fragment. 42,43 In the enamide, the NR 1 unit is a σ-donor and the olefinic unit is a π-donor. The orientation of the four substituents on C β −C γ is not coplanar, as the NC β C γ H β torsion angle (ω1) is between 29 and 40°while the C(Me)C β C γ H α torsion angle (ω2) is between 2 and 8°.…”