Nucleophilic Addition to the η²-Alkyne Ligand in [CpFe(CO)₂(η²-R−C⋮C−R)]⁺. Dependence of the Alkenyl Product Stereochemistry on the Basicity of the Nucleophile

Abstract

Reaction of the cationic η 2 -alkyne iron complex, [Fp(η 2 -Ph-CtCPh)]BF 4 (1) [Fp ) (η 5 -C 5 H 5 )-Fe(CO) 2 ], with various O-, N-, and C-nucleophiles afforded products containing the Ph-Cd C-Ph linkage. i.e., cis-(3) and trans-alkenyl complexes (4), Fp-C(Ph)dC(Ph)-Nu, and metallacycles, Cp(CO)Fe-C(Ph)dC(Ph)-C(dO)-Nu [coordinated at dO (5) or Nu (6)]. Crystallographic determination of the configuration of the CdC moiety in 3 and 4 and analysis of the product distribution revealed that basic nucleophiles bearing alkyl substituents produced the cis-alkenyl complex (3) and/or the metallacycles (5, 6), whereas less basic nucleophiles bearing aryl substituents afforded the trans-alkenyl complex (4). Correlation between the product distribution and pK b values of the nucleophiles leads to the conclusions that (1) products 3, 5, and 6 arise from acyl intermediates formed by initial nucleophilic addition of basic nucleophiles at the CO ligand, (2) trans-alkenyl complexes 4 result from addition of less basic nucleophiles to the η 2 -alkyne ligand from the exo-side, and (3) the equilibrium of the CO addition governed by the basicity of the nucleophile is the key step of the diversity of the stereochemistry of the alkenyl products. Reactions of 2-hexyne complex, [Fp(η 2 -Et-CtC-Et)]BF 4 (2), were also examined.

Synthesis and characterization of amine complexes of the cyclopentadienyliron dicarbonyl complex cation, [Cp(CO)2Fe]+

“…This mode of packing is observed in related structures [47,56] which aims to minimize steric repulsion. Hence, a set of molecules in a layer of 2a are related to the subsequent layer via a screw axis, while 2b favored a packing arrangement in which the bulky Cp ligands of subsequent layers occupy the spaces created due to the mutual repulsion of the flexible alkyl chains of adjacent neighboring molecules.…”

“…+ [47]. The N-C bond length was found to fall in the range 1.479-1.491 Å, slightly longer than the C-N bond (1.47 Å) observed for free methylamine [57] and shorter than the N-C bond length ( [47].…”

“…+ was also reported [47] but no physical or analytical details were given. Cyclopentadienyliron dicarbonyl complexes of secondary [48], tertiary [35] and cyclic [49] amines are established.…”

“…The N-C bond length was found to fall in the range 1.479-1.491 Å, slightly longer than the C-N bond (1.47 Å) observed for free methylamine [57] and shorter than the N-C bond length ( [47]. The C-C bond lengths in the alkyl chain of the alkylamine fall within the range 1.511-1.527 Å which is in close agreement with that of typical C-C bond lengths (1.53 Å) of alkane chains, indicating that the alkylamine suffered no distortion.…”

Reactions of N-heterocyclic ligands with substitutionally labile organometallic complexes, [(η5-C5R5)Fe(CO)2E]BF4

“…The IR spectra of compounds 3a-4b show two strong absorption bands in the range 2058-1982 cm À1 assignable to the two terminal carbonyls as expected for cationic amine complexes [44,[48][49][50]. The IR spectra of 4a and 4b exhibited characteristic v(CO) stretching vibrations at wavenumbers lower than those of 3a and 3b by ca.…”