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Conversion of σ,η-allenyl group into a σ,σ-(Diphenylphosphino)allyl and a hexa-1,3,5-triene-2,6-diyl ligand at a diiron center

Lookup NU author(s): Dr Simon DohertyORCiD

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Abstract

Thermolysis of a toluene solution of [Fe2(CO)5(PPh3)CH-PPh 2){μ-η1:η2 α,β-(H)Cα=Cβ=CγR2}] (R = H, 4a; R = Me, 4b), obtained via CO substitution in [Fe2(CO)6(μ-PPh2){μ-η 1:η2 α,β-(H)Cα=Cβ=CγR2}] (R = H, 1a; R = Me, 1b), results in regiospecific 1,2-migration of the phosphido bridge to the α-carbon of the σ,η-allenyl group to give [Fe2(CO)5(PPh3){μ-η 1:η3-(Ph2P)HCCCR2}] (R = H, 5a; R = Me, 5b) and, in the case of R = H, [Fe2(CO)4(PPh32{μ-η 1:η3(PPh2)HCCCH2}] (R = H, 6). Single-crystal X-ray analysis of 6 revealed that the Ph2PC(H)C= CH2 fragment can be considered as a 1-diphenylphosphino-substituted propene-l,2-diyl group which functions as a six-electron donor. Thermolysis of a toluene solution of [Fe2(CO)6(μ-PPh2){μ-η 1:η2 α,β-(H)Cα==Cβ=CγMe2}] (1b) results in a similar transformation to give [Fe2(CO)6{μ-η1:η 3-(Ph2P)HCCCMe2}] (7). In contrast, in the absence of PPh3, thermolysis of a toluene solution of [Fe2(CO)6(μ-PPh2){μ-η 1:η2α,β-(H)Cα=C̀=C̀H 2}] (1a) results in intermolecular carbon-carbon coupling of two /μ-η1:η2-allenyl groups to give [Fe4(CO)11(μ-PPh2) 2{μ-η1:η2η 1:η2:η2-(H)C=CHCHCHC=CH2}] (8). Deuterium labeling studies have been used to identify the most likely regioselectivity of carbon-carbon coupling and hydrogen migration. A single-crystal X-ray study of 8 revealed a polyunsaturated hexa-l,3,5-trien-2,6-diyl chain, coordinated to one bimetallic unit as a μ-η1:η2-alkenyl group and to the other as a μ-η1η2: η2-butadien-2-yl group. Solution NMR studies reveal that both 4a and 4b exist as rapidly interconverting mixtures of two isomers (4a1 + 4a2 and 4b1 + 4b2). The free energy of activation associated with this exchange (ΔG = 15.9 ± 0.2 kcal mol-1, 4a; ΔG = 16.1 ± 0.2 kcal mol-1, 4b) is consistent with a flipping motion of the allenyl ligand that interchanges the isomer with the triphenylphosphine attached to the σ-bonded iron with that in which the phosphine is attached to the η2-bonded iron. A value of 1.2 kcal mol-1 has been estimated for the difference in free energy between isomers 4a1 and 4a2. Full structural details of the compounds 4a1, 4b2, and 6-8 are reported.

Publication metadata

Author(s): Doherty S

Publication type: Article

Publication status: Published

Journal: Organometallics

Year: 2000

Volume: 19

Issue: 26

Pages: 5696-5708

ISSN (print): 0276-7333

ISSN (electronic): 1520-6041

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/om000702x

DOI: 10.1021/om000702x

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