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Comptes Rendus Chimie
Volume 8, n° 8
pages 1174-1185 (août 2005)
Doi : 10.1016/j.crci.2004.11.036
Received : 30 June 2004 ;  accepted : 16 November 2004
Solid-state characterisation of the [(η2-dppe)(η5-C5 Me5 )FeCO]+ cation: an unexpected ‘oxidation' product of the [(η2-dppe)(η5-C5 Me5 )FeC≡C(C6 H4 )NMe2 ]+ radical cation

Frédéric Paul a, , Loic Toupet b , Thierry Roisnel c , Paul Hamon a, Claude Lapinte a
a Organométalliques et catalyse : chimie et électrochimie moléculaires, UMR CNRS 6509, Institut de chimie, université Rennes-1, campus de Beaulieu, bât. 10C, 35042 Rennes cedex, France 
b Groupe « Matière condensée et matériaux », UMR CNRS 6626, Institut de chimie, université Rennes-1, campus de Beaulieu, 35042 Rennes cedex, France 
c Laboratoire de chimie du solide et inorganique moléculaire, UMR CNRS 6511, Institut de chimie, université Rennes-1, campus de Beaulieu, 35042 Rennes cedex, France 

*Corresponding author.

Several attempts to grow crystals of the [(η2-dppe)(η5-C5 Me5 )Fe-C≡C-1,4-(C6 H4 )NMe2 ][PF6 ] radical cation (1+PF6 -) resulted in the isolation of crystals of the known iron(II) [(η2-dppe)(η5-C5 Me5 )FeCO][PF6 ] carbonyl adduct (2+PF6 -). We report here the solid-state structures of two polymorphs of this unexpected product, as well as the solid-state structure of a tetrafluoroborate analogue of 2+ for comparison purposes. Our investigations lead to a mechanistic proposal rationalising its formation from 1+PF6 -. In this respect, this work reveals one possible ‘decomposition' pathway undergone by electron-rich functional arylacetylide radical cations bearing electron-donating groups, when exposed to air. To cite this article: F. Paul et al., C. R. Chimie 8 (2005) .

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Plusieurs tentatives pour cristalliser le complexe radical cation [(η2-dppe)(η5-C5 Me5 )Fe-C≡C-1,4-(C6 H4 )NMe2 ][PF6 ] (1+PF6 -) ont abouti à l'obtention de cristaux du complexe carbonyle de Fe(II) [(η2-dppe)(η5-C5 Me5 )FeCO][PF6 ] (2+PF6 -) connu. Nous rapportons les structures à l'état solide de deux polymorphes de ce composé inattendu, ainsi que la structure d'un analogue de 2+ possédant un contre-anion tétrafluoroborate. Nos investigations permettent de comprendre comment ce complexe a été formé à partir de 1+PF6 -. Ce travail met notamment en lumière un mécanisme possible de « décomposition » des complexes acétylures d'aryle fonctionnels possédant des substituants riches en électrons, lorsque ceux-ci sont exposés à l'air. Pour citer cet article : F. Paul et al., C. R. Chimie 8 (2005) .

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Keywords : Organoiron, Fe(III) Arylacetylide, Radical cation, Fe(II) Carbonyl, Oxidation, Kinetics, X-rays

Mots clés : Organofers, Radical cation, Acétylure d'aryle de Fe(III), Carbonyles de Fe(II), Oxydation, Cinétique, Cristallographie

1  In line with the data reported for [(η2-dppm) (η5-C5 H5 )FeCO][PF6 ], [19] the carbonyl adduct 2+ exhibits only a weak absorption at 380 nm in its UV spectrum with a tail extending in the visible range. Accordingly, the red cubes (structure A) or pale-orange plates (structure B) of 2+PF6 - give pale yellow solutions upon dissolution in dichloromethane that become more and more red upon concentration. In contrast, 1+PF6 - is much more strongly coloured in reason of a strong absorption at 880 nm in dichloromethane [23].
2  Also, the CO-bond length/ν (CO) correspondence does not hold among the various molecules of 2+ constituting the asymetric units in these polymorph (Table 3). For instance, the molecules in 2+PF6 -(A) present a significantly shorter bond length than that of 2+PF6 -(B) (1.119±0.012 Å and 1.088±0.012 Å vs. 1.153±0.021 Å) and should therefore exhibit both higher wave numbers for the carbonyl stretch. Notably, this deviation cannot be ascribed to the existence of hydrogen-bond interactions involving the carbonyl ligand of one of the molecules of the asymmetric unit in 2+PF6 -(A), since the ligand in this molecule curiously exhibits the shorter C-O bond (1.088 Å) [18].

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