Syntheses and X-ray crystal structures of five- and six-coordinated iron(I) and iron(II) complexes with the same (η5-C5Me5)Fe(dppe) framework

Article abstract of DOI:10.1021/om950766l

The one electron reduction of Cp*Fe(dppe)-(OSO₂CF₃) (2) with cobaltocene gave the d⁷ iron(I) Cp*Fe(dppe) (3, 90%) as thermally stable orange crystals, whereas the one-electron oxidation of 3 with ferrocenium hexafluorophos

Full text of DOI:10.1021/om950766l

10
Organometallics 1996, 15, 10-12
Syn th eses a n d X-r a y Cr ysta l Str u ctu r es of F ive- a n d
Six-Coor d in a ted Ir on (I) a n d Ir on (II) Com p lexes w ith th e
Sa m e (η5-C5Me5)F e(d p p e) F r a m ew or k
Paul Hamon,† Lo¨ıc Toupet,‡ J ean-Rene´ Hamon,*,† and Claude Lapinte*,†
Laboratoire de Chimie des Complexes de Me´taux de Transition et Synthe`se Organique,
URA CNRS 415, and Groupe Matie`re Condense´e et Mate´riaux, URA CNRS 804,
Universite´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
Received September 26, 1995X
Sch em e 1
Summary: The one electron reduction of Cp*Fe(dppe)-
(OSO2CF3) (2) with cobaltocene gave the d7 iron(I)
Cp*Fe(dppe) (3, 90%) as thermally stable orange crys-
tals, whereas the one-electron oxidation of 3 with ferro-
cenium hexafluorophosphate afforded the new d6 iron(II)
16-electron complex [Cp*Fe(dppe)][PF6] (4, 98%) isolated
as an stable orange powder. The X-ray crystal structures
of 3 and 4 show that their major structural difference
concerns the “pyramidalization” of the metal center.
Organometallic radicals and coordinatively unsatur-
ated transition metal complexes are recognized to play
an important role in material science1 as well as in
catalytic processes.2 However, despite considerable
efforts, no stable d7 iron(I)2,3 and d6 iron(II)4 compounds
in the wide [(C5R5)FeL2] series (R ) H, alkyl, aryl; L )
two-electron ligand) have ever been isolated and X-ray
characterized.5 In this communication, we report the
synthesis, the isolation, the X-ray characterization, and
the electrochemical behavior of the neutral 17-electron
iron(I) [Cp*Fe(dppe)] (3) and the 16-electron iron(II)
[Cp*Fe(dppe)][PF6] (4) complexes [Cp* ) η5-C5Me5, dppe
) η2-ethylenebis(diphenylphosphine)]. We also report
on two related 18-electron iron derivatives, namely the
triflate adduct [Cp*Fe(dppe)(OSO2CF3)] (2) and the
organometallic iron aqua complex [Cp*Fe(dppe)(OH2‚
2THF)][PF6] (5), which both exist in solution as an
equilibrium with the 16-electron cation [Cp*Fe(dppe)]+
(4, Scheme 1).
quickly turned to an air- and moisture-sensitive green
solid. Recrystallization from a THF/pentane mixture
afforded the triflate complex Cp*Fe(dppe)(OSO2CF3) (2)
in 85% yield as dark green crystals.7 The triflate was
demonstrated to be covalently bound to the metal center
in the solid state by X-ray crystallographic analysis.8
As expected, the Fe-O bond distance of 2.128(4) Å is
longer than the Fe-O bond length measured in a
previously reported iron triflate compound [Cp*Fe(CO)2-
(OSO2CF3), 2.007(3) Å]9 and rather long in comparison
with the data available in the literature.10
Treatment of a diethyl ether solution of the iron
hydride Cp*Fe(dppe)H (1)6 with an excess of methyl
triflate caused the formation of a yellow suspension that
† URA CNRS 415.
‡ URA CNRS 804.
X Abstract published in Advance ACS Abstracts, November 15, 1995.
(1) (a) Parshall, G. W. Organometallics 1987, 6, 687. (b) Miller, J .
S.; Epstein, A. J . Angew. Chem., Int. Ed. Engl. 1994, 33, 385.
(2) (a) Kochi, J . K. Organometallic Mechanisms and Catalysis;
Academic Press: New York, 1978. (b) Baird, M. C. Chem. Rev. 1988,
88, 1217. (c) Trogler, W. C., Ed. Organometallic Radical Processes;
Elsevier: New York, 1989. (d) Astruc, D. Chem. Rev. 1988, 88, 1189.
(3) (a) Bianchini, C.; Laschi, F.; Masi, D.; Ottaviani, F. M.; Pastor,
A.; Peruzzini, M.; Zanello, P.; Zanobini, F. J . Am. Chem. Soc. 1993,
115, 2723. (b) MacNeil, J . H.; Chiverton, A. C.; Fortier, S.; Baird, M.
C.; Hynes, R. C.; Williams, A. J .; Preston, K. F.; Ziegler, T. J . Am.
Chem. Soc. 1991, 113, 9834. (c) Fortier, S.; Baird, M. C.; Preston, K.
F.; Morton, J . R.; Ziegler, T.; J aeger, T. J .; Watkins, W. C.; MacNeil,
J . H.; Watson, K. A.; Hensel, K.; Le Page, Y.; Charland, J .-P.; Williams,
A. J . J . Am. Chem. Soc. 1991, 113, 542.
Treatment of 2 with 0.9 equiv of cobaltocene in THF
at -80 °C, followed by removal of the solvent under
vacuum at -20 °C and crystallization from pentane,
gave Cp*Fe(dppe) (3) as thermally stable orange crys-
(7) Analytical data for 2-5 are as follows. Anal. Calcd for C37H39F3-
FeO3P2S (2): C, 60.17; H, 5.32. Found: C, 59.82; H, 5.33. Calcd for
C36H39FeP2 (3): C, 73.35; H, 6.67. Found: C, 73.76; H, 6.82. Calcd
for C36H39F6FeP3 (4): C, 58.87; H, 5.35. Found: C, 58.55; H, 5.63.
Calcd for C44H57F6FeO3P3 (5): C, 58.94; H, 6.41. Found: C, 58.97; H,
6.49.
(8) Hamon, P.; Toupet, L.; Hamon, J .-R.; Lapinte, C. Work in
progress.
(9) Humphrey, M. B.; Lamanna, W. M.; Brookhart, M.; Husk, G. R.
Inorg. Chem. 1983, 22, 3355.
(4) (a) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry;
J ohn Wiley and Sons: New York, 1988; p 716. (b) Kazlauskas, R. J .;
Wrighton, M. S. Organometallics 1982, 1, 602.
(5) During the reviewing process of this paper, a d6 iron(II) complex
was published: Leal, A. D.; Tenorio, M. J .; Puerta, M. C.; Valerga, P.
Organometallics 1995, 14, 3839.
(6) (a) Roger, C.; Marseille, P.; Salus, C.; Hamon, J .-R.; Lapinte, C.
J . Organomet. Chem. 1987, 336, C13. (b) Roger, C.; Hamon, P.; Toupet,
L.; Rabaaˆ, H.; Saillard, J .-Y.; Hamon, J .-R.; Lapinte, C. Organometal-
lics 1991, 10, 1045.
(10) (a) Hamon, P.; Toupet, L.; Hamon, J .-R.; Lapinte, C. J . Chem.
Soc., Chem. Commun. 1994, 931. (b) Churchill, M. R.; Wormald, J .
Inorg. Chem. 1983, 22, 3355. (c) Darensbourg, D. J .; Day, C. S.; Fisher,
M. B. Inorg. Chem. 1981, 20, 3577.
0276-7333/96/2315-0010$12.00/0 © 1996 American Chemical Society

Communications
Organometallics, Vol. 15, No. 1, 1996 11
causes a lowering of the symmetry as it can be pointed
out by comparison with the structure of 4 (see below).
The complex 3 constitutes the first neutral half-
sandwich 17-electron iron-centered radical to be isolated
and structurally characterized.3,13
The 17-electron complex 13 reacts with ferrocenium
hexafluorophosphate to afford the new half-sandwich
iron(II) 16-electron complex [Cp*Fe(dppe)][PF6] (4) iso-
lated as an orange powder in 98% yield. X-ray-quality
crystals of 4 were grown by slow diffusion of pentane
into a THF solution of 4. The 16-electron structure of
the organometallic cation 4 was established by an X-ray
crystal structure (Figure 1B).7,14 The molecule is close
to the C2v symmetry. Examination of the structural
data reveals that the one-electron oxidation of 3 to 4
significantly affects the bonding pattern of the Cp*Fe-
(dppe) framework. In particular, an increase of the
average Fe-P bond distance by 0.11 Å and a slight
increase of the Fe-Cp* centroid of 0.04 Å are observed.15
Whereas the P-Fe-P bond angles are the same in 3
and 4 (∼86.5°), the major structural difference between
these two five-coordinated iron complexes concerns the
“pyramidalization” of the metal center. Indeed, the
angle defined by the ring centroid, the Fe atom, and the
middle of the P(1)-P(2) vector is close to 180° in 4
(175.2°), indicating that the coordination around the
metal atom is pseudo-trigonal bipyramidal (hybridiza-
tion dsp3). In the 17-electron neutral complex 3 a
distortion of the trigonal bipyramidal environment
toward an octahedral one is observed. The correspond-
ing angle of 167.8° is intermediate between those
expected for a trigonal bipyramid (180°) and an ideal
octahedral geometry (144.7°).16 Moreover, it is note-
worthy that the shortest distance between the iron
center and the ortho carbon atoms of the phenyl groups
of the dppe is 3.50 Å, definitely excluding an agostic
interaction between the electron-deficient metal center
and a C-H bond. In THF solution, the complex 4
exhibits a magnetic moment µeff ) 3.3 µB (310 K, Evans’
method)17 corresponding to two unpaired electrons. The
planar structure of 4, usually observed for d8 18-electron
systems such as CpCo(CO)2,18 is a consequence of its
high-spin electronic configuration. In contrast, it was
predicted on the basis of MO calculations that the
diamagnetic d6 complexes of the [CpML2] type having
only 16-electron valence should be pyramidal com-
pounds,19 in agreement with experimental data.20
F igu r e 1. Solid-state molecular structures of 3 (A) and 4
(B), with 50% probability thermal ellipsoids. The phenyl
rings of the dppe ligand are omited for clarity. Selected
interatomic distances (Å) and angles (deg): 3, Fe-P(1) )
2.126(3), Fe-P(2) ) 2.148(3), Fe-Cp*centroid ) 1.722(4),
P(1)-Fe-P(2) ) 86.87(8); 4, Fe-P(1) ) 2.259(4), Fe-P(2)
) 2.231(4), Fe-Cp*centroid ) 1.777(4), P(1)-Fe-P(2) ) 86.5-
(2).
(13) For rare examples of in situ ESR-characterized CpFe(L)2
radicals see: (a) Ruiz, J .; Lacoste, M.; Astruc, D. J . Am. Chem. Soc.
1990, 112, 5471. (b) J onas, K.; Klusmann, P.; Goddard, R. Z.
Naturforsch. 1995, 50b, 394.
(14) Crystal data for [Cp*Fe(dppe)][PF6] (4): C36H39FeF6P3, mono-
clinic, Cc, a ) 19.612(10) Å, b ) 8.422(6) Å, c ) 20.513(9) Å, â ) 113,-
88(5)°, V ) 3098(2) Å3, Z ) 4, Dcalc ) 1.264 g cm-3, R(F) ) 5.7% for
2162 independent reflections (2θmax ) 50°, scan ω/2θ ) 1). All non-
hydrogen atoms were found by direct methods with the SHELX-86
program. Hydrogen atoms were idealized, and the whole structure
was refined.
tals in 90% yield (µeff ) 1.5 µB). The molecular structure
of 3 determined by a single-crystal X-ray diffraction
analysis consists of discrete monomeric 17-electron d7
Fe(I) complex.7,11,12 The geometry of 3 is roughly
intermediate between a C2v pyramidal structure with
a vacant site and a distorted C2v planar polyhedron,
assuming that the pentamethylcyclopentadienyl group
occupies three ligand sites and the dppe the two others
(Figure 1A). The presence of the seventeenth electron
(15) Fe-P bond distance variation upon one-electron oxidation could
be interpreted in terms of a π-back-bonding contribution: Orpen, A.
G.; Connelly, N. G. Organometallics 1990, 9, 1206.
(16) Note that in the pseudo-octahedral complexes 2 and 5 the
(11) Details of the structure determinations are provided as Sup-
porting Information.
corresponding values are both equal to 153.2°.8
(17) Isostructural diamagnetic complexes of Ru were characterized
in situ by NMR: (a) Kirchner, K.; Mauthner, K.; Mereiter, K.; Schmid,
R. J . Chem. Soc., Chem. Commun. 1993, 892. (b) J oslin, F. L.; Pontier
J ohnson, M.; Mague, J . T.; Roundhill, D. M. Organometallics 1991,
10, 2781.
(18) Byers, L. R.; Dahl, L. F. Inorg. Chem. 1980, 19, 277.
(19) Hoffmann, P. Angew. Chem., Int. Ed. Engl. 1977, 16, 536.
(12) Crystal data for Cp*Fe(dppe) (3): C36H39FeP2, monoclinic, P21/
n, a ) 10.522(2) Å, b ) 15.772(4) Å, c ) 24.107(5) Å, â ) 91.95(2)°, V
) 3998(2) Å3, Z ) 4, Dcalc ) 1.337 g cm-3, R(F) ) 6.2% for 2217
independent reflections (2θmax ) 50°, scan ω/2θ ) 1). All non-hydrogen
atoms were found by direct methods with the SHELX-86 program.
Hydrogen atoms were idealized, and the whole structure was refined.

12 Organometallics, Vol. 15, No. 1, 1996
Communications
[Cp*Fe(dppe)]• y-θz [Cp*Fe(dppe)]+ y-θz
The coordinatively unsaturated complex 4 could be
regarded as a strong Lewis acid. It exhibits a rich
chemistry with many solvents and small molecules. To
illustrate this ability a THF solution of complex 4 was
treated with 5 equiv of H2O. Slow diffusion of pentane
into the solution provided a diamagnetic green crystal-
line material which was recovered in 90% yield and
identified as [Cp*Fe(dppe)(H2O‚THF)][PF6] (5) by an
X-ray crystal structure determination.7,8,21,22 Orga-
noiron aquo compounds were previously suggested to
be putative reaction intermediates, and only the com-
plex [Cp*Fe(CO)2(H2O)][OSO2CF3] was spectroscopically
identified.23 Crystallization of complexes 4 and 5 from
a THF solution strongly indicates that the formation of
the THF derivative [Cp*Fe(dppe)(THF)[PF6] is unlikely.
The initial scan in the cyclic voltammograms of the
four complexes 2-5, from -1.5 to +0.5 V, are identical
and display two reversible oxidation waves in THF (0.1
M nBu4N+PF6-, 20 °C, 0.1 V/s) with the E0 potential at
-0.73 ( 0.01 V and +0.24 ( 0.01 V (vs SCE). The
chemical reversibility at the platinum electrode is
established for each redox system by the ratio of the
anodic to cathodic peak current ipa/ipc ) 1. As a
consequence of the same E0 values observed for the
compounds 2-5, it must be concluded that (i) the same
redox systems described by eq 1 are involved in the four
cases and (ii) in THF the oxygen-containing ligands of
the complexes 2 and 5 are dissociated (eq 2). Indeed,
much more negative reduction potentials are expected
3
4
[Cp*Fe(dppe)]•2+ (1)
4+
[Cp*Fe(dppe)(L)]+ y-θz [Cp*Fe(dppe)]+ + L
L ) OSO2CF3, H2O
(2)
for the one-electron reduction of an isostructural 18-
electron cation into a 19-electron entity than for a 16-
electron cation into a 17-electron compound.24 On the
other hand, as for 2, the one-electron reduction of 5 with
0.9 equiv of cobaltocene in THF at -80 °C gave 3 (90%).
In THF solution, the complexes 2 and 5 are paramag-
netic and their magnetic moments determined by NMR
are 1.9 and 2.9 µB, respectively. These values clearly
support a strong dissociation of the triflato and aquo
adducts 2 and 5 in solution.
In summary, we have prepared by electron transfer
reactions and X-ray characterized two new and rare
complexes displaying unusual electronic and structural
flexibility at a metal center with the same Cp*Fe(dppe)
backbone, which constitute a unique family of 18-, 17-,
and 16-electron compounds. Efforts are currently un-
derway to explore the chemistry of these complexes.
Ack n ow led gm en t. We thank Prof. J .-Y. Saillard
(Rennes, France) for perceptive discussions.
Su p p or tin g In for m a tion Ava ila ble: For 3 and 4, text
describing X-ray procedures, complete tables of atomic coor-
dinates and B values and their estimated standard deviations,
bond lengths and angles, and anisotropic temperature factors,
and ORTEP diagrams (31 pages). Ordering information is
given on any current masthead page.
(20) (a) Winter, C. H.; Arif, A. M.; Gladysz, J . A. J . Am. Chem. Soc.
1987, 109, 7560. (b) Brookhart, M.; Lincoln, D. M.; Volpe, A. F., J r.;
Schmidt, G. F. Organometallics 1989, 8, 1212. (c) Ko¨lle, U.; Kossa-
kowski, J .; Raabe, G. Angew. Chem., Int. Ed. Engl. 1990, 29, 773.
(21) The role of hydrogen bonding is important in stabilizing the
solid-state structure of aquo transition metal complexes: (a) Stang,
P. J .; Cao, D. H.; Poulter, G. T.; Arif, A. M. Organometallics 1995, 14,
1110. (b) Braga, D.; Grepioni, F. Acc. Chem. Res. 1994, 27, 51.
(22) For a review on organometallics aqua complexes of transition
metal see: Koelle, U. Coord. Chem. Rev. 1993, 135/ 136, 623.
(23) Tahiri, A.; Guerchais, V.; Toupet, L.; Lapinte, C. J . Organomet.
Chem. 1990, 381, C47.
OM950766L
(24) The irreversible reduction of [Cp*Fe(dppe)(CO)][PF6] occurs at
-1.65 V vs SCE. Hamon, J .-R.; Lapinte, C. Unpublished results.