Electron-rich piano-stool iron σ-acetylides. Electronic structures of arylalkynyl iron(III) radical cations

Article abstract of DOI:10.1021/om0505108

The present study reports the isolation and the structural (X-ray), UV-vis-near-IR, and ESR characterization of a series of Fe(III) complexes of formula [(η²-dppe)(η⁵-C₅Me ₅)Fe(C=CC₆H₄X)][PF₆] (1a[PF ₆]-1k[PF₆]; with X = NO₂, CN, CF₃, Br, F, H, Me, ^tBu, OMe, NH₂, NMe₂). The electronic substituent effect of the remote X group on the electronic structure is experimentally evidenced by means of correlations with electronic substituent parameters (ESPs) and is theoretically investigated. The spectroscopic and ESR data are discussed in connection with the DFT computations. A consistent picture of the electronic structure of these Fe(III) radical cations is obtained. Notably, the near-IR absorption observed for 1a[PF₆]-1k[PF ₆] is assigned to a SOMO-2/SOMO electronic transition.

Full text of DOI:10.1021/om0505108

5464
Organometallics 2005, 24, 5464-5478
Electron-Rich Piano-Stool Iron σ-Acetylides. Electronic
Structures of Arylalkynyl Iron(III) Radical Cations†
Fre´de´ric Paul,*,‡ Loic Toupet,§ Jean-Yves The´pot,‡ Karine Costuas,*,|
Jean-Franc¸ois Halet,| and Claude Lapinte‡
Organome´talliques et Catalyse: Chimie et Electrochimie Mole´culaires, UMR CNRS 6509,
Institut de Chimie, Universite´ de Rennes I, Campus de Beaulieu, Baˆt. 10C, 35042 Rennes
Cedex, France, Groupe Matie`re Condense´e et Mate´riaux, UMR CNRS 6626, Universite´ de
Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France, and Laboratoire de Chimie du
Solide et Inorganique Mole´culaire, UMR CNRS 6511, Institut de Chimie,
Universite´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
Received June 21, 2005
The present study reports the isolation and the structural (X-ray), UV-vis-near-IR, and
ESR characterization of a series of Fe(III) complexes of formula [(η2-dppe)(η5-C5Me5)Fe-
(CtCC6H4X)][PF6] (1a[PF6]-1k[PF6]; with X ) NO2, CN, CF3, Br, F, H, Me, tBu, OMe, NH2,
NMe2). The electronic substituent effect of the remote X group on the electronic structure is
experimentally evidenced by means of correlations with electronic substituent parameters
(ESPs) and is theoretically investigated. The spectroscopic and ESR data are discussed in
connection with the DFT computations. A consistent picture of the electronic structure of
these Fe(III) radical cations is obtained. Notably, the near-IR absorption observed for
1a[PF6]-1k[PF6] is assigned to a SOMO-2/SOMO electronic transition.
Chart 1
Introduction
Mononuclear organometallic acetylide complexes fea-
turing one or several unpaired electrons exhibit usually
a very rich chemistry and,6 when kinetically stable,
always constitute a very appealing class of complexes
to study from a structural and theoretical viewpoint.7,8
Nowadays, such species also possess a tremendous
potential for molecular electronics, since redox-active
organometallic complexes stable under several redox
states can be envisioned either as electron or spin
reservoirs.9 This is particularly true for species contain-
ing the “(η2-dppe)(η5-C5Me5)Fe(CtC)-” fragment (dppe
) 1,2-bis(diphenylphosphino)ethane),1-5 owing to the
potential these compounds offer for redox switching
when incorporated in molecular devices.10,11 We have
indeed shown in several instances that this remarkable
fragment allows for efficient switching of various elec-
tronic properties of complexes, such as fluorescence12
and nonlinear optical (NLO) hyperpolarizability.3 This
effect evidently results from the change in redox state
of the iron center which is “transmitted” to the rest of
the molecule through the alkynyl spacer.
† For previous contributions on similar Fe(II) and Fe(III) alkynyls
see refs 1-5.
‡ Organome´talliques et Catalyse, UMR CNRS 6509. Te´l: (33) 02
23 23 59 62. Fax: (33) 02 23 23 56 37.
§ Groupe Matie`re Condense´e et Mate´riaux, UMR CNRS 6626. Te´l:
(33) 02 23 23 64 97. Fax: (33) 02 23 23 52 92.
| Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR
CNRS 6511. Te´l: (33) 02 23 23 69 73. Fax: (33) 02 23 23 51 98.
(1) Denis, R.; Toupet, L.; Paul, F.; Lapinte, C. Organometallics 2000,
19, 4240-4251.
(2) Paul, F.; Mevellec, J.-Y.; Lapinte, C. Dalton Trans. 2002, 1783-
1790.
(3) Paul, F.; Costuas, K.; Ledoux, I.; Deveau, S.; Zyss, J.; Halet, J.-
F.; Lapinte, C. Organometallics 2002, 21, 5229-5235.
(4) Courmarcel, J.; Le Gland, G.; Toupet, L.; Paul, F.; Lapinte, C.
J. Organomet. Chem. 2003, 670, 108-122.
Mononuclear Fe(II) compounds such as (η2-dppe)(η5-
C5Me5)Fe(CtCC6H4X) (1a-k) have been extensively
studied in our group as model complexes to understand
the substituent influence on the bonding properties
within the metal-acetylide backbone (Chart 1).1-5,13
(5) Costuas, K.; Paul, F.; Toupet, L.; Halet, J.-F.; Lapinte, C.
Organometallics 2004, 23, 2053-2068.
(6) (a) Rigaut, S.; Touchard, D.; Dixneuf, P. H. Coord. Chem. Rev.
2004, 248, 1585-1601. (b) Le Narvor, N.; Lapinte, C. J. Chem. Soc.,
Chem. Commun. 1993, 357-359. (c) Beddoes, R. L.; Bitcon, C.; Grime,
C. W.; Ricalton, A.; Whiteley, M. W. J. Chem. Soc., Dalton Trans. 1995,
2873-2883. (d) Fernandez, F. J.; Blacque, O.; Alfonso, M.; Berke, H.
Chem. Commun. 2001, 1266-1267. (e) Fernandez, F. J.; Venkatesan,
K.; Blacque, O.; Alfonso, M.; Schmalle, H.; Berke, H. Chem. Eur. J.
2003, 9, 6192-6209. (f) Venkatesan, K.; Blacque, O.; Fox, T.; Alfonso,
M.; Schmalle, H. W.; Berke, H. Organometallics 2004, 23, 1183-1186.
(7) (a) Krivykh, V. V.; Eremenko, I. L.; Veghini, D.; Petrunenko, I.
A.; Poutney, D. L.; Unseld, D.; Berke, H. J. Organomet. Chem. 1996,
511, 111-114. (b) Bianchini, C.; Laschi, F.; Masi, D.; Ottaviani, F. M.;
Pastor, A.; Peruzzini, M.; Zanello, P.; Zanobini, F. J. Am. Chem. Soc.
1993, 115, 2723-2730. (c) Beddoes, R. L.; Bitcon, C.; Whiteley, M. W.
J. Organomet. Chem. 1991, 402, 85-96. (d) Doherty, J. C.; Ballem, K.
H. D.; Patrick, B. O.; Smith, K. M. Organometallics 2004, 23, 1487-
1489.
(9) (a) Venkatesan, K.; Blacque, O.; Fox, T.; Alfonso, M.; Schmalle,
H.; Kheradmandan, S.; Berke, H. Organometallics 2005, 24, 920-932.
(b) Astruc, D. Electron Transfer and Radical Processes in Transition-
Metal Chemistry; VCH: New York, 1995. (c) Kahn, O. Molecular
Magnetism; VCH: New York, 1993.
(10) Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178/180, 427-505.
(11) Paul, F.; Lapinte, C. In Unusual Structures and Physical
Properties in Organometallic Chemistry; Gielen, M., Willem, R.,
Wrackmeyer, B., Eds.; Wiley: San Francisco, 2002; pp 219-295.
(12) Wong, K. M.-C.; Lam, S. C.-F.; Ko, C.-C.; Zhu, N.; Yam, V. W.-
W.; Roue´, S.; Lapinte, C.; Fathallah, S.; Costuas, K.; Kahlal, S.; Halet,
J.-F. Inorg. Chem. 2003, 42, 7086-7097.
(8) Adams, C. J.; Pope, S. J. A. Inorg. Chem. 2003, 43, 3492-3499.
10.1021/om0505108 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 09/17/2005

Electron-Rich Piano-Stool Iron σ-Acetylides
Organometallics, Vol. 24, No. 22, 2005 5465
Chart 2
Scheme 1
Previous investigations on this particular family of
complexes have notably revealed that, depending on the
X substituent, sizable changes in local polarization take
place for the alkynyl linker before and after oxidation
of the Fe(II) center.2 In addition, it was established that
only minor geometrical changes could be expected upon
mono-oxidation and a dominant iron-centered structure
was proposed for this radical species.5 However, only
limited experimental data were available for the Fe(III)
parents.14 Considering that such stable organometallic
piano-stool radical cations are reasonably scarce species
and deserve some special interest,15-17 and also in order
to better understand the influence of the oxidation upon
the electronic properties of such electron-rich Fe(II)
acetylides,18-20 we have now isolated here the complete
family of corresponding radical Fe(III) cations [(η2-
dppe)(η5-C5Me5)Fe(CtCC6H4X)][PF6] (1a[PF6]-1k[PF6])
from 1a-k (Chart 1).
We thus complete here the spectroscopic character-
ization of these Fe(III) alkynyl complexes by means of
electron spin resonance (ESR) and ultraviolet-visible
(UV-vis) and near-infrared (near-IR) spectroscopy,19,20
as well as the solid-state structures and the magnetic
measurements (Evans method) for selected representa-
tives. The electronic structures of the entire Fe(III)
family are discussed with complementary density func-
tional theory calculations (DFT). To determine the
impact of the acetylide ligand on these properties, we
also gathered data for closely related mononuclear
complexes without such a ligand, i.e., the known chloro
and hydride complexes [(η2-dppe)(η5-C5Me5)FeCl][PF6]
(2[PF6])21 and [(η2-dppe)(η5-C5Me5)FeH][PF6] (3[PF6])22
(Chart 2).23 Actually, the present contribution consti-
tutes a logical continuation of the previous theoretical/
experimental investigation on the corresponding Fe(II)
congeners5 and also usefully complements an older
contribution from some of us on closely related σ-alkyl
Fe(III) complexes.21
Results
Synthesis and Solid-State Structures. The com-
plexes 1a[PF6]-1k[PF6] (X ) NO2, CN, CF3, F, Br, H,
Me, tBu, OMe, NH2, NMe2) were isolated after chemical
oxidation of their known Fe(II) parents using ferroce-
nium hexafluorophosphate, following the workup previ-
ously given for the complexes with the nitro (1a) and
amino (1j) substituents (Scheme 1).1 The isolated samples
were characterized by infrared spectroscopy. Their
purity was checked by cyclic voltammetry, and satisfac-
tory elemental analyses were obtained in most cases.
Notably, we have shown recently that the cationic
carbonyl complex 4+ (Chart 2) forms quantitatively upon
reaction of molecular oxygen with 1a[PF6]-1k[PF6], the
reaction being favored by electron-releasing substitu-
ents.24 Clearly, these Fe(III) radical cations, although
isolable at ambient temperature, are still quite reactive
species and should be checked periodically when stored.
In addition to 1a[PF6], previously characterized by
X-ray diffraction,1 crystals could be grown for several
t
other representatives (X ) CN, Br, H, Me, Bu, OMe,
NH2) and their solid-state structures were solved (Fig-
ure 1 and Tables 1 and 2). As reported for 1a[PF6], all
complexes crystallize in the monoclinic systems (P21/n
space groups in most cases) with one molecule per
asymmetric unit, except for 1f[PF6]. The latter crystal-
lizes in the orthorhombic system and presents two
molecules in the asymmetric unit. We also unexpectedly
realized with 1b[PF6] that some of these Fe(III) com-
plexes could be stable toward water; two water mol-
ecules act as solvates in this particular case.
(13) Denis, R.; Weyland, T.; Paul, F.; Lapinte, C. J. Organomet.
Chem. 1997, 545/546, 615-618.
(14) Connelly, N. G.; Gamasa, M. P.; Gimeno, J.; Lapinte, C.; Lastra,
E.; Maher, J. P.; Le Narvor, N.; Rieger, A. L.; Rieger, P. H. J. Chem.
Soc., Dalton Trans. 1993, 2575-2578.
(15) (a) Smith, K. M. Organometallics 2004, 24, 778-784. (b)
Legzdins, P.; McNeil, W. S.; Batchelor, R. J.; Einstein, F. W. B. J. Am.
Chem. Soc. 1995, 117, 54-66. (c) Le Bras, J.; Jiao, H.; Meyer, W. E.;
Hampel, F.; Gladysz, J. A. J. Organomet. Chem. 2000, 616, 54-66 and
references therein.
(16) (a) Fortier, S.; Baird, M. C.; Morton, K. F. P. J. R.; Ziegler, T.;
Jaeger, 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, 542-551. (b) Pike, R. D.; Rieger, A. L.; Rieger, P. H. J.
Chem. Soc., Faraday Trans. 1 1989, 85, 3913-3925.
(17) Cooley, N. A.; Baird, M. C.; Morton, J. R.; Preston, K. F.; Le
Page, Y. J. Magn. Reson. 1988, 76, 325-330.
In comparison to the Fe(II) parents, the geometrical
changes reveal the expected expansion of the coordina-
tion sphere of the iron center upon oxidation. For the
complete series of Fe(III) compounds, only very slight
geometrical differences take place in the aryl acetylide
fragment when X is varied. The bond length with the
(18) Weyland, T.; Costuas, K.; Mari, A.; Halet, J.-F.; Lapinte, C.
Organometallics 1998, 17, 5569-5579.
(19) Le Stang, S.; Paul, F.; Lapinte, C. Inorg. Chim. Acta 1999, 291,
403-425.
(20) Weyland, T.; Costuas, K.; Toupet, L.; Halet, J.-F.; Lapinte, C.
Organometallics 2000, 19, 4228-4239.
(23) (a) Tilset, M.; Fjeldahl, I.; Hamon, J.-R.; Hamon, P.; Toupet,
L.; Saillard, J.-Y.; Costuas, K.; Haynes, A. J. Am. Chem. Soc. 2001,
123, 9984-10000. (b) Tilset, M.; Hamon, J.-R.; Hamon, P. Chem.
Commun. 1998, 765-766.
(24) Paul, F.; Toupet, L.; Roisnel, T.; Hamon, P.; Lapinte, C. C. R.
Chim. 2005, 8, 1174-1185.
(21) Roger, C.; Hamon, P.; Toupet, L.; Rabaaˆ, H.; Saillard, J.-Y.;
Hamon, J.-R.; Lapinte, C. Organometallics 1991, 10, 1045-1054.
(22) Hamon, P.; Hamon, J.-R.; Lapinte, C. J. Chem. Soc., Chem.
Commun. 1992, 1602-1603.

5466 Organometallics, Vol. 24, No. 22, 2005
Paul et al.
Figure 1. ORTEP representations of the complexes 1b[PF6] (a), 1d[PF6] (b), 1f[PF6] (c), 1g[PF6] (d), 1h[PF6] (e), 1i[PF6]
-
(f), and 1j[PF6] (g) at the 50% probability level (PF6 counterions not shown).
substituent (C42-X) relative to that in the correspond-
ing Fe(II) parent changes significantly only in the case
of the amino complex 1j[PF6] (1.370(6) vs 1.406(5) Å).
This shortening is in contrast with the lengthening
previously stated for 1a[PF6]1 and could now originate
from an increased π-bonding between the amine center
and the aryl ring in the Fe(III) complex. The changes
in this bond length for other complexes are not really
significant (1.435(5) vs 1.441(8) Å, 1.904(4) vs 1.904(4)
Å, 1.510(7) vs 1.512(4) Å, and 1.370(3) vs 1.382(3) Å for
X ) CN, Br, CH3, OMe, respectively). The CtN bond
of the cyano substituent appears also quite affected by
oxidation (1.152(5) vs 1.130(7) Å).26a Finally, considering
only the complexes crystallizing in the P21/n space
(25) Szafert, S.; Gladysz, J. Chem. Rev. 2003, 103, 4175-4205.

Electron-Rich Piano-Stool Iron σ-Acetylides
Organometallics, Vol. 24, No. 22, 2005 5467
Table 1. Crystal Data and Data Collection and Refinement Parameters for 1b[PF6], 1d[PF6], 1f[PF6],
1g[PF6], 1h[PF6], 1i[PF6], and 1j[PF6]
1b+ (X ) CN)
1d+ (X ) Br)
1f+ (X ) H) 1g+ (X ) Me) 1h+ (X ) tBu) 1i+ (X ) OMe) 1j+ (X ) NH2)
formula
C
45H43NP2Fe-
C
44H43BrP2Fe-
C
88H88F12P4-
Fe2(2PF6)
C
45H46P2Fe-
(PF6)
C
48H52P2Fe-
(PF6)
C
45H46OP2Fe-
(PF6)
C
44H45NP2Fe-
(PF6)‚2H2O
(PF6)‚CH3OH
(PF6)‚CH2Cl2
fw
896.6
120(2)
946.50
120(2)
monoclinic
P21/n
15.3126(2)
16.3293(2)
16.7496(2)
90.0
94.382(1)
90.0
1671.10
120(1)
orthorhombic monoclinic
Pca21
25.1849(4)
16.6927(3)
18.8160(4)
90.0
90.0
90.0
849.58
293(2)
891.66
120(1)
865.58
120(1)
935.50
120(2)
temp (K)
cryst syst
space group
a (Å)
monoclinic
P21/n
15.2639(3)
16.6781(3)
16.5813(3)
90.0
93.128(1)
90.0
4214.9(1)
4
1.413
monoclinic
P21/n
11.7138(1)
21.1255(2)
17.4624(2)
90.0
94.087(1)
90.0
4321.24(7)
4
1.371
monoclinic
P21/n
14.0681(3)
17.2547(3)
17.0021(3)
90.00
100.594(1)
90.00
4056.76(13)
4
1.417
monoclinic
P21/n
13.0259(2)
15.1369(3)
23.4141(5)
90.0
105.934(1)
90.0
4439.22(15)
4
1.400
C2/c
27.4521(6)
13.0301(3)
23.5369(4)
90.0
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
92.237(1)
90.0
8417.3(3)
8
1.341
4175.9(1)
4
1.495
7910.3(3)
4
1.403
Z
Dcalcd (g cm-3
)
cryst size (mm)
0.42 × 0.40 × 0.22 × 0.15 ×
0.42 × 0.32 × 0.50 × 0.35 × 0.40 × 0.32 × 0.44 × 0.34 × 0.30 × 0.30 ×
0.35
1860
0.12
1940
Kappa CCD
0.30
3464
0.35
3528
Kappa CCD
0.30
1860
Kappa CCD
0.31
1796
Kappa CCD
0.12
1932
Kappa CCD
F(000)
diffractometer
(Nonius)
Kappa CCD
Kappa CCD
radiation
Mo KR
0.537
Mo KR
1.495
Mo KR
0.563
Mo KR
0.530
Mo KR
0.520
Mo KR
0.553
Mo KR
0.626
abs coeff (mm-1
data collection
2θmax (deg)
no. of frames
)
54
227
2.0
54
54
54
54
54
54
117
186
225
262
122
203
Ω rotation (deg)
scan rate (s/frame) 20
θ range (deg)
hkl ranges
2.0
1.2
1.4
1.6
2.0
1.5
80
25
12
110
30
73
1.73-27.48
0-19
0-21
2.14-27.47
0-19
0-21
-21 to +21
39 693
9536
1.22-27.48
0-32
0-21
0-24
59 451
9340
7217
1.48-27.47
0-35
0-16
-30 to +30
40 464
9571
2.25-27.54
0-15
0-22
-27 to +27
76 089
19 482
9916
1.70-27.52
0-18
0-22
-22 to +22
41 000
9309
1.62-27.48
0-16
0-19
-30 to +29
51 541
10 175
6523
-21 to +21
69 005
9636
total no. of rflns
no. of unique rflns
no. of obsd rflns
(I > 2σ(I))
7353
7986
4798
7362
no. of restraints/
params
0/514
0/513
0/973
0/506
0/524
0/506
0/529
w ) 1/[σ2(Fo)2 +
(aP)2 + bP]a
a ) 0.0920
a ) 0.0826
a ) 0.1266
a ) 0.1143
a ) 0.0656
a ) 0.0657
a ) 0.1120
b ) 12.6000
0.0664
b ) 22.2426
0.0681
0.1704
0.0818
0.1821
0.994
b ) 3.8822
0.0713
0.1687
0.0993
0.1921
1.073
b ) 0.0000
0.0670
0.1752
0.1540
0.2176
1.020
b ) 3.1905
0.0435
0.1180
0.0497
0.1236
1.022
b ) 3.8619
0.0449
0.1146
0.0634
0.1277
1.024
b ) 5.9585
0.0712
0.1840
0.1211
0.2158
1.042
final R
final Rw
0.1690
R (all data)
Rw (all data)
0.0903
0.1904
goodness of fit/F2 (Sw) 1.008
∆Fmax (e Å-3
)
1.2
2.8
1.8
0.56
0.86
0.65
1.85
largest diff peak,
1.824, -1.258 2.816, 2.467
2.821, -0.557 0.564, -0.463 0.867, -0.526 0.652, -0.679 1.850, -0.871
hole (e Å-3
)
aP ) [Fo + Fc ]/3.
2
2
group, the data reveal a noticeable deviation from
linearity of the Fe-CtC fragment when the X substitu-
ent becomes more and more electron-releasing.
1f[PF6]. Note, however, that these experimental values
were obtained with relatively large uncertainties. More-
over, since 1f[PF6] presents two molecules in the asym-
metric unit, this discrepancy could also originate from
solid-state effects induced by the unusual packing of this
complex.25 We now have closely examined the Fe-C37-
C38 and the C37-C38-C39 angles in DFT-computed
structures obtained for 1a-H+-1k-H+ from optimiza-
tion, to see if the acetylide “bending” experimentally
observed could have a intramolecular origin. No clear
trend was apparent from theoretical data, most angles
being quite close to 180°.
As discussed in our previous contribution on these
compounds,5 the structural features experimentally
determined for the Fe(III) complexes are overall in good
accordance with the optimized geometries that were
computed for the corresponding model complexes by
means of density functional theory (DFT). Notably, the
trend found for the C42-X bond length is well in line
with theoretical computations previously reported.5,26b
The largest discrepancies are presently observed for
Evans Measurements. Evans measurements were
performed on selected Fe(III) complexes in chloroform
or in dichloromethane (Table 3). The Fe(III) compounds
proved to decompose slowly in chloroform solutions,
especially with electron-releasing X substituents, and
the measurements had to be performed rapidly after
dissolution of the complexes. With several of these
(26) (a) For the cyano complex (1b[PF6]), the νCN value observed in
the solid-state infrared spectrum of this complex suggests that the
cyano bond is strengthened upon oxidation, not weakened. A shorten-
ing and not a lengthening of this bond should therefore be observed in
the crystal structure. Whether this discrepancy is an artifact origi-
nating from the comparably large uncertainty in these bond lengths
or results from a specific vibronic coupling involving the CN unit is
presently unknown. (b) We thank a reviewer for this interesting
comment.

5468 Organometallics, Vol. 24, No. 22, 2005
Paul et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1b[PF6], 1d[PF6], 1f[PF6], 1g[PF6], 1h[PF6], 1i[PF6],
and 1j[PF6]
1b
(X ) CN)
1d
(X ) Br)
1f
(X ) H)
1g
(X ) Me)
1h
1i
1j
(X ) tBu)
(X ) OMe)
(X ) NH2)
Selected Bond Lengths
1.766/1.784
Fe-(Cp*)centroid
Fe-P1
1.774
1.779
1.772
1.773
1.776
1.773
2.2739(10)
2.2554(10)
1.892(4)
1.219(5)
1.443(5)
1.405(5)
1.378(5)
1.397(5)
1.397(5)
1,390(5)
1.398(5)
1.435(5)
1.152(5)
2.2640(11)
2.2519(11)
1.876(4)
1.230(7)
1.431(6)
1.410(6)
1.382(6)
1.381(6)
1.386(6)
1.379(7)
1.403(7)
1.904(4)
2.2599(18)/2.2266(19)
2.2535(19)/2.2453(18)
1.919(7)/1.852(7)
1.207(10)/1.262(10)
1.438(10)/1.425(10)
1.412(10)/1.396(10)
1.373(10)/1.363(11)
1.385(10)/1.402(11)
1.375(11)/1.384(11)
1.396(11)/1.375(10)
1.360(10)/1.406(9)
/
2.2595(11)
2.2682(13)
1.889(4)
1.207(6)
1.438(6)
1.390(6)
1.391(6)
1.359(7)
1.383(7)
1.370(7)
1.383(7)
1.510(7)
2.2718(5)
2.2484(5)
1.891(2)
1.218(3)
1.443(3)
1.396(3)
1.385(3)
1.399(3)
1.397(3)
1.375(3)
1.408(3)
1.536(3)
1.547(4)
1.530(4)
1.545(4)
2.2402(6)
2.2628(6)
1.895(2)
1.220(3)
1.436(3)
1.408(3)
1.382(3)
1.396(3)
1.387(4)
1.393(3)
1.398(3)
1.370(3)
1.431(3)
2.2497(11)
2.2355(12)
1.865(4)
1.219(6)
1.432(6)
1.407(6)
1.379(6)
1.387(6)
1.420(6)
1.355(6)
1.407(6)
1.370(6)
Fe-P2
Fe-C37
C37-C38
C38-C39
C39-C40
C40-C41
C41-C42
C42-C43
C43-C44
C44-C39
C42-X
X-Y1
X-Y2
X-Y3
Selected Bond Angles
83.18(7)/84.71(7)
85.8(2)/90.6(2)
P1-Fe-P2
83.53(3)
90.06(11)
84.21(11)
175.6(3)
176.2(4)
119.2(3)
120.5(3)
120.2(3)
119.4(3)
178.8(5)
83.36(4)
90.52(14)
84.87(13)
174.1(4)
177.6(6)
118.0(4)
122.7(4)
120.0(3)
117.3(4)
84.27(4)
84.36(12)
87.71(14)
176.8(4)
176.0(5)
117.8(4)
117.4(4)
121.7(5)
120.9(6)
83.801(19)
89.24(6)
85.38(6)
174.29(17)
174.7(2)
117.67(19)
117.5(2)
122.9(2)
119.7(2)
109.9(2)
111.9(2)
108.4(2)
95.5
82.37(2)
86.84(7)
90.67(7)
171.0(2)
176.7(2)
117.9(2)
120.1(2)
115.8(2)
124.1(2)
117.1(2)
84.69(4)
90.65(13)
84.93(12)
171.9(4)
176.2(5)
117.0(4)
117.8(4)
121.1(4)
120.9(4)
P1-Fe-C37
P2-Fe-C37
89.2(2)/86.3(2)
Fe-C37-C38
C37-C38-C39
C40-C39-C44
C41-C42-C43
C41-C42-X
C43-C42-X
C42-X-Y1
171.0(6)/174.5(6)
176.2(8)/175.2(7)
118.6(7)/119.4(7)
120.3(7)/119.4(7)
C42-X-Y2
C42-X-Y3
Fe-(Cp*)centroid/C39-C40a
91.6
90.3
41.0/38.3
62.3
96.7/10.6b
23.6/0b
a Dihedral angle (Cp* ) pentamethylcyclopentadienyl ligand). b Dihedral angle (aryl mean plane)/(substituent mean plane or axis).
Table 3. Experimental Molecular Magnetic
Moments (in µB) from Evans Measurements in
CDCl3 at 297 K for Selected Compounds among
1a[PF6]-1k[PF6]
Table 4. ESR Spectroscopic Dataa for
[(η2-dppe)(η5-C5Me5)FeCtCAr][PF6] Complexes
Ar
g1
g2
g3
∆g
g
ref
p-(C6H4)NO2
p-(C6H4)CN
p-(C6H4)CF3
p-(C6H4)Br
p-(C6H4)F
1.973 2.028 2.497 0.524 2.167 this work
1.970 2.028 2.499 0.529 2.166 this work
1.970 2.028 2.496 0.527 2.165 this work
1.974 2.034 2.475 0.501 2.161 this work
1.976 2.032 2.448 0.472 2.152 this work
1.975 2.033 2.464 0.489 2.157 14
[Fe]CtC(C6H4)X
X ) NO2
X ) CN
X ) H
X ) OMe
X ) NH2
µ (µB)
1.57
1.45b
1.87
1.56a,b
1.16b
a In CD2Cl2. b Slight decomposition observed in the NMR tube.
p-(C6H5)
p-(C6H4)Me
1.979 2.034 2.424 0.445 2.145 this work
1.978 2.032 2.444 0.466 2.151 this work
p-(C6H4)tBu
samples, partial decomposition could not be avoided
during the measurements, and generated signals for an
unidentified side product (less than 20% in general). The
latter likely corresponds to the diamagnetic Fe(II)
complex 4[PF6] (Chart 2).24 With respect to the experi-
mental uncertainties associated with these measure-
ments, the paramagnetic complexes apparently show a
molecular magnetic moment close to 1.7 µB, which
corresponds to the spin-only value calculated for one
unpaired electron. Thus, as expected from previous
investigations,10,14 this establishes that these Fe(III)
derivatives are low-spin complexes in solution and that
the unpaired electron experiences only a slight spin-
orbit coupling.
ESR Measurements. ESR measurements on the
1a[PF6]-1k[PF6] radicals were performed in 1,2-dichlo-
roethane/dichloromethane (1:1) in frozen glasses at 77
K. Under such conditions, rhombic spectra were ob-
tained for all compounds (Table 4). The Fe(III) radical
cations were either generated in situ from neutral Fe-
(II) complexes or obtained from isolated Fe(III) com-
plexes. In contrast to a previous ESR investigation on
similar compounds, no hyperfine structure correspond-
ing to a coupling to equivalent 31P atoms could be
p-(C6H4)OMe 1.983 2.036 2.379 0.396 2.133 this work
p-(C6H4)NH2 1.992 2.037 2.310 0.318 2.113 this work
p-(C6H4)NMe2 1.993 2.039 2.263 0.270 2.098 this work
Cl
H
1.987 2.055 2.454 0.467 2.165 this work
1.994 2.043 2.449 0.455 2.162 23a
a At 77 K in a CH2Cl2-C2H4Cl2 (1:1) glass.
resolved.14 Given the relative width of the observed
features, such a coupling (ca. 15-20 G) could easily be
concealed underneath. Notably, for all complexes, a
significantly larger half-width is observed for the low-
field component of the g tensor (ca. 70 vs ca. 30 G). A
similar observation was also made by Rieger and co-
workers on the isoelectronic radical (η6-C6Me6)Cr(PEt3)-
(CO)2 a few years ago, and an explanation was proposed
for this phenomenon.27 More importantly, it appears
clearly that the anisotropy (∆g) of the ESR spectra
increases with the electron-withdrawing capability of
the X substituent.
Variable-temperature measurements performed on
1a[PF6], 1f[PF6], 1i[PF6], and 1j[PF6] reveal that any
signal is lost in solution, above 180 K for the first two
(27) Castellani, M. P.; Connelly, N. G.; Pike, R. D.; Rieger, A. L.;
Rieger, P. H. Organometallics 1997, 16, 4369-4376.

Electron-Rich Piano-Stool Iron σ-Acetylides
Organometallics, Vol. 24, No. 22, 2005 5469
850 nm spectral range and only one for 1k[PF6]. These
are bathochromically shifted and gain in intensity as
the X substituent becomes more and more electron
releasing. The substituent effect is opposite to the trend
previously reported for the corresponding Fe(II) parents5
and would be in favor of these transitions being LMCT
in character. For the chloro (2[PF6]) and hydride Fe-
(III) complexes (3[PF6]), the low-energy absorptions
resemble those of the complex 1a[PF6] but are blue-
shifted relative to them and present lower extinction
coefficients.31
Measurements in the near-IR range indicate that
most Fe(III) radicals were generated in situ from the
neutral Fe(II) parents using ferrocenium hexafluoro-
phosphate as an oxidant, but we checked for several
representatives (1a[PF6], 1f[PF6], 1i[PF6], 1k[PF6], and
3[PF6]) that similar spectra are obtained from isolated
Fe(III) samples. A weak absorption is detected for all
acetylide complexes 1a[PF6]-1k[PF6] in the 1500-2000
nm range. As observed with 1a[PF6] or 1f[PF6], the
extinction coefficient of this electronic absorption was
not sensitive to dilution, in line with its intramolecular
nature, nor to the nature of the solvent used (see the
Supporting Information). This absorption is, however,
quite sensitive to the nature of the X substituent. It
gains in intensity and is hypsochromically shifted when
the para substituent becomes more and more electron-
releasing (Figure 4). This substituent effect is opposite
to the effect stated on the low-energy visible absorptions.
The chloride and the hydride complexes (2[PF6] and
3[PF6]) also present a similar absorption in the near-
IR range near 1500 nm.31 For all these Fe(III) radicals,
the presence of these near-IR absorptions was confirmed
in the solid state by means of diffuse scattering on
powdered samples. Actually, the very weak extinction
coefficient for this near-IR absorption is reminiscent of
a forbidden metal-centered ligand field (LF) electronic
transition. This hypothesis would also be in line with
its apparently poor sensitivity to the solvent nature and
with the fact that a similar low-energy absorption is
observed for 2[PF6] and 3[PF6], which both lack the
acetylide ligand. Notably, similar absorptions were
previously observed in mono- and polynuclear Fe(III)
acetylide complexes.18,19 Theoretical computations were
carried out in order to check these tentative statements.
DFT Computations. Theoretical computations on
the Fe(III) model complexes 1a-H+ (X ) NO2), 1b-H+
(X ) CN), 1d-H+ (X ) Br), 1f-H+ (X ) H), 1i-H+ (X )
OMe), and 1j-H+ (X ) NH2), with the chelating dppe
ligand replaced by two PH3 ligands and C5Me5 replaced
by C5H5, were previously performed.5 They indicated
that the electron is lost from a delocalized molecular
orbital heavily weighted on the metal center in the Fe-
(II) parent molecules. The effect of oxidation on the
energies of the molecular orbitals is depicted in Figure
5 for 1j and 1j-H+ (X ) NH2) using a spin-restricted
representation, to simplify the discussion. As can be
seen in this diagram, the HOMO-LUMO gap slightly
increases upon oxidation.
Figure 2. Variable-temperature ESR spectra of the 1j[PF6]
radical (X ) NH2) in 1,2-CH2Cl2/C2H4Cl2 (1:1). Please note
the changes in relative intensity (the spectrum at 77 K was
recorded with half of the accumulations of the other
spectra).
compounds. On the other hand, in the last two cases, a
very weak (ca. 200 times less intense) and large isotropic
signal is still observable at ambient temperature at g
) 2.007, 2.088, respectively, with a peak-to-peak sepa-
ration of ca. 500 G (Figure 2). This suggests that spin
relaxation might be somewhat slower with the more
electron-releasing substituents in this series of com-
pounds. As shown in Figure 2, upon heating, the
rhombic signal transforms transiently in a pseudoaxial
signal around 190 K, before giving a large and weak
isotropic signal above 220 K, upon melting of the solvent
glass. This behavior is reminiscent of that observed with
related piano-stool d5 radicals such as (η5-C5Ph5)Cr-
(PMe3)(CO)2, which was previously attributed to the
rotation of the ligand along the main g-component axis
in the melting solvent glass.28 The relative constancy
of the average g value upon variation of the temperature
shows that the spectral changes are due neither to the
presence of more than one paramagnetic species in the
sample nor to a change in spin state.29
Under similar conditions, the ESR spectra of the
known [(η2-dppe)(η5-C5Me5)FeCl][PF6] (2[PF6]) and [(η2-
dppe)(η5-C5Me5)FeH][PF6] (3[PF6]) radical cations were
also recorded.23a The Fe(III) hydride complex 3[PF6] is
unstable in the solvent system used and transforms into
another unknown metal-centered radical species, ex-
hibiting three tensors at g ) 2.002, 2.044, 2.339 with a
lesser anisotropy (∆g ) 0.337). This is not very surpris-
ing, considering the instability already reported for the
Fe(II) hydride parent in dichloromethane.30
Electronic Spectroscopy. Electronic spectroscopy
can provide a wealth of information on the electronic
structure of radical cations. We therefore recorded the
electronic spectra of 1a[PF6]-1k[PF6] in dichloromethane
solutions, in the UV-vis (Table 5) and in the near-
infrared (near-IR) range (Table 6). The UV-vis spectra
presented several absorptions which were found for all
Fe(III) acetylide complexes (Figure 3). Thus, for 1a[PF6]-
1j[PF6], two distinct peaks are apparent in the 600-
(31) As precised above, a slow decomposition of 3[PF6] was observed
in dichloromethane, evidenced by the progressive disappearance of the
corresponding electronic absorptions. The spectra of this sensitive
sample were therefore acquired several times with concentrated
solutions and over short time scales to ascertain that the observed
absorptions did indeed correspond to 3[PF6]. The reported extinction
coefficients remain nevertheless approximate.
(28) Rieger, A. L.; Rieger, P. H. Organometallics 2004, 2004, 154-
162.
(29) Connelly, N. G.; Kitchen, M. D. J. Chem. Soc., Dalton Trans.
1977, 931-937.
(30) Belkova, N. V.; Revin, P. O.; Epstein, L. M.; Vorontsov, E. V.;
Bakhmutov, V. I. J. Am. Chem. Soc. 2003, 125, 11106-11115.

5470 Organometallics, Vol. 24, No. 22, 2005
Table 5. UV-Vis Data for [(η2-dppe)(η5-C5Me5)FeCtCAr][PF6] Complexes in CH2Cl2 Solution
Ar
abs (nm) (10-3ꢀ (M-1 cm-1)) ∆ν (cm-1
Paul et al.
a
)
Cl
H
266 (sh, 16.3); 284 (sh, 16.8); 466 (2.4); 550 (sh, ca. 1.0)
302 (sh, 4.9); 382 (1.6); 536 (sh, 0.3)
p-(C6H4)NO2
p-(C6H4)CN
p-(C6H4)CF3
p-(C6H4)Br
p-(C6H4)F
267 (sh, 85.4); 310 (sh, 16.1); 379 (15.4); 504 (sh, 2.3); 562 (sh, 1.6); 650 (1.2)
263 (sh, 40.5); 326 (sh, 12.5); 394 (sh, 2.8); 458 (sh 1.4); 501 (1.1); 564 (1.0); 652 (0.9)
261 (sh, 44.9); 298 (sh, 26.0); 342 (sh, 5.2); 379 (sh, 2.7); 430 (sh 1.8); 515 (1.5); 557 (1.7); 635 (1.5)
356 (sh, 13.8); 391 (sh, 11.2); 579 (sh, 1.8); 670 (2.4)
2410
2390
2210
2350
2230
2290
2480
2480
2330b
4130
349 (sh, 5.6); 388 (sh, 3.1); 576 (2.4); 661 (3.4)
p-(C6H5)
261 (sh, 32.6); 280 (sh, 27.4); 301 (sh, 18.8); 342 (sh, 5.9); 379 (sh, 3.6); 575 (sh, 2.3); 662 (3.1)
262 (40.7); 350 (sh, 5.6); 397 (sh, 2.9); 589 (sh, 2.1); 686 (4.2)
p-(C6H4)Me
p-(C6H4)tBu
p-(C6H4)OMe
p-(C6H4)NH2
p-(C6H4)NMe2
259 (73.0); 299 (sh, 42.7); 344 (sh, 6.3); 395 (sh, 3.5); 584 (sh, 2.3); 683 (3.8)
265 (46.2); 323 (sh, 10.0); 352 (6.5) 394 (sh, 3.6); 440 (sh, 1.5); 519 (1.3); 615 (1.9); 718 (6.1)
268 (sh, 34.5); 287 (8.8); 413 (sh, 6.0); 595 (1.2); 789 (9.7)
263 (sh, g34.1); 278 (67.0); 414 (19.8); 894 (24.7)
a Energetic difference ((100 cm-1) between the two low-energy transitions. b A significantly larger value (2760 cm-1) was found by
deconvolution of the peaks in this case.
Table 6. Near-IR Data for
[(η2-dppe)(η5-C5Me5)FeX][PF6]a Complexes
solnb
solidc
f
d
f
X
νmaxd (ꢀe)
ν1/2
νmax
ν1/2
Cl
H
1572/6361 (11)
1384/7225 (26)
1300 1610/6210 1400
1900 1322/7560 1850
CtC(C6H4)NO2
CtC(C6H4)CN
CtC(C6H4)CF3
CtC(C6H4)F
CtC(C6H4)Br
CtC(C6H4)
CtC(C6H4)Me
CtC(C6H4)tBu
CtC(C6H4)OMe
CtC(C6H4)NH2
CtC(C6H4)NMe2
1936/5165 (70)
1916/5219 (75)
1900/5263 (68)
1824/5482 (93)
1500 1912/5230 1400
1400 1908/5240 1400
1400 1894/5280 1400
1500 1818/5500 1400
1864/5364 (113) 1500 1757/5691 1500
1846/5417 (94)
1808/5531 (99)
1808/5531 (124) 1700 1835/5450 1700
1752/5707 (139) 1900 1792/5580 1600
1664/6203 (243) 2300 1672/5980 2000
1588/6297 (427) 2500 1588/6295 2000
1500 1865/5360 1400
1600 1835/5450 1400
Figure 4. Near-IR spectra for selected Fe(III) complexes
(X ) NO2, H, NH2, NMe2) in dichloromethane.
g
a Unless precised; ν in nm/cm-1 and ꢀ in M-1 cm-1 b Conditions:
.
CH2Cl2, absorbance mode, 20 °C (Cary 5). c Conditions: KBr,
diffuse reflectance mode, 20 °C (Bruker IFS 28). d Values (20 nm/
(50 cm-1 e Values (5 M-1 cm-1
. .
f Values (200 cm-1 g No
.
changes upon dilution (×3) of the solution.
Figure 5. Orbital diagram for [(H3P)2(η5-C5H5)Fe(CtC)-
1,4-(C6H4)NH2]0/+ (1j-H/1j-H+) showing the effect of oxida-
tion on the energy levels. The arrows indicate the possible
excitations leading to MLCT, LMCT, or LF transitions in
the visible range. Transitions in parentheses were not
observed in the visible spectrum (400-800 nm).
Figure 3. UV-vis spectra for selected Fe(III) complexes
(X ) NO2, CN, H, OMe, NH2) in dichloromethane.
The LUMO for these electron-rich compounds has a
are unoccupied and four are occupied) are shown in
Figure 6. Except for 1f-H+, the metal contribution of
the higher lying spin MOs among these is dominated
by one type of d atomic orbital (Table 7). In contrast,
the metal contribution of the lower lying spin MOs is
more accurately described by a mixture of two d AOs.
Notably, in the case of complex 1a-H+, the filled spin
MOs are interspersed by three aryl- and nitro-based
MOs (not represented in Figure 6).
For all compounds, the highest occupied R-spin MO
has a dominant dyz character. This is not the case for
the lowest empty â-spin MOs. Actually, depending on
the X substituent in 1a-H+-1f-H+, different trends are
dominant dxy character, whereas the LUMO + 1 has a
2
dominant dz character. As previously described, these
MOs correspond to the eg set of an homoleptic octahedral
complex, while the SOMO and the two MOs below
correspond to the t2g set.5 The SOMO and SOMO - 1
exhibit an important metal dyz or dxz character, as well
as an important acetylide character (Table 7).32
Spin-unrestricted calculations were next carried out.
The energies of the six frontier spin MOs (R and â)
possessing a large metal character in 1a-H+-1j-H+ (two
(32) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. J. Am.
Chem. Soc. 1993, 115, 3276-3285.

Electron-Rich Piano-Stool Iron σ-Acetylides
Organometallics, Vol. 24, No. 22, 2005 5471
Table 7. Decomposition of (PH3)2(η5-C5H5)Fe(CtCC6H4X) Model Complexes (X ) NO2, H, NH2) for Selected
Frontier MOs
1a-H+ (X ) NO2)
MO
53a(R)
53a(â)
52a(R)
52a(â)
51a(R)
51a(â)
48a(R)
50a(â)
46a(R)
46a(â)
44a(R)
44a(â)
ꢀ (eV)
occ
% Fe
% Cp
% P
% CR
% Câ
% C(Ph)
% NO2
-6.59
-6.30
-7.19
-6.87
-8.62
1
18
0
0
18
17
36
3
-8.29
-9.39
-8.47
1
27
8
2
16
16
28
3
-9.73
-9.44
-10.01
-9.77
0
0
0
0
0
1
1
1
1
1
48
14
7
11
1
3
2
53
19
6
10
1
0
0
46
18
20
0
51
24
19
0
56
5
0
7
21
0
0
37
4
0
75
11
0
79
1
54
0
48
0
2
0
0
12
28
0
0
0
1
5
0
0
0
0
3
0
0
0
0
0
28
2
32
2
0
0
0
0
0
1f-H+ (X ) H)
MO
45a(R)
45a(â)
44a(R)
44a(â)
43a(R)
43a(â)
42a(R)
42a(â)
41a(R)
41a(â)
39a(R)
39a(â)
ꢀ (eV)
occ
% Fe
% Cp
% P
% CR
% Câ
% C(Ph)
-5.94
-5.71
-6.53
-6.29
-8.22
1
29
1
0
15
18
26
-7.83
0
45
3
0
10
19
15
-8.60
-8.01
1
43
2
0
10
20
14
-9.02
-8.73
-9.62
-9.24
0
0
0
0
1
1
1
1
1
50
20
8
10
1
0
52
18
9
10
1
0
46
26
20
0
0
0
51
23
18
0
0
0
37
4
1
9
21
13
76
6
74
7
55
0
50
0
2
2
0
0
0
0
5
7
0
0
4
0
0
0
21
32
1j-H+ (X ) NH2)
MO
48a(R)
48a(â)
47a(R)
47a(â)
46a(R)
46a(â)
45a(R)
45a(â)
44a(R)
44a(â)
43a(R)
43a(â)
ꢀ (eV)
occ
% Fe
% Cp
% P
% CR
% Câ
% C(Ph)
% NH2
-5.30
-5.17
-5.83
-5.65
-7.69
1
17
0
0
16
10
37
13
-7.21
0
29
0
0
12
13
28
10
-7.72
-7.54
-8.20
-7.93
-8.87
-8.44
0
0
0
0
1
1
1
1
1
1
49
19
7
11
1
0
0
51
19
8
11
1
0
0
49
25
19
0
53
19
18
0
56
5
0
6
23
0
0
58
5
0
5
22
0
0
76
5
66
7
53
0
52
0
2
0
0
0
0
2
0
2
0
0
0
0
0
3
0
0
0
2
24
12
20
12
0
0
0
2
observed for the energies of the frontier R-spin and
â-spin MOs containing the unpaired electron. Thus,
starting from 1f-H+, the R-spin filled πy and πx MOs
separate in energy when the substituent becomes more
and more electron withdrawing, while the corresponding
â-spin πy and πx MOs become closer in energy and
eventually cross for X ) NO2. This is in fact an avoided
crossing, since admixture between these spin MOs is
allowed under C1
symmetry. Actually, admixture clearly
happens in 1f-H+ (X ) H), where the two filled frontier
R-spin MOs (Figure 7) extend over both the πy and πx
manifold on the acetylide linker. The admixture be-
tween the two occupied frontier R-spin MOs has quite
important consequences for the unpaired spin distribu-
tion, as discussed below, and illustrates the decisive
X-substituent influence on the SOMO in these com-
pounds.
The spin distribution computed for 1a-H+-1j-H+ is
given in Table 8. The largest positive spin density is
computed on the metal and then on the â-carbon atom
of the acetylide. It is strongly influenced by the X
substituent, being larger on these atoms for the most
electron-withdrawing substituents. The spin delocal-
ization on the aryl acetylide is clearly favored for the
most electron-releasing substituents, spreading more
and more on the aryl ring. As a result of the avoided
crossing mentioned above, the unpaired spin density is
located in the π manifold conjugated with the aryl ring
for the compounds bearing the electron-releasing sub-
stituents and in the perpendicular π manifold for the
electron-accepting X substituents, while for compounds
with a moderate substituent such as 1f-H+, the spin
density is located around the acetylide bridge, in both
πx and πy manifolds. This shows that the spatial location
of the unpaired spin density in the arylacetylide is
Figure 6. Energy of the frontier molecular spin orbitals
of the [(H3P)2(η5-C5H5)Fe(CtC)-1,4-(C6H4)X]+ (1a-H+-1j-
H+) model complexes (X ) NO2, CN, Br, H, OMe, NH2),
presenting a strong metallic character. The dominant
metal-centered AOs are represented on the left.

5472 Organometallics, Vol. 24, No. 22, 2005
Paul et al.
Figure 7. Plots of the highest occupied spin MO (left), of the lowest unoccupied spin MO (center) and of the total spin
density (right) of (H3P)2(η5-C5H5)Fe(CtC)-1,4-(C6H4)X complexes: (a-c) X ) NO2; (d-f) X ) H; (g-i) X ) NH2. The contour
values are 0.05 [e/bohr3]1/3
.
Table 8. Calculated Unpaired Spin Densities for
[(PH3)2(η5-C5H5)Fe(CtCC6H4X)]+ Complexes (X )
NO2, CN, Br, H, OMe, NH2)
motion of an electron from lower lying MOs in the
SOMO (no transition is strictly symmetry forbidden in
such compounds under C1 point-group symmetry). Thus,
the observed near-IR transition must correspond either
to the SOMO - 1/SOMO or to the SOMO - 2/SOMO
transition. Actually, the energy difference between
SOMO and SOMO - 1 levels of the â-spin orbitals
increases as the X substituent becomes more and more
electron releasing (Figure 6), in line with the observed
substituent effect on the near-IR absorption. However,
the energy gaps do not match with the energies of the
near-IR transitions of 1a+-1j+, being roughly one-
fourth to one-third of the energies experimentally found.
The energy gaps between the SOMO and the next
1a-H+
1b-H+ 1d-H+
1f-H+
(X )
H)
1i-H+
1j-H+
(X )
(X )
(X )
(X )
(X )
NO2)
CN)
Br)
OMe)
NH2)
Fe
0.799
0.734
0.598
0.047
0.209
0.210
0.637
0.042
0.241
0.151
0.508
0.108
0.147
0.315
0.427
0.134
0.133
0.382
CR
-0.054 -0.020
Câ
0.306
0.012
0.266
0.079
C6H4X
C5H5
2PH3
-0.036 -0.035 -0.036 -0.037 -0.041 -0.039
-0.027 -0.024 -0.028 -0.034 -0.037 -0.037
strongly modulated by the substituent. The positive
values found for the unpaired spin density on the
arylacetylide ligand suggest that the spin density is
mostly delocalized by Fermi contact. Comparatively, the
spin delocalization in the η5-C5H5 and phosphane ligands
is lower and overall negative, suggesting that spin
polarization mechanisms are also operative there.
In the absence of TD-DFT calculations, the discussion
of the electronic transitions of 1a+-1k+ has to be based
on the ground-state energy diagrams established for 1a-
H+-1j-H+. Such an approach is at best qualitative, since
no account is given to electronic relaxation in the excited
state.33 Accordingly, assignment of the lowest energy
(near-IR) transitions proves rather problematic using
these diagrams. Nevertheless, some general statements
can be made. The SOMO-LUMO gap computed for the
R-spin MOs in all complexes being much larger than
the energies of the low-energy electronic transitions
experimentally measured, these must involve the pro-
2
2
metal-centered MOs which possess a strong dx -y
character (Figure 6) are actually much closer but are
calculated to follow a reverse energetic trend depending
on the substituent. However, further evidence for the
latter assignment comes from the ESR data (see Discus-
sion).
Comparison between MOs derived for 1a-H+-1j-H+
and 1a-H-1j-H indicate that the MLCT transitions
computed for Fe(II) parents should be shifted toward
higher energies in the corresponding Fe(III) complexes
and might, therefore, no longer be detected above 300
nm for most compounds (Figure 5). Conversely, several
LMCT transitions, previously very energetic in Fe(II)
complexes, are shifted to lower energies and might now
become observable with Fe(III) complexes, since several
occupied levels with a dominant acetylide or phenyl
character are located 2.1-2.3 eV below the SOMO.
These levels (or some of lower energy) could typically
give rise to a LMCT transition that would appear in the
visible range (Figure 5). Alternatively, the low-energy
transitions observed in the visible range could also
(33) (a) Powell, C. E.; Cifuentes, M. P.; Morall, J. P.; Stranger, R.;
Humphrey, M. G.; Samoc, M.; Luther-Davies, B.; Heath, G. A. J. Am.
Chem. Soc. 2003, 125, 602-610. (b) Morrall, J. P.; Powell, C. E.;
Stranger, R.; Cifuentes, M. P.; Humphrey, M. G.; Heath, G. A. J.
Organomet. Chem. 2003, 670, 248-255.

Electron-Rich Piano-Stool Iron σ-Acetylides
Organometallics, Vol. 24, No. 22, 2005 5473
Figure 8. Plots of the (a) ESR principal and mean g values and (b) g tensor anisotropy vs σ+ ESPs for [(η2-dppe)(η5-C5-
t
Me5)Fe(CtC)-1,4-(C6H4)X][PF6] complexes (X ) NO2, CN, CF3, F, Br, H, Me, Bu, OMe, NH2, NMe2).
correspond to transitions between the SOMO and the
LUMO or LUMO + 1. However, according to Laporte’s
rule,34 these transitions are expected to show up less
intensely than the (symmetry allowed) LMCT men-
tioned above, since they take place between MOs
possessing a strong metal d character.
1a[PF6]-1j[PF6] are low-spin radical cations possessing
one unpaired electron (S ) 1/2). Despite the small
structural changes evidenced by X-ray studies, DFT
calculations indicate that sizable spin delocalization
takes place within the arylacetylide ligand (Table 8).5
Indeed, the quite large spin density computed on the
â-carbon is in line with a recent study on the reactivity
of these compounds with dioxygen.24 As expected for
such piano-stool d5 radicals, rhombic ESR spectra were
obtained for all of them, with three principal g tensor
ESP Correlations with ESR or UV-Vis/Near-IR
Spectrochemical Data. Significant linear correlations
have previously been reported between electronic sub-
stituent parameters (ESP) and infrared data for these
Fe(III) complexes. We have checked here if similar
correlations could also be obtained with characteristic
ESR or spectral data. The three lowest electronic
absorptions observed for these compounds as well as the
various g tensors recorded by ESR give significant linear
correlations (R2 > 0.95) when plotted against the
Hammett or σ+ ESP sets (see the Supporting Informa-
tion). The correlations with ESR data are shown in
Figure 8. Notably, a good linear correlation is also
obtained with the g tensor anisotropy (∆g). As previ-
ously stated with infrared data,2 better results were
obtained with the σ+ parameters than with the regular
Hammett σp set for single ESP correlations. This clearly
evidences an electronic substituent effect with a domi-
nant mesomeric contribution. Following the method of
Jiang,35 we next checked if some improvements in the
fits could be obtained when an ESP set reflects the spin
delocalization properties of the substituent, such as σJJ
for instance. However, no linear correlations at all were
found with this set alone, while only slight improve-
ments on the fits were stated in dual correlations
including σJJ and an additional ESP set reflecting the
polar character of the substituent (such as σ+). Thus,
substituent-centered spin-delocalization effects are ap-
parently not determining regarding this electronic effect
of the X substituent on the ESR and UV-vis-near-IR
data.
values in the order gmin < ge e gint < gmax.
14 As is often
observed for Fe(III)-centered radicals, the spin relax-
ation of these species is too fast to allow for the detection
of the hyperfine structure.10 The ESR signal is usually
very sensitive to the symmetry of the SOMO and to its
energy separation from the other metal-based frontier
MOs in these organometallic radicals.37 To rationalize
the experimental data obtained for 1a[PF6]-1j[PF6], the
ESR g tensor values were derived from the frontier MOs
calculated for 1a-H+-1j-H+.
Frontier MOs of 1a-H+-1j+-H and g Tensor
Anisotropy in 1a[PF6]-1j[PF6]. In complexes 1a-H+-
1j+-H, the σ and π MOs of the acetylide fragment
overlap significantly with the t2g d AOs of the metal
center.5 As discussed above, this has a strong influence
on the metal d composition and the energy ordering of
the frontier MOs.38 These parameters are determinant
for the ESR g anisotropy. Notably, our data differ
somewhat from those of previous EHMO calculations
on related d5 piano-stool radicals containing electron
poorer ligands, such as CO in place of PH3,39 or R
ligands without π orbitals.21 Our calculations compare
better with recent DFT computations reported for Ru-
(III) analogues by Rousseau et al.40 They show that,
depending on the X substituent, the SOMO location in
these Fe(III) compounds changes. Thus, it extends
mainly on the acetylide bridge and to a lesser extent
on the π manifold of the functional aryl group for
Discussion
(37) Rieger, P. H. Coord. Chem. Rev. 1994, 135/136, 203-286.
(38) Powell, C. E.; Cifuentes, M. P.; MacDonagh, A.; Hurst, S. K.;
Lucas, N. T.; Delfs, C. A.; Stranger, R.; Humphrey, M. G.; Houbrechts,
S.; Asselberghs, I.; Persoons, A.; Hockless, D. C. Inorg. Chim. Acta
2003, 352, 9-18.
In accordance with previous data on closely related
complexes,10,14,21,36 Evans measurements confirm that
the functionalized arylacetylide Fe(III) complexes
(39) (a) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. J.
Am. Chem. Soc. 1979, 101, 585-591. (b) Kostic, N. M.; Fenske, R. F.
Organometallics 1982, 1, 974-982. (c) Sponsler, M. B. Organometallics
1995, 14, 1920-1927.
(40) Koentjoro, O. F.; Rousseau, R.; Low, P. J. Organometallics 2001,
20, 4502-4509.
(34) Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University
Press: Oxford, U.K., 1998; p 501.
(35) Jiang, X.-K. Acc. Chem. Res. 1997, 30, 283-289.
(36) Morrow, J.; Catheline, D.; Debois, M.-H.; Manriquez, J.-M.;
Ruiz, J.; Astruc, D. Organometallics 1987, 6, 2605-2607.

5474 Organometallics, Vol. 24, No. 22, 2005
Paul et al.
Chart 3
In all cases, gzz corresponds to g3 in the experimental
ESR data obtained for 1a[PF6]-1j[PF6] (g1 < ge e g2 <
g3). The main component of the magnetic anisotropy (g3)
is therefore along the z direction with electron-releasing
substituents and becomes somewhat tilted in the yz
plane for electron-withdrawing substituents. The g
anisotropy in 1a[PF6]-1j[PF6] is therefore different
from that of related Cr(I) and Mn(II) piano-stool d5
radicals of Cs symmetry previously studied, such as 5
and 6 (Chart 3), for which the main component of the g
tensor was found to be along the cyclopentadienyl
centroid, somewhat tilted toward the most electron-rich
ligand among the piano stool “legs”.16,17
Figure 9. Schematic spin-restricted representation of the
frontier MOs for d5 piano-stool radicals such as [(PH3)2(η5-
C5H5)Fe(CtCC6H4X)]+ with C1 symmetry.
It is clear from the gii values found in the general case
that a single d-d transition cannot determine g1 or g2.
Accordingly, very poor linear fits (R2 < 0.76) are
obtained between g2 and the wavelength of all low-
energy absorptions. In contrast, fairly linear correlations
can be found (R2 ) 0.92) between the wavelength of the
second band in the visible spectrum and g1 (Supporting
Information). Significant linear correlations are also
found between g3 or g1 and the wavelength of the near-
IR transition (Supporting Information). However, the
near-IR transition is too energetic to be the SOMO -
1/SOMO transition (∆1) (see below) and is not energetic
enough to be ∆4, according to our calculations. Thus,
these “unexpected” correlations evidence that these g
tensor components and the corresponding electronic
transitions experience a similar electronic substituent
effect, rather than expressing any dependence through
equations such as (1).
If, as assumed here, ESR anisotropy arises essentially
from spin-orbit coupling, the more the unpaired elec-
tron is ligand-centered, the less anisotropic the ESR
signal will be, its giso value becoming concomitantly
closer and closer to the ge value (ge ) 2.0023). Accord-
ingly, the g values for a free σ radical obtained from
phenylacetylene (PhCtC•)44 or for a related π radical
anion45 are quite close to ge. On the basis of this simple
reasoning, we had previously proposed that the anisot-
ropy of the ESR signal in dinuclear mixed-valence
complexes was related to the “metal vs bridge” character
of the unpaired electron.46 In this work, our calculations
on 1a-H+-1k-H+ confirm that the anisotropy decreases
with the decrease of the total spin density on the iron
atom and the Hirschfeld charge on the metal center.
However, no strict linear dependence can be evidenced
between ∆g and these quantities (Figure 10). In line
with this statement, the asymmetry recorded for the
chloride and hydride Fe(III) complexes, 2[PF6] and
3[PF6] (ca. 0.450), compares to that obtained for 1h[PF6]
compounds with electron-releasing substituents. The
SOMO presents a dominant dyz metal character in these
complexes, while the SOMO - 1 is mostly dxz and the
LUMO mostly dxy in character (Figure 7).18,41 For
unsubstituted complexes, the SOMO is a mixture of dyz
and dxz and becomes purely dxz with electron-withdraw-
ing groups (Figure 7).
The atomic composition given in Figure 9 has there-
fore been considered for the five frontier MOs presenting
a large metallic contribution (Figure 6), using a spin-
restricted formalism, and the various gii components of
the tensor have been derived using a perturbative
approach to the first order (eq 1).28 Several classic
g ) g δ + 2ú m*0( 0|Iˆi|m m|Iˆj|0 )/(E0 - Em) (1)
∑
ij
e ij
simplifying assumptions have been made to obtain this
expression.42 Notably, the influence of spin-orbit cou-
pling from the carbon atoms of the arylacetylene ligand
has been neglected.43
The expressions of the gii values are quite complicated
in the general case (Supporting Information), but an
important simplification takes place in the case of
electron-releasing substituents (eqs 2-4), since the g
tensor becomes diagonal in the referential chosen. In
gxx ) ge + 2ú(c0c2)2/(∆2) -
6ú(c0c5)2/(∆5) + 2ú(c0c3′)2/(∆3) (2)
gyy ) ge - 2ú(c0c4)2/(∆4)
gzz ) ge + 2ú(c0c1)2/(∆1)
(3)
(4)
these equations, ge stands for the free electron g value,
cn values are the coefficients of the d AOs in the frontier
MOs, according to Figure 9, and ∆n values are the
corresponding transition energies.
(44) Coleman, J. S.; Hudson, A.; Root, K. D. J.; Walton, D. R. M.
Chem. Phys. Lett. 1971, 11, 300-301.
(41) Weyland, T.; Lapinte, C.; Frapper, G.; Calhorda, M. J.; Halet,
J.-F.; Toupet, L. Organometallics 1997, 16, 2024-2031.
(42) Stone, A. J. Proc. R. Soc. (London), Ser. A 1963, 271, 424-434.
(43) Neese, F.; Solomon, E. I. Inorg. Chem. 1998, 37, 6568-6582.
(45) Evans, A. G.; Evans, J. C.; Phelan, T. J. J. Chem. Soc., Perkin
Trans. 2 1974, 1216-1219.
(46) Le Stang, S.; Paul, F.; Lapinte, C. Organometallics 2000, 19,
1035-1043.

Electron-Rich Piano-Stool Iron σ-Acetylides
Organometallics, Vol. 24, No. 22, 2005 5475
tected in the near-IR range (Table 6) cannot correspond
to the SOMO - 1/SOMO transition (∆1). Indeed, con-
sidering a maximum value for the spin-orbit coupling
of a d5 Fe(III) ion (ú ) 460 cm-1 47
and considering c0 )
)
c1 ) 1 in eq 4, maximum values can be derived for ∆1
(1940-3400 cm-1), which remain far below the energies
of the transition detected in the near-IR domain for each
compound. Most probably, the observed near-IR transi-
tion does correspond to the SOMO - 2/SOMO transition
(∆2). As experimentally stated, its transition moment
is expected to increase for the most electron-releasing
substituents, in line with an increasing MLCT character
imparted to this LF transition. Indeed, the SOMO
becomes slightly more delocalized on the arylacetylide
ligand for electron-releasing substituents, while the
SOMO - 2 remains largely localized on the iron center.
In related d5 piano-stool radicals of Cs symmetry such
as [CpMn(CO)2(PPh3)]+ (6+) and (C5Ph5)Cr(CO)3 (7)
(Chart 3), presenting similar symmetry representations
for the two occupied upper frontier MOs,16 the SOMO
- 1/SOMO transition also determines the strongest
contribution to the g tensor for these organometallic
radicals and could be detected at 5710 cm-1 (ꢀ ca. 100
cm-2 M) and near 4000 cm-1 (ꢀ ca. 62 cm-2 M) for 6+
and 7, respectively.48 This result led to the proposal that
the increase of the largest g value for isoelectronic
radicals which varies in the sequence Fe(III) > Mn(II)
> Cr(I) was a consequence of the diminished energetic
separation between the 2A′ and 2A′′ states, itself mostly
determined by the SOMO/SOMO - 1 gap.14 Despite the
different expression previously found for gzz with 6+ and
7, our present work confirms this proposal. However,
the near-IR absorption detected now with 1a+-1k+
corresponds to the SOMO - 2/SOMO transition, rather
than to the SOMO - 1/SOMO transition.
Figure 10. Plot of the of the ESR g tensor anisotropy for
[(η2-dppe)(η5-C5Me5)Fe(CtC)-1,4-(C6H4)X][PF6] complexes
vs spin density computed for [(PH3)2(η5-C5H5)Fe(CtC)-1,4-
(C6H4)X]+ model complexes (X ) NO2, CN, Br, H, OMe,
NH2).
Chart 4
bearing the 4-tert-butyl-1-phenylacetylene ligand,
whereas computations suggest a far more pronounced
metallic character for the SOMOs in the latter com-
pounds, as well as a larger unpaired spin and charge
densities at the metal center, than for 1h[PF6].23a This
discrepancy possibly partly originates from the different
nature of the frontier MOs in these compounds featuring
a R ligand less π bonding than arylacetylides. These
complexes might also present a slightly different ani-
sotropy of the g tensor.21 We therefore want to stress
here that the anisotropy of the rhombic ESR signal for
[(η2-dppe)(η5-C5Me5)FeR][PF6] radicals can be used to
derive the relative metallic character of the SOMOs only
for complexes with structurally close R groups (i.e.
arylacetylide complexes).
Electronic Transitions. LF transitions such as ∆1-
∆3 (Figure 9) constitute the lowest energy transitions
in these complexes. Such forbidden transitions will not
induce large changes in the bonding within the Fe-Ct
C core in the excited state and might therefore not be
detected experimentally (Laporte’s rule). MO diagrams
suggest that the excited states in these transitions are
in agreement with the valence-bond (VB) structure E1
(Chart 4).
Among these LF transitions, the SOMO - 1/SOMO
transition (∆1) is particularly important since it deter-
mines the strongest diagonal g tensor component (eq
4). DFT computations indicate that this electronic
transition corresponds to the promotion of one electron
delocalized in the πx manifold of this fragment into the
perpendicular πy manifold (Figure 6), except for 1a-H+,
in which the reverse transition takes place. From the
expression of gzz established above (eq 4) and from the
experimental values (g3) determined for 1a[PF6]-
1k[PF6], one can derive the magnitude of the SOMO -
1/SOMO transition (∆1). Obviously, the transition de-
Transitions from lower lying MOs dominantly local-
ized on the acetylide bridge will be higher in energy
(Figure 5). Notably, most of these MOs still possess a
weak contribution of d AOs and cannot therefore be
considered as pure LMCT transitions, although we will
call them LMCT in the following. They should appear
in the same energy range as LF transitions between the
SOMO and the LUMO or the LUMO + 1, such as ∆4-
∆5 (Figure 9), but should have much larger transition
moments. These LMCT transitions certainly correspond
to the most intense transition(s) detected in the low-
energy part of the visible range (650-900 nm), whereas
∆4 or ∆5, if detected, could be the weaker absorption
detected at slightly higher energies. Similar LMCT
assignments have previously been proposed for the low-
energy transitions at the near-IR/visible edge in the
complexes [Ru(16-TMC)(CtCC6H4X-p)2]+ (8+) and [Ru-
(Me2bipy)(PPh3)2Cl(CtCR)]+ (9+) (Chart 5).8,33,49 How-
ever, for these compounds, the weak shoulder on the
high-energy side of this absorption was attributed to a
vibronic progression involving the acetylide stretch, the
difference between respective maxima being typically
below 1800 cm-1 8,49,50
Presently, the energetic differ-
.
ences (∆E) between the two low-energy absorptions in
the visible range are around 2300 cm-1 for 1a[PF6]-
(47) Dunn, T. M. Trans. Faraday Soc. 1961, 57, 1441-1444.
(48) Atwood, C. G.; Geiger, W. E. J. Am. Chem. Soc. 1994, 116,
10849-10850.
(49) Choi, M.-Y.; Chan, M. C.-W.; Zhang, S.; Cheug, K.-K.; Che, C.-
M.; Wong, K.-Y. Organometallics 1999, 2074, 4-2080.

5476 Organometallics, Vol. 24, No. 22, 2005
Paul et al.
Chart 5
Scheme 2
1h[PF6] (Table 5). These differences are too large to
correspond to a similar vibronic progression, and we
tentatively suggest that the weak absorption near
16 670 cm-1 (600 nm) corresponds to the SOMO/LUMO
(LF) transition ∆4 (Figure 9). Notably, both LMCT and
LF transitions experience a related substituent-induced
shift. The substituent effect on their intensity is also in
accordance with the composition of the frontier MOs
discussed above, since the partial localization of the
SOMO on the substituent imparts a slight LMCT
character to ∆4 or ∆5, in contrast to the MLCT character
imparted to ∆1-∆3.
Whereas the VB description (E1) can also be used to
represent excited states resulting from LF transitions
∆4-∆5, the VB representation E2 (Chart 4) should be
used to depict the LMCT excited state. In this VB
structure, the metal is formally in the reduced Fe(II)
oxidation state and presents a decreased acetylide bond
order in the excited state, in accord with the depopula-
tion of the acetylide π (bonding) levels.8,49 E2 therefore
constitutes a better representation of the MLCT state
than E3, previously proposed,1-3 in which the metal
center was formally in an Fe(I) oxidation state (19-
electron metal center).
low-lying LMCT absorption in the visible range (such
as E2 or E3) remains qualitatively correct, especially
regarding the most electron-releasing substituents. The
negative sign previously found for most â values con-
firms that the GS of these Fe(III) complexes is admixed
with excited states having diminished dipole moments
along the acetylide axis in comparison to the GS under
the influence of the laser light beam. As briefly dis-
cussed by Humphrey and co-workers,33 low-lying (near-
IR) transitions might have a much more profound effect
on third-order NLO properties of these compounds.
Electronic Substituent Effects. The linear correla-
tions obtained between the ESR data and σ+ ESPs
confirm the existence of electronic substituent effects,
with a strong “mesomeric” character, transmitted from
the aryl ring to the metal center through the alkynyl
spacer. The failure to improve the linear fits by use of
ESP sets tailored to analyze substituent-centered spin-
delocalization effects suggests that the substituent
influence in these Fe(III) complexes originates mostly
from polarity changes induced by the X substituent. In
accordance with related correlations on infrared data,2
the substituent influence in the GS can be described
using the VB scheme given in Scheme 2.
Hyperpolarizabilities and VB Schemes. NLO
hyperpolarizabilities of several of these Fe(III) com-
plexes were previously measured by EFISH.3 The lowest
energy excited states are usually determinant for this
electronic property.51 A few words are needed here, since
we have previously overlooked the existence of the near-
IR absorption while EFISH measurements were re-
corded under near-resonant conditions relative to this
transition for several Fe(III) complexes. The simple two-
level model used to rationalize the NLO data was
therefore not fully appropriate, and a perturbational
approach involving more excited states should have
been considered to rationalize the static hyperpolariz-
abilities (â0) for these Fe(III) compounds. Indeed, the
existence of low-lying (near-IR) LF excited states helps
in understanding the relatively weak hyperpolarizabili-
ties (in absolute magnitude) observed for these Fe(III)
complexes in comparison to those for the Fe(II) parents.
However, considering the weak transition moments of
these transitions along with the fact that they probably
do not induce a large charge transfer along the metal-
acetylide axis (see E1 in Chart 4), our former VB
interpretation of the NLO properties based on a single
The substituent effect on the ESR g tensor compo-
nents (Figure 7) is operative following different mech-
anisms (eqs 2-4 for instance) and cannot be rationalized
in a simple way. Depending on its nature, the X group
modifies the energies and the atomic composition of the
frontier MO’s. Our computations suggest that the
former effect prevails along the 1a-H+-1k-H+ series.
Qualitatively, the g3 tensor component is principally
responsible for the deviation of the giso value from the
free electron value, and this deviation can be traced back
to the fact that the SOMO and SOMO - 1 are close in
energy in these compounds.
Conclusion. We report here the isolation and char-
acterization of several members of a family of Fe(III)
radical cations of the formula [(η2-dppe)(η5-C5Me5)Fe-
(CtC)-1,4-(C6H4)X][PF6] (1a[PF6]-1j[PF6]). These com-
1
pounds are S ) /2 compounds, with the unpaired spin
residing mostly on the {Fe-(CtC)-1,4-(C6H4)X} frag-
ment. DFT computations on [(H3P)2(η5-C5H5)Fe(CtC)-
1,4-(C6H4)X]+ model complexes indicate that a change
in the X substituent induces a sizable change in the
unpaired spin distribution within this moiety. Accord-
ingly, the ESR data reveal that the unpaired electron
presents an increasing “arylacetylide” (organic) char-
acter when the X substituent becomes more and more
electron donating. The anisotropy (∆g) of the ESR signal
can therefore be conveniently used to approximate the
“metallic” character of the SOMO in families of struc-
turally related complexes. The electronic substituent
effect is also experimentally evidenced by means of
(50) In the corresponding LMCT excited states of 8+ or 9+, an
electron is removed from an acetylide π-bonding MO to be promoted
either into an acetylide antibonding π* MO49 or into a nonbonding
metal-centered d MO,8 resulting in a diminished acetylide bond order
and ν(CtC) value.
(51) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994, 94,
195-242. di Bella, S.; Fragala, I.; Marks, T. J.; Ratner, M. A. J. Am.
Chem. Soc. 1996, 118, 12747-12751.

Electron-Rich Piano-Stool Iron σ-Acetylides
Organometallics, Vol. 24, No. 22, 2005 5477
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)Br][PF6] (1d-
[PF6]). Yield: 87%. Color: dark brown. Crystals of the complex
were obtained by slow diffusion of n-pentane in a dichlo-
romethane solution of the complex (layer/layer). Traces of
methanol were also present in the n-pentane. Anal. Calcd for
C45H43BrF6P3Fe‚CH3OH: C, 57.13; H, 5.11. Found: C, 56.88;
H, 5.02. IR (ν, KBr/Nujol, cm-1): 2021, 1993 (w, CtC).
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)F][PF6] (1e-
[PF6]). Yield: 81%. Color: dark brown. Anal. Calcd for
C45H43F7P3Fe‚H2O: C, 60.61; H, 5.21. Found: C, 60.30; H, 5.06.
The origin of the water solvate(s) is unknown, possibly aerial
contamination during recrystallization. IR (ν, KBr/Nujol,
cm-1): 2015 (m, CtC).
correlations between σ+ ESPs and the ESR rhombic g
tensors, which emphasize the “mesomeric” origin of this
phenomenon. Electronic transitions appear to be quite
substituent dependent as well. Notably, a weak absorp-
tion is detected in the near-IR range for all Fe(III)
complexes. We assign it to a SOMO - 2/SOMO elec-
tronic transition presenting a slight but increasing
MLCT character for the most electron-releasing X
substituents. Similar LF transitions also show up at
higher energies in the visible range, as well as intense
LMCT transitions. In conclusion, this study rationalizes
the electronic properties of electron-rich mononuclear
Fe(III) radicals such as 1a+-1k+ and will now consti-
tute a valuable benchmark to analyze spectral data for
related polynuclear complexes containing [(η2-dppe)(η5-
C5Me5)Fe(CtC)-1,4-(C6H4)]+-fragments.
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)Me][PF6] (1g-
[PF6]). Yield: 83%. Color: pale brown. Crystals of the complex
were obtained by slow diffusion of diethyl ether in a dichlo-
romethane solution of the complex (layer/layer). Anal. Calcd
for C45H46F6P3Fe‚H2O: C, 62.30; H, 5.58; P, 10.71. Found: C,
62.70; H, 5.53; P, 10.57. IR (ν, KBr/Nujol, cm-1): 2001 (w, Ct
C).
Experimental Section
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)tBu][PF6] (1h-
[PF6]). Yield: 91%. Color: pale brown. IR (ν, KBr/Nujol, cm-1):
1996 (w, CtC).
General Data. All manipulations were carried out under
an inert atmosphere. Solvents or reagents were used as
follows: Et2O and n-pentane, distilled from Na/benzophenone;
CH2Cl2, distilled from P2O5 and then Na2CO3 and purged with
argon; HN(iPr)2, distilled from KOH and purged with argon;
aryl bromides (Acros, >99%), opened/stored under Ar. The [(η5-
C5H5)2Fe][PF6] ferrocenium salt,52 the Fe(II) acetylide com-
plexes (η5-C5Me5)(η2-dppe)FeCtC(C6H4)X (X ) NO2, CN, CF3,
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)OMe][PF6] (1i-
[PF6]). Yield: 91%. Color: red-brown. Crystals of the complex
were obtained by slow diffusion of n-pentane in a dichlo-
romethane solution of the complex (layer/layer). Anal. Calcd
for C45H46F6OP3Fe: C, 62.44; H, 5.36, F, 13.17. Found: C,
62.48; H, 5.36; F, 11.53. IR (ν, KBr/Nujol, cm-1): 1990 (m, Ct
C).
t
Br, F, H, Me, Bu, OMe, NH2, NMe2; 1a-k), and some of the
Fe(III) acetylide complexes [(C5Me5)(η2-dppe)FeCtC(C6H4)X]-
[PF6] (X ) NO2, H, NH2) were prepared by previously
published procedures.1,4,14 Transmittance FTIR spectra were
recorded using a Bruker IFS28 spectrometer (400-4000 cm-1).
Near-infrared (near-IR) spectra were recorded using a Bruker
IFS28 spectrometer, using a Nernst Globar source and a KBr
separator with a DTGS detector (400-7500 cm-1) or tungsten
source and a quartz separator with a Peltier-effect detector
(5200-12500 cm-1). Liquid near-IR spectra were recorded on
a Cary 5 spectrometer. UV-visible spectra were recorded on
an UVIKON XL spectrometer. EPR spectra were recorded on
a Bruker EMX-8/2.7 (X-band) spectrometer. Elemental analy-
ses were performed at the “Service central d’analyses” (USR
CNRS 59 at Lyon-Vernaison) and at the “Centre Regional de
Mesures Physiques de l’Ouest” (CRMPO, University of Rennes
1).
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)NH2][PF6] (1j-
[PF6]). Crystals of the known complex were obtained by slow
diffusion of n-pentane in a dichloromethane solution of the
complex (layer/layer).
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)NMe2][PF6] (1k-
[PF6]). The oxidation reaction, the precipitation, and the
washings of the Fe(III) acetylide complex were done at -70
°C. Yield: 70%. Color: dark brown. IR (ν, KBr/CH2Cl2, cm-1):
1959/1962 (vs, CtC).
ESR Measurements. The Fe(III) complexes were ground
with a slight excess of [(η5-C5H5)2Fe][PF6] and introduced in a
ESR tube under an argon-filled atomsphere and a 1:1 mixture
of degassed dichloromethane/1,2-dichloroethane was trans-
ferred to dissolve the solid, just before being frozen at 77 K,
and the tubes were sealed and transferred to the ESR cavity.
The spectra were immediately recorded at that temperature.
Computational Details. DFT calculations were carried out
using the Amsterdam Density Functional (ADF) program.53
The model compounds Fe(η5-C5H5)(PH3)2(CtC-1,4-C6H4X)n+ (X
) NO2, CN, H, OMe, NH2; n ) 0, 1) were used in order to
reduce computational effort. Electron correlation was treated
within the local density approximation (LDA) in the Vosko-
Wilk-Nusair parametrization.54 The nonlocal corrections of
Becke55 and of Perdew56 were added to the exchange and
correlation energies, respectively. The numerical integration
procedure applied for the calculations was developed by te
Velde et al.53 The basis set used for the metal atom was a
triple-ú Slater-type orbital (STO) basis for Fe 3d and 4s and a
single-ú function for 4p of Fe. A triple-ú STO basis set was
employed for H 1s and for 2s and 2p of C, N, and O, extended
Synthesis of the Fe(III) Acetylide Complexes. [Fe(η5-
C5H5)2][PF6] (0.95 equiv; 0.120 g, 0.361 mmol) was added to a
solution of the corresponding Fe(II) parent (0.380 mmol) in
15 mL of dichloromethane, resulting in an instantaneous
darkening of the solution. Stirring was maintained for 1 h at
room temperature, and the solution was concentrated in vacuo
to approximately 5 mL. Addition of 50 mL of of n-pentane
allowed precipitation of a dark solid. Decantation and subse-
quent washing with 3 × 3 mL portions of toluene followed by
3 × 3 mL of diethyl ether and drying under vacuum yielded
the desired [(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)X][PF6]
complex as an analytically pure sample.
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)CN][PF6] (1b-
[PF6]). Yield: 92%. Color: dark brown. Crystals of the complex
were obtained by slow diffusion of diethyl ether in a dichlo-
romethane solution of the complex (layer/layer). The origin of
the water solvate(s) is unknown: possibly aerial contamination
during diffusion. Anal. Calcd for C45H43F6NP3Fe‚H2O: C,
61.52; H, 5.16; N, 1.59. Found: C, 61.30; H, 5.01; N, 1.65. IR
(ν, KBr/Nujol, cm-1): 2222 (w, CtN); 2042, 2021 (vw, CtC).
[(η2-dppe)(η5-C5Me5)Fe-(CtC)-1,4-(C6H4)CF3][PF6] (1c-
[PF6]). Yield: 87%. Color: brown. Anal. Calcd for C45H43F9P3-
Fe: C, 59.82; H, 4.80; F, 18.92. Found: C, 59.60; H, 4.77; F,
16.71. IR (ν, KBr/Nujol, cm-1): 2041 (vw, CtC).
(53) (a) te Velde, G.; Bickelhaupt, F. M.; Fonseca Guerra, C.; van
Gisbergen, S. J. A.; Baerends, E. J.; Snijders, J.; Ziegler, T. Theor.
Chim. Acc. 2001, 22, 931-967. (b) Fonseca Guerra, C.; Snijders, J.; te
Velde, G.; Baerends, E. J. Theor. Chim. Acc. 1998, 99, 391-403. (c)
ADF2002.01. In Theoretical Chemistry, Vrije Universiteit: Amsterdam;
2002 ed.; SCM: Amsterdam, The Netherlands, 2002.
(54) Vosko, S. D.; Wilk, L.; Nusair, M. Can. J. Chem. 1990, 58,
1200-1211.
(55) (a) Becke, A. D. J. Chem. Phys. 1986, 84, 4524-4529. (b) Becke,
A. D. Phys. Rev. A 1988, 38, 3098-3100.
(56) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822-8824. (b) Perdew,
J. P. Phys. Rev. B 1986, 34, 7406.
(52) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910.

5478 Organometallics, Vol. 24, No. 22, 2005
Paul et al.
with a single-ú polarization function (2p for H; 3d for C, N,
and O) for X groups. The valence orbitals of the atoms of the
other groups (C5H5, PH3) were described by a double-ú STO
basis set. Full geometry optimizations (assuming C1 symmetry)
were carried out on each complex, using the analytical gradient
method implemented by Verluis and Ziegler.57 Spin-unre-
stricted calculations were performed for all the considered
open-shell systems. The representations of the molecular
orbitals were done using MOLEKEL4.1.58
Crystallography. Crystals of 1b[PF6], 1d[PF6], 1f[PF6],
1g[PF6], 1h[PF6], 1i[PF6], and 1j[PF6] were obtained as
described above. The samples were studied on a NONIUS
Kappa CCD with graphite-monochromatized Mo KR radiation.
The cell parameters are obtained with Denzo and Scalepack
with 10 frames (ψ rotation: 1° per frame).59 The data collec-
tion60 (2θmax, number of frames, Ω rotation, scan rate, and hkl
range as given in Table 1) gives 69 005, 39 693, 59 451, 40 464,
76 089, 41 000, and 51 541 reflections for 1b[PF6], 1d[PF6],
1f[PF6], 1g[PF6], 1h[PF6], 1i[PF6], and 1j[PF6], respectively.
Data reduction with Denzo and Scalepack59 gave the indepen-
dent reflections (Table 1). The structures were solved with SIR-
97, which reveals the non-hydrogen atoms.61 After anisotropic
refinement, the remaining atoms were found by a Fourier
difference map. The whole structures were then refined with
SHELXL9762 by full-matrix least-squares techniques (use of
F2 magnitude; x, y, z, âij for Fe, P, C, N, and/or O atoms, x, y,
z in riding mode for H atoms with variables “N(var)”, observa-
tions and “w” used as defined in Table 1).
Atomic scattering factors were taken from the literature.63
ORTEP views of 1b[PF6], 1d[PF6], 1f[PF6], 1g[PF6], 1h[PF6],
1i[PF6], and 1j[PF6] were realized with PLATON98.64 All of
the calculations were performed on a Pentium NT Server
computer.
Acknowledgment. Thanks are expressed to P.
Hamon for kindly providing spectral data as well as
starting material for the hydride complex. The CNRS
is acknowledged for financial support.
Supporting Information Available: Tables of solvent-
induced shifts on the near-IR absorption for 1f[PF6], tables of
R2 indices for various correlations between spectroscopic data
and ESP sets, derivation of the expression for the g tensor
components, selected plots of the energy vs g tensor diagonal
components, and full details of the X-ray structure of 1b[PF6],
1d[PF6], 1f[PF6], 1g[PF6], 1h[PF6], 1i[PF6], and 1j[PF6], in-
cluding tables of atomic positional parameters, bond distances
and angles, anisotropic and isotropic thermal displacement
parameters; X-ray data are also given as CIF files. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(57) Verluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322-328.
(58) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. Swiss Center
for Scientific Computing (CSCS), 2000-2001.
OM0505108
(59) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter,
C. W., Sweet, R. M., Eds.; Academic Press: London, 1997; Vol. 276,
pp 307-326.
(62) Sheldrick, G. M. SHELX97-2: Program for the Refinement of
Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany,
1997.
(63) Reidel: D. International Tables for X-ray Crystallography;
Kynoch Press: Birmingham, U.K., 1974; Vol. IV (present distributor:
D. Reidel, Dordrecht, The Netherlands).
(64) Spek, A. L. PLATON. A Multipurpose Crystallographic Tool;
Utrecht University, Utrecht, The Netherlands, 1998.
(60) Nonius, B. V. Kappa CCD Software; Enraf-Nonius, Delft, The
Netherlands, 1999.
(61) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
J. Appl. Chem. 1998, 31, 74-77.