Piano-stool iron(II) complexes as probes for the bonding of N-heterocyclic carbenes: Indications for π-acceptor ability

Article abstract of DOI:10.1021/om060637c

A series of new piano-stool iron(II) complexes comprising mono- and bidentate chelating N-heterocyclic carbene ligands [Fe(cp)(CO)(NHC)(L)]X have been prepared and analyzed by spectroscopic, electrochemical, crystallographic, and theoretical methods. Selectively substituting the L site with a series of ligands going from carbene to pyridine to CO suggests that CO is the strongest π acceptor, while the behavior of pyridine and carbene is nearly identical. This suggests that in these complexes comprising an electron-rich iron(cp)(carbene) fragment, N-heterocyclic carbenes are not pure σ donors but also moderate π acceptors. Theoretical calculations support this bonding model and indicate charge saturation at the metal as key for π back-bonding to N-heterocyclic carbenes. On the basis of voltammetric measurements, the Lever electrochemical parameter of these carbenes has been determined: E_L = +0.29. Systematic substitution of the wingtip groups of the carbene revealed only subtle changes in the electronic properties of the iron center, thus providing a suitable methodology for ligand-induced fine-tuning of the coordinated metal.

Full text of DOI:10.1021/om060637c

5648
Organometallics 2006, 25, 5648-5656
Piano-Stool Iron(II) Complexes as Probes for the Bonding of
N-Heterocyclic Carbenes: Indications for π-Acceptor Ability
Laszlo Mercs,† Gae¨l Labat,‡ Antonia Neels,‡ Andreas Ehlers,§ and Martin Albrecht*,†
Department of Chemistry, UniVersity of Fribourg, Ch. du Muse´e 9, CH-1700 Fribourg, Switzerland,
Institute of Microtechnology, UniVersity of Neuchaˆtel, Rue Emile Argand 11,
CH-2009 Neuchaˆtel, Switzerland, and Scheikundig Laboratorium, Vrije UniVersiteit Van Amsterdam,
De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
ReceiVed July 14, 2006
A series of new piano-stool iron(II) complexes comprising mono- and bidentate chelating N-heterocyclic
carbene ligands [Fe(cp)(CO)(NHC)(L)]X have been prepared and analyzed by spectroscopic, electrochemi-
cal, crystallographic, and theoretical methods. Selectively substituting the L site with a series of ligands
going from carbene to pyridine to CO suggests that CO is the strongest π acceptor, while the behavior
of pyridine and carbene is nearly identical. This suggests that in these complexes comprising an electron-
rich iron(cp)(carbene) fragment, N-heterocyclic carbenes are not pure σ donors but also moderate π
acceptors. Theoretical calculations support this bonding model and indicate charge saturation at the metal
as key for π back-bonding to N-heterocyclic carbenes. On the basis of voltammetric measurements, the
Lever electrochemical parameter of these carbenes has been determined: EL ) +0.29. Systematic
substitution of the wingtip groups of the carbene revealed only subtle changes in the electronic properties
of the iron center, thus providing a suitable methodology for ligand-induced fine-tuning of the coordinated
metal.
Introduction
The discovery of N-heterocyclic carbenes (NHCs) as ligands
for transition metals has greatly stimulated the development of
efficient catalysts.1 Often, these carbene complexes surpass the
activity of their corresponding phosphine analogues.2 Their
catalytic performance may be optimized by ligand tuning, since
steric and electronic influences of a large number of ligands
have been tabulated. In organometallic chemistry, Tolman’s
electronic parameters (ν) are typically used,3 while in coordina-
tion chemistry ligands are more often classified according to
Lever’s electrochemical parameters (EL).4 Recently, a theoretical
model has been proposed, which relates these two parameters
via computed electronic parameters. This allows the classifica-
tion also of ligands that have not been considered thus far.5
For example, NHCs have been suggested to be some of the
Figure 1. Direct and indirect probes for ligand tuning in Fe(II)
carbene complexes.
strongest neutral donors available to date, having donor strengths
only slightly weaker than anionic I-. Some IR investigations
on Rh and Ir complexes6 confirmed the theoretical prediction
that heteroatom-stabilized carbenes are generally stronger
ligands than the most basic phosphines.
We were interested to combine the probes of Lever and
Tolman in order to classify the basicity of carbenes quantita-
tively. Half-sandwich iron carbonyl complexes are particularly
attractive, as they combine a redox-active FeII center and a CO
ligand for Lever- and Tolman-type parametrization, respectively.
This allows the quantification of the effects of both the wingtip
group R and the donor E. Due to the conformational rigidity of
these complexes, the ligand basicity may be measured eventually
also by NMR spectroscopy, using the carbon and proton nuclei
of the cyclopentadienyl (cp) ligand as probes.
Here we report on a series of new iron(II) carbene complexes
in which the ligands can be varied systematically (Figure 1).
Steric effects of the wingtip groups have been investigated
through substitution of R ) Me to R ) iPr, and electronic
influences by including mesityl (Mes)-substituted NHCs. In
addition, variation of the donor site E from carbene to different
ligands such as pyridine, iodide, and CO allows a quantitative
* To whom correspondence should be addressed. E-mail:
martin.albrecht@unifr.ch. Fax: +41-263009738.
† University of Fribourg.
‡ University of Neuchaˆtel.
§ Vrije Universiteit van Amsterdam.
(1) (a) Wanzlick, H-W.; Scho¨nherr, H-J. Angew. Chem., Int. Ed. Engl.
1968, 7, 141. (b) O¨ fele, K. J. Organomet. Chem. 1968, 12, P42. (c)
Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113,
361. (d) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV.
2000, 100, 39. (e) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(f) Ce´sar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. ReV. 2004,
33, 619. (g) Cavell, K. J.; McGuinness, D. S. Coord. Chem. ReV. 2004,
248, 671.
(2) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron
Lett. 1999, 40, 2247. (b) Huang, J.; Schanz, H. Z.; Stevens, E. D.; Nolan,
S. P. Organometallics 1999, 18, 5375. (c) Perry, M. C.; Cui, X.; Powell,
M. T.; Hou, D-R.; Reibenspies, J. H.; Burgess, K. J. Am. Chem. Soc. 2003,
125, 113. (d) Navarro, O.; Marion, N.; Mei, J.; Nolan, S. P. Chem. Eur. J.
2006, 12, 5142. (e) O’Brien, C. J.; Kantchev, E. A. B.; Valente, C.; Hadei,
N.; Chass, G. A.; Lough, A.; Hopkinson, A. C.; Organ, M. G. Chem. Eur.
J. 2006, 12, 4743.
(5) Perrin, L.; Clot, E.; Eisenstein, O.; Loch, J.; Crabtree, R. H. Inorg.
Chem. 2001, 40, 5806.
(3) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(4) (a) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271. (b) Lever, A. B. P.
Inorg. Chem. 1991, 30, 1980.
(6) (a) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree,
R. H. Organometallics 2003, 22, 1663. (b) Viciano, M.; Mas-Marza, E.;
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10.1021/om060637c CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/12/2006

Piano-Stool Iron(II) Complexes as Probes
Organometallics, Vol. 25, No. 23, 2006 5649
a
Scheme 1
a Reagents and conditions: (i) KOtBu, thf, RT 1.5 h or BuLi, thf, -78 °C; then [FeI(cp)(CO)2], toluene, RT, 20 h; (ii) AgBF4, CH2Cl2, RT, 3
h; (iii) hν, CH2Cl2, 16 h.
comparison of the donor properties of these ligands in a well-
defined environment.
complexes, we have applied a free carbene route according to
a recently established procedure (Scheme 1).11 Deprotonation
of the imidazolium salt with a strong base such as KOtBu or
nBuLi gave the corresponding carbene, which was metalated
in situ with [FeI(cp)(CO)2] as iron(II) precursor. The formed
complexes 1-9 are air-stable when kept in the solid state for
several weeks. In CDCl3 solution, they decompose to a
paramagnetic compound typically within a few hours. In DMSO
or acetone solution, the cationic monodentate carbene complexes
1 and 2 gradually lose CO to form the corresponding neutral
complexes. This reaction is accelerated by UV irradiation and
is accompanied by a characteristic color change of the com-
plexes from yellow to green.
Complexation with picolyl-carbenes gave mixtures of two
compounds. Spectroscopic analyses identified these products
as the monodentate coordinating carbene complex 7 (νs ) 2049
cm-1, νas ) 2002 cm-1) and the desired chelate 8 (ν ) 1960
cm-1).12 UV irradiation of the product mixture induced CO
dissociation in 7, thus affording pure 8. Efforts to prepare 7
selectively by performing the metalation under strict exclusion
of light failed, and instead mixtures of 7 and 8 were obtained
again. Similarly, exposure of 8 to excess CO (up to 2 bar) did
not induce pyridine dissociation.
Our results unexpectedly suggest a similar basicity of NHCs
and pyridine ligands in these piano-stool iron(II) carbene
complexes. Given the stronger σ donation of NHC versus
pyridine, the π-acceptor properties of NHCs must be at least as
significant as in pyridines and not, as often quoted,1d,e negligible.
Theoretical studies on these complexes provide further insight
in the carbene bonding mode and reveal significant iron-to-
carbene π back-bonding interactions when the metal center is
electron-rich.7 In addition, the ligand basicity in the Fe
complexes investigated shows very little dependence on the
wingtip groups, which corroborates previous studies on square-
planar complexes.8 These results may be particularly relevant
for identifying ligand positions that affect the ligand donor
properties and hence the catalytic activity of the coordinated
metal center.
Results and Discussion
Synthesis of Fe Carbene Complexes. Iron(II) complexes
containing a monodentate NHC ligand have been prepared
previously by Lappert and co-workers in the course of their
pioneering studies on the reactivity of electron-rich ene-
tetramines9 and by heterocycle formation at the metal.10 Since
both these methods provide only restricted access to chelated
Formation of the desired complexes is indicated by the
expected imidazolium/cp proton ratio in the pertinent 1H NMR
spectra. Chelation of the bidentate dicarbene ligand in 5 and
the pyridine-carbene in 8 is evident from the two AX doublets
(7) On the basis of computational studies, the relevance of π interactions
in carbene bonding has been discussed controversely. For examples, see:
(a) Tafipolsky, M.; Scherer, W.; O¨ fele, K.; Artus, G.; Pedersen, B.;
Herrmann, W. A.; McGrady, G. S. J. Am. Chem. Soc. 2002, 124, 5865. (b)
Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics 2003, 22, 612.
(c) Lee, M-T.; Hu, C-H. Organometallics 2004, 23, 976. (d) Nemcsok, D.;
Wichmann, K.; Frenking, G. Organometallics 2004, 23, 3640. (e) Scott,
N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.; Nolan, S. P. J.
Am. Chem. Soc. 2005, 127, 3516.
(8) (a) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo,
L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485. (b) Scott,
N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815.
(9) (a) Cetinkaya, B.; Dixneuf, P.; Lappert, M. F. J. Chem. Soc., Dalton
Trans. 1974, 1827. (b) Lappert, M. F. J. Organomet. Chem. 1972, 358,
185.
(10) (a) McCormick, F. B.; Angelici, R. J. Inorg. Chem. 1979, 18, 1231.
(b) Rieger, D.; Lotz, S. D.; Kernbach, U.; Andre´, C.; Bertran-Nadal, J.;
Fehlhammer, W. P. J. Organomet. Chem. 1995, 491, 135.
(11) (a) Raubenheimer, H. G.; Scott, F.; Cronje, S.; Rooyen, P. H.; Psotta,
K. J. Chem. Soc., Dalton Trans. 1992, 1009. (b) Buchgraber, P.; Toupet,
L.; Guerchais, V. Organometallics 2003, 22, 5144.
(12) Formation of a neutral carbene complex comprising a coordinated
iodide and a monodentate coordinating carbene with a dangling picolyl
wingtip substituent has been discarded on the basis of variable-temperature
NMR experiments. No indication of a dynamic behavior due to an
interconversion to the chelating complex with a noncoordinating iodide anion
has been observed in the -80 to +20 °C temperature range (acetone-d6
solution).

5650 Organometallics, Vol. 25, No. 23, 2006
Mercs et al.
due to the methylene protons. Apparently, the boat-like con-
formation of the six-membered metallacycle is rigid on the NMR
time scale. The CH3 protons of the iPr wingtip substituents of
the chelating ligands appear as two distinct doublets, thus
indicating a restricted wingtip rotation about the N-Cpropyl bond.
1
In the H NMR spectrum of 4b, four different CH3 groups
appear for the iPr wingtip groups. This suggests that rotation
about both the Fe-Ccarbene and the N-CiPr bonds is slow on
the NMR time scale. No coalescence is observed up to 90 °C
(DMSO solution), which corresponds to an activation energy
∆Gq > 90 kJ mol-1 13,14
In contrast, complex 4a, containing
.
smaller Me wingtip groups, is fluxional at room temperature.
A low-temperature limiting spectrum is reached at T ) -20
°C, which correlates to an approximate free energy of activation
∆Gq ) 59 kJ mol-1 for the rotation of the carbene about the
Fe-C bond.15
Figure 2. ORTEP representation of 3b (50% probability ellipsoids;
H atoms and BF4 counterion omitted for clarity).
Infrared spectroscopy provided valuable information on the
donor strength of the carbene ligand. Notably, the CO absorption
energies appear to be virtually independent of the wingtip
substituents. This points to a rather limited influence of those
groups on the electronic properties of the metal center. Interest-
ingly, the CO vibrations in the cationic monocarbene complexes
2 (νs ) 2049 and νas ) 2001 cm-1) are similar to those in the
precursor complex [FeI(cp)(CO)2] (2041 and 1997 cm-1).
Hence, the donor strength of the formally neutral carbene ligand
to the [Fe(cp)(CO)2]+ fragment is comparable to that of the
anionic iodide. When bound to the [Fe(cp)(CO)(carbene)]+
fragment, however, iodide is a stronger donor (ν ) 1935 cm-1
in 4) than carbenes (ν ) 1950 cm-1 in 5). This illustrates that
the ligand donor power is not an intrinsic parameter but strongly
depends on the metal fragment. The data further suggest a
similar donor strength for carbene and pyridine ligands (ν )
1960 cm-1 in 8), the carbene being slightly more donating. Such
a conclusion is also supported by the 13C NMR resonance
frequencies of the cylopentadienyl carbons, which appear at
nearly identical frequencies (δC 81.9 ( 0.2 in 5 vs 82.3 ( 0.1
in 8).
Figure 3. ORTEP representation of the molecular structure of 5c
(30% probability level; H atoms and the nonbonding iodide have
been omitted for clarity).
Solid-State Structures. Crystals of 3b suitable for a structure
determination were grown by slow diffusion of Et2O into a CH2-
Cl2 solution. The molecular structure (Figure 2) reveals the
characteristic piano-stool arrangement of half-sandwich iron
complexes. The Fe-carbene bond distance Fe-C6 1.970(3) Å
fits well in the 1.97-1.99 Å range of related monodentate
carbene-iron bonds in (pseudo-)octahedral geometries11,17 yet
being significantly shorter than in tetrahedral high-spin com-
plexes (typically 2.07-2.13 Å).18 The CO-Fe-CO bond angle
is 92.8(2)° and thus slightly smaller than the CO-Fe-carbene
angles (CO-Fe-C6 94.2(1)° and 94.4(2)°, respectively), pre-
sumably due to the steric impact of the carbene ligand.
The molecular structure of 5c (Figure 3) provides unambigu-
ous evidence for the chelating bonding mode of the dicarbene
with a bite angle C2-Fe1-C14 of 86.1(2)°. The metallacycle
adopts a boat-like conformation, which issaccording to the
magnetic inequivalence of these methylene protonssalso present
in solution. The Fe-C bonds (1.952(5) and 1.955(5) Å) are
slightly shorter than in the monocarbene complex 3b and similar
to those in related iron(II) complexes containing chelating
carbene ligands.19 Apparently, the Fe-C bond length in these
complexes is determined predominantly by the steric constraints
of the chelate rather than being a consequence of the bond
strength. Similarly, the CO bond length cannot be used as a
probe for the electron donor properties of the carbene ligand.
In 5c, containing two carbene donors, Fe back-bonding into the
CO π* orbital is expected to be larger than in 3b, with only
one carbene ligand. Contrary to this hypothesis, the C-O bond
length is shorter in 5c (1.113(7) Å) than in 3b (1.142(4) Å).
AgBF4-mediated exchange of the noncoordinating anion from
I to BF4 afforded complexes 3, 6, and 9 in good yields (Scheme
1). As expected for substitutions in the outer coordination sphere,
the electronic properties of the metal center are not strongly
affected. For example, the IR spectroscopic data for the CO
vibrations are identical to those of the parent iodide complexes.
In the 1H NMR spectra, a distinct high-field shift of one
heterocyclic and the low-field methylene proton is diagnostic
for the anion exchange. This may be explained by weak
interactions of acidic ligand protons with the anion.16 Such
-
interactions are expected to be weak for the soft BF4 anion
(δH 7.94 and 6.95 in 6c, CDCl3 solution), though stronger for
iodide (δH 8.36 and 7.97 in 5c). Notably, no such anion-
dependent signal shift is seen when the measurements are
performed in polar solvents such as DMSO-d6.
(13) Faller J. W. In Encylopedia of Inorganic Chemistry; King, R. B.,
Ed.; John Wiley: New York, 2005; p 5270.
(14) Instead, metal dissociation is indicated by the rapid evolution of
signals assigned to the imidazolium ligand precursor. Decomposition of
4b in DMSO-d6 solution is temperature dependent, with t1/2 ≈ 24 h at 25
°C and t1/2 ≈ 20 min at 80 °C.
(15) On the basis of this large energy difference, we have discarded an
alternative fluxional process involving ligand dissociation and inversion of
configuration at the Fe center. For a discussion on the configurational
stability of related complexes, see: (a) Brunner, H.; Wallner, G. Chem.
Ber. 1976, 109, 1053. (b) Brunner, H. Eur. J. Inorg. Chem. 2001, 905.
(16) For related ion-pairing effects, see: (a) Dupont, J.; Suarez, P. A.
Z.; De, Souza, R. F.; Burrow, R. A.; Kintzinger, J-P. Chem. Eur. J. 2000,
6, 2377. (b) Filipponi, S.; Jones, J. N.; Johnson, J. A.; Cowley, A. H.;
Grepioni, F.; Braga, D. Chem. Commun. 2003, 2716.
(17) Capon, J-F.; El, Hassnaoui, S.; Gloaguen, F.; Schollhammer, P.;
Talarmin, J. Organometallics 2005, 24, 2020.
(18) (a) Louie, J.; Grubbs, R. H. Chem. Commun. 2000, 1479. (b) Chen,
M-Z.; Sun, H-M.; Li, W- F.; Wang, Z-G.; Shen, Q.; Zhang, Y. J.
Organomet. Chem. 2006, 691, 2006.
(19) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E.
Organometallics 2004, 23, 166.

Piano-Stool Iron(II) Complexes as Probes
Organometallics, Vol. 25, No. 23, 2006 5651
Table 1. Hydrogen Bond Interactions in the
Pyridine-Carbene Complexes 8a-c
distance (Å)
H‚‚‚X C‚‚‚X
angle (deg)
sym translation
for X
C-H‚‚‚X
C-H‚‚‚X
8a
C26-H26B‚‚‚I2 3.04 3.941(10)
8b
151
-1/2 + x, -3/2-y, z
C8-H8‚‚‚I1
C10-H10A‚‚‚I1 3.00 3.930(9)
2.97 3.910(9)
C28-H28A‚‚‚I2 3.06 3.980(9)
8c
3.02 3.957(9)
170
158
170
156
-1 + x, y, z
-x, 1-y, 1-z
-1 + x, -1 + y, z
-x, 1-y, -z
C26-H26‚‚‚I2
C3-H3‚‚‚O1
2.42 3.231(3)
2.91 3.824(3)
143
161
176
149
-1-x, 1-y, -z
x, 1 + y, z
-x, y, 1-z
1 + x, y, z
C8-H8‚‚‚I1
C10-H10B‚‚‚I1 2.94 3.925(3)
C13-H13‚‚‚I1
3.05 3.890(3)
Table 2. Vibrational and Electrochemical Data of Fe(II)
Complexes
E
1/2 vs
E
1/2 vs
E
entry complex bonding mode ν(CO)a
SCEb
NHEc (obs)d
1
2
3
4
5
6
7
8
9
4a
4b
4ce
6a
6b
6c
9a
9b
9c
C-monodentate 1936 +0.48
C-monodentate 1935 +0.46
C-monodentate 1938 +0.41
+0.72 +0.78
+0.70 +0.78
+0.65 +0.78
+1.36
+1.34 +1.36
+1.39 +1.36
+1.34 +1.43
+1.40 +1.32
+1.42 +1.32
C,C-bidentate
C,C-bidentate
C,C-bidentate
C,C-bidentate
C,C-bidentate
C,C-bidentate
1950 +1.14 (irrev.)f
1948 +1.10
1956 +1.15
1964 +1.10
1964 +1.16
1966 +1.18
Figure 4. ORTEP representation (50% probability) of the molec-
ular structures of 8a (a), 8b (b), and 8c (c). Hydrogen atoms,
cocrystallized solvent molecules, and the noncoordinating iodide
ligand have been omitted for clarity. Only one of the two
crystallographically independent molecules of 8a and 8b are shown.
a In cm-1 measured in CH2Cl2. In V vs SCE (E1/2 Fc+/Fc at +0.46 V),
b
measured in CH2Cl2, 0.1 M [Bu4N][PF6] electrolyte, sweep rate 200 mV
s-1. Calculated based on E(SCE) ) 0.24 V. Calculated according to eq 2
c
d
e
f
with EL(NHC) ) +0.29, EL(cp) ) +0.04. From ref 11b. Epa of irreversible
Notably, the crystal structure of 5c displays short contacts
between the iodide anion and the methylene proton H1A
(H1A‚‚‚I 2.97 Å, C1‚‚‚I 3.936(5) Å, C1-H1A‚‚‚I 166°). A
similar hydrogen-bonding motif, albeit much weaker, may also
exist in solution (vide supra).
oxidation.
The solid-state structures of 8a-c have been determined in
order to study wingtip group effects. The molecular structures
of these complexes confirm the C,N-bidentate chelating bonding
mode deduced from solution measurements (Figure 4a-c).
Neither Fe-Ccarbene nor carbonyl C-O bond length analyses
show any correlation for wingtip-dependent iron-carbene bond
lengths. The largest differences are seen for the pyridine-
carbene bite angle, which is smaller for the mesityl-substituted
ligand. This may be due to the conformational rigidity of the
mesityl group, since wingtip change from Me (8a) to sterically
more demanding though flexible iPr (8b) does not influence
the ligand bite angle similarly.
In all three complexes, short contacts are observed between
the noncoordinating iodide anion and the acidic CH2 protons
of the methylene that interlinks the two heterocycles (Table 1).
In addition, 8c reveals Ccp-H‚‚‚OCO hydrogen bonds in the solid
state, thus resulting in a dimeric structure (Figure 5). The
H‚‚‚O distance is 2.42 Å and hence rather short, though the
carbonyl C-O bond is not exceedingly stretched when com-
pared with 8a and 8b, which do not form similar dimers.
Electrochemical Measurements. Electrochemical analysis
of the FeIII/FeII redox potential E0 offers a useful probe of the
ligand basicity.4 Thus, low oxidation potentials and hence a
better stabilization of iron(III) rather than iron(II) centers is
expected for ligands that are strongly donating. Electrochemical
measurements have been carried out on the neutral complex 4
and all ionic BF4 complexes in CH2Cl2 containing 0.1 M nBu4-
PF6 as supporting electrolyte. The data are compiled in Table 2
and correspond to standard potentials for fully reversible Fe
oxidation, except for 6a, whose oxidation wave appeared to be
Figure 5. Intermolecular C-H‚‚‚O hydrogen bonding in 8c
provides a dimeric solid-state structure (C3‚‚‚O1_a 3.231(3) Å, C3-
H3‚‚‚O1_a 143°). The indices represent the following symmetry
translations: a ) -1 - x, 1 - y, -z; b ) -x, -y, 1 - z; c ) 1
+ x, y, z; d ) x, 1 + y, z.
irreversible. In addition, the monocarbene complex 3 did not
provide any useful signal. According to our calculations using
additive electrochemical ligand parameters, a very high oxidation
potential is estimated for Fe oxidation in 3 (1.84 V vs SCE).
As expected, the oxidation potentials for the neutral com-
plexes 4 are significantly lower than for the ionic complexes.
Furthermore, the wingtip substituents hardly affect the redox
potential. Interestingly, only minor differences between the
dicarbene complexes 6 and the pyridine-carbenes 9 are
observed. This indicates a similar electronic configuration of
all these Fe centers and thus closely related donor properties of
carbenes and pyridines in these complexes. Such a conclusion

5652 Organometallics, Vol. 25, No. 23, 2006
Mercs et al.
Figure 6. Calculated d orbital energies (in eV) of a series of Fe(cp) complexes.
is also confirmed by the pertinent CO stretching bands of these
complexes.20
donation of carbenes is generally accepted to be stronger than
for pyridines, our results suggest that also π back-bonding must
be stronger in carbenes than in pyridines in order to balance
the net charge transfer. This conclusion is particularly relevant
when considering that π back-bonding to pyridine ligands is
well established.23
DFT Calculations and Energy Decomposition Analyses.
Independent DFT calculations have been carried out for six
different [Fe(cp)(CO)L2]+ cations, A-F, in the gas phase
(Figure 6). While the cations C, E, and F are simplified
analogues of the complexes 3, 6, and 9, respectively, the
structures B and a chelating bipyridine version of D have been
described in the literature.24 Complex A provides a valuable
starting point to discuss primary ligand effects.
Geometry optimization gave bond distances that corroborate
in most cases the experimental values.25 Exceptions are the
calculated structural analogues of 6 and 9, for which a slightly
longer Fe-CO bond distance is predicted from calculations.
This may be rationalized by the chelate effect, which organizes
the ligand position more rigidly than in a nonchelating system.
The scaled IR stretch vibrations corroborate the experimental
observations and reflect an increasing ligand basicity, CO ,
pyr e NHC.25
In order to further describe the ligand bonding mode, an
energy decomposition analysis has been carried out for the
cations of type [Fe(cp)(CO)2L]+, A, B, and C (Table 3), with
L and [Fe(cp)(CO)2]+ as the two fragments. The calculated bond
dissociation energies (BDEs) clearly confirm that the carbene
bonding is stronger than that of pyridine and CO, the latter being
decreased due to Pauli repulsion. Analysis of the orbital
interactions ∆Eorb suggests similar π interactions in pyridine
and carbene bonding. The π contribution to the Fe-L bond
strength in the pyridine-containing cation B is 17.8% and for
NHC in C 15.4%, while in the tricarbonyl cation A, this
The electrochemical data help determine Lever’s electro-
chemical parameter EL for the NHC ligand.4 If the ligand
contributions are supposed to be additive, that is, if synergistic
and steric factors are ignored in a first approximation, the redox
potential E(obs) of the low-spin FeIII/FeII couples follows the
least-square equation
E(obs) ) 1.11( E ) - 0.43
(2)
∑
L
The observed standard potentials E(obs) have been corrected
to NHE references and averaged for each set of donors. Using
eq 2 and the parameters for pyridine (EL ) +0.25), iodide (EL
) -0.24), and CO (EL ) +0.99) gives for N-heterocyclic
carbenes EL ) +0.29 and EL ) +0.04 for the cp ligand.21 This
value may be translated into Hammett substituent parameter σ22
and also into Tolman’s electronic parameters (ν),3 the latter
being frequently used in phosphine chemistry. According to the
correlation proposed by Clot and co-workers5
ν (cm-1) ) 76.82EL (V) + 2049
(3)
the Tolman parameter of NHC ligands is determined as ν )
2071 cm-1. While this value is at the higher end when compared
to previously calculated parameters,5 it places carbenes in a
donor range similar to that of the most basic phosphines. This
corroborates preceding investigations on carbene donor proper-
ties using IR stretch vibrations.6
The Lever parameters of pyridine and carbene indicate similar
ligand donor properties in these iron(II) complexes. Since σ
(20) The 1H and 13C NMR shifts of the cp signals follow a similar trend.
While often, NMR chemical shifts are a consequence of various influences,
in this case they apparently provide additional support for the electronic
configuration at the metal center.
(21) The cp parameter has been confirmed by examining the redox
potential of [FeI(cp)(CO)2]. The measured value (+1.24 V vs SCE) is in
good agreement with the calculated potential (+1.30 V vs SCE for EL(cp)
) +0.04 V), indicating that the Lever model is applicable for this kind of
complexes. Previously, a slightly higher value has been determined for cp
(EL ) +0.08 V), albeit based on anodic peak potentials only. See: Jia, G.;
Lough, A. J.; Morris, R. H. Organometallics 1992, 11, 161.
(23) (a) Lindoy, L. F.; Livingstone, S. E. Coord. Chem. ReV. 1967, 2,
173. (b) Smith, A. P.; Fraser, C. L. In ComprehensiVe Coordination
Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Oxford, UK,
2004; p 1.
(24) Treichel, P. M.; Shubkin, R. L.; Barnett, K. W.; Reichard, D. Inorg.
Chem. 1966, 5, 1177.
(22) Masui, H.; Lever, A. B. P. Inorg. Chem. 1993, 32, 2199.
(25) See the Supporting Information for details on the calculations.

Piano-Stool Iron(II) Complexes as Probes
Organometallics, Vol. 25, No. 23, 2006 5653
Table 3. Energy Decomposition Analyses for the Cations
wingtip substitution in these piano-stool complexes has only a
minor impact on the electronic nature of the central metal. Such
groups may therefore be more important for modifying the
accessibility rather than the activity of the metal center.
These results corroborate earlier theoretical studies and may
contribute to a more refined application of carbenes as ligands.
While our results suggest that carbenes do not behave as pure
σ-donor ligands, further studies are certainly warranted to
generalize this bonding model beyond the iron(II) complexes
presented here. Notably, the donor power and π-bonding
character depend not just on the ligand but also on the metal
fragment, so these results may not be reliably transferable to
other systems comprising, for example, electron-poorer metal
centers.
A-Ca
A
B
C
∆Esteric
33.2
-1.9
-5.4
∆Eoi (a′)
∆Eoi (a′′)
π contribution
BDE
-66.0
-21.1
48.5%
-52.8
-41.2
-8.9
-66.4
-12.1
15.4%
-81.4
17.8%
-50.0
a In kcal mol-1; π contribution is fraction of ∆Eoi(a′′)/sum(∆Eoi) except
for A, where due to symmetry π contribution is a fraction of 2∆Eoi(a′′)/
sum(∆Eoi).
Table 4. Charge Transfer in Cations A-Ca
A
B
C
∆e σ
-0.48
+0.36
-0.12
-0.35
+0.04
-0.31
-0.62
+0.10
-0.52
∆e π
∆e (total)
Experimental
a In electrons, negative values indicate charge transfer to the metal
(LMCT).
General Comments. All reactions have been performed using
standard Schlenk techniques under a nitrogen atmosphere unless
stated otherwise. Toluene, THF, and CH2Cl2 were dried by passage
through solvent purification columns; all other reagents were used
without further purification. The syntheses of the imidazolium salts27
contribution is, as expected, considerably larger (48.5%). These
percentages are in line with earlier theoretical investigations26
and represent metal-NHC π interactions that are substantial
rather than negligible, as often quoted.1d,e The orbitals that are
predominantly involved in these π interactions have been
identified as the HOMO of the [Fe(cp)(CO)2]+ fragment as
donor and the ligand’s aromatic π* orbitals (LUMO for pyridine
and LUMO+1 for NHC) as acceptor orbitals, thus unambigu-
ously suggesting metal-to-ligand back-bonding. On the basis
of the amount of transferred charge (Table 4), the NHC ligand
is in absolute terms a better π acceptor than pyridine. Yet, the
calculations clearly reveal that the metal-carbene bonding is
dominated by σ-type ligand-to-metal charge transfer. The σ
donation occurs from the carbene lone pair, which is signifi-
cantly higher in energy (-4.75 eV) than the nitrogen lone pair
of pyridine (-5.97 eV). As a result, the energies of the occupied
d orbital of the complexes A-F are shifted up to higher levels
with increasing number of σ-donor ligands (Figure 6). Obvi-
ously, also the π back-bonding of the pyridine and NHC ligands
increases in this series (synergistic effect), leading to comparable
orbital energies for E and F. This is supported by a further
increase of π contribution to 28% for cations [Fe(cp)(L)3]+ (L
) py, NHC).
1
and complexes 2c and 4c11b are described elsewhere. All H and
13C{1H} NMR spectra were recorded at 25 °C unless stated
otherwise and referenced to residual solvent 1H or 13C resonances
(δ in ppm, J in Hz). Assignments are based either on distortionless
enhancement of polarization transfer (DEPT) experiments or on
homo- and heteronuclear shift correlation spectroscopy. IR spectra
were recorded on a Mattson 5000 FTIR instrument in CH2Cl2
solution. Elemental analyses were performed by the Microanalytical
Laboratory of Ilse Beetz (Kronach, Germany). UV irradiation was
performed by using a commercial Hg lamp.
Electrochemical Measurements. Electrochemical studies were
carried out using an EG&G Princeton Applied Research Potentiostat
Model 273A employing a gastight three-electrode cell under an
argon atmosphere. A Pt disk with a 3.14 mm2 surface area was
used as the working electrode and was polished before each
measurement. The reference was a saturated calomel electrode
(SCE); the counter electrode was a Pt wire. Bu4NPF6 (0.1 M) in
dry CH2Cl2 was used as a base electrolyte with analyte concentra-
tions of approximately 1 × 10-3 M. The redox potentials were
measured against the ferrocenium/ferrocene (Fc+/Fc) redox couple,
which was used as an internal standard (E1/2 ) 0.46 V vs SCE).28
General Procedure for the Preparation of the Monocarbene
Complexes 1-3. To a suspension of the imidazolium salt (1.0 molar
equiv) in dry THF (15 mL) was added KOtBu (1.2 molar equiv).
After 1 h, this solution was added to a solution of [FeI(cp)(CO)2]
(0.9 molar equiv) in dry toluene (40 mL). After stirring for 16 h,
the formed precipitate was separated by centrifugation, washed once
with dry toluene (30 mL), and then extracted with dry CH2Cl2 (2
× 30 mL). Evaporation of the solvent gave the crude product, which
was recrystallized by slow diffusion of Et2O into a CH2Cl2 solution
to give an analytically pure sample.
Conclusions
Electrochemical, IR spectroscopic, and theoretical studies of
piano-stool Fe(II) carbene complexes have been carried out in
order to qualitatively and quantitatively analyze the donor
strength of N-heterocyclic carbenes. Our studies, which are also
supported by NMR chemical shift analyses of the cp spectator
ligand, suggest that the donor properties of NHC and pyridine
are highly similar when coordinated to the Fe(cp)(carbene)
fragment. The Lever electrochemical parameter for carbene (EL
) 0.29) has been determined for the first time and relates well
to that of pyridine (EL ) 0.25). The comparable donor strength
of these ligands has been explained by considerable π back-
bonding from the electron-rich iron(II) center to the carbene
ligand. While metal-carbene π bonding contrasts with the
general assumption that carbenes are pure σ donors with
negligible π contribution, DFT calculations suggest moderate
π-acceptor properties of these NHC ligands. Furthermore,
Synthesis of 1a. This complex was prepared from dimethylimi-
dazolium iodide (1.12 g, 5 mmol), KOtBu (0.67 g, 6 mmol), and
[FeI(cp)(CO)2] (1.44 g, 4.8 mmol). The crude product was obtained
as a brownish powder (1.12 g, 58%). 1H NMR (CDCl3, 400
MHz): δ 7.32 (s, 2H, im-H), 5.51 (s, 5H, cp), 3.94 (s, 6H, CH3).
13C{1H} NMR (CDCl3, 100 MHz): δ 211.4 (CO), 164.0 (im-C2),
(27) (a) Gardiner, M. G.; Herrmann, W. A.; Reisinger, C-P.; Schwarz,
J.; Spiegler, M. J. Organomet. Chem. 1999, 572, 239. (b) Albrecht, M.;
Miecznikowski, J. R.; Samuel, A.; Faller, J. W.; Crabtree, R. H. Organo-
metallics 2002, 21, 3596. (c) Tulloch, A. A. D.; Danopoulos, A. A.; Winston,
S.; Kleinhenz, S.; Eastham, G. J. Chem. Soc., Dalton Trans. 2000, 4499.
(d) McGuinness, D. S.; Cavell, K. J. Organometallics 2000, 19, 741. (e)
Gru¨ndemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R.
H. J. Am. Chem. Soc. 2002, 124, 10473.
(26) (a) Frenking, G.; Sola, M.; Vyboishchikov, S. J. Organomet. Chem.
2005, 690, 6178. (b) Termaten, A. T.; Schakel, M.; Ehlers, A. W.; Lutz,
M.; Spek, A. L.; Lammertsma, K. Chem. Eur. J. 2003, 9, 3577.
(28) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.

5654 Organometallics, Vol. 25, No. 23, 2006
Mercs et al.
127.1 (im-C4,5), 87.4 (cp), 40.8 (CH3). IR (CH2Cl2, cm-1): 2048,
2001 ν(CO). Anal. Calcd for C12H13FeIN2O2 (399.99): C 36.03,
H 3.28, N 7.00. Found: C 35.94, H 3.36, N 7.12.
) 6.4 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz): δ 223.8
(CO), 181.9 (im-C2), 120.4, 119.2 (im-C4,5), 79.8 (cp), 54.2, 52.2
(2 × CHMe2), 24.8, 24.2, 23.9, 23.7 (4 × CH(CH3)2). IR (CH2Cl2,
cm-1): 1935 ν(CO). Anal. Calcd for C15H21FeIN2O (428.09): C
42.08, H 4.94, N 6.54. Found: C 42.07, H 4.94, N 6.45.
General Procedure for the Preparation of Dicarbene Com-
plexes 5. To a suspension of the corresponding bisimidazolium salt
(1 molar equiv) in dry THF (15 mL) was added KOtBu (2.4 molar
equiv) at RT or nBuLi (2 molar equiv) at -78 °C. The mixture
was stirred at RT for 1 h and then added to a solution of [FeI(cp)-
(CO)2] (0.9 molar equiv) in dry toluene (40 mL). After stirring for
16 h, the formed precipitate was separated by centrifugation, washed
once with dry toluene (30 mL), and then extracted with dry CH2-
Cl2 (2 × 30 mL). The crude product was obtained by evaporating
the solvent.
Synthesis of 1b. This complex was prepared from (N-isopropyl-
N′-methyl)imidazolium iodide (0.50 g, 2 mmol), KOtBu (0.27 g,
2.4 mmol), and [FeI(cp)(CO)2] (0.55 g, 1.8 mmol). The crude
product was obtained as a yellow powder (1.09 g, 67%). 1H NMR
(CDCl3, 500 MHz): δ 7.44 (s, 1H, im-H4), 7.30 (s, 1H, im-H5),
5.48 (s, 5H, cp), 4.86 (septet, 1H, 3JHH ) 6.5 Hz, CHMe2), 3.96 (s,
3H, NCH3), 1.50 (d, 6H, 3JHH ) 6.5 Hz, CH(CH3)2). 13C{1H} NMR
(CDCl3, 125 MHz): δ 211.2 (CO), 162.7 (im-C2), 128.4 (im-C4),
121.3 (im-C5), 87.4 (cp), 53.7 (NCH3), 41.0 (CHMe2), 24.0 (CH-
(CH3)2). IR (CH2Cl2, cm-1): 2049, 2001 ν(CO). Anal. Calcd for
C14H17FeIN2O2 (428.05): C 39.28, H 4.00, N 6.54. Found: C 39.22,
H 4.10, N 6.58.
Synthesis of 1c. This complex was prepared from (N-mesityl-
N′-methyl)imidazolium iodide (0.66 g, 2 mmol), KOtBu (0.27 g,
2.4 mmol), and [FeI(cp)(CO)2] (0.55 g, 1.8 mmol). The crude
Synthesis of 5a. This complex was prepared from the methyl-
enedi(N-methyl)imidazolium diiodide (1.30 g, 3 mmol), nBuLi (1.6
M in hexanes, 3.8 mL, 6 mmol), and [FeI(cp)(CO)2] (0.82 g, 2.7
mmol). The product was obtained as a brown powder (0.78 g, 64%).
Recrystallization from CH2Cl2/Et2O gave an analytically pure
1
product was obtained as a green powder (0.41 g, 45%). H NMR
(CDCl3, 400 MHz): δ 7.70 (s, 1H, im), 7.01 (s, 3H, im and mes-
H3,5), 5.35 (s, 5H, cp), 4.14 (s, 3H, NCH3), 2.35 (s, 3H, p-CH3),
1.89 (s, 6H, o-CH3). 13C{1H} NMR (CDCl3, 100 MHz): δ 210.2
(CO), 167.0 (im-C2), 140.9 (mes-C4), 135.8 (mes-C2,6), 135.7 (mes-
C1), 129.8 (mes-C3,5), 128.9, 126.2 (im-C4,5), 87.0 (cp), 41.8
(NCH3), 21.2 (p-CH3), 18.1 (o-CH3). IR (CH2Cl2, cm-1): 2049,
2004 ν(CO). Anal. Calcd for C20H21FeIN2O2 (504.14): C 47.65,
H 4.20, N 5.56. Found: C 47.76, H 4.16, N 5.50.
Synthesis of 2b. This complex was prepared from (N,N′-
diisopropyl)imidazolium iodide (0.56 g, 2 mmol), KOtBu (0.27 g,
2.4 mmol), and [FeI(cp)(CO)2] (0.55 g, 1.8 mmol). The crude
product was obtained as a yellow powder (0.71 g, 87%). 1H NMR
(CDCl3, 500 MHz): δ 7.41 (s, 2H, im), 5.47 (s, 5H, cp), 4.94
(septet, 2H, 3JHH ) 6.7 Hz, CHMe2), 1.52 (d, 12H, 3JHH ) 6.7 Hz,
CH(CH3)2). 13C{1H} NMR (CDCl3, 125 MHz): δ 211.0 (CO),
161.4 (im-C2), 122.6 (im-C4,5), 87.3 (cp), 53.7 (CHMe2), 24.1 (CH-
(CH3)2). IR (CH2Cl2, cm-1): 2049, 2001 ν(CO). Anal. Calcd for
C16H21FeIN2O2 (456.11): C 42.13, H 4.64, N 6.14. Found: C 42.29,
H 4.73, N 6.16.
1
2
sample. H NMR (CDCl3, 500 MHz): δ 7.96 (d, 2H, JHH ) 1.7
Hz, im), 7.44 (low-field part of AX d, 1H, 2JHH ) 12.8 Hz, CH2),
2
7.06 (d, 2H, JHH ) 1.7 Hz, im), 5.91 (high-field part of AX d,
2
1H, JHH ) 12.8 Hz, CH2), 4.73 (s, 5H, cp), 3.76 (s, 6H, CH3).
13C{1H} NMR (CDCl3, 125 MHz): δ 219.2 (CO), 182.9 (im-C2),
124.2, 124.0 (im-C4,5), 82.1 (cp), 62.8 (CH2), 37.8 (CH3). IR (CH2-
Cl2, cm-1): 1949 ν(CO). Anal. Calcd for C15H17FeIN4O (452.07):
C 39.85, H 3.79, N 12.39. Found: C 39.93, H 3.85, N 12.43.
Synthesis of 5b. This complex was prepared from methylenedi-
(N-isopropyl)imidazolium diiodide (1.46 g, 3 mmol), nBuLi (1.6
M in hexanes, 3.8 mL, 6 mmol), and [FeI(cp)(CO)2] (0.82 g, 2.7
mmol). The product was obtained as a brown powder (0.75 g, 55%).
Recrystallization from acetone/pentane gave an analytically pure
1
3
sample. H NMR (CDCl3, 400 MHz): δ 8.08 (d, 2H, JHH ) 2.2
Hz, im), 7.55 (low-field part of AX d, 1H, 2JHH ) 12.9 Hz, CH2),
7.05 (d, 2H, 3JHH ) 2.2 Hz, im), 5.85 (high-field part of AX d, 1H,
2JHH ) 12.9 Hz, CH2), 4.84 (septet, 2H, JHH ) 6.7 Hz, CHMe2),
3
3
4.69 (s, 5H, cp), 1.46, 1.42 (2 × d, 12H, JHH ) 6.7 Hz, CH-
(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz): δ 219.5 (CO), 180.3
(im-C2), 125.3, 118.6 (im-C4,5), 82.1 (cp), 68.3 (CH2), 52.2
(CHMe2), 23.8, 23.5 (2 × CH(CH3)2). IR (CH2Cl2, cm-1): 1948
ν(CO). Anal. Calcd for C19H25FeIN4O (508.18) × C3H6O: C 46.66,
H 5.52, N 9.89, Fe 9.86. Found: C 47.17, H 5.27, N 9.86, Fe 9.62.
Synthesis of 3b. Complex 2b (0.20 g, 0.4 mmol) and AgBF4
(0.10 g, 0.5 mmol) were stirred in dry CH2Cl2 (10 mL) under
exclusion of light. The solution was stirred for 3 h in the dark,
then filtered through Celite. Evaporation of the solvent gave 0.12
g of 7 as a brown powder (69%). 1H NMR (acetone-d6, 360
MHz): δ 7.83 (s, 2H, im), 5.62 (s, 5H, cp), 5.13 (septet, 2H, 3JHH
Synthesis of 5c. This complex was prepared from methylenedi-
(N-mesityl)imidazolium diiodide (1.28 g, 2 mmol), KOtBu (0.52
g, 4.6 mmol), and [FeI(cp)(CO)2] (0.55 g, 1.8 mmol). The product
was obtained as a greenish powder (0.68 g, 54%). Recrystallization
3
) 6.6 Hz, CHMe2), 1.52 (d, 12H, JHH ) 6.6 Hz, CH(CH3)2). IR
(CH2Cl2, cm-1): 2050, 2002 ν(CO). Anal. Calcd for C16H21BF4-
FeN2O2 (416.00): C 46.20, H 5.09, N 6.73. Found: C 46.29, H
5.16, N 6.67.
1
from CHCl3/pentane gave an analytically pure sample. H NMR
(CDCl3, 500 MHz): δ 8.36 (s, 2H, im), 7.97 (low-field part of AX
Synthesis of 4a. A solution of 2a (0.4 g, 1 mmol) in dry CH2-
Cl2 (15 mL) was irradiated for 16 h, upon which the initially brown
solution became green. Evaporation of the solvent gave the crude
product as a green powder (0.32 g, 86%) that was analytically pure
after filtration through Celite and subsequent recrystallization from
acetone/pentane. 1H NMR (acetone-d6, 400 MHz, -20 °C): δ 7.45,
7.34 (2 × s, 2H, im), 4.57 (s, 5H, cp), 4.29, 3.86 (2 × s, 6H, Me).
13C{1H} NMR (acetone-d6, 100 MHz, -20 °C): δ 225.7 (CO),
184.6 (im-C2), 125.5, 125.3 (im-C4,5), 81.0 (cp), 41.9, 39.5 (2 ×
Me). IR (CH2Cl2, cm-1): 1936 ν(CO). Anal. Calcd for C11H13-
FeIN2O (371.98): C 35.52, H 3.52, N 7.53. Found: C 35.59, H
3.57, N 7.50.
2
d, 1H, JHH ) 12.9 Hz, CH2), 6.97, 6.94 (2 × s, 4H, mes-H3,5),
2
6.90 (s, 2H, im), 6.08 (high-field part of AX d, 1H, JHH ) 12.9
Hz, CH2), 4.36 (s, 5H, cp), 2.33 (s, 6H, p-CH3), 2.03, 1.70 (2 × s,
12H, o-CH3). 13C{1H} NMR (CDCl3, 125 MHz): δ 218.8 (CO),
186.2 (im-C2), 139.7 (mes-C1), 136.3, 135.5, 134.4 (mes-C2,4,6),
129.5, 129.1 (mes-C3,5), 125.2, 124.7 (im-C4,5), 81.7 (cp), 63.0
(CH2), 21.2 (p-CH3), 18.4, 18.1 (2 × o-CH3). IR (CH2Cl2, cm-1):
1956 ν(CO). Anal. Calcd for C31H33FeIN4O (660.37) × CHCl3: C
49.29, H 4.40, N 7.19. Found: C 49.23, H 4.48, N 7.05.
General Procedure for the Preparation of BF4 Complexes 6.
All BF4 salts were prepared from the corresponding iodide complex
(1 molar eqiuv) and AgBF4 (1.2 molar equiv) in dry CH2Cl2 (10
mL) under exclusion of light. The solution was stirred for 3 h in
the dark and then filtered through Celite. Evaporation of the solvent
gave the desired compound.
Synthesis of 4b. This complex was prepared in a manner similar
to that for 4a using 2b (0.46 g, 1 mmol). Evaporation of the solvent
gave the crude product as a green powder (0.40 g, 93%). Filtration
through Celite and recrystallization from toluene at -20 °C gave
an analytically pure sample. 1H NMR (CDCl3, 400 MHz): δ 7.17,
1
Characterization of 6a. H NMR (CDCl3, 360 MHz): δ 7.66
3
7.06 (2 × s, 2H, im), 6.33, 5.35 (2 × septet, 2H, JHH ) 6.4 Hz,
(s, 2H, im), 7.01 (s, 2H, im), 6.66 (low-field part of AX d, 1H,
2
CHMe2), 4.45 (s, 5H, cp), 1.60, 1.52, 1.41, 1.28 (4 × d, 12H, 3JHH
2JHH ) 12.9 Hz, CH2), 5.84 (high-field part of AX d, 1H, JHH
)

Piano-Stool Iron(II) Complexes as Probes
Organometallics, Vol. 25, No. 23, 2006 5655
2
12.9 Hz, CH2), 4.71 (s, 5H, cp), 3.76 (s, 6H, Me). IR (CH2Cl2,
field part of AX d, 1H, JHH ) 15.9 Hz, CH2), 5.70 (high-field
cm-1): 1950 ν(CO).
part of AX d, 1H, 2JHH ) 15.9 Hz, CH2), 4.57 (s, 5H, cp), 2.36 (s,
3H, p-CH3), 2.00, 1.78 (2 × s, 6H, o-CH3). 13C{1H} NMR (CDCl3,
50 MHz): δ 218.6 (CO), 183.1 (im-C2), 160.1 (py-C2), 159.1 (py-
C6), 139.1 (py-C4), 139.9 (mes-C1), 136.5, 135.5, 134.7 (mes-C2,4,6),
129.7 (mes-C3/5), 129.1 (im), 127.6 (py-C3), 126.6 (im), 125.1 (mes-
C5/3), 123.7 (py-C5), 82.2 (cp), 55.7 (CH2), 21.3 (p-CH3), 18.4, 17.9
(2 × o-CH3). IR (CH2Cl2, cm-1): 1965 ν(CO). Anal. Calcd for
C24H24FeIN3O (553.22): C 52.11, H 4.37, N 7.60. Found: C 52.10,
H 4.44, N 7.70.
1
Characterization of 6b. H NMR (CDCl3, 360 MHz): δ 7.70
(d, 2H, 3JHH ) 1.8 Hz, im), 7.06 (d, 2H, 3JHH ) 1.8 Hz, im), 6.62
2
(low-field part of AX d, 1H, JHH ) 13.2 Hz, CH2), 5.82 (high-
2
field part of AX d, 1H, JHH ) 13.2 Hz, CH2), 4.84 (septet, 2H,
3JHH ) 6.8 Hz, CHMe2), 4.68 (s, 5H, cp), 1.46, 1.43 (2 × d, 12H,
3JHH ) 6.8 Hz, CH(CH3)2). IR (CH2Cl2, cm-1): 1948 ν(CO).
1
Characterization of 6c. H NMR (CDCl3, 360 MHz): δ 7.94
(s, 2H, im), 6.98-6.90 (m, 7H, im, mes-H,3,5 low-field part of CH2),
6.08 (high-field part of AX d, 1H, 2JHH ) 12.7 Hz, CH2), 4.36 (s,
5H, cp), 2.33 (s, 6H, p-CH3), 2.04, 1.72 (2 × s, 12H, o-CH3). 13C-
{1H} NMR (CDCl3, 90 MHz): δ 219 (CO), 186.3 (im-C2), 139.6
(mes-C1), 136.4, 135.6, 134.6 (mes-C2,4,6), 129.5, 129.1 (mes-C3,5),
125.1, 124.8 (im), 81.7 (cp), 62.5 (CH2), 21.3 (p-CH3), 18.5, 18.1
(2 × o-CH3). IR (CH2Cl2, cm-1): 1956 ν(CO).
Synthesis of 9. The procedure was identical to the one described
for the synthesis of 6.
1
Characterization of 9a. H NMR (acetone-d6, 400 MHz): δ
9.11 (d, 1H, JHH ) 5.6 Hz, py-H6), 7.95 (t, 1H, JHH ) 7.6 Hz,
3
3
py-H4), 7.74 (s, 1H, im), 7.71 (d, 1H, 3JHH ) 7.6 Hz, py-H3), 7.51
3
(s, 1H, im), 7.30 (t, 1H, JHH ) 6.6 Hz, py-H5), 5.81 (low-field
2
General Procedure for the Preparation of Pyridine-Carbene
Comlpexes 8. The procedure was identical to the preparation of
the dicarbene complexes 5, starting from the imidazolium salt and
a slight excess of KOtBu or 1.0 molar equiv of nBuLi. After
extraction with CH2Cl2 the solution was irradiated for 16 h and
subsequently evaporated to dryness to give the desired complex 8.
part of AB d, 1H, JHH ) 15.8 Hz, CH2), 5.59 (high-field part of
2
AB d, 1H, JHH ) 15.8 Hz, CH2), 5.00 (s, 5H, cp), 3.85 (s, 3H,
CH3). IR (CH2Cl2, cm-1): 1964 ν(CO).
1
Characterization of 9b. H NMR (CDCl3, 360 MHz): δ 8.81
(d, 1H, 3JHH ) 5.9 Hz, py-H6), 7.78 (m, 3H, py-H3, py-H4 and im),
3
3
7.16 (d, 1H, JHH ) 1.8 Hz, im), 7.09 (t, 1H, JHH ) 5.4 Hz, py-
Synthesis of 8a. This complex was prepared from (N-methyl-
N′-2-picolyl)imidazolium bromide (0.51 g, 2 mmol), KOtBu (0.25
g, 2.2 mmol), and [FeI(cp)(CO)2] (0.55 g, 1.8 mmol), affording
0.64 g of product (79%). Recrystallization from CH2Cl2/Et2O gave
H5), 5.85 (low-field part of AB d, 1H, 2JHH ) 16.1 Hz, CH2), 5.42
2
(high-field part of AB d, 1H, JHH ) 16.1 Hz, CH2), 4.73 (s, 5H,
3
cp), 4.56 (septet, 1H, JHH ) 6.6 Hz, CHMe2), 1.51, 1.44 (2 × d,
6H, 3JHH ) 6.6 Hz, CH(CH3)2). IR (CH2Cl2, cm-1): 1964 ν(CO).
1
8a as brown crystals. H NMR (CDCl3, 500 MHz): δ 8.81 (dd,
1
Characterization of 9c. H NMR (CDCl3, 500 MHz): δ 8.79
1H, 3JHH ) 5.7 and4JHH ) 1.1 Hz, py-H6), 8.05 (m, 2H, py-H3 and
(d, 1H, JHH ) 5.0 Hz, py-H6), 7.97 (br, 1H, im-H), 7.89 (br, 1H,
3
3
4
im), 7.79 (td, 1H, JHH ) 7.7 and JHH ) 1.6 Hz, py-H4), 7.13 (d,
py-H3), 7.81 (br, 1H, py-H4), 7.10 (br, 1H, py-H5), 7.03, 7.01 (2 ×
s, 2H, mes-H3,5), 6.97 (s, 1H, im), 6.02 (low-field part of AB, br,
1H, CH2), 5.61 (high-field part of AB, br, 1H, CH2), 4.55 (s, 5H,
cp), 2.38 (s, 3H, p-CH3), 2.04, 1.80 (2 × s, 6H, o-CH3). 13C{1H}
NMR (CDCl3, 125 MHz): δ 218.7 (CO), 183.2 (im-C2), 160.2 (py-
C2), 159.1 (py-C6), 139.3 (py-C4), 139.9 (mes-C1), 136.5, 135.6,
134.8 (mes-C2,4,6), 129.7 (mes-C3/5), 129.1 (im), 127.6 (py-C3),
126.6 (im), 125.1 (mes-C5/3), 123.7 (py-C5), 82.3 (cp), 55.6 (CH2),
21.3 (p-CH3), 18.4, 18.0 (2 × o-CH3). IR (CH2Cl2, cm-1): 1966
ν(CO).
1H, JHH ) 2.0 Hz, im), 7.12-7.09 (m, 1H, py-H5), 6.47 (low-
field part of AX d, 1H, JHH ) 16.1 Hz, CH2), 5.55 (high-field
3
2
part of AX d, 1H, 2JHH ) 16.1 Hz, CH2), 4.79 (s, 5H, cp), 3.75 (s,
3H, CH3). 13C{1H} NMR (CDCl3, 125 MHz): δ 219.0 (CO), 181.0
(im-C2), 159.7 (py-C2), 159.1 (py-C6), 139.1 (py-C4), 127.6 (py-
C3), 125.8 (im), 124.4 (im), 123.6 (py-C5), 82.4 (cp), 55.4 (CH2),
37.8 (CH3). IR (CH2Cl2, cm-1): 1963 ν(CO). Anal. Calcd for
C16H16FeIN3O (449.07): C 42.79, H 3.59, N 9.36. Found: C 42.81,
H 3.71, N 9.25.
Synthesis of 8b. This complex was prepared from (N-isopropyl-
N′-2-picolyl)imidazolium bromide (0.56 g, 2 mmol), KOtBu (0.29
g, 2.6 mmol), and [FeI(cp)(CO)2] (0.55 g, 1.8 mmol). The crude
product was obtained as a brown powder (0.71 g, 74%). Recrys-
tallization from CH2Cl2/Et2O gave the title compound as orange
crystals. 1H NMR (CDCl3, 500 MHz): δ 8.82 (dd, 1H, 3JHH ) 5.7
DFT Calculations. All DFT calculations have been performed
with the parallelized ADF suite of programs, release 2004.01.29
Geometry optimizations were carried out with the generalized
gradient approximation, using nonlocal corrections to exchange by
Becke30 and to correlation by Perdew31 (BP86) The Kohn-Sham
MOs were expanded in a large, uncontracted basis set of Slater-
type orbitals (STOs), of a triple-ú + polarization functions quality
(TZP), within the frozen-core approximation using a small core
for Fe. An auxiliary set of STOs was used to fit the density for the
Coulomb-type integrals.29a The IR frequencies are scaled by a factor
of 0.95332 using B3LYP33 geometries with a 6-31G* (C, N, H)
basis set and the quasirelativistic LANL2DZ pseudopotentials and
basis set for Fe,34 employing the Gaussian 03 suite of programs.35
3
and 4JHH ) 1.4 Hz, py-H6), 8.12 (d, 1H, JHH ) 1.8 Hz, im), 8.08
3
3
4
(d, 1H, JHH ) 7.8 Hz, py-H3), 7.78 (td, 1H, JHH ) 7.8 and JHH
) 1.4 Hz, py-H4), 7.17 (d, 1H, 3JHH ) 1.8 Hz, im), 7.13-7.10 (m,
1H, py-H5), 6.52 (low-field part of AX d, 1H, JHH ) 15.7 Hz,
2
2
CH2), 5.53 (high-field part of AX d, 1H, JHH ) 15.7 Hz, CH2),
4.76 (s, 5H, cp), 4.55 (septet, 1H, JHH ) 6.8 Hz, CHMe2), 1.50,
3
1.44 (2 × d, 6H, 3JHH ) 6.8 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3,
125 MHz): δ 219.3 (CO), 178.8 (im-C2), 160.0 (py-C2), 159.1 (py-
C6), 139.1 (py-C4), 127.6 (py-C3), 126.5 (im), 123.7 (py-C5), 119.0
(im), 82.4 (cp), 55.1 (CH2), 52.3 (CHMe2), 23.8, 23.6 (2 × CH-
(CH3)2). IR (CH2Cl2, cm-1): 1961 ν(CO). Anal. Calcd for C18H20-
FeIN3O (477.12) × 1/3 CH2Cl2: C 43.57, H 4.12, N 8.31. Found:
C 43.48, H 4.36, N 8.62.
Bonding analysis of the metal-ligand interactions is accom-
plished with the extended transition state method (ETS).36 Accord-
ing to the ETS scheme, the bond energy (negative bond dissociation
(29) (a) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(b) te Velde, G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84. (c) Fonseca-
Guerra, C.; Visser, O.; Snijders, J. G.; te Velde, G.; Baerends, E. J. In
METECC-9; Clementie, E., Corongiu, C., Eds.; Cagliari, 1995; p 303. (d)
te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; van
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22,
931.
Synthesis of 8c. This complex was prepared from (N-mesityl-
N′-2-picolyl)imidazolium bromide (0.72 g, 2 mmol), nBuLi (1.6
M in hexanes, 1.3 mL, 2 mmol), and [FeI(cp)(CO)2] (0.55 g, 1.8
mmol). The product was isolated as an orange powder (0.44 g,
44%). Recrystallization of the combined toluene washings from
CH2Cl2/pentane gave another crop of orange crystals (0.09 g, total
(30) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
1
3
(31) Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
yield 53%). H NMR (CDCl3, 360 MHz): δ 8.80 (d, 1H, JHH
)
(32) Zhou, M.; Andrews, L.; Bauschlicher, C. W. Chem. ReV. 2001, 101,
5.4 Hz, py-H6), 8.36 (s, 1H, im), 8.16 (d, 1H, JHH ) 7.2 Hz, py-
3
1931.
H3), 7.81 (t, 1H, JHH ) 7.3 Hz, py-H4), 7.13-7.10 (m, 1H, py-
3
(33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(34) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
H5), 7.02, 7.00 (2 × s, 2H, mes-H3,5), 6.96 (s, 1H, im), 6.77 (low-

5656 Organometallics, Vol. 25, No. 23, 2006
Mercs et al.
energy, BDE) between the two fragments is decomposed in the
following interaction terms:
squares on F2 with SHELXL-97.39 The hydrogen atoms were
included in calculated positions and treated as riding atoms using
SHELXL-97 default parameters. All non-hydrogen atoms were
refined anisotropically. No absorption correction was applied for
3b (µ < 1 mm-1). For all other structures a semiempirical
absorption correction was applied using MULABS as implemented
in PLATON.40 Complex 8b crystallized with one disordered and
partially occupied CH2Cl2 molecule (occupancy 0.5 for C37, Cl1,
and Cl2; occupancy 0.25 for C38, Cl3, and Cl4) in the asymmetric
Bond Energy ) ∆Etot ) ∆Eprep + ∆EPauli + ∆Eelstat + ∆Eoi (4)
The total interaction energy equals the bond energy and is
decomposed into several terms. The first term, ∆Eprep, is the energy
required to deform the fragments into the geometries they possess
in the complex. ∆EPauli quantifies the Pauli repulsion between the
electron densities of the two fragments. The electrostatic attraction
∆Eelstat describes the attraction between the nuclei of one fragment
and the electron density of the other fragment (vice versa). ∆EPauli
and ∆Eelstat are usually summarized as ∆Esteric. ∆Eoi represents the
orbital interaction term, which quantifies the energy gain upon
mixing of the orbitals of the two fragments, and is generally
dominated by the HOMO-LUMO interactions. This term can be
further dissected into the different symmetry classes, which are A′
and A′′ in the cases we consider here.
unit. The highest final residual electron density in 8b (2.4 e A-3
)
was found next to the disordered CH2Cl2 molecule. Selected bond
lengths and angles and crystallographic details are collected in
Tables S1-S4 of the Supporting Information. All calculations and
graphical illustrations were performed with the PLATON03 pack-
age.40 Crystallographic data (excluding structure factors) for the
structures 3b, 5c, 8a, 8b, and 8c have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion nos. CCDC 614248-644252. Copies of the data can be
obtained free of charge on application to CCDS, 12 Union Road,
Cambridge CB2 1EZ, UK [fax: (int.) +44-1223-336-033; e-mail:
deposit@ccds.cam.ac.uk].
Crystal Structure Determinations. Data were collected on a
Stoe imaging plate diffractometer system37 equipped with a graphite
monochromator. Data collection was performed at -100 °C using
Mo KR radiation (λ ) 0.71073 Å). The structures were solved by
direct methods using SHELXS-9738 and refined by full matrix least-
Acknowledgment. We thank Mr. F. Fehr for technical
assistance and Prof. R. H. Crabtree for stimulating discussions.
M.A. gratefully acknowledges the Alfred Werner Foundation
for an Assistant Professorship. This work has been financially
supported by the Swiss National Science Foundation (grant
200021-101891).
(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
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N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
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Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Supporting Information Available: Crystallographic data for
3b, 5c, 8a, 8b, and 8c in CIF format, tables with selected bond
lengths and angles, CV of 6a, and details on the DFT geometry
optimization and energy decomposition analysis. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM060637C
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