Behavior of group 6 fischer aminocarbene complexes in a supercharged medium: A single electron transfer-H atom transfer process

Article abstract of DOI:10.1021/om900159h

The ESI-MS study of the ionization of a series of aryl, alkenyl, and alkynyl (Fischer) aminocarbene complexes is described. The process requires the initial capture of one electron from the ESI source, followed by the transfer of a hydrogen atom to the additive (TTF or Cs₂CO₃). Regardless of their structure, both NH and N,N-disubstituted aminocarbenes can effectively ionize under ESI-MS conditions, as long as they have abstractable H atoms. Overall, the process can be envisaged as a single electron transfer-H atom transfer process. The experimental findings are supported by DFT calculations. The ESI-MS ionization of bis-alkynylaminocarbene complexes follows the same patterns. However, the [M - H]^- ions evolve by arylacetylene extrusion together with the usual sequence of CO loss. The process requires the presence of a free NH group and occurs by intramolecular hydrogen transfer from the NH to the metal center. Bis-aminocarbenes having rigid tethers between the nitrogen atoms or lacking free NH groups do not produce this fragmentation.

Full text of DOI:10.1021/om900159h

2762
Organometallics 2009, 28, 2762–2772
Behavior of Group 6 Fischer Aminocarbene Complexes in a
Supercharged Medium: A Single Electron Transfer-H Atom
Transfer Process
´
Marta L. Lage, Mar´ıa J. Manchen˜o, Roberto Mart´ınez-Alvarez, Mar Go´mez-Gallego,
Israel Ferna´ndez, and Miguel A. Sierra*
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad Complutense,
28040-Madrid, Spain
ReceiVed February 27, 2009
The ESI-MS study of the ionization of a series of aryl, alkenyl, and alkynyl (Fischer) aminocarbene
complexes is described. The process requires the initial capture of one electron from the ESI source,
followed by the transfer of a hydrogen atom to the additive (TTF or Cs2CO3). Regardless of their structure,
both NH and N,N-disubstituted aminocarbenes can effectively ionize under ESI-MS conditions, as long
as they have abstractable H atoms. Overall, the process can be envisaged as a single electron transfer-H
atom transfer process. The experimental findings are supported by DFT calculations. The ESI-MS ionization
of bis-alkynylaminocarbene complexes follows the same patterns. However, the [M - H]- ions evolve
by arylacetylene extrusion together with the usual sequence of CO loss. The process requires the presence
of a free NH group and occurs by intramolecular hydrogen transfer from the NH to the metal center.
Bis-aminocarbenes having rigid tethers between the nitrogen atoms or lacking free NH groups do not
produce this fragmentation.
reduction of the charged droplets due to the high voltage applied,
and production of very small drops from which ions in the gas
phase are generated. During the last years, we have demon-
strated10 that the ESI-reactivity of neutral carbene complexes
lacking acidic positions or basic centers can be studied in the
presence of additives such as tetrathiafulvalene (TTF) or
hydroquinone (HQ). We proposed the term electron carriers10a
for TTF and HQ since, apparently, they efficiently transferred
one electron from the ESI source to the carbene, leading to the
formation of a radical anion species that evolves in the ESI
conditions to the detected [M - H]- species (Scheme 1).
Introduction
Single electron transfer (SET)1 reactions in organic chemistry
are well-established, and their synthetic utility has been profusely
demonstrated during the last century.2 However, knowledge of
SET processes in organometallic complexes or about the
interaction between organometallic complexes and organic or
organometallic additives in a nonconventional media (electro-
spray ionization sources (ESI)) is less extensive.3 Mechanisms
beginning with SET processes followed by H atom transfer
(HAT)4,5 are a subclass of the general proton-coupled electron
transfer processes (PCET),6 which have gained increasing
interest in the fields of experimental and theoretical chemistry
in the past years.7 In this context, transition metal Fischer
carbene complexes8 are paramount to study both the chemical
and ESI-induced SET processes,3 since the electron-withdrawing
effect of the M(CO)5 moiety in these compounds renders the
carbene carbon very electrophilic, and hence an extremely good
electron acceptor.6
(8) Selected reviews in the chemistry and synthetic applications of
Fischer carbene complexes: (a) Do¨tz, K. H.; Fischer, H.; Hofmann, P.;
Kreissel, R.; Schubert, U.; Weiss, K. Transition Metal Carbene Complexes;
Verlag Chemie: Deerfield Beach, FL, 1983. (b) Do¨tz, K. H. Angew. Chem.,
Int. Ed. Engl. 1984, 23, 587. (c) Wulff, W. D. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol.
5, p 1065. (d) Schwindt, M. A.; Miller, J. R.; Hegedus, L. S. J. Organomet.
Chem. 1991, 413, 143. (e) Rudler, H.; Audouin, M.; Chelain, E.; Denise,
B.; Goumont, R.; Massoud, A.; Parlier, A.; Pacreau, A.; Rudler, M.; Yefsah,
Due to the fact that ESI is a technique9 that allows the transfer
of ions from solution to the gas phase as isolated entities, it is
an exceptional media to study SET processes to organometallic
compounds in nonconventional media. In this technique, transfer
of ions from solution to the gas phase occurs through three main
steps: production of charged droplets at the capillary, size-
´
R.; Alvarez, C. F.; Delgado-Reyes, Chem. Soc. ReV. 1991, 20, 503. (f)
Gro¨tjahn, D. B.; Do¨tz, K. H. Synlett. 1991, 381. (g) Wulff, W. D. In
ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 12, p 470. (h) Hegedus,
L. S. In ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 12, p 549. (i)
Harvey, D. F.; Sigano, D. M. Chem. ReV. 1996, 96, 271. (j) Hegedus, L. S.
Tetrahedron 1997, 53, 4105. (k) Aumann, R.; Nienaber, H. AdV. Organomet.
Chem. 1997, 41, 163. (l) Sierra, M. A. Chem. ReV. 2000, 100, 3591. (m)
Barluenga, J.; Fan˜an´as, F. J. Tetrahedron 2000, 56, 4597. (n) Go´mez-
Gallego, M.; Manchen˜o, M. J.; Sierra, M. A. Acc. Chem. Res. 2005, 38,
44. (o) Herndon, J. W. Coord. Chem. ReV. 2006, 250, 1889. (p) Sierra,
M. A.; Ferna´ndez, I.; Coss´ıo, F. P. Chem. Commun. 2008, 4671.
(9) Gaskell, S. J. Mass Spectrom. 1997, 32, 677.
* Corresponding author. E-mail: sierraor@quim.ucm.es.
(1) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, 2001; Vols. 1-5.
(2) (a) Kochi, J. K. Angew. Chem., Int. Ed. 1988, 27, 1227. (b) Molander,
G. A.; Harris, C. R. Chem. ReV. 1996, 96, 307.
´
(3) Sierra, M. A.; Go´mez-Gallego, M.; Martínez-Alvarez, R. Chem.-
Eur. J. 2007, 13, 736.
(4) Mayer, J. M.; Hrovat, D. A.; Thomas, J. L.; Borden, W. T. J. Am.
Chem. Soc. 2002, 124, 11142.
(5) In the HAT process both electrons and protons are transferred from
the same chemical bond in a concerted manner.
(6) Huynh, M. H. V.; Meyer, T. J. Chem. ReV. 2007, 107, 5004.
(7) Mayer, J. M. Annu. ReV. Phys. Chem. 2004, 55, 363.
(10) (a) Sierra, M. A.; Go´mez-Gallego, M.; Manchen˜o, M. J.; Martínez-
´
Alvarez, R.; Ramírez-Lo´pez, P.; Kayali, N.; Gonza´lez, A. J. Mass Spectrom.
´
2003, 38, 151. (b) Martínez-Alvarez, R.; Go´mez-Gallego, M.; Ferna´ndez,
I.; Manchen˜o, M. J.; Sierra, M. A. Organometallics 2004, 26, 4647. (c)
´
Wulff, W. D.; Korthals, K. A.; Martínez-Alvarez, R.; Go´mez-Gallego, M.;
Ferna´ndez, I.; Sierra, M. A. J. Org. Chem. 2005, 70, 5269.
10.1021/om900159h CCC: $40.75
2009 American Chemical Society
Publication on Web 04/16/2009

Group 6 Fischer Aminocarbene Complexes
Organometallics, Vol. 28, No. 9, 2009 2763
Scheme 2. Synthesis of Complexes 1-3
Scheme 1. Proposed Mechanism for the SET Process from
Hydroquinone (HQ) to r,ꢀ-Unsaturated Chromium(0)
(Fischer) Carbene Complexes10
Classical deuterium labeling experiments carried on substrate,
solvent, and additive were used to study the different mecha-
nisms for the evolution of the [M - H]- ions. These experiments
were conclusive proof for the suitability of this methodology
to study the reactivity of group 6 metal-carbene complexes in
supercharged media.10c
Nevertheless, the interaction between the additive and the
complex (that is, the actual mechanism of the ionization process)
remains an open matter. In this article we report a detailed study
on the ionization process of group 6 mono- and bis-aminocar-
bene complexes (in which both metal centers are tethered by
the amino groups). Unseen reaction pathways for the evolution
of the molecular anions will be discussed, and a different
mechanism for the interaction between the additive and the
complex, involving a HAT process, will be proposed on
experimental and computational grounds.
Just by comparison of their structures, the different behavior
observed for aminocarbenes 1a and 1b could suggest that the
ionization process requires the presence of an NH group attached
to the carbene carbon, which may be involved in an acid-base
process. However, the ionization of 1b takes place in the
presence of the scarcely basic TTF as additive, but the use of
a stronger base (CO32-) did not change the outcome of the
process. These results point to a process somewhat more
complex than a simple acid-base proton transfer and rule out
the alternative role of the additive as an electron carrier
responsible for the ionization of the complex.10 It is much more
likely to consider the initial formation of radical anions 8a,b,
derived from the capture of one electron (from the source) by
the aminocarbene complex. Then, in order to produce a
detectable molecular ion, the loss of a hydrogen atom is required
and, at this point, the additive must be playing a main role.
Overall, the ionization of 1b can be envisaged as a SET-HAT
process (Scheme 3). The formation of a [M - H]- ion is not
possible in the case of aminocarbene 1a, which lacks ab-
stractable H atoms.
Results and Discussion
Complexes 1-3 were prepared by reaction of the correspond-
ing alkoxycarbene complexes 4-6 with amines 7a-e. For the
synthesis of R,ꢀ-unsaturated aminocarbene complexes, forma-
tion of the 1,2-adducts over the 1,4-adducts was achieved at
low temperature, according to previously reported procedures
(Scheme 2).11
Complexes 1 were studied first. Aminocarbene 1a was solved
in CHCl3 (1.5 × 10-5 mol/L) and pumped through a PEEK
capillary into the ESI source (5.0 kV). Simultaneously, a 25 mM
solution of tetrathiafulvalene (TTF) in MeOH/CHCl3 (1:1) was
introduced. No ionization was detected under these conditions.
However, when complex 1b was treated in the same way, a nice
[M - H]- ion at m/z 310 was obtained. The [M - H]- ions were
isolated and their MSn spectra12 were recorded using the collision-
induced dissociation (CID) method (He). Together with a sequential
CO loss, also observed for alkoxychromium(0) carbene com-
plexes,13 1b produced a new type of fragmentation, extruding
methyl isocyanide (detected in MS3).
Once the [M - H]- ion 9 is formed from 1b, it evolves by
sequential CO loss to generate the unsaturated species 10,
followed by extrusion of methyl isocyanide, as observed in the
MS2 and MS3 spectra (Table 1).13 This process leads to a new
species, 11, in which the moiety formerly attached to the carbene
carbon is now joined to the metal center.14 The coordinatively
unsaturated character of the metal should favor this sequence
of events (this is further supported by the fact that the evolution
of the isocyanide did not occur from the molecular ion, requiring
at least extrusion of one of the CO ligands) (Scheme 3).
(11) (a) Duetsch, M.; Stein, F.; Lackmann, R.; Pohl, E.; Herbst-Irmer,
R.; De Meijere, A. Organometallics 1993, 12, 2556. (b) Aumann, R.;
Hinterding, P. Chem. Ber. 1992, 125, 3765. (c) Imwinkelried, R.; Hegedus,
L. S. Organometallics 1988, 7, 702. (d) Hegedus, L. S.; Schwindt, M. A.;
De Lombaert, S.; Imwinkelried, R. J. Am. Chem. Soc. 1990, 112, 2264. (e)
Hegedus, L. S. Pure Appl. Chem. 1983, 55, 1745. (f) Fischer, E. O.; Kalder,
H. J. J. Organomet. Chem. 1977, 131, 57. (g) Aumann, R.; Hinterding, P.
Chem. Ber. 1993, 126, 421. (h) Moreto´, J.; Ricart, S.; Do¨tz, K. H.; Molins,
E. Organometallics 2001, 20, 62.
(12) Through this work MSn (n ) 1) refers to spectra of the molecular
ion, while the value of n (n * 1) denotes the successive collision-induced
dissociation of isolated MS1ions.
(13) (a) Mu¨ller, J.; Connor, J. A. Chem. Ber. 1969, 102, 1148. (b)
Fischer, E. O.; Leopold, M.; Kreiter, C. G.; Mu¨ller, J. Chem. Ber. 1972,
105, 1150. (c) Careri, A.; Mangia, P.; Manini, P.; Predieri, G.; Licandro,
E.; Papagni, A. Rapid Commun. Mass Spectrom. 1997, 11, 51.
(14) The alternative and simplest way to explain the formation of
isocyanide from intermediate 10 is the extrusion of the phenyl moiety as a
Ph- fragment. However, we have not observed this extrusion in the many
experiments carried out through this work.

2764 Organometallics, Vol. 28, No. 9, 2009
Lage et al.
attached to the carbene carbon, gave nice molecular [M - H]-
ions at m/z 336, 334, and 376, respectively. A mechanism
analogous to that depicted in Scheme 3 could explain their
ionization. However, once formed, the molecular ions follow
different fragmentation patterns. Thus, for complexes 3b and
3c the correspondent isocyanide (methyl or butyl) is observed
in the MS2 and MS3 spectra, whereas for complex 2b only the
usual CO loss cascade is observed (Table 1).13 The fragmenta-
tion patterns were similar with either TTF or CO32- as additive.
A different reaction outcome was observed for conjugated
N,N-disubstituted aminocarbenes 2a and 3a. In spite of the lack
of NH bonds in its structure, alkenyl carbene 2a ionized to yield
the corresponding [M - H]- ion (m/z 350), whereas alkynyl
carbene 3a did not ionize at all. These results suggest that for
all the alkenyl complexes 2 a different ionization mechanism
is operative, as the H• capture must take place from a CH bond.
The proposed pathway for the ionization of 2a is depicted in
Scheme 5.
Scheme 3. Proposed Mechanism for Ionization of
NH-Substituted Aminocarbene Complexes 1
To support the mechanism of ionization proposed for
compound 1b, DFT calculations were carried out for the HAT
process. The reaction of radical anion 8b and TTF was computed
at the uB3LYP/6-31+G(d)&LanL2DZ level. The computed
spin-density in 8b (0.60 au) clearly shows that the former
carbene carbon atom acts as the acceptor center of the electron
provided by the ESI source. Transfer of the H atom from the
N-H group of 8b to TTF occurs via the transition state TS1,
which has developed the TTF-H bond at the expense of the
TTF CdC double-bond breakage (Scheme 4). The activation
barrier of the process is 18.8 kcal/mol. The reaction products
are the expected anion 9b and the radical TTF-H• 12 (whose
spin-density is concentrated in the carbon atom, 0.86 au) through
a slightly endothermic process (reaction energy of 0.3 kcal/mol).
Therefore, the computational results are compatible with the
SET-HAT mechanism proposed in Scheme 3.
Scheme 5. Proposed Mechanism for Ionization of Complex
2a
Scheme 4. DFT-Computed Mechanism for the Ionization of
8ba
Thus, the formation of the radical anion 13 proposed as the
first step must be now followed by the H-transfer from the
C-HR bond of the carbene to the additive, leading to
the detected molecular ion 14. The preferential breakage of the
C-HR bond during the ESI-MS ionization of alkenyl alkoxy
carbene complexes was previously demonstrated by us.10c
Clearly, the lack of abstractable hydrogens in the alkynyl
aminocarbene 3a makes the ionization of this compound not
possible. To support the proposal in Scheme 5, DFT calculations
were made using radical anion 15 as a model for complex 2a.
In this case, the spin-density is shared by the former carbene
carbon atom (0.64 au) and the double-bond terminal carbon atom
(0.66 au) as a consequence of the electronic π-delocalization.
The C-HR transfer to TTF was analogous to that calculated
from a NH group (Scheme 6), but in this case, the energy barrier
involved to reach TS2 was considerably higher (32.3 kcal/mol),
as it would be expected considering the well-known higher
strength of a C-H bond compared with an N-H bond.
Furthermore, the computed NBO-Wiberg bond orders agree with
the higher strength of the C-H bond in 15 (0.91 au) compared
to the N-H bond in 8b (0.80 au). After the hydrogen atom
transfer, the reaction products will be the anion 16 and the
radical TTF-H• 12 in an endothermic process (reaction energy
of 14.5 kcal/mol).
a
All structures correspond to fully optimized uB3LYP/6-
31+G(d)&LanL2DZ geometries. Bond distances and energies are given
in Å and kcal/mol, respectively. Numbers close to the arrows correspond
to the relative energies between the corresponding structures. Zero-
point vibrational energy corrections have been included (kcal/mol).
Computed spin-densities (in au) are given in brackets.
Considering these results, it could be postulated that the
ionization of an aminocarbene complex (and the further detec-
tion of [M - H]- ions) does not depend on the substitution of
the N atom, but on the presence of any abstractable H atoms in
Conjugated aminocarbenes 2 and 3 were studied next. Alkenyl
and alkynyl complexes 2b, 3b, and 3c, bearing an NH group

Group 6 Fischer Aminocarbene Complexes
Organometallics, Vol. 28, No. 9, 2009 2765
Scheme 6. DFT-Computed Mechanism for the Ionization of
16a
Scheme 7. Proposed Mechanism for Ionization of Complexes
3f and 3g
a
See Scheme 4 for caption details.
the structure. To check this point, complexes 3f and 3g were
prepared by reaction of complex 6a with ethylamine or
n-butylamine, followed by alkylation with EtI or nBuBr,
respectively, in the presence of Cs2CO3 (see Scheme 2). The
study of the ESI ionization in the above conditions revealed
that while compound 3f (R ) Et) led to the [M - H]- ion at
m/z 348, no ionization was observed for 3g (Table 1). Addition-
ally, the fragmentation for 3f revealed an additional loss of
ethylene together with the expected loss of CO. The formation
of ethylene was further confirmed by high-resolution mass
spectrometry (HRMS), which discriminates between the extru-
sion of a CO (27.9951 Da) and a CH2dCH2 (28.0323 Da). In
fact, the MS1 and MS2 spectra of the [M - H]- ion showed
clear extrusions of fragments at 28.0323 Da (CH2dCH2) and
27.9951 Da (CO), respectively. The detection of ethylene gives
valuable information to understand the mechanism of the
ionization of 3f. Thus, once the radical anion 17 is formed, the
transference of the H atom to the additive should take place
from the ethyl group to form the allenyl anion 18, detected as
[M - H]- ion (m/z 348) (Scheme 7). Extrusion of ethylene could
take place from 18 leading to anion 19. This sequence of events
is analogous to that proposed for the ionization of alkynyl-ethoxy
chromocarbenes.10c The analogous allenyl anion 20 derived from
n-butyl aminocarbene 3g was not detected. The impossibility
of extruding a stable ethylene molecule could be the reason for
this behavior (Scheme 7).
bearing both a Cr and a W metallic center), aminocarbene 22k
was prepared by reacting equimolecular amounts of complexes
6a and 6b with 1,4-diaminobutane. The product was obtained
together with the corresponding homometallic bis-carbene
complexes 22f and 22g. The mixture of the three compounds was
submitted to ESI-MS analysis, and the molecular ion [M - H]-
ion corresponding to 22k at m/z 827 was detected. Furthermore,
the expected loss of phenylacetylene from this molecular ion was
clearly observed in the MS2 spectra of the mixture (Table 2).
Scheme 8. Synthesis of Bis-aminocarbene Complexes 22
Once we had a clearer picture about the factors influencing
the ionization of mono- and disubstituted aminocarbene com-
plexes, our next goal was to study the reactivity of bis-alkynyl
aminocarbene complexes 22. In these compounds, the effect of
additional factors, such as the structure and length of the tether
between the nitrogen atoms and the nature of the metal carbene,
can be established. The synthesis of compounds 22 was carried
out by reaction of the corresponding alkoxycarbene complexes
6 and diamines 21 (Scheme 8). Both chromium(0) and
tungsten(0) bis-carbene complexes 22 efficiently ionized in the
presence of TTF, showing intense [M - H]- ions that extruded
arylacetylene (MS2) in all cases (Table 2). This reactivity pattern
was independent both of the length of the tether joining the
metal-carbene centers and of the nature of the metals. To study
the behavior of mixed bis-aminocarbenes (namely, complexes
Being structurally related as they are, the differences in
reactivity of the molecular ions [M - H]- derived from alkynyl
aminocarbenes 3b,c and bis-aminocarbenes 22 should arise from
the presence of a second NH group in the molecule, possibly
involved in an interaction with the metallic fragment once the

2766 Organometallics, Vol. 28, No. 9, 2009
Lage et al.
Table 1. ESI-MS Experiments of Complexes 1-3
a No ionization detected.
[M - H]- ion is formed. The involvement of an NH group
was established with complexes 23a,b prepared by alkylation
of complexes 22d,e with Cs2CO3/MeI (Scheme 9). These
complexes did not ionize under the conditions used for
aminocarbenes 22 (CHCl3/MeOH), which suggests some kind
of intramolecular H-transfer from the free NH group to the metal
once the [M - H]- ion is formed. Furthermore, the extrusion
of arylacetylene does not depend on the nature of the metal
(Cr, W) and is detected in bis-aminocarbenes having tethers of
different lengths (up to five CH2). To establish the intramolecular
nature of the process and to evaluate the need of a flexible chain
between the nitrogen atoms, complex 22j, bearing a rigid
m-xylylidene tether, was ionized under the same conditions. In
this case, only the cascade of CO fragmentations was observed,
and no extrusion of phenylacetylene was detected.
ionization revealed that complexes 3d and 3e both ionized
showing the usual CO cascade, as well as the corresponding
isocyanide formation, but only 3d extruded phenylacetylene in
the MSn experiments, which is in agreement with the proposed
H-transfer from the free NH group in bis-aminocarbenes 22
(Table 3).
Scheme 9. Synthesis of Complexes 23, and Complexes 3d,
3e, and 3h
An additional evidence for the involvement of an intramo-
lecular H-transfer came from the study of aminocarbenes 3d,
3e, and 3h (Scheme 9). Aminocarbene 3h was prepared by
alkylation of 3e with Cs2CO3/MeI in the above conditions.
Complex 3d incorporates an additional OH group in the
structure, and hence an abstractable hydrogen atom, whereas
this possibility has been precluded in complex 3e, structurally
related but bearing a OMe group. Finally, complex 3h has no
abstractable H atoms, since both the OH and the NH groups
are blocked by a methyl substituent. The study of the ESI-MS
Considering the experimental evidence, a reasonable
explanation for the formation of arylacetylenes in the

Group 6 Fischer Aminocarbene Complexes
Organometallics, Vol. 28, No. 9, 2009 2767
Table 2. ESI-MS Experiments of Bis-aminocarbene Complexes 22 and 23

2768 Organometallics, Vol. 28, No. 9, 2009
Lage et al.
Table 2. Continued
a No ionization detected.
Table 3. ESI-MS Experiments of Complexes 3d, 3e, and 3h
a No ionization detected.
fragmentation of the [M - H]- ions derived from bis-
aminocarbenes 22 is depicted in Scheme 10. Thus, after the
nCO loss, an intramolecular H-transfer from the free NH
group to the metal could take place, leading to a new metal
hydride 24. This species experiences a reductive elimination
upon arylacetylene migration, leading to the observed loss
of neutral arylacetylene. The absence of the additional NH
center (23a,b) or geometrical restraints (22j) are able to
inhibit the process.
required. Overall, the ionization can be envisaged as a SET-
HAT process. Both NH and N,N-disubstituted aminocarbenes
can effectively ionize under ESI-MS conditions, as long as
they have abstractable H atoms in their structures. The
experimental findings are supported by DFT calculations, and
the evolution of the [M - H]- species has been studied by
isolation and determination of their MSn spectra. On the other
hand, the ESI-MS ionization of bis-alkynylaminocarbene
complexes follows the same patterns. However, the evolution
of the [M - H]- ions is by arylacetylene extrusion together
with the usual sequence of CO loss. The process requires
the presence of a free NH group and occurs by intramolecular
hydrogen transfer from the NH to the metal center. Bis-
aminocarbenes having rigid tethers between the nitrogen
atoms or lacking free NH groups do not produce this
fragmentation. Further computational and experimental work
Conclusions
The ESI-MS ionization of aminocarbene (Fischer) com-
plexes requires the initial capture of one electron (from the
ESI source) by the aminocarbene, with subsequent formation
of a radical anion. Then, to produce a detectable molecular
ion, the capture of a hydrogen atom by the additive is

Group 6 Fischer Aminocarbene Complexes
Organometallics, Vol. 28, No. 9, 2009 2769
spectra reported are the averages of 15 scans using 450 ms as the
accumulation time. MSn spectra were carried out using collision-
induced dissociation (CID) with helium after isolation of the
appropriate precursor ions. An isolation width of 4.0 m/z was used,
and the fragmentation voltage amplitude was maintained at 0.60 V
with a fragmentation time of 40 ms. The HRMS spectra for
compound 3f were carried out using a QSTAR pulsar i spectrometer
(Applied Bisosystems) with a QTOF hybrid analyzer in the negative
mode of detection, using the same dilution and potentials as those
used for the other ESI-MS experiments. The tables summarize the
results obtained for carbene complexes in the presence of different
additives.
to understand the mechanisms of evolution of metal-carbene
and bis-carbene complexes in the supercharged ESI droplets
is currently underway in our laboratories.
Scheme 10. Proposed Mechanism for the Ionization of
Complexes 22
Computational Details. All the calculations reported in this paper
were obtained with the GAUSSIAN 03 suite of programs.17 Electron
correlation was partially taken into account using the unrestricted
(u)B3LYP18 functional in combination with the 6-31G(d)19 and
LanL2dz for Cr.20 Zero-point vibrational energy (ZPVE) corrections
were computed at the B3LYP/def2-SVP level and were not scaled.
Reactants and products were characterized by frequency calculations21
and have positive definite Hessian matrices. Transition structures (TSs)
show only one negative eigenvalue in their diagonalized force constant
matrices, and their associated eigenvectors were confirmed to cor-
respond to the motion along the reaction coordinate under consideration
using the intrinsic reaction coordinate (IRC) method.22 Bond orders
have been computed using the natural bond orbital (NBO) method.23
Pentacarbonyl[(N-methyl)(phenyl)carbene]chromium(0), 1b.
To a solution of complex 4 (500 mg, 1.53 mmol) in THF (20 mL)
was added dropwise a solution of methylamine in THF (0.76 mL,
1.53 mmol). The reaction mixture was stirred under argon for 48 h,
after which the solvent was evaporated under vacuum and the crude
purified (SiO2, hexanes/ethyl acetate, 8:1) to give 1b (119 mg, 25%)
Experimental Section
General Procedures. All reactions were carried under an argon
atmosphere. All solvents used in this work were purified by
distillation and were freshly distilled immediately before use.
Tetrahydrofuran (THF) was distilled from sodium/benzophenone
and dichloromethane (DCM) from calcium hydride. Flame-dried
glassware and standard Schlenk techniques were used for moisture-
sensitive reactions. Silica gel for flash column chromatography
purification of crude mixtures was purchased from Merck (230-400
mesh), and the identification of products was made by thin-layer
chromatography (Kiesegel 60F-254). UV light (λ ) 254 nm) and
5% phosphomolybdic acid solution in 95% EtOH were used to
develop the plates. NMR spectra were recorded at 22 °C in CDCl3,
on Bruker Avance 300 (300 MHz for 1H, 75 MHz for 13C) or Bruker
AM-500 (500 MHz for 1H, 125 MHz for 13C). Chemical shifts are
given in ppm relative to TMS (1H, 0.0 ppm) or CDCl3 (13C, 77.0
ppm). IR spectra were taken on a Bruker Tensor 27 (MIR
8000-400 cm-1) spectrometer in CHCl3 solution. All commercially
available products were used without further purification. Com-
pounds 1a,11c 2a,11d 2b,11e 3a,11f 3b,11g 4,15 5,16 6a,11g 6b,11g
22a,11h and 22b11h were prepared and identified by procedures
previously reported in the literature.
1
as a yellow solid. H NMR (300 MHz, CDCl3, 22 °C): δ 2.97 (d,
J ) 5.1 Hz, 3H; CH3), 6.82 (d, J ) 7.2 Hz, 2H; ArH), 7.26 (t, J )
7.2 Hz, 1H, ArH), 7.44 (t, J ) 7.7 Hz, 2H, ArH); 9.10 ppm (s, 1H,
NH). 13C NMR (75 MHz, CDCl3, 22 °C): δ 37.8 (CH3), 118.9,
126.9, 128.7, 149.4 (CAr), 217.1 (CO cis), 223.2 (CO trans), 283.7
ppm (CdCr). IR (CHCl3): ν 1923, 2054, 2177, 3385 cm-1. Anal.
(%) Calcd for C13H9CrNO5 (311.21): C 50.17, H 2.91, N 4.50.
Found: C 50.47, H 3.11, N 4.70.
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, ; Scalmani, M.; 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, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
ReVision D. 01; Gaussian, Inc.: Wallingford, CT, 2004.
(18) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1998, 37, 785. (c) Vosko, S. H.; Wilk, L.;
Nusair, M. Can. J. Phys. 1980, 58, 1200.
(19) Hehre, W. J.; Radom, L.; Scheleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986; p 76, and the pertinent
references therein.
(20) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(21) McIver, J. W.; Komornicki, A. K. J. Am. Chem. Soc. 1972, 94,
2625.
ESI-MS Experiments. All ESI-MS experiments were carried
out using an ESQUIRE-LC (Bruker Daltonik, Bremen, Germany)
ion trap spectrometer in the negative mode of detection. A syringe
pump (model 74900, Cole-Parmer, Vernon Hills, IL) was used to
deliver chloroform solutions (1.5 × 10-5 mol L-1) of the corre-
sponding Fischer carbene complex through a short length of PEEK
tubing of 254 µm i.d. (Upchurch Scientific, Oak Harbor, WA) with
a flow rate of 3 µL min-1. The stainless steel capillary was held at
a potential of 5.0 kV. Nitrogen was used as nebulizer gas with a
-1
flow rate of 3.98 L min
(nebulizer pressure 11 psi) at 150 °C.
Solutions (chloroform/methanol, 1:1 v/v) containing 25 mM
tetrathiafulvalene (TTF)/cesium carbonate (Cs2CO3) were used. The
(22) Gonza´lez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
(23) (a) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211.
(b) Reed, A. E.; Weinhold, F. J. J. Chem. Phys. 1985, 83, 1736. (c) Reed,
A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (d)
Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(15) Fischer, E. O.; Do¨tz, K. H. J. Organomet. Chem. 1972, 36, C4.
(16) Aumann, R.; Heinen, H. Chem. Ber. 1987, 120, 537.

2770 Organometallics, Vol. 28, No. 9, 2009
Lage et al.
General Procedure for the Preparation of Compounds 3c-e.
To a solution of complex 6a (1 equiv) in THF was added dropwise
a solution of the correspondent amine (1 equiv) at -100 °C. The
reaction mixture was stirred under argon at this temperature until
total consumption of the starting material. The solvent was then
evaporated in Vacuo and the resulting crude reactions purified by
flash column chromatography (SiO2) under argon pressure using
the mixture of solvents indicated in each case as eluent.
stirred overnight, and the crude product thus obtained purified using
a mixture hexanes/ethyl acetate (10:1) as eluent. After evaporation
of the solvent, compound 22c was obtained as an unstable red solid
1
(48.1 mg, 53%). H NMR (500 MHz, CDCl3, 300 K): δ 2.43 (s,
6H; CH3), 4.27-4.13 (m, 4H; CH2), 7.21 (d, J ) 7.9 Hz, 4H; ArH),
7.41 (d, J ) 7.9 Hz, 4H; ArH), 8.88 ppm (s, 2H; NH). IR: ν 1934,
1981, 2054, 2160, 3387, 3447 cm-1
.
Bis-carbene Complex 22d. Following the general procedure,
complex 6a (200 mg, 0.57 mmol) was reacted with 1,3-diamino-
propane (21.5 mg, 0.29 mmol). After 36 h, the crude was purified
(hexanes/ethyl acetate, 8:1) to yield compound 22d as an orange-
Pentacarbonyl[1-(N-butylamine)-3-phenyl-2-propinylidine-
carbene]chromium(0), 3c. According to the general procedure,
complex 6a (100 mg, 0.28 mmol) (solution in 5 mL of THF) was
reacted with butylamine (20.9 mg, 0.28 mmol in 5 mL of THF)
for 18 h. Purification by flash chromatography (hexanes/ethyl
acetate, 8:1) gave an orange solid identified as compound 3c (40.8
mg, 38%). 1H NMR (300 MHz, CDCl3, 22 °C): δ 1.03 (d, J ) 7.1
Hz, 3H; CH3), 1.48 (sex, J ) 6.9 Hz, 2H; CH2), 1.76 (q, J ) 6.8
Hz, 2H; CH2), 3.82 (c, J ) 6.04 Hz, 2H; CH2), 7.46 (m, 3H; ArH),
7.55 (m, 2H; ArH), 8.76 ppm (s, 1H; NH). 13C NMR (75 MHz,
CDCl3, 22 °C): δ 13.6 (CH3), 19.7, 31.4, 52.9 (CH2), 88.9 (Ct C-
Ar), 121.7 (Ct C-Ar), 128.7, 130.7, 131.2, 132.1 (CAr), 217.2 (CO
cis), 223.4 (CO trans), 254.9 ppm (CdCr). IR (CHCl3): ν 1912,
1929, 2055, 2169, 3375 cm-1. Anal. (%) Calcd for C18H15CrNO5
(377.31): C 57.03, H 4.01, N 3.71. Found: C 57.43, H 3.90, N
3.74.
1
red solid (121 mg, 61%). H NMR (300 MHz, CDCl3, 22 °C): δ
2.15 (m, 2H; CH2), 3.86 (m, 4H; CH2), 7.49-7.29 (m, 10H; ArH),
8.84 ppm (s, 2H; NH). 13C NMR (75 MHz, CDCl3, 22 °C): δ 29.6
(CH2), 49.6 (NCH2), 88.6 (Ct C-Ar), 121.2 (Ct C-Ar), 128.8,
131.0, 132.1, 133.8 (CAr), 217.0 (CO cis), 223.3 (CO trans), 259.2
ppm (CdCr). IR (CHCl3): ν 1919, 2058, 2162, 3354 cm-1. Anal.
(%) Calcd for C31H18Cr2N2O10 (682.47): C 54.56, H 2.66, N 4.10.
Found: C 54.16, H 2.48, N 4.30.
Bis-carbene Complex 22e. Complex 6b (800 mg, 1.70 mmol)
was reacted with 1,3-diaminopropane (63.1 mg, 0.85 mmol)
following the general procedure. After 24 h, the solvent was
evaporated under vacuum and the crude product purified (hexanes/
ethyl acetate, 10:1). Evaporation of the solvent yielded compound
1
22e as an unstable reddish-orange solid (500 mg, 62%). H NMR
Pentacarbonyl[1-(N-ethanolamine)-3-phenyl-2-propinylidine-
carbene]chromium(0), 3d. According to the general procedure,
complex 6a (100 mg, 0.28 mmol, in 5 mL of THF) was reacted
with ethanolamine (17 mg, 0.28 mmol in 5 mL of THF). The
reaction mixture was stirred under argon for 1 h 30 min, and the
solvent was then removed in Vacuo. After purification by flash
column chromatography (hexanes/ethyl acetate, 2:1), compound 3d
was obtained as an orange solid (62 mg, 59%). 1H NMR (300 MHz,
CDCl3, 22 °C): δ 3.96-3.77 (m, 4H; CH2), 7.45 (m, 3H; ArH),
7.53 (m, 2H; ArH), 9.38 ppm (s, 1H; NH). 13C NMR (75 MHz,
CDCl3, 22 °C): δ 54.3 (CH2), 60.6 (CH2), 89.0 (Ct C-Ar), 121.6
(Ct C-Ar), 128.7, 130.7, 131.5, 132.0 (CAr), 217.2 (CO cis), 223.5
(CO trans), 256.1 ppm (CdCr). IR (CHCl3): ν 1913, 2055, 2172,
3396 cm-1. Anal. (%) Calcd for C16H11CrNO6 (365.26): C 52.61,
H 3.04, N 3.83. Found: C 52.20, H 2.86, N 3.80.
(CDCl3, 300 MHz, 22 °C): δ 2.33-2.20 (m, 2H, CH2), 3.96-3.84
(m, 4H, NCH2), 7.59-7.36 (m, 10H, ArH), 8.74 ppm (s, 2H, NH).
Bis-carbene Complex 22f. Complex 6a (504 mg, 1.44 mmol)
and 1,4-diaminobutane (63.5 mg, 0.72 mmol) were stirred under
argon for 48 h according to the general procedure. After purification
of the crude (hexanes/ethyl acetate, 4:1), compound 22f was
1
obtained as an orange solid (343 mg, 68%). H NMR (300 MHz,
CDCl3, 22 °C): δ 1.88 (m, 4H; CH2), 3.91-3.89 (m, 4H; CH2),
7.53-7.38 (m, 10H; ArH), 8.80 ppm (s, 2H; NH). 13C NMR (75
MHz, CDCl3, 22 °C): δ 26.6 (CH2), 52.1 (NCH2), 88.8 (Ct C-Ar),
121.3, (Ct C-Ar), 128.8, 131.0, 132.1, 132.6 (CAr), 217.1 (CO cis),
223.4 (CO trans), 257.4 ppm (CdCr). IR (CHCl3): ν 1923, 1985,
2054, 2166, 3391 cm-1. Anal. (%) Calcd for C32H20Cr2N2O10
(696.5): C 55.18, H 2.89, N 4.02. Found: C 55.54, H 3.08, N 4.20.
Bis-carbene Complex 22g. Following the general procedure,
complex 6b (489 mg, 1.04 mmol) was reacted with 1,4-diami-
nobutane (45.8 mg, 0.52 mmol) over 24 h. The crude obtained was
purified by flash chromatography (hexanes/ethyl acetate, 5:1) to
give an orange solid identified as compound 22g (130 mg, 26%).
1H NMR (300 MHz, CDCl3, 22 °C): δ 2.06-1.82 (m, 4H; CH2),
3.88-3.77 (m, 4H; CH2), 7.57-7.37 (m, 10 H; ArH), 8.70 ppm
(s, 2H; NH). 13C NMR (75 MHz, CDCl3, 22 °C): δ 26.3 (CH2),
43.6 (NCH2), 91.3 (Ct C-Ar), 121.2 (Ct C-Ar), 128.9, 131.0, 132.0,
144.6 (CAr), 194.2 (CO cis), 198.4 (CO trans), 234.2 ppm (CdW).
IR (CHCl3): ν 1906, 2067, 2174, 3390 cm-1. Anal. (%) Calcd for
C32H20N2O10W2 (960.19): C 40.03; H 2.10; N 2.92. Found: C 40.40,
H 2.30, N 3.04.
Pentacarbonyl[1-(N-(2-methoxyethylamine))-3-phenyl-2-pro-
penylidinecarbene]chromium(0), 3e. Following the general pro-
cedure described above, complex 6a (200 mg, 0.57 mmol) was
reacted with 2-methoxyethylamine (42.8 mg, 0.57 mmmol) for 6 h.
The crude thus obtained was purified by flash column chromatog-
raphy (4:1 hexanes/ethyl acetate) to give compound 3e as an orange
solid (143 mg, 66%). 1H NMR (300 MHz, CDCl3, 22 °C): δ 3.47
(s, 3H; CH3), 3.65 (t, 3J(H,H) ) 5.19 Hz, 2H; CH2), 3.96 (c, 3J(H,H)
) 5.20 Hz, 2H; CH2), 7.43 (m, 3H; ArH), 7.54 (m, 2H; ArH),
9.10 ppm (s, 1H; NH). 13C NMR (75 MHz, CDCl3, 22 °C): δ 52.2
(CH2), 59.2 (CH3), 70.1 (CH2), 88.9 (Ct C-Ar), 121.5 (Ct C-Ar),
128.7, 130.7, 131.4, 132.0 (CAr), 217.1 (CO cis), 223.6 (CO trans),
255.8 ppm (CdCr). IR (CHCl3): ν 1907, 2055, 2172, 3382 cm-1
.
Anal. (%) Calcd for C17H13CrNO6 (379.28): C 53.83, H 3.45, N
3.69. Found: C 53.49, H 3.40, N 3.72.
Bis-carbene Complex 22h. Complex 6a (493 mg, 1.41 mmol)
was reacted with 1,5-diaminopentane (71.9 mg, 0.70 mmol) over
5 days, after which the crude product was purified by flash
chromatography (hexanes/ethyl acetate, 4:1) to yield compound 22h
as an orange solid (301 mg, 61%). 1H NMR (300 MHz, CDCl3, 22
°C): δ 1.57-1.55 (m, 2H; CH2), 1.88-1.84 (m, 4H; CH2),
3.85-3.83 (m, 4H; CH2), 7.54-7.43 (m, 10H; ArH), 8.76 ppm (s,
2H; NH). 13C NMR (75 MHz, CDCl3, 22 °C): δ 23.5 (CH2), 29.0
(CH2), 52.6 (NCH2), 88.8 (Ct C-Ar), 121.4 (Ct C-Ar), 128.8,
130.8, 131.9, 132.0 (CAr), 217.2 (CO cis), 223.5 (CO trans), 255.9
General Procedure for the Preparation of Bis-aminocar-
bene Complexes 22. To a solution of the correspondent alkoxy-
carbene complex 6a-c (1 equiv) in THF (5 mL/mmol carbene) at
-78 °C was added dropwise a solution of the corresponding
diamine (0.5 equiv) in THF (2.5 mL/mmol amine). The reaction
mixture was stirred under argon at this temperature until total
consumption of the starting material. The solvent was evaporated
in Vacuo and the crude products purified by flash column chroma-
tography (SiO2) under argon pressure using the mixture of eluents
indicated for each complex.
ppm (CdCr). IR (CHCl3): ν 1933, 1979, 2055, 2167, 3375 cm-1
.
Anal. (%) Calcd for C33H22Cr2N2O10 (710.53): 55.78, H 3.12, N
3.94. Found: C 56.16, H 3.32, N 4.04.
Bis-carbene Complex 22i. Following the general procedure,
complex 6b (484 mg, 1.03 mmol) was reacted with 1,5-diamino-
Bis-carbene Complex 22c. Complex 6c (947 mg, 0.26 mmol)
and ethylenediamine (7.8 mg, 0.13 mmol) were reacted according
to general procedure described above. The reaction mixture was

Group 6 Fischer Aminocarbene Complexes
Organometallics, Vol. 28, No. 9, 2009 2771
pentane (52.1 mg, 0.51 mmol) for 24 h. The crude product thus
obtained was purified by flash column chromatography (hexanes/
ethyl acetate, 5:1) to yield compound 22i as an orange solid (80
the product obtained after evaporation of the solvent was im-
mediately solved in DMF, ethyl iodide (280.8 mg, 1.8 mmol) and
Cs2CO3 (758.6 mg, 2.15 mmol) were added, and the mixture was
stirred under argon overnight. After purification by flash chroma-
tography (hexanes/ethyl acetate, 8:1), compound 3f was obtained
1
mg, 16%). H NMR (300 MHz, CDCl3, 22 °C): δ 1.30-1.23 (m,
2H; CH2), 1.92 (m, 4H; CH2), 3.8 (m, 4H; CH2), 7.57-7.39 (m,
10H; ArH), 8.64 ppm (s, 2H; NH). 13C NMR (75 MHz, CDCl3, 22
°C): δ 23.7 (CH2), 29.7 (CH2), 52.4 (NCH2), 91.3 (Ct C-Ar), 121.3
(Ct C-Ar), 128.3, 128.8, 131.0, 132.3 (CAr), 198.4 (CO cis), 203.5
(CO trans), 233.0 (CdW). IR (CHCl3): ν 1905, 1984, 2062, 2171,
3375 cm-1. Anal. (%) Calcd for C33H22N2O10W2 (974.22): C 40.68,
H 2.28, N 2.88. Found: C 40.94, H 2.40, N 2.69.
1
as an orange solid (73.4 mg, 27%). H NMR (300 MHz, CDCl3,
22 °C): δ 1.39 (t, J ) 6.59 Hz, 3H; CH3), 1.49 (t, J ) 6.62 Hz,
3H; CH3), 4.02 (c, J ) 6.93 Hz, 2H; CH2), 4.27 (c, J ) 6.78 Hz,
2H; CH2), 7.43 (m, 3H; ArH), 7.51 ppm (m, 2H; ArH). 13C NMR
(75 MHz, CDCl3, 22 °C): δ 14.1, 14.3 (CH3), 51.5, 54.1 (CH2),
90.6 (Ct C-Ar), 122.2 (Ct C-Ar), 128.6, 128.9, 130.1, 131.4 (CAr),
217.2 (CO cis), 224.2 (CO trans), 246.0 ppm (CdCr). IR (CHCl3):
ν 1905, 1977, 2055, 2170 cm-1. Anal. (%) Calcd for C18H15CrNO5
(377.31): C 57.30, H 4.01, N 3.71. Found: C 57.59, H 4.20, N
3.58.
Bis-carbene Complex 22j. Complex 6a (200 mg, 0.57 mmol)
was reacted with m-xylylendiamine (40 mg, 0.29 mmol) following
the general procedure described above. After purification of the crude
reaction by flash chromatography (hexanes/ethyl acetate, 6:1), an
orange solid was isolated and identified as compound 22j (164 mg,
Pentacarbonyl[1-(N,N′-dibutylamine)-3-phenyl-2-propinylidine-
carbene]chromium(0), 3g (Method A). Complex 6a (400 mg, 1.14
mmol) was reacted with butylamine (83.5 mg, 1.14 mmol) in THF,
and the resulting crude product (after evaporation of the solvent)
was immediately solved in DMF and mixed with butyl bromide
(390.5 mg, 2.85 mmol) and Cs2CO3 (1.11 g, 3.42 mmol). Purifica-
tion of the crude product yielded compound 3g as an orange solid
1
76%). H NMR (300 MHz, CDCl3, 22 °C): δ 5.01 (m, 4H; CH2),
7.70-7.39 (m, 14 H; ArH), 8.93 ppm (s, 2H; NH). 13C NMR (75
MHz, CDCl3, 22 °C): δ 56.8 (CH2), 89.6 (Ct C-Ar), 121.8 (Ct C-
Ar), 129.5, 131.1, 132.0, 135.8 (CAr), 217.5 (CO cis), 223.7 (CO trans),
258.4 ppm (CdCr). IR (CHCl3): ν 1915, 1942, 1980, 2056, 2167,
3382 cm-1. Anal. (%) Calcd for C36H20Cr2N2O10 (744.54): C 58.07,
H 2.71, N 3.76. Found: C 58.44, H 2.90, N 3.60.
1
(115 mg, 23%). H NMR (300 MHz, CDCl3, 22 °C): δ 1.02 (m,
Bis-carbene Complex 22k. To a solution of complex 6a (50
mg, 0.14 mmol) and complex 6b (65.8 mg, 0.14 mmol) in THF at
-78 °C was added dropwise a solution of 1,4-diaminobutane (12.3
mg, 0.14 mmol), and the mixture was stirred under argon at -78
°C for 24 h. The solvent was then evaporated in Vacuo and the
crude purified (hexanes/ethyl acetate, 6:1). The product thus
obtained was identified as a mixture of complexes 22f, 22g, and
22k, and spectral data for complex 22k were identified by
comparison of the spectra of the mixture to those of complexes
6H; CH3), 1.41 (m, 2H; CH2), 1.52 (m, 2H; CH2), 1.81 (m, 4H;
CH2), 4.00 (t, J ) 7.29 Hz, 2H; CH2), 4.16 (t, J ) 7.9 Hz, 2H;
CH2), 7,44 (m, 3H; ArH), 7.51 ppm (m, 2H; ArH). 13C NMR (75
MHz, CDCl3, 22 °C): δ 13.1, 13.7 (CH3), 19.5, 19.9 (CH2), 30.9,
31.1 (CH2), 57.1, 59.8 (CH2), 90.8 (Ct C-Ar), 122.3 (Ct C-Ar),
128.7, 128.9, 130.1, 131.4 (CAr), 217.3 (CO cis), 224.2 (CO trans),
246.2 ppm (CdCr). IR (CHCl3): ν 1915, 2055, 2068, 2173 cm-1
.
Anal. (%) Calcd for C22H23CrNO5 (433.42): C 60.97, H 5.35, N
3.23. Found: C 61.03, H 5.40, N 3,46.
1
22f and 22g. H NMR (500 MHz, CDCl3, 22 °C): δ 1.94-1.91
Complex 23a (Method B). Complex 22d (115 mg, 0.17 mmol)
was reacted with MeI (96.5 mg, 0.68 mmol) in the presence of
Cs2CO3 (332 mg, 1.02 mmol) in DMF for 4 days, yielding (hexanes/
ethyl acetate, 6:1) an orange solid identified as compound 23a (40
(m, 4H; CH2), 3.90-3.67 (m, 4H; CH2), 7.53-7.39 (m, 10H; ArH),
8.65 (s, 1H; NH), 8.75 ppm (s, 1H; NH). 13C NMR (125 MHz,
CDCl3, 22 °C): δ 26.6 (CH2), 29.8 (CH2), 43.9 (NCH2), 51.6
(NCH2), 88.8 (Ct C-Ar), 91.3 (Ct C-Ar), 121.2, 128.8, 130.2,
132.5, 144.5 (CAr), 194.2, 195.9, 198.3, 203.4 (COW), 215.9, 223.3
(COCr), 234.5 (CdW), 258.7 ppm (CdCr). IR (CHCl3): ν 1906,
1
mg, 33%). H NMR (300 MHz, CDCl3, 22 °C): δ 2.32 (m, 2H;
CH2), 3.96 (s, 6H; CH3), 4.17 (m, 4H; CH2), 7.40-7.54 ppm (m,
10H; ArH). 13C NMR (75 MHz, CDCl3, 22 °C): δ 26.5 (CH2),
47.3 (CH3), 57.7 (CH2), 90.0 (Ct C-Ar), 121.6 (Ct C-Ar), 128.8,
130.5, 131.3, 131.5 (CAr), 217.1 (CO cis), 223.8 (CO trans), 252.6
ppm (CdCr). IR (CHCl3): ν 1906, 1969, 2054, 2168 cm-1. Anal.
(%) Calcd for C33H22Cr2N2O10 (710.53): C 55.78, H 3.12, N 3.94.
Found: C 56.10, H 3.30, N 4.10.
Complex 23b (Method B). Complex 22e (270 mg, 0.28 mmol)
was reacted with MeI (161.8, 1.14 mmol) and Cs2CO3 (557 mg,
1.71 mmol) in DMF for 48 h. After purification of the crude product
(hexanes/ethyl acetate, 6:1), compound 23b was obtained as an
orange solid (90 mg, 33%). 1H NMR (300 MHz, CDCl3, 22 °C): δ
2.34 (m, 2H; CH2), 3.81 (s, 6H; CH3), 4.12 (t, J ) 7.64 Hz, 4H;
CH2), 7.44-7.31 ppm (m, 10H; ArH). 13C NMR (75 MHz, CDCl3,
22 °C): δ 26.5 (CH2) 49.6 (CH3), 56.5 (CH2), 92.0 (Ct C-Ar), 121.4
(Ct C-Ar), 127.7, 128.8, 130.6, 131.5 (CAr), 198.3 (CO cis), 203.9
(CO trans), 231.6 ppm (CdW). IR (CHCl3): ν 1897, 1961, 2062,
2169 cm-1. Anal. (%) Calcd for C33H22N2O10W2 (974.22): C 40.68,
H 2.28, N 2.88. Found: C 41.04, H 2.40, N 2.50.
2067, 2172, 3370 cm-1
.
General Procedures for the N-Alkylation of NH-Carbene
Complexes. Method A (compounds 3f, 3g). To a solution of
compound 6a (1 equiv) in THF (10 mL/mmol compound) was
added dropwise a solution of the corresponding amine (ethylamine/
butylamine) (1 equiv) in THF (5 mL/mmol of amine) at -100 °C,
and the reaction mixtures were stirred under argon until total
consumption of starting material. The solvent was then evaporated
in Vacuo, and the crudes thus obtained were solved in DMF (10
mL/mmol of complex) and Cs2CO3 (3 equiv). The corresponding
halide (ethyl iodide, butyl bromide) (2.5 equiv) was added. The
crudes were stirred until completion and then extracted in ethyl
acetate, the organic extracts were washed with brine and water and
dried over magnesium sulfate, the solvent was evaporated in Vacuo,
and the products were purified by flash column chromatography
under argon pressure.
Method B (compounds 23a, 23b, 3h). To a solution of the
corresponding carbene complex (22d/22e/3e) (1 equiv) in DMF
(10 mL/mmol of complex) were added Cs2CO3 (6 equiv) and MeI
(4 equiv). The reaction mixture was stirred under argon at rt
overnight, and then the crude was extracted in ethyl acetate. The
organic extracts were washed with brine and water and dried over
magnesium sulfate, and the solvent was evaporated in Vacuo. The
products thus obtained were purified by flash column chromatog-
raphy under argon pressure.
Pentacarbonyl[1-(N-methyl,N′-(2-methoxyethylamine))-3-phe-
nyl-2-propenylidinecarbene]chromium(0), 3h (Method B). A
mixture of complex 3e (270 mg, 0.71 mmol), MeI (251.2 mg, 1.77
mmol), and Cs2CO3 (640 mg, 2.13 mmol) in DMF was stirred under
argon for 24 h, after which the crude was worked up following the
general procedure and purified (hexanes/ethyl acetate, 8:1) to yield
an orange solid identified as compound 3h (120 mg, 43%) (mixture
of isomers syn/anti (m/M) in the ratio 1:2.5). 1H NMR (300 MHz,
CDCl3, 22 °C): δ 3.41 (s, 3H; OCH3) (M), 3.50 (s, 3H; OCH3)
(m), 3.63 (m, 2H; CH2) (M), 3.74 (s, 3H; NCH3) (M), 3.82 (m,
2H; CH2) (m), 3.96 (s, 3H; NCH3) (m), 4.21 (m, 2H; CH2) (M),
Pentacarbonyl[1-(N,N′-diethylamine)-3-phenyl-2-propinylidine-
carbene]chromium(0), 3f(Method A). Complex 6a (250 mg, 0.71
mmol) was reacted with ethylamine (45.1 mg, 0.71 mmol) in THF,

2772 Organometallics, Vol. 28, No. 9, 2009
Lage et al.
4.42 (m, 2H; CH2) (m), 7.43 (m, 6H; ArH) (m+M), 7.50 (m, 4H;
ArH) (m+M). 13C NMR (75 MHz, CDCl3, 22 °C): δ 49.4 (CH3),
59.1 (CH3), 60.2, 60.3 (CH2), 69.7, 70.8 (CH2), 90.3 (Ct C-Ar),
122.0 (Ct C-Ar), 128.6, 129.8, 130.2, 131.4, 132.3 (CAr), 217.1
(CO cis), 224.1 (CO trans), 249.6 ppm (CdCr). IR (CHCl3): ν 1912,
2054, 2067, 2172 cm-1. Anal. (%) Calcd for C18H15CrNO6 (393.31):
C 54.97, H 3.84, N 3.56. Found: C 54.67, H 3.80, N 3.18.
CCG07-UCM/PPQ-2596 from the Comunidad de Madrid is
gratefully acknowledged. M.L.L. thanks the MEC (Spain)
for a FPU fellowship. I.F. is a Ramo´n y Cajal fellow.
1
Supporting Information Available: H and 13C NMR spectra
for all complexes, Cartesian coordinates (in Å), and total energies
(in au, noncorrected zero-point vibrational energies included) of
all the stationary points discussed in the text are available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. Support for this work under grants
CTQ2007-67730-C02-01/BQU and CSD2007-0006 (Pro-
grama Consolider-Ingenio 2010) from the MEC (Spain) and
OM900159H