Influence of the N-N Coligand: C-C coupling instead of formation of imidazol-2-yl complexes at {Mo(η3-allyl)(CO)2} fragments. theoretical and experimental studies

Article abstract of DOI:10.1021/ic502729z

New N-methylimidazole (N-MeIm) complexes of the {Mo(η³-allyl)(CO)₂(N-N)} fragment have been prepared, in which the N,N-bidentate chelate ligand is a 2-pyridylimine. The addition of a strong base to the new compounds deprotonates the central CH group of the imidazole ligand and subsequently forms the C-C coupling product that results from the nucleophilic attack to the imine C atom. This reactivity contrasts with that previously found for the analogous 2,2′-bipyridine compounds [Mo(η³-allyl)(CO)₂(bipy)(N-RIm)]OTf [N-RIm = N-MeIm, N-mesitylimidazole (N-MesIm, Mes= 2,4,6-trimethylphenyl); OTf = trifluoromethanesulfonate) which afforded imidazol-2-yl complexes upon deprotonation. Density Functional Theory (DFT) computations uncover that the reactivity of the imine C atom along with its ability to delocalize electron density are responsible for the new reactivity pattern found for the kind of molybdenum complexes reported herein.

Full text of DOI:10.1021/ic502729z

Article
pubs.acs.org/IC
Influence of the N−N Coligand: C−C Coupling Instead of Formation
of Imidazol-2-yl Complexes at {Mo(η3‑allyl)(CO)2} Fragments.
Theoretical and Experimental Studies
Andrea Cebollada,† Maialen Espinal Viguri,‡ Julio Perez,‡,§ Jesus Díaz,*, ∥ Ramon Lopez,*,⊥
́
́
́
́
and Lucía Riera*,§
†Departamento de Química Inorgan
‡Departamento de Química Organ
ica e Inorgan
§Centro de Investigacion
en Nanomateriales y Nanotecnología (CINN), CSIC-Universidad de Oviedo-Principado de Asturias,
Avenida de la Vega 4-6, 33940 El Entrego, Spain
∥Departamento de Química Organ
ica e Inorganica, Universidad de Extremadura, Avda de la Universidad s/n, 10071 Cac
⊥Departamento de Química Física y Analítica, Universidad de Oviedo, Julian
Clavería, 8, 33006 Oviedo, Spain
́
ica-ISQCH, Universidad de Zaragoza-CSIC, Pedro Cerbuna, 12, 50009 Zaragoza, Spain
ica, Universidad de Oviedo, Julian Clavería, 8, 33006 Oviedo, Spain
́
́
́
́
́
́
́
eres, Spain
́
S
* Supporting Information
ABSTRACT: New N-methylimidazole (N-MeIm) complexes of the {Mo(η3-allyl)-
(CO)2(N−N)} fragment have been prepared, in which the N,N-bidentate chelate ligand is
a 2-pyridylimine. The addition of a strong base to the new compounds deprotonates the
central CH group of the imidazole ligand and subsequently forms the C−C coupling product
that results from the nucleophilic attack to the imine C atom. This reactivity contrasts with
that previously found for the analogous 2,2′-bipyridine compounds [Mo(η3-allyl)(CO)2-
(bipy)(N-RIm)]OTf [N-RIm = N-MeIm, N-mesitylimidazole (N-MesIm, Mes= 2,4,6-
trimethylphenyl); OTf = trifluoromethanesulfonate) which afforded imidazol-2-yl complexes
upon deprotonation. Density Functional Theory (DFT) computations uncover that the
reactivity of the imine C atom along with its ability to delocalize electron density are
responsible for the new reactivity pattern found for the kind of molybdenum complexes
reported herein.
with N-alkylimidazole ligands, trying to generate, in a similar
way, a new type of NHC species with a metal fragment as
substituent of one of the imidazole nitrogen atoms.5 In this
research area we have found that the deprotonation of the
central C−H group of a coordinated N-alkylimidazole affords
very reactive species with reaction outcomes strongly depend-
ing on the nature of the imidazole substituent and the ancillary
ligands.6 In particular, for Mo(II) cationic compounds of
formula [Mo(η3-C4H7)(bipy)(CO)2(N-RIm)]OTf (R = Me,
Mes) the addition of a strong base led to imidazol-2-yl
complexes resulting from the tautomerization of the hetero-
cycle from N- to C-coordinated once the deprotonation had
occurred. The subsequent addition of electrophilic reagents
(HOTf, MeOTf, or EtOTf) afforded molybdenum NHC
compounds (Scheme 1) by protonation or alkylation of the
nonsubstituted nitrogen.7
INTRODUCTION
■
The imidazole moiety is very often encountered in biological
systems. The importance of this function is mainly due to its
presence in the side chain of the amino acid histidine, and its
role as a metal binding site in metalloenzymes.1 The relatively
small size and the electronic properties (σ-donor and π-
aceptor) make imidazole and its derivatives good ligands for a
variety of metal fragments.2 Whereas the coordination
chemistry of imidazoles has been therefore extensively studied,
little attention has been paid to the analogy between cationic
metal complexes with N-alkylimidazole (N-RIm) ligands and
N,N′-dialkylimidazolium salts instead.3 In fact, the former can
be regarded as N-metalated analogous of the latter (Figure 1).
The deprotonation of an N,N′-dialkylimidazolium salt affords
a N-heterocyclic carbene (NHC).4 Therefore, we focused our
attention in the deprotonation of transition metal complexes
In agreement with the experimental results found for the
deprotonation of [Mo(η3-C4H7)(bipy)(CO)2(N-RIm]OTf
compounds, a DFT study showed that the most favorable
reaction mechanism was reminiscent of the one found for
[Mn(bipy)(CO)3(N-RIm)]OTf compounds,5b,7 involving an
Figure 1. Analogy between imidazolium salts (a) and cationic metal
complexes with N-alkylimidazole ligands (b).
Received: November 12, 2014
Published: February 27, 2015
© 2015 American Chemical Society
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Scheme 1. Synthesis of Imidazol-2-yl and NHC Complexes from N-Alkylimidazole Derivatives
initial attack of the imidazole deprotonated carbon atom onto a
cis-CO ligand, to afford in a second step imidazol-2-yl species.
Furthermore, these calculations pointed out that the difference
in energy between this pathway and the one that would lead to
C−C coupling products (analogous to the reactivity pattern
found for [Re(bipy)(CO)3(N-RIm)]+ complexes) is only 3.5
kcal/mol.7 This result prompted us to investigate the feasibility
of inverting the reactivity pattern in {Mo(η3-C4H7)(CO)2}
complexes, to obtain C−C coupling products.
has been proposed that the κ2-(N,O) coordination of the
aldehyde activates the carbonyl group toward the condensation
(enhancing its electrophilic character) making the reaction
more favorable.10
The employment of 4-methylaniline and 3,5-dimethylaniline
afforded, in the same way, complexes 1b and 1c, respectively. In
contrast, ortho-substituted anilines (such as 2,4,6-trimethylani-
line or 2,6-diisopropylaniline) precluded the formation of the
desired molybdenum iminopyridine derivatives, probably due
to the steric hindrance. The new compounds 1a−c were
obtained in good yields, as the only products of the reactions,
and their spectroscopic data in solution were in agreement with
the proposed stoichiometry and geometry showed in Scheme
2,11 and similar to related pyridylimino compounds of the
fragment {MoCl(η3-allyl)(CO)2}.9d
The addition of a few drops of acetonitrile to a 1:1 mixture of
AgOTf and [MoCl(η3-C4H7)(CO)2(py-2-CHNAr)] (Ar
= C6H5 1a, C6H4-4-Me 1b, C6H3-3,5-Me2 1c) in dichloro-
methane led immediately to the precipitation of a white solid
(AgCl), and the formation of the nitrile compounds [Mo(η3-
C4H7)(CO)2(NCMe)(py-2-CHNAr)]OTf. From these
species the labile MeCN ligand is easily substituted by addition
of the equimolar amount of N-methylimidazole (N-MeIm)
affording imidazole compounds [Mo(η3-C4H7)(CO)2(N-
MeIm)(py-2-CHNAr)]OTf (2a−c, see Scheme 3).
Herein we report the synthesis and reactivity of [Mo(η3-
C4H7)(CO)2(N-MeIm)(py-2-CHN-R)]OTf (R= C6H5,
C6H4-4-Me, C6H3-3,5-Me2, iPr, tBu) compounds, showing
that C−C coupling products are obtained upon deprotonation.
The DFT computations are in agreement with the experimental
findings, and show that the replacement of the α-diimine bipy
by the iminopyridines employed in this work results in that the
pathway leading to the C−C coupling products becomes
preferred.
RESULTS AND DISCUSSION
■
The labile complex [MoCl(η3-C4H7)(CO)2(NCMe)2] reacts
with the equimolar amounts of 2-pyridylcarboxaldehyde and
aniline in THF to afford complex 1a as shown in Scheme 2. Its
Scheme 2. One-Pot Synthesis of 2-Pyridylimino
Chlorocomplexes 1a−e
Scheme 3. Synthesis of Cationic Imidazole 2-Pyridylimino
Complexes 2a−e
spectroscopic data in solution, and the full characterization of
some of its reaction products, discussed below, show that 1a
possesses an iminopyridyl ligand coordinated as bidentate
chelate to the {MoCl(η3-C4H7)(CO)2} fragment.
The metal-free reaction of the aldehyde and aniline takes 12
h in refluxing toluene to reach completion.8 In contrast, the
three-component reaction described above produces 1a in less
than 1 h at room temperature, indicating that the metal exerts a
significant templating effect, as previously noted by others.9 It
These reactions were followed by IR spectroscopy, showing
first the formation of cationic nitrile species (the IR νCO bands
showed the typical pattern for cis-{MoII(CO)2} fragments, and
the values changed, from 1951, 1876 cm−1 in the case of 1a, to
1964, 1885 cm−1), and afterward the substitution of the nitrile
by the more σ-donor N-MeIm ligand (IR νCO values
downshifted to 1949, 1869 cm−1 for 2a).
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1H and 13C NMR spectra in solution of compounds 2a−c
showed the asymmetry of the new molecules (as the
iminopyridine ligands are nonsymmetric) and the incorpo-
ration of one N-MeIm ligand per metallic fragment. The solid
state structure of 2c, determined by X-ray diffraction (see
Figure 2),12 shows that the molybdenum atom displays a
pseudo-octahedral coordination geometry.
Scheme 4. Reactivity of Cationic Imidazole 2-Pyridylimino
Complexes 2a−e
length was typical of a double bond (1.283(3) Å) as expected
for an imine moiety. The originally imino nitrogen atom (N2)
displays in 4a a tetrahedral geometry, which is consistent with
the protonation of this atom by the HOTf forming an amino
group. The high quality of the results of the structural
determination of compound 4a allowed the hydrogen atom on
N2 to be refined. Finally, it is interesting to note that in the
solid state structure of the cationic complex of 4a the pyridyl
moiety is in trans disposition to the η3-methallyl ligand, whereas
in the starting compound (2c) the imidazole ligand is
occupying this position. This indicates that the C−C coupling
reaction is accompanied by a rotation of the trigonal face
formed by the three nitrogen ligands, probably in order to
minimize the steric hindrance in the resulting product.
Figure 2. Molecular structure of the cation of compound 2c.
The 2-pyridylimino chelate is coplanar with the two CO
ligands, and the N-methylimidazole is trans to the η3-allyl
ligand. The pyridine and imine moieties of the new bidentate
ligand are virtually coplanar reflecting conjugation, with the
dihedral angle being 4.43°. The Mo−N(imidazole) bond
distance, of 2.220(2) Å, is very close to that found for the
only other crystallographically characterized N-alkylimidazole
complex of the fragment {Mo(η3-allyl)(CO)2}, of 2.219(8) Å.7
The reaction of [Mo(η3-C4H7)(CO)2(N-MeIm)(py-2-CH
N-Ph)]OTf (2a) with a slight excess of KN(SiMe3)2 in THF at
−78 °C afforded, as indicated by the large shift to lower
wavenumbers of the νCO bands in the IR spectrum (from 1949,
1869 to 1932, 1835 cm−1), the formation of a neutral species.
The low stability of this deprotonated derivative precluded its
isolation and, upon addition of trifluoromethanesulfonic acid in
CH2Cl2, the stable compound 4a was obtained as the main
product of the reaction (Scheme 4). The IR νCO bands of the
new compound 4a, at 1950, 1969 cm−1, indicated the
protonation reaction had occurred and a cationic complex
was formed.
The molecular structure of the cation of compound 4a,
determined by X-ray diffraction,13 is depicted in Figure 3a
showing that a tridentate N-donor ligand is now coordinated to
the {Mo(η3-methallyl)(CO)2} fragment in a facial disposition.
This tripodal ligand results from the C−C coupling between
the central C atom of the imidazole (C2) and the imine C atom
of the pyridylimino bidentante ligand (C1). The bond distance
C1−C2, of 1.497(8) Å, is typical for a single C−C bond.
Therefore, the C1 atom is sp3 hybridized, the angles around it
being consistent with an approximately tetrahedral geometry
(C2−C1−N2 104.0(5)°, C6−C1−N2 110.9(5)°, C2−C1−C6
107.0(5)°). The C1−N2 bond distance is in agreement with a
single bond (1.507(8) Å), whereas in the solid state structure of
its precursor 2c (determined by X-ray diffraction) this bond
The 1H and 13C NMR data in solution of the new compound
4a are in accordance with the structure found in the solid state.
The 1H NMR spectrum undoubtedly shows the deprotonation
of the imidazole central CH group (as only two signals at 7.17
and 6.90 ppm that correspond to imidazole CH moieties are
observed), and the C−C coupling with the imine NCH unit
(as evidenced by the disappearance of the signal at 9.08 ppm
1
assigned to that group in the H NMR spectrum of 2a).
1
Moreover, the H NMR spectrum of 4a includes a singlet at
5.65 ppm that corresponds to the Csp3−H group formed as a
consequence of the C−C coupling reaction. Unfortunately, the
signal of the new NH group is not observed, which can be
attributed to the acidic character of this hydrogen. The 13C
NMR spectrum of 4a corresponds to an asymmetric complex,
showing for example two low intensity signals at 227.7 and
226.8 ppm for the two carbonyl ligands, and a signal at 64.3
ppm assigned to the new Csp3 atom.
As we have discussed above, the solid state structure of
compound 4a showed that the formation of the tridentate
ligand is accompanied by a rotation of the original N-donor
1
ligands. Accordingly, the H NMR spectra of 4a in CD2Cl2 at
variable temperature (from 298 to 213 K) showed the existence
of a dynamic process. The data are in accordance with a
trigonal twist rearrangement (that would involve a rotation of
the triangular face formed by the tripodal ligand relative to the
face formed by the allyl and the two carbonyl groups),14
a
process that has been frequently found for complexes of the
fragment {Mo(η3-allyl)(CO)2}.15,16
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Figure 3. (a) Molecular structure of the cation of compound 4a; (b) molecular structure of the cation of compound 4b.
The reaction of the related compounds 2b and 2c toward the
strong base KN(SiMe3)2 showed the same reactivity pattern
discussed above for 2a, but for these species the neutral
derivatives, 3b and 3c, respectively (Scheme 4), could be
After numerous attempts, only poor quality crystals were
obtained for compound 4c. The X-ray structure, although of
low quality,19 clearly shows that the connectivity of the
molecule is analogous to those described previously for
compounds 4a and 4b.
1
isolated and characterized in solution by means of IR and H
NMR. The IR νCO bands, at approximately 20 cm−1 lower
wavenumbers than those of the imidazole starting compounds,
are typical of neutral complexes displaying a good σ-donor
The H NMR spectra of the crude of all the protonated
1
compounds 4a−c show the existence of two sets of signals
attributed to the presence of two different diastereomers. The
diastereomer that could be crystallyzed in each case was found
to be the more abundant (for example in the case of the
formation of 4b the ratio of diasteromers is of 3.85). The
existence of diasteromers is due to the presence of three
stereocenters: the metal, the carbon atom that is the site of the
nucleophilic attack, and the N atom that is protonated. As the
stereochemistry of the metal and the C atom are prefixed by the
geometry of the starting compounds, it is the final protonation
step that leads to the formation of two diasteromers (each a
pair of enantiomers). A NOESY experiment was carried out for
compound 4a, showing that the η3-allyl ligand is oriented with
its open face toward the carbonyl ligands, i.e. in the same
orientation found in the solid state structures of compounds
4a−c, and also found in its precursor 2a.11 This orientation of
the η3-allyl is the one most frequently found for complexes with
the {Mo((η3-allyl)(CO)2}fragment.20
1
ligand (such as the deprotonated imidazole). The H NMR
spectra of 3b and 3c show characteristic features similar to
those described for the spectrum of compound 4a: i.e., only
two N-MeIm CH groups are observed (two singlets at 7.02 and
6.19 ppm for 3b), there is no longer an imine NCH signal
(at 9.03 ppm in the precursor 2b), and instead there is a signal
(at 5.76 ppm for 3b) for the new Csp3−H group originated as a
consequence of the C(imidazolyl)−C(imine) bond formation.
Unfortunately, the stability of complexes 3b and 3c was not
enough to obtain the 13C NMR spectra, as they decomposed
after several hours in CD2Cl2 solution into a mixture of
unidentified products.
The reaction of 3b or 3c intermediates with a slight excess of
HOTf led to the formation of the cationic protonated products
4b or 4c, respectively (Scheme 4). The stability of the
protonated products is considerably higher than that of the
neutral species, which can be attributed to the presence of a
highly reactive amido group17 in 3b or 3c that, upon
protonation, is transformed into a more stable amine moiety
in 4b or 4c. The spectroscopic data of the new compounds are
analogous to those of 4a, and the solid state X-ray diffraction
structures were determined for both compounds. In Figure 3b
is depicted the molecular structure of the cation of 4b,18
showing the formation of the C−C coupling product. The
central carbon atom of the N-MeIm moiety (C2) is therefore
bonded to the imine carbon (C1), and the resulting tridentate
ligand occupies one face of the octahedral coordination sphere
of the molybdenum atom. The other face is occupied by the
two CO ligands and the η3-methallyl moiety.14 As discussed for
compound 4a, the formation of the tridentate ligand implies a
rotation of the N-donor ligands so that the 2-pyridyl group, and
not the 2-imidazolyl, is trans to the methallyl ligand in 4b.
Finally, to test the generality of the reactivity pattern
previously described, we extended our study to alkyl- (instead
of aryl-) amines in the Schiff condensation reaction. As a result,
the pyridylimino bidentate ligands would be more electron rich,
and therefore the nucleophilic attack onto the NCH group of
the imidazol-2-yl ligand would be less favorable.
Compounds [Mo(η3-C4H7)(CO)2(N-MeIm)(py-2-CHN-
R)]OTf (R= iPr, 2d; tBu, 2e) were prepared from the
corresponding chloroderivatives (1d and 1e) as described for
compounds 2a−c (see Scheme 3).11 The addition of the
equimolar amount of KN(SiMe3)2 to a previously cooled THF
solutions of 2d or 2e afforded immediately the formation of
neutral species resulting, presumably, from the deprotonation
of the central CH group of the imidazole ligand. The
subsequent addition of the electrophilic reagent HOTf led to
more stable metal complexes,21 4d and 4e, respectively, that
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Figure 4. (a) Molecular structure of the cation of compound 4d; (b) molecular structure of the cation of compound 4e.
Figure 5. CPCM-B3LYP-D3/6-311++G(d,p) (LANL2DZ + f for Mo)//CPCM-B3LYP/6-31+G(d) (LANL2DZ + f for Mo) Gibbs energy profiles
in THF solution of the mechanisms located for the formation of an imidazol-2-yl product starting from I. Relative Gibbs energies in THF solution, in
kcal/mol, are given in parentheses. Most relevant distances are also included in angstroms.
were isolated and purified by crystallization. The more
characteristic features of the NMR (1H and 13C) spectra of
the new compounds are analogous to the pyridyliminoaryl
compounds discussed above, and clearly indicate that (i) the
strong base has deprotonated the central CH group of the N-
methylimidazole ligand, and (ii) the generated nucleophile
attacked the imine carbon atom to form a CC bond,
resulting in the formation of a nitrogen-donor tridentate
ligand.22 The solid state structures were determined by X-ray
diffraction (Figure 4) and are in agreement with the structures
proposed from the spectroscopic data in solution.23,24
A tridentate N-donor ligand, constituted by imidazolyl,
pyridyl, and amine arms, is bonded in a facial disposition to the
{Mo(η3-methallyl)(CO)2} fragment. The disposition of this
ligand is different from that found in the aryl derivatives 4a−c,
since the amine function is trans to the allyl group. The fact that
substituents iso-propyl and tert-butyl are notably bulkier than
the aryl derivatives can explain this different orientation of the
tripodal ligand.
To confirm that the C−C coupling reaction described in the
present work proceeds through an intramolecular nucleophilic
attack, a crossover experiment was conducted employing two
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Figure 6. CPCM-B3LYP-D3/6-311++G(d,p) (LANL2DZ + f for Mo)//CPCM-B3LYP/6-31+G(d) (LANL2DZ + f for Mo) Gibbs energy profiles
in THF solution of the mechanisms located for the formation of C−C coupling products starting from I. Relative Gibbs energies in THF solution (in
kcal/mol) and NBO charges of the numbered atoms are given in parentheses and in square brackets, respectively. Most relevant distances are also
included in angstroms.
complexes with different imines (iso-propyl vs para-tolyl) and
different N-alkylimidazole ligands (methyl vs ethyl). The results
showed that the only products obtained were those that
resulted from an intramolecular reaction, with the cross-
coupled products not being observed.11
molybdenum center in I is analogous to that found in 2c.
Nonetheless, we also found an isomer of I (Ii in the Supporting
Information) that differs from 2c in having the central C atom
of the N-MeIm (namely Cimidazole) pointing approximately
toward the bisector of the angle defined by the two Mo−CO
bonds, instead of toward the chelate ligand, as in I and 2c. Ii is
only 1.3 kcal/mol in Gibbs energy in THF solution more stable
than I, and no TS was found for such an isomerization. As a
consequence, hereafter, unless otherwise stated, Gibbs energies
in THF solution will be referred to I.
First, we evaluated the generation of an imidazol-2-yl species.
As previously found for related complexes,7 this compound can
be generated through two ways (see Figure 5). I can directly
transform into the imidazol-2-yl product II (−15.3 kcal/mol)
via the TS TSIIa (12.7 kcal/mol) in which Cimidazole and the
initially coordinated nitrogen atom interact simultaneously with
the molybdenum center. Alternatively, II can be achieved
through the attack of Cimidazole onto each of the carbonyl ligands
via a two-step pathway in both cases. The attack on the
carbonyl ligand trans to the pyridyl group of the chelate ligand
is found to be more favorable than the other; therefore, we only
describe here that one for brevity (see the Supporting
Information for the other carbonyl attack). The first step of
this mechanism involves an energy jump of 9.1 kcal/mol
controlled by the TS TSIIb11 to afford the intermediate IIb1
(−1.3 kcal/mol). In the second step, IIb1 evolves into II via the
TS TSIIb12 (7.5 kcal/mol). As in related Mo and Re
complexes,7 the two-step mechanism is the one most favorable.
Next, we analyzed the formation of different C−C coupling
species between Cimidazole and the pyridylimine ligand (see
Figure 6). The attack of Cimidazole on the two ortho carbon atoms
of the pyridine ring and the Cimine atom were studied (C1, C2,
DFT Computations. The deprotonation of [Mo(η3-C4H7)-
(bipy)(CO)2(N-RIm)]OTf (R = Me, Mes) compounds leads
to the formation of imidazol-2-yl species via the attack of the
imidazole deprotonated carbon atom (Cimidazole) on a cis-CO
ligand.7 In contrast, the deprotonation of [Mo(η3-C4H7)-
(CO)2(N-MeIm)(py-2-CHN-R′)]OTf (R′ = C6H5 (2a),
C6H4-4-Me (2b), C6H3-3,5-Me2 (2c), iPr (2d), tBu (2e))
affords CC coupling species. Furthermore, these products
differ from the CC coupling species obtained in the
deprotonation of [Re(CO)3(bipy)(N-RIm)]+ (R = Me, Mes)
complexes.7 In the rhenium compounds, the CC coupling
takes place between Cimidazole and an ortho carbon atom of the
bipy ligand, while in the present reaction it occurs between
Cimidazole and the imine carbon atom (Cimine) of the py-2-CH
N-R′ ligand. Aiming at understanding the reasons for the
difference, we investigated the deprotonation of the [Mo(η3-
C4H7)(CO)2(N-MeIm)(py-2-CHN-R′)]OTf complexes at
the CPCM-B3LYP-D3/6-311++G(d,p) (LANL2DZ + f for
Mo)//CPCM-B3LYP/6-31+G(d) (LANL2DZ + f for Mo)
level of theory (see Computational Methods for details).
Specifically, we first studied in detail the reaction mechanism
for the Mo complex with R′ = C6H3-3,5-Me2 (2c in Scheme 4).
Next, we analyzed the influence of replacing this aryl group by
tBu (2e in Scheme 4) on the nature of the product.
As in previous studies,6c,d,7 the deprotonated form of 2c has
been taken as the starting critical structure (I in Figures 5 and
6). The relative orientation of the ligands around the
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Figure 7. CPCM-B3LYP/6-31+G(d) (LANL2DZ + f for Mo) optimized structures of the different Cimidazole−Cimine coupling conformers formed for
the C6H3-3,5-Me2 derivative and the TS connecting them. CPCM-B3LYP-D3/6-311++G(d,p) (LANL2DZ + f for Mo)//CPCM-B3LYP/6-
31+G(d) (LANL2DZ + f for Mo) Gibbs energies in THF solution referred to Va (in kcal/mol) are given in parentheses. Most relevant distances are
also included in angstroms.
and C3, respectively in Figure 6). The formation of C−C
coupling products between Cimidazole and the ipso carbon atom
and the ortho carbon atoms of the aryl ring (C5, C4, and C6,
respectively, in Figure 6) was also considered, but as expected,
they are notably less stable than those mentioned above (see
the Supporting Information). As shown in Figure 6, the
formation of the Cimidazole−C1 and Cimidazole−C2 coupling
products (III, −6.8 kcal/mol, and IV, −2.0 kcal/mol,
respectively) requires the surmounting of energy barriers of
11.8 (TSIII) and 8.9 (TSIV) kcal/mol, respectively. An NBO
analysis of I reveals that C2 (0.17 e) has a larger positive atomic
charge than C1 (0.06 e), which makes it more susceptible to a
nucleophilic attack, and, consequently, TSIV is lower in energy
than TSIII. The attack on the Cimine atom leads to the
formation of a very stable Cimidazole−Cimine coupling species (Va
in Figure 6, −26.4 kcal/mol). Unlike in the generation of III
and IV, no TS was found for the formation of Va after an
extensive search. This can be ascribed to the high reactivity of
Cimine due to its electrophilic character (atomic charge of 0.16 e
at I) combined with its ability to delocalize electron density
toward the originally imine nitrogen atom, which in the C−C
coupling product is part of an amido ligand.
Taking into account the previous discussion, we conclude
that the generation of an imidazol-2-yl product (II) and of the
Cimidazole−C1 (III) and Cimidazole−C2 (IV) coupling products is
kinetically accessible as they show energy barriers of 9.1, 11.8,
and 8.9 kcal/mol, respectively. However, the evolution of I to
the Cimidazole−Cimine coupling product (Va) is even much more
favorable due to the nonexistence of a kinetic barrier for such a
molecular rearrangement, along with the fact that Va is a much
more stable product. The transformation of I into Va involves
the formation of a new C−C bond without any bond breaking,
while the original Mo−Nimidazole bond is replaced by the Mo−
Cimidazole bond for the process from I to II. This can explain the
higher stability of Va compared to II. In addition, the
generation of the Cimidazole−Cimine bond at Va does not require
dearomatization of the pyridine ring, in contrast with what
occurs when going from I to III or IV.
Despite the high stability of Va (−26.4 kcal/mol), this
species can evolve into two isomers that are also very stable, Vb
(−30.2 kcal/mol) and Vc (−26.3 kcal/mol) (see Figure 7).
These three isomers present a pseudo-octahedral geometry.
The two carbonyl ligands and the methallyl group occupy one
triangular face of the pseudo-octahedron (hereafter denoted as
A), while the other (henceforth referred as B) is determined by
the tridentate N-donor ligand, consisting of imidazolyl, pyridyl,
and amido arms (see for instance Va Figure 7). As in 2c and I,
Va has the imidazole group trans to the methallyl group, so the
amido and pyridyl groups are both trans to the carbonyl ligands.
In Vb and Vc the methallyl ligand is trans to the amido and
pyridyl group, respectively, and hence, the N-donor groups are
trans to the carbonyls. The interconversion between Va, Vb,
and Vc could proceed via a trigonal-twist mechanism.15 Starting
from Va, a counterclockwise rotation of about 120° of B over A
would lead to Vb, which in turn become Vc via another similar
rearrangement. Alternatively, the clockwise rotation of B over A
would directly transform Va into Vc. However, as shown in
Figure 7, all the TSs (TSVa-b, TSVb-c, and TSVa-c) for the
rearrangement of the tridentate ligand (Va → Vb, Vb → Vc,
and Va → Vc, respectively) are pentacoordinated owing to the
cleavage of the Mo−pyridyl bond. Accordingly, an NBO
analysis of Va confirms that the Mo−Npyridyl bond is the
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Inorganic Chemistry
Article
weakest Mo−N interaction between Mo and the tridentate
ligand. Specifically, the Mo−Npyridyl bond has a bond order of
0.3857 while values of 0.5169 and 0.5649 were found for the
Mo−Nimidazole and Mo−Nimide bonds, respectively. The breaking
of one of the Mo−N bonds seems to be needed in order to
favor the interconversion of the conformers containing the
tridentate ligand rearrangement when bulky ligands are present
in Mo complexes.25 This could change in the absence of bulky
ligands as reported in a theoretical study on octahedral
vanadium complexes wherein no bond breaking was found at
the TS located for trigonal twist.26 Theoretical studies on this
trigonal twist in molybdenum octahedral center are scarce,25a
although it is common to attribute the dynamics deduced from
variable-temperature NMR experiments to such processes.27
Concerning the energetics implied in the transformations Va
→ Vb → Vc, our results reveal Gibbs energy barriers of 9.6,
20.2, and 22.4 kcal/mol (measured from Va, Vb, and Va,
respectively) for the interconversions Va → Vb, Vb → Vc, and
Va → Vc, respectively. The highest energy barriers obtained are
similar to those reported (∼22 kcal/mol) for a dissociative
trigonal twist mechanism of other six-coordinate Mo complex-
es.25a Therefore, we conclude that Va, Vb, and Vc are
continuously interconverting, and consequently, the complex
should show a fluxional behavior. Vb is the most stable
conformer in THF solution, but is only 3.8 and 3.9 kcal/mol
more stable than Va and Vc, respectively. The protonation of
these three stable conformers leads to the appearance of two
diastereomers per deprotonated species, identified as VxH with
x = a, b, c. As seen in Table 1, a diastereomer from Vc, (RS)-
presents a Gibbs energy barrier still higher (28.5 kcal/mol,
TSV′a-c). All these energy barriers are higher than those for the
arylimino derivative, with differences in the 0.7−6.1 kcal/mol
range. This can be mainly ascribed to the steric effect of the tBu
group. The results obtained for the tBu-imino complex also
suggest a fluxional dynamics between V′a and V′b, and between
V′b and V′c, but not directly between V′a and V′c. Besides, the
significant stability of V′b compared to V′a and V′c agrees with
the finding of a solid state structure of a protonated complex
(4e) with the tBu-substituted nitrogen atom trans to the
methallyl ligand (see Figure 4b). Accordingly, the analysis of
the relative stability of the diastereomers derived from the
protonation of V′a, V′b, and V′c confirms that the protonated
conformers of V′b, (RR)-V′bH and (RS)-V′bH, are the most
stable (see Table 1).
CONCLUSIONS
■
The reaction of the new cationic complexes of [Mo(η3-
C4H7)(CO)2(N-MeIm)(py-2-CHN-R)]OTf (R= C6H5,
C6H4-4-Me, C6H3-3,5-Me2, iPr, tBu) with the equimolar
amount of the strong base KN(SiMe3)2 at low temperature
leads to the deprotonation of the central CH group of the N-
methylimidazole ligand. The neutral species so formed are very
reactive and immediately afford the C−C coupling products
that result from the nucleophilic attack of the deprotonated
imidazole C2 atom onto the imine CH moiety. The resulting
N-donor tridentate ligand occupies one face of the pseudo-
octahedral geometry of the complex, and its disposition
depends on the nature of the substituent of the amine N
atom. Whereas for aryl derivatives the pyridyl group is trans to
the methallyl moiety, for the alkyl derivatives it is the amine
function that occupies this position, probably in order to
minimize steric congestion.
Table 1. Relative Gibbs Energy, in kcal/mol, of the Different
Diastereomers of the Protonated Cimidazole−Cimine Coupling
Products Obtained from the Deprotonation of 2c and 2e at
the CPCM-B3LYP-D3/6-311++G(d,p) (LANL2DZ + f for
Mo)//CPCM-B3LYP/6-31+G(d) (LANL2DZ + f for Mo)
For this family of Mo(II) pyridylimino compounds the
formation of the C−C coupling products is preferred over the
formation of the imidazol-2-yl species, that had been previously
found in the related 2,2′-bipyridine complexes [Mo(η3-
C4H7)(bipy)(CO)2(N-RIm]OTf (R= Me, Mes). DFT calcu-
lations have shown, in agreement with the experimental results,
that the formation of the C−C coupling product is
undoubtedly the most favorable way of evolution of the
deprotonated species, both kinetically (no TS was found for
such molecular rearrangement) and thermodynamically (Va is
16.9 kcal/mol more stable than the 2-imidazolyl structure II).
In that respect, the electrophilicity of the imine C atom along
with its ability to delocalize electron density toward nitrogen
are crucial.
a
Level of Theory
conformation x = a conformation x = b conformation x = c
(RR)-VxH
(RS)-VxH
(RR)-V′xH
(RS)-V′xH
0.0
0.7
0.0
7.7
1.5
0.2
2.1
−0.2
5.5
−1.0
−0.7
−0.4
a
The most stable diastereomer in conformation a is taken as a
reference in both cases.
VcH, becomes slightly the most stable species. This isomer is
also the one found in the solid state structure of 4c, in which
the pyridine ligand is trans to the methallyl group, and of the
analogous complexes 4a and 4b.
EXPERIMENTAL SECTION
On the basis of the results discussed above, it is also possible
to rationalize the product obtained for the deprotonation of the
tBu-imine Mo complex (2e in Scheme 4). To that end, we
focused our attention on the interconversion among the
conformers analogous to Va, Vb, and Vc wherein the aryl group
is replaced by tBu (denoted as V′a, V′b, and V′c, respectively).
As seen in Supporting Information Figure S3, the TSs for the
transformations V′a → V′b, V′b → V′c, and V′a → V′c
(TSV′a-b, TSV′b-c, and TS′Va-c, respectively) are analogous
to those found for the aryl complexes. Concerning the
energetics, V′a becomes V′b via a Gibbs energy barrier of
10.3 kcal/mol (TSV′a-b). A value of 21.5 kcal/mol (TSV′b-c)
was found for the transformation V′b → V′c because V′b is 7.0
kcal/mol more stable than V′a. The interconversion V′a → V′c
■
General. All manipulations were carried out under a nitrogen
atmosphere using Schlenk techniques. Solvents were distilled from Na
(toluene and hexanes), Na/benzophenone (THF), and CaH2
(CH2Cl2). Compound [MoCl(η3-C4H7)(CO)2(NCMe)2]28 was pre-
pared as previously reported. Deuterated dichloromethane (Cam-
bridge Isotope Laboratories, Inc.) was stored under nitrogen in a
Young tube and used without further purification. H NMR and 13C
1
NMR spectra were recorded on a Bruker Avance 300 or DPX-300
spectrometer. NMR spectra are referred to the internal residual solvent
peak for 1H and 13C{1H} NMR. IR solution spectra were obtained in a
PerkinElmer FT 1720-X spectrometer using 0.2 mm CaF2 cells. NMR
samples were prepared under nitrogen using Kontes manifolds
purchased from Aldrich. Full experimental details of all compounds
are given in the Supporting Information, whereas herein only a set of
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Article
complexes displaying each type of ligand (alkyl or aryl derivatives) is
included as representative examples.
(CH3 η3-C4H7). Anal. Calcd for C19H19ClMoN2O2: C 52.01, H 4.36,
N 6.38. Found: C 51.98, H 4.32, N 6.18.
Synthesis of [MoCl(η3-C4H7)(CO)2(py-2-CHN-tBu)] (1e). Com-
pound 1e was prepared as described above for compound 1b, starting
from [MoCl(η3-C4H7)(CO)2(NCMe)2] (100 mg, 0.308 mmol),
pyridine-2-carboxaldehyde (29 μL, 0.308 mmol), and tertbutylamine
(32 μL, 0.308 mmol). Compound 1e was obtained as a dark blue solid.
Yield: 161 mg (87%). IR (CH2Cl2, cm−1): 1949, 1864 (νCO). 1H NMR
(CD2Cl2): δ 8.76 (m, 1H, py), 8.54 (s, 1H, NCH), 8.01 (m, 1H,
py), 7.81 (m, 1H, py), 7.54 (m, 1H, py), 3.13 (m, 1H, Hsyn η3-C4H7),
2.77 (sbr, 1H, Hsyn η3-C4H7), 1.59 (s, 9H, tBu), 1.34 (s, 3H, CH3 η3-
C4H7), 1.31 (s, 1H, Hanti η3-C4H7), 1.27 (s, 1H, Hanti η3-C4H7).
13C{1H} NMR (CD2Cl2): δ 228.3, 227.1 (CO), 163.9 (NCH),
Crystal Structure Determination. General Description. For
Compounds 1c, 4a, 4b, and 4e. Crystal data were collected on a
Bruker APPEX II diffractometer using graphite-monochromated Mο
Kα radiation (λ = 0.710 73 Å) from a fine-focus sealed tube source at
100 K. Computing data and reduction were made with the APPEX II
software.29 In all cases empirical absorption corrections were applied
using SADABS.30 For compound 4e: data collection was performed at
150 K on an Oxford Diffraction Xcalibur Nova single crystal
diffractometer, using Cu Kα radiation (λ= 1.5418 Å). Images were
collected at a 65 mm fixed crystal-detector distance, using the
oscillation method, with 1° oscillation and variable exposure time per
image (4−16 s). Data collection strategy was calculated with the
program CrysAlisPro CCD.31 Data reduction and cell refinement was
performed with the program CrysAlisPro RED.31 An empirical
absorption correction was applied using the SCALE3 ABSPACK.31
In all cases the structures were solved using SIR9232 and finally refined
by full-matrix, least-squares based on F2 by SHELXL.33 Molecular
graphics were made with ORTEP 3.34
Computational Methods. Quantum chemical computations were
carried out with the Gaussian 09 series of programs.35 Full geometry
optimizations of stable species and TS were performed in THF
solution from the outset with the Conductor-like Polarizable
Continuum Model (CPCM)36 and the Universal Force Field (UFF)
radii37 in conjunction with the hybrid density functional B3LYP38 and
the 6-31+G(d)39 basis set for nonmetal atoms together with the
LANL2DZ40 with f polarization functions41 of exponent 1.043 for Mo
and by using the standard Schlegel’s algorithm.42 A relative
permittivity of 7.58 was assumed in the calculations to simulate
THF as the solvent experimentally used for all the cases. To further
refine the quality of the above-mentioned energies, we also performed
single-point CPCM-B3LYP/6-311++G(d,p)39 (LANL2DZ + f for
Mo) and B3LYP-D343 dispersion calculations on the CPCM-B3LYP/
6-31+G(d) geometries. For the latter computations, we used the
Becke−Johnson damping function44 to avoid near singularities for
small interatomic distances. The nature of the stationary points was
verified by analytical computations of harmonic vibrational frequen-
cies. Intrinsic reaction coordinate (IRC) calculations with the
Gonzalez and Schlegel method45 were carried out to check the two
minimum energy structures connecting each TS except for the species
involved in the trigonal twist mechanism. The complexity of these
molecular rearrangements prevented the use of the IRC algorithm; we
instead analyzed the transition vector to verify the TS connectivity.
Besides this, we also checked how the TS was finally reached from an
initial geometry wherein the methallyl ligand is in front of the pyridyl
ligand, the imidazole ligand, or the nitrogen atom bearing the Ar or
tBu group. Thermodynamic magnitudes (ΔH, ΔS, and ΔG) were also
calculated within the ideal gas, rigid rotor, and harmonic oscillator
approximations at a pressure of 1 atm and a temperature of 195.15
K.46 For interpretation purposes, a natural bond orbital (NBO)
analysis was also performed.47
154.1, 151.6, 138.6, 128.8, 127.2 (py), 83.4 (C2 η3-C4H7), 70.8 (tBu),
64.0 (C1 or C3 η3-C4H7), 32.5 (CH3 tBu), 19.1 (CH3 η3-C4H7). The
signal of C1 or C3 of the methallyl ligand is overlapped with the solvent
residual. Anal. Calcd for C16H21ClMoN2O2: C 47.48, H 5.23, N 6.92.
Found: C 47.49, H 4.99, N 6.83.
Synthesis of [Mo(η3-C4H7)(CO)2(N-MeIm)(py-2-CHNC6H4-4-
Me)]OTf (2b). To a solution of [MoCl(η3-C4H7)(CO)2(py-2-CH
NC6H4-4-Me)] (1b) (50 mg, 0.114 mmol) in CH2Cl2 (20 mL)
were added AgOTf (35 mg, 0.136 mmol) and MeCN (3 mL), and the
mixture was stirred in the dark for 1 h. The resulting slurry was filtered
off the white solid (AgCl), and the solvent was evaporated to dryness.
The dark red residue was redissolved in CH2Cl2 (20 mL), N-MeIm
(10 μL, 0.125 mmol) was added, and the reaction mixture was allowed
to stir for 1 h. The solvent was concentrated under reduced pressure to
a volume of 7−10 mL, and addition of hexane (15 mL) caused the
precipitation of a dark red solid, which was washed with hexane (2 ×
20 mL) and dried under vacuum. Yield: 63 mg (87%). IR (CH2Cl2,
1
cm−1): 1951, 1869 (νCO). H NMR (CD2Cl2): δ 9.05 (s, 1H, N
CH), 8.77 (m, 1H, py), 8.41 (m, 1H, py), 8.16 (m, 1H, py), 7.61 (m,
1H, py), 7.43 (s, 1H, NCHN), 7.31 (d (J = 8.4), 2H, C6H4), 7.25 (d (J
= 8.4), 2H, C6H4), 7.10 (sbr, 1H, CH N-MeIm), 6.97 (sbr, 1H, CH N-
MeIm), 3.65 (s, 3H, CH3 N-MeIm), 3.05 (m, 1H, Hsyn η3-C4H7), 2.59
(m, 1H, Hsyn η3-C4H7), 2.42 (s, CH3 C6H4-4-Me), 1.59 (s, 1H, Hanti η3-
C4H7), 1.34 (sbr, 4H, CH3 and Hanti η3-C4H7). 13C{1H} NMR
(CD2Cl2): δ 226.0, 225.8 (CO), 166.1 (NCH), 153.8, 152.4, 147.3,
141.1, 140.6, 139.9, 132.0, 130.9, 128.9, 122.9, 122.7 (py-2-CHN
C6H4-4-Me and N-MeIm), 86.1 (C2 η3-C4H7), 58.1, 56.3 (C1 and C3
η3-C4H7), 35.2 (CH3 N-MeIm), 21.4 (CH3 C6H4-4-Me), 18.9 (CH3
η3-C4H7). Anal. Calcd for C24H25F3MoN4O5S: C 45.43, H 3.97, N
8.83. Found: C 45.91, H 4.06, N 8.96.
Synthesis of [Mo(η3-C4H7)(CO)2(N-MeIm)(py-2-CHN-tBu)]OTf
(2e). Compound 2e was prepared following the procedure described
for the synthesis of 2b, starting from [MoCl(η3-C4H7)(CO)2(py-2-
CHN-tBu)] (1e) (50 mg, 0.124 mmol), AgOTf (35 mg, 0.136
mmol), and N-MeIm (10 μL, 0.125 mmol). Compound 2e was
obtained as a dark red microcrystalline solid. Yield: 64 mg (87%). IR
(CH2Cl2, cm−1): 1947, 1864 (νCO). 1H NMR (CD2Cl2): δ 8.97 (s, 1H,
NCH), 8.67 (m, 1H, py), 8.38 (m, 1H, py), 8.22 (m, 1H, py), 7.64
(m, 1H, py), 7.22 (s, 1H, NCHN), 6.98 (sbr, 1H, CH N-MeIm), 6.91
(sbr, 1H, CH N-MeIm), 3.68 (s, 3H, CH3 N-MeIm), 3.42 (m, 1H, Hsyn
η3-C4H7), 2.94 (m, 1H, Hsyn η3-C4H7), 1.62 (s, 1H, Hanti η3-C4H7),
1.42 (s, 1H, Hanti η3-C4H7), 1.38 (s, 9H, CH3 tBu), 1.23 (s, 3H, CH3
η3-C4H7). 13C{1H} NMR (CD2Cl2): δ 227.1, 226.2 (CO), 166.8 (N
CH), 154.5, 151.7, 140.6, 140.3, 131.8, 131.1, 128.6, 122.4 (py-2-
CHN-tBu and N-MeIm), 86.5 (C2 η3-C4H7), 64.9 (tBu), 58.7, 56.2
(C1 and C3 η3-C4H7), 34.9 (CH3 N-MeIm), 31.9 (CH3 tBu), 18.6
(CH3 η3-C4H7). Anal. Calcd for C21H27F3MoN4O5S: C 42.00, H 4.53,
N 9.33. Found: C 41.81, H 4.87, N 9.02.
Synthesis of [MoCl(η3-C4H7)(CO)2(py-2-CHN−C6H4-4-Me)] (1b).
Pyridine-2-carboxaldehyde (29 μL, 0.308 mmol) and p-toluidine
(0.033 g, 0.308 mmol) were added to a solution of [MoCl(η3-
C4H7)(CO)2(NCMe)2] (0.100 g, 0.308 mmol) in THF (20 mL), and
the reaction mixture was stirred for 30 min. The solvent was
evaporated under reduced pressure to dryness, and the resulting violet
powder was washed with hexane (2 × 20 mL) and dried under
vacuum. Compound 1b was obtained as a violet powder. Yield: 105
mg (78%). IR (CH2Cl2, cm−1): 1951, 1875 (νCO). 1H NMR
(CD2Cl2): δ 8.79 (m, 1H, py), 8.42 (s, 1H, NCH), 7.99 (m, 1H,
py), 7.87 (m, 1H, py), 7.54 (m, 1H, py), 7.38 (d (J = 8.2), 2H, C6H4),
7.29 (d (J = 8.2), 2H, C6H4), 2.78 (m, 1H, Hsyn η3-C4H7), 2.41 (s, 3H,
CH3 C6H4-4-Me), 2.30 (m, 1H, Hsyn η3-C4H7), 1.35 (sbr, 4H, CH3 and
Reaction of [Mo(η3-C4H7)(CO)2(N-MeIm)(py-2-CHNC6H4-4-
Me)]OTf (2b) with KN(SiMe3)2. Synthesis of 3b. KN(SiMe3)2 (0.15
mL of a 0.5 M solution in toluene, 0.076 mmol) was added to a
solution of compound 2b (40 mg, 0.063 mmol) in THF (15 mL),
previously cooled to −78 °C. The color of the solution changed
immediately from dark red to orange, and the solvent was evaporated
to dryness under reduced pressure. The residue was extracted with
CH2Cl2 (20 mL) and filtered via canula, and the resulting solution was
concentrated to a volume of 5 mL. Addition of hexane (15 mL) caused
H
anti η3-C4H7), 1.11 (s, 1H, Hanti η3-C4H7). 13C{1H} NMR (CD2Cl2):
δ 227.1, 226.8 (CO), 162.5 (NCH), 153.6, 152.3, 148.7, 139.9,
138.8, 130.4, 129.3, 127.5, 122.4 (py-2-CHNC6H5), 83.8 (C2 η3-
C4H7), 54.2, 52.7 (C1 and C3 η3-C4H7), 23.2 (CH3 C6H4-4-Me), 19.3
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Article
the precipitation of a dark yellow solid that was washed with hexane (2
× 15 mL). Yield: 19 mg (63%). IR (CH2Cl2, cm−1): 1929, 1833 (νCO).
1H NMR (CD2Cl2): δ 8.57 (m, 1H, py), 7.77 (m, 1H, py), 7.53 (m,
1H, py), 7.17 (m, 1H, py), 6.95 (m, 5H, C6H4 and CH N-MeIm), 6.71
(sbr, 1H, CH N-MeIm), 5.72 (s, 1H, Csp3H), 3.71 (s, 3H, CH3 N-
MeIm), 3.31 (m, 2H, Hsyn η3-C4H7), 2.20 (s, CH3 C6H4-4-Me), 1.90
(s, 3H, CH3 η3-C4H7) 1.24 (s, 1H, Hanti η3-C4H7), 1.08 (s, 1H, Hanti η3-
C4H7). Anal. Calcd for C23H24MoN4O2: C 57.03, H 4.99, N 11.57.
Found: C 57.26, H 4.66, N 2.13.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jdal@unex.es.
*E-mail: rlopez@uniovi.es.
*E-mail: l.riera@cinn.es.
Notes
The authors declare no competing financial interest.
Reaction of 3b with HOTf. Synthesis of 4b. HOTf (8 μL, 0.091
mmol) was added to a solution of compound 3b (0.040 g, 0.083
mmol) in CH2Cl2, and the reaction mixture was stirred for 15 min at
room temperature. The solvent was evaporated to a volume of 5 mL,
and addition of hexane caused the precipitation of a red solid which
was washed with hexane (2 × 15 mL). Slow diffusion of hexane (15
mL) into a concentrated solution of 4b in CH2Cl2 (5 mL) afforded red
crystals, one of which was employed for an X-ray structure
determination. Yield: 36 mg (69%). IR (KBr, cm−1): 3456 (νNH),
ACKNOWLEDGMENTS
■
́
Financial support from the Ministerio de Economia y
Competitividad (Projects CTQ2010-18231, CTQ2012-37370-
C02-01, and CTQ2012-37370-C02-02) is gratefully acknowl-
edged. J.D. thanks the Fundacion Computacion y Tecnologias
Avanzadas de Extremadura (COMPUTAEX) for computing
resources.
́
́
́
1
1947, 1858 (νCO). IR (CH2Cl2, cm−1): 1949, 1860 (νCO). H NMR
REFERENCES
■
(CD2Cl2): δ 9.66 (m, 1H, py), 8.03 (m, 1H, py), 7.71 (m, 1H, py),
7.57 (m, 1H, py), 7.20 (s, 1H, CH N-MeIm), 7.05 (d (J = 8.2), 2H,
C6H4), 6.93 (s, 1H, CH N-MeIm), 6.87 (d (J = 8.2), 2H, C6H4), 5.64
(s, 1H, Csp3H), 3.81 (s, 3H, CH3 N-MeIm), 3.53 (m, 1H, Hsyn η3-
C4H7), 2.71 (m, 1H, Hsyn η3-C4H7), 2.31 (s, CH3 C6H4-4-Me), 1.98 (s,
3H, CH3 η3-C4H7), 1.52 (s, 1H, Hanti η3-C4H7), 1.31 (s, 1H, Hanti η3-
C4H7). 13C{1H} NMR (CD2Cl2): δ 227.7, 226.9 (CO), 156.7, 146.9,
142.1, 141.8, 137.7, 130.6, 130.3, 127.8, 126.2, 124.2, 123.4, 122.7 (py-
2-CHNC6H4-4-Me and N-MeIm), 85.5 (C2 η3-C4H7), 64.5
(Csp3), 58.0 (C1 or C3 η3-C4H7), 34.8 (CH3 N-MeIm), 20.8, 20.6
(CH3 C6H4-4-Me and η3-C4H7). The signal of C1 or C3 of the
methallyl ligand is overlapped with the solvent residual peak. Anal.
Calcd for C24H25F3MoN4O5S: C 45.43, H 3.97, N 8.83. Found: C
45.76, H 4.10, N 9.05.
(1) This subject has been the topic of several reviews, see for
́
́
̈
example: (a) Sov
́
AGO, I.; Var
́
nagy, K.; Osz, K. Comments Inorg. Chem.
2002, 23, 149−178. (b) Gaggelli, E.; Kozlowsky, H.; Valensin, D.;
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(5) As far as we know, prior to our work there are just a couple of
examples of this type of reaction: (a) Muller, J.; Stock, R. Angew.
Reaction of [Mo(η3-C4H7)(CO)2(N-MeIm)(py-2-CHN-tBu](2e)
with KN(SiMe3)2 Followed by Addition of HOTf. Synthesis of 4e.
KN(SiMe3)2 (0.200 mL of a 0.5 M solution in toluene, 0.100 mmol)
was added to a solution of compound 2e (50 mg, 0.083 mmol) in
THF (20 mL) cooled to −78 °C. The reaction mixture was allowed to
reach room temperature and stirred for 40 min. The solvent was
evaporated to dryness, the residue was extracted with CH2Cl2 (20
mL), and HOTf (9 μL, 0.101 mmol) was added. After 15 min stirring
at room temperature the reaction mixture was filtered via canula, the
solvent evaporated under reduced pressure, and the resulting yellow
solid was washed with hexane (2 × 20 mL) and dried under vacuum.
Yield: 31 mg (62%). IR (KBr, cm−1): 3451 (νNH), 1945, 1856 (νCO).
IR (CH2Cl2, cm−1): 1950, 1861 (νCO). 1H NMR (CD2Cl2): δ 9.07 (m,
1H, py), 7.97 (m, 1H, py), 7.81 (m, 1H, py), 7.50 (m, 1H, py), 7.16 (s,
1H, CH N-MeIm), 6.90 (s, 1H, CH N-MeIm), 5.71 (s, 1H, Csp3H),
4.56 (sbr, 1H, NH), 3.83 (s, 3H, CH3 N-MeIm), 3.50 (m, 1H, Hsyn η3-
C4H7), 3.45 (m, 1H, Hsyn η3-C4H7), 1.80 (s, 3H, CH3 η3-C4H7), 1.44
(sbr, 1H, Hanti η3-C4H7), 1.35 (sbr, 10H, tBu and Hanti η3-C4H7).
13C{1H} NMR (CD2Cl2): δ 222.7, 226.9 (CO), 156.0, 152.9, 146.9,
̈
Chem., Int. Ed. 1983, 22, 993−993. (b) Ruiz, J.; Perandones, B. F. J.
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Bonavia, G. J. Organomet. Chem. 1999, 589, 29−37. (b) Herrick, R. S.;
Wrona, I.; McMicken, N.; Jones, G.; Ziegler, C. J.; Shaw, J. J.
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141.7, 127.3, 125.5, 124.1, 123.4 (py-2-CHN-tBu), 83.1 (C2 η3-
C4H7), 60.0, 56.1 (C1 and C3 η3-C4H7), 57.7 (Csp3), 35.0 (CH3 N-
MeIm), 29.6 (CH3 tBu), 19.1 (CH3 η3-C4H7). Anal. Calcd for
C21H27F3MoN4O5S: C 42.00, H 4.53, N 9.33. Found: C 42.11, H 4.69,
N 8.97.
(10) (a) Alberto, R.; Wang, W.; Spingler, B. Inorg. Chim. Acta 2003,
355, 386−393. (b) Miguel, D.; Alvarez, C. M.; García-Rodríguez, R.
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(11) See Supporting Information for further experimental details.
(12) Selected crystallographic data for 2c: C25H27F3MoN4O5RS, M =
648.51, monoclinic, P21/c, a = 10.890(5) Å, b = 17.561(5) Å, c =
14.168(5) Å, α = 90°, β = 93.464(5)°, γ = 90°, 100.0(1) K, V =
2704(2) Å3, Z = 4. 5335 reflections measured, 5145 independent (Rint
= 0.0275). R1 = 0.0289, wR2 = 0.0762 (all data).
ASSOCIATED CONTENT
́
■
S
* Supporting Information
Full experimental details for all compounds, X-ray crystallo-
graphic data for compounds 2c and 4a, 4b, 4d, and 4e in CIF
format, discussion about the mechanisms not included in the
paper, and full DFT details. This material is available free of
charge via the Internet at http://pubs.acs.org
2589
DOI: 10.1021/ic502729z
Inorg. Chem. 2015, 54, 2580−2590

Inorganic Chemistry
Article
(13) Selected crystallographic data for 4a: C23H23F3MoN4O5S·
CH2Cl2, M = 705.38, monoclinic, P21/c, a = 8.519(5) Å, b = 11.186(5)
Å, c = 30.020(5) Å, α = 90°, β = 93.456(5)°, γ = 90°, 100.0(1) K, V =
2856(2) Å3, Z = 4. 7087 reflections measured, 6136 independent (Rint
= 0.0470). R1 = 0.0511, wR2 = 0.1306 (all data).
(14) In these complexes it is customary to describe the structures
considering that the η3-allyl ligand occupies one coordination site, see
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Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
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Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
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Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
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(16) It has to be noted that for the starting compound [Mo(η3-
C4H7)(CO)2(N-RIm)(py-2-CHN-Ph)]OTf (2a) the NMR dynam-
ic process is not observed in CD2Cl2 at low temperature (198 K). This
behavior is similar to that of related allyldicarbonyl molybdenum(II)
complexes with diimine chelate ligands, such as 2,2′-bipyridine or 1,10-
phenanthroline, see: Davis, R.; Kane-Maguire, L. A. P. Comprehensive
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon: Oxford, U.K., 1982; Vol. 8, pp 1156−1159.
(17) Destabilizing interactions between the amido N lone pair and
Mo filled d orbitals (a feature imposed by their saturated, 18-electron,
nature) are responsible of the high instability of these species. See:
́
Morales, D.; Perez, J.; Riera, L.; Riera, V.; Miguel, D.; Mosquera, M. E.
G.; García-Granda, S. Chem.Eur. J. 2003, 9, 4132−4143 and
references therein.
(18) Selected crystallographic data for 4b: C24H25F3MoN4O5S, M =
634.48, monoclinic, P21/n, a = 14.315(5) Å, b = 11.167(3) Å, c =
16.931(6) Å, α = 90°, β = 108.88(2)°, γ = 90°, 100.0(1) K, V =
2560(1) Å3, Z = 4. 5532 reflections measured, 5245 independent (Rint
= 0.0401). R1 = 0.0342, wR2 = 0.0808 (all data).
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(19) The molecular structure of the cation of compound 4c is
displayed in the Supporting Information.
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(21) For complexes 2d and 2e the stability of the neutral derivatives
resulting from the deprotonation reaction (3d and 3e, respectively)
will be even lower than in the aryl derivatives 3a−c, as the lone pair on
the amide type nitrogen would not have the possibility of
delocalization towards an aryl substituent.
(22) NOESY spectra of tert-buyl derivatives 2d and 4d showed that,
as evidenced for the arylimine complexes, the η3-allyl ligand is oriented
with the open face towards the carbonyls. See the Supporting
Information for further experimental details.
(23) Selected crystallographic data for 4d: C16H12F3N2O8ReS, M =
635.54, monoclinic, P21/c, a = 8.7249(1) Å, b = 15.1603(3) Å, c =
14.7401(3) Å, α = 90°, β = 91.822(2)°, γ = 90°, 100.0(1) K, V =
1948.74(6) Å3, Z = 4. 10 569 reflections measured, 3662 independent
(Rint = 0.0431). R1 = 0.0324, wR2 = 0.0889 (all data).
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(24) Selected crystallographic data for 4e: C21H27F3MoN4O5S·
CH2Cl2, M = 685.35, triclinic, P1, a = 7.953(3) Å, b = 12.934(5) Å, c =
14.373(5) Å, α = 76.750(3)°, β = 87.895(3)°, γ = 76.286(3)°, 152(4)
K, V = 1397(1) Å3, Z = 2. 9654 reflections measured, 5059
independent (Rint = 0.0224). R1 = 0.0535, wR2 = 0.01353 (all data).
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