Coupling of isocyanide and μ-aminocarbyne ligands in diiron complexes promoted by hydride addition

Article abstract of DOI:10.1021/om500502h

The diiron μ-aminocarbyne complexes [Fe₂{μ-CN(Me)(R)} (μ-CO)(CO)(CNR′)(Cp)₂][SO₃CF₃] (R = R′ = Xyl, 2a; R = Xyl, R′ = Me, 2b; R = Xyl, R′ = Bu ^t, 2c; R = Xyl, R′ = p-C₆H₄CF

Full text of DOI:10.1021/om500502h

Article
pubs.acs.org/Organometallics
Coupling of Isocyanide and μ‑Aminocarbyne Ligands in Diiron
Complexes Promoted by Hydride Addition
Fabio Marchetti,‡ Stefano Zacchini,† and Valerio Zanotti*,†
†Dipartimento di Chimica Industriale “Toso Montanari”, Universita
̀
di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
‡Dipartimento di Chimica e Chimica Industriale, Universita
̀
di Pisa, Via Risorgimento 35, I-56126 Pisa, Italy
S
* Supporting Information
ABSTRACT: The diiron μ-aminocarbyne complexes [Fe2{μ-CN(Me)(R)}(μ-
CO)(CO)(CNR′)(Cp)2][SO3CF3] (R = R′ = Xyl, 2a; R = Xyl, R′ = Me, 2b; R
= Xyl, R′ = But, 2c; R = Xyl, R′ = p-C6H4CF3, 2d; R = Me, R′ = Xyl, 2e; Xyl =
2,6-Me2C6H3), containing an isocyanide ligand, have been obtained via CO
replacement with the appropriate CNR′ ligand from [Fe2{μ-CN(Me)(R)}(μ-
CO)(CO)2(Cp)2][SO3CF3] (R = Xyl, 1a; R = Me, 1b). Compound 2a, upon
treatment with NaBH4 and heating at reflux temperature in THF solution, is
transformed into the aminocarbene−aldimine [Fe2{μ-η1(C):η1(N)-CN(Me)(Xyl)CHN(Xyl)}(μ-CO)2(Cp)2] (3) in
moderate yield. The reactions occurs via formation of the formimidoyl complex [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO){C-
(H)NXyl}(Cp)2] (4a), which has been isolated by reacting 2a with NaBH4 at 0 °C. Likewise, the formimidoyl complex
[Fe2{μ-CN(Me)(R)}(μ-CO)(CO){C(H)NR′}(Cp)2] (R = Xyl, R′ = Me, 4b; R = Me, R′ = Xyl, 4c) have been obtained from
2b,e, respectively, upon reaction with NaBH4, but these complexes do not convert into the aminocarbene−aldimine complexes
analogous to 3. Reactions of 2a with other nucleophiles have been investigated, without obtaining any product similar to 3.
Complex 2a reacts with NaCN or with LiPPh2, resulting in isocyanide displacement and formation of [Fe2{μ-CN(Me)(Xyl)}(μ-
CO)(CO)(CN)(Cp)2] (5) and [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO)(PPh2)(Cp)2] (6), respectively. Addition of the
organocopper reagent Li2CuCNMe2 to 2a affords the acyl complex [Fe2{μ-CN(Me)(Xyl)}(μ-CO){C(O)Me}(CNXyl)(Cp)2]
(7). The thiocarbyne complex [Fe2{μ-CSMe}(μ-CO)(CO)(CNXyl)(Cp)2][SO3CF3] (8), which shows analogies with 2a, reacts
with NaBH4, affording the carbene derivative [Fe2 μ-{C(SMe)(H)}(μ-CO)(CO)(CNXyl)(Cp)2] (9). The molecular structures
of 2a and 3 have been determined by X-ray diffraction studies.
can be regarded as models for the active site of [FeFe]ase
natural enzymes.9 We have previously published two different
approaches to C−C bond formation involving diiron complexes
and isocyanides. One method (Scheme 1a) consists of alkyne
insertion into the Fe−C bond of a bridging isocyanide.10 The
second example pertains to the addition of isocyanide to a
bridging vinyliminium, affording a ketenimine (Scheme 1b).11
In this case metal activation concerns the bridging organic
frame and isocyanide is used as a reagent rather than a ligand.
In light of these results, we decided to extend studies to
diiron complexes of the type [Fe2{μ-CN(R)Me}(μ-CO)(CO)-
(CNR′)(Cp)2]+ (R = Me, Xyl; Xyl = 2,6-Me2C6H3) containing
both isocyanide (as a terminally bound ligand) and activated
bridging ligands. In particular, we investigated μ-aminocarbyne
ligands in that they exhibit a remarkable electrophilic character
which, in theory, should be exploited to promote assembly with
terminally bound ligands. For example, we have previously
reported that the aminocarbyne diiron complexes [Fe2(μ-
CNMe2)(μ-CO)(CO)(L)(Cp)2][SO3CF3] (L = nitriles,12
imines13) undergo attack by acetylides (LiCCR) at the
unsaturated L ligands, which consequently rearrange and
undergo coupling with the bridging aminocarbyne. In these
INTRODUCTION
■
Isocyanides are extremely valuable building blocks for the
construction of complex molecular structures and, in particular,
of heterocycles. Ever since the Passerini and Ugi reactions were
discovered, multicomponent reactions (MCRs) based on
isocyanides have had a continuous development.1 In addition,
the past decade has witnessed a remarkable growth of interest
for transition-metal-promoted isocyanide insertion, providing
further possibilities for the use of isocyanides as C1 synthons.2
This field has been so far dominated by palladium,3 but it is
reasonable to assume that there is potential for expanding
metal-promoted isocyanide transformations. This is suggested
by the fact that isocyanides have long proved to be both
excellent ligands and reactive species in a variety of transition-
metal complexes.4
Our specific interest concerns diiron complexes and is aimed
at finding new reactions, based on Fe, potentially able to
replace, or provide alternatives to, the use of rare, expensive, or
toxic metals.5 Indeed, examples of isocyanide insertion into
Fe−C bonds are known,6 but catalytic applications are very
limited.7 On the other hand, diiron complexes with isocyanide
ligands have long been known, and these ligands, like carbonyls,
can assume both bridging and terminal coordination.8 More
recently, interest toward diiron isocyanide complexes has
resurged as a consequence of the fact that these compounds
Received: May 12, 2014
Published: July 24, 2014
© 2014 American Chemical Society
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Scheme 1. Examples of Isocyanide Incorporation into
Bridging Ligands
Figure 1. Molecular structure of 2a, with key atoms labeled (the
counteranion SO3CF3 and all H atoms have been omitted).
Displacement ellipsoids are at the 30% probability level.
−
examples, assembly of three components (aminocarbyne,
acetylide, and unsaturated N-containing ligands) gave rise to
the formation of a more complex molecular fragment, bridging
the two Fe atoms.12,13
Fe2(μ-C)2 plane, as previously found for analogous diiron and
diruthenium complexes. Moreover, the bulky Xyl group in the
bridging ligand μ-CN(Me)(Xyl) points toward the opposite
side of the terminal CNXyl ligand, in order to minimize steric
repulsions.
Here, we report on the results obtained with a similar
approach, aimed at assembling isocyanides and bridging ligands,
initiated by nucleophilic addition. The final target is to find out
new C−C bond forming reactions assisted by iron.
RESULT AND DISCUSSION
The IR spectra of 2a−e (in CH2Cl2 solution) exhibit the
usual ν(CO) pattern consisting of terminal and bridging
carbonyl absorptions (e.g., for 2b at 1989 and 1823 cm−1,
respectively). Complexes 2 should, in theory, display different
isomeric forms: cis,trans, due to the mutual position of the Cp
ligands with respect to the Fe−Fe bond, and E,Z, associated
with the different orientations that the N substituents Me and
Xyl (Xyl = 2,6Me2C6H3) can assume with respect to the
nonequivalent Fe atoms, as a consequence of the double-bond
character of the μ-C−N interaction. Usually, complexes of the
type [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO)(L)(Cp)2] (L =
halides, cyanides, alkyl, acyl, etc.) in chlorinated solvents
preferentially adopt the cis isomeric form with E configuration,
which is the same observed in the solid state for 2a.15,13 NMR
data show that the isomeric composition of the isocyanide
complexes 2a−d is consistent with that usually found in
analogous diiron complexes. In particular, 1H NMR spectra and
NOE experiments indicate that 2c,d are formed exclusively as
cis isomers with the isocyanide ligand and the N-methyl on the
same side (E configuration). Conversely, complexes 2a,b exist
■
As a first step, we synthesized a few diiron μ-aminocarbyne
complexes containing a terminally bound isocyanide ligand (see
Scheme 2, complexes 2a−e). To the best of our knowledge,
Scheme 2. CO Displacement by Isocyanides
in both E and Ζ forms, but the E isomer is more abundant. 13
C
complexes 2a−e have been not reported in the literature, in
spite of the fact that the analogous [Fe2(μ-CNMe2)(μ-
CO)(CO)(CNMe)(Cp)2]+ has been described by Manning
and co-workers several years ago.14 Therefore, their preparation
and properties are detailed in the Experimental Section.
Complexes 2a−e have been conveniently obtained from their
aminocarbyne precursors 1a upon replacement of CO with the
appropriate isocyanide, using Me3NO to favor CO displace-
ment (Scheme 2).
All of the compounds have been characterized by IR and
NMR spectroscopy and elemental analysis. The structure of 2a
has been ascertained by X-ray diffraction studies (Figure 1 and
Table 1). The Cp ligands adopt a cis geometry relative to the
NMR spectra of 2a−e show the typical low-field resonance due
to the aminocarbyne center (e.g., at δ 332.4 ppm for 2b) and
the signal due to the isocyanide carbon (at δ 116.3 ppm for 2b).
Having obtained and characterized the new complexes 2a−e,
we then investigated their reactions with nucleophiles, aimed at
transforming the isocyanide ligands into more reactive species,
potentially able to further rearrange and undergo coupling with
the bridging aminocarbyne. This strategy, which implies
multicomponent assembly of isocyanide, aminocarbyne, and
nucleophile, was successfully accomplished only in one case.
Indeed, treatment of 2a with an excess of NaBH4 (or LiHBEt3)
in THF solution at room temperature leads to the consumption
of 2a (monitored by IR spectroscopy) and the formation of
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 2a
Fe(1)−Fe(2)
Fe(2)−C(11)
Fe(1)−C(12)
Fe(2)−C(12)
Fe(1)−C(13)
Fe(2)−C(13)
Fe(1)−C(23)
Fe(1)−C(12)−Fe(2)
Fe(1)−C(13)−Fe(2)
2.5260(16)
1.754(10)
1.888(8)
1.981(8)
1.866(7)
1.884(7)
1.812(9)
81.5(3)
C(11)−O(11)
C(12)−O(12)
C(13)−N(1)
N(1)−C(14)
N(1)−C(15)
C(23)−N(2)
N(2)−C(24)
Fe(1)−C(23)−N(2)
C(23)−N(2)−C(24)
1.137(9)
1.157(9)
1.303(9)
1.487(10)
1.450(10)
1.162(9)
1.428(11)
176.2(7)
166.0(8)
84.7(3)
products which, upon heating in THF at reflux temperature for
about 20 h, afforded the complex 3 in moderate yield (Scheme
3).
CO)(CO){N(H)C(CCTol)CMe3}(Cp)2] which contains a
pure σ-Fe−N(imine) interaction (1.964(3) Å).13 The short-
ening of the Fe−N interaction in 3 can be explained by
assuming that a strong bond is formed due to extensive overlap
which develops between metal and nitrogen orbitals when the
nitrogen is in the delocalized electron system of the ligand.
Moreover, the bond can be further strengthened by back-
donation from iron to the antibonding π-orbital of the imine. In
agreement with this, the C(14)−N(2) interaction (1.326(8) Å)
is considerably elongated in comparison to a normal CN
bond. The strong π character of the Fe(1)−N(2) interaction is
also evidenced by the asymmetry of both μ-CO bridges
(Fe(1)−C(11), 1.912(7) Å; Fe(2)−C(11), 1.865(7) Å;
Fe(1)−C(12), 1.919(7) Å; Fe(2)−C(12), 1.888(6) Å), which
show shorter contacts to the more electron rich Fe(2) center.
Finally, the C(13)−C(14) interaction (1.422(8) Å) presents
some double-bond character, indicating delocalization between
the aminocarbene and the imino termini of the bridging ligand.
Scheme 3. Hydride Addition and Rearrangement
Complex 3 has been characterized by spectroscopy and X-ray
diffraction: the ORTEP molecular diagram is shown in Figure
2, whereas relevant bond angles and distances are reported in
1
The H NMR spectrum of 3 shows two sets of resonances,
which indicate the presence of the two isomeric forms. These
are associated with the presence of the aminocarbene ligand
and with the different orientation that the N substituents (Me
and Xyl) can adopt. The most relevant features in the NMR
spectra include the 1H NMR resonances attributable to the C−
H in the bridging C3 frame (at 6.79 and 5.72 ppm, for the two
isomers, respectively) and the 13C NMR resonances of the
aminocarbene and the iminium carbon (at about 230 and 160
ppm, respectively).
The formation of 3 exhibits some peculiar features that
deserve to be underlined. First, the reaction is a unique example
of coupling of isocyanide and aminocarbyne ligands, initiated
by hydride addition. It affords an aminocarbene ligand
containing an imidoyl group. Analogies can be envisaged with
reductive coupling of two isocyanide ligands in the
dimolybdenum complex [Mo2Cp2(μ-SMe)3(XylNC)2][BF4]
reported by Petillon, Schollhammer and co-workers.18 Also in
that case, a coupling reaction was initiated by a nucleophile (the
hydrosulfide anion) and occurred via C−C bond formation,
yielding the aminocarbene derivative [Mo2Cp2(μ-SMe)3{μ-
η1(C):η1(C)-C(NHXyl)C(NXyl)}]. However, both the ligand
and the coordination mode are different from those observed in
3.
A second observation concerns the reaction mechanism,
which presumably proceeds via hydride addition at the
isocyanide ligand to generate a formimidoyl intermediate.
Indeed, it has been relatively easy to isolate and characterize
such an intermediate by performing the reaction of 2a with
NaBH4 (or LiHBEt3) in THF solution at 0 °C. Compound 4a
(Scheme 4) has been purified by chromatography and fully
characterized by IR and NMR spectroscopy and elemental
analysis. Details are reported in the Experimental Section. The
Figure 2. Molecular structure of 3, with key atoms labeled (all H
atoms, except H(14), have been omitted). Displacement ellipsoids are
at the 30% probability level. Only the main image of the disordered Cp
ring bonded to Fe(1) is reported.
Table 2. The molecule contains the bridging aminocarbene−
aldimine ligand μ-η1:η1-CN(Me)(Xyl)C(H)N(Xyl) coordi-
nated to a Fe2(μ-CO)2(Cp)2 unit, with the Cp ligands in cis
positions with respect to the Fe2(μ-CO)2 plane. The Fe(2)−
C(13) distance (1.911(6) Å) is comparable with the values
found in analogous diiron complexes which contain a terminal
two-electron-donor aminocarbene ligand,16 even though the
C(13)−N(1) interaction (1.386(7) Å) shows some elongation;
nonetheless, N(1) presents the expected sp2 hybridization (sum
of angles 359.5(8)°), suggesting a π interaction between the
nitrogen and the carbene carbon. The Fe(1)−N(2) distance
(1.864(5) Å) is one of the shortest found for an iron−imine
nitrogen interaction17 and is, for instance, 0.1 Å shorter than
that in the diiron imino complex [Fe2{μ-C{N(Me)(Xyl)}(μ-
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 3
Fe(1)−Fe(2)
Fe(1)−C(11)
Fe(2)−C(11)
Fe(1)−C(12)
Fe(2)−C(12)
Fe(1)−N(2)
Fe(2)−C(13)
2.5056(13)
1.912(7)
1.865(7)
1.919(7)
1.888(6)
1.864(5)
1.911(6)
C(11)−O(11)
C(12)−O(12)
C(13)−N(1)
C(13)−C(14)
C(14)−N(2)
N(1)−C(16)
N(2)−C(24)
N(1)−C(15)
C(13)−N(1)−C(15)
C(13)−N(1)−C(16)
C(15)−N(1)−C(16)
C(14)−N(2)−Fe(1)
C(14)−N(2)−C(24)
Fe(1)−N(2)−C(24)
1.209(8)
1.189(8)
1.386(7)
1.422(8)
1.326(8)
1.429(7)
1.428(7)
1.464(8)
123.4(5)
124.0(5)
112.1(5)
122.5(4)
114.1(5)
123.4(4)
Fe(1)−C(11)−Fe(2)
Fe(1)−C(12)−Fe(2)
Fe(2)−C(13)−C(14)
Fe(2)−C(13)−N(1)
N(1)−C(13)−C(14)
C(13)−C(14)−N(2)
83.1(3)
82.3(3)
117.4(4)
130.9(4)
111.7(5)
122.8(5)
Scheme 4. Formimidoyl Complexes
ligands and the diiron frame remain unchanged upon hydride
addition and do not allow bridging coordination of the
formimidoyl.
As mentioned above, the formation of 3 was unique in that
only hydride, among the nucleophiles investigated, initiates the
observed rearrangement and assembly. Reactions of 2a with
NaCN or with LiPPh2 resulted in the displacement of the
isocyanide, affording the complexes 5 and 6, respectively
(Scheme 5). In contrast, the reaction of 2a with Li2CuCNMe2
afforded the acyl derivative [Fe2{μ-CN(Me)(Xyl)}(μ-CO){C-
(O)Me}(CNXyl)(Cp)2] (7) (Scheme 5).
Scheme 5. Reactions with Nucleophiles
reaction also affords, as a minor product, the bridging hydride
complex [Fe2{μ-CN(Me)(Xyl)}(μ-H)(CO)2(Cp)2], which is
presumably formed by hydride displacement of the isocyanide
and has been identified by comparison of its spectroscopic data
with those reported in the literature.19
Interestingly, other complexes of type 2 (namely 2b,e)
undergo hydride addition to form the corresponding
formimidoyl complexes 4b,c (Scheme 4).
In order to demonstrate that the formimidoyl complex 4a
really corresponds to an intermediate step in the formation of
3, a sample of pure 4a has been treated in THF at reflux
temperature for 20 h. The reaction afforded 3 in good yelds
(about 80%). Conversely, analogous treatment of 4b,c gave
extensive decomposition and failed to produce compounds
analogous to 3.
The formation of 4a−c is interesting in that both dinuclear20
and trinuclear21 formimidoyl complexes are normally obtained
by reaction of isocyanides with μ-hydride complexes, via
isocyanide insertion into metal−hydride bonds, whereas the
complementary route (i.e., hydride addition to isocyanide
ligands) is less common.22 A further point of interest is that
formimidoyl ligands usually adopt a bridging coordination in
dinuclear complexes, acting as three-electron donors via C and
N coordination.20,21 Conversely, 4a−c exhibit a σ-coordinated
formimidoyl, which is normally observed only in mononuclear
complexes.23 In our case, this is most probably a consequence
of coordinative saturation, which does not provide access to
bridging coordination. Indeed, IR spectra of 4a−c show the
usual ν(CO) band pattern consistent with the presence of one
terminally bonded and one bridging CO. Moreover, NMR
spectra indicate the presence of the bridging aminocarbyne
ligand (e.g., 13C NMR resonance of the μ-carbyne carbon at δ
337.6 for 4a) and of the two Cp ligands as well. Thus, ancillary
Selective nucleophilic addition at the CO is consistent with
previously reported reactions of the analogous aminocarbyne
complex [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO)2(Cp)2]-
[SO3CF3] with organocopper reagents, which also afforded
acyl derivatives.24 Complex 2a resulted unreactive toward other
nucleophiles such as MeOH, NH2Bun, and NaN3, whereas
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reactions with MeLi and PhLi produced extensive decom-
position.
addition and thermal treatment. The reaction proceeds via
hydride attack at the isocyanide to yield a formimidoyl, which
subsequently undergoes coupling with the bridging carbyne
ligand. The resulting fragment acts as a bridging amino-
carbene−aldimine ligand, coordinated through the carbene
carbon and imine nitrogen.
Considering that the μ-aminocarbyne is simply obtained by
methylation (with MeSO3CF3) of a isocyanide ligand, the
observed aminocarbyne−formimidoyl coupling can be regarded
as the assembly of two isocyanide ligands, via C−C coupling,
provided that one isocyanide has been activated by electrophilic
addition (transformation into aminocarbyne) and the other has
been activated by nucleophilic addition (transformation into
formimidoyl) (Scheme 7). This “double activation” is a
Complexes 5−7 have been characterized by spectroscopy
and elemental analysis. Complex 5 has been identified by
comparison of its spectroscopic properties with those reported
in the literature.19 Details are reported in the Experimental
Section. It has to be remarked that complex 6 exists, in solution,
as mixture of E and Ζ isomers, with prevalence of the former,
1
whereas the H NMR spectrum of 7 exhibits a single set of
resonances. NOE investigations performed on 7 have
evidenced a significant NOE effect between the resonance
related to the N-methyl group (at δ 4.29 ppm) and the
resonance attributed to the equivalent methyls of the terminal
CNXyl moiety (at 2.05 ppm), indicating that these groups are
placed on the same side (as shown in Scheme 5).
Finally, we have investigated the reactivity of the thiocarbyne
complex [Fe2{μ-CSMe}(μ-CO)(CO)(CNXyl)(Cp)2]-
[SO3CF3] (8), which differs from 2a in that it displays the
bridging thiocarbyne ligand μ-CSMe in place of the amino-
carbyne μ-CN(Me)(Xyl). It has been shown that μ-thiocarbyne
and μ-aminocarbyne diiron complexes exhibit a few similarities
but also some distinct features.25 Here, we have found that 8
reacts with NaBH4, but neither the isocyanide nor the CO
undergoes nucleophilic attack. In this case, hydride addition
takes place selectively at the carbyne carbon, yielding the
bridging thiocarbene complex [Fe2{μ-C(SMe)(H)}(μ-CO)-
(CO)(CNXyl)(Cp)2] (9) (Scheme 6).
a
Scheme 7. Sequential Steps in Isocyanide Activation
Scheme 6. Hydride Addition at the Thiocarbyne Complex
a
Ancillary ligands have been omitted for clarity.
multistep process, significantly different from the “reductive
coupling” of isocyanides, which is so far the major approach to
the metal-mediated assembly of isocyanides.16,27
Unfortunately the reaction does not have a general character,
in that other nucleophiles (NaCN, LiPPh2 Li2CuCNMe2) do
not give addition at the isocyanide and fail to induce the
“double activation” mentioned above. Nevertheless, the
reaction offers an unprecedented example of C−C bond
formation promoted by diiron complexes.
Complex 9 closely resembles the thiocarbene [Fe2 {μ-
C(SMe)(H)}(μ-CO)(CO)2(Cp)2], which was analogously
obtained by treatment of the cationic thiocarbyne [Fe2(μ-
CSMe)(μ-CO)(CO)2(Cp)2]+ with NaBH4.26
Compound 9 was characterized by IR and NMR spectros-
copy and elemental analysis. The IR spectrum of 9 (in CH2Cl2
solution) exhibits absorptions due to terminal and bridging
carbonyls (at 1944 and 1758 cm−1) and an absorption
EXPERIMENTAL SECTION
■
General Data. All reactions were routinely carried out under a
nitrogen atmosphere, using standard Schlenk techniques. Solvents
were distilled immediately before use under nitrogen from the
appropriate drying agents. Chromatography separations were carried
out on columns of deactivated alumina (4% w/w water). Glassware
was oven-dried before use. Infrared spectra were recorded on a
PerkinElmer Spectrum 2000 FT-IR spectrophotometer, and elemental
analyses were performed on a ThermoQuest Flash 1112 Series EA
instrument. NMR spectra were recorded on a Mercury Plus 400
instrument. Unless otherwise stated, NMR spectra were recorded at
1
attributable to the CN group (2075 cm−1). The H NMR
spectrum shows a unique set of resonances: the μ-CH proton
resonates at low field (δ 11.63 ppm), as expected for a bridging
thiocarbene ligand. The SMe protons resonate at 2.82 ppm,
whereas the methyl protons of the Xyl group give rise to a
single resonance at 2.28 ppm. The thiocarbene carbon
resonates at 166.7 ppm in the 13C NMR spectrum, whereas
the resonance of the isocyanide carbon is found at 176.0 ppm.
No reaction was observed between the isocyanide and the
bridging carbene ligand upon thermal treatment of 9 (in THF
at reflux) for 20 h.
1
298 K. Chemical shifts for H and 13C were referenced to internal
TMS. Spectra were fully assigned via DEPT experiments and H,13C
1
correlation measured through gs-HSQC and gs-HMBC experiments.
NOE measurements were recorded using the DPFGSE-NOE
sequence. NMR signals due to the minor isomeric form (where it
has been possible to detect) are italicized. All of the reagents were
commercial products (Aldrich) of the highest purity available and were
used as received. [Fe2(CO)4(Cp)2] was purchased from Strem and
used as received. The compounds [Fe2{μ-CN(Me)(R)}(μ-CO)-
(CO)2(Cp)2][SO3CF3] (R = Xyl, 1a; R = Me, 1b)14a,28 and [Fe2(μ-
CONCLUSIONS
■
Assembly of bridging carbyne and isocyanide ligands, in the
complex [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO)(CNR′)-
(Cp)2][SO3CF3] (2a), has been obtained upon hydride
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CSMe)(μ-CO)(CO)2(Cp)2][SO3CF3]29 were prepared as described
in the literature.
Crystals suitable for X-ray analysis were obtained by a Et2O/n-pentane
(1/3) solution, at −20 °C. Anal. Calcd for C31H32Fe2N2O2: C, 64.61;
H, 5.60; N, 4.86. Found: C, 64.62; H, 5.55; N, 4.78. Data for 3 are as
Synthesis of [Fe2{μ-CN(Me)(R)}(μ-CO)(CO)(CNR′)(Cp)2]-
[SO3CF3] (2a−e). A solution of [Fe2{μ-CN(Me)(Xyl)}(μ-CO)-
(CO)2(Cp)2][CF3SO3] (1a; 100 mg, 0.161 mmol), in THF (15
mL), was treated with CNXyl (23 mg, 0.175 mmol) and then with
Me3NO (16 mg, 0.213 mmol). The mixture was stirred for 3 h.
Solvent removal under reduced pressure gave a solid residue which
was dissolved in CH2Cl2 and chromatographed on alumina. Elution
with MeCN afforded a red-brown band corresponding to 2a (R = R′ =
Xyl) (yield 104 mg, 90%; E/Ζ ratio 3/1, determined by integration of
NMR signals) Crystals of E-2a suitable for X-ray analysis were
obtained by a CH2Cl2 solution layered with diethyl ether at −20 °C.
Anal. Calcd for C32H31F3Fe2N2O5S: C, 53.06; H, 4.31; N, 3.87. Found:
C, 53.12; H, 4.22; N, 3.99. IR (CH2Cl2) ν(CN) 2120 (vs), ν(CO)
1
follows. IR (CH2Cl2): ν(CO) 1734 (vs) cm−1. H NMR (CDCl3): δ
7.30, 7.28, 7.04, 6.88, 6.87 (s, 6 H, Me2C6H3); 6.79, 5.72 (s, 1 H, CH);
5.13, 4.41, 4.36, 4.23 (s, 10 H, Cp); 3.52, 2.76 (s, 3 H, NMe); 2.30,
2.21, 1.99, 1.95 (s, 6 H, Me2C6H3). E/Z ratio 13/10. 13C{1H} NMR
(CDCl3): δ 291.2, 290.3 (μ-CO); 232.0, 229.2 (Ccarbene); 162.7, 159.6
(CHNXyl); 149.6, 148.0, 144.0, 141.8 (Cipso‑Xyl); 135.9, 133.2,
129.6−125.1 (Carom); 89.2, 88.9, 88.4, 88.3 (Cp); 47.2, 43.8 (NMe);
18.4, 17.9, 17.7, 17.6 (Me2C6H3).
Synthesis of [Fe2{μ-CN(Me)(R)}(μ-CO)(CO){C(H)NR′}(Cp)2]-
[SO3CF3] (4a−c). Complex 2a (150 mg, 0.207 mmol) was dissolved
in THF (15 mL) and treated with NaBH4 (40 mg, 1.05 mmol) at 0
°C. The solution was stirred for 30 min, and then the mixture was
warmed to room temperature and was filtered on an alumina pad. The
solvent was removed, and the residue, dissolved in CH2Cl2, was
chromatographed on alumina. A first band, corresponding to the
complex [Fe2{μ-CN(Me)(Xy)}(μ-H)(CO)2(Cp)2], was obtained in
low yield (23 mg, 25%). Then, complex 4a was isolated as a green
fraction by using THF as eluent. Yield: 79 mg, 66%. Anal. Calcd for
C31H32Fe2N2O2: C, 64.61; H, 5.60; N, 4.86. Found: C, 64.55; H, 5.51;
N, 4.93. Data for 4a (R = R′ = Xyl) are as follows. IR (CH2Cl2):
ν(CO) 1955 (vs), 1785 (s), ν(CN) 1538 (s), cm−1. 1H NMR
(CDCl3): δ 9.08, 8.49 (s, 1 H, CH); 7.30−6.67 (m, 6 H, Me2C6H3);
4.95, 4.92, 4.38, 4.35 (s, 10 H, Cp); 4.30, 4.21 (s, 3 H, NMe); 2.65,
2.60, 2.18 (s, 6 H, Me2C6H3); 2.00, 1.91 (s, 6 H, Me2C6H3). E/Ζ ratio
6/1. 13C{1H} NMR (CDCl3): δ 337.6 (μ-CN); 271.2 (μ-CO); 215.2
(CO); 208.6 (CN); 147.9 (Cipso‑Xyl); 134.1−121.3 (Carom); 90.0, 89.6,
88.3, 86.1 (Cp); 52.3, 51.6 (NMe); 18.9, 18.6, 18.5, 18.0, 17.7
(Me2C6H3).
1
1991 (vs), 1824 (s), cm−1. H NMR (CDCl3): δ 7.37−7.01 (m, 6 H,
Me2C6H3); 5.33, 5.29, 4.69, 4.65 (s, 10 H, Cp); 4.54, 4.41 (s, 3 H,
NMe); 2.74, 2.70, 2.07, 1.88 (s, 6 H, μ-CNMe2C6H3); 2.27, 2.10 (s, 6
H, CNMe2C6H3). 13C{1H} NMR (CDCl3): δ 332.0, 330.9 (μ-CN);
258.8, 259.2 (μ-CO); 210.5, 210.2 (CO); 167.3, 166.9 (CN); 148.0,
147.8 (Cipso‑Xyl); 134.8−127.9 (Carom); 89.4, 89.1, 88.5, 88.4 (Cp);
54.9, 54.8 (NMe); 18.6, 17.4 18.4, 17.3 (Me2C6H3).
Complexes 2b−e were prepared by the same procedure described
for 2a by reacting 1a,b with the appropriate isocyanide CNR′,
respectively.
2b: R = Xyl, R′ = Me; yield 72%. Anal. Calcd for
C25H25F3Fe2N2O5S: C, 47.34; H, 3.97; N, 4.42. Found: C, 47.41; H,
4.02; N, 4.33. Data for 2b are as follows. IR (CH2Cl2): ν(CN) 2182
1
(vs), ν(CO) 1989 (vs), 1823 (s), cm−1. H NMR (CDCl3): δ 7.38−
7.26 (m, 3 H, Me2C6H3); 5.22, 5.16, 4.50, 4.29 (s, 10 H, Cp); 4.56
4.46 (s, 3 H, μ-CNMe); 3.20, 3.11 (s, 3 H, CNMe); 2.46, 2.39, 2.13,
2.12 (s, 6 H, Me2C6H3). E/Ζ ratio 3/1. 13C{1H} NMR (CDCl3) δ
334.0, 332.4 (μ-CN); 259.6, 258.4 (μ-CO); 210.9, 208.9 (CO); 148.6,
147.7 (Cipso‑Xyl); 134.0−128.9 (Carom); 122.5, 119.3 (CN); 90.5, 89.1,
88.0, 87.8 (Cp); 55.0, 54.6 (μ-CNMe); 31.1 30.8 (CNMe); 18.6, 18.5,
18.2, 17.2 (Me2C6H3).
Compounds 4b,c were prepared by the same procedure described
for 4a, by reacting NaBH4 with 2b,e, respectively.
4b: R = Xyl, R′ = Me; yield 45%. Anal. Calcd for C24H26Fe2N2O2:
C, 59.29; H, 5.39; N, 5.76. Found: C, 59.30; H, 5.48; N, 5.61. Data for
4b are as follows. IR (CH2Cl2): ν(CO) 1952 (vs), 1762 (s), ν(CN)
1
1590 (s) cm−1. H NMR (CDCl3): δ 8.01 (br, 1 H, CH); 7.50−7.13
2c: R = Xyl, R′ = But; yield 85%. Anal. Calcd for
C28H31F3Fe2N2O5S: C, 49.73; H, 4.62; N, 4.14. Found: C, 49.81; H,
4.55; N, 4.10. Data for 2c are as follows. IR (CH2Cl2): ν(CN) 2149
(m, 3 H, Me2C6H3); 4.89, 4.33 (s, 10 H, Cp); 4.20, 4.08 (s, 6 H,
NMe); 2.61, 2.14 (s, 6 H, Me2C6H3). 13C{1H} NMR (CDCl3): δ 336.7
(μ-CN); 269.9 (μ-CO); 257.5 (CN); 216.5 (CO); 147.9 (Cipso‑Xyl);
133.1, 133.0, 129.8, 128.3, 128.2 (Carom); 87.9, 86.2 (Cp); 50.2 (μ-
CNMe); 41.5 (CNMe); 18.3, 17.4 (Me2C6H3).
1
(vs), ν(CO) 1988 (vs), 1822 (s), cm−1. H NMR (CDCl3) δ 7.54−
7.23 (m, 3 H, Me2C6H3); 5.24, 4.64 (s, 10 H, Cp); 4.50 (s, 3 H,
NMe); 2.72, 2.16 (s, 6 H, Me2C6H3); 1.29 (s, 9 H, CMe3). 13C{1H}
NMR (CDCl3) δ 332.4 (μ-CN); 259.1 (μ-CO); 210.8 (CO); 147.7
(Cipso‑Xyl); 133.1, 131.5, 129.7, 128.9, 128.8 (Carom); 116.3 (CN); 88.4,
88.0 (Cp); 58.8 (NMe); 49.7 (CMe3); 30.0 (CMe3); 18.6, 17.2
(Me2C6H3).
4c: R = Me, R′ = Xyl; yield 49%. Anal. Calcd for C24H26Fe2N2O2:
C, 59.29; H, 5.39; N, 5.76. Found: C, 59.40; H, 5.22; N, 5.74. Data for
4c are as follows. IR (CH2Cl2): ν(CO) 1927 (vs), 1746 (s), ν(CN)
1
1586 (s), cm−1. H NMR (CDCl3): δ 9.62 (s, 1 H, CH); 7.11, 6.91
(m, 3 H, Me2C6H3); 4.56, 4.51 (s, 10 H, Cp); 3.77, 3.08 (s, 6 H,
NMe); 2.39 (s, 6 H, Me2C6H3). 13C{1H} NMR (CDCl3): δ 331.4 (μ-
CN); 264.3 (μ-CO); 214.9 (CO); 207.5 (CN); 145.6 (Cipso‑Xyl);
138.5, 127.9, 121.7 (Carom); 87.4, 85.4 (Cp); 53.8, 44.9 (NMe); 19.0
(Me2C6H3).
Synthesis of [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO)(CN)(Cp)2] (5).
Complex 2a (95 mg, 0.130 mmol) was dissolved in THF (15 mL)
and treated with NBut4CN (42 mg, 0.157 mmol). The solution was
heated to reflux temperature for 30 min, and then it was filtered on an
alumina pad. Solvent removal gave complex 5 as a green powder.
Yield: 42 mg, 68%.
2d: R = Xyl, R′ = p-C6H4CF3; yield 72%. Anal. Calcd for
C31H26F6Fe2N2O5S: C, 48.72; H, 3.43; N, 3.67. Found: C, 48.70; H,
3.31; N, 3.80. Data for 2d are as follows. IR (CH2Cl2): ν(CN) 2119
1
(vs), ν(CO) 1985 (vs), 1824 (s), cm−1. H NMR (CDCl3) δ 7.71−
7.13 (m, 7 H, Me2C6H3 and C6H4CF3); 5.27, 4.63 (s, 10 H, Cp); 4.36
(s, 3 H, NMe); 2.68, 1.99 (s, 6 H, Me2C6H3). 13C{1H} NMR (CDCl3)
δ 330.3 (μ-CN); 257.5 (μ-CO); 209.6 (CO); 168.9 (CN); 147.9
(Cipso‑Xyl); 133.3−118.6 (Carom); 115.4 (CF3); 89.2, 88.5 (Cp); 52.0
(μ-CNMe); 18.4, 18.1 (Me2C6H3).
2e: R = Me, R′ = Xyl; yield 85%. Anal. Calcd for
C25H25F3Fe2N2O5S: C, 47.34; H, 3.97; N, 4.42. Found: C, 47.30; H,
3.99; N, 4.21. Data for 2e are as follows. IR (CH2Cl2): ν(CN) 2119
Synthesis of [Fe2{μ-CN(Me)(Xyl)}(μ-CO)(CO)(PPh2)(Cp)2] (6).
Complex 2a (95 mg, 0.130 mmol), was dissolved in THF (15 mL) and
treated at room temperature with LiPPh2 (0.18 mmol) in THF
solution (2.0 mL). The solution was stirred for 20 min, and then the
solvent was removed under reduced pressure. Chromatography of the
residue on an alumina column with a mixture of THF and MeOH (9/
1 v/v) as eluent gave an emerald green band corresponding to 6. Yield:
56 mg, 68%. Anal. Calcd for C34H32Fe2NO2P: C, 64.89; H, 5.13; N,
2.23. Found: C, 64.92; H, 5.06; N, 2.15. Data for 6 are as follows. IR
1
(vs), ν(CO) 1987 (vs), 1819 (s) cm−1. H NMR (CDCl3) δ 7.07−
6.98 (m, 3 H, Me2C6H3); 5.21, 5.18 (s, 10 H, Cp); 4.37, 4.27 (s, 6 H,
NMe); 2.18 (s, 6 H, Me2C6H3). 13C{1H} NMR (CDCl3) δ 322.1 (μ-
CN); 260.8 (μ-CO); 210.1 (CO); 167.7 (CN); 134.5, 128.5, 127.9
(Carom); 88.8, 88.6 (Cp); 53.7 (NMe); 18.4 (Me2C6H3).
Synthesis of [Fe2{μ-CN(Me)(Xyl)C(H)N(Xyl)}(μ-CO)2(Cp)2]
(3). Complex 2a (150 mg, 0.207 mmol) in THF (20 mL) was treated
with NaBH4 (40 mg, 1.05 mmol) at reflux for 20 h. Then, the solvent
was removed and the residue was dissolved in Et2O and filtered
through an alumina pad. Solvent removal and chromatography of the
residue on an alumina column with a mixture (1/1 v/v) of CH2Cl2 and
Et2O as eluent gave a red-violet band containing 3. Yield: 64 mg, 54%.
1
(CH2Cl2): ν(CO) 1981 (vs), 1786 (s), cm−1. H NMR (CDCl3): δ
7.72−7.22 (m, 16 H, Me2C6H3 and PPh2); 4.95, 4.71, 4.42, 4.25 (s, 10
H, Cp); 4.67 (s, 3 H, NMe); 2.69, 2.64, 2.23 (s, 6 H, Me2C6H3). E/Ζ
ratio: 5/1. 13C{1H} NMR (CDCl3): δ 333.0 (μ-C); 267.6 (μ-CO);
3995
dx.doi.org/10.1021/om500502h | Organometallics 2014, 33, 3990−3997

Organometallics
Article
213.1, 212.0 (CO); 151.0−125.6 (Carom); 89.0, 87.4, 87.3, 87.0 (Cp);
51.4 (NMe); 18.6, 18.5, 18.4, 18.2 (Me2C6H3). 31P{1H} NMR
(CDCl3): δ 34.7, 33.6.
ASSOCIATED CONTENT
* Supporting Information
A table, figures, and CIF files giving NMR spectra for all new
compounds and crystal data for 2a and 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
Synthesis of [Fe2{μ-CN(Me)(Xyl)}(μ-CO){C(O)Me}(CNXyl)(Cp)2]
(7). Complex 2a (250 mg, 0.345 mmol) was dissolved in THF (22
mL) and treated at −50 °C with Li2CuCNMe2 (0.40 mmol) in THF
solution (2.5 mL), freshly prepared by reacting CuCN and MeLi in a
1/2 ratio. The resulting mixture was stirred for 50 min, and then the
solvent was removed under reduced pressure. The solid residue was
dissolved in CH2Cl2 and filtered on alumina. Solvent removal gave 7 as
a green microcrystalline solid. Yield: 153 mg, 75%. Anal. Calcd for
C32H34Fe2N2O2: C, 65.11; H, 5.81; N, 4.75. Found: C, 65.04; H, 5.88;
N, 4.62. Data for 7 are as follows. IR (CH2Cl2): ν(CN) 2071 (vs),
AUTHOR INFORMATION
Corresponding Author
*E-mail for F.M.: fabio.marchetti1@unipi.it. E-mail for V.Z.:
valerio.zanotti@unibo.it.
■
Notes
1
ν(CO) 1749 (s), 1591 (ms) cm−1. H NMR (CDCl3): δ 7.25−6.89
The authors declare no competing financial interest.
(m, 6 H, Me2C6H3); 4.80, 4.19 (s, 10 H, Cp); 4.29 (s, 3 H, NMe);
2.63, 2.26 (s, 6 H, Me2C6H3); 2.32 (s, 3 H, MeCO); 2.05 (s, 6 H,
Me2C6H3). 13C{1H} NMR (CDCl3): δ 340.2 (μ-C); 277.4 (MeCO);
274.0 (μ-CO); 178.9 (CN); 148.5 (Cipso‑Xyl); 135.3, 134.0, 133.7,
129.4, 127.8, 127.5, 157.4, 125.8 (Carom); 88.1, 85.3 (Cp); 51.4
(NMe); 46.5 (MeCO); 18.7, 18.4, 17.4 (Me2C6H3).
ACKNOWLEDGMENTS
We thank the Ministero dell’Universita
Scientifica e Tecnologica (MIUR) and the University of
Bologna for financial support.
■
̀
e della Ricerca
Synthesis of [Fe2{μ-CSMe}(μ-CO)(CO)(CNXyl)(Cp)2][SO3CF3]
(8). Complex [Fe2{μ-CSMe}(μ-CO)(CO)2(Cp)2][SO3CF3] (300
mg, 0.562 mmol), in THF solution (15 mL), was treated with C
NXyl (81 mg, 0.618 mmol) and with Me3NO (51 mg, 2.13 mmol).
The mixture was stirred for 1 h, and then the solvent was removed.
The residue was subsequently chromatographed on alumina, and
elution with MeOH gave a dark red band corresponding to 8. Yield:
286 mg, 80%. Anal. Calcd for C24H22F3Fe2NO5S2: C, 45.23; H, 3.48;
N, 2.20. Found: C, 45.20; H, 3.35; N, 2.19. IR (CH2Cl2): ν(CN)
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occupancy parameter per disordered group. The crystals of 2a are very
small and of low quality, although different attempts at recrystallization
have been made. While the overall connectivity of 2a is certain,
structural parameters, however, should be taken with caution.
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