Synthesis of NHC complexes by template controlled cyclization of β-functionalized isocyanides

Article abstract of DOI:10.1039/b915033a

Starting from complexes of type [Ru(Cp)Cl(P-P)] (P-P = 2PPh₃, 3a; P-P = 2PMe₃, 3b: P-P = dppe, 3c; P-P = dppp, 3d) isocyanide complexes [Ru(Cp)(P-P)(CNR) 4a-4d (CNR = CN-CH₂-CH₂N ₃, 1) and 7a-7d (CN-C

Full text of DOI:10.1039/b915033a

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www.rsc.org/dalton | Dalton Transactions
Synthesis of NHC complexes by template controlled cyclization
of b-functionalized isocyanides†
Aaro´n Flores-Figueroa, Oliver Kaufhold, Kai-Oliver Feldmann and F. Ekkehardt Hahn*
Received 24th July 2009, Accepted 1st September 2009
First published as an Advance Article on the web 24th September 2009
DOI: 10.1039/b915033a
Starting from complexes of type [Ru(Cp)Cl(P-P)] (P-P = 2PPh3, 3a; P-P = 2PMe3, 3b: P-P = dppe, 3c;
P-P = dppp, 3d) isocyanide complexes [Ru(Cp)(P-P)(CNR) 4a–4d (CNR = CN–CH2–CH2N3, 1) and
7a–7d (CN–C6H4–2N3, 2) have been prepared. Reduction of the azido functions of the coordinated
isocyanide ligands with Zn/NH4Cl/H2O in methanol leads to coordinated 2-amino functionalized
isocyanides which cyclize to yield the complexes with a saturated NH,NH-stabilized NHC ligand 5a–5d
or a benzannulated NH,NH-stabilized NHC ligand 8a–8d. The Zn/NH4Cl/H2O reduction method is
of general applicability and allowed the generation of complex 11 bearing three saturated
NH,NH-stabilized NHC ligands.
its in situ liberation to initiate the NHC formation. We have
Introduction
used 2-trimethylsiloxyphenyl isocyanide which, after coordination
to a suitable metal center and cleavage of the O–SiMe3 bond,
spontaneously cyclizes to give complexes with a benzoxazolin-
2-ylidene NHC ligand.9 We have also studied complexes of
b-azido functionalized isocyanides which upon transformation of
the azido function into a primary amine by the Staudinger reaction
followed by hydrolysis also cyclize to give NH,NH-stabilized NHC
ligands.6,7
Recently we noticed that the reduction of the azido function in
ruthenium coordinated b-azidoethyl isocyanide by the Staudinger
reaction and hydrolysis did not lead to the 2-amino functionalized
isocyanide required for cyclization. Further complications arise if
the phosphine used in the Staudinger reaction preferably attacks
the transition metal center and not the azido group of the
coordinated isocyanide ligand. An alternative reduction protocol
using a mixture of FeCl3 and NaI was employed in that case.10
Since the reduction of the azido function in coordinated b-azido
isocyanides appears to depend on both the type of isocyanide
(aromatic or aliphatic) and the metal center the isocyanide is
coordinated to, we initiated a systematic study of this reaction.
Here we report on the reduction of coordinated 2-azidoethyl iso-
cyanide 1 and 2-azidophenyl isocyanide 2 using a Zn/NH4Cl/H2O
mixture to give complexes with NH,NH-stabilized NHC ligands
(Scheme 1)
Metal complexes containing N-heterocyclic carbene (NHC) lig-
ands are currently studied as scaffolds for transition metal
catalysts1 and supramolecular frameworks.2 Due to the gen-
eral applicability, most syntheses of NHC complexes rely on
the C2-deprotonation of N,N¢-disubstituted azolium salts fol-
lowed by coordination of the formed NHC to a suitable metal
center.3a–c Other methods like the transmetallation from silver
complexes,3d the in situ generation of the NHC ligands by
a-elimination from 2-alkoxy-4,5-imidazolines3e or imidazolium-
2-carboxylates3f and the reductive desulfurization of imidazolin-
2-thiones3g or benzimidazolin-2- thiones3h–i have also been suc-
cessfully employed. If the C2 deprotonation is attempted with
a monosubstituted azole the formation of the complex with the
NH,NR-stabilized NHC ligand competes with the coordination
of the ligand through the unsubstituted nitrogen atom of the
heterocycle.4
An alternative method for the preparation of NHC com-
plexes with NH,O-stabilized heterocyclic carbene ligands is the
cycloaddition of haloalcohols to isocyanide complexes.5 In an
extension of this approach, complexes with NH,NH-stabilized
NHC ligands can be obtained by intramolecular cyclization of
b-amino functionalized isocyanides at a suitable template metal.6
Such complexes contain reactive NHC ligands which, for example,
allow the linkage of the NHC to other ligands coordinated to
the metal center via alkylation of the nitrogen atoms of the
heterocycle.7
Results and discussion
The free isocyanides containing an -OH or -NHR nucleophile
in b-position to the isocyanide function tend to cyclize under
formation of the corresponding oxazoles or azoles.8 The use of
such isocyanides in metal template controlled cyclization reactions
requires the protection of the intramolecular nucleophile and
Screening of reduction methods
Since we were particularly interested in generating NH,NH-
stabilized NHC ligands at RuII for the subsequent preparation
of RuII complexes with cyclic [11]ane-P2CNHC ligands,7,10 we
initially studied the reduction and cyclization of RuII coordinated
2-azidoethyl isocyanide.
Organic azides are common precursors for amines since the
azido group is easily introduced into different molecules and a
large number of protocols for the azide reduction to a primary
amine have been described.11 From these, suitable procedures for
Institut fu¨r Anorganische und Analytische, Chemie, Westfa¨lische Wilhelms-
Universita¨t Mu¨nster, Corrensstraße 30, D-48149 Mu¨nster, Germany.
E-mail: fehahn@uni-muenster.de; Fax: +49-251-8333108; Tel: +49-251-
8333110
† CCDC reference number 742312. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b915033a
9334 | Dalton Trans., 2009, 9334–9342
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©

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observed together with other minor components which were not
further characterized due to their low abundance and irrelevance
for our purposes.
All reduction reactions were carried out over 48 h after which
the most efficient reduction methods could be identified. The
1
composition of the product mixture was determined by H and
1
31P{ H} NMR spectroscopy which allowed the determination of
the relative amounts of complexes 5a and 6.
Analysis of the raw reaction mixtures showed the predominance
of the two ruthenium complexes 5a and 6 as reaction products if
conversion was achieved. Some well established reducing agents
did not lead to conversion in the case of the metal coordinated
2-azidoethyl isocyanide. For example, the reactions of 4a with
FeCl3/hydrazine, SiMe3Cl/NaI or [Sn(bdt)2]/NaBH4 in MeOH
did not yield complex 5a or any other reduction products (entries
1–3). Using catalytic amounts of [Sn(bdt)2] in basic or neutral
media led to the reduction of the azido function but also produced
a considerable amount of the vinyl isocyanide complex 6 (entries
4 and 5). Complex 6 was also found to be the main reaction
product in the reduction of 4a with CuSO4·5H2O/NaBH4, (entry
6) and a minor product in the reaction of 4a with NHEt3[Sn(SPh)3]
(entry 7).
Scheme 1 Template-controlled formation of NH,NH-stabilized NHC
ligands from 2-azidoethyl isocyanide 1 or 2-azidophenyl isocyanide 2.
Table 1 Azide reduction in complex 4a using different reducing agentsa
The best results in terms of conversion and selectivity were
observed for the reduction of 4a with stoichiometric amounts of
In/NH4Cl, Zn/NH4Cl and in the Staudinger reaction (entries
10–12). While the Staudinger reaction and the reduction with
Zn/NH4Cl proceeded satisfactorily in MeOH, the reduction with
In/NH4Cl gave only in EtOH an acceptable yield. It was also
observed that the reductions with indium or zinc gave almost
quantitative conversion to5a when carried out in refluxing solvents
(vide infra).
Entry Reducing agent
Solvent
5a (%) 6 (%) Ref.
1
2
3
FeCl3/H2NNH2
SiMe3Cl/NaI
MeOH
CH3CN
MeOH
—
5
—
—
—
17
17c
40
5d
12
13
14
14
14
15
16
17
17
17
6c
18
[Sn(bdt)2]b/NaBH4
—
4
5
6
7
[Sn(bdt)2]b/NaBH4, pH = 7 MeOH/H2O 59
[Sn(bdt)2]b/NaBH4, pH = 10 MeOH/H2O 27
CuSO4·5H2O/NaBH4
NHEt3[Sn(SPh)3]
In/NH4Cl
MeOH
MeOH
MeOH
MeOH/H2O —
EtOH
MeOH
MeOH/H2O 85
—
33
10
8
—
—
—
—
—
9
In/NH4Cl
In spite of the successful reduction of the azido function
in 4a under Staudinger conditions, we noticed previously that
the Staudinger reaction at the isocyanide ligand of [RuCl2(p-
cymene)(1)] did not lead to the desired 2-aminoethyl isocyanide
or its cyclization product.10 In addition, complex 4a contains
two phosphine ligands which are not easily substituted by PMe3
used to initiate the Staudinger reaction (Table 1, entry 11).
Complexes with more labile ligands may react more readily with
the Staudinger agent PR3 which makes the Staudinger reaction
not always the method of choice for the reduction of the azido
group in coordinated b-azido functionalized isocyanide.
We therefore became interested in the azide reduction with
alternative reducing agents like indium or zinc. Due to the lower
cost of the metal, the reduction of ruthenium(II) coordinated
2-azidoethyl isocyanide with Zn/NH4Cl was subsequently studied
in detail and the results were compared to the results obtained by
the Staudinger reaction (Table 2). For that purpose the reported
conditions for the Staudinger reaction6a,c were slightly modified.
The protocol employed for the reduction of the azido group in
4a by a Staudinger reaction involved the dropwise addition of a
methanolic solution of PMe3 over 12 h to a methanol solution
of complex 4a. The resulting phosphinimine was not isolated
but directly hydrolyzed with triflic acid/water at the end of the
reaction.
10
11
12
In/NH4Cl
80
90
PMe3/H3O+
Zn/NH4Cl
a The anion X is dependent on the reducing agent. b Catalytic amount,
bdt = benzene-o-dithiolato anion. c A control reaction using only NaBH4
and pH = 10 buffer gave no conversion. d An unidentified byproduct
(m/z = 776) was obtained in a yield of 40%.
the azide reduction in a metal coordinated 2-azidoethyl isocyanide
had to be selected. Limiting factors are the reaction temperature
and the use of a reducing agent which would not require chemical
quenching during workup, both of which are important to avoid
isocyanide complex decomposition. In addition, a diamagnetic
ruthenium template center facilitating complex analysis by NMR
and MS spectrometry had to be used.
With these limitations in mind [Ru(Cp)Cl(PPh3)2] 3a was
selected as the metal precursor for the initial studies. The reaction
of 3a with a slight excess of isocyanide 1 in boiling methanol
for 12 h produced complex 4a in good yield. Complex 4a was
1
1
1
characterized by H, 13C{ H} and 31P{ H} NMR spectroscopy
and by HRMS spectrometry. Reduction of the azido function in
4a was attempted under the conditions and with the reagents listed
in Table 1.
In the case of a successful reduction, the formed 2-aminoethyl
isocyanide ligand cyclized by intramolecular attack of the amine
function at the isocyanide carbon atom and the NHC complex 5a
was obtained as the reaction product.6 Under certain conditions,
the formation of complex 6 with the vinyl isocyanide ligand was
Table 2 illustrates that omission of any of the components of the
reduction mixture Zn/NH4Cl/H2O reduces the yield in the azide
reduction (entries 7–9). Absence of water in the reaction mixture
allowed the reduction to proceed only in moderate yield (entry 6).
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Table 2 Azide reduction in complex 4a under varying reaction
7a–d (with the aromatic 2-azidophenyl isocyanide) (Scheme 2).
Coordination of the isocyanides gave stable ruthenium complexes
in all cases except for the complexes with the PMe3 ligand 4b and
7b. The instability of complex [Ru(Cp)Cl(PMe3)2] has been noticed
previously.20b In fact, PMe3 dissociated from the ruthenium center
reacted with the azido function of the isocyanide ligand leading to
the Staudinger product and prevented the isolation of analytically
pure samples of the two isocyanide complexes 4b and 7b.
conditionsa
Entry Reducing agent
Amount of H2O Time/h Yield 5a (%)
1
2
3
4
5
6
7
8
9
PMe3/TfOH
Zn, NH4Cl
Zn, NH4Cl
Zn, NH4Cl
Zn, NH4Cl
Zn, NH4Cl
Zn
Excess
Excess
0.5 eq.
1 eq.
2 eq.
—
—
Excess
Excess
48
48
24
24
24
48
48
48
48
100 (77)
100 (90)
60
86
17
70
—
—
—
Complexes 4a–d and 7a–d were identified by NMR and IR
spectroscopy as well as by mass spectrometry and, except for
NH4Cl
Zn
1
4b and 7b, by microanalytical data. 13C{ H} NMR spectra of
complexes 4a–d exhibit the resonance for the isocyanide carbon
atom at d ª 154 ppm with a characteristic 2JC–P coupling constant
of about 20 Hz. The chemical shift of the isocyanide carbon atom
in complexes 4a–d is only slightly shifted upfield compared to the
value recorded for the free ligand6c (d = 160.3 ppm). Even smaller
differences in the chemical shift were measured for the isocyanide
carbon resonance in complexes 7a–d (range d = 165.6 to d =
170.3 ppm) compared to the resonance for the free ligand 2 (d =
168.8 ppm).6a
a 0.12 mmol 4a, 0.16 mmol Zn, 0.27 mmol NH4Cl, 20 mL MeOH
refluxing. Yields were determined by NMR spectroscopy, isolated yields
in parentheses.
To elucidate the role of the water, three reductions were carried
out with varying amounts of water present (entries 3–5). No clear
picture emerged from these experiments and we assume that there
might exist an interplay between the water and the surface of
the zinc particles as has been reported for the reduction of other
substrates with zinc.19
Addition of an excess of water and a reaction time of 48 h,
however, led to a reproducible reduction of the azido function
in very good yield (entry 2). We concluded that the complete
reduction of the azido group using zinc and ammonium chloride
required an excess of water and should be performed in refluxing
methanol over a period of 1–2 d. While the Staudinger reaction
also works for the reduction of the azido group in 4a (Table 2,
entry 1), the reduction of the azido function of ruthenium(II) co-
ordinated 2-azidoethyl isocyanide using Zn/NH4Cl/H2O emerged
as an interesting alternative procedure which does not require the
addition of a ligand like PR3 potentially causing undesired side
reactions at the metal center.
≡
The IR stretching frequencies for the C N groups in complexes
of types 4 and 7 serve as an indicator for the propensity of the
coordinated isocyanide ligand to undergo a nucleophilic attack
at the isocyanide carbon atom. Reduction of the azido groups in
these complexes leads to complexes with a 2-amino substituted
isocyanide which can cyclize by an intramolecular nucleophilic
attack of the amino group at the isocyanide carbon atom to give
complexes with NH,NH-stabilized NHC ligands (Scheme 1). We
have previously studied this type of cyclization with complexes
bearing the 2-hydroxyphenyl isocyanide ligand.9 It was observed
that the intramolecular nucleophilic attack is obstructed when
the isocyanide ligand is deactivated by metal to ligand d → p*
backbonding which can be detected by a drop in the wavenumber
≡
for the C N stretching vibration for the coordinated isocyanide
relative to the free isocyanide ligand.9e It can be expected that
in electron-rich ruthenium complexes of types 4 and 7 such
backbonding from the metal to the isocyanide ligand also exists
which might deactivate the isocyanide ligand for cyclization after
reduction of the azido group.
Synthesis of complexes with 2-azido substituted isocyanides
In order to test the versatility of the Zn/NH4Cl/H2O reduction
protocol, a systematic variation of the phosphine ligands at
ruthenium and of the 2-azido substituted isocyanide ligands
was carried out. A total of four different ruthenium complexes
3a–d were prepared by displacement of the two triphenylphos-
phine ligands in [Ru(Cp)Cl(PPh3)2] 3a by PMe3 (3b), dppe (3c) and
dppp (3d) ligands (Scheme 2). The substitution of the chloro ligand
in complexes 3a–d is an established procedure for the introduction
of an isocyanide ligand.20 This method was used for the generation
of complexes 4a–d (with the aliphatic 2-azidoethyl isocyanide) and
Table 3 lists the wavenumbers for the isocyanide stretching
vibrations found for complexes 4a–d and 7a–d. For all complexes
of type 7 bearing the aromatic 2-azidophenyl isocyanide ligand,
≡
a significant drop in the wavenumber for the C N stretching
vibration upon coordination of the isocyanide was observed in
accord with the good p-acceptor properties of aryl isocyanides. For
the complexes with the aliphatic isocyanide ligand 1, a significant
drop in the wavenumber was observed for the particularly electron-
rich complexes 4b and 4d. The adverse effects of Ru → Cisocyanide
Table 3 Wavenumbers of the IR stretching vibrations for the isocyanides
1 and 2 and isocyanide complexes 4a–d and 7a–da
-1
-1
≡
≡
Compound
n(C N)/cm
Compound
n(C N)/cm
1
2152b
2155
2122
2154
2141
2
2142
2122
2124
2122
2123
4a
4b
4c
4d
7a
7b
7c
7d
a Measured in KBr. b Measured in CH2Cl2.
Scheme 2 Synthesis of isocyanide complexes 4a–d and 7a–d.
9336 | Dalton Trans., 2009, 9334–9342
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d–p* backbonding in complexes of type 4 and 7 which could
obstruct the cyclization after reduction of the azido group in these
complexes may, however, become offset by the high nucleophilicity
of the liberated amino group.
Synthesis of a tris(NHC) complex
To further test the applicability of the azide reduction in
complexes bearing 2-azido functionalized isocyanides using
Zn/NH4Cl/H2O, the type of template metal and the co-ligands
at the metal center were modified. The strong donors (Cp and
phosphines) in complexes of type 4 and 7 were substituted by
three strong p-acceptor ligands (CO) and two strong s-donating
carbene ligands and the template metal center was changed from
divalent ruthenium(II) to monovalent rhenium(I). These changes
led us to the preparation of the previously described complex
9 (Scheme 4). Complex 9 was obtained by the elegant method
described by Liu et al. involving the deoxygenation reaction of two
carbonyl ligands with a phosphinimine followed by cyclization of
the intermediate isocyanide ligand.21 Abstraction of the bromo
ligand in 9 with AgBF4 followed by coordination of 2-azidoethyl
isocyanide yielded the mixed biscarbene isocyanide complex 10
(Scheme 4).
Synthesis of NHC complexes
The azido groups in complexes 4a–d and 7a–d were reduced
quantitatively in refluxing methanol over a period of 1–2 d using
Zn/NH4Cl/H2O as the reducing agent. Following the reduction,
cyclization of the isocyanide by intramolecular nucleophilic attack
at the isocyanide carbon atom to give complexes 5a–d and 8a–d
bearing the NH,NH-stabilized carbene ligands was observed in all
cases (Scheme 3).
Scheme 3 Reduction of the azido groups in complexes 4a–d and 7a–d
followed by cyclization of the isocyanide ligand.
Reduction of the azido groups in complexes 4a and 7a with
the sterically demanding PPh3 ligands took 34 h while the
reduction for the other complexes was completed in 24 h. These
reaction times are longer than those observed for the reduction
of related free organic azides (10–120 min)11 suggesting that the
steric conditions around the azido function determine the ease and
rate of reduction. As was observed for the isocyanide complexes 4
and 7 the carbene complexes 5 and 8 are air stable solids with the
exception of complexes 5b and 8b with the PMe3 ligands.20b
Formation of the carbene complexes was confirmed by NMR
and IR spectroscopy. The IR spectra were free of absorptions for
isocyanide or azido groups and instead showed the absorptions of
Scheme 4 Synthesis of the isocyanide complex 10 followed by reduction
of the azido group and cyclization to give 11.
1
1
the N–H vibrations around n = 3400 cm-1. The 13C{ H} NMR
Complex 10 was identified by the 13C{ H} NMR resonance for
resonances for the isocyanide carbon atoms in complexes of type
4 (d ª 154 ppm) and 7 (d = 165.6 to d = 170.3 ppm) were absent in
the carbene complexes and new resonances for the carbene carbon
atoms were found in the range of d = 202.6 to d = 206.3 ppm
(for complexes of type 5) and d = 183.4 to d = 188.7 ppm (for
complexes of type 8). Conversion of the isocyanide to a NH,NH-
stabilized NHC ligand leads also to a slight upfield shift for the
the carbene carbon atoms (d = 191.6 ppm in DMSO-d6, d =
211.1 ppm in CD3OD) and the isocyanide carbon atom (d =
141.2 ppm in CD3OD). The resonance for the isocyanide carbon
atom could not be detected in DMSO-d6. The wavenumbers for
the isocyanide stretching modes (n = 2212 and 2200 cm-1), which
are higher than the value for the free ligand (n = 2152 cm-1)
indicate an activation of the isocyanide for a nucleophilic attack
upon coordination to the ReI center in spite of the reduced formal
charge at the metal atom in 10 compared to the ruthenium(II)
complexes.
1
1
resonances in the 31P{ H} NMR spectra. The H NMR spectra
exhibit the signals for the NH protons in the expected range of d =
6.63–7.08 ppm (for complexes of type 5) and d = 10.53–11.37 ppm
(for complexes of type 8).
Consequently, reduction of the azido function in 10 with
Zn/NH4Cl/H2O gave the 2-aminoethyl isocyanide ligand which
spontaneously cyclized leading to the tris(NHC) complex 11. The
IR spectrum of 11 exhibits only absorptions for the CO ligands
and for the NH groups of the carbene ligands. Only one resonance
(d = 202.2 ppm) was detected for all three carbene carbon atoms
of 11 in CD3OD.
Complexes of type 5 and 8 are difficult to crystallize as the
reaction conditions allow for the formation of chloride and
tetrachloro zincate salts. This also caused problems regarding
reproducible microanalytical data. Analytically pure samples of
compounds 5a–d and 8a–d can be obtained by anion exchange
with NaBPh4.
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Compound 11 was crystallized as a chloride salt by diffusion
of diethyl ether into a methanol solution of the salt. The
molecular structure was determined by X-ray diffraction showing
the formation of a tris(NHC) complex cation (Fig. 1). The complex
cation resides on a three-fold crystallographic axis.
reaction mixture was stirred for 12 h under reflux. The solvent was
then removed in vacuo. The resulting yellow solid was washed with
a minimum amount of diethyl ether (7 mL) to yield compounds 4a–
4d as yellow powders and 7a–7d as brown powders. All complexes
containing PMe3 ligands are unstable and were difficult to isolate
in pure form. Yields for these complexes were therefore determined
by NMR spectroscopy and satisfactory microanalytical data could
not be obtained.
[Ru(Cp)(PPh3)2(1)]Cl 4a. Yield: 0.288 g, 85%. 1H NMR
(400 MHz, CDCl3, 25 ◦C): d 7.32 (m, 6 H, PPh3), 7.21 (m, 12 H,
3
PPh3), 7.04 (m, 12 H, PPh3), 4.69 (s, 5 H, Cp), 4.13 (t, JH–H
=
3
5.2 Hz, 2 H, CNCH2), 3.52 (t, JH–H = 5.2 Hz, 2 H, CH2N3).
◦
1
2
13C{ H} NMR (100 MHz, CDCl3, 25 C): d 153.9 (t, JC–P
=
≡
20.2 Hz, C N), 135.4 (PPh3), 132.9 (PPh3), 130.1 (PPh3), 128.2
(PPh3), 87.5 (Cp), 50.0 (CH2N3), 45.8 (CNCH2). 31P{ H} NMR
1
(162 MHz, CDCl3, 25 ◦C): d 45.39. IR (KBr): n 2155 (s, CN),
2103 (s, N3) cm-1. MS (MALDI): m/z (%) 787 (100) [M - Cl]+.
HRMS (ESI, positive ions): m/z 787.1696 [M - Cl]+, calcd for
C44H39N4P2Ru 787.1700.
Fig. 1 Molecular structure of the cation of 11. The molecule resides
on a crystallographic three-fold axis and the asymmetric unit contains
◦
˚
1/3 of the cation. Selected bond lengths (A) and bond angles ( ): Re–C1
2.183(2), Re–C4 1.950(2), N1–C1 1.322(3), N2–C1 1.335(3); C1–Re–C1*
86.0(7), C1–Re–C4 92.09(8), Cl–Re–C4* 93.38(8), C1–Re–C4** 177.57(8),
C4–Re–C4* 89.49(9), N1–C1–N2 106.5(2).
[Ru(Cp)(PMe3)2(1)]Cl 4b. Yield: 70◦% (by NMR spec-
troscopy). 1H NMR (400 MHz, CDCl3, 25 C): d 4.83 (s, 5 H, Cp),
3.95 (t, 3JH–H = 4.9 Hz, 2 H, CNCH2), 3.55 (t, 3JH–H = 4.9 Hz, 2 H,
The metric parameters in the cation of 11 compare well to
those observed for related rhenium(I) complexes bearing NH,NH-
stabilized saturated NHC ligands.7c,21 Due to steric reasons, values
larger than 90◦ are observed for the Ccarbene–Re–Ccarbene angles and
values of about 90◦ for the OC–Re–CO angles.
1
CH2N3), 1.46–1.36 (m, 18 H, PMe3). 13C{ H} NMR (100 MHz,
◦
2
≡
CDCl3, 25 C): d 157.0 (t, JC–P = 20.0 Hz, C N), 83.8 (Cp),
50.4 (CH2N3), 44.8 (CNCH2), 22.7 (dd, JC–P = 18.0 Hz, JC–P
=
◦
15.8 Hz, PMe3). 31P{ H} NMR (162 MHz, CDCl3, 25 C): d 8.79.
IR (KBr): n 2122 (s, CN), 2097 (s, N3) cm-1. MS (MALDI): m/z
(%) 415 (100) [M - Cl]+.
1
Experimental
[Ru(Cp)(dppe)(1)]Cl 4c. Yield: 0.191 g, 78%. 1H NMR
(400 MHz, CDCl3, 25 ◦C): d 7.71–7.62 (m, 4 H, PPh2), 7.54–
7.44 (m, 6 H, PPh2), 7.41–7.32 (m, 6 H, PPh2), 7.22–7.13 (m,
General
Caution organic azides might be explosive! Although compound 1
has been heated up to 100 ◦C without observation of any violent
decomposition and 2 has been shown to be stable up to 60 ◦C, care
must be taken when handling organic azides.
3
4 H, PPh2), 4.94 (s, 5 H, Cp), 3.38 (t, JH–H = 5.1 Hz, 2 H,
3
CNCH2), 2.81 (t, JH–H = 5.1 Hz, 2 H, CH2N3), 2.75–2.64 (m,
1
2 H, PCHHCHHP), 2.64–2.63◦(m, 2 H, PCHHCHHP). 13C{ H}
2
NMR (100 MHz, CDCl3, 25 C): d 152.5 (t, JC–P = 19.9 Hz,
All reactions were carried out under an argon atmosphere
using conventional Schlenk techniques. Solvents were dried and
degassed by standard methods prior to use. Isocyanides 16a and
26c were prepared as described. Complexes 3a–d20 and 921 were
synthesized by literature methods. NMR spectra were recorded
≡
C N), 137.3, 133.0, 132.7, 131.1, 130.6, 130.6, 129.0, 128.9 (PPh2),
84.9 (Cp), 49.6 (CH2N3), 44.7 (CNCH2), 28.6 (dd, JC–P = 24.1 Hz,
1
JC–P = 21.8 Hz, PCH2CH2P). 31P{ H} NMR (162 MHz, CDCl3,
25 ◦C): d 79.90. IR (KBr): n 2154 (s, CN), 2091 (s, N3) cm-1. MS
(MALDI): m/z (%) 661 (100) [M - Cl]+. HRMS (ESI, positive
ions): m/z 661.1226 [M - Cl]+, calcd for C34H33N4P2Ru 661.1228.
Anal. calcd for C34H33N4ClP2Ru: C, 58.66; H, 4.78; N, 8.05. Found:
C, 58.37; H, 4.59; N, 8.22.
1
with a Bruker Avance I 400 NMR spectrometer. 1H and 13C{ H}
NMR chemical shifts are reported as parts per million relative
1
to tetramethylsilane, 31P{ H} NMR chemical shifts are reported
relative to 85% H3PO4. IR spectra were measured with a Bruker
Vector 22 spectrometer. Mass spectra were recorded on a Finnigan
MAT 4200S, a Bruker Daltonics Micro TOF, a Waters-Micromass
QuatroLCZ (ESI) or a Varian MAT 212 (MALDI). Elemental
analysis was performed with an Elementar, Vario EL III microan-
alyzer. In the description of the carbene complexes SNHC refers
to the saturated imidazolidin-2-ylidene ligand and BNHC to the
benzannulated benzimidazolin-2-ylidene ligand.
[Ru(Cp)(dppp)(1)]Cl 4d. Yield: 0.210 g, 85%. 1H NMR
(400 MHz, CDCl3, 25 ◦C): d 7.45–7.32 (m, 12 H, PPh2), 7.25–
7.19 (m, 4 H, PPh2), 7.18–7.10 (m, 4 H, PPh2), 4.87 (s, 5 H, Cp),
4.34 (t, 3JH–H = 5.1 Hz, 2 H, CNCH2), 3.61 (t, 3JH–H = 5.1 Hz, 2 H,
CH2N3), 2.73–2.59 (m, 2 H, PCHHCH2CHHP), 2.58–2.46 (m,
3 H, PCHHCHHCHHP), 1.74–1.55 (m, 1 H, CH2CHHCH2).
◦
1
2
13C{ H} NMR (100 MHz, CDCl3, 25 C): d 153.8 (t, JC–P
=
≡
19.3 Hz, C N), 138.5, 136.1, 132.3, 131.3, 130.5, 130.3, 128.7,
General procedure for the synthesis of isocyanide complexes 4a–4d
and 7a–7d
128.4 (PPh2), 86.1 (Cp), 49.5 (CH2N3), 46.1 (CNCH2), 28.1 (m,
CH2CH2CH2), 20.6 (CH2CH2CH2). 31P{ H} NMR (162 MHz,
1
A slight excess (0.37 mmol, 1.05 eq.) of the isocyanide ligand
was added to a suspension of the appropriate [Ru(Cp)(Cl)P–P]
precursor complex (0.35 mmol, 1.0 eq.) in methanol (20 mL). The
CDCl3, 25 ◦C): d 39.43. IR (KBr): n 2137 (s, CN), 2096 (s, N3) cm-1.
MS (MALDI): m/z (%) 675 (100) [M - Cl]+. HRMS (ESI, positive
ions): m/z 675.1384 [M - Cl]+, calcd for C35H35N4P2Ru 675.1384.
9338 | Dalton Trans., 2009, 9334–9342
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The Royal Society of Chemistry 2009
©

View Online
Anal. calcd for C35H35N4ClP2Ru: C, 59.19; H, 4.97; N, 7.89. Found:
C, 58.88; H, 5.22; N, 4.79.
General procedure for the synthesis of carbene complexes 5a–d and
8a–d
A mixture of one of the isocyanide complexes 4a–d or 7a–d
(0.25 mmol, 1.0 eq.), fine powdered zinc (0.32 mmol, 1.3 eq.)
and NH4Cl (0.57 mmol, 2.3 eq.) were suspended in a mixture of
methanol (20 mL) and degassed water (0.1 mL). The mixture was
then stirred under reflux for 24 h (4b–d, 7b–d) or 34 h (4a, 7a).
At the end of the selected reaction time (conversion was mon-
itored by MALDI MS), the solvent was removed in vacuo and
dichloromethane (40 mL) was added to the residue. The obtained
suspension was filtered and the solvent was removed in vacuo.
Spectroscopic data were recorded for the compounds obtained this
way. Due to the presence of chloride and tetrachloro zincate anions
it was difficult to obtain satisfactory microanalytical data. Anion
exchange with NBPh4 gave the tetraphenyl borate salts. Yields and
microanalytical data are therefore given for the tetraphenyl borate
salts. The tetraphenyl borate anions would give a large number
of NMR resonances in the aromatic region and were therefore
not used for the spectroscopic characterization of the compounds.
X in the formulae therefore corresponds to the chloride and/or
tetrachloro zincate anion. All complexes containing PMe3 ligands
are unstable and were difficult to isolate in pure form. Yields for
these complexes were therefore determined by NMR spectroscopy
and satisfactory microanalytical data could not be obtained.
[Ru(Cp)(PPh3)2(2)]Cl 7a. Yield: 0.304 g, 90%. 1H NMR
(400 MHz, CDCl3, 25 ◦C): d 7.41–7.31 (m, 7 H, CNPhN3 and
PPh3), 7.25–7.04 (m, 26 H, CNPhN3 and PPh3), 6.91 (m, 1 H,
1
CNPhN3), 4.82 (s, 5 H, Cp). 13C{ H} NMR (100 MHz, CDCl3,
◦
25 C): d 167.6 (t, 2JC–P = 20.7 Hz, C N), 136.0 (CNPhN3), 135.0,
≡
133.0, 130.5 (PPh3), 129.8 (CNPhN3), 128.4 (PPh3), 127.0, 125.8,
120.0, 119.0 (CNPhN3), 88.6 (Cp). 31P{ H} NMR (162 MHz,
1
CDCl3, 25 ◦C): d 44.65. IR (KBr): n 2122 (s, CN), 2085 (s, N3) cm-1.
MS (MALDI): m/z (%) 835 (100) [M - Cl]+. HRMS (ESI, positive
ions): m/z 835.1691 [M - Cl]+, calcd for C48H39N4P2Ru 835.1701.
Anal. calcd for C48H39N4ClP2Ru: C, 66.24; H, 4.52; N, 6.44. Found:
C, 64.69; H, 4.63; N, 6.92.
[Ru(Cp)(PMe3)2(2)]Cl 7b. Yield: 73% (determined by NMR
◦
1
spectroscopy). H NMR (400 MHz, CDCl3, 25 C): d 7.31–7.01
(m, 4 H, CNPhN3), 5.06 (s, 5 H, Cp), 1.56 (d, JH–P = 8.9 Hz,
◦
18 H, PMe3). 13C{ H} NMR (100 MHz, CDCl3, 25 C): d 170.3
1
(t, 2JC–P = 19.4 Hz, C N), 135.7, 134.9, 129.0, 126.9, 125.6, 118.9
≡
(CNPhN3), 85.2 (Cp), 22.9 (dd, JC–P = 18.3 Hz, JC–P = 15.9 Hz,
◦
PMe3). 31P{ H} NMR (162 MHz, CDCl3, 25 C): d 8.66. IR (KBr):
n 2124 (s, CN), 2075 (s, N3) cm-1. MS (MALDI): m/z (%) 463 (100)
[M - Cl]+.
1
[Ru(Cp)(PPh3)2(SNHC)]◦X 5a. Yield: 0.246 g, 90%. 1H NMR
(400 MHz, DMSO-d6, 25 C): d 7.52–7.41 (m, 6 H, PPh3), 7.35
(m, 12 H, PPh3), 7.11–6.96 (m, 12 H, PPh3), 6.64 (s, 2 H, NH), 4.55
[Ru(Cp)(dppe)(2)]Cl 7c. Yield: 0.210 g, 81%. 1H NMR
(400 MHz, CDCl3, 25 ◦C): d 7.80–7.68 (m, 4 H, PPh2), 7.49–
7.36 (m, 12 H, PPh2), 7.29–7.19 (m, 4 H, PPh2), 7.18–7.11 (m,
1 H, CNPhN3), 6.93–6.82 (m, 2 H, CNPhN3), 6.30–6.24 (m, 1 H,
1
(s, 5 H, Cp), 3.09 (s, 4 H, CH2CH2). 13C{ H} NMR (100 MHz,
DMSO-d6, 25 ◦C): d 202.6 (t, 2JC–P = 16.3 Hz, NCN), 137.0–136.1
(PPh3), 132.9, 129.1, 128.2, (PPh3), 86.2 (◦Cp), 44.6 (CH2CH2). d
1
31P{ H} NMR (162 MHz, DMSO-d6, 25 C): d 48.44. IR (KBr):
CNPhN3), 5.07 (s, 5 H, Cp), 2.96–2.70 (m, 4 H, PCH2CH2P).
n 3442 (w, NH) cm-1. MS (MALDI): m/z (%) 761 (100) [M -
Cl]+. HRMS (ESI, positive ions): m/z 761.1790 [M - Cl]+, calcd
for C44H41N2P2Ru 761.1795. Anal. calcd for C68H61N2BP2Ru: C,
75.62; H, 5.69; N, 2.59. Found: C, 75.38; H, 5.75; N 2.21.
◦
1
2
13C{ H} NMR (100 MHz, CDCl3, 25 C): d 165.6 (t, JC–P
=
≡
19.0 Hz, C N), 137.0 (PPh2), 135.4 (CNPhN3), 133.0, 132.8,
131.1, 130.8, 130.7, (PPh2), 129.2 (CNPhN3), 129.1, 128.9 (PPh2),
127.0, 125.8, 125.2, 118.6 (CNPhN3), 86.0 (Cp), 29.0 (dd, JC–P
=
24.0 Hz, JC–P = 21.8 Hz, PCH2CH2P). 31P{ H} NMR (162 MHz,
CDCl3, 25 ◦C): d 78.59. IR (KBr): n 2122 (s, CN), 2090 (s, N3) cm-1.
MS (MALDI): m/z (%) 709 (100) [M - Cl]+. HRMS (ESI, positive
ions): m/z 709.1232 [M - Cl]+, calcd for C38H33N4P2Ru 709.1229.
Anal. calcd for C38H33N4ClP2Ru: C, 61.33; H, 4.47; N, 7.53. Found:
C, 60.33; H, 4.66; N, 7.40.
1
[Ru(Cp)(PMe3)2(SNHC)]X 5b. Yield: 73% (determined by
NMR spectroscopy). H NMR (400 MHz, DMSO-d6, 25 ◦C):
1
d 7.08 (s br, 2 H, NH), 4.80 (s, 5 H, Cp), 3.40 (s, 4 H, CH2CH2),
1
1.45–1.40 (m, 18 H, PMe3). 13C{ H} NMR (100 MHz, DMSO-
d6, 25 ◦C): d 206.3 (t, JC–P = 15.9 Hz, NCN), 82.2 (Cp),
2
44.4 (NCH2CH2N), 22.5 (dd, JC–P = 16.02, JC–P = 14.05 Hz,
PMe3). 31P{ H} NMR (162 MHz, DMSO-d6, 25 ◦C): d 11.17.
1
[Ru(Cp)(dppp)(2)]Cl 7d. Yield: 0.240 g, 90%. 1H NMR
HRMS (ESI, positive ions): m/z 389.850 [M - Cl]+, calcd. for
◦
(400 MHz, CDCl3, 25 C): d 7.47–7.14 (m, 24 H, CNPhN3 and
C14H29N2P2Ru 389.0848.
PPh2), 5.03 (s, 5 H, Cp), 2.89–2.78 (m, 2 H, CHHCH2CHH),
1
2.76–2.49 (m,
1
H, CH2CHHCH2), 2.45–2.32 (m,
2
H,
[Ru(Cp)(dppe)(SNHC)]X 5c. Yield: 0.222 g, 93%. H NMR
PCHHCH2CHHP), 1.87–1.68 (m, 1 H, CH2CHHCH2). 13C{ H}
(400 MHz, DMSO-d6, 25 ◦C): d 7.74–7.57 (m, 4 H, PPh2), 7.52–
7.38 (m, 6 H, PPh2), 7.37–7.19 (m, 6 H, PPh2), 7.19–7.02 (m, 4 H,
PPh2), 6.40 (s, 2 H, NH), 4.68 (s, 5 H, Cp), 3.07–2.86 (m, 2 H,
PCHHCHHP), 2.86–2.68 (m, 2 H, PCHHCHHP), 2.64 (s, 4 H,
1
◦
2
NMR (100 MHz, CDCl3, 25 C): d 167.2 (t, JC–P = 19.5 Hz,
≡
C N), 138.1 (PPh2), 136.0 (CNPhN3), 135.1, 132.4, 131.4, 131.0,
130.7 (PPh2), 129.8 (CNPhN3), 128.8, 128.7 (PPh2), 127.5, 126.0,
1
125.1, 119.1 (CNPhN3), 87.3 (Cp), 28.4 (m, CH2CH2CH2), 20.7
NCH2CH2N). 13C{ H} NMR (100 MHz, DMSO-d6, 25 ◦C): d
◦
(CH2CH2CH2). 31P{ H} NMR (162 MHz, CDCl3, 25 C): d 38.32.
IR (KBr): n 2123 (s, CN), 2083 (s, N3) cm-1. MS (MALDI):
m/z (%) 723 (100) [M - Cl]+. HRMS (ESI, positive ions): m/z
723.1383 [M - Cl]+, calcd for C39H35N4P2Ru 723.1385. Anal. calcd
for C39H35N4P2Ru: C, 61.78; H, 4.65; N, 7.39. Found: C, 61.03; H,
4.95; N, 7.39.
203.0 (t, 2JC–P = 15.2 Hz, NCN), 141.8, 134.1, 133.0, 130.3, 129.9,
1
129.1, 128.3, 128.1, (PPh2), 85.1 (Cp), 44.0 (NCH2CH2N), 28.1
◦
1
(m, PCH2CH2P). 31P{ H} NMR (162 MHz, DMSO-d6, 25 C):
d 90.73. IR (KBr): n 3448 (m, NH) cm-1. MS (MALDI): m/z
635 (100) [M - Cl]+. HRMS (ESI, positive ions): m/z 635.1331
[M - Cl]+, calcd for C34H35N2P2Ru 635.1323. Anal. calcd for
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 9334–9342 | 9339
©

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C58H55N2BP2Ru: C, 73.03; H, 5.81; N, 2.94. Found: C, 72.74; H,
5.79; N, 2.07.
128.1–127.7 (PPh2), 121.7, 109.9 (BNHC), 86.0 (Cp), 25.6 (m,
1
CH2CH2CH2), 20.6 (CH2CH2CH2). 31P{ H} NMR (162 MHz,
DMSO-d6, 25 ◦C): d 42.47. IR (KBr): n 3424 (m, NH) cm-1. MS
(MALDI): m/z (%) 697 (100) [M - Cl]+. HRMS (ESI, positive
ions): m/z 697.1480 [M - Cl]+, calcd for C39H37N2P2Ru 697.1480.
Anal. calcd for C63H57N2BP2Ru: C, 74.48; H, 5.65; N, 2.76. Found:
C, 73.25; H, 5.78; N 2.76.
1
[Ru(Cp)(dppp)(SNHC)]X 5d. Yield: 0.212 g, 88%. H NMR
(400 MHz, DMSO-d6, 25 ◦C): d 7.51–7.38 (m, 16 H, PPh2), 7.27–
7.19 (m, 4 H, PPh2), 6.63 (s, 2 H, NH), 4.65 (s, 5 H, Cp), 3.01 (s,
4 H, NCH2CH2N), 2.71–2.29 (m, 5 H, PCH2CHHCH2P), 1.89–
1.78 (m, 1 H, PCH2CHHCH2P). 13C NMR (100 MHz, DMSO-
d6, 25 ◦C): d 202.7 (t, JC–P = 15.3 Hz, NCN), 142.1, 138.0,
2
Preparation of the vinyl isocyanide complex 6
131.9, 131.3, 129.5, 129.3, 128.0, 127.0 (PPh2), 85.8 (Cp), 44.5
(NCH2CH2N), 25.4 (m, PCH2CH2CH2P), 20.6 (PCH2CH2CH2P).
31P NMR (162 MHz, DMSO-d6, 25 ◦C): d 43.3. IR (KBr): n
3422 (m, NH) cm-1. MS (MALDI): m/z (%) 649 (100) [M -
Cl]+. HRMS (ESI, positive ions): m/z 649.1492 [M - Cl]+, calcd
for C35H37N2P2Ru 649.1485. Anal. calcd for C59H57N2BP2Ru: C,
73.21; H, 5.93; N, 2.89. Found: C, 73.34; H, 5.85; N, 1.87.
Compound 6 was identified as the product in the reduction of
4a using CuSO4/NaBH4 as the reducing agent. It was identified
in the reaction mixture by NMR spectroscopy and MALDI MS
spectrometry. An authentic sample of 6 was obtained by stirring
50 mg (0.0636 mmol, 1 eq.) of 4a and 7 mg (0.0636 mmol, 1 eq.) of
KOtBu in THF (20 mL) for 14 h. After removal of the solvent the
solid residue was dried in vacuo, redissolved in CH2Cl2 and filtered
over Celite. Removal of the solvent gave compound 6 as a yellow
solid. Yield: 0.44 g, 90%. 1H NMR (400 MHz, CDCl3, 25 ◦C): d
[Ru(Cp)(PPh3)2(BNHC)]X 8a. Yield: 0.250 g, 89%. 1H NMR
(400 MHz, DMSO-d6, 25 ◦C): d 10.62 (s br, 2 H, NH), 7.51–
1
6.93 (m, 34 H, PPh2 + BNHC), 4.74 (s, 5 H, Cp). 13C{ H} NMR
7.35–6.24 (m, 30 H, PPh3), 6.38 (dd, 1H, 3JH–H = 15.3 Hz, 3JH–H
=
(100 MHz, DMSO-d6, 25 ◦C): d 183.4 (t, 2JC–P = 17.3 Hz, NCN),
3
8.0 Hz, CHCH2), 5.13 (d, 1 H, JH–H = 8.0 Hz, CHCHH), 5.08
136.3 (PPh3), 133.3 (BNHC), 132.8, 129.9, 128.3 (PPh3), 122.2,
3
1
(d, 1 H, JH–H = 15.3 Hz, CHCHH), 4.81 (s, 5 H, Cp). 13C{ H}
1
110.0 (BNHC), 86.4 (Cp). 31P{ H} NMR (162 MHz, DMSO-d6,
◦
2
NMR (100 MHz, CDCl3, 25 C): d 162.3 (t, JC–P = 20.8 Hz,
25 ◦C): d 47.64. IR (KBr): n 3414 (m, NH) cm-1. MS (MALDI):
m/z (%) 809 (100) [M - Cl]+. HRMS (ESI, positive ions): m/z
809.1802 [M - Cl]+, calcd for C48H41N2P2Ru 809.1802. Anal. calcd
for C72H61N2BP2Ru: C, 76.66; H, 5.45; N, 2.48. Found: C, 76.63; H,
5.43; N 2.53.
≡
C N), 135.1, 133.0, 130.4, 128.5 (PPh3), 122.1 (CHCH2),◦118.3
(CHCH2), 88.3 (Cp). 31P{ H} NMR (162 MHz, CDCl3, 25 C): d
1
45.17 (PPh3). IR (KBr): n 2107 (m, CN), 2036 (s, CN) 2008 (m,
CN) cm-1. MS (MALDI): m/z (%) 744 (100) [M - Cl]+.
Synthesis of the tris(NHC) complex 11
[Ru(Cp)(PMe3)2(BNHC)]X 8b. Yield: 90% (determined by
NMR spectroscopy). H NMR (400 MHz, DMSO-d6, 25 ◦C):
1
fac-[Re(CO)3(SNHC)2(1)]BF4 10. To a mixture of 100 mg of
biscarbene complex 9 (0.20 mmol, 1 eq.) and 40 mg of AgBF4
(0.20 mmol, 1 eq.) was added acetonitrile (40 mL). The reaction
mixture was stirred for 12 h in the dark. The solids were removed by
filtration and solid residue was washed with acetonitrile (5 mL). To
the combined acetonitrile fractions was added isocyanide 1 (23 mg,
0.25 mmol, 1.2 eq.). The reaction mixture was stirred for 16 h
at ambient temperature. Subsequently, all solvents were removed
in vacuo and the solid residue was dissolved in dichloromethane
(5 mL). Compound 10 precipitated from this solution upon
addition of diethyl ether (20 mL) and cooling to -20 ◦C overnight.
d 11.37 (s br, 2 H, NH), 7.51–7.39 (m, 2 H, BNHC), 7.13–7.06
(m, 2 H, BNHC), 4.92 (s, 5 H, Cp), 1.48–1.40 (m, 18 H, PMe3).
◦
1
13C{ H} NMR (100 MHz, DMSO-d6, 25 C): d 188.7 (t, 2JC–P
=
16.7 Hz, NCN), 134.1, 121.6, 110.0 (BNHC), 82.6 (Cp), 22.7 (dd,
JC–P = 15.7 Hz, JC–P = 14.6 Hz, PMe3). 31P{ H} NMR (162 MHz,
1
CDCl3, 25 ◦C): d 10.77 (PMe3). HRMS (ESI, positive ions): m/z
(%) 437.0844 [M - Cl]+, calcd for C18H29N2P2Ru, 437.0849.
1
[Ru(Cp)(dppe)(BNHC)]X 8c. Yield: 0.233 g, 93%. H NMR
(400 MHz, DMSO-d6, 25 ◦C): d 10.53 (s, 2 H, NH), 7.63–7.55
(m, 4 H, PPh2), 7.49–7.40 (m, 6 H, PPh2), 7.31–7.19 (m, 10 H,
PPh2), 7.04–6.98 (m, 2 H, BNHC), 6.96–6.89 (m, 2 H, BNHC),
4.87 (s, 5 H, Cp), 3.30–3.08 (m, 2H, PCHHCHHP), 3.05–2.84
1
Yield: 0.105 g, 88%. H NMR (400 MHz, DMSO-d6): d 8.20
3
(s br, 4 H, NH), 4.05 (t, JH–H = 5.3 Hz, 2 H, CNCH2), 3.71 (t,
3JH–H = 5.3 Hz, 2 H, CH2N3), 3.49 (s, 8 H, NCH2CH2N). 13C{ H}
1
1
(m, 2 H, PCHHCHHP). 13C{ H} NMR (100 MHz, DMSO-d6,
25 ◦C): d 184.3 (t, JC–P = 15.9 Hz, NCN), 141.6, 133.7 (m,
NMR (100 MHz, DMSO-d6): d 191.6 (NCN), 191.4, 189.8 (CO),
48.9 (CH2N3), 44.4 (NCH2CH2N), 44.1 (CNCH2); the resonance
2
PPh2), 133.4 (BNHC), 132.9, 130.1, 129.9, 129.2, 128.3, 127.8
1
(PPh2), 121.3, 109.4 (BNHC), 85.4 (Cp),◦28.0 (m, PCH2CH2P).
for the isocyanide carbon atom could not be detected. 13C{ H}
1
31P{ H} NMR (162 MHz, DMSO-d6, 25 C): d 88.74. IR (KBr):
NMR (100 MHz, CD3OD): d 211.1 (NCN), 193.8, 193.0 (CO),
141.2 (br, CN), 51.7 (CH2N3), 46.9 (NCH2CH2N), 46.1 (CNCH2).
19F NMR (376 MHz, DMSO-d6): d -148.3 (d, BF4). IR (KBr): n
3478, 3452, 3404 (NH), 2212 (m, CN), 2200 (m, CN), 2144 (s, N3),
2108 (s, N3), 2018 (m, CO), 1945 (m, CO), 1922 (m, CO) cm-1. MS
(MALDI): m/z 507 (100) [M - BF4]+.
n 3404 (m, NH) cm-1. MS (MALDI): m/z (%) 683 (100) [M -
Cl]+. HRMS (ESI, positive ions): m/z 683.1314 [M - Cl]+, calcd
for C38H35N2P2Ru 683.1324. Anal. calcd for C62H55N2BP2Ru: C,
74.32; H, 5.53; N, 2.79. Found: C, 76.02; H, 5.66; N, 2.28.
1
[Ru(Cp)(dppp)(BNHC)]X 8d. Yield: 0.230 g, 94%. H NMR
(400 MHz, DMSO-d6, 25 ◦C): d 10.67 (s, 2 H, NH), 7.50–7.37
(m, 6 H, PPh2), 7.36–7.28 (m, 2 H, BNHC), 7.27–7.18 (m, 14 H,
PPh2), 7.09–7.03 (m, 2 H, BNHC), 4.81 (s, 5 H, Cp), 2.79–1.72
fac-[Re(CO)3(SNHC)3]Cl 11. A Schlenk flask was charged
with 240 mg of compound 10 (0.41 mmol, 1 eq.), 35 mg
of zinc powder (0.53 mmol, 1.3 eq.) and 50 mg of NH4Cl
(0.93 mmol, 2.3 eq.). Methanol (20 mL) and degassed water
(0.1 mL) were added to the mixture. The resulting suspension
was heated under reflux for 24 h. Subsequently, all solids were
1
(m, 6 H, PCH2CH2CH2P). 13C{ H} NMR (100 MHz, DMSO-
d6, 25 ◦C): d 184.0 (t, JC–P = 16.1 Hz, NCN), 141.6, 138.0
2
(PPh2), 134.1 (BNHC), 131.7–131.3 (PPh2), 129.6–129.3 (PPh2),
9340 | Dalton Trans., 2009, 9334–9342
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removed by filtration and the resulting solution was brought
to dryness under reduced pressure. The solid residue obtained
was thoroughly washed with dichloromethane (30 mL). Filtration
and removal of the solvent gave the tris(NHC) complex 11. Salt
the reduction under Staudinger conditions could lead to an
undesirable reaction of the required phosphines with the iron(III)
template.
11 could contain different anions (Cl-, [ZnCl4]2-, BF4 ). This
-
Acknowledgements
prevented the microanalytical characterization of the compound.
Crystallization of 11 from methanol/diethyl ether gave exclusively
the chloride salt [Re(CO)3(SNHC)3]Cl which was characterized
We thank the Graduate School of Chemistry Mu¨nster
(GSC-MS) and DGRI-SEP Mexico for a predoctoral grant to
A. F. F. and GSC-MS for a predoctoral grant to K. O. F. Our work
was supported by the Deutsche Forschungsgemeinschaft (IRTG
1444) through a predoctoral grant for O. K.
1
by X-ray diffraction in 60%. H NMR (400 MHz, CD3OD): d
4.87 (s br, 6 H, NH), 3.63 (s, 8 H, 2 ¥ NCH2CH2N), 3.61 (s,
1
2 H, 1 ¥ NCH2CH2N). 13C{ H} NMR (100 MHz, CD3OD): d
204.2 (NCN), 198.2, 194.7 (CO), 46.0 (2 ¥ NCH2CH2N), 45.8 (1 ¥
NCH2CH2N). IR (KBr): n 3416 (NH), 1999 (m, CO), 1914 (m,
CO) cm-1. MS (MALDI): m/z (%) 481 (100) [M - Cl]+.
Notes and references
1 N-Heterocyclic Carbenes in Transition Metal Catalysis: Top.
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Crystal structure determinations
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X-Ray diffraction data were collected with a Bruker AXS APEX
diffractometer equipped with a rotation anode at 153(2) K using
˚
graphite monochromated Mo Ka radiation, l = 0.71073 A.
Diffraction data were collected over the full sphere and were
corrected for absorption. The data reduction was performed
with the Bruker SMART22 program package. Structure solutions
were found with the SHELXS-97 package23 using the heavy-atom
method and were refined with SHELXL-9724 against F2 using first
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hydrogen atoms. Hydrogen atoms were added to the structure
models on calculated positions.
Crystal data for 11. Formula C12H28N6ClO3Re, M = 519.97,
¯
colorless crystal, 0.16 ¥ 0.16 ¥ 0.09 mm, cubic, space group Pa3,
a = 14.8438(3), V = 3270.66(11) A , rcalc = 2.096 g cm-3, m =
3
˚
7.617 mm-1, w- and f-scans, 35 498 measured intensities (4.6 ≤
2q ≤ 59.1◦), semi-empirical absorption correction (0.3453 ≤ T ≤
0.5034), 1541 independent (Rint = 0.031) and 1383 observed (I ≥
2s(I)) intensities, Z = 8, R(all) = 0.0185, wR(all) = 0.0356,
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Conclusions
We have described a new method for the reduction/cyclization
of b-azido functionalized ethyl and phenyl isocyanides using a
mixture of Zn/NH4Cl/H2O as the reducing agent. The method
is of general applicability and a total of eight new cationic
[RuII(Cp)(P–P)(NHC)]+ complexes and [Re(CO)3(NHC)3]Cl have
been obtained by this method from the corresponding isocyanide
complexes. The new reduction method can be employed instead
of the Staudinger reaction which has been demonstrated to
fail in the reduction of ruthenium(II) coordinated 2-azidoethyl
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the Zn/NH4Cl/H2O reduction. The major drawback of the
Zn/NH4Cl/H2O reduction in the synthesis of charged complexes
is the formation of salt mixtures containing Cl- and [ZnCl4]2-
anions. These anions, however, can be exchanged. We are currently
studying the reduction of b-azido functionalized isocyanides
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9342 | Dalton Trans., 2009, 9334–9342
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©