Substituent R-dependent regioselectivity switch in nucleophilic addition of N-phenylbenzamidine to PdII-and PtII-complexed isonitrile RN-C giving aminocarbene-like species

Article abstract of DOI:10.1021/om101041g

Metal-mediated coupling between one or two isonitrile ligands in cis-[MCl₂(C-NR)₂] [M = Pd, Pt; R = 2,6-Me ₂C₆H₃ (Xyl), Bu^t, cyclohexyl (Cy)] and N-phenylbenzamidine, HN-C(Ph)NHPh, proceeds

Full text of DOI:10.1021/om101041g

Organometallics 2011, 30, 863–874 863
DOI: 10.1021/om101041g
Substituent R-Dependent Regioselectivity Switch in Nucleophilic
Addition of N-Phenylbenzamidine to PdII- and PtII-Complexed Isonitrile
RNtC Giving Aminocarbene-Like Species
Alexander G. Tskhovrebov,† Konstantin V. Luzyanin,†,‡ Maxim L. Kuznetsov,‡
Viktor N. Sorokoumov,† Irina A. Balova,† Matti Haukka,§ and Vadim Yu. Kukushkin*,†,^
†St. Petersburg State University, Universitetskii Pr. 26, 198504 Stary Petergof, Russian Federation,
‡
´
ꢀ
Centro de Quımica Estrutural, Complexo Interdisciplinar, Instituto Superior Tecnico, TU Lisbon,
Av. Rovisco Pais, 1049-001 Lisbon, Portugal, §Department of Chemistry, University of Eastern Finland,
P.O. Box 111, FI-80101, Joensuu, Finland, and ^Institute of Macromolecular Compounds of Russian
Academy of Sciences, Bolshoii Pr. 31, 199004 St. Petersburg, Russian Federation
Received November 4, 2010
Metal-mediated coupling between one or two isonitrile ligands in cis-[MCl2(CtNR)2] [M = Pd,
Pt; R = 2,6-Me2C6H3 (Xyl), But, cyclohexyl (Cy)] and N-phenylbenzamidine, HNdC(Ph)NHPh,
proceeds with different regioselectivity upon varying R group. When the aromatic isonitrile is used
(R = Xyl), N-phenylbenzamidine is coordinated to a metal by the HNdC moiety, and the nucleophilic
attack proceeds via the NHPh center of the benzamidine giving [MCl{C(N(Ph)C(Ph)dNH)d
NXyl}(CtNXyl)]. For R = But, HNdC(Ph)NHPh is coordinated to a metal by the NHPh center,
and the addition occurs via the HNdC center of the nucleophile to afford [MCl{C(NC(Ph)dNPh)d
NBut}(CtNBut)]. With R = Cy, a mixture of two products that are derived from the addition of
N-phenylbenzamidine by two nucleophilic centers was detected. The substituent R dependent
reactivity was explored using theoretical (DFT) methods and interpreted as a result of the steric
repulsions in one of the regioisomers of the addition products, when R = Cy and But. All prepared
species were fully characterized by elemental analyzes (C, H, N), high resolution ESIþ-MS, IR, 1D
(1H, 13C{1H}) and 2D (1H,1H-COSY, 1H,13C-HMQC/1H,13C-HSQC, 1H,13C-HMBC) NMR spec-
troscopies, and by X-ray diffraction for four complexes. The latter studies indicate that in the case of
XylNC, the corresponding carbene species possess distinct consequence of double and single bonds in
the MCN2 fragment and, therefore, belong to a novel family of aminocarbene-like ligands, while in the
case of ButNC, the carbene ligand is of the classical diaminocarbene type with bond delocalization in
the MCN2 functionality. The catalytic properties of systems based on two representative species, i.e.,
[PdCl{C(N(Ph)C(Ph)dNH)dNXyl}(CtNXyl)] (10) and [PdCl{C(NC(Ph)dNPh)dN(H)But}
(CtNBut)] (16);that are derived from the addition of HNdC(Ph)NHPh to a PdII-bound isonitrile
by its different nucleophilic centers ; in Sonogashira cross-coupling of 4-NO2C6H4I with oct-1-yne
(in EtOH as a solvent, K2CO3 as a base, 60 ꢀC) yielding 1-nitro-2-(oct-1-ynyl)benzene, were evaluated
indicating that the system involving 16 exhibit slightly higher catalytic efficiency (yields up to 99%,
TONs up to 2000; TOFs up to 280) as compared to the system based on 10 (yields up to 99%, TONs
up to 1400; TOFs up to 120). Moreover, catalytic activities of both systems are substantially higher
than the conventional one based on [PdCl2(PPh3)2] (yield 40%, TON 400; TOF 22). We also
employed 16 for the synthesis of 1-(dodeca-1,3-diyn-1-yl)-2-nitrobenzene from 1-iodo-2-nitroben-
zene and dodeca-1,3-diyne (in EtOH as a solvent, K2CO3 as a base, 50 ꢀC), and found that
aminocarbene complex 16 is significantly more efficient precatalysts for Sonogashira reaction with
diynes [as compared to the previously used Pd(OAc)2] providing 1-(dodeca-1,3-diynyl)-2-nitroben-
zene in 97% yield with TON up to 1400.
Introduction
alternative to the commonly used phosphine complexes.1-3
However, NHCs possess certain disadvantages mostly re-
garding to the fact that the preparation of the vast majority
of these species (and/or their precursors) for the further
Metal complexes with N-heterocyclic carbenes (heterocyclic
aminocarbenes or NHCs, Scheme 1A) are widely employed
in catalysis of various organic transformations as a powerful
ꢀ
(2) Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009,
253, 862–892.
(3) Herrmann, W. A.; Elison, M.; Fischer, J.; Koecher, C.; Artus,
*Corresponding author. kukushkin@VK2100.spb.edu.
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r
2011 American Chemical Society
Published on Web 01/28/2011
pubs.acs.org/Organometallics

864 Organometallics, Vol. 30, No. 4, 2011
Tskhovrebov et al.
Scheme 1. Examples of Metal Complexes with NHCs (A) and
Acyclic Aminocarbenes (B) and Preparation of Acyclic Amino-
carbene Complexes via a Metal-Mediated Nucleophilic Addition
to Isonitriles (C)
via an efficient reaction based on a metal-mediated nucleophilic
addition to isonitriles (Scheme 1C).18,19
As far as the generation of acyclic aminocarbene metal
species is concerned, most of the known protocols are based
on the addition of monofunctional (amines, alcohols) or
bifunctional (hydrazines, diamines, or amidines) nucleo-
philes to RNC ligands (see Results and Discussion).19 How-
ever, addition of unsymmetrical bifunctional nucleophiles,
i.e. containing two distinct nucleophilic centers in one mole-
cule, to coordinated isonitriles providing two regioisomers
was never previously reported. It is important that the
coupling with such nucleophiles, e.g., unsymmetrically sub-
stituted amidines, may proceed via different routes affording
structurally different products (Scheme 2; the chelates are
conditionally represented in the classical diaminocarbene
form with bond delocalization in the MCN2 fragment).
In order to obtain the first example of regioselectivity in
the addition to metal-activated isonitriles, we decided to
employ the unsymmetrical amidine, viz., N-phenylbenzami-
dine, for the coupling with both aromatic and aliphatic
isonitriles ligated to palladium- and platinum centers. Our
goals were 3-fold: (i) to study the regioselectivity of the
coupling between PdII and PtII-bound isonitriles and N-
phenylbenzamidine and to understand factors affecting the
coupling and its regioselectivity at both experimental and
theoretical levels, (ii) to verify structural features of thus
prepared aminocarbene species, and (iii) to evaluate the
catalytic properties of the aminocarbenes in Sonogashira
cross-coupling and to compare catalytic efficiencies for
complexes derived from the addition of N-phenylbenzami-
dine by its different nucleophilic sites. Our results are con-
secutively uncovered in this article.
coordination to metals represents a harsh synthetic task.4-6
In particular, when the target NHC species are unsymme-
trically substituted,7 preparative difficulties limit the possi-
bilities for the precise tuning of the steric properties for these
ligands, as it may be performed for the phosphines. In
addition, despite the fact that NHC ligands are able to
exhibit donor abilities comparable to phosphines,8-12 ra-
tional tuning of prepared metallocarbenes toward selected
catalytic application, is mostly achieved via the empiric
variation of the carbene structure. The latter may be justified
by the complexity and insufficient knowledge of mechanisms
(the so-called “black-box” character) of cross-coupling pro-
cesses. Hence, a continuous search for novel carbene-based
species and optimization of their properties toward selected
catalytic applications is an important task.
Results and Discussions
Surprisingly, acyclic aminocarbene species, structurally
related to cyclic NHCs, (Scheme 1B) have not yet attracted
the same attention in the catalysis despite their very long
history.13 It is important to point out that acyclic aminocar-
benes possess similar electronic stabilization as nonaromatic
NHCs, but their wider range of net electron-donor properties
and steric flexibility as compared to NHCs allow greater
control of both donor and steric properties.14-17 In addition,
complexes with acyclic aminocarbene ligands can be prepared
The predominate part of reports on generation of acyclic
aminocarbene metal species via nucleophilic addition to
metal-activated isonitriles deal with the coupling of RNC
ligands and nucleophiles containing one or two equal sp3-N-
or O-donor centers, viz. amines/hydrazines and alcohols,
resulting in diaminocarbene C(NR1R2)N(H)R and oxyamino-
carbene C(OR1)N(H)R ligands, correspondingly.18,19 Re-
cently, we also provided examples for the addition of sp2-N
nucleophiles such as acyclic (benzophenone imine20) and
heterocyclic (3-iminoisoindolin-1-one21) imines, and also
nucleophiles having an intermediate sp3/sp2-N hybridization
of the N donor site (hydrazones22) affording novel amino-
carbene-like complexes.
Regarding to addition of amidines to metal-bound isoni-
triles, the known reactions of [M(CNMe)4]2þ (M = PdII,
PtII) species with symmetrical amidines (such as acetamidine
or N,N0-dimethylformamidine) allows the generation of the
chelating aminocarbene ligands (Scheme 3).23 Note, despite
the fact that amidines given in Scheme 3 are shown as
(4) Glorius, F. Top. Organomet. Chem. 2007, 21, 1–20.
(5) Marion, N.; Nolan, S. P., Acc. Chem. Res. 2008, 41, 1440-1449,
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(10) Kelly, R. A., III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens,
E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P.
Organometallics 2008, 27, 202–210.
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(11) Diez-Gonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251,
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(18) Vignolle, J.; Catton, X.; Bourissou, D. Chem. Rev. 2009, 109,
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(14) Rosen, E. L.; Sanderson, M. D.; Saravanakumar, S.; Bielawski,
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(20) Luzyanin, K. V.; Guedes da Silva, M. F. C.; Kukushkin, V. Y.;
Pombeiro, A. J. L. Inorg. Chim. Acta 2009, 362, 833–888.
(21) Luzyanin, K. V.; Pombeiro, A. J. L.; Haukka, M.; Kukushkin,
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Article
Organometallics, Vol. 30, No. 4, 2011 865
Scheme 2. Routes for the Regioselective Addition of the Unsymmetrically Disubstituted Amidines to Metal-Bound Isonitriles
Scheme 3. Reported Examples for the Coupling between Isoni-
trile Ligands and Amidines
NH2 form (Scheme 2A) by both 1D and 2D NMR methods.
This observation is in complete agreement with the previous
studies on tautomerism of amidines,33-35 demonstrating that
for N-substituted species (bearing an electron-acceptor sub-
stituent at one of the N atoms) the tautomerinwhichtheCdN
bond is conjugated with this substituent, i.e. the NH2 form,
usually predominates.34,35
Reaction of Metal-Complexed Isonitriles with N-Phenyl-
benzamidine. We started our studies from an attempt to per-
form the reaction between an uncomplexed isonitrile (Ct
NCy) and 7. In this blank experiment, no reaction was observed
upon reflux of the reactants in CHCl3 for 2 d, and this indicates
that the addition of 7 along the CtN triple bond of an
isonitrile described below and depicted in Schemes 4-6, has
a metal-mediated character.
unsymmetrical species, two N centers are symmetric due to
the tautomeric effect (see later). In addition, structural fea-
tures of the product were not verified23 and it conditionally
represented in Scheme 3 as the classical aminocarbene with
the bond delocalization in the CN2 fragment.
It is known, that the reactivity of metal-bound RNtC
species toward nucleophiles strongly depends on a metal
center. Previously, we observed that the coupling of sp2-N
nucleophiles (imines) with both palladium(II) and platinum-
(II)-bound isonitriles, proceeds faster and with the higher
selectivity for the palladium-mediated reaction.21 Thus, we
decided to study in details the reaction between both PdII and
PtII-complexed isonitriles, viz., cis-[MCl2(CtNR)2] (1-6)
and 7 at different temperatures and varying ratios of the
reactants (Schemes 4-6).
(i). Reaction of cis-[MCl2(CtNXyl)2] [M = Pd (1), Pt (4)]
with 7 (Scheme 4). After addition of 1 equiv of 7 to
cis-[MCl2(CtNXyl)2] (M=Pd (1), Pt (4)) (Route A), the
reaction mixtures rapidly (ca. 5 min, 20-25 ꢀC) changed
their color from pale yellow to yellow. In the case of PdII,
the progress of the reaction was monitored by IR,
ESI-MS, and 1H NMR (for the latter method CDCl3
was used instead of CHCl3) and, after ca. 30 min, the
monitoring allowed the identification of only one formed
complex 8 (ca. 95% NMR yield); 8 originates from the
replacement of a chloride with deprotonated 7. Complex 8
undergoes slow decomposition, when this reaction mix-
ture was left to stand for 1 d at 20-25 ꢀC giving a mixture
of yet unidentified species (6 spots on TLC). In the case of
PtII, we observed a similar substitution reaction but it
proceeds slower (2 h for platinum complex 4 vs 5 min for
palladium complex 1) and with a lower selectivity as
compared to the palladium(II) system. Thus, 9 is formed
in ca. 80% NMR yield, in a mixture with two unidentified
compounds (detected by TLC).
For our study we addressed, on one hand, the known
palladium(II) and platinum(II) isonitrile complexes cis-[M-
Cl2(CtNR)2] [M=Pd, R=C6H3(2,6-Me2) (Xyl) (1), But (2),
cyclohexyl (Cy) (3); M=Pt, R=Xyl (4), But (5), Cy (6)]24-29
and, on the other hand, usymmetrical monosubstituted
amidine containing aromatic substituents, viz., N-phenyl-
benzamidine (7) [HNdC(Ph)NHPh, IUPAC name:30 N-
phenylbenzimidamide], previously employed for metal-
mediated coupling with nitrile ligands.31,32 The latter species
represents an easily accessible solid amidine, which can be
handled as a stable free base, whereas most of amidines are
only available as amidinium salts.
It is well-documented that amidines exist in a tautomeric
equilibrium in solution, which is believed to be an intramolecular
process of proton transfer between two N centers (Scheme 2A),
and the presence of these forms was unambiguously proved by a
series of chemical and physicochemical experiments.33,34 The
1H NMR spectrum of 7 in CDCl3 at 20-25 ꢀC indicated the
presence of a single tautomer, which was identified as the
(24) Bonati, F.; Minghetti, G. J. Organomet. Chem. 1970, 24, 251–
256.
(25) Crociani, B.; Boschi, T.; Belluco, U. Inorg. Chem. 1970, 9, 2021–
2025.
(26) Bertani, R.; Mozzon, M.; Michelin, R. A. Inorg. Chem. 1988, 27,
2809–2815.
(27) Michelin, R. A.; Zanotto, L.; Braga, D.; Sabatino, P.; Angelici,
R. J. Inorg. Chem. 1988, 27, 85–92.
(28) Michelin, R. A.; Zanotto, L.; Braga, D.; Sabatino, P.; Angelici,
R. J. Inorg. Chem. 1988, 27, 93–99.
(29) Bertani, R.; Mozzon, M.; Michelin, R. A.; Benetollo, F.;
Bombieri, G.; Castilho, T. J.; Pombeiro, A. J. L. Inorg. Chim. Acta
1991, 189, 175–187.
(30) http://www.iupac.org/nomenclature/index.html
(31) Bokach, N. A.; Kuznetsova, T. V.; Simanova, S. A.; Haukka,
M.; Pombeiro, A. J. L.; Kukushkin, V. Y. Inorg. Chem. 2005, 44, 5152–
5160.
Evaporation of the latter reaction mixtures led to the
solids, that were subjected to ESIþ-MS, IR, and 1H NMR
spectroscopy (upon redissolution in CDCl3) and the physico-
chemical data allows the formulation of amidinato complexes
8 and 9 (see Supporting Information for details). However,
(32) Sarova, G. H.; Bokach, N. A.; Fedorov, A. A.; Berberan-Santos,
ꢀ
M. N.; Kukushkin, V. Y.; Haukka, M.; Frausto da Silva, J. J. R.;
Pombeiro, A. J. L. Dalton Trans. 2006, 3798–3805.
(33) Shriner, R. L.; Neumann, F. W. Chem. Rev. 1944, 35, 351–425.
(34) Granik, V. G. Russ. Chem. Rev. 1983, 52, 377–393.
(35) Cook, M. J.; Katritzky, A. R.; Nadji, S. J. Chem. Soc., Perkin
Trans. 2 1976, 211–214.

866 Organometallics, Vol. 30, No. 4, 2011
Scheme 4. Reaction of cis-[MCl2(CtNXyl)2] (M = Pd (1), Pt (4)) with 7
Tskhovrebov et al.
Scheme 5. Reaction of cis-[MCl2(CtNBut)2] (M = Pd (2), Pt (5)) with 7
we were unable to isolate analytically pure 8 and 9 due to
their insufficient stability even at room temperature gradu-
ally converting to 10 and 11.
reflux of a mixture of 10 (or 11) with 2 equiv of 7 in CHCl3 for
ca. 8 h (Route D).
(ii). Reaction of cis-[MCl2(CtNBut)2] [M = Pd (2), Pt (5)]
with 7 (Scheme 5). In this case, both PdII- and PtII-mediated
reactions proceed significantly slower than the correspond-
ing processes with complexes 1 and 4 bearing XylNC (see
above). Thus, when a mixture of the equimolar amounts of
cis-[MCl2(CtNBut)2] (M = Pd (2), Pt (5)) and 7 in CHCl3
were used, the solution gradually (ca. 30 min for 2 and 3 h for
5; 20-25 ꢀC) changed their color from pale yellow to yellow.
In the case of PdII, the monitoring demonstrated the presence
of only one complex 14 (ca. 95% NMR yield) derived from
the replacement of one chloride ligand with deprotonated 7
(Route F); no further changes in the system were observed
even after its keeping for 1 d at 20-25 ꢀC. In the case of
platinum complex 5, corresponding product 15 was obtained
in ca. 75% NMR yield in a mixture with two unidentified
compounds (detected by TLC). Complexes 14 and 15 were
further refluxed in CHCl3 for 4 h accomplishing 16 and 17 in
Complexes 10 and 11 were also prepared by a single-pot
procedure upon reflux of a mixture of the equimolar
amounts of cis-[MCl2(CtNXyl)2] and 7 in CHCl3 for ca.
4 h (Route E). When a 1:2 mixture of cis-[MCl2(CtNXyl)2]
(M = Pd (1), Pt (4)) and 7 in CHCl3 was used, no
significant difference in the reaction rates for Route A
was observed at 20-25 ꢀC, but formed 8 and 9 (ca. 85%
and 75% yields, respectively) were strongly contaminated
with starting 7, its hydrochloride, and also with 10 and 11
(ca. 5% each), along with some other yet unidentified
species.
When a 1:4 mixture of cis-[MCl2(CtNXyl)2] and 7 was
refluxed in CHCl3 for ca. 8 h, the formation of bright yellow
complexes 12 (or 13) was accomplished in ca. 65% yield,
accompanied by precipitation of 7•HCl (Route C). Slightly
higher yields of 12 and 13 (ca. 70%) were achieved upon

Article
Organometallics, Vol. 30, No. 4, 2011 867
Scheme 6. Coupling of cis-[MCl2(CtNCy)2] (M = Pd (3), Pt (6)) with 7
ca. 85% isolated yield (Route G). Complexes 16 and 17 were
also prepared without intermediate isolation of 14 and 15
upon reflux of cis-[MCl2(CtNBut)2] and 7 in CHCl3 for ca.
4 h (Route H). When a 1:2 (or 1:4) mixture of cis-[MCl2(Ct
NBut)2] and 7 in CHCl3 was used, no significant difference in
the reaction rate for Route F was observed, but formed 14
and 15 (ca. 85% and 75% yields, respectively) were strongly
contaminated with an excess of starting 7, 7•HCl, and other
yet unidentified species. Attempts to prepare complexes struc-
turally relevant to 12 and 13 (Scheme 4), were unsuccessful even
upon a broad modification of reaction conditions, e.g., reflux in
either CHCl3 or MeNO2 and variation of molar ratios of
reagents up to 1:8.
(iii). Reaction of cis-[MCl2(CtNCy)2] [M = Pd (3), Pt (6)]
with 7 (Scheme 6). When a mixture of the equimolar amounts
of the complexes cis-[MCl2(CtNCy)2] (M = Pd (3), Pt (6))
and 7 in CHCl3 were used, the reaction mixtures rapidly at
20-25 ꢀC (ca. 5 min for palladium complex 3 and 1 h for
platinum complex 6) changed their color from pale yellow to
yellow. The monitoring after 1 h indicated the presence of a
mixture of two complexes, i.e. 18 and 20 (for PdII) or 19 and
21 (for PtII), formed in ca. 95% overall NMR yield and in ca.
1:1 ratio. (Route I).
Attempts of the isolation of 18-21 as pure species failed
due to their slow transformation even at 20-25 ꢀC to
aminocarbene complexes 22-25. At room temperature the
reactions are rather slow (ca. 5% of 22-25 were formed for
3 h), and do not accomplish high yields of the target species
even after 1 week at 20-25 ꢀC. In order to increase the rate,
the reaction mixtures containing 18-21 were further re-
fluxed in CHCl3 giving 22-25 in ca. 65% isolated yield after
ca. 4 h (Route J) in a mixture with other yet unidentified
compounds (4 spots on TLC). When a 1:2 mixture of
cis-[MCl2(CtNCy)2] and 7 in CHCl3 was used, no signifi-
cant difference in the reaction rate for Route I was observed,
formation of a bright yellow mixture of 26 and 27 was
accomplished after 8 h in ca. 45% isolated yield, accompa-
nied by precipitation of 7 HCl (Route K) and formation of
3
other yet unidentified species (4 spots on TLC). Our efforts
to obtain the corresponding platinum-based products via
Route K were unsuccessful. Moreover, no conversions of any
complexes 22-25 to 26 and 27 (expected Route L) were
observed under various reaction conditions (2-4 equivs 7,
reflux, 12-24 h) and molar ratios of reagents (up to 1:8).
Thus, upon reflux of 22-25, only gradual decomposition of
these species providing a broad mixture of yet unidentified
species (6 spots on TLC) was detected.
Theoretical Calculations. The substituent R dependent re-
activity of the ligated isonitriles in cis-[MCl2(CtNR)2] toward7
prompted us to use quantum-chemical methods (DFT, B3LYP
functional) for the estimate of thermodynamic characteristics of
the monochelated products derived from the coupling. The
calculations were performed for possible products trans-/cis-
1AR, trans-/cis-2AR, trans-/cis-1BR, and trans-/cis-2BR (R =
Xyl, But, Cy) depicted in Scheme 7.
For each reaction, two regioisomers (types A and B)
with various location of the amidine H atom (i.e., at the
endocyclic or exocyclic N atoms) and both trans- and cis-
configuration of the ligands were calculated. The results
indicate the following. First, for each R, the most stable
isomers of type A are trans-2AR. These isomers corre-
spond to product 16 isolated experimentally upon the
reaction of 2 (R=But) with 7. Second, the most stable
isomers of type B are cis-1BR corresponding to com-
plex 10 obtained upon the reaction of 1 (R=Xyl) with 7.
Third, the relative stability of trans-2AR vs cis-1BR clearly
decreases along the sequence of R = But > Cy > Xyl
(Scheme 8) well correlating with the experimental observa-
tions. Such a trend may be explained by a destabilization
of cis-1BCy and, in particular, cis-1BBut due to steric
repulsions between hydrogen(s) atom(s) of R and the metal-
bound carbon atom of the isonitrile ligand (Figure 1) (see
Supporting Information for details).
but the precipitate of 7 HCl was isolated.
3
When a 1:4 mixture of cis-[MCl2(CtNCy)2] (M = Pd 3)
and 7 in CHCl3 was used under reflux conditions, the

868 Organometallics, Vol. 30, No. 4, 2011
Scheme 7. Possible Products of the Reaction between cis-[PdCl2(CNR)2] and 7a
Tskhovrebov et al.
a The ΔGs values (in kcal/mol) are indicated relative to the most stable isomer.
steric repulsions between hydrogen atoms at the endocyclic
N atom and the R group (see Supporting Information). In
contrast, the structures of type B having the protonated end-
ocyclic nitrogen (trans-/cis-1BR) were found to be more
stable than trans-/cis-2BR, in agreement with X-ray data
for 10 and 11.
Fifth, for the structures with the protonated exocyclic
nitrogen, trans-isomers trans-2AR and trans-2BR are more
stable than corresponding cis-isomers cis-2AR and cis-2BR.
For the complexes with the protonated endocyclic nitrogen,
the opposite was found. This correlates with the experimen-
tal isolation of cis-10 and explains the observed geometrical
isomerization upon the reaction of cis-2 with 7 giving trans-16.
Characterization of Synthesized Complexes. Diaminocar-
bene and aminocarbene-like (10-13, 16, 17, and 22-27)
species were fully characterized by elemental analyzes (C, H,
N), high-resolution ESIþ-MS, IR, and 1D 1H and 13C{1H}
and 2D (1H,1H-COSY, 1H,13C-HMQC/1H,13C-HSQC, 1H,13-
C-HMBC) NMR spectroscopies (for detailed description of
this characterization as well as characterization of the inter-
mediate amidinato species (8, 9, 14, 15, 18-21) see Supporting
Information). Four complexes (10, 11, 16, and 28) were
characterized by a single crystal X-ray diffraction (Figures 2
and 3; Figures 1S and 2S and Tables 1S-3S of Supporting
Information). A rather poor quality of structure of 16 (crystals
are heavily twinned) allowed just the confirmation of the basic
structure of 16, but not precise inspection of its geometric
parameters. However, we succeeded in isolation from the same
reaction (Scheme 5, Route G) a few X-ray quality crystals of a
minor product (28), i.e., the complex obtained by the replace-
ment of the isonitrile ligand in 16 with one molecule of 7. High
crystallographic quality structure of 28 allows the qualitative
comparison of bond lengths and angles.
Figure 1. Equilibrium structure of cis-1BCy. The steric repul-
sion between the C and the H atoms (indicated by the arrow)
provides the destabilization of the structure.
Fourth, as can be inferred from the inspection of the X-ray
data for 10 and 11, the hydrogen atom in the structures of
type B is localized at the endocyclic N atom. However, the
site of the hydrogen location in the complexes of type A was
not elucidated by the X-ray diffraction (see below). The
theoretical calculations allow to clarify this situation and
indicate that in the complexes of type A the protonation of
the exocyclic N is preferable (structures trans-/cis-2AR)
except pair cis-1AXyl-cis-2AXyl with the same stability of
both species. This is conceivably accounted for by intramo-
lecular interactions. Namely, in the pairs trans-1AR/trans-
2AR, structures trans-2AR are stabilized by the intramole-
cular H-bond with the Cl atom. In the pairs cis-1AR/cis-
2AR, structures cis-1ABut and cis-1ACy are destabilized by

Article
Organometallics, Vol. 30, No. 4, 2011 869
Figure 3. View of 28 with the atomic numbering schemes.
Thermal ellipsoids are drawn with the 50% probability. Hydro-
gen labels were omitted for simplicity, and only one molecule
from the dimeric unit is shown.
Figure 2. View of 10 with the atomic numbering schemes.
Thermal ellipsoids are drawn with the 50% probability. Hydro-
gen labels were omitted for simplicity, and only one molecule
from the dimeric unit is shown.
of internal alkynes starting from the terminal CtC
species.39,40 Although many palladium catalysts bearing
nitrogen and phosphorus ligands have been studied as
catalysts for this reaction, only few systems involving PdII
species with NHCs have been employed.41-52 Moreover,
despite evident resemblance between N-heterocyclic and
acyclic diaminocarbenes (see Introduction), examples for
Sonogashira reaction catalyzed by acyclic diaminocarbene
palladium complexes are scarce and represented by only
one report emerged until the current study.53
The catalytic part consists of two subsections. First, we
employed a model Sonogashira reaction to evaluate the
difference in catalytic properties for systems based on two
types of palladium aminocarbene species (represented by 10
and 16). Second, we applied the most efficient precatalyst for
the cross-coupling involving diynes; these difficult-to-obtain
substrates are very little explored in Sonogashira reaction.
Our results are disclosed in two subsections that follow.
(i). Systems Based on 10 and 16 in the Model Sonogashira
Cross-Coupling. Many recent studies have demonstrated that
the cyclization of substituted aryl- and hetarylacetylenes (in
particular, those containing ortho-substituents in the aromatic
Figures 2 (and 1S, Supporting Information) and 3 (and
2S) represent two types of complexes generated from the
nucleophilic addition of 7 to the isonitrile complexes. The
first type (compounds 10-13; Scheme 4, Figure 2) includes
the complexes formed via the addition by the NHPh center,
while the second type comprises complexes (16, 17, and 28;
Scheme 2, Figure 3) generated via the addition by the
HNdC center. All these structural data support our ideas
on the substituent dependent regioselectivity of the coupling.
Detailed description of X-ray data can be found in Support-
ing Information and here one should stress one important
issue. When complexes with the aromatic XylNC were
employed, obtained carbene ligands exhibit distinct conse-
quence of the double and the single bonds in the MCN2
fragment. We previously observed this unusual phenomenon
for the relevant [MCl{C(NdC(C6R1R2R3R4CON))d
N(H)R}(CtNR)] (M=Pd, Pt; R=Cy, But, Xyl; R1-R4=
H; R1, R3, R4 = H, R2 = Me; R1, R4 = H, R2, R3 = Cl)
complexes21 and those compounds along with the current
examples provide a novel family of aminocarbene-like species
(Scheme 8). For aliphatic ButNC, corresponding carbene
possess bond delocalization in the MCN2 functionality and
belong to the classical diaminocarbene systems (Scheme 8).
Evaluation of the Catalytic Properties of Systems on the
Basis of the Novel Diaminocarbene/Aminocarbene-Like Spe-
cies in Sonogashira Cross-Coupling. Having two structural
types of aminocarbene ligands at our disposal, it was tempt-
ing to get initial data on their catalytic activity in some
organic transformations. Taking into account our experi-
ence in the alkyne chemistry,36-38 it is not surprising that
Sonogashira reaction was selected as the starting point for
these studies. Previously palladium-catalyzed Sonogashira
cross-coupling system has been widely used for the synthesis
(39) Sonogashira, K., Metal-Catalyzed Cross-Coupling Reactions. In
Stang, P. J.; Diederich, F., Eds. Wiley: Weinheim, Germany, 1998; pp
203-229.
(40) Anastasia, L.; Negishi, E.-I. Chem. Rev. 2003, 103, 1979–2018.
(41) Gu, S.; Chen, W. Organometallics 2009, 28, 909–914.
(42) Ray, L.; Barman, S.; Shaikh, M. M.; Ghosh, P. Chem.;Eur. J.
2008, 14, 6646–6655.
(43) Kim, J. H.; Lee, D. H.; Jun, B. H.; Lee, Y., S. Tetrahedron Lett.
2007, 48, 7079–7084.
(44) Altenhoff, G.; Wurtz, S.; Glorius, F. Tetrahedron Lett. 2006, 47,
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2925–2928.
(45) Sommer, W., J.; Weck, M. Adv. Synth. Catal. 2006, 348, 2101–
2113.
(46) Tykwinski, R. R. Angew. Chem., Int. Ed. 2003, 42, 1566–1568.
(47) Mas-Marza, L.; Segarra, A., M.; Claver, C.; Peris, E.; Fernandez,
E. Tetrahedron Lett. 2003, 44, 6595–6599.
(48) Eckhardt, M.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 13642–
13643.
(49) Batey, R. A.; Shen, M.; Lough, A. J. Org. Lett. 2002, 4, 1411–
1414.
(50) Yang, C.; Nolan, S., P. Organometallics 2002, 21, 1020–1022.
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748.
(52) Herrmann, W. A.; Reisinger, C. P.; Spiegler, M. J. Organomet.
Chem. 1998, 557, 93–96.
(36) Balova, I. A.; Morozkina, S. N.; Sorokoumov, V. N.; Vinogradova,
O. V.; Knight, D. W.; Vasilevsky, S. F. Eur. J. Org. Chem. 2005,
882–888.
(37) Vinogradova, O. V.; Sorokoumov, V. N.; Vasylevsky, S. F.;
Balova, I. A. Tetrahedron Lett. 2007, 48, 4907–4909.
(38) Vinogradova, O. V.; Sorokoumov, V. N.; Balova, I. A. Tetra-
hedron Lett. 2009, 50, 6358–6360.
(53) Dhudshia, B.; Thadani, A. N. Chem. Commun 2006, 668–670.

870 Organometallics, Vol. 30, No. 4, 2011
Tskhovrebov et al.
˚
Scheme 8. Bond Distances (A) in Classical Diaminocarbene vs Novel Aminocarbene-like Species
Scheme 9. Model Sonogashira Catalytic System Employed for Our Studies
Table 1. Evaluation of the Catalytic Properties of 10 and 16 in Sonogashira Coupling
Entry
1
Catalyst
10
Catalyst loading, mol %
0.07
Solvent
THF
Base
Et3N
Time, h
96
Yield (isolated yield), %
99 (94)
TON
1400
TOF, h-1
15
2
3
4
5
6
7
8
9
10
11
12
13
16
10
16
10
10
16
10
16
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.1
THF
THF
THF
Et3N
Et3N
Et3N
68
24
12
24
24
24
14
5
99
99
99
20
-
10
99 (96)
99
40
1400
1400
1400
290
-
140
1400
1400
400
990
990
15
60
120
12
-
6
100
280
22
55
250
110
THF
Et3N
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
[PdCl2(PPh3)]2
18
18
4
10
16
16
0.1
0.1
0.05
99
99
99
18
1980
For all runs, 1-iodo-2-nitrobenzene (0.610 g, 2.45 mmol) and oct-1-yne (0.550 g, 5.00 mmol) were used. For all runs, except 6
and 7, CuI (0.045g, 9.60 mmol %) was added as co-catalyst, and for all runs except 5, PPh3 (0.030 g, 4.60 mmol %) was used as
the catalyst activator. All runs were performed at 60 ꢀC, except 1 and 2 (those were performed at 20 ꢀC) until disappearance of the
starting 1-iodo-2-nitrobenzene. Yields are based on 1H NMR integration.
ring, e.g. 2-nitroarylacetylens54) represents an efficient route
for the preparation a wide range of pharmacologically im-
portant heterocycles such as indoles, indolo[1,2-c]quina-
zolines, quinolines/isoquinolines, furopyridine, and pyra-
zolopyridines (for a recent review see ref55). This fact
provided a motivation to investigate an interplay between
1-iodo-2-nitrobenzene and oct-1-yne accomplishing 1-ni-
tro-2-(oct-1-ynyl)benzene as a model reaction for our catalytic
studies (Scheme 9). As components of catalytic systems we
addressed 10 and 16, derived from the addition of 7 via its
different nucleophilic centers to (RNC)PdII species as depicted
in Schemes 4 and 5.
With palladium complexes 10 and 16 in hand, we first
examined their catalytic activity for the coupling of the men-
tioned substrates under traditionally employed conditions
[THF as solvent, Et3N as a base, and CuI as a cocatalyst,
and PPh3 as a precatalyst activator41] (Table1, entries1-5) and
observed the full conversion of the starting 1-nitro-2-iodoben-
zene within 96 h (for 10) or 68 h (for 16) at room temperature to
furnish the target 1-nitro-2-(oct-1-ynyl)benzene in 99% yield
(entries 1 and 2). Increase in the reaction temperature up to
60 ꢀC resulted in 99% product yield accomplished for 24 h
(for 10) and 12 h (for 16) (entries 3 and 4). We also found that
the presence of PPh3 in the system is essential for the coupling
and without the added PPh3 1-nitro-2-(oct-1-ynyl)benzene was
obtained in only 20% yield after 24 h (entry 5).
We have recently reported that acyclic diaminocarbene
precatalysts operate efficiently in the Suzuki-Miyaura cross-
coupling under sustainable conditions (EtOH as solvent, K2-
CO3 as a base).22 Inspired by these results, we conducted the
model reaction in EtOH as solvent and using K2CO3 as a base
(Table 1, entries 6-13). The catalytic test without addition of
CuI (entries 6 and 7) afforded no target 1-nitro-2-(oct-1-
ynyl)benzene (for 10) or it was obtained in low (10%) yield (for
16) after 24 h at 60 ꢀC, thus suggesting that the catalytic cycle
for both 10 and 16 involves CuI as the cocatalyst. Indeed, in the
presence of CuI, 1-nitro-2-(oct-1-ynyl)benzene was obtained at
60 ꢀC in 99% yield after 14 h (for 10, entry 8) or after 5 h (for 16,
entry 9).
As the next step, we also compared the catalytic properties
of 10- and 16-based systems with the widely used one based
on [PdCl2(PPh3)2]56 (entries 10-12). After 3 h at 60 ꢀC,
(54) Kim, J. S.; Han, J. H.; Lee, J. J.; Jun, Y. M.; Lee, B. M.; Kim,
B. H. Tetrahedron Lett. 2008, 49, 3733–3738.
(55) Kirsch, G.; Hesse, S.; Comel, A. Curr. Org. Synth. 2004, 1, 47–63.
(56) Bailar, J. C. J.; Itatani, H. Inorg. Chem. 1965, 4, 1618–1620.

Article
Organometallics, Vol. 30, No. 4, 2011 871
Scheme 10. Sonogashira Reaction with Dodeca-1,3-diyne Furnishing 1-(Dodeca-1,3-diynyl)-2-nitrobenzene
[PdCl2(PPh3)2] allowed the formation of the target product
in ca. 35% yield, approaching the total 40% yield after
next 15 h (entry 10). Under similar conditions, 10-based
system demonstrated slightly better catalytic activity pro-
viding ca. 40% conversion within 3 h, and increasing to ca.
99% after next 15 h (entry 11). At the same time, for 16 nearly
quantitative conversion (99%) of substrates was achieved in 3 h
(entry 12), and this explicitly indicates that 16-based system is
more catalytically efficient for the coupling as compared to
both 10- and the conventionally used one based on [PdCl2-
(PPh3)2]. Finally, we also searched to optimize TON for more
active complex 16 and found that maximum TON of 1980 (at
99% conversion of the substrates) is accomplished with [16]=
0.05 mol %.
(ii). Sonogashira Cross-Coupling with Terminal Diyne
Catalyzed by 16-based System. As it was indicated previously,
cyclization of the ortho-substituted aryl- and hetarylacetyl-
enes allows the preparation of various heterocyclic species,
while corresponding cyclization reactions of ortho-diynyl-
substituted arenes and hetarenes are practically unexplored
due to a lack of synthetic procedures and suitable catalysts (or
precatalysts) for their synthesis. Following the project
on chemistry of functionalized diynes conducted by two of
us,37,38 we employed 16-based system for the synthesis of
1-(dodeca-1,3-diynyl)-2-nitrobenzene from 1-iodo-2-nitroben-
zene and dodeca-1,3-diyne (Scheme 10; see Supporting Infor-
mation for detailed procedure). Reaction was performed under
conditions indicated in Table 1 (entry 9), but due to a poor
stability of the diyne, the catalytic run was performed at lower
(50 ꢀC) temperature. The starting 1-iodo-2-nitrobenzene was
fully transformed to the target 1-(dodeca-1,3-diynyl)-2-nitro-
benzene (97% isolated yield) for 6 h and this reaction is
characterized with TON = 1400. These results indicate that
16, as precatalyst, gives significantly more efficient catalytic
species for Sonogashira reaction with diynes as compared to the
previously used36 Pd(OAc)2 [in previous work36 withPd(OAc)2,
1-(dodeca-1,3-diynyl)-2-nitrobenzene was obtained in 80%
yield and high precatalyst loading (10 mol %)].
fragment and belong to a novel family of aminocarbene-like
species. The carbene derived from ButNC, exhibits the
delocalization in the MCN2 functionality and belong to the
classical diaminocarbene systems (Scheme 1, B). Further
studies on the aminocarbene-like type of ligands performed in
our group will be aimed on extending number of these species,
inspection of their geometrical parameters, and understanding
factors that determine the preference in formation of amino-
carbene-like vs. classical diaminocarbene structure.
Second, we detected the first example of regioselectivity in
addition of bifunctional nucleophiles to isonitriles. Thus, the
interplay between N-phenylbenzamidine and a metal-bound
isonitrile is strongly affected by the nature of the substituent
R of the isonitrile. When aromatic isonitrile is used (R=Xyl),
N-phenylbenzamidine is coordinated to a metal by the HNd
C moiety (Scheme 4), and the nucleophilic attack proceeds
via the NHPh center of the benzamidine. For R=But, N-
phenylbenzamidine is coordinated to a metal by the NHPh
center, and the addition occurs via the HNdC center of the
nucleophile (Scheme 5). When R=Cy, we observed a mixture
of two regioisomers that are derived from the addition of
N-phenylbenzamidine bytwonucleophiliccenters(Scheme6).
This experimentally observed substituent R dependent reac-
tivity of isonitriles in cis-[MCl2(CtNR)2] toward N-phenyl-
benzamidine was explored using the theoretical (DFT)
methods and interpreted as a result of the steric repulsions
in one of the regioisomers of the addition products, when R=
Cy and But. The thermodynamic stability of various isomers
of the reaction products was calculated and analyzed. It is
important to point out that examples for the metal-mediated
regioselective addition to CtN triple bond are scarce, and the
formation of two different products was observed only for the
addition of N,N0-diphenylguanidine to metal-bound dialkyl-
cyanamides, R2NCtN.57 To the best of our knowledge, no
regioselective additions to isonitriles RNC were reported
before this study.
Third, we evaluated catalytic properties for systems
based on two types of palladium aminocarbene species,
i.e. 10 and 16, derived from the regioselective addition of
N-phenylbenzamidine to PdII-bound isonitrile by its dif-
ferent nucleophilic centers. We found that both systems
exhibit catalytic activities in Sonogashira cross-coupling
of 4-NO2C6H4I with oct-1-yne yielding 1-nitro-2-(oct-1-
ynyl)benzenes (yields are up to 99%, TONs up to 2000),
under relatively mild and sustainable conditions (in non-
dried EtOH as a solvent, K2CO3 as a base, air, 60 ꢀC).
Moreover, we found that 16-based system exhibits slightly
higher catalytic efficiency (yields up to 99%, TONs up to
2000; TOFs up to 280) as compared to one based on 10
Final Remarks. The results of this work may be considered
from the following perspectives. First, searching for novel
acyclic aminocarbene species, we succeeded in preparation of
metal complexes containing chelating diaminocarbene or
aminocarbene-like ligands via the reaction between the un-
symmetrical amidine, i.e., N-phenylbenzamidine, and one or
two isonitriles in cis-[MCl2(CtNR)] (M=PdII, PtII; R=Xyl,
But, Cy) (see Schemes 4-6). The formation of these species
occur via the two-step process, involving the initial coordi-
nation of N-phenylbenzamidinate to a metal center followed
by an intramolecular attack of the remote nucleophilic center
of the coordinated amidinate on the CN triple bond of the
isonitrile. The products derived from XylNC exhibit distinct
consequence of the double and the single bonds in the MCN2
(57) Gushchin, P. V.; Kuznetsov, M. L.; Haukka, M.; Wang, M.-J.;
Gribanov, A. V.; Kukushkin, V. Y. Inorg. Chem. 2009, 48, 2583–2592.

872 Organometallics, Vol. 30, No. 4, 2011
Tskhovrebov et al.
(yields up to 99%, TONs up to 1400; TOFs up to 120), and
substantially higher activity than for the conventionally
used system based on [PdCl2(PPh3)2] (yield 40%, TONs up
to 400; TOFs up to 22). We also employed 16 for the
synthesis of 1-(dodeca-1,3-diynyl)-2-nitrobenzene from
1-iodo-2-nitrobenzene and dodeca-1,3-diyne (in EtOH as
a solvent, K2CO3 as a base, 50 ꢀC), and found that 16 is
significantly more efficient precatalysts for Sonogashira
reaction with diynes [as compared to the previously used
Pd(OAc)2] providing 1-(dodeca-1,3-diyn-1-yl)-2-nitro-
benzene in 97% yield with one of the best reported TONs
of 1400. It is important to notice that the observed
difference in the catalytic activity between 10 and 16 could
be justified by either different steric/donor properties of
the R group of the ligands and/or by the different structure
of these ligands.
Finally, it is worth mentioning, that the catalytic system
based on 16 or 10 is not sensitive to air and moisture, operate
under mild and sustainable conditions (nondried EtOH
having low environmental impact) and accomplishes the
target compounds in high yields. We believe that upon
proper derivatization of the carbene catalyst (e.g., by em-
ploying of other substituted amidines toward isonitriles),
and further optimization of the reaction conditions, sustain-
able catalytic systems for Sonogashira coupling may be
further developed.
were collected on a Nonius KappaCCD diffractometer using
59
˚
Mo KR radiation (λ=0.710 73 A). The Denzo-Scalepack or
EvalCCD60 program packages were used for cell refinements
and data reductions. The structures were solved by direct methods
using the SIR9761 or SHELXS-9762 program with the WinG463
graphical user interface. A semiempirical64 (10) or analytical65 (11)
absorption correction was applied to data. Structural refine-
ments were carried out using SHELXL-97.62 In 10 and 11, the
NH hydrogen atoms were located from difference Fourier
maps and refined isotropically. In 28, the NH hydrogen atoms
were also located from the difference Fourier map but con-
strained to ride on their parent atom Uiso = 1.2-1.5 Ueq
-
(parent atom). Other hydrogen atoms were positioned
geometrically and constrained to ride on their parent atoms,
˚
with C-H = 0.95-0.98 A and Uiso = 1.2-1.5 Ueq(parent
atom). The crystallographic details are summarized in Table
S1 of the Supporting Information. CCDC 795512-795514
contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Synthetic Work. General Procedure for Reaction of
cis-[MCl2(CtNR)2] (1-6) and HNdC(Ph)NHPh (1 equiv).
Solid 7 (0.049 g, 0.250 mmol) was added to a solution (R =
But, Cy) or a suspension (R=Xyl) of cis-[MCl2(RNC)2] (0.250
mmol) in CHCl3 (5 mL). The reaction mixture was refluxed for
4 h. During the reaction time, the color of the mixture turned
from light orange to bright yellow. After 4 h, the reaction mix-
ture was filtered off from some undissolved material and then
evaporated to dryness at 40-45 ꢀC, and the solid residue was
extracted with two 5 mL portion of CH2Cl2. The bright yellow
solution was evaporated to dryness at room temperature,
washed with three 1 mL of Et2O, and dried in air at 20-25 ꢀC.
Yields were 65-85%, based on the metal.
10. Anal. Calcd for C31H29N4ClPd: C, 62.19; H, 4.88; N, 9.36.
Found: C, 62.47; H, 4.31; N, 9.12. HRMS (ESIþ, 105 V,
MeCN): found, 564.1532 [M - Cl]þ; calcd for C31H29N4Pd,
564.1521. IR (KBr, selected bands, cm-1): ν(N-H) 3362 (m),
ν(C-H) 2955 (m), ν(CtN) 2191 (s), ν(NdCcarbene, NdC) 1635
(s), 1588 (s), δ(C-H from Ar) 751 (s), 695 (s). 1H NMR (CDCl3,
δ): 7.35-7.28 (m), 7.24-6.94 (m), 6.66 (d), 6.11 (t, 17H, CdNH,
aryls), 2.21 (s), 2.15 (s, 12H, Me’s). 13C{1H} NMR (CDCl3, δ):
171.2 (CdN from carbene), 163.8 (CdNH), 150.6, 139.3 (C
from Ph’s), 134.3-123.3 (C and CH from aryls), 19.4 and 18.6
(Me’s).
11. Anal. Calcd for C31H29N4ClPt: C, 54.13; H, 4.25; N, 8.15.
Found: C, 54.36; H, 4.38; N, 7.80. HRMS (ESIþ, 105 V,
MeCN): found, 689.1530 [M]þ; calcd for C31H29N4ClPt,
689.1753. IR (KBr, selected bands, cm-1): ν(N-H) 3437 (m),
ν(C-H) 2977 (m), ν(CtN) 2185 (s), ν(NdCcarbene, NdC) 1610
(s), 1584 (s), δ(C-H from Ar) 756 (s), 693 (s). 1H NMR (CDCl3,
δ): 7.53 (s, 1H, CdNH), 7.39-7.27 (m), 7.25-6.95 (m), 6.63 (d),
6.05 (t, 16H, aryls), 2.26 (s), 2.13 (s, 12H, Me’s). 13C{1H} NMR
(CDCl3, δ): 171.7 (CdN from carbene), 162.0 (CdNH), 151.0,
139.0 (C from Ph’s), 134.5-122.9 (C and CH from aryls), 19.3
and 18.6 (Me’s).
Experimental Section
Materials and Instrumentation. Solvents, PdCl2, K2[PtCl4], all
isonitriles and all reagents for catalytic studies were obtained
from commercial sources and used as received, apart from
chloroform that was purified by conventional distillation over
calcium chloride. N-Phenylbenzamidine was prepared by the
known protocol based on a nucleophilic addition of PhNH2 to
PhCN in the presence of AlCl3.58 The complexes cis-[PdCl2-
(RNC)2] (R=Xyl (1), But (2), Cy (3)) and cis-[PtCl2(RNC)2]
(R = Xyl (4), But (5), Cy (6)) were prepared as previously
reported.24-29 C, H, and N elemental analyses were carried
out by the Department of Organic Chemistry of St. Petersburg
State University on a 185B Hewlett-Packard carbon hydrogen
nitrogen analyzer. Electrospray ionization mass spectra were
obtained on a Bruker micrOTOF spectrometer equipped with
electrospray ionization (ESI) source. The instrument was oper-
ated in positive ion mode using a m/z range of 50-3000. The
capillary voltage of the ion source was set at -4500 V
(ESIþ-MS) and the capillary exit at 70-150 V. The nebulizer
gas flow was 0.4 bar and drying gas flow 4.0 L/min. For ESI
species were dissolved in MeCN. In the isotopic pattern, the
most intensive peak is reported. Mass calibration for data
system acquisition was achieved using CsI. Infrared spectra
(4000-400 cm-1) were recorded on a Shimadzu FTIR 8400S
instrument in KBr pellets. 1H and 13C{1H} NMR spectra were
measured on a Bruker-DPX 300 spectrometer at ambient tem-
perature, while 2D (1H,1H-COSY, 1H,13C-HMQC, 1H,13C-
16. Anal. Calcd for C23H29N4ClPd: C, 54.96; H, 5.82; N,
11.15. Found: C, 54.93; H, 5.83; N, 11.19. HRMS (ESIþ, 105 V,
MeCN): found, 506.1295 [M þ H]þ; calcd for C23H30N4ClPd,
506.1288. IR (KBr, selected bands, cm-1): ν(N-H) 3283 (m),
1
HSQC, and H,13C-HMBC) NMR spectra were acquired on
Bruker Avance IIþ 400 MHz (UltraShield Magnet) and Bruker
Avance IIþ 500 MHz (UltraShield Plus Magnet) spectrometers
at ambient temperature.
X-ray Structure Determinations. The crystals of 10, 11, and 28
were immersed in cryo-oil, mounted in a Nylon loop, and
measured at a temperature of 100 K. The X-ray diffraction data
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Article
Organometallics, Vol. 30, No. 4, 2011 873
ν(C-H) 2983 (m), ν(CtN) 2210 (s), ν(NdCcarbene, NdC) 1537
(s), 1506 (s), δ(C-H from Ar) 700 (s). 1H NMR (CDCl3, δ): 7.85
(s, 1H, CdNH), 7.30-7.27 (m), 7.24-6.93 (m, 10H, Ph’s), 1.49
(s) and 1.17 (s, 18H, Me’s). 13C{1H} NMR (CDCl3, δ): 190.4
(CdN from carbene), 180.6 (CdNH), 150.9, 146.4, 133.1-122.2
(C and CH from Ph’s), 56.1 and 54.9 (C from But’s), 30.2, 29.7,
29.3, and 29.0 (Me’s).
13. Anal. Calcd for C44H40N6Pt: C, 62.33; H, 4.75; N, 9.91.
Found: C, 62.35; H, 4.72; N, 10.12. HRMS (ESIþ, 70 V,
MeCN): found, 847.2958 [M]þ; calcd for C44H40N6Pd, 847.
2962. IR (KBr, selected bands, cm-1): ν(N-H) 3442 (m),
ν(C-H) 2912 (m), ν(NdCcarbene, NdC) 1641 (s), 1578 (s),
1
δ(C-H from Ar) 782 (s), 740 (s), 688 (s). H NMR (CDCl3,
δ): 8.02 (s, br, 2H, CdNH), 7.62-7.29 (m), 7.22-6.50 (m, 26H,
CdNH, aryls), 2.54 (s) and 2.32 (s, 12H, Me’s). 13C{1H} NMR
(CDCl3, δ): 172.1 (CdN from carbenes), 164.0 (CdNH), 145.5
and 134.7 (C from Ph’s), 132.9-123.5 (C and CH from aryls),
19.6 and 17.6 (Me’s).
Mixture of 26 and 27. Anal. Calcd for C40H44N6Pd: C, 67.17;
H, 6.20; N, 11.75. Found: C, 67.46; H, 6.27; N, 11.58. HRMS
(ESIþ, 105 V, MeCN): found, 714.2648 [M]þ; calcd for
C40H44N6Pd, 714.2662. IR (KBr, selected bands, cm-1): ν(N-
H) 3325 (m), ν(C-H) 2932 (s), ν(NdCcarbene, NdC) 1642 (s),
1585 (s), δ(C-H from Ar) 702 (s). 1H NMR (CDCl3, δ): 7.92 (s,
br, 4H, CdNH), 7.56-7.30 (m), 7.26-6.34 (m, 40H, Ph’s), 5.72
(s, br, 4H, CdNH), 4.32 (s, 2H) and 4.22 (s, 2H, CH), 2.22-1.30
(m, 40H, CH2). 13C{1H} NMR (CDCl3, δ): 178.2 and 171.5
(CdN from carbene), 168.5 and 166.4 (CdNH), 157.8, 152.7,
146.4, 142.3 (C from Ph’s), 134.1-121.2 (Ph’s), 52.4 and 51.0
(CH from Cy’s), 34.8-21.2 (CH2).
Computational Details. The full geometry optimization of all
structures has been carried out at the DFT/HF hybrid level of
theory using Becke’s three-parameter hybrid exchange func-
tional in combination with the gradient-corrected correlation
functional of Lee, Yang, and Parr (B3LYP)66,67 with the help of
the Gaussian-0368 program package. This functional was found
to be a quite reasonable one for the investigation of reactivity,
structural and spectral properties of Pt and Pd complexes with
nitriles and isonitriles.69-72 No symmetry operations have been
applied. The geometry optimization was carried out using a
quasi-relativistic Stuttgart pseudopotential that described 28
core electrons and the appropriate contracted basis set
(8s7p6d)/[6s5p3d]73 for the palladium atom and the 6-31G basis
set for other atoms. Then, single-point calculations were per-
formed on the basis of the equilibrium geometries found using
the 6-31þG(d) basis set for nonmetal atoms. The Hessian matrix
was calculated analytically in order to prove the location of
correct minima (no imaginary frequencies) and to estimate the
thermodynamic parameters, the latter being calculated at 25 ꢀC.
17. Anal. Calcd for C23H29N4ClPt: C, 46.68; H, 4.94; N, 9.47.
Found: C, 47.05; H, 5.14; N, 9.16. HRMS (ESIþ, 70 V, MeCN):
found, 557.2376 [M - Cl]þ; calcd for C23H30N4ClPt, 557.2142.
IR (KBr, selected bands, cm-1): ν(N-H) 3445 (m), ν(C-H)
2978 (m), 2931 (m), ν(CtN) 2199 (s), ν(NdCcarbene, NdC) 1625
(s), 1519 (s), δ(C-H from Ph) 696 (s). 1H NMR (CDCl3, δ): 7.88
(s, 1H, CdNH), 7.14-6.64 (m, 10H, Ph’s), 1.62 (s) and 1.55 (s,
18H, Me’s). 13C{1H} NMR (CDCl3, δ): 180.6 (CdN from car-
bene), 162.7 (CdNH), 150.9 and 146.5 (C from Ph’s), 130.7-122.9
(CH from Ph’s), 30.3, 29.7, 29.3, and 29.1 (C from But’s).
Mixture of 22 and 24. Anal. Calcd for C27H33N4ClPd: C,
58.46; H, 6.00; N, 10.10. Found: C, 58.64; H, 6.17; N, 10.03.
HRMS (ESIþ, 105 V, MeCN): found, 557.1547 [M]þ; calcd for
C27H33N4ClPd, 557.1522. IR (KBr, selected bands, cm-1):
ν(N-H) 3331 (m), ν(C-H) 2929 (s), 2853 (s), ν(CtN) 2221
(s), ν(NdCcarbene, NdC) 1628 (s), 1593 (s), δ(C-H from Ar) 694
(s). 1H NMR (CDCl3, δ): 7.88 (s, 1H, CdNH), 7.54-7.31 (m),
7.23-6.24 (m, 20H, Ph’s), 5.75 (s, 1H, CdNH), 4.23 (m, 2H),
4.02 (m, 1H), 3.46 (m, 1H, CH), 2.15-1.20 (m, 40H, CH2).
13C{1H} NMR (CDCl3, δ): 181.2 and 179.8 (CdN from
carbene), 169.5 and 165.2 (CdNH), 157.6, 150.8, 146.4 (C from
Ph’s), 133.1-121.7 (Ph’s), 54.9, 54.7, 53.2, 52.4 (CH from Cy’s),
33.9-14.2 (CH2).
Mixture of 23 and 25. Anal. Calcd for C27H33N4ClPt: C,
50.37; H, 5.17; N, 8.71. Found: C, 50.18; H, 5.42; N, 8.92.
HRMS (ESIþ, 105 V, MeCN): found, 608.2477 [M - Cl]þ; calcd
for C27H33N4Pt, 608.2377. IR (KBr, selected bands, cm-1):
ν(N-H) 3428 (m), ν(C-H) 2928-2853 (s), ν(CtN) 2212 (s),
ν(NdCcarbene, NdC) 1620 (s), 1569 (s), δ(C-H from Ar) 693 (s).
1H NMR (CDCl3, δ): 7.87 (s, br, 1H, CdNH from carbene),
7.54-7.29 (m), 7.24-6.52 (m), 6.30 (d 20H, Ph’s), 4.28 (m) and
4.00-3.42 (m, 4H, CH), 2.15-1.17 (m, 40H, CH2); HNdC(Ph)
is not resolved. 13C{1H} NMR (CDCl3, δ): 182.0 and 169.6
(CdN from carbene), 165.2 and 164.1 (CdNH), 148.5 and 141.3
(C from Ph’s), 130.8-123.8 (Ph’s), 55.1, 54.8, 52.7, and 52.9 (CH
from Cy’s), 33.9-14.2 (CH2).
General Procedure for the Reaction between cis-[MCl2-
(CtNR)2] (1-6) and HN=C(Ph)NHPh (4 equiv). Solid 7
(0.100 g, 0.510 mmol) was added to a solution (R=Cy, But)
or a suspension (R=Xyl) of cis-[MCl2(RNC)2] (0.127 mmol) in
CHCl3 (3 mL). The reaction mixture was then refluxed for ca. 8 h
under vigorous stirring. During the reaction time, the color of
the mixture turned from light orange to bright yellow and then
to dark yellow. After 8 h, the reaction mixture was filtered off
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Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
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Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
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Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
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from the released 7 HCl and then evaporated to dryness at
3
90 ꢀC, and the solid residue was extracted with two 5 mL por-
tions of CH2Cl2. The dark yellow solution was filtered off to
remove some insoluble material, the filtrate was evaporated to
dryness at room temperature, washed with five 1 mL of cold
(5 ꢀC) Et2O, and dried in air at 20-25 ꢀC. Yields were 45-65%,
based on the metal.
12. Anal. Calcd for C44H40N6Pd: C, 69.63; H, 5.31; N, 11.08.
Found: C, 70.05; H, 5.22; N, 11.04. HRMS (ESIþ, 70 V,
MeCN): found, 759.2383 [M]þ; calcd for C44H40N6Pd, 759.-
2443. IR (KBr, selected bands, cm-1): ν(N-H) 3455 (m), ν(C-
H) 2917 (m), ν(NdCcarbene, NdC) 1633 (s), 1589 (s), δ(C-H
from Ar) 777 (s), 749 (s), 697 (s). 1H NMR (CDCl3, δ): 7.97 (s,
br, 2H, CdNH), 7.70-7.28 (m), 7.23-6.57 (m, 26H, CdNH,
aryls), 2.56 (s) and 2.37 (s, 12H, Me’s). 13C{1H} NMR (CDCl3,
δ): 171.2 (CdN from carbenes), 163.4 (CdNH), 145.3, 134.3 (C
from Ph’s), 131.8-122.5 (C and CH from aryls), 19.4 and
18.6 (Me’s).
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874 Organometallics, Vol. 30, No. 4, 2011
Tskhovrebov et al.
The starting geometries were based on the experimental X-ray
structures of 10 and 16 (this work).
relatively to the peak of the standard. In some cases, the products
were isolated by extraction of the residue after evaporation of the
reaction mixture with CH2Cl2, followed by column chromatog-
raphy on silica gel (10:1 hexane/ethyl acetate, v/v).
Solvent effects (δEs) were taken into account at the single-
point calculations on the basis of the gas-phase geometries at the
CPCM-B3LYP/6-31þG(d)//gas-B3LYP/6-31G level of theory
using the polarizable continuum model in the CPCM
version74,75 with CHCl3 taken as solvent. The UAKS model
was applied for the molecular cavity. The enthalpies and Gibbs
free energies in solution (Hs and Gs) were estimated by addition
of the solvent effect δEs to the gas-phase values Hg and Gg.
General Procedure for the Catalytic Sonogashira Cross-Cou-
pling (Specific Conditions Provided in the Legend to Table 2 and
Supporting Information). Oct-1-yne (0.550 g, 5.00 mmol),
1-iodo-2-nitrobenzene (0.610 g, 2.45 mmol), CuI (0.045 g, 9.60
mmol %) and PPh3 (0.030 g, 4.60 mmol %) and selected base
(K2CO3, 2.0 g, 14.5 mmol; Et3N, 1.5 g, 14.5 mmol), were mixed
in a round-bottom flask, followed by addition of a solution of
the catalyst in EtOH (1 mL). All the reactions were performed
under air. The flask was placed in a preheated oil bath at 60 ꢀC
and stirred for the required time. After cooling to 25 ꢀC, the
reaction mixture was evaporated to dryness under a stream of
dinitrogen followed by addition of 1.0 equiv of 1,2-dimethox-
yethane (NMR internal standard), and extraction of the reac-
tion mixture with three 0.20 mL portions of CDCl3. All fractions
were joined and analyzed by 1H NMR spectroscopy. The
product peak assignments were based on authentic samples or
on published data,37,38,76,77 while quantifications were per-
formed upon integration of the selected peak of the product
Acknowledgment. This project has been supported by
the Federal Aimed Program “Personnel”, direction “In-
organic and Coordination Chemistry, Analytical Chem-
istry of Inorganic Compounds”, in the framework of
Subprogram 1.2.1 (Contracts P676 from 20.05.2010 and
P1294 from 09.07.2010), Saint-Petersburg State Univer-
sity for research grant (2011), Russian Fund of Basic
Research (Grants 09-03-00065 and 09-03-12173-ofi_m),
and RAS Presidium Subprogram coordinated by Acad.
N. T. Kuznetsov (Grant No. 18P). K.V.L. and M.L.K.
~
^
express gratitude to the Fundac-ao para a Ciencia e a
Tecnologia (FCT), Portugal (Project PTDC/QUI-QUI/
098760/2008), and to both the FCT and the Instituto
ꢀ
Superior Tecnico (IST) for the research contracts
(Ciencia 2007 and 2008 Initiatives). The authors thank
^
the Portuguese NMR Network (IST-UTL NMR Center)
for providing access to the NMR facilities. Prof. V. P.
Boyarskiy is thanked for stimulating discussions and
valuable comments.
Supporting Information Available: Characterization of the
amidinato and aminocarbene/aminocarbene-like species, X-ray
crystal structure determinations of the PdII and PtII complexes
with aminocarbene/aminocarbene-like ligands (including a cif
file), catalytic procedures, and theoretical calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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5752-5755 and references therein.
(77) Liang, Y.; Xie, Y.-X.; Li, J.-H., J. Org. Chem. 2006, 71, 379-381
and references therein.