Synthesis of the first rNHC (remote N-Heterocyclic Carbene) complexes with no heteroatom in the carbene carbon-containing ring

Article abstract of DOI:10.1021/ic060963k

Two cationic carbene complexes with no heteroatom in the ring containing the carbene carbon, trans-bromo(2-methyl-2,6-dihydroisoquinolin-6-ylidene) bis(triphenylphosphine)palladium(II) triflate (3) and trans-chloro(1,2-dimethyl- 1,7-dihydroquinolin-7-ylid

Full text of DOI:10.1021/ic060963k

Inorg. Chem. 2006, 45, 7997−7999
Synthesis of the First rNHC (Remote N-Heterocyclic Carbene)
Complexes with No Heteroatom in the Carbene Carbon-Containing Ring
Oliver Schuster and Helgard G. Raubenheimer*
Department of Chemistry and Polymer Science, UniVersity of Stellenbosch,
7602 Stellenbosch, South Africa
Received June 1, 2006
Two cationic carbene complexes with no heteroatom in the ring
containing the carbene carbon, trans-bromo(2-methyl-2,6-dihy-
droisoquinolin-6-ylidene)bis(triphenylphosphine)palladium(II) triflate
(3) and trans-chloro(1,2-dimethyl-1,7-dihydroquinolin-7-ylidene)bis-
(triphenylphosphine)palladium(II) triflate (4), were synthesized by
oxidative substitution of Pd(PPh3)4 with N-methylated 6-bromoiso-
quinolinium and 7-chloro-2-methylquinolinium cations, respectively.
Compound 3 was also prepared by methylation of neutral trans-
bromo(2-methylisoquinolin-6-yl)bis(triphenylphosphine)palladium-
(II) (5). All complexes were unambiguously characterized by NMR
and X-ray crystallographic studies.
Figure 1. Ligand precursors 1 and 2 with the triflate counterions omitted.
(triphenylphosphine)palladium(II) triflate (3) and trans-chloro-
(1,2-dimethyl-1,7-dihydroquinolin-7-ylidene)bis(triphenyl-
phosphine)palladium(II) triflate (4) were obtained by oxida-
tive addition according to Schemes 1 and 2, when the ligand
precursors 1 and 2 were reacted with Pd(PPh3)4 at 70 °C in
toluene overnight. Filtration and washing with toluene
afforded the complexes as off-white powders. Recrystalli-
zation from dichloromethane/pentane gave colorless crystals
in excellent (91%) and satisfying (66%) yields, respectively.
In recent investigations, we have observed that complexes
bearing N-heterocyclic carbene ligands with remote hetero-
atoms (rNHC) are superior in their performance compared
to their “classic” NHC analogues in C,C-coupling catalysis.1,2
Computational studies have shown that these new ligands
are even stronger σ donors than their well-known counter-
parts.2,3
While compound 3 could also be prepared easily and in
good yield by changing the sequence of the synthetic steps
(i.e., first oxidative addition and second alkylation,
Scheme 2), the attempted synthesis of 4 following this route,
by reacting 7-chloro-2-methylquinoline with Pd(PPh3)4,
surprisingly yielded exclusively palladium black and free
PPh3 at a variety of temperatures.
In a further development of this concept, we herein present
the first rNHC complexes with no heteroatom in the carbene
carbon-containing ring; i.e., the nitrogen is located in an
adjacent annealed aromatic ring. Existing literature proce-
dures were followed or slightly modified to prepare the target
ligand precursors 6-bromo-2-methylisoquinolinium triflate
(1) and 7-chloro-1,2-dimethylquinolinium triflate (2)
(Figure 1).4 A slight excess of methyl triflate was used to
alkylate the corresponding substrates in dichloromethane. The
crude product was washed with cold tetrahydrofuran to give
compounds 1 and 2 in excellent yields.
The complexes 3-5 are soluble in polar solvents such as
dichloromethane or chloroform and insoluble in pentane or
toluene. Even under moist atmospheric conditions, decom-
position in solution is slow. Solids need to be heated to over
170 °C for 4 and even 190 °C for 3 and 5 before a purple
color indicates the formation of elemental palladium.
The single resonances observed for the complexes 3-5
in the 31P NMR spectrum, lying in a narrow range between
24.6 and 25.4 ppm, show the equivalence of the two
phosphorus ligands in each compound, indicating a rigid trans
configuration in solution because no evidence of a fast ligand
exchange could be observed.
Following Stone’s method,5 the cationic complexes
trans-bromo(2-methyl-2,6-dihydroisoquinolin-6-ylidene)bis-
* To whom correspondence should be addressed. E-mail: hgr@sun.ac.za.
Fax: +27 21 808 3849. Tel: +27 21 808 3850.
(1) Schneider, S. K.; Rentzsch, C. F.; Kru¨ger, A.; Raubenheimer, H. G.;
Herrmann, W. A. J. Mol. Catal. A: Chem. 2006, submitted for
publication.
(2) Schneider, S. K.; Roembke, P.; Julius, G. R.; Loschen, C.; Rauben-
heimer, H. G.; Frenking, G.; Herrmann, W. A. Eur. J. Inorg. Chem.
2005, 2973-2977.
(3) Schneider, S. K.; Julius, G. R.; Loschen, C.; Raubenheimer, H. G.;
Frenking, G.; Herrmann, W. A. Dalton Trans. 2006, 1226-1233.
(4) Meyer, W. M.; Deetlefs, M.; Pohlman, M.; Scholz, R.; Esterhuysen,
M. W.; Julius, G. R.; Raubenheimer, H. G. Dalton Trans. 2004, 413-
420.
(5) Fraser, P. J.; Roper, W. R.; Stone, F. G. A. J. Chem. Soc., Dalton
Trans. 1974, 760-764.
10.1021/ic060963k CCC: $33.50
Published on Web 09/01/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 20, 2006 7997

COMMUNICATION
Scheme 1. Sythesis of 4 (Triflate Counterions Omitted for Clarity)
Scheme 2. Sythesis of 3 via Two Different Routes (Triflate
Counterions Omitted for Clarity)
Figure 2. Molecular structure of compound 3 (ORTEP drawing with 50%
probability ellipsoids; H atoms, counterions, and solvent molecules omitted
for clarity). Selected bond lengths [Å] and angles [deg]: Pd1-Br1 2.5193-
(5), Pd1-P1 2.3301(11), Pd1-P2 2.3249(10), Pd1-C6 2.005(4); Br1-
Pd1-P1 92.15(3), Br1-Pd1-P2 91.63(3), Br1-Pd1-C6 178.73(11), P1-
Pd1-P2 173.32(4), P1-Pd1-C6 88.55(11), P2-Pd1-C6 87.78(11).
The carbene carbon atoms of 3 (6-C) and 4 (7-C) resonate
at 187.0 and 180.7 ppm, respectively, in their 13C NMR
spectra. Downfield shifts of 52.5 and 37.4 ppm, with
reference to the analogous carbon resonances for the alky-
lated ligand precursors, occur upon oxidative addition. Both
signals appear as triplets with coupling constants of 4.8 Hz,
again emphasizing the trans arrangement of the phosphines
in solution. Carbene formation can be assumed because the
alkylation of 5 gives rise to a severe 20.5 ppm downfield
shift of the 6-C resonance (166.5 ppm), while the other
carbon resonances are much less affected [average 3(2) ppm],
although they are nearer to the region of change. Moreover,
only a singlet is observed in compound 5 opposed to the
triplet in 3 (vide supra), indicating a change in the mode of
binding.
The molecular structures of 3-5 were determined by X-ray
diffraction analysis.6-8 As can be seen in Figures 2-4, the
palladium ions in the cationic complexes occur in a distorted
square-planar environment with the two phosphines trans to
each other and bending away from the halides a little. This
distortion is, for obvious reasons, more prominent in the
bromide-containing examples. The steric demand of the
rNHC ligands is minimal because the angles between the
planes, defined through the cyclic carbene ligands, on the
Figure 3. Molecular structure of compound 4 (ORTEP drawing with 50%
probability ellipsoids; H atoms, counterions, and solvent molecules omitted
for clarity). Selected bond lengths [Å] and angles [deg]: Pd1-Cl1 2.4005-
(12), Pd1-P1 2.3380(11), Pd1-P2 2.3175(11), Pd1-C7 1.998(4); Cl1-
Pd1-P1 92.35(4), Cl1-Pd1-P2 90.27(4), Cl1-Pd1-C7 179.16(10), P1-
Pd1-P2 176.57(4), P1-Pd1-C7 88.39(10), P2-Pd1-C7 89.01(10).
(6) Crystal data for 3: C47H39BrF3NO3P2PdS, M ) 1003.10, monoclinic,
space group P2(1)/n, a ) 13.0765(14) Å, b ) 27.951(3) Å, c )
16.4035(17) Å, R ) 90°, â ) 100.537(2)°, γ ) 90°, V ) 5894.1(11)
Å3, Z ) 4, Dc ) 1.130 g cm-3, F(000) ) 2014, µ ) 11.23 cm-1, T
) 100(2) K, 533 parameters refined with 11 083 [Rint ) 0.0357] unique
reflections to R1 [I > 2σ(I)] ) 0.0509.
(7) Crystal data for 4: C50H45Cl5F3NO3P2PdS, M ) 1142.52, triclinic,
space group P1h, a ) 11.033(3) Å, b ) 13.351(4) Å, c ) 18.598(5)
Å, R ) 80.788(5)°, â ) 75.116(5)°, γ ) 72.122(5)°, V ) 2510.0(13)
Å3, Z ) 2, Dc ) 1.512 g cm-3, F(000) ) 1160, µ ) 7.95 cm-1, T )
100(2) K, 597 parameters refined with 9277 [Rint ) 0.0235] unique
reflections to R1 [I > 2σ(I)] ) 0.0432.
Figure 4. Molecular structure of compound 5 (ORTEP drawing with 50%
probability ellipsoids; H atoms, counterions, and solvent molecules omitted
for clarity). Selected bond lengths [Å] and angles [deg]: Pd1-Br1 2.5198-
(5), Pd1-P1 2.3259(15), Pd1-P2 2.3315(15), Pd1-C6 2.014(4); Br1-
Pd1-P1 92.25(4), Br1-Pd1-P2 92.61(4), Br1-Pd1-C6 172.66(11), P1-
Pd1-P2 173.93(3), P1-Pd1-C6 88.2(2), P2-Pd1-C6 86.5(2).
(8) Crystal data for 5: C48H42BrCl6NP2Pd, M ) 1093.78, monoclinic,
space group P2(1), a ) 10.1637(9) Å, b ) 22.087(2) Å, c ) 10.9083-
(17) Å, R ) 90°, â ) 110.8640(10)°, γ ) 90°, V ) 2288.2(4) Å3, Z
) 2, Dc ) 1.588 g cm-3, F(000) ) 1100, µ ) 17.35 cm-1, Flack )
0.412(10), T ) 100(2) K, 533 parameters refined with 8303 [Rint
0.0249] unique reflections to R1 [I > 2σ(I)] ) 0.0509.
)
7998 Inorganic Chemistry, Vol. 45, No. 20, 2006

COMMUNICATION
solvent providing said columnar space. In agreement with
this microscopic observation, single-crystalline blocks of 3
immediately break apart to give thin weathered plates when
removed from the mother liquor at room temperature.
In summary, 3 and 4 represent the first reported rNHC
complexes with no heteroatom in the ring that contains the
carbene carbon. They are accessible in good yields following
straightforward, established routes and can be handled for a
reasonable amount of time under atmospheric conditions,
even in solution. While the heteroatom in 4 is not more
remote than that in known 1,4-dihydropyridin-4-ylidene or
1,4-dihydroisoquinolin-4-ylidene complexes (three bonds),1-4
3 pushes the boundaries of this new class of ligands a little
further (five bonds). At what stage of heteroatom remoteness
such complexes become immonium salts rather than carbene
complexes and especially how precisely discrimination can
be established require further attention. At the moment, NMR
measurement seems to be the only method to identify carbene
complex formation because bond distances do not discrimi-
nate effectively. In future studies, we will determine whether
these compounds are suitable as catalysts for C,C-coupling.
To better understand the nature of these ligands and their
relationship to standard NHCs, quantum mechanical calcula-
tions will accompany these investigations. We believe that
this new class of ligands provides many possibilities for
application, especially if they could be attached to a range
of metal centers.
Figure 5. Projection of the crystal structure of compound 3 along the ac
vector (arbitrary radii; solvent molecules omitted).
one hand, and the coordination sphere, on the other hand,
very closely approach 90°. The Pd-C distances of all
complexes fall within a very narrow range between 1.998
and 2.014 Å. Hence, with respect to their metal-carbon
bonds, there is no difference between carbene 3 and pro
carbene complex 5. This is in harmony with CCSD data,9
where complexes of the type trans-(aryl)(PPh3)2PdX and
trans-(NHC)(PPh3)2PdX (X ) Br, Cl) cannot be discrimi-
nated by their Pd-C or Pd-X bond lengths. The variations
in the CsC/CdC bond lengths within the corresponding
rings are also negligible. Therefore, molecular structural data
are not useful probes to confirm or dismiss carbene forma-
tion.
All compounds crystallize with at least two solvent
molecules per formula unit. This is mostly due to the cavities
provided between the carbene ligand and the phenyl rings
of the phosphines. In the case of complex 3, no less than
five severely distorted dichloromethane molecules have been
identified in the asymmetric unit. These are mainly located
in channels along the ac vector made available by the ionic
network (Figure 5). Anions and cations pack in layers along
the ac plane, with the former sharing the plane with the
Acknowledgment. We gratefully acknowledge the fi-
nancial assistance provided by the Alexander von Humboldt
Foundation (to O.S.).
Note Added after ASAP Publication. There was an
error in Figure 1 in the version published ASAP
September 1, 2006; the corrected version was published
ASAP September 5, 2006.
Supporting Information Available: Synthetic procedures for
3-5 and their precursors as well as crystallographic data for 3-5
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(9) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.;
McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2002, B58, 389-
397.
IC060963K
Inorganic Chemistry, Vol. 45, No. 20, 2006 7999