Pyridine functionalised N-heterocyclic carbene complexes of palladium

Article abstract of DOI:10.1039/b002645j

The pyridine functionalised N-heterocyclic carbene complexes (C- N)PdMeBr, C-N = 3-R-1-(2-picolyl)imidazolin-2-ylidene, (R = Bu(t) 1a, R = mes 1b, mes = mesityl) are described; they are excellent catalysts for the Heck arylation; 1b is monomeric with a chelating C-N ligand; the product obtained by interaction of 1a with AgO₂CCF₃ shows a [Pd(C-N)]₄ framework with bridging C-N ligands.

Full text of DOI:10.1039/b002645j

Pyridine functionalised N-heterocyclic carbene complexes of palladium
Arran A. D. Tulloch,a Andreas A. Danopoulos,*a Robert P. Tooze,b Sean M. Cafferkey,a Sven Kleinhenza and
Michael B. Hursthouse*a
a Department of Chemistry, University of Southampton, Highfield, Southampton, UK SO17 1BJ.
E-mail: ad1@soton.ac.uk
b Ineos Acrylics, Wilton, PO Box 90, Middlesbrough, Cleveland, UK TS90 8JE
Received (in Basel, Switzerland) 28th March 2000, Accepted 26th May 2000
Published on the Web 20th June 2000
The pyridine functionalised N-heterocyclic carbene com-
plexes (C–N)PdMeBr, C–N = 3-R-1-(2-picolyl)imidazolin-
2-ylidene, (R = But 1a, R = mes 1b, mes = mesityl) are
described; they are excellent catalysts for the Heck arylation;
1b is monomeric with a chelating C–N ligand; the product
obtained by interaction of 1a with AgO2CCF3 shows a
[Pd(C–N)]4 framework with bridging C–N ligands.
N-heterocyclic carbenes have recently emerged as a new family
of ligands with electronic characteristics similar to those of the
phosphines.1 The great interest in their complexes with
transition metals has been stimulated by the realisation that they
can act as catalysts or catalyst precursors to important
transformations, such as Pd-catalysed Heck and Suzuki cou-
plings, CO–ethylene copolymerisations, Ru-catalysed olefin
metathesis and Rh catalysed hydrosilylations.2 Furthermore,
electronic and steric optimisation of the catalytic site should be
achievable via ligand design. Recent reports on the synthesis of
functionalised hemilabile N-heterocyclic carbene complexes3,4
and their use as catalyst precursors prompted us to communicate
our results describing the full characterisation of three novel
pyridine functionalised N-heterocyclic carbene complexes of
palladium, including two crystal structures and preliminary
studies on their catalytic activities in Heck coupling and
amination reactions.
The complexes (C–N)PdMeBr, C–N = 3-R-1-(2-picolyl)-
imidazolin-2-ylidene, (R = But 1a, R = mes 1b, Scheme 1)
were prepared in good yields by careful deprotonation at low
temperatures of 3-R-1-(2-picolyl)imidazolium bromide, [H(C–
N)] Br, (R = But, mes), with LiNPri2 in THF, followed by
trapping of the in situ formed carbene with (cod)PdMeBr.
Complexes 1a,b are air-stable solids and 1b can be crystallised
from CH2Cl2 by slow evaporation.† The structure of 1b was
determined by X-ray crystallography and is shown in Fig. 1.‡
The chelating ligand is coordinated to the square planar
palladium centre with the carbene end disposed trans to the
bromide. The carbene plane forms an angle of 64.2° with the
square plane of the palladium. The resulting six-membered ring
is puckered to release conformational strain.
Fig. 1 Molecular structure of 1b. Selected bond lengths (Å) and angles (°):
Pd1–C1 1.964(4), Pd1–C10, 2.147(3), Pd11–N3 2.183(3), Pd1–Br1
2.4969(6); Br1–Pd1–C1 177.5(1), Br1–Pd1–C10 90.51(8), Br1–Pd1–N3
92.59(8).
obtain crystallographic data. In the absence of such data for
comparison, we note with interest that there are several other
aspects of their paper, which point towards important differ-
ences between their complexes and the ones reported herein.
Mention is made of the unselective reaction of bases with the
precursor imidazolium salts, which precludes the use of the
simple synthetic technique employed by us to synthesise the
palladium complexes. They also note that their complexes once
formed are unstable towards strong bases,which limits their use
in amination catalysis. Both problems are believed to arise from
Scheme 1
Abstraction of the halide from 1a by Ag(O2CCF3) in
acetonitrile results in isolation of complex 2† the structure of
which is shown in Fig. 2.‡ In this case the C–N ligand is
bridging with carbene and pyridine ends occupying mutually
trans positions. The four Pd atoms occupy the corners of a
distorted tetrahedron and the ligands bridge four of the six sides
in a way which leaves two opposite sites free.
Fig. 2 Molecular structure of 2. Selected bond lengths (Å): Pd–N
1.99(2)–2.11(2), Pd–O 2.145(15)–2.170(15), Pd–Cmethyl 1.94(2)–2.10(2),
Pd–Ccarbene 1.89(3)–1.96(3).
Compounds analogous to 1 (R = Me) have recently been
reported by McGuinness and Cavell,4 however, they could not
DOI: 10.1039/b002645j
Chem. Commun., 2000, 1247–1248
This journal is © The Royal Society of Chemistry 2000
1247

Table 1 Selected results of Heck reactions catalysed by 1a
Aryl halidea
Alkeneb/amine
t/h
T/°C
Catalyst (mol %)
Base
Yield (%)d
TON
12 900
121 400
135 700
142 900
285 800
1400 000
2858 000
190
PhI
PhI
PhI
PhI
PhI
PhI
PhI
mac
mac
mac
mac
mac
mac
mac
bac
3
1
2
3
6
18
38
18
18
24
75
152
130
130
130
130
130
140
140
140
140
65
7 3 1023
7 3 1024
7 3 1024
7 3 1024
3.5 3 1024
7 3 1025
3.5 3 1025
0.5
Na2CO3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NaOAc
NaOAc
NEt3
90
85
95
100
100
98
100
95
4-NO2C6H4Br
4-MeCOC6H4Br
PhBrc
PhBr
PhBr
bac
0.5
100
10
10
200
14 300
1 400
PhNHMe
mac
mac
7 3 1024
7 3 1023
7 3 1023
130
130
NEt3
NEt3
20
2 900
a ArX, 5 mmol; alkene/amine, 6 mmol; NEt3, 7 mmol; NaOAc, 7 mmol; Na2CO3, 3.5 mmol; N-methylpyrrolidone used as solvent. b mac = Methyl acrylate;
bac = n-butyl acrylate. c Determined by GC, based on the aryl halide. d THF used as solvent.
2: MS (ES): m/z 377, [Pd(C–N)Me(MeCN)]+. 1H NMR (300 MHz,
undesired deprotonation of the methylene protons linking the
CD3CN): 8.8 (d, 1H, a-N-pyridyl H), 7.5 (dt, 1H, g-N-pyridyl H), 7.2 (m,
1H, b-N-pyridyl H), 7.1 (d, 1H, b-N-pyridyl H), 7.0 (d, 1H, HCCH), 6.4 (d,
1H, HCCH), 5.3 (br, CH2), 0.56 (s, 3H, PdCH3), 1.7 [s, 9H, C(CH3)3].
13C{1H} NMR (100.5 MHz, CDCl3): 174.0 (NCN), 158.5 (pyridyl C),
151.5 (PdCO2CF3), 150.5, 138.2 (pyridyl C), 123.3, 122.8, 121.4, 120.3
(pyridyl C, NCCN), 97.6 (CF3), 58.4 (NCH2), 56.1 [C(CH3)3], 31.8
[C(CH3)3], 29.2 (PdCH3).
carbene to the heterocyclic donor, or even the methyl sub-
stituent of the carbene. We believe these initial observations
may presage structure–property relationships in similar cata-
lysts.
Complexes 1a and 1b are excellent precatalysts for the Heck
coupling and show good activity for amination reactions (see
Table 1). The activity of 1a does not seem to decrease with time,
implying high thermal stability under the reaction conditions. It
is highest in N-methylpyrrolidone and N,NA-dimethylacetamide
but is dependent on reaction temperature. The highest turnover
frequencies are observed when triethylamine is used as base;
other bases such as Na2CO3 or NaO2CMe have also been used
successfully but require longer reaction times. The results for
the coupling of aryl iodides with acrylates are comparable to the
best systems known.5 Although the mechanism and the nature
of the active species in the Heck reaction is far from clear,6 our
results show that (i) highly active palladium catalysts are
obtained by using hemilabile carbene complexes, (ii) the
presence of other labile ligands in the coordination sphere of the
metal makes predictions of the nature of the catalytic species
difficult and (iii) higher nuclearity complexes such as 2 may be
precursors to the active catalyst which could be obtained by
dissociation of the labile end of the ligand. Catalysis by higher
nuclearity complexes, especially under Heck conditions, is less
likely.
‡ Crystal data: for 1b: C19H22BrN3Pd, M = 478.71, rhombohedral, space
group R3h (no. 148), a = 24.521(4), c = 20.201(4) Å, U = 10519(3) Å3,
T
= 150 K, Z = 18, (Mo-Ka) =
2.507 mm21, 25874 reflections
measured, 4782 unique (Rint = 0.047) which were used in all calculations.
The final wR(F2) was 0.1142 (all data) and R = 0.0439 [F > 2 (F)]. The
structure contains highly disordered solvent CH2Cl2 which is located in
channels along the c axis (1035 e cell21) and was treated in the manner
described by Sluis and Spek.7
For 2: crystals were obtained by layering of CH2Cl2 solution of 2 with
ether: C60H80F12N12O8Pd4·(x THF (x ≈ 4), Mr = 1749.04, triclinic, space
group P1 (no. 2), a = 9.852(2), b = 20.026(4), c = 21.056(4) Å,
=
89.95(3), = 90.19(3), = 90.04(3) °, U = 4154.2(14) Å3, Z = 2, T = 150
K, = 0.935, 34221 reflections measured, 10208 reflections observed, R =
0.1095, Rw = 0.2539. The crystals were of particularly poor quality, and the
data reported is the best of four data collections and refinements tried.
Recognising the approximate C2 symmetry of the molecule and or-
thorhombic cell geometry, we have explored the possibility of higher
symmetry structure. Whilst strong data merge reasonably well for
orthorhombic (Rint = 0.13), monoclinic (Rint = 0.062; cf. Rint = 0.056 for
triclinic) we were not able to solve or refine in the higher symmetries.
CCDC 182/1666. See http://www.rsc.org/suppdata/cc/b0/b002645j/ for
crystallographic files in .cif format.
Studies on the reactivity of the new complexes, extension of
the methodology to other transition metals and the synthesis of
other functionalised N-heterocyclic carbene ligands with a
variety of other donor functionalities is under way.
1 Review: W. A. Herrmann and C. Køcher, Angew. Chem., Int. Ed. Engl.,
1997, 36, 2162.
We are indebted to Ineos Acrylics, EPSRC and the University
of Southampton for support.
2 For Heck and Suzuki coupling reactions, see: W. A. Herrmann, in
Applied Homogeneous Catalysis with Organometallic Compounds, ed.
B. Cornils and W. A. Herrmann, Wiley-VCH, Weinheim, 2000, p. 725;
C. Zhang, J. Huang, M. L. Trudell and S. P. Nolan, J. Org. Chem., 1999,
64, 3804; for CO–ethylene copolymerisations, see: M. G. Gardiner,
W. A. Herrmann, C.-P. Reisinger, J. Schwarz and M. Spiegler,
J. Organomet. Chem., 1999, 572, 239; for olefin metathesis reactions,
see: T. Weskamp, W. C. Schattenmann, W. C. Spiegler and W. A.
Herrmann, Angew. Chem., Int. Ed., 1998, 37, 2490; M. Scholl, T. M.
Trnka, J. P. Morgan and R. H. Grubbs, Tetrahedron Lett., 1999, 40, 2247;
for catalytic hydrosilations, see: W. A. Herrmann, L. J. Goossen, C.
Køcher and G. R. J. Artus, Angew. Chem., Int. Ed. Engl., 1996, 35,
2805.
3 W. A. Herrmann, C. Køcher, L. Goossen and G. R. J. Artus, Chem. Eur.
J., 1996, 2, 1627; W. A. Herrmann, L. Goossen and M. Spiegler,
Organometallics, 1998, 17, 2162.
4 D. S. McGuinness and K. J. Cavell, Organometallics, 2000, 19, 741.
5 M. Ohff, A. Ohff and D. Milstein, Chem. Commun., 1999, 357.
6 B. L. Shaw, S. D. Perera and E. A. Staley, Chem. Commun., 1998, 1361;
B. L. Shaw, New J. Chem., 1998, 22, 77.
Notes and references
† Spectroscopic data: 1a: MS (ES): m/z 377, [Pd(C–N)Me(MeCN)]+. 1H
NMR (300 MHz, CDCl3): 9.14 (d, 1H, a-N-pyridyl H), 7.70 (dt, 1H, g-N-
pyridyl H), 7.33 (t, 1H, b-N-pyridyl H), 7.24 (d, 1H, HCCH), 7.14 (d, 1H,
HCCH), 6.66 (m, 1H, b-N-pyridyl H), 5.23 (m, 1H,NCHH), 5.92 (m, 1H,
NCHH), 1.95 [s, 9H, C(CH3)3], 0.56 (s, 3H, PdCH3). 13C{1H} NMR (100.5
MHz, CDCl3): 175.2 (NCN), 159.7, 152.7, 137.9 (pyridyl C), 123.6,
122.2, 120.4, 119.1 (pyridyl C, NCCN), 59.3 (NCH2), 54.3 [C(CH3)3], 30.2
[C(CH3)3], 27.5 (PdCH3).
1b: MS (ES): m/z 439, [Pd(C–N)Me(MeCN)]+. 1H NMR (300 MHz,
CDCl3): 9.33 (d, 1H, a-N-pyridyl H), 7.76 (dt, 1H, g-N-pyridyl H), 7.50
(d, 1H, b-N-pyridyl H), 7.34 (m, 1H, b-N-pyridyl H), 7.27 (d, 1H, HCCH),
6.93 (s, 2H, mes CH), 6.78 (d, 1H, HCCH), 5.50 (br, 2H, CH2), 2.31 (s, 3H,
mes CH3), 2.08 (s, 6H, mes CH3), 0.22 (s, 3H, PdCH3). 13C{1H} NMR
(100.5 MHz, CDCl3): 174.4 (NCN), 153.3, 152.8, 138.7 (pyridyl C),
135.6, 134.9 (mes C), 129.2, 128.8 (mes CH), 124.5, 124.2, 122.2, 121.7
(pyridyl C, NCCN), 55.5 (NCH2), 21.2, 18.5 (mes CH3), 213.9 (PdCH3).
7 P. van der Sluis and A. L. Spek, Acta Crystallogr., Sect. A., 1990, 46,
194.
1248
Chem. Commun., 2000, 1247–1248