The synthesis of a di-N-heterocyclic carbene-amido complex of palladium(II)

Article abstract of DOI:10.1039/b314814a

A diimidazolium salt incorporating a secondary amine moiety has been used to prepare a palladium(II) di-N-heterocyclic carbene amino complex that can be deprotonated with NaH to give the first example of a transition metal NHC-amide.

Full text of DOI:10.1039/b314814a

The synthesis of a di-N-heterocyclic carbene-amido complex of
palladium(II)†
a
a
b
Richard E. Douthwaite,* Jennifer Houghton and Benson M. Kariuki
a
Department of Chemistry, University of York, Heslington, York, UK YO10 5DD. E-mail: red4@york.ac.uk;
Fax: +44 (0)1904 432516; Tel: +44 (0)1904 434183
Department of Chemistry, University of Birmingham, Edgbaston, Birmingham, UK B15 2TT
b
Received (in Cambridge, UK) 11th November 2003, Accepted 19th January 2004
First published as an Advance Article on the web 13th February 2004
A diimidazolium salt incorporating a secondary amine moiety
has been used to prepare a palladium(II) di-N-heterocyclic
carbene amino complex that can be deprotonated with NaH to
give the first example of a transition metal NHC–amide.
2 2
Reaction between 4 and [PdCl (MeCN) ] in dichloromethane at
room temperature gives the sparingly soluble complex
[(tBuCN(H)CtBu) PdCl] Cl2 (5). Crystals of complex 5‡ suitable for
an X-ray diffraction study were grown from dichloromethane and
diethyl ether and the molecular structure of the cationic fragment is
shown in Fig. 2.
Metal complexes of hybrid ligand sets derived from a mixture of
electronically diverse donor atoms have been extensively investi-
gated for their application to catalysis and activation of small
molecules. Of the hybrid ligand sets known, phosphorus–nitrogen
complexes possibly exhibit the most diverse range of reactivities.
The geometry at the palladium atom is square planar with the
NHC moieties in a trans configuration. The proton H(3) is
calculated but its existence is inferred from the presence of a
1
chloride counter anion and the H NMR spectrum (vide infra). In
1
2
These include N
lytic coupling reactions.
2
activation, polymerizations and various cata-
the solid state the cation of 5 is chiral by virtue of atropisomerism
engendered by the ‘S’-configuration of the CH groups that link the
2
3
N-Heterocyclic carbenes (NHC) have often been compared to
tertiary phosphines and have come to prominence mainly because
of their application to catalysis. However there is also growing
amine and NHC nitrogen atoms. Related di-NHC complexes
incorporating a bridging pyridine functionality exhibit similar
4
interest in using NHC as a basis for new ligand platforms that
display unusual reactivity. Several hybrid ligand sets based on
5
6
NHC have been prepared including those of N-donors. However
a notable exception is that an NHC–amido complex has not been
reported. We have recently been interested in developing the
chemistry of NHC–amine ligands and an initial goal was to prepare
an example of an NHC–amido complex. Here we wish to report the
synthesis and characterisation of a di-NHC–secondary amino
complex of palladium(II) and its transformation to the correspond-
ing di-NHC amide.
Our initial target ligand set was based on a di-NHC secondary
amine shown in Fig. 1, because it was envisaged that chelating
NHC moieties would favour metal–nitrogen interaction.
Fig. 1 Target ligand.
Scheme 1 i) BnBr, MeCN, 25 °C, 4 h; ii) tert-butyl imidazole, 1,4-dioxane,
However preliminary experiments showed that preparation of
the NHC-precursor diimidazolium salt from reaction between
bis(2-dichloroethyl)amine and two equivalents of tert-butyl imida-
zole were largely thwarted by competitive oligomerisation of the
amine. It was therefore found necessary to protect the secondary
amine with a benzyl moiety giving the tertiary amine 1 as shown in
Scheme 1.† Subsequent reaction between 1 and two equivalents of
tert-butyl imidazole proceeded smoothly to give the diimidazolium
1
2
2
20 °C, 18 h; iii) 10%Pd/C, 1 atm H
2
, EtOH, 60 °C, 18 h; iv) Ag
2
O, CH
2 2
Cl ,
2 2
5 °C, 18 h; v) [PdCl (MeCN) ], CH Cl , 25 °C, 1 h; NaH, CH
5 °C, 24 h.
2
2
2
Cl /THF,
2
tBu
tBu
2
tertiary amine salt ( C(H)N(Bn)C(H) )2Cl (2). Removal of the
benzyl group could then be achieved by palladium catalysed
hydrogenation to give the dimidazolium secondary amine salt
tBu
tBu
2
(
C(H)N(H)C(H) )2Cl (3).
Silver( ) halide complexes of NHC have been shown to be useful
as NHC ligand transfer agents to metals, particularly palladium,
I
7
tBu
tBu
and therefore we prepared ( C(AgCl)N(H)C(AgCl) ) 4 from
reaction between 3 and Ag O in dichloromethane. Characteristic
H and C NMR data for complex 4 include two triplet signals at
d 3.02 and 4.23 ppm for the CH
protons and in the 13C NMR
spectrum, a single NHC carbon signal at d 178.0 ppm.
2
1
13
2
Fig. 2 Molecular structure of cation of 5. Selected bond lengths (Å) and
angles (°) Pd(1)–C(1) 2.057(15), Pd(1)–C(10) 2.087(15), Pd(1)–N(3)
2.058(12), Pd(1)–Cl(1) 2.341(4), C(1)–Pd(1)–C(10) 171.4(7), Cl(1)–Pd(1)–
N(3) 173.4(4), C(1)–Pd(1)–N(3) 82.1(5), C(10)–Pd(1)–N(3) 89.7(6).
†
Electronic supplementary information (ESI) available: synthesis and
characterising data of compounds 1–6. See http://www.rsc.org/suppdata/cc/
b3/b314814a/
6
98
C h e m . C o m m u n . , 2 0 0 4 , 6 9 8 – 6 9 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

structures and have been shown to undergo atropisomerisation,
In conclusion we have prepared the first example of a NHC–
secondary amine complex and its corresponding amide by
deprotonation of the metal coordinated amine. We are currently
investigating the coordination chemistry of 2 and 3 and the
synthesis of s-block amido–NHC complexes for ligand transfer to
transition metals.
The authors would like to thank the University of York for
financial support and Danielle Schott for copious help with the
NMR line shape analysis.
8
interconverting between C
2
symmetric enantiomers. In contrast
the presence of H(3) renders the cationic fragment of 5 C
1
1
symmetric and this is reflected in the low temperature H NMR
1
spectrum. At 25 °C the only clearly discernable signals in the H
NMR spectrum are a single signal at d 1.95 ppm attributable to the
t
two Bu groups, two signals at d 7.02 and 7.09 ppm for the four
NHC protons and a single broad signal at d 8.50 ppm corresponding
to the proton of the amine group. The protons of the CH groups are
2
broadened into the baseline of the spectrum. These observations are
attributed to interconversion of the atropisomers via a ‘H-
windscreen wiper’ motion as shown in eqn. (1) at a rate
commensurate with the NMR timescale. On cooling to 240 °C the
rate of interconversion is sufficiently slow to distinguish a complex
Notes and references
‡
2 5
Crystal data for 5: C18H31Cl N Pd, M = 494.78, monoclinic, space
set of 8 resolved signals for the CH
attributable to two Bu, four NHC and an amine proton.
2
protons in addition to signals
group P2 /c, a = 12.5628(15), b = 10.3135(15), c = 17.2307(18) Å, b =
1
t
23
9
K
2.159(7)° U = 2230.9(5), Z = 4, D = 1.473 g cm , T = 273(2), m (Mo–
a
) = 8.998 mm , 4802 reflections measured, 1407 [R(int) = 0.1059]
21
2
independent reflections, wR(F all data) = 0.1820 and R (F > 2s(F)) =
0.0905. CCDC 225003. See http://www.rsc.org/suppdata/cc/b3/b314814a/
for crystallographic data in .cif or other electronic format.
(1)
Synthesis and spectroscopic data for 6: To a dichloromethane solution
(10 ml) of 5 (62 mg, 0.125 mmol) was added a THF solution (5 mL) of
sodium hydride (30 mg, 1.25 mmol) and the mixture stirred for 24 h at 25
°C. The volatiles were removed under reduced pressure and the resulting
yellow solid extracted with toluene (2 3 20 ml). The volatiles were removed
Reaction between 5 and various bases was then investigated in an
attempt to prepare the corresponding di-NHC amido complex
1
tBu
tBu
to give 6 as a pale yellow solid (Yield = 35 mg, 61%). H NMR (300 MHz,
[
(
CNC )PdCl] (6). We found that NEt
3
and Py gave no
t
CD
2
Cl
CH
2
, 240 °C): 1.98 (18H, s, CH
3
), 3.54 (2H, m, CH
2
CH
CH
H–H = 2.59, CHNCH); C{ H}
, 25 °C) 32.0 (CH ), 58.4 (C(CH ), 58.5
CH ), 118.1 (CHNCH), 119.8 (CHNCH), 120.8
2
), 3.90 (2H, m,
apparent reaction even on heating and KO Bu lead to decomposi-
CH
2
2
), 4.63 (2H, m, CH CH ), 5.32 (2H, m, CH
2
2
2
2
), 6.88 (2H, d,
2 2
tion. However in a mixture of THF and CH Cl reaction between 5
and NaH gave 6 in 61% yield.‡ In contrast to 5, compound 6 is
3
3
13
1
J
H–H = 2.59, CHNCH), 6.95 (2H, d, J
NMR (75 MHz, CD
2
Cl
2
3
3 3
)
readily soluble in aromatic and ether solvents, consistent with an
(CH
2
CH
2
), 59.9 (CH
2
2
1
uncharged molecule. Over the range 25 to 240 °C the H NMR
+
(
CHNCH), 122.5 (CHNCH), 165.7 (CPd); MS (TOF ES ) m/z 422 (100, [M
) ; Anal. [found (calc.)] for C18H30N PdCl: C 46.97 (47.17), H, 6.60
spectra of 6 differs significantly from 5 displaying signals
2 Cl]
(6.40) N 15.00 (15.28)%.
+
5
consistent with atropisomerisation of a C
2
symmetric compound.
At all temperatures a single Bu resonance and two NHC protons
are observed. The CH protons appear as four very broad peaks at
5 °C resolving to four complex multiplets at 240 °C that do not
t
1
2
(a) M. D. Fryzuk, C. M. Kozak, M. R. Bowdridge, B. O. Patrick and S.
J. Rettig, J. Am. Chem. Soc., 2002, 124, 8389; (b) M. D. Fryzuk, J. B.
Love, S. J. Rettig and V. C. Young, Science, 1997, 275, 1445.
(a) D. S. McGuinness, P. Wasserscheid, W. Keim, C. H. Hu, U. Englert,
J. T. Dixon and C. Grove, Chem. Commun., 2003, 334; (b) K. R. Reddy,
W. W. Tsai, K. Surekha, G. H. Lee, S. M. Peng, J. T. Chen and S. T. Liu,
J. Chem. Soc., Dalton Trans., 2002, 1776; (c) G. A. Luinstra and P. H. P.
Brinkmann, Organometallics, 1998, 17, 5160; (d) A. Aeby, A. Gsponer
and G. Consiglio, J. Am. Chem. Soc., 1998, 120, 11000.
2
2
change on further cooling and the resonance at d 8.50 ppm
attributed to the amine proton of 5 is absent. In addition the IR
spectrum of 5 shows a stretch at 3152 cm2 consistent with an N–H
vibration, which is absent for 6. Elemental analysis also confirmed
the bulk composition. Unfortunately to date growth of a single
crystal of 6 suitable for an X-ray diffraction study has not been
successful.
1
3 (a) G. Helmchen and A. Pfaltz, Acc. Chem. Res., 2000, 33, 336; (b) M.
Qadir, T. Mochel and K. K. Hii, Tetrahedron, 2000, 56, 7975.
4 W. A. Herrmann, Angew. Chem. Int. Ed., 2002, 41, 1291.
1
Line shape analysis of the variable temperature H NMR spectra
‡
‡
over the range 25 to 240 °C allowed estimates of DH and DS for
5
(a) P. L. Arnold, A. C. Scarisbrick, A. J. Blake and C. Wilson, Chem.
Commun., 2001, 2340; (b) J. J. van Veldhuizen, S. B. Garber, J. S.
Kingsbury and A. H. Hoveyda, J. Am. Chem. Soc., 2002, 124, 4954.
(a) A. A. D. Tulloch, A. A. Danopoulos, S. Winston, S. Kleinhenz and G.
Eastham, J. Chem. Soc., Dalton Trans., 2000, 4499; (b) A. A. D. Tulloch,
A. A. Danapoulos, R. P. Tooze, S. M. Cafferkey, S. Kleinhenz and M. B.
Hursthouse, Chem. Commun., 2000, 1247; (c) E. Peris, J. A. Loch, J.
Mata and R. H. Crabtree, Chem. Commun., 2001, 201; (d) A. M. Magill,
D. S. McGuinness, K. J. Cavell, G. J. P. Britovsek, V. C. Gibson, A. J. P.
White, D. J. Williams, A. H. White and B. W. Skelton, J. Organomet.
Chem., 2001, 617, 546; (e) M. T. Powell, D. R. Hou, M. C. Perry, X. H.
Cui and K. Burgess, J. Am. Chem. Soc., 2001, 123, 8878; (f) K. S.
Coleman, H. T. Chamberlayne, S. Turberville, M. L. H. Green and A. R.
Cowley, Dalton Trans., 2003, 2917; (g) L. G. Bonnet, R. E. Douthwaite
and B. M. Kariuki, Organometallics, 2003, 22, 4187; (h) X. Hu, I. Castro-
Rodriguez and K. Meyer, J. Am. Chem. Soc., 2003, 125, 12237.
atropisomerisation of 5 and 6 and the data are shown in Table 1.
Given the errors in DS the only meaningful conclusion is that the
mechanism of interconversion between atropisomers for 5 and 6 is
likely to be similar. Within the errors DH for 5 and 6 are lower than
3
that obtained for a di-NHC pyridine complex in CDCl (51.6 (1.9)
kJ mol ) but are similar to those observed for the ring inversion of
‡
6
‡
8
b
2
1
N,N,NA,NA-tetramethylpiperazinium dichloride (37.6 (5.0) kJ
2
1 9
mol ). For 5 and 6 the data are consistent with a mechanism
where the nitrogen donor atom remains coordinated to the
palladium atom as shown in eqn. (1) and also that the counter anion
does not play a significant role in the interconversion mecha-
nism.8
c
Table 1 Thermodynamic parameters for atropisomerism of 5 and 6
7
H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972.
5
6
8 (a) A. A. D. Tulloch, A. A. Danopoulos, G. J. Tizzard, S. J. Coles, M. B.
Hursthouse, R. S. Hay-Motherwell and W. B. Motherwell, Chem.
Commun., 2001, 1270; (b) S. Grundemann, M. Albrecht, J. A. Loch, J. W.
Faller and R. H. Crabtree, Organometallics, 2001, 20, 5485; (c) J. R.
Miecznikowski, S. Grundemann, M. Albrecht, C. Megret, E. Clot, J. W.
Faller, O. Eisenstein and R. H. Crabtree, Dalton Trans., 2003, 831.
‡
21 a
DG (kJ mol
)
57.44 (0.37)
32.5 (4.9)
54.8 (0.41)
39.0 (5.4)
DH (kJ mol21)
‡
DS (J K mol21)
‡
21
288 (18)
256 (10)
a
at 281 K.
9
R. J. Abraham and D. B. Macdonald, J. Chem. Soc. D, 1966, 188.
C h e m . C o m m u n . , 2 0 0 4 , 6 9 8 – 6 9 9
699