The first example of a doubly orthometallated aryl bis(phosphinite) ligand

Article abstract of DOI:10.1039/b305136f

The reaction of 2-methylresorcinolbis(diphenyl)phosphinite with Pd(TFA)₂ gives a dimeric tetranuclear complex which contains two of the ligands, each di-orthopalladated; the X-ray structure of the complex has been determined and its reactivity briefly examined.

Full text of DOI:10.1039/b305136f

View Article Online / Journal Homepage / Table of Contents for this issue
The first example of a doubly orthometallated aryl bis(phosphinite)
ligand
Robin B. Bedford,*a Michael E. Blake,a Simon J. Coles,b Michael B. Hursthouseb and P. Noelle
Scullyc
a School of Chemistry, University of Exeter, Exeter, UK EX4 4QD.
E-mail: r.bedford@ex.ac.uk; Fax: ϩ44 (0) 1392 263434
b Department of Chemistry, EPSRC National Crystallography Service,
University of Southampton, Highfield Road, Southampton, UK SO17 1BJ
c Department of Chemistry, Trinity College Dublin, Dublin 2, Ireland
Received 7th May 2003, Accepted 18th June 2003
First published as an Advance Article on the web 18th June 2003
The reaction of 2-methylresorcinolbis(diphenyl)phosphin-
ite with Pd(TFA)2 gives a dimeric tetranuclear complex
which contains two of the ligands, each di-ortho-
palladated; the X-ray structure of the complex has been
determined and its reactivity briefly examined.
Orthometallated P-donor ligands have recently attracted
attention due to the high activity they show as catalysts in
carbon–carbon bond formation. Shaw and co-workers demon-
strated that the palladated naphthyl phosphine complexes 1
and 2 could be used as catalysts in the Heck reaction,1 while
Gibson, Cole-Hamilton and co-workers showed that the
orthometallated benzyl phosphine-containing complexes 3
could be used in the Heck and Suzuki coupling of aryl
bromides and even activated aryl chlorides.2 Our interest has
been in the synthesis of orthopalladated complexes of more
π-acidic ligands such as triaryl phosphite and aryl phosphinites.
We have found that catalysts of the type 4 show extremely high
activity in the coupling reactions of aryl bromides,3 while tri-
cyclohexylphosphine adducts of 4 and 5 can be used to excel-
lent effect in the Suzuki and Stille couplings of aryl chlorides.4
High catalytic activity is not limited to κ2-P,C complexes but is
also seen for κ3-P,C,P-pincer complexes. This was initially
shown by Milstein and co-workers when they reported that the
complexes 6 and 7 show good activity in the Heck coupling of
aryl iodides.5 Later Shibasaki and co-workers showed that
complex 8a shows even better activity.6 We found that the
P,C,P-bis(phosphinite) pincer complexes 8b and c give very
good activity in the Suzuki coupling of aryl bromides and can
even be used to couple activated aryl chlorides.7 The more
electron rich analogue 8d was shown by Morales-Morales and
co-workers to be an active catalyst for the Heck couling of aryl
chlorides.8
Given the attention paid to the generation and exploitation
of such orthopalladated structural types, we were interested
to see whether the introduction of a 2-methyl group into a
resorcinol bis(phosphinite) ligand and subsequent reaction
with an appropriate palladium source would lead to a carb-
ometallation, and whether this would be an orthometallation
of the aryl ring or the C–H activation of the 2-methyl group.
While the second process is in principle more difficult than the
first, it may be facilitated by the fact that strong chelation of
the bisphosphine ligand would effectively pre-organise the
transition state for C–H activation.
The ligand 2-methylresorcinolbis(diphenyl)phosphinite, 9,
was prepared in 90% yield by the reaction of 2-methylresorcinol
with 2 equivalents of chlorodiphenylphosphine in the presence
of triethylamine in toluene at reflux temperature (Scheme 1).†
The 31P-{1H} NMR spectrum of 9 shows a singlet at δ 111.7
ppm, essentially identical to those obtained for the related free
resorcinol bis(phosphinite) ligands used in the synthesis of
complexes 8b and c.7 When ligand 9 is reacted with one
equivalent of palladium trifluoroacetate in THF at room
temperature, the 31P-{1H} NMR spectrum of the product
shows it to be a complex mixture of species. However when the
reaction is repeated using a 2 : 1 palladium : ligand ratio then
one major species, complex 10, is observed in the crude reaction
mixture with a singlet in the 31P-{1H} NMR spectrum at δ 149.3
ppm.‡ In addition there are small amounts (<10%) of
additional phosphorus-containing complexes with peaks in the
range δ 146.9–150.1 ppm. The close similarity of the 31P data
for the impurities with that of complex 10 suggest they may be
oligomeric complexes related to 10. Similar polymeric species
with bridging chlorides are formed on double orthopalladation
of N,N,NЈ,NЈ-tetraethylbenzene-1,3-bis(methylamine).9 Com-
plex 10 was obtained microanalytically pure by passing it
through a plug of silica (CH2Cl2 eluent) followed by recrystal-
lisation. A re-examination of the 31P-{1H) NMR spectrum of
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 3
D a l t o n T r a n s . , 2 0 0 3 , 2 8 0 5 – 2 8 0 7
2805

View Article Online
Scheme 1 Conditions: (i) 2 ClPPh2, excess Et3N, toluene, ∆, 18 h. (ii) Pd(TFA)2, THF, r.t., 18 h. (iii) PPh3, CH2Cl2, r.t., 18 h. (iv) dppe, CH2Cl2, r.t.,
18 h.
the reaction performed in a 1 : 1 ratio of palladium to ligand
shows that complex 10 is one of the main compounds in the
product mixture. The 31P NMR data indicate that both phos-
phorus donors are in an identical environment and that they are
both coordinated to palladium. The upfield shift of ca. 40 ppm
compared to the free ligand is typical for an orthopalladated
phosphinite ligand.10 The 1H NMR shows a singlet for the
methyl group at δ 2.18 ppm and, while the shift is not partic-
ularly informative, the integration implies that all three protons
are present and the lack of multiplicity due to coupling to the
31P nuclei indicates that the carbon is not metallated. By con-
trast the protons on the metallated sp3-methylene carbon in the
complexes 7 are seen as triplets coupling to the equivalent
phosphorus donors.10 Integration of the aromatic region indi-
cates the presence of only one residual proton on the resorcinol
ring. The 13C-{1H} NMR spectrum of 10 confirms the presence
of a methyl rather than a metallated methylene with a singlet
seen at δ 12.3 ppm, however due to poor solubility, many of the
quaternary carbons could not be identified. The 19F NMR spec-
Fig. 1 Molecular structure of complex 10ؒ2CHCl3. Selected inter-
atomic lengths (Å) and angles (Њ): Pd1–C19 1.996(3), Pd2–C17 1.992(3),
trum shows two singlets of equal intensity at δ Ϫ74.8 and Ϫ75.7
Pd1–P1 2.1756(9), Pd2–P2 2.1690(9), Pd1–O3i 2.141(2), Pd2–O4
ppm. This implies that there are two distinct types of TFA
(trifluoroacetate) ligand.
2.145(2), Pd1–O5i 2.149(2), Pd2–O6 2.151(2), Pd1 ؒ ؒ ؒ Pd2i 3.2190(3),
P1–Pd1–C19 77.28(9), P2–Pd2–C17 80.44(10), C19–Pd1–O3i 95.06(11),
C17–Pd2–O4 96.33(11), O3i–Pd1–O5i 87.23(9), O4–Pd2–O6 88.15(9),
P1–Pd1–O5i 99.53(7), P2–Pd2–O6 95.09(7), C19–Pd1–O5i 175.02(11),
C17–Pd2–O6 175.49(11), P1–Pd1–O3i 165.66(7), P2–Pd2–O4
165.07(7).
Taken together the spectroscopic data suggest that the ligand
has been doubly orthometallated by two equivalent palladium
centres (Scheme 1). This suggestion is confirmed unequivocally
by a single crystal X-ray structure determination.§ The molec-
ular structure of complex 10 is shown in Fig. 1. As can be seen
each ligand has two palladium centres bonded through both
a P-donor ligand and the ortho-carbon. The molecule is a
tetranuclear dimeric complex in which half the dimer contains
one-dipalladated bisphosphinite ligand. The two halves of the
dimer are held together by µ2-TFA ligands, giving a ‘hinge-like’
structure. In addition there is a twist away from the con-
formation in which the methyl groups would be eclipsed, giving
a C2-axis which renders the complex chiral.
To the best of our knowledge this is a unique example
of a di-orthometallated P-donor ligand. Indeed there are very
few reports of structurally characterised di-orthometallated
aryl ligands supported by any donor groups. Those that exist
tend to be based on N,N,NЈ,NЈ-tetraalkylbenzene-1,3-bis-
(methylamine)—C6H4-1,3-(CH2NR2)2—and related N-donor
ligands.9,11
the palladated carbon, as evidenced by the magnitude of the
2JPP coupling (28.4 Hz). This is in contrast with similar reac-
tions with doubly orthopalladated and -platinated N,N,NЈ,NЈ-
tetraethylbenzene-1,3-bis(methylamine) complexes which give
adducts with PPh3 cis to the metallated carbon.9,11a
The analogous reaction of complex 10 with 1,1Ј-bis(diphenyl-
phosphino)ethane (dppe) gives the binuclear complex 12. The
best evidence for the formation of complex 12 is provided by
the 31P NMR spectra which consists of a doublet of doublets
corresponding to the phosphite ligands at δ 149.5 ppm with
couplings to the cis and trans phosphine donors of 24 and 379
Hz respectively, and two doublets of doublets at δ 51.6 and 46.0
ppm corresponding to the phosphine groups trans and cis to the
2
phosphite respectively with a mutual JPPcis of 34 Hz. In this
case there is evidence for a second, minor species with similar
shifts and coupling patterns in the 31P NMR spectrum.¶ It is
possible that this second species is a tetranuclear dimeric
complex with bridging dppe ligands. Further work is ongoing
to establish its identity.
We thank EPSRC (PDRA for MEB) and Forbairt/Enterprise
Ireland (studentship for PNS) for funding and Johnson
Matthey for the loan of palladium salts.
Complex 10 reacts with 1 equivalent of triphenylphosphine
per palladium in dichloromethane at room temperature to give
the complex 11 (Scheme 1). The 31P-{1H} NMR spectrum
consists of two doublets at δ 154.5 and 18.8 ppm corresponding
to the phosphinite and phosphine donors respectively. Only one
of the three potential isomers is formed, namely that with the
PPh3 ligands cis to the phosphinite donors and thus trans to
D a l t o n T r a n s . , 2 0 0 3 , 2 8 0 5 – 2 8 0 7
2806

View Article Online
2
¶
31P-{1H) NMR (121 MHz) of second species: δ 150.1 (dd, JPPtrans
=
Notes and references
2
2
371.5 Hz, JPPcis = 26.1 Hz, phosphite), 55.05 (dd, JPPtrans = 371.9 Hz,
2
† Synthesis of ligand 9: To a solution of 2-methylresorcinol, (2.5 g,
20.1 mmol) and chlorodiphenylphosphine (7.3 mL, 40.3 mmol) in
toluene (40 mL) was added triethylamine (7.0 mL, 50.0 mmol) drop-
wise. The mixture was heated at reflux temperature for 18 h, cooled and
the solvent removed under reduced pressure. The residue was extraced
with THF (20 mL), the solution filtered through celite and the celite
washed with THF (2 × 20 mL). The solvent was removed from the
combined THF fractions to give the product as a pale yellow solid that
2JPPcis = 33.2 Hz, phosphine trans to phosphite), 43.0 (dd, JPPcis = 33.2
Hz, 2jPPcis = 24.2 Hz, phosphine cis to phosphite).
1 B. L. Shaw, S. D. Perera and E. A. Staley, Chem. Commun., 1998,
1362.
2 S. Gibson, D. F. Foster, G. R. Eastham, R. P. Tooze and D. J.
Cole-Hamilton, Chem. Commun., 2001, 779.
3 (a) D. A. Albisson, R. B. Bedford, S. E. Lawrence and P. N. Scully,
Chem. Commun., 1998, 2095; (b) D. A. Albisson, R. B. Bedford
and P. N. Scully, Tetrahedron Lett., 1998, 39, 9793; (c) R. B. Bedford
and S. L. Welch, Chem. Commun., 2001, 129.
4 (a) R. B. Bedford, C. S. J. Cazin and S. L. Hazelwood, Angew. Chem.
Int. Ed., 2002, 41, 4120; (b) R. B. Bedford, M. E. Limmert and
S. L. Hazelwood, Chem. Commun., 2002, 2610; (c) R. B. Bedford,
C. S. J. Cazin and S. L. Hazelwood, Chem. Commun., 2002,
2608.
5 M. Ohff, A. Ohff, E. van der Boom and D. Milstein, J. Am. Chem.
Soc., 1997, 119, 11687.
6 F. Miyazaki, K. Yamaguchi and M. Shibasaki, Tetrahedron Lett.,
1999, 40, 7379.
7 R. B. Bedford, S. M. Draper, P. N. Scully and S. L. Welch,
New J. Chem., 2000, 24, 745.
8 D. Morales-Morales, R. Redón, C. Yung and C. M. Jensen, Chem.
Commun., 2000, 1619.
1
was not purified further (8.9 g, 90%). NMR data (CDCl3, 25 ЊC): H
NMR (300 MHz): δ 2.78 (s, 3H, CH3), 6.85 (d, 2H, JHH = 8.0 Hz, H4
3
and H6 resorcinol ring), 6.97 (t, 3JHH = 8.0 Hz, H5 resorcinol ring), 7.38
(m, 12H, Ph), 7.62 (m, 8H, Ph). 31P-{1H} NMR (121 MHz): δ 111.7.
‡ A solution of palladium trifluoroactetate (1.00 g, 0.30 mmol) and
ligand 9 (0.74 g, 0.15 mmol) in THF (25 mL) was stirred under an
atmosphere of nitrogen for 18 h. The solvent was removed in vacuo,
CHCl3 (20 mL) added, the residue was collected on a plug of silica
which was washed with CHCl3 (2 × 15 mL) and the residual yellow
precipitate on the silica was dissolved in and eluted with dichloro-
methane. The solvent was removed from the CH2Cl2 solution in vacuo
and the residue was recrystallised from CH2Cl2/EtOH to give 10 as a
yellow solid (0.92 g, 66%). Crystals suitable for X-ray analysis were
grown from CHCl3/EtOH. Calc. for (C70H48F12O12P4Pd4)ؒ2CHCl3: C,
41.23; H, 2.40%. Found: C, 41.0; H, 2.3%. 1H NMR (CD2Cl2, 300
MHz, 25 ЊC): δ 7.70–7.54 (m, 12H, Ar), 7.51–7.43 (m, 8H, Ar),
7.18–7.06 (m, 4H, Ar), 7.02–6.96 (m, 2H, Ar), 6.90–6.79 (m, 8H, Ar),
6.78–6.70 (m, 8H, Ar), 2.10–2.02 (m,6H, CH3) ppm.
9 S. Chakladar, P. Paul, A. K. Mukherjee, S. K. Dutta, K. K. Nanda,
D. Podder and K. Nag, J. Chem. Soc., Dalton Trans., 1992,
3119.
§ Crystallographic data for C72H50Cl6F12O12P4Pd4: Monoclinic, space
group C2/c, a = 33.4010(5), b = 14.0306(3), c = 20.3106(3) Å, β =
126.461(1)Њ, U = 7655.2(2) Å3, Dc = 1.82 Mg mϪ3, Z = 4, T = 120(2) K,
yellow rod, 0.2 × 0.08 × 0.06 mm3. Data collection was carried out
using an Enraf Nonius KappaCCD area detector and SHELXS-97 and
SHELXL-9712 programs were used for structure solution and refine-
ment. 38540 reflections collected, 8745 independent [R(int) = 0.0763]
which were used in all calculations. R1 = 0.0387 for observed unique
reflections [F2 > 2σ(F2)] and wR2 = 0.0905 for all data. The max. and
min. residual electron densities on the final difference Fourier map were
0.883 and Ϫ0.903 e ÅϪ3, respectively. CCDC reference number 210006.
See http://www.rsc.org/suppdata/dt/b3/b305136f/ for crystallographic
data in CIF or other electronic format.
10 R. B. Bedford and S. L. Hazelwood, unpublished results.
11 For examples see: (a) M. D. Meijer, A. W. Kleij, M. Lutz, A. L. Spek
and G. van Koten, J. Organomet. Chem., 2001, 640, 166; (b)
B. O’Keefe and P. J. Steel, Organometallics, 1998, 17, 3621;
(c) J.-P. Djukic, A. Maisse, M. Pfeffer, A. de Cian and J. Fischer,
Organometallics, 1997, 16, 657; (d ) P. Steenwinkel, S. L. James,
D. M. Grove, H. Kooijman, A. L. Spek and G. van Koten,
Organometallics, 1997, 16, 513 and references therein.
12 (a) G. M. Sheldrick, SHELXS-97, Program for solution of crystal
structures, University of Göttingen, Germany, 1997; (b) G. M.
Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Göttingen, Germany, 1997.
D a l t o n T r a n s . , 2 0 0 3 , 2 8 0 5 – 2 8 0 7
2807