High-Yield Synthesis of a C∧N∧C Tridentate Platinum Complex

Article abstract of DOI:10.1021/om981006o

2,6-Diphenylpyridine is metalated twice by potassium tetrachloroplatinate in acetic acid to give in high yield a complex with a tridentate ligand bound to the metal via a C∧N∧C donor set. This complex and three derivatives have been characterized, including the single-crystal X-ray analysis of one derivative.

Full text of DOI:10.1021/om981006o

Organometallics 1999, 18, 1801-1803
1801
High -Yield Syn th esis of a C∧N∧C Tr id en ta te P la tin u m
Com p lex
Gareth W. V. Cave, Nathaniel W. Alcock, and J onathan P. Rourke*
Department of Chemistry, Warwick University, Coventry, U.K. CV4 7AL
Received December 10, 1998
Summary: 2,6-Diphenylpyridine is metalated twice by
potassium tetrachloroplatinate in acetic acid to give in
high yield a complex with a tridentate ligand bound to
the metal via a C∧N∧C donor set. This complex and
three derivatives have been characterized, including the
single-crystal X-ray analysis of one derivative.
metalation of two ligands by one metal center is also
known.19-22 What is not common are tridentate C∧N∧C
or C∧P∧C donor sets where two cyclometalated rings
have been formed via two C-H activations. One paper
reports the isolation, in low yield, of the dicyclometa-
lated platinum complex of 2,6-diphenylpyridine.23 The
subject of this paper is the high-yield synthesis of such
tridentate C∧N∧C compounds of platinum.
In tr od u ction
Cyclometalation is a reaction that has been both
widely used1,2 and widely studied.3,4 Typically, a ligating
species coordinates to a metal center and a proximal
C-H bond is activated, generating a five- or six-
membered chelate ring.3 Our own use of the reaction
has been to generate novel cyclopalladated metallo-
mesogens5-7 and some unusual platinum complexes.8
Tridentate cyclometalated species where two coordinat-
ing groups hold a C-H bond close to the metal and this
bond becomes activated are relatively common; thus,
N∧C∧N9 donor sets,10 P∧C∧P donor sets,11-14 or S∧C∧S
donor sets15 are well-known. Indeed, some groups have
used two ligating groups to induce C-C activation16 or
C-Si activation.17 In addition, the use of a chelating
N∧N donor set to yield N∧N∧C tridentate cyclometa-
lated species has been reported.18 The double cyclo-
Resu lts a n d Discu ssion
The reaction of potassium tetrachloroplatinate with
diphenylpyridine in acetic acid to give a complex we
tentatively formulate as 2 is essentially quantitative
(Scheme 1). While we could get no solution data on 2,
as it either is insoluble or reacts with solvent, solid-
state characterization is consistent with this formula-
tion. Thus, elemental analysis and mass spectrometry
(both positive and negative ion) confirm our hypothesis.
On dissolution in dimethyl sulfoxide (dmso) 2 gives
complex 3, which has been fully characterized (NMR,
elemental analysis, single-crystal X-ray diffraction).
Characterization of 3 provides additional support for our
formulation of 2.
The synthesis of 2 is simplicity itself and has been
used for monocycloplatination before.24 Complex 2 is a
robust yellow powder stable to air and water, which
decomposes without melting at 320-370 °C (TGA
analysis shows this decomposition process to be com-
plete, leaving only platinum). Our major use of 2 is in
the synthesis of complexes 3 and 4. Complex 3 was
isolated in essentially quantitative yield by the simple
act of dissolution in dmso. Thus, the isolated yield of 3,
starting from our platinum source, is 98%. The NMR
spectrum of 3 is unexceptional, and crystals suitable for
X-ray analysis were grown. The X-ray structure itself
is as expected (Figure 1): three molecules are found
within the asymmetric unit, with little to distinguish
among them. The diphenylpyridine and the Pt are
essentially flat, though the dmso is bent out of plane
with the N-Pt-S angles being 170.9(2), 173.3(2), and
173.8(2)° in the three molecules in the asymmetric unit.
The C1-Pt-S-O torsion angles also differ, being -14.1,
13.2, and 6.6° in the three molecules. Quite why the
* To whom correspondence should be addressed. E-mail: j.rourke@
warwick.ac.uk.
(1) Ryabov, A. D. Synthesis 1985, 233.
(2) Pfeffer, M. Pure Appl. Chem. 1992, 64, 335.
(3) Ryabov, A. D. Chem. Rev. 1990, 90, 403.
(4) Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem. Eur. J .
1998, 4, 759.
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7, 403.
(6) Lydon, D. P.; Rourke, J . P. Chem. Commun. 1997, 1741.
(7) Cave, G. W. V.; Lydon, D. P.; Rourke, J . P. J . Organomet. Chem.
1998, 555, 81.
(8) Cave, G. W. V.; Hallett, A. J .; Errington, W.; Rourke, J . P. Angew.
Chem. 1998, 37, 3466; Angew. Chem., Int. Ed. Engl. 1998, 37, 3270.
(9) The abbreviation N∧C∧N refers to a tridentate ligand bonding
through two N donors and one C donor, with the ligand conectivity
being N linked to C linked to N. Thus, N∧N∧C also represents a
tridentate ligand bonding through two N donors and one C donor, but
this time the ligand conectivity is N linked to N linked to C.
(10) Terheijden, J .; van Koten, G.; Muller, F.; Grove, D. M.; Vrieze,
K.; Nielsen, E.; Stam, C. H. J . Organomet. Chem. 1986, 315, 401.
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1020.
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451, 249.
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Chem. 1996, 35, 1792.
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1989, 1715.
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Turtle, B. L. J . Chem. Soc., Chem. Commun. 1973, 128.
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23, 4249.
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Chem. Soc. 1996, 118, 12406.
(17) Steenwinkel, P.; Gossage, R. A.; Maunula, T.; Grove, D. M.; van
Koten, G. Chem. Eur. J . 1998, 4, 763.
(18) Constable, E. C.; Henney, R. P. G.; Leese, R. A.; Tocher, D. A.
J . Chem. Soc., Chem. Commun. 1990, 513.
(21) van der Boom, M. E.; Liou, S.-Y.; Shimon, L. J .; Ben-David, Y.;
Milstein, D. Organometallics 1996, 15, 2562.
(22) Thorn, D. L. Organometallics 1998, 17, 348.
(23) Cornioley-Deuschel, C.; Ward, T.; von Zelewsky, A. Helv. Chim.
Acta 1988, 71, 130.
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10.1021/om981006o CCC: $18.00 © 1999 American Chemical Society
Publication on Web 03/31/1999

1802 Organometallics, Vol. 18, No. 9, 1999
Notes
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) in th e Cr ysta l Str u ctu r e of 3
n ) 1
n ) 2
n ) 3
Pt(n)-N(n8)
Pt(n)-C(n1)
Pt(n)-C(n14)
Pt(n)-S(n)
2.004(7)
2.068(9)
2.082(9)
2.201(2)
1.469(6)
1.772(9)
1.773(9)
2.014(7)
2.069(9)
2.106(9)
2.190(2)
1.474(6)
1.766(9)
1.777(9)
2.016(7)
2.074(9)
2.085(9)
2.197(2)
1.485(6)
1.772(9)
1.775(10)
S(n)-O(n)
S(n)-C(n01)
S(n)-C(n02)
N(n8)-Pt(n)-C(n1)
N(n8)-Pt(n)-C(n14)
C(n1)-Pt(n)-C(n14)
N(n8)-Pt(n)-S(n)
C(n1)-Pt(n)-S(n)
C(n14)-Pt(n)-S(n)
O(n)-S(n)-Pt(n)
C(n01)-S(n)-Pt(n)
C(n02)-S(n)-Pt(n)
C(n6)-C(n1)-Pt(n)
C(n9)-N(n8)-Pt(n)
C(n7)-N(n8)-Pt(n)
C(n13)-C(n14)-Pt(n)
79.8(3)
80.6(3)
159.8(3)
170.9(2)
98.4(2)
101.7(2)
117.7(3)
114.6(3)
110.1(3)
112.8(6)
118.7(6)
118.6(6)
110.8(6)
80.3(3)
80.6(3)
160.4(3)
173.8(2)
98.6(3)
100.9(2)
119.3(3)
112.2(3)
109.9(3)
111.5(6)
118.1(6)
118.5(6)
110.5(6)
80.0(3)
79.8(3)
159.2(3)
173.3(2)
100.3(2)
100.3(3)
119.3(3)
112.7(3)
109.8(3)
111.9(6)
118.5(6)
117.7(6)
111.6(7)
F igu r e 1. Crystal structure of 3 (50% thermal ellipsoids).
The dicyclometalated platinum complex of 2,6-diphe-
nylpyridine has been reported previously: a synthetic
procedure involving the double lithiation of diphenylpy-
ridine followed by reaction with PtCl2(SEt2)2 gave 4 (L
) SEt2) in 10% yield;23 the same paper reports the
analogous palladium complex in 1.4% yield. Other than
this report there are only three structurally character-
ized species containing a C∧N∧C or a C∧P∧C donor set.
The first reported example was a platinum phosphine
complex, though this C∧P∧C,P complex is unstable with
respect to the (P∧C)2 isomer;19 the other examples are
palladium species metalated to a pyridine with pendant
â-dicarbonyls, where the reaction is with a fairly acidic
hydrogen.25,26
Sch em e 1
We are at present investigating the reactivity of both
2 and 3 and seeking to broaden the scope of this reaction
to other metals (for instance, preliminary studies indi-
cate that the palladium analogue of 2 may be isolated
in a yield of around 20%).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All chemicals were used as
supplied; unless noted otherwise, the stilbazole was synthe-
sized via the published route.27 All elemental analyses were
performed by Warwick Analytical Service.
Syn th esis of K+[(2,6-d ip h p yP t)2Cl]- (2). Potassium tet-
rachloroplatinate (374 mg, 0.900 mmol) was added to a
solution of 2,6-diphenylpyridine (233 mg, 1.00 mmol) in glacial
acetic acid (250 mL). The reaction mixture was stirred at 80
°C until the red platinum salt was no longer visible (3 days).
The reaction mixture was filtered, yielding the product as an
insoluble yellow powder which was washed with water,
acetone, and ether. Yield: 407 mg (98%, 0.441 mmol). Anal.
Found (calcd): C, 43.7 (44.2); H, 2.6 (2.4); N, 3.0 (3.0). MALDI
MS (2,5-dihydroxybenzoic acid matrix): negative ion peaks at
m/z 35 and 37 indicating Cl-; positive ion peaks at m/z 39 and
425 corresponding to K+ and (diphpy)Pt.
dmso is not in plane is unclear: presumably it arises
through packing constraints, constraints which cannot
be equal for all three molecules, given the 3° variation
in the N-Pt-S angle. All other bond lengths and angles
are normal. Selected bond lengths and angles are listed
in Table 1.
Complex 3 may be reacted with other ligating species
to give complexes 4 (Scheme 1). A sample of 3 in an
NMR tube was treated with 1 equiv of trimethylphos-
phine, whereupon an instantaneous reaction took place
to give 4a in quantitative yield. The complex may be
isolated by the simple expedient of solvent removal.
Likewise, the pyridine-based stilbazole ligand reacts to
give 4b. Hence, 3 is a very convenient source of the
C∧N∧C-Pt moiety.
(25) Newkome, G. R.; Kawato, T.; Kohli, D. K.; Puckett, W. E.;
Oliver, B. D.; Chiari, G.; Fronczek, F. R.; Deutsch, W. A. J . Am. Chem.
Soc. 1981, 103, 3423.
(26) Newkome, G. R.; Kohli, D. K.; Fronczek, F. R. J . Am. Chem.
Soc. 1982, 104, 994.
(27) Bruce, D. W.; Dunmur, D. A.; Lalinde, E.; Matilis, P. M.;
Styring, P. Liq. Cryst. 1988, 3, 385.

Notes
Syn th esis of [(2,6-d ip h p y)P t(d m so)] (3). The yellow
Organometallics, Vol. 18, No. 9, 1999 1803
Syn th esis of [(2,6-diph py)P t(P Me3)] (4a). Trimethylphos-
phine (9 mg, 0.12 mmol) was added to a solution of [(2,6-
diphpy)Pt(dmso)] (56 mg, 0.108 mmol) in chloroform (1 mL).
All solvents were removed in vacuo to give a yellow product.
Yield: 54 mg (99%, 0.107 mmol). Anal. Found (calcd): C, 48.2
(48.0); H, 4.0 (4.0); N, 3.0 (2.8). NMR data: δP (161.9 MHz,
K+[(2,6-diphpyPt)2Cl]- produced above was dissolved in hot
dmso (1 mL), the solution was cooled, and precipitation was
induced with water (1 mL) to give a yellow crystalline product.
Yield: 443 mg (98%, 0.882 mmol). Anal. Found (calc): C, 45.0
(45.4); H, 3.4 (3.4); N, 2.8 (2.8). NMR data: δH (250 MHz,
CDCl3) 7.80 (2H, dd, 3J ) 7.6, 4J ) 1.2, 3J (Pt-H) ) 24 Hz,
phenyl ortho to Pt), 7.62 (1H, t, 3J ) 7.3 Hz, central pyridine),
1
CDCl3) -20.98 (s, J (Pt-P) ) 1606 Hz); δH (250 MHz, CDCl3)
8.38 (1H, t, J ) 8.5 Hz, central pyridine), 8.11 (2H, d, 3J )
3
3
4
3
4
8.5 Hz, pyridine), 7.93 (2H, dd, J ) 7.6, J ) 1.2 Hz, phenyl
7.47 (2H, dd, J ) 7.6, J ) 1.2 Hz, phenyl ortho to pyridine,
ortho to pyridine, meta to Pt), 7.80 (2H, dd, 3J ) 7.6, 4J ) 1.2,
3
meta to Pt), 7.30 (2H, d, J ) 7.3 Hz, pyridine), 7.28 (2H, td,
3J (Pt-H) ) 16 Hz, phenyl ortho to Pt), 7.41 (2H, td, J ) 7.6,
3
3J ) 7.6, J ) 1.2 Hz, phenyl), 7.21 (2H, td, J ) 7.6, J ) 1.2
4
3
4
4J ) 1.2 Hz, phenyl), 7.34 (2H, td, 3J ) 7.6, 4J ) 1.2 Hz,
phenyl), 3.68 (6H, d, 2J (P-H) ) 1.8, 3J (Pt-H) ) 64.5 Hz,
PMe3).
3
Hz, phenyl), 3.68 (6H, s, J (Pt-H) ) 26.8 Hz, dmso).
X-r a y Cr ysta llogr a p h ic Stu d y of 3. Crystals suitable for
structural analysis were grown from dmso. A golden yellow
prism (dimensions 0.2 × 0.06 × 0.06 mm) was mounted with
oil on a thin glass fiber. Data were collected at 180(2) K using
a Siemens SMART CCD area-detector diffractometer. Crystal
data for 3: C19H17NOSPt, Mr ) 502.49, monoclinic, space group
P21/c, a ) 17.9050(10) Å, b ) 9.9528(5) Å, c ) 27.658(2) Å, â
) 97.885(5)°, V ) 4882.2(5) Å3, Z ) 12, D(calcd) ) 2.051 Mg/
m3. Refinement was by full-matrix least squares on F2 for 7378
reflection positions using SHELXL-9628 with additional light
atoms found by Fourier methods. Hydrogen atoms were added
at calculated positions and refined using a riding model with
freely rotating methyl groups. Anisotropic displacement pa-
rameters were used for all non-H atoms; H atoms were given
isotropic displacement parameters equal to 1.2 (or 1.5 for
methyl hydrogen atoms) times the equivalent isotropic dis-
placement parameter of the atom to which the H atom is
attached. The weighting scheme was calculated using w )
1/[σ2(Fo2) + (0.0520P)2], where P ) (Fo2+2Fc2)/3. The goodness
of fit on F2 was 0.914, R1 (for 7378 reflections with I > 2σ(I))
) 0.0481, wR2 ) 0.1076. The data/parameter ratio was 11 487/
628. The largest difference Fourier peak and hole were 1.912
and -1.431 e Å-3; the only large peaks are near the Pt atoms.
The asymmetric unit contains three essentially identical
molecules. Selected bond lengths and angles are listed in
Table 1.
Syn t h esis of [(2,6-d ip h p y)P t (st ilb a zole)] (4b ). Octyl-
oxystilbazole (33 mg, 0.119 mmol) was added to a solution of
[(2,6-diphpy)Pt(dmso)] (56 mg, 0.108 mmol) in chloroform (1
mL). The mixture was stirred (30 min), solvents were removed
in vacuo, and the residue was washed with ether to give a
yellow product. Yield: 76 mg (99%, 0.108 mmol). Anal. Found
(calcd): C, 61.0 (61.3); H, 4.8 (4.9); N, 4.0 (4.0). NMR data:
3
δH (400 MHz, CDCl3) 8.92 (2H, AA′XX′, J (Pt-H) ) 43.5 Hz,
stilbazole pryidine, ortho to N), 7.53 (2H, AA′XX′, stibazole
phenyl ring), 7.51 (1H, t, 3J ) 8 Hz, central pyridine), 7.46
(2H, m, phenyl ortho to Pt), 7.44 (2H, AA′XX′, stilbazole
3
pyridine), 7.41 (1H, d, J ) 16.6 Hz, stilbazole double bond),
3
3
4
7.23 (2H, d, J ) 8 Hz, pyridine), 7.20 (2H, dt, J ) 7.5, J )
3
4
1.3 Hz, phenyl), 7.05 (2H, dt, J ) 7.5, J ) 1.3 Hz, phenyl),
3
4
7.00 (2H, dd, J ) 7.5, J ) 1.3 Hz, phenyl ortho to pyridine),
6.93 (2H, AA′XX′, stilbazole), 6.91 (1H, d, 3J ) 16.6 Hz,
3
stilbazole double bond), 3.99 (2H, t, J ) 6.5 Hz, OCH2), 1.80
(2H, m), 1.46 (2H, m), 1.31 (8H, m), 0.89 (3H, t, J ) 6.5 Hz).
3
Ack n ow led gm en t. We thank the EPSRC for a
studentship (G.W.V.C.) and J ohnson-Matthey for a loan
of precious metal salts.
Su p p or tin g In for m a tion Ava ila ble: Tables of positional
parameters, bond distances, bond angles, and anisotropic
parameters for the structural analysis of 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
(28) Sheldrick, G. M. SHELXL-96, Program for Crystal Structure
Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1996.
OM981006O