Synthesis and crystallographic characterization of a novel platinocycle

Article abstract of DOI:10.1021/om0004904

The efficient synthesis and characterization of the trans-platinum complex (Ph₃P)₂Pt(C₂₅H₁₃N) by coupling the dialkyne 3,5-bis-(2-ethynyl-phenylethynyl)-pyridine with cis-PtCl₂(PPh₃)₂ is described. The X-ray crystal structure shows that the platinum center is essentially square planar with the two phosphine ligands mutually perpendicular to the planar organic ligand.

Full text of DOI:10.1021/om0004904

5522
Organometallics 2000, 19, 5522-5524
Syn th esis a n d Cr ysta llogr a p h ic Ch a r a cter iza tion of a
Novel P la tin ocycle
Eric Bosch*
Department of Chemistry, Southwest Missouri State University, Springfield, Missouri 65804
Charles L. Barnes
Department of Chemistry, University of Missouri, Columbia, Missouri 65211
Received J une 13, 2000
Summary: The efficient synthesis and characterization
of the trans-platinum complex (Ph3P)2Pt(C25H13N) by
coupling the dialkyne 3,5-bis-(2-ethynyl-phenylethynyl)-
pyridine with cis-PtCl2(PPh3)2 is described. The X-ray
crystal structure shows that the platinum center is
essentially square planar with the two phosphine ligands
mutually perpendicular to the planar organic ligand.
Stang9 and Youngs10 independently utilized four cis-
platinum diacetylide moieties in the synthesis of dif-
ferent large organometallic macrocycles.
Resu lts a n d Discu ssion
The synthesis of the platinum complex 5 is shown in
Scheme 1. The dialkynyl ligand 4 was synthesized by
repetitive applications of the Sonagshira coupling11 of
aryl halides with terminal alkynes. Thus palladium-
catalyzed coupling of 3,5-dibromopyridine with 2.2 equiv
of trimethylsilylacetylene in refluxing triethylamine
followed by basic hydrolysis yielded 3,5-diethynylpyrin-
dine, 2, in 86% overall isolated yield. Compound 2 was
coupled with an excess (3.5 equiv) of 1,2-diiodobenzene
to form 3,5-bis(2-iodophenylethynyl)pyridine, 3, in mod-
est yield (63%). Palladium-catalyzed coupling of diiodide
3 with trimethylsilylacetylene followed by basic hy-
drolysis, as before, afforded the organic ligand 3,5-bis-
(2-ethynylphenylethynyl)pyridine, 4, in moderate yield
(77%). The dialkyne 4 was then reacted with 1 equiv of
bis(triphenylphosphine)platinum dichloride at a con-
centration of 1.6 × 10-3 M in refluxing diethylamine
for 10 h. The solvent was concentrated, diluted with
hexane, and passed through a short silica column to
yield essentially pure complex 5 in 74% yield.12 The
colorless solid yielded crystals suitable for X-ray analy-
sis when recrystallized from hexane.13
In tr od u ction
The synthesis of platinum acetylides has recently
been directed toward the preparation of specialty ma-
terials including nonlinear optical materials,1 building
blocks in the preparation of large organoplatinum
macrocycles,2 liquid crystalline materials,3 or the syn-
thesis of conjugated metal-containing polymers.4 Our
current research efforts are directed to the synthesis of
metal-containing macrocycles. In this paper we describe
the synthesis and characterization of a small, strained,
platinocycle formed by coupling of a preformed dialkynyl
ligand with cis-bis(triphenylphosphine)platinum dichlo-
ride in an amine as solvent and base.5 Small plati-
nocycles have recently been synthesized using a similar
platinum σ-acetylide motif. Thus Haley6 reported the
synthesis and crystallographic characterization of a
strained cyclic trans-platinum diacetylide complex while
Youngs7 reported the characterization of a relatively
unstrained cyclic cis-platinum diacetylide. The meth-
odology has been extended to the synthesis of larger
macrocycles; for example Takahashi8 synthesized a
macrocycle containing four trans-palladium diacetylides.
Although the reaction utilized cis-bis(triphenylphos-
phine)platinum dichloride, the resultant product 5 has
mutually trans phosphine ligands as shown in Figure
1. In this regard it should be noted that although there
are several examples of stereospecific syntheses of
platinum diacetylides, i.e., cis-platinum dichlorides
yielding cis products,14 other reports indicate that cis/
trans mixtures are obtained when diethylamine is used
(1) (a) Myers, L. K.; Ho, D. M.; Thompson, M. E.; Langhoff, C.
Polyhedron 1995, 14, 57. (b) Long, N. J . Angew. Chem., Int. Ed. Engl.
1995, 34, 826. (c) Nguyen, P.; Lesley, G.; Marder, T. B.; Ledoux, I.;
Zyss, J . Chem. Mater. 1997, 9, 406.
(2) Whiteford, J . A.; Lu, C. V.; Stang, P. J . J . Am. Chem. Soc. 1997,
119, 2524.
(3) Takahashi, S.; Takati, H.; Morimoto, K.; Sonagshira, K.; Hagi-
hara, N. Mol. Cryst. Liq. Cryst. 1982, 32, 139.
(4) (a) Khan, M. S.; Davies, S. J .; Kakkar, A. K.; Schwartz, D.; Lin,
B.; J ohnson, B. F. G.; Lewis, J . J . Organomet. Chem. 1992, 424, 87.
(b) Onitsuka, K.; Harada, Y.; Takei, F.; Takahashi, S. Chem. Commun.
1998, 643. See also: Kingsborough, R. P.; Swager, T. M. Prog. Inorg.
Chem. 1999, 48, 123.
(9) Manna, J .; Whiteford, J . A.; Stang, P. J .; Muddiman, D. C.;
Smith, R. D. J . Am. Chem. Soc. 1996, 118, 8731.
(10) AlQuaisi, S. M.; Galat, K. J .; Chai, M.; Ray, D. G., III; Rinaldi,
P. L.; Tessier, C. A.; Youngs, W. J . J . Am. Chem. Soc. 1998, 120, 12149.
(11) Takahashi, S.; Kuroyama, Y.; Sonagshira, K.; Hagihara, N.
Synthesis 1980, 14, 627.
(5) (a) Sonagshira, K.; Yatake, Y.; Tohda, Y.; Takahashi, S.; Hagi-
hara, N. J . Chem. Soc., Chem. Commun. 1977, 291. (b) Russo, M. V.;
Furlani, A. J . Organomet. Chem. 1979, 165, 101.
(6) Pak, J . J .; Weakley, T. J . R.; Haley, M. M. Organometallics 1997,
16, 4505.
(7) Bradshaw, J . D.; Guo, L.; Tessier, C. A.; Youngs, W. J . Organo-
metallics 1996, 15, 2582. See also: Zhang, D.; McConville, D. B.;
Tessier, C. A.; Youngs, W. J . Organometallics 1997, 16, 824.
(8) Onitsuka, K.; Yamamoto, S.; Takahashi, S. Angew. Chem., Int.
Ed. 1999, 38, 174.
(12) TLC Analysis indicated that a single product was formed, and
we found no evidence for the formation of oligomeric or polymeric
products. Future studies will consider the role of concentration on the
nature of the products formed.
(13) Crystal data: 5 (C61H41NP2Pt), M ) 1044.98, triclinic space
group P-1, a ) 10.8302(16), b ) 14.743(2), and c ) 16.083(2) Å, R )
81.856(2), â ) 80.155(2), and γ ) 68.650 (2)°, V ) 2347.7(6) Å3, Z ) 2,
Dcalcd ) 1.478 Mg/m3, λ ) 0.71073 Å, T ) 173(2) K.
(14) Sonagshira, K.; Fujikura, Y.; Yatake, T.; Toyoshima, N.; Ta-
kahashi, S.; Hagihara, N. J . Organomet. Chem. 1978, 145, 101.
10.1021/om0004904 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 11/11/2000

Notes
Organometallics, Vol. 19, No. 25, 2000 5523
Sch em e 1. Syn th esis of P la tin u m Com p lex 5a
a
Reaction conditions: (a) 2.2 equiv of (CH3)3SiC2H, PdCl2(PPh3)2/CuI catalyst, refluxing NEt3; (b) K2CO3/CH3OH; (c) 3.5 equiv
of 1,2-diiodobenzene, PdCl2(PPh3)2/CuI catalyst, refluxing NEt3; (d) PtCl2(PPh3)2, catalytic CuI, refluxing Et2NH.
F igu r e 2. View of the platinum-bound ligand. The two
phosphine ligands and all hydrogen atoms except H12 have
been omitted for clarity.
quently there is significantly less strain when the phos-
phine ligands are rotated out of the plane of the organic
ligand into a trans configuration mutually perpendicular
F igu r e 1. Crystal structure of 5.20 The thermal elipsoids
to the organic ligand as shown in Figure 1.
are drawn at the 35% level. All hydrogen atoms are omitted
The geometry about the platinum is slightly distorted
for clarity. Selected bond lengths (Å): Pt(1)-C(26) )1.995-
square planar with the phosphine ligands tilted away
(3); Pt(1)-C(1) )1.996(3); C(1)-C(2) ) 1.209(5); C(25)-
from the organic ligand. In addition the alkynes are bent
away from linearity to act as a pincer to bind the
C(26) ) 1.215(5); Pt(1)-P(2) ) 2.3048(8); Pt(1)-P(1) )
2.3103(9). Selected bond angles (deg): P(1)-Pt(1)-P(2) )
platinum atom as shown in Figure 2. Although the
organic ligand is essentially planar the bent alkynes
impart strain on the entire ligand, and this is manifest
in the bond lengths of the flanking benzene rings. For
example the inner benzene bonds C(24)-C(19) and
C(3)-C(8) are elongated to 1.419(2) and 1.417(5) Å,
respectively and the outer benzene C-C bonds are
compressed for example the bond length of C(6)-C(7)
is 1.369(5) Å for a difference in bond length around the
ring of 4.8 pm. Despite the strain in the organic ligand,
the platinum carbon bond lengths, the platinum phos-
phorus bond lengths, and the platinum-bonded C-C
triple bond lengths [C(1)-C(2) ) 1.209(5) and C(25)-
C(26) ) 1.215(5) Å] are similar to those observed in
other platinum diacetylides.17
170.68; C(1)-Pt(1)-C(26) ) 170.02; C(1)-Pt(1)-P(1) )
90.12; C(3)-C(2)-C(1) ) 173.6; C(2)-C(1)-Pt(1) ) 169.0;
C(24)-C(25)-C(26) ) 171.4; C(25)-C(26)-Pt ) 167.5.
as solvent and base.15 In addition it has been reported
that metal salts, including copper iodide, catalyze the
isomerization of the less stable cis-platinum diacetylides
to the corresponding trans-platinum diacetylides.16 In
this case we believe, however, that the steric environ-
ment of the small dialkyne ligand dictates the formation
of the trans compound 5. Indeed a cis square planar
complex would have the phosphine ligands coplanar
with the organic ligand and this is prohibited by the
large bite angle of the dialkynyl ligand (>120°) and the
large cone angle of the triphenylphosphines. Conse-
It is noteworthy that the distortion of the alkynes
holds the platinum atom relatively close to H12 on the
(15) Russo, M. V.; Licoccia, S.; Furlani, A.; Villa, A. C.; Guastini, C.
J . Chem. Soc., Dalton Trans. 1984, 2197.
(16) (a) Cross, R. J .; Davidson, M. F. Inorg. Chim. Acta 1985, 97,
L35. (b) Cross, R. J .; Davidson, M. F. J . Chem. Soc., Dalton Trans.
1986, 1987. (c) Harriman, A.; Hissler, M.; Ziessel, R.; De Cain, A.;
Fisher, J . J . Chem. Soc., Dalton Trans. 1995, 4067.
(17) See, for example: (a) Manna, J .; J ohn, K. D.; Hopkins, M. D.
Adv. Organomet. Chem. 1995, 38, 79. (b) Nast, R. Coord. Chem. Rev.
1982, 47, 89 and references therein.

5524 Organometallics, Vol. 19, No. 25, 2000
Notes
as described above, sealed, and heated at 80 °C for 8 h. The
tube was allowed to cool to room temperature, diluted with
ethyl acetate, and washed with water and saturated aqueous
sodium chloride. The organic phase was separated and dried
with magnesium sulfate, and the solvent was removed in
vacuo. The crude product was subjected to flash chromatog-
raphy with hexane:ethyl acetate (10:1) as eluant and the
desired compound obtained as an off-white solid (581 mg, 63%).
The material was recrystallized from ether/hexane as colorless
crystals: mp 86 °C; 1H NMR δ 8.76 (d, J ) 2.0 Hz, 2 H), 8.03
(t, J ) 2.0 Hz, 1 H), 7.90 (dd, J ) 1,2, 7.7 Hz, 2 H), 7.56 (dd,
J ) 1.8, 7.7 Hz, 2H), 7.36 (dt, J ) 1.8, 7.7 Hz, 2 H), 7.06 (dt,
J ) 1.8, 7.7 Hz, 2H); 13C NMR δ 150.0, 139.4, 137.8, 131.6,
128.1, 127.8, 126.9, 118.8, 100.1, 94.3, 87.6. Anal. Calcd for
C21H11NI2: C, 47.50; H, 2.09; N, 2.64. Found: C, 47.78; H, 2.28;
N, 2.41.
central pyridine ring as shown in Figure 2. The calcu-
lated distance between this proton and the platinum is
3.00 A and this close nonbonded interaction is manifest
in a downfield shift of the 1H NMR signal of this proton.
1
This proton is readily identified in the H NMR spec-
trum, appearing as a triplet with J ≈ 2 Hz18 and
exhibits a chemical shift of 8.35 ppm in complex 5 as
compared to a chemical shift of 7.98 ppm in the free
ligand 4.
We intend to extend this study to the preparation of
larger platinocycles and also to explore the complexation
of other transition metals with the ligand 4 and related
dialkynes.
Exp er im en ta l Section
3,5-Bis(2-eth yn ylp h en yleth yn yl)p yr id in e (4) was syn-
thesized as described for 2 starting with 3 (350 mg, 0.66 mmol)
and 2.5 equiv (160 mg, 1.63 mmol) of trimethylsilylacetylene.
The intermediate bis-trimethylsilyl-protected compound was
essentially pure by 1H NMR: δ 8.72 (d, J ) 2 Hz, 2H), 7.99 (t,
J ) 2 Hz, 1H), 7.55-7.51 (m, 4H), 7.33-7.28 (m, 4H), 0.29 (S,
18H). After base-promoted deprotection as before an off-white
solid was isolated and purified by flash chromatography with
hexane:ethyl acetate (10:1) as eluant. Compound 4 was
obtained as colorless crystals (189 mg, 77%): mp 151 °C; IR
1
Gen er a l. The H NMR spectra were recorded at 200 MHz
and the 13C NMR spectra were recorded at 50 MHz. Elemental
analyses were performed by Atlantic Microlab, Atlanta, GA.
Ma ter ia ls. Diethylamine, triethylamine, 3,5-dibromopyri-
dine, trimethylsilyl acetylene, cis-PtCl2(PPh3)2, triphenylphos-
phine copper iodide (ACROS), and 1,2-diiodobenzene (Oak-
wood) were used as received.
3,5-Dieth yn ylp yr id in e19 (2) was synthesized according to
Sonagshira’s11 coupling procedure. Thus a solution of 3,5-
dibromopyridine (2.5 g, 10.6 mmol), trimethylsilyl acetylene
(3.2 mL, 23.3 mmol), triphenyl phosphine (120 mg, 0.74 mmol),
copper iodide (8 mg, 0.04 mmol), and bis(triphenylphosphine)-
palladium(II) chloride (78 mg, 0.11 mmol) in triethylamine (60
mL) was prepared in a pressure flask. Argon was bubbled
through the solution for 5 min, and the vessel was sealed with
a Teflon stopcock and heated at 70 °C for 8 h. The flask was
allowed to cool to room temperature, diluted with hexane:ethyl
acetate (1:1), and washed three times with water and finally
with saturated aqueous sodium chloride. The organic phase
was dried with magnesium sulfate, and the solvent was
removed in vacuo to yield an off-white solid. The 1H NMR
spectrum indicated that this material was essentially pure 3,5-
bis(trimethylsilanylethynyl)pyridine: 1H NMR δ 8.62 (d, J )
2 Hz, 2H), 7.83 (t, J ) 2 Hz, 1H), 0.29 (s, 18H). The solid was
thus deprotected without further purification. The crude solid
was dissolved in methanol (100 mL), sodium carbonate (0.5
g) was added, and the heterogeneous mixture was stirred at
room temperature for 2 h. The mixture was diluted with water
and extracted twice with hexane/ethyl acetate (1/1, 2 × 70 mL).
The organic extracts were washed with water, saturated
aqueous sodium chloride, and dried over magnesium sulfate.
The solvent was evaporated in vacuo to yield 3,5-diethynylpy-
ridine as a white solid (1.14 g, 86%): mp 78 °C; IR (KBr) 3280,
1
(KBr) 3275, 3031, 1580 cm-1; H NMR δ 8.73 (d, J ) 2.0 Hz,
2 H), 7.98 (t, J ) 2.0 Hz, 1H), 7.55 (m, 4 H), 7.34 (m, 4H), 3.40
(s, 2H); 13C NMR δ 150.08, 139.62, 131.66, 130.88, 127.61,
124.24, 123.87, 118.97, 90.61, 88.10, 80.84, 80.53. Anal. Calcd
for C25H13N: C, 91.72; H, 4.00; N, 4.27. Found: C, 91.43; H,
4.03; N, 3.99.
P la tin u m Com p lex (5). Bis(triphenylphosphine)platinum
dichloride (274 mg, 0.35 mmol) and 4 (118 mg, 0.36 mmol) and
were added to dry diethylamine (220 mL) in a round-bottom
flask fitted with a Schlenk adapter. Argon was bubbled
through the flask for 10 min, a condenser was attached, and
the mixture was refluxed under an argon atmosphere for 10
h. A distillation adapter was then fitted, and the diethylamine
was distilled until the mixture was concentrated to 70 mL.
Hexanes (100 mL) were added to the concentrate, and the
resultant heterogeneous mixture was passed through a short
column of silica gel. The column was eluted with a mixture of
hexane and ethyl acetate (4:1). The solvent was evaporated
under vacuum, and the product was isolated as an off-white
solid (267 mg, 74%). A sample suitable for crystallographic
analysis was recrystallized from hexane: 1H NMR δ 8.42 (d,
J ) 2.0 Hz, 2 H), 8.35 (t, J ) 2.0 Hz, 1 H), 7.83 (br m, 12 H),
7.35 (m, 2 H), 7.20 (m, 15 H), 7.01 (m, 4 H), 6.74 (m, 2 H); 13
C
NMR δ 149.18, 145.31, 135.44, 135.32, 132.37, 131.79, 131.20,
130.57, 129.17, 128.35, 128.23, 128.12, 128.02, 125.01, 124.52,
121.47, 95.41, 88.66. Anal. Calcd for C61H41NP2Pt: C, 70.11;
H, 3.95. Found: C, 69.84; H, 3.95.
1
3216, 2104, 1581 cm-1; H NMR δ 8.61 (d, J ) 2.0 Hz, 2 H),
7.82 (t, J ) 2.0 Hz, 1H), 3.24 (s, 2H); 13C NMR δ 150.80, 140.80,
117.88, 80.42, 78.30. Anal. Calcd for C9H5N: C, 85.02; H, 3.96;
N, 11.02. Found: C, 84.66; H, 4.09; N, 10.91.
Ack n ow led gm en t. E.B. acknowledges receipt of a
Faculty Research Grant and a Summer Faculty Fellow-
ship from the Graduate College of Southwest Missouri
State University.
3,5-Bis(2-iod op h en yleth yn yl)p yr id in e (3). A solution of
3,5-diethynylpyridine (220 mg, 1.7 mmol), 1,2-diiodobenzene
(1.78 g, 5.4 mmol), triphenylphosphine (30 mg), copper iodide
(5 mg), and bis(triphenylphosphine)palladium(II) chloride (36
mg) in triethylamine (20 mL) in a pressure flask was deaerated
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data and structure refinement, atomic coordinates, bond
lengths and angles, anisotropic displacement parameters, and
hydrogen coordinates for 5. This material is available free of
charge via the Internet at http://pubs.acs.org.
(18) Macomber, S. A Complete Introduction to Modern NMR Spec-
troscopy; Wiley-Interscience: New York, 1998; p 168.
(19) Shvartsberg, M. S.; Moroz, A. A.; Kozhevnikova, A. N. Izv. Akad.
Nauk SSSR, Ser. Khim. 1978, 4, 875-9.
(20) Drawn using ORTEP-3: Farrugia, L. J . J . Appl. Cryst. 1997,
30, 565.
OM0004904