Synthesis and structures of platinum(0) alkyne complexes with extended π-conjugated systems

Article abstract of DOI:10.1021/om800163p

1,2-Bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobuta[l]phenanthrene (DPCB-phen) as a low-coordinated phosphorus ligand forms the platinum(0) alkyne complexes [Pt(alkyne)-(DPCB-phen)] with extended π-conjugated systems. The colors of the complexes a

Full text of DOI:10.1021/om800163p

1970
Organometallics 2008, 27, 1970–1972
Synthesis and Structures of Platinum(0) Alkyne Complexes with
Extended π-Conjugated Systems
Kyohei Hayashi,† Mitsuharu Nakatani,† Akito Hayashi,† Masato Takano,†
Masaaki Okazaki,† Kozo Toyota,‡ Masaaki Yoshifuji,‡,§ and Fumiyuki Ozawa*,†
International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto
UniVersity, Uji, Kyoto 611-0011, Japan, and Department of Chemistry, Graduate School of Science,
Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan
ReceiVed February 21, 2008
clobuta[l]phenanthrene (DPCB-phen)5 as an auxiliary ligand
Summary: 1,2-Bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cy-
clobuta[l]phenanthrene (DPCB-phen) as a low-coordinated phos-
phorus ligand forms the platinum(0) alkyne complexes [Pt(alkyne)-
(DPCB-phen)] with extended π-conjugated systems. The colors of
the complexes are highly dependent on alkyne ligands, showing a
marked Variation from reddish orange to teal.
(eq 1), which is a 1,2-fused compound of 1,2-diphenyl-3,4-
bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene
(DPCB).6 As has been already demonstrated for group 9 and
10 metal complexes, DPCB as a low-coordinated phosphorus
compound possesses extremely low-lying π* orbitals and acts
as a strong π-acceptor toward transition metals.7,8 It has been
found that this unique electronic property effectively induces
π-conjugation over the molecules.
Organotransition-metal complexes with extended π-conjugated
systems have attracted continuous research interest.1 Considering
the great utility of organic molecules with extensive pπ-pπ
interactions in materials science,2 organometallic complexes with
effective dπ-pπ interactions between transition metals and un-
saturated hydrocarbon ligands are of particular interest.3 Although
platinum alkyne and alkynyl species are frequently employed as
the key components of such complexes, dπ-pπ interaction between
the dπ orbital of platinum and the out-of-plane π orbitals (i.e., the
π orbitals perpendicular to the coordination plane; denoted as π )
of alkyne and alkynyl ligands is insignificant in most cases because
the dπ orbital is commonly occupied and undergoes repulsive
interaction with the π orbital.4
(1)
We herein describe that platinum(0) alkyne complexes with
extended π-conjugated systems are successfully prepared
using 1,2-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cy-
Complexes 1a–d were synthesized from [Pt(cod)2] (cod )
1,5-cyclooctadiene) by stepwise displacement of the cod
ligands.9 Thus, the alkyne ligands were initially introduced, and
then the DPCB-phen ligand was introduced to the resulting
[Pt(alkyne)(cod)] complexes.10 Although the ligand exchange
* To whom correspondence should be addressed. E-mail: ozawa@
scl.kyoto-u.ac.jp.
† Kyoto University.
(5) Nakamura, A.; Kawasaki, S.; Toyota, K.; Yoshifuji, M. Chem. Lett.
2004, 33, 1570.
‡ Tohoku University.
§ Present address: Department of Chemistry, The University of Alabama,
Tuscaloosa, AL 35487-0336.
(6) Appel, R.; Winkhaus, V.; Knoch, F. Chem. Ber. 1987, 120, 243.
(7) (a) Ozawa, F.; Yoshifuji, M. C. R. Chim. 2004, 7, 747. (b) Ozawa,
F.; Yoshifuji, M. Dalton Trans. 2006, 4987.
(1) Mashima, K.; Nakamura, A. In Organometallic Conjugation; Na-
kamura, A., Ueyama, N., Yamaguchi, K., Eds.; Kodansha-Springer: Tokyo,
2002; p 161.
(8) For related studies on low-coordinated phosphorus compounds, see:
(a) Le Floch, P. Coord. Chem. ReV. 2006, 250, 627. (b) Yoshifuji, M. Pure
Appl. Chem. 2005, 77, 2011. (c) Mathey, F. Angew. Chem., Int. Ed. 2003,
42, 1578. (d) Weber, L. Angew. Chem., Int. Ed. 2002, 41, 563.
(9) A typical procedure is reported for 1c. To a Schlenk tube containing
[Pt(cod)2] (78 mg, 0.19 mmol) was added hexane (4 mL). The heterogeneous
mixture was cooled to -20 °C, and a solution of 1,2-bis(4-methoxyphe-
nyl)ethyne (tolan-OMe; 45 mg, 0.19 mmol) in Et2O (2 mL) was added
dropwise with stirring. After 1 h, the pale yellow precipitate of [Pt(tolan-
OMe)(cod)] that formed in the system was collected by filtration, washed
with hexane (2 mL × 3), and dried under vacuum (77 mg, 75%). This
product was used in the next step without purification. The complex
[Pt(tolan-OMe)(cod)] (22 mg, 0.040 mmol) was placed in a Schlenk tube
and dissolved in CH2Cl2 (1.5 mL). A solution of DPCB-phen (30 mg, 0.040
mmol) in CH2Cl2 (1 mL) was added, and the resulting mixture was stirred
at room temperature for 3 h, giving a deep purple solution. Completion of
the ligand displacement was confirmed by 31P{1H} NMR spectroscopy.
The solution was concentrated to dryness under reduced pressure, and the
resulting solid was washed with hexane (1 mL × 3) to afford a purple
powder of [Pt(tolan-OMe)(DPCB-phen)] (1c; 28 mg, 59%). Complexes
1a,b,d were similarly prepared in 60, 58, and 63% yields, respectively.
(10) Boag, N. M.; Green, M.; Grove, D. M.; Howard, J. A. K.; Spencer,
J. L.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1980, 2170.
(2) (a) Roncali, J. Chem. ReV. 1997, 97, 173. (b) Kraft, A.; Grimsdale,
A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (c) Carroll,
R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378. (d) Heeger,
A. J. Angew. Chem., Int. Ed. 2001, 40, 2591. (e) Hissler, M.; Dyer, P. W.;
Réau, R. Coord. Chem. ReV. 2003, 244, 1. (f) Hoeben, F. J. M.; Jonkheijm,
P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491. (g)
Coppo, P.; Turner, M. L. J. Mater. Chem. 2005, 15, 1123. (h) Roncali, J.;
Blanchard, P.; Frère, P. Mater. Chem. 2005, 15, 1589. (i) Yamaguchi, S.;
Xu, C. J. Synth. Org. Chem. Jpn. 2005, 63, 1115.
(3) (a) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999,
48, 123. (b) Nguyen, P.; Gómez-Elipe, P.; Manners, I. Chem. ReV. 1999,
99, 1515. (c) Long, N, J.; Williams, C. K. Angew. Chem., Int. Ed. 2003,
42, 2586. (d) Ceccon, A.; Santi, S.; Orian, L.; Bisello, A. Coord. Chem.
ReV. 2004, 248, 683. (e) Weder, C. Chem. Commun. 2005, 5378. (f) Wong,
W.-Y.; Ho, C.-L. Coord. Chem. ReV. 2006, 250, 2627. (g) Lang, H.;
Packheiser, R.; Walfort, B. Organometallics 2006, 25, 1836. (h) Williams,
K. A.; Boydston, A. J.; Bielawski, C. W. Chem. Soc. ReV. 2007, 36, 729.
(4) (a) Frenking, G.; Fröhlich, N. Chem. ReV. 2000, 100, 717. (b)
Massera, C.; Frenking, G. Organometallics 2003, 22, 2758. (c) Piacenza,
M.; Rakow, J.; Hyla-Kryspin, I.; Grimme, S. Eur. J. Inorg. Chem. 2006,
213. and references cited therein.
10.1021/om800163p CCC: $40.75
2008 American Chemical Society
Publication on Web 04/01/2008

Communications
Organometallics, Vol. 27, No. 9, 2008 1971
Figure 1. Absorption spectra of 1a–d in CHCl3.
reactions may be operative in the opposite order, the intermedi-
ate [Pt(cod)(DPCB-phen)] is poorly reactive and the use of
excess alkyne was required in the subsequent reaction. All
complexes were isolated as crystalline (1a–c) or powdery (1d)
solids and characterized by NMR spectroscopy and elemental
analysis. The 31P{1H} NMR signals appeared at δ 150.7 (1a,
1JPtP ) 3913 Hz), 155.7 (1b, 1JPtP ) 3455 Hz), 156.6 (1c, 1JPtP
) 3436 Hz), and 157.4 (1d, 1JPtP ) 3406 Hz), respectively; the
chemical shifts are higher than that of DPCB-phen (δ 173.8).
The 13C{1H} NMR signals of acetylenic carbons were observed
at δ 121.4–122.3; the chemical shifts are somewhat higher than
10
those of the dppe11 and PPh3 analogues of 1b (δ 135.6 and
Figure 2. Chem3D view of the X-ray structure of 1c. Selected bond
distances (Å) and angles (deg): C1-C2 ) 1.299(3), Pt-C1 )
2.038(2), Pt-C2 ) 2.053(2), Pt-P1 ) 2.2884(6), Pt-P2 )
2.3117(6); P1-Pt-P2 ) 83.29(2), C1-Pt-C2 ) 37.02(9),
C2-C1-C3 ) 151.7(2), C1-C2-C10 ) 147.0(2).
127.9, respectively) but are still in the normal range for alkyne
complexes with a d10 metal center.12
Platinum(0) alkyne complexes are usually almost colorless,
showing no significant absorption in the visible region. On the
other hand, the present alkyne complexes 1a–d bearing DPCB-
phen ligands exhibited vivid colors that vary remarkably with
alkyne ligands. Thus, the dimethyl acetylenedicarboxylate
(dmad) complex 1a is reddish orange, whereas the complexes
coordinated with nonsubstituted (tolan) and para-substituted
diphenylacetylenes (tolan-Y; Y ) OMe, NMe2) are dark red
(1b), mulberry (1c), and teal (1d), respectively.
Figure 1 shows the UV–vis spectra of 1a–d, where the
characteristic absorption bands of medium or strong intensity
are observed at 450–700 nm. In accordance with the marked
color variation of the complexes, the absorption maximum is
shifted significantly depending on the alkyne ligands: λmax (nm)
492 (1a, log ꢀ ) 3.82), 558 (1b, 4.18), 573 (1c, 4.14), 607 (1d,
4.15). Since no solvatochromic shift was observed in cyclo-
hexane, CHCl3, acetone, and DMF, these absorptions are
assignable to π-π* transitions, not to MLCT. Hence, the
occurrence of strong dπ-pπ interaction between the [Pt(DPCB-
phen)] moiety and the alkyne ligands causing notable extension
of π-conjugated systems is evidenced. The strong dependence
of the absorption maxima upon para substituents of dipheny-
lacetylene ligands in 1b–d is consistent with this consideration.
Figure 2 depicts the top and side views of the X-ray crystal
structure of 1c,13 which adopts a square-planar configuration
around platinum, taking the two phosphorus atoms of DPCB-
phen and the two acetylenic carbons of tolan-OMe as coordina-
tion atoms. The bond length between the acetylenic carbons
(C1-C2 ) 1.299(3) Å) is similar to those of [Pt(tolan)(dppbn)]
(1.301(4) Å)14 and [Pt(tolan)(PPh3)2] (1.280(5) Å).15 The Pt-C
bond distances (2.038(2), 2.053(2) Å) are also comparable to
those of dppbn (2.038(3), 2.041(3) Å) and PPh3 (2.047(3),
2.048(3) Å) complexes. On the other hand, the bent angles of
the phenyl groups against the C1-C2 axis (28.3(2), 33.1(2)°)
are somewhat smaller than those of dppbn (34.6(3), 33.6(3)°)
and PPh3 (37.7(3), 37.4(3)°) complexes.
The most striking feature of the structure of 1c is the
extremely planar arrangements of the diphenylacetylene, plati-
num, and 1,2-diphosphinidenecyclobuta[l]phenanthrene units,
while the 2,4,6-tri-tert-butylphenyl groups (Mes*) on the
phosphorus atoms are almost perpendicular to the coordination
(13) Crystallographic data for 1c: C68H80O2P2Pt, fw 1186.35, monoclinic,
space group P21/c (No. 14), a ) 14.684(3) Å, b ) 22.008(4) Å, c )
18.693(3) Å, ꢀ ) 107.720(2)°, V ) 5754.3(18), Å3, Z ) 4, dcalcd ) 1.369
g cm-3, µ(Mo KR) ) 2.538 mm-1, 2θ < 55.0°. Of the 45340 reflections
collected, 13140 were independent (Rint ) 0.0347) and used for the
refinement of 898 parameters: R1 ) 0.0319, wR2 ) 0.0610, GOF ) 1.085
for all data (R1 ) 0.0266 for I > 2σ(I)).
(14) (a) Petzold, H.; Görls, H.; Weigand, W. J. Organomet. Chem. 2007,
692, 2736. (b) dppbn ) 1,2-bis(diphenylphosphino)benzene.
(15) Harris, K. J.; Bernard, G. M.; McDonald, C.; McDonald, R.;
Ferguson, M. J.; Wasylishen, R. E. Inorg. Chem. 2006, 45, 2461.
(11) (a) Ros, R.; Michelin, R. A.; Bataillard, R.; Roulet, R. J. J.
Organomet. Chem. 1978, 161, 75. (b) The 13C NMR data were obtained in
this study.
(12) (a) Rosenthal, U.; Oehme, G.; Burlakov, V. V.; Petrovskii, P. V.;
Shur, V. B.; Vol’pin, M. E. J. Organomet. Chem. 1990, 391, 119. (b)
Rosenthal, U.; Nauck, C.; Arndt, P.; Pulst, S.; Baumann, W.; Burlakov,
V. V.; Giirls, H. J. Organomet. Chem. 1994, 484, 81. (c) Templeton, J. L.;
Ward, B. C. J. Am. Chem. Soc. 1980, 102, 3288.

1972 Organometallics, Vol. 27, No. 9, 2008
Communications
Table 1. Relationship between HOMO-LUMO Gaps (HLG) and
Ep,a Values
UV–visa
complex
λmax (nm)
λedge (nm)
HLG (eV)
CVb Ep,a (V)
1a
1b
1c
1d
492
558
573
607
553
603
623
682
2.24
2.06
1.99
1.82
0.755
0.337
0.289
-0.023
b
a In CHCl3 at room temperature. Ep,a values vs Fc/Fc+, in CH2Cl2
(0.1 M Bu4NBF4, 100 mV s-1) at room temperature.
the HOMO levels should be dependent predominantly on the
electronic properties of the alkyne ligands. Therefore, we next
examined oxidation potentials of 1a–d by cyclic voltammetry.
The cyclic voltammograms measured in CH2Cl2 showed
quasi-reversible (1a,b) or reversible (1c,d) one-electron oxida-
tion waves. As given in Table 1, the oxidation potentials ([1]/
[1]+) were shifted clearly to the negative side as the π-electron
density of alkyne ligands increased. The Ep,a values showed a
good linear correlation (r2 ) 0.982) with the HOMO-LUMO
gaps (HLG) estimated from the absorption edge wavelengths
(λedge): HLG ) 0.54(5) × Ep,a + 1.84(2).
As described in the introductory paragraph, the interaction
of the dπ orbital of platinum with the out-of-plane π orbital
of the alkyne ligand is generally negligible.4 Actually, [Pt-
(tolan)L2] complexes bearing tertiary phosphine ligands as L
show no notable absorption in the visible region. In this context,
the strong π-π* transition observed for 1a–d is remarkable.
As seen from Figure 3, the π orbital of tolan-OMe and the dπ
orbital of platinum interact with each other in an antibonding
manner. Furthermore, the 13C{1H} NMR chemical shifts of the
acetylenic carbons are in a typical range of alkyne ligands that
serve as two-electron donors.12 Nevertheless, the π orbitals are
delocalized widely over the molecule, as typically seen in the
HOMO and LUMO of 1c, showing the establishment of efficient
interaction between the two pπ orbital systems (i.e., DPCB-
phen and alkynes) through the platinum dπ orbital. It is
convincing that the combination of the electron-poor pπ system
of DPCB-phen and the electron-rich pπ system of alkyne
facilitates this orbital interaction, providing the extended
π-conjugated systems with small HOMO-LUMO gaps.
Figure 3. HOMO and LUMO of the model compound of 1c.
plane. The torsion angle between C1-C2 (tolan-OMe) and
C26-C32 (DPCB-phen) is 5.3(2)°, and the dihedral angles
between the PtC1C2 plane A and benzene rings B and C of
tolan-OMe are 7.6(1) and 5.3(1)°, respectively. Although there
is a possibility that steric repulsion between the benzene rings
(B and C) and the t-Bu groups of the Mes* substituents causes
the highly planar orientation of the benzene rings, DFT
calculations for the model compound without the t-Bu groups
also suggested the stability of the planar geometry.16
Figure 3 illustrates the HOMO and LUMO of the model
compound. While both the frontier orbitals, which correspond
to π and π* orbitals, respectively, are delocalized on the whole
molecular plane, the HOMO is distributed over the [Pt(tolan-
OMe)] moiety to a considerable extent, whereas the LUMO is
located mainly on the [Pt(DPCB-phen)] moiety. In this situation,
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research on Priority Areas (No.
18064010, “Synergy of Elements”) from the Ministry of
Education, Culture, Sports, Science, and Technology of
Japan.
(16) (a) The primary structure for geometry optimization was based on
the X-ray structure of 1c, where the t-Bu groups of the Mes* substituents
were omitted and the dihedral angles of the phenyl groups with the P1-C17
and P2-C18 axes (see Figure 3) were fixed to the X-ray values. Selected
bond distances (Å) and angles (deg) for the optimized structure: C1-C2 )
1.30, Pt-C1 ) Pt-C2 ) 2.05, Pt-P1 ) Pt-P2 ) 2.38; P1-Pt-P2 )
81.6, C1-Pt-C2 ) 37.1, C2-C1-C3 ) C1-C2-C10 ) 147.8. (b) DFT
calculations were carried out with the Gaussian 98 program (Revision A.9,
Gaussian, Inc., 1998) using B3LYP in conjunction with the standard
LanL2DZ basis set and effective core potentials for Pt and 6-31G(d) basis
set for other atoms.
Supporting Information Available: Text giving experimental
procedures and analytical data and a CIF file giving crystallographic
data for 1c. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM800163P