Luminescent heterometallic branched alkynyl complexes of rhenium(I)-palladium(II): Potential building blocks for heterometallic metallodendrimers

Article abstract of DOI:10.1021/om049696l

The photophysical and electrochemical behavior of a novel series of luminescent rhenium(I) complexes bearing a trialkynyl ligand, [1,3-(HC≡C)₂-5-{(N∧N)(CO)₃ReC≡C}C ₆H₃] (N∧N = ^tBu₂bpy 1, Me ₂bpy 2, bpy 3), and their heterometallic branched complexes, [1,3-{Cl(PEt₃)₂PdC≡C}₂-5-{(Nand;N)(CO) ₃ReC≡C}C₆H₃] (N∧N = Me₂bpy 4, bpy 5), have been studied; the X-ray crystal structures of 2 and 5 have been determined. Density functional theory calculations have also been carried out in order to probe the bonding in these compounds.

Full text of DOI:10.1021/om049696l

4924
Organometallics 2004, 23, 4924-4933
Lu m in escen t Heter om eta llic Br a n ch ed Alk yn yl
Com p lexes of Rh en iu m (I)-P a lla d iu m (II): P oten tia l
Bu ild in g Block s for Heter om eta llic Meta llod en d r im er s
Samuel Hung-Fai Chong, Sally Chan-Fung Lam, Vivian Wing-Wah Yam,*
Nianyong Zhu, and Kung-Kai Cheung
Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based Materials Research, and the
Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, People’s Republic of China
Sofiane Fathallah, Karine Costuas, and J ean-Franc¸ois Halet*
Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR 6511 CNRS, Universite´ de
Rennes 1, Institut de Chimie de Rennes, F-35042 Rennes, France
Received April 29, 2004
The photophysical and electrochemical behavior of a novel series of luminescent rhe-
nium(I) complexes bearing a trialkynyl ligand, [1,3-(HCtC)2-5-{(N∧N)(CO)3ReCtC}C6H3]
t
(N∧N ) Bu2bpy 1, Me2bpy 2, bpy 3), and their heterometallic branched complexes,
[1,3-{Cl(PEt3)2PdCtC}2-5-{(N∧N)(CO)3ReCtC}C6H3] (N∧N ) Me2bpy 4, bpy 5), have been
studied; the X-ray crystal structures of 2 and 5 have been determined. Density functional
theory calculations have also been carried out in order to probe the bonding in these
compounds.
In tr od u ction
number of examples featuring this core unit have
recently been reported. Stang and co-workers reported
the synthesis of [1,3,5-{trans-I(Et3P)2Pt(CtC)}3C6H3]
and [1,2,4,5-{trans-I(Et3P)2Pt(CtC)}4C6H2], which serve
as versatile precursors to platinum-containing dendri-
mers.9 Takahashi and co-workers reported the synthesis
of a series of platinum-alkynyl dendrimers using this
triethynylbenzene core as bridging ligand, and den-
drimers containing up to 189 platinum atoms have also
been synthesized.10 Long and co-workers reported on a
series of complexes featuring trans-[Ru(dppm)2Cl], trans-
[Os(dppm)2Cl], or [Ru(η-C5H5)(PPh3)2] and ferrocenyl
[(C5H5FeC5H4)] units unsymmetrically arranged around
the periphery of a 1,3,5-triethynylbenzene core, the
spectroscopic characterizations and electrochemical
properties of which have also been studied in detail.11
Although an increasing number of works that involve
dendrimers with this trifunctional alkynyl core molecule
There is a constant search for organometallic den-
drimers with unique and interesting properties that are
inaccessible by their organic counterparts.1 Multidi-
mensional alkynyl ligands, such as 1,3,5-triethynylben-
zene, are of particular interest for the construction of
organometallic dendrimers due to their geometry and
active coordination sites, which enable the extension of
the core in three directions. Such three-dimensional
hyperbranched polymers or dendrimers usually have a
lower intrinsic viscosity and better solubility than the
corresponding linear polymers with similar molecular
weights. Currently, dendritic molecules have attracted
much attention of numerous researchers because of
their interesting electrochemical,2 catalytic,3 and energy
transfer properties.4 Indeed, the organometallic den-
drimers are useful in building carbon-rich networks,5
supramolecular polymetallic assemblies,6 nanoarchitec-
tures for materials science,7 and fabricating nanoscale
(6) (a) Venkataraman, D.; Lee, S.; Moore, J . S.; Zhang, P.; Hirsch,
K. A.; Gardner, G. B. A.; Covey, C.; Prentice, C. L. Chem. Mater. 1996,
8, 2030. (b) Irwin, M. J .; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt,
R. J .; Yufit, D. S. Chem. Commun. 1997, 219.
optical and electronic devices for nanotechnology.4a,8
A
* To whom correspondence should be addressed. E-mail: wwyam@
hku.hk (V.W.-W.Y); halet@univ-rennes1.fr (J .-F.H).
(1) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Mol-
ecules: Concepts, Synthesis, Perspectives, VCH: Weinheim, Germany,
1996. (b) Cuadrado, I.; Mora´n, M.; Casado, C. M.; Alonso, B.; Losada,
J . Coord. Chem. Rev. 1999, 193-195, 395.
(7) (a) Schumm, J . S.; Pearson, D. L.; Tour, J . M. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1360. (b) Campagna, S.; Denti, G.; Serroni, S.;
J uris, A.; Venturi, M.; Riceunto, V.; Balzani, V. Chem. Eur. J . 1995,
1, 211.
(8) (a) Xu, Z.; Kahr, M.; Walker, K. L.; Wilkins, C. L.; Moore, J . S.
J . Am. Chem. Soc. 1994, 116, 4537. (b) Xu, Z.; Moore, J . S. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1354. (c) Devadoss, C.; Bharathi, P.;
Moore, J . S. Macromolecules 1998, 31, 8091.
(2) Long, N. J .; Angela, A. J .; de Biani, F. F.; Zanello, P. J . Chem.
Soc., Dalton Trans. 1998, 2017.
(3) Ko¨llner, C.; Pugin, B.; Togni, A. J . Am. Chem. Soc. 1998, 120,
10274.
(4) (a) Revadoss, C.; Bharathi, P.; Moore, J . S. J . Am. Chem. Soc.
1996, 118, 9635. (b) Hu, Q. S.; Pugh, V.; Sabat, M.; Pu, L. J . Org. Chem.
1999, 64, 7528.
(5) (a) Diederich, F. Nature 1994, 369, 149. (b) Diederich, F. Modern
Acetylene Chemistry; VCH: Weinheim, 1995; p 443. (c) Faust, R.;
Diederich, F.; Gramlich, V.; Seiler, P. Chem. Eur. J . 1995, 1, 111.
(9) Leininger, S.; Stang, P. J .; Huang, S. Organometallics 1998, 17,
3981.
(10) (a) Ohshiro, N.; Takei, F.; Onitsuka, K.; Takahashi, S. J .
Organomet. Chem. 1998, 569, 195. (b) Onitsuka, K.; Fujimoto, M.;
Ohshiro, N.; Takahashi, S. Angew. Chem., Int. Ed. 1999, 38, 689. (c)
Onitsuka, K.; Shimizu, A.; Takahashi, S. Chem. Commun. 2003, 280.
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10.1021/om049696l CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/14/2004

Branched Alkynyl Complexes of Re(I)-Pd(II)
Organometallics, Vol. 23, No. 21, 2004 4925
Sch em e 1. Syn th etic Sch em e of th e Heter on u clea r Rh en iu m (I) Alk yn yl Com p lexesa
a
Reaction conditions: (i) 1,3,5-(HCtC)3C6H3, AgOTf, Et3N, THF; (ii) trans-[Pd(PEt3)2Cl2], CuCl, Et3N, THF.
Syn th eses of Re(I) Alk yn yl P r ecu r sor s a n d Re(I)-P d -
(II) Mixed -Meta l Alk yn yl Com p lexes. All reactions were
performed under anaerobic and anhydrous conditions using
standard Schlenk techniques under an inert atmosphere of
nitrogen.
are reported in the literature, hyperbranched molecules
containing the tricarbonyl rhenium(I) diimine lumino-
phore are still not known. Heterometallic dendrimeric
molecules are also remarkably rare.11 As an extension
of our previous efforts on the study of luminescent
branched Pd(II) and Pt(II) alkynyl complexes,12 we have
launched a program to study heterometallic branched
alkynyl complexes based on the tricarbonyl rhenium(I)
diimine core. Herein are reported the synthesis, struc-
tural characterization, and luminescence behavior of a
series of Re(I) alkynyl precursors, [1,3-(HCtC)2-5-
[1,3-(HCtC)2-5-{(tBu 2bpy)(CO)3ReCtC}C6H3] (1). A mix-
ture of [Re(CO)3(tBu2bpy)Cl] (100 mg, 0.17 mmol), Et3N (52
mg, 0.51 mmol), AgOTf (48 mg, 0.19 mmol), and 1,3,5-
(HCtC)3C6H3 (26 mg, 0.17 mmol) in THF (70 mL) was allowed
to reflux under an inert atmosphere of nitrogen in the dark
for 24 h. After cooling to room temperature, the dark brown
suspension was filtered and the orange filtrate was reduced
in volume under reduced pressure. The residue was then
purified by column chromatography on silica gel using dichlo-
romethane-petroleum ether (bp 40-60 °C) (4:1 v/v) as the
eluent. After removal of the first band, which contains the
unreacted 1,3,5-triethynylbenzene, the second band was col-
lected, which gave the desired product. Subsequent recrystal-
lization from vapor diffusion of diethyl ether into a dichlo-
romethane solution of 1 gave orange crystals. Yield: 41 mg,
t
{(N∧N)(CO)3ReCtC}C6H3] (N∧N ) Bu2bpy 1, Me2bpy
2, bpy 3), and their trinuclear branched Re(I)-Pd(II)
mixed-metal alkynyl complexes, [1,3-{Cl(PEt3)2PdCtC}2-
5-{(N∧N)(CO)3ReCtC}C6H3] (N∧N ) Me2bpy 4, bpy 5)
(Scheme 1). DFT calculations have also been carried out
on selected metal complexes in order to probe the
bonding in this kind of compound.
1
35%. H NMR (300 MHz, acetone-d6, 298 K, relative to Me4-
t
Si): δ 1.40 (s, 18H, Bu), 3.40 (s, 2H, CtCH), 6.90 (d, 2H, J )
Exp er im en ta l Section
1.5 Hz, 4- and 6-H on C6H3), 7.10 (t, 1H, J ) 1.5 Hz, 2-H on
C6H3), 7.80 (dd, 2H, J ) 2.0 and 5.9 Hz, 5- and 5′-bipyridyl
H’s), 8.70 (d, 2H, J ) 2.0 Hz, 3- and 3′-bipyridyl H’s), 9.00 (d,
2H, J ) 5.9 Hz, 6- and 6′-bipyridyl H’s). Positive FAB-MS: ion
clusters at m/z 690 {M}+, 662 {M - CO}+, 539 {M - [1,3-
(HCtC)2-5-{CtC}C6H3]}+. IR (Nujol mull on KBr disk, ν/cm-1):
2120 (w), 2107 (w), 2088 (m) ν(CtC); 2004 (s), 1914 (s), 1888
(s) ν(CtO). Anal. Found: C 57.93, H 4.55, N 3.86. Calcd for
[1,3-(HCtC)2-5-{(tBu2bpy)(CO)3ReCtC}C6H3]‚1/2THF: C 58.20,
H 4.46, N 3.90.
Ma ter ia ls a n d Rea gen ts. [Re(CO)5Cl] was obtained from
Strem Chemicals, Inc. 2,2′-Bipyridine (bpy) and 4,4′-dimethyl-
2,2′-bipyridine (Me2bpy) were obtained from Aldrich Chemical
Co. 4,4′-Di-tert-butyl-2,2′-bipyridine (tBu2bpy) was prepared by
a slight modification of the reported procedure.13 Triethylamine
was purchased from Lancaster Synthesis Ltd. Silver trifluo-
romethanesulfonate and copper(I) chloride were purchased
from Aldrich Chemical Co. (1,3,5-(HCtC)3C6H3),10d [Re(CO)3-
(tBu2bpy)Cl],14 [Re(CO)3(Me2bpy)Cl],15 [Re(CO)3(bpy)Cl],15 and
trans-[Pd(PEt3)2Cl2]16 were synthesized according to the lit-
erature procedures. All solvents were purified and distilled
using standard procedures before use.17 All other reagents
were of analytical grade and were used as received.
[1,3-(HCtC)2-5-{(Me2bp y)(CO)3ReCtC}C6H3] (2). The
procedure was similar to that described for the preparation of
1, except [Re(CO)3(Me2bpy)Cl] (83 mg, 0.17 mmol) was used
in place of [Re(CO)3(tBu2bpy)Cl] to give orange crystals of 2.
Yield: 41 mg, 30%. 1H NMR (300 MHz, acetone-d6, 298 K,
relative to Me4Si): δ 2.50 (s, 6H, -Me), 3.40 (s, 2H, CtCH),
6.90 (d, 2H, J ) 1.5 Hz, 4- and 6-H on C6H3), 7.10 (t, 1H, J )
1.5 Hz, 2-H on C6H3), 7.60 (dd, 2H, J ) 1.9 and 5.9 Hz, 5- and
5′-bipyridyl H’s), 8.60 (d, 2H, J ) 1.9 Hz, 3- and 3′-bipyridyl
H’s), 8.80 (d, 2H, J ) 5.9 Hz, 6- and 6′-bipyridyl H’s). Positive
FAB-MS: ion clusters at m/z 605 {M}+, 576 {M - CO}+, 455
{M - [1,3-(HCtC)2-5-{CtC}C6H3]}+. IR (Nujol mull on KBr
disk, ν/cm-1): 2120 (w), 2107(w), 2087 (m) ν(CtC); 2004 (s),
1914 (s), 1882 (s) ν(CtO). Anal. Found: C 52.17, H 3.06, N
4.51. Calcd for [1,3-(HCtC)2-5-{(Me2bpy)(CO)3ReCtC}C6H3]:
C 52.13, H 2.91, N 4.29.
(11) Long, N. J .; Martin, A. J .; White, A. J . P.; Williams, D. J .;
Fontani, M.; Laschi, F.; Zanello, P. J . Chem. Soc., Dalton Trans. 2000,
3387.
(12) (a) Yam, V. W. W.; Zhang, L.; Tao, C. H.; Wong, K. M. C.;
Cheung, K. K. J . Chem. Soc., Dalton Trans. 2001, 1111. (b) Yam, V.
W. W.; Tao, C. H.; Zhang, L.; Wong, K. M. C.; Cheung, K. K.
Organometallics 2001, 20, 453.
(13) Hadda, T. B.; Bozec, H. L. Polyhedron 1988, 7, 75.
(14) Yam, V. W. W.; Lau, V. C. Y.; Cheung, K. K. Organometallics
1995, 14, 2749.
(15) (a) Caspar, J . V.; Sullivan, B. P.; Meyer, T. J . Inorg. Chem. 1984,
23, 2104. (b) Sullivan, B. P.; Meyer, T. J . Chem. Soc., Chem. Commum.
1984, 403.
(16) Okano, T.; Iwahara, M.; Konish, H.; Kiji, J . J . Organomet.
Chem. 1988, 346, 267.
(17) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: Oxford, U.K., 1988.
[1,3-(HCtC)2-5-{(bp y)(CO)3ReCtC}C6H3] (3). The pro-
cedure was similar to that described for the preparation of 1,
except [Re(CO)3(bpy)Cl] (79 mg, 0.17 mmol) was used in place

4926 Organometallics, Vol. 23, No. 21, 2004
Chong et al.
1
of [Re(CO)3(tBu2bpy)Cl] to give orange crystals of 3. Yield: 39
mg, 32%. 1H NMR (300 MHz, acetone-d6, 298 K, relative to
Me4Si): δ 3.40 (s, 2H, CtCH), 6.80 (d, 2H, J ) 1.5 Hz, 4- and
6-H on C6H3), 7.10 (t, 1H, J ) 1.5 Hz, 2-H on C6H3), 7.80 (t,
2H, J ) 6.1 Hz, 4- and 4′-bipyridyl H’s), 8.30 (t, 2H, J ) 7.9
Hz, 5- and 5′-bipyridyl H’s), 8.70 (d, 2H, J ) 8.2 Hz, 3- and
3′-bipyridyl H’s), 9.20 (d, 2H, J ) 5.3 Hz, 6- and 6′-bipyridyl
H’s). Positive FAB-MS: ion clusters at m/z 578 {M}+, 550
{M - CO}+, 427 {M - [1,3-(HCtC)2-5-{CtC}C6H3]}+. IR
(Nujol mull on KBr disk, ν/cm-1): 2120 (w), 2108 (w), 2077
(m) ν(CtC); 2011 (s), 1914 (s), 1883 (s) ν(CtO). Anal. Found:
C 51.69, H 1.91, N 4.53. Calcd for [1,3-(HCtC)2-5-{(bpy)-
(CO)3ReCtC}C6H3]‚1/10CH2Cl2: C 51.61, H 2.27, N 4.79.
[1,3-{Cl(P E t 3)2P d CtC}2-5-{(Me2b p y)(CO)3R eCtC}C6-
H3] (4). To a suspension of trans-[Pd(PEt3)2Cl2] (211 mg, 0.51
mmol), CuCl (0.50 mg, 0.0051 mmol), and Et3N (52 mg, 0.51
mmol) in THF (20 mL) was added dropwise a solution of 2
(103 mg, 0.17 mmol) in THF (20 mL). The reaction mixture
was allowed to stir at room temperature in an inert atmo-
sphere of nitrogen for 2 h. The orange suspension was filtered,
and the orange filtrate was reduced in volume under reduced
pressure. The residue was then purified by column chroma-
tography on silica gel using dichloromethane as eluent. After
removal of the first band, which contains unreacted 2, ethyl
acetate was used to elute out the second band, which contains
4 as the desired product. Subsequent recrystallization from
vapor diffusion of diethyl ether into a dichloromethane solution
of 4 gave orange crystals. Yield: 62 mg, 27%. 1H NMR (300
MHz, acetone-d6, 298 K, relative to Me4Si): δ 1.10 (m, 36H,
-CH3 on ethyl groups), 1.80 (m, 24H, -CH2 on ethyl groups),
2.50 (s, 6H, methyl H’s on Me2bpy), 6.50 (d, 2H, J ) 1.4 Hz, 4-
and 6-H on C6H3), 6.70 (t, 1H, J ) 1.4 Hz, 2-H on C6H3), 7.50
(dd, 2H, J ) 1.9 and 5.7 Hz, 5- and 5′-bipyridyl H’s), 8.50 (d,
2H, J ) 1.9 Hz, 3- and 3′-bipyridyl H’s), 8.80 (d, 2H, J ) 5.7
Hz, 6- and 6′-bipyridyl H’s). Positive FAB-MS: ion clusters at
m/z 1360 {M}+, 1324 {M - CO}+, 1098 {M - Pd(PEt3)2Cl}+,
455 {M - [1,3-{Cl(PEt3)2PdCtC}2-5-{CtC}C6H3]}+. IR (Nujol
mull on KBr disk, ν/cm-1): 2102 (m, br) ν(CtC); 2005 (s), 1910
(s), 1895 (s) ν(CtO). Anal. Found: C 46.19, H 5.81, N 1.96.
Calcd for [1,3-{Cl(PEt3)2PdCtC}2-5-{(Me2bpy)(CO)3ReCtC}-
C6H3]‚C4H8O: C 46.02, H 5.81, N 1.98.
[1,3-{Cl(P Et3)2P d CtC}2-5-{(bp y)(CO)3ReCtC}C6H3] (5).
The procedure was similar to that described for the prepara-
tion of 4, except [1,3-(HCtC)2-5-{(bpy)(CO)3ReCtC}C6H3] (98
mg, 0.17 mmol) was used in place of [1,3-(HCtC)2-5-{(Me2-
bpy)(CO)3ReCtC}C6H3]. Subsequent recrystallization from
dichloromethane-methanol afforded 5 as an orange solid.
Single crystals of 5 were obtained by recrystallization from
dichloromethane-n-hexane. Yield: 66 mg, 29%. 1H NMR (300
MHz, acetone-d6, 298 K, relative to Me4Si): δ 1.10 (m, 36H,
-CH3 on ethyl groups), 1.80 (m, 24H, -CH2 on ethyl groups),
6.50 (d, 2H, J ) 1.4 Hz, 4- and 6-H on C6H3), 6.70 (t, 1H, J )
1.4 Hz, 2-H on C6H3), 7.70 (t, 2H, J ) 5.6 Hz, 4- and
4′-bipyridyl H’s), 8.20 (t, 2H, J ) 7.9 Hz, 5- and 5′-bipyridyl
H’s), 8.60 (d, 2H, J ) 8.2 Hz, 3- and 3′-bipyridyl H’s), 9.10 (d,
2H, J ) 4.7 Hz, 6- and 6′-bipyridyl H’s). Positive FAB-MS: ion
clusters at m/z 1331 {M}+, 1247 {M - 3CO}+, 427 {M - [1,3-
{Cl(PEt3)2PdCtC}2-5-{CtC}C6H3]}+. IR (Nujol mull on KBr
disk, ν/cm-1): 2120 (m), 2104 (m, br) ν(CtC); 2005 (s), 1912
(s), 1891 (s) ν(CtO). Anal. Found: C 44.84, H 5.49, N 2.05.
Calcd for [1,3-{Cl(PEt3)2PdCtC}2-5-{(bpy)(CO)3ReCtC}C6H3]:
C 44.60, H 5.55, N 1.92.
recorded by using an optical Dewar sample holder. H NMR
spectra were recorded on a Bruker DPX-300 (300 MHz) Fourier
transform NMR spectrometer with chemical shifts recorded
relative to Me4Si. Positive-ion FAB mass spectra were recorded
on a Finnigan MAT95 mass spectrometer. Elemental analyses
for the metal complexes were performed on the Carlo Erba
1106 elemental analyzer at the Institute of Chemistry, Chinese
Academy of Sciences.
Emission lifetime measurements were performed using a
conventional laser system. The excitation source was the 355
nm output (third harmonic) of a Spectra-Physics Quanta-Ray
Q-switched GCR-150 pulsed Nd:YAG laser (10 Hz). Lumines-
cence decay signals were recorded on a Tektronix Model TDS-
620A (500 MHz, 2GS/s) digital oscilloscope and analyzed using
a program for exponential fits. All solutions for photophysical
studies were degassed on a high-vacuum line in a two-
compartment cell consisting of a 10 mL Pyrex bulb and a 1-cm
path length quartz cuvette and sealed from the atmosphere
by a Bibby Rotaflo HP6 Teflon stopper. The solutions were
subject to at least four freeze-pump-thaw cycles.
Cyclic voltammetric measurements were performed by using
CH Instruments, Inc. Model CHI 620 electrochemical
a
analyzer interfaced to a personal computer. The electrolytic
cell used was a conventional two-compartment cell. The salt
bridge of the reference electrode was separated from the
working electrode compartment by a Vycor glass. A Ag/AgNO3
(0.1 M in CH3CN) reference electrode was used. The ferroce-
nium-ferrocene couple was used as the internal standard in
the electrochemical measurements in acetonitrile (0.1 M nBu4-
NPF6).18 The working electrode was a glassy carbon (Atomergic
Chemetals V25) electrode with a platinum foil acting as the
counter electrode.
DF T Ca lcu la tion s. Calculations were carried out at the
BP86 level of density functional theory (DFT) as implemented
in the Gaussian-98 suite of programs.19 The complexes [1,3-
(HCtC)2-5-{(bpy)(CO)3ReCtC}C6H3] (3) and the hydrogen-
substituted model complex [1,3-{Cl(PH3)2PdCtC}2-5-{(bpy)-
(CO)3ReCtC}C6H3] (5-H), used to mimic compounds 1-3 and
4-5, respectively, were fully optimized with the LANL2DZ
basis set and an additional set of polarization functions as
introduced by Hay and Wadt, without any symmetry con-
straint.20 In all cases the structural arrangements obtained
were all very close to Cs symmetry. Therefore, Cs symmetry
was considered.
Cr ysta l Str u ctu r e Deter m in a tion . Crystals of 2 were
obtained by recrystallization from vapor diffusion of diethyl
ether into a dichloromethane solution of 2. [2(C27H17N2O3Re)‚
C5H12]; Mr ) 1279.45, triclinic, space group P1h (No. 2), a )
13.025(3) Å, b ) 14.515(5) Å, c ) 16.632(5) Å, R ) 113.00(3)°,
â ) 101.75(3)°, γ ) 106.82(3)°, V ) 2585(2) Å3, Z ) 2, Dc )
1.643 g cm-3, µ(Mo KR) ) 48.98 cm-1, F(000) ) 1252. An
orange crystal of dimensions 0.30 × 0.10 × 0.07 mm mounted
on a glass fiber was used for data collection at 28 °C on a
(18) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2854.
(19) Frisch, M. J . T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A., J r.;
Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J .; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.; Wong, M. W.;
Andres, J . L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople,
J . A. Gaussian98, revision A.5; Gaussian, Inc.: Pittsburgh, PA, 1998.
(20) (a) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 299. (b)
Dunning, T. H., J r.; Hay, P. J . In Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.; Plenum: New York, 1976. (c) Huzinaga, S.
A., J .; Klobukowski, M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H.
Gaussian Basis Sets for Molecular Calculations; Elsevier: Amsterdam,
1984.
P h ysica l Mea su r em en ts a n d In str u m en ta tion . UV-
visible spectra were obtained on a Hewlett-Packard 8452A
diode array spectrophotometer, IR spectra as Nujol mulls on
a Bio-Rad FTS-7 Fourier transform infrared spectrophotometer
(4000-400 cm-1), and steady-state excitation and emission
spectra on a Spex Fluorolog-2 Model F 111 fluorescence
spectrofluorometer equipped with a Hamamatsu R-928 pho-
tomultiplier tube. Low-temperature (77 K) spectra were

Branched Alkynyl Complexes of Re(I)-Pd(II)
Organometallics, Vol. 23, No. 21, 2004 4927
Ta ble 1. Cr ysta l a n d Str u ctu r e Deter m in a tion Da ta for Com p lexes 2 a n d 5
2
5
molecular formula
fw
crystal system
space group
a, Å
b, Å
c, Å
[2(C27H17N2O3Re)‚C5H12
1279.45
triclinic
P1h (No. 2)
13.025(3)
14.515(5)
16.632(5)
113.00(3)
101.75(3)
106.82(3)
2585(2)
]
[ReC50H77N2O5P4Cl2Pd2]
1379.92
monoclinic
C2/c (No. 15)
48.412(9)
8.390(2)
31.512(6)
90
102.52(3)
90
12495(4)
8
R, deg
â, deg
γ, deg
V, Å3
Z
2
µ, cm-1
48.98
301
27.30
293
T, K
diffractometer
cryst dimens, mm
λ/Å (graphite monochromated, Mo KR)
R
Rigaku AFC7R
0.30 × 0.10 × 0.07
0.71073
0.034a
MAR
0.40 × 0.10 × 0.04
0.7103
0.0516b
0.1452b
25006
8771
0.0383
8771
wR
0.050a
no. of data collected
no. of unique data
Rint
no. of data used in refinement, m
no. of parameters refined, p
9507
9117
0.020
6962
615
477
w ) 4Fo2/σ2(Fo)2, where σ2(Fo)2 ) [σ2(I) + (0.40Fo2)2] with I > 3σ(I). w ) 1/[σ2(Fo2) + (aP)2 + bP], where P ) [2Fc + Max(F02,0)]/3
a
b
2
with I > 2σ(I).
Rigaku AFC7R diffractometer with graphite-monochromatized
Mo KR radiation (λ ) 0.71073 Å) using ω-2θ scans with
ω-scan angle (0.73 + 0.35 tan θ)° at a scan speed of 8.0 deg
min-1 (up to 6 scans for reflections with I < 15σ(I)). Unit-cell
dimensions were determined on the basis of the setting angles
of 25 reflections in the 2θ range of 41.7-43.5°. Intensity data
(in the range of 2θmax ) 50°; h: -15 to 14; k: 0 to 17; l: -19
to 18 and three standard reflections measured after every 300
reflections showed decay of 7.86%) were corrected for decay
and for Lorentz and polarization effects, and empirical absorp-
tion corrections based on the ψ-scan of five strong reflections
(minimum and maximum transmission factors 0.252 and
1.000). A total of 9507 reflections were measured, of which
9117 were unique and Rint ) 0.020. 6962 reflections with I >
3σ(I) were considered observed and used in the structural
analysis. The space group was determined on the basis of a
statistical analysis of intensity distribution, and the successful
refinement of the structure was solved by Patterson methods
and expanded by Fourier methods (PATTY)21 and refined by
full-matrix least-squares using the software package TeXsan22
on a Silicon Graphics Indy computer. One crystallographic
asymmetric unit consists of two independent complex mol-
ecules and one n-pentane molecule. In the least-squares
refinement, all 66 non-H atoms of the complex molecules were
refined anisotropically, the five C atoms of the solvent molecule
were refined isotropically, and 46 H atoms at calculated
positions with thermal parameters equal to 1.3 times that of
the attached C atoms were not refined. Convergence for 615
variable parameters by least-squares refinement F with w )
4Fo2/σ2(Fo2), where σ2(Fo2) ) [σ2(I) + (0.040 Fo2)2] for 6962
reflections with I > 3σ(I), was reached at R ) 0.034 and wR )
0.050 with a goodness-of-fit of 1.74. (∆/ σ)max ) 0.04 except for
the C atoms of the solvent molecule. The final difference
Fourier map was featureless, with maximum positive and
negative peaks of 1.38 and 0.74 e Å-3, respectively. Crystal
and structure determination data for 2 are summarized in
Table 1. The final agreement factors for 2 are given in Table
1. Selected bond distances and angles are summarized in Table
Ta ble 2. Selected Bon d Dista n ces (Å) a n d Bon d
An gles (d eg) for 2 a n d 5, w ith Estim a ted Sta n d a r d
Devia tion s (esd ’s) Given in P a r en th eses
2
Re(1)-N(1)
Re(1)-C(1)
Re(1)-C(3)
C(4)-C(5)
C(14)-C(15)
C(2)-O(2)
2.194(5)
1.954(8)
1.921(8)
1.199(9)
1.17(1)
Re(1)-N(2)
Re(1)-C(2)
Re(1)-C(4)
C(12)-C(13)
C(1)-O(1)
C(3)-O(3)
2.193(5)
1.904(8)
2.161(7)
1.16(1)
1.133(9)
1.134(9)
1.154(8)
N(1)-Re(1)-N(2)
N(2)-Re(1)-C(3)
C(4)-C(5)-C(6)
74.1(2) N(1)-Re(1)-C(2)
174.2(2) C(1)-Re(1)-C(4)
172.2(2)
174.6(3)
176.9(7) C(8)-C(12)-C(13) 178(1)
C(10)-C(14)-C(15) 177(1)
Re(1)-C(4)-C(5)
177.3(6)
5
Re(1)-N(1)
Re(1)-C(11)
Re(1)-C(13)
C(14)-C(15)
C(24)-C(25)
C(12)-O(2)
Pd(1)-C(23)
2.138(6)
Re(1)-N(2)
Re(1)-C(12)
Re(1)-C(14)
C(22)-C(23)
C(11)-O(1)
C(13)-O(3)
Pd(2)-C(25)
2.171(8)
1.934(12)
1.895(13)
1.200(11)
1.181(14)
1.163(13)
1.944(10)
1.887(12)
2.143(10)
1.207(12)
1.159(13)
1.152(12)
1.924(11)
N(1)-Re(1)-N(2)
74.7(3)
C(13)-Re(1)-N(1) 172.5(4)
C(11)-Re(1)-C(14) 174.2(5)
C(12)-Re(1)-N(2) 171.6(5)
C(14)-C(15)-C(16) 175.8(10) C(23)-C(22)-C(18) 176.4(10)
C(25)-C(24)-C(20) 177.0(11) C(15)-C(14)-Re(1) 170.7(8)
P(1)-Pd(1)-Cl(1)
P(3)-Pd(2)-Cl(2)
P(3′)-Pd(2)-Cl(2)
89.28(11) P(2)-Pd(1)-Cl(1)
88.8(2) P(4)-Pd(2)-Cl(2)
91.89(19) P(4′)-Pd(2)-Cl(2)
94.71(11)
90.9(2)
91.83(19)
2. The atomic coordinates and thermal parameters are given
as Supporting Information.
Crystals of 5 were obtained by recrystallization from dichlo-
romethane-n-hexane. [C50H77Cl2N2O5P4Pd2Re]: Mr ) 1379.92,
monoclinic, space group C2/c (No. 15), a ) 48.412(9) Å, b )
8.390(2) Å, c ) 31.512(6) Å, â ) 102.52(3)°, V ) 12495(4) Å3,
Z ) 8, Dc ) 1.467 g cm-3, µ(Mo KR) ) 2.730 mm-1, F(000) )
5536. An orange crystal of dimensions 0.40 × 0.10 × 0.04 mm
mounted on a glass fiber was used for data collection at 20 °C
on a MAR diffractometer with a 300 nm image plate detector
using graphite-monochromatized Mo KR radiation (λ ) 0.71073
Å). Data collection was made with 2° oscillation step of æ, 600
s exposure time, and scanner distance at 120 mm. Ninety
images were collected. The images were interpreted and
(21) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J . M. M.; Smykalla, C. The
DIRDIF program system; Technical Report of the Crystallography
Laboratory; University of Nijmegen: The Netherlands, 1992.
(22) TeXsan, Crystal Structure Analysis Package; Molecular Struc-
ture Corporation: The Woodlands, TX, 1985 & 1992.

4928 Organometallics, Vol. 23, No. 21, 2004
Chong et al.
intensities interpreted using the program DENZO.23 A total
of 25 006 reflections were measured, of which 8771 were
unique and Rint ) 0.0383; 5582 reflections with larger than
4σ(Fo) were considered observed and used in the structural
analysis. The space group was determined on the basis of a
statistical analysis of intensity distribution and the successful
refinement of the structure. The structure was solved by direct
methods employing the SIR-97 program24 on a PC and refine-
ment by full-matrix least-squares using the software package
SHELXL-9725 on a PC. One crystallographic asymmetric unit
consists of one water and one methanol solvent molecule. Two
P atoms of the triethylphosphine ligands on Pd(2) were
distorted each into two positions, which were separated by 0.80
and 0.54 Å, respectively; the ethyl groups on them were also
distorted. In the least-squares refinement, all non-H atoms of
the complex molecules were refined anisotropically, and the
distorted atoms and the atoms mentioned above were refined
isotropically. H atoms were generated by the program SHELXL-
97.25 The positions of H atoms were calculated on the basis of
a riding mode with thermal parameters equal to 1.2 times that
of the associated C atoms and participated in the calculation
of final R-indices. Convergence for 477 variable parameters
by full-matrix least-squares refinement on F2 with (∆/σ)max
)
0.002 (av 0.001) was reached at R1 ) 0.0516 and wR2 ) 0.1452
with a goodness-of-fit of 0.946. The final difference Fourier map
showed maximum rest peaks and holes of 0.991 and -0.761 e
Å-3, respectively. Crystal and structure determination data
for 5 are summarized in Table 1. The final agreement factors
for 5 are given in Table 1. Selected bond distances and angles
are summarized in Table 2. The atomic coordinates and
thermal parameters are given as Supporting Information.
F igu r e 1. Perspective drawing of 2 with the atomic
numbering scheme. Hydrogen atoms on the bipyridyl and
phenyl rings have been omitted for clarity. Thermal el-
lipsoids are shown at the 40% probability level.
Resu lts a n d Discu ssion
Pd(II) centers in the heterometallic complexes 4 and 5.
The aryl protons on the 2,4,6-positions of the central
phenylene ring were found to show an upfield shift upon
the coordination of the alkynyl groups to Pd(II). Such
an upfield shift in the C6H3 signals in 4 and 5 compared
with that in 1-3 may be suggestive of the electron-
richness of the Pd(PEt3)2Cl moieties, leading to a
reduced electron donation from the alkynyl unit.
The IR spectra of 1-3 exhibit three weak to medium
intensity bands at ca. 2120-2077 cm-1, corresponding
to the two a1 and one b1 IR-active ν(CtC) modes of 1-3
assuming a C2v microsymmetry of the core and charac-
teristic of ν(CtC) stretches in metal alkynyl complexes.
The medium intensity band at ca. 2087-2077 cm-1 in
2 and 3 shows an obvious reduction in intensity upon
coordination to the palladium(II) metal centers in the
trinuclear mixed-metal complexes 4 and 5. Attempts to
locate this third IR-active band in 4 and 5 were not
possible since it was too weak to be observed. However,
a slight shift of the weak ν(CtC) band at ca. 2107 cm-1
in 2 and 3 to ca. 2103 cm-1 in 4 and 5 was observed.
This may be suggestive of some weak back π-donation
from the electron-rich Pd centers to the alkynyl units,
causing a slight weakening of the CtC bond.
Reaction of a mixture of [Re(CO)3(N∧N)Cl], Et3N,
AgOTf, and 1,3,5-(HCtC)3C6H3 in THF under reflux
conditions in an inert atmosphere of nitrogen for 24 h
afforded [1,3-(HCtC)2-5-{(N∧N)(CO)3ReCtC}C6H3], iso-
lated as orange crystals after purification by column
chromatography on silica gel, followed by recrystalliza-
tion from dichloromethane-diethyl ether. On the other
hand, reaction of a mixture of [1,3-(HCtC)2-5-{(N∧N)-
(CO)3ReCtC}C6H3], trans-[Pd(PEt3)2Cl2], CuCl, and
Et3N in THF at room temperature in an inert atmo-
sphere of nitrogen for 2 h afforded [1,3-{Cl(PEt3)2Pd-
CtC}2-5-{(N∧N)(CO)3ReCtC}C6H3], isolated as orange
crystals after purification by column chromatography
on silica gel, followed by subsequent recrystallization
from dichloromethane-diethyl ether, with the exception
of 5 from a dichloromethane-n-hexane mixture. Addi-
tion of the THF solution of [1,3-(HCtC)2-5-{(N∧N)-
(CO)3ReCtC}C6H3] to the reaction mixture in a drop-
wise manner was required to avoid the self-coupling
reaction of the rhenium(I) alkynyl complexes. The
identities of 1-5 have been confirmed by satisfactory
elemental analyses, 1H NMR, IR, and positive FAB-MS,
and for complexes 2 and 5 also by X-ray crystallography.
1
The H NMR spectra of the mononuclear rhenium(I)
Figures 1 and 2 depict the perspective drawings of 2
and 5 with atomic numbering, respectively. Both struc-
tures show a slightly distorted octahedral geometry
about Re with three carbonyl ligands arranged in a
facial fashion. In both complexes, the N-Re-N bond
angles of 74.1(2)° and 74.7(3)° for 2 and 5, respectively,
are less than 90°, as required by the bite distance
exerted by the steric demand of the chelating bipyridine
ligands.
complexes 1-3 revealed a singlet at δ 3.40 ppm,
corresponding to the acetylenic protons, which disap-
peared upon coordination of the alkynyl units to the
(23) Otwinowski, Z.; Minor, W. In Methods in Enzymology, Vol.
276: Macromolecular Crystallography, part A; Carter C. W., Sweet,
R. M., J r., Eds.; Academic Press: New York, 1997; pp 307-326.
(24) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
A new tool for crystal structure determination and refinement. J . Appl.
Crystallogr. 1998, 32, 115.
The three CtC bond lengths are 1.199(9), 1.16(1), and
1.17(1) Å for 2 and 1.200(11), 1.207(12), and 1.181(14)
(25) Sheldrick, G. M. SHELX97, Programs for Crystal Structure
Analysis (Release 97-2); University of Goetingen: Germany, 1997.

Branched Alkynyl Complexes of Re(I)-Pd(II)
Organometallics, Vol. 23, No. 21, 2004 4929
F igu r e 2. Perspective drawing of 5 with the atomic numbering scheme. Hydrogen atoms on the bipyridyl, phenyl rings,
and ethyl groups have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
Å for 5, which are comparable to those found for other
organometallic triethynylbenzene systems.10a,b,d The
Re-CtC unit for 2 is essentially linear, with a bond
angle of 177.3(6)°, similar to those found for other
related rhenium(I) alkynyl systems,14,26 while the Re-
CtC bond in 5 shows a deviation from linearity with a
bond angle of 170.7(8)°, which may be a result of the
steric bulk of the two Pd(PEt3)2Cl moieties.
In complex 5, both palladium centers show a slightly
distorted square-planar coordination geometry, with the
P-Pd-Cl bond angles in the range 88.8(2)-94.71(11)°.
It is conceivable that a slight distortion from an ideal
square-planar arrangement, which has also been ob-
served in the crystal structure of trans-[PtCl(CtCPh)-
(PEt2Ph)2],27 could help to relieve steric interactions
between the bulky groups. The coordination plane
around Pd(1) forms an interplanar angle of 50.57° with
the central phenylene ring, while that around Pd(2) are
10.50° and 26.48° due to the disorder of the two
triethylphosphines on Pd(2).
tively, were optimized at the DFT level (see computa-
tional details). The pertinent computed distances are
compared to the corresponding experimental distances
of the crystallographically characterized complexes 2
and 5 in Figure 3. A rather good agreement is found.
The largest deviation concerns the M-C bond distances
computed, which are ca. 0.05-0.07 Å longer than that
observed in 2 and 5. This is mainly due to the fact that
the relativistic effect is not completely taken into
account in the calculations.28 Although they are slightly
longer overall, the computed C-C separations in the
organic spacer are in a good agreement with the
corresponding experimental values, which were mea-
sured with rather large uncertainties. They fall in the
range of C-C distances generally measured experimen-
tally for this kind of complex.10a,b,d The angles are
reproduced at high accuracy, with deviations of less
than 1°, except for the Re-CR-Câ angle, which deviates
in both 3 and 5-H by 7° from the experimental values
(178.6 and 180.0 vs 177.3(6)° in 2 and 170.7(8)° in 5).
The Pd environment in model 5-H is nearly ideally
square-planar with a P-Pd-Cl angle of 89°. Thus, the
slight distortion observed experimentally in 5 can be
unambiguously attributed to steric interactions. The
computed dihedral angle between the central phenyl
ring and the plane formed by the palladium and the
phosphorus atoms is 45°. The corresponding experimen-
tal values are 51° around Pd(1) and 10° and 26° around
Pd(2). This suggests that the bulky ligands present in
The complex [1,3-(HCtC)2-5-{(bpy)(CO)3ReCtC}-
C6H3] (3) and the hydrogen-substituted model complex
[1,3-{Cl(PH3)2PdCtC}2-5-{(bpy)(CO)3ReCtC}C6H3] (5-
H), used to mimic compounds 1-3 and 4-5, respec-
(26) (a) Yam, V. W. W.; Lau, V. C. Y.; Cheung, K. K. Organometallics
1996, 15, 1740. (b) Yam, V. W. W.; Chong, S. H. F.; Cheung, K. K.
Chem. Commun. 1998, 2121. (c) Yam, V. W. W.; Lo, K. K. W.; Wong,
K. M. C. J . Organomet. Chem. 1999, 578, 3. (d) Yam, V. W. W.; Chong,
S. H. F.; Ko, C. C.; Cheung, K. K. Organometallics 2000, 19, 5092. (e)
Yam, V. W. W. Chem. Commun. 2001, 789. (f) Yam, V. W. W.; Wong,
K. M. C.; Chong, S. H. F.; Lau, V. C. Y.; Lam, S. C. F.; Zhang, L.;
Cheung, K. K. J . Organomet. Chem. 2003, 670, 205.
(27) Cardin, C. J .; Cardin, D. J .; Lappart, M. F. K.; Muir, W. J .
Chem. Soc., Dalton Trans. 1978, 46.
(28) Koch, W.; Holthausen M. C. A Chemist’s Guide to Density
Functional Theory; Wiley-VCH: Weinheim, Germany, 2000.

4930 Organometallics, Vol. 23, No. 21, 2004
Chong et al.
As expected, the LUMO in both complexes is exclusively
located at the bpy ligand tethered to the Re center.
Substitution of the hydrogen atoms of the alkynyl
units in 3 by Pd(PH3)2Cl moieties in 5-H perturbs
slightly the electronic structure of the latter with respect
to that of the former. In particular, MOs of the HOMO
region are somewhat destabilized due to some interac-
tion between π components of the triethynylbenzene
moiety and the low-lying dπ FMOs of the palladium
units in 5-H. The orbital 1a′′ of 3, exclusively localized
on the arylacetylide unit, is the most affected and
becomes the HOMO-1 (3a′′) of 5-H with 12% Pd
character. The energy destabilization of the HOMO
region in 5-H with respect to that of complex 3 leads to
a smaller HOMO-LUMO gap in the former (0.95 vs
1.13 eV).
The Mulliken atomic net charges computed for 3 and
5-H are given in Figure 5. An increase of the electronic
density of the Re-alkynyl moiety is calculated upon
substitution of the hydrogen atoms of the alkynyl
ligands by Pd(PH3)2Cl moieties. The Re center is more
electron-rich, gaining 0.27 electron. This reflects the
stronger electron-donating ability of the Cl(PH3)2PdCtC
fragment with respect to HCtC.
The electronic absorption spectra of 1-5 in THF show
intense absorption bands at ca. 410-430 nm, with
extinction coefficients on the order of 103 dm3 mol-1
cm-1. Table 3 summarizes the electronic absorption data
of the complexes. The higher energy absorption bands
at ca. 284-372 nm, which are also found in the free
bipyridine and alkynyl ligands, are assigned as π f π*
intraligand transitions of bipyridine and the alkynyl
ligands. With reference to previous spectroscopic work
on rhenium(I) diimine systems,26 the intense low-energy
absorptions at ca. 410-430 nm for 1-5 are tentatively
assigned as the dπ(Re) f π*(diimine) metal-to-ligand
charge transfer (MLCT) transition, probably with some
mixing of an alkynyl-to-diimine [π(CtC) f π*(diimine)]
ligand-to-ligand charge transfer (LLCT) character. This
is also supported by DFT calculations on complex 3 and
the hydrogen-substituted model complex 5-H, which
show a LUMO of bpy π* character and HOMOs of
rhenium and/or alkynyl character (see above). The
occurrence of the MLCT absorption band of 1-5 at lower
energies than that of their related precursor complexes,
[Re(CO)3(N∧N)Cl], is suggestive of the higher-lying
orbital energy of the dπ(Re) orbital in the alkynyl
complexes, as a result of the better σ- and π-electron-
donating abilities of the alkynyl ligand than the chloro
counterpart, which renders the Re(I) center more elec-
tron-rich.
F igu r e 3. Pertinent computed distances (in bold) for
complex 3 (a) and the hydrogen-substituted model complex
5-H (b). Experimental distances measured for 2 and 5 are
given for comparison, respectively.
5 prevent the structure from adopting a more sym-
metrical arrangement.
A comparison of the computed geometries of 3 and
5-H indicates a slight shortening of the CtC distance
of the alkynyl unit bound to Re by 0.006 Å upon going
from 3 to 5-H. On the other hand, a slight lengthening
of the same amount is computed for the CtC distance
of the alkynyl units bound to Pd in 5-H with respect to
the corresponding alkynyls in 3. It is conceivable that
some weak back π-donation from the electron-rich Pd
centers to the alkynyl units is at work in 5-H. Despite
rather high uncertainties in the distances measured
experimentally for 2 and 5, the slight shift of the ν(CtC)
stretch to lower frequency upon coordination to pal-
ladium is also supportive of this point.
With different diimine ligands, a trend in the low-
energy absorption was observed. The higher MLCT
absorption energies of 1 (408 nm) and 2 (408 nm) than
that of 3 (418 nm) are consistent with the higher π*
The DFT molecular orbital (MO) diagrams of complex
3 and the hydrogen-substituted model complex 5-H are
compared in Figure 4. A large HOMO-LUMO gap (1.13
eV) is computed for 3. The HOMO and HOMO-1
orbitals of 3 show an important contribution of the
alkynyl unit bound to Re and to a lesser extent of the
rhenium atom. They result from antibonding interac-
tions between dπ FMOs of the rhenium unit and
in-plane and out-of-plane orbitals of the triethynylben-
zene moiety. This is a common feature for organome-
tallic complexes containing this sort of carbon ligand.29
t
orbital energies of Bu2bpy and Me2bpy than bpy as a
result of its poorer π-accepting abilities derived from the
presence of the more electron-rich tert-butyl and methyl
substituents on the bipyridine ligands. Similarly, the
(29) See for example: (a) Weyland, T.; Costuas, K.; Toupet, L.; Halet,
J .-F.; Lapinte, C. Organometallics 2000, 19, 4228, and references
therein. (b) Wong, K. M.-C.; Lam, S. C.-F.; Ko, C.-C.; Zhu, N.; Yam, V.
W.-W.; Roue´, S.; Lapinte, C.; Fathallah, S.; Costuas, K.; Kahlal, S.;
Halet, J .-F. Inorg. Chem. 2003, 42, 7086.

Branched Alkynyl Complexes of Re(I)-Pd(II)
Organometallics, Vol. 23, No. 21, 2004 4931
F igu r e 4. DFT MO diagrams of complex 3 (left) and the hydrogen-substituted model complex 5-H (right). The bpy/Re/
carbon spacer (3) and bpy/Re/carbon spacer/Pd (5-H) percentage contributions of the MOs are given in parentheses.
complex 5-H, which indicate a smaller HOMO-LUMO
gap for the complexes containing palladium centers (see
above).
Upon photoexcitation of 1-5 at λ > 400 nm, strong
orange luminescence was observed both in the solid
state and in fluid solutions at room temperature,
3
attributed to the MLCT phosphorescence. The MLCT
emission energies of 1 (625 nm) and 2 (625 nm) in
tetrahydrofuran are higher than that of the correspond-
ing analogue 3 (636 nm). This could be ascribed to the
electron-donating effect of the tert-butyl or methyl
substituents on the bipyridine ligand, which would raise
t
the π* orbital energies of Bu2bpy or Me2bpy over that
of bpy. Similarly, the MLCT emission energy of 4 (628
nm) is higher than that of 5 (639 nm). The rhenium(I)
alkynyl complexes 1 and 2 with tert-butyl and methyl
substituents on the bipyridine ligands, respectively,
show MLCT emission energies similar to the electron-
donating abilities of the tert-butyl and methyl groups,
and hence the π-accepting abilities of tBu2bpy and Me2-
bpy are quite similar.
With different alkynyl ligands, a trend in the MLCT
emission energy was also observed. Although the dif-
ference in the emission energies is small, it could be
suggested that the slightly higher MLCT emission
energy in 2 (625 nm) compared to 4 (628 nm) and also
3 (636 nm) compared to 5 (639 nm) is consistent with
the trends observed in their electronic absorption stud-
ies. Indeed, similar but more obvious trends can be
observed in the solid state at room temperature, as
shown in Figure 6.
F igu r e 5. Pertinent Mulliken atomic net charges for
complex 3 (a) and the hydrogen-substituted model complex
5-H (b).
MLCT absorption energy of 4 (416 nm) is higher than
that of 5 (426 nm) for the same reason.
The cyclic voltammograms of 1-5 in acetonitrile (0.1
mol dm-3 nBu4NPF6) display an irreversible oxidation
wave at ca. +1.00 to +1.08 V, a quasi-reversible oxida-
tion couple at ca. +1.75 to +1.82 V, and a quasi-
reversible reduction couple at ca. -1.43 to -1.52 V vs
SCE. An additional irreversible oxidation wave at ca.
+1.40 V vs SCE was observed in the heterometallic
complexes 4 and 5. The electrochemical data are sum-
marized in Table 4.
On the other hand, a red shift in the absorption
energy on going from 2 (408 nm) to 4 (416 nm) was
observed upon the incorporation of palladium(II) metal
centers into the rhenium alkynyl unit. This is consistent
with the stronger electron-donating ability of Cl(PEt3)2-
PdCtC than that of HCtC, which renders the Re(I)
center more electron-rich and raises the dπ(Re) orbital
energy, leading to a lower MLCT energy. A similar trend
of MLCT absorption energy is also observed in the
related analogues, in which 3 (418 nm) is found to show
a higher MLCT absorption energy than that of 5 (426
nm). This is in agreement with DFT calculations carried
out on complex 3 and the hydrogen-substituted model
With reference to electrochemical studies on other
related rhenium(I) diimine complexes,26a,b the first
oxidation wave that occurred at ca. +1.0 to 1.1 V vs SCE
was tentatively assigned as a Re(I) to Re(II) oxidation.
All the complexes 1-5 showed a less positive potential

4932 Organometallics, Vol. 23, No. 21, 2004
Ta ble 3. Electr on ic Absor p tion a n d Em ission Da ta for 1-5
Chong et al.
emission
electronic absorption
a
complex
λ
max/nm (ꢀ/dm3mol-1cm-1
)
medium (T/K)
THF (298)
λ
em/nm (τo/µs)
1
240 (51640), 294 (39680), 334 sh (12100), 408 (2840)
244 (52540), 294 (41180), 334 sh (12940), 408 (3020)
252 (30410), 294 (32010), 332 sh (12740), 418 (2590)
236 (57080), 284 (103890), 336 sh (16240), 416 (2250)
238 (78150), 284 (99590), 334 sh (21590), 426 (2050)
625 (<0.1)
solid (298)
solid (77)
EtOH-MeOH glass (4:1 v/v) (77)
THF (298)
solid (298)
solid (77)
EtOH-MeOH glass (4:1 v/v) (77)
THF (298)
solid (298)
solid (77)
EtOH-MeOH glass (4:1 v/v) (77)
THF (298)
solid (298)
solid (77)
EtOH-MeOH glass (4:1 v/v) (77)
THF (298)
570 (0.26)
573 (0.36, 0.21)b
538 (2.56, 0.70)b
625 (<0.1)
2
3
4
5
573 (0.20)
559 (0.38)
555 (3.36, 1.00)b
636 (<0.1)
568 (0.28)
564 (1.25, 0.22)b
568 (3.27, 0.70)b
628 (<0.1)
600 (<0.1)
589 (0.73)
546 (4.39, 0.73)b
639 (<0.1)
solid (298)
598 (0.26)
solid (77)
EtOH-MeOH glass (4:1 v/v) (77)
594 (0.62)
565 (3.13, 0.78)b
a
b
In THF at 298 K. Double exponential decay.
3 (+1.08 V vs SCE), respectively, suggestive of a slight
increase in the electron-donating ability of the alkynyl
ligand upon incorporation of the Pd(PEt3)2Cl moieties,
which renders the Re(I) center slightly easier to oxidize.
This is supported by DFT calculations that show HO-
MOs higher in energy in the case of the palladium-
substituted complex. The relatively small shifts in the
oxidation potential upon changing the electron-richness
of the alkynyl ligands are supportive of a metal-centered
oxidation rather than an alkynyl ligand-centered one,
which otherwise would give rise to a more obvious
change in the oxidation potential.
F igu r e 6. Solid-state emission spectra of [1,3-(HCtC)2-
5-{(Me2bpy)(CO)3 ReCtC}C6H3] (2) (s) and [1,3-{Cl(PEt3)2-
PdCtC}2-5-{(Me2bpy)(CO)3ReCtC}C6H3] (4) (‚‚‚) at room
temperature.
The oxidation wave that occurred at ca. +1.80 V vs
SCE was found to be relatively insensitive to the nature
of the alkynyl ligand but to vary with the nature of the
diimine ligand. An EC mechanism was proposed for this
oxidation wave,30 in which a weakening of the metal-
carbon bond between the rhenium center and the 1,3,5-
triethynylbenzene ligand occurred after the first oxida-
tion. This resulted in the loss of an alkynyl radical to
give the coordinative unsaturated intermediate [Re-
(CO)3(N∧N)]+, which then readily picked up an aceto-
nitrile molecule from the solvent to form the correspond-
ing solvento complex, [Re(CO)3(N∧N)(MeCN)]+. There-
fore, this oxidation process of the rhenium(I) alkynyl
complexes 1-5 was tentatively assigned as the oxida-
tion of Re(I) to Re(II) of the corresponding solvento
analogues, [Re(CO)3(N∧N)(MeCN)]+, in acetonitrile. The
observation that [1,3-(RCtC)2-5-{(Me2bpy)(CO)3Re-
CtC}C6H3] (R ) H, 2, or Pd(PEt3)2Cl, 4) showed a less
positive potential for this oxidation wave than their
corresponding counterparts, [1,3-(RCtC)2-5-{(bpy)-
(CO)3ReCtC}C6H3] (R ) H, 3, and Pd(PEt3)2Cl, 5),
respectively, is in line with the presence of the more
electron-rich and less π-accepting Me2bpy ligand, which
would make the Re(I) in [Re(CO)3(Me2bpy)(MeCN)]+
more easily to oxidize than that in [Re(CO)3(bpy)-
(MeCN)]+. Complexes 1 (+1.06 V vs SCE) and 2 (+1.05
V vs SCE) showed similar oxidation potentials for this
Ta ble 4. Electr och em ica l Da ta for 1-5
E1/2b/V vs SCE (∆Ep/mV)
complexa
oxidation
reduction
1
+1.06c
-1.52 (98)
+1.80 (102)
2
3
4
5
+1.05c
-1.52 (93)
-1.44 (85)
-1.52 (84)
-1.43 (77)
+1.78 (100)
+1.08c
+1.82 (131)
+1.00c
+1.38,c +1.75 (131)
+1.03c
+1.40,c +1.81 (104)
a
In acetonitrile (0.1 mol dm-3 nBu4NPF6). Working electrode:
glassy carbon; ∆Ep of Fc+/Fc ranges from 62 to 64 mV; scan rate,
100 mV s-1
.
E1/2 ) (Epa + Epc)/2; Epa and Epc are the anodic and
b
cathodic peak potentials, respectively. ∆Ep ) |Epa - Epc|. c Irre-
versible oxidation wave. The potential refers to Epa, which is the
anodic peak potential.
for their first oxidation wave than their chloro counter-
parts, [Re(CO)3(N∧N)Cl], consistent with the better σ-
and π-donating ability of the alkynyl ligand than the
chloro group, which would render the rhenium(I) metal
center more electron-rich and hence increased its ease
of oxidation. With the same diimine ligand, 4 (+1.00 V
vs SCE) and 5 (+1.03 V vs SCE) showed a slightly less
positive potential for their first oxidation wave than
their corresponding analogues, 2 (+1.05 V vs SCE) and
(30) (a) Breikss, A. I.; Abruna, H. D. J . Electroanal. Chem. 1986,
201, 347. (b) Lau, V. C. Y. Ph.D. Thesis, The University of Hong Kong,
1997.

Branched Alkynyl Complexes of Re(I)-Pd(II)
Organometallics, Vol. 23, No. 21, 2004 4933
oxidation wave, consistent with the similar electron-
donating ability of the methyl and tert-butyl groups.
An additional irreversible oxidation wave at ca. +1.40
V vs SCE for the heterometallic complexes 4 and 5 was
tentatively assigned as Pd(II) metal-centered oxidation.
The observation that complexes 4 and 5 show a less
positive potential than the trans-[Pd(PEt3)2Cl2] precur-
sor (+1.52 V vs SCE) is consistent with the better
σ-donating ability of the alkynyl moiety than the chloro
group, which would render the palladium(II) metal
center more electron-rich and hence increases its ease
of oxidation. However, the possible involvement of a
1,3,5-triethynylbenzene ligand-centered oxidation can-
not be completely excluded since the free ligand also
shows an irreversible oxidation wave at ca. +1.22 V vs
SCE. A similar result was also found in the palladium-
(II) branched alkynyl complexes.31
The quasi-reversible reduction couple, which was
found to vary with the nature of the diimine ligand, was
tentatively assigned as a bipyridyl ligand-centered
reduction. Indeed, DFT calculations on the monoanion
of 3 and 5-H indicate that the added unpaired electron
is localized on the bpy ligand. The observation that 1
(-1.52 V vs SCE) and 2 (-1.52 V vs SCE) showed a
more negative reduction potential than 3 (-1.44 V vs
SCE) is in line with the attachment of the electron-
donating tert-butyl or methyl substituents on the bipyr-
idine ligand, which would render the bipyridine ligand
more electron-rich and poorer π-accepting, thus decreas-
ing its ease of reduction.
tures of 2 and 5 have also been determined. Their
photophysical and electrochemical properties have been
studied. All complexes exhibit an intense low-energy
absorption at ca. 410-430 nm, tentatively assigned as
the [dπ(Re) f π*(diimine)] MLCT transition, probably
with some mixing of an alkynyl-to-diimine [π(CtC) f
π*(diimine)] LLCT character. Such an assignment has
been supported by DFT calculations. Red shifts in the
absorption and emission energies were observed upon
incorporation of the Cl(PEt3)2PdCtC moiety.
Ack n ow led gm en t. V.W.-W.Y acknowledges support
from the University of Hong Kong Foundation for
Educational Department and Research Limited and the
receipt of a Croucher Senior Research Fellowship from
the Croucher Foundation. The work has been supported
by the Research Grants Council of the Hong Kong
Special Administrative Region, China (Project No. HKU
7123/00P). S.H.-F.C. acknowledges the receipt of a
postgraduate studentship, administered by The Uni-
versity of Hong Kong, S.C.-F.L. the receipt of a post-
graduate studentship, a Li Po Chun Scholarship, and a
Sir Edward Youde Memorial Fellowship, administered
by The University of Hong Kong, the Li Po Chun
Charitable Fund, and Sir Edward Youde Memorial
Fund Council, respectively, and N.Z. the receipt of a
University Postdoctoral Fellowship, administrative by
The University of Hong Kong. S.F. thanks the French
Ministry of Education and Research for a postgraduate
studentship. J .-F.H. and K.C. thank the Poˆle de Calcul
Intentif de l’Ouest (PCIO) of the University of Rennes
for computing facilities. V.W.-W.Y. and J .-F.H. acknowl-
edge the award of a travel grant by the RGC/CNRS
under the PROCORE: Hong Kong-France J oint Re-
search Scheme.
Con clu sion
A series of luminescent mononuclear and heterome-
tallic rhenium(I) complexes containing the hyper-
branched 1,3,5-triethynylbenzene ligand have been
successfully synthesized, and the X-ray crystal struc-
Su p p or tin g In for m a tion Ava ila ble: This material is
available free of charge via the Internet at http://pubs.acs.org.
(31) Yam, V. W. W.; Tao, C. H. Unpublished results.
OM049696L