MLCT and LMCT transitions in acetylide complexes. structural, spectroscopic, and redox properties of ruthenium(II) and -(III) Bis(σ-arylacetylide) complexes supported by a tetradentate macrocyclic tertiary amine ligand

Article abstract of DOI:10.1021/om990009d

Ruthenium(II) complexes trans-[Ru(16-TMC)(C≡CC₆H₄X-p₂] (X = OMe (1), Me (2), H (3), F (4), Cl (5); 16-TMC = 1,5,9,13-tetramethyl-l,5,9,13-tetraazacyclohexadecane) are prepared by the reaction of [Ru^III(16-TMC)Cl₂]Cl with the corresponding alkyne and NaOMe in the presence of zinc amalgam. Low v(C≡C) stretching frequencies are observed for 1-5 and are attributed to the σ-donating nature of 16-TMC. The molecular structures of 1, 3, and 5 have been determined by X-ray crystal analyses, which reveal virtually identical Ru-C and C≡ C bond distances (mean 2.076 and 1.194 A?, respectively). The cyclic voltammograms of 1-5 show quasi-reversible Ru^III/II and Ru^IV/III oxidation couples. Oxidative cleavage of the acetylide ligand in 3 by dioxygen affords [Ru( 16-TMC)(C≡CPh)(CO)]⁺ (6). Ruthenium(III) derivatives trans-[Ru(16-TMC)(C≡CC₆H₄X-p)₂] ⁺ are generated in situ by electrochemical oxidation in dichloromethane or by chemical oxidation of 1-5 with Ce(IV). Their UV-visible absorption spectra show a vibronically structured absorption band with λ^max at 716-768 nm. The vibrational progressions, which range from 1730 to 1830 cm^-1, imply that the electronic transition involves distortion of the acetylide ligand in the excited state. An assignment of p_π(ArC≡C) →d_π*(Ru^III) charge transfer is proposed for this transition.

Full text of DOI:10.1021/om990009d

2074
Organometallics 1999, 18, 2074-2080
MLCT a n d LMCT Tr a n sition s in Acetylid e Com p lexes.
Str u ctu r a l, Sp ectr oscop ic, a n d Red ox P r op er ties of
Ru th en iu m (II) a n d -(III) Bis(σ-a r yla cetylid e) Com p lexes
Su p p or ted by a Tetr a d en ta te Ma cr ocyclic Ter tia r y
Am in e Liga n d
Mei-Yuk Choi,† Michael Chi-Wang Chan,† Suobo Zhang,† Kung-Kai Cheung,†
Chi-Ming Che,*,† and Kwok-Yin Wong‡
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, and
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Kowloon, Hong Kong
Received J anuary 11, 1999
Ruthenium(II) complexes trans-[Ru(16-TMC)(CtCC6H4X-p)2] (X ) OMe (1), Me (2), H (3),
F (4), Cl (5); 16-TMC ) 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane) are prepared
by the reaction of [RuIII(16-TMC)Cl2]Cl with the corresponding alkyne and NaOMe in the
presence of zinc amalgam. Low ν(CtC) stretching frequencies are observed for 1-5 and are
attributed to the σ-donating nature of 16-TMC. The molecular structures of 1, 3, and 5 have
been determined by X-ray crystal analyses, which reveal virtually identical Ru-C and Ct
C bond distances (mean 2.076 and 1.194 Å, respectively). The cyclic voltammograms of 1-5
show quasi-reversible RuIII/II and RuIV/III oxidation couples. Oxidative cleavage of the acetylide
ligand in 3 by dioxygen affords [Ru(16-TMC)(CtCPh)(CO)]+ (6). Ruthenium(III) derivatives
trans-[Ru(16-TMC)(CtCC6H4X-p)2]+ are generated in situ by electrochemical oxidation in
dichloromethane or by chemical oxidation of 1-5 with Ce(IV). Their UV-visible absorption
spectra show a vibronically structured absorption band with λmax at 716-768 nm. The
vibrational progressions, which range from 1730 to 1830 cm-1, imply that the electronic
transition involves distortion of the acetylide ligand in the excited state. An assignment of
pπ(ArCtC) f dπ*(RuIII) charge transfer is proposed for this transition.
In tr od u ction
description of the metal-acetylide bond by spectroscopic
and theoretical means.7-9 For electron-rich metal cen-
ters, M f (CCR) π back-bonding interactions with
The chemistry of σ-acetylide transition-metal com-
plexes has experienced intense scrutiny.1,2 In principle,
the linear CtC moiety allows delocalization of electron
density and thus electronic communication between
metal centers and remote functional groups. The nature
of the metal-acetylide interaction is manifested in
specific properties which are applicable to several areas
of material science and fundamental research, including
nonlinear optical materials,3 liquid crystals,4 polymeric
conductors,5 and conjugated carbon-rich chains and
molecular wires.6 Significantly, several recent reports
have provided valuable insights toward a comprehensive
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1997, 119, 775. (d) Lavastre, O.; Plass, J .; Bachmann, P.; Guesmi, S.;
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P.; Wu, W. F.; Ng, W. S.; Williams, I. D.; J ia, G. Organometallics 1997,
16, 2940. (f) Coat, F.; Guillevic, M.-A.; Toupet, L.; Paul, F.; Lapinte,
C. Organometallics 1997, 16, 5988. (g) Harriman, A.; Ziessel, R. Chem.
Commun. 1996, 1707. (h) Bunz, U. H. F. Angew. Chem., Int. Ed. Engl.
1996, 35, 969. (i) Bruce, M. I.; Ke, M.; Low, P. J . Chem. Commun. 1996,
2405. (j) Yam, V. W. W.; Lau, V. C. Y.; Cheung, K. K. Organometallics
1996, 15, 1740. (k) Pirio, N.; Touchard, D.; Dixneuf, P. H.; Fettouhi,
M.; Ouahab, L. Angew. Chem., Int. Ed. Engl. 1992, 31, 651.
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† The University of Hong Kong.
‡ The Hong Kong Polytechnic University.
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Chem. Mater. 1997, 9, 406. (b) Whittall, I. R.; Humphrey, M. G.; Samoc,
M.; Luther-Davies, B. Angew. Chem., Int. Ed. Engl. 1997, 36, 370. (c)
McDonagh, A. M.; Whittall, I. R.; Humphrey, M. G.; Hockless, D. C.
R.; Skelton, B. W.; White, A. H. J . Organomet. Chem. 1996, 523, 33.
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Chem., Int. Ed. Engl. 1995, 34, 21. (f) Whittall, I. R.; Humphrey, M.
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10.1021/om990009d CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/28/1999

MLCT and LMCT Transitions in Acetylide Complexes
Organometallics, Vol. 18, No. 11, 1999 2075
electron-poor acetylide groups typically prevail and lead
to delocalization of the metal dπ electrons into the
acetylide π* orbital. Alternatively, π back-bonding is
negligible with electron-deficient metal ions, but metal-
carbon multiple-bonding interactions in the M-CtCR
linkage are possible. This would result in considerable
acetylide-to-metal charge transfer, yet this observation
is rarely made.
Exp er im en ta l Section
All reactions were performed under a nitrogen atmosphere
using standard Schlenk techniques unless otherwise stated.
Solvents were purified by standard methods. All reagents were
used as received. [Ru(16-TMC)Cl2]Cl (16-TMC ) 1,5,9,13-
tetramethyl-1,5,9,13-tetraazacyclohexadecane) was prepared
according to the published procedure.14
1H and 13C{1H} NMR spectra were recorded on J EOL 270,
Bruker 300 DPX, and Bruker 500 DRX FT-NMR spectrom-
eters. Peak positions were calibrated with Me4Si as internal
standard. In the 13C{1H} NMR spectra, multiple resonances
for 16-TMC are observed because of the flexible nature of the
propylene groups, and the peaks listed correspond to the most
intense signals. Fast atom bombardment (FAB) mass spectra
were obtained on a Finnigan MAT 95 mass spectrometer with
a 3-nitrobenzyl alcohol matrix. Infrared spectra were recorded
as KBr plates on a Bio-Rad FT-IR spectrometer. Raman
spectra were recorded on a Bio-Rad FT Raman spectrometer.
UV-visible spectra were recorded on Perkin-Elmer Lambda
19 and Milton Roy (Spectronic 3000 Array) diode array
spectrophotometers. Elemental analyses were performed by
the Butterworth Laboratories Ltd., Teddington, U.K.
Cyclic voltammetry was performed with a Bioanalytical
Systems (BAS) Model 100 B/W electrochemical analyzer. A
conventional two-compartment electrochemical cell was used.
The glassy-carbon electrode was polished with 0.05 µm alu-
mina on a microcloth, sonicated for 5 min in deionized water,
and rinsed with dichloromethane before use. An Ag/AgNO3 (0.1
M in CH3CN) electrode was used as reference electrode. All
solutions were degassed with argon gas before experiments.
E1/2 values are the average of the cathodic and anodic peak
potentials for the oxidative and reductive waves. The E1/2 value
of the ferrocenium/ferrocene couple (Cp2Fe+/0) measured in the
same solution was used as an internal reference (+0.23 mV
in CH2Cl2). Thin-layer UV-vis spectroelectrochemistry was
performed by using the HP 8452A diode array spectrophotom-
eter and Princeton Applied Research Model 273A potentiostat,
a thin-layer quartz cell of path length 0.5 mm with a platinum-
gauze working electrode, a platinum-wire counter electrode,
and a Ag/AgNO3 reference electrode.
Ruthenium is an established π donor in the +2
oxidation state but is also known to form metal-ligand
multiple-bonded species in oxidation states g4.10 Nu-
merous studies have appeared describing the reactivity
of ruthenium σ-acetylide complexes supported by mono-
and bidentate phosphine ligands.11 Nitrogen donors
have been incorporated to a lesser extent, although such
endeavors can yield rewarding results.12,13 We became
interested in (σ-acetylide)ruthenium complexes of the
macrocyclic tertiary amine 1,5,9,13-tetramethyl-1,5,9,-
13-tetraazacyclohexadecane (16-TMC). Generation of
trans-bis(acetylide) derivatives would facilitate linear
rigid-rod applications. This tetradentate amine and its
congeners (14- and 15-TMC) are optically transparent
in the UV-visible spectral region, and this permits
investigation of the metal-to-ligand or ligand-to-metal
charge transfer (MLCT or LMCT, respectively) elec-
tronic transitions associated with the Ru-CtCR moiety
by optical spectroscopy. This class of ligands constitutes
strong σ donors, is resistant to oxidation, and can
accommodate reactive but isolable high-valent oxoru-
thenium complexes.14,15 We envisaged that the 16-TMC
ligand would maximize Ru(II) f CCR π back-bonding
and stabilize organoruthenium species in high oxidation
states.
We now describe the synthesis and structural and
redox properties of a series of trans-bis(σ-arylacetylide)-
ruthenium complexes, which are the first to contain a
macrocyclic N-donor ligand, namely 16-TMC. Because
of the optical transparency of the tertiary amine, we
have been able to probe the MLCT and LMCT transi-
tions associated with the [RuII(CtCAr)2] and [RuIII(Ct
CAr)2]+ cores, respectively.
Syn th eses. tr a n s-[Ru (16-TMC)(CtCC6H4X-p)2]. Sodium
metal (0.05 g, 2.20 mmol) was added to a solution of HCt
CC6H4X-p (0.5 mmol) in methanol (20 cm3), and the mixture
was stirred until all the sodium was consumed. [Ru(16-TMC)-
Cl2]Cl (0.10 g, 0.20 mmol) and zinc amalgam were added, and
this mixture was heated at reflux for 1 h to yield a yellow
precipitate. After the system was cooled to room temperature,
the resultant solid was collected and dissolved in a minimum
amount of benzene. Slow diffusion of n-hexane into this
solution afforded bright yellow crystals.
(10) Che, C. M.; Yam, V. W. W. Adv. Inorg. Chem. 1992, 39, 233.
(11) For example, see: (a) Touchard, D.; Haquette, P.; Daridor, A.;
Romero, A.; Dixneuf, P. H. Organometallics 1998, 17, 3844. (b)
Touchard, D.; Haquette, P.; Guesmi, S.; Le Pichon, L.; Daridor, A.;
Toupet, L.; Dixneuf, P. H. Organometallics 1997, 16, 3640 and
references therein. (c) Yi, C. S.; Liu, N.; Rheingold, A. L.; Liable-Sands,
L. M. Organometallics 1997, 16, 3910. (d) Lee, H. M.; Yao, J .; J ia, G.
Organometallics 1997, 16, 3927. (e) de los R’os, I.; J ime´nez-Tenorio,
M.; Puerta, M. C.; Valerga, P. J . Am. Chem. Soc. 1997, 119, 6529. (f)
Ting, P. C.; Lin, Y. C.; Lee, G. H.; Cheng, M. C.; Wang, Y. J . Am. Chem.
Soc. 1996, 118, 6433. (g) Albertin, G.; Antoniutti, S.; Bordignon, E.;
Cazzaro, F.; Ianelli, S.; Pelizzi, G. Organometallics 1995, 14, 4114. (h)
Esteruelas, M. A.; Lahoz, F. J .; Lo´pez, A. M.; On˜ate, E.; Oro, L. A.
Organometallics 1994, 13, 1669. (i) Trost, B. M.; Flygare, J . A. J . Am.
Chem. Soc. 1992, 114, 5476.
(12) (a) Slugovc, C.; Mereiter, K.; Schmid, R.; Kirchner, K. J . Am.
Chem. Soc. 1998, 120, 6175. (b) Slugovc, C.; Mauthner, K.; Kacetl, M.;
Mereiter, K.; Schmid, R.; Kirchner, K. Chem. Eur. J . 1998, 4, 2043.
(c) Santos, A.; Lo´pez, J .; Gala´n, A.; Gonza´lez, J . J .; Tinoco, P.;
Echavarren, A. M. Organometallics 1997, 16, 3482. (d) Bianchini, C.;
Innocenti, P.; Peruzzini, M.; Romerosa, A.; Zanobini, F. Organometal-
lics 1996, 15, 272.
Complex 1 (X ) OMe): yield 0.09 g, 67%. Anal. Calcd for
C
34H50N4O2Ru: C, 63.03; H, 7.78; N, 8.65. Found: C, 62.85;
H, 7.92; N, 8.41. 1H NMR (300 MHz, C6D6): δ 0.78, 0.83, 1.22-
1.55 (m, 16H, CH2), 2.41 (s, 12H, NCH3), 3.40 (s, 6H, OCH3),
3
4.02-4.14 (m, 8H, CH2), 6.98 (d, 4H, J HH ) 8.7 Hz, aryl H),
7.73 (d, 4H, J HH ) 8.8 Hz, aryl H). 13C{1H} NMR (126 MHz,
3
C6D6): δ 22.2, 22.6 (NCH2CH2), 50.0 (NCH3), 55.0 (OCH3), 60.7,
68.1 (NCH2), 109.0 (Câ), 114.3, 126.1, 131.8 (aryl C), 156.2 (Cp),
158.7 (CR). IR (cm-1): ν(CtC) 2002. FAB-MS: m/z 648 [M+].
Complex 2 (X ) Me): yield 0.08 g, 69%. Anal. Calcd for
C
34H50N4Ru: C, 66.31; H, 8.18; N, 9.10. Found: C, 66.47; H,
1
8.18; N, 9.09. H NMR (300 MHz, C6D6): δ 0.76, 0.81, 1.19-
1.56 (m, 16H, CH2), 2.24 (s, 6H, CH3), 2.38 (s, 12 H, NCH3),
3
3.96-4.23 (m, 8H, CH2), 7.73 (d, 4H, J HH ) 8.0 Hz, aryl H),
4 aryl H obscured by C6D6 solvent. 13C{1H} NMR (68 MHz,
C6D6): δ 21.4 (CH3), 22.1, 22.5, (NCH2CH2), 50.0 (NCH3), 60.6,
68.0 (NCH2), 110.0 (Câ), 129.1, 130.2, 131.0, 131.3, (aryl C),
161.2 (CR). IR (cm-1): ν(CtC) 2003. FAB-MS: m/z 616 [M+].
Complex 3 (X ) H): yield 0.08 g, 68%. Anal. Calcd for
(13) Yang, S. M.; Chan, M. C. W.; Cheung, K. K.; Che, C. M.; Peng,
S. M. Organometallics 1997, 16, 2819.
(14) Che, C. M.; Wong, K. Y.; Poon, C. K. Inorg. Chem. 1986, 25,
1809.
(15) Che, C. M.; Poon, C. K. Pure Appl. Chem. 1988, 60, 495.
C32H46N4Ru: C, 65.39; H, 7.89; N, 9.53. Found: C, 65.07; H,

2076 Organometallics, Vol. 18, No. 11, 1999
Choi et al.
Ta ble 1. Cr ysta l Da ta
1
3
5
formula
fw
C
34H50N4O2Ru
C32H46N4Ru
587.81
C32H44N4Cl2Ru
656.70
647.87
cryst size, mm
cryst syst
space group
a, Å
b, Å
c, Å
0.35 × 0.10 × 0.07
triclinic
P1h (No. 2)
8.399(2)
9.670(2)
10.292(2)
89.12(2)
73.23(2)
88.85(2)
800.1(3)
1
0.30 × 0.15 × 0.10
monoclinic
P21/a (No. 14)
8.617(3)
17.563(3)
9.381(2)
0.30 × 0.20 × 0.10
monoclinic
P21/a (No. 14)
8.460(2)
16.975(2)
10.904(2)
R, deg
â, deg
91.96(2)
94.61(2)
γ, deg
V, Å3
1418.9(5)
2
1.376
5.80
1560.8(5)
2
1.397
7.01
Z
Dc, g cm-3
1.344
5.26
342
µ, cm-1
F(000)
620
684
2θmax, deg
no. of unique rflns
no. of obsd rflns
no. of variables
51
46
51
2761 (Rint ) 0.031)
2676 (I > 3σ(I))
187
0.037, 0.050
1.75
2104 (Rint ) 0.017)
1434 (I > 3σ(I))
169
0.030, 0.035
2.03
2831 (Rint ) 0.055)
2037 (I > 3σ(I))
178
0.039, 0.051
1.73
R,a Rw
b
GOFc
residual peaks, e Å-3
+0.47, -0.66,
+0.45, -0.29
+0.60, -0.55
a
b
2
R ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
c GOF ) [∑w(|Fo| - |Fc|)2/(n - p)]1/2
.
1
7.70; N, 9.48. H NMR (500 MHz, C6D6): δ 0.76, 0.79, 1.19-
1.71 (m, 16H, CH2), 2.36 (s, 12H, NCH3), 3.94-4.05 (m, 8H,
Diffraction experiments were performed at 301 K on MAR
(1 and 5) and Enraf-Nonius CAD4 (3) diffractometers using
graphite-monochromatized Mo KR radiation (λ ) 0.710 73 Å).
The structures were solved by direct methods (1 and 5,
SIR9216) and by Patterson methods (3, PATTY17), expanded
by Fourier techniques and refined by full-matrix least squares
using the TeXsan18 program on a Silicon Graphics Indy
computer. For 1, a crystallographic asymmetric unit consists
of a half-molecule with the Ru atom at the origin. All 21 non-H
atoms were refined anisotropically, and 25 H atoms at
calculated positions with thermal parameters equal to 1.3
times that of the attached C atoms were not refined. Atoms
marked with asterisks have coordinates at -x, -y, -z. For 3,
a crystallographic asymmetric unit consists of a half-molecule
with the Ru atom at the origin. All 19 non-H atoms were
refined anisotropically, and 23 H atoms at calculated positions
were not refined. Atoms marked with asterisks have coordi-
nates at -x, -y, -z. For 5, a crystallographic asymmetric unit
3
3
CH2), 7.07 (t, 2H, J HH ) 7.3 Hz, Hp), 7.33 (t, 4H, J HH ) 7.6
Hz, Hm), 7.78 (d, 4H, J HH ) 7.1 Hz, Ho). 13C{1H} NMR (126
3
MHz, C6D6): δ 22.4, 22.8, (NCH2CH2), 50.2 (NCH3), 61.0, 68.4
(NCH2), 110.0 (Câ), 122.8, 127.9, 131.5, 133.3, (aryl C), 163.5
(CR). IR (cm-1): ν(CtC) 2012. FAB-MS: m/z 588 [M+].
Complex 4 (X ) F): yield 0.06 g, 48%. Anal. Calcd for
C
32H44N4F2Ru: C, 61.62; H, 7.11; N, 8.98. Found: C, 61.37;
1
H, 6.97; N, 8.70. H NMR (500 MHz, C6D6): δ 0.70-1.71 (m,
16H, CH2), 2.33 (s, 12H, NCH3), 3.91-3.99 (m, 8H, CH2), 6.99,
7.53 (m, 8H, aryl H). 13C{1H} NMR (68 MHz, C6D6): δ 21.9,
22.3 (NCH2CH2), 49.2 (NCH3), 61.0, 67.5 (NCH2), 109.3 (Câ),
1
115.1 (d, J FC ) 21.8 Hz, aryl C), 132.0 (aryl C), 159.8 (d, J FC
) 242 Hz, Cp), 161.2 (CR). IR (cm-1): ν(CtC) 2006. FAB-MS:
m/z 624 [M+].
Complex 5 (X ) Cl): yield 0.07 g, 53%. Anal. Calcd for
32H44N4Cl2Ru: C, 58.53; H, 6.75; N, 8.53. Found: C, 58.73;
C
1
1
H, 6.94; N, 8.56. H NMR (300 MHz, C6D6): δ 0.73-1.48 (m,
16H, CH2), 2.28 (s, 12H, NCH3), 3.79-3.93 (m, 8H, CH2), 7.30
(d, 4H, 3J HH ) 8.5 Hz, aryl H), 7.50 (d, 4H, 3J HH ) 8.5 Hz, aryl
H). 13C{1H} NMR (126 MHz, C6D6): δ 22.0, 22.4 (NCH2CH2),
49.8 (NCH3), 60.6, 68.0 (NCH2), 109.7 (Câ), 128.6, 131.0, 132.1
(aryl C), 165.5 (CR). IR (cm-1): ν(CtC) 2004. FAB-MS: m/z
658 [M+].
consists of a half-molecule with the Ru atom at 1/2, /2, 0. All
20 non-H atoms were refined anisotropically, and 22 H atoms
at calculated positions were not refined. Atoms marked with
asterisks have coordinates at 1 - x, 1 - y, -z.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . Treatment of the
conveniently prepared ruthenium(III) complex [Ru(16-
TMC)Cl2]Cl14 with zinc amalgam, NaOMe, and alkyne
afforded the desired Ru(II) trans-bis(σ-arylacetylide)
derivatives as yellow crystalline solids (Scheme 1).
Reduction of the Ru(III) precursor yields the [RuII(16-
TMC)Cl2] species, which is substitutionally labile and
facilitates ligand exchange. Complexes 1-5 are stable
for several days in the solid state and in deoxygenated
solutions. Under aerobic conditions, solutions of 1-5 are
unstable and a color change from yellow to green is
detectable within 3 h (see Oxidation).
[Ru (16-TMC)(CtCP h )(CO)]P F 6 (6). Oxygen gas was in-
troduced into a 1,2-dichloroethane solution (15 cm3) of 3 (0.10
g, 0.20 mmol) and NH4PF6 (0.16 g, 1.0 mmol) for 3 h, during
which time the color of the solution changed from yellow to
green and eventually to yellow with the appearance of a yellow
precipitate. The solid was filtered, washed with cold water and
diethyl ether, and air-dried. Recrystallization from CH2Cl2/
diethyl ether yielded yellow crystals (yield 0.09 g, 68%). Anal.
Calcd for
C25H41N4ORuF6P: C, 45.52; H, 6.26; N, 8.49.
1
Found: C, 45.42; H, 5.93; N, 8.30. H NMR (300 MHz, CD2-
Cl2): δ 1.50-1.68, 1.93-2.11, 2.40-2.48, 3.18-3.38, 3.98-4.18
(m, 24H, CH2), 2.65, 2.67 (two singlets, 12H, NCH3), 7.12-
7.13, 7.30-7.38 (m, 5H, C6H5). 13C{1H} NMR (68 MHz, CD3-
CN): δ 21.5, 21.8 (NCH2CH2), 52.5 (NCH3), 63.7, 71.9 (NCH2),
126.6, 129.3, 130.0, 131.6, 136.4, 143.7 (aryl C, CR, Câ), 202.8
(CO). IR (cm-1): ν(CtC) 2080 w, ν(CtO) 1916 s. FAB-MS: m/z
515 [M+].
(16) SIR92: Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi,
A.; Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994,
27, 435.
(17) PATTY: Beurskens, P. T.; Admiraal, G.; Beursken, 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, Nijmegen, The Netherlands, 1992.
(18) TeXsan: Crystal Structure Analysis Package; Molecular Struc-
ture Corp., The Woodlands, TX, 1985 and 1992.
X-r a y Cr ysta llogr a p h y. Crystals of 1, 3, and 5 were
obtained by diffusion of n-hexane into benzene solutions.
Crystal data and details of collection and refinement are
summarized in Table 1.

MLCT and LMCT Transitions in Acetylide Complexes
Organometallics, Vol. 18, No. 11, 1999 2077
Ta ble 2. UV-Visible Absor p tion Da ta for
[Ru (16-TMC)(CtCC6H4X-p)2] (1-5) a n d 6
Sch em e 1
λ
max, nm (log ꢀ)
complex
CH2Cl2
CH2Cl2 + Ce(IV)/MeOHa
1 (X ) OMe) 248 (4.54), 379 (4.52)
2 (X ) Me) 250 (4.64), 392 (4.57)
675 (sh), 768
648 (sh), 730
646 (sh), 729
633 (sh), 716
638 (sh), 722
3 (X ) H)
4 (X ) F)
5 (X ) Cl)
6
250 (4.46), 394 (4.48)
246 (4.48) 388 (4.45)
255 (4.55), 408 (4.58)
256 (4.10), 271 (3.97, sh)
a
Vibronic bands. See Figure 6 for full spectrum of 1+.
The bis(acetylide) complexes have been characterized
by various spectroscopic techniques. The IR spectra of
1-5 are comparable, with an intense band for the
asymmetric CtC stretch, νas(CtC) at 2002-2012 cm-1
.
These values are among the lowest reported for ruthe-
nium acetylide derivatives.2 For example, the νas(CtC)
value for trans-[Ru(16-TMC)(CtCPh)2] (3) is 2012 cm-1
(KBr), which is lower than in related complexes with
phosphine auxiliary ligands, such as trans-[Ru(Ct
CPh)2(PEt3)2(CO)2] (2093 cm-1 in CH2Cl2)19 and trans-
[Ru(CtCPh)2(depe)2] (2054 cm-1 in CH2Cl2, depe )
Et2PCH2CH2PEt2).20 The FT Raman spectrum of com-
plex 3 shows an intense symmetric CtC stretch at 2033
cm-1, which again is distinctly lower than that for trans-
[Ru(CtCPh)2(PEt3)2(CO)2] (2101 cm-1).21 In the 13C{1H}
NMR spectra of complexes 1-5, a singlet is observed
at 158.7-165.5 ppm (C6D6) for the R-acetylide carbon.
This resonance for 3 at 163.5 ppm is significantly
downfield from that for the phosphine analogues trans-
[Ru(CtCPh)2(depe)2] (130.6 ppm in C6D6)22 and trans-
[Ru(CtCPh)2(PMe3)4] (132.5 ppm in CD2Cl2).23 In con-
trast, the signals for the â-acetylide carbons in these
complexes are similar (111.0, 112.8, and 108.2 ppm,
respectively). The low νas(CtC) and νs(CtC) stretching
frequencies and the downfield shifts of CR in 1-5,
relative to derivatives with phosphine donors, are
consistent with the greater σ-donating ability of the
tertiary amine ligand 16-TMC. This in turn promotes
increased metal-to-acetylide π back-bonding from the
electron-rich Ru(II) center.
F igu r e 1. UV-vis absorption spectrum of 2 in CH2Cl2 at
room temperature.
is apparent that the MLCT transition red-shifts in
energy as the electron-withdrawing ability of the para
substituents on the arylacetylide groups increases.
Hence, an electron-withdrawing substituent stabilizes
the π*(CCAr) orbital and lowers the energy of the MLCT
transition. The exception in this regard is complex 4,
where the large resonance effect of the fluoride group
can destabilize π*(CCAr) and increase the MLCT en-
ergy. We note that the Ru(II) f π*(py) MLCT transition
of cis-[Ru([14]aneN4)(py)2]2+, bearing a macrocyclic tet-
radentate amine ligand, occurs at 378 nm (H2O).24 This
is comparable in energy to the MLCT transitions of
complexes 1-5, despite electronic differences between
the pyridine group and the anionic arylacetylide moi-
eties. Furthermore, the Ru(II) f π*(py) MLCT transi-
tion of cis-[Ru(NH3)5(py)]2+ appears at 407 nm (CH3-
CN).25
Cr ysta l Str u ctu r es. Figures 2 and 3 show perspec-
tive views of complexes 1 and 5, respectively (for 3, see
the Supporting Information). Selected bond distances
and angles are listed in Table 3.
The UV-vis absorption data of the bis(arylacetylide)
complexes are summarized in Table 2. The absorption
spectrum of 2 is shown in Figure 1. The absorption
spectra of 1-5 in CH2Cl2 contain a high-energy band
(λmax 248-255 nm) and an intense band with λmax in
the 379-408 nm region (ꢀmax > 104 dm3 mol-1 cm-1).
We assign the latter absorption to a dπ(Ru) f π*(CCAr)
metal-to-ligand charge transfer (MLCT) transition. It
The ruthenium atom in each structure resides in an
octahedral environment and is coordinated by four
equatorial nitrogen atoms of 16-TMC and two σ-ary-
lacetylide ligands in a trans arrangement. The six-
membered chelate rings of the Ru(16-TMC) fragment
are all in chair forms, with the N-methyl groups
adopting the “two up, two down” configuration as in
(19) Sun, Y.; Taylor, N. J .; Carty, A. J . J . Organomet. Chem. 1992,
423, C43.
(20) Atherton, Z.; Faulkner, C. W.; Ingham, S. L.; Kakkar, A. K.;
Khan, M. S.; Lewis, J .; Long, N. J .; Raithby, P. R. J . Organomet. Chem.
1993, 462, 265.
trans-[Ru(16-TMC)O2]2+ 26
The Ru-C (1, 2.077(3) Å; 3,
.
2.077(4) Å; 5, 2.073(4) Å) and CtC (1, 1.195(4) Å; 3,
(21) Sun, Y.; Taylor, N. J .; Carty, A. J . Organometallics 1992, 11,
4293.
(22) Field, L. D.; George, A. V.; Hockless, D. C. R.; Purches, G. R.;
White, A. H. J . Chem. Soc., Dalton Trans. 1996, 2011.
(23) Rappert, T.; Yamamoto, A. Organometallics 1994, 13, 4984.
(24) Che, C. M.; Kwong, S. S.; Poon, C. K.; Lai, T. F.; Mak, T. C. W.
Inorg. Chem. 1985, 24, 1359.
(25) Creutz, C.; Chou, M. H. Inorg. Chem. 1987, 26, 2995.
(26) Mak, T. C. W.; Che, C. M.; Wong, K. Y. J . Chem. Soc., Chem.
Commun. 1985, 986.

2078 Organometallics, Vol. 18, No. 11, 1999
Choi et al.
F igu r e 2. Perspective view of trans-[Ru(16-TMC)(Ct
CC6H4OMe-p)2] (1) (50% probability ellipsoids, hydrogen
atoms omitted for clarity).
F igu r e 3. Perspective view of trans-[Ru(16-TMC)(Ct
CC6H4Cl-p)2] (5) (50% probability ellipsoids, hydrogen
atoms omitted for clarity).
1.198(6) Å; 5, 1.190(5) Å) bond lengths in this work are
typical for ruthenium(II) bis(acetylide) complexes.1,2 The
latter distances are evidently insensitive to the nature
of the para substituent. By comparison, the CtC
distances in trans-[Ru(CtCPh)2(dppe)2] are 1.194(7) and
1.207(7) Å,20 while that in trans-[Ru(CtCPh)2(PEt3)2-
(CO)2] is 1.200(4) Å.19 The virtual linearity of the Ar-
CtC-Ru-CtC-Ar fragment is maintained in these
structures. For example, the Ru-C(1)-C(2) and C(1)-
C(2)-C(3) angles in complex 1 are 175.9(3) and 174.9-
(3)°, respectively.
Electr och em istr y. Cyclic voltammetry was used to
examine the electrochemistry of the bis(σ-arylacetylide)
complexes (Table 4). The cyclic voltammograms of
complexes 1-5 reveal two quasi-reversible oxidation
couples at E1/2 ) -0.83 to -0.68 V and E1/2 ) +0.54 to
+0.80 V vs Cp2Fe0/+ (Figure 4 for 2). For complex 2, ∆Ep
and the Ep,a/Ep,c ratio of the first couple approaches 0.07
V and 0.90, respectively, and a one-electron process was
established by constant-potential electrolysis. We assign
this couple to RuIII/II, and the half-reaction is
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg)
[Ru(16-TMC)(CtCC6H4OMe-p)2] (1)
Ru-N(1)
Ru-N(2)
N(1)-C(17)
2.271(2)
2.275(3)
1.486(4)
Ru-C(1)
C(1)-C(2)
C(2)-C(3)
2.077(3)
1.195(4)
1.439(4)
N(1)-Ru-N(2)
N(1)-Ru-N(2*)
N(1)-Ru-C(1)
N(1)-Ru-C(1*)
89.3(1)
90.7(1)
92.2(1)
87.8(1)
N(1)-Ru-N(1*)
Ru-C(1)-C(2)
C(1)-C(2)-C(3)
180.0
175.9(3)
174.9(3)
[Ru(16-TMC)(CtCC6H4Cl-p)2] (5)
Ru-N(1)
Ru-N(2)
N(1)-C(10)
2.273(3)
2.274(3)
1.492(6)
Ru-C(1)
C(1)-C(2)
C(2)-C(3)
2.073(4)
1.190(5)
1.440(5)
N(1)-Ru-N(2)
N(1)-Ru-N(2*)
N(1)-Ru-C(1)
N(1)-Ru-C(1*)
89.2(1)
90.8(1)
91.1(1)
88.9(1)
N(1)-Ru-N(1*)
Ru-C(1)-C(2)
C(1)-C(2)-C(3)
180.0
177.4(4)
174.2(4)
influence the π* orbital and hence the MLCT energy.
Similarly, if there is a significant dπ(Ru) f π*(CCAr)
back-bonding interaction in the trans-[Ru(CtCAr)2]
moiety, the Ru(II) electron density and hence the
E°(RuIII/II) value would also be affected by the energy
of the π*(CCAr) orbital. It is interesting to note that
the E1/2(RuIII/II) value of trans-[Ru(16-TMC)Cl2]+ (-0.60
[RuIII(16-TMC)(CtCAr)2]+ + e- f
[RuII(16-TMC)(CtCAr)2] (1)
The variation of the E1/2(RuIII/II) values for 1-5
corresponds to a red shift of the MLCT transition (except
for 4; see the discussion on absorption data). Changing
the substituents on the acetylide groups is expected to
V vs Cp2Fe0/+ 14
) is partially higher than those for 1-5.
In terms of σ-donor strength, ArCtC- is superior to Cl-
for the stabilization of Ru(III) species because of the
covalency of the RuIII-CCAr bond. Nevertheless, the

MLCT and LMCT Transitions in Acetylide Complexes
Organometallics, Vol. 18, No. 11, 1999 2079
Ta ble 4. Electr och em ica l Da ta for
[Ru (16-TMC)(CtCC6H4X-p)2]a
complex
E1/2(RuIV/III
)
E1/2(RuIII/II
)
1 (X ) OMe)
2 (X ) Me)
3 (X ) H)
4 (X ) F)
+0.54
+0.70
+0.76
+0.76
+0.80
-0.83
-0.80
-0.74
-0.72
-0.68
5 (X ) Cl)
a
E1/2 (V vs Cp2Fe0/+, scan rate 100 mV/s) in CH2Cl2 at 298 K
with 0.1 M NBun4PF6 as supporting electrolyte.
F igu r e 5. UV-vis spectral changes during electrochemical
oxidation (at -0.47 V vs Cp2Fe0/+) of 5 in 0.1 M NBun4PF6/
CH2Cl2 solution: (s) initial trace; (‚‚‚) trace after 10 min;
(-‚‚-) trace after 50 min.
implies that, for ruthenium(IV) species, ArCtC- is a
better donor than Cl- and metal-ligand π-bonding
interactions are presumably more prevalent for the
former. The electrochemical data also demonstrate the
merit of macrocyclic tertiary amine ligands for the
kinetic stabilization of highly oxidizing metal acetylide
complexes. In this study, the 16-TMC ligand may also
provide steric protection for the [Ru(CtCAr)2] moiety
from nucleophilic attack.
Oxid a tion . Introduction of dioxygen into a 1,2-
dichloroethane solution of 3 containing NH4PF6 afforded
[Ru(16-TMC)(CtCC6H5)(CO)]PF6 (6). Transformation of
an acetylide to a carbonyl ligand by oxidative cleavage
has precedent.13,27 The mass spectrum of 6 reveals a
cluster at m/z 515 which corresponds to the parent
cation. A low-field singlet at 202.8 ppm in the 13C NMR
spectrum and a strong IR absorption at 1916 cm-1 are
characteristic of a terminal carbonyl ligand.
F igu r e 4. Cyclic voltammogram of 2 vs Cp2Fe0/+ in CH2-
Cl2 at room temperature.
former is also a better π-acceptor than Cl- and can
accommodate RuII ions through increased π back-
bonding interactions.
The trans-[RuIII(16-TMC)(CtCAr)2]+ species can be
generated by electrolysis of the Ru(II) precursors 1-5
using thin-layer spectroelectrochemistry. Figure 5 de-
picts the spectral changes associated with the electro-
chemical oxidation of 5 at -0.47 V (the initial trace
shows that partial oxidation has occurred under the
reaction conditions). Isosbestic spectral changes are
recorded: the MLCT absorption band disappears and
is replaced by a new band at λmax 722 nm. As discussed
in the following section, this low-energy band is assigned
to a pπ(ArCC) f dπ*(RuIII) LMCT transition.
The complexes 1-5 can be chemically oxidized to
[RuIII(16-TMC)(CtCAr)2]+. Addition of methanolic (NH4)2-
Ce(NO3)6 to a dichloromethane solution of 1-5 caused
an immediate color change from yellow to green. As
shown in Figure 6, the UV-vis spectrum of trans-[RuIII
-
The second couple (+0.54 V for 1, +0.70 to +0.80 V
for 2-5) is less reversible, especially with slower scan
rates (<100 mV s-1). Because the peak currents for the
first and second couples are similar, we infer that the
latter also originates from a one-electron oxidation,
(16-TMC)(CtCC6H4OMe-p)2]+ (1+) obtained by Ce(IV)
oxidation closely resembles that by electrochemical
oxidation of 5 (Figure 5). Intense vibronically structured
absorptions with λmax varying from 716 to 768 nm are
observed for 1-5, with vibrational progressions in the
range 1730-1830 cm-1 (Table 2). These values are lower
than the ground-state ν(CtC) value but are consistent
with reduced CtC stretching frequencies associated
with an acetylide-to-Ru(III) charge transfer excited
state. We therefore tentatively assign these bands to a
pπ(ArCC) f dπ*(RuIII) ligand-to-metal charge transfer
(LMCT) transition. It is pertinent to compare these
absorptions with the LMCT transition energies of trans-
[Ru(14-TMC)(NCS)2]+ (λmax 594 nm in H2O),28 trans-
[Ru([14]aneN4)I2]+ (λmax 601 nm in DMSO),29 and trans-
[Ru(en)2I2]+ (λmax 573 nm in MeOH, en ) 1,2-diamino-
namely RuIV/III
[RuIV(16-TMC)(CtCAr)2]2+ + e- f
[RuIII(16-TMC)(CtCAr)2]+ (2)
:
This assignment is supported by the fact that the E1/2
values for complexes 2-5 are comparable, despite
varying the phenyl para substituents (CH3, H, F, and
Cl). The 16-TMC ligand is known to be electrochemically
inactive upon coordination to a metal ion.15 The notice-
ably lower E1/2(RuIV/III) value for complex 1 can be
rationalized by the mesomeric effect of the methoxy
substituent in the electrochemically generated [RuIV(16-
TMC)(CtCC6H4OMe-p)2]2+ species, where π-bonding
between RuIV and the acetylide group is stabilized by
(27) (a) Oro, L. A.; Ciriano, M. A.; Campo, M.; Foces-Foces, C.; Cano,
F. H. J . Organomet. Chem. 1985, 289, 117. (b) Bruce, M. I.; Swincer,
A. G.; Wallis, R. C. J . Organomet. Chem. 1979, 171, C5.
(28) Che, C. M.; Kwong, S. S.; Poon, C. K. Inorg. Chem. 1985, 24,
1601.
electron donation from the OMe moiety. The E1/2(RuIV/III
)
values for 1-5 are significantly lower than that for
trans-[Ru(16-TMC)Cl2]2+ (1.16 V vs Cp2Fe0/+).14 This
(29) Poon, C. K.; Che, C. M. Inorg. Chem. 1981, 20, 1640.

2080 Organometallics, Vol. 18, No. 11, 1999
Choi et al.
material chemistry and electronic communication. In
this work, a systematic series of ruthenium(II) bis(σ-
arylacetylide) complexes supported by 16-TMC has been
synthesized from [RuIII(16-TMC)Cl2]Cl. Trends in spec-
troscopic, structural, and electrochemical data have
been examined. Low ν(CtC) stretches are observed in
the IR spectra, while the cyclic voltammograms of the
bis(acetylide) derivatives reveal two quasi-reversible
oxidation couples, namely RuIII/II and RuIV/III. The role
of the tertiary amine ligand 16-TMC is apparent: its
strong σ-donating strength is manifested in substantial
metal-to-acetylide π back-bonding, while its ability to
kinetically stabilize highly oxidized species, in this case
[RuIV(CtCAr)2]2+, is exemplified. Oxidation of the Ru-
(II) acetylide complexes by Ce(IV) or by electrochemical
means yielded low-energy vibronic UV-vis absorption
bands, which are tentatively assigned as pπ(ArCC) f
dπ*(RuIII) LMCT transitions.
F igu r e 6. UV-vis absorption spectrum of 1 in CH2Cl2
after addition of methanolic (NH4)2Ce(NO3)6 at room tem-
perature.
ethane).30 Since the natures of the ancillary amine
ligands are similar, these data suggest that the optical
electronegativity of the pπ electrons in ArCtC- is
comparable to those in NCS- and I-. Attempts to isolate
the trans-[RuIII(16-TMC)(CtCAr)2]+ complexes were
unsuccessful, presumably because the +3 oxidation
state in this system readily transforms to other Ru(II)
species. For example, we have identified the Ru(II)
carbonyl complex 6 as one of the products in the
oxidation of the phenyl derivative 3.
Ack n ow led gm en t. We thank the University of
Hong Kong (for a postgraduate studentship to M.-Y.C.)
and the Hong Kong Research Grants Council for sup-
port. We are grateful to Dr. V. M. Miskowski for helpful
discussions.
Su p p or tin g In for m a tion Ava ila ble: A figure giving the
ORTEP plot of 3 and tables of crystal data, atomic coordinates,
calculated hydrogen coordinates, anisotropic displacement
parameters, and bond distances and angles for 1, 3, and 5.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Con clu sion . trans-Bis(acetylide)metal complexes have
potential applications as linear rigid-rod motifs in
(30) Poon, C. K.; Che, C. M. J . Chem. Soc., Dalton Trans. 1980, 756.
OM990009D