Axial butadiynyl adducts on a tetrakis(di(m-methoxyphenyl)formamidinato)diruthenium core: First examples of M-M bonded complexes containing σ-poly-ynyl ligand

Article abstract of DOI:10.1021/ic0012930

The transmetalation reaction between chlorotetrakis(di(m-methoxyphenyl)formamidinato)diruthenium(II,III) and excess LiC-C-C-CSiMe₃ resulted in the concurrent formation of complexes beating one (1) and two (2) axial C-C-C-CSiMe₃ ligands.

Full text of DOI:10.1021/ic0012930

Inorg. Chem. 2001, 40, 2925-2927
2925
Scheme 1a
Axial Butadiynyl Adducts on a Tetrakis-
(di(m-methoxyphenyl)formamidinato)diruthenium
Core: First Examples of M-M Bonded
Complexes Containing σ-Poly-ynyl Ligand
a I, simple rods with [Ru2(ap)4] termini, m ) 1 and 2. II,
oligometallayne rigid rods; m and n ) integers.
Guolin Xu and Tong Ren*
chain in I, a desirable feature for the construction of an
oligometallayne (II in Scheme 1). However, our synthetic effort
toward II was hampered by the low solubility of [Ru2(µ-ap)4]+-
containing species. In the search for more soluble diruthenium
species that retain the essential structural features of I, we inves-
tigated the reactivity of Ru2(DArF)4Cl toward Li(CtC)mSiR3
reagents. Details of the isolation and characterization of both
mono- and bisbutadiynyl adducts of the tetrakis(di(m-meth-
oxyphenyl)formamidinato)diruthenium core (Ru2(DmAniF)4,
DmAniF ) di-m-methoxyphenylformamidinate) are presented.
Department of Chemistry, University of Miami,
P.O. Box 249118, Coral Gables, Florida 33124-0431
ReceiVed NoVember 15, 2000
Interest in the synthesis and characterization of M-M bonded
species bearing σ-alkynyl ligands continues from the first such
example reported by Cotton in 1986,1 [Ru2(ap)4](CtCPh),
where ap is 2-anilinopyridinate and the phenylethynyl group is
axially bonded to the Ru(II,III) core. Subsequent work by Bear
et al. demonstrated that a variety of axial alkynyl complexes of
both dirhodium and diruthenium cores can be isolated,2,3 and
both the mono adduct and bis adduct may be obtained with a
diruthenium core. Coordination of alkynyls at equatorial posi-
tions of the M2 core has been demonstrated by Hopkins et al.,4
where a significant δ(MM)-π(CC) conjugation was noted.
Our contributions to this field include the elucidation of the
linear free-energy relationship in Ru2(DArF)4(CtCR)n com-
pounds (DArF ) diarylformamidinate and n ) 1 and 2)5 and
the syntheses and characterization of Ru2(ap)4(CtCR) with R
) SiMe3, H, and CH2OCH3.6 More recently, we published the
first examples of a carbyne chain capped by bimetallic termini,
[Ru2(ap)4]2(µ-(CtC)m) (I in Scheme 1),7 where the degree of
electronic delocalization across the carbyne bridges is compa-
rable with the highest degree of electronic delocalization
reported for carbyne chains capped with mononuclear termini.8
The diruthenium terminus is advantageous over the mononuclear
termini as a result of its extraordinary redox flexibility and the
availability of an open axial site trans to the existing carbyne
Results and Discussion
Treating Ru2(DmAniF)4Cl9 with 3 equiv of LiC4SiMe3 and
exposing the reaction mixture to air afforded Ru2(DmAniF)4-
(C4SiMe3) (1) and Ru2(DmAniF)4(C4SiMe3)2 (2) in a combined
yield of 70%. As elaborated in earlier work on phenylethynyl
adducts,3,5 there exists an equilibrium between the mono ad-
duct and the anionic form of the bis adduct ([Ru2(DmAniF)4-
(C4SiMe3)2]1- in this study) when the reaction mixture is
maintained under an inert atmosphere. The anion of the bis
adduct is subsequently converted to the neutral form when the
reaction mixture is exposed to air. While the yields of mono-
phenylethynyl adducts and bisphenylethynyl adducts of a
Ru2(DArF)4 core are dependent on the postsynthesis treatment
of the reaction mixture,3,5 the yields of 1 and 2 are independent
of the workup procedure. Unlike the phenylethynyl analogues,3,5
both compounds 1 and 2 are thermally stable under aerobic
conditions. Molecule 1 has an effective magnetic moment of
4.09 µB at room temperature, indicative of the S ) 3/2 ground
state common to this class of Ru2(II,III) species.10 Molecule 2,
a Ru2(III,III) species, displays a well-resolved 1H NMR spectrum
and is diamagnetic. Formulation of monobutadiynyl adduct and
bisbutadiynyl adduct is confirmed by the study of crystal and
molecular structures of 1 and 2, as shown in Figures 1 and 2,
respectively. The relevant topological parameters for molecules
1 and 2 are listed in Tables 1 and 2.
While the coordination geometry of the bridging formamidi-
nates around the Ru2 core in 1 is similar to that in the parent
compound Ru2(DmAniF)4Cl,9 the Ru-Ru bond distance (2.5060-
(5) Å) is increased by 0.12 Å from that of the parent compound
(2.3855(8) Å).9 In contrast, the elongation of the Ru-Ru bond
upon formation of Ru2(DArF)4(CtCPh)3,5 and Ru2(ap)4-
(CtCR) (R ) Ph and SiMe3)1,6 is generally less than 0.04 Å.
The significant elongation of the Ru-Ru bond length in 1 is
attributed to the strong electron-withdrawing nature of the
additional CtC bond, which siphons the σ-bonding elec-
tron density away from the Ru2 core and hence weakens the
σ(Ru-Ru) bond.
* Corresponding author tel: (305)284-6617; fax: (305)284-1880;
e-mail: tren@miami.edu.
(1) Chakravarty, A. R.; Cotton, F. A. Inorg. Chim. Acta 1986, 113, 19.
(2) (a) Yao, C. L.; Park, K. H.; Khokhar, A. R.; Jun, M. J.; Bear, J. L.
Inorg. Chem. 1990, 29, 4033. (b) Bear, J. L.; Han, B.; Huang, S. J.
Am. Chem. Soc. 1993, 115, 1175. (c) Li, Y.; Han, B.; Kadish, K. M.;
Bear, J. L. Inorg. Chem. 1993, 32, 4175. (d) Bear, J. L.; Li, Y. L.;
Han, B. C.; VanCaemelbecke, E.; Kadish, K. M. Inorg. Chem. 1997,
36, 5449.
(3) Bear, J. L.; Han, B.; Huang, S.; Kadish, K. M. Inorg. Chem. 1996,
35, 3012.
(4) (a) Stoner, T. C.; Dallinger, R. F.; Hopkins, M. D. J. Am. Chem. Soc.
1990, 112, 5651. (b) Stoner, T. C.; Geib, S. J.; Hopkins, M. D. J. Am.
Chem. Soc. 1992, 114, 4201. (c) Stoner, T. C.; Geib, S. J.; Hopkins,
M. D. Angew Chem., Int. Ed. Engl. 1993, 32, 409. (d) Stoner, T. C.;
Schaefer, W. P.; Marsh, R. E.; Hopkins, M. D. J. Cluster Sci. 1994,
5, 107. (e) John, K. D.; Geib, S. J.; Hopkins, M. D. Organometallics
1996, 15, 4357. (f) John, K. D.; Stoner, T. C.; Hopkins, M. D.
Organometallics 1997, 15, 44948.
(5) (a) Lin, C.; Ren, T.; Valente, E. J.; Zubkowski, J. D. J. Chem. Soc.,
Dalton Trans. 1998, 571. (b) Lin, C.; Ren, T.; Valente, E. J.;
Zubkowski, J. D. J. Organomet. Chem. 1999, 579, 114.
(6) Zou, G.; Alvarez, J. C.; Ren, T. J. Organomet. Chem. 2000, 596, 152.
(7) Ren, T.; Zou, G.; Alvarez, J. Chem. Commun. 2000, 1197.
(8) (a) Le Narvor, N.; Toupet, L.; Lapinte, C. J. Am. Chem. Soc. 1995,
117, 7129. (b) Brady, M.; Weng, W.; Zou, Y.; Seyler, J. W.; Amoroso,
A. J.; Arif, A. M.; Bohme, M.; Frenking, G.; Gladysz, J. A. J. Am.
Chem. Soc. 1997, 119, 775. (c) Kheradmandan, S.; Heinze, K.;
Schmalle, H. W.; Berke, H. Angew Chem., Int. Ed. 1999, 38, 2270.
(d) Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J.-F.; Best, S. P.;
Heath, G. A. J. Am. Chem. Soc. 2000, 122, 1949.
In the bis adduct of 2, the Ru-Ru distance is 2.5990(3) Å,
a slight increase from that in Ru2(DArF)4(CtCPh)2 (2.55-2.56
(9) (a) Lin, C.; Ren, T.; Valente, E. J.; Zubkowski, J. D.; Smith, E. T.
Chem. Lett. 1997, 753. (b) Lin, C. Ph.D. Thesis, Florida Institute of
Technology, 1997.
(10) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms;
Oxford University Press: Oxford, 1993.
10.1021/ic0012930 CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/04/2001

2926 Inorganic Chemistry, Vol. 40, No. 12, 2001
Notes
Table 2. Crystallographic Data for Compounds 1 and 2
1
2
chemical formula
fw
space group
a, Å
b, Å
c, Å
C67H69N8O8SiRu2
1344.53
Pbca (No. 61)
25.7964(13)
16.4755(8)
30.6770(15)
90
90
90
13038(1)
8
C74H78N8O8Si2Ru2
1465.7
P1h (No. 2)
10.8663(4)
10.8991(4)
16.0803(6)
92.728(1)
90.921(1)
103.121(1)
1851.9(1)
1
R, deg
â, deg
γ, deg
vol, Å3
Z
T, °C
27
0.71073
1.370
5.41
0.0385, 0.0892
0.0733, 0.1197
27
0.71073
1.314
4.98
0.0296, 0.0732
0.0366, 0.0764
λ(Mo KR), Å
Figure 1. ORTEP plot of 1 at 20% probability level.
D
calcd, g/cm-3
µ, cm-1
R1, wR2a (I > 2σ(I))
R1, wR2 (all data)
2
2
a R1 ) [∑w(Fo - Fc)2/∑wFo ]1/2, wR2 ) [∑w(Fo - Fc2)2/
∑w(Fo )2]1/2
.
2
Figure 2. ORTEP plot of 2 at 20% probability level.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 1 and 2
1
2
Ru(1)-Ru(2)
Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-N(3)
Ru(1)-N(4)
Ru(2)-N(5)
Ru(2)-N(6)
Ru(2)-N(7)
Ru(2)-N(8)
Ru(1)-C(1)
C(1)-C(2)
2.5060(5) Ru(1)-Ru(1A)
2.054(3) Ru(1)-C(1)
2.065(3) Ru(1)-N(2)
2.060(3) Ru(1)-N(3)
2.064(3) Ru(1)-N(1)
2.039(3) Ru(1)-N(4)
2.019(3) C(1)-C(2)
2.032(4) C(2)-C(3)
2.031(3) C(3)-C(4)
2.027(5)
2.5990(3)
1.947(2)
2.0062(18)
2.027(2)
2.0709(19)
2.1203(18)
1.206(3)
Figure 3. Cyclic voltammograms of compounds 1 and 2 recorded at
a scan rate of 100 mV/s in 0.20 M (n-Bu)4NPF6 solution (THF, N2-
degassed) with a glassy carbon working electrode, a Pt-wire auxiliary
electrode, and an Ag/AgCl reference electrode.
1.371(4)
1.196(4)
1.207(6)
1.377(7)
1.205(7)
mediated by the filled π-π interaction across the Ru-Ru bond.
Similar to the phenylethynyl analogues, the arrangement of the
bridging formamidinate ligands around the Ru2 core is severely
distorted, which was attributed to a second-order Jahn-Teller
effect.5 The butadiynyl ligands are bent away from the Ru-Ru
vector, but the Ru-Ru-CR angle (164°) is closer to 180° than
that found in phenylethynyl adducts (ca. 159°).
C(2)-C(3)
C(3)-C(4)
N(1)-Ru(1)-Ru(2) 88.33(9)
N(2)-Ru(1)-Ru(2) 88.10(9)
N(3)-Ru(1)-Ru(2) 86.00(9)
N(4)-Ru(1)-Ru(2) 85.13(9)
N(5)-Ru(2)-Ru(1) 86.84(9)
N(1)-Ru(1)-Ru(1A) 82.59(5)
N(2)-Ru(1)-Ru(1A) 95.10(5)
N(3)-Ru(1)-Ru(1A) 90.66(5)
N(4)-Ru(1)-Ru(1A) 78.55(5)
C(1)-Ru(1)-Ru(1A) 164.34(7)
N(6)-Ru(2)-Ru(1) 86.96(10) C(2)-C(1)-Ru(1)
N(7)-Ru(2)-Ru(1) 89.43(10) C(1)-C(2)-C(3)
N(8)-Ru(2)-Ru(1) 90.39(10) C(4)-C(3)-C(2)
C(1)-Ru(1)-Ru(2) 175.34(12)
175.2(2)
177.3(3)
179.2(5)
The rich redox characteristics of compounds 1 and 2 were
revealed with the measurement of cyclic voltammograms (CV,
Figure 3). The bis adduct of 2 undergoes two reversible one-
electron reductions at -0.32 V (B) and -1.32 V (C) and one
irreversible oxidation at 1.17 V (A). When the potential range
is limited within 1.20 V, the mono adduct of 1 displays two
reversible one-electron processes, an oxidation at 0.63 V (B)
and a reduction at -0.63 V (C). However, the reversible waves
disappear when the range of potential sweep is extended beyond
1.2 V, indicating an oxidative degradation of 1 in the high
potential range. The observed redox couples in both 1 and 2
are assigned to the formal oxidation states of the Ru2 core as
follows:
C(2)-C(1)-Ru(1) 175.9(4)
C(1)-C(2)-C(3)
C(4)-C(3)-C(2)
176.4(6)
178.5(7)
Å).3,5 The Ru-CR bond length (1.947(2) Å) is decreased from
that of the phenylethynyl analogues (1.99(1) Å). These structural
changes are consistent with an enhancement of Ru-CR bonding
at the expense of Ru-Ru bonding. Since trimethylsilylbutadiy-
nyl is a weaker nucleophile than phenylethynyl, the stronger
Ru-CR bonding in 2 is likely due to a significant filled π-π
interaction.11 As additional evidence of the presence of filled
π-π interactions, 2 exhibits a very intense CtC stretch in the
IR spectrum. Since most σ-alkynyl complexes exhibit only a
weak CtC stretch,12 the observed intense CtC stretch implies
a constructive interference between two trans butadiynyls
(11) (a) Lichtenberger, D. L.; Renshaw, S. K.; Wong, A.; Tagge, C. D.
Organometallics 1993, 12, 3522. (b) Lichtenberger, D. L.; Renshaw,
S. K.; Bullock, R. M. J. Am. Chem. Soc. 1993, 115, 3276.
(12) Manna, J.; John, K. D.; Hopkins, M. D. AdV. Organomet. Chem. 1995,
38, 79.

Notes
Inorganic Chemistry, Vol. 40, No. 12, 2001 2927
-
-
- e z
y
- e z
mmetric analyzer with a glassy carbon working electrode, a Pt wire
auxiliary electrode, and a Ag/AgCl reference electrode in 0.2 M
n-Bu4NPF6 in THF. Single crystals of X-ray quality were obtained for
both 1 and 2 by slow evaporation of fractions from column chroma-
tography purification (hexanes/ethyl acetate). Diffraction data were
collected on a Bruker SMART1000 CCD diffractometer at ambient
temperature, and the structures were solved and refined using the Bruker
SHELXTL (Version 5.1) software package.15
Synthesis of Compounds 1 and 2. To a solution of 0.291 g (1.5
mmol) of bis(trimethylsilyl)butadiyne in 20 mL of THF at -78 °C
was added 0.63 mL of LinBu (1.6 M in hexane). The reaction was
allowed to warm to room temperature to yield an off-white solution,
which was transferred to a 40-mL THF solution of Ru2(DmAniF)4Cl
(0.630 g, 0.5 mmol). The reaction mixture was stirred at room
temperature overnight and became a clear deep purple solution. TLC
analysis (4:1 hexane/ethyl acetate) revealed the formation of two major
products: 1 (Rf ) 0.60) and 2 (Rf ) 0.80) and the absence of the starting
ruthenium compound. After being exposed to air for 20 min. the solvent
was removed in vacuo, and the residue was purified by flash column
chromatography using a linear gradient eluent (hexanes/ethyl acetate,
95:5-5:1, v/v) to yield 1 (0.283 g, 42% based on Ru) and 2 (0.205 g,
28%) as purple crystalline solids.
Data for 1. Anal. Calcd for C67H69N8O8SiRu2: C, 59.73; H, 5.13;
N, 8.32. Found: C, 59.47; H, 5.25; N, 8.07. MS-FAB (m/z): [M+]
calcd for 101Ru, 1346. UV-vis λmax, nm (ꢀ): 771.0 (sh), 564.9 (7682.5).
IR (cm-1, KBr disk): 2154.5 (w), 2103.9 (m).
Data for 2. Anal. Calcd for C74H78N8O8Si2Ru4: C, 60.53; H, 5.35;
N, 7.63. Found: C, 60.33; H, 5.41; N, 7.42. MS-FAB (m/z): [M+]
calcd for 101Ru, 1467. 1H NMR δ: 8.19 (s, 4H, -NCHN), 7.13 (t, 8H,
Ph-H), 6.70 (q, 8H, Ph-H), 6.56 (t, 8H, Ph-H), 6.19 (d, 8H, Ph-H),
3.48 (s, 24H, CH3O), 0.09 (s, 18H, (CH3)3Si). UV-vis λmax, nm (ꢀ):
920.03 (sh), 575.42(11 089), 516.19 (12 053). IR (cm-1, KBr disk):
2183.5 (m), 2118.7 (s).
-
- e
y
Ru2(II,II)
Ru2(II,III)
Ru2(III,III)
Ru (IV,III)
8
2
+ e-
+ e-
A
C
B
It is clear that the potentials of the B and C couples in molecule
2 have been cathodically shifted by about 0.95 and 0.69 V from
that of 1, and the cathodic shifts reflect the further stabilization
of the Ru2 core by an additional butadiynyl ligand, a strong
nucleophile. It is noteworthy that the first reduction potential
of 2 is nearly zero, indicating a high electron affinity in solution.
As pointed out by Michl et al.,13 high electron affinity is an
essential prerequisite for the modules of type II rods as
molecular wires.
Molecules 1 and 2 represent the first examples of M-M
bonded compounds bearing poly-ynyl ligands and exhibit
significant differences from alkynyl analogues in both structure
and thermal stability. Synthetic methodologies for 1 and 2 may
be extended to analogues bearing longer poly-ynyls, such as
1,3,5-hexatriynyl and 1,3,5,7-octatetraynyl, which are being
explored in our laboratory. Preliminary studies indicate that the
terminal trimethylsilyl groups in both 1 and 2 can be easily
removed to yield the unprotected terminal alkyne (tCsH),
which may undergo further oxidative coupling to yield extended
rods (II in Scheme 1) under the Glaser-Hay-Eglinton condi-
tions.14
Experimental Section
n-Butyllithium (1.6 M in hexane) and 1,4-bis(trimethylsilyl)-1,3-
butadiyne were purchased from Aldrich. Syntheses were generally
performed in a dry argon atmosphere using standard Schlenk-line
techniques. NMR (1H and 13C), IR, and UV/Vis spectra were recorded
on a Bruker AVANCE300 NMR spectrometer, a Perkin-Elmer 2000
FT-IR spectrometer (using KBr disks), and a Perkin-Elmer Lambda900
UV/Vis/NIR spectrophotometer, respectively. Magnetic susceptibility
was measured with a Johnson Matthey MarkII magnetic susceptibility
balance. Cyclic voltammograms were recorded on a CHI620A volta-
Acknowledgment. We thank the University of Miami for
generous support (startup grant, fund for diffractometer, and
General Research Award).
Supporting Information Available: X-ray crystallographic files,
in CIF format, for the structure determination of both 1 and 2. This
material is available free of charge via the Internet at http:/pubs.acs.org.
IC0012930
(13) Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. ReV. 1999, 99, 1863.
(14) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed.
2000, 39, 2632.
(15) SHELXTL 5.03 (WINDOW-NT Version), Program Library for
Structure Solution and Molecular Graphics; Bruker-AXS Inc., 1998.