Binuclear ruthenium macrocycles formed via the weak-link approach

Article abstract of DOI:10.1021/ic048975y

The weak-link approach for the synthesis of metallomacrocycles has been used to synthesize a series of novel Ru(II) macrocycles in high yield. RuCl₂(PPh₃)₃ has been reacted with two different phosphino-alkyl-ether hemilabile ligands, 1,4-(PPh₂(CH ₂)₂O)₂C₆H₄ and 1,4-(PPh₂(CH₂)₂OCH₂) ₂C₆H₄. The hemilabile bidentate ligand coordinates to Ru(II) centers through both the P and O atoms to form bimetallic condensed intermediates . The weak Ru-O bonds have been selectively cleaved with CO, 1,2-diaminopropane, and pyridine to yield large open macrocycles. This is the first example of the weak-link approach employed to synthesize macrocycles with Ru, and metal centers in general that have more than four coordination sites.

Full text of DOI:10.1021/ic048975y

Inorg. Chem. 2005, 44, 496−501
Binuclear Ruthenium Macrocycles Formed via the Weak-Link Approach
Meisa S. Khoshbin,† Maxim V. Ovchinnikov,† Chad A. Mirkin,*,† Lev N. Zakharov,‡ and
Arnold L. Rheingold‡
Department of Chemistry and Institute for Nanotechnology, Northwestern UniVersity,
2145 Sheridan Road, EVanston, Illinois 60208, and the Department of Chemistry and
Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093
Received July 28, 2004
The “weak-link approach” for the synthesis of metallomacrocycles has been used to synthesize a series of novel
Ru(II) macrocycles in high yield. RuCl2(PPh3)3 has been reacted with two different phosphino-alkyl-ether hemilabile
ligands, 1,4-(PPh2(CH2)2O)2C6H4 and 1,4-(PPh2(CH2)2OCH2)2C6H4. The hemilabile bidentate ligand coordinates to
Ru(II) centers through both the P and O atoms to form bimetallic “condensed intermediates”. The weak Ru−O
bonds have been selectively cleaved with CO, 1,2-diaminopropane, and pyridine to yield large open macrocycles.
This is the first example of the weak-link approach employed to synthesize macrocycles with Ru, and metal centers
in general that have more than four coordination sites.
Scheme 1. Weak Link Approach
Introduction
Over the past decade, major advances have been made in
the field of supramolecular chemistry. A significant com-
ponent of the field is coalescing around the challenge of
developing synthetic routes for rationally synthesizing struc-
tures with well-defined shape, chirality, and function, and
then using such control for preparing molecules with unusual
and potentially useful catalytic and molecular sensing
properties.1 Most recently, a novel type of allosteric catalyst
was reported that behaves like a synthetic metalloenzyme,
which demonstrates the type of sophisticated molecular
function possible via the supramolecular coordination chem-
istry approach.2
ogy, metallomacrocycles are prepared in high yield from
transition metal precursors and flexible organic ligands
containing strong and weak metal bonding groups capable
of chelating to a metal center of interest to template the
formation of a condensed intermediate.3,4 The condensed
intermediate can be subsequently opened into a targeted
macrocyclic structure through ligand substitution reactions
involving small molecules that can break the weak metal-
ligand interactions while keeping the strong ones intact. This
approach has predominantly focused on the preparation of
complexes with four-coordinate, square-planar transition
metals, namely rhodium(I), palladium(II), and iridium(I), and
more recently tetrahedral copper(I) complexes.3a,g,k,p
Our group has developed the “weak-link approach” to
supramolecular structures, Scheme 1.2,3 With this methodol-
* Author to whom correspondence should be addressed. E-mail:
chadnano@northwestern.edu.
† Northwestern University.
‡ University of California, San Diego.
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1991. (d) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000,
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By including higher-coordinate metal centers into this
approach, one could, in principle, take advantage of the
additional coordination sites in stoichiometric and catalytic
reactions. Herein, we introduce the use of ruthenium(II), a
metal that often prefers an octahedral coordination sphere,
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496 Inorganic Chemistry, Vol. 44, No. 3, 2005
10.1021/ic048975y CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/11/2005

Weak-Link Approach to Binuclear Ruthenium Macrocycles
Scheme 2. Synthetic Outline for Closed Ru(II) Macrocycles 2 and 3
Scheme 3. Synthetic Outline for Open Ru(II) Macrocycles 4 and 5
at room temperature.11 All other solvents were purified by published
methods.12 Deuterated solvents were purchased from Cambridge
Isotope Laboratories Inc. and used as received. 1,4-(2-Diphenyl-
phosphinoethoxy) benzene3c and R,R′-dichloroethoxy-p-xylene13
were prepared according to literature methods. RuCl2(PPh3)3 was
purchased from Strem Chemicals and used as received. All other
chemicals were used as received from Aldrich Chemical Co.
into the weak-link approach, and a variety of ancillary ligands
that can be used to tailor the electronic and steric properties
of the metal centers within the resulting macrocyclic
structures (Scheme 2). Mononuclear ruthenium(II) systems
with hemilabile phosphine-ether ligands have been exten-
sively studied.5-10 It has been shown that the weak Ru-O
bondcanbeeasilycleavedbyavarietyofsmallmolecules,7a,h,i,8
including CO5,6,7c-g,10c and nitrogen-based ligands.7j This
knowledge has been used to synthesize two new bimetallic
condensed macrocycles via the weak link approach (Schemes
3 and 4).
1
Physical Measurements. H NMR spectra were recorded on a
Varian Gemini 300 MHz FT-NMR spectrometer and referenced
relative to residual proton resonances in CDCl3 or CD2Cl2. 31P{1H}
NMR spectra were recorded on a Varian Gemini 300 MHz FT-
NMR spectrometer at 121.53 MHz and referenced relative to an
external 85% H3PO4 standard. All chemical shifts are reported in
ppm. FT-IR spectra were obtained in solution with a Thermo Nicolet
Nexus 670 FT-IR with NaCl cells with 0.1-mm spacers or CsI cells
with 0.5-mm spacers for far IR work. They were collected in the
Experimental Section
General Procedures. Unless otherwise noted, all reactions were
carried out under a nitrogen atmosphere in reagent grade solvents,
using standard Schlenk techniques or an inert atmosphere glovebox
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Sommer, R. D.; Rheingold, A. L. Organometallics, 2002, 21, 5713-
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Inorganic Chemistry, Vol. 44, No. 3, 2005 497

Khoshbin et al.
Scheme 4. Synthetic Outline for Open Ru(II) Macrocycles 6 and 7
solid state with the same instrument using a KBr pellet or a CsI
pellet for far IR work. Electrospray mass spectra (ESMS) were
recorded on a Micromas Quatro II triple quadrapole mass spectro-
meter. Fast atom bombardment (FAB) and electron ionization mass
spectra (EIMS) were recorded on a Fisions VG 70-250 SE mass
spectrometer. Elemental analyses were performed by Quantitative
Technologies, Inc., Whitehouse, NJ.
Synthesis of [r,r′-Bis(2-diphenylphosphinoexthoxy)-p-xylene]2-
Ru2Cl4 (3). A solution of 1 (29 mg, 0.052 mmol) in CH2Cl2 (25
mL) was added dropwise over 30 min to a solution of RuCl2(PPh3)3
(50 mg, 0.052 mmol) in CH2Cl2 (25 mL). The solution was stirred
for an additional 2 h. The deep red solution was concentrated to
approximately 2 mL under reduced pressure. Addition of Et2O (80
mL) caused the precipitation of solid 3, which was collected by
filtration and washed with additional Et2O (5 mL). This process
was repeated once to completely remove excess PPh3. Yield: 60.4
Synthesis of r,r′-Bis(2-diphenylphosphinoexthoxy)-p-xylene
(1). A solution of KPPh2 in THF (0.5 M, 25.5 mL, 12.7 mmol)
was added dropwise to a stirring solution of R,R′-dichloroethoxy-
p-xylene (1.68 g, 6.4 mmol) in THF (50 mL). The orange solution
was passed through a column of alumina to remove the KCl, and
the solvent was removed in vacuo. The crude product was washed
with EtOH (20 mL) and then recrystallized from CH2Cl2 and
hexanes. The product was dried, yielding 1.78 g (49% yield) of
analytically pure white powder. 1H NMR: (CDCl3) δ 2.43 (t, 4H,
CH2P, JH-H ) 7.8 Hz), 3.60 (q, 8H, CH2CH2O, JH-H ) 7.2 Hz),
4.46 (s, 4H, CH2O), 7.22-7.45 (complex m, 24H, Ph-H). 31P{1H}
NMR: (CDCl3) δ -21.07 (s). C36H36O2P2: calcd 76.85% C, 6.45%
H; found 76.91% C, 6.34% H. EI-MS: [M] calcd: 562.2, expt:
562.2 m/z.
1
mg, 0.041 mmol, 79%. H NMR: (CD2Cl2) δ 2.98 (br m, 8H,
CH2P), 4.05-4.12 (br m, 8H, CH2CH2O), 5.13 (s, 8H, CH2O),
7.08-7.30 (complex m, 48H, Ph-H). 31P{1H} NMR: (CD2Cl2) δ
64.2 (s). C72H72Cl4O4P4Ru2: calcd 58.86% C, 4.94% H; found
58.53% C, 4.62% H. ESMS: [M - 2 Cl-]2+ calcd ) 700.1, expt
) 700.3 m/z.
Synthesis of [1,4-Bis(2-diphenylphosphinoethoxy)benzene]2Ru2-
Cl4(CO)4 (4). A solution of complex 2 (10.0 mg, 0.007 mmol) in
CD2Cl2 (0.7 mL) was bubbled with CO (1 atm) until a color change
from bright pink to bright yellow occurred. Compound 4 precipi-
tated from solution as a pale yellow powder. Yield: 9.6 mg, 0.006
mmol, 90%. 1H NMR: (CDCl3) δ 3.13 (br m, 8H, CH2P), 4.14 (br
m, 8H, CH2O), 6.58 (s, 8H, C6H4), 7.19-7.26 (m, 24H, P(C6H5)2),
7.60-7.78 (m, 16H, P(C6H5)2). 31P{1H} NMR: (CD2Cl2) δ 15.2
(s). FTIR: (KBr pellet used for CO stretches; CsI pellet used for
Ru-Cl stretch) ν(CO) 2002 cm-1 (s), ν(CO) 1944 cm-1 (m), ν(Ru-
Cl) 333 cm-1. C72H64Cl4O8P4Ru2: calcd 56.69% C, 4.23% H; found
56.17% C, 4.15% H. FAB-MS: [M - CO - Cl]+ calcd ) 1463.1,
expt ) 1463.6 m/z; [M - 2CO - Cl]+ calcd ) 1435.0, expt )
1434.5 m/z.
Synthesis of [1,4-Bis(2-diphenylphosphinoethoxy)benzene]2-
Ru2Cl4 (2). A solution of 1,4-(2-di-phenylphosphinoethoxy)-
benzene (28 mg, 0.052 mmol) in CH2Cl2 (25 mL) was added
dropwise over 30 min to a solution of RuCl2(PPh3)3 (50 mg, 0.052
mmol) in CH2Cl2 (25 mL). The solution was stirred for an additional
2 h. The deep red solution was concentrated to approximately 2
mL under reduced pressure. Addition of Et2O (80 mL) caused the
precipitation of solid 2, which was collected by filtration and washed
with additional Et2O (5 mL). This process was repeated twice to
completely remove excess PPh3. Yield: 60.2 mg, 0.043 mmol; 82%.
1H NMR: (CD2Cl2) δ 2.85-2.98 (br m, 8H, CH2P), 4.52-4.59
(br m, 8H, CH2O), 6.99 (s, 8H, C6H4), 7.07-7.13 (m, 24H,
P(C6H5)2), 7.30-7.38 (m, 16H, P(C6H5)2). 31P{1H} NMR: (CD2Cl2)
δ 64.1 (s). C68H64Cl4O4P4Ru2: calcd 57.79% C, 4.57% H, 9.91%
Cl, 8.77% P; found 58.30% C, 4.54% H, 10.04% Cl, 9.32% P.
ESMS: [M - 2 Cl-]2+ calcd ) 671.1, expt ) 671.0 m/z. Crystals
of 2 were grown by slow diffusion of Et2O vapor into a dilute
solution of 2 in CH2Cl2 at room temperature.
Synthesis of [r,r′-Bis(2-diphenylphosphinoexthoxy)-p-xylene]2-
Ru2Cl4(CO)4 (5). A solution of complex 3 (10 mg, 0.007 mmol)
in CD2Cl2 (0.7 mL) was bubbled with CO (1 atm) until a color
change from bright pink to bright yellow occurred. The solvent
was removed in vacuo to yield a pale yellow powder. Yield: 10.2
1
mg, 0.006 mmol, 92%. H NMR: (CD2Cl2) δ 3.05 (br m, 8H,
CH2P), 3.60 (br m, 8H, CH2CH2O), 4.31 (s, 8H, CH2O), 7.05 (br
s, 8H, Ph-H), 7.41-7.72 (complex m, 40H, Ph-H). 31P{1H} NMR:
(CD2Cl2) δ 14.6 (s). FTIR: (NaCl solution cell used for CO
stretches; CsI solution cell used for Ru-Cl stretch; CH2Cl2)
498 Inorganic Chemistry, Vol. 44, No. 3, 2005

Weak-Link Approach to Binuclear Ruthenium Macrocycles
Figure 1. ORTEP diagram of 2, [1,4-bis(diphenylphosphinoethoxy)benzene]2Ru2Cl4, showing the labeling scheme of selected atoms and thermal ellipsoids
at 50% probability. Hydrogen atoms and solvent molecules are omitted for clarity.
ν(CO) 2006 cm-1(s), ν(CO) 1948 cm-1(m), ν(Ru-Cl) 333 cm-1
C76H72Cl4O8P4Ru2: calcd 57.73% C, 4.59% H; found 57.73% C,
3.97% H. FAB-MS: [M - CO - Cl]+ calcd: 1520.1, expt: 1520.6
m/z; [M - 2CO - Cl]+ calcd: 1491.1, expt: 1490.6 m/z.
.
chelating phosphine-ether ligand to form the bimetallic
complexes. Due to the affinity of PPh3 for Ru(II), all
precautions must be taken to remove even trace amounts of
this ligand to avoid any subsequent reactions with it. Indeed,
when a CH2Cl2 solution of complex 2 was charged with CO
in the presence of trace quantities of PPh3, ligand substitution
occurred, resulting in the formation of trace quantities of
the mononuclear complex, Ru(CO)2Cl2(PPh3)2. This known
compound14 readily crystallizes, and its identity was con-
firmed by X-ray crystallography. To completely remove the
PPh3, successive recrystallizations of 2 and 3 from CH2Cl2
and Et2O were carried out, leaving pure condensed inter-
mediates, 2 and 3. Compounds 2 and 3 have been fully
characterized in solution (see Experimental Section), and 2
has been characterized in the solid state by a single-crystal
X-ray diffraction study, Figure 1.
Synthesis of [1,4-Bis(2-diphenylphosphinoethoxy)benzene]2Ru2-
Cl4(1,2-diaminopropane)2 (6). A solution of 1,2-diaminopropane
(3 µL, 0.036 mmol) in CH2Cl2 (20 mL) was added dropwise over
30 min to a solution of 2 (20 mg, 0.014 mmol) in CH2Cl2 (20 mL).
The mixture was stirred for an additional hour to give a clear yellow
solution. The solvent was removed in vacuo to give a yellow
1
powder. Yield: 19.2 mg, 0.012 mmol, 88%. H NMR: (CD2Cl2)
δ 0.90 (d, 6H, JH-H ) 6.3 Hz, CHCH3), 2.47-2.85 (m, 20H, NH2,
CH2P, NH2CH2), 3.00-3.10 (br m, 2H, CHCH3), 3.67 (br m, 8H,
CH2O), 6.33 (s, 8H, C6H4), 7.16-7.58 (complex m, 40H, P(C6H5)2).
31P{1H} NMR: (CD2Cl2) 36.8 (s). C74H84Cl4O4N4P4Ru2: calcd
56.93% C, 5.42% H, 3.59% N; found 57.45% C, 5.51% H, 3.51%
N. FAB-MS: [M] calcd ) 1560.2, expt ) 1560.8 m/z; [M - Cl-]+
calcd ) 1525.3, expt ) 1525.8 m/z.
The 31P{1H} NMR spectra of these complexes are analo-
gous to the spectra for similar mononuclear Ru(II) complexes
with phosphine-ether ligands.7f,h,8,10c Unlike condensed in-
termediates incorporating four-coordinate rhodium(I) and
palladium(II), closed macrocycles with octahedral ruthe-
nium(II) centers can form as two geometric isomers.
However, the 31P{1H} NMR spectra of 2 and 3 exhibit
singlets at 64.1 and 64.2 ppm, respectively, indicative of the
formation of only a single isomer of the condensed inter-
mediate, with the phosphines in identical magnetic environ-
ments. Solid-state characterization of 2 suggests that the
isomer formed is the trans-chloro, cis-phosphine, cis-ether
isomer, Figure 1. NMR spectroscopy of the crystals after
they have been redissolved in CD2Cl2 show that no additional
isomerization has taken place.
Solid State Characterization of 2‚4(CH2Cl2)‚4(Et2O).
Pink needle single crystals of 2 were grown by slow diffusion
of Et2O into a dilute solution of 2‚4(CH2Cl2)‚4(Et2O) in
CH2Cl2. The structure of 2‚4(CH2Cl2)‚4(Et2O) was deter-
mined by single-crystal X-ray diffraction methods. Diffrac-
tion intensity data were collected with a Bruker Smart Apex
CCD diffractometer at 150 K. Crystal, data collection, and
refinement parameters are given in Table 1. The structures
were solved using the Patterson function, completed by
subsequent difference Fourier syntheses, and refined by full
matrix least-squares procedures on F2. SADABS15 absorption
Synthesis of [r,r′-Bis(2-diphenylphosphinoexthoxy)-p-xylene]2-
Ru2Cl4(pyridine)4 (7). Pyridine-d5 (15 µL, 0.199 mmol) was added
to a solution of 3 (70 mg, 0.048 mmol) in CH2Cl2. Upon addition
of the pyridine-d5, a color change from red to yellow/green
occurred. The solvent was removed in vacuo to give a yellow/green
powder, as a mixture of cis-chloro-cis-phosphine-trans-pyridine and
1
all trans isomers. Yield: 74.6 mg, 0.041 mmol, 87%. H NMR:
(CD2Cl2) δ 2.41 (br m, 8H, CH2P), 2.90 (br m, 8H, CH2CH2O),
4.02 (br s, 8H, CH2O), 7.03-7.78 (complex m, 48H, Ph-H). 31P{1H}
NMR: (CD2Cl2) δ 35.07 (s), 35.80 (s). C92H92Cl4O4N4P4Ru2: calcd
61.88% C, 5.19% H, 3.14% N, 7.94% Cl; found 61.84% C, 4.98%
H, 3.33% N, 7.91% Cl. FAB-MS: [M] calcd ) 1618.2, expt )
1617.6 m/z.
Results and Discussion
Ligand Synthesis. Ligand 1 was synthesized via a
substitution reaction between R,R′-dichloroethoxy-p-xylene
and KPPh2 in THF. The ligand is a mildly air-sensitive white
solid, which has been characterized by 1H and 31P{1H} NMR
spectroscopies, mass spectrometry, and elemental analysis.
All data are consistent with the assigned structure.
Condensed Macrocycles 2 and 3. The ruthenium con-
densed intermediates, 2 and 3, were synthesized via a route
analogous to those previously published for rhodium and
palladium macrocycles, Scheme 2.3b,g The ligand 1,4-bis(2-
diphenylphosphino-ethoxy)benzene and ligand 1, dissolved
in CH2Cl2, were added dropwise to solutions of RuCl2(PPh3)3
in CH2Cl2 to form condensed intermediates 2 and 3,
respectively. In this reaction, PPh3 is replaced by the
(14) Collman, J. P.; Roper, W. R. J. Am. Chem. Soc. 1965, 87, 4008-
4009.
Inorganic Chemistry, Vol. 44, No. 3, 2005 499

Khoshbin et al.
Table 1. Crystallographic Data for 2
formula
(C68H64O4P4Cl4Ru2)‚4(CH2Cl2)‚4(C4H10O)
formula weight
space group
a, Å
2049.20
I4h
42.5605(18)
42.5605(18)
9.7061(8)
b, Å
c, Å
V, Å3
17582(2)
Z
8
crystal color, habit
crystal sizes (mm)
D(calc), g cm-3
µ(Mo KR), cm-1
temp, K
red, block
0.15 × 0.10 × 0.05
1.548
8.37
150(2)
Bruker Smart Apex CCD
Mo KR (0.71073) Å
53897
20119 [Rint)0.0584]
6.05
Figure 2. Possible geometric isomers about Ru(II) center for macrocycles
4 and 5.
diffractometer
radiation
reflections measured
reflections ind.
R(F), %a
compared with that of phosphorus.5 The Ru(1)‚‚‚Ru(2)
distance, 8.5503(6) Å, and the arene-arene distance, 4.02
Å, as well, are slightly longer than those in rhodium and
palladium based analogues.3c,g
R(wF2),%b
13.38
corrections were applied to all data (Tmin/Tmax ) 0.801). In
the crystal structure, there are four CH2Cl2 and four Et2O
solvate molecules. They are highly disordered and were
treated by SQUEEZE.16 Corrections of the X-ray data by
SQUEEZE (350 electrons per symmetrically independent
part) were close to the required values (328 electrons). Non-
hydrogen atoms were refined with anisotropic displacement
coefficients. The anisotropic thermal parameters of the O(1),
O(2), and C(33) atoms were negative and these atoms were
refined with isotropic thermal parameters. Hydrogen atoms
were treated as idealized contributions. The found Flack
parameter was 0.48(4), and the structure was refined finally
as a racemic twin structure. All software and sources of
scattering factors are contained in the SHELXTL (5.10)
program package (G. Sheldrick, Bruker XRD, Madison, WI).
The coordination environments around the Ru atoms are
distorted octahedral, with the chlorine atoms pointed inward
toward the cavity, with the Cl-Ru-Cl angles of 162.49(6)°
and 162.83(6)°, respectively, at the Ru(1) and Ru(2) atoms.
The same distortion was found in mononuclear Ru(II)
complexes with hemilabile phosphine-ether ligands, where
the chlorine atoms shift toward a square planar arrangement.
Such slight distortion of the Cl-Ru-Cl angles from an ideal
octahedral environment serves to compensate for the weakly
coordinating oxygen atoms and minimizes their steric
interactions with the phenyl groups on the phosphines.5,6a-b,9
The average Ru-O bond distance of 2.31(2) Å compares
well with mononuclear complexes with analogous phosphine-
alkoxy ligands.5,6a The Ru-O distances in 2 are significantly
longer than the sum of the covalent radii of these two atoms
(1.99 Å),17 suggesting the weak coordination of the O atoms.
The average of the cis Ru-P bond lengths, 2.228(1) Å, is
considerably shorter than those observed in a number of six-
coordinate Ru(II) complexes with mutually trans phosphines,
where values commonly range from 2.41 to 2.44 Å.18 This
effect is presumably due to the low trans influence of O atom
Neutral Macrocycles 4 and 5. Macrocycle complexes 4
and 5 were synthesized by bubbling a CH2Cl2 solution of 2
or 3 with CO, Scheme 3. Based on 31P{1H} NMR and FTIR
data, only single isomers of open complexes 4 and 5 are
formed. These new binuclear macrocycles have been char-
1
acterized by H, 31P{1H} NMR, and FTIR spectroscopies,
mass spectrometry, and elemental analysis. Complexes 4 and
5 each yield two bands indicative of CO stretches in the IR,
1944, 2002, and 1948 cm-1, 2006 cm-1, respectively.
Although the frequencies of these stretches fall in the range
of similar mononuclear compounds,5,6a,7c,e,f,h,10c,19 the relative
intensities of the bands do not match theory to dictate a cis
or trans arrangement of COs.19 To determine which of the
five possible isomers formed (Figure 2), further FTIR and
NMR experiments were conducted. The far-FTIR spectra of
complexes 4 and 5 each display a band at 333 cm-1,
associated with a Ru-Cl stretch, and indicative of trans-Cl
groups.20,21 The following ranges have been quoted for
ν(Ru-Cl) in octahedral Ru(II) complexes: Cl trans to Cl,
347-299 cm-1, Cl trans to CO, 311-266 cm-1, and Cl trans
to PR3, 262-229 cm-1.20,21 These data rule out all geometric
isomers with mutually cis-chlorides, Figure 2a-c. To
distinguish between the final two possible geometric isomers,
13CO was used to open the closed macrocycles. The 31P{1H}
NMR spectrum displayed one triplet with JP-C ) 12.5 Hz,
indicative of magnetically equivalent phosphines cis to the
13C nuclei.22 Taken together, these data lead to the conclusion
that the geometric isomer formed is the all trans isomer,
Figure 2e. The arrangement of the ligands about the Ru
center in these complexes is similar to that in analogous
mononuclear ruthenium complexes.5,7c,e,f,h,10c In many of these
mononuclear examples, reaction with CO results in uptake
of only one equivalent of CO, yielding a monocarbonyl
species.5,6a,7c,e,f,h After prolonged exposure to CO, the kineti-
cally stable all trans RuCl2(CO)2(P∼O)2 product is obtained
(15) Sheldrick, G. M. SADABS (2.01), Bruker/Siemens Area Detector
Absorption Correction Program; Bruker AXS: Madison, WI, 1998.
(16) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, A46,
194-201.
(19) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; John Wiley & Sons: New York, 1988.
(20) Bennett, M. A.; Johnson, R. N.; Tomkins, I. B. Inorg. Chem. 1974,
13, 346-350.
(21) Lupin, M. S.; Shaw, B. L. J. Chem. Soc. A. 1968, 741-749.
(22) Singewald, E. T.; Shi, X.; Mirkin, C. A.; Schofer, S. J.; Stern, C. L.
Organometallics 1996, 15, 3062-3069.
(17) Pauling, L. Nature of the Chemical Bond, 3rd ed.; Cornell University
Press: Ithaca, NY, 1960; pp 224-249.
(18) Haymore, B. L.; Ibers, J. A. Inorg. Chem. 1974, 14, 3060-3070.
500 Inorganic Chemistry, Vol. 44, No. 3, 2005

Weak-Link Approach to Binuclear Ruthenium Macrocycles
for all mononuclear complexes studied thus far. Over time,
through heating, or upon removal of the CO atmosphere, an
isomerization to the thermodynamically stable cis-CO, cis-
chloride, trans-phosphine isomer occurs.5,7c,e,10c Interestingly,
complexes 4 and 5 form immediately as the all trans isomers
but do not isomerize over time. Any attempts at heating these
complexes to drive this isomerization resulted in the forma-
tion of a variety of decomposition products.
Addition of pyridine to macrocycle 3 yields a mixture of
two geometric isomers: the all trans isomer, as well as the
cis-chloro, cis-phospine, trans-pyridine isomer (as evidenced
by 31P{1H} NMR spectroscopy), Scheme 4. To eliminate the
possibility of the formation of isomers with cis pyridines,
2,2′-bipyridine was used as the amine and the reaction was
monitored by 31P{1H} NMR spectroscopy. The open com-
plexes formed and displayed 31P{1H} NMR chemical shifts
significantly different from the shifts associated with the
pyridine opened complex, 7, indicating pyridines in a cis
arrangement would have 31P{1H} NMR chemical shifts
different from those in a trans arrangement about the
ruthenium metal center. The reaction between 2 and pyridine
gave a mixture of many products, including the expected
open macrocycle. The side products are most likely due to
the more constrained geometry and smaller cavity size of
macrocycle 2 making it difficult to incorporate the bulky
pyridine groups.
Neutral Macrocycles 6 and 7. In contrast to the rhodium
and palladium based macrocycles previously synthesized,3b,g
the weak ruthenium-oxygen bonds in 2 and 3 can be cleaved
with certain types of amines. For example, addition of a
stoichiometric or excess amount of 1,2-diaminopropane or
pyridine to CH2Cl2 solutions of condensed macrocycles 2
and 3 results in the clean formation of complexes 6 and 7,
respectively (Scheme 4). The ruthenium-oxygen bonds are
displaced by one equivalent per metal of the incoming
bidentate 1,2-diaminopropane (complex 6) or by two equiva-
lents of pyridine per metal center (complex 7). These
Conclusion
1
macrocycles have been characterized by H and 31P{1H}
NMR spectroscopies, mass spectrometry, and elemental
analysis.
Herein, we have reported that the weak-link approach has
been successfully applied to the synthesis of ruthenium
containing macrocycles. This approach has now been made
more general with the inclusion of a d6 transition metal with
an octahedral coordination environment, allowing for more
reactive sites at the metal center. The chemistry of ruthe-
nium(II) is different from previous four-coordinate metal
centers used in this approach, allowing for new molecules,
such as 1,2-diaminopropane, to be used to open the con-
densed intermediates. Although the additional coordination
sites provided by the octahedral coordination geometry of
ruthenium(II) are beneficial for conducting further chemistry,
they also introduce the possibility of mixtures of geometric
isomers. Characterizing and separating these mixtures adds
a degree of difficulty not encountered with four-coordinate
metal centers.
Due to the octahedral coordination environment of ruthe-
nium(II), the formation of different geometric isomers of the
open complex is possible. Initially, the condensed macrocycle
was combined with a variety of symmetric diamines, to yield
a mixture of two products, as evidenced by the 31P{1H}
NMR. To establish these products as geometric isomers, an
asymmetric diamine, 1,2-diaminopropane, was chosen to
open the condensed intermediates. Unlike the macrocycles
formed from symmetric diamines, the 31P{1H} NMR for the
different geometric isomers should display coupling of the
inequivalent phosphorus atoms, establishing the different
products as isomers of one another. Macrocycle 2 reacted
with 1,2-diaminopropane to form complex 6 with trans-
phosphines. This is in contrast to similar mononuclear
examples, where the trans-chloro isomer is the most favored
geometry in solution.7j Addition of 1,2-diaminopropane also
introduces an element of chirality into the macrocycle.
Though experiments in this study were conducted with a
racemic mixture of the diamine, enantiomerically pure
diamine can be used to cleave the ruthenium-oxygen bond
to create a chiral macrocycle. Phosphorus NMR is consistent
with the formation of a single isomer, but a mixture of
diastereomers cannot be completely ruled out.
Acknowledgment. C.A.M. acknowledges the NSF for
support of this research.
Supporting Information Available: Detailed X-ray structural
data including a summary of crystallographic parameters, atomic
coordinates, bond distances and angles, anisotropic thermal param-
eters, and H atom coordinates for 2 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC048975Y
Inorganic Chemistry, Vol. 44, No. 3, 2005 501