Ruthenium(II) dichloride complexes of chiral, tetradentate aminosulfoxide ligands: Stereoisomerism and redox-induced linkage isomerism

Article abstract of DOI:10.1021/ic302188y

Ruthenium(II) dichloride complexes of two chiral tetradentate aminosulfoxide ligands, varying only in the N-N linker, were synthesized. With each ligand, two major isomers formed, and these were structurally assigned and characterized through a combination of NMR and UV-vis spectroscopies, X-ray crystallography, and density functional theory calculations. The cis-β geometric isomer was formed by each ligand, whereas the trans and cis-α geometric isomers were significant components for one ligand only. Cyclic voltammetry studies show that only the cis-β isomers undergo linkage isomerism upon oxidation to ruthenium(III), whereas the trans and cis-α isomers show simple reversible redox couples.

Full text of DOI:10.1021/ic302188y

Communication
pubs.acs.org/IC
Ruthenium(II) Dichloride Complexes of Chiral, Tetradentate
Aminosulfoxide Ligands: Stereoisomerism and Redox-Induced
Linkage Isomerism
Peter O. Atolagbe,† Krista N. Taylor,† Samantha E. Wood,† Arnold L. Rheingold,‡ Lenora K. Harper,§
Craig A. Bayse,§ and Tim J. Brunker*,†
†Department of Chemistry, Jess and Mildred Fisher College of Science and Mathematics, Towson University (TU), 8000 York Road,
Towson, Maryland 21252, United States
‡Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
§Department of Chemistry and Biochemistry, Old Dominion University, Hampton Boulevard, Norfolk, Virginia 23529, United States
S
* Supporting Information
Chart 1. Tetradentate Aminosulfoxide Ligands Synthesized
ABSTRACT: Ruthenium(II) dichloride complexes of two
chiral tetradentate aminosulfoxide ligands, varying only in
the N−N linker, were synthesized. With each ligand, two
major isomers formed, and these were structurally assigned
and characterized through a combination of NMR and
UV−vis spectroscopies, X-ray crystallography, and density
functional theory calculations. The cis-β geometric isomer
was formed by each ligand, whereas the trans and cis-α
geometric isomers were significant components for one
ligand only. Cyclic voltammetry studies show that only the
cis-β isomers undergo linkage isomerism upon oxidation to
ruthenium(III), whereas the trans and cis-α isomers show
simple reversible redox couples.
Showing Absolute Configurations (top) and Possible
Geometric Isomers of Ru(L)Cl2 Complexes of Tetradentate
Aminosulfoxide Ligands (bottom) and Complexes of Each
Type Isolated
xamples of linkage isomerism of sulfoxide ligands
E
[especially dimethyl sulfoxide (DMSO)], in which a
sulfoxide ligand may coordinate through either its sulfur or
oxygen donor atom, are prevalent in ruthenium sulfoxide
complexes.1,2 The sulfur-bound form is favored on ruthenium-
(II), Ru2+[S], and in certain complexes, linkage isomerism may
be induced photochemically or upon oxidation of ruthenium-
(II) to ruthenium(III) (redox-induced linkage isomerism or
RILI): In each case, a metastable oxygen-bound form results,
Run+[O].3−10 Manipulation of the coordination mode of the
sulfoxide ligand by an external stimulus is of interest for
applications as molecular switches. Complexes containing
labile, monodentate sulfoxides are susceptible to substitution
by solvents or other donors; however, certain complexes of
chelating sulfoxides retain RILI and photoisomerism behav-
ior.11−15 Although these chelating sulfoxides contain sulfur
stereocenters, only racemic ligands have been utilized to date. A
complex containing a resolved chiral sulfoxide ligand could
interconvert between two chiral linkage isomers and thus be of
interest in chiroptical switching. We have investigated the
synthesis of complexes of chiral, tetradentate aminosulfoxide
ligands and report on the synthesis, structures, and electro-
chemistry of ruthenium(II) dichloride complexes of two such
ligands (see Chart 1). Several complexes of the type RuCl2(N-
donor)2(DMSO)2 are known to display RILI behavior although
only by one of the possible metal complex geometric isomers in
each case.7,9,10 We demonstrate that, in our case, all three
possible metal complex geometric isomers (shown in Chart 1)
are synthetically accessible,16 but only one displays the desired
RILI behavior.17
The two single-enantiomer ligands, L1 and L2 in Chart 1,
were synthesized by the conjugate addition of the correspond-
ing secondary diamines to (R)-p-tolylvinyl sulfoxide.18 The
reaction of L1 with RuCl2(PPh3)3 in toluene at reflux
precipitated two isomers of RuCl2(L1) (1-L1 and 2-L1) in a
4:5 ratio, as determined by 1H NMR spectroscopy, separable by
solubility differences in ethanol. The less-soluble, pink-orange
isomer, 1-L1, displayed a C2-symmetric structure in solution, as
evidenced by equivalent p-tolyl rings and N-methyl resonances
1
in the H NMR spectrum. The yellow isomer, 2-L1, displayed
two sets of peaks in both of these regions and is assigned as the
cis-β isomer as discussed below. 1-L1 was identified as the cis-α
isomer by single-crystal X-ray diffraction (Figure 1). It contains
Received: October 9, 2012
Published: January 16, 2013
© 2013 American Chemical Society
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largest ligand-field splitting in the cis-β complexes is in
agreement with the reported UV−vis spectra for
RuCl2(DMSO)2L2 (L = imidazole and NH3).20 The intensities
of electronic transitions determined by time-dependent DFT
calculations are in good agreement with experimental UV−vis
data. The trans and cis-β isomers show major transitions at
shorter wavelengths (trans, 464 and 327 nm; cis-β: 430 and 337
nm). The cis-α structure has an additional transition at longer
wavelengths (515, 391, and 341 nm). Each of these transitions
is from a t2g-type molecular orbital (MO) to a Ru−S
antibonding MO heavily mixed with phenyl π* character.
The chiroptical properties of all complexes were also
characterized by circular dichroism (CD) spectroscopy (see
the SI for spectra).
To probe potential RILI behavior, cyclic voltammograms
(CVs) were recorded in CH2Cl2 solution. 1-L1 and 1-L2
display couples at +0.21 and +0.41 V, respectively (vs Cp2Fe+/
Cp2Fe) that display reversibility similar to that of the internal
Cp2Fe standard at all scan rates measured. These are assigned
to Ru3+[S,S]/Ru2+[S,S] couples; i.e., neither complex under-
goes linkage isomerism upon oxidation on the cyclic
voltammetry time scale. On the other hand, CVs of the cis-β
complexes show more features and scan rate dependence. CVs
of 2-L2 (see Figure 2) and 2-L1, with an initial anodic scan
Figure 1. ORTEP plots of X-ray crystal structures of trans-1-L2 (left)
and cis-α-1-L1 (right) with hydrogen atoms omitted for clarity.
a ruthenium stereocenter of Δ configuration, and both nitrogen
stereocenters are of R configuration. 1-L1 is an unusual
example of a ruthenium(II) complex containing trans sulfur-
bound sulfoxide ligands. This arrangement is generally
disfavored because of the substantial trans influence of sulfur-
bound sulfoxides.2,7,19
The reaction of L2 with RuCl2(PPh3)3 in refluxing
tetrahydrofuran precipitated two C1-symmetric isomers of
RuCl2(L2) (1-L2 and 2-L2) in a 1:1 ratio, as determined by
1H NMR spectroscopy, separable by solubility differences in
acetonitrile. A single-crystal X-ray diffraction structure of the
orange isomer, 1-L2, identified it as the trans isomer (Figure 1).
The nitrogen atoms adopt R,S configurations such that the N-
methyl groups lie in a cis-like relationship, consistent with the
lack of rotational symmetry observed by NMR. The second,
yellow isomer, 2-L2, is also assigned as the cis-β isomer.
Each geometric isomer gives rise to characteristic chemical
1
shift patterns for the p-tolyl ring protons in the H NMR
spectra: Only the cis-β isomers, 2-L1 and 2-L2, show
resonances that are shielded relative to the free ligands. A
third stereoisomer with the L1 ligand, 3-L1, was also isolated in
small amounts from an alternate procedure (see the Supporting
Information), is similar to that of 1-L2, and so is also assigned
as the trans isomer. By IR spectroscopy, the S−O stretching
bands lie between 1060 and 1080 cm−1 and are shifted to
slightly higher energies compared to the free ligands (1040−
1050 cm−1), as is consistently observed for metal complexes
with sulfur-bound sulfoxides.4
To rationalize the stabilities of the various isomers, density
functional theory (DFT) calculations were performed on
isomers of a RuCl2-L1′ complex in which the p-tolyl groups
of L1 are truncated to phenyl groups. Bond distances and
angles for the trans and cis-α structures were in agreement with
the X-ray structures of 1-L2 and 1-L1, respectively. The lowest-
energy cis-β structure was determined by a conformation search
of the chelate rings. The cis-α isomer is the most stable of the
three with the trans and cis-β isomers roughly equal in energy
(ΔG = 7.9 and 9.3 kcal/mol, respectively). Common to these
less-stable structures is the R,S conformation of the diamine,
which suggests that the preference for the trans isomer with the
L2 ligand can be attributed to the steric constraints of the six-
membered ring. The calculated lowest-energy cis-β stereo-
isomer (Λ-RNSN) is consistent with 1H−1H NOESY NMR data
obtained for 2-L1 (see the Supporting Information, SI). For the
L1 ligand then, different relative nitrogen configurations appear
to be necessary to adopt the cis-α (R,R) or cis-β (R,S)
geometries.
Figure 2. CV of 2-L2 with an initial cathodic scan direction (recorded
in CH2Cl2, 50 mV/s, 0.1 M TBA·PF6): solid line, 0−1.2 V; dashed
line, 0−0.6 V vs Ag+/Ag reference electrode. Inset: ECEC square
scheme for RILI including parameter definitions.
direction, reveal waves corresponding to Ru3+/Ru2+ couples
with ipc/ipa < 1 (E1/2 = 0.62 and 0.65 V vs Cp2Fe+/Cp2Fe,
respectively, at 50 mV/s). In each returning cathodic scan, an
additional lower potential wave is also observed (cathodic shifts
of 0.54 and 0.70 V, respectively, at 50 mV/s).21 CVs recorded
around the region of the second peak only are featureless in
both cases, establishing that the second reduction is of a species
formed after oxidation to Ru3+. Multiple cycles do not reveal
the corresponding anodic wave for the lower potential couple
in the case of 2-L2, although it is observed for 2-L1. These data
can be interpreted on the basis of an ECEC square scheme (see
the inset to Figure 2), in which linkage isomerism follows
oxidation to Ru3+ (Ru3+[S,S] to Ru3+[S,O]). The Ru3+[S,O]
species is then reduced at a lower potential than the Ru3+[S,S]
species. Rapid isomerism of Ru2+[S,O] to Ru2+[S,S] follows this
reduction. Further evidence for a Ru3+[S,O] isomer is given by
the IR spectrum of the bulk oxidation product of 2-L2 with
In UV−vis spectra, each complex shows several low-intensity
bands between 300 and 550 nm (ε in the range 10−500 M−1
cm−1), which we assign as d−d bands (cis-α-1-L1, 510 nm;
trans-1-L2, 450 nm; cis-β-2-L1 and -2-L2, below 400 nm). The
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Communication
[NO]+, which displays a band at 924 cm−1 consistent with an
oxygen-bound sulfoxide.3 UV−vis spectroelectrochemical data
were also obtained for the oxidized forms of 1-L1, 1-L2, and 2-
L2: The visible spectra are dominated by a moderately intense
Cl(pπ) → Ru(dπ) LMCT transition in each case.
Various parameters were quantitatively determined for 2-L2
following earlier treatments of similar data: CVs with an initial
anodic scan direction were recorded at a variety of scan rates
(see the SI).10 The equilibrium constant, KIIIO→S, was calculated
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank both the Research Corporation for Science
Advancement (Cottrell College Science Award) and the NSF
(MRI 0923051) for support. We also thank the Jess and
Mildred Fisher College of Science and Mathematics at TU and
TU Faculty Development and Research Committee for
financial support. Dr. S. Stitzel is thanked for assistance in
obtaining the spectroelectrochemical data.
to be 1.33
0.06, indicating marginally greater stability of
Ru3+[S,S] versus Ru3+[S,O]. KIIO→S was then determined to be
1.91 × 109; i.e., the Ru2+[S,S] form is highly favored versus
Ru2+[S,O]. The rate constants kIII
and kIII
were also
S→O
REFERENCES
O→S
■
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peak currents in CVs of 2-L1 precluded similar analysis, the
forms of the CVs imply that both Ru3+[S,O] and Ru2+[S,O]
have greater stability than that in 2-L2.
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Figure 3. Calculated HOMOs of the three isomers.
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character. Only in the cis-β isomer is the Ru−S bond also in this
plane. We propose that depopulation of this HOMO upon
oxidation leads to increased “hardness” in this plane, resulting
in the tendency of this sulfoxide to isomerize in the cis-β
isomer. Ongoing investigations will seek to further clarify the
electronic origin of RILI in these complexes and other
outstanding questions, including the relationship of the ligand
structure to the metal complex geometric isomer distribution.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, complete exper-
imental details and characterization data for the synthesis of
ligands and ruthenium complexes, UV−vis and CD spectral
data, CV data and X-ray crystallographic data for solution and
refinement, and table of selected bond lengths and angles, and
details of DFT calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tbrunker@towson.edu. Phone: 410-704-3118.
■
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