Ligand selective monosubstitution with complete enantiomeric retention in ruthenium bis(bipyridine) complexes

Article abstract of DOI:10.1021/ic015515l

Ligand selective monosubstitution with complete enantiomeric retention in Δ and Λ-cis-[Ru(bpy)₂(DMSO)(Cl)]PF₆ has been achieved photochemically. Irradiation in the presence of various ligands, including 4,4′-bipyridine, results in th

Full text of DOI:10.1021/ic015515l

2478
Inorg. Chem. 2001, 40, 2478-2479
Ligand Selective Monosubstitution with Complete Enantiomeric Retention in Ruthenium Bis(bipyridine)
Complexes
Dusan Hesek,† Guy A. Hembury,† Michael G. B. Drew,‡ Seiji Taniguchi,† and Yoshihisa Inoue*,†
Inoue Photochirogenesis Project, ERATO, JST, 4-6-3 Kamishinden, Toyonaka 560-0085, Osaka, Japan, and
Department of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, U.K.
ReceiVed February 19, 2001
Ruthenium bis(bipyridine) compounds of the type Ru(bpy)2-
(X)(Y) possess extensive coordination and photo/electrochemical
properties,1 along with varied and complex chiral attributes such
as: the interconversion between cis- and trans- isomers,2 dia-
stereomerically controlled synthesis,3 and the generation and
control of atropisomeric chirality.4 However, despite such efforts
by ourselves and other research groups, the ability to retain what
may be seen as the most essential chiral feature of these systems,
the ∆ or Λ chirality around the ruthenium center on mono-ligand
substitution without racemization, has remained elusive.
This inability to perform enantioretaining monosubstitution lies
essentially in the fact that conventional thermal methods are not
(typically) bond selective. Previously we described the synthesis
and stereochemistry associated with the molecule cis-[Ru(bpy)2-
(DMSO)(Cl)]PF6,3 and from our previous experience with these
compounds, we were aware that we could selectively excite the
Figure 1. Variation in the CD spectra on substitution of the DMSO
ligand of the Λ isomer of 1 for Cl-.
ruthenium-sulfoxide bond over the adjacent ruthenium-chloride
bond. Thus we felt it would be possible, in principle, to selectively
substitute the sulfoxide group for a new ligand via a photochemi-
cal process.
Scheme 1. Bond Selective Photoinduced Substitution of the
Sulfoxide Ligand in ∆-cis-Ru(bpy)2(DMSO)(Cl)]PF6 (1) by
4,4′-bpy, F-, Cl-, I-, and SCN- with Enantioretention (the
Product Charge is Omitted for Clarity)
The work detailed below reveals a new concept, that the
sulfoxide moiety (DMSO) of enantiopure ∆- or Λ-cis-[Ru(bpy)2-
(DMSO)(Cl)]PF6 (1) can be specifically monosubstituted for a
variety of ligands via irradiation of either the LC (288 nm) or
MLCT (420 nm) bands, resulting in new products with complete
∆/Λ enantiomeric retention around the ruthenium center. The
enantioretaining nature of these reactions can be seen by the
retention of the 1st positive and 2nd negative Cotton effects (for
the Λ isomer) in the LC region (250-330 nm) of the CD spectra
for all ligands studied, which corresponds to the orientation of
the two bipyridine groups around the Ru center (Figure 1).
Additionally, it is noted that the outcome of these reactions
further resulted in inversions of the bisignate Cotton effects arising
from the MLCT transitions (350-550 nm). This is not related to
the ∆/Λ configuration around the metal center (as verified by
the retention the bisignate CD coupling in the LC region), but
indicates a subtle change in the stereochemistry of these transi-
tions.
group; then at longer irradiation times, the chloride is also replaced
as a result of the new ligands excess of approximately 105:1.
However, when an I- ligand was employed in a 1:1 ratio under
the same conditions, only the enantiopure monosubstituted product
was formed, showing that monosubstitution may be achieved,
without bis-substitution contamination, by the amount of new
ligand used. In contrast to this new photochemical method, the
conventional thermal reaction as typified by the reaction with I-
results in the bisiodide substituted racemic product, rac-Ru(bpy)2-
(I)2. With the success of these simple ligands, we investigated
the pyridyl ligand 4,4′-bipyridine (4,4′-bpy), which possesses two
coordination sites. In particular, the ability to coordinate pyridyl
groups with complete enantiomeric control is crucial, as pyridyl
ligand coordination to numerous transition metals, including
ruthenium, has shown great potential for the generation of useful
molecular architectures5,6 and bio-active species;7 this reaction
Initially, the anions F-, Cl-, I-, and SCN- were employed as
new ligands to test if this photochemical approach to enantiore-
taining substitution was possible (Scheme 1). Remarkably all of
these species were found to replace the sulfoxide ligand, showing
quantitative conversion of 1 with complete retention of config-
uration. The reactions proceed by initially replacing the sulfoxide
* To whom correspondence should be addressed.
† Inoue Photochirogenesis Project.
‡ The University of Reading.
(1) Keene, R. F. Coord. Chem. ReV. 1997, 166, 121-159.
(2) Hesek, D.; Everitt, S. L. R.; Inoue, Y. Chem. Lett. 1999, 2, 109-110.
(3) Hesek, D.; Inoue, Y.; Everitt, S. L. R.; Ishida, H.; Kunieda, M.; Drew,
M. G. B. Inorg. Chem. 2000, 39, 317-324.
(4) (a) Velders, A. H.; Hotze, A. C. G.; Haasnoot, J. G.; Reedijk, J. Inorg.
Chem. 1999. 38, 2762-2763. (b) Hesek, D.; Hembury, G. A.; Drew,
M. G. B.; Taniguchi, S.; Inoue, Y. J. Am. Chem. Soc. 2000, 41, 10236.
(5) (a) Fujita, M. Chem. Soc. ReV. 1998, 27, 417-425. (b) Scherer, M.;
Caulder, D. L.; Johnson, D. W.; Raymond, K. N. Angew. Chem., Int.
Ed. 1999, 38, 11, 1588-1592.
10.1021/ic015515l CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/18/2001

Communications
Inorganic Chemistry, Vol. 40, No. 11, 2001 2479
was also found to proceed with complete retention of the
enantiomeric configuration, in high chemical yield, to give ∆-cis-
[Ru(bpy)2(4,4′-bpy)(Cl)]+ (2).
The essential synthetic procedure is very simple. To 3 mL
solutions of Λ-cis-[Ru(bpy)2(DMSO)(Cl)]+ (1) at 10-6 M in either
1,2-dichloroethane, MeCN, or THF/MeOH were added the new
ligands (F-, Cl-, I-, and SCN). In the weak intensity irradiation
conditions employed for these reactions, MeCN was not found
to be a competing coordinating ligand. These concentrations were
used so that the reactions could be conveniently followed by either
UV-vis or CD spectroscopy, or by chiral phase HPLC. The
reaction mixtures were then irradiated using a Xe lamp (0.5 mW
cm-2) fitted with a monochromator to give wavelengths of either
288 or 420 nm ((5 nm); the enantioretaining nature of the
reactions was found to be independent of the excitation wave-
length. In all cases, after reaction times of 17-60 min, it was
found that the reaction proceeded with 100% conversion of the
starting material (as revealed by CD, UV-vis spectroscopy and
HPLC analysis) to form the monosubstituted product (with small
amounts of the bisproduct in the cases of ligand excesses of 105:
1, as discussed previously). With the confirmation that this
methodology worked at “analytical concentrations”, a conven-
tional reaction concentration of 0.01 M of 1 was utilized for the
more “practical” 4,4′-bpy ligand to evaluate if concentration
affected the outcome of the reaction and, thus, the procedures
applicability at these concentrations. The reaction was found to
occur not only with complete enantioretention, but also with
excellent efficiency, with the weak light source giving complete
conversion from 1 to 2 in 10 h.
Notably, the 4,4′-bpy ligand only displaces the sulfoxide and
never the chloride, even at long reaction times. This monosub-
stitution arises from a 4,4′-bpy excess of only 10:1 rather than
the 105:1 previously employed. Further, and crucially, the 4,4′-
bpy was only coordinated through one of the nitrogens, leaving
the other free for further coordination to other species, as
confirmed by X-ray analysis (Figure 2).8 In this case, the crystal
was grown from a racemic sample of 1 irradiated under the same
conditions in the presence of 4,4′-bpy. This is a result of far-
reaching consequence for the subsequent facile, rational design
of chirally sophisticated molecules and materials.
Figure 2. The ORTEP view of cis-rac-[Ru(bpy)2(4,4′-bpy)(Cl)]PF6 at
50% probability level. The counteranion (PF6) is omitted for clarity.
determined from CD and UV-vis spectral changes, carried out
in deaerated 1,2-dichloromethane solution, and was found to be
0.6. Light intensity for irradiation experiments were determined
by using ferrioxalate actinometry.9 The quantum yield for
replacement of the Ru-chloride in this molecule could not be
determined due to its photoinertness, cf. the Ru-sulfoxide bond.
This clearly shows that the photoreactivity of the Ru-chloride
bond is significantly lower than that of the Ru-sulfoxide bond.
Confirmation of quantitative enantioretention for these reactions
is obtained by comparing the values of the molar CDs for
enantiopure Λ-cis-Ru(bpy)2(Cl)210 with that of the Λ-cis-Ru(bpy)2-
(Cl)2 formed in the photoreaction of Λ-cis-[Ru(bpy)2(DMSO)-
(Cl)]+ with Bu4NCl; these are found to be identical, confirming
quantitative enantioretention. The efficiency of this reaction is
such that the reaction carried out at 1 M using cis-[Ru(bpy)2-
(DMSO)(Cl)]PF6 (1) and an Xe light source, with a λ > 420 nm
filter, went to completion in 4 h.
The wider significance of this result is enhanced by the
monocoordination of 4,4′-bpy, as this leaves a reactive site for
subsequent chirally controlled or racemic reactions. The mech-
anism for these reactions is likely to proceed via a two step
process; first, photoinduced intramolecular isomerization from the
thermodynamically stable S-bound DMSO species to the kineti-
cally stable O-bound DMSO species,11 second, rapid exchange
of the O-bound DMSO for the new ligand.12 The unambiguous
determination of the reaction mechanism for the present case is
currently being undertaken.
The photoinduced bond-selective substitution of the Ru-S bond
over the Ru-Cl bond in [Ru(bpy)2(DMSO)(Cl)]+ (1) was
evaluated by determination of the quantum yield (Φ) for the
replacement of the sulfoxide by SCN-. Quantum yields were
Supporting Information Available: 1H and 13C NMR spectra, CD
and UV spectra, HPLC chromatogram charts and X-ray tables. This
material is available free of charge via the Internet at http://pubs.acs.org.
(6) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: Chichester, U.K., 1991. (b) Juris, A.; Barigelletti, S.;
Campagna, S.; Balzani, V,; Besler, P.; von Zelewsky, A. Coord. Chem.
ReV. 1988, 84, 85.
IC015515L
(7) Erkkila, K. E.; Odom, D. T.; Barton, J. K.; Chem. ReV. 1999, 99, 2777.
(8) Data for 2 was collected on a Rigaku AFC7R four-circle diffractometer
with filtered Mo KR radiation and a rotating anode generator using the
w scan technique, and the structure was solved using direct methods.
Data for compound 2: Monoclinic space group P21/n; a ) 13.0154-
(19), b ) 14.068(2), c ) 17.453(2) Å, _ ) 99.703(12), V ) 3154.0(8)
Å3, Z ) 4, R ) 0.0601, Rw ) 0.1652 for 4504 observed data.
Crystallographic data for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Center.
(9) Murov, S. L.; Carmichael, I.; Hug, G. Handbook of Photochemistry;
Marcel Dekker: New York, 1993; p 299.
(10) Yamagishi, A.; Naing, K.; Goto, Y.; Taniguchi, M.; Takahashi, M. J.
Chem. Soc., Dalton Trans. 1994, 2085-2089.
(11) Yeh, A.; Scott, N.; Taube, H. Inorg. Chem. 1982, 21, 2542-2545.
(12) Smith, M. K.; Gibson, J. A.; Young, C. G.; Broomhead, J. A.; Junk, P.
C.; Keene, F. R. Eur. J. Inorg. Chem. 2000, 1365-1370.