Aggregation and dna intercalation properties of di(isocyano) rhodium(i) Diimine complexes

Article abstract of DOI:10.1021/om200723j

A series of di(isocyano) rhodium(I) diimine complexes has been synthesized and characterized. Owing to the aggregation affinity of these complexes, they were found to exhibit thermochromism. To provide further insights into the aggregation affinity of these complexes, the enthalpy (δH) and entropy (δS) changes of dimerizations of some of the complexes have been determined. In addition, the DNA intercalation properties of these complexes have also been investigated by the DNA unwinding assay.

Full text of DOI:10.1021/om200723j

Article
pubs.acs.org/Organometallics
Aggregation and DNA Intercalation Properties of Di(isocyano)
Rhodium(I) Diimine Complexes
Larry Tso-Lun Lo, Wing-Kin Chu, Chun-Yat Tam, Shek-Man Yiu, Chi-Chiu Ko,* and Sung-Kay Chiu*
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, People's
Republic of China
S
* Supporting Information
ABSTRACT: A series of di(isocyano) rhodium(I) diimine
complexes has been synthesized and characterized. Owing to
the aggregation affinity of these complexes, they were found to
exhibit thermochromism. To provide further insights into the
aggregation affinity of these complexes, the enthalpy (ΔH)
and entropy (ΔS) changes of dimerizations of some of the
complexes have been determined. In addition, the DNA
intercalation properties of these complexes have also been
investigated by the DNA unwinding assay.
intercalators,7 which can be potentially useful for the
INTRODUCTION
■
development of antitumoral chemotherapeutic drugs, the
The studies of noncovalent intermolecular interactions, such as
DNA intercalation properties of these complexes have also
hydrogen bonding, van der Waals force, and π−π, hydrophobic,
been investigated.
and dipolar interactions, have led to the development of many
1
Physical Measurements and Instrumentation. H
different important applications.1 Apart from these weak
NMR spectra were recorded on a Bruker AV400 (400 MHz)
FT-NMR spectrometer. Chemical shifts (δ, ppm) were
interactions, metal−metal interactions, in particular those of
d10 and d8 metal complexes with linear and square-planar
reported relative to tetramethylsilane (Me4Si). All positive-ion
geometry, were also extensively investigated.2 The intriguing
ESI mass spectra were recorded on a PE-SCIEX API 150 EX
and unique photophysical properties associated with the
metal−metal interactions have also been utilized for the
development of sensors, light-emitting devices, and molecular
single quadrupole mass spectrometer. Elemental analyses of all
compounds were performed on an Elementar Vario MICRO
wires.3 Although the aggregation properties of square-planar
Cube elemental analyzer. Electronic absorption spectra were
recorded on a Hewlett-Packard 8452A diode array spectropho-
Rh(I) complexes and their distinguishable photophysical
tometer. The variable-temperature control was performed on
properties between the monomeric and the dimeric or
an Oxford Instrument Optistat DN liquid nitrogen optical
oligomeric species have been known for decades, many of
spectroscopy cryostat. Steady-state emission and excitation
these studies mainly focused on the binuclear systems and
spectra were recorded on a Horiba Jobin Yvon Fluorolog-3-
corresponding studies of the aggregation of mononuclear Rh(I)
complexes are relatively less explored.4
TCSPC spectrofluorometer. Measurements of the EtOH−
MeOH (4:1, v/v) glass samples at 77 K were carried out with
the dilute EtOH−MeOH sample solutions contained in a
quartz tube inside a liquid nitrogen-filled quartz optical Dewar
flask. Luminescence lifetimes of the samples were measured
using the time-correlated single photon counting (TCSPC)
technique on the TCSPC spectrofluorometer in a Fast MCS
mode with a NanoLED-375LH excitation source, which has its
excitation peak wavelength at 375 nm and pulse width shorter
than 750 ps. The photon counting data were analyzed by
Horiba Jobin Yvon decay analysis software.
With our interest in designing readily tunable polypyridyl
isocyano metal complexes,5 we have designed a series of
di(phenylisocyano) Rh(I) diimine complexes with interesting
themochromic behavior attributable to the aggregation of the
mononuclear Rh(I) complex units.6 Our preliminary results
showed that the aggregation affinity of these mononuclear
Rh(I) complexes could be tuned through the slight
modifications of the isocyanide ligands.6 To provide insights
into the structural relationship of the thermochromic and
aggregation properties, herein we report the detailed syntheses,
characterization, and photophysical and aggregation properties
of a new series of di(phenylisocyano) Rh(I) diimine complexes
with various isocyanide and diimine ligands of diverse
electronic and steric natures. As many square-planar transition
metal complexes have been shown to be effective DNA
X-ray Crystal Structure Determination. The crystal
structures were determined on an Oxford Diffraction Gemini
Received: August 3, 2011
Published: October 18, 2011
© 2011 American Chemical Society
5873
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Organometallics
Article
S Ultra X-ray single-crystal diffractometer using graphite-
monochromatized Cu-Kα radiation (λ = 1.54178 Å). The
structure was solved by direct methods employing the
SHELXL-97 program8 on a PC. Rh and many non-H atoms
were located according to the direct methods. The positions of
other non-hydrogen atoms were found after successful
refinement by full-matrix least-squares using the SHELXL-97
program8 on a PC. In the final stage of least-squares refinement,
all non-hydrogen atoms were refined anisotropically. H atoms
were generated by the program SHELXL-97.8 The positions of
H atoms were calculated on the basis of the riding mode with
thermal parameters equal to 1.2 times that of the associated C
atoms and participated in the calculation of final R-indices.
DNA Unwinding Assay (ref 9). The level of DNA
intercalation was determined by incubating different concen-
trations of the complexes from a DMSO stock solution into a
pH 7.9 buffered aqueous solution containing supercoiled
plasmid (pEGFP-C1, 0.5 μg), 50 mM potassium acetate, 20
mM Tris-acetate, 10 mM magnesium acetate, 1 mM
dithiothreitol, and 50 μg of bovine serum albumin at 37 °C
for 10 min. After the incubation, it was mixed with 2 U of E.
coli DNA topoisomerase I for 15 min, during which
supercoiling in the plasmid without any intercalator was fully
and irreversibly relaxed. As DNA intercalators change the
supercoiling of DNA by unwinding, only the residual
supercoiling without intercalation in partially intercalated
DNA would be irreversibly relaxed by the enzyme. On the
contrary, there is no enzymatic relaxation in fully intercalated
DNA, as no remaining supercoiling is present. Subsequently,
the DNA topoisomerase reactivity was stopped by the addition
of proteinase K (300 μg/mL) in 20 mM EDTA and 0.1% SDS
solution to digest the DNA topoisomerase. When the
intercalator was removed, the supercoiling of the intercalated
portion would be reinstated. The resulting supercoiling status
of the DNA, which reflects the level of DNA intercalation, was
then analyzed using 0.8% agarose gel electrophoresis. The DNA
bands representing the plasmid DNA with different super-
coiling remaining (topoisomers) were then visualized with
staining with GelRed DNA dye (Phenix Research Products,
NC, USA) using an LAS-4000 luminescent image analyzer
(Fujifilm Life Science).
SYNTHESIS AND CHARACTERIZATION
■
Materials and Reagents. 4,4′-Di-tert-butyl-2,2′-bipyridine
(tBu2bpy), 4,4′-dimethoxy-2,2′-bipyridine ((MeO)2bpy), 4,4′-dimeth-
yl-2,2′-bipyridine (Me2bpy), 2,2′-bipyridine (bpy), 1,5-cyclooctadiene
(cod), silver tetrafluoroborate (AgBF4), and rhodium chloride hydrate
were purchased from Strem Chemical Company and used without
further purification. 4,4′-Dichloro-2,2′-bipyridine (Cl2bpy) was ob-
tained from Aldrich Chemical Co. 5,6-Diboromo-1,10-phenanthroline
(Br2phen) was prepared according to a previously published
procedure.11 Substituted phenyl isocyanide ligands, 4-ClC6H4NC,
2,4,6-Cl3C6H2NC, 2,4,6-Br3C6H2NC, C6H5NC, 2,6-iPr2C6H3NC, and
2,6-(CH3)2C6H3NC, were prepared from the corresponding sub-
stituted anilines using the synthetic methodology developed by Ugi
and co-workers.12 [Rh(cod)Cl]2 and rhodium(I) 1,5-cyclooctadieyl
diimine complexes [Rh(bpy)(cod)]BF4, [Rh(Me2bpy)(cod)]BF4,
[Rh(Cl2bpy)(cod)]BF4, [Rh((MeO)2bpy)(cod)]BF4, [Rh(tBu2bpy)-
(cod)]BF4, and [Rh(Br2phen)(cod)]BF4 were synthesized according
to a modified literature procedure for related complexes.13 All solvents
were of analytical reagent grade and used as received.
General Synthetic Procedure for Di(isocyano) Rhodium(I)
Diimine Complexes, [Rh(CNR)2(N−N)]BF4 (1−11). To a suspension
of [Rh(N−N)(cod)]BF4 (0.5 mmol) in THF (10 mL) was added in a
dropwise manner substituted phenylisocyanide (1.0 mmol) in THF
(10 mL). The resulting solution was stirred at room temperature for 3
h. After removal of the solvent by rotary evaporation, the residue was
washed thoroughly with copious amounts of diethyl ether to remove
the displaced 1,5-cyclooctadiene and a trace amount of unreacted
isocyanide ligand. Two different types of crystalline solids with red and
green colors were obtained by the slow diffusion of diethyl ether vapor
into a concentrated acetone, acetonitrile, or THF solution of the
complexes.
1: Yield: 57%. 1H NMR (400 MHz, CDCl3, 298 K): δ 1.21 (d, 24H,
J = 5.7 Hz, iPr), 1.30−1.35 (m, 4H, iPr), 1.42 (s, 18H, tBu), 7.31−7.46
(m, 6H, phenyl H’s), 7.74 (dd, 2H, J = 1.2, 5.8 Hz, 5,5′-bpy H’s), 8.67
(s, br, 2H, 3,3′-bpy H’s), 8.81 (d, 2H, J = 5.8 Hz, 6,6′-bpy H’s). ESI-
MS: m/z 745 [M − BF4]+. IR (KBr disk, ν/cm−1): 1076 ν(B−F),
2090, 2136 ν(NC). Anal. Calcd for C44H58N4RhBF4·1/2CHCl3
(found): C 59.89 (60.16), H 6.61 (6.65), N 6.28 (6.37).
1
2: Yield: 52%. H NMR (400 MHz, CDCl3, 298 K): δ 7.54−7.75
(m, 10H, phenyl H’s), 7.77 (t, br, 2H, J = 6.5 Hz, 4,4′-bpy H’s), 8.32
(td, 2H, J = 1.2, 7.8 Hz, 5,5′-bpy H’s), 8.62 (d, 2H, J = 7.8 Hz, 3,3′-bpy
H’s), 8.96 (d, 2H, J = 6.5 Hz, 6,6′-bpy H’s). ESI-MS: m/z 465 [M −
BF4]+. IR (KBr disk, ν/cm−1): 1084 ν(B−F), 2100, 2144 ν(NC).
Anal. Calcd for C24H18N4RhBF4 (found): C 49.49 (49.77), H 3.22
(3.14), N 9.42 (9.75).
3: Yield: 59%. 1H NMR (400 MHz, CDCl3, 298 K): δ 7.83 (td, 2H,
J = 1.2, 5.3 Hz, 4,4′-bpy H’s), 8.04 (s, 4H, phenyl H’s), 8.38 (td, 2H, J
= 1.4, 7.9 Hz, 5,5′-bpy H’s), 8.67 (d, 2H, J = 7.9 Hz, 3,3′-bpy H’s), 9.03
(d, br, 2H, J = 5.3 Hz, 6,6′-bpy H’s). ESI-MS: m/z 672 [M − BF4]+. IR
(KBr disk, ν/cm−1): 1080 ν(B−F), 2074, 2136 ν(NC). Anal. Calcd
for C24H12N4Cl6RhBF4·CH2Cl2 (found): C 35.58 (35.45), H 1.67
(1.77), N 6.64 (6.97).
DNA Binding: Absorption Titration Study. The base-
pair concentration of CT-DNA, purchased from Calbiochem
Company, was determined on the basis of the molar extinction
coefficient of 6600 M−1 cm−1 at 260 nm.10 The titration
experiment was performed in a 5% DMF/5 mM Tris-HCl/50
mM NaCl buffer solution at pH 7.1 with a constant
concentration of the complex and varying the CT-DNA
concentrations (from 1 to 10 mol equiv). The UV−vis
absorption spectra were recorded after thermal equilibration,
which is about 10 min subsequent to the addition of the CT-
DNA solution. The intrinsic binding constant, K, was
determined from the linear least-squares fitting on the plot of
{[CT-DNA]/(εa − εf)} versus [CT-DNA] according to eq 1,
1
4: Yield: 58%. H NMR (400 MHz, CDCl3, 298 K): δ 8.02−8.05
(m, 6H, phenyl H’s and 5,5′-bpy H’s), 8.96 (d, 2H, J = 4.1 Hz, 3,3′-bpy
H’s), 8.99 (d, 2H, J = 1.6 Hz, 6,6′-bpy H’s). ESI-MS: m/z 741 [M −
BF4]+. IR (KBr disk, ν/cm−1): 1080 ν(B−F), 2070, 2140 ν(NC).
Anal. Calcd for C24H10N4Cl8RhBF4·1/2H2O (found): C 34.45
(34.20), H 1.33 (1.29), N 6.70 (6.58).
1
5: Yield: 52%. H NMR (400 MHz, CDCl3, 298 K): δ 8.06 (s, 4H,
phenyl H’s), 8.23 (dd, 2H, J = 5.1, 8.5 Hz, 3,8-phen H’s), 9.08 (dd,
2H, J = 1.2, 8.5 Hz, 4,7-phen H’s), 9.42 (d, 2H, J = 5.1 Hz, 2,9-phen
H’s). ESI-MS: m/z 854 [M − BF4]+. IR (KBr disk, ν/cm−1): 1080
ν ( B − F ) , 2 0 8 0 , 2 1 4 6 ν ( N  C ) . A n a l . C a l c d f o r
C26H10N4Cl6Br2RhBF4·CH2Cl2 (found): C 31.62 (31.42), H 1.18
(1.44), N 5.46 (5.51).
(1)
where [CT-DNA] is the base-pair concentration of CT-DNA,
6: Yield: 65%. 1H NMR (400 MHz, CDCl3, 298 K): δ 2.49 (s, 12H,
Me), 4.25 (s, 6H, OMe), 6.98 (dd, 2H, J = 2.6, 6.4 Hz, 5,5′-bpy H’s),
7.15 - 7.27 (m, 6H, phenyl H’s), 8.22 (d, 2H, J = 2.6 Hz, 3,3′-bpy H’s),
8.50 (dd, 2H, J = 1.2, 6.4 Hz, 6,6′-bpy H’s). ESI-MS: m/z 581 [M −
εa is the apparent extinction coefficient calculated from (A310
/
[complex]), and εf and εb are the extinction coefficients for the
complex in its free form and in the fully bound form at 310 nm.
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Organometallics
Article
Scheme 1. Synthetic Route to Di(isocyano) Rhodium(I) Diimine Complexes 1−11
BF4]+. IR (KBr disk, ν/cm−1): 1080 ν(B−F), 2082, 2136 ν(NC).
tetra(isocyano) rhodium complex, the isocyanide ligands
should be diluted in THF solution and slowly added, and an
excess amount of isocyanide ligands should be avoided.
Consequently, the purity of the isocyanide ligand plays an
important role in the control of the yield and the purity of the
crude product. Recrystallization of the crude complexes by slow
diffusion of diethyl ether into concentrated solutions of 1−11
gave the analytically pure crystalline solids. Except complex 1,
two types of distinguishable orange-red and green crystalline
solids were obtained during the recrystallization of all the
complexes (Figure S1, Supporting Information). For complex
1, only orange-red crystals were obtained on recrystallization.
As demonstrated in our earlier communication,6 the orange-red
crystals were attributed to the monomeric form of the rhodium
complexes, whereas the green crystals were considered to be
the dimeric form of the rhodium complexes. The nonexistence
of the green crystal of 1 can be explained by the significant
steric repulsion between the complex monomers due to the
presence of the bulky tert-butyl and isopropyl substituents on
the diimine and the isocyanide ligands, respectively.
Anal. Calcd for C30H30N4O2RhBF4 (found): C 53.92 (53.56), H 4.52
(4.53), N 8.38 (8.61).
7: Yield: 56%. 1H NMR (400 MHz, CDCl3, 298 K): δ 2.49 (s, 12H,
phenyl Me), 2.65 (s, 6H, bipyridyl Me), 7.15−7.24 (m, 6H, phenyl
H’s), 7.31 (dd, 2H, J = 1.1, 5.6 Hz, 5,5′-bpy H’s), 8.57 (s, br, 2H, 3,3′-
bpy H’s), 8.60 (d, 2H, J = 5.6 Hz, 6,6′-bpy H’s). ESI-MS: m/z 549 [M
− BF4]+. IR (KBr disk, ν/cm−1): 1080 ν(B−F), 2082, 2136 ν(NC).
Anal. Calcd for C30H30N4RhBF4 (found): C 56.63 (56.67), H 4.75
(4.68), N 8.81 (9.09).
8: Yield: 54%. 1H NMR (400 MHz, CDCl3, 298 K): δ 2.51 (s, 12H,
Me), 7.17−7.28 (m, 6H, phenyl H’s), 7.59 (ddd, 2H, J = 1.2, 5.5, 8.0
Hz, 4,4′-bpy H’s), 8.31 (td, 2H, J = 1.8, 5.5 Hz, 5,5′-bpy H’s), 8.69 (d,
2H, J = 8.0 Hz, 3,3′-bpy H’s), 8.82 (d, 2H, J = 5.5 Hz, 6,6′-bpy H’s).
ESI-MS: m/z 521 [M − BF4]+. IR (KBr disk, ν/cm−1): 1080 ν(B−F),
2086, 2140 ν(NC). Anal. Calcd for C28H26N4RhBF4 (found): C
55.30 (55.67), H 4.31 (4.16), N 9.21 (9.44).
9: Yield: 48%. 1H NMR (400 MHz, CDCl3, 298 K): δ 2.54 (s, 12H,
Me), 7.16−7.28 (m, 6H, phenyl H’s), 8.24 (dd, 2H, J = 5.1, 8.6 Hz,
3,8-phen H’s), 9.06 (d, 2H, J = 8.6 Hz, 4,7-phen H’s), 9.27 (d, 2H, J =
5.1 Hz, 2,9-phen H’s). ESI-MS: m/z 703 [M − BF4]+. IR (KBr disk,
ν/cm−1): 1080 ν(B−F), 2093, 2142 ν(NC). Anal. Calcd for
C30H24N4Br2RhBF4·1/2CH3CN (found): C 45.93 (45.79), H 3.17
(2.99), N 7.78 (7.52).
1
All complexes were characterized by H NMR spectroscopy,
IR spectroscopy, mass spectrometry, and elemental analyses. All
complexes show two intense IR absorptions in the regions of
2135−2146 and 2070−2095 cm−1, corresponding to the
symmetric and asymmetric CN stretches of isocyanide
ligands in a cis-conformation of a square-planar geometry with
C2v symmetry. These absorption bands are attributed to the A1
and B2 stretching vibrational modes.14 For complexes with the
same isocyanide ligands, the CN stretches follow the order 9
(2093, 2142) > 8 (2086, 2139) > 7 (2082, 2136) ≈ 6 (2082,
2136); 5 (2080, 2146) > 4 (2076, 2140) > 3 (2074, 2134),
which is in the line with the π-accepting ability of the
substituted diimine ligands. This trend is attributed to the
competition between the isocyanides and diimine ligands for
the π-back-bonding.
1
10: Yield: 52%. H NMR (400 MHz, CDCl3, 298 K): δ 7.61−7.77
(m, 10H, phenyl H’s and 4,4′-bpy H’s), 8.32 (td, 2H, J = 1.6, 5.8 Hz,
5,5′-bpy H’s), 8.60 (d, 2H, J = 8.0 Hz, 3,3′-bpy H’s), 8.95 (d, 2H, J =
5.8 Hz, 6,6′-bpy H’s). ESI-MS: m/z 533 [M − BF4]+. IR (KBr disk, ν/
cm−1): 1084 ν(B−F), 2097, 2144 ν(NC). Anal. Calcd for
C24H16N4Cl2RhBF4·1/2CH3CN (found): C 46.80 (46.52), H 2.75
(2.72), N 9.82 (9.83).
1
11: Yield: 60%. H NMR (400 MHz, CDCl3, 298 K): δ 7.62 (ddd,
2H, J = 1.2, 5.6, 7.8 Hz, 4,4′-bpy H’s), 7.86 (s, 4H, phenyl H’s), 8.34
(td, 2H, J = 1.6, 5.6 Hz, 5,5′-bpy H’s), 8.72 (d, 2H, J = 7.8 Hz, 3,3′-bpy
H’s), 8.94 (d, 2H, J = 5.6 Hz, 6,6′-bpy H’s). ESI-MS: m/z 937 [M −
BF4]+. IR (KBr disk, ν/cm−1): 1080 ν(B−F), 2074, 2132 ν(NC).
Anal. Calcd for C24H12N4Br6RhBF4·CH3CN (found): C 29.28
(29.03), H 1.42 (1.66), N 6.57 (6.23).
X-ray Crystal Structure. We have previously reported the
crystal structures of both the orange-red and green crystals of
complex 7,6 which show different intermolecular arrangements,
orientations, and rhodium−rhodium metal distances. Although
crystalline solids for all the complexes were obtained on
recrystallization, most of them are polycrystalline or poor-
quality single crystals not suitable for X-ray structure
determination. The crystal structure of the red crystal of 6
shows a very similar structural arrangement to the red crystal of
7.6 In the red crystal of 6, the orientations of two closest
neighboring complex cation ([Rh(CNC6 H3 Me2 -
2,6)2((MeO)2bpy)]+) units are related by a 180° rotation
with respect to the adjacent units (Figure 1). In these complex
cation units, the rhodium metal center adopted a square-planar
RESULT AND DISCUSSION
■
Syntheses and Characterization. The synthetic route for
the di(isocyano) rhodium(I) diimine complexes is summarized
in Scheme 1. The precursor complexes [Rh(N−N)(cod)]BF4
were synthesized by stirring the 1:1:1 mixture of [Rh(cod)Cl]2,
diimine ligand (N−N), and AgBF4 in dichloromethane at room
temperature for 2 h. Subsequent substitution reaction of these
precursor complexes with 2.0 mol equiv of the substituted
isocyanide ligands afforded the target di(isocyano) rhodium(I)
diimine complexes 1−11 in moderate yields. As the diimine
ligand could be further substituted on stirring with excess
isocyanide ligands to give the tetra(isocyano) rhodium complex
[Rh(CNR)4]+, thus to minimize the formation of undesirable
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Organometallics
Article
of 240−330 nm, one to two moderately intense absorptions
with λabs peaking at about 353−388 nm, and a weak absorption
shoulder in the region of 455−490 nm (Table 3). With
Table 1. Crystal Structure Determination Data for 6
formula
C33H36BF4N4O3Rh
726.38
Mr
T/K
133(2)
a/Å
8.18060(13)
12.82265(19)
31.1555(4)
90
b/Å
c/Å
α/deg
β/deg
91.7411(13)
90
γ/deg
V/Å3
3266.61(8)
yellow
cryst color
cryst syst
monoclinic
P21/c
space group
Z
4
F(000)
Dc/g cm3
1488
1.477
cryst dimens/mm
λ/Å (graphite monochromated, Cu Kα)
absorption coeff/mm−1
collection range
0.6 × 0.1 × 0.1
1.541 80
4.767
3.735° ≤ θ ≤ 66.98°
(h: −9 to 9;
k: −15 to 10;
l: −37 to 37)
99.9%
completeness to theta
no. of data collected
15 093
no. of unique data
5811
no. of data used in refinement, m
no. of parameters refined, p
5036
438
a
R
0.0430
a
wR
0.0905
goodness-of-fit, S
1.101
Figure 1. Ortep drawings of (a) complex cation of 6 with the atomic
numbering scheme and (b) complex cations of 6 showing the
intermolecular packing. Thermal ellipsoids are shown at the 50%
probability level.
maximum shift, (Δ/σ)max
0.001
residual extrema in final diff map, eÅ−3
0.787, −0.606
a
w = 1/[σ2(Fo ) + (ap)2 + bP], where P = [2Fc2 + max(Fo ,0)]/3.
2
2
Table 2. Selected Bond Distances (Å) and Angles (deg) with
Estimated Standard Deviations (esd's) in Parentheses for 6
geometry with one coplanar isocyanide and one noncoplanar
isocyanide ligand, in which the phenyl ring is tilted by 51°
relative to the plane of the complexes. The shortest Rh−Rh
distance between two closest complex cation molecules is ca.
4.76 Å, which is much longer than the typical Rh−Rh distances
of 3.2−3.9 Å in stacked Rh(I) complexes with significant
metal−metal interactions.9 Therefore, this crystal can be
considered as the monomeric form of the complex. As with
other chelating diimine complexes,10 the angle subtended by
the nitrogen atoms of the bipyridine at the Rh center, N−Rh−
N, was about 78.6°, which is much smaller than the ideal angle
of 90° in the ideal square-planar geometry. The bending of the
two isocyanide ligands as reflected from the CN−C bond
angles of 169.0° and 173.8° can be attributed to the π-back-
bonding interaction between the isocyanide ligands and the
rhodium center, which is commonly observed in other metal−
isocyanide complexes.5,15
Bond Distances
Rh(1)−C(1)
Rh(1)−C(2)
C(1)−N(1)
C(3)−N(1)
1.889(3)
1.902(4)
1.163(4)
1.401(4)
Rh(1)−N(3)
Rh(1)−N(4)
C(2)−N(2)
C(11)−N(2)
2.076(3)
2.080(3)
1.168(5)
1.405(5)
Bond Angles
N(3)−Rh(1)−N(4)
C(2)−Rh(1)−N(4)
Rh(1)−C(1)−N(1)
C(1)−N(1)−C(3)
78.63(10) C(1)−Rh(1)−C(2)
96.94(12) C(1)−Rh(1)−N(3)
88.34(14)
96.08(12)
178.5(3)
169.0(4)
177.6(3)
Rh(1)−C(2)−N(2)
C(2)−N(2)−C(11)
173.8(3)
reference to the previous spectroscopic study of related
rhodium(I) complexes,16 the very intense absorptions are
attributed to the ligand-centered (LC) π → π* transitions of
isocyanide and diimine ligands mixed with the MLCT
transition [dπ(Rh) → π*(RNC)]. For the lowest energy
moderately intense absorption and weak absorption tailing in
the visible region, they are ascribed to the spin-allowed and
spin-forbidden MLCT [dπ(Rh) → π*(N−N)] transitions. The
UV−Vis Absorption and Emission Spectroscopy.
Dissolution of complexes 1−11 in acetone and CH2Cl2 gave
yellow to orange solutions with similar UV−vis absorption
spectra (Figure S2). The electronic absorption spectra of 1−11
in CH2Cl2 show several very intense absorptions in the region
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Table 3. Photophysical Data for 1−11
a
emission
absorption
complex
λem/nm (τo/μs)
medium
T/K
λabs/nm (ε/dm3 mol−1cm−1
)
1
565 (0.47, 1.74)
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
CH2Cl2
acetone
acetone
298
298
178
298
298
178
298
298
178
298
298
178
298
298
178
298
298
178
298
298
178
298
298
178
298
298
178
298
298
178
298
298
178
244 (34145), 290 (28415), 318 sh (11585), 375 (9525), 487 (840)
368 (9670), 458 (850)
b
−
2
577 (0.43, 1.53)
568 (0.55, 2.15)
579 (0.27, 1.15)
580 (0.54, 2.19)
587 (0.51, 1.43)
574 (0.51, 1.52)
580 (0.48, 1.61)
588 (0.52, 1.74)
570 (0.55, 1.87)
565 (0.82, 2.30)
241 (29015), 291 (28755), 324 sh (9690), 379 (11250), 485 (1250)
375 (12605), 472 (1290)
c
d
e
f
705 (4405 ), 903 , 990
3
246 (25315), 302 (20435), 325 sh (10100), 370 (9230), 470 (1000)
363 (11175), 453 (1010)
c
d
e
662 (2960 ), 834
4
275 (36870), 298 (36790), 382 (16490), 495 (1160)
373 (17125), 485 (1070)
c
d
683 (4990 )
5
249 (22410), 289 (22390), 358 (6410), 391 (5940), 496 (630)
353 (14510), 380 (12825), 482 (1255)
c
d
701 (4000 )
6
256 (42070), 286 (32840), 304 sh (16615), 365 (14255), 463 (1170)
360 (13940), 455 (855)
cg
,
587
7
242 (31135), 290 (31050), 321 sh (9835), 374 (10975), 482 (960)
369 (10760), 462 (975)
c
d
626 (2265 )
8
241 (25430), 291 (25840), 323 sh (8000), 383 (8460), 488 (800)
375 (10055), 475 (841)
cg
,
655
9
241 (38170), 283 (36585), 362 (7215), 401 (7860), 519 (625)
362 (10200), 388 (10365), 490 (880)
c
d
675 (1600 )
10
11
248 (19870), 295 (8545), 311 sh, (6915), 375 (4790), 483 (380)
372 (11550), 470 (960)
ch
,
678
269 (16560), 302 (14685), 324 sh (7990), 370 (7710), 463 (675)
366 (11285), 455 (850)
c
d
657 (6275 )
a
b
Measured in EtOH−MeOH (4:1 v/v) glass at 77 K upon excitation at 400 nm. Emission maxima are uncorrected values. Absorption of dimeric
unit was not observed. Absorption of the dimeric complexes [Rh(CNR)2(N−N)]22+. Determined from equilibrium analysis.4e,f,h,6 Absorption of
c
d
e
f
g
the trimeric complexes [Rh(CNR)2(N−N)]33+. Absorption of the tetrameric complexes [Rh(CNR)2(N−N)]4
. The absorptivity cannot be
4+
determined due to low aggregation affinity, in which the absorption of the dimeric complexes appears only at high concentration of the complex.
h
The absorptivity cannot be determined due to the precipitation of the aggregated complexes.
MLCT [dπ(Rh) → π*(N−N)] assignments are further
supported by their absorption energy dependence on the
electronic properties of the diimine and isocyanide ligands as
revealed by the order of the absorption maxima (λ abs), 3 (370,
470) < 4 (382, 495) < 5 (391, 496); 6 (365, 463) < 7 (374,
482) < 8 (383, 488) < 9 (401, 519), which is in the line with
the π-accepting ability of the diimine ligand, for complexes with
the same isocyanide ligand, and 3 (370, 470) ≈ 11 (370, 463) <
10 (375, 483) < 2 (379, 485) < 8 (383, 488), which is in the
reverse order of the π-accepting ability of the isocyanide ligand,
for the bipyridyl complexes.
As demonstrated in our previous communication,6 the
solutions of all complexes except 1 exhibit thermochromism
with their color gradually changed from yellow to dark green on
cooling to 178 K. The representative UV−vis absorption
spectral changes of these complexes on gradual decrease of the
temperature are revealed in Figure 2. For 4−11, a new
absorption band evolved (Figure 2a) on cooling with λabs in the
range 587 to 701 nm at 178 K (Table 3). With reference to our
previous study, this new absorption band is attributed to the
formation of the dimeric complex {[Rh(CNR)2(N−N)]2}2+
Figure 2. Overlaid UV−vis absorption spectra of the acetone solutions
of (a) 4 (1.01 mM), (b) 3 (0.95 mM), and (c) 2 (0.98 mM) at
different temperatures.
(Scheme 2). The dimer assignment is further validated by the
1/2
1/2
linear relationship between [Rh]T/(Aabs
)
and (Aabs)
(Figure 2a, inset) at a constant temperature,4e,f,h,6,17 where
[Rh]T is the total concentration of the rhodium complex and
Aabs is the absorbance at λabs of the new absorption band.
Complexes 2 and 3 also show similar UV−vis absorption
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Scheme 2. Dimerization of the Rhodium(I) Complexes
spectral changes with the evolution of one new absorption band
in the same region (λabs = 705 nm for 2 and 662 nm for 3 at
178 K) at the early stage of cooling when they are in a diluted
solution (<0.35 mM). In contrast, at lower temperature or in a
more concentrated solution, 3 shows an additional lower
energy absorption band (λabs = 834 nm at 178 K), whereas 2
shows two additional absorption bands (λ abs = 903 and 990 nm
at 178 K). The evolutions of the second and the third lower
energy absorption bands are attributed to the formation of the
trimeric and tetrameric complexes [Rh(CNR)2(N−N)]nn+ (n =
3 and 4) as similarly observed in the tetra(isocyano)
rhodium(I) complexes.4e,f,h,16 The formation of the trimeric
and tetrameric forms in 2 can be attributed to the highest
aggregation affinity of 2 among 1−11, as reflected by enthalpy
change of dimerization (see below) due to the absence of bulky
substituents on the diimine and isocyanide ligands.
Comparing the absorption of the dimeric form of the
complexes with bpy (2, 3, 8, 10, 11) and Br2phen (5, 9)
ligands, a red shift of the absorption maximum for complexes
with phenylisocyanide ligands containing no substituent or less
sterically bulky substituents is clearly noted, as reflected by the
order of the λabs [2 (705 nm) > 10 (678 nm) > 3 (662 nm) >
11 (657 nm) > 8 (655 nm); 5 (701 nm) > 9 (675 nm)]. These
absorption maxima do not show correlation with the electronic
effect of the isocyanide ligands. On the contrary, the absorption
maxima of the dimeric complexes for those with identical
isocyanide ligands show a clear correlation with the electronic
effect of the diimine ligand and follow the order 5 (701 nm) >
4 (683 nm) > 3 (662 nm); 9 (675 nm) > 8 (655 nm) > 7 (626
nm) > 6 (587 nm), which is in line with the π-accepting ability
of the diimine ligand. On the basis of these absorption energy
dependences together with the previous spectroscopic work on
the dinuclear rhodium complexes with bridging isocyanide
ligands,4f,j,l these absorption bands are assigned to the
transitions from dσ*[dz2*(Rh)] to pσ[pz(Rh)] mixed with
the π* orbital of the diimine ligand [π*(N−N)]. The
absorptions of the trimeric and tetrameric complexes in the
lower energy region are also similarly assigned. The observation
of the red-shifted absorptions of the aggregated forms for
complexes with less steric bulky ligands can be explained by the
shorter Rh−Rh distance and thus stronger Rh−Rh interaction
in their aggregated form.4 Similarly, the slight red shifts of the
absorptions of the dimeric, trimeric, and tetrameric complexes
with decreasing temperature can also be ascribed to the
shortening of the Rh−Rh distance in the aggregated forms as
the temperature decreased.
Table 4. Summary of Thermodynamic Parameters for the
Dimerization of 2−5 and 11
complex
ΔH/kJ mol−1
ΔS/J mol−1 K−1
2
−40.6
−34.1
−29.1
−32.5
−22.0
−127.4
−115.9
−85.3
−79.7
−55.5
3
4
5
11
order of magnitude for the dimerizations of related square-
planar rhodium(I) and platinum(II) complexes.4j,v,18 The
negative values of all these ΔH for the dimerization reactions
indicated that the dimeric form is thermodynamically more
stable than the monomeric complex. On the basis of the
enthalpy change of dimerization, the relative stability of the
dimeric form compared to its monomeric form is found to be
significantly affected by the steric properties of the isocyanide
ligands as reflected by the magnitude of the enthalpy change,
|ΔH|, in the order of 2 (40.6) ≫ 3 (34.1) ≥ 5 (32.5) ≥ 4
(29.1) ≫ 11 (22.0). The negative entropy changes of these
dimerization reactions in the range −55.5 to −127.4 J mol−1
K−1 are consistent with the decrease of the degrees of freedom
of two monomeric complex molecules in the formation of the
dimer. Although complexes 6−9 also exhibit thermochromism
through the dimerization reactions, the thermodynamic
parameters for these dimerization reactions were not
determined since they started to show thermochromism at
much lower temperature or higher concentration; thus, the
van’t Hoff analysis cannot be accurately performed due to the
limiting data in narrow ranges of temperature and concen-
tration.
The emission properties of these complexes were also
investigated. The solids and solutions of these complexes are
nonemissive at room temperature, but they become lumines-
cent with emission maxima in the range 565−588 nm (Figure
To provide further insights into the aggregation affinity of
these complexes, the enthalpy (ΔH) and entropy (ΔS) changes
of dimerizations of 2−5 and 11 (Table 4) are determined
through the analysis of the temperature dependence of the
equilibrium constants for the dimerizations using the van’t Hoff
equation. The ΔH of the dimerizations of these complexes
range from −22.0 to −40.6 kJ mol−1, which are in the same
Figure 3. Overlaid normalized emission spectra of complexes 1, 2, 7,
8, 9, and 11 in EtOH−MeOH (4:1 v/v) glass at 77 K.
3, Table 3) in EtOH−MeOH, 4:1 (v/v), glass at 77 K. Since
the emissions are higher in energy compared to the absorptions
of the dimeric complexes, they should be derived from the
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Organometallics
Article
monomeric form of the complexes. In view of the relatively
long-lived emission lifetimes in the submicrosecond time scale
and the emission energy showing similar trends to the MLCT
[dπ(Rh) → π*(N−N)] absorption bands, these emissions are
This is further supported by the similar intercalation ability of 1
(Kint ≈ 10 μM) even though it shows no intermolecular
aggregation due to the steric repulsion and the nonplanarity of
the complex as a result of sterically demanding 2,6-diisopropyl
substituents on the isocyanide ligands. These observations
suggested that the diimine ligand of the complexes is the
intercalating moiety as typically reported for metal polypyridyl
complexes.19 Consequently, as Br2phen is a more extensively
conjugated intercalating ligand than bpy, 5 displayed the
strongest ability in intercalation as reflected by the smallest Kint
of less than 5 μM in this series of complexes.
The DNA intercalation of 1 to the CT-DNA was also
investigated by a UV−vis absorption titration study. On
addition of CT-DNA to 1 in 5% DMF/5 mM Tris-HCl/50
mM NaCl buffer solution, the absorption in the UV region,
corresponding to the LC π → π* transitions, exhibited
hypochromism with a very slight red shift (ca. 2 nm) of the
λ abs (Figure 5). This observation is suggestive of an
3
tentatively assigned as derived from the MLCT [dπ(Rh) →
π*(N−N)] excited-state origin.
DNA Intercalation Study. Many important carcinogens
and antitumoral chemotherapeutic drugs act on DNA by
inserting themselves into the space between base pairs and
unwinding the intercalated DNA. As many square-planar
transition metal complexes, in particular those of platinum(II)
complexes, have been shown to be effective DNA intercalators,7
the capability of these rhodium(I) diimine complexes for DNA
intercalation has also been investigated using a well-established
DNA unwinding assay.9 In this assay, plasmid DNA molecules,
interacting with or without intercalator, were relaxed with DNA
topoisomerase I. Subsequently, the plasmid DNA was then
analyzed for their superhelicity or their range of topoisomers
remaining in the population by agarose gel electrophoresis.
Plasmid DNA without any intercalated molecules was fully
relaxed and moved slowly in the gel (Figure 4, lane 2), while
Figure 5. UV−vis absorption spectral change of 1 (30.5 μM) in a 5%
DMF/5 mM Tris-HCl/50 mM NaCl buffer solution at pH 7.1 upon
addition of different amounts (0, 0.5, 1.5, 2.5, 4.5, 6.5, 8.5, 10, 12.5,
14.5, and 17 mol equiv) of CT-DNA. The inset shows the plot of [CT-
DNA]/(εa(310) − εf(310)) versus [CT-DNA] and its linear least-squares
fit ().
Figure 4. Complex 3 intercalated into supercoiled DNA. Supercoiled
plasmid DNA was incubated with different concentrations of 3 for 10
min, then relaxed by DNA topoisomerase I and separated in agarose
gel. The top of the figure is the negative electrode of the
electrophoresis tank. Lane 1 contained supercoiled plasmid DNA
without any treatment, which appears at the lower part of the gel.
Lanes 2−7 contained plasmid DNA incubated with increasing
concentration of 3 (from 0 to 100 μM). DNA topoisomers with
different degrees of complex intercalation were formed and appeared
as DNA bands in lane 5.
intercalative DNA binding interaction between 1 and CT-
DNA20 and is typically observed in the DNA titration study of
other intercalators.20,21 Using the equation for CT-DNA
binding and from the linear least-squares fitting of the plot of
[CT-DNA] vs [CT-DNA]/(εa − εf) (Figure 5, inset),22 the
binding constant for 1 was determined to be 2.40 × 104 M−1,
which is higher than that for other octahedral metal-
lointercalators of tris(phenanthroline) metal complexes,21
indicating a higher DNA intercalating affinity for these
square-planar Rh(I) diimine complexes. Compared to classical
organic intercalators,23 this binding constant is similar to that of
proflavine hemisulfate (2.2 × 104 M−1)23a but is ca. 2 orders of
magnitude lower than that for ethidium bromide (1.25 × 106
M−1)23b,c under similar experimental conditions. This trend is
consistent with the results found in the DNA unwinding assay.
the more supercoiled or intercalated DNA molecules migrated
much faster and therefore located in the lower part of the gel
(Figure 4, lanes 6, 7). All complexes from 1 to 500 μM were
incubated with 0.5 μg of plasmid DNA, and the remaining
superhelicity of the plasmid molecules was then analyzed. Due
to the low solubility of 2 and 10 and the poisoning effects of 6−
9 on DNA topoisomerase I and proteinase K as reflected by the
observation of smear located above the DNA topoisomers in
the gel (Figure S3, Supporting Information), the DNA
intercalation properties of these complexes cannot be
determined by this assay. For other complexes, they were
found to intercalate into DNA molecules with Kint, which is
defined as the concentration of the complex at which half of the
supercoiled DNA was intercalated with the complex, and
display a full-range of topoisomers in agarose gel electro-
phoresis, ranging from 5 to 35 μM (Table S1, Supporting
Information). The close resemblance of the Kint values (∼10
μM) for 3 and 11, which contain the same bpy ligand and
different isocyanide ligands, indicated that the intercalating
ability of these complexes is relatively insensitive to the changes
of the isocyanide ligands and the planarity of the complexes.
CONCLUSION
■
A new series of thermochromic di(isocyano) rhodium(I)
diimine complexes, [Rh(CNR)2(N−N)]+, with different
isocyanide and diimine ligands has been successfully synthe-
sized and characterized, and their photophysical and
thermochromic properties have been studied. The detailed
study revealed that the thermochromic properties of these
complexes can be significantly modified through the change of
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Organometallics
Article
15, 2714. (c) Yam, V. W.-W.; Lu, X.-X.; Ko, C.-C. Angew. Chem., Int.
Ed. 2003, 42, 3385. (d) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-
C.; Ko, C.-C.; Cheung, K.-K. Inorg. Chem. 2001, 40, 571.
the substituents of the isocyanide ligands. To provide insights
into the aggregation affinity and the future design of thermally
responsive molecular devices based on the square-planar
rhodium(I) complexes, the enthalpy (ΔH) and entropy (ΔS)
changes of the dimerizations of some of the complexes have
been determined. In addition, the DNA intercalation properties
of these rhodium(I) complexes have also been investigated by
the DNA unwinding assay, which showed that these complexes
can intercalate DNA at micromolar range. Further modifica-
tions of the diimine ligands on these complexes to enhance
their DNA intercalation ability and the study of their potential
applications as antitumoral chemotherapeutic drugs are now in
progress.
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ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving crystal data of 6, photographs showing two
different types of crystals obtained during the course of
recrystallization, overlaid UV−vis spectra of selected complexes,
and table summarizing the Kint in the DNA intercalation study.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vinccko@cityu.edu.hk; kaychiu@cityu.edu.hk. Fax:
(+852)-3442-0522. Tel: (+852)-3442-6958.
■
ACKNOWLEDGMENTS
■
The work described in this paper was supported by a RGC
GRF grant from the Research Grants Council of Hong Kong
(Project No. CityU 101510) and a grant from City University
of Hong Kong (Project No. 7002688). L.T.-L.L. and W.-K.C.
acknowledge the receipt of a Postgraduate Studentship and a
Research Tuition Scholarship, both administrated by City
University of Hong Kong.
(7) Liu, H.-K.; Sadler, P.-J. Acc. Chem. Res. 2011, 44, 349.
(8) Sheldrick, G. M. SHELX-97: Programs for Crystal Structure
Analysis (Release 97-2); University of Goettingen: Germany, 1997.
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Osheroff, N., Eds.; 1999; Vol. 94, pp 149−160. (b) Peixoto, P.; Bailly,
C.; David-Cordonnier, M. H. Methods Mol. Biol. 2010, 613, 235.
(10) Reichmann, M. E.; Rice, S. A.; Thomas, C. A.; Doty, P. J. Am.
Chem. Soc. 1954, 76, 3047.
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