Thermochromic and aggregation properties of bis(phenylisocyano) rhodium(I) diimine complexes

Article abstract of DOI:10.1021/om900161f

A series ofbis(phenylisocyano) rhodium(I) diimine complexes with thermally mediated aggregation properties have been synthesized, and their photophysical and thermochromic properties have been studied.

Full text of DOI:10.1021/om900161f

Organometallics 2009, 28, 3597–3600 3597
DOI: 10.1021/om900161f
Thermochromic and Aggregation Properties of Bis(phenylisocyano)
Rhodium(I) Diimine Complexes
Larry Tso-Lun Lo, Chi-On Ng, Hua Feng, and Chi-Chiu Ko*
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong, People’s Republic of China
Received February 28, 2009
Summary: A series of bis(phenylisocyano) rhodium(I) diimine
complexes with thermally mediated aggregation properties have
been synthesized, and their photophysical and thermochromic
properties have been studied.
and tuning the aggregation properties of mononuclear Rh
complexes and polypyridyl Rh(I) complexes have been much
less explored.4 With our recent interest in designing readily
tunable polypyridyl isocyano metal complexes,5 we believe
that the incorporation of isocyanide ligands into a Rh(I)
diimine complex would generate square-planar d8 metal
complexes wherein the planarity, physical, aggregation,
and excited-state properties would be readily tuned by alter-
ing the substituent of the isocyanide nitrogen atom. Herein,
we report the synthesis, characterization, and photophysical
properties of a new series of bis(phenylisocyano) Rh(I)
diimine complexes with tunable aggregation properties and
themochromic behavior.
The intriguing excited-state properties as well as the
extraordinary properties associated with the weak metal-
metal interaction of square-planar d8 metal polypyridyl
complexes1 have attracted tremendous attention for decades.
The use of the aggregation properties, controlled through
the elegant design of square-planar polypyridyl Pt(II)
complexes, has revealed many interesting photophysical
properties, which have inspired the development of com-
plexes that serve as chemosensors and molecular devices.2
Despite the great successes in controlling the Rh-Rh dis-
tance by using bridging ligands with different spacers in the
binuclear systems and the unique properties associated with
aggregation of mononuclear Rh(I) complexes,3 controlling
The reaction of [Rh(cod)Cl]2 with AgBF4 in the presence
of 1 equiv of diimine ligand in dichloromethane, followed by
a substitution reaction with a substituted phenylisocyanide,
afforded the BF4- salts of the desired bis(phenyisocyano)
rhodium(I) diimine complexes, [Rh(CNR)2(N-N)]BF4,
in moderate yield (Scheme 1). Complexes 1-5 were char-
1
acterized by H NMR, IR, and ESI-MS and gave satisfac-
*To whom correspondence should be addressed. E-mail: vinccko@
cityu.edu.hk. Fax: (+852)-2788-7406. Tel: (+852)-3442-6958.
(1) (a) Morgan, G. T.; Burstall, F. H. J. Chem. Soc. 1934, 965. (b)
Jennette, K. W.; Gill, J. T.; Sadownick, J. A.; Lippard, S. J. J. Am. Chem.
Soc. 1976, 98, 6159. (c) Miskowski, V. M.; Houlding, V. H. Inorg. Chem.
1989, 28, 1529. (d) Connick, W. B.; Henling, L. M.; Marsh, R. E.; Gray,
H. B. Inorg. Chem. 1996, 35, 6261.
(2) (a) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625. (b)
Exstrom, C. L.; Sowa, J. R.Jr.; Daws, C. A.; Janzen, D.; Mann, K. R.;
Moore, G. A.; Stewart, F. F. Chem. Mater. 1995, 7, 15. (c) Yam,
V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Ko, C.-C.; Cheung, K.-K.
Inorg. Chem. 2001, 40, 571. (d) Yam, V. W. W.; Wong, K. M.-C.; Zhu,
N. J. Am. Chem. Soc. 2002, 124, 6506. (e) Chakraborty, S.; Wadas, T. J.;
Hester, H.; Flaschenreim, C.; Schmehl, R.; Eisenberg, R. Inorg. Chem.
2005, 44, 6284. (f) Kui, S. C.-F.; Chui, S. Sin-Yin; Che, C.-M.; Zhu, N. J.
Am. Chem. Soc. 2006, 128, 8297.
tory elemental analyses. Recrystallization of these complexes
from slow diffusion of diethyl ether vapor into concentrated
solutions of 1-5 in common organic solvents, such as
acetone, acetonitrile, and THF, gave two types of solids or
crystals with a red or dark green color, similar to other
related square-planar d8 transition metal complexes.3 The
solutions of these two different colored solids or crystals gave
identical 1H NMR and UV-vis absorption spectra. The
X-ray crystal structures6 of the two forms of 1 were also
determined.
(4) Indelli, M. T.; Chiorboli, C.; Scandola, F. Top. Curr. Chem. 2007,
280, 215, and references therein.
(3) (a) Mann, K. R.; Gordon, J. G.II; Gray, H. B. J. Am. Chem. Soc.
1975, 97, 3553. (b) Lewis, N. S.; Mann, K. R.; Gordon, J. G.II; Gray, H.
B. J. Am. Chem. Soc. 1976, 98, 7461. (c) Balch, A. L. J. Am. Chem. Soc.
1976, 98, 8049. (d) Mann, K. R.; Lewis, N. S.; Williams, R. M.; Gray, H.
B. Inorg. Chem. 1978, 17, 828. (e) Miskowski, V. M.; Sigal, I. S.; Mann,
K. R.; Gray, H. B. J. Am. Chem. Soc. 1979, 101, 4383. (f) Rice, S. F.;
Gray, H. B. J. Am. Chem. Soc. 1981, 103, 1593. (g) Fordyce, W. A.;
Brummer, J. G.; Crosby, G. A. J. Am. Chem. Soc. 1981, 103, 7061. (h)
Miskowski, V. M.; Rice, S. F.; Gray, H. B.; Milder, S. J. J. Phys. Chem.
1993, 97, 4277. (i) Fortin, D.; Drouin, M.; Harvey, P. D. J. Am. Chem.
Soc. 1997, 119, 531. (j) Tejel, C.; Ciriano, M. A.; Lopez, J. A.; Lahoz, F.
J.; Oro, L. A. Angew. Chem., Int. Ed. 1998, 37, 1542. (k) Tejel, C.;
Ciriano, M. A.; Oro, L. A. Chem.;Eur. J. 1999, 5, 1131. (l) Heyduk, A.
F.; Macintosh, A. M.; Nocera, D. G. J. Am. Chem. Soc. 1999, 121, 5023.
(m) Bradley, P. M.; Bursten, B. E.; Turro, C. Inorg. Chem. 2001, 40,
1376. (n) Bera, J. K.; Dunbar, K. R. Angew. Chem., Int. Ed. 2002, 41,
4453. (o) Cooke, M. W.; Hanan, G. S.; Loiseau, F.; Campagna, S.;
Watanabe, M.; Tanaka, Y. Angew. Chem., Int. Ed. 2005, 44, 4881. (p)
Tran, N. T.; Stork, J. R.; Pham, D.; Chancellor, C. J.; Olmstead, M. M.;
Fettinger, J. C.; Balch, A. L. Inorg. Chem. 2007, 46, 7998. (q) Tran, N. T.;
Stork, J. R.; Pham, D.; Olmstead, M. M.; Fettinger, J. C.; Balch, A. L.
Chem. Commun. 2006, 1130.
(5) Ng, C.-O.; Lo, L. T.-L.; Ng, S.-M.; Ko, C.-C.; Zhu, N. Inorg.
Chem. 2008, 47, 7447.
(6) Crystal data for green form of 1: [C32H35BF4N4O0.5Rh] (1 1/
3
˚
2Et2O), Mw = 673.36, monoclinic, C2/c (No. 15), a = 19.0965(4) A, b =
˚
˚
20.8077(4) A, c = 15.4552(3) A, R = 90°, β = 96.555(2)°, γ = 90°, V =
3
6101.0(2) A , Z = 8, Dc = 1.466 g cm-3, μ(Cu-KR) = 4.993 mm-1
,
˚
6424 reflections collected, 3581 were unique (Rint = 0.0419) and 2681
were observed with I g 2σ(I) in the ranges of -19 e h e 20, -22 e k e 21,
-15 e l e 15 with 2θmax equal to 55.00°. F(000) = 2760, T = 173(2) K,
R1 = 0.0468 and wR2 = 0.1051 with a GOF on F2 = 0.996. Data/
restraints/parameters: 6424/0/391. Crystal data for red form of 1:
[C33H36BF4N4ORh] (1 (CH3)2CO), Mw = 694.38, monoclinic, P21/c
˚
3
˚
˚
(No. 14), a = 14.7171(5) A, b = 13.9287(4) A, c = 16.6506(5) A, R =
3
˚
90°, β = 109.209(3)°, γ = 90°, V = 3223.18(17) A , Z = 4, Dc
=
1.431 g cm-3, μ(Cu-KR) = 4.756 mm-1, 9263 reflections collected,
5498 were unique (Rint = 0.0193) and 4669 were observed with I g 2σ(I)
in the ranges -17 e h e 14, -16 e k e 16, -19 e l e 19 with 2θmax equal
to 67.48°. F(000) = 1424, T = 293(2) K, R1 = 0.0355 and wR2 = 0.0942
with a GOF on F2 = 1.045. Data/restraints/parameters: 9263/0/405. For
crystallographic data in CIF and perspective drawings of these struc-
tures, see the Supporting Information.
r
2009 American Chemical Society
Published on Web 05/19/2009
pubs.acs.org/Organometallics

3598 Organometallics, Vol. 28, No. 13, 2009
Communication
which is only slightly longer than the formal Rh-Rh bond
˚
distance of 2.8-3.2 A, between two adjacent [Rh-
(CNR)2(bpy)]+ units is suggestive of a weak metal-metal
interaction. In the red crystalline material, the shortest
distance between two neighboring [Rh(CNR)2(bpy)]+ units
˚
is 4.39 A, which is much longer than the typical Rh-Rh
I
˚
distances of 3.2-3.9 A in stacked Rh complexes with
significant metal-metal interactions.3 Therefore, the green
and the red crystals can be considered as dimeric and
monomeric forms of the complex. The dimeric nature of
the units in the green crystal was further supported by π-π
interactions between the bipyridyl units of neighboring
complexes. Although different solvent molecules were found
in the X-ray crystal structures of the red form and the green
form, the significant difference in the color of these forms is
not the result of the interaction between the complex and the
specific solvent molecule since the red- and green-colored
solids could also be obtained by the recrystallization of these
complexes in other solvent systems. As with other chelating
diimine ligands,5 the angles subtended by the nitrogen atoms
of the bipyridine at the Rh center, N-Rh-N, were about
78.2° in both forms, which is much smaller than the ideal
angle of 90° in the square-planar geometry.
The two forms of the solids or crystals of 1-5 dissolved in
acetone to give yellow to orange solutions with identical
absorption spectra.
A moderately intense absorption
shoulder at 455-490 nm (Table 1), with the absorption
energy following the trend of 1 (462 nm) > 2 (475 nm) >
3 (490 nm) for the same 2,6-dimethylphenyl isocyanide
ligands and 5 (455 nm) > 4 (470 nm) > 2 (475 nm) for the
same bipyridine ligands, was also observed. This absorption
energy trend is suggestive of the metal-to-ligand charge
transfer (MLCT) transition of [dπ(Rh) f π*(N-N)], but it
probably includes some mixing from metal-centered and li-
gand-to-ligand charge transfer (LLCT) [π(RNC) f π*(N-N)]
transitions. In general, the presence of electron-donating
substituents on the diimine ligand should raise the π* orbital
energies of the ligand, whereas a better stabilized dπ(Rh)
orbital should result in the presence of better π-accep-
ting isocyanide ligands, both of which would lead to a
higher MLCT energy. Consequently, using the same bipyr-
idine ligand, the absorption energy of this absorption
shoulder is in line with the different π-accepting abilities of
the isocyanide ligands: Br3-C6H2NC > Cl-C6H4NC >
(CH3)2C6H3NC.5
Figure 1. Thermal ellipsoid plot (40% probability level) of the
crystal structures of both(a) the dark green and (b) red forms of 1.
Scheme 1. Synthetic Route to Bis(phenylisocyano) Rhodium(I)
Diimine Complexes
Upon cooling, the yellow solutions of 1-5 changed to a
green color with the evolution of a new absorption band at
ca. 629-678 nm, which grew in intensity and showed a slight
red shift as temperature decreased (Figure 2a). The close
resemblance of this absorption band with the green crystal’s
absorption (Figure S5, Supporting Information) suggested
The Rh atom in both forms of 1 adopted a square-planar
geometry with the phenyl ring of one of the isocyanide
ligands twisted at an interplanar angle of ca. 51° with respect
to the plane of the complexes. One crystallographic asym-
metric unit of the red form consists of 1 and an acetone,
whereas that of the green form consists of 1 and half a diethyl
ether. The [Rh(CNR)2(bpy)]+ units in the crystal of the dark
green material are stacked with their orientations alternately
rotated by 90° and 180° with respect to the adjacent units.
The 90° and 180° rotations correspond to the shorter
that this new absorption band be ascribed to the absorption
2+
of the dimeric form of the complex, [Rh(CNR)2(N-N)]2
.
The assignment is further supported by the linear relation-
ship between [5]T/(A655
1/2 and (A655 1/2 (Figure 2a, inset) at a
)
)
constant temperature,3d,7 where [5]T is the total concentra-
tion of 5 and A655 is the absorbance at λmax of the new
absorption band. At a concentration of 0.97 mM in acetone
solution, this new absorption of 4 and 5 evolved at ca. 253 K,
whereas for 1, 2, and 3, it evolved at ca. 203 K. This may be
attributed to the more sterically demanding nature of the
˚
Rh-Rh distances (3.48 A) and the longer Rh-Rh distances
˚
(4.68 A), respectively (Figure 1a). The observation of the
(7) See the Supporting Information for the equation used in equili-
brium analysis.
˚
relatively short Rh-Rh distance in the range of 3.2-3.9 A,

Communication
Organometallics, Vol. 28, No. 13, 2009 3599
Table 1. Photophysical Data for Complexes 1-5
complex
emissiona λem/ nm (τo/μs)
T/K
absorptionb λabs/nm (ε/dm3 mol-1 cm-1
)
1
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)
298
183
298
183
298
183
298
183
298
183
369 (10 760), 462 (975)
629c
375 (10 055), 475 (840)
2
3
4
5
654c
362 (10 200), 388 (10 365), 490 (880)
670c
372 (11 550), 470 (960)
678c
366 (11 285), 455 (850)
655c
a Measured in EtOH/MeOH (4:1 v/v) glass at 77 K upon excitation at 460 nm. Emission maxima are uncorrected values. b In acetone. c Absorption of
2+
.
the dimeric unit of the complexes [Rh(CNR)2(N-N)]2
Figure 2. (a) UV-vis absorption spectral changes of a 0.97 mM acetone solution of 5 at different temperatures. The inset shows the
1/2
plot of [5]T/(A655
(c) Dimerization of 5.
)
vs (A655)
1/2. (b) Overlaid uncorrected emission spectra of 1-3 and 5 in EtOH/MeOH 4:1 (v/v) glass at 77 K.
2,6-dimethylphenylisocyanide ligands, in which at least one
of the phenyl moieties is twisted with respect to the plane of
the Rh complexes and exerts a steric repulsion on the other
monomer in the dimeric form, rendering the dimeric unit less
stable. In light of previous spectroscopic work on binuclear
Rh(I) complexes with bridging isocyanide ligands,3b-3f,3h
this absorption is tentatively assigned to the transition from
2 (654 nm) and 5 (655 nm). This may be attributed to
the decreased steric bulk of the isocyanide ligands in
4 (Cl–C6H4NC), rendering it easier to achieve the shorter
Rh-Rh distance in the dimeric unit and resulting in a red-
shifted absorption energy.3a-3h Similarly, a slight red shift of
this absorption band with decreasing temperature may be
ascribed to the possible shortening of the Rh-Rh distance in
the dimer units as the temperature decreased.
The solids and solutions of these complexes are non-
emissive at room temperature, but they become strongly
luminescent with emission maximum of 565-588 nm
(Figure 2b, Table 1) upon excitation into the absorption
shoulder at ca. 460 nm in EtOH/MeOH 4:1 (v/v) glass at
77 K. This emission was attributed to the emission of the
monomeric complex, as the emission energy is higher than
the lowest-energy absorption of the dimeric form. This
emission showed an energy dependence similar to the low-
est-energy absorption shoulder in the acetone solution, in
which the emission maximum is consistent with the
π-accepting ability of the diimine ligand and is in the reverse
dσ*[dz2*(Rh)] to pσ[pz(Rh)] mixed with some π*(N-N)
2+
character from the dimeric complex [Rh(CNR)2(N-N)]2
.
Thus, the absorption energy of this new absorption band is
sensitive to both the steric effects from the phenylisocyanide
ligands and the π-accepting ability of the diimine ligands.
Consequently, among complexes with the same isocyanide
ligands, this new absorption for 3 (678 nm) is significantly
red-shifted relative to 2 (654 nm), as the diimine ligand of
3 (5,6-Br2phen) has a better π-accepting ability. On the other
hand, the absorption energy of this band also showed
significant dependence on the steric factors from the iso-
cyanide ligands. With the same bipyridine ligand, the new
absorption of 4 (670 nm) is red-shifted compared to those of

3600 Organometallics, Vol. 28, No. 13, 2009
Communication
order of the π-accepting ability of the isocyanide ligands.
Thus, these emissions were also tentatively assigned to
originate from the MLCT [dπ(Rh) f π*(N-N)] excited-
the aggregation and photophysical properties and to design
functional molecular devices are now in progress.
3
state origin. The extended, submicrosecond excited-state
lifetime observed in glass medium at 77 K also supports its
triplet origin.
Acknowledgment. The work described in this paper
was supported by a RGC GRF grant from the Research
Grants Council of Hong Kong (Project No. CityU
101008) and a grant from City University of Hong Kong
(Project No. 7002435). H.F. acknowledges the receipt of a
postgraduate studentship from City University of Hong
Kong.
In summary, a series of bis(phenylisocyano) rhodium(I)
diimine complexes with thermally mediated aggregation
properties have been synthesized, and their photophysical
and thermochromic properties have been studied. By varying
the substituent on the nitrogen atom of the isocyanide
ligands, the electronic and steric nature of the isocyanide
ligands and the subsequent photophysical, thermochromic,
and aggregation properties of the complexes could be readily
tuned. These studies will be useful in designing Rh(I) poly-
pyridyl complexes for thermally responsive molecular de-
vices and Rh(I) complexes with controllable aggregation
properties. Further modifications of this system to elucidate
Supporting Information Available: Synthetic procedures
and characterization for 1-5, derivation of equation for
equilibrium analysis, solid-state reflectance spectrum of 5,
decay analysis of TCSPC data, Ortep drawings, and CIF
files giving crystal data for both forms of 1. This material
is available free of charge via the Internet at http://pubs.
acs.org.