Fac-tricarbonyl rhenium(I) azadipyrromethene complexes

Article abstract of DOI:10.1021/om900552e

Rhenium(I) complexes ofazadipyrromethene ligands are reported; three have been characterized crystallographically. The free ligands and their metallo-complexes undergo reductive electrochemistry. Red-light absorption results from optically allowed transit

Full text of DOI:10.1021/om900552e

Organometallics 2009, 28, 5837–5840 5837
DOI: 10.1021/om900552e
fac-Tricarbonyl Rhenium(I) Azadipyrromethene Complexes
David V. Partyka,† Nihal Deligonul,‡ Marlena P. Washington,‡ and Thomas G. Gray*,‡
‡Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, and †Creative
Chemistry L.L.C., Cleveland, Ohio 44106
Received June 25, 2009
Summary: Rhenium(I) complexes of azadipyrromethene ligands
are reported; three have been characterized crystallographi-
cally. The free ligands and their metallo-complexes undergo
reductive electrochemistry. Red-light absorption results from
optically allowed transitions to a ligand-localized LUMO.
as luminescence probes; when bound to fac-[99mTc(CO)3]þ,
as potential radioimaging agents. Technetium and rhenium
are strongly homologous, and the design strategy extends to
target-selective probes. The substitutional inertness of
rhenium(I) assists biological applications. A variety of donor
atoms stably bind Re(I) and enable tagging with bioactive
moieties. Among such ligands are isonitriles,16 imidazoles,17
and thioethers.18
The diimine carbonyls of rhenium(I) support varied
applications.19-23 However, limited visible absorption re-
stricts their photochemical uses. Absorption typically sets in
below 450 nm. In a series of 20 rhenium(I) tricarbonyl
diimine complexes, MLCT absorption maxima range from
330 nm (ε=4000 M-1 cm-1) to 402 nm (ε=810 M-1 cm-1).24
Combining the triplet photoproperties of rhenium carbonyl
diimines with intense uptake of long-wavelength visible light
(g600 nm) creates appealing prospects.
Mononuclear rhenium(I) carbonyl complexes draw con-
tinuing scrutiny for their ground- and excited-state proper-
ties. Tricarbonyl rhenium(I) bipyridine and phenanthro-
line complexes, all with facial stereochemistry, emit from
triplet excited states. Excitation into metal-to-ligand charge-
transfer absorptions yields submicrosecond luminescence at
room temperature, with longer lifetimes upon cooling.1-11
Emission from these complexes obeys the approximate
energy-gap law;12,13 luminescence lifetimes, maxima, and
quantum yields respond to changes in the metal-ion coordi-
nation sphere or along the diimine perimeter.
Facial rhenium(I) carbonyls are valuable apart from their
photochemical properties. 186Re and 188Re are beta emitters
under clinical consideration, and rhenium complexes are
often sought as cold analogues of γ-emitting 99mTc species.14
Azadipyrromethenes are a broad ligand set25-30 with
remarkable red-light-absorbing capabilities. Recent work
on boron adducts has produced efficient fluorophores and
drug candidates for human photodynamic therapy.31 The
optically allowed electronic transitions of azadipyrro-
methene complexes are ligand centered,28,29 and the li-
gands’ absorption profile is preserved in metallo-complexes.
þ
The fac-M(CO)3 (M=Tc, Re) core recurs in many com-
plexes having real or potential utility in medical imaging.
Zubieta and co-workers have disclosed chelating ligands
bearing quinoline, tryptophan, and benzimidazole donors.15
When bound to the fac-[Re(CO)3]þ core, the complexes act
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*To whom correspondence may be addressed.
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2009 American Chemical Society
Published on Web 09/29/2009
pubs.acs.org/Organometallics

5838 Organometallics, Vol. 28, No. 20, 2009
Partyka et al.
Azadipyrromethenes are chelating nitrogen-donor ligands
with superficial resemblances to bipyridines and phenan-
throlines. These observations, combined with the favorable
excited-state properties of rhenium(I) diimines, prompt in-
vestigation of (azadipyrromethene)rhenium(I) chromo-
phores.
Boron azadipyrromethenes are fluorescent; triplet emission
is not apparent despite oxygen sensitization that must result
from a triplet excited state.31,32 Azadipyrromethene complexes
of d10 metals fluoresce with quantum yields below 1.5% at
room temperature; group 12 bis(azadipyrromethenes) are
virtually nonluminescent in the visible region. Thus, emission
from d6 complexes of these ligands is a question in itself.
Scheme 1 illustrates general preparative methods and a
numbering scheme. THF complex 1 is a synthon for azadi-
pyrromethene complexes of other ligands; displacement of
the bound THF is facile. Attempts at recrystallizing 1 by
vapor diffusion of n-pentane into neat pyridine or tetrahy-
drothiophene afford the respective solvate complexes in 99%
and quantitative yields, respectively. For complex 4 the
ligand is reacted with 1 in dichloromethane solution.
Reaction of a methylene chloride solution of 1 with neat
tert-butylisonitrile yields 5 (73% isolated). Complexes 1-4
are prepared in the open air, and 5 under a nitrogen atmos-
phere. All new compounds are isolated as air-stable blue or
blue-violet solids.
Diffraction-quality crystals have been obtained for com-
plexes 2, 3, and 5.33 A thermal ellipsoid projection of 2
appears in Figure 1; those for 3 and 5, in the Supporting
Information. Rhenium azadipyrromethene-nitrogen bond
˚
˚
lengths range from 2.174(3) A in 2 to 2.199(2) A in 3; that to
˚
the pyridine nitrogen of 2 is 2.223(3) A. These interatomic
distances are normal. For comparison, the crystallographi-
cally characterized salt fac-[Re(CO)3(1,10-phenanthroline)-
(pyridine)][Co(CO)4] shows Re-Nphen distances of 2.177(13)
34
˚
˚
and 2.185(13) A and a Re-Npyridine distance of 2.21(2) A.
Figure 1. (Top) Thermal ellipsoid projection (100 K, 50%
probability) of pyridine complex 2. Hydrogen atoms are omitted
for clarity. Unlabeled atoms are carbon. (Bottom) Packing
diagram of 2 along b. A π-stacking interaction between pyridine
ligands is evident.
ꢀ
(32) Palma, A.; Tasior, M.; Frimannsson, D. O.; Vu, T. T.; Meallet-
Renault, R.; O’Shea, D. F. Org. Lett. 2009, 11, 3638–3641.
(33) (a) Crystallographic data for 2: crystal dimensions 0.37 ꢀ 0.09 ꢀ
3
0.08 mm , space group P1, a=10.3966(12) A, b=11.9293(13) A, c=
˚
˚
˚
13.3425(15) A; R=67.4560(10)°, β=85.7890(10)°, γ=86.3640(10)°; V=
3
1523.1(3) A ; Z=2; Fcalc=1.740 Mg/m3; μ(Mo KR)=4.038 mm-1; data
˚
Scheme 1
measured on a Bruker AXS SMART APEX II CCD-based diffract-
˚
ometer (Mo KR, λ=0.710 73 A) at 100 ( 2 K; structure solved by direct
methods, 17 493 reflections collected, 6637 unique reflections (Rint =
0.0247), data/restraints/parameters 6637/0/433, final R indices (I >
2σ(I)) R1=0.0210 and wR2=0.0491, R indices (all data) R1=0.0238
and wR2=0.0563; largest difference peak and hole 0.876 and -1.190 e
˚
A
0.10 mm , space group P1, a=11.9231(13) A, b=11.9357(13) A, c=
-3. (b) Crystallographic data for 3: crystal dimensions 0.47 ꢀ 0.16 ꢀ
3
˚
˚
˚
12.8357(14) A; R=63.9600(10)°, β=74.7280(10)°, γ=83.4900(10)°; V=
1583.3(3) A ; Z=2; Fcalc=1.693 Mg/m3; μ(Mo KR)=3.948 mm-1; data
3
˚
measured on a Bruker AXS SMART APEX II CCD-based diffract-
˚
ometer (Mo KR, λ=0.710 73 A) at 100 ( 2 K; structure solved by direct
methods, 19 031 reflections collected, 7219 unique reflections (Rint =
0.0180), data/restraints/parameters 7219/0/424, final R indices (I >
2σ(I)) R1=0.0215 and wR2=0.0550, R indices (all data) R1=0.0226
and wR2=0.0555; largest difference peak and hole 1.196 and -0.894 e
˚
A
0.32 mm , space group P21/n; a=15.722(20) A, b=12.3980(18) A, c=
-3. (c) Crystallographic data for 5: crystal dimensions 0.39 ꢀ 0.34 ꢀ
3
˚
˚
3
˚
˚
17.657(3) A; β=105.1090(10)°; V=3322.7(8) A ; Z=4; Fcalc=1.603 Mg/
m3; μ(Mo KR)=3.702 mm-1; data measured on a Bruker AXS SMART
˚
APEX II CCD-based diffractometer (Mo KR, λ=0.710 73 A) at 100 ( 2
K; structure solved by direct methods, 28 209 reflections collected, 5806
unique reflections (Rint = 0.0402), data/restraints/parameters 5806/0/
436, final R indices (I > 2σ(I)) R1=0.0378 and wR2=0.1268, R indices
(all data) R1=0.0404 and wR2=0.1324; largest difference peak and hole
The mean plane of the azadipyrromethene core tilts relative
to the equatorial CCO-Re-CCO plane by 36.7° (CCO repre-
sents the two cis carbonyl carbons) in pyridine complex
2. Similar canting occurs for 3 and 5 and in the crystal
-3
˚
0.898 and -1.688 e A
.
(34) Lucia, L. A.; Abboud, K.; Schanze, K. S. Inorg. Chem. 1997, 36,
6224–6234.

Communication
Organometallics, Vol. 28, No. 20, 2009 5839
Figure 2. Absorption spectra of rhenium(I) complexes 1-5
(solid) and free HLa (dashed) in CHCl3 at room temperature.
Figure 3. Cyclic voltammograms (100 mV s-1) of 1.0 mM 1 and
HLa (inset) with 0.10 M Bu4NPF6 supporting electrolyte. Peak
potentials vs the saturated calomel electrode are indicated.
structures of azadipyrromethenes bound to planar trigonal
metal sites.28 The rhenium-thioether sulfur bond in THT
complex 3 is 2.5415(7) A, somewhat longer than the 2.502-
˚
35
˚
Table 1. Voltammetric Data for New Complexes in THF
(100 mV s-1, 0.1 M Bu4NPF6)
(2) A Re-S bonds found in fac-[Re(CO)3(THT)3]. The
rhenium-isonitrile carbon bond length in 5 is 2.108(6) A. In
a mixed-ligand rhenium(I) tris(tert-butylisonitrile) complex,
˚
compound
E1/2 (V vs SCE)
˚
1.927(8), 1.974(8), and 2.003(8) A Re-C bonds were mea-
sured.36 Observed rhenium-carbon and carbon-oxygen
HLa
-0.705, -1.325
-0.812, -1.529a
-0.831, -1.670a
-0.822, -1.541a
-0.926, -1.751a
-0.893, -1.648a
1
2
3
4
5
bond lengths are unexceptional.37-42
Figure 2 depicts the absorption spectra of rhenium
complexes 1-5 in chloroform solution. The spectral signa-
tures of azadipyrromethenes are apparent: an intense (ε =
40 000-55 000 M-1 cm-1) structureless absorption peak at
ca. 590 nm and pronounced absorptions (ε > 20 000 M-1
cm-1) near 290 nm. For comparison, the absorption spec-
trum of the ligand also appears (dashed line). The visible
absorption maximum is more intense relative to that of free
HLa. Higher-energy absorptions in the ultraviolet also in-
tensify. Absorption spectra of other rhenium(I) azadipyrro-
methenes are similar. In contrast to diimine analogues,
emission has not been detected below 800 nm in the new
azadipyrromethene complexes at room temperature or 77 K.
This observation echoes earlier findings in studies of d10
complexes of azadipyrromethene ligands. In these cases,
emission is minimal, and the low Stokes shift indicates singlet
parentage.28
Tricarbonyl rhenium(I) azadipyrromethene complexes are
electroactive. Figure 3 depicts cyclic voltammograms of free
tetraphenylazadipyrromethene and of 1 in 0.1 M Bu4NPF6
in THF, relative to the saturated calomel electrode. All
voltammograms were measured with a 0.001 M concentra-
tion of azadipyrromethene or of complex. The ferrocene/
ferrocenium couple appeared as a reversible redox wave at
0.545 V. The free ligand HLa undergoes cleanly reversible
a Irreversible; cathodic peak maxima are indicated.
reductions at -0.705 and -1.325 V. The voltammogram of 1
is more complicated, with the second reduction being irre-
versible and smaller features tentatively assigned as the redox
events of decomposition products. The first reduction shifts
cathodically by 107 mV in 1 compared to the free ligand,
indicating that binding to the fac-[Re(CO)3(solv)]þ moiety
widens the HOMO-LUMO gap, relative to free HLa.
Table 1 gathers electrode potentials measured for all new
compounds; similar conclusions apply.
Density-functional theory calculations find that the aza-
dipyrromethene dominates the frontier orbitals of 2. Figure
S3 (Supporting Information) depicts an orbital correlation
diagram of optimized 2. A harmonic frequency calculation
verifies that the converged structure is a local minimum of
the potential energy hypersurface. The highest-occupied
Kohn-Sham orbital (HOMO) of 2 is 95% azadipyrro-
methene localized; the lowest-unoccupied orbital (LUMO)
has 98% azadipyrromethene character. Percentage compo-
sitions derive from a Mulliken population analysis43 of the
electron density. The calculated HOMO-LUMO gap is
1.45 eV, and a 0.95 eV energy gap separates the LUMO
from the LUMOþ1, which is a pyridine π* combination. For
comparison, the HOMO-LUMO gap of optimized HLa is
narrower, 1.32 eV. The low-lying, isolated LUMO suggests
electrochemical reversibility, as observed. The HOMOs-1,
-2, and -3 have appreciable rhenium density that derives
from the metal dxy, dyz, and dxz orbitals.
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predict three optical transitions with sizable oscillator
strengths at wavelengths > 550 nm. Singlet excited states
are calculated at 1.86 eV (668 nm), 1.87 eV (662 nm), and 2.21
eV (562 nm). These consist of one-electron promotions that
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5840 Organometallics, Vol. 28, No. 20, 2009
Partyka et al.
undergo configuration interaction. The major contributors
in each are LUMOrHOMO, LUMOrHOMO-1, and
LUMOrHOMO-2 excitations. The most intense is the
transition calculated at 562 nm, which consists mainly of
LUMOrHOMO-2 (38%) and LUMOrHOMO (26%)
transitions. Gaussian convolution44 (fwhm 3000 cm-1) of
all transitions beyond 500 nm yields a single asymmetric
peak at 563 nm, in fair agreement with the absorption
maximum of 2 (591 nm) in CHCl3. Figure S3, Supporting
Information, plots frontier orbitals of 2 as insets at right. All
such orbitals have preponderant (g52%) azadipyrro-
methene character. The HOMO-1 and HOMO-2 bear
29% and 26% rhenium character, respectively. The visible
absorptions of (tricarbonyl)azadipyrromethene complexes
are primarily ligand centered with some metal-to-ligand
charge-transfer admixture.
In summary, we have prepared fac-tricarbonyl rhenium(I)
azadipyrromethene complexes by reaction of fac-[Re(CO)3-
(H2O)3]Cl with the free chromophore in the presence of base.
The absorption features of azadipyrromethenes and their
boron chelates carry over to rhenium(I) complexes. A fifth
ligand completes the coordination sphere: this can be an
oxygen, nitrogen, carbon, or sulfur donor. Absorption pro-
files are insensitive to this fifth ligand. Its relative lability
promotes rhenium attachment to Lewis bases in varied
environments. Rhenium azadipyrromethenes and the
free ligand undergo reductive electrochemistry. A second
reduction event that is reversible for the free ligand is
irreversible in metallo-complexes, likely indicating participa-
tion of the fac-[Re(CO)3(L)]þ moiety. Time-dependent DFT
calculations indicate that transitions to the LUMO account
for red-light absorption of azadipyrromethene complexes.
The optical and electrochemical properties of rhenium(I)
azadipyrromethenes, including their use in light capture, are
subjects of continuing investigation.
Acknowledgment. The authors thank the National
Science Foundation (grant CHE-0749086 to T.G.G.)
for support. M.P.W. was supported by National Science
Foundation grant CHE-0748982 to J. D. Protasiewicz.
The diffractometer at Case Western Reserve was funded
by NSF grant CHE-0541766. T.G.G. is an Alfred P.
Sloan Foundation Fellow. N.D. holds a Republic of
Turkey Ministry of National Education fellowship. We
thank Mr. T. J. Robilotto, CWRU, and Mr. T. S. Teets,
Massachusetts Institute of Technology, for experimental
assistance; Professor J. D. Protasiewicz, CWRU, for
access to instrumentation; and Dr. S. Y. Reece, Sun
Catalytix, for valuable discussion.
Supporting Information Available: Experimental procedures
and characterization data for new compounds. Thermal ellips-
oid diagrams of 3 and 5; optimized Cartesian coordinates,
partial Kohn-Sham orbital energy-level diagram of 2, and
optimized bond lengths; crystallographic data in CIF format.
This material is available free of charge via the internet at http://
pubs.acs.org.
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