Luminescent rhenium(I) phenanthroline complexes with a benzoxazol-2-ylidene ligand: Synthesis, characterization, and photophysical study

Article abstract of DOI:10.1021/om300526e

A series of luminescent rhenium(I) phenanthroline complexes containing benzoxazol-2-ylidene ligands with the general formula {Re(CO) ₃(phen)[CN(X)C₆H₄-2-O]}⁺ and cis,trans-{Re(CO)₂(phen)(L)[CN(H)C₆H₄-2-O]} ⁺ (X = H, methyl; phen = 1,10-phenanthroline; L = PPh₃, PPh₂Me, P(OEt)₃) have been synthesized and characterized. The X-ray crystal structures of most of the carbene complexes and some of their synthetic precursors have also been determined. A new synthetic methodology for the preparation of dicarbonyl rhenium diimine synthetic precursors with a labile acetonitrile ligand, [Re(CO)₂(phen)(PR₃)(MeCN)] ⁺, was developed. Photophysical study shows that these carbene complexes display a green to red ³MLLCT [dπ(Re) → π*(N-N)] phosphorescence at room temperature. The N-deprotonations of the benzoxazol-2-ylidene ligand in these complexes were investigated.

Full text of DOI:10.1021/om300526e

Article
pubs.acs.org/Organometallics
Luminescent Rhenium(I) Phenanthroline Complexes with
a Benzoxazol-2-ylidene Ligand: Synthesis, Characterization,
and Photophysical Study
Chi-Chiu Ko,* Chi-On Ng, and Shek-Man Yiu
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong,
People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of luminescent rhenium(I) phenanthroline complexes containing benzoxazol-2-ylidene ligands with the
general formula {Re(CO)3(phen)[CN(X)C6H4-2-O]}+ and cis,trans-{Re(CO)2(phen)(L)[CN(H)C6H4-2-O]}+ (X = H, methyl;
phen = 1,10-phenanthroline; L = PPh3, PPh2Me, P(OEt)3) have been synthesized and characterized. The X-ray crystal structures
of most of the carbene complexes and some of their synthetic precursors have also been determined. A new synthetic
methodology for the preparation of dicarbonyl rhenium diimine synthetic precursors with a labile acetonitrile ligand,
[Re(CO)2(phen)(PR3)(MeCN)]+, was developed. Photophysical study shows that these carbene complexes display a green to
red 3MLLCT [dπ(Re) → π*(N−N)] phosphorescence at room temperature. The N-deprotonations of the benzoxazol-2-ylidene
ligand in these complexes were investigated.
diimine complexes with NHC ligands. It is anticipated that these
NHC rhenium(I) diimine complexes will possess better photo-
and thermal stability, as NHC ligands are much less susceptible
to nucleophilic attack compared to isocyanide.3m,7 Herein, we
report the synthesis, structures, photophysics, and electro-
chemistry of a series of carbonyl rhenium(I) phenanthroline
complexes with a benzoxazol-2-ylidene ligand. A new synthetic
methodology for dicarbonyl rhenium(I) diimine synthetic
precursors, cis,trans-[Re(CO)2(diimine)(PR3)(MeCN)]+, was
also described.
INTRODUCTION
■
Transition metal complexes with N-heterocyclic carbene
(NHC) were first documented in 1968.1 However, they have
not received significant attention until the first report of a
stable NHC ligand2 and the subsequent application studies of
these complexes for various catalytic properties.3 Compared with
free NHC ligands, the carbene ligand in transition metal
complexes is much more stable, as its lone-pair electrons on the
carbene carbon become unavailable after coordination. More-
over, upon coordination to metal centers with d-electrons, the
pπ orbital of the carbene C is stabilized by delocalization of the
filled pπ orbital of the adjacent heteroatom and filled dπ orbital
of its coordinated metal center. In addition to their unique
catalytic properties,3 many NHC transition metal complexes also
display interesting photophysical and luminescent properties.4
The ease of functionalization, unique electronic properties, and
robustness of NHC ligands have made them ideal ligands in the
design of tunable luminescent NHC transition metal complexes
with enhanced stability and novel photophysical properties.4
As metal isocyanide complexes are excellent synthetic precursors
for NHC metal complexes,5 we have extended our recent
work on luminescent isocyano rhenium(I) diimine complexes6
to synthesize a new class of tunable luminescent rhenium(I)
1
Physical Measurements and Instrumentation. H and
13C NMR spectra were recorded on a Bruker AV400 (400 MHz)
FT-NMR spectrometer. Chemical shifts (δ, ppm) were reported
relative to tetramethylsilane (Me4Si). IR spectra were obtained
from KBr discs by using a Perkin-Elmer Spectrum 100 FTIR
spectrophotometer. All positive-ion ESI mass spectra were
recorded on a PE-SCIEX API 150 EX single quadrupole mass
spectrometer. Elemental analyses of all compounds were
performed on an Elementar Vario MICRO Cube elemental
analyzer.
Received: June 12, 2012
Published: October 2, 2012
© 2012 American Chemical Society
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2-Trimethylsiloxyphenyl isocyanide was prepared according to a
reported procedure.13 2-Methoxyphenyl isocyanide (2-MeOC6H4NC)
was synthesized from 2-methoxyphenyl formamide14 using the
synthetic methodology developed by Ugi and co-workers.15 THF
and benzene were distilled over sodium prior to use. All other reagents
and solvents were of analytical grade and used as received.
General Procedure. All reactions were performed under strictly
anaerobic and anhydrous conditions in an inert atmosphere of argon
using standard Schlenk techniques.
Electronic absorption spectra were recorded on a Hewlett-
Packard 8452A diode array spectrophotometer. Steady-state
emission and excitation spectra at room temperature and at
77 K were recorded on a SPEX FluoroLog 3-TCSPC spectro-
fluorometer. Solutions were rigorously degassed on a high-
vacuum line in a two-compartment cell with no less than four
successive freeze−pump−thaw cycles. 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. The emission lifetimes were measured in the MCS or Fast
MCS lifetime mode with NanoLED-375LH (λex = 375 nm; pulse
width <750 ps) as the excitation source. The photon counting data
were analyzed by Horiba Jobin Yvon decay analysis software.
Luminescence quantum yields were measured by the optically
dilute method described by Demas and Crosby8 with an aqueous
solution of [Ru(bpy)3]Cl2 (ϕem = 0.0429 with 436 nm excitation)
as reference. Cyclic voltammetry measurements were performed
by using a CH Instruments, Inc., model CHI 620 electrochemical
analyzer. Electrochemical measurements were performed in
Synthesis of cis,trans-[Re(CO)2(phen)(PPh3)(MeCN)](CF3SO3).
To a solution of [Re(CO)3(phen)(PPh3)](CF3SO3) (100 mg, 116 μmol)
in MeCN (30 mL) was added trimethylamine N-oxide dihydrate
(19.3 mg, 174 μmol). The resulting solution was refluxed for 18 h.
After removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel using dichloromethane−
acetone−acetonitrile (20:10:1 v/v/v) as eluent to give analytically pure
complex as an orange solid. Yield: 67.4 mg, 77.0 μmol; 66.4%. 1H NMR
(400 MHz, CDCl3, 298 K): δ 8.77 (dd, 2H, J = 5.1, 1.4 Hz, 2,9-phen
H’s), 8.59 (dd, 2H, J = 8.2, 1.4 Hz, 4,7-phen H’s), 8.14 (s, 2H, 5,6-phen
H’s), 7.56 (dd, 2H, J = 8.2, 5.1 Hz, 3,8-phen H’s), 7.23 (m, 3H, phenyl
H’s), 7.12 (m, 6H, phenyl H’s), 7.03 (m, 6H, phenyl H’s), 2.14 (s, 3H,
methyl H’s). Positive-ion ESI-MS: m/z 726 [M − CF3SO3]+. IR (KBr
disk): ν/cm−1 1942, 1871 (CO). Anal. Calcd (%) for C35H26F3N3-
O5PReS (874.84): C 48.05, H 3.00, N 4.80. Found: C 47.86, H 3.32,
N 4.41.
Synthesis of cis,trans-[Re(CO)2(phen)(PPh2Me)(MeCN)]-
(CF3SO3). The complex was prepared according to a procedure
similar to that for cis,trans-[Re(CO)2(phen)(PPh3)(MeCN)]-
(CF3SO3) except [Re(CO)3(phen)(PPh2Me)](CF3SO3) (92.8 mg,
116 μmol) was used in place of [Re(CO)3(phen)(PPh3)](CF3SO3).
Yield: 57.9 mg, 71.2 μmol; 61.4%. 1H NMR (400 MHz, CDCl3, 298 K):
δ 8.98 (dd, 2H, J = 5.2, 1.3 Hz, 2,9-phen H’s), 8.61 (dd, 2H, J = 8.1, 1.3
Hz, 4,7-phen H’s), 8.12 (s, 2H, 5,6-phen H’s), 7.69 (dd, 2H, J = 8.1, 5.2
Hz, 3,8-phen H’s), 7.21 (td, 2H, J = 7.4, 1.9 Hz, phenyl H’s), 7.09 (td, 4H,
J = 7.9, 1.9 Hz, phenyl H’s), 6.90 (m, 4H, phenyl H’s), 2.16 (s, 3H,
methyl H’s), 1.57 (d, 3H, JPH = 8.3 Hz, methyl H’s). Positive-ion ESI-MS:
m/z 664 [M − CF3SO3]+. IR (KBr disk): ν/cm−1 1934, 1861 (CO).
Anal. Calcd (%) for C30H24F3N3O5PReS (821.77): C 44.33, H 2.98, N
5.17. Found: C 44.31, H 2.71, N 4.97.
Synthesis of cis,trans-{Re(CO)2(phen)[P(OEt)3](MeCN)}-
(CF3SO3). The complex was prepared according to a procedure
similar to that for cis,trans-[Re(CO)2(phen)(PPh3)(MeCN)]-
(CF3SO3) except {Re(CO)3(phen)[P(OEt)3]}(CF3SO3) (88.8 mg,
116 μmol) was used in place of [Re(CO)3(phen)(PPh3)](CF3SO3).
Yield: 63.3 mg, 81.3 μmol; 70.1%. 1H NMR (400 MHz, CDCl3, 298 K,
TMS): δ 9.31 (dd, 2H, J = 5.1, 1.4 Hz, 2,9-phen H’s), 8.74 (dd, 2H, J =
8.3, 1.4 Hz, 4,7-phen H’s), 8.19 (s, 2H, 5,6-phen H’s), 7.95 (dd, 2H,
J = 8.3, 5.1 Hz, 3,8-phen H’s), 3.66 (q, 6H, J = 7.0 Hz, ethyl H’s), 0.90
(t, 9H, J = 7.0 Hz, ethyl H’s). Positive-ion ESI-MS: m/z 630 [M −
CF3SO3]+. IR (KBr disk): ν/cm−1 1949, 1869 (CO). Anal. Calcd
(%) for C23H26F3N3O8PReS (778.71): C 35.47, H 3.37, N 5.40.
Found: C 35.52, H 3.61, N 5.34.
n
acetonitrile solutions with 0.1 M Bu4NPF6 as the supporting
electrolyte at room temperature with a glassy carbon electrode
(CH Instruments, Inc.) as working electrode, a platinum wire as
the counter electrode, and Ag/AgNO3 (0.01 M in acetonitrile)
(CH Instruments, Inc.) as the reference electrode. The working
electrode surface was polished with a 1 μm α-alumina slurry and
then a 0.3 μm α-alumina slurry (Linde) on a microcloth (Buehler
Co.). The ferrocenium/ferrocene couple (FeCp2+/0) was used as
the internal reference. All solutions for electrochemical studies
were deaerated with prepurified argon gas prior to measurements.
The acid dissociation constants (pKa) of 2 and 4−6 (0.04 mM) in
n
acetonitrile/water (9:1, v/v, 0.05 M Bu4NPF6) were determined
by using a potentiometric titration described in the literature.10
Crystal Structure Determinations. Single crystals of 1−3
and 5−7 suitable for X-ray diffraction studies were obtained by
slow diffusion of diethyl ether vapor into acetone solutions of
the complexes. The crystal structures were determined on an
Oxford Diffraction Gemini S Ultra X-ray single-crystal
diffractometer using graphite-monochromatized Cu Kα radia-
tion (λ = 1.5418 Å) or Mo Kα radiation (λ = 0.7107 Å). The
experimental parameters for the structure determination data
are summarized in Table 1. All structures were solved by direct
methods employing the SHELXS-97 program.11 Rhenium and
many non-hydrogen 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 program.11 In the final stage of least-
squares refinement all non-hydrogen atoms were refined
anisotropically. The positions of hydrogen atoms were
calculated on the basis of the riding mode with thermal
parameters equal to 1.5 times that for methyl H atoms and 1.2
times that of the associated carbon or nitrogen and participated
in the calculation of final R-indices.
Synthesis of [Re(CO)3(phen)(CNC6H4-2-O)] (1) and {Re-
(CO)3(phen)[CN(H)C6H4-2-O]}(PF6) (2). [Re(CO)3(phen)(MeCN)]-
(CF3SO3) (100 mg, 156 μmol) and 2-trimethylsiloxyphenyl isocyanide
(35.8 mg, 187 μmol) were dissolved in THF (50 mL). The resulting
solution was refluxed overnight. After removal of the solvent under
reduced pressure, the residue was purified by column chromatography
on alumina using dichloromethane−acetone (9:1 v/v) as eluent to give
analytically pure complex 1, which is the N-deprotonated form of 2, as
a yellow crystalline solid. 1H NMR (400 MHz, CDCl3, 298 K): δ 9.61
(dd, 2H, J = 5.1, 1.3 Hz, 2,9-phen H’s), 8.50 (dd, 2H, J = 8.2, 1.3 Hz,
4,7-phen H’s), 7.98 (s, 2H, 5,6-phen H’s), 7.86 (dd, 2H, J = 8.2, 5.1
Hz, 3,8-phen H’s), 7.24 (dd, 1H, J = 7.0, 1.9 Hz, phenyl H’s), 7.02 (dd,
1H, J = 7.0, 1.8 Hz, phenyl H’s), 6.86 (m, 2H, phenyl H’s). 13C NMR
(100 MHz, CDCl3, 298 K): δ 214.5, 199.6, 198.3, 153.5, 152.7, 147.3,
137.1, 130.4, 127.4, 125.7, 121.4, 121.1, 117.3, 116.3, 108.5. Positive-ion
ESI-MS: m/z 570 [M + H]+. IR (KBr disk): ν/cm−1 2009, 1913, 1883
(CO). Anal. Calcd (%) for C22H12N3O4Re (569.04): C 46.47,
EXPERIMENTAL SECTION
■
Materials and Reagents. [Re(CO)5Br] was obtained from Strem
Chemicals, Inc. 1,10-Phenanthroline (phen), benzoxazole, triphenyl-
phosphine (PPh3), diphenylmethylphosphine (PPh2Me), triethylphos-
phite [P(OEt)3], dimethyl sulfate, and trimethylamine N-oxide
dihydrate (Me3NO·2H2O) were obtained from Aldrich Chemical
Co. [Re(CO)3(phen)(MeCN)](CF3SO3), [Re(CO)3(phen)(PPh3)]-
(CF3SO3), [Re(CO)3(phen)(PPh2Me)](CF3SO3), and {Re-
(CO)3(phen)[P(OEt)3]}(CF3SO3) were prepared by a slight modifi-
cation of a literature procedure for related rhenium complexes.12
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H 2.13, N 7.39. Found: C 46.47, H 2.11, N 7.53. Protonation of the
methanolic solution of 1 with hydrochloric acid (0.1% in MeOH) and
subsequent metathesis reaction with a saturated methanolic solution of
67.8%. 1H NMR (400 MHz, d6-DMSO, 298 K): δ 14.01 (s, 1H, NH’s),
9.23 (dd, 1H, J = 5.2, 1.2 Hz, 2,9-phen H’s), 8.74 (dd, 2H, J = 8.2, 1.2 Hz,
4,7-phen H’s), 8.15 (s, 2H, 5,6-phen H’s), 7.89 (dd, 2H, J = 8.2, 5.2 Hz,
3,8-phen H’s), 7.35 (d, 1H, J = 7.9 Hz, phenyl H’s), 7.32 (d, 1H, J = 8.1
Hz, phenyl H’s), 7.22 (m, 3H, phenyl H’s), 7.13 (m, 5H, phenyl H’s),
6.92 (m, 4H, phenyl H’s), 1.54 (d, 3H, J = 7.3 Hz, methyl H’s). 13C
NMR (100 MHz, d6-DMSO, 298 K): δ 207.9 (d, JPC = 62.2 Hz), 203.4
−
ammonium hexafluorophosphate gave complex 2 as a PF6 salt. Slow
diffusion of diethyl ether vapor into a concentrated acetone solution of
the complex gave 2 as a pale yellow crystalline solid. Yield: 74.1 mg,
104 μmol; 66.7%. 1H NMR (400 MHz, d6-DMSO, 298 K): δ 14.82 (s,
1H, NH’s), 9.56 (dd, 2H, J = 5.2, 1.4 Hz, 2,9-phen H’s), 9.00 (dd, 2H,
J = 8.3, 1.4 Hz, 4,7-phen H’s), 8.34 (s, 2H, 5,6-phen H’s), 8.16 (dd,
2H, J = 8.3, 5.2 Hz, 3,8-phen H’s), 7.46 (m, 2H, phenyl H’s), 7.32 (td,
1H, J = 7.7, 1.2 Hz, phenyl H’s), 7.26 (td, 1H, 7.7, 1.4 Hz, phenyl H’s).
13C NMR (100 MHz, DMSO, 298 K): δ 204.9, 195.2, 192.4, 154.7,
150.9, 146.3, 139.6, 130.5, 127.8, 126.8, 125.8, 125.6, 122.2, 113.1,
111.1. Positive-ion ESI-MS: m/z 570 [M − PF6]+;. IR (KBr disk):
ν/cm−1 2031, 1942, 1912 (CO), 839 (P−F). Anal. Calcd (%) for
C22H13F6N3O4PRe (714.53): C 36.98, H 1.83, N 5.88. Found: C
36.80, H 1.62, N 5.91.
(d, JPC = 7.6 Hz), 153.7, 151.6, 146.1, 138.5, 133.8, 133.3, 131.1 (d, JPC
=
10.6 Hz), 130.6, 130.0, 128.7 (d, JPC = 9.3 Hz), 128.1, 126.8, 125.7, 125.0,
112.5, 111.0, 13.9 (d, JPC = 30.1 Hz). Positive-ion ESI-MS: m/z 742
[M − PF6]+. IR (KBr disk): ν/cm−1 1933, 1861 (CO), 841 (P−F).
Anal. Calcd (%) for C34H26F6N3O3P2Re (886.73): C 46.05, H 2.96,
N 4.74. Found: C 45.83, H 3.13, N 4.73.
Synthesis of cis,trans-{Re(CO)2(phen)[P(OEt)3][CN(H)C6H4-2-O]}(PF6)
(6). The complex was prepared according to a procedure similar to
that of 4 except cis,trans-[Re(CO)2(phen)[P(OEt)3](MeCN)]-
(CF3SO3) (53.4 mg, 68.6 μmol) was used in place of cis,trans-
[Re(CO)2(phen)(PPh3)(MeCN)](CF3SO3). Yield: 43.1 mg, 50.5
Synthesis of {Re(CO)3(phen)[CN(Me)C6H4-2-O]}(PF6) (3). To a
suspension of complex 2 (60 mg, 84.0 μmol) and K2CO3 (116 mg,
840 μmol) in acetone (30 mL) was slowly added dimethyl sulfate
(79.6 μL, 840 μmol). The reaction mixture was then stirred at room
temperature for 18 h. After removal of the solvent under reduced
pressure, the residue was redissolved in chloroform and washed with
dilute hydrochloric acid (0.1 M) and water. The organic layer was then
dried over Na2SO4 and concentrated in vacuo. The residue was
purified by column chromatography on silica gel using dichloro-
methane−acetone (4:1 v/v) as eluent. It was recrystallized by slow
diffusion of diethyl ether vapor into a concentrated acetone solution of
the complex to give analytically pure complex 3 as a dark yellow
1
μmol; 73.7%. H NMR (400 MHz, d6-DMSO, 298 K): δ 14.20 (s,
1H, NH’s), 9.45 (dd, 2H, J = 5.2, 0.9 Hz, 2,9-phen H’s), 8.91 (dd, 2H,
J = 8.2, 0.9 Hz, 4,7-phen H’s), 8.30 (s, 2H, 5,6-phen H’s), 8.09 (dd,
2H, J = 8.2, 5.2 Hz, 3,8-phen H’s), 7.39 (m, 2H, phenyl H’s), 7.27 (td,
1H, J = 7.5, 1.2 Hz, phenyl H’s), 7.20 (td, 1H, J = 7.2, 1.2 Hz, phenyl
H’s), 3.62 (q, 6H, J = 7.0 Hz, ethyl H’s), 0.77 (t, 9H, J = 7.0 Hz,
methyl H’s). 13C NMR (100 MHz, d6-DMSO, 298 K): δ 209.0 (d,
JPC = 93.2 Hz), 201.4 (d, JPC = 9.8 Hz), 154.3, 151.6, 146.9, 138.9,
130.9, 130.7, 128.1, 126.8, 125.8, 125.2, 112.8, 111.2, 60.8 (d, JPC
=
5.5 Hz), 16.1 (d, JPC = 5.9 Hz). Positive-ion ESI-MS: m/z 709 [M −
PF6]+. IR (KBr disk): ν/cm−1 1940, 1859 (CO), 844 (P−F). Anal.
Calcd (%) for C27H28F6N3O6P2Re (852.67): C 38.03, H 3.31, N 4.93.
Found: C 38.15, H 3.45, N 4.93.
1
crystalline solid. Yield: 52.4 mg, 71.9 μmol; 85.6%. H NMR (400
MHz, d6-DMSO, 298 K): δ 9.58 (dd, 2H, J = 5.2, 1.4 Hz, 2,9-phen
H’s), 9.01 (dd, 2H, J = 8.3, 1.4 Hz, 4,7-phen H’s), 8.35 (s, 2H, 5,6-
phen H’s), 8.14 (dd, 2H, J = 8.3, 5.2 Hz, 3,8-phen H’s), 7.69 (dd, 1H,
J = 8.0, 0.9 Hz, phenyl H’s), 7.39 (td, 1H, J = 6.6, 1.8 Hz, phenyl H’s),
7.25 (m, 2H, phenyl H’s), 4.03 (s, 3H, methyl H’s). 13C NMR (100
MHz, d6-DMSO, 298 K): δ 206.6, 196.1, 192.9, 155.8, 151.6, 147.5,
140.6, 132.3, 131.5, 128.8, 127.6, 126.9, 126.6, 113.8, 111.9, 35.1.
Positive-ion ESI-MS: m/z 584 [M − PF6]+. IR (KBr disk): ν/cm−1
2024, 1919, 1909 (CO), 840 (P−F). Anal. Calcd (%) for
C23H15F6N3O4PRe (728.55): C 37.92, H 2.08, N 5.77. Found: C
37.71, H 2.18, N 5.53.
Synthesis of [Re(CO)3(phen)(2-MeOC6H4NC)](PF6) (7). [Re(CO)3-
(phen)(MeCN)](CF3SO3) (100 mg, 156 μmol) and 2-MeOC6H4NC
(25 mg, 187 μmol) were dissolved in THF (50 mL). The resulting
solution was refluxed overnight. After removal of the solvent under re-
duced pressure, the residue was further purified by column chromato-
graphy on alumina using dichloromethane−acetone (9:1 v/v) as eluent.
Subsequent metathesis reaction with a saturated methanolic solution of
ammonium hexafluorophosphate gave the target complex as a PF6− salt.
Slow diffusion of diethyl ether vapor into a concentrated acetone solution
of the complex gave 7 as a pale yellow crystalline solid. Yield: 92.8 mg, 127
μmol; 81.4%. 1H NMR (400 MHz, d6-DMSO, 298 K): δ 9.55 (dd, 2H, J =
5.2, 1.4 Hz, 2,9-phen H’s), 9.08 (dd, 2H, J = 8.3, 1.4 Hz, 4,7-phen H’s),
8.41 (s, 2H, 5,6-phen H’s), 8.20 (dd, 2H, J = 8.3, 5.2 Hz, 3,8-phen H’s),
7.38 (m, 2H, phenyl H’s), 7.02 (dd, 1H, J = 9.0, 1.1 Hz, phenyl H’s), 6.89
(td, 1H, J = 7.9, 1.1 Hz, phenyl H’s), 3.38 (s, 3H, methyl H’s). 13C NMR
(100 MHz, d6-DMSO, 298 K): δ 192.2, 189.1, 155.9, 155.8, 155.2, 146.8,
140.5, 132.8, 131.2, 128.5, 127.6, 127.5, 121.1, 114.2, 112.9, 56.6. Positive-
ion ESI-MS: m/z 584 [M − PF6]+. IR (KBr disk): ν/cm−1 2181 (C
N), 2046, 1974, 1943 (CO), 839 (P−F). Anal. Calcd (%) for
C23H15F6N3O4PRe (728.55): C 37.92, H 2.08, N 5.77. Found: C 37.74,
H 2.13, N 5.81.
Synthesis of cis,trans-{Re(CO)2(phen)(PPh3)[CN(H)C6H4-2-O]}(PF6)
(4). 2-Trimethylsiloxyphenyl isocyanide (15.7 mg, 82.3 μmol)
and cis,trans-[Re(CO)2(phen)(PPh3)(MeCN)](CF3SO3) (60 mg,
68.6 μmol) were dissolved in THF (30 mL). The resulting solution
was refluxed overnight. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography on alumina
using dichloromethane−acetone (9:1 v/v) as eluent. Subsequent
metathesis reaction with a saturated methanolic solution of ammonium
hexafluorophosphate gave the target complex as a PF6− salt. It was then
further purified by slow diffusion of diethyl ether vapor into a
concentrated acetone solution of the complex, giving 4 as a pale yellow
crystalline solid. Yield: 46.8 mg, 49.3 μmol; 71.9%. 1H NMR (400 MHz,
d6-DMSO, 298 K): δ 14.15 (s, 1H, NH’s), 9.09 (dd, 2H,
J = 5.2, 1.1 Hz, 2,9-phen H’s), 8.69 (dd, 2H, J = 8.2, 1.1 Hz, 4,7-phen
H’s), 8.16 (s, 2H, 5,6-phen H’s), 7.76 (dd, 2H, J = 8.2, 5.2 Hz, 3,8-phen
H’s), 7.37 (d, 1H, J = 7.5 Hz, phenyl H’s), 7.30 (m, 12H, phenyl H’s),
6.98 (m, 6H, phenyl H’s). 13C NMR (100 MHz, d6-DMSO, 298 K): δ
206.7 (d, JPC = 62.6 Hz), 203.3 (d, JPC = 5.5 Hz), 154.0, 151.5, 146.1,
RESULTS AND DISCUSSION
■
Synthesis and Characterization. 2-Trimethylsiloxyphenyl
isocyanide was prepared from benzoxazole according to a
literature method,13 whereas 2-methoxyphenyl isocyanide (2-
MeOC6H4NC) was prepared by dehydration of 2-methox-
yphenyl formamide commonly used for the synthesis of
isocyanide ligands.15 On the basis of our recently reported
Me3NO-mediated decarbonylation reactions for carbonyl-
containing rhenium(I) complexes,16 the dicarbonyl rhenium
phenanthroline acetonitrile precursor complexes, cis,trans-
[Re(CO)2(phen)(PR3)(MeCN)](CF3SO3), with different
phosphine ligands were also prepared. The observations of
138.5, 132.6 (d, JPC = 11.4 Hz), 132.2, 131.1, 130.4, 130.3, 128.9 (d, JPC
=
9.4 Hz), 128.1, 126.8, 125.8, 125.1, 112.7, 111.0. Positive-ion ESI-MS:
m/z 803 [M − PF6]+. IR (KBr disk): ν/cm−1 1937, 1850 (CO),
844 (P−F). Anal. Calcd (%) for C39H28F6N3O3P2Re (948.80): C 49.37,
H 2.97, N 4.43. Found: C 49.29, H 3.26, N 4.42.
Synthesis of cis,trans-{Re(CO)2(phen)(PPh2Me)[CN(H)C6H4-2-O]}(PF6)
(5). The complex was prepared according to a procedure similar to
that for 4 except cis,trans-[Re(CO)2(phen)(PPh2Me)(MeCN)]-
(CF3SO3) (55.7 mg, 68.6 μmol) was used in place of cis,trans-
[Re(CO)2(phen)(PPh3)(MeCN)](CF3SO3). Yield: 41.2 mg, 46.5 μmol;
1
one set of H NMR signals for the two pyridyl moieties in the
phenanthroline ligand of all precursor complexes confirm the
symmetrical environment of the phenanthroline ligand. This
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Scheme 1. Synthetic Routes to Complexes 1−7
suggests that the carbonyl ligand trans to the phosphine ligand
is selectively decarbonylated and substituted with acetonitrile.
The geometrical arrangements of ligands in these complexes
are further confirmed by the X-ray crystal structures of cis,trans-
[Re(CO)2(phen)(PPh3)(MeCN)](CF3SO3) and cis,trans-
[Re(CO)2(phen)(PPh3Me)(MeCN)](CF3SO3) (Figure S1,
Supporting Information). Although these complexes could
also be synthesized using the photochemical ligand substitution
reaction reported by Ishitani and co-workers,17 the current
thermal method allows for a larger scale synthesis and easy
control and optimization of reaction conditions.
For phosphino rhenium(I) complexes reported by Hahn and
co-workers,5d the 2-hydroxyphenyl isocyanide ligand coordinates
in an isocyanide form without conversion into the benzoxazol-
2-ylidene ligand. In contrast, reactions of 2-trimethylsiloxyphenyl
isocyanide with precursor acetonitrile complexes, [Re(CO)3(phen)-
(MeCN)](CF3SO3) and [Re(CO)2(phen)(PR3)(MeCN)]-
(CF3SO3), gave a series of luminescent carbene-containing
rhenium(I) phenanthroline complexes (1, 2, 4−6). This is
attributed to the weaker π-back-bonding interaction between
the metal center and the 2-hydroxyphenyl isocyanide ligand in
the carbonyl rhenium(I) complexes as a result of the competition
of strong π-accepting carbonyl ligands. Hence, the isocyanide C
is more susceptible to the intramolecular nucleophilic attack to
form the carbene ligand.5b−g,18 The weak π-back-bonding inter-
action between the Re(I) metal center and the isocyanide ligand
can be reflected by the high isocyanide stretching frequency
[ν(CN) ≈ 2184 cm−1] observed in the analogous complex 7
with the 2-methoxyphenyl isocyanide ligand. As suggested by Hahn
and co-workers, the coordinated 2-hydroxyphenyl isocyanide ligand
with such a high CN stretching frequency would prefer the
formation of a carbene ligand.5b−g,18 The absence of the isocyanide
CN stretch in the IR spectra of 1−6 and the observation of the
downfield 13C signals in the range 205 to 215 ppm, which is in the
typical range for carbene-13C signals of other benzoxazol-2-ylidene
complexes,5b−g,19 in their 13C NMR spectra are supportive of the
formation of a carbene ligand in 1−6.
In the ligand substitution reaction of [Re(CO)3(phen)-
(MeCN)](CF3SO3) with 2-trimethylsiloxyphenyl isocyanide,
neutral complex 1 with an anionic N-deprotonated carbene
ligand was obtained. On the other hand, similar reactions for
[Re(CO)2(phen)(PR3)(MeCN)](CF3SO3) gave cationic com-
plexes with neutral carbene ligands. This is attributed to the
stronger acidity of the NH proton in the carbene ligand of 2, as
reflected by its lower pKa value (see below). Acidification of 1
with hydrochloric acid followed by metathesis reaction with a satu-
rated methanolic solution of ammonium hexafluorophosphate
gave 2 as a PF6− salt. Similar to other NHC ligands, the benzoxazol-
2-ylidene ligand of 2 could be readily methylated by dimethyl sulfate
in the presence of K2CO3 to give 3. All complexes 1−7 were char-
acterized by 1H NMR, 13C NMR, and IR spectroscopy, positive-ion
ESI mass spectrometry, and satisfactory elemental analyses. The
structures of complexes 1−3, 5, 6, and the CF3SO3− salt of 7 were
also determined by X-ray crystallography.
X-ray Crystal Structure. The perspective drawings of com-
plex 1 and the complex cations of 2, 3, and 5−7 are depicted in
Figure 1. The experimental details for the crystal structure deter-
minations and selected bond distances and bond angles are sum-
marized in Tables 1 and 2, respectively. The rhenium metal centers
of all complexes adopted a distorted octahedral geometry with bite
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Figure 1. Perspective drawings of (a) 1 and the complex cations of (b) 2, (c) 3, (d) 5, (e) 6, and (f) 7 with the atomic numbering. Hydrogen atoms
have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
angles of the phenanthroline ligands from 74.5° to 75.8°, commonly
observed in other rhenium(I) phenanthroline complexes.16,20 In
the crystal structures of 1−3 and 7, the three carbonyl ligands
show a typical facial arrangement, whereas a cis conformation of
the carbonyl ligands in the dicarbonyl complexes can be confirmed
by the crystal structures of 5 and 6. This confirms the selectivity of
the decarbonylation and subsequent ligand substitution reactions.
For tricarbonyl complexes 1−3 and 7, the Re−C (carbonyl)
and C−O (carbonyl) bond lengths are in the ranges 1.92−1.98
and 1.12−1.15 Å, which are similar to those reported for other
rhenium(I) tricarbonyl diimine complexes.16,20 In the structures
of the dicarbonyl complexes 5 and 6, they show shorter average
Re−C (carbonyl) bond lengths (1.89 Å vs 1.94 Å) and slightly
longer C−O (carbonyl) bond lengths (1.17 vs 1.14 Å) compared
to those observed in the tricarbonyl complexes 1−3 and 7. This
can be explained by the stronger Re−CO π-back-bonding inter-
action in the dicarbonyl rhenium(I) complexes compared to
that in the tricarbonyl complexes, as phosphines are weaker π-
accepting ligands than carbonyl ligands. For benzoxazol-2-ylidene
ligands in these structures, the Re−C (carbene) bond lengths are
in the range 2.09−2.15 Å, which are in a typical range reported in
other rhenium(I) N-heterocyclic carbene complexes.4a,21 Sim-
ilarly, due to a stronger π-back-bonding interaction, the Re−C
(carbene) bond lengths in 5 and 6 (2.09−2.11 Å) are also shorter
than those in 1−3 (2.14−2.15 Å). Upon N-deprotonation of the
benzoxazol-2-ylidene ligand, the C−N−C angle in the oxazol-2-
ylidene moiety decreases from 111.7° in 2, which is typical
for benzoxazol-2-ylidene ligands (∼110−112°;19b,22 109.9° in 3,
111.8° in 5, and 110.6° in 6), to 106.3° in 1. This is probably due
to the increased electron repulsion for anionic lone-pair electrons
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Table 2. Selected Bond Lengths [Å] and Angles [deg] with Estimated Standard Deviations in Parentheses for Complexes 1−3
and 5−7
1
Re(1)−C(1)
Re(1)−C(2)
Re(1)−C(3)
Re(1)−N(1)
C(1)−Re(1)−C(3)
N(1)−Re(1)−N(2)
Re(1)−C(16)−O(4)
Re(1)−C(16)−N(3)
1.917(3)
1.952(3)
1.915(3)
2.165(3)
90.21(13)
75.77(10)
123.7(2)
124.9(2)
Re(1)−N(2)
Re(1)−C(16)
C(1)−O(1)
C(2)−O(2)
2.177(2)
C(3)−O(3)
C(16)−N(3)
C(16)−O(4)
1.148(4)
1.354(4)
1.372(4)
2.144(3)
1.154(4)
1.147(4)
N(3)−C(16)−O(4)
C(16)−O(4)−C(22)
C(16)−N(3)−C(17)
111.0(3)
106.2(2)
106.3(2)
2
3
5
6
7
Re(1)−C(1)
Re(1)−C(2)
Re(1)−C(3)
Re(1)−N(1)
C(1)−Re(1)−C(3)
N(1)−Re(1)−N(2)
Re(1)−C(16)−O(4)
Re(1)−C(16)−N(3)
1.930(4)
1.969(4)
1.924(4)
2.186(3)
89.37(17)
75.83(11)
123.9(3)
129.0(3)
Re(1)−N(2)
Re(1)−C(16)
C(1)−O(1)
C(2)−O(2)
2.176(3)
C(3)−O(3)
C(16)−N(3)
C(16)−O(4)
1.147(5)
1.328(5)
1.352(5)
2.151(4)
1.150(5)
1.139(6)
N(3)−C(16)−O(4)
C(16)−O(4)−C(17)
C(16)−N(3)−C(22)
107.1(3)
108.8(3)
111.7(3)
Re(1)−C(1)
Re(1)−C(2)
Re(1)−C(3)
Re(1)−N(1)
C(2)−Re(1)−C(3)
N(1)−Re(1)−N(2)
Re(1)−C(16)−O(4)
Re(1)−C(16)−N(3)
1.969(4)
1.922(4)
1.936(4)
2.168(3)
89.45(17)
75.77(12)
116.4(2)
135.3(3)
Re(1)−N(2)
Re(1)−C(16)
C(1)−O(1)
C(2)−O(2)
2.187(3)
C(3)−O(3)
C(16)−N(3)
C(16)−O(4)
1.146(5)
1.329(5)
1.375(5)
2.146(3)
1.122(5)
1.146(5)
N(3)−C(16)−O(4)
C(16)−O(4)−C(17)
C(16)−N(3)−C(22)
107.8(3)
108.1(3)
109.9(3)
Re(1)−C(1)
Re(1)−C(2)
Re(1)−P(1)
Re(1)−N(1)
C(1)−Re(1)−C(2)
N(1)−Re(1)−N(2)
Re(1)−C(3)−O(3)
Re(1)−C(3)−N(3)
1.861(19)
1.954(18)
2.436(6)
2.155(16)
88.1(5)
Re(1)−N(2)
C(1)−O(1)
C(2)−O(2)
2.202(15)
1.20(2)
Re(1)−C(3)
C(3)−O(3)
C(3)−N(3)
2.088(3)
1.370(3)
1.336(4)
1.13(2)
N(3)−C(3)−O(4)
C(3)−O(3)−C(4)
C(3)−N(3)−C(5)
106.3(2)
108.5(2)
111.8(2)
74.5(5)
122.95(18)
130.5(2)
Re(1)−C(1)
Re(1)−C(2)
Re(1)−P(1)
Re(1)−N(1)
C(1)−Re(1)−C(2)
N(1)−Re(1)−N(2)
Re(1)−C(3)−O(3)
Re(1)−C(3)−N(3)
1.890(11)
1.864(10)
2.419(10)
2.197(9)
88.7(5)
Re(1)−N(2)
C(1)−O(1)
C(2)−O(2)
2.184(8)
1.170(13)
1.177(13)
Re(1)−C(3)
C(3)−O(3)
C(3)−N(3)
2.105(10)
1.366(13)
1.343(12)
N(3)−C(3)−O(3)
C(3)−O(3)−C(4)
C(3)−N(3)−C(9)
107.2(8)
108.8(8)
110.6(9)
75.4(3)
122.2(7)
130.6(8)
Re(1)−C(1)
Re(1)−C(2)
Re(1)−C(3)
Re(1)−N(1)
C(1)−Re(1)−C(3)
N(1)−Re(1)−N(2)
1.935(4)
1.984(4)
1.936(4)
2.167(3)
90.91(14)
75.84(11)
Re(1)−N(2)
Re(1)−C(4)
C(4)−N(3)
2.183(3)
2.076(4)
1.150(5)
C(1)−O(1)
C(2)−O(2)
C(3)−O(3)
1.146(5)
1.133(5)
1.145(5)
Re(1)−C(4)−N(3)
C(4)−N(3)−C(17)
173.6(3)
167.5(4)
in the deprotonated N atom compared to the N−H or N−R
bond-pair electrons in benzoxazol-2-ylidene ligands. Moreover,
an enlarged O−C−N angle (111.0° in 1 vs 107.1° in 2), which is
similar to the deprotonated benzoxazol-2-ylidene ligands
structure of 7, the slightly bent C−N−C angle (167.5°) is
attributed to the π-back-bonding interaction between the
rhenium(I) metal and the isocyanide ligand, which is commonly
observed in different metal isocyanide complexes.16,20f,g,23
UV−Vis Absorption Spectroscopy. Complexes 1−7
dissolve in acetonitrile to give pale yellow to orange solutions,
observed in other dicarbene complexes,18 19b is also observed
c,
upon N-deprotonation. For the isocyanide ligand in the crystal
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Table 3. Photophysical Data for the Rhenium(I) Complexes
a
b
c
complex
medium (T/K)
emission λem /nm (τo/μs)
ϕem (%)
absorption λabs /nm (ε/dm3 mol−1 cm−1
)
1
MeCN (298)
d
602 (0.27)
520 (14.0)
1.10
10.2
6.72
1.01
0.83
0.76
14.5
261 (34 660), 283 (25 360), 380 (3940)
glass (77)
2
3
4
5
6
7
MeCN (298)
545 (1.66)
527 (11.4)
256 (32 590), 274 sh (37 675), 303 sh (17 935), 332 sh (9250), 373 (3535)
261 (44 285), 280 sh (33 585), 375 (5235)
d
glass (77)
MeCN (298)
546 (1.26)
522 (16.2)
d
glass (77)
MeCN (298)
620 (0.26)
558 (7.78)
267 (31 480), 292 (24 175), 318 (19 965), 349 sh (12 755), 420 (4420)
267 (34 215), 292 (23 940), 320 (19 550), 3475 sh (13 825), 421 (4820)
265 (40 975), 288 (25 195), 311 (18 985), 340 sh (12 100), 400 (5435)
256 (40 545), 268 (42 575), 295 sh (21 685), 306 sh (19 640), 365 (3380)
d
glass (77)
MeCN (298)
626 (0.13)
567 (5.96)
d
glass (77)
MeCN (298)
617 (0.21)
556 (5.44)
d
glass (77)
MeCN (298)
511 (5.59)
459, 490, 525 (304)
d
glass (77)
a
b
c
d
Excitation >400 nm. Luminescence quantum yield with excitation at 436 nm. In acetonitrile at 298 K. EtOH−MeOH (4:1 v/v).
depending on the position of their MLCT [dπ(Re) → π*(phen)]
absorption in the visible region. The electronic absorption data
of these complexes are summarized in Table 3. All these com-
plexes in acetonitrile show very intense intraligand (IL) π→π*
transitions of phenanthroline, carbene (1−6), and 2-methox-
yphenyl isocyanide (7) with molar extinction coefficients on
the order of 104 dm3 mol−1 cm−1 in the range 220−350 nm in
the UV region (Figure 2). Similar to other related rhenium(I)
diimine complexes,8 they all exhibit a moderately intense
MLCT [dπ(Re) → π*(phen)] absorption shoulder at ca. 365−
421 nm with molar extinction coefficients on the order of
103 dm3 mol−1 cm−1 and tailing to the visible region down to
about 500 nm (Figure 2). The observation of the absorption
also be observed in 4−6 (400−421 nm) compared to that in
2 (373 nm), as a carbonyl ligand in 2 is replaced by weaker
π-accepting phosphine ligands. A slightly higher MLCT
absorption energy in 6 compared to 4 and 5 is also observed,
as P(OEt)3 is a stronger π-accepting ligand compared to PPh3
and PPh2Me.24
Emission Spectroscopy. All complexes in acetonitrile
solution displayed long-lived photoluminescence with λem in
the range 511−626 nm and lifetime (τ) in the range 0.13−5.59 μs
(Figure 3, Table 3). These structureless emission bands show a
Figure 3. Overlaid normalized emission spectra of complexes 2−7 in
acetonitrile at 298 K.
significant energy dependence on the electronic properties of the
ancillary ligands as reflected by the trends of the λem in the order
7 (511 nm) ≪ 2 (545 nm) ≈ 3 (546 nm) ≪ 6 (617 nm) < 4
(620 nm) < 5 (626 nm), which is in line with the π-accepting
ability of the ancillary ligands and the MLCT absorption (see
above). Considerable red-shift of emission energy is observed
when the ancillary isocyanide ligand is replaced by a carbene ligand
or the carbonyl ligand is replaced by a phosphine ligand, as
reflected from the emissions of 7, 2, and 6 (Figure 3). Such
emission energy trends and the sub-microsecond emission lifetime
are suggestive of the 3MLCT [dπ(Re) → π*(phen)] excited-state
origin, probably mixed with some LLCT character.25
Figure 2. Overlaid UV−vis spectra of (a) 2, 3, and 7 and (b) 2, 4, 5,
and 6 in acetonitrile at 298 K.
energy in the order 7 (365 nm) > 2 (373 nm) ≈ 3 (375 nm) ≫
6 (400 nm) > 4 (420 nm) ≈ 5 (421 nm), which is in line with
the π-accepting ability of the ancillary ligands, is consistent with
the MLCT [dπ(Re) → π*(phen)] transition. As the dπ(Re)
orbital could be better stabilized with a stronger π-accepting
ligands, complex 7, with the strongest π-accepting ancillary
ligands, three carbonyl and one isocyanide ligands, shows the
highest MLCT [dπ(Re) → π*(phen)] energy among 1−7.
Similarly, significant red-shift of the MLCT absorption could
These complexes also displayed strong photoluminescence in
EtOH−MeOH (4:1, v/v) glass at 77 K (Table 3, Figure S2,
Supporting Information). With the exception of 7, the
emissions of 1−6 in 77 K EtOH−MeOH glassy medium are
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also structureless with a few microsecond emission lifetimes.
These emissions show similar emission energy trends to their
solution emissions but are all blue-shifted. They are also
3
ascribed to the MLLCT excited-state origin. The blue-shifted
MLLCT emission of related complexes in 77 K EtOH−MeOH
glass relative to the solution emission is commonly observed
and were coined as “luminescence rigidochromism”.26 In
contrast, 7 in 77 K EtOH−MeOH glassy medium displays a
highly structured emission band with vibrational progression
spacing of ca. 1380 cm−1 and much longer emission lifetime of
ca. 304 μs. The significant difference in emission characteristic
of 7 in 77 K EtOH−MeOH glass compared to its solution
emission as well as those of 1−6 and related MLLCT emission
of other rhenium(I) diimine complexes in the same medium is
suggestive of a different emission origin. In view of the long-
lived emission lifetimes and the structured emission bands, the
emission of 7 in 77 K glassy medium was ascribed as derived
from the triplet ligand-centered (3LC) excited state of the
phenanthroline ligand. The change in emissive excited state
3
3
origin from MLLCT at room temperature to LC at 77 K
3
glassy medium is probably due to the blue-shift of the MLCT
state, which becomes higher lying in energy than the 3LC state.
Similar observations have also been reported in other related
complexes.27
Acidity of N−H in the Carbene Ligand. The formation
of neutral complex 1 in our initial attempt to prepare carbene
complex 2 suggests a high acidity of the NH group in the carbene
ligand, which can be readily deprotonated. To study the acidity
of the benzoxazol-2-ylidene ligand in different complexes, spectro-
photometric titration studies of 2 and 4−6 with a non-nucleophilic
Figure 4. (a) UV−vis absorption and (b) emission spectral changes of
2 (0.0297 mM) in (0.1 M nBu4NPF6) acetonitrile solution upon
addition of Bu4NOH. The insets show the plot of (a) absorbance at
n
310 nm and (b) emission intensity at 547 nm as a function of mole
n
equivalence of Bu4NOH.
n
Table 4. Summary of pKa Values of the Complexes 2 and
4−6 in MeCN−H2O (9:1, v/v, 0.05 mol dm−3 nBu4NPF6) at
298 K
strong base of Bu4NOH were carried out. To avoid UV−vis
absorption spectral changes due to alternation of ionic strength
during the titration studies, all titrations were performed in 0.1 M
nBu4NPF6 in acetonitrile in order to maintain a fairly constant
ionic strength.
complex
pKa
2
4
5
6
6.3
8.2
8.5
8.1
n
Upon addition of Bu4NOH up to a stoichiometric amount
n
of 2 in acetonitrile solution (0.1 M Bu4NPF6), a blue-shift of
the intense IL ππ* absorptions and a red-shift of the MLCT
[dπ(Re) → π*(phen)] absorption with well-defined isosbestic
points at 271 and 359 nm were observed (Figure 4a). The
observation of well-defined isosbestic points together with the
identical UV−vis absorption spectrum compared to that of 1
much stronger electron-withdrawing effect of the trans-carbonyl
ligand, which can better stabilize the deprotonated anionic
carbene ligand, compared to that of the trans-phosphine ligands
in 4−6. The current deprotonation study shows that the N
atom of the benzoxazol-2-ylidene ligand in these complexes can
be readily functionalized in the presence of base.
n
after addition of one molar equivalent of Bu4NOH suggests a
clean N-deprotonation of the benzoxazol-2-ylidene ligand in 2.
The bathochromic shift of the MLCT absorption of 2 upon
deprotonation can be explained by the destabilization of the
dπ(Re) orbital due to a significant decrease of the π-accepting
ability of the benzoxazol-2-ylidene ligand upon conversion to
an anionic carbene ligand. As MLCT energy decreases, the
emission of 2 also shows a red-shift and decrease in intensity
upon deprotonation (Figure 4b). For analogous complexes
4−6 with a phosphine ligand, they also exhibit similar UV−vis
and emission spectral changes (Figures S3 and S4, Supporting
Electrochemistry. The electrochemical properties of 1−7
in acetonitrile (0.1 M nBu4NPF6) were studied by cyclic
voltammetry. The electrochemical data are collected in Table 5,
and representative cyclic voltammograms are shown in Figure 5.
Except for 1, all other complexes showed one irreversible or
quasi-reversible oxidation couple with oxidation potential in the
range +1.12 to +1.83 vs SCE, which is ascribed to the typical
ReI/II metal-centered oxidation similar to that observed in
related tricarbonyl rhenium(I) diimine complexes.28 The
observation of the oxidative potential of the oxidation couple
in the order 7 (+1.83 V) ≫ 2 (+1.67 V) ≈ 3 (+1.66 V) ≫ 6
(+1.20 V) > 4 (+1.16 V) > 5 (+1.12 V), which is in line with
the π-accepting ability of the ligands and hence the electron
richness of the rhenium metal center, is supportive of the ReI/II
metal-centered oxidation assignment. For 1, the second
oxidation wave is also assigned to the ReI/II metal-centered
oxidation, whereas the additional oxidation wave observed at
n
Information) in the titration with Bu4NOH. These changes
were also ascribed to the N-deprotonation of the benzoxazol-2-
ylidene ligand. To quantify the acidity of the NH proton in
the benzoxazol-2-ylidene ligand of these complexes, the acid
dissociation constants of the NH proton in 2 and 4−6 in
acetonitrile−water (9:1, v/v, 0.05 mol dm−3 nBu4NPF6) have
also been determined (Table 4). A significantly lower pKa for 2
compared to 4−6 is noted, suggesting a much more acidic NH
proton in the benzoxazol-2-ylidene ligand of 2. The much
stronger acidity of NH protons in 2 can be rationalized by the
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Article
precursor complexes have been structurally characterized by X-ray
crystallography. The photophysical study on these complexes
revealed that all these complexes display green to orange-red
MLLCT phosphorescence. In short, the conversion of isocyano
rhenium(I) phenanthroline complexes into luminescent carbene-
containing rhenium(I) phenanthroline complexes can be achieved.
The development of other luminescent rhenium(I) carbene
complexes from our previously reported isocyano rhenium(I)
diimine complexes and the study of their applications in
photocatalysis are currently in progress.
Table 5. Electrochemical Data for 1−7 in Acetonitrile
a
Solution (0.1 mol dm−3 nBu4NPF6) at 298 K
b
b
oxidation E1/2/V vs SCE
reduction E1/2/V vs SCE
complex
(Epa/V vs SCE)
(Epc/V vs SCE)
1
2
3
4
5
6
7
(+1.10), (+1.69)
(+1.67)
+1.66
−1.43
(−1.30)
−1.26
(−1.41)
(−1.42)
(−1.41)
+1.16
+1.12
+1.20
+1.83
−1.22
ASSOCIATED CONTENT
a
b
Working electrode, glassy carbon; scan rate, 100 mV s−1. E1/2 is
■
S
* Supporting Information
(Epa + Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials,
Tables, ORTEP drawings, CIF files, and experimental details
for the X-ray crystal structures of cis,trans-[Re(CO)2(phen)-
(PPh3)(MeCN)](CF3SO3), cis,trans-[Re(CO)2(phen)(PPh3Me)-
(MeCN)](CF3SO3), 1−3, and 5−7, emission spectra of 2−7 in
EtOH−MeOH (4:1, v/v) glassy medium at 77 K, UV−vis and
respectively.
n
emission spectral changes of 4−6 upon addition of Bu4NOH.
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. Fax: +(852) 3442-0522. Tel:
+(852) 3442-6958.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work described in this paper was supported by the Hong
Kong University Grants Committee Area of Excellence Scheme
(AoE/P-03/08) and a grant from City University of Hong
Kong (Project No. 7008183). The flash photolysis system was
supported by the Special Equipment Grant from the University
Grants Committee of Hong Kong (SEG_CityU02). C.-O. Ng
acknowledges the receipt of a postgraduate studentship from
City University of Hong Kong.
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