Synthesis, characterization, and photophysical and emission solvatochromic study of rhenium(I) tetra(isocyano) diimine complexes

Article abstract of DOI:10.1021/om200074t

A new series of rhenium(I) tetra(isocyano) diimine complexes ([Re(CNR) ₄(N-N)]⁺, where R = 4-IC₆H₄, 4-BrC₆H₄, 4-Br-2,6-Me₂C₆H ₂, 2,4,6-Cl₃C₆H₂, 2,4,6-Br ₃C₆H₂, 2,4-Cl₂-6-MeOC ₆H₂ and N-N = bpy, (MeO)₂bpy, ^tBu₂bpy, phen, 1-methyl-2-(2′-pyridyl)imidazole) with various isocyanide and diimine ligands of diverse electronic and steric natures has been synthesized and characterized. The X-ray crystal structures of four of the target complexes were also determined. These complexes were found to show orange to red MLCT (dπ(Re) → π*(N-N)) phosphorescence. Given the submicrosecond solution emission lifetime, nanosecond transient absorption spectroscopic studies have been performed. The emission solvatochromic behavior and the electrochemical properties of these complexes and their structural correlation have also been reported.

Full text of DOI:10.1021/om200074t

ARTICLE
pubs.acs.org/Organometallics
Synthesis, Characterization, and Photophysical and Emission
Solvatochromic Study of Rhenium(I) Tetra(isocyano) Diimine
Complexes
Chi-Chiu Ko,* Jacky Wai-Kit Siu, Apple Wai-Yi Cheung, 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
b
ABSTRACT: A new series of rhenium(I) tetra(isocyano) dii-
mine complexes ([Re(CNR)4(NꢀN)]þ, where R = 4-IC6H4,
4-BrC6H4, 4-Br-2,6-Me2C6H2, 2,4,6-Cl3C6H2, 2,4,6-Br3C6H2,
t
2,4-Cl2-6-MeOC6H2 and NꢀN = bpy, (MeO)2bpy, Bu2bpy,
phen, 1-methyl-2-(20-pyridyl)imidazole) with various isocyanide
and diimine ligands of diverse electronic and steric natures has
been synthesized and characterized. The X-ray crystal structures
of four of the target complexes were also determined. These
complexes were found to show orange to red MLCT (dπ(Re) f
π*(NꢀN)) phosphorescence. Given the submicrosecond solution emission lifetime, nanosecond transient absorption spectro-
scopic studies have been performed. The emission solvatochromic behavior and the electrochemical properties of these complexes
and their structural correlation have also been reported.
’ INTRODUCTION
photophysical and electrochemical properties of a new series of
rhenium(I) tetra(isocyano) diimine complexes with various isocya-
nide and diimine ligands of diverse electronic and steric natures.
Given the submicrosecond solution emission lifetime, nanosecond
transient absorption spectroscopic studies have also been carried out.
Isocyanides (CNR) are interesting ligands that are capable of
coordinating to many different metal centers to give stable orga-
nometallic complexes, as they possess excellent π-accepting and
σ-donating ability.1 Since their coordination abilities are similar to
those of the carbonyl ligand, the isocyanide analogues of many of
the metal carbonyl complexes can be prepared.1a However, the
synthetic routes for metal carbonyl and isocyanide complexes are
generally different.1a One of the advantages of using isocyanide
ligands over the carbonyl ligand in the design of metal complexes for
various applications is that the steric and electronic properties of
metal isocyanide complexes can be effectively tuned by changing
the substituent on the nitrogen atom. Such flexibility has been
demonstrated to be useful and beneficial in the design and study of
transition-metal isocyanide complexes for catalytic applications.2
Rhenium(I) tricarbonyl diimine complexes have been demon-
strated to show rich photophysical and photochemical behavior3 and
are useful in a wide variety of applications.4 On the basis of the
similarity of the carbonyl and the isocyanide ligands in the me-
talꢀligand interaction and taking advantage of the tunability of the
isocyanide ligands, we have recently communicated the synthesis and
the photophysical studies of a new class of readily tunable rhenium
(I) tetra(isocyano) diimine complexes, [Re(CNR)4(NꢀN)]þ.5
The preliminary study reported in our communication showed that
the emissions of some of the complexes are highly sensitive to the
change of the solvent environment.5 To elucidate the emissive
excited state and provide insights into the structure relationship of
the excited-state properties as well as the emission solvatochromism,
herein we report the detailed syntheses, characterization, and
’ EXPERIMENTAL SECTION
Materials and Reagents. Ammonium hexafluorophosphate, potas-
sium iodide, and hydrazine hydrate were obtained from Aldrich Chemical Co.
Rhenium powder (325 mesh) and thallium triflate (TlOTf) were obtained
from Strem Chemicals, Inc. Hydrogen iodide was obtained from International
Laboratory Chemical Co. The diimine ligands 1,10-phenanthroline (phen),
2,20-bipyridine (bpy), 4,40-di-tert-butyl-2,20-bipyridine (tBu2bpy), and 4,40-
dimethoxy-2,20-bipyridine ((MeO)2bpy) and substituted anilines 4-iodoa-
niline, 4-bromoaniline, 4-chloroaniline, 4-fluoroaniline, 2,6-dimethylani-
line, 2,4,6-trichloroaniline, 2,4,6-tribromoaniline, 2,4-dichloro-6-
methoxyaniline, and 4-bromo-2,6-dimethylaniline were purchased from
Aldrich Chemical Co. and used as received. 1-Methyl-2-(20-pyri-
dyl)imidazole (pimMe) and 1-phenyl-2-(20-pyridyl)imidazole (pimPh)
were prepared according to the literature procedure.6 The substituted
phenyl isocyanide ligands 4-chlorophenyl isocyanide, 2,4,6-trichlorophenyl
isocyanide, 4-iodophenyl isocyanide, 2,4-dichloro-6-methoxyphenyl iso-
cyanide, 4-fluorophenyl isocyanide, 4-bromo-2,6-dimethylphenyl isocya-
nide, 4-bromophenyl isocyanide, 2,4,6-tribromophenyl isocyanide, and
2,6-dimethylphenyl isocyanide were all prepared from the dehydration of
the corresponding substituted formamides, which can be readily obtained
Received: January 26, 2011
Published: April 18, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om200074t Organometallics 2011, 30, 2701–2711
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Organometallics
ARTICLE
from the reactions between the substituted anilines with formic acid or
formic acetic anhydride, using the synthetic methodology developed by
Ugi and co-workers.7 Precursor complexes such as [Re(CNC6H4F-4)5I],
[Re(CNC6H4Cl-4)5I], and [Re(CNC6H3Me2-2,6)5I], were prepared by
our previously communicated procedure.6 All solvents were of analytical
reagent grade and used without further purification.
mol equiv.) were mixed in 1,4-dioxane and heated to reflux for 2 days. The
resulting suspension was filtered to remove the precipitated TlI. After the
solvent was removed under reduced pressure, the residue was purified
by column chromatography on silica gel using dichloromethane/acetone
(8/2 v/v) as eluent. The residue was then redissolved in methanol (2 mL).
A subsequent metathesis reaction with a saturated methanolic solution of
ammonium hexafluorophosphate gave the target complex as a PF6ꢀ salt.
An analytically pure complex of 1 as a crystalline solid was obtained by
the slow diffusion of diethyl ether vapor into a concentrated dichloro-
methane solution of the complex. Yield: 97 mg, 0.068 mmol; 68%.
1H NMR (400 MHz, CDCl3, 298 K): δ 6.79 (d, 4H, J = 8.8 Hz,
phenyl H’s), 7.12 (d, 4H, J = 8.8 Hz, phenyl H’s), 7.58 (d, 4H, J = 8.8
Hz, phenyl H’s), 7.74 (d, 4H, J = 8.8 Hz, phenyl H’s), 8.05 (dd, 2H, J = 8.2,
5.1 Hz, 3,8-phen H’s), 8.19 (s, 2H, 5,6-phen H’s), 8.70 (dd, 2H, J = 8.2, 1.3
Hz, 4,7-phenH’s), 9.60 (dd, 2H, J= 5.1, 1.3 Hz, 2,9-phen H’s). ESI-MS: m/
z 1283 {M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1): 842 ν(PꢀF); 1880, 1986,
2056, 2133 ν(CtN). Anal. Calcd (found) for 1: C, 33.66 (33.69); H, 1.69
(1.89); N, 5.89 (5.77).
[Re(CNC6H4Br-4)5I]. To a methanolic solution of K2[ReI6] (500 mg,
0.49 mmol) was added 4-bromophenyl isocyanide (561 mg, 4.60 mmol).
The resulting mixture was stirred overnight. Thereafter, hydrazine hydrate
(2.5 mL) was added in a dropwise manner. After the mixture was stirred at
room temperature for 2 h, the precipitate was filtered by suction filtration and
washed with diethyl ether. Yield: 379 mg, 0.309 mmol; 63%. 1H NMR (400
MHz, CDCl3, 298 K): δ7.05 (d, 2H, J=7.4Hz, phenylH’s), 7.27 (d, 8H, J=
7.4 Hz, phenyl H’s), 7.41 (d, 2H, J = 7.4 Hz, phenyl H’s), 7.50 (d, 8H, J = 7.4
Hz, phenyl H’s). ESI-MS: m/z 1228 {M}þ, 1045 {M ꢀ CNC6H4Br}þ. IR
(KBr disk, ν/cmꢀ1): 2001, 2063 ν(CtN). Anal. Calcd (found) for
[Re(CNC6H4Br-4)5I]: C 34.37 (34.60); H, 1.65 (1.71); N, 5.73 (5.68).
[Re(CNC6H4I-4)5I]. This complex was synthesized according to a
procedure similar to that of [Re(CNC6H4Br-4)5I], except 4-iodophenyl
isocyanide was used in place of 4-bromophenyl isocyanide. Yield: 286 mg,
0.196 mmol; 40%. 1H NMR (400 MHz, CDCl3, 298K): δ 6.92 (d, 2H,
J = 7.4 Hz, phenyl H’s), 7.12 (d, 8H, J = 7.4 Hz, phenyl H’s), 7.60 (d, 2H,
J = 7.4 Hz, phenyl H’s), 7.70 (d, 8H, J = 7.4 Hz, phenyl H’s). ESI-MS: m/z
1458 {M}þ, 1229{Mꢀ CNC6H4I}þ. IR(KBrdisk,ν/cmꢀ1): 1993, 2059
v(CtN). Anal. Calcd (found) for [Re(CNC6H4I-4)5I]: C, 28.83 (28.75);
H, 1.38 (1.18); N, 4.80 (4.65).
[Re(CNC6H2Br-4-Me2-2,6)4(phen)]PF6 (2). This complex was
synthesized according to a procedure similar to that of 1, except
[Re(CNC6H2Br-4-Me2-2,6)5I] (145 mg, 0.1 mmol) was used in place
of[Re(CNC6H4I-4)5I] and dichloromethane/acetone (7/3 v/v) was used
as eluent in the column chromatography. Yield: 66 mg, 0.049 mmol; 49%.
1H NMR (400 MHz, CDCl3, 298 K): δ 1.83 (s, 12H, methyl H’s), 2.47
(s, 12H, methyl H’s), 7.09 (s, 4H, phenyl H’s), 7.52 (s, 4H, phenyl H’s),
7.99 (dd, 2H, J = 8.2, 5.1 Hz, 3,8-phen H’s), 8.22 (s, 2H, 5,6-phen H’s),
8.72 (dd, 2H, J = 8.2, 1.3 Hz, 4,7-phen H’s), 9.59 (dd, 2H, J = 5.1, 1.3 Hz,
2,9-phen H’s). ESI-MS: m/z 1207 {M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1):
834 ν(PꢀF); 1920, 1986, 2019, 2111 ν(CtN). Anal. Calcd (found) for
[Re(CNC6H2Br3-2,4,6)5I]. This complex was synthesized according
to a procedure similar to that of [Re(CNC6H4Br-4)5I], except 2,4,
6-tribromophenyl isocyanide was used in place of 4-bromophenyl iso-
cyanide. Yield: 453 mg, 0.225 mmol; 46%. 1H NMR (400 MHz, CDCl3,
298 K): δ 7.62 (s, 2H, phenyl H’s), 7.72 (s, 8H, phenyl H’s). ESI-MS: m/z
2013 {M}þ, 1673 {M ꢀ CNC6H2Br3-2,4,6}þ. IR (KBr disk, ν/cmꢀ1):
1896, 2027 ν (CtN). Anal. Calcd (found) for [Re(CNC6H2Br3-
2,4,6)5I]: C, 20.86 (20.89); H, 0.5 (0.77); N, 3.48 (3.46).
2
1/2CH2Cl2: C, 41.78 (42.03); H, 2.96 (3.03); N, 6.03 (6.15).
[Re(CNC6H2Br3-2,4,6)4(phen)]PF6 (3). This complex was
3
synthesized according to a procedure similar to that of 1, except
[Re(CNC6H2Br3-2,4,6)5I] (200 mg, 0.1 mmol) was used in place of
[Re(CNC6H4I-4)5I] and dichloromethane/acetone (1/1v/v) was usedas
eluent in the column chromatography. Yield: 105 mg, 0.056 mmol; 56%.
1H NMR (400 MHz, DMSO, 298 K): δ 7.92 (s, 4H, phenyl H’s), 8.07
(s, 4H, phenyl H’s), 8.19 (dd, 2H, J = 8.2, 5.0 Hz, 3,8-phen H’s), 8.35
(s, 2H, 5,6-phen H’s), 8.96 (dd, 2H, J = 8.2, 1.3 Hz, 4,7-phen H’s), 9.75
(d, 2H, J = 5.0 Hz, 2,9-phen H’s). ESI-MS: m/z 1726 {M ꢀ PF6}þ.
IR (KBr disk, ν/cmꢀ1): 842 ν(PꢀF); 1876, 1953, 2026, 2136 ν(CtN).
Anal. Calcd(found) for3: C, 25.68 (25.62);H, 0.86 (1.05); N, 4.49 (4.58).
[Re(CNC6H2Cl3-2,4,6)4(phen)]PF6 (4). This complex was
synthesized according to a procedure similar to that of 1, except
[Re(CNC6H2Cl3-2,4,6)5I] (135 mg, 0.1 mmol) was used in place of
[Re(CNC6H4I-4)5I] and dichloromethane/acetone (1/1 v/v) was used
as eluent in the column chromatography. Yield: 82 mg, 0.061 mmol;
61%. 1H NMR (400 MHz, CDCl3, 298 K): δ 7.25 (s, 4H, phenyl H’s),
7.45 (s, 4H, phenyl H’s), 8.03 (dd, 2H, J = 8.2, 5.1 Hz, 3,8-phen H’s),
8.26 (s, 2H, 5,6-phen H’s), 8.82 (dd, 2H, J = 8.2, 1.3 Hz, 4,7-phen H’s),
9.63 (dd, 2H, J = 5.1, 1.3 Hz, 2,9-phen H’s). ESI-MS: m/z 1192
{M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1): 853 ν(PꢀF); 1909, 1957,
2037, 2133 ν(CtN). Anal. Calcd (found) for 4: C, 35.93 (36.22); H,
1.21 (1.50); N, 6.28 (6.29).
[Re(CNC6H2Cl2-2,4-OMe-6)4(phen)]PF6 (5). This complex was
synthesized according to a procedure similar to that of 1, except
[Re(CNC6H2Cl2-2,4-OMe-6)5I] (132 mg, 0.1 mmol) was used in place
of[Re(CNC6H4I-4)5I] and dichloromethane/acetone (7/3 v/v) was used as
eluent in the column chromatography. Yield: 56 mg, 0.042 mmol; 42%. 1H
NMR (400 MHz, CDCl3, 298 K): δ 3.57 (m, 12H, OMe), 7.24 (s, 4H,
phenyl H’s), 7.43 (s, 4H, phenyl H’s), 8.02 (dd, 2H, J = 8.2, 5.1 Hz, 3,8-phen
H’s), 8.25 (s, 2H, 5,6-phen H’s), 8.81 (dd, 2H, J = 8.2, 1.3 Hz, 4,7-phen H’s),
9.67 (dd, 2H, J= 5.1, 1.3 Hz, 2,9-phen H’s). ESI-MS: m/z1174 {Mꢀ PF6}þ.
IR (KBr disk, ν/cmꢀ1): 849 ν(PꢀF); 1964, 1993, 2045, 2136 ν(CtN).
Anal. Calcd (found) for 5: C, 40.05 (40.33); H, 2.14 (1.95); N, 6.37 (6.26).
[Re(CNC6H2Cl3-2,4,6)5I]. This complex was synthesized according
to a procedure similar to that of [Re(CNC6H4Br-4)5I], except 2,4,
6-trichlorophenyl isocyanide was used in place of 4-bromophenyl isocya-
1
nide. Yield: 458 mg, 0.340 mmol; 70%. H NMR (400 MHz, CDCl3,
298 K): δ 7.28 (s, 2H, phenyl H’s), 7.38 (s, 8H, phenyl H’s). ESI-MS: m/z
1345 {M}þ, 1139 {M ꢀ CNC6H2Cl3-2,4,6}þ. IR (KBr disk, ν/cmꢀ1):
1883, 2037 ν(CtN). Anal. Calcd (found) for [Re(CNC6H2Cl3-2,4,6)5I]:
C, 31.25 (31.31); H, 0.75 (0.82); N, 5.21 (5.23).
[Re(CNC6H2Cl2-2,4-OMe-6)5I]. This complex was synthesized
according to a procedure similar to that of [Re(CNC6H4Br-4)5I],
except 2,4-dichloro-6-methoxyphenyl isocyanide was used in place of
4-bromophenyl isocyanide. Yield: 246 mg, 0.186 mmol; 38%. 1H NMR
(400 MHz, CDCl3, 298 K): δ 3.83ꢀ3.92 (m, 15H, OMe), 6.68ꢀ6.91 (m,
2H, phenyl H’s), 7.34ꢀ7.40 (m, 8H, phenyl H’s). ESI-MS: m/z 1323
{M}þ, 1121 {M ꢀ CNC6H2Cl2-2,4-OMe-6}þ. IR (KBr disk, ν/cmꢀ1);
1891, 1946, 2034 ν(CtN). Anal. Calcd (found) for [Re(CNC6H2Cl2-
2,4-OMe-6)5I]: C, 36.31 (36.28); H, 1.90 (2.01); N, 5.29 (5.38).
[Re(CNC6H2Br-4-Me2-2,6)5I]. This complex was synthesized ac-
cording to a procedure similar to that of [Re(CNC6H4Br-4)5I], except
4-bromo-2,6-dimethylphenyl isocyanide was used in place of 4-bromophe-
1
nyl isocyanide. Yield: 311 mg, 0.27 mmol; 55%. H NMR (400 MHz,
CDCl3, 298 K): δ 2.46 (s, 6H, CH3), 2.54 (s, 24H, CH3), 7.15 (s, 2H,
phenyl H’s), 7.21 (s, 8H, phenyl H’s), 7.34ꢀ7.40 (m, 8H, phenyl H’s). ESI-
MS: m/z1153 {M}þ, 943 {Mꢀ CNC6H2Br-4-Me-2,6}þ. IR(KBrdisk, ν/
cmꢀ1): 2037, 2062 ν(CtN). Anal. Calcd (found) for [Re(CNC6H2Br-4-
Me2-2,6)5I]: C, 39.64 (39.66); H, 2.96 (3.10); N, 5.14 (5.22).
[Re(CNC6H4I-4)4(phen)]PF6 (1). The reaction was performed
under anhydrous conditions and strictly inert atmosphere of argon using
standard Schlenk techniques. [Re(CNC6H4I-4)5I] (146 mg, 0.1 mmol),
phen (40 mg, 0.2 mmol, 2 mol equiv), and TlOTf (53 mg, 0.15 mmol, 1.5
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[Re(CNC6H4I-4)4(bpy)]PF6 (6). This complex was synthesized
according to a procedure similar to that of 1, except bpy (31 mg,
0.2 mmol) was used in place of phen and dichloromethane/acetone
(7/3 v/v) was used as eluent in the column chromatography. Yield:
80 mg, 0.057 mmol; 57%. 1H NMR (400 MHz, CDCl3, 298 K): δ 6.86
(d, 4H, J = 8.6 Hz, phenyl H’s), 7.04 (d, 4H, J = 8.6 Hz, phenyl H’s), 7.57
(ddd, 2H, J = 8.2, 5.1, 0.9 Hz, 4,4-bipyridyl H’s), 7.64 (d, 4H, J = 8.6 Hz,
phenyl H’s), 7.72 (d, 4H, J = 8.6 Hz, phenyl H’s), 8.16 (td, 2H, J = 5.1,
1.5 Hz, 5,50-bipyridyl H’s), 8.59 (d, 2H, J = 8.2 Hz, 3,30-bipyridyl H’s),
9.18 (dd, 2H, J = 5.1, 0.9 Hz, 6,60-bipyridyl H’s). ESI-MS: m/z 1258
{M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1): 834 ν(PꢀF); 1960, 1990, 2052,
2129 ν(CtN). Anal. Calcd (found) for 6: C, 32.52 (32.28); H, 1.72
(1.80); N, 5.99 (5.97).
[Re(CNC6H4Br-4)4(bpy)]PF6 (7). This complex was synthesized
according to a procedure similar to that of 1, except bpy (31 mg, 0.2 mmol)
and [Re(CNC6H4Br-4)5I] (136 mg, 0.1 mmol) were used in place of phen
and [Re(CNC6H4I-4)5I], respectively. Yield: 71 mg, 0.058 mmol; 58%. 1
H NMR (400 MHz, CDCl3, 298 K):δ7.02 (d, 4H, J= 8.7 Hz, phenylH’s),
7.19 (d, 4H, J = 8.7 Hz, phenyl H’s), 7.45 (d, 4H, J = 8.7 Hz, phenyl H’s),
7.54 (d, 4H, J = 8.7 Hz, phenyl H’s), 7.59 (ddd, 2H, J = 8.2, 0.9 Hz, 4,
40-bipyridyl H’s), 8.17 (td, 2H, J = 5.7, 1.5 Hz, 5,50-bipyridyl H’s), 8.59 (d,
2H, J = 8.2 Hz, 3,30-bipyridyl H’s), 9.21 (dd, 2H, J = 5.7, 0.9 Hz, 6,60-
bipyridyl H’s). ESI-MS: m/z 1074 {M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1):
842 ν(PꢀF); 1949, 2004, 2056, 2136 ν(CtN). Anal. Calcd (found) for 7:
C, 37.55 (37.48); H, 1.99 (2.07); N, 6.91 (6.88).
N-methyl H’s), 6.87 (d, 4H, J = 8.7 Hz, phenyl H’s), 6.99 (d, 4H, J = 8.7
Hz, phenyl H’s), 7.39ꢀ7.69 (m, 11H, phenyl H’s, 4-imidazoyl H’s,
5-imidazoyl H’s, and 50-pyridyl H’s), 8.24 (t, 1H, J = 8.2 Hz, 4-pyridyl
H’s), 8.32 (d, 1H, J = 8.2 Hz, 3-pyridyl H’s), 9.17 (d, 1H, J = 5.5 Hz,
6-pyridyl H’s). ESI-MS: m/z 1258 {M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1):
842 ν(PꢀF); 1909, 1990, 2048, 2129 ν(CtN). Anal. Calcd (found) for
11: C, 31.60 (31.73); H, 1.79 (1.84); N, 6.97 (6.94).
Re(CNC6H4Cl-4)4(pimMe)]PF6 (12). This complex was synthe-
sized according to a procedure similar to that of 1, except [Re(CNC6H4Cl-
4)5I] (100 mg, 0.1 mmol) and pimMe (32 mg, 0.2 mmol) were used in place
of [Re(CNC6H4I-4)5I] and phen, respectively. Yield: 50 mg, 0.048 mmol;
48%. 1H NMR (400 MHz, CDCl3, 298 K): δ 4.29 (s, 3H, N-methyl H’s),
7.11 (d, J = 8.6 Hz, 4H, phenyl H’s), 7.20ꢀ7.41 (m, 15H, phenyl H’s,
5-imidazoyl H’s, 4-imidazoyl H’s, 5-pyridyl H’s), 8.24 (t, 1H, J = 6.5 Hz,
4-pyridyl H’s), 8.42 (d, 1H, J=8.2Hz, 3-pyridylH’s), 9.16 (d, 1H, J=4.8Hz,
6-pyridyl H’s). ESI-MS: m/z 892 {M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1):
845 ν(PꢀF); 1850, 2001, 2056, 2125 ν(CtN). Anal. Calcd (found) for 12:
C, 42.70 (42.85); H, 2.42 (2.57); N, 9.42 (9.61).
Re(CNC6H2Br-4-Me2-2,6)4(pimMe)]PF6 (13). This complex was
synthesized according to a procedure similar to that of 1, except
[Re(CNC6H2Br-4-Me2-2,6)5I] (145 mg, 0.1 mmol) and pimMe (32 mg,
0.2 mmol) were used in place of [Re(CNC6H4I-4)5I] and phen, respec-
tively. Yield: 87 mg, 0.06 mmol; 60%. 1H NMR (400 MHz, CDCl3, 298 K):
δ 2.06 (s, 12H, methyl H’s), 2.42 (s, 12H, methyl H’s), 4.31 (s, 3H,
N-methyl H’s), 7.17 (s, 4H, phenyl H’s), 7.25 (s, 4H, phenyl H’s), 7.30
(s, 1H, 5-imidazoylH’s), 7.39ꢀ7.42 (m, 2H, 4-imidazoyl H’s, 5-pyridyl H’s),
8.24 (t, 1H, J = 8.0 Hz, 4-pyridyl H’s), 8.42 (d, 1H, J = 7.8 Hz, 3-pyridyl H’s),
9.16 (d, 1H, J = 5.4 Hz, 6-pyridyl H’s). ESI-MS: m/z 1182 {M ꢀ PF6}þ. IR
(KBr disk, ν/cmꢀ1): 842 ν(PꢀF); 1916, 1946, 2026, 2118 ν(CtN). Anal.
[Re(CNC6H2Br-4-Me2-2,6)4((MeO)2bpy)]PF6 (8). This com-
plex was synthesized according to a procedure similar to that of 1, except
[Re(CNC6H2Br-4-Me2-2,6)5I] (145 mg, 0.1 mmol) and (MeO)2bpy
(43 mg, 0.2 mmol) were used in place of [Re(CNC6H4I-4)5I] and phen,
respectively. Yield: 78 mg, 0.056 mmol; 56%. 1H NMR (400 MHz, CDCl3,
298 K): δ2.04 (s, 12H, methylH’s), 2.41 (s, 12H, methylH’s), 4.18 (s, 6H,
OMe), 7.00 (dd, 2H,J = 2.6, 6.4 Hz, 5,50-bipyridyl H’s), 7.18 (s, 4H, phenyl
H’s), 7.30 (s, 4H, phenyl H’s), 8.02 (d, 2H, J = 2.6 Hz, 3,30-bipyridyl H’s),
8.90 (d, 2H, J = 6.4 Hz, 6,60-bipyridyl H’s). ESI-MS: m/z 1242
{M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1): 845 ν(PꢀF); 1916, 1953, 2030,
1
Calcd (found) for 13 /2CH3COCH3: C, 41.08 (41.21); H, 3.26 (3.34); N,
3
7.21 (7.08).
[Re(CNC6H4Cl-4)4((MeO)2bpy)]PF6 (14). The complex has pre-
viously been communicated.6 Yield: 55%. 1H NMR (300 MHz, CDCl3,
298 K): δ 4.17 (s, 6H, OMe), 7.06ꢀ7.37 (m, 18H, phenyl H’s and 5,
50-bipyridyl H’s), 7.95 (d, 2H, J = 2.4 Hz, 3,30-bipyridyl H’s), 8þ.90 (d, 2H,
J = 6.6 Hz, 6,60-bipyridyl H’s). ESI-MS: m/z 953{M ꢀ PF6} . IR (KBr
disk, ν/cmꢀ1): 846 ν(PꢀF); 1936, 2005, 2058, 2134 ν(NtC). Anal.
2125 ν(CtN). Anal. Calcd (found) for 8 1/2Et2O: C, 42.15 (42.05); H,
3
3.47 (3.40); N, 5.90 (5.88).
Calcd (found) for 14 1/2Et2O: C, 44.46 (44.43); H, 2.93 (2.94); N, 7.41
[Re(CNC6H2Cl3-2,4,6)4((MeO)2bpy)]PF6 (9). This complex was
synthesized according to a procedure similar to that of 1, except
[Re(CNC6H2Cl3-2,4,6)5I] (134 mg, 0.1 mmol) and (MeO)2bpy
(43 mg, 0.2 mmol) were used in place of [Re(CNC6H4I-4)5I] and phen,
respectively. Yield: 87 mg, 0.063 mmol; 63%. 1H NMR (400 MHz,
CDCl3, 298 K): δ 4.20 (s, 6H, OMe), 7.00 (dd, 2H, J = 2.6, 6.4 Hz,
5,50-bipyridyl H’s), 7.32 (s, 4H, phenyl H’s), 7.39 (s, 4H, phenyl H’s), 8.05
(d, 2H, J = 2.6 Hz, 3,30-bipyridyl H’s), 8.94 (d, 2H, J = 6.4 Hz, 6,60-bipyridyl
H’s). ESI-MS: m/z 1228{M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1): 845
ν(PꢀF); 1843, 1924, 2030, 2136 ν(CtN). Anal. Calcd (found) for
3
(7.63).
[Re(CNC6H4Cl-4)4(tBu2bpy)]PF6 (15). The complex has pre-
viously been communicated.6 Yield: 57%. 1H NMR (300 MHz, CDCl3,
298 K): δ 1.57 (s, 18H, tBu), 7.09ꢀ7.14 (m, 4H, phenyl H’s), 7.17ꢀ7.45
(m, 12H, phenyl H’s), 7.65 (dd, 2H, J = 5.9, 1.5 Hz, 5,50-bipyridyl H’s),
8.38 (d, 2H, J = 1.5 Hz, 3,30-bipyridyl H’s), 9.16 (d, 2H, J = 5.9 Hz, 6,
60-bipyridyl H’s). ESI-MS: m/z 1003 {M ꢀ PF6}þ. IR (KBr disk, ν/
cmꢀ1): 846 ν(PꢀF); 1860, 2007, 2061, 2135 ν(CtN). Anal. Calcd
(found) for 15: C, 48.05 (48.18); H, 3.51 (3.45); N, 7.31 (7.52).
[Re(CNC6H4Cl-4)4(bpy)]PF6 (16). The complex has previously
9 Et2O: C, 36.51 (36.59); H, 2.09 (2.19); N, 5.81 (5.68).
3
1
Re(CNC6H4Cl-4)4(pimPh)]PF6 (10). This complex was synthe-
sized according to a procedure similar to that of 1, except [Re(CNC6H4Cl-
4)5I] (100 mg, 0.1 mmol) and pimPh (45 mg, 0.2 mmol) were used in place
of [Re(CNC6H4I-4)5I] and phen, respectively. Yield: 35 mg,
been communicated.6 Yield: 60%. H NMR (300 MHz, CDCl3, 298 K):
δ 7.07 (m, 4H, phenyl H’s), 7.31 (m, 12H, phenyl H’s), 7.57 (ddd, 2H,
J = 8.2, 5.1, 0.9 Hz, 4,40-bipyridyl H’s), 8.15 (td, 2H, J = 5.1, 1.5 Hz,
5,50-bipyridyl H’s), 8.59 (dd, 2H, J 0= 8.2, 1.5 Hz, 3,30-bipyridyl H’s), 9.21
(dd, 2H, J = 5.1, 0.9 Hz, 6,6 -bipyridyl H’s). ESI-MS: m/z 891
{M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1): 846 ν(PꢀF); 1926, 2003, 2055,
2126 ν(CtN). Anal. Calcd (found) for 16: C, 43.99 (44.03); H, 2.33 (2.56);
N, 8.10 (8.14).
[Re(CNC6H4F-4)4(bpy)]PF6 (17). The complex has previously been
communicated.6 Yield: 55%. 1H NMR (300 MHz, CDCl3, 298 K):
δ 6.96ꢀ7.34 (m, 16H, phenyl H’s), 7.56 (ddd, 2H, J = 8.4, 5.2, 0.9 Hz,
4,40-bipyridyl H’s), 8.16 (td, 2H, J = 5.4, 1.5 Hz, 5,50-bipyridyl H’s), 8.62 (dd,
2H, J = 8.4, 1.5 Hz, 3,30-bipyridyl H’s), 9.22 (dd, 2H, J = 5.6, 0.9 Hz, 6,
60-bipyridyl H’s). ESI-MS: m/z 827 {M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1):
848 ν(PꢀF); 1941, 2033, 2062, 2137 ν(CtN). Anal. Calcd (found) for 17:
C, 46.96 (47.36); H, 2.49 (2.55); N, 8.65 (8.91).
1
0.032 mmol; 32%. H NMR (400 MHz, CDCl3, 298 K): δ 7.01 (d, J =
8.3 Hz, 1H, 4-imidazoyl H’s), 7.16 (d, J= 8.7 Hz, 4H, phenyl H’s), 7.22ꢀ7.44
(m, 15H, phenyl H’s, 5-imidazoyl H’s, 5-pyridyl H’s), 7.56ꢀ7.70 (m, 5H,
phenyl H’s, 4-pyridyl H’s), 8.42 (td, 1H, J = 1.3, 7.3 Hz, 3-pyridyl H’s),
9.21(d, 1H, J = 5.3 Hz, 6-pyridyl H’s). ESI-MS: m/z 958 {M ꢀ PF6}þ. IR
(KBr disk, ν/cmꢀ1): 846 ν(PꢀF); 1860, 2004, 2058, 2136 ν(CtN). Anal.
Calcd (found) for 10: C, 45.75 (45.89); H, 2.47 (2.29); N, 8.89 (8.68).
[Re(CNC6H4I-4)4(pimMe)]PF6 (11). This complex was synthe-
sized according to a procedure similar to that of 1, except pimMe (32 mg,
0.2 mmol) was used in place of phen and dichloromethane/acetone (1/
1 v/v) was used as eluent in the column chromatography. Yield: 71 mg,
0.050 mmol; 50%. 1H NMR (400 MHz, CDCl3, 298 K): δ 4.31 (s, 3H,
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Scheme 1. Synthetic Routes to Rhenium(I) Tetra(isocyano) Diimine Complexes
[Re(CNC6H3Me2-2,6)4(bpy)]PF6 (18). The complex has pre-
viously been communicated.6 Yield: 61%. 1H NMR (300 MHz, CDCl3,
298 K): δ 2.02 (s, 12H, Me), 2.49 (s, 12H, Me), 6.96ꢀ7.12 (m, 12H,
phenyl H’s), 7.48 (ddd, 2H, J = 8.1, 5.5, 0.9 Hz, 4,40-bipyridyl H’s), 8.18
(td, 2H, J = 5.3, 1.5 Hz, 5,50-bipyridyl H’s), 8.69 (dd, 2H, J = 8.1, 1.5 Hz,
3,30-bipyridyl H’s), 9.29 (dd, 2H, J = 5.2, 0.9 Hz, 6,60-bipyridyl H’s). ESI-
MS: m/z 867 {M ꢀ PF6}þ. IR (KBr disk, ν/cmꢀ1): 844 ν(PꢀF); 1962,
1997, 2042, 2123 ν(CtN). Anal. Calcd (found) for 18: C, 54.49 (54.93);
H, 4.38 (4.22); N, 8.30 (8.59).
at room temperature and at 77 K were recorded on a Horiba Jobin Yvon
Fluorolog-3-TCSPC spectrofluorometer with a Hamamatsu R928 photo-
multiplier tube detector. Solutions were rigorously degassed on a high-
vacuum line in a two-compartment cell with no less than four successive
freezeꢀpumpꢀthaw cycles. Measurements of the EtOH/MeOH (4/1, v/v)
glass samples at 77 K were carried out with the diluted EtOH/MeOH sample
solutions contained in a quartz tube inside a liquid nitrogen filled quartz
optical Dewar flask. Luminescence quantum yields were determined using
the method described by Demas and Crosby8 with an aqueous solution of
[Ru(bpy)3]Cl2 (φem = 0.0429 with 436 nm excitation) as reference.
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 a pulse width
shorter than 750 ps. The photon counting data were analyzed by Horiba
Jobin Yvon decay analysis software. Transient absorption spectra at room
temperature were recorded using the spectral mode on a Edinburgh
Instruments LP920-KS equipped with an ICCD detector. The excitation
source for the transient absorption measurement was the third harmonic
output (355 nm; 6ꢀ8 ns fwhm pulse width) of a Spectra-Physics Quanta-
Ray Q-switched LAB-150 pulsed Nd:YAG laser (10 Hz).
Cyclic voltammetric measurements were performed by using a CH
Instruments, Inc., Model CHI 620 electrochemical analyzer. Electroche-
mical measurements were performed in acetonitrile solutions with 0.1 M
nBu4NPF6 as the supporting electrolyte at room temperature. The
reference electrode was an Ag/AgNO3 (0.1 M in acetonitrile) electrode,
and the working electrode was a glassy-carbon electrode (CH Instruments,
Inc.) with a platinum wire as the counter electrode. The working electrode
surface was polished with a 1 μm R-alumina slurry (Linde) and then a 0.3
μm R-alumina slurry (Linde) on a microcloth (Buehler Co.). The
ferrocenium/ferrocene couple (FeCp2þ/0) was used as the internal
reference. All solutions for electrochemical studies were deaerated with
prepurified argon gas prior to measurements.
General Procedure for Microwave-Assisted Synthesis. To a
10 mL reaction vial containing a suspension of [Re(CNR)5I] (0.1 mmol)
and diimine (0.2 mmol, 2 mol equiv) in THF (5 mL) was added TlOTf
(53 mg, 0.15 mmol, 1.5 mol equiv). The resulting suspension was degassed
by bubbling argon for 10 min. The vial was then sealed with a septum cap
and heated in a microwave reactor for 4 h with a maximum temperature
and pressure of 170 °C and 170 psi, respectively. After it was cooled to
room temperature, the reaction vial was uncapped and the resulting
suspension was filtered to remove the precipitated TlI. After the solvent
was removed under reduced pressure, the residue was purified by column
chromatography on silica gel using dichloromethane/acetone (7/3 v/v) as
eluent. The residue was then redissolved in methanol (2 mL). A
subsequent metathesis reaction with a saturated methanolic solution of
ammonium hexafluorophosphate gave the target complex as a PF6ꢀ salt.
Physical Measurements and Instrumentation. Microwave
reactions were performed using a CEM Discover SP single-mode micro-
wave instrument (CEM Corp., Matthews, NC). Microwave reactions were
performed in specially designed Pyrex tubes equipped with a stir bar and
sealed with a Teflon/silicon septum. 1H NMR spectra were recorded on a
Bruker AV400 (400 MHz) or a Varian (300 MHz) FT-NMR spectro-
meter. Chemical shifts (δ, ppm) werereportedrelativetotetramethylsilane
(Me4Si). All positive-ion ESI mass spectra were recorded on a PE-SCIEX
API300triplequadrupolemassspectrometer. Theelementalanalyseswere
performed on an Elementar Vario EL III analyzer.
Electronic absorption spectra were recorded on a Hewlett-Packard 8452A
diode array spectrophotometer. Steady-state emission and excitation spectra
Crystal Structure Determinations. The crystal structures of 1,
4, 7, and 12 were determined on an Oxford Diffraction Gemini S Ultra
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X-ray single-crystal diffractometer using graphite-monochromated Mo
KR radiation (λ = 0.710 73 Å) and Cu KR radiation (λ = 1.541 78 Å),
respectively. The structures were solved by direct methods employing
the SHELXL-97 program10 on a PC. Re and many non-H atoms were
located according to direct methods. The positions of other non-
hydrogen atoms were found after successful refinement by full-matrix
least squares using the SHELXL-97 program on a PC.10 In the final stage
of least-squares refinement, all non-hydrogen atoms were refined
anisotropically. H atoms were generated by the program SHELXL-
97.10 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.
’ RESULTS AND DISCUSSION
Synthesis and Characterization. With the dehydration meth-
od developed by Ugi and coworkers,7 isocyanide ligands with different
electronic and steric properties were successfully synthesized. The
reactions of K2[ReI6] with 8 mol equiv of substituted isocyanide
ligands (RNC) in a MeOH mixture gave [Re(CNR)5I].11 However,
in the synthesis of [Re(CNR)5I], where R = 4-IC6H4, 4-BrC6H4,
2,4,6-Cl3C6H2, 2,4,6-Br3C6H2, 6-OMe-2,4-Cl2C6H2, 2,6-(CH3)2-4-
BrC6H2, with electron-withdrawing substituents, the yield is very low
(<15%). The exceptionally low yield could be significantly increased
to 40ꢀ70% by the addition of the reducing agent hydrazine hydrate.
Substitution reactions of [Re(CNR)5I] with a 2-fold excess of diimine
ligands (NꢀN) in the presence of thallium triflate in 1,4-dioxane
under reflux conditions for 2 days gave the target complexes
[Re(CNR)4(NꢀN)]þ (Scheme 1). Since isocyanide ligand is
released in the substitution reaction, it also leads to the formation
of the rhenium(I) hexa(isocyano) complex [Re(CNR)6]þ as a major
byproduct and the yields of the target complexes are limited to
50ꢀ70%. These two complexes can be separated by column
chromatography. Attempts to improve the yield of the target com-
plexes by carrying out the substitution reactions with the microwave
reactor at elevated temperature and pressure (∼170 °C and 170 psi;
Figure S1 in the Supporting Information) were made. However, the
purity and yield of the target complexes are found to be similar to
those of reactions carried out by heating with an oil bath. However,
carrying out the substitution reaction in the microwave reactor can
significantly shorten the reaction time from 2 days to 4 h. Subsequent
metathesis reactions with ammonium hexafluorophosphate and
recrystallization by slow diffusion of diethyl ether into concentrated
dichloromethane solutions of the complexes gave the analytically pure
complexes as orange to red crystals depending on the π-accepting
ability of the isocyanide and diimine ligands.
Figure 1. The four IR active stretching modes (1A1, 2A1, B1, and B2
modes) of [Re(CNR)4(NꢀN)]þ with C2v symmetry.
1A1; 2020ꢀ2060 cmꢀ1 for B1; 1950ꢀ2000 cmꢀ1 for 2A1;
1880ꢀ1960 cmꢀ1 for B2. The high-energy vibration at ca.
2120ꢀ2140 cmꢀ1 is very close to the free ligand isocyanide
ν(CtN) stretch (∼2130ꢀ2140 cmꢀ1), indicating that there
was only a weak π-back-bonding interaction between the rhe-
nium metal and one or more isocyanide ligands. These observa-
tions are likely to be a result of the competition between the π-
back-bonding of the two isocyanide ligands trans to each other.
In addition, the hexafluorophosphate anions (PF6ꢀ) in com-
plexes 1ꢀ18 are also characterized by the observation of ν(PꢀF)
at ca. 840 cmꢀ1
.
X-ray Crystal Structures. Figure 2 depicts the perspective
drawings of the complex cations of 4, 7, and 12 and perchlorate
salt of 1 with their atomic numbering. The crystal and structure
determination data are collected in Table S1 (Supporting
Information), and selected bond distances and angles are sum-
marized in Table S2 (Supporting Information). The crystal
structures of these complexes show similar distorted-octahedral
structures with comparable bond lengths and angles. Due to the
steric requirements of the bidentate ligands, the bite angles
subtended by the nitrogen atoms of chelating diimine ligands
at the rhenium center of these complexes are ca. 75°, which is
commonly observed in other related complex systems.3ꢀ5 The
bond lengths of ReꢀC and CtN(isocyanide) are in the ranges
of 1.92ꢀ2.05 and 1.15ꢀ1.19 Å, respectively, similar to those
reported for other related rhenium(I) isocyanide complexes.13
Owing to the π-back-bonding interaction between the isocya-
nide ligands and the rhenium metal center, the isocyanide ligands
in these complexes are nonlinear and show different degrees of
bending.1b,14 In general, the trans isocyanide ligands are more
linear, as reflected by the averaged bond angle of CtNꢀC
(169.4° vs 162.9°) in comparison to those cis isocyanide ligands,
which are trans to the diimine ligand. These observations can be
rationalized by the competition between the two trans isocyanide
ligands for the π-back-bonding and the large trans influence of
isocyanide ligands.
1
Complexes 1ꢀ18 were characterized by H NMR, IR, and
ESI-MS and gave satisfactory elemental analyses. The X-ray
crystal structures of 4, 7, and 12 and the perchlorate salt of
1 were also determined. All complexes show four CtN stretches
in the 1850ꢀ2140 cmꢀ1 region, which is consistent with the IR
active modes for an octahedral complex with four isocyanide
ligands in a C2v symmetry. These four modes (Figure 1) include
two A1 modes involving the in-phase motion of the two trans
isocyanides (1A1) and another in-phase motion of the two
isocyanide ligands trans to the diimine ligand (2A1), a B1 mode
involving the out-of-phase motion of the trans isocyanides, and
another B2 mode involving the out-of-phase motion of the two
isocyanides trans to the diimine ligand. Similar vibrational modes
were also assigned for the related rhenium(I) tetracarbonyl
bipyridyl systems.12 On the basis of these vibrational modes,
the stretches are assigned to the modes: 2120ꢀ2140 cmꢀ1 for
UV Spectroscopy. All the complexes dissolve in dichloro-
methane to give orange to red solutions, which show intense
absorptions with molar extinction coefficients on the order of 104
dm3 molꢀ1 cmꢀ1 in the region of 230ꢀ370 nm(Table1, Figure 3).
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Figure 2. Perspective drawings of the complex cations of (a) 1, (b) 4, (c) 7, and (d) 12 with atomic numbering. Hydrogen atoms have been omitted for
clarity. Thermal ellipsoids are shown at the 30% probability level.
These absorptions are ascribed to the intraligand (IL) π f π*
transitions of the substituted phenyl isocyanide and diimine
moieties. In addition to intense IL absorptions in the UV region,
they also show two to three additional moderately intense absorp-
tion bands or shoulders at 370ꢀ470 nm. With reference to the
previous spectroscopic studies of Re(I) diimine complexes3ꢀ5,15
and the homoleptic metal phenylisocyano complexes of different
metal centers,16 which show metal-to-ligand charge transfer
(MLCT) transitions [dπ(M) f π*(RNC)] in the region from
250 to 500 nm depending on the nature of the metal center, these
moderately intense absorptions are assigned to the MLCT transi-
tions of [dπ(Re) f π*(RNC)] and [dπ(Re) f π*(NꢀN)],
probably with some mixing of a ligand-to-ligand charge transfer
(LLCT) transition of [π(RNC) f π*(NꢀN)]. The higher
energy absorptions at ca. 370ꢀ410 nm are assigned to the MLCT
[dπ(Re) f π*(RNC)] transition, whereas the lowest energy
absorption bands are assigned to the MLCT [dπ(Re) f
π*(NꢀN)] transition, since π* orbitals of substituted phenyl
isocyanides lie higher in energy than those of the diimine ligands.
Moreover, the lowest energy absorption also shows an absorption
energy dependence on the π-accepting ability of the diimine and the
isocyanide ligands, in line with the MLCT [dπ(Re) f π*(NꢀN)]
assignment. For complexes 3ꢀ5 and 8ꢀ15, which contain diimine
ligands of relatively higher lying π* orbitals such as those with
electron-donating substituents or a less extensively π-conjugated
methylimidazole moiety and/or isocyanide ligands of lower lying π*
orbital due to the presence of electron-withdrawing and π-accepting
substituents, the MLCT [dπ(Re) f π*(NꢀN)] and [dπ(Re) f
π*(RNC)] transitions are close in energy and therefore do not show
well-resolved absorption bands or shoulders. The observation of a
much higher extinction coefficient for the lowest energy absorption
in comparison to those for other complexes is supportive of the
mixing of the two MLCT transitions. In addition, all the complexes
also exhibit very weak absorption tailing at about 550ꢀ650 nm with
molar extinction coefficients on the order of 102 dm3 molꢀ1 cmꢀ1
,
which is ascribed to spin-forbidden 3MLCT transitions.
Emission Spectroscopy. Excitation of dichloromethane solu-
tions of complexes 1ꢀ18 at λ >350 nm produces orange to red
MLCT [dπ(Re) fπ*(NꢀN)] phosphorescence with the emission
maxima in the range of 612ꢀ738 nm (Table 1). These emission
bands are found to be sensitive to the nature of the isocyanide and
diimine ligands. For complexes with the same isocyanide ligands, a
significant blue shift in emission energy is observed when the
bipyridyl ligand is replaced by 1-methyl-2-(20-pyridyl)imidazole, in
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Table 1. Photophysical Data for Complexes 1ꢀ18
medium
emissiona
complex
(T/K)
λem/nm (τo/μs)
φem ꢁ 103
absorptionb λabs/nm (ε/dm3 molꢀ1 cmꢀ1
)
1
CH2Cl2 (298)
glassc (77)
674 (0.061)
608 (1.69)
686 (0.039)
610 (1.32)
616 (0.241)
4.6
269 (72 455), 290 (41 605), 331 (49 520), 385 (28 940), 410 sh (17 080), 465 sh (7890)
269 (80 765), 293 (53 240), 327 (68 020), 382 (33 410), 479 (8890)
2
3
CH2Cl2 (298)
glassc (77)
1.7
CH2Cl2 (298)
43.2
229 (92 790), 267 (89 820), 292 (45 950), 344 (63 475), 381 sh (45 495), 393 sh (42 910),
419 sh (25 665)
glassc (77)
566 (3.38)
614 (0.275)
558 (3.28)
629 (0.187)
570 (2.79)
684 (0.013)
614 (0.72)
686 (0.013)
612 (0.68)
683 (0.005)
584 (0.40)
612 (0.039)
540 (2.74)
645 (0.099)
574 (2.97)
631 (0.173)
560 (3.95)
632 (0.159)
570 (3.95)
648 (0.134)
566 (3.29)
681 (0.007)
586 (0.46)
671 (0.020)
586 (0.83)
690 (0.012)
615 (0.56)
698 (0.009)
629 (0.41)
738 (0.005)
642 (0.23)
4
CH2Cl2 (298)
glassc (77)
50.3
25.4
0.40
0.50
0.49
3.1
229 (90 810), 266 (88 270), 293 (45 010), 342 (62 495), 390 sh (43 080), 419 sh (26 195)
267 (85 705), 291 (43 810), 341 (58 645), 376 sh (43 965), 430 sh (18 925)
254 (58 780), 295 (72 400), 334 (67 420), 383 (38 020), 406 sh (21 155), 450 sh (9440)
244 (60 835), 296 (69 680), 331 (60 670), 380 (32 525), 412 sh (14 780), 451 (8205)
246 (72 160), 288 (56 440), 335 (74 515), 376 (41 255), 418 sh (16 990)
262 (55 525), 283 (41 925), 349 (51 155), 377 sh (37 850), 402 (30 840)
242 (59 790), 307 (68 010), 334 (58 405), 380 sh (30 495), 414 sh (18 365)
259 (60 170), 307 (64 100), 338 (61 540), 386 (37 605), 411 sh (23 690)
240 (54 710), 306 (61 810), 333 (54 475), 385 (30 105), 408 sh (19 840)
248 (54 885), 309 (55 005), 333 (53 460), 383 (28 350), 408 sh (17 500)
231 (72 610), 252 (52 390), 283 (53 615), 332 (59 270), 375 (33 105), 412 sh (16 160)
239 (55 965), 294 (66 050), 331 (57 320), 365 (33 980), 412 sh (15 020)
230 (65 215), 296 (78 970), 328 (66 405), 366 (38 610), 408 sh (17 450), 456 sh (9095)
235 (38 200), 296 (52 585), 315 (43 390), 358 (22 345), 400 sh (9430), 471 (5520)
242 (53 940), 298 (69 555), 322 (52 095), 360 (29 910), 410 sh (9830), 492 (6525)
5
CH2Cl2 (298)
glassc (77)
6
CH2Cl2 (298)
glassc (77)
7
CH2Cl2 (298)
glassc (77)
8
CH2Cl2 (298)
glassc (77)
9
CH2Cl2 (298)
glassc (77)
10
11
12
13
14
15
16
17
18
CH2Cl2 (298)
glassc (77)
19.1
27.9
22.6
13.8
0.57
2.9
CH2Cl2 (298)
glassc (77)
CH2Cl2 (298)
glassc (77)
CH2Cl2 (298)
glassc (77)
CH2Cl2 (298)
glassc (77)
CH2Cl2 (298)
glassc (77)
CH2Cl2 (298)
glassc (77)
1.1
CH2Cl2 (298)
glassc (77)
0.62
0.16
CH2Cl2 (298)
glassc (77)
a Emission maxima are uncorrected values. b At 298 K. c EtOH/MeOH (4/1 v/v).
comparison to the introduction of electron-donating substituents on
the bipyridyl ligand. This can be exemplified by the trends of
the emission maxima: 11 (631 nm) , 6 (684 nm); 12 (632 nm) <
10 (645 nm) , 15 (671 nm) < 14 (681 nm) < 16 (690 nm)
(Figure 4a). The increase in the MLCT emission energy could be
explained by the raising of the energy of the π* orbital as a result of
the introduction of electron-donating substituents or the decrease of
the extent of π-conjugation of the diimine ligand. In addition to the
direct modification of the LUMO [π*(NꢀN)] by changing the
diimine ligand to tune the emissive MLCT [dπ(Re) f π*(NꢀN)]
excited state, the perturbation of the HOMO [dπ(Re)] through the
change of isocyanide ligands and their corresponding interactions
with rhenium metal center are found to be very effective in tuning the
emissive excited state. This could be illustrated by the considerable
shift of the MLCT phosphorescence (Figure 4b) for rhenium
phenanthrolinyl complexes with different π-accepting isocyanide
ligands through the modification of the substituents on the phenyl
moiety: 4 (614 nm) ≈ 3 (616 nm) < 5 (629 nm) < 1 (674 nm) < 2
(686 nm) (ΔEem ≈ 1690 cmꢀ1 from Cl3C6H2NC to
BrMe2C6H2NC). This could be explained by the difference in the
stabilization of the dπ(Re) orbital as a result of the interaction
between the dπ(Re) and π*(RNC) orbitals. Such a π-back-bonding
interaction between the metal center and aryl isocyanide ligands, and
its variation with the aryl isocyanide ligands of different electronic
natures were also commonly observed in other metal isocyanide
complexes.5,15ꢀ17 Further tuning of the emissive excited state can be
anticipated by simple modification of substituents on the phenyl
group of the isocyanide ligands or the replacement of the phenyl
group with other functional moieties.
These complexes also display strong photoluminescence with a
structureless emission band in solid state and in 77 K EtOH/
MeOH (4/1 v/v) glassy medium. As these emissions show trends
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Figure 3. Overlaid absorption spectra of selected complexes 2, 5, 7, and
18 in CH2Cl2 solution at 298 K.
Figure 5. Normalized uncorrected emission spectra of (a) 4, (b) 9, and
(c) 10 in different solvents at 298 K.
Figure 4. Overlaid uncorrected emission spectra of (a) 12, 14ꢀ16 and
(b) 1ꢀ5 in CH2Cl2 solution at 298 K.
similar to those of the solution emissions, they are also tentatively
assigned as derived from 3MLCT [dπ(Re) f π*(NꢀN)] excited
state origin. The blue shift is attributed to the increased rigidity in
the glass, which is commonly observed in other related systems.
The submicrosecond excited-state lifetime observed in a glassy
medium at 77 K is also supportive of its triplet parentage.
Figure 6. Plot of emission energy of 6, 8, 9, and 16ꢀ18 in different
solvents versus Dimroth’s solvent parameter and their linear least-
squares fits (—), excluding the data points of methanol and ethanol.
The emission of complex 14 was found to be highly sensitive
to the solvent environment.5 To investigate the structureꢀprop-
erty relationship of the solvent dependence of the MLCT
emission of these complexes, the emissions of 1ꢀ18 in different
solvent media, such as benzene, dioxane, THF, CHCl3, CH2Cl2,
acetone, MeCN, ethanol, and methanol, were investigated and
summarized in Table S3 (Supporting Information). The emis-
sions of these complexes show different solvent dependences as
well as different degrees of sensitivity (Table S3, Figure 5).
Except for complexes with unsymmetrical diimine ligands
(pimMe and pimPh), the emissions of all other complexes show
a blue shift in emission energy with decreasing solvent polarity
from an acetonitrile solution to a benzene solution. Such a
solvent dependence may be due to the increase in the dipole
moment of the excited state, which is more stabilized in a polar
solvent medium, in comparison to its ground state. Moreover,
the emission energies of these complexes are found to correlate
well with Dimroth’s solvent parameter18 (Figure 6). Since these
complexes do not have any functional moiety capable of forming
a hydrogen-bonding interaction and the pyridinium phenoxide
dye used for the derivation of Dimroth’s solvent parameter18 can
have significant hydrogen-bonding interactions with the protic
solvents, the emission data of these complexes obtained in
methanol and ethanol deviate from the correlation.
tBu2bpy . bpy > phen (Figure 5). This is in the reverse order of
the π-accepting ability of the diimine ligand. Such a decrease of
the solvatochromic shifts with the increasing π-acceptor strength
of the diimine ligand is commonly observed in other related
metal polypyridyl MLCT chromophores19 and was attributed to
the decrease in charge transfer character as a result of the stronger
mixing of the dπ orbital of metal center and π* orbital of the
diimine ligand by better π-back-bonding interaction. In addition,
the degree of solvent dependence is also found to decrease for
complexes with 2,4,6-trisubstituted phenyl isocyanide ligands
compared to those with 4-monosubstituted phenyl isocyanide
ligands. The lower solvent sensitivity of these complexes may be
attributed to the better shielding of the excited portions of the
complexes in the presence of more sterically bulky isocyanide
ligands. Similar shielding effects of the excited state against
environmental perturbation were also reported in the emission
solvatochromic studies of related MLCT luminophores.20 Con-
sequently, the MLCT emissions of phenanthroline complexes
2ꢀ5 with different 2,4,6-trisubstituted phenyl isocyanide ligands
are almost insensitive and do not show any obvious emission
energy dependence to the polarity of the solvent media. Inter-
estingly, complexes 10ꢀ12 with unsymmetrically substituted
pyridine imidazole ligands show a different solvent dependence,
with the lowest energy emission in nonpolar solvents, such as
benzene and dioxane, compared to other polar solvents
(Figure 5c). This may be attributed to the significant variations
The degree of solvent dependence is found to vary with the
nature of diimine ligand and follow the order (MeO)2bpy ≈
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Figure 7. (a) Transient difference spectra of 4 in MeCN solution at 298
K obtained after 355 nm nanosecond laser excitation at different time
delays: 0, 60, 120, 180, 240, 300, 360, 420, 480, 540, and 600 ns.
(b) Overlaid absorption (solid line) and emission (dashed line) spectra
of 4 in MeCN solution. The insets show (a) the absorptionꢀtime profile
at 307 nm and (b) a luminescence intensity decay trace at 634 nm after
355 nm nanosecond laser excitation and their first-order exponential fit
(red line).
Figure 8. Cyclic voltammograms of (a) the oxidative scan and (b) the
reductive scan of 7 in MeCN solution (0.1 mol dmꢀ3 nBu4NPF6). Scan
rate: 100 mV sꢀ1. The red line shows the inset of the oxidative and
reductive scan.
of the dipole moments of the complexes in the ground state and
the excited state. However, these emissions do not show any
correlation with the solvent parameters.
Transient Absorption Spectroscopy. The solution emission
lifetime of 4 is on the submicrosecond time scale, which is
sufficiently long for a nanosecond transient absorption spectro-
scopic study. Transient different spectra on the nanosecond time
scale after 355 nm nanosecond laser excitation were recorded. Two
absorption features peaking at ca. 310 and 525 nm together with a
strong ground-state bleaching at ca. 396 nm, which decay in accord
with the first-order kinetics with a lifetime of 166 ns, were observed
(Figure 7). However, due to the spectral overlapping of the 3MLCT
phosphorescence, the excited-state absorptions at λ >570 nm of the
complexes cannot be determined. The close resemblance of the
lifetimes for these transient absorptions and the luminescence
intensity (τ = 170 ns in MeCN) is suggestive of the origin of these
transient absorptions being attributed to the emissive excited
species. On the basis of the reported absorption for phenanthroline
anion radical21 as well as the previous transient absorption studies of
[Re(CO)3(phen)Cl],3c which show similar absorptions, these
transient absorptions were also assigned to the absorptions of the
reduced phenanthroline. In contrast to the transient absorptions of
[Re(CO)3(phen)Cl],3c the transient absorption spectrum of
4 shows a much weaker absorption feature at ca. 310 nm and
strong ground-state bleaching peaking at ca. 397 nm. This ground-
state bleaching is most likely associated with MLCT [dπ(Re) f
π*(RNC)] and MLCT [dπ(Re) f π*(phen)] transitions, prob-
ably mixed with LLCT [π(RNC) f π*(NꢀN)]. The presence of
this significant ground-state bleaching for 4, which is absent in the
transient absorption spectrum of [Re(CO)3(phen)Cl], is consis-
tent with the much stronger MLCT absorptions for 4 as a result of
the mixing of different MLCT transitions ([dπ(Re) f π*(RNC)]
and [dπ(Re) f π*(phen)]). All these observations are in agree-
ment with MLCT phosphorescence assignments.
ꢀ1.77 V vs SCE was determined, as these complexes show
significant decomposition on scanning to more negative poten-
tial. The representative cyclic voltammograms of the oxidative
scan and reductive scan of 7 in MeCN solution are shown in
Figure 8. The electrochemical data of these complexes in
acetonitrile solution (0.1 mol dmꢀ3 nBu4NPF6) at ambient
temperature are summarized in Table 2.
The first quasi-reversible oxidation couple and the irreversible
oxidation wave are assigned to the metal-centered Re(I/II) and
Re(II/III) oxidation processes, respectively. These oxidation
potentials were found to vary with the π-accepting ability of
the diimine and the isocyanide ligands. With the same isocyanide
ligands, the potential for the oxidation is found to follow the
order 1 (þ0.74, þ1.42 V) ≈ 6 (þ0.74, þ1.40 V) > 11 (þ0.67,
þ1.27 V), which is in line with the π-accepting ability of the
diimine ligands: phen ≈ bpy > pimMe. Similarly, for the
phenanthroline complexes with different isocyanide ligands,
the trend for the oxidation potential (4 (þ1.05, þ1.72 V) ≈ 3
(þ1.05, þ1.72 V) > 5 (0.98, þ1.67 V) > 1 (0.74, þ1.42 V)) is
consistent with the π-accepting ability of the isocyanide
ligands: 2,4,6-Cl3C6H2NC ≈ 2,4,6-Br3C6H2NC >2,4-Cl2-6-
MeOC6H2NC > 4-IC6H4NC. The increasing metal-centered
oxidation potential for complexes with π-accepting ability of the
ligands can be attributed to the lower-lying dπ(Re) orbital as a
result of a better metalꢀligand interaction.
In the reductive scan, the first quasi-reversible couple or
irreversible wave of these complexes shows a noticeable cathodic
shift for complexes with more electron rich or less π-conjugated
diimine ligands and are relatively insensitive to the nature of
isocyanide ligands. Therefore, they are assigned to the ligand-
centered reduction of the diimine ligands. For the three addi-
tional irreversible reduction waves at more negative potentials
observed in complexes 7, 8, 10, 12, and 14ꢀ18, the first two
irreversible reduction waves are assigned to successive reductions
of the isocyanide ligands, as their potentials are highly sensitive to
the substituents as well as the π-accepting ability of the
Electrochemistry. 10, 12, and 14ꢀ18 show one quasi-rever-
sible reduction couple with E1/2 in the range of ꢀ1.40 to ꢀ1.70 V
vs SCE in their cyclic voltammograms in acetonitrile (0.1 mol
dmꢀ3 nBu4NPF6) and three irreversible reduction waves with Epc
in the ranges of ꢀ1.72 to ꢀ2.08 V, ꢀ2.05 to ꢀ2.54 V, and ꢀ2.29
to ꢀ2.72 V vs SCE, whereas for complexes 1ꢀ6, 9, 11 and 13,
only one irreversible reduction wave with Epc from ꢀ1.36 to
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Table 2. Electrochemical Data for Complexes 1ꢀ18 in
showing the microwave conditions used in the microwave-
assisted synthesis. This material is available free of charge via
the Internet at http://pubs.acs.org.
Acetonitrile Solution (0.1 mol dmꢀ3 nBu4NPF6) at 298 Ka
oxidation,b E1/2/V vs
SCE (Epa/V vs SCE)
reduction,b E1/2/V vs |
SCE (Epc/V vs SCE)
complex
’ AUTHOR INFORMATION
1
0.74 (1.42)c
0.72 (1.49)c
1.05 (1.72)c
1.05 (1.72)c
0.98 (1.67)c
0.74 (1.40)c
0.74 (1.39)c
0.67 (1.39)c
1.00 (1.62)c
0.67 (1.33)c
0.67 (1.27)c
0.66 (1.35)c
0.65 (1.39)c
0.69 (1.30)c
0.71 (1.36)c
0.73 (1.42)c
0.68 (1.43)c
0.63 (1.46)c
(ꢀ1.39)d,e
Corresponding Author
*E-mail: vinccko@cityu.edu.hk. Fax: þ(852) 3442-0522. Tel:
2
(ꢀ1.41)d,e
3
(ꢀ1.36)d,e
þ(852) 3442-6958.
4
(ꢀ1.37)d,e
5
(ꢀ1.39)d,e
6
(ꢀ1.45)d,e
’ ACKNOWLEDGMENT
7
ꢀ1.42 (ꢀ1.75)d (ꢀ2.09)d (ꢀ2.30)d
ꢀ1.53 (ꢀ2.05)d (ꢀ2.51)d (ꢀ2.72)d
(ꢀ1.47)d,e
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. 7008032). The flash photolysis system
was supported by a Special Equipment Grant from the University
Grants Committee of the Hong Kong (SEG_CityU02). J.W.-K.
S. and A.W.-Y.C. acknowledge the receipt of a Postgraduate
studentship from the City University of Hong Kong.
8
9
10
11
12
13
14
15
16
17
18
ꢀ1.63, (ꢀ1.89)d, (ꢀ2.27)d, (ꢀ2.63)d
(ꢀ1.72)d,e
ꢀ1.69 (ꢀ1.92)d (ꢀ2.30)d (ꢀ2.67)d
(ꢀ1.77)d,e
ꢀ1.52 (ꢀ1.87)d (ꢀ2.20)d (ꢀ2.53)d
ꢀ1.49 (ꢀ1.78)d (ꢀ2.18)d (ꢀ2.42)d
ꢀ1.42 (ꢀ1.72)d (ꢀ2.16)d (ꢀ2.29)d
ꢀ1.43 (ꢀ1.80)d (ꢀ2.20)d (ꢀ2.33)d
ꢀ1.46 (ꢀ2.03)d (ꢀ2.54)d (ꢀ2.70)d
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Re(0).
’ CONCLUSION
A new series of luminescent rhenium(I) tetra(isocyano) diimine
complexes, [Re(CNR)4(NꢀN)]þ, with isocyanide and diimine
ligands of different electronic and steric features has been success-
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electrochemical properties have been studied. The X-ray crystal
structures of four of the complexes were also determined. Detailed
photophysical studies revealed that all complexes displayed orange
to red MLCT [dπ(Re) f π*(NꢀN)] phosphorescence upon
3
excitation with λ >350 nm. The emissive MLCT [dπ(Re) f
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b
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