Synthesis and luminescence properties of Cr(III) complexes with cyclam-type ligands having pendant chromophores, trans-[Cr(L)Cl 2]Cl1

Article abstract of DOI:10.1021/ic048312g

The synthesis and spectroscopic properties of new cyclam-type ligands 5,7-dimethyl-6-R-1,4,8,11-tetraazacyclotetradecane (L), where R is a pendant chromophore such as an anthracene derivative, are reported. These ligands were prepared according to a nickel(II) template procedure, and the X-ray crystal structures of several Ni(II) intermediates are described. Reaction of the free base ligands L with CrCl₃·3THF resulted in facile formation of trans-[Cr(L)Cl₂]Cl complexes, and the structures and spectroscopic characterizations of these complexes are also described. Examination of the photophysical properties of trans-[Cr(L)Cl₂]Cl solutions at 77 K demonstrated the emission spectra to be dominated by phosphorescence from the ligand field doublet of the chromium(III) center. This also applies to the Cr(III) complex trans-[Cr(mac)Cl₂]Cl, where mac is the anthracene derivative 5,7-dimethyl-6-anthracenylcyclam. Excitation into the π-π* states of the anthracene leads to marked quenching of the fluorescence from this chromophore and sensitized phosphorescence from the metal-centered doublet state.

Full text of DOI:10.1021/ic048312g

Inorg. Chem. 2005, 44, 4166−4174
Synthesis and Luminescence Properties of Cr(III) Complexes with
Cyclam-Type Ligands Having Pendant Chromophores, trans-[Cr(L)Cl2]Cl1
Frank DeRosa, Xianhui Bu, Kristina Pohaku, and Peter C. Ford*
Department of Chemistry and Biochemistry, UniVersity of California,
Santa Barbara, California 93106
Received November 30, 2004
The synthesis and spectroscopic properties of new cyclam-type ligands 5,7-dimethyl-6-R-1,4,8,11-tetraazacyclotet-
radecane (L), where R is a pendant chromophore such as an anthracene derivative, are reported. These ligands
were prepared according to a nickel(II) template procedure, and the X-ray crystal structures of several Ni(II)
intermediates are described. Reaction of the free base ligands L with CrCl3‚3THF resulted in facile formation of
trans-[Cr(L)Cl2]Cl complexes, and the structures and spectroscopic characterizations of these complexes are also
described. Examination of the photophysical properties of trans-[Cr(L)Cl2]Cl solutions at 77 K demonstrated the
emission spectra to be dominated by phosphorescence from the ligand field doublet of the chromium(III) center.
This also applies to the Cr(III) complex trans-[Cr(mac)Cl2]Cl, where mac is the anthracene derivative 5,7-dimethyl-
6-anthracenylcyclam. Excitation into the π−π* states of the anthracene leads to marked quenching of the fluorescence
from this chromophore and sensitized phosphorescence from the metal-centered doublet state.
Introduction
Cr(III) complexes display visible range quartet and doublet
ligand field absorption bands, the extinction coefficients are
small. Therefore, it would be desirable to build systems with
ligand-based chromophores having higher extinction coef-
ficients that can serve as internal antennae and sensitizers
of metal-centered photoreactions.
Ongoing studies in this laboratory and others have been
concerned with the synthesis and photochemical character-
izations of new metal-based compounds having possible
applications as photochemically activated drugs.1-7 Materials
of interest in this regard include chromium(III) complexes
of macrocyclic tetraamine ligands, such as cyclam (cyclam
) 1,4,8,11-tetraazacyclotetradecane).8-10 Although such
We have recently reported the synthesis and characteriza-
tion of Cr(III) macrocyclic tetraamine complexes cis-
[CrL′Cl2]Cl with chromophores appended to the 1,8-nitrogens
of a cyclam ligand (L′).11 However, once formed these Cr-
(III) complexes proved to be resistant to isomerization. The
crystal structures showed the tertiary nitrogens to be posi-
tioned along the ligand folding axis, and the absence of
ionizable protons at those sites may inhibit the cis-trans
isomerization. In this context, it appeared desirable to prepare
macrocyclic complexes having the pendant chromophore
attached at a ring carbon instead. Described here are
preparations of several cyclam-based ligands of the type 5,7-
dimethyl-6-R-1,4,8,11-tetraazacyclotetradecane, where R is
a C-bound pendant chromophore. These ligands L were
prepared via nickel(II) template methods,12,13 and intermedi-
* Author to whom correspondence should be addressed. E-mail: ford@
chem.ucsb.edu.
(1) Taken in part from the Ph.D. Dissertation of F.D., University of
California, Santa Barbara, 2003.
(2) Modica-Napolitano, J. S.; Joyal, J. L.; Ara, G.; Oseroff, A. R.; Aprille,
J. R. Cancer Res. 1990, 50, 7876-7881.
(3) (a) Bourassa, J.; DeGraff, W.; Kudo, S.; Wink, D. A.; Mitchell, J. B.;
Ford, P. C. J. Am. Chem. Soc. 1997, 119, 2853-2860. (b) Ford, P.
C.; Bourassa, J.; Miranda, K.; Lee, B.; Lorkovic, I.; Boggs, S.; Kudo,
S.; Laverman, L Coord. Chem. ReV. 1998, 171, 185-202. (c) Conrado,
C. L.; Wecksler, S.; Egler, C.; Magde, D.; Ford, P. C. Inorg. Chem.
2004, 43, 5543-5549. (d) Wecksler, S.; Mikhailovsky, A.; Ford, P.
C. J. Am. Chem. Soc. 2004, 126, 13566-13567.
(4) Lorkovic, I. M.; Miranda, K. M.; Lee, B.; Bernhard, S.; Schoonover,
J. R.; Ford, P. C. J. Am. Chem. Soc. 1998, 120, 11674-11683.
(5) Works, C. F.; Ford, P. C. J. Am. Chem. Soc. 2000, 122, 7592-7593.
(6) Kawakami, M.; Koya, K.; Ukai, T.; Tatsuta, N.; Ikegawa, A.; Ogawa,
K.; Shishido, T.; Chen, L. B. J. Med. Chem. 1998, 41, 130-142.
(7) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G.
B. Chem. ReV. 2000, 100, 1973-2011 and references therein.
(8) DeLeo, M. A.; Ford, P. C. J. Am. Chem. Soc. 1999, 121, 1980-1981.
(9) DeLeo, M. A.; Bu, X.; Bentow, J.; Ford, P. C. Inorg. Chim. Acta
2000, 300, 944-950.
(11) DeRosa, F.; Bu, X.; Ford, P. C. Inorg. Chem. 2003, 42, 4171-4178.
(12) Katovic, V.; Taylor, L. T.; Busch, D. H. J. Am. Chem. Soc. 1969, 91,
2122-2123.
(13) (a) Curtis, N. F. Coord. Chem. ReV. 1968, 3, 3-47 and references
therein. (b) Comba, P.; Curtis, N. F.; Lawrance, G. A.; Sargeson, A.
M.; Skelton, B. W.; White, A. H. Inorg. Chem. 1986, 25, 4260-4267.
(10) DeLeo, M. A.; Ford, P. C. Coord. Chem. ReV. 2000, 208, 47-59.
4166 Inorganic Chemistry, Vol. 44, No. 12, 2005
10.1021/ic048312g CCC: $30.25
© 2005 American Chemical Society
Published on Web 03/31/2005

Synthesis and Luminescence Properties of trans-[Cr(L)Cl2]Cl
Scheme 1. Compounds Prepareda
tetrahydrofuran) was purchased from the Strem Chemical Co.
Dimethylformamide (DMF) was dried over 3 Å molecular sieves
before use. Acetonitrile was distilled over CaH2. All other solvents
were used as received.
Elemental analyses (C, H, N) were performed by the UCSB
Marine Science Institute Analytical Laboratory.
Electronic spectra were recorded using a Hewlett-Packard model
HP8452A diode array spectrophotometer or a Shimadzu model
2401PC spectrophotometer. Low-resolution mass spectra were
obtained using a VG Fisons Platform II single-quadrupole mass
spectrometer with an electrospray ionization source run with a
Fisons Masslinks data system. NMR spectra were obtained on
Varian 200 and 400 MHz spectrometers in CDCl3 (CHCl3 at 7.260
ppm). Infrared spectra (KBr pellets) were recorded using a BioRad
model FTS 60 SPC 3200 FTIR spectrometer.
Emission and excitation spectra were recorded utilizing a SPEX
Fluorolog 2 spectrofluorimeter equipped with a Hamamatsu R928-A
water-cooled PMT configured for photon counting and interfaced
with a computer running Spex DM3000f software. Emission spectra
were corrected for PMT response as well as for lamp intensity
variation by the ratio method with a Rhodamine-6G reference. A
cutoff filter was placed in front of the emission monochromator to
block scattered excitation light. All emissions were obtained in a
front-face configuration at 77 K using EtOH/MeOH glass matrixes
(4:1 v/v).
Emission lifetime data were recorded using the second (532 nm)
and third (355 nm) harmonics of a Nd:YAG pulsed laser system
(Continuum NY61). The samples were placed in a quartz dewar
filled with liquid nitrogen. Emission lifetimes at 77 K were
monitored by a PMT (RCA 8852) coupled to a digitalizing
oscilloscope (Tektronix TDS 540), and data were transferred to a
computer for subsequent analysis.
2. Syntheses of Ni(II) Complexes. [(5,7-Dimethyl-1,4,8,11-
tetraazacyclotetradeca-4,7-dienato)nickel(II)] Nitrate ([Ni(di-
enato)]NO3, 1) was prepared according to literature methods.20
Yield: 6.587 g (76.0%). ESI-MS: m/z 281 (M+). IR (KBr): 1558
cm-1 (CdC), 1537 cm-1 (CdN).
[(5,7-Dimethyl-6-benzyl-1,4,8,11-tetraazacyclotetradeca-4,7-
dienenickel(II)] Nitrate Bromide ([Ni(benzyl diene)]NO3,Br, 2a)
was prepared in 74.3% yield following a reported synthesis21 and
characterized by ESI-TOF mass spectrometry.
[(5,7-Dimethyl-6-anthracyl-1,4,8,11-tetraazacyclotetradeca-
4,7-diene)nickel(II)] Nitrate Iodide ([Ni(anthracyl diene)]I,NO3,
2b) was synthesized in a manner similar to that reported for 2a.21
A solution of 1 (0.434 g, 1.26 mmol) in 25 mL of absolute ethanol
was mixed with 1 equiv of 9-iodomethylanthracene (anthracyl
iodide, 1.26 mmol) in acetone via cannulation. The anthracyl iodide
had been prepared in situ from anthracyl chloride by the Finklestein
reaction.18 The acetone was allowed to boil off, and a condenser
was attached. The solution was refluxed for ∼4 h, affording a bright
yellow precipitate. The product was recrystallized via slow evapora-
tion of an aqueous solution. Yield: 0.62 g (73.3%). ESI-MS: m/z
236 (M2+). IR (KBr): 1663 cm-1 (CdN). An X-ray crystal structure
was obtained.
a The numerical labels for the complexes refer to various salts of the
cations as indicated in the text. The ligands bbc and abc were prepared by
modifying the pendant group after coordination to Cr(III).
ates along this scheme were characterized. The corresponding
Cr(III) complexes were formed by the direct reaction with
CrCl3, and the more stable14-16 trans-[Cr(L)Cl2]Cl isomers
were the first products isolated. The analogous reaction with
cyclam gives largely (>90%) the cis isomer as previously
reported.17 Spectroscopic characterization for all new species
is reported as well as several X-ray crystal structures. Scheme
1 illustrates the complexes and ligands prepared and the
shorthand notation used to designate them.
Experimental Section
1. Materials, Procedures, and Instrumentation. Syntheses were
carried out in the presence of air with the exception of the halide
exchange reaction.18 N,N′-Bis(2-aminoethyl)-1,3-propanediamine
(2,3,2-Tet) was purchased from Acros Organics, and 2,4-pentanedi-
one and p-xylene glycol were purchased from TCI Chemicals Co.
Sodium borohydride, concentrated HBr (48%), 1-hydroxybenza-
triazole, 9-chloromethylanthracene (anthracyl chloride), and NiCl2‚
6H2O were purchased from Aldrich and used without further
purification. The synthesis precursor p-(hydroxymethyl)benzyl
bromide was prepared from p-xylene glycol in excellent yield
1
following a reported preparation19 and was characterized by H
NMR and ESI-TOF mass spectrometry. CrCl3‚3THF (THF )
(14) Bosnich, B.; Poon, C. K.; Tobe, M. L. Inorg. Chem. 1965, 4, 1102-
1108.
(15) Whimp, P. O.; Bailey, M. F.; Curtis, N. F. J. Chem. Soc. A 1970,
1956-1963.
(16) Connolly, P. J.; Billo, E. J. Inorg. Chem. 1987, 26, 3224-3226.
(17) Tobe, J.; Ferguson, M. L. Inorg. Chim. Acta 1970, 4, 109-112.
(18) Finkelstein reaction: Dissolve x amount of NaI in acetone and add 1
equiV of alkyl chloride. Purge with nitrogen and stir for approximately
4 h in the dark. Results should yield desired alkyl iodide. March, J.
AdVanced Organic Chemistry, 4th ed.; John Wiley & Sons: New York,
1992; p 430.
[(5,7-Dimethyl-6-(p-hydroxymethylbenzyl)-1,4,8,11-tetraaza-
cyclotetradeca-4,7-diene)nickel(II)] Nitrate Bromide (2c) was
synthesized in a similar manner. A 4.00 g portion of 1 (11.6 mmol)
was partially dissolved in ∼80 mL of absolute ethanol with about
1 equiv of p-hydroxymethylbenzyl bromide (3.50 g, 17.4 mmol).
(20) Cummings, S. C.; Martin, J. G Inorg. Chem. 1973, 12, 1477-1482.
(21) Fabbrizzi, L.; De Santis, G.; Iacopino, D.; Poggi, A.; Perotti, A.;
Pallavicini, P. Inorg. Chem. 1997, 36, 827-832.
(19) Kang, S. K.; Kim, W. S.; Moon, B. H. Synthesis 1985, 1161-1162.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4167

DeRosa et al.
The solution was refluxed for 4 h, during which time a yellow
precipitate formed. After the solution was cooled, the precipitate
was collected by filtration and washed with cold absolute ethanol
followed by ether. Yield: 5.14 g (81.0%). ESI-MS: m/z 401 (M2+
- H+), 201 (M2+). IR (KBr): 1667 cm-1 (CdN), 3450 cm-1
(-OH).
g, 15.4 mmol) was added, and a banana-colored precipitate formed
immediately, presumed to be the cyano adduct. However, upon
bringing the solution to reflux, the mixture again became homo-
geneous. The solution was refluxed for 2 h, during which time the
orange color of the starting material had given way to the yellow
color of Ni(CN)42-. Once completed, the solution was cooled to
room temperature and 6 M aqueous NaOH s was added dropwise
until the pH was greater than 12. The solution was extracted with
CHCl3 (6 × 50 mL), and the organic layers were collected and
dried with MgSO4. The solution was filtered and the solvent
removed under reduced pressure to give a white precipitate. Yield:
0.198 g (80.7%). ESI-MS: m/z 319 (M + H+). 1H NMR (CDCl3):
δ 0.96 (d, 6H), 1.74 (q, 2H), 1.92 (t, 1H), 2.23 (s, 4H), 2.42 (td,
2H), 2.66 (m, 6H), 2.84 (m, 8H), 7.19 (m, 5H). Anal. Calcd for
C19H34N4‚0.5H2O: C, 69.67; H, 10.77; N, 17.11. Found: C, 69.24;
H, 10.12; N, 16.72.
[(5,7-Dimethyl-6-benzyl-1,4,8,11-tetraazacyclotetradecane)-
nickel(II)] Perchlorate ([Ni(mbc)](ClO4)2, 3a) was prepared
following an original procedure. A 0.500 g portion of 2a (9.71 ×
10-4 mol) was partially dissolved in ∼7 mL of methanol. A large
excess of NaBH4 (0.551 g, 14.6 mmol) was added, and the yellow
solution became immediately red, followed by the formation of a
pink precipitate. The mixture was allowed to stir at room temper-
ature for 2 h. Upon completion, the pink precipitate was collected
by filtration and redissolved in hot H2O. Saturated aqueous NaClO4
was added dropwise, affording a light orange powder. The solution
was cooled, and the powder was collected by filtration and washed
with ice-cold water and then with ether. Yield: 0.559 g (∼100%).
ESI-MS: m/z 375 (M2+ - H+), 475 (M2+ + ClO4-). IR (KBr):
3210 cm-1 (N-H). Anal. Calcd for NiC19H34N4Cl2O8‚H2O: C,
38.41; H, 6.10; N, 9.43. Found: C, 38.86; H, 5.71; N, 9.41. An
X-ray crystal structure was obtained. Note: Perchlorate salts may
be explosiVe and should be handled with care!
[(5,7-Dimethyl-6-anthracyl-1,4,8,11-tetraazacyclotetradecane)-
nickel(II)] Perchlorate ([Ni(mac)](ClO4)2, 3b) was synthesized
in a manner similar to that of 3a. A 0.500 g portion of 2b (7.55 ×
10-4 mol) was partially dissolved in ∼55 mL of methanol. To this
solution was added 15 equiv of NaBH4 (0.429 g, 11.3 mmol), during
which time the yellow solution became peach-colored, followed
by the formation of a pink precipitate. After being stirred at room
temperature for 2 h, the mixture was filtered and the resulting solid
redissolved in hot H2O. Saturated aqueous NaClO4 was added
dropwise, the solution was cooled, and the resulting orange powder
was collected by filtration and washed with ice-cold water and then
ether. Yield: 0.510 g (∼100%). ESI-MS: m/z 238 (M2+), 575 (M2+
+ ClO4-). IR (KBr): (loss of CdN peak at 1663 cm-1), 3215 cm-1
(N-H). Anal. Calcd for NiC27H38N4Cl2O8‚2H2O: C, 45.53; H, 5.94;
N, 7.86. Found: C, 45.55; H, 5.35; N, 7.68. An X-ray crystal
structure was obtained. Note: Perchlorate salts may be explosiVe
and should be handled with care!
5,7-Dimethyl-6-anthracyl-1,4,8,11-tetraazacyclotetradecane
(mac, 4b) was prepared from 3b following an original procedure.
[Ni(mac)](ClO4)2 (0.500 g, 7.39 × 10-4 mol) was dissolved in a
solution of H2O (7 mL) and DMSO (30 mL), and 30 equiv of NaCN
(1.087 g, 22.2 mmol) was added slowly. A banana-colored
precipitate formed immediately; however, the mixture became
homogeneous when heated to reflux. The solution was refluxed
for 2 h, during which time it became darker orange. The solution
was then cooled to room temperature and placed in a refrigerator
overnight. Large white crystals were collected by filtration from
the pale yellow solution and dried in vacuo for 2 days. Yield: 0.279
1
g (90.2%). ESI-MS: m/z 419 (M + H+). H NMR (CDCl3): δ
0.736 (d, 6H), 1.765 (quint, 2H), 2.366 (m, 9H), 2.366-2.768 (m,
8H), 2.899 (m, 4H), 7.464 (quint, 4H), 7.981 (d, 2H), 8.321 (s,
1H), 8.520 (d, 2H). 13C NMR: δ 19.72 (2C, -CH3), 29.52 (1C,
â-C), 38.74 (1C, â-C), 47.33 (2C, R-C), 50.35 (2C, R-C), 51.30
(4C, R-C), 61.91 (1C, CH2-arom), 124.90 (4C, arom), 125.35 (4C,
arom), 126.09 (1C, arom), 126.20 (1C, arom), 129.49 (4C, arom).
Anal. Calcd for C27H38N4‚1.5H2O: C, 72.77; H, 9.27; N, 12.58.
Found: C, 73.57; H, 9.05; N, 12.69.
5,7-Dimethyl-6-(p-hydroxymethylbenzyl)-1,4,8,11-tetraazacy-
clotetradecane, (hbc, 4c) was prepared from 3c using a procedure
modified from an earlier paper.22 [Ni(hbc)](ClO4)2 (1.86 g, 3.06
mmol) was dissolved in H2O (80 mL), and the solution was warmed
to ensure homogeneity. After 30 equiv of NaCN (4.50 g, 91.8
mmol) was added, the solution turned purple and then light yellow,
and a precipitate formed and then redissolved upon bringing the
solution to reflux. During the 2 h reflux, the orange color of the
starting material gave way to the yellow of Ni(CN)42-. The solution
was then cooled to room temperature, and a 6 M NaOH solution
was added dropwise until the pH was greater than 12. The solution
was extracted with CHCl3 (6 × 50 mL), the organic layers were
collected, dried with MgSO4, and filtered, and the solvent was
removed under reduced pressure, yielding a viscous oil. The oil
was placed under vacuum to yield a white crystalline precipitate,
which was Very hygroscopic. Yield: 0.972 g (91.1%). ESI-MS:
[(5,7-Dimethyl-6-(p-hydroxymethylbenzyl)-1,4,8,11-tetraaza-
cyclotetradecane)nickel(II)] Perchlorate (Ni(hbc)](ClO4)2, 3c)
was synthesized in the same manner as 3a. A portion of 2c (1.00
g, 1.83 mmol) was partially dissolved in ∼10 mL of CH3OH, and
then 15 equiv of NaBH4 (1.039 g, 27.5 mmol) was added. The
yellow solution became immediately red and then formed a pink
precipitate. After the solution was stirred at room temperature for
2 h, the pink precipitate was collected by filtration and redissolved
in hot H2O. Saturated aqueous NaClO4 in water was added
dropwise, affording a light orange powder. The solution was cooled,
and the hygroscopic orange powder was collected by filtration and
washed with ice-cold water, followed by ether. Yield: 1.110 g
(∼100%). ESI-MS: m/z 203 (M2+), 505 (M2+ + ClO4-). IR
(KBr): 3210 cm-1 (N-H), 3450 cm-1 (-OH). Note: Perchlorate
salts may be explosiVe and should be handled with care!
1
m/z 349 (M + H+). H NMR (CDCl3): δ 0.915 (d, 6H), 1.674 (t,
2H), 1.825 (t, 2H), 2.336-2.876 (m, 15H), 4.597 (s, 2H), 7.20 (d,
4H). 13C NMR: δ 20.28 (2C, -CH3), 29.22 (1C, â-C), 29.39 (1C,
â-C), 47.47 (2C, R-C), 50.05 (2C, R-C), 51.03 (2C, R-C), 52.17
(2C, R-C), 61.11 (1C, CH2-arom), 64.54 (1C, CH2OH), 127.04 (2C,
arom), 129.03 (2C, arom). Anal. Calcd for C20H36N4‚CHCl3: C,
53.90; H, 7.97; N, 11.97. Found: C, 53.97; H, 7.95; N, 11.35.
4. Synthesis of Chromium(III) Complexes. trans-[Dichloro-
(5,7-dimethyl-6-benzyl-1,4,8,11-tetraazacyclotetradecane)chro-
mium(III)] Chloride (trans-[Cr(mbc)Cl2]Cl, 5a) was prepared
from 4a by a procedure adapted from that reported by Tobe and
3. Synthesis of Free Ligands. 5,7-Dimethyl-6-benzyl-1,4,8,11-
tetraazacyclotetradecane (mbc, 4a) was prepared from 3a using
a modified procedure based on a previously reported synthesis.22
[Ni(mbc)](ClO4)2 (0.444 g, 7.7 × 10-4 mol) was dissolved in H2O
(125 mL) by warming the solution. A large excess of NaCN (0.755
(22) Barefield, E. K.; Wagner, F.; Hodges, K. D. Inorg. Chem. 1976, 15,
1370-1377.
4168 Inorganic Chemistry, Vol. 44, No. 12, 2005

Synthesis and Luminescence Properties of trans-[Cr(L)Cl2]Cl
Ferguson for the synthesis of the cyclam analogues.17 A solution
prepared from the macrocyclic ligand mbc (0.200 g, 6.28 × 10-4
mol) and CrCl3‚3THF (0.235 g, 6.27 × 10-4 mol) in ∼20 mL of
dry DMF was refluxed for ∼20 min, during which time the color
changed from a deep purple to a gray-brown, and then to green
upon cooling. The solvent was removed under reduced pressure,
and the residue was redissolved in a minimum amount of hot H2O
and placed in a refrigerator overnight to give light green needles.
Yield: 0.258 g (86.2%). ESI-MS: m/z 440 (M+). UV-vis (H2O)
{λmax, nm (ꢀ, M-1 cm-1)}: 242 (4.17 × 103), 372 (30.1), 420 (26.2)
sh, 572 (16.7). An X-ray crystal structure was obtained.
solution was cooled, and the solvent was removed under reduced
pressure. The residue was redissolved in a minimum amount of
methanol and filtered, and the volatiles were removed to give a
purple powder. Yield: 0.080 g (96.7%). ESI-MS: m/z 540 (M+).
UV-vis (H2O) {λmax, nm}: 336, 354, 370, 392, 540.
trans-[Dichloro(5,7-dimethyl-6-(p-bromomethylbenzyl)-1,4,8,11-
tetraazacyclotetradecane)chromium(III)] Bromide (trans-[Cr-
(bbc)Cl2]Br, 7) was prepared from 5c following an original
procedure. A 0.100 g portion of trans-[Cr(hbc)Cl2]Cl (1.97 × 10-4
mol) was partially dissolved in ∼5 mL of concentrated HBr (48%)
and the resulting solution stirred for 3 h at room temperature. The
resulting gray-purple precipitate was collected by filtration and
rinsed with acetone followed by ether. Yield: 0.121 g (∼100%).
ESI-MS: m/z 534 (M+). UV-vis (MeOH) {λmax, nm (ꢀ, M-1
cm-1)}: 240 (6.54 × 103), 382 (54.4), 424 sh (35.9), 562 (21.0).
trans-[Dichloro(5,7-dimethyl-6-(p-aminomethylbenzyl)-1,4,8,11-
tetraazacyclotetradecane)chromium(III)] Bromide Hydrobro-
mide (trans-[Cr(abc)Cl2]Br‚HBr, 8) was prepared from 7 fol-
lowing an original procedure. A 0.176 g portion of trans-
[Cr(bbc)Cl2]Br (2.87 × 10-4 mol) was dissolved in dry acetonitrile
(15 mL), and the resulting pink solution was treated with ∼15 drops
of concentrated NH4OH and stirred for 12 h at room temperature.
The solution was then filtered, and the solvent was removed under
reduced pressure. The pink precipitate was recrystallized via ether
diffusion into a concentrated methanol solution. Large purple
crystals were obtained. Yield: 0.145 g (80.1%). ESI-MS: m/z 469
(M+). UV-vis (H2O) {λmax, nm (ꢀ, M-1 cm-1)}: 242 (3.50 × 103),
372 (37.3), 420 sh (31.0), 570 (17.8). Anal. Calcd for C20H38N5‚CH3-
OH: C, 37.96; H, 6.37; N, 10.54. Found: C, 37.59; H, 5.79; N,
9.90. An X-ray crystal structure was obtained.
5. Crystal Growth and Structure Determination. Crystal
structures for compounds 2b, 3a, 3b, 5a, 5b, and 8 have been
determined. Crystals of 2b were grown via slow evaporation of an
aqueous solution to yield large yellow hexagonal crystals. Crystals
of 3a and 3b were grown via slow vapor diffusion of ether into a
concentrated acetone solution to yield orange rectangular crystals.
Crystals of 5a, 5b, and 8 were grown in a manner similar to that
of 3a and 3b only using methanol in place of acetone to yield long
purple needles.
Suitable crystals were mounted on thin glass fibers with epoxy
resin. Room-temperature (293 K) as well as low-temperature (180
K) single-crystal studies were carried out on a Bruker Smart 1000
diffractometer equipped with a normal-focus 2.4 kW sealed-tube
X-ray source (Mo KR radiation) operating at 50 kV and 40 mA
with a two-dimensional CCD detector. The crystals were solved
by direct methods followed by difference Fourier methods.
Hydrogen atoms attached to carbon atoms were calculated at
ideal positions and refined as riding atoms of the parent carbon
atoms. Calculations were performed using SHELXTL running on
Silicon Graphics Indy 5000. Final full-matrix refinements were
against F2. Further details are given in the Supporting Information.
trans-[Dichloro(5,7-dimethyl-6-anthracyl-1,4,8,11-tetraazacy-
clotetradecane)chromium(III)] Chloride (trans-[Cr(mac)Cl2]Cl,
5b) was prepared from 4b by a procedure modified from that
reported by Hay et al.23 The macrocyclic ligand mac (0.150 g, 3.58
× 10-4 mol) and CrCl3‚3THF (0.133 g, 3.57 × 10-4 mol) were
dissolved in ∼20 mL of dry DMF, and the solution was refluxed
for ∼30 min, during which time the color changed from a deep
purple to a gray-brown. The solution was reduced in volume by
∼2/3 by solvent evaporation, and then the mixture was refrigerated
overnight, during which time purple granular crystals formed.
Yield: 0.198 g (95.6%). ESI-MS: m/z 540 (M+). UV-vis (H2O)
{λmax, nm (ꢀ, M-1 cm-1)}: 336 (3.37 × 103), 354 (6.84 × 103),
372 (10.8 × 103), 392 (10.2 × 103), 572 (18.9). An X-ray crystal
structure was obtained.
trans-[Dichloro(5,7-dimethyl-6-(p-hydroxymethylbenzyl)-
1,4,8,11-tetraazacyclotetradecane)chromium(III)] Chloride (trans-
[Cr(hbc)Cl2]Cl, 5c) was synthesized in the same manner as 5b. A
solution of the macrocyclic ligand hbc (1.23 g, 3.50 mmol) and
CrCl3‚3THF (1.28 g, 3.41 mmol) in ∼30 mL of dry DMF was
refluxed for ∼20 min, during which time the color changed from
a deep purple to a gray-purple and a light purple precipitate began
to form. The solution was reduced in volume by ∼2/3 by solvent
evaporation, and then was refrigerated overnight. A light purple
powder formed and was collected by filtration, rinsed with acetone
and then ether, and dried under vacuum. Yield: 1.30 g (75.2%).
ESI-MS: m/z 470 (M+). UV-vis (H2O) {λmax, nm (ꢀ, M-1 cm-1)}:
242 (4.28 × 103), 374 (46.9), 420 sh (42.6), 568 (25.4). Anal.
Calcd for C20H36N4‚CH3OH: C, 46.79; H, 7.48; N, 10.39. Found:
C, 46.56; H, 7.37; N, 10.33.
cis-[Dichloro(5,7-dimethyl-6-benzyl-1,4,8,11-tetraazacyclotet-
radecane)chromium(III)] Chloride (cis-[Cr(mbc)Cl2]Cl, 6a) was
prepared following an original procedure. A solution of the
macrocyclic ligand mbc (0.054 g, 1.69 × 10-4 mol) and CrCl3‚
3THF (0.063 g, 1.69 × 10-4 mol) in ∼20 mL of dry DMF was
heated for 5 min at 100 °C, during which time the color changed
from magenta to deep purple. Upon disappearance of the starting
material (monitored via UV-vis spectroscopy), the solution was
cooled, and the solvent was removed under reduced pressure. The
residue was redissolved in a minimum amount of methanol and
filtered, and the volatiles were removed to give a purple powder.
Yield: 0.077 g (95.4%). ESI-MS: m/z 440 (M+). UV-vis (H2O)
{λmax, nm}: 242, 404, 542.
Results and Discussion
cis-[Dichloro(5,7-dimethyl-6-anthracyl-1,4,8,11-tetraazacy-
clotetradecane)chromium(III)] Chloride (cis-[Cr(mac)Cl2]Cl,
6b) was prepared by a procedure analogous to that used for the
preparation of 6a. A solution of the macrocyclic ligand mac (0.060
g, 1.43 × 10-4 mol) and CrCl3‚3THF (0.054 g, 1.43 × 10-4 mol)
in 20 mL of dry DMF was heated for 5 min at 100 °C, during
which the color changed from magenta to a deep purple. The
1. Synthesis and Characterization of Macrocyclic Ni-
(II) Complexes and Free Ligands. Scheme 2 outlines a
synthetic platform for preparing tetraamine macrocyclic
complexes with pendant chromophores attached to the carbon
backbone using Ni(II)-templated Schiff base condensa-
tions.20,21 Reaction of the R′CH2X with the dienatonickel-
(II) complex 1 is more facile if X is Br or I. For example,
this was unsuccessful when R′CH2X was anthracyl chloride,
but accomplished in good yields (2b, >70%) by the reaction
(23) House, D. A.; Hay, R. W.; Akbar Ali, M. Inorg. Chim. Acta 1983,
72, 239-245.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4169

DeRosa et al.
Scheme 2. Outline of the Synthetic Route to Pendant-Derivatized
Tetraamine Macrocycles 4a, 4b, and 4c
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Ni(II)
Complexes [Ni(anthracyl diene)]I2 (2b), [Ni(mbc)](ClO4)2 (3a), and
[Ni(mac)](ClO4)2 (3b)
2b
3a
3b
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-N(4)
N(1)-C(1)
N(2)-C(3)
N(3)-C(6)
N(4)-C(8)
1.886(2)
1.878(2)
1.919(2)
1.915(2)
1.275(4)
1.278(4)
1.482(4)
1.486(4)
1.960(5)
1.942(5)
1.937(5)
1.939(7)
1.487(10)
1.520(9)
1.484(9)
1.487(12)
1.952(3)
1.945(3)
1.935(3)
1.935(3)
1.504(4)
1.507(4)
1.485(4)
1.489(5)
N(1)-C(1)-C(2)
N(1)-C(1)-C(11)
N(2)-C(3)-C(2)
N(2)-C(3)-C(12)
N(1)-Ni(1)-N(3)
N(2)-Ni(1)-N(4)
C(2)-C(13)-C(14)
121.4(2)
123.8(3)
121.0(2)
123.9(3)
174.68(10)
174.98(11)
113.9(2)
110.9(8)
111.5(8)
110.4(6)
111.9(7)
178.9(4)
177.1(3)
118.9(7)
112.8(3)
111.6(3)
109.8(3)
113.2(3)
179.42(13)
177.12(12)
116.9(3)
N(2)-C(3) bond lengths are 1.487(10) and 1.520(9) Å and
1.504(4) and 1.507(4) Å, respectively, as expected for C-N
single bonds. In addition, the N(1)-C(1)-C(11) and N(2)-
C(3)-C(12) bond angles are 111.5(8)° and 111.9(7)° for 3a
and 111.6(3)° and 113.2(3)° for 3b, appropriate for sp3-
hybridized central atoms. For both complexes, the 5- and
7-methyl groups lie in macrocycle equatorial positions while
the pendant aromatic chromophore is axial with respect to
of 1 with anthracyl iodide. The latter was prepared from the
chloride in situ using the Finklestein reaction.18 X-ray
crystallographic (2b) and IR, MS, and elemental analysis
data confirmed the identity of the product. Analogous
procedures were used to form the p-hydroxymethylbenzyl
derivative [Ni(hbc diene)]Br,NO3 (2c) using p-hydroxy-
methylbenzyl bromide.
The reduction of the nickel(II) diimine complexes 2a and
2b presented several problems. Although analogous com-
pounds were reported to be reduced by excess NaBH4 in
aqueous pyridine/HCl buffer (0.1 M, pH 5.1),21 we found
that only one imine group was reduced. Quantitative reduc-
tion was achieved by partially dissolving the diimine
complex in a small amount of methanol and then adding a
∼15-20-fold excess of NaBH4. An immediate color change
of the solution from yellow to red was apparent, and a pink
precipitate began to form. After 2 h at room temperature
the precipitate was collected by filtration. This method was
effective for the synthesis of 3a, 3b, and 3c.
Crystal structures were determined for Ni(II) complexes
2b, 3a, and 3b. The coordination geometry in each case was
square planar. Selected bond lengths and angles are listed
in Table 1. Figure 1 shows the ORTEP diagram for the
diimine ligand complex [Ni(anthracyl diene)]2+ (2b). The
N(1)-C(1) and N(2)-C(3) bond lengths are 1.275(4) and
1.278(4) Å, respectively, typical for CdN double bonds,24
and the N(1)-C(1)-C(11) and N(2)-C(3)-C(12) angles are
123.8(3)° and 123.9(3)°, consistent with sp2 hybridization
at C(1) and C(3). The distance from the center of the
anthracene to the Ni(II) metal center is 3.55 Å, suggesting
weak attraction between the two centers as was reported for
the naphthyl analogue.24
Figure 1. Molecular structure and numbering of atoms for the cation of
[Ni(anthracyl diene)]I2 (2b). Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are drawn at the 30% level.
Figures 2 and 3 show ORTEP diagrams of the reduced
complexes [Ni(mbc)]2+ and [Ni(mac)]2+. The N(1)-C(1) and
Figure 2. Molecular structure and numbering of atoms for the cation of
[Ni(mbc)](ClO4)2 (3a). Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn at the 30% level.
(24) Fabbrizzi, L.; Lichelli, M.; Rospo, C.; Sacchi, D.; Zema, M. Inorg.
Chim. Acta 2000, 3002-302, 453-461.
4170 Inorganic Chemistry, Vol. 44, No. 12, 2005

Synthesis and Luminescence Properties of trans-[Cr(L)Cl2]Cl
Figure 4. Molecular structure and numbering of atoms for the cation of
trans-[Cr(mbc)Cl2]Cl (5a). Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn at the 30% level.
Figure 3. Molecular structure and numbering of atoms for the cation of
[Ni(mac)](ClO4)2 (3b). Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn at the 30% level.
tions between the methyl substituents in the 5 and 12
positions with coordinated chlorides in the cis formation,23
leading to accelerated isomerization of the initially formed
cis complex to the trans analogue. For the present systems
substituents in the 5, 6, and 7 positions of the macrocycles
may generate similar steric crowding.
the six-membered ring. This can be explained by attack of
BH4 at the unsaturated carbon from the less crowded
“proximal” side of the diimine functionality complex owing
to steric hindrance from the substituent.
-
Free Ligands. Cyanide removal of the Ni2+ from such
complexes gave the free macrocyclic ligands.22 For 3a and
3c, reaction in refluxing aqueous solution with a large excess
of CN- (20-30-fold) followed by chloroform extraction
recovered mbc and hbc (4a and 4c). With [Ni(mac)](ClO4)2
(3b), the same procedure gave an insoluble cyano adduct
that did not react further; however, in DMSO/H2O (4.5:1
v/v), the reaction proceeded smoothly. The anthracene-
derivatized mac ligand (4b) crystallized upon cooling of the
solution and was isolated by filtration. 1H and 13C NMR and
MS spectral data support the identity of all three ligands.
2. Synthesis and Characterization of Chromium(III)
Complexes. Reaction of the cyclam derivatives 4a, 4b, and
4c with CrCl3‚3THF in refluxing DMF gave cleanly the trans
products trans-[Cr(mbc)Cl2]Cl (5a), trans-[Cr(mac)Cl2]Cl
(5b), and trans-[Cr(hbc)Cl2]Cl (5c) (eq 1).
Further modification of the Cr(III)-coordinated tetraazo
macrocycles was accomplished by utilizing the p-hydroxy-
methylbenzyl derivative 5c as the starting point. The alcohol
functional group was easily converted to the bromo derivative
7 by the reaction of 5c with concentrated HBr (48%) (3 h at
ambient T). The heterogeneous mixture was filtered to give
trans-[Cr(bbc)Cl2]Br in near quantitative yields. Reaction of
7 with aqueous ammonia gave the amino derivative trans-
[Cr(abc)Cl2]Br‚HBr (8) for which the X-ray crystal structure
was determined (see below). The -CH2OH and -CH2NH2
functional groups on the pendant benzyl group were intro-
duced as an entry for the attachment of other chromophores.
For example, in preliminary studies,1 the dye molecule
gallocyanine (7-dimethylamino-4-hydroxy-3-oxo-3H-phe-
noxazine-1-carboxylic acid) was attached at the amine group
of trans-[Cr(abc)Cl2]+ by use of the coupling agent 1-ethyl-
3-propyldimethylaminocarbodiimide (EDC) to form an amide
bond. The resulting compound was characterized by mass
spectroscopy, displayed a strong absorption band at 610 nm
(ꢀ > 104 M-1 cm-1), and showed some promise as the
forerunner of new dye conjugates; however, purification
problems stymied its use in the photophysical studies
described below. Another application along this theme is
based on the hydroxymethyl derivative 5c. As described
elsewhere,1,32 this complex can be first converted to the trans-
dinitrito analogue trans-[Cr(hbc)(ONO)2]BF4 by reaction
with silver nitrite and then linked at the CH2OH site to
1-pyrenecarboxylic acid via an ester linkage.
Notably, reaction of unsubstituted cyclam with CrCl3‚
3THF gives cis-[Cr(cyclam)Cl2]Cl as the major product
(>90%) with only a small amount of the trans isomer under
comparable conditions.17 It is necessary to reflux solutions
of cyclam and CrCl3‚3THF in dry DMF for 6 h to effect
complete formation of the trans isomer. Samples of cis-[Cr-
(mbc)Cl2]Cl (6a) and cis-[Cr(mac)Cl2]Cl (6b) could be
isolated, but only by reducing the reaction time (5-7 min)
and temperature (∼100°C) while monitoring the solution
optical spectrum. These were obtained as purple powders
and purified by fractional recrystallization in MeOH. The
assignment of the cis configuration was based on the
similarity of the visible spectra to that of [cis-Cr(cyclam)-
Cl2]Cl. However, the low extinction coefficients suggested
that the isolated materials might not be pure.
Figures 4-6 are ORTEP diagrams for the Cr(III) cations
trans-[Cr(mbc)Cl2]+, trans-[Cr(mac)Cl2]+, and trans-[Cr-
(abc)Cl2]Br2, respectively. Selected bond lengths and angles
are listed in Table 2. The Cl-Cr-Cl bond angles of ∼180°
and Cr-Cl bond distances which fall in the range 2.319-
(2)-2.340(2) Å are similar to those reported for trans-[Cr-
(cyclam)Cl2]Cl.25 The carbon-substituted macrocycles are
coordinated in the trans-III conformation.14 The methyl
groups (C(11) and C(12)) in the ring 5 and 7 positions occupy
Ready formation of the trans isomer has also been seen
for the reaction between meso-5,7,7,12,14,14-hexamethyl-
1,4,8,11-tetraazacyclotetradecane (tet-A) and CrCl3‚3THF.23
This was interpreted in terms of unfavorable steric interac-
(25) Flores-Velez, L. M.; Sosa-Rivadeneyra, J.; Sosa-Torres, M. E.; Rosales-
Hoz, M. J.; Toscano, R. A. J. Chem. Soc., Dalton Trans. 1991, 3243-
3247.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4171

DeRosa et al.
Figure 5. Molecular structure and numbering of atoms for the cation of
trans-[Cr(mac)Cl2]Cl (5b). Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn at the 30% level.
Figure 6. Molecular structure and numbering of atoms for the cation of
trans- [Cr(abc)Cl2]Br2 (8). Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn at the 30% level.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for the Cr(III)
Complexes trans-[Cr(mbc)Cl2]Cl (5a), trans-[Cr(mac)Cl2]Cl (5b), and
trans-[Cr(abc)Cl2]Br2 (8)
5a
5b
8
Figure 7. (Top) Optical spectra for 5a, 5b, 5c, 8, and 10 in aqueous
solution (5 mM). The small signal at ∼660 nm is an instrumental artifact
(see Supporting Information Figure S1). (Bottom) Spectrum of 5b recorded
in dilute solution (50 µM).
Cr(1)-N(1)
Cr(1)-N(2)
Cr(1)-N(3)
Cr(1)-N(4)
Cr(1)-Cl(1)
Cr(1)-Cl(2)
2.076(4)
2.074(5)
2.063(4)
2.068(5)
2.3189(17)
2.3400(17)
2.0917(16)
2.0881(17)
2.0553(17)
2.0606(17)
2.3380(7)
2.3200(7)
2.073(4)
2.074(4)
2.065(4)
2.068(4)
2.3071(13)
2.3448(13)
trans position. The spectrum of the anthracene derivative
+
trans-Cr(mac)Cl2 differs primarily in that the antenna
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
N(2)-Cr(1)-N(3)
N(2)-Cr(1)-N(4)
N(3)-Cr(1)-N(4)
Cl(1)-Cr(1)-Cl(2)
95.00(18)
179.19(18)
85.82(18)
85.72(18)
178.90(17)
93.46(18)
178.97(6)
94.93(6)
179.40(6)
85.28(6)
85.67(6)
179.80(6)
94.13(7)
179.23(2)
94.53(14)
179.84(15)
85.99(15)
85.61(15)
179.07(14)
93.87(15)
177.97(5)
dominates the near-UV spectrum owing to the strong π-π*
band that obscures the quartet band in this region (Figure
7b). These spectra and those of several cis analogues are
summarized in Table 3. Notably, the spin-forbidden, doublet
absorptions predicted for chromium(III) complexes and found
at ∼700 nm for related cis-dichlorochromium(III) complexes
of cyclam derivatives with extinction coefficients of 1-9
M-1 cm-1 11 were not obvious in the spectra of the trans-
dichloro analogues described here. Spectra were recorded
for solutions near the solubility limits of the compound
(either 5 or 10 mM), and no sharp absorption bands typical
of Cr(III) Q0 to D0 transitions were seen that could be
discerned from the baseline noise (∆Abs ( 0.001). However,
a broad shoulder in the region 650-700 nm (ꢀ ≈ 1 M-1
cm-1) was apparent in this region for each of the compounds
described (see Supporting Information Figure S1).
the equatorial positions, while the aryl methyl substituents
occupy the axial position. This “equatorial, axial, equatorial”
configuration has been seen previously in analogous com-
plexes.24
3. Electronic Absorption and Emission Spectra. The
electronic spectra of the modified cyclam complexes trans-
Cr(L)Cl2+ each display a visible range, low-intensity quartet-
to-quartet absorption band with a λmax at ∼570 nm (Figure
7 and Supporting Information Figure S1) quite similar to
that of trans-[Cr(cyclam)Cl2]+. With the exception of the
anthracene derivative 5b, each of these spectra also showed
the expected second quartet absorption band (∼370-420 nm)
split as predicted for a d3 ion in a tetragonally distorted ligand
field.17,26-30 These visible range spectra demonstrate that the
substituents have only modest effects on the ligand field
strengths of these tetraazo macrocycles when bound in the
(26) Poon, C. K.; Pun, K. C. Inorg. Chem. 1980, 19, 568-569.
(27) Meyerstein, D.; Wasgestian, F.; Guldi, D. Inorg. Chim. Acta 1992,
194, 15-22.
(28) Bakac, A.; Espenson, J. H. Inorg. Chem. 1992, 31, 1108-1110.
(29) Kane-Maguire, N. A. P.; Wallace, K. C.; Miller, D. B. Inorg. Chem.
1985, 24, 597-605.
(30) Chen, Y.; Perkovic, M. W. Inorg. Chim. Acta 2001, 317, 127-132.
4172 Inorganic Chemistry, Vol. 44, No. 12, 2005

Synthesis and Luminescence Properties of trans-[Cr(L)Cl2]Cl
Table 3. Optical Spectra of cis- and trans-Dichlorochromium(III) Complexes in Aqueous Media
π-π* λ (nm)
Q2 λ (nm)
Q1 λ (nm)
complex
(ꢀ x 10-3, M-1 cm-1
)
(ꢀ, M-1 cm-1
)
(ꢀ, M-1 cm-1
)
ref
cis-[Cr(cyclam)Cl2]Cl
408 (123)
404 (106)
406
536 (122)
529 (111)
542
540
567 (20)
30
17
b
b
17
cis-[Cr(mbc)Cl2]Cl (6a)
cis-[Cr(mac)Cl2]Cl (6b)
trans-[Cr(cyclam)Cl2]Cl
242
336, 354, 372, 392
a
370 (34)
∼404 (30) sh
trans-[Cr(mbc)Cl2]Cl (5a)
242 (4.2)
370 (30)
567 (19)
b
∼420 (26) sh
a
372 (47)
trans-[Cr(mac)Cl2]Cl (5b)
trans-[Cr(hbc)Cl2]Cl (5c)
336 (3.4), 354 (6.8), 372 (10.8), 392 (10.2)
242 (4.3)
572 (19)
567 (25)
b
b
∼420 (43) sh
trans-[Cr(bbc)Cl2]Br (7)c
trans-[Cr(abc)Cl2]Br2 (8)
240 (6.5)
242 (3.5)
382 (54)
562 (21)
569 (18)
b
b
∼424 (36) sh
372 (35)
∼420 (30) sh
a Q2 band hidden by anthracene absorption. b This work. c Spectrum taken in methanol.
doublet ligand field (LF) excited state(s) (Do f Qo).31 The
behavior of 5b was different; excitation at 370 nm led to
weak emission at 416, 442, and 466 nm characteristic of the
π-π* fluorescence from an anthracene chromophore in
addition to the metal-centered doublet phosphorescence
(Figure 8). For comparison, emission from the mac free
ligand is 500-fold stronger under comparable conditions. This
did not diminish in intensity after repeated recrystallization
of 5b, so we attribute it to weak fluorescence from the
aromatic appendage of the coordinated mac ligand. Despite
this weak π-π* emission, it is clear that the singlet state of
the anthracene antenna is efficiently quenched (>99%) by
the Cr(III) center from which the doublet emission is
observed.
The excitation spectra of 5a and 5b (λmon ) 704 nm)
closely track the absorption spectra in a manner consistent
with efficient intramolecular intersystem crossing/internal
conversion from the π-π* states of the ligand chromophore
and from the Cr(III) quartet LF states to the emissive doublet
LF state(s). Notably, the bands in the excitation spectra
recorded in the 77 K glasses and attributed to the Q0 f Q1
absorptions are blue shifted (to 555 and 558 nm, respectively)
from those of the ambient temperature solution absorption
spectra. A similar pattern was seen for the excitation
spectrum of 10, where the Q1 λmax appeared at 552 nm and
has been described previously for several cis-dichloro(N,N′-
disubstituted cyclam)chromium(III) complexes.11
The phosphorescence lifetimes of 5a and 5b in (4:1 v/v)
EtOH/MeOH glassy solutions at 77 K were determined using
355 nm excitation and monitoring at 704 nm (Table 4). The
decay curves were exponential, and the lifetimes determined
under these conditions were 88.6 and 27.7 µs, respectively,
using 5 mM solutions. For the anthracenyl derivative 5b,
lowering the concentration to 50 µM had no affect on the
doublet lifetime; however, the weak anthracene-centered
fluorescence was too short-lived (<15 ns) to be measured
on our nanosecond laser system. As a reference point, the
Figure 8. (Top) Emission spectrum for a dilute solution of trans-[Cr-
(mac)Cl2]Cl (5b) (50 µM) in EtOH:MeOH (4:1 v/v) at 77 K (λex ) 370
nm) showing both the fluorescence from the aromatic chromophore and
phosphorescence from the Cr(III) doublet states. (Bottom) Comparison of
emission spectra from free mac (50 µM) in aqueous solution vs that from
trans-[Cr(mac)Cl2]Cl (50 µM) in 4:1 EtOH/MeOH.
The photoluminescence spectra and lifetimes of 5a and
5b were measured at 77 K in 4:1 EtOH/MeOH glasses (5
mM in each case). With 370 nm excitation, each of these
compounds displayed weak emission with sharp bands
around 700 nm (Figure 8 and Supporting Information Figure
S2). The emission spectra and qualitative intensities for both
complexes were very close to those we have recorded for
the luminescence from trans-[Cr(cyclam)Cl2]Cl (10) under
identical conditions (Supporting Information Figure S3). In
analogy to other Cr(III) compounds, these sharp spectra are
attributed to phosphorescence from the lowest energy,
(31) (a) Forster, L. S.; Mønsted, O. J. Phys. Chem. 1986, 90, 5131-5134.
(b) Porter, G. B. Chapter 2; Zinato, E. Chapter 4. In Concepts of
Inorganic Photochemistry; Adamson, A. W., Fleischauer, P. D., Eds.;
John Wiley and Sons: New York, 1975.
(32) DeRosa, F.; Bu, X.; Ford, P. C. Submitted for publication.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4173

DeRosa et al.
Table 4. Emission Lifetime Data for 10, 5a, and 5ba
In summary, in this paper we describe the preparation of
several new macrocyclic ligands L with pendant aromatic
chromophores R of the type 5,7-dimethyl-6-R-cyclam via a
Ni2+ template based synthetic scheme. The X-ray structures
of several of these Ni(II) intermediates have been determined.
The ligands L were then used to prepare chromium(III)
complexes with the geometry trans-[Cr(L)Cl2]Cl as deter-
mined by X-ray crystallography. Notably, the modified
cyclams give the trans Cr(III) coordination geometries much
more rapidly than does cyclam itself, and it is likely that
this is due to steric interactions from the substituents in the
5, 6, and 7 positions of the macrocycle. The photophysical
properties upon π-π* excitation of the anthracene antenna
tethered to the macrocyclic ligand of trans-[Cr(mac)Cl2]Cl
are characterized by nearly complete quenching of the
anthracene-centered fluorescence coupled to phosphorescence
from Cr(III)-centered ligand field doublet states. Notably,
the trans-[Cr(L)Cl2]Cl complexes described here are synthons
for various dinitrito complexes trans-[Cr(L)(ONO)2]BF4 that
are being studied as possible photochemical precursors for
nitric oxide delivery to biological targets.8,32 The syntheses
and photochemical reactions of several examples of the latter
compounds are reported elsewhere.1,32
complex
τ1 (µs)
ref
trans-[Cr(cyclam)Cl2]Cl (10)
86.9
81.1
88.6
27.7
27.5c
31
b
b
trans-[Cr(mbc)Cl2]Cl (5a)
trans-[Cr(mac)Cl2]Cl (5b)
b
a Lifetime data were obtained for solutions in EtOH/MeOH (4:1 v/v)
glass matrixes (77 K). λex ) 355 nm. λmon ) 704 nm. b This work. c Solution
diluted to 50 µM.
lifetime of 10 was measured as 81.1 µs under these
conditions, in reasonable agreement with that (86.9 µs)
reported by Forster and Mønsted31a under similar circum-
stances.
Near the conclusion of the present study,1 Funston et al.
reported33 the template synthesis of several cobalt(III)
complexes of cyclam-type ligands with a pendant anthracenyl
group, for example, trans-[Co(6-anthracenylcyclam)Cl2]Cl.
These workers also reported that the anthracene chromophore
fluorescence that is very strong for the free ligand is largely,
but not entirely, quenched by coordination to the metal
center, and attributed this quenching either to Forster
resonance energy transfer to metal-centered states and/or
electron-transfer mechanisms involving the cobalt(III) center.
As a consequence of irradiating at the wavelengths leading
to excitation of the anthracene chromophore, spectral changes
attributed to reactions at the cobalt(III) center were observed
that were uncharacterized but were consistent with substitu-
tions of the cobalt chloride. Under comparable conditions,
photolysis of analogous solutions of trans-[Co(cyclam)Cl2]+
gave no measurable photochemistry; thus, in that case as
well, the pendant anthracenyl group apparently serves as an
antenna to internally sensitize the excited-state reactions of
a metal complex.
Acknowledgment. These studies were supported by the
National Science Foundation (Grants CHE-0095144 and
CHE-0352650). The ESI/TOF mass spectral instrumentation
was purchased under Army Research Office Award No.
DAAD19-00-1-0026. We thank Ryan Absalonson for his
technical help.
Supporting Information Available: Three figures showing
emission data and listings of complete structure refinement details,
bond lengths and angles, anisotropic displacement parameters for
non-hydrogen atoms, and hydrogen coordinates and isotropic
displacement parameters (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(33) Funston, A. M.; Ghiggino, K. P.; Grannas, M. J.; McFadyen, W. D.;
Tregloan, P. A. Dalton Trans. 2003, 3704-3712
IC048312G
4174 Inorganic Chemistry, Vol. 44, No. 12, 2005