Synthesis of cis and trans Bis-alkynyl complexes of Cr(III) and Rh(III) supported by a tetradentate macrocyclic amine: A spectroscopic investigation of the M(III)-alkynyl interaction

Article abstract of DOI:10.1021/ic2009336

Alkynyl complexes of the type [M(cyclam)(CCR)₂]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane; M = Rh(III) or Cr(III); and R = phenyl, 4-methylphenyl, 4-trifluoromethylphenyl, 4-fluorophenyl, 1-naphthalenyl, 9-phenanthrenyl, and cyclohexyl) were prepared in 49% to 93% yield using a one-pot synthesis involving the addition of 2 equiv of RCCH and 4 equiv of BuLi to the appropriate [M(cyclam)(OTf)₂]OTf complex in THF. The cis and trans isomers of the alkynyl complexes were separated using solubility differences, and the stereochemistry was characterized using infrared spectroscopy of the CH₂ rocking and NH bending region. All of the trans-[M(cyclam)(CCR)₂]OTf complexes exhibit strong Raman bands between 2071 and 2109 cm^-1, ascribed to ν_s(CC). The stretching frequencies for the Cr(III) complexes are 21-28 cm^-1 lower than for the analogous Rh(III) complexes, a result that can be interpreted in terms of the alkynyl ligands acting as π-donors. UV-vis spectra of the Cr(III) and Rh(III) complexes are dominated by strong charge transfer (CT) transitions. In the case of the Rh(III) complexes, these CT transitions obscure the metal centered (MC) transitions, but in the case of the Cr(III) complexes the MC transitions are unobscured and appear between 320 and 500 nm, with extinction coefficients (170-700 L mol^-1 cm^-1) indicative of intensity stealing from the proximal CT bands. The Cr(III) complexes show long-lived (240-327 μs), structureless, MC emission centered between 731 and 748 nm in degassed room temperature aqueous solution. Emission characteristics are also consistent with the arylalkynyl ligands acting as π-donors. The Rh(III) complexes also display long-lived (4-21 μs), structureless, metal centered emission centered between 524 and 548 nm in degassed room temperature solution (CH₃CN).

Full text of DOI:10.1021/ic2009336

ARTICLE
pubs.acs.org/IC
Synthesis of cis and trans Bis-alkynyl Complexes of Cr(III) and Rh(III)
Supported by a Tetradentate Macrocyclic Amine: A Spectroscopic
Investigation of the M(III)ꢀAlkynyl Interaction
Chivin Sun,† Christopher R. Turlington,† W. Walsh Thomas,† James H. Wade,† Wade M. Stout,†
David L. Grisenti,† William P. Forrest,† Donald G. VanDerveer,‡ and Paul S. Wagenknecht*,†
†Department of Chemistry, Furman University, Greenville, South Carolina 29613, United States
‡Department of Chemistry, Clemson University, Clemson, South Carolina 29634, United States
S Supporting Information
b
ABSTRACT: Alkynylcomplexesofthetype[M(cyclam)(CCR)2]-
OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane; M =
Rh(III) or Cr(III); and R = phenyl, 4-methylphenyl, 4-trifluor-
omethylphenyl, 4-fluorophenyl, 1-naphthalenyl, 9-phenanthre-
nyl, and cyclohexyl) were prepared in 49% to 93% yield using a
one-pot synthesis involving the addition of 2 equiv of RCCH
and 4 equiv of BuLi to the appropriate [M(cyclam)(OTf)2]OTf
complex in THF. The cis and trans isomers of the alkynyl complexes were separated using solubility differences, and the
stereochemistry was characterized using infrared spectroscopy of the CH2 rocking and NH bending region. All of the trans-
[M(cyclam)(CCR)2]OTf complexes exhibit strong Raman bands between 2071 and 2109 cmꢀ1, ascribed to νs(CtC). The
stretching frequencies for the Cr(III) complexes are 21ꢀ28 cmꢀ1 lower than for the analogous Rh(III) complexes, a result that can
be interpreted in terms of the alkynyl ligands acting as π-donors. UVꢀvis spectra of the Cr(III) and Rh(III) complexes are
dominated by strong charge transfer (CT) transitions. In the case of the Rh(III) complexes, these CT transitions obscure the metal
centered (MC) transitions, but in the case of the Cr(III) complexes the MC transitions are unobscured and appear between 320 and
500 nm, with extinction coefficients (170ꢀ700 L molꢀ1 cmꢀ1) indicative of intensity stealing from the proximal CT bands. The
Cr(III) complexes show long-lived (240ꢀ327 μs), structureless, MC emission centered between 731 and 748 nm in degassed room
temperature aqueous solution. Emission characteristics are also consistent with the arylalkynyl ligands acting as π-donors. The
Rh(III) complexes also display long-lived (4ꢀ21 μs), structureless, metal centered emission centered between 524 and 548 nm in
degassed room temperature solution (CH3CN).
’ INTRODUCTION
Cr(cyclam)(CN)2+ (cyclam =1,4,8,11-tetraazacyclotetradecane)
have relatively long excited state lifetimes in a room temperature
fluid solution.15,16 Consequently, it might be possible to intercept
these excited states prior to deactivation, particularly if they can be
bridged to an appropriate acceptor through an alkynyl type ligand.
Given our interest in the photochemistry of such systems and
the possibility that the metallocyclam complexes might serve as
Assemblies comprising two or more metal centers bridged by
unsaturated organic ligands have been proposed as potential
building blocks for molecular electronic devices, including wirelike
structures. Alkynyl ligands such as CtC2ꢀ and CtCꢀCtC2ꢀ
are common building blocks in such assemblies,1ꢀ9 and these
ligands have, in many cases, been demonstrated to mediate
electronic communication between metal centers. The prospect
of inventing functional photochemical molecular devices (PMDs)10,11
has prompted research on intramolecular energy transfer be-
tween transition metal centers bridged by ligands that facilitate
such electronic communication.12 We have had a long-term
interest in the preparation and photophysical characteriza-
building blocks for metal-alkynyl polymers, we sought a general
n+
synthetic method for complexes of the type M(cyclam)(CCR)2
.
The presence of the macrocyclic cyclam ligand provides stereo-
chemical variability by virtue of the possibility of cis and trans
isomers. Accordingly, M(cyclam) complexes might be used as
building blocks for linear assemblies (as in the case of the trans
complexes) or as corners (as in the case of the cis complexes).
An additional motivation for developing synthetic routes to
+
tion of trans-Cr(N4)(CN)2 (where N4 = a tetraazamarocyclic
ligand) complexes13 and in the study of the rates of intermole-
cular electronic energy transfer between them.14 Our interest in
the alkynyl ligands stems from their promise as isoelectronic
substitutes for the CNꢀ ligand and the possibility of preparing
photoactive complexes with bridging alkynyl ligands that might
mediate intramolecular electron- and energy transfer. In addi-
n+
M(cyclam)(CCR)2 complexes concerns the question of
MꢀCCR π-bonding. By analogy with the CNꢀ ligand,17 the
alkynyl ligands were conventionally thought to be strong σ-donors
Received: May 3, 2011
Published: September 02, 2011
+
tion, the dicyano complexes trans-Rh(cyclam)(CN)2 and trans-
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
and modest π-acceptors (weaker than CO). However, this simple
analogy does not appear to apply in all cases.18 For example,
photoelectron spectroscopic measurements on FeCp(CCR)-
(CO)2 (R = H, Ph, t-Bu, and CCH) and FeCp*(CC-But)(CO)2
suggests that the alkynyl ligand acts as a weak π-donor,19 a result
that is supported by molecular orbital calculations.19,20 More
recently, a combination of electronic spectroscopy and density
functional theory calculations on [M(CCSiMe3)6]nꢀ, where M =
Cr(III), Fe(II), and Co(III), support the model that the alkynyl
ligand acts as a π-donor in these cases.21 Understanding MꢀCCR
π-bonding is important not only from a theoretical perspective but
also from a practical standpoint because π-delocalization is a
prerequisite for electrical conductivity.
acid (10 g) was sparged for 24 h with nitrogen. The product mixture was
poured into anhydrous ethyl ether (200 mL) that was being vigorously
stirred. The resulting pink precipitate was collected by suction filtration
on a coarse fritted glass funnel, washed with anhydrous ethyl ether, dried
under vacuum, and stored in a desiccator for further use (yield = 2.67 g,
91%). Anal. Calcd (found) for C13H24CrF9N4O9S3: C, 22.32 (21.85);
H, 3.46 (3.45); N, 8.01 (7.57).
[Cr(cyclam)(CCC6H5)2]OTf (1a). Using THF (5 mL), cis/trans-
[Cr(cyclam)(OTf)2]OTf (200 mg, 0.286 mmol), phenylacetylene
(62.8 μL, 0.572 mmol), and n-butyllithium (480 μL, 1.20 mmol) yielded
0.159 g (92.2%) of analytically pure cis/trans-[Cr(cyclam)(CCC6-
1
H5)2]OTf 1/2H2O. Anal. Calcd (found) for C27H34CrF3N4O3S
/
2
3
3
H2O: C, 52.93 (53.01); H, 5.76 (5.60); N, 9.15 (9.15). ESI-MS: intense
parent ion (M+) at m/z value of 454.21.
Herein we report a straightforward, high yield route to cis and
trans M(cyclam)(CCR)2n+ complexes along with characterization
by infrared, Raman, UVꢀvis, and emission spectroscopy. The
results are consistent with the alkynyl ligands acting as π-donors.
[Cr(cyclam)(p-CCC6H4CH3)2]OTf (1b). Using THF (5 mL), cis/
trans-[Cr(cyclam)(OTf)2]OTf (200 mg, 0.286 mmol), 4-ethynylto-
luene (72.5 μL, 0.572 mmol), and n-butyllithium (480 μL, 1.20 mmol)
yielded 0.148 g (82.0%) of analytically pure cis/trans-[Cr(cyclam)-
(CCC6H4CH3)2]OTf. Anal. Calcd (found) for C29H38CrF3N4O3S:
C, 55.14 (54.95); H, 6.06 (5.96); N, 8.87 (8.78). ESI-MS: intense
parent ion (M+) at m/z value of 482.36.
’ EXPERIMENTAL SECTION
[Cr(cyclam)(p-CCC6H4CF3)2]OTf (1c). Using THF (5 mL), cis/trans-
[Cr(cyclam)(OTf)2]OTf (200 mg, 0.286 mmol), 4-ethynyl-α,α,α-
trifluorotoluene (93.3 μL, 0.572 mmol)), and n-butyllithium (480 μL,
1.20 mmol) yielded 0.160 g (75.7%) of analytically pure trans-[Cr(cyclam)-
(CCC6H4CF3)2]OTf. Anal. Calcd (found) for C29H32CrF9N4O3S: C,
47.09 (47.06); H, 4.36 (4.29); N, 7.57 (7.44). ESI-MS: intense parent ion
(M+) at m/z value of 590.19.
[Cr(cyclam)(p-CCC6H4F)2]OTf (1d). Using THF (5 mL), cis/trans-
[Cr(cyclam)(OTf)2]OTf (200 mg, 0.286 mmol), 1-ethynyl-4-fluoro-
benzene (65.5 μL, 0.572 mmol), and n-butyllithium (480 μL, 1.20 mmol)
yielded 0.139 g (76.0%) of analytically pure cis/trans-[Cr(cyclam)-
(CCC6H4F)2]OTf. Anal. Calcd (found) for C27H32CrF5N4O3S: C,
50.70 (50.53); H, 5.04 (5.04); N, 8.76 (8.61). ESI-MS: intense parent
ion (M+) at m/z value of 490.29.
Materials and Methods. Tetrahydrofuran (THF) and diethyl
ether were dried and degassed using an Innovative Technology Inc.
solvent purification system before use. All other materials were reagent
grade and used as received. Cis/trans mixtures and isomerically pure
cis-[Cr(cyclam)Cl2]Cl were prepared by the method of Ferguson and
Tobe.22 Isomerically pure trans-[Cr(cyclam)Cl2]Cl was prepared by the
method of Bakac and Espenson.23 trans-[Rh(cyclam)Cl2]Cl and cis-
[Rh(cyclam)Cl2]Cl were prepared according to the method of Bounsall
and Koprich.24 Triflato complexes were prepared from these chloro
complexes by modifications of the literature procedures.25,26 All other
reactions were performed under a dry Ar atmosphere implementing
standard Schlenk procedures unless otherwise noted. UVꢀvis absorp-
tion spectra were recorded using a Cary-50 spectrophotometer. NMR
spectra were obtained using Varian INOVA 500 and 400-MR spectro-
meters. Raman spectra were recorded using a Thermo Scientific Nicolet
6700 FT-IR with an NXR FT-Raman module. Infrared spectra were
measured on solid samples using a Perkin-Elmer Spectrum 100 series
FT-IR spectrometer equipped with an ATR accessory. Emission spectra
were recorded using an SLM Aminco instrument running DataMax
software. Emission lifetimes were measured using as the excitation
source a Photon Technology International (PTI) GL-3300 pulsed
nitrogen laser fed into a PTI GL-302 dye laser. Data were collected
on an OLIS SM-45 EM fluorescence lifetime measurement system and
analyzed using OLIS SpectralWorks. Elemental analyses were per-
formed by Midwest Microlabs in Indianapolis, IN.
General Method for Synthesis of the Ethynyl Complexes. An oven-
dried two-neck round-bottom flask (50 mL) under a positive pressure of
Ar was charged with 1 equiv of [M(cyclam)(OTf)2]OTf, 2 equiv of the
appropriate acetylene, and THF (5 mL). After the mixture was cooled in
a dry ice/acetone bath for 10 min, 4 equiv of butyllithium (2.5 M
solution in hexanes) was added through a septum. The reaction mixture
was stirred in the cold bath for 5 min, then removed, and allowed to
warm to room temperature with stirring for 2 h. The mixture was
quenched with ∼0.5 mL of water, and the reaction mixture was loaded
directly onto silica gel (4 cm in a 60 mL fritted glass funnel) and eluted
using a 70:30 (v/v) mixture of methylene chloride and acetonitrile
(200 mL or until the eluate became colorless). The solvent was removed
using rotary evaporation. Cold diethyl ether (20 mL) was added to the
residue, and the mixture was sonicated and kept in an ice bath for 5 min to
maximize precipitation. The product was collected via vacuum filtration,
washed with cold diethyl ether (2 ꢁ 10 mL), and dried under vacuum.
Synthesis of the Chromium Complexes. [Cr(cyclam)(OTf)2]OTf. A
solutionof[Cr(cyclam)Cl2]Cl (1.50 g, 4.19 mmol) in trifluormethanesulfonic
[Cr(cyclam)(CCC6H11)2]OTf (1e). Using THF (5 mL), cis/trans-[Cr-
(cyclam)(OTf)2]OTf (200 mg, 0.286 mmol), cyclohexylacetylene (74.7 μL,
0.572 mmol), and n-butyllithium (480 μL, 1.20 mmol) yielded 0.109 g
(61.9%) of analytically pure trans-[Cr(cyclam)(CCC6H11)2]OTf. Anal.
Calcd (found) for C27H46CrF3N4O3S: C, 52.67 (52.47); H, 7.53 (7.39);
N, 9.10 (8.98). ESI-MS: intense parent ion (M+) at m/z value of 466.30.
[Cr(cyclam)(CCC10H7)2]OTf (1f). Using THF (5 mL), cis/trans-[Cr-
(cyclam)(OTf)2]OTf (200 mg, 0.286 mmol), 1-ethynylnaphthalene
(81.3 μL, 0.572 mmol), and n-butyllithium (480 μL, 1.20 mmol) yielded
0.161 g (80.0%) of analytically pure cis/trans-[Cr(cyclam)(CCC10H7)2]OTf.
Anal. Calcd (found) for C35H38CrF3N4O3S: C, 59.73 (59.46); H, 5.44 (5.45);
N, 7.96 (7.84). ESI-MS: intense parent ion (M+) at m/z value of 554.22.
[Cr(cyclam)(CCC14H9)2]OTf (1g). Using THF (5 mL), cis/trans-[Cr-
(cyclam)(OTf)2]OTf (200 mg, 0.286 mmol), 9-ethynylphenanthrene
(0.116 g, 0.572 mmol), and n-butyllithium (480 μL, 1.20 mmol)
yielded 0.213 g (92.7%) of analytically pure cis/trans-[Cr(cyclam)-
(CCC14H9)2]OTf H2O. Anal. Calcd (found) for C43H42CrF3N4O3S
3
3
H2O: C, 62.84 (62.50); H, 5.40 (5.32); N, 6.82 (6.88). ESI-MS: intense
parent ion (M+) at m/z value of 654.38.
Separation of cis and trans Isomers. A cis/trans mixture of
[Cr(cyclam)(CCR)2]OTf (100 mg) was dissolved in dry THF (2 mL
or desired amount). All dissolve completely except 1a, 1f, and 1g. To
precipitate the trans product, dry diethyl ether (8 mL) was added and the
mixture was sonicated. The trans product was collected via vacuum
filtration and washed with dry diethyl ether (2 ꢁ 10 mL). The filtrate
was evaporated to afford the cis product.
trans-[Cr(cyclam)(CCC6D5)2]OTf. The basic procedure for the
synthesis of trans-[Cr(cyclam)(CCC6H5)2]OTf was followed, except
that phenylacetylene-d6 was used.
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N-Deuteration of trans-[Cr(cyclam)(CCC6H5)2]OTf and trans-[Cr-
(cyclam)(CCC6D5)2]OTf. A solution of the complex (0.021 g, 0.035 mmol)
in anhydrous THF (1 mL) and D2O (0.3 mL) was stirred under a
nitrogen atmosphere for 24 h. After removal of the solvent under high
vacuum, the solid was dissolved in a minimum amount of anhydrous
THF (4 mL) and reprecipitated with anhydrous ether. The percent
deuteration was determined to be >95% by relative integration using
infrared spectroscopy (νND = 2401 cmꢀ1 and νNH = 3225 cmꢀ1).
Isolation of the Hydroamination Product of 1c. To a solution of
4-ethynyl-α,α,α-trifluorotoluene (1.4 mL, 8.6 mmol) in THF (15 mL)
at ꢀ78 °C was added n-BuLi (0.54 mL of a 2.5 M solution, 1.35 mmol).
After the solution was allowed to warm to room temperature, cis/
trans-[Cr(cyclam)(OTf)2]OTf (0.250 g, 0.358 mmol) was added and
the solution stirred for an additional hour. Following standard workup,
half of the residue was separated using column chromatography (silica
gel, 2% MeOH in CH2Cl2). The second (yellow) band was collected.
After removal of the solvent, the residue was redissolved in a minimum
of CH3CN and precipitated with ether yielding 22 mg of a yellow/
orange solid. ESI-MS: intense parent ion (M+) at m/z value of 930.68.
Crystals were grown by vapor diffusion of pentane/ether (1:1) into a
solution of the complex in CH3CN.
Synthesis of the Rhodium Complexes. [Rh(cyclam)(OTf)2]OTf.
[Rh(cyclam)Cl2]Cl (0.70 g, 1.71 mmol) in triflic acid (10.0 g) was
heated at 110 °C for 3 days under a steady flow of argon. The reaction
mixture was then added dropwise to a beaker of stirring anhydrous ethyl
ether (200 mL) under a blanket of Ar. After stirring for an additional
5 min, the contents were transferred to a glovebag and the precipitate was
collected by vacuum filtration using a course fritted glass funnel, washed
with anhydrous ethyl ether (2 ꢁ 100 mL), and dried under vacuum. The
product was stored in a desiccator due to its hygroscopic nature. Yield:
1.065 g (83.0%). Anal. Calcd (found) for C13H24F9N4O9RhS3: C, 20.81
(20.73); H, 3.22 (3.44); N, 7.47 (7.20).
(4.16); N, 7.09 (7.00). ESI-MS: intense parent ion (M+) at m/z value
of 641.38. trans-2c, 1H NMR (500 MHz, CD3CN) δ 1.49 (q, 2H), 2.05
(d, 2H), 2.63 (m, 4H), 3.00 (m, 12H), 4.54 (b, 4H), 7.56 (m, 8H).
[Rh(cyclam)(CCC6H4F)2]OTf (2d). Using THF (5 mL), trans-[Rh-
(cyclam)(OTf)2]OTf (175 mg, 0.233 mmol), 1-ethynyl-4-fluoroben-
zene (53.5 μL, 0.466 mmol), and BuLi (392 μL, 0.979 mmol) yielded
0.098 g (60.9%) of analytically pure [Rh(cyclam)(CCC6H4F)2]OTf.
Anal. Calcd (found) for C27H32F5N4O3RhS: C, 46.96 (46.47); H, 4.67
(4.58); N, 8.11 (7.91). ESI-MS: intense parent ion (M+) at m/z value of
541.29. trans-2d, H NMR (400 MHz, CD3CN) δ 1.48 (q, 2H), 2.04
(d, 2H), 2.62 (m, 4H), 3.01 (m, 12H), 4.50 (b, 4H), 7.01 (t, 4H), 7.38
(m, 4H).
Separation of cis and trans Isomers. The same procedure was
followed as for the Cr(III) complexes.
1
’ RESULTS AND DISCUSSION
General Synthesis. Recently we reported the syntheses of
three complexes of the form trans-[Cr(cyclam)(CCR)2]OTf,
1aꢀ1c (Figure 1, OTf = trifluoromethanesulfonate).27 These
were prepared by a method similar to that reported by Berben
and Long for the preparation of (Me3tacn)Cr(CCH)3
(Figure 2).28 In this synthetic method, cis/trans-[Cr(cyclam)-
(OTf)2]OTf was treated with the ethynyllithium reagent.
Following purification, 1a and 1b were obtained with isolated
yields in the range of 50ꢀ60% (Figure 3).27 Infrared spectros-
copy provided evidence for the trans geometry in complexes
1aꢀ1c. Confirmation of this geometry for 1a was obtained by
X-ray crystallography.27 Recently, this method has been
exploited by the groups of Nishi29 and Ren30 for applications
involving molecular magnetism and molecular wires. We have now
broadened the scope of this synthetic method, determined the
key side reactions in the synthesis, and discovered that the cis
product can also be isolated in most instances.
Hydroamination Side Reaction. A complex mixture of
species resulted when this synthetic method was performed
using arylacetylenes where the aryl group has either an electron
withdrawing group (i.e., CN or CF3) in the para position or
where the aryl group is one with extended aromaticity (i.e.,
naphthalene or phenanthrene). Mass spectrometry demon-
strated that the mixture consisted of complexes containing two
to five equivalents (n = 2ꢀ5) of the arylethynyl moiety. The
complex (with n = 4) resulting from the synthesis of 1c was
isolated and a poorly resolved X-ray crystal structure demon-
strated the connectivity shown in Figure 4. The additional two
arylethynyl units appear to have added by hydroamination of the
cyclam NHs across the arylethynyl CC triple bond.
Infrared spectroscopy confirms the presence of the alkene
functionalities (νCdC = 1611, 1647), and the calculated mass
spectrum (parent ion and isotopic distribution) matches that
predicted by the formula associated with this structure
(Supporting Information). Hydroamination of alkynes has re-
ceived considerable attention because of the synthetic utility of
this reaction.31 It is likely that the side-reaction here is an example
of a base catalyzed hydroamination, a reaction that has precedence
in the CsOH catalyzed hydroamination of phenylacetylene.32
The fact that the hydroamination reaction is most problematic
for the synthesis of 1c and 1fꢀ1h suggests that the presence of
either electron withdrawing groups or extended aromaticity
helps to stabilize charge buildup in the transition state. Mass
spectroscopy shows that trace amounts of the hydroamination
product were also formed for the other complexes, but these
small amounts were removed in the final precipitation step.
[Rh(cyclam)(CCC6H5)2]OTf (2a). Using THF (5 mL), [Rh(cyclam)-
(OTf)2]OTf (0.164 g, 0.219 mmol), phenylacetylene (48.0 μL,
0.437 mmol), and BuLi (367 μL, 0.918 mmol) yielded 0.098 g
(68.5%) of analytically pure [Rh(cyclam)(CCC6H5)2]OTf. Anal. Calcd
(found) for C27H34F3N4O3RhS: C, 49.54 (49.39); H, 5.24 (5.23); N,
8.56 (8.38). ESI-MS: intense parent ion (M+) at m/z value of 505.27.
NMR data: trans-2a, 1H NMR (500 MHz, CD3CN) δ 1.49 (q, 2H), 2.04
(d, 2H), 2.64 (m, 4H), 3.03 (m, 12H), 4.50 (b, 4H), 7.17 (t, 2H), 7.26
(t, 4H), 7.38 (d, 4H); 13C{1H} NMR (126 MHz, CD3CN) δ 30.5, 52.8,
54.5, 108.4 (d, JRhC = 6.4 Hz, RhꢀCCPh), 113.3 (d, JRhC = 38 Hz,
1
RhꢀCCPh), 126.6, 129.0, 129.1, 132.2. cis-2a, H NMR (500 MHz,
CD3CN) δ 1.81 (d, 2H), 2.60 (d, 2H), 2.71 (m, 4H), 3.01 (m, 8H), 3.35
(t, 2H), 3.47 (q, 2H), 4.44 (b, 2H), 4.88 (b, 2H), 7.15 (t, 2H), 7.25
(t, 4H), 7.33 (d, 4H); 13C{1H} NMR (126 MHz, CD3CN) δ 24.4, 49.0,
49.9, 51.3, 53.7, 103.9 (d, JRhC = 47 Hz, RhꢀCCPh), 104.0 (d, JRhC
=
8.7 Hz, RhꢀCCPh), 126.4, 129.21, 129.25, 131.9.
[Rh(cyclam)(CCC6H4CH3)2]OTf (2b). Using THF (5 mL), trans-
[Rh(cyclam)(OTf)2]OTf (0.224 g, 0.299 mmol), 4-ethynyltoluene
(75.7 μL, 0.597 mmol), and BuLi (501 μL, 1.254 mmol) yielded
0.100 g (49.1%) of analytically pure [Rh(cyclam)(CCC6H4CH3)2]-
OTf H2O. Anal. Calcd (found) for C29H38F3N4O3RhS H2O: C, 49.71
3
3
(49.98); H, 5.75 (5.47); N, 8.00 (7.78). ESI-MS: intense parent ion
(M+) at m/z value of 533.23. trans-2b. 1H NMR (400 MHz, CD3CN) δ
1.48 (q, 2H), 2.04 (d, 2H), 2.30 (s, 6H), 2.63 (m, 4H), 3.03 (m, 12H),
4.48 (b, 4H), 7.17 (m, 8H); 13C{1H} NMR (100 MHz, CD3CN) δ 21.3,
30.5, 52.8, 54.5, 108.2 (d, JRhC = 6.7 Hz, RhꢀCCPh), 111.9 (d, JRhC = 37
Hz, RhꢀCCPh), 126.1, 129.8, 132.1, 136.4.
[Rh(cyclam)(CCC6H4CF3)2]OTf (2c). Using THF (5 mL), trans-[Rh-
(cyclam)(OTf)2]OTf (0.175 g, 0.233 mmol), 4-ethynyl-α,α,α-trifluor-
otoluene (76 μL, 0.466 mmol), and BuLi (392 μL, 0.979 mmol) yielded
0.090 g (48.8%) of analytically pure [Rh(cyclam)(CCC6H4CF3)2]OTf.
Anal. Calcd (found) for C29H32F9N4O3RhS: C, 44.06 (44.39); H, 4.08
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Figure 1. [M(cyclam)(CCR)2]OTf complexes discussed in this paper. The abbreviations 1aꢀ1h and 2aꢀ2d are used to refer to the trans complexes.
The cis complexes will be referred to as cis-1aꢀ1h.
Figure 2. (Me3tacn)Cr(CCH)3.
Figure 4. Structure of the product resulting from hydroamination.
THF (and have trace solubility in water) and are air stable both in
the solid state and in solution. The preparation of 2aꢀ2d
demonstrates that this synthesis is not restricted to Cr(III) nor
to metals from the first transition series. Likewise, the synthesis of
1e demonstrates that the synthesis is not restricted to arylethynyl
complexes but can be applied to alkylethynyl complexes even
Figure 3. General synthetic scheme for 1aꢀ1c.
A likely mechanism for the hydroamination involves depro-
tonation of one of the cyclam NHs by the arylacetylide, yielding
an amide coordinated to Cr(III). This results in an arylacetylene
that is much more likely to undergo hydroamination than the
corresponding (electron rich) arylacetylide. Decreasing the
ethynyllithium to chromium ratio to 2:1 did give the desired
product, albeit in significantly lower yields (15ꢀ30%). The lower
yields for this stoichiometric ratio are consistent with the
consumption of much of the ethynyllithium reagent through
deprotonation of a macrocyclic amine. This suggested an alter-
nate approach, namely, using excess n-BuLi for the deprotona-
tion of the arylacetylene. When 1c was prepared using a 4:2:1
ratio of n-BuLi to arylacetylene to Cr, the desired product was
obtained in 75% yield, with the hydroamination product being
barely detectable by mass spectroscopy. Notably, all reagents
may be added in a single step with no decrease in yield, thus
obviating the need to separately perform the deprotonation of
the alkyne prior to the addition of the chromium complex.
This method has been successfully applied to the synthesis of
1aꢀ1g and 2aꢀ2d. The complexes are soluble in CH3CN and
though the alkylacetylides are much more basic than the corre-
sponding arylacetylides. The only complex for which this method
failed was 1h, where mass spectral data indicated n-BuLi attack
on the nitrile. Replacing n-BuLi with lithium diisopropylamide
resulted in the desired product, albeit contaminated with an
impurity that we have been unable to separate from 1h.
It is important to note that these ethynyl complexes can be
prepared in excellent yield using reagents that are very strong
bases, even in the presence of the macrocyclic secondary amines.
Consideration of relative pKa values and the evidence cited above
suggests that one or more macrocyclic amines are deprotonated
during the synthesis of the cyclam complexes (presumably by
the excess BuLi when a 4:2:1 ratio of BuLi to arylacetylene to Cr
is used). The previously mentioned synthesis of (Me3tacn)-
Cr(CCH)3 avoided this potential problem by using a macrocycle
with tertiary amines.28 Apparently, deprotonation of the macro-
cyclic amines does not inhibit the ethynyllithium reagent from
binding to the metal. This is not surprising inasmuch as one of
the mechanisms suggested for base hydrolysis of M(NH3)5X2+
complexes involves prior deprotonation of the amine. This
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retention of configuration (Supporting Information). For all of
the ethynyl complexes of Cr(III), we obtain greater than 60%
yield of the trans isomer after separation, suggesting that the
reaction from cis-[Cr(cyclam)(OTf)2]OTf proceeds with iso-
merization (eq 1).
½CrðcyclamÞCl2ꢂCl f ½CrðcyclamÞðOTfÞ2ꢂOTf
>90% cis
>90% cis
f ½CrðcyclamÞðCCRÞ2ꢂOTf
ð1Þ
>60% trans
Furthermore, starting with isomerically pure cis-[Cr(cyclam)-
(OTf)2]OTf for the synthesis of 1a results in a 2:1 ratio of trans
to cis product. Alternatively, starting with isomerically pure
trans-[Cr(cyclam)(OTf)2]OTf results in isomerically pure trans-
1a. Thus, the preparation of the ethynyl complexes proceeds with
retention of configuration when starting with the trans precursor,
whereas it proceeds with significant isomerization when starting
with the cis precursor. This is not surprising inasmuch as a similar
cis to trans isomerization occurs when preparing trans-Cr-
(cyclam)(CN)2 from a cis/trans mixture of Cr(cyclam)Cl2 .
For the synthesis of the trans ethynyl complexes, 1aꢀ1g, we find it
more facile to start with a cis/trans mixture of [Cr(cyclam)Cl2]Cl
and to performthe isomeric separationofthe ethynyl product than
to prepare isomerically pure trans-[Cr(cyclam)Cl2]Cl starting
material.
Figure 5. Infrared spectra in the CH2 rocking and NH bending region
for the cis and trans isomers of 1a.
+
+ 35
mechanism, termed SN1CB, then proceeds via dissociative ligand
exchange from this conjugate base (CB).33 However, given the
successful synthesis28 of (Me3tacn)Cr(CCH)3 (a case in which
there are no amine protons) under similar conditions as used for
the cyclam complexes herein, it is unlikely that NH deprotona-
tion is a requirement for OTf replacement.
Cis and trans Isomers for the Rh(III) Complexes. In addition
to IR spectroscopy,24,34 1H and 13C NMR spectroscopy have
been useful in determining stereochemistry for cis and trans
Cis and trans Isomers for the Cr(III) Complexes. The pre-
viously reported synthesis resulted in exclusive isolation of the
trans isomers of 1aꢀ1c.27 We have determined that this was due
to the rather surprising solubility of the cis isomers in ether or
mixtures of THF and ether. Because the final purification step
had involved precipitation of the product from THF solution by
the addition of ether, the trans isomer was obtained selectively. In
the purification reported here, the crude residue obtained after
elution through a silica gel plug is sonicated with a small amount
of ether, resulting in a fine solid that is collected by filtration. In
several cases, 1a, 1b, 1d, 1f, and 1g, the resulting solid is a mixture
of cis and trans isomers as demonstrated by IR spectroscopy. For
1c and 1e, the resulting solid is the trans isomer with the cor-
responding cis isomer remaining in the ether filtrate (as evi-
denced by mass spectrometry and IR spectroscopy) along with
other minor impurities.
For 1a, 1b, 1d, 1f, and 1g, the isomers are separated by
dissolving the mixture in THF and precipitating the trans
product with ether. The filtrate contains the cis isomer. Infrared
spectroscopy in the region of NꢀH bending and CH2 rocking
vibrations has been used to differentiate between cis and trans
isomers for cyclam complexes of Co(III).34 The infrared spectra
in this region for the cis and trans isomers of 1a (Figure 5) are
consistent with complete separation of the isomers.
+
Rh(cyclam)X2 complexes.36 Starting with isomerically pure
cis-[Rh(cyclam)Cl2]Cl results in isomerically pure cis-[Rh-
1
(cyclam)(OTf)2]OTf (Supporting Information). However, H
and 13C NMR demonstrate that the [Rh(cyclam)(CCPh)2]OTf,
2a, prepared from cis-[Rh(cyclam)(OTf)2]OTf contains ap-
proximately 15% of the trans isomer, indicating some cis to trans
isomerization in this final step (eq 2).
½RhðcyclamÞCl2ꢂCl f ½RhðcyclamÞðOTfÞ2ꢂOTf
pure cis
pure cis
f ½RhðcyclamÞðCCPhÞ2ꢂOTf
ð2Þ
∼15% trans
The stereochemical progression starting with trans-[Rh-
(cyclam)Cl2]Cl is less apparent by IR or NMR, due to what
appears to be multiple macrocyclic conformations of the cyclam
ligand for the trans isomer of the chloro and triflato complexes.37
In addition, 1H NMR demonstrates that the trans-[Rh-
(cyclam)Cl2]Cl starting material is contaminated with at least
a few percent of the cis isomer (Supporting Information).
Regardless, the ratio of trans to cis 2a prepared starting from
trans-[Rh(cyclam)Cl2]Cl is approximately 2:1 (eq 3).
½RhðcyclamÞCl2ꢂCl f ½RhðcyclamÞðOTfÞ2ꢂOTf
The most diagnostic feature is the doublet from the NꢀH
vibration centered at 878 cmꢀ1 for the trans isomer, which
gives way to a doublet centered at 853 cmꢀ1 for the cis isomer.
Similar spectra are observed for the other congeners (Supporting
Information), though these doublets are sometimes obscured by
the aromatic out of plane vibration(s).
∼95% trans
??
f ½RhðcyclamÞðCCPhÞ2ꢂOTf
ð3Þ
∼2:1 trans=cis
+
The tendency of cis-Rh(cyclam)I2+ (but not cis-Rh(cyclam)Br2
+
or cis-Rh(cyclam)Cl2 ) to isomerize in refluxing aqueous solu-
For the sake of simplicity, these syntheses begin with a cis/
trans mixture of [Cr(cyclam)Cl2]Cl, prepared by the method of
Ferguson and Tobe,22 which typically contains no more than
10% of the trans isomer. IR evidence demonstrates that ligand
exchange to the [Cr(cyclam)(OTf)2]OTf proceeds with
tion suggests a steric preference of bulkier ligands for the trans
configuration.24 This suggests that the isomerization shown in
eq 2 is due to steric preferences, implying that the isomerization
shown in eq 3 occurs during the (harsh) first step and is likely due
to an electronic preference.
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Like the analogous Cr(III) complexes, the cis and trans isomers
of the alkynyl complexes can be separated based on solubility.
However, isomerically pure cis-2aꢀ2d were more difficult to
obtain due to a greater solubility of their trans isomers (in ether/
THF) than the corresponding Cr(III) complexes. Even so, a
quantity of cis-2a sufficient for spectroscopic characterization was
obtained.
Table 1. Raman Vibrational Data (cmꢀ1
)
ν(CtC) complex
(parent alkyne)
Δ
1a
1b
1c
1d
1e
1f
2077 (2110)
2079 (2108)
2086 (2115)
2083 (2112)
2088 (2115)
2073 (2100)
2071 (2097)
2105 (2110)
2105 (2108)
2107 (2115)
2109 (2112)
33
29
29
29
27
27
26
5
1H and 13C NMR spectra of the ethynyl complexes of Rh(III)
confirm the cis and trans geometries and demonstrate that the cis
and trans isomers each exist in a single conformation. For
example, the 1H NMR spectrum of trans-2a shows a single NH
resonance indicative of four equivalent amine protons, whereas
cis-2a shows two such resonances. Furthermore, the 13C NMR
spectrum of trans-2a displays two resonances (52.8 and 54.5 ppm)
associated with N-bound methylene carbons and one assigned to
the central carbon of the three-carbon linkage (30.5), whereas
that of cis-2a displays four resonances (49.0, 49.9, 51.3, 53.7)
associated with N-bound methylene carbons and, again, one
assigned to the central carbon of the three-carbon linkage (24.4).
Alkynyl C;C Vibrational Frequency. Tables 1 and 2 list
ν(CtC) for all complexes along with ν(CtC) for the parent
alkyne. Intense Raman bands near 2100 cmꢀ1 are evident for
all complexes. All of the Rh(III) complexes also have IR active
stretches in this region, whereas ν(CtC) for the majority
of the Cr(III) complexes are IR silent, with the exception of
[Cr(cyclam)(CCC6H11)2]OTf for which this band is weak but
observable. For all trans complexes, the Raman bands can be
assigned to the symmetric CtC stretch and the IR bands to the
asymmetric CtC stretch.
1g
2a
2b
2c
2d
3
8
3
Table 2. IR Vibrational Data (cmꢀ1
)
ν(CtC)
2a
2094
cis-2a
2b
2110, 2123
2098
2c
2103
2d
2103
cis-2d
2107, 2120
For M-CCR complexes, the frequency of the CtC stretch
relative to ν(CtC) for the parent HCCR has been used to
determine information about the bond between the metal and
the alkynyl carbon. However, as pointed out by Manna et al.,
better reference compounds are those of the type R0CCR (R0 ¼ H)
because of the fact that ν(CtC) for the HCCR derivatives
contains contributions from the terminal CꢀH coordinate.18
For example, whereas ν(CtC) for HCCPh is 2110 cmꢀ1, the
bond weakening for the Cr(III) complexes. Note that, apart
from a slight ionic radius difference [66.5 pm for Rh(III) vs 60.5
pm for Cr(III)], the chief difference between these metals is in
the population of the orbitals of appropriate symmetry for π-
interaction with the ethynyl ligands. Namely, these orbitals are
half-filled for Cr(III) whereas they are filled for Rh(III). Thus, if
the chief π-interaction were π-backbonding, this interaction
would be favored for the Rh(III) complexes over the Cr(III)
complexes, suggesting that the Rh(III) complexes should have
the weaker CtC bonds. In fact, the opposite is observed, which
is consistent with π-donation from the ethynyl ligands to the half-
filled t2g orbitals of the Cr(III) complexes being more pro-
nounced than π-donation to the filled t2g orbitals of the Rh(III)
complexes. Furthermore, if the difference in M(III) ionic radius
here has any significant effect, it would be to weaken the CtC
bond of the Rh(III) complexes relative to the Cr(III) complexes,
serving only to partly mitigate the effect of the observed π-
interaction.
Though these spectroscopic results clearly indicate that the
alkynyl ligands act as π-donors for the Cr(III) complexes, the
nature of the interaction with the Rh(III) complexes is less
obvious, though clearly weak. For the related trans-Ru(16-
TMC)(CCAr)2 complexes (16-TMC = 1,5,9,13-tetramethyl-
1,5,9,13-tetraazacyclohexadecane), the Ru(II)-alkynyl interac-
tions are discussed in terms of the alkynyl ligands being π-
acceptors, albeit quite weak.7 Such π-backbonding would be
expected to be even weaker for Rh(III) complexes compared
with Ru(II) complexes, due to the higher positive charge on Rh.
Photophysical Characterization. Electronic Absorption Spec-
tra of the Cr(III) Complexes. As demonstrated in Figure 6, the
UVꢀvisabsorptionspectra ofthe trans-[Cr(cyclam)(CCR)2]OTf
complexes exhibit intense (ε ∼ 5 000ꢀ40 000 Mꢀ1 cmꢀ1) transi-
tions at wavelengths below 320 nm. These likely involve charge
corresponding R0CCPh can range from 2150 to 2280 cmꢀ1
.
However, for the sake of a common comparison we have in-
cluded the HCCR data.
A closer look at the Raman active bands for each of the trans
complexes reveals a slight reduction of ν(CtC) relative to the
analogous alkyne (Table 1), thus these ν(CtC) values should all
be significantly lower than the corresponding R0CCR com-
pounds. This is consistent with a weakening of the CtC bond.
Two explanations have been put forth for this weakening. The
first explanation involves π-interactions, either M f CCR
π-backbonding or RCC f M π-bonding. The second explana-
tion recognizes that the lone pair on the RCCꢀ anion is
antibonding with respect to the CtC bond. Thus, an increased
ionic character in the MꢀCCR bond will result in a weaker CtC
bond.18 For example, the alkali metal salts of the phenylethynyl
anion demonstrate decreased values for ν(CtC) down the
column (LiCCPh, 2030 cmꢀ1; NaCCPh, 2005 cmꢀ1; KCCPh,
1998 cmꢀ1).38 Presumably, the CtC bond weakening for the
Cr(III) and Rh(III) ethynyl complexes is partly due to the ionic
nature of the M(III)ꢀCCR bond. However, can any reduction in
bond order be ascribed to π-interactions and if so, π-bonding or
π-backbonding?
A comparison of ν(CtC) reveals that these values range from
21 to 28 cmꢀ1 higher for the Rh(III) complexes than the
analogous Cr(III) complexes (Table 1), indicating more CtC
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Figure 6. UVꢀvis absorption spectra (in CH3CN) comparing selected trans-[Cr(cyclam)(CCR)2]OTf complexes, 1aꢀ1c and 1e.
transfer (CT)39,40 and intraligand (IL) transitions. The absor-
bance spectra between 320 and 500 nm (typical region for
4
4
4
transitions from the A2g ground state to the T1g and T2g
(Oh) excited states) demonstrate extinction coefficients ranging
from 170 to 700 Mꢀ1 cmꢀ1 (Figures 6 and 7 and Table 3)
These molar extinction coefficients are much larger than is
typical for dꢀd transitions involving centrosymetric species. For
example, the isoelectronic trans-Cr(cyclam)(CN)2+ displays two
absorption bands of modest intensity (ε ∼ 60 L/mol cm) at
414 nm (4A2g f 4T2g) and 328 nm (4A2g f 4T1g).35 The larger
extinction coefficients for the arylethynyl complexes, along with
the fine structure apparent in the absorption spectra suggest
possible intensity stealing from the proximal CT band41 as has
been observed for other alkynyl complexes of Cr(III).21 Here,
this intensity stealing appears to require the aryl substituent,
inasmuch as the electronic absorption spectrum of the related
cyclohexylethynyl complex, 1e, lacks either the fine structure or
intensity observed for the arylethynyl complexes (Figure 6 and
Table 3). The fact that these visible absorption bands for 1e are
shifted to lower energy relative to those of the trans-Cr(cyclam)-
(CN)2+ cation also suggests that the ethynylcyclohexane ligand is
a weaker field ligand than CNꢀ. This conclusion is consistent
with the π-donor character of the alkynyl ligands that has
previously been inferred for Cr(III) complexes.21
Figure 7 also demonstrates that extended aromaticity of the
aryl group shifts the onset of absorption approximately 100 nm to
the red. Furthermore, as demonstrated in Table 3 for 1a vs cis-1a
(and shown in the Supporting Information for several additional
congeners), the cis isomers exhibit extinction coefficients ap-
proximately twice the magnitude of their corresponding trans
isomers, with absorption maxima shifted 7ꢀ10 nm to lower
energy. The larger extinction coefficients are consistent with the
loss of centrosymmetry.
Figure 7. UVꢀvis absorption spectra (in CH3CN) comparing 1a with
1f and 1g. Note that the expanded aromaticity shifts the onset of
absorption to longer wavelengths.
Steady State and Transient Emission Data for the Cr(III)
Complexes. The Cr(III) alkynyl complexes are emissive in the
red region of the visible spectrum, the emission being solvent and
concentration independent (Table 4). The excitation spectrum
for each complex resembles the UVꢀvis absorption spectrum,
indicating that emission is not due to an impurity.
+
By analogy with the isoelectronic trans-Cr(cyclam)(CN)2
complexes, emission from the alkynyl complexes could be expected
to originate from the 2Eg (Oh) excited state. However, the steady-
state room temperature emission spectra for most of these com-
plexes are broad and structureless (Supporting Information), with
emission maxima red-shifted relative to the 650ꢀ710 nm region
characteristic of 2Eg emission.43 Such spectra are characteristic of
4
2
2T1g f A2g (Oh) transitions.44 Typical T1g emitters of similar
structure (e.g., trans-[Cr(cyclam)(OH)2]+ and trans-[Cr(tet
a)F2]+ where tet a is C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-
tetraazacyclotetradecane) can be characterized as having axial
ligands that are both σ and π donors.45 However, assignment of
Electronic Absorption Spectra of the Rh(III) Complexes.
UVꢀvis spectra for the corresponding Rh(III) complexes,
2aꢀ2c and cis-2a, are shown in Figure 8. These are dominated
by CT transitions that are red-shifted approximately 40ꢀ70 nm
relative to the corresponding transition for the Rh(cyclam)-
2
the emission as originating from the T1g excited state does not
necessitate that the ligands be π-donors but only that there exists an
asymmetry of π effects (π-donating or π-withdrawing) between
the axial and equatorial ligands.46 Regardless, the vibrational data
discussed above clearly point toward the asymmetry arising from π-
donation from the alkynyl ligand.
(CN)2 complex.42 As a result of this red-shift, the metal
+
centered (MC) ligand field transitions for the Rh(III) alkynyl
complexes are obscured (for reference, the lowest energy MC
+
transition for Rh(cyclam)(CN)2 occurs at 267 nm). The MC
transitions are also likely to be red-shifted due to the alkynyl
ligand being a weaker field ligand than cyanide, but apparently
this shift is not large enough to reveal this transition.
The cyclohexylethynyl complex, 1e, on the other hand, displays
an emission spectrum that is not as red-shifted and that shows
evidence of fine structure (Supporting Information). This suggests
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Table 3. UVꢀVisible Spectral Data (Visible Region) for [Cr(cyclam)(CCR)2]OTf Complexes in CH3CNa
complex
λmax (ε)b
ref
1a
352 (528), 360 (554), 373 (550), 387 (560), 400 (597), 414 (380), 432 (343)
354 sh (753), 369 (794), 382 (943), 395 (1025), 408 (1091), 424 (669), 441 (582)
355 (620), 365 (635), 376 (629), 380 (631), 392(673), 404 (703), 421 (443), 436 (400)
348 (452), 382 (358), 394 (363), 406 (369), 424 (258), 438 (224)
359 (576), 373 (569), 385 (598), 397 (594), 414 (401), 428 (327)
354 (36), 438 (37)
27
cis-1a
1b
1c
this work
27
27
1d
1e
this work
this work
this work
this work
1f
434 (178), 466 sh (141), 500 (109), 528 (56), 542 (63)
1g
434 (183), 464 sh (158), 496 (139), 528 (80), 536 (85)
a The spectrum for 1a was also recorded in THF and water (Supporting Information), demonstrating that the spectra are largely solvent independent.
The Beer’s Law dependence was also demonstrated for 1a in CH3CN. b Absorption wavelengths are in nanometers, and the extinction coefficients are in
units of molarityꢀ1 centimetersꢀ1
.
Table 5. Effect of Deuteration on Lifetime (μs) of 1a
room
temperature
77 K
H2O/ D2O/
H2Oa D2Oa DMSO DMSO
trans-[Cr(cyclam)(CCC6H5)2]OTf
trans-[Cr(cyclam)(CCC6D5)2]OTf
trans-[Cr(d4-cyclam)(CCC6H5)2]OTf
trans-[Cr(d4-cyclam)(CCC6D5)2]OTf
293
290
318
303
343
367
1086
1019
1380
1420
a An ion-exchange resin, Dowex 2-X8, chloride form, 20-50 mesh, was
used to assist the solubility of the complexes in H2O and D2O.
Figure 8. UVꢀvis absorption spectra of Rh(III) alkynyl complexes in
CH3CN. The spectrum for trans-2a was also recorded in THF and water,
demonstrating that the spectra are largely solvent independent. The
Beer’s Law dependence was also demonstrated for trans-2a in CH3CN.
might occur) reveals two different lifetimes: 179 μs at 719 nm,
compared with 341 μs at 756 nm. At room temperature, the
excited state lifetime is independent of emission wavelength. If
these results indeed demonstrate at least some emission from the
2Eg (Oh) excited state, it implies that the cyclohexylethynyl
ligand is a poorer π-donor than any of the arylethynyl ligands,
an assertion that seems reasonable considering that the aryl ring
can contribute π-electron density to the alkynyl bond.
Table 4. Photophysical Data for trans-[Cr(cyclam)(CCR)2]+
λmax (nm)a
τ (μs)
The excited state lifetimes of the alkynyl Cr(III) complexes are
slightly solvent dependent and also sensitive to the presence of
oxygen (Table 4). The oxygen sensitivity stands in contrast
to trans-[Cr(cyclam)(CN)2]+ 16 but is consistent with observa-
tions for trans-[Cr(cyclam)(NCS)2]+.48 Such oxygen sensitivity
for the lifetimes of Cr(III) complexes has been attributed to the
presence of π-electron density on the coordinated ligand.49
Consistent with this, the excited state lifetime of 1e is the least
oxygen sensitive of the series (Table 4).
In an attempt to better understand the excited state deactivation
process, the lifetime of 1a was studied (under deaerated con-
ditions) as a function of deuteration of the macrocyclic amines
and/or the aryl ring (Table 5).
Because phosphorescence quantum yields are typically quite
small for Cr(III) complexes,50 excited state relaxation must be
dominated by nonradiative processes. Electronically coupled vibra-
tional modes (typically NꢀH and CꢀH vibrations) are typical
channels for nonradiative release of this excited state energy.11,51
Note that there is roughly a 4-fold increase in the excited state
lifetime of 1a upon macrocyclic NꢀH deuteration (both at 77 K
and at room temperature). Such an isotope effect indicates the
significant role that the NꢀH vibrations play in excited state
relaxation.11,51 Deuteration of the aryl ring, on the other hand,
20 °C
77 K
20 °C, air saturated (N2)
77 K
glass
complex CH3CN H2Ob glass
CH3CN
H2Ob
trans-1a
748
745
746
746
731
748
747
745c
746c
745c
735d,e
748d
750d
6 (225)
5 (259)
6 (203)
37 (293) 343c
30 (285) 340c
32 (313) 370c
trans-1b 749
trans-1c
trans-1e
trans-1f
trans-1g
748
727
749
750
51 (331) 187 (327) 179,d,f 341d,g
3 (250)
3 (238)
13 (240) 343d
h (273)
345d
a The emission spectra are concentration independent. b An ion-ex-
change resin, Dowex 2-X8, chloride form, 20-50 mesh, was used to assist
the solubility of the complexes in H2O. c H2O/DMSO glass. d CH3OH/
DMSO glass. e The spectrum exhibits significant fine structure with
peaks between 719 and 756 nm. f Emission monitored at 718 nm.
g Emission monitored at 756 nm. h Because of low solubility in water and
oxygen quenching, this lifetime could not be measured.
that emission in this case may originate from the 2Eg (Oh) excited
state or perhaps from both the 2T1g and 2Eg (Oh) excited states as is
observed for Cr(bpy)33+.47 Consistent with this latter suggestion,
an analysis of the lifetimes for 1e in a CH3OH/DMSO glass at 77
K (a temperature below that at which thermal equilibration
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Table 6. Photophysical Data for trans-[Rh(cyclam)(CCR)2]+
consistent with excited state population of the dx2-y2 orbital,
supporting the understanding that alkynyl ligands are stronger
σ-donors than the in-plane amines. In contrast to trans-Rh-
λmax (nm)a
τ (μs)
+
(cyclam)(CN)2 , 2aꢀ2c are quenched by oxygen, reminiscent
ambient
77 K
glassc
ambient
77 K
glassc
of the analogous Cr(III) complexes. The slight red shift of the
excitation spectrum from the absorption spectrum (<20 nm, e.
g., Figure 9) is likely an artifact from the precipitous drop in the
excitation lamp intensity at 270 nm.
complex
CH3CNb
H2Ob
CH3CNb
trans-2a
trans-2b
trans-2c
548
546
524
546
544
520
526
524
d
5.4
3.8
21
19
17
d
’ SUMMARY
a The emission spectra are concentration independent. b N2 purged.
c H2O/DMSO/CH3OH glass. d The 77 K emission spectrum is domi-
nated by an emission at 460 nm with a lifetime of 22 ms, typical of organic
triplet emission. We cannot rule out impurity emission at this time.
A high-yield one-pot synthesis for complexes of the type
[M(cyclam)(CCR)2]OTf has been developed. Both the cis
and trans complexes can be isolated, though the trans complexes
(which are typically more desirable as building blocks for
molecular electronic devices) are typically obtained in substan-
tially higher yields (due both to isomerization and solubility).
+
Like their corresponding trans-M(cyclam)(CN)2 complexes,
both the Cr(III) and Rh(III) complexes exhibit long-lived MC
phosphorescence. Analysis of ν(CtC) suggests that RCC f M
π-donation plays a significant role in the MꢀCCR bond for the
Cr(III) complexes but much less of a role for the corresponding
Rh(III) complexes. The MC electronic absorption bands of
arylalkynyl complexes of Cr(III) show significant fine structure
and unusually high intensities, indicative of intensity stealing from a
proximal charge transfer band. This behavior appears to require the
aryl ring, inasmuch as the MC absorption bands for the cyclohex-
ylethynyl complex, 1e, have normal intensities and lack fine
structure. This indicates that the presence of the aryl ring enhances
the electronic communication between Cr(III) and the alkynyl
ligand. The presence of these π-interactions suggests the possibility
of long-range electronic communication between metal centers
connected by arylalkynyl ligands. We are currently working on
extending this chemistry to the preparation of complexes with other
transition metals as well as the synthesis of dimers and trimers for
studies of intramolecular energy transfer.
Figure 9. Absorption, emission (λex = 290 nm), and excitation (λem
548 nm) spectra for 2a in degassed acetonitrile.
=
has no observable effect, clearly indicating that the aryl CꢀH bonds
are not electronically coupled to the metal in the 2T1g excited state.
The observation of a deuterium isotope effect upon amine
deuteration for Cr(III) complexes is commonly cited as evidence
for a weak coupling mechanism for excited state relaxation, that is, a
mechanism involving tunneling between nested electronic states.11
The independence of the emission energy of the arylethynyl
Cr(III) complexes as a function of the identity of the aromatic
group suggests that there is very little electronic communication
between Cr(III) and the aromatic ring in the doublet excited
state. This is supported by the absence of a deuterium isotope
effect upon aryl CꢀH bond deuteration.
’ ASSOCIATED CONTENT
S
Supporting Information. Characterization data for the
b
hydroamination product; infrared spectra and analysis support-
ing the stereochemical assignments for the Cr(III) and Rh(III)
starting materials and their alkynyl complex products; overlaid
UVꢀvis spectra of the cis and trans isomers of 1a, 1d, 1f, and 1g;
comparison of the UVꢀvis spectra of 1a in CH3CN, THF, and
H2O; room temperature and 77 K emission spectra of 1aꢀ1c
and 1eꢀ1g; and room temperature emission and excitation
spectra for 2b and 2c. This material is available free of charge
via the Internet at http://pubs.acs.org.
Steady State and Transient Emission Data for the Rh(III)
Complexes. Emission from acidoam(m)ine Rh(III) complexes in
room temperature fluid solutions is typically short-lived (1ꢀ50 ns),52
+
but trans-Rh(cyclam)(CN)2 exhibits long-lived (8 μs) broad
structureless emission centered at 470 nm in room temperature
aqueous solutions. This behavior is attributed to the presence
of stronger field CNꢀ ligands along the z-axis (both a better
σ-donor and π-acceptor than the in-plane amine ligands), which
’ AUTHOR INFORMATION
3
results in the A2g (D4h) state being the lowest energy MC
Corresponding Author
*E-mail: paul.wagenknecht@furman.edu.
excited state.15 This restricts nonradiative and photochemical
deactivation to the xy-plane (due to population of the dx2-y2
orbital), and such deactivation is inhibited by the rigid macro-
cyclic ligand.11,26 The isoelectronic Rh(III) alkynyl complexes,
2aꢀ2c, exhibit similar behavior, though emission is shifted to
’ ACKNOWLEDGMENT
This material is based upon work supported by the National
Science Foundation under Grant CHE-0709562. W.P.F. and
C.R.T. acknowledge summer support from the Furman Advan-
tage program. J.H.W. acknowledges support from the Arnold and
Mabel Beckman Foundation’s Beckman Scholars Program. The
authors thank Parth Thakker for experimental assistance.
+
lower energy relative to trans-Rh(cyclam)(CN)2 (Table 6,
Figure 9, and Supporting Information). The red-shift in emis-
sion demonstrates that the alkynyl ligands are weaker-field
ligands than cyanide, consistent with prior observations for the
Cr(III) complexes. At the same time, the emissive behavior is
9362
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Inorganic Chemistry
ARTICLE
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