Emissive chromium(III) complexes with substituted arylethynyl ligands

Article abstract of DOI:10.1021/ic801376p

Arylethynylchromium(III) complexes of the form trans-[Cr(cyclam)(CCC ₆H₄R)₂]OTf (where cyclam = 1,4,8,11- tetraazacyclotetradecane, R = H, CH₃, or CF₃ in the para position, and OTf = trifluoromethanesulfonate) have been prepared and characterized by IR spectroscopy and X-ray diffraction. The complexes are emissive with excited-state lifetimes in a deoxygenated fluid solution between 200 and 300 μs.

Full text of DOI:10.1021/ic801376p

Inorg. Chem. 2008, 47, 11452-11454
Emissive Chromium(III) Complexes with Substituted Arylethynyl Ligands
David L. Grisenti,† W. Walsh Thomas,† Christopher R. Turlington,† Matthew D. Newsom,†
Christopher J. Priedemann,† Donald G. VanDerveer,‡ and Paul S. Wagenknecht*,†
Department of Chemistry, Furman UniVersity, GreenVille, South Carolina 29613, and Department
of Chemistry, Clemson UniVersity, Clemson, South Carolina 29634-0973
Received July 22, 2008
Arylethynylchromium(III) complexes of the form trans-
[Cr(cyclam)(CCC6H4R)2]OTf (where cyclam ) 1,4,8,11-tetraaza-
cyclotetradecane, R ) H, CH3, or CF3 in the para position, and
OTf ) trifluoromethanesulfonate) have been prepared and
characterized by IR spectroscopy and X-ray diffraction. The
Figure 1. trans-[Cr(cyclam)(CCC6H4R)2]OTf.
complexes are emissive with excited-state lifetimes in a deoxy-
ical potential of metal alkynyl polymers, transition-metal
genated fluid solution between 200 and 300 µs.
alkynyl complexes have been intensely studied over the last
several decades.5 Herein, we present the synthesis and
characterization of chromium(III) cyclam complexes with
arylethynyl ligands in the trans positions (see Figure 1) and
highlight their significant differences with the isoelectronic
trans-[Cr(cyclam)(CN)2]+.
The prospect of inventing functional photochemical mo-
lecular devices has prompted research in the area of inter-
and intramolecular energy transfer between transition-metal
complexes.1,2 Recent efforts in our group have focused on
determining intermolecular energy-transfer rate constants that
approximate self-exchange rate constants between trans-
[Cr(N4)X2]n+ complexes (where N4 ) tetraazamacrocycle and
X ) CN-, NH3, F-).3 Most recently, we have become
interested in the effect of subtle thermodynamic effects on
energy-transfer rate constants. Thus, we have begun research
on the preparation of new arylethynyl complexes of the
chromium(III) cyclam moiety. Substituents on the arylethynyl
ligand may provide the abilty to tune the excited-state energy
and lifetime. Precedence for this type of excited-state tuning
includes studies by Eisenberg’s group on platinum(II) diimine
complexes with substituted arylethynyl ligands, which
demonstrate that replacement of a phenyl H with CH3, F, or
OCH3 caused significant deviations in both the excited-state
energies and emission lifetimes.4 Because of the technolog-
A
series of complexes of the form of trans-
[Cr(cyclam)(CCC6H4R)2]OTf [where R ) H (1), CH3 (2),
or CF3 (3) in the para position] have been prepared by a
method similar to that reported by Berben and Long6 for
the preparation of [(Me3tacn)Cr(CCH)3] (where Me3tacn )
N,N′,N′′-trimethyl-1,4,7-triazacyclononane). For example, 1
was prepared as follows: Under an argon atmosphere,
C6H5CCLi was prepared by the addition of n-BuLi (0.54 mL
of a 2.5 M solution in hexanes, 1.35 mmol) to a solution of
phenylacetylene (0.146 g, 1.43 mmol) in anhydrous tetrahy-
drofuran (THF; 8 mL) chilled to -78 °C. The mixture was
allowed to warm to room temperature and was stirred for
an additional
1
h. Upon the addition of
[Cr(cyclam)(OTf)2]OTf7 (0.250 g, 0.357 mmol), the color
of the mixture changed from pale yellow to dark brown and
the contents was stirred for an additional 1 h. The reaction
was then quenched with 5 drops of water, and the contents
of the flask was eluted through a 3 cm plug of silica gel (in
a 30 mL fritted glass funnel) using 30% (v/v) CH3CN in
CH2Cl2 (ca. 100 mL). After removal of the solvent, the oily
*
To whom correspondence should be addressed. E-mail:
paul.wagenknecht@furman.edu.
†
Furman University.
Clemson University.
‡
(1) (a) Balzani, V. Photochem. Photobiol. Sci. 2003, 2, 459–476. (b)
Callan, J. F.; de Silva, A. P.; Magri, D. C. Tetrahedron 2005, 61,
8551–8588.
(2) (a) Schlicke, B.; Belser, P.; De Cola, L.; Sabbioni, E.; Balzani, V.
J. Am. Chem. Soc. 1999, 121, 4207–4214. (b) Welter, S.; Salluce, N.;
Belser, P.; Groeneveld, M.; De Cola, L. Coord. Chem. ReV. 2005,
249, 1360–1371. (c) Polson, M.; Chiorboli, C.; Fracasso, S.; Scandola,
F. Photochem. Photobiol. Sci. 2007, 6, 438–443.
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N. Inorg. Chem. 2002, 41, 1229–1235.
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137.
(5) (a) Long, N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2003, 42,
2586–2617. (b) Manna, J.; John, K. D.; Hopkins, M. D. AdV.
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1982, 47, 89–124.
(6) Berben, L. A.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 11588–
11589.
(7) [Cr(cyclam)(OTf)2]OTf was prepared from cis/trans-[Cr(cyclam)Cl2]Cl
by a method analogous to that found in the literature. Wright-Garcia,
K.; Basinger, J.; Williams, S.; Hu, C.; Wagenknecht, P. S.; Nathan,
L. C. Inorg. Chem. 2003, 42, 4885–4890.
11452 Inorganic Chemistry, Vol. 47, No. 24, 2008
10.1021/ic801376p CCC: $40.75  2008 American Chemical Society
Published on Web 11/08/2008

COMMUNICATION
residue was dissolved in a minimum volume of 10% (v/v)
H2O in THF (ca. 6 mL) and the solution was slowly added
to cold diethyl ether (ca. 200 mL), resulting in a pale-yellow
precipitate. Reprecipitation of this product by the same
method yielded 107 mg (50%) of analytically pure 1.8
Derivatives 29 and 310 were prepared in analogous procedures
starting with 4-ethynyltoluene and 4-ethynyl-R,R,R-trifluo-
rotoluene, respectively. One interesting feature of this
reaction is that arylethynyllithium preferentially reacts at the
metal site in the presence of the secondary amine protons.
This is presumably because of the relatively low pKa value
of phenylacetylene (20),11 whereas secondary amines have
pKa values above 30. Apparently, ligation does not drop the
pKa value of the cyclam amines below those of the
arylacetylenes.
Another feature of the synthesis is that the reaction results
in only the trans- ethynyl complex. This is not surprising
given that the isoelectronic trans-[Cr(cyclam)(CN)2]+ is
produced stereochemically pure even though the starting
material is a cis/trans mixture of [Cr(cyclam)Cl2]+.12 Evi-
dence for the trans geometry in the ethynyl complexes is
provided using IR spectroscopy for complexes 1-3 and
X-ray diffraction data for 1. The patterns of vibrational bands
in the cyclam N-H bending region (∼900 cm-1) have been
shown by Poon to be indicative of either cis or trans
geometry for transition-metal cyclam complexes. Specifi-
cally, the trans complexes display a doublet near 900 cm-1,
whereas for the less symmetric cis complexes, the peak is
split into at least three peaks.13 Compounds 1-3 all show a
vibrational doublet in this region. In addition, the X-ray
structure of a yellow monoclinic crystal obtained by diffusion
of a 9:1 Et2O/hexanes solution into a concentrated solution
of [Cr(cyclam)(CCPh)2]OTf in acetonitrile demonstrates the
trans geometry (Figure 2).14 To establish that there had not
simply been a selective crystallization of the trans isomer
from a cis/trans mixture, powder X-ray diffraction analysis
was performed on a bulk sample, resulting in a pattern that
Figure 2. Thermal ellipsoid plot (50% probability level) of the complex
cation of [Cr(cyclam)(CCC6H5)2]OTf. Hydrogen atoms are omitted for the
sake of clarity. This is one of two crystallographically distinct complexes
in the unit cell (the other is on an inversion center). Selected interatomic
distances (Å) and angles (deg): Cr-C(11) 2.079(4), Cr-C(19) 2.067(4),
C(11)-C(12) 1.214(5), C(19)-C(20) 1.218(5); C(11)-Cr-C(19)
179.19(15).
closely matches the pattern predicted from the observed
crystal structure.15
The Cr-N bond lengths and the N-Cr-N bond angles
are comparable to related cyclam complexes.16 The average
Cr-C and Ct C bond lengths for the arylethynylchro-
mium(III) complexes reported herein are each ∼0.02 Å
longer than the values for [(Me3tacn)Cr(CCH)3] reported by
Berben and Long.6 Although Ct C bond lengths have been
used to suggest the extent of π interactions in metal alkynyl
complexes, Manna et al. have demonstrated that there is a
low sensitivity of the Ct C bond length to differences in
metal alkynyl bonding.5b Because the Ct C bond lengths for
trans-[Cr(cyclam)(CCPh)2]OTf fall within the normal range5b
for metal alkynyl complexes, no significant conclusions about
the type of bonding can be drawn from the structural data.
Vibrational data have also been used to determine the
extent of π interactions in metal complexes. Raman spec-
troscopy on complexes 1-3 reveal ν(Ct C) values of 2077,
2079, and 2086 cm-1, respectively. For comparison, we have
measured ν(Ct C) by Raman spectroscopy on neat samples
of the respective parent acetylenes: ethynylbenzene, 2110
cm-1; 4-ethynyltoluene, 2108 cm-1; 4-ethynyl-R,R,R-trif-
luorotoluene, 2115 cm-1. Thus, there is a common shift of
31 ( 2 cm-1 to lower energy for ν(Ct C) of the chro-
mium(III) complex versus the free parent acetylene. These
data suggest that there is no significant M f CCR π back-
bonding. If there were significant back-bonding, one would
have expected the significantly more electron-withdrawing
Ct CC6H4CF3 ligand17 to have a noticeably larger shift of
ν(Ct C) (relative to the parent acetylene) to lower energy
than the Ct CC6H4CH3 ligand. Thus, the ∼31 cm-1 shift to
(8) Characterization of 1. UV-vis (MeCN): λmax (ꢀM) 352 (528), 360
(554), 373 (550), 387 (560), 400 (597), 414 (380), 432 (343). Anal.
Calcd (found) for C27H34CrF3N4O3S: C, 53.72 (53.61); H, 5.68 (5.75);
N, 9.28 (9.13).
(9) Characterization of 2. UV-vis (MeCN): λmax (ꢀM) 355 (620), 365
(635), 376 (629), 380 (631), 392 (673), 404 (703), 421 (443), 436
(400). Anal. Calcd (found) for C29H38CrF3N4O3S: C, 55.14 (55.34);
H, 6.06 (6.09); N, 8.87 (8.87).
(10) (a) Characterization of 3. UV-vis (MeCN): λmax (ꢀM) 348 (452), 382
(358), 394 (363), 406 (369), 424 (258), 438 (224). Anal. Calcd (found)
for C29H32CrF9N4O3S: C, 47.09 (47.12); H, 4.36 (4.36); N, 7.57 (7.45).
(b) For this synthesis,
a 2:1 ratio of LiCCC6H4CF3 to
[Cr(cyclam)(OTf)2]OTf was used.
(11) Lin, A. C.; Chiang, Y.; Dahlberg, D. B.; Kresge, A. J. J. Am. Chem.
Soc. 1983, 105, 5380–5386.
(12) Kane-Maguire, N. A. P.; Bennet, J. A.; Miller, P. K. Inorg. Chim.
Acta 1983, 76, L123–L125.
(13) Poon, C. K. Inorg. Chim. Acta 1971, 5, 322–324.
(14) (a) Data were collected using a Rigaku AFC8S diffractometer equipped
with a CCD detector and using Mo KR radiation (λ ) 0.710 73 Å).
The structures were solved by direct methods and Fourier difference
maps, and refinements were performed by full-matrix least squares
on F2.14b Crystal and structure refinement parameters:
C27H34CrF3N4O3S, M ) 603.64, T ) 153(2) K, P2(1)/c, a ) 15.620(3)
Å, b ) 11.886(2) Å, c ) 22.794(5) Å, ꢀ ) 96.88(3)°, V ) 4201.7(15)
Å3, Z ) 6, Dcalcd ) 1.431 Mg/m3, µ ) 0.54 mm-1, R [F2 > 2σ(F2)]
) 0.070, Rw (F2) ) 0.206. (b) Sheldrick, G. M. SHELXTL, version
6.10. Acta Crystallogr. 2008, A64, 112-122.
(15) Mercury: visualization and analysis of crystal structures. Macrae, C. F.;
Edgington, P. R.; McCabe, P.; Pidcock, E.; Shields, G. P.; Taylor,
R.; Towler, M.; van de Streek, J. J. Appl. Crystallogr. 2006, 39, 453–
457.
(16) Wagenknecht, P. S.; Hu, C.; Ferguson, D.; Nathan, L. C.; Hancock,
R. D.; Whitehead, J. R.; Wright-Garcia, K.; Vagnini, M. T. Inorg.
Chem. 2005, 44, 9518–9526.
(17) Computational data suggest that 4-ethynyl-R,R,R-trifluorotoluene has
a pKa value of more than 7 units lower than either ethynylbenzene or
4-ethynyltoluene. Bo¨hm, S.; Par´ık, P.; Exner, O. New J. Chem. 2006,
30, 384–391.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11453

COMMUNICATION
excited state like trans-[Cr(cyclam)(CN)2]+. However, the
steady-state room temperature emission spectra for these
ethynyl complexes are broad and structureless, with emission
maxima red-shifted relative to the 650-710 nm region
21
2
observed for most Eg emitters. Such spectra are charac-
teristic of 2T1g f 4A2g (Oh) transitions.22 Typical 2T1g 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 char-
acterized as having axial ligands that are both σ and π
donors.23 Assignment of the emission as originating from
the 2T1g excited state only reflects an asymmetry of π effects
(π-donating or π-withdrawing) between the axial and
equatorial ligands.24 However, the Raman data and literature
precedence5,19 would suggest that these arylethynyl ligands
are acting as π donors toward the electron-poor chro-
mium(III). The excited-state lifetimes of these complexes
are solvent-dependent and also sensitive to the presence of
oxygen (Table 1). The oxygen sensitivity stands in contrast
to trans-[Cr(cyclam)(CN)2]+11 but is consistent with observa-
tions for trans-[Cr(cyclam)(NCS)2]+.25 Such oxygen sensi-
tivity for the lifetimes of chromium(III) complexes has been
attributed to the presence of π electron density on the
coordinated ligand.26
Figure 3. Absorption spectra taken in CH3CN for 1 (- - -), 2 (s), 3 (· · ·),
and trans[Cr(cyclam)(CN)2]+ (thick s) in an aqueous solution.
Table 1. Photophysical Data for trans-[Cr(cyclam)(CCC6H4R)2]+
b
emission λmax (nm)a
τ (µs)d
,
CH3CN
H2Oe
CH3CN
H2Oe
1
2
3
748
749
748
745
746
746
6 (225)b
5 (259)c
6 (203)b
37 (293)b
30 (285)c
32 (313)b
a The excitation spectrum for each complex closely resembles its UV-vis
b
spectrum, confirming that emission is not due to an impurity. λex ) 380
nm. c λex ) 440 nm. d Air (degassed). e An ion-exchange resin, Dowex 2-X8
Chloride Form 20-50 mesh, was used to assist the solubility of the
complexes in H2O.
lower energy relative to the free acetylenes is unlikely to be
due to π back-bonding and can likely be attributed to the
Mδ+-Cδ- bond polarization, as suggested by Manna et al.5b
The UV-vis absorption spectra of the ethynyl complexes
between 320 and 500 nm [typical region for transitions from
the 4A2g ground state to the 4T1g and 4T2 g (Oh) excited states]
demonstrate extinction coefficients ranging from 200 to 700
M-1 cm-1 (Figure 3 and refs 8-10). The molar extinction
coefficients are much larger than typical d-d transitions for
centrosymmetric species. The spectrum of the isoelectronic
trans-[Cr(cyclam)(CN)2]+ is shown for comparison and has
extinction coefficients of approximately 60 M-1 cm-1.11 Such
large extinction coefficients along with the fine structure
apparent in the absorption spectrum suggest possible intensity
stealing from a proximal charge-transfer band,18 as has been
observed for other alkynyl complexes of chromium(III).19
The observed fine structure and large extinction coefficients
are absent in the analogous alkylethynyl complexes (the
spectra of which look very similar to that of trans-
[Cr(cyclam)(CN)2]+),20 suggesting that these features are
associated with the aromaticity of the arylethynyl ligands.
Low-temperature absorption spectroscopy will be necessary
to correlate the fine structure with vibrational data.
In conclusion, we have demonstrated a simple and general
method for preparing arylethynyl complexes of the CrIII(cy-
clam) moiety. Significant differences between these com-
plexes and trans-[Cr(cyclam)(CN)2]+ are likely due to
differences in the ligand π characteristics. Current investiga-
tions include expansion of the range of complexes to the
related alkylethynyl complexes, along with full photophysical
characterization of the complexes.
Acknowledgment. This material is based upon work
supported by the National Science Foundation under Grant
CHE-709562 and by the Henry Dreyfus Teacher Scholar
Award Program.
Supporting Information Available: X-ray data for 1 in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org and from the Cambridge Crystallographic Data
Centre (CCDC 694282), obtained, upon request, from the Director,
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, U.K.
IC801376P
Photophysical data for complexes 1-3 are summarized
in Table 1. Given the comparisons between CN- and the
isoelectronic ethynyl ligands, we expected these arylethynyl
(21) Fucaloro, A. F.; Forster, L. S.; Glover, S. G.; Kirk, A. D. Inorg. Chem.
1985, 24, 4242–4246.
(22) (a) Forster, L. S.; Rund, J. V.; Fucaloro, A. F. J. Phys. Chem. 1984,
88, 5012–5017. (b) Vagnini, M. T.; Rutledge, W. C.; Hu, C.;
VanDerveer, D. G.; Wagenknecht, P. S. Inorg. Chim. Acta 2007, 360,
1482–1492.
2
complexes to demonstrate emission from their Eg (Oh)
(18) Fenske, R. F. J. Am. Chem. Soc. 1967, 89, 252–256.
(19) Berben, L. A.; Long, J. R. Inorg. Chem. 2005, 44, 8459–8468.
(20) The alkylethynyl complex trans-[Cr(cyclam)(CCC6H11)2]OTf (where
C6H11 ) cyclohexyl) has been prepared and has extinction coefficients
in the d-d region below 40 M-1 cm-1. Wagenknecht, P. S.;
Turlington, C. R.; Grisenti, D. L. Furman University, Greenville, SC,
unpublished work, 2008.
(23) (a) Forster, L. S.; Moensted, O. J. Phys. Chem. 1986, 90, 5131–5134.
(b) Kane-Maguire, N. A. P.; Wallace, K. C.; Speece, D. G. Inorg.
Chem. 1986, 25, 4650–4654.
(24) Hoggard, P. E. Z. Naturforsch. 1981, 36A, 1276–1288.
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11454 Inorganic Chemistry, Vol. 47, No. 24, 2008