Synthesis of trans bis-alkynyl complexes of Co(III) supported by a tetradentate macrocyclic amine: A spectroscopic, structural, and electrochemical analysis of π-interactions and electronic communication in the CCMCC structural unit

Article abstract of DOI:10.1016/j.ica.2013.11.036

Alkynyl complexes of the type trans-[Co(cyclam)(CCR)₂]OTf have been prepared and characterized by UV-Vis spectroscopy, ¹H NMR, vibrational spectroscopy (infrared and Raman), and cyclic voltammetry. Where appropriate the data is compared to the corresponding Cr(III) and Rh(III) complexes. Though the arylalkynyl ligands have been shown to act as π-donors for the corresponding Cr(III) complexes, vibrational spectroscopy suggests that the π-interactions between the arylalkynyl ligands and Co(III) are quite weak, and that the more electron withdrawing trifluoropropynyl ligand likely behaves as a weak π-acceptor toward Co(III). X-ray crystal structures for trans-[Co(cyclam)(CCCF₃)₂]OTf and trans-[Cr(cyclam) (CCCF₃)₂]OTf are also reported and analysis of the MC and CC bond lengths are consistent with this understanding of the trifluoropropynyl ligand. Cyclic voltammetry of the trans-[Co(cyclam)(CCR)₂]OTf complexes demonstrates that when R = C₆H₅ or p-C ₆H₄CH₃, the Co^III/II reduction wave is chemically irreversible. However, when R = p-C₆H ₄CF₃, p-C₆H₄CN, or CF₃, the Co^III/II reduction wave is chemically reversible. This suggests that the more electron withdrawing alkynyl ligands become π-acceptors toward the reduced form of cobalt. Finally, the ferrocenyl capped trans-[M(cyclam) (CCFc)₂]OTf complexes (where M = Co(III) and Rh(III)) were prepared and studied. Cyclic voltammetry shows only a single 2e^- wave for the ferrocenyl termini, indicating little to no electronic communication through the organometallic backbone.

Full text of DOI:10.1016/j.ica.2013.11.036

Accepted Manuscript
Synthesis of trans Bis-alkynyl Complexes of Co(III) Supported by a Tetraden-
tate Macrocyclic Amine: A Spectroscopic, Structural, and Electrochemical
Analysis of π-Interactions and Electronic Communication in the C≡C-M-C≡C
Structural Unit
Parth U. Thakker, Roberto G. Aru, Chivin Sun, William T. Pennington, Adam
M. Siegfried, Elyse C. Marder, Paul S. Wagenknecht
PII:
S0020-1693(13)00649-X
http://dx.doi.org/10.1016/j.ica.2013.11.036
ICA 15756
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 October 2013
26 November 2013
27 November 2013
Please cite this article as: P.U. Thakker, R.G. Aru, C. Sun, W.T. Pennington, A.M. Siegfried, E.C. Marder, P.S.
Wagenknecht, Synthesis of trans Bis-alkynyl Complexes of Co(III) Supported by a Tetradentate Macrocyclic
Amine: A Spectroscopic, Structural, and Electrochemical Analysis of π-Interactions and Electronic Communication
in the C≡C-M-C≡C Structural Unit, Inorganica Chimica Acta (2013), doi: http://dx.doi.org/10.1016/j.ica.
2013.11.036
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Synthesis of trans Bis-alkynyl Complexes of Co(III)
Supported by a Tetradentate Macrocyclic Amine: A
Spectroscopic, Structural, and Electrochemical
Analysis of π-Interactions and Electronic
Communication in the C≡C-M-C≡C Structural Unit
Parth U. Thakker,† Roberto G. Aru,† Chivin Sun,† William T. Pennington,‡
Adam M. Siegfried,‡ Elyse C. Marder,† and Paul S. Wagenknecht†*
†Department of Chemistry, Furman University, Greenville, SC 29613
‡Department of Chemistry, Clemson University, Clemson, SC 29634

Abstract
Alkynyl complexes of the type trans-[Co(cyclam)(CCR)2]OTf have been
prepared and characterized by UV-Vis spectroscopy, 1H NMR, vibrational
spectroscopy (infrared and Raman), and cyclic voltammetry. Where appropriate the
data is compared to the corresponding Cr(III) and Rh(III) complexes. Though the
arylalkynyl ligands have been shown to act as π-donors for the corresponding
Cr(III) complexes, vibrational spectroscopy suggests that the π-interactions
between the arylalkynyl ligands and Co(III) are quite weak, and that the more
electron withdrawing trifluoropropynyl ligand likely behaves as a weak π-acceptor
toward Co(III). X-ray crystal structures for trans-[Co(cyclam)(CCCF3)2]OTf and
trans-[Cr(cyclam)(CCCF3)2]OTf are also reported and analysis of the M-C and C≡C
bond lengths are consistent with this understanding of the trifluoropropynyl ligand.
Cyclic voltammetry of the trans-[Co(cyclam)(CCR)2]OTf complexes demonstrates
that when R = C6H5 or p-C6H4CH3, the CoIII/II reduction wave is chemically
irreversible. However, when R = p-C6H4CF3, p-C6H4CN, or CF3, the CoIII/II reduction
wave is chemically reversible. This suggests that the more electron withdrawing
alkynyl ligands become π-acceptors toward the reduced form of cobalt. Finally, the
ferrocenyl capped trans-[M(cyclam)(CCFc)2]OTf complexes (where M = Co(III) and
Rh(III)) were prepared and studied. Cyclic voltammetry shows only a single 2e–
wave for the ferrocenyl termini, indicating little to no electronic communication
through the organometallic backbone.
2

1. Introduction
The chemistry of transition metal σ-alkynyl complexes has been explored for
use in applications such as molecular wires [1-5], molecular magnetism [5 -9], and
non-linear optical materials [10]. Since the first reports of trans-bis(alkynyl) metal
complexes of macrocyclic tetraamine ligands [11-13] several groups have explored
this structural motif as a potential building block for molecular wires [3-4] and
molecular magnets [8-9], in part due to this framework facilitating rigid-rod type
applications. Our own interest in alkynyl complexes began with a desire to compare
the emissive behavior of the trans-[M(cyclam)(CCR)2]+ (cyclam = 1, 4, 8, 11-
tetraazacyclotetradecane, M = Cr3+ and Rh3+) cations with their isoelectronic
dicyano analogues. This led us to develop a streamlined synthesis of the trans-
[M(cyclam)(CCR)2]+ complexes [13-14] that has now been applied successfully to
the synthesis of Cr3+ [4, 8, 13-15], Rh3+ [14-15], Fe3+ [3], and now Co3+ (herein)
complexes with a host of alkynyl ligands.
Particularly important in the design of systems for the applications named
above is an understanding of the bonding interaction between the metal center and
the alkynyl ligand [16]. In particular, strong π-interactions may facilitate
communication between transition metal centers linked through an alkynyl bridging
ligand. By analogy with the CN– ligand [17], the alkynyl ligands were conventionally
thought be strong σ-donors and modest π-acceptors (weaker than CO). However,
this simple analogy does not appear to apply in all cases [16]. For example,
spectroscopic measurements and molecular orbital calculations for
3

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 [18-19] Likewise, electronic spectroscopy
and density functional theory calculations on [M(CCSiMe3)6]n–, where M = Cr(III),
Fe(II), and Co(III) support the model that the alkynyl ligand acts as a π-donor in
+
these cases [20]. We have recently shown for a series of trans-M(cyclam)(CCR)2
that RCC→M π-donation plays a significant role in the M-CCR bond for the Cr(III)
complexes, but much less of a role (if any) for the corresponding Rh(III) complexes
[14]. Thus, it is clear that the metal plays a major role in determining whether the
alkynyl ligand behaves as a π-donor or π-acceptor.
+
Herein we report the synthesis of a series of trans-Co(cyclam)(CCR)2
complexes along with their characterization by infrared, Raman, and UV-Vis
spectroscopy and by electrochemistry. In addition, we report single crystal X-ray
structures for trans-M(cyclam)(CCCF3)2+ (where M = Co(III) and Cr(III)). The data
suggest that the alkynyl ligands act as weak π-donors in the case of the Cr(III)
complexes but not for the low spin d6 Co(III) complexes. Co(III) complexes are also
unlikely candidates for π-backbonding with alkynes due to their electron deficient
nature. However, we observe some evidence for π-backbonding when the alkynyl
ligand is trifluoropropynyl. We also have tested the ability of the Co(III) and Rh(III)
complexes to act as organometallic molecular wires and observe no significant
communication through the -C≡C-M-C≡C- structural motif.
2. Experimental Section
2.1 Materials and Methods.
4

Tetrahydrafuran (THF) was dried and degassed using an Innovative
Technology Inc. solvent purification system. Lithium diisopropylamide (2.0 M
solution in THF and heptane) and n-butyllithium (2.5 M solution in hexanes) were
purchased from Aldrich. Infrared spectra were recorded on solid samples using a
Perkin-Elmer Spectrum 100 series FT-IR spectrometer equipped with an ATR
accessory. UV-Vis absorption spectra were collected using a Cary-50
spectrophotometer. NMR spectra were obtained using a Varian INOVA 400-MR
spectrometer. Raman spectra were recorded using either a Thermo Scientific
Nicolet 6700 FT-IR with an NXR FT-Raman module or a Spectra-Physics model 2017
with an argon ion laser, a Spex 750m monochrometer, and a Symphony CCD
Detector from Horiba. Cyclic voltammograms were recorded using a BAS εpsilon
electrochemical work station and a BASi cell stand. Cell resistance was measured by
the instrument and iR compensation applied accordingly. Parameters for iR
compensation were evaluated on solutions of ferrocene to avoid over compensation.
Elemental analyses were performed by Midwest Microlabs in Indianapolis, IN.
2.2 Syntheses
2.2.1 General synthetic methods.
The complexes trans-[Co(cyclam)Cl2]Cl [21], trans-[Co(cyclam)(CN)2]Cl [22],
trans-[Co(cyclam)(CCC6H5)Cl]Cl [9]; and trans-[Cr(cyclam)(CCCF3)2]OTf, 1e, and
trans-[Co(cyclam)(CCCF3)2]OTf, 2e [15], were prepared according to the literature
procedures. trans-[Co(cyclam)(CN)2]PF6 was prepared by anion metathesis in
water from the chloride salt. Alkynyl complexes of the type trans-
5

[M(cyclam)(CCR)2]OTf were prepared using a modification of the literature
procedure [14].
2.2.2 trans-[Co(cyclam)(OTf)2]OTf
A slight modification of the literature synthesis of [Co(en)2(OTf)2]OTf was
followed [23]. A solution of trans-[Co(cyclam)Cl2]Cl (1.0 g, 2.73 mmol) in
trifluoromethanesulfonic acid (10 g) was heated at 100°C for 3 hours under argon.
The resulting mixture was then added to 200 mL of anhydrous diethyl ether that
was being stirred vigorously. A light green precipitate formed which was then
collected by vaccum filtration. The solid was washed with anhydrous diethyl ether
(20 mL), dried under vacuum, and stored in a desiccator until further use. (Yield =
1.59 g, 82 %) Anal. Calcd (found) for CoC13N4F9S3O9H24: C, 22.10 (21.59); N, 7.93
(7.51); H, 3.42 (3.38); Cl, 0 (none found).
2.2.3 trans-[Co(cyclam)(CCC6H5)2]OTf (2a).
Using the general procedure [14], THF (5 mL), trans-[Co(cyclam)(OTf)2]OTf
(200 mg, 0.283 mmol), phenylacetylene (65 μL, 0.592 mmol), and n-butyllithium
(480 μL, 1.20 mmol) resulted in 0.104 g (60 %) of analytically pure trans-
[Co(cyclam)(CCC6H5)2]OTf. Anal. Calcd. (found) for CoC27N4F3SO3H34 : C, 53.13
(52.84); N, 9.18 (9.25); H, 5.61 (5.54). ESI-MS: intense parent ion (M+) at m/z value
of 461.51. IR: νa(C≡C) 2098 cm-1 Raman: νs(C≡C) 2102 cm-1. 1H NMR (400 MHz,
.
CD3CN) δ 1.33 (q, 2H), 1.86 (d, 2H), 2.54 (m, 12H), 2.83 (q, 4H), 4.19 (b, 4H), 7.23 (t,
2H), 7.34 (t, 4H), 7.51 (d, 4H). UV-Vis CH3CN: λMAX (ε) 272 (53,069), 354(sh) (450),
452 (106).
6

2.2.4 trans-[Co(cyclam)(p-CCC6H4CH3)2]OTf (2b)
Using the general procedure [14], THF (5 mL), trans-[Co(cyclam)(OTf)2]OTf
(200 mg, 0.283 mmol), 4-ethynyltoluene (78 μL, 0.615 mmol), and n-butyllithium
(480 μL, 1.20 mmol) resulted in 0.120 g (66 %) of analytically pure trans-
[Co(cyclam)(CCC6H4CH3)2]OTf. Anal. Calcd. (found) for CoC29N4F3SO3H38 : C, 54.54
(54.92); N, 8.77 (8.78); H, 5.99 (6.12). ESI-MS: intense parent ion (M+) at m/z value
of 489.56. IR: νa(C≡C) 2102 cm-1. Raman: νs(C≡C) 2104 cm-1. 1H NMR (400 MHz,
CD3CN) δ 1.30 (q, 2H), 1.86 (d, 2H), 2.33 (s, 6H), 2.50 (m, 12H), 2.82 (q, 4H), 4.16 (b,
4H), 7.13 (d, 4H), 7.40 (d, 4H). UV-Vis CH3CN: λMAX (ε) 272 (64,200), 455 (115).
2.2.5 trans-[Co(cyclam)(p-CCC6H4CF3)2]OTf (2c).
Using the general procedure [14], THF (5 mL), trans-[Co(cyclam)(OTf)2]OTf
(200 mg, 0.283 mmol), 4-ethynyl-α,α,α-trifluorotoluene (91 μL, 0.56 mmol), and n-
butyllithium (480 μL, 1.20 mmol) resulted in 0.143 g (68 %) of analytically pure
trans-[Co(cyclam)(CCC6H4CF3)2]OTf. Anal. Calcd. (found) for CoC29N4F9SO3H32 : C,
46.66 (46.61); N, 7.50 (7.47); H, 4.32 (4.30). ESI-MS: intense parent ion (M+) at m/z
value of 597.51. IR: νa(C≡C) 2106 cm-1. Raman: νs(C≡C) 2111 cm-1. 1H NMR (400
MHz, CD3CN) δ 1.30 (q, 2H), 1.85 (d, 2H), 2.46 (m, 8H), 2.60 (m, 4H), 2.75 (q, 4H),
4.18 (b, 4H), 7.66 (s, 8H). UV-Vis CH3CN: λMAX (ε) 289 (52,000), 447 (120).
2.2.6 trans-[Co(cyclam)(p-CCC6H4CN)2]OTf (2d)
8

Using the general procedure [14], THF (5 mL), trans-[Co(cyclam)(OTf)2]OTf
(200 mg, 0.283 mmol), 4-ethynylbenzonitrile (72 mg, 0.566 mmol), and lithium
diisopropylamide (600 μL, 1.20 mmol) resulted in a crude product that was purified
using silica gel column chromatography (70/30 CH2Cl2/CH3CN, 20 cm x 2.5 cm).
Yield: 0.103 g (55 %). Anal. Calcd. (found) for CoC29N6F3SO3H32 : C, 52.73 (52.92); N,
12.72 (12.77); H, 4.88 (4.93). ESI-MS: intense parent ion (M+) at m/z value of 511.53.
IR: νa(C≡C) 2104 cm-1. Raman: νs(C≡C) 2108 cm-1. 1H NMR (400 MHz, CD3CN)
δ 1.31 (q, 2H), 1.86 (d, 2H), 2.48 (m, 8H), 2.61 (m, 4H), 2.78 (q, 4H), 4.19 (b, 4H),
7.63 (dt, 4H), 7.69 (dt, 4H). UV-Vis CH3CN: λMAX (ε) 206 (52,000), 251 (24,000), 308
(79,000), 441 (157).
2.2.7 trans-[Co(cyclam)(CCFc)2]OTf (2f)
Using the general procedure [14], THF (7 mL), trans-[Co(cyclam)(OTf)2]OTf
(200 mg, 0.283 mmol), ethynylferrocene (119 mg, 0.57 mmol), and n-butyllithium
(460 µL, 1.15 mmol) resulted in 0.110 g (47 %) of analytically pure trans-
[Co(cyclam)(CCFc)2]OTf. Anal Calcd. (found) for CoFe2C35N4F3SO3H42 : C, 50.81
1
(50.52); N, 6.78 (6.48); H, 5.12 (5.16). IR: νa(C≡C) 2114 cm-1. H NMR (400 MHz,
CD3CN) δ 1.29 (q, 2H), 1.92 (d, 2H), 2.54 (m, 12H), 2.90 (q, 4H), 4.04 (b, 4H), 4.15 (s,
4H), 4.23 (s, 10H), 4.40 (d, 4H). UV-Vis CH3CN: λMAX (ε) 220 (77,000), 275 (26,000),
306 (21,000), 446 (766).
2.2.8 trans-[Rh(cyclam)(CCFc)2]OTf (3f)
The general procedure [14] using THF (7 mL), trans-[Rh(cyclam)Cl2]Cl (100
mg, 0.244 mmol), ethynylferrocene (103 mg, 0.490 mmol), and n-butyllithium (410
9

µL, 1.0 mmol) was applied, followed by a modified purification. The reaction
mixture was first eluted through a silica gel plug with 100 mL of 50/50
CH3CN/CH2Cl2 followed by 100 mL of pure CH3CN. The solvent was removed from
the second fraction resulting in 180 mg of crude chloride salt which was converted
to the triflate by metathesis with NaOTf in acetone. The crude product was washed
with water resulting in 0.127 g (71%) of analytically pure trans-
[Rh(cyclam)(CCFc)2]OTf•2H2O. Anal Calcd. (found) for RhFe2C35N4F3SO3H42•2H2O:
1
C, 46.38 (46.88); N, 6.18 (6.21); H, 5.12 (4.82). IR: νa(C≡C) 2110 cm-1. H NMR (400
MHz, CD3CN) δ 1.46 (q, 2H), 2.10 (d, 2H), 2.65 (m, 4H), 3.07 (q, 12H), 4.11 (s, 4H),
4.18 (s, 10H), 4.29 (s, 4H), 4.34 (b, 4H). UV-Vis CH3CN: λMAX (ε) 230 (36,000), 275
(18,000), 447 (634).
2.2.9 trans-[Co(cyclam)(CCC6H5)(CN)]PF6
An oven-dried, two-neck, round-bottom flask (100mL) was charged with a
stir bar, a filtered aqueous (50 mL) solution of trans-[Co(cyclam)(CCC6H5)Cl]Cl (308
mg, 0.714 mmol) and sodium cyanide (86 mg, 1.8 mmol). The reaction mixture was
heated at reflux for 4 hours during which the color changed from red to yellow. The
solution was then cooled to room temperature and the solvent volume was reduced
to 5 mL. The mixture was then cooled overnight and filtered to remove dark
impurities. To the filtrate was added an aqueous solution of ammoium
hexafluorophosphate and the solid was collected via vacuum filtration and washed
with diethyl ether (3 x 5 mL). The precipitate was eluted down a column of alumina
(15 x 1 in) using a 50:50 mixture of CH3CN/CH2Cl2. The yellow band was collected
10

and evaporated to dryness. Yield: 0.254g (66 %). ESI-MS: intense parent ion (M+) at
m/z value of 386.40. IR: νa(C≡C) 2129 cm–1. Anal Calcd. (found) for
1
CoC19N5F6PH29•H2O : C, 41.54 (41.75); N, 12.75 (12.58); H, 5.69 (5.27). H NMR (400
MHz, CD3CN) 1.34 (q, 2H), 1.89 (d, 2H), 2.52 (m, 12H), 2.83 (q, 4H), 4.27 (b, 4H), 7.25
(t, 1H), 7.34 (t, 2H), 7.51 (d, 2H). UV-Vis CH3CN: λMAX (ε) 218 (28,400), 254 (25,600),
264 (sh) (22,100), 356 (214), 445 (125).
2.3 X-ray structure determinations
Crystals of 1e and 2e were grown by slow evaporation of solvent from a
concentrated acetonitrile solution. All single crystal measurements were performed
on a Rigaku AFC8S diffractometer with a Mercury CCD detector at 173 ± 2 K, with
graphite-monochromated Mo Kα radiation ( λ = 0.71073 Å). Data were collected to
a maximum 2θ value of 52.8º in 0.5º oscillations (in ω) with two exposures (to
identify detector anomalies) of 15s (1e) or 30s (2e). Corrections for Lorentz and
polarization effects and absorption [24] were applied to the data. The structures
were solved by direct methods and refined by using full-matrix least-squares
techniques. The triflate anion is disordered about an inversion center, and an
acetonitrile solvent molecule shares occupancy with the anion. The trifluoromethyl
groups of the alkynyl ligands of the cation were rotationally disordered in both
compounds, with a major component occupancy in both compounds of 82.5%. The
major component fluorine atoms were refined with anisotropic displacement
parameters and the minor component fluorine atoms were refined isotropically. All
non-hydrogen atoms other than the minor component fluorine atoms were refined
11

anisotropically. Hydrogen atoms of the cyclam ligands were included in optimized
positions with riding displacement parameters (20% greater than Ueq of host atom).
Structure solution, refinement and the calculation of derived results were
performed with the SHELXTL-Plus package of computer programs [25].
Crystallographic parameters are reported in Table 1.
Table 1.
Crystal data
1e
2e
Formula
Mw
C19H27N5O3F9SCr
628.52
C19H27N5O3F9SCo
635.45
Crystal System
Monoclinic
P21/c (No. 14)
8.9746(18)
11.229(4)
14.081(4)
107.542(15)
1353.1(7)
2
Monoclinic
P21/c (No. 14)
8.9019(16)
11.179(3)
14.0286(19)
108.570(13)
1323.4(5)
2
Space Group
a, Å
b, Å
c, Å
β, (°)
V, Å3
Z
Dcalc, g cm-3
1.54
1.60
µ, mm-1
0.592
0.822
Transmission coefficients
Reflections collected
Reflections unique (Rmerge
0.76-1.00
10827
0.83-1.00
10227
)
2759 (0.114)
1331
2693 (0.0609)
1810
Reflections observed (I>2σ(I))
a
R1
0.0769 (0.1533)
0.1484 (0.1850)
0.0656 (0.0960)
0.1323 (0.1503)
b
wR2
aR1 = Σ||Fo| - |Fc||/ Σ|Fo| for observed data; number in parentheses is for all data.
2
2 2
2 2
bwR2 = {Σ[w(Fo – Fc ) ]/Σ[w(Fo ) ]}1/2 for observed data; number in parentheses is for all data.
12

3. Results and Discussion
3.1 General synthesis and characterization
The syntheses of 1a – 1f, 3a – 3c and 3e have been previously reported using
the procedure summarized in Figure 1 [4, 13-15]. The syntheses of the
corresponding Co(III) complexes, 2a – 2f and the Rh(III) complex, 3f, are herein
reported using this method. Though Figure 1 shows the starting material and
Figure 1. General synthesis of the complexes discussed in this paper
product in the trans configuration, for the Cr(III) and Rh(III) complexes, cis/trans
mixtures of the starting material result in cis/trans product mixtures which can be
separated by taking advantage of solubility differences [14]. In the case of the
Co(III) complexes, the trans-[Co(cyclam)Cl2]Cl precursor undergoes ligand exchange
to the triflato complex, trans-[Co(cyclam)(OTf)2]OTf, with retention of
configuration. Replacement of the triflato ligands with the alkyne ligands (Figure 1)
also proceeds with retention of configuration, thus requiring no separation of cis
and trans isomers. The isomeric purity was confirmed by analysis of the CH2
14

rocking and NH bending region of the IR spectra (supplementary material)
according to the method of Poon et al [26]. The products are air stable in the solid
state and in solution, with the triflate salts demonstrating good solubility in CH3CN
and THF.
Asymmetrically substituted trans-[Co(cyclam)(CCC6H5)CN]PF6 was prepared
by ligand substitution from trans-[Co(cyclam)(CCC6H5)Cl]Cl. The 1H NMR spectra of
all Co(III) complexes are well resolved, indicating the expected low-spin d6
configuration.
UV-Vis absorption spectra for 2a – 2e are compared with that of trans-
[Co(cyclam)(CN)2]PF6, 2g (Figure 2). For 2g, excitation from the 1A1g ground state
Figure 2. UV-Vis absorption spectra comparing the charge transfer bands (left) and
the metal centered bands (right) for the Co(III) complexes, 2a – 2e, and 2g.
into the 1T1g and 1T2g (Oh) ligand field excited states is observed. Likwise, both
ligand field absorptions are observed for the trifluoropropynyl complex, 2e, though
these absorptions are slightly red shifted [15]. For the corresponding arylethynyl
complexes, 2a – 2d, the higher energy 1A1g to 1T2g absorptions are obscured by the
tails of charge transfer bands with maxima between 272 nm and 308 nm. In
15

addition, the lowest energy ligand field absorption bands of the arylethynyl
complexes are two to three times as intense as that for the dicyano analogue, 2g.
Increased intensity was also observed for the corresponding arylethynyl Cr(III)
complexes, 1a – 1d, and attributed to intensity borrowing from the charge transfer
bands [14].
A comparison of the lowest energy absorption bands in Figure 2 suggests
that the arylethynyl ligands are weaker field than either trifluoropropynyl or cyano
ligands. It is also noteworthy that the addition of an electron withdrawing group to
the aryl ring (2c = –CF3; 2d = –CN) slightly blue-shifts the lowest energy absorption
band relative to 2a and 2b. This mild increase in ligand field strength for these
Co(III) complexes might reflect a slight reduction in π-donation from the alkynyl
ligand (or possibly switching on some π-acceptor nature).
3.2 Analysis of the C≡C stretching frequencies
All of the trans bis-ethynyl complexes discussed herein have ν(C≡C) near
2100 cm–1. For such complexes, one would expect νs(C≡C) to be Raman active and
νa(C≡C) to be IR active. In fact, all of the previously studied trans bis-ethynyl Cr(III)
and Rh(III) complexes studied show intense Raman bands attributed to νs(C≡C). In
addition, whereas νa(C≡C) for the Rh(III) complexes are observable by IR
spectroscopy, for most of the Cr(III) complexes these vibrations are too weak to
observe [14]. Not surprisingly, the Co(III) complexes synthesized herein show
behavior similar to the Rh(III) complexes in having Raman observable νs(C≡C), and
16

IR observable νa(C≡C). The following discussion will focus on the Raman data so
that cross comparisons can be made between all three metals.
The C≡C vibrational frequencies have often been used to determine relative
C≡C bond strengths and make determinations regarding the nature of metal alkynyl
bonding. As has been thoroughly discussed elsewhere [14, 16], the strength of the
C≡C bond in an alkynyl complex depends both on the ionic character of the M–CCR
bond, and on the metal alkynyl π-interactions. In our previous study comparing a
set of [M(cyclam)(CCR)2]+ complexes, the vibrational data suggested that the
differences in νs(C≡C) between complexes where M = Cr3+ and Rh3+ was chiefly due
to π-interactions, not the ionic character of the bond [14]. A comparison of νs(C≡C)
values as a function of ionic radius of the metal (Table 2) further supports this
contention. Namely, complexes of the two metal ions with the most disparate
Table 2. Raman vibrational data (cm–1) for trans-[M(cyclam)(CCR)2]OTf
νs(C≡C) (∆νa)
parent
alkyne
Cr(III) 1
Co(III) 2
Rh(III) 3
r = 62 pmb
r = 55 pmb
r = 67 pmb
[M(cyclam)(CCC6H5)2]OTf
(a) 2110
2077 (-33) 2102 (-8) 2105 (-5)
2079 (-29) 2104 (-4) 2105 (-3)
2086 (-29) 2111 (-4) 2107 (-8)
[M(cyclam)(CCC6H4CH3)2]OTf (b) 2108
[M(cyclam)(CCC6H4CF3)2]OTf (c) 2115
[M(cyclam)(CCC6H4CN)2]OTf (d) 2111
[M(cyclam)(CCCF3)2]OTf
a Difference in stretching frequencies = (metal complex – parent alkyne).
b Effective ionic radius [27].
2084 (-27) 2108 (-3)
2126 (-39) 2137 (-28)
-
-
(e) 2165
effective ionic radii, Co(III) and Rh(III), and thus the largest difference in ionic
character, have nearly identical νs(C≡C) values. In contrast, the Cr(III) complexes
have lower stretching frequencies than either the Rh(III) or Co(III) complexes by
approximately 25 cm–1. This suggests that it is the d-electron count of the metal
17

that has the greatest impact on νs(C≡C) and that the ionic character of the M-C bond
is of relatively little importance. The lower energy C≡C stretch for the d3
configuration compared with that of the low spin d6 configuration is consistent with
the alkynyl ligands acting as π-donors and that the π-donation to the half-filled t2g
orbitals of the Cr(III) complexes is more pronounced than that to the filled t2g
orbitals of the Rh(III) and Co(III) complexes.
Though π-donation to Cr(III) is clearly implicated, it is unclear whether there
is any significant π-donation to the low spin d6 metals, or if the alkynyl ligands might
even be behaving as very weak π-acceptors. Thus we also prepared trans-
[Co(cyclam)(CCC6H5)X]+, where X = CN–, to compare with the literature value for the
complex where X = Cl– [9]. Given that the CoIII/II reduction wave for the cyano
complex is more negative than the chloro complex by 240 mV (supplementary
material), the Co(III) center is clearly more electron rich when X = CN–, a very potent
σ-donor. Thus, if the alkynyl ligand were acting as a π-acceptor, ν(C≡C) should be
lower for the more electron-rich cyano complex than for the chloro complex. If the
alkynyl ligand is acting as a π-donor, the opposite should be observed. We observe a
slightly higher ν(C≡C) for the complex where X = CN– (2129 cm–1) than where X = Cl–
(2125 cm–1) [9]. Though this difference is in the direction consistent with the
alkynyl ligand acting as a π-donor, the difference in these frequencies is within
experimental error, and thus suggests there are very minimal π-interactions
between the arylethynyl ligand and Co(III).
Finally, in an attempt to increase the π-acceptor nature of the alkynyl ligand,
we also prepared complexes from 4-ethynyl-α, α, α -trifluorotoluene, 4-
18

ethynylbenzonitrile, and trifluoropropyne. However, as demonstrated in Table 2,
the CF3 and CN substituents on 2c and 2d did not reduce νs(C≡C) relative to the
parent alkyne (∆ν) any more than for 2a and 2b, indicating these substituents didn’t
significantly increase the π-acceptor nature of the ligand toward Co(III). Likewise,
∆ν for 1c and 1d is in line with that found for 1a and 1b. In contrast, the
trifluoropropynyl complexes behave differently. Namely, the magnitude of ∆ν is
notably increased relative to those of the arylalkynyl complexes. In addition, this
increase is markedly greater for the Co(III) complex, 2e, than for the Cr(III)
complex, 1e, which is consistent with turning on a slight π-acceptor nature of the
ligand for these complexes. Not surprisingly, this is more accentuated for the low
spin d6 configuration than the d3 configuration. The ability of trifluoropropynyl to
act as a π-acceptor is consistent with earlier studies showing that this ligand is a
very good electronic surrogate for cyanide [15]. However, this π-interaction is
expected to be relatively weak due to the electron deficient nature of M3+ ions.
3.3 Structural data for the Co and Cr complexes
Vibrational data suggests that the arylethynyl ligands act as weak π-donors
toward Cr(III) but likely have little to no π-interactions with Co(III), and that
trifluoropropynyl may behave as a weak π-acceptor, particularly toward the low
spin d6 Co3+ complex. To further understand the metal-ligand π-interactions in the
trifluoropropynyl complexes, X-ray structures (Figure 3, and supporting
information) of the two trifluoropropynyl complexes, 1e and 2e, were determined
19

in order to analyze the M-C and C≡C bond lengths (Table 3). Compounds 1e and 2e
are isomorphous, both crystallizing in the monoclinic space group, P21/c. For each,
the complex cation is situated upon an inversion center at (1/2 1/2 1/2). The triflate
anion is disordered about an inversion center at (1/2 0 1/2), and an acetonitrile
molecule shares occupancy with the anion, sitting opposite to it across the inversion
center.
Figure 3. Thermal ellipsoid plot at the 35% probability level for the cation of trans-
[Co(cyclam)(CCCF3)2]OTf, 2e. The structure of the corresponding Cr complex, 1e, is
quite similar and appears in the supplementary material.
20

Table 3. Selected bond lengths for trans-[M(cyclam)(CCR)2]+ cations
Compound
M-C length (Å)
C≡C length (Å)
+
Cr(cyclam)(CCPh)2 1a [13]
2.073a
1.216a
+
Cr(cyclam)(CCCF3)2 1e
2.071(5)
2.001(3)
1.917(4)
1.223(7)
1.113(4)
1.217(6)
+
Co(cyclam)(CCPh)2 2a [9]
+
Co(cyclam)(CCCF3)2 2e
a The Cr does not occupy a center of inversion so these values are an average of the
two distances; for Cr-C = 2.079(4) and 2.067(4); for C≡C = 1.214(5) and 1.218(5).
A simple comparison of the trifluoropropynyl complexes of Cr(III) and
Co(III), 1e and 2e, demonstrates that the C≡C bond lengths for these two complexes
are nearly identical. One explanation would be that there is no difference in the π-
interactions for these two complexes. However, another is that weakening of the
C≡C bond in 1e due to ligand-to-metal π-donation is of similar magnitude to the
weakening caused by metal-to-ligand π back bonding in 2e. Extracting such
information requires a comparison of the M-C and C≡C bond lengths of the two
trifluoropropynyl complexes, 1e and 2e, with those of the phenylethynyl complexes,
1a and 2a. For the Cr(III) complexes, there is less than a 0.01 Å change in the Cr-C
bond length when replacing CCPh with CCCF3 (Table 3). Likewise, there is less than
a 0.01 Å change in the respective C≡C bond lengths. It is also worth noting that the
C≡C bond lengths in the parent alkynes, HCCPh (1.188 Å)[28] and HCCCF3 (1.201 Å)
[29], also differ very little (< 0.02Å). However, for the Co(III) complexes there is a
0.1 Å lengthening of the C≡C bond upon the same replacement of the CCPh ligand
with the CCCF3 ligand, and a concomitant shortening of the M-C bond by nearly 0.1Å
(Table 3). Thus, this structural data at least hints that the CCCF3 ligand is acting as a
21

weak π-acceptor for Co(III), inasmuch as this shortening occurs only for the metal
with filled t2g orbitals, and thus is consistent with the vibrational data.
3.4 Voltammetry
The phenylethynyl, 2a, and tolylethynyl, 2b, complexes show irreversible
CoIII/II reduction waves (Figure 4).
Figure 4. Cyclic voltammograms for [Co(cyclam)X2]+ complexes (2-5mM) in CH3CN
/ 0.1 M Bu4NClO4 at a glassy carbon working electrode (3.0 mm). Scan rate = 100
mV/s; reference electrode = Ag/Ag+(0.005M) in CH3CN. Under these conditions the
Fc+/0 couple appears at +0.133 V.
The irreversible reduction for 2a was first reported by Shores’ group and likely
involves ligand dissociation from the resulting substitutionally labile Co(II) [9].
Addition of electron withdrawing groups on the aryl ring (2c and 2d) results in a
22

slight anodic shift, and chemical reversibility. Likewise, the bis-trifluoropropynyl
(2e) and dicyano (2g) [15] complexes show reversible CoIII/II reduction waves that
are further anodically shifted relative to the arylethynyl complexes. To test whether
reversibility is more a function of the electron density on the metal or the nature of
the ligand, the cyclic voltammograms of trans-[Co(cyclam)(CCC6H5)Cl]+ [9] and
trans-[Co(cyclam)(CCC6H5)CN]+ were also recorded (supplementary material).
Though the CoIII/II reduction waves for these complexes are anodically shifted
relative to 2c, and 2d, they are chemically irreversible. Thus, we conclude that it is
not the net electron density on the metal but rather the nature of the ligand that
confers reversibility. Those ligands that are more capable of acting as π-acceptors
–
–
(C≡N–, C≡CCF3 , C≡CC6H4CF3 , C≡CC6H4CN–) are not as labile upon reduction to Co(II).
This suggests that, even though these ligands are not terribly good π-acceptors
toward the electron deficient Co(III), they are, not surprisingly, much more capable
π -acceptors toward Co(II). A similar conclusion was drawn from the comparison of
Rh(III) and Rh(II) alkynyl complexes of the type Rh(PP3)(CCR)n+. [30]
Because of the strong Ru(II) mediated coupling observed between the
ferrocenyl termini in trans-Ru(dppm)2(CCFc)2 [31], we have been interested in
studying related cyclam complexes (Figure 5). For trans-Ru(dppm)2(CCFc)2 the
communication is measured electrochemically, and oxidation waves for the two
ferrocenyl termini are separated in the cyclic voltammogram by ∆E1/2 = 220 mV.
[31] Ren’s group first reported the Cr(III) complex, 1f, which demonstrated
minimal communication between the ferrocenyl termini [4]. We have now prepared
the Co(III) complex, 2f, and the Rh(III) complex, 3f, to determine whether or not
23

communication between the ferrocenyl termini could be increased by replacing the
d3 metal with a d6 metal.
Figure 5. Ethynylferrocene capped metal complexes reported herin (top), and the
model complex for which strong communication between the ferrocenyl termini has
been observed (bottom) [31].
In the cyclic voltammograms for 2f (Figure 6) and 3f (supplementary material) only
a single two-electron wave was observed for the ferrocenyl termini, thus
demonstrating little to no communication through the Co(III) or Rh(III) centers.
Differential pulse voltammograms [32] of the two-electron ferrocenyl oxidation for
both the Co(III) and Rh(III) complexes also show no evidence of splitting
(supplementary material).
24

Figure 6. Cyclic voltammogram of 2f showing the 2e– oxidation wave (A)
corresponding to the two ferrocenyl termini and the one electron CoIII/II reduction.
For experimental conditions, see Figure 4 caption.
4. Conclusions
Previous work in our group has suggested that arylalkynyl ligands behave as
π-donors towards Cr(III). Herein we have explored what, if any, π-interactions can
be demonstrated with low-spin d6 metals, chiefly Co(III). For the arylethynyl
ligands, there is little evidence to suggest that the arylalkynyl ligands are behaving
as either π-donors or π-acceptors toward Co(III). This is perhaps not surprising
given that the low-spin d6 configuration leaves little room for donation even though
the +3 oxidation state leaves the metal fairly electron deficient. Only in the case of
trifluoropropynyl as a ligand is there evidence for the ligand behaving as a weak π-
acceptor. The lack of π -interactions between Co(III) and the arylalkynyl ligands
inhibits π-conjugation through the metal and thus it should not be surprising that no
communication is observed between the ferrocenyl termini of 2f and 3f. Even for
the Cr(III) complex, 1f, there is no significant communication between the
ferrocenyl termini, even though this system likely involves π-donation from the
25

alkynyl ligand to Cr(III) [4, 14]. For trans-Ru(dppm)2(CCFc)2, the magnitude of
∆E1/2 is attributed to metal-to-ligand π-backbonding [31] which has clearly been
demonstrated with the more electron rich Ru(II) alkynyl complexes [11]. Our
electrochemical results suggest that there is metal to alkynyl ligand π-back bonding
in the Co(II) oxidation state, particularly for more electron withdrawing alkynyl
ligands. However, due to the irreversibility of the CoIII/II reduction wave for 2f, and
fact that this reduction is more negative than for Fc+/0 (Figure 6) it is not possible to
determine whether electronic communication occurs through the Co(II) form of 2f.
Finally, it may yet be possible to observe significant electronic coupling in systems
where the ethynylferrocene ligands are behaving as π-donors, but clearly the metal
center should be more electron poor than [Cr(cyclam)]3+.
Acknowledgements This material is based upon work supported by the National
Science Foundation under Grant CHE-0709562.
Appendix A. Supplementary material CCDC 964986 and 964987 contain the
supplementary crystallographic data for 1e, and 2e, respectively. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre at
www.ccdc.cam.ac.uk. Additional supplementary data for this article includes
infrared spectra for 2a-2d, 2f, and 3f; cyclic voltammograms of trans-
[Co(cyclam)(CCC6H5)(X)]+ and 3f; and a thermal ellipsoid plot for 1e. These can be
found in the online version at doi 10.1016/j.ica.xxxx.
26

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31