Stepwise acetylide ligand substitution for the assembly of ethynylbenzene-linked Co(III) complexes

Article abstract of DOI:10.1016/j.ica.2011.08.051

We describe the preparation and structural characterization of a family of chloro-ethynylbenzene complexes containing [(cyclam)Co]³⁺ units: [(cyclam)CoCl(CCPh)]X (X = Cl (1a), BPh₄ (1b)), [(cyclam) ₂Co₂Cl_2<

Full text of DOI:10.1016/j.ica.2011.08.051

Inorganica Chimica Acta 380 (2012) 174–180
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Stepwise acetylide ligand substitution for the assembly of ethynylbenzene-linked
Co(III) complexes
1
⇑
Wesley A. Hoffert , Md. Khayrul Kabir, Ethan A. Hill, Sara M. Mueller, Matthew P. Shores
Department of Chemistry, Colorado State University, Fort Collins, CO 80523-1872, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 7 September 2011
We describe the preparation and structural characterization of a family of chloro-ethynylbenzene com-
plexes containing [(cyclam)Co]3+ units: [(cyclam)CoCl(CCPh)]X (X = Cl (1a), BPh4 (1b)), [(cyclam)2Co2Cl2
(p-DEB)]X2 (X = Cl (2a), BPh4 (2b)), [(cyclam)3Co3Cl3(1,3,5-TEB)]X3 (X = Cl (3a), BPh4 (3b)), [(cyclam)2-
Co2Cl2(1,3,5-TEBH)]X2 (X = Cl (4a), BPh4 (4b)), where the bridging ligands p-DEB and TEB are the bis- and
tris-deprotonated anions of 1,4-diethynylbenzene and 1,3,5-triethynylbenzene, respectively. Cyclic vol-
tammetry of [(cyclam)3Co3Cl3(1,3,5-TEB)]Cl3 (3a) suggests that electrochemical reduction of the Co(III)
centers to Co(II) is accompanied by complex dissociation resulting from the increased lability of the Co(II)
ions. Inclusion of the tetraphenylborate anions in 1b–4b allows for the possibility of stepwise acetylide sub-
stitution by imparting complex cation solubility in tetrahydrofuran: trans-[(cyclam)Co(CCPh)2](BPh4) (5) is
prepared by mixing LiCCPh with 1b. Isolation of 5 demonstrates the feasibility of metallodendrimer synthe-
sis using octahedral first-row transition metal nodes, which awaits demonstration.
Young Investigator Award Special Issue
Keywords:
Cobalt complexes
Ethynylbenzene ligands
Synthesis
Ligand substitution
Metallodendrimer
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
candidates for incorporation into metallodendrimers, as many
examples of heteroleptic complexes exist for those ions.
Seeking a greater degree of synthetic control in inorganic and
organometallic materials, chemists have long been concerned with
being able to assemble transition metal clusters using a ‘‘building
block’’ approach [1–3]. Small ligands such as oxide [4,5], hydroxide
[6,7], and cyanide [8,9] have proven to be effective directors of
multimetallic cluster self-assembly, but there is also interest in
the synthesis of metal clusters bridged by a framework of larger li-
Interesting properties are predicted for first-row transition me-
tal analogs, especially for paramagnetic species. Considering first
row transition metal complexes bridged by TEB3À, this ligand
engenders ferromagnetic coupling between ligated metal ions be-
cause of the mutually meta bridged connectivity [18], and the
two-dimensional and C3-symmetric topology imparted by the rigid
ligand should enhance magnetic anisotropy [19]. Prior to attempt-
ing the synthesis of air- and moisture-sensitive paramagnetic
complexes, we sought to prepare a stable structural analog of a
first-row complex that could be easily studied with conventional
techniques such as 1H NMR, making CoIII-containing species an
ideal choice. Earlier work from Giese established that Grignard
reagents could be used to prepare monomeric CoIII chloro-acetylide
complexes [20]. The syntheses of mono- and bis-acetylide CoIII
complexes were also studied by Lewis and co-workers [21]. They
discovered that trialkylstannyl acetylene precursors combined
with neutral [CoIIIN4Cl2] complexes in the presence of catalytic
amounts of CuI form monomeric metal-acetylide compounds.
Depending on reaction stoichiometry, polymeric compounds could
also be prepared by heating equimolar amounts of the dichloro
CoIII precursor and ditopic para-bis(trimethyltin)diethynylbenzene
[21]. We note that diamagnetic FeII ethynylbenzene complexes
have also been the focus of considerable attention from Field and
co-workers. Among several possible synthetic protocols, the dehy-
drohalogenation route has been shown to be a versatile method for
preparing metal acetylide complexes containing [(dmpe)2FeCl]À
units (dmpe = 1,2-bis(dimethylphosphino)ethane) [22–24]. Noting
gands with delocalized
p orbitals [10–12]. Such molecules have
been suggested as components of non-linear optical devices [13],
and more recently, magnetic materials [14,15]. Polyethynylben-
zene bridging ligands are attractive as structural units that foster
communication between nodes. Takahashi and co-workers have
synthesized metallodendrimers with up to 45 square-planar PtII-
containing units bridged by the triply deprotonated trianion of
1,3,5-triethynylbenzene (H3TEB) [16]. Humphrey and co-workers
later reported the preparation and properties of TEB3À-based den-
drimers that incorporated nine octahedral RuII ions [17]. Both types
of molecules were synthesized by stepwise replacement of trans-
chloride ligands; isolation of products with good monodispersity
relied on the high inertness of the metal ions with mixed ligands
at sites trans to each other. For this reason, PtII and RuII are ideal
⇑
Corresponding author. Tel.: +1 970 491 7235; fax: +1 970 491 1801.
E-mail address: matthew.shores@colostate.edu (M.P. Shores).
Present address: Pacific Northwest National Laboratory, Richland, WA 99352,
1
USA.
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.08.051

W.A. Hoffert et al. / Inorganica Chimica Acta 380 (2012) 174–180
175
these precedents, our interest lies in synthetic routes that may allow
for the stepwise preparation of multinuclear complexes. Herein, we
present the preparations, X-ray structures, and spectroscopic
and electrochemical characterizations of a series of diamagnetic
CoIII(cyclam)-based chloro-acetylide complexes synthesized using
a dehydrohalogenation synthetic strategy. The stepwise preparation
of a di-substituted CoIII bis-acetylide complex is also described,
which raises the possibility of using related octahedral first-row
transition metal complexes as precursors for paramagnetic
metallodendrimers.
0.495 mmol) and freshly sublimed p-H2DEB (29.7 mg, 0.236 mmol)
in a 50 mL round-bottomed flask, causing the solution to darken.
The flask was fitted with a condenser tube and the solution was
heated with stirring at reflux for 24 h, during which time the solu-
tion color turned to red-orange. The solvent was removed by rotary
evaporation, and the resulting red-brown residue was washed with
diethyl ether (10 mL) and dried under vacuum to yield 196 mg of
crude 2a as an orange solid. Crystals of 2a suitable for X-ray dif-
fraction were grown by diffusion of diethyl ether vapor into a con-
centrated solution of the crude product in MeOH.
2.2.4. Trans,trans-[(cyclam)2Co2Cl2(l-p-DEB)](BPh4)2 (2b)
2. Experimental section
A solution of NaBPh4 (256 mg, 0.748 mmol) in ca. 3 mL methanol
was added to a solution of compound 2a (196 mg) in ca. 10 mL meth-
anol, causing 2b to precipitate as an orange solid. This solid was iso-
lated by filtration, washed with methanol (3 Â 3 mL) and diethyl
ether (3 Â 3 mL). The compound was recrystallized by slow diffu-
sion of diethyl ether into a concentrated solution of 2b in acetone
to afford 151 mg of the final product (0.112 mmol, 47% overall yield
based on p-H2DEB used to produce 2a). Absorption spectrum (ace-
2.1. Materials and methods
Unless otherwise noted, all manipulations were performed in air.
The compounds [(cyclam)CoCl2]Cl [25] and 1,3,5-triethynylbenzene
[26] (H3TEB) were prepared as described elsewhere. Triethylamine
was freshly distilled before use. All other reagents were purchased
commercially and were used without further purification. The
bridging ligand p-DEB2À is the doubly deprotonated anion of 1,4-
diethynylbenzene (p-H2DEB). The concentration of n-BuLi was veri-
fied by titration prior to the synthesis of compound 5.
tone): kmax
(e
M) 327 (3720), 386 (sh) (940) 488 (440 MÀ1 cmÀ1).
IR: mC„C 2133 cmÀ1
.
1H NMR ((CD3CN): d 7.36 (s, 4H, CC(Ar-H)CC),
7.27 (br, 20H, B-C6H5), 6.99 (t, 16H, B-C6H5), 6.84 (t, 8H, B-C6H5),
4.48 (br, 8H, N-H), 2.89-2.40 (m, 36H, CH2), 1.54–1.38 ppm (m, 4H,
exo-CH(2)). ESI+-MS (acetone): m/z 1031.47 ([2b-BPh4]+), 356.13
([2b-2BPh4]2+). Anal. Calc. for C84H104B2Cl2Co2N8O2 (2bÁ2C3H6O):
C, 68.72; H, 7.14; N, 7.63. Found: C, 68.56; H, 7.06; N, 7.36%.
2.2. Preparation of compounds
2.2.1. Trans-[(cyclam)CoCl(C2Ph)]Cl (1a)
Triethylamine (1.62 mL, 11.6 mmol) was added to a green
methanolic (10 mL) solution of [(cyclam)CoCl2]Cl (213 mg,
0.582 mmol) and phenylacetylene (61 lL, 0.55 mmol) in a
2.2.5. Trans,trans,trans-[(cyclam)3Co3Cl3(TEB)]Cl3 (3a)
Triethylamine (1.70 mL, 12.3 mmol) was added to a green
methanolic (10 mL) solution of [(cyclam)CoCl2]Cl (239 mg,
0.655 mmol) and freshly sublimed H3TEB (30.7 mg, 0.205 mmol)
in a 100 mL round-bottomed flask, causing the solution color to
darken. The flask was fitted with a condenser tube and the solution
was refluxed with stirring for 24 h, during which time the solution
color turned to red-orange. The solvent was removed by rotary
evaporation, and the resulting red-brown residue was washed with
diethyl ether (10 mL) and dried under vacuum to yield 271 mg of
crude 3a as an orange solid. Crystals of 3a suitable for X-ray dif-
fraction were grown by diethyl ether vapor diffusion into a concen-
trated solution of the crude product in MeOH.
100 mL round-bottomed flask, causing the solution color to darken.
The flask was fitted with a condenser tube, and the solution was
heated with stirring at reflux for 24 h, during which time the solu-
tion color turned to red-orange. The solvent was removed by rotary
evaporation, and the resulting red-brown residue was washed with
diethyl ether (10 mL) and dried under vacuum to yield 231 mg of
crude 1a as a red-brown solid. Crystals of 1a suitable for X-ray dif-
fraction were grown by diffusion of diethyl ether vapor into a con-
centrated solution of the crude product in tetrahydrofuran.
2.2.2. Trans-[(cyclam)CoCl(C2Ph)]BPh4 (1b)
A solution of NaBPh4 (184 mg, 0.536 mmol) in ca. 3 mL of meth-
anol was added to a solution of compound 1a (231 mg) in ca. 10 mL
of methanol, causing an orange solid to precipitate. This solid was
isolated by filtration, washed with methanol (3 Â 3 mL) and diethyl
ether (3 Â 3 mL) and dried in air to afford 251 mg of the final product
(0.334 mmol, 61% overall yield based on phenylacetylene used to
2.2.6. Trans,trans,trans-[(cyclam)3Co3Cl3(TEB)](BPh4)3 (3b)
A solution of NaBPh4 (326 mg, 0.953 mmol) in ca. 5 mL of meth-
anol was added to a solution of compound 3a (271 mg) in ca. 10 mL
methanol, causing a salmon-colored solid to precipitate. This solid
was isolated by filtration, washed with methanol (3 Â 3 mL) and
diethyl ether (3 Â 3 mL) and dried in air to afford 234 mg of the fi-
nal product (0.118 mmol, 57% overall yield based on H3TEB used to
produce 1a). Absorption spectrum (THF): kmax
258 (33300), 326 (sh, 10600), 381 (sh, 1400) 486 nm
(1100 MÀ1 cmÀ1). IR: C„C 2125 cmÀ1. 1H NMR ((CD3)2CO): d 7.37–
(eM) 227 (43500),
produce 3a). Absorption spectrum (acetone): kmax
(eM) 329 (3300),
384 (sh, 575) 485 nm (420 MÀ1 cmÀ1). IR: C„C 2114 cmÀ1. 1H NMR
m
m
7.22 (m, 13H, B-C6H5, CCC6H5), 6.93 (t, 8H, B-C6H5), 6.78 (t, 4H, B-
C6H5), 5.06 (br, 2H, N-H), 4.92 (br, 2H, N-H), 3.03–2.59 (m, 18H,
CH2), 1.74–1.64 ppm (m, 2H, exo-CH(2)). [Note: the integrations of
the most upfield resonances in the chloro-acetylide complexes are
not consistent with there being two equivalent H atoms on the ali-
phatic cyclam carbon. Rather, we assign these signals to the ‘‘exo’’
or Cl-pointing protons only.] ESI+-MS (acetone): m/z 395.20
([1aÀBPh4]+). Anal. Calc. for C42H49BClCoN4: C, 70.55; H, 6.91; N,
7.84. Found: C, 70.37; H, 7.14; N, 7.79%. Crystals of 1b suitable for
X-ray structural analysis were grown by diffusion of diethyl ether
vapor into a concentrated solution of 1b in tetrahydrofuran.
((CD3)2CO): d 7.34 (m, 27H, B-C6H5 and Ar-H), 6.93 (t, 24H, B-C6H5),
6.78 (t, 12H, B-C6H5), 4.93 (br, 12H, N-H), 3.07-2.54 (m, 56H, CH2),
1.74-1.58 ppm (m, 6H, exo-CH(2)). ESI-MS+ (acetone): m/z 1667.60
([3b-BPh4]+), 674.80 ([3b-2BPh4]2+), 343.90 ([3b-3BPh4]3+). Anal.
Calc. for C114H135B3Cl3Co3N12: C, 68.84; H, 6.84; N, 8.45. Found:
C, 68.58; H, 6.83; N, 8.19%.
2.2.7. Trans,trans-[(cyclam)2Co2Cl2(HTEB)]Cl2 (4a)
Triethylamine (0.34 mL, 2.42 mmol) was added to a green
methanolic (10 mL) solution of [(cyclam)CoCl2]Cl (233 mg,
0.637 mmol) and freshly sublimed H3TEB (45.5 mg, 0.303 mmol)
in a 100 mL round-bottomed flask, causing the solution color to
darken. The flask was fitted with a condenser tube and the solution
was refluxed with stirring for 24 h, during which time the solution
turned to orange-brown. The solvent was removed by rotary
2.2.3. Trans,trans-[(cyclam)2Co2Cl2(l-p-DEB)]Cl2 (2a)
Triethylamine (1.32 mL, 9.44 mmol) was added to a green
methanolic (10 mL) solution of [(cyclam)CoCl2]Cl (181 mg,

176
W.A. Hoffert et al. / Inorganica Chimica Acta 380 (2012) 174–180
evaporation, and the resulting red-brown residue was washed with
10 mL of absolute ethanol, causing an orange solid to precipitate.
The solid was isolated by filtration, washed with ethanol
(3 Â 3 mL) and diethyl ether (3 Â 3 mL) and dried in air to afford
92.1 mg of 4a. Single crystals suitable for X-ray analysis were
grown by diffusing diethyl ether vapor into a concentrated solution
of 4a in methanol.
refined anisotropically. Hydrogen atoms were assigned to ideal posi-
tions and were refined using a riding model where the isotropic dis-
placement parameters were set at 1.2 times those of the attached
carbon or nitrogen atoms (1.5 times for methyl protons). The final
structures have been deposited (in cif format) with the Cambridge
Crystallographic Data Centre (Table 1).
In the structure of 5Á2THF, the b-acetylenic carbon atoms in
each complex exhibited abnormally small thermal ellipsoids com-
pared to their nearest neighbors. For those atoms, the thermal
2.2.8. Trans,trans-[(cyclam)2Co2Cl2(HTEB)](BPh4)2 (4b)
A solution of NaBPh4 (ca. 1 g, 342 mmol) in 10 mL methanol was
added to a solution of compound 4a (145 mg) in 10 mL methanol,
causing an orange solid to precipitate. This solid was isolated by fil-
tration, washed with methanol (3 Â 3 mL) and diethyl ether
(3 Â 3 mL). The compound was recrystallized by diffusing diethyl
ether vapor into a concentrated solution of 4b in acetone. After
1 day, orange crystals were isolated by filtration, washed with
diethyl ether (3 Â 3 mL), and dried in air to afford 166 mg of the final
product (0.121 mmol, ꢀ67% for the step described above, 27% over-
all yield based on H3TEB used to produce 4a). Absorption spectrum
parameters along the bond axis with the a-acetylenic carbon
atoms were constrained to be the same as those of the
a-acetylenic
carbon atoms.
2.4. Other physical measurements
Absorption spectra were obtained in quartz cuvettes with a
Hewlett-Packard 8453 spectrophotometer. Infrared spectra were
measured with a Nicolet 380 FT-IR using the Smart Performer ZnSe
ATR accessory. 1H NMR spectra were recorded using a Varian INO-
VA instrument operating at 300 MHz. Cyclic voltammetry experi-
ments were performed in 0.1 M solutions of (Bu4N)PF6 in
tetrahydrofuran or acetonitrile. The voltammograms were re-
corded with either a CH Instruments 1230A or 660C potentiostat
using a 0.25 mm Pt disk working electrode, Ag/Ag+ reference elec-
trode, and a Pt mesh auxiliary electrode. All voltammograms
shown were measured with a scan rate of 0.1 V/s. Reported poten-
tials are referenced to the [Cp2Fe]+/[Cp2Fe] redox couple and were
determined by adding ferrocene as an internal standard at the con-
clusion of each electrochemical experiment. Elemental analyses
were performed by Robertson Microlit Laboratories in Madison, NJ.
(acetone): kmax
(e
M) 326 (1770), 333 (2340), 380 (sh, 360), 483 nm
(260 MÀ1 cmÀ1). IR: mC„C 2122 cmÀ1
,
mCC–H 3311 cmÀ1 1H NMR
.
((CD3)2CO): 7.42 (t, 1H, Ar-H), 7.38 (d, 2H, Ar-H), 7.34 (m, 16H,
BAr-H), 6.94 (t, 16H, BAr-H), 6.79 (t, 8H, BAr-H), 4.94 (br s, 8H, N-
H), 3.79 (s, 1H, CC-H), 2.99-2.52 (br m, 36H, CH2), 1.66 ppm (br m,
4H, exo-CH(2)). Anal. Calc. for C86H104B2Cl2Co2N8O2 (4bÁ2C3H6O): C,
69.22; H, 7.02; N, 7.51. Found: C, 68.78; H, 6.98; N, 7.71%.
2.2.9. Trans-[(cyclam)Co(C2Ph)2](BPh4) (5)
Under an atmosphere of dinitrogen, LiC2Ph was generated by
adding n-BuLi (0.07 mL of a 1.6 M solution in hexanes, 0.112 mmol)
to a solution of phenylacetylene (13.1 lL, 12.1 mg, 0.119 mmol) in
diethyl ether (3 mL). After stirring for 5 min, the ethereal solution
was added to an orange solution of 1b (77.3 mg, 0.108 mmol) in tet-
rahydrofuran (5 mL). After 1 h of stirring, the solvent was removed
in vacuo to afford a brown residue. Washing with pentane afforded
a yellow-brown solid, which was isolated by filtration and washed
with diethyl ether (3 Â 3 mL) to yield 88 mg of crude yellow solid.
The product was recrystallized by slow diffusion of diethyl ether va-
por into a concentrated solution of the crude product in tetrahydro-
furan. The yellow needle crystals that formed after one day were
isolated by filtration, washed with diethyl ether (3 Â 3 mL), and
dried under dinitrogen to afford 53.8 mg of the final product (64%).
3. Results and discussion
3.1. Syntheses and characterizations of CoIII acetylide complexes
Our approach to making ‘‘0th’’ generation metallodendrimers is
based on a dehydrohalogenation strategy that has been employed
in the synthesis of other CoIII-, FeII-, and RuII-containing metal-acet-
ylide complexes. For Ru-containing complexes, the reaction has
been shown to proceed by way of a metal–vinylidene intermediate,
which reacts readily with a base to afford the metal acetylide.
Shown in Scheme 1, trans-[Co(cyclam)Cl2]Cl reacts directly with
an arylacetylide ligand using triethylamine as the base, yielding
the chloro-acetylide complexes as orange solids in acceptable
yield. No changes in (visible) color or infrared spectra are observed
if the reaction is carried out in the absence of Et3N. In order to drive
the ligand substitution reactions in the syntheses of 1a, 2a, and 3a
to completion, it was necessary to use a slight stoichiometric ex-
cess [29] of [(cyclam)CoCl2]Cl. As evidenced by the isolation of
4a, CoIII-deficient reaction mixtures lead to ‘‘incomplete’’ substitu-
tion of the ethynylbenzene ligands. Obtaining pure bulk samples of
the chloride salts of all complexes presented herein was not
possible, likely owing to the similar solubilities of the acetylide
complexes, surplus [(cyclam)CoCl2]Cl, and triethylammonium
hydrochloride, which forms as the conjugate acid byproduct of
dehydrohalogenation. Although recrystallization of the chloride
salts could be used to obtain a few X-ray quality crystals, crystal-
line samples subjected to combustion analysis were not acceptably
pure. Occasionally, crops of crystals of 4a contained a green-col-
ored side product, consistent with excess [(cyclam)CoCl2]Cl. In or-
der to impart differential solubility between the product and
impurities, tetraphenylborate anion metathesis was performed,
allowing for workup to be performed in less polar solvents like
tetrahydrofuran. Slow diffusion of diethyl ether vapor into THF
solutions of the tetraphenylborate salts produced microcrystalline
Absorption spectrum (MeCN): kmax
(eM) 268 (28200), 363 (sh,
270), 463 nm (84 MÀ1 cmÀ1). IR: C„C 2100 cmÀ1. 1H NMR (CD3CN):
m
d 7.48 (d, 4H, Ar-H), 7.34-7.19 (m, 14H, B-C6H5 and Ar-H), 6.99 (t, 8H,
B-C6H5), 6.84 (t, 4H, B-C6H5), 4.16 (br, 4H, N-H), 2.86 (m, 4H, CH2),
2.58–2.40 (m, 12H, CH2), 1.87 (d, 2H, CH2), 1.32 (m, 2H, CH2). ESI-
MS+ (acetone): m/z 461.07 (5-BPh4+). Anal. Calc. for C56H66BCoN4O1.5
(5Á1.5C4H8O): C, 75.67; H, 7.48; N, 6.30. Found: C, 75.64; H, 7.26; N,
6.52%.
2.3. X-ray structure determinations
Structures were determined for the compounds listed in Table 1.
Single crystals were coated in Paratone oil, supported on cryoloops,
transferred to a Bruker Kappa Apex 2 CCD diffractometer, and cooled
under a stream of dinitrogen. All data collections were performed at
100 or 120 K with Mo Ka radiation (k = 0.71073 Å) and a graphite
monochromator. Initial lattice parameters were determined from
a minimum of 190 reflections harvested from 36 frames, and data
sets were collected targeting complete coverage and four-fold
redundancy. Data were integrated and corrected for absorption ef-
fects with the Apex 2 software package [27]. Structures were solved
by direct methods with the aid of successive Fourier difference maps
and were refined against all data using the SHELXTL software package
[28]. Displacement parameters for all non-hydrogen atoms were

W.A. Hoffert et al. / Inorganica Chimica Acta 380 (2012) 174–180
177
Table 1
Crystallographic dataa for compounds [(cyclam)CoCl(C2Ph)]BPh4 (1b), [(cyclam)2Co2Cl2(
[(cyclam)2Co2Cl2(HTEB)]Cl2Á2MeOH (4aÁ2MeOH), and [(cyclam)Co(C2Ph)2]BPh4Á2THF (5Á2THF).
l-p-DEB)]Cl2Á2MeOH (2aÁ2MeOH), [(cyclam)3Co3Cl3(TEB)]Cl3Á5MeOH (3aÁ5MeOH),
1b
2aÁ2MeOH
3aÁ5MeOH
4aÁ2MeOH
5Á2THF
Formula
Formula weight
Color, habit
T (K)
C
42H49BClCoN4
C32H60Cl4Co2O2N8
848.55
orange prism
100(2)
C47H95Cl6Co3 N12O5
1297.84
orange prism
C36H68Cl4Co2N8O4
911.01
orange prism
120(2)
C58H70BCoN4O2
924.92
yellow needle
120(2)
715.04
orange needle
120(2)
120(2)
ꢀ
Space group
P21/c
P21/c
C2/c
P21/c
P1
Z
4
2
2
4
4
a (Å)
b (Å)
c (Å)
16.0376(4)
21.5470(5)
21.1968(5)
90
92.266(1)
90
14.3852(3)
9.6749(2)
17.4624(4)
90
119.535(1)
90
8.617(1)
15.070(2)
24.676(3)
72.656(3)
89.289(4)
80.701(4)
3016.4(6)
1.429
18.4701(3)
9.7704(1)
28.8748(4)
90
94.850(1)
90
15.911(1)
18.464(1)
17.139(1)
90
97.391(2)
90
a
(°)
b (°)
c
(°)
V (Å3)
7319.1(3)
1.298
2114.52(8)
1.433
4472.8(1)
1.353
4993.8(5)
1.230
Dcalc (g/cm3)
Goodness-of-fit (GOF)
0.996
1.038
1.084
1.068
1.031
R1 (wR2)b, %c
4.58 (11.14)
6.96 (12.83)
830633
3.98 (9.83)
5.55 (10.49)
830634
5.21 (13.96)
7.01 (15.15)
830635
5.30 (14.19)
7.54 (15.67)
830636
4.63 (1128)
8.80 (13.27)
830637
R1 (wR2)b, %d
CCDC reference #
a
b
c
Obtained with graphite-monochromated Mo K
a
(k = 0.71073 Å) radiation.
P
P
P
P
||Fo| À |Fc||/ |Fo|, wR2 = {| [w(Fo À Fc2)2]/ [w(Fo2)2]|}1/2
.
2
R1
=
For data where I > 2
r(I).
d
For all data.
half-substituted complexes is important for the controlled, rational
preparation of larger assemblies, especially considering the pro-
pensity for polymeric metal acetylide compounds to form via other
synthetic methods [21].
The increased solubility of the tetraphenylborate salts 1b–4b in
nonpolar solvents like tetrahydrofuran allows for the application of
additional ligand substitution methods beyond dehydrohalogen-
ation. A common way to exchange halide ligands for acetylides
proceeds via salt metathesis involving alkali metal acetylides
[10], which are only stable in ethereal solvents. Thus, the synthesis
of 5 was achieved by combining a slight excess of lithium phenyl-
acetylide with 1b in THF (Scheme 2). The product was isolated
from the lithium chloride byproduct by recrystallization. The isola-
tion of 5 is the first known example of stepwise acetylide ligand
substitution in a CoIII system. Related to stepwise acetylide substi-
tution, Field previously demonstrated differential reactivity at a
low-spin FeII center by using sequential dehydrohalogenation and
photochemical reactions starting with [(dmpe)2FeCl(CH3)]
(dmpe = 1,2-bis(dimethylphosphino)ethane) [24].
Scheme 1. Synthetic conditions used for the preparation of Co3+ acetylide
complexes 1b, 2b, 3b, and 4b. Reaction conditions: a = MeOH, excess Et3N, reflux
24 h; b = MeOH, 4 eq. Et3N per Co3+, reflux 24 h; c = excess NaBPh4, MeOH.
The synthesis of dinuclear 4a was achieved by mixing 2.1 equiv-
alents of [(cyclam)CoCl2]Cl with H3TEB and four equivalents of tri-
ethylamine per Co in methanol solution. Attempts to synthesize 4a
using less triethylamine resulted in greatly diminished yields,
whereas a larger excess of base appears to favor the formation of
3a. Unlike trinuclear 3a, 4a is sparingly soluble in ethanol; rinsing
the product with ethanol essentially removes all triethylammo-
nium hydrochloride and undesired mono- and tri-nuclear Co3+
acetylide species. Presumably, mono-, di- and trinuclear complexes
are all formed during the reaction, but the reaction stoichiometry
favors the formation of 4a. The wedge-like topology of 4b would
samples that were found to be analytically pure. In most cases, the
BPh4-containing crystals were not suitable for X-ray analysis,
although a structure of the mononuclear complex 1b was obtained
(Section 2.2.2), and we recently reported a structure of the mixed
bridging ligand salt [1,3-{(cyclam)2Co2Cl2C2}-5-(C2H)0.88(Br)0.12
benzene](BPh4)2 [30].
-
The use of dehydrohalogenation conditions allows for the reten-
tion of one chloride ligand trans to the acetylide in this family of
complexes. Attempts to prepare the bis-alkynyl complex 5 directly
from trans-[(cyclam)CoCl2]Cl and excess phenylacetylene only af-
ford the mono-substituted 1a, as evidenced from mass spectra ob-
tained on the reaction mixtures. Based on the tetradentate
chelating ligand environment and the propensity for CoIII to under-
go dissociative-type ligand substitution reactions [31], and noting
Field’s observation of monosolvento species in related isoelectronic
FeII complexes [22], it seems reasonable that the substitution of
acetylide for chloride proceeds through a methanol complex. In
that case, we suspect bis-alkynyl complexes are not observed un-
der dehydrohalogenation conditions because the trans-[(cyclam)-
Co(C2R)Cl]+ species is less electron-rich compared to trans-
[(cyclam)CoCl2]+; the dissociation equilibrium to form the putative
trans-[(cyclam)Co(C2R)(MeOH)]2+ species is disfavored due to a
trans influence of the acetylide. Notwithstanding, isolation of
Scheme 2. Synthesis of bis-acetylide complex 5 from the chloro acetylide complex
1b. The tetraphenylborate anion allows the complex in 1b to be soluble in
tetrahydrofuran.

178
W.A. Hoffert et al. / Inorganica Chimica Acta 380 (2012) 174–180
at first appear to be useful in the schematic assembly of metallo-
dendrimers, for example when combined with one-third of an
equivalent of trinuclear 3b. However, attempts to lithiate the acet-
ylene moiety of 4b were thwarted by the presence of the amine
hydrogens on the cyclam ligands, which appear to be more reactive
toward n-BuLi than the acetylene hydrogen. Although a direct
comparison of the pKa values for the relevant protons is not avail-
able, a measured amine pKa for [(cyclam)Co(CO3)]+ in H2O (10.76)
is significantly lower than that for phenylacetylene in dimethyl-
sulfoxide (28.7) [32,33].
The IR spectra of 1b, 2b, and 3b each contain a single absor-
bance between 2114 and 2133 cmÀ1 that corresponds to the
stretching frequency of the acetylide ligand when coordinated to
the CoIII ion. In 4b, two separate absorbances should be visible
due to the different acetylene environments; however a single
broad peak centered at 2122 cmÀ1 masks the contribution from
the non-bridging acetylene, which absorbs at 2109 cmÀ1 for
H3TEB. However, IR evidence for the non-bridging acetylene in
4b is found in the sharp acetylenic C–H stretch that is observed
at 3313 cmÀ1. For the bis-acetylide complex 5, the acetylide
stretching frequency moves to lower energies and is found at
2100 cmÀ1
.
Fig. 1. X-ray structures of the complex cations in 1b (top) and 2aÁ2MeOH (bottom),
with thermal ellipsoids rendered at 40% probability. Hydrogen atoms have been
omitted for clarity. The complex in 2aÁ2MeOH resides on an inversion center. Red,
blue green, and gray ellipsoids represent cobalt, nitrogen, chloride, and carbon
atoms, respectively. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
3.2. X-ray crystal structures of CoIII acetylide complexes
Crystals of mononuclear 1b suitable for X-ray analysis were ob-
tained by slow diffusion of diethyl ether vapor into a concentrated
solution of 1b in tetrahydrofuran. The compound crystallizes in the
P21/c space group with two molecules in the unit cell. The coordi-
nation geometry about each Co3+ center is essentially octahedral,
with the equatorial positions occupied by the nitrogen atoms from
the cyclam ligand (Fig. 1). The chloride and acetylide ligands are
apically coordinated with metal-ligand distances of ca. 2.3 and
1.9 Å, respectively (Table 2).
Similar local coordination geometries are observed for the di-
and tri-nuclear complexes 2a, 4a, and 3a as for mononuclear 1b
(Figs. 1 and 2). Bond distances and angles measured for the com-
plex cation in the structure of 4a (Table 2) are statistically identical
nificant elongation in the Co–C bond length compared to 1a, along
with a concomitant contraction in the carbon–carbon triple bond
distance. Here, a decrease in the Co–C bonding interaction would
be expected to lead to a reduction in M–L
giving a shorter C–C bond [35,36].
p-backbonding, thereby
3.3. Cyclic voltammetry
to those measured for [1,3-{(cyclam)2Co2Cl2C2}-5-(C2H)0.88(Br)0.12
-
Although the CoIII complexes reported here are meant to serve
as structural models for future paramagnetic dendritic building
blocks, we note that there are several reported examples of CoII-
based single-molecule magnets [37–39]. Thus, we undertook a
study of the compounds’ electrochemical behaviors to probe the
possibility of reducing the CoIII salts to neutral, paramagnetic
CoII-containing analogs. To our knowledge, there are no reports
of electrochemical characterization of cobalt acetylide complexes
in the literature.
In acetonitrile solution, crystals of trinuclear 3a display an irre-
versible reduction during an initial cathodic scan at À1.52 V.
During subsequent scans, a quasi-reversible wave grows in at
E1/2 = À0.74 V while the wave at À1.52 is diminished (Fig. 4). We
benzene]2+ [30].
The structure of the mononuclear bis-acetylide complex 5 was
also determined by X-ray crystallography (Fig. 3). The coordination
geometry of the Co3+ center is similar to the chloro-acetylide com-
plex 1b with the exception of the second phenylacetylide ligand in
place of the chloride. Only minor differences in bond lengths and
angles are observed among the series of chloro-acetylide com-
plexes (Table 2). When compared to the X-ray structure of [Co(cy-
clam)Cl2]Cl [34], the Co–Cl bond in all of the chloro-acetylide
complexes studied herein is slightly elongated, which is likely
due to the trans influence of the acetylide ligand. Substitution of
the second chloride ligand for another acetylide in 5 causes a sig-
Table 2
Selected measured interatomic distances (Å) and angles (°) for the X-ray structures of [(cyclam)CoCl2]Cl [34], [(cyclam)CoCl(C2Ph)]BPh4 (1b), [(cyclam)2Co2Cl2(
l-p-
DEB)]Cl2Á2MeOH (2aÁ2MeOH), [(cyclam)3Co3Cl3(TEB)]Cl3Á5MeOH (3aÁ5MeOH), [(cyclam)2Co2Cl2(HTEB)]Cl2Á1.8MeOH (4aÁ2MeOH), and [(cyclam)Co(C2Ph)2]-BPh4Á2THF (5Á2THF).
[Co(cyclam)Cl2]Cl [34]
1ba
2aÁ2MeOH
3aÁ5MeOH
4aÁ2MeOH
5Á2THFa
1.983[2]
Co–N
Co–Cl
Co–C
C„C
C„CH
Co–C„C
C–Co–Cl
1.974[4]b
2.253(3)
–
–
–
–
–
1.975[2]
2.3089[5]
1.898[2]
1.160[3]
–
1.979[7]
2.3123(4)
1.885(2)
1.210(2)
–
1.979[6]
2.320[5]
1.881[4]
1.193[5]
–
1.978[3]
2.3149(7)
1.878(3)
1.195(4)
1.189(7)
174.6(3)
178.52(9)
–
2.001[3]
1.113[4]
–
175.5[2]
–
174.5[5]
179.0[3]
173.4(1)
179.17(5)
174.7[3]
178.6[1]
a
These structures each have two complex residues in the asymmetric unit; bond lengths and angles are averaged from both residues.
Square brackets [] represent standard uncertainties for averaged metric parameters and round brackets represent standard uncertainties obtained from a single
b
interatomic distance or angle.

W.A. Hoffert et al. / Inorganica Chimica Acta 380 (2012) 174–180
179
Fig. 3. X-ray structure of the complex cation in 5Á2THF, with thermal ellipsoids
rendered at 40% probability. Hydrogen atoms have been omitted for clarity. The
complex resides on an inversion center and unique atoms are labeled (online). The
color scheme is the same as Fig. 1. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Cyclic voltammogram obtained for 3a in acetonitrile. The arrow indicates
the starting point and direction of the first scan. The slight increase in current at
À0.75 V during the initial scan is likely due to a [(cyclam)CoCl2]Cl impurity. See text
for further explanation of the electrochemical behavior.
whether a less coordinating solvent would prevent ligand dissoci-
ation upon reduction, cyclic voltammograms of 1 and 2b in tetra-
hydrofuran were also recorded. However, irreversible reduction
peaks were also observed for these salts. The bis-acetylide complex
5 also exhibits non-reversible reduction behavior, indicating that
replacement of a good leaving group (chloride) with phenylacety-
lide does not promote reversible reduction behavior on even elec-
trochemical timescales.
Fig. 2. X-ray structures of the complex cations in 3aÁ5MeOH (top) and 4aÁ2MeOH
(bottom), with thermal ellipsoids rendered at 40% probability. With the exception
of the acetylenic H, hydrogen atoms have been omitted for clarity. The complex in
3aÁ5MeOH resides on a general position, while the complex in 4aÁ2MeOH resides on
a twofold rotation axis. The color scheme is the same as Fig. 1. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
As stated above, no electrochemically-characterized cobalt
acetylide complexes have been reported. The closest literature
comparison is Bianchini and co-workers’ study of the redox behav-
note that a small amount of this wave is detectable even in the first
scan. This redox behavior is consistent with a three electron reduc-
tion of tri-nuclear 3a at highly negative potentials, followed by dis-
sociation of the [(cyclam)CoCl]2+ units from the TEB3À ligand.
Increased lability of Co2+ ions relative to Co3+ is the probable rea-
son for this dissociation. In acetonitrile, the resulting vacant coor-
dination site at the Co2+ center is likely occupied by a solvent
molecule. This monomeric complex can then be reversibly oxi-
dized, accounting for the quasi-reversible wave at À0.74 V. Sup-
ior of trigonal bipyramidal RhI complexes with a NP3(
r-C2R) first
coordination sphere (NP3 = N(CH2CH2PPh2) [40]. Starting with the
RhI congener, electrochemical measurements indicate that the
one- and two-electron oxidized complexes are electrochemically
accessible, but the RhIII–containing species could only be observed
with fast electrochemical scan rates. Comparing the complexes, the
Rh species enjoys better metal-ligand overlap (through lower coor-
dination number and softer ligands) and is less labile as a divalent
species. In that regard, the lack of reversible electrochemical
behaviors for the CoIII acetylide complexes reported here is not
surprising.
porting this proposed electrochemical reaction,
a
cyclic
voltammogram of [(cyclam)CoCl2]Cl in acetonitrile exhibits a sin-
gle reversible Co3+/Co2+ redox couple at À0.73 V. To investigate

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W.A. Hoffert et al. / Inorganica Chimica Acta 380 (2012) 174–180
4. Conclusions and outlook
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Acknowledgements
This research was supported by Colorado State University and
the ACS Petroleum Research Fund (44691-G3). We thank NSF–
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CCDC 830633–830637 contain the supplementary crystallo-
graphic data for compounds 1–5, respectively. Color versions of
Figs. 1–3 (pdf). 1H NMR spectra of compounds 1b, 2b, 3b, 4b, and
5. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2011.08.051.