The Tetracyanopyrrolide Anion as Ligand in Transition Metal Complexes

Article abstract of DOI:10.1002/zaac.201400199

Two isotypic mononuclear discrete complexes [Co(MeCN)₄(tcp) ₂]·2MeCN (1) and [Ni(MeCN)₄(tcp) ₂]·2MeCN (2) containing the tetracyanopyrrolide anion [C ₄(CN)₄N]^- (tcp) were synthes

Full text of DOI:10.1002/zaac.201400199

ARTICLE
DOI: 10.1002/zaac.201400199
The Tetracyanopyrrolide Anion as Ligand in Transition Metal Complexes
Barbara Szafranowska[a] and Johannes Beck*[a]
Keywords: Tetracyanopyrrolide; Cobalt complex; Nickel complex; Magnetic properties
Abstract. Two isotypic mononuclear discrete complexes via the cyano groups is observed. Both complexes show nearly ideal
[Co(MeCN)4(tcp)2]·2MeCN (1) and [Ni(MeCN)4(tcp)2]·2MeCN (2) paramagnetic behavior according to the Curie law with magnetic mo-
containing the tetracyanopyrrolide anion [C4(CN)4N]– (tcp) were syn- ments of 4.98 μB for 1 and 3.09 μB for 2. In the presence of Cu2+ ions
thesized from [Me4N]tcp and the respective metal perchlorates in tcp reacts with traces of water under hydrolysis of one cyano group
acetone/acetonitrile. Tcp coordinates to the transition metal atoms in
η1 fashion via the nitrogen atom of the pyrrole ring. No coordination
to tricyanopyrrole-2-carboxamide (NC4(CN)3C(O)NH2)– (tcpc). From
solutions in DMF the complex [Cu(tcpc)2(DMF)2] (3) is isolated.
stitution of all hydrogen atoms of the underlying pyrrolide
C4H4N– anion with four electron-withdrawing CN groups
leads to dispersion the negative charge towards the peripheral
cyano groups. Tcp may thus behave as a multifunctional ligand
that gains particular interest in constructing metal-organic sys-
tems.
Introduction
Cyanocarbons are a class of compounds that consist solely
of C and N, known as neutral species and as anions. The func-
tionality is mainly dominated by the large number of present
cyano groups, which are strongly electron withdrawing and
participate by resonance with the π system of the underlying
carbon ring or chain. Cyanocarbon molecules are highly elec-
tron deficient and the respective anions weakly basic and
Results and Discussion
[2]
weakly coordinating.[1] Azole ring based polynitriles
and
cyano-substituted pyrroles [3] have been in the focus of theoret-
ical investigations, showing the aromaticity, the stability versus
oxidation and decreasing basicity with increasing number
of CN groups. The pentacyanocyclopentadienide anion
[C5(CN)5]– is a prominent member due to its high symmetry.
The coordination chemistry of [C5(CN)5]– has received sub-
stantial attention during the last years for the preparation of
metal-organic coordination polymers.[4–7] In contrast, the
chemistry of its congener, in which a nitrogen atom replaces
one ring carbon atom, namely the tetracyanopyrrolide anion
(tcp) (Scheme 2) is still scarcely explored. Although the tcp
anion was prepared already in 1980 by Webster et al. only one
report about its structural chemistry has been published.
H(tcp), Me(tcp), Me4N(tcp), Na(tcp)·2.33THF, and EMIM(tcp)
(EMIM = 1-ethyl-3-methylimidazolium) have been charac-
terized by single-crystal X-ray diffraction.[9] This cyanocarbon
ion attracted our attention because it has the potential for metal
coordination, maintaining a classical σ coordination behavior
either through CN groups, as has been found in the structure
of Na(tcp)[9] or via the nitrogen atom of the pyrrole ring. Sub-
Synthesis and Properties
Tetramethylammonium tetracyanopyrrolide [Me4N]tcp was
synthesized according to the method reported by Schulz et al.
and used as starting material in all attempts of preparation of
transition metal complexes. The synthetic method employed
for the formation of the complexes 1 and 2 is the metathesis
reaction of [Me4N]tcp with the perchlorates of Co2+ and Ni2+
as summarized in Scheme 1.
Scheme 1. The synthetic route for the formation of complexes 1 and
2.
Our experiments show that the coordination ability of tcp is
rather weak. It turned out as essential to use a weakly coordi-
nating anion with the used transition metal salts as essential
for a successful synthesis (Scheme 2).
Water-free metal perchlorates Co(ClO4)2 and Ni(ClO4)2 in
dry acetone as solvent were therefore used. On combining the
metal perchlorates with [Me4N]tcp, [Me4N]ClO4 precipitated
out of the reaction mixture leaving only M2+ (M = Ni, Co) and
the tcp anions in the solution. On recrystallization of the resi-
dues from acetonitrile the complexes [Co(MeCN)4(tcp)2]·
2MeCN (1) and [Ni(MeCN)4(tcp)2]·2MeCN (2) were formed,
* Prof. Dr. J. Beck
Fax: +49-228-735660
E-Mail: j.beck@uni-bonn.de
[a] Institut für Anorganische Chemie
Gerhard-Domagk-Str. 1
53121 Bonn, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201400199 or from the au-
thor.
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Z. Anorg. Allg. Chem. 2014, 640, (10), 1960–1964

Tetracyanopyrrolide Anion as Ligand in Transition Metal Complexes
If water is not strictly excluded, the tetracyanopyrrolide
anion can undergo hydrolysis under transformation of
one cyano substituent to a carboxamide group, transforming
tcp into tricyanopyrrole-2-carboxamide (tcpc). It is observed
that this reaction occurs in the presence of Cu2+ ions
and converts tcp within
3
d
at room temperature
completely. From DMF solutions, the copper(II) complex
[Cu{NC4(CN)3C(O)NH2}2(DMF)2] (3) could be isolated,
Scheme 2. Structural formulae and atom numbering scheme for (left)
tetracyanopyrrolide (tcp) and (right) 3,4,5-tricyanopyrrole-2-carbox- which incorporates the mononegative tricyanopyrrole-2-car-
amide (tcpc) anions.
boxamide as a chelating ligand. This copper(II)-assisted hy-
drolysis to the corresponding amide under mild conditions is
well known.[10,11] Complex 3 was characterized by X-ray dif-
characterized by elemental and thermal analyses and magnetic
measurements.
fraction only, since it was obtained as a by-product in small
amounts.
The thermal decomposition of the air stable complex 2 oc-
curs in two stages in the temperature range 25–420 °C. The
first mass loss of 12.6% occurs between 90–130 °C with a
DTA peak at 110.3 °C and is assigned to the release of the two
solvate acetonitrile molecules (calculated weight loss 12.9%).
Crystal Structures
Crystallographic data and details of structure determinations
The second mass loss of 22.0%, occurring between 140– of compounds 1, 2, and 3 are shown in Table 1. The crystal
190 °C (DTA peak at 176.1 °C) is attributed to the release of structures of 1 and 2 are isotypic. Both contain mononuclear
four coordinated MeCN molecules (calculated mass loss complexes with the central metal ions Co2+ and Ni2+ located
25.8%), leading to Ni(tcp)2. The crystals of 1 have a pro- in inversion centers. The metal atoms are each sixfold coordi-
nounced tendency to lose the incorporated solvate molecules. nated in a slightly distorted octahedral shape by two trans-
Just on exposure to air, freshly prepared orange crystals of positioned tcp ligands and four molecules of acetonitrile in the
1 become opaque after some minutes (Figure S3, Supporting equatorial plane (Figure 1). The two axially bound tcp ligands
Infomation). This limited stability of samples of 1 in air caused are coordinated by their nitrogen atoms of the pyrrole rings in
results of low accuracy with the thermal and elemental analy- η1 fashion. A sandwich structure with η5-pyrrolide ligands
ses in comparison to the nickel complex 2. Thermal decompo- as characteristic for bis(η5-pyrrolide)cobalt or bis(η5-
sition occurs in two stages with the first mass loss of 16.4% pyrrolide)nickel[12,13] is not present, which is obviously caused
beginning as low as 25 °C (DTA peak at 73.1 °C). The second by the low electron density in the central C4N ring due to the
detected mass loss of approx. 17% can be contributed to the strong electron-withdrawing effect of the nitrile substituents.
formation of Co(tcp)2.
The average angle M–N–centroid(tcp ring) equals 178°, indi-
Table 1. Crystallographic data and details of the structure refinements of compounds 1–3.
1
2
3
Formula
C24H12N14Co·2(C2H3N)
637.51
C24H12N14Ni·2(C2H3N)
637.29
C22H18CuN12O4
578.02
Mr /g·mol–1
Crystal system
Space group
a /Å
triclinic
triclinic
triclinic
¯
¯
¯
P1
P1
P1
7.8693(2)
9.9411(3)
10.4822(3)
96.171(2)
102.334(2)
90.110(1)
796.19(4)
1.330
7.8384 (1)
9.9108 (1)
10.4508 (1)
96.075 (1)
102.170 (1)
89.896 (1)
788.99 (2)
1.341
6.4860(3)
9.9804(5)
10.5548(6)
109.265(2)
94.697(4)
101.619(4)
623.58(6)
1.539
b /Å
c /Å
α /°
β /°
γ /°
V /Å3
Dcalc /mg·m–3
Z
1
0.59
1
0.66
1
0.93
μ /mm–1
Crystal size /mm
Absorption correction
Range of data collection /°
No. measured refl.
No. independent refl.
Data averaging Rint
Refined parameters
R (F2 Ͼ 2σ(F2))
R (F2) all refl.
wR (F2)
0.13ϫ0.10ϫ0.10
analytical
3.0 Ͻ θ Ͻ 27.6°
27031
3664
0.071
208
0.037
0.055
0.077
0.16ϫ0.11ϫ0.09
analytical
3.0 Ͻ θ Ͻ 30.1°
29462
4614
0.039
208
0.030
0.037
0.073
0.16ϫ0.07ϫ0.04
analytical
3.0 Ͻ θ Ͻ 27.6°
8797
2672
0.075
180
0.044
0.067
0.097
Goodness of fit, S
1.06
0.27 / –0.52
1.05
0.34 / –0.59
1.04
0.46 / –0.72
Δρmax/Δρmin /e·Å–3
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1961

B. Szafranowska, J. Beck
ARTICLE
reveal a good agreement with previously determined crystal
structures involving this anion and the optimized structure ob-
tained on basis of DFT calculations.[9] The tendency for delo-
calization of the π-electron system towards the peripheral cy-
ano groups is displayed in the exocyclic C–C bond lengths,
which are only slightly longer than those within the aromatic
ring. The two nitrile substituents in the positions 2 and 5 of
the pyrrole ring cause a high steric demand of this molecule
when bound in the η1-N mode, which becomes apparent in the
space filling representation of the molecular structure (Fig-
ure 1). Complexes 1 and 2 adopt a similar structural arrange-
ment as found for the complex [Co{C5(CN)5}2(H2O)2(thf)2]
[C5(CN)5)– = pentacyanocyclopentadienide anion], where the
CoII atom is coordinated by the nitrogen atoms of nitrile
groups of two trans positioned C5(CN)5 ligands.[5] In the crys-
tal of 1 and 2, the shortest distance between the centroids of
each two closest neighbored tcp ligands amounts to 4.36 Å,
which excludes any π-stacking interactions.
In the crystal structure of [Cu{NC4(CN)3C(O)-NH2}2(DMF)2]
(3), the central copper(II) atom is located in an inversion center
and adopts an octahedral coordination (Figure 2). The equato-
rial positions are occupied by two symmetry related tcpc li-
gands, forming five-membered chelate rings in one plane, co-
ordinating the central metal atom by the oxygen atom of the
amide group and the nitrogen atom of the pyrrole ring. Two
trans-positioned dimethylformamide molecules coordinate via
their oxygen atoms almost perpendicular to this plane and
complete the coordination arrangement around the central cop-
per atoms to an elongated octahedron. A pronounced Jahn-
Teller effect is present, manifested in largely differing bond
lengths Cu–N(equat) = Cu–O(equat) = 1.99 Å and Cu–O(axial)
= 2.55 Å. The tcpc ligand is essentially planar, the largest devi-
ation from the plane through all 16 atoms amounts to 0.13 Å.
Figure 1. The crystal structure of [Co(MeCN)4(tcp)2]·2 MeCN (1) in
a view of the unit cell along the a axis. Displacement ellipsoids are
drawn to enclose a 50% probability. On bottom a space filling model
of the complex is given to depict the high steric demand of the tcp
ligand. The solvate CH3CN molecules are omitted for clarity. Sym-
metry code: I = –x+1, –y+1, –z+1.
cating that the metal atoms are essentially in the plane of the
C4N ring, which is expected for a σ bonding situation. The
bonds M–N(pyrrole) amount to 2.15 Å for 1 and 2.11 Å for 2
and are shorter than previously observed in Na(tcp) but slightly
longer than those of the bonds M–NCMe in the equatorial pla-
nes (average M–N 2.12 Å for 1 and 2.07 Å for 2). The tcp
ligands are almost planar, the maximal deviation from the least
square planes through all 13 atoms amounts to 0.09 Å for 1
and 2. The C–N and C–C bond lengths within the pyrrole ring
Figure 2. Structure of the discrete complex [Cu{NC4(CN)3C(O)
NH2}2(DMF)2] (3). Displacement ellipsoids are drawn to enclose a
50% probability, hydrogen atoms are drawn as small spheres.
Magnetic Properties
Magnetic susceptibility data of [Ni(MeCN)4(tcp)2]·2MeCN
(1.362–1.409 Å), the exocyclic C–C bond lengths (1.423– (2) and, due to its sensitivity [Co(MeCN)4(tcp)2]·2MeCN (1)
1.436 Å) and the CϵN triple bond lengths (1.144–1.150 Å) with some loss of incorporated MeCN were recorded in the
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Z. Anorg. Allg. Chem. 2014, 1960–1964

Tetracyanopyrrolide Anion as Ligand in Transition Metal Complexes
range of 1.9–300 K. Both complexes show nearly ideal Curie Both complexes behave as diluted S = 3/2 (CoII, 1) and S = 1
behavior in the full temperature range and therefore (NiII, 2) spin systems and obey the Curie law. In the presence
display regular paramagnetism. From the equation μeff
=
of Cu2+ ions and even small amounts of water, tcp undergoes
2.828(χmolT)1/2 the calculated effective magnetic moment per rapid hydrolysis under transformation of the cyano substituent
Ni2+ ion in complex 2 at 300 K is 3.09 μB, which fits well in vicinal position to the ring nitrogen atom to a carboxamide
with the free Ni2+ ion spin-only value for a S = 1 system (μeff
=
group. The hydrolysis product tricyanopyrrolide-2-carbox-
2.83 μB) (see Supporting Information). According to the linear amide tcpc acts as a bidentate ligand as found in the complex
regression of the χ–1 = f(T) function for the cobalt(II) complex [Cu(tcpc)2(DMF)2] (3).
1 (Figure 3) the susceptibility is consistent with a high spin
complex of a magnetically isolated Co2+ ion in octahedral co-
ordination. The experimental moment μeff at 300 K equals to
Experimental Section
4.98 μB, which is substantially higher than the spin only value
Safety Note: Sodium cyanide is highly poisonous and liberates volatile
of μeff = 3.87 μB calculated for a S = 3/2 system. The enlarged
HCN when mixed with acids. Organic perchlorates like [Me4N]ClO4
effective magnetic moment is typical for Co2+ and is explained
have to be treated as potential explosives and must therefore be han-
dled with precautions; only small amounts should be prepared.
by the contribution of the orbital angular momentum to mag-
netic moment.[14] The large M···M separation between the dis-
crete complexes in the crystals excludes any significant metal–
metal magnetic interactions and pure paramagnetism is pres-
ent.
Co(ClO4)2·6H2O and Ni(ClO4)2·6H2O were dried under reduced pres-
sure at 50 °C for about 5 h. Acetonitrile and acetone were stirred over-
night with dry K2CO3 and then distilled.
Tetramethylammonium Tetracyanopyrrolide [Me4N]tcp: The pro-
cedure mainly follows that given in Ref. [8] and with some modifica-
tions in Ref. [9]. Details are given in the Supporting Information.
Synthesis of [Co(MeCN)4(tcp)2]·2 MeCN (1): Dry [Me4N]tcp
(60 mg, 0.25 mmol) was dissolved in dry acetone (10 mL) and mixed
with a solution of Co(ClO4)2 (33 mg, 12 mmol) in dry acetone
(10 mL). [Me4N]ClO4 precipitated as a white solid, which was filtered
and discarded. The filtrate was evaporated to dryness and the solid
residue was dissolved in dry acetonitrile. Slow evaporation of the solu-
tion (within about 6 d) produced orange crystals, which rapidly lost
included solvent when taken out of the solution (see Supporting Infor-
mation). Yield: 20 mg (20%). M.p. Ͼ420 °C under decomposition.
C24H12N14Co (555.39 g·mol–1): C 46.17 (calcd. 51.90); H 2.22 (2.18);
N 32.63 (35.31)%.
Synthesis of [Ni(MeCN)4(tcp)2]·2 MeCN (2): The synthetic pro-
cedure follows that of 1, but with the use of Ni(ClO4)2. The product
was obtained 33% yield as air stable violet crystals. M.p. Ͼ420 °C
under decomposition. C28H18N16Ni (637.27 g·mol–1): C 52.02 (calcd.
52.72); H 2.89 (2.84); N 34.98 (35.16)%.
Figure 3. Plots of inverse susceptibility χ–1(T) and the effective mag-
netic moments μeff as function of temperature for the high spin co-
balt(II) complex 1 (top) and nickel(II) complex 2 (bottom). The experi-
mental data correspond to open squares and cross points. The solid
line represents the best fit to the Curie–Weiss model χmol = C/(T – Θ)
with Θ = 0 K for 1 and 2.
Synthesis of [Cu{NC4(CN)3C(O)NH2}2(DMF)2] (3): Co(ClO4)
2·6H2O (23 mg, 0.06 mmol) was dissolved in acetone (10 mL). 5 mL
of a solution of [Me4N]tcp (30 mg, 0.12 mmol) in acetone were added.
[Me4N]ClO4 precipitated as a white solid, which was filtered off and
discarded. The filtrate was evaporated to dryness and dissolved in
DMF (3 mL). After 4 d, blue crystals in the shape of blocks appeared.
Crystal Structure Determinations: The crystals were selected im-
mersed in perfluorinated oil and transferred in this medium into the
cold nitrogen stream of the crystal cooling device of the diffractometer.
Data collections were performed at –150 °C (123 K) with a Bruker-
Nonius kappa-CCD diffractometer equipped with graphite monochro-
matized Mo-Kα radiation (λ = 0.71073 Å). The crystal structures were
solved by Direct Methods and refined using the SHELX97 program
suite.[15] Analytical absorption corrections were applied to all data
sets.[16] Hydrogen atoms were refined in idealized positions as riding
on their attached carbon atoms with isotropic displacement factors
fixed to the value 1.5 of the respective carbon atoms. Graphical repre-
Conclusions
The complexes [Co(MeCN)4(tcp)2]·2MeCN (1) and
[Ni(MeCN)4(tcp)2]·2MeCN (2) show that the tetracyanopyrrol-
ide anion acts as a monodentate ligand towards transition metal
ions. Only the η1-N-σ coordination mode via the lone electron
pair of the nitrogen atom of the pyrrole ring was found. The
other possible coordination modes, by the nitrogen atoms of
the nitrile substituents as present in the structure of Na(tcp)[9]
or the η5-π coordination mode, have not been observed so far. sentations were made using the program DIAMOND.[17]
Z. Anorg. Allg. Chem. 2014, 1960–1964
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1963

B. Szafranowska, J. Beck
ARTICLE
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[3] Rimarcˇík, V. Lukesˇ, E. Klein, M. Griesser, A.-M. Kelterer, Chem.
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perature range 20–420 °C.
Phys. 2008, 353, 177–184.
Magnetic Measurements: The magnetic susceptibility measurements
were performed with a Quantum Design PPMS Vibrating Sample
Magnetometer (VSM) in the temperature range 1.9 to 300 K under an
applied magnetic field of 10 kOe. Samples of weights 8.8 mg for 1
and 11.6 mg for 2 were used.
[4] R. J. Less, M. McPartlin, J. M. Rawson, P. T. Wood, D. S. Wright,
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Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.
Copies of the data can be obtained free of charge on quoting
the depository numbers CCDC-999746 for 1, CCDC-999747
for 2, and CCDC-999748 for 3 (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
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Supporting Information (see footnote on the first page of this article):
Detailed description of the synthesis and NMR spectra of [Me4N]tcp,
TG/DTA diagrams for complexes 1 and 2, bond lengths and angles for
the crystal structures of 1, 2, and 3.
Chem. Soc., Chem. Commun. 1991, 1368.
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Acknowledgements
many, 1999.
The support of this work by the Deutsche Forschungsgemeinschaft
within the Collaborative Research Centre SFB 813 is gratefully
acknowledged. The authors thank J. Daniels for recording the single
crystal diffraction data sets, N. Wagner for the magnetic measurements
and K. Armbruster for DTA/TG thermal investigations.
[15] G. M. Sheldrick, SHELX97 [includes SHELXS97, SHELXL97,
Ciftab], Programs for Crystal Structure Analysis, Universität
Göttingen, Germany, 1998.
[16] N. W. Alcock, Cryst. Computing 1977, 271.
[17] DIAMOND, Program for Crystal Structure Visualisation, Crystal
Impact Corp., Bonn, Germany, 2005.
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Received: April 28, 2014
Published Online: June 10, 2014
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Z. Anorg. Allg. Chem. 2014, 1960–1964