An investigation of the formation mechanism of copper(II) carbodiimide

Article abstract of DOI:10.1002/zaac.200900320

The formation mechanism of copper(II) carbodiimide in aqueous ammonia solution was studied by synthetic and crystallography means. The reduction of the starting Cu^II solution led to the synthesis of solid Cu ¹ intermediates, and the crystal structures of the two new compounds Cu(NCNH₂)Cl (a = 8.2925(4), b = 3.7275(1), c = 12.4534(4) A, Pnma) and the isostructural Cu(NCNH₂)Br (a = 8.406(3), b = 3.9229(9), c = 12.656(3) A, Pnma) were elucidated. A further increase in pH value by adding more ammonia led to the crystallization of two additional Cu^I intermediates, Cu₂NCN and the ammine adduct Cu₄(NCN) ₂NH₃. The former can be also made by the thermal decomposition of the latter whereas mild oxidation of Cu₄(NCN) ₂NH₃ led to the formation of perfectly crystalline CuNCN.

Full text of DOI:10.1002/zaac.200900320

ARTICLE
DOI: 10.1002/zaac.200900320
An Investigation of the Formation Mechanism of Copper(II) Carbodiimide
[a]
[a]
[a]
Xiaohui Liu, Hari Puspitosari, and Richard Dronskowski*
Dedicated to Professor Arndt Simon on the Occasion of His 70th Birthday
Keywords: Carbodiimides; Cyanamides; Copper; X-ray diffraction; Reaction mechanisms
Abstract. The formation mechanism of copper(II) carbodiimide in 8.406(3), b = 3.9229(9), c = 12.656(3) Å, Pnma) were elucidated. A
aqueous ammonia solution was studied by synthetic and crystallo- further increase in pH value by adding more ammonia led to the crys-
II
I
graphic means. The reduction of the starting Cu solution led to the tallization of two additional Cu intermediates, Cu
2
NCN and the am-
I
synthesis of solid Cu intermediates, and the crystal structures of the mine adduct Cu
4
(NCN)
)Cl (a = 8.2925(4), b = 3.7275(1), c = thermal decomposition of the latter whereas mild oxidation of
2.4534(4) Å, Pnma) and the isostructural Cu(NCNH )Br (a = Cu (NCN) NH led to the formation of perfectly crystalline CuNCN.
2 3
NH . The former can be also made by the
two new compounds Cu(NCNH
2
1
2
4
2
3
Introduction
The motivation for carrying out exploratory syntheses in the
fields of carbodiimides and cyanamides of the divalent mag-
5
9
netic transition metals (d –d ) is naturally linked with the
structural and physical properties of these novel types of inor-
ganic materials [1–7]. Among them, copper carbodiimide,
CuNCN, stands for the nitrogen-based analogue of CuO [2, 5], Scheme 1.
and CuNCN was firstly synthesized in large, phase-pure and
well-crystallized quantities only five years ago [2]. The syn-
thetic difficulty lies in the fact that aforementioned M2+ transi-
In order to come up with a less formal description, we have
now examined the reaction mechanism of CuNCN formation
in more detail. The syntheses and crystal structures of two new
solid-state copper(I) halide complexes have been accom-
tion-metal ions prefer to form cyanamide complexes in aque-
ous solution [8, 9] such that, as a characteristic example, Cu2+
cations merely form coordination complexes like
2
+
I
[
Cu(NCNH ) ] from which the protons can not be fully re-
2 4
plished, together with yet another new Cu cyanamide solid-
moved. The brute-force alternative of a (strong) basic aqueous
state material.
II
Cu solution reacting with anionic cyanamide units, however,
yields a black material whose amorphous nature renders its
structure determination impossible [10, 11]. As said before, we Results and Discussion
were fortunate to discover a new synthetic approach for per-
Reaction Procedure
fectly crystalline CuNCN earlier [2]. The two-step method is
I
essentially based on an ammine Cu cyanamide compound,
The reported synthesis of CuNCN is, in fact, a one-pot reac-
Cu (NCN) NH , which can be obtained from the mild reaction tion. In this study, the synthetic procedure has been interrupted
4
2
3
+
of [Cu(NH ) ] and H NCN under aqueous ammonia condi- after each proposed intermediate species. In order to character-
3
2
2
tions [12]. The likewise mild oxidation of Cu (NCN) NH
ize these intermediates, additional reactions were carried out.
4
2
3
yields the target phase CuNCN. Schematically, the entire pro-
cedure may be summarized as follows (Scheme 1):
At first, Cu2+ ions were reduced by Na SO in an excess
2
3
(two equimolar) of H NCN solution in an inert atmosphere.
2
For CuCl2 as starting substance, the pure intermediate
Cu(NCNH )Cl (1a) was obtained. In the case of CuBr , the
2
2
*
Prof. Dr. R. Dronskowski
bromine analogue Cu(NCNH )Br (1b) was found together with
2
E-Mail: drons@HAL9000.ac.rwth-aachen.de
a small amount of CuBr. This finding may be a consequence
[
a] Institute of Inorganic Chemistry
RWTH Aachen University
of the 25 times smaller solubility product of CuBr (Ksp = 4.15
–8
2
–2
–6
2
–2
52056 Aachen, Germany
×
10 mol ·L ) than that of CuCl (1.02 × 10 mol ·L ).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.200900320 or from the
author.
Fortunately, single crystals of 1b could be isolated for crystal
structure determination. Although 1a was not obtained as a
Z. Anorg. Allg. Chem. 2010, 636, 121–125
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
121

X. Liu, H. Puspitosari, R. Dronskowski
ARTICLE
single crystal, the XRPD patterns for 1a and 1b clearly show observed with the only exception of a slightly reduced yield
that they are isostructural.
of CuNCN from lower-concentrated H NCN solutions. If an
2
II
2+
After reducing the whole amount of Cu in H NCN solution equimolar relation of Cu and H NCN was used at the very
2
2
by using Na SO , a concentrated ammonia solution was added beginning, insoluble CuCl was found together with 1a; the lat-
2
3
until pH > 10, which resulted in the formation of colorless ter phase will not, however, influence the purity of the final
Cu (NCN) NH (2). By careful observation it appears that, product simply because CuCl later also dissolves in concen-
4
2
3
II
along with the addition of ammonia, a yellow precipitate forms trated ammonia solution. Taking into account that Cu must
at the very beginning, which rapidly changes to colorless. be tetracoordinate by electronegative atoms such as O and N –
2+
Thus, the attempt to isolate the yellow compound affords a for example, as given in the complex [Cu(NCNH ) ] cation
2
4
buffer solution, a one-molar NH ·NH Cl mixture with a pH [9] – we therefore tentatively conclude that the complex ion
3
4
2+
around 8–9, which leads to a solid mixture of 2 and Cu NCN [Cu(NCNH ) (H O) ] has been formed [Equation (1)].
2
2 2
2
2
(
3). The chemical similarity between 2 and 3 is obvious be-
cause the heat treatment of 2 (150 °C, vacuum, 48 h) leads to
a pure yellow powder of 3, together with NH gas inside the
3
cooling trap. Elemental analysis agrees with the theoretical
II
composition of Cu NCN. Comparing the infrared spectra of 3
The reduction of Cu in the presence of coordinating ligands
2
with the compilation of metal-carbodiimides/cyanamides pro- is an established method to obtain copper(I) complexes [13,
vided by Reckeweg and Simon [7], it is obvious that 3 not only 14], but such reactions are typically performed in organic sol-
exhibits strong carbodiimide-type asymmetrical vibrations, vents instead of aqueous solution because of the low equilib-
–1
I
ν (NCN) = 2138/2193 cm , plus a strong deformation vibra- rium concentrations of Cu and its instability with respect to
as
–1
I
0
II
tion, δ(NCN) = 655 cm , but also the symmetrical ν band disproportion (2 Cu = Cu + Cu ). The latter stability problem
s
–
1
(
1182 cm ), which would be allowed for the less symmetrical in aqueous solution depends very strongly on the nature of the
2–
+
cyanamide [N–C≡N] anion and IR-forbidden for the [N=C= ligand which should have π-acceptor character to stabilize Cu .
2–
N] carbodiimide unit. However, no good structural model has In this respect, H NCN is indeed a suitable ligand that is able
2
been found so far to allow for a reasonable crystal-structure to form the new solid-state halide complexes 1a or 1b under
refinement of the XRPD pattern.
strongly acidic conditions [Equation (2)] and even to allow for
In case that a small amount of air was allowed to enter the the existence of the ammonia complex 2 under strongly basic
mixture of 2 and the cyanamide solution, the color of the solu- conditions [Equation (4), see below].
tion changed rapidly to blue; the oxidation of the colorless
precipitate 2, however, proceeded very slowly to eventually
give black CuNCN. The latter phase forms with an exception-
ally good crystallinity overnight (Figure 1).
The adjustment of the pH value plays a very important role
in the entire procedure; in fact, the change of the pH value
leads to the formation of four different compounds. Whereas
the pH value of the starting reaction mixture is about 4, the
addition of the reducing agent leads to reaction 2, in which the
pH drops to about 1. Thus, the two solid-state complexes 1a
and 1b can only exist under acidic conditions. Upon the addi-
tion of ammonia to the reaction solution, which leads to
weakly basic conditions, the H NCN species is deprotonated
2
and compound 3 is formed as shown in Equation (3):
When the pH value exceeds 10, only compound 2 exists,
which is a consequence of the law of mass action. In other
Figure 1. SEM micrograph of CuNCN. All micro-crystals appear as
words, the structural stability of Cu (NCN) NH (2) at room
4
2
3
thin plates.
temperature exceeds the one of Cu NCN (3). Likewise, it is
2
not difficult to understand that the higher temperatures lead to
the removal of weakly coordinating NH from 2, thereby form-
3
Mechanism Discussion
ing 3 by thermal decomposition [Equation (4)].
We tried to reduce the surplus amount of H NCN from four
2
to two equimoles relative to that of Cu2 ions as described in
+
experiment 1. Nonetheless, essentially the same results were
1
22
www.zaac.wiley-vch.de
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 121–125

An Investigation of the Formation Mechanism of Copper(II) Carbodiimide
The final oxidation step takes place under strongly basic refined for 1b; for the 1a Rietveld-based structure, the carbon–
(
aqueous ammonia) conditions, which are more effective and nitrogen bond lengths had to be refined with restraints [1.10(4)
ensure the purity of the final product CuNCN by forming and 1.27(4) Å] but the structure of H NCN should not have
2
2
+
[
Cu(NH ) ] . As mentioned before, stable copper(I) com- too much deviation. As is evident from Table 1, there exist no
3 4
plexes are not very common although plenty of copper(I) com- strong hydrogen bonds in the crystals. Compared to much
pounds have been reported in recent years [13–17]. To the best shorter Cu–Cu distances of 2.979(1) Å in Cu(NH )Cl [14] and
3
of our knowledge, the use of a stable copper(I) complex to 2.453(1) Å in Cu (C N H ) [19], the Cu–Cu distances in 1a
2
7
3
12 2
synthesize a copper(II) target compound is very rare. The main and 1b are about 3.0 Å and indicate nonbonding Cu–Cu inter-
advantage of our method is that the oxidation of suspended 2 actions [20].
is taking place quite slowly in order to form well-crystallized
CuNCN [Equation (5)].
Crystal Structure
I
A large number of different structures for Cu halide com-
2
Figure 3. ORTEP drawing of Cu(NCNH )Br with thermal ellipsoids
plexes were identified depending on the nature of ligand as
at 30 % probability level and hydrogen atoms as circles of arbitrary
well as the composition [14–17]. In the present case, the struc- size. Cu(NCNH )Cl is iso-structural.
2
tures of 1a and 1b are of the same type, despite the fact that
Cu(NH )Cl forms linear NH –Cu–Cl units whereas
3
3
Cu(NH )Br does not exist; instead, the phase [Cu(NH ) ]Br Table 1. Hydrogen bond lengths /Å and angles /° for 1a and 1b.
3
3 2
has been described [14]. The structures of 1a and 1b may be
D–H···A
d(D–H) d(H···A) d(D···A) <(DHA)
similar to the one of copper(I) iodide pyridine, [CuI(NC H )] ,
5
5 4
I
1a N2–H2A···N1
0.89
0.882
2.58
3.04
3.346
3.55
144.0
134.63
with a tetracoordinate Cu ion, to which three halides and one
nitrogen atom from the ligand are coordinated [18]. In the case
of compounds 1a and 1b, one finds infinite one-dimensional
chains of [–Cu(NCNH )X–] (see Figure 2), which consist of
1
b N2–H2A···N1
It is quite interesting that the NCN cores of the cyanamide
2
n
I
tetracoordinate Cu ions that are coordinated to three halide molecules exhibit different structures in the two intermediates
atoms [2 × 2.47(3) and 1 × 2.40(3) Å for Cu–Cl in 1a; 2 × 1a and 2 relative to CuNCN. In Cu(NCNH )Cl, there is a cyan-
2
2
.536(2) and 1 × 2.510(3) Å for Cu–Br in 1b] and to one nitro- amide molecule with a single C–N and a triple C≡N bond (see
gen atom of a C–N single bond [Cu–N = 2.04(3) in 1a and above). The same is found for Cu (NCN) NH despite it con-
4
2
3
II
1
.92(2) Å in 1b] from the cyanamide molecule (see Figure 3). tains an anion. In CuNCN, the “harder” Cu atom is connected
The H NCN unit shows the typical cyanamide shape with one to the NCN2– unit, which exists in the carbodiimide shape with
2
single C–N bond of 1.35(2) Å and one triple bond of 1.11(2) Å two double C=N bonds. Obviously, the molecular shape de-
Figure 2. Packing diagram of the crystal structure of Cu(NCNH
2
)X with X = Cl and Br. The left part shows a view along the crystallographic
b axis, and the right part displays the quasi one-dimensional copper-halogen chain.
Z. Anorg. Allg. Chem. 2010, 121–125
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
123

X. Liu, H. Puspitosari, R. Dronskowski
ARTICLE
pends on the counter cation, the coordination motif and also Experiment 5: Compound 2 (0.1 g) was heated at 150 °C under vac-
uum for two days in one side of a long Schlenk tube. Compound 3
was obtained as a yellow precipitate. The presence of ammonia was
the crystal packing.
verified in the attached cooling trap. Compound 3 appeared to be very
well crystallized. XRPD analysis showed that 3 was a pure phase,
Conclusions
which could be indexed using an orthorhombic cell. Analytical data
–
1
(
(
2
%) for Cu
2
NCN (167.09 g·mol ): Cu 75.30 (calcd. 76.06); C 7.20
The reaction mechanism of the formation of copper(II) car-
bodiimide was analyzed by using chemical and crystallo-
graphic means. The results indicate the presence of at least
three new intermediates, two of which were characterized in
terms of their crystal structures. The crystal structure determi-
7.18); H 0.14 (0); N 17.0 (16.76). IR (KBr): νas(NCN) = 2138 and
193(s) cm , ν (NCN) = 1183 (s) cm and δ(NCN) = 655 (m) cm .
s
–
1
–1
–1
Experiment 6: Upon stirring the suspension from experiment 3 over-
night, compound 2 was slowly oxidized at room temperature because
of the presence of small amounts of atmospheric oxygen within the
reaction flask. Black crystalline CuNCN [2] was obtained.
nation of Cu NCN is still in progress. The entire reaction
2
scheme may be formulated as follows (Scheme 2).
X-ray Crystallography
Structure data of 1a: The X-ray diffraction data were recorded at
room temperature using a G670 Imageplate Guinier diffractometer
(Huber, Rimsting) equipped with a Johansson Ge monochromator for
Cu-Kα1 radiation and a flat sample holder in the 2θ range between 6
and 100°. The XRPD pattern of 1a indicated a pure phase such that
its structure was determined using Rietveld analysis based upon the
iso-structural model of 1b. The FULLPROF program package [21]
was used with a pseudo-Voigt profile function, and the background
data were manually subtracted by linear interpolation. Because of
strongly preferred orientation and likewise asymmetric Bragg peaks,
the atomic distances of Cu–N and C–N and the N–C–N angle had to
be soft-restrained to yield reasonable bond lengths and a more reliable
refinement. Likewise, all hydrogen positional parameters were fixed
Scheme 2.
2 2 r
to N–H distances of 0.89 Å. Empirical formula CH ClCuN , M =
1
1
0
41.04, orthorhombic, Pnma, a = 8.2925(4), b = 3.7275(1), c =
3
2.4534(4) Å, V = 384.48(2) Å , Z = 4. The residual values arrived at
p
.072 (R ), 0.102 (Rwp), and 0.18 (RBragg) for a total of 22 variables, 4
Experimental Section
In order to elucidate possible reaction pathways, alternative syntheses
were tested, namely a total of six different preparations.
restraints and 248 Bragg reflections.
Structure data of 1b: A complete set of X-ray intensities was col-
lected with a Bruker SMART-CCD diffractometer using graphite-
Experiment 1: A mixture of CuCl
amide H
water (10 mL) under argon. A solution of Na
water (10 mL) was added dropwise to the above mixture resulting in using the empirical SADABS method. The crystal structure was solved
colorless needle-like crystals of the new phase Cu(NCNH )Cl (1a), by Direct Methods and refined by alternating cycles of difference Fou-
which were filtered off and dried under vacuum. Analytical data (%) for rier syntheses and full-matrix least-squares refinements with SHELXS-
2 2
α
·2H O (1.71 g, 10 mmol) and cyan- monochromatized Mo-K radiation at T = 20(1) °C and with the aid
2
NCN (0.84 g, 20 mmol) was stirred at room temperature in of the SMART and SAINT software packages [22]. A total Ewald
SO (1.26 g, 10 mmol) in sphere was collected and carefully corrected with respect to absorption
2
3
2
–1
2
Cu(NCNH )Cl (141.04 g·mol ): Cu 44.76 (calcd. 45.05); C 8.32 (8.52);
H 2.01 (1.42); N 19.34 (19.86).
Table 2. Important structural parameters of 1a and 1b including char-
Experiment 2: It was carried out in analogy to experiment 1 but using
acteristic bond lengths /Å and angles /°.
CuBr
like crystals of the new phase Cu(NCNH
of a CuBr impurity detectable from X-ray powder diffraction data
2
(2.23 g, 10 mmol). The product crystallized as colorless needle-
2
)Br (1b) with a small amount
2 2
Cu(NCNH )Cl (1a) Cu(NCNH )Br (1b)
color, form
space group
a /Å
white needles
Pnma (no. 62)
8.2825(4)
white needles
(
XRPD). Fortunately, a single crystal of 1b was isolated, which turned
Pnma (no. 62)
8.406(3)
out suitable for single-crystal X-ray diffraction.
b /Å
3.7275(1)
3.9229(9)
12.656(3)
417.4(2)
Experiment 3: The addition of a 25 wt.-% ammonia solution
c /Å
12.4534(4)
384.48(2)
V /Å3
(
3 2
NH ·H O, 10 mL) to experiment 1 resulted in the precipitation of
pure white Cu
vacuum [12].
4 2 3
(NCN) NH (2), which was filtered off and dried under
Cu–X
2.47(3) (2×); 2.40(3) 2.536(2) (2×); 2.510(3)
Cu–N1
2.04(3)
1.916(15)
N1–C, C–N2
Cu–N1–C
N1–C–N2
1.10(4), 1.27(4)
177(2)
1.11(2), 1.35(2)
177.2(19)
180(2)
3 4
Experiment 4: The addition of a one-molar NH ·NH Cl buffer solu-
tion (10 mL) to experiment 1 yielded a pale-yellow precipitate, which
was filtered off and dried under vacuum. XRPD analysis showed that
177 (2)
N2–H2A, N2–H2B 0.89 (fixed)
0.882, 0.865 (restrained)
it contained a mixture of 2 and the new phase Cu
2
NCN (3).
1
24 www.zaac.wiley-vch.de
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 121–125

An Investigation of the Formation Mechanism of Copper(II) Carbodiimide
2
Table 3. Positional parameters (all atoms on 4c, y ≡ 1/4) and isotropic displacement parameters (Ueq in Å ) for 1a and 1b (X = Cl, Br) with
standard deviations in parentheses.
Cu(NCNH
2
)Cl (1a)
2
Cu(NCNH )Br (1b)
Atom
Cu
x
z
U
eq
x
z
U
eq
0.1211(3)
0.0468(5)
0.5593(2)
0.3727(3)
0.6110(11)
0.6340(18)
0.6665(11)
0.738
0.0894(15)
0.0169(14)
0.082(2)
0.082(2)
0.082(2)
0.088
0.1091(3)
0.0365(2)
0.5550(2)
0.36269(14)
0.5892(15)
0.6131(13)
0.6415(13)
0.7066
0.0758(10)
0.0552(7)
0.076(5)
0.056(4)
0.073(5)
0.087
X
N1
0.3555(10)
0.4833(11)
0.6290(10)
0.643
0.3312(17)
0.458(3)
0.6132(17)
0.6433
C
N2
H2A
H2B
0.696
0.611
0.088
0.6903
0.5943
0.087
9
7 and SHELXL-97 [23]. All non-hydrogen atoms were subjected to [8] X. Liu, P. Kroll, R. Dronskowski, Z. Anorg. Allg. Chem. 2001,
627, 1882.
anisotropic refinement. All hydrogen atoms were located by difference
Fourier synthesis and refined using a riding model with restrained N–
H distances of 0.86–0.89 Å and with Uiso(H) values of 1.2 × Ueq(N2).
[
[
[
9] X. Liu, R. Dronskowski, Z. Kristallogr. NCS 2002, 217, 118.
10] S. K. Deb, A. D. Yoffe, Trans. Faraday Soc. 1959, 55, 106.
11] K. M. Adams, M. J. Cooper, M. J. Sole, Acta Crystallogr. 1964,
Empirical formula CH
2
BrCuN
2
, M
r
= 185.49, orthorhombic, Pnma, a =
17, 1449.
3
8
.406(3), b = 3.9229(9), c = 12.656(3) Å, V = 417.4(2) Å , Z = 4,
[12] X. Liu, P. Müller, R. Dronskowski, Z. Anorg. Allg. Chem. 2005,
size = 0.15 × 0.05 × 0.04 mm, θrange = 2.9 to 24.8°, 3039 reflections
int = 0.126), GOF = 0.862, R [I > 2σ(I)) = 0.0639, wR
.130; CSD-420807. All important numerical details of the refinements
631, 1071.
(
0
R
1
2
[I > 2σ(I)) = [13] M. A. Galindo, D. Amantia, W. Clegg, R. W. Harrington, R. J.
Eyre, J. P. Goss, P. R. Briddon, W. McFarlane, A. Houlton, Chem.
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parameters are listed in Table 3.
[14] G. Margraf, J. W. Bats, M. Bolte, H. W. Lerner, M. Wagner,
Chem. Commun. 2003, 956.
[
15] C. Nather, J. Greve, I. Jess, Solid State Sci. 2002, 4, 813.
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Oldham, B. W. Skelton, M. E. Smith, A. H. White, Dalton Trans.
[
Supporting Information (see footnote on the first page of this article):
Diffraction and IR data for compounds 1–3.
[
[
2008, 13, 1710.
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Z. Anorg. Allg. Chem. 2010, 121–125
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
125