The complex amide K2[Zr(NH2)6]

Article abstract of DOI:10.1002/zaac.201100083

We present the low-temperature synthesis of potassium hexaamido zirconate(IV) from the transition metal tetrafluoride and thealkali metal dissolved in liquid ammonia at -40 °C. Potassium hexaamido zirconate(IV) K₂[Zr(NH₂)₆] is the first ternary amide reported for elements of group 4 of the periodic table It crystallizes with a novel structure type in the trigonal space group Rβar{3}χ with a = 6.5422(2) A, c = 32.824(2) A, V = 1216.66(9) A³, Z = 6 and c/a = 5.017. The structure can be derived from the K₂PtCl₆ type. The compound contains discrete D ₃-symmetric [Zr(NH₂)₆]^2- anions which differ significantly from octahedral shape. Quantum chemical calculations show the distortion to arise from a splitting of degenerate d-orbitals on the zirconium atom leading to a significant gain in energy. Copyright

Full text of DOI:10.1002/zaac.201100083

ARTICLE
DOI: 10.1002/zaac.201100083
The Complex Amide K2[Zr(NH2)6]
Florian Kraus,*[a] Sebastian A. Baer,[a] and Antti J. Karttunen[b]
Dedicated to Professor Hanskarl Müller-Buschbaum on the Occasion of His 80th Birthday
Keywords: Liquid ammonia; Amides; Zirconium; Structure elucidation; Quantum chemistry
Abstract. We present the low-temperature synthesis of potassium hexa- c = 32.824(2) Å, V = 1216.66(9) Å3, Z = 6 and c/a = 5.017. The
amido zirconate(IV) from the transition metal tetrafluoride and the structure can be derived from the K2PtCl6 type. The compound con-
alkali metal dissolved in liquid ammonia at –40 °C. Potassium hexa- tains discrete D3-symmetric [Zr(NH2)6]2– anions which differ signifi-
amido zirconate(IV) K2[Zr(NH2)6] is the first ternary amide reported cantly from octahedral shape. Quantum chemical calculations show
for elements of group 4 of the periodic table It crystallizes with a novel the distortion to arise from a splitting of degenerate d-orbitals on the
¯
structure type in the trigonal space group R3c with a = 6.5422(2) Å, zirconium atom leading to a significant gain in energy.
from the alkali metal amide and SnPh4 in liquid ammonia at
Introduction
The substance groups of amides, imides and nitrides are a
0 °C and single crystals of these compounds could also be
obtained at 385 K and 80 bar NH3 pressure within seven
classic field of preparative inorganic chemistry. Many pseudo
days.[4,5,9–13]
binary and ternary amides of the main group elements, the
Fluorides have been rarely used as starting materials for am-
transitions metals and the lanthanides have been reported. Pio-
ide, imide or nitride synthesis due to their very low solubility
neers in this field are Juza, Jacobs and their co-workers, for
in ammonia. Franklin and Kraus stated in 1898 “The fluorides
examples see.[1–5] Amides of zirconium have been synthesized
are insoluble”.[14] Thus, exploration of fluorine chemistry in
previously: Zr(NH2)4 was synthesized by ammonolysis of
ammonia might have been seriously hampered. Our previous
ZrCl4·4NH3 at 220–250 °C.[6] In an attempt to reproduce this
studies have shown that some carefully chosen fluorides are
result, Schmitz-DuMont only obtained the imide Zr(NH)2.[7] In
quite soluble in liquid ammonia. ZrF4 for example dissolves
a targeted synthesis of Ti(NH2)4 from K2[Ti(SCN)6] and KNH2
readily and forms [Zr(NH3)4F4] in solution. The compound
in liquid ammonia, only polymerization reactions were ob-
crystallizes from liquid ammonia at –40 °C as its monoammo-
served leading presumably to Ti(NH)2 and to potassium amido
niate [ZrF4(NH3)4]·NH3.[15] Uranium tetrafluoride forms the
diimido titanate(IV) K[Ti(NH2)(NH)2], depending on the stoi-
isotypic compound, and we have shown that the [UF4(NH3)4]
chiometry of the reaction.[7,8] So, the pure amides of group 4
molecule is stable up to 150 °C.[16] Here we report the low-
still seem to be elusive.
temperature synthesis of K2[Zr(NH2)6] (1) from ZrF4 and po-
Most of the syntheses targeting amides or imides used pres-
tassium metal dissolved in liquid ammonia and the characteri-
surized ammonia at temperatures ranging from ambient tem-
zation of the compound by single-crystal X-ray structure anal-
perature to several hundred °C and pressures up to 6 kbar.
ysis and quantum chemical methods.
Low-temperature synthesis of complex amides is also possible
using organometallic starting materials. For example, com-
pounds like M2[Sn(NH2)6] (M = Na–Cs) were synthesized
Results and Discussion
Alkali metals dissolved in liquid ammonia are among the
strongest reducing agents available. However, reactions of
metal halides with such solutions rarely lead to the metal in its
elemental state. In most of the cases insoluble, “uncharacter-
ized” or “uncharacterizable” residues have been obtained.[17]
In our case, potassium must have formed potassium amide in
situ (Equation (1)), which reacted with the dissolved ZrF4 with
formation of compound 1 (Equation (2)). Some colorless sin-
gle crystals were isolated at low temperature, and the structure
was determined (see Table 2) from the collected X-ray data.
* Dr. F. Kraus
Fax: +49-89-289-13762
E-Mail: fluorchemie@mytum.de
[a] AG Fluorchemie
Department Chemie
Technische Universität München
Lichtenbergstrasse 4
85747 Garching, Germany
[b] University of Eastern Finland
Department of Chemistry
P.O. Box 111
80101 Joensuu, Finland
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201100083 or from the
author.
2 K + 2 NH3 → 2 KNH2 + H2
(1)
1122
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 637, 1122–1130

The Complex Amide K2[Zr(NH2)6]
Table 1. Selected atom distances and angles of compound 1. Symmetry transformations for the generation of equivalent atoms: #1 –x, –x+y,
–z+1/2; #2 x–y, –y, –z+1/2; #3 y, x, –z+1/2; #4 –y, x–y, z; #5 –x+y, –x, z.
Distance /Å
Angle /deg
N(1)–H(1A)
N(1)–H(1B)
Zr–N
0.832(16)
0.848(15)
2.1931(9)
N(1)–Zr(1)–N(1)#1, N(1)#2–Zr(1)–N(1)#4, N(1)#3–Zr(1)–N(1)#5
N(1)–Zr(1)–N(1)#2, N(1)#3–Zr(1)–N(1)#4, N(1)#1–Zr(1)–N(1)#5
N(1)#1–Zr(1)–N(1)#2, N(1)#1–Zr(1)–N(1)#3, N(1)#2–Zr(1)–N(1)#3, N(1)–Zr(1)–N(1)#4,
N(1)–Zr(1)–N(1)#5, N(1)#4–Zr(1)–N(1)#5
165.72(5)
101.63(5)
89.22(4)
N(1)–Zr(1)–N(1)#3, N(1)#1–Zr(1)–N(1)#4, N(1)#2–Zr(1)–N(1)#5
81.76(5)
Table 2. Summary of crystallographic data of compound 1.
tigations whether this precipitate can be transformed to com-
pound 1 by prolonged exposure to KNH2 are still ongoing.
K2[Zr(NH2)6]
¯
K2[Zr(NH2)6] was found to crystallize in space group R3c.
Empirical formula
H12K2N6Zr
colorless plate
265.58
The asymmetric unit contains the zirconium atom Zr(1) at the
special Wyckoff position 6a, the potassium ion K(1) at 12c, the
nitrogen atom N(1) at the common site 36f, and the two hydro-
gen atoms H(1A) and H(1B) also at the common 36f position.
For atom coordinates, isotropic and anisotropic displacement
parameters see Table 3 and Table 4, respectively. Compound 1
contains the discrete [Zr(NH2)6]2– molecule anion. The zirco-
nium atom is surrounded by six symmetry-equivalent nitrogen
atoms forming a coordination sphere reminiscent of an octahe-
dron. The Zr–N distance with 2.1931(9) Å is similar to the
Zr–N bond length of 2.198(6) Å in the hexapyrrolyl complex
[Zr(η1-NC4H4)6]2–.[19] The N–Zr–N angles are 165.72(5),
101,63(5), 89.22(4) and 81.76(5)°. They deviate by 14.27(5)°
from 180°, and by 11.63(5)°, 8.24(5)°, and 0.78(4)° from 90°
(see Table 1). For comparison, the [Sn(NH2)6]2– anions of
M2[Sn(NH2)6] (M = K, Rb, Cs)[4,5,9,10,12] show Sn–N distances
of approximately 2.17 Å and angle deviations of 0° from the
180° angles, and approximately 2° from the 90° angles. The
hexaazido germanates(IV) and -titanates(IV) also show only
slight deviations from ideal Oh symmetry with less than 0.9
and 1.7°, respectively.[20,21] The point group symmetry of the
hexaamido zirconate(IV) anion of compound 1 is D3 as the
zirconium atom resides on Wyckoff position 6a with 32 sym-
metry.
Color and appearance
Molecular mass /g·mol–1
Crystal system
Space group
a /Å
trigonal
¯
R3c
6.5422(2)
32.824(2)
1216.66(9)
6
c /Å
V /Å3
Z
ρcalc /Mg·m–3
λ /Å
2.18
0.71073
T /K
123(2)
μ (Mo-Kα) /mm–1
θmax
2.32
39.26
hklmax
–7 ≤ h ≤ 11
–11 ≤ k ≤ 11
–56 ≤ l ≤ 57
0.05·0.05·0.01
0.0311, 0.0173
0.0172, 0.0288
0.0438, 0.0454
0.980
Size /mm
Rint, Rσ
R(F) (I ≥ 2σ (I), all data)
wR(F2) (I ≥ 2σ (I), all data)
S (all data)
Data, parameter, restraints
Δρmax, Δρmin /e·Å–3
Extinction coefficient
799, 24, 0
0.604, –0.588
0.00117(18)
6 KNH2 + ZrF4 → K2[Zr(NH2)6] + 4 KF
(2)
It is somewhat surprising that the mononuclear hexaamido
complex has been formed. Condensation reactions leading to
oligo- or polynuclear compounds analogous to the isopoly ac-
ids well known from aqueous chemistry could also be ex-
pected. In liquid ammonia solution, the hexammine cobalt(III)
ion is known to form dinuclear and trinuclear species upon
addition of KNH2, with the cobalt atoms bridged by amide
anions.[18] After evaporation of liquid ammonia from the reac-
tion vessel an X-ray powder pattern taken at room temperature
showed reflections of K, KNH2, and KF only. Thus, polycon-
densation of 1 may occur on warming to room temperature. It
seems that a high concentration of amide ions is needed at low
temperatures to shift the equilibrium to the monomeric species
obtained by us. So far we were not able to synthesize com-
pound 1 from a direct and stoichiometric reaction of a KNH2
Table 3. Atomic coordinates and equivalent isotropic displacement pa-
rameters /Å2·103 for compound 1.
x
y
z
Ueq
Zr(1)
N(1)
0
0
1/4
8(1)
0.3102(2)
0
0.1968(2)
0
0.2891(1)
0.3680(1)
0.2839(4)
0.3080(5)
17(1)
19(1)
21(3)
29(4)
K(1)
H(1A)
H(1B)
0.4340(30)
0.3600(30)
0.3220(20)
0.1440(20)
Table 4. Anisotropic displacement parameters /Å2·103 for compound
1.
is defined as exp(–2π2(a*2h2U11+b*2k2U22+c*2l2U33+
2b*c*klU23+2a*c*hlU13+2a*b*hkU12)).
U
Atom
Zr(1)
U11
U22
U33
U23
U13
U12
8(1)
12(1)
18(1)
8(1)
17(1)
18(1)
9(1)
17(1)
21(1)
0
0
4(1)
4(1)
9(1)
with ZrF4 in liquid NH3, as upon addition of KNH2 solution N(1)
–2(1)
0
–5(1)
0
K(1)
to a solution of ZrF4 – and vice versa – a colorless precipitate
is formed immediately. This colorless precipitate is amorphous,
and only reflections of KF are observed in the X-ray powder
As shown in Figure 1, the nitrogen atom is coordinated in
pattern. We assume it to be a polycondensation product. Inves- an almost planar environment as it resides only 0.1254(79) Å
Z. Anorg. Allg. Chem. 2011, 1122–1130
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F. Kraus, S. A. Baer, A. J. Karttunen
ARTICLE
above the H(1A)–H(1B)–Zr(1) plane. The Zr–H distances are
2.785(15) and 2.798(16) Å for H(1A) and H(1B), respectively.
The H–N–H bond angle is found to be 99.7(15)°. The Zr–N–
H bond angles are with 128.1(9)° for H(1A) and 128.2(10)°
for H(1B) are closer to 120°, suggesting sp2-hybridized nitro-
gen atoms. In contrast to this finding, the Sn–N–H and H–N–
H angles of the tin compounds mentioned above are closer to
the tetrahedral angle, and sp3-hybridization was suggested for
the nitrogen atoms.[4,5] The anion of compound 1 is surrounded
by eight potassium ions residing above the trigonal faces of
the coordination polyhedron around zirconium in the shape of
a distorted cube. An amide ion is thus surrounded by the zirco-
nium atom and four symmetry-equivalent potassium ions. The
four K–N distances are 3.0179(9), 3.1412(1), 3.529(1) and
3.560(1) Å. The hydrogen atoms are not oriented towards the
potassium ions but point in between them (see Figure 1b). The
closest K–H distance is 2.69(2) Å. If sp2-hybridized nitrogen
atoms are assumed, the p-orbital shaped lone pairs coordinate
to four symmetry-equivalent potassium ions and are additional
acceptors of N–H hydrogen bonds. It can be argued that the
distortion of the [Zr(NH2)6]2– ion from ideal symmetry might
arise from hydrogen bonding. From a structural point of view,
a hydrogen bond should be present if the distance of the hydro-
gen and the acceptor atom is shorter than the sum of their van-
der-Waals radii, and the D–H···A angle (D = donor, A = ac-
ceptor) is between 120 and 180°, approximately. Strong hydro-
gen bonds show small H···A distances and D–H···A angles
close to 180°. In the structure of compound 1, the H···N distan-
ces are only 0.6 % (N–H(1A)···N) and 1.3 % (N–H(1B)···N)
shorter than the sum of the respective van-der-Waals radii. The
D–H···A angles are 161.3(12) and 168.1(14)°, respectively.
Comparison of X-ray and neutron data shows the margin of
error to be typically within 0.7 % and 3° for hydrogen bonds
from X-ray structures. Thus and due to the very small H···A
shortening observed, it remains unclear if hydrogen bonds are
present. If hydrogen bonds are assumed to be present, each
symmetry-equivalent N(1) atom acts as a donor as well as an
acceptor of two hydrogen bonds via H(1A) and H(1B) and
their symmetry equivalents. These hydrogen bonds lead to the
interconnection of six amide anions of six symmetry-equiva-
lent [Zr(NH2)6]2– units in a shape similar to an octahedron
(Figure 2).
Figure 1. a) Projection of the [Zr(NH2)6]2– anion of compound 1. b)
Projection of the coordination sphere of the [Zr(NH2)6]2– anion by po-
tassium ions of compound 1. Thermal displacement parameters are
shown at 70 % probability level at 123 K. Symmetry transformations
for the generation of equivalent atoms: #1 –x, –x+y, –z+1/2; #2 x–y,
–y, –z+1/2; #3 y, x, –z+1/2; #4 –y, x–y, z; #5 –x+y, –x, z.
Figure 2. Projection of the empty polyhedron resulting from assumed
N–H···N hydrogen bonds (dashed) in the structure of compound 1.
Thermal displacement parameters are shown at 70 % probability level
at 123 K. D–H, H···A, D···A hydrogen bond lengths /Å and DHA an-
gles /deg (D = Donor, A = Acceptor): N(1)–H(1A)···N(1)#12:
0.832(15), 2.732(15), 3.5299(16), 161.3(12); N(1)–H(1B)···N(1)#9:
0.848(15), 2.714(16), 3.5477(16), 168.1(14). Symmetry transforma-
tions for the generation of equivalent atoms: #9 x–y+1/3, x–1/3,
–z+2/3; #12 –x+y+1, –x+1, z.
The potassium ions are reside above the trigonal faces of the
(NH2)6 polyhedron. Thus, the structure consists of
[Zr(NH2)6]2– units that are connected by N–H···N hydrogen
bonds to twelve other [Zr(NH2)6]2– moieties. Empty (NH2)6
and [Zr(NH2)6]2– polyhedra are sharing common corners and
the potassium ions reside in the remaining voids. The coordi-
nation number of the potassium cation is thus twelve. It is
surrounded by four [Zr(NH2)6]2– units which are arranged in
a shape similar to a tetrahedron with three amide ions each
coordinating to potassium. The potassium ion is not located in
the center of the resulting distorted cube-octahedron, however
(Figure 3). For comparison, in K2[Sn(NH2)6] the coordination
number of the potassium ion is only 6. It is also coordinated
by four [Sn(NH2)6]2– anions in a tetrahedral arrangement, how-
Figure 3. Projection of the coordination sphere of the potassium ion
of compound 1. It is similar to a cube-octahedron. Hydrogen atoms
omitted for clarity. Thermal displacement parameters are shown at
70 % probability level at 123 K. Symmetry transformations for the
generation of equivalent atoms: #6 –x+1/3, –y+2/3, –z+2/3; #10
ever only one of them interacts with three amide ions. The –y+2/3, –y–1/3, –z+2/3; #11 –x+1/3, –y–1/3, –z+2/3.
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The Complex Amide K2[Zr(NH2)6]
other three [Sn(NH2)6]2– units coordinate one amide anion electron densities close to them. Table 2 contains relevant crys-
each. The coordination sphere of the potassium ion in tallographic data of compound 1, Table 3 the x, y, and z coordi-
K2[Sn(NH2)6] is trigonal anti-prismatic.[4]
nates of the atoms and their isotropic displacement parameters,
From a molecular point of view it is not straightforward to and Table 4 shows the anisotropic displacement parameters of
understand why the [Zr(NH2)6]2– anion is distorted from ideal non-hydrogen atoms.
S6 symmetry, but crystallographically this distortion is a conse-
From a structural point of view it still remains unclear to
quence of the space group. The structure of compound 1 can what extent hydrogen bonding is present in compound 1, and
be derived from the K2PtCl6 structure type. The symmetry re- why the [Zr(NH2)6]2– anion is distorted. Thus, we decided to
lationship can be displayed using a Bärnighausen tree,[22] further elucidate the structure by quantum chemical calcula-
which is shown in Figure 4. A translationengleiche transition tions.
of index four leads from the K2PtCl6 space group type F4/
¯
¯
m32/m to space group type R32/m. Then, a klassengleiche tran-
Quantum Chemical Calculations
To explain the distortion of the [Zr(NH2)6]2– anion, we com-
pared its structure with the one of K2[Sn(NH2)6] by means of
¯
sition of index two leads to the space group type R32/c and to
the novel structure type of K2Zr(NH2)6, Wyckoff sequence f3ca
for all atoms and fca for non-hydrogen atoms.
The unit cell of compound 1 is shown in Figure 5a and its quantum chemical calculations (see Exp. Sect. for computa-
structural relation to the K2PtCl6-type in Figure 5b. tional details). The DFT-optimized solid-state structures of
¯
¯
The projection of the unit cell on the bc plane gives the K2[Zr(NH2)6] (R3c) and K2[Sn(NH2)6] (R3) are in good agree-
impression that the c axis should be halved, but a closer look ment with the experimental geometries (Table 5). The metal–
shows the different tilting of the [Zr(NH2)6]2– anions. The dif- N–H bond angles suggest sp2- and sp3-hybridized nitrogen at-
fraction patterns clearly show the presence of the long c axis. oms in K2[Zr(NH2)6] and K2[Sn(NH2)6], respectively. Based
Refinements in the smaller cell (a = 6.5433(1), c = on the partial atomic charges, the bonding in [Zr(NH2)6]2– ap-
16.4155(4) Å, c/a = 2.5087, V = 608.67(2) Å3, Z = 3) in space pears to be much more ionic in comparison to the one in
groups R3 or R3m lead to erroneous anisotropic displacement [Sn(NH2)6]2–, which is in line with the electronegativities of
¯
¯
parameters for the nitrogen atoms and unassignable residual the atoms (Zr: 1.33; Sn: 1.96; N: 3.04). The band gap of
Figure 4. Bärnighausen tree displaying the relationship of K2Zr(NH2)6 to the K2PtCl6 structure type.
Z. Anorg. Allg. Chem. 2011, 1122–1130
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F. Kraus, S. A. Baer, A. J. Karttunen
ARTICLE
Figure 5. a) Projection of the unit cell of K2[Zr(NH2)6] on the bc plane. b) Projection showing the similarity of compound 1 to the K2PtCl6
2–
structure type. Dark-gray polyhedra are Zr(NH2)6 anions, gray transparent polyhedra are (NH2)6 polyhedra. Thermal displacement parameters
are shown at 70 % probability level at 123 K.
K2[Zr(NH2)6] is also 1.0 eV larger than that for K2[Sn(NH2)6]. structure. The [Sn(NH2)6]2– anion shows an almost perfect
To compare the energetics of the two different structural mo- symmetrical coordination sphere with point group S6. Nota
¯
tifs, we also optimized the hypothetical K2[Zr(NH2)6] (R3) and bene: The point group symmetry is lowered from Oh due to the
¯
K2[Sn(NH2)6] (R3c) structures derived from the experimental presence of hydrogen atoms. Both molecular structures were
¯
¯
geometries of K2[Sn(NH2)6] (R3) and K2[Zr(NH2)6] (R3c), re- confirmed to be true local minima. Furthermore, we optimized
spectively. The hypothetical structural alternatives were also the structure of a hypothetical [Zr(NH2)6]2– anion that was con-
found to be true local minimum structures, but the experimen- strained to an octahedral geometry with point group S6. The
tally observed structures are clearly energetically preferred structure proved to be a true local minimum, but it was
over the hypothetical ones. For compound 1 space group R3c 18 kJ·mol–1 less stable than the distorted [Zr(NH2)6]2– unit.
¯
–1
¯
is favored over space group R3 by 40 kJ·mol , and for the tin
Furthermore, a detailed investigation of the electronic char-
compound space group R3 is favored over space group R3c by acteristics of the distorted and the octahedral [Zr(NH2)6]2– an-
¯
¯
52 kJ·mol–1. During the structural optimization, the hybridiza- ion provides a justification for the distortion (Figure 6).
tion of the nitrogen atoms changed from sp3 towards sp2 in the
A distorting of the perfectly octahedral S6 geometry breaks
hypothetical K2[Zr(NH2)6] (R3) and vice versa for the degeneracy of a threefold degenerate Zr(d)–N bonding
¯
¯
K2[Sn(NH2)6] (R3c).
MO. This improves the overlap between the Zr(d)- and N(p)-
To shed light on the structural and electronic characteristics orbitals, thus stabilizing the complex. The σ-bonds of the dis-
of K2[Zr(NH2)6], we also investigated the electronic structures torted complex are found at an MO energy of –0.314 a.u. They
of the molecular [Zr(NH2)6]2– and [Sn(NH2)6]2– anions in solu- are energetically destabilized a little compared to the S6-sym-
tion. During a full geometry optimization both anions retained metric molecule. At the MO energies of –0.219 and
the structural motifs encountered in the solid state, –0.197 a.u. the overlap of the lone pair on the nitrogen atom
[Zr(NH2)6]2– showing a distorted octahedral coordination with with d-orbitals of zirconium is shown, and a major amount of
point group D3. The Zr–N bond lengths are with 2.21 Å in energy is gained, which leads to the above mentioned stabiliza-
good agreement with the experimental data and the theoretical tion of 18 kJ·mol–1.
calculation of the bulk structure. The Zr–N–H bond angles are
The distortion of the octahedral symmetry of d0-configured
found in the range between 121.8 and 123.8°. The N–Zr–N ML6 complexes is not a common phenomenon, but it is known
bond angles of the anion are 159.8°, 106.2°, 89.6°, and 78.3°. for L = CH3 and for M = W6+, Re6+, Nb5+, Ta5+.[23–25] In
–
Thus, the unit is even more distorted than in the solid-state [Zr(Me)6]2–, a second-order Jahn–Teller effect leads to a lower-
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The Complex Amide K2[Zr(NH2)6]
Table 5. Comparison of experimentally determined parameters with theoretical results obtained at the PBE0-DFT/SVP level of theory.
Experimental parameters
(single-crystal X-ray)
Parameters obtained from theory
(difference to exp. in %)
¯
K2[Zr(NH2)6] (R3c, 123 K)
a /Å
6.5422(2)
32.824(2)
2.1931(9)
3.8726(4)
4.1012(2)
128.1(9)
6.483 (–0.91 %)
32.311 (–1.6 %)
2.20 (+0.5 %)
3.95 (+2.1 %),
4.01 (–2.2 %)
131.1 (+2.3 %)
121.5 (–5.3 %)
167.0 (+0.8 %)
100.3 (+1.3 %)
89.5 (+0.31 %)
82.2 (+0.54 %)
c /Å
R(Zr–N) /Å
R(Zr–K)
A(Zr–N–H(1A)) /°
A(Zr–N–H(1B)) /°
A(N–Zr–N)
128.2(10)
165.72(5)
101.63(5)
89.22(4)
81.76(5)
Band gap /eV
Charge of zirconium /e–
5.7
+1.67
Bond overlap
populations /e–
Zr–N
0.183
N–H
0.280, 0.275
N···H
N···K
0.044, 0.040
0.030, 0.021, 0.017, 0.013
¯
K2[Sn(NH2)6] (R3, 298 K)
a /Å
6.337(4)
19.769(5)
2.178(1)
3.574(6)
107(1)
6.408 (+1.12 %)
19.841 (+0.37 %)
2.20 (+1.0 %)
c /Å
R(Sn–N) /Å
R(Sn–K)
3.57 (+0.3 %)
A(Sn–N–H(A)) /°
A(Sn–N–H(B)) /°
106.5 (–0.5 %)
106.3 (+0.3 %)
106(1)
Band gap /eV
Charge of tin /e–
4.7
+0.96
Bond overlap
populations /e–
Sn–N
0.236
N–H
0.308, 0.297
0.006, 0.004
0.050, 0.028
N···H
N···K
NH3 (P213)
a /Å
5.1305(8)
5.07 (–1.2 %)
Bond overlap
populations /e–
N–H
0.337
0.040
N···H
ing in symmetry to D3h and hence to an electronic mized solid-state structure of ammonia with its distinctive hy-
stabilization.[26] In the case of [Sn(NH2)6]2–, there are no empty drogen bonding as a reference structure (Table 4).[27] In solid
metal d-orbitals available for the Sn–N(sp3) bonding, i.e. the ammonia, the covalent N–H bonds have a bond overlap popu-
sp3-hybridized lone pair is not able to overlap with the filled lation of 0.337 e–, whereas the overlap population for the hy-
d-orbitals. Thus, distortion of the octahedral structure would drogen bonds of 0.040 e– is one order of magnitude smaller.
not lead to a stabilization of the complex. In addition to the The overlap populations for the N···H hydrogen bonds in com-
differences arising from the electronic configuration of the pound 1 are equivalent to those in solid ammonia (0.044 e–
central metal atoms, the N···H hydrogen bonds and N···K inter- and 0.040 e–), which clearly suggests the presence of hydrogen
actions are expected to play a role in determining the structural bonding. For comparison, the N···H overlap populations in
characteristics of K2[Zr(NH2)6] and K2[Sn(NH2)6]. When con- K2[Sn(NH2)6] are one order of magnitude smaller, and N···H
sidering the N···H hydrogen bonding, we used the DFT-opti- hydrogen bonding does not take place. A similar analysis of
Z. Anorg. Allg. Chem. 2011, 1122–1130
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1127

F. Kraus, S. A. Baer, A. J. Karttunen
ARTICLE
Figure 6. The molecular orbitals of the [Zr(NH2)6]2– anion involving bonding Zr–N interactions (isodensity value 0.04 a.u.). The S6-symmetric
unit is shown on the left, the D3-symmetric one on the right. Distortion of the octahedral arrangement breaks the degeneracy of the threefold
degenerate Zr(d)–N bonding MOs and clearly stabilizes the Zr(d)–N bonding MOs.
the N···K overlap populations shows that the N···K interactions atoms, to a lesser extent by hydrogen bonding, and of course
are somewhat stronger in K2[Sn(NH2)6] than in K2[Zr(NH2)6] by Coulomb interaction of anions and cations.
as well. Thus, the structure of compound 1 is strongly influ-
enced by electronic effects of the d-shell and to a lesser extent
by hydrogen bonding.
Experimental Section
All work was carried out excluding moisture and air in an atmosphere
of dried and purified argon (Westfalen AG) using high-vacuum glass
lines. Liquid ammonia (Air Liquide, 99.98 %) was dried and stored
over sodium in a special high-vacuum glass line. All crystals were
Conclusions
We have synthesized K2[Zr(NH2)6] (1) from ZrF4 with ele-
mental potassium dissolved in liquid ammonia as colorless sin-
gle crystals and have shown that low-temperature synthesis of
amides is possible using fluorides as starting materials. Com-
pound 1 crystallizes in a novel structure type in space group
handled in cooled, perfluorinated ether under nitrogen at temperatures
below –50 °C and mounted using the Crystal Cap system.
Preparation of K2[Zr(NH2)6] (1)
ZrF4 was synthesized by reaction of ZrO2Cl2·8H2O with 100 % F2.
The temperature was raised from room temperature to 400 °C within
one week. A flame-dried reaction vessel was charged with ZrF4
(381 mg, 2.28 mmol, 1 equiv.) and distilled potassium(315 mg,
8.06 mmol, 3.5 equiv.). Dry liquid ammonia (≈ 10 mL) was condensed
at –78 °C onto the starting materials, which led to a deep-blue solution
¯
R3c, which can be deduced from the K2PtCl6 structure type.
Hydrogen bonding in compound 1 can be assumed from its
structural data and quantum chemical calculations show hydro-
gen bonding to be present in an amount comparable to that in
solid NH3. Interestingly, the discrete [Zr(NH2)6]2– molecules
do not display octahedral geometry but are distorted to D3 of the alkali metal. A colorless slurry remained at the bottom of the
reaction vessel. After four weeks of storage at –40 °C colorless crystals
of compound 1 were obtained in the still deep-blue solution and sub-
jected to low-temperature single-crystal X-ray analysis with an Oxford
Xcalibur3 system (Table 2).
symmetry. This distortion was traced back to the electronic
structure of the zirconium complex by quantum chemical cal-
culations. The results show the distorted molecule to be
18 kJ·mol–1 more stable compared to the “ideal” octahedral S6
structure. The structure of compound 1 is thus dominated
The diffraction data were reduced with the CRYSALISRED software,
mainly by electronic effects of the d-shell of the zirconium and an empirical absorption correction using spherical harmonics with
1128 www.zaac.wiley-vch.de
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Z. Anorg. Allg. Chem. 2011, 1122–1130

The Complex Amide K2[Zr(NH2)6]
SCALE3 ABSPACK was applied.[28] The structure was solved using
Direct Methods[29] and refined on F2.[30] All non-hydrogen atoms were
localized by Fourier cycling methods and refined anisotropically, hy-
drogen atoms were also localized from the difference Fourier map and
were refined isotropically.
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Computational Details
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sets composed of Gaussian-type functions were used in the theoretical
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Acknowledgement
A.J.K. gratefully acknowledges financial support from the Academy of
Finland (grant 138560/2010). S.A.B. and F.K. would like to thank the
Fonds der Chemischen Industrie for their stipends, M. Bentlohner for
his aid in conducting some experiments, Prof. Dr. B. G. Müller and
Prof. Dr. R. Hoppe for many, many chemicals and equipment, and
Solvay Fluor for a generous donation of elemental fluorine. The Deut-
sche Forschungsgemeinschaft is acknowledged for financial support.
We thank Dr. B. Wahl, Dr. A. Schier, and Prof. Dr. H. Schmidbaur for
helpful discussions.
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Received: February 18, 2011
Published Online: April 4, 2011
1130
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Z. Anorg. Allg. Chem. 2011, 1122–1130