Monomeric homoleptic (2-pyridylmethyl)(tert-butyldimethylsilyl)amido complexes of the divalent metals Mg, Mn, Fe, Co, and Zn

Article abstract of DOI:10.1002/zaac.200600307

The transamination reaction of M[N(SiMe₃)₂] ₂ with (2-pyridylmethyl)(tert-butyldimethylsilyl)amine yields the corresponding homoleptic metal bis[(2-pyridylmethyl)(tert-butyldimethylsilyl) amides] of Mg (1), Mn (2), Fe (3), Co (4) and Zn (5). All these compounds crystallize from hexane isotypic in the space group C2/c. From toluene the zinc derivative precipitates as toluene solvate 5·toluene. The molecular structures of these compounds are very similar with large NMN angles to the amide nitrogen atoms with NMN values of 148° (1) and 150° (5) for the diamagnetic compounds and 156° for the paramagnetic derivatives 2 and 3. The Co derivative 4 displays a rather small NCoN angle of 142°. Different synthetic routes have been explored for compound 3 which is also available via the metallation reaction of bis(2,4,6-trimethyl phenyl)iron with (2-pyridylmethyl)(tert-butyldimethylsilyl)amine and via the metathesis reaction of lithium (2-pyridylmethyl)(tertbutyldimethylsilyl)amide with [(thf) ₂FeCl₂]. In course of the metathesis reaction, an equimolar amount of lithium (2-pyridylmethyl)-(tert-butyldimethylsilyl)amide and [(thf)₂FeCl₂] yields heteroleptic (2-pyridylmethyl)(tert- butyldimethylsilyl)amido iron(II) chloride (6) which crystallizes as a centrosymmetric dimeric molecule. The oxidative C-C coupling reaction of 5 with Sn[N(SiMe₃)₂]₂ leads to the formation of tin(II) 1,2-bis(2-pyridyl)-1,2-bis(tert-butyldimethylsilylamido)ethane, tin metal and Zn[N(SiMe₃)₂]₂.

Full text of DOI:10.1002/zaac.200600307

Articles
DOI: 10.1002/zaac.200600307
Monomeric Homoleptic (2-Pyridylmethyl)(tert-butyldimethylsilyl)amido
Complexes of the Divalent Metals Mg, Mn, Fe, Co, and Zn
Christian Kocha, Astrid Malassaa, Christine Agthea, Helmar Görlsa, Ralf Biedermannb, Harald Krautscheidb,
and Matthias Westerhausena,*
a Jena, Germany, Friedrich-Schiller-Universität, Institute of Inorganic and Analytical Chemistry
b Leipzig, Germany, University, Institute of Inorganic Chemistry
Received November 2nd, 2006.
Abstract. The transamination reaction of M[N(SiMe3)2]2 with
(2-pyridylmethyl)(tert-butyldimethylsilyl)amine yields the corre-
sponding homoleptic metal bis[(2-pyridylmethyl)(tert-butyldime-
thylsilyl)amides] of Mg (1), Mn (2), Fe (3), Co (4) and Zn (5). All
these compounds crystallize from hexane isotypic in the space
group C2/c. From toluene the zinc derivative precipitates as toluene
solvate 5·toluene. The molecular structures of these compounds are
very similar with large NMN angles to the amide nitrogen atoms
with NMN values of 148° (1) and 150° (5) for the diamagnetic
compounds and 156° for the paramagnetic derivatives 2 and 3. The
Co derivative 4 displays a rather small NCoN angle of 142°. Diffe-
rent synthetic routes have been explored for compound 3 which is
also available via the metallation reaction of bis(2,4,6-trimethyl-
phenyl)iron with (2-pyridylmethyl)(tert-butyldimethylsilyl)amine
and via the metathesis reaction of lithium (2-pyridylmethyl)(tert-
butyldimethylsilyl)amide with [(thf)2FeCl2]. In course of the meta-
thesis reaction, an equimolar amount of lithium (2-pyridylmethyl)-
(tert-butyldimethylsilyl)amide and [(thf)2FeCl2] yields heteroleptic
(2-pyridylmethyl)(tert-butyldimethylsilyl)amido iron(II) chloride
(6) which crystallizes as a centrosymmetric dimeric molecule. The
oxidative C-C coupling reaction of 5 with Sn[N(SiMe3)2]2 leads to
the formation of tin(II) 1,2-bis(2-pyridyl)-1,2-bis(tert-butyldi-
methylsilylamido)ethane, tin metal and Zn[N(SiMe3)2]2.
Keywords: Amides; C-C Coupling reactions; Homoleptic complexes;
Metallation reactions; Transamination reactions
Introduction
2-Pyridylmethylamines are easily deprotonated at the amine
unit by dialkylzinc to give compound A according to equa-
tion (1) [1]. However, at elevated temperatures an oxidative
C-C coupling reaction is observed yielding bis(alkylzinc)
1,2-bis(2-pyridyl)-1,2-bis(amido)ethane (B) [1, 2]. Depending
on the substituent R at the amide nitrogen atom, an equilib-
rium of this complex with its dissociation product C forms
for R ϭ tBu via a homolytic C-C bond cleavage [3Ϫ5]. If
R resembles a trialkylsilyl ligand no C-C bond cleavage is
detected [2].
(1)
Regardless of the stoichiometry of the first metallation
step, only heteroleptic compounds A are obtained, the syn-
thesis of homoleptic zinc bis(2-pyridylmethylamide)s fails.
The unwillingness of the RЈZn moieties in alkylzinc amides
to undergo metallation reactions was observed earlier also
with smaller amide ligands [6, 7]. Also in the dinuclear com-
plex B the alkyl substituents RЈ show an extreme low reac-
tivity [8]. Therefore, alterations at the metal atom are hard
to perform and a different pathway has to be found in order
to prepare the homoleptic complexes.
The second C-C coupling step strongly depends on the
standard potential E0(M/M2ϩ) of the metal M. Whereas the
reaction of Sn[N(SiMe3)2]2 [E0(Sn/Sn2ϩ) ϭ Ϫ0.14 V] with
(2-pyridylmethyl)(tert-butyldimethylsilyl)amine gives the
corresponding C-C coupling reaction already at room tem-
perature, elevated temperatures are necessary for the similar
reaction with dialkylzinc [E0(Zn/Zn2ϩ) ϭ Ϫ0.76 V] [1, 2].
The reaction mechanism is not yet fully understood, how-
ever, the initial reaction is the intramolecular deprotonation
of the methylene moiety under loss of the aromatic charac-
ter of the pyridyl fragment according to equation (2) thus
* Prof. Dr. M. Westerhausen
Institut für Anorgan. und Analyt. Chemie
Friedrich-Schiller-Universität
August-Bebel-Str. 2
D-07743 Jena
Fax: ϩ49 (0) 3641 948110
e-mail: m.we@uni-jena.de
Z. Anorg. Allg. Chem. 2007, 633, 375Ϫ382
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
375

C. Koch, A. Malassa, C. Agthe, H. Görls, R. Biedermann, H. Krautscheid, M. Westerhausen
giving a bisamide species. This divalent anion was isolated
In order to investigate the possibility to employ other
synthetic routes for the preparation of the iron complex 3,
bis(2,4,6-trimethylphenyl)iron(II) (dimesityliron) [19] was
used as a metallating reagent toward (2-pyridylmethyl)(tert-
butyldimethylsilyl)amine. Another access route is the meta-
thesis reaction of [(thf)2FeCl2] with lithium (2-pyridyl-
methyl)(tert-butyldimethylsilyl)amide according to equation
(4). The paramagnetic compounds 2, 3 and 4 (M ϭ Mn,
Fe, Co) are extremely air and moisture sensitive, far more
than the diamagnetic complexes 1 and 5 (M ϭ Mg, Zn).
as a dimeric magnesium (D) [2] [E0(Mg/Mg2ϩ) ϭ Ϫ2.36 V]
and as an octameric lithium derivative (E) [9] [E0(Li/Liϩ) ϭ
Ϫ3.04 V] because the reduction of these very electropositive
metals was impossible in course of a C-C coupling reaction.
(2)
(4)
Here we investigate the reaction of (2-pyridylmethyl)(tert-
butyldimethylsilyl)amine with the homoleptic bis(trimethyl-
silyl)amides of the divalent metals Mg, Mn [E0(Mn/
Mn2ϩ) ϭ Ϫ1.18 V], Fe [E0(Fe/Fe2ϩ) ϭ Ϫ0.44 V], Co
[E0(Co/Co2ϩ) ϭ Ϫ0.28 V], and Zn in order to obtain the
For all preparative procedures of 3 the yields are rather
low. Compound 3 is hard to crystallize and the addition of
small amounts of TMEDA or DME can initiate the crystal-
lization process, even though no TMEDA or DME adducts
are observed. The crystals of 2, 3 and 4 often are covered
with dark oil and repeated recrystallization procedures from
hydrocarbons are necessary and lead to a decrease of the
yield.
homoleptic
ides.
(2-pyridylmethyl)(tert-butyldimethylsilyl)am-
Results and Discussion
Synthesis
The metathesis reaction of [(thf)2FeCl2] with lithium
(2-pyridylmethyl)(tert-butyldimethylsilyl)amide
proceeds
Transamination reactions were investigated earlier with
support of theoretical considerations [10]. In a similar pro-
cedure, the employment of zinc bis[bis(trimethylsilyl)amide]
[11] allows the synthesis of zinc bis[(2-pyridylmethyl)(tert-
butyldimethylsilyl)amide] (5) with high yield according to
equation (3). An excess of amine does not lead to an en-
hancement of the coordination number. Addition of di-
alkylzinc to 5 yields the well-known heteroleptic alkylzinc
(2-pyridylmethyl)(tert-butyldimethylsilyl)amide [1, 2]. In or-
der to verify the generality of this synthesis and in order to
compare the molecular structures in dependency of the
radius and the d-electron count of the metal, (2-pyridyl-
methyl)(tert-butyldimethylsilyl)amine is deprotonated by
the homoleptic bis(trimethylsilyl)amides of magnesium [12,
13], manganese [13Ϫ15], iron [16, 17], and cobalt [15, 18]
according to equation (3).
stepwise and therefore the isolation of (2-pyridylmethyl)-
(tert-butyldimethylsilyl)amido iron(II)chloride (6) is possible
according to equation (5). This heteroleptic iron complex 6
dimerizes and precipitates in the shape of pale yellow
needles. An excess of lithium (2-pyridylmethyl)(tert-butyldi-
methylsilyl)amide yields the homoleptic derivative 3.
(5)
The similarity of the molecular structures of 1 to 5 is
supported by mass spectrometric investigations. These de-
rivatives show a very intense Mϩ signal besides [M-Bu]ϩ. A
weaker mass signal was detected for [M-Me]ϩ. All of these
signals show the isotopic pattern of silicon and the metals.
In addition, the IR spectra are also very similar and sup-
port the conclusion that the bis(amides) of Mg, Mn, Fe, Co,
and Zn precipitate with very similar molecular structures.
(3)
376
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Z. Anorg. Allg. Chem. 2007, 375Ϫ382

(2-Pyridylmethyl)(tert-butyldimethylsilyl)amido Complexes of the Divalent Metals
Molecular Structures
The compounds 1 to 5 crystallize isotypic and a compari-
son of the structural parameters is given in Table 1, for
comparison reasons (2-pyridylmethyl)(tert-butyldimethyl-
silyl)amine is included as a complex at dimeric LiI [20]. As
representative examples, Figures 1 and 2 show the molecu-
lar structures of 3 and 5, whereas a superposition of 2 and
4 is displayed in Figure 3.
Table 1 Comparison of selected bond lengths (average values, pm)
and angles /° of the homoleptic complexes 1 to 5. The radii r(M2ϩ
)
(pm) refer to tetrahedrally coordinated metal cations, the metal
radii r(Mmetal) refer to the metal structures with twelve-coordinate
metal atoms.
a)
[LiI·L]2
Mg (1)
Mn (2) Fe (3)
Co (4)
Zn (5)
r(M2ϩ
r(Mmetal
M-N2
)
73 (Liϩ) 71
157 160
80
137
77
126
72
125
74
134
)
213.6(5) 198.9(2) 202.9(5) 196.1(2) 192.9(2) 192.6(2)
202.4(5) 213.4(2) 221.0(5) 212.3(2) 206.6(2) 211.7(2)
Fig 2 Molecular structure of zinc bis[(2-pyridylmethyl)(tert-butyl-
dimethylsilyl)amide] (5). The ellipsoids represent a probability of
40 %, hydrogen atoms are not shown for clarity reasons. Symmetry
related atoms (Ϫx, y, Ϫzϩ0.5) are marked with “A“.
M-N1
N2-M-N2Ј
N2-M-N1
N2-M-N1Ј
N1-M-N1Ј
N2-Si
N2-C6
N1-C1
N1-C5
C1-C2
C2-C3
C3-C4
Ϫ
83.6(2)
Ϫ
Ϫ
148.2(2) 155.7(3) 155.5(1) 141.9(1) 149.95(7)
82.65(9) 80.2(2) 82.21(6) 84.16(6) 83.84(7)
114.6(1) 113.5(2) 111.79(6) 115.19(7) 113.48(7)
116.4(1) 113.8(3) 112.54(8) 120.51(9) 111.71(7)
177.3(2) 169.5(2) 169.3(5) 170.3(2) 169.7(2) 170.4(2)
146.5(3) 144.9(4) 144.0(7) 145.3(2) 144.9(2) 144.6(3)
133.5(4) 133.3(4) 134.0(7) 134.7(2) 134.0(3) 135.0(3)
133.6(3) 134.5(4) 132.9(7) 133.8(2) 134.1(2) 133.2(3)
137.4(5) 137.2(4) 138.2(9) 137.2(3) 137.2(3) 137.1(3)
136.9(5) 137.8(5) 136.4(9) 138.8(3) 138.1(4) 139.6(3)
137.4(5) 136.7(4) 139.3(8) 137.5(3) 136.7(4) 138.6(3)
138.5(4) 139.0(4) 138.4(8) 139.4(3) 139.2(3) 139.8(3)
150.3(4) 151.3(4) 150.6(8) 150.4(3) 149.2(3) 151.5(3)
123.7(2) 125.6(1) 123.4(3) 124.12(9) 123.74(8) 124.90(9)
C4-C5
C5-C6
M-N2-Si
M-N2-C6 103.8(2) 114.0(2) 115.0(4) 115.0(1) 113.7(1) 114.2(1)
Si-N2-C6 114.9(2) 120.1(2) 121.5(4) 120.8(1) 122.3(1) 120.3(1)
a) L resembles the neutral bidentate ligand (2-pyridylmethyl)(tert-butyldi-
methylsilyl)amine) at lithium (H2-N2-Li 97(2), H2-N2-Si 107(3), H2-N2-C6
109(3) pm).
Fig. 3 Superposition of the molecular structures of manganese (2)
(empty bonds) and cobalt bis[(2-pyridylmethyl)(tert-butyldimethyl-
silyl)amide] (4) (solid bonds). The atoms are drawn with arbitrary
radii, H atoms are omitted for clarity reasons. Symmetry related
atoms (Ϫx, y, Ϫzϩ0.5) are marked with “A“.
The neutral ligand (2-pyridylmethyl)(tert-butyldimethyl-
silyl)amine, which is coordinated at a lithium cation, shows
larger N2-Si and N2-C6 distances than the deprotonated
amide ligand of the compounds 1 to 5. The smaller N2-C6
bond of the amide substituent is a consequence of the larger
s-orbital participation of the planarily coordinated N2
atom. The shortening of the N2-Si bond after depro-
tonation is much more significant and has several reasons:
(i) larger s-orbital participation of the sp2-hybridized N2
atom, (ii) enhanced electrostatic attraction between the
negative N2 and the partly positive silicon atom and (iii)
Fig. 1 Molecular structure of iron bis[(2-pyridylmethyl)(tert-
butyldimethylsilyl)amide] (3). The ellipsoids represent a probability
of 40 %, H atoms are omitted for clarity reasons. Symmetry related
atoms (Ϫx, y, Ϫzϩ0.5) are marked with “A“.
Z. Anorg. Allg. Chem. 2007, 375Ϫ382
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377

C. Koch, A. Malassa, C. Agthe, H. Görls, R. Biedermann, H. Krautscheid, M. Westerhausen
backdonation from the pz(N) lone pair into a σ*(Si-C) or-
bital.
The complexes 1 to 5 display N2-M-N2Ј angles between
142° (1) and 156° (2). The coordination sphere lies between
a distorted tetrahedron and a linear arrangement with two
additional loose contacts to the pyridyl units. Derivative 1
(M ϭ Mg, d0 system) and the Zn compound 5 (d10 system)
are very much alike, even though Mg usually prefers a tetra-
hedral and Zn often a linear coordination sphere. This ob-
servation can be interpreted that the ligands play the key
role and perform a coordination gap which can be filled by
any metal ions of the appropriate size and that the elec-
tronic state of the 3d orbitals is of minor importance.
In order to verify this interpretation, paramagnetic com-
plexes are isolated for Mn (2, d5), Fe (3, d6) and Co (4, d7).
The paramagnetic compounds 2 and 3 display larger N2-
M-N2Ј angles than the Mg and Zn derivatives. This obser-
vation can be explained by involvement of the partly filled
d-orbitals in the σ-bonding situation. A linearization would
maximize the orbital interactions between the ligand and
the metal center. However, any kind of a π-bond between
the nitrogen lone pair and the metal atom can be excluded
because this interaction should lead to an increase of the
N2-Si bond which has not been found. The cobalt deriva-
tive 4 displays the smallest N2-M-N2Ј angle of these com-
pounds. Cobalt is the smallest metal atom of our examples
and therefore, the intramolecular steric strain induced by
the bulky trialkylsilyl groups is enhanced which leads to a
distortion towards a tetrahedron. Whereas the derivatives
of Mg, Mn, Fe, and Zn are very much alike, the deviation
of the cobalt-containing compound is displayed in Figure 3
as a superposition of 2 and 4.
A comparison of 1 to 5 with M[N(SiMe3)2]2 and its di-
mers is given in Table 2 and shows that the M-N2 bond
lengths compare fairly well with the terminal distances of
the dimeric molecules with three-coordinate metal atoms.
In comparison to the monomeric molecules expectedly an
elongation of the M-N bond is observed. In the solid state
monomeric molecules are found for zinc bis[bis(trimethyl-
silyl)amide] and therefore no comparison data can be given.
However, anionic tris[bis(trimethylsilyl)amino]zincates are
known and display Zn-N values of 197 pm [24]. The
manganese derivatives display the largest bond lengths as
can be expected for high spin complexes. Andersen et al. [16]
Fig. 4 Molecular structure of the centrsymmetric (2-pyridyl-
methyl)(tert-butyldimethylsilyl)amido iron(II) chloride dimer (6).
Symmetry related atoms (Ϫxϩ1, Ϫy, Ϫzϩ2) are marked with “A“.
The ellipsoids represent a probability of 40 %, hydrogen atoms are
neglected for clarity reasons. Selected bond lengths /pm:
Fe-N1 208.0(2), Fe-N2 210.7(2), Fe-N2Ј 201.2(2), Fe-Cl 226.18(7), N2-Si
175.5(2), N2-C6 148.7(3), Fe···FeЈ 273.63(7); angles (°): N1-Fe-Cl 102.31(6),
N1-Fe-N2 81.82(8), N1-Fe-N2Ј 128.60(8), N2-Fe-Cl 131.31(6), Cl-Fe-N2Ј
115.31(6), N2-Fe-N2Ј 96.76(7).
showed that even monomeric Mn[N(SiMe3)2]2 is high spin
what most probably is also the case for the isostructural
iron derivative. The fact that the M-N bond lengths are in
accordance with the M-Nt distances of dimeric
M[N(SiMe3)2]2 support the interpretation that the com-
pounds 1 to 5 show a coordination behaviour between a
tetrahedron and a linear arrangement.
The crystallization behaviour of 3 can be favoured by the
addition of small amounts of TMEDA or DME. The for-
mation of an adduct was not observed. This fact demon-
strates the effective shielding of the metal center by the
(2-pyridylmethyl)(tert-butyldimethylsilyl)amide anions. Lee
et al. [25] showed that TMEDA can serve as a base with a
flexible coordination behaviour. Thus, TMEDA can bind in
a monodentate or a bidentate as well as in a bridging
fashion. In the heteroleptic complexes [(TMEDA)FeL2]
with L as a bulky bidentate benzamidinate ligand the iron
atom is penta-coordinated. In [L2Mn(µ-TMEDA)MnL2]
the TMEDA molecule acts as a N,NЈ-bridging Lewis base.
Homoleptic compounds with bulky azaallyl substituents of
iron(II) show a favoured η3-coordination, however, extreme
bulkiness leads to a two-coordinate iron center with a bent
N-Fe-N unit [26]. In the compounds 2 and 3 Lewis bases
such as ethers or amines are unable to form complexes due
to steric reasons. However, small molecules such as oxygen
or water react immediately with these derivatives which ex-
plains the extreme sensitivity towards air and moisture.
Table 2 Comparison of M-N bond lengths in compounds with
the metals M ϭ Mg, Mn, Fe, Co, and Fe in dependency of the
coordination number C.N.(M) of the metal M.
M
1 Ϫ 5
M[N(SiMe3)2]2
{(Me3Si)2Nt-M[µ-Nbr(SiMe3)2]}2
a)
a)
M-N2/pm Method
M-N/pm Method
M-Nt/pm M-Nbr/pm
Mg 198.8
Mn 202.9
Fe 196.1
Co 192.9
Zn 192.6
GED [21] 191
GED [16] 195
GED [16] 184
GED [16] 184
X-ray [23] 183
X-ray [22] 198
215
217
208
206
X-ray [15] 200
X-ray [17] 193
X-ray [15] 192
a) Electron diffraction at the gaseous phase (GED), X-ray structure determi-
nation at a single crystal (X-ray).
378
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Z. Anorg. Allg. Chem. 2007, 375Ϫ382

(2-Pyridylmethyl)(tert-butyldimethylsilyl)amido Complexes of the Divalent Metals
rotation barrier is possible based on the coalescence tem-
perature (Tc ϭ 285 K for 1 and Tc ϭ 310 K for 5 at
200.129 MHz) and the line separation at low temperatures
(∆δ ϭ 10.5 Hz for 1 and ∆δ ϭ 12.4 Hz for 5). With these
data rotation barriers of 67.4 and 62.1 kJ·molϪ1, respec-
tively, have been estimated for the Mg (1) and Zn (5) com-
pounds. The slight difference of 5.3 kJ·molϪ1 is in agree-
ment with the results of the X-ray structures: The slight
elongation of the metal-nitrogen bonds in the magnesium
derivative 1 leads to decreasing steric strain and conse-
quently to a smaller rotation barrier around the Si-N bond
of the tert-butyldimethylsilyl substituent.
Reactivity studies. Even though the metal centers of the
complexes 1 to 5 are shielded quite effectively by the bulky
ligands, reactions at the periphery should be possible. These
(2-pyridylmethyl)(tert-butyldimethylsilyl)amide substituents
offer the possibility to form a C-C bond by a metal me-
diated oxidation reaction similar to the reaction shown in
equation (1). As a representative example, the diamagnetic
zinc derivative 5 was reacted with tin(II) bis[bis(trimethyl-
silyl)amide] allowing a NMR spectroscopic characterization
of the products. In accordance to the expected reaction
pathway we observed the C-C coupling product and the
formation of metal powder. X-ray diffraction experiments
showed that only tin metal precipitated. However, the
tin(II) derivative does not only initiate a C-C coupling reac-
tion, but also a metal-metal exchange reaction (transamin-
ation). Thus, the already well-known mononuclear tin(II)
1,2-bis(2-pyridyl)-1,2-bis(tert-butyldimethylsilylamido)-
ethane [1, 2] was isolated according to equation (6). Simul-
taneously Zn[N(SiMe3)2]2 was formed, because the newly
formed C-C bond leads to an opening of the coordination
sphere at zinc, thus enabling this transamination step.
Fig. 5 DNMR studies of compound 5 (solvent: [D8]toluene). Only
the 1H-NMR spectra of the resonances of the silicon-bound methyl
groups are shown at various temperatures at 345 K, 318 K, 308 K,
300 K, and 280 K (from the top to the bottom spectrum).
The molecular structure and numbering scheme of the
centrosymmetric (2-pyridylmethyl)(tert-butyldimethylsilyl)-
amido iron(II)chloride dimer (6) is displayed in Figure 4.
The structural parameters differ from those of the homo-
leptic derivative 3 due to the coordination number of four
for the amide nitrogen atom N2. Due to this fact the N2
atom is in a distorted tetrahedral environment und therefore
the back donation of charge from the pz(N) orbital into the
σ*(Si-C) orbital is not possible. Consequently, the N2-Si
bond length increases significantly and shows a value of
175.5(2) pm. The lower s-orbital participation in the Fe-N
bond also leads to an elongation of the Fe-N2 distances.
(6)
The reaction of the iron compound
3
with
Sn[N(SiMe3)2]2 also led to the precipitation of metal pow-
ders. However, a powder diffractogram confirmed that iron
and tin were formed simultaneously. The solution was
decanted and all volatile materials were removed under
vacuum. Mass spectrometric investigations of the residue
confirmed the formation of the C-C coupled ligand. How-
ever, an isolation of a metal complex with the C-C coupled
tetradentate ligand from the oily residue failed thus far.
It was not possible to isolate bis[(2-pyridylmethyl)(tert-
butyldimethylsilyl)amido]iron(III) chloride from the meta-
thesis reaction of lithium (2-pyridylmethyl)(tert-butyldi-
methylsilyl)amide with FeCl3. Instead of an iron(III) com-
plex, compound 6 was obtained with a yield of 17 %. Mass
spectrometric investigations showed that C-C coupling re-
actions have occurred. This observation is plausible regard-
ing the standard potential E0(Fe2ϩ/Fe3ϩ) of ϩ0.77 V. Al-
DNMR studies
Due to the paramagnetism of the coordination compounds
of Mn (2), Fe (3) and Co (4), NMR studies were only pos-
sible with the diamagnetic magnesium (1) and zinc com-
plexes (5). At room temperature the resonance of the sili-
con-bound methyl groups of 1 and 5 are broadened. Tem-
perature-dependent NMR studies show that the chemical
shift is temperature-dependent and that the signal becomes
narrow at high temperatures and splits into two resonances
at low temperatures as shown in Figure 5. This dynamic
behaviour results from a hindered rotation of the tert-butyl-
dimethylsilyl group around the Si-N bond. With the
Gutowsky-Holm equation the exact determination of the
Z. Anorg. Allg. Chem. 2007, 375Ϫ382
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
379

C. Koch, A. Malassa, C. Agthe, H. Görls, R. Biedermann, H. Krautscheid, M. Westerhausen
week at Ϫ10 °C led to the precipitation of colorless crystals. Yield:
1.33 g (2.85 mmol, 95 %). Elemental analysis (C24H42MgN4Si2,
467.11): calcd. C 61.71, H 9.06, N 11.99; found: C 60.09, H 9.06,
N 11.60 %. M.p. 136 °C (dec.).
1H NMR (C6D6): δ ϭ 7.91 Ϫ 7.89 (d, H1), 6.91 Ϫ 6.83 (m, H3), 6.69 Ϫ 6.66
(d, H4), 6.43 Ϫ 6.37 (m, H2), 4.91 (s, H6), 1.29 (s, CCH3), 0.29 (s, SiCH3).
13C{1H} NMR (C6D6): δ ϭ 170.4 (C5), 146.4 (C1), 138.7 (C3), 122.6 (C2),
121.1 (C4), 54.3 (C6), 28.4 (CCH3), 20.9 (CCH3), Ϫ3.27 and Ϫ3.39 (SiCH3).
IR (Nujol, KBr, cmϪ1): 1590 s, 1570 s, 1462 s, 1377 s, 1251 s, 1121 s, 935 m,
830 s, 775 s, 662 m, 628 w.
ready thirty years ago also Bradley and Chisholm [27] no-
ticed that dialkylamides of iron(III) are not stable but they
form C-C coupling products.
The rather low yields of 3 and 4 can also be understood
by partial subsequent formation of C-C coupling products
during the transamination reaction. The raw materials of 3
and 4 were covered with an oily substance after the com-
mon work-up procedures and had to be recrystallized. The
comparison of the standard potentials of Sn [E0(Sn/Sn2ϩ) ϭ
Ϫ0.14 V] with Mg [E0(Mg/Mg2ϩ) ϭ Ϫ2.36 V], Mn [E0(Mn/
Mn2ϩ) ϭ Ϫ1.18 V], Fe [E0(Fe/Fe2ϩ) ϭ Ϫ0.44 V], Co
Synthesis of manganese bis[(2-pyridylmethyl)(tert-butyldimethyl-
silyl)amide] (2). Mn[N(SiMe3)2]2 (0.375 g, 1.0 mmol) was dissolved
in 10 mL of hexane. At r.t. 0.222 g of (2-pyridylmethyl)(tert-butyl-
dimethylsilyl)amine (1.0 mmol) was added dropwise. The pink solu-
tion turned purple immediately. All solid material was removed. At
r.t. yellow crystals precipitated within several hours. This com-
pound is extremely sensitive towards air and moisture. Upon iso-
lation the crystals liquefied and turned into a brown oily substance.
Therefore the determination of the melting point gave no reproduc-
ible results. The yield was approximately 20 %. Elemental analysis
(C24H42MnN4Si2, 497.74): calcd. C 57.92, H 8.51, N 11.26; found:
C 58.00, H 8.18, N 11.21 %.
[E0(Co/Co2ϩ
) ϭ ) ϭ
Ϫ0.28 V] and Zn [E0(Zn/Zn2ϩ
Ϫ0.76 V] explains why for the paramagnetic complexes 3
and 4 C-C coupling side reactions occur whereas for
magnesium only transamination reactions have to be con-
sidered.
Summary and Perspectives
The synthesis of homoleptic metal bis[(2-pyridylmethyl)(tri-
alkylsilyl)amides] of Mg, Mn, Fe, Co, and Zn succeeds via a
transamination reaction. Due to similar radii of these metal
cations between 70 and 80 pm, these compounds crystallize
isotypic with strongly widened NMN bond angles. These
isotypic structures suggest that the packing of the ligands
preforms the coordination gap which is occupied by the me-
tal atom. The steric shielding allows the synthesis of homo-
leptic alkylamides even of the paramagnetic metal cations
manganese, iron and cobalt.
The open shell systems (paramagnetic complexes) are
much more sensitive towards moisture and air than the dia-
magnetic compounds of Mg and Zn. The addition of
Sn[N(SiMe3)2]2 leads to a C-C coupling reaction, however,
also transamination reactions take place. Therefore, no ana-
lytically pure product with a C-C coupled tetradentate
1,2-bis(2-pyridyl)-1,2-bis(tert-butyldimethylsilylamido)ethane
ligand of Mg, Mn, Fe, Co, or Zn was available from this
reaction as of yet.
EPR (298 K): g ϭ 2.0298. IR (Nujol, KBr, cmϪ1): 1592 m, 1570 m, 1462 vs,
1406 m, 1377 s, 1252 s, 1124 s, 1047 w, 1006 w, 830 vs, 769 s, 662 m. MS
(DEI, m/z): 497 (Mϩ, 100 %), 482 ([M-Me]ϩ, 8 %), 440 ([M-Bu]ϩ, 69 %).
Synthesis of iron bis[(2-pyridylmethyl)(tert-butyldimethylsilyl)-
amide] (3). Method A: (2-Pyridylmethyl)(tert-butyldimethylsilyl)-
amine (0.222 g, 1.0 mmol) was added dropwise at Ϫ20 °C to a solu-
tion of 0.376 g of Fe[N(SiMe3)2]2 (1.0 mmol). During the addition
the green solution darkens immediately. After complete addition
the reaction mixture was filtered and the solution cooled to
Ϫ20 °C. Slightly yellow single crystals precipitated from this dark
green mother liquor. After isolation and during drying the crystals
liquefied and turned into a dark oily substance. If the crystalliza-
tion failed all volatile materials were removed in vacuum till an oily
residue remained. Then addition of a small amount of DME or
TMEDA and cooling to Ϫ20 °C led to the precipitation of crystals
of 3.
Method B: To
a suspension of 0.26 g of dimesityliron(II)
(0.9 mmol) in 7 ml of diethylether, a solution of (2-pyridylmethyl)-
(tert-butyldimethylsilyl)amine in 5 ml of Et2O was added at
Ϫ78 °C. After warming to r.t. the red-brown solution was filtered
and then all volatile materials were removed. The red-brown resi-
due was redissolved in boiling THF. Slow cooling to Ϫ20 °C led to
the precipitation of colorless crystals of 3 covered with red oil.
Experimental Part
General procedure: All manipulations were carried out in an anhy-
drous argon atmosphere and the solvents were thoroughly dried.
Starting M[N(SiMe3)2]2 with M ϭ Mg, Mn, Fe, Co, and Zn as well
as FeMes2 were prepared according to literature procedures. The
N and C values of the elemental analysis are too small due to
nitride and carbonate formation during combustion.
Method C: At Ϫ78 °C 1.4 ml of a 1.6M BuLi solution in hexane
were dropped to a solution of 0.5 g of (2-pyridylmethyl)(tert-butyl-
dimethylsilyl)amine (2.2 mmol) in 5 ml of THF. This solution was
added dropwise at Ϫ78 °C to a suspension of 0.26 g of (thf)2FeCl2
in 5 ml of THF. After complete addition the red-brown reaction
mixture was slowly warmed to r.t. After removal of all volatile ma-
terials the residue was dissolved in 10 ml of pentane. After filtration
0.10 g of colorless crystals of 3 (0.2 mmol, 18 %) precipitated at
Ϫ20 °C. Elemental analysis (C24H42FeN4Si2, 498.65): calc.: C
57.81, H 8.49, N 11.24; found: C 57.12, H 8.97, N 9.69 %.
M.p. 58 °C (dec.).
Synthesis of magnesium bis[(2-pyridylmethyl)(tert-butyldimethylsilyl)-
amide] (1). 1,1,1,3,3,3-Hexamethyldisilazane (0.97 g, 6.01 mmol)
was dissolved in 4 mL of toluene. Dibutylmagnesium dissolved in
toluene (3 mL of a 1.0 molar solution) was added at r.t. After com-
plete addition the reaction mixture was heated to 40 °C and the
evolution of butane was observed for the next 2 h. Thereafter (2-
pyridylmethyl)(tert-butyldimethylsilyl)amine (0.68 g, 3.06 mmol)
was added and the initially light yellow solution turned brown to
purple. The volatile materials were removed under vacuum. The
residue was dissolved in n-hexane. Storage of this solution for one
IR (nujol, KBr, cmϪ1): 1602 m, 1568 w, 1480 m, 1278 w, 1241 s, 1152 w,
1054 m, 1037 s, 1003 vs, 956 m, 826 vs, 772 s, 762 s, 661 m, 631 m, 516 m.
MS (DEI, m/z): 498 (Mϩ, 93 %), 483 ([M-Me]ϩ, 22 %), 441 ([M-Bu]ϩ,
100 %), 223 (18 %).
380
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 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2007, 375Ϫ382

(2-Pyridylmethyl)(tert-butyldimethylsilyl)amido Complexes of the Divalent Metals
IR (Nujol, KBr, cmϪ1): 3371 w, 2925 s, 2854 s, 1592 s, 1570 s, 1463 s, 1377 s,
Synthesis of cobalt bis[(2-pyridylmethyl)(tert-butyldimethylsilyl)-
amide] (4). At r.t. 0.44 g of (2-pyridylmethyl)(tert-butyldimethyl-
silyl)amine (2.0 mmol) was added to 0.76 g of cobalt bis[bis(tri-
methylsilyl)amide] (2.0 mmol) in 10 mL of hexane. After complete
addition all solids were removed from this dark green solution by
filtration. Cooling to Ϫ18 °C afforded the precipitation of dark
brown crystals of 4. After isolation the crystals slowly liquefied and
decomposed. Therefore, no melting point can be given. Elemental
analysis (C24H42MnN4Si2, 501.73): calc.: C 57.45, H 8.44, N 11.17;
found: C 56.02, H 8.40, N 10.77 %.
1252 s, 1124 s, 937 m, 830 s, 776 s, 662 m, 628 w, 593 w. MS (EI): 508 (Mϩ,
8 %). MS (DEI, m/z): 506 (Mϩ, 16 %), 491 ([M-Me]ϩ, 3 %), 449 ([M-Bu]ϩ,
100 %), 163 (95 %).
Synthesis
of
(2-pyridylmethyl)(tert-butyldimethylsilyl)amido
iron(II)chloride (6). At Ϫ78 °C a solution of nBuLi in hexane
(1.6M, 1.0 mL) was added to 0.36 g of 2-pyridylmethylamine
(1.6 mmol) in 5 mL of THF. After 5 minutes of stirring and warm-
ing to r.t. this solution was dropped at Ϫ78 °C to a suspension of
0.38 g of (thf)1.5FeCl2 in 5 ml of THF. The reaction mixture turned
red immediately. During the warm-up after complete addition the
solution changed its color to yellow-brown. At r.t. the solution was
allowed to stand without stirring. During 17 h a pale yellow pow-
der precipitated. Reduction of the volume of the mother liquor and
cooling to 5 °C gave another crop of 6. Yield: 0.21 g (0.34 mmol;
43 %). M.p. 179 °C (dec.). Elemental analysis (C24H42Cl2Fe2N4Si2,
625.71): Calcd. C 46.07, H 6.77, N 8.95, found: C 45.43, H 6.59,
N 8.81 %.
IR (nujol, KBr, cmϪ1): 1592 m, 1570 w, 1463 vs, 1405 m, 1378 m, 1251 s,
1122 s, 1046 w, 1006 w, 937 w, 890 w, 830 vs, 772 m, 762 m, 753 m, 661 m.
MS (DEI, m/z): 501 (Mϩ, 100 %), 486 ([M-Me]ϩ, 12 %), 444 ([M-Bu]ϩ,
94 %), 223 (4 %), 163 (18 %).
Synthesis of zinc bis[(2-pyridylmethyl)(tert-butyldimethylsilyl)-
amide] (5). A solution of 1.79 g of (2-pyridylmethyl)(tert-butyldi-
methylsilyl)amine (8.05 mmol) in 5 mL of toluene or hexane was
stirred at r.t. Then a solution of 1.56 g of Zn[N(SiMe3)2]2
(4.04 mmol) was added dropwise. After stirring over night a small
amount of colorless precipitate formed and was removed. The vol-
ume of the solution was reduced to 2/3 of the original volume.
After 3 d at r.t. colorless crystals of 5 were collected. After re-
duction of the volume of the mother liquor another crop of crystals
was isolated. The solid material was dissolved in 10 mL of toluene.
After 2 weeks at Ϫ10 °C colorless crystals of 5·1/2 toluene (M.p.
75 °C) were isolated. Recrystallization from hexane gave 1.83 g of
solvent-free crystalline 5 (3.60 mmol, 90 %). M.p. 110 °C. Elemen-
tal analysis (C24H42N4Si2Zn, 508.17): Calcd. C 56.73, H 8.33, N
11.03, found: C 56.01, H 8.22, N 10.87 %.
IR (Nujol, KBr, cmϪ1): 1608 m, 1569 w, 1487 m, 1286 w, 1255 m, 1155 w,
1104 w, 1026 m, 991 m, 965 w, 899 w, 823 s, 780 vs, 754 w, 660 m, 638 w,
525 m, 461 w. MS (EI): 625 (Mϩ, 1 %), 567 ([MϩBu]ϩ, 1 %), 498
([M-FeCl2]ϩ ϭ 3ϩ, 4 %), 165 (100 %).
Structure Determinations. The intensity data of 5 were collected on
a Stoe IPDS-2T diffractometer and of 1, 2, 3, 4, and 5·toluene on
a Nonius Kappa CCD diffractometer using graphite-monochrom-
˚
ated Mo-Kα radiation (λ ϭ 0.71073 A). Data was corrected for
Lorentz polarization and for absorption effects [28Ϫ30]. All iso-
typic compounds 1 to 5 crystallize in the monoclinic space group
C2/c. The crystal data and refinement details are summarized in
Table 3. The structures were solved by direct methods (SHELXS
1H NMR (C6D6): δ ϭ 7.96 Ϫ 7.93 (d, H1), 6.94 Ϫ 6.86 (m, H3), 6.69 Ϫ 6.65
(d, H4), 6.46 Ϫ 6.40 (m, H2), 4.93 (s, H6), 1.23 (s, CCH3), 0.27 (s, SiCH3).
13C{1H} NMR (C6D6): δ ϭ 166.6 (C5), 149.2 (C1), 136.9 (C3), 122.3 (C2),
121.4 (C4), 54.6 (C6), 28.3 (CCH3), 21.6 (CCH3), Ϫ3.02 and Ϫ2.87 (SiCH3).
2
[31]) and refined by full-matrix least squares techniques against Fo
(SHELXL-97 [32]).
Table 3 Crystal data and refinement details for the X-ray structure determinations of the isotypic compounds 1 to 5 as well as 5·1/2
toluene and 6.
Compound
1
2
3
4
5
5·1/2 toluene
6
formula
fw (g·molϪ1
T/K
crystal system
space group
a/pm
C24H42MgN4Si2 C24H42MnN4Si2 C24H42FeN4Si2 C24H42CoN4Si2 C24H42N4Si2Zn C27.5H46N4Si2Zn C24H42Cl2Fe2N4Si2
)
467.11
183(2)
monoclinic
C2/c (Nr. 15)
1805.2(2)
826.12(8)
2062.3(2)
90
112.030(6)
90
2850.9(5)
4
497.74
183(2)
498.65
183(2)
501.73
183(2)
monoclinic
C2/c (Nr. 15)
2199.6(1)
793.36(5)
2050.1(1)
90
118.63(3)
90
2832.9(3)
4
508.17
293(2)
554.23
183(2)
triclinic
625.71
183(2)
monoclinic
C2/c (Nr. 15)
1803.21(6)
819.85(3)
2074.67(8)
90
111.769(2)
90
2848.4(2)
4
monoclinic
C2/c (Nr. 15)
1786.71(7)
823.59(3)
2071.18(8)
90
112.016(2)
90
2825.5(2)
4
monoclinic
C2/c (Nr. 15)
1777.6(1)
829.74(8)
2050.1(1)
90
112.165(5)
90
2800.3(4)
4
monoclinic
P21/n (Nr. 14)
848.81(1)
1740.27(7)
1095.03(5)
90
94.444(3)
90
1612.7(1)
2
¯
P1 (Nr. 2)
1053.44(4)
1267.38(6)
1271.08(6)
82.081(2)
88.955(3)
67.221(3)
1548.6(1)
2
b/pm
c/pm
Ͱ /°
β /°
γ /°
3
˚
V/A
Z
σ /g·cmϪ3
µ /mmϪ1
measured data
data with I > 2σ(I)
1.088
0.164
1.161
0.565
1.172
0.636
1.176
0.707
1.205
0.980
1.189
0.892
1.288
1.158
9333
1797
9248
2660
9318
2509
9689
2539
9026
2474
10471
5677
6958
2604
unique data (Rint
)
3194
3252
0.1026
0.0396
1.024
0.408/-0.479
CCDC-609815
3240
3225
2740
6847
0.1397
0.0509
1.028
3698
0.0954
0.0395
1.005
0.288/-0.459
CCDC-609820
wR2 (all data, on F2)a) 0.1587
0.0990
0.0383
1.008
0.0989
0.0371
1.021
0.0937
0.0347
1.089
R1 (I > 2σ(I))a)
0.0636
1.019
0.450/-0.241
CCDC-609814
sb)
Ϫ3
˚
Res. dens./e·A
0.330/-0.372
CCDC-609816 CCDC-609817
0.326/-0.337
0.944/-0.739
CCDC-609818 CCDC-609819
0.961/-0.530
CCDC No.
a) Definition of the R indices: R1 ϭ (ΣʈFoΗ-ΗFcʈ)/ΣΗFoΗ
wR2 ϭ {Σ[w(Fo2-Fc2)2]/Σ[w(Fo2)2]}1/2 with wϪ1 ϭ σ2(Fo2) ϩ (aP)2.
b) s ϭ {Σ[w(Fo2-Fc2)2]/(No-Np)}1/2
.
Z. Anorg. Allg. Chem. 2007, 375Ϫ382
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
381

C. Koch, A. Malassa, C. Agthe, H. Görls, R. Biedermann, H. Krautscheid, M. Westerhausen
Acknowledgement. This work was supported in the collaborative
[13] D. C. Bradley, M. B. Hursthouse, A. A. Ibrahim, K. M. Abdul
Malik, M. Motevalli, R. Möseler, H. Powell, J. D. Runnacles,
A. C. Sullivan, Polyhedron 1990, 9, 2959Ϫ2964.
[14] (a) H. Bürger, U. Wannagat, Monatsh. Chem. 1964, 95,
1099Ϫ1102. (b) B Horvath, R. Möseler, E. G. Horvath, Z.
Anorg. Allg. Chem. 1979, 450, 165Ϫ177.
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[16] R. A. Andersen, K. Faegri, J. C. Green, A. Haaland, M. F.
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research initiative of the DFG (SFB 436) and we thank the
Deutsche Forschungsgemeinschaft (DFG, Bonn-Bad Godesberg,
Germany) for generous funding.
Supporting Information available: Crystallographic data (excluding
structure factors) has been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication CCDC-
609814 for 1, CCDC-609815 for 2, CCDC-609816 for 3, CCDC-
609817 for 4, CCDC-609818 for 5, CCDC-609819 for 5·toluene,
and CCDC-609820 for 6. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [E- mail: deposit@ccdc.cam.ac.uk].
[19] B. Machelett, Z. Chem. 1976, 16, 116Ϫ117; see also: H.
Müller, W. Seidel, H. Görls, J. Organomet. Chem. 1993, 445,
133Ϫ136.
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382
www.zaac.wiley-vch.de
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2007, 375Ϫ382