Silylamide complexes of chromium(II), manganese(II), and cobalt(II) bearing the ligands N(SiHMe2)2 and N(SiPhMe2) 2

Article abstract of DOI:10.1021/ic5002682

Bis(dimethylsilyl)amide and bis(dimethylphenylsilyl)amide complexes of the divalent transition metals chromium, manganese, and cobalt were synthesized. Dimeric, donor-free {Mn[N(SiHMe₂)₂]₂} ₂ could be obtained via two different pathways, a salt metathesis route (utilizing MnCl₂(thf)_1.5 and LiN(SiHMe ₂)₂) and a transsilylamination protocol (utilizing Mn[N(SiMe₃)₂]₂(thf) and HN(SiHMe ₂)₂). Addition of 1,1,3,3-tetramethylethylendiamine (tmeda) to {Mn[N(SiHMe₂)₂]₂}₂ yielded the monomeric adduct Mn[N(SiHMe₂)₂] ₂(tmeda). The syntheses of Cr[N(SiHMe₂)₂] ₂(tmeda), Co[N(SiMe₃)₂][N(SiHMe ₂)₂](tmeda), and Co[N(SiHMe₂)₂] ₂(tmeda) were achieved by transsilylamination from Cr[N(SiMe ₃)₂]₂(tmeda) and {Co[N(SiMe₃) ₂]₂}₂(μ-tmeda), respectively. Bis(dimethylphenylsilyl)amide complexes Mn[N(SiMe₂Ph) ₂]₂, Cr[N(SiMe₂Ph)₂]₂, and Co[N(SiMe₂Ph)₂]₂(thf) were obtained via salt metathesis employing MCl₂(thf)_x (M = Cr, Mn, Co) with equimolar amounts of LiN(SiMe₂Ph)₂ in n-hexane. Treatment of CrCl₂ with LiN(SiMe₂Ph)₂ in thf gave Cr[N(SiMe₂Ph)₂]₂(thf)₂, featuring an almost square planar trans-coordination. All complexes were examined by elemental analyses, DRIFT and UV-vis spectroscopy, as well as X-ray structure analysis, paying particular attention to secondary M - -SiH β-agostic and M - -π(arene) interactions. Magnetic moments were determined by Evans' method.

Full text of DOI:10.1021/ic5002682

Article
pubs.acs.org/IC
Silylamide Complexes of Chromium(II), Manganese(II), and Cobalt(II)
Bearing the Ligands N(SiHMe2)2 and N(SiPhMe2)2
Sonja N. Konig, Christoph Schadle, Cacilia Maichle-Mossmer, and Reiner Anwander*
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̈
̈
̈
Institut fur Anorganische Chemie, Universitat Tubingen, Auf der Morgenstelle 18, 72076 Tubingen, Germany
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̈
̈
̈
S
* Supporting Information
ABSTRACT: Bis(dimethylsilyl)amide and bis(dimethylphenylsilyl)amide complexes of the divalent transition metals chromium,
manganese, and cobalt were synthesized. Dimeric, donor-free {Mn[N(SiHMe2)2]2}2 could be obtained via two different
pathways, a salt metathesis route (utilizing MnCl2(thf)1.5 and LiN(SiHMe2)2) and a transsilylamination protocol (utilizing
Mn[N(SiMe3)2]2(thf) and HN(SiHMe2)2). Addition of 1,1,3,3-tetramethylethylendiamine (tmeda) to {Mn[N(SiHMe2)2]2}2
yielded the monomeric adduct Mn[N(SiHMe2)2]2(tmeda). The syntheses of Cr[N(SiHMe2)2]2(tmeda), Co[N(SiMe3)2][N-
(SiHMe2)2](tmeda), and Co[N(SiHMe2)2]2(tmeda) were achieved by transsilylamination from Cr[N(SiMe3)2]2(tmeda) and
{Co[N(SiMe3)2]2}2(μ-tmeda), respectively. Bis(dimethylphenylsilyl)amide complexes Mn[N(SiMe2Ph)2]2, Cr[N(SiMe2Ph)2]2,
and Co[N(SiMe2Ph)2]2(thf) were obtained via salt metathesis employing MCl2(thf)x (M = Cr, Mn, Co) with equimolar
amounts of LiN(SiMe2Ph)2 in n-hexane. Treatment of CrCl2 with LiN(SiMe2Ph)2 in thf gave Cr[N(SiMe2Ph)2]2(thf)2, featuring
an almost square planar trans-coordination. All complexes were examined by elemental analyses, DRIFT and UV−vis
spectroscopy, as well as X-ray structure analysis, paying particular attention to secondary M---SiH β-agostic and M---π(arene)
interactions. Magnetic moments were determined by Evans’ method.
complexes M[N(SiMe3)2]2, and in particular iron and cobalt
derivatives, were comprehensively studied as precursors for
magnetic nanoparticles.9−11
INTRODUCTION
■
Silylamido ligands of type −N(SiR3)2 (R = any organic group)
are known to ideally stabilize low coordination numbers of
transition metal complexes; especially the archetypal bis-
(trimethylsilyl)amido ligand (−N(SiMe3)2) has been frequently
used.1 The respective complexes M[N(SiMe3)2]x (x = 1, 2, or
3) have been fully characterized for the 3d-transition metals
except for nickel.2 Furthermore, their syntheses are straightfor-
ward with moderate to high yields, and the resulting complexes
are soluble in hydrocarbon solvents such as n-hexane. Crucially,
complexes M[N(SiMe3)2]x qualify as precursors for other
metallorganic species since protic exchange yields only the free
silylamine as a side product (pKa[HN(SiMe3)2] = 25.8),2,3
which can be removed very easily under vacuum. For example,
clusters exhibiting magnetic properties can be synthesized from
Mn[N(SiMe3)2]2, Fe[N(SiMe3)2]2, and Co[N(SiMe3)2]2 by
protic exchange of the silylamido ligand with 8-aminoquinoline
or hexahydropyrimidopyrimidine.4 Complexes M[N(SiMe3)2]2
(M = Fe, Co) and their donor adducts were successfully
employed as homogeneous catalysts, e.g., for the hydrosilylation
of carbonyl compounds,5 as well as to design single-molecule
magnets.6 Transition metal silylamide complexes have also been
used as molecular precursors for metals, metal alloys, carbides,
nitrides, oxides, and silicates according to chemical vapor
deposition2o,7 and ammonolysis techniques.8 More recently,
Our group has been investigating metal silylamides not only
as synthesis precursors for homogeneous catalysts according to
silylamine12 and silylamide elimination protocols13,14 but also
as reactants for the surface functionalization of silica materials.
Metal silylamides engage in efficient mild surface grafting if
compared to metal alkyls and metal alkoxides, which tend to
undergo multifunctional surface reactions and lead to
incomplete consumption of the silanol groups, respectively.15
Silylamide complexes employed so far for the functionalization
of silica surfaces comprise the ligands [N(SiMe3)2], [N-
(SiHMe2)2], and [N(SiMe2Ph)2].15,16 For example, Cr[N-
(SiMe3)2]2(thf)2 can be immobilized on silica gel and applied in
ethylene polymerization.17 By grafting Ca[N(SiMe3)2]2 onto
Aerosil 380 silica, the mostly unwanted Schlenk equilibrium can
be prevented (the latter occurs when aiming at heteroleptic
complexes of the type Ca[N(SiMe3)2][OSi(OtBu)3]); more-
over, these heterogenized “heteroleptic” catalysts show
promising activity in the hydrosilylation of alkenes.18 Ln[N-
(SiMe3)2]3/SBA-15 hybrid materials (Ln = Y, La, Nd; SBA-15
Received: February 3, 2014
Published: April 24, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis Routes and Schematic Drawing of Compound 1
= high-surface periodic mesoporous silica) exhibit better
performance than in the intramolecular hydroamination
cyclization of aminoalkenes than the molecular counterparts.19
Grafting of lanthanide and aluminum bis(dimethylsilyl)amide
complexes M[N(SiHMe2)2]3(thf)x and subsequent ligand
exchange against 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oc-
tanedione generates surface species that are active in the
hetero-Diels−Alder cyclization (Danishefsky transformation).20
The surface chemistry of bis(dimethylphenylsilyl)amide com-
plexes is less exploited. Only Mg[N(SiMe2Ph)2]2 and Fe[N-
(SiMe2Ph)2]2 have been grafted onto cage-like periodic
mesoporous silica until now.21
The transsilylamination route offers a straightforward
protocol for the synthesis of bis(dimethylsilyl)amide complexes
(pKa[N(SiHMe2)2] = 22.8).22,23 Moreover, the ligand is an
excellent IR/NMR spectroscopic probe,15 allowing for an easy
monitoring of ligand exchange and surface reactions. Bis-
(dimethylsilyl)amide complexes have been synthesized for most
rare-earth metals23−26 and alkaline-earth metals,21b,27 as well as
group 13 metals.28 In contrast, only a few transition metal
bis(dimethylsilyl)amide complexes have been described in the
literature so far; namely, homoleptic Hf[N(SiHMe2)2]4,7e
Figure 1. ORTEP view of complex 1. The atomic displacement
parameters are set at the 50% probability level; hydrogen atoms of the
methyl groups are omitted for clarity.
Table 1. Selected Interatomic Distances and Angles of
21a
Compound 1 and {Fe[N(SiHMe2)2]2}2
1
{Fe[N(SiHMe2)2]2}2
Bond Lengths/Interatomic Distances(Å)
U[N(SiHMe2)2]4,29 {Fe(II)[N(SiHMe2)2]2}2,21a and Zn[N-
M1−N1
M1−N2
M1−N2′
M1---M1′
M1---Si1
M1---Si2
M1---Si3
M1---Si4
1.984(4)
2.129(4)
2.123(4)
2.849(2)
2.994(2)
3.218(2)
3.000(2)
3.354(3)
Angles (deg)
1.903(2)
2.017(2)
2.050(2)
2.6733(6)
3.0821(8)
3.0794(7)
3.1324(7)
3.2573(7)
30
(SiHMe2)2]2
and heteroleptic Mo2(O2CR)2[N-
t
(SiHMe2)2]2(PR′3)2 (R = Me, Bu; PR′3 = PMe3, PMe2Ph,
PEt3),31 M[N(SiHMe2)2]3(NSiHMe2) (M = Nb, Ta),32
Zr[N(SiHMe2)2]2Cl2,33 Ti[N(SiHMe2)2](NMe2)3,34 and Fe-
21a
(III)[N(SiHMe2)2]3(μ-Cl)Li(thf)3 are known.
Exchanging the hydrogen atom in a Si−H group for a phenyl
group leads to the similar ligand bis(dimethylphenyl)silylamide
(−N(SiMe2Ph)2). This ligand is more bulky but able to
establish π-interactions with the metal centers. Only five
homoleptic metal amides with this ligand have been synthesized
to date, {LiN(SiMe2Ph)2}2,35 Fe[N(SiMe2Ph)2]2,36 {Cu[N-
(SiMe2Ph)2]}4,37 Cd[N(SiMe2Ph)2]2,38 and Sn[N-
(SiMe2Ph)2]2,39 illustrating that this ligand can stabilize
monomeric species of divalent metals.
In this work, we present the synthesis of Cr(II), Mn(II), and
Co(II) bis(dimethylsilyl)amide and bis(dimethylphenylsilyl)-
amide complexes and their analytical characterization including
X-ray structure analysis.
N1−M1−N2
M1−N2−M1′
N2−M1−N2′
Si1−N1−Si2
Si3−N2−Si4
Mn1−N1−Si1
Mn1−N1−Si2
Mn1−N2−Si3
Mn1−N2−Si4
132.4(2)
84.2(1)
134.96(7)
82.18(6)
97.82(6)
126.49(1)
117.43(9)
116.8(1)
116.7(1)
111.57(9)
118.93(9)
95.9(1)
129.5(3)
123.2(2)
109.1(3)
121.4(3)
101.7(2)
120.1(3)
to the corresponding bis(trimethylsilyl)amide complex of
manganese(II) the terminal Mn−N distances are very similar
(1.984 vs 1.994 Å)2k and the Mn−N distances to the bridging
ligands are slightly shorter by ca. 0.06 Å (2.123 vs 2.184 Å),2k
which apparently arises from the lower steric demand of the
bis(dimethysilyl)amido ligand. This phenomenon has also been
observed for the dimeric bis(dimethylsilyl)amide and bis-
(trimethylsilyl)amide complexes of iron(II).21a Furthermore,
the Mn1−N2−Mn1′ angle of 84.2° is wider than in
{Mn[N(SiMe3)2]2}2 (81.1°),2k giving an almost square planar
core for complex 1, while the distances between the manganese
atoms Mn1---Mn1′ are almost the same in both complexes
(Δd: 0.008 Å). The crystal structure of the iron(II)
RESULTS AND DISCUSSION
■
Synthesis of Bis(dimethylsilyl)amide Complexes. Ho-
moleptic manganese(II) bis(dimethylsilyl)amide 1 can be
synthesized via a salt metathesis route from MnCl2(thf)1.5 or
via transsilylamination from Mn[N(SiMe3)2]2(thf).40 In both
cases, crystallization from n-hexane affords the donor-free
dimeric structure of {Mn[N(SiHMe2)2]2}2 (1, Scheme 1).
The molecular structure of 1 (Figure 1, Table 1, Table 5) is
2k
similar to those of {Mn[N(SiMe3)2]2}2 and {Fe[N-
(SiHMe2)2]2}2,21a revealing a dimeric structure with two
bridging and two terminal silylamido moieties, causing a
distorted trigonal configuration of the metal centers. Compared
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Table 2. Selected Interatomic Distances and Angles of Compounds 4, 5, 6, and 7
4 (Cr)
7 (Mn)
5 (Co)
6 (Co)
Bond Angles (deg)
2.0470(7)
M1−N1
M1−N2
M1−N3
M1−N4
M1---Si1
M1---Si2
M1---Si3
M1---Si4
2.046(2)
2.048(2)
2.209(2)
2.225(2)
3.1430(7)
3.2197(7)
3.1484(7)
3.2759(7)
1.9750(8)
1.9761(8)
2.2075(8)
2.1842(9)
3.2146(3)
3.1284(3)
3.1485(3)
3.2043(3)
1.968(1)
1.975(1)
2.184(1)
2.184(1)
2.0475(7)
2−2901(7)
2.3020(7)
3.2915(3)
3.0873(4)
3.2196(5)
3.2698(5)
2.9382(4)
3.0930(3)
3.2306(3)
3.2345(3)
Bond Lengths/Interatomic Distances (Å)
124.57(3)
N1−M1−N2
N1−M1−N3
N1−M1−N4
N2−M1−N3
N2−M1−N4
N3−M1−N4
M1−N1−Si1
M1−N1−Si2
M1−N2−Si3
M1−N2−Si4
100.85(7)
91.25(7)
163.58(7)
163.78(7)
91.21(7)
79.29(7)
114.19(8)
118.72(8)
114.35(8)
121.81(9)
117.32(3)
101.92(3)
121.33(3)
124.27(3)
105.55(4)
82.93(3)
116.33(5)
106.68(5)
121.19(5)
118.76(5)
106.69(5)
83.50(5)
119.21(3)
102.09(3)
102.80(3)
120.13(3)
80.37(3)
122.90(4)
121.73(5)
116.31(4)
116.89(4)
120.87(5)
114.31(6)
122.43(7)
125.81(7)
105.81(6)
110.94(3)
118.79(4)
119.12(4)
36
Table 3. Selected Bond Lengths and Angles of Compounds 8, 9, 10, 11, and Fe[N(SiMe2Ph)2]2
36
9 (Cr)
11 (Cr)
8 (Mn)
Fe[N(SiMe2Ph)2]2
10 (Co)
Bond Lengths/Interatomic Distances (Å)
M1−N1
M1−N2
M1---C3
M1---C11
M1---C19
M1---C27
M1−O1
2.0031(8)
1.9550(8)
3.8699(9)
2.379(1)
3.191(1)
3.113(1)
2.078(1)
1.968(2)
1.976(2)
3.046(2)
3.279(2)
3.068(2)
3.099(2)
1.896(2)
1.909(2)
3.013(3)
3.227(3)
2.956(3)
3.234(4)
1.925(1)
1.922(1)
4.388(1)
3.920(1)
3.889(1)
4.219(1)
2.0549(9)
3.866(1)
4.622(1)
2.1083(9)
Bond Angles (deg)
N1−M1−N2
M1−N1−Si1
M1−N1−Si2
M1−N2−Si3
M1−N2−Si4
N1−M1−O1
N2−M1−O1
N1−M1−O1′
140.89(4)
133.19(5)
99.85(4)
180.00
172.66(7)
115.47(9)
119.03(9)
115.81(9)
116.01(8)
172.1(1)
113.9(1)
119.5(1)
113.2(1)
119.1(1)
146.49(4)
110.15(5)
124.79(5)
125.41(5)
107.77(5)
104.68(4)
108.83(4)
116.36(5)
118.35(5)
116.29(4)
115.89(5)
91.38(3)
88.62(3)
protons are arranged in an eclipsed manner and face the metal
center. In the case of {Fe[N(SiHMe2)2]2}2, the silicon groups
of the bridging ligand are residing in a gauche conformation,
meaning one Si−H proton is pointing away from the iron atom.
Dimeric structures of bis(dimethylsilyl)amide complexes with
two bridging bis(dimethylsilyl)amido ligands have also been
observed for the trivalent rare-earth metals {Y[N-
Table 4. Effective Magnetic Moments Determined by the
Evans’ Method
compound
μeff (μB)
{Mn[N(SiHMe2)2]2}2 (1)
Cr[N(SiMe3)2]2(tmeda) (2)
{Co[N(SiMe3)2]2}2(μ-tmeda) (3)
Cr[N(SiHMe2)2]2(tmeda) (4)
Co[N(SiHMe2)2]2(tmeda) (6)
Mn[N(SiHMe2)2]2(tmeda) (7)
Mn[N(SiMe2Ph)2]2 (8)
3.26
4.82
5.09
4.38
4.50
5.97
5.74
4.69
5.20
4.54
24d
(SiHMe2)2]3}2 and {La[N(SiHMe2)2]3}2.24g
In the case of chromium(II) and cobalt(II), we were not able
to access the donor-free bis(dimethylsilyl)amide complexes.
Instead, degradation was observed during the attempted
syntheses, via either salt metathesis or transsilylamination.
Apparently, even the donor abilities of thf are too weak to
stabilize this kind of compound. Adding the more powerful,
potentially chelating donor tetramethylethylendiamine (tmeda)
Cr[N(SiMe2Ph)2]2 (9)
Co[N(SiMe2Ph)2]2(thf) (10)
Cr[N(SiMe2Ph)2]2(thf)2 (11)
bis[bis(dimethylsilyl)amide] exhibits shorter metal−nitrogen
and metal---metal distances (Table 1), which probably arises
from the smaller ionic radius of iron(II)41 compared to
manganese(II). In the manganese(II) complex (1), all Si−H
2g
to Cr[N(SiMe3)2]2(thf)2 and Co[N(SiMe3)2]2(thf)9a,42 gives
Cr[N(SiMe3)2]2(tmeda) (2) and {Co[N(SiMe3)2]2}2(μ-
tmeda) (3, Scheme 2), respectively, which thereinafter enable
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Table 5. Crystallographic Data of Bis(dimethylsilyl)amide Complexes 1, 4, 5, 6, and 7
1 (Mn)
4 (Cr)
5 (Co)
6 (Co)
7 (Mn)
fw (g/mol) 639.25
432.89
173(2)
467.87
100(2)
439.82
100(2)
435.83
103(2)
T (K)
293(2)
triclinic P1
space
monoclinic P21/c
monoclinic P21
monoclinic P21/n
monoclinic P21/n
̅
group
a, b, c (Å) 7.975(3), 10.783(3),
9.9205(8), 13.7329(8),
18.819(2)
9.3416(3), 17.7835(4),
10.2637(3)
10.0324(2), 17.8533(4),
14.3726(4)
11.0741(4), 15.4342(5),
14.9391(5)
11.595(3)
α, β, γ
100.84(2), 106.62(2),
92.48(2)
94.381(7)
104.198(1)
95.706 (1)
96.043(2)
(deg)
V (Å3)
933.4(5)
1
2556.3(4)
4
1374.14(7)
2561.5(1)
4
2539.2(2)
4
Z
2
dcalc
1.137
1.125
1.131
1.140
1.140
(g/cm3)
a
R1
0.0777
0.2198
1.085
0.0389
0.0967
0.0171
0.0470
0.0277
0.0707
0.0195
0.0541
1.061
b
wR2
c
GOF
1.153
1.033
1.020
a
b
c
2
2
2
R1 = ∑(|Fo| − |Fc|)/∑|Fo|, Fo > 2σ(Fo). wR2 = {∑[w(Fo − Fc2)2/∑[w(Fo )2]}1/2. GOF = [∑w(Fo − Fc2)2/(n0 − np)]1/2
.
Scheme 2. Syntheses and Schematic Drawings of Compounds 2 and 3
the syntheses of the bis(dimethylsilyl)amide complexes. Later
on, the stabilizing effect of the newly introduced donor tmeda
on the transsilylamination reactions to give the respective
bis(dimethylsilyl)amido compounds was exploited in one-pot-
reactions, without preceding isolation of the tmeda adducts 2
and 3. Nevertheless, we were interested in the structural
chemistry of both compounds. As illustrated in Scheme 2, the
cobalt(II) complex 3 is binuclear with one bridging tmeda
molecule and consequently three-coordinate for each cobalt
atom, whereas the chromium(II) complex 2 is monomeric and
four-coordinate. This finding probably arises from the different
ionic radii of the two metals, with Cr(II) being the larger ion41
and hence being able to coordinate both donor atoms of tmeda.
The DRIFT (diffuse reflectance infrared Fourier transform)
spectra of 2 and 3 show only slight differences (Figure 2). The
C−H valence vibration of the Si−CH3 methyl groups (3030−
2890 cm−1), the C−H stretching vibration of the N−CH3
Figure 2. DRIFT spectra of compounds 2 and 3.
methyl groups (2850−2800 cm−1), the C−H bending vibration
of the methylene and methyl groups (1480−1460 cm−1), the
C−N vibration at about 1250 cm−1, and the Si−CH3 valence
vibration around 1240 cm−1 can be readily assigned. The
asymmetric metal−nitrogen stretching vibrations are located at
377 (2) and 365 (3) cm−1 (Figures S1 and S2).
respectively. Overall, in all three structures the metal centers
adopt a distorted tetrahedral geometry. As expected, the M1−
N(SiMe3)2 and M1−NMe2 distances are somewhat longer for
the previously described complexes (Yb(II): 2.34(1) and
2.61(1) Å; Ca: 2.315(1) and 2.592(2) Å).43,44
The core of {Co[N(SiMe3)2]2}2(μ-tmeda) (3, Figure 4) has
an almost linear zigzag pattern propagating from N2 via the
bridging tmeda to N3. The coordination geometry of the cobalt
centers is distorted trigonal planar, and the M1−N bond
lengths are shorter than in complex 2. Other examples of tmeda
bridging between two identical organometallic moieties have
The molecular structure of 2 is depicted in Figure 3. Similar
structures have been reported for calcium(II)43 and ytterbium-
(II),44 but in contrast to compound 2, the latter display a center
of symmetry (C2 axis). Moreover, the angle between the N1−
Cr1−N2 plane and the N3−Cr1−N4 plane is much smaller in
the case of complex 2, with 35.86(7)° compared to 53.66° and
54.5(4)° for the calcium(II) and ytterbium(II) compounds,
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Applying tmeda as stabilizing donor, the transsilylamination
protocol can be followed to generate the bis(dimethylsilyl)-
amide complexes Cr[N(SiHMe2)2]2(tmeda) (4), Co[N-
(SiMe3)2][N(SiHMe2)2](tmeda) (5), and Co[N-
(SiHMe2)2]2(tmeda) (6, Scheme 3). The reaction temperature
of the transsilylamination of {Co[N(SiMe3)2]2}2(μ-tmeda)
with HN(SiHMe2)2 seems to be crucial, giving the mixed
bis(trimethylsilyl)bis(dimethylsilyl)amide at ambient temper-
ature (5) and the bis[bis(dimethylsilyl)amide)] at elevated
temperatures (6). Since −N(SiHMe2)2 is sterically less
demanding than −N(SiMe3)2, coordination of both donor
atoms of tmeda to one cobalt center becomes feasible in 5 and
6. For comparison only, the monomeric tmeda adduct of
{Mn[N(SiHMe2)2]2}2 was synthesized by simply adding tmeda
to a stirred solution of 1 in n-hexane. The only other known
bis(dimethylsilyl)amide complex with additional tmeda coor-
dination is the trivalent La[N(SiHMe2)2]3(tmeda).48
Comparing the angles between the planes spanned by the
metal amido (N1−M1−N2) and the metal tmeda coordination
(N3−M1−N4), complex 4 stands out with only 17.46(7)°,
leading to a distorted square planar configuration of the
chromium center. The interplanar angles for the cobalt (5, 6)
and manganese (7) are 74.38(3)°, 79.21(4)°, and 75.84(3)°,
respectively, creating a distorted tetrahedral environment of the
metal centers. The M1−N distances decrease with decreasing
ionic radius of the metal (Mn1−N > Cr1−N > Co1−N). The
terminal Mn1−N1 bond in 1 is shorter than the Mn1−N bonds
in the tmeda adduct 7, but the bridging Mn1−N2 and Mn1−
N2′ bonds are slightly longer. The Si−H moieties in complexes
1 and 4−7 exhibit distinct orientations with respect to the
metal centers. In some cases, somewhat shorter metal−silicon
distances can be noticed, which is also reflected in slightly
smaller M−N−Si angles. This is the case for Mn1---Si1 and
Mn1---Si3 in 1, Co1---Si4 in 6, and Mn1---Si1 in 7 (Table 1,
Table 2). These observations can denote weak β-agostic
interactions between some of the Si−H bonds and the
respective metal centers. Bis(trimethylsilyl)amide complexes
of Mn(II) and Co(II) with additional coordination of 2,2′-
bipyridyl (2,2′-bipy)49 or two donor molecules such as pyridine
(py),50 pyrazine (prz),50 and thf51 are known in the literature.
Similar to complexes 2 and 4−7, the N−M−N angles spanned
by the silylamido ligands are wider than the D−M−D angles
spanned by the donor molecules (M = Mn, do = 2,2′-bipy:
128.24(17)° vs 72.91(18)°,49 M = Mn, do = py: 127.18(3)° vs
86.77(3)°,50 M = Co, do = py: 123.17(4)° vs 90.1(4)°,50 M =
Mn, do = prz: 130.82(7)° vs 96.23(7),50 and M = Mn, do = thf:
131.7(2)° vs 86.6°51).
Figure 3. ORTEP view of complex 2 with the atomic displacement
parameters set at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr1−
N1 2.080(2), Cr1−N2 2.073(2), Cr1−N3 2.254(2), Cr1−N4
2.273(2), N1−Cr1−N2 106.76(8), N1−Cr1−N3 92.63(8), N1−
Cr1−N4 152.01(8), N2−Cr1−N3 149.97(8), N2−Cr1−N4 92.40(8),
N3−Cr1−N4 79.29(8).
Figure 4. ORTEP view of complex 3 with the atomic displacement
parameters set at the 50% probability level. Hydrogen atoms are
omitted, and the methyl groups attached to silicon are shown in the
ball and stick representation for clarity. Selected bond lengths/
distances (Å) and angles (deg): Co1−N1 1.910(1), Co1−N2
1.915(1), Co1−N6 2.147(2), Co2−N3 1.910(1), Co2−N4 1.907(1),
Co2−N5 2.151(1), Co1---Co2 7.3844(3), N1−Co1−N2 140.33(6),
N1−Co1−N6 110.62(6), N2−Co1−N6 109.05(6), N3−Co2−N4
138.51(6), N3−Co2−N5 109.61(6), N4−Co2−N5 111.88(6).
been reported in the literature, for instance, {Co[N(SitBuMe2)-
(2-C5H3N-6-Me)]2}2(μ-tmeda),45 {Co[PhC(NSiMe3)-
(NAr)]2}2(μ-tmeda) (Ar = 2,6-Me2C6H3),46 [Al(SiMe3)3]2(μ-
tmeda),47 or [H2GaN(SiHMe2)]2(μ-tmeda).28c
Scheme 3. Syntheses of Compounds 4, 5, 6, and 7 via a One-Pot tmeda Addition/Transsilylamination
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The most prominent difference between the DRIFT spectra
of 1, 4, 5, 6, and 7 (Figure 9) and the bis(trimethylsilyl)amide
Figure 8. ORTEP view of complex 7 with the atomic displacement
parameters set at the 50% probability level. C−H protons are omitted
for clarity.
Figure 5. ORTEP view of complex 4 with the atomic displacement
parameters set at the 50% probability level. C−H protons and the
disorder of C9, C10, C13, and C14 are omitted for clarity.
Figure 9. DRIFT spectra of compounds {Mn[N(SiHMe2)2]2}2 (1),
Mn[N(SiHMe2)2]2(tmeda) (7), Cr[N(SiHMe2)2]2(tmeda) (4), Co-
[N(SiMe3 )2 ][N(SiHMe2 )2 ](tmeda) (5), and Co[N-
(SiHMe2)2]2(tmeda) (6).
Figure 6. ORTEP view of complex 5 with the atomic displacement
parameters set at the 50% probability level. C−H protons are omitted
for clarity.
weakened and therefore the stretching mode is shifted to
smaller wavenumbers.33,52 For lanthanide and alkaline-earth
metal bis(dimethylsilyl)amide complexes it has been found that
the Si−H valence vibration can be shifted to values below 2000
cm−1 in the presence of significant β-agostic interac-
tions.23,25 48 Such low-energy Si−H valence vibrations were
b,
not observed for the bis(dimethylsilyl)amide complexes 1, 4, 5,
6, and 7 (all bands lie above 2020 cm−1), but the bands appear
well resolved, e.g., revealing three distinct stretching modes for
complex 5. The discrepancy in the arrangement of the Si−H
moieties that is observed in the solid-state structures is most
likely caused by different effects including crystal packing. The
asymmetric metal−nitrogen stretching vibrations of the
monomeric tmeda adducts are located below 400 wavenumbers
(Figures S3−S5) with 380 cm−1 for 4, 351 cm−1 for 6, and 343
cm−1 for 7. In contrast, the asymmetric Mn−N stretching
vibration in 1 can be found at 450 cm−1 (Figure S6).
Figure 7. ORTEP view of complex 6 with the atomic displacement
parameters set at the 50% probability level. C−H protons are omitted
for clarity.
Syntheses of Bis(dimethylphenylsilyl)amide Com-
plexes. The reaction of MnCl2(thf)1.5 and CoCl2(thf)1.25
with 2 equiv of LiN(SiMe2Ph)2 in n-hexane gave donor-free
Mn[N(SiMe2Ph)2]2 (8) and donor-coordinated Co[N-
(SiMe2Ph)2]2(thf) (10), respectively. Applying the same
method for CrCl2(thf)x yielded the donor-free complex
Cr[N(SiMe2Ph)2]2 (9) and a minor amount of Cr[N-
(SiMe2Ph)2]2(thf)2 (11). The two compounds could be
complexes 2 and 3 (Figure 2) is the Si−H valence vibration
appearing in the region from 2130 to 2020 cm−1. The Si−H
deformation vibration can be found in the fingerprint region
between 1010 and 700 cm−1. The assignment of the remaining
characteristic bands is equivalent to 2 and 3. Infrared
spectroscopy is a valuable tool to determine β-agostic
interactions, since the heteroatom−hydrogen bond gets
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separated by fractional crystallization from n-hexane, giving first
purple crystals of the donor adduct and then pale green crystals
of the donor-free product. The former (11) can also be
synthesized directly from CrCl2 and 2 equiv of LiN(SiMe2Ph)2
in thf. The syntheses are straightforward (Scheme 4) and give
the desired complexes in moderate yields.
Scheme 4. Syntheses of Compounds 8 to 11 via Salt
Metathesis
With manganese(II) being the largest of the three ions,41 all
four phenyl rings can be arranged around the metal center to
saturate the coordination sphere (8, Chart 1). Chromium(II) is
Figure 10. DRIFT spectra of compounds 8, 9, 10, and 11 in the range
from 1600 to 400 cm−1.
respectively (Figures S8−S10). The asymmetric Cr−N
stretching vibration in square planar complex 11 is located at
413 cm−1 (Figure S11).
Chart 1. Schematic Drawings of Compounds 8, 9, 10, and 11
The molecular structure of Mn[N(SiMe2Ph)2]2 (8) is
depicted in Figure 11. The N1−Mn1−N2 angle is almost
the second largest metal ion;41 here only three phenyl rings face
the metal center, while the fourth phenyl ring points in the
opposite direction (9). Cobalt(II) is the smallest of the metal
ions, and the phenyl rings exhibit rather large distances to the
metal center (10). In this case, one thf molecule is needed to
saturate the coordination sphere of a monomeric species. In
contrast, the donor-coordinated Cr(II) complex (11) contains
two thf molecules, probably because the bigger ionic radius
requires one more thf to saturate the coordination sphere.
The fingerprint regions of the recorded DRIFT spectra of
compounds 8 to 11 differ greatly, presumably depending on the
interactions of the phenyl rings with the metal center and thf
coordination (Figure 10). The characteristic peaks, being the
C−H valence vibration of the aromatic rings and the methyl
groups (3130−2995 and 2953−2894 cm−1, respectively), the
C−H bending vibration at around 1425 cm−1, the Si−CH3
valence vibration at 1245 cm−1, and the aromatic C−H bending
vibration at 1109 cm−1, can be assigned (Figure S7). The bands
of the asymmetric metal−nitrogen stretching vibration are
similar for complexes 8−10, at 382, 390, and 386 cm−1,
Figure 11. ORTEP view of molecule 1 of compound 8. The atomic
displacement parameters are set at the 50% probability level, hydrogen
atoms are omitted for clarity, and the carbon atoms of the phenyl rings
are shown in the ball and stick representation for clarity.
linear at 172.66(7)° and is very similar to the angle of the
36
equivalent iron(II) complex Fe[N(SiMe2Ph)2]2 (see Table
3). Bending of the L−M−L moiety has been observed for most
two-coordinate open-shell transition metal complexes and can
be due to intramolecular interactions between the metal center
and the ligand backbone, packing forces, ligand field or
hybridization effects, and Renner−Teller distortions.53
The Mn−N bonds in 8 are slightly longer in the
corresponding iron compound, which is caused by the larger
ionic radius of manganese(II) compared to iron(II).41 The
corresponding manganese(II) bis(diphenylmethylsilyl)amide
Mn[N(SiMePh2)2]2 exhibits longer Mn1−N distances
(1.989(3) and 1.988(3) Å).36 The same trend is observed for
the Fe−N bond lengths in the iron(II) complexes Fe[N-
(SiMe2Ph)2]2 and Fe[N(SiMePh2)2]2, and the authors state
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that this arises from the sterically more demanding ligand
possessing two phenyl groups.36 The Si−N−Si angles of 8 are a
bit smaller than in Mn[N(SiMePh2)2]2 (125.4(1)° and
128.08(9)° vs 127.7(2)° and 131.8(2)°, respectively), which
is probably also due to the less bulky ligand. The distances from
the manganese center in 8 to the ipso carbon atoms of the
phenyl rings are only differing by 0.397 Å, with an average value
of dav = 3.123 Å. For one of the phenyl rings, the ipso carbon
(C3) does not show the shortest distance to the metal center,
but the carbon atom in ortho position (C8) does (3.046(2) and
2.945(3) Å, respectively). Although the distances of the phenyl
rings to the metal center are rather large, the arrangement of
the ligands suggests secondary interactions. A pseudo-
octahedral configuration geometry has also been reported for
the respective iron(II) complex.36
C12 seem to additionally coordinate to the chromium center in
9 in an η2-coordination mode.
The N1−Co1−N2 angle of the cobalt complex Co[N-
(SiMe2Ph)2]2(thf) (10) is very similar to the angle of the
chromium complex 9 (Figure 13, Table 3), but here an
The bending of N1−M−N2 in Cr[N(SiMe2Ph)2]2 (9,
140.89(4)°) is stronger than in Mn[N(SiMe2Ph)2]2 (8,
172.66(7)°) and Fe[N(SiMe2Ph)2]2 (172.1(1)°)36 but far less
pronounced compared to other two-coordinate Cr(II) amides
53
with bent structures, e.g., Cr[N(H)ArMe ]2 (120.9(5)°) or
6
Cr[N(Ph)BMes2]2 (Mes = 2,4,6-trimethylphenyl, 110.8(1)°)54
Figure 13. ORTEP view of complex 10. The atomic displacement
parameters are set at the 50% probability level; hydrogen atoms and
disorder of C35 are omitted for clarity.
(Table 3, Figure 12).55 Only three phenyl rings are directed
additional thf molecule is coordinated to the metal center. For
Co[N(SiMePh2)2]2, Chen et al.36 state that the d7 Co(II) ion
prefers a pseudotetrahedral geometry, and thus the angle is
bent. The Co1−N bonds of complex 10 are longer than in the
equivalent bis(diphenylmethylsilyl)amide complex (1.898(3)
and 1.904(3) Å, respectively), which is the reverse of what was
observed for the manganese(II) and iron(II) complexes.36 Most
probably the additional coordination of thf gives rise to this
bond elongation. The Co1---Cipso distances in 10 are longer
than in 8 and 9 (Table 3). Three-coordinate cobalt(II) centers
in silylamide complexes have already been observed for
Co[N(SiMe3)2]2(do) (do = thf,9a,42 py,50 or PPh361), and the
N−Co−N angles in these compounds are even narrower than
in 10 (141.80(16)°/141.89(9)°,9a,42 140.7(2)°,50 and
130.7(7)°,61 respectively). While the Co−N bonds in the
phosphane-coordinated complex are similar to 10 (1.93(1) and
1.92(1) Å),61 Co[N(SiMe3)2]2(thf) and Co[N(SiMe3)2]2(py)
exhibit slightly shorter Co−N bond lengths (1.898(2)/
1.9000(15)9a,42 and 1.904(3) Å,50 respectively).
Figure 12. ORTEP view of complex 9. The atomic displacement
parameters are set at the 50% probability level; hydrogen atoms are
omitted for clarity.
Owing to the coordination of two thf molecules, the
molecular structure of Cr[N(SiMe2Ph)2]2(thf)2 (11, Figure
14) is completely different from the aforementioned com-
pounds (8, 9, and 10). Through the coordination of two thf
molecules, an almost square planar coordination geometry of
the chromium center is achieved, similarly to the bis-
toward the chromium center in 9. Furthermore, this complex
exhibits larger variations in the Cr---Cipso distances and the
Cr1−N−Si angles (cf. Table 3). The Cr1−N1−Si1 angle is the
widest and comes with the longest Cr1---C3 contact. Likewise,
the shortest Cr---Cipso distance corresponds to the smallest
angle (Cr---C11 and Cr1−N1−Si2, cf. Table 3). In the latter
case, the carbon atom in ortho position (C12) also exhibits a
short contact to the chromium center (C11: 2.379(1) Å and
C12: 2.524(1) Å). The distances are in the range of reported
secondary interactions of phenyl rings with the metal center in
two-coordinate arylamide complexes of divalent chromium,
manganese, iron, cobalt, and nickel.53,55−58 Short contacts to
the ipso as well as the ortho carbon atom of phenyl rings in γ-
position to the metal center have been reported for
2g
(trimethylsilyl)amide complex Cr[N(SiMe3)2]2(thf)2 (cf.,
Table 3). Moreover, the chromium atom depicts a center of
symmetry as an inversion center. Compared to its donor-free
homologue 9, the Cr1−N1 bonds are longer, most likely
originating from the additional coordination of thf.
Electronic Spectra. The UV−vis spectra of the manganese-
(II) compounds in n-hexane solution (1, 7, and 8) are
essentially featureless, which is in accordance with their pale
colors. The chromium(II) complexes (2, 4, 9, and 11) mainly
exhibit two bands in the UV−vis spectra, one in the range of
260−330 nm and the second above 620 nm (see Figures S12−
S15). Two bands have as well been observed for other Cr(II)
59
{Mn[CH2CMe2Ph]2}2 and tricarbonyl{2-[(1,2-η2),κCα-2-
(phenylmethoxymethylene)phenyl]pyridine-κN}manganese-
(I)60 with distances of 2.728 and 2.639 Å or 2.240 and 2.438 Å,
respectively.59,60 From this it can be concluded that C11 and
i
i
compounds, for example, Cr[N(H)ArX]2 (X = Me6, Pr4, Pr6)
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4.50 μB can be observed, which lie below the spin-only value in
the case of chromium(II) (4.90 μB) and above the spin-only
value for cobalt(II) (3.87 μB). These observations have also
been made in the literature for Cr(II) and Co(II) complexes
and have been ascribed to spin−orbit coupling.50,55,58 The
bis(trimethylsilyl)amide complex of chromium(II), Cr[N-
(SiMe3)2]2(tmeda) (2), shows a magnetic moment of 4.82
μB, which is much closer to the spin-only value than in the
bis(dimethylsilyl)amide complex 4 (vide supra). Square planar,
thf-coordinated Cr[N(SiMe3)2]2(thf)2 exhibits a slightly higher
magnetic moment of 4.93 μB.2g In contrast, the μB-value of also
square planar Cr[N(SiMe2Ph)2]2(thf)2 (11) is significantly
lower (4.54 μB). Compared to the donor-coordinated version,
the magnetic moment in Cr[N(SiMe2Ph)2]2 (9) is slightly
higher (4.69 μB). Again, μB-values below the spin-only value
have been observed in the literature for two-coordinate, bent
chromium(II) complexes, due to spin−orbit coupling.53,55 The
magnetic moment of the analogous manganese(II) complex
Mn[N(SiMe2Ph)2]2 (8) was determined to be 5.74 μB, which is
lower than in complex 7 and than the spin-only value (vide
supra). Similar magnetic moments have been observed for
corresponding Mn[N(SiMePh2)2]2 (5.72 μB)36 and Mn[N(H)-
Ar#]2 (5.73 μB, Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2).57 The
dinuclear cobalt(II) complex {Co[N(SiMe3)2]2}2(μ-tmeda)
(3), with three-coordinate cobalt centers, exhibits a magnetic
moment of μeff = 5.09 μB (for the calculation of μeff, the
molecular weight was divided by 2). A similar μB-value can be
found for Co[N(SiMe2Ph)2]2(thf) (10) (5.20 μB), where the
metal centers are also three-coordinate. Both are significantly
higher than for complex 6, with a four-coordinate cobalt center
(vide infra). The bis(trimethylsilyl)amide complex of cobalt(II)
with additional coordination of triphenylphosphane features a
lower magnetic moment of 4.84 μB.61 In contrast, magnetic
moments of 5.883(3) and 5.269(5) μB for Co[N-
(SiMe3)2]2(thf) and Co[N(SiMe3)2]2(py), respectively, were
determined by SQUID (superconduction quantum interference
device) measurements.42 The bis(diphenylmethylsilyl)amide
complex Co[N(SiMePh2)2]2 exhibits a relatively small μB-value
of 4.42 μB.36
Conclusion. Bis(dimethylsilyl)amide and bis-
(dimethylphenylsilyl)amide complexes of chromium(II),
manganese(II), and cobalt(II) were synthesized in moderate
to high yields. In the case of the bis(dimethylsilyl)amide
derivatives, a salt metathesis protocol (utilizing chloride
precursors) was successful only for the synthesis of donor-
free {Mn[N(SiHMe2)2]2}2. For chromium(II) and cobalt(II),
transsilylamination (utilizing bis(trimethylsilyl)amide precur-
sors in n-hexane) is a viable protocol, requiring, however, the
presence of a (stabilizing) chelating donor such as tmeda. For
the smallest metal center Co(II) complete transilylamination
and formation of Co[N(SiHMe2)2]2(tmeda) were observed
only at elevated reaction temperatures. The IR spectra of the
bis(dimethylsilyl)amide complexes under study feature charac-
teristic SiH stretch vibration patterns in the nonagostic range.
Monometallic bis(dimethylphenylsilyl)amide complexes M(II)-
[N(SiMe2Ph)2]2(thf)x (M = Cr, x = 0, 2; M = Mn, x = 0; M =
Co, x = 1) are straightforwardly accessible from chloride and
lithium silylamide precursors. Depending on the metal size and
the presence of donor solvent, secondary M---π(arene)
interactions cause distinct coordination environments.
Figure 14. ORTEP view of complex 11. The atomic displacement
parameters are set at the 50% probability level; hydrogen atoms are
omitted for clarity.
or Cr[N(Ar)BMes2]2 (Ar = Ph, Mes), but positioned in
different ranges (333−345 and 400−417 nm for the former;
672/624 and 800 nm for the latter).53,54 In the case of
bis(dimethylphenylsilyl)amide complexes 9 and 11, the π→π*
transition of the phenyl rings at around 261 nm can be
additionally assigned (Figure S16). The UV−vis spectrum of
cobalt complex Co[N(SiMe3)2]2(μ-tmeda) (3) with three-
coordinate cobalt centers features four bands at 330, 414, 600,
and 666 nm (Figure S17), which is very similar to
Co[N(SiMe3)2]2(py) (318, 383, 647, 696 nm).50 The UV−
vis spectrum of dimeric {Co[N(SiMe3)2]2}2 in the liquid phase
also showed four bands at accordant wavelengths (319, 410,
588, 680 nm).2m In n-hexane solution, however, the maxima are
shifted to higher wavelengths (410, 585, 685, 1538 nm).2m The
bis(dimethylsilyl)amide complex Co[HN(SiHMe2)2]2(tmeda)
(6) exhibits five bands (225, 316, 473, 632, 710 nm; Figure
S18); unlike Co[N(SiMe3)2]2(py)2, which gave only three
bands.50 The last four bands in the UV−vis spectrum of 6 are
positioned in similar ranges to those for 3 and Co[N-
(SiMe3)2]2(py) (vide supra). For Co[N(SiMe2Ph)2]2(thf) (10,
Figure S19), the π→π* transition of the phenyl rings can be
found at 263 nm. Further bands are located at 360, 520, 629,
and 791 nm, which is close to what was observed for
corresponding Co[N(SiMePh2)2]2 (526, 634, 802 nm).36
Magnetic Studies. Investigation of the magnetic moments
has been carried out by Evans’ method62 in benzene/deuterated
benzene and hexamethyldisiloxane as reference. In general, the
determined μB-values are in accordance with d4, d5, and d7 high-
spin configurations (Table 4). Monomeric Mn[N-
(SiHMe2)2]2(tmeda) (7) exhibits a magnetic moment of μeff
= 5.97 μB, which is almost identical with the spin-only value for
a d5 high-spin electron configuration (5.92 μB). In contrast, the
donor-free manganese complex {Mn[N(SiHMe2)2]2} (1)
shows an effective magnetic moment of only 3.26 μB. This
low value suggests antiferromagnetic coupling of the two metal
centers probably via the bridging nitrogen atoms, proving that
the dimeric structure is retained in solution. This phenomenon
has also been observed for {Mn[N(SiMe3)2]2}2 with a μB-value
of 3.34.2k Furthermore, the analogous {Fe[N(SiHMe2)2]2}2
complex has been proven to be dimeric in solution, as well.21a
For Cr[N(SiHMe2 )2 ]2 (tmeda) (4) and Co[N-
(SiHMe2)2]2(tmeda) (6), magnetic moments of 4.38 and
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reaction was stirred at ambient temperature for 3.5 h. Then the
volatiles were removed under vacuum; crystallizing from n-hexane
yielded turquoise green crystals. Yield: 111 mg, 92%. Anal. Calcd for
C18H52N4Si4Co: C 41.15, H 10.13, N 9.60. Found: C 41.24, H 9.77, N
9.53. DRIFT (KBr, cm−1): 3004 (w), 2947 (m), 2893 (w), 2846 (w),
2810 (w), 2800 (w), 1478 (w), 1462 (w), 1278 (sh), 1257 (s), 1244
(s), 1015 (w), 967 (s), 885 (m), 847 (s), 831 (s), 786 (m), 753 (m),
705 (w), 666 (m), 613 (w), 521 (w), 419 (w). IR (Nujol, cm−1): 667
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out under a
dry argon atmosphere using standard Schlenk or glovebox techniques
(MBraun, MB250B, < 0.1 ppm of H2O, O2). The solvents were dried
over Grubbs columns (MBraun, Solvent Purification System) and
stored inside the glovebox. Elemental analyses were carried out on an
Elementar Vario MICRO instrument. DRIFT spectra were measured
on a Thermo Scientific Nicolet 6700 FTIR spectrometer using KBr
powder and a DRIFT cell equipped with KBr windows. The spectra
were recorded with 256 scans from 4000 to 400 cm−1 with a resolution
of 2 cm−1. FIR spectra were collected with a Vertex 70 spectrometer
from PerkinElmer using Nujol mulls and CsI plates. The spectra were
recorded from 680 to 200 cm−1 with 256 scans and a resolution of 2
cm−1. UV−vis spectra of n-hexane solutions of the compounds have
been measured with a Lambda 35 spectrophotometer from
PerkinElmer. The spectra were collected from 1000 to 200 nm with
a scan speed of 480 nm/min. The Evans’ method62 has been carried
out on a Bruker AVII+500 at 298 K in benzene/deuterated benzene
with hexamethyldisiloxane as reference. Concentrations of the
complexes in benzene solution ranged from 6 to 9 mg/mL.
Manganese(II) chloride (97%), cobalt(II) chloride (99%), chromium-
(II) chloride (97%), chromium(III) chloride (99.9%), 1,1,3,3-
tetramethyldisilazane (97%), and 1,1,1,3,3,3-hexamethyldisilazane
(98%) were purchased from ABCR. Sodium amide (95%), potassium
bis(trimethylsilyl)amide (sublimed at 130 °C under HV prior to use),
n-butyllithum (2.5 M in n-hexane), and tetramethylethylendiiamine
(99.5%) were obtained from Sigma-Aldrich. 1,3-Diphenyl-1,1,3,3-
tetramethyldisilazane (96%) was acquired from Fluka. LiN-
(SiHMe2)2,63 LiN(SiMe2Ph)2,35,36 NaN(SiMe3)2,64 Mn[N-
(SiMe3)2]2(thf),65 Cr[N(SiMe3)2]2(thf)2,2g and Co[N-
(SiMe3)2]2(thf)9a,42 were synthesized according to literature proce-
dures. Activated manganese(II) chloride and cobalt(II) chloride were
generated by stirring the corresponding metal chloride in thf at
ambient temperature; the thf content was calculated from elemental
analyses.66
Synthesis of {Mn[N(SiHMe2)2]2}2 (1). Pathway A: MnCl2(thf)1.5
(184 mg, 0.77 mmol) was suspended in n-hexane, and LiN(SiHMe2)2
(216 mg, 1.55 mmol) in n-hexane was added. The mixture turned pink
gradually. After stirring for 6 h at ambient temperature, LiCl was
separated by centrifugation. The combined n-hexane fractions were
dried under vacuum, yielding a rose solid. Crystallization from n-
hexane gave rose crystals. Yield: 198 mg, 80%. Pathway B:
Mn[N(SiMe3)2]2(thf) (102 mg, 0.23 mmol) was dissolved in n-
hexane, and HN(SiHMe2)2 (67 mg, 0.50 mmol) was added. The pink
solution was stirred for 4.5 h at ambient temperature, and then the
solvent was removed under vacuum. Crystallization from n-hexane
yielded rose crystals. Yield: 51 mg, 70%. Anal. Calcd for
C16H56N4Si8Mn2: C 30.06, H 8.83, N 8.77. Found: C 29.98, H 9.03,
N 8.61. DRIFT (KBr, cm−1): 2959 (m), 2897 (w), 2125 (m), 2086
(b), 1174 (w), 1414 (w), 1255 (m), 1176 (w), 1043 (m), 930 (s), 890
(s), 837 (m), 785 (m), 770 (m), 733 (w), 680 (w), 628 (w), 591 (w),
454 (w). IR (Nujol, cm−1): 629 (m), 594 (w), 450 (m).
(w), 612 (vw), 521 (w), 365 (m). UV−vis (n-hexane solution, λmax
nm): 330, 414, 600 (sh), 666.
,
Synthesis of Cr[N(SiHMe2)2]2(tmeda) (4). Cr[N(SiMe3)2]2(thf)2
(104 mg, 0.20 mmol) was dissolved in n-hexane, and tmeda (47 mg,
0.40 mmol) and HN(SiHMe2)2 (54 mg, 0.40 mmol) were added. The
blue solution was stirred at ambient temperature for 3 h. Removing the
solvent under vacuum yielded a purple solid, which gave purple
crystals from toluene. Yield: 82 mg, 93%. Anal. Calcd for C14H44
N4Si4Cr: C 38.85, H 10.25, N 12.94. Found: C 37.21, H 10.94, N
12.07 (unfortunately, we were not able to obtain better microanalysis
data for this compound). DRIFT (KBr, cm−1): 3013 (w), 2946 (m),
2894 (m), 2845 (w), 2807 (w), 2083 (m), 2047 (m), 2018 (m), 1468
(m), 1439 (w), 1356 (w), 1284 (m), 1240 (s), 1191 (w), 1164 (w),
1123 (w),1047 (s), 1025 (s), 948 (s), 920 (s), 891 (s), 789 (s), 756
(m), 704 (m), 672 (m), 628 (m), 485 (w). IR (Nujol, cm−1): 660 (m),
609 (w), 380 (m). UV−vis (n-hexane solution, λmax, nm): 282 (sh),
303, 793.
Synthesis of Co[N(SiMe3)2][N(SiHMe2)2](tmeda) (5). Co[N-
(SiMe3)2]2(thf) (101 mg, 0.22 mmol) was dissolved in n-hexane,
and tmeda (39 mg, 0.34 mmol) and HN(SiHMe2)2 (63 mg, 0.47
mmol) were added. The green solution turned dark green while
stirring at ambient temperature for 3 h. Then the volatiles were
removed under vacuum; crystallizing from n-hexane yielded dark green
crystals. Yield: 67 mg, 64%. Anal. Calcd for C16H48N4Si4Co: C 41.08,
H 10.34, N 11.98. Found: C 41.29, H 9.82, N 12.00. DRIFT (KBr,
cm−1): 3021 (w), 2952 (m), 2894 (m), 2850 (w), 2803 (w), 2121
(m), 2085 (m), 1466 (m), 1433 (w), 1282 (w), 1245 (s), 1044 (s),
1021 (m), 955 (s), 928 (s), 889 (vs), 836 (s), 787 (s), 757 (s), 704
(w), 674 (w), 629 (w), 421 (w).
Synthesis of Co[N(SiHMe2)2]2(tmeda) (6). Co[N(SiMe3)2]2(thf)
(160 mg, 0.35 mmol) was dissolved in n-hexane, and tmeda (53 mg,
0.35 mmol) and HN(SiHMe2)2 (99 mg, 0.74 mmol) were added. The
green solution was heated at 70 °C for 1 h in a pressure tube, turning
dark brown toward the end. After cooling, the volatiles were removed
under vacuum, and crystallizing from n-hexane yielded green crystals.
Yield: 92 mg, 59%. Anal. Calcd for C14H44N4Si4Co: C 38.23, H 10.08,
N 12.74. Found: C 38.26, H 9.76, N 12.52. DRIFT (KBr, cm−1): 3011
(w), 2946 (m), 2893 (m), 2844 (w), 2799 (w), 2120 (m), 2078 (m),
1464 (m), 1443 (w), 1281 (w), 1241 (s), 1122 (w), 1043 (w), 1007
(m), 972 (s), 941 (s), 894 (s), 883 (s), 872 (s), 839 (s), 798 (s), 755
(m), 718 (m), 704 (m), 669 (m), 618 (m), 494 (w), 414 (w). IR
(Nujol, cm−1): 631 (w), 351 (w). UV−vis (n-hexane solution, λmax
nm): 225 (sh), 316, 473, 632, 710.
,
Synthesis of Mn[N(SiHMe2)2]2(tmeda) (7). Mn[N(SiHMe2)2]2 (86
mg, 0.27 mmol) was dissolved in n-hexane, and tmeda (63 mg, 0.54
mmol) was added. The colorless solution was stirred at ambient
temperature for 1.5 h. Removing the solvent under vacuum yielded a
white solid, which gave colorless crystals from n-hexane. Yield: 111 mg,
93%. Anal. Calcd for C14H44N4Si4Mn: C 38.58, H 10.18, N 12.86.
Found: C 38.41, H 10.22, N 12.96. DRIFT (KBr, cm−1): 3006 (w),
2949 (m), 2893 (m), 2847 (w), 2803 (w), 2049 (s), 1471 (m), 1446
(w), 1457 (m), 1434 (w), 1419 (w), 1409 (w), 1286 (w), 1239 (m),
1188 (w), 1163 (w), 1122 (w), 1033 (m), 1010 (m), 947 (s), 923 (s),
890 (s), 835 (s), 791 (s), 756 (s), 708 (m), 676 (m), 629 (m), 585
(w), 481 (w), 441 (w), 418 (w). IR (Nujol, cm−1): 627 (m), 476 (w),
417 (w), 343 (m).
Synthesis of Mn[N(SiMe2Ph)2]2 (8). MnCl2(thf)1.5 (100 mg, 0.43
mmol) was suspended in n-hexane, and LiN(SiMe2Ph)2 (264 mg, 0.85
mmol) in n-hexane was added. The mixture turned colorless. After
stirring for 4 h at ambient temperature, no visible reaction had
occurred and hence the reaction mixture was transferred into a
pressure tube and heated to 70 °C for 6 h. LiCl was separated by
Synthesis of Cr[N(SiMe3)2]2(tmeda) (2). Cr[N(SiMe3)2]2(thf)2 (87
mg, 0.17 mmol) was dissolved in n-hexane, and tmeda (28 mg, 0.24
mmol) was added. The color of the solution went from dark blue to
light blue. After having stirred for 2.5 h at ambient temperature, the
solvent was removed under vacuum, yielding an azur solid, which
could be recrystallized from n-hexane. Yield: 68 mg, 83%. Anal. Calcd
for C18H52N4Si4Cr: C 44.21, H 10.71, N 11.46. Found: C 43.27, H
10.61, N 11.17. DRIFT (KBr, cm−1): 3025 (w), 3008 (w), 2957 (m),
2942 (m), 2896 (m), 2847 (w), 2807 (w), 1472 (m), 1440 (w), 1281
(w), 1247 (m), 1237 (s), 1187 (w), 1163 (w), 1116 (w), 1065 (w),
1016 (m), 1003 (w), 961 (s), 890 (m), 864 (s), 828 (s), 801 (m), 780
(m), 751 (m), 705 (w), 659 (m), 609 (w), 490 (w). IR (Nujol, cm−1):
658 (m), 608 (w), 489 (vw), 390 (sh), 377 (m), 365 (sh), 226 (vw).
UV−vis (n-hexane solution, λmax, nm): 269 (sh), 630.
Synthesis of {Co[N(SiMe3)2]2}2(μ-tmeda) (3). Co[N(SiMe3)2]2(thf)
(125 mg, 0.27 mmol) was dissolved in n-hexane, and tmeda (64 mg,
0.55 mmol) was added. No color change was observed, and the
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Inorganic Chemistry
Article
centrifugation after cooling. The combined n-hexane fractions were
dried under vacuum, yielding a yellowish solid. Crystallization from n-
hexane gave colorless crystals. Yield: 205 mg, 76%. Anal. Calcd for
C32H44N2Si4Mn: C 61.59, H 7.11, N 4.49. Found: C 61.76, H 6.96, N
4.57. DRIFT (KBr, cm−1): 3128 (w), 3067 (w), 3051 (w), 3013 (w),
2995 (w), 2951 (m), 2894 (w), 1980 (w), 1961 (w), 1906 (w), 1891
(w), 1831 (w), 1776 (w), 1586 (w), 1563 (w), 1425 (m), 1406 (w),
1321 (w), 1305 (w), 1245 (s), 1190 (w), 1157 (w), 1109 (s), 1030
(w), 1002 (s), 923 (s), 933 (w), 842 (s), 791 (s), 772 (s), 732 (s), 705
(s), 675 (w), 642 (w), 476 (m), 463 (m). IR (Nujol, cm−1): 646 (vw),
474 (m), 462 (m), 382 (m), 368 (sh), 299 (w).
Synthesis of Cr[N(SiMe2Ph)2]2 (9). CrCl2 (52 mg, 4.21 mmol) was
suspended in thf and stirred for 40 min at ambient temperature. The
solvent was evaporated off under vacuum, and the solid was suspended
in n-hexane. Then LiN(SiMe2Ph)2 (245 mg, 8.42 mmol) in n-hexane
was added, and the resulting green mixture was stirred at ambient
temperature for 4 h. Afterward, LiCl was separated by centrifugation,
and the combined n-hexane fractions were dried under vacuum.
Recrystallization from pentane gave purple crystals first (Cr[N-
(SiMe2Ph)2]2(thf)2, 60 mg, 19%). Crystallization of the mother liquor
yielded light green crystals. Yield: 158 mg, 60%. Anal. Calcd for
C32H44N2Si4Cr: C 61.89, H 7.14, N 4.51. Found: C 61.89, H 7.19, N
4.35. DRIFT (KBr, cm−1): 3064 (w), 3045 (w), 2993 (w), 3010 (w),
2951 (m), 2895 (w), 1962 (vw), 1949 (vw), 1890 (vw), 1879 (vw),
1586 (w), 1485 (w), 1426 (m), 1403 (w), 1303 (w), 1246 (m), 1108
(m), 1091 (w), 1040 (m), 1028 (w), 998 (w), 987 (m), 912 (w), 830
(s), 811 (s), 801 (s), 773 (s), 733 (s), 701 (s), 679 (m), 638 (m), 489
(w), 473 (m), 448 (w). IR (Nujol, cm−1): 646 (m), 473 (m), 390 (m).
UV−vis (n-hexane solution, λmax, nm): 261 (sh, π → π* benzene ring),
323, 868.
Synthesis of Co[N(SiMe2Ph)2]2(thf) (10). CoCl2(thf)1.25 (99 mg,
0.45 mml) was suspended in n-hexane, and LiN(SiMe2Ph)2 (262 mg,
0.90 mmol) in n-hexane was added. The mixture turned teal. After
stirring for 6 h at ambient temperature, LiCl was separated by
centrifugation. The combined n-hexane fractions were dried under
vacuum, yielding a green solid. Crystallization from n-hexane gave
green needles. Yield: 199 mg, 64%. Anal. Calcd for CoN2Si4C32H44: C
61.67, H 7.49, N 4.00. Found: C 61.68, H 6.54, N 4.09. DRIFT (KBr,
cm−1): 3131 (w), 3065 (w), 301 (w), 3017 (w), 2996 (w), 2952 (m),
2895 (w), 1980 (w), 1961 (w), 1906 (w), 1891 (w), 1831 (w), 1776
(w), 1586 (w), 1484 (w), 1425 (m), 1405 (w), 1321 (w), 1302 (w),
1246 (s), 1182 (w), 1109 (s), 1005 (s), 992(s), 931 (m), 835 (s),
800(s), 779 (s), 718 (s), 700 (s), 675 (m), 648 (m), 470 (m), 441
(w). IR (Nujol, cm−1): 675 (w), 640 (w), 469 (m), 441 (m), 405 (m),
386 (m), 300 (w). UV−vis (n-hexane solution, λmax, nm): 263 (sh, π
→ π* benzene ring), 360, 520, 629, 791.
was solved by using SHELXS68 and refined with SHELXL68 against F2.
Numerical absorption correction has been done applying X-SHAPE/
X-RED69 based on an optimized crystal shape. Data collection for 2, 5,
and 8 was performed on a Bruker SMART APEX II diffractometer
using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). X-
ray data for 3, 6, 7, 9, 10, and 11 were collected on a Bruker APEX II
DUO diffractometer equipped with a multilayer monochromator and
Mo Kα radiation (λ = 0.71073 Å). Raw data were collected by using
the program COSMO70 and integrated and reduced with the program
SAINT.71 Corrections for adsorption effects were applied with
SADABS.72 The structures were solved by direct methods and refined
with standard difference Fourier techniques (SHELXS/SHELXL).68
All CIF files were checked at http://www.checkcif.iucr.org/. For
further experimental details on refinement and crystallographic data
see Tables 5−7. All plots were generated utilizing the programs
Table 6. Crystallographic Data for Bis(trimethylsilyl)amide
Complexes 2 and 3
2 (Cr)
3 (Co)
fw (g/mol) 489.00
T (K) 100(2)
space group monoclinic C2/c
875.64
200(2)
triclinic P1
̅
a, b, c (Å)
17.434(2), 8.597(1),
38.458(5)
9.1717(2), 14.4022(3),
21.3290(4)
α, β, γ (deg) 92.534(6)
71.411(1), 86.059(1),
84.980(1)
V (Å3)
5758.2(13)
8
2657.69(9)
2
Z
dcalc (g/cm3) 1.128
1.094
a
R1
0.0448
0.1033
1.334
0.0341
0.0913
1.027
b
wR2
c
GOF
a
b
2
R1 = ∑(|Fo| − |Fc|)/∑|Fo|, Fo > 2σ(Fo). wR2 = {∑[w(Fo − Fc2)2/
c
∑[w(Fo )2]}1/2. GOF = [∑w(Fo − Fc2)2/(n0 − np)]1/2
.
2
2
Table 7. Crystallographic Data of
Bis(dimethylphenylsilyl)amide Complexes 8, 9, 10, and 11
8 (Mn)
623.99
9 (Cr)
621.05
10 (Co)
700.09
11 (Cr)
765.26
fw
(g/mol)
T (K)
153(2)
100(2)
100(2)
100(2)
space
group
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
Synthesis of Cr[N(SiMe2Ph)2]2(thf)2 (11). CrCl2 was suspended in
thf, and LiN(SiMe2Ph)2 in thf was added, giving a teal suspension.
After stirring for 3.5 h at ambient temperature, LiCl was separated by
centrifugation. The thf was evaporated almost completely, and then
pentane was added. Extraction with toluene and crystallization gave
purple crystals. Yield: 255 mg, 47%. Anal. Calcd for C40H60N2O2Si4Cr:
C 62.78, H 7.90, N 3.66. Found: C 62.58, H 7.94, N 3.66. DRIFT
(KBr, cm−1): 3130 (w), 3066 (w), 3047 (w), 3016 (w), 2996 (w),
2953 (m), 2897 (w), 1948 (w), 1889 (w), 1819 (w), 1587 (w), 1486
(m), 1426 (m), 1405 (w), 1303 (w), 1248 (s), 1190 (w), 1109 (s),
1037 (m), 980 (m), 909 (s), 835 (s), 803 (s), 774 (s), 731 (s), 701
(s), 678 (s), 637 (m), 473 (m), 408 (m). IR (Nujol, cm−1): 636 (w),
473 (m), 447 (w), 413 (m), 390 (sh), 366 (w), 347 (w), 288 (w), 246
(w). UV−vis (n-hexane solution, λmax, nm): 262 (sh, π → π* benzene
ring), 325, 871.
a, b, c (Å) 22.819(2),
13.306(1),
11.5550(4),
13.4514(5),
22.0416(7)
18.6495(4),
11.8450(2),
18.0184(4)
11.4798(6),
17.8748(8),
10.6447(5)
23.498(2)
β (deg)
V (Å3)
Z
100.335(4)
7019.1(9).
8
99.178(1)
3382.1(2)
4
109.597(1)
3749.8(1)
4
111.877(2)
2027.0(2)
2
dcalc
1.181
1.220
1.240
1.254
(g/cm3)
a
R1
0.0415
0.1038
1.015
0.0264
0.0720
1.063
0.0248
0.0669
0.0273
0.0832
b
wR2
c
GOF
1.028
b
1.088
2
a
R1 = ∑(|Fo| − |Fc|)/∑|Fo|, Fo > 2σ(Fo). wR2 = {∑[w(Fo − Fc2)2/
c
∑[w(Fo )2]}1/2. GOF = [∑w(Fo − Fc2)2/(n0 − np)]1/2
.
2
2
Crystallographic Data Collection and Refinement. Crystals of
1 to 11 were grown by standard techniques from saturated solutions
using n-hexane (1, 3, 4, 5, 6, 8), toluene (2, 7, 10, 11), or pentane (9)
at −38 °C. Single crystals suitable for X-ray structure analyses were
selected in a glovebox and coated with Paratone-N (Hampton
Research). X-ray data for compounds 1 and 4 were collected on a Stoe
IPDS II 2T diffractometer using Mo Kα radiation (λ = 0.71073 Å) and
were corrected for Lorentz and polarization effects. Cell refinement
and data reduction were performed by using X-Area.67 The structure
Diamond 3.2i73 and POV-Ray.74 CCDC 976114 (10), 976115 (11),
976116 (1), 976117 (2), 976118 (3), 976119 (4), 976120 (5), 976121
(6), 976122 (7), 976123 (8), and 976123 (9) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Article
M.; Schwarzmaier, C.; Tuna, F. Chem. Commun. 2011, 47, 10623.
(e) Zadrozny, J. M.; Atanasov, M.; Bryan, A. M.; Lin, C.-Y.; Rekken, B.
D.; Power, P. P.; Neese, F.; Long, J. R. Chem. Sci. 2013, 4, 125.
(7) (a) Baxter, D. V.; Chisholm, M. H.; Gama, G. J.; Hector, A. L.;
Parkin, I. P. Chem. Vap. Deposition 1995, 1, 49. (b) Suh, S.; Hoffman,
D. M.; Atagi, L. M.; Smith, D. C. Chem. Vap. Deposition 2001, 7, 81.
(c) Hubert-Pfalzgraf, L. G.; Touati, N.; Pasko, S. V.; Vaissermann, J.;
Abrutis, A. Polyhedron 2005, 24, 3066. (d) Abrutis, A.; Hubert-
Pfalzgraf, L. G.; Pasko, S.; Touati, N.; Kazlauskiene, V. Vacuum 2006,
ASSOCIATED CONTENT
* Supporting Information
IR and UV−vis spectra as well as CIF files giving full
crystallographic data for complexes 1−11. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: reiner.anwander@uni-tuebingen.de.
Notes
■
́
81, 13. (e) Jimenez, E. d. L.; Javed, S.; Hoffman, D. M. Inorg. Chim.
Acta 2009, 362, 385.
(8) Baxter, D. V.; Chisholm, M. H.; Gama, G. J.; DiStasi, V. F.;
Hector, A. L.; Parkin, I. P. Chem. Mater. 1996, 8, 1222.
(9) (a) Cormary, B.; Dumestre, F.; Liakakos, N.; Soulantica, K.;
Chaudret, B. Dalton Trans. 2013, 42, 12546. (b) Amiens, C.;
Chaudret, B.; Ciuculescu-Pradines, D.; Colliere, V.; Fajerwerg, K.;
Fau, P.; Kahn, M.; Maisonnat, A.; Soulantica, K.; Philippot, K. New J.
Chem. 2013, 37, 3374.
(10) (a) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes,
P. Science 2004, 303, 821. (b) Desvaux, C.; Amiens, C.; Fejes, P.;
Renaud, P.; Respaud, M.; Lecante, P.; Snoeck, E.; Chaudret, B. Nat.
Mater. 2005, 4, 750. (c) Lacroix, L.-M.; Lachaize, S. B.; Falqui, A.;
Respaud, M.; Chaudret, B. J. Am. Chem. Soc. 2008, 131, 549.
(d) Liakakos, N.; Cormary, B.; Li, X.; Lecante, P.; Respaud, M.;
Maron, L.; Falqui, A.; Genovese, A.; Vendier, L.; Koïnis, S.; Chaudret,
B.; Soulantica, K. J. Am. Chem. Soc. 2012, 134, 17922.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Special thanks go to Daniel Werner at Monash University for
solving the structure of Co[N(SiHMe2)2]2(tmeda) (6). Further
thanks go to Dr. Klaus Eichele for technical assistance with the
measurements of the magnetic moments and to Dr. Wolfgang
Leis for help with UV−vis and FIR spectroscopy.
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