Formation of specific configurations at stereogenic nitrogen centers upon their coordination to Zinc and mercury

Article abstract of DOI:10.1021/ic301089h

The coordination of (R,R)-tetramethylcyclohexane-1,2-diamine derivatives with stereogenic nitrogen centers to zinc and mercury halides is investigated. It is shown that the resulting complexes display one specific configuration at the stereogenic nitrogen centers. This fact is unusual due to the fast inversion of nitrogen centers but highly desirable as the stereoinformation of the ligands is brought closer to the metal centers of the potential catalysts. A combination of NMR studies and quantum chemical calculations gives insight into the selective formation of one specific configuration at the stereogenic nitrogen centers of the zinc complexes.

Full text of DOI:10.1021/ic301089h

Article
pubs.acs.org/IC
Formation of Specific Configurations at Stereogenic Nitrogen
Centers upon Their Coordination to Zinc and Mercury
Prisca K. Eckert,† Viktoria H. Gessner,‡ Michael Knorr,§ and Carsten Strohmann*,†
†Anorganische Chemie, Technische Universitat Dortmund, Otto-Hahn-Str. 6, 44227 Dortmund, Germany
̈
‡Institut fur Anorganische Chemie, Am Hubland, 97074 Wurzburg, Germany
̈
̈
§Institut UTINAM UMR CNRS 6213, Universite
́
de Franche-Comte,
́
16 Route de Gray, La Bouloie, 25030 Besanco
̧
n, France
S
* Supporting Information
ABSTRACT: The coordination of (R,R)-tetramethylcyclohexane-
1,2-diamine derivatives with stereogenic nitrogen centers to zinc
and mercury halides is investigated. It is shown that the resulting
complexes display one specific configuration at the stereogenic
nitrogen centers. This fact is unusual due to the fast inversion of
nitrogen centers but highly desirable as the stereoinformation of the
ligands is brought closer to the metal centers of the potential
catalysts. A combination of NMR studies and quantum chemical
calculations gives insight into the selective formation of one specific configuration at the stereogenic nitrogen centers of the zinc
complexes.
INTRODUCTION
■
Coordination compounds containing chiral cyclohexane-1,2-
diamine derivatives and transition metals play an active role as
catalysts in stereoselective synthesis. The ligand (R,R)-
tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA, 1] for
example, can be coordinated to titanium(II) chloride and
cobalt chloride to enhance the stereoselectivity of pinacol
coupling and cross-coupling reactions, respectively.1 As the
metal center of such coordination compounds constitutes the
active site of the catalyst, the stereoselectivity of a reaction
might be even further enhanced if the stereogenic information
of the ligand were brought closer to the metal. In the case of
(R,R)-TMCDA (1), this would mean that the stereo-
information of the (R)-configured carbon centers would have
to be passed on to the coordinating nitrogen centers. This
could be achieved if one of the methyl groups at the nitrogen
centers was substituted by a different group. Then, upon
coordination of the ligand to a metal, the formation of three (in
the case of two stereogenic nitrogen centers containing the
same alkyl substitutents) or two (in the case of one stereogenic
nitrogen center) diastereomers is possible (Figure 1). If one of
the diastereomers was formed selectively, its use as a catalyst in
preparative synthesis might lead to enhanced stereoselectivity.
Achieving this is quite a challenge, however, as nitrogen centers
tend to undergo a fast inversion process2 which is usually only
avoided by using ligands that fix the configuration at the
nitrogen centers by enclosing them in rigid carbocycle ring
structures (the most commonly employed example for this
principle is (−)-sparteine3). If this fast interconversion were
slowed down, resulting in a fixed equilibrium of diastereomers,
a useful catalyst for preparative synthesis would be formed.
In the past, transition metal coordination compounds of
cyclohexane-1,2-diamine derivatives containing one proton and
Figure 1. Different diastereomers that can be formed upon
coordination of an (R,R)-TMCDA derivative with two stereogenic
nitrogen centers to a transition metal.
one alkyl group at the stereogenic nitrogen centers have already
been used in asymmetric synthesis with mixed results.4,5 Crystal
structure analysis of some compounds revealed nickel, copper,
and zinc complexes with (S)-configuration at both stereogenic
nitrogen centers,5,6 while crystal structures of cobalt and
palladium compounds displayed (R)-configuration at one and
(S)-configuration at the other stereogenic nitrogen center.7,8
For tertiary bidentate cyclohexane-1,2-diamine derivatives,
however, little is known about the configuration of the
stereogenic nitrogen centers, although the configuration plays
an important role in some proposed reaction mechanisms.9 We
recently noted the specific formation of (S)-configuration at the
stereogenic nitrogen center in coordination compounds with
silyl substituted TMCDA derivatives.10 Extensive literature
research revealed only one additional crystal structure of a
copper complex with (S)-configuration at all stereogenic
nitrogen centers.11,12 NMR studies by Rafii et al., however,
Received: May 24, 2012
Published: July 25, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
Table 1. Data Collection and Structure Refinement Details for Coordination Compounds 7−10
7
8
9
10
formula
C11H24Br2N2Zn
409.51
C12H26Br2N2Zn
423.54
C14H30Cl2N2Zn
362.67
C12H26Br2 N2Zn
423.54
fw [g·mol−1
temp [K]
]
173(2)
173(2)
173(2)
173(2)
wavelength [Å]
cryst syst
space group
a [Å]
0.71073
0.71073
0.71073
0.71073
orthorhombic
P212121
orthorhombic
P212121
monoclinic
P21
orthorhombic
P212121
8.2007(9)
13.2412(14)
14.1628(15)
7.8253(4)
12.4256(6)
16.7554(8)
12.3903(18)
12.8524(19)
12.6936(19)
116.785(2)
1804.5(5)
4
9.9604(15)
12.2299(18)
13.610(2)
b [Å]
c [Å]
β [deg]
vol [Å3]
1537.9(3)
1629.20(14)
4
1657.9(4
Z
4
4
calcd density [Mg·m−3
]
1.769
1.727
1.335
1.697
μ (Mo Kα) [mm−1
]
6.773
6.397
1.648
6.286
F(000)
816
848
768
848
cryst dimensions [mm]
Θ range [deg]
index ranges
0.40 × 0.30 × 0.20
2.11−26.99
−10 ≤ h ≤ 10
−16 ≤ k ≤ 16
−18 ≤ l ≤ 18
42365
0.40 × 0.30 × 0.30
2.43−25.00
−9 ≤ h ≤ 9
−14 ≤ k ≤ 14
−19 ≤ l ≤ 19
10634
0.40 × 0.30 × 0.20
1.80−27.00
−15 ≤ h ≤ 15
−16 ≤ k ≤ 16
−16 ≤ l ≤ 16
40541
0.50 × 0.20 × 0.20
2.24−25.99
−12 ≤ h ≤ 12
−15 ≤ k ≤ 15
−16 ≤ l ≤ 16
16054
reflns collected
independent reflns
data/restraints/paramsr
goodness-of-fit on F2
final R indices
3353 [Rint = 0.0592]
3353/0/149
1.067
2873 [Rint = 0.0487]
2873/0/158
0.961
7884 [Rint = 0.0318]
7884/1/351
1.026
3268 [Rint = 0.1192]
3268/0/158
1.019
R1 = 0.0229
wR2 = 0.0557
R1 = 0.0250
wR2 = 0.0568
−0.004(10)
0.675, −0.475
R1 = 0.0260
wR2 = 0.0329
R1 = 0.0356
wR2 = 0.0335
−0.010(11)
0.389, −0.391
R1 = 0.0251
wR2 = 0.0629
R1 = 0.0275
wR2 = 0.0642
−0.009(6)
0.379, −0.168
R1 = 0.0482
wR2 = 0.0898
R1 = 0.0595
wR2 = 0.0925
0.01(2)
[I > 2σ(I)]
R indices
(all data)
absolute structure param
largest diff. peak and hole [e/Å−3
]
0.846, −0.775
details are shown in Tables 1 and 2. Quantum chemical calculations
were carried out using the programs Gaussian 03, revision E.0113 and
Turbomole V6.3.14 The synthesis of (R,R)-TMCDA (1), (R,R)-
dimethylcyclohexane-1,2-diamine (2), and N-ethyl-N,N′,N′-trimethyl-
(1R,2R)-cyclohexane-1,2-diamine (3) has already been reported.12,15
The lithiation step in the synthesis of the ligands 5a and 5b required
an inert argon atmosphere, dry solvents, and the use of standard
Schlenk techniques.
indicate that upon coordination of such a tertiary cyclohexane-
1,2-diamine derivative to PdCl2, a mixture of all three possible
stereoisomers is formed.8 With such little information known
about the configuration of the nitrogen centers of tertiary
cyclohexane-1,2-diamines in coordination compounds with
transition metals and considering the possibility of a broad
number of applications if the targeted complexes indeed
showed a selective configuration at the coordination sites, we
set out to investigate these compounds.
N-Propyl-N,N′,N′-trimethyl-(1R,2R)-cyclohexane-1,2-diamine
(5a). To a solution of 1 (3.0 g, 17.6 mmol) in pentane (10 mL) was
added tert-butyllithium (14 mL, 26.4 mmol, 1.9 M solution in
pentane) at −55 °C. The solution was stirred at room temperature for
6 h, and 1-bromoethane (2.7 mL, 36.3 mmol, dried over calcium
hydride and freshly distilled) was then added at −65 °C. After stirring
for 12 h at room temperature, sodium hydroxide solution (13 mL, 2 M
solution) was added until the reaction mixture was basic. The mixture
was then extracted with diethylether (five 20 mL portions). The
combined organic phases were dried over sodium sulfate and filtered,
and the solvent was removed. Subsequent Kugelrohr distillation (80
°C, 3.7 × 10−1 mbar) yielded the pure ligand 5a as a clear liquid. Yield:
39% (1.35 g, 6.81 mmol). 1H NMR (CDCl3): δ 0.87 (t, 3JHH = 7.5 Hz,
3H; C2H4CH3), 1.05−1.19 (m, 4H; CH2), 1.42−1.51 (m, 2H;
CH2CH2CH3), 1.69−1.82 (m, 4H; CH2), 2.19 (s, 3H; NCH3), 2.29 [s,
6H; N(CH3)2], 2.38−2.48 (m, 4H; C2H4CH3 + CHN). 13C NMR
(CDCl3): δ 12.2 (C2H4CH3), 21.6 (CH2CH2CH3), 24.0, 24.7, 25.9
(CH2), 36.2 (NCH3), 40.5 [N(CH3)2], 57.0 (NCH2C2H5), 62.7, 63.8
(CHN). GC(+)-MS: tR = 4.93 min; m/z 198 (100) [M·+], 127 (57)
[C6H11N(CH3)2·+], 112 (90) [C6H11NHCH2+], 84 (76)
EXPERIMENTAL SECTION
■
̈
Instruments, Measurements, and Chemicals. Melting points:
Buchi B-545. GC-MS, gas chromatography: HP 6890 [column: J. &
W. Scientific, 25 m length, ID 0.2; gas: helium, 2.06 bar; temperature
program: 50 °C (1 min) −40 °C/min −300 °C (5 min)]. EI-MS: HP
Selective Detector 5973 (70 eV). The selected m/z values refer to the
isotopes 1H, 12C, and 14N. Elemental analysis: Leco Instrument
CHNS-932. Powder diffraction data: Siemens D5005, Cu tube, λ =
1.54178 Å, 1.00 kV, 40 mA, steps 0.02°. Measuring time: 1 s per step.
X-ray structure determination: (1) Bruker APEX-CCD, T = 173 K.
Programs used for data collection: Smart V. 5622, SaintPlus V. 6.02,
Sadabs V. 2.01. (2) Oxford Diffraction Xcalibur S, T = 173 K.
Programs used for data collection: CrysAlis (Oxford, 2008), CrysAlis
RED (Oxford, 2008). The structures were solved using direct methods
(SHELXS90); structural refinement was done with SHELXL97. The
H atoms were refined on a riding model in their ideal geometric
positions. Crystallographic data (excluding structure factors) have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication CCDC 873207 (7), 873208 (8), 873209
(9), 873210 (10), 873203 (11), 873204 (12), 873205 (13), 873206
(14a), and 880491 (14b). Data collection and structure refinement
+
+
[CH2CHCHN(CH3)2 ], 72 (48) [CH3CHN(CH3)2 ], 58 (48)
+
[CH2N(CH3)2 ]. Anal. Calcd: C, 72.66; H, 13.21; N, 14.12. Found:
C, 72.7; H, 13.0; N, 13.9.
N-Pentyl-N,N′,N′-trimethyl-(1R,2R)-cyclohexane-1,2-diamine
(5b). To a solution of 1 (6.0 g, 35.2 mmol) in pentane (20 mL) was
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Inorganic Chemistry
Article
Table 2. Data Collection and Structure Refinement Details for Coordination Compounds 11−14
11
12
13
14a
14b
formula
C12H26Cl2N2Zn
334.62
C10H22Br2HgN2
530.71
C12H26Br2HgN2
558.76
C14H30Br2HgN2
586.81
C14H30Br2HgN2
586.81
fw [g·mol−1
temp [K]
]
173(2)
173(2)
173(2)
173(2)
173(2)
wavelength [Å]
cryst syst
space group
a [Å]
0.71073
0.71073
0.71073
0.71073
0.71073
orthorhombic
P212121
orthorhombic
P212121
orthorhombic
P212121
monoclinic
P21
monoclinic
P21
10.2752(11)
14.4299(16)
21.668(3)
9.9374(6)
11.5949(6)
12.8313(6)
8.6139(4)
13.1839(7)
14.3439(8)
12.0059(11)
8.2368(5)
19.3045(10)
99.499(7)
1882.9(2)
4
8.7499(6)
15.8447(9)
13.4635(11)
92.018(8)
1865.4(2)
4
b [Å]
c [Å]
β [deg]
vol [Å3]
3212.8(6)
1478.46(14)
4
1628.96(15)
4
Z
8
calcd density [Mg·m−3
]
1.384
2.384
2.278
2.070
2.089
μ (Mo Kα) [mm−1
]
1.845
15.792
14.339
12.411
12.527
F(000)
1408
984
1048
1112
1112
cryst dimensions [mm]
0.50 × 0.50 × 0.30
1.70−27.00
−13 ≤ h ≤ 12
−18 ≤ k ≤ 18
−27 ≤ l ≤ 27
33644
0.50 × 0.20 × 0.20
2.37−25.00
−12 ≤ h ≤ 12
−13 ≤ k ≤ 14
−15 ≤ l ≤ 15
8698
0.50 × 0.20 × 0.10
2.76−26.00
−10 ≤ h ≤ 10
−16 ≤ k ≤ 13
−17 ≤ l ≤ 16
8125
0.40 × 0.30 × 0.20
2.14−25.00
−14 ≤ h ≤ 14
−9 ≤ k ≤ 9
−22 ≤ l ≤ 22
20225
0.30 × 0.10 × 0.10
2.33−27.00
−11 ≤ h ≤ 11
−20 ≤ h ≤ 20
−14 ≤ h ≤ 17
15591
Θ range [deg]
index ranges
reflns collected
independent reflns
data/restraints/paramsr
goodness-of-fit on F2
final R indices
7008 [Rint = 0.0524]
7008/0/315
1.041
2912 [Rint = 0.0323]
2912/0/140
1.027
3207 [Rint = 0.0337]
3207/0/158
1.057
6620 [Rint = 0.0389]
6620/1/351
1.013
8041 [Rint = 0.0496]
8036/1/342
1.030
R1 = 0.0272
wR2 = 0.0504
R1 = 0.0310
wR2 = 0.0513
0.005(7)
R1 = 0.0644
wR2 = 0.1522
R1 = 0.0706
wR2 = 0.1540
−0.01(4)
R1 = 0.0676
wR2 = 0.1551
R1 = 0.0779
wR2 = 0.1576
0.01(3)
R1 = 0.0314
wR2 = 0.0493
R1 = 0.0417
wR2 = 0.0501
−0.005(9)
1.758, −2.130
R1 = 0.0563
wR2 = 0.1060
R1 = 0.0909
wR2 = 0.1106
0.010(14)
3.307, −1.562
[I > 2σ(I)]
R indices
(all data)
absolute structure param
largest diff. peak and hole [e/Å−3
]
0.353, −0.347
3.678, −0.707
2.797, −0.727
added tert-butyllithium (28 mL, 47.6 mmol, 1.9 M solution in
pentane) at −50 °C. The solution was stirred at room temperature for
22 h, and then 1-chlorobutane (7.4 mL, 71.3 mmol, 6.6 g) was added
at −50 °C. After stirring for 22 h at room temperature, sodium
hydroxide solution (25 mL, 2 M solution) was added until the reaction
mixture was basic. The mixture was then extracted with diethylether
(five 20 mL portions). The combined organic phases were dried over
sodium sulfate and filtered, and the solvent was removed. Subsequent
Kugelrohr distillation (95 °C, 4 ·10−1 mbar) yielded the pure ligand 5b
as a clear liquid. Yield: 47% (3.74 g, 16.5 mmol). 1H NMR (CDCl3): δ
0.88 (t, 3JHH = 8.00 Hz, 3H; C4H8CH3), 1.06−1.17 (m, 4H; C4H8CH3
+ CH2), 1.24−1.34 (m, 4H; C3H6CH2CH3 + CH2), 1.42−1.48, 1.70−
1.72, 1.76−1.82 (m, 2H; CH2), 2.20 (s, 3H; NCH3), 2.30 [s, 6H;
N(CH3)2], 2.39−2.50 (m, 4H; CHN + CH2C4H11). 13C NMR
(CDCl3): δ 14.3 (C4H8CH3), 22.9 (C4H8CH3), 24.1, 24.6, 25.9 (CH2),
28.3, 30.1 (C4H8CH3), 36.3 (NCH3), 40.5 [N(CH3)2], 55.0
(C4H8CH3), 62.7, 63.8 (CHN). GC(+)-MS: tR = 5.52 min; m/z 226
(10) [M·+], 169 (71) [M+−C4H9], 124 (100) [CH2(CH)5N-
removed. Subsequent Kugelrohr distillation (74 °C, 4 × 10−1 mbar)
yielded the pure ligand 6 as a clear liquid. Yield: 76% (2.65 g, 13.4
mmol). 1H NMR (CDCl3): δ 0.96−1.24 (m, 4H; CH2), 1.05 (t, 3JHH
=
7.20 Hz, 6H; CH2CH3), 1.62−1.73 (m, 2H; CH2), 1.76−1.83 (m, 2H;
CH2), 2.24 (s, 6H; NCH3), 2.47−2.66 (m, 6H; CHN + CH2CH3). 13
C
NMR (CDCl3): δ 13.7 (CH2CH3), 25.1, 25.8 (CH2), 36.3 (NCH3),
48.0 (CH2CH3), 61.9 (CHN). GC(+)-MS: tR = 4.84 min; m/z 198
(90) [M·+], 169 (14) [M+−CH3−CH2], 138 (33) [C6H11N(CH2)-
+
+
CHCH2 ], 114 (43) [C4H9CHN(CH3)2 ], 98 (100) [CH3(CH)3N-
+
+
(CH3)2 ], 72 (71) [CH3CHN(CH3)2 ]. Anal. Calcd: C, 72.66; H,
13.21; N, 14.12. Found: C, 72.5; H, 13.2; N, 14.2.
Zinc Coordination Compounds (General Procedure). The zinc
halide was diluted in acetone and 1 equivalent of ligand was added.
The solvent was allowed to evaporate slowly, and the resulting
colorless crystals were washed with cold acetone.
1
(SN)-[ZnBr23] (7). Yield: 86%; mp: 221 °C. H NMR (CDCl3): δ
1.13−1.50 (m, 4H; CH2), 1.38 (t, 3JHH = 7.5 Hz, 3H; NC2CH3), 1.87−
1.95, 2.00−2.14 (m, 2H; CH2), 2.40, 2.47, 2.66 (s, 3H; NCH3), 2.56−
2.91 (m, 3H; CHN + NCH2CH3), 2.95−3.11 (m, 1H; NCH2CH3).
13C NMR (CDCl3): δ 13.3 (NCH2CH3), 23.0, 23.1, 24.5, 24.6 (CH2),
36.7, 42.3, 47.8 (NCH3), 52.3 (NCH2CH3), 64.7, 65.1 (CHN). Anal.
Calcd: C, 32.26; H, 5.91; N, 6.84. Found: C, 32.4; H, 5.9; N, 6.7.
+
(CH3)2+H+], 84 (24) [CH2CHCHN(CH3)2 ], 58 (43) [CH2N-
(CH3)2 ]. Anal. Calcd: C, 74.27; H, 13.36; N, 12.37. Found: C,
74.2; H, 13.2; N, 12.3.
N,N′-Diethyl-N,N′-dimethyl-(1R,2R)-cyclohexane-1,2-diamine (6).
To a solution of (R,R)-dimethylcyclohexane-1,2-diamine (2; 2.5 g,
17.6 mmol) in toluene (25 mL) were added triethylamine (5.4 mL,
3.94 g, 38.7 mmol) and bromoethane (2.9 mL, 4.22 g, 38.7 mmol).
The reaction mixture was refluxed for 14 h. Then, hydrochloric acid (3
M solution) was added until the mixture was acidic. The aqueous
phase was separated, and the organic phase was washed with water (25
mL). To the combined aqueous phases was added sodium hydroxide
until the reaction mixture was basic. The reaction mixture was then
extracted with diethyl ether (four 50 mL portions). The combined
organic phases were dried over sodium sulfate, and the solvent was
1
(SN)-[ZnBr25a] (8). Yield: 91%; mp: 188 °C. H NMR (CDCl3): δ
3
0.93 (t, JHH = 12.0 Hz 6H; NC2H4CH3), 1.12−1.23, 1.28−1.39 (m,
2H; CH2), 1.57−1.69 (m, 1H; CH2), 1.82−2.00 (m, 3H; CH2), 2.03−
2.11 (m, 2H; CH2), 2.40, 2.46 (s, 3H; NCH3), 2.60−2.73 (m, 3H;
CH2 + CHN), 2.65 (s, 3H; NCH3), 2.77−2.84 (m, 1H; CHN). 13C
NMR (CDCl3): δ 11.6 (NCH2CH3), 21.5, 23.0, 23.1, 24.5, 24.6
(NCH2), 37.3, 42.3, 47.8 (NCH3), 59.7 (CH2), 64.5, 65.1 (CHN).
Anal. Calcd: C, 34.03; H, 6.19; N, 6.61. Found: C, 33.8; H, 6.2; N, 6.5.
1
(SN)-[ZnCl25b] (9). Yield: 40%; mp: 124 °C. H NMR (CDCl3): δ
3
0.89 (t, JHH = 8.0 Hz, 3H; NC4H8CH3), 1.10−1.44 (m, 8H; NCH2),
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Inorganic Chemistry
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1.56−1.70 (m, 1H; NCH2), 1.81−1.95 (m, 3H; NCH2), 2.01−2.11
(m, 2H; NCH2), 2.37, 2.43 (s, 3H; NCH3), 2.54−2.84 (m, 4H; CHN
+ CH2), 2.64 (s, 3H; NCH3). 13C NMR (CDCl3): δ 14.2
(NC4H8CH3), 22.5, 22.9, 23.1, 24.5, 24.6, 27.6, 29.7 (CH2), 36.7,
41.1, 47.1 (NCH3), 58.1 (CH2), 64.6, 65.3 (CHN). Anal. Calcd: C,
46.36; H, 8.34; N, 7.72. Found: C, 46.0; H, 8.4; N, 7.6.
Scheme 1. Synthesis of (R,R)-TMCDA Derivatives with One
(3, 5a,5b) or Two (6) Stereogenic Nitrogen Centers
(SN,SN)-[ZnBr26] (10). Yield: 63%; mp: 185 °C. 1H NMR (CDCl3):
3
δ 1.12−1.26, 1.32−1.43 (m, 2H; CH2), 1.37 (t, JHH = 10.0 Hz, 6H;
NCH2CH3), 1.83−1.94, 2.02−2.10 (m, 2H; CH2), 2.42 (s, 6H;
NCH3), 2.69−2.77 (m, 2H; CHN), 2.79−2.87, 2.97−3.06 (m, 2H;
CH2). 13C NMR (CDCl3): δ 13.1 (NCH2CH3), 23.6, 24.6 (CH2), 37.5
(NCH3), 52.7 (NCH2CH3), 64.7 (CHN). Anal. Calcd: C, 34.03; H,
6.19; N, 6.61. Found: C, 34.1; H, 6.1; N, 6.5.
1
(SN,SN)-[ZnCl27] (11). Yield: 40%; mp: 157 °C. H NMR (CDCl3):
3
δ 1.11−1.27, 1.30−1.44 (m, 2H; CH2), 1.36 (t, JHH = 6.0 Hz, 6H;
NCH2CH3), 1.82−1.94, 2.00−2.09 (m, 2H; CH2), 2.38 (s, 6H;
NCH3), 2.52−3.01 (m, 6H; CHN + NCH2CH3). 13C NMR (CDCl3):
δ 12.9 (NCH2CH3), 23.1, 24.4 (CH2), 36.3 (NCH3), 52.1
(NCH2CH3), 64.4 (CHN). Anal. Calcd: C, 34.07; H, 7.83; N, 8.37.
Found: C, 34.0; H, 8.0; N, 8.4.
Coordination Compounds. To examine the coordination
behavior of the ligands and the configuration of the stereogenic
nitrogen centers, we chose the transition metals zinc (a harder
lewis acid) and mercury (a soft lewis acid),17 as complexes with
these metals were expected to be stable in the air and solution
as well as diamagnetic, thereby allowing for a broad variety of
characterization techniques. As coordination compounds with
tertiary diamines and zinc halides can be used as catalysts for
lactide polymerization,18,19 we decided to employ both zinc
chloride and zinc bromide for a wider variety. Crystals of the
coordination compounds with zinc chloride and zinc bromide
were grown from acetone solutions.
Mercury Coordination Compounds (General Procedure). A total
of 1−1.2 equivalents of ligand were added to a suspension of mercury
bromide in toluene. The mixture was stirred for 1 min, and the
solution was then decanted from the remaining mercury salt. The
solvent was allowed to evaporate slowly, and the resulting colorless
crystals were washed with diethyl ether.
1
(SN)-[HgBr21] (12). Yield: 97%; mp: 131 °C. H NMR (CDCl3): δ
0.35−0.49, 1.05−1.21 (m, 4H; CH2), 1.79 (s, 3JHHg = 30 Hz, 8H; CH3
3
+ CHN), 1.95 (s, 6H, JHHg = 30 Hz; CH3). 13C NMR (CDCl3): δ
22.0, 24.5 (CH2), 40.2, 64.1 [N(CH3)2], 63.0 (CHN).
1
With the ligands 3 and 5a, coordination compounds 7 and 8
were obtained with zinc bromide. Both coordination
compounds crystallize in the orthorhombic crystal system,
space group P212121. In both monomeric structures, the zinc
center displays a distorted tetrahedral coordination sphere with
N1−Zn−N2 angles of only 87.93(9)° in 7 and 86.95(14)° in 8.
The angle Br1−Zn1−Br2 is significantly larger [116.49(2)° in 7
and 117.60(3)° in 8] (Figure 2). This distortion of the
tetrahedral environment at the zinc center has already been
reported for [ZnBr2(tmeda)].20 While the chelating bond
lengths are quite similar in 7 [2.081(2) Å (N1−Zn1), 2.087(2)
Å (N2−Zn1)], they differ significantly in 8 [2.102(3) Å (N1−
Zn1), 2.069(3) Å (N2−Zn1)]. The coordination compounds 7
and 8 display only one diastereomer with (S)-configuration at
the stereogenic nitrogen center: In each complex, the alkyl
group avoids the sterical demand of the cyclohexane backbone.
This is also made clear by comparing the distance of the α-
carbon atom of the alkyl group and the carbon atom of the
methyl group to the sterically closest carbon atom of the
cyclohexane ring (Figure 3): In 7, C9−C7 is shorter [2.850(4)
Å] than C10−C7 [3.185(4) Å], and in 8, C9−C7 is shorter
[2.988(5) Å] than C10−C7 [3.022(5) Å]. For all zinc
complexes, powder diffraction data were collected to ascertain
that all crystals contain complexes with (S)-configuration at the
stereogenic nitrogen center exclusively. This is explained at the
end of the section. We were pleasantly surprised that even with
ligand 3, (S)-configuration is formed selectively upon
coordination to zinc bromide, although the ethyl group at the
stereogenic nitrogen center is only slightly longer than the
original methyl group.
(SN)-[HgBr24] (13). Yield: 64%; mp: 142 °C. H NMR (CDCl3): δ
0.74−0.99 (b, 3H; C2H4CH3), 1.02−1.55, 1.59−2.05 (m, 5H; CH2),
2.12−2.81 (m, 4H; CH2 + CHN), 2.31, 2.34, 2.53 (s, 3H; NCH3). 13
C
NMR (CDCl3): δ 11.7 (C2H4CH3), 22.9, 23.5, 24.8, 24.9 (CH2), 27.3,
41.3, 47.2 (NCH3), 59.6 (NCH2), 63.4, 64.2 (CHN).
1
(SN)-[HgBr25] (14). Yield: 38%; mp: 127 °C. H NMR (CDCl3): δ
3
0.88 (t, JHH = 15.0 Hz, 3H; C4H8CH3), 1.00−2.94 (m, 18H; CH2 +
CHN), 2.13, 2.34, 2.53 (s, 3H; NCH3). 13C NMR (CDCl3): δ 14.2
(C4H8CH3), 22.5 (C4H8CH3), 22.9, 23.5, 24.9 (CH2), 29.2, 29.6
(C4H8CH3), 37.3, 41.2, 47.2 (NCH3), 58.0 (C4H8CH3), 63.4, 64.2
(CHN).
(SN,SN)-[HgBr26] (15). Yield: 44%; mp: 154 °C. 1H NMR (CDCl3):
3
δ 1.09−1.27 (m, 4H; CH2), 1.36 (t, JHH = 8.0 Hz, 6H; CH2CH3),
1.77−1.89, 1.94−2.09 (m, 2H; CH2), 2.33 (s, 6H; NCH3), 2.49−3.07
(m, 6H; CHN + CH2CH3). 13C NMR (CDCl3): δ 14.2 (CH2CH3),
23.8, 25.0 (CH2), 37.3 (NCH3), 52.3 (CH2CH3), 63.8 (CHN).
RESULTS AND DISCUSSION
■
Ligand Synthesis. We synthesized four (R,R)-tetramethyl-
cyclohexane-1,2-diamine [(R,R)-TMCDA] derivatives (3, 5a,
5b, 6) containing one or two stereogenic nitrogen centers
(Scheme 1). The synthesis of ligand 3 is described in the
literature.12 For the synthesis of the derivatives 5a and 5b
containing one stereogenic nitrogen center, (R,R)-TMCDA (1)
was deprotonated with tert-butyllithium according to literature
methods.16 The resulting lithiated product 4 was then reacted
with bromoethane and 1-chlorobutane, resulting in the
asymmetric ligands 5a and 5b, respectively. The known
synthesis of ligand 6 starts from the primary cyclohexane-1,2-
diamine to which the ethyl groups are added in a two-step
procedure first, while the methyl groups are added afterward.12
We decided to synthesize ligand 6 by reacting the more widely
used secondary (R,R)-dimethylcyclohexane-1,2-diamine with
2.2 equivalents of bromoethane in the presence of triethyl-
amine. All ligands were subsequently coordinated to transition
metal halides.
By reacting ligand 5b with zinc chloride, the coordination
compound 9 was obtained. It crystallizes in the monoclinic
crystal system, space group P21. The asymmetric unit contains
two molecules of 9 which are so similar that only one of them is
discussed and displayed in Figure 4. Similar to the coordination
compounds [ZnCl2(tmeda)]21 and [ZnCl2(tmcda)],22 the zinc
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similar to 2.086(2) Å (N1−Zn1) and 2.076(2) Å (N2−Zn1).
As observed in complexes 7 and 8, the sterically more
demanding pentyl group evades the cyclohexane backbone,
resulting in (S)-configuration of the stereogenic nitrogen
center. Therefore, the C1−C8 distance is shorter than the
C2−C8 distance [2.898(3) Å compared to 3.181(3) Å].
Finally, the coordination compounds 10 and 11 were
obtained by reacting ligand 6 with zinc bromide and zinc
chloride. Both complexes crystallize in the orthorhombic crystal
system, space group P212121. The asymmetric unit of 11
contains two molecules which are so similar that only one of
them is discussed and displayed in Figure 5. Both coordination
Figure 2. Molecular structures and numbering scheme of the
coordination compounds 7 and 8. Selected angles [deg] and bond
lengths [Å] of 7: N1−Zn1−N2 87.93(9), Br1−Zn1−Br2 116.49(2),
N1−Zn1 2.081(2), N2−Zn1 2.087(2), C9−C7 2.850(4), C10−C7
3.185(4). Selected angles [deg] and bond lengths [Å] of 8: N1−Zn1−
N2 86.95(14), Br1−Zn1−Br2 117.60(3), N1−Zn1 2.102(3), N2−Zn1
2.069(3), C9−C7 2.988(5), C10−C7 3.022(5).
Figure 5. Molecular structures and numbering scheme of the
coordination compounds 10 and 11. Selected angles [deg] and
bond lengths [Å] of 10: N1−Zn1−N2 88.03(19), Br1−Zn1−Br2
117.10(4), N1−Zn1 2.081(5), N2−Zn1 2.085(5), C10−C8
2.893(10), C1−C5 2.892(11), C11−C8 3.110(10), C2−C5
3.201(12). Selected angles [deg] and bond lengths [Å] of 11: N1−
Zn1−N2 88.03(7), Cl1−Zn1−Cl2 118.33(3), N1−Zn1 2.085(2),
N2−Zn1 2.088(2), C10−C8 2.865(4), C1−C5 2.888(4), C11−C8
3.194(4), C2−C5 3.144(4).
Figure 3. The sterically more demanding alkyl group evades the
sterical demand of the cyclohexane ring.
compounds display a pseudotetrahedral surrounding of the zinc
center. As in 7, the angle N1−Zn1−N2 is small [88.03(19)° in
10 and 88.03(7)° in 11] while the angle comprising the two
halide groups is significantly larger [10, 117.10(4)° (Br1−
Zn1−Br2); 11, 118.33(3)° (Cl1−Zn1−Cl2)]. The N−Zn
bond lengths are also quite similar [10, 2.081(5) Å (N1−Zn1),
2.085(5) Å (N2−Zn1); 11, 2.085(2) Å (N1−Zn1), 2.088(2) Å
(N2−Zn1)]. As in the other coordination compounds with zinc
halides and the copper coordination compound with ligand
6,11,12 the stereogenic nitrogen center is (S)-configured, and the
ethyl groups evade the sterical demand of the cyclohexane
backbone. This can also be shown by comparing the distances
C10−C8 [10, 2.893(10) Å; 11, 2.865(4) Å] and C1−C5 [10,
2.892(11) Å; 11, 2.888(4) Å] to the distances C11−C8 [10,
3.110(11) Å; 11, 3.194(4) Å] and C2−C5 [10, 3.201(12) Å;
11, 3.144(4) Å]. Again, the α-carbon atom of the ethyl group is
further away from the sterically demanding cyclohexane
backbone than the carbon atom of the methyl group.
Figure 4. Molecular structure and numbering scheme of the
coordination compound 9. Selected angles [deg] and bond lengths
[Å]: N1−Zn1−N2 87.95(7), Cl1−Zn1−Cl2 120.18(3), N1−Zn1
2.086(2), N2−Zn1 2.076(2), C1−C8 2.898(3), C2−C8 3.181(3).
center displays a pseudotetrahedral environment with a small
N1−Zn1−N2 angle [87.95(7)°] and a large Cl1−Zn1−Cl2
angle [120.18(3)°]. The coordinating bond lengths are quite
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Coordination compounds with mercury bromide were
obtained from toluene solutions. The reaction was first tested
with the parent compound (R,R)-TMCDA (1) to give crystals
of complex 12 and then used for the other ligands. Complexes
13 and 14 with the ligands 5a and 5b were obtained as crystals,
while the reaction with ligand 3 yielded no product, and crystals
of the coordination compound 15 obtained with ligand 6 were
of very poor quality.
Complex 12 crystallizes in the orthorhombic crystal system,
space group P212121. The mercury center of the coordination
compound displays a distorted tetrahedral environment with a
N1−Hg−N2 angle [76.3(4)°] that is smaller than the
corresponding angles in the zinc complexes. The Br1−Hg1−
Br2 angle is very large, amounting to 134.05(6)°. As expected,
the N−Hg bond lengths N1−Hg1 [2.406(14) Å] and N2−Hg1
2.381(15) Å] are distinctly longer than the coordinating bonds
in the zinc complexes (Figure 6).
group and the cyclohexane ring (C9−C7) is shorter [2.923(32)
Å] than the distance between the α-carbon atom of the propyl
group and the cyclohexane ring [C10−C7, 3.198(32) Å].
Similarly, C9−C7 [2.832(10) Å] and C23−C21 [2.541(26) Å]
in 14a are shorter than C10−C7 [3.110(25) Å] and C24−C21
[3.134(28) Å], and C9−C7 [2.964(23) Å] and C23−C21
[2.917(27) Å] in 14b are shorter than C10−C7 [3.183(40) Å]
and C24−C21 [3.138(24) Å].
In conclusion, both the zinc and the mercury complexes with
ligands 3, 5a, 5b, and 6 display (S)-configuration selectively. To
ascertain that all crystals obtained from the reaction mixtures
exhibit the same configuration at the stereogenic nitrogen
centers, powder diffractograms of all the crystals obtained from
the reaction mixture were measured and compared to the
patterns calculated from the crystal structures (this was not
possible for complex 14 as the compound formed two different
kinds of crystals). As can be seen from the data included in the
Supporting Information, both patterns overlap.
NMR Studies. NMR spectra of the zinc and mercury
coordination compounds showed only one set of signals, even
after heating or cooling to −50 °C, another indication that only
one diastereomer is formed selectively. In a first attempt to
better understand this selective formation of one specific
configuration, further NMR studies were carried out to
determine whether the complexes are kinetic or thermody-
namic products: To the solution of the zinc compound 11, the
free ligand 5b was added. After 10 min, NMR spectra revealed a
slow equilibrium of both possible coordination compounds and
both free ligands, which had formed a stable ratio after 1 h. This
indicates that the zinc coordination compounds are thermody-
namic products. The same experiment with the mercury
complexes did not show any exchange, not even after heating
the sample. However, when adding an excess of ligand 6 to a
solution of complex 14, the products 15 and 5b were formed
after 10 min, and no signals of complex 14 remained. An
equilibrium was formed in solution, which in this case was
shifted completely toward the products (Scheme 2). The
experiment indicates that the mercury coordination compounds
are thermodynamic products, and therefore an equilibrium
between the different configurations at the stereogenic nitrogen
centers must exist and seems to be shifted toward the (S) or
(S,S)-configured isomer.
Quantum Chemical Calculations. The NMR studies have
shown thermodynamic equilibria between complexes with
different ligands in solution. Therefore, a thermodynamic
equilibrium between the different configurations of the
stereogenic nitrogen center must exist as well and be shifted
to the thermodynamically most stable compound with (S)-
configuration at the nitrogen centers. To confirm this
assumption, DFT studies can be performed to calculate the
energies of all possible isomers. These calculations were only
carried out for the zinc complexes though, as the many
electrons of the mercury atoms make calculations difficult and
time-consuming.
Figure 6. Molecular structures of the coordination compounds 12.
Selected angles [deg] and bond lengths [Å]: N1−Hg1−N2 76.3(4),
Br1−Hg1−Br2 134.05(6), N1−Hg1 2.406(14), N2−Hg1 2.381(15).
Complex 13 crystallizes in the orthorhombic crystal system,
space group P212121. Complex 14 forms two different crystals
(a and b) (Figure 7). Both times, it crystallizes in the
monoclinic crystal system, space group P21. In both crystal
structures a and b, the asymmetric unit contains two molecules
of 14 which differ in the geometry of the pentyl group. The
carbon atoms of this group are eclipsed in 14a and one
molecule in 14b and stacked in the second molecule of 14b.
Additionally, the second molecule in 14a displays a disorder at
the terminal methyl moiety of the pentyl group.
In both coordination compounds, 13 and 14, the N−Hg−N
angle is very small [13, 76.3(5)° (N1−Hg1−N2); 14a,
77.1(2)° (N1−Hg1−N2), 76.1(3)° (N3−Hg2−N4); 14b,
76.8(5)° (N1−Hg1−N2), 78.2(4)° (N3−Hg2−N4)] while
the angle Br−Hg−Br is quite large [13, 127.56(8)° (Br1−
Hg1−Br2); 14a, 135.45(3)° (Br1−Hg1−Br2), 127.84(4)°
(Br3−Hg2−Br4); 14b, 132.77(6)° (Br1−Hg1−Br2),
131.08(7)° (Br3−Hg2−Br4)]. Not only the angles surrounding
the mercury center but also the N−Hg bond lengths are
comparable to those in complex 12 [13, 2.393(16) Å (N1−
Hg1), 2.406(14) Å (N2−Hg1); 14a, 2.399(5) Å (N1−Hg1),
2.391(6) Å (N2−Hg1), 2.404(5) Å (N3−Hg2), 2.367(7) Å
(N4−Hg2); 14b, 2.386(14) Å (N1−Hg1), 2.351(11) Å (N2−
Hg1), 2.275(15) Å (N3−Hg2), 2.416(11) Å (N4−Hg2) ]. As
in the zinc complexes with stereogenic nitrogen centers, the
alkyl groups in 13 and 14 evade the sterical demand of the
cyclohexane ring, resulting in (S)-configuration at the stereo-
genic nitrogen centers. In 13, the distance between the methyl
Initial gas phase calculations with the B3LYP/6-31+G(d)
method provided by the program Gaussian 03 of the zinc
compound 7 showed great deviations in the geometries of the
calculated compound and crystal structure. The Br−Zn−Br
angle, especially, was too large [128.84° compared to
116.36(2)° in the crystal structure]. An extensive evaluation
of several different methods and basis sets showed that the
combination of BP86/def-TZVP and the additional use of
dispersion and the COSMO solvation model provided by the
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Figure 7. Molecular structures and numbering scheme of the coordination compounds 13 and 14 (with the crystal structures 14a and 14b). The
asymmetic units of 14a and 14b contain two molecules which are placed next to each other. Selected angles [deg] and bond lengths [Å] of 13: N1−
Hg1−N2 76.3(5), Br1−Hg1−Br2 127.56(8), N1−Hg1 2.393(16), N2−Hg1 2.406(14), C9−C7 2.923(32), C10−C7 3.198(32). Selected angles
[deg] and bond lengths [Å] of 14a: N1−Hg1−N2 77.1(2), N3−Hg2−N4 76.1(3), Br1−Hg1−Br2 135.45(3), Br3−Hg2−Br4 127.84(4), N1−Hg1
2.399(5), N2−Hg1 2.391(6), N3−Hg2 2.404(5), N4−Hg2 2.367(7), C9−C7 2.832(10), C24−C21 3.100(13), C10−C7 3.197(9), C23−C21
2.871(10). Selected angles [deg] and bond lengths [Å] of 14b: N1−Hg1−N2 76.8(5), N3−Hg2−N4 78.2(4), Br1−Hg1−Br2 132.77(6), Br3−
Hg2−Br4 131.08(7), N1−Hg1 2.386(14), N2−Hg1 2.351(11), N3−Hg2 2.275(15), N4−Hg2 2.416(11), C9−C7 2.964(23), C23−C21 2.917(27),
C10−C7 3.110(25), C24−C21 3.134(28).
program Turbomole V6.3 is much better suited for calculations
of the zinc complexes. This can be seen not only from the Br−
Zn−Br angle (116.58°) but also from the RMSD values of the
whole molecule, which amount to 0.2 Å in the second
calculation method compared to 0.4 Å in the “standard”
method B3LYP/6-31+G(d).
Scheme 2. NMR Studies Showing That the Zinc and
Mercury Complexes Are Thermodynamic Products
Using this method, the different isomers of all coordination
compounds were calculated. A comparison of selected bond
lengths and angles of the crystal structures as well as the
calculated values of the diastereomers are presented in the
Supporting Information. The calculated bond lengths and
angles fit nicely. A comparison of the calculated energies shows
that the (S)- or (S,S)-configured isomer is indeed the
thermodynamically most stable structure, while the other
isomers are considerably higher in energy. Therefore, the
calculations explain the experimentally observed specific
configuration at the stereogenic nitrogen center(s) (Table 3).
Table 3. Calculated Energies [kJ/mol] Using the Method
BP86/def-TZVP/COSMO/Dispersion of the Different
Possible Diastereomers of Compounds 7−11 in Relation to
the Most Stable Isomer
(S)-isomer
(R)-isomer
7
8
9
0
0
0
8
7
7
(S,S)-isomer
(R,S)-isomer
(R,R)-isomer
10
11
0
0
8
8
16
15
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Y.-M.; Kang, S. K.; Kima, Y.- I.; Choi, S.-N. Acta Crystallogr. 2002,
C58, m453−m454.
CONCLUSION
■
In this paper, we present a series of zinc and mercury
complexes that display a specific configuration at the stereo-
genic nitrogen center(s). In all cases, the sterically more
demanding alkyl group at the nitrogen center evades the sterical
demand of the cyclohexane backbone and is found in a
pseudoequatorial position in the five-membered ring formed by
the coordination of both nitrogen centers to the metal center.
We were able to show, both by NMR spectra and by combined
single crystal and powder diffractometry, that indeed only one
of the possible diastereomers is formed upon coordination to
the transition metal halide. Therefore, the complexes are
interesting as catalysts for asymmetric reactions or lactide
polymerization.18,19
NMR studies indicate that the zinc and mercury compounds
are the result of a thermodynamically controlled reaction. By
employing quantum chemical calculations including a solvent
model, we were able to explain the formation of only one
specific diastereomer in the zinc complexes by comparing the
energies of the different isomers. Future studies will focus on
the question, whether the highly desired selective formation of
only one diastereomer is also found in coordination
compounds with transition metals that are more widely used
in catalysis, such as diethyl zinc and copper chloride.
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ASSOCIATED CONTENT
■
S
* Supporting Information
ORTEP23 plots with thermal ellipsoids of the crystal structures
of complexes 7−14, powder diffractograms of complexes 7−11
and 13, MOLEKEL24 plots, input coordinates and output
coordinates of the calculated complexes, and an overview of the
quantum chemical calculations. This material is available free of
charge via the Internet at http://pubs.acs.org/.
(14) TURBOMOLE, V6.3; University of Karlsruhe and Forschungs-
zentrum Karlsruhe GmbH (1989−2007): Karlsruhe, Germany;
TURBOMOLE GmbH (since 2007): Karlsruhe, Germany, 2011.
Available from http://www.turbomole.com (accessed July 2012).
(15) (a) Kizirian, J.-C.; Cabello, N.; Pinchard, L.; Caille, J.-C.;
Alexakis, A. Tetrahedron 2005, 61, 8939−8946. (b) Larrow, J. F.;
Jacobsen, E. N. J. Org. Chem. 1994, 59, 1939−1942. (c) Benson, S. C.;
Cai, P.; Colon, M.; Haiza, M. A.; Tokles, M.; Snyder, J. K. J. Org.
Chem. 1988, 53, 5335−5341.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mail@carsten-strohmann.de.
Notes
(16) (a) Strohmann, C.; Gessner, V. H. Angew. Chem. 2007, 119,
The authors declare no competing financial interest.
8429−8432. (b) Gessner, V. H.; Frohlich, B.; Strohmann, C. Eur. J.
̈
Inorg. Chem. 2010, 5640, 5649.
ACKNOWLEDGMENTS
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Patent WO 2010136544 A1 20101202, 2010.
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The authors thank Cornelia Werner for her valuable help in the
lab and Michaela Schulte for measuring the powder diffracto-
grams. Financial support by the Fonds der Chemischen
Industrie and the DFG is acknowledged. V.H.G. thanks the
Fonds der Chemischen Industrie, the Alexander-von-Hum-
boldt-Foundation, and the DFG for financial support.
(20) Citeau, H.; Conrad, O.; Giolando, D. M. Acta Crystallogr. 2001,
E57, m5−m6.
(21) Chen, D.-M.; Ma, X.-J.; Tu, B.; Feng, W.-J.; Jin, Z.-M. Acta
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