Syntheses and X-ray structures of zinc amidinate complexes

Article abstract of DOI:10.1002/zaac.200700457

Reactions of ZnX₂ (X = Cl, Br) with equimolar amounts of Li[t-BuC(NR)₂] (R = i-Pr, Cy) yielded mono-amidinate complexes [{t-BuC(NR)₂}ZnX]₂ (X = Cl, R = i-Pr 1, Cy 2; X = Br, R = i-Pr 3, Cy 4), whereas reactions with two equivalents of Li-amidinate resulted in the formation of the corresponding bis-amidinate complexes [t-BuC(NR) ₂]₂Zn (R = i-Pr 5, Cy 6). 1 - 6 were characterized by elemental analyses, IR, mass and multinuclear NMR spectroscopy (¹H, ¹³C), and single crystal X-ray analysis (1, 2, 3, 6). In addition, the single crystal X-ray structure of [t-BuC(NCy)₂] ZnBr·LiBr(OEt₂)₂ 7, which was obtained as a byproduct in low yield from re-crystallization experiments of 4 in Et ₂O, is reported.

Full text of DOI:10.1002/zaac.200700457

DOI: 10.1002/zaac.200700457
Syntheses and X-Ray Structures of Zinc Amidinate Complexes
Tamara Eisenmann, Jayaprakash Khanderi, Stephan Schulz*, and Ulrich Flörke
Paderborn, Universität, Chemistry Department
Received August 16th, 2007.
Abstract. Reactions of ZnX2 (X ϭ Cl, Br) with equimolar amounts of
Li[t-BuC(NR)2] (R ϭ i-Pr, Cy) yielded mono-amidinate complexes
[{t-BuC(NR)2}ZnX]2 (X ϭ Cl, R ϭ i-Pr 1, Cy 2; X ϭ Br, R ϭ i-Pr 3,
Cy 4), whereas reactions with two equivalents of Li-amidinate resul-
ted in the formation of the corresponding bis-amidinate complexes
[t-BuC(NR)2]2Zn (R ϭ i-Pr 5, Cy 6). 1 - 6 were characterized by ele-
mental analyses, IR, mass and multinuclear NMR spectroscopy
(1H, 13C), and single crystal X-ray analysis (1, 2, 3, 6). In addition, the
single crystal X-ray structure of [t-BuC(NCy)2]ZnBr·LiBr(OEt2)2 7,
which was obtained as a byproduct in low yield fromre-crystallization
experiments of 4 in Et2O, is reported.
Keywords: Zinc; Amides; Bridging Ligands; Coordination Modes;
X-Ray Structures
Introduction
copolymerization of CO2 and epoxides, respectively [7].
Moreover, low-valent organozinc compounds of the general
type LZn-ZnL containing a direct Zn-Zn bond, which
were initially prepared by Carmona et al. [8], can be stabil-
ized by use of chelating organic substituents. (L ϭ
[{(2,6-i-Pr2C6H3)N(Me)C}2CH] [9], 1,2-Bis[(2,6-diisopro-
pylphenyl)imino]acenaphthene [10]). Surprisingly, zinc
guanidinate and amidinate complexes have been studied to
a far lesser extend, which is in remarkable contrast to well
known beryllium [11] and magnesium complexes [12]. Coles
et al. reported on the synthesis of a very few guanidinate
complexes and their application in ROP catalysis [13],
whereas, to the best of our knowledge, zinc amidinate com-
plexes haven’t been structurally characterized except for a
very few formamidinate complexes [14]. In addition, Edel-
mann et al. described the synthesis of a bis-benzamidinate
zinc complex [15] and very recently, Chivers et al. reported
on the synthesis and X-ray crystal structure of bis-boraami-
dinate zinc complexes [16].
N,NЈ-chelating organic ligands have attracted growing inter-
est in organometallic chemistry in the last decade and sev-
eral systems including β-diketiminate I [1], guanidinate II
[2], and amidinate anions III [3], have been investigated in-
tensely in both main group and transition metal chemistry
[4]. Steric and electronic properties of these N,NЈ-chelating
substituents can simply be modified by use of different or-
ganic substituents R and RЈ, allowing fine-tuning of the
chemical properties of the resulting metal complexes.
Consequently, a large variety of unforeseen low-coordinate
organometallic compounds in low oxidation states have
been synthesized and structurally characterized.
Herein, we report on the synthesis and single crystal X-
ray structures of mono- and bis-amidinate zinc complexes,
which were obtained from reactions of zinc halides ZnX2
(X ϭ Cl, Br) with either one or two equivalents of the cor-
responding Li-amidinate Li[t-BuC(NR)2] (R ϭ i-Pr, Cy).
Scheme 1 Typical N,NЈ chelating ligands: β-diketiminate I, guani-
dinate II and amidinate anions III.
In case of organozinc chemistry, the interest has focused
on β-diketiminato complexes of the general type LZnR
(L ϭ β-diketiminate) [5], which were shown to be active
catalysts in the metal catalyzed ring-opening polymerization
(ROP) of cyclic esters, in particular lactides [6], and in
Experimental Section
General Procedure
All manipulations were performed in a glovebox under an N2
atmosphere or with standard Schlenk techniques. Solvents were
dried over sodium/potassium or by use of a solvent purification
system (MBRAUN) and degassed prior to use. Li-amidinates
Li[t-BuC(NR)2] (R ϭ i-Pr, Cy) were prepared by reaction of t-BuLi
with the corresponding carbodiimide RNϭCϭNR according to a
procedure described by Jordan et al. [17]. A Bruker Avance 500
spectrometer was used for NMR spectroscopy. 1H and 13C{1H}
NMR spectra were referenced to internal C6D5H (1H: δ ϭ 7.154;
* Prof. Dr. Stephan Schulz
Anorganische Chemie
Universitätsstrasse 5Ϫ7
Raum S05 T05 B84
Universität Duisburg-Essen 45117 Essen
Fax: 0201/1833830
E-mail: stephan.schulz@uni-due.de
Z. Anorg. Allg. Chem. 2008, 634, 507Ϫ513
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
507

T. Eisenmann, J. Khanderi, S. Schulz, U. Flörke
13C: δ ϭ 128.0) and THF-d8 (1H: δ ϭ 1.73; 3.58; 13C: δ ϭ 25.2;
67.4). Mass spectra (EI) were recorded on a Finnigan MAT 8230
spectrometer. Melting points were measured in sealed capillaries
and were not corrected. Elemental analyses were performed at the
Elementaranalyse Labor of the University of Paderborn.
General Synthesis of [t-BuC(NR)2]2Zn (R ؍ i-Pr, Cy): 5 mmol Li[t-
BuC(NR)2] (R ϭ i-Pr 0.951 g; Cy 1.352 g) dissolved in 30 mL of
Et2O was slowly added at Ϫ78 °C to a suspension of 2.5 mmol
ZnX2 (X ϭ Cl 0.341; Br 0.562 g) in 40 mL of Et2O. The resulting
suspension was warmed up to ambient temperature and stirred for
12h. Thereafter, Et2O was distilled of and the remaining solid sus-
pended in n-hexane. Filtration yielded a clear solution, which was
concentrated to 5 mL and stored at -30 °C. Colorless crystals were
formed within 24h. Yields are given for the crystalline complexes.
General Synthesis of [{t-BuC(NR)2}ZnX]2 (X ؍ Cl, Br; R ؍ i-Pr,
Cy): 5 mmol Li[t-BuC(NR)2] (R ϭ i-Pr 0.951 g; Cy 1.352 g) dis-
solved in 30 mL of Et2O was slowly added at Ϫ78 °C to a suspen-
sion of 5 mmol ZnX2 (X ϭ Cl 0.681 g; Br 1.125 g) in 40 mL Et2O
and stirred for 1h. After warming up to room temperature the reac-
tion mixture was allowed to stir additional 12h. Et2O was removed
at reduced pressure and the remaining yellowish solid was sus-
pended in 30 mL of n-hexane and filtered. The residue was dis-
solved in 20 mL of Et2O, filtered and then the Et2O removed at
reduced pressure. The resulting solid was re-crystallized from solu-
tions in toluene / Et2O at Ϫ30 °C, yielding colorless crystals of 1
Ϫ 4. Yields are given for the isolated crystalline materials.
[t-BuC(Ni-Pr)2]2Zn 5: Yield 0.85 g (80 %). Mp: > 220 °C. Anal.
C22H46N4Zn (432.00 g/mol): found (calcd): H, 10.71 (10.73); C:
61.13 (61.17) %.
3
1H NMR (500 MHz, C6D6, 25 °C):
δ
1.20 (d, JHH
ϭ
5.9 Hz, 12H,
3
CH(CH3)2), 1.31 (s, 9H, t-Bu), 4.16 (sep, JHH ϭ 5.9 Hz, 2H, CH(CH3)2).
3
1H NMR (500 MHz, THF-d8, 25 °C): δ 1.03 (d, JHH ϭ 6.1 Hz, 12H,
3
CH(CH3)2), 1.40 (s, 9H, t-Bu), 4.14 (sep, JHH ϭ 6.1 Hz, 2H, CH(CH3)2).
13C NMR (125 MHz, C6D6, 25 °C): δ 27.6 (CHMe2); 30.3 (CCMe3), 39.5
(CCMe3), 46.0 (CHMe2), 175.9 (CCMe3). 13C NMR (125 MHz, THF-d8,
25 °C): δ 27.8 (CHMe2); 30.6 (CCMe3), 40.1 (CCMe3), 46.5 (CHMe2), 176.4
(CCMe3). EI-MS (70 eV, 100 °C): m/z (%): 43 (90) [i-Pr]ϩ, 57 (90) [t-Bu]ϩ,
84 (95) [t-BuC(Ni-Pr)2Ϫt-BuϪi-Pr]ϩ, 126 (90) [t-BuC(Ni-Pr)2Ϫt-Bu]ϩ, 183
(100) [t-BuC(Ni-Pr)2]ϩ, 247 (85) [MϪt-BuϪ3i-Pr]ϩ, 304 (20) [MϪ3i-Pr]ϩ,
345 (5) [MϪ2i-Pr]ϩ, 388 (80) [MϪi-Pr]ϩ, 430 (80) [M]ϩ. IR (nujol): ν 2960,
[{t-BuC(Ni-Pr)2}ZnCl]2 1: Yield 0.78 g (55 %). Mp: > 220 °C. Anal.
C22H46Cl2N4Zn2 (568.28 g/mol): found (calcd): H, 8.11 (8.16); C:
46.23 (46.50) %.
2924, 2857, 1455, 1424, 1378, 1316, 1264, 1186, 1119, 1041, 866, 793 cmϪ1
.
3
1H NMR (500 MHz, THF-d8, 25 °C): δ 1.01 (d, JHH ϭ 6.1 Hz, 12H,
3
CH(CH3)2), 1.36 (s, 9H, t-Bu), 4.11 (sep, JHH ϭ 6.1 Hz, 2H, CH(CH3)2).
13C NMR (125 MHz, THF-d8, 25 °C): δ 27.8 (CHMe2); 30.5 (CCMe3), 40.0
(CCMe3), 46.5 (CHMe2), 176.3 (CCMe3). EI-MS (70 eV, 150 °C): m/z (%):
43 (100) [i-Pr]ϩ, 57 (70) [t-Bu]ϩ, 84 (90) [t-BuC(Ni-Pr)2Ϫt-BuϪi-Pr]ϩ, 126
(50) [t-BuC(Ni-Pr)2Ϫt-Bu]ϩ, 183 (100) [t-BuC(Ni-Pr)2]ϩ, 282 (5) [M/2]ϩ. IR
[t-BuC(NCy)2]2Zn 6: Yield 1.30 g (83 %). Mp: > 220 °C. Anal.
C34H62N4Zn (592.26 g/mol): found (calcd): H, 10.52 (10.55); C:
68.73 (68.95).
1H NMR (500 MHz, C6D6, 25 °C): δ 1.09-1.19 (m, 2H, H(4)ax), 1.23Ϫ1.36
(m, 4H, H(3/5)ax), 1.37 (s, 9H, t-Bu), 1.38Ϫ1.49 (m, 4H, H(2/6)ax), 1.57 (d
(br), 2H, H(4)eq), 1.76 (d (br), 4H, H(3/5)eq), 2.04 (d (br), 4H, H(2/6)eq), 3.77
(tt, 3JHH ϭ 10.1 Hz, 3JHH ϭ 3.8 Hz, 2H, H(1)ax). 1H NMR (500 MHz, THF-
d8, 25 °C): δ 1.06-1.15 (m, 2H ϩ 4H, H(4)ax ϩ H(3/5)ax), 1.23-1.30 (m, 4H,
H(2/6)ax), 1.38 (s, 9H, t-Bu), 1.58 (d (br), 2H, H(4)eq), 1.69 (d (br), 4H, H(3/
(nujol): ν 2916, 2717, 1460, 1378, 1259, 1087, 1026, 881, 798, 731 cmϪ1
.
[{t-BuC(NCy)2}ZnCl]2 2: Yield 0.64 g (42 %). Mp: > 220 °C. Anal.
C34H62Cl2N4Zn2 (728,54 g/mol): found (calcd): H, 8.43 (8.58); C:
55.78 (56.05) %.
3
3
5)eq), 1.78 (d (br), 4H, H(2/6)eq), 3.60 (tt, JHH ϭ 10.2 Hz, JHH ϭ 3.8 Hz,
2H, H(1)ax). 13C NMR (125 MHz, C6D6, 25 °C): δ 26.1 (C3/C5); 26.3 (C4),
30.4 (CMe3), 38.7 (C2/C6), 39.5 (CMe3), 54.9 (N-C1), 176.2 (CCMe3). 13C
NMR (125 MHz, THF-d8, 25 °C): δ 26.5 (C3/C5); 26.9 (C4), 30.6 (CMe3),
39.1 (C2/C6), 40.0 (CMe3), 55.3 (N-C1), 176.5 (CCMe3). EI-MS (70 eV,
100 °C): m/z (%): 57 (45) [t-Bu]ϩ, 84 (100) [CyH]ϩ, 181 (30) [t-
BuC(NCy)2ϪCy]ϩ, 263 (100) [t-BuC(NCy)2]ϩ, 327 (20) [t-BuC(Ni-Pr)2Zn]ϩ,
507 (20) [MϪCy]ϩ, 590 (15) [M]ϩ. IR (nujol): ν 2956, 2842, 2733, 2668, 1651,
1H NMR (500 MHz, THF-d8, 25 °C): δ 1.08Ϫ1.14 (m, 4H, CH2), 1.26Ϫ1.37
(m, 8H, CH2), 1.36 (s, 9H, t-Bu), 1.68Ϫ1.74 (m, 4H, CH2), 1.75Ϫ1.84 (m,
4H, CH2), 3.21Ϫ3.33 (m, 2H, CH). 13C NMR (125 MHz, THF-d8, 25 °C): δ
26.4 (C3/C5); 26.7 (C4), 30.5 (CMe3), 38.7 (C2/C6), 49.9 (CMe3), 55.1 (N-
C1), 176.4 (CCMe3). EI-MS (70 eV, 150 °C): m/z (%): 43 (85) [t-BuϪMe]ϩ,
57 (90) [t-Bu]ϩ, 84 (100) [CyH]ϩ, 109 (65) [CyNC]ϩ, 166 (90) [t-BuC(NCy)]ϩ,
181 (90) [t-BuC(NCy)2ϪCy]ϩ, 263 (95) [t-BuC(NCy)2]ϩ, 327 (10) [M/
2ϪCl]ϩ, 362 (15) [M/2]ϩ. IR (nujol): ν 2717, 2360, 1465, 1372, 1263, 1087,
1460, 1379, 1341, 1297, 1172, 1129, 884 cmϪ1
.
1015, 793, 720 cmϪ1
.
[t-BuC(NCy)2ZnBr x LiBr(OEt2)2] 7:
[{t-BuC(Ni-Pr)2}ZnBr]2 3: Yield 0.78 g (35 %) Mp: > 220 °C. Anal.
C22H46Br2N4Zn2 (657.18 g/mol): found (calcd): H, 6.98 (7.05); C:
40.11 (40.21) %.
1H NMR (500 MHz, THF-d8, 25 °C): δ 1.06-1.15 (m, 4H, Cy), 1.12 (t,
3JHH ϭ 6.9 Hz, 6H, OCH2CH3), 1.20-1.29 (m, 8H, Cy), 1.34 (s, 9H, t-Bu),
3
1.55 (m, 2H, Cy), 1.66 (m, 4H, Cy), 1.77 (m, 2H, Cy), 3.39 (q, JHH
ϭ
6.9 Hz, 6H, OCH2CH3), 3.58 (m, 2H, Cy). 13C NMR (125 MHz, THF-d8,
25 °C): δ 15.7 (OCH2CH3), 26.4 (C3/C5); 26.8 (C4), 30.6 (CMe3), 38.5 (C2/
C6), 39.1 (CMe3), 55.6 (N-C1), 66.3 (OCH2CH3), 176.5 (CCMe3). EI-MS
(70 eV, 150 °C): m/z (%): 29 (15) [Et]ϩ, 57 (60) [t-Bu]ϩ, 84 (100) [CyH]ϩ, 98
(50) [NCyH]ϩ, 167 (30) [t-BuC(NCy)H]ϩ, 181 (90) [t-BuC(NCy)2ϪCy]ϩ, 264
(65) [t-BuC(NCy)2H]ϩ. IR (nujol): ν 2960, 2846, 2670, 1460, 1372, 1258,
3
1H NMR (500 MHz, THF-d8, 25 °C): δ 1.02 (d, JHH ϭ 6.1 Hz, 12H,
3
CH(CH3)2), 1.38 (s, 9H, t-Bu), 4.16 (sep, JHH ϭ 6.1 Hz, 2H, CH(CH3)2).
13C NMR (125 MHz, THF-d8, 25 °C): δ 27.3 (CHMe2); 30.4 (CCMe3), 40.1
(CCMe3), 46.7 (CHMe2), 176.4 (CCMe3). EI-MS (70 eV, 150 °C): m/z (%):
43 (80) [i-Pr]ϩ, 58 (90) [t-BuH]ϩ, 83 (100) [t-BuC(Ni-Pr)2Ϫt-BuϪi-Pr]ϩ, 126
(40) [t-BuC(Ni-Pr)2Ϫt-Bu]ϩ, 183 (75) [t-BuC(Ni-Pr)2]ϩ, 247 (10) [M/2ϪBr]ϩ,
328 (15) [M/2]ϩ. IR (nujol): ν 2955, 2918, 2852, 1646, 1563, 1465, 1374, 1263,
1087, 1025, 798 cmϪ1
.
1155, 1103, 1015, 818 cmϪ1
.
X-ray Structure Solution and Refinement
[{t-BuC(NCy)2}ZnBr]2 4: Yield 1.13 g (46 %). Mp: > 220 °C. Anal.
C34H62Br2N4Zn2 (817.44 g/mol): found (calcd): H, 7.55 (7.64); C:
49.77 (49.96) %.
Bond lengths and angles for 1, 2, 3, 6 and 7 are summarized in
Table 1. Crystallographic data of 1, 2 and 3 are given in Table 2,
those of 6 and 7 in Table 3. Figures 2Ϫ5 show ORTEP diagrams
of the solid state structures of 2, 3, 6 and 7. Data were collected on
a Bruker-AXS SMART APEX CCD. The structures were solved by
Direct Methods (SHELXS-97) [18] and refined by full-matrix least-
squares on F2 (SHELXL-97) [19]. Multi-scan absorption correc-
tions were applied for 1, 2, 3, 6 and 7. All non-hydrogen atoms
were refined anisotropically and hydrogen atoms by a riding model.
The crystallographic data of 1, 2, 3, 6 and 7 (excluding structure
1H NMR (500 MHz, THF-d8, 25 °C): δ 1.02-1.16 (m, 2H ϩ 4H), 1.20-1.31
(m (br), 2H ϩ 4H, CH2), 1.34 (s, 9H, t-Bu), 1.51-1.59 (m (br), 2H, CH2),
1.63Ϫ1.71 (m (br), 4H, CH2), 1.75Ϫ1.80 (m (br), 2H, CH2), 3.62 (m, 2H,
CH2). 13C NMR (125 MHz, THF-d8, 25 °C): δ 26.7 (C3/C5); 26.9 (C4), 30.6
(CMe3), 38.5 (C2/C6), 40.1 (CMe3), 55.7 (N-C1), 176.5 (CCMe3). EI-MS
(70 eV, 150 °C): m/z (%): 43 (75) [t-BuϪMe]ϩ, 57 (100) [t-Bu]ϩ,
84 (90) [CyH]ϩ, 109 (35) [CyNC]ϩ, 166 (55) [t-BuC(NCy)]ϩ, 181 (95)
[t-BuC(NCy)2ϪCy]ϩ, 206 (25) [t-BuC(NCy)2Ϫt-Bu]ϩ, 263 (85) [t-
BuC(NCy)2]ϩ, 327 (5) [M/2ϪBr]ϩ, 408 (3) [M/2]ϩ. IR (nujol): ν 2717, 1620,
460, 1372, 1264, 1093, 865, 803 cmϪ1
.
508
www.zaac.wiley-vch.de
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2008, 507Ϫ513

Zinc Amidinate Complexes
factors) have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC-648547 (1),
CCDC-644935 (2), CCDC-646888 (3), CCDC-644934 (6), CCDC-
644936 (7). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge, CB21EZ (Tele-
fax: (ϩ44) 1223/336033; E-mail: deposit@ccdc.cam-ak.uk).
Results and Discussion
Equimolar amounts of ZnX2 (X ϭ Cl, Br) and Li[t-BuC-
(NR)2] (R ϭ i-Pr, Cy) dissolved in Et2O react at
ambient temperature with elimination of LiX and
subsequent formation of mono-amidinate complexes
[{t-BuC(NR)2}ZnCl]2 (R
ϭ i-Pr 1, Cy 2) and [t-
BuC(NR)2ZnBr]2 (R ϭ i-Pr 3, Cy 4). Reactions of ZnCl2
with two equivalents Li-amidinate yielded [t-BuC(NR)2]2Zn
(R ϭ i-Pr 5, Cy 6). Pure 1Ϫ6 were obtained after re-crystal-
lization from solutions in Et2O/toluene (1Ϫ4) and hexane
(5, 6), respectively, at Ϫ30 °C as colorless crystalline solids.
In addition, [t-BuC(NCy)2]ZnBr·LiBr(OEt2)2 7 was ob-
tained as a byproduct in low yield from re-crystallization
experiments of 4 in Et2O.
Table 1 Crystallographic details for 1, 2 and 3.
1
2
3
empirical formula
molecular mass
cryst syst
C22H46Cl2N4Zn2 C41H70Cl2N4Zn2 C22H46Br2N4Zn2
568.27
820.65
657.19
monoclinic
P21/n (no.14)
9.4749(11)
10.5814(12)
13.3037(15)
92.461(2)
1332.6(3)
2
monoclinic
P21/c (no.14)
12.2207(15)
17.269(2)
10.2976(13)
104.425(3)
2104.7(5)
2
monoclinic
P21/n (no.14)
9.6188(12)
10.6682(13)
13.4013(16)
91.751(3)
1374.5(3)
2
space group
˚
a /A
˚
b /A
˚
c /A
β /deg
3
˚
V /A
Z
T /K
120(2)
120(2)
120(2)
˚
radiation (λ/A)
Mo-Kα (0.71073) Mo-Kα (0.71073) Mo-Kα (0.71073)
μ /mmϪ1
2.017
1.416
56
1.299
1.295
56
4.668
1.588
56
Dcalcd. /g cmϪ3
2θmax /deg
cryst dimens. /mm
no. of reflns
no. of unique reflns 3171
0.42 x 0.41 x 0.33 0.26 x 0.20 x 0.13 0.28 x 0.22 x 0.21
11465
18369
5013
11783
3268
Rmerg
0.0245
136 / 0
0.0452
235 / 0
0.0397
136 / 0
no. of param.
refined/restraints
R1a
0.0245
0.0634
1.035
0.477 / Ϫ0.300
0.0429
0.1062
1.100
0.783 / Ϫ0.295
0.0284
0.0684
1.033
0.769 / Ϫ0.381
wR2b
goodness of fitc
final max/min.
Scheme 2 Synthesis of Zn-amidinate complexes 1 Ϫ 6.
Ϫ3
˚
Δρ, e A
2
Mass spectra of 1Ϫ4 show the monomeric unit of the
complexes (M/2ϩ), whereas those of 5 and 6 show the mo-
lecular ion peak indicating them to be stable under these
specific conditions. 1H and 13C NMR spectra of 5 and 6 in
C6D6 and THF-d8 each show one set of resonances for the
i-Pr and Cy groups as expected for a Zn complex with dis-
torted tetrahedral environment (C2ν symmetry). The Cy
group in 6 does not undergo ring inversion reactions in
solution as can clearly be seen from the 1H NMR spectrum.
The resonances of the axial H atoms (Hax) are well resolved
with large coupling constants to the axial (dihedral angle
about 180°) and small coupling constants to the equatorial
H atoms (dihedral angle about 60°) of adjacent CH2
groups. The different coupling constants can clearly be seen
for the methine H atom (C1Hax; triplet of triplet) showing
a significantly smaller (3.8 Hz) coupling constant to the
equatorial H atoms than to the axial ones (10.1 Hz) as de-
picted in figure 1. The equatorial H atoms only show large
[a] R1 ϭ Σ(ʈFoΗ Ϫ ΗFcʈ)/ΣΗFoΗ (for I > 2σ(I)). Ϫ [b] wR2 ϭ {Σ[w(Fo Ϫ Fc2)2]/
Σ[w(Fo2)2]}1/2
Nparams)}1/2
.
Ϫ
[c] Goodness of fit
ϭ
{Σ[w(ΗFo2Η
Ϫ
ΗFc2Η)2]/(NobservnsϪ
.
Table 2 Crystallographic details for 6 and 7.
6
7
empirical formula
molecular mass
cryst syst
C34H62N4Zn
592.25
C25H51Br2LiN2O2Zn
643.81
monoclinic
P21/n (no.14)
10.961(7)
17.100(11)
21.196(12)
117.78(3)
3515(4)
monoclinic
P21/n (no.14)
9.4466(11)
17.505(2)
18.741(2)
94.019(3)
3091.4(6)
4
space group
˚
a /A
˚
b /A
˚
c /A
β /deg
3
˚
V /A
Z
T /K
4
293(2)
120(2)
˚
radiation (λ/A)
Mo-Kα (0.71073) Mo-Kα (0.71073)
μ /mmϪ1
0.725
1.119
56
3.401
1.383
56
Dcalcd. /g cmϪ3
2θmax /deg
2
geminal JHH couplings yielding broad doublets with the
cryst dimens /mm
no. of reflns
no. of unique reflns
Rmerg
0.41 x 0.35 x 0.22 0.43 x 0.24 x 0.18
28357
8352
0.1124
27008
7357
0.0455
298 / 0
0.0333
0.0803
1.006
broadening resulting from small 3JHH couplings to the mag-
netically non-equivalent axial and equatorial H atoms of
the adjacent CH2 groups (spectrum of higher order). The
assignment of the 1H resonances of 6 is based on HH-
no. of param. refined/restraints 352 / 0
R1a
0.0772
0.2157
1.008
0.957 / Ϫ0.929
wR2b
1
COSY and DPFGSE-NOE spectroscopy studies. The H
goodness of fitc
Ϫ3
˚
NMR spectra of 1Ϫ4 in THF-d8 also show only one set of
resonances due to the organic substituents in the expected
ratio despite the non-equivalence of the i-Pr and Cy groups
bonded to the three- and four-coordinated N atoms.
final max/min. Δρ, e A
0.534 / Ϫ0.335
2
[a] R1 ϭ Σ(ʈFoΗ Ϫ ΗFcʈ)/ΣΗFoΗ (for I > 2σ(I)). Ϫ [b] wR2 ϭ {Σ[w(Fo Ϫ Fc2)2]/
Σ[w(Fo2)2]}1/2. Ϫ [c] Goodness of fit ϭ {Σ[w(ΗFo2Η Ϫ ΗFc2Η)2]/(NobservnsϪ
Nparams)}1/2
.
Z. Anorg. Allg. Chem. 2008, 507Ϫ513
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
509

T. Eisenmann, J. Khanderi, S. Schulz, U. Flörke
Figure 1 1H NMR spectrum of 6 including the assignment of the proton resonances of the Cy groups.
The solid-state structures of compounds 1, 2, 3 and 6
were determined by single crystal X-ray diffraction. Suitable
crystals were obtained from solutions in toluene/Et2O (95:5;
1, 2, 3) and Et2O (6), respectively, at Ϫ30 °C. In addition,
very few crystals of 7 were obtained from re-crystallization
experiments at Ϫ30 °C of a solution of 3 prepared in Et2O.
1, 3, 6 and 7 crystallize in the monoclinic space group
P(2)1/n, 2 in P21/c. 1Ϫ3 are centrosymmetric and form di-
meric complexes in the solid state with bridging N-centers
and the center of symmetry in the Zn2N2 ring. In contrast,
the comparable Mg complex [HC(NCy)2Mg(thf)(μ-Cl)2(μ-
0.5thf)]2 contains chlorine bridges between the two Mg cen-
ters [20]. 6 and 7 are monomeric in the solid state with the
Lewis acidic Zn atom in 7 coordinated by one additional
Br atom and [Li(OEt2)2]ϩ as counter ion as was previously
observed in [Li(thf)2(μ-Br)2Zn{C(SiMe3)(SiMe2NPhMe)}]
[21] and [Li(OEt2)2(μ-Cl)2Zn{N(Dipp)C(Me)}2CH] [5b].
The Zn atoms in 1Ϫ7 adopt distorted tetrahedral coordi-
nation spheres, the Zn-X bond lengths (X ϭ Cl: 2.205(1) 1,
˚
2.201(1) 2; X ϭ Br, 2.339(1) 3 A) are within typical ranges
Figure 2 Molecular structure and atom numbering scheme of 2;
thermal ellipsoids are drawn at the 50 % probability level. H atoms
have been omitted for clarity.
Figure 3 Molecular structure and atom numbering scheme of 3;
thermal ellipsoids are drawn at the 50 % probability level. H atoms
have been omitted for clarity.
510
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© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2008, 507Ϫ513

Zinc Amidinate Complexes
Figure 5 Molecular structure and atom numbering scheme of 7;
thermal ellipsoids are drawn at the 50 % probability level. H atoms
have been omitted for clarity.
Figure 4 Molecular structure and atom numbering scheme of 6;
thermal ellipsoids are drawn at the 50 % probability level. H atoms
have been omitted for clarity.
˚
Table 4 Selected bond lengths /A and angles /° of 6 and 7.
of terminal Zn-X bond lengths [22]. The Zn-Br bond
lengths in 7 (2.407(1), 2.424(1) A) are comparable to those
˚
6
7
in [Li(thf)2(μ-Br)2Zn{C(SiMe3)(SiMe2NPhMe)}] (2.403(1),
˚
Zn1-N1 / Zn1-N1A
Zn1-N2
2.045(3)
2.023(3)
2.020(3) / 2.054(3)
2.015(2)
2.009(2)
2.441(1) A) [21].
The most remarkable structural difference between 1, 2
and 3 on the one hand and 6 and 7 on the other hand is
reflected by the different coordination modes of the amidin-
ate ligand. The N atoms in the bis-amidinate complexes 6
and 7 are threefold coordinated and the Zn-N bond lengths
are almost equal (2.020(3), 2.023(3), 2.045(3), 2.054(3) 6;
Zn1-N3 / Zn1-N4
Zn1-X (X ϭ Cl, Br)
Li1-Br1 / Li1-Br2
2.424(1) / 2.407(1)
2.592(4) / 2.570(4)
1.342(3) / 1.326(3)
C1-N1 / C1-N2
1.349(4) / 1.355(5)
1.364(5) / 1.338(5)
1.480(4) / 1.474(4)
1.462(5) / 1.476(5)
65.6(2) / 65.7(2)
131.6(2) / 124.9(2)
109.2(3) / 109.7(3)
126.7(3) / 123.9(3)
121.2(3) / 121.2(3)
131.6(2) / 135.5(2)
C6-N3 / C6-N4
N1-C11 / N2-C21
N3-C31 / N4-C41
N1-Zn1-N2 / N3-Zn1-N4
N1-Zn1-N3 / N1-Zn1-N4
N1-C1-N2 / N3-C6-N4
C2-C1-N1 / C2-C1-N2
C7-C6-N3 / C7-C6-N4
C11-N1-Zn1 / C21-N2-Zn1
1.459(3) / 1.462(3)
65.5(1)
˚
2.009(2), 2.015(2) A 7). The sum of the bond angles of the
109.2(2)
122.9(2) / 127.8(2)
central carbon atom of the amidinate ligand (C1 359.7, C6
359.9 6; 359.9° 7) and of the nitrogen atoms (N1 353.6, N2
359.5; N3 359.7, N4 359.8 6; N1 358.4, N2 359.0° 7) prove
the amidinate ligand to be nearly planar with sp2-hy-
bridized C and N atoms. Moreover, the almost equal C-
135.4(2) / 132.3(2)
N bond lengths as observed in 6 (C1-N1 1.349(4), C1-N2
˚
˚
1.355(5), C6-N3 1.364(5), C6-N4 1.338(5) A) and 7 (C1-N1
Table 3 Selected bond lengths /A and angles /° of 1, 2 and 3.
˚
1.342(3), C1-N2 1.326 (3) A) are in between typical values
1
2
3
for C-N single and CϭN double bonds. These structural
findings clearly point to an almost perfect delocalization
of the π-electrons within the CN2 backbone of the four-
membered metallacycle.
Zn1-N1 / Zn1-N1A
Zn1-N2
2.146(2) / 2.038(2) 2.118(2) / 2.049(2) 2.145(2) / 2.039(2)
2.078(2)
2.205(1)
2.076(2)
2.201(1)
2.077(2)
2.339(1)
Zn1-X (X ϭ Cl, Br)
Li1-Br1 / Li1-Br2
1: C7-N1 / C7-N2
2, 3: C1-N1 / C1-N2
1: N1-C1 / N2-C4
2: N1-C11 / N2-C21
3: N1-C9 / N2-C6
N1-Zn1-N2 / N3-Zn1-N4
1.432(2) / 1.292(2) 1.428(3) / 1.293(3) 1.432(3) / 1.291(3)
1.499(2) / 1.479(2) 1.500(3) / 1.476(3) 1.495(3) / 1.481(3)
In sharp contrast, the Zn-N bond lengths in 1, 2 and 3
vary significantly in accordance with the different coordi-
nation numbers of the N-atoms (three vs. four). As is ex-
pected, the Zn-N bond lengths of the four-coordinate N-
64.2(1)
69.5(1)
64.4(1)
N1-Zn1-N1A / Zn1-N1-Zn1A 92.2(1) / 87.9(1)
91.2(1) / 88.8(1)
110.6(2)
92.5(1) / 87.6(1)
111.3(2)
˚
1: N1-C7-N2
111.0(2)
atoms (Zn1-N1 2.146(2) 1; 2.118(2) 2; 2.145(2) A 3) are
2, 3: N1-C1-N2 / N3-C6-N4
1: C8-C7-N1 / C8-C7-N2
2, 3: C2-C1-N1 / C2-C1-N2
CX-N1-Zn1 / CY-N2-Zn1
1: X ϭ 1, Y ϭ 4
2: X ϭ 11, Y ϭ 21
3: X ϭ 9, Y ϭ 6
elongated compared to those of the three-coordinate N-
123.1(2) / 125.7(2) 122.7(2) / 126.0(2) 122.9(2) / 125.6(2)
117.6(1) / 136.7(2) 118.9(2) / 136.5(2) 118.2(2) / 136.7(2)
˚
atom (Zn1-N2 2.078(2) 1; 2.076(2) 2; 2.077(2) A 3). Surpris-
ingly, the Zn1-N1A bond lengths in 1Ϫ3 are the shortest
ones even though the N atoms are fourfold-coordinate. In
addition, the amidinate ligand significantly differs from
Z. Anorg. Allg. Chem. 2008, 507Ϫ513
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
511

T. Eisenmann, J. Khanderi, S. Schulz, U. Flörke
We gratefully acknowledge financial support by the Deutsche
Forschungsgemeinschaft (DFG). We also like to thank Dr. Hans
Egold, University of Paderborn, for recording COSY and NOE
spectra of compound 6.
planarity as shown by the sum of the bond angles. The
three-coordinate carbon and nitrogen atoms in 1 (C7 359.8;
N2 359.6°), 2 (C1 359.3; N2 359.9°) and 3 (C1 359.8; N2
359.6°) are almost planar, indicating sp2-hybridized centers,
whereas significantly smaller sum of bond angles are ob-
served for the four-coordinate nitrogen atoms (N1 319.7 1;
325.5 2; 320.6° 3) [23]. Moreover, the C-N bond lengths
within the amidinate moiety of 1 (C7-N1 1.432(2), C7-N2
References
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˚
˚
1.292(2) A), 2 (C1-N1 1.428(3), C1-N2 1.293(3) A) and 3
˚
˚
(C1-N1 1.432(3), C1-N2 1.291(3) A) vary by almost 0.14 A,
clearly revealing the π-electrons to be rather localized at N1,
consequently allowing the intermolecular coordination to
Zn1A. According to these structural findings, the coordi-
nation mode of the amidinate moiety within the complexes
1Ϫ3 is better described with the resonance structure B (fig-
ure 6), whereas that within 6 and 7 better agrees with reson-
ance structure A.
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Figure 6 Different resonance structures of the amidinate moieties
in complexes 1 Ϫ 6.
The almost identical amidinate bite angles (endocyclic
N-Zn-N bond angle: 64.2(1) 1; 64.5(1) 2; 64.4(1) 3; 65.6(2),
65.7(2) 6, 65.5(1)° 7) are within the typical range
(53.8Ϫ84.5°) of amidinate complexes containing four-coor-
dinate metal centers (based on 100 entries in the CSD data-
base), but slightly smaller than the mean value of 68.2° [24].
The rather acute endocyclic bond angles indicate consider-
able ring strain to be present in these complexes. Conse-
quently, the exocyclic N1/2-Zn1-X1/2 bond angles (average
values: 114.2 1; 113.3 2; 114.1 3; 134.3 6; 121.6° 7) are sig-
nifically widened [25]. In particular the large exocyclic
N-Zn-N bond angles in 6 indicates high steric pressure
within this bis-amidinate complex. The exocyclic C-N-C
and C-C-N bond angles of the amidinate backbones of 1,
2, 3, 6 and 7 are very similar.
[8] a) I. Resa, E. Carmona, E. Gutierrez-Puebla, A. Monge, Sci-
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ence 2004, 305, 1136; b) D. del Rıo, A. Galindo, I. Resa, E.
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B. King, P. v. R. Schleyer, H. F. Schaefer, III, G. H. Robinson,
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b) J. A. R. Schmidt, J. Arnold, J. Chem. Soc., Dalton Trans.
2002, 2890Ϫ2899.
Conclusion
Four mono-amidinate and two bis-amidinate complexes
have been prepared by reactions of zinc halides with the
corresponding Li-amidinate and characterized by single
crystal X-ray analysis. The mono-amidinate complexes,
which forms N-bridged dimers in the solid state, may be
valuable starting reagents for further substitution reactions
(exchange of the halide substituents) as well as for re-
duction reactions, probably yielding Zn-Zn bonded species.
Further investigations are currently in progress.
[13] a) M. P. Coles, P. B. Hitchcock, Eur. J. Inorg. Chem. 2004,
2662Ϫ2672; b) S. J. Birch, S. R. Boss, S. C. Cole, M. P. Coles,
R. Haigh, P. B. Hitchcock, A. E. H. Wheatley, J. Chem. Soc.,
Dalton Trans. 2004, 3568Ϫ3574.
512
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© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2008, 507Ϫ513

Zinc Amidinate Complexes
[14] a) F. A. Cotton, L. M. Daniels, L. R. Falvello, J. H. Matonic,
C. A. Murillo, X. Wang, H. Zhou, Inorg. Chim. Acta 1997,
266, 91Ϫ102; b) M. L. Cole, D. J. Evans, P. C. Junk, L. M.
Louis, New J. Chem. 2002, 26, 1015Ϫ1024.
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1997, 16, 5183Ϫ5194.
[22] Based on more than 1000 entries in the CSD database, ter-
minal Zn-Cl bond lengths of four-coordinated Zn atoms range
˚
˚
from 2.068 Ϫ 2.416 A (mean bond length of 2.248 A), whereas
˚
terminal Zn-Br bond lengths range from 2.278 Ϫ 2.548 A
˚
(mean bond length of 2.38 A based on more than 200 entries).
Cambridge Structural Database (CSD) Version 5.28, Nov-
ember 2006 including update Mai 2007).
[23] The sum of bond angles is defined as Σ(C1-N1-C9/11 ϩ C1-
N1-Zn1 ϩ C9/11-N1-Zn1.
[24] 560 amidinate complexes with metal centers in different coor-
dination environments were found in the CSD database. The
bite angle ranges from 52 Ϫ 86° with a mean value of 62.1°.
Cambridge Structural Database (CSD) Version 5.28, Nov-
ember 2006 including update Mai 2007).
[18] G. M. Sheldrick, SHELXS-97, Program for Structure Solu-
tion: Acta Crystallogr. 1990, A46, 467.
[19] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, Universität Göttingen, 1997.
[20] F. A. Cotton, S. C. Haefner, J. H. Matonic, X. Wang, C. A.
Murillo, Polyhedron 1997, 16, 541Ϫ550.
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Hitchcock, J. D. Smith, J. Organomet. Chem. 2004, 689,
1718Ϫ1722.
[25] 1: X1 ϭ Cl1, X2 ϭ N1A; 2: X1 ϭ Cl1, X2 ϭ N1A; 3: X1 ϭ
Br1, X2 ϭ N1A; 6: X1 ϭ N3, X2 ϭ N4; 7: X1 ϭ Br1,
X2 ϭ Br2.
Z. Anorg. Allg. Chem. 2008, 507Ϫ513
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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