A general high-yield route to bis(salicylaldimine) zinc(II) complexes: Application to the synthesis of pyridine-modified salen-type zinc(II) complexes

Article abstract of DOI:10.1021/ic010090o

A general, direct, and high-yield synthesis of bis(salicylaldimine) zinc complexes from the ligands and Et₂Zn is reported. This synthetic method is particularly valuable, not only because it allows the efficient preparation of salen-type complexes of zinc but also because it can be used to prepare bifunctional pyridine-modified zinc(II) bis(salicylidene) complexes, which are potentially useful compounds for applications in asymmetric catalysis and materials chemistry. The synthesis and complete structural characterization of a new series of pyridine-modified zinc(II) bis(salicylidene) ligands is discussed.

Full text of DOI:10.1021/ic010090o

3222
Inorg. Chem. 2001, 40, 3222-3227
A General High-Yield Route to Bis(salicylaldimine) Zinc(II) Complexes: Application to the
Synthesis of Pyridine-Modified Salen-Type Zinc(II) Complexes
Gregory A. Morris, Hongying Zhou, Charlotte L. Stern, and SonBinh T. Nguyen*
Department of Chemistry and Institute for Environmental Catalysis, Northwestern University,
2145 Sheridan Road, Evanston, Illinois 60208
ReceiVed January 23, 2001
A general, direct, and high-yield synthesis of bis(salicylaldimine) zinc complexes from the ligands and Et2Zn is
reported. This synthetic method is particularly valuable, not only because it allows the efficient preparation of
salen-type complexes of zinc but also because it can be used to prepare bifunctional pyridine-modified zinc(II)
bis(salicylidene) complexes, which are potentially useful compounds for applications in asymmetric catalysis and
materials chemistry. The synthesis and complete structural characterization of a new series of pyridine-modified
zinc(II) bis(salicylidene) ligands is discussed.
Introduction
led us to investigate salen complexes of zinc-containing pendant
Lewis base groups. Our primary target was zinc(II) salen
complex 1, featuring a pair of pyridyl groups oriented at 180°
with respect to each other. By reacting the bifunctional zinc
salen “edges” with transition-metal “corners” we envisioned the
coordinative self-assembly of such a complex into larger
assemblies, e.g., molecular squares, triangles, rectangles, and
so on.
Though there are several reports on the preparation of zinc-
(II) salen complexes from either Zn(OAc)214,15 or ZnCl2/Et3N,7
we had difficulties isolating pure metalated product following
these routes. In our hands, using the common salen-type ligand
2a, the ZnCl2/Et3N method gave a product mixture con-
taining equal amounts of Schiff base ligand and zinc salen
complex that could not be readily separated. This observation
is consistent with literature reports in which a related zinc Schiff
base complex was eventually isolated by a recrystallization
procedure that involved cooling the mixture to -30 °C for 2
weeks.7
As a potential solution to the aforementioned synthetic
problem, we considered the direct reaction of the salen ligand
and Et2Zn as an alternative method for the preparation of zinc-
(II) salen-type complexes. Given the highly acidic nature of the
ligand’s phenolic protons and the highly reactive nature of
Et2Zn, we predicted that the only byproduct for this reaction
would be ethane, which could be easily removed to afford pure
zinc salen complex in high yield. To our surprise, there have
been no direct reports of such a strategy to prepare isolated zinc
salen complexes.16 Herein, we report a general procedure for
the direct synthesis of zinc(II) salen-type complexes using this
method. We also disclose the successful application of this
methodology to the synthesis of the bifunctional complex 1.
Salen-type ligands derived from salicylaldehydes and di-
amines have emerged as one of the most important ligand
systems in asymmetric catalysis. Over the past decade, the
number of applications for metal salen1 complexes has grown
rapidly to encompass an extremely broad range of chemical
transformations, including the asymmetric ring-opening of
epoxides, aziridination, cyclopropanation, and the epoxidation
of olefins.2 This versatility in chemical reactivity and selectivity
is a result of the ability of salen-type ligands to complex a
variety of metals with a large number of oxidation states in an
easily tunable chiral environment. As such, there has been
considerable interest in the synthesis of new salen-type com-
plexes of transition3-5 and main group6,7 metals to further
develop applications in both catalysis and materials chemistry.
Recently there has been a surge of interest in the use of chiral
zinc salen complexes in asymmetric Lewis acid-catalyzed
reactions.8-10 Furthermore, our own search for a bifunctional
salen-type template to be used in the synthesis of cyclic
supramolecular structures of the Fujita11,12 and Stang13 varieties
* Author to whom correspondence should be addressed. E-mail: stn@
chem.nwu.edu.
(1) For the remainder of this article, the term salen will be used to describe
the general class of bis(salicylaldimine) ligands and complexes.
(2) Canali, L.; Sherrington, D. C. Chem. Soc. ReV. 1999, 28, 85-93.
(3) Leung, W. H.; Chan, E. Y. Y.; Chow, E. K. F.; Williams, I. D.; Peng,
S. M. J. Chem. Soc., Dalton Trans. 1996, 1229-1236.
(4) Stinziano-Eveland, R. A.; Nguyen, S. T.; Liable-Sands, L. M.;
Rheingold, A. L. Inorg. Chem. 2000, 39, 2452-2455.
(5) Bohle, D. S.; Zafar, A.; Goodson, P. A.; Jaeger, D. A. Inorg. Chem.
2000, 39, 712-718.
(6) Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 5315-
5316.
(7) Singer, A. L.; Atwood, D. A. Inorg. Chim. Acta 1998, 277, 157-
Experimental Section
162.
(8) Meng, Q.; Li, Y.; He, Y.; Guan, Y. Tetrahedron: Asymmetry 2000,
11, 4255-4261.
(9) Mino, T.; Oishi, K.; Yamashita, M. Synlett 1998, 965-966.
(10) Cozzi, P. G.; Papa, A.; Umani-Ronchi, A. Tetrahedron Lett. 1996,
37, 4613-4616.
(11) Fujita, M.; Ogura, K. Coord. Chem. ReV. 1996, 148, 249-264.
(12) Fujita, M. Chem. Soc. ReV. 1998, 27, 417-425.
(13) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853-
907.
General Information. Melting points were uncorrected and were
determined on a Fisher-Johns melting point apparatus. 1H and 13C NMR
(14) O’Conner, M. J.; West, B. O. Aust. J. Chem. 1967, 20, 2077.
(15) Bately, G. E.; Graddon, D. P. Aust. J. Chem. 1967, 20, 885-891.
(16) We note that a Zn salen complex has been postulated to be a catalytic
intermediate in the enantioselective addition of Et2Zn to aldehydes in
the presence of salen ligands. See ref 10.
10.1021/ic010090o CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/24/2001

Route to Bis(salicylaldimine) Zinc(II) Complexes
Inorganic Chemistry, Vol. 40, No. 13, 2001 3223
spectra were recorded on either a Varian Inova 500 (499.773 MHz for
(d, 2H, J ) 1.8 Hz), 3.45 (m, 2H), 2.09-1.48 (m, 8H), 1.44 (s, 18H).
13C NMR (125 MHz, CDCl3): δ 165.6, 161.8, 150.0, 148.3, 138.5,
128.4, 128.1, 127.4, 121.0, 118.9, 72.5, 35.2, 33.0, 29.4, 24.4. EIMS:
m/z 588 (M+, 100), 446 (24), 334 (86), 279 (13), 239 (23), 211 (14),
83 (22). HREIMS: exact mass calcd for [C38H44N4O2]+ 588.3464, found
588.3460. Anal. Calcd for C38H44N4O2: C, 77.52; H, 7.53; N, 9.52.
Found: C, 77.28; H, 7.54; N, 9.38.
1
1H, 125.669 MHz for 13C) or a Mercury 400 (400.178 MHz for H)
1
spectrometer. H NMR data are reported as follows: chemical shift
(multiplicity (b ) broad, s ) singlet, d ) doublet, t ) triplet, q )
1
quartet, and m ) multiplet), integration). H and 13C chemical shifts
are reported in ppm downfield from tetramethylsilane (TMS, δ
scale) with the solvent resonances as internal standards. IR data were
collected on a Nicolet 5PC FT-IR spectrometer with PC-IR soft-
ware. Mass spectra were obtained from the Mass Spectrometry
Laboratory, University of Illinois (Urbana, IL). Elemental analyses
were provided by Atlantic Microlab, Inc. (Norcross, GA). All reactions
were carried out under a dry nitrogen atmosphere using standard
Schlenk techniques unless otherwise noted. Flash column chromatog-
raphy was carried out with 230-400 mesh silica gel, purchased from
Merck.
Materials. Schiff base ligands 2a,17 2b,18 2d,19 1,1,2,2-tetramethyl-
1,2-ethylenediamine,20 and aldehyde 621 were synthesized according
to published procedures. (1R,2R)-(-)-1,2-diaminocyclohexane was
obtained from Aldrich (at >98% ee). Dichloromethane was distilled
over calcium hydride. Tetrahydrofuran (THF) and diethyl ether were
distilled over sodium/benzophenone. All solvents were distilled under
nitrogen and saturated with nitrogen prior to use. Deuterated solvents
were purchased from Cambridge Isotope Laboratories and used without
further purification. All other reagents were purchased from the Aldrich
Chemical Co. and used without further purification, unless otherwise
noted.
N,N′-Bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,2-diaminoben-
zene (5b): an orange solid (mp 268-269 °C). Yield ) 95%. IR
(KBr): 3467, 2946, 1611, 1592, 1571, 1438, 1254, 1168, 821, 749,
1
625 cm-1. H NMR (500 MHz, CDCl3): δ 8.79 (s, 2H), 8.64 (d, 4H,
J ) 4.7 Hz), 7.69 (s, 2H), 7.58 (s, 2H), 7.50 (d, 4H, J ) 4.7 Hz), 7.40
(m, 2H), 7.33 (m, 2H), 1.49 (s, 18H). 13C NMR (125 MHz, CDCl3):
δ 164.2, 162.0, 150.3, 148.1, 124.3, 139.2, 129.3, 129.3, 128.2, 121.2,
120.0, 119.6, 35.4, 29.5. EIMS: m/z 582 (M+, 16), 446 (100), 418
(17), 387 (15), 331 (37). HREIMS: exact mass calcd for [C38H38N4O2]+
582.2995, found 582.2986. Anal. Calcd for C38H38N4O2: C, 78.32; H,
6.57; N, 9.60. Found: C, 78.38; H, 6.48; N, 9.59.
N,N′-Bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,1,2,2-tetramethyl-
1,2-diaminoethane (5c): a yellow solid (mp 264-265 °C). Yield )
94%. IR (KBr): 3427, 3020, 2988, 2947, 2910, 2866, 1627, 1593, 1466,
1
1442, 1274, 1256, 819, 622 cm-1. H NMR (500 MHz, CDCl3): δ
14.83 (s, 2H), 8.60 (d, 4H, J ) 5.2 Hz), 8.46 (s, 2H), 7.58 (s, 2H),
7.43 (d, 4H, J ) 5.2 Hz), 7.42 (s, 2H), 1.48 (s, 12H), 1.46 (s, 18H).
13C NMR (125 MHz, CDCl3): δ 162.5, 162.4, 150.3, 148.2, 138.7,
128.7, 128.0, 122.2, 121.0, 119.0, 65.5, 35.2, 29.4, 23.4. FABMS (NBA)
m/z 591 (MH+, 100), 477 (12), 296 (51), 255 (11), 154 (33). HRFABMS
(NBA): exact mass calcd for [C38H47N4O2]+ 591.3699, found 591.3700.
Anal. Calcd for C38H46N4O2: C, 77.25; H, 7.85; N, 9.48 Found: C,
77.00; H, 7.87; N, 9.50.
General Procedure for the Synthesis of 1a-c and 3a-e. To a
stirred solution of Schiff base ligand (0.91 g, 1.5 mmol) in THF (100
mL) at room temperature was added Et2Zn (159 µL, 1.5 mmol) in a
glovebox under N2 atmosphere. Compounds 3a-e were prepared
following the same procedure but with hexane as the solvent. The
mixture was stirred at room temperature for 12 h, after which the
precipitate was filtered and washed with water (200 mL). The solid
was dried under reduced pressure.
General Procedure for the Synthesis of 2c, 2e, and 5a-c. A
mixture of aldehyde (2.5 g, 9.8 mmol), diamine (0.5 equiv), and EtOH
(25 mL) was heated to reflux in a 100 mL round bottom flask equipped
with a water-cooled West condenser for 1 h before being allowed to
cool to room temperature. If the product precipitated out from the
reaction mixture, it was isolated by filtration. If the product was soluble
in ethanol, the solvent was removed in vacuo to isolate the product.
N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,1,2,2-tetramethyl-1,2-di-
aminoethane (2c): a yellow solid (mp 190.5-192.0 °C). Yield ) 86%.
IR (KBr): 2987, 2958, 2904, 2866, 1628, 1592, 1468, 1438, 1389,
1
1374, 1359, 1272, 1250, 1170, 1128, 976 cm-1. H NMR (400 MHz,
CD2Cl2): δ 14.41 (s, 2H), 8.43 (s, 2H), 7.37 (d, 2H, J ) 2.0 Hz), 7.16
(d, 2H, J ) 2.0 Hz), 1.44 (s, 18H), 1.42 (m, 12H), 1.31 (s, 18H). 13C
NMR (125 MHz, CD2Cl2): δ 163.1, 158.7, 140.2, 136.8, 127.1, 126.7,
118.4, 65.5, 35.5, 34.6, 31.8, 29.8, 23.6. FABMS (NBA): m/z 548
(M+, 74), 533 (51), 274 (100), 258 (15), 234 (18), 218 (15). Anal.
Calcd for C36H56N2O2: C, 78.78; H, 10.28; N, 5.10. Found: C, 78.70;
H, 10.32; N, 5.12.
(R,R)-(-)-1,2-Cyclohexanediamino-N,N′-bis(3-tert-butyl-5-(4-py-
ridyl)salicylidene) zinc(II) (1a): a yellow solid (mp 315-316 °C).
Yield ) 92%. IR (KBr): 3427, 2941, 2860, 1629, 1590, 1548, 1527,
1401, 1383, 1351, 1335, 1297, 1288, 1221, 1159, 815, 794, 773
1
cm-1. H NMR (500 MHz, pyridine-d5): δ 8.83 (d, 4H, J ) 5.5 Hz),
8.62 (s, 2H), 8.01 (d, 2H, J ) 2.0 Hz), 7.88 (d, 2H, J ) 2.0 Hz),
7.76 (d, 4H, J ) 5.5 Hz), 3.13 (m, 2H), 2.37-1.25 (m, 8H), 1.83 (s,
18H). 13C NMR (125 MHz, pyridine-d5): δ 173.6, 166.5, 151.2, 149.3,-
143.9, 133.6, 128.8, 121.2, 120.8, 120.7, 66.0, 36.5, 30.3, 28.6, 24.9.
FABMS (NBA): m/z 651 (MH+, 50), 589 (50), 307 (20), 289 (15),
154 (100), 136 (82). HRFABMS (NBA): exact mass calcd for
[C38H43N4O2Zn]+ 651.2677, found 651.2676. Anal. Calcd for C38H42N4O2-
Zn‚1/2H2O: C, 69.03; H, 6.56; N, 8.47. Found: C, 69.12; H, 6.47; N,
8.36.
N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,3-diaminobenzene (2e):
a yellow solid (mp 209-210 °C). Yield ) 89%. IR (KBr): 2954, 2905,
2867, 1619, 1571, 1476, 1467, 1438, 1360, 1272, 1250, 1200, 1174,
1
965 cm-1. H NMR (400 MHz, CD2Cl2): δ 13.66 (s, 2H), 8.77 (s,
2H), 7.51 (m, 2H), 7.32-7.26 (m, 2H), 7.31 (m, 2H), 1.49 (s, 18H),
1.36 (s, 18H). 13C NMR (125 MHz, CD2Cl2): δ 164.8, 158.6, 150.1,
141.1, 137.2, 130.7, 128.7, 127.5, 119.8, 118.7, 114.3, 35.6, 34.7, 31.8,
29.8. FABMS (NBA): m/z 541 (MH+, 100), 525 (72), 485 (18), 154
(18), 136 (15). HRFABMS (NBA): exact mass calcd for [C36H48N2O2]+
540.3716, found 540.3717. Anal. Calcd for C36H48N2O2: C, 79.96; H,
8.95; N, 5.18. Found: C, 80.04; H, 9.03; N, 5.24.
1,2-Benzenediamino-N,N′-bis(3-tert-butyl-5-(4-pyridyl)sali-
cylidene) zinc(II) (1b): a yellow solid (mp 327.5-328.5 °C). Yield )
86%. IR (KBr): 3429, 1595, 1577, 1544, 1526, 1383, 1257, 1158, 819,
1
795, 772, 710 cm-1. H NMR (400 MHz, CDCl3): δ 8.81 (s, 2H),
(R,R)-(-)-N,N′-Bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,2-di-
aminocyclohexane (5a): recrystallized from 5:1 hexane/CH2Cl2 to give
a white solid. An analytical sample was prepared by flash column
chromatography [CH2Cl2/MeOH 10:1 (6 × 10 cm silica gel)] to give
a white solid (mp 212-213 °C). Yield ) 73%. IR (KBr): 3400, 3066,
3022, 2936, 2860, 1628, 1592, 1465, 1441, 1273, 1170, 1075, 819,
8.31 (d, 4H, J ) 5.5 Hz), 7.62 (m, 2H), 7.58 (s, 2H), 7.45 (s, 2H),
7.30 (d, 4H, J ) 5.5 Hz), 7.33 (m, 2H), 1.52 (s, 18H). 13C NMR (125
MHz, CDCl3): δ 174.3, 162.3, 150.0, 149.0, 144.2, 140.0, 133.1, 129.2,
127.7, 120.7, 120.5, 119.7, 115.9, 35.9, 29.6. FABMS (NBA): m/z
645 (MH+, 14), 583 (98), 346 (22), 309 (11), 240 (13), 195 (16), 155
(42), 135 (50), 119 (100). HRFABMS (NBA): exact mass calcd for
[C38H37N4O2Zn]+ 645.2208, found 645.2209. Anal. Calcd for C38H36N4O2-
Zn‚1/2H2O: C, 69.67; H, 5.69; N, 8.55. Found: C, 69.60; H, 5.61; N,
8.40.
1
623 cm-1. H NMR (500 MHz, CDCl3): δ 8.57 (d, 4H, J ) 5.6 Hz),
8.35 (s, 2H), 7.51 (d, 2H, J ) 1.8 Hz), 7.35 (d, 4H, J ) 5.6 Hz), 7.27
(17) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J.
Am. Chem. Soc. 1991, 113, 7063-7064.
1,1,2,2-Tetramethyl-1,2-ethanediamino-N,N′-bis(3-tert-butyl-5-(4-
pyridyl)salicylidene) zinc(II) (1c): a yellow solid (mp 324-325 °C).
Yield ) 89%. IR (KBr): 3429, 1595, 1577, 1544, 1526, 1383, 1257,
(18) Herrmann, W. A.; Rauch, M. U.; Artus, G. R. J. Inorg. Chem. 1996,
35, 1988-1991.
1
1158, 819, 795, 772, 710 cm-1. H NMR (500 MHz, CDCl3): δ 8.50
(19) Hill, M. S.; Atwood, D. A. Eur. J. Inorg. Chem. 1998, 67-72.
(20) Sayre, P. J. Am. Chem. Soc. 1955, 77, 6689-6690.
(21) Morris, G. A.; Nguyen, S. T. Tetrahedron Lett. 2001, 42, 2093-2096.
(s, 2H), 8.31 (d, 4H, J ) 4.5 Hz), 7.61 (s, 2H), 7.49 (d, 4H, J ) 4.5
Hz), 7.46 (s, 2H), 1.43 (s, 18H), 1.28 (s, 12H). 13C NMR (125 MHz,

3224 Inorganic Chemistry, Vol. 40, No. 13, 2001
Morris et al.
CDCl3): δ 173.4, 165.4, 150.1, 149.1, 143.8, 132.7, 127.9, 120.1, 119.8,
65.2, 35.8, 29.5, 23.9. FABMS (NBA): m/z 653 (MH+, 100), 294 (15),
278 (13), 262 (15), 237 (14), 195 (16), 155 (42), 135 (50), 119 (100).
HRFABMS (NBA): exact mass calcd for [C38H45N4O2Zn]+ 653.2834,
found 653.2834.
MHz, CD2Cl2): δ 170.0, 169.9, 150.9, 142.2, 136.4, 131.5, 130.3, 130.0,
120.6, 118.1, 114.5, 35.7, 34.0, 31.3, 29.3. Anal. Calcd for C33H50N2O2-
Zn‚CH2Cl2: C, 64.91; H, 7.17; N, 3.98. Found: C, 64.67; H, 7.05; N,
4.05.
X-ray Structure Determination for rac-3a‚pyridine. Single crystals
of rac-3a‚pyridine were obtained by slow evaporation from pyri-
dine. X-ray diffraction analysis was carried out on a Bruker SMART-
1000 CCD area detector with graphite-monochromated Mo KR radia-
tion. The data were collected at a temperature of -120 °C to a
maximum 2θ value of 56.6°. Data were collected in 0.30° oscillations
with 15.0 s exposures. The crystal-to-detector distance was 50.00
mm. The detector swing angle was 28.00°. The final unit cell was
obtained by the least-squares refinement of 7047 reflections. The
structure was solved by direct methods (SHELXS97) and expanded
using Fourier techniques (DIRDIF94). All non-hydrogen atoms were
refined anisotropically. All calculations were performed using the
teXsan crystallographic software package of Molecular Structure
Corporation. The ORTEP drawing of the molecule was realized with
thermal ellipsoids at the 50% probability level for non-hydrogen
atoms.
X-ray Structure Determination for 1b‚piperidine. Single crystals
of 1b‚piperidine were obtained in piperidine/hexane (8:1). X-ray
diffraction analysis was carried out on a Bruker SMART-1000 CCD
area detector with graphite-monochromated Mo KR radiation. The data
were collected at a temperature of -120 °C to a maximum 2θ value
of 56.5°. Data were collected in 0.30° oscillations with 25.0 s exposures.
The crystal-to-detector distance was 50.00 mm. The detector swing
angle was 28.00°. The final unit cell was obtained by the least-squares
refinement of 4911 reflections. The structure was solved by direct
methods (SHELXS97) and expanded using Fourier techniques
(DIRDIF94). All non-hydrogen atoms were refined anisotropically. All
calculations were performed using the teXsan crystallographic software
package of Molecular Structure Corporation. The ORTEP drawing of
the molecule was realized with thermal ellipsoids at the 50% probability
level for non-hydrogen atoms.
(R,R)-(-)-1,2-Cyclohexanediamino-N,N′-bis(3,5-di-tert-butylsali-
cylidene) zinc(II) (3a). This compound was isolated from the reaction
mixture by the removal of the solvent under reduced pressure and
subsequent recrystallization with CH2Cl2 to give a yellow solid. Yield
) 40%. IR (KBr): 2952, 2904, 2864, 2360, 2336, 1614, 1530, 1460,
1
1434, 1411, 1327, 1271, 1255, 1165, 1074, 836, 788, 537 cm-1. H
NMR (500 MHz, pyridine-d5): δ 8.50 (s, 2H), 7.72 (s, 2H), 7.36 (s,
2H), 3.04 (s, 2H), 2.28-1.17 (m, 8H), 1.83 (s, 9H), 1.43 (s, 9H). 13C
NMR (125 MHz, pyridine-d5): δ 171.0, 166.8, 142.4, 134.1, 130.5,
128.6, 119.6, 66.2, 36.8, 34.7, 32.5, 30.8, 28.9, 25.3. EIMS: m/z 608
(M+, 10), 600 (20), 205 (100), 177 (18), 145 (21), 72 (45). HREIMS:
exact mass calcd for [C36H52N2O2Zn]+ 608.3320, found 608.3313. Anal.
Calcd for C36H52N2O2Zn: C, 70.86; H, 8.59; N, 4.59. Found: C, 71.16;
H, 8.68; N, 4.57.
rac-1,2-Cyclohexanediamino-N,N′-bis(3,5-di-tert-butylsali-
cylidene) zinc(II) (rac-3a). This compound was isolated from the
reaction mixture by the removal of the solvent under reduced pressure
and subsequent recrystallization with CH2Cl2 to give a yellow solid.
Yield ) 71%.
1,2-Ethanediamino-N,N′-bis(3,5-di-tert-butylsalicylidene) zinc(II)
(3b). This compound was isolated from the reaction mixture by the
removal of the solvent under reduced pressure and subsequent recrys-
tallization with CH2Cl2 to give a yellow solid. Yield ) 72%. IR
(KBr): 2954, 2904, 2866, 2360, 2335, 1608, 1547, 1528, 1459, 1430,
1411, 1384, 1349, 1323, 1253, 1153, 985, 834, 788, 744, 539
cm-1 1H NMR (400 MHz, CD2Cl2): δ 7.91 (s, 2H), 7.41 (d, 2H,
.
J ) 2.0 Hz), 6.60 (d, 2H, J ) 2.0 Hz), 4.06 (d, 2H, J ) 12.8 Hz),
3.75 (d, 2H, J ) 12.8 Hz), 1.40 (s, 18H), 1.27 (s, 18H). 13C NMR
(125 MHz, CDCl3): δ 174.2, 168.3, 141.0, 135.4, 130.3, 129.8, 117.5,
57.6, 35.6, 34.1, 31.5, 29.5, 18.9. EIMS: m/z 554 (M+, 82), 539 (100),
492 (85), 446 (25), 259 (26), 216 (16). HREIMS: exact mass calcd
for [C32H46N2O2Zn]+ 554.2851, found 554.2852. Anal. Calcd for
C32H46N2O2Zn: C, 69.11; H, 8.34; N, 5.04. Found: C, 69.02; H, 8.38;
N, 5.00.
Further details on the crystal structure investigations are available
on request from the Director of the Cambridge Crystallographic Data
Centre, 12 Union Road, GB-Cambridge, U.K., on quoting the full
journal citation.
1,1,2,2-Tetramethyl-1,2-ethanediamino-N,N′-bis(3,5-di-tert-butyl-
salicylidene) zinc(II) (3c): a yellow solid. Yield ) 64%. IR (KBr):
2949, 2902, 2864, 2360, 2336, 1627, 1547, 1532, 1464, 1427, 1393,
1381, 1360, 1309, 1256, 1155, 979, 876, 828, 786, 743, 530 cm-1. 1H
NMR (400 MHz, CDCl3): δ 8.40 (s, 2H), 7.45 (d, 2H, J ) 2.0 Hz),
7.03 (d, 2H, J ) 2.0 Hz), 1.48 (s, 18H), 1.33 (m, 12H), 1.32 (s, 18H).
13C NMR (125 MHz, CDCl3): δ 168.3, 165.8, 142.0, 135.5, 128.9,
128.8, 117.6, 65.1, 36.1, 34.3, 31.3, 31.2, 29.9. EIMS: m/z 610 (M+,
100), 595 (35), 554 (15), 538 (18). HREIMS: exact mass calcd for
[C36H54N2O2Zn]+ 610.3476, found 610.3479. Anal. Calcd for C36H54N2O2-
Zn‚1/4H2O: C, 70.04; H, 8.84; N, 4.54. Found: C, 70.07; H, 8.92; N,
4.69.
1-Methyl-1,2-ethanediamino-N,N′-bis(3,5-di-tert-butylsali-
cylidene) zinc(3d): a yellow solid. Yield ) 53%. IR (KBr): 2957,
2904, 2868, 2360, 2336, 1600, 1531, 1457, 1428, 1413, 1360, 1320,
1272, 1256, 1166, 1099, 835, 788, 744, 541 cm-1. 1H NMR (500 MHz,
pyridine-d5): δ 8.50 (s, 1H), 8.38 (s, 1H), 7.70 (s, 2H), 7.22 (s, 2H),
3.70(m, 2H), 3.20 (m, 1H), 1.80 (s, 18H), 1.41 (s, 18H), 1.44 (d, 3H,
J ) 6.0 Hz). 13C NMR (125 MHz, pyridine-d5): δ 171.0, 170.9, 170.3,
168.9, 142.3, 142.2, 134.0, 133.9, 129.9, 129.6, 128.6, 128.5, 119.4,
119.3, 62.9, 60.3, 36.6, 34.5, 32.3, 30.5, 20.8. EIMS: m/z 568 (M+,
100), 553 (95), 506 (55), 275 (31), 216 (18), 160 (15). HREIMS: exact
mass calcd for [C33H48N4O2Zn]+, 568.3007, found 568.3002. Anal.
Calcd for C33H48N2O2Zn‚1.5H2O: 66.37; H, 8.61; N, 4.69. Found: C,
66.75; H, 8.43; N, 4.65.
Results and Discussion
We report a general, direct, and high-yield synthesis of zinc
salen complexes from salen-type ligands and Et2Zn. The zinc
metalation procedure was first tested on a series of common,
easily obtainable salicylidene ligands (2a-e) prepared from a
variety of diamine backbones. The synthesis of ligands 2a,
2b, and 2d are reported in the literature. Two new ligands,
2c and 2e, were synthesized by combining aldehyde 4 (2
equiv) with the required diamine (1 equiv) in refluxing ethanol
(Scheme 1).
The zinc salen-type complexes 3a-e were then obtained by
addition of Et2Zn to the free diimine ligands 2a-e in hexane.
After stirring for 12 h at room temperature, the complexes were
isolated by a simple filtration procedure in moderate to good
yield (40-84%).22 Carrying out reactions at -78 °C for 4 h
and then gradually warming to room temperature over 12 h did
not significantly affect yields.
The structure of rac-3a‚pyridine was confirmed by a single-
crystal X-ray diffraction study. An ORTEP representation is
shown in Figure 1. Relevant X-ray diffraction data and selected
bond lengths and angles are listed in Tables 1 and 2, respec-
tively. The Zn(II) center lies 0.43 Å above the coordination plane
in a square pyramidal geometry with the pyridine ligand
occupying the axial position. The Zn-N(py) distance (2.108-
1,3-Benzenediamino-N,N′-bis(3,5-di-tert-butylsalicylidene) zinc-
(II) (3e). The isolated crude product was washed with cold CH2Cl2 to
afford a yellow solid. Yield ) 84%. IR (KBr): 2952, 2904, 2864, 2360,
2336, 1614, 1530, 1460, 1434, 1411, 1327, 1271, 1255, 1165, 1074,
1
836, 788, 537 cm-1. H NMR (400 MHz, THF-d8): δ 8.41 (s, 2H),
(22) Compounds 3a,b were soluble in hexane and were isolated by the
removal of solvent under reduced pressure and subsequent recrystal-
lization.
7.51 (d, 2H, J ) 2.0 Hz), 7.16 (m, 1H), 7.07 (d, 2H, J ) 2.0 Hz), 6.85
(m, 2H), 5.94, (s, 1H), 1.48 (s, 18H), 1.31 (s, 18H). 13C NMR (125

Route to Bis(salicylaldimine) Zinc(II) Complexes
Inorganic Chemistry, Vol. 40, No. 13, 2001 3225
Figure 1. An ORTEP diagram of rac-3a‚pyridine showing the atom-labeling scheme. Thermal ellipsoids are shown at 50% probability. Hydrogen
atoms and 3,3′-tBu groups are omitted for clarity.
Scheme 1
Table 1. Crystal Data and Structure Refinement Parameters for
rac-1,2-Cyclohexanediamino-N,N′-bis(3,5-di-tert-butylsalicylidene)
Zinc(II)‚pyridine (rac-3a‚pyridine) and
Table 2. Selected Bond Distances and Angles for
rac-1,2-Cyclohexanediamino-N,N′-bis(3,5-di-tert-butylsalicylidene)
Zinc(II)‚pyridine (rac-3a‚pyridine)
1,2-Benzenediamino-N,N′-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)
Zinc(II)‚piperidine (1b‚piperidine)
Bond Distances (Å)
Zn(1)-O(1)
Zn(1)-N(1)
Zn(1)-N(3)
1.963(3)
2.087(3)
2.108(3)
Zn(1)-O(2)
Zn(1)-N(2)
1.953(3)
2.089(3)
1b‚piperidine
rac-3a‚pyridine
empirical formula
fw
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
C48H56N6O2Zn
814.39
153(2)
0.71069
triclinic
P1h
10.588(2)
13.021(3)
16.951(3)
85.157(3)
74.477(3)
79.690(3)
2213.7(7)
2
C48.5H64.5N4.5O2Zn
807.95
153(2)
0.71069
monoclinic
P21/n
14.3373
16.1799
19.3730
Bond Angles (deg)
N(1)-Zn(1)-N(2)
N(1)-Zn(1)-O(1)
78.95(12) N(2)-Zn(1)-O(2)
87.93(12)
92.05(13)
96.39(11)
89.58(12) N(2)-Zn(1)-N(3)
N(1)-Zn(1)-O(2) 147.14(13) O(2)-Zn(1)-O(1)
N(1)-Zn(1)-N(3) 101.11(13) O(2)-Zn(1)-N(3) 109.42(12)
N(2)-Zn(1)-O(1) 164.46 (13) O(1)-Zn(1)-N(3) 100.51(13)
b (Å)
c (Å)
Having demonstrated that the zinc metalation procedure
worked well with ordinary salen-type ligands, we were interested
in applying this technology to a new series of bifunctional
pyridine-modified salen ligands 5a-c. The preparation of
pyridyl-containing salen-type ligands required the synthesis of
a highly functionalized salicylaldehyde 6.21 Aldehyde 6 (2 equiv)
was combined with a series of diamines (1 equiv) in refluxing
ethanol (Scheme 2) to produce pyridine-modified ligands 5a-c
in good yield (73-89%).
R (deg)
â (deg)
101.1090
γ (deg)
V (Å3)
4409.86(0)
4
1.217
0.600
3.30
0.060
0.122
Z
F (calcd) (g/cm3)
1.222
0.599
1.65
0.064
µ (mm-1
)
GOF on F2
R1a
wR2b
0.120
Since these salen ligands contain Lewis basic functionalities,
we were concerned that self-coordination-induced aggregation23
might lead to problems with the solubility during the synthesis
of the zinc complexes; however, none of these problems was
observed. Bifunctional zinc(II) complexes 1a-c were obtained
in high yield (86-92%) through a variation of the above
procedure where the free diimine ligand was combined with
Et2Zn in THF at room temperature. THF was necessary as a
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑(w(Fo2 - Fc2)2/∑w(Fo )2]1/2
.
2
(3) Å) is longer than the Zn-N(ligand) distance (2.087(3) Å).
This is similar to what was observed for a (salophen)Zn-
pyridine (salophen ) N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-
diaminobenzene) complex in which the Zn-N(py) distance was
2.103(2) Å, the Zn-N(ligand) distance was 2.099(1) Å, and
the Zn atom was displaced by 0.40 Å from the coordination
plane.7
(23) Fleischer, E. B.; Shachter, A. M. Inorg. Chem. 1991, 30, 3763-3769.

3226 Inorganic Chemistry, Vol. 40, No. 13, 2001
Morris et al.
Figure 2. An ORTEP diagram of 1b‚piperidine showing the atom-labeling scheme. Thermal ellipsoids are shown at 50% probability. Hydrogen
t
atoms and Bu groups are omitted for clarity.
Scheme 2
Table 3. Selected Bond Distances and Angles for
1,2-Benzenediamino-N,N′-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)
Zinc(II)‚piperidine (1b‚piperidine)
complexes insoluble or poorly soluble in non-coordinating
organic solvents, but this is not the case as 1b,c can be
redissolved readily in chloroform.
Bond Distances (Å)
We suspect that these THF-free zinc salen complexes still
contain five-coordinate zinc centers in the solid state but that
not all of them have the distal pyridine groups interacting with
the zinc centers. It is likely that some adventitious water
coordinates to the axial site of a number of the zinc(II) centers
and renders the resulting amorphous and complex mixture,
containing some (pyridine)zinc salen aggregation and some
(water)zinc salen complex, more readily redissolvable in
chloroform.15 A five-coordinate water-ligated zinc salen complex
has been described.24 The presence of water in the solid state
is supported by the elemental analysis results for THF-free 1a,b
where a submolar amount of water (0.5 equiv) is present. This
miniscule amount of water may be enough to disrupt long-range
intermolecular aggregation (between the distal pyridines and
the zinc center) in the solid state. As a comparison, a 15-fold
molar excess of THF is sufficient to compete effectively with
pyridine-zinc salen ligation in solution.25
Zn(1)-O(1)
Zn(1)-N(1)
Zn(1)-N(3)
1.986(4)
2.089(5)
2.119(5)
Zn(1)-O(2)
Zn(1)-N(2)
1.982(4)
2.094(5)
Bond Angles (deg)
N(1)-Zn(1)-N(2)
N(1)-Zn(1)-O(1)
N(1)-Zn(1)-O(2)
N(1)-Zn(1)-N(3)
N(2)-Zn(1)-O(1)
77.2(2)
87.6(2)
145.3(2)
109.1(2)
155.0(2)
N(2)-Zn(1)-O(2)
N(2)-Zn(1)-N(3)
O(2)-Zn(1)-O(1)
O(2)-Zn(1)-N(3)
O(1)-Zn(1)-N(3)
87.8(2)
99.9(2)
93.9(2)
104.1(2)
103.8(2)
solvent to ensure the reasonable solubility of the ligands. After
stirring for 12 h, the solid was filtered and then dried under
reduced pressure to yield the compounds as five-coordinate
complexes where a THF molecule was bound axially to the zinc
1
center, as evidenced by H NMR integration.
The THF ligand could be exchanged through the dissolution
of 1b in piperidine to form 1b‚piperidine. The five-coordinate
structure of 1b‚piperidine was confirmed by a single-crystal
X-ray diffraction study (Figure 2, Tables 1 and 3). The Zn(II)
center lies 0.49 Å above the coordination plane in a square
pyramidal geometry with the piperidine ligand occupying the
axial position. The Zn-N(piperidine) distance (2.119(5) Å) is
longer than the Zn-N(ligand) distance (2.094(5) Å). The
dihedral angles between the plane of the pyridine and the
salicylidene aromatic ring are 37.9° and 27.5°.
The THF molecule could be removed from 1b,c‚THF by
heating at 80 °C under high vacuum to give the solvent-free
complexes as identified by 1H NMR. We initially expected that
removal of the THF molecules would lead to intermolecular
aggregation due to the interaction between the distal pyridine
groups and the highly electrophilic zinc center, rendering the
The tBu group in the THF-free complexes 1b,c appears as a
1
singlet around 1.5 ppm in the H NMR spectrum, close to the
value observed for the free ligands. The imine protons for each
compound are singlets in the range 8.5-8.8 ppm, implying that
the structures are symmetrical in solution. Unsymmetric oxygen-
bridged dimeric structures, seen in the case of the more flexible
t
salan ligand,26 give two singlets each for the Bu and imine
groups.7 We do not observe clear evidence for coordinated water
1
in the H NMR spectrum, presumably due to facile exchange.
Surprisingly, pyridyl salen-type complex 1a, with the flexible
trans-1,2-cyclohexanediyl backbone, is insoluble in the majority
(24) Hall, D.; Moore, F. H. Proc. Chem. Soc. 1960, 256.
(25) Splan, K. E.; Hupp, J. T. Unpublished results.
(26) Salan ligands are derived by hydrogenating the appropriate salen ligand.

Route to Bis(salicylaldimine) Zinc(II) Complexes
Inorganic Chemistry, Vol. 40, No. 13, 2001 3227
of common organic solvents and could only be characterized
in basic solvents such as pyridine that are capable of axially
ligating the Zn(II) center.
Acknowledgment. This work was supported by the National
Science Foundation’s Partnership in Nanotechnology Initiative
(NSF Grant No. CHE-9811334) and by the EMSI program of
the National Science Foundation and the Department of Energy
(NSF Grant No. CHE-9810378) at the Northwestern University
Institute for Environmental Catalysis. S.T.N. is an Alfred P.
Sloan Research Fellow. We thank the Dreyfus Foundation, the
DuPont Company, the Beckman Foundation, and the Packard
Foundation for partial financial support. We acknowledge Fron-
tier Scientific, Inc., for providing samples of 4-pyridylboronic
acid used to prepare pyridyl-substituted ligands.
In conclusion, the direct reaction of a salen-type ligand and
Et2Zn appears to be a versatile method for the synthesis of zinc
salen complexes containing a variety of functionalities in good
to high yield (40-92%). To the best of our knowledge, this
direct metalation method has not been used previously for the
preparation of isolated five-coordinate salen complexes of zinc.
This new method is significant, in part, because it offers an
efficient route to otherwise difficult to access Zn(II) salen
complexes containing pendant Lewis base groups, a series of
which has been fully characterized. We are currently investigat-
ing both the application of chiral zinc salen complexes in
asymmetric Lewis acid catalysis and the use of bifunctional
pyridine-modified Zn(II) salen complexes as components for
the assembly of discrete cyclic nanostructures.27
Supporting Information Available: Relevant NMR spectra for
representative complexes and several different crystallographic views.
Two X-ray crystallographic files in CIF format for the structure
determination of rac-3a‚pyridine and 1b‚piperidine. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC010090O
(27) Morris, G. A.; Nguyen, S. T.; Hupp, J. T. Manuscript in preparation.