Synthesis of azolyl carboximidamides as ligands for Zn(II) and Cu(II): Application of the Zn(II) complexes as catalysts for the copolymerization of carbon dioxide and epoxides

Article abstract of DOI:10.1021/jo052056n

A series of novel S,N-heterocyclic (thiazolyl) substituted carboximidamides 3 and 4 was synthesized in yields up to 82% from specific triazinium salts 1 and primary or secondary amines 2 which additionally bear pyridine or imidazole units. These carboximi

Full text of DOI:10.1021/jo052056n

Synthesis of Azolyl Carboximidamides as Ligands for Zn(II) and
Cu(II): Application of the Zn(II) Complexes as Catalysts for the
Copolymerization of Carbon Dioxide and Epoxides1
Martin Walther,† Kurt Wermann,‡ Marion Lutsche,‡ Wolfgang Gu¨nther,‡ Helmar Go¨rls,§ and
Ernst Anders*,‡
Institut fu¨r Bioanorganische und Radiopharmazeutische Chemie, Forschungszentrum Rossendorf e.V.,
PF 51 01 19, D-01314 Dresden, Germany, Institut fu¨r Organische Chemie und Makromolekulare
Chemie der Friedrich-Schiller-UniVersita¨t, Humboldtstrasse 10, D-07743 Jena, Germany, and
Institut fu¨r Anorganische und Analytische Chemie der Friedrich-Schiller-UniVersita¨t, August-Bebel-Str. 2,
D-07743 Jena, Germany
Ernst.Anders@uni-jena.de
ReceiVed September 30, 2005
A series of novel S,N-heterocyclic (thiazolyl) substituted carboximidamides 3 and 4 was synthesized in
yields up to 82% from specific triazinium salts 1 and primary or secondary amines 2 which additionally
bear pyridine or imidazole units. These carboximidamides are used as tailor-made ligands for the
complexation of Cu(II) and Zn(II). The coordination behavior of 3 and 4 and the properties of the resulting
metal complexes are affected a significant extent especially by the nature of these amine substituents.
The most important structural feature of the novel complexes is that the ligation of the metal cations is
achieved by a 1,3,5-triazapentadienyl anion system, compare the X-ray structure of the model complex
Cu-4d. Analogous Zn(II) complexes 5, 6a, 6b, 6c, 7a, and 7b were obtained from carboximidamides 3,
4a, 4b, 4c, 4d, and 4e after reaction with diethylzinc. Interestingly, these Zn(II) complexes possess an
intrinsic activity to catalyze the copolymerization of cyclohexene oxide and carbon dioxide to give
polycarbonates 15 (TON up to 113; Turn Over Number: moles of substrate 14 consumed per moles of
zinc. Molecular weights: up to 206‚103 Da). Contaminations of 15 by polyethers are produced only in
remarkably small amounts.
Introduction
iaries in asymmetric synthesis4 as well as superbases catalyzing
ring formations from carbon dioxide and suitable amines.5 Thus,
the development of reagents which allow the syntheses of this
class of compounds is in constant progress;6 this includes some
solid-phase syntheses of guanidine derivatives.7 Despite their
high importance in the field of biochemistry and their interest-
attracting properties as anion receptors, the metal coordination
chemistry of guanidines is barely known and appears to be
The azolyl carboximidamide (commonly known as amidine)
and the guanidine functionality, respectively, are frequent con-
stituents in numerous natural compounds.2 Structures containing
such units often play an essential role because of their biological
activity.3 Recently, guanidines have been used as chiral auxil-
* To whom correspondence should be addressed. Tel: +49-(0)3641 948210;
fax: +49-(0)3641-948212.
† Institut fu¨r Bioanorganische und Radiopharmazeutische Chemie.
‡ Institut fu¨r Organische Chemie und Makromolekulare Chemie der Friedrich-
Schiller-Universita¨t.
(3) (a) The Pharmacological Basis of Therapeutics, 7th ed.; Gilman, A.
G., Goodman, L. S., Rall, T. W., Murad, F., Eds.; Pergamon Press: New
York, 1990; p 899. (b) Hu, L. Y.; Guo, J.; Magar, S. S.; Fischer, J. B.;
Burke-Howie, K. J.; Durant, G. J. J. Med. Chem. 1997, 40, 4281-4289.
(4) (a) Ishikawa, T.; Isobe, T. Chem.sEur. J. 2002, 8, 552-557. (b)
Isobe, T.; Fukuda, K.; Araki, Y.; Ishikawa, T. Chem. Commun. 2001, 243-
244.
(5) Costa, M.; Chiusoli, G. P.; Taffurelli, D.; Dalmonego, G. J. Chem.
Soc., Perkin Trans. 1 1998, 1541-1546.
(6) See ref 13e and literature cited therein.
§ Institut fu¨r Anorganische und Analytische Chemie der Friedrich-Schiller-
Universita¨t.
(1) Bis-(1,2,4-thiadiazolo)-1,3,5-triazinium halides. 9. For part 8, see:
Wermann, K.; Walther, M.; Gu¨nther, W.; Go¨rls, H.; Anders, E. Tetrahedron
2005, 61, 673-685.
(2) (a) Berlinck, R. G. S. Nat. Prod. Rep. 2002, 19, 617-649. (b)
Berlinck, R. G. S. Nat. Prod. Rep. 1999, 16, 339-409.
10.1021/jo052056n CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/17/2006
J. Org. Chem. 2006, 71, 1399-1406
1399

Walther et al.
SCHEME 1. Synthesis of Azolyl Carboximidamides 3 and 4a-k
surprisingly unexplored.8 A recent communication reports the
(2-guanidyl)ethyl-cyclen to be an interesting ligand capable of
forming a 1:1 Zn2+ complex.9 As recently demonstrated, zinc
guanidinate complexes are useful catalysts in the ring-opening
polymerization of lactide and further point to the clear possibility
of other catalytically active metal â-diketiminates and amidi-
nates.10 Specific requirements are necessary for the catalytic
abilities of zinc â-diketiminates in polycarbonate synthesis,
described in detail in the review recently published by Coates.11
Metal complexes with sterically demanding triazapentadienyl
anions12 (Figure 1) reveal a coordination pattern similar to that
of the â-diketiminate system. Furthermore, they are comparable
with our new carboximidamidate complexes.
The focus of our research presented here is a strategy to
synthesize azolyl carboximidamides 3 and 4 using our novel
bis-(1,3,4-thiadiazolo)-1,3,5-triazinium halides 1 (so-called SNS-
heterocycles, which can be understood as a shorthand to indicate
the key heteroatom composition of the three fused rings; see
Scheme 1) as precursors. 3 and 4 may also be viewed as
guanidines because of the attached cyclic ligands. Compounds
1 are reacted with a variety of primary or secondary amines
2a-j (aliphatic, heterocyclic, aromatic).13a-e Such fused tricyclic
structures 1 are easily accessible in the course of a three-
component reaction of an aldehyde, a thionyl halide, and a
pyridine which primarily leads to N-(1-haloalkyl)pyridinium
halides.14a,b These salts were reacted with 2-amino-5-alkyl-1,3,4-
thiadiazoles to generate the triazinium salts 1 in excellent
yields.15
FIGURE 1. 1,3,5-Triazapentadienylmetal complexes, cf. ref 12.
(7) (a) Manimala, J.; Anslyn, E. V. Eur. J. Org. Chem. 2002, 3909-
3922. (b) Zapf, C. W.; Creighton, C. J.; Tomioka, M.; Goodman, M. Org.
Lett. 2001, 3, 1133-1136. (c) Guisado, O.; Martinez, S.; Pator, J.
Tetrahedron Lett. 2002, 43, 7105-7109.
(8) (a) Wittmann, H.; Raab, V.; Schorn, A.; Plackmeyer, J.; Sundermeyer,
J. Eur. J. Inorg. Chem. 2001, 1937-1948. (b) Wittmann, H.; Schorn, A.;
Sundermeyer, J. Z. Anorg. Chem. 2000, 625, 1583-1590. (c) Muche, M.
S.; Go¨bel, M. W. Angew. Chem., Int. Ed. Engl. 1996, 35, 2126-2129. (d)
Deslongchamps, G.; Galan, A.; de Mendoza, J.; Rebek, J., Jr. Angew. Chem.,
Int. Ed. Engl. 1992, 31, 61-63.
(10) Coles, M. P.; Hitchcock, P. B. Eur. J. Inorg. Chem. 2004, 2662-
2672.
(11) Coates, G. W.; Moore, D. R. Angew. Chem. 2004, 116, 6784-6806.
(12) (a) Siedle, A. R.; Webb, J. R.; Behr, F. E.; Newmark, R. A.; Weil,
D. A.; Erickson, K.; Naujok, R.; Brostrom, M.; Mueller, M.; Chou, S.-H.;
Young, V. G., Jr. Inorg. Chem. 2003, 42, 932-934. (b) Dias, H. V. R.;
Singh, S. Inorg. Chem. 2004, 43, 5786-5788. (c) Dias, H. V. R.; Singh, S.
Inorg. Chem. 2004, 43, 7396-7402.
(9) Aoki, S.; Iweida, K.; Hanamoto, N.; Shiro, M.; Kimura, E. J. Am.
Chem. Soc. 2002, 124, 5256-5257.
1400 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis of Azolyl Carboximidamides as Ligands
In general, the designation “guanidines” often is connected
with the concept of compounds which are characterized by their
highly basic properties. The observed relatively low basicity of
compounds 3 and 4 is a consequence of the specific kind of
heterocyclic substitution, which results in a less expressed ability
to form stable delocalized guanidinium ions.
Moreover, the deprotonation of the NH function in com-
pounds such as 3 and 4a-f with metal organyls should result
in the formation of remarkably stable 1,3,5-triazapentadienyl-
anion system (vide infra).
FIGURE 2. Imidothioate intermediate Z.
In this paper, we describe (a) our studies on the synthesis of
novel carboximidamides 3 and 4a-k, (b) the formation of a
novel Cu(II) and several Zn(II) complexes (Cu-4d, 5-11) from
these carboximidamides, and (c) successful attempts to use some
of these Zn(II) complexes as catalysts for the copolymerization
of carbon dioxide and epoxides.16 Chemical fixation of CO2 is
of great interest in connection with the development of truly
environmentally tolerable processes in which catalysis is
achieved by transition-metal complexes.
Results and Discussion
The novel fused SNS-heterocycle 1c, which was used in
addition to the previously described compounds 1a and 1b in
conjunction with the carboximidamide syntheses presented here,
was generated from 1-[chloro-(3-hydroxyphenyl)methyl]pyri-
dinium chloride and 2-amino-5-tert-butyl-1,3,4-thiadiazole17 in
73% yield according to the procedure reported in the literature.13a,b
The overall reaction of 1 with amines 2 to give the carboxamides
3 and 4a-k (Scheme 1) can be understood in terms of an
interesting example of an SN(ANRORC) mechanism.18 Because
of their remarkable electrophilicity at the C3a and C4a positions
of the central triazinium ring,19 the tricyclic compounds 1 are
able to react with specifically selected primary and secondary
amines 2a-k. The next steps are characterized by fast ring-
opening and ring-closure steps which finally yield the products
3 and 4.1,13e In more detail and as previously discussed,13e,20
after the nucleophilic attack of the amine nitrogen of 2 at the
C3a position of the cations 1 and a fast deprotonation step, the
consecutive ring transformations are controlled by the influence
of negative hyperconjugation (nN(ex.)/σC(3a)-S(5) interaction). This
effect causes the formation of the triazinium-imidothioate
FIGURE 3. Crystal structure of the Cu(II) complex Cu-4d; except
for H-O(1), hydrogen atoms are omitted for clarity; selected bond
lengths [Å] and angels [°]: Cu-N(1) 2.003, Cu-N(2) 1.990, Cu-
N(4) 1.958 Cu-O(2) 2.002, Cu-O(4) 2.323, N(2)-C(8) 1.305, C(8)-
N(3) 1.349, N(3)-C(9) 1.335, C(9)-N(4) 1.320, C(8)-N(6) 1.407,
O(1)-C(26) 1.361, N(2)-Cu-O(4) 109.14, N(4)-Cu-O(4) 89.97,
N(4)-Cu-N(2) 89.74, N(2)-C(8)-N(3) 127.6, N(3)-C(9)-N(4)
130.4.
(13) (a) Wermann, K.; Walther, M.; Gu¨nther, W.; Go¨rls, H.; Anders, E.
J. Org. Chem. 2001, 66, 720-726. (b) Walther, M.; Wermann, K.; Go¨rls,
H.; Anders, E. Synthesis 2001, 1327-1330. (c) Wermann, K.; Walther,
M.; Anders, E. ARKIVOC (Gainesville, FL) 2002, 10, 24-33. (d) Wermann,
K.; Walther, M.; Go¨rls, H.; Anders, E. Synlett 2003, 1459-1462. (e)
Wermann, K.; Walther, M.; Gu¨nther, W.; Go¨rls, H.; Anders, E. Eur. J.
Org. Chem. 2003, 1389-1403.
zwitterions Z (Figure 2, ring-opening step) as reactive inter-
mediates. Accelerated by the specific reaction conditions
(temperature and reaction time), compounds Z react to give
structures 3 or 4 (ring-closure step).
(14) (a) Anders, E.; Vanden Eynde, J.-J.; Wermann, K. AdV. Heterocycl.
Chem. 2000, 77, 183-219. (b) Anders, E.; Opitz, A.; Wermann, K.; Wiedel,
B.; Walther, M.; Imhof, W.; Go¨rls, H. J. Org. Chem. 1999, 64, 3113-
3121.
(15) Anders, E.; Wermann, K.; Wiedel, B.; Gu¨nther, W.; Go¨rls, H. Eur.
Org. Chem. 1998, 2923-2930.
(16) Scott, D. A.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 2002, 124, 1484-1485, and literature cited therein.
(17) Matti, J.; Ledoux, C.; Kesler, E. Bull. Soc. Chim. Fr. 1959, 477-
479.
(18) Van der Plas, H. C. In AdV. in Heterocycl. Chem.: Degenerate
Transformations of Heterocyclic Compounds; Katritzky, A. R., Ed.;
Academic Press: New York, 1999; p 87.
The general procedure described here is quite successful as
it allows the synthesis of the carboximidamides in yields up to
95%. Interestingly and fortunately, the heterocyclic nitrogen
atoms in 2 which are incorporated in the substituents R1 or R2
do not cause any synthetic problems. In the products 4g-k,
these heteroatoms may serve as additional coordination sites
for metal ions compared with the properties of our previously
synthesized standard carboximidamides starting from less critical
amines.13a-e
To prove the participation of such a pyridine nitrogen in the
coordination pattern, we synthesized (from 4d and Cu(II)
acetate) and X-rayed the copper complex Cu-4d (Figure 3).
The Cu(II) center is surrounded by a square pyramidal
coordination sphere with three different nitrogens and one
(19) Results of a natural population analysis for a model cation 1b: R
) 4-MeC6H4 and Me instead of t-Bu: qC3a ) qC4a ) +0.30 e B3LYP/
6-311++D(d,p) opt.
(20) Nilsson Lill, S. O.; Rauhut, G.; Anders, E. Chem. Eur. J. 2003, 9,
3143-3153.
J. Org. Chem, Vol. 71, No. 4, 2006 1401

Walther et al.
SCHEME 2. Synthesis of Zn Complexes 5-11
acetate anion in the plane and one water molecule forming the
top of the pyramid. The six-membered 1,3,5-triazapentadienyl
metal chelate is vitally important for structural and catalytic
properties of the complex and the analogous Zn(II) compounds.
The easily feasible deprotonation of the central NH-function
deserves particular interest as it was performed in the presence
of a hydroxyaryl function and the water molecule. Both the OH
substituents survive without deprotonation. The characteristic
six Cu-N and N-C distances of the copper-triazapentadienyl
anion unit of Cu-4d (Cu-N: 1.990, 1.958 Å; N-C: 1.305,
1.349, 1.335, and 1.320 Å) are in good agreement with the
reported values of a triazapentadienyl-Cu(I) complex (Cu-N:
1.939, 1.941 Å, N-C: 1.303, 1.338, 1.335, 1.301 Å).12b Further,
the relatively large 2-ethyl-2-pyridinyl substituent (cf. R2 4d)
was selected with the intention to favor the formation of
monomeric ligand-to-metal 1:1 complexes. This was success-
fully realized in this Cu(II) complex.
because these reaction paths give rise to the formation of a
second chiral center in the newly formed dihydrothiadiazole ring.
Both the racemic diastereomers 4f/1 and 4f/2 have been
separated by means of fractional crystallization from ethyl
acetate and recrystallization from MeOH and were characterized
by X-ray crystal structure analyses (cf. Figure 7; Supporting
Information).
The arrangement of the thiadiazole and the dihydrothiadiazole
ring shows the expected E-stereochemistry with respect to the
exocyclic imine double bond. Characteristic bond lengths and
bond angles are in agreement with the expected properties of a
guanidine subunit which has been substituted by the moieties
which stem from the synthetically involved nucleophiles 2 and
the SNS heterocycles 1. The conversion of the cations 1 into
the uncharged carboximidamides 4 caused a shift in the NMR
signals toward higher fields (1H ≈ 1 ppm; 13C ≈ 10-15 ppm).
All of the 1H and 13C NMR signals of the rearrangement
products 4 have been unambiguously assigned to the newly
formed dihydrothiadiazole, the aromatic thiadiazole, and the
guanidine unit via HMQC, HMBC, COSY, and TOCSY
Structural assignments of the novel carboximidamides 3 and
1
4a-k are based on H and 13C NMR data and MS and IR
spectra as well as elemental analyses. As expected, the reaction
of the asymmetric pyridylamine 2f21 and the tricyclic 1b yielded
the formation of diastereomeric carboximidamides 4f/1 and 4f/2
(21) Michelsen, K. Acta Chem. Scand. 1974, A28, 428-434.
1402 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis of Azolyl Carboximidamides as Ligands
SCHEME 3. Proposed Structure of Coordination Polymer
of 12
SCHEME 4. Proposed Structure of the Binuclear Species of
13 in Solution
experiments. As expected, the diastereomeric carboximidamides
4f/1 and 4f/2 show slightly different NMR and IR spectra.
The alternating bonds of the 1,3,5-triazapentadiene system
N(4)-C(1)-N(1)-C(2)-N(2) are well recognizable in the mol-
ecular structure of 4f/1 (cf. the bond lengths: 1.342, 1.320,
1.361, 1.314 Å) and show the expected differences to the data
of the Cu(II)-complex. The bond between carboximidamide C(1)
and the dihydrothiadiazole N(6) (1.369 Å) is significantly shorter
than the corresponding bond in the model complex Cu-4d (1.407
Å).
The syntheses of the carboximidamide zinc complexes 5-11
(Scheme 2) are easily achieved by reaction of diethyl zinc as
the appropriate zinc reagent (1 equiv) with the carboximidamides
3 or 4a-k (1 equiv) according to the method described in the
literature.22 The deprotonation of 3 already starts at -30 °C
(DEE as solvent) and is finished after warming to room
temperature for 1 h. The Zn(II) carboximidamide complex 5 is
formed in yields up to 80% as a white precipitate. Unfortunately,
a complete structure characterization of 5 has not been achieved
because of both the lack of suitable crystals for X-ray experi-
ments and the insolubility of 5 in appropriate solvents for NMR
measurements. Therefore, we cannot rule out the formation
dimers of 5, but because of the bulky nature of the carboximi-
damide ligand 3 and particularly with regard to the X-ray
structure of the model Cu(II) complex Cu-4d synthesized for
comparison (cf. Figure 3), we tend to assume the existence of
intramolecular N,N six-ring chelate systems as described for
metformin23 or ethoxy guanidine zinc complexes.24
products. From the 1:1 and the 1:0.5 ratio experiments, the same
compound 12 (in the latter case together with the unaltered
ligand 4d) was obtained. In contrast, the 1:2 ratio experiment
in THF or pyridine results in a clear solution of a binuclear
ligand-(ZnEt)2 species 13 without any precipitation (Scheme
4). In DEE as solvent, the same 1:2 experiment again results in
the formation of 12 which slowly precipitated from the solution.
Thus, repetition of this experiment in an NMR tube in pyridine-
d5 gains precise information regarding the zinc coordination.
1
(e.g., on the basis of the complete H and 13C NMR spectra;
see Supporting Information).
The 1H NMR spectrum of 13 shows between 3 and 5 ppm a
group of signals with various intensities and resolutions belong-
ing to the ethylene bridge in the ligand. Two small, well-resolved
multiplets in the range of 4-4.3 ppm can be assigned to a little
amount of free ligand and the other signals are those of two
complexed species exhibiting diastereotopic splittings of the
ethylene bridge CH2 protons caused by complexation. The 13
C
NMR shows the presence of two different ethyl residues from
two differently coordinated Zn-ethyl species).
In comparison with 3, the carboximidamides 4a-k possess
a hydroxyphenyl group at the C9 position which stems from
the aldehydes used for the synthesis of the SNS heterocycles.
Thus, they have a further acidic substituent which, especially
after deprotonation, will increase the number of donor functions
for the Zn(II) complexation. After reaction of 4a-f with diethyl
zinc (mole ratio 1:1) in DEE, the two protons of the OH and
NH groups are split off and are replaced by the zinc coordina-
tion. The immediate formation of a precipitate together with
its bad solubility in common solvents could indicate the
existence of coordination polymers, for instance, compound 12
(cf. Scheme 3).
The complexes 6a-c and 7a-c (yields up to 87%) do not
show any OH and NH bond absorptions in the IR spectra and
further prove the participation of one DEE molecule in the
coordination sphere. NMR investigations were not feasible
because of the insolubility of these compounds. For this reason,
further experiments with varying molar ratios of 4d and ZnEt2
(1:0.5 and 1:2) were performed especially to yield soluble
FIGURE 4. Calculated structure of 8a (B3LYP/6-31+G(d,p)). The sub-
stituent R ) tBu is replaced by H. Distances in [Å]. Zn-O: 1.852 Å.
Furthermore, to study the reactivity of this kind of soluble
complexes toward CO2, experiments with 13C labeled CO2 were
carried out in the NMR tube and are discussed below (cf. Figure
5). For all the isolated and insoluble Zn(II) complexes 5, 6,
and 7, the results of molecular weight determinations and
(22) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
1998, 120, 11018-11019.
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Acta 1987, 136, 127-131.
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J. Org. Chem, Vol. 71, No. 4, 2006 1403

Walther et al.
FIGURE 5. 13C NMR difference spectrum of 13 and 13 + CO2 in pyridine-d5. (The small triplet at 149 and the two dispersion signals 135 and
123 ppm are residual resonances of the pyridine-d5.)
elemental analyses are in agreement with the assumed structures
(cf. Scheme 2 and the experimental part).
ized. Nevertheless, a DFT calculation at the B3LYP/6-31+G-
(d,p) level25 of the complex 8a (cf. Figure 4) shows a persuasive
agreement with structural data of other zinc-phenoxide crys-
tals.26,27 Without consideration of the very weak interaction to
imidazole ring N atom, in this model, the four zinc ligands are
arranged in a slightly distorted tetrahedral configuration. As-
suming that a correlation exists between the Zn-O bond lengths
and the degree of nucleophility,28 for such Zn(II) complexes a
certain reactivity can be expected toward heterocumulenes such
as CO2. This is confirmed by our NMR experiments: Inspection
and interpretation of our 13C NMR spectra of the compounds
7a and 8b in DMSO-d6 and 13 in pyridine-d5 which are based
on previous investigations26,29 doubtless show the presence of
An additional type of carboximidamide/zinc complexes
(structures 8-11) which does not belong to the anionic 1,3,5-
triazapentadienyl chelate species was synthesized by reaction
of diethyl zinc and the carboximidamides 4g-k. These com-
pounds do not possess acidic NH functionalities, and just the
phenolic OH groups at the C9 positions are still present.
Therefore, cationic zinc carboximidamide complexes 8-11 are
generated which finally are stabilized by the coordination of
different anions which stem from external acidic compounds
(alcohols, phenols, or carbonic acids) as a consequence of
subsequent addition reactions. The elemental composition of
these complexes was confirmed via mass spectra investigations
and elemental analyses. The compounds 8a- c, 10a, and 10b
feature m/z values from 90 to 100% which is an indicator for
the remarkably high complex stability. Because of improved
solubility in DMSO-d6, in some cases (8a, 9b, 11a, b) it was
possible to achieve analyzable 1H and 13C NMR signals which
are in good agreement with the designed structures 8-11 (cf.
example Figure 8; Supporting Information).
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(33) (a) Coates, G. W. J. Chem. Soc., Dalton Trans. 2002, 467-475.
(b) Cheng, M.; Darling, N. A.; Lobkovsky, E. B.; Coates, G. W. Chem.
Commun. 2000, 2007-2008.
(25) (a) DFT-calculations were performed using the GAUSSIAN 98 suite
of programs, revision A.11: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A. J.; Cheeseman, R.; Zakrzewski, V. G.;
Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam,
J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. GAUSSIAN 98, revision A.11; Gaussian Inc.: Pittsburgh,
PA, 1998. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (c) Lee,
C.; Yang, W.; Parr, R. G. Phys. ReV. B. 1988, 37, 785-789.
1404 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis of Azolyl Carboximidamides as Ligands
TABLE 1. Formation of the Copolymer 15 from Cyclohexene
a
Oxide 14 and CO2
time
yield
Mn‚103
Mw‚103
ratio 15/
polyether
catalyst (h) TONb TOFc (g) 15 (g‚mol-1
)
(g‚mol-1
)
5
21
20
20
109.1
104.0
108.3
3.5
113.2
11.5
7.4
5.2
5.2
5.4
1.4
5.6
0.5
1.2
2.6
3.93
5.34
5.06
1.10
3.64
1.23
0.91
3.61
14.5
21.6
23
215.8
241.4
204.8
39.9
238.1
199.6
43.1
96.4
95.5
99.1
95.5
96.5
99.1
99.1
99.1
6a
6b
6c
6c
7ad
7a
7b
8a
2.5
8.0
20
24
6
20
20
14.8
43.8
16.5
26.4
FIGURE 6. Efficient zinc catalysts A (four-coordinate initiator, see
ref 32) and B (â-diimine ligand; single-site catalyst, see ref 22).
52.9
206.0
SCHEME 5. Copolymerization of Cyclohexene Oxide 14
and CO2
a Conditions: 85 °C, 20 bar CO2, 10 mL 14. b Mole 15/mole Zn). c Mole
15/(mole Zn‚h). d 50 °C.
The catalytic properties of our zinc complexes 5, 6a, 6b, 6c,
7a, 7b, and 8a, which were selected for these tests, are very
different with regard to their efficiency (Table 1). For the
compound 8a which represents an example for guanidine
complexes without the NH unit, no catalytic copolymerization
reactivity was observed. Such carboximidamide complexes
apparently do not exhibit the structural requirements for the
formation of a so-called single-site catalyst (cf. ref 22, 33a and
b and Figure 6). The badly soluble neutral complexes 5 and
6a-6c achieved TOF values from 5.2 to 5.6; their effectiveness
thus can be compared with that of some phenoxide catalysts
previously published by Darensbourg.32
a zinc-phenoxide-CO2 adduct (compound 13 + CO2, δ ) 169
ppm, Figure 5) and a zinc-carbamate equilibrium (structures a
and i; 13 + CO2, δ ) 159 ppm). This is a property which
necessarily must be expressed in systems which might be
appropriate for catalytic CO2 activation and transfer reactions.
Worldwide, there is a continuing interest in the activation
and utilization of CO2 as a C1 building block in organic
synthesis.30 An important application is the alternating CO2
copolymerization with epoxides to produce aliphatic polycar-
bonates with interesting material properties. This has been
extensively investigated particularly with regard to the prepara-
tion of Zn(II) complexes which finally can be used as catalysts
for that reaction.16,22,30-33
Figure 6 shows two exemplary efficient zinc catalysts for
the ring-opening copolymerization of cyclohexene oxide 14 and
CO2. They have been constructed on the basis of bis(phenoxide)
(Darensbourg et al.)32 and anionic â-diimine-type ligands
(Coates et al.)33 which are able to achieve turnover frequencies
up to 235 (Turn Over Frequency (TOF): moles of substrate 14
consumed per moles of zinc per hour).
The newly synthesized polycarbonates 15 have Mw up to 206
× 103 Da and possess only minor amounts of polyether (1-
1
5%, analyzed by H NMR spectra) which indicate the high-
catalyst selectivity of our complexes for such polymer insertion
reactions. Further, 13C NMR investigations of the polymers 15
show that our guanidine zinc complexes have a notable influence
on the tacticity of the growing polymer chain because com-
pounds 15 are substantially syndiotactic polymers.31b,c
These results have motivated us to design further metal
complexation reactions of carboximidamides derived from novel
SNS and NNS compounds and especially enantiomeric amines.
These investigations are presently underway.
Conclusion
From this investigation, we conclude that the general reaction
sequence which starts with the synthesis of cationic SNS
heterocycles and ends with the formation of the title compounds
is a very useful procedure. The products (various highly
substituted carboximidamides functionalized by different elec-
tron donor moieties) can be constructed in a very efficient
manner which opens paths to tailor-made novel ligands for
Cu(II) and Zn(II) cations. In case of carboximidamides such as
3 and 4a-f, the built-in NH function allows deprotonation and
formation of negatively charged ligands which belong to the
class of 1,3,5-triazapentadienylanions. This is confirmed by the
X-ray crystal structure analysis of the Cu(II) complex Cu-4d
from the ligand precursor 4d which reveals that the phenolic
OH (the R substituent in 1b and 4d; cf. Scheme 1) survives
while the NH deprotonation yields the delocalized 1,3,5-
triazapentadienyl anion. Some of their Zn(II) complexes possess
promising catalytic properties for the copolymerization of CO2
and 14 to give polycarbonates 15.
A typical copolymerization run (Scheme 5) was carried out
in a 100-mL autoclave which was loaded with a zinc complex/
14 mixture and then was placed under 20 bar of carbon dioxide
and was heated to 85 °C for 20 h (cf. the Experimental Section).
(34) Taveras, A. G.; Doll, R. J.; Cooper, A. B.; Ferreira, J. A.; Guzi, T.;
Rane, D. F.; Girijavallabhan, V. M.; Aki, C. J.; Chao, J.; Alvarez, C.; Kelly,
J. M.; Lalwani, T.; Desai, J. A.; Wang, J. J-s. (Schering Corp., USA) U.S.
6372747, 2002; Chem. Abstr. 2002, 136, 309918.
(35) MOLEN, An InteractiVe Structure Solution Procedure; Enraf-
Nonius: Delft, Netherlands, 1990.
(36) COLLECT, Data Collection Software; Nonius B. V.: Delft,
Netherlands, 1998.
(37) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology; Carter, C. W.,
Sweet, R. M., Eds.; Macromolecular Crystallography, Part A; Academic
Press: San Diego, CA, 1997; Vol. 276, pp 307-326.
(38) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(39) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(40) CCDC 282561 (Cu-4d) and 282562 (4f/2) contains the supplemen-
tary crystallographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
United Kingdom; fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
Experimental Section
General methods, analytical data, and crystal structure determi-
nation of the novel compounds are assembled in the Supporting
Information.34-40
J. Org. Chem, Vol. 71, No. 4, 2006 1405

Walther et al.
Supporting Information Available: Details of experimental
Acknowledgment. We thank Stephan Schenk (University
of Jena) for useful discussions of DFT results. Financial support
by the Deutsche Forschungsgemeinschaft (Collaborative Re-
search Centre 436, University of Jena, Germany), the Fonds
der Chemischen Industrie (Germany), and the Thu¨ringer Min-
isterium fu¨r Wissenschaft, Forschung und Kultur (Erfurt,
Germany) is gratefully acknowledged. We thank Prof. Manfred
Do¨ring (Forschungszentrum Karlsruhe, Germany) for support
and helpful discussion of the general procedure of the copo-
lymerization with carbon dioxide.
1
procedures, elemental and H and 13C NMR analytical data, MS,
and crystal structure determination of the novel compounds,
especially Figure 7 (crystal structures of the diastereomeric car-
boximidamides 4f/1 and 4f/2) and Figure 8 (1H NMR spectra of
zinc complex 9b). A general procedure for a typical copolymeri-
zation of cyclohexene oxide 14 and carbon dioxide. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO052056N
1406 J. Org. Chem., Vol. 71, No. 4, 2006