A sterically hindered N,N,O tripod ligand and its zinc complex chemistry

Article abstract of DOI:10.1021/ic001204j

The new ligand bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amine (HL) was prepared from bis(2-picolyl)-amine and 2,4-di-tert-butyl-6-(chloromethyl)phenol. It acts as a tetradentate N,N,O tripod ligand ensuring 5-fold coordination in all its zinc complexes L·Zn-X. The central complex of the series was [L·Zn(OH₂)]ClO₄ (1) obtained from zinc perchlorate. Together with the more labile complex L·Zn-C₂H₅ (2), obtained from diethyl zinc, it was used as a starting material for ligand substitutions. In the presence of bases, 1 was converted to L·Zn-OH (3), [L·Zn(py)]ClO₄ (4), and [(L·Zn)₃(μ3-CO₃)]ClO₄ (5). Metathetical reactions produced the neutral complexes L·Zn-X with X = Br (6), OAc (7), OC₆H₅ (8), SC₆H₅ (9), OP(O)(OPh)₂ (10), p-nitrophenolate (11), l-methyluracilate (12), o-formylphenolate (13), and o-hydroxymethylphenolate (14). Structure determinations of 1, 5, 7, 10, 11, 13, and 14 confirmed the strictly monodentate attachment of all units X in L·Zn-X. The hydrolytic cleavage of tris(p)-nitrophenyl) phosphate by 1 was investigated preparatively and kinetically. L·Zn-OH was found to be the hydrolytically active nucleophile. The second-order rate constant for the cleavage reaction was found to be slightly lower than the values for related systems, reflecting the steric hindrance in the tert-butyl-substituted ligand L.

Full text of DOI:10.1021/ic001204j

Inorg. Chem. 2001, 40, 2305-2311
A Sterically Hindered N,N,O Tripod Ligand and Its Zinc Complex Chemistry
Alexander Tro1sch and Heinrich Vahrenkamp*
2305
Institut fu¨r Anorganische und Analytische Chemie der Universita¨t Freiburg, Albertstrasse 21,
D-79104 Freiburg, Germany
ReceiVed NoVember 1, 2000
The new ligand bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amine (HL) was prepared from bis(2-picolyl)-
amine and 2,4-di-tert-butyl-6-(chloromethyl)phenol. It acts as a tetradentate N,N,O tripod ligand ensuring 5-fold
coordination in all its zinc complexes L‚Zn-X. The central complex of the series was [L‚Zn(OH2)]ClO4 (1)
obtained from zinc perchlorate. Together with the more labile complex L‚Zn-C2H5 (2), obtained from diethyl
zinc, it was used as a starting material for ligand substitutions. In the presence of bases, 1 was converted to
L‚Zn-OH (3), [L‚Zn(py)]ClO4 (4), and [(L‚Zn)3(µ3-CO3)]ClO4 (5). Metathetical reactions produced the neutral
complexes L‚Zn-X with X ) Br (6), OAc (7), OC6H5 (8), SC6H5 (9), OP(O)(OPh)2 (10), p-nitrophenolate (11),
1-methyluracilate (12), o-formylphenolate (13), and o-hydroxymethylphenolate (14). Structure determinations of
1, 5, 7, 10, 11, 13, and 14 confirmed the strictly monodentate attachment of all units X in L‚Zn-X. The hydrolytic
cleavage of tris(p-nitrophenyl) phosphate by 1 was investigated preparatively and kinetically. L‚Zn-OH was
found to be the hydrolytically active nucleophile. The second-order rate constant for the cleavage reaction was
found to be slightly lower than the values for related systems, reflecting the steric hindrance in the tert-butyl-
substituted ligand L.
The zinc model complex chemistry with respect to catalytic
pyrazolylborate ligands19 but also with tripods possessing central
carbon,20 nitrogen,21 phosphorus, or arsenic atoms22 and cyclo-
hexane rings.23
or bioinorganic systems has reached a considerable level of
sophistication today. Encapsulating ligands have become the
ligands of choice for mimicking the donor environment of the
metal and to restrain the number of available “active” coordina-
tion sites to one or two. Among these the tripodal ligands are
the most popular ones. The majority of research groups active
in a preparative zinc model complex chemistry have put efforts
in the design or use of tripodal ligands, as evidenced by recent
publications.1-18 We have contributed to this mostly with
While the well-established tripod ligands possess three
identical arms (more correctly feet), sophistication has been
achieved by making them unsymmetrical or heteroleptic.
Induced by Nature’s predominating mixed N/S or N/O donor
sets (provided by cysteinate, histidine, tyrosinate, glutamate, and
aspartate), mixed N/S or N/O tripod ligands have become
popular. Recent examples of N2S or NS2 tripods with central
carbon,24 boron,5,25,26 or nitrogen atoms18 underline this, includ-
ing examples from our research group.27
This paper deals with tripods containing a N2O donor set.
The motivation for their design and use comes from the fact
that important hydrolytic enzymes such as carboxypeptidase A,28
thermolysine,29 alkaline phosphatase,30 L-fuculose 1-phosphate
(1) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. J. Chem.
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Chem. Commun. 1992, 814.
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A. L. J. Chem. Soc., Chem. Commun. 1999, 159.
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10.1021/ic001204j CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/11/2001

2306 Inorganic Chemistry, Vol. 40, No. 10, 2001
Tro¨sch and Vahrenkamp
aldolase,31 or the peptidase astacin32 contain the catalytically
active zinc ion in a NxOy donor environment. Major contribu-
tions to the ligand design and zinc complex chemistry of N,N,O
tripods based on a central N atom (type A) were made by
Fenton.33,34 But also Carrano,35 Parkin,36 and others37,38 pub-
lished new ways of obtaining and applying such tripods,
including those with a central noncoordinating atom X (type
B).
Figure 1. Molecular structure of 1. (One cationic complex is shown;
in one of them the water ligand is hydrogen-bonded to two water
molecules, and in the other it is bonded to one water and one THF
molecule). Important bond lengths (Å) and angles (deg) for both
independent cations: Zn-O1 1.945(6)/1.924(6), Zn-N1 2.099(7)/
2.073(8), Zn-N2 2.061(7)/2.059(8), Zn-N3 2.178(7)/2.219(7), Zn-
O2 2.059(6)/2.079(7); O2-Zn-O1 99.8(3)/94.0(3), O2-Zn-N1 91.8-
(3)/95.4(3), O2-Zn-N2 98.3(3)/97.6(3), O2-Zn-N3 165.0(3)/
173.2(3).
We made our entry into the field of N,N,O tripods with the
reaction chemistry of dipicoylglycinate complexes of zinc39 and
a report on the first chiral N,N,O tripod,40 followed by some
studies of the zinc complex chemistry of dipicolylalanate and
dipicolyl-2-oxybenzylate.41 Like our competitors3,14,33 we could
accumulate some evidence that the reactive species in water-
containing solutions of these complexes is the monoaqua cation
[(ligand)Zn(OH2)]+. Yet so far neither of us had been able to
prepare or isolate this elusive species ot its even more interesting
deprotonated form (ligand)Zn-OH.
picolyl)amine42 and 2,4-di-tert-butyl-6-(chloromethyl)phenol.40
Chromatographic purification yielded 70% HL as a yellowish
solid which is soluble in organic solvents but insoluble in water.
HL is easily identified by its NMR spectrum and its charac-
teristic pyridine IR band at 1591 cm-1
.
We therefore resorted to a technique that had been so
successful for us in (pyrazolylborato)zinc chemistry,19 the
introduction of voluminous substituents on the ligand L, which
favor lower coordination numbers of zinc and induce a
hydrophobic environment of the metal. Both effects should
enhance the stability of a L‚Zn(OH2) or L‚Zn-OH complex.
This paper reports the synthesis of the so designed ligand bis-
(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amine (HL) and
the reaction chemistry of its zinc aqua complex [L‚Zn(H2O)]+.
The latter is the first zinc monoaqua complex of a N,N,O tripod.
It has allowed to prove that the hydrolytically active species in
its reactions is actually the hydroxide L‚Zn-OH.
Starting Complexes. It had been shown that the simplest
way of generating a Zn-OH2 or Zn-OH function in the pocket
of an encapsulating tripodal ligand is the reaction between the
ligand and the hydrated zinc salt of a noncoordinating anion.
This had worked for us for the tris(pyrazolyl)borate-Zn-OH
and tris(benzimidazolylmethyl)amine-Zn-OH2 complexes19,21
but failed for others and ourselves when applied to ligands
analogous to HL.3,14,33,34,39-41 It worked now for HL due to
the presence of the tert-butyl substituents. Complex 1 resulted
from HL, KOH, and Zn(ClO4)2‚6H2O in methanol. Its charac-
teristic spectroscopic features are a proton resonance for the
aqua ligand at 3.34 ppm in CDCl3 and the typically shifted
pyridine IR band at 1610 cm-1
.
L‚Zn-C2H5
[L‚Zn(OH2)]ClO4
2
1
While 1 served the purpose of functionalization by replace-
ment of the aqua ligand in polar media (see below), another
starting complex for derivatizations by proteolytic cleavage in
nonpolar media was found in 2. As experienced for a similar
system before,40 2 was obtained from HL and diethylzinc in
hydrocarbon solvents. Its lability and its high solubility in
nonpolar media prevented its purification, but it was easily
identified by its proton resonances for the Zn-C2H5 unit. For
derivatizations 2 was prepared and used in situ.
Results and Discussion
Ligand HL. The ligand synthesis was straightforward,
combining HL from the easily available components bis(2-
(30) Kim, E. E.; Wyckof, H. W. J. Mol. Biol. 1991, 218, 449.
(31) Dreyer, M. K.; Schulz, G. E. J. Mol. Biol. 1993, 231, 549.
(32) Bode, W.; Goumis-Ru¨th, F.; Huber, R.; Zwilling, R.; Sto¨cker, W.
Nature 1992, 358, 164.
Proof for the existence and structure of 1 was obtained by a
X-ray analysis; see Figure 1. 1 crystallizes with two formula
units/asymmetric unit which differ in the hydrogen-bonding
patterns of the ligated water molecules but not in the gross
molecular features. The coordination geometry is roughly
trigonal-bipyramidal with a typical distortion toward a pseudot-
etrahedral ligation (long Zn-N bonds to the apical nitrogen atom
and O(axial)-Zn-O,N(equatorial) angles above 90°.
(33) Adams, H.; Bailey, N. A.; Fenton, D. E.; He, Q.-Y. J. Chem. Soc.,
Dalton Trans. 1996, 2857.
(34) Rodriguez de Barbarin, C. O.; Bailey, N. A.; Fenton, D. E.; He, Q.-
Y. J. Chem. Soc., Dalton Trans. 1997, 161.
(35) Higgs, T. C.; Spartalian, K.; O’Connor, C. J.; Matzanke, B. F.; Carrano,
C. J. Inorg. Chem. 1998, 37, 2263.
(36) Ghosh, P.; Parkin, G. J. Chem. Soc., Dalton Trans. 1998, 2281.
(37) Otero, A.; Fernandez-Baeza, J.; Tejeda, J.; Antin˜olo, A.; Carillo-
Hermosilla, F.; Diez-Barra, E.; Lara-Sanchez, A.; Fernandez-Lopez,
M. J. Chem. Soc., Dalton Trans. 2000, 2367.
(38) Mao, Z.-W.; Yu, K.-B.; Chen, D.; Han, S.-Y.; Sui, Y.-X.; Tang, W.-
X. Inorg. Chem. 1993, 32, 3104.
There are some trigonal-bipyramidal Zn-OH2 complexes
(39) Abufarag, A.; Vahrenkamp, H. Inorg. Chem. 1995, 34, 2207.
(40) Abufarag, A.; Vahrenkamp, H. Inorg. Chem. 1995, 34, 3279.
(41) Tro¨sch, A.; Vahrenkamp, H. Eur. J. Inorg. Chem. 1998, 827.
(42) Romary, J. K.; Zachariasen, R. D.; Bargar, J. D.; Schiesser, H. J. Chem.
Soc. 1968, 2884.

A Sterically Hindered N,N,O Tripod Ligand
Inorganic Chemistry, Vol. 40, No. 10, 2001 2307
with N3O ligands in the literature,43,44 but their N3O ligands
differ too much from L to allow useful comparisons of
molecular details. The geometry of 1, including the Zn-O(H2O)
distance, compares well, however, with that of the tris-
(benzimidazolylmethyl)amine-Zn-OH2 complex.45 Compared
to all other L‚Zn-OX complexes described below, the axial
Zn-O bond in 1 is long, possibly reflecting the cationic nature
of 1. An important comparison is that of 1 with the zinc
environment in the peptidase astacin, containing a Zn-OH2 unit
ligated in a trigonal-bipyramidal fashion by a tyrosine oxygen
and three histidine nitrogens.32 The similarity in terms of bond
lengths and angles is satisfying, with the exception that the
phenol-derived ligand in the enzyme is in an axial position
(Zn-O ) 2.6 Å) and the water ligand in the enzyme is
equatorial (Zn-O ) 2.1 Å).
Reactions with Bases. While complexes of the type L‚Zn-
OH2 are analogues of the resting state of hydrolytic zinc
enzymes, complexes L‚Zn-OH are models of the active
enzymes. We have shown this extensively for the pyrazolylbo-
rate-Zn-OH complexes,19 but it was also shown indirectly for
the zinc complex chemistry of ligands analogous to L. We
therefore hoped to identify or isolate the deprotonated state of
1, the neutral comple 3.
Figure 2. Coordination of the three zinc ions in 5. Important bond
lengths (Å) and angles (deg): C-O(carbonate) 1.288(9), 1.281(9),
1.305(8); Zn-O(carbonate) 2.000(5), 2.003(5), 1.985(5); Zn-O(phen-
olate) 1.967(5), 1.951(5), 1.957(5); Zn-N(amine) 2.163(6), 2.169(6),
2.153(6); Zn-N(pyridine) 2.118(7)/2.144(6), 2.140(6)/2.138(6), 2.162(6)/
2.157(7); N(pyridine)-Zn-N(pyridine) 150.0(3), 150.6(2), 150.2(3);
N(amine)-Zn-O(carbonate) 144.1(2), 144.0(2), 147.5(2); Zn-O-
C(carbonate) 107.3(4), 105.9(4), 107.7(4).
to the observation by Karlin11 that an isolated dinuclear Zn-
OH complex binds CO2 to form a carbonato complex analogous
to 5.
L‚Zn-OH
3
The structure of 5 (complete figure in the Supporting
Information; for a schematic drawing, see Figure 2) displays
the tris-monodentate attachment of the carbonate ligand to the
three L‚Zn units. Unlike in all other complexes in this paper
the coordination of the three zinc ions is square-pyramidal,
which is also unique in comparison to the other reported
trinuclear zinc carbonato complexes.11,14,46,47 For each L‚Zn unit
the phenolate oxygen atom occupies the apical position. As a
consequence, all Zn-N bonds have about the same length, while
the Zn-O(carbonate) bonds are slightly longer than the Zn-
O(phenolate) bonds. Figure 3 is meant to display the coordina-
tion pattern. Otherwise there are no unusual features of 5 in
comparison to the other complexes of this paper or the other
(µ3-carbonato)zinc complexes. The crystals of 5 contain three
water molecules/formula unit of 5 which, however, are not
located in the vicinity of the zinc ions.
Metathetical Reactions. The replacement of the water
molecule in 1 and the proteolytic removal of the ethyl group
from 2 were applied to introduce other ligands at the L‚Zn
moiety which are either standard ligands in zinc coordination
chemistry or bear some relevance to the bioinorganic chemistry
of zinc. Bromide, acetate, phenolate, and thiophenolate were
chosen for the first group of these. Bromide and acetate could
be introduced as 6 and 7 by treating 1 with their sodium salts.
Alternatively and more conveniently, 6 was prepared from
deprotonated HL and ZnBr2. Phenolate and thiophenolate could
be introduced as 8 and 9 by reacting 2 with phenol and
thiophenol.
Ligand HL and its complex 1 are not suitable for potentio-
metric titrations in aqueous solution due to solubility problems.
1 was therefore treated with various bases in organic or organo-
aqueous solutions. Complex 3 could not be isolated from such
reactions, but spectra and reactivity support its existence. We
propose that 3 is formed from equimolar amounts of 1 and
triethylamine in chloroform. The impure solid obtained has a
NMR spectrum that differs from that of 1, specifically by the
absence of the H2O resonance. The assumption that 3 is an
enzyme model, i.e., its Zn-OH function is a hydrolytically
active nucleophile, is borne out by its phosphate ester cleavage
described below which does not take place with nondeprotonated
1.
Treatment of 1 with a stoichiometric amount of pyridine in
methanol resulted in the replacement of the water ligand by
the pyridine donor. The resulting complex 4 was identified by
its spectra, the most notable features of which are the absence
of water signals in the IR as well as in the NMR.
[L‚Zn(py)]ClO4 [(L‚Zn)3(µ3-CO3)]ClO4
4
5
When a methanolic solution of 1 was treated with 1 equiv of
NaOH and left to stand in an open vessel, it absorbed CO2 from
the atmosphere, resulting in the crystallization of the trinuclear
carbonato complex 5. 5 is another member of this group of
complexes having three zinc units with polydentate ligands
attached to the three carbonate oxygen atoms.11,14,46,47 Again
the formation of 5 is evidence for the existence of 3, in analogy
L‚Zn-OC(O)CH3
L‚Zn-Br
6
7
(43) Kratochvil, B.; Ondracek, J.; Novotny, J. Acta Crystallogr. C 1991,
47, 2207.
(44) Kimura, E.; Koike, T.; Toriumi, K. Inorg. Chem. 1988, 27, 3687.
(45) Brandsch, T.; Schell, F. A.; Weis, K.; Ruf, M.; Mu¨ller, B.; Vahren-
kamp, H. Chem. Ber. 1997, 130, 283.
L‚Zn-OC6H5 L‚Zn-SC6H5
8
9
(46) Bazzicalupi, C.; Benini, A.; Bianchi, A.; Corana, F.; Fusi, V.; Giorgi,
C.; Paoli, P.; Paoletti, P.; Valtancoli, B.; Zanchini, C. Inorg. Chem.
1996, 35, 5540.
Complexes 6-9 are molecular species with a good solubility
in nonpolar organic solvents, thereby indicating their mono-
nuclear nature. Their similarity is underlined by their spectro-
(47) Schrodt, A.; Neubrand, A.; van Eldik, R. Inorg. Chem. 1997, 36, 4579.

2308 Inorganic Chemistry, Vol. 40, No. 10, 2001
Tro¨sch and Vahrenkamp
scopic features. The coligands X in these species L‚Zn-X are
monodentate. Only the acetate ligand in 7 has the alternative to
be bidentate. Therefore, 7 was chosen for a structure determi-
nataion which is documented in the Supporting Information. It
confirmed the strictly monodentate acetate attachment (Zn-
O-C ) 138°). Thus the four simple complexes 6-9 can be
assigned a structure like 1 with the coligand X on an apical
position of a distorted trigonal-bipyramidal ZnON3X ligand set.
The second group of coligands chosen contained diphenyl
phosphate and p-nitrophenolate which are substrate and product
analogues of zinc enzyme catalyzed phosphate ester hydrolyses.
At the same time they were meant to provide a structural
background for the mechanistic investigation of the phosphate
ester cleavage by 3 described below. Complexes 10 and 11 were
obtained from 1 by addition of base and the reagent (diphe-
nylphosphoric acid or p-nitrophenol). Both reaction courses
(deprotonation of 1 before addition of the reagent or deproto-
nation of the reagent before addition to 1) are feasible.
Alternatively, 10 and 11 are accessible from the ethyl complex
2 and the reagent.
by the zinc-containing alcoholdehydrogenase enzymes. We had
observed before52,53 that the enzyme-substrate interactions of
these enzymes can be modeled by zinc complexes in which the
aldehydic and alcoholic functions are attached to zinc in a
chelating fashion. As the second donor of the chelating alcohol
or aldehyde ligands we had used pyridine nitrogen52 or phenolate
oxygen,53 and even in the pocket within encapsulating pyra-
zolylborate ligands could the increase of the coordination
number of zinc be realized. With this in mind, complexes L‚
Zn-OC6H4-o-CHO (13) and L‚Zn-O-C6H4-o-CH2OH (14)
were synthesized from 1 and the potassium salts of o-
formylphenol and o-(hydroxymethyl)phenol.
L‚Zn-OC6H4-p-NO2
L‚Zn-OP(O)(OPh)2
11
10
The relevant spectroscopic data for 13 and 14 [ν(CO-
aldehyde) and δ(CHO) as well as δ(CH2OH)] do not differ
significantly from those of the free substrates and hence speak
against a zinc coordination by the CHO or CH2OH groups.
Structure determinations confirmed this conclusion. Both 13 and
14 (details in the Supporting Information) have a zinc coordina-
tion which is virtually identical to that of 11 and related
complexes.40,48 The aldehyde donor in 13 is turned away from
the zinc ion. The alcoholic function in 14 is bent inward but
not enough to have a bonding interaction with zinc (Zn‚‚‚O )
4.27 Å). Instead it is connected to the phenolate oxygen by a
hydrogen bond (O‚‚‚O ) 2.66 Å), as displayed below. The
major conclusion from these findings is that the steric situation
of ligand L is such that it unambiguously enforces 5-fold
coordination in its zinc complexes.
Solubility and spectroscopic properties confirm the mono-
nuclear and molecular nature of 10 and 11. One significant
spectroscopic feature of 11 is the position of the UV absorption
band due to the p-nitrophenyl group (λmax ) 385 nm) which
allows one to distinguish 11 from free p-nitrophenolate (λmax
) 400 nm), which is important for the mechanistic investigation
described below. Both 10 and 11 were subjected to crystal
structure determinations which are documented in the Supporting
Information. They confirmed the distorted trigonal-bipyramidal
ZnN3O2 coordination with axial angles of 167° in 10 and 160°
in 11 and with Zn-O(substrate) bond lengths of 1.99 Å in 10
and 1.98 Å in 11. The other molecular details of 10 and 11
correspond closely to those reported by us39-41 for similar
phosphate complexes and by Fenton48 and us40 for related
phenolate complexes, and the ligand arrangements in 11 and
13/14 (see below) are also very similar.
The third group of coligands contained those which are
potentially bidentate and relevant in the context of nonhydrolytic
zinc enzymes. We had observed that nucleobases, which are
synthesized and metabolized inter alia by zinc enzymes, are
bound to zinc preferably via their deprotonated NH functions
and can use their additional donor functions for chelation.49,50
We tested this here for 1-methyluracil as the simplest analogue
of uridine and the uridine nucleotides. Its reaction with 2 yielded
the uracilate 12. The spectroscopic data for 12 support the
formulation given without coordination of the CdO functions
to zinc. The main indicator is the ν(CO) absorption in the IR at
1644 cm-1 which exactly corresponds to that of the pyra-
zolylborate complex TpCum,MeZn-1-methyluracilate which we
have structurally characterized.51
Phosphate Ester Cleavage. The abundance of zinc-contain-
ing phosphatases and the biological importance of phophate
transfer have induced numerous mechanistic studies of zinc
complex mediated phosphate ester cleavages.14,33,54-56 We have
contributed to this with preparative and detailed kinetic inves-
tigations involving (pyrazolylborato)zinc complexes.57-59 Our
o-Formylphenolate (the anion of salicylic aldehyde) and
o-hydroxymethylphenolate were applied with the clear expecta-
tion that they might act as bidentate ligands. Their formyl and
hydroxymethyl groups represent the oxidized and reduced forms
of the substrates (aldehyde/alcohol) which are interconverted
(52) Mu¨ller, B.; Schneider, A.; Tesmer, M.; Vahrenkamp, H. Inorg. Chem.
1999, 38, 1900.
(53) Walz, R.; Ruf, M.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2001, 139.
(54) Gellmann, S. H.; Petter, R.; Breslow, R. J. Am. Chem. Soc. 1986,
108, 2388.
(55) Koike, T.; Kimura, E. J. Am. Chem. Soc. 1991, 113, 8935.
(56) Jurek, P.; Martell, A. E. Inorg. Chim. Acta 1999, 287, 47.
(57) Weis, K.; Rombach, M.; Ruf, M.; Vahrenkamp, H. Eur. J. Inorg.
Chem. 1998, 263.
(58) Weis, K.; Vahrenkamp, H. Eur. J. Inorg. Chem. 1998, 271.
(59) Rombach, M.; Maurer, C.; Weis, K.; Keller, E.; Vahrenkamp, H. Chem.
Eur. J. 1999, 5, 1013.
(48) Rodriguez de Barbarin, C. O.; Bailey, N. A.; Fenton, D. E.; He, Q.-
Y. Inorg. Chim. Acta 1994, 219, 205.
(49) Ruf, M.; Weis, K.; Vahrenkamp, H. Inorg. Chem. 1997, 36, 2130.
(50) Koppenho¨fer, A.; Hartmann, U.; Vahrenkamp, H. Chem. Ber. 1995,
128, 779.
(51) Badura, D.; Vahrenkamp, H. Unpublished results.

A Sterically Hindered N,N,O Tripod Ligand
Inorganic Chemistry, Vol. 40, No. 10, 2001 2309
Conclusions
preparative work proved that the Zn-OH unit is the hydrolyti-
cally active nucleophile. In our kinetic work we observed for
the first time the strongly negative activation entropies indicative
of the four-center association of the Zn-OH and PdO functions
in the rate-determining step. The availability of the Zn-OH
complex 3 in this work provided a chance to verify the previous
conclusions for the L‚Zn complexes. Work by Fenton had
already provided indirect evidence for the involvement of a Zn-
OH complex with a tripodal ligand similar to L.33
As usual for these studies, tris(p-nitrophenyl) phosphate
(TNP) was chosen for the cleavage reactions which we
performed in chloroform because several of the uncharged
molecular species involved are insoluble in water. We observed
that the aqua complex 1 does not react with TNP but the hydroxo
complex 3 does, thereby underlining the essentiality of the Zn-
OH function. The reaction of 2 equiv of 3 with 1 equiv of TNP
produces a mixture of the phosphate complex L‚Zn-OPO-
(OC6H4-p-NO2)2 (15) and the nitrophenolate complex 11. The
individual components of this mixture were prepared from 3
and bis(p-nitrophenyl) phosphate (15) or p-nitrophenol (11; see
above), respectively. 15 could not be obtained in a pure form
but was easily identified by its 31P NMR signal at -15.4 ppm.
The individual preparations from 3 showed that the formation
of 15 is about 2 orders of magnitude slower than the formation
of 11. The nucleophilic strength of 3 is not high enough to effect
hydrolytic cleavage of bis(p-nitrophenyl) phosphate.
The chemistry of the new ligand L has fulfilled the expecta-
tions put into it. It is easy to synthesize, it makes stable zinc
complexes of great variety, the complexes L‚Zn-X are easy to
handle due to their molecular nature, they are strictly and reliably
five-coordinate, and the functionality of the Zn-X unit can be
exploited in the intact L‚Zn environment. All these properties
provide the L‚Zn moiety with the same advantages for five-
coordinate zinc that the pyrazolylborate-Zn moiety has for four-
coordinate zinc.
The ZnN3O coordination pattern provided by L corresponds
to the ligation of zinc in several hydrolytic enzymes. In addition
to this structural analogy there exists the functional analogy in
the form of the stable complex [L‚Zn(OH2)]+ (1). 1 is the first
isolated monoaquazinc complex for the class of tripodal ligands
such as L. Its relevance could be demonstrated by the prepara-
tion and use of the complex L‚Zn-OH (3), resulting from
deporotonation of 1. 3 is a general base, being able to
deprotonate species HX which then form complexes L‚Zn-X.
It is also strong enough a base to bind CO2 from the air forming
the trinuclear carbonato complex [(L‚Zn)3(µ3-CO3)]+ (5).
The functional analogy of 1 or 3 with the hydrolytic zinc
enzymes could be verified by phosphate ester cleavage. The
kinetic data of this reaction show that, although the bulky ligand
L limits access to the hydrolytically active Zn-OH center, the
reactivity of the complex is only slightly lower than that of
related systems. Complexes [L‚Zn(OH2)]+ and L‚Zn(OH)
thereby suggest themselves for further biomimetic studies
concerning the hydrolysis of phosphates, esters, peptides, and
CO2.
All these observations were favorable for the kinetic study
under pseudo-first-order conditions, i.e., with a large excess of
3 for the cleavage of TNP. It can be assumed that the cleavage
initially produces 15 and free p-nitrophenol. The latter is then
converted by a fast reaction with excessive 3 into 11. The
concentration of 11 can be monitored by UV spectroscopy using
its unobscured absorption band at 385 nm. It is a measure of
the concentration of TNP, and from the UV extinctions I the
pseudo-first-order rate constant can be derived using the equation
Experimental Section
General Information. The general working techniques were as
described previously.60 Reactions involving ZnEt2 were performed in
dehydrated solvents and in a nitrogen atmosphere. Starting materials
were obtained commercially. Bis(picolyl)amine42 and 2,4-di-tert-butyl-
6-(chloromethyl)phenol40 were prepared according to the published
procedures. The term hexanes is used for petroleum ether boiling
between 60 and 70 °C. In those cases where several preparations yielded
the same complex only the best procedure is described.
ln[1 - (It/I∞)] ) -kobs
t
The values of kobs were determined this way for six different
concentrations of 3 in the presence of 0.1 mM TMP. The
resulting plot is given in the Supporting Information. The
linearity of the plot points to a clean second-order reaction, as
expected. The slope yields a second-order rate constant k′′ of
0.27 s-1 M-1. The least-squares line does not pass through the
origin. This means that hydrolysis occurs also in the absence
of 3, e.g. by traces of water in the reaction system. As mentioned
above, the aqua complex 1 is not hydrolytically active. The
intercept of the ordinate is small enough to ensure that complex
3 is the essential hydrolytically active species.
Of the previous investigations of phosphate ester hydrolysis
with zinc complexes few were done in solvents other than water.
As these reations are generally much faster in water, our rate
constant can only be compared with those obtained in similar
solvents. We found k′′ values between 0.45 and 1.55 s-1 M-1
for TNP cleavage by pyrazolylborate-Zn-OH complexes.59
Fenton found k′′ values between 0.90 and 2.21 s-1 M-1 for the
cleavage by Zn-OH complexes of ligands similar to L.33 The
k′′ value of 0.27 s-1 M-1 observed here for L‚Zn-OH has the
same order of magnitude but is considerably smaller than the
others. We invoke steric hindrance by L to explain this. Just
like L prevents all its stable zinc complexes from being
octahedral, it should make it more difficult than the other ligands
to bring in the phosphate substrate and form the four-center
Zn-OH/PdO aggregate which is a reaction intermediate or the
transition state of the cleavage reaction.
Ligand HL. A solution of bis(picolyl)amine (4.00 g, 20.1 mmol)
and triethylamine (7.26 g, 71.8 mmol) in dioxane (30 mL) was treated
with a solution of 2,4-di-tert-butyl-6-(chloromethyl)phenol (5.12 g, 20.1
mmol) in dioxane (20 mL). After stirring of the mixture for 1 day, the
triethylammonium chloride was filtered off and the filtrate evaporated
to dryness. Chromatography with ethanol/ethyl acetate (7:3) over a 6
× 30 cm silica gel column using UV detection allowed us to separate
the product HL with a Rf value of 0.78. A 5.89 g (70%) amount of HL
remained as a yellowish solid, mp 96 °C. IR (KBr): 3210 (m, br) (OH),
1
1591 (s) (CdN). H NMR (CDCl3): 1.19 [s, 9H, t-Bu], 1.38 [s, 9H,
t-Bu], 3.73 [s, 2H, CH2-phenol], 3.80 [s, 4H, CH2-py], 6.80-7.60
[m, 8H, aromatic], 8.48 [d, J ) 4.9 Hz, 2H, HR], 10.58 [s, 1H, OH].
Anal. Calcd for C27H35N3O (Mr ) 417.6): C, 77.66; H, 8.45; N,
10.06. Found: C, 77.12; H, 8.49; N, 9.44.
Complex 1. A solution of HL (300 mg, 0.72 mmol) and KOH (40
mg, 0.72 mmol) in methanol (20 mL) was treated with a solution of
Zn(ClO4)2‚6H2O (268 mg, 0.72 mmol) in methanol (10 mL). After
stirring of the mixture for 30 min, the KClO4 precipitate was filtered
off and the filtrate evaporated to dryness. Crystallization from benzene
yielded 259 mg (60%) of 1 as colorless crystals, mp 250 °C. IR
(KBr): 3434 (m, br) (OH), 1610 (s) (CdN), 1105 (vs) (ClO4). 1H NMR
(CDCl3): 1.22 [s, 9H, t-Bu], 1.30 [s, 9H, t-Bu], 3.34 [s, 2H, H2O],
3.82 [s, 2H, CH2-phenol], 3.97 [d, J ) 16.3 Hz, 2H, CH2-py], 4.12
(60) Fo¨rster, M.; Burth, R.; Powell, A. K.; Eiche, T.; Vahrenkamp, H. Chem.
Ber. 1993, 126, 2643.

2310 Inorganic Chemistry, Vol. 40, No. 10, 2001
Tro¨sch and Vahrenkamp
[d, J ) 16.3 Hz, 2H, CH2-py], 6.81 [d, J ) 2.6 Hz, 1H, Hc], 7.10 [d,
J ) 2.6 Hz, 1H, Ha], 7.32-7.90 [m, 6H, aromatic], 8.95 [d, J ) 4.5
Hz, 2H, HR].
Anal. Calcd for C27H36ClN3O6Zn (Mr ) 599.4): C, 54.10; H, 6.05;
N, 7.01. Found: C, 54.29; H, 6.11; N, 6.32.
Anal. Calcd for C29H37N3O3Zn‚CH2Cl2 (Mr ) 541.0 + 84.9): C,
57.57; H, 6.28; N, 6.71. Found: C, 57.24; H, 6.13; N, 6.95.
Complex 8. To a solution of HL (300 mg, 0.72 mmol) in toluene
(10 mL) were added 0.72 mmol (0.72 mL of a 1 M solution in n-hexane)
of diethylzinc and a solution of phenol (68 mg, 0.72 mmol) in toluene
(5 mL). After the mixture was stirring for a few minutes, the product
precipitated. Recrystallization from methanol yielded 341 mg (78%)
Complex 2. A solution of ZnEt2 (1 M, 0.72 mmol, 0.72 mL) in
n-hexane was added to a solution of HL (300 mg, 0.72 mmol) in
toluene/n-hexane (5:1) and stirred for 1 h. Filtration and removal of
the solvent from the filtrate left behind 316 mg (86%) of impure 2 as
1
of 8 as a colorless solid, mp 225 °C. IR (KBr): 1606 (s) (CdN). H
NMR (CDCl3): 1.23 [s, 9H, t-Bu], 1.36 [s, 9H, t-Bu], 3.39 [s, 3H,
methanol], 3.80 [s, 2H, CH2-phenol], 3.88 [d, J ) 15.8 Hz, 2H, CH2-
py], 4.11 [d, J ) 15.8 Hz, 2H, CH2-py], 6.81 [m, 1H, phenolate],
7.12 [d, J ) 2.6 Hz, 1H, Hc], 7.18 [d, J ) 2.6 Hz, 1H, Ha], 7.20-7.78
[m, 10H, aromatic], 9.06 [d, J ) 5.0 Hz, 2H, HR].
1
a yellowish solid which decomposes within hours. H NMR (C6D6):
0.76 [q, J ) 8.2 Hz, 2H, Zn-CH2], 1.37 [s, 9H, t-Bu], 1.71 [t, J ) 8.2
Hz, 3H, ethyl-CH3], 1.85 [s, 9H, t-Bu], 3.45 [s, 2H, CH2-phenol], 3.74
[d, J ) 15.0 Hz, 2H, CH2-py], 4.04 [d, J ) 15.0 Hz, 2H, CH2-py],
6.40-7.40 [m, 8H, aromatic], 8.19 [d, J ) 4.6 Hz, 2H, HR].
Complex 3. A mixture of 1 (100 mg, 0.17 mmol) and triethylamine
(17 mg, 0.17 mmol) in chloroform (5 mL) was stirred for 10 min.
Removal of the solvent in vacuo left behind a mixture of triethylam-
monium perchlorate and 3 as a colorless solid. 1H NMR (CDCl3): 1.10
[t, J ) 7.2 Hz, 9H, CH3-ethyl], 1.26 [s, 9H, t-Bu], 1.35 [s, 9H, t-Bu],
2.66 [q, J ) 7.2 Hz, 6H, N-CH2], 3.83 [s, 2H, CH2-phenol], 4.12 [d,
J ) 16.6 Hz, 2H, CH2-py], 4.23 [d, J ) 16.6 Hz, 2H, CH2-py], 6.87
[d, J ) 2.4 Hz, 1H, Hc], 7.17-7.83 [m, 7H, aromatic], 8.85 [d, J )
4.8 Hz, 2H, HR].
Anal. Calcd for C33H39N3O2Zn‚CH3OH (Mr ) 575.1 + 32.0): C,
67.26; H, 7.14; N, 6.92. Found: C, 67.72; H, 6.90; N, 6.70.
Complex 9. This was made as was 8 from HL (300 mg, 0.72 mmol),
with 0.72 mmol of diethylzinc and thiophenol (79 mg, 0.72 mmol).
Yield: 294 mg (69%) of 9 as a colorless solid, mp 179 °C. IR (KBr):
1
1604 (s) (CdN). H NMR (CDCl3): 1.13 [s, 9H, t-Bu], 1.29 [s, 9H,
t-Bu], 3.65 [s, 2H, CH2-phenol], 3.90 [d, J ) 15.7 Hz, 2H, CH2-py],
4.06 [d, J ) 15.7 Hz, 2H, CH2-py], 6.68-7.70 [m, 13H, aromatic],
8.97 [d, J ) 4.9 Hz, 2H, HR].
Anal. Calcd for C33H39N3OSZn (Mr ) 591.2): C, 67.05; H, 6.65;
N, 7.11. Found: C, 66.56; H, 6.70; N, 6.86.
Complex 4. A solution of pyridine (40 mg, 0.50 mmol) in methanol
(5 mL) was added to a solution of 1 (300 mg, 0.50 mmol) in methanol
(15 mL). After the solution was stirred for 1 h, the solvent was removed
in vacuo and the residue crystallized from ethanol, yielding 116 mg
(35%) of 4 as a colorless solid, mp 196 °C. IR (KBr): 1608 (s) (Cd
Complex 10. A solution of diphenyl phosphate (168 mg, 0.67 mmol)
and NaOH (27 mg, 0.67 mmol) in methanol (15 mL) was added to a
solution of 1 (400 mg, 0.67 mmol) in methanol (20 mL). After the
solution was stirred for 1 h, the solvent was removed in vacuo and the
residue crystallized from acetone, yielding 159 mg (30%) of 10 as
colorless crystals, mp 217 °C. IR (KBr): 1607 (s) (CdN), 1281 (s),
1
N), 1098 (vs) (ClO4). H NMR (CDCl3): 1.15 [s, 9H, t-Bu], 1.30 [s,
9H, t-Bu], 3.74 [s, 2H, CH2-phenol], 4.12 [d, J ) 16.0 Hz, 2H, CH2-
py], 4.40 [d, J ) 16.0 Hz, 2H, CH2-py], 6.78 [d, J ) 2.4 Hz, 1H, Hc],
7.03 [d, J ) 2.4 Hz, 1H, Ha], 7.19-8.02 [m, 11H, aromatic], 9.15 [d,
J ) 4.6 Hz, 2H, HR].
1
1211 (s) (PdO). H NMR (CDCl3): 1.22 [s, 9H, t-Bu], 1.43 [s, 9H,
t-Bu], 2.09 [s, 6H, acetone], 3.74 [s, 2H, CH2-phenol], 3.81 [d, J )
16.0 Hz, 2H, CH2-py], 4.08 [d, J ) 16.0 Hz, 2H, CH2-py], 6.77 [d,
J ) 2.4 Hz, 1H, Hc], 7.04-7.82 [m, 17H, aromatic], 9.08 [d, J ) 5.2
Hz, 2H, HR].
Anal. Calcd for C32H39ClN4O5Zn (Mr ) 660.5): C, 58.19; H, 5.95;
N, 8.48; Zn, 9.90. Found: C, 60.21; H, 6.23; N, 8.18; Zn, 9.47.
Complex 5. A solution of 1 (500 mg, 0.83 mmol) in methanol (20
mL) was treated with a solution of NaOH (33 mg, 0.83 mmol) in
methanol (10 mL), stirred for 1 h, and left standing in an open beaker
for 2 days. A 763 mg (57%) amount of 5 precipitated as colorless
crystals, mp 195 °C, which were filtered off and washed with hexanes.
IR (KBr): 1708 (w) (CdOas), 1605 (s) (CdN), 1262 (s) (CdOsym),
Anal. Calcd for C39H44N3O5PZn‚(CH3)2CO (Mr ) 731.2 + 58.0):
C, 63.92; H, 6.39; N, 5.32. Found: C, 64.51; H, 6.18; N, 5.33.
Complex 11. A solution of p-nitrophenol (93 mg, 0.68 mmol) in
chloroform (5 mL) was added to a solution of 1 (400 mg, 0.67 mmol)
and triethylamine (68 mg, 0.67 mmol) in chloroform (15 mL). After
the soution was stirred for 5 min, the solvent was removed in vacuo
and the residue crystallized from acetone, yielding 224 mg (54%) of
11 as yellow crystals, mp 238 °C. IR (KBr): 1609 (s) (CdN), 1287
(vs) (NdO). 1H NMR (CDCl3): 1.17 [s, 9H, t-Bu], 1.27 [s, 9H, t-Bu],
3.76 [s, 2H, CH2-phenol], 3.83 [d, J ) 16.0 Hz, 2H, CH2-py], 4.07
[d, J ) 16.0 Hz, 2H, CH2-py], 6.74 [d, J ) 2.5 Hz, 1H, Hc], 7.08 [d,
J ) 2.5 Hz, 1H, Ha], 7.19-8.09 [m, 10H, aromatic], 8.83 [d, J ) 5.0
Hz, 2H, HR].
1
1101 (vs) (ClO4). H NMR (CDCl3): 1.32 [s, 9H, t-Bu], 1.55 [s, 9H,
t-Bu], 3.74 [s, 2H, CH2-phenol], 3.99 [s, 4H, CH2-py], 6.76 [d, J )
2.5 Hz, 1H, Hc], 7.04 [d, J ) 2.5 Hz, 1H, Ha], 7.13-7.80 [m, 6H,
aromatic], 9.26 [d, J ) 5.2 Hz, 2H, HR].
Anal. Calcd for C82H102ClN9O10Zn3‚3H2O (Mr ) 1605.4 + 54.0):
C, 58.71; H, 6.61; N, 7.52. Found: C, 58.50; H, 6.30; N, 6.96.
Complex 6. A solution of ZnBr2 (162 mg, 0.72 mmol) in methanol
(10 mL) was added to a solution of HL (300 mg, 0.72 mmol) and
NaOH (29 mg, 0.72 mmol) in methanol (20 mL). After the solution
was stirred for 2 h, diethyl ether (50 mL) was added, precipitating 6.
Recrystallization from methanol yielded 289 mg (64%) of 6 as colorelss
Anal. Calcd for C33H38N4O4Zn (Mr ) 620.1): C, 63.92; H, 6.18; N,
9.04. Found: C, 63.70; H, 6.23; N, 8.87.
Complex 12. To a solution of HL (300 mg, 0.72 mmol) in
acetonitrile (10 mL) were added 0.72 mmol of diethylzinc (0.72 mL
of a 1 M solution in n-hexane). After 5 min of stirring and subsequent
heating to 50 °C, a solution of 1-methyluracil (91 mg, 0.72 mmol) in
acetonitrile (10 mL) was added. After being stirred for 1 h at 50 °C,
the mixture was allowed to cool to room temperature upon which the
product was precipitated. Recrystallization from methanol/water (6:1)
yielded 142 mg (31%) of 12 as a colorless solid, mp 120 °C. IR
(KBr): 1644 (vs) (CdO), 1605 (s) (CdN). 1H NMR (DMSO-d6): 1.09
[s, 9H, t-Bu], 1.30 [s, 9H, t-Bu], 3.16 [s, 3H, CH3-N], 3.47 [s, 2H,
CH2-phenol], 4.22 [d, J ) 15.5 Hz, 2H, CH2-py], 4.38 [d, J ) 15.5
Hz, 2H, CH2-py], 5.30 [d, J ) 7.4 Hz, 1H, uracil], 6.64 [d, J ) 2.5
Hz, 1H, Hc], 6.66 [d, J ) 2.5 Hz, 1H, Ha], 7.27-7.83 [m, 7H, aromatic
+ uracil], 8.32 [d, J ) 4.7 Hz, 2H, HR].
Anal. Calcd for C32H39N5O3Zn‚1.5H2O (Mr ) 607.1 + 27.0): C,
60.61; H, 6.68; N, 11.04. Found: C, 60.75; H, 6.48; N, 11.04.
Complex 13. A solution of potassium 2-formylphenolate (89 mg,
0.67 mmol) in methanol (15 mL) was added to a solution of 1 (400
mg, 0.67 mmol) in methanol (20 mL). After the mixture was stirred
for 1 h, the KClO4 precipitate was filtered off and the filtrate evaporated
to dryness. The residue was taken up in 5 mL of THF and layered
1
crystals, mp 268 °C. IR (KBr): 1607 (s) (CdN). H NMR (CDCl3):
1.16 [s, 9H, t-Bu], 1.35 [s, 9H, t-Bu], 3.39 [s, 6H, methanol], 3.69 [s,
2H, CH2-phenol], 3.80 [d, J ) 15.9 Hz, 2H, CH2-py], 4.04 [d, J )
15.9 Hz, 2H, CH2-py], 6.71 [d, J ) 2.5 Hz, 1H, Hc], 7.04 [d, J ) 2.5
Hz, 1H, Ha], 7.16-7.79 [m, 6H, aromatic], 9.48 [d, J ) 5.0 Hz, 2H,
HR].
Anal. Calcd for C27H34BrN3OZn‚2CH3OH (Mr ) 561.9 + 64.1):
C, 55.65; H, 6.76; N, 6.71. Found: C, 55.43; H, 6.74; N, 6.72.
Complex 7. A solution of glacial acetic acid (40 mg, 0.67 mmol)
and NaOH (27 mg, 0.67 mmol) in methanol (15 mL) was added to a
solution of 1 (400 mg, 0.67 mmol) in methanol (20 mL) and stirred
for 1 h. The solvent was removed in vacuo and the residue taken up in
5 mL of dichloromethane. Layering with hexanes yielded, within 3
days, 180 mg (43%) of 7 as colorless crystals, mp 221 °C. IR (KBr):
1
1609 (vs) (CdO and CdN). H NMR (CDCl3): 1.08 [s, 9H, t-Bu],
1.28 [s, 9H, t-Bu], 2.02 [s, 3H, acetate], 3.58 [s, 2H, CH2-phenol],
4.06 [s, 4H, CH2-py], 5.32 [s, 2H, CH2Cl2], 6.62 [d, J ) 2.6 Hz, 1H,
Hc], 6.83 [d, J ) 2.6 Hz, 1H, Ha], 7.00-7.66 [m, 6H, aromatic], 8.72
[d, J ) 5.0 Hz, 2H, HR].

A Sterically Hindered N,N,O Tripod Ligand
Inorganic Chemistry, Vol. 40, No. 10, 2001 2311
with hexanes. After 3 days, 199 mg (44%) of 13 had separated as
yellowish crystals, mp 128 °C. IR (KBr): 1635 (s) (CdO), 1606 (vs)
(CdN). 1H NMR (CDCl3): 1.13 [s, 9H, t-Bu], 1.25 [s, 9H, t-Bu], 1.85
[broad, 4H, THF], 3.75 [broad, 4H, THF], 3.78 [s, 2H, CH2-phenol],
3.93 [d, J ) 15.6 Hz, 2H, CH2-py], 4.08 [d, J ) 15.6 Hz, 2H, CH2-
py], 6.68 [d, J ) 2.4 Hz, 1H, Hc], 6.93 [d, J ) 2.4 Hz, 1H, Ha], 7.09-
7.69 [m, 10H, aromatic], 8.78 [d, J ) 4.0 Hz, 2H, HR], 10.01 [s, 1H,
CHO].
was adjusted to 5.00, 10.00, 15.00, 20.00, and 25.00 mM, corresponding
to a 50-, 100-, 150-, 200-, and 250-fold excess.
The absorption intensities were recorded for 5t1/2. The I value at
this time was taken as I∞. Reaction times for 90% conversion were
2-8 h. Up to a conversion of 75% the reactions were cleanly of first
order with a correlation coefficient greater than 0.997. For the
computations the measurements up to 2t1/2 were included. The
reproducibility of the I values was within 10%. Each data point in Figure
8 represents the average of three measurements.
Anal. Calcd for C34H39N3O3Zn‚C4H8O (Mr ) 603.1 + 72.1): C,
67.60; H, 7.02; N, 6.22; Zn, 9.68. Found: C, 66.42; H, 7.05; N, 6.04;
Zn 9.43.
Structure Determinations. Crystals of 5, 10, 11, 13, and 14 were
obtained directly from the preparations, those of 1 and 9 by layering
THF solutions with hexanes. They were immersed in fluorinated
polyether oil for the measurements on a Nonius CAD4 diffractometer
with graphite-monochromatized Mo KR radiation (λ ) 0.7107 Å) at
180 K (room temperature for 13 and 14). No absorption corrections
were applied. The structures were solved with direct methods and
refined anisotropically with the SHELX program suite.61 Hydrogen
atoms were included with fixed distances and isotropic temperature
factors 1.5 times those of their attached atoms. Parameters were refined
against F2. Complexes 1, 5, and 7 show a rotational disorder of one of
their tert-butyl groups; complex 10 has one of its phosphate phenyl
groups disordered over two positions. Drawings were produced with
SCHAKAL.62 The crystallographic data are listed in the Supporting
Information.
Complex 14. Like 13 from 1 (400 mg, 0.67 mmol) and potassium
2-(hydroxymethyl)phenolate (109 mg, 0.67 mmol). The residue after
filtration was picked up in 5 mL of acetone and exposed to an
atmosphere of diethyl ether. After 4 days, 302 mg (68%) of 14 had
separated as yellowish crystals, mp 207 °C. IR (KBr): 1605 (s) (Cd
1
N). H NMR (CDCl3): 1.23 [s, 9H, t-Bu], 1.38 [s, 9H, t-Bu], 2.09 [s,
6H, acetone], 3.82 [s, 2H, CH2-phenol], 3.91 [d, J ) 16.0 Hz, 2H,
CH2-py], 4.14 [d, J ) 16.0 Hz, 2H, CH2-py], 4.86 [s, 2H, CH2OH],
6.56-7.79 [m, 12H, aromatic], 8.93 [d, J ) 5.2 Hz, 2H, HR].
Anal. Calcd for C34H41N3O3Zn‚(CH3)2CO (Mr ) 605.1 + 58.1): C,
67.01; H, 7.14; N, 6.34. Found: C, 67.09; H, 7.12; N, 6.24.
Reaction of 3 with TNP. A solution of 0.17 mmol of 3 in 15 mL
of chloroform was prepared as described above. Tris(p-nitrophenyl)
phosphate (39 mg, 0.085 mmol) was added with stirring and the
resulting solution monitored by 31P NMR and UV. After 2 h the 31P
NMR resonance of TNP (-20.0 ppm) had completely disappeared and
been replaced by the resonance of 15 (-15.4 ppm). The solution had
turned yellow, and the UV spectrum confirmed the formation of 11 by
the unperturbed absorption band at 385 nm. When an equimolar amount
of bis(p-nitrophenyl) phosphate (58 mg, 0.17 mmol) was added to the
solution of 3, the UV spectrum showed no sign of the formation of
p-nitrophenol or 11, and the NMR spectrum showed the quantitative
formation of 15.
Kinetic Data. Measurements were performed on a JASCO V-570
UV spectrometer by continually recording the 385 nm absorption of
11. Chloroform (UV quality) was used as a solvent. Samples of TNP
were taken from a 2.00 mM stock solution. Stock solutions of 3 (0.100
M) were prepared prior to use as described above from 1 and
triethylamine. The measuring chamber and the solutions were thermo-
stated to 25.0 °C for 30 min prior to the measurements and during the
kinetic runs. Reagents were mixed in the quartz cuvettes. The
concentration of TNP was 0.100 mM for all measurements; that of 3
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen Indus-
trie. We are indebted to Drs. W. Deck, M. Tesmer, and M.
Rombach for assistance during the measurements and their
interpretation.
Supporting Information Available: A table with all crystal-
lographic details, fully labeled ORTEP plots for all seven structure
determinations, a plot of the rate constants of the phosphate cleavage,
and seven crystallographic files, in CIF format. This information is
available free of charge via the Internet at http://pubs.acs.org.
IC001204J
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