Phenoxyl radical complexes of zinc(II)

Article abstract of DOI:10.1021/ja970417d

A series of phenoxyl radical complexes of zinc(II) have been generated in solution and, in one instance, isolated as solid material (5) in order to study their spectroscopic features by EPR, resonance Raman, and UV-vis spectroscopy. They serve as model complexes for the active form of the copper containing fungal enzyme galactose oxidase. The complexes [Zn(L¹H₂)]BF₄·H₂O (1), [Zn(L²H₂)]BF₄·H₂O (2), [Zn(L²H)] (2a), [Zn(L³)(Ph₂acac)] (3), [Zn(L⁴)(Ph₂acac)] (4), and [Zn(L⁴)(Me-acac)] (6) were synthesized from solutions of Zn(BF₄)₂·4H₂O and the corresponding ligand (L¹H₃ = 1,4,7-tris(3,5-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane; L²H₃ = 1,4,7-tris-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)- 1,4,7-triazacyclononane; L³H = 1,4-dimethyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1, 4,7-triazacyclononane; L⁴H 1,4-dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7 -triazacyclononane, Ph₂acac^- = 1,3-diphenyl-1,3-propanedionate, and Me-acac^- = 3-methyl-2,4-pentanedionate). Complexes 2, 3·0.5 toluene·1n-hexane, and 4 were structurally characterized by single-crystal X-ray crystallography. An electrochemical investigation of these complexes in CH₃CN and/or CH₂Cl₂ solution revealed that the coordinated phenolate ligands undergo reversible one-electron oxidations with formation of coordinated phenoxyl radicals. Synthetically, the microcrystalline, paramagnetic (μ(eff) = 1.7 μ(B)), solid material of [Zn(L⁴)(Ph₂acac)]PF₆ (5) was produced by one electron oxidation of 4 by 1 equiv of ferrocenium hexafluorophosphate in dry CH₂Cl₂. Oxidation of coordinated phenol pendent arms in 1, 2, and 2a occurs at significantly higher potentials and is irreversible. Electronic (UV-vis), electron paramagnetic resonance (EPR), and resonance Raman (RR) spectra of the radicals have been studied in solution and allow the description of the electronic structure of these coordinated phenoxyl radical complexes.

Full text of DOI:10.1021/ja970417d

J. Am. Chem. Soc. 1997, 119, 8889-8900
8889
Phenoxyl Radical Complexes of Zinc(II)
Achim Sokolowski, Jochen Mu1ller, Thomas Weyhermu1ller, Robert Schnepf,
Peter Hildebrandt, Knut Hildenbrand, Eberhard Bothe, and Karl Wieghardt*
Contribution from the Max-Planck-Institut fu¨r Strahlenchemie, P.O. Box 10 13 65,
D-45413 Mu¨lheim an der Ruhr, Germany
ReceiVed February 7, 1997X
Abstract: A series of phenoxyl radical complexes of zinc(II) have been generated in solution and, in one instance,
isolated as solid material (5) in order to study their spectroscopic features by EPR, resonance Raman, and UV-vis
spectroscopy. They serve as model complexes for the active form of the copper containing fungal enzyme galactose
oxidase. The complexes [Zn(L1H2)]BF4‚H2O (1), [Zn(L2H2)]BF4‚H2O (2), [Zn(L2H)] (2a), [Zn(L3)(Ph2acac)] (3),
[Zn(L4)(Ph2acac)] (4), and [Zn(L4)(Me-acac)] (6) were synthesized from solutions of Zn(BF4)2‚4H2O and the
corresponding ligand (L1H3 ) 1,4,7-tris(3,5-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane; L2H3 ) 1,4,7-tris-
(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane; L3H ) 1,4-dimethyl-7-(3,5-di-tert-butyl-2-hy-
droxybenzyl)-1,4,7-triazacyclononane; L4H ) 1,4-dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-
triazacyclononane, Ph2acac- ) 1,3-diphenyl-1,3-propanedionate, and Me-acac- ) 3-methyl-2,4-pentanedionate).
Complexes 2, 3‚0.5 toluene‚1n-hexane, and 4 were structurally characterized by single-crystal X-ray crystallography.
An electrochemical investigation of these complexes in CH3CN and/or CH2Cl2 solution revealed that the coordinated
phenolate ligands undergo reversible one-electron oxidations with formation of coordinated phenoxyl radicals.
Synthetically, the microcrystalline, paramagnetic (µeff ) 1.7 µB), solid material of [Zn(L4)(Ph2acac)]PF6 (5) was
produced by one electron oxidation of 4 by 1 equiv of ferrocenium hexafluorophosphate in dry CH2Cl2. Oxidation
of coordinated phenol pendent arms in 1, 2, and 2a occurs at significantly higher potentials and is irreversible.
Electronic (UV-vis), electron paramagnetic resonance (EPR), and resonance Raman (RR) spectra of the radicals
have been studied in solution and allow the description of the electronic structure of these coordinated phenoxyl
radical complexes.
Introduction
directlysthe electronic spectrum of the active form resembles
features characteristic of phenoxyl radicals, i.e., a strong
absorption at 445 nm (ꢀ > 5000 M-1 cm-1) which is not present
in the one-electron reduced form;12 (iii) the RR spectrum13,14
of active GO shows features which closely resemble those of
genuine phenoxyl radicals, i.e., a ν(C-O•) and a ν(CdC)
stretching frequency at 1487 and 1595 cm-1, respectively, and
(iv) the X-ray absorption Cu K-edge spectra of the active and
one-electron reduced Cu(II) state of GO do not show a shift of
the Cu K-edge energy excluding an oxidation state change of
Cu(III) f Cu(II).15
The crystal structure determination of the inactive form of
GO 9,10 has revealed an active site as depicted in Figure 1 where
the Cu(II) ion is in square pyramidal environment of an axially
and an equatorially bound tyrosine (or tyrosinate), two histidine
residues, and an acetate (or water) ligand. The equatorially
bound Tyr272 is modified by a covalent C-S bond to Cys228,
which lowers the redox potential for radical formation. In the
catalytic cycle, intriguing protonation-deprotonation steps have
been invoked where bound and uncoordinated tyrosine (phenol)
ligands and their coordinated tyrosinate (phenolate) analogs as
well as the oxidized, bound tyrosyl form play an interesting
role.16
The occurrence of tyrosyl radicals has been established in a
number of metalloproteins1-4 by spectroscopic methods (elec-
tron paramagnetic resonance (EPR), resonance Raman (RR),
absorption (UV-vis), and in one instance by X-ray crystal-
lography. Well-characterized examples include the R2 subunit
of ribonucleotide reductase with a persistent uncoordinated
tyrosyl radical in close vicinity to a diferric µ-oxo bridged
cluster5-7 and a tyrosyl derivative coordinated to a copper(II)
ion in the fungal enzyme galactose oxidase (GO).8-10 In the
latter case the assignment of oxidation state as Cu(II)-tyrosyl
Versus Cu(III)-tyrosinate is based on spectroscopic features of
the chromophore: (i) the active form of GO is EPR-silent,11
probably due to intramolecular antiferromagnetic coupling
between the tyrosyl radical and the Cu(II) (d9) center; (ii)smore
X Abstract published in AdVance ACS Abstracts, September 1, 1997.
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S0002-7863(97)00417-4 CCC: $14.00 © 1997 American Chemical Society

8890 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Sokolowski et al.
Figure 2. Infrared spectra of 4 and 5 (KBr-disks) in the region 1000-
1800 cm-1
.
triazacyclononane backbone and one, two, or three phenolate
pendent arms.25-29 These tetra-, penta-, and hexadentate ligands
bind very strongly to di- and trivalent transition metal ions.30,31
Here we use the macrocycles L1H3, L2H3, L3H, and L4H shown
in Scheme 1.
Figure 1. Schematic representation of the active site of the inactive
form of Galactose oxidase (pH ) 4.5) using atom coordinates from
the Brookhaven National Laboratory Protein Data Bank.
We have synthesized their complexes with zinc(II) for a
variety of reasons. Zinc(II) with a d10 electronic configuration
is redox-innocent in a wide potential range. Thus, all observable
redox processes of its complexes must be ligand centered.
Secondly, if phenoxyl radical species are generated, they are
paramagnetic (S ) 1/2) and should be ideally suited for EPR
spectroscopic characterization. In addition, the electronic spectra
of such species are unperturbed by d-d transitions. Therefore,
these ligand centered chromophores can be readily investigated
by RR spectroscopy. Finally, zinc(II) is a much weaker Lewis
acid than the trivalent metal ions Ga(III) and Sc(III). It was
hoped that coordinated phenol complexes might also become
synthetically accessible which was not the case for [Ga(L1,2)]
and [Sc(L1,2)] complexes.21
A zinc complex containing an uncoordinated phenoxyl
radical, namely [Zn(BIDPhE)Cl2] (BIDPhE ) 1,1-bis[2-(1-
methylimidazolyl]-1-(3,5-di-tert-butyl-4-oxylphenyl)ethane, has
recently been described.32 Its spectroscopic properties (UV-
vis, EPR, and RR) provide a useful basis for the interpretation
of our data on complexes containing coordinated phenoxyl
radicals.
While low-molecular weight, crystallographically character-
ized transition metal complexes with coordinated phenolate
ligands including those of copper(II)17 are well known, their
analogs containing coordinated phenol ligands are by far less
well understood,18 and the chemistry of coordinated phenoxyl
radicals is still in its infancy.19 Only very recently, we and
others have been able to produce such complexes of Fe,20,21
Ga,21 Sc,21 Cr,22 and Cu,23 and only in one instance such a
species has been structurally characterized by X-ray crystal-
lography, namely [CrIII(L2)](ClO4).22 Model complexes of this
kind are important because they allow to establish definitively
the spectroscopic features of coordinated Vs uncoordinated
phenoxyl radicals and to understand their chemical reactivity.
The synthesis of transition metal complexes containing
phenoxyl radicals has been hampered by the inherent reactivity
of the ligand.24 Stable phenoxyl radicals are available only when
the ortho- and para-positions at the aromatic ring are protected
by bulky nonoxidizable groups, e.g., a tert-butyl group. Oth-
erwise a variety of side reactions including dimerization (radical
coupling) will destroy the radical. Secondly, the transition metal
ion-to-phenoxyl bond is thermodynamically less stable22 than
the corresponding bond to a phenolate. This leads to a situation
where monodentate phenoxyl ligands will rapidly dissociate.
In order to circumvent both synthetic problems we and others
have made use of pendent arm macrocycles with a 1,4,7-
Results
Syntheses. The syntheses of the macrocyclic pendent arm
ligands L1H3 and L2H3 (Scheme 1) have been described
previously as have their isotopomers deuterated at the benzylic
groups, d6-L1H3 and d6-L2H3.21 The ligand 1,4-dimethyl-7-(3,5-
di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L3H)
has been obtained from the reaction of 1 equiv of 1,4-dimethyl-
1,4,7-triazacyclononane with 1 equiv of 2-hydroxy-3,5-di-tert-
(16) Wachter, R. M.; Branchaud, B. P. J. Am. Chem. Soc. 1996, 118,
2782.
(17) Whittaker, M. M.; Chuang, Y.-Y.; Whittaker, J. W. J. Am. Chem.
Soc. 1993, 115, 10029.
(18) Auerbach, U.; Eckert, U.; Wieghardt, K.; Nuber, B.; Weiss, J. Inorg.
Chem. 1990, 29, 938.
(19) Goldberg, D. P.; Lippard, S. J. In Mechanistic Bioinorganic
Chemistry; Thorp, H. H., Pecoraro, V. L., Eds.; Advances in Chemistry
246; American Chemical Society: Washington, D.C., 1995; p 61.
(20) Hockertz, J.; Steenken, S.; Wieghardt, K.; Hildebrandt, P. J. Am.
Chem. Soc. 1993, 115, 11222.
(21) Adam, B.; Bill, E.; Bothe, E.; Goerdt, B.; Haselhorst, G.; Hilden-
brand, K.; Sokolowski, A.; Steenken, S.; Weyhermu¨ller, T.; Wieghardt, K.
Chem. Eur. J. 1997, 3, 308.
(25) Moore, D. A.; Fanwick, P. E.; Welch, M. J. Inorg. Chem. 1989,
28, 1504.
(26) Auerbach, U.; Weyhermu¨ller, T.; Wieghardt, K.; Nuber, B.; Bill,
E.; Butzlaff, C.; Trautwein, A. X. Inorg. Chem. 1993, 32, 508.
(27) Martell, A. E.; Motekaitis, R. J.; Welch, M. J. J. Chem. Soc., Chem.
Commun. 1990, 1748.
(28) Flassbeck, C.; Wieghardt, K. Z. Anorg. Allg. Chem. 1992, 608, 60.
(29) Stockheim, C.; Hoster, L.; Weyhermu¨ller, T.; Wieghardt, K.; Nuber,
B. J. Chem. Soc., Dalton Trans. 1996, 4409.
(22) Sokolowski, A.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Wieghardt,
K. J. Chem. Soc., Chem. Commun. 1996, 1671.
(23) (a) Halfen, J. A.; Young, V. G.; Tolman, W. B. Angew. Chem. 1996,
108, 1832; Angew. Chem., Int. Ed. Engl. 1996, 35, 1687. (b) Halfen, J. A.;
Jazdzewski, B. A.; Mahapatra, S.; Berreau, L. M.; Wilkinson, E. C.; Que,
Jr., L.; Tolman, W. B. J. Am. Chem. Soc. 1997, in press.
(24) Altwicker, E. R. Chem. ReV. 1967, 67, 475.
(30) Clarke, E. T.; Martell, A. E. Inorg. Chim. Acta 1991, 186, 103.
(31) Motekaitis, M. J.; Sun, Y.; Martell, A. E. Inorg. Chim. Acta 1992,
198-200, 421.
(32) Goldberg, D. P.; Watton, S. P.; Maschelein, A.; Wimmer, L.;
Lippard, S. J. J. Am. Chem. Soc. 1993, 115, 5346.

Phenoxyl Radical Complexes of Zinc(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8891
Scheme 1. Ligands and Complexes
butyl-benzylbromide in dry toluene in the presence of KOH.
Sodium [1,4-dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxyben-
zyl)-1,4,7-triazacyclononane], Na[L4], was obtained from a
Mannich reaction of 1,4-dimethyl-1,4,7-triazacyclononane,
paraformaldehyde, (CH2O)n, and 2-tert-butyl-4-methoxyphenol
in methanol. After exchange of the solvent methanol for dry
tetrahydrofuran and addition of 1 equiv of NaH the sodium salt
Na[L4] was obtained as pale-yellow solid. By using (CD2O)n
and CH3OD for the above synthesis the compound Na[d2-L4]
selectively deuterated at the benzylic group was obtained.
The reaction of Zn(BF4)2‚4H2O in acetonitrile with the
macrocyclic pendent arm ligands L1H3 and L2H3 in the ratio
1:1, respectively, affords colorless precipitates of [Zn(L1H2)]-
BF4‚H2O (1) and [Zn(L2H2)]BF4‚H2O (2). Treatment of a
methanolic solution of 2 with KOH results in the formation of
a colorless precipitate of the neutral compound [Zn(L2H)] (2a).
From similar reaction mixtures of L3H and [L4]Na with Zn-
(BF4)2‚4H2O in methanol (1:1) to which 1 equiv of potassium
1,3-diphenyl-1,3-propanedionate (Ph2acac) or, alternatively,
3-methyl-2,4-pentanedionate (Me-acac) has been added, yellow,
microcrystalline precipitates of [Zn(L3)(Ph2acac)] (3), [Zn-
(L4)(Ph2acac)] (4), and [Zn(L4)(Me-acac)] (6) formed, respec-
tively.
It is possible to oxidize complex 4 by one electron with 1
equiv of ferrocenium hexafluorophosphate in dry CH2Cl2
solution. From the solution a green-brown microcrystalline
precipitate of [Zn(L4)(Ph2acac)]PF6 (5) was obtained in 24%
yield. Measurement of the temperature-dependent magnetic
susceptibility (100-300 K) of a powdered sample revealed that
5 is paramagnetic with an effective magnetic moment, µeff (298
K), of 1.7 µB. Complex 5 is quite stable in the solid state and
in solution, but it is easily reduced by an appropriate reducing
agent to the colorless neutral species 4.
Figure 3. (a) Average structure of the cation in crystals of 2 (without
hydrogen atoms). (b) Possible hydrogen bonding scheme between the
cation and a water molecule of crystallization. Note that atoms Zn and
O(20) lie on a 3-fold crystallographic axis. Small open circles represent
hydrogen atoms.
(1627-1648 cm-1). These bands are assigned to ν(CdC)
stretching frequencies (see below) in accordance with a quinone-
like resonance structure of coordinated phenoxyls. The ν(C-
O•) stretching frequency expected for phenoxyl radicals at
∼1500 cm-1 has not been unambiguously identified in the
spectrum of 5 due to ν(CdO) bands of the coordinated Ph2acac-
ligand in both 4 and 5.
The 1H NMR spectra of 3, 4, and 6 in CDCl3 solution at 20
°C show unambiguously that a stable conformation of the six-
membered chelate ring, Zn-O-C-C-C-N, of the coordinated
phenolate pendent arm is retained in solution since the protons
of the benzyl group are diastereotopic.
In contrast, in the corresponding spectra of 1 and 2 in CD3-
CN solution at 20 °C only a singlet is observed for the six
benzylic protons; these protons are magnetically equivalent on
the time scale of a 1H NMR experiment (∼10-4 s). This implies
that under these experimental conditions the phenol pendent
arms undergo rapid deprotonation-protonation. In the spectrum
of 2a in CD2Cl2 at ambient temperature the six benzylic protons
are again diastereotopic giving rise to two doublets at δ ) 2.97
and 4.24 (J ) 10.78 Hz). This indicates that the two phenolates
and the phenol pendent arm are coordinated but equivalent due
to a rapid deprotonation-protonation equilibrium. The spec-
trum of [Zn(L2)]- in CD2Cl2 solution also shows two doublets
for six benzylic protons.
Figure 2 shows a comparison of the infrared spectra of 4 and
5 in the range 1800-1000 cm-1. Two features are remarkable.
The ν(C-O) stretching frequency of the coordinated phenolate
in 4 is observed at 1284 cm-1 but is absent in 5. On the other
hand, 4 does not show bands >1605 cm-1, but in the spectrum
of the oxidized form, 5, new features appear in this region
Crystal Structures. The structures of 2, 3‚0.5 toluene‚1n-
hexane, and 4 have been determined by single-crystal X-ray
crystallography at 173(2) (2) and 100(2)K (3, 4). Figures 3-5
show the structures of the monocation [Zn(L1H2)]+ in 2 and of

8892 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Sokolowski et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) of 2,
3‚0.5Toluene‚1n-Hexane, and 4
Complex 2
N1-C3
Zn-N1
Zn-O1
O1-C5
O1-H
2.160(2)
2.173(2)
1.384(3)
0.85(3)
1.492(3)
1.495(3)
1.419(4)
1.530(3)
1.505(4)
N1-C1
O2-C10
C1-C2
C3-C4
O2-C8
1.375(4)
N1-Zn-N1′
N1-Zn-O1
N1-Zn-O1′
N1-Zn-O1′′
82.64(8)
Zn-O1-H
C5-O1-H
Zn-O1-C5
115(3)
118(3)
127.0(2)
85.95(7)
164.60(7)
106.15(7)
Complex 3‚0.5Toluene‚1n-Hexane
Figure 4. Structure of the neutral complex in crystals of 3‚0.5toluene‚-
1n-hexane.
Zn-O1 1.981(2) Zn-O2 2.038(2) O1-C11 1.312(3)
Zn-O3 2.091(2) Zn-N1 2.177(2) O2-C24 1.273(3)
Zn-N2 2.249(2) Zn-N3 2.279(2) O3-C32 1.260(3)
O1-Zn-O2
O2-Zn-O3
O2-Zn-N1
O1-Zn-N2
O3-Zn-N2
O1-Zn-N3
O3-Zn-N3
N2-Zn-N3
C24-O2-Zn
95.08(7)
89.35(7)
170.86(7)
100.56(8)
166.91(8)
172.14(8)
88.28(8)
78.77(9)
124.9(2)
O1-Zn-O3
O1-Zn-N1
O3-Zn-N1
O2-Zn-N2
N1-Zn-N2
O2-Zn-N3
N1-Zn-N3
C11-O1-Zn
C32-O3-Zn
92.00(8)
92.08(7)
96.09(8)
93.14(7)
79.93(8)
92.78(8)
80.08(8)
129.2(2)
123.9(2)
Complex 4
Zn-O1
Zn-O4
Zn-N3
O1-C11
O2-C20
O4-C29
1.962(3)
2.124(3)
2.210(3)
1.322(5)
1.432(5)
1.271(5)
Zn-O3
Zn-N1
Zn-N2
O2-C14
O3-C21
2.069(3)
2.172(3)
2.265(4)
1.384(5)
1.269(5)
Figure 5. Structure of the neutral complex in crystals of 4.
O1-Zn-O3
O3-Zn-O4
O3-Zn-N1
O1-Zn-N3
O4-Zn-N3
O1-Zn-N2
O4-Zn-N2
N3-Zn-N2
C29-O4-Zn
96.8(1)
87.5(1)
170.2(1)
97.4(1)
170.6(1)
171.3(1)
91.1(1)
79.5(1)
119.3(3)
O1-Zn-O4
O1-Zn-N1
O4-Zn-N1
O3-Zn-N3
N1-Zn-N3
O3-Zn-N2
N1-Zn-N2
C11-O1-Zn
C21-O3-Zn
91.9(1)
91.9(1)
96.9(1)
92.1(1)
82.1(1)
91.5(1)
79.7(1)
130.6(3)
123.6(3)
the neutral complexes in crystals of 3 and 4, respectively. Table
1 summarizes selected bond distances and angles.
The monocation in 2 possesses crystallographically imposed
C3 symmetry which is not compatible with the fact that it
contains two coordinated phenolic and one bound phenolate
pendent arm of the monoanionic macrocycle [L1H2]-. As a
consequence, a static disorder is observed which has been
successfully modeled. One water molecule of crystallization
is in close vicinity to the three facial oxygen donor atoms of
the cation via hydrogen bonding contacts (O1‚‚‚O(20) 2.661-
(4) Å). The oxygen atom O(20) of this H2O molecule lies on
a crystallographic C3 axis. In the difference Fourier map two
residual electron density maxima per asymmetric unit were
located within bonding distance to this oxygen. In addition, a
further hydrogen atom was located in close proximity to the
coordinated phenol/phenolate oxygen O(1). These positions
were satisfactorily refined without restraints by using the
occupancy factor 0.33 for each hydrogen atom of H2O and 0.66
for those bound to the phenol oxygen. Figure 3b shows that
this can be interpreted as a 3-fold superposition where two
coordinated phenol groups form hydrogen bonds to the oxygen
atom of the water molecule (Ophenol-H‚‚‚Owater) and one
Table 2. Redox Potentials of Complexesa Vs Fc+/Fc
E1/2, V
complex solvent
Epox, V
1
2
2a
CH3CN
CH3CN
0.39 (rev)
0.17 (rev)
+1.40 (irr)
+0.94 (irr)
+0.80 (irr)
CH2Cl2 -0.28 (rev) -0.05 (rev)
[Zn(L2)]- b CH2Cl2 -0.63 (rev) -0.28 (rev) -0.06 (rev)
3
4
5
6
CH2Cl2 -0.09 (rev)
CH2Cl2 -0.28 (rev)
CH2Cl2 -0.28 (rev)
CH2Cl2 -0.32 (rev)
a Conditions: 0.10 M [N(n-but)4]PF6 supporting electrolyte; glassy-
carbon working electrode; T ) 298 K; reference electrode: Ag/AgCl
(LiCl/C2H5OH) or Ag/AgNO3. Cvs were recorded at scan rates 20-
500 mV s-1; square-wave voltammograms were recorded at 30 Hz
frequency and 25 mV pulse height. b This species was generated from
2a by addition of 1 equiv of K[OC(CH3)3].
hydrogen bonding contact is of the type Ophenolate‚‚‚H-Owater
.
Due to this disorder it is not possible to distinguish the bonding
distances between a zinc-to-phenol and that of zinc-to-phenolate
bond, but the average distance of 2.173(2) Å is significantly
longer than genuine zinc-phenolate bonds in 3 and 4 at 1.981-
(2) and 1.962(2) Å. This is in agreement with the fact that
phenolates are better ligands than phenols.
Electrochemistry. The electrochemistry of complexes has
been investigated by cyclic and square-wave voltammetry as
well as coulometry in CH2Cl2 or CH3CN solution containing
0.10 M [N(n-but)4]PF6 as supporting electrolyte. All potentials
are referenced Vs the ferrocenium/ferrocene (Fc+/Fc) couple;
the results are summarized in Table 2.
For the sake of clarity we will first discuss the cyclic
voltammograms (cv) of 3, 4, and 6. The insets of Figure 6
show the cvs of 3, 4, and 6. In the potential range +0.5 to
-2.0 V the three species display a single reversible one-electron
transfer wave. Coulometry at a constant potential of +0.10 V
and ambient temperature established that 3, 4, and 6 undergo a
In all structures the zinc ion is six coordinate in a distorted
octahedral environment of the three facially bound amine
nitrogen donors and three oxygen donors which are phenol,
phenolate, or 1,3-diphenyl-1,3-propanedionate derived.
Interestingly, in structures 3 and 4 the phenolate is very
strongly bound (Zn-Ophen ) 1.981(2) in 3 and 1.962(3) Å in
4) and exerts a significant trans-influence on the Zn-N bond
in trans-position.

Phenoxyl Radical Complexes of Zinc(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8893
Figure 6. Electronic absorption spectra of complexes 3 (top), 4
(middle), and 6 (bottom) in CH2Cl2 (dashed lines) and their electro-
chemically generated one-electron oxidized forms (full lines) [3]•+, [4]•+,
and [6]•+, respectively. The insets show the respective cyclic voltam-
mograms recorded at scan rates 500, 400, 200, 100, and 50 mV s-1 in
CH2Cl2 (0.10 M [N(n-but)4]PF6).
one-electron oxidation, eq 1, respectively. During this oxidation
3 (4, 6) h [3]•+ ([4]•+, [6]•+) + e
(1)
the color of the solution changed from yellow to green. A cv
of such green CH2Cl2 solution showed that no decomposition
of the oxidized species had occurred during the coulometry
experiment. Thus, solutions of [3]•+, [4]•+, and [6]•+ are stable
at room temperature. Complex 3 is more difficult to oxidize
than 4 by 0.19 V.
Interestingly, the cvs of 1 and 2 in CH3CN (Figure 7a) which
each contain two coordinated phenols and one phenolate pendent
arm also display only one reversible one-electron transfer wave
at 0.39 and 0.17 V, respectively. We assign this process to the
reversible formation of one coordinated phenoxyl radical as in
eqs 2 and 3. The potential difference of 220 mV for 1 and 2
which is nearly the same as between 3 and 4 reflects the differing
oxidizability of tert-butyl and methoxy substituted phenol-
ates in [L1H2]- and [L2H2]-, respectively. At potentials >0.9
V the cv reveals an irreversible oxidation which is probably
due to the irreversible oxidation of the coordinated phenol
groups.
Figure 7. (a) Square-wave (30 Hz, pulse height 25 mV) and cyclic
(inset) voltammograms of 2 in CH3CN (0.10 M [N(n-but)4]PF6) at 298
K at a glassy carbon working electrode (scan rates: 200, 100, 50, and
20 mV s-1). (b) Square-wave and cyclic (inset) voltammogram of 2a
in CH3CN (0.10 M [N(n-but)4]PF6) at 298 K (scan rates: 500, 200,
100, 50, 20 mV s-1). Other experimental conditions are as above. (c)
Square-wave and cyclic (inset) voltammogram of 2a in CH2Cl2 (0.10
M [N(n-but)4]PF6) to which 1 equiv of K [OC(CH3)3] has been added
(scan rates as in b)). Other experimental conditions are as above.
that 3 is easier to oxidize than 1 (or 4 than 2) by 480 mV (or
450 mV). We interpret this as a purely electrostatic effect since
oxidation of the monocation in 1 yields a dication, whereas the
neutral species 3 is oxidized to afford a monocation. Differences
in the respective solvation energies may account for the potential
differences. This has previously been shown to be the case for
neutral [Ga(L1)] and [Ga(L2)], where the three identical
coordinated phenolate pendent arms are successively oxidized
to give the respective mono-, di-, and trication.21 The redox
potentials E11/2, E21/2, and E3 for these one-electron transfer
1/2
[Zn(L1H2)]+ h [Zn(L1H2)]•2+ + e
(2)
(3)
processes differ by 200-270 mV per electron transfer step.
The cv of the neutral species 2a dissolved in CH2Cl2 is shown
in Figure 7b. Clearly, two reversible one-electron transfer
processes are observed at -0.28 and -0.05 V which are
assigned to the successive oxidation of two coordinated
phenolate pendent arms. The coordinated phenol is irreversibly
oxidized at Epox ) +0.80 V. Thus, generation of the first
2
[Zn(L2H2)]+ [Zn(L H2)]
h
•2+ + e
colorless
yellow-green
Considering the fact that in complexes 1 and 3 (or 2 and 4)
the same Zn-O-R unit is oxidized, it is at first sight surprising

8894 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Sokolowski et al.
Scheme 2. Electrochemically Identified Equilibria of 2 (and
2a)a
a The potentials refer to experiments in CH2Cl2 (0.1 M [TBA]PF6)
and are referenced Vs the Fc+/Fc couple. Dotted arrows indicate
equilibria which have not been established experimentally; species in
{ } have only been generated by cyclic voltammetry; they are not stable
in solution at 20 °C.
Figure 8. X-band EPR spectrum of electrochemically generated [3]•+
(top) and [d2-3]•+ (bottom) in CH2Cl2 (0.10 M [N(n-but)4]PF6) at 298
K (microwave power 0.8 mW; modulation amplitude 0.06 mT, a
decrease to 0.002 mT did not allow to resolve any hyperfine splitting).
Solid lines are experimental spectra; broken lines represent simulations.
coordinated phenoxyl radical in 2a is easier by 450 mV than in
2 which again reflects the difference in solvation energies for
the two processes.
We have also generated the monoanionic form of 2, [Zn(L2)]-,
in CH2Cl2 solution from 2a by addition of 1 equiv of potassium
tert-butyloxide and recorded its cv (Figure 7c). This monoanion
contains three coordinated phenolate pendent arms, and, con-
sequently, three reversible one-electron transfer waves are
observed at E1/2 ) -0.63, -0.28, and -0.05 V which were
assigned to the formation of the mono-, bis-, and trisphenoxyl
radical complexes as in eq 4, respectively. Scheme 2 sum-
-e
[Zn(L2)]- y-+ee
\
z [Zn(L2)]• y-+ee
\
z [Zn(L2)] z [Zn(L2)]•••2+
y\
••+
+e
(4)
Figure 9. X-band EPR spectra of electrochemically generated [4]•+
(a) and [d2-4]•+ (b) in CH2Cl2 at 298 K (microwave power 0.4 mW,
modulation amplitude 0.06 mT; decreasing the modulation amplitude
to 0.002 mT did not improve the resolution). Solid lines: experimental
spectra; broken lines: simulations).
marizes the species generated from 2, 2a via protonation-
deprotonation and one-electron oxidation reactions.
EPR Spectroscopy. X-band EPR spectroscopy on CH3CN
or CH2Cl2 solutions at 298 K of the electrochemically one-
electron oxidized forms of 1, 2, 3, 4, 6 and of [Zn(L2)]- proved
to be informative because they all contain one coordinated
phenoxyl radical (S ) 1/2) ligand. At 298 K the six radical
species display signals at giso ) 2.0045 ( 0.0002 which is
characteristic for phenoxyl radicals.24 Table 3 summarizes the
results.
Figure 8 displays the EPR spectra of [3]•+ and [d2-3]•+. The
spectra were satisfactorily simulated by taking into account
hyperfine coupling with one benzylic proton only. Hyperfine
coupling to the other benzylic proton, to the amine nitrogen, to
the two aromatic protons in meta-position, or to the 67Zn (I )
5/2) isotope (4.1% natural abundance) was not resolved. The
fact that coupling to only one benzylic proton is observed is in
line with the RR data (see below) which suggest that the
phenoxyl radical is coordinated to the Zn ion. In this case,
magnetically inequivalent benzylic protons are expected to be
present, as is observed in the 1H NMR spectrum of diamagnetic
3.
arises from hyperfine coupling with this position (Figure 9b).
The smaller doublet did not collapse upon deuteration. From
a comparison with the identical spectrum of [6]•+ it is obvious
that this is not due to coupling to the methine proton of the
acac ligand, and, therefore, we assign it to one of the aromatic
protons of the coordinated phenoxyl radical. Simulation of the
spectrum of [d2-4]•+ shows that a further small proton coupling
(aH ) 0.034 mT, probably due to the other aromatic ring proton)
and a nitrogen coupling (aN ) 0.053 mT) have to be taken into
account.
The EPR spectra of [Zn(L1H2)]•2+ and of its benzyl deuterated
form [Zn(d6-L1H2)]•2+ are very similar (Table 2) to those of
[3]•+ and [d2-3]•+, respectively. Hyperfine coupling to only one
benzylic proton is detected which suggests that the phenoxyl
pendent arm is coordinated to zinc(II) and that the spin is
delocalized over one aromatic ring only. The two coordinated
phenol groups do not carry significant spin density.
Identical spectra consisting of eight signals were obtained
for [4]•+ and [6]•+ (Figure 9a). They were analyzed in terms
of a quartet due to the three methoxy protons (aH ) 0.200 mT)
which is split into a doublet (aH ) 0.616 mT) of doublets (aH
) 0.139 mT). Changes in the spectrum induced by selective
deuteration of the benzylic position prove that the larger doublet
The corresponding spectra of [Zn(L2H2)]•2+ and [Zn(d6-
L2H2)]•2+ are shown in Figure 10. From the simulations
hyperfine coupling to three methyl protons of one methoxy
group, to two aromatic protons in meta position, to both
magnetically inequivalent benzyl protons, and to one nitrogen

Phenoxyl Radical Complexes of Zinc(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8895
Scheme 4. A Model for the Phenoxyl Radicals in [2]•2+ and
[4]•+a
a Left: side view; right hand side: end view, looking along the Câ-
C1 bond of the phenoxyl. The dihedral angle for the more strongly
coupled proton, Θ1, is ∼36.5° for [2]•2+ and ∼26.5° for [4]•+
Table 3. EPR Simulation Parameters
.
Figure 10. X-band EPR spectra of [Zn(L2H2)]•2+ (top) and [Zn(d6-
L2H2)]•2+ in CH3CN (0.10 M [N(n-but)4]PF6) at 298 K (microwave
power 0.5 mW; modulation amplitude 0.016 mT). Solid lines are
experimental spectra; broken lines represent simulations.
aH,a
mT
aN,a
mT
aD,a
mT
∆ν,b
complex
solvent
mT
[Zn(L1H2)]•2+ [1]•2+ CH3CN 0.586 (1H)
0.24
[Zn(d6-L1H2)]•2+
CH3CN
0.09(1D) 0.22
0.062
[Zn(L2H2)]•2+ [2]•2+ CH3CN 0.500(1H) 0.067(1N)
Scheme 3. Hyperfine Coupling Constants in mT of
[Zn(L2H2)]•2+ [2]•2+, [Zn(d6-L2H2)]•2+ [d6-2]•2+, and of [4]•+
[d2-4]•+.
0.054(1H)
0.069(2H)
0.218(3H)
[Zn(d6-L2H2)]•2+
CH3CN 0.069(2H) 0.067(1N) 0.077(1D) 0.045
0.218(3H)
CH2Cl2 0.709(1H)
CH2Cl2
CH2Cl2 0.200(3H) 0.053(1N)
0.616(1H)
0.008(1D)
[3]•+
0.200
[d2-3]•+
[4]•+
0.109(1D) 0.187
0.05
0.139(1H)
0.034(1H)
[d2-4]•+
[6]•+ c
CH2Cl2 0.200(3H) 0.053(1N) 0.095(1D) 0.04
0.139(1H)
0.034(1H)
CH2Cl2
a Hyperfine coupling constant. b Line width. c The spectrum and
simulation parameters of [6]•+ are identical to those given above for
[4]•+
.
radicals remain Zn-bound) and (ii) a McConnell-type, angle-
dependent relationship for the hyperfine coupling to the benzylic
proton exists where the magnitude of aH depends on cos2Θ1,
then the ratio of aH for [2]•2+ and [4]•+ should be very similar
to that of the cos2Θ1 values. Experimentally, the ratio of the
coupling constants is 0.812, in good agreement with the above
assumptions. We take this as further corroboration that the
phenoxyl radicals are coordinated in both complexes.
Electronic Spectra. Two-phase oxidation of colorless tris-
2,4,6-tert-butylphenol34 or 2,4-di-tert-butyl-4-methoxyphenol
dissolved in CCl4 with an alkaline aqueous solution of
[Fe(CN)6]3- yields the corresponding persistent yellow phenoxyl
radical in the CCl4 phase. The electronic spectra of these
radicals characteristically display a fairly weak absorption
maximum at 500-800 nm (ꢀ ∼ 200-500 L mol-1 cm-1) and
two intense maxima at 380-440 nm (ꢀ > 103).34 These features
have finger-print character; they have been identified in proteins
such as ribonucleotide reductase35 and its model complex33
where the tyrosyl radicals are not bound to a metal ion and in
the active form of galactose oxidase.12
is detected. The values of these coupling constants are close
to those observed for [4]•+. The assignments are shown in
Scheme 3.
Like in the case of [4]•+ the spin of the coordinated phenoxyl
radical is localized on one ring. Treatment of a CH2Cl2 solution
of [Zn(L2H2)] with potassium tert-butylate yields [Zn(L2)]• and
results in a feature consisting of the multiline spectrum shown
in Figure 10 superimposed on a broad unresolved signal. This
result is in line with RR data showing the coexistence of
protonated and deprotonated species. The broad background
signal reminds of the spectra reported for the radicals [Ga(L1)]•+
and [Sc(L1)]•+ which are isostructural with [Zn(L2)]•.21 Since
we had shown previously that the spin in [Ga(L1)]•+ and [Sc-
(L1)]•+ is delocalized over all three aromatic rings, we suggest
that a similar spin distribution prevails in [Zn(L2)]•.
The EPR spectra of radical complexes [2]•2+ and [4]•+ display
strong hyperfine coupling to one benzylic proton at aH ) 0.500
and 0.616 mT, respectively. Since the phenoxyl radical in both
complexes is the same, it is tempting to ascribe the numerical
difference between these hyperfine coupling constants, aH, to
small structural differences of the coordinated radicals in [2]•2+
and [4]•+. Since the crystal structures of the reduced forms [2]
and [4] have been determined, it is possible to calculate the
dihedral angle Θ1 defined in Scheme 4 of the benzyl protons
of the coordinated benzyl phenolates as 36.5° for [2] and 26.5°
for [4]. The ratio of the two cos2Θ1 values is 0.807. If one
accepts the following two assumptions that (i) the dihedral
angles do not change upon one-electron oxidation (i.e., the
The electronic spectra of electrochemically generated zinc-
bound phenoxyl radicals have been recorded and are sum-
marized in Table 4. For the sake of clarity and simplicity we
will first discuss the spectra of colorless 1 and 2 and of their
(33) Goldberg, D. P.; Koulougliotis, D.; Brudvig, G. W.; Lippard, S. J.
J. Am. Chem. Soc. 1995, 117, 3134.
(34) (a) Land, E. J.; Porter, G. Trans. Faraday Soc. (London) 1961, 57,
1885. (b) Land, E. J.; Porter, G. Trans. Faraday Soc. (London) 1963, 59,
2016.
(35) (a) Mann, G. J.; Gra¨slund, A.; Ochiai, E.-I.; Ingemarson, R.;
Thelander, L. Biochemistry 1991, 30, 1939. (b) Sjo¨berg, B.-M.; Gra¨slund,
A. AdV. Inorg. Biochem. 1983, 5, 87.

8896 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Table 4. Electronic Spectra of Complexes
Sokolowski et al.
complex
solvent
λ )
max, nm (ꢀ, L mol-1 cm-1
1
2
2a
3
4
CH3CN
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
222 (1.6 × 104), 248 sh (3.4 × 103), 280 (4.7 × 103), 300 sh (1.1 × 103)
231 (1.6 × 104), 248 sh (8.1 × 103), 294 (8.3 × 103), 316 sh (3.7 × 103)
252 (1.8 × 104), 320 (1.0 × 104)
251 (2.4 × 104), 314 (9.2 × 103), 362 (1.85 × 104)
251 (1.9 × 104), 337 sh (1.3 × 104), 362 (1.6 × 104)
252 (8.1 × 103), 316 (1.2 × 104)
6
[Zn(L1H2)]•2+ a
[Zn(L2H2)]•2+ a
[Zn(L2)]- a
[Zn(L2)]• a
[3]•+ a
228 (2.1 × 104), 284 (1.6 × 104), 298 (1.2 × 104), 394 sh (3.2 × 103), 408 (3.5 × 103), 700 (320)
228 sh (1.3 × 104), 303 (1.6 × 104), 334 sh (9.5 × 103), 408 sh (6.9 × 103), 425 (8.3 × 103), 561 (370)
255 (1.4 × 103), 322 (8.4 × 103)
248 (7.6 × 103), 306 (1.5 × 104), 338 sh, 402 sh, 419 (6.0 × 103), 522 (350)
254 (2.0 × 104), 283 (1.5 × 104), 361 (2.5 × 104), 408 (3.8 × 103), 672 (320)
252 (1.6 × 104), 307 (1.5 × 104), 350 (2.3 × 104), 419 (6.1 × 103), 543 (310)
305 (2.3 × 104), 401 sh (4.8 × 103), 418 (6.0 × 103), 545 (280)
[4]•+ a
[6]•+ a
a Species generated by controlled potential electrolysis in the indicated solvent containing 0.10 M [N(n-but)4]PF6.
[N-(n-butyl)4]PF6. The spectra were recorded at 20 °C by using
excitation lines between 413 and 432 nm, coincident with the
phenoxyl radical π f π* transition.
The RR spectrum of [Zn(L4)(Me-acac)]•+ ([6]•+) shown in
Figure 12a displays two prominent bands at 1511 and 1611 cm-1
which constitute the characteristic vibrational signature of para-
substituted phenoxyl radicals36a,37 and which, in turn, differs
significantly from that of the corresponding phenolates.37 These
two bands are readily assigned to the modes ν7a (1511 cm-1
)
and ν8a (1611 cm-1) which predominantly include the C-O
stretching and the Cortho-Cmeta stretching coordinate, respec-
tively. For the uncoordinated 2,6-di-tert-butyl-4-methoxyphe-
noxyl radical the ν8a mode is found at a substantially lower
frequency (1590 cm-1) and with a significantly weaker RR
activity.38 Thus, the RR spectrum confirms the notion that one-
electron oxidation of 6 generates a phenoxyl radical ligand,
which is coordinated to the Zn ion.
This conclusion is also true for [Zn(L4)(Ph2acac)]•+ ([4]•+)
since its RR spectrum reveals a similar picture (Figure 12b)
with two strong bands at 1509 and 1611 cm-1. The slightly
lower frequency of the ν7a mode in [4]•+ as compared to [6]•+
results from the effect of the ligand exchange, Ph2acac- Vs Me-
acac-, on the coordinated phenoxyl, which should be most
sensitively reflected by the ν7a mode. The Ph2acac- ligand,
which exhibits a strong π f π* (intraligand) transition at ca.
380 nm, is also the origin of the bands at 1286, 1314, and 1492
cm-1 as well as of the weak shoulder at 1600 cm-1 in the
spectrum of [4]•+, which are preresonance enhanced with 417
nm excitation. The contribution of these bands, relative to those
of the phenoxyl radical, is significantly increased in the RR
spectrum of [3]•+ (Figure 12c). Due to the blue-shift of the π
f π* phenoxyl transition which overlaps with that of Ph2acac-,
a selective enhancement of the phenoxyl compared to the
Ph2acac- modes is not possible. Choosing an excitation line
on the high-energy side of the 380 nm π f π* transition at
351 nm, however, provides a selective enhancement of the RR
bands of Ph2acac- (Figure 12d). The comparison with the RR
spectrum in Figure 12c clearly shows that the only band which
can unambiguously be attributed to the phenoxyl radical in [3]•+
is the 1511 cm-1 band, whereas the band at 1600 cm-1
originates from a Ph2acac- mode. This latter band most likely
obscures the ν8a phenoxyl mode which, for the p-tert-butyl-
substituted species, is found in the range between 1591 and 1601
cm-1 and generally exhibits a relatively weak RR intensity.38
Figure 11. Electronic spectra of 1 and 2 and their one-electron oxidized
radicals in CH3CN at 298 K.
one-electron oxidized, yellow-green radicals [Zn(L1H2)]•2+ and
[Zn(L2H2)]•2+ which are shown in Figure 11. The spectral
changes observed upon oxidation of the colorless monocations
of 1 and 2 to dications are dramatic and clearly indicate the
formation of phenoxyl radicals. The one-electron oxidation of
[Zn(L2)]- yielding the neutral radical [Zn(L2)]• is accompanied
by similar spectral changes. A comparison of the spectra of
[Zn(L2H2)]•2+ and [Zn(L2)]• shows that protonation of the latter
shifts the two intense maxima at ∼410 nm bathochromically
and increases their intensities (Table 4). Complexes 3, 4, and
6 have two chromophores, namely the phenolate pendent arm
and the 1,3-diphenyl-1,3-propanedionate or 3-methyl-2,4-pen-
tanedionate anion, which display very intense π f π* transitions
at <400 nm. Upon one-electron oxidation new maxima at ∼415
nm and ∼600 nm again indicate the formation of phenoxyl
radicals (see Figure 6).
The electronic spectra of the above coordinated phenoxyl
complexes do not differ much from that reported for [Zn-
(BIDPhE)Cl2] which contains an uncoordinated radical.32 It is
therefore not possible to discern unequivocally between these
forms by their electronic spectra alone.
Resonance Raman Spectroscopy. In order to gain further
information on characteristic spectroscopic features of coordi-
nated phenoxyl radicals we have measured RR spectra of various
electrochemically generated Zn phenoxyl radical complexes in
acetonitrile or dichloromethane solutions containing 0.1 M
(36) (a) Tripathi, G. N. R.; Schuler, R. H. J. Phys. Chem. 1988, 92, 5129.
(b) Johnson, C. R.; Ludwig, M.; Asher, S. A. J. Am. Chem. Soc. 1986,
108, 905.
(37) Mukherjee, A.; McGlashen, M. L.; Spiro, T. G. J. Phys. Chem. 1995,
99, 4912.
(38) Schnepf, R.; Sokolowski, A.; Mu¨ller, J.; Bachler, V.; Wieghardt,
K.; Hildebrandt, P. J. Am. Chem. Soc. Submitted for publication.

Phenoxyl Radical Complexes of Zinc(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8897
Figure 13. RR spectra of electrochemically generated radical species
[Zn(L2H2)]•2+, [Zn(L2H)]•+, and [Zn(L2)]• in (A) CH3CN (+ K[OC-
(CH3)3]), (B) CH3CN with [Zn(L2H)] as starting material, (C) CH3CN
with [Zn(L2H2)]+ as starting material. The component spectra of
individual species are indicated by the dashed ([Zn(L2H2)]•2+), dotted
([Zn(L2H)]•+), and dashed-dotted ([Zn(L2)]•) lines.
Figure 12. RR spectra of (a) [6]•+ (λexc ) 413 nm); (b) [4]•+ (λexc
)
417 nm); (c) [3]•+ (λexc ) 413 nm); (d) [4]•+ (λexc ) 351 nm); (e) [4]
(λexc ) 351 nm) measured in CH3CN solution containing 0.1 M [N(n-
but)4]PF6, except [6]•+ in CH2Cl2.
Finally, it is instructive to compare the RR spectrum of the
Ph2acac- ligand in [4]•+ with that of the reduced complex [4],
measured under the same conditions (Figure 12d,e). In [4]•+,
all the Ph2acac- bands are found at higher frequencies (by 2-7
cm-1) compared to [4]. Evidently, the oxidation of the
phenolate exerts a subtle influence on the electron density
distribution and the structure of the Ph2acac- ligand.
The assignment of the component spectra to the individual
species was straightforward by taking into account that [Zn-
(L2H2)]•2+ is the prevailing form in samples prepared from [Zn-
(L2H2)]+ in the absence of added base (Figure 13c), whereas
[Zn(L2)]• is expected to be the main component in the presence
of high base concentrations (Figure 13a,b). The ν8a and ν7a
frequencies of the various species obtained are listed in Table
5. The high ν8a frequencies as well as the frequency difference
(ν8a-ν7a) are characteristic for metal-coordinated phenoxyl
radicals. On the other hand, the variation of the frequencies,
in particular the frequency difference (ν8a-ν7a), from [Zn(L2)]•
(95 cm-1) to [Zn(L2H)]•+ (102 cm-1), and [Zn(L2H2)]•2+ (115
cm-1) indicates that the stepwise protonation of the phenolates
affects the electron distribution in the coordinated phenoxyl.
Attempts to obtain the RR spectrum of the corresponding
radical complex derived from [1] was aggravated by its
significantly lower stability. Thus, it was only possible to detect
The Zn trisphenolato complexes as well as their one-electron
oxidation products are involved in acid-base equilibria includ-
ing [Zn(L2)]-/[Zn(L2)]•, [Zn(L2H)]/[Zn(L2H)]•+, and [Zn(L2H2)]+/
[Zn(L2H2)]•2+ (see Scheme 2). We have measured a series of
RR spectra from radical complexes in different solvents and in
the absence and presence of the proton-accepting base tert-butyl
oxide; however, none of these species could be obtained in a
pure form. Thus, the measured spectra include contributions
from all three radical species, [Zn(L2H2)]•2+, [Zn(L2H)]•+, and
[Zn(L2)]•, albeit with different relative concentrations. This is
most clearly reflected in the ν7a band region (1490-1530 cm-1
as shown by a selection of the spectra in Figure 13.
)
the ν7a and ν8a modes of [Zn(L1H2)]•2+ at 1506 and 1593 cm-1
.
This indicates that the replacement of the methoxy- by the tert-
butyl-substituent lowers the ν8a frequency, thereby reflecting a
decrease of the Cortho-Cmeta bond strength.
In order to determine the most relevant spectral parameters
of the radicals [Zn(L2)]•, [Zn(L2H)]•+, and [Zn(L2H2)]•2+, i.e.,
ν8a and ν7a, we have focused onto the spectral range between
1470 and 1650 cm-1 by employing a component analysis.39 This
approach takes into account that the frequencies and half widths
of the ν8a and ν7a bands of a given species are independent of
its relative concentration and the excitation line. In a good
approximation, also the ν8a/ν7a intensity ratio can be regarded
as constant for the various excitation lines which were employed
(418-432 nm). Thus, the eight measured spectra were fitted
by a superposition of component spectra rather than of
independent bands, as shown in Figure 13. In this fashion, a
satisfactory fit for all measured spectra was achieved with an
overall error of (0.5 and (1.6 cm-1 for the frequencies and
half widths, respectively, and of 15.5% for the relative intensi-
ties.
Discussion and Conclusion
In the present study we have shown that pendent arm
macrocycles of the type 1,4,7-tris(2-hydroxybenzyl)-1,4,7-
triazacyclononane form stable octahedral complexes with zinc-
(II). It is possible to reversibly protonate one or two coordinated
phenolate groups affording the corresponding coordinated
phenols. Protonation weakens the resulting Zn-O(H)R bond
with respect to the corresponding Zn-OR bond.
We have established that appropriate bulky substituents at
the coordinated phenolate in ortho and para positions allow a
reversible one-electron oxidation of the ligand with formation
of stable, coordinated phenoxyl radical ligands.

8898 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Sokolowski et al.
Table 5. Comparison of RR Spectral Data of Coordinated and Uncoordinated Phenoxyl Radicals
compound
ν8a, cm-1
ν7a, cm-1
∆(ν8a-ν7a), m-1
ref
phenoxyl
tyrosyl
1557
1565
1590
1619
1612
1615
1593
1611
1505
1510
1511
1504
1510
1520
1506
1511
1509
1511
52
55
79
115
102
95
87
100
102
36a
36b
2,6-di-tert-butyl-4-methoxyphenoxyl
this work
this work
this work
this work
this work
this work
this work
this work
[Zn(L2H2)]•2+
[Zn(L2H)]•+
[Zn(L2)]•
[Zn(L1H2)]•2+
[6]•+
[4]•+
1611
not detected
[3]•+
Table 6. Crystallographic Data for 2, 3‚0.5Toluene‚1n-Hexane, and 4
2
3‚0.5Toluene‚1n-Hexane
4
formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
C42H64BF4N3O7Zn
875.14
cubic
I 23
20.636(2)
C47.5H69N3O3Zn
795.43
monoclinic
P21/n
12.118(2)
19.723(3)
19.266(3)
104.23(2)
4463(1)
4
1.184
1716
0.592
0.56 × 0.60 × 0.76
0.71073
100(2)
34664
7829
C35H45N3O4Zn
637.11
monoclinic
P21/c
15.389(3)
11.236(2)
18.449(3)
100.70(3)
3135(1)
4
1.350
1352
0.827
0.14 × 0.07 × 0.21
0.71073
100(2)
12243
4497
8787(2)
8
1.323
3712
0.627
0.32 × 0.53 × 0.60
0.71073
173(2)
20073
2724
188
0.0352
0.61
F
calc, g cm-3
F (000)
µ, mm-1
cryst dimens, mm
λ, Å
T, K
no. of total data collected
no. of unique obsd. data
no. of parameters refined
Ra
493
0.048
0.51
394
0.049
0.43
largest residual electron density e Å-3
a R ) ∑(|Fo| - |Fc|)/∑|Fo|.
The phenoxyl-zinc complexes exhibit spectroscopic features
which allow their unambiguous identification.
difference (ν8a-ν7a) is between 95 and 115 cm-1. In these
spectra, the RR intensity ratio of the modes ν8a and ν7a, I(ν8a)/
I(ν7a), is g 1. In contrast, the uncoordinated radical (2,6-di-
tert-butyl-4-methoxyphenoxyl) exhibits the ν8a mode at a lower
frequency (1590 cm-1), accompanied by a substantial lowering
of the RR intensity, i.e., I(ν8a)/I(ν7a) < 0.1, and a decrease of
(ν8a-ν7a) below 80 cm-1. These findings are in line with the
RR spectroscopic results obtained from Lippard’s complex [Zn-
(BIDPhe)Cl2] with an uncoordinated (dangling) phenoxyl arm.32
These authors only observed the ν7a mode (at the same position
as for the free radical BIDPhe), whereas the ν8a band was
apparently not detectable. Also, for ribonucleotide reductase
(uncoordinated tyrosyl) only the ν7a mode was detected.40 On
the other hand, the RR spectrum of the active form of GO allows
the identification of both modes at 1595 (ν8a) and 1487 cm-1
(i) Their electronic spectra resemble closely those of unco-
ordinated phenoxyl radicals. One or two intense absorption
maxima in the visible range at ∼400 nm are bathochromically
shifted upon coordination and their intensity increases. A third
less intense absorption maximum at 500-800 nm is also
detectable in the metal ion bound and uncoordinated species.
(ii) The EPR spectra of zinc phenoxyl complexes show that
on the time scale of such an experiment the spin of the unpaired
electron is delocalized not only over the phenoxyl aromatic ring
but also over the metal ion and neighboring coordinated
phenolates but not over bound phenol ligands. It is conceivable
that on the EPR time scale a rapid electron hopping process
occurs as shown in eq 5.
(ν7a), corresponding to a frequency difference of 108 cm-1
,
which is characteristic for a coordinated tyrosyl.14 This conclu-
sion is also true for the related enzyme glyoxal oxidase, which
exhibits an active site similar to GO as reflected, inter alia, by
the same ν8a and ν7a frequencies.41
(5)
Experimental Section
(iii) RR spectroscopy is a very powerful tool for the
identification and characterization of (coordinated) phenoxyl
radicals. Upon excitation in resonance with the π f π*
transition of the phenoxyl, the RR bands originating from the
Physical Measurements. Electronic spectra were recorded on a
Perkin Elmer Lambda 19 (range: 220-1400 nm) or on a Hewlett
Pakard HP 8452A diode array spectrophotometer (range: 220-820
nm). Cyclic voltammograms, square wave voltammograms, and
modes ν7a (∼1500 cm-1; C-O stretching) and ν8a (∼1600 cm-1
;
CdC stretching) are clearly detectable. The exact positions of
these bands as well as their RR intensity ratio can be used to
distinguish between coordinated and uncoordinated phenoxyls.
For Zn-coordinated para-methoxy-substituted phenoxyls, the ν8a
mode is found between 1610 and 1620 cm-1, and the frequency
(39) Do¨pner, S.; Hildebrandt, P.; Mauk, A. G.; Lenk, H.; Stempfle, W.
Spectrochem. Acta 1996, A51, 573.
(40) Backes, G.; Sahlin, M.; Sjo¨berg, B.-M.; Loehr, T. M.; Sanders-
Loehr, J. Biochemistry 1989, 28, 1923.
(41) Whittaker, M. W.; Kersten, P. J.; Nakamura, N.; Sanders-Loehr,
J.; Schweizer, E. S.; Whittaker, J. W. J. Biol. Chem. 1996, 271, 681.

Phenoxyl Radical Complexes of Zinc(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8899
coulometric experiments were performed with EG&G equipment
(Potentiostat/Galvanostat Model 273A). EPR spectra of complexes
(10-3 M, CH3CN/CH2Cl2 solutions containing 0.10 M [N(n-butyl)4]-
PF6) were measured on a Varian E-9 X-band spectrometer with 100
kHz modulation frequency at 298 K in a quartz cell (d ) 0.3 mm).
The data were digitized by means of the data station Stelar DS-EPR
(Stelar s.n.c., Mede, Italy). The spectra were simulated by iteration of
the isotropic hyperfine coupling constants and line widths. We thank
Dr. F. Neese (Abteilung Biologie der Universita¨t Konstanz) for a copy
of his EPR simulation program. All NMR spectra were recorded on a
400 MHz Bruker AMX series spectrometer.
RR spectra were recorded with a U1000 spectrograph (2400/mm
holographic gratings) equipped with a liquid nitrogen-cooled CCD
detector (Instruments S.A.). The output of a dye laser (stilbene 3;
Coherent 899-01) pumped by an argon ion laser (multiline UV;
Coherent Innova 400) served as excitation source. The laser power at
the sample was about 50 mW. In order to avoid photoinduced
degradation, the sample which exhibits an optical density of ca. 1.5 at
the excitation wavelength was deposited in a rotating cell. The Raman
scattered light was detected at 90° with a scrambler placed in front of
the entrance slit of the spectrometer to account for the polarization-
sensitivity of the gratings. The spectral slit width was 2.8 cm-1. The
spectra, measured with an acquisition time of 15 s, were linearized in
wavenumbers yielding an increment of 0.24 cm-1 and a total spectral
range of ca. 200 cm-1. Thus, several spectra covering different but
overlapping ranges are combined to give the whole spectra displayed
in this work. In these spectra, the contributions of the solvent and the
supporting electrolyte are subtracted.
(CDCl3, 400 MHz): δ 6.76 (d, J ) 2.9 Hz, 1H), 6.39 (d, J ) 2.9 Hz,
1H), 3.73 (s, 2H), 3.72 (s, 3H), 2.88 (t, 4H), 2.63 (t, 4H), 2.51 (s, 4H),
2.33 (s, 6H), 1.39 (s, 9H) ppm. 13C{1H} (CDCl3, 100 MHz): δ 151.3,
150.8, 137.6, 123.5, 112.2, 111.0, 61.8, 58.2, 57.9, 55.5, 53.7, 46.5,
34.7, 29.3 ppm.
[Zn(L1H2)]BF4‚H2O (1) and [Zn(L2H2)]BF4‚H2O (2). A solution
of H3L1 or H3L2 (1.0 mmol) in acetonitrile (50 mL) and Zn(BF4)2‚-
4H2O (0.31 g; 1.0 mmol) was heated to reflux for 2 h. From the cooled
solution colorless microcrystals precipitated which were filtered off;
yield: ∼70%. Anal. Calcd for C51H82BF4N3O4Zn: C, 64.2; H, 8.7;
N, 4.4. Found: C, 63.4; H, 8.5; N, 4.4. Anal. Calcd for C42H63-
BF4N3O7Zn: C, 57.6; H, 7.2; N, 4.8. Found: C, 58.1; H, 7.3; N, 4.9.
The deuterated isotopomers [Zn(d6-L1H2)]BF4‚H2O (d6-1) and [Zn(d6-
L2H2)]BF4‚H2O (d6-2) were prepared analogously by using the deu-
terated ligands d6-H3L1 or d6-H3L2 for the synthesis. 1: 1H NMR
(CD3CN, 400 MHz): δ 7.42 (d, J ) 2.28 Hz, 1H); 7.40 (d, J ) 2.39
Hz, 2H); 7.12 (d, J ) 2.39 Hz, 2H); 7.09 (d, J ) 2.28 Hz, 1H); 3.99
(s, 6H); 2.8 (m, 6H); 2.6 (m, 6H); 1.47 (s, 9H); 1.44 (s, 18H); 1.28 (s,
27H) ppm. 13C{1H} NMR (CD3CN, 100 MHz): δ 152.9, 144.3, 137.4,
127.2, 126.2, 121.6, 60.3, 51.8, 35.5, 35.0, 31.7, 30.9, 30.4 ppm. 2:
1H NMR (CD3CN, 400 MHz): δ 6.92 (d, J ) 3.04 Hz, 3H); 6.68 (d,
J ) 3.04 Hz, 3H); 3.89 (s, 6H); 2.7 (m, 6H); 2.5 (m, 6H); 1.46 (s,
21H); 1.41 (s, 6H). 13C{1H} NMR (CD3CN, 100 MHz): 154.4, 148.8,
139.6, 123.6, 115.4, 114.4, 62.3, 56.2, 51.7, 35.5, 30.7, 30.2 ppm.
[Zn(L2H)] (2a). To a solution of 2 (0.43 g; 0.5 mmol) in CH3OH
(70 mL) was added KOH (0.10 g; 1.7 mmol) at ambient temperature.
Upon stirring for a few minutes a colorless precipitate formed which
was collected by filtration and recrystallized from CH2Cl2 solution. (0.22
g; 56%). Anal. Calcd for C42H61N3O6Zn: C, 65.6; H, 8.0; N, 5.5.
Found: C, 64.8; H, 8.1; N, 5.4. 1H NMR (CD2Cl2, 400 MHz): δ 6.79
(d, J ) 3.20 Hz, 3H); 6.44 (d, J ) 3.20 Hz, 3H); 4.24 (d, J ) 10.78
Hz, 3H); 3.68 (s, 9H); 2.97 (d, J ) 10.78 Hz, 3H); 2.62 (m, 6H); 2.05
(m, 6H); 1.46 (s, 27H) ppm. 13C{1H} (CD2Cl2, 100 Hz): δ 162.6,
146.8, 136.7, 123.7, 114.9, 114.1, 63.5, 57.1, 56.4, 48.2, 34.9, 30.4
ppm.
Treatment of a solution of 2a in CD2Cl2 (∼10-2 M) with 1 equiv of
potassium tert-butyloxide generates a solution of [Zn(L2)]- and tert-
butylhydroxide. The NMR data of 2a are as follows: 1H NMR (CD2-
Cl2, 400 MHz): δ 6.80 (d, J ) 3.28 Hz, 3H); 6.44 (d, J ) 3.28 Hz,
3H); 4.24 (d, J ) 10.78 Hz, 3H); 3.68 (s, 9H); 2.97 (d, J ) 10.78 Hz,
3H); 2.05-2.62 (m, 12H); 1.46 (s, 27H) ppm. 13C{1H} (CD2Cl2, 100
MHz): 162.6, 146.8, 136.7, 123.7, 114.9, 114.1, 63.5, 57.1, 56.5, 48.2,
34.9, 30.4 ppm.
[Zn(L3)(Ph2acac)] (3). To a solution of L3H (0.38 g; 1.0 mmol) in
methanol (30 mL) was added Zn(BF4)2‚4H2O (0.31 g; 1.0 mmol). After
30 min of stirring at room temperature K[Ph2acac] (0.26 g; 1.0 mmol)
was added. Within a few hours, a microcrystalline yellow precipitate
formed which was collected by filtration and recrystallized from diethyl
ether. Recrystallization from a toluene/n-hexane mixture (1:1) produced
yellow single crystals of 3‚0.5toluene‚1n-hexane. The isotopomer [Zn-
(d2-L3)(Ph2acac)] was prepared by using d2-L3H as starting material.
Anal. Calcd for C38H51N3O3Zn: C, 68.8; H, 7.75; N, 6.3. Found: C,
68.8; H, 7.7; N, 6.3. FAB-MS (MNBA): m/z (rel intensity %) 661
{[Zn(L3)(Ph2acac)]+, 55}, 438 {[Zn(L3)]+, 100}; 663 {[d2-Zn(L3)(Ph2-
acac)]+, 55}, 440 {[d2-Zn(L3)]+, 100}. 1H NMR (CDCl3, 400 MHz):
δ 7.90 (m, 4H); 7.36 (m, 6H); 7.08 (d, J ) 2.80 Hz, 1H); 6.73 (d, J )
2.80 Hz, 1H); 6.53 (s, 1H); 4.66 (d, J ) 11.54 Hz, 1H); 3.37 (d, J )
11.54 Hz, 1H), 2.80 (s, 3H); 2.42 (s, 3H); 2.12-3.65 (m, 12H); 1.28
(s, 9H); 1.24 (s, 9H) ppm. 13C{1H} (CDCl3, 100 MHz): δ 186.02,
185.28, 166.74, 142.94, 141.70, 136.49, 130.58, 130.01, 129.68, 127.90,
127.86, 127.15, 127.08, 125.52, 123.00, 119.79, 93.09, 64.36, 57.04,
54.42, 54.26, 52.06, 50.34, 47.20, 47.16, 47.13, 35.08, 33.64, 32.02,
29.75 ppm.
Syntheses. The ligands H3L1 and H3L2 and their isotopomers
deuterated at the benzyl groups have been prepared as described in ref
21.
1,4-Dimethyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triaza-
cyclononane (L3H). To a solution of 2,4-di-tert-butylphenol (30 g;
0.145 mol) in methanol (40 mL) was added dropwise with stirring at
room temperature a suspension of paraformaldehyde (4.5 g; 0.15 mol)
and LiOH‚H2O (0.5 g; 0.012 mol) in methanol (40 mL). The mixture
was heated to reflux for 12 h. The solvent was removed by rotary
evaporation, and the orange-brown viscous residue was dissolved in
n-hexane (20 mL). Upon filtration and storage of the solution at 0 °C
for 12 h, a colorless precipitate of 3,5-di-tert-butyl-2-hydroxybenzyl
alcohol formed (17.5 g; 51%).
The crude product was dissolved in CHCl3 (60 mL), and a solution
of PBr3 (8.1 g; 0.03 mol) in CHCl3 (60 mL) was added dropwise. After
stirring the resulting solution for 1 h at 20 °C water (100 mL) was
added. The organic phase was quickly washed three times with water
and dried over MgSO4, and the solvent was removed by evaporation.
The resulting pale-yellow viscous oil crystallized at 0 °C within a few
days (3,5-di-tert-butyl-2-hydroxybenzyl bromide) (19.5 g; 88%).
To a mixture of 1,4-dimethyl-1,4,7-triazacyclononane (2.0 g; 12.7
mmol) and KOH (1.1 g; 20 mmol) in dry toluene (30 mL) was added
dropwise a solution of 3,5-di-tert-butyl-2-hydroxybenzyl bromide (3.8
g; 12.7 mmol) in toluene (30 mL). The solution was heated to 70 °C
for 6 h. The cooled solution was filtered, and the solvent was removed
by rotary evaporation. A yellow-brown viscous oil of the desired ligand
L1H was obtained which was not further purified but used for the
preparation of complexes. EI-Ms (pos. Ion) m/z 375 (M+) calcd for
C23H40N3O 374.6. 1H NMR (CDCl3, 400 MHz): δ 7.18 (d, J ) 2.52
Hz, 1H), 6.81 (d, J ) 2.52 Hz, 1H), 3.77 (s, 2H), 2.94-2.54 (m, 12H),
2.36 (s, 6H), 1.42 (s, 9H), 1.27 (s, 9H). 13C{1H} (CDCl3, 100 MHz):
154.6, 140.0, 135.4, 123.3, 122.5, 122.0, 62.0, 58.3, 58.0, 53.5, 46.7,
34.8, 34.1, 32.7, 29.6 ppm.
Sodium [1,4-Dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxyben-
zyl)-1,4,7-triazacyclononane] Na(L4). A solution of 1,4-dimethyl-
1,4,7-triazacyclononane (3.0 g; 19.0 mmol) and paraformaldehyde (0.57
g; 19.0 mmol) in methanol (50 mL) was heated to reflux for 1 h. To
the then yellow solution was added 2-tert-butyl-4-methoxyphenol (3.42
g; 0.019 mol), and heating to reflux was continued for 12 h. The solvent
was removed by rotary evaporation, and the orange-brown viscous
residue was dissolved in dry THF. To this solution was added a small
amount of NaH (0.46 g; 0.019 mol) (caution: very exothermic reaction).
The reaction volume was reduced to one-half by evaporation of THF
and dry diethyl ether (10 mL) and dry n-pentane (30 mL) were added.
A pale-yellow precipitate of Na(L4) formed (2.75 g; 39%). 1H NMR
[Zn(L4)(Ph2acac)] (4). Yellow crystals of 4 were obtained following
the procedure given above for 3 by using Na(L4) as ligand. Yield:
0.25 g (39%). Single crystals for X-ray crystallography were obtained
by recrystallization from acetonitrile/water (1:1) mixture. The isoto-
pomer [Zn(d2-L4)(Ph2acac)] was prepared by using [d2-L4]Na as starting
material. Anal. Calcd for C35H45N3O4Zn: C, 66.0; H, 7.1; N, 6.6.
Found: C, 65.8; H, 7.0; N, 6.6. FAB-MS (MNBA) m/z (rel intensity
%) 636 {[Zn(L4)(Ph2acac)]+, 35}; 412 {[Zn(L4)]+, 100}; 637.4 {[Zn(d2-
L4)(Ph2acac)]+, 50}; 414 {[Zn(d2-L4)]+, 100}. 1H NMR (CDCl3, 400
MHz): δ 7.89 (m, 4H); 7.36 (m, 4H); 6.77 (d, J ) 3.20 Hz, 1H); 6.54

8900 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Sokolowski et al.
(s, 1H); 6.41 (d, J ) 3.20 Hz, 1H); 4.64 (d, J ) 11.60 Hz, 1H); 3.69
(s, 3H); 3.33 (d, J ) 11.60 Hz, 1H); 2.79 (s, 3H); 2.42 (s, 3H); 2.12-
3.62 (m, 12H); 1.27 (s, 9H) ppm. 13C{1H} (CDCl3, 100 MHz): δ 186.0,
185.6, 164.0, 145.0, 142.9, 141.9, 138.3, 130.0, 129.7, 127.9, 127.8,
127.1, 127.0, 120.0, 114.2, 113.6, 93.2, 64.0, 57.0, 56.5, 54.4, 54.2,
52.0, 50.2, 47.2, 47.1, 47.0, 35.0, 29.5 ppm.
out. The structures were solved by conventional Patterson and
difference Fourier and direct methods and refined with anisotropic
thermal parameters for all non-hydrogen atoms. The Siemens program
package SHELXTL PLUS (G. M. Sheldrick, Universita¨t Go¨ttingen)
was used throughout. All methyl, methylene, methine, and aromatic
hydrogen atoms were placed at calculated positions and were refined
with isotropic temperature factors. The function minimized during full-
matrix least-squares refinement was ∑w(|Fo| - |Fc|)2.
[Zn(L4)(Ph2acac)]PF6 (5). To a solution of 4 (0.20 g; 0.54 mmol)
in dry deoxygenated CH2Cl2 was added ferrocenium hexafluorophos-
phate (0.18 g; 0.54 mmol) under an argon blanketing atmosphere at
room temperature. A color change from orange to green was observed.
After stirring for 1 h deoxygenated dry diethyl ether (10 mL) was added
to the solution. Upon storage of this solution at 0 °C for 1 day a green-
brown microcrystalline precipitate formed which was collected by
filtration: 0.10 g (24%). µeff (298 K) ) 1.7 µB. Anal. Calcd for
C35H45F6N3O4PZn: C, 53.75; H, 5.80; N, 5.37. Found: C, 53.5; H,
5.8; N, 5.4.
In the structure of 3‚0.5toluene‚1n-hexane both solvent molecules
of crystallization were found to be disordered. The toluene molecule
lies on a crystallographic center of symmetry, and two positions (1:1)
were successfully refined (with an occupancy factor 0.5 for C and H
atoms, respectively). The disorder of the n-hexane molecule was also
successfully modeled by a split atom model over three positions with
occupancy factors given in the Supporting Information.
[Zn(L4)(Me-acac)] (6). A white microcrystalline precipitate of 6
was obtained following the procedure given above for 4 by using Me-
acac as ligand. Yield: 0.29 g (55%). Anal. Calcd for C26H43N3O4-
Zn: C, 59.3; H, 8.2; N, 8.0. Found: C, 59.2; H, 8.2; N, 8.0. FAB-
MS (MNBA) m/z (rel intensity %) 527 {[Zn(L4)(Me-acac)]+, 30}; 527
{[Zn(L4)]+, 100}. 1H NMR (CDCl3, 250 MHz): δ ) 6.77 (d, J )
3.20 Hz, 1H); 6.36 (d, J ) 3.20 Hz, 1H); 4.42 (d, J ) 11.49 Hz, 1H);
3.69 (s, 3H); 3.20 (d, J ) 11.49 Hz, 1H); 2.52 (s, 3H); 2.39 (s, 3H);
2.10-3.45 (m, 12H); 2.011 (s, 3H); 1.89 (s, 3H); 1.79 (s, 3H); 1.34 (s,
9H) ppm. 13C{1H} (CDCl3, 63 MHz): δ ) 164.0, 144.0, 138.0, 120.0,
114.3, 113.9, 102.7, 98.4, 77.2, 63.7, 56.8, 56.4, 54.3, 54.1, 51.9, 50.1,
47.5, 47.1, 35.0, 29.4, 28.6, 27.9 ppm.
Crystallography. Details of the crystal data, data collection, and
refinement are summarized in Table 6. Intensities and lattice parameters
of a colorless crystal of 2, a pale-yellow crystal of 3‚0.5toluene‚1n-
hexane, and a pale-yellow crystal of 4 were measured on a Siemens
SMART system by using Mo-KR radiation at 173(2), 100(2), and 100-
(2) K, respectively. No corrections for absorption effects were carried
Acknowledgment. We thank the Fonds der Chemischen
Industrie for financial support. P.H. thanks the Deutsche
Forschungsgemeinschaft for granting a Heisenberg fellowship.
We also thank a reviewer for his constructive criticism and
valuable suggestions.
Supporting Information Available: Tables of crystal-
lographic data and structure refinement data, atom coordinates
and Ueq, bond lengths and angles, anisotropic thermal param-
eters, and calculated positional parameters of hydrogen atoms
for 2, 3‚0.5toluene‚1n-hexane, and 4 and figures of the resolved
static disorder in 2 and of the resolved disorder of solvent
molecules in 3 (23 pages). See any current masthead page for
ordering and Internet access instructions.
JA970417D