Copper(I) complexes with N,O-donor schiff base ligands related to intermediate forms of the TPQ cofactor in amine oxidases

Article abstract of DOI:10.1016/S0020-1693(99)00612-X

Neutral complexes (PPh₃)₂Cu(L) were obtained from (PPh₃)₂Cu(NO₃) and the deprotonated forms of the three Schiff base ligands 2-(benzylideneimino)phenol (BimOH), 4-(benzylideneimino)resorcinol (Bim(OH)₂) and N-(2,4-dihydroxy-5-isopropylphenyl)acetamide (DipaH₃). The ligands were designed to model aminated intermediate forms of the topaquinone (TPQ) cofactor in the enzymatic cycle of copper-dependent amine oxidases. The ligands L are coordinated to the metal centers through imine-N and phenolate-O donor atoms to form five-membered chelate rings, in contrast to the six-membered chelate rings of salen-type Schiff base ligands. Compound (Bim(OH)O)Cu(PPh₃)₂·0.5CH₃OH was structurally analyzed, it exhibits intramolecular aryl-aryl interactions between the ligands and intermolecular hydrogen bonds involving the free phenolic hydroxyl substituent, the coordinated phenolate oxygen atom and the methanol solvate molecule. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00612-X

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Inorganica Chimica Acta 300–302 (2000) 762–768
Copper(I) complexes with N,O-donor Schiff base ligands related to
intermediate forms of the TPQ cofactor in amine oxidases
Torsten Sixt, Wolfgang Kaim *
Institut fu¨r Anorganische Chemie, Uni6ersita¨t Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
Received 28 October 1999; accepted 10 December 1999
Abstract
Neutral complexes (PPh3)2Cu(L) were obtained from (PPh3)2Cu(NO3) and the deprotonated forms of the three Schiff base
ligands 2-(benzylideneimino)phenol (BimOH), 4-(benzylideneimino)resorcinol (Bim(OH)2) and N-(2,4-dihydroxy-5-isopropy-
lphenyl)acetamide (DipaH3). The ligands were designed to model aminated intermediate forms of the topaquinone (TPQ) cofactor
in the enzymatic cycle of copper-dependent amine oxidases. The ligands L are coordinated to the metal centers through imineꢀN
and phenolateꢀO donor atoms to form five-membered chelate rings, in contrast to the six-membered chelate rings of salen-type
Schiff base ligands. Compound (Bim(OH)O)Cu(PPh3)2·0.5CH3OH was structurally analyzed, it exhibits intramolecular aryl–aryl
interactions between the ligands and intermolecular hydrogen bonds involving the free phenolic hydroxyl substituent, the
coordinated phenolate oxygen atom and the methanol solvate molecule. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Copper complexes; Crystal structures; Hydrogen bonding; Phenols; Schiff base ligands
1. Introduction
The catalysis of a two-electron process such as Eq.
(1) requires a corresponding catalytic site. Since biolog-
ical copper usually cycles between the oxidation states I
and II, the mononuclear ‘type 2’ copper center [5,6] in
the subunit requires coupling with a redox-active cofac-
tor. For some time, that quinonoid cofactor had been
assumed to be pyrroloquinoline quinone (PQQ,
methoxatin) [7], however, subsequent work has led to a
reformulation, revealing the 2,5-quinone form (to-
paquinone, TPQ) of 2,4,5-trihydroxyphenylalanine as
covalently linked, post-translationally modified cofactor
[1–5,8].
Copper-dependent amine oxidases are ubiquituous
enzymes which catalyze the oxidation of amines to
aldehydes (Eq. (1)) [1–4].
R-CH2NH3+ +O2+H2O“R-CHO+H2O2+NH4+
(1)
Amine oxidases are typically homodimeric enzymes
(Mr=70–90 kDa) with one Cu center per subunit.
Their biological roles within the general amine
metabolism include developmental functions such as
connective tissue maturation, cell wall cross-linking and
lignification, growth regulation and cell differentiation;
stress response and defence functions via H2O2 produc-
tion are also discussed [1–3]. Among the biogenic
amine substrates for amine oxidases are also neuro-
transmitters, hormones and allergens. The two-electron
oxidation of primary amines to aldehydes is set off by
the two-electron reduction of dioxygen, O2, to
metastable H2O2 (Eq. (1)).
The electronic coupling and mechanistic cooperation
between the single copper center and the quinonoid
cofactor is facilitated by the close proximity of the
metal and TPQ as evident from structural analyses
[9–11]. Protein crystal structures were reported of cop-
* Corresponding author. Tel.: +49-711-685 4170/4171; fax: +49-
711-685 4165.
E-mail address: kaim@iac.uni-stuttgart.de (W. Kaim)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00612-X

T. Sixt, W. Kaim / Inorganica Chimica Acta 300–302 (2000) 762–768
763
per-dependent amine oxidases from procaryotic (Es-
cherichia coli ) and eucaryotic sources (pea seedling,
yeast). They agree in placing the topaquinone and the
metal in close proximity in the active site, however, the
actual arrangements differ in details. In the structure of
the enzyme from E. coli [9] an ‘active’ crystal form
shows triply histidine-coordinated copper Cu(II)(His)3
with two additional water ligands very close to TPQ
while an ‘inactive’ crystal form contains TPQ directly
coordinated to the Cu(II)(His)3 group via the oxygen
atom in 4-position. In the structure of an enzyme from
pea seedling [10] there is also a loose connection be-
tween the metal and the quinonoid ring, suggesting
some flexibility in the metal–cofactor interaction.
Whereas the triple histidine coordination of the copper
center remains a constant feature [9–11], the obvious
flexibility of TPQ with regard to metal bonding is
probably crucial for enzymatic catalysis [10]. Recent
crystallographic studies [11a] as well as EXFAS results
of Cu(I) and Cu(II) forms have confirmed the metal-
cofactor interaction [11b].
librium had indeed been deduced from EPR spectro-
scopic studies of substrate-reduced forms of amine
oxidases from various sources which revealed a low-
temperature Cu(II) EPR signal and a narrow EPR line
at higher temperatures; the latter was attributed to a
Cu(I)/semiquinone, i.e. an organic radical species [15].
Added cyanide was shown to stabilize the copper(I)/
semiquinone form. Detailed studies of enzyme kinetics
confirmed that the copper(I)/semiquinone state is a
viable intermediate [3c,15,16].
I
3
(L)CuII(Q2−) X (L)Cu (Q −) (reactive towards O2)
’
(2)
Q, quinone state; Q −, semiquinone state; Q2−, aro-
matic catecholate state.
’
In addition to that intramolecular electron transfer
step which has been modelled recently [17,18], the
overall enzymatic mechanism [1–5] involves O2 addi-
tion and its reduction by Cu(I), the oxidation of the
aromatic 5-amino derivative of TPQ to the quinonoid
species with formation of H2O2 and ammonium ion
(deamination step), the reaction of an activated car-
bonyl group at the generated quinone with the primary
amine substrate, and the conversion of the
quinoneimine and Schiff base intermediates to the alde-
hyde and the aromatic form, a 4-amino-6-alkylresor-
cinol molecule (Fig. 1).
Concerning the coordination chemistry of the inter-
esting 1,2,4-trioxybenzene parent system with its alter-
native electron transfer reactivity to o- and p-quinonoid
structures [17] we have previously studied the 2,4,5-tri-
hydroxytoluene ligand and its ruthenium(II) complex in
dia- and paramagnetic forms [17,19]. Due to the labile
binding of copper ions by exclusively O-donor ligands
we have now studied the possible binding of the cop-
per(I) complex fragment [(PPh3)2Cu]+ by three N,O-
donor containing ligands, viz, the deprotonated forms
of 2-(benzylideneimino)phenol (BimOH), 4-(benzyli-
deneimino)resorcinol (Bim(OH)2) and N-(2,4-dihy-
droxy-5-isopropylphenyl)acetamide (DipaH3).
Remarkably, the very formation of TPQ from ty-
rosine after translation also requires copper [3c,12], as
may be anticipated from this metal’s role in hydroxyla-
tion of activated aromatics (e.g. via tyrosinase enzymes)
[5,13].
The requirement for a metal such as copper (or a
flavin cofactor in metal-free amine oxidases) comes
from the necessity to activate the dioxygen co-substrate
3
in its triplet ground state, O2. In biology, there are
only two common metal states available for this task,
viz., high-spin iron(II) and copper(I) [14]. To generate
Cu(I) starting from the enzyme resting state which
involves Cu(II) and the aromatic 5-amino form of the
quinone (Fig. 1) would require an intramolecular elec-
tron transfer (Eq. (2)) from the Cu(II)/catecholate form
to a Cu(I)/semiquinone state [5,15]. Such an equi-
BimOH [20] and the new Bim(OH)2 bear some re-
semblance to the ‘product Schiff base’ intermediate in
the amine oxidase enzymatic reaction cycle (Fig. 1)
[3,4,11a]; they provide an anionic N,O-donor chelate
site for metal binding after deprotonation at the pheno-
Fig. 1. Substrate binding and conversion by the TPQ cofactor of
copper-dependent amine oxidases.

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T. Sixt, W. Kaim / Inorganica Chimica Acta 300–302 (2000) 762–768
3
lic hydroxyl group neigbouring the benzylideneimino
function. The structural and chemical activity of the
non-chelating hydroxyl substituent of the Bim(OH)O
anionic ligand will be particularly addressed. The
DipaH3 ligand serves as model for the fully reduced
form of TPQ (Fig. 1) but may also be written as the
iminol tautomer [21] in the deprotonated form.
4-OH), 8.16 (d, 2H, J=7.5 Hz), 7.71–7.53 (m, 3H,
H-3%/H-4%/H-5%), 7.10 (d, 1H, H-6, J=8.6 Hz), 6.25 (dd,
3
4
1H, H-5, J=8.6 Hz, J=2.5 Hz), 6.31 (m, 1H, H-3).
3.1.2. 4-Nitroso-6-isobutylresorcinol
An amount of 400 mg (2.41 mmol) 4-isobutylresor-
cinol [24] was dissolved in 15 ml HCl-saturated ethanol.
Slow addition at −5°C of 166 mg (2.41 mmol) NaNO2
in 1 ml H2O produced an ochre-coloured precipitate
which was collected by filtration, washed with water
and dried under vacuum (280 mg, 59%). 1H NMR
(acetone-d6): l 0.88 (d, 6H, C(CH3)2, 3J=6.6 Hz), 1.83
(sept, 1H, CH(CH3)2, 3J=6.6 Hz), 2.26 (d, 2H,
As to the copper(I) complex fragment (PPh3)2Cu+,
previous results for complexes with N,O-donor ligands,
including those with biochemical relevance, have shown
that the phosphine co-ligands stabilize the CuI oxida-
tion state and facilitate crystallization [7b,22,23].
3
PhꢀCH2ꢀ, J=6.9 Hz,), 2.85 (s, br, OH), 5.71 (s, 1H,
phenyl), 7.35 (s, 1H, phenyl), 8.94 (s, br, 1-OH), 12.31
(s, br, 3-OH).
2. Experimental
2.1. Instrumentation
3.1.3. N-(2,4-Dihydroxy-5-isopropylphenyl)acetamide
(DipaH3)
NMR spectra were taken on Bruker AC 250 or AC
400 spectrometers, infrared spectra were obtained using
a Perkin–Elmer Paragon 1000 PC instrument. UV–
Vis/NIR absorption spectra were recorded on a Bruins
Instruments Omega 10 spectrophotometer. Cyclic
voltammetry was carried out at 100 mV s−1 standard
scan rate in dichloromethane/0.1 M Bu4NPF6 using
a three-electrode configuration (glassy carbon elec-
trode, Pt counter electrode, Ag/AgCl reference) and
a PAR 273 potentiostat and function generator.
The decamethylferrocene–decamethylferrocenium cou-
ple served as internal reference, potentials are given in
V versus ferrocene–ferrocenium (DE=0.55 V).
A suspension of 100 mg (0.51 mmol) 4-nitroso-6-
isobutylresorcinol and 15 mg Pd/C in 2 ml acetic acid
and 1 ml acetic anhydride was treated with 0.1 bar
H2/0.9 bar Ar for 20 min. Filtration over celite, reduc-
tion to dryness, treatment with chloroform in an ultra-
sonic bath and drying under vacuum gave 82 mg (73%)
1
of a colourless residue. H NMR (CD3OD): l 0.88 (d,
3
3
6H, J=6.6 Hz, CH(CH3)2), 1.84 (sept, 1H, J=6.6
Hz, CHCH3)2), 2.13 (s, 3H, C(O)CH3), 2.35 (d, 2H,
3J=6.6 Hz, ꢀCH2ꢀ), 6.34 (s, 1H, H-3), 6.95 (s, 1H,
1
H-6); H NMR (acetone-d6): l 0.86 (d, 6H, CH(CH3)2,
3J=6.6 Hz), 1.87 (sept, 1H, CHCH3)2, J=6.6 Hz),
3
2.15 (s, 3H, C(O)CH3), 2.35 (d, 2H, ꢀCH2ꢀ, J=7.13
Hz), 6.39 (s, 1H, H-3), 6.80 (s, 1H, H-6), 8.00 (s,
exchangeable with MeOD), 9.20 (s, br, exchangeable
with MeOD), 9.37 (s, exchangeable with MeOD). 13C
NMR (CD3OD): l 22.82 (C(O)CH3), 22.95 (CH(CH3),
29.96 (CH(CH3), 39.82 (ꢀCH2ꢀ), 104.89 (C-3), 118.44
(C-1), 120.84 (C-5), 126.87 (C-6), 149.34 (C-2), 155.10
(C-4), 172.18 (C(O)CH3).
3. Syntheses
3.1. Ligands
3.1.1. (4-Benzylideneamino)resorcinol hydrochloride
Bim(OH)2·HCl
A suspension of 500 mg (3.1 mmol) 4-aminoresor-
cinol hydrochloride (Aldrich) and 1 ml (9.8 mmol)
benzaldehyde in 30 ml acetonitrile was treated with
methanol until a homogeneous mixture had formed.
Stirring overnight and reduction of the volume to 5 ml
gave a yellowish precipitate which was dried to yield
3.2. Complexes
3.2.1. [(BimO)Cu(PPh3)2]
A
suspension of 164 mg (0.253 mmol)
Cu(PPh3)(NO3) [25] in 5 ml methanol was treated with
50 mg (0.253 mmol) 2-benzylideneaminophenol [20]
and 2.5 ml of 0.1 N H3CONa/CH3OH in 5 ml CH3OH.
After clearing, the volume of the solution was reduced
to yield 145 mg (73%) of an orange–red precipitate.
Anal. Calc. for C49H40CuNOP2 (784.36 g mol−1): C,
73.35; H, 5.28; N, 1.75. Found: C, 73.31; H, 5.41; N,
1
675 mg (87%) of the product. H NMR (acetone-d6): l
3
10.05 (s, 1H, NꢁCH), 7.91 (d, 2H, H2%/H6%, J=7.5
Hz), 7.69–7.61 (m, 3H, H3%/H4%/H5%), 7.10 (d, 1H, H6,
3J=8.7 Hz), 6.88 (d, 1H, H3, J=2.5 Hz), 6.47 (dd,
3
5
1
1H, J=8.5 Hz, J=2.5 Hz); H NMR (DMSO-d6),
closed form: l N···HꢀO: 10.01 (s, 1H, NꢁCH), 9.60 (s,
br, 4-OH), 8.98 (s, br, 1H, N···HꢀO), 8.16 (d, 2H,
3J=7.5 Hz), 7.71–7.53 (m, 3H, H-3%/H-4%/H-5%), 7.34
1
1.76%. H NMR (acetone-d6): l 8.73 (s, 1H, NꢁCHꢀ),
3
8.01 (d, 1H, H2%/H6%, J=7.5 Hz), 7.18–7.36 (m, 2H,
3
3
(d, 1H, H-6, J=8.6 Hz), 6.34 (dd, 1H, H-5, J=8.6
Hz, 4J=2.5 Hz), 6.31 (m, 1H, H-3); open form: l 10.50
(s, br, 1H, 2-OH···N), 10.01 (s, 1H, NꢁCH), 9.60 (s, br,
H3%/H4%/H5%), 6.86 (m, 2H, H-4/H-5), 6.64 (d, 1H, H-6,
3J=8.6 Hz), 6.17 (t, 1H, H-3, J=7.3 Hz). 31P NMR
3
(CDCl3): l −2.27 ppm.

T. Sixt, W. Kaim / Inorganica Chimica Acta 300–302 (2000) 762–768
765
,
0.71073 A, graphite monochromator). Two crystallo-
graphically independent complex molecules and one
methanol molecule were found. Crystal data for
[(PPh3)2Cu(Bim(OH)O)]2·CH3OH (C99H80Cu2N2O5P4,
1628.6 g mol−1): monoclinic P21, a=12.047(2), b=
,
20.009(4), c=17.400(4) A, i=92.37(3)°, V=4191(2)
3
A , Z=2, Dcalc=1.291 g cm−3, v=0.639 mm−1
;
,
1.69°BqB24.03°, −13BhB13, −22BkB22,
−19BlB19; 27175 reflections measured, 12725 inde-
pendent reflections, full-matrix least-squares refine-
ment, R1=0.0459, wR2=0.0776 [I\2|(I)], GOF on
F2 0.624; largest difference peak and hole: 0.352 and
−0.370 e A−3. Data were refined using the programs
,
SHELXT-PLUS and SHELX-93 [26]. The non-hydrogen
atoms were refined anisotropically; the hydrogen atoms
were introduced at the appropriate positions and
refined isotropically. For additional information see
Section 5.
Fig. 2. Synthesis of DipaH3.
3.2.2. [(Bim(OH)O)Cu(PPh3)2]
A suspension of 143 mg (0.22 mmol) Cu(PPh3)(NO3)
[25] in 10 ml methanol was treated with 55 mg (0.22
mmol) (4-benzylideneamino)resorcinol hydrochloride
and 4.4 ml of 0.1 N H3CONa/CH3OH in 5 ml CH3OH.
After clearing, the volume of the red solution was
reduced until orange–red microcrystals formed (137
mg, 78%). Anal. Calc. for C49H40CuNO2P2 (800.36 g
mol−1)·CH3OH, C, 72.05; H, 5.35; N, 1.68. Found: C,
4. Results and discussion
4.1. Synthesis, NMR and IR spectroscopy
The two new ligands Bim(OH)2 and DipaH3 were
synthesized according to established methodology, as
indicated for the latter in Fig. 2.
The reaction of the deprotonated ligands with com-
mon copper(I) precursors such as [Cu(PPh3)4](BF4) did
not produce pure materials, however, the reaction with
(PPh3)2Cu(NO3) [25] in methanol/methanolate gave the
neutral complexes (PPh3)2Cu(L) in analytically pure
form.
1
72.02; H, 5.31; N, 1.66%. H NMR (acetone-d6): l 8.65
3
(s, 1H, NꢁCH), 7.97 (d, 2H, H-2%/H-6%, J=6.2 Hz),
7.11–7. 36 (m, 3H, H-3%/H-4%/H-5%, 6.81 (t, 1H, H-6,
3J=7.8 Hz), 6.20 (m, 1H, H-5), 5.88 (d, 1H, H-3,
3J=8.5 Hz). 31P NMR (acetone-d6): l −3.34 ppm.
3.2.3. [(DipaH2)Cu(PPh3)2]
56 mg (0.25 mmol) of N-(2,4-dihydroxy-5-isopropy-
lphenyl)acetamide were dissolved in 2.5 ml of a 0.1 N
solution of H3CONa in methanol. This mixture was
added to a suspension of 162 mg (0.25 mmol)
Cu(PPh3)2(NO3) in 20 ml methanol. After 5 days stir-
ring the red solution was reduced in volume, treated
with 1:1 water–ethylene glycol and the resulting precip-
itate dried under vacuum to yield 138 mg (68%). Anal.
Calc. for C48H45CuNO3P2 (809.38 g mol−1)·2H2O: C,
68.19; H, 5.84; N, 1.66. Found: C, 68.46; H, 5.67; N,
1.66%. 1H NMR (acetone-d6): l 0.85 (d, 6H,
3
ꢀCH(CH3)2, J=6.6 Hz), 1.85 (sept, 1H, CH(CH3)2,
3
3J=6.6 Hz), 2.20 ( d, 2H, ꢀCH2ꢀ, J=6.6 Hz), 3.54 (s,
3H, C(O)CH3), 5.44 (s, 1H, H-3), 6.54 (s, 1H, H-6),
7.23–7.34 (m, 30H, PPh3), 10.77 (s, 1H, OH). 31P NMR
(acetone-d6): l −1.11 ppm.
3.3. Crystallography
Orange–red single crystals of (PPh3)2Cu(Bim(OH)-
O)·0.5CH3OH were obtained from the saturated reac-
tion solution. Data were collected for a specimen of
0.1×0.08×0.05 mm size on an IPDS diffractometer
with a Stoe area detector at 293 K (Mo Ka radiation,
Both the NMR and IR spectra (Table 1) reveal
characteristic shifts induced by copper(I) coordination
to the deprotonated ligands. In particular, the carbonyl

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T. Sixt, W. Kaim / Inorganica Chimica Acta 300–302 (2000) 762–768
Table 1
Selected IR frequencies w (cm−1) of ligands and complexes in KBr
Compound
w(CꢁX)
(X=O or N)
w(OꢀH) a
BimOH
[(BimO)Cu(PPh3)2]
Bim(OH)2
[(Bim(OH)O)Cu(PPh3)2]
DipaH3
1625
1581
1656
1608
1619 c
3324
3030–3220
b
3372, 3314
(NꢀH, OꢀH)
3226
[(DipaH2)Cu(PPh3)2]
1533
Fig. 3. Two crystallographically independent complex molecules in
[(Bim(OH)O)Cu(PPh3)2]·0.5CH3OH (phenyl rings of PPh3 ligands
omitted for clarity).
a
Additional bands through metal coordination.
Not clearly discernible.
X=O.
b
c
Table 2
Selected bond lengths (A) of (Bim(OH)O)Cu(PPh3)2·0.5CH3OH
,
Cu(1)ꢀO(1)
Cu(1)ꢀN(1)
Cu(1)ꢀP(1)
2.062(6)
2.092(7)
2.256(3)
2.260(3)
2.081(6)
2.104(7)
2.253(3)
2.264(3)
1.313(8)
1.364(9)
1.320(10)
1.365(11)
1.287(9)
1.434(9)
1.311(10)
1.439(10)
Cu(1)ꢀP(2)
Cu(2)ꢀO(101)
Cu(2)ꢀN(101)
Cu(2)ꢀP(102)
Cu(2)ꢀP(101)
O(1)ꢀC(2)
O(2)ꢀC(4)
O(101)ꢀC(102)
O(102)ꢀC(104)
N(1)ꢀC(7)
N(1)ꢀC(1)
N(101)ꢀC(107)
N(101)ꢀC(101)
Fig. 4. Hydrogen bonding in the crystal of [(Bi-
m(OH)O)Cu(PPh3)2]·0.5CH3OH.
stretching vibration experiences a low-frequency coordi-
nation shift of about 40–50 cm−1 for the two benzyl-
ideneaminophenol compounds but of 86 cm−1 for
(DipaH2)Cu(PPh3)2. This result as well as the addi-
tional 3052 cm−1 band, the disappearance of the amide
Table 3
Selected bond angles (°) of (Bim(OH)O)Cu(PPh3)2·0.5CH3OH
O(1)ꢀCu(1)ꢀN(1)
O(1)ꢀCu(1)ꢀP(1)
N(1)ꢀCu(1)ꢀP(1)
O(1)ꢀCu(1)ꢀP(2)
N(1)ꢀCu(1)ꢀP(2)
P(1)ꢀCu(1)ꢀP(2)
O(101)ꢀCu(2)ꢀN(101)
O(101)ꢀCu(2)ꢀP(102)
N(101)ꢀCu(2)ꢀP(102)
O(101)ꢀCu(2)ꢀP(101)
N(101)ꢀCu(2)ꢀP(101)
P(102)ꢀCu(2)ꢀP(101)
C(2)ꢀO(1)ꢀCu(1)
C(102)ꢀO(101)ꢀCu(2)
C(7)ꢀN(1)ꢀC(1)
C(7)ꢀN(1)ꢀCu(1)
83.1(3)
107.7(2)
113.5(2)
103.2(2)
115.0(2)
124.57(10)
82.1(3)
104.9(2)
120.2(2)
109.8(2)
105.7(2)
125.19(11)
111.6(5)
111.4(5)
121.7(7)
129.3(6)
107.9(5)
119.3(8)
129.1(7)
109.3(5)
1
vibration at 1542 cm−1 and the H NMR resonance at
10.77 ppm suggest an iminol tautomer form [21] of the
coordinated ligand DipaH−2 whereas free DipaH3 is
clearly a carboxamide species. Similar amide“iminol
tautomerization on coordination of a low-valent metal
center was structurally established for a platinum(II)
compound [27] whereas Pt(IV) and Cu(II) prefer the
amide form [27,28]. Obviously, the better p accepting
and ‘softer’ imine coordination site is energetically fa-
voured over the carboxamide alternative by p electron-
rich metal centers such as Pt(II) or Cu(I).
4.2. Structure of (Bim(OH)O)Cu(PPh3)2·0.5CH3OH
C(1)ꢀN(1)ꢀCu(1)
C(107)ꢀN(101)ꢀC(101)
C(107)ꢀN(101)ꢀCu(2)
C(101)ꢀN(101)ꢀCu(2)
Complex (Bim(OH)O)Cu(PPh3)2 could be crystallized
with two crystallographically independent molecules
and one equivalent of methanol in the unit cell for

T. Sixt, W. Kaim / Inorganica Chimica Acta 300–302 (2000) 762–768
767
single crystal structure analysis. Results of the structure
determination at 293 K are summarized in Tables 2 and
3, Figs. 3 and 4 illustrate the molecular structure and
intermolecular interactions.
rate to 2000 mV s−1 or lowering the temperature to 193
K did not render the oxidation processes reversible;
reduction processes were not observed at potentials
E\ −1 V.
In contrast to the common Schiff base ligands of the
salen type with their six-membered chelate arrange-
ments [29–31], the Bim(OH)O− ligand forms close to
planar five-membered chelate rings in the copper(I)
complex, the ‘envelope angles’ are 7.4° for O(1)ꢀ
Cu(1)ꢀN(1)/N(1)ꢀC(1)ꢀ(2)ꢀO(1) and 3.9° for O(101)ꢀ
Cu(2)ꢀN(101)/N(101)ꢀC(101)ꢀC(102)ꢀO(101).Themetal
is coordinated in typical distorted tetrahedral fashion
with small NꢀCuꢀO bite angles of about 83° and
two largely equivalent PPh3 ligands. The metal binding
by the unsymmetrical chelate Bim(OH)O− occurs with
slightly longer bonds of Cu(I) to the imine N (ca.
The absorption spectra of all three compounds are
characterized by a typically [32] broad metal-to-ligand
charge transfer (MLCT) band in the visible (m:5×103
M−1 cm−1) and a strong p“p* transition in the
ultraviolet region (260–270 nm, m:30×103 M−1
cm−1). The MLCT absorption maxima (in CH2Cl2
solution) at 458 nm for (BimO)Cu(PPh3)2, 450 nm for
(Bim(OH)O)Cu(PPh3)2 and 515 nm for (DipaH2)Cu-
(PPh3)2 confirm the better p acceptor behaviour of the
DipaH−2 ligand which may tentatively be attributed to
optimum p conjugation.
Expectedly [33,34], the MLCT transition shifts to
higher energies on deprotonation of the non-coordi-
nated but conjugated phenol group: Addition of the
DBU base to a dichloromethane solution of (Bim-
(OH)O)Cu(PPh3)2 shows disappearance of the 450 nm
band and emergence of a shoulder at about 350 nm.
Summarizing, we have demonstrated through three
TPQ model complexes how metal coordination to hy-
droxide containing chelate ligands can affect hydrogen
bonding, tautomerization equilibria and spectroscopic
properties even in the absence of redox reactions.
,
,
2.10 A) than to phenolate O center (ca. 2.07 A).
,
Compared to values of B2.02 A for more classical
Schiff base/phenolate complexes of copper(I) in a six-
membered chelate arrangement [29–31] the cop-
per(I)ꢀimine bond is rather long in the case of
(Bim(OH)O)Cu(PPh3)2 which is attributed to the
different p conjugation in the five-membered chelate
situation found here.
The presence of phenyl rings from the PPh3 ligands
and from the benzylidene part of Bim(OH)O− gives rise
to weak intramolecular p–p contacts, the closest atom
,
distance being 3.59 A between C(11) and C(35). As a
result, the phenyl planes of the benzylidene moieties
exhibit dihedral angles of 31.7° (molecule 1) and 35.3°
(molecule 2) with the plane involving the imine and
phenol p systems.
The intramolecular hydrogen bonding network link-
ing the molecules in the crystal is illustrated in Fig. 4. It
involves the free phenolic hydroxyl group and the
methanol solvate molecule as H-donor and the metal-
coordinated phenolate and again the CH3OH molecule
as H-acceptor. The shortest O(ꢀH)ꢀO% distance is at
5. Supplementary material
A full list of data collection and refinement details,
table of positional parameters for all atoms, bond
distances and angles, anisotropic temperature factors as
well as calculated and observed structure factor ampli-
tudes, have been deposited and can be obtained from
the Cambridge Crystallographic Data Center, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336-
033; e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk) on quoting the deposition
number CCDC-136025 and the full journal citation.
,
2.607(8) A between O01(CH3OH) and O1b(phenolate);
,
the other distances of 2.673(8) A between O2a(phenol-
,
H) and O01(methanol-O) and 2.759(9) A between
O2b(phenol-H) and O1a(phenolate) are slightly longer
(a refers to molecule 1 and b to molecule 2 within the
asymmetric unit). Calculated H positions of the hydro-
gen bridges lead to OꢀHꢀO% angles between 174 and
180°, in agreement with the presence of at least moder-
ately strong hydrogen bonding.
Acknowledgements
Support for this work from the Deutsche
Forschungsgemeinschaft and the Fonds der Chemis-
chen Industrie is gratefully acknowledged. We also
thank Dr F. Lissner for the crystallographic
measurements.
4.3. Electrochemistry and absorption spectra
The three copper(I) complexes described here are
irreversibly oxidized in CH2Cl2/0.1 M Bu4NPF6 at an-
odic peak potentials of 1.14 V for (BimO)Cu(PPh3)2,
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