A Copper(II) tris-imidazolylphosphine complex as a functional model of flavonol 2,4-dioxygenase

Article abstract of DOI:10.1016/j.molstruc.2019.02.078

Reaction of tetrakis(acetonitrile)copper(I) perchlorate ([Cu(NCCH₃)₄][ClO₄]), tris-1-ethyl-4-methylimidazolylphosphine (T1Et4MeIP) (1) and 3-hydroxyflavone (flavH) under ambient conditions produces an in-situ generated flavonol bound copper(II) complex, which converts to a stable green complex that formulates to [Cu(T1Et4MeIP)(flav)][ClO₄] (2). The crystal structure of 2 was determined by X-ray diffraction and crystallizes in a triclinic system (P1ˉ) with unit cell dimensions of a = 14.537(15) ?, b = 15.794(14) ?, c = 17.044(17) ?, α = 65.58(3)?, β = 86.80(5)?, γ = 73.34(4)?, V = 3376(6) ?³ and Z = 2. While the five-coordinate copper(II) complex is stable under ambient conditions in the solid state, it undergoes oxidative scission of the flavonol pyrone ring under photolytic (300 nm) and/or thermal (120 °C) conditions in the presence of molecular oxygen. The degradative process produces the corresponding methylated products: methylbenzoate, methyl salicylate and N,N-dimethylbenzamide. In addition, the previously undisclosed single crystal X-ray structure of tris-1-ethyl-4-methylimidazolylphosphine (1), T1Et4MeIP, is also reported.

Full text of DOI:10.1016/j.molstruc.2019.02.078

Accepted Manuscript
A Copper(II) tris-imidazolylphosphine complex as a functional model of flavonol 2,4-
dioxygenase
Will E. Lynch, Delana Nivens, Brandon Quillian, Clifford W. Padgett, Anthony Petrillo,
Nathan Peek, John Stone
PII:
S0022-2860(19)30208-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.02.078
MOLSTR 26235
Reference:
To appear in: Journal of Molecular Structure
Received Date: 14 September 2018
Revised Date: 31 January 2019
Accepted Date: 20 February 2019
Please cite this article as: W.E. Lynch, D. Nivens, B. Quillian, C.W. Padgett, A. Petrillo, N. Peek, J.
Stone, A Copper(II) tris-imidazolylphosphine complex as a functional model of flavonol 2,4-dioxygenase,
Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.02.078.
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ACCEPTED MANUSCRIPT
A Copper(II) Tris-imidazolylphosphine Complex as a Functional Model of Flavonol 2,4-
Dioxygenase.
Will E. Lynch,* Delana Nivens, Brandon Quillian, Clifford W. Padgett, Anthony Petrillo, Nathan Peek,
John Stone
Department of Chemistry and Biochemistry, Georgia Southern University, Savannah, Georgia 31419.
•
•
Corresponding author. Department of Chemistry and Biochemistry, Georgia Southern
University, 11935 Abercorn Street, Savannah, GA 31419.
E-mail address: wlynch@georgiasouthern.edu
Keywords:
Copper complex
Dioxygenase
X-ray Structure
3-hydroxyflavone
Abstract
Reaction of tetrakis(acetonitrile)copper(I) perchlorate ([Cu(NCCH3)4][ClO4]), tris-1-ethyl-4-
methylimidazolylphosphine (T1Et4MeIP) (1) and 3-hydroxyflavone (flavH) under ambient conditions
produces an in-situ generated flavonol bound copper(II) complex, which converts to a stable green
complex that formulates to [Cu(T1Et4MeIP)(flav)][ClO4] (2). The crystal structure of 2 was determined
by X-ray diffraction and crystallizes in a triclinic system (P1̅) with unit cell dimensions of a = 14.537(15)
Å, b = 15.794(14) Å, c = 17.044(17) Å, α = 65.58(3)˚, β = 86.80(5)˚, γ = 73.34(4)˚, V = 3376(6) Å3 and Z
= 2. While the five-coordinate copper(II) complex is stable under ambient conditions in the solid state, it
undergoes oxidative scission of the flavonol pyrone ring under photolytic (300 nm) and/or thermal (120
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˚C) conditions in the presence of molecular oxygen. The degradative process produces the corresponding
methylated products: methylbenzoate, methyl salicylate and N,N-dimethylbenzamide. In addition, the
previously undisclosed single crystal X-ray structure of tris-1-ethyl-4-methylimidazolylphosphine (1),
T1Et4MeIP, is also reported.
Introduction
Dioxygenases are an important class of enzymes capable of inserting two oxygen atoms into a
substrate. They play an important biological role in the reduction of aromatic and heteroaromatic
compounds to non-aromatic products in biosyntheses, detoxification, and catabolism of various
metabolites.[1-4] Early reports on the purification, characterization, and structure of this genre of
enzymes, revealed a metal-containing active site bound by three histidine residues as the general motif
(Fig. 1).[5-11] A protein structure possessing this motif (quercetin 2,3-dioxygenase), isolated from the
Aspergillus japonicus fungus without bound substrate, has been reported to have copper in its active
site.[9] The copper(II) dependent metalloenzyme catalyzes the conversion of flavonol derivatives to the
corresponding carboxylic acid ester (depside) with loss of carbon monoxide (Fig. 2). Corresponding
electron paramagnetic resonance spectroscopy studies on the active site of the enzyme suggested a
mononuclear type II copper center in two geometric forms.[12] The first geometric form was found to be
a distorted tetrahedron with copper bound to three histidine residues and a water molecule. The second
form correlated to a trigonal bipyramidal copper center with histidines and a glutamate ligation.[13]
Studies have indicated that in the enzyme substrate active site, five coordinate geometry is proposed with
glutmate and monodentate coordination of quercetin is observed.10 Early biomimetic studies showed
coordination complexes with similar metal centers were also capable of flavonol oxygenation.[14-22]
These reports have shed insight into the substrate metal interaction and have shown that the complexes
are capable of degrading 3-hydroxyflavone at elevated temperatures. Recent reports have also expanded
the model systems capable of mimicking this chemistry to include alternative metal atoms and extended
conjugated flavonol derivatives.[23-26]
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Figure 1. General active site for copper containing dioxygenases showing distorted tetrahedral and
trigonal bipyramidal coordination geometries about copper.
Figure 2. Oxidative decarbonylation reaction of 3-hydroxyflavones.
These reports prompted our investigation into incorporating a biomimetic ligand with three
imidazoles to represent the three bound histidine residues in the protein. Herein, we report the synthesis,
characterization and single-crystal X-ray structure of a flavonol (flavH) bound model copper(II) complex
supported by
a
facially coordinating tris-imidazolylphosphine ligand
(tris-1-ethyl-4-
methylimidazolylphosphine, T1Et4MeIP) and its reaction with dioxygen. The use of imidazolyl ligation
instead of other O, N or P donor ligands should more closely model the binding site in flavonol
dioxygenase (FDO) than those previously reported.[12-24] To our knowledge, this is the first structurally
characterized copper(II) complex supported by a tris-imidazolyphosphine ligand and flavonol.
2. Experimental
2.1 Materials.
The ligand tris-1-ethyl-4-methylimidazolylphosphine (T1Et4MeIP; 1) was prepared by a
previously described procedure.[27] The tetrakis(acetonitrile)copper(I) perchlorate complex was prepared
as outlined previously with perchloric acid substituted for hexafluorophosphoric acid.[28]
All other
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commercial reagents, standards (Aldrich Chemical Company) and solvents (Fischer Scientific) were used
without further purification.
2.2 Physical Measurements.
C, H, and N analyses were performed by Atlantic Microlab. Infrared spectra (IR) were recorded
on a Perkin Elmer Spectrum One using a diffuse reflectance attachment and KBr mixtures. Electronic
spectra were recorded on a Hewlett Packer 8453 Diode Array spectrophotometer. GC-MS measurements
were recorded on a Hewlett Packard 6890 Gas Chromatograph coupled with a Hewlett Packard 5973
Mass Selective Detector. Fluorescence spectra were obtained using a Shimadzu RF 5301 PC
Spectrofluorophotometer. UV lamp intensity was measured using a Fisher Scientific UV light meter. For
excitation spectra, the emission wavelength was 644 nm; while for emission spectra, the excitation
wavelength was 430 nm. Single-crystal X-ray structure determinations were performed on a Rigaku
XtaLAB mini diffractometer.
2.3 X-ray Diffraction.
Crystals suitable for single-crystal X-ray analysis for 1 were grown by slow evaporation of
hexane solution of the complex at room temperature and mounted on a Mitigen micromount. Isolated
single crystals appropriate for X-ray analysis of 2 were grown by vapor diffusion of diethyl ether into a
dichloromethane solution of the copper complex followed by mounting on a Mitigen micromount.
Single-crystal X-ray diffraction data for compounds 1 and 2 were collected at 173 K on a Rigaku XtaLAB
mini diffractometer equipped with Mo-Kα radiation (λ = 0.71073 Å) and processed using CrystalClear
(Rigaku) software.[29] The structures were solved using direct methods and refined by using full matrix
least squares refinement using SHELXT and SHELXL2017.[30, 31] All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in ideal positions and refined as riding atoms with
relative isotropic displacement parameters. The crystal data and refinement details for compounds 1 and 2
are listed in Table 1.
Crystallographic data of the structures have been deposited with the Cambridge Crystallographic
Data Center as supplementary publication CCDC 1867305 for (1) and CCDC 1867306 for (2). Copies of
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the data are available from CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033;
email: deposit@ccdc.cam.ac.uc.uk or www: http://www.ccdc.cam.ac.uk).
Table 1. X-ray experimental details for 1 and 2
(1)
(2)
Crystal data
Chemical formula
Mr
C18H27N6P
C33H36CuN6O3P·ClO4
758.65
358.42
Crystal system, space group
Monoclinic, P21/c
Triclinic, P1
̅
Temperature (K)
a, b, c (Å)
α, β, γ (°)
173
173
8.396 (7), 11.365 (8), 21.088 (17) 14.537 (6), 15.794 (7), 17.044 (8)
90, 98.858 (11), 90
64.58 (3), 86.80 (5), 73.34 (4)
V (Å3)
1988 (3)
3376 (3)
Z
4
4
Radiation type
µ (mm−1)
Crystal size (mm)
Mo Kα
Mo Kα
0.15
0.83
0.57 × 0.26 × 0.26
0.41 × 0.14 × 0.14
Data collection
Diffractometer
Rigaku XtaLAB mini
Rigaku XtaLAB mini
Absorption correction
Multi-scan
Multi-scan
(REQAB; Rigaku, 1998)
(REQAB; Rigaku, 1998)
Tmin, Tmax
0.507, 100
0.720, 1.00
No. of measured, independent and 8067, 4440, 2725
29925, 12212, 9690
observed [I > 2σ(I)] reflections
Rint
0.072
0.650
0.070
0.600
(sin θ/λ)max (Å−1)
Refinement
R[F2 > 2σ(F2)], wR(F2), S
No. of reflections
No. of parameters
H-atom treatment
0.076, 0.234, 1.01
0.056, 0.153, 1.06
4440
12212
232
923
H-atom parameters constrained
0.51, −0.56
H-atom parameters constrained
1.47, −1.28
ꢀρmax, ꢀρmin (e Å−3)
Computer programs: [27],[28],[37]
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2.4 Synthesis of [Cu(T1Et4MeIP)(flav)][ClO4](2).
The compound 2 was prepared by mixing stoichiometric amounts of [Cu(CH3CN)4][ClO4] (0.091
g, 0.280 mmol) and tris-1-ethyl-4-methylimidzolylphosphine (0.100 g, 0.280 mmol) in a minimal amount
of dichloromethane under ambient conditions. The solution gradually turned green over 10 minutes, at
which time, 3-hydroxyflavone (0.066 g, 0.280 mmol) was added. After 10 minutes, a green solid was
precipitated by dropwise addition of diethyl ether. The solid was collected on a glass frit, washed
successively with water and diethyl ether and subsequently dried under vacuum (0.144 g, 68 %). X-ray
quality crystals of 2 were grown by vapor diffusion of a saturated dichloromethane solution of the title
compound with diethyl ether. IR (KBr): 2982, 2923, 1551, 1095, 623 cm-1. UV-vis (MeCN) λmax / nm
(ε/M-1cm-1): 257 (50900), 412 (18900), 428 (21400), 676 (150). Anal. Calc’d for C33H36N6O7PClCu: C,
52.24; H, 4.79; N, 11.08. Found: C, 52.18; H, 4.78; N, 11.08.
2.5 Reactions of 2 with Dioxygen.
a.) Photolysis. UV-photolysis experiments were carried out with a solution (0.036 mM) of 2 in
dichloromethane over 6 hours. While vigorously stirring, the sample was exposed to light at 300 ± 10 nm
with a power setting of 2.9 µw/cm2 from a 150 W Xenon lamp. Periodically, the UV-vis spectrum was
recorded over the range of 300 - 550 nm to determine reaction completion. Upon reaction completion, the
products were methylated with diazomethane in diethyl ether to lower their boiling points for facile
characterization by GC-MS. The products were analyzed in-situ via GC-MS and identified by comparing
retention times and mass spectra with corresponding standards.
b.) Thermal Degradation. Thermal degradation experiments were carried out in a typical
procedure by preparing a solution of 2 in DMF (22 ꢁM). The sample was then heated to 120 ˚C and the
reaction progress was monitored every 10 minutes via UV-vis spectroscopy, surveying absorbance
changes within the range of 300 - 550 nm. Products were determined by GC-MS and/or GC analysis as
outlined above.
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3. Results and Discussion.
3.1 Synthesis and Characterization of 2.
The reaction of equimolar quantities of tetrakis(acetonitrile)copper(I) perchlorate, 1 and 3-
hydroxyflavone in dichloromethane under ambient conditions quantitatively yielded 2 as green crystals
(eq 1).
The IR spectrum of the complex displays peaks at 2982 and 2923 cm-1, which are characteristic of
aliphatic C-H stretches of T1Et4MeIP ligand. The band observed at 1551 cm-1 is assigned to the νC=O
stretch of the deprotonated 3-hydroxyflavone ligand, and suggests strong delocalization of the
π-bond.
This is likely the result of the deprotonated -hydroxy groups. Other bands assigned to perchlorate are
α
observed at 1195 and 623 cm-1. The electronic spectrum displays transitions at 412 and 428 nm that are
assigned as intraligand π−π* processes in the flavonolate ligand.[15] A low energy d-d transition
characteristic of a copper(II) center is observed at 676 nm.
3.2 X-ray Structure of T1Et4MeIP (1) .
The ligand, compound 1, crystallizes in the monoclinic space group, P21/c (Fig. 3). The geometry
about the phosphorous atom was found to have strained trigonal pyramid geometry with C-P-C bond
angles of 101.90 (15)o, 101.46 (16) o and 103.37 (15) o. The average C–P–C bond angle in 1 (102.24 o) is
slightly
larger
than
those
found
in
other
structurally
characterized
o
tris(imidazolyl/thiazolyl)phosphines,[32-36] but is substantially smaller than that (107.1 ) of the tri-
cationic, tris(1,3,4,5-tetramethylimidazolium-2-yl)phosphine. [37] A list of C–P–C bond angles and P–C
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lengths, including their averages, of previously reported structurally characterized
tris(imidazolyl/thiazolyl)phosphines is shown in Table 2. Two of the imidazolyl units in 1 have methyl
groups pointed towards the trigonal cavity, whereas the third unit shows the methyl group pointing away
from the cavity and towards backside of the molecule. The C–P bond distances in 1 (P1–C1 =1.820 (4)
Å; P1–C7 = 1.810 (4) Å; P1–C13 =1.827 (4) Å) compares well with those previously reported for
reported tris(imidazolyl/thiazolyl)phosphines (see Table 2). There are no hydrogen bonding (< 3.0 Å) or
π-stacking interactions present in the packing of compound 1 and only Van der Waals contacts are
observed (See Supporting Information for image of packing).
Table 2. Structural data for 1 and previously reported tris(imidazolyl/thiazolyl)phosphines
P-C bond Length
(Å)
C-P-C Bond
angle (°)
Ligand
Ref.
[32]
tris(2-imidazolyl)phosphine hydrate
1.811 (4)
1.814 (4)
1.807 (4)
1.811
102.6 (2)
100.9 (2)
101.4 (2)
101.6
Avg.
[36]
[34]
[33]
tris(4-methylimidazol-2-yl)phosphine
1.8289
104.45
1.8167
98.66
1.8203
1.8220
99.85
100.57
avg.
tris[2-(vinyl)imidazolyl]phosphine
1.813 (2)
1.825 (2)
1.827 (2)
1.822
101.73 (10)
102.24 (10)
98.39 (10)
100.57
avg.
tris[2-methyl-4-tolylimidazolyl)phosphine
1.841 (5)
1.824 (5)
96.8 (2)
103.5 (2)
1.813 (5)
1.826
102.2 (2)
100.8
avg.
tris(1,3,4,5-tetramethylimidazolium-2-
1.813 (2)
106.8 (2)
yl)phosphine
[37]
1.818 (2)
107.2 (2)
1.825 (2)
1.819
107.2 (2)
107.1
avg.
[35]
This
tris(benzothiazol-2-yl)phosphine
1.82 (2)
101.87 (7)
tris-1-ethyl-4-methylimidazolylphosphine (1)
1.820 (4)
101.90 (15)
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work
1.810 (4)
1.827 (4)
1.819
101.46 (16)
103.37 (15)
102.24
avg.
Figure 3. Molecular structure of T1Et4MeIP compound (1). Thermal ellipsoids have been drawn at the
50% probability level with hydrogen atoms omitted for clarity. Selected bond lengths (Å): P1–C1 = 1.820
(4); P1–C7 = 1.810 (4); P1–C13 =1.827 (4). Selected bond angles (°):C1–P1–C13 101.90 (15), C7–P1–
C1 = 101.46 (16); C7–P1–C13 = 103.37 (15)o.
3.3 X-ray Structure of [Cu(T1Et4MeIP)(flav)][ClO4](2).
Compound 2 crystallizes in the triclinic space group (P1̅). The structure was refined to a final R
value of 5.63%. There was disorder associated with two of the perchlorate oxygen atoms located on Cl2
(53% and 47% occupancy). There are two independent, five-coordinate, copper(II) centers per unit cell
with minor variances in the bond distances surrounding the copper atom and a more noticeable difference
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in apical Cu-N bond distances (vide infra), which is likely due to lattice stacking. The coordination
geometry of the two copper centers is best described as square pyramidal with slightly different τ values
(Cu(1) = 0.094 and Cu(2) = 0.17). [23] Due to similarities between the two structures, a single unique
structure is shown (Fig. 4).
Figure 4. Molecular structure of the cationic portion of [Cu(II)(T1Et4MeIP)(flav)]ClO4, (2). Thermal
ellipsoids have been drawn at the 50% probability level with hydrogen atoms omitted for clarity.
The five-coordinate copper(II) centers of 2 are supported by a facially coordinated T1Et4MeIP
ligand that occupies three coordination sites through nitrogen donors and κ2-bonding of the two oxygen
atoms of the deprotonated flavone ligand. The square planar base of 2 is formed by two of the three
nitrogen donor atoms (N(3) and N(5) for Cu(1) and N(9) and N(11) for Cu(2)) of T1Et4MeIP and the
oxygen atoms (O(1), O(2) and O(4), O(5) for Cu(1) and Cu(2), respectively) of the deprotonated 3-
hydroxyflavone chelate. The third imidazole ring occupies the axial position. The bond distances are very
similar as noted in Table 4. The largest bond deviation is 0.052 A from the copper center to N1 and N7 in
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the two complexes. The other five bond distances vary by less than 0.021 AA comparison of the
coordination spheres in structure of the model complex 2 with the structurally characterized enzymatic
active site of A. Japonicus reveals a number of similarities and disparities. While the copper coordination
spheres in complex 2 and the type II center found in A. Japonicus are both five-coordinate, they differ in
geometry (complex 2 = square pyramidal [τ values (Cu(1) = 0.094 and Cu(2) = 0.17)}) A. Japonicus
exhibits a distorted trigonal bipyramidal copper center with three histidine residues, a glutamate and a
monodentate substrate binding.[11] This disparity in coordination geometries may be due to the rigid tris-
imidizolylphosphine ligand in 2, which maintains acute bond angles and the fact that no other Lewis base
ligands are available in our reaction scheme. Moreover, unlike that observed in 2, which displays κ2
bidentate binding of flavonolate substrate, utilizing the carbonyl and -hydroxyl groups, a manually
modeled enzyme-substrate (ES) complex of the FDO active site suggests only monodentate binding of
quercetin through its -hydroxyl group. However, models of the FDO-ES complex with kojic acid
-
α
α
suggests similar κ2-bidentate binding as observed in 2. Bidentate flavonol binding has also been observed
with a number of synthetic copper-containing FDO-ES mimics with various ligands supporting
copper,[13] and has been suggested the source of their retarded oxidative reactivity as compared to the
native enzyme. Previously reported, structurally characterized, synthetic FDO-ES complexes have
displayed coordination geometries between distorted square-pyramidal and trigonal-bipyramidal with
τ values ranging from 0.12-0.61.[13, 14, 38-40] A list of selected synthetic FDO-ES complexes with
geometrical parameters and structural data is shown in Table 3. The apical Cu–N bond distances (N(1)
‒
Cu(1) = 2.244 (4) Å; N(7) Cu(2) =2.296 (3) Å) in the two independent units of 2 are slightly different
‒
and are significantly longer than the Cu–N bonds found in other structurally characterized synthetic FDO-
ES complexes (see Table 3), suggesting a very weak bind interaction. The two Cu–O bonds in both units
of 2 are asymmetric. In both independent units, the Cu
‒O bond distance (Cu(1)–O(1) = 1.999 (3) Å and
Cu(2)–O(4) =1.998(3)Å) involving the carbonyl is somewhat longer than that of the deprotonated
α
-
alkoxide (Cu(1)–O(2) = 1.944 (3) Å and Cu(2)–O(5) = 1.956 (3) Å). These C–O bond distances in 2, as
11

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expected, are significantly longer than those reported for either glutamate or water coordination in the
type II copper center in the FDO active site of A. Japonicus (Glutamate Cu-O bond distance = 2.1 Å; H2O
Cu–O bond distance = 2.4 Å). The Cu‒N bond distances of the copper-imidazole interactions in the
square pyramid basal plane of 2 are all slightly over 2.0 Å (Cu1-N3 = 2.023(3) Å; Cu1-N5 = 2.019(4) Å;
Cu2-N11 = 2.029(3) Å; Cu2-N9 = 2.002(3) Å) for both copper centers and are significant shorter than the
apical Cu–N bond distances by more than 0.2 Å. Both unique copper atoms of 2 rest slightly less than 0.2
Å above the N2O2 basal plane (Cu(1) is 0.194 Å and Cu(2) is 0.170 Å). Bond distances and angles around
the copper atoms for 2 are reported in Table 4.[14, 27, 41] The molecular packing diagram for 2 shows
no intermolecular interactions of significant consequence between the molecules (see supporting
information).
Table 3. Geometric and structural data for previously reported structurally characterized synthetic FDO-
ES complexes.
Structurally
Characterized Synthetic
FDO-ES Complexes
Bond
Distance
(Å)
Basal plane
atoms
Bond
distance (Å)
Apical
atom(s)
τ
values Geometry
Distorted square
pyramidal
Ref
[38]
Cu(Ind)(mco)
0.38
Cu-N
Cu-N
Cu-N
2.0418(16)
2.0410(15)
1.904(8)
Cu-O(carbonyl)
2.2966
Cu-O(hydroxyl) 1.9586(14)
Distorted
trigonal
pyramidal
[39]
[Cu(Fla)-(idapH)]ClO4
Cu(Fla)(Bz-TAC)
0.61
0.12
Cu-N
Cu-N
2.137(3)
2.112(4)
C-O (hydroxyl)
Cu-N
1.918(3)
2.006(4)
Cu-O(carbonyl) 2.210(3)
Distorted square
pyramidal
Cu-N
Cu-N
2.040(30
2.027(3)
Cu-N
2.231(3)
[40]
Cu-O(hydroxyl) 1.917(3)
Cu-O(carbonyl) 2.012(3)
Trigonally
distorted
[14]
CuII(Fla)(Ind)
0.5
bipyramidal
Cu-N
Cu-N
2.039(6)
2.031(7)
Cu-N
1.903(6)
Cu-O(hydroxyl) 1.942(5)
Cu-O(carbonyl) 2.2065(5)
12

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0.094,
0.17
Distorted square
pyramidal
1.999(3),
1.998(3)
1.944(3),
1.956(3)
2.023(3),
2.003(3)
2.019(4),
2.029(3)
2.244(4),
2.296
This
work
Compound 2
C-O(carbonyl)
Cu-O
C-N
(hydroxyl)
Cu-N
Cu-N
Ind = 1,3-bis(2-pyridylimino)isoindolide; mco = 3-hydroxy-(4H)-benzopyran-4-one; idpaH = 3,3'-iminobis(N,N-
dimethylpropylamine ;Bz-TAC = N,N,N-tribenzyl-1,4,7, triazacyclononane
Table 4. Geometric parameters (Å, °) for (2) at the copper centers.
Bonds
Distances
2.244 (4)
1.999 (3)
1.944 (3)
2.023 (3)
2.019 (4)
Angles
Bonds
Distances
2.296 (3)
1.998 (3)
1.956 (3)
2.002 (3)
2.029 (3)
Angles
Cu1—N1
Cu2—N7
Cu2—O4
Cu2—O5
Cu2—N9
Cu2—N11
Atoms
Cu1—O1
Cu1—O2
Cu1—N3
Cu1—N5
Atoms
O1—Cu1—N1
O1—Cu1—N3
O1—Cu1—N5
O2—Cu1—N1
O2—Cu1—O1
O2—Cu1—N3
O2—Cu1—N5
N3—Cu1—N1
N5—Cu1—N1
N5—Cu1—N3
105.40 (13) O4—Cu2—N7
162.97 (12) O4—Cu2—N9
96.63 (13)
172.55 (11)
91.14 (13)
82.79 (12)
103.66 (13)
93.92 (13)
166.91 (12)
90.64 (13)
90.69 (14)
88.51 (13)
91.36 (14)
95.22 (13)
83.20 (13)
94.80 (13)
O4—Cu2—N11
O5—Cu2—O4
O5—Cu2—N7
O5—Cu2—N9
173.15 (12) O5—Cu2—N11
91.62 (14)
90.23 (13)
89.18 (14)
N9—Cu2—N7
N9—Cu2—N11
N11—Cu2—N7
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3.3 Luminescence of 2.
Free and metal ion bound hydroxyflavones are known to be highly fluorescent with a typical blue
excitation (300-430 nm) and corresponding green emission of 510-560 nm with λmax and peak shape
dependent upon solvation and substitution or degree of deprotonation.[42-46] Compound 2 is
luminescent in both the solid and solution states. The photochemistry of flavonoids has been studied
extensively and reviewed due to their ubiquity in biological processes.[35] In inert atmosphere, the
compounds generally undergo cyclization and rearrangement, whereas in the presence of oxygen, it is
commonly observed that ring opening occurs with concomitant depside formation with loss of carbon
monoxide.[36]
The excitation and emission spectra of the free flavone (3-hydroxyflavone) and complex 2 are
shown in Figure 5. For reference, 3-hydroxyflavone excitation spectrum shows a broad absorbance
(dashed gray). Using 430 nm excitation, the emission spectrum (solid gray) is observed. Similarly, the
430 nm excitation observed in 2 (dashed black) results in an emission spectrum observed in solid black
with the major green emission observed at 644 nm.
In solution, the complex presumably dissociates to some extent. This slight dissociation of
flavonol from the complex results in the weak luminescence of the complex being obscured. This is
primarily due to the high quantum efficiency of the flavonol. This observation has been previously
reported.[46] In the solid state, compound 2 (Fig. 5) was observed to be luminescent at room temperature
with a sharp λex = 430 nm and λem’s = 644, 690 and 715 nm. The emission is likely due to the ligand
based transitions of the flavonol that are lost upon solution formation due to the proposed dissociation of
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the flavonol. Additional excitation bands, if present, are obscured by the overlap of the broad 3-
hydroxyflavone excitation.
3.4 Reactions of Dioxygen with [Cu(II)(T1Et4MeIP)(flav)]ClO4.
1000
800
600
400
200
0
340
440
540
640
740
Wavelength (nm)
Figure 5. Solid-state luminescence spectra of [Cu(II)(T1Et4MeIP)(flav)]ClO4 and 3-hydroxyflavone.
[Cu(II)(T1Et4MeIP)(flav)]ClO4: excitation spectrum (dashed black); emission spectrum using 430 nm
excitation (solid black). 3-hydroxyflavone: excitation spectrum (dashed gray) using 520 nm emission; 3-
hydroxyflavone emission spectrum (solid gray) using 430 nm excitation.
a. Photolysis of 2. For photolytic degradation of the complex, a wavelength range of 300 nm (±
10 nm) was chosen. Dichloromethane samples of the title complex were prepared and illuminated for 6
hours. As shown (Fig. 6), the photolysis was followed via UV-vis at 1-hour intervals of exposure. After
the flavonol absorbance at 428 nm decreased to 5 % of the starting absorbance (indicating degradation of
the substrate), the solution was methylated using diazomethane. The transition excited by irradiation at
300 nm is assigned as the metal to ligand charge transfer process. This implies that the initiation step of
degradation of a bound flavonol is a redox process at the metal center which has been proposed
previously in model complexes.[14] The product distribution for the degradation reaction was determined
to be 0.25 mole equivalent methylbenzoate (A), 0.24 mole equivalent methyl salicylate (B) (Fig. 7).
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These products also correspond to ring opening oxidation products of 3-hydroxyflavone.[14] It is noted
here that if the complex is excited with 430 ± 10 nm light, corresponding to an intraligand charge transfer
process, the ligand is not degraded but dissociated from the metal.
1.2
1
t = 0 h
0.8
0.6
t = 1 h
0.4
t = 2 h
0.2
t = 3 h
t = 4 h
0
280
330
380
430
480
Wavelength(nm)
Figure 6. UV-vis spectra of [Cu(II)(T1Et4MeIP)(flav)]ClO4 in dichloromethane during photolysis.
Reaction progress was monitored for 4 hours at 1 hour intervals.
b. Thermal Degradation of 2. Thermal degradation studies involved a DMF solution of the title
complex, sealed in a glass vial under ambient conditions. Samples were extracted and monitored via UV-
vis. After the reaction reached completion, the solution was methylated in the same manner as in the
photolysis studies. The product distribution for the reaction was determined to be 0.26 mole equivalent
methylbenzoate (A), 0.37 mole equivalent methyl salicylate (B) and 0.12 mole equivalent N,N-
dimethylbenzamide (C) (See Fig. 7). These products correspond to ring opening oxidation products of 3-
hydroxyflavone.[9] The N,N-dimethylbenzamide product is believed to arise from a reaction with the
solvent DMF. This product was not seen in photolysis experiments carried out in dichloromethane.
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Figure 7. Degradation products observed for oxidation of 3-hydroxyflavone. Note compound C is
only observed in degradation studies conducted in dimethylformamide.
4. Conclusion
The tridentate ligand, tris-1-ethyl-4-methylimidazolylphosphine supports copper flavonol
complex formation and functions analogous to the binding site of 2,3-quercetin dioxygenase. The
imidazole ligation is a more accurate structural model for the 3 histidine residues found in the protein.
These complexes are also models for examining the reactivity of flavonol bound copper complexes.
Ring-opening reactions occur via photolysis at 300 nm or highly elevated temperatures. The products
observed are relevant degradation products to that of the metalloenzyme and evidence indicates that these
degradation reactions are initiated by a charge transfer process involving the copper atom. Further work
is being carried out on the mechanism of the degradation and dissociation processes, the intermediates
formed in the reactions as well as electronic and substituent group effects on both the flavonol and
trisimidazolylphosphine ligand.
Acknowledgements
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Financial Support from the American Chemical Society Petroleum Research Fund (30550-GB3 to
W. E. L.) and NSF Improvement in Laboratory Instruction grants (DUE 9451465) for the Gas
Chromatograph Mass Spectrometer and (DUE 975103) for the Gas Chromatograph.
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Highlights:
•
•
•
•
•
Structural characterization of a copper(II) tris-imidazolylphosphine ligand/flavonol complex
New synthetic enzyme-substrate model of flavonol dioxygenase
Photo and thermal degradation of Flavanol
Structural characterization of tris-1-ethyl-4-methylimidazolylphosphine
Luminescence assay of a copper(II) tris-imidazolylphosphine ligand/flavonol complex