Iron(II)-8-quinolinol/MCM-41-Catalyzed Phenol Hydroxylation and Reaction Mechanism

Article abstract of DOI:10.1006/jcat.1997.1612

Iron(II)-8-quinolinol/MCM-41 is prepared. Its catalysis is studied in phenol hydroxylation using H₂O₂ (30 %) as oxidant. The experiment shows that Iron(II)-8-quinolinol/MCM-41 has good catalytic activity and desired stability. Based on cyclic voltammetry, ESR, and UV-visible spectra studies of iron(II)-8-quinolinol complex in liquid phase, a radical substitution mechanism is proposed and used to demonstrate the experimental facts clearly.

Full text of DOI:10.1006/jcat.1997.1612

JOURNAL OF CATALYSIS 168, 35–41 (1997)
ARTICLE NO. CA971612
Iron(II)-8-quinolinol/MCM-41-Catalyzed Phenol Hydroxylation
and Reaction Mechanism
Chibiao Liu, Yongkui Shan, Xiangguang Yang, Xingkai Ye, and Yue Wu1
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China
Received March 14, 1996; revised December 6, 1996; accepted January 21, 1997
tant oxidations including phenol hydroxylation under mild
conditions; however, the results have been disappointing
because of the limited pore size of these catalysts, which
makes it difficult for substrates to diffuse into and products
to diffuse out of the narrow channel of the microporous
inorganic solids (19).
Up to now, some mesoporous solids including silica
(20), kanemite (21), modified layered materials (22), and
MCM-41 (23) have been produced and are characterized
by pore diameters that could be adjusted to between 1.8
and 20.0 nm. MCM-41 mesoporous molecular sieve pos-
sesses not only regular arrays of uniform channels, but also
high surface area and exceptionally high sorption capacity
of cyclohexane and benzene (23). MCM-41 thus could be
used as a support for metal complexes to solve the draw-
back existing with the microporous zeolite-supported metal
complexes.
Iron(II)-8-quinolinol/MCM-41 is prepared. Its catalysis is stud-
ied in phenolhydroxylation usingH2O2 (30%)asoxidant. Theexper-
iment shows that Iron(II)-8-quinolinol/MCM-41 has good catalytic
activity and desired stability. Based on cyclic voltammetry, ESR,
and UV–visible spectra studies of iron(II)-8-quinolinol complex in
liquid phase, a radical substitution mechanism is proposed and used
c
to demonstrate the experimental facts clearly.
ꢀ 1997 Academic Press
INTRODUCTION
Diphenol production through phenol hydroxylation has
been studied extensively since the 1970s. Cu2+, Au3+, Co2+
(1), Fe3+ (2, 3), and Fe2+ (4–6) have been investigated with
respect to their catalysis in the hydroxylation of phenol.
Although these simple metal ions have some activity
in catalyzing this reaction, the reaction rate and selec-
tivity to diphenols are not desirable. Therefore, scien-
tists turn to studying the catalysis of metal complexes.
EXPERIMENTAL
–
Some complexes, such as metal–phthalocyanine (M Pc)
Catalyst Preparation
–
–
(7) [M = Sb, Bi, V, Mo, Sn, Sn Mo, and Sn Sb (7); M =
Cu2+, Co2+, Mn2+, Fe2+ (8)], Fe(III)–bipyridine (9), Co(II,
III)–Schiff base (10), and Co(II)bis-[3(salicylideneconine)
propyl]methylamine, as well as the porphyrin complexes
of group IA–VIA, IVB–VII, and VIII metal cations (11),
have been investigated as catalysts in phenol hydroxylation.
Although they have better catalytic activity and selectivity
than the simple metal ions, these metal complex catalysts
are not desirable for industrial application. This is due to
the shortcomings of homogeneous catalysis, such as difficult
recovery of catalysts and difficult continuous operation.
To find desirable catalysts for phenol hydroxyla-
tion, many heterogeneous catalysts have been pre-
pared. FePc–zeolite X or Y (12), iron(III)- and man-
ganese(III) tetramethylporphyrin–zeolite Y (13), zeolite
Y-encapsulated Mn-salen [salen = N,N0-ethylenebis (sali-
cylideneaminato)] (14), TS-1 (15, 16), and TS-2 (17, 18)
have been used to catalyze a variety of synthetically impor-
Until now, there have been no reports of MCM-41 as
a support for iron(II)–8-quinolinol. A sample of MCM-41
(Si/Al = 20) was synthesized following several patents (23).
For the as-synthesized product, interplanar spacings d100
=
4.46 nm and d110 = 2.74 nm were observed. The correspond-
ing powder X-ray diffraction patterns of the bulk sample
are shown in Fig. 1a. The BET surface area of the prepared
sample is 972.4 m2 gꢁ1
.
Iron(II)–8-quinolinol/MCM-41 was prepared according
to the following steps. First, 5.0 g MCM-41 was mixed with
200 ml of a 0.05 mol/liter [Fe(8-quinolinol)3]Cl2 solution
of alcohol. The mixture was then stirred for 24 h at am-
bient temperature, filtered, and washed with alcohol until
no [Fe(8-quinolinol)3]Cl2 was detected in the filtrate. Dur-
ing this process, the complexes on the surface of MCM-41
particles and those poorly immobilized in the pore are re-
moved. Only those complex molecules immobilized well
would remain in the channels of this support.
1
The BET area of the fresh iron(II)–8-quinolinol/
To whom correspondence should be addressed. Fax: 011-86-0431-
5685653. E-mail: peifk@sun.ihep.ac.cn.
MCM-41 is 886.5 m2/gꢁ1. Its X-ray diffractogram, in
35
0021-9517/97 $25.00
c
Copyright ꢀ 1997 by Academic Press
All rights of reproduction in any form reserved.

36
LIU ET AL.
Fig. 1b, shows that MCM-41 undergoes no structural
change during preparation of the catalyst. According to
the infrared spectra of [Fe(8-quinolinol)3]Cl2 (Fig. 2a),
iron(II)–8-quinolinol/MCM-41 (Fig. 2b), and MCM-41
(Fig. 2c), MCM-41 has no IR signals in the region
1300–1600 cmꢁ1, but iron(II)–8-quinolinol/MCM-41 and
[Fe(8-quinolinol)3]Cl2 have the similar infrared spectra in
this region. This shows that the metal complex was im-
mobilized in MCM-41 successfully. According to chemi-
cal analysis, the supported catalyst contains 7% mass of
iron(II)–8-quinolinol. The good immobilization ofiron(II)–
8-quinolinol could be attributed to the adsorption and static
coulombic functions between [Fe(Qx)3]2+ species and the
pore wall of MCM-41.
Reaction Conditions and Analytical Methods
Phenol hydroxylation was carried out in a 25-ml glass
reactor equipped with a thermostatic jacket, reflux con-
denser, submerged cooler, thermometer, and magnetic stir-
rer. Catalyst, phenol, and solvent were added successively
into the reactor. The reaction was initiated by adding H2O2
(30 mass% ) at the set temperature. Products were ana-
lyzed on a gas chromatograph using a flexible glass capillary
column coated with XE-60. Programmed temperature was
adopted for product analysis, with the initial temperature
100ꢂC and the final temperature 190ꢂC.
FIG. 2. Infrared spectra of [Fe(Qx)3]Cl2 (a), fresh iron(II)–8-quino-
linol/MCM-41 (b), and MCM-41(c).
Infrared spectra were recorded on
a
Bio-Rad
FTS-7(USA) IR spectrophotometer; X-ray diffraction
powder patterns were recorded with a Rigaku D/max-qB
X-ray diffractometer (Japan). The UV–visible (UV–Vis)
studies were carried out with a Specord UV–Vis (Zeiss,
Jena). The ESR study was carried out with a JES-FE3AX
ESR spectrometer (Japan). Cyclic voltammetry was carried
on a Model 500 AC. Impedance Analyzer (CH Instrument
Co.), which was equipped with a cell stand, a Ag/AgCl
reference electrode, and a platinum wire counter elec-
trode. The working electrode used was a glassy carbon disk
electrode.
RESULTS AND DISCUSSIONS
Catalytic Activity
The catalytic activities of iron(II) ion, iron(II)–8-
quinolinol, iron(II)–8-quinolinol/MCM-41, MCM-41, and
TS-1 in phenol hydroxylation were studied. The results, in
Table 1, show that phenol and H2O2 cannot react without
the participation of catalysts under the experimental con-
ditions. It was also observed that Fe2+ (Fenton reagent)
and iron(II)–8-quinolinol had poor average turnover fre-
quency. Moreover, more by-products of benzoquinone and
oxygen were also not good for diphenol production when
Fe2+ (Fenton reagent) was used as catalyst. When iron(II)–
8-quinolinol was immobilized in MCM-41, the aver-
age turnover frequency of iron(II)–8-quinolinol/MCM-41
was about two times that of iron(II)–8-quinolinol. On
FIG. 1. XRD patterns of MCM-41 (a), fresh iron(II)–8-quinolinol/
MCM-41 (b), and the used iron(II)–8-quinolinol/MCM-41 (c) after
10 runs.

Fe(II)-8-QUINOLINOL/MCM-41-CATALYZED PHENOL HYDROXYLATION
37
TABLE 1
TABLE 2
Catalytic Activity Comparison of Iron(II)–8-quinolinol/
MCM-41, Iron(II)–8-quinolinol, MCM-41, and Other Catalysts in
the Phenol Hydroxylationa
Effects of Reaction Medium on the Phenol Hydroxylation
Catalyzed by Iron(II)–8-quinolinol/MCM-41a
Phenol
conversion
(% )
O2
evolved TOFb
(ml)
Product selectivity (% )
Phenol
conversion
(% )
O2
Reaction
medium
Product selectivity (% )
evolved TOFb
CAT
HQ
PBQ
(hꢁ1
)
Catalyst
CAT
HQ
PBQ
(ml)
(hꢁ1
)
Water
Acetonitrile
Acetone
48.2
26.4
3.5
57.5
56.8
56.3
—
41.8
30.7
20.1
—
0.7
12.5
23.6
—
8
5
1
0
33.9c
18.6c
2.5c
0c
No catalyst
8-Quinolinol
MCM-41
Fe2+ (Fenton
reagent)
Fe(II)-Qx
TS-1d
Fe(II)–
0.0
0.0
0.0
—
—
—
—
—
—
0
0
0
0
0
0
Cyclohexane
0.0
—
—
—
20.5
68.7
18.9
12.4
35
17.6c
CAT, catechol; HQ, hydroquinone; PBQ, p-benzoquinone.
a
Reaction time: 6 h; temperature: 50ꢂC; reaction medium: water;
pH 7.0; concentration of phenol: 0.35 mol/liter; molar ratio of phenol/
H2O2: 1.0; volume of reaction mixture: 15 ml; iron(II)–8-quinolinol/
MCM-41: 100 mg.
25.8
27.0
48.2
65.3
53.0
57.5
32.7
47.0
41.8
2.0
0.0
0.7
25
20
8
18.1c
13.8e
33.9c
Qx/MCM-41
b Average turnover frequency.
c The active site is [Fe(Qx)3]Cl2.
CAT, catechol; HQ, hydroquinone; PBQ, p-benzoquinone; Fe(II)–Qx,
iron(II)–8-quinolinol.
a
Reaction time: 6 h; temperature: 50ꢂC; reaction medium: water; pH
droxylation catalyzed by iron(II)–8-quinolinol/MCM-41
(Table 3).
7.0; concentration of phenol: 0.35 mol/liter; molar ratio phenol/H2O2:
1.0; volume of reacting mixture: 15 ml. Catalyst used: MCM-41 molec-
ular sieve, 100 mg; Fe(II)–Qx/MCM-41, 100 mg (containing 7 mg
[Fe(Qx)3]Cl2); [Fe(Qx)3]Cl2, 7 mg; Fe2+, same moles as for 7 mg
[Fe(Qx)3]Cl2; 8-quinolinol, 7 mg.
Mechanism of the Phenol Hydroxylation Catalyzed
by Free and Supported Iron(II)-8-quinolinol
b Average turnover frequency.
c The active site is Fe2+ or [Fe(Qx)3]Cl2.
The typical voltammogram of iron(II)–8-quinolinol is
shown in Fig. 3, which indicates that Epc and Epa are 1.0 and
0.93 V, respectively. Based on their difference 1E (70 mV),
this redox reaction could be considered reversible. Accord-
ing to the equation E0 = 1/2(Epa + Epb), the potential of the
redox couple iron(II/III)–8-quinolinol is 0.965 (V). The po-
tential of the redox couple H2O2/H2O is 1.77 V (24). The
large potential difference of these two redox couples can
easily lead to a redox reaction between H2O2 and iron(II)–
8-quinolinol.
d
For TS-1 [Ti/(Ti + Si) = 0.021]: phenol/catalyst (g/g) = 10; other con-
ditions are the same as those for the above-mentioned catalysts.
e The active site is Ti4+ in TS-1.
one hand, this increase may be due to the concentration
of substrates in the channels of MCM-41; on the other
hand, it might be attributed to the distortion of iron(II)–
8-quinolinol by the pore wall of MCM-41. The distortion
might activate substratesand/or catalyst, which would make
the phenol hydroxylation proceed well. Table 1 also shows
that iron(II)–8-quinolinol/MCM-41 has a higher average
turnover frequency than the commercial catalyst TS-1.
Iron(II)–8-quinolinol/MCM-41 was used in 10 runs to
catalyze phenol hydroxylation with little decrease in cata-
lytic activity; phenol conversion in the 10th run was about
85% that in the first run. The X-ray diffractogram of the
catalyst used for the 10 runs is shown in Fig. 1c, and is simi-
lar to those offresh iron(II)–8-quinolinol/MCM-41(Fig. 1b)
and MCM-41 (Fig. 1a). According to chemical analyses, loss
of the immobilized metal complex during the 10 runs of re-
action was 9.5% . The preceding indicates that the support
MCM-41 did not change its structure during phenol hydrox-
ylation and iron(II)–8-quinolinol had good immobilization
in MCM-41.
Generally, H2O2 is considered to have a skew chain struc-
ture. The bond energies between two atoms in hydrogen
peroxide are helpful in predicting the production of active
TABLE 3
Effects of pH on the Phenol Hydroxylation Catalyzed
by Iron(II)–8-quinolinol/MCM-41a
Phenol
conversion
(% )
O2
evolved
(ml)
Product selectivity (% )
TOFb
(hꢁ1
pH
CAT
HQ
PBQ
)
1
2
4
7
9
20
58.2
56.0
56.3
57.5
57.9
0.0
40.9
36.8
38.6
41.8
42.5
0.0
0.9
7.2
5.1
0.7
0.0
0.0
5
10
9
8
15
20
14.1c
40.4c
37.2c
33.9c
6.0c
57.5
53.2
48.2
8.5
When phenol hydroxylation catalyzed by iron(II)–8-
quinolinol/MCM-41 was carried out in different solvents, 12
such as water, acetonitrile, acetone, and cyclohexane, dif-
ferent results were obtained (shown in Table 2).
0.0
0c
For abbreviations, see Table 2.
a The reaction conditions are the same as those in Table 2.
b Average turnover frequency.
Meanwhile, it was observed that the pH of the re-
action system also had great effects on the phenol hy-
c The active site is [Fe(Qx)3]Cl2.

38
LIU ET AL.
FIG. 3. Cyclic voltammogram of iron(II)–8-quinolinol in 0.1 M KCl
solution. Scan rate: 1 mV/s.
FIG. 4. UV–visible spectra of iron(III)–8-quinolinol (a), 8-quinolinol
(b), iron(II)–8-quinolinol (c), and H2O2 (d) in the visible region.
–
–
species in oxidation. The bond energies of O H and O O
in H2O2 can be obtained from the energies of a number of
reactions (25).
–
Table 4 shows that the O O bond is more easily bro-
–
ken than the O H bond in the H2O2 molecule. The p or-
–
bital along the O O bond direction in the H2O2 molecule
would thus get one electron from another substance to pro-
duce ꢀOH. In the phenol hydroxylation catalyzed by free
and supported iron(II)–8-quinolinol, on one hand, H2O2
may receive one electron from iron(II)–8-quinolinol to pro-
duce iron(III)–8-quinolinol and active species ꢀOH; on the
other hand, iron(III)–8-quinolinol could also be reduced to
iron(II)–8-quinolinol. This can be confirmed by ESR and
UV–Vis spectra studies of iron(II)–8-quinolinol.
The UV–Vis spectra of iron(II/III)–8-quinolinol com-
plexes are shown in Fig. 4 and are in accordance with those
reported in the literature (26). The spectralchangesinduced
by addition of H2O2 the solution of iron(II)–8-quinolinol
are shown in Fig. 5. In Figs. 4 and 5, the transforma-
tion of iron(II)–8-quinolinol into iron(III)–8-quinolinol is
TABLE 4
Energies of a Number of Reactions Related to the Breaking
of the Bonds of H2O2
Energy needed to break
Reaction
Reaction
bonds (kcal/mol)
1
2
3
4
H2O2 → 2 ꢀH + 2 ꢀO
H2O2 → 2 ꢀOH
254.9
52.6
136.6
90
FIG. 5. UV–visible spectral change of iron(II)–8-quinolinol
(2 ꢃ 10ꢁ4 mol/liter) + H2O2 (2 ꢃ 10ꢁ4 mol/liter) + H2O system (50ꢂC)
(time was recorded at the beginning of addition of H2O2).
H2O2 → 2 ꢀH + O2
ꢀ
H2O2 → ꢀH + HO2

Fe(II)-8-QUINOLINOL/MCM-41-CATALYZED PHENOL HYDROXYLATION
39
FIG. 7. ESR spectra of [Fe(Qx)3]3+ (10ꢁ4 mol/liter) (a) and
[Fe(Qx)3]2+ (10ꢁ4 mol/liter) (b).
liquid–solid phases. Therefore, iron(II)–8-quinolinol and
iron(II)–8-quinolinol/MCM-41 should have the same cata-
lytic reaction mechanism in phenol hydroxylation.
Based on the above studies of catalytic activity and ESR
and UV–Vis spectra, a reaction mechanism was proposed
for phenol hydroxylation catalyzed by free and supported
iron(II)–8-quinolinol (Scheme 1).
This mechanism shows that steps 1 and 3 are very impor-
tant for phenol hydroxylation, which has been confirmed
by UV–Vis (Figs. 5, 6) and ESR (Figs. 8, 9) spectral studies.
For step 2, ꢀOH is an electrophile, which differs from the
nucleophile OHꢁ, so it tends to have an electrophilic, attack
at the ortho and para positions of the phenol molecules to
FIG. 6. UV–visible spectral change of the system described in Fig. 5,
when phenol (4 ꢃ 10ꢁ4 mol/liter) is added to this system at 30 min.
demonstrated clearly. In Fig. 6, when phenol was added to
the iron(II)–8-quinolinol + H2O2 + H2O system at 30 min,
it was observed that iron(III)–8-quinolinol was slowly re-
duced to iron(II)–8-quinolinol again. According to gas
chromatographic analysis, catechol and hydroquinone were
produced in this process.
The transformation of iron(II)–8-quinolinol into
iron(III)–8-quinolinol in the iron(II)–8-quinolinol + H2O2
+ H2O system was also confirmed by ESR. Water solutions
of iron(II)–8-quinolinol, iron(III)–8-quinolinol, and the
iron(II)–8-quinolinol + H2O2 + H2O system were individ-
ually transferred into capillary tubes and then used for
ESR studies. The ESR spectra of iron(II)–8-quinolinol
and iron(III)–8-quinolinol, in Fig. 7, show that iron(II)–8-
quinolinol has no ESR signals, but iron(III)–8-quinolinol
does. When H2O2 was added to the iron(II)–8-quinolinol
solution, the ESR spectrum of iron(III)–8-quinolinol
appeared (Fig. 8), and its peak height also increased with
time. This means that part of the iron(II)–8-quinolinol
was oxidized to iron(III)–8-quinolinol. When phenol
was added 30 min after H2O2 was, the spectral change
shown in Fig. 9 occurred, confirming the transformation of
iron(III)–8-quinolinol into iron(II)–8-quinolinol.
Table 1 shows that MCM-41 and ligand 8-quinolinol have
no catalytic activity for phenol hydroxylation, but iron(II)–
8-quinolinol and supported iron(II)–8-quinolinol/MCM-41
do. This means that iron(II)–8-quinolinol is the key sub-
FIG. 8. ESR spectral change of [Fe(Qx)3]2+ (2 ꢃ 10ꢁ4 mol/liter)
stance in catalyzing this reaction in both homogeneous and + H2O2 (2 ꢃ 10ꢁ4 mol/liter) + H2O system.

40
LIU ET AL.
produce catechol and hydroquinone (28, 29). No resorci-
nol was detected in the phenol hydroxylation. Steps 4 and
5 are side reactions for diphenol production; the quanti-
ties of the by-products O2 and benzoquinone change under
different reaction conditions. By choosing suitable reaction
conditions, good results can be obtained for phenol hydrox-
ylation.
Table 2 shows that phenol hydroxylation proceeds bet-
ter in water than in acetonitrile, acetone, and cyclohex-
ane. The reason may be that phenol and H2O2 can reach
the active sites more easily in water than in the organic
solvents. In this way, the active species ꢀOH is produced
(step 1) and dispersed more easily, which could acceler-
ate step 2. In cyclohexane, no reaction occurred; this may
be due to the fact that no H2O2 could be dissolved in this
solvent. In acetonitrile and acetone, it was also observed
that hydroquinone instead of catechol was further oxidized
(Table 2). It can also be confirmed with the following study
that hydroquinone is more easily oxidized than catechol. In
the hydroquinone + catechol + [Fe(Qx)]23+ + H2O2 + H2O
reaction system, the concentration changes of some sub-
stances are shown in Fig. 10. Figure 10 also shows that hy-
droquinone can be easily further oxidized to benzoquinone
through step 5; however, catechol cannot.
The pH value of the reaction medium has a great effect
on phenol hydroxylation. When phenol hydroxylation was
carried out in a weak acid medium, the average turnover
frequency was higher than that in neutral and basic media
SCHEME 1. Mechanism of the phenol hydroxylation catalyzed by
iron(II)–8-quinolinol and iron(II)–8-quinolinol/MCM-41. Qx = 8-quino-
linol.
(Table 3). One reason might be that in the acidic reaction
system, H+ can accelerate step 1, thus producing more ꢀOH
than in the neutral and basic reaction systems. Another
reason for the higher average turnover frequency in acidic
medium may be due to the acid-catalyzed hydroxylation
not involving radicals; however, when the pH was too low
(e.g., pH 1), catalytic activity would decrease (Table 3), pos-
sibly because step 3 was greatly inhibited by H+. For the
reaction in basic medium, the poor activity might have two
causes: (1) Step 1 is inhibited by OH; not enough ꢀOH ac-
tive species are produced for further reaction. (2) In basic
medium, Fe(Qx)33+ has poor oxidizability, which makes it
difficult for Fe(Qx)33+ to get electrons from the delocalized
cyclohexadienyl radicals to produce catechol and hydro-
quinone (step 3).
CONCLUSION
The experiments show that MCM-41 is a good catalyst
support for [Fe(Qx)3]Cl2. The good immobilization could
FIG. 9. ESR spectral change of the system described in Fig. 8 when
phenol (4 ꢃ 10ꢁ4 mol/liter) is added to this system at 30 min.

Fe(II)-8-QUINOLINOL/MCM-41-CATALYZED PHENOL HYDROXYLATION
41
The successful hydroxylation of phenol and better ac-
tivity compared with TS-1 (Table 1) demonstrated here
mean that we can establish a new route for diphenol
production.
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FIG. 10. Concentration change of catechol (a), hydroquinone (b), and
benzoquinone (c). Reaction conditions: medium, water; pH 7.0; 50ꢂC;
time, 6 h; volume of reaction mixture, 15 ml; catechol = hydroquinone =
5 ꢃ 10ꢁ3 mol; molar ratio: catechol = hydroquinone/H2O2/catalyst (molar-
ity of the support complex) = 1000/1000/1.
be due to the adsorption and static coulombic function be-
tween the pore wall of MCM-41 and [Fe(Qx)3]2+ species.
Iron(II)–8-quinolinol/MCM-41 has better catalytic activ-
ity than iron(II)–8-quinolinol and Fe2+ (Fenton agent).
The better catalytic activity of the support iron(II)–8-
quinolinol/MCM-41 might be attributed to the higher con-
centration of catalyst and substrates in the channels of
MCM-41 and the distortion of iron(II)–8-quinolinol by the
pore wall of MCM-41.
Moreover, the channels are large enough not only to al-
low phenol and H2O2 to approach catalytic centers freely,
but also to allow products to move out easily. In this way, we
could solve the problems caused by the limited pore sizes
of microporous zeolite-encapsulated metal complexes. It
might be possible for the larger ligand metal complexes to
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