First-row early transition metal complexes with a highly sterically demanding triisopropylphenyl amino triphenolate ligand: Synthesis and applications

Article abstract of DOI:10.1039/c9dt00456d

Amino triphenolate ligands have been widely used for the synthesis of various transition metal complexes aiming at various applications such as ring-opening polymerization, olefin polymerization, and sulfoxidation. However, the introduction of highly ster

Full text of DOI:10.1039/c9dt00456d

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DOI: 10.1039/C9DT00456D
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ARTICLE
First-row early transition metal complexes with a highly sterically
demanding triisopropylphenyl amino triphenolate ligand:
synthesis and applications
Received 00th January 20xx,
Accepted 00th January 20xx
Dae Young Bae,ab Gyeong Su Park,c Nakeun Ko,a Kyung-sun Son,*c Eunsung Lee *abd
DOI: 10.1039/x0xx00000x
Amino triphenolate ligands have been widely used for the synthesis of various transition metal complexes aiming at various
applications such as ring-opening polymerization, olefin polymerization, and sulfoxidation. However, the introduction of
highly sterically demanding aromatic substituents, such as triisopropylphenyl (TRIP), to the amino triphenolate ligand has
not been previously reported probably due to the synthetic difficulty. In six-step reactions using commercial materials, a
highly sterically demanding amino triphenolate ligand was successfully synthesized, and early transition metal complexes
(Ti, V, Cr, Mn) supported by the ligand were also obtained and fully characterized. In addition, titanium and chromium
complexes were further used for catalytic sulfoxidation, and polymerization of ethylene, respectively.
www.rsc.org/
Introduction
Ligands in transition metal chemistry control the environment of the Synthetic routes A and C were applied to access a highly
metal and are thus key players in a variety of catalytic reactions.1 sterically demanding triisopropylphenyl amino triphenolate
Therefore, tremendous efforts have been made to modulate the (TRIP-ATP) ligand under various conditions,7 but all attempts
electronic and steric properties of the coordination geometry of were unsuccessful. Therefore, we set out to develop a new
metal centers. Multidentate ligands have been widely investigated synthetic method to prepare TRIP-ATP. Herein, we report its
for securing robustness and stability to transition metal catalysts. synthesis and early transition metal complexes, and
Among various multidentate ligands, amino triphenolate ligands applications of the titanium and chromium complexes. The key
have recently emerged as new candidates for transition metal to the successful preparation of the bulky ligand was to use an
catalysts. For example, the metal complexes supported by amino allyl protection group, which did not affect the subsequent
triphenolates were successfully used for various applications, such as deprotection step.
ring-opening polymerization (ROP),2 olefin polymerization,3 and
sulfoxidation.4 Therefore, in the last two decades, a variety of their
Synthetic Route A
OH
R
complexes have been synthesized and applied to a wide variety of
N
R
OH
110 °C
transition metals3e, 4c, 5 and main group elements.6
N
+
N
N
2-7 days
N
R'
Although there are various precedents to sterically
demanding amino triphenolates, amino triphenolate ligands
decorated by highly substituted aromatic groups have less
studied. At first glance, the synthesis of amino triphenolate
substituted by functionalized aromatic groups seems facile
because several synthetic routes have been reported (Scheme
1). However, such known precedents were not effective with a
highly bulky aromatic group, such as triisopropylphenyl (TRIP).
R'
3
Synthetic Route B
OMe
OMe
OH
1. base, heating
2. AlCl3, reflux
NH2
+
Br
N
2
3
Synthetic Route C
R
OBn
R
OH
1. BnBr, K2CO3
H2, Pd/C
OH
R
N
N
2. NaBH(OAc)3
NH4OAc
CHO
3
3
a. Department of Chemistry, Pohang University of Science and Technology, 77
Cheongam-Ro, 37673 Pohang, Republic of Korea.
iPr
b. Center for Self-assembly and complexity, Institute for Basic Science (IBS), 77
Cheongam-Ro, 37673, Pohang, Republic of Korea.
Our attempts
TRIP
TRIP =
iPr
iPr
OBn
OH
c. Department of Chemistry, Chungnam National University, 34134 Daejeon,
Republic of Korea.
N
TRIP
TRIP
OH
+
N
N
N
N
d. Division of Advanced Materials Science, Pohang University of Science and
Technology, 77 Cheongam-Ro, 37673, Pohang, Republic of Korea.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
N
various
conditions
various
3
conditions
3
2
Scheme 1. Synthetic approaches to the amino triphenolate ligand, and our
attempts to introduce triisopropylphenyl (TRIP) to amino triphenolate.
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to the vial. The mixture was stirred at 23°C for 30 min. VCl3(THF)3
We investigated the complexation of TRIP-ATP with first-
row early transition metals because early transition metals
typically have a strong affinity toward phenolate ligands. Early
transition metal or oxophilic metal complexes have been
applied to various reactions including amination,8 olefin
polymerization,9 cyclic esters/carbonates ROP2d, 10 and ketone
hydrosilylation.11 Thus, we also explored their catalytic activities
for these reactions. A titanium complex with TRIP-ATP was
found to be efficient for selective sulfoxidation of thioanisoles.
A chromium complex with TRIP-ATP was found to be efficient
for the preparation of high density polyethylene (HDPE) with a
high melting point, which may have resulted from the highly
steric effect of the ligand.
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(39.6 mg, 0.106 mmol, 1.0 equiv.) was then added to the vial. The
DOI: 10.1039/C9DT00456D
resulting mixture was stirred at 60 °C for 12 h. The solvents were
removed in vacuo. The solid residue was extracted with pentane (4 x
3 mL). The mixture was filtered through a pad of Celite, and dried in
vacuo to afford 6-V as a yellow solid (93.2 mg, 83 %). X-ray quality
single crystals were grown from a saturated pentane solution at ‒40
°C over several days. Anal. Calcd. For C70H92VNO4(pentane)1.2: C,
79.44; H, 9.33; N, 1.22; found: C, 79.98; H, 8.83; N, 1.44. Solution
magnetic moment (C6D6): 2.66 B. IR (KBr, cm-1): 2958 (s, C–H), 2930
(s, C–H), 2868 (s, C–H), 1594 (m), 1430 (m), 1360 (s), 1243 (m),
1070(m), 861 (m), 756 (s).
6-Cr. NaH (7.9 mg, 3.1 equiv.) and 6 (100.0 mg, 0.106 mmol, 1.0
equiv.) were added to a 20 mL vial. Dry THF (10 mL) was added to the
vial. The resulting mixture was stirred at 23 °C for 30 min. CrCl3(THF)3
(39.8 mg, 0.106 mmol, 1.0 equiv.) was then added to the vial. The
mixture was stirred at 60 °C for 24 h. The solvents were removed in
vacuo. The solid residue was extracted with pentane (4 x 3 mL). The
extracted solution was removed in vacuo to afford 6-Cr as a green
solid (75.6 mg, 67 %). X-ray quality single crystals were grown from a
saturated pentane solution at ‒40 °C over several days. Anal. Calcd.
For C70H92CrNO4H2O: C, 77.74; H, 8.76; N, 1.30; found: C, 77.42; H,
8.86; N, 1.31. Solution magnetic moment (C6D6): 3.76 B. IR (KBr, cm-
1): 2958 (s (br), C–H), 2927 (s, C–H), 2868 (s, C–H), 1589 (s), 1430 (m),
1360 (s), 1274(m), 1043(m), 859 (s), 752 (s).
Experimental Section
The synthesis of the ligand precursors (1-5), density functional
theory (DFT) calculations for vanadium and chromium
complexes, and details of sulfoxidation and ethylene
polymerization are described in the Supplementary
Information.
Tris{[2-(hydroxy)-2',4',6'-triisopropyl-3-biphenylyl]methyl}amine
(6). K2CO3 (3.63 g, 26.2 mmol, 3.0 equiv.), Pd(PPh3)4 (0.707 g, 0.07
equiv.) and 5 (9.29 g, 8.74 mmol, 1.0 equiv.) were added to a 350-mL
heavy wall round-bottom flask. Dry tetrahydrofuran (THF; 90 mL) and
methanol (60 mL) were added to the flask. The resulting mixture was
stirred in the sealed flask at 50 °C for 24 h. Distilled water was then
added to the flask and the organic layer was extracted with pentane
(4 × 30 mL). The combined organic layers were filtered through a pad
of Celite. The layers were then filtered through a pad of silica. The
combined organic layers were dried to afford 6 as a colorless solid
(6.43 g, 78 %). 1H NMR (500 MHz, CDCl3, 23 °C, δ): 7.22 (dd, J = 7.7,
1.6 Hz, 3H), 7.06 (s, 6H), 6.94 (dd, J = 7.5, 1.6 Hz, 3H), 6.85 (t, J = 7.6
Hz, 3H), 6.68 (s, 3H), 3.84 (s, 6H), 2.93 (sept, J = 6.9 Hz, 3H), 2.56 (sept,
J = 6.8 Hz, 6H), 1.30 (d, J = 6.6 Hz, 18H), 1.01 (dd, J = 6.9, 5.1 Hz, 36H).
13C NMR (125 MHz, CDCl3, 23 °C, δ): 153.2, 148.9, 148.1, 130.7,
130.2, 129.9, 126.6, 122.8, 121.3, 119.5, 53.9, 34.5, 30.6, 24.6, 24.2,
24.0. Anal. Calcd. for C66H87NO3: C, 83.77; H, 9.63; N, 1.21; found: C,
84.12; H, 9.31; N, 1.49.
6-Mn. NaH (7.9 mg, 0.329 mmol, 3.1 equiv.) and 6 (100.0 mg, 0.106
mmol, 1.0 equiv.) were added to a 20 mL vial. Dry THF (10 mL) was
added to the vial. The resulting mixture was stirred at 23 °C for 30
min. Mn(acac)3 (37.4 mg, 0.106 mmol, 1.0 equiv.) was then added to
the vial and the resulting mixture was stirred at 60 °C for 12 h. The
solvent was removed in vacuo. The solid residue was extracted with
diethyl ether (4 x 3 mL). The mixture was filtered through a pad of
Celite, and dried in vacuo to afford 6-Mn as a green solid (33.8 mg,
26 %). X-ray quality single crystals were grown from a saturated
diethyl ether solution at ‒40 °C over several days. Anal. Calcd. For
C75H99MnNNa2O6(H2O)0.6: C, 73.70; H, 8.26; N, 1.15; found: C, 73.33;
H, 8.15; N, 1.07. Solution magnetic moment (C6D6): 6.00 B. IR (KBr,
cm-1): 2960 (s, C–H), 2927 (s, C–H), 2868 (s, C–H), 1585 (s), 1503 (m),
1428 (m), 1276 (m), 1070(m), 940 (s), 853 (m), 750 (s).
6-Ti. 6 (100.0 mg, 0.106 mmol, 1.0 equiv.) was added to a 20 mL vial
followed by addition of 6 mL of dry benzene. Ti(OiPr)4 (35.3 µL, 0.117
mmol, 1.1 equiv.) was added to the vial. The mixture was stirred at
23°C for 12 h. The solution was filtered through a pad of Celite and
volatiles were removed in vacuo to obtain 6-Ti as a colorless solid
(94.1 mg, 85 %). X-ray quality single crystals were grown by vapor
Result and Discussions
2-bromophenol
NaH
cat. Pd(acac)2
Br
iPr
TRIP
Mg
iPr
iPr
1,2-dibromoethane
1
diffusion (C6H6/pentane) at ‒40 °C over several days. H NMR (500
MgBr
iPr
OH
MHz, C6D6, 23 °C, δ): 7.16 (s, 3H), 7.14–7.12 (m, 6H) 6.94 (d, J = 1.50
Hz, 1H), 6.92 (d, J = 1.53 Hz, 2H), 6.88 (t, J = 7.40 Hz, 3H) 4.16 (d, J =
13.76 Hz, 3H), 3.98 (sept, J = 6.18 Hz, 1H), 2.91–2.80 (m, 6H), 2.66 (d,
J = 13.9 Hz, 3H), 2.55 (sept, J = 6.83 Hz, 3H), 1.34 (d, J = 6.70 Hz, 9H),
1.27 (d, J = 6.97 Hz, 9H), 1.26 (d, J = 6.97 Hz, 9H), 1.24 (d, J = 6.83 Hz,
9H), 1.08 (d, J = 6.83 Hz, 9H), 1.07 (d, J = 6.97 Hz, 9H), 0.58 (d, J = 6.18
Hz, 3H), 0.54 (d, J = 6.18 Hz, 3H). 13C NMR (125 MHz, C6D6, 23 °C, δ):
161.0, 148.1, 147.2, 147.2, 133.6, 131.4, 124.6, 120.6, 120.4, 120.4,
80.4, 58.1, 34.9, 31.2, 31.1, 25.5, 24.7, 24.6, 24.5, 24.4, 24.4, 24.3.
Anal. Calcd. for C69H91TiNO4·(H2O)1.3: C, 77.47; H, 8.82; N, 1.31; found:
C, 77.37; H, 8.74; N, 1.32. IR (KBr, cm-1): 3054 (s (br), C–H), 2958 (s,
C–H), 2927 (s, C–H), 2866 (s, C–H), 1445 (s), 1433 (s), 1241(s),
1011(m), 874 (s), 709 (m), 858 (m).
THF, 85 °C (12 h)
75 %
THF, rt (24 h)
sonication
iPr
iPr
1
2
MgCl2
Et3N
CH2O
TRIP
TRIP
K2CO3
allyl bromide
O
OH
O
O
THF, 85 °C (12 h)
96 %
MeCN, 110 °C (4.5 h)
97 %
H
4
H
3
K2CO3
cat. Pd(PPh3)4
OH
NH4OAc
NaB(OAc)3H
O
TRIP
TRIP
N
N
THF/MeOH (3:2)
50 °C (24 h)
THF, rt, (16 h)
70 %
3
3
78 %
5
6
Scheme 2. Synthesis of a highly sterically demanding amino triphenolate ligand (6).
6-V. Et3N (89.0 µL, 6.0 equiv.) and 6 (100.0 mg, 0.106 mmol, 1.0
equiv.) were added to a 20 mL vial. Dry toluene (10 mL) was added
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Synthesis of triisopropylphenyl amino triphenolate (TRIP-ATP). The
bulky amino triphenolate ligand was synthesized in six steps from 1-
bromo-2,4,6-triisopropylbenzene, with an overall yield of 38%
(Scheme 2). From a practical point of view, it is quite efficient that
the six-step synthesis of the pure complex ligand was achieved
without requiring column chromatography. The simple washing,
recrystallization, and sublimation of the as-synthesized products
provided analytically pure compounds. The facile purification
process enables multi-gram scale synthesis of the ligand 6. The
synthesis involved the palladium-catalyzed Kumada coupling
reaction of the Grignard reagent (1)12 and 2-bromophenol,13
followed by ortho formylation of the phenol.14 Protection of phenol
by allyl group and amination were proceeded to afford 5,15 of which
was further deprotected to obtain the final amino triphenol ligand
6.16 The successful preparation of other bulky amino triphenol via
allyl protection was also patented by Kazuki et al.15 However, other
phenol protecting groups such as benzyl16 and silyl groups that used
for the synthesis of other amino triphenolates were not successful
because the deprotection of benzyl group and the clean silyl
protection were not successful in the ligand system. Therefore, allyl
protection seems to be useful for the preparation of amino
triphenolates with highly bulky ortho substituents on phenols. All
new compounds were fully characterized.
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6-Mn
6-Ti
6-V
6-Cr
DOI: 10.1039/C9DT00456D
Empirical
formula
Formula
weight
Temperatu
re/K
C75H99.06MnN
Na2O6
C90H112NO4Ti
C70H92NO4V
C72.5H98CrNO4
1099.51
100
1319.70
100
1062.38
100
1211.52
100
Crystal
system
Space
trigonal
R-3
monoclinic
C2/c
triclinic
P-1
orthorhombic
Pbca
group
a/Å
23.4039(2)
23.4039(2)
24.9083(5)
90
37.206(7)
14.294(3)
24.650(5)
90
14.1640(11)
14.4132(11)
18.4865(14)
108.161(3)
91.379(3)
110.604(3)
3318.4(4)
2
18.612(4)
27.311(6)
30.616(6)
90
b/Å
c/Å
α/°
β/°
90
95.33(3)
90
90
γ/°
120
90
Volume/Å3
Z
11815.5(3)
6
13053(5)
8
15562(5)
8
MoKα (λ =
0.71073)
synchrotron
(λ = 0.70000)
MoKα (λ =
0.71073)
synchrotron
(λ = 0.70000)
Radiation
Synthesis of transition metal complexes
R1 [I>=2σ
(I)]
Most early transition metal complexes (6-V, 6-Cr, and 6-Mn) of TRIP-
ATP were synthesized by phenol deprotonation in the presence of
metal salts, except a titanium complex 6-Ti (Scheme 3). That complex
was directly synthesized by transalkoxylation of the ligand 6 with
Ti(OiPr)4 in benzene solution at room temperature. The analytically
pure colorless complex (6-Ti) was characterized by 1H and 13C nuclear
magnetic resonance (1H and 13C NMR) spectroscopy. The Iso-
propoxide and methylene protons of the complex were successfully
assigned using 1H NMR spectroscopy and 1H–1H correlated
spectroscopy (COSY) (see Supplementary Information). In the case of
vanadium and chromium complexes, THF was coordinated to each
metal ion, as evidenced by X-crystallographic analysis. In the
synthesis of a manganese complex with 6, several manganese salts,
such as hydrated Mn(acac)3 (acac = acetylacetonate), MnF3, and
dried Mn(acac)3, were evaluated for complexation under various
conditions. However, only dried Mn(acac)3 afforded the desired and
analytically pure complex with 6. MnF3 did not react with 6, and
hydrated Mn(acac)3 afforded a poorly crystalline material, whose
structure was not confirmed by X-ray analysis. Anhydrous Mn(acac)3
afforded 6-Mn, where the coordinated acac anion interacted with
two sodium cations and THF to increase the complexity of the
manganese metal environment (Scheme 3). Interestingly, the net
oxidation state of Mn was +2, which possibly originates from the
disproportionation of manganese(III) complexes, as evidenced by X-
ray crystallography and its solution magnetic moment (6.00 µB). We
believe that the low isolation yield (26%) of 6-Mn is associated with
the complexity of the Mn coordination geometry and the
disproportionation of Mn(III) intermediates. Additionally, almost
colorless 6-Mn shows very weak peaks in the visible region based on
UV-Vis spectra, which indicates a spin forbidden character of the
Mn(II) with a high spin state (see Supplementary Information). When
NaH was replaced by Et3N, complexation of Mn with 6 was not
observed.
0.0784
0.2231
0.0862
0.2447
0.0499
0.1315
0.0682
0.2089
wR2 [I>=2σ
(I)]
Solid-state structures of 6-Ti, 6-V, 6-Cr, and 6-Mn. X-ray single
crystals of each complex were grown in a proper solvent system at
‒40 °C. The structures of the monomeric units of the complexes 6-Ti,
6-V, 6-Cr and 6-Mn are shown in Figure 1. As described in the
literature,17 all complexes adopted a trigonal bipyramidal (TBP)
geometry having high geometry index ( ≥ 0.87); most of the early
transition metal amino triphenolate complexes have the same
geometry. It is interesting that even the highly sterically demanding
TRIP does not change the preferential geometry of TBP in the early
transition metal complexes, while some cases show octahedral
geometry at the metal sites. In the 6-Ti complex, due to the highly
symmetric system (trigonal), the iso-propoxide molecule was
disordered in three positions. The Ti–O(2) and Ti–N(4) bond
distances at the axial position are 1.780 and 2.319 Å, respectively.
The Ti–O(1) distance in the equatorial position is 1.843 Å (Figure 1).
These bond distances are in good agreement with the values of other
amino triphenolate titanium complexes that have been reported.4d,18
Notably, the equatorial Ti–O(1) distance in 6-Ti is almost the same as
the Ti–O bond distance in the other titanium amino triphenolate
complexes with the less sterically hindered methyl group.5o This
indicates that the TRIP substituent is just adequate for the TBP
geometry and provides a hydrophobic environment around the
titanium center.
Table 1. Selected crystal data and refinement details.
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agreement with the experimental values. Molecular orbital (MO)
analysis of both complexes (6-V and 6-Cr) DshOoI:w1s0.1th03aV9ti/etCwh9eADrtTeic0llee0cO4tn5rl6oinDne
densities of the filled frontier orbitals are localized in the metal sites,
while those of the lowest empty frontier orbitals are localized in the
ligand phenyl rings (Figures S12, S13 and S14).
TRIP
TRIP
O
O
Ti(OiPr)4
O
TRIP
Ti
O
C6H6, rt
85%
N
6-Ti
TRIP
TRIP
Et3N (6 eq.)
VCl3(THF)3
O
O
O
TRIP
V
N
O
PhMe, 60 °C
83%
OH
6-V
TRIP
N
TRIP
3
TRIP
NaH (3.1 eq.)
CrCl3(THF)3
O
6
O
O
TRIP
Cr
N
O
THF, 60 °C
67%
6-Cr
O
O
O
TRIP
TRIP
Na
Na
NaH (3.1 eq.)
Mn(acac)3
O
O
TRIP
Mn
O
THF, 60 °C
26%
N
Figure 1. X-ray crystal structures of 6-Ti, 6-Cr, 6-V and 6-Mn showing thermal ellipsoids
at the 50% probability level. Hydrogen atoms were omitted for clarity. Geometry index
() values were noted. Selected bond lengths (Å) (computed bond lengths by DFT) and
angles (°) for the complexes: 6-Ti, Ti1–O1 1.843, Ti1–O2 1.780, Ti1–N4 2.319, O1–Ti–O1’
118.02, N4–Ti–O2 173.8 N4–Ti1–O1 81.85; 6-V, V1–O1 1.874(2) (1.878), V1–O2 1.866(2)
(1.873), V1–O3 1.869(2) (1.872), V1–O4 2.102(2) (2.123), V1–N6 2.145(3) (2.137) O1–
V1–O2 120.16(9) N6–N1–O4 179.3(1) N6–V1–O1 90.94(9); 6-Cr, Cr1–O1 1.878(2) (1.895),
Cr1–O2 1.856(2) (1.888), Cr1–O3 1.890(2) (1.897), Cr1–N1 2.067(1) (2.059) N1–Cr1–O5
176.59(7) O1–Cr1–O2 108.43(7) N1–Cr1–O1 91.06(7); 6-Mn, Mn1–O1 2.093(2), Mn1–O2
2.006(2), Mn1–O3 2.091(2), Mn1–N1 2.238(2) N1–Mn1–O4 172.42(8) O1–Mn1–O2
125.63(8) N1–Mn1–O1 89.88(8).
6-Mn
Scheme 3. Synthesis of first-row early transition metal complexes with 6.
In the vanadium structure (6-V), a THF molecule is coordinated in
the axial position at a distance of 2.102(2) Å (V–O(4)), while the V–
N(6) bond distance is 2.145(3) Å, which is shorter than that of 6-Ti
due to the weaker trans influence of THF. The V–O bond distances in
the equatorial position are 1.866(2), 1.874(2) and 1.869(2) Å,
affording a typical TBP geometry. These bond distances are in good
agreement with the values of the previously reported amino
triphenolate vanadium THF complex.19
Structural studies of manganese with an amino triphenolate are
also rare, probably due to the synthetic difficulty that we
experienced. The first X-ray crystal structure was reported by the
Meyer group in 2014.5d The group successfully isolated and
characterized a manganese amino triphenolate complex with an
empty coordinate site at the axial position by X-ray crystallography.
The key to the successful isolation was to utilize an adamantly group
as a bulky substituent and bis(triphenylphosphine)iminium (PNP)
cation. Due to the highly soluble character of 6, unstable single
crystals of 6-Mn were analyzed using a synchrotron X-ray source.
Although the manganese oxidation state of 6-Mn is somewhat
confused due to the different oxidation state of the precursor
(Mn(acac)3), the X-ray crystal structure of 6-Mn clearly indicates that
manganese has the +2 oxidation state, evidenced by the sum (-2) of
amino triphenolate, one acac, and two sodium cations (Figure 2). In
the core structure, two sodium cations interact tightly with the two
benzene rings of 6, acac, and one THF molecule, while Mn adopts a
TBP geometry with relatively long equatorial Mn–O distances
(2.091(2), 2.006(2), and 2.093(2) Å). The axial distances to Mn are
2.147(2) Å (Mn–O(4)) and 2.238(2) Å (Mn–N(1)).
Although numerous amino triphenolate complexes with a variety
of transition metal ions have been reported,7 there have been few
structural studies of chromium metal with amino triphenolates. The
first X-ray structural study of an amino triphenolate chromium
complex was reported in 2018 to the best of our knowledge.2c Unlike
the published octahedral geometry of the chromium complex
supported by amino triphenolate and two 4-dimethylaminopyridine
(DMAP) molecules, 6-Cr shows a TBP geometry, where the ligand and
chromium bond distances are similar to those in the 6-V complex. A
THF molecule and nitrogen atom of 6-Cr are coordinated to Cr(III) in
a trans position at distances of 2.041(1) Å (Cr–O(5)) and 2.067(1) Å
(Cr–N(1)); these values are slightly shorter than those of 6-V due to
the higher number of bonding orbital electrons of Cr(III) (d3) than
those of V(III) (d2). The Cr–O bond distances in the equatorial position
are 1.856(2), 1.878(2) and 1.890(2) Å.
Both 6-V and 6-Cr complexes, which have a high spin state based
on measurements of their magnetic moments using the Evans
method,20 were also analyzed by DFT calculations using the
Gaussian09 program.21 Not surprisingly, the bond distances obtained
from the optimized structures at the B3PW91 functional with
appropriate basis sets (see Supplementary Information) are in good
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catalytic sulfoxidation regarding faster reaction time, increased
View Article Online
DOI: 10.1039/C9DT00456D
conversion rate and selectivity.
Table 3. Ethylene polymerization catalyzed by 6-Cr.a
yield
(g)
Tm
χ
Mw
PDI
entry
solvent
activityb
e,f
(°C)c
(%)d (kg/mol)e
1
2h
3
MeCyg
MeCyg
PhMeg
PhMeg
0.183
0.006
0.120
0.015
73.2
2.4
134.6
81
-
152
13.4
Figure 2. X-ray crystal structures of the 6-Mn core with thermal ellipsoids at the 50%
probability level. Hydrogen atoms were omitted for clarity. Selected bond lengths (Å) for
6-Mn, Mn1–O4 2.147(2), O1-Na1 2.336(2), O3-Na2 2.242(2), O5-Na2 2.369(3), O5-Na1
2.506(2), Na1-O4 2.368(2).
-
135.4
-
-
996
-
-
93.2
-
48.0
6.0
51
-
4h
a General reaction conditions: 5 µmol 6-Cr, 250 equivalents DMAO, 50 mL of
Solution behaviors of 6-Ti, 6-V, 6-Cr, and 6-Mn. Diamagnetic 6-Ti
was analyzed by 1H and 13C NMR spectroscopy in benzene solution.
6-Ti presented two doublets at 4.16 and 2.66 ppm corresponding to
two benzyl protons; such a large difference was already noted by the
Brown group.18 1H–1H COSY NMR data clearly show the correlation
of methine and isopropyl protons of the iso-propoxide. Unlike
diamagnetic 6-Ti, magnetic properties of 6-V, 6-Cr and 6-Mn in the
solution phase were measured by the Evans method. The solution
magnetic moment of 6-V in C6D6 was 2.66 B, which is consistent with
an S = 1 complex having the d2 configuration of V3+. The magnetic
moments of 6-Cr and 6-Mn were 3.76 B and 6.00 B, respectively,
indicating high spin states in both complexes.
b
solvent, 20 bar ethylene pressure, 40 °C, 30 min. In units of [kg of PE·(mol
Cr)–1·h–1. c Determined by differential scanning calorimetry. d Crystallinity (%)
= (ΔHm/ΔHm*) x 100, where ΔHm = measured melting enthalpy, ΔHm* =
e
theoretical value for purely crystalline polyethylene (ca. 293 J/g).
f
Determined by high-temperature gel permeation chromatography.
g
Polydispersity index (PDI) = Mw/Mn. MeCy = methylcyclohexane. PhMe =
toluene Control experiment carried out in the absence of 6-Cr. Not
h
determined (-).
Polymerization of ethylene catalyzed by 6-Cr. Most homogeneous
chromium complexes generate a mixture of ethylene oligomers
and/or low density polyethylene (PE) with many branches.22 To date,
only a few examples of homogeneous chromium catalysts producing
HDPE have been reported.23
A chromium(III)-based amino
Table 2. Sulfoxidation of thioanisoles catalyzed by 6-Ti.
triphenolate complex has been investigated for the copolymerization
of epoxides and cyclic anhydrides,2c but not for ethylene
polymerization. We expected that the bulky structure of 6-Cr would
produce a highly linear polyethylene by suppressing chain-walking
(re-insertion of β-eliminated olefins).24 Thus, we carried out ethylene
polymerization with 6-Cr activated by dry methylaluminoxane
(DMAO), and the representative results are summarized in Table 3.
Notably, activation of 6-Cr/DMAO in methylcyclohexane as the
reaction solvent produced PE of high melting temperatures (134.6
°C) with high crystallinity 81% (entry 1; Figure S3). Also, 13C NMR
analysis of this polymer indicated that the PE was highly linear and
had almost no branches (Figure S5 (b)). These results suggest that the
bulky ligand retards chain walking, resulting in highly linear PE in
accordance with our hypothesis.
O
O
O
S
S
S
6-Ti (1 mol%)
H2O2 (1.0 equiv.)
+
solvent, 30 °C
0.25 M
R
R
R
R = H, NO2
SO
SO2
entry
R
H
H
solvent
time
3 h
10 min
3 h
6 h
1 h
conv. (%)
SO:SO2
64:36
99:1
85:15
66:34
64:36
90:10
98:2
1
2
3
4
5
6
7
CD3CN
MeOD
CDCl3
C6D6
CD3CN
MeOD
CDCl3
81
97
44
43
93
99
34
H
NO2
NO2
NO2
NO2
In addition, we observed that the molecular weight and
polydispersity index (PDI) of the resulting PE varied dramatically with
the solvent used, whereas the thermal properties of PE were
insensitive to the reaction solvent. For example, ethylene
polymerization using 6-Cr/DMAO in methylcyclohexane yielded PE
with Mw of 152 kg/mol and PDI of 13.4 (entry 1). In contrast, the same
catalytic system using toluene as the solvent produced PE with a high
melting temperature of 135.4 °C (Figure S4), but with much broader
polydispersity (PDI = 93.2, Mw = 996 kg/mol; entry 3). A nearly
unimodal distribution was observed for PE obtained using
methylcyclohexane, while the PE produced in toluene displayed a
clearly multimodal distribution with a high molecular weight fraction
being dominant (Figure S5 (a)). The probable source of the broad
multimodal distribution of PE is the formation of different active
species after activation by the aluminum cocatalyst. The catalytic
performance of 6-Cr was confirmed by the control experiments
carried out in the absence of 6-Cr, but under otherwise identical
conditions (entries 1 vs. 2, entries 3 vs. 4).
40 min
3 h
Conversion and selectivity were measured by 1H NMR spectroscopy using 1,2-
dichloroethane as an internal standard. Without 6-Ti, no conversion was
observed for both thioanisoles.
Sulfoxidation catalyzed by 6-Ti. To study the catalytic behaviour of
the transition complexes, we first investigated 6-Ti for the oxidation
of thioanisole by aqueous hydrogen peroxide. The Licini group
reported the sulfoxidation of thioanisoles catalyzed by a titanium
complex supported by simple amino triphenolates.4a Inspired by this
work, we wanted to evaluate the steric effect of 6-Ti to assess the
selectivity in sulfoxidation of thioanisole and overoxidation to
sulfone. The sulfoxidation of thioanisoles was very efficient in
methanol, as the reaction rate, conversion, and selectivity of
sulfoxidation in methanol were superior to that in other solvents
(Table 2). Especially, sulfoxidation of 4-nitrothioanisole, which
possesses an electron-withdrawing group, afforded 99% conversion
with 9:1 selectivity (entry 6). Compared with the results from the
previous report,4a the bulky titanium complex 6-Ti showed better
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9500; (g) K. Nomura, U. Tewasekson anVdiew AYrt.icle TOankliniie,
Organometallics, 2015, 34, 3272-3281; (Dh)OYI:.1T0a.1k0ii3,9A/.CI9nDaTg0a0k4i a56nDd
K. Nomura, Dalton Trans., 2013, 42, 11632-11639.
Conclusion
In summary, we successfully demonstrated the synthesis and
characterization of the highly sterically demanding amino
triphenolate ligand and its early-transition metal complexes. All
metal complexes adopted the TBP geometry and the oxygen
atom of iso-propoxide, THF or acac was bound to the metal in
the axial position. However, the manganese complex, which
was presumably obtained by disproportionation of
manganese(III) complexes, was more complex, with sodium
cations that bound to the oxygen atoms of acac and the ligand.
The titanium complex was used for sulfoxidation of thioanisoles,
which showed high conversion and selectivity. Additionally, the
bulkiness of the chromium complex enabled the preparation of
a highly linear PE, presumably due to suppression of chain
walking. We believe that this new bulky ligand system provides
novel reactivity and is a robust catalytic platform for various
organic transformations. Applications to additional catalytic
reactions and the synthesis of other transition metal complexes
are being studied.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Institute for Basic Science (IBS)
[IBSR007-D1], a National Research Foundation of Korea (NRF)
grant funded by the Korean government [Ministry of Science,
ICT
and
Future
Planning
(MSIP);
No.
NRF-
2015H1A2A1030999—Global Ph.D. Fellowship Program], and
research fund of Chungnam National University. The X-ray
crystallography analysis with synchrotron radiation was
performed at the Pohang Accelerator Laboratory (PLS-II BL2D
SMC).
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Table of Contents
i-Pr
Highly sterically demanding
amino triphenolate complexes
TRIP ≡
 Ligand synthesis without column chromatography
 First-row early transition metal complexes
 Selective sulfoxidation
TRIP
L
i-Pr
i-Pr
 HDPE polymerization
O
O
TRIP
M
N
O
M: Ti, V, Cr, Mn
L: OiPr, THF, or acac
A highly sterically demanding amino triphenolate ligand was
successfully synthesized, and early transition metal complexes (Ti,
V, Cr, and Mn) supported by the ligand were also obtained and fully
characterized.

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DOI: 10.1039/C9DT00456D