New tridentate ligands based on 2-tert-butyl-4-methylphenol: synthesis and structure

Article abstract of DOI:10.1007/s11172-021-3222-3

New sterically hindered bis(3-tert-butyl-2-hydroxy-5-methylbenzyl)amine was synthesized based on 2-tert-butyl-4-methylphenol. The reaction of the latter with N, N-dimethylformamide affords N, N-bis(3-tert-butyl-2-hydroxy-5-methylbenzyl)formamide. Accordin

Full text of DOI:10.1007/s11172-021-3222-3

Russian Chemical Bulletin, International Edition, Vol. 70, No. 7, pp. 1349—1355, July, 2021
1349
New tridentate ligands based on 2-tert-butyl-4-methylphenol:
synthesis and structure
N. F. Lazarevaa and E. A. Zelbstb
aA. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences,
1 ul. Favorskogo, 664033 Irkutsk, Russian Federation.
Е-mail: nataly_lazareva@irioch.irk.ru
bDepartment of Physics, Pedagogical Institute, Irkutsk State University,
1 ul. K. Marksa, 664003 Irkutsk, Russian Federation
New sterically hindered bis(3-tert-butyl-2-hydroxy-5-methylbenzyl)amine was synthesized
based on 2-tert-butyl-4-methylphenol. The reaction of the latter with N,N-dimethylformamide
affords N,N-bis(3-tert-butyl-2-hydroxy-5-methylbenzyl)formamide. According to the X-ray
diffraction data, this reaction product occurs in the crystal as a dimer stabilized by a bifurcated
...
hydrogen bond. The intermolecular component of the C=O H—O hydrogen bond (l = 1.990 Å)
between the carbonyl oxygen atom of one molecule and the phenolic hydroxy group of an-
other molecule ensures the formation of dimers. Another component is the intramolecular
...
C=O H—O hydrogen bond (l = 1.754 Å) between the carbonyl oxygen atom and the phenolic
hydroxy group of the same molecule. In the crystal, the dimers are linked via C—H…π interac-
tions to form long chains (2.741 Å).
Key words: bis(3-tert-butyl-2-hydroxy-5-methylbenzyl)amine, N,N-bis(3-tert-butyl-2-
hydroxy-5-methylbenzyl)formamide, X-ray diffraction.
2-Aminophenol and its derivatives are widely used as
ligands.1—5 Particular attention is given to polydentate
ligands. Their stereoelectronic characteristics can be
controlled and, consequently, their physicochemical
properties and reactivity can be varied by changing the
nature of substituents both at the nitrogen atom and at the
aromatic ring. Tetradentate ligands based on 2-hydroxy-
benzylamine N(CH2C6HnR4–nOH)3 are being extensively
investigated and are commonly employed in the synthesis
of transition metal complexes (see the study6 and refer-
ences cited therein) and hypervalent compounds of main
group elements (Si, Al, P).7—15
organic synthesis and polymer chemistry.16—25 The syn-
thesis of bis(3,5-di-tert-butyl-2-hydroxybenzyl)amine (A,
R = H, R´ = Rʺ = But) has recently been reported.23,26
The free NH group in tridentate ligands not only increases
their synthetic potential but also influences their confor-
mational behavior due to the formation of intra- and/or
intermolecular hydrogen bonds. Therefore, the develop-
ment of methods for the synthesis of bis(2-hydroxybenzyl)-
amine derivatives and investigation of their structures are
of interest. The goals of this work are to develop a method
for the synthesis of bis(3-tert-butyl-2-hydroxy-5-methyl-
benzyl)amine (1) and to study its structure and properties.
Tridentate ligands based on 2-hydroxybenzylamine
(A), which form complexes with P, Ti, Cu, Fe, Zn, Mo,
W, Zr compounds, have received much less attention.
Results and Discussion
The reaction of 2,4-di-tert-butylphenol with hexa-
methylenetetramine in 85% formic acid affords bis(3,5-
di-tert-butyl-2-hydroxybenzyl)amine.23,26 This reaction
proceeds through a benzoxazine derivative as an interme-
diate, which was also isolated and characterized.23 We
applied this procedure26 to prepare bis(3-tert-butyl-2-
hydroxy-5-methylbenzyl)amine (1). 2-tert-Butyl-4-meth-
ylphenol reacts with hexamethylenetetramine to form
compound 1 in 37% yield (Scheme 1, method A).
R = H, Me, PhCH2, PyCH2, CH2C(O)OH
R´, Rʺ = Me, But
These unique complexes are not only of interest for
research in theoretical and structural chemistry but are
also of great practical importance as efficient catalysts for
We expected that an alternative method (B) based on
the reaction of 3-tert-butyl-2-hydroxy-5-methylbenzyl
chloride (2) with 3-tert-butyl-2-hydroxy-5-methylbenzyl-
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1349—1355, July, 2021.
1066-5285/21/7007-1349 © 2021 Springer Science+Business Media LLC

Lazareva and Zelbst
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Russ. Chem. Bull., Int. Ed., Vol. 70, No. 7, July, 2021
Scheme 1
structures of all the synthesized compounds were con-
firmed by NMR spectroscopy; their composition, by
elemental analysis.
We obtained crystals of compound 4 suitable for X-ray
diffraction. The interatomic distances of formamide 4 are
given in Fig. 1. In the crystal, formamide 4 occurs as

a dimer formed through intermolecular C=O H—O
hydrogen bonds (l = 1.990 Å) between the carbonyl oxy-
gen atom of one molecule and the phenolic hydroxy group
of another molecule (Fig. 2). This bond is the intermo-
lecular component of the bifurcated hydrogen bond.
Another component of this bond is the intramolecular
Reagents and conditions: 1) 85% HCOOH; 2) HCl—HOCH2-
CH2OH; 3) KOH.

amine (3) would allow the synthesis of amine 1 in higher
yield (Scheme 2). Compound 2 was prepared by the
chloromethylation of 2-tert-butyl-4-methylphenol in
toluene. Unfortunately, the amination of compound 2
with aqueous ammonia was not selective and gave a mix-
ture of compound 3 and bis(3-tert-butyl-2-hydroxy-5-
methylbenzyl)amine (1). Hence, we synthesized 3-tert-
butyl-2-hydroxy-5-methylbenzylamine 3 through a mul-
tistep process (see Scheme 2). The reaction of benzylamine
3 with benzyl chloride 2 in acetonitrile in the presence of
potassium carbonate as a base produced compound 1 in
72% yield.
However, an attempt to improve the yield of compound
1 using another solvent led to an unexpected result. The
heating of equimolar amounts of compounds 2 and 3 in
DMF affords N,N-bis(3-tert-butyl-2-hydroxy-5-methyl-
benzyl)formamide (4) (68% yield) as the formylation
product of compound 1 (see Scheme 2). Actually, the
heating of compound 1 with DMF in a sealed tube for 10 h
at 150 C resulted in the formylation of compound 1, and
formamide 4 was isolated in a similar yield (62%). The
C=O H—O hydrogen bond (l = 1.754 Å) between the
carbonyl oxygen atom and the phenolic hydroxy group of
the same molecule. The oxygen atoms that link two mol-
ecules of the dimer, O(1) and O(3), lie almost in one plane;
the distance between these atoms is 2.705(3) Å. The intra-
molecular distance between the oxygen atoms, O(1)…O(2),
is 2.643(4) Å, and the intramolecular O(1)…H—O(2)
angle is 172.3. Both the intra- and intermolecular contacts
are smaller than the sum of the van der Waals radii of the
corresponding atoms. The formamide molecule contains
a trivalent nitrogen atom, which lies nearly within the
plane formed by the carbon atoms and deviates from it
only slightly (by 0.013 Å). The deviation of the nitrogen
atom from the plane passing through three carbon atoms
of N-(3,5-di-tert-butyl-2-hydroxyphenyl)-N-methylform-
amide27 is even smaller, and it differs from the correspond-
ing distance in the N,N-diformylformamide (triform-
amide) molecule N[C(O)H]3, in which the nitrogen atom
is within the CHO plane.28 The interatomic N—C dis-
tances between the nitrogen atom and the carbonyl groups
of the dimer are similar within the experimental error
Scheme 2
Reagents and conditions: i. 1) HCl—PhCH3, 2) (CH2O)n; ii. 1) Me3SiCl, Et3N, PhCH3; 2) (Me3Si)2NLi; 3) MeOH, , 10 h.

Tridentate ligands based on 2-But-4-Me-C6H3OH
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 7, July, 2021
1351
H(21B)
H(21A)
С(21)
H(21C)
1.508
H(18A)
H(23A)
С(18)
С(17)
H(24A)
H(23B)
H(24C)
H(16A)
C(23)
H(23C)
С(22)
H(9C)
H(9B)
С(16)
С(15)
С(19)
H(4A)
H(14A)
1.541
C(24)
H(24B)
C(25)
1.517
С(9)
С(20)
1.371
H(9A)
С(14)
С(4)
H(14B)
1.476
H(2A)
С(5)
H(25A)
N(1)
O(3)
С(2)
H(3)
H(25C)
H(25B)
С(3)
1.326
1.471
H(6A)
1.514
H(1A)
С(6)
С(1)
H(2B)
С(8)
1.251
H(11A)
1.367
С(7)
H(13A)
1.537
С(10)
H(11B)
C(11)
H(2)
O(1)
O(2)
H(13C)
H(12A)
С(12)
H(12C)
С(13)
H(12B)
H(13B)
H(11C)
Fig. 1. Molecular structure of formamide 4.
O(2)
H(2)
1.754
1.990
O(1)
H(3)
O(3)
O(3)
O(1)
1.754
1.990
H(3)
H(2)
O(2)
Fig. 2. Dimer of formamide 4.

Lazareva and Zelbst
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Russ. Chem. Bull., Int. Ed., Vol. 70, No. 7, July, 2021
(1.471 and 1.476 Å; 2σ = 0.006—0.008 Å). The N—C=O
and C=O bond lengths are 1.326 and 1.251 Å, respectively,
and are in the range typical of the amide moiety of carb-
oxamides. Thus, the N—C=O and C=O bond lengths of
3-phenylpropionic acid N,N-dibenzylamide are 1.350 and
1.244 Å, respectively.29 The hydrogen bond in compound
4 leads to a decrease in the N—C=O bond length and an
increase in the C=O bond length compared to the cor-
responding values in the compound PhCH2CH2C(O)-
N(CH2Ph)2.
The nitrogen atom in compound 4 is bound to two
almost equivalent benzyl moieties and the formyl group
HC=O. The benzene carbon atoms lie in one plane, the
deviations of the carbon atoms from the plane being within
0.001—0.007 Å. The angle between the planes is 31.83.
The dimers of the formamide are linked to form infinite
polymer chains (Fig. 3) through interactions between the
methylene groups of the C(O)N—CH2 moiety and the π
system of the aromatic ring of the adjacent molecule. The

CH π distance between the H atom and the centroid of
the aromatic ring is 2.741 Å (Fig. 4). Such interactions are
described in the literature (see, e.g., the study30).
In summary, we synthesized new sterically hindered
bis(3-tert-butyl-2-hydroxy-5-methylbenzyl)amine (1)
based on 2-tert-butyl-4-methylphenol. Compound 1 reacts
with N,N-dimethylformamide on heating to form N,N-bis-
(3-tert-butyl-2-hydroxy-5-methylbenzyl)formamide (4).
According to the X-ray diffraction data, compound 4 oc-
curs in the crystal as a dimer formed through an intermo-

lecular component of the bifurcated C=O H—O hydro-
Fig. 3. Chains of dimers of compound 4.
2.741
2.741

Fig. 4. The CH π contacts in compound 4.

Tridentate ligands based on 2-But-4-Me-C6H3OH
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 7, July, 2021
1353
Table 1. X-ray diffraction data collection and structure refine-
ment statistics for compound 4
gen bond between the carbonyl oxygen atom of one
molecule and the phenolic hydroxy group of another
molecule. The second component of the bifurcated hy-

Parameter
Value
drogen bond is the intramolecular C=O H—O hydrogen
bond between the carbonyl oxygen atom and the phenolic
hydroxyl group of the same molecule. The dimers are
Molecular formula
М
T/K
Crystal system
Space group
Unit cell parameters
a/Å
C
25H35NO3
397.54
100

linked by CH π interactions (2.741 Å) to form long
Triclinic
chains. Based on the literature data on the structures and
properties of related compounds, it can be suggested that
the new compounds are potential tridentate ligands.
P–1
9.030(2)
b/Å
c/Å
10.181(3)
14.160(4)
92.284(6)
103.511(5)
15.707(5)
1125.5 (1)
2
Experimental
α/deg
/deg
The 1H and 13C NMR spectra were recorded on a Bruker DPX
400 spectrometer (400.13 and 100.61 MHz, respectively) in
CDCl3 with SiMe4 as the internal standard. The solvents were
prepared by standard procedures31 and stored over 4 Å molecu-
lar sieves, which were activated for 6 h on heating (290 10 C
(1 Torr)).
X-ray diffraction study of the crystal and molecular structure
of formamide 4 was performed on a Bruker automated diffrac-
tometer. The X-ray diffraction data collection statistics are
given in Table 1. The refinement of the unit cell parameters and
preliminary X-ray diffraction data processing were performed
using the SHELX TL, version 5.1, program package. The struc-
ture was refined by the full-matrix least-squares method with
anisotropic displacement parameters for nonhydrogen atoms.
All hydrogen atoms were found in difference Fourier maps and
refined using a riding model to R = 0.063. The CIF file contain-
ing the complete structural data was deposited with the Cambridge
Crystallographic Data Centre (CCDC 2032153) and is available
at www.ccdc.cam.ac.uk/data_request/cif.
(3-tert-Butyl-2-hydroxy-5-methylbenzyl)amine (1). Method A.
A mixture of 2-tert-butyl-4-methylphenol (1.64 g, 10 mmol) and
hexamethylenetetramine (2.8 g, 20 mmol) in 85% formic acid
(100 mL) was refluxed for 2 h, resulting in the formation of two
layers. The lower layer (yellowish oil) was separated by decanta-
tion and evacuated (~3 Torr) at room temperature for 3 h. The
residue was dissolved in ethylene glycol (100 mL), and concen-
trated hydrochloric acid (40 mL) was added. The mixture was
warmed in an oil bath (130 5 C) for 12 h and then cooled.
The precipitate was filtered off and washed with distilled
water (2×10 mL). The resulting hydrochloride of compound 1
was added portionwise with vigorous stirring to a mixture of
a KOH solution (3.5 g in 30 mL of water) and diethyl ether
(50 mL). After 30 min, the ethereal layer was separated
and dried with MgSO4. The solvent was removed using
a rotary evaporator, and the residue was evacuated (~2 Torr) at
room temperature for 4 h. Yellowish viscous oil was obtained
in a yield of 1.37 g (37%); attempts to crystallize this com-
pound failed.
Method B. Equimolar amounts of 3-tert-butyl-2-hydroxy-5-
methylbenzyl chloride (2) (0.43 g, 2 mmol) and 3-tert-butyl-2-
hydroxy-5-methylbenzylamine (3) (0.39 g, 2 mmol) were dis-
solved in MeCN, and K2CO3 (1 g, 7.2 mol, excess) was added.
The reaction mixture was refluxed with vigorous stirring for 30 h
and then poured to cold water (10 mL). Compound 1 was ex-
tracted with diethyl ether (3×5 mL). The solvent was removed
using a rotary evaporator, and the residue was evacuated
γ/deg
V/Å3
Z
dcalc/g cm–3
μ/mm–1
Crystal size/mm
min—max/deg
Tmin/Tmax
F(000)
1.173
0.076
0.10×0.08×0.03
2.25—26.02
0.9925/0.9977
432
λ/Ǻ
0.71073
2θmax/deg
Ranges of indices
52
–11≤ h ≤ 11,
–12≤ k ≤ 10,
–17≤ l ≤ 17
Number of reflections
measured
8374
4353
2365
(0.0617)
272
0.948
unique
with I > 2(I)
(Rint
)
Number of refined parameters
GOOF
R factors based on F2 > 2(F2)
R1
wR2
0.0633
0.1278
R factors based on all reflections
R1
wR2
0.1318
0.1533
Residual electron density
–0.236/0.300
(Δρmin/Δρmax)/e Å–3
(~1 Torr) at room temperature for 6 h. Yellowish viscous oil
was obtained in a yield of 0.53 g (72%).
1H NMR (CDCl3), δ: 1.36 (s, 18 H, 2 But); 2.22 (s, 6 H,
2 Me); 3.56 (s, 4 H, 2 CH2); 5.64 (br.s, 1 H, NH); 6.72 (s, 2 H,
2 CHAr); 6.96 (s, 2 H, 2 CHAr); 8.54 (br.s, 2 H, 2 OH). 13C NMR
(CDCl3), δ: 24.8 (Me); 30.7 (Me3C); 36.2 (CMe3); 52.4 (CH2);
122.6, 123.8, 124.8, 137.4, 142.3, 151.8 (Ar). Found (%): C, 77.82;
H, 9.39; N, 3.86. C24H35NO2. Calculated (%): C, 78.00; H, 9.55;
N, 3.79.
3-tert-Butyl-2-hydroxy-5-methylbenzyl chloride (2). Hydr-
ogen chloride was bubbled through a solution of 2-tert-butyl-4-
methylphenol (3.28 g, 20 mmol) in toluene (25 mL), cooled to
–5 2 C, for 15 min. Then paraformaldehyde (0.75 g, 25 mmol,
excess) was added with stirring. The reaction mixture was
gradually warmed to room temperature, and hydrogen chloride

Lazareva and Zelbst
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Russ. Chem. Bull., Int. Ed., Vol. 70, No. 7, July, 2021
was bubbled for two hours. Water (10 mL) was added to the reac-
tion mixture. The organic layer was separated, washed with
water (3×5 mL), and dried with K2CO3. The solvent was re-
moved using a rotary evaporator, and the residue was evacuated
(~3 Torr) at room temperature for 6 h. Yellowish oil was ob-
tained in a yield of 3.78 g (89%) and was used without addi-
tional purification. 1H NMR (CDCl3), δ: 1.37 (s, 9 H, But); 2.26
(s, 3 H, Me); 4.62 (s, 2 H, CH2); 5.43 (br.s, 1 H, OH); 6.92
(s, 1 H, CHAr); 7.14 (s, 1 H, CHAr). 13C NMR (CDCl3), δ: 20.6
(Me); 31.4 (Me3C); 35.8 (CMe3); 42.7 (CH2); 123.9, 124.6,
128.3, 138.7, 143.5, 152.4 (Ar). Found (%): C, 67.93; H, 8.25.
The results of NMR spectroscopy and elemental
analysis were obtained using equipment of the Baikal
Analytical Center for Collective Use of the Siberian Branch
of the Russian Academy of Sciences.
This paper does not contain descriptions of studies on
animals or humans.
The authors declare no competing interests.
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3-tert-Butyl-2-hydroxy-5-methylbenzylamine (3). A solution
of Me3SiCl (1.08 g, 10 mmol) in toluene (10 mL) was slowly
added dropwise with vigorous stirring to a solution of compound
2 (2.13 g, 10 mmol) and triethylamine (1.50 g, 15 mmol, excess)
in anhydrous toluene (30 mL) under an argon atmosphere. The
reaction mixture was stirred at room temperature for 30 min,
refluxed for 1 h, cooled, and filtered off (back filtration). The
precipitate was washed with toluene (2×5 mL), and a solution
of (Me3Si)2NLi (2 g, 12 mmol) in toluene was slowly added
dropwise to the combined filtrate (–78 C). The reaction mixture
was stirred at this temperature for 1 h. Then the mixture was
slowly warmed to room temperature and kept for 16 h. To remove
the LiCl precipitate, the solution was filtered through a ~1-cm
thick Celite 521 layer. Methanol (10 mL) was added to the filtrate,
and the reaction mixture was refluxed for 8 h. Volatile sub-
stances were removed using a rotary evaporator, and the residue
was evacuated (~1 Torr) at room temperature for 6 h. Viscous
oil was obtained in a yield of 1.58 g (82%) and was used without
additional purification. 1H NMR (CDCl3), δ: 1.38 (s, 9 H, But);
2.24 (s, 3 H, Me); 4.29 (s, 2 H, CH2); 5.84 (br.s, 2 H, NH2);
6.94 (s, 1 H, CHAr); 7.02 (s, 1 H, CHAr); 8.12 (br.s, 1 H, OH).
13C NMR (CDCl3), δ: 20.8 (Me); 31.7 (Me3C); 35.9 (CMe3);
44.3 (CH2); 127.3, 127.9, 129.5, 138.8, 142.2, 153.2 (Ar).
Found (%): C, 74.68; H, 10.06; N, 7.19. C12H19NO. Calculat-
ed (%): C, 74.57; H, 9.91; N, 7.25.
N,N-Bis(3-tert-butyl-2-hydroxy-5-methylbenzyl)formamide
(4). Compound 1 (0.74 g, 2 mmol) and DMF (1 mL) were
added with a syringe to a tube pre-filled with argon. The tube
was degassed, evacuated, and sealed. The sealed tube was placed
in a protective jacket and heated at 150 C for 10 h. Then the
tube was cooled and opened. The content was poured to a mix-
ture of water (5 mL) and o-xylene (10 mL). The organic layer
was separated and dried over 4 Å molecular sieves. The solvent
was removed using a rotary evaporator, and the residue was
crystallized from ethanol. Compound 4 was obtained in a yield
1
of 0.49 g (62%). H NMR (CDCl3), δ: 1.38 (s, 9 H, But); 1.40
(s, 9 H, But); 2.21 (s, 3 H, Me); 2.31 (s, 3 H, Me); 4.26 (s, 2 H,
CH2); 4.42 (s, 2 H, CH2); 5.18 (br.s, 1 H, OH); 6.66 (s, 1 H,
CHAr); 6.89 (s, 1 H, CHAr); 7.03 (s, 1 H, CHAr); 7.08 (s, 1 H,
CHAr); 8.22 (s, 1 H, HC=O); 8.97 (s, 1 H, OH). 13C NMR
(CDCl3), δ: 20.72, 20.87 (Me); 29.62, 30.31 (Me3C); 33.80,
34.89 (CMe3); 42.94, 47.78 (CH2); 122.14, 122.57, 127.45,
127.93, 128.14, 128.96, 129.65, 129.75, 135.85, 137.99, 151.18,
152.51 (Ar); 165.04 (C=O). Found (%): C, 75.69; H, 8.92;
N, 3.44. C25H35NO3. Calculated (%): C, 75.53; H, 8.87; N, 3.52.
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Received November 26, 2020;
in revised form January 26, 2021;
accepted January 31, 2021