First structurally characterised metal complex of a 2H-benzimidazole derivative. Copper mediated synthesis of 2,2-dimethyl-2H-benzimidazole from 1,2-phenylenediamine and acetone

Article abstract of DOI:10.1039/b007410l

2,2-Dimethyl-2H-benzimidazole (1) was synthesised by exploiting the reaction of acetone with bis(1,2-phenylenediamine)copper(II) perchlorate. Additionally, the X-ray crystal structure of [Cu¹(1)(PPh₃)₂]ClO₄ is reported.

Full text of DOI:10.1039/b007410l

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First structurally characterised metal complex of a 2H-benzimidazole
derivative. Copper mediated synthesis of 2,2-dimethyl-2H-
benzimidazole from 1,2-phenylenediamine and
acetone
Sanchita Hati,a Goutam K. Patra,a Jnan P. Naskar,a Michael G. B. Drewb and
Dipankar Datta*a
L e t t e r
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India. E-mail: icdd=mahendra.iacs.res.in
b Department of Chemistry, University of Reading, W hiteknights, Reading RG6 6AD, UK
Received (in Montpellier, France) 8th September 2000, Accepted 7th November 2000
First published as an Advance Article on the web 4th January 2001
2,2-Dimethyl-2H-benzimidazole (1) was synthesised by exploit-
ing the reaction of acetone with bis(1,2-phenylenediamine)cop-
per(II) perchlorate. Additionally, the X-ray crystal structure of
[CuI(1)(PPh ) ]ClO is reported.
less. The identity of the organic fragment in 2 is corroborated
by the X-ray crystal structure (Fig. 1) of [Cu(1)(PPh ) ]ClO
3 2
4
(4), which is obtained by the smooth reaction of PPh with 2
3
at room temperature in a 2 : 1 molar ratio in methanol.
3 2
4
Complex 4 is indeÐnitely stable in air in the solid state as
well as in solution. As revealed by the X-ray crystal structure,
2H-Benzimidazole is a tautomer of 1H-benzimidazole and has
an ortho-quinonoid electronic structure. Our AM1
calculations1 using the standard MOPAC package (version
1.1) shows that in the gas phase 2H-benzimidazole is energeti-
cally less stable than 1H-benzimidazole by 27.5 kcal mol~1.
While 1H-benzimidazole is not a rare ligand in transition
metal chemistry, nothing is known about the metal binding of
2H-benzimidazole. Herein we describe, for the Ðrst time, two
copper(I) complexes of the 2,2-dimethyl derivative of 2H-
benzimidazole. The structure of one of them has been deter-
mined. We also describe a novel viable synthesis of 2,2-
dimethyl-2H-benzimidazole (1).
the copper(I) centre in 4 has a trigonal NP coordination
2
sphere with the interligand angles very close to 120¡. The
coordination sphere in most tricoordinate copper(I) complexes
is found to be somewhat T-shaped. For an example, see ref. 5.
Fig. 1 describes the Ðrst structurally characterised metal
complex of a 2H-benzimidazole. It is noted that the corre-
sponding 1H-benzimidazole complex is not known. Our bond
valence sum calculations6,7 show that the CuÈN bond dis-
tance [1.997(3) A] in 4 is quite close to that expected (1.98 A)
for a symmetric CuN ` moiety. In this context, we mention
3
that while some 2,2-dialkyl derivatives of 2H-imidazole are
Reaction of acetone with bis(ethylenediamine)copper(II) per-
chlorate gives rise to the cationic copper(II) complex of CurtisÏ
macrocycle A.2 In an attempt to generate B, the 1,2-phenyl-
enediamine (OPDA i.e. ortho-phenylenediamine) counterpart
of A, which is not yet known, we have reacted acetone with
Cu(OPDA) (ClO ) . It is found that this reaction in air at
known,8 no metal complexes have so far been reported.
Interestingly, when OPDA is reÑuxed in acetone, one
obtains 2-methyl-2-isobutenylbenzimidazoline.9 On the other
hand, reaction of acetone with OPDA at room temperature in
2
4 2
room temperature does not yield B; it leads instead to a
reddish-brown copper(I) complex of 1 of the formulation
Cu(1)ClO (2), sparingly soluble in acetone, in almost quanti-
4
tative yield and from the Ðltrate is obtained the perchloric
acid salt3,4 of 2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiaze-
pine (3) in ca. 35% yield (eqn. 1).
[Cu(OPDA) ](ClO ) ] acetone ]
2
4 2
2 ] 0.5 H ] H` ] OPDA ] ClO ~ (1a)
2
4
H` ] OPDA ] ClO ~ ] 2 acetone ] 3 É HClO (1b)
4
4
Later we have found that reaction 1 also occurs in the absence
of air. Complex 2 is stable in air in the solid state for about
two weeks. However, in methanol 2 is stable in air for about 2
h only; on standing in air, the reddish-brown colour of the
methanol solution slowly disappears becoming almost colour-
Fig. 1 The structure of the cation in 4 with ellipsoids at 30% occu-
pancy. Selected bond distances (A) and angles (¡): Cu1ÈN1 1.997(3),
Cu1ÈP1 2.287(3), Cu1ÈP2 2.280(2), N1ÈC2 1.499(5), C2ÈN3 1.469(5),
N3ÈC4 1.286(5), C4ÈC5 1.453(6), C5ÈC6 1.326(7), C6ÈC7 1.436(7),
C7ÈC8 1.364(7), C8ÈC9 1.425(6), C9ÈN1 1.308(5), C4ÈC9 1.469(5);
N1ÈCu1ÈP1 118.09(12), N1ÈCu1ÈP2 119.83(11), P1ÈCu1ÈP2 120.00(4).
218
New J. Chem., 2001, 25, 218È220
DOI: 10.1039/b007410l
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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Experimental
Syntheses
Reaction 1. Silky violet Cu(OPDA) (ClO ) (3 g, 6.27
2
4 2
mmol), prepared by a reported procedure,15 was suspended in
75 ml of puriÐed acetone and stirred for 6 h at room tem-
perature with a CaCl (fused) guard tube Ðtted to the reaction
2
vessel. The reaction mixture gradually turned intense red
during stirring. The reddish-brown Cu(1)ClO (2) that
4
appeared was Ðltered o†, washed thoroughly with acetone and
dried in vacuo over fused CaCl ; yield: 1.9 g (93%). Anal. calc.
2
for C H ClCuN O : C, 34.94; H, 3.26; N, 9.06; Cu, 20.55%.
Scheme 1
9
10
2 4
Found: C, 34.90; H, 3.30; N, 9.01; Cu, 20.49%. FTIR (KBr): l
the presence of an acid gives the acid salt of 3.3,4 Thus, the
copper atom clearly plays an important role in the formation
of 1 here. Since reaction 1 also occurs in an N atmosphere,
we can say that copper(II) acts as a one-electron oxidant in
1620s,br (C2N), 1135vs, 1100vs, 1070vs (ClO ) cm~1. Dia-
4
magnetic: K (CH OH) 83 cm2 )~1 mol~1 (1 : 1 electrolyte).
M
3
max
UV-VIS (nujol): j
460, 358, 280 nm. UV-VIS (CH OH;
2
3
c \ 0.50 ] 10~3 mol dm~3): j
(e/dm3 mol~1 cm~1) 414
max
reaction 1 and it is not a case10h12 of copper-mediated aerial
(1200), 352 (2800), 270 (3300), 218 (8100) nm. 1H NMR (300
oxidation.
MHz, (CD ) SO, TMS): d 1.57 (s, methyl, 6H), 7.29È7.36 (br,
3 2
Demetallation of 2 by aqueous ammonia a†ords 1 in the
free state. The overall synthetic procedure is summarised in
Scheme 1. The overall yield of 1 in Scheme 1 is ca. 60%. Com-
pound 1 is a light reddish-brown liquid. Its 1H and 13C NMR
spectra in a 4 : 1 CDCl ÈCCl mixture (Fig. 2) show that 1 has
m, phenyl, 4H). 13C NMR (300 MHz, (CD ) SO, TMS): d
3 2
22.71, 104.54, 124.87, 137.06, 159.27.
The Ðltrate obtained after the isolation of 2 was evaporated
on a water bath to obtain a dark mass. It was dissolved in 10
ml of hot water and Ðltered. To the red Ðltrate, cooled to
saturated aqueous solution of
NaClO É H O was added dropwise with stirring until a crys-
talline precipitate started to appear. The reaction mixture was
left in air for 1 h. The deposited orange-red diamond-like crys-
tals were Ðltered o†, washed with 5 ml of cold water and dried
3
4
a C axis, as expected. It is extremely unstable in air; it
room temperature,
a
2
rapidly turns red and solidiÐes with decomposition. From the
4
2
decomposed red solid, crystals of OPDA can be isolated by
trituration with hot hexane (65È70 ¡C fraction from
petroleum). Previously, 1 has been synthesised by reacting
benzofurazan N-oxide with 2-nitropropane under basic
catalysis, followed by reduction of the resulting 2,2-dimethyl-
2H-benzimidazole bis-N-oxide with NaBH .13,14 However,
this procedure is not straightforward and the overall yield was
not reported. Moreover,13,14 1 had not been fully character-
in vacuo over fused CaCl ; yield: 0.6 g (34%). The crystals
2
were characterised as 3 É HClO .3,4
4
4
[Cu(1)(PPh ) ]ClO (4). To a suspension of 0.32 g (1
3 2
4
mmol) of 2 in 15 ml of methanol, 0.52 g (2 mmol) of PPh ,
3
ised.
dissolved in 20 ml of methanol, was added dropwise with con-
Because of ““blocked tautomerisationÏÏ, 2,2-dimethyl-2H-
benzimidazole is more stable than 2H-benzimidazole towards
tautomerisation. Our AM1 calculations show that 1 is ener-
getically less stable than its 1H tautomer by only 15.6 kcal
mol~1. This indicates, within the framework of the AM1
method, a gain of 11.9 kcal mol~1 in stability for 1 towards
tautomerisation in comparison with 2H-benzimidazole.
Thus, we have found a new and easy synthesis of 2,2-
dimethyl-2H-benzimidazole (1) that exploits the reaction
between acetone and bis(1,2-phenylenediamine)copper(II) per-
chlorate. Though in the free state 1 is very unstable in air, it
becomes quite stable in air upon binding copper(I). At present
we are engaged in an examination of the generality of Scheme
1 and in developing the transition metal chemistry of 2H-
benzimidazole.
stant stirring to obtain a clear yellowish-orange solution. The
reaction mixture was left in air overnight. The yellowish-
brown compound 4 that precipitated was Ðltered o†, washed
with 5 ml of cold methanol and dried in vacuo over fused
CaCl ; yield: 0.45 g (54%). An additional crop of crystals
2
could be obtained by adding diethyl ether to the Ðltrate. Anal.
calc. for C
H
ClCuN O P : C, 64.80; H, 4.84; N, 3.36; Cu,
45 40
2 4 2
7.62%. Found: C, 64.72; H, 4.92; N, 3.36; Cu, 7.67%. FTIR
(KBr): l 1610s,br (C2N), 1150È1075vs,br (ClO ) cm~1. Dia-
4
magnetic: K (CH OH) 91 cm2 )~1 mol~1 (1 : 1 electrolyte).
M
3
UV-VIS (nujol): j
465, 360, 290 nm. UV-VIS (CH OH;
max
3
c \ 0.05 ] 10~3 mol dm~3): j
(e/dm3 mol~1 cm~1) 345
max
(7050), 250 (25 800), 222 (66 900) nm. 1H NMR (300 MHz,
(CD ) SO, TMS): d 1.37 (s, methyl, 6H), 7.07È7.50 (br, m,
3 2
phenyl, 34H). 13C NMR (300 MHz, (CD ) SO, TMS): d
3 2
22.05. 104.66, 125.83, 129.27, 129.41, 130.76, 132.42, 132.98,
133.86, 135.73, 159.33.
2,2-Dimethyl-2H-benzimidazole (1). 2 (3 g, 9.3 mmol) was
taken up in 75 ml of chloroform and stirred for 5 min. To the
resulting dark brown suspension, 50 ml of 25% aqueous
ammonia was added and stirred for 1 h at room temperature,
after which it was Ðltered. The blue aqueous layer of the Ðl-
trate was discarded. The light reddish-brown layer of the Ðl-
trate was collected and evaporated at room temperature
under reduced pressure to ca. 5 ml, which was then loaded on
a neutral alumina column (25 ] 2.5 cm) and eluted with a
1 : 9 diethyl etherÈpetroleum ether mixture. The yellowish
fraction (ca. 200 ml) was collected, which upon removal of the
solvent at room temperature under reduced pressure yielded 1
as a light reddish-brown liquid. (The whole process was com-
pleted within 8 h). Yield: 0.9 g (63%). Anal. calc. for
C H N : C, 73.93; H, 6.90; N, 19.16%. Found: C, 73.91; H,
Fig. 2 (a) 1H and (b) 13C NMR spectra (300 MHz; TMS reference)
of 1 in a 4 : 1 CDCl ÈCCl mixture. For the labelling scheme of the C
9
10 2
6.88; N, 19.13%. Density: 0.93 g cm~3. EI-MS: m/z 146.1 (1`,
3
4
atoms, see Fig. 1.
100), 131.1 (1` [ CH , 90), 105.1 (1` [ C H , 28), 90.1
3
3 5
New J. Chem., 2001, 25, 218È220
219

View Article Online
(1` [ NC H , 42), 78.1 (C H `, 12), 63.0 (C H `, 6), 51.0
acknowledges the Ðnancial support received from the Depart-
ment of Science and Technology, New Delhi, India.
6
3
6
6
5 3
(C H `, 8), 41.0 (C H `, 15%). UV-VIS (CH OH): j
4
3
3
5
3
max
(e/dm3 mol~1 cm~1) 354 (2500), 263 (2800), 217 (6000) nm. 1H
NMR (300 MHz, CDCl ÈCCl , TMS): d 1.54 (s, methyl, 6H),
3
4
6.99È7.02 (br, phenyl, 2H), 7.18È7.21 (br, phenyl, 2H). 13C
NMR (300 MHz, CDCl ÈCCl , TMS): d 21.33, 104.14,
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Acknowledgements
M. G. B. D. thanks EPSRC (UK) and the University of
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220
New J. Chem., 2001, 25, 218È220