Reduction of azobenzene by neodymium(II), dysprosium(II), and thulium(II) diiodides

Article abstract of DOI:10.1016/S0022-328X(03)00785-X

The diiodides of divalent neodymium (1), dysprosium (2), and thulium (3) LnI₂ readily reduce azobenzene under ambient conditions to give binuclear complexes [LnI(THF)₂]₂(N₂Ph₂) ₂ (Ln=Nd (4)

Full text of DOI:10.1016/S0022-328X(03)00785-X

Journal of Organometallic Chemistry 682 (2003) 218Á223
/
www.elsevier.com/locate/jorganchem
Reduction of azobenzene by neodymium(II), dysprosium(II), and
thulium(II) diiodides
Marina A. Katkova, Georgy K. Fukin, Anatolii A. Fagin, Mikhail N. Bochkarev *
Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Tropinina 49, 603950 Nizhny Novgorod, Russia
Received 14 April 2003; received in revised form 23 June 2003; accepted 23 June 2003
Abstract
The diiodides of divalent neodymium (1), dysprosium (2), and thulium (3) LnI2 readily reduce azobenzene under ambient
conditions to give binuclear complexes [LnI(THF)2]2(N2Ph2)2 (LnꢀNd (4), Dy (5), Tm (6)). Magnetic measurements and X-ray
/
structural analyses of 5 and 6 revealed that the bridging N2Ph2 ligands are dianions bounded to the trivalent lanthanoid. The
complexes displayed photochromic properties. Ytterbium iodide YbI2(THF)2 reacts with N2Ph2 at 80 8C yielding the known
[YbI(THF)3]2(N2Ph2)2 (7).
# 2003 Elsevier B.V. All rights reserved.
Keywords: Lanthanoid complexes; Azobenzene; Diiodides; Dysprosium; Neodymium; Thulium
1. Introduction
In continuation of these investigations we have
studied the reactions of the indicated iodides with
azobenzene. In the past, azobenzene was involved in
the reactions with (C5Me5)2Sm(THF)2, Cp2Yb(THF)
[2], the samarium pyrazolylborate complex Tp2Sm
It has previously been shown that divalent neody-
mium (1), dysprosium (2), and thulium (3) diiodides are
powerful reducing agents [1]. Addition of alkyl halides
to ketones in the presence of 2 and 3 proceeds faster and
more easily that with SmI2 [1a,1b]. Oxidative addition to
3 was observed in the reactions with 3,6-di(tert-bu-
tyl)catechole, 3,6-di(tert-butyl)benzoquinone-1,2, [1c],
cyclooctatetraene, acenaphthylene [1d] and calix[4]arene
[1e]. Dysprosium diiodide 2 adds to naphthalene and
diphenylacetylene [1a]. Interaction of 2 or 3 with
(Tpꢀ
/
hydrotris(3,5-dimethylpyrasol-1-yl)borate) [3], a
Sm, Yb) [4], and naphtha-
mixture of Ln and I2 (Ln ꢀ
/
lene complexes of samarium and ytterbium
C10H8Ln(THF)3 [5]. In the reactions with metallocenes
the complexes of the type (Cp2Ln)2N2Ph2, Cp2LnN2Ph2
or (CpLn)2(N2Ph2)2 were isolated depending on a
reagents ratio. The products revealed different arrange-
ment of Ln-azobenzene fragments: A, B, or C, respec-
tively.
acetonitrile is accompanied by CÁ
formation of novel bis(imine)amine complexes [{(HNꢀ
/
C coupling and
/
The samarium pyrazolylborate complex afforded the
product (Tp)2Sm(N2Ph2) with the A type of central
fragment whereas a mixture of lanthanoid metals and
iodine gave the binuclear complex [SmI(THF)3]2(m-
h2:h2-trans-N2Ph2)2 of C type. In all the cases azoben-
zene was subjected to one- or two-electron reduction.
Quite different tetranuclear clusters [Ln4(m2-h2:h2-Ph-
CMe)2MeCNH2}Ln(MeCN)6]I3 [1f]. In most reactions
the triiodides LnI3(solv.)x were isolated as the second
product in high yield indicative of disproportionation of
the starting LnI2 or initially formed products of the type
RLnI2. The data obtained in the reactions of 1 and 2
with benzene and other aromatic compounds in THF
solutions support the first mechanism [1g].
NꢀN)4(m3-PhN)2(THF)x were obtained in the lantha-
/
noid naphthalenide reactions, which were accompanied
by combined two- and four-electron reduction. It
seemed interesting to study the reduction of Ph2N2 by
divalent iodides of neodymium, dysprosium and thu-
lium whose reduction power is much higher than that of
* Corresponding author. Tel.: ꢁ
1497.
/
7-831-266-6795; fax: ꢁ7-831-266-
/
E-mail address: mboch@imoc.sinn.ru (M.N. Bochkarev).
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00785-X

M.A. Katkova et al. / Journal of Organometallic Chemistry 682 (2003) 218Á
/223
219
4LnI2 ꢁ2PhNꢀNPh T0HF[LnI(THF)2]2(N2Ph2)2
ꢁ2LnI3(THF)3
cyclopentadienyl, pyrazolylborate and iodide com-
pounds of Sm(II) or Yb(II) but probably is comparable
with the potential of naphthalenelanthanoids.
LnꢀNd (4); Dy (5); Tm (6)
Extraction of the solid mixture with warm THF allows
separation of the azobenzene derivatives from LnI3, the
2. Results and discussion
former being isolated as greenish-yellow solids in 78Á
/
82% yield after removal of a solvent from the extract.
Their composition was confirmed by elemental analysis,
IR spectroscopy and hydrolysis, which afforded quanti-
tatively diphenylhydrazine. The molecular iodide
TmI2(THF)5 reacts with azobenzene similarly to diio-
dide 3 affording 6 in 81% yield.
Recrystallization of the obtained complexes from
THF does not give good crystals, however the single
crystals of 5 and 6 suitable for X-ray analysis were
obtained directly from the reaction mixture when the
reactions were carried out in dilute solutions. It was
found that the dysprosium compound 5 is isostructural
It was found that all three diiodides 1, 2, and 3 react
easily with azobenzene in THF solution to give,
immediately after mixing under ambient conditions,
yellowish solutions. After several days a mixture of the
triiodide and the product [LnI(THF)2]2(N2Ph2)2 (Lnꢀ
/
Nd (4), Dy (5), Tm (6)) precipitated from the solution,
respectively.
Table 1
˚
Selected bond lengths (A) and angles (8) for [TmI(THF)2]2(N2Ph2)2 (6)
Bond lengths
Tmꢁ
Tm(1)ꢁ
Tm(1)ꢁ
Tm(1)ꢁ
Tm(1)ꢁ
/
I
2.986(1)
2.219(2)
2.460(2)
2.489(2)
2.227(2)
1.471(3)
1.393(3)
1.398(3)
/
N(1)
/N(2)
/N(1A)
/N(2A)
N(1)ꢁ
N(1)ꢁ
N(2)ꢁ
/
N(2)
C(1)
C(7)
/
/
Bond angles
C(1)N(1)N(2)
119.2(2)
C(1)N(1)Tm(1)
N(2)N(1)Tm(1)
C(1)N(1)Tm(1A)
N(2)N(1)Tm(1A)
Tm(1)N(1)Tm(1A)
C(7)N(2)N(1)
156.99(18)
80.88(13)
110.76(16)
62.32(12)
87.93(8)
119.9(2)
C(7)N(2)Tm(1A)
N(1)N(2)Tm(1A)
C(7)N(2)Tm(1)
N(1)N(2)Tm(1)
Tm(1A)N(2)Tm(1)
152.39(18)
81.87(13)
115.81(16)
62.94(11)
88.48(8)
Fig. 1. ORTEP view of the structure of the complex 6 with atom
numbering scheme (thermal ellipsoids at 40% probability level).

220
M.A. Katkova et al. / Journal of Organometallic Chemistry 682 (2003) 218Á223
/
with its thulium analogue 6, but the poor quality of the
data for the former prevented a detailed discussion of its
structural features. The molecular structure of 6,
including the atom numbering scheme, is shown in
Fig. 1. The most relevant bond distances and angles are
collected in Table 1. Complex 6 consists of two
TmI(THF)2 units joined by two bridging N2Ph2 ligands.
The Tm atoms have a trigonal-bipyramidal coordina-
tion. The I atom and one h2-N2Ph2 ligand occupy the
apical positions but oxygen atoms of two THF mole-
cules and the second h2-N2Ph2 ligand are located in the
equatorial positions. The structural feature of 6 is quite
similar to that of [SmI(THF)3]2(m-h2:h2-trans-N2Ph2)2
(6a) [4], [(C5Me5)(THF)Sm]2(m-h2:h2-trans- N2Ph2)2
(6b) [2], [(C5H5)(THF)Yb]2(m-h2:h2-trans-N2Ph2)2 (6c)
[2].
The dihedral angle in h2-N2Ph2 unit between the
Fig. 2. Electronic spectra of azobenzene (dotted line), reaction
solution of 6 freshly prepared (broken line) and after irradiation by
sunlight (full line) in THF.
planes of the phenyl rings is 68.68 for 6 (768 for 6b, 45.78
˚
N(2) distance is 1.471(3) A which is
for 6c). The N(1)ꢁ
/
˚
nearly equal to the similar distances in 6a (1.477(12) A),
˚
˚
6b (1.44(1) A) and 6c (1.470(6) A). These distances are
much closer to the NꢁN single bond distance in
hydrazine (1.45 A [6]) than to that of the NꢀN bond
most characteristic for the thulium compound 6, a
/
solution of which in 20Á
deep green and then deep blue. In 5Á
temperature the blue solution irreversibly changes color
to light brown. UVÁvis spectroscopic monitoring of the
/
30 min under sunlight turned
˚
/
/
8 h at ambient
˚
in azobenzene (1.25 A [6]). The geometry around each
nitrogen atom is consistent with an sp3 center. Therefore
we can assume the presence of the N2Ph22ꢂ ligand. The
/
reaction in THF solution (Fig. 2) revealed that in a few
minutes after mixing of reagents the initial absorption
band of azobenzene at 447 nm disappeared, which can
be assigned to formation of a short-living intermediate
in which the azobenzene ligand has lost its initial trans
configuration. The initial absorption bands of TmI2 at
420, 581 and 650 nm also are absent in these spectra.
After sunlight irradiation two new bands at 383 and 568
nm occur. It should be noted that the green color is
typical of Tm(II) compounds but the magnetic measure-
ments of the reaction solution on the green and blue
same situation was found for 6aÁ
The Tm(1)ꢁN(1), Tm(1)ꢁN(2) distances are 2.219(2)
A, 2.460(2) A respectively and a little bit shorter than
/
6c.
/
/
˚
˚
˚
the Tm(1)ꢁ
/
N(1A) (2.489(2) A) and Tm(1)ꢁ
/
N(2A)
(2.227(2) A) distances. It should be noted that the
difference between the TmꢁN distances in 6 and Smꢁ
N in 6a is bigger than the difference in ionic radii of the
Tm and Sm [7]. Note, the relative LnꢁN distances in the
compounds 6Á6b are close. A significant difference in
˚
/
/
/
/
the same distances was found only in 6c. The distances
from the Yb atom in 6c up to the first N2Ph2 unit are
˚
2.206(5) and 2.188(5) A and up to the second N2Ph2 unit
stage indicated the trivalent state of the metal (meff
7.4mB). We believe that the intermediate colors could be
ꢀ
/
˚
are 2.487(5) and 2.572(5) A.
attributed to metal-to-ligand charge-transfer (MLCT)
All these data clearly show a trivalent state of Tm in 6
that is confirmed also by magnetic measurements (meff
7.4mB).
transitions involving the azobenzene p*(Nꢀ/N) orbital as
ꢀ
/
was suggested in a photochemical study of dark-green
Pt-azobenzene complexes (432 and 592 nm), in which
cis-azobenzene binds two platinum fragments [9]. Un-
fortunately our attempts to isolate the green or blue
product (which are probably intermediate complexes
with non-reduced azobenzene ligand) in crystalline form
for X-ray analysis failed.
Our attempts to prepare single crystals of the
neodymium complex 4 for X-ray diffraction investiga-
tions failed but the data from elemental analysis,
hydrolysis and comparison of its IR spectrum with
those of 5 and 6 strictly confirm a similarity of
molecular structure of all these compounds.
The reduction potential of divalent ytterbium (ꢂ
/
1.15
It is well known that azobenzene derivatives are one
of the representative photochromic compounds and are
used for preparation of various dyes and photo-switch-
ing molecular devices [8]. The same feature was found
for the complexes 4, 5 and 6. Irradiation by sunlight of
the yellowish reaction solutions formed immediately
after mixing of initial reagents for several minutes
caused the color change to green. This property is the
V) is much lower than that of Tm(II) (ꢂ2.22 V [10])
/
therefore the reaction of YbI2(THF)2 with azobenzene
proceeds under more drastic conditions (80 8C, 15 h) but
yields
a
product
of
similar
composition:
[YbI(THF)3]2(N2Ph2)2 (7). A structure of the same
compound, obtained by the reaction of N2Ph2 with
ytterbium metal and iodine was established previously

M.A. Katkova et al. / Journal of Organometallic Chemistry 682 (2003) 218Á
/223
221
[4]. Interestingly, the reaction solution of the complex 7
in contrast to the 4, 5 and 6 analogues does not display
any color change under sunlight irradiation.
measurements of solutions were carried out on a Bruker
DPX 200 instrument by Evans’ NMR method [15].
Thus, in spite of high reduction potentials the
diiodides of neodymium(II), dysprosium(II) and thuliu-
m(II) do not reduce azobenzene to the imido ligand.
Under given conditions only two-electron reduction
takes place in all the cases. From two possible routes
of the reactions (via disproportionation of starting LnI2
or initially generated [(LnI2)2(N2Ph2)]) the latter seems
more plausible since 4, 5, and 6 are formed in high yield.
A similar scheme has been proposed before for the
reactions of azobenzene with (C5Me5)2Sm(THF)2 and
Cp2Yb(THF) [2]. If the reaction would include the
disproportionation of LnI2, inevitable side reactions of
the generated monovalent species LnI with solvent
would greatly decrease the yield of the obtained com-
plexes.
3.1. Preparation of NdI2 (1)
A mixture of neodymium shavings (100Á150 mesh)
3.06 g (21.2 mmol) and 3.63 g (14.3 mmol) of powdered
iodine was placed in an ampoule A connected via rubber
/
tube C to a Pyrex ampoule B (20ꢃ70 mm), containing a
few shavings of Nd (Fig. 3). The system was evacuated,
vacuum line was disconnected and the bottom of
/
ampoule B was heated by gas-burner up to 200Á
300 8C. Heating was stopped and a portion of 0.1Á0.2
g of Nd/I mixture from ampoule A was added to
ampoule B. From a hot bottom of ampoule B the
/
/
mixture ignited and burned for 2Á
burning an additional portion of 0.1Á
/
3 s. At the end of the
0.2 g of the
/
mixture was added to ampoule B. The operation was
repeated until all Nd/I mixture was transferred to
ampoule B and burned. The formed black solid was
3. Experimental
heated for 3Á
/
5 min at 400Á500 8C under vacuum and
/
allowed to cool. Gentle tapping of ampoule B allowed
crushing of the black solid to a powder, which was
transferred in vacuum a line to another ampoule. Most
of the excess neodymium was left in the ampoule B as
frozen drops. According to the data of complexonome-
try (Found: Nd, 49.99) and hydrolysis the obtained
black powder (6.12 g) consisted of NdI2 (4.8 g, 78%) and
Nd metal (1.32 g, 22%). This mixture without additional
purification was used in the reaction with azobenzene.
The iodides 2 and 3 were prepared and analyzed
similarly.
All manipulations were carried under vacuum using
Schlenk techniques. Azobenzene was purchased from
Aldrich and recrystallized from hexane. NdI2 (1), DyI2
(2) and TmI2 (3) were prepared by modified procedures
[11] as described below. TmI2(THF)5 and YbI2(THF)2
were synthesized according to Refs. [12] and [13],
respectively. THF was refluxed continuously over mol-
ten alkali metals and collected immediately prior to use.
Melting points were taken in sealed capillaries and are
uncorrected. IR spectra were obtained on a PerkinÁ
/
Elmer 577 spectrometer and recorded from 4000 to
450 cmꢂ1 as a Nujol mull on KBr plates. Magnetic
measurements of solid samples were carried out at room
temperature as described previously [14], and magnetic
3.2. Synthesis of [NdI(THF)2]2(N2Ph2)2 (4)
To a Nd/NdI2 mixture containing 390 mg (0.98 mmol)
of NdI2 was added with stirring at ambient temperature
a solution of azobenzene (89 mg, 0.49 mmol) in 20 ml of
THF. The mixture immediately turned pale brown.
After 5 min the mixture was filtered through a glass
filter and placed in a freezer (ꢂ20 8C) for 2 days. The
/
formed mixture of crystals of 4 and NdI3(THF)3 were
separated from the solution by decantation and washed
by hexane. The obtained solid mixture was extracted
with warm THF (2ꢃ
from the extract in vacuum gave 4 (241 mg, 82%) as
yellowish crystals; m.p.ꢀ181 (dec.). Anal. Calc. for
/10 ml). Removal of the solvent
/
C40H52I2N4Nd2O4: C, 40.20; H, 4.39; Nd, 24.14. Found:
C, 40.09; H, 4.47; Nd, 24.17%. IR bands (cmꢂ1) 1580
(s), 1295 (m), 1210 (s), 1170 (s), 1075 (m), 1020 (s), 930
(w), 870 (s), 845 (w), 790 (m), 750 (s), 705 (m), 670 (w),
605 (w), 520 (w).
Fig. 3. Diagram of the solid-state apparatus: (A), (B) pirex ampoules;
(C) connecting rubber tube; (D) rubber tube to vacuum line.

222
M.A. Katkova et al. / Journal of Organometallic Chemistry 682 (2003) 218Á223
/
3.3. Synthesis of [DyI(THF)2]2(N2Ph2)2 (5)
cmꢂ3, mꢀ
mm, 1.915
which 6626 were independent [Rint
ture was solved by direct method and refined on F2 by
full-matrix least-squares calculations with SHELX 97 [16].
The phenyl H atoms was found from Fourier synthesis
and refined isotropically. A riding model was applied to
/
4.781 mmꢂ1, crystal size 0.15ꢃ
u 529.038, 29 793 reflections collected of
0.0374]. The struc-
/
0.10ꢃ/0.06
/
/
The compound was prepared similarly to 4 from 400
mg (0.96 mmol) of DyI2 and 87 mg (0.48 mmol) of
ꢀ
/
azobenzene. Yield of 5: 231 mg, 78%; m.p.ꢀ182 (dec.).
/
Anal. Calc. for C40H52Dy2I2N4O4: C, 39.01; H, 4.26;
Dy, 26.39. Found: C, 38.91; H, 4.41; Dy, 26.43%. IR
bands (cmꢂ1) 1600 (s), 1310 (m), 1250 (s), 1160 (m),
1070 (s), 1020 (m), 920 (w), 890 (m), 860 (m), 750 (s), 695
(s), 645 (w), 515 (m).
H atoms of THF molecules, placed at calculated
˚
0.99 A and isotropic Uꢀ1.2Ueq
position with Cꢁ
/
Hꢀ
/
/
of attached atoms. Final R indices R1ꢀ
/
0.0294, wR2ꢀ
/
0.0627 [I ꢀ2s(I)].
/
3.4. Synthesis of [TmI(THF)2]2(N2Ph2)2 (6)
To a solution of TmI2(THF)5 (478 mg, 0.61 mmol) in
THF (10 ml) a solution of azobenzene (55 mg, 0.30
mmol) in THF (10 ml) was added with stirring at
ambient temperature. The green color of the solution
immediately turned to yellowish upon addition of
azobenzene. The solution was placed in a freezer
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 207634 for compound 6.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
(ꢂ20 8C) for 2 days. The precipitated mixture of green-
/
ish-yellow crystals of 6 and colorless crystals of
TmI3(THF)3 was separated by filtration and washed
with hexane. To separate 6 and TmI3(THF)3 the mixture
Cambridge CB2 1EZ, UK (Fax: ꢁ44-1223-336033; E-
mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
/
was extracted with warm THF (2ꢃ
the solvent from the extract gave 6 as a yellow
microcrystalline solid. Yield 152 mg (80%); m.p.ꢀ185
/
10 ml). Removal of
/
Acknowledgements
(dec.). Anal. Calc. for C40H52I2N4O4Tm2: C, 38.60; H,
4.21; Tm, 27.15. Found: C, 38.52; H, 4.31; Tm, 27.19%.
IR bands (cmꢂ1) 1605 (m), 1515 (w), 1330 (m), 1280
(w), 1210 (m), 1170 (m), 1040 (m), 1000 (ws), 920 (s), 830
The research described in this publication was made
possible in part by Award No. RC1-2322-NN-02 of the
U.S. Civilian Research & Development Foundation for
the Independent States of the Former Soviet Union
(CRDF).
(ws), 670 (m), 570 (w). meff
ꢀ7.4mB.
/
3.5. Synthesis of [YbI(THF)3]2(N2Ph2)2 (7)
To a solution of YbI2(THF)2 (452 mg, 0.79 mmol) in
THF (10 ml) was added a solution of azobenzene (71
mg, 0.39 mmol) in THF (10 ml) with stirring. After
stirring the mixture for 15 h at 80 8C dark red crystals of
7 had formed and settled out of the solution. Yield of 7:
References
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102 mg, 37%; m.p.ꢀ191 (dec.). Anal. Calc. for
/
C48H68I2N4O6Yb2: C, 41.27; H, 4.91; Yb, 24.77. Found:
C, 40.09; H, 5.12; Yb, 24.75%. IR bands (cmꢂ1) 1600
(s), 1300 (m), 1220 (m), 1170 (m), 1070 (s), 1020 (m), 920
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(1999) 466;
(m), 880 (m), 860 (m), 650 (w), 580 (s). meff
ꢀ4.3mB.
/
3.6. X-ray crystallography of [TmI(THF)2]2(N2Ph2)2
(6)
(f) M.N. Bochkarev, G.V. Khoroshenkov, H. Schumann, S.
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Intensity data were collected at 100 K temperature on
a Smart Apex diffractometer with graphite monochro-
˚
[2] W.J. Evans, D.K. Drummond, L.R. Chamberlain, R.J. Doedens,
S.G. Bott, H. Zhang, J.L. Atwood, J. Am. Chem. Soc. 110 (1988)
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mated MoÁ
/
Ka radiation (lꢀ
/
0.71073 A) in the 8 Á
/
v
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metallics 12 (1993) 4286.
scan mode (vꢀ
[TmI(THF)2]2(N2Ph2)2×
C48H68I2N4O6Tm2, Mꢀ
group Pbcn, aꢀ17.744(1), bꢀ
/
0.38, 10
2THF,
1390.72, orthorhombic, space
13.2641(8), cꢀ
5030.9(5) A , Zꢀ4, rcalc 1.833 g
s
on each frame).
/
empirical formula:
[4] Y. Nakayama, A. Nakamura, K. Mashima, Chem. Lett. (1997)
803.
/
/
/
/
[5] (a) N.S. Emelyanova, M.N. Bochkarev, H. Schumann, J. Loebel,
L. Esser, Koord. Khim. (Russ.) 20 (1994) 789;
3
˚
˚
21.3756(12) A, Vꢀ
/
/
ꢀ
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