Isolation, X-ray structural characterization, and reactivities of diiodosamarium(II) complexes bearing amide compounds as ligands

Article abstract of DOI:10.1246/bcsj.74.1417

The structures of diiodosamarium(II) complexes with amide compounds have been investigated basing on single crystal X-ray structural analyses. The reaction of [SmI₂(thf)₂] with 2 equivalents of tetramethylurea (TMU) gave [SmI₂(thf)₂(tmu)₂] (1). The three different kinds of ligands in 1 attach to the samarium atom in a trans orientation. [SmI₂(thf)₂] reacted with 4 equivalents of 1,3-dimethyl-2-imidazolidione (DMI) to afford [Sm_I2(dmi)₄] (2), forming a typical hexa-coordinated octahedral structure. Diiodosamarium complex containing four N,N-dimethylacetamide (DMA) ligands [SmI₂(dma)₄] (3) was prepared by treatment of [SmI₂(thf)₂] with 4 equivalents of DMA. The reaction of [SmI₂(thf)₂] with 6 equivalents of 1,3-dimethyl-3,4,5,6-tetrahydro-2-(1H)-pyrimidinone (dimethylpropyleneurea (DMPU)) gave [Sm(dmpu)₆]I₂ (4) quantitatively. This complex experiences a unique coordination fashion, in which six DMPU ligands coordinate to the samarium atom to overlap each other. [SmI₂(dmpu)₃(thf)] (5) was isolated from the reaction of [SmI₂(thf)₂] with 2 equivalents of DMPU. The reducing ability of these complexes was evaluated by reactions with benzylacetone or 9-haloanthracene. The structural analyses and reactivity studies of these complexes indicate that DMPU and DMA ligands are the most effective among the amide compounds to enhance the reducing ability of samarium diiodide.

Full text of DOI:10.1246/bcsj.74.1417

c. Jpn.
e Chemical © 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1417–1424 (2001) 1417
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Isolation, X-ray Structural Characterization, and Reactivities of Di-
iodosamarium(Ⅱ) Complexes Bearing Amide Compounds as Ligands
Diiodosamarium(Ⅱ) Complexes Bearing Amide Compounds
M. Nishiura et al.
Masayoshi Nishiura, Kosuke Katagiri, and Tsuneo Imamoto*
01
1
2
7
4
Department of Chemistry, Faculty of Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522
(Received January 17, 2001)
SJA8
09-2673
018
The structures of diiodosamarium(Ⅱ) complexes with amide compounds have been investigated basing on single
crystal X-ray structural analyses. The reaction of [SmI
2
(thf)
] (1). The three different kinds of ligands in 1 attach to the samarium atom in a trans orientation.
] reacted with 4 equivalents of 1,3-dimethyl-2-imidazolidione (DMI) to afford [SmI (dmi) ] (2), forming a
2
] with 2 equivalents of tetramethylurea (TMU) gave
01
[SmI
SmI
2
(thf)
(thf)
2
(tmu)
2
[
2
2
2
4
typical hexa-coordinated octahedral structure. Diiodosamarium complex containing four N,N-dimethylacetamide
(
[
DMA) ligands [SmI
SmI (thf) ] with 6 equivalents of 1,3-dimethyl-3,4,5,6-tetrahydro-2-(1H)-pyrimidinone (dimethylpropyleneurea (DM-
]I (4) quantitatively. This complex experiences a unique coordination fashion, in which six
DMPU ligands coordinate to the samarium atom to overlap each other. [SmI (dmpu) (thf)] (5) was isolated from the re-
action of [SmI (thf) ] with 2 equivalents of DMPU. The reducing ability of these complexes was evaluated by reactions
2 4 2 2
(dma) ] (3) was prepared by treatment of [SmI (thf) ] with 4 equivalents of DMA. The reaction of
0
1
2
2
2
PU)) gave [Sm(dmpu)
6
2
.
2
3
2
2
with benzylacetone or 9-haloanthracene. The structural analyses and reactivity studies of these complexes indicate that
DMPU and DMA ligands are the most effective among the amide compounds to enhance the reducing ability of samari-
um diiodide.
1
After the pioneering work of Kagan and coworkers in 1977,
diiodosamarium(Ⅱ) (SmI
coordination bond lengths; reactions of benzylacetone and 9-
haloanthracene with these samarium complexes have been ex-
amined in order to elucidate their reducing abilities.
2
)-mediated organic reactions have
been extensively investigated. This reagent has become one of
the most important and useful one-electron reductants in syn-
2
Experimental
thetic organic chemistry. It is well known that the addition of
hexamethylphosphoric triamide (HMPA) to a solution of SmI
2
General Methods. All reactions were carried out under ar-
gon atmosphere by using Schlenk techniques or under nitrogen at-
mosphere in an Mbraun glovebox. The nitrogen in the glovebox
was constantly circulated through a copper/molecular sieves (4A)
catalyst unit. The oxygen and moisture were constantly moni-
in tetrahydrofuran (THF) dramatically increases the reducing
3
ability of divalent samarium species. The molecular struc-
tures of HMPA coordinated diiodosamarium complexes,
4
SmI
2
(hmpa)
4
and [Sm(hmpa)
6
]I
2
, have also been determined
5
by X-ray analyses, and the structural information has become
an important guide for the utilization of HMPA. Although the
HMPA ligand reveals a prominent effect, one is reluctant to
2 2
tored by an O /H O Combi-Analyzer to ensure that both were al-
1
ways below 1 ppm. H NMR spectra were recorded on JNM-
1
LA400 (400.05 MHz for H) spectrometer. Chemical shifts are re-
6
ported in parts per million downfield from tetramethylsilane. Ele-
mental analyses were carried out at The Institute of Physical and
Chemical Research (RIKEN). THF, hexane, and toluene were dis-
tilled from sodium benzophenone ketyl, degassed by the freeze–
thaw method (three times), and dried over fresh Na chips. Tetra-
use it owing to its suspected carcinogenicity. Recently, some
urea derivatives and amide compounds, which are strong elec-
tron-donating ligands like HMPA, have been employed as al-
7
–9
ternative additives. The effect of these amide complexes for
the reduction enhancement of SmI has been observed in many
reactions, but the structure-reactivity relationships of the SmI
2
methylurea,
,4,5,6-tetrahydro-2(1H)-pyrimidinone, and N,N-dimethylacet-
amide were purchased from Wako Pure Chemical Industries Co,
dried by stirring with CaH for 24 h, distilled under reduced pres-
1,3-dimethyl-2-imidazolidinone,
1,3-dimethyl-
2
3
1
0
complexes generated in situ have not yet been elucidated.
We consider that the characterization of the molecular struc-
tures of the diiodosamarium complexes with these additives
and the evaluation of their reactivities are indispensable for
further development of samarium diiodide-promoted organic
reactions. In this paper, we report the X-ray structural analy-
ses of diiodosamarium complexes bearing amide compounds
such as tetramethylurea (TMU), 1,3-dimethyl-2-imidazolidi-
none (DMI), N,N-dimethylacetamide (DMA), and 1,3-dimeth-
yl-3,4,5,6-tetrahydro-2-(1H)-pyrimidinone (dimethylpropyl-
eneurea (DMPU)) as ligands. Furthermore, the coordination
abilities of the amide compounds are discussed based on their
2
sure, and degassed by the freeze–thaw method (three times). Sa-
marium metal was obtained from Nippon Yttrium Co.
11
[
SmI
SmI
to a blue-green solution of [SmI
2
(thf)
2
] was prepared according to the literature.
[
2
(thf)
2
(tmu)
2
] (1). Addition of TMU (0.48 mL, 2 mmol)
(thf) ] (1.10 g, 2 mmol) in 20 mL
2
2
of THF at room temperature caused an immediate color change to
dark-purple. After 2 h, the solution was filtered through a frit and
the solvent was removed under reduced pressure to give 1 as dark-
1
purple powder (1.50 g, 96%). H NMR (C D , 400 MHz, 25 °C):
6
6
3 2 2
δ 0.70 (d, 3H, CH ), 1.38 (s, 2H, CH ), 3.58 (s, 2H, CH ). (THF-

1
418 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)
Diiodosamarium(ꢀ) Complexes Bearing Amide Compounds
d
8
, 400 MHz, 25 °C): δ 2.71 (t, 2H, CH
), 5.40 (d, 3H, CH ). IR (Nujol) 1584 (s), 1538 (s),
412 (m), 1158 (m). 1037 (m), 912 (w), 878 (w), 766 (w), 748
2
), 3.10 (s, 3H, CH
3
), 3.57
meter with gaphite-monochromated Mo Kα radiation. A laser-
stimulated fluorescence image plate was used as a two-dimension-
al area detector. Because of the instability of the crystals, rapid
analysis was required. The distance between the crystal and the
detector was 85 mm. Thus, 27 frames were recorded at intervals
of 6° and each exposure lasted for 5 min (ca. 135 min for the total
data collection). Molecular structures of complexes 1, 2 and 4
(
d, 2H, CH
2
3
1
(
m), 722 (w), 670 (w), 566 cm− (m). Found: C, 27.54; H, 5.18;
N, 7.46%. Calcd for C18 Sm: C, 27.69; H, 5.16; N,
.18%. Slow cooling of the THF solution gave X-ray quality sin-
gle crystals of [SmI (thf) (tmu) •(THF)0.67], and its crystal solvent
was easily removed in vacuo for 6 h to give [SmI (thf) (tmu) ].
SmI (dmi) ] (2). DMI (0.86 mL, 8 mmol) was added to a so-
lution of [SmI (thf) ] (1.10 g, 2 mmol) in 20 mL of THF at room
1
40 2 4 4
H I N O
7
2
2
2
22
2
2
2
were solved by direct methods using the programs SIR 92, and
[
2
4
that of complex 3 was solved by direct methods using the pro-
23
2
2
grams SAPI 91. Full matrix less-squares refinements were car-
2
temperature. After 2 h, the solvent was removed under reduced
pressure, and the residue was washed with toluene to give 2 as a
dark-purple powder (1.7 g, 99%). H NMR (C
ried out by minimizing the function Σω(|F
O
| − |F
C
|) where F
O
and F were observed and calculated structure factors. The non-
C
1
6
D
6
, 400 MHz, 25
). (THF-d , 400 MHz,
), 1.25 (s, 3H, CH ), 2.23 (s, 2H, CH ),
). IR (Nujol) 1660 (s), 1524 (s), 1408 (m), 1286
hydrogen atoms were refined anisotropically. Hydrogen atoms
were included but not refined. The residual electron densities
were of no chemical significance. Crystallographic data have
been deposited at the CCDC, 12 Union Road, Cambridge CB2
1EZ, UK and copies can be obtained on request, free of charge, by
quoting the publication citation and the deposition numbers
162183-162187. The data are also deposited as Document No.
74042 at the Office of the Editor of Bull. Chem. Soc. Jpn.
°
2
2
C) δ 2.40 (s, 2H, CH
2
), 2.52 (s, 3H, CH
5 °C) δ 0.85 (s, 3H, CH
.41 (s, 2H, CH
3
8
3
3
2
2
−
1
(
s), 1245 (s), 1088 (w), 1041 (w), 967 (w), 778 (m), 758 cm (w).
Found: C, 28.08; H, 4.58; N, 12.76%. Calcd for C20 Sm:
C, 27.91; H, 4.68; N, 13.02%. Single crystals for X-ray structural
analysis were obtained by slow addition of DMI to a solution of
40 2 8 4
H I N O
SmI
SmI
solution of [SmI
room temperature. After 2 h, the solvent was removed under re-
duced pressure to give 3 as a dark-purple powder (14.6 g, 97%).
2
in THF.
X-ray Structural Determination and Refinement for Com-
plex 5. X-ray data collection was carried out as described above.
The molecular structure was solved by direct methods using the
programs SIR 92. The two iodide ions and one DMPU molecule
are ordered, while two DMPU molecules and one THF molecule
are disordered. Although the overall geometry of the complex has
been established, location and refinement of all of the light atoms
(C, N) of the disordered molecules were impossible. The best re-
[
2
(dma)
4
] (3). DMA (7.43 mL, 80 mmol) was added to a
(thf) ] (11.0 g, 20 mmol) in 200 mL of THF at
2
2
1
H NMR (C
H, CH ), 2.61 (s, 3H, CH
in THF-d solution. IR (Nujol) 1618 (m), 1460 (s), 1380 (s), 1022
m), 962 (w), 721 (m), 598 cm− (w). Found: C, 25.73; H, 4.84;
N, 7.24%. Calcd for C16 Sm: C, 25.53; H, 4.82; N,
.44%. Single crystals for X-ray structural analysis were obtained
by slow addition of DMA to a solution of SmI in THF.
Sm(dmpu) ]I (4). DMPU (1.93 mL, 16 mmol) was added
to a solution of [SmI (thf) ] (1.46 g, 2.67 mmol) in 20 mL of THF
6
D
6
, 400 MHz, 25 °C) δ 1.66 (s, 3H, CH
3
), 2.08 (s,
3
3
3
). Assignable peaks were not observed
8
1
(
1
finement of the model yielded R = 11%. The space group is P2 /
H
36
I
2
N
4
O
4
n with cell parameters of a = 10.421(5) Å, b = 15.980(10) Å, c =
3
7
19.95(1) Å, β = 99.01(5)°, V = 3281(2) Å , and Z = 4 for Dcalcd
−
3
2
= 1.742 g cm .
[
6
2
General Procedure for Reactions of Benzylacetone with Di-
iodosamarium(Ⅱ) Complexes (Table 6). Benzylacetone (0.15
mL, 1 mmol) was added to a solution (0.1 M, 2.2 mmol) of diio-
dosamarium(Ⅱ) complex in THF at room temperature. The reac-
tion mixture was treated with air and passed through a short col-
umn of silica gel using ethylacetate as the eluent. The solvent was
removed on a rotary evaporator, and pinacol coupling product 6
and secondary alcohol 7 were isolated by preparative TLC ethyl-
acetate/hexane (1:5) as the developing solvent.
2
2
at room temperature. The pure solution was stirred at room tem-
perature for 2 h. The solvent was removed under reduced pressure
and the residue was washed with toluene to give a purple powder
1
of 4 (3.0 g, 97%). H NMR (C
6
D
6
, 400 MHz, 25 °C) δ 0.03 (s,
), 1.48 (s, 2H, CH ). (THF-d , 400
MHz, 25 °C) δ 0.92 (t, 2H, CH ), 1.14 (s, 2H, CH ), 1.31 (s, 3H,
CH ), 2.01 (s, 3H, CH ), 2.52 (s, 2H, CH ). IR (Nujol) 1596 (s),
548 (s), 1485 (m), 1417 (m), 1325 (m), 1253 (m), 1234 (w), 1065
2
H, CH
2
), 1.12 (s, 3H, CH
3
2
8
2
2
3
3
2
1
−
1
Results and Discussion
(
w), 943 (w), 893 (w), 746 (m), 712 cm (w). Found: C, 36.95;
H, 6.16; N, 13.92%. Calcd for C36 Sm: C, 36.86; H,
.19; N, 14.33%. Slow addition of hexane to a concentrated THF
solution of 4 gave single crystals for X-ray analysis.
SmI (dmpu) (thf)] (5). Method A: addition of DMPU (0.48
mL, 4 mmol) to a solution of [SmI (thf) ] (1.10 g, 2 mmol) in 20
72 2 12 6
H I N O
2 2 2
Synthesis and Structure of [SmI (thf) (tmu) ] (1). We
first attempted the isolation and structural characterization of a
diiodosamarium(Ⅱ) complex with TMU, which was used in
6
[
2
3
2
SmI -promoted [3 + 2] cycloaddition reaction of carbonyl
2
2
7
2 2
ylide. The reaction of [SmI (thf) ] with 4 equivalents of TMU
mL of THF gave a dark-purple solution, which was stirred at room
temperature for 2 h. The solution was concentrated and cooled to
in THF gave 1. Removal of the solvent gave a purple powder,
and the resultant complex was recrystallized from THF to give
dark-purple prisms. The molecular structure of complex 1 was
determined by X-ray structural analysis. There are three mole-
cules of the samarium complexes and a crystal solvent of two
THF molecules in the crystallographic unit. Figure 1 shows
the structure of one of the samarium complexes. The other
complexes and the crystal solvent are omitted for clarity. The
selected bond lengths and angles are listed in Table 1. Com-
plex 1 contains two iodide ions, two TMU molecules, and two
THF molecules. The ligand–Sm–ligand angles are in the range
of 87°–93°, showing an ordinary octahedral structure. Three
different kinds of ligands are located at the trans positions to
−20 °C. After 2 days, purple crystals were formed. Method B:
the reaction of 4 (587 mg, 0.5 mmol) with [SmI (thf) ] (548 mg, 1
2
2
mmol) in 20 mL of THF also gave 5 (480 mg, 56% based on DM-
1
PU). In H NMR analysis, no assignable peak was observed in
THF-d
8
solution. Found: C, 30.52; H, 5.18; N, 9.57%. Calcd for
C H I N O Sm: C, 30.70; H, 5.15; N, 9.76%. Single crystals for
22 44 2 6 4
X-ray analysis were obtained by keeping a concentrated THF so-
lution of 5 at −20 °C.
X-ray Crystallographic Analyses. The crystals were han-
dled in a glovebox under a microscope (MZ6, Leica) which was
mounted on the glovebox window, and were sealed in glass capil-
laries. Data collections were performed on a R-AXISII diffracto-

M. Nishiura et al.
Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1419
Fig. 1.
SmI
Thermal ellipsoid plot of the structure of
(tmu) ] (1).
[
2
(thf)
2
2
2 4
Fig. 2. Molecular structure of [SmI (dmi) ] (2).
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
SmI (thf) (tmu) ] (1)
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
SmI (dmi) ] (2)
[
2
2
2
[
2
4
Sm(1)–I(1)
Sm(3)–I(3)
Sm(1)–O(2)
Sm(2)–O(4)
Sm(3)–O(6)
3.3169(9)
Sm(2)–I(2)
Sm(1)–O(1)
Sm(2)–O(3)
Sm(3)–O(5)
3.312(2)
2.458(8)
2.436(7)
2.444(7)
Sm(1)–I(1)
Sm(1)–O(2)
Sm(2)–O(3)
3.3578(9)
2.59(1)
2.455(9)
Sm(1)–O(1)
Sm(2)–I(2)
Sm(2)–O(4)
2.473(8)
3.3450(9)
2.396(6)
3.3061(8)
2.541(8)
2.533(8)
2.509(9)
I(1)–Sm(1)–I(1*)
I(1)–Sm(1)–O(2)
O(1)–Sm(1)–O(2)
O(2)–Sm(1)–O(2*)
Sm(1)–O(2)–C(4)
180
90
87.7(3)
180
172(1)
I(1)–Sm(1)–O(1)
O(1)–Sm(1)–O(1*)
O(1)–Sm(1)–O(2*)
Sm(1)–O(1)–C(1)
90
180
92.3(3)
174.9(8)
I(1)–Sm(1)–I(1*)
I(1)–Sm(1)–O(2)
O(1)–Sm(1)–O(2)
O(2)–Sm(1)–O(2*)
180
O(1)–Sm(1)–O(1)
O(1)–Sm(1)–O(1*)
O(1)–Sm(1)–O(2*)
Sm–O(1)–C(1)
87.6(2)
180
86.2(3)
171.7(8)
87.4(2)
93.8(3)
180
each other. Bond angles of I–Sm–I, O(THF)–Sm–O(THF),
and O(TMU)–Sm–O(TMU) are just 180°, for there is a sym-
metry center on the samarium atom. The bond length of Sm–
O(TMU) (2.446(7) Å) is shorter than that of Sm–O(THF)
bond length is 2.48(1) Å; however, the difference between the
shortest bond length and the longest bond length is significant-
ly large (0.2 Å), though the reason for this difference is not
clear. The ligand–Sm–ligand angles are almost 90°. It is noted
that all DMI ligands coordinate to the samarium atom in regu-
larly perpendicular directions to the equatorial plane formed
by four oxygen atoms of the DMI ligands. There is a mirror
plane in the equatorial plane; therefore, the DMI ligands are
2
.528(9) Å, indicating that TMU molecules coordinate to the
samarium atom more strongly than THF molecules.
Although 4 equivalents of TMU were added to [SmI
2
2
(thf) ],
the isolated complex contained only two TMU molecules, and
the reaction of [SmI (thf) ] with 2 equivalents of TMU gave
complex 1 quantitatively. The treatment of [SmI (thf) ] with a
2
2
divided into two equal parts crystallographically. [SmI
was reacted with DMI by changing the molar equivalent of the
ligand. It was found that [SmI (dmi) ] was always produced
even through 10 equivalents of DMI was added.
Synthesis and Structure of [SmI (dma) ] (3). Inanaga et
al. have investigated the selective reduction of α,β-unsaturated
2 2
(thf) ]
2
2
large excess of TMU (10 and 60 equivalents) also afforded
2
4
complex 1. These results are clearly different from those of
4
,5
HMPA coordinated diiodosamarium complexes.
HMPA coordinated samarium complex SmI (hmpa)
tained by the reaction of [SmI (thf)
HMPA. Moreover, a diiodosamarium complex bearing six
HMPA molecules [Sm(hmpa) ]I was also obtained by the re-
action of [SmI (thf) ] with 10 equivalents of HMPA. These re-
Four
2
4
2
4
was ob-
9
2
2
] with 4 equivalents of
2
esters and amides with SmI . The use of some additives such
as dimethylformamide (DMF), DMA, TMU, N,N,Nꢀ,Nꢀ-tet-
ramethylethylenediamine (TMEDA), and N,N,Nꢀ,Nꢀ-tetrameth-
ylpropylenediamine (TMPDA) promote effectively this conju-
gate reduction; DMA is the most effective ligand among these
amides and amines. Therefore, we tried to isolate the DMA-
6
2
2
2
sults suggest that the coordination ability of the TMU ligand is
weaker than that of HMPA.
Synthesis and Structure of [SmI
tion of [SmI (thf) ] with 4 equivalents of DMI gave 2 quantita-
tively. Single crystals were obtained by slow addition of DMI
to SmI solution in THF, because of the poor solubility of com-
plex 2 in this solvent. Its molecular structure was determined
by X-ray structural analysis, as illustrated in Figure 2. The se-
lected bond lengths and angles are shown in Table 2. This
complex contains four DMI ligands and two iodide ions to
form a regular hexa-coordinated octahedral structure. There
are two molecules in the crystallographic unit, and four differ-
ent bond lengths of Sm–O(DMI) are observed. The average
2
(dmi)
4
] (2). The reac-
coordinated diiodosamarium complex.
[SmI (thf) ] with 4 equivalents of DMA gave a dark-blue solu-
tion. Single crystals were obtained by slow addition of DMA
to a SmI solution in THF due to the poor solubility of the
complex 3. The crystal structure of [SmI (dma) ] (3) was de-
termined by X-ray structural analysis. The ORTEP drawing of
it is shown in Figure 3. The selected bond lengths and angles
are summarized in Table 3. The central samarium atom is co-
ordinated by the two iodide anions and the four DMA ligands
to form a distorted octahedral structure. The four DMA
ligands in the equatorial plane form a unique coordination
The reaction of
2
2
2
2
2
2
2
4

1
420 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)
Diiodosamarium(ꢀ) Complexes Bearing Amide Compounds
4
shows the selected bond lengths and angles. There are two
molecules in the unit cell, and the central samarium atom is
hexacoordinated by six DMPU molecules. This complex has
high symmetry; therefore, six DMPU ligands are crystallo-
graphically equivalent. The two iodide ions do not coordinate
to the samarium atom, thus making the complex dicationic. To
the best of our knowledge, dicationic divalent samarium com-
plexes are very rare, and only a few complexes such as
2
+
2–9,12
[
[
Sm(hmpa)
Sm(dime)
6
]I
2
, [Sm(thf)
7
] [Zn
4
(µ-SePh)
6
(SePh) and
4
]
3
][Co(CO)
4
]
2
{dime = (MeOCH
2
CH ) O} are
2 2
1
3
known. The coordination number of the complex 4 is six, as
in complexes 1, 2, and 3; however, the coordination geometry
of the central samarium atom is not octahedral but has a dis-
torted trigonal antiprism structure. The large deviation in the
ligand–Sm–ligand angle (O1–Sm–O1*) (76°) from 90° sup-
ports this coordination geometry. A samarium complex bear-
ing six DMPU ligands could be isolated, but the corresponding
complexes were not obtained when TMU, DMI, and DMA
were used as the ligand. These facts suggest that coordination
ability of DMPU is stronger than those of the other amide
ligands. It is noteworthy that all DMPU ligands coordinate to
the samarium atom while overlapping each other. The average
bond angle of Sm–O–C (carbonyl carbon) (152°) is consider-
ably different from that of complex 1 (171°) and complex 2
2 4
Fig. 3. ORTEP drawing of [SmI (dma) ] (3).
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
SmI (dma) ] (3)
[
2
4
Sm(1)–I(1)
Sm(1)–O(1)
3.309(1)
2.449(9)
Sm(1)–O(1)
2.45(1)
I(1)–Sm(1)–I(1*)
I(1)–Sm(1)–O(2)
O(1)–Sm(1)–O(2)
Sm(1)–O(2)–C(5)
179.11(4)
90.8(3)
177.4(4)
143(1)
I(1)–Sm(1)–O(1)
O(1)–Sm(1)–O(1*)
Sm(1)–O(1)–C(1)
91.5(3)
85.1(5)
138(1)
fashion.
Synthesis and Structure of [Sm(dmpu)
a well-known and less toxic alternative to HMPA, has been
6
2
]I (4). DMPU,
(
175°), implying that some interaction exists between the up-
per ligands and lower ligands. However, the atomic distance
between C1 and C19 (3.54(1) Å) is slightly longer than the
sum of the van der Waals radii (ca. 3.0 Å), indicating that there
is no intramolecular ligand interaction in the complex.
8
employed as an additive in SmI
In order to isolate and clarify the molecular structure of DMPU
ligand-coordinated diiodosamarium complex, [SmI (thf) ] was
2
-promoted organic reactions.
2
2
allowed to react with 4 equivalents of DMPU. The isolated
crystals were subjected to X-ray structural analyses. The mo-
lecular structure of the complex is depicted in Figure 4. Table
6 2
A packing diagram for 4 indicates that [Sm(dmpu) ]I units
of the backside position themselves so as to minimize the
space between the front side units and the backside units (Fig-
ure 5). It is noted that the closest intermolecular contact be-
tween the methyl carbon of adjacent molecules (3.35(1) Å) is
significantly less than the sum of the corresponding van der
Waals radii (ca. 4.0 Å).
2 3
Synthesis and Structure of [SmI (dmpu) (thf)] (5). The
reaction of [SmI (thf) ] with 2 equivalents of DMPU provided
2
2
a new diiodosamarium complex with DMPU ligands. The mo-
lecular structure of this complex 5 was determined and is
shown in Figure 6. The crystallographic data reveal that three
6 2
Fig. 4. Molecular structure of [Sm(dmpu) ]I (4).
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
SmI (dmpu) ]I (4)
[
2
6
2
Sm(1)–O(1)
2.475(7)
Sm(2)–O(2)
2.488(9)
Fig. 5. Packing diagram of complex 4 looking down the “c”
axis. Molecules indicated in black are front units and mol-
ecules indicated in white are back units.
O(1)–Sm(1)–O(1ꢁ)
O(1)–Sm(1)–O(1$)
Sm(1)–O(1)–C(1)
76.7(3)
167.2(3)
151.1(7)
O(1)–Sm(1)–O(1ꢀ)
O(1)–Sm(1)–O(1*)
94.6(2)
94.6(2)

M. Nishiura et al.
Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1421
amide compounds are strong donor ligands. Elongation of the
Sm–I bonds was observed due to the strong coordination of
amide ligands. The bond distances between the samarium ion
and the iodide ion Sm–I (av 3.312(1) Å) in complex 1, (av
3
.3514(9) Å) in complex 2, and 3.310(1) Å in complex 3 are
slightly shorter than that of SmI (hmpa) (3.390(2) Å), but they
are longer than those of other diiodosamarium complexes such
as SmI (dme) (thf) 3.246(1) Å, SmI (dme)(thf) av 3.233(1) Å,
Sm(µ-I)(NCCMe 3.260(1) Å and 3.225(1) Å, trans-
2
4
2
2
2
3
15
[
[
[
[
(
3 2 ∞
) ]
1
5
SmI
SmI
2
{O(CH
{O(CH
{N(SiMe
2
CH
CH
2
OMe)
2
}
2
]
3.265(1)
] 3.332(1) Å and 3.333(1) Å,
(µ-I) av 3.3486(9)
(µ-I)(N-Meim) ] (N-Meim = N-methylimi-
Å,
cis-
1
8
2
2
2
OMe)
}(dme)
2
}
2
Sm
2
3
)
2
2
(thf)
2
2
]
Å
1
7
bridge), [Sm
dazole) 3.237(1) Å (terminal), and av 3.294(1) Å (bridge).
2
I
2
6
1
9
Fig. 6. Ball and stick plot of [SmI
2
(dmpu)
3
(thf)] (5).
In general, the coordination numbers of divalent samarium
atoms are 7–9. In this case, it is noteworthy that the coordina-
tion numbers of all diiodosamarium complexes bearing amide
ligands are six. The bond angles of O–C–N (Sm–O(THF)–C
(1) 128.5°, O(TMU)–C–N (1) 119°, O(DMI)–C–N (2) 124.2°,
O(DMA)–C–N (3) 102°, O(DMPU)–C–N (4) 124.9°, O(DM-
PU)–C–N (5) 123°) suggest that these amide ligands are rela-
tively bulky, and therefore the coordination numbers of these
complexes are comparatively small.
DMPU ligands, one THF molecule, and two iodide ions coor-
dinate to the samarium atom to form a six-coordinated neutral
complex, whereas 4 forms a dicationic complex. This complex
was also obtained by the reaction of 4 with 2 equivalents of
[
2 2
SmI (thf) ], clearly indicating that ligand redistribution oc-
curred in the reaction. Unfortunately, further discussion of
bond distances and angles was hampered due to the limited
quality of the crystallographic data.
Coordination Bond Lengths. In order to estimate the
Reactions of Various SmI
tone. The isolated complexes were dissolved in THF-d
benzene-d and their NMR spectra were measured. It is noted
2
Complexes with Benzylace-
Ⅲ
Ⅱ
Sm /Sm reduction potential of complexes 1, 2, 3, and 4, the
coordination bonds were compared with other divalent diio-
dosamarium complexes (Table 5). The bond distances be-
tween the samarium ion and the oxygen atoms of the amide
compounds Sm–O(TMU) (av. 2.446(8) Å) in 1, Sm–O(DMI)
8
or
6
that no peaks based on free amide ligands were observed.
These results suggest that the molecular structures of the com-
plexes in solution resemble those of the crystal system. There-
fore, we considered that the reactivity of the complexes in so-
lution can be correlated to their crystal structures. Based on
this idea, we evaluated their reducing ability using benzylace-
tone as a representative carbonyl compound (Scheme 1). The
reactions of the samarium(Ⅱ) complexes with benzylacetone
were carried out under the same reaction conditions. The ob-
(
av 2.48(1) Å) in 2, Sm–O(DMA) (av 2.465(9) Å) in 3, and
Sm–O(DMPU) (av 2.482(9) Å) in 4 are much shorter than
those of the corresponding coordination bonds of the divalent
samarium complexes bearing oxygen donor ligands; e.g. Sm–
O(DME) av 2.618(5) Å and Sm–O(THF) 2.530(5) Å in
1
4
[
SmI
2 2
(dme) (thf)], Sm–O(DME) av 2.641(4) Å and Sm–
1
4
O(THF) 2.571(4) Å in [SmI
2
(dme)(thf)
CH OMe)
and Sm–O(THF) av 2.592(6) Å
{N(SiMe }(dme)
], and they are almost the same or slightly shorter
than those between the samarium atom and the oxygen atoms
of HMPA (av 2.500(6) Å) in SmI (hmpa) and (av 2.53(1) Å)
in [Sm(hmpa) ]I These results clearly indicate that these
3
], Sm–O(diglyme) av
} ], Sm–O(DME) av
2 2
tained results are summarized in Table 6. SmI
about 6 h to complete the reaction (entry 1). The reactivity of
the complex 1 was nearly equal to that of SmI itself, but the
major product was 6 (entry 2). The reaction of SmI with 10
equivalents of TMU also afforded the complex 1. Therefore,
these facts suggest that the reactivity of SmI in the presence of
2
itself required
1
5
2
2
.699(4) Å in [SmI
.67(1) Å in [SmI
2
{O(CH
2
2
1
6
2
(dme)
3
]
2
and Sm–O(DME) av 2.685(8) Å in [Sm
2
3
)
2
2
-
2
17
(
thf)
2
(µ-I)
2
2
10 equivalents of TMU resembles that of complex 1. In prac-
tice, however, this reductant revealed lowering of the reactivity
and decreased selectivity of product 6 (entry 3). Flowers et al.
2
4
6
2
.
Table 5. Sm–I and Sm–O Bond Distances (Å) for Various
Samarium Diiodide Complexes
2
reported that the oxidation potential of SmI reached maxi-
2
1
mum when 60 equivalents of TMU was added. In this reac-
tion, SmI with 60 equivalents of TMU showed low reactivity,
2
Bond distances, Å
and the major product was secondary alcohol 7 (entry 4). It
was thought that excess TMU ligands in situ inhibited the reac-
tivity and production of pinacol-coupling product 6. The reac-
Sm–I
3.312(1)
Sm–O
2.446(8) 2.528(9)
Sm–O(THF)
a)
SmI
SmI
SmI
2
2
2
(tmu)
(dmi)
(dma)
2
(thf)
2
a)
4
3.3514(9) 2.48(1)
2
tivity of the complex 2 was rather inferior to that of SmI itself
a)
4
3.310(1)
2.449(9)
2.484(9)
2.500(6)
2.530(5)
2.571(4)
[
Sm(dmpu)
6
]I
2
SmI
SmI
SmI
2
2
2
(hmpa)
(dme)
(dme)(thf)
4
3.390(2)
3.246(1)
3.233(1)
2
(thf)
3
trans-SmI
O(CH CH
2
[
2
2
OMe)
2
]
2
3.265(1)
2.699(4)
a) These bond distances are average data of Table 1–4.
Scheme 1.

1
422 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)
Diiodosamarium(ꢀ) Complexes Bearing Amide Compounds
Table 6. Reactions of SmI Complexes with Benzylacetone
2
Entry
Reductant
Time
Yield/%
b)
Conversion (6:7)
92 (34:66)
88 (82:18)
75 (74:26)
79 (19:81)
73 (79:21)
76 (32:68)
47 (0:100)
100 (93:7)
100 (93:7)
97 (92:8)
a)
1
2
3
4
5
6
7
8
9
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
SmI
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
(thf)
2
6 h
2 h
2 h
110 h
2 h
18 h
a)
Scheme 2.
(tmu)
2
+ 10 equiv TMU
+ 60 equiv TMU
Table 7.
anthracene
2
Reactions of SmI Complexes with 9-Chloro-
+ 2 equiv DMI
a)
(dmi)
4
a)
+ 10 equiv DMI
+ 2 equiv DMA
48 h
Entry
Reductant
Time
Yield/%
30 min
5 min
15 min
2 h
15 min
5 min
15 min
15 min
2 h
1
2
3
4
5
6
7
8
SmI
SmI
SmI
SmI
2
2
2
2
(thf)
(tmu)
(dmi)
2
24 h
24 h
24 h
4 h
4 h
4 h
no reaction
a)
(dma)
4
2
5
2
94
34
96
10
11
12
13
14
15
16
17
18
19
20
+ 6 equiv DMA
+ 10 equiv DMA
+ 1 equiv DMPU
+ 2 equiv DMPU
+ 3 equiv DMPU
+ 4 equiv DMPU
4
70 (66:34)
82 (81:19)
100 (85:15)
100 (81:19)
100 (84:16)
71 (75:25)
61 (41:59)
64 (13:87)
100 (42:58)
79 (14:86)
(dma)
4
[Sm(dmpu)
6
]I
2
SmI
SmI
SmI
2
2
2
+ 4 equiv DMPU
+ 2 equiv DMPU
4 h
1 min
65
> 99
+ 5 equiv DMPU
(hmpa)
4
a)
[Sm(dmpu)
6
]I
2
2 h
1
a) Determined by H NMR.
SmI
SmI
SmI
2
2
2
+ 30 equiv DMPU
(hmpa)
+ 6 equiv HMPA
110 h
15 min
1 h
a)
4
of 9-bromoanthracene, but this reaction was not good for com-
parison of ligand effects. Therefore, 9-chloroanthracene was
used as substrate because its reactivity is lower than 9-bro-
moanthracene (Scheme 2). The results are shown in Table 7.
a) Isolated complex was disolved in THF, and the solution
0.1 M) was used for the reaction.
(
1
b) Determined by H NMR.
2
No anthracene was obtained in the reaction of SmI itself (en-
(
entry 6). On the other hand, the complex 3 demonstrated high
reactivity, and the reaction was completed within 5 minutes
entry 9). To our surprise, reactivities of the SmI /DMPU sys-
tems greatly depended on the amounts of DMPU (entry 12–
8). One equivalent of DMPU per SmI was not sufficient to
show high reactivity (entry 12). The additions of 2–4 equiva-
lents of DMPU to SmI revealed high reactivities (entry 13–
5). The reactivity of the complex 4 was clearly different from
try 1), and complexes 1 and 2 afforded trace amounts of an-
thracene (entry 2 and 3). The complex 3 showed moderate re-
activity to afford anthracene in high yield (entry 4). The reac-
(
2
tivities of SmI
DMPU as observed in the reactions of benzylacetone (entry 5–
7); 4 equivalents of DMPU per SmI revealed the highest reac-
tivity (entry 6). [SmI (hmpa) ] exhibited high reactivity to
give anthracene quantitatively (entry 8). The reactivities of
SmI with amide ligand are a little lower than that of the SmI
HMPA system. However, enough reactivity could be obtained
when using 4 equivalents of DMPU or DMA to SmI
Recently, the effect of oxygen donor ligands on the oxida-
tion potential of SmI was examined by voltammetric analy-
2
/DMPU systems depended on the amounts of
1
2
2
2
2
4
1
that of SmI
dation potential of SmI
2
with 2 equivalents of DMPU (entry 17). The oxi-
reached a maximum when 30 equiva-
2
2
/
2
2
1
lents of DMPU added, but it showed a low reactivity (entry
8). Hou and Wakatsuki have carried out the X-ray structural
analysis of [Sm(hmpa) ]I and clearly demonstrated that six
2
.
1
6
2
2
2
1
Ⅲ
Ⅱ
HMPA molecules could coordinate to a samarium atom. On
the other hand, Flowers et al. have recently investigated in de-
tail the structure of a samarium diiodide-HMPA complex in
THF solution by UV-vis spectroscopy, isothermal titration
sis. The reduction potential (Sm /Sm ) of SmI
2
/DMPU sys-
tem (−2.21 V) was lower than that of the SmI /TMU system
2
(−2.04 V). These facts also suggest that DMPU is a stronger
electron-donating ligand than TMU ligand.
2
0
(
ITC), and vapor pressure osmometry (VPO). They conclud-
Diiodosamarium(Ⅱ) complexes bearing amide compounds
have been isolated and structurally characterized by X-ray dif-
fraction analysis (Table 8). The coordination number of these
complexes is six, which is smaller than those (7–9) of the usual
divalent samarium complexes, indicating that amide com-
pounds are strong electron-donating and bulky ligands.
DMPU and DMA ligands are shown to be most effective to en-
ed that a four HMPA coordinated samarium complex was sta-
ble but a six HMPA coordinated complex was thermodynami-
cally unstable under standard reaction conditions. In the case
of SmI
observed because a major complex is [SmI
solution even in the presence of excess HMPA. In contrast
with HMPA, the great reactivity change in SmI /DMPU sys-
2
/HMPA system, no prominent reactivity change was
2
(hmpa) ] in THF
4
hance the reducing power of SmI
complexes are slightly lower than that of [SmI
thermore, it is found that the reactivities of SmI
ligands, in particular DMPU, greatly depend on the amounts of
amide ligand per SmI
2
, but the reactivities of these
(hmpa) ]. Fur-
with amide
2
tem reveals that more than five DMPU ligands can coordinate
to a samarium atom.
2
4
2
Reduction of 9-Chloroanthracene. It was reported that
the addition of HMPA to SmI
2
dramatically promoted the re-
2
.
3
duction of organic halides. In order to evaluate their reducing
abilities, the reactions of 9-haloanthracene with diiodosamari-
um complexes bearing amide ligands were carried out at room
temperature. The amide ligands were effective in the reduction
This work was suported by “Research for the Future” Pro-
gram, the Japan Society for the Promotion of Science.

M. Nishiura et al.
Table 8. Crystallographic Data for SmI
Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1423
2
(thf)
2
(tmu)
2
(1), SmI
2
(dmi)
4
(2), SmI
2
(dma)
4
(3), and [Sm(dmpu)
6
]I (4)
2
1
2
3
4
Formula
Fw
Crystal system
Color of crystal
Space group
a, Å
C
62
H
136
I
6
N
12
O
14Sm
3
C
20
H
860.80
monoclinic
blue
40
I
2
N
8
O
4
Sm
C
16
H
752.69
tetragonal
purple
3 1
P4 2 2
36
I
2
N
4
O
4
Sm
C
36
H
72
I
2
N
12
O
6
Sm
2486.46
triclinic
purple
P1
1173.25
trigonal
purple
P321
C2/m
13.442(10)
13.409(6)
13.402(5)
99.44(3)
99.45(3)
98.96(3)
2308(2)
1
14.265(7)
15.147(6)
14.25(2)
10.011(4)
12.965(2)
b, Å
c, Å
26.132(7)
16.333(7)
α, deg
β, deg
89.59(7)
γ, deg
3
V, Å
3080(4)
4
2619(1)
4
2377(1)
2
Z
T/K
D
173
1.788
39.50
173
1.856
39.53
173
1.909
46.30
110
1.639
25.90
calcd, g cm−
3
−
1
µ(Mo Kα)/cm
No. of obsd rflns
6783 (I > 2.0σ(I))
0.085(0.136)
2.24
1893 (I > 2.0σ(I))
0.075(0.127)
2.07
1199 (I > 2.0σ(I))
0.054(0.080)
1.87
1495 (I > 2.0σ(I))
0.082(0.119)
1.70
a)
R(R
GOF
w
)
2
2 1/2
a) R = Σ||F
O
| − |F
C
||/Σ|F
O
|, R
w
= [Σw(|F
O
| − |F
C
|) /ΣwF
O
] .
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