Serendipitous Isolation of the First Example of a Mixed-Valence Samarium Tripyrrole Complex

Article abstract of DOI:10.1021/om030155q

The transamination reaction of Sm{N(SiMe₃)₂} ₂(THF)₂ with two dipyrrole ligands of formula RR′C(α-C₄H₃NH)₂ (R = R′ = Et; R = Me, R′ = Ph) has been examined. The reactions in THF and under N₂ gave two similar tetranuclear dinitrogen complexes, {[Et ₂C(α-C₄H₃N)₂Sm} ₄(THF)₂](μ-N₂)·2THF and [{[(C ₆H₅)(CH₃)C(α-C₄H ₃N)₂]Sm}₄(DME)₂]μ-N₂), where the dinitrogen unit has undergone a four-electron reduction via cooperative attack of four divalent samarium atoms and remained coordinated both side-on and end-on between the four coplanar metal centers. The same reaction carried out with diethyldipyrrolylmethane under an argon atmosphere afforded the divalent samarium macrocyclic cluster {[Et ₂C(α-C₄H₃N)₂]Sm} ₈(THF)₄·4THF. In the case of the reaction with the methylphenyldipyrrolylmethane ligand under N₂, the unprecedented hexanuclear mixed-valence Sm^II/Sm^III cluster [([(C ₆H₅)(CH₃)C(α-C₄H ₃N)₂][(C₆H₅)(CH ₃)C(α-C₄H₃N)(??-C₄H ₃N)]{[(C₆H₅)(CH₃)C] ₂(α-C₄H₃N) ₂(α,α′-C₄H₂N)}Sm ₃)(THF)₃]₂, containing tripyrrolide, N-confused , and regular dipyrrolide ligands was serendipitously formed by reaction with the impurities contained in the nonpurified ligand.

Full text of DOI:10.1021/om030155q

3742
Organometallics 2003, 22, 3742-3747
Ser en d ip itou s Isola tion of th e F ir st Exa m p le of a
Mixed -Va len ce Sa m a r iu m Tr ip yr r ole Com p lex
Christian D. Be´rube´,† Mohammad Yazdanbakhsh,‡ Sandro Gambarotta,*,† and
Glenn P. A. Yap†
Departments of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada, and
Fardowsi University of Mashad, Mashad, Iran
Received March 4, 2003
The transamination reaction of Sm{N(SiMe3)2}2(THF)2 with two dipyrrole ligands of
formula RR′C(R-C4H3NH)2 (R ) R′ ) Et; R ) Me, R′ ) Ph) has been examined. The reactions
in THF and under N2 gave two similar tetranuclear dinitrogen complexes, {[Et2C(R-C4H3N)2-
Sm}4(THF)2](µ-N2)‚2THF and [{[(C6H5)(CH3)C(R-C4H3N)2]Sm}4(DME)2](µ-N2), where the
dinitrogen unit has undergone a four-electron reduction via cooperative attack of four divalent
samarium atoms and remained coordinated both side-on and end-on between the four
coplanar metal centers. The same reaction carried out with diethyldipyrrolylmethane under
an argon atmosphere afforded the divalent samarium macrocyclic cluster {[Et2C(R-C4H3N)2]-
Sm}8(THF)4‚4THF. In the case of the reaction with the methylphenyldipyrrolylmethane
ligand under N2, the unprecedented hexanuclear mixed-valence SmII/SmIII cluster [([(C6H5)-
(CH3)C(R-C4H3N)2][(C6H5)(CH3)C(R-C4H3N)(â-C4H3N)]{[(C6H5)(CH3)C]2(R-C4H3N)2(R,R′-C4H2N)}-
Sm3)(THF)3]2, containing tripyrrolide, “N-confused”, and regular dipyrrolide ligands was
serendipitously formed by reaction with the impurities contained in the nonpurified ligand.
In tr od u ction
where the nuclearity (either hexa- or octanuclear) seems
to be controlled by the dipyrrole ligand substituents.10
Thus, in an attempt to identify the factors that
promote the ability of these systems to interact with N2,
Exciting recent work has substantially expanded the
frontiers of low-valent lanthanides by making available
new, highly reactive divalent salts.1,2 However, divalent
samarium still offers, for molecular activation purposes,
the best combination of high reactivity and availability
of well-characterized systems. The wealth of transfor-
mations displayed by the samarocenes3 provides a
uniquely various and rich chemistry with a large series
of diversified substrates.4,5 Among the most exciting
reactions, the reversible dinitrogen fixation performed
by Cp*2Sm featured not only an attractive side-on
bonding mode but also a puzzling minimal extent of
N-N triple-bond reduction.5 In contrast, the use of
ancillary ligands such as calix-tetrapyrrole has afforded
four-electron reduction of N2, the mechanism of which
has been fully elucidated.6,7 Alkali-metal cations, un-
avoidably present in the structures of calix-tetrapyr-
roles, play a substantial role in determining the occur-
rence of N2 fixation. Their undesirable presence was
eliminated by using dipyrroles, affording tetranuclear
dinitrogen complexes that conclusively demonstrated
the occurrence of a simultaneous cooperative interaction
of four metal centers on the same dinitrogen molecule.9
The dipyrrole ligands have also been successfully used
to characterize the Sm(II) precursors to dinitrogen
activation that appeared to be large polynuclear clusters
(3) (a) Girard, P.; Namy, J . L.; Kagan, H. B. J . Am. Chem. Soc. 1980,
102, 2693. (b) Evans, W. J .; Bloom, I.; Hunter, W. E.; Atwood, J . L. J .
Am. Chem. Soc. 1983, 105, 140. (c) Evans, W. J .; Engerer, S. C.; Piliero,
P. A.; Wayda, A. L. J . Chem. Soc., Chem. Commun. 1979, 1007. (d)
Evans, W. J .; Engerer, S. C.; Piliero, P. A.; Wayda, A. L. Fundamentals
of Homogeneous Catalysis; Tsutsui, M., Ed.; Plenum Press: New York,
1979. (e) Evans, W. J .; Ansari, M. A.; Ziller, J . W.; Khan, S. I.
Organometallics 1995, 14, 3. (f) Evans, W. J .; Ulibarri, T. A.; Ziller, J .
W. J . Am. Chem. Soc. 1990, 112, 219. (g) Evans, W. J .; Hughes, L. A.;
Hanusa, T. P. J . Am. Chem. Soc. 1984, 106, 4270. (h) Evans, W. J .;
Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5, 1285. (i) Evans,
W. J .; Drummond, D. K.; Bott, S. G.; Atwood, J . L. Organometallics
1986, 5, 2389. (j) Evans, W. J .; Keyer, R. A.; Zhang, H.; Atwood, J . L.
J . Chem. Soc., Chem. Commun. 1987, 837. (k) Evans, W. J .; Drum-
mond, D. K.; Grate, J . W.; Zhang, H.; Atwood, J . L. J . Am. Chem. Soc.
1987, 109, 3928. (l) Evans, W. J .; Drummond, D. K. J . Am. Chem. Soc.
1989, 111, 3329. (m) Evans, W. J .; Keyer, R. A.; Rabe, G. W.;
Drummond, D. K.; Ziller, J . W. Organometallics 1993, 12, 4664. (n)
Evans, W. J .; Grate, J . W.; Doedens, R. J . J . Am. Chem. Soc. 1985,
107, 1671. (o) Evans, W. J .; Ulibarri, T. A. J . Am. Chem. Soc. 1987,
109, 4292. (p) Evans, W. J .; Grate, J . W.; Hughes, L. A.; Zhang, H.;
Atwood, J . L. J . Am. Chem. Soc. 1985, 107, 3728. (q) Evans, W. J .;
Drummond, D. K. J . Am. Chem. Soc. 1988, 110, 2772. (r) Evans, W.
J .; Drummond, D. K. Organometallics 1988, 7, 797. (s) Evans, W. J .;
Drummond, D. K.; Chamberlain, L. R.; Doedens, R. J .; Bott, S. G.;
Zhang, H.; Atwood, J . L. J . Am. Chem. Soc. 1988, 110, 4983. (t) Evans,
W. J .; Drummond, D. K. J . Am. Chem. Soc. 1986, 108, 7440. (u) Evans,
W. J .; Ulibarri, T. A.; Ziller, J . W. J . Am. Chem. Soc. 1990, 112, 2314.
(v) Zhang, X.; Loppnow, G. R.; McDonald, R.; Takats, J . J . Am. Chem.
Soc. 1995, 117, 7828. (w) Desurmont, G.; Li, Y.; Yasuda, H.; Maruo,
T.; Kanehisa, N.; Kai, Y. Organometallics 2000, 19, 1811.
(4) Pikramenou, Z. Coord. Chem. Rev. 1990, 99, 172.
(5) Evans, W. J .; Ulibarri, T. A.; Ziller, J . M. J . Am. Chem. Soc. 1988,
110, 6877.
(6) J ubb, J .; Gambarotta, S. J . Am. Chem. Soc. 1994, 117, 4477.
(7) (a) Guan, J .; Dube, T.; Gambarotta, S.; Yap, G. P. A. Organo-
metallics 2000, 19, 4820. (b) Dube, T.; Gambarotta, S.; Yap, G. P. A.
Organometallics 2000, 19, 817.
(8) Dube, T.; Conoci, S.; Gambarotta, S.; Yap, G. P. A. Organome-
tallics 2000, 19, 1182.
† University of Ottawa.
‡ Fardowsi University of Mashad.
(1) Bochkarev, M. N.; Fedushkin, I. L.; Dechert, S.; Fagin, A. A.;
Schumann, H. Angew. Chem., Int. Ed. 2001, 40, 3176.
(2) Bochkarev, M. N.; Fagin, A. A. Chem. Eur. J . 1999, 10, 5.
10.1021/om030155q CCC: $25.00 © 2003 American Chemical Society
Publication on Web 07/25/2003

A Mixed-Valence Samarium Tripyrrole Complex
Organometallics, Vol. 22, No. 18, 2003 3743
in vacuo to give a white solid which was further purified by
sublimation (110 °C, 50 mmHg) to yield analytically pure
product as a colorless crystalline solid (5.2 g, 22.0 mmol, 30%).
MS-EI (positive ion): m/e 236. 1H NMR (500 MHz, C6D6,
25 °C): δ 7.05 (m, 5H, C-H phenyl), 6.25 (q, 4H, C-H pyr),
6.02 (q, 2H, C-H pyrrole), 3.44 (very br, 2H, N-H pyrrole),
1.79 (s, 3H, CH3). 13C NMR (125.72 MHz, C6D6, 25 °C): δ 148.1
(quartenary C phenyl), 137.4 (quartenary C pyrrole), 128.4 (CH
phenyl), 128.0 (CH phenyl), 126.7 (CH phenyl), 117.2 (CH
pyrrole), 108.4 (CH pyrrole), 106.9 (CH pyrrole), 45.0 (quar-
we are now extending this study to other dipyrrolide
dianions bearing different substituents. The aims is
2-fold: (1) we wish to understand how different sub-
stituents could modify the metal redox potential and
hopefully to enable complete N-N cleavage and (2) we
wish to gather further information about the ability of
ligand substituents to control the nuclearity of large
clusters. Herein we describe our findings.
tenary C), 29.0 (CH3 methyl). Anal. Calcd (found) for C16
16N2: C, 81.32 (80.99); H, 6.82 (6.85); N, 11.85 (11.74). Mp:
221 °C.
-
Exp er im en ta l Section
H
All operations were performed under an inert atmosphere
of a nitrogen-filled drybox or by using Schlenk-type glassware
in combination with a nitrogen-vacuum line.11 Solvents were
dried by passing through a column of activated Al2O3 under
inert atmosphere prior to use, degassed in vacuo, and trans-
ferred and stored under an inert atmosphere. DME was dried
by distillation over LiAlH4. Sm{N(SiMe3)2}2(THF)2 was pre-
pared according to literature procedures.12 NMR spectra were
recorded on a Bruker AMX-500 spectrometer. Infrared spectra
were recorded on a Mattson 3000 FTIR instrument from Nujol
mulls prepared inside a drybox. Samples for magnetic sus-
ceptibility measurements were carried out at room tempera-
ture using a Gouy balance (J ohnson Matthey) and corrected
for underlying diamagnetism.13 Elemental analyses were
carried out using a Perkin-Elmer Series II CHN/O 2400
analyzer.
P r ep a r a tion of Dieth yld ip yr r olylm eth a n e. A catalytic
amount of methanesulfonic acid (0.5 mL) was added under
nitrogen flow to a stirred mixture of 2-pentanone (7.6 mL, 70
mmol) and pyrrole (20 mL, 300 mmol). After the mixture was
stirred for 30 min, it changed from clear yellow to dark green.
The removal of excess pyrrole in vacuo afforded a green paste,
which was dissolved in a small volume of ethanol (25 mL). A
small quantity of water was added to the solution until it
became cloudy. The heterogeneous mixture was placed at
4 °C for 24 h, upon which colorless crystalline needles of
diethyldipyrrolylmethane were obtained (3.6 g, 17.8 mmol,
25%). MS-EI (positive ion): m/e 202. 1H NMR (500 MHz, C6D6,
25 °C): δ 6.84 (br, 2H, N-H pyrrole), 6.22 (m, 4H, C-H
pyrrole), 6.12 (q, 2H, C-H pyrrole), 1.75 (q, 4H, CH2 ethyl),
0.615 (t, 6H, CH3 ethyl). 13C NMR (125.72 MHz, C6D6, 25 °C):
δ 136.9 (quaternary C pyrrole), 116.9 (CH pyrrole), 107.0 CH
pyrrole), 106.1 (CH pyrrole), 43.6 (quaternary C), 29.3 (CH2
ethyl), 8.01 (CH3 ethyl). Anal. Calcd (found) for C13H18N2: C,
77.18 (76.91); H, 8.97 (8.85); N, 13.84 (13.54). Mp: 193 °C.
P r ep a r a tion of Meth ylp h en yld ip yr r olylm eth a n e. Neat
pyrrole (25 mL, 360 mmol) was placed in a 250 mL flask under
a nitrogen atmosphere, fitted with a magnetic stirrer and a
dropping funnel, and cooled with an ice bath. While the
mixture was stirred, a solution of acetophenone (8.4 mL, 70
mmol) in ethanol (30 mL) containing a catalytic amount of
methanesulfonic acid (0.5 mL) was added over a period of 20
min. The solution was allowed to react overnight, after which
the solvent and the excess pyrrole were removed in vacuo. The
crude residual solid was solubilized in methylene chloride (20
mL) and filtered over a short column of silica to remove
polypyrrole contaminants. Methylene chloride was removed
Syn th esis of {[Et2C(r-C4H3N)2]Sm }8(THF )4‚4THF (1). A
solution of Sm{N(SiMe3)2}2(THF)2 (2.0 g, 3.3 mmol) in anhy-
drous THF (300 mL) was treated with diethyldipyrrolyl-
methane (0.7 g, 3.3 mmol) at room temperature and under an
argon atmosphere. After 5 days at room temperature very air-
sensitive red needles of 1 were formed (1.1 g, 0.32 mmol, 79%).
Once crystallized, the complex is nearly insoluble in THF. IR
(Nujol mull, cm-1): ν 3082 (m), 3064 (m), 2722 (w), 2503 (w),
2394 (w), 1685 (w), 1639 (w), 1594 (w), 1543 (m), 1459 (s), 1423
(s), 1377 (s), 1341 (m), 1325 (m), 1284 (m), 1260 (s), 1172 (s),
1132 (s), 1086 (s), 1038 (s), 951 (s), 925 (m), 883 (s), 831 (s),
799 (s), 751 (s), 665 (w), 632 (m). Anal. Calcd (found) for
C
136H192N16O8Sm8 C, 48.29 (48.14); H, 5.72 (5.69); N, 6.63
(6.58). µeff ) 10.6 µB. Mp: 69 °C (loss of solvent), 245 °C dec.
Syn th esis of {[Et2C(r-C4H3N)2]Sm }4(THF)2](µ-N2)‚2THF
(2). Meth od A. A solution of Sm{N(SiMe3)2}2(THF)2 (1.1 g,
1.8 mmol) in anhydrous THF (75 mL) was treated with
diethyldipyrrolylmethane (0.4 g, 1.9 mmol) at room temper-
ature and under a nitrogen atmosphere. After 3 days at room
temperature, dark red, large, air-sensitive crystals of 2 sepa-
rated (0.72 g, 0.42 mmol, 93%). Once crystallized, the complex
is nearly insoluble in THF. IR (Nujol mull, cm-1): ν 3092 (w),
2729 (w), 2207 (w), 2057 (w), 1698 (w), 1627 (w), 1562 (w),
1460 (s), 1428 (s), 1406 (m), 1376 (s), 1338 (m), 1326 (m), 1306
(m), 1272 (s), 1254 (s), 1172 (s), 1133 (s), 1069 (s), 1035 (s),
961 (s), 927 (s), 877 (s), 835 (s), 756 (s), 726 (s), 670 (w), 632
(s), 568 (s). Anal. Calcd (found) for C68H96N10O4Sm4: C, 47.51
(47.44); H, 5.63 (5.59); N, 8.15 (8.08). µeff ) 3.6 µB. Mp: 71 °C
(loss of solvent), 237 °C dec.
Meth od B. A suspension of 1 (0.35 g, 0.1 mmol) in THF
(15 mL) was exposed to a N2 atmosphere and stirred overnight.
Analytically pure 2 was isolated as a microcrystalline solid in
nearly quantitative yield (0.31 g, 0.18 mmol, 90%).
Syn th esis of [{[(C6H5)(CH3)C(r-C4H3N)2]Sm }4(DME)2]-
(µ-N2) (3). A solution of Sm{N(SiMe3)2}2(THF)2 (1.2 g, 2.0
mmol) in anhydrous DME (175 mL) was treated with methy-
phenyldipyrrolylmethane (0.5 g, 2.0 mmol) at room tempera-
ture and under a nitrogen atmosphere. Small air-sensitive red
crystals of 3 separated after standing for 2 days at room
temperature (0.58 g, 0.33 mmol, 66 %). Once crystallized, the
complex is nearly insoluble in both THF and DME. IR (Nujol
mull, cm-1): ν 3090 (w), 3058 (w), 1599 (m), 1491 (s), 1464 (s),
1426 (m), 1411 (m), 1378 (m), 1363 (m), 1315 (w), 1280 (w),
1260 (m), 1230 (w), 1192 (w), 1180 (m), 1145 (m), 1104 (s),
1079 (s), 1062 (s), 1032 (s), 971 (w), 946 (m), 936 (w), 916 (w),
866 (s), 838 (m), 795 (s), 773 (s), 730 (m), 725 (m), 705 (m),
700 (m), 640 (m), 635 (m), 622 (w), 569 (s), 554 (s), 529 (s).
Anal. Calcd (found) for C72H76N10O4Sm4: C, 49.50 (49.47); H,
4.38 (4.33); N, 8.02 (7.97). µeff ) 3.4 µB. Mp: 98 °C (loss of
solvent), 256 °C dec.
Isola tion of [([(C6H5)(CH3)C(r-C4H3N)2][(C6H5)(CH3)C-
(r-C4H3N)(â-C4H3N)]{[(C6H5)(CH3)C]2(r-C4H3N)2(r,r′-C4H2-
N)}Sm 3)(THF )3]2 (4). A solution of [Sm{N(SiMe3)2}2(THF)2]
(1.0 g, 1.6 mmol) in anhydrous THF (50 mL) was treated with
nonsublimed methyphenyldipyrrolylmethane (0.39 g, 1.7 mmol)
at room temperature and under a nitrogen atmosphere. After
it stood 4 days at room temperature, the solution was
evaporated to a small volume and diluted with DME (25 mL).
(9) (a) Dube, T.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int.
Ed. 1999, 38, 1432. (b) Guan, J .; Dube, T.; Gambarotta, S.; Yap, G. P.
A. Organometallics 2000, 19, 4820. (c) Dube, T.; Ganesan, M.; Conoci,
S.; Gambarotta, S.; Yap, G. P. A. Organometallics 2000, 19, 3716.
(10) Ganesan, M.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int.
Ed. 2001, 40, 766.
(11) Errington, R. J . Advanced Practical Inorganic and Metalorganic
Chemistry; Blackie Academic & Professional: London, 1997.
(12) Evans, W. J .; Drummond, D. K.; Zhang, H.; Atwood, J . L. Inorg.
Chem. 1988, 27, 575.
(13) (a) Mabbs, M. B.; Machin, D. Magnetism and Transition Metal
Complexes; Chapman and Hall: London, 1973. (b) Foese, G.; Gorter,
C. J .; Smits, L. J .; Constantes Selectionnes, Diamagnetisme, Paramag-
netisme, Relaxation Paramagnetique; Masson; Paris, 1957.

3744 Organometallics, Vol. 22, No. 18, 2003
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e An a lysis Resu lts
Be´rube´ et al.
1
2
3
4
formula
C
136H192N16O8Sm8
C
68H96N10O4Sm4
C
72H76N10O4Sm4
C
74H80N7O3.5Sm3
fw
3381.86
orthorhombic
32.986(2)
28.819(2)
33.030(2)
90
90
90
31400(4)
8
1718.95
triclinic
1746.83
monoclinic
9.0634(11)
13.5294(17)
26.030(3)
90
99.334(2)
90
3149.6(7)
2
1574.50
space group
a (Å)
b (Å)
c (Å)
monoclinic
12.6987(10)
31.230(2)
18.8470(15)
90
90.700(2)
90
7327.1(10)
4
11.1020(10
12.4004(11)
13.5795(12)
101.4330(10)
91.272(2)
113.313(2)
1672.2(3)
1
R (deg)
â (deg)
γ (deg)
V (Å3)
Z′
radiation (Mo KR) (Å)
0.710 73
293(2)
1.431
2.993
13440
0.710 73
203(2)
1.707
3.514
854
0.710 73
293(2)
1.842
3.734
1716
0.710 73
203(2)
1.427
2.421
3148
T (K)
Dcalcd (g cm-3
)
µcalcd (mm-1
)
F000
R1, wR2,a GOF
0.0598, 0.1602, 1.056
0.0340, 0.0571, 1.038
0.0589, 0.1273, 1.033
0.0563, 0.1375, 1.018
R1 ) ∑Fo - Fc/∑Fo; wR2 ) [(∑(Fo - Fc)2/∑wFo2)]1/2
.
a
Ta ble 2. Selected Bon d Dista n ces (Å) a n d
An gles (d eg)
After further filtration, the solution was layered with anhy-
drous hexane (20 mL). Once the layering was complete, the
solid mass separated was isolated and examined with a
polarized optical microscope and found to consist of two
different crystalline products. Red crystals of 3 were the major
component of the mixture, and their identity was elucidated
by comparison of the IR spectrum of a solid crystalline sample,
from which large crystals of 4 were manually removed, with
that of an analytically pure sample. Complex 4 was the minor
compound, and it was isolated as dark red hexagonal prisms.
While the crystals were suitable for X-ray analysis, the
physical separation from 3 for other analytical determinations
proved to be unfeasible.
X-r a y Cr ysta llogr a p h y. Suitable crystals were selected,
mounted on thin, glass fibers using paraffin oil, and cooled to
the data collection temperature. Data were collected on a
Bruker AX SMART 1k CCD diffractometer using 0.3° ω-scans
at 0, 90, and 180° in φ. Unit cell parameters were determined
from 60 data frames collected at different sections of the Ewald
sphere. Semiempirical absorption corrections based on equiva-
lent reflections were applied.14 No symmetry higher than
triclinic was observed for 2‚2THF, and refinement in the
centrosymmetric space group option yielded computationally
stable and chemically reasonable results of refinement. Sys-
tematic absences in the diffraction data and unit cell param-
eters were uniquely consistent with the reported space groups
for 1‚4THF, 3 and 4‚3THF. The structures were solved by
direct methods, completed with difference Fourier syntheses,
and refined with full-matrix least-squares procedures based
on F2.
The compound molecules of 2‚2THF, 3, and 4‚3THF reside
on an inversion center. One, eight (half-occupancy), and three
(half-occupancy) molecules of tetrahydrofuran solvent were
located cocrystallized in the asymmetric units of 2‚2THF, 1‚
4THF, and 4‚3THF, respectively. The half-occupied, cocrys-
tallized THF molecules were found to be severely disordered,
and the atoms having the smallest isotropic parameter per ring
were assigned oxygen atom identities. All non-hydrogen atoms
were refined with anisotropic displacement parameters. All
hydrogen atoms were treated as idealized contributions. All
scattering factors are contained in the SHEXTL 5.10 program
library (G. M. Sheldrick, Bruker AXS, Madison, WI, 1997).
Crystal data and relevant bond distances and angles are given
in Tables 1 and 2.
Compound 1
Sm(1)-N(1)
Sm(1)-N(2)cent
Sm(1)-N(16)
2.617(10)
2.582(10)
2.600(11)
Sm(2)-N(2)
Sm(2)-N(3)
Sm(2)-O(1)
2.625(10)
2.644(10)
2.599(10)
N(1)-Sm(1)-N(16)
N(2)-Sm(2)-O(1)
N(2)-Sm(2)-N(3)
99.1(3) N(3)-Sm(2)-O(1)
85.4(3)
77.3(3) N(2)-Sm(2)-N(4)cent 101.8(3)
162.7(3) N(2)-Sm(2)-N(1)cent
85.1(3)
Compound 2
N(1)-N(1A)
Sm(1)-N(1)
Sm(2)-N(1)
Sm(2)-N(1A)
1.415(6)
2.145(3)
2.342(3)
2.342(3)
Sm(1)-O(1)
Sm(1)-N(2)
Sm(1)-N(3)cent
Sm(2)-N(5)
2.525(3)
2.698(4)
2.693(4)
2.579(4)
N(4)-Sm(1)-N(3)cent 170.5(13) O(1)-Sm(1)-N(1) 145.56(13)
N(2)-Sm(1)-N(5)cent 170.3(13) N(1)-Sm(1)-N(2) 77.16(13)
Sm(1)-N(1)-N(1A) 156.64(13) N(2)-Sm(1)-N(4) 76.83(13)
Compound 3
N(1)-N(1A)
Sm(1)-N(1)
Sm(2)-N(1)
Sm(2)-N(1A)
Sm(1)-O(1)
1.42(2)
Sm(1)-O(2)
Sm(1)-N(2)
Sm(2)-N(2)
Sm(1)-N(3)
Sm(1)-N(5A)
2.629(10)
2.585(12)
2.828(12)
2.586(12)
2.817(14)
2.149(11)
2.316(13)
2.316(13)
2.588(10)
Sm(1)-N(1)-N(1A) 167.0(13) N(1)-Sm(1)-N(3) 80.8(4)
N(2)-Sm(1)-N(3)
66.3(4)
Compound 4
Sm(1)-N(1)
2.583(6)
2.670(6)
4.042
2.708(6)
2.771(6)
2.481(6)
2.392(6)
Sm(2)-N(5)
Sm(2)-N(7)
Sm(3)-N(6)
Sm(3)-N(5)cent
Sm(3)-O(1)
Sm(3)-O(2)
2.544(6)
2.495(6)
2.602(6)
2.581(6)
2.575(6)
2.574(6)
Sm(1)-N(2)cent
Sm(1A)-N(1)cent
Sm(1)-N(3)cent
Sm(2)-N(2)
Sm(2)-N(3)
Sm(2)-N(4)
N(2)-Sm(2)-N(4)
N(2)-Sm(2)-N(7)
84.1(2)
N(4)-Sm(2)-N(5) 159.5(2)
78.6(19) O(1)-Sm(3)-O(2)
N(6)-Sm(3)-O(2)
77.3(2)
N(3)-Sm(2)-N(7) 155.2(2)
77.36(2)
N(3)-Sm(2)-N(4)
74.4(2)
forming a planar SmII macrocyclic complex. Each sa-
marium atom is both σ-bonded and π-bonded to two
pyrrolyl rings of the same ligand (Sm(1)-N(1) ) 2.617-
(10) Å, Sm(1)-N(2)cent ) 2.582(10) Å) that in turn are
also σ- and π-bonded to the next Sm center. The
presence of four THF molecules coordinated to every
second metal atom (Sm(2)-O(1) ) 2.599(10) Å) gives
rise to two sets of samarium atoms. The samarium
atoms that bear the THF molecule adopt a distorted-
trigonal-bipyramidal conformation with the solvent’s
donor atom and two π-bonded pyrrolyl centroids occupy-
Descr ip tion of Cr ysta l Str u ctu r es
Complex 1 (Figure 1) is formed by eight samarium
atoms bridged by eight diethyldipyrrolylmethane ligands,
(14) Blessing, R. Acta Crystallogr. 1995, A51, 33.

A Mixed-Valence Samarium Tripyrrole Complex
Organometallics, Vol. 22, No. 18, 2003 3745
F igu r e 3. Partial thermal ellipsoid plot of 3. Thermal
ellipsoids are drawn at the 30% probability level.
F igu r e 1. Partial thermal ellipsoid plot of 1. Thermal
ellipsoids are drawn at the 30% probability level.
bearing one coordinated molecule of THF (Sm(1)-
O(1) ) 2.525(3) Å). These metals display a distorted-
pseudo-octahedral geometry (O(1)-Sm(1)-N(1) ) 145.56-
(13)°, N(4)-Sm(1)-N(3)cent ) 170.5(14)°, N(2)-Sm(1)-
N(5)cent ) 170.3(14)°) defined by one of the two nitrogen
atoms of the N2 unit, the O atom of THF, the two N
atoms of two σ-bonded pyrrolyl rings, and the two
centroids of the two π-bonded rings. The second pair of
samarium atoms are bound in a side-on fashion to the
dinitrogen molecule and have slightly longer Sm-N
bond lengths (Sm(2)-N(1) ) 2.328(3) Å).
Complex 3 is similar to complex 1 (Figure 3), the main
difference being the different substituents of the dipyr-
rolide dianion and the presence of coordinated DME
replacing the two THF molecules. Again, the nitrogen-
nitrogen distance (N(1)-N(1A) ) 1.42(2) Å) indicates
four-electron reduction of the dinitrogen molecule. The
major differentiation with 1 resides in the different
bonding mode adopted by the ligand system. Two of the
methylphenyldipyrrolylmethane ligands bear a pyrrole
ring that does not form the expected σ and π bonds but
has a nitrogen atom that bridges two contiguous sa-
marium centers. These nitrogen atoms do not interact
equally between both metal atoms (Sm(1)-N(2) ) 2.585-
(12) Å, Sm(2)-N(2) ) 2.828(12) Å) and form a shorter
σ-bond with one of the two metal centers.
The unit cell of complex 4 consists of two crystallo-
graphically independent but chemically equivalent units
(Figure 4). Each molecule is composed by six samarium
atoms forming a symmetry-generated sinusoidal type
of array. The three independent samarium atoms are
bridged by three different pyrrole-based ligands. The
first ligand is an “N-confused” 15 methylphenyldipyrro-
lylmethane, where one of the pyrrole rings is attached
to the central carbon by using the â position instead of
the R position. This ligand acts as a tetradentate
F igu r e 2. Thermal ellipsoid plot of 2. Thermal ellipsoids
are drawn at the 30% probability level.
ing the equatorial positions and the two N atoms of the
σ-bonded rings occupying the axial positions (N(2)-Sm-
(2)-N(3) ) 62.7(3)°, O(1)-Sm(2)-N(2) ) 77.3(3)°,
N(4)cent-Sm(2)-N(2)
) 101.8(3)°, N(1)cent-Sm(2)-
N(2) ) 85.1(3)°). The other samarium atoms, which are
not coordinated to solvent molecules, adopt a severely
distorted-pseudo-tetrahedral geometry defined by the
σ-bonded N atoms and the centroids of the π-bonded
rings [N(2)cent-Sm(1)-N(1) ) 84.9(3)°, N(2)cent-Sm(1)-
N(16) ) 111.9(3)°, N(2)cent-Sm(1)-N(15)cent ) 126.8(3)°).
Complex 2 is comprised of four samarium atoms,
bridged by four diethyldipyrrolylmethane ligands, that
are coplanar with a dinitrogen molecule (N(1)-N(1A)
) 1.415(4) Å (Figure 2). Each samarium is both σ-bond-
ed and π-bonded to the pyrrolyl rings of two adjacent
dipyrrole ligands. Each ligand acts as a tetradentate
dianion, with each pyrrole ring being both σ-bonded to
one samarium (Sm(1)-N(2) ) 2.698(4) Å) and π-bonded
to another adjacent metal (Sm(1)-N(3)cent ) 2.693(4)
Å). Two of the samarium atoms are end-on bonded to
the dinitrogen molecule (Sm(1)-N(1) ) 2.145(3) Å), each
(15) (a) Depraetere, S.; Smet, M.; Dehaen, W. Angew. Chem., Int.
Ed. 1999, 38, 3359 and references therein. (b) Gale, P. A.; Sessler, J .
L. Chem. Commun. 1998, 1 and references therein. (c) Sessler, J . L.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1348. (d) Furuta, H.; Asano,
T.; Ogawa, T. J . Am. Chem. Soc. 1994, 116, 767. (e) Chmielewski, P.
J .; Latos-Grazynski, L.; Rachlewicz, K.; Glowiak, T. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 779. (f) Liu, B. Y.; Bruckner, C.; Dolphin, D.
J . J . Chem. Soc., Chem. Commun. 1996, 2141.

3746 Organometallics, Vol. 22, No. 18, 2003
Be´rube´ et al.
Sch em e 1
F igu r e 4. Partial thermal ellipsoid plot of 4. Thermal
ellipsoids are drawn at the 30% probability level.
dianion, being σ-bonded to two different samarium
atoms (Sm(1)-N(1) ) 2.583(6) Å, Sm(2)-N(2) ) 2.464-
(6) Å), π-bonded to one of these two metal atoms (Sm-
(1)-N(2)cent ) 2.670(6) Å), and also π-bonded to another
samarium from the symmetry-generated unit (Sm(1A)-
N(1)cent ) 4.042 Å). The second ligand is a normal
methylphenyldipyrrolylmethane, and it is similar to the
previous ligand in its bonding mode, with both pyrrole
rings σ-bonded to the same samarium (Sm(2)-N(3) )
2.481(6) Å, Sm(2)-N(4) ) 2.392(6) Å) and forming
π-bonds to the next samarium atom (Sm(1)-N(3)cent
)
2.708(6) Å). The third ligand is an unprecedented
trianionic bis(phenylmethyl)tripyrrolide acting as a
hexadentate ligand. The two terminal pyrrolyl rings are
σ-bonded to the same samarium (Sm(2)-N(5) ) 2.544-
(6) Å, Sm(2)-N(7) ) 2.495(6) Å) and also form π-bonds
to the Sm(3) atom (Sm(3)-N(5)cent ) 2.581(6) Å). The
central pyrrole ring forms a σ-bond to the third sa-
marium atom (Sm(3)-N(6) ) 2.602(6) Å) while π-bond-
ing to the Sm(2) atom (Sm(2)-N(6)cent ) 2.514(6) Å).
The Sm(1) atom is thus π-bonded to four pyrrole rings
and σ-bonded to a fifth. The metal adopts a highly
distorted trigonal bipyramidal geometry where the
nitrogen atom of the σ-bonded rings of the N-confused
ligand occupies one of the two axial positions. The
second axial position is occupied by the centroid of
another bridging pyrrolide ring. The three equatorial
positions are defined by the centroids of the three
π-bonded rings from the three ligands surrounding the
metal (N(1)-Sm(1)-N(4)cent ) 167.0(2)°, N(1)-Sm(1)-
N(1A)cent ) 94.8(2)°, N(1)-Sm(1)-N(2)cent ) 83.4(2)°,
N(1)-Sm(1)-N(3)cent ) 103.9(2)°). The Sm(2) atom is
distorted octahedral, being σ-bonded to one axial and
four equatorial pyrrolyl rings. The centroid of a sixth
π-bonded pyrrole ring occupies the second axial position
(N(2)-Sm(2)-N(6)cent ) 164.1(2)°, N(4)-Sm(2)-N(5) )
159.5(2)°, N(3)-Sm(2)-N(7) ) 155.2(2)°). Finally, the
Sm(3) atom is σ-bonded to a single pyrrole nitrogen,
π-bonded to two pyrrole rings, and coordinatively bound
by two THF molecules (Sm(3)-O(1) ) 2.575(6) Å, Sm-
(3)-O(2) ) 2.574(5) Å), which confers a distorted-
trigonal-bipyramidal geometry to the metal center
(N(6)-Sm(1)-O(1)cent ) 174.2(2)°, N(6)-Sm(1)-O(2)cent
) 99.9(2)°, N(6)-Sm(3)-N(5)cent ) 83.4(2)°, N(6)-Sm-
(3)-N(7)cent ) 81.1(2)°).
at room temperature and under Ar, affording rapid
precipitation of divalent 1 as a dark red microcrystalline
powder (Scheme 1). Suitable crystals for X-ray analysis
were grown upon carrying out reactions in dilute
solutions and allowing the reaction flask to rest in a
vibration-free environment for 5 days. Compound 1 is
an octameric macrocycle (Figure 1) formed by eight
divalent samarium dipyrrolide units. The magnetic
moment is nearly as expected for eight divalent sa-
marium centers, thus indicating that there is minimal
magnetic exchange within the ring.
Exposure of 1 to nitrogen gas or more simply carrying
out the above preparation directly under an N2 atmo-
sphere afforded large red crystals of the dinitrogen
complex [{[(C2H5)2C(R-C4H3N)2]Sm}4(THF)2](µ-N2)]‚2THF
(2) in good yield (Scheme 1). The long N-N distance
(N(1)-N(1A) ) 1.415(4) Å) showed by the X-ray crystal
structure (Figure 2) indicated that the dinitrogen mol-
ecule has undergone a four-electron reduction. In turn,
this implies that each metal center was oxidized to the
trivalent state. Accordingly, the coordination of N2 was
irreversible, with the complex remaining unchanged
when exposed to vacuum at high temperature. As a
feature common to the other very similar dinitrogen
complexes of other dipyrrolides previously reported,7
complex 2 displays poor solubility in the most common
solvents.
Use of the methylphenyldipyrrolylmethane ligand and
the usual reaction conditions afforded neither the dini-
trogen nor the macrocyclic complex but yielded instead
intractable compounds. However, the same reaction in
a small volume of anhydrous DME gave the dinitrogen
complex 3 as red crystals. This compound is very similar
to 2 as far as the Sm4N2 core (Figure 3) and fits in what
turns out to be a trend in the behavior of divalent Sm
dipyrrolide compounds.7 However, there are slight
Resu lts a n d Discu ssion
The trans-amination reaction of alkali-metal- and
halogen-free Sm{N(SiMe3)2}2(THF)2 with diethyldipyr-
rolylmethane was performed in a small volume of THF

A Mixed-Valence Samarium Tripyrrole Complex
Organometallics, Vol. 22, No. 18, 2003 3747
Sch em e 2
variations in the bonding mode of the ligand. Whether
these variations are due to change in the solvent
denticity (DME versus THF) or to ligand backbone
asymmetry is unclear. Attempts to reproduce the diva-
lent octacyclic samarium precursor were unsuccessful
with this particular ligand system.
to approximately 10% of the dipyrromethane methyl
peak at 1.79 ppm. Because of the resemblance of all the
methylphenyl-polypyrrolide ligands, this was found to
be the only evidence for the presence of unwanted
analogues besides TLC separation. Attempts to perform
chromatographic separation have been unsuccessful so
far. In any event, sublimation of the ligand eliminated
the impurities and systematically gave clean formation
of 3 without the presence of contaminants. Therefore,
the formation of 4 may be attributed only to the ability
of divalent samarium to scavenge the impurities pro-
duced during the formation of the particular meth-
ylphenyldipyrrolylmethane. This is still puzzling though,
given that the presence of 4, although a minor product,
was rather substantial in the crystalline mass. Fur-
thermore, the partial oxidation of the metal center
during this reaction remains unexplained.
When the reaction affording 3 was carried out with
recrystallized (nonsublimed) methylphenyldipyrrolyl-
methane, two distinct types of crystals were formed
upon reducing the volume of the reaction mixture,
diluting with DME, and layering with hexanes. The
major component consisted of small reddish brown
crystals that, after manual separation under a stereo-
microscope, were rapidly identified as compound 3 by
the identity of their IR spectrum with that of an
analytically pure sample. The minor component con-
sisted of large red hexagonal prisms that were invari-
ably contaminated by 3 but were still of sufficient
quality to be analyzed by X-ray crystallography (Figure
4). Complex 4 is very unusual and interesting (Scheme
2). Its hexameric structure with a sinusoidal arrange-
ment of the metals clearly shows the presence of three
different ligand systems. In addition to the presence of
two normal ligands, the complex contains two “N-
confused”15 dipyrrolide units: i.e., a dipyrrolide where
the attachment of one of the two pyrrolyl rings was
switched from the R- to the â-position. The last consists
of two attractive tripyrrole moieties, thus making 4 the
first metal complex of a tripyrrolide ligand. Regrettably,
purification or isolation of the crystalline 4 in analyti-
cally pure form proved unfeasible, and therefore no
other analytical determination could be carried out. This
is even more unfortunate, given that the complex is a
mixed-valence Sm(II)/Sm(III) species.
Con clu sion
Two new dinitrogen complexes from the reactions of
divalent samarium and dipyrrolides have been prepared
and characterized, as well as the octameric cyclic cluster
precursor of one of them. Complexes 1-3 are in line
with the behavior of other dipyrrolide ligands,8-10 and
no evidence was found that this ligand system may
support N-N cleavage, despite assembling large octa-
nuclear clusters. However, the serendipitous isolation
of 4 indicates that N-confused dipyrrole and tripyrroles
may also be versatile and exciting ligand systems for
assembling noncyclic cluster structures. Further work
to rationally prepare complexes of these new ligand
systems is in progress at the moment.
Ack n ow led gm en t. This work was supported by the
Natural Sciences and Engineering Council of Canada
(NSERC).
Questions arise about the generation of both the
tripyrrole and the N-confused ligand. Although the
mixed valence of 4 might suggest that some complex
redox transformation could be at the origin of their
formation, a careful inspection of the 500 MHz 1H NMR
of the ligand used for the reaction revealed the presence
of two unexpected ligands in the starting material. Two
small peaks between 1.77 and 1.78 ppm were integrated
Su p p or tin g In for m a tion Ava ila ble: Listings of atomic
coordinates, thermal parameters, and bond distances and
angles for structures 1-4. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM030155Q