Monomeric and octameric divalent ytterbium complexes of diphenylmethyl dipyrrolyl dianion

Article abstract of DOI:10.1021/om990798z

Dipyrrolide dianion [Ph₂C(C₄H₃N)₂]^2- readily reacts with YbCl₃(THF)₃ to form a trivalent precursor, which was further reduced by using either lithium or sodium. The result of the reaction was determined by the nature of the alkali metal since octameric anionic clusters and neutral monomeric complexes were isolated in the two cases.

Full text of DOI:10.1021/om990798z

Organometallics 2000, 19, 209-211
209
Mon om er ic a n d Octa m er ic Diva len t Ytter biu m
Com p lexes of Dip h en ylm eth yl Dip yr r olyl Dia n ion
Tiffany Dube, Dominique Freckmann, Sabrina Conoci, Sandro Gambarotta,* and
Glenn P. A. Yap
Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
Received October 7, 1999
Summary: Dipyrrolide dianion [Ph2C(C4H3N)2]2- readily
reacts with YbCl3(THF)3 to form a trivalent precursor,
which was further reduced by using either lithium or
sodium. The result of the reaction was determined by
the nature of the alkali metal since octameric anionic
clusters and neutral monomeric complexes were isolated
in the two cases.
versatile for assembling polynuclear structures via
formation of additional M-N σ-bonds. Given that a low-
valent lanthanide atom may perform only one-electron
redox reactions, the presence of two or more metal
centers in the same molecular frame may be desirable
for performing molecular activation processes through
cooperative attack on the same substrate when avail-
ability of several electrons is required. A preliminary
reactivity study carried out with this particular ligand
system has so far revealed a promising feature in the
chemistry of divalent samarium, having shown the
ability of this ligand to assemble tetranuclear clusters
able to carry out four-electron reduction of dinitrogen.4
Thus, in an attempt to better understand the chemistry
of this particular ligand system and its ability to both
assemble polynuclear structures and tune the redox
properties of low-valent lanthanide complexes, the less
reducing and NMR accessible Yb2+ ion was chosen for
this study. In this paper, we describe the preparation
of an unprecedented octameric cluster and monomeric
complex of divalent Yb of the diphenylmethyl dipyrrolyl
dianion obtained via reduction of an Yb3+ precursor.
In tr od u ction
The unique versatility of the cyclopentadienyl-based
ligand system has been a significant factor in contribut-
ing to the spectacular development experienced by
lanthanide chemistry in the early 1980s. However,
despite the caliber of transformations discovered for low-
valent lanthanide complexes during the last two dec-
ades,1 information concerning the chemical reactivity
of low-valent species supported by different ligand
systems remains relatively scarce and fragmentary.2
Recently, we have been investigating the chemistry of
low-valent samarium supported by diphenylmethyl
dipyrrole3 dianions which bear some electronic and
steric resemblance to bent-metallocene-type ligands.
However, the presence of a hard nitrogen donor atom
in the five-membered rings renders these anions more
Resu lt a n d Discu ssion
Treatment of a solution of the lithium salt of the
diphenyl dipyrromethanyl dianion with YbCl3(THF)3
followed by reduction with metallic lithium afforded
dark-red crystals of {[Ph2C(C4H3N)2Yb]8(µ3-Cl)2}{Li-
(THF)4}2‚10THF (1), which were isolated in 61% yield
(Figure 1). The connectivity of 1, which was elucidated
by an X-ray crystal structure, showed a dianionic cluster
formed by eight [Ph2C(C4H3N)2Yb] units with the eight
metal atoms arranged to form a large macrocyclic
structure. The two negative charges of the octameric
cluster are balanced by two identical cations located in
the interstitial space and unconnected to the cluster.
Both cations consist of two lithium atoms, each solvated
by four molecules of THF. Similar to the tetranuclear
samarium dinitrogen cluster of the same ligand system,4
each set of adjacent ytterbium atoms is bridged by one
dipyrrolide dianion, with the pyrrole rings being in turn
π-bonded to one ytterbium and σ-bonded to a second.
As a result, each ytterbium atom is in an ytterbocene-
like environment defined by two π-bonded pyrrolyl rings
from two ligands. Two chlorides reside within the
framework of eight Yb atoms each bridging a set of three
Yb atoms located at opposite ends of the cavity such that
two ytterbium atoms remain uncoordinated to a chlo-
ride.
(1) (a) Evans, W. J .; Hozbor, M. A. J . Organomet. Chem. 1987, 326,
299, and references therein. (b) Schaverien, C. J . Adv. Organomet.
Chem. 1994, 36, 283, and references therein. (c) Marks, T. J .; Ernst,
R. D. In Comprehensive Organometallic Chemistry; Pergamon: Oxford,
1982; Chapter 21. (d) Evans, W. J . Adv. Organomet. Chem. 1985, 24,
131. (e) Ephritikhine, M. Chem. Rev. 1997, 21933. (f) Hubert-Pfalzgraf,
L. G. New J . Chem. 1995, 19, 727. (g) Ernst, R. D.; Marks, T. J . J .
Organomet. Chem. 1987, 318, 29. (h) Ernst, R. D. J . Organomet. Chem.
1990, 392, 51. (i) Meese-marktscheffel, J . A.; Esser, L.; Schumann, H.
Chem. Rev. 1995, 95, 865.
(2) (a) Takats, J .; Zhang, X. W. Organometallics 1993, 12, 4286. (b)
Xie, Z.; Chiu, K.; Wu, B.; Mak, T. C. W. Inorg. Chem. 1996, 35, 5957.
(c) Takats, J .; Zhang, X.; Lopnow, G. R.; McDonald, R. J . Am. Chem.
Soc. 1995, 117, 7828. (d) Evans, W. J .; Drummond, D. K.; Zhang, H.;
Atwood, J . L. Inorg. Chem. 1988, 27, 575. (e) Reger, D. L.; Knox, S. J .;
Lindeman, J . A.; Leboida, L. Inorg. Chem. 1990, 29, 416. (f) Recknagel,
A.; Steiner, A.; Noltemeyer, M.; Brooker, S.; Stalke, D.; Edelmann, F.
T. J . Organomet. Chem. 1991, 414, 327. (g) Schumann, H.; Keitsch,
M. R.; Winterfeld, J .; Demtschuk, J . J . Organomet. Chem. 1996, 525,
279. (h) Swamy, S. J .; Leobel, J .; Schumann, H. J . Organomet. Chem.
1989, 379, 51 (i) Nief, F.; Ricard, L. J . Organomet. Chem. 1997, 529,
357. (j) Hou, Z.; Fujita, A.; Yoshimura, T.; J esorka, A.; Zhang, Y.;
Yamazaki, H.; Wakatsuki, Y. Inorg. Chem. 1996, 35, 7190. (k) Rabe,
G. W.; Riede, J .; Schier, A. Organometallics 1996, 15, 439. (l) Minhas,
R.; Ma, Y.; Gambarotta, S. Inorg. Chem. 1996, 35, 1866. (m) Song, J .;
Gambarotta, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 2141. (n)
Mashima, K.; Nakayama, Y.; Shibahara, T.; Fukumoto, H.; Nakamura,
A. Inorg. Chem. 1996, 35, 93. (o) Sen, A.; Chebolu, V.; Rheingold, A.
L. Inorg. Chem. 1987, 26, 1821. (p) Rabe, G. W.; Schier, A. Inorg. Chem.
1996, 35, 2680.
(3) (a) Bru¨ckner, C.; Sternberg, E. D.; Boyle, R. W.; Dolphin, D.
Chem. Commun. 1997, 1689, and references therein. (b) Lee, C. H.;
Lindsey, S. Tetrahedron 1994, 50, 11427. (c) Brown, W. H.; Hutchinson,
B. J .; MacKinnon, M. H. Can. J . Chem., 1971, 49, 4018.
(4) Dube´, T.; Conoci, S.; Gambarotta, S.; Yap, G. P. A.; Vasapollo,
G. Angew. Chem., Int. Ed., in press.
10.1021/om990798z CCC: $19.00 © 2000 American Chemical Society
Publication on Web 12/28/1999

210 Organometallics, Vol. 19, No. 2, 2000
Notes
sphere of each sodium atom is completed by the nitrogen
atom of one of the two pyrrole rings π-bonded to the
ytterbium atom.
1
Complex 2 is diamagnetic. The H NMR spectrum is
characterized by three poorly solved bands for the three
nonequivalent protons of the pyrrolyl ring, while the
phenyl groups display two sharp multiplets and one
poorly resolved band. The simplicity of the NMR spec-
tra, showing only one set of pyrrole and phenyl rings
despite the variety of interactions with sodium and
ytterbium observed in the solid-state structure, may be
taken as an indication of fluxional behavior. However,
spectra recorded at -60 °C were basically unchanged
except for a small yet significant broadening of the
spectral lines.
The structural arrangement of the ligand in complex
2 around the Yb and Na atoms is almost identical to
that observed in 1. The difference arises from the fact
that the Yb atom in 2 bears two molecules of ligand.
However, regardless of the stoichiometry employed, 2 is
the only species isolated, and we found no evidence so
far for the formation of higher nuclearity compounds.
As well, NMR spectra of the residues from the dried
mother liquors did not indicate the presence of other
species. This implies that, if the ligand is exclusively
used for the formation of 2, another ytterbium com-
pound should necessarily be present. Unfortunately,
attempts to analyze the composition of the small amount
of insoluble residue obtained from the reduction were
unsuccessful. A stoichiometry ratio of two ligands per
one ytterbium atom optimized the reaction yield to 75%
of isolated crystalline material.
F igu r e 1. Thermal ellipsoid plot of 1. Thermal ellipsoids
are drawn at the 30% probability level. Phenyl groups were
omitted for clarity.
The apparent role of the alkali metal in determining
the result of the reaction is surprising and is exclusively
determined by the nature of the alkali cation. Further-
more, the fact that the ytterbium atom in 2 retains two
ligands even when reactions were carried out in a 1:1
stoichiometric ratio (although yields are significantly
lower in this case) indicates that (1) the dipyrrolyl ligand
cannot act as a regular mononucleating chelating ligand
but works instead as a bi- or polynucleating ligand and
(2) the formation of a metallocenic-type structure around
the lanthanide may be achieved with only two pyrrolyl
rings from two ligands and is a driving force for
determining the stoichiometry and the overall result of
the reaction.
F igu r e 2. Thermal ellipsoid plot of 2. Thermal ellipsoids
are drawn at the 30% probability level.
Complex 1 is a divalent ytterbium species and is
diamagnetic in the solid state. Unfortunately, the
insolubility of this ionic compound in the most common
organic solvents prevented NMR characterization. Fi-
nally, the octameric frame of complex 1 appears to be
rather robust, and no reactivity was observed during
attempts to replace the halogen atoms with more
reactive functions such as hydride and alkyls.
Exp er im en ta l Section
The fact that complex 1 is the result of the formal
addition of two LiCl units to an octameric cluster
prompted us to attempt similar reaction by using
sodium instead of lithium. The reduction reaction using
finely dispersed metallic sodium was carried out in THF
under Ar and afforded dark-red crystals of the mono-
meric {[Ph2C(C4H3N)2]Na (THF)2}2Yb (2) in 75% yield
(Figure 2). The monomeric nature of 2 was elucidated
by an X-ray crystal structure. The complex consists of
a single ytterbium atom π-bonded to two pyrrole rings
of two ligands assuming an overall bent-ytterbocene
arrangement very similar to that observed in complex
1. The second pyrrole ring of each ligand is both
σ-bonded to ytterbium and engaged in π-coordination
with one of the sodium atoms, which are, in turn,
coordinated to two molecules of THF. The coordination
All operations were performed under an inert atmosphere
of a nitrogen-filled drybox or by using standard Schlenk-type
glassware in combination with a nitrogen-vacuum line. Sol-
vents were dried by passing through a column of Al2O3 under
an inert atmosphere prior to use, degassed in vacuo, and
transferred and stored under inert atmosphere. Ph2C(C4H3-
NH)23 and YbCl3(THF)35 were prepared according to literature
procedures. THF-d8 was dried over Na/K alloy, vacuum-
transferred into ampules, and stored under nitrogen prior to
use. NMR spectra were recorded on a Bruker AMX-500
spectrometer using vacuum-sealed NMR tubes prepared inside
a drybox. Infrared spectra were recorded on a Mattson 3000
FTIR instrument from Nujol mulls prepared inside the drybox.
Elemental analyses were carried out using a Perkin-Elmer
Series II CHN/O 2400 analyzer.
(5) Talor, M. D.; Carter, C. P. J . Inorg. Nucl. Chem. 1962, 24, 387.

Notes
P r epar ation of {[P h 2C(C4H3N)2Yb]8(µ3-Cl)2}{Li(THF)4}2‚
Organometallics, Vol. 19, No. 2, 2000 211
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e An a lysis
Resu lts
10THF (1). A solution of MeLi (15.0 mL, 1.4 M, 20.8 mmol)
in diethyl ether was added at room temperature to a solution
of diphenyldipyrromethane (3.1 g, 10.4 mmol) in THF (150 mL)
under Ar. After stirring for 15 min, YbCl3(THF)3 (5.2 g, 10.4
mmol) was added, and the resulting suspension was stirred
for 24 h at room temperature, during which time the color
changed to yellow-orange. The suspension was subsequently
treated with excess metallic lithium (0.1 g, 14.4 mmol). Within
minutes the color began to deepen to red. Stirring proceeded
for 16 h, resulting in a dark-red solution. The mixture was
heated gently to dissolve the red microcrystalline solid that
had accumulated, and a filtration of the hot mixture was
carried out to separate unreacted lithium. The solution was
concentrated to 100 mL and layered with hexanes (15 mL).
After 48 h at room temperature, dark-red blocks of 1 separated
(4.0 g, 0.8 mmol, 61%). IR (Nujol mull, cm-1): 3051(w), 1597-
(w), 1491(m), 1462(s), 1415(m), 1377(s), 1261(m), 1232(w),
1182(w), 1155(s), 1078(m, br), 1038(s), 980(w), 964(w), 918-
(w), 892(m), 850(m), 800(w), 762(s), 742(s), 702(s), 659(w), 637-
(m). Anal. Calcd (Found) for C240H272N16Yb8O18Cl2Li2: C 56.10
(55.92), H 5.34 (5.39), N 4.36 (4.32). Very low solubility in
solvents including THF and pyridine precluded NMR charac-
terization.
1
2
formula
fw
space group
a (Å)
b (Å)
c (Å)
C240H272N16Yb8Cl2Li2O18 C58H64N4Na2O4Yb
5137.84
triclinic, P1h
17.254(4)
19.145(4)
19.659(4)
61.167(5)
84.757(4)
74.114(4)
5465(2)
1100.15
triclinic, P1h
10.758(4)
10.987(4)
24.968(9)
91.809(5)
96.237(5)
116.248(5)
2621(2)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
radiation
(KR, Å)
T (K)
1
2
0.71073
0.71073
233
1.561
3.476
2568
238
1.394
1.850
1128
D
calcd (g cm-3
)
µcalcd (cm-1
)
F000
R, Rw2, GoFa
0.0580, 0.1211, 1.041
0.0518, 0.1021, 1.055
R ) ∑(|Fo| -|Fc|)/∑|Fo|. Rw ) [(∑(|Fo| - |Fc|)2/∑wFo2)]1/2.
a
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg)
1
2
P r ep a r a tion of {[P h 2C(C4H3N)2]Na (THF )2}2Yb (2). A
solution of diphenyldipyrromethane (4.0 g, 13.4 mmol) in
anhydrous THF (100 mL) was treated with NaH (0.6 g, 26.8
mmol), resulting in effervescence. Stirring proceeded for 1 h,
after which time YbCl3(THF)3 (3.3 g, 6.7 mmol) was added.
The resulting pale yellow suspension was stirred for 16 h. After
filtration to remove a small amount of insoluble material and
subsequent addition of finely dispersed, metallic sodium (0.15
g, 6.7 mmol), a vigorous stirring produced a color change to
red after 5 min. After 16 h, the suspension was filtered and
the deep-red filtrate was concentrated to 25 mL. Layering with
hexanes (25 mL) and standing at room temperature for 2 days
resulted in the separation of dark-red plates of 2 (5.5 g, 5.0
mmol, 75%). IR (Nujol mull, cm-1): 3095(w), 3057(w), 2727-
(w), 2684(w), 1597(m), 1491(s), 1462(vs), 1443(s), 1417(m),
1377(m), 1261(w), 1233(w), 1178(w), 1153(w), 1095(w), 1049-
(vs), 978(w), 918(w), 893(m), 850(m), 841(w), 798(w), 766(s),
746(vs), 730(vs), 706(vs), 658(m), 636(s). Anal. Calcd (Found)
for C58H64O4N4Na2Yb: C 63.32 (63.16), H 5.86 (5.82), N 5.09
Yb(1)-Cl ) 2.707(6)
Yb-N(1) ) 2.401(8)
Yb(2)-Cl ) 2.797(6)
Yb-N(2) ) 2.719(8)
Yb(4a)-Cl ) 3.066(6)
Yb-N(3) ) 2.420(8)
Yb-N(4) ) 2.693(8)
Yb(1)-N(1) ) 2.588(17)
Yb(1)-N(2) ) 2.729(16)
Yb(1)-N(3) ) 2.704(16)
Yb(1)-N(4) ) 2.535(17)
Yb(1)-C(5) ) 2.776(19)
Yb(1)-C(7) ) 2.81(2)
Yb-C(5) ) 2.743(10)
Yb-C(6) ) 2.760(9)
Yb-C(7) ) 2.713(9)
Yb-C(8) ) 2.695(9)
Na(1)-N(1) ) 2.624(9)
Na(1)-C(1) ) 2.920(12)
Na(1)-C(2) ) 3.118(12)
Na(1)-C(3) ) 2.869(11)
Na(1)-C(4) ) 2.605(11)
Na(1)-O(1) ) 2.281(9)
Na(1)-O(2) ) 2.278(9)
Na(1)-N(2) ) 2.460(9)
N(1)-Yb-N(3) ) 122.5(3)
O(1)-Na(1)-O(2) ) 90.4(4)
O(1)-Na(1)-N(2) ) 101.0(3)
O(2)-Na(1)-N(2) ) 131.1(4)
Yb(1)-C(8) ) 2.71(2)
Yb(2)-N(3) ) 2.579(16)
Yb(2)-N(4) ) 2.694(16)
Yb(2)-N(5) ) 2.865(16)
Yb(2)-N(6) ) 2.514(15)
Yb(3)-N(5) ) 2.455(16)
Yb(3)-N(6) ) 2.670(16)
Yb(3)-N(7) ) 2.656(15)
Yb(3)-N(8) ) 2.455(16)
Yb(1)-Cl-Yb(2) ) 86.91(16)
Yb(1)-Cl-Yb(4a) ) 83.62(16)
Yb(2)-Cl-Yb(4a) ) 169.8(2)
N(1)-Yb(1)-N(4) ) 154.9(5)
N(1)-Yb(1)-Cl ) 76.8(4)
N(4)-Yb(1)-Cl ) 78.2(4)
Cl-Yb(2)-N(3) ) 81.2(4)
Cl-Yb(2)-N(6) ) 131.7(4)
N(5)-Yb(3)-N(8) ) 123.2(5)
1
(5.07). H NMR (THF-d8, 500 MHz, 60 °C): δ 10.00 (br s, 1H,
pyrrolyl), 7.16 (m, 2H, phenyl), 6.93 (m, 2H, phenyl), 6.68 (br
s, 1H, phenyl), 5.89 (s, 1H, pyrrolyl), 5.71 (br s, 1H, pyrrolyl),
3.58 (br s, THF), 1.73 (br s, THF). 13C NMR (THF-d8, 125.72
MHz, 23 °C): δ 127.35, 104.57 (C-H pyrrolyl), 128.70, 127.35,
125.60 (C-H phenyl), 68.21 (THF coord), 26.37 (THF coord),
149.90, 131.33, 60.02 (quaternary C).
located. However only the ytterbium, nitrogen, and chloride
atoms were refined anisotropically due to the insufficient
number of observations. All hydrogen atoms were treated as
idealized contributions. All scattering factors and anomalous
dispersion factors are contained in the SHEXTL 5.03 program
library (Sheldrick, 1997, WI.). Crystal data are summarized
in Table 1, while relevant bond distances and angles are given
in Table 2.
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.6
Ack n ow led gm en t. This work was supported by the
Natural Sciences and Engineering Council of Canada
(NSERC) and NATO.
Systematic absences in the diffraction data and unit-cell
parameters were uniquely consistent with the reported space
group. The structures were solved by direct methods, com-
pleted with difference Fourier syntheses, and refined with full-
matrix least-squares procedures based on F2. All non-hydrogen
atoms were refined with anisotropic displacement parameters
for complex 2. In the case of 1 all non-hydrogen atoms were
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 1 and 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
(6) Blessing, R. Acta Crystallogr. 1995, A51, 33.
OM990798Z