f-block-element-f-block-element NMR spin-spin coupling

Article abstract of DOI:10.1002/ejic.200700679

This is a report on the synthesis and structure of novel dinuclear Yb ^II complexes. In one of the described compounds, the two Yb ^II centres are in close proximity and chemically different. Thus, an f-block-element-f-block-element co

Full text of DOI:10.1002/ejic.200700679

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700679
f-Block-Element–f-Block-Element NMR Spin–Spin Coupling
Anna M. Dietel,[a] Oleg Tok,[a] and Rhett Kempe*[a]
Dedicated to Professor Bernd Wrackmeyer on the occasion of his 60th birthday
Keywords: Amido ligands / Bimetallic complexes / Dynamic N ligands / NMR spectroscopy / Ytterbium
This is a report on the synthesis and structure of novel dinu-
clear YbII complexes. In one of the described compounds, the
two YbII centres are in close proximity and chemically dif-
ferent. Thus, an f-block-element–f-block-element coupling
pattern was observed. The low-field 171Yb signal consists of
a central singlet and two satellites with integral intensities of
about 7% each. This 14% approximately corresponds to the
natural abundance of the 171Yb isotope and was assigned as
a doublet arising from 1J(171Yb, 171Yb) spin–spin coupling
with a magnitude of 76.1 Hz. Furthermore, the relevance of
such studies in understanding dimer equilibria and the re-
arrangement chemistry of rather bulky structural ensembles
is discussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
more, in systems undergoing dynamic processes such as
bridge–terminal exchange or monomer–dimer equilibria,
coupling constants will be time-averaged and thus may be
Introduction
The last decade has seen a dramatic increase in the im-
portance of the organometallic chemistry of lanthanide ele-
ments. Solution-state NMR, one of the organometallic
chemist’s most informative tools, has been applied infre-
quently to the direct observation of compounds of f-block
elements. This omission is probably because the majority of
the complexes (except those of LaIII, YbII and LuIII) are
paramagnetic, and many of the NMR-active f-block nuclei
have large quadrupole moments. I71Yb is a spin-1/2 nucleus,
with a natural abundance of 14.31% and a moderately
sized, positive gyromagnetic ratio (4.735ϫ107 radT–1 s–1);
these features combine to give a receptivity about 4.5 times
greater than that of 13C.[1] The first direct high-resolution
NMR observation of an f-block element was reported by
Lawless, Lappert, et al.[2] In principle, 171Yb–171Yb coup-
ling should be observable as well. Dinuclear and polynu-
clear Yb complexes are a well-documented class of com-
pounds. The metal centres in many of these complexes are
close enough (3.5 Å was used as the upper limit)[3] to ob-
serve f-block-element–f-block-element spin–spin coupling.
Complexes that meet this requirement can consist of
1
unobservable.[1] On the other hand, J(171Yb,171Yb) coup-
ling patterns, if observed, could be a highly sensitive probe
on the dynamic aspect of f-block-element–f-block-element
interactions in solution, an area of chemistry difficult to
access by other methods. We started recently a research pro-
gram in which we used very bulky aminopyridinato li-
gands[8] in order to avoid ligand redistribution in low-valent
and organolanthanide chemistry.[9] We report here on the
synthesis and structure of a novel dinuclear YbII complex
in which the two YbII centres are in close proximity and
chemically different within the NMR time scale, as well as
the 171Yb NMR spectroscopy of this species.
Results and Discussion
Bimetallic YbII complexes should be accessible by the in-
troduction of a sterically encumbered extension at an ami-
nopyridinato ligand, which leads to bipyridylamide ligands,
and the nonsymmetrical bulk could lead to homobimetallic
complexes having YbII in different chemical environments.
The “simple” bipyridylamide ligand was already used suc-
cessfully by Müller-Buschbaum to stabilise, for instance,
multinuclear YbIII complexes.[10] The sterically demanding
dipyridylamide ligand precursor 1 was synthesised by Pd-
catalysed aryl amination (Scheme 1) in good yield.
YbIII [4] exhibit mixed oxidation states (II/III)[5] or might
,
be diamagnetic due to the presence of YbII[6,7] ions only.
Nonsymmetrical polynuclear YbII complexes are rare,[6]
and metal centres in different chemical environments are a
prerequisite to observe J(171Yb,171Yb) coupling. Further-
1
[a] Lehrstuhl für Anorganische Chemie II, Universität Bayreuth,
95440 Bayreuth, Germany
E-mail: kempe@uni-bayreuth.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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A. M. Dietel, O. Tok, R. Kempe
SHORT COMMUNICATION
ric bridges (N5 and N8) and one nonsymmetrical (via N2)
bridge. N2 seems to provide the anionic function of the
amido ligand dominantly to Yb1, which is perfectly in ac-
cordance with the facts that the additional neutral ligand
(thf) coordinates Yb1 and the anionic iodido ligand binds
Yb2.
The bimetallic nature of complex 2 in solution was con-
firmed by 171Yb NMR spectroscopy. In C6D6 there are two
signals at 585.6 (broad, ν1/2 = 92.0 Hz) and 646.6 (sharp,
ν1/2 = 8.5 Hz) ppm. The broad signal belongs to the Yb
atom bonded directly to iodine, and the broadening arises
from unresolved 171Yb–127I scalar coupling. The low-field
signal consists of a central singlet and two satellites with an
integral intensity of about 7% each. This 14% approxi-
mately corresponds to the natural abundance of the 171Yb
isotope and was assigned as a doublet arising from
1J(171Yb, 171Yb) spin–spin coupling with a magnitude of
76.1 Hz. The chemical shift for YbII amides ranges from
δ(171Yb) = 614 ppm to δ(171Yb) = 1228 ppm, and the line
width associated with 171Yb resonances of these com-
pounds ranges from ∆ν1/2 = 70 Hz to ∆ν1/2 = 550 Hz.[2]
nJ(171Yb,X) couplings have been observed for a variety of
nuclei. Coupling constants usually cover a large range, for
instance from 8627–9479 Hz (X = 119Sn) and 8254–9058 Hz
(X = 117Sn),[13] 758 Hz (X = 125Te),[14] 233–265 Hz (X =
13C; resulting from alkyl carbon atoms)[15] up to 73 Hz (X
Scheme 1. Synthesis of 1 and 2.
The reaction of 1-H with KH and subsequent treatment
with [YbI2(thf)4][11] leads to the bimetallic complex 2 with
a residual iodido ligand to guarantee asymmetry. The mo-
lecular structure of complex 2 (Figure 1) was investigated
by X-ray crystal structure analysis[12] and NMR spec-
troscopy.
=
31P).[16] Well-resolved 3J(171Yb,19F) and 4J(171Yb,19F)
couplings have been recorded for perfluorophenyl ytter-
bium complexes. Coupling constants range from 13 to
48 Hz.[17]
Figure 1. Molecular structure of 2 (ORTEP plot for all non-C,H-
atoms). Substituents at the pyridine rings are plotted in wire frame
style for clarity. Selected bond lengths [Å] and angles [°]: Yb1–Yb2
3.4200(3), N1–Yb2 2.469(3), N2–Yb1 2.538(3), N2–Yb2 2.640(3),
N3–Yb1 2.611(3), N4–Yb2 2.476(3), N5–Yb1 2.568(3), N5–Yb2
2.578(3), N6–Yb1 2.681(3), N7–Yb2 2.451(4), N8–Yb1 2.604(3),
N8–Yb2 2.622(3), N9–Yb1 2.613(3), O1–Yb1 2.431(3), I1–Yb2
3.0714(4); N5–Yb1–N6 52.19(10), N2–Yb1–N3 53.32(10), N8–
Yb1–N9 52.55(10), N4–Yb2–N5 53.61(10), N7–Yb2–N8 53.58(10),
N1–Yb2–N2 53.28(10).
The two YbII ions in complex 2 are triply bridged by the
amido N atoms of 1, which results in a short metal–metal
distance [3.4200(3) Å]. Shorter YbII–YbII distances have
been observed for carbon- and oxygen-bridged dimers.[5,6]
Figure 2. 171Yb{1H} NMR spectra (69.96 MHz, [D8]thf) of com-
The Yb–Namido bond lengths are indicative of two symmet- plex 2 (A) and complex 3 (B).
Scheme 2. Observed rearrangement reactions of 2 in thf (L = 1).
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Eur. J. Inorg. Chem. 2007, 4583–4586

f-Block-Element–f-Block-Element NMR Spin–Spin Coupling
Although complex 2 is stable in C6D6 for days, the 171Yb
The X-ray crystal structure analysis of 3 revealed[11] it to
NMR spectrum in [D8]thf solution consists of two already be a bimetallic complex in which two deprotonated bipyr-
noted resonances (δ = 585.6 and 646.6 ppm) together with idylamide ligands act as tridentate bridging ligands and two
1
a third signal at δ = 679.8 ppm (ν1/2 = 25.5 Hz). In the H of them as terminal bidentate aminopyridinates (Figure 3).
NMR spectrum at room temperature there is an additional
set of broad signals. At low temperature (from 0 °C and
Conclusion
lower), the signal set which belongs to the monoiodido
complex 2 did not change, but the broad lines were more
narrow and gave two sets (in a ratio of ca. 1:3) where the
chemical shifts of the major set were completely identical
to the 1H NMR spectrum of binuclear complex 3
(Scheme 2). In the 171Yb NMR spectrum of 3 in [D8]thf,
the signal at δ = 681.9 ppm was observed (Figure 2B). This
value is very close to the chemical shift of the third signal
in the spectrum presented in Figure 2A. Both 1H and 171Yb
NMR spectroscopic data suggest that in thf solution com-
plex 2 can dissociate to give cationic compound 2(thf),
where one coordination position is occupied by thf instead
of iodine (Scheme 2). Complex 2(thf) seems not to be stable
and rearranges with the formation of neutral compound 3
and (most likely) dicationic 4. The signal for complex 4 was
not observed because of the low solubility of the complex
(precipitation was observed).
We observed the first f-block-element–f-block-element
coupling pattern in solution and have shown the relevance
of such investigations in understanding dimer equilibria
and rearrangement chemistry of rather bulky structural en-
sembles. Chemical transformations of low-valent lanthanide
species has hitherto been difficult to follow.
Experimental Section
NMR Investigations: The 171Yb NMR spectra were recorded by
using a single-pulse sequence with pulse: approx. 30°, relaxation
delay: 1 s, digital FID resolution: 0.3–0.5 Hz before zero filling and
with proton decoupling during acquisition. The suitable signal-to-
noise ratio was reached after 2–4 h. The chemical shifts are given
relative to 0.171  [Yb(η5-C5Me5)(thf)2] in thf [δ (171Yb) = 0 ppm
for Ξ (171Yb) = 17.499306 MHz].
In order to confirm the identity of complex 3, it was
independently synthesised by the reaction of 2 with the po-
tassium salt of 1-H, generated by treatment of 1-H with
KH (Scheme 3).
Synthesis of Complex 2:
In a glove box, [YbI2(thf)4][18] (2.13 g, 2.98 mmol) and 1.90 g
(3.82 mmol) of 1-K were combined in a flask and thf (100 mL) was
added at 0 °C. The resulting dark red reaction mixture was stirred
at room temperature for 16 h. After separation of the solution from
the precipitated potassium iodide by filtration, the solvent was re-
moved under reduced pressure. The resulting solid was dissolved in
toluene (100 mL) and filtered. The filtrate was reduced to dryness,
and a dark red-black product was obtained. Crystals of 2 were
grown from a concentrated toluene solution. Yield 2.06 g (72%).
M.p. 190 °C. C82H104IN9OYb2·3C4H8O (1921.06): calcd. C 58.77,
H 6.72, N 6.56; found C 58.77, H 6.67, N 6.93. 1H NMR
Scheme 3. Synthesis of 3 [Tipp = tri(isopropyl)phenyl].
3
(250 MHz, C6D6, 323 K): δ = 0.67–1.24 (m, J = 6.7 Hz, 54 H, 22-
H, 23-H, 24-H, 25-H, 26-H, 27-H), 1.43 (br., 4 H, β-CH2, thf), 2.38
3
(sept, 3J = 6.9 Hz, 3 H, 15-H), 2.60 (sept, J = 7.0 Hz, 3 H, 14-H),
3
2.66 (s, 9 H, 21-H), 2.78 (sept, J = 6.7 Hz, 3 H, 13-H), 3.53 (br.,
3
4 H, α-CH2, thf), 6.00 (d, J = 6.7 Hz, 3 H, 19-H), 6.16 (d, 3J =
6.6 Hz, 3 H, H5), 6.83–7.16 (m, 18 H, 3-H, 4-H, 9-H, 11-H, 17-H,
18-H) ppm. 13C NMR (62.9 MHz, C6D6, 298 K): δ = 24.0 (s, C-
22, C-23, C-26, C-27), 24.1 (s, C-22, C-23, C-26, C-27), 24.2 (s, C-
22, C-23, C-26, C-27), 24.3 (s, C-22, C-23, C-26, C-27), 26.1 (s, β-
CH2, thf), 26.8 (s, C-24, C-25), 27.1 (s, C-24, C-25), 29.8 (s, C-13,
C-14), 30.0 (s, C-13, C-14), 34.6 (s, C-15), 67.7 (s, α-CH2, thf), 109.5
(s, C-19), 113.4 (s, C-17), 114.5 (s, C-5), 116.2 (s, C-3), 121.0 (s, C-
9, C-11), 135.8 (s, C-4), 137.5 (s, C-7, C-18), 137.6 (s, C-7, C-18),
146.9 (s, C-8, C-12), 147.1 (s, C-8, C-12), 148.5 (s, C-10), 156.8 (s,
C-2, C-16), 157.4 (s, C-2, C-16), 164.2 (s, C-20), 169.1 (s, C-6) ppm.
171Yb NMR (70 MHz, C4H8O, 298 K): δ = 646.6 [s, 1J(171Yb,
171Yb) = 76.1 Hz, Yb-thf], 585.6 (br., Yb-I) ppm.
Figure 3. Molecular structure of 3 (ORTEP plot for all non-C,H-
atoms). Substituents at the pyridine rings are plotted in wire frame
style for clarity. Selected bond lengths [Å] and angles [°]: Yb1–O1
2.457(6); Yb1–N3 2.462(7), Yb1–N5 2.488(7), Yb1–N1 2.517(8),
Yb1–N4 2.586(8), Yb1–N2 2.607(8), Yb1–N1Ј 2.681(7), Yb1–Yb1Ј
3.8685(11); N1–Yb1–N2 53.6(2), N3–Yb1–N1 53.0(2), N5–Yb1–
N4 52.7(2).
Eur. J. Inorg. Chem. 2007, 4583–4586
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A. M. Dietel, O. Tok, R. Kempe
SHORT COMMUNICATION
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Acknowledgments
We thank the DFG (SPP 1166 “Lanthanide Specific Functionali-
ties in Molecules and Materials”) and the Fonds der Chemischen
Industrie for financial support, as well as Germund Glatz and
Christian Döring for their support in the X-ray lab.
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Received: June 29, 2007
Published Online: September 7, 2007
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Eur. J. Inorg. Chem. 2007, 4583–4586