Group 3 metal stilbene complexes: Synthesis, reactivity, and electronic structure studies

Article abstract of DOI:10.1039/c3cc47505k

Group 3 metal (E)-stilbene complexes supported by a ferrocene diamide ligand were synthesized and characterized. Reactivity studies showed that they behave similar to analogous naphthalene complexes. Experimental and computational results indicated that the double bond was reduced and not a phenyl ring, in contrast to a previously reported uranium (E)-stilbene complex. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc47505k

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Group 3 metal stilbene complexes: synthesis,
reactivity, and electronic structure studies†
Cite this: Chem. Commun., 2014,
50, 5221
Wenliang Huang, Paul M. Abukhalil, Saeed I. Khan and Paula L. Diaconescu*
Received 1st October 2013,
Accepted 15th November 2013
DOI: 10.1039/c3cc47505k
www.rsc.org/chemcomm
Group 3 metal (E)-stilbene complexes supported by a ferrocene La, Lu, x = 1), contain a naphthalene dianion bridging the two
diamide ligand were synthesized and characterized. Reactivity metal centers through different phenyl rings. The metal biphenyl
studies showed that they behave similar to analogous naphthalene complexes, with the general formula ([(NNTBS)M](m-Z6:Z6-C6H5Ph))-
complexes. Experimental and computational results indicated that [K(solvent)]2 (M2K2-biph, M = Sc, Y, La, Lu, solvent = toluene,
the double bond was reduced and not a phenyl ring, in contrast to a tetrahydrofuran, diethyl ether, or 18-crown-6), contain a biphenyl
previously reported uranium (E)-stilbene complex.
tetraanion bridging the two rare-earth centers through the same
phenyl ring. While the negative charges in M2-naph are equally
Group 3 metals (scandium, yttrium, lanthanum, and lutetium) distributed over the naphthalene fragment, the four electron
usually show chemistry representative of all rare earths and their reduction is mainly localized on the coordinating phenyl ring
compounds are easier to characterize because of their diamag- in M2K2-biph and results in a 6C,10p-electron aromatic system.
netic nature.1,2 Their classification with lanthanides rather than DFT calculations on the naphthalene and biphenyl complexes
with transition metals is supported by the fact that, with few showed p overlap for the former and d overlap for the latter
exceptions,2 their complexes contain the metal in the +3 oxida- between the metal orbitals and arene p* orbitals.
tion state, while transition metals display multiple oxidation
This bonding dichotomy is in sharp contrast to diuranium
states. Recently, our group reported the synthesis of group 3 inverse sandwich arene complexes of biphenyl, p-terphenyl,
metal naphthalene3–5 and biphenyl complexes6 supported by a naphthalene, and (E)-stilbene supported by a ketimide ligand
ferrocene diamide ligand (NNTBS = 1,10-fc(NSitBuMe2)2)7 (Chart 1a). (Chart 1b).8 Despite the different nature of the arene, the
The metal naphthalene complexes, with the general formula resulting complexes shared analogous electronic structures,
[(NNTBS)M(THF)x](m-Z4:Z4-C10H8) (M2-naph, M = Sc, x = 0; M = Y, featuring d overlap between LUMOs of one phenyl ring and
the two uranium centers. The related diuranium inverse
sandwich benzene or toluene complexes have been syn-
thesized using various ancillary ligands and present a similar
bonding character.9–14 As mentioned, the d interaction is also
present in M2K2-biph,6 but to a lesser extent than in the
uranium compounds. Since group 3 metal arene complexes
supported by the ferrocene diamide ligand showed discrepancy
in the binding mode of naphthalene and biphenyl, we became
interested in synthesizing the corresponding (E)-stilbene com-
plexes; the presence of multiple sites for reduction and bind-
ing, i.e., the CQC bond and the phenyl rings, will offer insight
into the binding preference for rare-earths in comparison with
Chart 1 (a) Rare-earth arene complexes supported by a ferrocene diamide
uranium.
In spite of the abundance of rare-earth naphthalene com-
ligand; (b) uranium arene complexes supported by a ketimide ligand.
plexes in the literature,1,15,16 (E)-stilbene complexes are rare.
Evans et al. reported the synthesis of [(C5Me5)2Sm]2((E)-stilbene)
from (C5Me5)2Sm and (E)-stilbene.17 The structure of [(C5Me5)2Sm]2-
((E)-stilbene) was established based on connectivity data derived
from X-ray crystallography; however, the poor quality of the data
Department of Chemistry and Biochemistry, University of California, Los Angeles,
California 90095, USA. E-mail: pld@chem.ucla.edu
† Electronic supplementary information (ESI) available: Synthesis, characteriza-
tion, NMR spectra, X-ray crystallographic data and DFT calculations. CCDC
963318–963320, 963896, 963897 and 969772. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c3cc47505k
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Fig. 2 Molecular structure of Y-stilbene-K-crown. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms and disordered
counterparts were omitted for clarity.
Scheme 1 Synthesis of M2-stilbene, Y-stilbene-K, and transformation of
Y2-stilbene to Y-stilbene-K.
prevented an accurate interpretation of the structural para-
meters. The analogous samarium styrene and butadiene com-
plexes showed two electron reduction of the CQC bond and
concomitant oxidation of Sm(II) to Sm(III).18 Related yttrium
and lutetium complexes of the readily available tetraphenyl-
ethylene dianion have been reported with similar structural
features.19,20
but rather exclusion of one (NNTBS)Y(THF) fragment to form
Y-stilbene-K (Scheme 1). In addition, Y-stilbene-K could be
generated from the reaction of Y2K2-biph and (E)-stilbene (see
the ESI† for details). Attempts to obtain single crystals of
Y-stilbene-K were not successful due to incorporation of labile
potassium ions. However, by using 18-crown-6, single crystals
of [(NNTBS)Y(THF)](E-stilbene)[K(18-crown-6)] (Y-stilbene-K-crown)
were grown from a hexane solution and its molecular structure
was determined by X-ray crystallography (Fig. 2).
Addition of 1.25 equiv. KC8 to a pre-mixed THF solution of
(NNTBS)YI(THF)2 and 0.5 equiv. (E)-stilbene at ꢀ78 1C resulted in an
immediate color change to dark green (Scheme 1). After stirring at
1
0 1C for 1 h, the color gradually changed to red. The H NMR
Disorder in the molecular structures of M2-stilbene (M = Y, La)
is caused by flipping the central C–C bond; the resulting two
conformations were solved separately and only one is shown in
Fig. 1 (see the ESI† for details and additional parameters). Since
they are isostructural, Y2-stilbene will be discussed as a represen-
tative. The bridging (E)-stilbene ligand coordinates equally to the
two yttrium centers in an Z3-fashion through the central C–C
bond and one ipso-carbon, with Y–C distances of 2.60, 2.58, and
2.74 Å, respectively. An additional contact between Y and one
ortho-carbon of 2.99 Å is also present but is longer than the sum
of the covalent radii of yttrium and carbon.21 This symmetrical
coordination mode is different from the asymmetrical coordi-
nation mode suggested for [(C5Me5)2Sm]2((E)-stilbene),17 but
is reasonable with the less sterically demanding NNTBS. The
C1–C1A distance of 1.52 Å is consistent with the single bond
character, while the shortened C1–C3 distances of 1.43 Å and
the elongated C3–C4 distance of 1.46 Å indicate charge deloca-
lization. Y-stilbene-K-crown exhibits a similar coordination
mode for yttrium as Y2-stilbene, while K+ is Z6-coordinated to
one of the phenyl rings (Fig. 2).
spectrum of the crude reaction mixture indicated the formation of
a single product. Crystals suitable for single X-ray diffraction were
grown from a toluene solution layered with hexanes. The molecular
structure (Fig. 1) unambiguously established the formation of a
yttrium (E)-stilbene complex with the formula [(NNTBS)Y(THF)]2-
(m-Z3:Z3-(E)-stilbene) (Y2-stilbene). The analogous lanthanum
complex was synthesized following the same protocol and structu-
rally characterized (see the ESI† for details, Fig. SX4). The synthesis
of M2-stilbene (M = Y and La) mimics that of M2-naph: when excess
KC8 was used, the formation of a heterobimetallic complex,
[(NNTBS)Y(THF)]((E)-stilbene)[K(THF)] (Y-stilbene-K), took place
(Scheme 1). This is different from the synthesis of M2K2-biph:
M2K2-biph was the only observed rare-earth product regardless of
the KC8 stoichiometry employed. It is interesting to note that
adding KC8 to isolated Y2-stilbene did not afford further reduction
Since the synthesis of Y2-stilbene echoed that of M2-naph,
we were interested to determine the relative reducing strength
of Y2-stilbene with respect to that of the other rare-earth arene
complexes. Based on arene exchange experiments (Schemes 2a
and b), we found that the reducing power decreases in the
order Y2-naph > Y2-stilbene > Y2-anth. Compound Y2-anth
([(NNTBS)Y(THF)]2(m-C14H10)) was synthesized from (NNTBS)YI(THF)2
and anthracene following a similar protocol to that reported for
the corresponding scandium complex.5
Fig. 1 Molecular structure of Y2-stilbene. Thermal ellipsoids are drawn at
the 50% probability level. Hydrogen atoms and disordered counterparts
were omitted for clarity.
The arene exchange results prompted us to explore the reactivity
of Y2-stilbene toward organic substrates to compare its behavior
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Fig. 3 HOMO plot for Y2-stilbene (left) and La2-stilbene (right).
In summary, we successfully synthesized group 3 metal
(E)-stilbene complexes through reduction of (E)-stilbene by the (NNTBS)-
MI(THF)x–KC8 system. The resulting complexes, M2-stilbene,
showed similar reactivity to M2-naph complexes. Both experi-
mental and computational data suggest that the reduction takes
place at the CQC bond instead of the phenyl ring. This contrasts
the case of uranium and indicates different binding preferences
for rare-earths that are similar to those of transition metals and
not actinides. Our synthetic route also allows access to rare-earth
alkene complexes, which were previously limited to samarium.23
This work was supported by NSF (CAREER Grant 0847735 to
PLD and CHE-1048804 for NMR spectroscopy). The authors
thank the Kaner group (UCLA) for generous gifts of KC8.
Scheme 2 Relative reducing strength of Y2-stilbene (a, b) and its reactivity
with organic substrates (c, d).
Notes and references
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with that of M2-naph. The reaction of Y2-stilbene and 2,20-bipyridine
(bipy) or phenylacetylene (PhCCH) resembled the reactivity of
Sc2-naph and yielded (NNTBS)Y(THF)(bipy) and [(NNTBS)Y(THF)]-
[(NNTBS)Y](CCPh)2 with the concomitant formation of (E)-stilbene
and bibenzyl, respectively (Scheme 2c and d).5 However, addition
of excess pyridine did not yield the corresponding C–C coupled
complex but rather an intractable mixture of products; no
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´
´
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