κ2-Coordination of 18-crown-6 to Ce(iii) cations: Solution dynamics and reactivity

Article abstract of DOI:10.1039/c4cc00448e

The coordination of 18-crown-6 to Ce[N(SiMe₃)Ph ^F]₃ (Ph^F = pentafluorophenyl) results in a κ²-18-crown-6 complex, a unique coordination mode for an f-block cation. The κ²-18-crown-6 complex showed exchange with free 18-crown-6 in solution and facile rearrangement of the crown ligand into a κ⁶-18-crown-6 cerium complex.

Full text of DOI:10.1039/c4cc00448e

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j2-Coordination of 18-crown-6 to Ce(III) cations:
solution dynamics and reactivity†
Cite this: Chem. Commun., 2014,
50, 3470
Haolin Yin, Jerome R. Robinson, Patrick J. Carroll, Patrick J. Walsh and
Eric J. Schelter*
Received 18th January 2014,
Accepted 9th February 2014
DOI: 10.1039/c4cc00448e
www.rsc.org/chemcomm
The coordination of 18-crown-6 to Ce[N(SiMe3)PhF]3 (PhF = penta-
fluorophenyl) results in a j2-18-crown-6 complex, a unique coordina-
tion mode for an f-block cation. The j2-18-crown-6 complex showed
exchange with free 18-crown-6 in solution and facile rearrangement of
the crown ligand into a j6-18-crown-6 cerium complex.
The coordination chemistry of f-block cations with macrocyclic
polyethers (MCPE) is important historically in f-block chemistry and
has drawn attention for the development of new extractants, NMR
shift reagents, and model complexes for natural ionophores.1 As a
readily available MCPE prototype, 18-crown-6 exhibits larger stability
constants in forming 1 : 1 lanthanide complexes than its smaller or
acyclic counterparts. The log b1 is 8.75 for La3+ with 18-crown-6
in anhydrous propylene carbonate, compared to 6.38 and 5.30 with
15-crown-5 and 18-podand-6, respectively.2 The flexibility of the
polyether moiety allows 18-crown-6 to bind lanthanide cations
throughout the series exclusively in the k6-coordination mode.1a,3
Previous work by Lappert and co-workers on the coordination
chemistry of 18-crown-6 with amide- and cyclopentadienide-
lanthanide complexes demonstrated that diverse coordination
chemistry could be achieved by varying the steric bulkiness of
the lanthanide supporting ligands, as well as the size of the
cations.3f,g Herein, we report a divergent coordination chemistry
Scheme 1 Synthesis of k2- and k6-coordinated 18-crown-6 complexes
2 and 3, transformation from 2 to 3, and substitution reaction of 2.
of 18-crown-6 with Ce[N(SiMe3)PhF]3 (ꢀPhF = pentafluorophenyl) species could be achieved through multiple labile C–F-M
(Scheme 1), featuring the first example of an 18-crown-6 moiety interactions (M = Ln, U), even for ligands with relatively low
k2-coordinated to an f-block cation. We also show that the steric demand. This type of C–F-M interaction has been
k2-18-crown-6 cerium(III) complex has rich solution behaviour; observed crystallographically for electrophilic metal complexes.6
k2-coordination of the 18-crown-6 provides a kinetic product, The displacement of such weak C–F-M interactions leads to
which can be readily converted to k6-coordination through unconventional coordination chemistry including the binding of
ligand substitution with choice of solvent.
neutral arene molecules in the solid state4a and unusual coordina-
Previous work by our group4 and others5 on the coordination tion geometries.4 We have also shown that C–F-M interactions
chemistry of fluorinated ligands revealed that monometallic and substrate binding can be readily identified with solution
19F and H NMR spectroscopy using paramagnetic Ce(III) (4f1)
1
cations.4a In the current work, we have reduced the number of
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of
Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, USA.
labile Ce–F interactions and increased the steric congestion
E-mail: schelter@sas.upenn.edu
around the metal center to achieve a more crowded coordina-
† Electronic supplementary information (ESI) available: Full synthetic details,
tion sphere at the cerium cation by using {N[(SiMe3)PhF]}ꢀ
NMR data. CCDC 978666–978670. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4cc00448e
ligands instead of [N(PhF)2]ꢀ ligands.4,5b
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Fig. 1 Thermal ellipsoid plot of 1 at the 30% probability level.
Fig. 2 Thermal ellipsoid plot of 2 at the 30% probability level.
Stirring a concentrated pentane solution of Ce[N(SiMe3)2]3
with HN(SiMe3)PhF for 1 week led to the precipitation of white TiCl4(k2-18-crown-6), where multiple proton resonances are
Ce[N(SiMe3)PhF]3 (1) in 83% yield. An X-ray diffraction study on suggestive of tightly coordinated 18-crown-6 to the smaller and more
the colorless crystals obtained from a cold n-pentane solution Lewis acidic Ti4+ cation.9f A variable temperature NMR experiment
of 1 revealed a monomeric nine-coordinate cerium(III) amide was performed on 2 in toluene-d8 from 300–200 K and decoalescence
structure with three Ce–N distances averaging 2.397(2) Å, three of the coordinated 18-crown-6 proton resonances was observed
Ce–F contacts averaging 2.6248(16) Å and three Ceꢁ ꢁ ꢁMe close at 240 K; decoalescence of 19F resonances due to C–F-Ce
contacts (Ce–C distances averaging 3.084 Å) (Fig. 1). The interactions was observed at 210 K (Fig. S8 and S9, ESI†).
compound is isostructural with Sm[N(SiMe3)PhF]3 reported by
Storing a C6D6 solution of 2 at room temperature led to
Watkin and co-workers.5b Highly shifted proton resonance from gradual appearance of two new sets of pentafluorophenyl ring
the –SiMe3 group at ꢀ9.15 ppm, compared to Ce[N(SiMe3)2]3 at signals as well as two sets of –SiMe3 proton resonances in a 1 : 2
1
ꢀ3.10 ppm in C6D6,7 and broad ortho-F resonance at ꢀ173.09 ppm, ratio over several hours by 19F and H NMR spectroscopies. Facile
implicated their proximity to the paramagnetic Ce(III) ion in conversion to the new product was accomplished upon treating 2
solution. Variable temperature data (Fig. S6 and S7, ESI†) with polar solvents, including tetrahydrofuran, dimethoxyethane,
collected in toluene-d8 showed at least eight 19F resonances pyridine, or DCM (Scheme 1). X-ray analysis of colorless crystals
below the coalescence temperature, which likely arose from obtained from thf–hexanes layering determined the product 3
an interplay between multiple C–F-Ce interactions4,6 and to comprise charge separated cations and anions in the form
Si–g-C–H - Ce agostic interactions.8
of {Ce(k6-18-crown-6)[N(SiMe3)PhF]2}+ and {Ce[N(SiMe3)PhF]4}ꢀ
Reaction of 1 with 18-crown-6 in n-pentane, toluene, or (Fig. 3). No C–F-Ce interactions were observed in the solid-state
+
diethyl ether produced its 18-crown-6 adduct 2 as indicated structure for the cationic moiety, Ce(k6-18-crown-6)[N(SiMe3)PhF]2 .
by NMR spectroscopy. A crystallographic study on 2 revealed The sharp doublet attributed of the ortho-F atom of the cation in
that the 18-crown-6 moiety was coordinated to the metal cation its 19F NMR spectrum also supported the absence of C–F-Ce
in a k2-fashion with the associated ethylene moiety folded back
into the ring cavity (Fig. 2). The k2-coordination of 18-crown-6
has been documented with small metal cations,9 for example
MCl3(H2O)(k2-18-crown-6) (M = V, Cr),9a,b MCl4(k2-18-crown-6)
(M = Ti, Sn),9c (TiF4)2(k2-18-crown-6),9d [PPh4][(VCl4)2(k2-18-
crown-6)],9b [In(k2-18-crown-6)I2][InI4],9e but not for f-block
cations with larger ionic radii and more available binding sites.
The Ce3+ cation, at 101 pm, is comparable to Na+ at 102 pm.10
This unexpected k2-coordination mode can be attributed to the
interplay between steric hindrance of the N[(SiMe3)PhF]ꢀ
ligands in 1 and their masking of the CeIII ion by weak
C–F-Ce interactions.
The solution 19F spectrum of 2 showed three resonances at
room temperature indicating a symmetric environment for the
amide ligands on the NMR timescale. Only one broad symmetric
proton resonance was observed for the bound 18-crown-6
Fig. 3 Thermal ellipsoid plot of the anionic (left) and cationic (right)
fragments of 3 at the 30% probability level (left: CeIII[N(SiMe3)PhF]4ꢀ, right:
molecule at ꢀ1.30 ppm, implicating a fluxional process at the
bound ether moiety. This is in contrast to the observation in CeIII(k6-18-crown-6)[N(SiMe3)PhF]2+).
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interactions in solution. On the other hand, the {CeIII[N(SiMe3)PhF]4}ꢀ Award No. DE-SC0006518. We also thank the University of
anion exhibits two C–F-Ce interactions in the solid state at an Pennsylvania and the Research Corporation for Science
average of 2.680(2) Å. These are evident by the broad ortho-F Advancement (Cottrell Scholar Award to E.J.S.) for financial
resonance in the solution 19F NMR spectrum (Fig. S11, ESI†).
support and the NSF for support of the X-ray diffractometer
Complex 3 could also be directly prepared by reacting (Grant CHE-0840438).
0.5 equiv. of 18-crown-6 with 1 in THF, DME, pyridine, or
Notes and references
DCM in near quantitative yield (Scheme 1). In contrast, 1 reacts
1
with neat DME to form a 1 : 1 adduct, as judged by H and 19F
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We have demonstrated the presence of C–F-Ce interactions
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3472 | Chem. Commun., 2014, 50, 3470--3472
This journal is ©The Royal Society of Chemistry 2014