Organolanthanoid-halide synthons - A new general route to monofunctionalized lanthanoid(ii) compounds?

Article abstract of DOI:10.1039/c0cc01317j

New Eu^II and Yb^II complexes [Ln(Ph ₂pz)I(thf)₄] were synthesized from the corresponding metals and 3,5-diphenylpyrazole (HPh₂pz) using iodobenzene as oxidizing agent. In the absence of the pyrazole, the charge separated Yb ^II/Yb^III complex [{Yb(dme)₄}{YbPh ₄(dme)}₂] has been isolated. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0cc01317j

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Organolanthanoid-halide synthons—a new general route to
monofunctionalized lanthanoid(II) compounds?w
Michal Wiecko,* Glen B. Deacon and Peter C. Junk*
Received 10th May 2010, Accepted 2nd June 2010
First published as an Advance Article on the web 16th June 2010
DOI: 10.1039/c0cc01317j
New EuII and YbII complexes [Ln(Ph2pz)I(thf)4] were synthe-
sized from the corresponding metals and 3,5-diphenylpyrazole
(HPh2pz) using iodobenzene as oxidizing agent. In the absence
of the pyrazole, the charge separated YbII/YbIII complex
[{Yb(dme)4}{YbPh4(dme)}2] has been isolated.
Scheme 1
The +2/+3 mixed valence species formed by the oxidation of
the lanthanoid (Ln) metals Eu, Sm and Yb by organic halides
the desired LnII complexes [Ln(Ph2pz)I(thf)4] (Ln = Eu (1),
such as PhI, MeI or 2,6-Me2C6H3I1 have been investigated in a
number of organic and inorganic transformations.2 In many
cases they show a similar reactivity towards electrophiles and
acids as the well known Mg based reagents3 and are therefore
formulated as Grignard analogues ‘‘RLnI’’.4 However
the thermal lability of the Ln–C s-bond5 has limited
pure, structurally defined products maintaining the Ln–C s
bond to compounds derived from bulky iodides. To our
knowledge, the only crystallographically characterized
examples are the dimeric [Yb{C(SiMe3)2(SiMe2X}I(OEt2)]2
(X = CHQCH2, OMe)6 and both [Yb(2,6-Ph2C6H3)(thf)3I]
and [Eu(2,6-Ph2C6H3)2(thf)2] reported by Niemeyer, who also
described a Schlenk equilibrium in these systems.7 For
calcium, which has much similar chemistry and a comparable
ion size to YbII,8 more success has been achieved with the
isolation and characterization of ArCaX (X = Br, I) and
Ar2Ca type complexes9 through the use of activated Ca metal
and benefiting from the absence of Ca2+ redox chemistry.
Nevertheless, putative ‘‘PhLnI’’ (Ln = Eu, Yb) derivatives are
a potential source of functionalized Ln(L)I (L = monoanionic
ligand) compounds by protolysis reactions. Following initial
results,10 we now report syntheses and structures of
[Ln(Ph2pz)I(thf)4] (Ln = Eu, Yb; Ph2pz = 3,5-diphenyl-
pyrazolate), as well as isolation of a charge separated phenyl-
ytterbium(II/III) complex from reaction of Yb metal with
iodobenzene in 1,2-dimethoxyethane (dme). The former
demonstrates the synthetic potential of these systems in
metal–organic chemistry whilst the latter demonstrates the
mixed oxidation state nature of ‘‘PhYbI’’.
Yb (2))z occurred which were isolated after 12 h (Scheme 1).
Both compounds were isolated in very good yields from the
reaction mixture by filtration from some residual metal and
crystallization from concentrated thf solution. By contrast, on
attempting to obtain YbII and EuII complexes of a general
composition [Ln(L)I(solvent)x], redistribution reactions giving
solvated LnL2 and LnI2 are often observed.11 X-Ray crystallo-
graphic studiesy show that 1 and 2 are isostructural in the solid
state (Fig. 1).
The structures exhibit some surprising features since,
especially for complexes with the larger Eu2+ cation, formation
of halide6,12 or ligand13 bridged dimers is usually favoured
over additional solvent coordination. Such is not the case for 1
and 2. The compounds crystallize in monomeric units with a
high degree of metal centre solvation having four coordinating
thf molecules. The formally seven coordinate complex has a cis
pseudo octahedral arrangement. A monomeric structure is
favoured by steric effects. The sum of the ligand steric
coordination numbers14 is a reasonable 7.5 for 1 and 2,
whereas an iodide bridged eight coordinate dimer would have
a value of 8.5 indicating crowding. The observed Z2-coordination
Sonication of a mixture of Yb or Eu metal and PhI at
ꢀ78 1C in thf results in a quick formation of dark red or red
brown solutions. When 3,5-diphenylpyrazole was added
immediately to this mixture, straightforward formation of
Fig. 1 Solid state structure of 1. Drawn with ellipsoids at 50%
probability and hydrogen atoms omitted. Selected bond distances
[A] and angles [1] for 1 and isostructural 2: 1: Eu–N1 2.5353(17),
Eu–O1 2.5770(14), Eu–O2 2.616(2), Eu–O3 2.558(2), Eu–I 3.1983(2),
N1–Eu–N10 31.58(8), O1–Eu–O10 155.81(7), O1–Eu–O3 86.64(4),
O3–Eu–I 168.42(5), N1–Eu–I 108.60(4). 2: Yb–N1 2.433(3), Yb–O1
2.473(3), Yb–O2 2.518(4), Yb–O3 2.441(4), Yb–I 3.0966(4), N1–N10
1.383(6); N1–Yb–N10 33.02(15), O1–Yb–O10 156.09(13), O1–Yb–O3
86.91(7), O3–Yb–I 168.48(9), N1–Yb–I 106.42(7).
School of Chemistry, Monash University, Clayton, Victoria 3800,
Australia. E-mail: peter.junk@sci.monash.edu.au,
mwiecko@freenet.de; Fax: +61 (03)99054597;
Tel: +61 (03)99054570
w Electronic supplementary information (ESI) available: Details of the
structure determination for 1–3. CCDC 776742–776744. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc01317j
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of the {Ph2pz}ꢀ ligand is highly symmetric, as indicated by the
crystallographic mirror plane bisecting the N–N vector.
The reaction depicted in Scheme 1 presumably proceeds via
an initial formation of the +2/+3 mixed valent, temperature
sensitive reagent ‘‘PhLnI(thf)n’’ followed by a ligand exchange
of Phꢀ with HPh2pz with concomitant formation of C6H6 and
a subsequent reduction of remaining LnIII species by residual
Ln metal. To further understand the nature of ‘‘PhYbI(thf)n’’
we decided to study the reaction mixtures in more detail.
Components of ‘‘PhYbI(thf)n’’ were crystallized by addition
of a few mL of n-hexane. Along with amorphous material,
abundant crystals of [YbPh3(thf)3]15 and [YbI2(thf)4]16 were
found in the resulting solid. To identify decomposition
or side products we also investigated hydrolysis products
[{YbII(dme)4}{YbIIIPh4(dme)}2] (3) in low isolated yield
(Scheme 2 and Fig. 2). Mixed oxidation state rare earth(II/III)
metal–organic compounds are uncommon, especially charge
separated complexes.19
In the solid state 3 consists of distinct ionic units with the
phenyl ligands attached to the YbIII centres which is presumably
favoured by the ionic character of the Yb–C bonds and by the
higher charge and smaller ionic radius of Yb3+. The Yb–C
bond distances ranging from 2.41 to 2.48 A are slightly longer
than in [YbPh3(thf)3],15 presumably due to the negative
charge on the fragment. Compound 3 is chemically related
to [Yb2Ph5(thf)4]20 which also contains Yb in both stable
oxidation states forming
a molecular dinuclear species
whereas 3 contains solvent separated ionic units. A comparison
of the ligand geometry of the {Yb(dme)4}2+ cation (average
Yb–O 2.52 A) with the only literature example
[Yb(dme)4][Hg2(C6H5Se)6] (hYb–Oi = 2.51 A)21 verifies the
assignment of the oxidation states of this cation in 3.
The 171Yb-NMR spectrum of 3 at ꢀ30 1C shows a sharp
resonance at d = 476 ppm for the {Yb(dme)4}2+ ion while
resonances for the anions containing paramagnetic YbIII could
not be observed. While solid 3 can be stored at low temperature
for several days, solutions in dme slowly decompose at ꢀ30 1C
forming a dark viscous solid.
of ‘‘PhYbI(thf)n’’ and
a concentrated filtrate following
isolation of 2, by GC/MS. Qualitatively the data show the
expected benzene and the byproduct biphenyl. While some
components could not be identified the data indicate that some
phenyl-tetrahydrofuran was formed, which could be a Wurtz
coupling product of short-lived a-metallated thf with PhI.17
With the goal of stabilizing PhYbI or YbPh2 type species
with a bidentate donor, we alternatively performed the
oxidation of Yb metal with PhI in dme. Fractional crystal-
lization of ‘‘PhYbI(dme)n’’ from dme/n-hexane at ꢀ25 1C
gave [YbI2(dme)3]18 and subsequently the charge separated
As shown by GC/MS experiments performed after hydro-
lysis of the dme reaction mixture a much more complicated
mixture of byproducts is formed (ESIw).22 Polyaromatic
compounds such as 1,4-Ph2C6H4 or 1,3,5-triphenyl-benzene
indicate a complex and as yet unclear decomposition mechanism
of ‘‘PhYbI(dme)n’’ even if handled at ꢀ78 1C.
The formation of 3 supports the view that both LnII and
LnIII species are present in solutions of ‘‘PhLnI’’. However,
the isolation of 1 and 2 shows that this evidently does not
diminish the value of ‘‘PhLnI’’ reagents as precursors for
functionalized LnII iodide species, which can then be used in
further reactions, e.g. as reducing agents or metathesis reagents.
To increase the reaction scope of the organolanthanoid-halide
route we are currently investigating other ligand systems and
alternative commercially available organic halides in analogous
reactions.
Scheme 2
This work was supported by the Australian Research
Council (ARC DP 0984775). M. Wiecko thanks the Deutsche
Forschungsgemeinschaft (DFG) for a research fellowship.
Notes and references
z Preparation of 1 and 2: Eu metal filings (380 mg, 2.5 mmol) were
suspended in thf (20 mL) and at ꢀ78 1C PhI (408 mg, 2.0 mmol) was
added. The mixture was sonicated for 10 s developing a red-brown
colour. Solid HPh2pz (440 mg, 2.0 mmol) was added and the mixture
stirred at ꢀ78 1C for another 3 h and at rt overnight. Filtration
through a pad of Celite and concentration resulted in the formation of
large yellow crystals. Further concentration of the mother liquor
yielded 1.21 g (76%) 1. IR (Nujol): 1600 (m), 1531 (m), 1512 (w),
1260 (m), 1172 (w), 1073 (m), 1041 (s), 878 (m), 774 (m), 702 (w). Anal.
calcd for C27H35IN2O3Eu (1–THF): C, 45.39; H, 4.94; N, 3.92%.
Found: C, 45.40; H, 5.17; N, 3.75%. 2 was prepared accordingly
Fig. 2 Solid state structure of 3. Drawn with ellipsoids at 50%
probability and hydrogen atoms omitted. Selected bond distances
[A] and angles [1] for the anion around Yb2: Yb2–O9 2.441(5),
Yb2–O10 2.481(6), Yb2–C21 2.458(9), Yb2–C27 2.427(9), Yb2–C33
2.432(8), Yb2–C39 2.475(9); O9–Yb2–O10 66.94(18), C21–Yb2–O9
85.2(2), C27–Yb2–O9 95.4(2), C33–Yb2–O9 157.2(3), C39–Yb2–O9
1
yielding 1.40 g (87%) as orange crystals. H-NMR (300 MHz, C6D6,
81.3(2),
C33–Yb2–O10 91.5(2), C39–Yb2–O10 87.8(2), C21–Yb2–C27
90.9(3), C21–Yb2–C33 99.7(3), C21–Yb2–C39 165.5(3),
C21–Yb2–O10
82.1(3),
C27–Yb2–O10
161.3(2),
25 1C): d = 1.27 (br, 16H, THF), 3.52 (br, 16H, THF), 7.10 (t, J = 6.7 Hz,
2H, p-H), 7.14 (s, 1H, pz-H), 7.24 (dd, J = 6.7, 7.4 Hz, 4H, m-H),
8.08 (d, J = 7.4 Hz, 4H, o-H). 13C-NMR (100.4 MHz, C6D6, 25 1C):
25.7 (THF), 68.5 (THF), 102.3 (pz-CH), 125.8, 126.6, 129.1, 136.3
(Ph). IR (Nujol): 1600 (m), 1512 (m), 1172 (m), 1039 (s), 874 (s),
C27–Yb2–C33 106.8(3), C27–Yb2–C39 95.6(3), C33–Yb2–C39
90.8(3).
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774 (s), 703 (m). Anal. calcd for C31H43IN2O4Yb: C, 46.10; H, 5.37; N,
3.47%. Found: C, 45.39; H, 5.26; N, 3.57%. Preparation of 3: Yb
metal (430 mg, 2.5 mmol) was suspended in dme (15 mL) and at
ꢀ78 1C PhI (408 mg, 2.0 mmol) was added. The mixture was placed in
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a
sonic bath immediately developing a yellow-brown colour.
Sonication for 3 h at around ꢀ78 1C resulted in a dark red-brown
mixture which was pump-filtered through a glass frit to remove
residual metal and colourless precipitate containing [YbI2(dme)3].
The solution was topped with 5 mL of n-hexane. Upon storage at
ꢀ25 1C overnight some colourless crystals of [YbI2(dme)3] (unit cell
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;
171Yb-NMR (dme), 52.6 MHz, ꢀ30 1C: d = 380 ppm)
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d
C72H100O12Yb3: Yb 30.96. Found: Yb 31.34% (too unstable for
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C,H,N microanalysis).
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y Crystal data for 1–3: Bruker X8 Apex II CCD, MoKa radiation,
l = 0.71073 A. 1: C31H43EuIN2O4, M = 786.53, yellow prism,
1.00 ꢂ 0.50 ꢂ 0.50 mm, orthorhombic, Pmn21 (No. 31), a =
17.7513(3), b = 8.7645(2), c = 10.3139(2) A, V = 1604.65(6) A3,
Z = 2, Dc = 1.628 g cmꢀ3, F000 = 782, T = 123(1) K, 2ymax = 55.01,
13 764 reflections, 3616 unique (Rint = 0.0204). Final GooF = 1.060,
R1 [I > 2s(I)] = 0.0136, wR2 (all data) = 0.0319. 2: C31H43IN2O4Yb,
M = 807.61, orange prism, 0.50 ꢂ 0.50 ꢂ 0.30 mm, orthorhombic,
Pmn21 (No. 31), a = 17.6761(8), b = 8.7327(4), c = 10.1877(5) A,
V = 1572.57(13) A3, Z = 2, Dc = 1.706 g cmꢀ3, F000 = 796,
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This journal is The Royal Society of Chemistry 2010
5078 | Chem. Commun., 2010, 46, 5076–5078