Heteroleptic ytterbium(II) complexes supported by a bulky β-diketiminato ligand

Article abstract of DOI:10.1039/b818969b

Heteroleptic β-diketiminatoytterbium(ii) complexes [{Yb(L)(μ-I)(thf)}₂] [1: L = {N(Ar*)C(Me)}₂CH = L¹, 2: L = {N(SiMe₃)C(Ph)}₂CH = L²; Ar* = C₆H₃ⁱPr₂

Full text of DOI:10.1039/b818969b

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www.rsc.org/dalton | Dalton Transactions
Heteroleptic ytterbium(II) complexes supported by a bulky
b-diketiminato ligand†
Peter B. Hitchcock, Alexei V. Khvostov, Michael F. Lappert* and Andrey V. Protchenko*
Received 27th October 2008, Accepted 18th December 2008
First published as an Advance Article on the web 10th February 2009
DOI: 10.1039/b818969b
Heteroleptic b-diketiminatoytterbium(II) complexes [{Yb(L)(m-I)(thf)}2] [1: L =
{N(Ar*)C(Me)}2CH = L1, 2: L = {N(SiMe3)C(Ph)}2CH = L2; Ar* = C6H3 Pr2-2,6] were prepared by a
i
salt elimination [from YbI2(thf)2 and KL] or by a ligand redistribution [from YbI2(thf)2 and YbL2]
reaction. Complexes 1 and 2 were convenient precursors to some new Yb(II) heteroleptic compounds.
Thus, reaction of 1 and 2 with KN(SiMe3)2 yielded the mononuclear amido compounds
[Yb(L){N(SiMe3)2}(thf)] (3: L = L1 and 4: L = L2). Similarly, 1 with K{N(H)Ar*} gave
[Yb(L1){N(H)Ar*}(thf)] (5). Complex 3 was also obtained via a protolytic reaction of
[Yb{N(SiMe3)2}2(thf)2] with HL1 at an elevated temperature. Treatment of 1 with slightly less than a
stoichiometric amount of K{CH(SiMe3)2} afforded [Yb(L1){CH(SiMe3)2}(thf)] (6), while using even a
small excess of the alkyl reagent resulted in the deprotonation of the b-diketiminato ligand and the
3
3
¯
¯
formation of the known dimer [{Yb(L )(thf)}2] [7, L = N(Ar*)C(Me)C(H)C(CH2)N(Ar*)].
Compounds 1 or 2 reacted with KCPh3 in thf yielding [Yb(L1){h -C6H5CPh2}(thf)] (8) or
5
[Yb(CPh3)2(thf)2] (9), respectively; the latter has two differently coordinated CPh3 ligands. The
molecular structures of 1, 2, 4, 5, 6, 8 and 9 were determined by X-ray diffraction.
Homoleptic bis(b-diketiminate)s YbL2 were described for type
Introduction
A (various R2 and R3 substituents)8a,8b and B (a single example)8d
b-Diketiminato ligands (shown in delocalised monoanionic form
L) have recently received significant attention in Group 3 metal
and lanthanide chemistry as an attractive alternative to the
ubiquitous cyclopentadienyl ligands.1 Two distinct types of b-
diketiminato ligands, different by their methods of preparation
and electronic and steric properties, were first introduced into
lanthanide chemistry in 1994 by us (type A)2 and Drees and Magull
(type B).3 Type A ligands are redox-active, undergoing facile
one- or two-electron reduction chemically or electrochemically.4
With the more resistant to reduction type B ligand (R1 = R4 =
CH2CH2NEt2; R2 = R3 = Me), isolation of a crystalline ScI
compound was achieved.5 Most of the recent studies on the
rare earth metal b-diketiminates deal with metal(III) complexes
of Sc,6 Y and the lanthanides;7 less explored are lanthanide(II)
b-diketiminato complexes, which have only been described for Sm
(a single example)8a and Yb.2,4,8
ligands. More promising as potential reagents or catalysts are
heteroleptic complexes. A few such compounds with a type
B ligand (L = L1: R1 = R4 = C6H3 Pr2-2,6; R2 = R3
=
i
Me) have recently been reported.8c,8e Mononuclear cyclopen-
tadienyl, indenyl and aryloxo derivatives [Yb(L1)R(thf)] (R =
5
5
t
h -C5H4Me, h -C9H7, OC6H2 Bu2-2,6-Me-4) were obtained by
Na/K alloy reduction of the corresponding YbIII chlorides,8c
while the amide [Yb(L1){N(SiMe3)2}(thf)] was prepared by a one-
pot reaction from YbI2, HL1 and 2KN(SiMe3)2,8e the method
originally employed for the synthesis of the Ca analogue.9
This ytterbium(II) amide was then used as a starting material
for dinuclear hydrido and hydroxo compounds in reactions
with PhSiH3 and H2O, respectively.8e Attempted synthesis of a
heteroleptic Yb(II) alkyl from Yb(L1)2 and YbR¢2(thf)2 (R¢ =
CH(SiMe3)C6H4NMe2-2) resulted in deprotonation of one of
the Me substituents of the b-diketiminato ligand backbone.8d
Mono-b-diketiminatoytterbium(II) halides remained unknown. It
is interesting, that with a related guanidinato ligand, homoleptic
lanthanide(II) compounds Ln(Giso)2 [Ln = Sm, Eu, Yb; Giso =
i
{N(C6H3 Pr2-2,6)}2CN(C6H11)2] have been isolated from both 1 : 2
and 1 : 1 reactions of LnI2 and K(Giso).10 Only with Ln = Yb was
a small amount (6%) of a monoguanidinatoytterbium(II) iodide
available; the latter redistributed readily in thf solution to give the
homoleptic complex and YbI2(thf)x.10
The Chemistry Department, University of Sussex, Brighton, U.K. BN1 9QJ.
E-mail: m.f.lappert@sussex.ac.uk; Fax: +44 1273 677196; Tel: +44 1273
678316
† Electronic supplementary information (ESI) available: PGSE NMR data
for complexes 5 and 8. CCDC reference numbers 706860–706866. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b818969b
The aim of the present study was to develop synthetic methods
for the preparation of b-diketiminatoytterbium(II) iodides using
both type A [L1 = {N(C6H3 Pr2-2,6)C(Me)}2CH] and B [L2 =
i
{N(SiMe3)C(Ph)}2CH] ligands and explore their potential as
starting materials for various mixed-ligand YbII compounds.
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◦
˚
Table 1 Selected bond distances (A) and angles ( ) for 1 and 2
Results and discussion
1a
2b
Synthesis and structures of b-diketiminatoytterbium(II)
iodides 1 and 2
˚
Bond distances/A
Reaction of YbI2(thf)2 with one equivalent of KL1 or KL2 in
thf afforded the b-diketiminatoytterbium(II) iodides [{YbL(m-
I)(thf)}2] [L = L1 (1) or L2 (2)] in good yields. Complexes 1 and 2
were also obtained via the ligand redistribution route from Yb(L1)2
or Yb(L1)2, respectively, Scheme 1. It is noteworthy that LiL1 failed
to react with YbI2(thf)2 in refluxing thf.
Yb–N1
Yb–N2
Yb–O
Yb–I
2.414(4)
2.398(4)
2.364(4)
3.1418(5)
3.1031(5)
Yb–N1
Yb–N2
Yb–O
Yb–I
2.342(4)
2.385(4)
2.414(4)
3.2063(4)
3.0993(5)
2.847(6)
2.885(5)
2.899(6)
Yb–I¢
Yb–I¢
Yb–C1
Yb–C3
Yb–C2
Bond angles/◦
N1-Yb–N2
I–Yb–I¢
O–Yb–N1
O–Yb–N2
78.77(14)
83.595(12)
99.08(15)
97.33(16)
N1-Yb–N2
I–Yb–I¢
O–Yb–N1
O–Yb–N2
79.63(15)
89.89(1)
134.78(15)
84.75(15)
a Symmetry transformation to generate equivalent atoms: ¢ -x + 1/2,
-y + 3/2, -z + 1. b Symmetry transformation to generate equivalent atoms:
¢ -x, -y, -z.
i
Scheme 1 Compounds 1 and 2 (L1: R1 = C6H3 Pr2-2,6, R2 = Me; L2:
R1 = SiMe3, R2 = Ph).
Complex 1 was fairly soluble in aromatic hydrocarbons or hot
thf. Its EI mass-spectrum revealed a dinuclear molecular ion peak
(with the loss of thf ligands), which suggests that the dimeric
structure of 1, determined by X-ray crystallography (vide infra),
is quite stable. Coordinated thf was removed from 1 by heating
◦
at 150 C (10-2 Torr) for 1 h; the resulting compound showed a
very similar 1H NMR spectrum to that of 1 (the thf signals were
absent), but its solubility in aromatic solvents was significantly
lower. Complex 2 was insoluble in hydrocarbons or Et2O but
dissolved well in thf at room temperature, which may be attributed
to dissociation of the dimer in this donor solvent. While freshly
prepared samples of 2 in thf-d8 showed one set of signals for
the b-diketiminato ligand (in 1H, 13C and 29Si NMR spectra), the
second set (at ca. 2 : 5 ratio) appeared either after storing ◦for
several days at room temperature or by heating for 2 h at 60 C.
The presence of only one signal in the 171Yb NMR spectrum
(d 1018 ppm) excludes a ligand redistribution, as [Yb(L2)2] has
a very characteristic downfield shift of d 2434 ppm.8b
The crystalline compounds 1 and 2 have dimeric structures
through bridging iodide atoms, as shown in Fig. 1 (skeletal
view); their selected geometrical parameters are given in Table 1.
The most significant difference between the two complexes is
Fig. 1 Molecular structure of compounds 1 (top) and 2 (bottom) (50%
thermal ellipsoids; all substituents of the b-diketiminato ligands and the
CH2 groups of thf ligands are omitted for clarity).
2
that the b-diketiminato ligand is k -bonded and nearly planar
5
˚
(maximum deviation 0.055 A) in 1, but h -bonded in 2 (the
Reactions of b-diketiminatoytterbium(II) iodides 1 and 2
˚
C2 atom is out of the N1C1C3N2 plane by 0.221 A). The Yb
atom in 1 is in a distorted square pyramid (thf in the apical
position; Yb off the basal plane by 0.413 A towards the thf
To evaluate the suitability of compounds 1 and 2 as het-
eroleptic YbII precursors, their reactions with selected potassium
reagents were studied (Schemes 2 and 3). Thus, treatment of 1
with KN(SiMe3)2 in thf at room temperature readily produced
the known8e ytterbium(II) amide [Yb(L1){N(SiMe3)2}(thf)] (3).
The latter was also obtained by the protolytic reaction of
[Yb{N(SiMe3)2}2(thf)2] with HL1. However, the activity of HL1
as a protic reagent was very low; no reaction was observed
in either hexane or thf at room temperature and the complete
transformation of HL1 to 3 required refluxing the mixture in thf
for 2 d.
˚
ligand). The Yb–N bond lengths are similar to those in the
1
8d
˚
homoleptic b-diketiminate [Yb(L )2] (2.382(2) and 2.398(2) A)
or in the heteroleptic guanidinate [{Yb(Giso)(m-I)(thf)}2] (2.373(3)
and 2.426(3) A),10 while the Yb–O bond in 1 is much shorter than
˚
˚
in the latter (2.441(3) A). In complex 2 the Yb–N bonds are slightly
shorter but the Yb–O are longer than the corresponding distances
in 1. The bridging Yb–I distances in 1 and 2 are very close to
each other and to such distances in YbII guanidinato10,11 or bulky
cyclopentadienyl12 complexes.
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Scheme 2 Reactions of compound 1 with selected potassium amides and hydrocarbyls [L1 = {N(C6H3 Pr2-2,6)C(Me)}2CH].
i
Scheme 3 Synthesis of complexes 4 and 9 [L2 = {N(SiMe3)C(Ph)}2CH].
The metathetical reaction of
1
and K{N(H)Ar*} gave
[Yb(L1){N(H)Ar*}(thf)] (5) (Scheme 2). This compound was
originally isolated in low yield as a by-product from the synthesis
of [Yb(L1)2] when K(L1) was prepared in situ by metallation of
HL1 with K metal. Apparently, the reductive cleavage of a
C–N bond in HL1 was the source of K{N(H)Ar*}; similar bond
cleavage was observed upon reduction of, for example, [Ti(L1)Cl2]
or [Ti(C5Me4H)(L1)Cl] with K/Na alloy,13a and a MnII analogue
of complex 5 (with an NHC carbene ligand in place of thf) was
isolated from a reaction of [Mn(L1)I(NHC)] and Na/K alloy.13b
Fig. 2 Molecular structure of 5 (20% thermal ellipsoids; only the anilido
H atom is shown; benzene solvate molecule is omitted for clarity).
˚
or Ca ◊ ◊ ◊ C36 are longer than 3.2 A). The Yb–N3 (anilido) bond is
only slightly shorter than the Yb–N1 or Yb–N2 (b-diketiminato)
bond and may be compared to that in the 4-coordinate YbII Me3Si-
15a
˚
substituted anilides [Yb{N(SiMe3)Ar*}2(thf)2] (2.354(7) A)
and [Yb{N(SiMe3)Ph}{m-N(SiMe3)Ph}(thf)]2 (terminal bonds,
15b
˚
2.326(4) and 2.353(4) A). The b-diketiminato ligand in 5 is
1
According to H and 13C NMR spectroscopic studies in C6D6
delocalised and nearly planar, as in 1.
solution, compound 5 is stable towards ligand redistribution (in
contrast to the Ca analogue14); a broad peak at d 3.51 ppm had
distinct Yb satellites and thus it was assigned to the primary anilido
NH group.
The reaction of 1 with KCH(SiMe3)2 in a 1 : 1.9 molar
ratio (Scheme 2) yielded the expected heteroleptic ytterbium(II)
alkyl [Yb(L1){CH(SiMe3)2}(thf)] (6). However, the outcome
of the reaction depended dramatically on the chosen reagent
stoichiometry: when a small excess of the potassium alkyl
was taken (1 : 2.3), a product was the known8d compound
[{Yb(L3)(thf)}2] (7) containing the deprotonated ligand L3 [L3 =
The molecular structure of the crystalline 5 is shown in Fig. 2;
the essential geometrical parameters are given in Table 2. The
Yb coordination environment can be described as distorted
tetrahedral, but there is an additional contact between the Yb atom
and a Me group of one of the isopropyl substituent of the anilido
¯
¯
N(Ar*)C(Me)C(H)C(CH2)N(Ar*)] as dark green crystals (a dark
red colour was reported in ref. 8d). The isolated complex 7 had the
1H and 13C NMR solution spectra and crystal unit cell parameters
14
˚
ligand (Yb ◊ ◊ ◊ C38, 2.998 A; cf., in the Ca analogue Ca ◊ ◊ ◊ C38
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Table 2 Selected bond distances (A) and angles ( ) for the b-diketiminatoytterbium compounds 5, 6, 8 and 4
5 (X = N3)
6 (X = C30 [C30B])a
8 (X = M1)b
4 (X = N3)
˚
Bond distances/A
Yb–N1
Yb–N2
Yb–O
2.382(2)
2.340(2)
2.399(2)
2.327(2)
2.404(5) [2.405(5)]
2.401(5) [2.401(5)]
2.383(4) [2.371(5)]
2.562(8) [2.549(7)]
2.342(9)
2.345(9)
2.366(8)
2.451(1)
2.403(3)
2.392(3)
2.422(3)
2.341(3)
Yb–X
Bond angles/◦
N1–Yb–N2
N1–Yb–X
N2–Yb–X
O–Yb–X
O–Yb–N1
O–Yb–N2
80.67(6)
133.29(7)
98.83(7)
125.28(6)
99.91(6)
101.99(6)
80.57(18) [80.85(17)]
128.3(2) [125.2(2)]
125.5(2) [128.0(2)]
118.7(2) [116.6(2)]
95.66(18) [99.56(18)]
98.65(18) [98.37(18)]
80.7(3)
121.3(3)
122.5(3)
121.0(3)
99.5(3)
83.46(10)
117.27(11)
113.03(11)
138.58(10)
95.64(10)
94.05(10)
103.2(3)
a Data for the second independent molecule of 6 are given in square brackets. b M1 is the centroid of the C2 to C7 ring.
1
almost identical to the reported8d values. The H and 13C NMR
spectra of complex 6 revealed the alkyl ligand CH group signals
at d -1.65 (with 171Yb satellites, J(1H–171Yb) = 28.9 Hz) and
22.9 ppm, respectively, which are similar to the corresponding
data for the dialkyl complex [Yb{CH(SiMe3)2}2(OEt2)2] (d -1.55
and 27.34 ppm, J(1H–171Yb) = 30 Hz).2 In contrast to the
latter, the 171Yb NMR spectrum of 6 showed only a singlet at
d 1021 ppm (the 171Yb–1H coupling was not resolved due to line
broadening).
In C6D6 solution, the heteroleptic complex 6 demonstrated
a remarkable stability towards (i) the deprotonation of a Me
group of the b-diketiminato ligand by the alkyl group (cf., the
8d
lability of Yb(L1){CH(SiMe3)C6H4NMe2-2} ) and (ii) ligand
redistribution. Thus, no changes in the 1H and 13C NMR spectra
of 6 were observed after storing for several months at ambient
temperature in a sealed tube, while heating such a sample for
3 h at 70 ◦C resulted in only 20% conversion (decomposi-
tion pathway ii) to the homoleptic compound [Yb(L1)2] (the
second expected product, Yb{CH(SiMe3)2}2(thf)2, decomposed
under the reaction conditions giving CH2(SiMe3)2 as the only
detectable species). No complex 7 or any other complex con-
taining the deprotonated ligand L3 (decomposition pathway i)
was found in this experiment. It is noteworthy that in the
group 3 and the lanthanide series L1 was easily deprotonated
to L3. Thus, complexes containing L3 were isolated from the
reactions of [Y(L1)I2(dme)] and KCH2SiMe3,7r TmI3(thf)3.5 and
K(L1)7p or [La(L1)Br2(thf)2] and KCH2Ph,7s the latter two bases
being apparently weaker than [CH(SiMe3)2]-. Similar transfor-
mation was also observed in b-diketiminatotitanium(IV) (e.g.,
in the reaction of [Ti(L1)(=CHtBu)(OTf)] with KCH2Ph)16a
and calcium chemistry (decomposition of the thermally unsta-
ble [Ca(L1){CH(SiMe3)C6H4NMe2-2}], cf. ref. 8d for the Yb
analogue).16b The decomposition pathway via deprotonation of
an aryl isopropyl group (found in [Sc(L1)(CH2R)2] R = Ph, But or
SiMe36c) has been ruled out for 6.
Fig. 3 Molecular structure of 6 (20% thermal ellipsoids; all H atoms and
the second independent molecule of 6 are omitted for clarity).
b-diketiminato ligand core (C2 is out of the N1C1C3N2 plane
˚
by ca. 0.13 A). The low quality of the structural data does not
warrant a detailed discussion of the geometric parameters of 6
(selected data are given in Table 2).
The reaction of 1 with KCPh3 in thf or toluene (Scheme 2)
5
cleanly produced the heteroleptic trityl compound [Yb(L1){h -
C6H5CPh2}(thf)] (8), which is the first structurally characterised
(Fig. 4) lanthanide complex featuring a metal–trityl bond (pre-
viously reported YbII complexes [Yb(thf)6][CPh3]2 and [{Yb(m-
17b
Cl)(thf)4}2][CPh3]2,17a as well as [Ca(thf)6][CPh3]2
had ion-
separated structures). The Yb coordination environment in 8
can be described as distorted tetrahedral considering that the
Ph ring bound to the Yb atom (five Yb–C distances ranging
The structure of compound 6 was further confirmed by a single-
crystal X-ray diffraction study (Fig. 3) showing the distorted
tetrahedral geometry around the Yb atom and the nearly planar
˚
from 2.734(10) to 2.878(11) A) occupies a single coordination
site. A similar geometry around the Yb atom was observed
2386 | Dalton Trans., 2009, 2383–2391
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Fig. 4 Molecular structure of 8 (20% thermal ellipsoids; all H atoms and
the isopropyl substituents of the b-diketiminato ligand are omitted for
clarity).
Fig. 5 Molecular structure of 4 (20% thermal ellipsoids; all H atoms and
the isopropyl substituents of the b-diketiminato ligand are omitted for
clarity).
in b-diketiminatoytterbium(II) complexes with cyclopentadienyl
and indenyl ligands.8c The trityl ligand bonding mode and the
arrangement of the Ph rings relative to the central C atom are close
[Yb{N(SiMe3)2}2(dmpe)]; the latter has a closely similar Yb–N
19
˚
amido bond length of 2.331(13) A.
5
to those in the anionic Ti complex [K(15-crown-5)2][Ti(CO)4(h -
The reaction of 2 with KCPh3 was not so straightforward as in
the case of 1. While the precipitation of the KI by-product was
observed, the desired heteroleptic trityl complex did not crystallise
upon removal of the solvent. Prolonged storing led to ligand
redistribution and crystallisation of the sparingly soluble bis-trityl
compound [Yb(CPh3)2(thf)2] (9), Scheme 3.
C6H5CPh2)] (and the C6H4OMe-4 analogue).18a Based on this
5
analogy, the ligand in 8 is depicted schematically as h -dienyl
(Scheme 2), although the negative charge is more delocalised than
in the Ti complex.18b
1
The H and 13C NMR spectra of complex 8 showed only one
set of Ph resonances at 233–293 K, in contrast to the solid-
state structure (the Ti-trityl complex18a showed two sets of signals
at 0 ◦C). Such a result would be consistent with the presence
of either a solvent-separated ligand (as in the ionic compounds
[M(thf)6][CPh3]2, M = Yb17a or Ca17b) or a contact ion pair (CIP)
with a fast (on the NMR time-scale) exchange between the three
Ph groups of the [CPh3]- ligand. The 1H pulsed gradient spin-echo
(PGSE) NMR diffusion measurements on both toluene and thf
solutions of 8 showed the strongly pronounced CIP behaviour (see
ESI†) similar to that of complex 5.
An X-ray diffraction study shows that 9 contains two inde-
pendent molecules in the unit cell, which have similar geometric
parameters (Table 3). The two CPh3 ligands are differently bonded
to the Yb atom: one interacts mainly via its central carbon atom
(C1 in Fig. 6), while the other is h -bonded via one of the Ph
rings (C22 to C26 atoms in Fig. 6) similar to that found in
5
complex 8. The Yb–Ccentral bond lengths in the two independent
˚
molecules of 9 (2.619(7) and 2.596(7) A) can be compared to Yb–C
˚
distances in the alkyl complexes 6 (2.562(8) and 2.549(7) A) and
Similarly to compound 1, the bis(trimethylsilyl)-b-diketimina-
toytterbium(II) iodide 2 reacted cleanly with KN(SiMe3)2 in thf
producing the expected amido complex [Yb(L2){N(SiMe3)2}(thf)]
(4) as dark green crystals, Scheme 3. The latter was highly soluble
in hydrocarbons, demonstrating the appropriate signals in its
1H and 13C NMR spectra with broadened peaks for the thf
ligand.
◦
a
˚
Table 3 Selected bond distances (A) and angles ( ) for 9
˚
Bond distances/A
Yb–O1
Yb–O2
Yb–M1
Yb–C1
2.369(5)
2.374(7)
2.501(7)
2.619(7)
Yb1B–O1B
Yb1B–O2B
Yb1B–M1B
Yb1B–C1B
2.382(5)
2.381(5)
2.507(7)
2.596(7)
The molecular structure of 4 was determined by single-crystal
X-ray diffraction. The coordination environment of the Yb atom
Bond angles/◦
in 4 can be described as distorted tetrahedral (Fig. 5), with a
2
˚
nearly planar L ligand core and the Yb atom 0.480(5) A out
O1–Yb–O2
O1–Yb–C1
O2–Yb–C1
O1–Yb–M1
O2–Yb–M1
M1–Yb–C1
90.3(2)
115.7(2)
98.7(2)
108.3(2)
106.0(2)
128.9(2)
O1B–Yb1B–O2B
O1B–Yb1B–C1B
O2B–Yb1B–C1B
O1B–Yb1B–M1B
O2B–Yb1B–M1B
M1B–Yb1B–C1B
94.5(2)
113.0(2)
101.0(2)
106.1(2)
101.8(2)
132.5(2)
˚
of the N1C1C2C3N2 plane (cf., Yb is 1.712(5) A out of the
5
N1C1C3N2 plane in the h -bonded compound 2). The Yb–N and
Yb–O distances in 4 are slightly longer than those in 5 apparently
due to the presence of two long range Yb ◊ ◊ ◊ C(Me) contacts
˚
(av. 3.06 A) to the SiMe3 substituents of the b-diketiminato
a M1 and M1B are the centroids of the C21 to C26 and C21B to C26B
rings, respectively.
ligand. Such Yb ◊ ◊ ◊ C(Me) intramolecular contacts are common
˚
in complexes containing a YbNSiMe3 moiety, e.g., 3.06 A in
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Experimental
General methods
All manipulations were carried out in an atmosphere of dry argon
using standard Schlenk tube techniques or under vacuum in a
sealed all-glass apparatus. Solvents were dried from the appropri-
ate drying agent, distilled, degassed and stored over a potassium
mirror. The NMR spectra were recorded on a DPX 300 or AM-
500 Bruker spectrometer with residual solvent signals as internal
1
reference (1H and 13C{ H}) or with an external reference (SiMe4
for 29Si and [Yb(C5Me5)2(thf)] for 171Yb). Elemental analyses were
carried out at the London Metropolitan University (satisfactory
elemental analyses for 5, 8 and 9 were not obtained due to their
high air- and moisture-sensitive nature). The compounds HL1,22
K(L2),4c and KN(H)(C6H3 Pr2-2,6)23 were prepared by published
i
procedures. Electron impact mass spectra were taken from solid
samples using a Kratos MS 80 RF instrument. Melting points
were measured in sealed capillaries.
Fig. 6 Molecular structure of 9 (20% thermal ellipsoids; all H atoms and
the second independent molecule of 9 are omitted for clarity).
Synthesis of K(L1)
t
˚
[Yb(Tp){CH(SiMe3)2}] (2.55(6) A; Tp = HB{N2C3H Bu-3-Me-
HL1 (4.40 g, 10.5 mmol) was dissolved in hexane (60 mL) and
nBuLi (6.6 mL of 1.6 M solution in hexanes) was added at room
temperature. Sublimed KOtBu (1.18 g, 10.52 mmol) was added
quickly to this warm solution with vigorous stirring producing a
yellow solution. After stirring for 5 h at ambient temperature, a
light-yellow microcrystalline solid precipitated, which was washed
with hexane (2 ¥ 20 cm3) and dried in a vacuum yielding KL
5}3)20 or in the Yb-benzyl compound [Yb{CH(SiMe3)C6H4NMe2-
8e
˚
2}2(thf)2] (2.640(3) and 2.661(3) A). The overall coordination
environment of the Yb atom in 9 can be described as dis-
torted tetrahedral with each CPh3 ligand occupying a single
site.
Various metal–trityl coordination modes were previously ob-
served in alkali metal complexes; this fact was explained
by the flat potential energy surfaces of ion pairs involving
highly delocalised carbanions.21 Such considerations can also
now be applied to the ytterbium(II)-trityl complexes with CIP
structures.
1
(4.08 g, 84%). H NMR (C6D6, 293 K): d 7.16–7.13 (m, 4 H, m-
H), 7.03 (m, 2 H, p-H), 4.79 (s, 1 H, middle CH), 3.33 (septet,
4 H, J = 6.9 Hz, CHMe2), 1.87 (s, 6 H, NCMe), 1.25 (d, 12 H, J =
6.9 Hz, CHMe2), 1.07 ppm (d, 12 H, J = 6.9 Hz, CHMe2).
Room temperature H and 13C NMR spectroscopy in C7D8
1
Synthesis of [{Yb(l-I)(L1)(thf)}2] (1). YbI2(thf)2 (2.23 g,
3.90 mmol) and K(L1) (1.74 g, 3.81 mmol) were stirred in thf
(50 mL) at 60 ◦C for 2 h. The solvent was removed in a
vacuum and the residue was extracted with toluene (2 ¥ 20 mL).
Slow evaporation of toluene using the vacuum-transfer technique
(fast crystallisation resulted in the formation of a wool-like
polycrystalline material) produced brick-red crystals of 1, which
were washed with hexane and dried in a vacuum (2.08 g, 69%)
(found: C 49.9, H 6.49, N 3.53. C66H98I2N4O2Yb2 (MW 1579.36)
requires C 50.2, H 6.25, N 3.55%). Melting was not observed up to
310 ◦C. EI MS m/z (%, assignment): 1433 (0.1, [M - 2thf]+), 1008
(0.8, [Yb(L1)2]+), 718 (10, [M/2 - thf]+), 403 (70, [HL1 - Me]+),
showed that both ligands and all the Ph substituents in 9 are
equivalent, whereas the low solubility in aromatic hydrocarbons
did not permit a low-temperature study. In thf, the dark brown
complex 9 immediately converted into the red ionic compound
[Yb(thf)6][CPh3]2 and the reverse transformation was achieved by
heating the latter in toluene (9 crystallised upon cooling). The Ca
analogue of 9 and its conversion to the structurally characterised
ionic compound [Ca(thf)6][CPh3]2 were reported, but no details on
the former was given.17b
202 (100, [N(C6H3 Pr2-2,6)CCH3]+). H NMR (C6D6, 293 K): d
7.19–7.15 (br s, 6 H, CHarom overlapping with solvent), 4.80 (s,
2 H, CHmiddle), 3.63 (br s, 8 H, thf), 3.30 (septet, 8 H, CHMe2), 1.62
(s, 12 H, NCMe), 1.34 (d, 24 H, CHMe2), 1.22 ppm (d, 24 H,
i
1
Conclusions
We have shown that two bulky b-diketiminatoytterbium(II) iodides
1 and 2 are readily prepared in a good yield by the salt
elimination or ligand redistribution method. These compounds
were convenient precursors to heteroleptic amides (including
the primary anilide 5 and the bis(trimethylsilyl)amide 4) and
hydrocarbyls (including the bis(trimethylsilyl)methyl compound
6 and the trityl derivative 8). In reactions with such strong bases
as KCH(SiMe3)2, an excess of the potassium reagent should be
avoided to prevent deprotonation of the b-diketiminato ligand
substituents (with the formation of 7). Considering the striking
resemblance of the behaviour of Ca and Yb b-diketiminates,8d,8e
a chemistry similar to that reported here can be expected in the
b-diketiminatocalcium series.
CHMe2 + br s, 8 H, thf). 13C{ H} NMR (C6D6, 293 K): d 165.3
1
(NCMe), 146.3 (ipso-C), 141.9 (o-C), 124.7 (p-C), 123.9 (m-C),
95.1 (Cmiddle), 70.1 (thf), 28.5 (CHMe2), 26.0 (CHMe2), 25.1 (thf),
25.0 (NCMe), 24.7 ppm (CHMe2). 171Yb{ H} NMR (C6D6/C6H6,
1
298 K): d 729.2 ppm.
Synthesis of [{Yb(l-I)(L2)(thf)}2] (2). YbI2 (0.95 g, 2.23 mmol)
was added to a stirred solution of K(L2) (0.90 g, 2.22 mmol) in
thf (100 mL). The green-brown suspension was stirred for 24 h
and filtered. The filtrate was concentrated to yield upon cooling
green crystals of 2 (1.42 g, 87%) (found: C 39.9, H 5.19, N 3.65.
2388 | Dalton Trans., 2009, 2383–2391
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C50H74I2N4O2Si4Yb2 (MW 1475.37) requires C 40.7, H 5.06, N
3.80%). The following NMR spectra were recorded on a freshly
prepared sample of 2. 1H NMR (thf-d8, 293 K; integration is given
for a monomer): d 7.42 (m, 4 H, o-H of Ph), 7.29 (m, 6 H, m-
and p-H of Ph), 5.10 (s, 1 H, CHmiddle), 3.62 (m, 4 H, thf), 1.77
110.1 (p-C of amido-Ar*), 95.1 (Cmiddle), 69.4 (thf), 29.4 (CHMe2
of amido-Ar*), 28.3 (CHMe2 of L1), 25.2 (thf), 25.0, 24.8, 24.6
(CHMe2 and NCMe of L1), 22.7 ppm (CHMe2 of amido-Ar*).
Synthesis
of
[Yb(L1){CH(SiMe3)2}(thf)]
(6). Solid
KCH(SiMe3)2 (0.142 g, 0.72 mmol) was added to a suspension
of compound 1 (0.604 g, 0.38 mmol) in thf (10 mL) at -40 ◦C.
Upon stirring the mixture for a few minutes at room temperature,
1 dissolved and KI precipitated to give a red-brown solution.
The reaction mixture was evaporated to dryness in a vacuum
and the residue was extracted with pentane. Slow evaporation
of the extract produced violet crystals of 6, which were washed
with cold pentane and dried in a vacuum (0.369 g, 59% based
on Yb), mp 135–138 ◦C (with dec.) (found: C 58.6, H 8.25, N
3.54. C40H68N2OSi2Yb (MW 822.19) requires C 58.4, H 8.34, N
1
(m, 4 H, thf), 0.01 ppm (s, 18 H, SiMe3). 13C{ H} NMR (thf-d8,
293 K): d 173.17 (NCPh), 148.17 (ipso-C of Ph), 128.82 (p-CH
of Ph), 128.43 and 128.34 (m- and o-CH of Ph), 101.07 (CHmiddle),
1
68.10 (thf), 26.25 (thf), 3.16 ppm (SiMe3). 29Si{ H} NMR (thf-d8,
1
298 K): d -4.43 ppm (SiMe3). 171Yb{ H} NMR (C7D8/thf, 298 K):
d 1018.1 ppm. UV-vis. (thf): l/nm 407 s, 663 w.
Synthesis of [Yb(L1){N(SiMe3)2}(thf)] (3). Solid KN(SiMe3)2
(0.062 g, 0.31 mmol) was added to a suspension of compound
1 (0.254 g, 0.16 mmol) in thf (10 mL) at -40 ◦C. Upon stirring
the mixture for a few minutes at room temperature, 1 dissolved
and KI precipitated to give a purple-red solution. The reaction
mixture was evaporated to dryness in a vacuum and the residue
was extracted with pentane. Slow evaporation of the extract
1
3.41%). H NMR (C6D6, 293 K): d 7.16-7.14 (br s, 3 H, CHarom
overlapping with solvent), 4.76 (s, 1 H, CHmiddle), 3.50 (br s, 4 H,
thf), 3.23 (br m, 4 H, CHMe2), 1.62 (s, 12 H, NCMe), 1.34 (br s,
12 H, CHMe2), 1.22 (d, 12 H, CHMe2 + br s, 4 H, thf), 0.15 (s,
18 H, SiMe3), -1.65 ppm [s, with 171Yb satellites, J(1H–171Yb) =
1
produced purple microcrystalline 6 (0.169 g, 60%). The H and
1
1
13C{ H} NMR spectra (C6D6, 293 K) were similar to those in the
28.9 Hz, 1 H, CH(SiMe3)2]. 13C{ H} NMR (C7D8, 293 K): d
literature.8e
165.4 (NCMe), 146.4 (ipso-C), 141.2 (o-C), 124.9 (p-C), 124.1
(m-C), 93.0 (Cmiddle), 69.3 (thf), 28.4 (CHMe2), 25.4 (CHMe2), 25.3
(NCMe), 25.0 (thf), 24.5 (CHMe2), 22.9 [CH(SiMe3)2], 5.96 ppm
Synthesis of [Yb(L2){N(SiMe3)2}(thf)] (4). Solid KN(SiMe3)2
(0.070 g, 0.35 mmol) was added to a solution of compound 2
(0.245 g, 0.17 mmol) in thf (10 mL) at -40 ◦C. The mixture
was allowed to warm up to room temperature and was stirred
for 2 h. The solvent was removed in a vacuum and the dark
green residue was extracted with pentane (2 ¥ 5 mL). The extract
was concentrated and stored at -27 ◦C for◦1 week yielding green
crystals of 4 (0.189 g, 73%), mp 140–141 C (with dec.) (found:
C 48.1, H 7.07, N 5.51. C31H55N3OSi4Yb (MW 771.17) requires
C 48.3, H 7.19, N 5.45%). 1H NMR (C7D8, 293 K): d 7.26 (m, 4 H,
o-H of Ph), 6.98 (m, 6 H, m- and p-H of Ph), 5.01 (s, 1 H,
CHmiddle), 3.66 (br s, 4 H, thf), 1.39 (br s, 4 H, thf), 0.47 (s,
18 H, SiMe3 of amide), 0.16 ppm (s, 18 H, SiMe3 of L2). 13C NMR
(C7D8, 293 K): d 174.6 (NCPh), 148.7 (ipso-C of Ph), 127.8, 127.4
and 126.6 (CH of Ph), 107.5 (CHmiddle), 69.4 (thf), 25.4 (thf), 5.64
(SiMe3 of amide), 2.68 ppm (SiMe3 of L2).
1
(SiMe3). 29Si{ H} NMR (C6D6/C6H6, 298 K): d -7.26 ppm
(SiMe3). 171Yb NMR (C6D6/C6H6, 298 K): d 1021.3 ppm (br s,
Dw1/2 ~ 60 Hz; no detectable 171Yb–1H coupling).
Isolation of [{Yb(L3)(thf)}2] (7). Solid KCH(SiMe3)2 (0.075 g,
0.37 mmol) was added to a cold suspension of compound 1
(0.245 g, 0.16 mmol) in thf (10 mL). After stirring at room
temperature overnight, the initially formed dark red solution
(as above) became pale with precipitation of a dark green
microcrystalline solid. The supernatant liquid was removed by
decantation and the solid was extracted with hot thf (2 ¥ 20 mL;
extraction was incomplete). Storing of the concentrated extracts
at room temperature yielded 7 as small◦dark green rhombic plates
and blocks (0.098 g, 46%), mp 145–150 C (with dec.). The 1H and
1
13C{ H} NMR spectra (thf-d8, 293 K) were similar to those in the
literature.8d
Synthesis of [Yb(L1){N(H)Ar*}(thf)] (5). Solid KN(H)-
i
C6H3 Pr2-2,6 (0.231 g, 1.07 mmol) was added to a suspension
Synthesis of [Yb(L1){g5-C6H5CPh2}(thf)] (8). Solid KCPh3
(0.075 g, 0.26 mmol) was added to a suspension of compound
1 (0.166 g, 0.12 mmol) in thf (5 mL) at -40 ◦C. Upon stirring the
mixture for a few minutes at room temperature, 1 dissolved and
KI precipitated to give a very dark brown solution. The reaction
mixture was stirred for 2 h and filtered, the filtrate was evaporated
to dryness in a vacuum and the residue was extracted with toluene
(2 ¥ 3 mL). The toluene solution was concentrated, covered with
hexane and stored at -10 ◦C overnight yielding black crystals of 8
(0.182 g, 83%). 1H NMR (C7D8, 293 K): d 7.13–7.00 (br m, 6 H,
CHarom overlapping with the solvent), 6.97 (d, 6 H, J = 7.7 Hz,
o-H), 6.59 (t, 6 H, J = 7.5 Hz, m-H), 6.19 (t, 3 H, J = 7.0 Hz,
p-H), 4.65 (s, 1 H, CHmiddle), 3.89 (br s, 4 H, thf), 3.31 (br m, 2 H,
CHMe2), 2.81 (br m, 2 H, CHMe2), 1.52 (s, 6 H, NCMe), 1.34 (br
of compound 1 (0.835 g, 0.53 mmol) in thf (20 mL) at -40 ◦C.
Upon stirring the mixture for a few minutes at room temperature,
1 dissolved and KI precipitated to give a brown solution. The
reaction mixture was stirred for 2 h, evaporated to dryness in a
vacuum and the residue was extracted with toluene (2 ¥ 5 mL). The
toluene solution was concentrated, covered with hexane and stored
at -10 ◦C overnight yielding black crystals of 5·PhMe (0.450 g,
46%). Recrystallisation of this material from benzene–pentane
produced X-ray quality crystals of 5·C6H6. 1H NMR (C6D6,
293 K): d 7.16–7.15 (two s, CHarom of L1 + solvent and C6H6),
7.08 (d, 2 H, m-CH of amido-Ar*), 6.64 (t, 1 H, p-CH of amido-
Ar*), 4.77 (s, 1 H, CHmiddle), 3.52 (br s, 1 H, NH), 3.38 (br m,
8 H, thf), 3.27 (septet, 4 H, CHMe2 of L1), 2.57 (br septet, 2 H,
CHMe2 of amido-Ar*), 1.62 (s, 6 H, NCMe), 1.23–1.14 ppm (two
1
s, 4 H, thf) 1.20–1.08 ppm (br m, 12 H, CHMe2). 13C{ H} NMR
1
d, 36 H, CHMe2 + br s, 4 H, thf). 13C{ H} NMR (C6D6, 293 K):
(C7D8, 293 K): d 165.9 (NCMe), 146.5 (ipso-C of Ar*), 144.0 (ipso-
C of Ph), 141.5 (o-C of Ar*), 129.2 (m-CH of Ph), 125.2 (p-CH
of Ar*), 123.9 (m-CH of Ar*), 123.2 (o-CH of Ph), 114.1 (p-CH
of Ph), 100.3 (CPh3), 94.2 (Cmiddle), 70.5 (thf), 28.6 (CHMe2), 25.6
d 165.1 (NCMe), 156.9 (ipso-C of amido-Ar*), 146.1 (ipso-C of
L1), 141.6 (o-C of L1), 130.1 (o-C of amido-Ar*), 128.5 (C6H6),
124.8 (p-C of L1), 124.1 (m-C of L1), 122.4 (m-C of amido-Ar*),
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Table 4 Crystal data and structure refinement for 1, 2, 4, 5, 6, 8 and 9
Compound
1
2
4
5
6
8
9
Empirical formula
C66H98I2N4-
O2Yb2
C50H74I2N4O2-
Si4Yb2
C31H55N3-
OSi4Yb
771.18
Monoclinic
P21/c (no. 14)
9.8514(2)
21.2659(4)
19.0709(4)
90
101.922(1)
90
3909.15(14)
4
C45H67N3-
OYb·C6H6
917.16
C40H68N2-
OSi2Yb
822.18
C52H64N2OYb
C46H46O2-
Yb·C7H8
896.00
Formula weight
Crystal system
Space group
1579.36
Monoclinic
C2/c (no. 15)
28.3326(4)
12.4318(2)
21.8144(4)
90
116.695(1)
90
6864.6(2)
4
1475.37
Monoclinic
P21/c (no. 14)
15.0479(4)
15.8523(7)
12.3722(6)
90
96.760(3)
90
2930.8(2)
2
906.09
Monoclinic
P21/c (no. 14)
23.6608(5)
9.0274(2)
21.7827(4)
90
100.391(1)
90
4576.4(2)
4
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1 (no. 2)
P1 (no. 2)
P1 (no. 2)
˚
a/A
10.1873(1)
12.2671(1)
20.0249(2)
79.262(1)
80.762(1)
76.620(1)
2373.81(4)
2
1.28
2.01
956
46 538
13.6465(2)
16.2031(2)
19.7654(3)
98.537(1)
97.971(1)
98.946(1)
4279.39(10)
4
1.28
2.27
1712
44 846
10.1293(2)
20.7321(4)
21.6554(5)
111.214(1)
96.485(1)
91.056(1)
4203.73(15)
4
1.42
2.27
1832
63 001
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
U/A
Z
D
calcd/Mg m-3
1.53
3.65
3136
1.67
4.35
1440
1.31
2.54
1584
1.32
2.08
1872
m/mm-1
F(000)
Reflections collected
Independent
reflections
31 737
6020 [Rint
0.064]
14 868
5121 [Rint
0.056]
33 366
6902 [Rint
0.068]
32 206
6287 [Rint
0.089]
=
=
=
10 787 [Rint
0.056]
=
14 833 [Rint
0.053]
=
=
15 191 [Rint
0.071]
=
Reflections with I >
5009
4096
5605
9950
12 434
4811
12 460
2s(I)
Data/restraints/
parameters
6020/0/345
5121/0/289
6902/0/361
10 787/0/538
14 833/0/833
6287/0/505
15 191/28/952
Goodness-of-fit on F2
Final R indices [I >
2s(I)]
1.019
1.097
1.034
1.018
1016
1.231
1.154
R1 = 0.038,
wR2 = 0.077
R1 = 0.052,
wR2 = 0.083
1.25 and -1.49
R1 = 0.037,
wR2 = 0.083
R1 = 0.053,
wR2 = 0.089
0.90 and -1.07
R1 = 0.029,
wR2 = 0.060
R1 = 0.042,
wR2 = 0.065
0.41 and -0.80
R1 = 0.024,
wR2 = 0.056
R1 = 0.028,
wR2 = 0.058
0.84 and -0.87
(near Yb)
R1 = 0.054,
wR2 = 0.138
R1 = 0.065,
wR2 = 0.148
5.66 and -1.81
R1 = 0.075,
wR2 = 0.134
R1 = 0.102,
wR2 = 0.1458
1.06 and -1.44
R1 = 0.059,
wR2 = 0.133
R1 = 0.075,
wR2 = 0.141
2.11 and -1.74
(near Yb)
R indices (all data)
Largest diffraction
peak and hole/e A-3
(thf), 25.1, 24.9, 24.7, 24.4, 24.0 ppm (NCMe and CHMe2, not
fully resolved).
peaks which have no chemical sense (the separation between these
peaks mirrors that of the two independent Yb atoms like “ghost”
images at very low occupancy). Complex 8 crystallised as very thin
needles having weak and rather fuzzy diffraction. In 9 there are
two independent molecules of the Yb complex. There are also two
independent molecules of toluene solvate, one ordered and one
disordered; both solvate molecules were refined with isotropic C
atoms, and the disordered solvate molecule had SADI constraints
and H atoms omitted; attempts to refine the non-disordered
solvate anisotropically resulted in unacceptable ellipsoids and were
abandoned. In all the figures only the larger occupancy orientation
is shown for disordered ligands. Absorption corrections using
MULTISCAN were applied. The structures were refined by a full-
matrix least-squares procedure based on F2, using SHELXL-97.24
Further details are in Table 4.
Synthesis of [Yb(CPh3)2(thf)2] (9). Solid KCPh3 (0.150 g,
0.52 mmol) was added to a solution of compound 2 (0.372 g,
0.25 mmol) in thf (20 mL) at -40 ◦C. The mixture was allowed to
warm up to room temperature and was stirred for 2 h. The solvent
was removed in a vacuum and the residue (a dark brown tar) was
extracted with toluene (2 ¥ 5 mL). The extract was concentrated,
covered with hexane and stored at -10 ◦C for 1 week yielding black
crystals of 9·PhMe (0.175 g, 39% based on Yb). 1H NMR (C7D8,
300 K): d 7.18 (d, 12 H, J = 7.7 Hz, o-H), 6.79 (t, 12 H, J = 7.5 Hz,
m-H), 6.32 (t, 6 H, J = 7.0 Hz, p-H), 3.32 (br s, 8 H, thf), 1.25 ppm
(br s, 8 H, thf) (peaks of solvate PhMe were overlapping with the
1
solvent signals). 13C{ H} NMR (C7D8, 300 K): d 145.1 (ipso-C of
Ph), 129.6 (m-CH of Ph), 123.5 (o-CH of Ph), 115.4 (p-CH of Ph),
94.4 (CPh3), 70.3 (thf), 25.4 ppm (thf).
Acknowledgements
We thank EPSRC for financial support; we acknowledge Prof.
Paul S. Pregosin and Dr Ignacio Ferna´ndez for PGSE NMR
measurements (complexes 5 and 8) and the late Dr Anthony G.
Avent for multinuclear NMR studies (1, 2, 6 and 7).
Crystal data and refinement details for 1, 2, 4, 5, 6, 8 and 9
Diffraction data were collected on a Nonius Kappa CCD diffrac-
tometer using monochromated MoKa radiation, l 0.71073 A at
173(2) K. Crystals were coated in oil and then directly mounted
˚
on the diffractometer under a stream of cold nitrogen gas.
Notes and references
In 4 the C30 atom of the thf ligand is disordered and the two
alternative positions were left isotropic. In 5 the H atom bound
to N3 (H3n) was refined, all the other H atoms were placed on
calculated positions and were refined in a riding mode. For 6
a somewhat low quality of crystals resulted in several problems
with the structure solution, including the presence of four residual
1 (a) L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031; (b) W. E. Piers and D. J. H. Emslie, Coord. Chem. Rev., 2002,
233, 131.
2 P. B. Hitchcock, S. A. Holmes, M. F. Lappert and S. Tian, J. Chem.
Soc., Chem. Commun., 1994, 2691.
3 D. Drees and J. Magull, Z. Anorg. Allg. Chem., 1994, 620, 814.
2390 | Dalton Trans., 2009, 2383–2391
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4 (a) A. G. Avent, A. V. Khvostov, P. B. Hitchcock and M. F. Lappert,
Chem. Commun., 2002, 1410; (b) O. Eisenstein, P. B. Hitchcock, A. V.
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