Divalent heteroleptic ytterbium complexes - Effective catalysts for intermolecular styrene hydrophosphination and hydroamination

Article abstract of DOI:10.1021/ic4027859

New heteroleptic Yb(II)-amide species supported by amidinate and 1,3,6,8-tetra-tert-butylcarbazol-9-yl ligands [2-MeOC₆H ₄NC(tBu)N(C₆H₃-iPr₂-2,6)] YbN(SiMe₃)₂(THF) (6) and [1,3,6

Full text of DOI:10.1021/ic4027859

Article
pubs.acs.org/IC
Divalent Heteroleptic Ytterbium Complexes − Effective Catalysts for
Intermolecular Styrene Hydrophosphination and Hydroamination
Ivan V. Basalov,† Sorin Claudiu Rosca,‡ Dmitry M. Lyubov,† Alexander N. Selikhov,† Georgy K. Fukin,†
̧
Yann Sarazin,‡ Jean-Francois Carpentier,*,‡ and Alexander A. Trifonov*,†
̧
†G. A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Nizhny Novgorod, Russia
‡Organometallics: Materials and Catalysis, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS − Universite
́
de Rennes 1,
Rennes, France
S
* Supporting Information
ABSTRACT: New heteroleptic Yb(II)−amide species supported by amidi-
nate and 1,3,6,8-tetra-tert-butylcarbazol-9-yl ligands [2-MeOC6H4NC(tBu)N-
(C6H3-iPr2-2,6)]YbN(SiMe3)2(THF) (6) and [1,3,6,8-tBu4C12H4N]Yb[N-
(SiMe3)2](THF)n (n = 1 (7), 2 (8)) were synthesized using the amine
elimination approach. Complex 6 features an unusual κ1-N,κ2-O,η6-arene
coordination mode of the amidinate ligand onto Yb(II). Complexes 7 and 8
represent the first examples of lanthanide complexes with π-coordination of
carbazol-9-yl ligands. Complexes 6 and 7, as well as the amidinate−Yb(II)−
amide [tBuC(NC6H3-iPr2-2,6)2]YbN(SiMe3)2(THF) (5), are efficient pre-
catalysts for the intermolecular hydrophosphination and hydroamination of
styrene with diphenylphosphine, phenylphosphine, and pyrrolidine to give
exclusively the anti-Markovnikov monoaddition product. For both types of
reaction, the best performances were observed with carbazol-9-yl complex 7 (TONs up to 92 and 48 mol/mol at 60 °C,
respectively).
ligands in intermolecular hydrophosphination and hydro-
amination of styrene.
INTRODUCTION
■
Extensive research activity pursued over the past three decades
in the field of organic derivatives of trivalent lanthanides has
revealed their high potential as catalysts (or precatalysts) for
various transformations involving unsaturated substrates, e.g.,
polymerization,1 hydrogenation,2 hydrosilylation,3 hydroamina-
tion,4 hydrophosphination,5 hydroalkoxylation,6 hydrothiola-
tion,6 and hydroboration.7 Besides, the catalytic properties of
divalent lanthanide complexes still remain poorly explored8
despite larger ionic radii9 and pronounced reductive proper-
ties,10 which make them promising candidates for catalytic
applications. Among these, hydroelementation reactions, that is,
the addition of E−H (E = N, P...) functionalities across C−C
multiple bonds, are atom-economic processes for the synthesis
of valuable compounds that may be one of the prospective
application fields of such divalent lanthanide complexes. Much
progress has been done in the area of lanthanide-promoted
intramolecular hydroamination and hydrophosphination; how-
ever, success is somewhat elusive for the intermolecular version
of these transformations.11 Recent achievements in intermo-
lecular hydroelementation reactions promoted by calcium
complexes,12 whose ionic radius is similar to that of Yb(II),9
prompted us to evaluate the catalytic activity of heteroleptic
Yb(II)−amides in intermolecular olefin hydrophosphination
and hydroamination. Herein, we report on the synthesis,
characterization, and catalytic activity of a series of heteroleptic
Yb(II)−amides supported by various multidentate N-based
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Divalent Ytterbium
Complexes. Amine elimination from Yb(II)−bis-
(trimethylsilylamide), [(Me3Si)2N]2Yb(THF)2 (1)13a read-
ily available precursor that can be prepared on a multigram
scalewas used for the synthesis of a series of heteroleptic
Yb(II)−amide complexes supported by N-containing ligands
(Scheme 1). The reactions of equimolar amounts of 1 and
proligands 2−4 were carried out in toluene at ambient
temperature or under moderate heating (70 °C). The synthesis
and characterization of [tBuC(NC6H3-iPr2-2,6)2]Yb[N-
(SiMe3)2](THF) (5) have been published recently.14 All
reactions occur with a release of one equivalent of HN(SiMe3)2,
1
which was eventually detected and quantified by H NMR
spectroscopy and GLC. Evaporation of volatiles under a
vacuum and subsequent recrystallization of the residual solids
from hexane afforded the new complexes 6 and 7 in 78% and
90% yields, respectively. Slow cooling of a solution of the
reaction product of 1 and 4 in a toluene−THF (20:1) mixture
at −30 °C allowed for the isolation of bis-THF adduct 8 in 63%
yield.
Received: November 6, 2013
© XXXX American Chemical Society
A
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Scheme 1
Complexes 6−8 are highly air- and moisture-sensitive
crystals, well soluble in aliphatic and aromatic hydrocarbons.
As evidenced by NMR spectroscopy, complexes 6−8 are
diamagnetic, in line with the divalent oxidation state of the
ytterbium atom.
Complex 6 crystallizes with two crystallographically
independent molecules in the asymmetric unit as established
by an X-ray diffraction study. The solid-state structure of 6
reveals that introduction of an additional Lewis base group (i.e.,
OMe) into one of the aryl substituents of the NCN core leads
to a dramatic change in the coordination mode of the amidinate
ligand (Figure 1). In fact, in complex 5, the bidentate amidinate
ligand adopts a classic κ2-N,N′-chelating coordination mode,14
while in 6 the monoanionic tridentate amidinate ligand
coordinates to Yb(II) in an unusual κ1-N,κ2-O,η6-arene fashion.
κ1-N,η6-arene coordination to ytterbium has been previously
documented for Yb(II) complexes with bulky guanidinate and
amidinate ligands14,15 while, to our knowledge, κ1-N,κ2-O,η6-
arene coordination fashion is described here for the first time.
Besides the tridentate amidinate ligand, the Yb(II) center in 6 is
bound to one monoanionic N(SiMe3)2− group and the oxygen
atom of the THF molecule. The formal coordination number of
Yb(II) in 6 is thus seven. The Yb−Namidinate and Yb−Arcentroid
distances in 6 (2.452(2) and 2.695(3) Å) are expectedly longer
compared to those in the related six-coordinated Yb(II)
complexes with κ1-N,η6-arene coordination [{tBuC(NC6H3-
2,6-iPr2)2}Yb(μ-H)]2 (2.329(3), 2.420(4) Å),14 [{tBuC-
(NC6H3-2,6-iPr2)2}Yb(μ-SCH2Ph)]2 (2.386(4), 2.408(4)
Å),15b and [{Cy2NC(NAr2)}Yb(μ-I)]2 (2.360(3), 2.424(4)
Å).15a No other examples of seven-coordinate Yb(II)
complexes featuring κ1-N,η6-arene coordination of the
amidinate ligand are known so far, limiting possible
comparisons. The Yb−Namide bond length in 6 (2.368(2) Å)
is noticeably shorter than those in seven-coordinate Yb(II)−
amide complexes [Na(THF)]+[Cp*2YbN(SiMe3)2]− (2.41(2)
Å)16 and {[(Me3Si)2N]Yb(μ-DAC)}2Yb (DAC = 4,13-diaza-
18-crown-6) (2.43(3)−2.44(3) Å).17 The Yb−Carene distances
fall into a rather large range (2.886(2)−3.157(3) Å, average =
3.034(3) Å).18 The Yb−O coordination bonds in complex 6
have similar lengths: Yb(1A)−O(1A) = 2.483(2) and Yb(1A)−
O(1SA) = 2.458(2) Å.
Figure 1. Molecular structure of [2-MeOC6H4NC(tBu)N(C6H3-iPr2-
2,6)]Yb[N(SiMe3)2](THF) (6). Only one of the two independent
molecules is shown. Hydrogen atoms, methyl fragments of the iPr
groups, and methylene fragments of the THF molecule are omitted for
clarity; thermal ellipsoids drawn at the 30% probability level. Bond
lengths (Å) and angles (deg): Yb(1)−N(3) 2.368(2), Yb(1)−N(2)
2.452(2), Yb(1)−O(1S) 2.458(2), Yb(1)−O(1) 2.483(2), Yb(1)−
C(6) 2.886(2), Yb(1)−C(7) 2.944(3), Yb(1)−C(8) 3.038(3),
Yb(1)−C(11) 3.043(2), Yb(1)−C(9) 3.138(3), Yb(1)−C(10)
3.157(3), Yb(1)−Si(2) 3.4788(8), Yb(1)−Si(1) 3.5501(8), N(1)−
C(1) 1.299(3), N(2)−C(1) 1.379(3); N(3)−Yb(1)−N(2) 122.43(7),
N(3)−Yb(1)−O(1S) 107.04(7), N(2)−Yb(1)−O(1S) 115.52(7),
N(3)−Yb(1)−O(1) 89.75(7), N(2)−Yb(1)−O(1) 64.60(6), O-
(1S)−Yb(1)−O(1) 78.13(6).
1
In the H NMR spectrum of 6 recorded in toluene-d8, the
methyl hydrogens of the amidinate isopropyl groups appear as a
broadened multiplet at δ 1.27−1.33 ppm overlapping with the
β-CH2 hydrogens of the THF molecule, and the methine
hydrogens of the isopropyl groups give one broad multiplet at δ
3.46 ppm. The α-CH2 THF hydrogens appear as a broadened
multiplet at δ 3.28 ppm. A singlet at δ 3.51 ppm is assigned to
B
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the methoxy group hydrogens of the amidinate ligand, while
the signals arising from the aromatic hydrogens of the 2-
methoxyphenyl and 2,6-diisopropylphenyl moieties give rise to
a multiplet in the region δ 6.36−6.80 ppm. The methyl
hydrogens of the silylamide ligand appear as a characteristic
sharp, high-field shifted singlet at δ 0.24 ppm. Variable
1
temperature H and 13C NMR spectra of complex 6 in the
range −60 to 50 °C were recorded in toluene-d8 and THF-d8;
however, they did not provide informative clues about the
actual arene coordination mode in solution, notably if the η6-
coordination observed in the solid state is maintained.
The X-ray diffraction studies of complexes 7 and 8 (Figures 2
and 3) revealed a very unusual π-coordination of the central
Figure 3. Molecular structure of [1,3,6,8-tBu4C12H4N]Yb[N-
(SiMe3)2](THF)2 (8). Hydrogen atoms, methyl fragments of the
tBu groups, and methylene fragments of the THF molecules are
omitted for clarity; thermal ellipsoids drawn at the 30% probability
level. Bond lengths (Å) and angles (deg): Yb(1)−N(1) 2.631(3),
Yb(1)−N(2) 2.323(3), Yb(1)−C(1) 2.706(4), Yb(1)−C(6) 2.899(4),
Yb(1)−C(7) 2.979(4), Yb(1)−C(12) 2.840(4), Yb(1)−O(1)
2.428(3), Yb(1)−O(2) 2.407(3), N(1)−Yb(1)−N(2) 121.3(1),
N(2)−Yb(1)−O(1) 95.9(1), N(2)−Yb(1)−O(2) 107.2(1), carbazo-
leCentr−Yb(1)−N(2) 136.8(4).
just slightly longer than the analogous Yb−C bonds in the
bis(fluorenyl)-ytterbium complex (C13H9)2Yb(THF)2
(2.852(7)−2.952(7) Å24). Curiously, the difference of coordi-
nation numbers of the metal centers in 7 and 8 does not affect
much the Yb−C distances (average Yb−C bond length: 7,
2.854(3); 8, 2.856(4) Å) while, at the same time, the Yb−
Figure 2. Molecular structure of [1,3,6,8-tBu4C12H4N]Yb[N-
(SiMe3)2](THF) (7). Hydrogen atoms, methyl fragments of the tBu
groups, and methylene fragments of the THF molecule are omitted for
clarity; thermal ellipsoids drawn at the 30% probability level. Bond
lengths (Å) and angles (deg): Yb(1)−N(1) 2.490(3), Yb(1)−N(2)
2.289(3), Yb(1)−O(1S) 2.368(3), Yb(1)−C(1) 2.678(4), Yb(1)−
C(6) 2.985(3), Yb(1)−C(7) 3.047(3), Yb(1)−C(12) 2.708(3);
N(1)−C(1) 1.373(5), N(1)−C(12) 1.386(5), C(1)−C(6) 1.434(5),
C(6)−C(7) 1.431(5), C(7)−C(12) 1.435(5); N(1)−Yb(1)−N(2)
130.2(1), N(1)−Yb(1)−O(1S) 125.2(1), N(2)−Yb(1)−O(1S)
99.1(1), Si(1)−N(2)−Si(2) 128.2(2), Si(1)−N(2)−Yb(1) 103.9(2),
Si(2)−N(2)−Yb(1) 128.0(2), carbazoleCentr−Yb(1)−N(2) 145.9(3).
N
carbazole distance in 8 (2.631(3) Å) is substantially longer than
in 7 (2.490(3) Å). Overall the geometric parameters of
complexes 7 and 8 suggest η5-coordination of the 1,3,6,8-tetra-
tert-butylcarbazol-9-yl ligands with a noticeable tilting toward
the η3-fashion, as observed in (C13H9)2Ln(THF)2 (Ln = Sm,
Yb) complexes.24,25 The Yb−NAmido bond lengths in 7 and 8
(2.289(3) and 2.323(3) Å, respectively) fall in the range of
values normally measured in Yb(II) complexes with terminal
N(SiMe3)2 ligands.13,17,26 In complex 7, the short contacts
Yb(1)−Si(1) (3.1465(11) Å) and Yb(1)−C(29) (2.841(4) Å),
together with noticeable distortion of the geometry around the
nitrogen (Si(1)−N(2)−Yb(1), 103.9(2)°, and Si(2)−N(2)−
Yb(1), 128.0(2)°) and silicon (C(29)−Si(1)−N(2),
107.5(2)°) atoms are indicative of an agostic interaction
between the metal center and one of the methyl groups of the
silylamide ligand.27 In complex 8, which contains a more
coordinatively saturated Yb(II) center, no such agostic
interaction was observed.
five-membered ring of 1,3,6,8-tetra-tert-butylcarbazol-9-yl li-
gands to the Yb(II) centers. Unlike lanthanide complexes of
unsubstituted19 or 1,8-disubstituted20 carbazol-9-yl ligands that
feature metal−ligand σ-interaction through the amide N-donor,
the ytterbium atoms in 7 and 8 are disposed above the pyrrolyl
ring (Yb−centroid distances = 2.517(8) Å in 7 and 2.545(7) Å
in 8).21 Along the Yb−NCarbazole bonds (7, 2.490(3); 8,
2.631(3) Å), bonding of the ytterbium center with two
neighboring pyrrolyl carbons are detected in both complexes.
The lengths of these two Yb−C bonds (7, 2.678(4) and
2.708(3) Å; 8, 2.706(4) and 2.840(4) Å) are comparable to
those observed in related Yb(II) cyclopentadienyl complexes
[for comparison, see {(C5Me5)Yb[μ-N(SiMe3)2]}2: Yb−Caverage
= 2.75(2) Å;22 (C5Me5)Yb[N(SiMe3)2](THF)2, Yb−C =
2.675(4)−2.727(4) Å23]. Moreover, two somewhat longer
contacts between the ytterbium center and the two remaining
distal carbons of the pyrrolyl ring are detected (7, 2.985(3) and
3.047(3) Å; 8, 2.899(4) and 2.979(4) Å). These distances are
1
In the H NMR spectrum of 7 recorded in C6D6 solution at
ambient temperature, the tBu groups of the carbazolyl ligand
give rise to a slightly broadened singlet at δ 1.48 ppm; the
aromatic hydrogens of the carbazolyl moiety appear as two
doublets at δ 7.59 and 8.23 ppm (4JH−H = 1.4 Hz). A sharp,
high-field shifted singlet at δ 0.37 ppm corresponds to the
silylamide ligand. The hydrogens of the THF molecule give two
1
slightly broadened singlets at δ 1.27 and 3.53 ppm. The H
C
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NMR spectrum of 8 is similar to that of 7 with the only
difference being the integral intensity of the signals
corresponding to the THF hydrogens.
Hydrophosphination Catalysis. Ytterbium(II) complexes
5−7 were investigated as catalyst precursors in the hydro-
phosphination reaction between styrene and Ph2PH or PhPH2,
chosen as model reactions. Representative results are reported
in Table 1. All reactions were found to be regioselective,
paramagnetic Yb(III) species, especially at high styrene
concentrations.29 This putative oxidation of divalent ytterbium
with styrene, in the presence of phosphine (or amine, vide
infra), is reminiscent of our recent results with related divalent
lanthanide systems.28
More detailed kinetic data about this reaction were sought
after using 7 as a precatalyst, with relatively large catalyst
loadings (>6.6 mol-%) to prevent as much as possible problems
connected to the above-mentioned catalyst decay. To
determine the partial order in Ph2PH, its conversion over
time was studied using a large excess of styrene, at three
different [phosphine]/[7] ratios (see the Supporting Informa-
tion, Figures S6−S8). In all cases, similar values of kapp were
obtained (0.107 s−1, 0.133 s−1, and 0.125 s−1, that is, 0.120
0.013 s−1), thus suggesting a zeroth order dependence on
phosphine. Another interesting observation in these experi-
ments is the induction period which was observed in all cases.
In fact, the larger the amount of Ph2PH, the shorter the
induction period: 3 equiv, 98 s; 7 equiv, 81 s; 15 equiv, 36 s
(data obtained by extrapolation of least-squares linear
regression laws, see Figures S6−S8). This may suggest that
the Yb(II)−amide complex reacts with the phosphine to
produce the real catalytic species, tentatively assigned as a
Yb(II)−PPh2 species. Unfortunately, attempts to prepare this
putative phosphide species, through independent reactions of
Ph2PH with 7, have been unsuccessful thus far.
In order to determine the partial order in the catalyst, the
conversion of the phosphine was studied using a large excess of
styrene at different catalyst concentrations (over a 10-fold
range, in the range 6.25−63 mM) while maintaining the total
volume of reactants and solvent constant. The apparent rate
constants (kapp) were thus determined for substrate conversion
below 70% (see the Supporting Information Table S1). The
plot of ln(kapp) vs ln([7]0) gave a straight line (R2 = 0.957) with
a slope of 0.5 corresponding to the apparent kinetic order for
the precatalyst (Figure 4; see also Figure S9).
Table 1. Hydrophosphination of Styrene with Ph2PH and
a
PhPH2 Catalyzed by Yb(II)−Amide Complexes 5−7
time
[h]
conversion
[%]
entry
complex [mol %]
5 (2.0)
phosphine
1
Ph2PH
Ph2PH
Ph2PH
Ph2PH
Ph2PH
Ph2PH
Ph2PH
Ph2PH
PhPH2
PhPH2
Ph2PH
0.25
2
27
55
17
27
31
71
92
15
30
61
24
2
5 (2.0)
6 (2.0)
6 (2.0)
7 (2.0)
7 (2.0)
7 (1.0)
7 (0.5)
7 (2.0)
7 (2.0)
3
0.25
2
4
5
0.25
2
6
7
4
8
8
9
10
60
2
10
11
[(Me3Si)2N]2Yb(THF)2
(2.0)
a
Reaction conditions: neat substrates in a 1:1 ratio, 60 °C. Conversion
1
determined by H NMR spectroscopy in C6D6
forming exclusively the anti-Markovnikov addition product.
Complex 7 was found to be the most productive over the series
(compare entries 2, 4, and 6), although all three complexes
featured performance of the same magnitude. The catalytic
activity of 5−7 in the hydrophosphination of styrene with
diphenylphosphine, especially that of 7, is remarkable and is
commensurate to that which we recently disclosed for an
Yb(II)−amide complex supported by an anilido−imine
ligand.28 Notably, the performance of 5 and 7 is clearly
superior to that of the simple precursor [(Me3Si)2N]2Yb-
(THF)2 (entry 11). The reactivity of phenylphosphine with 7
proved considerably lower than that of diphenylphosphine
(entries 9−10). Of note, only the primary hydrophosphination
product was formed in the latter reactions with PhPH2; i.e., no
(PhCH2CH2)2PPh eventually resulting from subsequent
reaction of PhCH2CH2PHPh with styrene was observed.
Besides experiments using 50 equiv of styrene and Ph2PH,
the limit of complex 7 was evaluated by increasing the
substrates loading up to 100 and 200 equiv (with [styrene]/
[Ph2PH] = 1:1). The neat reaction using 100 equiv of
substrates reached 92% conversion after 4 h at 60 °C. However,
in the presence of 200 equiv of substrates, only 15% conversion
was observed after 8 h, suggesting catalyst decay. Actually, in
the experiments performed over 2 h at such low catalyst
loadings, we noticed that the color of the solution gradually
changed from deep red to pale yellow; this is consistent with an
oxidation of Yb(II) to Yb(III). This hypothesis is also
corroborated by the observation of broad resonances in all
NMR spectra of the crude reaction mixtures, diagnostic of
Figure 4. Plot of ln(kapp) vs ln([7]0) for the intermolecular
hydrophosphination of styrene and Ph2PH catalyzed by 7 at different
catalyst concentrations (6.25, 13.0, 25.0, 34.0, 47.0, and 63.0 mM).
Reaction conditions: 60 °C, [styrene]0 = 1.34 M, [styrene]0/[Ph2PH]0
= 10:1, C6D6 + Ph2PH + styrene =0.6 mL.
Further experiments were conducted to determine the partial
order in styrene, using a constant concentration of the catalyst
and the phosphine and varying the concentration of styrene in
the reaction mixture in a ca. 10-fold range (in the range 0.33−
2.61 M).30 Similar treatment as that performed above gave an
apparent partial order in styrene of 0.72 (R2 = 0.985). However,
because of the narrow concentration range (due to
D
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experimental constraints),30 a substantial error exists, and the
values are also quite consistent with a partial first-order on
styrene (R2 = 0.962; see Figures S10−S12 and Table S2). Since
a first order is usually observed for intermolecular hydro-
phosphination of styrene with group 2 and 3 divalent
metals,11,12,28 we assume this is also the case here.
hydroamination took place under the same conditions: this
complex immediately decomposed (likely oxidized) in the
presence of styrene and pyrrolidine (also evidenced by the
rapid color change from purple to light yellow concomitant
with the formation of a precipitate), and only the starting
reagents were observed.28
On the basis of previous results with related Yb(II) catalytic
systems,28 we anticipated to find a zeroth order for Ph2PH and
a first order for styrene and the catalyst, respectively. In fact, the
data with 7 are consistent with a zeroth order in phosphine and
a first order in styrene but indicate a 0.5 order for the catalyst,
suggesting that the precatalyst (putatively a Yb(II)-phosphide)
has a dimeric association in solution or is involved in a
reversible dimer−monomer aggregation phenomenon. In order
to obtain additional evidence for a dimeric structure of the real
catalytic species in solution, the NMR-tube reactions of
complexes 5−7 with equimolar amounts of Ph2PH (C6D6,
r.t.) were carried out. All the reactions were found to occur with
CONCLUSIONS
■
The amine elimination reactions of [(Me3Si)2N]2Yb(THF)2
with amidine 2-MeOC6H4NHC(tBu)N(C6H3-iPr2-2,6) and
1,3,6,8-tetra-tert-butylcarbazol pro-ligands allowed for the
synthesis of new heteroleptic Yb(II) amido complexes which
were isolated in high yields. The solid state structures of
complexes 6−8 revealed profound changes of coordination
modes of conventional ligands provoked by modification of
their denticity and steric bulkiness. The introduction of an
additional Lewis base group (i.e., OMe) into one of the aryl
substituents of the amidinate ligand leads to an unprecedented
κ1-N,κ2-O,η6-arene coordination, instead of the classic κ2-N,N′
mode. Similarly, replacing a simple carbazol-9-yl by 1,3,6,8-
tetra-tert-butylcarbazol-9-yl ligand results in the switch of
coordination mode from σ to π.
The Yb(II)−amide complexes 5−7 all proved able to
promote intermolecular hydroelementation of styrene under
mild conditions; however, their catalytic performances are quite
contrasted. The carbazol-9-yl-Yb(II)-amide revealed superior
abilities for both hydrophosphination and hydroamination, as
compared to amidinates 5 and 6. This can be tentatively related
to the lower (formal) coordination number of complex 7, as in
this type of catalysis, activity tends to increase when the C.N. of
the catalyst decreases. The high activity of complex 7 in
hydrophosphination is commensurate to that of a recently
reported {anilido-imine}Yb(II)-amide,28 but it is remarkable
that it also promotes efficiently the addition of pyrrolidine onto
styrene, while the {anilido-imine}Yb(II)-amide proved com-
pletely unable to do so, due to rapid decomposition/oxidation
in the presence of these reagents. This clearly evidences that
the reactivity and stability of those classes of divalent lanthanide
complexes can be largely tuned with appropriate ligand
frameworks. Research along these lines is currently under
progress in our laboratories.
the release of amine and formation of precipitates, albeit the 31
P
and 171Yb spectra of the reaction mixtures turned out to be
uninformative. In particular, no coupling patterns were
observed in the 31P and 171Yb spectra.
Hydroamination Catalysis. Complexes 5−7 have been also
investigated briefly in the intermolecular hydroamination of
styrene and pyrrolidine. Experiments were performed in neat
reagents, using a 2 mol loading of the precatalyst at 60 °C.
Representative results are reported in Table 2. In all cases, only
Table 2. Intermolecular Hydroamination of Styrene with
Pyrrolidine Promoted by 5−7
entry
complex [mol %]
5 (2.0)
time [h]
conv. [%]
1
2
3
4
31
31
31
31
12
85
96
37
6 (2.0)
7 (2.0)
[(Me3Si)2N]2Yb(THF)2 (2.0)
the anti-Markovnikov hydroamination product was observed.
As for hydrophosphination reactions, complex 7 proved to be
the most productive, with nearly complete conversion after 31
h of reaction, somewhat better than with 6, and much better
than with 5 or the simple precursor [(Me3Si)2N]2Yb(THF)2. In
fact, with the latter complexes 5 and [(Me3Si)2N]2Yb(THF)2,
the color of the reaction mixture turned from red to yellow
within minutes, indicating rapid catalyst decay (oxidation); in
these cases, and in contrast to the hydroamination reactions
conducted with 6 and 7 (see Figure S13), the NMR spectra of
the crude reaction mixtures featured rather broad resonances,
indicating the presence of Yb(III) species; also, formation of an
unidentified precipitate was noted when [(Me3Si)2N]2Yb-
(THF)2 was used as a catalyst precursor. It is important to
note that the yet unidentified species resulting from the
oxidation of 5 under the reaction conditions is (are) not active
(or at least significantly less than 5). This provides indirect
evidence that the true active species in these hydroelementation
reactions are indeed divalent ytterbium species.
EXPERIMENTAL SECTION
■
General Considerations. All experiments were performed in
evacuated tubes, using standard Schlenk-flask or glovebox techniques,
with rigorous exclusion of traces of moisture and air. After drying over
KOH, THF and DME were purified by distillation from sodium/
benzophenone ketyl, hexane, and toluene by distillation from sodium/
triglyme benzophenone ketyl prior to use. C6D6 toluene-d8 and THF-
d8 were dried with sodium/benzophenone ketyl and condensed in a
vacuum prior to use. Yb[N(SiMe3)2]2(THF)2 was prepared according
to literature procedures.13 PhSiH3 was purchased from Aldrich and
was dried over CaH2 and condensed in a vacuum prior to use.
(PhCH2S)2 and Ph3SnCl were purchased from Acros and were used
without additional purification. Styrene and pyrrolidine were
purchased from Aldrich or Acros and were vacuum-distilled over
CaH2 and then were degassed by freeze−pump−thaw methods.
Diphenylphosphine and phenylphosphine were purchased from
Aldrich and used as received.
Instruments and Measurements. NMR spectra were recorded
on a Bruker DPX 200 or Bruker Avance DRX-400 spectrometer.
Chemical shifts for 1H and 13C1{H} spectra were referenced internally
using the residual solvent resonances and are reported relative to TMS.
Lanthanide metal analysis was carried out by complexometric
The catalytic performance of precatalysts 6 and 7 are
remarkable since with another Yb(II)−amide complex
supported by an monoanionic anilido−imine ligand,28 no
E
dx.doi.org/10.1021/ic4027859 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Crystallographic Data and Structure Refinement Details for 6−8
6
7
8
formula
Mr
C34H59N3O2Si2Yb
771.06
C38H66N2OSi2Yb
796.15
C46H82N2O3Si2Yb
940.36
cryst size, mm
cryst syst
space group
a, Å
0.19 × 0.15 × 0.09
triclinic
0.23 × 0.14 × 0.11
orthorhombic
P2(1)2(1)2(1)
9.3830(3)
12.5151(5)
34.574(1)
90
0.28 × 0.10 × 0.08
orthorhombic
Pna2(1)
18.663(1)
24.031(2)
10.9921(8)
90
P1
̅
10.3636(6)
17.943(1)
20.441(1)
79.836(2)
88.185(2)
87.923(2)
3737.7(4)
4
b, Å
c, Å
α, deg
β, deg
90
90
γ, deg
90
90
cell volume, Å
Z
4060.0(3)
4
4929.8(6)
4
D
calc, g/cm3
1.370
1.302
1.267
μ, mm−1
2.598
2.391
1.983
F000
1592
1656
1976
2θ range, °
index ranges
54
54
52
−13 ≤ h ≤ 13
−22 ≤ k ≤ 22
0 ≤ l ≤ 26
−11 ≤ h ≤ 11
−15 ≤ k ≤ 15
−44 ≤ l ≤ 43
37676
−23 ≤ h ≤ 23
−29 ≤ k ≤ 29
−13 ≤ l ≤ 13
41419
reflns collected
independent reflns
Rint
26106
8796
9682
0.0646
0.0809
completeness to θ
data/restraints/params
GoF
98.5
99.4
99.9
26106/0/786
1.010
8796/1/403
1.039
9682/22/518
0.978
R1 (I > 2σ(I))
0.0298
0.0356
0.0414
wR2 (all data)
0.0674
0.0749
0.0778
max. and min transmission
absolute structure parameter
largest diff. peak and hole, e/Å3
0.7998/0.6381
0.7789/0.6093
0.8575/0.6067
−0.019(9)
1.056/−0.785
2.063/−0.948
1.428/−0.567
titration.31 C, H, and N elemental combustion analysis was performed
in the microanalytical laboratory of IOMC.
[1,3,6,8-tBu4C12H4N]Yb[N(SiMe3)2](THF) (7). A solution of 1,3,6,8-
tBu4C12H4NH (0.502 g, 1.29 mmol) in toluene (10 mL) was added to
a solution of [(Me3Si)2N]2Yb(THF)2 (0.820 g, 1.29 mmol) in toluene
(15 mL) at room temperature. The reaction mixture was stirred at 70
°C for 3 days, volatiles were removed in a vacuum, the solid residue
was dissolved in hexane (20 mL), and the resulting solution was
centrifuged. Slow concentration of the hexane solution at room
temperature and prolonged cooling of the concentrated solution at
−30 °C resulted in the formation of complex 7 as bright orange
crystals. The mother liquid was decanted, and the crystals were washed
with cold hexane and dried in a vacuum for 30 min. Complex 7 was
[2-MeOC6H4NC(tBu)N(C6H3-iPr2-2,6)]YbN(SiMe3)2(THF) (6). A sol-
ution of 2-MeOC6H4NHC(tBu)N(C6H3-iPr2-2,6) (0.360 g, 0.98
mmol) in toluene (10 mL) was added to a solution of
[(Me3Si)2N]2Yb(THF)2 (0.628 g, 0.98 mmol) in toluene (10 mL)
at room temperature. The reaction mixture was stirred for 1 h, and
then volatiles were removed in a vacuum. The solid residue was
dissolved in hexane (40 mL), and the resulting solution was
centrifuged. Slow concentration of the hexane solution at room
temperature resulted in the formation of complex 6 as dark red
crystals. The mother liquid was decanted, and the crystals were washed
with cold hexane and dried in a vacuum for 30 min. Complex 2 was
1
isolated in 90% yield (0.921 g, 1.16 mmol). H NMR (400 MHz,
C7D8, 293 K): δ 0.37 (s, 18H, SiMe3), 1.27 (br m, 4H, β-CH2 THF),
1
1.48 (s, 36H, CH3 tBu), 3.53 (br m, 4H, α-CH2 THF), 7.59 (d, 4JHH
=
isolated in 78% yield (0.588 g, 0.76 mmol). H NMR (400 MHz,
4
C7D8, 293 K): δ 0.24 (s, 18H, SiMe3), 1.27−1.33 (complex m, 16H,
CH3 iPr and β-CH2 THF), 1.65 (s, 9H, CH3 tBu), 3.28 (br s, 4H, α-
CH2 THF), 3.46 (br m, 2H, CH iPr), 3.51 (s, 3H, OMe), 6.36 (br d,
3JHH = 7.3 Hz, 1H, CH C6H4OMe), 6.50 (complex m, 2H, CH
C6H4OMe), 6.80 (complex m, 4H, CH C6H4OMe and C6H3iPr2)
ppm. 13C{1H} NMR (100 MHz, C7D8, 293 K): δ 5.7 (s, SiMe3), 21.2
(s, CH3 iPr), 21.5 (s, CH3 iPr), 23.4 (s, CH3 iPr), 24.9 (s, β-CH2
THF), 28.3 (s, CH3 iPr), 28.4 (s, CH iPr), 29.1 (s, CH iPr), 31.7 (s,
CH3 tBu), 41.1 (s, C tBu), 56.4 (s, CH3 OMe), 68.6 (s, α-CH2 THF),
110.4 (s, CH C6H4OMe), 115.9 (s, CH C6H4OMe), 120.3 (s, CH
C6H4OMe), 121.9 (s, CH C6H3iPr2), 122.6 (s, CH C6H4OMe), 123.3
(s, CH C6H3iPr2), 123.6 (s, CH C6H3iPr2), 136.8 (s, C ipso-
NC6H4OMe), 140.6 (s, C orto-NC6H3iPr2), 142.6 (s, C orto-
NC6H3iPr2), 147.4 (s, C orto-NC6H4OMe), 154.2 (s, C ipso-
NC6H3iPr2), 174.0 (s, C N−CN) ppm. Elem. anal. calcd for
C34H59N3O2Si2Yb (771.08 g/mol): C, 52.96; H, 7.71; N, 5.45; Yb,
22.44. Found: C, 53.34; H, 7.53; N, 5.89; Yb, 22.70.
1.4 Hz, 2H, CH C12H4N), 8.23 (d, JHH = 1.4 Hz, 2H, CH C12H4N)
ppm. 13C{1H} NMR (100 MHz, C7D8, 293 K): δ 5.3 (s, SiMe3), 24.7
(s, β-CH2 THF), 30.1 (s, CH3 tBu), 31.9 (s, CH3 tBu), 34.4 (s, C tBu),
34.7 (s, C tBu), 69.3 (s, α-CH2 THF), 114.3 (s, CH C12H4N), 120.0
(s, CH C12H4N), 124.6 (s, C C12H4N), 131.7 (s, C C12H4N), 135.6 (s,
C C12H4N), 1420 (s, C C12H4N) ppm. Elem. anal. calcd for
C38H66N2OSi2Yb (796.17 g/mol): C, 57.33; H, 8.36; N, 3.52; Yb,
21.74. Found: C, 57.68; H, 8.70; N, 3.29; Yb, 21.90.
[1,3,6,8-tBu4C12H4N]Yb[N(SiMe3)2](THF)2 (8). A solution of 1,3,6,8-
tBu4C12H4NH (0.550 g, 1.41 mmol) in toluene (10 mL) was added to
a solution of [(Me3Si)2N]2Yb(THF)2 (0.899 g, 1.41 mmol) in toluene
(15 mL) at room temperature. The reaction mixture was stirred at 70
°C for 3 days, and volatiles were then removed in a vacuum. Complex
8 was obtained after recrystallization of the solid reaction products
from a toluene/THF (10:1) mixture by slow concentration at room
temperature as bright orange cubic crystals. The mother liquid was
decanted, and the crystals were washed with a small amount of cold
F
dx.doi.org/10.1021/ic4027859 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
hexane. Complex 8 was isolated in 85% yield (1.040 g, 1.20 mmol). 1H
NMR (400 MHz, C7D8, 293 K): δ 0.37 (s, 18H, SiMe3), 1.27 (br m,
8H, β-CH2 THF), 1.48 (s, 36H, CH3 tBu), 3.53 (br m, 8H, α-CH2
4511. (c) Gromada, J.; Carpentier, J.-F.; Mortreux, A. Coord. Chem.
Rev. 2004, 248, 397−410.
(2) (a) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.;
Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091−8103.
(b) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T.
J. J. Am. Chem. Soc. 1985, 107, 8103−8110.
4
4
THF), 7.59 (d, JHH = 1.4 Hz, 2H, CH C12H4N), 8.23 (d, JHH = 1.4
Hz, 2H, CH C12H4N) ppm. 13C{1H} NMR (100 MHz, C7D8, 293 K):
δ 5.3 (s, SiMe3), 24.7 (s, β-CH2 THF), 30.1 (s, CH3 tBu), 31.9 (s, CH3
tBu), 34.4 (s, C tBu), 34.7 (s, C tBu), 69.3 (s, α-CH2 THF), 114.3 (s,
CH C12H4N), 120.0 (s, CH C12H4N), 124.6 (s, C C12H4N), 131.7 (s,
C C12H4N), 135.6 (s, C C12H4N), 1420 (s, C C12H4N) ppm. Elem.
anal. calcd for C42H74N2O2Si2Yb (868.27 g/mol): C, 58.10; H, 8.59;
N, 3.23; Yb, 19.93. Found: C, 58.18; H, 8.70; N, 3.29; Yb, 19.90.
Hydrophosphination/Hydroamination Experiments. In a
typical hydrophosphination experiment, the complex was loaded in
an NMR tube in the glovebox. Styrene and Ph2PH were then added in
and the reaction time started after heating the NMR tube at 60 °C in a
preheated oil bath. After the desired reaction time, C6D6 was added to
the reaction mixture, and the 1H NMR spectrum was recorded shortly
after at regular time intervals. Conversion was determined by
integrating the remaining styrene and the newly formed addition
product.
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X-Ray Crystallography. The X-ray data for 6−8 were collected on
a Smart Apex diffractometer (graphite monochromated, MoKα
radiation, ω-scan technique, λ = 0.71073 Å, T = 100(2) K). The
structures were solved by direct methods and were refined on F2 using
the SHELXTL package.32 All non-hydrogen atoms were found from
Fourier syntheses of electron density and were refined anisotropically.
All hydrogen atoms were placed in calculated positions and were
refined in the riding model. SADABS33 was used to perform area-
detector scaling and absorption corrections. The X-ray data for
complex 6 were refined using the HKLF5 method. The details of
crystallographic, collection, and refinement data are shown in Table 3,
and corresponding CIF files are available as Supporting Information.
CCDC-969365 (6), -969366 (7), and -969367 (8) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via ccdc.cam.ac.uk/products/csd/request.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Representative 1H and 13C NMR spectra of ytterbium
complexes; crystallographic data for 6, 7, and 8 as CIF files;
and representative NMR spectra and kinetic monitoring for
hydrophosphination reactions. This material is available free of
charge via the Internet at http://pubs.acs.org
(14) Basalov, I. V.; Lyubov, D. M.; Fukin, G. K.; Shavyrin, A. S.;
Trifonov, A. A. Angew. Chem., Int. Ed. 2012, 51, 3444−3447.
(15) (a) Heitmann, D.; Jones, C.; Junk, P. C.; Lippert, K. A.; Stasch,
A. Dalton Trans. 2007, 187−189. (b) Basalov, I. V.; Lyubov, D. M.;
Fukin, G. K.; Cherkasov, A. V.; Trifonov, A. A. Organometallics 2013,
32, 1507−1516.
(16) Hou, Z.; Zhang, Y.; Tezuka, H.; Xie, P.; Tardif, O.; Koizumi, T.;
Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc. 2000, 122, 10533−
10543.
AUTHOR INFORMATION
Corresponding Authors
*Fax: (+33)(0)2 23 23 6939. E-mail: jean-francois.carpentier@
univ-rennes1.fr.
■
*Fax:(+7)8314621497. E-mail: trif@iomc.ras.ru.
Notes
The authors declare no competing financial interest.
(17) Lee, L.; Berg, D. J.; Bushnell, G. W. Inorg. Chem. 1994, 33,
5302−5308.
ACKNOWLEDGMENTS
■
(18) (a) Niemeyer, M. Eur. J. Inorg. Chem. 2001, 1969−1981.
(b) Deacon, G. B.; Forsyth, C. M.; Junk, P. C. Eur. J. Inorg. Chem.
2005, 817−821. (c) Evans, W. J.; Champagne, T. M.; Ziller, J. W.
Organometallics 2007, 26, 1204−1211.
The study was supported the Russian Foundation for Basic
Research (12-03-93109-HUHNΠ_a), Program of the Presi-
dium of the Russian Academy of Science (RAS), RAS
Chemistry and Material Science Division, and the French
ANR GreenLAkE project (grant ANR-11-BS07-009-01 to
SCR). The authors also thank the Groupe de Recherche
International (GDRI) CNRS-RAS “Homogeneous Catalysis for
Sustainable Development” for support.
(19) (a) Muller-Buschbaum, K.; Quitmann, C. C. Eur. J. Inorg. Chem.
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2004, 4330−4337. (b) Evans, W. J.; Rabe, G. W.; Ziller, J. W.
Organometallics 1994, 13, 1641−1645.
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67.
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Chem.Eur. J. 2011, 17, 5381−5386. (b) Moorhouse, R. S.; Moxey,
G. J.; Ortu, F.; Reade, T. J.; Lewis, W.; Blake, A. J.; Kays, D. L. Inorg.
Chem. 2013, 52, 2678−2683. (c) Blake, A. J.; Lewis, W.; McMaster, J.;
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