C-H bond activation and isoprene polymerization by rare-earth-metal tetramethylaluminate complexes bearing formamidinato N-ancillary ligands

Article abstract of DOI:10.1021/om301010m

The bimetallic formamidinate complexes Ln(Form)(AlMe₄) ₂ (Ln = Y, Form (ArNCHNAr) = EtForm (Ar = 2,6-Et₂C ₆H₃), MesForm (Ar = 2,4,6-Me₃C ₆H₂), DippForm (Ar = 2,6-iPr₂C ₆H₃), tBuForm (Ar = 2-tBuC₆H₄); Ln = La, Form = DippForm, tBuForm) were obtained in high yield by protonolysis reactions between formamidines (FormH) and homolepticrare-earth-metal tetramethylaluminates Ln(AlMe₄)₃. Y(Form)(AlMe ₄)₂ (Form = EtForm, DippForm) were also prepared by treatment of Y(Form)[N(SiHMe₂)₂]₂(thf) with trimethylaluminum after the former were prepared by the protonolysis of Y[N(SiHMe₂)₂]₃(thf)₂ complexes with EtFormH or DippFormH. The monomeric six-coordinate complexes Ln(Form)(AlMe ₄)₂ (Ln = Y, Form = EtForm, MesForm, DippForm, tBuForm; Ln = La, Form = DippForm, tBuForm) show similar molecular structures with distorted-octahedral geometry and bidentate (N,N′) Form and AlMe ₄ ligands. The complex [La(EtFormAlMe₃)(AlMe ₄)₂](C₇H₈)_1.5 from a protonolysis reaction between La(AlMe₄)₃ and EtFormH has the EtForm ligand adopting a configuration in which one nitrogen and one aryl substituent are coordinated to the eight-coordinate lanthanum center in an η¹(N):η⁶(arene) manner. From the reaction of La(AlMe₄)₃ with MesFormH, C-H bond activation of an o-methyl group of the mesityl moiety occurred, yielding [La{η¹(N) :η⁶(Ar)-Me₂CH₂FormAlMe₃} (AlMe₃)(AlMe₄)][La(Me₂CH ₂FormAlMe₃)(AlMe₃)(AlMe₄)](C ₆H₁₄)_1.5 (Me₂CH₂Form = MesForm-H(o-Me)), in which two linkage isomers of Me₂CH ₂Form were observed. Investigations were carried out on the compounds [Ln(Form)(AlMe₄)₂] (Ln = Y, La; Form = EtForm, DippForm) as precatalysts activated by [Ph₃C][B(C₆F ₅)₄] or [PhNMe₂H][B(C₆F ₅)₄] in isoprene polymerization. While the lanthanum complexes showed narrower molecular weight distributions (PDI < 1.2), a stereodirecting role was evidenced for the cocatalysts (trityl borate, maximum 87% trans-1,4-selectivity; anilinium borate, maximum 82% cis-1,4-selectivity).

Full text of DOI:10.1021/om301010m

Article
pubs.acs.org/Organometallics
C−H Bond Activation and Isoprene Polymerization
by Rare-Earth-Metal Tetramethylaluminate Complexes
Bearing Formamidinato N‑Ancillary Ligands
Shima Hamidi,† Lars N. Jende,‡ H. Martin Dietrich,‡ Cacilia Maichle-Mossmer,‡ Karl W. Tornroos,§
̈
̈
̈
Glen B. Deacon,*,† Peter C. Junk,*,∥ and Reiner Anwander*,‡
†School of Chemistry, Monash University, 3800 Clayton, Victoria, Australia
‡Institut fur Anorganische Chemie, Universitat Tubingen, Auf der Morgenstelle 18, D-72076 Tubingen, Germany
̈
̈
̈
̈
§Department of Chemistry, University of Bergen, Alleg
́
aten 41, N-5007 Bergen, Norway
∥School of Pharmacy & Molecular Sciences, James Cook University, Townsville, Queensland 4811, Australia
S
* Supporting Information
ABSTRACT: The bimetallic formamidinate complexes Ln-
(Form)(AlMe4)2 (Ln = Y, Form (ArNCHNAr) = EtForm (Ar =
2,6-Et2C6H3), MesForm (Ar = 2,4,6-Me3C6H2), DippForm (Ar =
2,6-iPr2C6H3), tBuForm (Ar = 2-tBuC6H4); Ln = La, Form =
DippForm, tBuForm) were obtained in high yield by protonoly-
sis reactions between formamidines (FormH) and homoleptic
rare-earth-metal tetramethylaluminates Ln(AlMe4)3. Y(Form)-
(AlMe4)2 (Form = EtForm, DippForm) were also prepared by
treatment of Y(Form)[N(SiHMe2)2]2(thf) with trimethylaluminum after the former were prepared by the protonolysis of
Y[N(SiHMe2)2]3(thf)2 complexes with EtFormH or DippFormH. The monomeric six-coordinate complexes Ln(Form)(AlMe4)2
(Ln = Y, Form = EtForm, MesForm, DippForm, tBuForm; Ln = La, Form = DippForm, tBuForm) show similar molecular
structures with distorted-octahedral geometry and bidentate (N,N′) Form and AlMe4 ligands. The complex [La(EtFormAlMe3)-
(AlMe4)2](C7H8)1.5 from a protonolysis reaction between La(AlMe4)3 and EtFormH has the EtForm ligand adopting a con-
figuration in which one nitrogen and one aryl substituent are coordinated to the eight-coordinate lanthanum center in an
η1(N):η6(arene) manner. From the reaction of La(AlMe4)3 with MesFormH, C−H bond activation of an o-methyl group of the
mesityl moiety occurred, yielding [La{η1(N):η6(Ar)-Me2CH2FormAlMe3}(AlMe3)(AlMe4)][La(Me2CH2FormAlMe3)(AlMe3)-
(AlMe4)](C6H14)1.5 (Me2CH2Form = MesForm-H(o-Me)), in which two linkage isomers of Me2CH2Form were observed.
Investigations were carried out on the compounds [Ln(Form)(AlMe4)2] (Ln = Y, La; Form = EtForm, DippForm) as
precatalysts activated by [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4] in isoprene polymerization. While the lanthanum com-
plexes showed narrower molecular weight distributions (PDI < 1.2), a stereodirecting role was evidenced for the cocatalysts
(trityl borate, maximum 87% trans-1,4-selectivity; anilinium borate, maximum 82% cis-1,4-selectivity).
Variation of the substituents on the aryl rings can modulate the
steric and electronic effects of the ligands as well as their solubili-
ties.8,35−37 A range of sterically different tris(formamidinato)
rare-earth-metal(III) complexes has been reported previously,
mostly synthesized by the redox transmetalation/protonoly-
sis (RTP) route.8,9,13 However, the bulkiest ligand caused C−F
bond activation to give [Ln(Form)2F(thf)] complexes, derived
from tetrafluorobenzyne elimination from a putative [Ln-
(Form)2C6F5(thf)] intermediate.8,9
The steric tunability of formamidinato ligands would make
mixed rare-earth-metal/aluminum formamidinate complexes struc-
turally interesting, as N-donors have a strong affinity for both
metals,38−40 and such bimetallics could serve as precatalysts in
INTRODUCTION
■
Among the approaches to replace the cyclopentadienyl family
of ligands in rare-earth-metal chemistry, N-coordinating ancil-
lary ligands have attracted considerable attention during the past
few years.1−7 For instance, bidentate amidinato and guanidinato
ligands can be exploited in so-called post-metallocene (or non-
cyclopentadienyl) complexes.8−23 Significantly, a number of com-
plexes containing N-coordinating ligands, e.g. guanidinato,3 benz-
amidinato,12,15,24 aminopyridinato,25 diamido pyridine and imino-
amido pyridine,26 nacnac,27 and others,28−34 activated by coca-
talysts such as organoaluminum or organoboron compounds,
show promising catalytic activity in the polymerization of 1,
3-dienes and α-olefins, relatively similar to cyclopentadienyl
complexes.
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: October 26, 2012
The formamidine proligands N,N′-bis(aryl)formamidines,
ArNCH-NHAr (Ar = aryl), can be prepared in high yields
from addition of triethyl orthoformate to a substituted aniline.
© XXXX American Chemical Society
A
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olefin polymerization. We now report syntheses, structures, and
catalysis in isoprene polymerization of a series of formamidi-
nato rare-earth-metal tetramethylaluminate complexes. In one
instance, C−H bond activation was observed. The homoleptic
Ln(AlMe4)3 (Ln = La, Nd, Y, Lu) complexes are convenient
precursors for generating a variety of heterobimetallic
Ln/Al complexes, currently used to build a LLnIIIbis-
(tetramethylaluminate)-based post-metallocene library (L =
ancillary ligand).41 The latter aluminate route also features
dianionic38,42 and monoanionic N-ancillary ligands such as
tris(pyrazolyl)borato (Tp)[NNN]−,43 benzamidinato [RNC(Ph)-
NR]− (Chart 1, I),24,44 triazenido [RNNNR]− (Chart 1, II),45
Scheme 1. Synthesis of [Ln(Form)(AlMe4)2] Complexes
Chart 1. Structurally Characterized Rare-Earth-Metal
Bis(tetramethylaluminate) Complexes with N-Ancillary
Ligands
(1c) by a double protonolysis of Y[N(SiHMe2)2]3(thf)2
(Scheme 1, (iii)).
As a general route to Ln(Form)(AlMe4)2 (Ln = La, Y)
51
complexes, use of homoleptic Ln(AlMe4)3 species proved
more satisfactory. Thus, protonolysis of these complexes with
several formamidines gave Ln(Form)(AlMe4)2 (Ln = Y, Form =
EtForm (2a), MesForm (3a), DippForm (4a), tBuForm (5a);
Ln = La, Form = DippForm (4b), tBuForm (5b); MesForm =
N,N′-bis(2,4,6-trimethylphenyl)formamidinate, tBuForm = N,
N′-bis(2-tert-butylphenyl)formamidinate) (Scheme 1, (iv)).
The strong Lewis acid Al3+ has a high affinity for nitrogen
donor ligands.38,52,53 Competition between the Lewis acidic
Al3+ and the rare-earth-metal ions for N-donor ligands is well
documented.38,41,43,45 However, no isolated byproducts of
formamidinato aluminum species such as [Me2Al{μ-Form}(μ-
Me)AlMe2] and [Al(Form)Me2] were observed in any case,
thereby simplifying isolation of pure complexes.
All aluminate compounds were obtained in reasonable yields
(65−72%), and their purity was established by elemental analy-
sis and proton and carbon NMR as well as IR spectroscopy,
except that satisfactory hydrogen analyses could not be
obtained for 2b, 3bc , 4a, and 5a nor a carbon analysis for 5a.
All complexes (except for 1c) were crystallized from hexane or
toluene and their structures determined by single-crystal X-ray
diffraction.
The yttrium centers in Y(Form)[N(SiHMe2)2]2(thf) com-
plexes 1a,b reveal a similar coordination environment involving
four nitrogen atoms, two from the formamidinato and two from
the bis(dimethylsilyl)amido ligand, and one tetrahydrofuran
molecule (Figure 1). The X-ray structures of mono- and bis-
(silylamido) rare-earth-metal compounds with ancillary nitro-
gen-donor ligands coordinated to the metals are well-
established.10,12,14,15,20−23 The coordination geometry of the
yttrium center in both cases is best described as distorted
trigonal bipyramidal. The two nitrogen atoms of the silylamido
ligands are approximately coplanar with the yttrium atom and
atom N1 of the formamidinato ligand. The oxygen atom of the
thf molecule and the N2 atom of the formamidinato ligand are
quinolyl-substituted cyclopentadienyl (CpQ) (Chart 1, III),46
and iminopyrrolyl (Chart 1, IV).47
Rare-earth-metal silylamide complexes also provide suitable
reagents for the stepwise synthesis of Ln/Al complexes,48,49 since
a number of complexes with mixed silylamido/N-coordinating
ligands have been reported.14,23,28,40,50 Both of these routes
have been employed in the present study.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Formamidinato
Rare-Earth-Metal(III) Tetramethylaluminate Complexes.
From the rare-earth metals, La and Y were chosen as repre-
sentative of larger and smaller sizes, respectively, as their Ln3+
ions are diamagnetic, facilitating NMR characterization. First,
protonolysis of tris{bis(dimethylsilyl)amido}bis(tetrahydrofuran)-
yttrium49 with N,N′-bis(2,6-diethylphenyl)formamidine (Et-
FormH) and N,N′-bis(2,6-diisopropylphenyl)formamidine (Dipp-
FormH) yielded the corresponding Y(Form)[N(SiHMe2)2]2(thf)
complexes (Form = EtForm (1a), DippForm (1b); thf = tetra-
hydrofuran) (Scheme 1, (i)). Treatment of 1a,b with trimethyl-
aluminum led to formation of the corresponding Y(Form)-
(AlMe4)2 complexes (Form = EtForm (2a), DippForm (4a))
with elimination of [AlMe2{N(SiHMe2)2}]49 (Scheme 1, (ii)).
It was also possible to prepare [Y(EtForm)2{N(SiHMe2)2}]
B
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Figure 1. Perspective ORTEP views of the molecular structures of 1a (left) and 1b (right). Atomic displacement parameters are set at the 30% level.
Hydrogen atoms (except Si−H) are omitted for clarity. The disorders of the tetrahydrofuran ligand (1a) and of the methyl groups at Si4 (1b) are
not shown. For selected bond lengths and angles, see Table 1.
located above and below the plane (O1−Y−N2 = 142.1(1)°
in 1a and 143.4(2)° in 1b), similar to the arrangement in
Y[PhC(N-C6H3iPr2-2,6)2](CH2SiMe3)2(thf), (O1−Y−N2 =
142.97(9)°).15 The silylamido ligands are asymmetrically coor-
dinated to the metal center, owing to Y···(Si−H) interactions50
of both silylamido ligands, in a manner similar to that for
(aminopyridinato)bis(dimethylsilylamido)scandium com-
plexes.28 There is one shorter Y···Si distance for each ligand
(Y···Si2 = 3.107(2) Å and Y···Si3 = 3.144(2) Å vs Y···Si1 =
3.526(2) Å and Y···Si4 = 3.550(2) Å in 1a; Y···Si2 = 3.052(2) Å
and Y···Si3 = 3.134(2) Å vs Y···Si1 = 3.663(2) Å and Y···Si4 =
3.658(2) Å in 1b). Due to these asymmetrical β-agostic inter-
actions, the Ln−N−Si angles within each amido ligand are
different (Y−N3−Si1 = 126.7(2)° vs Y−N3−Si2 = 103.8(2)°
and Y−N4−Si3 = 105.1(2)° vs Y−N4−Si4 = 127.3(2)° in 1a;
Y−N3−Si1 = 134.0(2)° vs Y−N3−Si2 = 100.0(2)° and Y−
N4−Si3 = 103.7(2)° vs Y−N4−Si4 = 132.9(3)° in 1b). In both
1a and 1b, the N−C(backbone) bond lengths and N1−C−N2
backbone angles are very similar (Table 1). The distances be-
tween yttrium and the Form nitrogen atoms (Y−N = 2.373(4) Å
and 2.368(4) Å in 1a; Y−N = 2.361(4) and 2.423(4) Å in 1b)
are consistent with the Y−N(amidinato) bond lengths in Y-
[PhC(N-C6H3iPr2-2,6)2](CH2SiMe3)2(thf) (2.339(3) and
2.369(2) Å) and Y[CyC(N-C6H3Me2-2,6)2](CH2SiMe3)2(thf)2
(2.375(7) Å)10,15 and also in Y[pTolC(N-C6H3iPr2-2,6)2]
[N(SiHMe2)2]2(thf) (2.412(3) and 2.351(2) Å).14 The N1−Y−
N2 bite angles (1a, 57.2(1)°; 1b, 56.5(2)°) are similar to those of
Y[PhC(N-C6H3iPr2-2,6)2](CH2SiMe3)2(thf)] (57.27(9)°) and
Y[CyC(N-C6H3Me2-2,6)2](CH2SiMe3)2(thf)2 (55.8(3)°).
Table 1. Selected Interatomic Distances (Å) and Angles (deg)
of 1a,b
a
1a
molecule 1
molecule 2
1b
Bond Lengths
3.526(2)
3.107(2)
3.144(2)
3.550(2)
2.373(4)
2.368(4)
2.240(4)
2.249(4)
2.374(4)
1.322(6)
Y···Si1(5)
Y···Si2(6)
Y···Si3(7)
Y···Si4(8)
Y−N1(5)
Y−N2(6)
Y−N3(7)
Y−N4(8)
Y−O1(2)
C11−N1(5)
C13−N1
3.600(2)
3.052(2)
3.140(2)
3.641(2)
2.350(3)
2.375(3)
2.257(4)
2.261(4)
2.360(4)
1.316(6)
3.663(2)
3.052(2)
3.134(2)
3.658(2)
2.423(4)
2.361(4)
2.259(4)
2.269(4)
2.354(4)
1.322(6)
1.318(7)
C11−N2(6)
C13−N2
1.325(6)
1.326(6)
Bond Angles
117.1(1)
57.2(1)
N3(7)−Y−N4(6)
N1(5)−Y− N2(6)
N1(5)−Y−N3(7)
N2(6)−Y−N4(8)
N2(6)−Y−O1(2)
Y−N3(7)−Si1(5)
Y−N3(7)−Si2(6)
Y−N4(8)−Si3(7)
Y−N4(8)−Si4(8)
N1(5)−C11−N2(6)
N1−C13−N2
130.3(2)
57.1(1)
120.3(2)
56.5(2)
111.9(1)
115.2(1)
142.1(1)
126.7(2)
103.8(2)
105.1(2)
127.3(2)
118.0(5)
113.8(1)
115.9(1)
139.9(1)
130.7(2)
104.3(2)
100.3(2)
133.3(2)
117.5(4)
111.9(1)
115.2(1)
143.4(2)
134.0(2)
100.0(2)
103.7(2)
132.9(3)
All bis(tetramethylaluminate) rare-earth metal complexes 2a,
3a, 4a,b, and 5a,b show similar molecular arrangements, with
yttrium (4a) and lanthanum (4b) DippForm derivatives (mono-
clinic space group P21/n) being isotypic, as are yttrium (5a) and
118.2(5)
a
Where pairs of atom numbers are given, those without parentheses
are for molecule 1 and those in parentheses are for molecule 2.
lanthanum (5b) tBuForm derivatives (triclinic space group P1).
̅
In each complex the rare-earth-metal atom and each of the two
Al atoms are bridged by two methyl groups (Figure 2). The six-
coordinate rare-earth metal is ligated by two nitrogen atoms
of the formamidinato ligand and two methyl carbons of each of
the η2-coordinated tetramethylaluminato moieties and adopts a
distorted-octahedral geometry. In all the above complexes both
[AlMe4] ligands coordinate in the commonly observed η2 fashion
to form an almost planar heterobimetallic LnC2Al unit.
All Ln−C bond lengths are in the expected range (Table 2
averages: 2a, 2.52 Å; 3a, 2.54 Å; 4a, 2.55 Å; 5a, 2.54 Å; 4b, 2.69 Å;
5b, 2.69 Å).51,54 These averages for Y−C are marginally larger than
those reported for the triazenide complex Y[(Tph)2N3](AlMe4)2
(2.49 Å (av))45 and closer to those for the benzamidinate
complex Y[PhC(N-C6H3iPr2-2,6)2](AlMe4)2 (2.53 Å (av))
(see also Chart 1, II and I).24 The Ln−N bonds in 4b and 5b
are on average 0.14 Å longer than those in 2a−5a. This value
C
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Figure 2. Perspective ORTEP views of the molecular structures of 2a (top, left), 3a (top, right), 4b (bottom, left), and 5b (bottom, right). Atomic
displacement parameters are set at the 30% level. Hydrogen atoms and the solvent molecules are omitted for clarity. The disorder of one of the
isopropyl groups of the DippForm ligand of complex 4b is not shown. Not depicted are complexes 4a and 5a, which show similar molecular
structures. For selected bond lengths and angles, see Table 2.
corresponds well with the 0.13 Å difference between the ionic
radii of yttrium and lanthanum.55 Furthermore, the La−C
bonds in 4b and 5b are on average 0.15 Å longer than the
respective Y−C bonds. In most complexes, chelation of the
Form ligand is symmetrical. A noteworthy exception is 2a,
which shows a considerable 0.13 Å difference between the
Y−N bond lengths and a larger N1−Y−N2 bite angle
(65.4(1)°). The bond lengths and angles in 3a−5a are also
relatively close to those of Y[PhC(N-C6H3iPr2-2,6)2]-
(AlMe4)2 (Y−N = 2.3157(15) Å, N1−Y−N2 = 57.51(8)°),24
while the N1−La−N2 angles of 4b and 5b are close to the related
angle of the triazenido ligand in Y[(Tph)2N3](AlMe4)2 (N1−Y−
N3 = 54.6(1)°).45,56 In both yttrium and lanthanum complexes
the two N−C(backbone) bond lengths (∼1.32 Å) and N1−C−
N2 backbone angles (∼118°) are almost equivalent. The
latter are larger than the corresponding angles in the triazenide
(109.9(3)°) and benzamidinate compounds (112.0(2)°).
a metal-coordinated formamidinato group is detected at
1503 cm−1. The IR spectra of 1a−c show the presence of two
distinct ν(SiH) bands at 2084/2074/2061 and 1929/1931/
1926 cm−1, respectively, as observed for other rare-earth-
metal bis(dimethylsilyl)amide complexes.49 The strong band
at around 690 cm−1 in all aluminate moieties is assigned to
an Al−C stretching absorption.57
In the IR spectra of all tetramethylaluminate complexes
CH asymmetrical stretching, CH symmetrical stretching, and a
Fermi resonance mode of methyl groups attached to aluminum
centers appear as three bands (or two broad overlapping) at
2900−2790 cm−1.58
The ambient-temperature 1H NMR spectra of 1a−c and 2a−
5a show a doublet for the CH backbone resonance due to the
scalar H−89Y coupling (3JYH = 4.6 Hz).50 Furthermore, the H
NMR spectra of 2a, 4a,b, and 5a,b at ambient temperature exhibit
a sharp resonance for the aluminum−methyl groups
(δ −0.24, −0.24, 0.00, −0.05, and −0.05 ppm, respectively), con-
sistent with rapid bridge−terminal exchange at ambient temper-
ature. In the case of the yttrium derivatives (2a, 4a, and 5a) the
1
1
The N−H stretching absorption of the formamidines at
3300−3100 cm−1 is not observed in the IR spectra of the
complexes, indicating complete deprotonation, and a strong
absorption assignable to the C−C stretching vibration of
1
observed splitting of the H methyl resonance for the [AlMe4]
D
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Table 2. Selected Interatomic Distances (Å) of 2a,b, 3a, 4a,b, and 5a,b
2a
3a
4a
5a
2b
4b
5b
Ln−C1
Ln−C2
Ln−C5
Ln−C6
Ln−N1
Ln−N2
Ln···Al1
Ln···Al2
Al1−C1
Al1−C2
Al1−C3
Al1−C4
Al2−C5
Al2−C6
Al2−C7
Al2−C8
2.403(4)
2.606(5)
2.710(5)
2.343(4)
2.428(4)
2.293(3)
3.075(2)
2.875(2)
2.091(5)
2.051(5)
2.042(6)
1.989(6)
2.002(5)
2.255(6)
2.038(6)
1.903(5)
1.403(5)
1.464(6)
2.542(7)
2.540(7)
2.518(6)
2.548(6)
2.338(5)
2.337(5)
3.091(2)
3.098(2)
2.067(7)
2.078(8)
1.958(8)
1.982(7)
2.076(7)
2.082(6)
1.965(8)
1.972(7)
1.320(8)
1.339(8)
2.548(5)
2.553(5)
2.555(5)
2.531(5)
2.343(4)
2.345(4)
3.084(2)
3.083(2)
2.079(5)
2.075(6)
1.973(6)
1.952(7)
2.077(6)
2.091(5)
1.963(6)
1.975(6)
1.313(6)
1.326(6)
2.568(2)
2.510(2)
2.526(2)
2.541(2)
2.340(2)
2.332(2)
3.080(7)
3.092(7)
2.091(2)
2.070(2)
1.972(3)
1.960(3)
2.083(2)
2.063(2)
1.960(3)
1.959(3)
1.324(3)
1.331(3)
2.705(4)
2.782(4)
2.715(4)
2.703(4)
3.861(3)
2.524(3)
3.301(2)
3.296(2)
2.068(4)
2.055(4)
1.968(4)
1.981(5)
2.082(5)
2.078(5)
1.971(5)
1.970(6)
1.316(4)
1.324(4)
2.694(4)
2.710(3)
2.693(4)
2.669(4)
2.478(2)
2.482(2)
3.172(2)
3.196(2)
2.080(4)
2.066(4)
1.980(5)
1.957(5)
2.076(4)
2.081(4)
1.955(4)
1.988(5)
1.324(3)
1.323(4)
2.702(3)
2.670(3)
2.680(3)
2.720(3)
2.500(2)
2.457(2)
3.234(2)
3.255(2)
2.091(3)
2.077(3)
1.972(3)
1.964(3)
2.093(3)
2.065(3)
1.966(3)
1.963(4)
1.324(3)
1.335(3)
a
a
a
C(backbone)−N1
C(backbone)−N2
a
Not bonding.
moieties is clearly attributable to a 1H−89Y scalar coupling (2JYH
=
the lanthanum atom in an η1(N):η6(arene) manner to give
eight-coordination, in contrast to the normal N,N′-chelating
mode in six-coordinate 2a with the smaller Y3+, which gives
more stable complexes in comparison to La3+ because of the
higher charge/size value. In addition the η6(arene)−La inter-
action may provide greater steric saturation than a single N−La
bond. The second nitrogen donor binds to aluminum. This
provides a rare case in the present study where Al competes
with a rare-earth metal for a formamidinato nitrogen atom.
Although η6(arene) interactions are known in lanthanum
complexes,59 this rearranging of amidinato ligands has not been
observed previously for rare-earth metals1 and is quite rare for
guanidinato ligands,60 but it is common in group 1, 2, and 13
(In and Tl) derivatives.37,61 The La−C(arene) distances (Figure 3)
fall in the same ranges of those of La(OC6H3Ph2-2,6)3
(3.107(7)−3.274(7) Å) and La2(NHiPr2C6H3-2,6)6 (2.972(3)−
3.209(4) Å).59a,b
3 Hz) as well. These resonances are shifted to lower frequencies in
comparison with the corresponding homoleptic precursors.51,54
When the temperature was lowered from +25 to −80 °C in
toluene-d8, the signals of yttrium complex 2a in a similar
manner to that for half-sandwich bis(tetramethyl)aluminate
complexes41,48 and the complex Y[(Tph)2N3](AlMe4)2]45
did not reveal any further resolution and did not show the
decoalescence of the methyl resonance.
The relatively larger backbone angle along with the different
donor abilities of amidinato ligands in comparison to triazenido
ligands are plausible reasons why formamidinato aluminum com-
pounds were not isolated in contrast to the triazenide system,45,56
and these factors facilitated the isolation of formamidinato
bis(tetramethylaluminato) rare-earth-metal complexes.
In contrast to the preceding formamidinate complexes, [La-
(EtFormAlMe3)(AlMe4)2](C7H8)1.5 (2b), which was obtained
following a similar protonolysis reaction from EtFormH
and La(AlMe4)3, has a metal−π-arene interaction (Scheme 2).
Chelation (η1, η6) of lanthanum by the EtForm ligand is
accompanied by C−N bond lengths suggestive of delocalization
(1.316(4) and 1.324(4) Å), rather similar to the case for alkali-
metal amidinate complexes with similar ligands.37,62 The NCN
backbone angle of 2b (126.1(3)°) also correlates well with
those of alkali-metal complexes with η1(N):η6(arene)-bonded
ligands (e.g., K[(η6-Mes)NC(H)N(Mes)][(η6-Mes)NHC(H)
N(Mes)] N−C(backbone)−N = 126.3(2)°)62 and is larger
than for 4b and 5b. The La−N2 interaction (2.524(3) Å) and the
average La−C(aluminato) bond length (2.73 Å) are slightly longer
than those of 4b and 5b (Table 2).
Scheme 2. Contrasting Outcomes of the Syntheses of 2a,b
1
The H NMR spectrum of 2b in toluene-d8 at ambient
temperature shows three discrete sets of CH2 signals caused by
a shift of the CH2 hydrogen atoms of the ethyl groups attached
to the asymmetrically coordinated aromatic ring. The signal
at 2.25 ppm is assignable to CH2 groups belonging to the
uncoordinated aromatic ring (C(9)−C(14)), while resonances
shifted to higher values (2.46 and 3.22 ppm) are attributable to
CH2 groups of the ring π-bonded to lanthanum. There are two
single resonances (−0.46 and −0.53 ppm) for the aluminum−
methyl groups of [AlMe4] and AlMe3, respectively. A low-
1
temperature H NMR spectrum (−52 °C) of complex 2b in
In complex 2b the EtForm ligand adopts a configuration in
which one nitrogen and one aryl substituent are coordinated to
toluene-d8 showed decoalescence of the signals of the eight
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resulted in a residue of the pure La[η1(N):η6(Ar)-
Me2CH2FormAlMe3](AlMe3)(AlMe4) (3b) (Scheme 3). Com-
plexes 3b,c are linkage isomers, and the Me2CH2Form ligand in
3b is in a configuration similar to that of compound 2b with η6
coordination of an aryl group to the lanthanum atom and a
metal-to-arene centroid distance of 2.768 Å, relatively close to
that of 2b (2.774 Å). However, compound 3c exhibits a different
form of Me2CH2Form (Table 3).
The coordination number in 3b is 10, the complex con-
taining the η1(N):η6(arene) binding mode of Me2CH2Form,
while 3c has 9-coordination, with AlMe3 bridging a nitrogen
donor atom and the lanthanum atom via two methyl groups.
The structures of 3b,c differ from that of 2b (and 2a) owing to
C−H bond activation of an o-methyl group of the mesityl
−
moiety. The resulting (RCH2 ) moiety bridges Al and La in a
μ2 fashion, creating a pseudo-aluminate species.
Each of the structures of 3b,c exhibits three methylaluminum
moieties: namely, [AlMe4], [AlMe3RCH2], and AlMe3. In each
molecule the [AlMe4]− ligand is coordinated to the La atom in
the common η2 fashion to form a [La(μ-CH3)2Al] unit with a
La1−C1−Al1−C2 torsion angle of 4.14(3)° in 3b and a smaller
La2−C30−Al4−C31 angle of 0.58(4)° in 3c. The [AlMe3RCH2]−
moiety coordinates through a methylene and two bridging
methyl groups to the lanthanum metal center. The La−(μ-
CH2) bond lengths (3b, La1−C5 = 2.851(7) Å; 3c, La2−C34 =
2.815(7) Å) are shorter than the La−(μ-CH3) bond lengths
(Table 3). The La−(μ-Me) bonds of this η3-coordinated
[AlMe3RCH2] ligand (average 2.92 Å in 3b and 2.96 Å in 3c)
are longer than the bonds of the η2-coordinated [AlMe4] ligand
(average 2.75 Å in 3b and 2.73 Å in 3c), consistent with
observations for homoleptic La(AlMe4)3 (average η3-coor-
dinated, 2.88 Å; average η2-coordinated, 2.70 Å).51 Remarkably,
the La−N and La−CH2 bonds anchor two aromatic carbons in
possible bonding positions. This rare multihapto NC3 bonding
of the lanthanum atoms has only been reported in a lanthanum
complex with chelating o-(dimethylamino)benzyl ligands.63
The La−C bond lengths of the anchored aromatic carbon
atoms (C9, C10/C38, C43) are consistent with or shorter than
typical La−π(C) values.59 For comparison, the smaller yttrium-
(III) was previously shown to engage in a respective NC2
bonding involving a η2-coordinated [AlMe3RCH2] ligand.38
The nature of the AlMe3 group is different in 3b,c. In the former
there is N−AlMe3 bonding similar to that in 2b and no inter-
action of the methyl groups of AlMe3 with the La center. How-
ever, in 3c the N−AlMe3 group binds via two bridging methyl
groups to the La atom.
Figure 3. Perspective ORTEP view of the molecular structure of 2b.
Atomic displacement parameters are set at the 30% level. Hydrogen
atoms and the solvent molecules are omitted for clarity. Selected bond
distances (Å): La1−C20 = 3.068(3), La1−C21 = 3.084(3), La1−C22
= 3.090(4), La1−C23 = 3.110(4), La1−C25 = 3.143(3), La1−C24 =
3.144(4), Al3−N1 = 1.989(3), Al3−C30 = 1.980(5), Al3−C31 =
1.984(5), Al3−C32 = 1.976(4) Å. For further selected bond lengths
and angles, see Table 2.
methyl groups of the two [AlMe4] ligands but one sharp signal
for the nitrogen bound AlMe3 (Experimental Section).
Addition of the homoleptic precursor La(AlMe4)3 to 1 equiv
of MesFormH in hexane gave a yellow solution, from which the
complex [La{η1(N):η6(Ar)-Me2CH2FormAlMe3}(AlMe3)-
(AlMe4)][La(Me2CH2FormAlMe3)(AlMe3)(AlMe4)](C6H14)1.5
(3bc; Me2CH2Form = MesForm-H(o-Me)) crystallized in the
triclinic space group P1 (Scheme 3). Compound 3bc exhibits
̅
Scheme 3. Syntheses of 3b,c Showing Distinct Me2CH2Form
Coordination
The most striking structural characteristic of compounds
3b,c is the presence of a methylene ligand, which increases the
coordination saturation of the lanthanum center and helps
anchor the η2 binding of two aromatic carbon atoms. The struc-
tures show the product of a ligand metalation involving σ-bond
metathesis between Al−CH3 and a C−H bond of the aromatic
methyl group together with loss of methane and the formation
of four- and five-membered metallacycles. The La−CH2
contacts are nearly equidistant (La1−C5 = 2.851(7) Å in 3b
and La2−C34 = 2.815(7) Å in 3c) and are in the range of the
previously observed La−CH2 bonds.43b,64−67 For lanthanum
and yttrium mono-C5Me5 complexes, single and multiple C−H
bond activation was established when using tetramethylalumi-
nate functionalities.66,68 Furthermore, C−H bond activation and
metalation of the isopropyl methyl groups have been documented
in the case of the pyridine-supported complexes (BDPPpyr-H)-
Y[(μ-Me)AlMe2]2 and (BDPPpyr-H)Lu[(μ-Me)AlMe2](thf)
cocrystallization of two discrete molecules (3b + 3c, 1:1) in the
unit cell with 1.5 hexane molecules in the lattice (Figure 4).
Isolation of the crystals and evaporation of the yellow filtrate
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Figure 4. Perspective ORTEP views of the molecular structures of 3b (left) and 3c (right). Atomic displacement parameters are set at the 30% level.
Hydrogen atoms and the solvent molecules are omitted for clarity. For selected bond lengths and angles, see Table 3.
(H2BDPPpyr = 2,6-bis-(((2,6-diisopropylphenyl)amino)methyl)-
pyridine),38 and also in scandium complexes supported by nacnac
(β-diketiminato) ligands containing bulky 2,6-diisopropylphenyl
substituents.69
we carried out preliminary tests on compounds 2a,b and 4a,b as
precatalysts activated by [Ph3C][B(C6F5)4] (A) or [PhNMe2H]-
[B(C6F5)4] (B) in isoprene polymerization. Furthermore, 1 equiv
of AlMe3 was added to the binary catalyst system to determine
the effect of excess trialkylaluminum. The polymerization results
are summarized in Table 4. For all activations of complexes 2a,b
and 4a,b colorless solutions changed to orange independent of
the boron activator, a common feature in such systems.
The NCN backbone angle of 3b (122.6(6)°) is less than that
of 2b (126.2(3)°), most likely due to the less sterically demand-
ing character of Me2CH2Form in comparison with EtForm.
The CNC backbone bond lengths in both 3b and 3c (Table 3)
are in the routinely observed range (1.316(7)−1.333(5) Å).8,9,13,23
In the 1H NMR spectrum of complex 3bc in C6D6, at ambient
temperature, six separate peaks (−0.56, −0.41, −0.28, −0.20,
−0.17, −0.14 ppm) can be assigned to [AlMe3]/[AlMe4]
moieties. The residue after isolation of 3bc is considered to be
pure 3b because of the observation of only three peaks at low
frequencies (−0.56, −0.41, −0.28 ppm) similar to the corre-
sponding chemical shifts (−0.53, −0.46 ppm) of 2b. The
The stereoselectivity of the polymerization reactions per-
formed with yttrium aluminate precatalysts (2a and 4a) was
highly affected by the choice of cocatalyst. Initiators applying
the cocatalyst trityl borate [Ph3C][B(C6F5)4] (A) display
relatively high trans-1,4-selectivities up to 84% (Table 4, runs 1
and 5), while activation with anilinium borate [PhNMe2H][B-
(C6F5)4] (B) exhibits less selective polymers with moderate cis-1,4-
content (Table 4, runs 2 and 6). When 1 equiv of AlMe3 was added
to the trityl borate (A) activated catalysts, no significant effect in
stereoselectivity was observed (Table 4, runs 3 and 7). In contrast,
the addition of trimethylaluminum to the binary catalyst systems
2a/B and 4a/B increases the trans-1,4-selectivity from 13.0% to
44.5% and from 15.9% to 87.0%, respectively (Table 4, runs 4 and
8). However, all polymers produced from yttrium aluminates show
comparatively broad molecular weight distributions (1.53−2.69).
The activation of the lanthanum complexes 2b and 4b with
[Ph3C][B(C6F5)4] (A) led to catalysts producing polyisoprene
with mainly trans-1,4-content (runs 9 and 13), whereby a
higher selectivity for the smaller EtForm complexes is observed,
comparable with that of the yttrium species (2a vs 4a). In
contrast, a switch from trans-1,4-polymer (2b, 69.6% trans;
Table 4, run 10) to cis-1,4-polymer (4b, 81.8% cis; Table 4,
run 14) could be reproducibly observed when anilinium borate
(B) was used as a cocatalyst. The addition of trimethylalumi-
num had no significant effect on the trans-1,4-selective catalyst
systems but changes the cis-1,4-selective system 4b/[PhNMe2H]-
[B(C6F5)4] toward trans-1,4-selectivity (81.8% cis versus 61.1%
trans; Table 4, runs 14 and 16). The trans-1,4-selectivity of 2b,
which already has an AlMe3 molecule coordinated to a nitrogen
atom in the structure (Figure 3), does not change significantly
after addition of trimethylaluminum (Scheme 2, Table 4, runs
9−12). The increases in trans formation for 2a/B, 4a/B, and 4b/
B on addition of AlMe3 (runs 4, 8, and 16) might be a result of the
formation of a precatalyst species similar to 2b (Scheme 2),
whereby the additional AlMe3 is complexed by one nitrogen
1
characteristic pattern of four methylene doublets in the H
NMR spectrum of 3bc in C6D6 and two methylene doublets in
3b is clearly indicative of the outcome of this reaction. The
doublets at 2.65, 2.34, 1.27, and 1.23 ppm (2JHH = 15.5 Hz) in
3bc and 2.34 and 1.27 ppm in 3b (2JHH = 15.5 Hz) are
assignable to the diastereotopic La−CH2 methylene protons.
The 13C NMR spectrum of compound 3bc in C6D6 indicates
two distinctive peaks belonging to CH2 at 24.9 and 27.6 ppm,
while in 3b merely 27.6 ppm is observed, consistent with the
proposed products of this reaction.
Polymerization of Isoprene. The new post-metallocene
catalyst families based on donor-functionalized chelating non-
cyclopentadienyl ligands (e.g., imines, amides) have received
significant attention in the field of polymer science during the
past few years.2−5,70,71 In the field of organo−rare-earth-metal
chemistry, most of these are bis(alkyl) (R = CH2SiMe3) com-
plexes of the [LnIII(Solv)(L)R2] type containing monoanionic
N-ancillary ligands (L−).7 For isoprene polymerization, the com-
plexes are activated by treatment with boron cocatalysts, often in
the presence of trialkylaluminum.32−34,72,73 Surprisingly, even
when the formation of a heterobimetallic complex as a pre-
catalyst is proposed, isolation of the discrete bis(aluminate)
post-metallocene complexes is scarce. To our knowledge, the
complexes [PhC(NC6H3iPr2-2,6)2]Y(AlMe4)2,24 [(Tph)2N3]-
Ln(AlMe4)2 (Ln = La, Nd, Y),45 and (CpQ)Ln(AlMe4)246 repre-
sent the only complexes of this type tested in isoprene poly-
merization (Chart 1, I−III). In order to extend these investigations,
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stereoselectivity. One typical example is given in Table 4, run
17, representing the reproducibility of these polymerizations.
The influence of addition of alkylaluminum cocatalysts has been
investigated previously for complexes including N-coordinating
ancillary ligands.24,28,32,33 Due to the formation of [AlR4]−
moieties as active species upon addition of alkylaluminum
derivatives to the initiators, a switch to cis-1,4-selectivity was
mostly observed, especially for less sterically saturated metal
centers.24,28 For complexes 2a,b and 4a,b already having
[AlMe4]− moieties in the structure, the switch between cis and
trans species is independent of the size of the metal cation or
the bulkiness of the ligand. Although the complexes under
investigation display high polymerization activity when acti-
vated with borate cocatalysts, the stereoselectivities did not exceed
90%. Table 5 summarizes important polymerization performances
of the related rare-earth-metal bis(tetramethylaluminate) com-
plexes depicted in Chart 1. Clearly, the implications of the metal
size for the cis/trans selectivity deserve special attention. While
a metal center as small as Sc(III) gives high cis-1,4-selectivity
(run 1, 94%), polyisoprene with high trans-1,4-selectivity is
favorably produced at the large La(III) center (run 7, 92%).
The cis-directing behavior of the complex [PhC(NC6H3iPr2-
2,6)2]Y(AlMe4)2 (IY) (Table 5, run 2, C6H5Cl)24 is striking in
comparison with that of Y(DippForm)(AlMe4)2 (4a) (Table 4,
run 5, toluene), which produced mainly trans-1,4-polyisoprene.
These two complexes differ only in that the former carries a
phenyl group in the ligand backbone. Generally, the high trans
stereoselectivities observed for the half-sandwich bis(aluminate)/
cocatalyst systems (C5Me5)La(AlMe4)2/A (trans-1,4 content
95.2%, Mw/Mn = 1.05, −30 °C preformation) and (C5Me5)La-
(AlMe4)2/B(C6F5)3 (trans-1,4 content 99.5%, Mw/Mn = 1.18)
remain unmatched.41
Table 3. Selected Interatomic Distances (Å) and Angles (deg)
of 3b,c
3b
3c
a
Bond Lengths
La1(2)−C1(30)
La1(2)−C2(31)
La1(2)−C5(34)
La1(2)−C6(35)
La1(2)−C8(36)
La1−C18
La1−C19
La1−C20
La1−C21
La1−C22
2.702(7)
2.807(7)
2.851(7)
2.959(7)
2.942(7)
3.036(6)
3.078(6)
3.092(7)
3.158(7)
3.131(6)
3.109(6)
3.087(6)
3.233(7)
2.719(7)
2.744(7)
2.815(7)
3.080(8)
2.976(7)
La1−C23
La1(2)−C9(38)
La1(2)−C10(43)
La2−C56
2.899(6)
2.984(7)
3.081(8)
2.850(8)
2.434(5)
3.295(3)
3.0514(3)
3.3832(3)
1.916(6)
2.063(8)
2.059(8)
1.970(9)
1.978(8)
2.095(7)
2.023(8)
1.928(9)
2.038(8)
2.013(9)
2.037(9)
1.320(8)
1.325(8)
La2−C57
La1(2)−N1(3)
La1(2)···Al1(4)
La1(2)···Al2(5)
La1(2)···Al3(6)
N2(4)−Al3(6)
Al1(4)−C1(30)
Al1(4)−C2(31)
Al1(4)−C3(32)
Al1(4)−C4(33)
Al2(5)−C5(34)
Al2(5)−C6(35)
Al2(5)−C7(37)
Al2(5)−C8(36)
Al6−C56
2.467(5)
3.314(3)
2.999(3)
5.784(3)
1.971(6)
2.053(7)
2.046(7)
1.972(7)
1.967(8)
2.068(7)
2.038(7)
1.950(7)
2.030(7)
CONCLUSIONS
■
Al6−C57
C(backbone)−N1(N3)
C(backbone)−N2(N4)
Formamidines (FormH) feature a suitable and versatile N-ancil-
lary ligand set for the synthesis of heteroleptic LnAl bimetallic
hydrocarbyl complexes. Post-metallocene-type complexes Ln-
(Form)(AlMe4)2 (Form (ArNCHNAr) = EtForm (Ar = 2,6-
Et2C6H3), MesForm (Ar = 2,4,6-Me3C6H2), DippForm (Ar =
2,6-iPr2C6H3), tBuForm (Ar = 2-tBuC6H4)) can be obtained
favorably via methane elimination utilizing homoleptic Ln-
(AlMe4)3 or by successive silylamine elimination/silylamido
elimination employing Ln[N(SiHMe2)2]3(thf)2 and AlMe3,
respectively. While the isolated yttrium complexes show η1-
(N):η1(N′)-coordinated Form ligands throughout, the lantha-
num congeners revealed distinct coordination chemistry depend-
ing on the substituents on the aryl rings of the Form ligands. Less
bulky aryl substituents (Me, Et) favor a η1(N):η6(arene) ligation,
as observed in La(EtFormAlMe3)(AlMe4)2. This configurational
switch is probably caused by the successful competition
of released Lewis acidic trimethylaluminum for a N(Form)
coordination site, meaning that the large and hence mildly Lewis
acidic La(III) center is abandoning η1(N):η1(N′) bonding in
favor of the η1(N):η6(arene) mode. Further, the lanthanum
derivatives can engage in C−H bond activation reactions of the
aryl substituents, as shown for La(Me2CH2FormAlMe3)(AlMe3)-
(AlMe4) (Me2CH2Form = MesForm-H(o-Me)). These C−H
activation reactions are facilitated by the high mobility of the
aluminate methyl groups. Although there is a potential competi-
tion between Al(III) and La(III) for the nitrogen atoms of the
formamidinato ligands, no byproducts of formamidinato alumi-
num species were isolated in any case. On activation with the
borate [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4], the
1.313(8)
1.313(8)
Bond Angles
N1(3)−La1(2)−N2(4)
Al1(4)−C1(30)−La1(2)
Al1(4)−C2(31)−La1(2)
Al2(5)−C5(34)−La1(2)
Al2(5)−C6(35)−La1(2)
Al2(5)−C8(36)−La1(2)
Al6−C56−La2
122.6(6)
87.3(2)
84.6(2)
73.2(2)
71.1(2)
71.5(2)
120.9(6)
85.9(2)
85.4(2)
75.3(2)
70.0(2)
72.3(2)
80.3(3)
85.9(3)
77.2(2)
111.5(3)
68.5(2)
107.7(3)
69.0(3)
112.3(3)
120.9(6)
Al6−C57−La2
C1(30)−La1(2)−C2(31)
C1(30)−Al1(4)−C2(31)
C5(34)−La1(2)−C6(35)
C5(34)−Al2(5)−C6(35)
C56−La2−C57
76.1(2)
111.8(3)
67.32(19)
103.4(3)
C56−Al6−C57
N1(3)−C(H)−N2(4)
122.6(6)
a
Where pairs of atom numbers are given, those without parentheses
are for 3b and those in parentheses are for 3c.
from the formamidinato ligand and the Ln metal center is now
changed to η1(N) and η6(arene) coordination. All polymers
produced by lanthanum complexes reveal narrow molecular
weight distributions lower than 1.3 and suggest a living fashion
by complete consumption of the monomer. Moreover, selected
polymerizations were repeated with 1 h reaction times, afford-
ing lower Mw/Mn values down to 1.07 without affecting the
H
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Table 4. Isoprene Polymerization of Formamidinate Complexes under Study
a
b
c
c
c
d
d
entry
precat.
cocat.
yield (%)
cis-1,4
3,4
trans-1,4
Mn (×104)
Mw/Mn
1
2
3
4
Y(EtForm)(AlMe4)2 (2a)
Y(EtForm)(AlMe4)2 (2a)
Y(EtForm)(AlMe4)2 (2a)
Y(EtForm)(AlMe4)2 (2a)
A
>99
>99
>99
>99
8.8
58.2
16.4
42.7
7.0
28.8
6.8
84.2
13.0
76.8
44.5
8.3
11.7
6.9
1.71
2.69
1.61
2.00
B
A/AlMe3
B/AlMe3
12.8
9.0
5
6
7
8
Y(DippForm)(AlMe4)2 (4a)
Y(DippForm)(AlMe4)2 (4a)
Y(DippForm)(AlMe4)2 (4a)
Y(DippForm)(AlMe4)2 (4a)
A
>99
>99
>99
>99
15.6
53.6
10.4
5.2
8.1
30.5
8.7
76.3
15.9
80.9
87.0
2.6
7.7
5.3
6.3
1.96
2.05
1.91
1.53
B
A/AlMe3
B/AlMe3
7.8
9
La(EtFormAlMe3)(AlMe4)2 (2b)
La(EtFormAlMe3)(AlMe4)2 (2b)
La(EtFormAlMe3)(AlMe4)2 (2b)
La(EtFormAlMe3)(AlMe4)2 (2b)
A
>99
>99
>99
>99
9.7
17.8
17.0
17.9
8.6
12.6
15.2
12.0
81.6
69.6
67.8
70.1
14.6
9.4
1.18
1.13
1.13
1.14
10
11
12
B
A/AlMe3
B/AlMe3
13.7
9.8
13
14
15
16
La(DippForm)(AlMe4)2 (4b)
La(DippForm)(AlMe4)2 (4b)
La(DippForm)(AlMe4)2 (4b)
La(DippForm)(AlMe4)2 (4b)
A
>99
>99
>99
>99
31.5
81.8
32.3
35.1
5.9
5.7
5.7
3.8
62.6
12.5
62.0
61.1
7.2
8.4
8.7
8.5
1.19
1.27
1.18
1.19
B
A/AlMe3
B/AlMe3
e
17
La(EtFormAlMe3)(AlMe4)2 (2b)
B/AlMe3
>99
16.3
16.3
67.4
8.2
1.07
a
b
Conditions: 0.02 mmol of precatalyst, [Ln]/[cocat] = 1/1, 8 mL of solvent toluene, 20 mmol of isoprene, 24 h. Cocatalyst: A, [Ph3C][B(C6F5)4];
c
d
B, [PhNMe2H][B(C6F5)4]. Determined by 13C NMR spectroscopy in CDCl3. Determined by means of size-exclusion chromatography (SEC)
against polystyrene standards. The polymerization reaction was quenched after 1 h.
e
Table 5. Isoprene Polymerization Promoted by Selected Rare-Earth-Metal Bis(tetramethylaluminate) Complexes Bearing
N-Ancillary Ligands (cf. Chart 1)
a
b
c
c
c
d
d
entry
precat.
cocat.
yield (%)
cis-1,4
3,4
trans-1,4
Mn (×104)
Mw/Mn
ref
e
1
[PhC(NC6H3iPr2-2,6)2]Sc(AlMe4)2 (ISc)
[PhC(NC6H3iPr2-2,6)2]Y(AlMe4)2 (IY)
[PhC(NC6H3iPr2-2,6)2]Y(AlMe4)2 (IY)
[(Tph)2N3]Y(AlMe4)2 (IIY)
[(Tph)2N3]Y(AlMe4)2 (IIY)
[(Tph)2N3]La(AlMe4)2 (IILa)
[(Tph)2N3]La(AlMe4)2 (IILa)
(CpQ)Y(AlMe4)2 (IIIY)
A
98
>99
>99
>99
>99
>99
>99
>99
>99
19
94
5
22
6
1
8.03
6.8
7.5
8.0
6.0
6.0
7.0
74
1.56
1.8
44
24
24
45
45
45
45
46
46
46
46
46
46
f
2
A
77.8
92
0.2
f
3
A/AlMe3
2
1.6
4
5
6
7
A
B
A
B
A
B
A
B
B
B
49.6
57.5
16.1
6.3
1.3
3.8
-
9.4
11.1
3.0
41.0
31.4
81.0
91.7
85.9
74.7
90.1
85.9
70.9
93.1
1.41
1.52
1.28
1.30
1.14
1.12
1.91
1.14
1.21
1.11
2.1
g
8
g
12.8
21.4
9.9
9
(CpQ)Y(AlMe4)2 (IIIY)
35
h
10
(CpQ)Y(AlMe4)2 (IIIY)
61
h
11
(CpQ)Y(AlMe4)2 (IIIY)
>99
>99
>99
1.3
19.6
2.1
12.8
9.5
74
i
12
(CpQ)La(AlMe4)2 (IIILa)
39
j
15
(CpQ)La(AlMe4)2 (IIILa)
4.9
93
a
Conditions unless otherwise specified: 0.02 mmol of precatalyst, [Ln]/[cocat] = 1/1, 8 mL of solvent toluene, 20 mmol of isoprene, 24 h.
b
c
d
Cocatalyst: A, [Ph3C][B(C6F5)4]; B, [PhNMe2H][B(C6F5)4]. Determined by 13C NMR spectroscopy in CDCl3. Determined by means of size-
e
f
exclusion chromatography (SEC) against polystyrene standards. Conditions: 10 mmol of isoprene, 2 min, 25 °C. Conditions: C6H5Cl, 1.022 g of
isoprene, 10 min, 25 °C, Tg = −57 °C. Conditions: 2 h. Conditions: hexane, 2 h. Conditions: 2 h. Conditions: hexane, 24 h.
g
h
i
j
purified by using Grubbs columns (MBraun SPS, solvent purification
system) and stored in a glovebox. The starting materials Ln(AlMe4)3
(Ln = La, Y),51,54 Y[N(SiHMe2)2]3(thf)2,74 and formamidine com-
pounds (EtFormH, DippFormH, MesFormH, and tBuFormH)35,36
were prepared by the literature methods. C6D6 and toluene-d8 were
obtained from Aldrich, dried over Na for 24 h, and filtered. CDCl3 and
AlMe3 were purchased from Aldrich and used as received. Isoprene
was obtained from Aldrich, degassed, dried over activated 4 Å mole-
cular sieves, and vacuum-transferred three times prior to use. The
borate cocatalysts [Ph3C][B(C6F5)4] (A) and [PhNMe2H][B(C6F5)4]
(B) were purchased from Boulder Scientific Co. and were used with-
out any further purification. The NMR spectra of air- and moisture-
sensitive compounds in C6D6 or toluene-d8 solutions were recorded
by using J. Young valve NMR tubes at +25/−52 °C on a Bruker
AVANCE-DMX400 (1H, 400.13 MHz; 13C, 100.62 MHz) or Bruker
complexes Y(EtForm)(AlMe4)2, Ln(DippForm)(AlMe4)2
(Ln = Y, La), and La(EtFormAlMe3)(AlMe4)2 efficiently
initiated the polymerization of isoprene. In particular, the
lanthanum-based catalysts produced polyisoprene of narrow molec-
ular weight distribution (PDI < 1.2) at ambient temperature. The
general tendency for trans-1,4-selectivity is clearly seen in the pres-
ence of the trityl borate cocatalyst (maximum 87%), while the
anilinium borate cocatalyst favors cis-1,4-selectivity (maximum 82%).
EXPERIMENTAL SECTION
■
General Considerations. Syntheses and catalytic operations were
carried out under an inert atmosphere of dry argon using standard
Schlenk, high-vacuum, and glovebox techniques (MBraun MBLab;
<1 ppm O2, <1 ppm H2O). Hexane, toluene, and tetrahydrofuran were
I
dx.doi.org/10.1021/om301010m | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
BIOSPIN-AV500 instrument (1H, 500.13 MHz; 13C, 125.77 MHz).
1H and 13C chemical shifts are referenced to internal solvent res-
onances and reported relative to TMS. Elemental analyses (C, H, N)
were performed using an Elementar Vario MICRO cube. IR spectra
were recorded between 4000 and 400 cm−1 on a Nicolet 6700 FTIR
spectrometer using a DRIFT chamber with dry KBr/sample mixtures
and KBr windows. The collected data were converted using the
Kubelka−Munk refinement. The molar masses (Mw and Mn) of the
polymers were determined by size-exclusion chromatography (SEC).
Sample solutions (1.0 mg of polymer/mL of thf) were filtered through
a 0.2 μm syringe filter prior to injection. SEC was performed with a
pump supplied by Viscotek (GPCmax VE 2001), employing
ViscoGEL columns. Signals were detected by means of a triple detec-
tion array (TDA 302) and calibrated against polystyrene standards
(Mw/Mn < 1.15). The flow rate was set to 1.0 mL min−1. The micro-
from hexane yielded 48 mg (36%) of 1c as colorless crystals not suitable
for X-ray crystallography. DRIFT (KBr, cm−1): 2965 s, 2931 m, 2872 w,
2060 m, 1925 w, 1523 s, 1448 s, 1374 w, 1323 w, 1280 s, 1245 m, 1199 m,
1
1105 w, 946 w, 894 s br, 840 w, 797 w, 759 m, 689 w, 610 w. H NMR
(400.13 MHz, C6D6, 25 °C): δ = 8.01 (d, 3JYH = 4.6 Hz, 2H, NC(H)N),
3
7.16 (s br, 12H, Ar), 5.16 (m, JHH = 2.9 Hz, 2H, SiH), 2.77 (q, 16H,
CH2), 1.23 (t, 24H, CH3), 0.24 (s br, 3JHH = 2.9, 12H, Si(CH3)2) ppm. 13
C
NMR (100.62 MHz, C6D6, 25 °C): δ 170.77 (NC(H)N), 145.8 (Ar-C),
137.6 (Ar-C), 125.6 (Ar-CH), 124.4 (Ar-CH), 25.2 (CH2), 14.72 (CH3),
2.29 (Si(CH3)2) ppm. Anal. Calcd for C46H68N5Si2Y: C, 66.08; H, 8.20; N,
8.38. Found: C, 65.92; H, 8.12; N, 8.33.
Y(EtForm)(AlMe4)2 (2a). Following the procedure described above,
Y(AlMe4)3 (150 g, 0.43 mmol) and EtFormH (140 mg, 0.45 mmol)
were reacted. Recrystallization of the product from hexane yielded
184 mg (65%) of 2a as colorless crystals not suitable for X-ray crys-
tallography. DRIFT (KBr, cm−1): 2966 m, 2926 m, 2885 w, 1507 s,
1448 m, 1374 w, 1327 w, 1274 m, 1191 w, 1107 w, 772 w, 719 s, 696 s,
611 w, 571 w, 550 w. 1H NMR (400.13 MHz, C6D6, 25 °C): δ 7.63 (d,
1
structure of the polyisoprenes was examined by means of H and 13C
NMR experiments on the AV400 and AV500 spectrometers at am-
bient temperature, using CDCl3 as solvent. Caution! Aluminate com-
pounds and volatiles containing trimethylaluminum react violently
when exposed to air; concentrated solutions of organoaluminum and
fluorinated boron compounds and their solid reaction products are
highly shock sensitive.41 For some of the tetramethylaluminate
complexes, we could not obtain satisfactory carbon (5a) and hydrogen
analyses (2b, 3bc , 4a, and 5a).
General Procedure for the Synthesis of Complexes 1−5. In a
glovebox, a solution of Y[N(SiHMe2)2]3(thf)2 or Ln(AlMe4)3 (Ln = Y,
La) in 5 mL of hexane was added dropwise to a suspension of the
formamidine reagents HL in 5 mL of hexane with vigorous stirring.
Instant gas formation was observed in the case of Ln(AlMe4)3. The
clear solution was stirred overnight at ambient temperature and then
evaporated to dryness under oil pump vacuum. The powder was
dissolved in hexane or toluene (only for compounds 2b and 3bc) and
crystallized at −40 °C. The pure crystalline product was generally
obtained after 1 day. All the characterizations were performed on the
crystalline compounds.
3JY,H = 4.6 Hz, 1H, NC(H)N), 6.94−6.99 (m, 6H, Ar), 2.56 (q, 3JHH
7.4 Hz, 8H, CH2), 1.12 (t, 3JHH = 7.4 Hz, 12H, CH3), −0.24 (d, 3JYH
=
=
3 Hz, 24 H, Al(CH3)4) ppm. 13C NMR (100.62 MHz, C7D8, 25 °C): δ
172.8 (NC(H)N), 144.0 (Ar-C), 137.6 (Ar-C), 126.4 (Ar-CH), 125.3
(Ar-CH), 25.2 (CH2), 15.2 (CH3), 1.8 (s br, Al(CH3)4) ppm. Anal.
Calcd for C29H51N2Al2Y: C, 61.04; H, 9.01; N, 4.91. Found: C, 60.25;
H, 8.46; N, 4.96. This compound was also formed by addition of
AlMe3 (4.03 mg, 0.056 mmol) to 1a (10 mg, 0.014 mmol) in C6D6 in
a J. Young valve NMR tube at 25 °C. When the temperature was
lowered from +25 to −80 °C in toluene-d8, the signals of complex 2a
did not reveal any decoalescence of the [Al(CH3)4] resonance.
[La(EtFormAlMe3)(AlMe4)2](C7H8)1.5 (2b). Following the procedure
described above, La(AlMe4)3 (145 mg, 0.35 mmol) and EtFormH
(112 mg, 0.36 mmol) were reacted. Recrystallization of the product
from toluene yielded 160 mg (56%) of 2b as colorless crystals suit-
able for X-ray crystallography from an orange solution. DRIFT (KBr,
cm−1): 2969 m, 2919 m, 2889 w, 1536 s, 1442 m, 1362 w, 1330 w,
1189 m, 1106 w, 841 w, 762 w, 694 s br, 587 m, 568 w, 545 w, 520 w.
Y(EtForm)[N(SiHMe2)2]2(thf) (1a). Following the procedure des-
cribed above, Y[N(SiHMe2)2]3(thf)2 (100 mg, 0.16 mmol) and
EtFormH (48 mg, 0.16 mmol) were reacted. Recrystallization of the
product from hexane yielded 45 mg (38%) of 1a as colorless crystals
suitable for X-ray crystallography. DRIFT (KBr, cm−1): 2961 m br,
2927 m, 2896 w, 2084 m, 1929 w, 1520 s, 1446 s, 1374 w, 1325 w,
1279 m, 1243 m, 1199 m, 1105 w, 1047 m br, 900 s br, 838 m, 789 w,
1H NMR (500.13 MHz, C7D8, 25 °C): δ 7.64 (s br, 1H, NC(H)N),
3
7.03 - 6.76 (m, 6H, Ar), 3.22 (q, JHH = 7.4 Hz, 2H, CH2), 2.46 (q,
,
3
3JHH = 7.4 Hz, 2H, CH2), 2.25 (q, JHH = 7.4 Hz, 4H, CH2), 1.06 (t,
3JHH = 7.4 Hz, 6 H, CH3), 0.98 (t, 3JHH = 7.4 Hz, 6H, CH3), −0.46 (s
br, 24H, Al(CH3)4), −0.53 (s, 9H, Al(CH3)3) ppm. 1H NMR (500.13
MHz, C7D8, −52 °C): δ 7.64 (s br, 1H, NC(H)N), 7.18 (d, 1H, m-Ar-
H), 6.87 (t, 1H, p-Ar-H), 6.76 (d, 1H, m-Ar-H), 6.67 (d, 1H, m-Ar-H),
6.62 (t, 1H, p-Ar-H), 6.12 (d, 1H, m-Ar-H), 3.22 (m, 2H, CH2), 2.46
(m, 2H, CH2), 2.33 (m, 2H, CH2), 2.26 (m, 2H, CH2), 1.06 (t, 6H,
CH3), 0.98 (t, 6H, CH3), −0.12 (s br, 3H, Al(CH3)4), −0.13 (s br, 3H,
Al(CH3)4), −0.28 (s br, 3H, Al(CH3)4), −0.31 (s, 9H, Al(CH3)3),
−0.45 (s br, 3H, Al(CH3)4), −0.46 (s br, 3H, Al(CH3)4), −0.50 (s br,
3H, Al(CH3)4), −0.51 (s, 3H, Al(CH3)4), −0.68 (s, 3H, Al(CH3)4)
ppm. 13C NMR (125.77 MHz, C7D8, 25 °C): δ 169.0 (NC(H)N),
155.5 (Ar), 146.7 (Ar), 141.9 (Ar), 139.4 (Ar), 131.4 (Ar), 130.0 (Ar),
129.2 (Ar), 128.3 (Ar), 26.1 (CH2), 25.1 (CH2), 16.22 (CH3), 12.9
(CH3), −7.1 (s br, Al(CH3)3 and Al(CH3)4) ppm. Anal. Calcd for
C32H60N2Al3La·(C7H8)1.5: C, 61.44; H, 8.73; N, 3.37. Anal. Calcd for
C32H60N2Al3La·C7H8: C, 59.68; H, 8.73; N, 3.57. Found: C, 59.13; H,
7.57; N, 3.61 (microanalysis revealed that half a toluene molecule in
the lattice was lost upon drying the sample in vacuo).
1
762 m, 683 w, 609 w. H NMR (400.13 MHz, C7D8, 25 °C): δ 7.88
3
(d, JYH = 4.6 Hz, 1 H, NC(H)N), 7.00−7.13 (m, 6H, Ar), 5.06 (m,
3JHH = 2.9 Hz, 4H, SiH), 3.76 (t, 4H, α-CH2, thf), 2.65 (q, 8H, CH2),
1.36 (t, 4H, β-CH2, thf), 1.13 (t, 12H, CH3), 0.29 (s br, 24H,
Si(CH3)2) ppm. 13C NMR (100.62 MHz, C7D8, 25 °C): δ 177.9
(NC(H)N), 145.2 (Ar-C), 137.9 (Ar-C), 125.4 (Ar-CH), 123.9 (Ar-
CH), 69.1 (α-CH2, thf), 25.1 (β-CH2, thf), 25.0 (CH), 14.5 (CH3),
2.1 (s br, Si(CH3)2) ppm. Anal. Calcd for C33H63N4OSi4Y: C, 54.06;
H, 8.66; N, 7.64. Found: C, 53.45; H, 9.09; N, 7.32.
Y(DippForm)[N(SiHMe2)2]2(thf) (1b). Following the procedure
described above, Y[N(SiHMe2)2]3(thf)2 (100 mg, 0.16 mmol) and
DippFormH (57 mg, 0.16 mmol) were reacted. Recrystallization of the
product from hexane yielded 50 mg (40%) of 1b as colorless crystals
suitable for X-ray crystallography. DRIFT (KBr, cm−1): 2959 s, 2923
m, 2867 m, 2074 m, 1931 w, 1664 s, 1587 w, 1526 m, 1461 m, 1382 w,
1321 w, 1286 m, 1254 m, 1182 w, 1018 m br, 897 s, 838 m, 800 s br,
1
Y(MesFormAlMe3)(AlMe4)2 (3a). Following the procedure described
above, Y(AlMe4)3 (100 mg, 0.28 mmol) and MesFormH
(80 mg, 0.28 mmol) were reacted. Recrystallization of the product
from hexane yielded 132 mg (65%) of 3a as colorless crystals suitable
for X-ray crystallography. DRIFT (KBr, cm−1): 2922 m br, 2886 m,
2864 m, 1510 s, 1477 s, 1376 w, 1304 w, 1267 s, 1224 m, 1187 m,
1161 w, 1031 w, 968 w, 922 w, 854 m, 720 s, 692 s br, 572 m, 533 w.
766 m, 678 w, 610 w. H NMR (400.13 MHz, C6D6, 25 °C): δ 8.22
(d, 3JYH = 4.6, 1H, NC(H)N), 7.06−7.14 (m, 6H, Ar), 5.07 (sep,, 3JHH
=
2.9 Hz, 4H, SiH), 3.76 (t, 4H, α-CH2, thf), 3.66 (sep, 3JHH = 6.9 Hz, 4H,
CH), 1.36 (t, 4H, β-CH2, thf), 1.19 (br, 24H, CH3), 0.29 (s br, 3JHH
=
2.9 Hz, 24H, Si(CH3)2) ppm. 13C NMR (100.62 MHz, C6D6, 25 °C): δ
170.1 (NC(H)N), 143.1 (Ar-C), 124.9 (Ar-CH), 123.9 (Ar-CH), 65.07
(β-CH2, thf), 27.9 (CH), 24.8 (s br α-CH2, thf + CH3), 3.3 (s br,
Si(CH3)2) ppm. Anal. Calcd for C37H71N4OSi4Y: C, 56.31; H, 9.07; N,
7.09. Found: C, 56.40; H, 8.99; N, 7.03.
3
1H NMR (400.13 MHz, C6D6, 25 °C): δ 7.74 (d, JYH = 4.6 Hz, 1H,
NC(H)N), 6.74 (br s, 4H, Ar), 2.13 (s, 6H; p-CH3), 2.11 (s, 12H;
3
o-CH3), −0.09 (d, JYH = 3 Hz, 24H, Al(CH3)4) ppm. 13C NMR
[Y(EtForm)2{N(SiHMe2)2}] (1c). Following the procedure described
above, Y[N(SiHMe2)2]3(thf)2 (100 mg, 0.16 mmol) and EtFormH
(96 mg, 0.31 mmol) were reacted. Recrystallization of the product
(100.62 MHz, C6D6, 25 °C): δ 172.9 (NC(H)N), 143.1 (Ar-C),
134.06 (Ar-C), 131.43 (Ar-C), 129.52 (Ar-C), 20.5 (CH3), 19.8
J
dx.doi.org/10.1021/om301010m | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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(CH3), 1.89 (br Al(CH3)4). Anal. Calcd for C27H47N2Al2Y: C, 59.77;
H, 8.73; N, 5.16. Found: C, 59.52; H, 8.25; N, 5.08.
9.49; N, 4.47. Found: C, 62.83; H, 8.61; N, 4.56. This compound was
also formed by addition of AlMe3 (3.65 mg, 0.051 mmol) to 1b (10
mg, 0.0126 mmol) in C6D6 in a J. Young valve NMR tube at 25 °C.
La(DippForm)(AlMe4)2 (4b). Following the procedure described
above, La(AlMe4)3 (0.12 g, 0.29 mmol) and DippFormH (0.11 g, 0.31
mmol) were reacted. Recrystallization of the product from hexane
yielded 140 mg (66%) of 4b as colorless crystals suitable for X-ray
crystallography. DRIFT (KBr, cm−1): 2961 s, 2926 m, 2882 m, 1512 s,
1460 m, 1442 m, 1382 w, 1362 w, 1315 m, 1271 s, 1189 m, 1100 w,
804 m, 773 w, 759 w, 697 s br, 615 w, 576 w. 1H NMR (400.13 MHz,
C6D6, 25 °C): δ 8.17 (s br, 1H, NC(H)N), 7.06 (s br, 6H, Ar), 3.27
(sep, 3JHH = 6.9 Hz, 4H, CH), 1.12 (s br, 24H, CH3), 0.00 (s br, 24H,
Al(CH3)4) ppm. 13C NMR (100.62 MHz, C6D6, 25 °C): δ 175.3
(NC(H)N), 143.8 (Ar-C), 142.8 (Ar-C), 126.1 (Ar-CH), 124.1 (Ar-
CH), 29.0 (CH), 25.3 and 23.9 (CH3), 4.8 (s br, Al(CH3)4) ppm.
Anal. Calcd for C33H59N2Al2La: C, 58.57; H, 8.79; N, 4.14. Found: C,
58.57; H, 8.96; N, 4.09.
[La{η1(N):η6(Ar)-Me2CH2FormAlMe3}(AlMe3)(AlMe4)][La-
(Me2CH2FormAlMe3)(AlMe3)(AlMe4)](C6H14)1.5 (3bc) and La-
[η1(N):η6(Ar)-Me2CH2FormAlMe3](AlMe3)(AlMe4) (3b). Following the
procedure described above, La(AlMe4)3 (100 mg, 0.25 mmol) and
MesFormH (70 mg, 0.25 mmol) were reacted. Crystallization of the
product from toluene yielded first 76 mg (44%) of 3bc as colorless
crystals suitable for X-ray crystallography from a yellow solution. After
filtering, drying of the yellow filtrate in vacuo gave 57 mg (35%) of 3b
as a residue of the pure compound. Characterization data for 3bc are
as follows: DRIFT (KBr, cm−1) 2921 m br, 2882 m, 1552 s, 1468 m,
1379 w, 1322 m, 1210 m br, 1186 m br, 1106 w, 1032 w, 968 w, 941 w,
1
854 m, 692 s br, 570 m br, 533 w, 506 w. H NMR (400.13 MHz,
C6D6, 25 °C): δ 7.74 (s, 1H, NC(H)N, 3b), 6.80 (s, 1H, Ar, 3c), 6.75
(s, 2H, Ar, 3b), 6.73 (s, 1H, Ar, 3c), 6.63 (s, 1H, Ar, 3b), 6.58 (s, 1H,
Ar), 6.57 (s, 1H, NC(H)N, 3c), 6.53 (s, 1H, Ar, 3b), 6.47 (s, 1H, Ar,
2
2
3c), 2.65 (d, JHH = 15.5 Hz, 1H, CH2, 3c), 2.34 (d, JHH = 15.5 Hz,
1H, CH2, 3b), 2.26 (s, 3H; CH3, 3b), 2.25 (s, 3H, CH3, 3b), 2.12 (s,
3H, CH3, 3c), 2.09 (s, 3H, CH3, 3c), 2.00 (s, 3H, CH3, 3c), 1.99 (s,
3H, CH3, 3c), 1.97 (s, 3H, CH3, 3c), 1.93 (s, 3H, CH3, 3b), 1.81 (s,
Y(tBuForm)(AlMe4)2 (5a). Following the procedure described
above, Y(AlMe4)3 (100 mg, 0.28 mmol) and tBuFormH (86 mg,
0.28 mmol) were reacted. Recrystallization of the product from hexane
yielded 124 mg (77%) of 5a as colorless crystals suitable for X-ray
crystallography. DRIFT (KBr, cm−1): 2958 m br, 2927 m, 2889 w,
1502 s, 1480 s, 1438 m, 1390 w, 1357 w, 1291 s, 1277 m, 1213 m br,
1085 w, 1055 w, 946 w, 774 m, 754 s, 720 s, 699 s br, 569 m, 518 w.
2
3H, CH3, 3c), 1.77 (d, JHH = 15.5 Hz, 1H, CH2, 3c), 1.73 (s, 3H,
CH3, 3b), 1.27 (d, 1H, CH2, 2JHH = 15.5 Hz, 3b), 1.23 (m, 8H, CH2 of
hexane), 0.88 (t, 6H, CH3 of hexane), −0.14 (s br, 9H, Al(CH3)3, 3c),
−0.18 (s br, 12H, Al(CH3)4, 3c), −0.20 (s br, 9H, Al(CH3)3, 3c),
−0.28 (s br, 9H, Al(CH3)3, 3b), −0.41 (s br, 9H, Al(CH3)3, 3b),
−0.56 (s br, 12H, Al(CH3)4, 3b) ppm. 13C NMR (100.62 MHz, C6D6,
25 °C): δ 166.1 (NC(H)N, 3b), 164.4 (NC(H)N, 3c), 154.9−127.2
(Ar-C, 3b + 3c), 32.3 (CH3, hex), 27.6 (CH2, 3b), 24.9 (CH2, 3c),
23.4 (CH2, hex), 21.8 (CH3, 3b), 21.3 (br, 2 CH3, 3c), 21.2 (br, CH3,
3b), 20.3 (CH3, 3c), 19.6 (CH3, 3b), 19.5 (br, 2 CH3, 3c), 19.2 (CH3,
3c), 18.9 (CH3, 3b), 14.7 (CH2, hex), 4.9 (br, Al(CH3)4, 3c), 2.9
(br, Al(CH3)4, 3b), −0.1 (br, Al(CH3)3, 3b), −1.6 (Al(CH3)3, 3c),
−1.7 (Al(CH3)3, 3c), −6.3 (br, Al(CH3)3, 3a) ppm. Anal. Calcd for
C58H104N4Al6La2·C6H14: C, 55.57; H, 8.60; N, 4.05. Calcd for
C58H104N4Al6La2·1.5C6H14: C, 56.41; H, 8.83; N, 3.93. Found: C,
56.20; H, 7.09; N, 4.29 (the microanalysis suggests 1.5 hexane
molecule in the lattice, in accordance with the crystal structure, while
3
1H NMR (400.13 MHz, C6D6, 25 °C): δ 7.66 (d, JY,H = 4.6 Hz, 1H,
NC(H)N), 7.29 (dd, 2H, Ar), 7.10 (td, 2H, Ar), 7.01 (td, 2H, Ar),
3
6.69 (dd, 2H, Ar), 1.34 (s br, 18 H, CH3), −0.05 (d, JYH = 3 Hz, 24
H, Al(CH3)4) ppm. 13C NMR (100.62 MHz, C6D6, 25 °C): δ 169.3
(NC(H)N), 144.7 (Ar-C), 144.2 (Ar-C), 127.2 (Ar-CH), 127.1 (Ar-CH),
126.6 (Ar-CH), 125.8 (Ar-CH), 35.4 (C), 31.7 (CH3), 2.5 (s br,
Al(CH3)4) ppm. Anal. Calcd for C29H51N2Al2Y: C, 61.04; H, 9.01; N,
4.91. Found: C, 62.46; H, 8.29; N, 5.24.
La(tBuForm)(AlMe4)2 (5b). Following the procedure described
above, La(AlMe4)3 (0.05 g, 0.12 mmol) and tBuFormH (38 mg, 0.12
mmol) were reacted. Recrystallization of the product from hexane
yielded 68 mg (72%) of 5b as colorless crystals suitable for X-ray
crystallography. DRIFT (KBr, cm−1): 2966 m br, 2922 m, 2888 w,
1503 s, 1478 s, 1438 m, 1388 w, 1354 w, 1291 s, 1279 m, 1210 m br,
1085 w, 1055 w, 942 w, 773 m, 754 m, 716 s, 697 s br, 610 w, 569 m,
518 w. 1H NMR (400.13 MHz, C6D6, 25 °C): δ 7.96 (s, 1H,
NC(H)N), 7.13 (dd, 2H, Ar), 7.10 (td, 2H, Ar), 7.00 (td, 2H, Ar),
6.69 (dd, 2H, Ar), 1.37 (s, 18H, CH3), −0.05 (s, 24H, Al(CH3)4)
ppm. 13C NMR (100.62 MHz, C6D6, 25 °C): δ 166.1 (NC(H)N),
148.6 (Ar-C), 143.7 (Ar-C), 127.4 (Ar-CH), 127.1 (Ar-CH), 126.6
(Ar-CH), 125.5 (Ar-CH), 35.5 (C), 31.8 (CH3), 4.9 (s br, Al(CH3)4)
ppm. Anal. Calcd for C29H51N2Al2La: C, 56.12; H, 8.28; N, 4.51.
Found: C, 55.75; H, 8.52; N, 4.38.
Polymerization of Isoprene. A detailed polymerization proce-
dure (Table 4, run 1) is described as a typical example. [Ph3C]-
[B(C6F5)4] (A, 18 mg, 0.02 mmol, 1 equiv) was added to a solution of
2a (11 mg, 0.02 mmol) in toluene (8 mL), and the mixture was aged
at ambient temperature for 30 min. After the addition of isoprene
(2.0 mL, 20 mmol), the polymerization was carried out at ambient
temperature for 24 h. The reaction was terminated by pouring the
polymerization mixture into a large quantity of acidified 2-propanol
containing 0.1% (w/w) 2,6-di-tert-butyl-4-methylphenol as a stabilizer.
The polymer was washed with 2-propanol and dried under vacuum at
ambient temperature to constant weight. The polymer yield was
determined gravimetrically.
X-ray Crystallography and Crystal Structure Determination
of 1a,b, 2a,b, 3bc, 3a, 4a,b, and 5a,b. Crystals suitable for
diffraction experiments were selected in a glovebox and mounted in
Paratone-N oil inside a nylon loop. Data collection was done at 173(2)
K on a STOE IPD II diffractometer using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å), performing ω scans in two φ
positions. Structure solutions and refinements were performed using
the programs SHELXS-9775 and SHELXL-9776 through the graphical
interface X-Seed, which was also used to generate the figures. All CIF
files were checked at http://www.checkcif.iucr.org/. For further experi-
mental details on refinement and crystallographic data see Table 6.
1
the H NMR of 3bc showed that half of a hexane molecule from the
lattice was lost upon drying the sample in vacuo prior to dissolution).
Characterization data for 3b are as follows: DRIFT (KBr, cm−1)
2914 m br, 2881 w, 1541 s, 1468 m, 1380 w, 1327 m, 1211m, 1184 m,
1150 w, 1032 w, 968 w, 941 w, 851 w, 692 s br, 574 m, 533 w, 522 w,
503 w; 1H NMR (400.13 MHz, C6D6, 25 °C): δ 7.74 (s, 1H,
NC(H)N), 6.75 (s, 1H, Ar), 6.63 (s, 1H, Ar), 6.53 (s, 1H, Ar), 6.47 (s,
1H, Ar), 2.34 (d, 2JHH = 15.5 Hz, 1H, CH2), 2.26 (s, 3H, p-CH3), 2.25
(s, 3H, p-CH3), 1.99 (s, 3H, o-CH3), 1.93 (s, 3H, o-CH3), 1.73 (s, 3H,
2
o-CH3), 1.27 (d, JHH = 15.5 Hz, 1H, CH2), −0.28 (s br, 9H,
Al(CH3)3), −0.41 (s br, 9H, Al(CH3)3), −0.56 (s br, 12H, Al(CH3)4)
ppm; 13C NMR (100.62 MHz, C6D6, 25 °C): δ 166.14 (NC(H)N),
154.9 (Ar-C), 148.1 (Ar-C), 140.7 (Ar-C), 140.08 (Ar-C), 138.8 (Ar-
C), 138.5 (Ar-CH), 135.1 (Ar-CH), 134.8 (Ar-CH), 133.5 (Ar-C),
131.6 (Ar-C), 129.8 (Ar-CH), 127.2 (Ar-CH), 27.6 (CH2), 21.8
(CH3), 21.2 (CH3), 20.3 (CH3), 19.6 (CH3), 18.9 (CH3), 2.9 (br
Al(CH3)4), −0.1 (Al(CH3)3), −6.3 (s br, Al(CH3)3) ppm. Anal. Calcd
for C29H52N2Al3La: C, 53.70; H, 8.08; N, 4.32. Found: C, 53.34; H,
7.19; N, 4.38.
Y(DippForm)(AlMe4)2 (4a). Following the procedure described
above, Y(AlMe4)3 (164 mg, 0.46 mmol) and DippFormH (171 mg,
0.46 mmol) were reacted. Recrystallization of the product from hexane
yielded 185 mg (64%) of 4a as colorless crystals suitable for X-ray
crystallography. DRIFT (KBr, cm−1): 2961 s, 2925 m, 2888 m, 1513 s,
1456 m, 1442 m, 1383 w, 1362 w, 1316 m, 1271 s, 1189 m, 1100 w,
805 m, 774 w, 759 w, 697 s br, 616 w, 575 w. 1H NMR (400.13 MHz,
3
C6D6, 25 °C): δ 8.05 (d, JYH = 4.6, 1H, NC(H)N), 7.06 (s br, 6H,
Ar), 3.24 (sep, 3JHH = 6.9 Hz, 4H, CH), 1.12 (d, 3JHH = 78.4 Hz, 24H,
CH3), −0.24 (s, 3JYH = 3 Hz, 24H, Al(CH3)4) ppm. 13C NMR (100.62
MHz, C6D6, 25 °C): δ 172.4 (NC(H)N), 142.5 (Ar-C), 142.3 (Ar-C),
125.9 (Ar-CH), 124.1 (Ar-CH), 28.6 (CH), 25.5 and 23.5 (CH3), 2.1
(s br, Al(CH3)4) ppm. Anal. Calcd for C33H59N2Al2Y: C, 63.24; H,
K
dx.doi.org/10.1021/om301010m | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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dx.doi.org/10.1021/om301010m | Organometallics XXXX, XXX, XXX−XXX

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Article
Variata are as follows. 1a: Two discrete molecules were observed in the
asymmetric unit. One thf was modeled as disordered with C atom
position C31 (refined occupancy 0.54:0.46). DELU and SIMU res-
trains were applied on C63, C64, C65 and C66. 1b: Methyl carbons on
Si4 were modeled as disordered on C30, C30A, C31, and C31A
(refined occupancy 0.64:0.36), and EADP and DFIX commands were
applied on them. 2a: SIMU and DELU commands were applied for
C9, C10, C11, C12, C13, and C14. The EADP command was applied
on C28 and C29. The DFIX command was used for C17 and C18 and
also C25 and C28. The DFIX command was used for C19 and H19,
C1 and H1A, and also C6 and H6A. 2b: 1.5 toluene molecules in the
lattice were modeled. SIMU and DELU were applied on C101 C102,
C103, C10a, C10b, C10c, C100, and C40 as well as C26 and C31. The
ISOR command was applied on C26, C31, and C40. 3a: Two discrete
molecules were observed in the asymmetric unit. TWIN and BASF
commands were applied. 3bc: Two discrete molecules were observed
in the asymmetric unit as discussed above.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving full crystallographic data for complexes 1a,b,
2a,b, 3bc, 3a, 4a,b, and 5a,b and a table giving selected bond
angles of 2a,b, 3a, 4a,b, and 5a,b. This material is available free
of charge via the Internet at http://pubs.acs.org. Crystallo-
graphic data have also been deposited at the Cambridge
Crystallographic Data Centre under CCDC reference numbers
907682−907691 and can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: glen.deacon@monash.edu (G.B.D.); peter.junk@jcu.
edu.au (P.C.J.); reiner.anwander@uni-tuebingen.de (R.A.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was performed with the support of ARC grant
DP0984775. S.H. is grateful to the School of Chemistry at
Monash for a scholarship. We also acknowledge support from
the German Science Foundation.
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