Synthesis of group 4 complexes that contain the tridentate diamido ligands [(ArNCH2CH2)2O]2- (Ar = C6H3-Me2-2,6 or C6H3Pri-2,6)

Article abstract of DOI:10.1039/a706452g

Complexes of Zr and Hf of the type [(2,6-R₂C₆H₃NCH₂CH₂) ₂O]ZrMe₂ (R = Me or Prⁱ) can be activated by [PhNMe₂H][B(C₆F₅)₄] or [Ph₃C][B(C₆F₅)₄] in chlorobenzene or bromobenzene to yield catalysts for the polymerization of hex-1-ene, but analogous titanium catalysts are relatively poorly behaved.

Full text of DOI:10.1039/a706452g

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Synthesis of group 4 complexes that contain the tridentate diamido ligands
[(ArNCH2CH2)2O]22 (Ar = C6H3-Me2-2,6 or C6H3Pri2-2,6)
Richard R. Schrock,* Florian Schattenmann, Michael Aizenberg and William M. Davis
Department of Chemistry 6-331, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
Complexes
of
Zr
and
Hf
of
the
type
become equivalent on the NMR timescale via a rapid ‘flipping’
of the ligand backbone through an intermediate with structure
A.
[(2,6-R2C6H3NCH2CH2)2O]ZrMe2 (R = Me or Pri) can be
activated by [PhNMe2H][B(C6F5)4] or [Ph3C][B(C6F5)4] in
chlorobenzene or bromobenzene to yield catalysts for the
polymerization of hex-1-ene, but analogous titanium cata-
lysts are relatively poorly behaved.
Ar
Rax
Ar
N
N
Ar
R
R
M
O
Req
O
M
N
Significant advances have been made in the synthesis of ‘well
defined’ alkene polymerization catalysts of the early metals
(largely group 4) in the last 15 years, the vast majority of which
contain two cyclopentadienyl-like rings.1–3 (See, however, ref.
4, and references therein.) Recently group 4 complexes that
contain chelating diamido ligand systems have received
increased attention as potential alkene polymerization cata-
lysts.5–13 The most successful so far appear to be dialkyl
complexes of Ti that contain diamido ligands such as
[ArNCH2CH2CH2NAr]22 (Ar = C6H3Pri2-2,6),5,6 and dialkyl
complexes of Zr that contain the ‘diamido/donor’ ligand,
[(o-[2H6]ButNC6H4)2O]22 ([NON]22).10 Both types can be
activated to yield (presumably) cationic catalysts for the living
polymerization of hex-1-ene. We report here Ti, Zr and Hf
complexes that contain potentially interesting ‘diamido/donor’
N
Ar
A
B
An X-ray study of ZrL1aMe2 (Fig. 1)¶ showed it to have
structure A with ‘axial’ Zr–N bond distances of 2.084(3) Å and
a Zr–O(2) bond length of 2.336(3) Å. The compound has a
crystallographic mirror plane defined by Zr, O(2), C(5) and C(6).
The planes of the aryl rings are perpendicular to the
ZrO(2)N(3)N(4) pseudo-plane. The structure is closely related
to that of [(2,6-Et2C6H3NCH2)2NC5H3]ZrMe2 [N–Zr–N
139.6(2)° and C–Zr–C 102.4(3)°]8 and contrasts markedly with
that of {[(o-[2H6]ButNC6H4)2O]ZrMe}{MeB(C6F5)3},10 in
which B(C6F5)3 has partially removed Meax. The two Me groups
are technically inequivalent in the solid state structure of
ZrL1aMe2 [C(8)–O(2)–C(18) 113.7(4)°] as a consequence of
C(18) and C(8) not lying in the ZrN2 plane.
variations
of
the
[NON]22
ligand,
[(2,6-R2C6H3NCH2CH2)2O]22 (R = Me, Pri), and preliminary
experiments concerning their use as catalysts for the poly-
merization of hex-1-ene.
Reactions similar to those employed to prepare Zr compounds
can be employed to prepare HfL1a(NMe2)2,‡ HfL1aCl2,‡ and
HfL1aMe2 3a.‡ The Hf compounds are virtually identical to the
Zr compounds in physical properties and NMR character-
istics.
The diamines, (2,6-Me2C6H3NHCH2CH2)2O (H2L1a) and
(2,6-Pri2C6H3NHCH2CH2)2O (H2L1b), can be prepared readily
from (TsOCH2CH2)2O (TsO = tosylate) and 2 equiv. of
LiNH(C6H3Me2-2,6) or LiNH(C6H3Pri2-2,6), respectively, in
thf.† Compound H2L1a can be obtained in 60% yield as colorless
crystals upon crystallization from pentane.‡,§ Compound H2L1b
is obtained in 80% yield as an oil that has crystallized in some
instances upon standing for several days, and that can be
crystallized from pentane in low yield at 230 °C.†,‡ Crude
H2L1b can be used in subsequent reactions.
Addition of Li2L1a to TiCl4(thf)2 in diethyl ether at 230 °C led
to formation of a deep red precipitate, which could be
recrystallized from CH2Cl2 to yield TiL1aCl2 in 57% yield.
Alkylation of TiL1aCl2 in ether with MeMgCl in thf at 230 °C
gave TiL1aMe2 4a in 53% yield as a yellow powder from
pentane. Red, crystalline TiL1a(CH2Ph)2 5a was prepared in low
yield (ca. 15%) via the reaction between Ti(CH2Ph)4 (2 mmol)
The reaction between H2L1a or H2L1b and Zr(NMe2)4 in
pentane at room temp. yields crystalline ZrL1a(NMe2)2‡ or
ZrL1b(NMe2)2‡ readily. The dimethylamido compounds react
with excess SiMe3Cl in ether overnight at room temp. to yield
ZrL1aCl2‡ and ZrL1bCl2‡ as colorless, relatively insoluble
crystals. NMR spectra of ZrL1b(NMe2)2 and ZrL1bCl2 show two
sets of isopropyl methyl groups, but only one methine resonance,
consistent with restricted rotation of the aryl ring about the
N–Cipso bond, a phenomenon that was also noted by McConville
and co-workers5–8,14 in complexes containing the
[ArNCH2CH2CH2NAr]22 and related ligands. Only a single
amido methyl resonance is observed in all dimethylamido
compounds at room temp. in C6D6.
C(19)
N(4)
C(18)
O(2)
C(5)
C(6)
Zr
C(8)
N(3)
Alkylation of ZrL1aCl2 with MeMgBr in ether gives ZrL1aMe2
1a‡ in good yield, while alkylation of ZrL1bCl2 with Me or Bui
Grignard reagents in ether yields ZrL1bMe2 1b‡ or
ZrL1b(CH2CHMe2)2 2b,‡ respectively, in good yields as
colorless cubes. NMR spectra are again consistent with slow
rotation around the N–Cipso bond on the NMR timescale. In all
dialkyl compounds the alkyl groups are equivalent, consistent
either with a pseudotrigonal bipyramidal structure of type A or
a pseudotrigonal bipyramid of type B in which the alkyl groups
C(9)
Fig. 1 A view of the structure of ZrL1aMe2 nearly opposite the Zr–O bond
in the ZrC2 plane. Bond lengths (Å) and angles (°): Zr–C(5) 2.253(6);
N(4)–Zr–N(3) 139.1(2), C(8)–O(2)–C(18) 113.7(4), Zr–N(3)–C(9)
120.2(2), C(5)–Zr–C(6) 100.0(2)°.
Chem. Commun., 1998
199

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Table 1 GPC and yield data for poly(hex-1-ene)a
b
b
b
Complex
Activator
Mn(theory)
Mn
Mw
Mw/Mn
Yield (%)
ZrL1bMe2
[NHMe2Ph]+
[CPh3]+
[NHMe2Ph]+
[CPh3]+
13 440
16 800
16 800
16 800
2 670
2 460
2 650
3 560
4 220
8 550
1.33
1.72
3.23
1.55
96
96
94
100
ZrL1bMe2
ZrL1b(CH2CHMe2)2
ZrL1bMe2
11 700
18 000
a Dialkyl complex (44 mmol), activator (40 mmol), 200 equiv. hex-1-ene (except first entry = 160 equiv.), 10 ml chlorobenzene. b Determined by GPC coupled
with on-line light scattering (Wyatt Technology).
R. R. S. thanks the Department of Energy (DEFG02-
86ER13564) and Exxon for supporting this research, and M. A.
is grateful for a Rueff-Wormser Postdoctoral Fellowship.
C(21)
Footnotes and References
C(2)
† Solid (TsOCH2CH2)2O (5 g, 12.0 mmol) was added to a chilled (230 °C)
solution of 2,6-Pri2C6H3NHLi (4.53 g, 24.8 mmol) in thf (30 ml). After 24
C(31)
h all solvents were removed in vacuo and the residue was extracted with
pentane. The pentane was removed in vacuo to yield an orange oil (4.2 g,
82%). An analytically pure sample was obtained by recrystallization from a
C(41)
C(1)
Ti
N(1)
N(2)
concentrated pentane solution at 230 °C: 1H NMR (C6D6) d 7.18–7.14 (m,
6, Haryl), 3.60 (t, 2, NH), 3.48 (spt, 4, CHMe2), 3.35 (t, 4, OCH2), 3.07 (q,
4, CH2N), 1.06 (d, 24, CHMe2); (CDCl3) d 7.18–7.05 (m, 6, Haryl), 3.70 (t,
4, OCH2), 3.57 (br, 2, NH), 3.36 (spt, 4, CHMe2), 3.14 (br t, 4, CH2N), 1.27
(d, 24, CHMe2).
2.224(5)
C(11)
O
C(6)
C(4)
‡ Satisfactory elemental analyses have been obtained (C, H, N).
§
1H NMR (C6D6) d 3.15 (t, 4, OCH2), 2.95 (dt, 4, CH2N), 2.24 (s, 12,
ArMe).
Fig. 2 A view of the structure of TiL1a(CH2Ph)2. Bond lengths (Å) and
¶ Crystallographic data: ZrL1aMe2: Siemens SMART/CCD, orthorhombic,
space group Pnma, a = 12.741(3), b = 22.663(5), c = 7.518(2) Å,
U = 2170.9(8) Å3, Z = 4, Dc = 1.321 g cm23, R = 0.0368, Rw = 0.1009,
GOF = 1.179. For TiL1a(CH2Ph)2: Siemens SMART/CCD, orthorhombic,
space group Fdd2, a = 32.803(8), b = 42.342(13), c = 8.547(4) Å,
U = 11872(7) Å3, Z = 16, Dc = 1.210 g cm23, R = 0.0609, Rw = 0.1269,
GOF = 1.095. CCDC 182/679.
∑ In a typical experiment a chilled (230 °C) solution of 1b (19 mg, 44 mmol)
in chlorobenzene (3 ml) was added to a suspension of [PhNMe2H][B(C6F5)4]
(32 mg, 40 mmol) in chlorobenzene (6 ml) at 230 °C. The reaction mixture
was allowed to warm to room temp. and was stirred for 15 min. The reaction
mixture was cooled to 0 °C and hex-1-ene (1.0 ml, 8.0 mmol) was added all
at once. After stirring for 1 h at 0 °C the reaction was quenched with HCl
(1.0 m in ether, 4 ml). In a typical experiment employing [Ph3C][B(C6F5)4]
the reaction mixture was stirred for 2 min at 230 °C and allowed to
equilibrate at 0 °C before hex-1-ene (1.0 ml, 8.0 mmol) was added all at
once.
angles (°): Ti–C(1) 2.129(10), Ti–N(1) 1.962(6), C(2)–Ti–O 169.3(3).
and H2L1a (2 mmol) in pentane (ca. 15 ml) in 40 h. In 4a and 5a
the alkyl groups are equivalent on the NMR timescale at room
temp., although several proton and carbon resonances in 5a are
broad at 22 °C. The structure of TiL1a(CH2Ph)2 (Fig. 2)¶ is best
described as type B in which O and C(2) are in the apical position
and N(1), N(2), and C(1) are in the equatorial positions of a
pseudotrigonal bipyramid. Surprisingly, the benzyl groups are
1
bound only in an h fashion; if two Ti–N p bonds can form, the
metal should be viewed as having a 14 electron count, an electron
2
3
deficiency that has led in many situations to h or h binding of
benzyl ligands.15
Activation of alkyl complexes 1a,b, 2b and 3a by either
[PhNMe2H][B(C6F5)4] or [Ph3C][B(C6F5)4] yields initiators for
the polymerization of hex-1-ene in chlorobenzene at 0 °C.∑
Polymerization yields are high, but the molecular masses of the
polymers were found to be significantly smaller than expected
when ZrL1bMe2 and ZrL1b(CH2CHMe2)2 were employed (Table
1). These results suggest that chain termination competes with
propagation, but that the catalyst remains active thereafter and
polymerization of a new chain is initiated. The [PhNMe2H]+ and
[Ph3C]+ activators seem to be equally efficient. The number
average molecular mass of poly(hex-1-ene) prepared using
ZrL1aMe2 is approximately that expected if no significant chain
termination is taking place. In all cases polymerization activity
ceases when 2,4-dimethylpyridine is added to the catalyst
mixture, consistent with competitive binding of the nitrogen
base to a cationic metal center. Analogous experiments
employing HfL1aMe2 yielded results similar to those employing
ZrL1aMe2, but use of TiL1aMe2 (activated by
[PhNMe2H][B(C6F5)4] in [2H5]bromobenzene) led to only ca.
75% consumption of 25 equiv. of hex-1-ene to give poly(hex-
1-ene).
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We conclude that [(2,6-R2C6H3NCH2CH2)2O]22 ligands
produce stable five-coordinate dialkyl complexes of Ti, Zr and
Hf, and that ‘cations’ formed by protonation or oxidative
cleavage of an alkyl group from Zr or Hf are initiators for the
polymerization of hex-1-ene. These results should be contrasted
with results obtained by McConville and co-workers5,6 where Ti
is the most successful when the diamido ligand does not contain
an additional donor.
Received in Bloomington, IN, USA, 3rd September 1997; 7/06452G
200
Chem. Commun., 1998