The synthesis and electropositive metal (Y, Lu, La, Zr, Hf) chemistry of
two rigid dianionic xanthene-based ligands, 4,5-bis(2,4,6-triisopropylanilido)-
-2,7-di-tert-butyl-9,9-dimethylxanthene (XN2) and 4,5-bis(2,4,6-triisopropylphenylphosphido)-
2,7-di-tert-butyl-9,9-dimethylxanthene (XP2) have been explored. The reaction of the pro-ligand H2XN2 with [Y(CH2SiMe2R)3(THF)2] (R =
Me or Ph) produced the monoalkyl yttrium complexes [(XN2)Y(CH2SiMe3)-
(THF)].(O(SiMe3)2)x (3, x = 1-1.5) and [(XN2)Y(CH2SiMe2Ph)(THF)].(O-
(SiMe3)2) (4). Neutral 3 reacted with excess AlMe3 to yield [(XN2)Y{(m-
Me)2AlMe2}(THF)].O(SiMe3)2 (5.O(SiMe3)2), which is thermally robust, and
transfer of the XN2 ligand to aluminum was not observed. However, [(XN2)-
AlMe].(O(SiMe3)2)0.5 (6.(O(SiMe3)2)0.5) was synthesized via the reaction of
H2XN2 with AlMe3. Compounds 3, 5 and 6 were characterized by X-ray crystallography,
and neutral 3, while being poorly active for ethylene polymerization,
was highly active for both intra- and inter-molecular hydroamination
with a variety of substrates. The synthesis of the pro-ligand H2XP2 was achieved via reduction of 4,5-bis(2,4,6-triisopropylphenylchlorophosphino)-2,7-di-tert-butyl-9,9-dimethylxanthene
(XP2Cl2; 7). Double deprotonation of H2XP2 (8) with excess KH
yielded the potassium salt, [K2XP2(DME)2.5] (9), which when stirred in THF
followed by recrystallization from hexanes, produced the tetrametallic complex,
[K4(XP2)2(THF)4] (10) featuring a central K4P4 cage. The reaction
of [K2XP2(DME)2.5] (9) with [YI3(THF)3.5] yielded a mixture of products including [(XP2)YI(THF)2] (11) and tris(2,4,6-triisopropylphenylphosphinidene)
(P3Tripp3); pure 11 could be isolated in low yield by extraction with
a minimum volume of hexanes or O(SiMe3)2. In the solid state, complex
11 reveals a face-capped trigonal bipyramidal geometry at yttrium, in which
the xanthene backbone is planar and adopts a large angle (85 degrees) between the
P(1)/C(4)/C(5)/P(2) and P(1)/Y/P(2) planes. Due to the successful synthesis and hydroamination catalysis achieved with
the XN2 ligand in combination with yttrium, the chemistry of XN2 was further
explored using both smaller (Lu) and larger (La) rare earth elements. The
alkane elimination reaction of H2XN2 with [Lu(CH2SiMe3)3(THF)2], followed
by crystallization from O(SiMe3)2, yielded [(XN2)Lu(CH2SiMe3)(THF)].(O-
(SiMe3)2)1.5 (12.(O(SiMe3)2)1.5). By contrast, lanthanum complexes of the
XN2 dianion were prepared by salt metathesis; treatment of H2XN2 with excess
KH in DME produced the dipotassium salt, [K2(XN2)(DME)x] (2; x =
2-2.5), and subsequent reaction with [LaCl3(THF)3] afforded [{(XN2)LaCl-
(THF)}x].(O(SiMe3)2)0.25x (13.(O(SiMe3)2)0.25x; x = 1 or 2) after crystallization
from O(SiMe3)2. Compound 13.(O(SiMe3)2)0.25x reacted with two equivalents
of LiCH2SiMe3, to form the dialkyl-`ate' complex, [Li(THF)x][(XN2)-
La(CH2SiMe3)2].Toluene.LiCl (14.Toluene.LiCl; x = 3). Both 12 and 14 (x
= 4) were structurally characterized by X-ray crystallography, and were evaluated
as catalysts for intramolecular hydroamination. While compound 14
showed poor activity, the neutral lutetium alkyl complex, 12, is highly active
for both intramolecular hydroamination and more challenging intermolecular
hydroamination. Like the yttrium analogue, 3, reactions with unsymmetrical
alkenes yielded Markovnikov products. Additionally, it is noteworthy that the activity of 12 surpassed that of 3 in the reaction of diphenylacetylene with
4-tert-butylbenzylamine. The reaction of H2XN2 with [Zr(NMe2)4], followed by crystallization from
O(SiMe3)2, yielded [(XN2)Zr(NMe2)2].(O(SiMe3)2)0.5 (15.(O(SiMe3)2)0.5). The
zirconium dimethyl complex [(XN2)ZrMe2] (16) was accessed via two routes;
either by treatment of 15.(O(SiMe3)2)0.5 with excess AlMe3, or by reaction of
15.(O(SiMe3)2)0.5 with excess Me3SiCl, affording [(XN2)ZrCl2] (17), followed
by the subsequent reaction of 17 with 2 equivalents of MeLi. The reaction of 16
with one equivalent of B(C6F5)3 or [CPh3][B(C6F5)4] yielded cationic [(XN2)-
ZrMe][MeB(C6F5)3] (18) and [(XN2)ZrMe(arene)][B(C6F5)4] (19; arene =
n6-benzene, n6-toluene or bromobenzene), respectively. Both 18 and 19 are active
for ethylene polymerization under 1 atm of ethylene at 24 and 80 degree Celcius in
toluene, with activities ranging from 23.5{883 kg/(mol.atm.h), yielding polymers
with weight average molecular weights (Mw) of 71{88 kg/mol and polydispersities
(Mw/Mn) of 3.94-4.67. / Thesis / Doctor of Philosophy (PhD) / Pincer ligands are defined as meridionally-coordinating tridentate ligands,
and are typically mono-, di- or tri-anionic. This thesis is focused on the synthesis
and reactivity of rigid dianionic pincer ligands with an NON- or POP-donor
array, with particular emphasis on rare earth and group 4 transition
metal complexes. This work explores the effect that these rigid ligands have
on the reactivity of the resulting metal complexes and the thermal stability
of the solid state structures. Both neutral and cationic mono alkyl complexes
have been isolated, and several are highly active catalysts for intra- and intermolecular
hydroamination or ethylene polymerization.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/22085 |
| Date | 11 1900 |
| Creators | Motolko, Kelly |
| Contributors | Emslie, David, Chemistry |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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