Uranium() alkyl cations: synthesis, structures, comparison with thorium() analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity
Reaction of [(XA 2 )U(CH 2 SiMe 3 ) 2 ] ( 1 ; XA 2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di- tert -butyl-9,9-dimethylxanthene) with 1 equivalent of [Ph 3 C][B(C 6 F 5 ) 4 ] in arene solvents afforded the arene-coordinated uranium alkyl cations, [(XA 2 )U(CH 2 SiMe 3 )(η n -arene)][B(C 6 F 5 ) 4 ] {a...
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| Published in: | Chemical science (Cambridge) 2022-11, Vol.13 (46), p.13748-13763 |
|---|---|
| Main Authors: | , , , , , , , , , , , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | Reaction of [(XA
2
)U(CH
2
SiMe
3
)
2
] (
1
; XA
2
= 4,5-bis(2,6-diisopropylanilido)-2,7-di-
tert
-butyl-9,9-dimethylxanthene) with 1 equivalent of [Ph
3
C][B(C
6
F
5
)
4
] in arene solvents afforded the arene-coordinated uranium alkyl cations, [(XA
2
)U(CH
2
SiMe
3
)(η
n
-arene)][B(C
6
F
5
)
4
] {arene = benzene (
2
), toluene (
3
), bromobenzene (
4
) and fluorobenzene (
5
)}. Compounds
2
,
3
, and
5
were crystallographically characterized, and in all cases the arene is π-coordinated. Solution NMR studies of
2-5
suggest that the binding preferences of the [(XA
2
)U(CH
2
SiMe
3
)]
+
cation follow the order: toluene benzene > bromobenzene > fluorobenzene. Compounds
2-4
generated in C
6
H
5
R (R = H, Me or Br, respectively) showed no polymerization activity under 1 atm of ethylene. By contrast,
5
and
5-Th
(the thorium analogue of
5
) in fluorobenzene at 20 and 70 °C achieved ethylene polymerization activities between 16 800 and 139 200 g mol
−1
h
−1
atm
−1
, highlighting the extent to which common arene solvents such as toluene can suppress ethylene polymerization activity in sterically open f-element complexes. However, activation of [(XA
2
)An(CH
2
SiMe
3
)
2
] {M = U (
1
) or Th (
1-Th
)} with [Ph
3
C][B(C
6
F
5
)
4
] in
n
-alkane solvents did not afford an active polymerization catalyst due to catalyst decomposition, illustrating the critical role of PhX (X = H, Me, Br or F) coordination for alkyl cation stabilization. Gas phase DFT calculations, including fragment interaction calculations with energy decomposition and ETS-NOCV analysis, were carried out on the cationic portion of
2′-Th
,
2′
,
3′
and
5′
(analogues of
2-Th
,
2
,
3
and
5
with hydrogen atoms in place of ligand backbone methyl and
tert
-butyl groups), providing insight into the nature of actinide-arene bonding, which decreases in strength in the order
2′-Th
>
2′
3′
>
5′
.
Uranium(
iv
) alkyl cations have been prepared, and arene solvent coordination was shown to have a major impact on ethylene polymerization activity. Actinide-arene binding was studied experimentally in the solid state and solution, and computationally. |
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| ISSN: | 2041-6520 2041-6539 |