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Transcript of 18 March 2010
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Polymetallic Uranium Azide
and Nitride ComplexesTanya Kumanova Todorova
Justin R. Walensky and William J. EvansUniversity of California, Irvine
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Uranium nitride vs. oxide
U
I
I
O O
O
+ 3NaN 36
- 18THF
U I
I
I
I
U
U
I
I
E2 + other products
E = N, 1; O , 2
(1)
Product [(C5Me5)U(-I)2]3(3-E) is identified by X-ray crystallography
E = N: U4+ nitride complex derived fromconversion of azide to nitride
E = O: (U4+ )2(U3+ )mixed valence compound derived from adventitious
source of oxygen
W. J. Evans et al.Inorg. Chem. 46, 8008 (2007)
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Uranium nitride vs. oxide
The two compounds have different color, crystallographic space group
and NMR spectrum: these are two different complexes.
UI
I
I
I
U
U
I
I
O(2)
3 UI3 + 3 KC5Me5- 3 KI
2
Hitchcock et al. Chem. Commun. 1, 82 (2008)
Is theory capable in differentiating between the two pairs (nitridevs. oxide in polymetalic uranium complexes)?
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Uranium nitride vs. oxide
U B + KC8
N2
THF
E
U
E
E
E
E
UU
E
U
U
E
E
E
UU
(4)
[(C5Me5)U(3-E)8not identified by X-ray crystallography
U4+ nitride orU3+ oxide complex?
It could also be mixed oxide/nitride with a mixture of uraniumvalencesSingle crystals are not obtainablecharacterization not possible
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Polymetallic Uranium Azide/Nitride
Compounds dense in uranium and nitrogen: interesting as molecular
precursors to high-purity UN, a potential nuclear fuel of the future
W. J. Evans, S. A. Kozimor, W. J. ZillerScience 309, 1835 (2005)W. J. Evans et al.Inorg. Chem. 46, 8008 (2007)
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Computational Approach
1. DFT optimization and comparison with X-ray crystallography data2. CASSCF/CASPT2 investigations of the electronic structure
PBE functional & def-TZVP basis sets on all atoms (ECP on I and U) NBO analysis Turbomole 5.10 package
ANO-RCC-VDZP on U, N, O, B, I; ANO-RCC-MB on F, C and H CAS: 6/21 for [(C5Me5)U(-I)2]3(3-N)] and [(C5Me5)2UN3(-N3)]3
7/21 for ([(C5Me5)U(-I)2]3(3-O)
14/14 for [(C6F5)3BNU(N[Me]Ph)3 MOLCAS 7.3 package
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Uranium nitride vs. oxide
septet octet
At CASSCF/CASPT2:Septet, quintet, triplet and singlet states are almost degenerate for
[(C5Me5)U(-I)2]3(3-N)]
Octet, sextet and quartet states are almost degenerate for[(C5Me5)U(-I)2]3(3-O)]
At DFT level:
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Uranium nitride vs. oxide
Very good agreement with the experimental X-ray data
The only difference is in the U E bond lengths
U Obond lengths are 2.208, 2.203 and 2.193 , being longerthanthe U N ones (2.145, 2.149 and 2.154 )
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Uranium nitride vs. oxide
[(C5Me5)U(-I)2]3(3-N)]:
NBO analysis: U1(5f2.03 ), U2(5f2.09 ), U3(5f2.09 )CASSCF/CASPT2: U1(5f1.99 ), U2(5f1.99 ), U3(5f1.99 )
5f2U4+ centersU4+ nitride complex
[(C5Me5)U(-I)2]3(3-O)]:
NBO analysis: U1(5f2.34 ), U2(5f2.31 ), U3(5f2.34 )CASSCF/CASPT2: U1(5f2.97 ), U2(5f1.99 ), U3(5f1.99 )
U3+ , U4+ , U4+ (U4+ )2(U3+ )mixed valence complex
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Uranium nitride
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Uranium oxide
Both structures are very ionic: no U N and U O interactions
Electrons are localized only on U: linear combinations of 5f
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Uranium azide: [(C5Me5)2UN3(-N3)]3
Trimetallic U(IV) complex
Each U is attached to two bridging azideligands and one terminal azide
Insights into U N (azide) bondingexisting in [(C5Me5)2U(-N)U(-N3)(C5Me5)2]4
Very good agreement with the experimental X-ray data
Eight septets and eight quintets are almost degenerate in energy
Very ionic structure
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Uranium azide: [(C5Me5)2UN3(-N3)]3
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Uranium nitride vs. oxide
DFT only!!! Required CAS: 16 electrons in 56 orbitals Differences in the U E and U Cnt bond lengths
Experimental structure is E = N
U B + KC8
N2
THF
E
U
E
E
E
E
UU
E
U
U
E
E
E
UU
(4)
E = N E = O Experiment
U E 2.145 2.359 2.184 2.499 2.142 2.405
U Cnt 2.557 2.600 2.488 2.539 2.554 2.573
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[(C5Me5)2U(-N)(C5Me5)2U(-N3)]4
Mixed nitride azide complex Crystallizes with a 24 atom (UNUNNN)4 ring
Near linear UNU linkages X-ray crystallographic studies indicate that it crystallizes both in a
pseudo-boat and a pseudo-chair form DFT only!!! CASSCF/CASPT2 not plausible
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Boat or Chair?
Chair Boat
Very good agreement with experiment The two conformations are very flexible in their geometries DFT favors the boat structure by only ca. 5 kcal/mol The chair conformation is a local minimum
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[(C6F5)3BNU(N[Me]Ph)3
First nitridoborate complex of uranium Nearly linear B N U core The system is an analogue of the formally
triply bonded U(VI) organoimido complexes(bond multiplicity value is 2.83)
CASSCF/CASPT2 study to investigate the U N type of bonding andto compare it with the bonding in [(C5Me5)U(-I)2]3(3-N) and
[(C5Me5)2U(-N)U(-N3)(C5Me5)2]4
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[(C6F5)3BNU(N[Me]Ph)3
The uranium nitride bonding in this complex has much more pronouncedcovalent character than any of the polymetallic uranium complexes.