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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.