1) The document describes research into the reactivity of uranium(III) and digermyne complexes. Experiments showed uranium(III) catalyzes the reduction of carbon monoxide and carbon dioxide to form carbocycles and oxocarbons. Digermyne was found to activate C-H bonds in cyclopentadiene and cyclopentene through oxidative addition and insertion into the Ge-H bond. 2) Mechanistic studies revealed uranium(III) reduces carbon monoxide to a cyclo-reduced product through insertion into a uranium-carbon bond. Carbon dioxide is reduced to squaric and mellitic acid derivatives in a catalytic cycle. 3) For digermyne, C

1. Owen SummerscalesOwen Summerscales
Los Alamos National LaboratoryLos Alamos National Laboratory
Owen SummerscalesOwen Summerscales
Los Alamos National LaboratoryLos Alamos National Laboratory
Curious Chemistry at
the Corners of the
Periodic Table

2. OverviewOverviewOverviewOverview
2

3. OverviewOverviewOverviewOverview
3
Uranium Germanium
 f-block introduction
 uranium(III) introduction
 unexpected cyclization of CO
 transformation of CO2
 mechanism
 p-block introduction
 heavy alkyne analogues
 cycloaddition
 C-H activation
 mechanisms
 low-valent complexes
 small molecule activation → catalysis
 redox chemistry

4. Small molecule activationSmall molecule activationSmall molecule activationSmall molecule activation
4
Low valent
Reducing agent
Small molecule Oxidized complex M(n+2)+
Reduced substrate 2 H-
Olefin hydrogenation
Haber-Bosch

5. The f-elementsThe f-elementsThe f-elementsThe f-elements
Ln3+
Trivalent state dominates
Ln2+
…exceptions exist
(e.g. SmI2)
U(VI) f0
U(V) f1
U(IV) f2
U(III) f3
Th(IV) f0
- Stable with hard ligands (e.g. N & O donors)
- Stable with soft ligands (e.g. carbocycles)
5

6. • Highly oxophilic
• ƒ-orbitals are core-like
4ƒ
5ƒ
The f-elementsThe f-elementsThe f-elementsThe f-elements
6

7. Uranium: research objectivesUranium: research objectivesUranium: research objectivesUranium: research objectives
7
 explore fundamental reactivity of uranium(III)
 small molecule activation with fundamental
substrates:
 carbon monoxide
 carbon dioxide
 dinitrogen
 synthesize a new type of U(III) complex to
provide window into this reactivity
 examine catalytic potential

8. Uranium carbonylUranium carbonylUranium carbonylUranium carbonyl
Brennan, JG; Andersen, RA; Robbins, JL J. Am. Chem. Soc. 1986, 108, 33; Cloke, FGN; Kuchta MC; Harker RM; Hitchcock
PB; Parry JS; Carmona E; Coles S J. Am. Chem. Soc. 1995 117, 2649; Evans, WJ et al J. Am. Chem. Soc. 2003 125,13831.
8
Manhatten project
1940s
1980s

9. U(III) mixed-sandwich designU(III) mixed-sandwich designU(III) mixed-sandwich designU(III) mixed-sandwich design
9
→ “Replace” 2 Cp-
for COT2-
→ Use bulky R = Sii
Pr3 groups
→ Crystallinity
→ Solubility
→ R = H analog known
(Sattelberger & Clark et al. 1993)
→ Too insoluble to handle

10. U – O(THF) 2.695(4) Ǻ
U(III) mixed-sandwich synthesisU(III) mixed-sandwich synthesisU(III) mixed-sandwich synthesisU(III) mixed-sandwich synthesis
10

11. - THF
40% (isolated)
CO cyclo-reductionCO cyclo-reductionCO cyclo-reductionCO cyclo-reduction
Summerscales, O.T.; Cloke, F.G.N.; Hitchcock, P.B.; Green, J.C.; Hazari, N. Science 2006, 829.
11

12. Core structure
Crystal & DFT analysisCrystal & DFT analysisCrystal & DFT analysisCrystal & DFT analysis
12
Summerscales, O.T.; Cloke, F.G.N.; Hitchcock, P.B.; Green, J.C.; Hazari, N. Science 2006, 829.
(C3O3)2-

13. Oxocarbons (CO)Oxocarbons (CO)nn
2-2-Oxocarbons (CO)Oxocarbons (CO)nn
2-2-
West, R. Oxocarbons, Academic Press New York 1980.
13

14. Calculated SCF energies from DFT
(gas phase)
Oxocarbon analysisOxocarbon analysisOxocarbon analysisOxocarbon analysis
14

15. U – O(THF) 2.737(4) Ǻ
Ligand modificationLigand modificationLigand modificationLigand modification
15

16. 66% isolated
Squarate synthesisSquarate synthesisSquarate synthesisSquarate synthesis
Summerscales, O.T.; Cloke, F.G.N.; Hitchcock, P.B.; Green, J.C.; Hazari, N. J. Am. Chem. Soc. 2006, 128, 9602.
16
U – C (av.) 3.045 Å
Identical with structure of
squarate unit in K2C4O4.H2O

17. CO couplingCO couplingCO couplingCO coupling
17
Bercaw et al. Acc. Chem. Res. 1980, 121; Wayland et al. J. Am. Chem. Soc. 1988, 6063; Evans et al. J. Am. Chem. Soc. 1985,
3728; J. Am. Chem. Soc. 2006, 14176.
Bercaw et al.
Wayland et al.
Evans et al.

18. i.e. 10:8 (5:4) ratio of [U]:CO2 required
+ CO
U(III) COU(III) CO22 reductionreductionU(III) COU(III) CO22 reductionreduction
18
Summerscales, O.T.; Frey, A.S.P.; Cloke, F.G.N.; Hitchcock, P.B. Chem. Commun. 2009, 198.

19. Synthetic cycle: deltateSynthetic cycle: deltateSynthetic cycle: deltateSynthetic cycle: deltate
19

20. Synthetic cycle: squarateSynthetic cycle: squarateSynthetic cycle: squarateSynthetic cycle: squarate
20
Tsoureas, N.; Summerscales, O.T.; Cloke, F.G.N.; Roe, S.M. Organometallics 2013, 32, 1353.

21. Mechanism: initial proposalMechanism: initial proposalMechanism: initial proposalMechanism: initial proposal
21

22. U2
U1
ν(CO) = 1920 cm-1
Mechanism: revised proposalMechanism: revised proposalMechanism: revised proposalMechanism: revised proposal
Frey, A.S.; Cloke, F.G.N.; Hitchcock, P.B.; Day, I.J.; Green, J.C.; Aitken, G. J. Am. Chem. Soc. 2008, 13816.
22

23. Post-scriptPost-scriptPost-scriptPost-script
23
Liddle et al. 2012
P. Arnold et al. 2011
Gardner, B.M.; Stewart, J.C.; Davis, A.L.; McMaster, J.; Lewis, W.; Blake, A.J.; Liddle, S.T. Proc. Natl. Acad. Sci. 2012 109, 9265;
Arnold, P.L. et al. J. Am. Chem. Soc. 2011 133, 9036; Chem. Sci. 2011 2, 77; Braunschweig, H. et al. Nat. Chem. 2013 5, 1025.
Braunschweig et al. 2013

24. Uranium summaryUranium summaryUranium summaryUranium summary
24
 discovery of an organometallic route to carbocycles from CO
 demonstration that different oxocarbons can be accessed with
same system
 isolation of C4O4
2-
salt from CO2 reduction

25. The p-block: group 14The p-block: group 14The p-block: group 14The p-block: group 14
25
Alkyne - linear
Dimetallyne - bent
Alkene
Dimetallene
E = Si “disilene”
= Ge “digermene”
= Sn “distannene”
E = Si “disilyne”
= Ge “digermyne”
= Sn “distannyne”Diradical
state

26. Dimetallynes: heavy alkyne analoguesDimetallynes: heavy alkyne analoguesDimetallynes: heavy alkyne analoguesDimetallynes: heavy alkyne analogues
26
Diradical
state
Arynes

27. Dimetallynes: heavy alkyne analoguesDimetallynes: heavy alkyne analoguesDimetallynes: heavy alkyne analoguesDimetallynes: heavy alkyne analogues
Power, P.P. Nature 2010 463, 171; Power, P. P. Organometallics 2007 26, 4362. * Calculated for R = Me
27
R-E E-R≡ Bond order *
Diradical
Character *
Carbon 2.99 5%
Silicon 2.20 - 2.37 17%
Germanium 1.74 - 2.32 15%
Tin 1.73 - 1.87 4%
Lead 1.51 - 1.65 8%
Diradical
state

28. Dimetallynes: reducing agentsDimetallynes: reducing agentsDimetallynes: reducing agentsDimetallynes: reducing agents
28
- First example of main group H2
activation
- “Metal-like” behavior
- Catalytic potential?
Spikes, GH; Fettinger, JC; Power, PP J. Am. Chem. Soc. 2005 127,
12232.

29. Diels-Alder
Sila-Diels-Alder
Cycloadditions: [4+2]Cycloadditions: [4+2]Cycloadditions: [4+2]Cycloadditions: [4+2]
Nakadaira, Y.; Otsuka, T.; Sakurai, H. Tet. Lett. 1981, 22, 2417; Wiberg, N.; Vasisht, S. K.; Fischer, G.; Mayer, P. Z. Anorg.
Allg. Chem. 2004, 630, 1823
29

30. Cycloadditions: [2+2]Cycloadditions: [2+2]Cycloadditions: [2+2]Cycloadditions: [2+2]
Peng, Y.; Ellis, B. D.; Wang, X.; Fettinger, J. C.; Power, P. P. Science 2009 325, 1668.
30

31. Cycloadditions: [2+2]Cycloadditions: [2+2]Cycloadditions: [2+2]Cycloadditions: [2+2]
Kinjo, R.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Sumimoto, M.; Nagase, S. J. Am. Chem. Soc. 2007, 129, 7766.
31
Mechanism for “[2+2]” addition:

32. Germanium: research objectivesGermanium: research objectivesGermanium: research objectivesGermanium: research objectives
32
 explore fundamental reactivity of digermyne
 examine possible catalytic reactions
 hydrides for hydrogenation
 olefin cycloadditions
Digermyne

33. Bond lengths: Ge(1)-C(31) 2.1827(14),
Ge(1)-C(32) 2.3272(14), Ge(1)-C(35)
2.531(2)
Average C-C distance (C5 ring) = 1.410(2)
Bond angles: C(1)-Ge(1)-C(31) 95.87(5),
CyclopentadieneCyclopentadieneCyclopentadieneCyclopentadiene
Summerscales, O.T.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2011, 133, 11960.
33
Bond dissociation energy (BDE) CpH = 81.2(2) kcal mol-1
?

34. (1) Oxidative addition of C-H bond across E-E bond:
(2) Deprotonation of C-H bond by Ge/Sn hydride:
Cyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanism
34
pKa = 18

35. Cyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanism
Summerscales, O.T.; Caputo, C.A.; Knapp, C.E.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2012, 134, 14595.
35
1 – Identification of an insertion olefin ‘probe’ to detect hydrides:
2 – Ensure probe does not react with digermyne:

36. Cyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanismCyclopentadiene: C-H activation mechanism
36
Summerscales, O.T.; Caputo, C.A.; Knapp, C.E.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2012, 134, 14595.
→ indicates existence of hydrides
→ hydride acts as base towards CpH

37. CpH C-H activation mechanism recapCpH C-H activation mechanism recapCpH C-H activation mechanism recapCpH C-H activation mechanism recap
37
Summerscales, O.T.; Caputo, C.A.; Knapp, C.E.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2012, 134, 14595.

38. Green crystals Ar(H)Ge=Ge(C5H9)Ar
1
H NMR (C6D6): Ge-H at 5.87 ppm
IR (Ge-H): 2010 cm-1
Ge-Ge 2.3098(5) Å
UV-vis 406 nm (ε = 1500)
CyclopenteneCyclopenteneCyclopenteneCyclopentene
38
Summerscales, O.T.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2011 133, 11960; Summerscales, O.T.; Caputo, C.A.;
Knapp, C.E.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2012, 134, 14595.
BDE c-C5H8 = 82.3 kcal mol-1
, pKa ≈ 46
?

39. Cyclopentene: triple C-H activationCyclopentene: triple C-H activationCyclopentene: triple C-H activationCyclopentene: triple C-H activation
39
Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1979, 101, 7738; Baudry, D.; Ephritikhine, M. J. Chem. Soc.,
Chem. Commun. 1980, 249.
Crabtree et al.
Ephritikhine et al.

40. (1) Dehydrogenation
via double C-H activation
(2) Insertion
into Ge-H bond
3) Dehydrogenation
of CpH
Cyclopentene: C-H activation mechanismCyclopentene: C-H activation mechanismCyclopentene: C-H activation mechanismCyclopentene: C-H activation mechanism
40

41. Cyclopentene: C-H activation mechanismCyclopentene: C-H activation mechanismCyclopentene: C-H activation mechanismCyclopentene: C-H activation mechanism
41
Summerscales, O.T.; Caputo, C.A.; Knapp, C.E.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2012, 134, 14595.
→ Indicates existence of hydrides
→ proves cyclopentene insertion

42. cc-C-C55HH99 C-H activation mechanism summaryC-H activation mechanism summarycc-C-C55HH99 C-H activation mechanism summaryC-H activation mechanism summary
42
Summerscales, O.T.; Caputo, C.A.; Knapp, C.E.; Fettinger, J.C.; Power, P.P. J. Am. Chem. Soc. 2012, 134, 14595.

43. Reductive elimination?Reductive elimination?Reductive elimination?Reductive elimination?
43

44. “Non-metal” reactivity
Cycloaddition vs. CH activationCycloaddition vs. CH activationCycloaddition vs. CH activationCycloaddition vs. CH activation
44
“Metallic” reactivity
• DFT predicts cycloaddition reaction, ie. Incorrect!
• Distannyne diradical character 4%, digermyne 15%

45. Germanium: objectives revisitedGermanium: objectives revisitedGermanium: objectives revisitedGermanium: objectives revisited
45
 explore fundamental reactivity of digermyne
 examine possible catalytic reactions
 hydrides for hydrogenation
 olefin cycloadditions
Digermyne
 new C-H activation reactions
 demonstrated role of transient hydrides

46. AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements
46
Prof. Geoff Cloke FRS
Dr. Peter Hitchcock (crystallography)
Prof. Jenny Green (Oxford, DFT)
Prof. Nilay Hazari (Oxford, DFT)
Prof. Phil Power FRS
Dr. Jim Fettinger (crystallography)
Prof. Gabriel Merino (Merida, DFT)
Dr. John Gordon
LDRD for Director’s Fellowship

48. U(III) U(IV)
Uranium(III)Uranium(III)Uranium(III)Uranium(III)
Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2005, 11242.
48
+ CO

49. U(III) U(IV)
Uranium(III)Uranium(III)Uranium(III)Uranium(III)
Cloke, F.G.N; Hitchcock, P.B. J. Am. Chem. Soc. 2002, 9352.
49

50. UIUI33 preparationpreparationUIUI33 preparationpreparation
50
The easy way
...the hard way...

51. 13
C NMR spectra
[UCOT1,4-R
Cp*]2
13
C3O3
O
OO
UU
O
O
O UU
FluxionalityFluxionalityFluxionalityFluxionality
51
Summerscales, O.T.; Cloke, F.G.N.; Hitchcock, P.B.; Green, J.C.; Hazari, N. Science 2006, 829.

52. 13
C NMR
194.500 194.000 193.500 193.000 192.500 192.000 191.500 191.000 190.500 190.000 189.500 189.000 188.500 188.000 187.500 187.000 186.500
Calculated spectrum: AA’BB’ spin system
A B
A’ B’
J(AA’) = 93.6 Hz
J(BB’) = 49.2 Hz
J(AB) = J(A’B’) = 49.1 Hz
J(AB’) = J(A’B) = 48.3 Hz
1313
CC44OO44(SiMe(SiMe33))22
1313
C{C{11
H} NMRH} NMR1313
CC44OO44(SiMe(SiMe33))22
1313
C{C{11
H} NMRH} NMR
Tsoureas, N.; Summerscales, O.T.; Cloke, F.G.N.; Roe, S.M. Organometallics 2013, 32, 1353.
52

53. - Cleavage of E-E (E = Ge, Sn) bond to give E(II) inverse sandwich complex
- First π-bound COT complexes of p-block
- Fluxional behavior, single COT resonance:
1
H at δ/ppm 5.36 (Sn) 5.32 (Ge) free COT 5.79 Li2COT 5.73
13
C at δ/ppm 96.4 (Sn) 100.0 (Ge) free COT 132.4 Li2COT 87.5
- No change down to -60°C
Cyclooctatetraene: ReductionCyclooctatetraene: ReductionCyclooctatetraene: ReductionCyclooctatetraene: Reduction
Summerscales, O.T.; Wang, X.; Power , P.P. Angew. Chem., Int. Ed. 2010, 49, 4788; Summerscales, O.T.; Jiménez-Halla,
J.O.C.; Merino, G.; Power, P.P. J. Am. Chem. Soc. 2011, 133, 180.
53

54. - Orange crystals; mp 200°C (dec) - Yellow crystals; mp 180°C
- UV-vis = 340 nm = 290 nm
- Almost planar COT rings with almost identical C-C distances -> 10π aromatic:
C-C(av.) Å = 1.411(6) = 1.417(3) (1.40 in K2COT)
Internal angles av. ° = 134(4) = 133.2(2) (135 in K2COT)
Sn Ge
Cyclooctatetraene: ReductionCyclooctatetraene: ReductionCyclooctatetraene: ReductionCyclooctatetraene: Reduction
54
Summerscales, O.T.; Wang, X.; Power , P.P. Angew. Chem., Int. Ed. 2010, 49, 4788; Summerscales, O.T.; Jiménez-Halla,
J.O.C.; Merino, G.; Power, P.P. J. Am. Chem. Soc. 2011, 133, 180.

55. Isomerization occurs in solution to give thermodynamic product with first order kinetics:
Activation parameters ΔH‡
= 14.9 kcal mol-1
and ΔS‡
= -6.2 cal mol-1
K-1
Reversible isomerization of (ArGe)Reversible isomerization of (ArGe)22((μμ22
-η-η22
:η:η22
--cot)cot)Reversible isomerization of (ArGe)Reversible isomerization of (ArGe)22((μμ22
-η-η22
:η:η22
--cot)cot)
Summerscales, O.T.; Jiménez-Halla, J.O.C.; Merino, G.; Power, P.P. J. Am. Chem. Soc. 2011, 133, 180.
55

56. - Colourless crystals; mp 120°C (dec)
- Ge1-Ge2 2.566 Å
- Ge1-C1 1.956 Å Ge2-C8 1.956 Å
- Two olefinic bonds:
C1-C2 1.323 Å C7-C8 1.330 Å
- Ge-C(terphenyl): Ge1-C39 2.004
(Å) Ge2-C9 1.969
(ArGe)(ArGe)22(C(C88HH88))(ArGe)(ArGe)22(C(C88HH88))
56
Summerscales, O.T.; Jiménez-Halla, J.O.C.; Merino, G.; Power, P.P. J. Am. Chem. Soc. 2011, 133, 180.

57. Mechanism: a Cope rearrangement?Mechanism: a Cope rearrangement?Mechanism: a Cope rearrangement?Mechanism: a Cope rearrangement?
57
Elschenbroich, C.; Heck, J.; Massa, W.; Schmidt, R. Angew. Chem. Int. Ed. Engl. 1983 22, 330.

58. Mechanism: a Cope rearrangement?
• DFT shows a kinetically viable route from syn-{(Ar)Ge}2(µ2:η2
:η2
-cot) (Int1) to
product observed via Cope rearrangement.
• N.B. Tin product shown is hypothetical: inverse sandwich isomer 37 kcal/mol
more stable
Mechanism: a Cope rearrangement?Mechanism: a Cope rearrangement?Mechanism: a Cope rearrangement?Mechanism: a Cope rearrangement?
58
Summerscales, O.T.; Jiménez-Halla, J.O.C.; Merino, G.; Power, P.P. J. Am. Chem. Soc. 2011, 133, 180.