Unusual Reactivity of Dimetallynes Towards Hydrocarbons

1. • Heavy alkyne analogues R-E≡E-R contain formal triple-bonds
( E = heavy group 14 element )
• Increasingly bent geometry as group is descended
• Derives from increasing s/p energy gap
• Corresponds to decreasing bond order
Introduction
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%
• Allows unusual reactivity with unsaturated species, e.g.
Woodward-Hoffmann forbidden [2 + 2] cycloadditions:
C-H Activation Under Mild Conditions
References
1. O. T. Summerscales, J. O. C. Jiménez-Halla, G.
Merino, P. P. Power
J. Am. Chem. Soc. 2011 133, 180
2. O. T. Summerscales, X. Wang, P. P. Power
Angew. Chem., Int. Ed. 2010 49, 4788.
3. P. P. Power Chem. Commun. 2003, 2091
4. Y. Peng, B. Ellis, X. Wang, J. C. Fettinger, P. P. Power
Science 2009, 325, 1668.
5. Y. Jung, M. Brynda, P. P. Power, M. Head-Gordon
J. Am. Chem. Soc. 2006, 128, 7185.
6. R. Pettit, J. S. McKennis, L. Brener, J. S. Ward
J. Am. Chem. Soc. 1971, 93, 4957.
Reduction of cyclopentadiene (Bond dissociation
energy (BDE) = 81.2  2 kcal mol-1, pKa = 18)
Gain in aromaticity drives reaction
Triple Csp3-H activation of cyclopentene (BDE =
82.3  1.1 kcal mol-1, pKa ≈ 46)
Allylic activation and partial hydrogenation of 1,4-
cyclohexadiene (BDE = 75  2 kcal mol-1)
Hydride (ArGeH)2 implicated as a possible
intermediate in these processes
Acknowledgments
We thank the National Science Foundation (CHE-
0948417) for support of this work and the UC Davis
Postdoctoral Scholars Association for a travel award
(OTS).
EPR Studies
Reversible C-C Activation
Symmetry-Forbidden Cycloadditions
Olefin (ArGe)2 (ArSn)2
Ethylene [2 + 2] [2 + 2]
Propene  
2,3-Dimethyl-1,3-butadiene [1 + 4] [1 + 4]
1,6-Heptadiene [2 + 2] -
Cyclopentadiene C-H C-H
Cyclopentene C-H 
1,4-Cyclohexadiene C-H 
Cyclohexene  
Fluorene  
9,10-Dihydroanthracene  
Cycloheptatriene  
Norbornene  
Norbornadiene [2 + 2] [2 + 2]
1,5-Cyclooctadiene  
Cyclooctatetraene [2 + 2] [2 + 2]
1,6-Heptadiene used as a “radical clock”
Concerted mechanism to give symmetry-forbidden
[2 + 2] product
Dimetallynes react with olefins via cycloaddition or
C-H activation
Ethylene cycloaddition with distannyne reversible
Increasing bulk inhibits activation reactions
Higher BDE of cyclohexene (85 kcal mol-1)
precludes C-H activation
High olefin strain energy of norbornadiene (17.2
kcal mol-1) drives cycloadditions
Aromatic driving force for cot reactions
Structures
Left: 1 Ge(1)-C(31) 2.1827(14),
Ge(1)-C(32) 2.3272(14), Ge(1)-
C(35) 2.531(2).
Isomerization speculated to proceed via
anti- to syn- rearrangement followed by
[3,3] sigmatropic shift.
DFT studies (left) describe hypothetical
tin product; however, inverse sandwich
isomer is 37 kcal mol-1 more stable
Above: co-crystallized rotamers in 3 C(1)-C(2) 1.546(3), C(5)-C(6)
1.330(3), C(7)-C(8) 1.537(7), C(8)-C(9) 1.46(3), C(9)-C(10) 1.504(9),
C(10)-C(11) 1.501(8), C(11)-C(12) 1.33(2), C(7)-C(12) 1.522(7).
ArGeGeAr ArSnSnAr
Summary & Conclusions
Degenerate Cope rearrangement
Thermal ellipsoid (50%) plots of 1-6 with
selected bond lengths [Å] and bond angles [°].
Hypostrophene C10H10
Left: 5 Sn1-C39 2.231(4),
Sn1-C2 2.366(4), Sn1-C1
2.506(4), Sn2-C9 2.242(4),
Sn2-C4 2.541(4), Sn2-C5
2.364(4), Sn2-C6 2.590(4).
Right: 6 Ge1-Ge2 2.566(1), Ge1-C1
1.956(2), Ge2-C8 1.950(2), Ge1-C4
1.979(2), Ge2-C5 1.985(2), C1-C2 1.323(2),
C7-C8 1.330(2), C2-C3 1.510(2), C7-C6
1.495(2), C3-C6 1.602(2), C3-C4 1.558(2),
C4-C5 1.561(2), C5-C6 1.556(2).
First p-block π-complexes of
cyclooctatetraene (cot)
Ge compound 4 isomerizes to strained
cage species 6 via Cope rearrangement
(ΔH‡ = 14.9 kcal mol-1 and ΔS‡ = -6.2 cal
mol-1 K-1) C-H activation
Between dimetallynes and cyclic olefins cyclopentadiene,
cyclopentene and 1,4-cyclohexadiene; dehydroaromatization yields
cyclopentadienyl and benzene products
Literature precedence & mechanism
Known in solution using strongly basic iridium or rhenium hydrides at
elevated temperatures or with an additional hydrogen acceptor
The low basicity of dimetallynes suggests radical abstraction
mechanism more likely
Isolation of (ArGeH)2 from the reaction with 1,4-cyclohexadiene
implicates its role as a reaction intermediate
Pericyclic reactions
Reduction of cot (E0 = -1.99 V) gives the first p-block -bound
derivatives with complete cleavage of the E≡E bonds following initial
[2 + 2] cycloaddition
Inverse sandwich species 4 undergoes a remarkable isomerisation to
cage complex 6, a heavy analogue of hypostrophene, via a reversible
C-C cleavage
Radical clock 1,6-heptadiene reveals a concerted (non-radical)
mechanism for the [2 + 2] symmetry-forbidden cycloaddition of the
type already known for dimetallynes
High reactivity of the E≡E triple bond is enhanced by partial
diradicaloid character and influenced strongly by steric pressures
Spectroscopy
EPR demonstrate unpaired spin density on the heavy group 14
element centers in (ArGe)2 and (ArSn)2, i.e. triplet rather than singlet
diradicaloid states that have been previously proposed
* Calculated for R = Me
Dimetallynes synthesized with
Mg to avoid radical anion
K(ArEEAr) impurity
Triplet diradicals observed in frozen solution
Bulk solids are diamagnetic (SQUID magnetic measurements)
Distannyne triplet from low-lying
excited state
Ge diradical from ground state
mixing of lone pair and π orbital
Unusual Reactivity of Dimetallynes Towards Hydrocarbons
Owen T. Summerscales, James C. Fettinger, Jamie A. Stull, R. David Britt, Philip P. Power*
Department of Chemistry, University of California, Davis, CA 95616, USA
Reaction coordinate
RelativeEnergy(kcalmol-1)
6
6-Sn
M = Sn
M = Ge