Exchange-Enhanced Open-Shell States: Hund's rule for Bioinorganic Species Applied to H-Abstraction

D. Janardanan, D. Usharani, S. Shaik Angewandte Chemie International Edition 2012, 51, 4421-4425 (Paywall)

In a Perspective in Nature Chemistry last year¹ Sason Shaik and co-workers described bond activation by metal-oxo enzymes and synthetic reagents. In it, they argued that Hund's rule of maximum multiplicity (valid for atoms) has an analogue for reactions and kinetics of (bio)inorganic species: the exchange-enhanced reactivity (EER). Pathways that increase the number of unpaired and spin-identical electrons on a metal center will be favored by exchange interactions, and hence are favored over pathways that keep the same number (or less) of exchange interactions.

In this recent paper in Angewandte Chemie International Edition Shaik and co-workers apply their EER principle on H-abstraction reactions, and show how dramatic axial ligand effects can be explained by it. The systems under study are [(Cz)(X)Mn^VO] (Cz: corrolazinato^3-, X=None, F^-, CN^-) complexes (see Figure below), which probably have a singlet ground state (X=None) or a triplet ground state (X=F^-, CN^-).

More important than the ground state of the reactant is however the spin state of the transition states (TSs). The hydrogen abstraction involves a proton-coupled electron transfer (PCET): the migrating H· radical transfers its radical to the d-block of the metal, while at the same time the proton makes the O-H bond. For the singlet state, one obtains at the TS an open-shell singlet with an alpha electron on the metal and a beta electron on the substrate. In the exchange-enhanced triplet state, there are now three alpha electrons on the metal (with favorable exchange interactions) that gives the dramatic decrease in barrier (from 32 kcal·mol^-1 for the singlet to 23.0 kcal·mol^-1 for the triplet).

The axial ligand effect has two origins: the exchange interactions become stronger (11.1, 12.4 and 12.9 kcal·mol^-1 for the three complexes) while at the same time the d-orbitals become closer in energy (smaller excitation energy).

References

(1) S. Shaik, H. Chen, D. Janardanan, "Exchange-enhanced reactivity in bond activation by metal-oxo enzymes and synthetic reagents", Nature Chem. 2011, 3, 19-27, DOI: 10.1038/NChem.943


This work is licensed under a Creative Commons Attribution 3.0 Unported License. 

Effects of Ethynyl Substitution on Cyclobutadiene

B. J. Esselman, R. J. McMahon Journal of Physical Chemistry A 2012, 116, 483-490 (Paywall)
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission

Cyclobutadiene is the prototypical antiaromatic compound. McMahon has examined the effect of ethynyl substitution on this ring, with a long term eye towards the possibility of these types of species being involved in the synthesis of fullerenes.¹

All of the possible ethynyl-substituted cyclobutadiene species (1-7) were optimized at B3LYP/6-31G(d) and CCSD/cc-pVDZ in their singlet and triplet states.

The structures of singlet and triplet 7 are shown in Figure 1. The geometries provided by the two different methods are quite similar. They show a rectangular form for the singlets and a delocalized, nearly square ring for the triplets.

The computed singlet-triplet gap decreases with each ethynyl substituent. B3LYP, which overestimates the stability of triplets, predicts that 6 and 7 will be ground state triplets, while CCSD predicts a singlet ground state for all 7 species, with the gap decreasing steadily from 11.5 to 8.2 kcal mol^-1, a value that is also probably underestimated.

This change in the singlet-triplet gap reflects a stronger stabilizing effect of each ethynyl group to the cycnobutadiene ring for the triplet than for the singlet state. This is seen in the homodesmotic stabilization energies.

Lastly, NICS(1)_zz values are positive for all of the singlets and negative for the triplets. The positive values for the singlets reflect their antiaromatic character, also seen in the alternant bond distances around the ring. The NICS values of the singlets decrease with increasing substitution. The negative NICS values of the triplets reflects aromatic character, as seen in the non-alternant distances around the ring. Interestingly, the triplet NICS values decrease with increasing ethynyl substitution, suggesting decreased aromaticity, even though the homodesmotic reactions suggest increasing stabilization with substitution.


References

(1) Esselman, B. J.; McMahon, R. J., "Effects of Ethynyl Substitution on Cyclobutadiene," J. Phys. Chem. A 2012, 116, 483-490, DOI: 10.1021/jp206478q


This work is licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License.

Regiochemical Substituent Switching of Spin States in Aryl(trifluoromethyl)carbenes

M.-G. Song and R.S. Sheridan Journal of the American Chemical Society 2011, 133, 19688 (Paywall)
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry (where interactive models can be found) with permission

Can a remote substituent affect the singlet-triplet spin state of a carbene? Somewhat surprisingly, the answer is yes. Sheridan has synthesized and characterized the meta and para methoxy-substituted phenyltrifluoromethyl)carbenes 1 and 2. The UV-Vis spectrum of 1 is consistent with a triplet as its EPR and reactivity with oxygen. However, the para isomer 2 gave no EPR signal and failed to react with oxygen or hydrogen, suggestive of a singlet.

The conformations of 1 and 2 were optimized at B3LYP/6-31+G(d,p) and the lowest energy singlet and triplet conformers are shown in Figure 1. The experimental spectral features of 1 match up best with the computed features of the triplet, and the same is true for the singlet of 2.

The triplet of 1 is estimated to be about 4 kcal mol^-1 below that of the singlet – larger than the general overestimation of the stability of triplets that beleaguer B3LYP. For 2, B3LYP predicts a singlet ground state.

The isodesmic reactions 1 and 2 help understand the strong substituent effect. For 1, the meta substituent destabilizes both the singlet and triplet by a small amount. For 2, the para methoxy group stabilizes the triplet slightly, but stabilizes the singlet by a large amount. This stabilization is likely the result of the contribution of a second resonance structure 2b. A large rotational barrier for both the methyl methyl and the trifluoromethyl groups supports the participation of 2b.

ΔE_singlet = -0.8 kcal mol^-1

ΔE_triplet = -0.6 kcal mol^-1

ΔE_singlet = -5.8 kcal mol^-1

ΔE_triplet = -1.1 kcal mol^-1

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