[Fe(IV)═O(TBC)(CH3CN)]2+: comparative reactivity of iron(IV)-oxo species with constrained equatorial cyclam ligation.
[Fe(IV)═O(TBC)(CH(3)CN)](2+) (TBC = 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclotetradecane) is characterized, and its reactivity differences relative to [Fe(IV)═O(TMC)(CH(3)CN)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are evaluated in hydrogen atom (H-atom) abstraction and oxo-transfer reactions. Structural differences are defined using X-ray absorption spectroscopy and correlated to reactivities using density functional theory. The S = 1 ground states are highly similar and result in large activation barriers (~25 kcal/mol) due to steric interactions between the cyclam chelate and the substrate (e.g., ethylbenzene) associated with the equatorial π-attack required by this spin state. Conversely, H-atom abstraction reactivity on an S = 2 surface allows for a σ-attack with an axial substrate approach. This results in decreased steric interactions with the cyclam and a lower barrier (~9 kcal/mol). For [Fe(IV)═O(TBC)(CH(3)CN)](2+), the S = 2 excited state in the reactant is lower in energy and therefore more accessible at the transition state due to a weaker ligand field associated with the steric interactions of the benzyl substituents with the trans-axial ligand. This study is further extended to the oxo-transfer reaction, which is a two-electron process requiring both σ- and π-electron transfer and thus a nonlinear transition state. In oxo-transfer, the S = 2 has a lower barrier due to sequential vs concerted (S = 1) two electron transfer which gives a high-spin ferric intermediate at the transition state. The [Fe(IV)═O(TBC)(CH(3)CN)](2+) complex is more distorted at the transition state, with the iron farther out of the equatorial plane due to the steric interaction of the benzyl groups with the trans-axial ligand. This allows for better orbital overlap with the substrate, a lower barrier, and an increased rate of oxo-transfer.
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
Samuel A
SA
Wilson
Junying
J
Chen
Seungwoo
S
Hong
Yong-Min
YM
Lee
Martin
M
Clémancey
Ricardo
R
Garcia-Serres
Takashi
T
Nomura
Takashi
T
Ogura
Jean-Marc
JM
Latour
Britt
B
Hedman
Keith O
KO
Hodgson
Wonwoo
W
Nam
Edward I
EI
Solomon
D002614:Chelating Agents / Q000737:chemistry; D058085:Iron Compounds / Q000737:chemistry*; D047029:Lactams, Macrocyclic / Q000737:chemistry*; D008958:Models, Molecular; D056928:X-Ray Absorption Spectroscopy
D002614:Chelating Agents; D058085:Iron Compounds; D047029:Lactams, Macrocyclic