TY - JOUR
T1 - Intramolecular gas-phase reactions of synthetic nonheme oxoiron(IV) ions
T2 - Proximity and spin-state reactivity rules
AU - Mas-Ballesté, Rubén
AU - McDonald, Aidan R.
AU - Reed, Dana
AU - Usharani, Dandamudi
AU - Schyman, Patric
AU - Milko, Petr
AU - Shaik, Sason
AU - Que, Lawrence
PY - 2012/9/10
Y1 - 2012/9/10
N2 - The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N4 ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe IV(O)(L)(OTf)]+ (OTf=trifluoromethanesulfonate) and [FeIV(O)(L)]2+ were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L8Py 2) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)- 1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [FeIV(O)(L)(OTf)]+ cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe IV(O)(L)]2+ cations. DFT calculations on [Fe IV(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C-H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σCH-σ* z 2 overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa- and pentacoordinate complexes.
AB - The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N4 ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe IV(O)(L)(OTf)]+ (OTf=trifluoromethanesulfonate) and [FeIV(O)(L)]2+ were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L8Py 2) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)- 1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [FeIV(O)(L)(OTf)]+ cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe IV(O)(L)]2+ cations. DFT calculations on [Fe IV(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C-H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σCH-σ* z 2 overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa- and pentacoordinate complexes.
KW - bioinorganic chemistry
KW - density functional calculations
KW - exchange enhanced reactivity
KW - hydrogen transfer
KW - oxoiron complexes
UR - http://www.scopus.com/inward/record.url?scp=84865726817&partnerID=8YFLogxK
U2 - 10.1002/chem.201200105
DO - 10.1002/chem.201200105
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C2 - 22837063
AN - SCOPUS:84865726817
SN - 0947-6539
VL - 18
SP - 11747
EP - 11760
JO - Chemistry - A European Journal
JF - Chemistry - A European Journal
IS - 37
ER -