Abstract
Dual-mode electron paramagnetic resonance (EPR), in which an oscillating magnetic field is alternately applied parallel or perpendicular to the static magnetic field, is a valuable technique for studying both half-integer and integer electron spin systems and is particularly useful for studying transition metals involved in redox chemistry. We have applied this technique to the characterization of the Mn(III) salen (salen = N,N′-ethylene bis(salicylideneaminato)) complex [(R,R)-(-)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2- cyclohexanediaminomanganese(III)], with an S = 2 integer electron spin system. Furthermore, we have used dual-mode EPR to study the Mn salen complex during the Mn(III) salen-catalyzed epoxidation of cis-β-methylstyrene. Our study shows that the additives N-methylmorpholine N-oxide (NMO) and 4-phenylpyridine-N-oxide (4-PPNO), which are used to improve epoxidation yields and enantioselection, bind to the Mn(III) center prior to the epoxidation reaction, as evidenced by the alteration of the Mn(III) parallel mode EPR signal. With these additives as ligands, the axial zero-field splitting values and 55Mn hyperfine splitting of the parallel mode signal are indicative of an axially elongated octahedral geometry about the Mn(III) center. Since the dual-mode EPR technique allows the observation of both integer and half-integer electron spin systems, Mn oxidation states of II, III, IV, and potentially V can be observed in the same sample as well as any radical intermediates or Mn(III,IV) dinuclear clusters formed during the Mn(III) salen-catalyzed epoxidation reaction. Indeed, our study revealed the formation of a Mn(III,IV) dinuclear cluster in direct correlation with expoxide formation. In addition to showing the possible reaction intermediates, dual-mode EPR offers insight into the mechanism of catalyst degradation and formation of unwanted byproducts. The dual-mode technique may therefore prove valuable for elucidating the mechanism of Mn(III) salen catalyzed reactions and ultimately for designing optimum catalytic conditions (solvents, oxidants, and additives such as NMO or 4-PPNO).
| Original language | English (US) |
|---|---|
| Pages (from-to) | 5710-5719 |
| Number of pages | 10 |
| Journal | Journal of the American Chemical Society |
| Volume | 123 |
| Issue number | 24 |
| DOIs | |
| State | Published - 2001 |
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Dual-mode EPR study of Mn(III) salen and the Mn(III) salen-catalyzed epoxidation of cis-β-methylstyrene. / Campbell, K. A.; Lashley, M. R.; Wyatt, J. K.; Nantz, M. H.; David Britt, R.
In: Journal of the American Chemical Society, Vol. 123, No. 24, 2001, p. 5710-5719.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Dual-mode EPR study of Mn(III) salen and the Mn(III) salen-catalyzed epoxidation of cis-β-methylstyrene
AU - Campbell, K. A.
AU - Lashley, M. R.
AU - Wyatt, J. K.
AU - Nantz, M. H.
AU - David Britt, R.
PY - 2001
Y1 - 2001
N2 - Dual-mode electron paramagnetic resonance (EPR), in which an oscillating magnetic field is alternately applied parallel or perpendicular to the static magnetic field, is a valuable technique for studying both half-integer and integer electron spin systems and is particularly useful for studying transition metals involved in redox chemistry. We have applied this technique to the characterization of the Mn(III) salen (salen = N,N′-ethylene bis(salicylideneaminato)) complex [(R,R)-(-)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2- cyclohexanediaminomanganese(III)], with an S = 2 integer electron spin system. Furthermore, we have used dual-mode EPR to study the Mn salen complex during the Mn(III) salen-catalyzed epoxidation of cis-β-methylstyrene. Our study shows that the additives N-methylmorpholine N-oxide (NMO) and 4-phenylpyridine-N-oxide (4-PPNO), which are used to improve epoxidation yields and enantioselection, bind to the Mn(III) center prior to the epoxidation reaction, as evidenced by the alteration of the Mn(III) parallel mode EPR signal. With these additives as ligands, the axial zero-field splitting values and 55Mn hyperfine splitting of the parallel mode signal are indicative of an axially elongated octahedral geometry about the Mn(III) center. Since the dual-mode EPR technique allows the observation of both integer and half-integer electron spin systems, Mn oxidation states of II, III, IV, and potentially V can be observed in the same sample as well as any radical intermediates or Mn(III,IV) dinuclear clusters formed during the Mn(III) salen-catalyzed epoxidation reaction. Indeed, our study revealed the formation of a Mn(III,IV) dinuclear cluster in direct correlation with expoxide formation. In addition to showing the possible reaction intermediates, dual-mode EPR offers insight into the mechanism of catalyst degradation and formation of unwanted byproducts. The dual-mode technique may therefore prove valuable for elucidating the mechanism of Mn(III) salen catalyzed reactions and ultimately for designing optimum catalytic conditions (solvents, oxidants, and additives such as NMO or 4-PPNO).
AB - Dual-mode electron paramagnetic resonance (EPR), in which an oscillating magnetic field is alternately applied parallel or perpendicular to the static magnetic field, is a valuable technique for studying both half-integer and integer electron spin systems and is particularly useful for studying transition metals involved in redox chemistry. We have applied this technique to the characterization of the Mn(III) salen (salen = N,N′-ethylene bis(salicylideneaminato)) complex [(R,R)-(-)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2- cyclohexanediaminomanganese(III)], with an S = 2 integer electron spin system. Furthermore, we have used dual-mode EPR to study the Mn salen complex during the Mn(III) salen-catalyzed epoxidation of cis-β-methylstyrene. Our study shows that the additives N-methylmorpholine N-oxide (NMO) and 4-phenylpyridine-N-oxide (4-PPNO), which are used to improve epoxidation yields and enantioselection, bind to the Mn(III) center prior to the epoxidation reaction, as evidenced by the alteration of the Mn(III) parallel mode EPR signal. With these additives as ligands, the axial zero-field splitting values and 55Mn hyperfine splitting of the parallel mode signal are indicative of an axially elongated octahedral geometry about the Mn(III) center. Since the dual-mode EPR technique allows the observation of both integer and half-integer electron spin systems, Mn oxidation states of II, III, IV, and potentially V can be observed in the same sample as well as any radical intermediates or Mn(III,IV) dinuclear clusters formed during the Mn(III) salen-catalyzed epoxidation reaction. Indeed, our study revealed the formation of a Mn(III,IV) dinuclear cluster in direct correlation with expoxide formation. In addition to showing the possible reaction intermediates, dual-mode EPR offers insight into the mechanism of catalyst degradation and formation of unwanted byproducts. The dual-mode technique may therefore prove valuable for elucidating the mechanism of Mn(III) salen catalyzed reactions and ultimately for designing optimum catalytic conditions (solvents, oxidants, and additives such as NMO or 4-PPNO).
UR - http://www.scopus.com/inward/record.url?scp=0034810834&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0034810834&partnerID=8YFLogxK
U2 - 10.1021/ja0027463
DO - 10.1021/ja0027463
M3 - Article
C2 - 11403603
AN - SCOPUS:0034810834
VL - 123
SP - 5710
EP - 5719
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 24
ER -