Cotmpyp-naf 2009

An N-Bound Peroxynitro-Cobalt Intermediate? Computational and Experimental Evidence John A. Goodwin, Donald F. Kavanagh, Tigran S. Kurtikyan, Jean Standard, Patrick Desrochers Coastal Carolina University, Conway, SC Molecular Structure Research Centre NAS, Yerevan, Armenia Illinois State University, Normal IL University of Central Arkansas, Conway, AR

Background (1): Oxygen Activation/ Secondary Oxo Transfer Catalysis 



(py)CoTPP(NO2)/(with Lewis acids) – since late ’70’s (Tovrog, Diamond, Mares J. Am. Chem. Soc. 1979, 101, (1), 270-272) Nitro/nitrosyl complexes and Oxygen Activation with N-bound peroxynitro intermediate (Clarkson, Basolo Inorg. Chem. 1973, 12, (7), 1528-1534)  Co(NO) + O2 → Co{(N=O)(O2)} + Co(NO) → 2 Co(NO2) (comproportionation mechanism) and/or  Co{(N=O)(O2)} + X → Co(NO2) +XO  Co{(N=O)(O2)}

 Co(NO2) 

+ X → Co(NO) + XO

overall: 2 X + O2 → 2 XO

Background (2): Five-coordinate (nitro)cobalt porphyrins vs. sixcoordinate

 Much

more reactive than the hexacoordinate derivatives  Lewis acids remove trans ligand such as pyridine  Catalytically oxidize alkenes  May involve a peroxynitro intermediate  Oxidation competitive with comproportionation  Goodwin, et al. Inorg. Chem., 2001, 40, 42174225.

Background (3): Proposed oxygen activation/oxo-transfer XO Epoxides, aldehydes

X non-radical L4Co(II)NO

L4Co(III)NO2 C-H insertion: alcohols

XO O2 radical L4Co(II)NO(O-O) X

No comproportionation

Overall: 2X + O2

2 XO

Background (4): Other examples of metal-bound peroxynitrite intermediates

Hemes

(Hb and Mb - O-bound: FeO2 + NO (Nitric oxide dioxygenation) Olson, Foley, et al. Free Rad. Biol. Med. 2004, 36, 685-697. (N-bound: MbNO + O2) Arnold, Bohle, 213th ACS Nat’l Meeting, San Francisco, 1997.

Other Iron (N-bound: Fe(II)(CN)5NO + O2) Videla, Olabe, et al. J. Am. Chem. Soc. 2007, 129, 278-279.

Copper (CuO2 + NO) Maiti, Karlin, et al. J. Am. Chem. Soc. 2008, 130, 6700-6701.

Background (5): Other examples of metal-bound peroxynitrite Cobalt (O-bound: CoO2 + NO) Wick, Kissner, Koppenol, Helv. Chim. Acta 2000, 83, 748–754. Thyagarajan, Incarvito, Rheingold, Theopold, Inorg. Chim. Acta 2003, 345, 333–339. Not observed.

Project Goals and Objectives  Adapt

solution-phase catalysis as a heterogeneous catalysts  Use

cationic porphyrins electrostatically immobilized on Nafion films  Use gas-phase reactants  Probe

catalytic mechanism

 immobilized

nitro complex to avoid comproportionation  Computational study to compare plausible intermediates  Goodwin,

J.A. et al. Inorg. Chem. 2008, 47, (17), 7852-7862.

Structures H3C

N

N

+

H3C

N N

+

NO2

Co N N

+

N CH3 Pentacoordinate (nitro)cobalt porphyrin, CoTMpyP(2), with cationic pyridinium groups.

N

+

CH3

Schematic of a Nafion monomeric unit The Nafion films are transparent and the reaction chemistry on these films parallels the solution chemistry – so visible spectra can be used to identify derivatives on films.

Preparation of [CoTMpyP(NO2)]4+ on Nafion 

 





[(H2O)CoTMpyP(2)Br]Br4 is prepared by synthesis of tetra(pyridyl(2))porphyrin, followed by methylation and metal insertion. Irreversible adsorption onto sodium-form films from aqueous solution gives [(H2O)CoTMpyP(2)Br]/Naf Treatment with sodium nitrite solution gives [(H2O)CoTMpyP(NO2)]/Naf (Soret shift from 430 nm to 428 nm) Treatment with absolute ethanol gives [(EtOH)CoTMpyP(NO2 )]/Naf (Soret shift from 428 to 425 nm) UV-vis spectra parallel changes observed in solution that were verified by FTIR. FTIR not feasible in the Nafion system.

Preparation of [CoTMpyP(NO2)]4+ on Nafion  Reduction

of [(H2O)CoTMpyP(2)Br]/Naf with Bu4NBH4 in DCM under nitrogen atmosphere (glove box) gives air-sensitive [(H2O)CoTMpyP(2)]/Naf. (Soret shift from 430 nm to 408 nm with EPR signal of five coordinate cobalt(II))  Heating [(EtOH)CoTMpyP(NO2 )]/Naf gives pentacoordinate [CoTMpyP(NO2 )]/Naf (Soret shift from 428 nm to 419 nm)

Preparation of [CoTMpyP(NO2)]4+ on Nafion H2O N

H2O

Co

N

III

N

+NO2-

N

N Br-

N

CH3CH2OH N

III

Co

N

NO2

BH4H 2O

N

N N

+ NO2

N

NO2 Heat and Vacuum

H2 O CoII

N

Co

N

III

H2O

Br

N

N

+EtOH

??

EtOH N N

N

III

Co

N

NO2

Reaction Chemistry  Treatment

of [(EtOH)CoTMpyP(NO2 )]/Naf with Ph3P in DCM or Ethanol under N2 gives [CoTMpyP(NO)]/Naf  Ph3P

is a very strong O-atom acceptor, Soret shift to 417 nm  Ph3P can form six-coordinate nitro complex, but Soret at 432 nm)  Treatment

of [(EtOH)CoTMpyP(NO2 )]/Naf with Ph3P in ethanol under O2 gives catalytic formation of Ph3PO (1 cm x 2 cm film, 3 mL of 1mM Ph3P, 12 hours)  Catalysis

is sustained by isolated porphyrin, turnover of 66 with turnover frequency of 5.5 hr-1 at room temp

Reaction Chemistry  Treatment

of [CoTMpyP(NO2 )]/Naf with gas phase cyclohexene under N2 gives [CoTMpyP(NO)]/Naf (85°C)  Five-coordinate

strength

nitro complex retains O-atom transfer

 Treatment

of [CoTMpyP(NO2 )]/Naf with cyclohexene and O2  catalytic formation of cyclohexene oxide (7-oxabicycloheptane), 2-cyclohexene-1-one, and 2cyclohexene-1-ol (85°C) (product analysis by GCMS)  Catalysis is sustained by isolated porphyrin with gasphase O-atom acceptor  Quantitative measurements not available yet

Reaction Chemistry H2O N

Co

N

N

N

III

N

CH3CH2OH N

III

Co

N

N

+EtOH

N

N

Co

N

III

N

H2O

NO2

NO2

NO2 + Ph3P

Ph3PO + H2O

+

EtOH + Ph3PO N N

O

II

Co

N N

NO O

OH

Both processes are catalytic in the presence of O2

Proposed Catalytic Pathway L N

Proposed coupled pathways of Six coordinate (nitro) complexes with triphenylphosphine and cyclohexene

N

N CoIII N NO2

L = EtOH or H2O

Ph3PO

Ph3P Ph3P EtOH + Ph3PO

N N

L CoII

N

O2 + L

N

N

N

NO

N CoIII N N

O O

Goodwin, et al. Inorg. Chem. 2008, 47(17) 7852-7862

O

O +

+ O

OH

N N

N CoIII N NO2

O +

+ O

OH

Computational Chemistry - 1 CoPO + NO + 1/2 O2



Calculated values of standard free energy changes (at 298 K, determined by DFT-BP-6-31G* methods) for conversion of fivecoordinate [CoTMpyP(NO2)]4+ to some possible intermediates in the oxygen activation catalysis.

CoP + NO2 + 1/2 O2

191.9 kJ/mol

203.58 kJ/mol

CoPNOO2

111.11 kJ/mol

CoPNO + O2 14.92 kJ/mol

CoPNO2 + 1/2 O2 -9.24 kJ/mol

CoPONO + 1/2 O2

Computational Chemistry - 2 Co(II)P + NO + O2

Plotted values of standard free energy changes (at 298 K, determined by DFT BP 6-31G* methods) for possible catalytic dioxygen activation by [CoTMpyP(NO2)]4+ drawn approximately to scale.

38.4 kJ/mol

40.9 kJ/mol

CoPO + 1/2 O2 + NO

Co(II)P + NO2 + 1/2 O2

-133.8 kJ/mol 191.9 kJ/mol 82.7 kJ/mol

CoPNO2O CoP(O2) + NO -43.5 kJ/mol CoPOONO (singlet) -3.4 kJ/mol CoPOONO (triplet) 203.6 kJ/mol

111.1 kJ/mol CoPNO + O2 14.9 kJ/mol CoPNO2 + 1/2 O2 -9.2 kJ/mol

CoPONO + 1/2 O2

Computational Chemistry - 3 CoPNO2O + H2O

Calculated values of standard free energy changes (at 298 K, determined by DFTBP-6-31G* methods) for possible catalytic dioxygen activation by [CoTMpyP(NO2)]4+ and its aqua complexes.

44.5 lK/mol (H2O)CoPNO2O -126.0 kJ/mol 111.1 kJ/mol 102.0 kJ/mol

CoPNO2 1 + /2 O 2 + H2 O

CoPNO + O2 + H2O +14.9 kJ/mol -82.4 kJ/mol

-35.6 kJ/mol (H2O)CoPNO + O2 +61.7 kJ/mol -126.0 kJ/mol

(H2O)CoPNO2 + 1/2 O2

An N-bound peroxynitro intermediate? O2

XO CoP(NO)

Proposed solution phase oxygen activation/oxotransfer cycle (without comproportionation step)

X CoP(NO2)

PCo N

XO overall:

2 X + O2

X 2 XO

O O O

Acknowledgements  NFSAT-CRDFARC2

-3231-YE-04  NSF OISO/IRES-0622810  CCU students: Jennifer Coor; Don Kavanagh; Mathieu Sabbagh; James Howard; John Adamec; Deidre Parmley; Emily Tarsis; Astghik Hovhannisyan  Tigran Kurtikyan (Molecular Structure Research Center, Yerevan, Armenia)  Patrick Desrochers (U Central Arkansas, epr)