Tunable Electrochemical and Catalytic Features of BIAN- and BIAO-Derived Ruthenium Complexes.
This article deals with a class of ruthenium-BIAN-derived complexes, [Ru(II)(tpm)(R-BIAN)Cl]ClO4 (tpm = tris(1-pyrazolyl)methane, R-BIAN = bis(arylimino)acenaphthene, R = 4-OMe ([1a]ClO4), 4-F ([1b]ClO4), 4-Cl ([1c]ClO4), 4-NO2 ([1d]ClO4)) and [Ru(II)(tpm)(OMe-BIAN)H2O](2+) ([3a](ClO4)2). The R-BIAN framework with R = H, however, leads to the selective formation of partially hydrolyzed BIAO ([N-(phenyl)imino]acenapthenone)-derived complex [Ru(II)(tpm)(BIAO)Cl]ClO4 ([2]ClO4). The redox-sensitive bond parameters involving -N═C-C═N- or -N═C-C═O of BIAN or BIAO in the crystals of representative [1a]ClO4, [3a](PF6)2, or [2]ClO4 establish its unreduced form. The chloro derivatives 1a(+)-1d(+) and 2(+) exhibit one oxidation and successive reduction processes in CH3CN within the potential limit of ±2.0 V versus SCE, and the redox potentials follow the order 1a(+) < 1b(+) < 1c(+) < 1d(+) ≈ 2(+). The electronic structural aspects of 1a(n)-1d(n) and 2(n) (n = +2, +1, 0, -1, -2, -3) have been assessed by UV-vis and EPR spectroelectrochemistry, DFT-calculated MO compositions, and Mulliken spin density distributions in paramagnetic intermediate states which reveal metal-based (Ru(II) → Ru(III)) oxidation and primarily BIAN- or BIAO-based successive reduction processes. The aqua complex 3a(2+) undergoes two proton-coupled redox processes at 0.56 and 0.85 V versus SCE in phosphate buffer (pH 7) corresponding to {Ru(II)-H2O}/{Ru(III)-OH} and {Ru(III)-OH}/{Ru(IV)═O}, respectively. The chloro (1a(+)-1d(+)) and aqua (3a(2+)) derivatives are found to be equally active in functioning as efficient precatalysts toward the epoxidation of a wide variety of alkenes in the presence of PhI(OAc)2 as oxidant in CH2Cl2 at 298 K, though the analogous 2(+) remains virtually inactive. The detailed experimental analysis with the representative precatalyst 1a(+) suggests the involvement of the active {Ru(IV)═O} species in the catalytic cycle, and the reaction proceeds through the radical mechanism, as also supported by the DFT calculations.
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
Arijit Singha
AS
Hazari
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
Ankita
A
Das
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
Ritwika
R
Ray
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
Hemlata
H
Agarwala
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
Somnath
S
Maji
‡Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Indore 452017, India.
Shaikh M
SM
Mobin
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
Goutam Kumar
GK
Lahiri
D000073:Acenaphthenes / Q000737:chemistry*; D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry*; D055664:Electrochemical Techniques; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011522:Protons; D011720:Pyrazoles / Q000737:chemistry*; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*
D000073:Acenaphthenes; D056831:Coordination Complexes; D011522:Protons; D011720:Pyrazoles; C523936:tris(1-pyrazolyl)methane; D012428:Ruthenium; D008697:Methane