Resonance Raman spectra of tetrapyrrole complexes of oxomolybdenum(V)

Solvent effects on the electronic and vibrational properties of high-valent oxomolybdenum(V) 5,10,15-triphenylcorrole probed by UV-visible and resonance Raman spectroscopy

Solution UV-visible electronic absorption and resonance Raman (RR) scattering spectra in 25 aprotic organic solvents have been measured and analyzed for a high-valent molybdenum( V ) oxo complex of 5,10,15-triphenylcorrole, (MoVO)CorPh3 . Optical absorption data in the visible region show a significant positive solvatochromism upon increasing solvent polarizability (ca. 660 cm-1 for the violet Soret band on going from acetonitrile to carbon disulfide), described by an excellent linear correlation between the bathochromic shifts of corrole π → π* electronic transitions and the solvent refractive index [Formula: see text]. Isotope labeling of the terminal oxo group with 18 O revealed the oxo-Mo(V) stretch of (MoVO)CorPh3 to vibrate at frequency ν(MoVO) ≈ 970 cm-1 [Δ(16O-18O) = 46 cm-1] , consistent with a MoV≡O triple bond. This vibration is greatly enhanced in the RR spectra with excitations within the Soret absorption band and its frequency is sensitive to solute-solvent interactions that weaken the MoV≡O triple bond by inhibiting O2- → Mo5+ electron donation. The ν(MoVO) frequency decreases in direct proportion to the Swain's solvent polarity parameter (A + B), from 969.4 cm-1 in n-hexane solution down to 955.6 cm-1 in dimethyl sulfoxide solution. However, only weak linear correlation was found when the ν(MoVO) frequencies were plotted as a function of the Gutmann's solvent acceptor numbers (ANs). A molecular association occurs between chloroform and (MoVO)CorPh3 through MoV≡O⋯H-CCl3 hydrogen-bonding interactions, manifested as an upshift of the ν(MoVO) RR band in deuteriochloroform solution. The ν(MoVO) stretching frequencies of (MoVO)CorPh3 and its fluorinated derivative (MoVO)Cor(Ph(CF3)2)3 {Cor(Ph(CF3)2)3 = 5,10,15-tris-[3,5-bis(trifluoromethyl)phenyl]corrole} are compared, and the increased strength of the MoV≡O bond in the latter oxo-Mo(V) species is attributed to the cis effect of the electron-withdrawing -C6H3(CF3)2 groups, as judged by a 6 cm-1 elevation of the ν(MoVO) frequency. Raman excitation profiles of ν(MoVO) and corrole skeletal vibrations show the presence of charge-transfer and π → π* electronic transitions within the Soret absorption band of oxomolybdenum( V ) corroles.

Solvent-Dependent Resonance Raman Spectra of High-Valent Oxomolybdenum(V) Tris[3,5-bis(trifluoromethyl)phenyl]corrolate

UV-visible, infrared (IR), and resonance Raman (RR) spectra were measured and analyzed for a high-valent molybdenum(V)-oxo complex of 5,10,15-tris[3,5-bis(trifluoromethyl)phenyl]corrole (1) at room temperature. The strength of the metal-oxo bond in 1 was found to be strongly solvent-dependent. Solid-state IR and RR spectra of 1 exhibited the MoVO stretching vibration at nu(MoVO)=969 cm(-1). It shifted up by 6 cm(-1) to 975 cm(-1) in n-hexane and then gradually shifted to lower frequencies in more polar solvents, down to 960 cm(-1) in dimethyl sulfoxide. The results imply that stronger acceptor solvents weaken the MoVO bond. The 45-cm(-1) frequency downshifts displayed by 1 containing an 18O label in the molybdenum(V)-oxo unit confirmed the assignments for the observed IR and RR nu(MoVO) bands. The solvent-induced frequency shift for the nu(MoVO) RR band, measured in a series of 25 organic solvents ranging from n-hexane (AN=0.0) to N-methylformamide (AN=32.1), did not decrease in direct proportion to Gutmann's solvent acceptor numbers (ANs). However, a good linear correlation of the nu(MoVO) frequency was found against an empirical "solvent polarity" scale (A+B) of Swain et al. J. Am. Chem. Soc. 1983, 105, 502-513. A molecular association was observed between chloroform and oxomolybdenum(V) corrole 1 through MoO...H/CCl3 hydrogen-bonding interactions. This association manifested itself as a shift of the nu(MoVO) RR band of 1 in CDCl3 to a higher frequency compared to that in CHCl3.