Molybdenum(VI) Dioxo and Oxo-Imido Complexes of Fluorinated β-Ketiminato Ligands and Their Use in OAT Reactions

Abstract

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)NC(Me)CH(2)CO(CF(3)) (HL(H), Ar = C(6)H(5); HL(Me), A r= 2,6-Me(2)C(6)H(3); HL(iPr), Ar = 2,6-(i)Pr(2)C(6)H(3)). Subsequent complexation to the [MoO(2)](2+) core leads to the formation of novel complexes of general formula [MoO(2)(L(R))(2)] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO(N(t)Bu)(L(Me))(2)] (4) has also been synthesized. Complexes 1-4 were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo(IV) complexes [MoO(PMe(3))(L(R))(2)] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo(N(t)Bu)(PMe(3))(L(Me))(2)] (8), were isolated. All complexes have been characterized by NMR spectroscopy, and 1-4 also by cyclic voltammetry. A positive shift of the Mo(VI)-Mo(V) reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single crystal X-ray diffraction analysis. Complex 8 represents a rare example of a Mo(IV) phosphino-imido complex. Kinetic measurements by UV-vis spectroscopy of the OAT reactions from complexes 1-4 to PMe(3) showed them to be more efficient than previously reported nonfluorinated ones, with ligand L' = (Ar)NC(Me)CH(2)CO(CH(3)) [MoO(2)(L')(2)] (9) and [MoO(N(t)Bu)(L')(2)] (10), respectively. Thermodynamic activation parameters ΔH(‡) and ΔS(‡) of the OAT reactions for complexes 2 and 4 have been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term ΔS(‡) is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

“…Forward OAT (to phosphanes) and backward-OAT from DMSO has been thoroughly studied. Fluorinated ˇ-ketiminato ancillary ligands shift the Mo VI/V reduction to more positive values and lower the barrier for OAT to phosphanes due to their electron withdrawing effect [77]. The oxido phosphane molybdenum(IV) complexes react with dry O 2 to give an oxido peroxido molybdenum(VI) complex Mo(L NO ) 2 O( 2 -O 2 ) underlining their sensitivity toward O 2 [78].…”

Oxygen activation and catalytic aerobic oxidation by Mo(iv)/(vi) complexes with functionalized iminophenolate ligands

“…3,4 The ease of such molybdenum compounds to undergo oxygen atom transfer made them also interesting in industrial applications. [6][7][8]9 In that course, iminophenolate based complexes incorporating different functionalities were developed. 5 In general, these catalysts use H 2 O 2 or organic peroxides such as tert-butyl hydroperoxide (TBHP) as terminal oxidants.…”

Electronic Fine‐Tuning of Oxygen Atom Transfer Reactivity of cis‐Dioxomolybdenum(VI) Complexes with Thiosemicarbazone Ligands

“…Evidence for the formation of the imine bond was provided by the appearance of a singlet resonance for the imine proton at 8.32-8.38 ppm and loss of the resonance due to the aldehyde proton in the 1 H NMR spectra ([D 6 ]DMSO). Corroborating evidence was obtained by 13 C NMR and FTIR spectroscopy. Subsequently, the six ligands H 2 L R were coordinated to a cis-dioxo Mo VI center and the resulting [MoO 2 (L R )MeOH] complexes were isolated and characterized.…”

“…[46] Triphenylphosphine was recrystallized from hot ethanol prior to use. 1 H and 13 C{ 1 H} decoupled NMR spectra were recorded with a Jeol ECS400 instrument ( 1 H NMR 400 MHz, 13 C NMR 100.6 MHz). Assignment of resonances was confirmed by COSY, DEPT135 and HSQC spectra.…”

“…[2][3][4][5][6] Whereas bidentate dithiolene ligands model the S,S donor set found in the molybdopterin cofactor of the enzymes, [7,8] bidentate ligands with N,S, N,N and N,O donor have also been found to catalyze OAT. Examples include pyridine thiolates, [9] iminopyrrolato, [10,11] diketiminate (NacNac), [12] and β-ketiminato [13] ligands. In addition, dioxo-molybdenum complexes with ligands of higher denticity and mixed relationships between Hammett constants and key properties of the complexes, including the molybdenum-phenolate bond lengths and the coordination shift of the imine proton resonance.…”