Substituent Effect on Oxygen Atom Transfer Reactivity from Oxomolybdenum Centers: Synthesis, Structure, Electrochemistry, and Mechanism

Abstract

Dioxo molybdenum complexes of general formula Tp*MoO(2)(S-p-RC(6)H(4)) (1), where Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and R = OMe, Me, SMe, NHCOMe, H, Cl, CF(3), NO(2), were reacted with trimethyl phosphine (PMe(3)) to convert into complexes of general formula Tp*MoO(S- p-RC(6)H(4))(OPMe(3)) (2) (where R = OMe, Me, SMe, H, Cl, and CF(3)). These complexes were isolated and characterized by NMR, IR, UV/vis, and single crystal X-ray crystallography. Electronic and NMR spectra, as well as redox potentials vary as a function of substituent on the thiophenolato ligand. When viewed entirety of the oxygen atom transfer (OAT) reactivity, the reaction of Tp*MoO(2)(S-p-RC(6)H(4)) with PMe(3) shows a biphasic behavior, indicating the formation of at least one intermediate. The kinetics of the both steps, that is, the formation of the phosphoryl intermediate and the formation of the solvent coordinated species have been investigated by UV-vis spectroscopy. The first step follows a second order process, first order with respect to both the complex and PMe(3), and the overall second order rate constants at 25 degrees C range from 98.2 (+/-0.01) x 10(-2) M(-1) s(-1) (for R = OMe) to 223.0 (+/-0.20) x 10(-2) M(-1) s(-1) (for R = CF(3)); activation parameters were in the ranges DeltaH(++) = 49.3(+/-4.1) kJ x mol(-1) (for R = OMe) to 34.0 (+/-7.5) kJ x mol(-1) (for R = CF(3)), DeltaS(++) = -154.0 (+/-14.2) J x mol(-1) x K(-1) (for R = OMe) to -184.3 (+/-26.1) J x mol(-1) K(-1) (for R = CF(3)), and DeltaG(++) = 95.0 kJ x mol(-1) (for R = OMe) to 88.7 kJ x mol(-1) (for R = CF(3)). Formation of the acetonitrile complex from the phosphoryl complex follows a first order process with respect to the complex. The first order rate constants at 25 degrees C range from 3.60 (+/-0.01) x 10(-4) sec(-1) (for R = OMe) to 6.32 (+/-0.11) x 10(-4) sec(-1) (for R = CF(3)), and the enthalpy of activation and entropy of activation show variation; DeltaH(++) = 62.5 (+/-2.2) to 67.8 (+/-1.0) kJ x mol(-1), DeltaS(++) = -82.5 (+/-3.3) to -101.3 (+/-7.5) J x mol(-1) x K(-1), but the free energy of activation remains constant DeltaG(++) approximately 92 (+/-1) kJ x mol(-1). Large entropies of activation associated with both steps are consistent with associative transition states. The comparable magnitude of the activation energy of the two steps underscores the difficulty in identifying the rate-limiting step in the overall OAT reaction. The first step, however, is more sensitive toward the substituent effects than the second step. Therefore, a change in the substituent can play an important role in deciding the rate-limiting step involved in a two-step OAT reaction.

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

“…In addition, if EWGs lower the energy of the transition state by ameliorating negative charge build-up, they give rise to faster reaction rates. Similar observations have previously been made with other ligands, including other tridentate Schiff base ligands, such as those obtained by condensation of substituted salicylaldehydes with ortho-aminobenzenethiol, [17] substituted thiophenolate, [44] substituted dithiolenes, [45] and salan-type ligands. [21,41] A particularly pronounced effect of the NO 2 substituent was also observed by Britovsek and Gibson et al with dioxomolybdenum complexes with para-substituted salan-type ligands.…”

“…Commonly studied tridentate ligands include facially coordinating hydrotris(pyrazolyl)borates (e.g., Tp*, 1 [14][15][16] ) and meridionally coordinating Schiff base derivatives (e.g., ssp, 2 [17] and tsc, 3 [18,19] ). Whereas tridentate ligands can occupy only three of the four available coordination sites on the cisMoO 2 center, tetradentate ligands, including tripodal (N3O, 4 [20] ) or linear (salan [21,22] ) ligands (5) form coordinatively saturated complexes.…”

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

“…46 In this case a 2-fold increase in the reaction rate was observed in going from the H to CF 3 substituted ligand, in contrast to our 92-fold increase, consistent with the shorter distance of the CF 3 group from the metal oxo core.…”

Mechanistic Insight into the Reactivity of Oxotransferases by Novel Asymmetric Dioxomolybdenum(VI) Model Complexes

“…Complexes 1, 2, 5, and 6 exhibit two successive cath-A C H T U N G T R E N N U N G odA C H T U N G T R E N N U N G ic waves as well as their corresponding anodic waves (Table 4 and Supporting Information, Figure S1), which are assigned to the Mo VI !Mo V and Mo V !Mo IV redox processes, respectively. [29,31,[47][48][49] However, complexes 3 and 4 show only one cathodic and one anodic wave (Table 4) ) plot into the Randles-Sevcik equation [50] are of the same order as those observed for metal complexes undergoing two consecutive one-electron reduction processes. [31,37,39,41] The E 1/2 values of the Mo VI /Mo V redox potentials of the complexes from CV vary in the order 6 > 5 > 1 > 2 > 3 > 4 (Table 4), suggesting a decrease in Lewis acidity (ten- (Figure 2).…”