Remote Charge Effects on the Oxygen-Atom-Transfer Reactivity and Their Relationship to Molybdenum Enzymes

Abstract

We report the syntheses, crystal structures, and characterization of the novel cis-dioxo molybdenum(VI) complexes [Tpm*Mo VI O 2 Cl](MoO 2 Cl 3) (1) and [Tpm*Mo VI O 2 Cl](ClO 4) (2), which are supported by the charge-neutral Tpm* ligand (Tpm* = tris(3,5-dimethyl-1pyrazolyl)methane). A comparison between isostructural [Tpm*Mo VI O 2 Cl] + and Tp*Mo VI O 2 Cl (Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) reveals the effects of one unit of overall charge difference on their spectroscopic and electrochemical properties, geometric and electronic structures, and OAT reactivities, providing new insight into pyranopterin molybdoenzyme OAT reactivity. Computational studies of these molecules indicate that the delocalized positive charge lowers the LUMO energy of cationic [Tpm*MoO 2 Cl] + relative to Tp*MoO 2 Cl. Despite their virtually identical geometric structures revealed by crystal structures, the Mo VI /Mo V redox potential of 2 is increased by 350 mV relative to Tp*Mo VI O 2 Cl. This LUMO stabilization also contributes to an increased effective electrophilicity of [Tpm*MoO 2 Cl] + relative to Tp*MoO 2 Cl, resulting in a more favorable resonant interaction between the Mo complex LUMO and the HOMO of the PPh 3 substrate. This leads to a greater thermodynamic driving force, an earlier transition state, and a lowered activation barrier for the orbitally controlled first step of the OAT reaction in the Tpm* system relative to the Tp* system. An Eyring plot analysis shows that this initial step yields an O≡Mo IV-OPPh 3 intermediate via an associative TS, and the reaction is ~500-fold faster for 2 than for Tp*MoO 2 Cl. The second step of the OAT reaction entails the solvolysis of the O≡Mo IV-OPPh 3 intermediate to afford the solvent-substituted Mo IV product, and is 750-fold faster for the Tpm* system at-15 °C compared to the Tp* system. The observed rate enhancement for the second step is ascribed to a switch of reaction mechanism from a dissociative pathway for the Tp* system, to an alternative associative pathway for the Tpm* system. This is due to a more Lewis acidic Mo(IV) center in the Tpm* system.

Gas Adsorption of Mixed-Valence Trinuclear Oxothiomolybdenum Glycolates

“…The valences of the metal Mo atoms are 4+ and 6+, respectively. The interval valence states distinguish from the most reported molybdenum complexes, where the Mo has the same or adjacent valences. , [MoO 2 (glyc) 2 ] 2– is a mononuclear glycolatodioxomolybdate­(VI) anion, as reported in K 2 [MoO 2 (glyc) 2 ]·H 2 O . [Mo 3 (μ 3 -S)­(μ 2 -O) 3 (glyc) 3 (4-mim) 3 ] 2– is an anion of trinuclear 4-methylimidazole oxothiomolybdenum­(IV) glycolate, as shown in Figure b, which is similar to the anionic structure of [Mo 3 (μ 3 -S)­(μ 2 -O) 3 (glyc) 3 (2-mim) 3 ] 2– in 1 , while 2-methylimidazole has been substituted by 4-methylimidazole.…”

“…The interval valence states distinguish from the most reported molybdenum complexes, where the Mo has the same or adjacent valences. 33,34 [MoO 2 (glyc) 2 ] 2− is a mononuclear glycolatodioxomolybdate(VI) anion, as reported in K 2 [MoO 2 (glyc) 2 ]•H 2 O. 35 S3, are formed between the N atom on the protonated 4-methylimidazoles and the O atom on the glycolates…”

Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands

“…Molybdenum is the only second-row transition metal that is required by most living organisms, where it is found as a mononuclear metal center in the active site of many enzymes. , With the exception of nitrogenase and related proteins, the active site of all molybdoenzymes contains a pyranopterin-dithiolene cofactor in which the metal is coordinated by the dithiolene moiety. − The dimethyl sulfoxide reductase (DMSOR) superfamily is structurally and catalytically the largest and most diverse family of molybdoenzymes, and reactions catalyzed by its members frequently involve oxygen atom transfer (OAT). − With these naturally occurring systems as inspiration, dioxidomolybdenum­(VI) complexes have been extensively investigated in catalytic OAT reactions. − Since an aqueous environment has not been feasible for the majority of those complexes, a widespread model reaction, which was developed in the 1980s, is the OAT from Me 2 SO to PPh 3 , yielding Me 2 S and POPh 3 . − The ligands utilized in this model reaction are either dithiolene or non-dithiolene type systems. , Although bidentate non-dithiolene ligands with S,N, O,O, or N,O donor sets are structurally obviously different from the molybdopterin cofactor present in molybdoenzymes, they were found to be generally more suitable for molybdenum-catalyzed OAT reactions. − Even more different tri- and tetradentate ligands with various donor sets − have been successfully utilized, with the class of scorpionate ligands being outstanding. − Furthermore, various theoretical studies have been performed to assess the influence of the protein ligand , or charge differences and to compare active sites containing either molybdenum or tungsten. , Apart from sulfoxides, molybdenum complexes could also be applied in the deoxygenation of more challenging substrates: nitrate is reduced by naturally occurring nitrate reductases belon...…”

Metal–Dithiolene Bonding Contributions to Pyranopterin Molybdenum Enzyme Reactivity

“…Namely, the change in ligand charge from −2 to −1 leads to an asymmetric reduction in the charge donated by the monoanionic ligand compared to the dianionic dithiolene. Charge effects on oxygen atom transfer catalysis have recently been explored in model compounds showing dramatic rate enhancements in the oxidative half reaction that leads to substrate reduction [ 53 ]. This reactivity correlates with a large shift in the Mo(VI/V) reduction potential between cationic [Tpm*MoO 2 Cl] + (−660 mV vs. Fc + /Fc) and charge neutral Tp*MoO 2 Cl (−1010 mV vs. Fc + /Fc) [ 53 ].…”

“…Charge effects on oxygen atom transfer catalysis have recently been explored in model compounds showing dramatic rate enhancements in the oxidative half reaction that leads to substrate reduction [ 53 ]. This reactivity correlates with a large shift in the Mo(VI/V) reduction potential between cationic [Tpm*MoO 2 Cl] + (−660 mV vs. Fc + /Fc) and charge neutral Tp*MoO 2 Cl (−1010 mV vs. Fc + /Fc) [ 53 ]. The same effect on redox potential and reactivity would be expected in enzymes that could adopt an oxidized PDT with a thiol-thione configuration.…”