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Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mn(V) -oxo complex.

ABSTRACT: Addition of an anionic donor to an Mn(V) (O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn(V) (O)(TBP8 Cz)(CN)](-) was generated from addition of Bu4 N(+) CN(-) to the 5-coordinate Mn(V) (O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives Mn?O=1.53?Å, Mn?CN=2.21?Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN(-) complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e(-) -reduced Mn(III) (CN)(-) complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24?000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives ?H(?) =14?kcal?mol(-1) , ?S(?) =-10?cal?mol(-1) ?K(-1) . Computational studies fully support the structures, spin states, and relative reactivity of the 5- and 6-coordinate Mn(V) (O) complexes.

SUBMITTER: Neu HM

PROVIDER: S-EPMC4321347 | biostudies-literature | 2014 Nov

REPOSITORIES: biostudies-literature

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Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mn(V) -oxo complex.

Neu Heather M HM Quesne Matthew G MG Yang Tzuhsiung T Prokop-Prigge Katharine A KA Lancaster Kyle M KM Donohoe James J DeBeer Serena S de Visser Sam P SP Goldberg David P DP

Chemistry (Weinheim an der Bergstrasse, Germany) 20140926 45

Addition of an anionic donor to an Mn(V) (O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn(V) (O)(TBP8 Cz)(CN)](-) was generated from addition of Bu4 N(+) CN(-) to the 5-coordinate Mn(V) (O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives MnO=1.53 Å, MnCN=2.21 Å. In combination with computational studies these distances were shown to ...[more]

PMID: 25256417

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Project description:We report the syntheses, crystal structures, and characterization of the novel cis-dioxomolybdenum(VI) complexes [Tpm*MoVIO2Cl](MoO2Cl3) (1) and [Tpm*MoVIO2Cl](ClO4) (2), which are supported by the charge-neutral tris(3,5-dimethyl-1-pyrazolyl)methane (Tpm*) ligand. A comparison between isostructural [Tpm*MoVIO2Cl]+ and Tp*MoVIO2Cl [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 O-atom-transfer (OAT) reactivities, providing new insight into pyranopterin molybdoenzyme OAT reactivity. Computational studies of these molecules indicate that the delocalized positive charge lowers the lowest unoccupied molecular orbital (LUMO) energy of cationic [Tpm*MoO2Cl]+ relative to Tp*MoO2Cl. Despite their virtually identical geometric structures revealed by crystal structures, the MoVI/MoV redox potential of 2 is increased by 350 mV relative to that of Tp*MoVIO2Cl. This LUMO stabilization also contributes to an increased effective electrophilicity of [Tpm*MoO2Cl]+ relative to that of Tp*MoO2Cl, resulting in a more favorable resonant interaction between the molydenum complex LUMO and the highest occupied molecular orbital (HOMO) of the PPh3 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≡MoIV-OPPh3 intermediate via an associative transition state, and the reaction is ∼500-fold faster for 2 than for Tp*MoO2Cl. The second step of the OAT reaction entails solvolysis of the O≡MoIV-OPPh3 intermediate to afford the solvent-substituted MoIV 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 the 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 MoIV center in the Tpm* system.

Dataset Information

Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mn(V) -oxo complex.

Publications

Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mn(V) -oxo complex.

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