Synthesis of a Bench-Stable Manganese(III) Chloride Compound: Coordination Chemistry and Alkene Dichlorination

Abstract

The complex [MnCl 3 (OPPh 3 ) 2 ] (1) is a bench-stable and easily prepared source of MnCl 3 . It is prepared by treating acetonitrile solvated MnCl 3 (2) with Ph 3 PO and collecting the resulting blue precipitate. 1 is useful in coordination reactions by virtue of the labile Ph 3 PO ligands, and this is demonstrated through the synthesis of {Tpm*}MnCl 3 (3). In addition, methodologies in synthesis that rely on difficult or cumbersome to prepare solutions of reactive MnCl 3 can be accomplished using 1 instead. This is demonstrated through alkene dichlorinations in a wide range of solvents, open to air, and with good substrate scope. Lightaccelerated halogenation and radical sensitive experiments support a radical mechanism involving stepwise Cl-atom transfer(s) from 1.

Experimental and Computational Determination of a M–Cl Homolytic Bond Dissociation Free Energy: Mn(III)Cl-Mediated C–H Cleavage and Chlorination

“…For comparison, we also conducted electrochemical analysis of 1 and the neutral trichloride complex Tpm Me MnCl 3 (6) (Tpm Me = tris(pyrazolyl)methane). 3 The DPV of 6 contained a well-defined peak at 0.37 V. By contrast, the electrochemistry of 1 was complex and difficult to interpret. MeCN solutions of 1 were blue, but under the electrochemical conditions with 0.2 M [Bu 4 N]PF 6 in rigorously dry MeCN, the solutions lost their blue color, turning pale green.…”

“…To illustrate, the DFT-calculated BDFE Mn–Cl of 1 and 3 H dictates that both reactions should proceed because the first step of C–H activation is thermodynamically favorable (Scheme , rxns 4–7). Note that C–H activation is in competition with alkene dihalogenation forming 4,5-dichlorocyclohexene as a co-product …”

“…Note that C−H activation is in competition with alkene dihalogenation forming 4,5-dichlorocyclohexene as a coproduct. 3 The reaction of 1 and 0.5 equivalents of hexamethylbenzene (HMB, BDFE C−H = 81.0 kcal/mol, MeCN) 28 afforded 1chloromethyl-pentachlorobenzene in 93% yield in 7 h at r.t., whereas the same reaction with 3 H yielded virtually no product in the same time (9% in 24 h at r.t.). These observations are likewise in accordance with the DFT and experimentally derived predictions (Scheme 6).…”

“…11 These intermediates are sufficiently long-lived for further reactions with {Mn II Cl} (reverse of rxn 10) or additional equivalents of {Mn III Cl}, forming HCl and new aryl C−Cl bonds (rxn 11). 3 The overall process results in net C−H chlorination (rxn 12). Note that this mechanism differs from electrophilic aromatic substitution, which relies on a metal catalyst.…”

“…The results from the coordination chemistry alone more than double the number of {Mn III Cl} complexes with facially coordinating tridentate ligands, 3,14 which is an important motif in the bioinorganic chemistry of Mn. 15,16 Some challenges in Mn(III) coordination chemistry are the intrinsic oxidative nature of Mn(III), 17,18 (1) 3 and solvated MnCl 3 ([MnCl 3 (MeCN) x ]) 19 to overcome the stated challenges.…”