Electronic versus steric control in manganese-hydroxamate chemistry:synthesis and characterization of Mn(II)and Mn(III)-hydroxamate complexes and catalytic epoxidation of olefins using the Mn(III)complexes as catalyst and H2O2 as terminal oxidant

摘要

Secondary hydroxamic acids,viz.Nbenzoyl Nphenyl/m-tolyl/p-tolyl hydroxamic acids react with Mn(OAc)2·4 H2O to afford light yellow polymeric paramagnetic(mu_(eff)=5.9 BM per molecular unit)materials having general composition[Mn~(II)(hydroxamate)2]_n.They all show a broad isotropic EPR signal with_(av)=2.04.However,when CPHAH(N-cinnamoyl N-phenyl hydroxamic acid)is used as a ligand,slightly brownish solid having the composition[Mn~(II)(CPHA)2]_n,is obtained,whose EPR spectrum displays most of the feature of a rhombic Mn(II)compound with1,2 and3 being 2.07,2.01 and 1.98,respectively.All the six ~(55)Mn hyperfines are resolved and_(iso)~(Mn)=109 G.Interestingly,when the hydroxamate=N-benzoyl N-o-tolyl/o-chlorophenyl hydroxamic acid,intensely colored Mn~(III)complexes,such as[Mn(B0THA)3](2)and[Mn(BOCPHA)3](3)(BOTHAH=JV-benzoyl N-o-tolyl hydroxamic acid and BOCPHAH=N-benzoyl N-o-chlorophenyl hydroxamic acid)are obtained which are EPR silent and mu_(eff)=4.88 and 4.83 BM,respectively,typical for high-spin Mn~(III)-complexes.2 and 3 show quasireversible Mn~(III)<=>Mn~(IV)response at E_(298)~0=1.14 V vs SCE.Though the peak for molecular ion is much less intense,the other vitally important peaks of the fragments in the electrospray mass spectra of 2 and 3 in positive mode support the composition of the complexes.Both 2 and 3 are efficient catalyst in epoxidation of olefins with H2O2 as oxidant and NaHCO3 as catalyst promoter.