Mechanistic Investigation of the Oxygen-Atom-Transfer Reactivity of Dioxo-molybdenum(VI) Complexes

摘要

The oxygen-atom-transfer (OAT) reactivity of [L~(iPr)MoO_2(OPh)] (1,L~(iPr)=hydrotris(3-isopropylpyrazol-l-yl)borate) with the tertiary phos-phines PEt_3 and PPh_2Me in acetonitrile was investigated.The first step,[L~(iPr)MoO_2(OPh)]+PR_3-> [L~(iPr)MoO(OPh)(OPR_3)],follows a second-order rate law with an associa-tive transition state (PEt_3,DELTA H~(not=)=48.4 (+-1.9)kJmol~(-1),DELTA S~(not=)=-149.2 (+-6.4) J mol~(-1) K~(-1),DELTA G~(not=)=92.9 kJmor~(-1);PPh_2Me,DELTA H~(not=)=73.4 (+-3.7) kJmol~(-1),DELTA S~(not=)=-71.9 (+-2.3) Jmol~(-1) K~(-1),DELTA G~(not=)=94.8 kJ moL~(-1)).With PMe_3 as a model substrate,the geometry and the free energy of the transition state (TS) for the formation of the phosphine oxide-coordinated intermediate were calcu-lated.The latter,95 kJ mol~(-1),is in good agreement with the experimental values.An unexpectedly large O-P-C angle calculated for the TS suggests that there is significant O-nucLeophiLic attack on the P-C sigma* in addition to the expected nucleophilic attack of the P on the Mo=O pi*.The second step of the reaction,that is,the exchange of the coordinated phosphine oxide with acetonitrile,[L~(iPr)MoO(OPh)(OPR_3)]+MeCN-> [L~(iPr)MoO(OPh)(MeCN)]+OPR_3,follows a first-order rate law in MeCN.A dissociative interchange (I_d) mechanism,with activation parameters of AH*=93.5 (±0.9)kJmol~(-1) DELTA S~(not=)=18.2 (±3.3)Jmol~(-1) K-1,DELTA G~(not=)=SS.l kJ mol~(-1) and DELTA H~(not=)=91.9 (+-3.4) kJmol-1, DELTA S~(not=)=47.3 (+-11.8) J mol~(-1) K~(-1),DELTA G~(not=)=83.8 kJ mol~(-1),for [L~(iPr)MoO(OPh)(OPEt_3)] (2 a) and [L~(iPr)MoO(OPh)(OPPh_2Me)] (2 b),re-spectively,is consistent with the experi-mental data.Although gas-phase calcu-lations indicate that the Mo-OPMe_3 bond is stronger than the Mo-NCMe bond,solvation provides the driving force for the release of the phosphine oxide and formation of [L~(iPr)MoO-(OPh)(MeCN)] (3).