Synthesis,Structure,and Reactivity of O-Donor Ir(lll)Complexes:C-H Activation Studies with Benzene

摘要

Various new thermally air-and water-stable alkyl and aryl analogues of(acac-O,O)_2lr(R)(L),R-lr-L(acac-O,O = kappa~2-O,O-acetylacetonate,-Ir-is the trans-(acac-O,O)_2lr(lll)motif,R = CH_3,C_2H_5,Ph,PhCH_2CH_2,L = Py)have been synthesized using the dinuclear complex [lr(mu-acac-O,O,C~3)-(acac-O,O)(acac-C~3)]_2,[acac-C-lr]_2,or acac-C-lr-H_2O.The dinuclear Ir(III)complexes,[lr(mu-acac-O,O,C~3)-(acac-O,O)(R)]_2(R = alkyl),show fluxional behavior with a five-coordinate,16 electron complex by a dissociative pathway.The pyridine adducts,R-lr-Py,undergo degenerate Py exchange via a dissociative mechanism with activation parameters for Ph-lr-Py(DELTAH* = 22.8(+-)0.5 kcal/mol;DELTAS* = 8.4(+-)1.6 eu;DELTAG*_(298K)= 20.3(+-)1.0 kcal/mol)and CH_3-lr-Py(DELTAH* = 19.9(+-)1.4 kcal/mol;DELTAS* = 4.4(+-)5.5 eu;DELTAG*_(298K)= 18.6(+-)0.5 kcal/mol).The trans complex,Ph-lr-Py,undergoes quantitatively trans-cis isomerization to generate cis-Ph-lr-Py on heating.All the R-lr-Py complexes undergo quantitative,intermolecular CH activation reactions with benzene to generate Ph-lr-Py and RH.The activation parameters(DELTDELTAS* =11.5(+-)3.0 eu;DELTAH* = 41.1(+-)1.1 kcal/mol;DELTAG*_(298K)= 37.7(+-)1.0 kcal/mol)for CH activation were obtained using CH_3-lr-Py as starting material at a constant ratio of [PyJ/IC_6D_6] = 0.045.Overall the CH activation reaction with R-lr-Py has been shown to proceed via four key steps:(A)pre-equilibrium loss of pyridine that generates a frans-five-coordinate,square pyramidal intermediate;(B)unimolecular,isomerization of the frans-five-coordinate to generate a cis-five-coordinate intermediate,cis-R-lr-D;(C)rate-determining coordination of this species to benzene to generate a discrete benzene complex,cis-R-lr-PhH;and(D)rapid C-H cleavage.Kinetic isotope effects on the CH activation with mixtures of C_6H_6/C_6D_6(Kl E = 1)and with 1,3,5-C_6H_3D_3(Kl E ~3.2 at 110 deg C)are consistent with this reaction mechanism.