Abstract
The present work embodies outward oxygen atom transfer (OAT) reaction from pyridylbenzoxazole (L1) chelated oxorhenium(V) complex of type [ReOCl3(L1)], 1 towards oxophilic PMe2Ph resulting in the formation of reduced Re(III) derivative [Re(OPMe2Ph)Cl3(L1)], 4 containing oxidised phosphine trans to azole nitrogen. Another possible isomer (4a) having oxidised phosphine trans to pyridyl nitrogen has not been isolated. Structural elucidation of 4 reveals meridional disposition of three Cl atoms around the metal featuring distorted octahedral geometry. DFT analysis of isomers of type [Re(OPMe2Ph)Cl3(L1)] (4 and 4a) confirms isoenergetic nature in gas phase, however, stereospecific binding of L1 to Re(III) centre affording 4 as exclusive product is attributed to crystal packing force. 1 → 4 redox transformation has also been attended with the change in singlet → triplet spin state of the metal. Triplet state of 4 is more stable by ca. 15 kcal mol−1 compared to the singlet state in gas phase. Two electron paramagnetic nature (2.13 μB) in solid state affirms triplet ground state characterised with strong orbital coupling. Kinetic data for 1 → 4 transformation suggests associative pathway (ΔS≠ = ∼−36 J/K/mol) with no major structural reorganisation in the primary coordination sphere of the substrate and the reagent (ΔH≠ = ∼+10 kcal mol−1) to reach the transition state. Pyridylbenzthiazole (L2) chelated oxorhenium(V) complex of type [ReOCl3(L2)], 1b also reacts with PMe2Ph in a same manner to from [Re(OPMe2Ph)Cl3(L2)], 4b where the pyridyl nitrogen lies trans to phosphineoxide moiety unlike 4. The second order rate constants of OAT to PMe2Ph for [ReOCl3(L1)] are nearly two times higher than that of [ReOCl3(L2)] at all recorded temperatures due to difference in azole heteroatom electronegativity, ReVO bond length and ReVI/ReV reduction potential.
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