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Thermodynamic Studies of [HPt(EtXantphos)2]+ and [(H)2Pt(EtXantphos)2]2+

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posted on 24.05.2004, 00:00 authored by Alex Miedaner, James W. Raebiger, Calvin J. Curtis, Susie M. Miller, Daniel L. DuBois
[HPt(EtXantphos)2](PF6) (where EtXantphos is 9,9-dimethyl-4,5-bis(diethylphosphino)xanthene) can be prepared by the reduction of Pt(COD)Cl2 (where COD is 1,4-cyclooctadiene) with hydrazine in the presence of 2 equiv of the diphosphine ligand followed by exchange of Cl- with PF6-. Deprotonation of [HPt(EtXantphos)2](PF6) (pKa = 27.3 in acetonitrile) leads to the formation of Pt(EtXantphos)2, which has been characterized by an X-ray diffraction study. Pt(EtXantphos)2 has a distorted tetrahedral geometry. The average chelate bite angle is 108.2°, and the dihedral angle between the two planes formed by the phophorus atoms of each diphophine ligand and platinum is 80.4°. Protonation of [HPt(EtXantphos)2]+ results in the formation of [(H)2Pt(EtXantphos)2]2+, which has a pKa of 6.8 in acetonitrile. Oxidation of Pt(EtXantphos)2 with ferrocenium tetrafluoroborate produces [Pt(EtXantphos)2]2+. [Pt(EtXantphos)2]2+ undergoes two reversible one-electron reductions (E1/2(II/I) = −0.81 V versus ferrocene and E1/2(I/0) = −0.97 V), and [HPt(EtXantphos)2]+ undergoes a reversible one-electron oxidation (E1/2(II/III) = +0.23 V). These half-wave potentials and the pKa values of [HPt(EtXantphos)2]+ and [(H)2Pt(EtXantphos)2]2+ have been used to calculate five additional homolytic and heterolytic bond-dissociation free energies for these two hydride species and for [HPt(EtXantphos)2]2+. The extensive thermodynamic characterization of this hydride system provides useful insights into the factors controlling the reactivity of these complexes.

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