Date on Master's Thesis/Doctoral Dissertation

12-2008

Document Type

Master's Thesis

Degree Name

M.S.

Department

Chemistry

Committee Chair

Grapperhaus, Craig

Subject

Chemical bonds

Abstract

This thesis reports the reactivity of the rhenium-thiolate complex, tris(2-diphenylphosphinobenzenethiolato)rhenium(III), [Re(DPPBT) 3 ] ( 1 ) and its oxidized derivatives with ethylene. The reactivity has been studied by electrochemical, spectroelectrochemical, and chemical methods. Based on the cyclic voltammetric data, ( 1 ) shows two reversible one electron oxidations and a single reduction. The three redox events are observed at potentials of 420, -340 and -1620 mV versus a ferrocene reference. The events span formal oxidation states from Re(II) to Re(V) although significant ligand participation in the redox events makes these formal assignments misleading with respect to the electronic structure of the complexes. Bulk oxidation of ( 1 ) (E applied = +23 mV) in the presence of ethylene yields {[(ethane-1,2-diylbis(thio-2,1-phenylene)diphenyl-phosphino)(2-diphenylphosphinobenzenethiolato)]rhenium(III)} ( 6 ) from the addition of the alkene across cis sulfur sites. Electronic spectra recorded during the oxidation reveal two stages. The first stage is assigned as the one electron oxidation of ( 1 ) to tris(2-diphenylphosphinobenzenethiolato)rhenium(IV) ( 3 ), which is indicated by the intensity increases at 390 and 581 nm. During the second stage, a reaction occurs between ( 3 ) and ethylene that yields {[(ethane-1,2-diylbis(thio-2,1-phenylene)diphenyl-phosphino)(2-diphenylphosphinobenzenethiolato)]rhenium(II)} ( 5 ), which is oxidized to ( 6 ) as shown by the intensity loss at 390 and 581 nm and simultaneous intensity gain at 484 nm. The formal Re(III)/Re(II) reduction potential of ( 6 ) is shifted approximately +1520 mV consistent with the formation of two thioether donors. Complex ( 6 ) is stable in solution, but reduction at an applied potential of -977 mV initiates C-S bond cleavage and release of ethylene. The spectroscopic results reveal the pathway to be the reverse of the C-S bond formation. Oxidation of ( 1 ) using AgPF 6 followed by an ethylene purge yields (6)[PF 6 ] . The +ESI-MS of (6)[PF 6 ] shows a parent ion peak at m/z = 547.0710 (z = 2). The complex (6)[PF 6 ] crystallizes as a long thin orange plate in the monoclinic space group C2/c with unit cell dimensions of a = 29.009(18) Å; b = 22.577(18) Å; c = 43.99(3) Å; and ß = 96.182(17)°. The kinetic and equilibrium parameters associated with C-S bond formation/cleavage were extracted from cyclic voltammograms at multiple scan rates using the DigiSim software package. The rate constants for C-S formation between ( 3 ) and ethylene, k f , and for C-S bond cleavage for ( 5 ), k r , were extracted from simulation of the CV data at 7 scan rates ranging from 100 to 1000 mV/s for 3 independent trials. Average values for k f and k r are (1.2 ± 0.2) × 10 -1 M -1 s -1 and (3.0 ± 0.4) × 10 -2 s -1 , respectively. From these, K 2 was calculated as 4.0 ± 0.8 in agreement with predictions from the UV-visible study. K 1 and K 3 are equilibrium constants for ethylene binding/release between ( 1 ) and {[(ethane-1,2-diylbis(thio-2,1-phenylene)diphenyl-phosphino)(2-diphenyl-phosphinobenzenethiolato)]rhenium(I)} ( 8 ), and tris(2-diphenylphosphinobenzenethiolato)rhenium(V) ( 4 ) and ( 6 ), respectively. From the redox potentials and the equilibrium constant K 2 , values for K 1 and K 3 were determined. K 1 has a calculated value of (1.9 ± 0.4) × 10 -11 consistent with observation of an unstable C-S bond. In contrast, the calculated value of K 3 , (2.5 ± 0.9) × 10 9 , is large and is consistent with the observed stability of ( 6 ). The large differences in equilibrium constants as a function of oxidation state provide a means to easily gate ethylene addition/release.

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