Formation and reactivity of a persistent radical in a dinuclear molybdenum complex.
The reactivity of the S-H bond in Cp*Mo(mu-S) 2(mu-SMe)(mu-SH)MoCp* ( S 4 MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(mu-S) 3(mu-SMe)MoCp* ( S 4 Me*), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, Delta H (double dagger) and Delta S (double dagger) were determined to be 1.54 +/- 0.25 kcal/mol and -25.5 +/- 0.8 cal/mol K, respectively, giving k abs = 1.3 x 10 (6) M (-1) s (-1) at 25 degrees C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo 2S 4 core was found to be the benzyl cross-termination product, Cp*Mo(mu-S) 2(mu-SMe)(mu-SBz)MoCp* ( S 4 MeBz), consistent with the Fischer-Ingold persistent radical effect. S 4 Me* was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(mu-S) 2(mu-SMe)MoCp*) 2(mu-S 2) ( ( S 4 Me) 2 ). The radical-dimer equilibrium constant has been determined to be 5.7 x 10 (4) +/- 2.1 x 10 (4) M (-1) from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 x 10 (3) s (-1) at 25 degrees C, based on variable temperature (1)H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 x 10 (7) M (-1) s (-1) in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for ( S 4 Me) 2 , S 4 MeBz, and the cationic Cp*Mo(mu-S 2)(mu-S)(mu-SMe)MoCp*(OTf) ( S 4 Me ( + )), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S 4 core is presented.
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA.
Aaron M
AM
Appel
Suh-Jane
SJ
Lee
James A
JA
Franz
Daniel L
DL
DuBois
M Rakowski
MR
DuBois
Jerome C
JC
Birnbaum
Brendan
B
Twamley
D000475:Alkenes / Q000737:chemistry*; D001593:Benzyl Compounds / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D008982:Molybdenum / Q000737:chemistry*; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry; D013457:Sulfur Compounds / Q000737:chemistry*; D013816:Thermodynamics
D000475:Alkenes; D001593:Benzyl Compounds; D005609:Free Radicals; D013438:Sulfhydryl Compounds; D013440:Sulfides; D013457:Sulfur Compounds; D008982:Molybdenum