Synthesis and Characterization of Neutral Thiocyanato-and Selenocyanato-Bridged Cobaloximes
Pseudohalides react with alkylcobaloximes to produce labile complexes [Co(dmgH)2(CNX)(R)]- (X=O, S, Se) where the linkage isomer preference is dependent upon X, the alkyl group R, and the solvent. Isomer equilibration occurs rapidly in solution and is not influenced by cdbalt(II) catalysis. The inert pseudohalide complexes Co(dmgH)2(CNS)(B) and Co(dmgH)2(SeCN)(B) were found to react with alkylcobaloximes Co(dmgH)2(R)(H2O) and Co(dmgH)2(R) to form neutral pseudohalide-bridged dicobaloximes (R)Co(dmgH)2-(CNS)-Co(dmgH)2(B) and (R)Co(dmgH)2-(NCSe)-Co(dmgH)2(B) for various combinations of alkyl groups R and nitrogen donor ligands B. The corresponding cyanato-bridged dicobaloximes could not be prepared. Product characterization is by 1H NMR and IR spectroscopies, vaporpliase osmometry, and elemental analysis. Pseudohalide bridging is established to occur by remote bonding of the two cobaloximes to the N and S or Se atoms of the pseudohalide. Both linkage isomers of the thiocyanato-bridged system have been prpared and characterized. Only the isomer shown above has been prepared and characterized for the selenocyanato system. This is consistent with the existence of a single isomer for the inert terminally bound compounds Co(dmgH)2(SeCN)(B). The ligand-bridged dicobaloximes were found to partially dissociate in chloroform according to the reaction (CNX-=NCS-, SCN-, NCSe-) (R)Co(dmgH)2-(CNX)-Co(dmgH)2(B)→Co(dmgH)2(R)+Co(dmgH)2(CNX)(B). the degree of dissociation is dependent on R, CNX, and B; more stable dimers are formed with R=CF3- than CH3- and with B=py than pip. Comparison with the previously reported analogous cyano-bridged dicobaloximes establishes the following order of decreasing stability with respect to the dissociation reaction (the line indicates the donor atom bonded to the Co(dmgH)2(B) end of the dimer):[FORMULA OMMITED] In the presence of trace quantities of cobalt(II) the reaction of Co-(dmgH)2(CNS)(B) or Co(dmgH)2(SeCN)(B) with Co(dmgH)2(R) produces oligomers of the form (R)Co(dmgH)2-[-(CN)Co(dmgH)2-]n(CNX)Co(dmgH)2(B) (n=2, 3; X=S, Se). Similar oligomer formation is observed when solid samples of the ligand-bridged dimer are dissolved in chloroform in the presence of trace quantities of cobalt(II). A mechanism involving cobalt(II) catalysis is proposed for the oligomer formation reaction. © 1979, American Chemical Society. All rights reserved.
Ciskowski, JM; Crumbliss, AL
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