Frozen-Out Rotamers of Mixed Cobaltacarborane Complexes

Abstract

Although the existence of frozen-out rotamers in solution had been shown for mixed cobaltacarborane complexes forming adducts with BF3 at low temperature. The existence of either one or more frozen-out conformations of these complexes in solution at room temperature, or otherwise the presence of free rotation of the pyrrolyl unit showing an average behavior, is still obscure. The reaction of 1,2-C2B10H12 and 1-C6H5-1,2-C2B10H11 with a suspension of K[NC4(CH3)2H2] in THF and anhydrous CoCl2 in 1:12:5 ratio yielded, respectively, closo-[3-Co(η5-NC4(CH3)2H2)-1,2-C2B9H11] (2) and closo-[3-Co(η5-NC4(CH3)2H2)-1-C6H5-1,2-C2B9H10] (3). 1H NMR studies performed on 2, 3, and other mixed complexes incorporating one pyrrolyl and one dicarbollide unit (closo-[3-Co(η5-NC4(CH3)2H2)-1-CH3-1,2-C2B9H10] (4), closo-[3-Co(η5-NC4H4)-1,2-C2B9H11] (1), closo-[3-Co(η5-NC4H4)-1-C6H5-1,2-C2B9H10] (5), and closo-[3-Co(η5-NC4H4)-1-CH3-1,2-C2B9H10] (6)) showed the presence of resonances with atypical chemical shifts that suggested the existence of stable frozen-out conformations. This abnormal displacement of 1H NMR resonances was assigned to electronic ring effects due to the presence of the phenyl unit; this electronic ring effect of the electrons of the phenyl ring on neighboring hydrogen atoms has been seen in mixed complexes for the very first time. ZINDO semiempirical calculations have been used to calculate the theoretical energy profile of the different rotamers. The obtained profiles furnish an explanation for experimental results with good qualitative concordance in all cases.