Restricted Rotation in Unbridged Sandwich Complexes:Rotational Behavior of closo-[Co(eta~5-NC_4H_4)(C_2B_9H_(11))] Derivatives

摘要

Rotation about the centroid/metal/centroid axis in ferrocene is facile;the activation energy is 1-5 kcalmol~(-1).The structurally similar sandwich complexes derived from closo-[3-Co(eta~5-NC_4H_4)-l,2-C_2B_9H_(11)] (1) have a different rotational habit.In 1,the cis rotamer in which the pyrrolyl nitrogen atom bisects the carboranyl cluster atoms is 3.5 kcal mol~(-1) more stable in energy than the rotamer thatis second lowest in energy.This cisrotamer is wide,spanning 216~°,and may be split into three rotamers of almost equal energy by substituting the N and the carboranyl carbon atoms adequately.To support this statement,closo-[3-Co(eta~5-NC_4H_4)-l,2-(CH_3)_2-l,2-C_2B_9H_9](2), closo-[3-Co(eta~5-NC_4H_4)-l,2-(mu- CH_2)_3-1,2-C_2B_9H_9] (3),2->BF_3,and 3->BF_3 have been prepared.Two rotamers are found at low temperature for 2->BF_3 and 3->BF_3.Compounds 2,3,and 1->BF_3 behave similarly to 1.Rotational energy barriers and the relative populations of the different energy states are calculated from ~1H DNMR spectroscopy (DNMR,dynamic NMR).These results agree with those of semiempirical calculations.Without exception,the cis rotamer is energetically the more stable.The fixed conformation of 1 assists in elucidating the rotational preferences of the [3,3'-Co(l,2-C_2B_9H_(11))_2]~- ion in the absence of steric hindrance;the [3,3'-Co(l,2-C_2B_9H_(11))_2]~- ion is commonly accepted to present a cisoid orientation.Complex 1 is electronically similar to the [3,3'-Co(l,2-C_2B_9H_(11))_2]~- ion.Both have heteroatoms in the it ligands,and they have the same electronegativity difference between the constituent atoms.This leads to a view of the [NC_4H_4]~- as [7,8-C_2B_9H_(11)]~(2-) ion,with no steric implications.Therefore the [3,3'-Co(l,2-C_2B_9H_(11))_2]~- ion should be considered to have a cisoid structure,and the different rotamers observed to be the result of steric factors and of the interaction of the counterion with either B-H groups and/or ancillary ligands.The rotamer adopted is the one with the atoms holding the negative charges furthest apart.