6533b7d6fe1ef96bd1265d45

RESEARCH PRODUCT

Restricted rotation in unbridged sandwich complexes: rotational behavior of closo-[Co(eta 5-NC4H4)(C2B9H11)] derivatives.

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description

Rotation about the centroid/ metal/centroid axis in ferrocene is facile; the activation energy is 1-5 kcal mol - 1 . The structurally similar sandwich complexes derived from closo-[3-Co(η 5 -NC 4 H 4 )-1,2-C 2 B 9 H 1 1 ] (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 that is second lowest in energy. This cis rotamer 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(η 5 -NC 4 H 4 )-1,2-(CH 3 ) 2 -1,2-C 2 B 9 H 9 ] (2), closo-[3-Co(η 5 -NC 4 H 4 )-1,2-(μ-CH 2 ) 3 -1,2-C 2 B 9 H 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 1 H 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(1,2-C 2 B 9 H 1 1 ) 2 ] - ion in the absence of steric hindrance; the [3,3'-Co(1,2-C 2 B 9 H 1 1 ) 2 ] - ion is commonly accepted to present a cisoid orientation. Complex 1 is electronically similar to the [3,3'-Co(1,2-C 2 B 9 H 1 1 ) 2 ] - ion. Both have heteroatoms in the π ligands, and they have the same electronegativity difference between the constituent atoms. This leads to a view of the [NC 4 H 4 ] - as [7,8-C 2 B 9 H 1 1 ] 2 - ion, with no steric implications. Therefore the [3,3'-Co(1,2-C 2 B 9 H 1 1 ) 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.