The effect of a paramagnetic metal ion within a molecule: comparison of the structurally identical paramagnetic [3,3-Fe(1,2-C2B9H11)2]− with the diamagnetic [3,3-Co(1,2-C2B9H11)2]− sandwich complexes

Abstract

Derivatives of the ferrabisdicarbollide [3,3′-Fe(1,2-C₂B₉H₁₁)₂]⁻ have been produced starting from the zwitterion [3,3′-Fe(8-(OCH₂CH₂)₂-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)], 1, upon reaction with nucleophiles: alkoxides, halides and hydrosulfide ions HS⁻. The result has been the preparation of [3,3′-Fe(8-(OCH₂CH₂)₂R/X-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)] (R = OMe, [2]⁻; OEt, [3]⁻; OCH₂CH₂OCH₃, [4]⁻; and X = Cl, [5]⁻; Br, [6]⁻; I, [7]⁻; and SH, [8]⁻). The reaction behavior of 1 is comparable to the well-studied cobalt equivalent, [3,3′-Co(8-(OCH₂CH₂)₂-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)], and the yields and stability of the resulting complexes are similar. These results are relevant taking into account that [3,3′-Fe(1,2-C₂B₉H₁₁)₂]⁻ is a paramagnetic anion. Implications of this are observed in the ¹¹B-, ¹H and ¹³C NMR spectra of [3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻ and [3,3′-Fe(1,2-C₂B₉H₁₁)₂]⁻ that having identical sandwich molecular structures and the same negative charge have absolutely different widths of the NMR field, between 15 and −25 ppm for [3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻ and in the range 150 to −550 ppm for [3,3′-Fe(1,2-C₂B₉H₁₁)₂]⁻. The sharpness of both spectra is on the other hand comparable, although no B–H couplings are observed in the Fe metallacarborane or its derivatives. Remarkable is the comparative influence vs. [3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻ of replacing Co by Fe on the elements of the cluster layer nearest to the metal. The two equivalent C cluster (C_c) atoms are influenced at 36 840 Hz, the two equivalent B atoms that are adjacent to the two C_c are influenced at 38 157 Hz and the single B that is adjacent to the two B atoms is influenced at 44 062 Hz. Remarkable is the similar influence on B and on C, taking into account that the values have been obtained from two distinct NMR spectra of ¹¹B and ¹³C. The {¹¹B–¹¹B} COSY NMR and {¹H–¹¹B} HETEROCOSY NMR spectra of [3,3′-Fe(1,2-C₂B₉H₁₁)₂]⁻ have been obtained and are reported for the first time.