Probing Conformational Strain in Multinuclear Lewis Acids: Synthesis, Spectroscopic and Structural Characterization of the Dinuclear Ferroceneboronic Ester (η5-C5H5)Fe(η5-C5H4)BO2C5H8O2B(η5-C5H4)Fe(η5-C5H5)

Abstract

The facile linking of ferrocene units into multinuclear frameworks via boronic ester condensation chemistry is demonstrated through the synthesis of (η5-C5H5)Fe(η5-C5H4)BO2C5H8O2B(η5-C5H4)Fe(η5-C5H5), which has been characterized by standard spectroscopic techniques and its structure (as the chloroform solvate) determined crystallographically. The crystal structure reveals the following properties: monoclinic, space group P21/n with a = 22.5093(3), b = 5.8951(3), c = 22.9770(5) A, β = 119.110(1)°, Z = 4, R = 0.059 and wR2 = 0.123 for observed reflections I > 2σ(I). The molecular structure is characterized by two planar, three-coordinate boron centres (Σangles at boron = 360° within the standard 3σ limit) featuring B–O bond lengths of 1.358(6)–1.371(6) A and B–C distances of 1.539(7) and 1.553(7) A. The pentaerythritol linker ensures that the core of the molecule is non-linear, with the B(1)···C(1)···B(2) angle being 140.1°. Moreover, the corresponding angle measured for the macrocycle [(η5-C5H4)Fe(η5-C5H4)BO2C5H8O2B]2 is only slightly narrower (137.6°), implying that the formation of such a macrocyclic ring does not necessitate a large structural distortion, and hence that the degree of resulting ring strain is likely to be minor. The facile linking of ferrocene units into multinuclear frameworks via boronic ester condensation chemistry is demonstrated through the synthesis of (η5-C5H5)Fe(η5-C5H4)BO2C5H8O2B(η5-C5H4)Fe(η5-C5H5), which has been characterized by standard spectroscopic techniques and its structure (as the chloroform solvate) determined crystallographically.