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)

The facile linking of ferrocene units into multinuclear frameworks via boronic ester condensation chemistry is demonstrated through the synthesis of (eta(5)-C5H5)Fe(eta(5)-C5H4)BO2C5H8O2B(eta(5)-C5H4)Fe(eta(5)-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 P2(1)/n with a = 22.5093(3), b = 5.8951(3), c = 22.9770(5) a<<, beta = 119.110(1)A degrees, Z = 4, R = 0.059 and wR2 = 0.123 for observed reflections I > 2 sigma(I). The molecular structure is characterized by two planar, three-coordinate boron centres (I angles pound at boron = 360A degrees within the standard 3 sigma 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)center dot center dot center dot C(1)center dot center dot center dot B(2) angle being 140.1A degrees. Moreover, the corresponding angle measured for the macrocycle [(eta(5)-C5H4)Fe(eta(5)-C5H4)BO2C5H8O2B](2) is only slightly narrower (137.6A degrees), 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.