A bimetallic 1,1’-bis(boratabenzene) complex: unprecedented intramolecular metal-metal communication through a B-B bond

We report the synthesis of the first 1,1’-bis (boratabenzene) species by tetrabromodiborane(4) induced ring-expansion reactions of cobaltocene. Six equivalents of cobaltocene are required as the species plays the dual role of reagent as well as reductant to yield [{(?5-C5H5)Co}2{µ:?6,?6-(BC5H5)2}]. X-ray crystallographic analysis shows that the compound consists of coplanar BC5H5 rings which are coordinated on opposite faces to two cyclopentadienyl cobalt moieties, resulting in a trans-configured bimetallic complex. The formally dianionic bis(boratabenzene) moiety with a boron-boron single bond can be viewed as a symmetric dimer of the parent boratabenzene anion as well as the first example of a diboron analogue of biphenyl. The solution electrochemistry of the bimetallic complex shows four stepwise redox events, indicating significant intramolecular interaction between the cobalt ions across the 1,1’-bis(boratabenzene) unit. The magnetic properties, as investigated by variable-temperature SQUID magnetometry, reveal weak intramolecular antiferromagnetic interactions (J = -6.0 cm-1). Density Functional Theory calculations support the experimental results and add insights into the various electronic states of the complex.