Triboranate Derivatives of Calcium and Strontium

This contribution reports the alkaline earth-mediated formation of homonuclear sigma bonding between the metalloid element, boron. The resultant triborane anions result from the formal dismutation of bis(pinacolato)diboron (B2pin2) and the generation of putative heavier group 2 boryl intermediates. Initial reactions of the heavier alkaline earth bis-(trimethylsilyl)amides, [Ae{N(SiMe3¬)2}2] (Ae = Ca or Sr), with an equimolar quantity of bis(pinacolato)diboron (B2pin) provided the 4-coordinate group 2 silazide diborane adducts [Ae(B2pin2){N(SiMe3¬)2}2] in which the diborane acts as a bidentate ligand via 2-O,O' coordination. Reaction of the calcium species at 60 C with a further three molar equivalents of B2pin2 induced the elimination of pinBN(SiMe¬3)2 and the formation of the bis-triboranate derivative, [Ca(B3pin3)2], in which the 6-coordinate calcium centre is bound in a 3-O,O',O''-coordination mode ¬by the three contiguous pinacolate oxygen atoms of both catena-B(sp2)-B(sp3)-B(sp2) bonded anions. Although analogous treatment of [Ae(B2pin2){N(SiMe3¬)2}2] provided similar triboranate anion formation, in this case two equivalents of the strontium silazide were retained within the structure of the trimetallic species, [Sr(B3pin3)2(Sr{N(SiMe3)2}2)2]. Despite these contrasting outcomes both triboranate complexes provide rare examples of catena-boron species beyond the common diboranes, (R2¬B-BR2), in which the homologated unit is propagated by electron precise and otherwise unsupported B-B bonds.