Zerovalent transition-metal inverse-sandwich complexes of a diborataanthracene dianion.

The 9,10-dihydro-9,10-diboraanthracene (9,10-DHDBA) 2,3,6,7,9,10-Me6-9,10-DHDBA (2) reacted with 1 equiv. [(MeCN)3Cr(CO)3] to yield the unsymmetrical half-sandwich complex [(η 6-2)Cr(CO)3] (2-Cr), in which the Cr(CO)3 unit binds exclusively to one DHDBA benzo ring. Reactions of 2 with 2 equiv. [(MeCN)3M(CO)3] (M = Cr, Mo, W) yielded the centrosymmetric slipped inverse-sandwich complexes [{µ-(η 6,η'6-2)}{M(CO)3}2] (2-M2), in which each M(CO)3 unit binds to one of the DHDBA benzo rings, one above and one below the DHDBA plane. The twofold reduction of 2 with Li sand afforded the corresponding diborataanthracene (DBA) dianion, isolated as the C4B2-bound inverse-sandwich dilithio complex [{µ-(η 6,η 6-2)}{Li(thf)2}2] (3). The latter reacted with 2 equiv. [(MeCN)3Cr(CO)3] (M = Cr, Mo) to yield the first stable zerovalent transition-metal inverse-sandwich complexes of a [9,10-DBA]2- dianion, complexes [{µ-(η 6,η 6-2)}{M(CO)3Li(thf)3}2] (3-M2). All new compounds were characterised by multinuclear NMR, UV-vis and IR spectroscopy, and their solid-state structures ascertained by single-crystal X-ray diffraction analyses. Furthermore, DFT calculations provide insight into the electronic structure of the 9,10-DHDBA and [9,10-DBA]2- fused ring systems, as well as metal-(DH)DBA bonding in the chromium complexes 2-Cr, 2-Cr2 and 3-Cr2.