Prospects for making organometallic compounds with BF ligands: fluoroborylene iron carbonyls

The fluoroborylene ligand (BF), isoelectronic with CO, was recently (2009) realized experimentally by Vidović and Aldridge in Cp(2)Ru(2)(CO)(4)(mu-BF). In this research the related iron carbonyl fluoroborylene complexes Fe(BF)(CO)(n) (n = 4, 3), Fe(2)(BF)(CO)(8), and Fe(2)(BF)(2)(CO)(n) (n = 7, 6) are compared with the isoelectronic Fe(CO)(n+1) and Fe(2)(CO)(n+2) as well as the thiocarbonyls Fe(CS)(CO)(n) and Fe(2)(CS)(2)(CO)(n) using density functional theory. For Fe(BF)(CO)(4) the axially and equatorially substituted trigonal bipyramidal structures are predicted to be nearly degenerate as is the case for Fe(CS)(CO)(4). The lowest energy structures for Fe(BF)(CO)(3) are derived from the trigonal bipyramidal Fe(BF)(CO)(4) structures by removal of CO groups. For the binuclear derivatives Fe(2)(BF)(CO)(8) and Fe(2)(BF)(2)(CO)(n) (n = 7, 6) structures with BF bridges are preferred energetically over structures with CO bridges. However, no structures for the unsaturated Fe(2)(BF)(2)(CO)(6) are found with four-electron donor eta(2)-mu-BF groups. This differs from the corresponding Fe(2)(CS)(2)(CO)(6) where structures with eta(2)-mu-CS groups and formal Fe-Fe single bonds are preferred over structures with only two electron donor CO and CS groups and formal Fe=Fe double bonds. The lowest energy structure for Fe(2)(BF)(2)(CO)(7) is thus predicted to be similar to the well-known triply bridged Fe(2)(CO)(9) structure but with two bridging BF groups and one bridging CO group. However, the dissociation energy of Fe(2)(BF)(2)(CO)(7) into mononuclear fragments is much higher than that of Fe(2)(CO)(9). Removal of the bridging CO group from this lowest energy Fe(2)(BF)(2)(CO)(7) structure leads to the doubly BF-bridged global minimum structure for Fe(2)(BF)(2)(CO)(6).