Prospects for Making Organometallic Compounds with BF Ligands: Fluoroborylene Iron Carbonyls

Abstract

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).

Terminal coordination of diatomic boron monofluoride to iron

“…This difference in chemical shift signifies that p-donation from fluorine to boron in 2 is substantially diminished relative to B-O p-bonding in the oxyboryl group and is reflective of the greater electronegativity of the fluorine atom in the BF unit. Complex 2 also gives rise to a 19 F NMR resonance at d F = +1.6 ppm, which is significantly downfield relative to that found in the dinuclear bridging BF complex (m 2 -BF)[CpRu(CO) 2 ] 2 (d F = -185.0 ppm) (20) and BF 3 •Et 2 O (d F = -153 ppm). Rapid quadrupolar relaxation of the I = 3/2 11 B nucleus prevented a determination of the 1 J BF coupling constant in both the one-dimensional (1D) 11 B and 19 F NMR spectra of 2.…”

Variational Forward–Backward Charge Transfer Analysis Based on Absolutely Localized Molecular Orbitals: Energetics and Molecular Properties

“…Boron monofluoride is also isoelectronic to CO and N 2 , and was predicted to be both a stronger σ-donor and π-acceptor compared against CO. [84][85][86] It possesses a lower bond order and like CO, the dipole moment of BF is in the opposite direction of intuition, with the B atom as the negative pole. 87,88 However, BF is only stable under extreme conditions, rendering its coordination compounds difficult to prepare.…”

Stabilities, Electronic Structures, and Bonding Properties of Iron Complexes (E1E2)Fe(CO)2(CNArTripp2)2(E1E2=BF, CO, N2, CN−, or NO+)**

“…However, the trigonal bipyramidal geometry in (BF)Fe(CO) 2 (CNAr Tripp2 ) 2 has an obvious distortion and the C1-FeÀ C2 bond angle is only 157.5°, which may come from the better σ-acceptor properties of BF. [18,20] The trigonal bipyramidal geometry in [(CN)Fe(CO) 2 (CNAr Tripp2 ) 2 ] À has been obviously distorted, in which the bond angle of C1À FeÀ C2 in is 144.3°while the C N (equatorial) À FeÀ C O (equatorial) is as large as 167.6°. Besides, the E 1 À E 2 bond in (E 1 E 2 )Fe (CO) 2 (CNAr Tripp2 ) 2 is elongated moderately by about 0.02 Å with respect to the free E 1 E 2 except for the distorted [(NO)Fe (CO) 2 (CNAr Tripp2 ) 2 ] + .…”

“…[15][16][17] Fluoroborylene iron carbonyls and homoleptic fluoroborylenes complexes have also been proposed theoretically by Hoffmann and Schaefer and King. [18][19][20][21][22][23][24] Very recently, Figueroa et al successfully realized the isolation of the iron complex (BF)Fe(CO) 2 (CNAr Tripp2 ) 2 [Ar Tripp2 = 2,6-(2,4,6-(iso-propyl) 3 C 6 H 2 ) 2 C 6 H 3 ] with a terminal BF ligand, along with the isoelectronic dinitrogen and CO complexes (N 2 ) Fe(CO) 2 (CNAr Tripp2 ) 2 and Fe(CO) 3 (CNAr Tripp2 ) 2 . [25] Further singlecrystal x-ray diffraction, spectroscopic, and electron-density topology calculation studies demonstrated that the terminal BF ligand possesses particularly strong σ-donor and π-acceptor properties.…”