The carbonylation of the 14-electron high-spin iron(II) dialkyls FeR2(dippe), dippe = 1,2-bis(diisopropylphosphino)ethane, leads to formation of the zerovalent iron carbonyl Fe(CO)3(dippe) (1) and, for R = CH2CMe3, 2,2,6,6-tetramethyl-4-heptanone. When monoalkyl derivatives FeRX(dippe) are carbonylated, acyl-containing species are formed, and products containing unidentate or bidentate acyls have been isolated depending on the CO pressure and the identity of the alkyl group. Thus, when the carbonylation of Fe(CH2CMe3)X(dippe) is caried out by using approximately 2 equiv of CO, Fe(772-COCH2CMe3)X(CO)(dippe), 2a (X = Br) and 2b (X - Cl), are formed. The products 2a and 2b contain a bidentate acyl (I≫Cor = 1591 and 1596 cm-1, respectively) and only one terminal carbonyl ligand. Reaction of Fe(CH2CMe3)X(dippe) with excess CO at 3 atm gives Fe(7/1-COCH2CMe3)X(CO)2(dippe), 4a (X = Br) and 4b (X = Cl), each of which contains a unidentate acyl (Pcor = 1634 and 1630 cm-1, respectively) and two carbonyl ligands. The major isomer formed in the latter reaction has been determined by 13C and 31P NMR spectroscopy to possess mutually trans acyl and halide ligands while the minor isomer has mutually trans carbonyl and halide ligands. Carbonylation of the iron(II) mesityl Fe(C6H2Me3)Br(dippe) gives only the bidentate acyl product Fe-(η2-COC6H2Me3)Br(CO) (dippe) (3) even with excess CO. The infrared absorption at 1525 cm-1 for the bidentate mesityl acyl is much lower in frequency than for the bidentate neopentyl acyls due to conjugation with the aromatic ring. Of the five isomers possible for this stoichiometry, 3 adopts a structure in which the carbonyl ligand is trans to the oxygen atom of the acyl group, as shown by 13C and 31P NMR data. This is confirmed by the X-ray crystal structure of 3, which shows that the Fe center is six-coordinate and that the acyl group is indeed bidentate, with Fe-Cacyl= 1.85 (1) Å and Fe-Oacyl= 1.99 (1) Å. Compound 3 crystallizes in the monoclinic space group P21/n with cell dimensions a = 10.470 (10) Å, b = 17.659 (16) Å, c = 15.259 (18) Å, β = 100.34 (8)°, V = 2775 (5) Å3, Z = 4, RF= 8.2%, and RwF = 8.1% for 1969 independent reflections. A review of structural parameters for bidentate acyls of transition metals is given, and a comparison of the bonding in 3 with other molecules reveals that the Fe-0 distance is unusually short as a result of steric “compression” from one of the ortho methyl groups of the mesityl substituent.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry