Preparation, spectroscopic properties, and crystal structures of Fe2(CO)6(μ-CO)(μ-CF2)2, Fe2(CO)6(μ-CO)2(μ-CF2), and Fe2(CO)6(μ-CF2)(PPh3)2 - Theoretical studies of methylenic vs. carbonyl bridges in diiron complexes

Article abstract of DOI:10.1002/1521-3749(200108)627:8<1859::AID-ZAAC1859>3.0.CO;2-P

The reaction of Na₂[Fe(CO)₄] with Br₂CF₂ in n-pentane generates a mixture of the compounds (CO)₃Fe(μ-CO)_3-n(μ-CF₂) _nFe(CO)₃ (2, n = 2; 3, n = 1) in low yields with 3 as the main product. 3 is obtained free from 2 by reacting Br₂CF₂ with Na₂[Fe₂(CO)₈]. The non-isolable monomeric complex (CO)₄Fe=CF₂ (1) can probably considered as the precursor for 2. 3 reacts with PPh₃ with replacement of two CO ligands to form Fe₂(CO)₆(μ-CF₂)(PPh₃)₂ (4). The complexes 2-4 were characterized by single crystal X-ray diffraction. While the structure of 2 is strictly similar to that of Fe₂(CO)₉, the structure of 3 can better be described as a re suiting from superposition of the two enantiomers 3a and 3b with two semibridging CO groups. Quantum chemical DFT calculations for the series (CO)₃Fe(μCO)_3-n(μ-CF₂) _nFe(CO)₃ (n = 0, 1, 2, 3) as well as for the corresponding (μ-CH₂) derivatives indicate that the progressively larger σ donor and π acceptor properties for the bridging ligands, in the order CO < CF₂ < CH₂, favor a stronger Fe-Fe bond. WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001.