Heteroleptic Four-Coordinate Tris(oxazolinyl)borato Iron(II) Compounds

The reaction of FeBr₂ and 1 equiv of thallium tris(4,4-dimethyl-2-oxazolinyl)phenylborate (TlTo^M) in THF provides To^MFeBr (1), whereas FeBr₂ and 2 equiv of TlTo^M react to give (To^M)₂Fe (2). Two ν_CN bands at 1604 and 1548 cm^-1 indicated bidentate coordination of To^M to iron in 2. Homoleptic 2 and FeBr₂ react in THF overnight through an unusual ligand exchange process to give compound 1, which is apparently the thermodynamic product. The salt metathesis reaction of 1 and KCH₂Ph affords To^MFeCH₂Ph (3). The effective magnetic moments of compounds 1-3 range from 4.9 to 5.4 μ_B, and these values are consistent with high-spin iron(II) ( S = 2). A single ¹H NMR signal assigned to the methyl groups of the To^M ligand suggested tridentate coordination of the To^M ligand to iron in 1 and 3. X-ray crystallography studies of 1-3 establish their structure as four-coordinated tetrahedral iron complexes. To^MFeBn and CO (1 atm) react to afford isolable To^MFe{C(═O)Bn}(CO)₂ (4) as a yellow solid. Complex 4 is diamagnetic ( S = 0), and the three distinct methyl signals in the ¹H NMR spectrum are consistent with a six-coordinate, C _s-symmetric species. This assignment is supported by its IR spectrum, which revealed intense bands at 2004 and 1935 cm^-1 (symmetric and asymmetric ν_CO), at 1680 and 1662 cm^-1 (acyl rotamers, ν_CO), and at 1593 and 1553 cm^-1 (ν_CN) and is confirmed by a single-crystal X-ray diffraction study.