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

The reaction of FeBr 2 and 1 equiv of thallium tris(4,4-dimethyl-2-oxazolinyl)phenylborate (TlTo M ) in THF provides To M FeBr (1), whereas FeBr 2 and 2 equiv of TlTo M react to give (To M ) 2 Fe (2). Two ν CN bands at 1604 and 1548 cm -1 indicated bidentate coordination of To M o iron in 2. Homoleptic 2 and FeBr 2 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 KCH 2 Ph affords To M FeCH 2 Ph (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 1 H 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 M FeBn and CO (1 atm) react to afford isolable To M Fe{C(═O)Bn}(CO) 2 (4) as a yellow solid. Complex 4 is diamagnetic ( S = 0), and the three distinct methyl signals in the 1 H 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.