Synthesis and Structural Characterization of Lithium and Trimethyltin Complexes of 2,6-Bis(oxazolinyl)phenyl

The treatment of 2,6-bis(oxazolinyl)phenyl bromide (Phebox-Br) with n - BuLi affords a Phebox-Li complex. Subsequent transmetalation with [SnClMe 3 ] affords a Phebox-Sn complex. The Phebox ligand can coordinate to a transition metal in various terdentate fashions; both the oxazoline oxygen and the imine nitrogen are perfectly positioned for chelation; “NCN”, “OCO”, or mixed terdentate coordination modes are theoretically possible using this ligand. The structural properties and NMR spectra of [Sn(Me,Me-Phebox)Me 3 ] ( 2 ) and [Li(R,R‘-Phebox)] complexes 3a (R = R‘ = Me), 3b (R = i Pr, R‘ = H), and 3c (R = t Bu, R‘ = H) were investigated. It was found that 2 exhibits no chelation of the Phebox ligand to the Sn center in this case. The [Li(R,R‘-Phebox)] complex 3a has been crystallographically characterized and is in the form of a molecular dimer (i.e. [Li(Phebox)] 2 ), containing two formally three-center−two-electron bonds in a four-membered Li 2 C 2 ring. The formal Phebox anion is bonded to the lithium cation via the two ortho imine N centers and the intraannular aromatic C atom. The 13 C{ 1 H} NMR signal of C ipso, being a seven-line pattern with coupling constant 1 J ( 13 C− 7 Li) = 18 Hz, confirms that the dimeric structure is maintained in solution at room temperature. Variable-temperature (VT) NMR studies of 3a indicate that a fluxional process is occurring at room temperature, which can be frozen out below −16 °C (Δ G ⧧ = 56 kJ/mol). This fluxional process is not observed in VT-NMR studies on 3b, c . This is likely due to the presence of bulky ( i Pr or t Bu) substituents that effectively shut down the pathways to rapid inversion of the puckering of the five-membered chelate ring.