The structures of three ortho-lithiated phenyloxazolines, the parent (1), the para-tert-butyl analogue (2), and the para-chloro analogue (3), were studied in solution. All three compounds are mixtures of monomers and dimers in THF/ether mixed solvents, with 2 the most aggregated and 3 the least. They are converted to monomers with HMPA and PMDTA. In the PMDTA complexes, the lithium appears to still be chelated to the oxazoline ring. Single-crystal X-ray structures were obtained for 1 and 2. Both are centrosymmetric dimers of B-type (each lithium coordinated to one oxazoline ring).

Chelation and aggregation in phenyllithium reagents with potential 6- and 7-ring chelating amine (2, 3) and 5-, 6-, and 7-ring chelating ether (4, 5, 6) ortho substituents have been examined utilizing variable temperature (6)Li and (13)C NMR spectroscopy, (6)Li and (15)N isotope labeling, and the effects of solvent additives. The 5- and 6-ring ether chelates (4, 5) compete well with THF, but the 6-ring amine chelate (2) barely does, and 7-ring amine chelate (3) does not. Compared to model compounds (e.g., 2-ethylphenyllithium 7), which are largely monomeric in THF, the chelated compounds all show enhanced dimerization (as measured by K = [D]/[M](2)) by factors ranging from 40 (for 6) to more than 200 000 (for 4 and 5). Chelation isomers are seen for the dimers of 5 and 6, but a chelate structure could be assigned only for 2-(2-dimethylaminoethyl)phenyllithium (2), which has an A-type structure (both amino groups chelated to the same lithium in the dimer) based on NMR coupling in the (15)N, (6)Li labeled compound. Unlike the dimer, the monomer of 2 is not detectably chelated. With the exception of 2-(methoxymethyl)phenyllithium (4), which forms an open dimer (12) and a pentacoordinate monomer (13), the lithium reagents all form monomeric nonchelated adducts with PMDTA.

[reaction: see text] The solution and chelation properties of 2-thienyllithium reagents with potential amine and ether chelating groups in the 3-position and related model systems have been investigated using low temperature 6Li, 7Li, 13C, and 31P NMR spectroscopy, 15N-labeling, and the effect of solvent additives. In THF-ether mixtures at low temperature 3-(N,N-dimethylaminomethyl)-2-thienyllithium (4) is ca. 99% dimer (which is chelated) and 1% monomer (unchelated), whereas 3-(methoxymethyl)-2-thienyllithium (5) is <10% dimer. Compound 5 crystallizes as a THF-solvated dimer, but there is no indication that the ether side chain is chelated in solution. Both 4 and 5 form PMDTA-complexed monomers almost stoichiometrically, similar to the model compound 2, in sharp contrast to phenyl analogues, which show very different behavior. The barriers to dimer interconversion are ca. 2 kcal/mol lower and chelation is significantly weaker in the 2-thienyllithium reagents than in their phenyl analogues.

The tripodal tetradentate ligand H 3 btap coordinates to V V , Mo VI , and Ni II Via three different bonding modes to yield three complexes with unique ligand-based oxidation chemistry. For V V and Mo VI (1), all four of the heteroatom donors are coordinated to the metal ion forming a trigonal bipyramidal complex with the oxovanadium-(V) ion, V V O 3+ , and an octahedral complex with the cis-dioxomolybdenum(VI) ion, [MoO 2 ] 2+ . Only three of the heteroatom donors of H 3 btap are used to coordinate to Ni II (2), two thiolate sulfurs and the amine nitrogen, yielding a dimeric structure in which each nickel(II) ion has NS 3 coordination. The ability of V V O(btap) to form η 2 -sulfenates, while [MoO 2 (btap)]does not form stable η 2 -sulfenates, has been ascribed to the electron-deficient, π-accepting nature of V V O 3+ relative to [Mo VI O 2 ] 2+ . Crystal data for 1 (C 11 H 16 NO 4 S 2 KMo): space group Pbcn, a ) 6.6596(9) Å, b ) 13.7446(9) Å, c ) 32.992(2) Å, R ) β ) γ ) 90°, Z ) 8. Crystal data for 2 (C 24 H 38 N 2 O 4 S 4 -Ni): space group Pbcn, a ) 12.0841(3) Å, b ) 14.4948(4) Å, c ) 16.7751(4) Å, R ) β ) γ ) 90°, Z ) 4.