CONSPECTUSNuclear magnetic resonance (NMR) is the most powerful and widely utilized technique for determining molecular structure. Although traditional NMR data analysis involves the correlation of chemical shift, coupling constant, and NOE interactions to specific structural features, a largely overlooked method introduced more than 40 years ago, pulsed gradient spinecho (PGSE), measures diffusion coefficients of molecules in solution, thus providing their relative particle sizes. In the early 1990s, the PGSE sequence was incorporated into a twodimensional experiment, dubbed diffusion-ordered NMR spectroscopy (DOSY), in which one dimension represents chemical shift data while the second dimension resolves species by their diffusion properties. This combination provides a powerful tool for identifying individual species in a multicomponent solution-earning the nickname "chromatography by NMR." In this Account, we describe our efforts to utilize DOSY techniques to characterize organometallic reactive intermediates in solution in order to correlate structural data to solidstate crystal structures determined by X-ray diffraction and to discover the role of aggregate formation and solvation states in reaction mechanisms.In 2000, we reported our initial efforts to employ DOSY techniques in the characterization of reactive intermediates such as organolithium aggregates. Since then, we have explored DOSY experiments with various nuclei beyond 1 H, including 6 Li, 7 Li, 11 B, 13 C, and 29 Si. Additionally, we proposed a diffusion coefficient-formula weight relationship to determine formula weight, aggregation number, and solvation state of reactive intermediates. We also introduced an internal reference system to correlate the diffusion properties of unknown reactive intermediates with known inert molecular standards, such as aromatic compounds, terminal olefins, cycloolefins, and tetraalkylsilanes. Furthermore, we utilized DOSY to interpret the role of aggregation number and solvation state of organometallic intermediates in the reactivity, kinetics, and mechanism of organic reactions. By utilizing multinuclear DOSY methodologies at various temperatures, we also correlated solid-state X-ray structures with those in solution and discovered new reactive complexes, including a monomeric boron enolate, a product-inhibition aggregate, and a series of intermediates in the vinyl lithiation of allyl amines. As highlighted by our efforts, DOSY techniques provide practical and feasible NMR procedures and hold the promise of even more powerful insights when extended to threedimensional experiments.*To whom correspondence should be addressed. E-mail address: pgw@brown.edu.. To establish the usefulness of the DOSY technique in identification of organometallic intermediates, our initial studies focused on n-BuLi in THF solution since n-BuLi is easily accessible and its solution structure is well characterized. n-BuLi was shown to exist in equilibrium between tetrasolvated dimeric and tetrasolvated tetrameric aggregates in THF soluti...

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low T(g) polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the "sandwich" α-diimines 3-6 yielded semicrystalline "polyethylene" comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events.

The synthesis and reactivity of the end-on bound dinitrogen complex [(η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 Ti] 2 (µ 2 ,η 1 ,η 1 -N 2 ) is described. The solid state structure of the dinitrogen compound reveals a weakly activated end-on bound N 2 ligand with an N-N bond length of 1.164(5) Å. Displacement of the N 2 ligand by organic azides has been used to prepare monomeric, basefree titanocene imido complexes, (η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 TidNR (R ) SiMe 3 , 2,4,6-Me 3 -C 6 H 2 ). While unreactive toward C-H bonds, the Ti-N linkage is readily hydrogenated and participates in group transfer reactions with unsaturated organic molecules such as carbon monoxide and benzophenone. Reaction of the N 2 complex with Ph 2 CN 2 allowed isolation of (η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 Ti(N 2 CPh 2 ), which exists as a mixture of interconverting η 2 and η 1 isomers in solution. The diazoalkane complex also participates in "imido-like" reactivity, producing (η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 Ti(NHNdCPh 2 )H upon addition of H 2 . Changing the diazoalkane to Me 3 SiCHN 2 resulted in isolation of the double cyclometalated titanocene (η 5 -C 5 H 3 -η 1 -1-SiMe 2 CH 2 -3-SiMe 3 ) 2 Ti, arising from facile intramolecular C-H activation of the cyclopentadienyl substituent by a transient titanocene alkylidene.

A family of isolable, well-defined bis-indenyl zirconium sandwich complexes, (eta(5)-C(9)H(5)-1,3-R(2))(eta(9)-C(9)H(5)-1,3-R(2))Zr (R = silyl, alkyl), have been prepared by either alkane reductive elimination or alkali metal reduction of a suitable zirconium(IV) dihalide precursor. Crystallographic characterization of two of these derivatives, R = SiMe(2)CMe(3) and CHMe(2), reveals unprecedented eta(9) coordination of one of the indenyl ligands. Variable-temperature and EXSY NMR studies establish that the eta(5) and eta(9) rings are rapidly interconverting in solution. The sandwich complexes serve as effective sources of low-valent zirconium reacting rapidly with both olefins and alkynes at ambient temperature. In contrast to bis-cyclopentadienyl chemistry, the olefin adducts of the bis-indenyl zirconium sandwiches undergo preferential C-H activation to yield the corresponding allyl hydride compounds, although reaction with excess olefin proceeds through the eta(2)-olefin adduct, forming the corresponding zirconacyclopentane.