Publications

Dihapto-coordination of aromatic molecules promotes numerous organic transformations that are not observed for the free aromatics. The development of osmium, rhenium, tungsten and molybdenum complexes that are capable of such binding is described in this perspective. The stability of these complexes strongly correlates to the metal d⁵/d⁶reduction potential and electrochemical data has played a central role in their design.

A series of complexes having the general formulas [TpM(NO)(L)Br] and [TpmM(NO)(L)Br]BF₄ (M = Mo or W and L = a moderate σ-donor) have been prepared. Variations in L provide access to M(I) complexes having a broad range of electronic and steric properties. Reduction of a limited number of these complexes in the presence of aromatic molecules produces novel complexes with the general formula [TpM(NO)(L)(η²-aromatic)]ⁿ⁺ (n = 0, 1).

By density functional methods we characterize the bonding and charge distribution in complexes of benzene with dearomatizing agents tpReCO(L), tpMoNO(L), and tpWNO(L), where tp = hydrido Tris (pyrazolyl)borate), for a range of ligands L. Our LSDA and B3LYP density functional calculations use the Spartan LACVP+ basis and pseudopotential on Re, Mo, and W and 6-31G* on light atoms. The binding energy is strongly dependent on the nature of the ligand L, being greatest for L = ammonia and N-methylimidazole and weakest for CH₃NC and CO. We find a correlation between strength of binding and electron transfer from the dearomatizing agents toward benzene. For the most strongly bound systems we find substantial (up to 500 millielectrons) charge transfer towards benzene, while for the most weakly bound systems charge is withdrawn from benzene. Structural details illustrate the ability of Re, Mo, and W species to dearomatize complexed benzene, which is extensive for all but the most weakly bound species with L = MeNC and CO. Re and W dearomatizing agents, which are computed and observed to form stable complexes with benzene, may be economic alternatives to osmium dearomatizing agents. © 2004 Wiley Periodicals, Inc. J Comput Chem 26: 194–200, 2005

A new generation of dearomatizing π-bases having the general form {TpM(L)(π-acid)} (where Tp = hydridotris(pyrazolyl)borate, M = rhenium, molybdenum, tungsten, L = variable ligand, and π-acid = CO, NO⁺) has been developed. These fragments show reactivity patterns similar to those of their pentaammineosmium(II) predecessor, binding aromatic molecules across two carbons and subsequently activating these ligands toward electrophilic addition and cycloaddition reactions. In many cases, these second-generation dearomatization agents show a greater degree of π-back-bonding than was observed for the analogous pentaammineosmium(II) complexes. A number of unprecedented reaction pathways have been discovered for these second-generation systems, most notably a broad array of cyclization reactions. In addition, these chiral-at-metal dearomatization agents are readily resolved and have been utilized in reaction sequences to give highly enantioenriched polycyclic molecules from common aromatic precursors.

Complexes of the form TpW(NO)(PMe₃)(η²-thiophene) have been synthesized in 31−40% yield by reduction of TpW(NO)(PMe₃)(Br) in the presence of the thiophene ligand. The dynamics of protonation and the subsequent deprotonation for the corresponding 2H-thiophenium complexes have been investigated. Hydrogenation of the uncoordinated double bond was accomplished, and sulfur−carbon, sulfur−oxygen, and carbon−carbon bond-forming reactions were explored with these complexes.