The first group VI η 2 -coordinated benzene complex, TpW(NO)(PMe 3 )(η 2 -benzene), is reported. The bound arene is activated toward Diels-Alder cycloaddition, and substitution of the arene with DMF yields a rare example of a η 2 -coordinated amide.Over the past decade, we have endeavored to develop π-basic metal fragments with the ability to bind aromatic molecules and activate them toward otherwise inaccessible organic transformations, with the ultimate goal of expanding the array of reactions available for this ubiquitous class of molecules. 1-3 These electronrich transition-metal fragments such as {Os(NH 3 ) 5 } 2+ and {TpRe(CO)(MeIm)} (Tp ) hydridotris(pyrazolyl)-borate, MeIm ) 1-methylimidazole) coordinate aromatic molecules across only two atoms of the aromatic ring and thereby localize the π-bonds of the uncoordinated portion, rendering the molecule highly nucleophilic. 2,4,5 Recently, the first group VI dearomatization fragment, {TpMo(NO)(MeIm)}, was synthesized 6 by using a strategy of matching d 5 /d 6 reduction potentials with previously developed rhenium(I) and osmium(II) systems. Although this complex forms stable η 2 complexes with naphthalene and furan, complexes of benzene or pyrrole could not be isolated. We hypothesized that a heavymetal analogue of the form {TpW 0 (NO)(L)} could more effectively back-bond with aromatic molecules, thereby strengthening the metal-arene bond to the point that a full range of complexes with aromatic molecules could be isolated and characterized.The difference in d 5 /d 6 reduction potentials between TpRe(CO) 3 7 and TpW(NO)(CO) 2 8 suggests that the fragment {TpW(NO)} is approximately 300 mV more reducing than {TpRe(CO)}. 9 This observation, along with a comparison of electrochemical data for the series of complexes {TpRe(CO)(L)} (L ) NH 3 , MeIm, py, PMe 3

The mechanisms for the interconversion of facial diastereomers of a variety of TpRe(CO)(L)(eta(2)-L(Ar)) complexes [L = (t)BuNC, pyridine (py), PMe(3), or 1-methylimidazole (MeIm); L(Ar) = benzene, anisole, naphthalene, 1-methylpyrrole, furan, or thiophene; Tp = hydridotris(pyrazolyl)borate] have been investigated by (1)H NMR spin saturation experiments. In addition, the rates and free energies of activation for these processes were calculated from spin saturation experiments and T(1) measurements. The operative mechanisms for interconversion of the pi diastereomers were found to be nondissociative, undergoing either an interfacial or intrafacial linkage isomerization. A comparison of the kinetic parameters for isomerization of related eta(2)-olefin complexes of the [TpRe(CO)(PMe(3))] and [CpRe(NO)(PPh(3))](+) fragments is also presented.

The π-basic metal fragment {TpRe(CO)-(MeIm)} (MeIm ) 1-methylimidazole; Tp ) hydridotris-(pyrazolyl)borate) binds a variety of aromatic molecules, including benzene, 2,6-lutidine, and 1-methylpyrrole, in an η 2 fashion. Although TpRe(CO)(MeIm)(η 2 -benzene) as a solid shows no decomposition over several months at 25 °C under a nitrogen atmosphere, the complex has proven to be a valuable precursor to a variety of other aromatic complexes through ligand exchange reactions in solution.

A variety of rhenium asymmetric π-basic dearomatization agents of the general formula TpRe(CO)(L)(L π ) (Tp ) hydridotris(pyrazolyl)borate; L ) t BuNC, pyridine, PMe 3 , 1-methylimidazole, or NH 3 ; L π ) dihapto-coordinated ligand) have been synthesized via three different routes. By varying the ligand, L, the steric and electronic properties of these complexes can be tuned, and thus the stability and selectivity of the η 2 -aromatic systems can be adjusted. Comparisons among the various rhenium complexes are presented as well as comparisons to the established pentaammineosmium(II) system. Scheme 2. Synthesis of Re(I) η 2 -Aromatic Complexes

Compounds having the form TpMo(NO)(1-methylimidazole)(eta(2)-L(pi)) (Tp = hydridotris(pyrazolyl)borate; L(pi) = cyclohexene, naphthalene, furan, thiophene, and acetone) were synthesized in 31-41% yield by the reduction of TpMo(NO)Br(2) in the presence of 1-methylimidazole and the respective pi-acidic organic ligand. The structure of the naphthalene complex was confirmed by single-crystal X-ray diffraction. Degradation studies showed the bound aromatics to have half-lives of 37-236 h in acetone solution at 20 degrees C. A tandem addition across the bound ring of the naphthalene complex yielded a free 1,2-dihydronaphthalene in 80% yield following decomplexation.