Diverse Reactivity of a Rhenium(V) Oxo Imido Complex: [2 + 2] Cycloadditions, Chalcogen Metathesis, Oxygen Atom Transfer, and Protic and Hydridic 1,2-Additions

Abstract

We present a wide range of reactivity studies focused on the rhenium­(V) oxo imido complex (DippN)­(O)­Re­(BDI) (1, Dipp = 2,6-diisopropylphenyl and BDI = N,N′-bis­(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate). This complex, which was previously shown to possess a highly polarized Re oxo moiety, has proven to be a potent nucleophile and a valuable precursor to a variety of rare structural motifs in rhenium coordination complexes. For example, the Re oxo moiety of 1 undergoes [2 + 2] cycloadditions with carbodiimides, isocyanates, carbon dioxide, and isothiocyanates at room temperature. In the case of CO2, the cycloadduct with 1 (a carbonate complex) undergoes the facile ejection of CO2, demonstrating that this binding process is reversible. In the case of isothiocyanate, chalcogen metathesis with 1 takes place readily as the inclusion of a second equivalent of substrate in the reaction mixture rapidly yields a dithiocarbamate complex. This metathesis process was extended to the reactivity of 1 with phosphine chalcogenides, leading to the isolation of terminal sulfido imido and selenido imido complexes. Attempts to complete this series and generate the analogous terminal telluride led to the formation of a bidentate tritelluride (Te3 2–) complex. Triethylphosphine could only undergo oxygen atom transfer (OAT) with 1 under pressing thermal conditions that also led to C–N cleavage of the BDI ligand. In contrast, OAT between 1 and CO or 2,6-xylylisocyanide (XylNC) was found to be much more facile, proceeding within seconds at room temperature. While the addition of excess CO led to a rhenium­(III) imido dicarbonyl complex, we found that the addition of 2 equiv of XylNC was necessary to promote OAT, resulting in the isolation of a rare example of a stable metal isocyanate complex. Our experimental observations of CO and XylNC and their OAT reactions with 1 inspired a mechanistic computational study to probe the intermediates and kinetic barriers along these reaction pathways. Finally, we describe 1,2-additions of both protic and hydridic substrates with the Re oxo moiety of 1, which most notably led to the syntheses of an uncommon example of a terminal rhenium hydroxide complex and an oxo-bridged Re–O–Zr hetero-bi-metallic complex that was generated using Schwartz’s reagent (Cp2ZrHCl). A brief discussion of a potential alternative route to 1 is also presented.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

“…We have observed similar [2 + 2] cycloaddition reactivity of CO 2 with the terminal oxo moiety of (BDI)Re(O)(NAr); however, the isolable carbonate complex formed in this system remains mononuclear in the solid and solution states, and readily releases CO 2 upon standing in solution under an N 2 atmosphere. 15 Thus, while the reactivity patterns of (BDI)Nb(O)(NAr) and (BDI)Re(O)(NAr) with carbon dioxide are similar, key structural and thermodynamic differences prevail in the resulting carbonate products.…”

“…To the best of our knowledge, 16 represents the rst structurally characterized example of a group 5 transition metal complex bearing both a terminal imido and a terminal oxo group (although many examples of related group 6, 7 and 8 complexes have been reported). 15,16,[87][88][89][90][91][92][93][94] In contrast to the 1 H NMR spectrum of 14, that of 16 displayed sharp signals at room temperature, consistent with a more strongly bound DMAP ligand. The X-ray crystal structure of 16 (Fig.…”

“…Single crystals of 1, 3a, 4,5,6,7,8,9,10,11,12,13,15,16,18, and 20 were coated in Paratone-N oil, mounted on a Kaptan loop, transferred to a Bruker APEX CCD area detector, 101 centered in the beam, and cooled by a nitrogen ow lowtemperature apparatus. Preliminary orientation matrices and cell constants were determined by collection of 36 10s frames, followed by spot integration and least-squares renement.…”

“…In terms of the rst challenge-unreactive spectator ligands-a second multiply-bound ligand serves to electronically saturate the symmetrically available p-bonding orbitals on the metal (a strategy termed ploading), thus activating the p-bonded ligands by polarizing the electron density away from the metal center. [7][8][9][10][11][12][13][14][15][16] Furthermore, the addition of a second p-donor ligand-specically, one with a sterically demanding substituent attached-can impede deleterious dimerization, thus avoiding the formation of unreactive species.…”

“…Mp: 143 C (dec). (k 1 -BDI)Nb(Py)(NAr)-(m-O) 2 -(NAr)Nb(BDI)(15) Compound 12 (10.0 mg, 0.013 mmol) was dissolved in C 6 D 6 (0.4 mL) in a 4 mL vial and transferred to a J. Young NMR tube.…”