Applications of Low-Valent Transition Metalates: Development of a Reactive Noncarbonyl Rhenium(I) Anion

Abstract

Conspectus Low-valent transition metalatesanionic, electronic-rich organometallic complexescomprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M–E bond formation, among others. The inherent reactivity of such electron-rich metal centers has necessitated the widespread use of strong backbonding ligands, particularly carbonyls, to aid in the isolation and handling of metalate reagents, albeit sometimes at the expense of partially masking their full reactivity. However, recent synthetic explorations into transition-metalate complexes devoid of archetypic back-bonding ligands have led to the discovery of highly reactive metalates capable of performing a variety of novel chemical transformations. Building on our group’s long-standing interest in reactive organometallic species, a series of rational progressions in early-to-middle transition-metal chemistry ultimately led to our isolation of a rhenium­(I) β-diketiminate cyclopentadienide metalate that displays exceptional reactivity. We have found this Re­(I) metalate to be capable of small-molecule activation; notably, the complex reversibly binds dinitrogen in solution and can be utilized to trap N2 for the synthesis of functionalized diazenido species. By employing isolobal analogues to N2 (CO and RNC), we were able to thoroughly monitor the mechanism of activation and conclude that the metalate’s sodium counterion plays an integral role in promoting dinitrogen activation through a novel side-on interaction. The Re­(I) metalate is also used in forming a variety of M–E bonds, including a series of uncommon rhenium-tetrylene (Si, Ge, and Sn) complexes that display varying degrees of multiple bonding. These metal tetrylenes act to highlight deviations in chemical properties within the group 14 elements. Our metalate’s utility also applies to metal–metal bond formation, as demonstrated through the synthesis of a hetero­tetrametallic rhenium–zinc dimer. In this reaction, the Re­(I) metalate performs a dual role as a reductant and metalloligand to stabilize a transient Zn2 2+ core fragment. Finally, the metalate displays unique reactivity with uranium­(III) to yield the first transition metal–actinide inverse-sandwich bonds, in this case with three rhenium fragments bound through their Cp moieties surrounding the uranium center. Notably, throughout these endeavors we demonstrate that the metalate displays reactivity at multiple locations, including directly at the rhenium metal center, at a Cp carbon, through a Cp-sandwich mode, or through reversibly bound dinitrogen. Overall, the rhenium­(I) metalate described herein demonstrates utility in diverse applications: small-molecule activation, the stabilization of reduced and/or unstable species, and the formation of unconventional M–E/M–M bonds or heterometallic complexes. Moving forward, we suggest that the continued discovery of noncarbonyl, electron-rich transition-metal anions featuring new or unconve...

Heterobimetallic-Mediated Dinitrogen Functionalization: N–C Bond Formation at Rhenium-Group 9 Diazenido Complexes

“…The divergent syntheses of 1-M versus 2-M depending on reaction conditions expands the synthetic potential of the Re(I) metalate starting material 37 and left us with an interesting prospect for further investigating the dinitrogen binding ability of Re-group 9 bimetallic species. We were curious if it were possible to abstract the hydride in 2-M under an N 2 atmosphere and, if so, whether abstraction might enforce either a direct M−M interaction or, alternatively, lead to the reincorporation of dinitrogen to form a diazenido species analogous to 1-M.…”

“…Our group has developed a highly reducing rhenium­(I) metalate, Na­[Re­(η 5 -Cp)­(BDI)] (Cp = cyclopentadienide, BDI = N , N ′-bis­(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate), capable of reversibly binding dinitrogen in solution at low temperatures, with the Na + counterion playing a crucial role in the binding and activation process. , Although there is some precedent for dinitrogen functionalization by rhenium complexes, both in monometallic complexes , and via the splitting of dinitrogen by homobimetallic species, ,− we were interested in the unexplored frontier of heterobimetallic dinitrogen functionalization. Through salt elimination reactions between the above-mentioned Na­[Re­(η 5 -Cp)­(BDI)] with either iridium­(I) or rhodium­(I) reagents, we report the synthesis, characterization, and initial N–C bond formation reactivity of Re-group 9 heterobimetallic dinitrogen species.…”

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

“…7,8,65,71,72 Examples of complexes obtained using such strategy are [Li 2 (thf) 4 {Ni(η 2 -cod)(η 4 -cod)}] (9) or the alkenemanganate 7, shown in Scheme 1. 7,68 The use of other alkali metals (sodium, potassium, or their Hg amalgams) to reduce a transition metal salt or precursor is also a common synthetic route (see Scheme 1, reaction type 1b), 73 with examples including the reduction of a Co(II) precursor with sodium to afford [(N 2 )Co(PEt 2 Ph) 3 ] − (10), or the formation of bis(arene)titanates(−I), K[Ti(arene) 2 ] [arene = benzene (6, Figure 1, vide supra), toluene (11)] from their Ti(0) parent compounds. 66,67,69 Reducing agents such as KC 8 74−82 and cobaltocene (Cp 2 Co) 83 can also be employed.…”

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten

“…Thus, these ligands have found widespread applications all over the periodic table spanning from main group chemistry, covering both s- − and p-block − elements, to d-block transition metal chemistry, − culminating in f-block chemistry. − Focusing on their chemistry with the early transition metals, these breakthroughs included the isolation of a plethora of reactive metal–ligand multiple bonds. Mindiola and co-workers for example reported the isolation of titanium nitrido , and phosphinidene , complexes or a vanadium phosphinidene complex, while Arnold and co-workers recently explored the versatile chemistry of niobium − and rhenium BDI complexes. − The latter included a rare example of a noncarbonyl stabilized rhenium­(I) cyclopentadiene complex, , which has also been proven to be a potent candidate for the formation of new metal–metal bonds, e.g., a Re–Zn–Zn–Re complex, or Cp bridged tetranuclear actinide complexes . Apart from that, a plethora of catalytic reactions have been elucidated using BDI supported early transition metal complexes, e.g., nitrene transfer reactions to yield carbodiimides or hydrodefluorinations ,, both catalyzed via low-valent niobium­(III) − or titanium­(III) complexes.…”