Abstract
The reaction of [TaCpRX4] (CpR = η5-C5Me5, η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) with SiH3Ph resulted in the formation of the dinuclear hydride tantalum(IV) compounds [(TaCpRX2)2(μ-H)2], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [TaCpRX2(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta(η5-C5Me5)X2}2(μ-H)2] derivatives and the cyclic diazo reagent benzo[c]cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta(η5-C5Me5)X2}2(μ-NC6H4C6H4N)] along with the release of molecular hydrogen. When the compounds [(TaCpRX2)2(μ-H)2] (CpR = η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCpRX)2{μ-(η2,η2-NC6H4C6H4N)}] (CpR = η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) as intermediates in the N=N bond cleavage process. DFT calculations provide insights into the N=N cleavage mechanism, in which the ditantalum(IV) fragment can promote two-electron reductions of the N=N bond at two different metal–metal bond splitting stages.
Highlights
IntroductionThe study of N−N bond cleavage reactions is of great interest for the development of synthetic transformations using azo compounds as precursors of [NR] fragments[1−3] and, more importantly, for a mechanistic understanding of the industrial and biological dinitrogen (N2) reduction to ammonia (NH3).[4,5] Among the variety of metal compounds capable of promoting these reactions,[6] low-valent early transition metals have attracted significant attention because they lead to metal imide fragments, which are important intermediates for a variety of catalytic processes such as nitrogen transfer, hydroamination, and metathesis reactions.[7]The reduced metallic compounds required for the multielectron N−N scission process can be accessed by three different pathways
The highest occupied molecular orbital (HOMO) consists of a bonding combination of atomic d-type orbitals centered at both tantalum centers, which is a clear indication of a σ Ta−Ta bonding between two Ta(IV) centers
Combining SiH3Ph and the mononuclear tantalum(V) derivatives [TaCpRX4] (CpR = η5-C5Me5, η5-C5H4SiMe3, η5C5HMe4; X = Cl, Br), we have developed a one-step methodology for the synthesis of dinuclear tantalum(IV) hydrides [(TaCpRX2)2(μ-H)2] (CpR = η5-C5Me5, η5C5H4SiMe3, η5-C5HMe4; X = Cl, Br) in high yields
Summary
The study of N−N bond cleavage reactions is of great interest for the development of synthetic transformations using azo compounds as precursors of [NR] fragments[1−3] and, more importantly, for a mechanistic understanding of the industrial and biological dinitrogen (N2) reduction to ammonia (NH3).[4,5] Among the variety of metal compounds capable of promoting these reactions,[6] low-valent early transition metals have attracted significant attention because they lead to metal imide fragments, which are important intermediates for a variety of catalytic processes such as nitrogen transfer, hydroamination, and metathesis reactions.[7]The reduced metallic compounds required for the multielectron N−N scission process can be accessed by three different pathways. The study of N−N bond cleavage reactions is of great interest for the development of synthetic transformations using azo compounds as precursors of [NR] fragments[1−3] and, more importantly, for a mechanistic understanding of the industrial and biological dinitrogen (N2) reduction to ammonia (NH3).[4,5] Among the variety of metal compounds capable of promoting these reactions,[6] low-valent early transition metals have attracted significant attention because they lead to metal imide fragments, which are important intermediates for a variety of catalytic processes such as nitrogen transfer, hydroamination, and metathesis reactions.[7]. Mashima[10] has reported a salt-free methodology employing a series of organic reducing reagents
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