The 4-Electron Cleavage of a N═N Double Bond by a Trimetallic TiNi₂ Complex

Abstract

The synthesis and reactivity of a new trimetallic complex Ti­(NP)4Ni2 (NP = 2-diphenylphosphinopyrrolide) (3) is reported. Single-crystal X-ray diffraction and X-ray absorption studies point to a unique bonding motif: a d10–d10, Ni0–Ni0 bond stabilized by a proximal d0 TiIV metal center. The coordination chemistry of 3 with a variety of L (L = isocyanide and alkyne) donors has also been explored. In the case of isocyanide coordination, the Ni–Ni bond is broken, while diphenylacetylene binding results in a symmetric butterfly μ2-κ2-alkyne bridge across the Ni–Ni moiety. Finally, complex 3 is capable of the 4-electron cleavage of the NN double bond in benzo­[c]­cinnoline, the first example of NN bond cleavage by Ni. The resulting product, 7, has been characterized structurally and spectroscopically, and the mechanistic implications are discussed in the context of metal–metal cooperativity.

Reductive Cleavage of Azoarene as a Key Step in Nickel-Catalyzed Amidation of Esters with Nitroarenes

“…39 For nickel, the cleavage of azobenzene has not been reported to the best of our knowledge, although Tonks recently reported the NN cleavage of benzo[c]cinnoline by a Ni 2 Ti complex. 40 The reverse reaction, i.e., the formation of an azoarene from two NiNAr moieties, has been reported by Hillhouse and co-workers: they showed that a diphosphineligated terminal nickel imide complex (dtbpe)NiNMes (5, dtbpe = 1,2-bis-(di-tert-butylphosphino)ethane, Mes = 2,4,6trimethylaryl) eliminated an azoarene at 80 °C (Scheme 4B). 41 This example suggests that the reverse reaction, the 4-electron reductive cleavage of an azoarene on Ni (as in Scheme 4A), is possible, given the appropriate ligand environment and reaction conditions (e.g., ZnCl 2 promotion).…”

“…For nickel, the cleavage of azobenzene has not been reported to the best of our knowledge, although Tonks recently reported the NN cleavage of benzo­[ c ]­cinnoline by a Ni 2 Ti complex . The reverse reaction, i.e., the formation of an azoarene from two NiNAr moieties, has been reported by Hillhouse and co-workers: they showed that a diphosphine-ligated terminal nickel imide complex (dtbpe)­NiNMes ( 5 , dtbpe = 1,2-bis-(di-tert-butylphosphino)­ethane, Mes = 2,4,6-trimethylaryl) eliminated an azoarene at 80 °C (Scheme B) .…”

Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands

“…The electronic structure of 45 was investigated by X-ray absorption spectroscopy and theoretical calculations. 209 The formally Ni 0 −Ni 0 complex, which contains a Ti IV center, is fluxional in solution, and its reaction with 2 equiv of CNtBu led to a rearrangement of the metal core and cleavage of the Ni−Ni bond to give the Ni−Ti−Ni chain complex 46. 209 A N,P donor set bridges each Ti−Ni bond while the two other N,P bidentate ligands triply bridge the metal chain, with the N atom adopting a μ 2 bonding mode.…”

“…209 The formally Ni 0 −Ni 0 complex, which contains a Ti IV center, is fluxional in solution, and its reaction with 2 equiv of CNtBu led to a rearrangement of the metal core and cleavage of the Ni−Ni bond to give the Ni−Ti−Ni chain complex 46. 209 A N,P donor set bridges each Ti−Ni bond while the two other N,P bidentate ligands triply bridge the metal chain, with the N atom adopting a μ 2 bonding mode. It is noteworthy that 45 was the first Ni complex capable of breaking the NN double bond in benzo[c]cinnoline, and the mechanism by which this fourelectron cleavage reaction occurs was discussed in terms of metal−metal cooperativity.…”

“…It is noteworthy that 45 was the first Ni complex capable of breaking the NN double bond in benzo[c]cinnoline, and the mechanism by which this fourelectron cleavage reaction occurs was discussed in terms of metal−metal cooperativity. 209 A Sn−M−Sn (M = Pd, Pt) chain complex was structurally characterized in which a M−Sn bond is doubly bridged by the N,P ligand. 2 1 0 Using the ligand bis(2-dipyridyl)phenylphosphine, a CuAu 2 complex, related to 12 (Chart 9), was stabilized in which each Cu−Au bond is doubly bridged, with the P donor being coordinated to Au and the central Cu center coordinated by four N donors.…”