Nickel-catalyzed synthesis of Zn(I)-Zn(I) bonded compounds

Reductive Dimerization of Alkenes and Allenes Enabled by Photochemically Activated Zinc-Zinc Bonded Compounds

Metal radicals have shown versatile reactivity in modern synthetic chemistry. However, the use of zinc radicals for molecular synthesis has been barely explored. Here, we show that a transient zinc radical can be formed through photoactivation of a zinc-zinc bonded compound, which is able to mediate the selective dimerization of alkenes and allenes. Treatment of dizinc compounds [L2Zn2] [L = CH3C(2,6-iPr2C6H3N)CHC(CH3)(NCH2CH2PR2); R = Ph (LPh) or iPr (LiPr)] with a diverse array of aromatic alkenes under UV irradiation (365 nm) facilely afforded the head-to-head coupling products, i.e., 1,4-dizinciobutanes in high yields. In addition, arylallenes could also be selectively dimerized by the dizinc compound to give 2,5-dizincyl-functionalized 1,5-hexadienes under the same conditions. Control reactions of [LPh2Zn2] in the presence of UV irradiation isolated a zinc phenyl complex and a trimeric zinc phosphide complex resulting from C-P bond cleavage at the tridentate ligand. Reactions of photoactivated dizinc compounds with organic spin traps, i.e., 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 2,2'-bipyridine (2,2'-bpy), successfully isolated zinc radical trapping products [LZnOTEMP] and [LPhZn(2,2'-bpy)·-], respectively. The profile of alkene dimerization was elucidated by density functional theory calculations, which confirmed that a transient zinc radical [LZn·] was initially generated through homolytic Zn-Zn bond cleavage via photoactivation followed by single-electron transfer and radical dimerization. The unique selectivity of the current reaction was also studied computationally.

Dihydrogen Cleavage by a Zinc-Zinc Bond of a Heteroleptic Dizinc(I) Cation

Oxidative addition of dihydrogen across a metal-metal bond to form reactive metal hydrides in homogeneous catalysis is known for transition metals but not for zinc(I)-zinc(I) bond as found in Carmona's eponymous dizinconene [Zn2Cp*2] (Cp* = η5-C5Me5). Dihydrogen reacted with the heteroleptic zinc(I)-zinc(I) bonded cation [(L2)Zn-ZnCp*][BAr4F] (L2 = TMEDA, N,N,N',N'-tetramethylethylenediamine, TEEDA, N,N,N',N'-tetraethylethylenediamine; ArF = 3,5-(CF3)2C6H3) under 2 bar at 80 °C to give the zinc(II) hydride cation [(L2)ZnH(thf)][BAr4F] along with zinc metal and Cp*H derived from the intermediate [Cp*ZnH]. DFT calculations show that the cleavage of dihydrogen occurs through a highly unsymmetrical transition state. Mechanistic studies agree with a heterolytic cleavage of dihydrogen as a result of the cationic charge and unsymmetrical ligand coordination. To explore the existence of zinc(I) hydride, thermally unstable hydridotriphenylborate complexes of zinc(I) [(L2)Zn(HBPh3)-ZnCp*] (L2 = TMEDA, TEEDA; TMPDA, N,N,N',N'-tetramethyl-1,3-propylenediamine) have been prepared by salt metathesis and were shown to undergo fast exchange with both BPh3 and [HBPh3]-.

Synthesis and Reactivity of a Zinc Diazoalkyl Complex: [3+2] Cycloaddition Reaction with Carbon Monoxide

This work reports the synthesis, characterization, and reactivity of the first example of a well-defined zinc α-diazoalkyl complex. Treatment of zinc(I)-zinc(I) bonded compound L2 Zn2 [L=CH3 C(2,6-i Pr2 C6 H3 N)CHC(CH3 )(NCH2 CH2 PPh2 )] or zinc(II) hydride LZnH with trimethylsilyldiazomethane affords zinc diazoalkyl complex LZnC(N2 )SiMe3 . This complex liberates N2 in the presence of a nickel catalyst to form an α-zincated phosphorus ylide by reacting with the pendant phosphine. It selectively undergoes formal [3+2] cycloaddition with CO2 or CO to form the corresponding product with a five-membered heterocyclic core. Notably, the use of CO in such a [3+2] cycloaddition reaction is unprecedented, reflecting a novel CO reaction mode.

Reversible Oxidative Addition of Zinc Hydride at a Gallium(I)-Centre: Labile Mono- and Bis(hydridogallyl)zinc Complexes

In the presence of TMEDA (N,N,N',N'-tetramethylethylenediamine), partially deaggregated zinc dihydride as hydrocarbon suspensions react with the gallium(I) compound [(BDI)Ga] (I, BDI={HC(C(CH3 )N(2,6-iPr2 -C6 H3 ))2 }- ) by formal oxidative addition of a Zn-H bond to the gallium(I) centre. Dissociation of the labile TMEDA ligand in the resulting complex [(BDI)Ga(H)-(H)Zn(tmeda)] (1) facilitates insertion of a second equiv. of I into the remaining Zn-H to form a thermally sensitive trinuclear species [{(BDI)Ga(H)}2 Zn] (2). Compound 1 exchanges with polymeric zinc dideuteride [ZnD2 ]n in the presence of TMEDA, and with compounds I and 2 via sequential and reversible ligand dissociation and gallium(I) insertion. Spectroscopic and computational studies demonstrate the reversibility of oxidative addition of each Zn-H bond to the gallium(I) centres.

Synthesis and Reactivity of Triangular Heterometallic Complexes Containing Zn-Zn Bond

This work provides a facile access to a series of triangular [Zn₂M] (M = group 10 and 11 metals) clusters. Treatment of Zn-Zn-bonded compounds [LZn-ZnL] (L = CH₃C(2,6-ⁱPr₂C₆H₃N)CHC(CH₃)(NCH₂CH₂PR₂); R = Ph, ⁱPr) with zero-valent transition-metal reagents selectively afforded the corresponding triangular clusters [Zn₂M], where M = Ni(0), Pd(0), and Pt(0). Notably, the isoelectronic triangular clusters [Zn₂M]⁺, where M = Ag(I) and Cu(I), could also be obtained by reactions of [LZn-ZnL] with AgOTf and CuOTf, respectively. The [Zn₂Ag]⁺ complex containing elusive Zn-Ag bonds was investigated by density functional theory analysis, showing a 3c-2e bonding feature in the metallic ring. The electrochemical behaviors of [Zn₂M] complexes were examined and revealed the donation of electron density from the Zn-Zn σ-bond to the metal centers. Reaction of the [Zn₂Ni] complex with isocyanide gave heterometallic species by coordination of isocyanide to the nickel center, keeping the trimetallic ring core structure intact. In contrast, the Zn-Zn bond was rapidly cleaved upon treatment of the [Zn₂Ni] complex with dihydrogen or phenyl acetylene, generating the hydride- or acetylide-bridged heterotrimetallic complex.

Synthesis and reactivity of heterometallic complexes containing Mg- or Zn-metalloligands

Heteronuclear metal complexes comprising main group metals and transition metals have attracted widespread attention from researchers due to their applications in stoichiometric and catalytic activation of small molecules with possible cooperative effects. Herein, the advances of heterometallic complexes containing Mg- or Zn-metalloligands over the past ten years are reviewed. They consist of two parts: (i) synthetic approaches to heterometallic complexes. Only a brief discussion is made on the different Mg/Zn precursors since they have been summarized before. (ii) Stoichiometric and catalytic reactivities of heterometallic complexes containing Mg/Zn metalloligands. The exploration of the cooperative catalytic reaction of heterometallic complexes is still in its infancy, promising but challenging; thus, further investigations are required in the future.