Heterobimetallic ruthenium–zinc complexes with bulky N-heterocyclic carbenes: syntheses, structures and reactivity

Isolobal Cationic Iridium Dihydride and Dizinc Complexes: A Dual Role for the ZnR Ligand Enhances H2 Activation

The reaction of [Ir(IPr)2H2][BArF4] (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; BArF4 = B{C6H3(3,5-CF3)2}4) with ZnMe2 proceeds with CH4 elimination to give [Ir(IPr)(IPr')(ZnMe)2H][BArF4] (3, where (IPr') is a cyclometalated IPr ligand). 3 reacts with H2 to form tetrahydride [Ir(IPr)2(ZnMe)2H4][BArF4], 4, that loses H2 under forcing conditions to form [Ir(IPr)2(ZnMe)2H2][BArF4], 5. Crystallization of 3 also results in the formation of its noncyclometalated isomer, [Ir(IPr)2(ZnMe)2][BArF4], 2, in the solid state. Reactions of 1 and CdMe2 form [Ir(IPr)2(CdMe)2][BArF4], 6, and [Ir(IPr)(IPr')(CdMe)2H][BArF4], 7, which reacts with H2 to give [Ir(IPr)2(CdMe)2H4][BArF4], 8, and [Ir(IPr)2(CdMe)2H2][BArF4], 9. Structures of 2-8 are determined crystallographically. Computational analyses show the various hydrides in 3-5 sit on a terminal to bridging continuum, with bridging hydrides exhibiting greater Znδ+···Hδ- electrostatic interaction. The isolobal analogy between H and ZnMe ligands holds when both are present as terminal ligands. However, the electrostatic component to the Znδ+···Hδ- unit renders it significantly different to a nominally isolobal H···H moiety. Thus, H2 addition to 3 is irreversible, whereas H2 addition to 1 reversibly forms highly fluxional [Ir(IPr)2(η2-H2)2H2][BArF4], 11. Computed mechanisms for cyclometalation and H2 addition showcase the role of the bridging Znδ+···Hδ- moiety in promoting reactivity. In this, the Lewis acidic ZnMe ligand plays a dual role: as a terminal Z-type ligand that can stabilize electron-rich Ir centers through direct Ir-ZnMe bonding, or by stabilizing strongly hydridic character via Znδ+···Hδ- interactions.

A Paramagnetic Nickel-Zinc Hydride Complex

Reaction of a molecular zinc-hydride [{(ArNCMe)2CH}ZnH] (Ar=2,6-di-isopropylphenyl) with 0.5 equiv. of [Ni(CO)Cp]2 led to the isolation of a nickel-zinc hydride complex containing a bridging 3-centre,2-electron Ni-H-Zn interaction. This species has been characterized in the solid-state by single crystal X-ray diffraction. DFT calculations are consistent with its formulation as a σ-complex derived from coordination of the zinc-hydride to a paramagnetic nickel(I) fragment. Continuous-wave and pulse EPR experiments suggest that this species is labile in solution. Further experiments show that in the presence of catalytic quantities of nickel(I) precursors, zinc-hydride bonds can undergo either H/D-exchange with D2 or dehydrocoupling to form Zn-Zn bonds. In combination, the data support the activation and functionalisation of zinc-hydride bonds at nickel(I) centres.

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.

A Planar Nickelaspiropentane Complex with Magnesium-Based Metalloligands: Synthesis, Structure, and Synergistic Dihydrogen Activation

The nature of transition-metal-olefin bonding has been explained by the Dewar-Chatt-Duncanson model within a continuum of two extremes, namely, a π-complex and a metallacyclopropane. The textbook rule suggests that a low-spin late-transition-metal-ethylene complex more likely forms a π-complex rather than a metallacyclopropane. Herein, we report a low-spin late-transition-metal-bis-ethylene complex forming an unprecedented planar metalla-bis-cyclopropane structure with magnesium-based metalloligands. Treatment of LMgEt (L = [(DippNCMe)₂CH]^-, Dipp = 2,6-ⁱPr₂C₆H₃) with Ni(cod)₂ (cod = 1,5-cyclooctadiene) formed the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂, which features a linear Mg-Ni-Mg linkage and a planar coordination geometry at the nickel center. Both structural features and computational studies strongly supported the Ni(C₂H₄)₂ moiety as a nickelaspiropentane. The exposure of (LMg)₂Ni(C₂H₄)₂ to 1 bar H₂ at room temperature produced a four-hydride-bridged complex (LMg)₂Ni(μ-H)₄. The profile of H₂ activation was elucidated by density functional theory calculations, which indicated a novel Mg/Ni cooperative activation mechanism with no oxidation occurring at the metal center, differing from the prevailing mono-metal-based redox mechanism. Moreover, the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂ catalyzed the hydrogenation of a wide range of unsaturated substrates under mild conditions.

Zinc-Promoted ZnMe/ZnPh Exchange in Eight-Coordinate [Ru(PPh3 )2 (ZnMe)4 H2 ]

The syntheses, reactivity and electronic structure analyses of [Ru(PPh3 )2 (ZnMe)4 H2 ], 1 a, and [Ru(PPh3 )2 (ZnPh)4 H2 ], 2 b, are reported. 1 a exhibits an 8-coordinate Ru centre with axial phosphines and a symmetrical (2 : 2) arrangement of ZnMe ligands in the equatorial plane. The ZnMe ligands in 1 a undergo facile, sequential exchange with ZnPh2 to give 2 b, which shows a 3 : 1 arrangement of ZnPh ligands. Both 1 a and 2 b exist in equilibrium with their respective 3 : 1 and 2 : 2 isomers. Mechanisms for ZnMe/ZnPh exchange and isomerisation are proposed using DFT calculations. The relationships of these {Ru(ZnR)4 H2 } species to isoelectronic Group 8 transition metal polyhydrides and related Schlenk equilibria in the Negishi reaction are discussed.

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.

Bonding and Reactivity of a Pair of Neutral and Cationic Heterobimetallic RuZn2 Complexes

A combined experimental and computational study of the structure and reactivity of two [RuZn2Me2] complexes, neutral [Ru(PPh3)(Ph2PC6H4)2(ZnMe)2] (2) and cationic [Ru(PPh3)2(Ph2PC6H4)(ZnMe)2][BArF4] ([BArF4] = [B{3,5-(CF3)2C6H3}4]) (3), is presented. Structural and computational analyses indicate these complexes are best formulated as containing discrete ZnMe ligands in which direct Ru-Zn bonding is complemented by weaker Zn···Zn interactions. The latter are stronger in 2, and both complexes exhibit an additional Zn···Caryl interaction with a cyclometalated phosphine ligand, this being stronger in 3. Both 2 and 3 show diverse reactivity under thermolysis and with Lewis bases (PnBu3, PCy3, and IMes). With 3, all three Lewis bases result in the loss of [ZnMe]+. In contrast, 2 undergoes PPh3 substitution with PnBu3, but with IMes, loss of ZnMe2 occurs to form [Ru(PPh3)(C6H4PPh2)(C6H4PPhC6H4Zn(IMes))H] (7). The reaction of 3 with H2 affords the cationic trihydride complex [Ru(PPh3)2(ZnMe)2(H)3][BArF4] (12). Computational analyses indicate that both 12 and 7 feature bridging hydrides that are biased toward Ru over Zn.

Impact of the Novel Z-Acceptor Ligand Bis{(ortho-diphenylphosphino)phenyl}zinc (ZnPhos) on the Formation and Reactivity of Low-Coordinate Ru(0) Centers

The preparation and reactivity with H₂ of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C₆H₄PPh₂)₂) are described. Ru(ZnPhos)(CO)₃ (2) and Ru(ZnPhos)(IMe₄)₂ (4; IMe₄ = 1,3,4,5-tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh₃)(C₆H₄PPh₂)₂(ZnMe)₂ (1) or Ru(PPh₃)₃HCl/LiCH₂TMS/ZnMe₂ with CO and IMe₄, respectively. Structural and electronic structure analyses characterize both 2 and 4 as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru(ZnPhos)(CO)₂ that then adds H₂ over the Ru-Zn bond to form Ru(ZnPhos)(CO)₂(μ-H)₂ (3). In contrast, 4 reacts directly with H₂ to set up an equilibrium with Ru(ZnPhos)(IMe₄)₂H₂ (5), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L₂ intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H₂ to add across the Ru-Zn bond, but for L = IMe₄, this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong π-acceptor ability of CO compared to IMe₄. Steric effects associated with the larger IMe₄ ligands are not significant. Species 4 can be considered as a Ru(0)L₄ species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L₄ species devoid of strong π-acceptor ligands.

Zn-Promoted C-H Reductive Elimination and H2 Activation via a Dual Unsaturated Heterobimetallic Ru-Zn Intermediate

Reaction of [Ru(PPh₃)₃HCl] with LiCH₂TMS, MgMe₂, and ZnMe₂ proceeds with chloride abstraction and alkane elimination to form the bis-cyclometalated derivatives [Ru(PPh₃)(C₆H₄PPh₂)₂H][M′] where [M′] = [Li(THF)₂]⁺ (1), [MgMe(THF)₂]⁺ (3), and [ZnMe]⁺ (4), respectively. In the presence of 12-crown-4, the reaction with LiCH₂TMS yields [Ru(PPh₃)(C₆H₄PPh₂)₂H][Li(12-crown-4)₂] (2). These four complexes demonstrate increasing interaction between M′ and the hydride ligand in the [Ru(PPh₃)(C₆H₄PPh₂)₂H]⁻ anion following the trend 2 (no interaction) < 1 < 3 < 4 both in the solid-state and solution. Zn species 4 is present as three isomers in solution including square-pyramidal [Ru(PPh₃)₂(C₆H₄PPh₂)(ZnMe)] (5), that is formed via C–H reductive elimination and features unsaturated Ru and Zn centers and an axial Z-type [ZnMe]⁺ ligand. A [ZnMe]⁺ adduct of 5, [Ru(PPh₃)₂(C₆H₄PPh₂)(ZnMe)₂][BAr^F₄] (6) can be trapped and structurally characterized. 4 reacts with H₂ at −40 °C to form [Ru(PPh₃)₃(H)₃(ZnMe)], 8-Zn, and contrasts the analogous reactions of 1, 2, and 3 that all require heating to 60 °C. This marked difference in reactivity reflects the ability of Zn to promote a rate-limiting C–H reductive elimination step, and calculations attribute this to a significant stabilization of 5 via Ru → Zn donation. 4 therefore acts as a latent source of 5 and this operational “dual unsaturation” highlights the ability of Zn to promote reductive elimination in these heterobimetallic systems. Calculations also highlight the ability of the heterobimetallic systems to stabilize developing protic character of the transferring hydrogen in the rate-limiting C–H reductive elimination transition states.

Transforming PPh3 into bidentate phosphine ligands at Ru-Zn heterobimetallic complexes

The reaction of [Ru(PPh₃)₃Cl₂] with excess ZnMe₂ led to P-C/C-H bond activation and P-C/C-C bond formation to generate a chelating diphenylphosphinobenzene ligand as well as a cyclometallated (diphenylphosphino)biphenyl group in the final product of the reaction, [Ru(dppbz)(PPh₂(biphenyl)')(ZnMe)] (1; dppbz = 1,2-bis(diphenylphosphino)benzene); PPh₂(biphenyl)' = cyclometallated PPh₂(biphenyl). The mechanism of reaction was studied and C-C coupling to give a bidentate 2,2'-bis(diphenylphosphino)biphenyl (BIPHEP) ligand was suggested to be one of the key steps of the process. This was confirmed by the reaction of [Ru(BIPHEP)(PPh₃)HCl] with ZnMe₂, which also gave 1. An analogous set of steps took place upon addition of ZnMe₂ to [Ru(rac-BINAP)(PPh₃)HCl] (rac-BINAP = racemic(2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) to give [Ru(dppbz)(PPh₂(binaphthyl)')ZnMe] (3). H₂ and the C-H bond of PhC[triple bond, length as m-dash]CH added across the Ru-Zn bond of 1, and also reversed the phosphine cyclometallation, to give [Ru(dppbz)(Ph₂P(biphenyl))(H)₂(H)(ZnMe)] (4) and [Ru(dppbz)(Ph₂P(biphenyl))(C[triple bond, length as m-dash]CPh)₂(H)(ZnMe)] (5) respectively.