Zn···Zn interactions at nickel and palladium centers

Synthesis and Modular Reactivity of Low Valent Al/Zn Heterobimetallics Supported by Common Monodentate Amides

Recent advances on low valent main group metal chemistry have shown the excellent potential of heterobimetallic complexes derived from Al(I) to promote cooperative small molecule activation processes. A signature feature of these complexes is the use of bulky chelating ligands which act as spectators providing kinetic stabilization to their highly reactive Al-M bonds. Here we report the synthesis of novel Al/Zn bimetallics prepared by the selective formal insertion of AlCp* into the Zn-N bond of the utility zinc amides ZnR2 (R=HMDS, hexamethyldisilazide; or TMP, 2,2,6,6-tetramethylpiperidide). By systematically assessing the reactivity of the new [(R)(Cp*)AlZn(R)] bimetallics towards carbodiimides, structural and mechanistic insights have been gained on their ability to undergo insertion in their Zn-Al bond. Disclosing a ligand effect, when R=TMP, an isomerization process can be induced giving [(TMP)2AlZn(Cp*)] which displays a special reactivity towards carbodiimides and carbon dioxide involving both its Al-N bonds, leaving its Al-Zn bond untouched.

Theoretical Analysis of Polynuclear Zinc Complexes Isolobally Related to Hydrocarbons

Based on the isolobal analogy of ZnCp (Cp = η⁵-C₅H₅) and ZnR (R = alkyl or aryl group) fragments with hydrogen atom and fragment [Zn(CO)₂] with a CH₂ carbene, the following complexes [(ZnCp)₂{µ-Zn(CO)₂}], 1, [(ZnPh)₂{µ-Zn(CO)₂}], 2, [(ZnPh){µ-Zn(CO)₂}(ZnCp)], 3, [(ZnCp)₂{µ-Zn₂(CO)₄}], 4, [(ZnPh)₂{µ-Zn₂(CO)₄}], 5, [(ZnPh){µ-Zn(CO)₂}₂(ZnCp)], 6, [Zn₃(CO)₆], 7 and [Zn₅(CO)₁₀], 8, were built. These polynuclear zinc compounds are isolobally related to simple hydrocarbons (methane, ethane, cyclopropane and cyclopentane). They have been studied by density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) to compare the nature and topology of the Zn-Zn bond with previous studies. There are bond critical points (BCPs) between each pair of adjacent Zn centers in complexes 1-8 with Zn-Zn distances within the range 2.37-2.50 Å. The nature of the Zn-Zn bond in these complexes can be described as polar rather than pure covalent bonds. Although in a subtle way, the presence of different ligands and zinc oxidation states introduces asymmetry and polarity in the Zn-Zn bond. In addition, the Zn-Zn bond is delocalized in nature in complex 7 whereas it can be described as a localized bond for the remaining zinc complexes here studied.

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.

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.

Propane Dehydrogenation Catalyzed by Isolated Pt Atoms in ≡SiOZn-OH Nests in Dealuminated Zeolite Beta

Atomically dispersed noble metal catalysts have drawn wide attention as candidates to replace supported metal clusters and metal nanoparticles. Atomic dispersion can offer unique chemical properties as well as maximum utilization of the expensive metals. Addition of a second metal has been found to help reduce the size of Pt ensembles in bimetallic clusters; however, the stabilization of isolated Pt atoms in small nests of nonprecious metal atoms remains challenging. We now report a novel strategy for the design, synthesis, and characterization of a zeolite-supported propane dehydrogenation catalyst that incorporates predominantly isolated Pt atoms stably bonded within nests of Zn atoms located within the nanoscale pores of dealuminated zeolite Beta. The catalyst is stable in long-term operation and exhibits high activity and high selectivity to propene. Atomic resolution images, bolstered by X-ray absorption spectra, demonstrate predominantly atomic dispersion of the Pt in the nests and, with complementary infrared and nuclear magnetic resonance spectra, determine a structural model of the nested Pt.

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.

Reactivity of [Pt(P t Bu3)2] with Zinc(I/II) Compounds: Bimetallic Adducts, Zn-Zn Bond Cleavage, and Cooperative Reactivity

Metal-only Lewis pairs (MOLPs) based on zinc electrophiles are particularly interesting due to their relevance to Negishi cross-coupling reactions. Zinc-based ligands in bimetallic complexes also render unique reactivity to the transition metals at which they are bound. Here we explore the use of sterically hindered [Pt(P t Bu3)2] (1) to access Pt/Zn bimetallic complexes. Compounds [(P t Bu3)2Pt → Zn(C6F5)2] (2) and [Pt(ZnCp*)6] (3) (Cp* = pentamethylcyclopentadienyl) were isolated by reactions with Zn(C6F5)2 and [Zn2Cp*2], respectively. We also disclose the cooperative reactivity of 1/ZnX2 pairs (X = Cl, Br, I, and OTf) toward water and dihydrogen, which can be understood in terms of bimetallic frustration.

Nickel(0)-Induced β-H Elimination of Magnesium Alkyls: Formation and Reactivity of Heterometallic Hydrides

We report the synthesis and reactivity of heterometallic Mg-Ni complexes with bridging hydrides. Treatment of magnesium monoalkyl complexes, which are supported by a tridentate β-diketiminato ligand bearing a pendent phosphine group, with nickel(0) reagent Ni(COD)₂ (COD: 1,5-cyclooctadiene) at a molar ratio of 2:1 resulted in the formation of a heterotrimetallic hydride-bridged [Mg-Ni-Mg] complex via facile elimination of the corresponding alkenes. A heterobimetallic hydride-bridged [Mg-Ni] complex served as an intermediate species for the formation of the [Mg-Ni-Mg] complex. Computational studies revealed that the reaction was initiated by coordination of nickel to magnesium followed by an alkyl group transfer. β-H elimination at the nickel center subsequently occurred to give the heterometallic hydride-bridged complex. Density functional theory analysis also highlighted a three-center two-electron interaction for the Mg-H-Ni unit. The hydride-bridged [Mg-Ni-Mg] complex showed diverse reactivity toward unsaturated small molecules. For instance, reactions with isocyanides provided heterometallic species by coordination of isocyanides to the nickel center, with no subsequent reduction detected. Isocyanides could also be dissociated at 80 °C. In contrast, hydromagnesiation occurred upon treatment of the heterotrimetallic hydride with carbodiimides, affording C₃-symmetric complexes with three heteroleptic magnesium mixed β-diketiminate/amidinate moieties. The hydride-bridged heterotrimetallic complex underwent dehydrogenation reaction with phenyl acetylene to produce an acetylide-bridged [Mg-Ni-Mg] complex.

Rationalizing the AlI -Promoted Oxidative Addition of C-C Versus C-H Bonds in Arenes

The factors controlling the oxidative addition of C-C and C-H bonds in arenes mediated by Al^I have been computationally explored by means of Density Functional Theory calculations. To this end, we compared the processes involving benzene, naphthalene and anthracene which are promoted by a recently prepared anionic Al^I -carbenoid. It is found that this species exhibits a strong tendency to oxidatively activate C-H bonds over C-C bonds, with the notable exception of benzene, where the C-C bond activation is feasible but only under kinetic control reaction conditions. State-of-the-art computational methods based on the combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis have been used to rationalize the competition between both bond activation reactions as well as to quantitatively analyze in detail the ultimate factors controlling these transformations.

Contrasting Structure and Bonding of a Copper-Rich and a Zinc-Rich Intermetalloid Cu/Zn Cluster

Reaction of the Cu(I) sources, [Cu₅](Mes)₅ and [(ⁱDipp)CuO^tBu] (Mes = mesityl; ⁱDipp = 1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-2-ylidene) with the Zn(I) complex [Zn₂](Cp*)₂ leads to a mixture of intermetallic Cu/Zn clusters with a distribution of species that is dependent on the stoichiometric ratio of the reactants, the reaction time, as well as the temperature. Systematic and careful investigation of the product mixtures rendered the isolation of two new clusters possible, i.e., the Zn-rich, red cluster 1, [CuZn₁₀](Cp*)₇ = [Cu(ZnZnCp*)₃(ZnCp*)₄], as well as the Cu-rich, dark-green cluster 2 [Cu₁₀Zn₂](Mes)₆(Cp*)₂. Structure and bonding of these two species was rationalized with the help of density functional theory calculations. Whereas 1 can be viewed as an 18-electron Cu center coordinated to four ZnCp* and three ZnZnCp* one-electron ligands (with some interligand bonding interaction), compound 2 is better to be described as a six-electron superatom cluster. This unusual electron count is associated with a prolate distortion from a spherical superatom structure. This unexpected situation is likely to be associated with the ZnCp* capping units that offer the possibility to strongly bind to the top and the bottom of the cluster in addition to the bridging mesityl ligands stabilizing the Cu core of the cluster.

Structural, Spectroscopic, and Chemical Bonding Analysis of Zn(II) Complex [Zn(sal)](H2O): Combined Experimental and Theoretical (NBO, QTAIM, and ELF) Investigation

The Zn(II) complex of salen-like scaffold [Zn(sal)](H2O) was synthesized and characterized by elemental analysis, IR, UV–Vis, and 1H-NMR spectroscopic techniques. The structure of complex was confirmed by single crystal X-ray diffraction studies. In the complex, Zn (II) was placed in the inner N2O2 compartment of the salen scaffold in square planar geometry and crystallized in the monoclinic space group P21/n. DFT and TDDFT calculations were performed to reproduce the experimentally observed structural and spectroscopic (IR and UV–vis) findings. The bonding of the Zn(II) framework in the [Zn(sal)](H2O) complex was explored in depth. The theoretical approaches employed were perturbation theory within the context of the natural bond orbital (NBO) framework, and quantum theory of atoms in molecule (QTAIM) and electron localization function (ELF) analysis. The study begins by delineating the difference between the NBO and QTAIM approaches. This paper thus exhibits the supportive nature of NBO theory and QTAIM in discussion of the bonding in the [Zn(sal)](H2O) complex, when both the methodologies are used in combination.

Understanding the role of frustrated Lewis pairs as ligands in transition metal-catalyzed reactions

The role of frustrated Lewis pairs (FLPs) as ligands in gold(i) catalyzed-reactions has been computationally investigated by using state-of-the-art density functional theory calculations. To this end, the nature of (P,B)-FLP-transition metal interactions in different gold(i)-complexes has been first explored in detail with the help of the energy decomposition analysis method, which allowed us to accurately quantify the so far poorly understood AuB interactions present in these species. The impact of such interactions on the catalytic activity of gold(i)-complexes has been then evaluated by performing the Au(i)-catalyzed hydroarylation reaction of phenylacetylene with mesitylene. With the help of the activation strain model of reactivity, the factors governing the higher activity of Au(i)-complexes having a FLP as a ligand as compared to that of the parent PPh₃ system have also been quantitatively identified.

Combined Experimental and Theoretical Study on Hampered Phosphine Dissociation in Heteroleptic Ni/Zn Complexes

Heterometallic Ni/Zn complexes can serve as molecular models for the semihydrogenation of acetylene catalyzed by heterogeneous Ni/Zn phases. Pursuing this target, we present the synthesis of the series [Ni(ZnCp*)_n(ZnMe)_n(PEt₃)_4-n] (n = 1-3; 1, 2, 3) which is obtained via E/Zn exchange from [Ni(ECp*)_n(PEt₃)_4-n] (n = 1-3, E = Al, Ga; P1, P2, P3). The isolation of the intermediate compound [Ni(GaCp*)(ZnCp*)(ZnMe)(PEt₃)₂] (2a) supports the assumption of a stepwise Ga/Zn exchange in the formation of 3. The dissociation behavior of PEt₃ in 2 and 3 was investigated experimentally using variable temperature (VT) UV-vis spectroscopy indicating suppressed phosphine dissociation in both cases. For comparison, the absorption spectra of the saturated and unsaturated compounds were calculated using time dependent DFT calculations (TDDFT). Energy decomposition analysis with the natural orbital for chemical valence extension (EDA NOCV) calculations shows a bond strengthening of the Ni-P bond by successive substitution of the phosphines with (ZnR)₂ units. The influence of different phosphines (PMe₃, PEt₃, PPh₃, P(OEt)₃) on Ni-P bond length and on Zn-Zn interactions in [Ni(ZnR)_2n(PR')_4-n] (R = Cp*, Me; R' = Me, Et, Ph, OEt) was also studied by DFT calculations. A correlation of increasing sterical demand of the phosphine ligand and a shortening of the Zn-Zn distances is observed.

Formation of Zn-Zn and Zn-Pd Bonded Complexes by Reactions of Terminal Zinc Hydrides with Pd(II) Species

Divalent palladium-induced homocoupling of terminal zinc hydrides to zinc-zinc bonded complexes was achieved herein. Reactions of zinc hydrides [LZnH] (L = CH₃C(2,6-ⁱPr₂C₆H₃N)CHC(CH₃)(N(CH₂)_nCH₂PPh₂); 1a: n = 1; 1b: n = 2) with 0.5 equiv of allyl(cyclopentadienyl)palladium(II) afforded heterotrinuclear [Zn₂Pd] complexes 3 containing direct Zn-Zn and Zn-Pd bonds, with concomitant elimination of propylene and cyclopentadiene. Complexes 3 were also accessed by the reactions of zinc hydrides 1 with allylpalladium(II) chloride with release of propylene and hydrogen chloride. Treatment of zinc hydrides 1 with 1 equiv of allyl(cyclopentadienyl)palladium(II) gave Zn-Pd bonded complex 5 by elimination of propylene, which can be transformed into heterotrinuclear complex 3 by further reaction with one additional molar equivalent of zinc hydrides. Heterobimetallic Zn-Pd complex 5b was found to be an effective catalyst in the hydrosilylation of benzaldehyde and its derivatives. Reaction of 5b with silane reagent Ph₂SiH₂ produced [Pd₂Si₂H₂] complex 8 with cleavage of the Pd-Zn bond, which served as an initiating species in the catalytic reaction. Complexes 4b, 5, and 8 in this study were characterized by X-ray diffraction.

All-zinc coordinated nickel-complexes as molecular mimics for NiZn catalyst surfaces, a density functional theory study

A prospective connection between Hume-Rothery inspired TM/E (TM = transition metal; E = Al, Ga, Zn) complexes and clusters with the related solid-state intermetallic TM/E compounds is presented with respect to the industrially relevant catalytic semihydrogenation of acetylene. The theoretical study dealing with [Ni(ER)_n(C₂H_x)_4-n] (x = 2, 4; R = CH₃, C₅Me₅,) calculated on the DFT level of theory shows intriguing structural and electronic properties of the examined complexes. Different Ni-E complexes show preferred binding of C₂H₂ over C₂H₄ in bridging positions between Ni and E depending on the [Ni(ER)_n] fragment. These findings render molecular TM/E systems, such as Ni/Zn, promising candidates to mimic key intermediates of intermetallic catalysts applied in heterogeneous hydrogenation reactions. We put these findings into the context of existing synthetic results and illustrate different experimental approaches to obtain compounds of the general formula [TM_aE_b](Cp*)_c(UHC)_d (UHC = unsaturated hydrocarbon ligands) as potential surface models.

Transition metal-induced dehydrogenative coupling of zinc hydrides

Transition metal-induced dehydrogenative homocoupling of zinc(ii) hydrides to a zinc–zinc bonded complex has been achieved.

Highly efficient palladium-catalysed carbon dioxide hydrosilylation employing PMP ligands

A series of zero-valent Pd complexes featuring PMP ligands (M = Li^I, Cu^I, Zn^II) is reported. An excellent activity in chemoselective CO₂ hydrosilylation producing silyl formate is observed (TOF_1/2 = 3000 h⁻¹).

Suppressed Phosphine Dissociation by Polarization Effects on the Donor-Acceptor Bonds in [Ni(PEt3)4- n(ECp*) n] (E = Al, Ga)

A series of heteroleptic complexes [Ni(PEt₃)_{4- n}(ECp*) _n] (E = Al, Ga, Cp* = pentamethylcyclopentadienyl, n = 0-4) was prepared and characterized by experimental as well as computational means. The series of compounds was studied with respect to ligand dissociation processes which are fundamental for reactivity. In contrast to the homoleptic complexes [Ni(PR₃) _n] phosphine dissociation is remarkably suppressed in the heteroleptic title complexes. Single crystal X-ray structures as well as density functional theory calculations reveal a gradual decrease of the Ni-PEt₃ distances with increasing number of coordinated group-13 ligands ECp*. Accordingly, variable-temperature UV-vis studies of [Ni(PEt₃)_{4- n}(AlCp*) _n] in solution indicate no ligand dissociation equilibrium for n ≥ 2. Energy decomposition analysis with the natural orbital for chemical valence extension shows higher Ni-P interaction energies for [Ni(PEt₃)_{4- n}(AlCp*) _n] than for [Ni(PEt₃)₄] which is mainly a result of an increase in columbic attraction forces induced by Ni-PEt₃ bond polarization upon ECp* coordination.

Stepwise synthesis of the heterotrimetallic chains [MRu2(dpa)4X2]0/1+ using group 7 to group 12 transition metal ions and [Ru2(dpa)4Cl]

The CoRu₂(dpa)₄Cl₂ (1) (dpa: 2,2'-dipyridylamide) is synthesized by the reaction of Ru₂(OAc)₄Cl and Co₃(dpa)₄Cl₂. By mixing 1 with NH₃, Co²⁺ can be removed and result in the formation of unique binuclear complex 4,0-Ru₂(dpa)₄Cl (2) featuring one coordination pocket supported by free pyridine groups. Hence, this complex can act as an outstanding precursor for the formation of heterotrimetallic chains with MRu₂ cores. A series of M-Ru₂⁵⁺ complexes (M = Co²⁺ (3), Ag⁺ (4), Mn²⁺ (5), Fe²⁺ (6), Zn²⁺ (7), Cd²⁺ (8), Pd²⁺ (9), Rh²⁺ (10), and Ir²⁺ (11)) were prepared and isolated, representing the most complete series of heterotrimetallic chains to date. All these metal string complexes are in a linear trimetallic framework helically wrapped by four dpa^- ligands, characterized by X-ray diffraction measurements. The bending of the trinuclear metal cores in RhRu₂ (10) and IrRu₂ (11) (∠Ru-Ru-Rh: 167.58° and ∠Ru-Ru-Ir: 167.61°) indicates that a heterometallic metal-metal bonds (Ru-Rh; Ru-Ir) are generated. The studies from DFT calculation of 10 and 11 coincide with the experimental results. Furthermore, the MRu₂⁵⁺ distances are regulated by the factors including the bonding force of M-pyridyl and the static repulsion between M and Ru₂⁵⁺ unit. Interestingly, the trend for these distances is in line with that observed in trans-M(py)₄Cl₂ complexes.