Mixed phosphine and group-13 metal ligator complexes [(PR3)aM(ECp*)b] (M = Mo, Ni; E = Ga, Al; R = C6H5, cyclo-C6H11, CH3)

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Metal-Metal Bonding in Late Transition-Metal [M2L5] Complexes: Exploring the Limits of the Isolobal Analogy between the CO and AlCp* Ligands

Late dinuclear transition-metal (especially group 10 and 11) homoleptic carbonyl complexes are elusive species and have so far not been isolated. A typical example is the 30-electron species [Ni₂(CO)₅], the structure and bonding of which is still debated. We show that, by using the AlCp* ligand (isolobal to CO), it is possible to isolate and fully characterize [Ni₂(AlCp*)₅] (1), which inspired us to revisit by DFT calculations, the bonding situation within [Ni₂L₅] (L = CO, AlCp*) and other isoelectronic species. The short Ni-Ni X-ray distance in 1 (2.270 Å) should not be attributed to the existence of a typical localized triple-bond between the metals, but rather to a strong through-bond interaction involving the three bridging ligands via their donating lone pairs and accepting π* orbitals. In contrast, in the isostructural 32-electron [Au₂(AlCp*)₅] (2) cluster an orbital with M-M antibonding and Al...Al bonding character is occupied, which is in accordance with the particularly long Au-Au distance (3.856 Å) and rather short Al...Al contacts between the bridging ligands (2.843 Å). This work shows that, unlike late transition-metal [M₂(CO)_x] species, stable [M₂(AlCp*)_x] complexes can be isolated, owing to the subtle differences between CO and AlCp*. We propose a similar approach for rationalizing the bonding in the emblematic 34 electron species [Fe₂(CO)₉].

A combinatorial coordination-modulated approach to all-hydrocarbon-ligated intermetallic clusters

The formation of Hume-Rothery-inspired intermetallic and all-hydrocarbon-ligated Ni/E clusters (E = Al, Ga) is studied. A library of organo-metallic complexes and small clusters is obtained when [Ni(cod)₂] is treated with ECp* in the presence of 3-hexyne (hex). While the alkyne reversibly coordinates side-on to the Ni/Ga species, it dimerizes at the Ni/Al species. The mass spectrometric monitoring of the reaction solutions provides insight into the chemical complexity generated by a combinatorial, coordination-modulated approach to control cluster nucleation and growth aiming at cluster size-focusing and selective synthesis of species such as [Ni₄Ga₄](Cp*)₄(hex)₂.

Formation of a Propeller-Shaped Ni4Ga3 Cluster Supported by Transmetalation of Cp* from Ga to Ni

The reactivity of GaCp* toward different Ni⁰ olefin complexes is investigated. The reaction of GaCp* with [Ni(cdt)] (cdt = all-trans-1,5,9-cyclododecatriene) leads to simple adduct formation and the 18 valence electron (ve) compound [Ni(GaCp*)(cdt)] (1). In contrast, [Ni₂(dvds)₃] (dvds = 1,1,3,3-tetramethyl-1,3-divinyldisiloxane) is converted to the undercoordinated and highly reactive 16 ve complex [Ni(GaCp*)(dvds)] (2), which represents an intermediate in the formation of the propeller-shaped M₇ cluster [Ni₄Ga₃](Cp*)₃(dvds)₂ (3). Extensive characterization of the latter compound by experimental and computational means reveals the Cp* transfer from Ga to Ni. Therefore, the title compound can be best expressed by the structural formula [(μ₂-GaCp*)(Ni₂)(μ₂-GaNiCp*)₂(dvds)₂]. The flexible dvds ligands stabilize this arrangement via alkene-Ni and O-Ga interactions. Furthermore, compound 2 exhibits a fast GaCp* ligand exchange with external GaCp*, which is rather unexpected for the [TM(ECp*)_a] compounds; they usually do not undergo substitution reactions with two electron donor ligands like CO, phosphines, or GaCp*.

An Alumanylyttrium Complex with an Absorption due to a Transition from the Al-Y Bond to an Unoccupied d-Orbital

The reaction between a dialkyl-substituted alumanyl anion and [Y(CH₂ SiMe₃ )₂ (thf)₃ ][BPh₄ ] resulted in the formation of (dialkylalumanyl)yttrium complex 2, which exhibits the first 2-center-2-electron (2 c-2 e) Al-Y bond. The ¹ H and ¹³ C NMR spectra of 2 together with an X-ray crystallographic analysis indicated a C_2v symmetrical structure. DFT calculations on 2 revealed that its LUMO consists of overlapping 3 p- and 4 d-orbitals of the Al and Y atoms, respectively, and that the HOMO-LUMO gap is narrow. The UV/Vis spectrum of 2 exhibited a visible absorption at 432 nm, which suggests that the strong σ-donating and π-accepting character of the three-coordinate dialkylalumanyl ligand generates a colored d⁰ -complex that does not contain any π-electrons.

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.

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.

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.

Zn···Zn interactions at nickel and palladium centers

The analogy between ZnR fragments and the hydrogen radical represents a fruitful concept in organometallic synthesis. The organozinc(ii) and -zinc(i) sources ZnMe₂ (Me = methyl) and [Zn₂Cp*₂] (Cp* = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp*), which can be trapped by transition metal complexes [L _a M], yielding [L _b (ZnR) _n ]. The addition of the dizinc compound [Zn₂Cp*₂] to coordinatively unsaturated [L _a M] by the homolytic cleavage of the Zn-Zn bond can be compared to the classic oxidative addition reaction of H₂, forming dihydride complexes [L _a M(H)₂]. It has also been widely shown that dihydrogen coordinates under preservation of the H-H bond in the case of certain electronic properties of the transition metal fragment. The σ-aromatic triangular clusters [Zn₃Cp*₃]⁺ and [Zn₂CuCp*₃] may be regarded as the first indication of this so far unknown, side-on coordination mode of [Zn₂Cp*₂]. With this background in mind the question arises if a series of complexes featuring the Zn₂M structural motif can be prepared exhibiting a (more or less) intact Zn-Zn interaction, i.e. di-zinc complexes which are analogous to non-classical dihydrogen complexes of the Kubas type. In order to probe this idea, a series of interrelated organozinc nickel and palladium complexes and clusters were synthesized and characterized as model compounds: [Ni(ZnCp*)(ZnMe)(PMe₃)₃] (1), [Ni(ZnCp*)₂(ZnMe)₂(PMe₃)₂] (2), [{Ni(CN ^t Bu)₂(μ₂-ZnCp*)(μ₂-ZnMe)}₂] (3), [Pd(ZnCp*)₄(CN ^t Bu)₂] (4) and [Pd₃Zn₆(PCy₃)₂(Cp*)₄] (5). The dependence of Zn···Zn interactions as a function of the ligand environments and the metal centers was studied. Experimental X-ray crystallographic structural data and DFT calculations support the analogy between dihydrogen and dizinc transition metal complexes.

The intermetalloid cluster [(Cp*AlCu)6H4], embedding a Cu6 core inside an octahedral Al6 shell: molecular models of Hume-Rothery nanophases

Defined molecular models for the surface chemistry of Hume-Rothery nanophases related to catalysis are very rare. The Al-Cu intermetalloid cluster [(Cp*AlCu)6H4] was selectively obtained from the clean reaction of [(Cp*Al)4] and [(Ph3PCuH)6]. The stronger affinity of Cp*Al towards Cu sweeps the phosphine ligands from the copper hydride precursor and furnishes an octahedral Al6 cage to encapsulate the Cu6 core. The resulting hydrido cluster M12H4 reacts with benzonitrile to give the stoichiometric hydrometalation product [(Cp*AlCu)6H3(N=CHPh)].

Colloidal nickel/gallium nanoalloys obtained from organometallic precursors in conventional organic solvents and in ionic liquids: noble-metal-free alkyne semihydrogenation catalysts

Efforts to replace noble-metal catalysts by low-cost alternatives are of constant interest. The organometallic, non-aqueous wet-chemical synthesis of various hitherto unknown nanocrystalline Ni/Ga intermetallic materials and the use of NiGa for the selective semihydrogenation of alkynes to alkenes are reported. Thermal co-hydrogenolysis of the all-hydrocarbon precursors [Ni(COD)2] (COD = 1,5-cyclooctadiene) and GaCp* (Cp* = pentamethylcyclopentadienyl) in high-boiling organic solvents mesitylene and n-decane in molar ratios of 1 : 1, 2 : 3 and 3 : 1 yields the nano-crystalline powder materials of the over-all compositions NiGa, Ni2Ga3 and Ni3Ga, respectively. Microwave induced co-pyrolysis of the same precursors without additional hydrogen in the ionic liquid [BMIm][BF4] (BMIm = 1-butyl-3-methyl-imidazolium) selectively yields the intermetallic phases NiGa and Ni3Ga from the respective 1 : 1 and 3 : 1 molar ratios of the precursors. The obtained materials are characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), IR, powder X-ray diffraction (PXRD) and atomic absorption spectroscopy (AAS). The single-source precursor [Ni(GaCp*)(PMe3)3] with a fixed Ni : Ga stoichiometry of 1 : 1 was employed as well. In comparison with the co-hydrogenolytic dual precursor source approach it turned out to be less practical due to inefficient nickel incorporation caused by the parasitic formation of stable [Ni(PMe3)4]. The use of ionic liquid [BMIm][BF4] as a non-conventional solvent to control the reaction and stabilize the nanoparticles proved to be particularly advantageous and stable colloids of the nanoalloys NiGa and Ni3Ga were obtained. A phase-selective Ni/Ga colloid synthesis in conventional solvents and in the presence of surfactants such as hexadecylamine (HDA) was not feasible due to the undesired reactivity of HDA with GaCp* leading to inefficient gallium incorporation. Recyclable NiGa nanoparticles selectively semihydrogenate 1-octyne and diphenylacetylene (tolan) to 1-octene and diphenylethylene, respectively, with a yield of about 90% and selectivities of up to 94 and 87%. Ni-NPs yield alkanes with a selectivity of 97 or 78%, respectively, under the same conditions.