Homoleptic hexa- and penta-coordinated gallium(I) amide complexes of ruthenium and molybdenum.

Reaction of neutral olefin complexes of ruthenium and molybdenum with GaTMP (TMP = 2,2,6,6-tetramethylpiperidinyl) by substitution leads to the formation of respective five- and six-coordinated homoleptic products. [Ru(GaTMP)5] (1) and [Mo(GaTMP)6] (2) were isolated and characterized. Core structure geometries were analyzed using continuous shape measure, and the complexes were subjected to DFT calculations unveiling competing π-interactions between the transition metal center and the amido substituent with the unoccupied pπ orbitals of the gallium.

All-Hydrocarbon-Ligated Superatomic Gold/Aluminum Clusters.

Key strategies in cluster synthesis include the use of modulating agents (e.g., coordinating additives). We studied the influence of various phosphines exhibiting different steric and electronic properties on the reduction of the Au(I) precursor to Au(0) clusters. We report a synthesis of the bimetallic clusters [Au6(AlCp*)6] = [Au6Al6](Cp*)6 (1) and [HAu7(AlCp*)6] = [HAu7Al6](Cp*)6 (2) (Cp* = pentamethylcyclopentadiene) using Au(I) precursors and AlCp*. The cluster [Au2(AlCp*)5] = [Au2Al5](Cp*)5 (3) was isolated and identified as an intermediate species in the reactions to 1 and 2. The processes of cluster growth and degradation were investigated by in situ 1H NMR and LIFDI-MS techniques. The structures of 1 and 2 were established by DFT geometry optimization. These octahedral clusters can both be described as closed-shell 18-electron superatoms.

Assignment of individual structures from intermetalloid nickel gallium cluster ensembles.

Poorly selective mixed-metal cluster synthesis and separation yield reaction solutions of inseparable intermetalloid cluster mixtures, which are often discarded. High-resolution mass spectrometry, however, can provide precise compositional data of such product mixtures. Structure assignments can be achieved by advanced computational screening and consideration of the complete structural space. Here, we experimentally verify structure and composition of a whole cluster ensemble by combining a set of spectroscopic techniques. Our study case are the very similar nickel/gallium clusters of M12, M13 and M14 core composition Ni6+xGa6+y (x + y ≤ 2). The rationalization of structure, bonding and reactivity is built upon the organometallic superatom cluster [Ni6Ga6](Cp*)6 = [Ga6](NiCp*)6 (1; Cp* = C5Me5). The structural conclusions are validated by reactivity tests using carbon monoxide, which selectively binds to Ni sites, whereas (triisopropylsilyl)acetylene selectively binds to Ga sites.

Catalytic Alkyne Semihydrogenation with Polyhydride Ni/Ga Clusters.

The bimetallic, decanuclear Ni3 Ga7 -cluster of the formula [Ni3 (GaTMP)3 (µ2 -GaTMP)3 (µ3 -GaTMP)] (1, TMP=2,2,6,6-tetramethylpiperidinyl) reacts reversibly with dihydrogen under the formation of a series of (poly-)hydride clusters 2. Low-temperature 2D NMR experiments at -80 °C show that 2 consist of a mixture of a di- (2Di ), tetra- (2Tetra ) and hexahydride species (2Hexa ). The structures of 2Di and 2Tetra are assessed by a combination of 2D NMR spectroscopy and DFT calculations. The cooperation of both metals is essential for the high hydrogen uptake of the cluster. Polyhydrides 2 are catalytically active in the semihydrogenation of 4-octyne to 4-octene with good selectivity. The example is the first of its kind and conceptually relates properties of molecular, atom-precise transition metal/main group metal clusters to the respective solid-state phase in catalysis.

Photochemically generated reactive sites at ruthenium/gallium complexes: catalysis vs. cluster growth.

Irradiation of [Ru(GaCp*)3(SiEt3)H3] (1) at 350 nm induces reductive elimination of dihydrogen and triethylsilane and generates unsaturated Ru/Ga species. This photochemically induced cascading reductive elimination processes generate the reactive intermediate [Ru(GaCp*)3], which can be trapped by diphosphine coordination to yield the stable complex [(dppe)Ru(GaCp*)3] (4). The photochemically generated RuGa3 species is catalytically active in the hydrogenation of alkynes, which is further investigated by 1H NMR and mass spectrometry. Formation of intermetallic Ru/Ga clusters is observed as a competing and for the catalytic activity of the species limiting side reaction.

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].

C-H and Si-H Activation Reactions at Ru/Ga Complexes: A Combined Experimental and Theoretical Case Study on the Ru-Ga Bond.

Treatment of [Ru(COD)(MeAllyl)2 ] and [Ru(COD)(COT)] with GaCp* under hydrogenolytic conditions leads to reactive intermediates which activate Si-H or C-H bonds, respectively. The product complexes [Ru(GaCp*)3 (SiEt3 )H3 ] (1) and [Ru(GaCp*)3 (C7 H7 )H3 ] (2) are formed with HSiEt3 or with toluene as the solvent, respectively. While 1 was isolated and fully characterized by NMR, MS, IR and SC-XRD, 2 was too labile to be isolated and was observed and characterized in situ by using mass spectrometry, including labelling experiments for the unambiguous assignment of the elemental composition. The structural assignment was confirmed by DFT calculations. The relative energies of the four isomers possible upon toluene activation at the ortho-, meta-, para- and CH3 -positions have been determined and point to aromatic C-H activation. The Ru-Ga bond was analyzed by EDA and QTAIM and compared to the Ru-P bond in the analogue phosphine compound. Bonding analyses indicate that the Ru-GaCp* bond is weaker than the Ru-PR3 bond.

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.

Intermetallic phases meet intermetalloid clusters.

In this article intermetalloid clusters of Cu-Zn, Cu-AI, Cu-Sn, and Cu-Pb are discussed. Intermetallic compounds based on these metal combinations are of the Hume-Rothery type with well-defined structures related to the valence electron count of the involved metals. Many Zintl-type and molecular clusters with these metals are known with remarkable structural parallels to the respective solid-state phases. On several examples, this article discusses intermetalloid clusters in terms of their metal core structures and relates them to structural principles in intermetallic solid-state phases. Also the syntheses of such clusters are addressed. Zintl-type and molecular clusters are most generally accessible from organometallic precursor complexes with redox processes between the different metals as an underlying synthesis concept.

Exploring Cu/Al cluster growth and reactivity: from embryonic building blocks to intermetalloid, open-shell superatoms.

Cluster growth reactions in the system [Cu5](Mes)5 + [Al4](Cp*)4 (Mes = mesitylene, Cp* = pentamethylcyclopentadiene) were explored and monitored by in situ LIFDI-MS and 1H-NMR. Feedback into experimental design allowed for an informed choice and precise adjustment of reaction conditions and led to isolation of the intermetallic cluster [Cu4Al4](Cp*)5(Mes) (1). Cluster 1 reacts with excess 3-hexyne to yield the triangular cluster [Cu2Al](Cp*)3 (2). The two embryonic [Cu4Al4](Cp*)5(Mes) and [Cu2Al](Cp*)3 clusters 1 and 2, respectively, were shown to be intermediates in the formation of an inseparable composite of the closely related clusters [Cu7Al6](Cp*)6 (3), [HCu7Al6](Cp*)6 (3H) and [Cu8Al6](Cp*)6 (4), which just differ by one Cu core atom. The radical nature of the open-shell superatomic [Cu7Al6](Cp*)6 cluster 3 is reflected in its reactivity towards addition of one Cu core atom leading to the closed shell superatom [Cu8Al6](Cp*)6 (4), and as well by its ability to undergo σ(C-H) and σ(Si-H) activation reactions of C6H5CH3 (toluene) and (TMS)3SiH (TMS = tris(trimethylsilyl)).

Enabling LIFDI-MS measurements of highly air sensitive organometallic compounds: a combined MS/glovebox technique.

A new setup combining a ThermoFisher Exactive Plus Orbitrap Mass Spectrometer with a liquid injection field desorption ionization (LIFDI) source directly connected to an inert atmosphere glovebox is presented. The described setup allows for the analysis of very air- and moisture sensitive samples. Furthermore, the soft nature of LIFDI ionization gives access to the molecular ions of fragile molecules. This new setup is therefore especially useful for sensitive organometallic complexes. The functionality of the new setup is tested against [(Cp)2TiCl]˙, which is known for its notorious sensitivity to air and moisture. Its drastic colour change from green to orange upon exposure to air further supports the easy detection of traces of oxygen during the experiment. In addition, we applied this setup to the mass spectrometric analysis of the qualitative composition of a Cu/Al cluster mixture, which is not accessible by other analytical methods.

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)CuOtBu] (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.

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*.

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(C2Hx)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 [TMaEb](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.

The Mackay-Type Cluster [Cu43 Al12 ](Cp*)12 : Open-Shell 67-Electron Superatom with Emerging Metal-Like Electronic Structure.

The paramagnetic cluster [Cu43 Al12 ](Cp*)12 was obtained from the reaction of [CuMes]5 and [AlCp*]4 (Cp*=η5 -C5 Me5 ; Mes=mesityl). This all-hydrocarbon ligand-stabilized M55 magic atom-number cluster features a Mackay-type nested icosahedral structure. Its open-shell 67-electron superatom configuration is unique. Three unpaired electrons occupy weakly antibonding jellium states. The situation prefigures the formation of a conduction band, which is in line with the measured temperature-independent magnetism. Steric protection by twelve Cp* ligands suppresses the intrinsic polyradicalar reactivity of the Cu43 Al12 core.

Embryonic brass: pseudo two electron Cu/Zn clusters.

The isoelectronic M7 clusters [Cu3Zn4](Cp*)5 (1) and {[Cu2Zn5](Cp*)5}+ (2) are described. While 1 can be isolated only as a minor side product from the reaction of Cu(CH3CO2) with equimolar amounts of [Zn2Cp*2] with the trigonal cluster [CuZn2](Cp*)3 as the major product, 2 is available in acceptable yields from the reaction of [CuZn2](Cp*)3 with the Cp*Zn2-transfer-reagent [Cp*Zn2(Et2O)3][BAr4 F]. The trigonal bipyramidal Cu/Zn-clusters exhibit exceptional bonding situations: with formally only one skeleton electron pair they can be regarded as highly electron deficient. However, a detailed DFT analysis reveals that the cluster bonding is supported by 3d orbital contributions of the trigonal metal base unit. The data contribute to the development of an advanced tool-box for synthesis of Hume-Rothery intermetallic (e.g. brass) inspired clusters.

Diverse Reactivity of ECp* (E = Al, Ga) toward Low-Coordinate Transition Metal Amides [TM(N(SiMe3)2)2] (TM = Fe, Co, Zn): Insertion, Cp* Transfer, and Orthometalation.

The reactivity of the carbenoid group 13 metal ligands ECp* (E = Al, Ga) toward low valent transition metal complexes [TM(btsa)2] (TM = Fe, Co, Zn; btsa = bis(trimethylsilyl)amide) was investigated, revealing entirely different reaction patterns for E = Al and Ga. Treatment of [Co(btsa)2] with AlCp* yields [Cp*Co(µ-H)(Al(κ2-(CH2SiMe2)NSiMe3)(btsa))] (1) featuring an unusual heterometallic bicyclic structure that results from the insertion of AlCp* into the TM-N bond with concomitant ligand rearrangement including C-H activation at one amide ligand. For [Fe(btsa)2], complete ligand exchange gives FeCp*2, irrespective of the employed stoichiometric ratio of the reactants. In contrast, treatment of [TM(btsa)2] (TM = Fe, Co) with GaCp* forms the 1:1 and 1:2 adducts [(GaCp*)Co(btsa)2] (2) and [(GaCp*)2Fe(btsa)2] (3), respectively. The tendency of AlCp* to undergo Cp* transfer to the TM center appears to be dependent on the nature of the TM center: For [Zn(btsa)2], no Cp* transfer is observed on reaction with AlCp*; instead, the insertion product [Zn(Al(η2-Cp*)(btsa))2] (4) is formed. In the reaction of [Co(btsa)2] with the trivalent [Cp*AlH2], transfer of the amide ligands without further ligand rearrangement is observed, leading to [Co(µ-H)4(Al(η2-Cp*)(btsa))2] (5).

Atom-Precise Organometallic Zinc Clusters.

The bottom-up synthesis of organometallic zinc clusters is described. The cation {[Zn10](Cp*)6 Me}(+) (1) is obtained by reacting [Zn2 Cp*2] with [FeCp2][BAr4 (F)] in the presence of ZnMe2. In the presence of suitable ligands, the high reactivity of 1 enables the controlled abstraction of single Zn units, providing access to the lower-nuclearity clusters {[Zn9 ](Cp*)6} (2) and {[Zn8 ](Cp*)5 ((t) BuNC)3}(+) (3). According to DFT calculations, 1 and 2 can be described as closed-shell species that are electron-deficient in terms of the Wade-Mingos rules because the apical ZnCp* units that constitute the cluster cage do not have three, but only one, frontier orbitals available for cluster bonding. Zinc behaves flexibly in building the skeletal metal-metal bonds, sometimes providing one major frontier orbital (like Group 11 metals) and sometimes providing three frontier orbitals (like Group 13 elements).