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*.

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.

Synthesis of nickel/gallium nanoalloys using a dual-source approach in 1-alkyl-3-methylimidazole ionic liquids.

NiGa is a catalyst for the semihydrogenation of alkynes. Here we show the influence of different dispersion times before microwave-induced decomposition of the precursors on the phase purity, as well as the influence of the time of microwave-induced decomposition on the crystallinity of the NiGa nanoparticles. Microwave-induced co-decomposition of all-hydrocarbon precursors [Ni(COD)2] (COD = 1,5-cyclooctadiene) and GaCp* (Cp* = pentamethylcyclopentadienyl) in the ionic liquid [BMIm][NTf2] selectively yields small intermetallic Ni/Ga nanocrystals of 5 ± 1 nm as derived from transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and supported by energy-dispersive X-ray spectrometry (EDX), selected-area energy diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). NiGa@[BMIm][NTf2] catalyze the semihydrogenation of 4-octyne to 4-octene with 100% selectivity towards (E)-4-octene over five runs, but with poor conversion values. IL-free, precipitated NiGa nanoparticles achieve conversion values of over 90% and selectivity of 100% towards alkene over three runs.

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.

Single-Site, Organometallic Aluminum Catalysts for the Precise Group Transfer Polymerization of Michael-Type Monomers.

Unlike different types of Lewis pairs as polymerization catalysts for acrylic monomers, organometallic aluminum(III) compounds are reported that show a surprisingly high polymerization activity even without an additional Lewis base. DFT calculations, end group analysis and kinetic investigations clearly suggest a main group element (MGE) group transfer polymerization (GTP) mechanism analogous to the known metal-mediated GTP mechanism. The novel catalysts perform a precision polymerization of a broad variety of monomers, ranging from 2-isopropenyl-2-oxazoline to tert-butylmethacrylate and N,N-dimethylacrylamide. Additionally, extended Michael-type structures like 4-vinyl pyridine are accessible. Especially the Al(III) half-metallocenes show an almost quantitative initiator efficiency, and, combined with the living character of the polymerization reactions, they enable the synthesis of block copolymers, even with unconventional monomers like vinyl phosphonates.

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.

Synthesis of Ester- and Phosphonate-Functionalized AuI -Imidazolylidene Chlorides through the Isonitrile Route.

Starting from DMSAuCl, isonitriles and functionalized propargylammonium salts in the presence of simple trimethylamine as auxiliary base, unsymmetrically substituted ester- and phosphonate-functionalized AuI -imidazolylidene complexes were synthesized in an easy-to-use modular one-pot template synthesis. In the course of the reaction, after the initial nucleophilic addition of the amine to the gold(I)-activated isonitrile, a Michael addition closes the N-heterocyclic carbene (NHC) ring. Then the remaining double bond migrates into the NHC ring, evidently a more stable position than the initial exocyclic double bond. These functional groups attached to the back bone of the NHC ligands represent ideal handles for a further modification of the system, for example an attachment to larger assemblies or heterogenization by an attachment to surfaces are conceivable.

The electronic structure of VO in its ground and electronically excited states: A combined matrix isolation and quantum chemical (MRCI) study.

The electronic ground and excited states of the vanadium monoxide (VO) molecule were studied in detail. Electronic absorption spectra for the molecule isolated in Ne matrices complement the previous gas-phase spectra. A thorough quantum chemical (multi-reference configuration interaction) study essentially confirms the assignment and characterization of the electronic excitations observed for VO in the gas-phase and in Ne matrices and allows the clarification of open issues. It provides a complete overview over the electronically excited states up to about 3 eV of this archetypical compound.