Moritz von Hopffgarten

Energy decomposition analysis

The energy decomposition analysis (EDA) is a powerful method for a quantitative interpretation of chemical bonds in terms of three major expressions. The instantaneous interaction energy ΔEint between two fragments A and B in a molecule A–B is partitioned in three terms, namely, (1) the quasiclassical electrostatic interaction ΔEelstat between the fragments, (2) the repulsive exchange (Pauli) interaction ΔEPauli between electrons of the two fragments having the same spin, and (3) the orbital (covalent) interaction ΔEorb, which comes from the orbital relaxation and the orbital mixing between the fragments. The latter term can be decomposed into contributions of orbitals with different symmetry, which makes it possible to distinguish between σ, π, and δ bonding. After a short introduction into the theoretical background of the EDA, we present illustrative examples of main group and transition metal chemistry. The results show that the EDA terms can be interpreted in a chemically meaningful way, thus providing a bridge between quantum chemical calculations and heuristic bonding models of traditional chemistry.

N-heterocyclic carbenes which readily add ammonia, carbon monoxide and other small molecules,

N-Heterocyclic carbenes (NHCs) are extremely valuable as nucleophilic organocatalysts. They are widely applied as ligands in transition-metal catalysed reactions, where they are known as particularly potent σ-donors. They are commonly viewed as workhorses exhibiting reliable, but undramatic, chemical behaviour. The N → Ccarbene π-donation stabilises NHCs at the expense of low reactivity towards nucleophiles. In contrast to NHCs, stable (alkyl)(amino)carbenes exhibit spectacular reactivity, allowing, for example, the splitting of hydrogen and ammonia and the fixation of carbon monoxide. NHCs have been judged to be electronically not suitable for showing similar reactivity. Here, we demonstrate that a ferrocene-based NHC is able to add ammonia, methyl acrylate, tert-butyl isocyanide, and carbon monoxide—reactions typical of (alkyl)(amino)carbenes, but unprecedented for diaminocarbenes. We also show that even the simplest stable diaminocarbene, C(NiPr2)2, adds CO. This reaction affords a β-lactam by a subsequent intramolecular process involving a C–H activation. Our results shed new light on the chemistry of diaminocarbenes and offer great potential for synthetic chemistry and catalysis.

Twelve One‐Electron Ligands Coordinating One Metal Center: Structure and Bonding of [Mo(ZnCH3)9(ZnCp*)3]

The highest coordination number for metal complexes of monodentate ligands has been nine since the days of Alfred Werner. [1] The term “complex” refers to a molecule [MLm] that features a central metal atom M that bonds to ligator atoms E of ligands L by donor–acceptor interactions to yield a core structure MEn. [2] Metal atoms can also become captured inside an electron-precise cage En. These compounds obey the Wade–Mingos rules and are referred to as endohedral clusters M@En, typically with n> 9. Examples for the latter are the recently synthesized [M@Pb10] 2 and [M@Pb12] 2 (M=Ni, Pd, Pt). Herein we describe the synthesis of an unprecedented molecule containing a MoZn12 core, which offers a novel linkage between coordination compounds and cluster molecules (Figure 1). At first glance, the icosahedral structure of the title molecule [MoZn12Me9Cp*3] (1; Me=CH3, Cp*=C5Me5) is reminiscent of the endohedral clusters described above. The actual bonding situation is, however, intriguingly different. Quantum chemical analysis revealed a unique situation best described as a perfectly sd-hybridized molybdenum atom that engages in six Mo Zn two-electron-three-center bonds. There are six high-lying valence molecular orbitals (MOs) occupied by 12 electrons that can clearly be identified as Mo Zn bonding. Another six electrons are delocalized over the Zn cage, evoking only weak Zn Zn interactions (Figures 2 and 3). Before discussing these aspects in detail, we briefly report the synthesis and the analytical and structural properties of 1. The title compound [MoZn12Me9Cp*3] (1) was reproducibly obtained in 82% yield by the treatment of [Mo(GaCp*)6] (2) with 14 equivalents of ZnMe2 in toluene at 110 8C over a period of 2 h. Two mixed Ga–Zn compounds [MoZn4Ga4Me4Cp*4] (3) and [MoZn8Ga2Me6Cp*4] (4) are intermediates of this reaction and were isolated in nearly quantitative yields using 4 and 8 equivalents of ZnMe2 (Scheme 1).

N-Heterocyclic carbenes versus transition metals for stabilizing phosphinyl radicals

It is shown that vanadium-iminato ligands are more efficient than imidazolidin-2-iminato substituents to delocalize the spin density from a phosphorus nucleus. However, the latter is stabilizing enough to allow for the isolation and characterization in the liquid and solid states of a neutral phosphinyl radical.

Molecular Alloys, Linking Organometallics with Intermetallic Hume-Rothery Phases: The Highly Coordinated Transition Metal Compounds [M(ZnR)n] (n ≥ 8) Containing Organo-Zinc Ligands

This paper presents the preparation, characterization and bonding analyses of the closed shell 18 electron compounds [M(ZnR)(n)] (M = Mo, Ru, Rh, Ni, Pd, Pt, n = 8-12), which feature covalent bonds between n one-electron organo-zinc ligands ZnR (R = Me, Et, eta(5)-C(5)(CH(3))(5) = Cp*) and the central metal M. The compounds were obtained in high isolated yields (>80%) by treatment of appropriate GaCp* containing transition metal precursors 13-18, namely [Mo(GaCp*)(6)], [Ru(2)(Ga)(GaCp*)(7)(H)(3)] or [Ru(GaCp*)(6)(Cl)(2)], [(Cp*Ga)(4)RhGa(eta(1)-Cp*)Me] and [M(GaCp*)(4)] (M = Ni, Pd, Pt) with ZnMe(2) or ZnEt(2) in toluene solution at elevated temperatures of 80-110 degrees C within a few hours of reaction time. Analytical characterization was done by elemental analyses (C, H, Zn, Ga), (1)H and (13)C NMR spectroscopy. The molecular structures were determined by single crystal X-ray diffraction. The coordination environment of the central metal M and the M-Zn and Zn-Zn distances mimic the situation in known solid state M/Zn Hume-Rothery phases. DFT calculations at the RI-BP86/def2-TZVPP and BP86/TZ2P+ levels of theory, AIM and EDA analyses were done with [M(ZnH)(n)] (M = Mo, Ru, Rh, Pd; n = 12, 10, 9, 8) as models of the homologous series. The results reveal that the molecules can be compared to 18 electron gold clusters of the type M@Au(n), that is, W@Au(12), but are neither genuine coordination compounds nor interstitial cage clusters. The molecules are held together by strong radial M-Zn bonds. The tangential Zn-Zn interactions are generally very weak and the (ZnH)(n) cages are not stable without the central metal M.

The Reactivity of [Zn2Cp*2]: Trapping Monovalent {.ZnZnCp*} in the Metal‐Rich Compounds [(Pd,Pt)(GaCp*)a(ZnCp*)4−a(ZnZnCp*)4−a] (a=0, 2)

Carmona s synthesis of [Zn2Cp*2] (Cp* = pentamethylcyclopentadienyl), the first molecular compound exhibiting a covalent Zn Zn bond, has generated much interest and stimulated research on low-coordinate (main-group) metal compounds. Other derivatives with the formula [M2L2], such as [Zn2{HC(CMeNAr)2}2] (Ar = 2,6-iPr2C6H3) or [Zn2Ar2] (Ar = 2,6-(2,6-iPr2C6H3)2C6H3) were subsequently obtained, and even magnesium analogues, such as [Mg2(DippNacnac)2] (DippNacnac = [(2,6-iPr2C6H3)N= CMe]2CH) have been reported. [4–7] Notably, Robinson s concept of using N-heterocyclic carbenes (NHCs) as neutral, soft, and very bulky ligands for stabilizing unusual bonding states, for example, [LDE=EDL] (E = Si, Ge; LD=DC[N(2,6-iPr2C6H3)CH]2) relates to this progress. [8] The quite well-developed coordination chemistry of the carbenoid Group 13 metal analogues of NHC ligands, ER (E = Al, Ga, In; R = Cp* and other bulky substituents) to metal centers complements this progress. 10] Nevertheless, not much is known on the chemistry of the compounds [M2L2] in general, [6,8] and only very few reports have appeared for reactions of the zinc dimers in particular. 5, 7] For example, [Zn2Cp*2] should behave as a natural source for the monovalent species CZnCp*, which in essence contains Zn. In fact, only a few transition metal (TM) complexes with one-electron ligands CZnR are known (R = Cp*, CH3). Recently, we established an access to very zinc-rich, highly coordinated [TM(ZnR)n] compounds (TM: Group 6–10 element, n = 8–12) bridging the gap between complexes, clusters, and Hume-Rothery intermetallic phases, with the icosahedral [Mo(ZnCp*)3(ZnMe)9] as prototype of a novel family. [11,12] The formation reaction starts from mononuclear complexes [TM(GaCp*)m] (m = 4–6) and ZnR2 (R = Me, Et) and involves Ga/Zn and Cp*/R exchange processes. In the course of the reaction, Zn is reduced to Zn by Ga, which ends up as Ga and causes the overall substitution of one two-electron GaCp* ligand by two one-electron ZnR ligands at the TM center. If inert co-ligands at TM are present, other unusual and high nuclearity clusters, such as [Mo4(CO)12Zn6(ZnCp*)4], may be formed, which reveal close similarities to structural motifs of Mo/Zn intermetallic phases. Herein, we present the first results of our ongoing study on reactions of [Zn2Cp*2] with [LaTMb(GaCp)*c]. Most interestingly, we found the fragment {ZnZnCp*} with the intact covalent Zn Zn linkage being trapped as a one-electron ligand in the coordination sphere of a transition metal. Treatment of [Pd(GaCp*)4] with four equivalents of [Zn2Cp*2] in toluene at 95 8C over a period of 2 h leads to the quantitative formation of a mixture of the six-coordinate complex [Pd(GaCp*)2(ZnCp*)2(ZnZnCp*)2] (1) and the eight-coordinate complex [Pd(ZnCp*)4(ZnZnCp*)4] (2) in a molar ratio of 6:1, as revealed by in situ NMR spectroscopy (Scheme 1). Orange crystals of 1 and red needle-shaped crystals of 2 deposit from a saturated toluene solution at

Chemical Bonding in the Inclusion Complex of He in Adamantane, He@adam: Antithesis and Complement

In the preceding paper in this issue Strenalyuk and Haaland discuss the bonding situation in the inclusion complex of He in adamantane, He@adam. Based on common chemical reasoning, the authors use immaculately valid arguments which let them conclude that the interatomic interactions between the helium atom and the carbon atoms of the adamantane cage are repulsive. This is in apparent contradiction to the statements which were made by Bader and Fang, who suggested that there are He C chemical bonds between the helium atom and the four tertiary CH carbon atoms of adamantane. In the following we want to show that the conflicting views arise from different perspectives which are valid within their own scope of definition. In order to make our arguments easy to follow we will use the notation and numbers which are introduced in the paper by Strenalyuk and Haaland. In the following we will thus assume that the reader is familiar with their work. For a discussion of the nature of the interatomic interactions and particularly for addressing the question if such interactions are attractive or repulsive, it is crucial to define an atom in a molecule. In the first part of their paper, Strenalyuk and Haaland do not explicitly give a definition of an atom. They rather discuss calculated energy values which show that the inclusion complex He@adam is 645 kJ mol 1 higher in energy than He + adam. They also show that the distortion of the adam cage from the equilibrium structure to the geometry in the complex is only 64 kJ mol 1 while the insertion of the He atom into the distorted adam requires 581 kJ mol . It follows that the higher energy of He@adam relative to He + adam comes mainly from the interactions between He and the adamantane cage. The same conclusion is reached when the helium atom is first inserted into the adam cage which then relaxes to He@adam. The authors divide the total destabilization energy of He@adam by four which gives a positive value of 161 kJ mol . This value is identified as the mean interaction energy He···C between helium and the tertiary carbon atoms which is, according to the work of Bader and Fang, however, attractive. The interaction of He···H3CH calculated with the same He···C distance as in He@adam gives a similar positive value of 178 kJ mol . Since the former species, which is not a minimum on the potential energy surface, spontaneously dissociates if the He–H3CH distance relaxes during the geometry optimization, the authors conclude that it is solely the presence of the C C bonds in He@adam which prevents spontaneous dissociation. In the following we will argue that the existence of He C bonds in He@adam does not contradict the statement that the spontaneous dissociation of He is prevented by the C C bonds of the adamantane cage, and that the instantaneous dissociation of He from He···H3CH also does not conflict with an attractive He–C interaction. In order to illustrate our case we first show in Figure 1a the contour line diagram of the Laplacian 51(r) of He@adam in the plane containing two tertiary CH carbon atoms, two secondary CH2 carbon atoms and helium. Figure 1b shows the Laplacian 51(r) of free adamantane in the same plane. Figure 1a nicely shows the zero-flux surface around He in the molecular plane which completely encapsulates the caged helium atom. The atomic basin within the boundaries of the zero-flux surface defines the atom in the molecule. Bader has shown that an atom in a molecule defined by the AIM obeys physical principles such as the virial theorem. Figure 1a shows also the He C bond paths which according to the AIM theory suggest that there are four helium carbon bonds in He@adam, two of them being displayed in Figure 1a. A visual comparison of the Laplacian of He@adam with that of free adamantane shown in Figure 1b does not indicate major changes which is misleading, however. In the preceding paper by Strenalyuk and Haaland it is shown (Table 1 in that paper) that the total energy of the helium atom calculated by the AIM is significantly lowered while the energy of the carbon atoms and to a lesser extent also the energy of the hydrogen atoms increase. The sum of [a] Dipl.-Chem. M. von Hopffgarten, Prof. Dr. G. Frenking Fachbereich Chemie, Philipps-Universit t Marburg Hans-Meerwein-Strasse, 35032 Marburg (Germany) Fax: (+49) 6421 2825566 E-mail : frenking@chemie.uni-marburg.de

Structure and Bonding of Metal-Rich Coordination Compounds Containing Low Valent Ga(I) and Zn(I) Ligands

Recent developments in the field of metal-rich low valent metal complexes with gallium(I)/zinc(I) ligands and their structural features are reviewed together with related theoretical calculations. Some emphasis is given to sterically encumbering NHC analogous ligands as well as the naked ions E+. The chemistry of organo Zn(I) ligands at transition metals is reviewed in the light of the recently discovered synthetic approach via suitable organo Ga(I) complexes as starting materials.

Oligonuclear Molecular Models of Intermetallic Phases: A Case Study on [Pd2Zn6Ga2(Cp*)5(CH3)3]

The synthesis, characterization, and theoretical investigation by means of quantum-chemical calculations of an oligonuclear metal-rich compound are presented. The reaction of homoleptic dinuclear palladium compound [Pd(2)(μ-GaCp*)(3)(GaCp*)(2)] with ZnMe(2) resulted in the formation of unprecedented ternary Pd/Ga/Zn compound [Pd(2)Zn(6)Ga(2)(Cp*)(5)(CH(3))(3)] (1), which was analyzed by (1)H and (13)C NMR spectroscopy, MS, elemental analysis, and single-crystal X-ray diffraction. Compound 1 consisted of two C(s)-symmetric molecular isomers, as revealed by NMR spectroscopy, at which distinct site-preferences related to the Ga and Zn positions were observed by quantum-chemical calculations. Structural characterization of compound 1 showed significantly different coordination environments for both palladium centers. Whilst one Pd atom sat in the central of a bi-capped trigonal prism, thereby resulting in a formal 18-valence electron fragment, {Pd(ZnMe)(2)(ZnCp*)(4)(GaMe)}, the other Pd atom occupied one capping unit, thereby resulting in a highly unsaturated 12-valence electron fragment, {Pd(GaCp*)}. The bonding situation, as determined by atoms-in-molecules analysis (AIM), NBO partial charges, and molecular orbital (MO) analysis, pointed out that significant Pd-Pd interactions had a large stake in the stabilization of this unusual molecule. The characterization and quantum-chemical calculations of compound 1 revealed distinct similarities to related M/Zn/Ga Hume-Rothery intermetallic solid-state compounds, such as Ga/Zn-exchange reactions, the site-preferences of the Zn/Ga positions, and direct M-M bonding, which contributes to the overall stability of the metal-rich compound.

Building a Bridge between Coordination Compounds and Clusters: Bonding Analysis of the Icosahedral Molecules [M(ER)12] (M = Cr, Mo, W; E = Zn, Cd, Hg)

The bonding situation of the icosahedral compounds [M(EH)(12)] (M = Cr, Mo, W; E = Zn, Cd, Hg), which are model systems for the isolated species [Mo(ZnCp*)(3)(ZnMe)(9)] possessing the coordination number 12 at the central atom M, have been analyzed with a variety of charge and energy decomposition methods (AIM, EDA-NOCV, WBI, MO). The results give a coherent picture of the electronic structure and the nature of the interatomic interactions. The compounds [M(EH)(12)] are transition metal complexes that possess 12 M-EH radial bond paths (AIM) that can be described as 6 three-center two-electron bonds (MO). The radial M-EH bonds come from the electron sharing interactions mainly between the singly occupied valence s and d AOs of the central atom M and the singly occupied EH valence orbitals (MO, EDA-NOCV). The orbital interactions provide ~42% of the total attraction, while the electrostatic attraction contributes ~58% to the metal-ligand bonding (EDA-NOCV). There is a weak peripheral E-E bonding in [M(EH)(12)] that explains the unusually high coordination number (MO). The peripheral bonding leads for some compounds [M(EH)(12)] to the emergence of E-E bond paths, while in others it does not (AIM). The relative strength of the radial and peripheral bonding in [Al(13)](-) and [Pt@Pb(12)](2-) is clearly different from the situation in [M(EH)(12)], which supports the assignments of the former species as cluster compounds or inclusion compounds (MO, WBI). The bonding situation in [WAu(12)] is similar to that in [M(EH)(12)].