Paul Jerabek

A Crystalline Singlet Phosphinonitrene: A Nitrogen Atom–Transfer Agent

N on P Nitrogen atoms form strong, relatively unreactive triple bonds with themselves (in N2) and with carbon (in cyanide and nitriles). In contrast, binding to transition metals often leaves an otherwise naked nitrogen center more prone to reactivity. Dielmann et al. (p. 1526) prepared a compound with nitrogen bound to divalent phosphorus, which acted more like a metal than a light element. Although the compound, formally a nitrene, was sufficiently stable to isolate at room temperature and characterize by x-ray diffraction, it transferred the nitrogen efficiently to unsaturated carbon compounds. A phosphorus fragment supports an otherwise uncoordinated nitrogen atom in a stable motif previously seen only with metals. A variety of transition metal–nitrido complexes (metallonitrenes) have been isolated and studied in the context of modeling intermediates in biological nitrogen fixation by the nitrogenase enzymes and the industrial Haber-Bosch hydrogenation of nitrogen gas into ammonia. In contrast, nonmetallic nitrenes have so far only been spectroscopically observed at low temperatures, despite their intermediacy in a range of organic reactions. Here, we report the synthesis of a bis(imidazolidin-2-iminato)phosphinonitrene, which is stable at room temperature in solution and can even be isolated in the solid state. The bonding between phosphorus and nitrogen is analogous to that observed for metallonitrenes. We also show that this nitrido phosphorus derivative can be used to transfer a nitrogen atom to organic fragments, a difficult task for transition metal–nitrido complexes.

Coinage metals binding as main group elements: structure and bonding of the carbene complexes [TM(cAAC)2] and [TM(cAAC)2](+) (TM = Cu, Ag, Au).

Quantum chemical calculations using density functional theory have been carried out for the cyclic (alkyl)(amino)carbene (cAAC) complexes of the group 11 atoms [TM(cAAC)2] (TM = Cu, Ag, Au) and their cations [TM(cAAC)2](+). The nature of the metal-ligand bonding was investigated with the charge and energy decomposition analysis EDA-NOCV. The calculations show that the TM-C bonds in the charged adducts [TM(cAAC)2](+) are significantly longer than in the neutral complexes [TM(cAAC)2], but the cations have much higher bond dissociation energies than the neutral molecules. The intrinsic interaction energies ΔEint in [TM(cAAC)2](+) take place between TM(+) in the (1)S electronic ground state and (cAAC)2. In contrast, the metal-ligand interactions in [TM(cAAC)2] involve the TM atoms in the excited (1)P state yielding strong TM p(π) → (cAAC)2 π backdonation, which is absent in the cations. The calculations suggest that the cAAC ligands in [TM(cAAC)2] are stronger π acceptors than σ donors. The trends of the intrinsic interaction energies and the bond dissociation energies of the metal-ligand bonds in [TM(cAAC)2] and [TM(cAAC)2](+) give the order Au > Cu > Ag. Calculations at the nonrelativistic level give weaker TM-C bonds, particularly for the gold complexes. The trend for the bond strength in the neutral and charged adducts without relativistic effects becomes Cu > Ag > Au. The EDA-NOCV calculations suggest that the weaker bonds at the nonrelativistic level are mainly due to stronger Pauli repulsion and weaker orbital interactions. The NBO picture of the C-TM-C bonding situation does not correctly represent the nature of the metal-ligand interactions in [TM(cAAC)2].

Isolation of Neutral Mononuclear Copper Complexes Stabilized by Two Cyclic (Alkyl)(amino)carbenes

Two (cAAC)2Cu complexes, featuring a two-coordinate copper atom in the formal oxidation state zero, were prepared by reducing (Et2-cAAC)2Cu(+)I(-) with metallic sodium in THF, and by a one-pot synthesis using Me2-cAAC, Cu(II)Cl2, and KC8 in toluene in a molar ratio of 2:1:2, respectively. Both complexes are highly air and moisture sensitive but can be stored in the solid state for a month at room temperature. DFT calculations showed that in these complexes the copper center has a d(10) electronic configuration and the unpaired electron is delocalized over two carbene carbon atoms. This was further confirmed by the EPR spectra, which exhibit multiple hyperfine lines due to the coupling of the unpaired electron with (63,65)Cu isotopes, (14)N, and (1)H nuclei.

Experimental Charge Density Study of a Silylone

An experimental and theoretical charge density study confirms the interpretation of (cAAC)2Si as a silylone to be valid. Two separated VSCCs present in the non-bonding region of the central silicon are indicative for two lone pairs. In the experiment, both the two crystallographically independent Si-C bond lengths and ellipticities vary notably. It is only the cyclohexyl derivative that shows significant differences in these values, both in the silylones and the germylones. Only by calculating increasing spheres of surrounding point charges we were able to recover the changes in the properties of the charge density distribution caused by weak intermolecular interactions. The nitrogen-carbene-carbon bond seems to have a significant double-bond character, indicating a singlet state for the carbene carbon, which is needed for donor acceptor bonding. Thus the sum of bond angles at the nitrogen atoms seems to be a reasonable estimate for singlet versus triplet state of cAACs.

The σ‐Aromatic Clusters [Zn3]+ and [Zn2Cu]: Embryonic Brass

The triangular clusters [Zn3Cp*3](+) and [Zn2CuCp*3] were obtained by addition of the in situ generated, electrophilic, and isolobal species [ZnCp*](+) and [CuCp*] to Carmonas compound, [Cp*Zn-ZnCp*], without splitting the ZnZn bond. The choice of non-coordinating fluoroaromatic solvents was crucial. The bonding situations of the all-hydrocarbon-ligand-protected clusters were investigated by quantum chemical calculations revealing a high degree of σ-aromaticity similar to the triatomic hydrogen ion [H3](+). The new species serve as molecular building units of Cu(n)Zn(m) nanobrass clusters as indicated by LIFDI mass spectrometry.

[3+2] Fragmentation of an [RP5Cl]+ Cage Cation Induced by an N‐Heterocyclic Carbene

The cage compound [DippP5 Cl][GaCl4 ] (Dipp=2,6-diisopropylphenyl) reacts with an NHC (N-heterocyclic carbene) by an unprecedented [3+2] fragmentation of the P5 (+) core. This yields an imidazoliumyl-substituted P3 species featuring a triphosphaallyl anion motif and a neutral P2 compound. The mechanism of the fragmentation reaction was elucidated by means of experimental and quantum chemical methods.

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.

Nearly Degenerate Isomers of C(BH)2: Cumulene, Carbene, or Carbone?

The ground electronic state of C(BH)2 exhibits both a linear minimum and a peculiar angle-deformation isomer with a central B-C-B angle near 90°. Definitive computations on these species and the intervening transition state have been executed by means of coupled-cluster theory including single and double excitations (CCSD), perturbative triples (CCSD(T)), and full triples with perturbative quadruples (CCSDT(Q)), in concert with series of correlation-consistent basis sets (cc-pVXZ, X=D, T, Q, 5, 6; cc-pCVXZ, X=T, Q). Final energies were pinpointed by focal-point analyses (FPA) targeting the complete basis-set limit of CCSDT(Q) theory with auxiliary core correlation, relativistic, and non-Born-Oppenheimer corrections. Isomerization of the linear species to the bent form has a minuscule FPA reaction energy of 0.02 kcal mol(-1) and a corresponding barrier of only 1.89 kcal mol(-1). Quantum tunneling computations reveal interconversion of the two isomers on a timescale much less than 1 s even at 0 K. Highly accurate CCSD(T)/cc-pVTZ and composite c~CCSDT(Q)/cc-pCVQZ anharmonic vibrational frequencies confirm matrix-isolation infrared bands previously assigned to linear C(BH)2 and provide excellent predictions for the heretofore unobserved bent isomer. Chemical bonding in the C(BH)2 species was exhaustively investigated by the atoms-in-molecules (AIM) approach, molecular orbital plots, various population analyses, local mode vibrations and force constants, unified reaction valley analysis (URVA), and other methods. Linear C(BH)2 is a cumulene, whereas bent C(BH)2 is best characterized as a carbene with little carbone character. Weak B-B attraction is clearly present in the unusual bent isomer, but its strength is insufficient to form a CB2 ring with a genuine boron-boron bond and attendant AIM bond path.

Dizinc Cation [Zn2]2+ Trapped In A Homoleptic Metalloid Coordination Environment Stabilized by Dispersion Forces: [Zn2(GaCp*)6][BAr4F]2

The synthesis and characterization of the cationic mixed metal Ga/Zn cluster [Zn2(GaCp*)6](2+) (1) is presented. The reaction of [Zn2Cp*2] with [Ga2Cp*][BAr4(F)] leads to the formation of the novel complex being the first example of a [Zn2](2+) core exclusively ligated by metalloid group-13 organyl-ligands. Compound 1 exhibits two different coordination modes: In the solid state, two of the six GaCp* ligands occupy bridging positions, whereas VT (1)H NMR indicates the coexistence of a second isomer in solution featuring six terminal GaCp* ligands. Quantum chemical calculations have been carried out to assign the gallium and zinc positions; the bonding situation in 1 is characterized and the importance of dispersion forces is discussed.

The Organozinc Rich Compounds [Cp*M(ZnR)5] (M = Fe, Ru; R = Cp*, Me, Cl, Br)

Organozinc (ZnR with R = Cp*, Me, Cl, Br) ligated transition metal (M) half-sandwich compounds of general formula [Cp*M(ZnR)5] (M = Fe, Ru) are presented in this work. The new compounds were obtained by treatment of various GaCp* ligated precursors with suitable amounts of ZnMe2 to exchange Ga against Zn. This exchange follows a strict Ga:Zn ratio of 1:2. Accordingly, a Ga/Zn mixed compound [{Cp*Ru(GaCp*)(ZnCp*)(ZnCl)2}2] can be obtained if the amount of ZnMe2 is reduced so that one GaCp* remains coordinated to the transition metal. All new compounds were characterized by elemental analysis, (1)H and (13)C NMR spectroscopy as well as by single crystal X-ray diffraction techniques, if applicable. The coordination polyhedra of [Cp*M(ZnR)5] can be derived from the pseudo homoleptic parent compound [Ru(ZnCp*)4(ZnMe)6], as emphasized by continuous shape measures analysis (CShM). Computational investigations at the density functional theory (DFT) level of theory were performed, revealing no significant attractive interaction of the zinc atoms and therefore these compounds are best described as classical complexes, rather than cluster compounds. The Ru-L bond strength follow the order Cp* > ZnCl > ZnMe > ZnCp*.