Sandwich-like compounds based on the all-metal aromatic unit Al(4)2- and the main-group metals M (M=Li, Na, K, Be, Mg, Ca)

Inspired by the pioneering experimental characterisation of the all-metal aromatic unit Al(4)2- in the bimetallic molecules MAl4- (M=Li, Na, Cu) and by the very recent theoretical design of sandwich-type transition-metal complexes [Al4MAl4]q- (q=0-2; M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W), we used density functional theory (DFT) calculations (B3LYP/6-311+G(d) to design a series of novel non-transition-metal sandwich complexes based on the all-metal aromatic unit Al4(2-) and the main-group metals M (M=Li, Na, K, Be, Mg, Ca). The traditional homo-decked sandwich compounds [Al4MAl4]q- (without counterions) and (nM)q+[Al4MAl4]q- (with counterions M) (q=2-3, M=Li, Na, K, Be, Mg, Ca), although some of them are truly energy minima, have a much higher energy than many fused isomers. We thus concluded that it seems unlikely for Al4(2-) to sandwich the main-group metal atoms in the homo-decked sandwich form. Alternatively, we proposed a new type of sandwich complex, namely hetero-decked sandwich compounds [CpMAl4]q-, that are the ground-state structures for each M both with and without counterions. It was shown that with the rigid Cp- partner, the all-metal aromatic unit Al(4)2- might indeed act as a "superatom". These new types of all-metal aromatic unit-based sandwich complexes await future experimental verification.     

Sandwich-like complexes based on "all-metal" (Al 4 2-) aromatic compounds

We report a new sandwich-like structure (Al4TiAl42-) based on the recently synthesized all-metal aromatic Al42- squares. This structure has two aromatic squares (-2 charged each) trapping a titanium +2 cation. These all-metal aromatic squares have been detected by various metallic counterions, with a global charge of -1. Thus, we have also characterized a similar sandwich-like structure interacting with one and two sodium atoms with global charges of -1 and 0. The properties of these compounds strongly support the idea that they may be formed by Al42- aromatic squares.     

Vibrational corrections to geometries of transition metal complexes from density functional theory

Zero-point vibrational corrections are computed at the BP86/AE1 level for the set of 50 transition-metal/ligand bonds that have recently been proposed as testing ground for DFT methods, because of the availability of precise experimental gas-phase geometries (Bühl and Kabrede, J Chem Theory Comput 2006, 2, 1282). These corrections are indicated to be transferable to a large extent between various density-functional/basis-set combinations, so that they can be used to estimate zero-point averaged r0g distances from re values optimized at other theoretical levels. Applying this approach to a number of popular DFT levels does not, in general, improve their overall accuracy in terms of mean and standard deviations from experiment. The hybrid variant of the meta-functional TPSS is confirmed as promising choice for computing structures of transition-metal complexes.     

Tungsten Biscorroles: New Chiral Sandwich Compounds

The oxidative metalation method, involving the interaction of free‐base meso‐triarylcorroles and W(CO)₆ in refluxing decalin, led to a set of three tungsten(VI) biscorroles, the first homoleptic sandwich compounds involving corroles. Single‐crystal X‐ray structures of two of the complexes revealed square‐antiprismatic coordination and strongly domed corroles with long W?N distances of 2.15–2.22 Å and a substantial displacement of ～1.17 Å of the metal relative to the mean N₄ planes of the ligands. The structures correspond to approximate C₂ symmetry and are thus chiral. DFT calculations strongly indicate that the enantiomers are configurationally stable and hence amenable to chiral resolution. Their other notable properties include a strongly blueshifted Soret band at (357±2) nm, a relatively intense π→W(d ) near‐IR feature at (781±3) nm, and a low electrochemical HOMO–LUMO gap of approximately 1.3 V. The results obtained herein suggest that metallobiscorroles may emerge as a new class of inherently chiral chromophores with novel optical and electrochemical properties.     

Sandwich Compounds of Transition Metals with Cyclopolyenes and Isolobal Boron Analogues

A series of sandwich compounds of transition metals (M=Ni, Fe, Cr) with cyclic hydrocarbon (M(CH)_n) and borane (M(BH₂)_n), ligands (including mixed hydrocarbon/borane sandwiches) has been studied using density functional theory (B3LYP/6‐311+G(df,p)). Multicenter bonding between the central metal atom and basal cycloborane rings provides stabilization to planar cycloborane species. Large negative NICS values allude to aromatic character in the cycloboranes similar to the analogous cyclic hydrocarbons. The ability of cycloborane sandwiches to stabilize attached carbocations, radicals and carbanions is also assessed.     

Double Friedel-Crafts acylation reactions on the same ring of a metallocene: synthesis of a 2,5-diacetylphospharuthenocene

The synthetic outcome of the Friedel-Crafts acylation of 1',2',3,3',4,4',5'-heptamethylphospharuthenocene reflects the nature of the acylating agent, with alkanoyl anhydride/trifluoromethanesulfonic acid (TfOH) reagents giving monosubstitution at the phospholyl ring, whereas alkanoyl chloride/AlCl(3) gives 2,5-disubstitution. DFT calculations indicate that this unusual double acylation can be facilitated by the intervention of the phosphorus atom at an early stage in the reaction trajectory, with the acyl group being delivered from the phosphorus atom into the ring 2- or 2,5-positions.     

Reaction mechanisms of dissociative chemisorption of HI, I2, and CH3I on a magic cluster Al13-

We have investigated the transition-state structures and reaction mechanisms for the dissociative chemisorption reactions of HI, I(2), and CH(3)I on the magic cluster Al(-) (13). The HI, I(2), and CH(3)I molecules approach Al(-) (13) with an end-on orientation rather than a side-on orientation because of the more effective orbital overlap in the end-on orientation. The reactions of Al(-) (13) with HI and I(2) would produce Al(13)HI(-) and Al(13)I(2) (-), respectively, because of large exothermic energy changes and relatively small activation energies. The reaction of Al(-) (13) with CH(3)I is unlikely to take place because of the low mobility of CH(3) on Al(-) (13) and the high activation barrier for the S(N)2-type reaction. The dissociative chemisorption reactions are preferred thermodynamically to the abstractive chemisorption reactions.     

Instability of the Al4(2-) "all-metal aromatic" ion and its implications

Al42 - is a prototype structural unit of a new class of "all-metal aromatic" molecules. Without stabilizing counterions this species is unstable with respect to electron autodetachment in the gas phase. We estimated the height of the repulsive Coulomb barrier to approximately 2.7 eV and calculated a lifetime of 9 fs. This is a short lifetime: The only way to study the isolated dianion experimentally is to use electron scattering techniques. Investigations of the validity of bound-state quantum chemical calculations on the isolated species show that the results suffer from significant admixture of continuum states to the bound-state wave function depending on the basis set. Calculations of molecular properties can therefore give essentially arbitrary results for this ill-defined system, as is demonstrated for the energy and nuclear magnetic shieldings. This substantiates that results from calculations on the isolated dianion should be approached with caution.     

Sandwich Double‐Decker Lanthanide(III) “Intracavity” Complexes Based on Clamshell‐Type Phthalocyanine Ligands: Synthesis,Spectral, Electrochemical,and Spectroelectrochemical Investigations

Phthalocyanine compounds of novel type based on a bridged bis‐ligand, denoted “intracavity” complexes, have been prepared. Complexation of clamshell ligand 1,1′‐[benzene‐1,2‐diylbis(methanediyloxy)]bis[9(10),16(17),23(24)‐tri‐tert‐butylphthalocyanine] (^clam,tBuPc₂H₄, 1 ) with lanthanide(III) salts [Ln(acac)₃] ? n H₂O (Ln=Eu, Dy, Lu; acetylacetonate) led to formation of double‐deckers ^clam,tBuPc₂Ln ( 2 a – c ). Formation of high molecular weight oligophthalocyanine complexes was demonstrated as well. The presence of an intramolecular covalent bridge affecting the relative arrangement of macrocycles was shown to result in specific physicochemical properties. A combination of UV/Vis/NIR and NMR spectroscopy, MALDI‐TOF mass‐spectrometry, cyclic voltammetry, and spectroelectrochemistry provided unambiguous characterization of the freshly prepared bis‐phthalocyanines, and also revealed intrinsic peculiarities in the structure–property relationship, which were supported by theoretical calculations. Unexpected NMR activity of the paramagnetic dysprosium complex 2 b in the neutral π‐radical form was observed and examined as well.     

Elongated dihydrogen versus compressed dihydride complexes: the temperature dependence of the H-D spin-spin coupling constant as a criterion to distinguish between them

To be able to propose experimental tests to distinguish elongated dihydrogen transition-metal complexes from compressed dihydride transition-metal complexes, a thorough density functional study of the electronic structure in combination with quantum nuclear dynamics calculations have been performed for complexes [Cp*Ru(H2PCH2PCH2(H2)]+ and [CpRe(CO)2H2]. The results of this study suggest that elongated dihydrogen complexes and compressed dihydride complexes have different properties and that it should be possible to distinguish between them experimentally. In particular, different behavior is predicted with respect to 1) the sign of the isotope geometric effect on the H-H distance at 0 K, 2) the temperature dependence of the H-H distance, and 3) the temperature dependence of the H-D spin-spin coupling constant in 1H NMR spectroscopy.     

Aromaticity of tri- and tetranuclear metal-carbonyl clusters based on magnetic criteria

Recently, the sigma-aromaticity model proposed for cyclopropane by Dewar was employed to account for the stability of Group 8 trinuclear metal-carbonyl compounds [M(3)(CO)(12)] (M=Fe, Ru, Os). This paper further examines this hypothesis and provides the first quantitative evidence for the sigma-aromatic/antiaromatic nature of the [M(3)(CO)(12)]/[M(4)(CO)(16)] species based on structural and nucleus-independent chemical-shift analysis. In addition, the extent of electron delocalization in tetrahedral [M(4)(CO)(14)] and butterfly [M(4)(CO)(15)] is analyzed and compared to prototype cycloalkanes. While remarkable analogies exist between metal-carbonyls and cycloalkanes, transition metals provide additional overlap possibilities that affect both the ring strain and the magnetic properties of metal-carbonyl rings and cages.     

Dimers of Nineteen‐Electron Sandwich Compounds: Crystal and Electronic Structures,and Comparison of Reducing Strengths

The dimers of some Group 8 metal cyclopentadienyl/arene complexes and Group 9 metallocenes can be handled in air, yet are strongly reducing, making them useful n‐dopants in organic electronics. In this work, the X‐ray molecular structures are shown to resemble those of Group 8 metal cyclopentadienyl/pentadienyl or Group 9 metal cyclopentadienyl/diene model compounds. Compared to those of the model compounds, the DFT HOMOs of the dimers are significantly destabilized by interactions between the metal and the central C?C σ‐bonding orbital, accounting for the facile oxidation of the dimers. The lengths of these C?C bonds (X‐ray or DFT) do not correlate with DFT dissociation energies, the latter depending strongly on the monomer stabilities. Ru and Ir monomers are more reducing than their Fe and Rh analogues, but the corresponding dimers also exhibit much higher dissociation energies, so the estimated monomer cation/neutral dimer potentials are, with the exception of that of [RhCp₂]₂, rather similar (?1.97 to ?2.15 V vs. FeCp₂^+/0 in THF). The consequences of the variations in bond strength and redox potentials for the reactivity of the dimers are discussed.