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.     

Stability and Nonlinear Optical Response of Alkalides that Contain a Completely Encapsulated Superalkali Cluster

Guided by density functional theory (DFT) computations, a new series of superalkali‐based alkalides, namely FLi₂⁺(aza222)K^?, OLi₃⁺(aza222)K^?, NLi₄⁺(aza222)K^?, and Li₃⁺(aza222)K^? were designed with various superalkali clusters embedded into an aza222 cage‐complexant. These species possess diverse isomeric structures in which the encapsulated superalkalis preserve their identities and behave as alkali metal atoms. The results show that these novel alkalides possess larger complexation energies and enhanced hyperpolarizabilities (β₀) compared with alkali‐metal‐based and previous superalkali‐based clusters. Especially, a prominent structural dependence of β₀ is observed for these studied compounds. Hence, the geometric factors that affect the nonlinear optical (NLO) response of such alkalides is elucidated in detail in this work. This study not only provides novel candidates for alkalides, it also offers an effective way to enhance the NLO response and stability of alkalides.     

New routes to polymetallic clusters: fluoride-based tri-, deca-, and hexaicosametallic MnIII clusters and their magnetic properties

The syntheses, structures and magnetic properties of three new MnIII clusters, [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1), [Mn(0O6(OH)2(bta)8(py)8F8] (2) and [NHEt3]2[Mn3O(bta)6F3] (3), are reported (bta=anion of benzotriazole), thereby demonstrating the utility of MnF3 as a new synthon in Mn cluster chemistry. The "melt" reaction (100 degrees C) between MnF(3) and benzotriazole (btaH, C6H5N3) under an inert atmosphere, followed by dissolution in MeOH produces the cluster [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1) after two weeks. Complex 1 crystallizes in the triclinic space group P1, and consists of a complicated array of metal tetrahedra linked by mu3-O2- ions, mu3- and mu2-OH- ions, mu2-MeO- ions and mu2-bta- ligands. The "simpler" reaction between MnF3 and btaH in boiling MeOH (50 degrees C) also produces complex 1. If this reaction is repeated in the presence of pyridine, the decametallic complex [Mn10O6(OH)2(bta)8(py)8F8] (2) is produced. Complex 2 crystallizes in the triclinic space group P1 and consists of a "supertetrahedral" [Mn(III)10] core bridged by six mu3-O2- ions, two mu3-OH- ions, four mu2-F- ions and eight mu2-bta- ions. The replacement of pyridine by triethylamine in the same reaction scheme produces the trimetallic species [NHEt3]2[Mn3O(bta)6F3] (3). Complex 3 crystallises in the monoclinic space group P2(1)/c and has a structure analogous to that of the basic metal carboxylates of general formula [M3O(RCO2)6L3]0/+, which consists of an oxo-centred metal triangle with mu2-bta- ligands bridging each edge of the triangle and the fluoride ions acting as the terminal ligands. DC magnetic susceptibility measurements in the 300-1.8 K and 0.1-7 T ranges were investigated for all three complexes. For each, the value of chi(M)T decreases with decreasing temperatures; this indicates the presence of dominant antiferromagnetic exchange interactions in 1-3. For complex 1, the low-temperature value of chi(M)T is 10 cm(3) K mol(-1) and fitting of the magnetisation data gives S=4, g=2.0 and D=-0.90 cm(-1). For complex 2, the value of chi(M)T falls to a value of approximately 5.0 cm(3) K mol(-1) at 1.8 K, which is consistent with a small spin ground state. For the triangular complex 3, the best fit to the experimental chi(M)T versus T data was obtained for the following parameters: Ja = -5.01 cm(-1), Jb = +9.16 cm(-1) and g=2.00, resulting in an S=2 spin ground state. DFT calculations on 3, however, suggest an S=1 or S=0 ground state with J(a)=-2.95 cm(-1) and J(b)=-2.12 cm(-1). AC susceptibility measurements performed on 1 in the 1.8-4.00 K range show the presence of out-of-phase AC susceptibility signals, but no peaks. Low-temperature single-crystal studies performed on 1 on an array of micro-SQUIDS show the time- and temperature-dependent hysteresis loops indicative of single-molecule magnetism behaviour.     

Low-lying excited states and primary photoproducts of [Os3(CO)10(s-cis-L)] (L=cyclohexa-1,3-diene, buta-1,3-diene)] clusters studied by picosecond time-resolved UV/Vis and IR spectroscopy and by density functional theory

Combined picosecond transient absorption and time-resolved infrared studies were performed, aimed at characterising low-lying excited states of the cluster [Os(3)(CO)(10)(s-cis-L)] (L=cyclohexa-1,3-diene, 1) and monitoring the formation of its photoproducts. Theoretical (DFT and TD-DFT) calculations on the closely related cluster with L=buta-1,3-diene (2') have revealed that the low-lying electronic transitions of these [Os(3)(CO)(10)(s-cis-1,3-diene)] clusters have a predominant sigma(core)pi*(CO) character. From the lowest sigmapi* excited state, cluster 1 undergoes fast Os-Os(1,3-diene) bond cleavage (tau=3.3 ps) resulting in the formation of a coordinatively unsaturated primary photoproduct (1 a) with a single CO bridge. A new insight into the structure of the transient has been obtained by DFT calculations. The cleaved Os-Os(1,3-diene) bond is bridged by the donor 1,3-diene ligand, compensating for the electron deficiency at the neighbouring Os centre. Because of the unequal distribution of the electron density in transient 1 a, a second CO bridge is formed in 20 ps in the photoproduct [Os(3)(CO)(8)(micro-CO)(2)(cyclohexa-1,3-diene)] (1 b). The latter compound, absorbing strongly around 630 nm, mainly regenerates the parent cluster with a lifetime of about 100 ns in hexane. Its structure, as suggested by the DFT calculations, again contains the 1,3-diene ligand coordinated in a bridging fashion. Photoproduct 1 b can therefore be assigned as a high-energy coordination isomer of the parent cluster with all Os-Os bonds bridged.     

Titanium imido complexes of cyclooctatetraenyl ligands

Reaction of [Ti(NR)Cl2(py)3] (R=tBu or 2,6-iPr2C6H3) with K(2)[COT] (COT=C8H8) or Li2[COT'] (COT'=1,4-C8H6(SiMe3)2) gave the monomeric complexes [Ti(NR)(eta8-COT)] or [Ti(NR)(eta8-COT')], respectively. The pseudo-two coordinate, "pogo stick" geometry for these complexes is unique in both early transition-metal and cyclooctatetraenyl ligand chemistry. In contrast, reaction of [Ti(N-2,6-Me2C6H3)Cl2(py)3] with K2[COT] gave the mu-imido-bridged dimer [Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2]. It appears that as the steric bulk of the imido and C8 ring substituents are decreased, dimerisation becomes more favourable. Aryl imido COT complexes were also prepared by imido ligand exchange reactions between anilines and [Ti(NtBu)(eta(8)-COT)] or [Ti(NtBu)(eta(8)-COT')]. The complexes [Ti(NtBu)(eta(8)-COT)], [Ti(N-2,6-iPr2C6H3)2(eta8-COT)] and [Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2] have been crystallographically characterised. The electronic structures of both the monomeric and dimeric complexes have been investigated by using density functional theory (DFT) calculations and gas-phase photoelectron spectroscopy. The most striking aspect of the bonding is that binding to the imido nitrogen atom is primarily through sigma and pi interactions, whereas that to the COT or COT' ring is almost exclusively through delta symmetry orbitals. A DFT-based comparison between the bonding in [Ti(NtBu)(eta8-COT)] and the bonding in the previously reported late transition-metal "pogo stick"complexes [Os(NtBu)(eta6-C6Me6)], [Ir(NtBu)(eta5-C5Me5)] and [Ni(NO)(eta5-C5H5)] has also been undertaken.     

Photoisomerizable bipyridine ligands and macroligands: absorption, photoisomerization properties and theoretical study.

Two 2,2'-bipyridines, substituted at the 4,4'-positions by p-dialkylaminophenylazostyryl moieties p-R2N-C6H4-N=N-C6H4-CH=CH-[6 a, R2N=nBu2N; 6 b, R2N=(nBu)(C4H8OTHP)N; 6 c, R2N=(nBu)(C4H8OH)N], were successfully synthesized by using Wadworth-Emmons reactions. The X-ray structure of 6 a has been determined. Esterification of 6 c with 2-bromoisobutyroylbromide afforded 6 d. This ligand was used as an initiator for the living radical polymerization of methylmethacrylate (MMA) and gave rise to macroligand 6 e. Thin films of good optical quality were obtained by the spin-coating technique. Photoisomerization experiments were carried out on 6 a in solution and on 6 e in both solution and film, and the kinetics of photochemical (E/Z) and thermal (Z/E) isomerization were investigated. They were found to show Z-E back isomerization typical of aminoazobenzene-type rather than of push-pull-type molecules. Density functional theoretical (TD-DFT) calculations were performed on model compound 6 a' (R2N=Me2N) to understand the structural and electronic transitions of the corresponding E-E, E-Z and Z-Z isomers. It was found that the E-E isomer is almost planar as observed experimentally by X-ray diffraction, whereas the Z-Z isomer, which is 35.4 kcal mol(-1) less stable than the E-E isomer, is nonplanar. The theoretical studies also reveal that several transitions of pi-pi*, n-pi* and charge-transfer (CT) types, are involved in the long-wavelength transition of 6 a (E-E). The same observations can be made for the (Z-Z) isomer, and the TD-DFT simulated spectrum fits quite nicely to the experimental, reproducing and explaining the apparition of a blue-shifted charge-transfer band at 390 nm.     

Protonation of water clusters induced by hydroperoxyl radical surface adsorption

We have investigated the HO(2) adsorption and acid dissociation process on the surface of (H(2)O)(20) and (H(2)O)(21) clusters by using quantum-chemistry calculations. Our results show that the radical forms a stable hydrogen-bond complex on the cluster. The HO(2) acid dissociation is more favorable in the case of the (H(2)O)(21) cluster, for which the inner water molecule plays a crucial role. In fact, acid dissociation of HO(2) is found to occur in two steps. The first step involves H(2) O autoionization in the cluster, and the second one involves the proton transfer from the HO(2) radical to the hydroxide anion. The presence of the HO(2) radicals on the surface of the cluster facilitates water autoionization in the cluster.     

Heptametallic,Octupolar Nonlinear Optical Chromophores with Six Ferrocenyl Substituents

New complexes with six ferrocenyl (Fc) groups connected to Zn^II or Cd^II tris(2,2′‐bipyridyl) cores are described. A thorough characterisation of their BPh₄^? salts includes two single‐crystal X‐ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(Fe^II)→π* metal‐to‐ligand charge‐transfer (MLCT) bands accompany the π→π* intraligand charge‐transfer absorptions in the near UV region. Each complex shows a single, fully reversible Fe^III/II wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper‐Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities β₀ reaching as high as approximately 10^?27 esu and generally increasing with π‐conjugation extension. Z‐scan cubic NLO measurements reveal high two‐photon absorption cross‐sections σ₂ of up to 5400 GM in one case. DFT calculations reproduce the π‐conjugation dependence of β₀, and TD‐DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature.     

From molecule to bulk material: optical properties of hydrogen-bonded dimers [C12H12N4O2AgPF6]2 and [C28H28N6O3AgPF6]2 depend on the arrangement of the oxime moieties

The dependence of the optical properties of [C(12)H(12)N(4)O(2)AgPF(6)](2) (dimer-1) and [C(28)H(28)N(6)O(3)AgPF(6)](2) (dimer-2) on the arrangement of the oxime moieties in the molecule and in bulk crystals was investigated by means of time-dependent density functional theory. Dimer-1 with simple pyridine oxime ligands and a wavy arrangement has a smaller dipole moment and larger transition energy between the two states, and thus smaller third-order polarizabilities and two-photon absorption cross sections. Dimer-2 with extended pyridine oxime ligands and a ladder arrangement has a larger dipole moment and smaller transition energy between the two states, and thus larger third-order polarizabilities and two-photon absorption cross sections. The lowest energy absorption band is red-shifted for dimer-2 as compared with dimer-1, due to more pronounced pi-pi delocalization interactions and weaker hydrogen bonding in dimer-2. The electronic absorption spectra, frequency-dependent third-order polarizabilities, and two-photon absorption cross sections involve significant contributions from charge transfers from pi/pi* orbitals of the pyridine oxime ligands but no contribution from PF(6) (-) ions or H(2)O molecules in the wavelength range studied for the monomers and dimers of the C(12)H(12)N(4)O(2)AgPF(6) and C(28)H(28)N(6)O(3)AgPF(6) molecules. The third-order susceptibilities and two-photon absorption coefficients of bulk solids were estimated on the basis of the optical properties of the corresponding dimers, and the bulk material constructed from dimer-2 has the larger optical parameters of the two.     

Terminally coordinated AsS and PS ligands

The terminal AsS and PS complexes [(N(3)N)W(ES)] (N(3)N=N(CH(2)CH(2)NSiMe(3))(3); E=P (3), As (4)) were synthesised by reaction of [(N(3)N)W[triple chemical bond]As] and [(N(3)N)W[triple chemical bond]P], respectively, with cyclohexene sulfide. Both complexes present very short W--E and E--S bond lengths. The bonding was investigated by density functional theory (DFT) calculations using the fragment calculation method and natural bond orbital (NBO) analysis. According to the fragment analysis, in which the complexes were separated in an ES and a (N(3)N)W fragment, the bonding in complexes 3, 4 and [(N(3)N)W(SbS)] (5) is realised over a set of two sigma (1 sigma and 2 sigma) and two degenerate pi molecular orbitals (MOs) (1 pi and 2 pi). The 1 sigma MO is a bonding MO extended over the N(ax)-W-E-S core, whereas the 2 sigma MO is localised mainly on the E-S fragment. The 1 pi set is a E-S localised bonding molecular orbital, whereas the 2 pi set is in phase with respect to W-E but in antiphase with respect to E-S. Both methods indicate bond orders around two for both the E--S and the W--E bonds. The polarity of the complexes was examined by Hirshfeld charge analysis. This shows that complexes 3 and 4 are only slightly polarised, whereas 5 is moderately polarised toward the sulphur. As suggested by the computational results, the pi system in complexes 3-5 is best described by two three-centre four-electron bonds.