Linear C(n) clusters: are they acetylenic or cumulenic?

Uncapped linear Cn clusters have been studied with hybrid density functional theory focusing on the geometry, HOMO-LUMO gap, and the longitudinal optical (LO) vibrational mode. The latter two correlate well with the bond length alternation (BLA) of the optimized geometry. Due to end effects, the BLA is not constant along the chains. The degree of BLA changes continuously with increasing n: starting with essentially nonalternating structures (cumulenic), then turning into strongly alternating (acetylenic) structures. This transition has not yet been described or characterized and occurs at relatively large values of n. The implications for the widely observed characteristic LO vibrational bands of linear carbon clusters are discussed.     

One-electron oxidized product of difluoroiron(III) porphyrin: is it iron(IV) porphyrin or iron(III) porphyrin π-cation radical?

The electronic structure of [Fe(TMP)F(2)], which is formally a one-electron oxidation equivalent above [Fe(III)(TMP)F(2)](-), has been examined in solution by (1)H NMR, UV-Vis, and M?ssbauer spectroscopy. In CD(2)Cl(2)-CD(3)OD solution at 193 K, the pyrrole-H and m-H signals appeared at 128.2 and 116.7 ppm, respectively. The UV-Vis spectrum showed broad absorption bands at 560-680 nm. The M?ssbauer spectrum taken in frozen toluene-methanol solution exhibited a very broad single line from which the IS and QS values were determined by computer simulation to be 0.50 and 0.14 mm s(-1), respectively. On the basis of these results, it was concluded that the one-electron oxidized product of [Fe(TMP)F(2)](-) should be formulated as the iron(III) radical cation [Fe(III)(TMP˙)F(2)], not as iron(IV) porphyrin [Fe(IV)(TMP)F(2)] as previously suggested.     

Structures of Pd(CN)2 and Pt(CN)2: intrinsically nanocrystalline materials?

Analysis and modeling of X-ray and neutron Bragg and total diffraction data show that the compounds referred to in the literature as "Pd(CN)(2)" and "Pt(CN)(2)" are nanocrystalline materials containing small sheets of vertex-sharing square-planar M(CN)(4) units, layered in a disordered manner with an intersheet separation of ~3.44 ? at 300 K. The small size of the crystallites means that the sheets' edges form a significant fraction of each material. The Pd(CN)(2) nanocrystallites studied using total neutron diffraction are terminated by water and the Pt(CN)(2) nanocrystallites by ammonia, in place of half of the terminal cyanide groups, thus maintaining charge neutrality. The neutron samples contain sheets of approximate dimensions 30 ? × 30 ?. For sheets of the size we describe, our structural models predict compositions of Pd(CN)(2)·xH(2)O and Pt(CN)(2)·yNH(3) (x ≈ y ≈ 0.29). These values are in good agreement with those obtained from total neutron diffraction and thermal analysis, and are also supported by infrared and Raman spectroscopy measurements. It is also possible to prepare related compounds Pd(CN)(2)·pNH(3) and Pt(CN)(2)·qH(2)O, in which the terminating groups are exchanged. Additional samples showing sheet sizes in the range ~10 ? × 10 ? (y ~ 0.67) to ~80 ? × 80 ? (p = q ~ 0.12), as determined by X-ray diffraction, have been prepared. The related mixed-metal phase, Pd(1/2)Pt(1/2)(CN)(2)·qH(2)O (q ~ 0.50), is also nanocrystalline (sheet size ~15 ? × 15 ?). In all cases, the interiors of the sheets are isostructural with those found in Ni(CN)(2). Removal of the final traces of water or ammonia by heating results in decomposition of the compounds to Pd and Pt metal, or in the case of the mixed-metal cyanide, the alloy, Pd(1/2)Pt(1/2), making it impossible to prepare the simple cyanides, Pd(CN)(2), Pt(CN)(2), or Pd(1/2)Pt(1/2)(CN)(2), by this method.     

Insertion of noble-gas atom (Kr and Xe) into noble-metal molecules (AuF and AuOH): are they stable?

The structure and the stability of a new class of insertion compounds of noble-gas atoms of the type AuNgX (Ng=Kr, Xe and X=F, OH) have been investigated theoretically through ab initio molecular-orbital calculations. All the species are found to have a linear structure with a noble-gas-noble-metal bond, the distance of which is comparable to covalent bond length except the AuKrOH system, for which it lies in between the covalent and van der Waals limits. The dissociation energies corresponding to the lowest-energy fragmentation products, AuX+Ng have been computed to be -166.2, -276.0, -194.4, and -257.6 kJ/mol for AuXeF, AuKrF, AuXeOH, and AuKrOH, respectively, at the MP2 level of theory. The respective barrier heights corresponding to the bent transition states (Au-Ng-X bending mode) have been calculated to be 119.1, 74.9, 160.7, and 141.6 kJ/mol. However, three of these species are found to be metastable in their respective potential-energy surface, and the dissociation energies corresponding to the Au+Ng+X fragments have been calculated to be 112.9, 3.0, and 18.7 kJ/mol for AuXeF, AuKrF, and AuXeOH, respectively, at the same level of theory. An analysis of the nature of interactions involved in the Au-Ng-X systems has been performed using Bader's topological theory of atoms-in-molecules (AIM). Geometric as well as energetic considerations along with AIM results suggest a partial covalent nature of Au-Ng bonds in these systems. This work might have important implications in the preparation of a new class of insertion compounds of noble-gas atoms containing noble-gas-noble-metal bond.     

PIXE and charge-induced X-rays (CHIX): Are they complementary or competitive?

Enhanced yields from CHIX studies on non-conducting samples have demonstrated conclusively that such yields are exceptionally high when compared to PIXE at low incident particle energies. This was confirmed with low energy¹⁴N⁺,¹⁶O⁺ and²⁰Ne⁺ ions whose X-ray production crosssections are negligibly small for PIXE yields. In particular, a combination of low incident energies and high X-ray yields could be useful for XSQR investigations and elemental studies with low energy accelerators. Furthermore, extended studies on pure metal targets revealed that the PIXE yield could be improved by insulating the targets thus making them suitable to produce the CHIX yield under identical experimental conditions. This paper discusses the complementary and competitive features of PIXE and CHIX.     

Studies of surface processes of electrocatalytic reduction of CO_2 on Pt(210), Pt(310) and Pt(510)

Surface processes of CO2 reduction on Pt(210), Pt(310), and Pt(510) electrodes were studied by cyclic voltammetry. Different surface structures of these platinum single crystal electrodes were obtained by various treatment conditions. The experimental results illustrated that the electrocatalytic activity of Pt single crystal electrodes towards CO2 reduction is decreased in an order of Pt(210)>Pt(310)>Pt(510), i.e., with the decrease of (110) step density on well-defined surfaces. When the surfaces were reconstructed due to oxygen adsorption, the catalytic activity of all the three electrodes has been enhanced to a cer- tain extent. Although the activity order remains unchanged, the electrocatalytic activity has been en- hanced more significantly as the density of (110) step sites is more intensive on the Pt single crystal surface. It has revealed that the more open the surface structure is, the more active the Pt single crystal electrode will be, and the easier for the electrode to be transformed into a surface structure that exhib- its higher activity under external inductions. However, the relatively ordered surfaces of Pt single crystal electrode are comparatively stable under the same external inductions. The present study has gained knowledge on the interaction between CO2 and Pt single crystal electrode surfaces at a micro- scopic level, and thrown new insight into understanding the surface processes of electrocatalytic re- duction of CO2.     

Potentially hexadentate bisazine dioximate ligands: “correct” synthetic procedure and encapsulation reactions of the iron(ii) ion

Condensation of diacetyl monoxime hydrazone with diacetyl, hexane-3,4-dione, and glyoxal in MeOH afforded potentially hexadentate bisazine dioximes. The crystal and molecular structure of the condensation product with diacetyl was established by X-ray diffraction analysis. The reactions of the resulting azine oximes with Fe²⁺ ions in the presence of Lewis acids were studied.     

Synthesis and Crystal Structure of Platinum Blue [(1,10-phen)Pt(μ-NHCOCH3)2Pt(1,10-phen)]2(NO3)4

The reaction of acetamide with PtphenCl₂ gave a mixed-valence black-brown-colored platinum complex Ptphen(NHCOCH₃)NO₃ (I), which was studied by X-ray diffraction. The monoclinic crystal (a = 16.389(6) Å, b = 19.664(6) Å, c = 11.049(4) Å; β = 122.8(3)°, V = 2993(18) ų, space group C2/m, Z = 4, R = 0.0432) is built of dimeric [Pt₂phen₂(NHCOCH₃)₂]²⁺ cations and NO ₃ ² . anions. Each platinum atom in the dimer is linked to two nitrogen or oxygen atoms of the two bridging (NHCOCH₃)^? groups and two phenanthroline nitrogen atoms. The Pt-Pt distance in the dimer is 2.8891(19) Å. In the crystal, the dimers form pairs (tetramers), the interdimer Pt…Pt distance being 3.167(2) Å. Four platinum atoms are arranged nearly linearly (the Pt(2)Pt(1)Pt(1)* angle is 178.71(4)°). The UV-Vis spectrum of an aqueous solution of compound I exhibits bands at 360, 480, 630, 680, and 880 nm in the visible region. The diffuse reflectance spectrum of a polycrystalline sample of I (in the 300–900 nm range) contains bands at ～360, ～500, ～600, ～690, and ～890 nm.     

A theoretical study of an unusual Y-shaped three-coordinate Pt complex: Pt(0) σ-disilane complex or Pt(II) disilyl complex?

The unusual Y-shaped structure of the recently reported three-coordinate Pt complex Pt[NHC(Dip)(2)](SiMe(2)Ph)(2) (NHC = N-heterocyclic carbene; Dip = 2,6-diisopropylphenyl) was considered a snapshot of the reductive elimination of disilane. A density functional theory study indicates that this structure arises from the strong trans influence of the extremely σ-donating carbene and silyl ligands. Though this complex can be understood to be a Pt(II) disilyl complex bearing a distorted geometry due to the Jahn-Teller effect, its (195)Pt NMR chemical shift is considerably different from those of Pt(II) complexes but close to those of typical Pt(0) complexes. Its Si···Si bonding interaction is ~50% of the usual energy of a Si-Si single bond. The interaction between the Pt center and the (SiMe(2)Ph)(2) moiety can be understood in terms of donation and back-donation interactions of the Si-Si σ-bonding and σ*-antibonding molecular orbitals with the Pt center. Thus, we conclude that this is likely a Pt(0) σ-disilane complex and thus a snapshot after a considerable amount of the charge transfer from disilane to the Pt center has occurred. Phenyl anion (Ph(-)) and [R-Ar](-) [R-Ar = 2,6-(2,6-iPr(2)C(6)H(3))(2)C(6)H(3)] as well as the divalent carbon(0) ligand C(NHC)(2) also provide similar unusual Y-shaped structures. Three-coordinate digermyl, diboryl, and silyl-boryl complexes of Pt and a disilyl complex of Pd are theoretically predicted to have similar unusual Y-shaped structures when a strongly donating ligand coordinates to the metal center. In a trigonal-bipyramidal Ir disilyl complex [Ir{NHC(Dip)(2)}(PH(3))(2)(SiMe(3))(2)](+), the equatorial plane has a similar unusual Y-shaped structure. These results suggest that various snapshots can be shown for the reductive eliminations of the Ge-Ge, B-B, and B-Si σ-bonds.     

Six-coordinate Co(2+) with H(2)O and NH(3) ligands: which spin state is more stable?

Octahedral, six-coordinate Co(2+) can exist in two spin states. For biological ligands, H(2)O and NH(3), the most stable spin state is high spin (S = (3)/(2)). The difference in energy between high and low spin is dependent upon the ligand mix and coordination stereochemistry. High spin optimized geometries for these model compounds give structures close to octahedral symmetry. Low spin permits significant Jahn-Teller distortion. H(2)O ligands preferentially assume axial positions. Continuum solvent has a greater effect on low spin Co(2+), and it reduces the energy difference between the two spin states. For some ligand combinations optimized in the presence of solvent, there is no significant difference in energy between spin states.     

Chemical and thermal destruction of the platinum blue {Pt(Bipy)(NHCOCH3)2NO3 · mH2O}. The molecular and crystal structures of new binuclear Pt(III) acetamide [Pt2(Bipy)2(NHCOCH3)2(μ-NHCOCH3)2](NO3)2 · H2O

The chemical reactivity of mixed-valence “platinum blue” of the composition [Pt(Bipy)(NHCOCH₃)₂NO₃ · mH₂O] _n (I) towards water, concentrated HCl, and ethanol was studied. The reaction of compound I with water gave previously unknown binuclear Pt(III) acetamide [Pt₂(Bipy)₂(μ-NHCOCH₃)₂(NHCOCH₃)₂](NO₃)₂ · H₂O (II) (yield 19%), which was studied by X-ray diffraction. Upon the reaction of “blue” I with HCl_conc, destruction took place to give PtBipyCl₃ and Pt(IV) oxonium hexachloroplatinate (H₃O)₂[PtCl₆] the structure of which was studied by X-ray diffraction; treatment of I with ethanol leads to partial reduction of the “blue” and destruction giving (according to X-ray diffraction data) the Pt(II) complex [Pt(Bipy)₂](NO₃)₂ · H₂O (III). Thermal decomposition of I under inert atmosphere was studied by DSC and TGA.     

Properties and structure of new volatile phenyl-containing β-diketonates of trimethylplatinum(IV): (CH3)3Pt(btfa)H2O, (CH3)3Pt(bac)Py, and initial complex [(CH3)3PtI]4

By interaction of trimethylplatinum(IV) iodide with phenyl-containing β-diketonates, the volatile monomeric complexes of trimethylplatinum(IV) based on benzoyltrifluoroacetone (Hbtfa) and benzoylacetone (Hbac) of the composition (CH₃)₃Pt(btfa)H₂O (I) and (CH₃)₃Pt(bac)Py (II) are obtained. Synthesis of the complexes is described; data of elemental analysis and IR spectra are reported; thermal characteristics are studied by thermogravimetry. For the first time, a single crystal X-ray diffraction study of complexes (I), (II), and the initial tetrameric complex [(CH₃)₃PtI]₄ (III) is performed.     

UV Spectra and Structure of 2-Diethylamino-3-phenylpropenal: Enamine or Enal?

According to the data of UV, IR, ¹H and ¹³NMR spectra and also quantum-chemical calculations (B3LYP/631G*), Z- and E-isomers of 2-diethylamino-3-phenylpropenal in inert environment exist as antiperiplanar conformers. The energy of electron transitions therein depends on the orientation both f the benzene ring and the unshared electron pair of the nitrogen with respect to the C = C bond. 2-Diethylamino-3-phenylpropenal is an enamine, but the extent of the p^* interaction in its prevailing Z-isomer is small.     

Stereospecific ligands and their complexes. IV: Synthesis,characterization and cytotoxicity of novel platinum(IV) complexes with ethylenediamine-N,N′-di-S,S-2-propanoate and halogenido ligands: Crystal structure of s-cis-[Pt(S,S-eddp)Cl2]·4H2O and uns-cis-[Pt(S,S-eddp)Br2]

The syntheses of two novel platinum(IV) complexes of formula [PtX₂(S,S-eddp)]·nH₂O (S,S-eddp = ethylenediamine-N,N′-di-S,S-2-propanoate ion, X = chlorido (1) or bromido (2), n = 4, 0) are reported. The complexes have been obtained by direct reaction of corresponding potassium hexahalogenidoplatinate(IV) with neutralized ethylenediamine-N,N′-di-S,S-2-propanoic acid (H₂-S,S-eddp). The complexes were characterized by elemental analysis, infrared, ¹H and ¹³C NMR spectroscopy. The spectroscopically predicted geometrical configurations of the obtained complexes were confirmed by X-ray analyses of the crystal structures of the s-cis-[Pt(S,S-eddp)Cl₂]·4H₂O and uns-cis-[Pt(S,S-eddp)Br₂]. These complexes displayed significantly lower in vitro cytotoxicity in comparison to cisplatin.     

Carbonyl substitution of the dicobalt-iron complex (μ3-S)FeCo2(CO)9 with monophosphane or diphosphane ligands

Eight new dicobalt-iron clusters have been synthesised and structurally characterized. Treatment of (μ₃-S)FeCo₂(CO)₉ (A) with monophosphane ligands tris(4-fluorophenyl)phosphane, tris(4-methoxyphenyl)phosphane, or tris(2-furyl)phosphane in the presence of Me₃NO?2H₂O afforded monosubstituted complexes (μ₃-S)FeCo₂(CO)₈L [L = P(4-C₆H₄F)₃, 1; P(4-C₆H₄OMe)₃, 3; P(2-C₄H₃O)₃, 5] and disubstituted complexes (μ₃-S)FeCo₂(CO)₇L₂ [L = P(4-C₆H₄F)₃, 2; P(4-C₆H₄OMe)₃, 4; P(2-C₄H₃O)₃, 6]. Reaction of complex A with Ph₂PN[CH(CH₃)₂]PPh₂ in refluxing toluene gave complex (μ₃-S)FeCo₂(CO)₇{Ph₂PN[CH(CH₃)₂]PPh₂} (7) with an intramolecular bridging diphosphane ligand. Reaction of complex A with trans-1,2-bis(diphenylphosphino)ethylene (trans-dppv) and Me₃NO?2H₂O yielded complex [(μ₃-S)FeCo₂(CO)₈]₂(trans-Ph₂PCH = CHPPh₂) (8) with an intermolecular bridging diphosphane ligand. The new complexes 1–8 were characterized by elemental analysis, IR, ¹H NMR, ³¹P{¹H} NMR, and ¹³C{¹H} NMR spectroscopy, particularly for 1, 3, and 6–8 by X-ray crystallography.     

Magic clusters MAu4 (M=Ti and Zr) and their dimers: how magic are they?

The stability of closed shell bimetallic magic clusters MAu(4) (M=Ti and Zr) is investigated theoretically through ab initio molecular orbital calculations. Both these clusters have tetrahedral structures and are found to be associated with large values of the ionization potential, HOMO-LUMO gap as well as the binding energies, which are characteristic of the magic clusters. However, the cluster-cluster interaction energy corresponding to a dimer formation is found to be unusually high ( approximately 5-7 eV) in contradiction to the usual properties of a magic cluster and is attributed to a 3-center-2-electron M-Au-M type bridge bonding as well as aurophilic attraction. Gross geometrical features of the individual clusters are, however, mostly retained in the dimer, thus satisfying the basic requirements for the cluster-assembled materials. This work would have important implications in the design of novel cluster-based nanomaterials for various nanoscale applications.     

Adsorption and decomposition of cyclohexanone (C6H10O) on Pt(111) and the (2 × 2) and (√3 × √3)r30°-Sn/Pt(111) surface alloys

Adsorption and decomposition of cyclohexanone (C(6)H(10)O) on Pt(111) and on two ordered Pt-Sn surface alloys, (2 × 2)-Sn/Pt(111) and (√3 × √3)R30°-Sn/Pt(111), formed by vapor deposition of Sn on the Pt(111) single crystal surface were studied with TPD, HREELS, AES, LEED, and DFT calculations with vibrational analyses. Saturation coverage of C(6)H(10)O was found to be 0.25 ML, independent of the Sn surface concentration. The Pt(111) surface was reactive toward cyclohexanone, with the adsorption in the monolayer being about 70% irreversible. C(6)H(10)O decomposed to yield CO, H(2)O, H(2), and CH(4). Some C-O bond breaking occurred, yielding H(2)O and leaving some carbon on the surface after TPD. HREELS data showed that cyclohexanone decomposition in the monolayer began by 200 K. Intermediates from cyclohexanone decomposition were also relatively unstable on Pt(111), since coadsorbed CO and H were formed below 250 K. Surface Sn allowed for some cyclohexanone to adsorb reversibly. C(6)H(10)O dissociated on the (2 × 2) surface to form CO and H(2)O at low coverages, and methane and H(2) in smaller amounts than on Pt(111). Adsorption of cyclohexanone on (√3 × √3)R30°-Sn/Pt(111) at 90 K was mostly reversible. DFT calculations suggest that C(6)H(10)O adsorbs on Pt(111) in two configurations: by bonding weakly through oxygen to an atop Pt site and more strongly through simultaneously oxygen and carbon of the carbonyl to a bridged Pt-Pt site. In contrast, on alloy surfaces, C(6)H(10)O bonds preferentially to Sn. The presence of Sn, furthermore, is predicted to make the formation of the strongly bound C(6)H(10)O species bonding through O and C, which is a likely decomposition precursor, thermodynamically unfavorable. Alloying with Sn, thus, is shown to moderate adsorptive and reactive activity of Pt(111).     

Studies pertaining to the mechanism of “Pt(PCy3)“ addition. The reaction of MPt(μ-H)(μ-PnPr2)Pt(CO)(PCy3) with Pt(C2H4)2(PCy3)

The reaction of Pt(C₂H₄)₂(PCy₃) with (OC)₄M(μ-H)(μ-PⁿPr₂)Pt(CO)(PCy₃, (1: M  Cr, Mo, W) occurs in a highly specific, kinetically controlled manner to give MPt₂(μ²_MPt-CO)(η²_PtPt-H)(μ²_MPt-PⁿPr₂)(CO)₄ (PCy₃)₂ (5), as the first formed trimer. The trimer 5 (M  Mo, W) isomerizes to give MPt₂(μ²_PtPt-CO) ((μ²_MPtH)(μ²_MPt-PⁿPr₂)(CO)₄)PCy₃)₂ (6) which in turn isomerizes to MPt₂μ²_MPtCO)(μ²_MPt (μ²_PtPt-PⁿPr₂)(CO)₄(PCy₃)₂ (7, as the final isolable product. These results provide a detailed insight into the mechanism of “Pt(PCy₃) addition”, a cluster assembly process.     

Synthesis of Platinum-Triruthenium Clusters Using the Zero-Valent Platinum Reagent Pt(nb)3: X-Ray Crystal Structures of Ru3Pt(CO)11(P-iPr3)2, Ru3Pt(μ-H)(μ3-η3-MeCCHCMe)(CO)9(P-iPr3), Ru3Pt(μ3-η2-PhCCPh)(CO)10(P-iPr3), Ru3Pt(μ-H)(μ4-N)(CO)10(P-iPr3) and Ru3Pt(μ-H)(μ4-η2-NO)(CO)10(P-iPr3)

The complexes Pt(nb)_3-n(P-iPr₃)_n (n=1, 2, nb=bicyclo[2.2.1]hept-2-ene), prepared in situ from Pt(nb)₃, are useful reagents for addition of Pt(P-iPr₃)_n fragments to saturated triruthenium clusters. The complexes Ru₃Pt(CO)₁₁(P-iPr₃)₂ (1), Ru₃Pt(-H)(₃-³-MeCCHCMe)(CO)₉(P-iPr₃) (2), Ru₃Pt(₃-²-PhCCPh)(CO)₁₀(P-iPr₃) (3), Ru₃Pt(-H)(₄-N)(CO)₁₀(P-iPr₃) (4) and Ru₃Pt(-H)(₄-²-NO)(CO)₁₀(P-iPr₃) (5) have been prepared in this fashion. All complexes have been characterized spectroscopically and by single crystal X-ray determinations. Clusters 1–3 all have 60 cluster valence electrons (CVE) but exhibit differing metal skeletal geometries. Cluster 1 exhibits a planar-rhomboidal metal skeleton with 5 metal–metal bonds and with minor disorder in the metal atoms. Cluster 2 has a distorted tetrahedral metal arrangement, while cluster 3 has a butterfly framework (butterfly angle=118.93(2)°). Clusters 4 and 5 posseses 62 CVE and spiked triangular metal frameworks. Cluster 4 contains a ₄-nitrido ligand, while cluster 5 has a highly unusual ₄-²-nitrosyl ligand with a very long nitrosyl N–O distance of 1.366(5) Å.