Synthesis of single-crystalline platinum nanorods within a soft crystalline surfactant-Pt(II) complex.

Single-crystalline platinum nanorods, monodisperse in diameter, are synthesized through a simple process at room temperature, in cetyltrimethyl ammonium bromide (CTAB) solution. The complexation of the CTA+ surfactant ion with tetrachloroplatinate in the presence of hexanol leads to the formation of a precipitate with a lamellar crystalline structure. The reduction of Pt(II) metal ions to Pt(0) is carried out using gamma radiolysis. Transmission electron microscopy (TEM) observations of the nanoparticles extracted from the solution, three weeks after radiolysis, revealed single-crystalline Pt nanorods, monodisperse in diameter (3-4 nm) and 20-60 nm long. By following the shape of the nanorods at various stages of the growth, it was found that the single-crystalline nanorods grow by coalescence of spherical seeds 3-4 nm in diameter. This suggests an aggregative mechanism similar to that recently observed for silver particles in solution.     

Imino‐Bridged Bisphosphaalkenes (2,4‐Diphospha‐3‐azapentadienes)

Deprotonation of aminophosphaalkenes (RMe₂Si)₂C?PN(H)(R′) (R=Me, iPr; R′=tBu, 1‐adamantyl (1‐Ada), 2,4,6‐tBu₃C₆H₂ (Mes*)) followed by reactions of the corresponding Li salts Li[(RMe₂Si)₂C?P(M)(R′)] with one equivalent of the corresponding P‐chlorophosphaalkenes (RMe₂Si)₂C?PCl provides bisphosphaalkenes (2,4‐diphospha‐3‐azapentadienes) [(RMe₂Si)₂C?P]₂NR′. The thermally unstable tert‐butyliminobisphosphaalkene [(Me₃Si)₂C?P]₂NtBu ( 4 a ) undergoes isomerisation reactions by Me₃Si‐group migration that lead to mixtures of four‐membered heterocyles, but in the presence of an excess amount of (Me₃Si)₂C?PCl, 4 a furnishes an azatriphosphabicyclohexene C₃(SiMe₃)₅P₃NtBu ( 5 ) that gave red single crystals. Compound 5 contains a diphosphirane ring condensed with an azatriphospholene system that exhibits an endocylic P?C double bond and an exocyclic ylidic P⁽⁺⁾? C^(?)(SiMe₃)₂ unit. Using the bulkier iPrMe₂Si substituents at three‐coordinated carbon leads to slightly enhanced thermal stability of 2,4‐diphospha‐3‐azapentadienes [(iPrMe₂Si)₂C?P]₂NR′ (R′=tBu: 4 b ; R′=1‐Ada: 8 ). According to a low‐temperature crystal‐structure determination, 8 adopts a non‐planar structure with two distinctly differently oriented P?C sites, but ³¹P NMR spectra in solution exhibit singlet signals. ³¹P NMR spectra also reveal that bulky Mes* groups (Mes*=2,4,6‐tBu₃C₆H₂) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the P? N bonds in persistent compounds [(RMe₂Si)₂C?P]₂NMes* ( 11 a , 11 b ). DFT calculations for the parent molecule [(H₃Si)₂C?P]₂NCH₃ suggest that the non‐planar distortion of compound 8 will have steric grounds.     

Synthesis and Characterization of Terminal [Re(XCO)(CO)2(triphos)] (X=N,P): Isocyanate versus Phosphaethynolate Complexes

The terminal rhenium(I) phosphaethynolate complex [Re(PCO)(CO)₂(triphos)] has been prepared in a salt metathesis reaction from Na(OCP) and [Re(OTf)(CO)₂(triphos)]. The analogous isocyanato complex [Re(NCO)(CO)₂(triphos)] has been likewise prepared for comparison. The structure of both complexes was elucidated by X‐ray diffraction studies. While the isocyanato complex is linear, the phosphaethynolate complex is strongly bent around the pnictogen center. Computations including natural bond orbital (NBO) theory, natural resonance theory (NRT), and natural population analysis (NPA) indicate that the isocyanato complex can be viewed as a classic Werner‐type complex, that is, with an electrostatic interaction between the Re^I and the NCO group. The phosphaethynolate complex [Re(P?C?O)(CO)₂(triphos)] is best described as a metallaphosphaketene with a Re^I–phosphorus bond of highly covalent character.     

Isolable tris(alkyne) and bis(alkyne) complexes of gold(I)

Golden trefoils: Tris(alkyne)gold complex [(coct)(3)Au][SbF(6)] (see picture; 1-SbF(6)) can be synthesized from cyclooctyne (coct) and AuSbF(6) generated in situ. Treatment of AuCl with cyclooctyne led to the bis(alkyne)gold complex [Au(coct)(2)Cl] (2). DFT analysis indicates that the cyclooctyne ligands are net electron donors in 1 but overall electron acceptors in 2. AuSbF(6) is shown to mediate [2+2+2] cycloaddition reactions of alkynes.     

Toward the assignment of the manganese oxidation pattern in the water-oxidizing complex of photosystem II: a time-dependent DFT study of XANES energies

Density functional theory (DFT) and time‐dependent DFT (TDDFT) calculations are reported on three sets of isomeric models of the {Mn₄/Ca} water‐oxidizing complex of photosystem II (PS II), which share the general formula [CaMn₄O₄(N₂C₃H₄)(RCOO)₆]^q?(H₂O)_n (R=H, CH₃; q=?1, 0, +1, +2; n=3, 4, 5, 6, 7). Comparison with the full range of available data on Mn K‐edge X‐ray absorption energy values determined for the photosystem allows us to validate the structures that correspond to the particular S states and to determine their Mn oxidation patterns. By using a new TDDFT procedure, it is shown that variations in the absolute K‐edge energy values for a particular S state, reported by different research groups, can be quantitatively explained by different geometries adopted by the Mn cluster, which demonstrates flexibility in the position of the fourth ‘dangling’ Mn atom in relation to a cubane structure created by the Ca atom and the three other Mn atoms. Computational results show that each step of the S cycle occurs by removal of one electron directly from the Mn cluster. This Mn‐centered oxidation still agrees with the small difference observed experimentally between the K‐edge energy values of the S₂ and S₃ states of the photosystem, thus resolving a controversy as to whether this represents ligand‐centered or metal‐centered oxidation. The overall oxidation state of Mn atoms in the tetramanganese cluster during functional turnover changes from 2.75 for S₀, 3.00 for S₁, and 3.25 for S₂ up to 3.50 for the S₃ state, which is systematically 0.50 lower than the previously proposed oxidation states of the cluster. The calculations give insight into why these earlier, purely empirical, assignments of the Mn oxidation levels in PS II could be in error.     

The first pure LambdaHT rotamer of a complex with a cis-[metal(nucleotide)2] unit: a cis-[Pt(amine)2(nucleotide)2] LambdaHT rotamer with unique molecular structural features

cis-[PtA2(nucleotide)2] complexes (A2 stands for two amines or a diamine) have been extensively investigated as model compounds for key cisplatin-DNA adducts. All cis-[metal(nucleotide/nucleoside)2] complexes with guanine and related purines characterized in the solid state thus far have the DeltaHT conformation (head-to-tail orientation of the two bases and right-handed chirality). In sharp contrast, the LambdaHT conformation (left-handed chirality) dominates in acidic and neutral aqueous solutions of cis-[PtA2(5'-GMP)2] complexes. Molecular models and solution experiments indicate that the LambdaHT conformer is stabilized by 5'-phosphate/N1H hydrogen-bond interactions between cis nucleotides with the normal anti conformation. However, this evidence, while compelling, is indirect. At last, conditions have been defined to allow crystallization of this elusive conformer. The structure obtained reveals three unique features not present in all other cis-[PtA2(nucleotide)2] solid-state structures: a LambdaHT conformation, very strong hydrogen-bond interactions between the phosphate and N1H of cis nucleotides, and a very small dihedral angle between the planes of the two guanines lying nearly perpendicular to the coordination plane. These new results indicate that, because there are no local base-base repulsions precluding the LambdaHT conformer, global forces rather than local interactions account for the predominance of the DeltaHT conformer over the LambdaHT conformer in the solid state and in both inter- and intrastrand HT crosslinks of oligonucleotides and DNA.     

Structure and Reactivity of a Unique Y‐Shaped Tricoordinate Bis(silyl)platinum(II)–NHC Complex

Y not? A unique, three‐coordinate Y‐shaped bis(silyl)platinum(II) complex was isolated and characterized (see structure; C light gray, N blue, Si pink, Pt dark gray). DFT studies on a model system shed light on the nature of this unusual coordination mode for platinum(II).

     

On the Molecular Structure and Bonding in a Lithium Bismuth Porphyrin Complex: LiBi(TPP)2

A new Bi? Li porphyrin sandwich compound, LiBi(TPP)₂ has been synthesized and characterized (TPP=tetraphenylporphyrin). The unique molecular structure of LiBi(TPP)₂ is such that the Bi sits between the porphyrins and is directed towards the Li. This complex was shown to remain intact in solution by temperature‐dependent 2D NMR spectroscopy. In order to investigate the potential interaction between these two metals, DFT calculations were used and showed a Bi 6s orbital polarized towards Li which could be indicative of a Bi? Li dative bond. This bond is remarkably short, 2.87 Å, and is among the shortest Bi? Li distances seen in a small molecule.     

Description of the ground-state covalencies of the bis(dithiolato) transition-metal complexes from X-ray absorption spectroscopy and time-dependent density-functional calculations

The electronic structures of [M(L(Bu))(2)](-) (L(Bu)=3,5-di-tert-butyl-1,2-benzenedithiol; M=Ni, Pd, Pt, Cu, Co, Au) complexes and their electrochemically generated oxidized and reduced forms have been investigated by using sulfur K-edge as well as metal K- and L-edge X-ray absorption spectroscopy. The electronic structure content of the sulfur K-edge spectra was determined through detailed comparison of experimental and theoretically calculated spectra. The calculations were based on a new simplified scheme based on quasi-relativistic time-dependent density functional theory (TD-DFT) and proved to be successful in the interpretation of the experimental data. It is shown that dithiolene ligands act as noninnocent ligands that are readily oxidized to the dithiosemiquinonate(-) forms. The extent of electron transfer strongly depends on the effective nuclear charge of the central metal, which in turn is influenced by its formal oxidation state, its position in the periodic table, and scalar relativistic effects for the heavier metals. Thus, the complexes [M(L(Bu))(2)](-) (M=Ni, Pd, Pt) and [Au(L(Bu))(2)] are best described as delocalized class III mixed-valence ligand radicals bound to low-spin d(8) central metal ions while [M(L(Bu))(2)](-) (M=Cu, Au) and [M(L(Bu))(2)](2-) (M=Ni, Pd, Pt) contain completely reduced dithiolato(2-) ligands. The case of [Co(L(Bu))(2)](-) remains ambiguous. On the methodological side, the calculation led to the new result that the transition dipole moment integral is noticeably different for S(1s)-->valence-pi versus S(1s)-->valence-sigma transitions, which is explained on the basis of the differences in radial distortion that accompany chemical bond formation. This is of importance in determining experimental covalencies for complexes with highly covalent metal-sulfur bonds from ligand K-edge absorption spectroscopy.