Pressure Induced Alteration and Stabilization of Intermolecular Stacking in Square-planar Osmium tetracarbonyl

Osmium carbonyls are well known to form stable 18-electron complexes like Os(CO)5 , Os2 (CO)9 and Os3 (CO)12 having both bridging and terminal carbonyls. For osmium tetra-carbonyl, Os(CO)4 solid-state packing significantly alters the ground-state structure. The gas-phase stable see-saw geometry converts to a square-planar structure in solid state. Highly efficient intermolecular stacking between Os(CO)4 units assists this transformation. Each Os(CO)4 molecule is stacked in a staggered orientation with respect to each other. Pressure induces a [Xe]4f14 5d6 6s2 (S=2)→[Xe]4f14 5d8 (S=0) electronic transition in osmium stabilize a square planar osmium tetra-carbonyl. Under the influence of isotropic pressure, the molecules not only come closer to each other but their relative orientations also get significantly altered. Calculations show that at P=1 GPa and above, the eclipsed orientation for the intermolecular stacking gets preferred over the staggered form. The staggered→eclipsed intermolecular stacking orientation under pressure is shown to be controlled by London dispersion interactions.

The borderless world of chemical bonding across the van der Waals crust and the valence region

The definition of the van der Waals crust as the spherical section between the atomic radius and the van der Waals radius of an element is discussed and a survey of the application of the penetration index between two interacting atoms in a wide variety of covalent, polar, coordinative or noncovalent bonding situations is presented. It is shown that this newly defined parameter permits the comparison of bonding between pairs of atoms in structural and computational studies independently of the atom sizes.

Emerging d-d orbital coupling between non-d-block main-group elements Mg and I at high pressure

d-d orbital coupling, which increases anisotropic and directional bonding, commonly occurs between d-block transition metals. Here, we report an unexpected d-d orbital coupling in the non-d-block main-group element compound Mg2I based on first-principles calculations. The unfilled d orbitals of Mg and I atoms under ambient conditions become part of the valence orbitals and couple with each other under high pressures, resulting in the formation of highly symmetric I-Mg-I covalent bonding in Mg2I, which forces the valence electrons of Mg atoms into the lattice voids to form interstitial quasi-atoms (ISQs). In turn, the ISQs highly interact with the crystal lattice, contributing to lattice stability. This study greatly enriches the fundamental understanding of chemical bonding between non-d-block main-group elements at high pressures.

Compression Produces a Square-Planar Iron Tetracarbonyl

Iron carbonyls are known to form 18-electron complexes like Fe(CO)₅, Fe₂(CO)₉, and Fe₃(CO)₁₂ having terminal or bridged Fe-CO bonding. Based on genetic algorithm-assisted density functional theory (DFT) calculations, it is predicted that at pressures above 2 GPa, iron tetracarbonyl, Fe(CO)₄, attains a square-planar geometry with a 16-electron count. Compression overcomes the [Ar]4s²3d⁶ (S = 2) → [Ar]4s⁰3d⁸ (S = 0) excitation energy to stabilize a closed-shell Fe(CO)₄ with a d⁸-configuration. Strong σ(4CO) → Fe (d_x²_-y²) bonding along with Fe(d_xz, d_yz) and Fe(d_xy) → π (CO)₄* back-bonding assists the formation of square-planar Fe(CO)₄ under pressure. Compression progressively flattens and destabilizes the ambient pressure C_2v structure of Fe(CO)₄, and beyond 2 GPa, it undergoes a sharp C_2v → D_4h transition with ΔV_unit-cell = 2.1% and trans-θ(OC-Fe-CO) = 180°. Realizing a square-planar geometry in a four-coordinated Fe-carbonyl complex shows the rich prospects of the new chemistry under pressure.

Shortwave infrared luminescent Pt-nanowires: a mechanistic study of emission in solution and in the solid state

Several complexes of "PtL2" composition containing two cyanoxime anions - 2-oximino-2-cyano-N-piperidineacetamide (PiPCO-) and 2-oximino-2-cyano-N-morpholylacetamide (MCO-) - have been obtained and characterized both in solution and in the solid state. Complexes exist as two distinct polymorphs: monomeric yellow complexes and dark-green [PtL2]n 1D polymers, while for the MCO- anion a red, solvent containing dimeric [Pt(MCO)2·DMSO]2 complex has also been isolated. The interconversion of polymorphs was investigated. The monomeric PtL2 units are arranged into anisotropic extended solid [PtL2]n polymers with the help of PtPt metallophilic interactions. Crystal structures of monomeric PtL2 (L = PiPCO-, MCO-) and red dimeric [Pt(MCO)2·DMSO]2 complexes were determined and revealed the cis-arrangement of cyanoxime anions. The Pt-Pt distance in the "head-to-tail" red dimer was found to be 3.133 Å. The structure of the polymeric [Pt(PiPCO)2]n compound was elucidated using the EXAFS method and evidenced the formation of Pt-wires with ∼3.15 Å intermetallic separation. The EPR spectra of both 1D polymers at variable temperatures indicate the absence of Pt(iii) species. Both pure dark-green [PtL2]n polymers showed a considerable room temperature electrical conductivity of 20-30 S cm-1, which evidences the formation of a mixed valence Pt(ii)/Pt(iv) system. We discovered that these 1D polymeric [PtL2]n complexes show an intense NIR fluorescence beyond 1000 nm, while yellow monomeric PtL2 complexes are not emissive at all. The room temperature excitation spectra of 1D polymeric [PtL2]n complexes demonstrated their strong emission beyond 1000 nm regardless of the used excitation wavelength between 350 and 800 nm, which is typical of systems with delocalized charge carriers. For the first time the formation of mixed valence "metal wires" held together by metallophilic interactions is directly linked both with an intense fluorescence in the NIR region of the spectrum and with the electrical conductivity. The effect of the concentration of [PtL2]n complexes dispersed in the dielectric salt matrix on the photoluminescence wavelength and intensity was investigated. Both polymers show a quantum yield that is remarkably high for this region of the spectrum, reaching ∼2%. Variable temperature emission of polymeric [PtL2]n in the -190-+60 °C range was studied as well.

Structure and bonding of [Pd2Sn18]4−: an interesting example of the mutual delocalisation phenomenon

The [Pd₂Sn₁₈]⁴⁻ cluster can be elucidated based on the mutual delocalisation of two identical [PdSn₉]²⁻ fragments. Other roles of the [E₉]²⁻ (E = Group 14 element) fragments were also examined.

2,4,6-Triarylphosphinines versus 2,4,6-triarylpyridines: an investigation of the differences in reactivity between structurally related aromatic phosphorus and nitrogen heterocycles

The novel atropisomeric pyridine derivative rac-10 has been synthesized and structurally characterized. In contrast to its phosphorus analogue 3, axially chiral 10 has a considerably lower rotational barrier as estimated by DFT calculations. However, the presence of the two enantiomers could be confirmed by means of chiral analytical HPLC analysis and by protonation experiments with a chiral acid. Compound rac-10 could be further dehydrogenated by treatment with DDQ to the benzo(h)quinoline derivative rac-12. This conversion failed for the phosphorus analogue rac-3. Interestingly, although 2,4,6-triarylphosphinines undergo facile CH activation with [Cp*IrCl2 ]2 in the presence of NaOAc, this reaction does not proceed with the corresponding pyridine derivatives. On the other hand, the latter ones can be selectively ortho-metalated with Pd(OAc)2 , leading to acetate-bridged dimeric species, which could be unambiguously confirmed by means of X-ray crystal structure analysis. The treatment of phosphinines with Pd(OAc)2 led instead to the formation of the unusual cofacial oxidative coupling products 16 and 17, which consist of a phosphorus-containing cage structure.

Electronic structures of Pd(II) dimers

The Pd(II) dimers [(2-phenylpyridine)Pd(mu-X)](2) and [(2-p-tolylpyridine)Pd(mu-X)](2) (X = OAc or TFA) do not exhibit the expected planar geometry (of approximate D(2h) symmetry) but instead resemble an open "clamshell" in which the acetate ligands are perpendicular to the plane containing the Pd atoms and 2-arylpyridine ligands, with the Pd atoms brought quite close to one another (approximate distance 2.85 A). The molecules adopt this unusual geometry in part because of a d(8)-d(8) bonding interaction between the two Pd centers. The Pd-Pd dimers exhibit two successive one-electron oxidations: Pd(II)-Pd(II) to Pd(II)-Pd(III) to Pd(III)-Pd(III). Photophysical measurements reveal clear differences in the UV-visible and low-temperature fluorescence spectra between the clamshell dimers and related planar dimeric [(2-phenylpyridine)Pd(mu-Cl)](2) and monomeric [(2-phenylpyridine)Pd(en)][Cl] (en = ethylenediamine) complexes that do not have any close Pd-Pd contacts. Density functional theory and atoms in molecules analyses confirm the presence of a Pd-Pd bonding interaction in [(2-phenylpyridine)Pd(mu-X)](2) and show that the highest occupied molecular orbital is a d(z(2)) sigma* Pd-Pd antibonding orbital, while the lowest unoccupied molecular orbital and close-lying empty orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other Pd(II)-Pd(II) dimers that have short Pd-Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(mu-X)](2), but quite different from that found for d(8)-d(8) dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd and may explain why Pd d(8)-d(8) dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species.

Supramolecular Structures and Columnar Mesophase Induction in Nondiscoid Pyrazoles by Complexation to Rhodium(I)

Several new cis-[RhCl(CO)2(Ln)] complexes have been prepared using different polycatenar pyrazole ligands (Ln) in order to obtain columnar liquid crystalline arrangements. The topology of the ligand plays an essential role, and a mesophase is induced at room temperature from a nonmesogenic pyrazole only when it is symmetrically substituted with six decyloxy chains. The single-crystal structure of a methoxy-substituted analogue, 3,5-bis(3,4,5-trimethoxyphenyl)pyrazole, is formed by globular tetrameric structures held together by H-bonding. However, parallel dimers are present in the corresponding cis-chlorodicarbonylrhodium(I) complex, a situation that explains the induction of a columnar mesophase in the decyloxy-substituted complex. The XRD pattern of the mesophase is consistent with a hexagonal symmetry in which the columns are formed by molecules assembled in an antiparallel mode. The crystal-to-mesophase transition was also detected by spectroscopic techniques as a shift in the IR carbonyl stretching bands and the appearance of a charge-transfer band in the absorption spectrum with thermochromic behavior.

Novel Luminescent Mixed-Metal PtTl-Alkynyl-Based Complexes: The Role of the Alkynyl Substituent in Metallophilic and η2(π⋅⋅⋅Tl)-Bonding Interactions

A novel series of [PtTl(2)(C[triple chemical bond]CR)(4)](n) (n = 2, R = 4-CH(3)C(6)H(4) (Tol) 1, 1-naphthyl (Np) 2; n = infinity, R = 4-CF(3)C(6)H(4) (Tol(F)) 3) complexes has been synthesized by neutralization reactions between the previously reported [Pt(C[triple chemical bond]CR)(4)](2-) (R = Tol, Tol(F)) or novel (NBu(4))(2)[Pt(C[triple chemical bond]CNp)(4)] platinum precursors and Tl(I) (TlNO(3) or TlPF(6)). The crystal structures of [Pt(2)Tl(4)(C[triple chemical bond]CTol)(8)]4 acetone, 14 acetone, [Pt(2)Tl(4)(C[triple chemical bond]CNp)(8)]3 acetone1/3 H(2)O, 23 acetone 1/3 H(2)O and [[PtTl(2)(C[triple chemical bond]CTol(F))(4)](acetone)S](infinity) (S = acetone 3 a; dioxane 3 b) have been solved by X-ray diffraction studies. Interestingly, whereas in the tolyl (1) and naphthyl (2) derivatives, the thallium centers exhibit a bonding preference for the electron-rich alkyne entities to yield crystal lattices based on sandwich hexanuclear [Pt(2)Tl(4)(C[triple chemical bond]CR)(8)] clusters (with additional Tlacetone (1) or Tlnaphthyl (2) secondary interactions), in the C(6)H(4)CF(3) (Tol(F)) derivatives 3 a and 3 b the basic Pt(II) center forms two unsupported Pt-Tl bonds. As a consequence 3 a and 3 b form an extended columnar structure based on trimetallic slipped PtTl(2)(C[triple chemical bond]CTol(F))(4) units that are connected through secondary Tl(eta(2)-acetylenic) interactions. The luminescent properties of these complexes, which in solution (blue; CH(2)Cl(2) 1,2; acetone 3) are very different to those in solid state (orange), have been studied. Curiously, solid-state emission from 1 is dependent on the presence of acetone (green) and its crystallinity. On the other hand, while a powder sample of 3 is pale yellow and displays blue (457 nm) and orange (611 nm) emissions, the corresponding pellets (KBr, solid) of 3, or the fine powder obtained by grinding, are orange and only exhibit a very intense orange emission (590 nm).

A pyrimidine thiolate Rh(I) complex: structure, bonding and one-dimensional interactions in solid and in solution

The reaction of [Rh(micro-Cl)(COD)]2 with 4,6-dimethyl-pyrimidinethiolate (Me2-pymt) and subsequent substitution of COD by CO yields [Rh(Me2-pymt)(CO)2]. The stacking pattern found in this compound is in contradiction with previously studied comparable square-planar complexes of type d8-[M(chelate)(monodentate)2] in which each ligand has different pi-acidic character. A theoretical study of the intermolecular interactions and conformation of the title compound has been carried out, combining semi-empirical band calculations on the real chains and ab initio(MP2 level) calculations on a model dimer. The combination of electronic and steric effects determines the rotation of the successive monomers and the deviation from linearity of the one-dimensional stacks. Its behaviour in solution is also special, developing a blue colour and forming micelles, when adding water to acetone solutions.

A New Class of Platinum(II) Vapochromic Salts

Luminescent chloride and hexaflurophosphate salts of Pt(Me2bzimpy)Cl+ (Me2bzimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) are reported. As solids, both compounds are vapochromic, undergoing pronounced and reversible changes of color and emission in the presence of volatile organic compounds. The chloride salt responds to vapors of methanol, chloroform, ethanol, and acetonitrile, undergoing a distinct change in color from yellow to red within seconds. The PF6- salt responded selectively to acetonitrile vapor, changing from yellow to violet while sorbing 1.0 +/- 0.1 equiv. For either salt, leaving vapor-exposed samples in air for several days or heating for several minutes restored the original color. UV-visible absorption spectra and solid-state room temperature and 77 K emission spectra are reported, and the accumulated data are consistent with a decrease in Pt...Pt separation accompanying vapor sorption.

Molecular structures of edge-sharing square-planar dinuclear complexes with unsaturated bridges

The dinuclear complexes of transition metal ions of type [M(2)(mu,eta(1)-XY)(2)L(4)], where XY is an unsaturated ligand that can act as a four-electron or a two-electron donor through the X atom, appear in two molecular conformations depending on whether the coordination planes around the two metal atoms are coplanar or bent. In both structures the geometry of the X atom is planar, corresponding to an sp(2) hybridization. An ab initio theoretical study on 43 representative complexes, complemented with a structural database analysis, provides a rationale for the experimentally observed structures.

Structural correlations and conformational preference in edge-sharing binuclear d8 complexes with XR2 bridges. A theoretical study

The binuclear complexes of d8 transition metal ions of the type [M2(mu-XR2)2L4] (where M = RhI, IrI, NiII, PdII, PtII, or AuIII; X = S, N, P, or As) appear in a variety of molecular conformations in which the coordination planes around the two metal atoms are sometimes coplanar, sometimes bent. For the bent compounds with asymmetric bridges, XR1R2, the substituents adopt different orientations relative to the metal framework and to each other. Ab initio theoretical studies on the different conformers of 30 representative complexes, complemented with a structural database analysis, have allowed the establishment of structural correlations in this family of compounds. The conformational choice results from a delicate balance of different interactions which are qualitatively analyzed, such as the changes in bond angles around the bridging atoms, the existence of weak metal...metal bonding in the bent structures, and steric interactions involving the terminal ligands and the substituents at the bridging atoms.