Angelo Sironi

Chemical bonding in transition metal carbonyl clusters: complementary analysis of theoretical and experimental electron densities

Abstract In the last few years, the quantum theory of atoms in molecules has become the paradigm for interpreting theoretical and experimental electron density distributions. Within this framework, the link between bonding modes and topological properties has been fully achieved for ‘light atom’ molecules. However, the derived correspondence rules cannot be extended straightforwardly to organometallic compounds since bonds to a transition metal display a different and much narrower spectrum of topological indexes. The complementary usage of theoretical computations on a set of prototype transition metal molecules and experimental determinations of the electron density in transition metal carbonyl clusters are discussed. Since these compounds are characterised by weakly bound metal cages and fluxional carbonyl ligands, the focus is on the nature of metal–metal and metal–carbonyl interactions as well as on the evolution of three-centre-four-electron M(μ-CO)M bonds along the (CO)M–M↔M(μ-CO)M↔M–M(CO) conversion path. The interpretation of the electron density distribution here proposed could be extended reasonably to a wider class of organometallic compounds.

Synthesis, structure and magnetism of homologous series of polycrystalline cobalt alkane mono- and dicarboxylate soaps.

Carboxylate-bridged chain complexes of Co(II) (the diaquacobalt(II) mono- and ,-dialkanoates) form two homologous series of layered compounds which have been fully characterised both structurally and magnetically. The crystal structures of two selected members, [Co[CH3(CH2)10COO]2(H2O)2] and [Co[CH3(CH2)18COO]2(H2O)2], have been solved by X-ray powder diffraction and selected-area electron diffraction methods, and refined by the Rietveld technique. Crystal data: monoclinic, P 2(1)/a; a=9.688(1), b=7.5495(9), c=37.281(5) A, =96.70(3) primary, Z=4; and monoclinic, P 2(1)/a; a=9.7260(7), b=7.5477(7), c=57.53(1) A, =94.66(4) primary, Z=4, respectively. Their isomorphous structures contain layers of octahedral diaquacobalt(II) ions bonded to two chemically inequivalent alkanoates, one chelating and one bridging two Co atoms about 6.3 A apart, thus confirming the rare anti-anti conformation mode of the -RCOO groups recently proposed for diaquacobalt(II) ,-dodecanedioate. Extensive magnetic characterisation allowed estimation of the feeble antiferromagnetic coupling, which is weaker in the mono- than in the dialkanoate series.

Interpenetrating diamondoid frameworks of silver( I ) cations linked by N , N ′-bidentate molecular rods

Three novel coordination polymers, [Ag(N–N)2]X (N–N = 4,4′-bipyridyl, X = CF3SO3––1; N–N = 4-cyanopyridine, X = BF4–, in two polymorphs, 2 and 3)], are prepared and investigated by single crystal X-ray analysis; 1 and 2 contain three-dimensional cationic frameworks, both consisting of four equivalent interpenetrating diamondoid lattices, while 3 is formed by two-dimensional layers of distorted squares.

Rhenium(V) oxide complexes. Crystal and molecular structures of the compounds trans-ReI2O(OR)(PPh3)2 (R = Et, Me) and of their hydrolysis derivative ReIO2(PPh3)2

Abstract The structure of the complex ReI2O(OEt)(PPh3)2·3/2C6H6 (I) has been investigated: the dark-yellow crystals are monoclinic, space group P21/n, with a = 17.410(8), b = 17.322(6), c = 15.039(5) A, β = 103.49(3)°, Z = 4. Least-squares refinements based on 3636 significant counter data led to a final R value of 0.059. The compound exhibits a slightly distorted octahedral coordination, with the two PPh3 groups trans between themselves (mean ReP 2.523 A), as well as the two iodide ligands (mean ReI 2.789 A). The ethoxo group (ReO 1.880(9) A) and the oxide ligand (ReO 1.715(9) A) occupy the other two coordination sites. Compound I in methanol transforms into ReI2O(OMe)(PPh3)2 (II), which has been characterized by X-ray analysis in crystals containing CHCl3 molecules of solvation. The crystals are triclinic, space group P 1 , a = 12.835(3), b = 13.067(4), c = 13.485(4) A, α = 89.71(2)°, β = 71.61(2)°, γ = 70.19(2)°, Z = 2. The refinements gave a final R value of 0.036, on the basis of 4524 significant counter intensities. The stereochemistry and the bond parameters are similar to those found in compound I. In particular the ReO (methoxo) and ReO (oxide) bond lengths are 1.859(5) and 1.698(5) A respectively. Hydrolysis reactions of both compounds I and II give the violet derivative ReIO2(PPh3)2 (III). The crystal data are: orthorhombic, space group Aba2, a = 19.834(5), b = 16.144(3), c = 10.560(2) A, Z = 4. 1435 significant counter reflections were used in the refinements the final R value being 0.039. The compound is an unusual example of a five coordinate Re(V) complex. It possesses a rigorous C2 crystallographic symmetry and exhibits a distorted trigonal bipyramidal stereochemistry, with the two PPh3 groups in an axial direction (ReP 2.488(3) A) and the two oxide and the iodide ligands in equatorial positions. The ReI and ReO interactions have values of 2.664(2) and 1.742(11) A respectively.

Highly Emitting Concomitant Polymorphic Crystals of a Dinuclear Rhenium Complex

The dinuclear complex [Re(2)(μ-Cl)(2)(CO)(6)(μ-4,5-(Me(3)Si)(2)pyridazine)] gives in the solid state two polymorphs (yellow, 1Y, and orange, 1O), which can be either concomitantly or separately obtained on varying the crystallization rate. Both crystal phases exhibit intense photoluminescence from the lowest lying triplet metal-to-ligand charge transfer state, much stronger than in solution (quantum yields 0.56 and 0.52, for 1O and 1Y respectively, vs 0.06 in toluene), likely due to the restricted rotation of the Me(3)Si groups in the solid state. A clean, irreversible 1O → 1Y single-crystal-to-single-crystal phase transition occurs at 443 K, as revealed by variable temperature X-ray diffraction analysis. In spite of the absence of any strong intermolecular interactions in both forms, 1O and 1Y show very different absorption and emission maxima (λ(abs) 370 and 393 nm, λ(em) 534 and 570 nm, for 1Y and 1O, respectively). This behavior highlights the importance of the local organization of molecular dipoles in perturbing the photophysical properties of the molecule in the crystal.

Polymorphic coordination networks responsive to CO2, moisture, and thermal stimuli: porous cobalt(II) and zinc(II) fluoropyrimidinolates.

The novel porous [{M(F-pymo)(2)}(n)]2.5n H(2)O coordination networks (M=Co, Zn; F-pymo=5-fluoropyrimidin-2-olate), possessing sodalitic topology, have been synthesised and structurally characterised by means of powder diffraction methods. Thermodiffractometry demonstrated their plasticity: when heated up to 363 K, they reversibly transform into three-dimensional dehydrated [{M(F-pymo)(2)}(n)] species, with significantly different lattice parameters. Further heating induces irreversible polymorphic transformations into layered phases, in which the original MN(4) coordination sphere changes into an MN(3)O one. A mixed-metal phase, [{Co(x)Zn(1-x)(F-pymo)(2)}(n)]2.5n H(2)O, was also prepared, showing that zinc is preferentially inserted, when starting from a Co/Zn reagent ratio of 1:1. The solid-gas adsorption properties of the anhydrous 3D frameworks have been explored towards N(2), H(2) (77 K) and CH(4), CO(2) (273 K). These results show that these materials permit the diffusion of CO(2) molecules only. Remarkably, the CO(2) adsorption process for the [{Co(F-pymo)(2)}(n)] network proceeds in two steps: the first step takes place at low pressures (<600 kPa) and the second one above a threshold pressure of 600 kPa. By contrast, the [{Zn(F-pymo)(2)}(n)] network only permits CO(2) diffusion by applying pressures above 900 kPa. This type of behaviour is typical of porous networks with gated channels. The high CO(2) selectivity of these systems over the rest of the essayed probe gases is explained in terms of flexibility and polarity of the porous network. Finally, the magnetic studies on the Co(II) systems reveal that the as synthesised [{Co(F-pymo)(2)}(n)]2.5n H(2)O material behaves as an antiferromagnet with a T(N) of about 29 K. At variance, the [{Co(F-pymo)(2)}(n)] layered phase shows an unusually weak ferromagnetic ordering below 17 K, arising from a spin-canting phenomenon.

UNIQUE FORMATION OF A CRYSTAL PHASE CONTAINING CYCLIC OLIGOMERS AND HELICAL POLYMERS OF THE SAME MONOMERIC FRAGMENT

The yellow, microcrystalline compound [Cu(pymo)] (Hpymo=2-hydroxypyrimidine) has been characterized with the newly emerging technique of ab initio X-ray powder diffraction. A unique and unprecedented crystal phase containing cyclic oligomers and infinite helical polymers (see picture) of the same monomeric fragment is selectively formed upon reaction of [Cu(CH3 CN)4 ][BF4 ] and Hpymo with NEt3 .

Structural studies of molecular-based nanoporous materials. Novel networks of silver(I) cations assembled with the polydentate N-donor bases hexamethylenetetramine and 1,3,5-triazine

Novel polymeric materials have been isolated from the reactions of silver(i) salts of poorly coordinating anions with the polydentate bases hexamethylenetetramine (hmt) and 1,3,5-triazine (trz), the one-dimensional polymer[Ag(hmt)] SbF 6 H 2 O 1, and the three-dimensional networked compounds [Ag 11 (hmt) 6 ][PF 6 ] 11 14H 2 O 2 and [Ag 6 (trz) 8 ][BF 4 ] 6 H 2 O 3, characterized by single-crystal X-ray analysis. Compound 1is monoclinic, space groupP2 1 /n, a=14.252(4), b=11.061(8), c=16.450(8) A, β=90.17(3)°. The structure contains a novel type of ribbon formed by hexagonal meshes of alternate Ag I ions and hmt molecules. The water molecules coordinated to the silver ions form hydrogen bonds which generate a three-dimensional network of unprecedented topology. Crystals of 2 are monoclinic, space group I2, a=19.552(6), b=10.856(6), c=25.749(11) A, β=92.83(3)°. It consists of an open three-dimensional network of complex topology with all the hmt molecules tetraconnected. Out of the eleven independent Ag I ions per formula unit, nine are biconnnected and two triconnected to the hmt bases. The large channels in the network contain many water molecules. Compound 3 is monoclinic, space group P2 1 /c, a=12.535(8), b=29.517(8), c=14.704(6) A, β=93.95(4)°. The structure contains six independent Ag ions and eight independent triazine molecules, which display varied connectivities to give a puzzling three-dimensional network.

X-RAY POWDER DIFFRACTION CHARACTERIZATION OF POLYMERIC METAL DIAZOLATES

ABSTRACT Polymeric metal diazolates typically appear as insoluble and intractable powders, the structure of which could only be retrieved by the extensive use of ab-initio X-ray powder diffraction (XRPD) methods from conventional laboratory data. A number of selected examples from the metal pyrazolate, imidazolate, pyrimidin-2-olate and pyrimidin-4-olate classes are presented, highlighting the specific crystallochemical properties, material functionality and methodological aspects of the structure determination process. Linear and helical one-dimensional polymers, layered systems and three-dimensional networks are described, with particular emphasis on polymorphism and on the thermal, optical, magnetic and sorption properties. A brief outline of the method, as it has been tailored in our laboratories during the last decade, is also offered.

Ab-initio X-ray powder diffraction structural characterization of co-ordination compounds: polymeric [{MX2(bipy)}n] complexes (M = Ni or Cu; X = Cl or Br; bipy = 4,4′-bipyridyl)

In the absence of crystals of suitable quality, the crystal structures of five [MX2(bipy)](M = Ni or Cu; X = Cl or Br; bipy = 4,4′-bipyridyl) phases were determined, ab initio, from powder diffraction data. The nickel compounds contain symmetrically bridging halides and trans-D4h octahedrally co-ordinated metal atoms; on the contrary, the co-ordination about the copper atoms in [CuBr2(bipy)] is square planar, with long Cu ⋯ Br contacts in the axial direction, due to Jahn–Teller distortion. Interestingly, [CuCl2(bipy)] is dimorphic; both in the orthorhombic and in the monoclinic phases, the copper co-ordination is substantially square planar as in the bromide analogue, but none of the two phases is isomorphous to it.