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Piero Macchi

Publications and source records attributed to Piero Macchi.

10 recordsLinked to original sources
PubMed2006

A Re(CO)5 fragment bonded to a polyhedral oligomeric hydroxysilsesquioxane as a model which gives further evidence to the existence of an unexpected Re(CO)5 surface species.

Reaction of Re(CO)5O3SCF3 with (c-C6H11)7Si8O12O-Li+ at 273 K under a CO atmosphere affords the [Re(CO)5OR] (R = (c-C6H11)7Si8O12) derivative (1). 1 is the first example of a rhenium pentacarbonyl bearing an OR ligand (R = alkyl, aryl, or silyl) stable enough to be characterized, and it represents also the first molecular model of the surface [Re(CO)5OSi] species formed by reductive carbonylation of silica-supported [Re(CO)3OH]4. At room temperature, 1 loses one carbonyl ligand and dimerizes to afford {Re(CO)4[(mu-O)O12Si8(c-C6H11)7]}2 (2), which has been characterized by X-ray diffraction and is the first reported example of a rhenium tetracarbonyl mu-oxo-bridged dimer of the type [Re(CO)4(mu-OR)]2.

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PubMed2005

The electron density of bridging hydrides observed via experimental and theoretical investigations on [Cr2(mu2-H)(Co)10]-.

The accurate experimental electron density distribution of [Cr2(mu2-H)(CO)10]- has been determined through X-ray diffraction at T = 28 K and gas phase theoretical calculations. The nature of the Cr-H-Cr bond has been investigated by means of the quantum theory of atoms in molecules, including the analysis of the Fermi hole distribution and the delocalization indicators from the pair density distribution. We were able to clarify not only the delocalization mechanism inside the three-center system but also the unexpected role of the equatorial carbonyls. In addition, the comparative study of a few Cr-H-X three center interactions has provided more insight into the nature of the M-H-M bond itself and has shed light on its peculiar stereochemical flexibility.

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PubMed2005

Using ring strain to inhibit a decomposition path: first synthesis of an Alkyl-BIAN ligand (Alkyl-BIAN = bis(alkyl)acenaphthenequinonediimine).

N-Alkyl imines of acenaphthenequinone are not stable because an isomerization occurs that releases part of the ring strain of the initially formed imine by changing the hybridization of one of the ring carbon atoms from sp(2) to sp(3); however, if an even more strained ring is present in the alkyl group, the isomerization becomes unfavorable and the compound is stable.

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PubMed2004

Synthesis of mixed Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine). Measurement of the coordination strength of hemilabile ligands with respect to their symmetric counterparts.

The synthesis of Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine) having different aryl groups bound to the two nitrogen atoms is reported for the first time. The ligands were obtained by two different strategies: (i) by a transimination reaction starting from a symmetric Ar,Ar-BIAN ligand having aryl groups bearing strongly electron-withdrawing substituents or (ii) by a two-step-one-pot sequence. The ligands synthesized have been chosen so that the electronic difference between the two aryl groups is very large, but the steric difference is variable and, in one case, the ligand is almost sterically symmetric. The coordination strength of the new ligands towards three palladium complexes has been measured by a competition experiment following a protocol previously described by us. The coordination strength of the mixed ligands is the mean of those of the corresponding symmetric counterparts. The X-ray crystal structure of a palladium pi-allyl complex of the electronically asymmetric-sterically symmetric ligand (3,5-(CF(3))(2)C(6)H(3)),(3,5-Me(2)C(6)H(3))-BIAN has been solved, together with those of the two symmetric analogues to allow a comparison. Despite the fact that the dodecafluorinated ligand has a K(eq) value about three orders of magnitude lower than the non-fluorinated counterpart, no notable difference is observed in the N-Pd and Pd-C(allyl) distances in the three complexes. Calculations at the density functional level confirm that Pd-BIAN distances are not strictly correlated to the coordination energies, which are in qualitative agreement with the spectroscopic evidence. The bond length is thus not a good indication of the bond strength in these cases.

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PubMed2004

Variable-temperature X-ray crystallographic studies: a complementary tool for charge-density investigation of soft (organometallic) bonds.

Comparative X-ray diffraction studies at different temperatures are used to complement the information usually extracted from 'single-point' charge-density studies. The response of soft bonds against (temperature driven) changes of the crystalline environment might be indicative of some intramolecular mode associated with an incipient bonding. This is in fact confirmed for the anion [FeCo(CO)(8)](-) in the [(Ph(3)P)(2)N](+) salt. In contrast, the [Cr(2)H(CO)(10)](-) anion, whose conformation is known to strongly depend on the counterion, is actually insensitive to T variations in the [K(C(18)H(36)N(2)O(6))](+) salt.

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PubMed2004

Electron transfer and CO addition to polynitrido cobalt carbonyl clusters: parallel pathways for conversion of the [Co10N2(CO)19]4- anion to the novel [Co11N2(CO)21]3- anion.

The redox aptitude of the dinitrido anion [Co10N2(CO)19]4- has been tested from both chemical and electrochemical points of view, together with its reactivity toward CO that induces disproportionation. In any case, through a remarkable overlapping of intermediate steps, the new anion [Co11N2(CO)21]3- (4) is eventually obtained. A detailed study of the pathways to 4 allowed the identification of three labile intermediates by their characteristic IR spectra as well as their electrochemical and paramagnetic properties. The unprecedented structure of trianion 4 has been studied in details in two different crystalline salts.

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PubMed2002

Electron density of semi-bridging carbonyls. Metamorphosis of CO ligands observed via experimental and theoretical investigations on [FeCo(CO)(8)](-).

The electron density distribution in a transition-metal dimer containing a semibridging carbonyl is determined through experimental X-ray diffraction and quantum chemical computations. The changes occurring during the evolution from terminal to bridging coordinations are described by a "structure-correlation-like" approach and by a theoretical investigation along the conversion path. The smooth continuum of conformations observed in the solid state is explained in terms of the mutual interplay of direct M-M and M-CO and indirect M- - -M and M- - -C interactions, which can be characterized by interatomic delocalization indexes, within the framework of Quantum Theory of Atoms in Molecules.

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PubMed2002

Synthesis and structural characterization of the [[Rh(5)(CO)(14)]-(H(2)N(CH(2))(4)NH(2))-[Rh(5)(CO)(14)])](2-) and [Rh(5)(CO)(13)(H(2)N(CH(2))(2)NH(2))](-) anions (as [PPh(4)](+) salts): an unprecedented example of carbonyl substitution by alkylamines in a homoleptic metal carbonyl cluster anion.

The substitution of one or two carbonyls by many different primary and secondary alkylamines and -diamines has been established for the first time in a homoleptic carbonyl cluster anion, the trigonal bipyramidal [Rh(5)(CO)(15)](-). Two derivatives, the bis-monosubstituted [[Rh(5)(CO)(14)]-(H(2)N(CH(2))(4)NH(2))-[Rh(5)(CO)(14)]](2-) dianion (1) and the disubstituted chelated [Rh(5)(CO)(13)(H(2)N(CH(2))(2)NH(2))](-) monoanion (2), have been structurally characterized, both in the solid state (as [PPh(4)](+) salts) and in solution, revealing that the sites of the substitution are the cluster apexes. (13)C NMR spectra of 2 revealed localized fluxionality of the CO ligands over the temperature range 298-183 K.

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