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Angelo Sironi

Publications and source records attributed to Angelo Sironi.

15 recordsLinked to original sources
PubMed2003

Synthesis and reactivity of (C6F5)3B-N-heterocycle complexes. 1. Generation of highly acidic sp3 carbons in pyrroles and indoles.

The reaction of pyrroles and indoles with B(C(6)F(5))(3) and BCl(3) produces 1:1 B-N complexes containing highly acidic sp(3) carbons, for example, N-[tris(pentafluorophenyl)borane]-5H-pyrrole (1) and N-[tris(pentafluorophenyl)borane]-3H-indole (2), that are formed by a new formal N-to-C hydrogen shift, the mechanism of which is discussed. With some derivatives, restricted rotation around the B-N bond and/or the B-C bonds was observed by NMR techniques, and some rotational barriers were calculated from experimental data. The acidity of the sp(3) carbons in these complexes is shown by their ability to protonate NEt(3), with formation of pyrrolyl- and indolyl-borate ammonium salts. The driving force for this reaction is given by the restoration of the aromaticity of the heterocycle.

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PubMed2003

Complexation of beryllium(II) ion by phosphinate ligands in aqueous solution. Synthesis and XRPD structure determination of Be[(PhPO2)2CH2](H2O)2.

Two bifunctional ligands, phenyl(carboxymethyl)phosphinate (ccp(2-) and P,P'-diphenylmethylenediphosphinate (pcp(2-)), have been tested as chelating agents of beryllium(II). Both ligands have the same charge and a similar chelating structure, but whereas the 1:1 adduct of pcp(2-), Be(pcp)(H(2)O)(2), could be isolated as a white powder, no pure compound could be isolated from solutions containing beryllium(II) and ccp(2-). Instead, the solutions were examined by means of potentiometry and (9)Be NMR spectroscopy. Analysis of the potentiometric titration data with the program HYPERQUAD suggested the formation of the complex species BeL, [BeHL](+), [BeL(2)](2-), and [BeHL(2)](-) (L = ccp). The formation constants for these species were determined at 25 degrees C and I = 0.5 mol dm(-3) NaClO(4). The (9)Be NMR spectra are consistent with this model. The formation constants found for the ccp(2-) complexes are lower than those reported for related phosphonate ligands. However, the effective stability constant (which gives a better indication of the intrinsic coordinating capacity of the ligand at a particular pH) of the complex [Be(ccp)(2)](2-) at pH < 4 is greater than the effective constants of the corresponding phosphonoacetate and methylenediphosphonate complexes. The structure of Be(pcp)(H(2)O)(2) was determined by X-ray powder diffraction methods and consists of discrete molecules interconnected by an extended 2D network of hydrogen bonds, resulting in a stacking of doublelayers with a polar core and a lipophilic surface. Crystal data: C(13)H(16)BeO(6)P(2), fw 339.21, monoclinic P2(1)/c, a = 16.174(1) A, b = 8.979(1) A, c = 10.929(1) A, beta = 90.398(9) degrees, V = 1587.2(3) A(3), Z = 4.

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PubMed2002

An intramolecular N-H...(mu-H)Re2 dihydrogen bond and a novel mu 3-eta 2 coordination mode of the pyrazolate anion on a triangular cluster face.

The quantitative addition of pyrazole (Hpz) to the 44 valence-electron, triangular cluster anion [Re3(mu 3-H)-(mu-H)3(CO)9]- gives the novel unsaturated anion [Re3(mu-H)4(CO)9(Hpz)]- (1, 46 valence electrons), which contains a pyrazole molecule that is terminally coordinated on a cluster vertex. Solidstate X-ray and IR analyses reveal a rather weak hydrogen-bonding interaction between the NH proton and one of the hydrides bridging the opposite triangular cluster edge (delta H degree = -3.1 kcal mol-1 from the Iogansen equation). Both IR and NMR data indicate that such a proton-hydride interaction is maintained in the major conformer present in CD2Cl2, but also provide evidence of the presence of minor conformers of 1 in which the NH proton is involved in an intermolecular hydrogen bond with the solvent. The mu-H...HN bond length evaluated in solution through the T1 minimum value (2.07 A) and that determined in the solid state by X-ray diffraction (2.05 A) are in good agreement. NMR experiments show that, in acetone, intermolecular N-H...solvent interactions replace the intramolecular dihydrogen bond. At room temperature in CH2Cl2, the pyrazole ligand in 1 is labile and 1 slowly "disproportionates" to [Re3(mu 3-H)-(mu-H)3(CO)9]- and [Re3(mu-H)3(CO)9-(mu-eta 2-pz)(Hpz)]-, with H2 evolution. Slow H2 evolution also leads to the formation of the anion [Re3(mu-H)3-(CO)9(pz)]- (5), in which the pyrazolate anion adopts a novel mu 3-eta 2-coordination mode, as revealed by a single-crystal X-ray analysis. The analysis of the bond lengths indicates that the pyrazolate anion in 5 acts as a six-electron donor, with loss of the aromaticity. The formation of 5 from 1 is much faster in solvents with a high dielectric constant, such as acetone or DMF. Anion 5 was also obtained from the reaction of pyrazole with [Re3(mu-H)3(CO)9(mu 3-CH3)]- through the intermediate formation of two isomeric addition derivatives and following CH4 evolution.

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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

One-dimensional polymers containing strictly collinear metal ions: synthesis and XRPD characterization of homoleptic binary metal pyrazolates.

The synthesis of a number of 3d transition metal binary pyrazolates in microcrystalline form, thus suitable for a full XRPD characterization, has been pursued. The crystal and molecular structures of the Fe(pz)3, Co(pz)2, Co(pz)3, and Ni(pz)2 polymers, together with the few congeners reported in the recent literature, show that these species tend to afford highly crystalline materials where strictly collinear chains of metal atoms are present. Depending on the synthetic strategy used, Ni(pz)2 has been found to crystallize as two different alpha (orthorhombic) and beta (monoclinic) phases, possessing nearly identical intramolecular features. Data for each compound follow. Fe(pz)3: C9H9FeN6, hexagonal, P63/m, a = 9.1745(3) A, c = 7.2191(4) A, Z = 2. Co(pz)2: C6H6CoN4, orthorhombic, Ibam, a = 7.5239(5) A, b = 14.3461(9) A, c = 7.4331(5) A, Z = 4. Co(pz)3: C9H9CoN6, hexagonal, P63/m, a = 9.1966(3) A, c = 7.1051(3) A, Z = 2. Alpha-Ni(pz)2: C6H6N4Ni, orthorhombic, Cmcm, a = 16.6758(11) A, b = 6.4872(4) A, c = 6.9423(6) A, Z = 4. Beta-Ni(pz)2: C6H6N4Ni, monoclinic, P21/m, a = 9.967(2) A, b = 6.975(1) A, c = 6.016(1), A, beta = 98.50(1)degrees, Z = 2. The thermal stability and the detailed structural properties of these model compounds have been evaluated, in the light of the technologically relevant crystal phases (the well-known metal-diazolates showing reversible spin-crossover or spin-transition behavior) obtainable upon doping, magnetic dilution, and ring substitution (in the 4-position).

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PubMed2002

NMR and DFT analysis of [Re(2)H(2)(CO)(9)]: evidence of an eta(2)-H(2) intermediate in a new type of fast mutual exchange between terminal and bridging hydrides.

Protonation of the anion [Re(2)H(CO)(9)](-) (1) with a strong acid at 193 K affords the neutral complex [Re(2)H(2)(CO)(9)] (2), that in THF above 253 K irreversibly loses H(2) to give [Re(2)(CO)(9)(THF)], previously obtained by room-temperature protonation of 1. Treatment of 2 with NEt(4)OH restores the starting anion 1. Variable temperature (1)H and (13)C NMR spectra as well as T(1) measurements agree with the formulation of 2 as a classical [Re(2)H(mu-H)(CO)(9)] complex, in which two dynamic processes takes place. The "windshield-wiper motion" observed in several related complexes equalizes the two carbonyls trans to the hydrides (E(a) = 44(1) kJ mol(-)(1)), while another much faster process equalizes bridging and terminal hydrides already at 172 K. The variable temperature behavior of the (1)H transverse relaxation times revealed also proton exchange between 2, water, and the parent anion 1 (due to the acidity of 2), but such a process is too slow to account for the fast hydrides exchange in 2. The nature of the latter process has been investigated both experimentally and theoretically. Kinetic data, obtained by the analysis of the variable temperature (1)H spectra (E(a) = 24.5(5) kJ mol(-1)), revealed a small normal kinetic isotope effect (ca. 1.5). The (2)H chemical shift of the fully deuterated isotopomer 2-d(2) was found isochronous with 2, thus ruling out the presence of a significant concentration of a nonclassical [Re(2)(eta(2)-H(2))(CO)(9)] tautomer, in fast exchange with the classical dihydride. Density functional theory (DFT) calculations, carried out at the B3LYP level, confirmed the formulation of [Re(2)H(2)(CO)(9)] as a classical complex. However, when DFT was used to obtain a detailed description of the dynamic behavior of 2 in solution, a new type of hydride fast exchange emerged, involving the nonclassical tautomer as a relatively high energy (12.7 kJ mol(-1)) intermediate. Isotopic perturbation of the equilibrium by partial deuteration of 2 indicated the preference of deuterium for the bridging sites, with Delta H degrees = -475(4) J mol(-1) and Delta S degrees = -0.80(2) J K(-1) mol(-1). The same preference was observed in the anion [Re(2)H(mu-H)Cl(CO)(8)](-).

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PubMed2002

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.

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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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PubMed1999

Metal Pyrazolato Complexes. Synthesis, Characterization, and X-ray Powder Diffraction Studies of Group 12 Coordination Polymers.

A number of polymeric pyrazolato complexes have been prepared and characterized by spectroscopy, thermal analyses (DSC and TGA), and X-ray powder diffraction (XRPD) methods. Ab initio XRPD studies showed that the (isomorphous) [Zn(pz)(2)](n)() and [Cd(pz)(2)](n)() species (Hpz = pyrazole) are 1-D polymers containing tetrahedrally coordinated metals and M(&mgr;-pz)(2)M (M = Zn, Cd) bridges, much alike [Cu(pz)(2)](n)() [orthorhombic, Ibam, a = 7.4829(4), b = 14.3844(6), c = 7.3831(3) Å (Zn) and a = 7.8591(6), b = 13.652(1), c = 7.9165(4) Å (Cd)]; differently, Hg(pz)(2) [triclinic, P&onemacr;, a = 7.4097(3), b = 9.4474(3), c = 5.8345(3) Å, alpha = 96.310(2), beta = 96.752(3), and gamma = 73.694(2) degrees ] is best described as a mononuclear complex, containing two monodentate pyrazolato ligands loosely interacting, through long(er) Hg.N contacts with neighboring molecules. During the synthesis of the latter, an intermediate phase was obtained, and characterized as Hg(pz)NO(3), which contains a polymeric polycation, [Hg(pz)](n)()(n)()(+), based on Hg(&mgr;-pz)Hg bridges, and uncoordinated NO(3)(-) groups (orthorhombic, Pcmn, a = 17.2985(9), b = 5.2538(3), c = 7.3912(4) Å). All structures were ultimately refined by the Rietveld method.

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PubMed1998

Mixed Ruthenium-Rhodium Carbonyl Cluster Complexes. Synthesis of the Anions [Ru(3)Rh(CO)(13)](-) and [RuRh(3)(CO)(12)](-) and Crystal Structures of Their [N(PPh(3))(2)](+) Salts(1).

The new anion [Ru(3)Rh(CO)(13)](-) (1) has been obtained by reaction of [Rh(CO)(4)](-) ([N(PPh(3))(2)](+) or [PPh(4)](+) salt) with Ru(3)(CO)(12); [RuRh(3)(CO)(12)](-) (2) has been derived by oxidative degradation of [RuRh(4)(CO)(12)](2-). Their salts, [N(PPh(3))(2)][Ru(3)Rh(CO)(13)] (I) and [N(PPh(3))(2)][RuRh(3)(CO)(12)] (II), were characterized by single-crystal X-ray diffraction. Data for I: space group P&onemacr;, a = 9.827(2) Å, b =14.911(2) Å, c = 18.735(3) Å, alpha = 110.53(1) degrees, beta = 99.79(2) degrees, gamma = 91.85(2) degrees, R1 = 0.0342, wR2 = 0.0926 for 7257 independent reflections with I > 3sigma(I). Data for II: space group P2(1)/c, a = 14.746(3) Å, b = 21.395(4) Å, c = 16.140(4) Å, beta = 102.95(2) degrees, R1 = 0.0582, wR2 = 0.1383 for 4381 independent reflections with I > 3sigma(I). Both anions have a tetrahedral metal frame; 1 has an idealized C(3) symmetry, with the Rh atom and its unique terminal carbonyl on the 3-fold axis; nine more carbonyls are terminally bonded to the three Ru atoms, while three bridge the Ru-Rh edges. In 2, which has an idealized C(s)() symmetry, three carbonyls are terminally bound on the Ru atom, and one, one, and two CO, respectively, on Rh1, Rh2, and Rh3; five more CO bridge all edges but the Ru-Rh3 edge. It is worthy of note that subtle details of the CO ligands stereochemistry allowed the correct labeling of metal centers, otherwise indistinguishable on the basis of good quality X-ray diffraction data only. Compound 1 reacts with PPh(3), yielding the rhodium-substituted [Ru(3)Rh(CO)(12)(PPh(3))](-) (1a) ((31)P NMR; 56.9 ppm (d), J(P-)(Rh) = 188 Hz). (13)C NMR spectrum of 1 is a doublet (207 ppm, J(C-Rh) = 17.5 Hz) consistently with a fluxional behavior with complete CO scrambling, from 295 down to 170 K.

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