Alessandro Fumagalli

Stereochemical non-rigidity of a metal polyhedron; carbon-13 and platinum-195 fourier transform nuclear magnetic resonance spectra of [Ptn(CO)2n]2- (n = 3, 6, 9, 12 or 15)

Abstract Platinum-195 spectra are reported for [Ptn(CO)2n]2- (n = 3, 6, 9, 12 and 15) and carbon-13 spectra are reported for n = 6, 9 and 12 over a range of temperatures. The spectra provide evidence for (a) intramolecular rotation of the Pt3-triangles about the principal three-fold axis, (b) inter-exchange of Pt3-triangles, (c) lack of terminal/edge carbonyl exchange within the Pt3(CO)3(μ-CO)3 group. Evidence is also presented for the formation of [Ni3Pt3(CO)12]2- on mixing [Pt6(CO)12]2- [Ni6(CO)12]2-.

Hydroformylation of olefins under mild conditions: Part I: the Co4−nRhn(CO)12 + x L (n = 0, 2, 4 ; x = 0 - 9) system and preformed Rh4(CO)12−xLx clusters (x = 1 – 4)

Abstract The hydroformylation of cyclohexene, 1-pentene and styrene under mild conditions (25–50 °C, 1 atm equimolar mixture of CO and H 2 ) has been investigated using as catalyst precursor either the Co 4- n Rh n (CO) 12 + x L ( n = 0, 2, 4; x = 0 – 9) system or preformed Rh 4 CO) 12− x L x ( x = 1 – 4) substituted clusters, where L is a trisubstituted phosphine or phosphite. The activity of these systems increases as a function of x , and reaches a maximum for a L/Co 4− n Rh n (CO) 12 ( n = 2, 4) molar ratio of ca . 5 – 6. A further increase in this ratio corresponds to a smooth decrease in the activity. This ratio has apparently a negligible effect on the regioselectivity in the hydroformylation of both 1-pentene and styrene. In contrast, both the activity and the regioselectivity are significantly affected by the nature of the ligand employed as cocatalyst. When working with Rh 4 (CO) 12 as well as Rh 6 (CO) 16 , and trisubstituted phosphites as ligands, infrared spectroscopy and 31 P NMR invariably show the presence of Rh 4 (CO) 9 L 3 as the most substituted rhodium carbonyl species present in solution, and there is no evidence of fragmentation of the tetranuclear cluster during the catalytic process. In contrast, when using phosphine ligands such as PPh 3 , evidence of fragmentation to Rh 2 (CO) 6 (PPh 3 ) 2 or to Rh 2 (CO) 4 (PPh 3 ) 4 species has been obtained at the higher PPh 3 /Rh 4 (CO) 12 molar ratios. Degradation of the ligand employed as cocatalyst, particularly the arylsubstituted phosphines, is observed, and this is probably at the origin of the loss of catalytic activity of some of these systems with time.

[Rh(CO)4]−, [Rh5(CO)15]−, and bimetallic clusters as catalysts for the carbonylation of nitrobenzene to methyl phenylcarbamate

Bimetallic clusters of the general formula [PPN]2[MRh4(CO)15] (M  Fe, Ru, or Os; PPN+ = (PPh3)2N+) are active catalysts in the presence of methanol for the carbonylation of nitrobenzene to methyl phenylcarbamate. The clusters are active even without co-catalyst, but the addition of bipy (2,2′-bipyridine) greatly enhances both rates and selectivities. In the absence of methanol the conversion was much lower, with a low selectivity in phenyl isocyanate. Interchanging Fe, Ru, and Os has a negligible effect on the activity and a moderate effect on the selectivity, with ruthenium being best. However, the monometallic rhodium cluster [Rh5(CO)15]− proved to be both more active and selective, and the monomer [Rh(CO)4]− was found to be even better. This last catalytic system has been optimized with respect to temperature, CO pressure and the bipy/Rh ratio. The influence of solvent was also examined.

Synthesis of the carbonyl cluster dianion [RuRh4(CO)9(μ2-CO)6]2− and X-ray crystal structure of its bis(triphenylphosphine)iminium salt

The dianion [RuRh4(CO)15]2− has been obtained by reductive carbonylation of mixtures of Rh4(CO)12 and Ru3(CO)12, and the bis(triphenylphosphine)iminium salt has been characterized by single-crystal X-ray diffraction techniques. Triclinic crystals, space group P1 (No. 2), a 11.459(3), b 14.171(5), c 26.083(8) A, α 84.32(3), β 83.46(2), γ 82.81(2)°, Z = 2; the salt is isomorphous with (PPN)2[RuIr4(CO)15]. Least-squares refinement based on 2849 significant reflections gave a final R value of 0.059. The mixed metal cluster is an elongated trigonal bipyramid with the Ru atom located in apical position, as confirmed by variable temperature 13C NMR spectroscopy. The mean values of the RheqRheq, RheqRhap and RheqRu bond lengths are 2.71, 3.03 and 2.99 A, respectively. The nine terminal and the six edge-bridging CO groups are disposed as in [RuIr4(CO)15]2−.

Synthesis and structural characterization of the [NiRh6(CO)16]2− dianion: a new mixed heptanuclear carbonyl cluster. Evidence for other NiRh mixed carbonyl cluster anions

Abstract The first heptanuclear mixed carbonyl cluster, the anion [NiRh6(CO)16]2− has been prepared by redox condensation of [Rh6(CO)15]2− with [Ni6(CO)12]2− or Ni(CO)4. The structure of the metallic framework is based on a distorted octahedron of Rh atoms capped on one face by a nickel atom and is closely related to that previously observed in the isoelectronic [Rh7(CO)16]3− anion. Evidence is also reported for the existence of [NiRh5(CO)15]− and [NiRh4-(CO)12]2−.

Mixed Rh–Pt carbonyl clusters: synthesis and X-ray crystallographic characterisation of [Rh5Pt(CO)15]–[(Ph3P)2N]+

The [Rh5Pt(CO)11(µ3-CO)4]– anion, which has been prepared by two independent routes, is shown to be structurally related to [Rh6(CO)12(µ3-CO)4][one Rh(CO)2 group is replaced by a Pt(CO) group] both in the solid state (X-ray) and in solution (n.m.r.); evidence is also presented for a further reduced mixed anion [Rh2Pt-(CO)x]n–.

Stereochemical non-rigidity of a metal polyhedron; fourier transform platinum-195 nuclear magnetic resonance spectra of [Ptn(Co)2n]2–(n= 3, 6, or 9)

195 Pt n.m.r. data are reported for [Ptn(CO)2n]2–(n= 3, 6, or 9) and, when n= 9, the spectra are consistent with rotation of the outer Pt3-triangles with respect to the middle triangle about the principal 3-fold axis; inter-exchange of Pt3-triangles occurs in mixtures of [Pt9(CO)18]2– and [Pt12(CO)24]2– at 25°C.

Synthesis and X-ray structure of the anion [Rh9(µ3-CO)3(µ-CO)9(CO)7]3– containing a cofacial bioctahedral rhodium cluster

The novel anion [Rh9(CO)19]3–, obtained by condensation between the anions [Rh4(CO)11]2– and [Rh5(CO)15]–, contains a Rh9 metal-atom cluster formed by two condensed octahedral sharing a face.

Synthesis and characterization of the tetranuclear anions [Rh4(CO)11(COOCH3)− and [Rh4(CO)11]2−

Abstract The orange [Rh 4 (CO) 11 (COOCH 3 )] − anion has been produced by reaction of sodium methoxide, or anhydrous sodium carbonate, with Rh 4 (CO) 12 in methanol. This anion reacts with potassium hydroxide to give the red [Rh 4 (CO) 11 ] 2− dianion, which can also be obtained directly from Rh 4 (CO) 12 and potassium hydroxide. Both these anions have been isolated as pure bis(triphenylphosphine)iminium salts. The carboalkoxy anion is very water sensitive, while the [Rh 4 (CO) 11 ] 2− dianion is air sensitive and reactive toward carbon monoxide. Both these anions react with Rh 4 (CO) 12 , to give [Rh 6 (CO) 5 (COOCH 3 )] − and [Rh 12 (CO) 30 ] 2− , respectively.

BiCo heteronuclear carbonyl compounds. Synthesis and crystal structure of the cubane-like species [Co(CO)3(μ3-Bil)]4

Abstract The compound [Co(CO) 3 (μ 3 -Bi)] 4 has been obtained by prolonged pyrolysis of [BiCo(CO) 4 3 ] in refluxing tetrahydrofuran. This species, isomorphous with the analogous [Co(CO) 3 (μ 3 -Sb)] 4 , crystallizes in the tetragonal space group I 4 d1 / amd (No. 141) with a 14.731(3), c 10.574(3) A and Z = 4. The structure was refined by full-matrix least-squares, on the basis of 447 significant unique reflections, to a final R value of 0.032. The compound has a cubane-like structure, with four tetrahedrally disposed Co(CO) 3 units triply bridged by four Bi atoms. The BiCo bonds have a mean value of 2.745 A. Each cobalt atom exhibits a highly distorted octahedral coordination, with BiCoBi and CCoC angles of mean value 75.48 and 101.1°. The Co…Co contacts are definitely non-bonding (mean 4.291 A). the non bonding Bi…Bi interactions (mean 3.360 A) are particularly short. The mean values of the CoC and CO bond lengths are 1.78 and 1.19 A, respectively.