Donatella Strumolo

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.

Heterometallic adducts of [Rh6(CO)15C]2–, including the novel double- and triple-decker sandwich compounds [Agn{Rh6(CO)15C}2](4–n)–(n= 1 and 3) and [Agn{Rh6(CO)15C}3](6–n)–(n= 2 and 4) and the X-ray structural characterisation of [PPh4]3[Ag{Rh6(CO)15C}2]

Multinuclear n.m.r. measurements have been used to establish the formation of the following new adducts of [Rh6(CO)15C]2– by capping the trigonal face of the Rh6 trigonal prism: [{M(PEt3)}{Rh6(CO)15C}]–(M = Ag or Au), [Agn{Rh6(CO)15C}2](4–n)–(n= 1 or 3), [Agn{Rh6(CO)15C}3](6–n)–(n= 2 or 4), [Agn{Rh6(CO)15C}n]n–(n > 3), and [Ag2{Rh6(CO)15C}]. There is no evidence for dissociation of any of these species on the n.m.r. time-scale, whereas n.m.r. spectra suggest that dissociation of [Cu(NCMe)]+ occurs in [{Cu(NCMe)}n{Rh6(CO)15C}](2–n)–(n= 1 or possibly 2) both at room and low temperature. X-ray crystallographic analysis of [Ag{Rh6(CO)15C}2]3– shows that a silver atom is sandwiched between trigonal faces of two staggered Rh6 trigonal-prismatic units and the nature of the silver–rhodium interaction is discussed.

New carbide clusters in the cobalt subgroup. Part 13. Synthesis and chemical characterization of the anions [Co6C(CO)14]–, [Co6C(CO)15]2–, and [Co8C(CO)18]2–, and crystal structure of µ6-carbido-ennea-µ-carbonyl-hexacarbonyl-polyhedro-hexacobaltate(2–) as its benzyltrimethylammonium salt; a comparison with isostructural species

The anions [Co6C(CO)15]2– and [Co8C(CO)18]2– have been prepared starting from compounds such as [Co3C(CO)9Cl], Na[Co(CO)4], and [Co4(CO)12], and [Co6C(CO)14]– has been obtained from [Co6C(CO)15]2– by oxidation with FeCl3 or iodine. The chemistry and relationships between these anions are discussed. The molecular structure of [Co6C(CO)15]2– has been determined by single-crystal X-ray crystallography. It is monoclinic, space group C2/c, with a= 21.752(2), b= 11.350(1), c= 18.099(2)A, β= 112.28(1)°, and Z= 4. The structure has been solved from 2 184 reflections, collected by counter methods, and refined by least-squares calculations to R= 0.074. The dianion has idealized D3d symmetry and contains a trigonal prism of cobalt atoms, whose cavity accommodates an interstitial carbon atom. The structural parameters are carefully compared with those of the isostructural anions [Rh6C(CO)15]2– and [M6N(CO)15]–(M = Co or Rh) to show how the metal atoms, the interstitial species, and the anionic charge affect the bond parameters.

Carbide cluster chemistry in the cobalt sub-group ☆: XI. Synthesis of the dodecanuclear carbido carbonyl cluster anions [Rh12C2(CO)23]n− (n = 3, 4) and X-ray characterization of the [Rh12C2(μ-CO)10(CO)13]3− trianion

Abstract The new [Rh12C2(CO)23]n− (n = 3, 4) anions have been obtained by reduction of [Rh12C2(CO)24]2−. The trianion is paramagnetic and rather unstable; an X-ray crystal structure determination shows that it has the same metal framework as the parent dianion.

ESR investigation of paramagnetic carbonyl-metal clusters of high nuclearity

Abstract An ESR study has been made of the high nuclearity paramagnetic metal cluster anions [Rh12(CO)13(μ2-CO)10(C)2]3-, [Co13(CO)12(μ2-CO)12(C)2]4- and [Co6(CO)8(μ2-CO)6C]-. The assignment of the HOMO is based on a mixed valence model which relates the g tensor components of cluster systems to those of an appropriate conventional paramagnetic center. With this model the HOMOs of [Rh12(CO)13(μ2-CO)10(C)2]3- and of [Co13(CO)12(μ2-CO)12(C)2]4- are found to be mainly comprised of metal dz2 atomic orbitals, while for [Co6(CO)8(μ2-CO)6C]- a large overlap between d atomic orbitals and ligand orbitals is suggested. The occupation of the valence molecular orbitals deduced from the ESR data is consistent with the variations in MM bond distance observed by X-ray analysis.

New carbide clusters in the cobalt sub-group. Part 4. Synthesis and crystallographic characterization of µ3-carbonyl-deca-µ-carbonyl-di-carbidotetradecacarbonyl-polyhedra-dodecarhodium

The title complex crystallizes in the orthorhombic space group P212121 with unit-cell dimensions a= 18.695(4), b= 18.604(4), c= 11 605(2)A, and Z= 4. The structure has been determined by direct methods from X-ray single-crystal counter data and refined by least-squares calculations to R 0.037 for 3 450 significant diffraction intensities. The complex [Rh12C2(CO)25] is asymmetric and the metal-atom cluster is an irregular closed polyhedron which can be described in terms of layer packing of atoms (mean Rh–Rh 2.79 A). The carbide carbon atoms occupy an irregular cavity as a C2 unit [C–C 1.48(2)A]; there are 14 Rh–C(carbide) contacts, nine short and five long (means 2.22 and 2.58 A respectively). Of the carbonyl ligands, 14 are terminal, 10 are edge-bridging, and one is face-bridging.

New carbide clusters in the cobalt sub-group. Part 10. Preparation and crystallographic characterization of dicarbido-octa-µ-carbonyl-hexadecacarbonyl-polyhedro-dodecarhodate(2–) as its bis(triphenyl-phosphine)iminium salt, [N(PPh3)2]2[Rh12C2(CO)24]

The title compound has been prepared by heating at 70 °C the dianion [Rh6C(CO)15]2– and H2SO4 in propan-2-ol. Its molecular structure has been determined by single-crystal X-ray crystallography: triclinic, space group P, with a= 17.190(4), b= 13.049(4), c= 11.581(3)A, α= 103.24(1), β= 88.69(1), γ= 102.24(1)°, and Z= 1. The structure has been solved from 3 421 diffraction intensities, collected by counter methods, and refined by least-squares calculations to R= 0.050. The dianion has idealized D2h-mmm symmetry and contains a Rh12 polyhedron based on a square–rhomb–square sequence of layers arranged in such a way as to form two prismatic cavities sharing one edge. Two carbon atoms are encapsulated in these cavities. Relevant average bond distances are Rh–C(carbide) 2.12 and Rh–Rh 2.81 A. Eight CO ligands bridge the edges of the square layers and 16 are terminal, one per metal atom in the outer squares and two for each atom of the central rhomb. The Rh–C and C–O average distances are 1.89, 1.10, and 2.04, 1.17 A for terminal and bridging groups, respectively.

New carbide clusters in the cobalt sub-group. Part III. Crystallographic characterization of the hydroxonium salt of tetradeca-µ-carbonyl-dicarbidotetradecacarbonyl-polyhedro-pentadecarhodate(1–)

The trtle compound crystallizes in the orthorhombic space group Pbcn, with cell constants a= 15.N (2). b= 17.34(2). c= 18.85(2)A, Z= 4. The structure was determined by direct methods from X-ray single-crystai counter data and refined by least-squares calculations to R 0.0267 for 2 387 significant reflections.The [Rh15(CO)28C2]– anion has precise C2 symmetry idealizable to C2v. The Rh15 metal atom cluster can be described as a centted and tetracapped pentagonal prism in which the central atom is twelve-co-ordinated in a non-crystallographicfashion, with Rh–Rh distances ranging from 2.738 to 3.332(3)A. The two carbide-atoms occupy octahedral cavities, mean Rh–C 2.04 A. The carbonyl ligands are bound on the cluster surface, fourteen linearly and fourteen edge-bridging such that all the metal atoms are thrice-connected to the ligands. The molecular architecture is discussed and a growth mechanism of the cluster is proposed.

Hydroformylation of olefins under mild conditions: Part II: The Co4−nRhn(CO)12 + xPPh2H + yL (n = 2, 4) system

Abstract The hydroformylation of cyclohexene, 1-pentene and styrene under mild conditions (25 – 50 °C, 1 atm equimolar CO and H2) in toluene or benzene using the system Co4−nRhn(CO)12 + xPPh3−mHm + yL (n = 2, 4; m = 1,2; L = trisubstituted phosphine or phosphite) as catalyst precursor has been investigated. This system was designed to generate in situ clusters containing a variable number of μ-PPh3−m bridging ligands. A set of experiments, in which x and y were independently and systematically varied in the range 1 – 4, points out that the Co4−nRhn(CO)12 + xPPh2H + yL system is an active hydroformylation catalyst for x = 1 – 2 and y = 2 – 4. Furthermore, the combined addition of PPh2H and a trisubstituted ligand L to Rh4(CO)12 apparently brings about a ‘synergetic effect’ which results in hydroformylation rates of 1-pentene and styrene often greater than those observed in the absence of PPh2H. An increase of x to 3 or 4 causes a sharp drop in the catalytic activity. Accordingly, preformed Rh3(μ2-PPh2)3(CO)5 and Rh4(μ2-PPh2)4(CO)6, both in the presence or in the absence of additional trisubstituted ligands, are substantially inactive. The Co4−nRhn(CO)12 + xPPhH2 + yL system in contrast is barely active for all tested values of x and y, although an increase in the temperature brings about some activity in a few cases. Unlike 1-pentene and styrene, the catalytic hydroformylation of cyclohexene could not be induced by combined addition of PPl2H and additional trisubstituted ligand to Rh4(CO)12 in any ratio. It is therefore conceivable that the formation of clusters containing the μ2-PPh2 group could be at the origin of the previously reported deactivation of the Rh4(CO)12 + xPPh3 system in the hydroformylation of cyclohexene.

Mixed-metal carbido carbonyl clusters. Part 1. Synthesis and structural characterization of Di-µ3-acetonitrilecuprio-carbido-ennea-µ-carbonyl-hexacarbonyl-polyhedro-hexarhodium, [Cu2Rh6(CO)15(NCMe)2]·0.5MeOH

The title complex has been prepared by direct reaction of the [Cu(NCMe)4]+cation with the [Rh6C(CO)15]2– anion in methanolic solution, and characterized by chemical and crystallographic methods. The species [Cu2Rh6(CO)15(NCMe)2].0.5MeOH crystallizes in the monoclinic space group C2/c with cell constants a= 18.21(1), b= 10.509(1), c= 20.69(1)A, β= 126.00(5)°, and Z= 4. 2 175 reflection intensities, collected by counter methods, have been used to solve and refine the structure down to R 0.041. The molecule contains a prism of rhodium atoms capped on the triangular faces by two Cu(NCMe) linear fragments, and the geometry of the CO ligands is the same as in the parent dianion. The idealized molecular symmetry is D3h. The Cu–Rh, Rh–Rh, and Rh–C(carbide) mean distances are 2.660, 2.78, and 2.127 A, respectively. The Cu–Rh bonds have been described in terms of σ and π dative interactions from the rhodium prism to the copper atoms.