Giovanni Tauzher

Organometallic derivatives of cobalt chelates of bis(acetylacetone) ethylendiamine

Abstract The preparations of the complexes Co II (BAE), [Co III (BAE)L 2 ]X, Co III (BAE)LX and of some new stable organocobalt complexes RCo III (BAE) and RCo III (BAE)·H 2 O are described (BAE = bis(acetylacetone) ethylendiamine; L = NH 3 , C 6 H 5 CH 2 NH 2 , C 6 H 5 N, (C 6 H 5 ) 3 P; X = Cl, Br). The UV and visible absorption spectra are reported. The compounds RCo(BAE) appear to be new examples of penta-coordinated organocobalt complexes and constitute further evidence for the stabilization of the cobalt-carbon bond by chelation. The labilization of the sixth ligand by the alkyl or aryl group in the trans position is pointed out.

Evidence of the interaction between steric and electronic influence in rhodoximes and cobaloximes. Synthesis of pyRh(DH)2I and X-ray structure of pyRh(DH)2Cl, pyCo(DH)2Cl and pyRh(DH)2I

Abstract The crystal structure of pyRh(DH)2Cl (I) and pyCo(DH)2Cl (II) are reported together with the synthesis and the crystal structure of pyRh(DH)2I (III).I: RhClO4N5C13H19; a=7.5090(8), b=8.588(1), c=14.479(2) A; α=96.45(8), β=87.07(8), γ=76.12(9)°; triclinic P 1 ; Z=2; R=0.022. II: CoClO4N5C13H19; a=14.738(2), b=8.606(1), c=13.369(2) A; β=96.30(1)°; monoclinic Cc; Z=4; R=0.022. III: IRhO4N5C13H19; a=8.875(1), b=14.741(2), c=14.163(3) A; β=97.47(8)°; monoclinic P21/c; Z=4; R=0.031. The Rh-py distance of 2.046(1) A in I, compared to that of 2.079(3) A in III, indicates that I is a slightly better trans-influencing ligand than Cl. The difference in the M-py distances when X=Cl and i-pr is 0.18 A for M=Rh and 0.14 A for M=Co. This result suggests that the interaction between steric and electronic factors is reduced in rhodoximes, owing to the larger size of RhIII.

Comparison between alkylrhodoximes and alkylcobaloximes. Solution studies of some alkylrhodoximes, pyRh(DH)2R, and crystal structure of the complexes with RCH2CF3, CH2Cl, n-Pr

Syntheses, kinetics and NMR measurements of pyRh(DH) 2 R complexes, where py=pyridine and DH=monoanion of dimethylglyoxime, with various R alkyl groups are described. X-ray crystal structures of the compounds with RCH 2 CF 3 ( I ), CH 2 Cl ( II ), n-Pr ( III ) are reported. Comparison of the geometry of the Rh(DH) 2 and Co(DH) 2 moieties shows significant differences in the MN, NO and O···O distances, with a decrease of the approximate symmetry from D 2h in cobaloximes to C 2h in rhodoximes. The axial Rh-C and Rh-N distances lengthen with increasing bulk and the σ-donating ability of R, respectively; the Rh-C distances are 2.059(5) A in I and 2.069(5) A in II , and the Rh-N(axial) distances are 2.145(3) ( I ), 2.178(3) ( II ) and 2.188(5) ( III ) A. The chemical shifts of the pyridine meta and para carbons decrease with the increase of the alkyl group σ-donating power; the linear relationship found between the δ values of the pyridine para carbon in rhodoximes and the EP values, derived from NMR data of cobaloximes, shows that the transmission of electronic charge from R to the neutral ligand through rhodium occurs in a way similar to that through cobalt. The rate constants for the pyridine dissociation increase with the σ-donating ability of the alkyl group, as measured by EP parameters. These results show that, as already reported for cobaloximes, trans -influence and trans -effect have the same trend in rhodoximes too.

Diphenylborylated derivatives of organocobaloximes and organorhodoximes: Synthesis, spectroscopic and structural characterisation

Abstract A series of organometallic complexes derived by organocobaloximes and organorhodoximes in which either one or both the hydrogen bridges have been replaced by BPh 2 groups, RM(DH)(DBPh 2 ) N -MeIm and RM((DBPh 2 ) 2 N -MeIm, respectively, have been synthesised and characterised, both in solution and in solid state. 1 H NMR spectra show that they assume different interconverting conformations in solution. With increasing steric bulk of R, the axial phenyls of the BPh 2 group tend to face N -MeIm, forcing the latter in an orientation which is quite unusual in organocobaloximes and causing a lengthening of the Co-N bond. Some possible implications on the strength of the trans Co-C bond are discussed.

New organo-cobalt complexes derived from cobaloximes with one or two diphenylboron moieties in the oxime bridges

Abstract The reaction between methylcobaloxime [CH 3 Co(DH) 2 L] and diphenylborinic anhydride affords derivatives containing either one or two diphenylboron bridges, depending on the ratio of complex: diphenylborinic anhydride. The crystal structures of [CH 3 Co(DH)(DBPh 2 )( N -MeIm)] ( Ib ) and [CH 3 Co(DBPh 2 ) 2 CH 3 OH] ( IId ) show that in Ib the axial phenyl of the BPh 2 moiety faces the axial Me, while in IId the axial phenyls are “trans”. The preferred conformations of these complexes in solution, as inferred from NMR spectra, are analogous to those found in the solid state.

Mechanistic informations from trans-effect in the anation reactions of some bis(dimethylglyoximato)Rh(III) complexes

Abstract The parallel lability trend for the anation reactions of RRh(DH)2H2O (RCH3, CH3CH2, CF3CH2, ClCH2) and RCo(DH)2H2O complexes suggests a dissociative activation process for the reactions of the organorhodoximes. The high lability of these complexes, arising from the stabilization of the transition state, is not entirely due to the trans-effect of the R group, but, at least partially, to the labilizing effect of the equatorial macrocycle.

Synthesis and structure of borylated organocobaloximes containing neutral nitrile ligands

Abstract The syntheses of the organometallic complexes RCo(DBPh2)2(MeCN), where R = Me (1), n-Pr (2), trans-β-styryl (β-Sty, 3), and MeCo (DBPh2)2TCNE (4) are described. The complexes were characterized by X-ray crystallography. Compound 1 is monoclinic, space group P2 1 /n, a = 9.081(2), b = 14.491(3), c = 14.178(3) A , β = 95.35(2)°, Z = 2, R = 0.053 compound 2 is monoclinic, space group P2 1 /a, a = 18.973(6), b = 9.369(2), c = 21.059(6) A , β = 106.65(2)°, Z = 4, R = 0.079 ; compound 3 is monoclinic, space group P2 1 /n, a = 15.130(2), b = 16.055(3), c = 15.591(3) A , β = 91.70(1)°, Z = 0.071 ; compound 4 is triclinic, space group P I , a = 15.390(6), b = 15.529(7), c = 18.038(7) A , α = 107.44(2)°, β = 103.54(2)°, γ = 101.19(2)°, Z = 4, R = 0.058 . Compound 1 is found to have an up-down conformation of the equatorial ligand, with one phenyl group facing the Me axial ligand and the other facing the axial MeCN ligand, whereas compounds 2–4 in the solid state show the down-down conformation, with the two axial phenyls of the equatorial ligand facing the neutral one. The present results sshow that the conformation adopted by these complexes is determined primarily by the difference in bulk between the axial ligands. When the bulk of R is similar to that of L. the π = π interactions become the conformational driving force.

Kinetics of aquation of trans-chloro-tertiary phosphine-CoIII-bisdimethylglyoximato complexes

Abstract The kinetics of the aquation reaction of trans- chloro-tertiary phosphine-Co III -bisdimethylglyomixato complexes and pKa of the corresponding aquo-complexes have been examined in order to determine the trans-effect of the tertiary phosphine on the chloro and aquo axial ligand respectively. The aquation rates, the pKa of aquo-complexes, and half-wave potentials of reduction of the aquo-complexes at d.m.e. correlated with the pKa of the phosphine. The results are discussed in term of σ-donor character of the phosphine and trasmission of electronic effect through the cobalt atom. The effect of the planar chelating system on the kinetic properties in model molecules of vitamin B 12 is discussed.

Self-Assembly of Cobaloximes and Rhodoximes with 3-Aminophenylboronic Acid: A Molecular Triangle and a Polymer

The reactions of methylaquacobaloxime and ethylaquacobaloxime with 3-aminophenylboronic acid in water produce supramolecular triangles 3a and 3b respectively, through the reaction of boron with the oxime oxygens of the alkylcobaloxime and the coordination of pyridine to cobalt. The reaction is reversible and strongly pH dependent. The complexes 3a and 3b precipitate immediately from the solution at pH 7, but dissolve quickly at both acidic (below 4) and basic (above 9) pH values. The aggregates are re-formed by readjusting the pH to neutrality and the precipitation-dissolution cycle may be repeated several times without an apparent decrease in yield. The complexes were fully characterised by 1H and 13C NMR spectroscopy. The X-ray structure shows that the neutral chiral supramolecule 3a is a triangle of cobalt atoms with an approximate C3 symmetry. The side of the triangle is about 9.5 A. The ring closure is ensured by the torsion angles around the B−C and C−NH2 bonds, which vary in the range 25−30° and 97−109°, respectively. The reaction of methylaquarhodoxime with 3-aminophenylboronic acid in water at pH 7 results in the displacement of H2O by 3-aminophenylboronic acid only. The polymeric form 4 precipitates from CD3OD in the NMR tube with time. The X-ray structure shows that 4 is a polymeric species with a zig-zag chain arrangement, with Rh···Rh distances of 9.2 A. The chain arrangement is characterized by torsion angles around the B−C and C-NH2 bonds of 61° and 92°, respectively. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Mechanism of formation of trans-chloro-bisdimethylglyoximato-tertiary phosphine cobalt(III) complexes and their isomers 1:1 electrolytes

Abstract The Tschugaeff reaction between cobalt chloride and dimethylglyoxime, in presence of tertiary phosphine or of nitrogen bases, in ethanol yields non charged trans- chlorobisdimethylglyoximato-tertiary phosphine cobalt(III) complexes and/or their isomers 1:1 electrolytes, depending of the nature of axial Lewis base. The mechanism of the reaction is discussed and interpreted in term of the labilizing (kinetic) trans-effect of the Lewis base.