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The reaction of a cis beta folded organocobalt derivative with a salen-type ligand, 1, isolated as racemic compound of Delta and Lambda enantiomers, with enantiomerically pure alpha-l amino acids is reported. The reaction between racemic 1 and l-tyrosine afforded a mixture of the two diastereoisomers Delta-2 and Lambda-2, which could be separated by fractional crystallization owing to the lower solubility of Delta-2. The absolute configuration of the two diastereomers was unequivocally assigned from the X-ray structure, using the known absolute configuration of the asymmetric carbon of the amino acid as internal reference. The reaction of racemic 1 with trans-4-hydroxy-l-proline afforded only the diastereoisomer with a Delta configuration of the tetradentate ligand, as proved by X-ray diffractometric analysis. For both l-tyrosine and trans-4-hydroxy-l-proline, the amino acid initially coordinates both to the Delta and to the Lambda enantiomers of 1, leading to an about equimolar mixture of diastereoisomers. In the case of l-tyrosine the diastereoisomers have about the same energy, so that the successive isomerization is negligible. In the case of trans-4-hydroxy-l-proline, Delta-3 is much more stable than Lambda-3, and the isomerization reaction Lambda-3 --> Delta-3 goes practically to completion.

The reduction of [Co(III)(tmsalen)py(2)](+)ClO(4)(-), where tmsalen = 4,4',7,7'-tetramethylsalen, with NaBH(4)/PdCl(2) in alkaline methanolic solution, followed by the oxidative addition of CH(2)ClI, leads to the expected trans organometallic dimeric species 1, [CH(2)ClCo(tmsalen)](2), provided that the product is recovered from the reaction mixture immediately after the completion of the reaction. If 1 is left for longer time in contact with the reaction mixture, the intramolecular reaction of the axial chloromethyl group with the equatorial chelate leads to the formation of the monocationic complex 2, containing a seven-membered ring. In this complex the novel tetradentate ligand coordinates Co in a cis fashion, the other two positions being occupied by one py and one water molecule. The resulting complex is chiral, even if the reaction product is a racemic compound. The unidentate ligands of 2 have been exchanged quantitatively for N-MeIm, and the resulting complex 3 still maintains the beta cis geometry. Therefore, 2 may be considered the precursor of a new class of organocobalt derivatives with a folded tetradentate ligand and two adjacent exchangeable sites. On the basis of the geometry of the tetradentate Schiff bases in complexes, where they adopt a planar geometry, it was suggested that there is a significant electron density delocalization involving the metal center over the two chemically equivalent moieties of Co(chel). Comparison of the geometry of the planar salicylaldiminate (sal) moiety with that of the cyclized methoxy-iminate one (imi) in 2 and 3 strongly supports that the delocalization, still present in sal, is essentially either lost or strongly reduced in imi.

The guest-induced synthesis of a molecular box from methylaquacobaloxime and 1,4-phenylenebisboronic acid, with pyrazine (pz) as guest, is described. The resulting supramolecular species was characterized by X-ray structural analysis, 1H and 13C NMR spectroscopy, and low-resolution electrospray ionization (ES) mass spectrometry. The assembly was monitored by a time dependent 1H NMR experiment, which showed that the guest thermodynamically drives the assembly of the host. The effectiveness of pyrazine in this role arises from its having both the correct geometry and a sufficiently low pKa value. Several other ditopic ligands were tested, but none led to formation of an analogous product. However, a second supramolecular species was formed in the case of ethylenediamine (en). X-ray structural analysis and 1H and 13C spectroscopy revealed that this is closely related to the first, with a phenyl side removed and the pz bridge substituted by the en bridge.

The complex mer-[Co(III)(L(1)Npy)(2)](+) (1') where the L(1)Npy(-) is the tridentate 3-[(2-pyridyl)methylimino]butan-2-one oximate ligand, gives alkyl-cobalt derivatives after reduction with NaBH(4)/Pd(2+) to the Co(I) and alkylation. The formation of the cobalt-carbon bond is accompanied by the reduction to the amino form of one or both imino ligands (depending on the experimental conditions) initially present in 1'. In one series of experiments, complexes of the type fac-[RCo(III)(L(1)Npy)(H-L(1)NHpy)](+) (R = Me, i-Pr, CH(2)Cl, CH(2)Br, CH(2)CF(3), and Bz) were obtained, in which only one of the two ligands was reduced to the amino form (H-L(1)NHpy). The saturation of one azomethine group causes the products to assume a fac configuration and induces the formation of one asymmetric carbon and one asymmetric nitrogen center in the chelating system. When an excess of reducing agent is used, both azomethine groups may be saturated, causing the introduction of one pair of chiral carbons and one pair of chiral nitrogens. Two isomers of the methyl derivative [MeCo(III)(L(1)NHpy)(H-L(1)NHpy)](+) were isolated. The X-ray analysis reveals that these isomers differ from one another in configuration of the C and N chiral centers. Possible reaction mechanisms leading to these different types of complexes are proposed.

The reaction of CH(3)Co(DH)(2)H(2)O with 4-pyridinyl boronic acid in methanol or water affords the dinuclear complexes [MeCo(DH)(DB(OR)(4-Py))](2), with R = Me (2) or H (3), respectively, through reaction of boron with the oxime oxygens of the alkylcobaloxime and coordination of the pyridinyl N to cobalt. The reaction is strongly pH dependent, and the formation of the complexes requires a neutral medium. The complexes have been fully characterized by (1)H and (13)C NMR spectroscopy, ESI-MS spectrometry, and elemental analysis. The X-ray structure shows that in 2, the pyridinyl groups are facing each other and nearly perpendicular both to the plane of the Co B Co1 B1 atoms and to the mean equatorial plane, so that the complex may be considered a molecular box. A dimeric arrangement has already been found in the related [MeCo(DH)(DB(OMe)(3-Py))](2) (1) complex, which forms a distorted molecular rectangle [Dreos, R.; Nardin, G.; Randaccio, L.; Tauzher, G.; Vuano, S. Inorg. Chem. 1997, 36, 2463]. The dimerization is possible in both cases, as the conformational freedom of the B bridge compensates for the different position (3- or 4-) of the pyridinyl N donor.

Treatment of R = -CH(2)X (X = halogen) derivatives of the type [RCo(III)(LNH-py)(HLNH-py)](+), where HLNH-py = 2-(2-pyridyl-ethyl)amino-3-butanone oxime and LNH-py its conjugated base with diluted NaOH, afforded a new complex containing a three-membered ring by a pathway involving the intramolecular nucleophilic addition of an equatorial nitrogen donor to the axial carbon. The X-ray analysis reveals a highly distorted structure. The C-Co-N angle is acute (42.8 degrees ) with the distortion of the coordination sphere concentrated in the Co-C axial and Co-N bonds, which move away from the pseudo-octahedral positions in the CH(2)X parent complex to form the C-N bond of the metallocycle. Kinetic studies of the formation of this novel complex starting from [(XCH(2)Co(III)(LNH-py)(HLNH-py)](+) (X = Cl,Br,I) showed that the metallocycle formation rates increase in the order Cl < Br < I. Kinetic data are consistent with a mechanism involving an intermediate species resulting from the deprotonation of an amine equatorial nitrogen in a rapid preequilibrium, followed by the slow step of the ring closure.