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Structural and 1 H NMR data have been obtained for cobaloximes with the bulkiest substituted pyridines reported so far. We have isolated in noncoordinating solvents the complexes CH 3 Co(DH) 2 L (methylcobaloxime, where DH = the monoanion of dimethylglyoxime) with L = sterically hindered N-donor ligands: quinoline, 4-CH 3 quinoline, 2,4-(CH 3 ) 2 Pyridine, and 2-R-pyridine (R = CH 3 , OCH 3 , CH 2 CH 3 , CH=CH 2 ). We have found that the Co-N ax bond is very long in the structurally characterized complexes. In particular, CH3Co(DH)2(4-CH3quinoline) has a longer Co-N ax bond (2.193(3) A) than any reported for methylcobaloximes. The main cause of the long bonds is unambiguously identified as the steric bulk of L by the fairly linear relationship found for Co-N ax distance vs CCA (calculated cone angle, CCA, a computed measure of bulk) over an extensive series of methylcobaloximes. The linear relationship improves if L basicity (quantified by pK a ) is taken into account. In anhydrous CDCl 3 at 25 °C, all complexes except the 2-aminopyridine adduct exhibit 1 H NMR spectra consistent with partial dissociation of L to form the methylcobaloxime dimer. 1 H NMR experiments at -20 °C allowed us to assess qualitatively the relative binding ability of L as follows: 2,4-(CH 3 ) 2 pyridine > 4-CH 3 quinoline ≈ quinoline ≈ 2-CH 3 pyridine > 2-CH 3 Opyridine > 2-CH 3 CH 2 pyridine > 2-CH 2 =CHpyridine. The broadness of the 1 H NMR signals at 25 °C suggests a similar order for the ligand exchange rate. The lack of dissociation by 2-aminopyridine is attributed to an intramolecular hydrogen bond between the NH 2 group and an oxime O atom. The weaker than expected binding of 2-vinylpyridine relative to the Co-N ax bond length is attributed to rotation of the 2-vinyl group required for this bulky ligand to bind to the metal center, a conclusion supported by pronounced changes in 2-vinylpyridine signals upon coordination.

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.

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 ClBrI. 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.

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.