Size discrimination in the coordination chemistry of an isoindoline pincer ligand with CdII and ZnII.

The reactions of Cd2+ and Zn2+ with the pyridine-arm isoindoline ligand 4'-MeLH = 1,3-bis[2-(4-methylpyridyl)imino]isoindoline produced the series of octahedrally coordinated complexes M(4'-MeL)2, [M(4'-MeLH)2]2+, and [M(4'-MeL)(4'-MeLH)]+. The complexes M(4'-MeL)2 resulted from reactions of the respective metal perchlorates with deprotonated ligand, whereas the complexes [M(4'-MeLH)2](ClO4)2 resulted from reactions with ligand in the absence of added base. The mixed-ligand complexes [M(4'-MeL)(4'-MeLH)]+ were generated in solution by reactions of equimolar quantities of M(4'-MeL)2 and [M(4'-MeLH)2]2+. Whereas [Cd(4'-MeL)(4'-MeLH)]+ is stable in solution, [Zn(4'-MeL)(4'-MeLH)]+ converts to and establishes equilibrium with the tetrahedrally coordinated, trinuclear complex [Zn3(4'-MeL)4]2+. The complexes Cd(4'-MeL)2 (1), Zn(4'-MeL)2 (2), and [Cd(4'-MeL)(4'-MeLH)]ClO4 (5) were characterized by single-crystal X-ray diffraction, with the latter complex being shown to contain 4'-MeLH coordinated as a protonated iminium zwitterionic ligand. The [M(4'-MeLH)2]2+ and [M(4'-MeL)(4'-MeLH)]+ complexes are tautomeric in solution because of the shuttling of the iminium protons between imine N atoms. The rate of prototropic tautomerism in [Cd(4'-MeLH)2]+ was followed by 1H NMR spectroscopy. Over the temperature range 276-312 K, a linear Eyring plot with the activation parameters DeltaG++ = 16.0 +/- 0.1 kcal/mol, DeltaH++ = 2.9 +/- 0.1 kcal/mol, and DeltaS++ = -44.0 +/- 0.3 cal/mol.K was obtained.

Cd(II), Zn(II), and Pd(II) complexes of an isoindoline pincer ligand: consequences of steric crowding.

The sterically crowded isoindoline pincer ligand, 6'-MeLH, prepared by condensation of 4-methyl-2-aminopyridine and phthalonitrile, exhibits very different reaction chemistry with Cd2+, Zn2+, and Pd2+. Three different ligand coordination modes are reported, each dependent upon choice of metal ion. This isoindoline binds to Cd2+ as a charge-neutral, zwitterionic, bidentate ligand using imine and pyridine nitrogen atoms to form the eight-coordinate fluxional complex, Cd(6'-MeLH)2(NO3)2. In the presence of Zn2+, however, loss of a pyridine arm occurs through solvolysis and tetrahedrally coordinated complexes are formed with coordination of pyrrole and pyridine nitrogen atoms. Reaction with Pd2+ produces the highly distorted, square planar complex Pd(6'-MeL)Cl in which a deprotonated isoindoline anion coordinates as a tridentate pyridinium NNC pincer ligand.

Fluxional behavior of a cadmium zwitterion complex: proton transport and tautomerism in methylene chloride solution.

The synthesis and structure of the tautomeric Cd(II) isoindoline zwitterion coordination compound [Cd(4'-MeLH)(NO(3))(2)].CH(3)OH (4'-MeLH = 1,3-bis[2-(4-methylpyridyl)imino]isoindoline) are reported. In methylene chloride solution, tautomer interconversion occurs as the N-H proton moves between the identical imine nitrogen atoms. We report the kinetics of proton transfer as followed by variable temperature (1)H NMR spectroscopy and demonstrate that methanol of solvation and adventitious water facilitate rapid proton transfer.

Syntheses and structures of isoindoline complexes of Zn(II) and Cu(II): an unexpected trinuclear Zn(II) complex.

Deprotonation of the tridentate isoindoline ligand 1,3-bis[2-(4-methylpyridyl)imino]-isoindoline, 4'-MeLH, and reaction with hydrated zinc(II) perchlorate produces an unexpected trinuclear Zn(II) complex, [Zn(3)(4'-MeL)(4)](ClO(4))(2).5H(2)O (1), whereas reaction with hydrated copper(II) perchlorate in methanol produces the expected mononuclear product, [Cu(4'-MeL)(H(2)O)(2)]ClO(4) (2). X-ray diffraction shows that the trinuclear Zn(II) complex (1) contains a linear zinc backbone, and the arrangement of ligands about the outer chiral zinc(II) atoms is helical. The two terminal zinc ions exhibit approximate C(2) site symmetry, with tetrahedral coordination by two pyrrole and two pyridyl nitrogen atoms of the potentially tridentate isoindoline ligands. The central zinc ion exhibits approximate tetrahedral symmetry, with coordination by four pyridyl nitrogen atoms of four different isoindoline ligands. Pyridyl-pyrrole intramolecular pi-stacking interactions contribute to the stability of the trinuclear cation. The structure of the mononuclear copper(II) complex cation in 2 is best described as a distorted trigonal bipyramid. The isoindoline anion binds Cu(II) in both axial positions and one of the equatorial positions; water molecules occupy the other two equatorial positions.

Synthesis and Structure of an Air-Stable, Free-Radical Cobalt(III) Semiquinone Complex.

The air-stable, free-radical, low-spin Co(III) complex, (Bu(4)N)(2) [3,5-Co(DBSQ)(CN)(4)].(1)/(2)H(2)O.(1)/(4)CH(2)Cl(2) (1), where 3,5-DBSQ is the semiquinone anion derived from the one-electron reduction of 3,5-di-tert-butyl-1,2-benzoquinone, has been synthesized by the reaction of the cobalt(II) tetramer [Co(3,5-DBSQ)(2)](4) with Bu(4)NCN in THF. This is a cyanide-induced redox reaction resulting in the formation of cobalt (II) and cobalt(III) products as follows, where 3,5-DBCat is the respective catecholate dianion: [Co(3,5-DBSQ)(2)](4) + 8CN(-) --> 2[Co(3,5-DBSQ)(CN)(4)](2)(-) + 2[Co(3,5-DBSQ)(2)(3,5-DBCat)](2)(-). The Co(III) product, (Bu(4)N)(2) [Co(3,5-DBSQ)(CN)(4)], is insoluble in THF while the Co(II) product remains in solution. Single-crystal X-ray diffraction of (1) reveals octahedrally coordinated cobalt(III) and C-O and C-C bond lengths indicative of semiquinone. The cyanide ligands occupy the remaining four sites with essentially linear Co-CN bond angles and average Co-C and Co-O bond distances of 1.89(2) Å and 1.97(2) Å, respectively. The complex has a magnetic moment of 1.80 &mgr;(B) and a typical semiquinone, S = (1)/(2), free-radical EPR signature (CH(2)Cl(2) solution, 293K) with g = 2.002, a(Co) = 8.8G, and a(H) = 2.6G. The identity of [Co(3,5-DBSQ)(2)(3,5-DBCat)](2)(-) (2) in the above reaction was confirmed by independent in situ generation of this anion from the reaction of [Co(3,5-DBSQ)(2)](4) with 3,5-DBCat(2)(-) solution.