High-spin Co(II) complexes are promising for development as paraCEST agents (paraCEST = paramagnetic chemical exchange saturation transfer) for magnetic resonance imaging applications. The first examples of Co(II) paraCEST agents with bound water ligands are presented here. Four Co(II) macrocyclic complexes based on 1,4,7-triazacyclononane and containing either pendent alcohol or pendent amide groups were prepared. Two of the macrocycles encapsulate the Co(II) and contain no water ligands as shown by X-ray crystallographic studies, and two complexes have macrocycles with only five ligand donor groups to leave an open coordination site for bound water. The ionization of alcohol, water, or amide groups in the complexes was characterized by using pH potentiometry. These data show that one of the complexes has a readily deprotonated group with a pK close to 6, which is assigned as an alcohol pendent. Amide pendents deprotonate at high pH (>8), and the water ligands of the Co(II) complexes are not deprotonated at neutral pH. All complexes produce CEST peaks through either alcohol OH or amide NH proton exchange. The water ligands exchange too rapidly to produce a CEST effect as shown by variable-temperature O NMR spectroscopy studies. The complexes with available coordination sites for inner-sphere water ligands produce large paramagnetic shifts and broadening of theO resonances of bulk water, whereas the encapsulated complexes show much less shifting and broadening of O resonances. All complexes produce substantial paramagnetic shifts of theH resonances of bulk water, which is promising for the development of supramolecular CEST agents.

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The synthesis of three five-coordinate nickel() complexes with pendant arm-containing macrocycles has been achieved by the reduction of C᎐ ᎐ N bonds in the Schiff base precursors derived from diacetyl-or diformyl-pyridine and a tripodal tetramine. Demetallation of the nickel() macrocycles yielded stable pentadentate ligands that were used for the preparation of the copper() complexes. The structures of three nickel() complexes and two copper() complexes were determined by X-ray crystallography. Protonation of the pendant arm (pK a = 6.3-6.6 for the nickel complexes, and 6.5-7.3 for the copper complexes) produced four-coordinate macrocycles, one of which was structurally characterized. The primary amino group of the pendant arm coordinated to the nickel() reacted with acetic anhydride or benzoyl chloride. The resulting mono-functionalized nickel() complexes and their copper() counterparts obtained by transmetallation displayed square-planar geometry in the solid state, as determined by X-ray crystallography, and remained four-coordinate in solutions below pH 11.

Noncovalent π-π interactions between chloroboron subphthalocyanine (1), 2,3-subnaphthalocyanine (3), 1,4,8,11,15,18-(hexathiophenyl)subphthalocyanine (4), or 4-tert-butylphenoxyboron subphthalocyanine (2) with C and C fullerenes were studied by UV-vis and steady-state fluorescence spectroscopy, as well as mass (APCI, ESI, and CSI) spectrometry. Mass spectrometry experiments were suggestive of relatively weak interaction energies between compounds 1-4 and fullerenes. The formation of a new weak charge-transfer band in the NIR region was observed in solution only for subphthalocyanine 4 when titrated with C and C fullerenes. Molecular structures of the subphthalocyanines 2 and 4 as well as cocrystallite of 4 with C fullerene (4···C) were studied using X-ray crystallography. One of the C fullerenes in the crystal structure of 4···C was found in the concave region between two subphthalocyanine cores, while the other three fullerenes are aligned above individual isoindole fragments of the aromatic subphthalocyanine. The excited-state dynamics in noncovalent assemblies were studied by transient absorption spectroscopy. The time-resolved photophysics data suggest that only electron-rich subphthalocyanine 4 can facilitate an electron-transfer to C or C fullerenes, while no electron-transfer from the photoexcited receptors 1-3 to fullerenes was observed in UV-vis and transient spectroscopy experiments. DFT calculations using the CAM-B3LYP exchange-correlation functional and the 6-31+G(d) basis set allowed an estimation of interaction energies for the noncovalent 1:1 and 1:2 (fullerene:subphthalocyanine) complexes. Theoretical data suggest that the weak (∼3.5-10.5 kcal/mol) van der Waals-type interaction energies tend to increase with an increase of the electron density at the subphthalocyanine core with compound 4 being the best platform for noncovalent interactions with fullerenes. DFT calculations also indicate that 1:2 (fullerene:subphthalocyanine) noncovalent complexes are more stable than the corresponding 1:1 assemblies.

Two macrocyclic complexes of 1,4,7-triazacyclononane (TACN), one with N-methyl imidazole pendants, [Fe(Mim)], and one with unsubstituted NH imidazole pendants, [Fe(Tim)], were prepared with a view toward biomedical imaging applications. These low-spin Fe complexes produce moderately paramagnetically shifted and relatively sharp H NMR resonances for paraSHIFT and paraCEST applications. The [Fe(Tim)] complex undergoes pH-dependent changes in NMR spectra in solution that are consistent with the consecutive deprotonation of all three imidazole pendant groups at high pH values. N-Methylation of the imidazole pendants in [Fe(Mim)] produces a complex that dissociates more readily at high pH in comparison to [Fe(Tim)], which contains ionizable donor groups. Cyclic voltammetry studies show that the redox potential of [Fe(Mim)] is invariant with pH ( E = 328 ± 3 mV vs NHE) between pH 3.2 and 8.4, unlike the Fe(III) complex of Tim which shows a 590 mV change in redox potential over the pH range of 3.3-12.8. Magnetic susceptibility studies in solution give magnetic moments of 0.91-1.3 cm K mol (μ value = 2.7-3.2) for both complexes. Solid-state measurements show that the susceptibility is consistent with a S = 1/2 state over the temperature range of 0 to 300 K, with no crossover to a high-spin state under these conditions. The crystal structure of [Fe(Mim)](OTf) shows a six-coordinate all-nitrogen bound Fe(III) in a distorted octahedral environment. Relativistic ab initio wave function and density functional theory (DFT) calculations on [Fe(Mim)], some with spin orbit coupling, were used to predict the ground spin state. Relative energies of the doublet, quartet, and sextet spin states were consistent with the doublet S = 1/2 state being the lowest in energy and suggested that excited states with higher spin multiplicities are not thermally accessible. Calculations were consistent with the magnetic susceptibility determined in the solid state.

New phenyliodine(III) sulfate (PhIO)3·SO3, which has a complex polymeric structure of the trimeric iodosylbenzene units linked by sulfate anions, can be conveniently prepared by treatment of (diacetoxy)iodobenzene with sodium bisulfate in the presence of water. This sulfate can find practical application as a readily available, stable, and water‐soluble hypervalent iodine reagent with a reactivity pattern similar tothat of iodosylbenzene. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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cis-3,4-Dicyano-3-hexene undergoes cyclotrimerization with BCl3 to form the new subtriazaporphyrin chloro[hexaethylsubtriazaporphyrinato]boron(III). The hydroxo derivative of this macrocycle has also been made, and the X-ray crystal structure of the hydroxy form was determined. Electronic absorption and magnetic circular dichroism spectra of the hydroxo monomer species were interpreted using time-dependent density functional theory calculations.