New complexes of chromium(III) containing organic π-radical ligands: an experimental and density functional theory study.

The electronic structures of a series of chromium complexes 1-7 have been experimentally investigated using a combination of X-ray crystallography, magneto- and electrochemistry, and Cr K-edge X-ray absorption and UV-vis spectroscopies. Reaction of the dimer [Cr(II)2(µ-CH3CO2)4](0) with 2,2'-bipyridine (bpy(0)) produced the complex [Cr(III)(bpy(0))(bpy(•))(CH3CO2)2](0) (S = 1) (1), but in the presence of isopropylamine ((i)PrNH2) [Cr(III)(bpy(•))((i)PrNH2)2(CH3CO2)2](0) (S = 1) (2) was obtained. Both 1 and 2 contain a Cr(III) ion and a single (bpy(•))(1-) ligand, so are not low-spin Cr(II) species. One-electron oxidation of 1 and 2 yielded [Cr(III)(bpy(0))2(CH3CO2)2]PF6 (S = 3/2) (3) in both cases. In addition, the new neutral species [Cr(III)(DAD(•))3](0) (S = 0) (4) and [Cr(III)(CF3AP(•))3](0) (S = 0) (5) have been synthesized. Both complexes contain three π-radical anion ligands, which derive from one electron reduction of 1,4-bis(cyclohexyl)-1,4-diaza-1,3-butadiene and one electron oxidation of 2-(2-trifluoromethyl)-anilino-4,6-di-tert-butylphenolate, respectively. Intramolecular antiferromagnetic coupling to d(3) Cr(III) gives the observed singlet ground states. Reaction of [Cr(II)(CH3CN)6](PF6)2 with 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (PDI(0)) under anaerobic conditions affords dark brown microcrystals of [Cr(III)(PDI(0))(PDI(•))](PF6)2 (S = 1) (6). This complex is shown to be a member of the electron transfer series [Cr(III)(PDI)2](3+/2+/1+/0), in which all one-electron transfer processes are ligand-based. By X-ray crystallography, it was shown that 6 possesses a localized electronic structure, such that one ligand is neutral (PDI(0)) and the other is a π-radical monoanion (PDI(•))(1-). Again, it should be highlighted that 6 is not a Cr(II) species. Lastly, the structure of [Cr(III)((Me)bpy(•))3](0) (S = 0) (7, (Me)bpy = 4,4'-dimethyl-2,2'-bipyridine) has been established by high resolution X-ray crystallography and clearly shows that three ((Me)bpy(•))(1-) radical anions are present. To further validate our electronic structure assignments, complexes 1-6 were investigated computationally using density functional theory (DFT) and found in all cases to contain a Cr(III) ion. This oxidation state assignment was experimentally confirmed for complexes 2, 4, 5, and 6 by Cr K-edge X-ray absorption spectroscopy.

Redox-switched oxidation of dihydrogen using a non-innocent ligand.

Organometallic complexes containing non-innocent ligands of the type Cp*Ir(tBAFPh)(1), where H2tBAFPh is 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, were found to activate H2 in a redox-switchable manner. The 16e- complex 1 was inert with respect to H2, CO, as well as conventional basic substrates until oxidation. Oxidation of 16-electron 1 with 1 equiv of Ag+ resulted in ligand-centered oxidation affording salts of [1]+, which were characterized by crystallographically, EPR, and elemental analyses. [1]+ was reduced to 1 in the presence of H2 and the sterically hindered base, 2,6-(tBu)2C5H3N, via a pathway that is first-order in both metal and dihydrogen. Compound [1]+ forms adducts with MeCN, which inhibits catalysis. The catalytic oxidation of H2 was established by electrochemical methods to be associated with the monocation.

Wiring terpyridine: approaches to alkynylthienyl 2,2':6',2''-terpyridines.

The reaction of triisopropylsilylethyne or trimethylsilylethyne with 4'-(5-bromo-2-thienyl)-2,2':6',2''-terpyridine () leads to 4'-(5-triisopropylsilylethynyl-2-thienyl)-2,2':6',2''-terpyridine () or 4'-(5-trimethylsilylethynyl-2-thienyl)-2,2':6',2''-terpyridine (). The latter compound may be deprotected to give 4'-(5-ethynyl-2-thienyl)-2,2':6',2''-terpyridine (). Treatment of the complexes [Ru()(2)][PF(6)](2), [Ru(tpy)()][PF(6)](2) and [Ru()(2)][PF(6)](2) ( = 4'-(4-bromo-2-thienyl)-2,2':6',2''-terpyridine) with triisopropylsilylethyne yields the corresponding homoleptic and heteroleptic alkynylthienyl-terminated ruthenium(ii) complexes, for which spectroscopic and electrochemical data are presented. The reaction of [RuCl(2)(DMSO)(4)] with leads to [Ru()(2)][PF(6)](2), in which the tpy ligands bear conjugated thienyl and alkynyl groups; the latter can also be accessed by deprotection of [Ru()(2)][PF(6)](2). The single-crystal structures of ligands and , and of [Ru()(tpy)][BPh(4)](2).1.3MeCN, [Ru()(2)][PF(6)](2).2MeNO(2), 2{[Ru()(2)][PF(6)](2)}.5MeCN, [Ru()(2)][PF(6)](2) and [Ru()(2)][PF(6)](2).MeCN are presented, and among the homoleptic complexes, pi-stacking interactions between thiophene and pyridine rings leading to pairs of associated cations provide a common structural motif.

Molecular and electronic structure of square planar complexes [PdII(tbpy)(L N,O IP)]0, [PdII(tbpy)(LN,O ISQ)](PF6), and [PdII(tbpy)(L N,O IBQ)](PF6)(BF4).2CH2Cl2: an o-iminophenolato based ligand centered, three-membered redox series.

Three Pd(II) complexes which are members of the same electron-transfer series have been synthesized. Refluxing of the reaction mixture containing equimolar amounts of PdCl(2), 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol (H(2)L(N,O)), 4,4'-di-tert-butyl-2,2'-dipyridyl ((t)bpy), and 3 equiv of triethylamine in MeOH under an argon atmosphere followed by exposure to air and addition of KPF(6) after cooling to room temperature yields reddish brown crystals of paramagnetic (S = 1/2) [Pd(L(N,O)(ISQ))((t)bpy)](PF6) (2). Reaction of 2 with one equiv of [CoCp2] in dry and degassed CH(2)Cl(2) using anaerobic conditions gives diamagnetic [Pd(L(N,O)(IP))((t)bpy)] (1), which is the one-electron reduced form of 2. One-electron oxidation of 2 in CH(2)Cl(2) under argon with one equiv of NOBF4 affords diamagnetic [Pd(L(N,O)(IBQ))((t)bpy)](PF6)(BF4).2CH(2)Cl(2) (3). Complexes 1, 2, and 3 constitute three members of the same electron-transfer series. They are ideally suited to distinctly distinguish the geometrical and spectroscopic features of the N,O-coordinated, closed-shell, diamagnetic o-iminophenolate (L(N,O)(IP))2-, the corresponding open-shell pi-radical o-iminobenzosemiquinonate (L(N,O)(ISQ))1-.(S(rad) = 1/2), and the closed-shell o-iminobenzoquinone (L(N,O)(IBQ))0 forms. All complexes were characterized by X-ray crystallography (100 K), cyclic voltammetry, EPR, and UV-vis spectroscopy. Complex 2 exhibits three reversible electron transfer waves in the cyclic voltammogram. Structural characterization of complex 3 reveals an interesting strong ion pairing between the BF4 anion and the complex dication with a short C-F distance of 2.7 A.