Synthesis and structural analysis of Bis(2-hydroxyphenyl) phenylamine, PhN(o-C6H4OH)2: Comparison with Tris(2-hydroxyphenyl)amine N(o-C6H4OH)3

The molecular structures of N(o-C₆H₄OH)₃, PhN(o-C₆H₄OH)₂, andp-TolN(o-C₆H₄OMe)₂ have been determined by X-ray diffraction, thereby indicating several structural differences. For example, whereas the nitrogen in N(o-C₆H₄OH)₃ is pyramidal with Σ_C–N–C = 348.3^∘, the nitrogen atoms in PhN(o-C₆H₄OH)₂ andp-TolN(o-C₆H₄OMe)₂ are trigonal planar with Σ_C–N–C = 359.9^∘ and Σ_C–N–C = 360.0^∘, respectively. The phenyl andp-tolyl groups of PhN(o-C₆H₄OH)₂ andp-TolN(o-C₆H₄OMe)₂ lie close to the trigonal plane, while theo-C₆H₄OH ando-C₆H₄OMe groups are almost orthogonal to this plane. The coplanar and orthogonal orientations of the aryl groups of PhN(o-C₆H₄OH)₂ andp-TolN(o-C₆H₄OMe)₂ are in marked contrast to those of the phenyl groups within Ph₃N, which exhibit dihedral angles in the range 38–52^∘ and approximateD₃ symmetry. The observed structures of PhN(o-C₆H₄OH)₂ andp-TolN(o-C₆H₄OMe)₂ may be rationalized in terms of maximizing delocalization of the nitrogen lone pair into the phenyl andp-tolyl groups, while minimizing unfavorable overlap with theo-C₆H₄OH ando-C₆H₄OMe groups due to the presence of π-donatingortho-substituents; the orthogonal orientation of theo-C₆H₄OH ando-C₆H₄OMe groups is also one that minimizes unfavorable steric interactions between theortho-substituents.     

Interaction between the type-3 copper protein tyrosinase and the substrate analogue p-nitrophenol studied by NMR

The interaction of the monooxygenating type-3 copper enzyme Tyrosinase (Ty) from Streptomyces antibioticus with its inhibitor p-nitrophenol (pnp) was studied by paramagnetic NMR methods. The pnp binds to oxidized Ty (Ty(met)) and its halide (F(-), Cl(-)) bound derivatives with a dissociation constant in the mM range. The Cu(2) bridging halide ion is not displaced upon the binding of pnp showing that the pnp does not occupy the Cu(2) bridging position. The binding of pnp to Ty(met) or Ty(met)Cl leads to localized changes in the type-3 (Cu-His(3))(2) coordination geometry reflecting a change in the coordination of a single His residue that, still, remains coordinated to Cu. The binding of pnp to Ty(met)Cl causes a decrease in the Cu(2) magnetic exchange parameter -2J from 200 cm(-)(1) in the absence to 150 +/- 10 cm(-)(1) in the presence of pnp. From the (1)H and (2)D NMR relaxation parameters of pnp bound to Ty(met), a structural model of pnp coordination to the Ty type-3 center could be derived. The model explains the absence of hydroxylase activity in the closely related type-3 copper protein catechol oxidase. The relevance of the experimental findings toward the Ty catalytic mechanism is discussed.     

Diorganoruthenium complexes incorporating noninnocent [C(6)H(2)(CH(2)ER(2))(2)-3,5](2)(2-) (E = N, P) bis-pincer bridging ligands: synthesis, spectroelectrochemistry, and DFT studies

The dinuclear complex [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 (bridging PCP-PCP = 3,3',5,5'-tetrakis(diphenylphosphinomethyl)biphenyl, [C6H2(CH2PPh2)2-3,5]22-) was prepared via a transcyclometalation reaction of the bis-pincer ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN = [C6H3(CH2NMe2)2-2,6]-) followed by a reaction with 2,2':6',2' '-terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 are compared with those of the closely related [(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2 (NCN-NCN = [C6H2(CH2NMe2)2-3,5]22-) obtained by two-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4. The molecular structure of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4 and one-electron oxidation of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 yielded the mixed-valence species [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ and [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+, respectively. The comproportionation equilibrium constants Kc (900 and 748 for [(tpy)RuIII(NCN-NCN)RuIII(tpy)]4+ and [(tpy)RuII(PCP-PCP)RuII(tpy)]2+, respectively) determined from cyclic voltammetric data reveal comparable stability of the [RuIII-RuII] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were employed to further characterize the different redox states with special focus on the mixed-valence species and their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ and [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ belong to strongly coupled borderline Class II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive delocalization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point to a decreased metal-metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge when strong sigma-donor NMe2 groups are replaced by weak sigma-donor, pi-acceptor PPh2 groups.