The linear quadridentate N2S2 donor ligand 1,7-bis(N-methylbenzimidazol-2'-yl)-2,6-dithiaheptane (bmdhp) forms mono-and di-hydrate 1 : 1 copper(i1) complexes which are significantly more stable toward autoreduction than those of the non-methylated analogue. The deep green monohydrate of the perchlorate salt crystallises as the mononuclear aqua-complex, [Cu(bmdhp)(OH,)] [CIO4I2, in the monoclinic space group P2,/n, with Z = 4, a = 18.459(3), & = 10.362(2), c = 16.365(3) A, and B = 1 1 7.1 4(1)". The structure was solved and refined by standard Patterson, Fourier, and least-squares techniques to R = 0.047 and R' = 0.075 for 3 343 independent reflections with l > 2 4 4 . The compound consists of [Cu(bmdhp) (OH2)I2+ ions and C104counter ions. The co-ordination around copper is intermediate between trigonal bipyramidal and square pyramidal, with CU-N distances of 1.950(4) and 1.997(4) A, Cu-O(water) 2.225(4) A, and Cu-S 2.328(1) and 2.337(1) A. In the solid state, the perchlorate dihydrate's co-ordination sphere may be a topoisomer of the monohydrate's. A new angular structural parameter, 7 , is defined and proposed as an index of trigonality, as a general descriptor of five-co-ordinate centric molecules. By this criterion, the irregular co-ordination geometry of [Cu(bmdhp)(OH2)I2+ in the solid state is described as being 48% along the pathway of distortion from square pyramidal toward trigonal bipyramidal. In the electronic spectrum of the complex, assignment is made of the S(thioether) -+ Cu charge-transfer bands by comparison with those of the colourless complex Zn(bmdhp)(OH) (C104). E.s.r. and ligand-field spectra show that the copper(ii) compounds adopt a tetragonal structure in donor solvents.Continuing interest in the chemistry of copper-thioether complexes has been promoted by the occurrence of the methionine side-chain as a structurally essential active-site component in a plastocyanin and two a ~u r i n s . ~-~ The additional presence of strongly bound imidazole nitrogen has stimulated the synthesis of various types of copper compounds with biomimetic donors,6-" which are intended to probe the consequences of the nitrogenlsulphur co-ordination and irregular geometry which are the hallmarks of the protein type-1 copper centres.We have prepared a series of chelating ligands with biomimetic donor atoms of the general structure shown below.l2-l5 These quadridentate ligands contain two imidazole nitrogendonor atoms, sterically hindered because they form part of the bulky benzimidazole groups, and two thioether sulphur atoms, with a varying number of carbon atoms

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The redox chemistry and e s r . spectra of a number of copper( 11) complexes with nitrogen-and sulphur-donor ligands have been examined, including complexes with 2,2'-bi-imidazole, histamine, and cyclic and acyclic saturated amine and thioether ligands. The tetrahedral Cu1'S4 centre generated by y-irradiation of single crystals of a thioacetamide complex of CuI has a low value of IAlll. Tetrahedral distortion of otherwise tetragonal copper(i1) centres increases gll and decreases lAlll for N-, S-, and O-donor ligands, and is a more effective source of reduction of lAlll than charge effects. The quotient gIIIIAill appears to be a convenient empirical index of distortion of the donor set from planar toward tetrahedral, indicating that most type I copper in proteins is tetrahedrally, rather than tetragonaily, co-ordinated.THE so-called ' blue ' (type I) copper proteins have attracted much interest recently. The active sites of this group of proteins exhibit anomalous spectroscopic and redox properties that common copper(I1) compounds of low molecular weight have seldom evinced.l Namely, (i) an intense blue colouration; the visible absorption spectra in the 600 nm region have E values ranging from lo00 to 5 0oO dm3 mol-l cm-l, which are ca. 50 times larger than those for ordinary low-molecularweight copper(I1) complexes ; (ii) anomalously small copper nuclear hyperfine (h.f.) coupling constants in the e.s.r. spectra, IAlll values being < ca. 90 x cm-l;(iii) rather high reduction potentials.Chemical modification and spectroscopic examinations 3-5 of blue copper proteins indicated the participation of nitrogen (histidine imidazole) and sulphur (cysteine and/or methionine) donors to CuII. In the case of oxidised Populus nigra italica plastocyanin these lines of reasoning have culminated in the X-ray crystallographic demonstration of a pseudo-tetrahedral CuN,S, chromophore.6 On the other hand, the recent results of Rorabacher and his co-workers7 appeared to point to the irrelevance of the co-ordination geometry as a prime prerequisite in generating the spectroscopic and redox properties of these proteins. Indeed, for individual examples within this class of proteins, the geometrical structures (necessarily idealised) proposed recently for the copper(I1) state of their active sites are rather diverse, including planar,8 trigonal bipyramidal? five-~o-ordinate,~ and distorted lo as well as flattened or pseudo-tetrahedra1.l' The diversity of the above models results mainly from the paucity of e.s.r. data for copper(I1)-sulphur complexes, especially for those with tetrahedral structures; in particular, the origin of the anomalously small h.f. coupling constants of this group of proteins is as yet uncertain. In view of this situation, it appeared most pertinent to investigate the e.s.r. properties of copper (11)-sulphur complexes having tetrahedral and square-planar structures.We have thus examined the e.s.r. spectra of a number of complexes, including those of the ligands below, containing Cu-S and Cu-N linkages, and compared ...

Procedures involving condensation of o-phenylenediamines with carboxylic acids, and reaction of bifunctional alkyl halides with bifunctional nucleophiles are described. Syntheses are reported of 2,6-bis-(2-benzimidazyl>pyridine, 1,3-bis(2-benzimidazyl)2-thiapropane, 1,7-bis(2-benzimidazyl)2,6-dithiaheptane, 2-hydroxymethyl-5,6-dimethylbenzimidazole, 2-chloromethyl-5,6-dimethylbenzimidazole hydrochloride, 1,7--bis(5,6-dimethyl-2-benzimidazyl)-2,6-dithiaheptane, 3,6-bis(l-pyrazolyl)pyridazine, 2-(2-hydroxy-3-rnethyl--phenyl)benzimidazole, 2-(2-hydroxyphenyl)benzimidazole, 5-(2.hydroxphenyl)3-methyl-l-phenylpyrazole, 3(5)-(2-hydroxyphenyl)5(3)methylpyrazole, 3(5)-(2-hydroxyphenyl>5(3)phenylpyrazole, and 1,3-bis((S-methyl-pyridy1)imino)isoindoline.

Visible absorption and ESR spectra and redox potentials have been examined for a series of pyrrole-2-carboxaldehyde Schiff base copper(II) complexes, whose coordination stereochemistry is distorted to varying extents from square-planar to pseudotetrahedral geometry. There are smooth correlations among d-d band energies, A\\, g]|, A0, and g0 values. As the dihedral angle between chelate rings increases from 0 through 90°, g|| increases, | ||| decreases in an antiparallel fashion, and redox potentials shift systematically to more positive values. These observations are consistent with tetrahedral distortion at the metal binding sites of blue copper proteins.

We report highly active iridium precatalysts, [Cp*Ir(N,N)Cl]Cl (1-4), for water oxidation that are supported by recently designed dihydroxybipyridine (dhbp) ligands. These ligands can readily be deprotonated in situ to alter the electronic properties at the metal; thus, these catalyst precursors have switchable properties that are pH-dependent. The pKa values in water of the iridium complexes are 4.6(1) and 4.4(2) with (N,N) = 6,6'-dhbp and 4,4'-dhbp, respectively, as measured by UV-vis spectroscopy. For homogeneous water oxidation catalysis, the sacrificial oxidant NaIO4 was found to be superior (relative to CAN) and allowed for catalysis to occur at higher pH values. With NaIO4 as the oxidant at pH 5.6, water oxidation occurred most rapidly with (N,N) = 4,4'-dhbp, and activity decreased in the order 4,4'-dhbp (3) > 6,6'-dhbp (2) ≫ 4,4'-dimethoxybipyridine (4) > bipy (1). Furthermore, initial rate studies at pH 3-6 showed that the rate enhancement with dhbp complexes at high pH is due to ligand deprotonation rather than the pH alone accelerating water oxidation. Thus, the protic groups in dhbp improve the catalytic activity by tuning the complexes' electronic properties upon deprotonation. Mechanistic studies show that the rate law is first-order in an iridium precatalyst, and dynamic light scattering studies indicate that catalysis appears to be homogeneous. It appears that a higher pH facilitates oxidation of precatalysts 2 and 3 and their [B(Ar(F))4](-) salt analogues 5 and 6. Both 2 and 5 were crystallographically characterized.

Nitrosyl complexes of the iron(III) forms of myoglobin, human hemoglobin, Glycera dibranchiata hemoglobins (Hbm and Hbh), and model iron(II) and iron(III) synthetic porphyrins including octaethylporphyrin (OEP) have been prepared. The iron(III) heme proteins are electron spin (paramagnetic) resonance (ESR) silent, while hexacoordinate solution structures are indicated for [Fe(OEP)(NO)2]ClO4 and for Hbm(II)NO, which has an ESR spectrum similar to that of Mb(II)NO and the hexacoordinate iron(II) model complex Fe(OEP)NO(BzIm). The splitting of the alpha- and beta-bands in the optical spectrum of Mb(III)NO and Hbh(III)NO contrasts markedly with the sharp, single bands observed in that of Hbm-(III)NO. The nondegeneracy of the dxz and dyz orbitals in Mb(III)NO and Hbh(III)NO is attributed to the influence of the distal histidine. Circular dichroism spectra were obtained for Hbm(III)NO, Hbm(II)NO, Hbh(III)NO, Hbh(II)NO, Mb(II)NO, and Mb(III)NO. The vicinal chiral center contribution that governs the heme protein CD leads to low Kuhn anisotropies, which have been used to assign certain electronic transitions. The Hb(III)NO spectrum is not stable but transforms into that of Hb(II)NO. This autoredox process follows kinetics that are first order in FeIIINO. The relative rates of autoreduction (25 degrees C, 1 atm NO) are Mb(III)NO less than Hbm(III)NO less than Hb alpha(III)NO less than HbA(III)NO. At high NO partial pressure or after "recycling" of HbA, the rates of reduction decrease. The first step in the reaction of NO with the ferric heme is the reversible formation of the formally iron(III) adduct. This reacts with another molecule of NO, generating the final heme(II)-NO via nitrosylation of NO itself or of an endogenous nucleophile. Kinetic and spectroscopic evidence shows involvement of trans-heme-(NO)2 in the reaction. The activation parameters delta H and delta S were determined. The overall reaction is photoenhanced.

·14H 2 O (2). Molecules in the structures of 1, 1a and 2 are held together by noncovalent interactions, and the lattices contain voids filled with water molecules. Compounds 1 and 1a differ in the mode of mphthalate coordination, solvent type, and crystal lattice content. Desolvated 1 and 2 sorb gaseous MeOH and EtOH. Whereas the MeOH sorption capacity was very similar for 1 and 2 (about 0.14 cm 3 /g at PP S -1 = 0.9, where P is the current methanol pressure and P S the saturation vapor pressure of