Contents 812 4.6.1. Trinuclear Complexes 812 4.6.2. Binuclear Complexes, Complexes Containing {M 2 S 6 } Frameworks, and Compounds with Related Structural/ Chemical Properties 813 5. Copper and Silver 816 5.1. Mononuclear Complexes 817 5.2. Dinuclear Complexes 817 5.3. Trinuclear Complexes 817 5.4. Tetranuclear Complexes 818 5.5. Penta-and Hexanuclear Complexes 819 5.6. Heptanuclear Complexes and Complexes with Higher Nuclearities 820 5.7. Neutral [M(SR)] n Compounds 821 6. Acknowledgment 822 7. References 822

A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone ( 2 L) and its Cu-O 2 chemistry is reported. [( 2 L)Cu I (MeCN)] 1+ complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(μ-oxo)dicopper(III) (O) species (2b), similar to the parent bis-guanidine ligand complex (1b) and permethylated-diamine ligand complex (3b). UVvis and X-ray absorption spectroscopy experiments confirm this assignment of 2b as an O species, and full formation of the 2:1 Cu-O 2 complex is demonstrated by an optical titration with ferrocenemonocarboxylic acid (FcCOOH). The UV-vis spectra of 1b and 2b with guanidine ligation show low-intensity visible features assigned as guanidine π → Cu 2 O 2 core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (1b-3b) with phenolate at 195 K is particularly insightful as only 2b hydroxylates 2,4-di-tert-butylphenolate to yield 3,5-di-tert-butylcatecholate (>95% yield) with the oxygen atom derived from O 2 , reminiscent of tyrosinase reactivity. 1b is unreactive, while 3b yields the C-C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the O complexes and increased phenolate accessibility to the Cu 2 O 2 core are attributes that correlate with phenolate hydroxylation reactivity observed in 2b. The comparative low-temperature reactivity of 1b-3b with FcCOOH (O-H BDE 71 kcal mol −1 ) to form the two-electron, two-proton reduced bis(μ-hydroxo) dicopper(II,II) complex is quantitative and presumably precedes through two sequential protoncoupled electron transfer (PCET) steps. Optical titrations along with DFT calculations support that the reduced complexes formed in the first step are more powerful oxidants than the parent O complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by 2b and 3b.

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Thiolates are presently a subject of great interest in the chemistry of complexes involving transition-metal elements and soft ligands. The manifold electronic and steric capabilities offered by the monodentate ligands RS" and the bidentate chelate ligands "SRSO have been used to stabilize a broad spectrum of mononuclear, oligomeric, and polymeric complexes with new and remarkable structures and properties. Impetus has especially been provided by the synthesis of polynuclear cagelike homo-and heteroleptic metal-sulfur frameworks, which can often be regarded as "molecular fragments" of the structures of inorganic sulfides. Thiolates and mixed sulfide-thiolates of the late open-and closed-shell 3d metals (Fe, Co, Ni, Cu, Zn) and some of their homologues (Au, Cd, Hg), as well as of Mo, are of particular importance as model complexes for biologically important metal centers coordinated by sulfur. They have played an important role in increasing our understanding of the structure, bonding, and function of the reactive centers in ferredoxins, rubredoxins, nitrogenases, blue copper proteins, metallothioneins, and antiarthritic drugs.

Salts of the tetrakis(trifluoromethyl)borate anion, M[B(CF3)4], M=Li, K, Cs, Ag, have been prepared by two different routes for the first time. The colorless compounds are thermally stable up to 425 C (Cs salt) and soluble in anhydrous HF, water, and most organic solvents. Single crystals of Cs[B(CF3)4] were grown from diethyl ether by diffusion of CH2Cl2 vapor into the solution. The molecular structure was obtained by single-crystal X-ray diffraction. Crystal data: rhombohedral space group R3m (no. 160); a =7.883(1), c=13.847(4) A: V=748.2 A3; Z=3; T=150K; R1=0.0118, wR2=0.0290. The internal bond parameters of the [B(CF3)4] ion were compared to those of the C(CF3)4 molecule. Due to a disorder of the anions in the cesium salt, it is not possible to distinguish between T and Td symmetry by X-ray diffraction experiments alone. However, a comprehensive IR and Raman study demonstrated that in the potassium and cesium salt as well as in aqueous solution, the anion exhibits T symmetry with all CF3 groups rotated off the staggered position required for Td symmetry. The vibrational study is supported by DFT calculations, which provide, in addition to the equilibrium structure and vibrational wavenumbers, estimates of IR and Raman band intensities. The anion is resistant against strong oxidizing (e.g., F2) as well as reducing agents (e.g., Na) and is not affected by nucleophiles like C2H5O or electrophiles such as H3O+. It is very weakly coordinating, as demonstrated by the low-equilibrium CO pressure over the [Ag(CO)x][B(CF3)4] (x=1, 2) co-adducts and the formation of [Ag(CO)x][B(CF3)4] (x=3,4) at higher CO pressure. The 11B, 13C, and 19F NMR data as well as the structural parameters of the anion are compared with those for other borates containing F, CN, and CF3 ligands.

Samples of the dithionite-reduced FeFe protein (the dinitrogenase component of the Fe-only nitrogenase) from Rhodobacter capsulatus have been investigated by 57Fe Mössbauer spectroscopy and by Fe and Zn EXAFS as well as XANES spectroscopy. The analyses were performed on the basis of data known for the FeMo cofactor and the P cluster of Mo nitrogenases. The prominent Fourier transform peaks of the Fe K-edge spectrum are assigned to Fe-S and Fe-Fe interactions at distances of 2.29 A and 2.63 A, respectively. A significant contribution to the Fe EXAFS must be assigned to an Fe backscatterer shell at 3.68 A, which is an unprecedented feature of the trigonal prismatic arrangement of iron atoms found in the FeMo cofactor of nitrogenase MoFe protein crystal structures. Additional Fe...Fe interactions at 2.92 A and 4.05 A clearly indicate that the principal geometry of the P cluster is also conserved. Mössbauer spectra of 57Fe-enriched FeFe protein preparations were recorded at 77 K (20 mT) and 4.2 K (20 mT, 6.2 T), whereby the 4.2 K high-field spectrum clearly demonstrates that the cofactor of the Fe-only nitrogenase (FeFe cofactor) is diamagnetic in the dithionite-reduced ("as isolated") state. The evaluation of the 77 K spectrum is in agreement with the assumption that this cofactor contains eight Fe atoms. In the literature, several genetic and biochemical lines of evidence are presented pointing to a significant structural similarity of the FeFe, the FeMo and and the FeV cofactors. The data reported here provide the first spectroscopic evidence for a structural homology of the FeFe cofactor to the heterometal-containing cofactors, thus substantiating that the FeFe cofactor is the largest iron-sulfur cluster so far found in nature.

The first tetrasilabuta‐1,3‐diene (2) was obtained as reddish‐brown crystals simply and unexpectedly by lithiation of disilene 1, partial bromination with an aryl bromide, and intermolecular cleavage of lithium bromide. The conjugation between the two SiSi double bonds of 2 was demonstrated by electron spectroscopy and X‐ray crystallography. Mes = 2,4,6‐Me3C6H2, Tip = 2,4,6‐iPr3C6H2.

A series of bis(guanidine) ligands designed for use in biomimetic coordination chemistry, namely bis(tetramethylguanidino)‐, bis(dipiperidinoguanidino)‐, and bis(dimethylpropyleno)propane (btmgp, DPipG2p and DMPG2p, respectively), has been extended to include bis(dimethylethyleneguanidino)propane (DMEG2p), which has both Namine atoms of each guanidine functionality connected by a short ethylene bridge, as a member. From this series, a family of novel bis(guanidine)copper(I) compounds – [Cu2(btmgp)2][PF6]2 (1), [Cu2(DPipG2p)2][PF6]2 (2), [Cu2(DMPG2p)2][PF6]2 (3), and [Cu2(DMEG2p)2][PF6]2·2MeCN (4) – has been synthesised. Single‐crystal X‐ray analysis of 1–4 demonstrated that these compounds contain dinuclear complex cations that contain twelve‐membered heterocyclic Cu2N4C6 rings with the Cu atoms being more than 4 Å apart. Each copper atom is surrounded by a set of two N‐donor functions from different ligands, resulting in linear N–Cu–N coordination sites. Depending on their individual substitution patterns, the guanidine moieties deviate from planarity by characteristic propeller‐like twists of the amino groups around their N–Cimine bonds. The influence of these groups on the reactivity of the corresponding complexes 1–4 with dioxygen was investigated at low temperatures by means of UV/Vis spectroscopy. The reaction products can be classified into μ‐η2:η2‐peroxodicopper(II) or bis(μ‐oxo)dicopper(III) complex cations that contain the {Cu2O2}2+ core portion as different isomers. The electronic properties of the specific bis(guanidine) ligands are discussed from the viewpoint of their σ‐donor and π‐acceptor capabilities, and it is shown that μ‐η2:η2‐peroxodicopper(II) complexes are stabilised relative to the bis(μ‐oxo)dicopper(III) ones if π conjugation within the guanidine moieties is optimised. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)