Synthesis and screening of catalytic activity of novel mono- and diruthenium carbene complexes 7a and 7b prepared from inexpensive Bisphenol S via Claisen rearrangement-isomerisation route is described. These catalysts constitute an excellent tool for ring-closing metathesis by combining high stability with increased catalytic activity as compared with the parent Hoveyda-Grubbs catalyst. (c) 2006 Elsevier B.V. All rights reserved.

The invention relates to a method for carrying out metathesis reactions, wherein the method is carried out continuously and a ruthenium-containing catalyst is used.

Mesitylcopper reacts with flavonol (flaH) in the presence of 1,3-bis(2-pyridylimino)isoindoline (indH) to yield the diamagnetic complex CuI(fla)(indH), which on reaction with molecular oxygen undergoes oxidative splitting of the C2-C3 bond of the pyranone ring of the flavonolate ligand to give CuI(indH)(O-bs) (O-bs = O-benzoylsalicylate) (orthorhombic, P1, a = 8.048(7) A, b = 8.969(9) A, c = 19.240(2) A, alpha = 85.69 degrees, beta = 80.24(7) degrees, gamma = 77.87(7) degrees, V = 1337(2) A3, Z = 2) and carbon monoxide. The reaction of [CuI(CH3CN)4]ClO4, flaH, and indH with dioxygen at room temperature affords the paramagnetic complex [CuII(fla)(indH)]ClO4 (mu = 2.10 mu B), and after elimination of HClO4, CuII(fla)(ind) (orthorhombic, Pbca, a = 8.888(2) A, b = 19.169(7) A, c = 33.614(10) A, alpha = beta = gamma = 90 degrees, V = 5727(3) A3, Z = 8) with mu = 1.86 mu B is formed. The latter undergoes cleavage of the pyranone ring on oxygenation at 80 degrees C to give CuII(ind)(O-bs) (mu = 1.87 mu B, nu(CO) = 1742 cm-1, and nu(CO2) = 1581, 1387 cm-1) and carbon monoxide. CuII(fla)(ind) and [CuII(fla)(indH)]ClO4 serve as good catalysts for the oxygenation of flavonol to O-benzoylsalicyclic acid.

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The tripodal ligand [CH3C(CH2C5H4) (CH2PPh2)(2)](-) reacts with RuCl2(PPh3)(3) to produce CH3C(CH2-eta(5)-C5H4) (CH2-eta(1)-PPh2)(2)RuCl, [tripodCpL(2)RuCl], 1. Complex 1 undergoes substitution of the chlorine function with various nucleophiles L' to produce [tripodCpL(2)RuL'](+). The carbonyl derivative (L' = CO) 2, isonitrile (L' = RNC) 3, nitrile compounds (L' = RCN) 4, and a tolane adduct (L' = eta(2)-PhC=CPh) 5 are obtained when 1 is treated with the appropriate Ligands in polar solvents. Halide accepters (e.g. TlPF6) are generally needed to promote these reactions. The cyanide derivative tripodCpL(2)RuCN (3a) is alkylated by F3CSO3CH3 to give the isonitrile derivative [tripodCpL(2)RuCNMe](+) 3b. Terminal alkynes HC=CR produce vinylidene compounds [tripodCpL(2)RuL'](+), where L' = C=CHR (R = tBu, 7b; R = Ph, 7c), or allenylidene derivatives, L' = C=C=CPh2 (6), depending on the nature of R (R = CPh2OH for synthesis of 6). Trimethylsilylacetylene gives the parent vinylidene species, L' = C=CH2 (7a), which is transformed to the Fischer-type carbene compound, L' = C(OMe)Me (8), upon treatment with methanol. The vinylidene species 7 are deprotonated by NaOMe to produce the alkynyl compounds tripodCpL(2)RuC=CR (9). Methylation of 9 with F3CSO3CH3 results in the vinylidene derivatives L' = C=C(Me)R (R - tBu, 7d; R = Ph, 7e), having two organic substituents at the terminal carbon centre. For all vinylidene compounds with two different substituents at their terminal carbon atom, hindered rotation of the single-faced vinylidene pi-ligand about its Ru-C bond is observed. Analysis by P-31- NMR spectroscopic coalescence measurements as well as line-shape analyses reveals activation enthalpies of around 40 kJmol(-1) for this rotation, with small activation entropies of around +/-10 Jmol(-1)K(-1). Solid-state structures of nine compounds of the type [tripodCpL(2)RuL'](+n) (n = 0, 1) demonstrate the remarkable conformational rigidity of the tripodCpL(2)Ru template. They also show that the possible strain imposed by linking the Cp Ligand and the two donor groups L in one and the same chelate scaffolding does not appear to impose a serious steric strain on these templates.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The synthesis of the superphanes 4-8 could be achieved from {(1,2,11,12-eta(4))-tricyclo[10.8.0.0(2,11)]eicosa-1, 11-diene-6,17-diyne}(eta(5)-cyclopentadienyl (15) and its methoxycarbonyl congener 14 by heating with R-CpCo(CO)(2) (R = H, CH3, (CH3)(5)). X-ray investigations on the superphanes 3, 6, and 8 reveal distances between the two cyclobutadiene units between 2.922 Angstrom (3) and 2.941 Angstrom (6). Investigations by means of cyclic voltammetry show a decrease of the first oxidation potentials with increasing number of CH3 substituents at the cyclopentadienyl ligands. The comparison between the first oxidation potentials of 2-6, 4-7, and 5-8 gives evidence for a partial charge delocalization over both CpCoCb fragments of the superphanes.

The preparation of novel dinucleating pyrazolate ligands H5L3 - H5L8 carrying chelating side arms with appending secondary amine functions is reported. Following different synthetic routes, either CH2CF3, C6H2F3 , or C6F5 moieties can be introduced as substituents at the terminal nitrogen atoms. These systems are reminiscent of two coupled coordination compartments of the potential triamidoamine-type. Crystallographic analyses of a series of bimetallic complexes of the CH2CF3 -substituted ligand H5L4 with NiCl2 and CoCI2 reveal manifold coordination modes in the solid state, resulting from the facile detachment of a single or several N-donor sites from the metal centers. Coordination number sets {4/6} (in H5L4Co2Cl4) and {5/6} (in H4L4Ni2Cl3, H4L4Co2 Cl3 and H5L4Ni2Cl4) are thus observed. In the non-deprotonated H5L-type systems the remaining protons are found to be scavenged by a pyrazolate-N (in H5L4Ni2Cl4) or an amine function of a ligand side arm (in H5L4Co2Cl4).

A series of symmetric and asymmetric pyrazolate-based dinuclear Ni(II) complexes relevant to the active site of urease is reported, which have acetate ions as secondary bridges and which feature variations in the type (N or S) and number of donor sites provided within the individual coordination compartments of the primary pyrazolate ligand matrixes. X-ray crystallographic structures of the acetone adduct [L(1)Ni(2)(&mgr;-OAc)(acetone)(2)](ClO(4))(2) (1) as well as of the urea adducts [L(1)Ni(2)(&mgr;-OAc)(benzylurea)(2)](ClO(4))(2) (2c), [L(2)Ni(2)(&mgr;-OAc)(urea)](ClO(4))(2) (3a), and [L(3)Ni(2)(&mgr;-OAc)(N,N'-dimethylurea)(2)(MeOH)(2)](ClO(4))(2) (4) have been determined. They reveal that the urea substrates are tied up with the bimetallic cores by both O-coordination to the metal centers and hydrogen bonding between the urea NH and the O atoms of the bridging acetate. In a related complex [L(3)Ni(2)(&mgr;-OAc)(OAc)(2)Na]BPh(4) (5) a sodium ion is associated with the dinickel framework via binding to one O atom of each of the three acetates. The nickel(II) ions in 1 and 2a are weakly antiferromagnetically coupled (J = -2.6 and -1.9 cm(-)(1)), where the magnitude of the coupling appears to correlate with the tilting of the acetate moiety with respect to the plane of the pyrazolate. The superexchange in 3a and 4 is even weaker. The ability of the new complexes to mediate the ethanolysis of urea is examined and is found to be dependent on the number and stereochemical arrangement of the accessible coordination sites at the dinuclear core: the asymmetric species 3a is not capable of inducing any solvolysis of the substrate, and the activity of the symmetric systems 1 and 2b is less than stoichiometric, whereas 4 displays higher activity, albeit this is still very low and possibly proceeds via a one metal ion mechanism.

The reaction of [Ti]Cl-2 (1) ([Ti]=(eta(5)-C5H4SiMe3)(2)Ti) with two equivalents of LiC=C-SiMe2-C=CSiMe3 (2) produces [Ti](C=C-SiMe2-C=CSiMe3)(2) (3). On treatment with [MX] (M = Cu: 4a X = Cl, 4b X = Br; M = Ag: 5a X = Cl, 5b X = Br) the tweezer complexes ([Ti](C=C-SiMe2-C=CSiMe3)(2)) MX (M = Cu: 6a X = Cl, 6b X = Br; M = Ag: 7a X = Cl, 7b X = Br) are formed in which the Ti-C=C-Si units are eta(2)-coordinated to a monomeric copper(I) or silver(I) halide moiety. When 6b is further reacted with [CuBr] (4b), oligomeric ([Ti](C=C-SiMe2-C=CSiMe3)(2)(CuBr)(3))(n) (8) is formed. This molecule contains a (eta(2)-TiC=CSi)(2)CuBr entity next to two (eta(2)-SiC=CSi)CuBr moieties, of which the latter building blocks are responsible for the oligomeric structure. In addition, 8 can be prepared by the direct reaction of 3 with an excess of 4b, respectively. However, when an excess of [AgX] is used, the only formed products are 7; no polymeric material is obtained. A Group 11 metal exchange reaction is noticed, when 7a or 7b are reacted with [CuX]: depending on the amount of [CuX] used, monomeric 6 or oligomeric 8 is produced. An explanation is given by a better bonding synergysmus for the alkyne-to-copper interaction. The result of the X-ray structure analysis of compound Tb is reported. The compound 7b crystallizes in the monoclinic space group C2/c with cell constants a = 25.097(8), b = 11.327(3), c = 19.014(6) Angstrom, beta = 122.36(3)degrees V = 4566(2) Angstrom(3) and Z = 4. The compound 7b contains a monomeric (eta(2)-alkyne)(2)AgBr moiety in which the silver(I) center possesses a trigonal-planar environment, caused by the eta(2)-coordinated TiC=CSi units as well as a eta(1)-bonded bromine atom. However this differs from the behavior of compounds 6 and 7 in solution, where all four C=C building blocks of the TiC=CSi as well as the SiC=CSi units are complexed by the transition metal entities MX (M = Cu, Ag X = Cl, Br). (C) 1999 Elsevier Science S.A. All rights reserved.

all-L-beta(3)-Penta-, hexa-, and heptapeptides with the proteinogenic side chains of valine, leucine, serine, cysteine, and methionine have been prepared by previously described procedures (12, 13, 14, 15; Schemes 2-5). Thioether cleavage with Na/NH3 in beta-HMet residues has also provided a beta(3)-hexapeptide with homocysteine (CH2CH2S) side chains (13e). The HS-(CH2)(n) groups were positioned on the beta-peptidic backbone in such a way that, upon disulfide-bridge formation, the corresponding beta-peptide was expected to maintain either a 3(1)-helical secondary structure (1, 2) (Fig. 1) or to be forced to adopt another conformation (3, 4). The 13-, 17-, 19-, and 21-membercd-ring macrocyclic disulfide derivatives and their open-chain precursors, as well as all synthetic intermediates, were purified (crystallization, flash or preparative HPL chromatography; Fig. 5) and fully characterized (m.p., [alpha](D), to, IR, NMR, FAB or ESI mass spectroscopy, and elemental analysis, whenever possible; Fig. 2 and Exper. Part). The structures in MeOH and H2O of the new beta-peprides were studied by CD spectroscopy (Figs. 3 and 4), where the characteristic 215-nm-trough/200-nm-peak pattern was used as an indicator for the presence or absence of (M)-3(1)-helical conformations. A CH2-S-2-CH2 and, somewhat less so, a (CH2)(2)-S-2-(CH2)(2) bracket between residues i and i + 3 (1 vs. 12d. and 2 vs. 13e in Fig. 3) give rise to CD spectra which are compatible with the presence of 3(1)-helical structures, while CH2-S-2-CH2 brackets between residues i and i+2 (3 vs. 14c) or i and i+4 (4 vs. 15c in Fig. 4) do not.

all-L-β3-Penta-, hexa-, and heptapeptides with the proteinogenic side chains of valine, leucine, serine, cysteine, and methionine have been prepared by previously described procedures (12, 13, 14, 15; Schemes 2 – 5). Thioether cleavage with Na/NH3 in β-HMet residues has also provided a β3-hexapeptide with homocysteine (CH2CH2S) side chains (13e). The HS−(CH2)n groups were positioned on the β-peptidic backbone in such a way that, upon disulfide-bridge formation, the corresponding β-peptide was expected to maintain either a 31-helical secondary structure (1, 2) (Fig. 1) or to be forced to adopt another conformation (3, 4). The 13-, 17-, 19-, and 21-membered-ring macrocyclic disulfide derivatives and their open-chain precursors, as well as all synthetic intermediates, were purified (crystallization, flash or preparative HPL chromatography; Fig. 5) and fully characterized (m.p., [α]D, CD, IR, NMR, FAB or ESI mass spectroscopy, and elemental analysis, whenever possible; Fig. 2 and Exper. Part). The structures in MeOH and H2O of the new β-peptides were studied by CD spectroscopy (Figs. 3 and 4), where the characteristic 215-nm-trough/200-nm-peak pattern was used as an indicator for the presence or absence of (M)-31-helical conformations. A CH2−S2−CH2 and, somewhat less so, a (CH2)2−S2−(CH2)2 bracket between residues i and i+3 (1vs. 12d, and 2vs. 13e in Fig. 3) give rise to CD spectra which are compatible with the presence of 31-helical structures, while CH2−S2−CH2 brackets between residues i and i+2 (3vs. 14c) or i and i+4 (4vs. 15c in Fig. 4) do not.

Starting from 4-methoxybenzylmalonic ester MeOC6H4CH2CH(COOEt)2, the synthesis of the tripod-ligand HOC6H4CH2C(CH2PPh2)3, 12, functionalized with a phenolic group at its backbone, is achieved in a few steps. Ether derivatives of 12 show the normal coordination behavior of RC(CH2PPh2)3. Thus MeOC6H4CH2C(CH2PPh2)3 forms the complexes [7·Fe(NCMe)3] (BF4)2 (8) and 7·Mo(CO)3 (9). The phenolate derived from 12 by deprotonation reacts with the CH2Cl groups of Merrifield resin to form covalently polymer fixed 12 with high efficiency. The polymer bound tripod ligands undergo tripod typical coordination reactions. In addition to the usual spectroscopic and analytical techniques, X-ray analyses of derivatives of 12 as well as of 8 and 9 are used to identify the products.

A series of pyrazolate-based dinuclear Ni(II) complexes relevant to the active site of urease are reported. Deprotonation of HL(1) [HL(1) = 3,5-bis(R(2)NCH(2))-pyzH; R(2)N = Me(2)N(CH(2))(3)NMe] by means of 1 equiv of BuLi and subsequent reaction with 2 equiv of [Ni(H(2)O)(6)](ClO(4))(2) in the presence of NEt(i)Pr(2) affords the dinuclear complex [L(1)Ni(2)(OH)(MeCN)(2)](ClO(4))(2) (1). This is shown crystallographically to contain two five-coordinate nickel ions bridged by both the pyrazolate and a hydroxide, with an acetonitrile solvent molecule bound to each metal center. When HL(2) is employed {HL(2) = 3,5-bis(R(2)NCH(2))-pyzH; R(2)N = [Me(2)N(CH(2))(3)](2)N}, the additional ligand side arms act as proton acceptors forming an intramolecular N.H.N bridge to yield the complex [HL(2)Ni(2)(OH)(MeCN)(2)](ClO(4))(3) (2), whose basic bimetallic framework is essentially identical to 1. The two Ni(II) centers in 2 exhibit strong antiferromagnetic coupling (J = -46.7 cm(-)(1)). The labile acetonitrile donors in 2 are easily replaced by either neutral ligands such as dmf or anions such as thiocyanate, giving rise to the formation of complexes [HL(2)Ni(2)(OH)(dmf)(2)](ClO(4))(3) (3) and [HL(2)Ni(2)(OH)(NCS)(2)](ClO(4)) (4), respectively, where the overall dinuclear framework of 2 remains unchanged upon the substitution reaction.

The chlorine functions of CH3C(CH2Cl)3, 1, may be replaced by pyrazolyl (pz) as well as imidazolyl (im) residues under the conditions of nucleophilic substitution leading to tripodal ligands CH3C(CH2X)3, X = pz, 2; X = im, 3. As a means of introducing two nitrogen donors and one phosphorus donor into a tripod ligand, substitution of the Br and OMs functions in O(CH2)2C(CH2Br)(CH2OMs), 8, by nitrogen nucleophiles and subsequent cleavage of the oxetane ring by a phosphide nucleophile to give HOCH2C(CH2PPh2)(CH2X)2 has been developed, furnishing 10a (X = pz) and 10d (X = NEt2), respectively. For the synthesis of 10a, K-pz was used as the nucleophile, while 10d was prepared using azide in the initial step, which then had to be transformed into NEt2 in two subsequent steps. The nucleophugic functions of the oxetane 8 undergo selective substitution by K-pz and KPPh2 in THF to produce O(CH2)2C(CH2PPh2)(CH2pz), 9b. Phosphide cleavage of the oxetane function leads to HOCH2C(CH2PPh2)(CH2PR2)(CH2pz), R = Ph, 10b; R = 3,5-Me2(C6H3), 10c. – The tris(pyrazolyl) tripod ligand 2 reacts with (MeCN)3Mo(CO)3to give 2 · Mo(CO)3(MeCN), 12a, in which only two of the three donor functions are coordinated. Upon reaction with 10a, the same reagent gives 10a · Mo(CO)4, 12b, with one pyrazolyl coordinated and the other involved in intramolecular hydrogen bonding to the CH2OH function (N···H–O distance 280 pm). Blocking of the OH function of 10a by etherification, i.e. to form EtOCH2C(CH2PPh2)(CH2pz)2, 11, does not dramatically affect the coordination capabilities with 11 · Mo(CO)3(MeCN), 12d, being formed upon treatment with (MeCN)3Mo(CO)3. Again only one pz function is coordinated to the metal. Bidentate coordination by two phosphorus donors of 10c is observed in 10c · Mo(CO)3(MeCN), 12d. The dangling arm pz donor function and the CH2OH group are intermolecularly hydrogen-bonded in this case. When the bulky P[3,5-Me2(C6H3)]2 substituent of 10c is replaced by the less sterically demanding PPh2 donor in 10b, η3-coordination is finally observed with the formation of 10b · Mo(CO)3, 13. The coordination capabilities of the new ligands are rationalized in terms of the size (six-, seven-, and eight-membered rings) and interference of the chelate cycles. All compounds have been characterized by the usual analytical and spectroscopic methods, with a complete assignment of the NMR data achieved by a combination of 2D-NMR techniques in some cases. The structures of the coordination compounds have additionally been deduced by X-ray methods.

Carbonylmetalate dianions react in thf with the group 13 chlorides XmECl3-m, (E = Al, Ga; X = Cl, Me, Et, Bu-i; m = 0, 1) to yield the monoanionic species [(CO)(n)M-EXmCl2-m](-) (M = Fe, Cr, Mo, W; n = 4, 5) as the primary products which could be isolated as solvent free salts after exchange with a non coordinating cation. After addition of a chelating Lewis base, e.g., tmeda, dme, and solvent exchange with dichloromethane the primary products undergo a second salt elimination reaction, yielding the neutral intermetallic systems (CO)(n)M-Ga[X(L)(2)] (M = Cr, Mo, W, Fe; n = 4, 5; X = Cl, Me, Et; L-2 = tmeda, dme, bipy, Bu-t-dab, thf(2)) (1-14) and (CO)(5)M-Al[X(L)(2)] (M = Cr, Mo, W; X = Cl, Et, Bu-i; L-2 = tmeda, tmpda) (15-20, 23, 24). The chloro derivatives can be converted to the corresponding hydride or tetrahydridoboranato species which is exemplarily shown by compounds 21 and 22. In the case of R2GaCl (R = Me, Et; 2 equiv) as starting compounds a ligand exchange reaction, generating GaR3, occurs, before the second salt elimination takes place. The new intermetallic systems were characterized by means of elemental analysis and IR, Raman, NMR, and mass spectroscopy. The complexes (CO)(5)Cr-Ga[Cl(tmeda)] (2), (CO)(5)W-Al[Et(tmeda)] (20), and (CO)(5)W-Al[Cl(tmpda)] (23) are also characterized by single-crystal X-ray diffraction. Compounds 2 and 20 crystallize in the monoclinic space group P2(1)/n, Z = 4. 2: a = 9.059(4) Angstrom, b = 16.084(7) Angstrom, c = 11.835(6) Angstrom, beta = 80.6(1)degrees, V = 1701(1) Angstrom(3), and R = 0.037 (R-w = 0.118). 20: a = 8.606(2) Angstrom, b = 16.463(6) Angstrom, c = 12.469(4) Angstrom, beta = 93.88(2)degrees, V = 1762(6) Angstrom(3), and R = 0.027 (R-w = 0.065). Complex 23 crystallizes in the orthorhombic space group Pccn, a = 23.990(6) Angstrom, b = 9.044(3) Angstrom, c = 15.871(4) Angstrom, V = 3445(1) Angstrom(3), and R = 0.044 (R-w = 0.088). Ab initio quantum chemical calculations at the MP2 level of theory of the model complexes (CO)(5)W-E[Cl(NH3)(2)] (E = B, Al, Ga, In, Tl), (CO)(5)W-Al[H(NH3)(2)], (CO)(5)W-AlH, and (CO)(5)W-AlCl are reported. The group-13 fragments E(R)L-2 behave as strong sigma-donors with significant acceptor capabilities. The W-E bonds are strong semipolar covalent bends with large ionic contributions (D-e(calc) between 70 and 120 kcal/mol). Only the W-Tl bond is comparatively weak (D-e(calc) = 48 kcal/mol).

The reaction of the novel mixed tripod ligands RCH2C(CH2X)(CH2Y)(CH2Z) 1–6 (X, Y, Z=PPh2, NR2, pyrazol-1-yl; R=H, OH) with [RhI(COD)Cl]2 is investigated. The resulting rhodium COD complexes [(1–6)Rh(COD)]PF6, 7 are characterized by NMR spectroscopy, mass spectra and elemental analysis. In addition, X-ray structure analysis is performed on several compounds 7, where in contrast to the behavior of the parent compound triphos [MeC(CH2PPh2)3], the potential tripod ligands 2–6 are found to coordinate in a bidentate mode. {η2-P,O-[HOCH2C(CH2PPh2)(CH2NEt2)2]Rh(COD)]}PF6, 7i exhibits the first structurally characterized example of an intramolecular hydrogen bond between a non-coordinated and a coordinated donor atom. The activities of the complexes 7 as catalyst precursors in the homogeneous hydrogenation of diphenylacetylene and (Z)-α-N-acetamidocinnamic acid are tested and rationalized with respect to a proposed reaction mechanism.

A series of pyrazole-based potential ligands bearing thioether substituents in 3- and 5-positions of the heterocycle was synthesized [3,5-bis(RSCH2)-pyzH; R = Ph (1aH), PhCH2 (1bH), iPr (1cH), tBu (1dH)]. These ligands afford oligonuclear Cu-I and Ag-I coordination compounds [LCu](x) (2a-c, L = 1a-c) and [LAg](x) (3a-d, L = 1a-d), respectively. The single crystal X-ray analysis of 3c shows the presence of trimeric planar arrays of N,N'-bridging pyrazolates and linear coordinated silver ions, with each two of the trinuclear moieties being linked by two unsupported short intermolecular Ag ... Ag contacts [3.041(1) Angstrom]. Molecular-weight determinations for 2a (THF) and 3c (toluene) indicate that hexanuclear entities are preserved in solution. Starting from 1bH the Cu-II complex [(1b)(2)Cu-2](BF4)(2) (4) was synthesized. According to an X-ray crystal structure analysis it consists of dinuclear molecules with two bridging pyrazolates, distorted square planar N2S2 coordination spheres for Cu-II and an axially bridging tetrafluoroborate. Magnetic susceptibility data reveal an antiferromagnetic exchange (J = -206 cm(-1)) that is among the highest found for doubly pyrazolate bridged dicopper(II) complexes, which is rationalized on the basis of the rather symmetric dinuclear core of 4. The irreversibility of the electrochemical reduction and oxidation processes for the Cu-II and Cu-I compounds, respectively, is explained by the inability of the respective coordination framework to adapt to different geometric preferences.

Reaction of tripod cobalt(II) templates [{CH3C(CH2PAr2)(3)}Co-II] with potentially bridging ligands L generates the dinuclear compounds [(tripod)-Co-L-Co(tripod)](2+). With L = oxalate (C2O42-) a biscobalt(II) complex (1) is Formed, while with L = C6H2O42-, the dianion derived from 2,5-dihydroxy-1,4-benzoquinone (anilic acid), two-electron transfer within the dimetallic unit occurs and a biscobalt(III) charge distribution results (2a), as shown by X-ray structural analyses of 1 and 2 a, NMR spectroscopy, and theoretical investigations by the INDO method, Complex 2a exhibits an unusually intense, low-energy absorption in its electronic spectrum; this is explained with a simple MO model. One-electron reduction of 2 a generates the corresponding mixed-valence complex, which is highly stabilised through extensive electron delocalisation. Substituents at the 3,6 posi tions of the bridging ligand (Cl, Br, I, NO2, Me, iPr. Ph: 2b-h) as well as alkyl substitution at the aromatic rings of the tripod ligands (3,4) influence the optical and electrochemical properties consistent with the proposed model of charge distribution, Formal replacement of one [(tripod)Co-III](3+) moiety by [CH2](2+) leads to the mononuclear complex 6, which is shown to be a typical [(tripod)Co-III-catecholato)](+) complex, Therefore the substantially different optical and electrochemical properties of the dinuclear complexes with respect to those of 6 result from strong metal-metal interactions mediated by the bridging ligand.