Formal Synthesis of Solanoeclepin A: Enantioselective Allene Diboration and Intramolecular [2+2] Photocycloaddition for the Construction of the Tricyclic Core.

An enantioselective synthesis of an intermediate in the Tanino total synthesis of solanoeclepin A has been developed. The key step was an intramolecular [2+2] photocycloaddition, which led to the tricyclo[5.2.1.0(1, 6)] decane core in six steps. The first photosubstrate, prepared through an indium-mediated Barbier-type reaction, gave an excellent [2+2] cycloaddition, but it could not be obtained in sufficient enantiopurity. The second photosubstrate, prepared through an asymmetric allene diborylation in high enantiomeric excess, gave the [2+2] cycloaddition product in high yield on irradiation at 365 nm on 20 g scale in a flow system. Other important steps were the replacement of a boronate group at the quaternary carbon by a vinyl group and diastereoselective cyclopropanation of an allylic alcohol.

Strong induced-fit binding of viologen and pyridine derivatives in adjustable porphyrin cavities.

The synthesis and binding properties of new porphyrin cage compounds consisting of a rigid diphenylglycoluril part, which is connected via flexible bis(ethyleneoxy) spacers to a (metallo)porphyrin "roof", are reported. Binding of viologen guests and pyridine ligands in these porphyrin cages are accompanied by significant conformational reorganizations of the hosts. Despite these structural changes, association constants are still very high, revealing that not only receptors that bind guests according to a lock-and-key mechanism but also those that bind guests by an induced-fit mechanism can exhibit strong binding.

Thermodynamics and kinetics of guest-induced switching between "basket handle" porphyrin isomers.

The synthesis and switching properties of two "basket handle" porphyrin isomers is described. The cis-oriented meso-phenyl groups of these porphyrins are linked at their ortho-positons via benzocrown-ether-based spacers, which as a result of slow atropisomerization are located either on the same side of the porphyrin plane (cis), or on opposite sides (trans). In solution, the cis-linked isomer slowly isomerizes in the direction of the thermodynamically more stable trans-isomer. In the presence of viologen (N,N'-dialkyl-4,4'-bipyridinium) derivatives, which have different affinities for the two isomers, the isomerization equilibrium could be significantly influenced. In addition, the presence of these guests was found to enhance the rate of the switching process, which was suggested to be caused by favorable interactions between the positively charged guest and the crown ethers of the receptor, stabilizing the transition state energies of the isomerization reaction between the two isomers.

Uncorrelated dynamical processes in tetranuclear carboxylate clusters studied by variable-temperature 1H NMR spectroscopy.

Tetranuclear carboxylate clusters with the general structural formula [M4(L)2(O2CR)4] (M = Cd, Zn; LH2 = 2,6-bis(1-(2-hydroxyphenyl)-iminoethyl)pyridine; R = CH3, C6H5) were studied by variable-temperature (VT) (1)H NMR spectroscopy. The dynamics of these clusters in solution can be described by two uncorrelated dynamical processes. The first dynamical process is the interconversion, both inter- as well as intramolecular, between syn-syn bridging and chelating carboxylate ligands. It is shown that this carboxylate interconversion mechanism is predominantly intramolecular for [Cd4(L)2(O2CCH3)4] (1a), whereas for [Zn4(L)2(O2CCH3)4] (2a) it is predominantly intermolecular. Two models for the second dynamic process, which involves the diiminepyridine ligand, are described. The first model comprises a nondissociative rotation around an internal axis, which changes the chirality of the cluster. The second model is based on the dissociation of the tetranuclear cluster into two dimeric species, which recombine again. This last model is supported by scrambling experiments between [Zn4(L)2(O2CCH3)4] (2a) and [Zn4(L3)2(O2CCH3)4] (5) (L3H2 = 2,6-bis(1-(2-hydroxyphenyl)-iminoethyl)4-chloropyridine).

Ligand effects of NHC-iridium catalysts for signal amplification by reversible exchange (SABRE).

SABRE hyperpolarizes substrates by polarization transfer from para-hydrogen in a metal complex. We have measured the signal enhancement of pyridine and its exchange rate in various [Ir(NHC)(Py)3(H)2](+) complexes to gain insight into their dependence on the N-Heterocyclic Carbene (NHC) ligand's steric and electronic properties.

Mononuclear and dinuclear manganese compounds stabilized by supramolecular interactions.

New manganese compounds [Mn(HphpzMe)(2)(H(2)phpzMe)(HCO(2))] (1), [Mn(2)(phpzMe)(2)(HphpzMe)(2)(OCH(3))]·2CH(3)OH (2), Na{[Mn(HphpzPh)(phpzPh)(MeOH)(2)](2)}(HCO(2)) (3), [Mn(HphpzPh)(2)(EtOH)(2)]ClO(4)·2EtOH (4) and [Mn(HphpzPh)(2)N(3)] (5) were synthesized and characterized with various techniques. 1, 4 and 5 are mononuclear manganese(III) compounds, 2 is a mixed-valence dinuclear manganese(III/IV) compound, and 3 is a trinuclear compound containing two manganese(III) ions and a sodium(I) ion. A remarkable feature is the spontaneous formation of the formate ion as a result of the methanol or methoxide oxidation in compounds 1 and 3. Using ethanol precludes the formation of the formate and compound 4 is obtained. The molecular structure of all compounds is stabilized by supramolecular interactions, including strong hydrogen bonding and π-π interactions.

Cationic heteroleptic cyclometalated iridium(III) complexes containing phenyl-triazole and triazole-pyridine clicked ligands.

Novel heteroleptic iridium complexes containing the 1-substituted-4-phenyl-1H-1,2,3-triazole (phtl) cyclometalating ligand have been synthesized. The 3+2 Huisgen dipolar cycloaddition method ('click' chemistry) was utilized to prepare a class of bidentate ligands (phtl) bearing different substituents on the triazole moiety. By using various ligands (phtl-R1 and pytl-R2) (R1 = adamantane, methyl and R2 = adamantane, methyl, beta-cyclodextrin, ursodeoxycholic acid), we prepared a small library of new luminescent ionic iridium complexes [Ir(phtr-R1)2(pytl-R2)]Cl and report on their photophysical properties. The flexibility of the clicking approach allows a straightforward control on the chemical-physical properties of the complexes by varying the nature of the substituent on the ligand.

High nuclearity manganese(III) compounds containing phenol-pyrazole ligands: the influence of the ligand on the core geometry.

Three high-nuclearity manganese(III) clusters have been synthesized and characterized: [Mn8(µ4-O)4(phpzH)8(thf)4] (1), [Mn8(µ4-O)4(phpzH)4(EtOH)4]·2EtOH (2), and [Mn6(µ3-O)4(µ3-Br)2(HphpzEt)6(phpzEt)] (3). Compounds 1 and 2 contain a [Mn8(µ4-O4)(phpzH)8] core in which antiferromagnetic interactions between the manganese(III) ions are found. Compound 3 is a hexanuclear manganese(III) cluster in which weak ferromagnetic interactions appear to be operative. The formation and the stability of the cluster cores in relation to the type of phenol-pyrazole ligand and the reaction conditions are discussed.

Ligand-controlled magnetic interactions in Mn(4) clusters.

A method is presented to design magnetic molecules in which the exchange interaction between adjacent metal ions is controlled by electron density withdrawal through their bridging ligands. We synthesized a novel Mn(4) cluster in which the choice of the bridging carboxylate ligands (acetate, benzoate, or trifluoroacetate) determines the type and strength of the three magnetic exchange couplings (J(1), J(2), and J(3)) present between the metal ions. Experimentally measured magnetic moments in high magnetic fields show that, upon electron density withdrawal, the main antiferromagnetic exchange constant J(1) decreases from -2.2 K for the [Mn(4)(OAc)(4)] cluster to -1.9 K for the [Mn(4)(H(5)C(6)COO)(4)] cluster and -0.6 K for the [Mn(4)(F(3)CCOO)(4)] cluster, while J(2) decreases from -1.1 K to nearly 0 K and J(3) changes to a small ferromagnetic coupling. These experimental results are further supported with density-functional theory calculations based on the obtained crystallographic structures of the [Mn(4)(OAc)(4)] and [Mn(4)(F(3)CCOO)(4)] clusters.

Ir(III) and Ru(II) complexes containing triazole-pyridine ligands: luminescence enhancement upon substitution with beta-cyclodextrin.

Novel 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine (pytl) ligands have been prepared by "click chemistry" and used in the preparation of heteroleptic complexes of Ru and Ir with bipyridine (bpy) and phenylpyridine (ppy) ligands, respectively, resulting in [Ru(bpy)(2)(pytl-R)]Cl(2) and [Ir(ppy)(2)(pytl-R)]Cl (R=methyl, adamantane (ada), beta-cyclodextrin (betaCD)). The two diastereoisomers of the Ir complex with the appended beta-cyclodextrin, [Ir(ppy)(2)(pytl-betaCD)]Cl, were separated. The [Ru(bpy)(2)(pytl-R)]Cl(2) (R=Me, ada or betaCD) complexes have lower lifetimes and quantum yields than other polypyridine complexes. In contrast, the cyclometalated Ir complexes display rather long lifetimes and very high emission quantum yields. The emission quantum yield and lifetime (Phi=0.23, tau=1000 ns) of [Ir(ppy)(2)(pytl-ada)]Cl are surprisingly enhanced in [Ir(ppy)(2)(pytl-betaCD)]Cl (Phi=0.54, tau=2800 ns). This behavior is unprecedented for a metal complex and is most likely due to its increased rigidity and protection from water molecules as well as from dioxygen quenching, because of the hydrophobic cavity of the betaCD covalently attached to pytl. The emissive excited state is localized on these cyclometalating ligands, as underlined by the shift to the blue (450 nm) upon substitution with two electron-withdrawing fluorine substituents on the phenyl unit. The significant differences between the quantum yields of the two separate diastereoisomers of [Ir(ppy)(2)(pytl-betaCD)]Cl (0.49 vs. 0.70) are attributed to different interactions of the chiral cyclodextrin substituent with the Delta and Lambda isomers of the metal complex.

Molecular recognition and self-assembly special feature: Squaring cooperative binding circles.

The cooperative binding effects of viologens and pyridines to a synthetic bivalent porphyrin receptor are used as a model system to study how the magnitudes of these effects relate to the experimentally obtained values. The full thermodynamic and kinetic circles concerning both activation and inhibition of the cage of the receptor for the binding of viologens were measured and evaluated. The results strongly emphasize the apparent character of measured binding and rate constants, in which the fractional saturation of receptors with other guests is linearly expressed in these constants. The presented method can be used as a simple tool to better analyze and comprehend the experimentally observed kinetics and thermodynamics of natural and artificial cooperative systems.

Synthesis and aromatisation of cyclic enediyne-containing amino acids.

A series of cyclic enediyne-containing amino acids with ring sizes varying from 10 to 12 atoms have been prepared starting from propargylglycine and homopropargylglycine. Their reactivity towards Bergman cyclisation under elevated temperatures has been explored. The enediynes displayed marked differences in cyclisation half-lives depending on the olefinic substituent and the ring size. A potential candidate for incorporation into peptides has been identified.

Processive rotaxane systems. Studies on the mechanism and control of the threading process.

The threading behavior of a zinc analogue of a previously reported processive manganese porphyrin catalyst onto a series of polymers of different lengths is reported. It is demonstrated that the speed of the threading process is determined by the opening of the cavity of the toroidal porphyrin host, which can be tuned with the help of axial ligands that coordinate to the metal center in the porphyrin.

Hydrogen-atom transfer in open-shell organometallic chemistry: the reactivity of Rh(II)(cod) and Ir(II)(cod) radicals.

A series of new metalloradical rhodium and iridium complexes [M(II)(cod)(N-ligand)](2+) in the uncommon oxidation state +II were synthesized by one-electron oxidation of their [M(I)(cod)(N-ligand)](+) precursors (M=Rh, Ir; cod=(Z,Z)-1,5-cyclooctadiene; and N-ligand is a podal bis(pyridyl)amine ligand: N,N-bis(2-pyridylmethyl)amine (dpa), N-(2-pyridylmethyl)-N-(6-methyl-2-pyridylmethyl)amine (pla), or N-benzyl-N,N-bis(6-methyl-2-pyridylmethyl)amine (Bn-dla). EPR spectroscopy, X-ray diffraction, and DFT calculations reveal that each of these [M(II)(cod)(N-ligand)](2+) species adopts a square-pyramidal geometry with the two cod double bonds and the two pyridine fragments in the basal plane and the N(amine) donor at the apical position. The unpaired electron of these species mainly resides at the metal center, but the apical N(amine) donor also carries a considerable fraction of the total spin density (15-18 %). Density functional calculations proved a valuable tool for the analysis and simulation of the experimental EPR spectra. Whereas the M(II)(olefin) complexes are quite stable as solids, in solution they spontaneously transform into a 1:1 mixture of M(III)(allyl) species and protonated M(I)(olefin) complexes (in the forms [M(I)(olefin)(protonated N-ligand)](2+) for M=Rh and [M(III)(H)(olefin)(N-ligand)](2+) for M=Ir). Similar reactions were observed for the related propene complex [M(II)(propene)(Me(2)tpa)](2+) (Me(2)tpa=N,N,N-tris(6-methyl-2-pyridylmethyl)amine). The decomposition rate of the [M(II)(cod)(N-ligand)](2+) species decreases with increasing N-ligand bulk in the following order: dpa>pla>Bn-dla. Decomposition of the most hindered [M(II)(cod)(Bn-dla)](2+) complexes proceeds by a second-order process. The kinetic rate expression v=k(obs)[M(II)](2) in acetone with k(obs)=k'[H(+)][S], where [S] is the concentration of additional coordinating reagents (MeCN), is in agreement with ligand-assisted dissociation of one of the pyridine donors. Solvent coordination results in formation of more open, reactive species. Protonation of the noncoordinating pyridyl group increases the concentration of this species, and thus [H(+)] appears in the kinetic rate expression. The kinetic data are in agreement with bimolecular hydrogen-atom transfer from M(II)(cod) to another M(II) species (DeltaH( not equal)=11.5+/-2 kcal mol(-1), DeltaS( not equal)=-27+/-10 cal K(-1) mol(-1), and DeltaG( not equal)(298 K)=19.5+/-5 kcal mol(-1)).

Rhodium-mediated stereoselective polymerization of "carbenes".

Unprecedented rhodium-catalyzed stereoselective polymerization of "carbenes" from ethyl diazoacetate (EDA) to give high molecular mass poly(ethyl 2-ylidene-acetate) is described. The mononuclear, neutral [(N,O-ligand)M(I)(cod)] (M = Rh, Ir) catalytic precursors for this reaction are characterized by (among others) single-crystal X-ray diffraction. These species mediate formation of a new type of polymers from EDA: carbon-chain polymers functionalized with a polar substituent at each carbon of the polymer backbone. The polymers are obtained as white powders with surprisingly sharp NMR resonances. Solution and solid state NMR data for these new polymers reveal a highly stereoregular polymer, with a high degree of crystallinity. The polymer is likely syndiotactic. Material properties are very different from those of atactic poly(diethyl fumarate) polymer obtained by radical polymerization of diethyl fumarate. Other diazoacetates are also polymerized. Further studies are underway to reveal possible applications of these new materials.

IrII(ethene): metal or carbon radical?

One-electron oxidation of [(Me(n)tpa)Ir(I)(ethene)]+ complexes (Me(3)tpa = N,N,N-tri(6-methyl-2-pyridylmethyl)amine; Me(2)tpa = N-(2-pyridylmethyl)-N,N,-di[(6-methyl-2-pyridyl)methyl]-amine) results in relatively stable, five-coordinate Ir(II)-olefin species [(Me(n)tpa)Ir(II)(ethene)](2+) (1(2+): n = 3; 2(2+): n = 2). These contain a "vacant site" at iridium and a "non-innocent" ethene fragment, allowing radical type addition reactions at both the metal and the ethene ligand. The balance between metal- and ligand-centered radical behavior is influenced by the donor capacity of the solvent. In weakly coordinating solvents, 1(2+) and 2(2+) behave as moderately reactive metallo-radicals. Radical coupling of 1(2+) with NO in acetone occurs at the metal, resulting in dissociation of ethene and formation of the stable nitrosyl complex [(Me(3)tpa)Ir(NO)](2+) (6(2+)). In the coordinating solvent MeCN, 1(2+) generates more reactive radicals; [(Me(3)tpa)Ir(MeCN)(ethene)](2+) (9(2+)) by MeCN coordination, and [(Me(3)tpa)Ir(II)(MeCN)](2+) (10(2+)) by substitution of MeCN for ethene. Complex 10(2+) is a metallo-radical, like 1(2+) but more reactive. DFT calculations indicate that 9(2+) is intermediate between the slipped-olefin Ir(II)(CH(2)=CH(2)) and ethyl radical Ir(III)-CH(2)-CH(2). resonance structures, of which the latter prevails. The ethyl radical character of 9(2+) allows radical type addition reactions at the ethene ligand. Complex 2(2+) behaves similarly in MeCN. In the absence of further reagents, 1(2+) and 2(2+) convert to the ethylene bridged species [(Me(n)tpa)(MeCN)Ir(III)(mu(2)-C(2)H(4))Ir(III)(MeCN)(Me(3)tpa)](4+) (n = 3: 3(4+); n = 2: 4(4+)) in MeCN. In the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxo), formation of 3(4+) from 1(2+) in MeCN is completely suppressed and only [(Me(3)tpa)Ir(III)(TEMPO(-))(MeCN)](2+) (7(2+)) is formed. This is thought to proceed via radical coupling of TEMPO at the metal center of 10(2+). In the presence of water, hydrolysis of the coordinated acetonitrile fragment of 7(2+) results in the acetamido complex [(Me(3)tpa)Ir(III)(NHC(O)CH(3)))(TEMPOH)](2+) (8(2+)).

Spontaneous disproportionation of rhodium(I) bisoxazolinates to rhodium(II).

[RhI(t-Bu2-boxate)(C2H4)2] spontaneously disproportionates to the mononuclear [RhII(t-Bu2-boxate)2], whereas [RhI(Ph2-boxate)(C2H4)2] is stable against disproportionation.

Coordination and oxidative addition at a low-coordinate rhodium(I) beta-diiminate centre.

The reaction of 14e [L(Me)Rh(coe)] (1; L(Me)[double bond]ArNC(Me)CHC(Me)NAr, Ar[double bond]2,6-Me(2)C(6)H(3); coe[double bond]cis-cyclooctene) with phenyl halides and thiophenes was studied to assess the competition between sigma coordination, arene pi coordination and oxidative addition of a C-X bond. Whereas oxidative addition of the C-Cl and C-Br bonds of chlorobenzene and bromobenzene to L(Me)Rh results in the dinuclear species [[L(Me)Rh(Ph)(micro-X)](2)] (X=Cl, Br), fluorobenzene yields the dinuclear inverse sandwich complex [[L(Me)Rh](2)(anti-micro-eta(4):eta(4)-PhF)]. Thiophene undergoes oxidative addition of the C-S bond to give a dinuclear product. The reaction of 1 with dibenzo[b,d]thiophene (dbt) in the ratio 1:2 resulted in the formation of the sigma complex [L(Me)Rh(eta(1)-(S)-dbt)(2)], which in solution dissociates into free dbt and a mixture of the mononuclear complex [L(Me)Rh(eta(4)-(1,2,3,4)-dbt)] and the dinuclear complex [[L(Me)Rh](2)(micro-eta(4)-(1,2,3,4):eta(4)-(6,7,8,9)-dbt)]. The latter could be obtained selectively by the 2:1 reaction of 1 and dbt. Reaction of 1 with diethyl sulfide produces [L(Me)Rh(Et(2)S)(2)], which in the presence of hydrogen loses a diethyl sulfide ligand to give [L(Me)Rh(Et(2)S)(H(2))] and catalyses the hydrogenation of cyclooctene.