Reversible Hydride Transfer to N,N'-Diarylimidazolinium Cations from Hydrogen Catalyzed by Transition Metal Complexes Mimicking the Reaction of [Fe]-Hydrogenase.

[Fe]-hydrogenase is a key enzyme involved in methanogenesis and facilitates reversible hydride transfer from H2 to N5,N10-methenyltetrahydromethanopterin (CH-H4MPT+). In this study, a reaction system was developed to model the enzymatic function of [Fe]-hydrogenase by using N,N'-diphenylimidazolinium cation (1+) as a structurally related alternative to CH-H4MPT+. In connection with the enzymatic mechanism via heterolytic cleavage of H2 at the single metal active site, several transition metal complex catalysts capable of such activation were utilized in the model system. Reduction of 1[BF4] to N,N'-diphenylimidazolidine (2) was achieved under 1 atm H2 at ambient temperature in the presence of an equimolar amount of NEt3 as a proton acceptor. The proposed catalytic pathways involved the generation of active hydride complexes and subsequent intermolecular hydride transfer to 1+. The reverse reaction was accomplished by treatment of 2 with HNMe2Ph+ as the proton source, where [(η5-C5Me5)Ir{(p-MeC6H4SO2)NCHPhCHPhNH}] was found to catalyze the formation of 1+ and H2 with high efficiency. These results are consistent with the fact that use of 2,6-lutidine in the forward reaction or 2,6-lutidinium in the reverse reaction resulted in incomplete conversion. By combining these reactions using the above Ir amido catalyst, the reversible hydride transfer interconverting 1+/H2 and 2/H+ was performed successfully. This system demonstrated the hydride-accepting and hydride-donating modes of biologically relevant N-heterocycles coupled with proton concentration. The influence of substituents on the forward and reverse reactivities was examined for the derivatives of 1+ and 2 bearing one para-substituted N-phenyl group.

Blue/red linear dichroic emission from a highly anisotropic crystal of triarylmethane dye conjugated with phenoxo-zinc complexes.

We have developed a novel triphenylmethane-based hexanuclear zinc complex that exhibits peculiar photochemical and photophysical properties. Upon UV irradiation, the compound turned from colorless to reddish purple, while the color of emission turned from blue to red. The color change was attributed to an oxidation of the ligand part. It was suggested that an intramolecular energy-transfer mechanism operates to give rise to the red emission. The UV treatment of a single crystal results in simultaneous emission of orthogonally polarized blue and red light. This color switching, namely linear dichroic emission was so distinct that one can recognize with by sight through optical microscope. The columnar arrangement of molecules in the crystal clearly accounts for the observed polarization of the emission.

Core conversion reactions of the cubane-type metal-sulfido clusters: shape shift, contraction, and expansion of the MM'Re2S4 Cubanes (M = Ir, Rh, Ru; M' = Pt, Pd).

Treatment of incomplete cubane-type clusters [(Cp*M){Re(L)}(2)(mu(3)-S)(mu(2)-S)(3)] (M = Ir (1a), Rh (1b); Cp* = eta(5)-C(5)Me(5); L = S(2)C(2)(SiMe(3))(2)) and [{(Pmb)Ru}{Re(L)}(2)(mu(3)-S)(mu(2)-S)(3)] (Pmb = eta(6)-C(6)Me(5)H) with 1 equiv of [Pt(PPh(3))(3)] gave tetranuclear tetra(sulfido) clusters having raft-type cores, [(Cp*M){Pt(PPh(3))(2)}{Re(L)}(2)(mu(3)-S)(4)] (M = Ir (3a), Rh) and [{(Pmb)Ru}{Pt(PPh(3))(2)}{Re(L)}(2)(mu(3)-S)(4)], which presents a sharp contrast to the reactions with [Pd(PPh(3))(4)] reported previously, affording the cubane-type clusters [(Cp*M){Pd(PPh(3))}{Re(L)}(2)(mu(3)-S)(4)] (M = Ir (2a), Rh) and [{(Pmb)Ru}{Pd(PPh(3))}{Re(L)}(2)(mu(3)-S)(4)]. The reactions of 2a with diphosphines P2 resulted in the conversion of its cubane-type core into the analogous raft-type frameworks, forming [(Cp*Ir){Pd(P2)}{Re(L)}(2)(mu(3)-S)(4)] (P2 = cis-Ph(2)PCH=CHPPh(2) (6), Ph(2)PCH(2)CH(2)PPh(2), Ph(2)PCH(2)CH(2)CH(2)PPh(2)). On the other hand, when 2 was allowed to react with Ph(2)PCH(2)PPh(2) (dppm) as P2, the trinuclear tri(sulfido) cluster [(Cp*Ir){Re(L)}(2)(mu(3)-S)(2)(mu(2)-S)(mu(2)-dppm)] (9a) was obtained. Alternatively, this cluster 9a and its Rh analogue 9b were derived from the incomplete cubane-type clusters 1a and 1b by treatment with dppm. It has also been found that further treatment of the cubane-type cluster 2a with excess [Pd(PPh(3))(4)] affords the heptanuclear tetra(sulfido) cluster [(Cp*Ir){Pd(PPh(3))}(4)Re(2)(mu(3)-L)(2)(mu(3)-S)(4)] (10). The detailed structures have been determined by the X-ray analyses for 3a, 6, 9a, and 10.

Theoretical study on activation and protonation of dinitrogen on cubane-type MIr3S4 clusters (M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, and W).

Density functional theory (DFT) calculations on cubane-type metal-sulfido clusters MIr(3)S(4) ligating N(2) (M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, and W) have been performed for the proposal of new clusters that can highly activate N(2) beyond the RuIr(3)S(4) cluster prepared by Mizobe and co-workers [Angew. Chem. Int. Ed. 2007, 46, 5431]. The degree of N(2) activation in the metal-N(2) complexes was evaluated based on the N-N bond distance and vibrational frequency and the gross atomic charge on N(2). The degree of N(2) activation strongly depends on the metal atoms at the N(2)-binding site, and the MoIr(3)S(4) and WIr(3)S(4) clusters exhibit significant N(2)-activation ability. The reactivity of the MIr(3)S(4)-N(2) complexes (M = Ru, Mo, and W) with a proton donor (lutidinium) has been discussed from a kinetic aspect by exploring a possible reaction pathway of proton transfer. The protonation of the Ru-N(2) complex would not occur due to a very high-activation barrier and to an instability of the Ru-NNH(+) complex, which is consistent with our present experimental result that the Ru-N(2) complex has not been protonated at room temperature. On the other hand, the protonation of the Mo-N(2) and W-N(2) complexes would proceed smoothly from DFT criteria. The result of calculations indicates that the Mo and W clusters are best suited for the protonation of N(2), which is the first step toward nitrogen fixation.

Heterolytic cleavage of hydrogen molecule by rhodium thiolate complexes that catalyze chemoselective hydrogenation of imines under ambient conditions.

The bis(thiolate)Rh(III) complex having a tris(3,5-dimethylpyrazolyl)borate (Tp(Me2)) coligand [Tp(Me2)Rh(SPh)(2)(MeCN)] reacts reversibly with H(2) to form the hydridothiolato complex [Tp(Me2)RhH(SPh)(MeCN)] and PhSH. The benzenedithiolate analogue [Tp(Me2)Rh(o-S(2)C(6)H(4))(MeCN)], which can also heterolytically activate H(2), catalyzes hydrogenation of imines under ambient temperature and pressure with high chemoselectivity.

DFT study on chemical N2 fixation by using a cubane-type RuIr3S4 cluster: energy profile for binding and reduction of N2 to ammonia via Ru-N-NHx (x = 1-3) intermediates with unique structures.

The N-N bond activation of the dinitrogen ligand in the cubane-type mixed-metal sulfido cluster, [(Cp*Ir) 3{Ru(tmeda)(N 2)}(mu 3-S) 4] (tmeda = Me 2NCH 2CH 2NMe 2), is investigated by using DFT calculations at the B3LYP level of theory. The elongated N-N bond distance, red-shifted N-N stretching, and negatively charged N 2 ligand indicate that the dinitrogen is reductively activated by complexation. The degree of the N-N bond activation is classified into the "moderately activated" category, [ Studt, F. ; Tuczek, F. J. Comput. Chem. 2006, 27, 1278 ] as in the Mo-triamidoamine complex that can catalyze N 2 reduction [ Yandulov, D. V. ; Schrock, R. R. Science 2003, 301, 76 ]. Availability of the RuIr 3S 4 cluster as a catalyst for N 2 reduction is discussed by optimizing possible intermediates in a catalytic cycle analogous to that proposed by Yandulov and Schrock. A calculated energy profile of the catalytic cycle demonstrates that the RuIr 3S 4 cluster can transform dinitrogen into ammonia in the presence of lutidinium cation and Cp* 2Co as proton and electron sources, respectively. The RuIr 3S 4 clusters with an NNH x ( x = 1-3) ligand, which are intermediates in the catalytic cycle, have a significantly bent Ru-N-N linkage, although precedent NNH x complexes generally adopt a linear M-N-N array. The unique structures of the nitrogenous ligands in these intermediates are interpreted in terms of the bonding interaction between the hydrogen atom bonded to the N 2 ligand and the adjacent iridium atom in the cuboidal RuIr 3S 4 framework.

N-Heterocyclic carbenes as supporting ligands in transition metal complexes of N2.

Recent developments have substantially expanded the scope of N-heterocyclic carbenes (NHCs) as ancillary ligands in coordination chemistry and homogeneous catalysis. This review provides a short overview of the emerging field of NHC-supported transition metal complexes of N2 and the possibilities to catalytically activate N2 in these complexes.

Two-dimensional metamagnet composed of cyano-bridged CuII-WV bimetallic assembly.

Two-dimensional cyanide-bridged copper(II) octacyanotungstates(V), [(Cu(3-CNpy)2(H2O))2(Cu(3-CNpy)2(H2O)2)(W(CN)8)2] (3-CNpy = 3-cyanopyridine) (1) and [(Cu(4-CNpy)2)2(Cu(4-CNpy)2(H2O)2)(W(CN)8)2] x 6H2O (4-CNpy = 4-cyanopyridine) (2), were prepared and these compounds exhibited metamagnetic behavior with Néel temperatures of 8.0 K (1) and 4.4 K (2).

Polymers with multishape memory controlled by local glass transition temperature.

A multishape memory polymer with flexible design capabilities is fabricated by a very simple method. Local glass transition temperatures of a loosely cross-linked polymer film are changed by immersing sections of the film in a cross-linker solution with a different concentration. Each section memorizes a temporary shape, which recovers its permanent shape at a different recovery temperature depending on the local glass transition temperature. As a base polymer, we chose a network polymer prepared by a Diels-Alder reaction between poly(2,5-furandimethylene succinate) (PFS) and 1,8-bis-maleimidotriethyleneglycol (M2). Quintuple shape memory behavior was demonstrated by a PFS/M film with four sections with distinct glass transition temperatures. The number of temporary shapes was determined by the number of different M2 solutions. Furthermore, owing to the reversibility of the Diels-Alder reaction, the permanent shape was rewritable.

Correction: Peptide-catalyzed kinetic resolution of planar-chiral metallocenes.

Correction for 'Peptide-catalyzed kinetic resolution of planar-chiral metallocenes' by Midori Akiyama et al., Chem. Commun., 2014, 50, 7893-7896.

Peptide-catalyzed kinetic resolution of planar-chiral metallocenes.

Kinetic resolution of racemic planar-chiral metallocenes was performed through the conjugate addition of a nucleophile to the enal part of substrates. While no enantiomeric discrimination was found with low-molecular-weight organocatalysts, a properly designed resin-supported peptide catalyzed the reaction in a highly selective manner.

A molybdenum complex bearing a tetraphosphine ligand as a precursor for heterobimetallic complexes.

The reactions of [CpMoH(κ(3)-P4)] (2; P4 = meso-o-C6H4(PPhCH2CH2PPh2)2) with protic acids gave [CpMo(κ(4)-P4)](+) (4(+)) via the intermediary formation of [CpMoH2(κ(3)-P4)](+). Treatment of 2 with iodine provided the cationic complex [CpMoHI(κ(3)-P4)][I] (5[I]). Early-late heterobimetallic complexes of the type [CpMoH(µ-P4-1κ(3):2κ)MLnCl] (MLn = Ru(Hmb)Cl (6), Ir(η(5)-C5Me5)Cl (7), Rh(cod), Ir(cod), Pd(η(3)-C3H5) (10); Hmb = η(6)-C6Me6, cod = η(4)-1,5-cyclooctadiene) were synthesised by reacting 2 with [MLn(µ-Cl)]2, and their structures were characterised by NMR spectroscopy. The hydride ligand in 6 was replaced by chloride in chlorinated organic solvents to give [CpMoCl(µ-P4-1κ(3):2κ)Ru(Hmb)Cl2] (11). Self-reaction of 10 produced allylbenzene and [CpMo(µ-H){µ-PhP(CH2)2P(Ph)-o-C6H4-P(Ph)(CH2)2PPh2-1κ(3):2κ(2)}PdCl] (12), in which the Mo-Pd edge was bridged by a hydride ligand and the phosphide moiety emerged from the loss of one phenyl group. The molecular structures of 4[Cl], 4[OTf], 5[I], 6, 7, 11, and 12 were established by single-crystal X-ray analysis.

Transformations of aryl isothiocyanates on tetraphosphine tungsten complexes and reactivity of the resulting dithiocarbonimidate ligand.

Treatment of [WH(4)(κ(4)-P4)] (3: P4 = meso-o-C(6)H(4)(PPhCH(2)CH(2)PPh(2))(2)) with aryl isothiocyanate ArNCS at 50 °C afforded the dithiocarbonimidate-isocyanide complex [W(κ(2)-S(2)CNAr)(CNAr)(κ(4)-P4)] (4) in moderate yields. The reaction also produced ArNHCH(3) and a small amount of ArNH(2). The yield of the hydrodesulfurization product ArNHCH(3) increased when the reaction was conducted under H(2) (up to 0.65 equiv. to 3 for Ar = p-MeC(6)H(4) (Tol)). Complex 4 was proposed to be formed via reductive disproportionation of two ArNCS molecules on a zero-valent W species generated by dissociation of H(2) from 3. The reaction of W(0) complex [W(dppe)(κ(4)-P4)] (dppe = Ph(2)PCH(2)CH(2)PPh(2)) with ArNCS also yielded 4 accompanied by free dppe, in contrast to that of [Mo(dppe)(κ(4)-P4)], which had been previously reported to undergo sulfur-atom transfer to phosphine ligands. The dithiocarbonimidate ligands in 4a (Ar = Tol) received the addition of electrophiles [PhMe(2)NH][BF(4)], MeI, and PhCOCl selectively at the N atom to afford the cationic dithiocarbamate complexes [W(κ(2)-S(2)CNHTol)(CNTol)(κ(4)-P4)][BF(4)] (6), [W{κ(2)-S(2)CN(Me)Tol}(CNTol)(κ(4)-P4)]I (7), and [W{κ(2)-S(2)CN(COPh)Tol}(CNTol)(κ(4)-P4)]Cl (8). Complexes 4a, 6, 7, and 8 have been characterized by spectroscopic and crystallographic methods, and the donor strengths of their κ(2)-dithio ligands are discussed.

Heterolytic H2 activation by rhodium thiolato complexes bearing the hydrotris(pyrazolyl)borato ligand and application to catalytic hydrogenation under mild conditions.

Thiolato complexes of Rh(III) bearing a hydrotris(3,5-dimethylpyrazolyl)borato ligand (Tp(Me2)) have been prepared, and their reactivity toward H(2) has been investigated. The bis(thiolato) complex [Tp(Me2)Rh(SPh)(2)(MeCN)] (1) reacted with 1 atm H(2) at 20 degrees C to produce the hydrido-thiolato complex [Tp(Me2)RhH(SPh)(MeCN)] (2) and PhSH via heterolytic cleavage of H(2). This process is reversible and in equilibrium in THF and benzene. The bis(selenolato) complex [Tp(Me2)Rh(SePh)(2)(MeCN)] (4) was also converted to [Tp(Me2)RhH(SePh)(MeCN)] and PhSeH under 1 atm H(2), but the equilibrium largely shifted to 4. Reaction of the dithiolato complex [Tp(Me2)Rh(bdt)(MeCN)] (3; bdt = 1,2-C(6)H(4)S(2)) with H(2) occurred in the presence of amine, giving the anionic hydrido complex [Tp(Me2)RhH(bdt)](-) and an equimolar amount of ammonium cations. Catalytic activity for hydrogenation has been examined under 1 atm H(2) at 20-50 degrees C. While 1, 2, and 4 slowly hydrogenated styrene at similar rates at 50 degrees C, activities for the hydrogenation of N-benzylideneaniline increased in the order, 2 < 1 < 4. Complex 3 was found to be the most active and selective catalyst for hydrogenation of imines, and thus a variety of imines were reduced at 20 degrees C under 1 atm H(2), with the C=C and C=O bonds in the substrate molecules completely preserved. An ionic mechanism was involved to explain such high chemoselectivity.

Preparation of a bis(hydrosulfido) complex of Mo having a tetraphosphine co-ligand and its transformation into MoRh2 and MoIr2 mixed-metal sulfido clusters.

The treatment of a Mo(0) complex with a tetraphosphine co-ligand [Mo(kappa(4)-P4)(Ph(2)PCH(2)CH(2)PPh(2))] (P4 = meso-o-C(6)H(4)(PPhCH(2)CH(2)PPh(2))(2)) with H(2)S gas in toluene at room temperature afforded [Mo(SH)(2)(kappa(4)-P4)] (1), which demonstrates the first Mo(II) bis(hydrosulfido) complex. Its trigonal-prismatic structure rarely observed for Mo(II) complexes has been determined by X-ray analysis. The reactions of 1 with [RhCl(CO)(2)](2) and [IrCl(CO)(2)(p-toluidine)] in the presence of NEt(3) resulted in the formation of the sulfido-bridged trinuclear clusters [Mo(kappa(4)-P4)(mu(3)-S)(2){Rh(CO)(2)}(2)] (4) and [Mo(kappa(4)-P4)(mu(3)-S)(2){Ir(CO)(2)}(2)] (5), respectively. The X-ray diffraction study has disclosed that the MoRh(2) cluster 4 in the solid state exists in two isomeric forms arising from the different orientation of P4 to the trigonal-prismatic Mo center, whereas the crystals of the Ir analogue 5 contains the molecules corresponding to only one of the two isomers observed for 4. The fluxional features of 1, 4, and 5 in solution have been confirmed by VT NMR studies.

Crystal structure of Ce(IV)/dipicolinate complex as catalyst for DNA hydrolysis.

The structures of Ce(4+) complexes that are active for DNA hydrolysis were determined for the first time by X-ray crystallography. The crystals were prepared from a 1:2 mixture of Ce(NH(4))(2)(NO(3))(6) and dipicolinic acid (2,6-pyridinedicarboxylic acid). Depending on the recrystallization conditions, three types of crystals were obtained. Some of the Ce(4+) ions in these complexes have enough coordinated water molecules that can directly and indirectly participate in the catalysis. The distances between the Ce(4+) and the dipicolinate ligand are considerably shorter than those in the corresponding La(3+) and Ce(3+) complexes. On the other hand, the distances between the Ce(4+) and its coordinated water are similar to those for the La(3+) and Ce(3+) complexes. In a proposed mechanism of DNA hydrolysis, the scissile phosphodiester linkage is notably activated by coordination to Ce(4+) and attacked by the Ce(4+)-bound hydroxide. The process is further assisted by acid catalysis of Ce(4+)-bound water.

Mo and W dihalide complexes with uncommon trigonal-prismatic geometry imposed by the linear tetraphosphine ancillary ligand and their reactivities toward diazoalkanes.

New Mo and W tetraphosphine-dihalide complexes [MX2(kappa4-P4)] (2, MX=MoCl, MoBr, WBr; P4=meso-o-C6H4(PPhCH2CH2PPh2)2) with uncommon trigonal-prismatic geometries have been prepared. Treatment of ethyl diazoacetate with 2 (MX=MoCl) resulted in catalytic carbenoid-group coupling to give diethyl maleate and fumarate, whereas reactions of 2 with trimethylsilyldiazoalkane formed the diazoalkane complexes trans-[MX(NN=CHSiMe3)-(kappa4-P4)]+ (3+) and cis,mer-[MoCl2(NN=CHSiMe3)(kappa3-P4)]. The molecular structures of 2 (MX=MoCl) and 3[PF6] (MX=WBr) were crystallographically determined.

Preparation of chalcogenolato-bridged dinuclear Tp*Rh-Cp*Ru complexes (Tp*= hydrotris(3,5-dimethylpyrazol-1-yl)borate, Cp*=eta(5)-pentamethylcyclopentadienyl) and binding of dioxygen to their Ru sites.

Reactions of [Tp*Rh(coe)(MeCN)](1; Tp*= hydrotris(3,5-dimethylpyrazol-1-yl); coe = cyclooctene) with one equiv of diphenyl dichalcogenides PhEEPh (E = Se, Te) afforded the mononuclear Rh(III) complexes [Tp*Rh(EPh)(2)(MeCN)](2b: E = Se; 2c: E = Te), as reported previously for the formation of [Tp*Rh(SPh)(2)(MeCN)](2a) from the reaction of 1 and PhSSPh. Complexes 2a-2c were treated with the Ru(II) complex [(Cp*Ru)(4)(mu(3)-Cl)(4)](Cp*=eta(5)-C(5)Me(5)) in THF at room temperature, yielding the chalcogenolato-bridged dinuclear complexes [Tp*RhCl(mu-EPh)(2)RuCp*(MeCN)](3). Complex 3a (E = S) in solution was converted slowly into a mixture of 3a and the sterically less encumbered dinuclear complex [Tp*RhCl(SPh)(mu-eta(1)-S-eta(6)-Ph)RuCp*](4a) at room temperature. In 4a, one SPh group binds only to the Rh center as a terminal ligand, while the other SPh group bridges the Rh and Ru atoms by coordinating to the former at the S atom and to the latter with the Ph group in a pi fashion. The Se analogue 3b also underwent a similar transformation under more forcing conditions, e.g. in benzene at reflux, whereas formation of the mu-eta(1)-Te-eta(6)-Ph complex was not observed for the Te analogue 3c even under these forcing conditions. When complexes 3 was dissolved in THF exposed to air, the MeCN ligand bound to Ru was substituted by dioxygen to give the peroxo complexes [Tp*RhCl(mu-EPh)(2)RuCp*(eta(2)-O(2))](5a: E = S; 5b: E = Se; 5c: E = Te). X-Ray analyses have been undertaken to determine the detailed structures for 2c, 3a, 3b, 4a, 5a, 5b, and 5c.