Celia Maya

Interconversion of Quadruply and Quintuply Bonded Molybdenum Complexes by Reductive Elimination and Oxidative Addition of Dihydrogen

Transition metal complexes that exhibit multiple metal–metal bonding are of fundamental importance in chemistry. Following decades of intense scrutiny of double, triple, and quadruple metal–metal bonds, in 2005 Power and co-workers made an outstanding discovery with the characterization of the first stable molecule with fivefold bonding between two chromium atoms. This report encouraged the search for further examples of quintuply bonded dimetal compounds and was followed by numerous experimental and computational studies. By and large, these studies have focused on chromium compounds. 2–8,11] However, Tsai and coworkers have extended their investigation of the Cr Cr quintuple bond 7,11] to analogous molybdenum complexes and have isolated two closely related compounds supported by monoanionic amidinate ligands, N,N’-disubstituted with 2,6-diisopropylphenyl groups. Recently, another compound with the Mo2 central unit reinforced by three amidinate ligands and a bridging lithium cation has also been reported by the same group of researchers. Identification of the above complexes with fivefold metalmetal bonding has prompted research directed toward understanding their bonding characteristics and the chemical reactivity of their central M M quintuple bond. So far, only a few reports have appeared in the literature concerning mainly the chemical properties of the Cr Cr quintuple bond. Thus, these complexes feature interesting reactivity toward unsaturated molecules like N2O, [19] alkenes, and alkynes, and are able to activate white phosphorous, yellow arsenic, and AsP3. [22] Similar to carbon carbon double and triple bonds, the Cr Cr quintuple bond of an aminopyridinate complex undergoes facile carboalumination, to generate the corresponding Cr Cr quadruple-bond derivative with formally monoanionic bridging CH3 and AlMe2 groups. [23] Disproportionation of Cr to Cr and Cr induced by the addition of [18]crown-6-ether to a Cr Cr quintuply bonded complex has also been demonstrated. During the preparation of this manuscript the reactivity of Mo Mo quintuply bonded compounds with alkynes has been shown. The above results illustrate the potential of quintuply bonded electron-rich M2 centers for bimetallic activation. Herein we describe that the quadruply bonded dimolybdenum dihydride complex [Mo2(H)2{HC(N-2,6-iPr2C6H3)2}2(thf)2] 2 undergoes reductive elimination of H2 from its [(H)Mo Mo(H)] core under UV irradiation (365 nm) to afford the known [Mo2{HC(N-2,6-iPr2C6H3)2}2], 3, [9] with fivefold Mo Mo bonding (Scheme 1B). Because a tetrahydrofuran (THF) solution of the latter reacts readily with H2 to reform 2, our results demonstrate that quadruply and quintuply bonded dimolybdenum complexes may interconvert by means of reductive elimination and oxidative

Zn–Zn‐Bonded Compounds that Contain Monoanionic Oxygen‐Donor Ligands

The last years have witnessed renewed interest in the study of molecular compounds that present a metal–metal bond between atoms of Group 12 metals. For zinc, following the initial report on the structural characterisation of decamethyldizincocene, [Zn2ACHTUNGTRENNUNG(h5-C5Me5)2] (1), a number of complexes of sterically demanding and in many cases chelating ligands have been prepared. Kinetic stabilisation of the Zn Zn bond to prevent disproportionation to Zn and Zn has been achieved by the use of different types of ligands. The initially employed bulky, substituted cyclopentadienyl units were followed by carbon-based m-terphenyl groups, as well as by a variety of chelating N-donor ligands, which have also proved useful to stabilise Mg Mg bonds. Moreover, recent work by Schulz and co-workers has disclosed interesting reactivity of compound 1 that occurs with preservation of the Zn Zn bond and proceeds with elimination of Cp*H (Cp*=C5Me5). [10] Subsequently, a unique Zn2 2+ ion stabilised by coordination of 4-dimethylaminopyridine (dmap), [Zn2ACHTUNGTRENNUNG(dmap)6]2+ , has been isolated and structurally characterised. Since so far Zn Zn bonded complexes with alkoxide or aryloxide ligands (RO ) are not known, we have decided to study the reactivity of 1 towards several alkyl and aryl alcohols. Herein, we present preliminary results on the synthesis and structural characterisation of metal–metal bonded dizinc species featuring Zn O bonds. As reported previously, complex 1 reacts with Me3COH to produce metallic zinc along with the alkoxide [{Zn ACHTUNGTRENNUNG(OtBu)2}x]. This result makes clear that bulkier RO groups are needed to stabilise the dizinc unit and accordingly the reactions of 1 with ArOH (2,6-(2,4,6-Me3C6H2)C6H3OH) and C5Me5OH have been investigated. Initial results were disappointing, as the low-temperature ( 208C) reaction of 1 with ArOH yielded an insoluble white solid that has eluded characterisation so far. Under similar conditions, C5Me5OH led to extensive decomposition, possibly as a result of disproportionation. We then considered providing further stabilisation to the Zn2 unit by carrying out corresponding reactions in the presence of an N-donor ligand. Since 1 is recovered unaltered when crystallised in the presence of an excess of pyridine (despite an evident colour change to yellow that suggests weak [Zn2ACHTUNGTRENNUNG(C5Me5)2]···pyridine interaction), while the more basic dmap provides the metal metal bonded adduct [Zn2 ACHTUNGTRENNUNG(C5Me5)2 ACHTUNGTRENNUNG(dmap)2],[9b] it is clear that sufficiently strong an electron donor is required (pKa values for pyridine and dmap in acetonitrile are 12.53 and 17.95, respectively). With this knowledge, 4-pyrrolidinopyridine (pyr-py, pKa = 18.33) and diaza-1,3-bicycloACHTUNGTRENNUNG[5.4.0]undecane (DBU, pKa = 24.34) have been chosen for this study. As discussed below, these results indicate that pyr-py appears to be more appropriate than DBU for this purpose. Low-temperature H NMR spectroscopic monitoring of the reactions of 1 with ArOH and C5Me5OH (represented in general as ROH), in presence of the aforementioned Ndonor L (pyr-py and DBU), reveals the consumption of both reactants, 1 and ROH, along with concomitant formation of C5Me5H and new zinc–zinc compounds of composition [Zn2 ACHTUNGTRENNUNG(h5-C5Me5)(OR)(L)x], in which a C5Me5 group has been replaced by RO. For instance, treatment of 1 with ArOH in the presence of pyr-py leads to compound 2 as a white solid in 80 % isolated yield (Scheme 1). In its H NMR spectrum in C6D6, the methyl groups of the h [a] M. Carrasco, Dr. R. Peloso, Dr. A. Rodr guez, Dr. E. lvarez, Dr. C. Maya, Prof. E. Carmona Instituto de Investigaciones Qu micas Departamento de Qu mica Inorg nica Universidad de Sevilla Consejo Superior de Investigaciones Cient ficas Avenida Am rico Vespucio 49, 41092 Sevilla (Spain) Fax: (+34) 954460565 E-mail : guzman@us.es Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001011.

Experimental and Theoretical Studies on Arene‐Bridged Metal–Metal‐Bonded Dimolybdenum Complexes

The bis(hydride) dimolybdenum complex, [Mo2(H)2{HC(N-2,6-iPr2C6H3)2}2(thf)2], 2, which possesses a quadruply bonded Mo2(II) core, undergoes light-induced (365 nm) reductive elimination of H2 and arene coordination in benzene and toluene solutions, with formation of the Mo(I)2 complexes [Mo2{HC(N-2,6-iPr2C6H3)2}2(arene)], 3⋅C6H6 and 3⋅C6H5Me, respectively. The analogous C6H5OMe, p-C6H4Me2, C6H5F, and p-C6H4F2 derivatives have also been prepared by thermal or photochemical methods, which nevertheless employ different Mo2 complex precursors. X-ray crystallography and solution NMR studies demonstrate that the molecule of the arene bridges the molybdenum atoms of the Mo(I)2 core, coordinating to each in an η(2) fashion. In solution, the arene rotates fast on the NMR timescale around the Mo2-arene axis. For the substituted aromatic hydrocarbons, the NMR data are consistent with the existence of a major rotamer in which the metal atoms are coordinated to the more electron-rich C-C bonds.

Reactivity Studies of Iridium Pyridylidenes [TpMe2Ir(C6H5)2(C(CH)3C(R)NH] (R=H, Me, Ph)

The reactivity of a series of iridiumpyridylidene complexes with the formula [Tp(Me2) Ir(C6 H5 )2 (C(CH)3 C(R)NH] (1 a-1 c) towards a variety of substrates, from small molecules, such as H2 , O2 , carbon oxides, and formaldehyde, to alkenes and alkynes, is described. Most of the observed reactivity is best explained by invoking 16 e(-) unsaturated [Tp(Me2) Ir(phenyl)(pyridyl)] intermediates, which behave as internal frustrated Lewis pairs (FLPs). H2 is heterolytically split to give hydridepyridylidene complexes, whilst CO, CO2 , and H2 CO provide carbonyl, carbonate, and alkoxide species, respectively. Ethylene and propene form five-membered metallacycles with an IrCH2 CH(R)N (R=H, Me) motif, whereas, in contrast, acetylene affords four-membered iridacycles with the IrC(CH2 )N moiety. C6 H5 (CO)H and C6 H5 CCH react with formation of IrC6 H5 and IrCCPh bonds and the concomitant elimination of a molecule of pyridine and benzene, respectively. Finally the reactivity of compounds 1 a-1 c against O2 is described. Density functional theory calculations that provide theoretical support for these experimental observations are also reported.

Synthetic, mechanistic, and theoretical studies on the generation of iridium hydride alkylidene and iridium hydride alkene isomers.

Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(Tp(Me2))Ir(H){=C(CH(2)R)ArO}] (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate; R = H, Me; Ar = substituted C(6)H(4) group), and their corresponding hydride olefin isomers, [(Tp(Me2))Ir(H){R(H)C=C(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(Tp(Me2))Ir(C(6)H(5))(2)] with o-C(6)H(4)(OH)CH(2)R, or with the substituted anisoles 2,6-Me(2)C(6)H(3)OMe, 2,4,6-Me(3)C(6)H(2)OMe, and 4-Br-2,6-Me(2)C(6)H(2)OMe. The reactions with the substituted anisoles require not only multiple C-H bond activation but also cleavage of the Me-OAr bond and the reversible formation of a C-C bond (as revealed by (13)C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140 degrees C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen-elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ir-carbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers.

Reactivity of Cationic Agostic and Carbene Structures Derived from Platinum(II) Metallacycles

This paper describes the formation of new platinacyclic complexes derived from the phosphine ligands PiPr2 Xyl, PMeXyl2 , and PMe2 Ar Xyl 2 (Xyl=2,6-Me2 C6 H3 and Ar Xyl 2=2,6-(2,6-Me2 C6 H3 )2 -C6 H3 ) as well as reactivity studies of the trans-[Pt(C^P)2 ] bis-metallacyclic complex 1 a derived from PiPr2 Xyl. Protonation of compound 1 a with [H(OEt2 )2 ][BArF ] (BArF =B[3,5-(CF3 )2 C6 H3 ]4 ) forms a cationic δ-agostic structure 4 a, whereas α-hydride abstraction employing [Ph3 C][PF6 ] produces a cationic platinum carbene trans-[Pt{PiPr2 (2,6-CH(Me)C6 H3 }{PiPr2 (2,6-CH2 (Me)C6 H3 }][PF6 ] (8). Compounds 4 a and 8 react with H2 to yield the same 1:3 equilibrium mixture of 4 a and trans-[PtH(PiPr2 Xyl)2 ][BArF ] (6), in which one of the phosphine ligands participates in a δ-agostic interaction. DFT calculations reveal that H2 activation by 8 occurs at the highly electrophilic alkylidene terminus with no participation of the metal. The two compounds 4 a and 8 experience C-C coupling reactions of a different nature. Thus, 4 a gives rise to complex trans-[PtH{(E)-1,2-bis(2-(PiPr2 )-3-MeC6 H3 )CHCH}] (7) that contains a tridentate diphosphine-alkene ligand, through agostic CH oxidative cleavage and C-C reductive coupling steps, whereas the C-C coupling reaction in 8 involves classical migratory insertion of its [PtCH] and [PtCH2 ] bonds promoted by platinum coordination of CO or CNXyl. The mechanisms of the CC bond-forming reactions have also been investigated by computational methods.

Synthesis, structural characterization, reactivity, and catalytic properties of copper(I) complexes with a series of tetradentate tripodal tris(pyrazolylmethyl)amine ligands.

Novel tris(pyrazolylmethyl)amine ligands Tpa(Me3), Tpa*(,Br), and Tpa(Br3) have been synthesized and structurally characterized. The coordination chemistries of these three new tetradentate tripodal ligands and the already known Tpa and Tpa* have been explored using different copper(I) salts as starting materials. Cationic copper(I) complexes [Tpa(x)Cu]PF6 (1-4) have been isolated from the reaction of [Cu(NCMe)4]PF6 and 1 equiv of the ligand. Complexes 2 (Tpa(x) = Tpa*) and 3 (Tpa(x) = Tpa(Me3)) have been characterized by X-ray studies. The former is a 1D helical coordination polymer, and the latter is a tetranuclear helicate. In both structures, the Tpa(x) ligand adopts a μ(2):κ(2):κ(1)-coordination mode. However, in solution, all of the four complexes form fluxional species. When CuI is used as the copper(I) source, neutral compounds 5-8 have been obtained. Complexes 6-8 exhibit a 1:1 metal-to-ligand ratio, whereas 5 presents 2:1 stoichiometry. Its solid-state structure has been determined by X-ray diffraction, revealing its 3D polymeric nature. The polymer is composed by the assembly of [Tpa2Cu4I4] units, in which Cu4I4 presents a step-stair structure. The Tpa ligands bridge the Cu4I4 clusters, adopting also a μ(2):κ(2):κ(1)-coordination mode. As observed for the cationic derivatives, the NMR spectra of 5-8 show the equivalence of the three pyrazolyl arms of the ligands in these complexes. The reactivities of cationic copper(I) derivatives 1-4 with PPh3 and CO have been explored. In all cases, 1:1 adducts [Tpa(x)CuL]PF6 [L = PPh3 (9-11), CO (12-15)] have been isolated. The crystal structure of [Tpa*Cu(PPh3)]PF6 (9) has been obtained, showing that the coordination geometry around copper(I) is trigonal-pyramidal with the apical position occupied by the tertiary amine N atom. The Tpa* ligand binds the Cu center to three of its four N atoms, with one pyrazolyl arm remaining uncoordinated. In solution, the carbonyl adducts 13-15 exist as a mixture of two isomers; the four- and five-coordinate species can be distinguished by means of their IR νCO stretching bands. Finally, the catalytic activities of complexes 1-4 have been demonstrated in carbene- and nitrene-transfer reactions.

Quadruply bonded dimolybdenum complexes with highly unusual geometries and vacant coordination sites

New quadruply bonded dimolybdenum complexes of the terphenyl ligand Ar(Xyl(2)) (Ar(Xyl(2)) = C(6)H(3)-2,6-(C(6)H(3)-2,6-Me(2))(2)) have been prepared and structurally characterized. The steric hindrance exerted by the Ar(Xyl(2)) groups causes the Mo atoms to feature unsaturated four-coordinate structures and a formal fourteen-electron count.

Synthesis and reactivity studies on alkyl–aryloxo complexes of nickel containing chelating diphosphines: cyclometallation and carbonylation reactions

Nickel alkyl–aryloxo complexes of composition Ni(R)(OC6H32,6-Me2)(PP) (R= CH2SiMe3 ,C H 3 ,C H 2CMe2Ph; PPPPr2 (CH2)nPPr2 , n=2 (dippe) or 3 (dippp)) have been synthesized. While the (trimethylsilyl)methyl and the methyl derivatives are stable in solution at room temperature, the bis-neophyl (R = CH2CMe2Ph) complexes undergo a cyclometallation reaction that leads to the metallacycles Ni(CH2CMe2-o-C6 H4)(PP) together with 2,6-dimethylphenol. The alkyl–aryloxo complexes cleanly react with carbon monoxide giving products resulting from CO insertion and reductive elimination, i.e. Ni(CO)2(PP) and the corresponding 2,6-dimethylphenyl carboxylates quantitatively.

The C-terminal extension of bacterial flavodoxin-reductases: Involvement in the hydride transfer mechanism from the coenzyme

To study the role of the mobile C-terminal extension present in bacterial class of plant type NADP(H):ferredoxin reductases during catalysis, we generated a series of mutants of the Rhodobacter capsulatus enzyme (RcFPR). Deletion of the six C-terminal amino acids beyond alanine 266 was combined with the replacement A266Y, emulating the structure present in plastidic versions of this flavoenzyme. Analysis of absorbance and fluorescence spectra suggests that deletion does not modify the general geometry of FAD itself, but increases exposure of the flavin to the solvent, prevents a productive geometry of FAD:NADP(H) complex and decreases the protein thermal stability. Although the replacement A266Y partially coats the isoalloxazine from solvent and slightly restores protein stability, this single change does not allow formation of active charge-transfer complexes commonly present in the wild-type FPR, probably due to restraints of C-terminus pliability. A proton exchange process is deduced from ITC measurements during coenzyme binding. All studied RcFPR variants display higher affinity for NADP(+) than wild-type, evidencing the contribution of the C-terminus in tempering a non-productive strong (rigid) interaction with the coenzyme. The decreased catalytic rate parameters confirm that the hydride transfer from NADPH to the flavin ring is considerably hampered in the mutants. Although the involvement of the C-terminal extension from bacterial FPRs in stabilizing overall folding and bent-FAD geometry has been stated, the most relevant contributions to catalysis are modulation of coenzyme entrance and affinity, promotion of the optimal geometry of an active complex and supply of a proton acceptor acting during coenzyme binding.