Riccardo Peloso

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.

Synthesis and Reactivity of a Cationic Platinum(II) Alkylidene Complex

The last decades have witnessed an explosive growth of interest in the chemistry of transition-metal carbenes, 2] which has mainly been due to their important catalytic applications. A great deal of effort has converged in recent years on platinumand gold-catalyzed reactions that occur with participation of carbene species as active reaction intermediates. For these systems F rstner and co-workers have established an analogy between metal carbenes and metal-stabilized carbocationic structures and proposed the term carbenoid to illustrate this ambiguity. 4] For the Group 10 elements, carbene complexes with carbocyclic-, N-heterocyclic-, and heteroatom-stabilizedcarbene ligands are known. However, related complexes that contain {M=C(R)(R’)} units, where R and R are hydrogen or hydrocarbyl groups are very rare. The first such nickel complex was reported by Mindiola and Hillhouse in 2002 and contains a Ni center bound to a C(Ph)2 carbene fragment. Interestingly, only a year later, a cationic Pd derivative of a non-heteroatom-stabilized carbene ligand C(p-tolyl)2 was prepared and structurally characterized by X-ray crystallography. The latter complex features a rather long Pd Ccarbene bond (1.976 ), which is indicative of only a minor stabilization of the carbene s donor by back-bonding from filled palladium dp orbitals. In view of the importance of cationic platinum(II) alkylidene species, [Pt=C(R)(R’)], in catalysis 5] we planned to generate and characterize some compounds of this kind. Over the years, we have succeeded in ascertaining the structural properties of related cationic Ir alkylidenes. Studies of the energetics of {Pt=CH2} + by ab initio calculations revealed a high bond energy that is largely due to relativistic effects. On the assumption that the bonding for a cationic {Pt=C(R)(R’)} unit follows the dative model, and taking into account that similar to Pd little backdonation from the cationic platinum(II) center to the alkylidene can be expected, it is reasonable to think that failure to isolate such species is most probably due to their high reactivity rather than to intrinsic weakness of the platinum–alkylidene bond. Indeed, Menj n, Forni s, and coworkers have demonstrated recently that, contrary to isolable [M]=CF2 derivatives of Group 8 and 9 metals, the analogous [Pt]=CF2 units are highly reactive and need extra stabilization by formation of a chelating pyridinium ylide structure. We envisaged that sterically protected platinum bis(metallacycles) containing five-membered rings as a result of metalation of aryl phosphine ligands (structures I and II in Scheme 1) could be suitable candidates for this enterprise, as cationic benzylidene structures could then be readily generated by a-hydride abstraction. Related platinum metallacycles have already been reported. 20] With the express intention of hindering C C coupling between the benzylidene and benzyl termini of the desired cationic alkylidene formulations, a trans geometry like II in Scheme 1 was chosen. Reaction of [PtCl2(cod)] (cod = 1,5-cyclooctadiene) with the bulky xylyl phosphine 21] PiPr2Xyl (Xyl = 2,6-Me2C6H3) under the conditions of Scheme 2 yielded the desired trans bis(metallacycle) 1 as a colorless microcrystalline solid, in yields of isolated product of close to 80%. Complex 1 was fully characterized by microanalysis and NMR spectroscopy (see the Supporting Information).

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 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.

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.

Structure and reactivity of new iridium complexes with bis(oxazoline)-phosphonito ligands

The synthesis and characterization of novel iridium(I) complexes bearing a neutral bis(oxazoline)phosphonite ligand, NOPON(Me(2)) (I), are reported. Numerous Ir(I) complexes have been isolated in high yields and characterized by spectroscopy and X-ray diffraction. [Ir(mu-Cl)(cod)](2) (cod = 1,5-cyclooctadiene) reacted with I to give the air-sensitive complex [IrCl(cod)(NOPON(Me(2)))] (1), which shows broad (1)H and (13)C{(1)H} NMR signals due to dynamic exchange equilibria involving the cod and the NOPON(Me(2)) ligands. Reaction between a solution of 1 and CO afforded the carbonyl complex [IrCl(CO)(NOPON(Me(2)))] (2), whose solid-state structure has been determined by X-ray diffraction. Cationic complexes have been obtained by using NaBAr(F) (BAr(F) = B[3,5-(CF(3))(2)C(6)H(3)](4)) as a chloride abstractor. The complex [Ir(cod)(NOPON(Me(2)))]BAr(F) (3) displays a mononuclear structure in the solid state with ligand I acting as a bidentate P,N chelating ligand. This complex is a precatalyst for the hydrogenation of alkenes. Oxidative addition of H(2) to 3 occurred either in solution or in the solid-state and this reaction allowed the isolation of the 32 electron, dinuclear dihydrido-bridged iridium(III) complex [IrH(mu-H)(NOPON(Me(2)))](2)(BAr(F))(2) (4), in which the NOPON(Me(2)) ligands exhibit a facial coordination mode. It contains only hydrides as bridging ligands and the Ir(2)(mu-H)(2) unit can be viewed as containing a formal Ir-Ir double bond or two 3c-2e bonds. Complex 3 has also been reacted with CO in solution and in the solid state, and this yielded the dicarbonyl derivative [Ir(CO)(2)(NOPON(Me(2)))]BAr(F) (5). A transmetalation reaction between 3 and [PdCl(2)(NCPh)(2)] afforded the cationic Pd(II) complex [PdCl(NOPON(Me(2)))]BAr(F) (6), which has been structurally characterized.

Lithium Di- and Trimethyl Dimolybdenum(II) Complexes with Mo–Mo Quadruple Bonds and Bridging Methyl Groups

New dimolybdenum complexes of composition [Mo2{μ-Me}2Li(S)}(μ-X)(μ-N^N)2] (3a-3c), where S = THF or Et2O and N^N represents a bidentate aminopyridinate or amidinate ligand that bridges the quadruply bonded molybdenum atoms, were prepared from the reaction of the appropriate [Mo2{μ-O2CMe}2(μ-N^N)2] precursors and LiMe. For complex 3a, X = MeCO2, while in 3b and 3c, X = Me. Solution NMR studies in C6D6 solvent support formulation of the complexes as contact ion pairs with weak agostic Mo-CH3···Li interactions, which were also evidenced by X-ray crystallography in the solid-state structures of the molecules of 3a and 3b. Samples of 3c enriched in (13)C (99%) at the metal-bonded methyl sites were also prepared and investigated by NMR spectroscopy employing C6D6 and THF-d8 solvents. Crystallization of 3c from toluene:tetrahydrofuran mixtures provided single crystals of the solvent separated ion pair complex [Li(THF)4] [Mo2(Me)2(μ-Me){μ-HC(NDipp)2}2] (4c), where Dipp stands for 2,6-iPr2C6H3. A computational analysis of the Mo2(μ-Me)2Li core of complexes 3a and 3b has been developed, which is consistent with a small but non-negligible electron-density sharing between the C and Li atoms of the mainly ionic CH3···Li interactions.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

A Cationic Unsaturated Platinum(II) Complex that Promotes the Tautomerization of Acetylene to Vinylidene

Complex [PtMe2 (PMe2 ArDipp2 )] (1), which contains a tethered terphenyl phosphine (ArDipp2 =2,6-(2,6-i Pr2 C6 H3 )2 C6 H3 ), reacts with [H(Et2 O)2 ]BArF (BArF- =B[3,5-(CF3 )2 C6 H3 ]4- ) to give the solvent (S) complex [PtMe(S)(PMe2 ArDipp2 )]+ (2⋅S). Although the solvent molecule is easily displaced by a Lewis base (e.g., CO or C2 H4 ) to afford the corresponding adducts, treatment of 2⋅S with C2 H2 yielded instead the allyl complex [Pt(η3 -C3 H5 )(PMe2 ArDipp2 )]+ (6) via the alkyne intermediate [PtMe(η2 -C2 H2 )(PMe2 ArDipp2 )]+ (5). Deuteration experiments with C2 D2 , and kinetic and theoretical investigations demonstrated that the conversion of 5 into 6 involves a PtII -promoted HC≡CH to :C=CH2 tautomerization in preference over acetylene migratory insertion into the Pt-Me bond.