Cooperative Reactivity Research Articles

Cooperative effects in the oxidation of CO by palladium oxide cations

Cooperative reactivity plays an important role in the oxidation of CO to CO(2) by palladium oxide cations and offers insight into factors which influence catalysis. Comprehensive studies including guided-ion-beam mass spectrometry and theoretical investigations reveal the reaction products and profiles of PdO(2)(+) and PdO(3)(+) with CO through oxygen radical centers and dioxygen complexes bound to the Pd atom. O radical centers are more reactive than the dioxygen complexes, and experimental evidence of both direct and cooperative CO oxidation with the adsorption of two CO molecules are observed. The binding of multiple electron withdrawing CO molecules is found to increase the barrier heights for reactivity due to decreased binding of the secondary CO molecule, however, reactivity is enhanced by the increase in kinetic energy available to hurdle the barrier. We examine the effect of oxygen sites, cooperative ligands, and spin including two-state reactivity.

Cooperative Catalysis with First‐Row Late Transition Metals

AbstractCooperative catalysis with first‐row transition metals holds much promise for future developments regarding sustainable, selective transformations, including e.g. alkenes, dienes and a variety of small molecules such as CO2, N2 and water. This non‐exhaustive analysis of the current state‐of‐the‐art aims to give a comprehensive overview of the various design strategies and applications of first‐row transition metal cooperative reactivity and to provide leads for new research initiatives in order to expand this emerging field. The main aspects covered involve bimetallic cooperativity, redox‐noninnocent ligands in combination with first‐row transition metal complexes, otherwise reactive or noninnocent scaffolds that can induce metal‐ligand bifunctional substrate activation and the design of adaptive ligands and complexes thereof, wherein hemilability is a key factor for selective reactivity. The metals under review are primarily the late transition metals Fe, Co, Ni and Cu.

Effect of metal ions on radical type and proton‐coupled electron transfer channel: σ‐Radical vs π‐radical and σ‐channel vs π‐channel in the imide units

The mechanism of proton transfer (PT)/electron transfer (ET) in imide units, and its regulation by hydrated metal ions, was explored theoretically using density functional theory in a representative model (a nearly planar and cisoid complex between uracil and its N(3)-dehydrogenated radical, UU). In UU (sigma-radical), PT/ET normally occurs via a seven-center, cyclic proton-coupled sigma-electron sigma-channel transfer (PC(sigma)E(sigma)T) mechanism (3.8 kcal/mol barrier height) with a N(3)-->N(3') PT and an O(4)-->O(4') ET. Binding of hydrated metal ions to the dioxygen sites (O(2)/O(2') or/and O(4)/O(4')) of UU may significantly affect its PT/ET cooperative reactivity by changing the radical type (sigma-radical <--> pi-radical) and ET channel (sigma-channel <--> pi-channel), leading to different mechanisms, ranging from PC(sigma)E(sigma)T, to proton-coupled pi-electron sigma-channel transfer (PC(pi)E(sigma)T) to proton-coupled pi-electron pi-channel transfer (PC(pi)E(pi)T). This change originates from an alteration of the ordering of the UU moiety SOMO/HDMO (the singly occupied molecular orbital and the highest doubly occupied molecular orbital), induced by binding of the hydrated metal ions. It is a consequence of three associated factors: the asymmetric reactant structure, electron cloud redistribution, and fixing role of metal ions to structural backbone. The findings regarding the modulation of the PT/ET pathway via hydrated metal ions may provide valuable information for a greater understanding of PT/ET cooperative mechanisms, and an alternative way for designing imide-based molecular devices, such as molecular switches and molecular wires.

Intramolecular C−H Bond Activation and Redox Isomerization across Two-Electron Mixed Valence Diiridium Cores

Metal−metal cooperativity enables the reaction of carbon-based substrates at diiridium two-electron mixed valence centers. Arylation of Ir20,II(tfepma)3Cl2 (1) (tfepma = bis[(bistrifluoroethoxy)phosphino]methylamine) with RMgBr (R = C6H5 and C6D5) is followed by C−H bond activation to furnish the bridging benzyne complex Ir2II,II(tfepma)3(μ-C6H4)(C6H5)H (2), as the kinetic product. At ambient temperature, 2 isomerizes to Ir2I,III(tfepma)3(μ-C6H4)(C6H5)H (3) (kobs = 9.57 ± 0.10 × 10−5 s−1 at 31.8 °C, ΔH⧧ = 21.7 ± 0.3 kcal/mol, ΔS⧧ = −7.4 ± 0.9 eu), in which the benzyne moiety is conserved and the IrIII center is ligated by terminal hydride and phenyl groups. The same reaction course is observed for arylation of 1 with C6D5MgBr to produce 2-d10 and 3-d10 accompanied by an inverse isotope effect, kh/kd= 0.44 (kobs = 2.17 ± 0.10 × 10−4 s−1 in C6D6 solution at 31.8 °C, ΔH⧧ = 24.9 ± 0.7 kcal/mol, ΔS⧧ = −6.4 ± 2.4 eu). 2 reacts swiftly with hydrogen to provide Ir2II,II(tfepma)3H4 as both the syn and anti isomers (4-syn and 4-anti, respectively). The hydrides of 4-syn were directly located by neutron diffraction analysis. X-ray crystallographic examination of 2, 2-d10, 3, and 4-syn indicates that cooperative reactivity at the bimetallic diiridium core is facilitated by the ability of the two-electron mixed valence framework to accommodate the oxidation state changes and ligand rearrangements attendant to the reaction of the substrate.

Oxazolinethiones and Oxazolidinethiones for the First Copper-Catalyzed Desulfurative Cross-Coupling Reaction and First Sonogashira Applications

Cyclic thionocarbamates, namely chiral oxazolidinethiones (OZT) and aromatic oxazolinethiones (OXT), were involved, for the first time, in Sonogashira cross-coupling. A cooperative effect of two different copper (I) species-CuI and CuTC-accounts for this new copper-catalyzed desulfurative carbon-carbon cross-coupling reaction. This cooperative reactivity could also be extended to other copper (I) catalysts.

Reactions of early–late heterobimetallics with oxiranes: New examples for cooperative reactivity

In the reaction of oxiranes and cobalt-containing early–late heterobimetallic (ELHB) compounds isolated or prepared in situ, acyl-cobalt complexes, Y 3M EOCHRCH 2C(O)Co(CO) 4 ( 1, M E = Ti, Zr, Hf), were obtained. Some of the complexes 1 were isolated in form of their derivatives substituted on the cobalt atom with PPh 3. We have demonstrated that the ELHB compounds containing Group 4 metals and silyl-cobalttetracarbonyls are analogous concerning their reactivity against oxiranes. The studied heterobimetallic systems mediated the rearrangement of terminal oxiranes to ketones.

Cooperative bimetallic reactivity: hydrogen activation in two-electron mixed-valence compounds.

Reversible dihydrogen uptake by a two-electron mixed-valence di-iridium complex is examined with nonlocal density-functional calculations. Optimized metrics compare favorably with crystal structures of isolated species, and the calculated activation enthalpy of acetonitrile exchange is accurate within experimental error. Dihydrogen attacks the Ir(2) core at Ir(II); the Ir(0) center is electronically saturated and of incorrect orbital parity to interact with H(2). Isomeric eta(2)-H(2) complexes have been located, and harmonic frequency calculations confirm these to be potential energy minima. A transition state links one such complex with the final dihydride; calculated atomic charges suggest a heterolytic H(2) bond scission within the di-iridium coordination sphere. This investigation also establishes a ligand-design criterion for attaining cooperative bimetallic reactivity, namely, that the supporting ligand framework has sufficient mechanical flexibility so that the target complex can accommodate the nuclear reorganizations that accompany substrate activation.

Towards co-operative reactivity in conjoint classical-organometallic heterometallic complexes: the co-ordination chemistry of novel ligands with triphenylphosphine and bis(pyridylethyl)amine or triazacyclononane domains

With a view towards later studies of co-operativity in heteronuclear complexes with hard classical (oxygen-activating) and soft organometallic (organic-substrate binding) metal centres, four novel ditopic N3P-donor ligands (L1–L4), each comprising triphenylphosphine tethered to an N,N′-bis(2-pyridyl-2-ethyl)amine (bpea) or a 1,4-diisopropyl-1,4,7-triazacyclononane (tacn*) N3-donor group, have been designed and prepared by reductive aminations of ortho- and meta-(diphenylphosphino)benzaldehydes with bpea (for L1 and L3) and tacn* (for L2 and L4). A range of κNn,κP-chelate mononuclear complexes have been isolated from the reactions of the ortho-substituted ligands, L1 and L2, with Cu(I), Zn(II) and Pt(II) sources, and the X-ray crystal structures of [Cu(L1)][PF6], [Cu(L2)][PF6] (communicated in: S. E. Watkins, D. C. Craig and S. B. Colbran, J. Chem. Soc., Dalton Trans., 1999, 1539) and [PtCl(L1)][PF6] have been determined. Six complexes with the phosphine of L1–L4 co-ordinated to a softer [Pt(II), Ir(I) or W(0)] metal centre and with dangling, metal-free N3-donor domains have been prepared: for the ortho-substituted ligands L1 and L2, it was necessary to protect the hard, more basic N3-donor domains by protonation (pH control) to prevent formation of κNn,κP-chelate mononuclear complexes; for the meta-substituted ligands L3 and L4, pH control was unnecessary as the phosphine group selectively binds to the softer metal ions. The complex trans-[IrCl(CO)(L3)2] reversibly forms a dioxygen adduct. An Ir(III)Cu(II)2 and four Pt(II)Cu(II)2 heterometallic complexes were prepared by adding hard Cu(II) ions to the Ir(I) and Pt(II) complexes with metal-free N3-donor domains, and the full characterisation of these is described. The tungsten(0) carbonyl complex [W(CO)5(L3)], with a metal-free N3-bpea domain, was prepared for a study of metal ion recognition. No perturbation of the carbonyl region of the IR spectrum was observed when metal ions were added. The effect of submolar quantities of heterometallic complexes, obtained by adding a first d-series metal(II) ion (2 equivalents) to [IrCl(CO)(L3)2], on the oxidation of styrene by oxygen in methylethyl ketone has been assayed: inhibition of the oxidation is observed with the %conversion and the product selectivity dependant on the metal(II) ion.

ChemInform Abstract: Highly Polar Metal-Metal Bonds in “Early-Late” Heterodimetallic Complexes

Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.

Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic Complexes.

Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.

Cooperative reactivity of early-late heterodinuclear transition metal complexes with polar organic substrates

A comprehensive investigation into the cooperative reactivity of two chemically complementary metal-complex fragments in early-late heterodinuclear complexes has been carried out. Reaction of the partially fluorinated tripodal amidozirconium complexes [HC-(SiMe2NR)3Zr(mu-Cl)2Li(OEt2)2] (R = 2-FC6H4: 2a, 2,3,4-F3C6H4: 2b) with K[CpM(CO)2] (M=Fe, Ru) afforded the stable metal-metal bonded heterodinuclear complexes [HC[SiMe2NR]3-Zr-MCp(CO)2] (3-6). Reaction of the dinuclear complexes with methyl isonitrile as well as the heteroallenes CO2, CS2, RNCO and RNCS led to insertion into the polar metal-metal bond. Two of these complexes, [HC[SiMe2N(2-FC6-H4)]3Zr(S2C)Fe(CO)2Cp] (9a) and [HC-[SiMe2N(2-FC2H4)]3Zr-(SCNPh)Fe(CO)2-Cp] (12), have been structurally characterized by a single crystal X-ray structure analysis, proving the structural situation of the inserted substrate as a bridging ligand between the early and late transition metal centre. The reactivity towards organic carbonyl derivatives proved to be varied. Reaction of the heterobimetallic complexes with benzyl and ethylbenzoate led to the cleavage of the ester generating the respective alkoxozirconium complexes [HC[SiMe2N(2-FC6H4)]3ZrOR] (R = Ph-CH2: 13a, Et: 13b) along with [CpFe-[C(O)Ph](CO)2], whereas the analogous reaction with ethyl formate gave 13b along with [CpFeH(CO)2]; this latter complex results from the instability of the formyliron species initially formed. Aryl aldehydes were found to react with the Zr-M complexes according to a Cannizzaro disproportionation pattern yielding the aroyliron and ruthenium complexes along with the respective benzoxyzirconium species. The transfer of the aldehyde hydrogen atom in the course of the reaction was established in a deuteriation experiment. [HC[SiMe2-N(2-FC6H4)]3Zr-M(CO)2Cp] reacted with lactones to give the ring-opened species containing an alkoxozirconium and an acyliron or acylruthenium fragment; the latter binds to the early transition metal centre through the acyl oxygen atom, as evidenced from the unusuallly low-field shifted 13C NMR resonances of the RC(O)M units. Ketones containing a-CH units react with the Zr-Fe complexes cooperatively to yield the aldol coupling products coordinated to the zirconium complex fragment along with the hydridoiron compound [CpFeH(CO)2], whereas 1,2-diphenylcyclopropenone underwent an oxygen transfer from the keto group to a CO ligand to give a linking CO2 unit and a cyclopropenylidene ligand coordinated to the iron fragment in [HC-[Si(CH3)2N(2,3,4-F3C6H2)]3Zr(mu-O2C)-Fe(CO)[C3Ph2)Cp] (19). The atom transfer was established by 17O and 13C labelling studies. Similar oxygen-transfer processes were observed in the reactions with pyridine N-oxide, dimethylsulfoxide and methylphenylsulfoxide.

Atom-transfer within the coordination sphere of early–late heterobimetallic complexes: rapid deoxygenation of sulfoxides at low temperatures

Extremely rapid oxygen transfer from sulfoxides to a carbonyl ligand to give the corresponding thioether and coordinated CO2 is observed in the reactions of unsupported Zr–Ru heterodimetallic complexes with sulfoxides; this is interpreted to arise from the cooperative reactivity of the two complementary metal complex fragments bound to each other through a highly polar metal–metal bond.

Cooperative Asymmetric Catalysis with Dimeric Salen Complexes

Cooperative reactivity between multiple metal centers is commonly postulated for enzymatic systems1 and has evolved into an intriguing design principle for synthetic catalysts.2 We reported recently that the asymmetric ring opening (ARO) of meso epoxides with TMS-N3 catalyzed by (salen)Cr complex 13 displays a second-order kinetic dependence on catalyst.4 This observation led to a mechanistic proposal for the ARO involving simultaneous activation of epoxide and azide by two different catalyst molecules (Figure 1). We have undertaken the design of catalysts that enforce this cooperative mechanism through the construction of covalently linked dimeric complexes, and we describe here the synthesis and successful application of such systems as highly enantioselective and efficient ARO catalysts. Kinetic and enantioselectivity data provide unambiguous evidence for a mechanism involving cooperative, intramolecular bimetallic catalysis.

Cooperative Reactivity of Unsupported Early−Late Heterobimetallics: Ring Opening and Subsequent Decarbonylation of Biaryllactones

Reaction of the heterodinuclear complex HC-{SiMe2N(2-FC6H3)}3Zr-FeC p(CO)2 (1) with 1,3-di-methyl-6H-benzobnaphtho1,2-dpyranone (2) and 1,3-dimethoxy-6H-benzob-naphtho-1,2-dpyranone (3) led to immediate ring opening of the lactones to yield products which combine the biaryl fragment with aryloxyzirconium and acyl-FeCp(CO)2 units. The latter slowly decarbonylate to yield the corresponding heterobimetallic complexes in which the {MCp(CO)2} fragment is directly bonded to the naphthalene ring.

Chemistry of electrophilic metal centres coordinated by silox ( tBu 3SiO), tritox ( tBu 3CO) and related binfunctional ligands

Bulky siloxide and alkoxide ligands, notably silox ( t Bu 3SiO), and tritox ( t Bu 3CO), have been employed as ancillary ligands in the chemistry of low-coordinate early transition metal complexes. Related sterically hindered difunctional ligands (e.g., alkoxyalkylphosphines) are utilized to link disparate early and late transition metal centres together in a quest for cooperative reactivity. This review focuses on our group's efforts in this area over the past 14 years. Emphasis is placed on the rationale for using bulky, hard, anionic donor ligands, and the synthesis and reactivity studies of this class of compounds.

Cooperative Reactivity in the Interactions of X-H Bonds with a Zirconium-Iridium Bridging Imido Complex

Cooperative Reactivity in the Interactions of X-H Bonds with a Zirconium-Iridium Bridging Imido Complex

Synthesis of an early-late heterobimetallic imido complex: cooperative reactivity with nitrogen-, oxygen-, and sulfur-hydrogen bonds and formation of a novel aminophosphine complex

Synthesis of an early-late heterobimetallic imido complex: cooperative reactivity with nitrogen-, oxygen-, and sulfur-hydrogen bonds and formation of a novel aminophosphine complex

ChemInform Abstract: Oxidative Photofragmentation of α,β‐Amino Alcohols via Single Electron Transfer: Cooperative Reactivity of Donor and Acceptor Ion Radicals in Photogenerated Contact Radical Ion Pairs

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Oxidative photofragmentation of .alpha.,.beta.-amino alcohols via single electron transfer: cooperative reactivity of donor and acceptor ion radicals in photogenerated contact radical ion pairs

The studies presented in this paper show that α,β-amino alcohols undergo a very clean C−C bond cleavage upon SET (single electron transfer) oxidation by photoexcited electron acceptors in a process which generally culminates in two-electron reduction of the acceptors. For a number of different α,β-amino alcohols, the oxidative fragmentation occurs in a high chemical yield (>90%), yet with low to medium quantum efficiencies (0.0001−0.1) which vary strongly depending on the properties of electron donor (D), acceptor (A), and solvent. The net quantum efficiency reflects the competition between back electron transfer and the chemical redox process. Detailed mechanistic studies were carried out to investigate the visible light induced oxidative fragmentation of α,β-amino alcohols in the presence of electron acceptors including thioindigo (TI), 9,10Dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene (TCA), and 1,4-dicyanonaphthalene (DCN)

Cooperative reactivity in photogenerated radical ion pairs: photofragmentation of amino ketones

A number of instances of fragmentation (or cleavage) have recently been noted for certain electron donors when activated by photoinduced single electron transfer (SET) reactions. Herein the authors report a novel reaction in which scission of a strong carbon-carbon bond in an unexcited donor can be brought about via quenching of an excited acceptor. In this case, the aar seem to function as a nucleophile is a displacement within the contact ion pair; the net reaction involves a chemically clean and yet unprecedented ketone to amide interconversion.