Roles of Iron Complexes in Catalytic Radical Alkene Cross-Coupling: A Computational and Mechanistic Study.

A growing and useful class of alkene coupling reactions involve hydrogen atom transfer (HAT) from a metal-hydride species to an alkene to form a free radical, which is responsible for subsequent bond formation. Here, we use a combination of experimental and computational investigations to map out the mechanistic details of iron-catalyzed reductive alkene cross-coupling, an important representative of the HAT alkene reactions. We are able to explain several observations that were previously mysterious. First, the rate-limiting step in the catalytic cycle is the formation of the reactive Fe-H intermediate, elucidating the importance of the choice of reductant. Second, the success of the catalytic system is attributable to the exceptionally weak (17 kcal/mol) Fe-H bond, which performs irreversible HAT to alkenes in contrast to previous studies on isolable hydride complexes where this addition was reversible. Third, the organic radical intermediates can reversibly form organometallic species, which helps to protect the free radicals from side reactions. Fourth, the previously accepted quenching of the postcoupling radical through stepwise electron transfer/proton transfer is not as favorable as alternative mechanisms. We find that there are two feasible pathways. One uses concerted proton-coupled electron transfer (PCET) from an iron(II) ethanol complex, which is facilitated because the O-H bond dissociation free energy is lowered by 30 kcal/mol upon metal binding. In an alternative pathway, an O-bound enolate-iron(III) complex undergoes proton shuttling from an iron-bound alcohol. These kinetic, spectroscopic, and computational studies identify key organometallic species and PCET steps that control selectivity and reactivity in metal-catalyzed HAT alkene coupling, and create a firm basis for elucidation of mechanisms in the growing class of HAT alkene cross-coupling reactions.

Catalyzed Radical Termination in the Presence of Tellanyl Radicals.

The decomposition of the diazo initiator dimethyl 2,2'-azobis(isobutyrate) (V-601), generating the Me2 C. (CO2 Me) radical, affords essentially the same fraction of disproportionation and combination in media with a large range of viscosity (C6 D6 , [D6 ]DMSO, and PEG 200) in the 25-100 °C range. This is in stark contrast to recent results by Yamago et al. on the same radical generated from Me2 C(TeMe)(CO2 Me) and on other X-TeR systems (X=polymer chain or unimer model; R=Me, Ph). The discrepancy is rationalized on the basis of an unprecedented RTe. -catalyzed radical disproportionation, with support from DFT calculations and photochemicaL V-601 decomposition in the presence of Te2 Ph2 .

Organometallic-Mediated Alternating Radical Copolymerization of tert-Butyl-2-Trifluoromethacrylate with Vinyl Acetate and Synthesis of Block Copolymers Thereof.

Organometallic-mediated radical polymerization (OMRP) has given access to well-defined poly(vinyl acetate-alt-tert-butyl-2-trifluoromethacrylate)-b-poly(vinyl acetate) and poly(VAc-alt-MAF-TBE) copolymers composed of two electronically distinct monomers: vinyl acetate (VAc, donor, D) and tert-butyl-2-trifluoromethacrylate (MAF-TBE, acceptor, A), with low dispersity (≤1.24) and molar masses up to 57 000 g mol-1 . These copolymers have a precise 1:1 alternating structure over a wide range of comonomer feed compositions. The reactivity ratios are determined as rVAc = 0.01 ± 0.01 and rMAF-TBE = 0 at 40 °C. Remarkably, from a feed containing >50% molar VAc content, poly(VAc-alt-MAF-TBE)-b-PVAc block copolymers are produced via a one-pot synthesis. Such diblock copolymers exhibit two glass transition temperatures attributed to the alternating and homopolymer sequences. The OMRP of this fluorine-containing alternating monomer system may provide access to a wide range of new polymer materials.

Olefin oxygenation by water on an iridium center.

Oxygenation of 1,5-cyclooctadiene (COD) is achieved on an iridium center using water as a reagent. A hydrogen-bonding interaction with an unbound nitrogen atom of the naphthyridine-based ligand architecture promotes nucleophilic attack of water to the metal-bound COD. Irida-oxetane and oxo-irida-allyl compounds are isolated, products which are normally accessed from reactions with H2O2 or O2. DFT studies support a ligand-assisted water activation mechanism.

Metal-ligand cooperation on a diruthenium platform: selective imine formation through acceptorless dehydrogenative coupling of alcohols with amines.

Metal-metal singly-bonded diruthenium complexes, bridged by naphthyridine-functionalized N-heterocyclic carbene (NHC) ligands featuring a hydroxy appendage on the naphthyridine unit, are obtained in a single-pot reaction of [Ru2(CH3COO)2(CO)4] with 1-benzyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (BIN⋅HBr) or 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (PIN⋅HBr), TlBF4, and substituted benzaldehyde containing an electron-withdrawing group. The modified NHC-naphthyridine-hydroxy ligand spans the diruthenium unit in which the NHC carbon and hydroxy oxygen occupy the axial sites. All the synthesized compounds catalyze acceptorless dehydrogenation of alcohols to the corresponding aldehydes in the presence of a catalytic amount of weak base 1,4-diazabicyclo[2.2.2]octane (DABCO). Further, acceptorless dehydrogenative coupling (ADHC) of the alcohol with amines affords the corresponding imine as the sole product. The substrate scope is examined with 1 (BIN, p-nitrobenzaldehyde). A similar complex [Ru2(CO)4(CH3COO)(3-PhBIN)][Br], that is devoid of a hydroxy arm, is significantly less effective for the same reaction. Neutral complex 1 a, obtained by deprotonation of the hydroxy arm in 1, is found to be active for the ADHC of alcohols and amines under base-free conditions. A combination of control experiments, deuterium labeling, kinetic Hammett studies, and DFT calculations support metal-hydroxyl/hydroxide and metal-metal cooperation for alcohol activation and dehydrogenation. The bridging acetate plays a crucial role in allowing ß-hydride elimination to occur. The ligand architecture on the diruthenium core causes rapid aldehyde extrusion from the metal coordination sphere, which is responsible for exclusive imine formation.

Ru-Zn heteropolynuclear complexes containing a dinucleating bridging ligand: synthesis, structure, and isomerism.

Mononuclear complexes in- and out-[Ru(Cl)(trpy)(Hbpp)](+) (in-0, out-0; Hbpp is 2,2'-(1H-pyrazole-3,5-diyl)dipyridine and trpy is 2,2':6',2″-terpyridine) are used as starting materials for preparation of Ru-Zn heterodinuclear out-{[Ru(Cl)(trpy)][ZnCl2](µ-bpp)} (out-2) and heterotrinuclear in,in- and out,out-{[Ru(Cl)(trpy)]2(µ-[Zn(bpp)2])}(2+) (in-3, out-3) constitutional isomers. Further substitution of the Cl ligand from the former complexes leads to Ru-aqua out,out-{[Ru(trpy)(H2O)]2(µ-[Zn(bpp)2])}(4+) (out-4) and the oxo-bridged Ru-O-Ru complex in,in-{[Ru(III)(trpy)]2(µ-[Zn(bpp)2(H2O)]µ-(O)}(4+) (in-5). All complexes are thoroughly characterized by the usual analytical techniques as well as by spectroscopy by means of UV-vis, MS, and when diamagnetic NMR. CV and DPV are used to extract electrochemical information and monocrystal X-ray diffraction to characterize complexes out-2, in-3, out-3, and in-5 in the solid state. Complex out-3 photochemically isomerizes toward in-3, as can be observed by NMR spectroscopy and rationalized by density functional theory based calculations.

Reactions of acids with naphthyridine-functionalized ferrocenes: protonation and metal extrusion.

Reaction of 1,8-naphthyrid-2-yl-ferrocene (FcNP) with a variety of acids affords protonated salts at first, whereas longer reaction time leads to partial demetalation of FcNP resulting in a series of Fe complexes. The corresponding salts [FcNP·H][X] (X = BF(4) or CF(3)SO(3) (1)) are isolated for HBF(4) and CF(3)SO(3)H. Reaction of FcNP with equimolar amount of CF(3)CO(2)H for 12 h affords a neutral complex [Fe(FcNP)(2)(O(2)CCF(3))(2)(OH(2))(2)] (2). Use of excess acid gave a trinuclear Fe(II) complex [Fe(3)(H(2)O)(2)(O(2)CCF(3))(8)(FcNP·H)(2)] (3). Three linear iron atoms are held together by four bridging trifluoroacetates and two aqua ligands in a symmetric fashion. Reaction with ethereal solution of HCl afforded [(FcNP·H)(3)(Cl)][FeCl(4)](2) (4) irrespective of the amount of the acid used. Even the picric acid (HPic) led to metal extrusion giving rise to [Fe(2)(Cl)(2)(FcNP)(2)(Pic)(2)] (5) when crystallized from dichloromethane. Metal extrusion was also observed for CF(3)SO(3)H, but an analytically pure compound could not be isolated. The demetalation reaction proceeds with an initial proton attack to the distal nitrogen of the NP unit. Subsequently, coordination of the conjugate base to the electrophilic Fe facilitates the release of Cp rings from metal. The conjugate base plays an important role in the demetalation process and favors the isolation of the Fe complex as well. The 1,1'-bis(1,8-naphthyrid-2-yl)ferrocene (FcNP(2)) does not undergo demetalation under identical conditions. Two NP units share one positive charge causing the Fe-Cp bonds weakened to an extent that is not sufficient for demetalation. X-ray structure of the monoprotonated FcNP(2) reveals a discrete dimer [(FcNP(2)·H)](2)[OTf](2) (6) supported by two N-H···N hydrogen bonds. Crystal packing and dispersive forces associated with intra- and intermolecular π-π stacking interactions (NP···NP and Cp···NP) allow the formation of the dimer in the solid-state. The protonation and demetalation reactions of FcNP and FcNP(2) with a variety of acids are reported.

Ligand-bridged dinuclear cyclometalated Ir(III) complexes: from metallamacrocycles to discrete dimers.

Metallamacrocycles 1, 2, and 3 of the general formula [{Ir(ppy)(2)}(2)(µ-BL)(2)](OTf)(2) (ppyH = 2-phenyl pyridine; BL = 1,2-bis(4-pyridyl)ethane (bpa) (1), 1,3-bis(4-pyridyl)propane (bpp) (2), and trans-1,2-bis(4-pyridyl)ethylene (bpe) (3)) have been synthesized by the reaction of [{(ppy)(2)Ir}(2)(µ-Cl)(2)], first with AgOTf to effect dechlorination and later with various bridging ligands. Open-frame dimers [{Ir(ppy)(2)}(2)(µ-BL)](OTf)(2) were obtained in a similar manner by utilizing N,N'-bis(2-pyridyl)methylene-hydrazine (abp) and N,N'-(bis(2-pyridyl)formylidene)ethane-1,2-diamine (bpfd) (for compounds 4 and 5, respectively) as bridging ligands. Molecular structures of 1, 3, 4, and 5 were established by X-ray crystallography. Cyclic voltammetry experiments reveal weakly interacting "Ir(ppy)(2)" units bridged by ethylene-linked bpe ligand in 3; on the contrary the metal centers are electronically isolated in 1 and 2 where the bridging ligands are based on ethane and propane linkers. The dimer 4 exhibits two accessible reversible reduction couples separated by 570 mV indicating the stability of the one-electron reduced species located on the diimine-based bridge abp. The "Ir(ppy)(2)" units in compound 5 are noninteracting as the electronic conduit is truncated by the ethane spacer in the bpfd bridge. The dinuclear compounds 1-5 show ligand centered (LC) transitions involving ppy ligands and mixed metal to ligand/ligand to ligand charge transfer (MLCT/LLCT) transitions involving both the cyclometalating ppy and bridging ligands (BL) in the UV-vis spectra. For the conjugated bridge bpe in compound 3 and abp in compound 4, the lowest-energy charge-transfer absorptions are red-shifted with enhanced intensity. In accordance with their similar electronic structures, compounds 1 and 2 exhibit identical emissions. The presence of vibronic structures in these compounds indicates a predominantly (3)LC excited states. On the contrary, broad and unstructured phosphorescence bands in compounds 3-5 strongly suggest emissive states of mixed (3)MLCT/(3)LLCT character. Density functional theory (DFT) calculations have been carried out to gain insight on the frontier orbitals, and to rationalize the electrochemical and photophysical properties of the compounds based on their electronic structures.

Bimetallic catalysis involving dipalladium(I) and diruthenium(I) complexes.

Dipalladium(I) and diruthenium(I) compounds bridged by two [{(5,7-dimethyl-1,8-naphthyridin-2-yl)amino}carbonyl]ferrocene (L) ligands have been synthesized. The X-ray structures of [Pd(2)L(2)][BF(4)](2) (1) and [Ru(2)L(2)(CO)(4)][BF(4)](2) (2) reveal dinuclear structures with short metal-metal distances. In both of these structures, naphthyridine bridges the dimetal unit, and the site trans to the metal-metal bond is occupied by weakly coordinating oxygen from the amido fragment. The catalytic utilities of these bimetallic compounds are evaluated. Compound 1 is an excellent catalyst for phosphine-free, Suzuki cross-coupling reactions of aryl bromides with arylboronic acids and provides high yields in short reaction times. Compound 1 is also found to be catalytically active for aryl chlorides, although the corresponding yields are lower. A bimetallic mechanism is proposed, which involves the oxidative addition of aryl bromide across the Pd-Pd bond and the bimetallic reductive elimination of the product. Compound 1 is also an efficient catalyst for the Heck cross-coupling of aryl bromides with styrenes. The mechanism for aldehyde olefination with ethyl diazoacetate (EDA) and PPh(3), catalyzed by 2, has been fully elucidated. It is demonstrated that 2 catalyzes the formation of phosphorane utilizing EDA and PPh(3), which subsequently reacts with aldehyde to produce a new olefin and phosphine oxide. The efficacy of bimetallic complexes in catalytic organic transformations is illustrated in this work.

Mapping the transformation [{Ru(II)(CO)(3)Cl(2)}(2)]-->[Ru(I) (2)(CO)(4)](2+): implications in binuclear water-gas shift chemistry.

The complete sequence of reactions in the base-promoted reduction of [{Ru(II)(CO)(3)Cl(2)}(2)] to [Ru(I) (2)(CO)(4)](2+) has been unraveled. Several mu-OH, mu:kappa(2)-CO(2)H-bridged diruthenium(II) complexes have been synthesized; they are the direct results of the nucleophilic activation of metal-coordinated carbonyls by hydroxides. The isolated compounds are [Ru(2)(CO)(4)(mu:kappa(2)-C,O-CO(2)H)(2)(mu-OH)(NP(F)-Am)(2)][PF(6)] (1; NP(F)-Am=2-amino-5,7-trifluoromethyl-1,8-naphthyridine) and [Ru(2)(CO)(4)(mu:kappa(2)-C,O-CO(2)H)(mu-OH)(NP-Me(2))(2)][BF(4)](2) (2), secured by the applications of naphthyridine derivatives. In the absence of any capping ligand, a tetranuclear complex [Ru(4)(CO)(8)(H(2)O)(2)(mu(3)-OH)(2)(mu:kappa(2)-C,O-CO(2)H)(4)][CF(3)SO(3)](2) (3) is isolated. The bridging hydroxido ligand in 1 is readily replaced by a pi-donor chlorido ligand, which results in [Ru(2)(CO)(4)(mu:kappa(2)-C,O-CO(2)H)(2)(mu-Cl)(NP-PhOMe)(2)][BF(4)] (4). The production of [Ru(2)(CO)(4)](2+) has been attributed to the thermally induced decarboxylation of a bis(hydroxycarbonyl)-diruthenium(II) complex to a dihydrido-diruthenium(II) species, followed by dinuclear reductive elimination of molecular hydrogen with the concomitant formation of the Ru(I)--Ru(I) single bond. This work was originally instituted to find a reliable synthetic protocol for the [Ru(2)(CO)(4)(CH(3)CN)(6)](2+) precursor. It is herein prescribed that at least four equivalents of base, complete removal of chlorido ligands by Tl(I) salts, and heating at reflux in acetonitrile for a period of four hours are the conditions for the optimal conversion. Premature quenching of the reaction resulted in the isolation of a trinuclear Ru(I) (2)Ru(II) complex [{Ru(NP-Am)(2)(CO)}{Ru(2)(NP-Am)(2)(CO)(2)(mu-CO)(2)}(mu(3):kappa(3)-C,O,O'-CO(2))][BF(4)](2) (6). These unprecedented diruthenium compounds are the dinuclear congeners of the water-gas shift (WGS) intermediates. The possibility of a dinuclear pathway eliminates the inherent contradiction of pH demands in the WGS catalytic cycle in an alkaline medium. A cooperative binuclear elimination could be a viable route for hydrogen production in WGS chemistry.

Proton-responsive naphthyridinone-based RuII complexes and their reactivity with water and alcohols.

We report the synthesis and reactivity of RuII complexes with a new naphthyridinone-substituted phosphine ligand, 7-(diisopropylphosphinomethyl)-1,8-naphthyridin-2(1H)-one (L-H), which contains two reactive sites that can potentially be deprotonated by a strong base: an NH proton of naphthyridinone and a methylene arm attached to the phosphine. In the absence of a base, the stable bis-ligated complex Ru(L-H)2Cl2 (1) containing two NH groups in the secondary coordination sphere is formed. Upon further reaction with a base, a doubly deprotonated, dimeric complex is obtained, [Ru2(L*-H)2(L)2] (2), in which two of the four ligands undergo deprotonation at the NH (L), while the other two ligands are deprotonated at the methylene groups (L*-H) as confirmed by an X-ray diffraction study; intramolecular hydrogen bonding is present between the NH group of one ligand and an O-atom of another ligand in the dimeric structure, which stabilizes the observed geometry of the complex. Complex 2 reacts with protic solvents such as water or methanol generating aqua Ru(L)2(OH2)2 (3) or methanol complexes Ru(L)2(MeOH)2 (4), respectively, both exhibiting intramolecular H-bonded patterns with surrounding ligands at least in the solid state. These complexes react with benzyl alcohols to give aldehydes via base-free acceptorless dehydrogenation.

Multifaceted coordination of naphthyridine-functionalized N-heterocyclic carbene: a novel "Ir(III)(C--N)(C--C)" compound and its evaluation as transfer hydrogenation catalyst.

The 1,8-naphthyridine-functionalized N-heterocyclic carbene 1-benzyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-2-ylidene (BIN) has been successfully coordinated to Pd(II), W(0), Rh(I), and Ir(III), exhibiting its diverse binding modes. Reaction of BIN x HBr with Ag(2)O, followed by transmetalation with PdCl(2)(COD)(2) provides a cis complex PdCl(2)(kappaC(2)-BIN)(2) (1). Treatment of BIN x HBr with W(CO)(4)(piperidine)(2) in acetonitrile affords a chelate complex W(CO)(4)(kappa(2)C(2),N(1)'-BIN) (2). Reaction of {RhCl(COD)}(2) with KO(t)Bu and subsequent treatment with BIN x HBr in 1:2 and 1:1 ratio results in the mono and dinuclear complexes [Rh(COD)Br(kappaC(2)-BIN)] (3) and [{Rh(COD)Br}(2)(kappaN(8)':kappaC(2)-BIN)] (4), respectively. In complex 3, the "Rh(COD)Br" unit is coordinated to the carbene center, whereas an additional "Rh(COD)Br" unit is attached to naphthyridine nitrogen in complex 4 in an anti arrangement. Under identical reaction condition, a novel Ir(III) complex [Ir(kappa(2)C(2),N(1)'-BIN)(kappa(2)C(3)',C(2)-BIN)(H(2)O)Br]Br (5) has been synthesized. Complex 5 is proved to be catalytically active in hydrogen transfer reaction from (i)PrOH. All complexes have been characterized by spectroscopic methods and X-ray crystallography.

A rare unsupported iridium(II) dimer, [IrCl2(CO)2]2.

Oxidative additions of dichloromethanes to a diiridium(i) core, bridged by 2-ferrocenyl-1,8-naphthyridines (NP-Fc), provide an iridium(II) dimer, [IrCl2(CO)2(eta 1-NP-Fc)]2, featuring an unsupported Ir-Ir single bond (2.7121(8) A).

A trinuclear bright red luminophore containing cyclometallated Ir(III) motifs.

The trinuclear complex [{Ir(ppy)(2)}(3)(L(1))(2)](OTf)(3) (1) is a bright red luminophore whereas the monomer [Ir(ppy)(2)L(2)](OTf) (2) exhibits emission in the green region.