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David Milstein

Publications and source records attributed to David Milstein.

32 records · Page 2Linked to original sources
PubMed2003

Dimethylsulfoxide as a ligand for RhI and IrI complexes--isolation, structure, and reactivity towards X-H bonds (X=H, OH, OCH3).

Novel neutral and cationic Rh(I) and Ir(I) complexes that contain only DMSO molecules as dative ligands with S-, O-, and bridging S,O-binding modes were isolated and characterized. The neutral derivatives [RhCl(DMSO)(3)] (1) and [IrCl(DMSO)(3)] (2) were synthesized from the dimeric precursors [M(2)Cl(2)(coe)(4)] (M=Rh, Ir; COE=cyclooctene). The dimeric Ir(I) compound [Ir(2)Cl(2)(DMSO)(4)] (3) was obtained from 2. The first example of a square-planar complex with a bidentate S,O-bridging DMSO ligand, [(coe)(DMSO)Rh(micro-Cl)(micro-DMSO)RhCl(DMSO)] (4), was obtained by treating [Rh(2)Cl(2)(coe)(4)] with three equivalents of DMSO. The mixed DMSO-olefin complex [IrCl(cod)(DMSO)] (5, COD=cyclooctadiene) was generated from [Ir(2)Cl(2)(cod)(2)]. Substitution reactions of these neutral systems afforded the complexes [RhCl(py)(DMSO)(2)] (6), [IrCl(py)(DMSO)(2)] (7), [IrCl(iPr(3)P)(DMSO)(2)] (8), [RhCl(dmbpy)(DMSO)] (9, dmbpy=4,4'-dimethyl-2,2'-bipyridine), and [IrCl(dmbpy)(DMSO)] (10). The cationic O-bound complex [Rh(cod)(DMSO)(2)]BF(4) (11) was synthesized from [Rh(cod)(2)]BF(4). Treatment of the cationic complexes [M(coe)(2)(O=CMe(2))(2)]PF(6) (M=Rh, Ir) with DMSO gave the mixed S- and O-bound DMSO complexes [M(DMSO)(2)(DMSO)(2)]PF(6) (Rh=12; Ir=in situ characterization). Substitution of the O-bound DMSO ligands with dmbpy or pyridine resulted in the isolation of [Rh(dmbpy)(DMSO)(2)]PF(6) (13) and [Ir(py)(2)(DMSO)(2)]PF(6) (14). Oxidative addition of hydrogen to [IrCl(DMSO)(3)] (2) gave the kinetic product fac-[Ir(H)(2)Cl(DMSO)(3)] (15) which was then easily converted to the more thermodynamically stable product mer-[Ir(H)(2)Cl(DMSO)(3)] (16). Oxidative addition of water to both neutral and cationic Ir(I) DMSO complexes gave the corresponding hydrido-hydroxo addition products syn-[(DMSO)(2)HIr(micro-OH)(2)(micro-Cl)IrH(DMSO)(2)][IrCl(2)(DMSO)(2)] (17) and anti-[(DMSO)(2)(DMSO)HIr(micro-OH)(2)IrH(DMSO)(2)(DMSO)][PF(6)](2) (18). The cationic [Ir(DMSO)(2)(DMSO)(2)]PF(6) complex (formed in situ from [Ir(coe)(2)(O=CMe(2))(2)]PF(6)) also reacts with methanol to give the hydrido-alkoxo complex syn-[(DMSO)(2)HIr(micro-OCH(3))(3)IrH(DMSO)(2)]PF(6) (19). Complexes 1, 2, 4, 5, 11, 12, 14, 17, 18, and 19 were characterized by crystallography.

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PubMed2003

C-C versus C-H activation and versus agostic C-C interaction controlled by electron density at the metal center.

Based on the PCN ligand 2, a remarkable degree of control over C-C versus C-H bond activation and versus formation of an agostic C-C complex was demonstrated by choice of cationic [Rh(CO)(n)(C(2)H(4))(2-n)] (n=0, 1, 2) precursors. Whereas reaction of 2 with [Rh(C(2)H(4))(2)(solv)(n)]BF(4) results in exclusive C-C bond activation to yield product 5, reaction with the dicarbonyl precursor [Rh(CO)(2)(solv)(n)]BF(4) leads to formation of the C-H activated complex 9. The latter process is promoted by intramolecular deprotonation of the C-H bond by the hemilabile amine arm of the PCN ligand. The mixed monocarbonyl monoethylene Rh species [Rh(CO)(C(2)H(4))]BF(4) reacts with the PCN ligand 2 to give an agostic complex 7. The C-C activated complex 5 is easily converted to the C-H activated one (9) by reaction with CO; the reaction proceeds by a unique sequence of 1,2-metal-to-carbon methyl shift, agostic interaction, and C-H activation processes. Similarly, the C-C agostic complex 7 is converted to the same C-H activated product 9 by treatment with CO.

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PubMed2003

Aromatic vs aliphatic C-H bond activation by rhodium(I) as a function of agostic interactions: catalytic H/D exchange between olefins and methanol or water.

The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)(2)(solv)(n)()]BF(4) (coe = cyclooctene, solv = solvent), producing the C-H activated complexes 4a-c (solv = (a). acetone, (b). THF, (c). methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through eta(2)-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.

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PubMed2003

Interplay between solvent and counteranion stabilization of highly unsaturated rhodium(III) complexes: facile unsaturation-induced dearomatization.

Abstraction of the chloride ligand from the PCN-based chloromethylrhodium complex 2 by AgX (X=BF(4)(-), CF(3)SO(3)(-)) or a direct C-C cleavage reaction of the PCN ligand 1 with [(coe)(2)Rh(solv)(n)](+)X(-) (coe=cyclooctene) lead to the formation of the coordinatively unsaturated rhodium(III) complexes 3. Compound 3 a (X=BF(4)(-)) exhibits a unique medium effect; the metal center is stabilized by reversible coordination of the bulky counteranion or solvent as a function of temperature. Reaction of [(PCN)Rh(CH(3))(Cl)] with AgBAr(f) in diethyl ether leads to an apparent rhodium(III) 14-electron complex 4, which is stabilized by reversible, weak coordination of a solvent molecule. This complex coordinates donors as weak as diethyl ether and dichloromethane. Upon substitution of the BF(4)(-) ion in [(PCN)Rh(CH(3))]BF(4) by the noncoordinating BAr(f)(-) ion in a noncoordinating medium, the resulting highly unsaturated intermediate undergoes a 1,2-metal-to-carbon methyl shift, followed by beta-hydrogen elimination, leading to the Rh-stabilized methylene arenium complex 5. This process represents a unique mild, dearomatization of the aromatic system induced by unsaturation.

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PubMed2003

Metallacarbenes from diazoalkanes: an experimental and computational study of the reaction mechanism.

PCP ligand (1,3-bis-[(diisopropyl-phosphanyl)-methyl]-benzene), and PCN ligand ([3-[(di-tert-butyl-phosphanyl)-methyl]-benzyl]-diethyl-amine) based rhodium dinitrogen complexes (1 and 2, respectively) react with phenyl diazomethane at room temperature to give PCP and PCN-Rh carbene complexes (3 and 5, respectively). At low temperature (-70 degrees C), PCP and PCN phenyl diazomethane complexes (4 and 6, respectively) are formed upon addition of phenyl diazomethane to 1 and 2. In these complexes, the diazo moiety is eta(1) coordinated through the terminal nitrogen atom. Decomposition of complexes 4 and 6 at low temperatures leads only to a relatively small amount of the corresponding carbene complexes, the major products of decomposition being the dinitrogen complexes 1 and 2 and stilbene. This and competition experiments (decomposition of 6 in the presence of 1) suggests that phenyl diazomethane can dissociate under the reaction conditions and attack the metal center through the diazo carbon producing a eta(1)-C bound diazo complex. Computational studies based on a two-layer ONIOM model, using the mPW1K exchange-correlation functional and a variety of basis sets for PCP based systems, provide mechanistic insight. In the case of less bulky PCP ligand bearing H-substituents on the phosphines, a variety of mechanisms are possible, including both dissociative and nondissociative pathways. On the other hand, in the case of i-Pr substituents, the eta(1)-C bound diazo complex appears to be a critical intermediate for carbene complex formation, in good agreement with the experimental results. Our results and the analysis of reported data suggest that the outcome of the reaction between a diazoalkane and a late transition metal complex can be anticipated considering steric requirements relevant to eta(1)-C diazo complex formation.

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PubMed2003

A new ligand system based on a bipyridine-functionalized calix[4]arene backbone leading to mono- and bimetallic complexes.

The synthesis of a new ligand system for mono- and bimetallic complexes based on a calixarene is described. Ligand BBPC (3, bis(bipyridine)-calix[4]arene) is obtained in three steps in 40% overall yield by first brominating one of the methyl groups of the 4,4'-dimethyl-2,2'-bipyridine in two steps and subsequently reacting it with p-tert-butylcalix[4]arene under basic conditions. Reaction of BBPC (3) with 2 equiv of [Rh(NBD)(2)]BF(4) or [Rh(NBD)(CH(3)CN)(2)]BF(4) (NBD = norbornadiene) produces the bimetallic compound BBPC[Rh(NBD)BF(4)](2) (4). Treatment of the ligand with PdCl(2)(CH(3)CN)(2) leads to the isolation of the bimetallic complex BBPC[PdCl(2)](2) (5). When the nickel precursor NiBr(2)(DME) (DME = dimethoxyethane) is reacted with BBPC, the bimetallic complex BBPC[NiBr(2)](2) (6) is isolated which, upon crystallization from methanol, gives the mononuclear bis(bipyridine) complex BBPC[NiBr(OMe)] (7). Full characterization includes X-ray structural studies of complexes 4, 5, and 7. The bimetallic compounds 4 and 5 show metal to metal distances of 4.334 A (for 4) and 3.224 A (for 5). For all three complexes, unique molecular packing arrangements were found, based on hydrophobic/hydrophilic interactions.

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PubMed2003

Selective ortho C-h activation of haloarenes by an Ir(I) system.

The cationic PNP-Ir(I)(cyclooctene) complex 1 (PNP = 2,6-bis-(di-tert-butyl phosphino methyl)pyridine) reacts with benzene at 25 degrees C to quantitatively yield the crystallographically characterized, square pyramidal, iridium phenyl hydride complex cis-(PNP)Ir(Ph)(H), 2, in which the hydride is trans to the vacant coordination site. The cationic complex 2 is stable to heating at 100 degrees C, in sharp contrast to the previously reported unstable neutral, isoelectronic (PCP)Ir(H)(Ph) (PCP = eta(3)-2,6-((t)()Bu(2)PCH(2))(2)C(6)H(3)). Heating of 2 at 50 degrees C with other arenes results in arene exchange. Complex 1 activates C-H bonds of chloro- and bromobenzene with no C-halide oxidative addition being observed. Selective ortho C-H activation takes place, the process being directed by halogen coordination and being thermodynamically and kinetically favorable. The meta- and para-C-H activation products are formed at a slower rate than the ortho isomer and are converted to it. NMR data and an X-ray crystallographic study of the ortho-activated chlorobenzene complex, which was obtained as the only product upon heating of 1 with chlorobenzene at 60 degrees C, show that the chloro substituent is coordinated to the metal center.

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PubMed2002

Oxidative addition of water and aliphatic alcohols by IrCl(trialkylphosphine)(3).

Oxidative addition of aliphatic alcohols to (C(8)H(14))IrCl(PMe(3))(3) in benzene yields the cis-hydrido-alkoxo products mer-cis-HIr(OR)Cl(PMe(3))(3) (R = Me, Et, 1-pentyl, 2-propyl). The analogous hydroxo complex is prepared by oxidative addition of water in THF. The addition rate depends on the nature of the alcohol (methanol > 1-pentanol >> 2-propanol and methanol > water). The reaction is retarded in polar media but accelerated by protic cosolvents. Anionic ligand redistribution involving chloride and alkoxide (or hydroxide) competes with the oxidative addition reaction. A detailed kinetic study suggests that the 16-electron IrCl(PMe(3))(3) is the species undergoing the oxidative addition, and mer-cis-HIr(OR)Cl(PMe(3))(3) is the kinetic product. The reaction proceeds by a single-step nucleophilic attack of the metal on the O-H proton. Pi-donation by chloride stabilizes the transition state and governs the stereochemical course of the reaction. Protic solvent aggregation in the transition state in an apolar medium is suggested. mer-cis-HIr(OH)Cl(PEt(3))(3), obtained by water addition to IrCl(PEt(3))(3), was crystallographically characterized, showing an unusual hydrophobic cage around the hydride ligand.

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PubMed2002

Oxidative addition of water to novel Ir(I) complexes stabilized by dimethyl sulfoxide ligands.

The oxidative addition of water to novel Ir(I) DMSO complexes is described. IrCl(DMSO)3 (1) is synthesized in 90% yield when treating a toluene slurry of [Ir2Cl2(COE)4] (COE = cyclooctene) with excess DMSO. Its dimer, [Ir2Cl2(DMSO)4] (2) is obtained in 95% yield starting from 1. The cationic complex [Ir(DMSO)4]PF6 (6) is prepared in situ from [Ir(COE)2(O=CMe2)2]PF6 (5). These complexes add water at room temperature, giving rise to the oxidative addition products syn-[(DMSO)2HIr(mu-OH)2(mu-Cl)IrH(DMSO)2] [IrCl2(DMSO)2] (3) and anti-[(DMSO)2(DMSO)HIr(mu-OH)2IrH(DMSO)2(DMSO)](PF6)2 (7), respectively. Reductive elimination in pyridine leads to quantitative isolation of mixed Ir(I) DMSO-pyridine complexes IrCl(py)(DMSO)2 (4) and [Ir(py)2(DMSO)2]PF6 (8), respectively. Compounds 1, 3, and 7 have been characterized by X-ray crystallography. 3 and 7 show dimeric structures with the hydroxo ligands bridging the iridium atoms and in 7 both O- and S-bonded DMSO ligands are present.

Journal Article↗
PubMed2001

Ligand-Controlled Chemoselectivity in the Classical Oxidative Addition Reactions of MeI and Aldehydes to Rhodium(I) Complexes This work was supported by the Israel Science Foundation, Jerusalem, Israel, by the MINERVA foundation, Munich, Germany, and by the Tashtiyot program of the Israeli Ministry of Science. D.M. is the holder of the Israel Matz Professorial Chair of Organic Chemistry. We thank Dr. L. Shimon and Dr. H. Rozenberg, Weizmann Institute of Science, for performing the X-ray structural analysis.

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PubMed1996

Formation and X-ray Structures of PCP Ligand Based Platinum(II) and Palladium(II) Macrocycles.

The coordination behavior prior to C-M bond formation of the chelating aromatic PCP substrate DPPMH (3; DPPMH = 1,3-bis((diphenylphosphino)methylene)mesitylene) has been studied in order to determine the factors which control the complex formation of such ligands. Reacting 3 with (RCN)(2)MCl(2) (R = Me, Ph; M = Pd, Pt) and (COD)PtX(2) (X = Cl, Me; COD = 1,5-cyclooctadiene) resulted in the formation of several 8- and 16-membered mono- and binuclear palladium(II) and platinum(II) macrocycles: trans-[(DPPMH)PdCl(2)](2) (5), trans-[(DPPMH)PtCl(2)](2) (6), cis-(DPPMH)PtCl(2) (7), cis-(DPPMH)PtMe(2) (8), and cis-[(DPPMH)PtMe(2)](2) (9). Compounds 5-9 were fully characterized using NMR, FAB-MS, FD-MS, elemental analysis, and X-ray crystallography. Thermolysis of the bimetallic trans-[(DPPMH)PtCl(2)](2) (6) results in the formation of the monomeric cis-(DPPMH)PtCl(2) (7). The product formation depends on the neutral- (nitriles or COD) and anionic ligands (Cl and CH(3)) of the metal precursor. The molecular structures of trans-[(DPPMH)PdCl(2)](2) (5) and cis-[(DPPMH)PtMe(2)](2) (9) have been determined by complete single-crystal diffraction studies. Crystal data for 5: monoclinic, space group P2(1)/n with a = 14.547(3) Å, b = 17.431(4) Å, c = 27.839 (5) Å, beta = 99.56(2) degrees, V = 6961(3) Å(3), and Z = 4. The structure converged to R = 0.048 and R(w) = 0.049. Crystal data for 9: monoclinic, space group P2(1)/n with a = 19.187(4) Å, b = 19.189(4) Å c = 20.705(2) Å, beta = 103.41(3) degrees, V = 7415(3) Å(3), and Z = 4. The structure refinement converged to R = 0.0977 and R(w) = 0.2212.

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