Active Precatalysts Research Articles

A Highly Active Manganese Precatalyst for the Hydrosilylation of Ketones and Esters

The reduction of ((Ph2PPr)PDI)MnCl2 allowed the preparation of the formally zerovalent complex, ((Ph2PPr)PDI)Mn, which features a pentadentate bis(imino)pyridine chelate. This complex is a highly active precatalyst for the hydrosilylation of ketones, exhibiting TOFs of up to 76,800 h(-1) in the absence of solvent. Loadings as low as 0.01 mol % were employed, and ((Ph2PPr)PDI)Mn was found to mediate the atom-efficient utilization of Si-H bonds to form quaternary silane products. ((Ph2PPr)PDI)Mn was also shown to catalyze the dihydrosilylation of esters following cleavage of the substrate acyl C-O bond. Electronic structure investigation of ((Ph2PPr)PDI)Mn revealed that this complex possesses an unpaired electron on the metal center, rendering it likely that catalysis takes place following electron transfer to the incoming carbonyl substituent.

Hydrogen-Transfer Catalysis with Cp*IrIIIComplexes: The Influence of the Ancillary Ligands

Fourteen Cp*IrIII complexes, bearing various combinations of N- and C-spectator ligands, are assayed in hydrogen-transfer catalysis from isopropyl alcohol to acetophenone under various conditions to investigate ligand effects in this widely used reaction. The new cationic complexes bearing monodentate pyridine and N-heterocyclic carbene (NHC) ligands were characterized crystallographically and by variable-temperature nuclear magnetic resonance (VT-NMR). Control experiments and mercury poisoning tests showed that iridium(0) nanoparticles, although active in the reaction, are not responsible for the high activity observed for the most active precatalyst [Cp*Ir(IMe)2Cl]BF4 (6). For efficient catalysis, it was found necessary to have both NHCs in monodentate form; tying them together in a bis-NHC chelate ligand gave greatly reduced activity. The kinetics of the base-assisted reaction showed induction periods as well as deactivation processes, and H/D scrambling experiments cast some doubt on the classical monohydride mechanism.

Enhanced Activity of [Ni(NHC)CpCl] Complexes in Arylamination Catalysis

Seven new air- and moisture-stable nickel complexes bearing flexible bulky NHC (N-heterocyclic carbene) ancillary ligands (NHC = IPr*, IPr*Tol, IPr*OMe, IPent) are reported. Using experimentally determined crystal structures, the steric environments of [Ni(NHC)CpCl] complexes were analyzed. A survey of their catalytic activity in Buchwald–Hartwig arylamination has been performed. In comparison to less sterically demanding analogues (NHC = IMes, SIMes, IPr, SIPr), an increase in the ligand bulkiness was found to correlate to a dramatic enhancement of the C–N bond formation efficiency. Finally, the catalytic activity of the most active precatalyst, [Ni(IPr*OMe)CpCl], was further explored and the scope and limitations of this complex were examined.

ChemInform Abstract: (IPr)Pd(pydc) (pydc = Pyridine‐2,6‐dicarboxylate) — A Highly Active Precatalyst for the Sterically Hindered C—N Coupling Reactions.

AbstractNovel N‐heterocyclic carbene complexes incorporating a pyridine‐2‐carboxylate or pyridine‐2,6‐dicarboxylate ligand are synthesized and successfully applied as catalysts in the Buchwald—Hartwig amination reaction of sterically hindered aryl chlorides.

Monomeric Pd(II) complexes with trans-chelated pyrazole ligands as effective pre-catalysts for Heck cross-coupling reaction under mild aerobic conditions

A new family of mononuclear palladium(II) complexes with bidentate trans-chelating pyrazole ligands [PdCl2(LR)] (R = H, 1; Et, 2; Ph, 3; CHPh2, 4), where new ligand LEt = 2-ethyl-1,3-bis-(3,5-dimethyl-pyrazol-1-ylmethyl)-2,3-dihydro-1H-perimidine, and a new mononuclear palladium(II) complex with monodentate trans-chelated pyrazole ligand [PdCl2(Ph-pz′)2] (5), where Ph-pz′ = 3,5-dimethyl-1-phenyl-1H-pyrazole, have been synthesized and characterized. Complexes 2–5 were also structurally confirmed by single crystal X-ray diffraction analysis. Remarkably, under very mild conditions (65–70 °C), complex 4 appeared to be an active pre-catalyst for the Heck coupling reactions of iodobenzene and t-butyl acrylate in MeOH. In the presence of co-catalyst [nNBu4]Br, however, complex 5 displayed the highest catalytic activity for the Heck reactions of activated bromobenzene and t-butyl acrylate.

New Phosphine-Functionalized NHC Ligands: Discovery of an Effective Catalyst for the Room-Temperature Amination of Aryl Chlorides with Primary and Secondary Amines

We report convenient and high-yielding syntheses of new phosphine-functionalized dihydroimidazolium salts and demonstrate their utility as ligand precursors for Buchwald–Hartwig amination. Several examples of the general formula [1-Mes-3-{2-(PR2)phenyl}imidazolidin-2-ylium][BF4] have been prepared, where phosphines of varying steric and electronic properties (R = Ph (9), Cy (10), 1-Ad (11)) are tethered by an o-phenylene group. The synthesis was not adaptable to N-aryl groups other than mesityl, giving unexpected phosphonium salt species instead. The synthesis was adapted to flexible benzyl-linked variants of the formula [1-Ar-3-{2-(PCy2)benzyl}imidazolidin-2-ylium][BF4], which allowed more steric variation of the dihydroimidazolium N-aryl group (Ar = Mes (21), Dipp (22)). A preliminary study of these hybrid NHC/P ligands in Buchwald–Hartwig amination catalysis (in situ precatalyst formation) revealed 11 to be the most active of the series. Premixing the isolated free NHC ligand 1-Mes-3-{2-(PAd2)phenyl}imidazolidin-2-ylidene (23) with [Pd(cinnamyl)Cl]2 provided a highly active precatalyst that performed well at room temperature and 1 mol % catalyst loading. The system was shown to have an unprecedented ability to arylate both primary alkylamines (monoarylation) and secondary dialkylamines with aryl chlorides at room temperature. Electron-rich and -poor aryl and heteroaryl halides, as well as those featuring ortho substitution, were well tolerated, while substrates featuring both primary and secondary amine groups were selectively arylated at the NH2 position. Furthermore, a preliminary examination of performance in ammonia arylation and acetone α-arylation showed promising results, giving good conversion and high selectivity for monoarylation in both cases.

Alkyne Hydroamination and Trimerization with Titanium Bis(phenolate)pyridine Complexes: Evidence for Low-Valent Titanium Intermediates and Synthesis of an Ethylene Adduct of Titanium(II)

A class of titanium precatalysts of the type (ONO)TiX_2 (ONO = pyridine-2,6-bis(4,6-di-tert-butylphenolate); X = Bn, NMe_2) has been synthesized and crystallographically characterized. The (ONO)TiX_2 (X = Bn, NMe_2, X_2 = NPh) complexes are highly active precatalysts for the hydroamination of internal alkynes with primary arylamines and some alkylamines. A class of titanium imido/ligand adducts, (ONO)Ti(L)(NR) (L = HNMe_2, py; R = Ph, ^tBu), have also been synthesized and characterized and provide structural analogues to intermediates on the purported catalytic cycle. Furthermore, these complexes exhibit unusual redox behavior. (ONO)TiBn_2 (1) promotes the cyclotrimerization of electron-rich alkynes, likely via a catalytically active Ti^(II) species that is generated in situ from 1. Depending on reaction conditions, these Ti^(II) species are proposed to be generated through Ti benzylidene or imido intermediates. A formally Ti^(II) complex, (ONO)Ti^(II)(η^2-C_2H_4)(HNMe_2) (7), has been prepared and structurally characterized.

Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions

Cobalt(II) alkyl complexes of aliphatic PNP pincer ligands have been synthesized and characterized. The cationic cobalt(II) alkyl complex [(PNHP(Cy))Co(CH2SiMe3)]BAr(F)4 (4) (PNHP(Cy) = bis[(2-dicyclohexylphosphino)ethyl]amine) is an active precatalyst for the hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols. To elucidate the possible involvement of the N-H group on the pincer ligand in the catalysis via a metal-ligand cooperative interaction, the reactivities of 4 and [(PNMeP(Cy))Co(CH2SiMe3)]BAr(F)4 (7) were compared. Complex 7 was found to be an active precatalyst for the hydrogenation of olefins. In contrast, no catalytic activity was observed using 7 as a precatalyst for the hydrogenation of acetophenone under mild conditions. For the acceptorless dehydrogenation of 1-phenylethanol, complex 7 displayed similar activity to complex 4, affording acetophenone in high yield. When the acceptorless dehydrogenation of 1-phenylethanol with precatalyst 4 was monitored by NMR spectroscopy, the formation of the cobalt(III) acetylphenyl hydride complex [(PNHP(Cy))Co(III)(κ(2)-O,C-C6H4C(O)CH3)(H)]BAr(F)4 (13) was detected. Isolated complex 13 was found to be an effective catalyst for the acceptorless dehydrogenation of alcohols, implicating 13 as a catalyst resting state during the alcohol dehydrogenation reaction. Complex 13 catalyzed the hydrogenation of styrene but showed no catalytic activity for the room temperature hydrogenation of acetophenone. These results support the involvement of metal-ligand cooperativity in the room temperature hydrogenation of ketones but not the hydrogenation of olefins or the acceptorless dehydrogenation of alcohols. Mechanisms consistent with these observations are presented for the cobalt-catalyzed hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols.

(IPr)Pd(pydc) (pydc = pyridine-2,6-dicarboxylate) – A highly active precatalyst for the sterically hindered C–N coupling reactions

A new class of well-defined NHC–Pd complexes incorporating a pyridine-2-carboxylate or pyridine-2,6-dicarboxylate ligand has been synthesized. These novel complexes exhibited prominent catalytic activity in the sterically hindered C–N coupling reactions at elevated temperature, but relatively inferior reactivity at low temperature. The distinctly different reactivity of these NHC–Pd complexes was presumed to be associated with their unique structures of ancillary ligands.

Iridium Dihydroxybipyridine Complexes Show That Ligand Deprotonation Dramatically Speeds Rates of Catalytic Water Oxidation

We report highly active iridium precatalysts, [Cp*Ir(N,N)Cl]Cl (1-4), for water oxidation that are supported by recently designed dihydroxybipyridine (dhbp) ligands. These ligands can readily be deprotonated in situ to alter the electronic properties at the metal; thus, these catalyst precursors have switchable properties that are pH-dependent. The pKa values in water of the iridium complexes are 4.6(1) and 4.4(2) with (N,N) = 6,6'-dhbp and 4,4'-dhbp, respectively, as measured by UV-vis spectroscopy. For homogeneous water oxidation catalysis, the sacrificial oxidant NaIO4 was found to be superior (relative to CAN) and allowed for catalysis to occur at higher pH values. With NaIO4 as the oxidant at pH 5.6, water oxidation occurred most rapidly with (N,N) = 4,4'-dhbp, and activity decreased in the order 4,4'-dhbp (3) > 6,6'-dhbp (2) ≫ 4,4'-dimethoxybipyridine (4) > bipy (1). Furthermore, initial rate studies at pH 3-6 showed that the rate enhancement with dhbp complexes at high pH is due to ligand deprotonation rather than the pH alone accelerating water oxidation. Thus, the protic groups in dhbp improve the catalytic activity by tuning the complexes' electronic properties upon deprotonation. Mechanistic studies show that the rate law is first-order in an iridium precatalyst, and dynamic light scattering studies indicate that catalysis appears to be homogeneous. It appears that a higher pH facilitates oxidation of precatalysts 2 and 3 and their [B(Ar(F))4](-) salt analogues 5 and 6. Both 2 and 5 were crystallographically characterized.

Synthesis, structure, and reactivity of tris(amidate) mono(amido) and tetrakis(amidate) complexes of group 4 transition metals

The syntheses of a series of tris(amidate) mono(amido) titanium and zirconium complexes are reported. The binding motif of the amidate ligand has been determined to depend on the size of the metal centre for these sterically demanding N,O-chelating ligands; the larger zirconium metal centre supports three κ(2)-(N,O) bound amidate ligands while the titanium analogue has one ligand bound in a κ(1)-(O) fashion to alleviate steric strain. Reactivity studies indicate that, despite high steric crowding about the tris(amidate) mono(amido) zirconium metal centre, transamination of the reactive dimethylamido ligand can be achieved using aniline. This complex is also an active precatalyst for intramolecular alkene hydroamination, in which protonolysis of one amidate ligand in the presence of excess amine is observed as an initiation step prior to catalytic turnover. Eight-coordinate homoleptic κ(2)-amidate complexes of zirconium and hafnium have also been prepared.

Synthesis and catalytic activity of iron complexes with bidentate NHC ligands

A family of well-defined Fe(II) complexes of the type {BnN(N-CH2(CH2)n-N'-tert-butyl-imidazole-2-ylidene)2}FeCl2 (Bn = benzyl; n = 1 (1) or 2 (2)), {BnN(N-CH2(CH2)n-N'-methylbenzimidazole-2-ylidene)2}FeCl2 (n = 1 (3) or 2 (4)) and {BnN(N-CH2CH2CH2-N'-methylbenzimidazole-2-ylidene)2}FeBr2 (5) has been synthesized. These complexes are rare examples of Fe species supported by bidentate NHC ligands. Complexes 2, 3, 4 and 5 were characterized by X-ray crystallography and in all cases a distorted tetrahedral geometry is observed around the Fe center. The magnetic data is consistent with the complexes containing non interacting high spin Fe(II) centers (S = 2) and indicates that a large zero-field splitting (D) is present. The new complexes are highly active pre-catalysts for the homo-coupling of Grignard reagents.

Hetero-dehydrocoupling of silanes and amines by heavier alkaline earth catalysis

The homoleptic alkaline earth hexamethyldisilazides, [M{N(SiMe3)2}2]2 (1: M = Mg; 2: M = Ca; 3: M = Sr), have been demonstrated as active pre-catalysts for the cross-dehydrocoupling of Si–H and N–H bonds under mild (25–60 °C) conditions. The reactions are applicable to the coupling of a wide variety of amine and silane substrates and are proposed to occur via a sequence of discrete Si–H/M–N and N–H/M–H metathesis steps. Whereas reactions of dialkyl group 2 species with 2,6-di-iso-propylaniline and phenylsilane delivered a series of well-defined compounds consistent with this rationale, kinetic analysis of the cross-coupling of diethylamine with diphenylsilane provided evidence for a more complex and subtly variable mechanistic landscape. Although reactions performed with all three pre-catalysts presented a number of common features, in every case the calcium species, 2, was found to provide notably superior catalytic activity, an order of magnitude higher than both 1 and 3 and in excess of many previously described benchmark transition metal- or f-element-mediated processes. Variations in overall reaction order, mode of pre-catalyst activation and the nature of the rate determining process are postulated to arise as a consequence of the marked change in M2+ radius and resultant charge density as group 2 is descended.

Imidazole Based Ruthenium(IV) Complexes as Highly Efficient Bifunctional Catalysts for the Redox Isomerization of Allylic Alcohols in Aqueous Medium: Water as Cooperating Ligand

Bis-allyl ruthenium(IV) complexes containing 1,3 azole β-N-H protic ligands [Ru(η3:η3-C10H16)Cl2L] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) (L = imidazole (1a), benzimidazole (1b)), and N-methylimidazole (1c) are highly active precatalysts in the redox isomerization of allylic alcohols into carbonyl compounds in aqueous medium and in the absence of base. A wide series of primary and secondary allylic alcohols can be isomerized into the corresponding carbonyl compounds. Remarkably, complex 1b has been found to be the most efficient catalyst reported to date for the isomerization of 1-octen-3-ol in water leading to a turnover frequency (TOF) value of 60 000 h–1. Moreover, catalyst 1b can be recycled remaining active up to seven cycles. Density functional theory (DFT) calculations give evidence that the hydroxo complexes derived from 1a–c species can be formed in aqueous solution and that they can act as the catalytic active species in a bifunctional catalyzed process. This study demonstrate that in water the participation of the β-N-H protic group of the 1,3-azole ligands in the bifunctional catalysis is not required, provided that a water molecule can act as cooperating ligand.

Enol-functionalized N-heterocyclic carbene lanthanide amide complexes: Synthesis, molecular structures and catalytic activity for addition of amines to carbodiimides

Reaction of LnCl3 (Ln = Y, Nd, Sm and Yb) with enol-functionalized imidazolium salt H2LBr (L = 4-OMe-C6H4COCH{C(NCHCHNiPr)}) and NaN(TMS)2 at a molar ratio of 1:4:1 in THF at room temperature afforded the corresponding novel enol-functionalized N-heterocyclic carbene (NHC) lanthanide amides L2LnN(TMS)2 (Ln = Y (1), Nd (2), Sm (3), Yb (4)) in 37–40% yields. Molecular structures of 1–4 were determined by X-ray structure analyses. All complexes adopt monomeric structures where each 5-coordinated metal is coordinated by two NHC ligands and one amido group.in distorted trigonal bipyramid geometry. All complexes, especially Yb complex (4), were found to be highly active precatalysts for the catalytic addition of amines to carbodiimides giving guanidines. The system with 4 shows good functional group tolerance and a wide scope of amines including primary and secondary cyclic amines.

Pd(IPr*)(acac)Cl]: An Easily Synthesized, Bulky Precatalyst for C–N Bond Formation

A very straightforward synthesis of [Pd(IPr*)(acac)Cl] has been developed from commercially available Pd(acac)2 and the easily prepared IPr*·HCl (acac = acetylacetonate; IPr* = N,N′-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene). The reactivity of the resulting complex [Pd(IPr*)(acac)Cl] (1) as a highly active PdII precatalyst for the Buchwald–Hartwig arylamination coupling has been explored. A wide range of substrates with varying electronic and steric demands of both coupling partners has been screened successfully. The chemoselectivity of the reaction was also explored by using aryl heterodihalides.

A neutral trianionic pincer [NCN]CrIV–Me complex as a highly active ethylene polymerization precatalyst

Trianionic pincer ligands are a new class of ligands that constrain three anionic metal–ligand bonds to the meridional plane and are capable of supporting highly reactive metal species exploitable for alkene polymerization. The previously reported NCN3- trianionic pincer CrIV–Me complex [2,6-iPrNCN]CrIV(Me)THF (1), upon activation by triisobutylaluminum (TIBA), catalyzes the polymerization of ethylene. Complex 1 (1 μmol) in the presence of 35 equiv. of TIBA polymerizes ethylene with an activity of 7.02 × 106 g PE(molCr)−1 h−1 (75 °C).

Synthesis, structures and ethylene polymerization activity of chromium(III) complexes with N,O,N′-tridentate ligands

This paper reports the synthesis and characterization of four neutral chromium(III) complexes bearing N,O,N′-tridentate ligands, [LCrCl3], (L=2-[(2-pyridylmethoxy)methyl]pyridine, 1; L=8-(2-pyridylmethoxy)quinoline, 2; L=8-[(2-pyridylmethoxy)methyl]quinoline, 3; L=8-(quinolin-8-ylmethoxy)quinoline, 4). Molecular structures of 1–3 have been determined by X-ray crystallography, which show that L1 and L3 exhibit facial configuration in 1 and 3 while L2 adopts meridional arrangement in 2. The ethylene polymerization experiments show that complexes 1–4 are moderately active precatalysts for ethylene polymerization with activation with MAO.

Cp* Iridium Precatalysts for Selective C–H Oxidation via Direct Oxygen Insertion: A Joint Experimental/Computational Study

A series of Cp*Ir complexes are active precatalysts in C–H oxidation of cis-decalin, cyclooctane, 1-acetylpyrrolidine, tetrahydrofurans, and γ-lactones. Moderate to high yields were achieved, and surprisingly, high selectivity for mono-oxidation of cyclooctane to cyclooctanone was observed. Kinetic isotope effect experiments in the C–H oxidation of ethylbenezene to acetophenone yield kH/kD = 15.4 ± 0.8 at 23 °C and 17.8 ± 1.2 at 0 °C, which are consistent with C–H oxidation being the rate-limiting step with a significant tunneling contribution. The nature of the active species was investigated by TEM, UV–vis, microfiltration, and control experiments. DFT calculations showed that the C–H oxidation of cis-decalin by Cp*Ir(ppy)(Cl) (ppy = o-phenylpyridine) follows a direct oxygen insertion mechanism on the singlet potential energy surface, rather than the radical rebound route that would be seen for the triplet, in good agreement with the retention of stereochemistry observed in this reaction.

Synthesis and catalytic application of palladium imidazol(in)ium-2-dithiocarboxylate complexes

The palladium(II) dimer, [Pd(C,N-C(6)H(4)CH(2)NMe(2))Cl](2) reacts with two equivalents of the NHC·CS(2) zwitterionic ligands [NHC = IPr (1,3-diisopropylimidazol-2-ylidene), ICy (1,3-dicyclohexylimidazol-2-ylidene), IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IDip (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene)] in the presence of NH(4)PF(6), to yield the cationic products [Pd(C,N-C(6)H(4)CH(2)NMe(2))(S(2)C·NHC)](+). In a similar fashion, the compounds [Pd(C,N-bzq)(S(2)C·NHC)](+) (bzq = benzo[h]quinolinyl, NHC = ICy, IMes, IDip) are obtained from the corresponding dimer [Pd(C,N-bzq)Cl](2). The bis(phosphine) compounds [Pd(S(2)C·NHC)(PPh(3))(2)](2+) (NHC = ICy, IMes, IDip, SIMes) are obtained on treatment of [PdCl(2)(PPh(3))(2)] with NHC·CS(2) zwitterions in the presence of NH(4)PF(6). The reaction of [PdCl(2)(dppf)] with IMes·CS(2) and NH(4)PF(6) provides the complex [Pd(S(2)C·IMes)(dppf)](2+). The complexes [Pd(S(2)C·NHC)(PPh(3))(2)](PF(6))(2) (NHC = IMes, IDip) were active pre-catalysts (1 mol% loading) for the conversion of benzo[h]quinoline to 10-methoxybenzo[h]quinoline in the presence of PhI(OAc)(2) and methanol. The intermediacy of [Pd(C,N-bzq)(S(2)C·NHC)](+) was supported by the high yield of 10-methoxybenzo[h]quinoline using [Pd(C,N-bzq)(S(2)C·IDip)](+) to promote the same reaction. Small amounts of 2,10-dimethoxybenzo[h]quinoline were also isolated from these reactions. Using [Pd(C,N-bzq)(S(2)C·IDip)](+) and N-chlorosuccinimide as the oxidant led to the formation of 10-chlorobenzo[h]quinoline in moderate yield from benzo[h]quinoline. The molecular structures of [Pd(S(2)C·IMes)(PPh(3))(2)](PF(6))(2) and [Pd(S(2)C·IMes)(dppf)](PF(6))(2) were determined crystallographically.