Oxidative addition of the imidazolium cation to zerovalent Ni, Pd, and Pt: a combined density functional and experimental study

Oxidative addition of different imidazolium cations to zerovalent group 10 metals, to afford heterocyclic carbene complexes, has been investigated by both density functional theory (DFT) and experimental studies. The theoretical analysis shows that addition of imidazoliums to Pt(0) and Ni(0) is more exothermic than to Pd(0), and Ni(0) is predicted to react with a much lower barrier than either Pt(0) or Pd(0). Strongly basic supporting ligands on the metal, as well as cis-chelating ligands, increase the exothermicity of the reaction and also lower the activation barrier. The addition of 2-H imidazoliums is easier and more exothermic than addition of 2-alkylimidazoliums, and a halo-imidazolium is expected to further lower the barrier to oxidative addition and increase the exothermicity. The DFT results show that all three of the metals should be able to oxidatively add imidazolium cations under appropriate conditions. Experimental studies confirmed that oxidative addition is possible, and a number of Pt- and Pd-carbene complexes were prepared via oxidative addition of imidazolium salts to M(0) precursors. Most significantly, oxidative addition of 2-H azolium salts was found to readily occur, and the reaction of 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPh(3))(2) and Pt(PCy(3))(2) affords [PtH(dmiy)(PPh(3))(2)]BF(4) (10) and [PtH(dmiy)(PCy(3))(2)]BF(4) (11), while reaction between 3,4-dimethylthiazolium tetrafluoroborate and Pt(PCy(3))(2) yields [PtH(dmty)(PCy(3))(2)]BF(4) (12) (dmiy = 1,3-dimethylimidazolin-2-ylidene, dmty = 3,4-dimethylthiazolin-2-ylidene). Addition of 2-iodo-1,3,4,5-tetramethylimidazolium tetrafluoroborate to Pt(PPh(3))(4) or Pd(dcype)(dba) yields [PtI(tmiy)(PPh(3))(2)]BF(4) (9) and [PdI(tmiy)(dcype)]BF(4) (14), respectively (tmiy = 1,3,4,5-tetramethylimidazolin-2-ylidene, dcype = 1,3-bis(dicyclohexylphosphino)ethane)). X-ray crystal structures are reported for complexes 9 and 11 (cis and trans). These studies clearly show for the first time that oxidative addition of imidazolium and thiazolium cations is possible, and the results are discussed in terms of the ramifications for catalysis in imidazolium-based ionic liquids with both carbene-based and non-carbene-based complexes.     

Kinetic and density functional studies on alkyl-carbene elimination from Pd(II) heterocylic carbene complexes: a new type of reductive elimination with clear implications for catalysis.

A number of new methyl-Pd(II) complexes of heterocyclic carbenes of the form [PdMe(tmiy)L(2)]BF(4) have been prepared, and their reaction behavior has been studied (tmiy = 1,3,4,5-tetramethylimidazolin-2-ylidene, L = cyclooctadiene (8), methyldiphenylphosphine (9), triphenyl phosphite (10), triphenylphosphine (11)). In common with other hydrocarbyl-M carbene complexes (M = Pd, Ni) the complexes are predisposed to a facile decomposition process. A detailed mechanism for the process and of the decomposition pathway followed is presented herein. All complexes decompose with first-order kinetics to yield 1,2,3,4,5-pentamethylimidazolium tetrafluoroborate and Pd(0) species. The kinetic investigations combined with density functional studies show that the complexes decompose via a mechanism of concerted reductive elimination of the methyl group and carbene. The reaction represents a new type of reductive elimination from transition metals and also represents a low-energy pathway to catalyst deactivation for catalysts based on heterocyclic carbenes. The theoretical studies indicate extensive involvement of the p(pi) orbital on the carbene carbon in the transition structure. Methods of stabilizing catalysts based on heterocyclic carbene complexes are suggested, and the possibility of involvement of carbene species during catalysis in ionic liquids is discussed.     

Tuning the electronic properties of N-heterocyclic carbenes

The electron-donating properties of N-heterocyclic carbenes ([N,N'-bis(2,6-dimethylphenyl)imidazol]-2-ylidene and the respective dihydro ligands) with 4,4'-R-substituted aryl rings (4,4'-R=NEt2, OC(12)H(25), Me, H, Br, S(4-tolyl), SO(4-tolyl), SO2(4-tolyl)) were studied. Twelve new N-heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (DeltaE1/2) and IR (nu (CO)) can be used to measure the electron-donating properties of the carbene ligands. Modifying the 4-positions with electron-withdrawing substituents (4-R=-SO(2)Ar, DeltaE1/2=+0.92 V) results in NHC ligands with virtually the same electron-donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (DeltaE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4-R=NEt2 (DeltaE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.     

Synthesis,characterization and catalytic activity in Suzuki–Miyaura coupling of palladacycle complexes with n‐butyl‐substituted N‐heterocyclic carbene ligands

A series of monomeric palladacycle complexes bearing n‐butyl‐substituted N‐heterocyclic carbenes, namely [Pd(NHC)X(dmba)] (dmba: dimethylbenzylamine and [Pd(NHC)X(ppy)]; NHC: 1‐n‐butyl‐3‐substituted benzylimidazol‐2‐ylidene; ppy: 2‐phenylpyridine), were prepared either by transmetallation from the corresponding silver carbene complexes or by the reaction of the corresponding acetate‐bridged palladacycle dimer with N‐heterocyclic carbene ligands in high yields. The palladium(II) complexes were characterized using elemental analyses, APCI‐MS, ¹H NMR and ¹³C NMR spectroscopies. These complexes are efficient in the Suzuki–Miyaura coupling reaction between phenylboronic acid and aryl bromides.     

A systematic study on the synthesis, reactivity and structure of ortho-palladated aryloximes, including the first cyclopalladated aryloximato and iminoaryloxime complexes

Complexes [Pd{C,N-Ar{C(Me)=NOH}-2}(μ-Cl)](2) (1) with Ar = C(6)H(4), C(6)H(3)NO(2)-5 or C(6)H(OMe)(3)-4,5,6, were obtained from the appropriate oxime, Li(2)[PdCl(4)] and NaOAc. They reacted with neutral monodentate C-, P- or N-donor ligands (L), with [PPN]Cl ([PPN] = Ph(3)P=N=PPh(3)), with Tl(acac) (acacH = acetylacetone), or with neutral bidentate ligands N^N (tetramethylethylenediamine (tmeda), 4,4'-di-tert-butyl-2,2'-bipyridine ((t)Bubpy)) in the presence of AgOTf or AgClO(4) to afford complexes of the types [Pd{C,N-Ar{C(Me)=NOH}-2}Cl(L)] (2), [PPN][Pd{C,N-Ar{C(Me)=NOH}-2}Cl(2)] (3), [Pd{C,N-Ar{C(Me)=NOH}-2}(acac)] (4) or [Pd{C,N-Ar{C(Me)=NOH}-2}(N^N)]X (X = OTf, ClO(4)) (5), respectively. Complexes 1 reacted with bidentate N^N ligands in the presence of a base to afford mononuclear zwitterionic oximato complexes [Pd{C,N-Ar{C(Me)=NO}-2}(N^N)] (6). Dehydrochlorination of complexes 2 by a base yielded dimeric oximato complexes of the type [Pd{μ-C,N,O-Ar{C(Me)[double bond, length as m-dash]NO}-2}L](2) (7). The insertion of XyNC into the Pd-C(aryl) bond of complex 2 produced the mononuclear iminoaryloxime derivative [Pd{C,N-C(=NXy)Ar{C(Me)=NOH}-2}Cl(CNXy)] (8) which, in turn, reacted with [AuCl(SMe(2))] to give [Pd{μ-N,C,N-C(=NXy)Ar{C(Me)=NOH}-2}Cl](2) (9) with loss of XyNC. Some of these complexes are, for any metal, the first containing cyclometalated aryloximato (6, 7) or iminoaryloxime (8, 9) ligands. Various crystal structures of complexes of the types 2, 3, 6, 7, 8 and 9 have been determined.     

Formation of [PtPd(2)(mu(3)-X)(2)(P-P)(dppmX)(2)](2+) (X = S, Se; P-P = dppe, 2 x PPh(3)) aggregates through activation of the chalcogen-rich [PtX(4)] ring by a Pd(I)-Pd(I) bond

Redox addition of the Pd-Pd bond in [Pd(2)Cl(2)(dppm)(2)] across S-S or Se-Se bond in [Pt(X(4)-kappa(2)X(1),X(4))(P-P)] (X = S, Se; P-P = dppe or 2 x PPh(3); dppm = bis(diphenylphosphino)methane, dppe = bis(diphenylphosphino)ethane) leads to the isolation of [PtPd(2)(mu(3)-X)(2)(P-P)(dppmX-kappa(2)X,P(4))(2)](2+) and represents an atom-economy process that converts chalcogen-rich complexes to heterometallic chalcogenide aggregates. Activation of the [PtX(4)] ring is achieved by tetrachalcogenide reduction and dual oxidation of palladium and phosphine.     

Synthesis of NHC complexes by oxidative addition of 2-chloro-N-methylbenzimidazole

The oxidative addition of 2-chloro-N-methylbenzimdazole to complexes of type [M(PPh(3))(4)] yields after N-protonation compounds with NH,NMe-substituted NHC ligands. For M = Pd complex compound trans-[3]BF(4) was obtained, while the oxidative addition for M = Pt yielded a mixture of cis-[4]BF(4) (major) and trans-[4]BF(4) (minor).     

Synthesis and characterization of [Cu(NHC)2]X complexes: catalytic and mechanistic studies of hydrosilylation reactions

The preparation of two series of [Cu(NHC)2]X complexes (NHC=N-heterocyclic carbene, X=PF6 or BF4) in high yields from readily available materials is reported. These complexes have been spectroscopically and structurally characterized. The activity of these cationic bis-NHC complexes in the hydrosilylation of ketones was examined, and both the ligand and the counterion showed a significant influence on the catalytic performance. Moreover, when compared with related [Cu(NHC)]-based systems, these cationic complexes proved to be more efficient under similar reaction conditions. The activation step of [Cu(NHC)2]X precatalysts towards hydrosilylation was investigated by means of 1H NMR spectroscopy. Notably, it was shown that one of the N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) ligands in [Cu(IPr)2]BF4 is displaced by tBuO(-) in the presence of NaOtBu, producing the neutral [Cu(IPr)(OtBu)]. This copper alkoxide is known to be a direct precursor of an NHC-copper hydride, the actual active species in this transformation. Furthermore, reagent loading and counterion effects have been rationalized in light of the species formed during the reaction.     

Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands

This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.     

One electron oxidation of chromium N,N-bis(diarylphosphino)amine and bis(diarylphosphino)methane complexes relevant to ethene trimerisation and tetramerisation

Complexes of the type [(diphosphine)Cr(CO)(4)] (diphosphine = Ph(2)PN(iPr)PPh(2), Ar(2)PN(Me)PAr(2) or Ar(2)PCH(2)PAr(2) (Ar = 2-C(6)H(4)(MeO)) have been synthesised. In the solid state, these complexes show tight phosphine bite angles in the range 67.82(4) degrees to 71.52(5) degrees and the nitrogen atom in N,N-bis(diarylphophino)amine ligands adopts an almost planar (sp(2)) geometry. All of the complexes are readily oxidised electrochemically or chemically to corresponding Cr(i) species. There is no evidence for coordination of the pendant ether group in derivatives with Ar = 2-MeO-C(6)H(4) in either Cr(0) or Cr(i) species. Treatment of the [(diphosphine)Cr(CO)(4)] complexes with [NO]BF(4) yields [(diphosphine)Cr(NO)(CO)(3)]BF(4). Removal of CO ligands to generate an oligomerisation-active species is not observed with amine oxides but triethyl aluminium is effective in this role, and active catalysts can be produced. The use of weakly coordinating anions seems crucial in achieving oligomerisation catalysis.     

Synthesis, structures, and solution dynamics of palladium complexes of quinoline-functionalized N-heterocyclic carbenes

A new type of quinoline-functionalized palladium N-heterocyclic carbene (NHC) complexes has been synthesized via silver transmetallation. The quinoline moiety was either directly attached to the imidazole ring or linked to it by a methylene group. NHCs with a methylene linker tend to form trans biscarbene complexes in the reaction of Pd(COD)Cl2, while NHCs without any linker form chelating NHC-quinoline (NHC-N) complexes. These two types of carbenes also react with [Pd(allyl)Cl]2 to give monodentate NHC palladium eta(3)-allyl chlorides [Pd(NHC)(allyl)Cl]. Fluxionality in the NMR time scale was observed for most complexes, and the origin of their dynamic behaviors was discussed for each type of structure. For [Pd(NHC)(allyl)Cl] with a relatively small wing tip group of the NHC, the fluxionality (selective line-broadening of (1)H NMR signals) is caused by selective eta(3)-eta(1)-eta(3) allyl isomerization. For NHC with a bulkier (t)Bu group, a different line-broadening pattern was observed and was ascribed to partially hindered Pd-C(carbene) bond rotation. For cationic chelating complexes [Pd(NHC-N)(allyl)]BF4, the dynamic exchange process likely originates from a dissociative boat-to-boat inversion of 7-membered palladacycles. Activation parameters were measured for this process. Crystal structures were reported for representative complexes in each category.     

Synthesis and structural characterisation of (aryl-BIAN)copper(I) complexes and their application as catalysts for the cycloaddition of azides and alkynes

A series of Ar-BIAN-based copper(I) complexes (where Ar-BIAN = bis(aryl)acenaphthenequinonediimine) were synthesised and characterised by (1)H and (13)C NMR spectroscopies, FT-IR spectroscopy, MALDI-TOF-MS spectrometry, cyclic voltammetry and single crystal X-ray diffraction. The bis-chelated complexes of general formula [Cu(Ar-BIAN)(2)]BF(4) (where Ar = C(6)H(5) (1), 4-iPrC(6)H(4) (3), 2-iPrC(6)H(4) (4)) were prepared by reaction of [Cu(NCMe)(4)]BF(4) with two equivalents of the corresponding Ar-BIAN ligands, in dichloromethane, while the mono-chelated complexes of the type [Cu(Ar-BIAN)L(2)]BF(4) (where Ar = 2,6-iPr(2)C(6)H(3), L = PhCN (6); Ar = 4-iPrC(6)H(4), L = PPh(3) (7)) were readily accessible by treatment of [Cu(NCR)(4)]BF(4) (R = Me, Ph) with one equivalent of the corresponding Ar-BIAN ligands in the absence or presence of two equivalents of PPh(3), in the same solvent. The structures of complexes 3, 4, 6 and 7 were obtained by single crystal X-ray diffraction, showing distorted tetrahedral geometries around the copper centres in all cases. The electrochemical studies of these complexes and of the already reported [Cu(2,4,6-Me(3)C(6)H(2)-BIAN)(2)]BF(4) (2) and [Cu(2,6-iPr(2)C(6)H(3)-BIAN)(NCMe)(2)] (5), demonstrated that the bis-chelated complexes 1-4 undergo a reversible one-electron reduction or oxidation processes on copper, while the mono-chelated complexes 5-7 show a partially reversible oxidation and an irreversible reduction feature. Both kinds of (Ar-BIAN)copper(I) complexes are active catalysts for the copper(I)-catalysed azide-alkyne cycloaddition reaction (CuAAC). Complex 7, bearing PPh(3) ligands, exhibits the highest catalytic activity, which is comparable with that of the typical CuSO(4)-sodium ascorbate catalyst system.     

Solution behavior and structural diversity of bis(dialkylphosphino)methane complexes of palladium

The preparation of dipalladium complexes containing sterically nondemanding diphosphine (P-P) ligands of the type R(2)PCH(2)PR(2) where R = Me (dmpm) or Et (depm) is reported. Variable-temperature (1)H NMR spectra of the Pd(I)(2) complexes Pd(2)X(2)(dmpm)(2) (X = Cl, Br, or I; the P-P ligands in the Pd(2) complexes are always bridged, but for convenience, the micro -symbol is omitted) show the complexes to be fluxional in solution, the barriers to a ring-flipping process being DeltaG( double dagger ) = 37.9, 39.0, and 43.2 +/- 0.9 kJ mol(-)(1) for the chloro, bromo, and iodo complexes, respectively. Treatment of Pd(2)X(2)(P-P)(2) (X = Cl or Br) with X(2) generates the stable, face-to-face Pd(II)(2) derivatives trans-Pd(2)X(4)(P-P)(2), while oxidation of Pd(2)I(2)(P-P)(2) complexes with I(2) generates a new type of symmetrically di-iodo-bridged, five-coordinate complexes Pd(2)I(2)(micro -I)(2)(dmpm)(2) and Pd(2)I(2)(micro -I)(2)(depm)(2). The molecular crystal structures of four dipalladium(II) complexes are described: trans-Pd(2)Cl(4)(dmpm)(2).2CHCl(3), trans-Pd(2)Br(4)(dmpm)(2), trans-Pd(2)Cl(4)(depm)(2), and Pd(2)I(2)(micro -I)(2)(dmpm)(2). Solution NMR and UV-vis absorption spectra are consistent with the solid-state structures determined by X-ray diffraction. The stability of the dimeric Pd(II) complexes is attributed primarily to ligand steric factors.     

Syntheses, crystal structures, reactivity, and photochemistry of gold(III) bromides bearing N-heterocyclic carbenes

Gold(I) complexes bearing N-heterocyclic carbenes (NHC) of the type (NHC)AuBr (3a/3b) [NHC = 1-methyl-3-benzylimidazol-2-ylidene (= MeBnIm), and 1,3-dibenzylimidazol-2-ylidene (= Bn(2)Im)] are prepared by transmetallation reactions of (tht)AuBr (tht = tetrahydrothiophene) and (NHC)AgBr (2a/2b). The homoleptic, ionic complexes [(NHC)(2)Au]Br (6a/6b) are synthesized by the reaction with free carbene. Successive oxidation of 3a/3b and 6a/6b with bromine gave the respective (NHC)AuBr(3) (4a/4b) and [(NHC)(2)AuBr(2)]Br (7a/7b) in good overall yields as yellow powders. All complexes were characterized by NMR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction. Reactions of the Au(III) complexes towards anionic ligands like carboxylates, phenolates and thiophenolates were investigated and result in a complete or partial reduction to a Au(I) complex. Irradiation of the Au(III) complexes with UV light yield the Au(I) congeners in a clean photo-reaction.     

Complexes of a gallium heterocycle with transition metal dicyclopentadienyl and cyclopentadienylcarbonyl fragments, and with a dialkylmanganese compound

The reactivity of several transition metal half sandwich complexes towards an anionic gallium(I) heterocyclic complex, [K(tmeda)][Ga{[N(Ar)C(H)]2}](Ar = C6H3Pri2-2,6), has been investigated. This has led to the anionic half sandwich complexes, [K(tmeda)][(C5H4R)M(CO)n[Ga{[N(Ar)C(H)]2}]](M = V, R = H, n= 3; M = Mn, R = Me, n= 2; M = Co, R = H, n= 1), which crystallographic studies show to form dimers (M = Mn and Co) or a polymer (M = V) through bridging potassium cations. The metal-gallium bond lengths in all complexes are very short which, combined with some spectroscopic evidence, is suggestive of M-Ga pi-bonding. Density functional theory studies of models of all complexes indicate that the level of back-bonding in these complexes is, however, minimal and of a similar order to that seen in analogous complexes incorporating neutral N-heterocyclic carbene ligands. Reactions of the metallocenes, [M(C5H4Me)2](M = V or Cr), with the digallane4, [Ga{[N(Ar)C(H)]2}]2, have afforded the neutral complexes, [M(C5H4Me)2[Ga{[N(Ar)C(H)]2}]], which are thought to be formed via an initial oxidative insertion of the transition metal centre into the Ga-Ga bond of the digallane. X-Ray crystallography shows the complexes to be monomeric. One (M = V) reacts with one equivalent of [K(tmeda)][Ga{[N(Ar)C(H)]2}] to give the crystallographically characterised, anionic bis(gallyl)-complex, [K(tmeda)][V(C5H4Me)2[Ga{[N(Ar)C(H)]2}]2]. For comparison, the reaction of [K(tmeda)][Ga{[N(Ar)C(H)]2}] with [Mn{CH(SiMe3)2}2] was carried out and gave the monomeric, anionic complex, [K(tmeda)][Mn{CH(SiMe3)2}2[Ga{[N(Ar)C(H)]2}]].     

A new tridentate pincer phosphine/n-heterocyclic carbene ligand: palladium complexes, their structures, and catalytic activities

A new imidazolium salt, 1,3-bis(2-diphenylphosphanylethyl)-3H-imidazol-1-ium chloride (2), for the phosphine/N-heterocyclic carbene-based pincer ligand, PC(NHC)P, and its palladium complexes were reported. The complex, [Pd(PC(NHC)P)Cl]Cl (4), was prepared by the common route of silver carbene transfer reaction and a novel direct reaction between the ligand precursor, PC(NHC)P.HCl and PdCl(2) without the need of a base. Metathesis reactions of 4 with AgBF(4) in acetonitrile produced [Pd(PC(NHC)P)(CH(3)CN)](BF(4))(2) (5). The same reaction in the presence of excess pyridine gave [Pd(PC(NHC)P)(py)](BF(4))(2) (6). The X-ray structure determination on 4-6 revealed the chiral twisting of the central imidazole rings from the metal coordination plane. In solution, fast interconversion between left- and right-twisted forms occurs. The twisting reflects the weak pi-accepting property of the central NHC in PC(NHC)P. The uneven extent of twisting among the three complexes further implies the low rotational barrier about the Pd-NHC bond. Related theoretical computations confirm the small rotational energy barrier about the Pd-NHC bond (ca. 4 kcal/mol). Catalytic applications of 4 and 5 have shown that the complexes are modest catalysts in Suzuki coupling. The complexes were active catalysts in Heck coupling reactions with the dicationic complex 5 being more effective than the monocationic complex 4.     

Synthesis,structural characterization and catalytic activity of benzimidazole‐functionalized Pd(II) N‐heterocyclic carbene complexes

Two Pd(II)–NHC complexes bearing benzimidazole and pyridine groups have been successfully prepared and fully characterized by NMR and X‐ray diffraction analysis. The structure of palladium complexes are a typical square‐planar with palladium surrounded by two pairs of trans‐arranged benzimidazole and carbene ligands. The Pd–NHC complexes have been proved to be a highly efficient catalyst for the Mizoroki–Heck coupling reaction of aryl halides with various substituted acrylates under mild conditions in excellent yields. Copyright © 2013 John Wiley & Sons, Ltd.     

Extending the coordination capabilities of tertiary phosphines and arsines: preparation, molecular structure, and reactivity of dinuclear rhodium complexes with PR3 and AsR3 in a doubly bridging coordination mode

The reactions of [Rh2(kappa2-acac)2(mu-CPh2)2(mu-PR3)] (PR3= PMe34, PMe2Ph 7, PEt38) with an equimolar amount of Me3SiX (X = Cl, Br, I) afforded the unsymmetrical complexes [Rh2X(kappa2-acac)(mu-CPh2)2(mu-PR3)]5, 9-12, which contain the phosphine in a semi-bridging coordination mode. From 4 and excess Me3SiCl, the tetranuclear complex [[Rh2Cl(mu-Cl)(mu-CPh2)2(mu-PMe3)]2]6 was obtained. In contrast, the reaction of 4 with an excess of Me3SiX (X = Br, I) yielded the dinuclear complexes [Rh2X2(mu-CPh2)2(mu-PMe3)]13, 14 in which, as shown by the X-ray crystal structure analysis of 14, the bridging phosphine is coordinated in a truly symmetrical bonding mode. While related compounds with PEt3 and PMe2Ph as bridging ligands were prepared on a similar route, the complex [Rh2Cl2(mu-CPh2)2(mu-PiPr3)]19 was obtained from the mixed-valence species [(PiPr3)Rh(mu-CPh2)2Rh(kappa2-acac)2]17 and HCl. The reaction of [Rh2(kappa2-acac)2(mu-CPh2)2(mu-SbiPr3)]3 with AsMe3 gave the related Rh(mu-AsMe3)Rh compound 21. With Me3SiCl, the acac ligands of 21 can be replaced stepwise by chloride to give [Rh2Cl(kappa2-acac)(mu-CPh2)2(mu-AsMe3)]23 and [[Rh2Cl(mu-Cl)(mu-CPh2)2(mu-AsMe3)]2]24, the latter being isomorphous to the phosphine-bridged dimer 6.     

Synthesis and characterization of Cu(I) and Cu(II) complexes containing ketiminate ligands

A series of Cu(I) and Cu(II) complexes containing substituted ketiminate ligands was synthesized. Reaction of CuCl₂ with 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] in toluene generated dark green solid of Cu[OC(Me)CHC(Me)N(Ar)]₂ (1). Similarly, Cu(I) complex, {Cu[OC(Me)CHC(Me)N(Ar)]Li[OC(Me)CHC(Me)N(Ar)]}₂ (2) was synthesized by reacting 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] with CuCl in toluene at room temperature for 12 h. While the reaction of CuCl with Li[OC(Me)CHC(Me)N(Ar)] in the presence of triphenylphosphine in THF solution at room temperature, a three-coordinated Cu[OC(Me)CHC(Me)N(Ar)](PPh₃) (3) can be isolated in high yield. Replacing the PPh₃ of 3 with N-heterocarbene (NHC) generates Cu[OC(Me)CHC(Me)N(Ar)](NHC) (4) in low yield. Complexes 2, 3, and 4 were characterized by ¹H and ¹³C NMR spectroscopies and all molecules were structurally characterized by X-ray diffractometry. Two coordination modes of ketiminate ligands were found in the molecular structure of 2, one of which is copper-coordinated terminal ketiminates and the other is lithium-copper-coordinated bridging ketiminates.     

Expanded ring N-heterocyclic carbenes: a comparative study of ring size in palladium (0) catalysed Mizoroki-Heck coupling

The synthesis and characterisation of a range of new 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) complexes of zero valent palladium dvtms (divinyltetramethyldisiloxane) 8-14 is reported, with the first comparative catalytic study probing the effects of large ring N-heterocyclic carbene ligands (6-Mes and 7-Mes) with their analogous 5-membered derivative (IMes) on catalytic performance. The catalytic performances of the [Pd(NHC)(dvtms)] complexes are evaluated in the Mizoroki-Heck coupling of 4-bromoacetophenone and n-butyl acrylate, providing a ready and effective comparison into the influence of ring size and steric congestion on catalytic activity in carbon-carbon cross coupling reactions.