Isolation of [Rh(diolefin)X2]- species and their reactions with P- or N-donor ligands,tin(II) halides and carbon monoxide

Anionic [Rh(diolefin)X₂]⁻ species (X = Cl, Br) have been prepared and their reactions studied. The reactions with monodentate ligands led to neutral tetracoordinated complexes, and with N-donor bidentate ligands (Rh : LL = 2 : 1) gave Rh(X)(diolefin)(LL), [Rh(diolefin)(LL)]⁺[Rh(diolefin)X₂]⁻, or [Rh(diolefin)(LL)]X compounds, depending on the nature of LL or X. Reactions with carbon monoxide involved diolefin displacement. A trichlorostannato complex was obtained from the [Rh(COD)Cl₂]⁻ species. Reactions of [Rh(COD)Br]₂ with bidentate N-donor ligands were also studied.     

Zur synthese von tetramesitylmolybdän-verbindungen

Hydrogen transfer from isopropanol to various ketones such as cyclohexanone, 4-t-butylcyclohexanone and acetophenone are catalyzed by cationic rhodium(I) complexes of the type [Rh(Diene)L₂]⁺ (Diene = 1,5-cyclooctadiene (COD) or norbornadiene (NBD); L₂ or L = mono- or bi-dentate phosphine ligands). The results indicate higher activities for complexes containing chelating ligands.     

Reactions of 1,4-diaza-3-methylbutadien-2-ylpalladium(II) derivatives with chloro-bridged rhodium(I) complexes

The reactions of the organometallic 1,4-diazabutadienes, RN=C(R′)C(Me)=NR″ [R = R″ = p-C₆H₄OMe, R′ = trans-PdCl(PPh₃)₂ (DAB); R = p-C₆H₄OMe, R″ = Me, R′ = trans-PdCl(PPh₃)₂ (DAB^I; R = R″ = p-C₆H₄OMe, R′ = Pd(dmtc)-(PPh₃), dmtc = dimethyldithiocarbamate (DAB^II); R = R″ = p-C₆H₄OMe, R′ = PdCl(diphos), diphos = 1,2-bis(diphenylphosphino)ethane (DAB^III)] with [RhCl(COD)]₂ (COD = 1,5-cyclooctadiene, Pd/Rh ratio = $\text{1}\text{2}$) depend on the nature of the ancillary ligands at the Pd atom in group R′. In the reactions with DAB and DAB^I transfer of one PPh₃ ligand from Pd to Rh occurs yielding [RhCl(COD)(PPh₃)] and the new binuclear complexes [Rh(COD) {RN=C(R?)-C(Me)=NR″}], in which the diazabutadiene moiety acts as a chelating bidentate ligand. Exchange of ligands between the two different metallic centers also occurs in the reaction with DAB^II. In this case, the migration of the bidentate dmtc anion yields [Rh(COD)Pdmtc] and [Rh(COD) {RN=C(R?)C(Me)=NR″}]. In contrast, the reaction with DAB^III leads to the ionic product [Rh(COD)- (DAB^III)][RhCl₂(COD)], with no transfer of ligands. The cationic complex [Rh(COD)(DAB^III)]⁺ can be isolated as the perchlorate salt from the same reaction (Pd/Rh ratio = 1/1) in the presence of an excess of NaClO₄. In all the binuclear complexes the coordinated 1,5-cyclooctadiene can be readily displaced by carbon monoxide to give the corresponding dicarbonyl derivatives. The reaction of [RhCl(CO)₂]₂ with DAB and/or DAB^I yields trinuclear complexes of the type [RhCl(CO)₂]₂(DAB), in which the diazabutadiene group acts as a bridging bidentate ligand. Some reactions of the organic diazabutadiene RN=C(Me)C(Me)=NR (R = p-C₆H₄OMe) are also reported for comparison.     

Heterobimetallic complexes as oxygen donor bidentate ligands: synthesis of early-late trinuclear complexes

The early-late heterometallic complexes [TiCp^∗((OCH₂)₂Py)(μ-O)M(COD)] (M = Rh, Ir) behave as four-electron donor ligands yielding the polynuclear cationic complexes [TiCp^∗(OCH₂)₂ Py(μ-O){M(COD)}₂]OTf (M = Rh (1), Ir (2)). The molecular structure of complex 1 has been established through an X-ray diffraction study.     

Conformational behaviour of dinuclear Rh(I) complexes of the open-chain tetrapyrrolic ligand 2,2′-bidipyrrin (H2BDP)

The reaction of 2,2′-bidipyrrins H₂BDP with the Rh^I complexes [Rh(COD)(μ-OMe)]₂ and [Rh(CO)₂(μ-Cl)]₂ yields the neutral species [{Rh(COD)}₂BDP] (7, 8) and [{Rh(CO)₂}₂BDP] (2, 9), respectively. Treatment of the COD complexes with carbon monoxide results in the exchange of all COD ligands against CO. Ligand exchange studies on the carbonyl complexes 2 and 9 with different phosphane donors reveal the regioselective exchange of one CO per metal ion. In most cases, the reaction is accompanied by a large conformational change of the tetrapyrrole from a syn to an anti conformation. This conformational change was resolved by a combination of NMR spectroscopy and X-ray diffraction studies.     

Electrochemical reduction of the bis(cyclo-octadiene)rhodium monocation: generation and fate of the 17-electron radical

The electrochemical reduction of the monocation of bis-(cyclo-octadiene)Rh[(COD)₂Rh⁺] has been studied in chlorinated hydrocarbons and d₆-acetone by cyclic voltammetry, chronoamperometry and exhaustive coulometry. Successive one-electron reductions are observed for the couples (COD)₂Rh⁺/(COD)₂Rh and (COD)₂Rh/(COD)₂Rh⁻ at -1.34 V vs. Fc and -1.93 V vs. Fc respectively. The 17-electron Rh(0) radical (COD)₂Rh abstracts a Cl atom from CH₂Cl₂ to give the dinuclear complex [(COD)Rh(μ-Cl)]₂ in high yield at 298 K. At subambient temperatures this reaction is suppressed and the dominant decomposition product is apparently (COD)Rh(C₈H₁₃), formed by H atom abstraction by (COD)₂Rh from solvent and/or adventitious water. Electrolysis products were characterized by electron spin resonance (ESR), nuclear magnetic resonance (NMR) and mass spectrometry. The reactivity of the radical is rationalized by a bonding model in which the lowest unoccupied molecular orbital (LUMO) is d_x²−y² with some diolefin mixing. ESR measurements are consistent with this model and suggest that the COD ligands form a ligand field around Rh which is closer to square planar than to tetrahedral.     

Schiff Base Complexes of Rhodium(I) with Tridentate N-Methyl-S-methyldithiocarbazate Ligands

Schiff bases derived from the condensation of β-diketones with N-methyl-S-methyldithiocarbazates yield cis dicarbonyl complexes Rh(CO)₂ (Schiff) on reaction with [Rh(μ-Cl)(CO)₂]₂. Those derived from aromatic aldehydes form trans dicarbonyl complexes. These complexes with excess of triphenylphosphine give only Rh(CO)(PPh₃)(Schiff). cis-1,5-cyclooctadiene (COD) reacts with cis dicarbonyl complexes to yield the carbonyl-free product Rh(COD)(Schiff); similar reactions have not been observed in the case of trans-dicarbonyl complexes. Oxidative addition of bromine to these complexes yields dibromo derivative in which the Schiff base acts as bidentate chelate. Rh(PPh₃)₂(Schiff) complexes have been obtained from the reaction of above Schiff bases with Rh(PPh₃)₃Cl. The structures of these new complexes have been determined based on IR and ¹H NMR spectra.     

Dicarboxylate rhodium complexes with diolefins and carbon monoxide

Dinuclear rhodium complexes of the type [Rh₂(C₂O₄)(diolefin)₂] (diolefin)₂  1,5-cyclooctadiene, 2,5-norbornadiene and tetrafluorobenzobarrelene) with bridging oxalate ligands have been obtained by reaction of [Rh(acac)(diolefin)] with oxalic acid (2: 1 mol ratio). The use of a 1 : 1 molar ratio affords [Rh(HC₂O₄)(COD)], that reacts with [Ir(acac)(COD)] yielding the heterodinuclear [(COD)Rh(C₂O₄)Ir(COD)] complex. Treatment of [Rh₂(C₂O₄)(diolefin)₂] complexes with phenanthroline type ligands leads to ionic complexes of formula [Rh(diolefin) (phen)][Rh(C₂O₄)(diolefin)]. Bubbling of carbon monoxide through solutions of the diolefin complexes leads to the formation of carbonylrhodium species of formula [Rh₂(C₂O₄)(CO)₂L₂] (L = CO, PPh₃t-BuNC) or [Rh(CO)₂(phen)] - [Rh(C₂O₄)(CO)₂]. Other related malonate complexes are also described.     

Synthesis and Structures of RhI and IrI Complexes Supported by N‐Heterocyclic Carbene‐Phosphinidene Adducts

N‐Heterocyclic carbene‐phosphinidene adducts of the type (IDipp)PR [R = Ph ( 5 ), SiMe₃ ( 6 ); IDipp = 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene] were used as ligands for the preparation of rhodium(I) and iridium(I) complexes. Treatment of (IDipp)PPh ( 5 ) with the dimeric complexes [M(μ‐Cl)(COD)]₂ (M = Rh, Ir; COD = 1,5‐cyclcooctadiene) afforded the corresponding metal(I) complexes [M(COD)Cl{(IDipp)PPh}] [M = Rh ( 7 ) or Ir ( 8 )] in moderate to good yields. The reaction of (IDipp)PSiMe₃ ( 6 ) with [Ir(μ‐Cl)(COD)]₂ did not yield trimethylsilyl chloride elimination product, but furnished the 1:1 complex, [Ir(COD)Cl{(IDipp)PSiMe₃}] ( 9 ). Additionally, the rhodium‐COD complex 7 was converted into the corresponding rhodium‐carbonyl complex [Rh(CO)₂Cl{(IDipp)PPh}] ( 10 ) by reaction with an excess of carbon monoxide gas. All complexes were fully characterized by NMR spectroscopy, microanalyses, and single‐crystal X‐ray diffraction studies.     

Cationic organometallic complexes with hexaalkylborazines: I. Synthesis and reactivity of cationic hexamethylborazinerhodium complexes with carbonyl,ethylene and diolefins as ligands

The PF₆ salts of the new cationic hexamethylborazinerhodium(I) complexes of general formula [Rh(Me₃B₃N₃Me₃)(LL′)]⁺ (LL′= 1,5-cyclooctadiene, norbornadiene, tetrafluorobenzobarrelene, trimethyltetrafluorobenzobarrelene, L = L′ = ethylene, CO) have been prepared from the reaction between [RhCl(LL′)]₂, Me₃B₃NMe₃, and AgPF₆ in dichloromethane. These complexes are very labile, undergoing rapid ring ligand exchange in solution with σ and π-donor ligands. The synthesis of [Rh(η⁶-naphthalene)(COD)]PF₆ is also described. The properties and NMR and IR spectroscopic characteristics of the new compounds are briefly discussed.     

Stereoselective synthesis of cationic heterobidentate C(NHC)/SR rhodium(I) complexes using stereodirecting N,N-dialkylamino groups

The synthesis of two different types of chiral C/S ligands based upon N-(N,N-dialkylamino)-substituted N-heterocyclic carbenes and thioether functionalities, along with their neutral [RhCl(CNH)(COD)] and cationic [Rh(I)(NHC/S)(COD)]⁺ complexes, has been accomplished. (S)-2-[(Phenylthio)methyl]pyrrolidine, carrying the thioether moiety, and (2S,5S)-2,5-diphenylpyrrolidine, combined with a thioether functionalized side chain, were studied as potential stereodirecting groups. Only the latter provided high selectivity in the formation of the neutral complex, leading to a single atropoisomer (de >98%) of the newly formed, configurationally stable C(NHC)–Rh bond. The synthesis of the corresponding cationic [Rh(I)(NHC/S)(COD)]⁺ complexes, however, resulted in the formation of single (R_a,S_S) and (S_a,S_S) diastereomers, respectively, of the four possible complexes in each case [combinations of the (R_a/S_a) C(NHC)–Rh axis and the (S_s/R_s) stereogenic S center formed upon coordination]. For the proline derivative, the resolution of the mixture of (R_a/S_a)-[RhCl(CNH)(COD)] neutral complexes proceeds via dynamic kinetic resolution through coordinatively unsaturated Rh(I) intermediates formed after halide abstraction. The absolute configurations of both types of cationic complexes were unequivocally assigned on the basis of X-ray diffraction analysis.     

Novel quasi‐scorpionate ligand structures based on a bis‐N‐heterocyclic carbene chelate core: synthesis,complexation and catalysis

A series of novel quasi‐scorpionate CNC donor ligands, MeC(2‐C₅H₄N){CH₂(imidazole‐R)} (R = methyl, n‐butyl, n‐propenyl), in which a chelating bis(NHC) core is supplemented by a hemi‐labile pyridyl donor, were prepared. The coordination chemistry of these ligands was investigated with silver, palladium, rhodium and iridium. The single crystal X‐ray structures of [Rh(NC₂^Me)(COD)]Cl 8a and [Ir(NC₂^Pr)(COD)]Br 9b were determined. The catalytic potential of the rhodium and iridium complexes was assessed in the transfer hydrogenation of ketones; the iridium complexes, which show superior performance, form very effective and stable catalysts. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis,characterization and catalytic applications of rhodium(I) organometallics with substituted tertiary phosphines

Organometallics of the type [Rh(COD)(L)Cl] (where, L = carboxyl/formyl/pyridyl tertiary phosphines) have been synthesized by treating the precursor [Rh(COD)Cl]₂ with substituted tertiary phosphines. [Rh(COD)(Ph₂P-2-C₆H₄COO)] and [Rh(COD)(Ph₂P-CH₂COO)] were synthesized by halide abstraction from the precursor [Rh(COD)Cl]₂ in the presence of AgPF₆ in tetrahydrofuran by involving 2-carboxy phenyl/carboxy methyl group of tertiary phosphines in coordination as bidentate ligands. Similarly, the cationic compounds of the type [Rh(COD)L₂]PF₆ were also synthesized by treating [Rh(COD)Cl]₂ in the presence of AgPF₆. All these compounds were characterized by elemental analysis, conductance measurements, IR, ¹H, ¹³C, ³¹P NMR, mass, and electronic spectral studies. [Rh(COD)(Ph₂-P-2-C₆H₄COOH)Cl] and [Rh(COD)(Ph₂-P-2-C₆H₄COO)] were studied on the catalytic reduction reactions of 2-nitroanisole, 3-nitro anisole, 4-nitroanisole, 2-nitrobenzoicacid, 3-nitrobenzoicacid, 4-nitrobenzoicacid under mild conditions and [Rh(COD)(Ph₂-P-2-C₆H₄COO)] was found to be more efficient than [Rh(COD)(Ph₂-P-2-C₆H₄COOH)Cl]. This article is dedicated to Dr. D. R. M Walton, who successfully completed his tenure as Editor-in-Chief, Transition Metal Chemistry.     

Binuclear complexes of rhodium bridged by some new bifunctional anionic ligands

Reaction of the binuclear complexes [Rh(μ-Cl)(COD)]₂ with the bifunctional anionic 8-hydroxyquinolinate, 2-mercaptoquinolinate, and 2-hydroxysalicylaldiminate groups yields the binuclear complexes [Rh(μ-XY)(COD)]₂, where XY⁻ are the anionic groups listed. The complexes have been fully characterized by ¹H and ¹³C NMR spectroscopy.     

Double Stereoselection in the Hydrogenation over Cationic Rh(I) Complexes with Two Different Chiral Ligands

The catalytic activity and stereoselectivity in the hydrogenation of itaconic and -(acetylamino)cinnamic acids were studied in the presence of the complex [Rh(COD)(L¹)₂]⁺ TfO^- (where COD is cyclooctadiene and L¹ is (1S,2S,5R)-(+)-neomenthyldiphenylphosphine] which was generated in situ. The optical yield of the hydrogenation of itaconic acid increases both on addition of chiral (4S,5S)-(+)-2,2-dimethyl-4,5-bis(dimethylaminomethyl)-1,3-dioxolane (L²) as an auxiliary ligand to the complex [Rh(COD)(L¹)₂]⁺ TfO^- and on addition of achiral and chiral tertiary phosphines to the complex [Rh(L²)₂]⁺ TfO^-. The result of joint action of two ligands can be regarded as "matched effect." Transformations of the complexes in a hydrogen atmosphere were examined by ¹H and ^{3 1}P NMR spectroscopy. It was found that at least three complexes: diamine complex [Rh(L²)₂]⁺ TfO^-, solvate complex [Rh(L¹)₂(solv)₂]⁺ TfO^-, and diamine-bis-phosphine complex [Rh(L¹)₂L²]⁺ TfO^- may be catalytic precursors.     

Amino acid dicarbonylrhodium(I) complexes

Summary Dicarhonyrhodium(I) complexes of eleven amino acids were prepared from [AcORh(COD)]₂ and the appropriate amino acid by carboxylato ligand exchange followed by treatment with CO. All complexes are light coloured substances of composition (XY)Rh(CO)₂ (where XYH = amino acid) with square planar geometry andcis-carbonyl ligands, The compounds are transformed by IICI into deep coloured hydrochlorides with more complex structures.COD - 1.5-cycloocladiene.     

Anionic and neutral rhodium(I) complexes containing trichlorostannato ligands

The complexes Et₄N[Rh(SnCl₃)₂(diolefin)(PR₃)] (diolefin = COD or NBD) have been isolated and their reactions studied. Reaction with arylic tertiary phosphines led to SnCl₃⁻ displacement and isolation of neutral pentacoordinated Rh(SnCl₃)(diolefin)(PR₃)₂ complexes. Reaction with carbon monoxide involved diolefin displacement when the diolefin was COD, thus giving Et₄N[Rh(SnCl₃)₂(CO)₂(PR₃)] compounds, but SnCl₃⁻ displacement when it was NBD, thus yielding Rh(SnCl₃)(CO)(NBD)(PR₃) complexes. The complexes [Rh(diolefin)Cl]₂ were found to react with triarylphosphines in the presence of SnCl₂ and with CO bubbling through the solution to give Rh(SnCl₃)(CO)(NBD)(PR₃) when the diolefin was NBD but Rh(Cl)(CO)(PR₃)₂ when the diolefin was COD.     

Darstellung und katalytische Eigenschaften von Rhodium(I)-Komplexsalzen des Typs [Rh(COD)o-Py(CH2)2P(Ph)(CH2)3ZR)]PF6 (Z = O,NH)

Preparation and Catalytic Properties of Rhodium(I) Complex Salts of the Type [Rh(COD)(o-Py(CH₂)₂ P(Ph)(CH₂)₃ZR)]PF₆ (Z = O, NH) . In dichloromethane solutions were reacted [Rh(COD)Cl]₂ (COD = cis,cis-1.5-cyclooctadiene) with each of the four new ligands of the type o-Py(CH₂)₂P(Ph)(CH₂)₃ZR in the presence of the halogen scavenger TIPF₆ at 0°C to complex salts [Rh(COD) (o-Py(CH₂)₂P(Ph)(CH₂)₃ZR]PF₆ (ZR = OC₂H₅, I ; OPh, II ; NHPh, III ; NHcyclo? C₆H₁₁, IV ). The Rh¹ complex cation in the obtained compounds I – IV coordinates besides the bedentate COD group the ligand donor atoms P und pyridinic N and the remaining donor atom Z is uncoodinated in an assumed square planar ligand geometry at the Rh central atom. In 1.4 dioxane solutions the complex catalysts I – IV polymerize at 25°C the substrate phenylacetylene (PA) to polyphenylacetylene (PPA): values of TON [h^?1] between 352 ( I ) and 876 ( IV ), and average molecular weights M_w (GPC measurements) between 238 000 ( I ) and 199 900 ( IV ). These given values exhibit a dependency on the ZR group in complexes I – IV . The microstructure of isolated PPA is cis-transoidal. It is formed stereospezific and, based on MNDO calculations, is thermodynamically favoured. For the purpose of comparison, from both the newly synthesized compounds of the type [Rh(COD)DBN- (or DBU)Cl] (DBN = 1.5-Diazabi-cyclo[4.3.0.]non-5-en, DBU = 1.8-Diazabicycl0[5.4.0]- undec-7-en) was obtained a larger value of TON with 1292 (or 1327) [h^?], but a lower value of M, with 166200 (or 131200). These catalysts including I –IV polymerize PA to PPA at a lower reaction temperature with improved selectivity and larger values of M_w as hitherto known catalyst systems.     

Rhodium and iridium complexes of N-heterocyclic carbenes: Structural investigations and their catalytic properties in the borylation reaction

Bridged and unbridged N-heterocyclic carbene (NHC) ligands are metalated with [Ir/Rh(COD)₂Cl]₂ to give rhodium(I/III) and iridium(I) mono- and biscarbene substituted complexes. All complexes were characterized by spectroscopy, in addition [Ir(COD)(NHC)₂][Cl,I] [COD = 1,5-cyclooctadiene, NHC =  1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] (1, 4), and the biscarbene chelate complexes 12 [(η⁴-1,5-cyclooctadiene)(1,1′-di-n-butyl-3,3′-ethylene-diimidazolin-2,2′-diylidene)iridium(I) bromide] and 14 [(η⁴-1,5-cyclooctadiene)(1,1′-dimethyl-3,3′-o-xylylene-diimidazolin-2,2′-diylidene)iridium(I) bromide] were characterized by single crystal X-ray analysis. The relative σ-donor/π-acceptor qualities of various NHC ligands were examined and classified in monosubstituted NHC-Rh and NHC-Ir dicarbonyl complexes by means of IR spectroscopy. For the first time, bis(carbene) substituted iridium complexes were used as catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives.