Supramolecular approaches to generate libraries of chelating bidentate ligands for homogeneous catalysis

The process of catalyst discovery and development relying on combinatorial methods has suffered so far from the difficult access to structurally diverse and large libraries of ligands, in particular the structurally more complex class of bidentate ligands. A completely new approach to streamline the difficult ligand synthesis process is to use structurally less complex monodentate ligands that self-assemble in the coordination sphere of a metal center through noncovalent attractive ligand-ligand interactions to generate bidentate, chelating ligands. When complementary attractive ligand-ligand interactions are employed, it is even possible to generate libraries of defined chelate-ligand catalysts by simply mixing two different monomeric ligands. This Minireview summarizes the first approaches and results in this new field of combinatorial homogeneous catalysis.     

Coordination chemistry of chelating nitrogen ligands with tungsten carbonyl nitriles

The reactions of a series of Schiff base ligands (1–4) prepared by condensation of pyridine-2-carboxaldehyde and 2-X-anilines (X = F, Cl, Br, OMe) with W(CO)₃(RCN)₃ results in the formation of deeply coloured complexes W(CO)₃(1–4)(NCR) which readily react with carbon monoxide to afford W(CO)₄(1–4). The ligand 5, prepared from pyridine-2-carboxaldehyde and N,N-dimethylethylenediamine, coordinates to W(CO)₃ through the conjugated pyridine—imine chelate to afford complexes with a pendant dimethylamino group which reacts with electrophiles. Of the ligands studied, only the bis-amine, pyridine ligand 8, coordinates to W(CO)₃ in a tridentate manner. These reactions provide insight into the geometric and electronic factors which influence coordination of nitrogen containing ligands at a fac-tricarbonyl metal centre.     

Synthesis of multiply-bonded dichromium complexes with a variety of formamidinate ligands

A series of dichromium tetraformamidinate complexes with (ArNCHNAr)⁻ where Ar is X₂C₆H₃ or XC₆H₄ and X is the remote substituent 3,5-Cl₂, 3,4-Cl₂, p-Cl, p-CF₃, m-CF₃, p-OCH₃ or m-OCH₃ has been synthesized, and the structures of four of the compounds determined by X-ray crystallography. The Cr–Cr bond distances of the complexes structurally characterized are 1.9072(10) Å [Cr₂((p-ClC₆H₄N)₂CH)₄], 1.9162(10) Å [Cr₂((3,5-Cl₂C₆H₃N)₂CH)₄], 1.9018((8) Å [Cr₂((m-CF₃C₆H₄N)₂CH)₄ ], and 1.9178(11) Å [Cr₂((m-OCH₃C₆H₄N)₂CH)₄]. ¹H NMR spectroscopy and electrochemistry were performed on the series. The diamagnetic anisotropy of the Cr–Cr bond was calculated from X-ray crystallographic and ¹H NMR data and correlates with the Hammett constant of the formamidine ligands.     

Complexes of diorganotins with bidentate ligands

The bidentate chelating agents 3-(2-pyridyl)-5,6- diphenyl-1,2,4-triazine, 1,10-phenanthroline and 2,2′-bipyridyl form 1:1 complexes with organotin dichlorides and diisothiocyanates. The complexes are assigned octahedral structures with trans hydrocarbon groups except for the diphenyltin diisothiocyanato complexes which are assigned octahedral structures with cis hydrocarbon groups and trans isothiocyanato groups. The assignments are made on the basis of electric dipole moment measurements.     

Synthesis of aluminum alkoxide and bis-alkoxide compounds containing bidentate pyrrolyl ligands

A series of aluminum alkoxide and bis-alkoxides compounds were synthesized and characterized. Reacting 1 with 1 and 2 equiv. of t-butanol in methylene chloride generates [C₄H₃N(CH₂NMe₂)-2]₂Al(O-t-Bu) (2) and [C₄H₃N(CH₂NMe₂)-2-H-C₄H₃N(CH₂NMe₂)-2]Al(O-t-Bu)₂ (3) in 47% and 54% yield, respectively. The ¹H NMR spectrum of 2 exhibits two singlets for NMe₂ and CH₂N at δ 2.52 and 3.84, respectively, representing the symmetrical manner of molecular structure 2 in a solution. Compound 3 is not thermal stable in solution which decompose into substituted pyrrole ligand C₄H₄N(CH₂NMe₂)-2 and unknown aluminum alkoxides. Reacting 1 with 2 equiv. of triphenylsilanol in methylene chloride generates a tetra-coordinated aluminum “ate” compound [C₄H₃N(CH₂NMe₂)-2-H- C₄H₃N(CH₂NMe₂)-2]Al(OSiPh₃)₂ (4) in 49% yield. The ¹H NMR spectra of 4 at room temperature show a broad signal at δ 1.57 for NMe₂ fragments and the signals for CH₂N were not observed. VT ¹H NMR spectra of 4 show the NMe₂ fragments became two singlets (δ 1.27 and 2.12) and the CH₂N exhibited two doublets (δ 2.44 and 3.56) at 240 K. The fluxional energy barrier (ΔG^‡) is estimated at ca. 50 kJ/mol. The molecular structures of compounds 3 and 4 are determined by single-crystal X-ray diffractometer.     

Synthesis and characterisation of nonclassical ruthenium hydride complexes containing chelating bidentate and tridentate phosphine ligands

The synthesis and characterisation of nonclassical ruthenium hydride complexes containing bidentate PP and tridentate PCP and PNP pincer-type ligands are described. The mononuclear and dinuclear ruthenium complexes presented have been synthesised in moderate to high yields by the direct hydrogenation route (one-pot synthesis) or in a two-step procedure. In both cases [Ru(cod)(metallyl)(2)] served as a readily available precursor. The influences of the coordination geometry and the ligand framework on the structure, binding, and chemical properties of the M--H(2) fragments were studied by X-ray crystal structure analysis, spectroscopic methods, and reactivity towards N(2), D(2), and deuterated solvents.     

Boron coordination compounds derived from 2-phenyl-benzimidazole and 2-phenyl-benzotriazole bidentate ligands

The syntheses and structural analyses of a series of boron heterocycles derived from 2-(1H-benzimidazol-2-yl)-phenylamine (1), 2-(1H-benzimidazol-2-yl)-phenol (2), 2-(1H-benzimidazol-2-yl)-benzenedisulfide (3), 2-[3-(1,1,1,3,-tetramethyl-butyl)-phenyl]-2H-benzotriazole (4), 2-[3,5-bis-(1-methyl-1-phenylethyl)-phenyl]-2H-benzotriazole (5) and (C₆H₅)₂BOH or BF₃·OEt₂ are reported. The new boron compounds: diphenyl-[2-(1H-benzimidazol-2-yl-κN)-phenylamide-κN]-boron (6), diphenyl-[2-(1H-benzimidazol-2-yl-κN)-phenolate-κO]-boron (7), diphenyl-[2-(1H-benzimidazol-2-yl-κN)-benzenethiolate-κS]-boron (8), diphenyl-[2-(2H-benzotriazol-2-yl-κN)-4-(1,1,3,3-tetramethyl-butyl)-phenolate-κO]-boron (9), diphenyl-[2-(2H-benzotriazol-2-yl-κN)-4,6-(1-methyl-1-phenylethyl)-phenolate-κO]-boron (10), difluoro-[2-(1H-benzimidazol-2-yl-κN)-phenolate-κO]-boron (11), difluoro-[2-(2H-benzotriazol-2-yl-κN)-4-(1,1,3,3-tetramethylbutyl)-phenolate-κO]-boron (12) and difluoro-[2-(2H-benzotriazol-2-yl-κN)-4,6-(1-methyl-1-phenylethyl)-phenolate-κO]-boron (13) have four fused rings, with boron included in a six-membered ring and bound to N, O or S atoms and strongly coordinated by a nitrogen atom from the imidazole or triazole rings. Their structures are zwitterionic, with a negative charge on the boron and a delocalized positive charge on the ligand. Compounds 6-12 were studied by NMR, IR, mass spectrometry, and 6-10 and 12 by X-ray diffraction analyses.     

Oxidative addition of organic halides to zerovalent palladium complexes containing rigid bidentate nitrogen ligands

Zerovalent complexes of the type Pd(Ar-BIAN)(alkene), i.e. complexes containing the rigid bidentate nitrogen ligands bis(arylimino) acenaphthene (Ar = p-Tol, p-MeOC₆H₄, o-Tol,o,o′-Me₂C₆H₃, o,o′-ⁱPr₂C₆H₃) and an electron-poor alkene have been shown to react with a variety of (organic) halides RX, including methyl, benzyl, aryl, acyl and allylic halides, to give the corresponding square planar divalent Pd(R)X(Ar-BIAN) or [Pd(η³-allyl)(Ar-BIAN)]X complexes. The new complexes obtained have been fully characterized and their fluxional behaviour in solution studied by ¹H NMR spectroscopy. The rate of oxidative addition of iodomethane to Pd(p-Tol-BIAN)(alkene) complexes was found to decrease with increasing Pd-alkene bond strength, i.e. dimethyl fumarate fumaronitrile, but oxidative addition to the fumaronitrile complex was accelerated by irradiation with a mercury lamp. Oxidative addition of allylic ha     

Complexes of osmium halides with monodentate and bidentate ligands

Complexes of osmium halides with donor ligands L (L = PPh₃, AsPh₃, SbPh₃, PMePh₂, C₂H₄(AsPh₂)₂, PPr₂ⁿPh, CH₂(AsPh₂)₂) have been prepared. The IR and NMR spectra of complexes of the type trans-[OsCl₄L₂], fac-[OsX₃L₃] and trans-[OsX₂L₄] are recorded and discussed. For the trans complexes the halogen sensitive modes occur near those of the related [OsX₄]²⁻ species. Tentative assignments for ν(OsY) (Y = P, As, Sb) are discussed on the basis of the experimental results. Raman and ESR spectra of selected complexes have been recorded and are discussed. NMR data indicates the presence of monodentate diarsine ligands in trans- (OsCl₄(CH₂(AsPh₂)₂)₂)·C₂H₅OH.     

4-Diphenylphosphinosydnone imines as bidentate ligands

The synthesis and characterization of new 3-substituted-4-diphenylphosphinosydnone imines (1,2,3-oxadiazolium-imines-5) are reported.     

Nitrogen- and oxygen-bridged bidentate phosphaalkene ligands

An overview is given on synthesis and structures of new bidentate phosphaalkene ligands [(RMe₂Si)₂CP]₂E (E = O, NR, N^?) and (RMe₂Si)₂CPN(R′)PR′′₂. Exceptional properties of these ligands, extending beyond predictable properties of phosphaalkenes are: (i) the NSi bond cleavage of [(iPrMe₂Si)₂CP]₂NSiMe₃ with Au^I and Rh^I chloro complexes under mild conditions leading to binuclear complexes of the 6π-delocalised imidobisphosphaalkene anion [(iPrMe₂Si)₂CP]₂N^?, and (ii) the chlorotropic formation of molecular 1:2 Pd^II and Pt^II metallochloroylid complexes with novel ylid-type ligands [(RMe₂Si)₂CP(Cl)N(R)PR₂]^?, and the transformation of a P-platina-P-chloroylid complex into a C-platina phosphaalkene by intramolecular chlorosilane elimination. Properties of the heavier congeners [(RMe₂Si)₂CP]₂E (E = S, Se, Te, PR, P^?, As^?) and (RMe₂Si)₂CPEPR′′₂ (E = S, Se, Te) are also described.     

Thermal isomerization of ruthenium hydride compounds containing asymmetric bidentate pyrrole‐imine ligands

A series of ruthenium hydride compounds containing substituted bidentate pyrrole‐imine ligands were synthesized and characterized. Reacting RuHCl(CO)(PPh₃)₃ with one equivalent of [C₄H₃NH(2‐CH=NR)] in ethanol in the presence of KOH gave compounds {RuH(CO)(PPh₃)₂[C₄H₃N(2‐CH=NR)]} where trans‐Py‐Ru‐H 1, R = CH₂CH₂C₆H₉; cis‐Py‐Ru‐H 2, R = Ph‐2‐Me; and cis‐Py‐Ru‐H 3, R = C₆H₁₁. Heating trans‐Py‐Ru‐H 1 in toluene at 70°C for 12 hr resulted a thermal conversion of the trans‐Py‐Ru‐H 1 into its cis form, {RuH(CO)(PPh₃)₂[C₄H₃N(2‐CH=NCH₂CH₂C₆H₉)]} (cis‐Py‐Ru‐H 1) in very high yield. The ¹H NMR spectra of trans‐Py‐Ru‐H 1, cis‐Py‐Ru‐H 2, cis‐Py‐Ru‐H 3, and cis‐Py‐Ru‐H 1 all show a typical triplet at ca. δ–11 for the hydride. The trans and cis form indicate the relative positions of pyrrole ring and hydride. The geometries of trans‐Py‐Ru‐H 1, cis‐Py‐Ru‐H 1, and cis‐Py‐Ru‐H 3 are relatively similar showing typical octahedral geometries with two PPh₃ fragments arranged in trans positions.     

Reduction of the pertechnetate anion with bidentate phosphine ligands

The reduction of ammonium pertechnetate with bis(diphenylphosphino)methane (dppm), and with diphenyl-2-pyridyl phosphine (Ph(2)Ppy), has been investigated. The neutral Tc(II) complex, trans-TcCl(2)(dppm)(2) (1), has been isolated from the reaction of (NH(4))[TcO(4)] with excess dppm in refluxing EtOH/HCl. Chemical oxidation with ferricinium hexafluorophosphate results in formation of the cationic Tc(III) analogue, trans-[TcCl(2)(dppm)(2)](PF(6)) (2). The dppm ligands adopt the chelating bonding mode in both complexes, resulting in strained four member metallocycles. With excess PhPpy, the reduction of (NH(4))[TcO(4)] in refluxing EtOH/HCl yields a complex with one chelating Ph(2)Ppy ligand and one unidentate Ph(2)Ppy ligand, mer-TcCl(3)(Ph(2)Ppy-P,N)(Ph(2)Ppy-P) (3). The cationic Tc(III) complexes, trans-[TcCl(2)(Ph(2)P(O)py-N,O)(2)](PF(6)) (4) and trans-[TcCl(2)(dppmO-P,O)(2)](PF(6)) (5) (Ph(2)P(O)py = diphenyl-2-pyridyl phosphine monoxide and dppmO = bis(diphenylphosphino)methane monoxide), have been isolated as byproducts from the reactions of (NH(4))[TcO(4)] with the corresponding phosphine. The products have been characterized in the solid state and in solution via a combination of single-crystal X-ray crystallography and spectroscopic techniques. The solution state spectroscopic results are consistent with the retention of the bonding modes revealed in the crystal structures.     

Metal uptake by a novel bidentate thiosemicarbazide chelating polymer

1-Cyanoethanoyl-4-acryloyl thiosemicarbazide (CEATS) has been prepared and polymerized by a free radical mechanism. The polymer PCEATS has chelating affinity, and metal-uptake capacities were determined for the chlorides of Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) in the pH range 1.04–6.0. The extraction experiments show high capacity for Cu(II), (0.26?mmol/g) at pH 5.34 and lower uptake capacities for the other divalent metal ions around pH 5 in buffered solutions, under noncompetitive conditions. However, competitive experiments, performed with solutions containing a mixture of metal chloride salts and acetate buffer, showed a high selectivity for Cu(II) and Cd(II) over other cations. Distribution coefficients determined for the polymer and the metal ions indicate that the Cu(II) complex is more stable than the Cd(II) are and suggest that the stability of the complex decreases rapidly with decreasing pH. Kinetic experiments have shown that uptake of Cu(II), is slow, with t _0.5?=?10?h. Ligand regeneration experiments for Cu(II)-loaded PCEATS performed with 2.00?M H₂SO₄ have shown that the capacity for Cu(II) stays at the same level after several cycles of consecutive loading and stripping, indicating that the polymer is chemically stable. ESR spectra of Cu(II)–CEATS resin show that there are two different coordination complexes present in the polymer. IR spectra of the Cu(II) complex confirms the bidentate behavior (S, O; N, O) of CEATS and PCEATS (it is supposed that the cations bridge vicinal CEATS ligands through S, O and N, O atoms); the acetate group completes the octahedral coordination. The obtained data suggest that the polymer behaves as a bidentate ligand via the thiocarbonyl, carbonyl and imide groups. PCEATS and its complexes have an inhibitory effect on both the bacterium Azotobacter and the fungus Fusarium oxysporium. The effect on the microorganisms is proportional to the amount of free ligand in the complex.     

Synthesis of new metallomesogens with enaminoketone chelating ligands

New liquid crystalline copper(II) enaminoketonates containing bridging ester groups were synthesized by hydrogenation of the heteroring in 3-(4-hydroxyphenyl)-5-pentylisoxazole to 1-amino-1-(4-hydroxyphenyl)oct-1-en-3-one, treatment of the latter with copper(II) acetate, and benzoylation of the coordination compound thus formed with 4-alkoxybenzoyl chlorides.     

Preparation of hydride complexes of ruthenium with bidentate phosphite ligands

Hydride complex RuH₂(PFFP)₂ (1) [PFFP = (CF₃CH₂O)₂PN(CH₃)N(CH₃)P(OCH₂CF₃)₂] was prepared by allowing the compound RuCl₄(bpy) · H₂O (bpy = 1,2-bipyridine) to react first with the phosphite PFFP and then with NaBH₄. Chloro-complex RuCl₂(PFFP)₂ (2) was also prepared, either by reacting RuCl₄(bpy) · H₂O with PFFP and zinc dust or by substituting triphenylphosphine with PFFP in the precursor complex RuCl₂(PPh₃)₃. Hydride derivative RuH₂(POOP)₂ (3) (POOP = Ph₂POCH₂CH₂OPPh₂) was prepared by reacting compound RuCl₃(AsPh₃)₂(CH₃OH) first with the phosphite POOP and then with NaBH₄. Depending on experimental conditions, treatment of carbonylated solutions of RuCl₃ · 3H₂O with POOP yields either the cis- or trans-RuCl₂(CO)(PHPh₂)(POOP) (4) derivative. Reaction of both cis- and trans-4 with LiAlH₄ in thf affords dihydride complex RuH₂(CO)(PHPh₂)(POOP) (5). Chloro-complex all-trans-RuCl₂(CO)₂(PPh₂OMe)₂ (6) was obtained by reacting carbonylated solutions of RuCl₃ · 3H₂O in methanol with POOP. Treatment of chloro-complex 6 with NaBH₄ in ethanol yielded hydride derivative all-trans-RuH₂(CO)₂(PPh₂OMe)₂ (7). The complexes were characterised spectroscopically and the X-ray crystal structures of complexes 1, 3, cis-4 and 6 were determined.     

The synthesis of some bidentate bismuth-containing ligands

The synthesis and properties including mass spectra of (o-diphenylphosphinophenyl)diphenylbismuthine, o-C₆H₄(PPh₂)(BiPh₂) and (o-diphenylarsinophenyl)-diphenylbismuthine, o-C₆H₄(AsPh₂)(BiPh₂) are described. The failure of attempts to prepare α, ω-bis(disphenylbismuthino)alkanes is discussed.     

Orthopalladation of phosphorus ylides in endo position with bidentate ligands

The ortho-metallated complexes [Pd₂{κ²(C,C)-C₆H₄(PPh₂CHC(O)C₆H₅R}₂(μ-Cl)₂] (R = Ph (1a), NO₂ (1b), Br (1c)) were prepared by refluxing equimolar mixtures of Ph₃PCHC(O)C₆H₅R, (R = Ph, NO₂, Br) and Pd(OAc)₂ in MeOH, followed by an excess of NaCl. The dinuclear complexes (1a-1c) react with silver trifluoromethylsulfonate and bidentate ligands [L = bipy (2,2′-bipyridine), phen (phenanthroline), dppe (bis(diphenylphosphino)ethane), dppp (bis(diphenylphosphino)propane)] giving the mononuclear stabilized orthopalladated complexes in endo position [Pd{κ²(C,C)-C₆H₄(PPh₂CHC(O)R}L](OTf) [R = Ph, L = phen (2a), bipy (3a), dppe (4a), dppp (5a); R = NO₂, L = phen (2b), bipy (3b), dppe (4b), dppp (5b); R = Br, L = phen (2c), bipy (3c), dppe (4c), dppp (5c); OTf = trifluoromethylsulfonate anion]. Orthometalation and ylidic C-coordination are demonstrated by an X-ray diffraction study of 2c and 3c. In the structures, the palladium atom shows a slightly distorted square-planar coordination geometry.     

New pinene-derived pyridines as bidentate chiral ligands

A synthesis of new bidentate pyridines 8a-d, 9, and 10 has been developed, starting from triflate 14, readily available from β-pinene 11. A copper complex of the pyridine-oxazoline ligands 8a has been found to catalyze asymmetric allylic oxidation of cyclic olefins 36a-c with good conversion rates and acceptable enantioselectivity (≤67% ee). The imidazolium salt 10 has been identified as a precursor of the corresponding N,N′-unsymmetrical N-heterocyclic carbene ligand, whose complex with palladium catalyzed the intramolecular amide enolate α-arylation leading to oxindole 45 in excellent yield but with low enantioselectivity.     

Symmetric Ni(II) dithiocarbamates with bidentate phosphines ligands

Ni(II) mononuclear dithiocarbamate complexes with bidentate P,P ligands of composition [Ni(R₂dtc)(P,P)]X {R?=?pentyl (pe), benzyl (bz); dtc?=?S₂CN^?; P,P?=?1,2-bis(diphenylphosphino)ethane (dppe), 1,4-bis(diphenylphosphino)butane (dppb), 1,1′-bis(diphenylphosphino) ferrocene (dppf); X?=?ClO₄, Cl, Br, NCS} and binuclear complexes of composition [Ni₂(μ-dpph)(R₂dtc)₂]X₂ with a P,P-bridging ligand {P,P?=?1,6-bis(diphenylphosphino)hexane (dpph); X?=?Cl, Br, NCS} have been synthesized. The complexes have been characterized by elemental and thermal analysis, IR, electronic and ³¹P{¹H}-NMR spectroscopy, magnetochemical and conductivity measurements. Single crystal X-ray analysis of [Ni(pe₂dtc)(dppf)]ClO₄ confirmed a distorted square planar coordination in the NiS₂P₂ chromophore. For selected samples, the catalysis of graphite oxidation was studied.