Cyclometallated platinum(II) complexes: oxidation to, and C-H activation by, platinum(IV)

A series of cyclometallated phenylpyridine platinum(II) complexes have been synthesised with a systematic variation in both the phenylpyridine and the ancillary ligand. Oxidation of one of the cyclometallated species leads to a number of isomeric platinum(IV) complexes, all of which eventually isomerize to a single compound. The route to these new compounds has been demonstrated to involve an initial slow oxidation followed by a rapid C-H activation to give doubly cyclometallated complexes. The solid state structures of a number of both the platinum(II) and the platinum(IV) species have been solved; many of the structures exhibited extended interactions that result in complex three dimensional packing.     

Benzene C-H activation by platinum(II) complexes of bis(2-diphenylphosphinophenyl)amide

The amido diphosphine complexes [PNP]PtMe and [PNP]PtOTf, where [PNP]- is bis(2-diphenylphosphinophenyl)amide, effectively activate the benzene C-H bond in the presence of an appropriate Lewis acid or base, leading to the formation of [PNP]PtPh quantitatively.     

Chiral palladium bis(phosphite)PCP-pincer complexes via ligand C-H activation

The synthesis of a range of chiral palladium bis(phosphite) pincer complexes has been achieved via C-H activation of the parent ligands and one of the complexes formed shows good activity in the catalytic allylation of aldehydes.     

Atropisomerization, C-H activation, and dissociative substitution at some biphenyl platinum(II) complexes

The reaction of 2,2'-dilithiumbiphenyl with cis-[PtCl(2)(SEt(2))(2)] at -10 degrees C in diethyl ether not only leads to the main product [Pt(2)(micro-SEt(2))(2)(bph)(2)], containing the planar 2,2'-biphenyl dianion (bph(2)(-)), but also forms a new dinuclear platinum(II) compound of formula [Pt(2)(micro-SEt(2))(2)(Hbph)(4)], 1a (Hbph(-) = eta(1)-biphenyl monoanion), in which each metal is in a square-planar environment. NMR spectroscopy and molecular mechanics (MMFF) calculations were used to characterize 1a. The results suggest that the favored conformation for the four Hbph biphenyl groups is alphabetabetaalpha. In chloroform solution, 1a undergoes atropisomerization to 1b (alphabetaalphabeta) (k(is) = 1.03 x 10(-)(4) s(-)(1), at 298 K) that subsequently cyclometalates (k(obs) = 4.48 x 10(-)(6) s(-)(1), at 298 K) to yield [Pt(2)(micro-SEt(2))(2)(bph)(2)] and biphenyl. Both processes, atropisomerization and C-H activation, presumably involve preliminary thioether bridge splitting. The dinuclear complex 1a has been shown to be a versatile and useful precursor to a variety of mononuclear eta(1)-biphenyl platinum(II) complexes. By reaction with diethyl sulfide, dimethyl sulfoxide, or with rigid dinitrogen containing ligands, such as 2,2'-bipyridine or 1,10-phenanthroline, complexes cis-[Pt(Hbph)(2)(dmso)(2)] 3, cis-[Pt(Hbph)(2)(SEt(2))(2)] 4, [Pt(Hbph)(2)(bpy)] 5, and [Pt(Hbph)(2)(phen)] 6 were obtained, respectively. The crystal structures of compounds 5 and 6 were determined. Only the head-to-tail isomer of these compounds was recognized in the solid state and in solution, where restricted rotation around the Pt-C bond prevents interconversion to the head-to-head form. A detailed kinetic study of ligand (dmso) exchange and substitution (by 2,2'-bipyridine and 1,10-phenanthroline) has been performed on complex 3 in CDCl(3) and toluene-d(8) by (1)H NMR magnetization transfer experiments, and in toluene by UV/vis spectroscopy, respectively. The rates of both processes show no dependence on ligand concentration, the rate of ligand substitution being in reasonable agreement with that of ligand exchange at the same temperature. The kinetics are characterized by largely positive entropies of activation. The results are consistent with a dissociative mode of activation analogous to the pattern already found for compounds with a similar [Pt(C,C)(S,S)] set of coordinating ligands. The role of ML(3) d(8) T-shaped 14-electron species, as elusive reaction intermediates or structurally characterized compounds, is discussed.     

Activation of C-H bonds of arenes: selectivity and reactivity in bis(pyridyl) platinum(II) complexes

The reaction of [PtMe2(NN)] and B(C6F5)3/H2O in CF3CH2OH with arenes Ar-H gives [PtAr{HOB(C6F5)3}(LL)] if the bis(pyridyl) ligand NN forms a six-membered, but not five-membered, chelate ring; methyl-substituted arenes give selectivity for metalation of meta > para > ortho, but methoxy-substituted arenes give ortho > meta, para.     

Unexpected selectivities in C-H activations of toluene and p-xylene at cationic platinum(II) diimine complexes. New mechanistic insight into product-determining factors.

The C-H activation of toluene and p-xylene at cationic Pt(II) diimine complexes (N-N)Pt(CH(3))(H(2)O)(+)BF(4)(-) (N-N = Ar-N=CMe-CMe=N-Ar; 1(BF(4)(-)), N(f)-N(f), Ar = 3,5-(CF(3))(2)C(6)H(3)); 2(BF(4)(-)), N'-N', Ar = 2,6-(CH(3))(2)C(6)H(3)) has been investigated. The reactions were performed at ambient temperature in 2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Pt-benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the reactions with toluene, the relative amounts of products resulting from aromatic C-H activation were found to decrease in the order (N-N)Pt(m-tolyl)(NCMe)(+) > (N-N)Pt(p-tolyl)(NCMe)(+) > (N-N)Pt(o-tolyl)(NCMe)(+) for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product (N'-N')Pt(benzyl)(NCMe)(+) were concurrently formed in the C-H activation of toluene at 2. The C-H activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios of (N-N)Pt(xylyl)(NCMe)(+) and (N-N)Pt(p-methylbenzyl)(NCMe)(+) were found to be 90:10 and 7:93 for reactions at 1 and 2, respectively. The elimination of toluene from (N(f)-N(f))Pt(Tol)(2) species (3a-c; a, Tol = o-tolyl; b, Tol = m-tolyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF(4) was investigated. Most notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (N(f)-N(f))Pt(m-tolyl)(NCMe)(+) (4b) and (N(f)-N(f))Pt(p-tolyl)(NCMe)(+) (4c) from all three isomeric bis(tolyl) complexes 3a-c. The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This behavior is explained by an associative mechanism for the product-determining displacement of toluene by the solvent. For the C-H activation reactions, our findings suggest the existence of a dynamic equilibrium between the isomeric intermediates (N-N)Pt(aryl)(CH(4))(+) (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate transition-state structures for the solvent-induced associative elimination of methane from these intermediates.     

Iridium-imine and -amine complexes relevant to the (S)-metolachlor process: structures, exchange kinetics, and C-H activation by Iri causing racemization

Iridium complexes of DMA-imine [2,6-dimethylphenyl-1'-methyl-2'-methoxyethylimine, 1 a) and (R)-DMA-amine [(1'R)-2,6-dimethylphenyl-1'-methyl-2'-methoxyethylamine, 2 a] that are relevant to the catalytic imine hydrogenation step of the Syngenta (S)-Metolachlor process were synthesized: metathetical exchange of [Ir2Cl2(cod)2] (cod=1,5-cyclooctadiene) with [Ag(1 a)2]BF4 and [Ag((R)-2 a)2]BF4 afforded [Ir(cod)(kappa2- -1 a)]BF4 (11) and [Ir(cod)(kappa2-(R)-2 a)]BF4 ((R)-19)), respectively. These complexes were then used in stopped-flow experiments to study the displacement of amine 2 a from complex 19 by imine 1 a to form the imine complex 11, thus modeling the product/substrate exchange step in the catalytic cycle. The data suggest a two-step associative mechanism characterized by k1=(2.6+/-0.3) x 10(2) M(-1) s(-1) and k2=(4.3+/-0.6) x 10(-2) s(-1) with the respective activation energies EA1=(7.5+/-0.6) kJ mol(-1) and EA2=(37+/-3) kJ mol(-1). Furthermore, complex 11 reacted with H2O to afford the hydrolysis product [Ir(cod)(eta(6-)-2,6-dimethylaniline)]BF4 (12), and with I2 to liberate quantitatively the DMA-iminium salt 14. On the other hand, the chiral amine complex (R)-19 formed the optically inactive eta6-bound compound [Ir(cod)(eta6-rac-2 a)]BF4 (rac-18) upon dissolution in THF at room temperature, presumably via intramolecular C-H activation. This racemization was found to be a two-step event with k'1=9.0 x 10(-4) s(-1) and k2=2.89 x 10(-5) s(-1), featuring an optically active intermediate prior to sp3 C-H activation. Compounds 11, 12, rac-18, and (R)-19 were structurally characterized by single-crystal X-ray analyses.     

Counterdisproportionation between cationic Ni(II) and phosphine Ni(0) complexes

The interaction of Ni(II) bis-tetrafluoroborate complexes [Ni(Dppe)₂](BF₄)₂ and [Ni(CH₃CN)₆](BF₄)₂ (where Dppe = 1,2-bis(diphenylphosphino)ethane)) with Ni(0) phosphine complexes Ni(Dppe)₂ and Ni(PPh₃)₄ in 1 : 1 mixture of toluene-acetonitrile was studied by the EPR method. The counter-disproportionation was shown to occur in a solution between the cationic Ni(II) complexes and the Ni(0) complexes to give Ni(I) complexes almost in quantitative amounts. The structures of the cationic Ni(I) complexes obtained were found to depend on both the solvent nature and the presence of a free phosphine in a solution.     

Tuning the electronic structures of platinum(II) complexes with a cyclometalating aryldiamine ligand

Triflate salts of four platinum(II) pyridyl complexes with a mer-coordinating tridentate pincer ligand, pip(2)NCN(-) (pip(2)NCNH = 1,3-bis(piperidylmethyl)benzene), are reported: Pt(pip(2)NCN)(L)(+) (2, L = pyridine; 3, L = 4-phenylpyridine; 5, L = 2,6-pyridinedimethanol) and [(Pt(pip(2)NCN))(2)(micro-4,4'-bipyridine)](2+) (4). The complexes have been fully characterized by (1)H NMR spectroscopy, elemental analysis, and X-ray crystallography. Compound 2(CF(3)SO(3)(-)): triclinic, P1, a = 9.7518(6) A, b = 12.0132(8) A, c = 12.6718(9) A, alpha = 114.190(2) degrees, beta = 100.745(3) degrees, gamma = 103.545(2) degrees, V = 1247.95(14) A(3), Z = 2. Compound 3(CF(3)SO(3)(-)): monoclinic, P2(1)/c, a = 15.550(2) A, b = 9.7386(11) A, c = 18.965(3) A, beta = 92.559(7) degrees, V = 2869.1(6) A(3), Z = 4. Compound 4(CF(3)SO(3)(-))(2).1/2(CH(3))(2)CO: monoclinic, I2/a, a = 21.3316(5) A, b = 9.6526(2) A, c = 26.1800(6) A, beta = 96.4930(10) degrees, V = 5356.0(2) A(3), Z = 4. Compound 5(CF(3)SO(3)(-)).3/2CHCl(3): monoclinic, P2(1)/n, a = 17.1236(10) A, b = 9.3591(5) A, c = 21.3189(11) A, beta = 96.11(3) degrees, V = 3397.2(3) A(3), Z = 4. The accumulated data indicate that the phenyl group of pip(2)NCN(-) labilizes the trans pyridyl ligand. The electronic structures were investigated using cyclic voltammetry, as well as UV-visible absorption and emission spectroscopies. Red emission from 2 in rigid media originates from a lowest triplet ligand field excited state, whereas yellow-green emissions from 3 and 4 originate from a lowest pyridyl ligand-centered triplet pi-pi state, indicating that substitution of the pyridyl ligand results in a dramatic change in the orbital character of the emissive state.     

Ligand effects on the Pt(II) → Pt(0) reduction of dichlorobis(tertiary phosphine)platinum(II) complexes: Correlations between electrochemical data,spectroscopic data,and electronic ligand parameters

Cyclic voltammetry has been employed to study the diffusive, irreversible platinum(II) → platinum(0) reduction of three sets of structurally related complexes: cis-[PtCl₂P{p-C₆H₄X}₃)₂] (X = H, CH₃, Cl, F, OCH₃, N(CH₃)₂); cis-[PtCl₂(PPh₂R)₂] (R = CH₃, n-C₃H₇, n-C₅H₁₁, n-C₆H₁₃, n-C₁₂H₂₅) and cis-[PtCl₂(PR₃)₂] (R = CH₃, C₂H₅, CH₂ch₂CN). Relationships between the peak potentials for the Pt(II) → Pt(0) reduction and thermodynamic parameters which measure the electronic properties of the ligands are shown to exist for complexes of P{p-C₆H₄X}₃ ligands, implying a thermodynamic origin for the sensitivity of the peak potentials to structural change. Complexes of both P{p-C₆H₄X}₃ and PPh₂R ligands show correlations between peak potentials for reduction and the ³¹P{¹H} NMR spectroscopic parameter, ¹J(¹⁹⁵Pt, ³¹P). Correlations with values of δ(³¹P) exist in both cases, but a correlation with the coordination chemical shift, Δδ(³¹P), exists for complexes of PPh₂R, and not for complexes of P{C₆H₄X}₃. Complexes of PR₃ ligands show no correlation between the peak potentials measured for the Pt(II) → Pt(0) reduction and electronic or spectroscopic parameters, except possibly ¹J(¹⁹⁵Pt, ³¹P).     

Phosphino-aminothiazoline platinum(II) and platinum(II)/gold(I) complexes: structural, chemical and vapoluminescent properties

Phosphino-amino-thiazolines and -thiazoles can exist in solution in two tautomeric forms, in which the N-H proton involves the endo-cyclic or exo-cyclic nitrogen atom. The two tautomers show different reactivities toward alcoholysis; the imino form degrades more rapidly. Their bischelated platinum complexes were studied in the solid state by single crystal X-ray diffraction. Thus, the unique stereoelectronic features of the [Pt(PN(th))] (PN(th)=diphenylposphino-aminothiazoline) moiety were revealed. The complex cis-[Pt(PN(th))(2)] reacts with gold(I) salts to yield dimetallic compounds, the molecular structures of which have been determined by X-ray diffraction. Solid cis-[Pt(PN(th))(2)] shows vapoluminescent properties if exposed to alcohol vapors. A combined photophysical and crystallographic investigation has been carried out to clarify the unprecedented rigidochromic role of the alcohol in this phenomenon.     

Synthesis and structural studies of binuclear platinum(II) complexes with a novel phosphorus-nitrogen-phosphorus ligand

The new pyridinediphosphinite ligand PONOP (1) was synthesized in one step from 2,6-pyridinedimethanol and diphenylchlorophosphane. Reaction of 1 with PtCl(2)(PhCN)(2) led to the neutral homobimetallic complex [Pt(2)Cl(4)(PONOP)(2)] (2), where the benzonitriles have been substituted by the phosphorus atoms of 1. The X-ray structure of 2 revealed a metallamacrocycle where the pyridines remained free. Addition of 2 equiv of [Cu(MeCN)(4)](BF(4)) to 2 led to CuCl and to the binuclear dicationic complex [Pt(2)Cl(2)(PONOP)(2)](BF(4))(2) (3), where chloro ligands have been substituted by pyridine groups. Conversely, reaction of 3 with chloride anions gave back complex 2. Solid-state (X-ray) and solution (NMR) studies indicated that after the transformation of 2 into 3, the platinum centers were brought much closer and the pyridinediphosphinite ligand was stiffened. The methylene NMR protons of 3 were strongly deshielded, and the corresponding proton-phosphorus coupling constants followed a Karplus-type relationship.     

Synthesis and structural characterization of palladium(II) and platinum(II) complexes containing a chiral P,N-donor iminophosphine ligand

A new chiral P,N-donor iminophosphine ligand was used to synthesize a series of palladium(II) and platinum(II) complexes. Single crystal X-ray analysis showed that in all cases the P,N ligand forms a six-membered chelate ring, and the square-planar coordination of the metal atom is completed by two halide ions, two methyl groups, or one methyl group and one halide ion. In the latter instance, the methyl group is cis to the phosphorus atom.     

Kinetic and thermodynamic preferences in aryl vs benzylic C-H bond activation with cationic Pt(II) complexes

Anhydrous cationic Pt(II) complexes [(NN)Pt(CH3)(CF3CD2OD)]+ (1, NN = ArN=C(Me)-C(Me)=NAr), which are obtained by reaction of (NN)Pt(CH3)2 with B(C6F5)3 in CF3CD2OD, activate C-H bonds of benzene and methylbenzenes, with enhanced reactivity compared to the previously prepared equilibrium mixtures with the (thermodynamically favored) aquo complexes. For methylbenzenes (toluene, p-xylene, mesitylene), activation at the aromatic and benzylic positions are kinetically competitive, but the product of the latter is strongly favored thermodynamically. This unusual trend is attributed to formation of eta3-benzyl structures, which can be observed spectroscopically for 1,4-diethylbenzene activation.     

Hemilability and structural considerations in complexes of platinum(II) comprising bis(diphenylphosphanyl)methane monoxide,monosulfide, and monoselenide

The structure of a platinum(II) complex containing (R)-(dimethylamino)ethylnapthyl and bis(diphenylphosphanyl)methane monosulfide ligands, namely, {(R)-1-[1-(dimethylamino)ethyl]napthyl-κ²N,C²}[(diphenylphosphanylmethyl)diphenylphosphine sulfide-κ²P,S]platinum(II) hexafluoridoantimonate dichloromethane monosolvate, [Pt(C₁₄H₁₆N)(C₂₅H₂₂P₂S)][SbF₆]·CH₂Cl₂, was determined. The structural features are compared with analogous platinum bis(diphenylphosphanyl)methane monoxide [dppm(O)] and bis(diphenylphosphanyl)methane monoselenide [dppm(Se)] complexes in relation to their potential hemilability and stereochemical nonrigidity.     

Thermal reactivity of cis- and trans-bis(triphenylgermyl)bis(tertiary phosphine)platinum(II)

The complex cis-Pt(Ph₃Ge)₂(PMe₂Ph)₂ underwent smooth isomerization to give the trans-isomer at room temperature via an associative five-coordinated intermediate. Thermodynamic parameters and activation energy for the cis to trans isomerization were obtained, ΔH^# = 105 kJ mol⁻¹, ΔS^# = 12.5 J mol⁻¹ K⁻¹, and Ea = 107 kJ mol⁻¹, respectively. Heating of trans-Pt(Ph₃Ge)₂(PMe₂Ph)₂ at 50 °C for 36 days produced trans-PtPh(Ph₃Ge)(PMe₂Ph)₂ followed by the formation of trans-PtPh₂(PMe₂Ph)₂, Pt(PMe₂Ph)₄, and Ph₄Ge finally via elimination of the phenyl group from Ph₃Ge ligand with liberation of the Ph₂Ge unit and subsequent reductive elimination of the remaining Ph₃Ge ligand at 80 °C for 1 month.     

Methyl- and phenyl-bis(tertiary phosphine) N-bonded carboxamido complexes of platinum(II)

Methyl- or phenylN-carboxamido-complexes of platinum(II) Pt(NHCOR')RL₂ (L = PEt₃, R = Me, R′ = Me, CH = CH₂; L = PEt₃, R = Ph, R′ = Me; L = PMe₂Ph, R = Ph, R′ = Me, Ph; L = PMePh₂, R = Ph, R′ =₃, R = Ph, R′ = Me) have been prepared by the reaction of KOH with cationic nitrile complexes [PtR(NCR′)L₂]BF₄. Thermally unstable hydrido-N-carboxamido-complexes could be detected spectroscopically. IR and NMR (¹H, ³¹P) spectra of some of the complexes indicate the existence of a solvent- and temperature-dependent equilibrium between syn-and anti-isomers arising from restricted rotation about the NC bond of the carboxamido-group. The anti-isomer is favoured by nonpolar solvents and by increasing bulk of L. In the complex [PtH(NCCH CH₂)(PEt₃)₂]BF₄, IR and NMR spectra show acrlonitrile to be bound through nitrogen, not through the olefinic CC bond.