Nitrile-amidine coupling at Pt(IV) and Pt(II) centers. An easy entry to imidoylamidine complexes

Treatment of trans-[PtCl4(RCN)2] (R = Me, Et, Ph, NEt2) with 2 equiv of the amidine PhC(=NH)NHPh in a suspension of MeCN (R = Me), CHCl3 (R = Et, Ph), or in CHCl3 solution (R = NEt2) results in the formation of the imidoylamidine complexes trans-[PtCl4{NH=C(R)N=C(Ph)NHPh}2] (1-4) isolated in good yields (66-84%). The reaction of soluble complexes 3 and 4 with 2 equiv of Ph3P=CHCO2Me in CH2Cl2 (40 degrees C, 5 h) leads to dehydrochlorination resulting in a chelate ring closure to furnish the platinum(IV) chelates [PtCl2{NH=C(R)NC(Ph)=NPh}2] (R = Ph, 5; R = NEt2, 6), accordingly, and the phosphonium salt [Ph3PCH2CO2Me]Cl. Treatment of 5 with 3 equiv of Ph3P=CHCO2Me at 50 degrees C for 5 d resulted in only a 30% conversion to the corresponding Pt(II) complex [Pt{NH=C(NEt2)NC(Ph)=NPh}2] (15). The reduction can be achieved within several minutes, when Ph2PCH2CH2PPh2 in CDCl3 is used. When the platinum(II) complex trans-[PtCl2(RCN)2] is reacted with 2 equiv of the amidine, the imidoylamidinato complexes [PtCl(RCN){NH=C(R)NC(Ph)=NHPh}] (8-11) and [PhC(=NH)NHPh] x HCl (7) are formed. The reaction of trans-[PtCl2(RCN)2] with 4 equiv of the amidine under a prolonged reaction time or treatment of [PtCl(RCN){NH=C(R)NC(Ph)=NHPh}] (8-11) with 2 more equiv of the amidine yields the complex bearing two chelate rings [Pt{NH=C(R)NC(Ph)=NHPh}2] (12-15). The treatment of cis-[PtCl2(RCN)2] (R = Me, Et) with the amidine gives ca. 50-60% yield of [PtCl2{NH=C(R)NHC(Ph)=NHPh}] (16 and 17). All of the platinum compounds were characterized by elemental analyses; FAB mass spectrometry; IR spectroscopy; 1H, 13C{1H}, and 195Pt NMR spectroscopies, and four of them (4, 6, 8, and 15) were also characterized by X-ray crystallography. The coupling of the Pt-bound nitriles and the amidine is metal-mediated insofar as RCN and PhC(=NH)NHPh do not react in the absence of the metal centers in conditions more drastic than those of the observed reactions. The nitrile-amidine coupling reported in this work constitutes a route to the synthesis of imidoylamidine complexes, some of them exhibiting luminescent properties.     

Kinetic and mechanistic study of the mercury(II)-catalyzed substitution of cyanide in hexacyanoruthenate(II) by pyrazine

The kinetics of mercury(II)-catalyzed substitution of cyanide ligand in hexacyanoruthenate(II) by pyrazine (Pz) has been investigated spectrophotometrically at 370 nm in aqueous medium. The reaction exhibits first-order dependence on [Pz] at low concentrations, then reaches a maximum value, and finally decreases at high [Pz]. The reaction has a variable-order dependence in [Ru(CN)₆ ⁴⁻], unity at lower [Ru(CN)₆ ⁴⁻], and fractional order, not tending to zero order at higher [Ru(CN)₆ ⁴⁻]. The effects of pH, ionic strength, concentration of catalyst, and temperature variations have been studied. The activation parameters for the reaction were calculated. We propose a solvent assisted interchange dissociative (I _d) mechanism for the reaction.     

Complexes of Pt(II) and Pt(IV) with disubstituted purines

Mixed-ligand complexes of Pt(II) and Pt(IV) with 2,6-diaminopurine and 6-thioguanine were synthesized and characterised. The complexes were prepared in acidic and basic media. The binding of the ligands to the metal ion varies according to the pH of the medium. Thus, in the complexes of 6-thioguanine, the ligand acts as a monodentate ligand coordinating through the neutral C₆-SH group in the acidic medium and in the basic medium as a bidentate ligand binding to the metal ion through C₆S^? and N₇, forming a five-membered chelate ring. In an acidic medium 2,6-diaminopurine forms mononuclear complexes with Pt(II) and Pt(IV) binding through N₇. In a basic medium binuclear hydroxobridged complexes are formed with Pt(IV) and the ligand is monodentate, coordinating through N₇.     

Hydrosilylation of Siloxanes in the Presence of Pt(II) and Pt(IV) Complexes

The catalytic activities of Pt(II) and Pt(IV) complexes with various ligands and counterions in hydrosilylation of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane with 1,1,3,3-tetramethyldisiloxanes are compared.     

First electrogeneration of a platinum(IV) porphyrin: elucidation of the Pt(II/IV) and Pt(IV/II) oxidation-reduction processes in nonaqueous media

The first example for electrogeneration of a Pt(IV) porphyrin from its Pt(II) form is presented and the Pt(II/IV) and reverse Pt(IV/II) oxidation-reduction processes are elucidated by electrochemistry and thin-layer UV-visible spectroelectrochemistry. Three products, [(TPP˙(+))Pt(II)](+), [(TPP)Pt(IV)](2+) and [(TPP˙(+))Pt(IV)](3+), produced by electrooxidation of the Pt(II) porphyrin have been characterized by in situ spectroelectrochemistry and ESR measurements after controlled potential bulk electrolysis. The first definitive evidence for the electrochemical conversion of a Pt(iv) porphyrin to its Pt(II) form is also presented. The potential for this electroreduction is highly dependent upon the nature of the anion, ClO(4)(-) or Cl(-). A mechanism for the reversible conversion between Pt(II) and Pt(IV) tetraphenylporphyrins is proposed.     

Kinetic and mechanistic aspects of metal ion catalysis in cerium(IV) oxidation

This review discusses the kinetic aspects of different metal ion catalysis in Ce(IV) oxidation of different types of organic and inorganic substrates in aqueous acid media. The reactions have been categorised with the metal ions acting as the catalysts. The nature of mechanism of catalysis in Ce(IV) oxidation depends on the nature of substrate for a particular metal ion catalyst and it also largely depends on the nature of acid medium used. The utility and scope of the catalytic oxidation by Ce(IV) in analytical chemistry are discussed.     

Synthesis and characterisation of mixed ligand Pt(II) and Pt(IV) oxadiazoline complexes

The nitrile ligands in trans-[PtX2(PhCN)2] (X = Cl, Br, I) undergo sequential 1,3 dipolar cycloadditions with nitrones R1R2C=N+(Me)-O(-) (R1 = H, R2 = Ph; R1 = CO2Et, R2 = CH2CO2Et) to selectively form the Delta4-1,2,4-oxadiazoline complexes trans-[PtX2(PhCN) (N=C(Ph)-O-N(Me)-CR1R2)] or trans-[PtX2(N=C(Ph)-O-N(Me)-CR1R2)2] in high yields. The reactivity of the mixed ligand complexes trans-[PtX2(PhCN)(N=C(Ph)-O-N(Me)-CR1R2)] towards oxidation and ligand substitution was studied in more detail. Oxidation with Cl2 or Br2 provides the Pt(IV) species trans-[PtX2Y2(PhCN)(N=C(Ph)-O-N(Me)-CH(Ph))] (X, Y = Cl, Br). The mixed halide complex (X = Cl, Y = Br) undergoes halide scrambling in solution to form trans-[PtX(4-n)Yn(PhCN)(N=C(Ph)-O-N(Me)-CH(Ph))] as a statistical mixture. Ligand substitution in trans-[PtCl2(PhCN)(N=C(Ph)-O-N(Me)-CR1R2)] allows for selective replacement of the coordinated nitrile by nitrogen heterocycles such as pyridine, DMAP or 1-benzyl-2-methylimidazole to produce mixed ligand Pt(II) complexes of the type trans- [PtX2(heterocycle)(N=C(Ph)-O-N(Me)-CR1R2)]. All compounds were characterised by elemental analysis, mass spectrometry, IR and 1H, 13C and 195Pt NMR spectroscopy. Single-crystal X-ray structural analysis of (R,S)-trans-[PtBr2(N=C(Ph)-O-N(Me)-CH(Ph))2] and trans-[PtCl2(C5H5N)(N=C(Ph)-O-N(Me)-CH(Ph))] confirms the molecular structure and the trans configuration of the heterocycles relative to each other.     

Kinetic and mechanistic study of the reactions of palladium(II) chlorocomplexes with ethylenediamine in aqueous acid medium

The complexes PdCl₄^2?, PdCl₃(H₂O)^?, and PdCl₃(Ac)^2?, in rapid equilibrium with each other under the adopted experimental conditions, react with ethylenediamine according to the experimental rate law A reaction scheme is proposed involving the reaction of enH⁺ with each of the above species, and the specific rate constants are computed. The activation parameters are given.     

Synthesis of acetimino complexes of Pt(II) and Pt(IV) and the first heteronuclear mu-acetimido complexes

The reactions of [Ag(NH=CMe2)2]ClO4 with cis-[PtCl2L2] in a 1:1 molar ratio give cis-[PtCl(NH=CMe2)(PPh3)2]ClO4 (1cis) or cis-[PtCl(NH=CMe2)2(dmso)]ClO4 (2), and in 2:1 molar ratio, they produce [Pt(NH=CMe2)2L2](ClO4)2 [L = PPh3 (3), L2= tbbpy = 4,4'-di-tert-butyl-2,2'-dipyridyl (4)]. Complex 2 reacts with PPh3 (1:2) to give trans-[PtCl(NH=CMe2)(PPh3)2]ClO(4) (1trans). The two-step reaction of cis-[PtCl2(dmso)2], [Au(NH=CMe2)(PPh3)]ClO4, and PPh3 (1:1:1) gives [SP-4-3]-[PtCl(NH=CMe2)(dmso)(PPh3)]ClO4 (5). The reactions of complexes 2 and 4 with PhICl2 give the Pt(IV) derivatives [OC-6-13]-[PtCl3(NH=CMe2)(2)(dmso)]ClO4 (6) and [OC-6-13]-[PtCl2(NH=CMe2)2(dtbbpy)](ClO4)2 (7), respectively. Complexes 1cis and 1trans react with NaH and [AuCl(PPh3)] (1:10:1.2) to give cis- and trans-[PtCl{mu-N(AuPPh3)=CMe2}(PPh3)2]ClO4 (8cis and 8trans), respectively. The crystal structures of 4.0.5Et2O.0.5Me2CO and 6 have been determined; both exhibit pseudosymmetry.     

Synthesis and Spectral Properties of Ni(II), Pd(II), Pt(II), and Pt(IV) Tetraphenyltetrabenzoporphyrinates

Complexation reactions of 5,10,15,20-tetraphenyltetrabenzoporphyrin and transmetallation of its cadmium complex with nickel(II) acetate, Ni(II), Pd(II), and Pt(II) chlorides in dimethylformamide and phenol have been studied. The corresponding Ni(II), Pd(II), and Pt(II) porphyrinates have been synthesized. Pt^IVBr₂ porphyrinate has been obtained by the treatment of Pt(II) 5,10,15,20-tetraphenyltetrabenzoporphyrinate with bromine in chloroform. The obtained compounds have been characterized by elemental analysis, electronic absorption and ¹H NMR spectroscopy and mass spectrometry.     

Concerted reductive coupling of an alkyl chloride at Pt(IV)

Oxidation of a doubly cyclometallated platinum(II) complex results in two isomeric platinum(IV) complexes. Whereas the trans isomer is robust, being manipulable in air at room temperature, the cis isomer decomposes at -20 °C and above. Reductive coupling of an alkyl chloride at the cis isomer gives a new species which can be reoxidised. The independence of this coupling on additional halide rules out the reverse of an S(N)2 reaction, leaving a concerted process as the only sensible reaction pathway.     

Amidines derived from Pt(IV)-mediated nitrile-amino alcohol coupling and their Zn(II)-catalyzed conversion into oxazolines

The reaction between the platinum(IV) complex trans-[PtCl(4)(EtCN)(2)] and the amino alcohols NH(2)CH(2)CH(2)OH, NH(2)CH(2)CH(Me)OH-(R)-(-), NH(2)CH(Ph)CH(2)OH-(R)-(-), NH(2)CH(Et)CH(2)OH-(R)-(-), NH(2)CH(Et)CH(2)OH-(S)-(+), and NH(2)CH(Pr(n)())CH(2)OH proceeds rapidly at room temperature in CH(2)Cl(2) to furnish the amidine complexes [PtCl(4)(HN=C(Et)NH(arcraise;)OH)(2)] (1-6) in good yield (70-80%). The related reaction between the platinum(II) complex trans-[PtCl(2)(EtCN)(2)] and monoethanolamine in a molar ratio of 1:2 in CH(2)Cl(2) results in the addition of 4 equiv of NH(2)CH(2)CH(2)OH per mole of complex to give [Pt(HN=C(Et)NHCH(2)CH(2)OH)(2)(NH(2)CH(2)CH(2)OH)(2)](2+) (7). Formulation of 1-6 is based upon satisfactory C, H, N elemental analyses, electrospray mass spectrometry, IR spectroscopy, and (1)H, (13)C((1)H), (15)N, and (195)Pt NMR spectroscopies, while the structures of trans-[PtCl(4)((Z)-NH=C(Et)NHCH(2)CH(2)OH)(2)] (1), trans-[PtCl(4)((Z)-NH=C(Et)NHCH(2)CH(Me)OH-(R)-(-))(2)] (2), and trans-[PtCl(4)((Z)-NH=C(Et)NHCH(Et)CH(2)OH-(R)-(-))(2)] (4) were determined by X-ray single-crystal diffraction. The Z-amidine configuration of the ligands is preserved in CDCl(3) solutions as confirmed by gradient-enhanced (15)N,(1)H-HMQC spectroscopy and NOE experiments. The amidines, formed upon Pt(IV)-mediated nitrile-amino alcohol coupling, were liberated from their platinum(IV) complexes 1, 3, and 4 by reaction with Ph(2)PCH(2)CH(2)PPh(2) (dppe) giving free NH=C(Et)NHCHRCH(2)OH (R = H 8, Et 9, Ph 10), with the substituents R of different types, and dppe oxides; the P-containing species were identified by (31)P((1)H) NMR spectroscopy. NOESY spectroscopy indicates that the liberated amidines retained the same configuration relative to the C=N double bond, i.e., syn-(H,Et)-NH=C(Et)NHCHRCH(2)OH. The liberated hydroxo-functionalized amidines 8-10 were converted into oxazolines (11-13) in the presence of a catalytic amount of ZnCl(2). A similar catalytic effect has also been reached using anhydrous MSO(4) (M = Cu, Co, Cd), CdCl(2), and AlCl(3).     

Pt(II)- and Pt(IV)-bridged cofacial diporphyrins via carbon-transition metal sigma-bonds

A directly Pt(IV)-bridged cofacial diporphyrin has been synthesized by the cyclometalation reaction of beta-pyridylporphyrin with a Pt(IV) salt. Upon treatment with methylhydrazine, the Pt(IV) bridge is reduced to the Pt(II) center, resulting in a Pt(II)-bridged cofacial dimer with a helicity inversion of the complex as well as change in electronic communication through the metal bridge.     

Reductive elimination/oxidative addition of carbon-hydrogen bonds at Pt(IV)/Pt(II) centers: mechanistic studies of the solution thermolyses of Tp(Me2)Pt(CH3)2H

Reductive elimination of methane occurs upon solution thermolysis of kappa(3)-Tp(Me)2Pt(IV)(CH(3))(2)H (1, Tp(Me)2 = hydridotris(3,5-dimethylpyrazolyl)borate). The platinum product of this reaction is determined by the solvent. C-D bond activation occurs after methane elimination in benzene-d(6), to yield kappa(3)-Tp(Me)2Pt(IV)(CH(3))(C(6)D(5))D (2-d(6)), which undergoes a second reductive elimination/oxidative addition reaction to yield isotopically labeled methane and kappa(3)-Tp(Me)2Pt(IV)(C(6)D(5))(2)D (3-d(11)). In contrast, kappa(2)-Tp(Me)2Pt(II)(CH(3))(NCCD(3)) (4) was obtained in the presence of acetonitrile-d(3), after elimination of methane from 1. Reductive elimination of methane from these Pt(IV) complexes follows first-order kinetics, and the observed reaction rates are nearly independent of solvent. Virtually identical activation parameters (DeltaH(++)(obs) = 35.0 +/- 1.1 kcal/mol, DeltaS(++)(obs) = 13 +/- 3 eu) were measured for the reductive elimination of methane from 1 in both benzene-d(6) and toluene-d(8). A lower energy process (DeltaH(++)(scr) = 26 +/- 1 kcal/mol, DeltaS(++)(scr) = 1 +/- 4 eu) scrambles hydrogen atoms of 1 between the methyl and hydride positions, as confirmed by monitoring the equilibration of kappa(3)-Tp(Me)()2Pt(IV)(CH(3))(2)D (1-d(1)()) with its scrambled isotopomer, kappa(3)-Tp(Me)2Pt(IV)(CH(3))(CH(2)D)H (1-d(1'). The sigma-methane complex kappa(2)-Tp(Me)2Pt(II)(CH(3))(CH(4)) is proposed as a common intermediate in both the scrambling and reductive elimination processes. Kinetic results are consistent with rate-determining dissociative loss of methane from this intermediate to produce the coordinatively unsaturated intermediate [Tp(Me)2Pt(II)(CH(3))], which reacts rapidly with solvent. The difference in activation enthalpies for the H/D scrambling and C-H reductive elimination provides a lower limit for the binding enthalpy of methane to [Tp(Me)2Pt(II)(CH(3))] of 9 +/- 2 kcal/mol.     

Aryl-halide versus aryl-aryl reductive elimination in Pt(IV)-phosphine complexes

Upon the addition of Br2 to complexes (P-P)Pt(Ar)2, two different products were observed, depending on the bite angle of the bidentate phosphine ligand: a Pt(II) aryl bromide complex, the product of C-Br reductive elimination, and Pt(IV) oxidative addition complex. At high temperatures, the latter exclusively gave the product of the C-C reductive elimination.     

Reaction of sterically hindered arylmercury compounds with Pt(IV), Pt(II), and Pd(II) chloride complexes

Conclusion Heating of a solution of the PtCl₆ ^2– ion and an arylmercury compound containing a substituent in the ortho position to mercury leads to the formation of a diaryl (in the case of -naphthylmercury) or arene (in the case of the mesityl or pentamethylphenyl mercury derivatives).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2374–2376, October, 1986.The authors express their gratitude to A. E. Shilov and A. K. Yatsimirskii for a useful discussion of these results.     

Kinetic and mechanistic study with optically active, four-coordinate nickel(II) complexes: stereoselectivity in ligand substitution

Conventional and rapid scan stopped-flow spectrophotometry as well as polarimetry was used to study the kinetics of ligand substitution in six chiral bis N-alkylsalicylaldiminato nickel(II) complexes NiA2 by different chiral salen-type ligands H2B, according to NiA2 + H2B --> NiB + 2HA, in acetone at 298 K and, partly, at variable temperature. In most cases ligand substitution was found to follow monophasic second-order kinetics, rate = k x [NiA2] x [H2B]. Second-order rate constant k, lying in the range 10(-2)-400 M(-1) s(-1) at 298 K, was determined for the various combinations of enantiomers in a given system NiA2/H2B, namely, R-NiA2/R-H2B, S-NiA2/R-H2B, R-NiA2/S-H2B, and S-NiA2/S-H2B. It was found that ligand substitution is subject to chiral discrimination. The ratio of second-order rate constants, kfast/kslow, with kfast being rate constant k for the faster reacting pair of enantiomers and vice versa, lies in the range 1.0-3.0, depending on the nature of the N-alkyl groups in NiA2 and organic groups attached to the ethylene bridge in the salen ligands H2B. The rate discrimination factor of 3.0, as obtained for NiA2 = bis[N-dehydroabietylsalicylaldiminato]nickel(II) reacting with the R- and with the S-enantiomer of H2B = N,N'-disalicylidene-1,2-diamino-4-methylpentane, appears to be the highest stereoselectivity reported so far for ligand substitution in nickel(II) complexes. With NiA2 = R- and S-bis[N-(1-phenylethyl)-5-nitrosalicylaldiminato]nickel(II) and H2B = R- and S-N,N-disalicylidene-1,2-diamino-4-methylpentane, the kinetics of ligand substitution are biphasic, describing initial adduct formation between NiA2 and H2B (equilibrium constant K) and stepwise loss of the two bidentate ligands HA (first-order rate constants k1 and k2). The data for K, k1, and k2 for one of the combinations of enantiomers were determined at variable temperature, and the corresponding activation parameters are presented.     

Reduction of (imine)Pt(IV) to (imine)Pt(II) complexes with carbonyl-stabilized phosphorus ylides

A novel method is reported for generation of the difficult-to-obtain (imine)Pt(II) compounds that involves reduction of the corresponding readily available Pt(IV)-based imines by carbonyl-stabilized phosphorus ylides, Ph3P=CHCO2R, in nonaqueous media. The reaction between neutral (imino)Pt(IV) compounds [PtCl4[NH=C(Me)ON=CR1R2]2] [R1R2 = Me2, (CH2)4, (CH2)5, (Me)C(Me)=NOH], [PtCl4[NH=C(Me)ONR2]2] (R = Me, Et, CH2Ph), (R1 = H; R2 = Ph or C6H4Me; R3 = Me) as well as anionic-type platinum(IV) complexes (Ph3PCH2Ph)[PtCl5[NH=C(Me)ON=CR2]] [R2 = Me2, (CH2)4, (CH2)5] and 1 equiv of Ph3P=CHCO2R (R = Me, Et) proceeds under mild conditions (ca. 4 h, room temperature) to give selectively the platinum(II) products (in good to excellent isolated yields) without further reduction of the platinum center. All thus prepared compounds (excluding previously described Delta4-1,2,4-oxadiazoline complexes) were characterized by elemental analyses, FAB mass spectrometry, IR and 1H, 13C[1H], 31P[1H] and 195Pt NMR spectroscopies, and X-ray single-crystal diffractometry, the latter for [PtCl2[NH=C(Me)ON=CMe2]2] [crystal system tetragonal, space group P4(2)/n (No. 86), a = b = 10.5050(10) A, c = 15.916(3) A] and (Ph3PCH2CO2Me)[PtCl3(NCMe)] [crystal system orthorhombic, space group Pna2(1) (No. 33), a = 19.661(7) A, b = 12.486(4) A, c = 10.149(3) A]. The reaction is also extended to a variety of other Pt(II)/Pt(IV) couples, and the ylides Ph3P=CHCO2R are introduced as mild and selective reducing agents of wide applicability for the conversion of Pt(IV) to Pt(II) species in nonaqueous media, a route that is especially useful in the case of compounds that cannot be prepared directly from Pt(II) precursors, and for the generation of systematic series of Pt(II)/Pt(IV) complexes for biological studies.     

Kinetic study of the reaction betweentrans-tetracyanodioxomolybdate(IV) and fluoride ions

Summary The kinetics of the reaction between [MoO₂(CN)₄]^4– and F^– have been studied in the pH range 8 to 11. The results indicated that the diprotonated form, [MoO(OH₂)(CN)₄]^2–, is the only reactive species and that the aqua-ligand is substituted by the F^– ion according to the following reaction. The k₁ and k_–1 values are 8.8(2) M^–1 s^–1 and 0.6(1)s^–1, respectively, at 15°C. A dissociative substitution process is proposed.