Five-coordinate Intermediates Research Articles

Oxidative C–C/C–X coupling in organometallic nickel complexes: insights from DFT

NiIII and NiIV-center complexes prefer direct reductive elimination than reacting through five-coordinate intermediates. 32+ complex in the presence of Cl− undergoes Cl–Csp2 elimination preferably over Cl–Csp3 and Csp3–Csp2 elimination.

Selectivity in carbon–carbon coupling reactions at palladium(IV) and platinum(IV)

The isomerization and reductive elimination reactions from octahedral organometallic complexes of palladium(IV) and platinum(IV) usually occur through five-coordinate intermediates that cannot be directly detected. This paper reports a computational study of five-coordinate complexes of formulae [PtMe3(bipy)]+, [PtMe2Ph(bipy)]+, and [PtMe(CH2CMe2C6H4)(bipy)]+ (M = Pd or Pt, bipy = 2,2′-bipyridine), particularly with respect to reactivity and selectivity in reductive elimination. All of the complexes are predicted to have square pyramidal structures with the bipy and two R groups in the equatorial positions and one R group in the axial position, and axial–equatorial exchange occurs by a pairwise mechanism, with the transition state having a pinched trigonal bipyramidal (PTBP) stereochemistry, with one nitrogen and two R groups in the trigonal plane. The activation energy for isomerization is lower than that for reductive elimination in all cases. For the complexes [MMe2Ph(bipy)]+, the activation energies for reductive elimination with Me–Me or Me–Ph coupling are similar. For the complexes [MMe(CH2CMe2C6H4)(bipy)]+, the reductive elimination with Me–C6H4 bond formation from the isomer with the methyl group in the axial position is predicted and is attributed to it having the best conformation of the Me and C6H4 groups for C–C bond formation. In all cases, the selectivity for reductive elimination is similar for M = Pd or Pt, but reactivity is higher for M = Pd. The relevance of this work to selectivity in catalysis is discussed.

Heterocycles meet and marry on phosphorus

Organic Chemistry Metals such as palladium are routinely used to link together carbon rings in pharmaceutical synthesis. However, the presence of nitrogen in both rings can trip up this process. Hilton et al. report a versatile alternative process in which phosphorus takes the place of the metal. The phosphorus binds successively to both rings at the sites opposite the nitrogen, and treatment with acidic ethanol then pushes them off, bound to each other. Theory implicates a five-coordinate phosphorus intermediate that kinetically favors coupling of the two nitrogen-bearing rings over reactions of the other all-carbon substituents. Science , this issue p. [799][1] [1]: /lookup/doi/10.1126/science.aas8961

Stereoretentive Ligand Exchange Reactions of N-Fused Porphyrin Ruthenium(II) Complexes.

The ligand exchange reactions of the ruthenium(II) complex of N-fused tetraphenylporphyrin, Ru(NFp)(CO)2Cl (2), with various anions were investigated. The chloride ligand of the isomers 2a-c was stereoretentively exchanged with bromide (Br-), iodide (I-), and acetate (AcO-) anions in toluene at 100 °C, structures of which were confirmed by 1H NMR as well as single crystal X-ray diffraction analysis. The silver (AgOAc, AgOTf) and boron (NaBPh4) reagents also afforded the corresponding stereoretentive products. On the other hand, the reaction with NaBH4 afforded the hydride complex Ru(NFp)(CO)2H (7) with low stereospecificity, showing a higher reactivity of 2c than other isomers. The ligand dissociation mechanism was proposed with the help of theoretical calculations on the plausible five-coordinated intermediates.

Quantum Mechanical Study on the Rate Determining Steps of the Reaction between 2-aminopyrimidine with Dichloro-[1-methyl-2-(naphthylazo) Imidazole] Palladium(II) Complex

In this work, using quantum mechanics, the mechanism of reaction between 2-aminopyrimidine with dichloro-[1-methyl-2-(naphthylazo) imidazole] palladium(II) complex was investigated. The reaction produces two five-coordinated intermediates which are converted into final products through two possible pathways (nucleophile-dependent pathway and solvent-dependent pathway). The activation energy and activation Gibbs free energy of two pathways have been calculated and compared with each other and experimental results. These activation energies for the solvent-dependent pathway are greater in comparison with the nucleophile-dependent pathway which demonstrates that the latter is the dominating route and makes the main contribution to the reaction. Considering solvent effects, the activation Gibbs free energy for the rate determining step of the nucleophile-dependent pathway was calculated, which was in good agreement with the experimental value. Although the theoretical activation energy shows a suitable behaviour qualitatively, it is not fully consistent with the experimental value. All of the calculations were performed using a hybrid density functional method (B3LYP) in the solution phase (PCM model).

Mechanistic Considerations for C–C Bond Reductive Coupling at a Cobalt(III) Center

The diamagnetic cobalt(III) dimethyl complex, cis,mer-(PMe(3))(3)Co(CH(3))(2)I, was found to promote selective C-C bond formation, affording ethane and triplet (PMe(3))(3)CoI. The mechanism of reductive elimination has been investigated by a series of kinetic and isotopic-labeling experiments. Ethane formation proceeds with a rate constant of 3.1(5) × 10(-5) s(-1) (50 °C) and activation parameters of ΔH(double dagger) = 31.4(8) kcal/mol and ΔS(double dagger) = 17(3) eu. Addition of free trimethylphosphine or coordinating solvent strongly inhibits reductive elimination, indicating reversible phosphine dissociation prior to C-C bond-coupling. EXSY NMR analysis established a rate constant of 9(2) s(-1) for phosphine loss from cis,mer-(PMe(3))(3)Co(CH(3))(2)I. Radical trapping, crossover, and isotope effect experiments were consistent with a proposed mechanism for ethane extrusion where formation of an unobserved five-coordinate intermediate is followed by concerted C-C bond formation. An unusual intermolecular exchange of cobalt-methyl ligands was also observed by isotopic labeling.

Molybdenum-Catalyzed Transformation of Molecular Dinitrogen into Silylamine: Experimental and DFT Study on the Remarkable Role of Ferrocenyldiphosphine Ligands

A molybdenum-dinitrogen complex bearing two ancillary ferrocenyldiphosphine ligands, trans-[Mo(N(2))(2)(depf)(2)] (depf = 1,1'-bis(diethylphosphino)ferrocene), catalyzes the conversion of molecular dinitrogen (N(2)) into silylamine (N(SiMe(3))(3)), which can be readily converted into NH(3) by acid treatment. The conversion has been achieved in the presence of Me(3)SiCl and Na at room temperature with a turnover number (TON) of 226 for the N(SiMe(3))(3) generation for 200 h. This TON is significantly improved relative to those ever reported by Hidai's group for mononuclear molybdenum complexes having monophosphine coligands [J. Am. Chem. Soc.1989, 111, 1939]. Density functional theory (DFT) calculations have been performed to figure out the mechanism of the catalytic N(2) conversion. On the basis of some pieces of experimental information, SiMe(3) radical is assumed to serve as an active species in the catalytic cycle. Calculated results also support that SiMe(3) radical is capable of working as an active species. The formation of five-coordinate intermediates, in which one of the N(2) ligands or one of the Mo-P bonds is dissociated, is essential in an early stage of the N(2) conversion. The SiMe(3) addition to a "hydrazido(2-)" intermediate having the NN(SiMe(3))(2) group will give a "hydrazido(1-)" intermediate having the (Me(3)Si)NN(SiMe(3))(2) group rather than a pair of a nitrido (≡N) intermediate and N(SiMe(3))(3). The N(SiMe(3))(3) generation would not occur at the Mo center but proceed after the (Me(3)Si)NN(SiMe(3))(2) group is released from the Mo center. The flexibility of the Mo-P bond between Mo and depf would play a vital role in the high catalysis of the Mo-Fe complex.

ChemInform Abstract: The Linkage Isomerism of Thiocyanate Bonded to Cobalt(III).

Abstract The structural differences between Co(III)-SCN and Co(III)-NCS bonding molecules are briefly reviewed as are attempts by Werner and his associates at the turn of the century to prepare (unsuccessfully) Co-SCN complexes. Work in the author's laboratory over a period of about 20 years is then reviewed. The thermal isomerisation of crystalline trans -[CO(NH 3 ) 4 ( 15 NH 3 )SCN](N 14 CS) 2 to give 40% trans -CoNCS 2+ , and 30% of each of cis -CoS 14 CN 2+ and cis -CoN 14 CS 2+ is discussed in terms of “frontside” intramolecular rearrangement to give the trans product, and “backside” attack of one specific lattice S 14 CN − to give both the cis 14 C-labelled products. In aqueous solution the thermal rearrangement of [Co(NH 3 ) 5 SCN] 2+ to give CoNCS 2+ also results in some 9% CoOH 2 3+ , and when carried out in the presence of S 14 CN − results in small amounts of CoS 14 CN 2+ and CoN 14 CS 2+ also being produced. The formation of these products is discussed in terms of reactive ion-pairs, with the Co 3+ ·SCN − ·S 14 CN − aq transition state incorporating small amounts of S 14 CN − and N 14 CS − in addition to OH 2 and NCS − . The Hg 2+ , Ag + and OH − catalysed reactions are also reviewed. Larger amounts of CoOH 2 3+ are produced in these reactions, but solvated anions (NO 3 − , ClO 4 − , CF 3 SO 3 − , N 3 − ) are also incorporated in addition to isomerised CoNCS 2+ . A transition state (or intermediate) containing pre-associated entering groups is required. There seems to be little discrimination between entering groups with stereochemical positioning and/or orientation of electron lone pairs being the most important factors in gaining entry. Very little activation for entry seems necessary, and the five-coordinate intermediate has no useful lifetime. Competition for entry only occurs between groups occupying adjacent stereochemical locations in the pre-associated reactant. A new mechanistic classification is suggested for substitution at a metal centre. This emphasises the timing of the breaking and making of ligand-metal bonds, and recognises the necessity for pre-association and organization of entering groups.

Ultrafast Ligand Exchange: Detection of a Pentacoordinate Ru(II) Intermediate and Product Formation

The ultrafast kinetics of ligand exchange of cis-[Ru(bpy)(2)(CH(3)CN)(2)](2+) were measured in H(2)O and CH(3)CN. The formation of the (3)MLCT excited-state and a five-coordinate intermediate are observed in both solvents within 2 ps after excitation (310 nm, fwhm approximately 300 fs). The (3)MLCT excited-state undergoes vibrational cooling (5-6 ps), then decays to regenerate the ground-state with a lifetime of approximately 50 ps. In CH(3)CN, ligand recombination takes place in 28 ps, while the formation of cis-[Ru(bpy)(2)(CH(3)CN)(H(2)O)](2+) in H(2)O takes place with tau = 77 ps.

Searching for Stable, Five-Coordinate Aquated Al(III) Species. Water Exchange Mechanism and Effect of pH

Density functional theory calculations have been performed for the water exchange mechanism of aquated Al(III). The effect of pH was considered by studying the exchange processes for [Al(H2O)6]3+ and its conjugated base, [Al(H2O)5OH]2+. Both complexes were found to exchange water in a dissociative way with activation energies (EA) of 15.9 and 10.2 kcal/mol, respectively. The influence of solvent molecules on the gas-phase cluster model was considered by the addition of up to four water molecules to the model system. The stabilizing effect of the solvent on the transition state decreases EA to 8.6 (hexa-aqua complex) and 7.6 (monohydroxo complex) kcal/mol, whereas EA for all hydroxo species is consistently significantly lower than those for the related aqua systems, which indicates a much faster water exchange rate. For the hydroxo complex, all calculated five-coordinate intermediates, nH2O.[Al(H2O)4(OH)]2+ (n = 1, 2, 3, 4, 5), are more stable than the corresponding six-coordinate reactants. Our results therefore suggest the presence of a stable five-coordinate species of aquated Al(III), namely, the [Al(H2O)4(OH)]2+ complex.

Protonolysis of Cationic Pt−C Bonds with Mild Acids: Can Ligand Torsional Effects Speed Associative Processes?

Tridentate phosphine pincer ligands facilitate the protonolysis of cationic Pt−CH3 compounds (to Pt dications and CH4) with the ammonium acid Ph2NH2·BF4. Large rate enhancements over nonpincer analogues (>50 000) are observed. A mechanism is proposed wherein ring strain present in the planar ground-state arrangement of three phosphine ligands is released in putative five-coordinate intermediates and transition states.

Thermodynamic and kinetic data for adduct formation, cis-trans isomerization and redox reactions of ML4 complexes: a case study with rhodium- and iridium-tropp complexes in d8, d9 and d10 valence electron configurations (tropp=dibenzotropylidene phosphanes).

The formation of adducts of the square-planar 16-electron complexes trans-[M(tropp(ph))(2)](+) and cis-[M(tropp(ph))(2)](+) (M=Rh, Ir; tropp(Ph)=5-diphenylphosphanyldibenzo[a,d]cycloheptene) with acetonitrile (acn) and Cl(-), and the redox chemistry of these complexes was investigated by various physical methods (NMR and UV-visible spectroscopy, square-wave voltammetry), in order to obtain some fundamental thermodynamic and kinetic data for these systems. A trans/cis isomerization cannot be detected for [M(tropp(ph))(2)](+) in non-coordinating solvents. However, both isomers are connected through equilibria of the type trans-[M(tropp(ph))(2)](+)+L<==>[ML(tropp(ph))(2)](n)<==>cis-[M(tropp(ph))(2)](+)+L, involving five-coordinate intermediates [ML(tropp(ph))(2)](n) (L=acn, n=+1; L=Cl(-), n=0). Values for K(d) (K(f)), that is, the dissociation (formation) equilibrium constant, and k(d) (k(f)), that is, the dissociation (formation) rate constant, were obtained. The formation reactions are fast, especially with the trans isomers (k(f)>1x10(5) m(-1) s(-1)). The reaction with the sterically more hindered cis isomers is at least one order of magnitude slower. The stability of the five-coordinate complexes [ML(tropp(ph))(2)](n) increases with Ir>Rh and Cl(-)>acn. The dissociation reaction has a pronounced influence on the square-wave (SW) voltammograms of trans/cis-[Ir(tropp(ph))(2)](+). With the help of the thermodynamic and kinetic data independently determined by other physical means, these reactions could be simulated and allowed the setting up of a reaction sequence. Examination of the data obtained showed that the trans/cis isomerization is a process with a low activation barrier for the four-coordinate 17-electron complexes [M(tropp(ph))(2)](0) and especially that a disproportionation reaction 2 trans/cis-[M(tropp(ph))(2)](0)-->[M(tropp(ph))(2)](+)+[M(tropp(ph))(2)](-) may be sufficiently fast to mask the true reactivity of the paramagnetic species, which are probably less reactive than their diamagnetic equilibrium partners.

Gas-Phase Ligand Exchange in a Square-Planar Rhodium(I) Complex Proceeding by Dissociative Exchange: ESI FT-ICR MS and DFT Evidence

Although the kinetics of square-planar d 8 ligand-substitution reactions have been explored extensively in solution, no such studies of gas-phase systems have been reported. Measurement of gas-phase ligand exchange offersthe advantage that the exchange mechanism is readily determined in the absence of solvent effects. The complex trans-Rh(PPh 3 ) 2 CO(4-picoline) + substitutes pyridine for its 4-picoline ligand readily in the gas phase but also loses CO during the reaction. The double-resonance experiments show that its primary substitution pathway proceeds dissociatively, unlike almost all solution ligand substitutions for Pt(II) and Pd(II). The density functional calculations reveal that the bond energies of Rh-4-picoline, Rh-pyridine, and Rh-CO are similar enough that CO loss might be expected. In support of the dissociative mechanism, the calculations also show five-coordinate intermediates for the associative pathway to be relatively high in energy.

Mechanisms of C-C and C-H alkane reductive eliminations from octahedral Pt(IV): reaction via five-coordinate intermediates or direct elimination?

The Pt(IV) complexes P(2)PtMe(3)R [P(2) = dppe (PPh(2)(CH(2))(2)PPh(2)), dppbz (o-PPh(2)(C(6)H(4))PPh(2)); R = Me, H] undergo reductive elimination reactions to form carbon-carbon or carbon-hydrogen bonds. Mechanistic studies have been carried out for both C-C and C-H coupling reactions and the reductive elimination reactions to form ethane and methane are directly compared. For C-C reductive elimination, the evidence supports a mechanism of initial phosphine chelate opening followed by C-C coupling from the resulting five-coordinate intermediate. In contrast, mechanistic studies on C-H reductive elimination support an unusual pathway at Pt(IV) of direct coupling without preliminary ligand loss. The complexes fac- P(2)PtMe(3)R (P(2) = dppe, R = Me, H; P(2) = dppbz, R = Me) have been characterized crystallographically. The Pt(IV) hydrides, fac-P(2)PtMe(3)H (P(2) = dppe, dppbz), are rare examples of stable phosphine ligated Pt(IV) alkyl hydride complexes.

Mechanism of ketone and alcohol formations from alkenes and alkynes on the head-to-head 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex.

Reactions of the head-to-head 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex having nonequivalent two platinum atoms, Pt(N(2)O(2)) and Pt(N(4)), with p-styrenesulfonate, 2-methyl-2-propene-1-sulfonate, 4-penten-1-ol, and 4-pentyn-1-ol were studied kinetically. Under the pseudo first-order reaction conditions that the concentration of the Pt(III) dinuclear complex is much smaller than that of olefin, a consecutive basically four-step reaction was observed: the olefin pi-coordinates preferentially to the Pt(N(2)O(2)) in the first step (step 1), followed by the second pi-coordination of another olefin molecule to the Pt(N(4)) (step 2). In the next step (step 3), the nucleophilic attack of water to the coordinated olefin triggers the pi-sigma bond conversion on the Pt(N(2)O(2)), and the second pi-bonding olefin molecule on the Pt(N(4)) is released. Finally, reductive elimination occurs to the alkyl group on the Pt(N(2)O(2)) to produce the alkyl compound (step 4). The first water substitution with olefin (step 1) occurs to the diaqua and aquahydroxo forms of the complex, whereas the second substitution (step 2) proceeds either on the coordinated OH(-) on the Pt(N(4)) (path a) or on the coordinatively unsaturated five-coordinate intermediate of the Pt(N(4)) (path b), in addition to the common substitution of H(2)O (path c). The reactions of p-styrenesulfonate and 2-methyl-2-propene-1-sulfonate proceed through paths b and c, whereas the reactions of 4-penten-1-ol and 4-pentyn-1-ol proceed through paths a and c. This difference reflects the difference of the trans effect and/or trans influence of the pi-coordinated olefins on the Pt(N(2)O(2)). The pentacoordinate state in path b is employed only by the sulfo-olefins, because these exert stronger trans effect. The steps 3 and 4 reflect the effect of the axial alkyl ligand (R) on the charge localization (R-Pt(IV)(N(2)O(2))-Pt(II)(N(4))) and delocalization (R-Pt(III)(N(2)O(2))-Pt(III)(N(4))-OH(2)); when R is p-styrenesulfonate having an electron withdrawing group, the charge localization in the dimer is less pronounced and the water molecule on the Pt(N(4)) atom is retained (R-Pt(III)(N(2)O(2))-Pt(III)(N(4))-OH(2)) in the intermediate state. In both routes, the alkyl group undergoes nucleophilic attack of water, and the oxidized products are released via reductive elimination.

Trends in Base Hydrolysis Rates of Chlorocobalt(III) Complexes of a Series of Related Pentaamine Ligands

Base hydrolysis kinetics of chloropentaaminecobalt(III) complexes of a range of acyclic and cyclic pentaamine ligands, incorporating the rigid -NH-CH 2 -C(CH 3 )(NH 2 )-CH 2 -NH- motif, which must coordinate facially in octahedral complexes, are analysed in terms of ground state structural influences on hydrolysis rates. For complexes without experimental structural data, molecular mechanics was used to predict Co-Cl and Co-N bond distances. The computed strain energies and structural parameters both isomers ( trans and cis ) of all compounds are analysed. Although sterically crowded ligands, which should favour a five-coordinate intermediate, exhibit larger k OH values, only a modest correlation between ground state Co-L distances and hydrolysis rates is found. This suggests that the structures of the transition state and/or of the five-coordinate intermediate is important.

Fluxional behavior of the dinitrogen ligand 2,9-dimethyl-1,10-phenanthroline in cationic methyl platinum(II) complexes.

The ionic methylplatinum(II) complexes [Pt(Me)(L)(dmphen)]X (dmphen = 2,9-dimethyl-1,10-phenanthroline, L = Me(2)SO, X = PF(6)(-) 1a, BF(4)(-) 1b, CF(3)SO(3)(-) 1c, ClO(4)(-) 1d, B(C(6)H(5))(4)(-) 1e, [B(3,5-(CF(3))(2)C(6)H(3))(4)](-) 1f; L = n-Bu(2)SO, X = CF(3)SO(3)(-) 1g; L = PPh(3), X = PF(6)(-) 2a, BF(4)(-) 2b, CF(3)SO(3)(-) 2c, ClO(4)(-) 2d, B(C(6)H(5))(4)(-) 2e, [B(3,5-(CF(3))(2)C(6)H(3))(4)](-) 2f; X = CF(3)SO(3)(-), L = CyNH(2) 3a, i-PrNH(2) 3b, 2,6-Me(2)py 3c, EtNH(2) 3d, AsPh(3) 3e, dimethylthiourea (Me(2)th) 3f and the uncharged [Pt(Me)(X)(dmphen)] (X = SCN(-) 4a, SeCN(-) 4b) complexes have been synthesized and fully characterized. In chloroform, as well as in acetone or methanol, complexes 1a-1g, 2a-2h (X = Cl(-) g, NO(2)(-) h, formed "in situ"), and 3e show dynamic behavior due to the oscillation of the symmetric chelating ligand dmphen between nonequivalent bidentate modes. All the other compounds feature a static structure in solution. The crystal structure of 2a shows a tetrahedral distortion of the square planar coordination geometry, a loss of planarity of the dmphen ligand, and, most notably, a rotation of the dmphen moiety, around the N1-N2 vector, to form a dihedral angle of 42.64(8) degrees with the mean coordination plane. The hexafluorophosphate ion lies on the side of the phenanthroline ligand. The interionic structures of 2a, 2b, and 2f were investigated in CDCl(3) at low temperature by (1)H-NOESY and (19)F[(1)H]-HOESY NMR spectroscopies. Whereas PF(6)(-) (2a) and BF(4)(-) (2b) show strong contacts with the cation [Pt(Me)(PPh(3))(dmphen)](+), being located preferentially on the side of the phenanthroline ligand, the [B(3,5-(CF(3))(2)C(6)H(3))(4)](-) (2f) ion does not form a tight ion pair. The dynamic process was studied by variable-temperature NMR spectroscopy for 1a-1f and 2a-2h in CDCl(3). The activation energies DeltaG(298) for the sulfoxide complexes 1a-1f are lower than those of the corresponding phosphine complexes 2a-2f by approximately 10 kJ mol(-)(1). The nature of the counteranion exerts a tangible influence on the fluxionality of dmphen in both series of complexes 1 and 2. The sequence of energies observed for 2a-2h encompasses an overall difference of about 16 kJ mol(-)(1), increasing in the order Cl(-) approximately NO(2)(-) << CF(3)SO(3)(-) < ClO(4)(-) < B(C(6)H(5))(4)(-) < BF(4)(-) approximately PF(6)(-) < B(3,5-(CF(3))(2)C(6)H(3))(4)(-). Acetone and methanol have an accelerating effect on the flipping. Concentration-dependent measurements, carried out in CDCl(3) for 2a with n-Bu(4)NPF(6) and the ligands dmphen, n-Bu(2)SO, sec-Bu(2)SO, and sec-Bu(2)S showed that the rate of the fluxional motion is unaffected by added n-Bu(4)NPF(6), whereas in the other cases this increases linearly with increasing ligand concentration, according to a pattern of behavior typical of substitution reactions. Dissociative and associative mechanisms can be envisaged for the observed process of flipping. Dissociation can be prevalent within the ion pair formed by a "noncoordinating" anion with the metallic cationic complex in chloroform. Among the possible associative mechanisms, promoted by polar solvents or by relatively strong nucleophiles, a consecutive displacement mechanism is preferred to intramolecular rearrangements of five-coordinate intermediates.

Cationic Four- and Five-Coordinate Nickel(II) Complexes: Insights into the Nickel(II)-Catalyzed Copolymerization of Ethylene and Carbon Monoxide

Using the dppp-derived Ni(II) catalyst system 2, several four- and five-coordinate intermediates relevant to the alternating copolymerization of ethylene and carbon monoxide have been characterized. In addition, the activation barriers for the migratory insertion steps corresponding to chain growth have been determined to be at or below ca. 10 kcal/mol, indicating that a strongly stabilized catalyst resting state is most likely responsible for the low activity of 2 for copolymerization.

Five-Coordinate Intermediates in Carbon−Carbon Reductive Elimination Reactions from Pt(IV)

Five-Coordinate Intermediates in Carbon−Carbon Reductive Elimination Reactions from Pt(IV)

Labile Hydrido Complexes of Iridium(III): Synthesis, Dynamic Behavior in Solution, and Reactivity toward Alkenes

The trisacetonitrile complexes [IrClH(PiPr3)(NCCH3)3]BF4 (1) and [IrH2(PiPr3)(NCCH3)3]BF4 (2) have been prepared in one-pot reactions with high yields by reaction of the iridium(I) dimers [Ir(μ-Cl)(coe)2]2 and [Ir(μ-OMe)(cod)2]2 with the phosphonium salt [HPiPr3]BF4. The rates of exchange between free acetonitrile and the labile acetonitrile ligands of complexes 1 and 2 have been measured by NMR spectroscopy. This kinetic study has shown that both complexes readily dissociate one acetonitrile ligand trans to hydride, giving rise to fluxional five-coordinate intermediates. Substitution products 3−7 have been obtained by treatment of complexes 1 and 2 with CO and PMe3. The structures determined for 3−7 can be rationalized on the basis of the steric requirements of the ligands, indicating that the products are formed by thermodynamic control. Ethene inserts reversibly into the Ir−H bond of 1 to give the compound [IrCl(Et)(PiPr3)(NCCH3)3]BF4 (8), which has been used for the preparation of the stable ethyliridium(III) complexes [IrCl(Et)(PiPr3)(Py)2(NCCH3)]BF4 (9) and [Ir(η2-O2CCH3)Cl(Et)(PiPr3)(NCCH3)3] (10), respectively. The molecular structure of 10 has been determined by X-ray crystallography. The reaction of 2 with ethene, at low temperature, results in the sequential formation of the ethene complex [IrH2(η2-C2H4)(PiPr3)(NCCH3)2]BF4 (11) and the diethyl derivative [Ir(Et)2(PiPr3)(NCCH3)3]BF4 (14). At room temperature in solution, 14 undergoes reductive elimination of ethane to form the iridium(I) species [Ir(PiPr3)(NCCH3)3]BF4 (15) and [Ir(PiPr3)(η2-C2H4)(NCCH3)2]BF4 (16). These cations readily react with H2 to regenerate 2, closing a cycle for ethene hydrogenation in which several participating species have been identified. The reaction of 2 with propene in solution also allows the characterization of products of propene coordination (17) and insertion (18). In this case, the species obtained after elimination of propane are products of allylic C−H activation: [IrH(η3-C3H5)(PiPr3)(NCCH3)2]BF4 (19) and [IrH(η3-C3H5)(η2-C3H6)(PiPr3)(NCCH3)]BF4 (20). The structure of complex 19 has been determined by X-ray diffraction, and the kinetics of dissociation of its two labile acetonitrile ligands have been studied by NMR spectroscopy. Complex 19 undergoes electrophilic activation of H2 to give propene and reform the starting complex 2.