Electron exchange reaction of bis(terpyridine)cobalt re-examined by NMR

The rate of the electron transfer self-exchange reaction between bis(terpyridine) cobalt(III) and bis(terpyridine)cobalt(II) has been reexamined by proton NMR. The rate constant of 4×10² M⁻¹ s⁻¹ at 50 °C is dependent on the identity of the anion. Average activation parameters of 32 kJ mol⁻¹ and −96 J K⁻¹ mol⁻¹ are in agreement with previous measurements by other techniques. There is no evidence for either spin restrictions or non-adiabtaticity in this and related cobalt(III)/(II) electron exchange reactions. An alternative explanation is offered for the anomalously negative volumes of activation reported elsewhere.     

Kinetics and mechanism of the axial substitution reactions of (ligand)bis(4,5-dichloro-1,2-benzo semiquinonediiminato)cobalt(III) complexes

Square-pyramidal (Ph₃X)bis(4,5-dichloro-1,2-benzosemiquinonediiminato)cobalt(III) complexes (X = As, Sb or P) have been synthesized. The kinetics of axial substitution for the triphenylantimony complex have been studied for 10 entering ligands (L*). The reaction is of reversible second-order in both directions for all complexes. Labile behavior is indicated by the rate constants in the range from 6.33 × 10³ (for L* = Ph₃P in MeOH) to 5.4 (L* = py in CH₂Cl₂) M⁻¹ s⁻¹. The kinetics is consistent with an I_a mechanism. The log of the second-order rate constant for axial substitution is a linear function of nucleophilic reactivity n_Pt°, which is due to the trans-labilizing effect of the entering ligand in the six-coordinate transition state.     

The dynamics of formation of the O2-Co bond in the cobalt(II) cyclidene complexes

The kinetics of O₂ binding to a vacant coordination site on the cobalt(II) ion have been determined, revealing a radical-like character for the reaction. Reversible oxygenation of Co(II) cyclidenes (C4, C5, C6, C8, C12-bridged and unbridged) was studied by a cryogenic stopped-flow method. In the presence of axial base, kinetic parameters are insensitive to the nature of the solvent, and negative entropies of activation suggest that dissociation of a solvent molecule is not the rate-determining step for the dioxygen binding process. This is in contrast to the behavior of previously studied Co(II) complexes. A very low activation energy (1–4 kcal mol⁻¹), typical of diffusion controlled processes, was found for dioxygen binding. The binding rate constants for the highest affinity complexes (10⁸ M⁻¹ s⁻¹) are comparable to the values for natural dioxygen carriers. The size of the lacuna primarily affects the dioxygen binding rates, while the axial bases influence the dioxygen dissociation rates.     

Stopped-flow mechanistic study of bromide substitution by an organonitrile at an iron(II) phosphinic centre; a π-electron driven process

The kinetics of the displacement reactions of the bromide ligands of trans-[FeBr₂(depe)₂] (depe = Et₂PCH₂CH₂PEt₂) by the organonitrile NCCH₂C₆H₄OMe-4, in tetrahydrofuran (either in the absence or in the presence of added Br⁻), to give the corresponding mono- and dinitrile complexes trans-[FeBr(NCCH₂C₆H₄OMe-4)(depe)₂]⁺ and trans-[Fe(NCCH₂C₆H₄OMe-4)₂(depe)₂]²⁺, have been investigated by stopped-flow spectrophotometry. The substitution reaction occurs by a mechanism involving rate-limiting dissociation of bromo ligands to form the unsaturated intermediates [FeBr(depe)₂]⁺ (k₁ = 1.52 ± 0.02 s⁻¹) and [Fe(NCR)(depe)₂]²⁺ (k₃ = 0.063 ± 0.008 s⁻¹) which add the nitrile ligand to form those nitrile complexes. The competition between the nitrile and Br⁻ for such metal centres has also been investigated and a stronger inhibiting effect of added Br⁻ is observed for the substitution of the second bromo ligand relative to the first one. The kinetic data are rationalized in terms of π-electronic effects of these unsaturated metal centres and of the bromide and nitrile ligands.     

Substitution kinetics of tetracarbonyl(η-alkene)ruthenium complexes: ALKENE = ethene and methyl acrylate

The kinetics in heptane of displacement of the alkene ligands ethene and methyl acrylate from Ru(CO)₄(η²-alkene) by P(OEt)₃ have been measured. The reactions occur by reversible dissociation of the alkenes, and activation parameters are compared with those for dissociation of CO from Ru(CO)₅ and for reactions of the corresponding Os complexes. A linear free energy relationship for ligand dissociation from Ru(CO)₅, Ru(CO)₄(C₂H₄) and Ru(CO)₄(MA) has a gradient close to unity, indicating virtually complete bond breaking in the transition states. Competition parameters for reactions of what is probably a solvated Ru(CO)₄S intermediate have been measured for the alkenes and P(OEt)₃, and for eleven other P-donor nucleophiles. Correlations with the electronic and steric properties of the P-donors show negligible dependence on the electron donicity of the nucleophiles and a small but significant dependence on their sizes. The sizes were quantified by Tolman cone angles or by ‘cone angle equivalents’ derived directly from Brown's ligand repulsion energies (E_r). These correlations compared with those, reported elsewhere, for reactions of the probably solvated intermediates Co₂(CO)₅(μ²-C₂Ph₂) and H₃Re₃(CO)₁₁ formed by ligand dissociative processes. In all cases the discrimination between nucleophiles by the intermediates is weak confirming their high reactivity and the borderline nature of the mechanisms of these bimolecular reactions between I_d and I_a.     

Intramolecular protonation and other mechanisms for substitution reactions of hydrido(thiolato) — and di(thiolato)-ruthenium(II) phosphine complexes

Reactions of cct-RuH(SR)(CO)₂(PPh₃)₂ (1) (cct = cis, cis, trans) with R′SH provide cct-RuH(SR′)(CO)₂(PPh₃)₂ (R = alkyl, aryl): based on described kinetic data, the proposed mechanism involves PPh₃ loss, coordination of R′SH, intramolecular protonation of RS⁻ by R′SH, and RSH elimination. The intramolecular protonation step circumvents a potentially slow RSH reductive elimination step. A similar mechanism is proposed for the thiol exchange reactions of cct-Ru(SR)₂(CO)₂(PPh₃)₂ (2). A corresponding dissociative mechanism is also proposed for the reaction of 1 with P(p-tolyl)₃, which gives cct-RuH(SR)(CO)₂(PPh₃)(P(p-tolyl)₃) and cct-RuH(SR)(CO)_2⁻ (P(p-tolyl)₃)₂. Other reactions described include the reactions of 1 with H₂, CO, HCl and PPh₃, and the reactions of 2 with P(p-tolyl)₃ and H₂. Exposure to light causes 2 to dimerize in solution.     

Kinetics and mechanism of the oxidation of ammineruthenium(II) complexes by bromine

The reactions of Ru(NH₃)₅py²⁺, Ru(NH₃)₄bpy²⁺, Ru₂(NH₃)₁₀pz⁵⁺, RuRh(NH₃)₁₀pz⁵⁺ and Ru(NH₃)₅pz²⁺ with bromine are first-order in ruthenium and first-order in bromine. The rates decrease with increasing bromide ion concentration and, except for Ru(NH₃)₅pz²⁺, are independent of hydrogen ion concentration. The reactions are postulated to proceed via outer-sphere, one-electron transfer from Ru(II) to Br₂ with the formation of Br₂⁻ as a reactive intermediate. The bromide inhibition is ascribed to the formation of Br₃⁻ which is unreactive in outer-sphere reactions because of the barrier imposed by the need to undergo reductive cleavage. The reaction of Ru(NH₃)₅pz²⁺ is inhibited by hydrogen ions. The hydrogen ion dependence shows that Ru(NH₃)₅pzH³⁺ has a pK_a of 2.49 and is at least 500 times less reactive than Ru(NH₃)₅pz²⁺. The reaction of Ru₂(NH₃)₁₀pz⁴⁺ with bromine is biphasic. The second phase has a rate identical to that of the Ru₂(NH₃)₁₀pz⁵⁺-Br₂ reaction. A detailed analysis shows that the reaction of Ru₂(NH₃)₁₀pz⁴⁺ with bromine proceeds by a sequence of one-electron steps, Br₂⁻ being produced as an intermediate. A linear free energy relationship between rate constants and equilibrium constants, obeyed for all the reactions studied, provides an estimate of 1.5 × 10² M⁻¹ s⁻¹ for the self-exchange rate constant of the Br₂/Br₂⁻ couple.     

The solvent-initiated photochemistry of tris(N,N-diethyldithiocarbamato)iron(III)

The photolysis of [Fe(Et₂dtc)₃], Et₂dtc = diethyldithiocarbamate to yield [Fe(Et₂dtc)₂Cl] proceeds under 313 nm irradiation through a metal complex excited state, as expected. Under 254 nm irradiation, however, the dominant pathway is through a solvent-initiated reaction in which radicals formed after absorption of light by CHCl₃ react thermally with [Fe(Et₂dtc)₃]. The initial rate varies linearly with the light intensity at 313 nm, but at 254 nm varies with the square root of the intensity.     

Kinetics and mechanism of the stoichiometric oxygenation of [Cu(fla)(idpa)]ClO4 [fla=flavonolate, IDPA=3,3′-imino-bis(N,N-dimethylpropylamine)] and the [Cu(fla)(idpa)]ClO4-catalysed oxygenation of flavonol

Oxygenation of [Cu^II(fla)(idpa)]ClO₄ (fla=flavonolate; IDPA=3,3′-iminobis(N,N-dimethylpropylamine)) in dimethylformamide gives [Cu^II(idpa)(O-bs)]ClO₄ (O-bs=O-benzoylsalicylate) and CO. The oxygenolysis of [Cu^II(fla)(idpa)]ClO₄ in DMF was followed by electronic spectroscopy and the rate law −d[{Cu^II(fla)(idpa)}ClO₄]/dt=k_obs[{Cu^II(fla)(idpa)}ClO₄][O₂] was obtained. The rate constant, activation enthalpy and entropy at 373 K are k_obs=6.13±0.16×10⁻³ M⁻¹ s⁻¹, ΔH^‡=64±5 kJ mol⁻¹, ΔS^‡=−120±13 J mol⁻¹ K⁻¹, respectively. The reaction fits a Hammett linear free energy relationship and a higher electron density on copper gives faster oxygenation rates. The complex [Cu^II(fla)(idpa)]ClO₄ has also been found to be a selective catalyst for the oxygenation of flavonol to the corresponding O-benzoylsalicylic acid and CO. The kinetics of the oxygenolysis in DMF was followed by electronic spectroscopy and the following rate law was obtained: −d[flaH]/dt=k_obs[{Cu^II(fla)(idpa)}ClO₄][O₂]. The rate constant, activation enthalpy and entropy at 403 K are k_obs=4.22±0.15×10⁻² M⁻¹ s⁻¹, ΔH^‡=71±6 kJ mol⁻¹, ΔS^‡=−97±15 J mol⁻¹ K⁻¹, respectively.     

Substitution reaction mechanisms of dihydrido-ruthenium(II) phosphine complexes: hydribe basicity and molecular hydrogen intermediates

Kinetic and activation parameter data for the reactions of cct-Ru(H)₂(CO)₂(PPh₃)₂ (1) (cct = cis, cis, trans) in THF with thiols, CO and PPh₃ to give cct-RuH(SR)(CO)₂(PPh₃)₂, Ru(CO)₃(PPh₃)₂ and Ru(CO)₂(PPh₃)₂, respectively, reveal a common, rate-determining step, the initial dissociation of H₂ from 1; the activated complex probably resembles the corresponding Ru(η²-H₂) species. Reaction of Ru(H)₂(dppm)₂ (2) (as a cis/trans mixture, DPPM = bis(diphenylphosphino)methane) with thiols initially generated cis- and trans- RuH(SR) (dppm)₂ with a rate that depends on both the type and concentration of thiol. The higher basicity of the hydride ligands in 2 (versus 1), which is demonstrated by deuterium exchange with CD₃OD, gives rise in the thiol reaction to an initial protonation step prior to loss of H₂. A species detected in the thiol reaction is possibly [RuH(η²-H₂ (dppm)₂]², the anticipated intermediate for this reaction and for the hydrogen exchange with alcohol. A longer reaction of 2 with PhCH₂SH gives solely cis-Ru(SCH₂Ph)₂(dppm)₂.     

Studies on the kinetics and mechanism of anation reactions of some six-coordinate complexes of chromium(III) in aqueous solution

Rates of stepwise anation of cis-Cr(ox)₂(H₂O₂)⁻ with SCN⁻/N₃⁻, Cr(acac)₂(H₂O)₂⁺ with SCN⁻ and Cr(atda)(H₂O)₂ with SCN⁻ have been investigated in weakly acidic aqueous solutions. Rate constants, k_I and k_II for the two steps in each system, are composite as k_x = k_x⁰+k_x^X[X⁻] (x = I, II; X⁻ = SCN⁻, N₃⁻). These rate constants have been evaluated also as the corresponding ΔH^≠ and ΔS^≠ values. The results obtained and the plausible I_d mechanism seem to suggest Cr---OOC bond dissociation (hence a strongly negative ΔS^≠) generating the transition state in each system with outer-sphere association forming the precursor complex in the X⁻ dependent paths.     

Ion-pair mechanism in square planar substitution. Reactivity of cationic platinum (II) complexes with negatively charged nucleophiles in solvents of high, medium and low polarity

The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen) (CH₃) (Me₂SO)]⁺ by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) mixture. The starting complex and the reaction products were characterized either as solids or in solution by their IR and ¹H NMR spectra. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sequence of reactivity observed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en) (NH₃)Cl]⁺ used as standard). The difference of reactivity between the first (t-BuNH₂) and the last (SeCN⁻) members of the series spans five orders of magnitude. The value measured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series of protic and dipolar aprotic solvents, using chloride ion as nucleophile, the rate of formation of [Pt (phen) (CH₃)Cl] is dominated by the extent of solvation of Cl⁻, as measured by its values of the Gibbs molar energy of transfer Δ_tG⁰. Conductivity measurements at 25°C in dichloromethane were fitted to the Fuoss equation and the values of the dissociation constants K_d for the ion pairs were calculated as follows: 2.27 × 10⁻⁵ M for Bu₄NCl, 2.75 × 10⁻⁵ M for Bu₄NSCN and 17.05 × 10⁻⁵ M for [Pt(phen) (CH₃) (Me₂SO)]PF₆. The pseudo-first-order rate constants k_obs for the reactions with Bu₄NCl, Bu₄NBr, Bu₄NSCN and Bu₄NI showed a curvilinear dependence on the concentration of the salt which levels off very soon (at concentrations higher than 0.005 M the kinetics are zero order in [Bu₄NX]). On addition of the inert electrolyte Bu₄NPF₆ the rates slow down and the kinetics follow the rate law k_obs = kK_ip[Bu₄NX]/[Bu₄NPF₆] + K_ip[Bu₄NX]). These findings fit well with a reaction scheme which involves a pre-equilibrium K_ip between ion pairs, followed by unimolecular substitution within the contact ion pair [Pt(phen) (CH₃) (Me₂SO)X]_ip. Values of the equilibrium constants K_ip for ion-pair exchange and of the internal substitution rates k were derived. The latter showed that the discrimination in reactivity between Cl⁻, Br⁻, SCN⁻ and I⁻ is greatly reduced with respect to aqueous solutions. The reason behind this may be desolvation of the ions coupled to the fact that a contact ion pair is already at a certain distance along the reaction coordinate in the direction of the transition state. Applications of the special salt effect and of ion pairing to synthesis are discussed.     

Monovalent copper as a potential catalyst for formation of acetaldehyde via the migration of methyl radicals to the coordinated carbonyl in the complex (CO)Cu-CH3

Cu_aq⁺ forms stable complexes with carbon monoxide in aqueous solutions. Furthermore it reacts very fast with aliphatic radicals. The reaction of Cu(CO)_maq⁺ with methyl radicals, ^•CH₃ was studied using the pulse-radiolysis technique. The results point out that methyl radicals react with Cu(CO)_aq⁺ to form an unstable intermediate with a Cu^II-C σ bond identified as (CO)Cu^II-CH₃⁺, k = (1.1±0.2) × 10⁹ M⁻¹ s⁻¹. This intermediate has a strong LMCT charge transfer band (λ_max = 385 nm, _max = 2500 M⁻¹ cm⁻¹) which is similar to the absorption bands of other transient complexes with Cu^II-alkyl σ bonds. The coordinated carbon monoxide in (CO)Cu^II-CH₃⁺ inserts into the copper—carbon bond (or rather the coordinated methyl migrates to the coordinated carbon monoxide ligand) at a rate of (3.0±0.8) × 10² s⁻¹ to form the copperacetyl complex (CO)_mCu^II-C(CH₃)=O⁺ (λ_max = 480 nm, _max = 2100 M⁻¹ cm⁻¹). The rate of formation of (CO)Cu^II-CH₃⁺ and of the insertion reaction are pH independent. The complex (CO)_mCu^II-C(CH₃)=O⁺ is also unstable and decomposes heterolytically to yield acetaldehyde and Cu_aq²⁺ as the final stable products. This reaction is slightly pH dependent. The same reactivity pattern has been observed for the Cu(CO_naq⁺ complexes (n = 2 or 3). The results clearly point out that CO remains coordinated to transient complexes of the type Cu^II-alkyl.     

Dissociative phosphine exchange for cyclopentadienylmolybdenum(III) systems. Bridging the gap between Werner-like coordination chemistry and low-valent organometallic chemistry

Several phosphine exchange processes on 17-electron CpMoCl₂(PR₃)₂ systems have been investigated. The exchange of two PPh₃ ligands with either two PMe₃ ligands or with Ph₂PCH₂PPh₂ (dppe) is complete within a few minutes at −80 °C. Equally fast is the exchange of two PEt₃ ligands with two PMe₃ ligands. On the other hand, the exchange of two PEt₃ ligands with dppe is much slower ( to a few hours at r.t.), with excess dppe accelerating the exchange and free PEt₃ retarding it. The self-exchange reaction of PMe₃ is extremely slow (less than 25% exchange at r.t. in 6 h at r.t.) and an analysis of the initial rate of this reaction shows a two-term rate law with one [PMe₃]-dependent and one independent term. Finally, PMe₃ self-exchange on Cp^*MoCl₂(PMe₃)₂ proceeds over one order of magnitude faster than for the corresponding Cp system, with a substantially [PMe₃]-independent rate law. All these data are indicative of a dominant dissociative exchange mechanism involving rupture of the Mo---PR₃ bond in the slow step and formation of a 15-electron intermediate. The rate of phosphine dissociation qualitatively correlates with the Mo---P distance in the 17-electron starting complex. Only for the Cp---MoCl₂(PMe₃)₂ system is phosphine dissociation sufficiently slowed down so that the alternative associative exchange pathway becomes competitive. Possible reasons for a low activation barrier in these dissociative exchanges are discussed.     

First- and second-order mechanisms for oxidative addition for bound methyl disulfide in the complex W(CO)3(phen)(MeSSMe)

The rate of oxidative addition of methyl disulfide in the complex W(CO)₃(1,10-phenanthroline) (MeSSMe) in methylene chloride has been studied. The dominant reaction pathway is second order in metal complex and inhibited by excess methyl disulfide. Formation of a dinuclear complex [W(CO)₃(phen)]₂(MeSSMe) is proposed to lead to the transition state for cleavage of the sulfur-sulfur bond in the second-order mechanism. In neat methyl disulfide, or in concentratred solutions of methyl disulfide at low metal complex concentrations, the reaction occurs at reduced rate and follows a first-order mechanism. Addition of Mo(CO)₃(1,10-phenanthroline) (MeSSMe) to the corresponding tungsten complex results in a ten-fold increase in the rate of oxidative addition of the tungsten complex and production of Mo(CO)₄(1,10-phenanthroline) as the sole molybdenum-containing product. The faster rate of reaction in the presence of the molybdenum complex is attributed to the faster formation of the heteronuclear dinuclear intermediate by initial loss of MeSSMe from the molybdenum versus tungsten center. Additional kinetic/mechanistic studies are described using a new flow-through FT-IR/microscope reaction system designed to allow convenient monitoring of small quantities of sensitive/hazardous reactants.     

Alkylcobalt chelates with Schiff bases derived from a β-diketone bearing both alkyl and aryl groups

Organocobalt(III) complexes with Schiff bases derived from a β-diketone bearing both an alkyl and an aryl group have been prepared. The template syntheses using benzoylacetone and ethylenediamine as complexing agents provide a route to alkylcobalt chelates with the corresponding tri- and tetradentate Schiff bases. However, if a β-diketone with two aryl groups, e.g. dibenzoylmethane, was employed as the starting ketoenol component, no organometallic products were detected; a new mixed-ligand ‘inorganic’ chelate of cobalt(II), [Co{O=C(Ph)CH=C(Ph)O}₂(en)], was isolated instead. Its structure as well as that of one of the alkylcobalt complexes with a tridentate Schiff base composed of benzoylacetone and ethylenediamine have been established by X-ray techniques. The current scope of the template synthesis of alkylcobalt complexes with Schiff bases is summarized.     

Model reactions of a carbonylation catalyst: phosphite induced migratory CO insertion in [MeIr(CO)2I3]

Carbonylation of the anionic iridium(III) methyl complex, [MeIr(CO)₂I₃]⁻ (1) is an important step in the new iridium-based process for acetic acid manufacture. A model study of the migratory insertion reactions of 1 with P-donor ligands is reported. Complex 1 reacts with phosphites to give neutral acetyl complexes, [Ir(COMe)(CO)I₂L₂] (L = P(OPh)₃ (2), P(OMe)₃ (3)). Complex 2 has been isolated and fully characterised from the reaction of Ph₄As[MeIr(CO)₂I₃] with AgBF₄ and P(OPh)₃; comparison of spectroscopic properties suggests an analogous formulation for 3. IR and ³¹P NMR spectroscopy indicate initial formation of unstable isomers of 2 which isomerise to the thermodynamic product with trans phosphite ligands. Kinetic measurements for the reactions of 1 with phosphites in CH₂Cl₂ show first order dependence on [1], only when the reactions are carried out in the presence of excess iodide. The rates exhibit a saturation dependence on [L] and are inhibited by iodide. The reactions are accelerated by addition of alcohols (e.g. 18× enhancement for L = P (OMe)₃ in 1:3 MeOH-CH₂Cl₂). A reaction mechanism is proposed which involves substitution of an iodide ligand by phosphite, prior to migratory CO insertion. The observed rate constants fit well to a rate law derived from this mechanism. Analysis of the kinetic data shows that k₁, the rate constant for iodide dissociation, is independent of L, but is increased by a factor of 18 on adding 25% MeOH to CH₂Cl₂. Activation parameters for the k₁ step are ΔH^≠ = 71 (±3) kJ mol⁻, ΔS^≠ = −81 (±9) J mol⁻¹ K⁻¹ in CH₂Cl₂ and ΔH^≠ = 60(±4) kJ mol⁻¹, ΔS^≠ = −93(± 12) J mol⁻¹ K⁻¹ in 1:3 MeOH-CH₂Cl₂. Solvent assistance of the iodide dissociation step gives the observed rate enhancement in protic solvents. The mechanism is similar to that proposed for the carbonylation of 1.     

Metal carbonyls of telluroether formed by the oxygen atom transfer reaction to M(CO)6 (M = Mo, W) in the presence of (p-CH3OC6H4)2TeO and relative kinetic investigation

A new synthetic process is reported for the preparation of two substituted metal carbonyls, (p-CH₃OC₆H₄)₂TeM(CO)₅ (M = Mo, W). In the presence of (p-CH₃OC₆H₄)₂TeO as O atom transfer reagent in tetrahydrofuran solvent, a CO ligand is replaced by telluroether when M(CO)₆ (M = Mo, W) is reacted with (p-CH₃OC₆H₄)₂TeO under very mild experimental conditions (r.t.). The products were characterized by elemental analysis, mass, IR and ¹H NMR spectroscopies. The spectra suggest that the coordination geometry is distorted from a regular octahedral structure due to an asymmetrical bulky telluroether ligand on the metal atom. Kinetics of these reactions of M(CO)₆ with (p-CH₃OC₆H₄)₂TeO show the reactions are first order in the concentration of M(CO)₆ and of Te oxide. The rates of reaction decrease in the order W(CO)₆>Mo(CO)₆>Cr(CO)₆, and the results obtained are discussed in term of a presumed mechanism.     

Kinetics of nucleophilic attack on coordinated organic moieties Part 29. Mechanism of addition of tertiary phosphines and phosphites to the tropylium ring of [M(CO)3(η-C7H7] (M = Cr, Mo, W) cations

Kinetic studies of the addition of a wide range of tertiary phosphines and phosphites to the tropylium ring of the cation [Cr(CO)₃(η⁷-C₇H₇]⁺ (1) reveal the two-term raw, k_obs = k₁[PR₃] + k₋₁. This is consistent with the reversible equilibrium process (i) which is also confirmed from IR and ¹H NMR studies. In the case of the highly basic nucleophiles PBu₃ⁿ and PEt₂Ph, the rate is dominated by the k₁ term and the equilibrium lies far to the right.

Full-size image

The first-order rate constants k₁, for addition to the tropylium ring decrease markedly down the series PBu₃ⁿ>PEt₂Ph>P(4-MeOC₆H₄)₃>P(4-MeC₆H₄)₃>P(C₆H_{11 3}>PPh₂(4-MeC₆H₄)>PPh₃>P(2-CNC₂H₄)₃>P(OBuⁿ)₃ (overall variation 10⁴). This reactivity order parallels the decreasing electron availability at the phosphorus centres, as confirmed by the linear correlation between log k₁ and the Tolman Σ_χ values for the nucleophiles. Excellent Hammett and Brønsted correlations are also observed for ring addition by a range of P(4-XC₆H₄)₃ nucleophiles. The Brønsted slope, , of 0.7 conirms the major importance of basicity in determining nucleophilicity towards cation 1. Kinetic studies of the related additions of PBu₃ⁿ to the cations [M(CO)₃(η⁷-C₇H₇]⁺ (M = Mo, W) reveal the rate law, Rate = k₁[M][PBu₃ⁿ, and show only a small dependence of k₁ on the nature of metal (Cr>WMo; 2:1.1:1). These data, together with the associated activation parameters, support a mechanism involving direct addition (k₁) of the phosphorus nucleophiles to the tropylium ring, and are inconsistent with initial rate-determining attack at the metal centre.