Strength of the metal-ligand bond in LCr(CO)5 measured by photoacoustic calorimetry

Yang, Gilbert K.; Peters, Kevin S.; Vaida, Veronica

doi:10.1016/0009-2614(86)87100-7

“…Recently, significant evidence has been provided for alkane σ-complexes mediating the solution-phase reductive elimination and oxidative addition of alkanes at transition metal centers. − For example, Bergman, Moore, and co-workers have obtained transient IR spectroscopic data directly supporting such an intermediate in the oxidative addition of alkanes at a Cp*Rh(CO) fragment. − The viability of alkane adducts has received additional support from a variety of other sources: theory − and gas phase, − condensed phase, − and low-temperature matrix studies. , Furthermore, analogous η 2 -dihydrogen and silane complexes along with 3-center-2-electron agostic C−H−M interactions are now well-known and many have been structurally characterized.…”

Section: Discussionmentioning

confidence: 99%

Exploring the Mechanism of Aqueous C−H Activation by Pt(II) through Model Chemistry: Evidence for the Intermediacy of Alkylhydridoplatinum(IV) and Alkane σ-Adducts

Stahl

¹

,

Labinger

²

,

Bercaw

³

1996

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The protonolysis mechanisms of several alkylplatinum(II) complexes [(tmeda)PtMeCl (2) (tmeda = N,N,N‘,N‘-tetramethylethylenediamine), (tmeda)Pt(CH2Ph)Cl (5), (tmeda)PtMe2 (11), and trans-(PEt3)2Pt(CH3)Cl (15)] in CD2Cl2 and CD3OD have been investigated. These reactions model the microscopic reverse of C−H activation by aqueous Pt(II). Each of the four systems (2 in CD3OD, 5 in CD2Cl2, 11 in CD3OD, and 15 in CD3OD) exhibits different behavior in the protonolysis reaction as observed by low-temperature 1H NMR spectroscopy. Protonolysis of 2 in methanol-d 4 proceeds with no observable intermediates. Reversible reaction between 5 and HCl in CD2Cl2 at −78 °C produces (tmeda)Pt(CH2Ph)(H)Cl2 (6), which undergoes reductive elimination of toluene at higher temperatures. Treatment of 11 with HCl in methanol at −78 °C generates (tmeda)PtMe2(H)Cl (12), which incorporates deuterium from solvent (CD3OD) into the methyl groups prior to reductive elimination of methane. Finally, 15 reacts with H+ in methanol to liberate methane with no intermediates observed. However, hydrogen/deuterium exchange takes place between the solvent (CD3OD) and Pt−Me prior to methane loss. Each of these reactions was evaluated further to determine the kinetics of the reaction, activation parameters, and isotope effects. Based on the results, a common mechanistic sequence is proposed to operate in all the reactions: (1) chloride- or solvent-mediated protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, five-coordinate platinum(IV) species, (3) reductive C−H bond formation producing a platinum(II) alkane σ-complex, and (4) loss of alkane either through an associative or dissociative substitution pathway. The characteristics of each system differ due to changes in the relative stabilities of the intermediates and/or transition states upon varying the solvent or alkylplatinum species.

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“…There have also been a number of reports attempting to prepare putatively “naked” nanoparticles that, in at least the literal interpretation of naked, would be devoid of any ligands, even solvent. ,− Although there have been claims of naked nanoparticles in the literature, − nanoparticle surfaces will of course never be truly naked in solution because even solvent-to-metal bond energies are 10 ± 3 kcal/mol for even the extreme case of metal–alkane bonds, at least in small-molecule organometallic complexes. − Solvent-only stabilized nanoparticles have been claimed in the literature, − but often anionic ligands (such as Cl – , OAc – , or other anions present in the synthetic precursor) are present but have not been tested, nor hence ruled out, as actual sources of nanoparticle stabilization. For example, in a recent report, Pt(0) nanoparticles generated in acetic acid are claimed to be naked, but the high probability of surface OAc – as a stabilizing ligand was not tested nor ruled out.…”

Section: Introductionmentioning

confidence: 99%

“Weakly Ligated, Labile Ligand” Nanoparticles: The Case of Ir(0)_n·(H⁺Cl^–)_m

Mondloch

¹

,

Özkâr

²

,

Finke

³

2018

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It is of considerable interest to prepare weakly ligated, labile ligand (WLLL) nanoparticles for applications in areas such as chemical catalysis. WLLL nanoparticles can be defined as nanoparticles with sufficient, albeit minimal, surface ligands of moderate binding strength to meta-stabilize nanoparticles, initial stabilizer ligands that can be readily replaced by other, desired, more strongly coordinating ligands and removed completely when desired. Herein, we describe WLLL nanoparticles prepared from [Ir(1,5-COD)Cl] 2 reduction under H 2 , in acetone. The results suggest that H + Cl – -stabilized Ir(0) n nanoparticles, herein Ir(0) n ·(H + Cl – ) a , serve as a WLLL nanoparticle for the preparation of, as illustrative examples, five specific nanoparticle products: Ir(0) n ·(Cl – Bu 3 NH + ) a , Ir(0) n ·(Cl – Dodec 3 NH + ) a , Ir(0) n ·(POct 3 ) 0.2 n (Cl – H + ) b , Ir(0) n ·(POct 3 ) 0.2 n , and the γ-Al 2 O 3 -supported heterogeneous catalyst, Ir(0) n ·(γ-Al 2 O 3 ) a (Cl – H + ) b . (where a and b vary for the differently ligated nanoparticles; in addition, solvent can be present as a nanoparticle surface ligand). With added POct 3 as a key, prototype example, an important feature is that a minimum, desired, experimentally determinable amount of ligand (e.g., just 0.2 equiv POct 3 per mole of Ir) can be added, which is shown to provide sufficient stabilization that the resultant Ir(0) n ·(POct 3 ) 0.2 n (Cl – H + ) b is isolable. Additionally, the initial labile ligand stabilizer HCl can be removed to yield Ir(0) n ·(POct 3 ) 0.2 n that is >99% free of Cl – by a AgCl precipitation test. The results provide strong support for the weakly ligated, labile ligand nanoparticle concept and specific support for Ir(0) ...

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“…For the RhCp(CO) complex, Bergman et al predicted the energy profile for the gas-phase methane reaction to be the following. First, on the basis of previous experimental work − for gas-phase equilibrium constants for, for example, the reaction between alkanes and W(CO) 5 , they conclude that the stability of the precursor complex between molecular methane and RhCp(CO) should be about 10 kcal/mol. From this precursor state, the C−H activation barrier was assumed to be the same as that measured for cyclohexane in liquid phase, i.e., 4.5 kcal/mol .…”

Section: Introductionmentioning

confidence: 99%

Comparison of the C−H Activation of Methane by M(C₅H₅)(CO) for M = Cobalt, Rhodium, and Iridium

Siegbahn

¹

1996

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The C−H activation reactions of methane by MCp(CO) (Cp = C5H5) for the metals cobalt, rhodium, and iridium have been studied using a variety of methods including a recently developed scaling scheme, different perturbation theory methods, and also hybrid density functional theory methods. The main chemical problems investigated include the recent finding that CoCp(CO) is entirely inert toward alkanes in contrast to the corresponding rhodium and iridium systems. Comparisons are made for CoCp(CO) between the reaction with methane and the reaction with CO, which is found experimentally to proceed fast. Also studied are the isotope effects on the reaction in relation to other recent experiments. At the highest level of treatment, good agreement is found with all present observations for these systems, but it should be pointed out that precise experimental information is lacking for many of the systems studied. Severe deviations between the results obtained at different computational levels are pointed out.

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Strength of the metal-ligand bond in LCr(CO)5 measured by photoacoustic calorimetry

Cited by 33 publications

References 9 publications

Exploring the Mechanism of Aqueous C−H Activation by Pt(II) through Model Chemistry: Evidence for the Intermediacy of Alkylhydridoplatinum(IV) and Alkane σ-Adducts

Exploring the Mechanism of Aqueous C−H Activation by Pt(II) through Model Chemistry: Evidence for the Intermediacy of Alkylhydridoplatinum(IV) and Alkane σ-Adducts

“Weakly Ligated, Labile Ligand” Nanoparticles: The Case of Ir(0)_n·(H⁺Cl^–)_m

Comparison of the C−H Activation of Methane by M(C₅H₅)(CO) for M = Cobalt, Rhodium, and Iridium

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Strength of the metal-ligand bond in LCr(CO)5 measured by photoacoustic calorimetry

Cited by 33 publications

References 9 publications

Exploring the Mechanism of Aqueous C−H Activation by Pt(II) through Model Chemistry: Evidence for the Intermediacy of Alkylhydridoplatinum(IV) and Alkane σ-Adducts

Exploring the Mechanism of Aqueous C−H Activation by Pt(II) through Model Chemistry: Evidence for the Intermediacy of Alkylhydridoplatinum(IV) and Alkane σ-Adducts

“Weakly Ligated, Labile Ligand” Nanoparticles: The Case of Ir(0)n·(H+Cl–)m

Comparison of the C−H Activation of Methane by M(C5H5)(CO) for M = Cobalt, Rhodium, and Iridium

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Product

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“Weakly Ligated, Labile Ligand” Nanoparticles: The Case of Ir(0)_n·(H⁺Cl^–)_m

Comparison of the C−H Activation of Methane by M(C₅H₅)(CO) for M = Cobalt, Rhodium, and Iridium