The effect of 19-electron formation constants on the electrochemistry and electron transfer induced substitution reactions of cyclopentadienylmetal halide complexes

The reduction of cyclopentadienylmetal halide complexes is generally considered to involve addition of an electron to an orbital that is antibonding with respect to the metal-halide bond. Subsequent metal-halide bond cleavage yields the halide and an organometallic radical. At inert electrodes, this radical is reduced further to an 18-electron anion. This series of reactions constitutes a prototypical ECE mechanism. Chemical reduction can be used to divert the radical into other pathways such as electron transfer chain catalyzed substitution. Attempts to initiate such reductively induced substitution reactions of CpFe(CO)₂I and Cp′Mo(CO)₃I give very different results, suggesting that these very similar complexes are reduced via substantially different mechanisms. Very likely, the molybdenum complex reacts via a DISP mechanism instead of ECE. The difference in electrochemical reduction mechanism as well as the different reactivity toward reductively induced substitution are explained in terms of a difference in the formation constants of 19-electron intermediates.     

19-electron intermediates in the ligand substitution of CpW(CO)3* with a Lewis base

The photochemical reactions of [CpW(CO)3]2 with the Lewis base P(OMe)3 are examined on the nanosecond and microsecond time scales using step-scan FTIR spectroscopy. Photolysis at 532 nm produces the 17-electron (17e) radicals CpW(CO)3*, which are in equilibrium with the 19-electron (19e) radicals CpW(CO)3P(OMe)3* on the nanosecond time scale. The reactions of the 19e radical are directly observed for the first time; the major reaction pathway is spontaneous loss of a carbonyl to form the 17e species CpW(CO)2P(OMe)3*, with a barrier of 7.6 +/- 0.3 kcal/mol for this process. The minor reaction pathway (<20%) at this concentration of P(OMe)3 (85 mM) is disproportionation to form the products CpW(CO)3P(OMe)3+ and CpW(CO)3-. On the microsecond time scale, the 17e radicals CpW(CO)2P(OMe)3* dimerize to form the ligand substitution product [CpW(CO)2P(OMe)3]2. These results indicate that the 19e species is a stable intermediate rather than transition state in the ligand substitution reaction, and this type of reactivity is likely to be typical of 17e organometallic radicals which undergo associative substitution mechanisms.     

Mutual Effect of Functional Groups and the Radical Center on the Reactivity of Nitroxyl Radicals

This review considers the correlation between the reactivity of nitroxyl radicals (piperidine, pyrroline, pyrrolidine, imidazoline, dihydroquinoline, tetrahydroquinoline, diphenyl nitroxide, etc.) and their chemical structure in terms of the rate constants of reactions between these radicals and hydrazobenzene. 4,4′-Di(tert-butyl)diphenyl nitroxyl has the highest reactivity, and the nitroxyl radical of benzoindolopyrrolidine is the least reactive (the difference is a factor of ∼10⁴). The effects of the metal atom in stable organometallic nitroxyl radicals and of the halogen atom in halogenated nitroxyl radicals on the reactivity of the nitroxyl center are considered. Data on the effect of the nitroxyl center on the reactivity of functional groups in the piperidine nitroxyl radical are generalized. Nitroxyl radicals with an activated double bond are shown by quantum chemical calculations to form cyclic transition complexes with amines, involving both the paramagnetic center and a double bond. This explains why the activated double bond in nitroxyl radicals is more reactive in nucleophilic additions of amines than the same bond in their diamagnetic analogues. The rate constants of nitroxyl reduction with hydrazobenzene and of nitroxyl oxidation with tetranitromethane are related to the σ_ESR constant derived from isotropic hyperfine coupling constants HFC_(aN), and their correlation with Hammett constants is demonstrated. The role of solvents in the reduction and oxidation of the nitroxyl radicals is considered. The influence of hydroxyl radical-polar solvent complexes and hydroxylamine-polar solvent H complexes on the course of reactions is considered for hydrogen atom transfer in systems of a sterically hindered nitroxyl radical and hydroxylamine.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 506–528.Original Russian Text Copyright © 2005 by Malievskii, Shapiro.     

Reductively induced homolytic carbon-carbon bond cleavage in Co(CO)3(PPh3)(COCF3)

The chemical or electrochemical reduction of the trifluoroacetyl complex Co(CO)₃(PPh₃)(COCF₃) involves a single electron transfer yielding trifluoromethyl radical and an anionic cobalt carbonyl complex. The mechanism is proposed to involve electron transfer followed by initial dissociation of either a carbonyl or phosphine ligand from the 19-electron [Co(CO)₃(PPh₃)(COCF₃)]⁻ anion. The resulting 17-electron intermediate undergoes subsequent one-electron reductive elimination of trifluoromethyl radical by homolytic cleavage of the carbon-carbon bond of the trifluoroacetyl group. The radical can be trapped by either benzophenone anion, forming the anion of α-(trifluoromethyl)benzhydrol, or Bu₃SnH, yielding CF₃H. The ultimate organometallic product is an 18-electron anion, either [Co(CO)₄]⁻ or [Co(CO)₃(PPh₃)]⁻, depending upon which ligand is initially lost. Fluorine-containing products were identified and quantitated by ¹⁹F NMR while cobalt-containing products were determined by IR.     

Reactions of 7,8‐Dithiabicyclo[4.2.1]nona‐2,4‐diene 7‐exo‐Oxide with Dodecacarbonyl Triiron Fe3(CO)12: A Novel Type of Sulfenato Thiolato Diiron Hexacarbonyl Complexes

The reaction of Fe₃(CO)₁₂ ( 13 ) with 7,8‐dithiabicyclo[4.2.1]nona‐2,4‐diene 7‐exo‐oxide ( 12 ) yields the sulfenato‐thiolato complex 14 , which is used as starting material for further reactions. The disulfenato complex 17 is obtained by using one equivalent of dimethyldioxirane (DMD), and the monoepoxide 18 is prepared by the oxidation of 14 with an excess of DMD. Complex 14 can be converted to the monophosphine complexes 19a and 19b by subsequent substitution of one CO ligand using trimethylaminoxide Me₃NO and triphenylphosphine PPh₃. Additional substitution reactions are done with 17 by using acetonitrile as a ligand to form 20a and 20b . In the electrochemical part of the paper, the reactions of the reduced iron species 14 , 15 , 17 , and 19a are studied.     

ETC: A Mechanistic Concept for Inorganic and Organic Chemistry

The concept of electron transfer catalysis (ETC), or more specifically “Double Activation Induced by Single Electron Transfer” (DAISET) gives an opportunity to connect experimental facts never previously correlated. The first activation results from the transfer of an electron to (or from) a molecular species; the second activation results from the build-up of a reaction chain able to reproduce the species formed in the first step. The starting point of this review is the S_RN 1 mechanism where principle and experimental diagnostic criteria are critically discussed. The thermal and photochemical exchange and substitution reactions of Pt^IV complexes are then reviewed together with the exchange reaction [AuCl₄]^?/Cl^?, reactions with Grignard reagents and other organometallic reagents, as well as the redox behavior of electronically excited organic compounds. Photochemical applications, including solar energy conversion are discussed. New aspects are also presented for the mechanistic problem “S_N 2 reaction or SET process?” Moreover, the concept has significance for S_H2 reactions at metal centers, molecule-induced homolyses, reactions of complexes, as well as electrochemical processes.–Unless otherwise specified, only double activation (DAISET) processes will be discussed in this article.     

Synthesis and Reactivity of Paramagnetic Nickel Polypyridyl Complexes Relevant to C(sp2)–C(sp3)Coupling Reactions

A number of new transition metal catalyzed methods for the formation of C(sp²)–C(sp³) bonds have recently been described. These reactions often utilize bidentate polypyridyl‐ligated Ni catalysts, and paramagnetic Ni^I halide or aryl species are proposed in the catalytic cycles. However, there is little knowledge about complexes of this type. Here, we report the synthesis of paramagnetic bidentate polypyridyl‐ligated Ni halide and aryl complexes through elementary reactions proposed in catalytic cycles for C(sp²)–C(sp³) bond formation. We investigate the ability of these complexes to undergo organometallic reactions that are relevant to C(sp²)–C(sp³) coupling through stoichiometric studies and also explore their catalytic activity.     

Organometallic dithiolene complexes of the group 8-10 metals: reactivities, structures and electrochemical behavior

Research progress in the organometallic dithiolene complexes such as [Cp(or Cp*)M(dithiolene)] (M = Co, Rh, Ir, Ni), [(C(6)R(6))Ru(dithiolene)] and [(C(4)R(4))Pt(dithiolene)] complexes during the past decade is described and the reactivities, structures and electrochemical behavior are summarized in this paper. The five-membered metalladithiolene ring (MS(2)C(2)) undergoes addition reactions to the M[double bond, length as m-dash]S bond to form 18-electron adducts by an imido, alkylidene, alkene or norbornene group and also undergoes dimerizations on the basis of the unsaturation in the ring. The aromaticity of the ring causes substitution reactions on the dithiolene carbon by a C-centered radical, S-centered radical or succinimide group when the ring has a C-H bond. Furthermore a dithiolene-dithiolene homo-coupling reaction by an acid or dithiolene-aryl cross-coupling occurs based on the aromaticity in the ring. Dissociations of the 18-electron adducts are observed by those thermolyses, photolyses, electrochemical redox reactions and other chemical reactions with tertiary phosphorus compounds. One representative example of them is the imido adduct dissociation with PR(3) under heating toward the intramolecular imido migration to a Cp ligand. Since all products are rearomatized by those adduct dissociations, it is concluded that the 'coexistence of aromaticity and unsaturation' in the metallacycle mediates the diverse chemical reactions.     

Coordinatively unsaturated organometallic system based on Tp ligand: tetrahedral TpM-R′ and TpM-M′Ln species

Chemistry of the highly coordinatively unsaturated, tetrahedral hydrocarbyl and dinuclear complexes bearing a hydrotris(pyrazolyl)borate (Tp^R) ligand, Tp^RM-R′ and Tp^RM-M′L_n, is reviewed. The organometallic Tp^R complexes are prepared by salt elimination between the corresponding halide and Grignard reagents or metalates and fully characterized by spectroscopic and crystallographic methods. Although the number of the valence electrons of the resultant species is much shorter than that expected for a coordinatively saturated species (for mononuclear species: 14-15e vs. 18e; for dinuclear species: 29-32e vs. 34e), they turn out to be thermally stable. In particular, the ethyl complexes Tp^iPr2M-CH₂CH₃ (M = Fe, Co) are stable with respect to β-hydride elimination. The tetrahedral structures of the obtained organometallic species cause a small ligand field splitting of the frontier orbitals to lead to a high spin configuration, which leaves no vacant coordination site, and this should be the origin of the thermal stability of the electron deficient species. Upon interaction with donors they are incorporated into the organometallic system via switching of the spin state, and selective reactions dependent on the nature of the donor molecules are observed for the dinuclear complexes. Thus the high spin species can be regarded as masked forms of coordinatively unsaturated intermediates, which are regarded as key intermediates of organometallic transformations.     

Redox properties of rhodium diene-cyclopentadienyl complexes

The electrochemical behavior of rhodium sandwich complexes containing η⁴-pentamethylcyclopentadiene or tetramethylfulvene fragments has been studied by cyclic voltammetry. The complexes undergo one-electron oxidation to give unstable 17-electron radical cations which are converted into rhodocenium salts as a result of elimination or uptake of hydrogen or C-C bond cleavage. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1802–1805, October, 1993.     

σ‐Noninnocence: Masked Phenyl‐Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of ?2 at a transition‐metal center. For a series of formal high‐valent Ni^IV complexes, aryl–CF₃ bond‐forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015 , 137, 8034–8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal Ni^IV centers are better described as approaching a +II oxidation state, originating from highly covalent metal–ligand bonds, a phenomenon attributable to σ‐noninnocence. A direct consequence is that the elimination of aryl–CF₃ products occurs in an essentially redox‐neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF₃ group is transferred to an electrophilic aryl group. The uncovered role of σ‐noninnocence in metal–ligand bonding, and of an essentially redox‐neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.     

19-electron intermediates and cage-effects in the photochemical disproportionation of [CpW(CO)3]2 with Lewis bases

The role of 19-electron intermediates in the photochemical disproportionation of [CpW(CO)3]2 (Cp = C5H5) with Lewis bases (PR3; R = OMe, Bu, Ph) is investigated on the ultrafast time scale using femtosecond VIS-pump, IR-probe spectroscopy. Formation of a 19-electron (19e) species CpW(CO)3PR3*by coordination of PR3 with photogenerated 17-electron (17e) radicals CpW(CO)3* is directly observed, and equilibrium is established between the 17e radicals and the 19e intermediates favoring 19e intermediates in the order: Bu > OMe > Ph. Steric effects dominate the 17e/19e equilibrium when the cone-angle of the Lewis base exceeds a certain limiting value (between 132 degrees and 145 degrees ), but below this value electronic properties of the Lewis base control the 17e/19e dynamics. Disproportionation occurs in less than 200 picoseconds by electron transfer between a solvent caged 17e radical and 19e, highly reducing species. The rate and extent of ultrafast disproportionation depends on both the identity and concentration of the Lewis base. In low concentrations of PR3 (typically 1-2 M or less) or with Lewis bases whose equilibrium heavily favors 17e radicals (e.g., PPh3), disproportionation is rate-limited by breakdown of the solvent cage. Density functional theory calculations on vibrational frequencies and charge distributions of the various complexes support the experimental results.     

The free-radical reaction of tri-n-butyltin hydride with disulfides

The kinetics and absolute rate constants for the free-radical chain reaction of tri-n-butyltin hydride with di-t-butyl disulfide have been measured in cyclohexane at 30°. The rate controlling step for chain propagation involves the cleavage of the disulfide bond by an attacking tributyltin radical. The rate constant for this bimolecular homolytic substitution at sulfur is ～8 × 10⁴ Mole^?1 sec^?1. Chain termination involves the self-reaction of two tributyltin radicals. The rate constants for attack of tributyltin radicals on some other disulfides and on elemental sulfur have also been measured. The results are compared with literature data for homolytic substitutions on these compounds by a variety of radicals which have their unpaired electron centered on carbon.     

Hydrogen binding property of Co- and Ni-based organometallic compounds

The binding property of hydrogen on organometallic compounds consisting of Co, and Ni transition metal atoms bound to C _m H _m rings (m = 4, 5) is studied through density functional theory calculation. CoC _m H _m and NiC _m H _m complexes can store up to 3.49 wt% hydrogen with an average binding energy of about 1.3 eV. The adsorption characteristics of hydrogen to organometallic compounds are investigated by analyzing vibrational spectra of CoC₄H₄(H₂) _n and NiC₄H₄(H₂) _n (n = 0, 1, 2). The kinetic stability of these hydrogen-covered organometallic complexes is assured by analyzing the energy gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals. It is also discussed the application of 18-electron rule in predicting maximum number of hydrogen molecules that could be adsorbed by these organometallic compounds.     

Oxidation of Organometallic Compounds

Stable organometallic compounds, notably of the later transition metals (groups VI–VIII), usually are characterized by closed shell electron configurations (typically 18-electron valence shells) which are destabilized by electron addition or removal. One-electron oxidation of such compounds results in the formation of unstable radical ions, whose characteristic reactivity patterns include susceptibility to nucleophilic attack, disproportionation, and metal-carbon bond dissociation. Two-electron oxidation may result in dissociation or oxidation of the organic ligand. In this review studies on the chemical and electrochemical oxidations of metal carbonyls, metal-olefin complexes, and alkyl transition-metal compounds are described. The studies encompass the following themes: (1) The kinetics and thermodynamics of the initial redox steps; (2) The characterization and reactivity patterns of the resulting oxidation products; (3) The synthetic and catalytic applications of organometallic redox processes.     

Electronic properties of polyoxometalate derivatives [(C2B9H11) M'M5O18]n‐ (M' = TiIV,MoVI, WVI; M = MoVI,WVI): Protonation,Electronic Spectra,and Redox Properties

Density functional theory is used to study the electronic structures and properties of Lindqvist‐type polyoxometalates‐supported organometallic compounds [LM'M₅O₁₈]^n– (L = [C₂B₉H₁₁]^2– (Cb), [C₅H₅]^– (Cp); M' = Ti^IV, Mo^VI, W^VI; M = Mo^VI, W^VI). [(Cb)M'M₅O₁₈]^n– are a series of novel compounds designed in this work, based on related experiment. The calculated results reveal that the Cb ligand is able to form a σ, 2π triple bond with M', which is similar to the bond character in [(Cp)M'M₅O₁₈]^n–. However, comparing with the protonation, electronic spectra and redox properties of [(Cp)M'M₅O₁₈]^n– and [M'M₅O₁₉]^n–, [(Cb)M'M₅O₁₈]^n– species show the advantageous electronic properties owning to the superior electron donating ability of the Cb ligand. © 2015 Wiley Periodicals, Inc.     

Intermediacy of Proton-Bound Dimers and Ion/Dipole Complexes in the Unimolecular Decompositions of Dialkyl-Peroxide Radical Cations: Evidence for a coupled proton and hydrogen-atom transfer

The unimolecular fragmentation reactions of the radical cations of diethyl, diisopropyl, dipropyl, isopropyl propyl, and di(tert-butyl) peroxide have been investigated by mass spectrometric and isotopic labeling techniques. Two competing pathways for unimolecular decomposition in the μs time regime (metastable ions) are observed: i) A combination of an α-C? C bond cleavage and a H migration gives rise to proton-bound dimers of two ketone or aldehyde molecules. ii) Ion/dipole complexes of alkyl cations and alkylperoxy radicals are generated by C–O bond cleavage. These complexes either exhibit direct losses of alkylperoxy radicals, or they rearrange via a coupled proton and H-atom transfer, this sequence of unprecedented isomerizations is completed by losses of alkyl radicals. Collisional activation experiments confirm that the ionic products of the latter process correspond to RR′C?OOH⁺; these ions can be regarded as protonated carbonyl oxides. In addition, we observe the elimination of alkenes leading to hydroperoxide radical cations and the expulsion of HO radicals. The latter process implies a C? C bond formation step between the two alkyl fragments leading to higher alkyl cations.     

An Isolable Radical Anion Featuring a 2-Center-3-Electron π-Bond without a Clearly Defined σ-Bond

Featuring an extra electron in the π* antibonding orbital, species with a 2-center-3-electron (2c3e) π bond without an underlying σ bond are scarcely known. Herein, we report the synthesis, isolation and characterization of a radical anion salt [K(18-C-6)]⁺{[(HCNDipp)₂Si]₂P₂}⋅⁻ (i.e. [K(18-C-6)]⁺ 3 ⋅⁻) (18-C-6=18-crown-6, Dipp=2,6-diisopropylphenyl), in which 3 ⋅⁻ features a perfectly planar Si₂P₂ four-membered ring. This species represents the first example of a Si- and P-containing analog of a bicyclo[1.1.0]butane radical anion. The unusual bonding motif of 3 ⋅⁻ was thoroughly investigated via X-ray diffraction crystallography, electron paramagnetic resonance spectroscopy (EPR), and calculations by density functional theory (DFT), which collectively unveiled the existence of a 2c3e π bond between the bridgehead P atoms and no clearly defined supporting P−P σ bond.     

Electron Transfer Chain Catalysis

Electron-transfer chain (ETC) catalysis belongs to the family of chain reactions where the electron is the catalyst. The ETC mechanism could be initiated by chemical activation, electrochemistry, or photolysis. If this pathway is applied to the preparation of organometallic complexes, it utilizes the greatly enhanced reactivity of organometallic 17e and 19e radicals. The chemical propagation is followed by the cross electron-transfer while the electron-transfer step is also followed by the chemical propagation, creating a loop in which reactants are facilely transformed into products. Interestingly the overall reaction is without any net redox change.     

Electrochemical reduction of Ru(η-arene)(κ-tris(pyrazolyl)methane) dicationic complexes

One-electron reduction of [Ru(η⁶-arene)(κ³-HCPz₃)]²⁺ complexes (arene = p-xylene, p-cymene, or hexamethylbenzene, Pz = pyrazolyl) was observed at approximately −1.4 V vs. ferrocene in nonaqueous media. Cyclic voltammetry data showed no direct evidence of a hapticity change for either the arene or the tris(pyrazolyl)methane (TPM) ligand in the electron-transfer step. The resulting 19-electron radicals underwent reactions at the arene, giving persistent cyclohexadienyl ruthenium complexes which could be oxidized back to the original complex in an overall chemically reversible process. The dominant reaction for the hexamethylbenzene-containing radical was shown to be an arene-based dimerization (k_dim = 4 (±3) × 10³ M⁻¹ s⁻¹) which occurs in competition with an H-atom capture, most likely from solvent. The behavior of the 19-electron radicals is similar to that of their all-carbocyclic mixed-sandwich analogues Ru(arene)(C₅R₅) [O.V. Gusev, M.A. Ievlev, T.A. Peganova, M.G. Peterleitner, P.V. Petrovskii, Y.F. Oprunenko, N.A. Ustynyuk, J. Organomet. Chem. 551 (1998) 93].