Characterization and Reactions of Previously Elusive 17-Electron Cations: Electrochemical Oxidations of (C6H6) Cr (CO) 3 and (C5H5) Co (CO) 2 in the Presence of [ … (original) (raw)
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Journal of The American Chemical Society, 2006
Anodic oxidation of the important half-sandwich compound CoCp(CO)2, 1, has been studied under gentle electrolyte conditions, e.g., chlorinated hydrocarbons with weakly coordinating anion (WCA) supporting electrolyte anions. The 17-electron cation 1 + produced at E1/2(1) ) 0.37 V vs FeCp2 0/+ undergoes a surprising reaction with neutral 1 to form the dimer radical cation [Co2Cp2(CO)4] + , 2 + , which has a metalmetal bond unsupported by bridging ligands. The dimer radical is oxidized at a slightly more positive potential (E1/2 ) 0.47 V) to the corresponding dication 2 2+ . Observation of the oxidation of 2 + is without precedent in confirming a radical-substrate (R-S) dimerization process by direct voltammetric detection of the R-
Organometallics, 2021
Recent reports have identified Cp*Co-based complexes as powerful catalysts for aromatic C-H bond activation under oxidative conditions. However, little is known about the speciation of Cp*Co species during catalysis. We now show that key intermediates, Cp*Co(III) metallacycles derived from 2-phenylpyridine (phpy-H), react swiftly in solution with one-electron oxidants to irreversibly collapse by a cyclocondensation of the organic ligands to afford cationic alkaloids in yields of >70 %. Low temperature EPR analysis of a mixture of cobaltacycle with the tritylium cation reveals the signatures of trityl and Co(IV)-centred radicals. Electrochemical analyses show that the oxidation of these cobaltacycles is irreversible and gives rise to several products in various amounts, among which the most salient ones are a cationic alkaloid resulting from the cyclocondensation of the phpy and Cp* ligands, and the dimeric cation {[Cp*Co] 2 (-I) 3 } +. DFT investigations of relevant noncovalent interactions using QTAIMbased NCI plots and Intrinsic Bond Strength Index suggest a ligand-dependent predisposition by "NCI-coding" for the Co(IV)templated cyclocondensation, the computed reaction network energy profile for which supports the key roles of a short lived Co(IV) metallacycle and of a range of triplet state organocobalt intermediates. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. Full experimental procedures and details, voltammograms, EPR, Mass and NMR spectra, energies and Cartesian coordinates, high resolution NCI figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Reduction potentials (E 0) of six mononuclear cobalt catalysts (1−6) for hydrogen evolution reaction and electron donating/withdrawing effect of nine X-substituents on their macrocyclic ligand are reported at solvation effect-included B3P86/ 6-311+G** level of density functional theory. The electrostatic potential at the Co nucleus (V Co) is found to be a powerful descriptor of the electronic effect experienced by Co from the ligand environment. The V Co values vary substantially with respect to the nature of macrocycle, type of apical ligands, nature of substituent and oxidation state of the metal center. Most importantly, V Co values of both the oxidized and reduced states of all the six complexes show strong linear correlation with E 0. The correlation plots between V Co and E 0 provide an easy-to-interpret graphical interpretation and quantification of the effect of ligand environment on the reduction potential. Further, on the basis of a correlation between the relative V Co and relative E 0 values of a catalyst with respect to the CF 3-substituted reference system, the E 0 of any X-substituted 1−6 complexes is predicted. ■ INTRODUCTION Mononuclear cobalt complexes of tetraazamacrocyclic ligands are promising class of hydrogen evolving electro catalysts, known to work at modest over potential. 1−10 Recently Solis and Hammes-Schiffer studied the effect of substituents on tuning the reduction potentials (E 0) of cobalt diglyoxime complex referred to here as 1-X. 11 Solis and Hammes-Schiffer showed that E 0 and pK a values of 1-X correlates linearly to the Hammett substituent constant. The E 0 becomes more negative with increase in the electron donating character of the substituent. Theoretical calculation of E 0 to the experimental accuracy is very difficult to achieve as it demands very accurate estimation of thermo-dynamic parameters for both oxidized and reduced forms of the complex. This becomes even more challenging for various substituted cases as the finer substituent effects may lead to subtle variations in E 0 values. Solis and Hammes-Schiffer's work suggests that E 0 for the 1-X complex can be predicted with a knowledge of substituent effect. Among the theoretically derived properties useful for the interpretation and quantification of substituent effects in molecular systems, the topographical and surface features of molecular electrostatic potential (MESP) have been widely used. The MESP can be experimentally determined from electron density data derived from X-ray diffraction studies on crystals whereas being a one electron property, its accurate calculation is rather easy with theoretical methods implemented in many of the standard ab initio/DFT program packages. The use of the theoretically derived MESP to understand molecular reactivity has been pioneered by the works of Tomasi, 12 Pullman, 13 Politzer, 14−19 and Gadre. 20−23 Recently the works of Wheeler and Houk 24−28 have contributed to the growth of this area. In many of the studies from our group, we have shown that MESP based analysis is useful to interpret and quantify resonance effect, 29 inductive effect, 30 substituent effects, 31,32 trans influence, 33 cation-π interactions, 34−36 lone pair-π interactions , 37 noncovalent interactions including a large variety of hydrogen bonds, 38 aromatic character of benzenoid hydrocarbons , 39,40 stereoelectronic features of ligands in organo-metallic/inorganic chemistry 41−46 etc. Very recently we have
Journal of the American Chemical Society, 1991
The substitution of pyridine by trimethyl phosphite in the complexes M(C0)3(PCy&(py) (M = Cr, Mo) has been studied by stopped-flow kinetics. Direct reactions of the proposed intermediate complexes M(CO),(PCy3), (M = Cr, Mo) have also been studied. The crystal structure of Cr(C0) (Pcy,), has been determined and shows an an agostic M-H-C interaction. Cell parameters: a = 10.127 (2) A, b = 12.524 (3) A, c = 15.329 (3) A, B = 90.93 ( 3 ) O , y = 103.28 (3)O, space group Pi,
The Unexpected Role of CO in C À H Oxidative Addition by a Cationic Rhodium(I) Complex
The activation of strong carbon-hydrogen bonds by transition metals is one of the fundamental fields of current organometallic chemistry. This process occurs by one of several possible pathways that are generally dependant on the electron density at the metal center. [1] For electron-rich, low-valent transition metals the typical pathway for C À H cleavage is oxidative addition, which leads to the corresponding alkyl or aryl hydride complexes and is accompanied by a formal twoelectron oxidation of the metal. Transition metals that lack the electron density necessary for oxidative addition, such as early transition metals or high-valent late transition metals, can activate C À H bonds by alternative routes, namely s-bond metathesis, radical activation, 1,2-addition, and electrophilic substitution. [1] It is widely accepted that both s-bond metathesis and oxidative addition processes take place via scomplexes or agostic intermediates. [2] As far as the oxidative addition of CÀH bonds is concerned, the requirement for high electron density means that strong p-acceptor ligands, such as carbon monoxide, are normally expected to inhibit oxidative addition processes by drawing electron density away from the metal center. Herein, however, we describe an electron-poor cationic Rh I system in which addition of a CO ligand can actually promote oxidative addition of a strong C À H bond. This unique reaction pathway is supported by both experimental and theoretical evidence.
Radical Pathways in Reactions of Transition Metal Organometallic Compounds
Annals of the New York Academy of Sciences, 1980
Nearly all transition metal organometallic compounds are spin-paired, closedshell, ground-state molecules. For the most part, they obey the so-called 18-electron rule, which means that the sum of valence shell electrons from the metal atom, and the electrons that may be considered as donated from the ligands in the u interaction with the metal, totals 18. It has long been recognized that coordinative unsaturation at the metal is a prerequisite for many of the most important reaction processes involving transition metal organometallic species. Loss of a ligand, e.g., CO, or a valence tautomeric equilibrium (e.g., q3-CsH5 C I q1-C3H5), results in formation of a 16-electron species in so1ution.t
Journal of the American Chemical Society, 1995
An infrared spectroscopic method was devised for measuring 6 in 1 8 f 6 organometallic complexes containing the chelating diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (L2) or 2,3-bis(diphe-nylphosphin0)-2-~yclopentene-1P-dione (L2'). (1 8 f 6 complexes are 19-electron complexes in which the unpaired 19th valence electron is primarily localized on a ligand; 6 represents the amount of the unpaired electron's charge on the metal.) 6 values for the Co(CO)3L2, Co(CO)3Li, Fe(CO)3L2-, and Fe(C0)3L2'-complexes fall in the range 0.01-0.25. The effects of the metal, the ligands, and the solvent on 6 were quantitatively evaluated. In addition, the effect of 6 on the reactivity was examined by studying the dissociative substitution reactions of Co(CO)3L2. The following principles emerged: (1) 6 is larger for complexes with a more electronegative metal center (e.g., Co(1) vs Fe(0)). (2) 6 is smaller for complexes containing the more electronegative L2 ligand than for those with the less electronegative L2'. (3) 6 increases with decreasing solvent polarity, but increases with increasing solvent donicity. (4) For those cases in which 6 is manipulated by changing the solvent, there is no simple correlation between 6 and the rate of a dissociative substitution reaction in an 18+6 complex. The latter two results are interpreted in terms of a model in which donor solvents increase the electronic population of the n* SOMO on the L2 ligand and acceptor solvents decrease the electron density in this orbital. Additional electron density in the n* orbital increases delocalization of the unpaired electron onto the Co fragment, causing 6 to increase and weakening the Co-CO bond. (The acceptor orbital on the Co fragment is Co-CO u antibonding.) There is no correlation between the rate constants and 6 because Asf effects are significant, especially in polar solvents. Nineteen-electron organometallic adducts, formed via the reactions of 17-electron radicals with 2-electron donors (eq l), are important intermediate's in many reactions.
The redox behavior of CpCo(CO) 2 , 1, (Cp = η 5-C 5 H 5) has been investigated in a number of organic solvents having different polarity and donor strength, in the presence of [B(C 6 F 5) 4 ]as the supporting electrolyte anion. Voltammetric oxidation in weakly-donor (donor number, DN solv , ~ 0) and low dielectric (ε) solvents such as benzotrifluoride (BTF, ε = 9.2), 1,2-dichloroethane (DCE, ε = 10.36), and 1,2-difluorobenzene (DFB, ε = 13.8) is identical to that observed in dichloromethane (DCM, ε = 8.9)/[NBu 4 ][B(C 6 F 5) 4 ] media. Thus, in these gentle solvent/electrolyte media, the electrochemically generated 17ecation, [CpCo(CO) 2 ] + , 1 + , is postulated to undergo "radical-substrate" (R-S) dimerization via reaction with its neutral counterpart, 1, to eventually form, upon further one-electron oxidation [Cp 2 Co 2 (CO) 4 ] 2+ , 2 2+. However, this R-S coupling reaction is eliminated when the strongly donor tetrahydrofuran (THF, DN solv = 20) or the highly polar nitromethane (NM, ε = 37.3) are used with [NBu 4 ][B(C 6 F 5) 4 ], owing to rapid attack by solvent molecules on 1 + and formation of solvated species that dissociate into different oxidation products such as Cp 2 Co. These results attest that the redox behavior of CpCo(CO) 2 is highly affected by the sequential changes in solvation energies and/or ion pairing, which can be manipulated via utilization of different solvent/electrolyte media.