Chem. Rev. 1996, 96, 877−910 877

Chemical Redox Agents for Organometallic Chemistry

Neil G. Connelly*,† and William E. Geiger*,‡

School of Chemistry, University of Bristol, U.K., and Department of Chemistry, University of Vermont, Burlington, Vermont 05405-0125

Received October 3, 1995 (Revised Manuscript Received January 9, 1996)

Contents I. Introduction 877 A. Scope of the Review 877 B. Benefits of Redox Agents: Comparison with 878 Electrochemical Methods 1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 The authors (Bill Geiger, left; Neil Connelly, right) have been at the forefront of organometallic electrochemistry for more than 20 years and have had 1. Radical Cations 891 a long-standing and fruitful collaboration. 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich 894 Neil Connelly took his B.Sc. (1966) and Ph.D. (1969, under the direction Compounds of Jon McCleverty) degrees at the University of Sheffield, U.K. Post- 4. Quinones 895 doctoral work at the Universities of Wisconsin (with Lawrence F. Dahl) 5. Other Organic Oxidants 896 and Cambridge (with Brian Johnson and Jack Lewis) was followed by an appointment at the University of Bristol (Lectureship, 1971; D.Sc. degree, III. Reductants 896 1973; Readership 1975). His research interests are centered on synthetic A. Inorganic 896 and structural studies of redox-active organometallic and coordination 1. Alkali Metals 896 complexes and have been presented in more than 150 publications. He is an active but unsuccessful orienteer and an optimistic but often 2. Complex Reducing Agents 898 disappointed supporter of Brentford Football Club. 3. Hydrazine 898 B. Organic 898 Bill Geiger received his B.S. from Canisius College (1965) and his Ph.D. 1. Metal Naphthalenides 898 from Cornell University (with David H. Geske, 1969) and did postdoctoral 2. Benzophenone Anion Radical 899 work at the University of California at Riverside with August H. Maki and 3. Triethylamine 900 at Northwestern University with Donald E. Smith before joining the faculty at Southern Illinois University in 1970. In 1974 he moved to the University 4. C2(NMe2)4 900 of Vermont. His main interest has been in the use of electrochemistry C. Organometallic 900 and spectroelectrochemistry to probe the molecular and electronic 1. Metal Sandwich Complexes 900 structures of odd-electron organometallic compounds. Born and raised 2. Metal Carbonylates 903 in Buffalo, he has been an avid fan of the Buffalo Bills since their inception, so he is familiar with near misses. IV. Conclusions 903 V. Appendix: Syntheses of Ferrocenium and 904 Diazonium Salts A. Ferrocenium Salts 904 I. Introduction 1. Method i. Ferrocenium Tetrafluoroborate, 904 [FeCp2][BF4] A. Scope of the Review 2. Method ii. Ferrocenium 904 As interest has grown in electron-transfer (ET) Hexafluorophosphate, [FeCp2][PF6] reactions of organometallic compounds, so too has the 3. Method iii. Acetylferrocenium 904 use of chemical oxidizing and reducing agents to Tetrafluoroborate, [Fe(η-C5H4COMe)Cp]- effect these transformations. The aim of this review [BF4] is to show how one-electron oxidants and reductants B. 4-Fluorobenzenediazonium 904 have been used in preparative chemistry (incorporat- Hexafluorophosphate, [N2C6H4F-4][PF6] ing both synthetic applications and generation of VI. List of Abbreviations 904 VII. Acknowledgments 904 † University of Bristol. VIII. References 904 ‡ University of Vermont. 0009-2665/96/0796-0877$25.00/0 © 1996 American Chemical Society

+ + 878 Chemical Reviews, 1996, Vol. 96, No. 2 Connelly and Geiger species for in situ characterization) in nonaqueous 1. Advantages of Chemical Redox Agents solutions, the usual media for organometallic ET reactions. It is hoped that the information in this a. Absence of Supporting Electrolyte. A post- review will also be useful to those interested in ET electrolysis solution contains a supporting electrolyte reactions of organic and inorganic systems, which salt in large (up to 1000-fold) excess over the desired have more often involved the use of labile metal ions electrode product. If the latter carries a charge, it such as M(III), M ) Fe, Tl1 (but see ref 2). may have a solubility similar to that of the support- ing electrolyte, making separation difficult. Perhaps A few words are in order about the reagents we the greatest advantage of chemical redox agents is have chosen to describe. Given the vast array of that the experimentalist is not confronted with this molecules and ions known to undergo one-electron separation problem in the workup of the reaction processes, some focus in coverage is necessary. We solution. have emphasized reagents in common usage and provide references to show typical applications rather b. Rapid, Large-Scale Preparations. Chemical than attempt comprehensive listings. We have also redox reactions generally yield larger quantities of included some less widely known reagents which are, products in briefer reaction times, and at higher in our opinion, worthy of further development. The concentrations, when compared with electrolytic subjective nature of such choices is obvious. Finally, preparations. Except for thin-layer electrochemical 7a,b we have not treated photochemically-generated cells, which are very limited in the quantities of reducing agents which, although generally transient reagent that can be produced, preparative electro- species, may have advantages in some applications.3 chemical cells have reaction times of tens of minutes, a time frame that may be troublesome if the desired The reader will note that in places we have referred product has limited stability. Homogeneous chemical to personal communications from colleagues. We are redox reactions, on the other hand, often proceed as grateful to them for providing many practical hints rapidly as solutions of the redox agent and the on the preparation and use of the particular reagents substrate are mixed. with which they are highly experienced. We would also appreciate correspondence from readers who use This time-scale advantage is compromised when other valuable oxidants or reductants which we may the chemical redox reaction is heterogeneous, as have inadvertently missed. occurs with dissolving-metal reductions, for example, or when the reagent is sparingly soluble, for example, B. Benefits of Redox Agents: Comparison with when ceric ammonium sulfate is used in CH2Cl2. The mass transfer factors which come into play are then Electrochemical Methods analogous to those encountered in electrolysis experi- Brief treatments have previously appeared con- ments. cerning applications of redox agents in metal dithio- Redox agents are easily adapted to low-temperature lene chemistry2a and in organic ET chemistry.2b In studies. Several applications are quoted below this section we emphasize the comparative benefits wherein the redox agent and the substrate are and shortcomings in chemical vs electrochemical allowed to dissolve and mix just above the melting approaches to preparative redox chemistry. point of a solvent, conditions very hard to duplicate Electrochemical methods are without rival for in practical electrochemical experiments. In elec- investigating the mechanisms of ET reactions.4 In trolysis experiments, lower diffusion coefficients and preparative applications, however, electrochemical higher ohmic effects lengthen reaction times at lower and nonelectrochemical approaches are closer in temperatures.7c relative merit. An experimental protocol making use c. Use of Nonpolar Solvents. The nature of the of the complementary aspects of both approaches is solvent medium influences the course of ET reactions. often more effective than either individual method. In electrochemistry, the use of solvents with polari- Controlled potential electrolysis (CPE) is the elec- ties significantly less than those of CH2Cl2 ( ) 9) trochemical method most often adopted for synthetic- and THF ( ) 7) is quite difficult because the scale redox reactions.4 Its versatility (wide range of electrolyte salt must dissolve and appreciably dis- accessible potentials) and precision ((1 mV or better sociate in order to render the solution conducting. in the absence of resistance effects) make CPE ideal Avoiding the requirement of a supporting electrolyte for forcing the uptake or release of one or more in chemical redox reactions opens opportunities for electrons from a substrate. the use of very low polarity solvents such as pentane The literature of CPE is well-developed for organic, and toluene in ET reactions. inorganic, and organometallic systems.5,6 When CPE Two potential major benefits arise from the use of is coupled with voltammetric methods such as cyclic low-polarity solvents such as aromatic and aliphatic voltammetry, polarography (in its various forms), and hydrocarbons, one relating to reactivity, the other to voltammetry at rotated electrodes, electrochemical solubility. Low-polarity solvents are often relatively methods can initiate and monitor the progress of noncoordinating and seldom displace ligands from simple or complex redox processes. Putting aside the primary redox product. For this reason, in- matters of availability of equipment and the inves- creased chemical stability may often be observed for tigator’s training in its use, what reasons might one organometallic complexes in these media. have, then, for using chemical redox agents to effect Rapid precipitation of highly reactive charged preparative-scale ET reactions? We now consider products is feasible using nonpolar solvents. A some relative advantages and disadvantages of chemi- particularly effective use of this strategy arises when cal vs electrochemical approaches. both the redox agent and the substrate are soluble

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 879 in nonpolar solvents; mixing the solutions or allowing Table 1. Formal Potentials (V) for the Ferrocene+1/0 them to diffuse together yields a precipitate of the Couple vs SCE in Selected Electrolytesa + b c d desired species (A in the case of the oxidation solvent [NBu4][PF6] [NEt4][PF6] [NBu4][ClO4] reaction of eq 1). The counterion of the desired nitromethane 0.35 0.31 0.34 + species is the redox byproduct, for example, [CoCp2] nitrobenzene 0.37 n when cobaltocene is used as a reductant and [Ni(tfd)2] propylene carbonate 0.38 0.36 0.34 (n )-1, tfd ) [S C (CF ) ]2-) when the neutral MeCN 0.40 0.38 0.38 2 2 3 2 DMSO 0.43 0.45 reagent (n ) 0) is used as an oxidant. DMF 0.45 0.46 0.47 CH2Cl2 0.46 0.48 A + Ox f [A][Red] (1) acetone 0.48 0.46 0.50 glyme 0.51 THF 0.56 0.53 others 2. Disadvantages of Chemical Redox Agents e H2O (0.1 M NaF) 0.16 V f MeCN (0.2 M Li[ClO4]) 0.31 V When compared with electrochemical methods, a Supporting electrolyte concentration, 0.1 M. b Data from chemical redox agents have a number of disadvan- W. E. Geiger’s laboratory, University of Vermont. c Chang, J. tages, listed here in anticipation of the discussion of P.; Fung, E. Y.; Curtis, J. C. Inorg. Chem. 1986, 25, 4233. individual reagents in sections II and III. d Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984, 23, 817. e Bond, A. M.; McLennan, E. A.; Stojan- a. Fixed Redox Strength. This is the greatest ovic, R. S.; Thomas, F. G. Anal. Chem. 1987, 59, 2853. limitation of chemical redox agents. A given reagent f Kuwana, T.; Bublitz, D. E.; Hoh, G. J. Am. Chem. Soc. 1960, in a given medium has a fixed formal potential so 82, 5811. that a range of reagents is required to provide a range of different redox strengths. b. Chemical Reagents May Be Noninnocent. C. Potentials in Nonaqueous Solvents In this context, an “innocent” reagent is one which engages exclusively in an outer-sphere ET reaction. Whereas electrodes are among the most benign Formal potentials, E°′, are given in this review vs reagents in synthetic applications, chemical redox the ferrocene/ferrocenium couple, as recommended by agents are more likely to engage in inner-sphere IUPAC.8 In most cases, conversions were required electron-transfer processes and to display coordina- from literature values referenced to other couples. tion tendencies, in addition to leaving a potentially Where a choice was involved, we used conversions reactive byproduct in solution. The nitrosonium from data obtained vs the aqueous saturated calomel + cation [NO] is, for example, prone toward inner- electrode (SCE), which is reasonably stable in non- sphere ET mechanisms which occasionally result in aqueous solutions.9 Conversions from the SCE scale coordination of the reagent (as NO) to a metal in the to [FeCp ]+/0 were based on values compiled in our oxidizable substrate. Related to the question of 2 laboratories or reported in the literature for different reagent innocence is the chemistry of the reagent solvents (Table 1). In some cases the conversions byproduct in the reaction medium. If Ox is employed were circuitous; the correction parameters are listed as the original redox agent, then the chemistry of Red, the reaction byproduct, must be anticipated or for each reagent included in Tables 2 (oxidants) and taken into account. The reagent byproduct may have 3 (reductants). We strongly urge workers to report ligating properties, be acidic or basic, or be able to the ferrocene/ferrocenium potential, vs their chosen initiate other reactions such as H-atom transfer, reference electrode, under the experimental conditions causing the desired electron-transfer product to react employed. further. For most couples, it was judged unnecessary to list c. Reagent Byproduct in Reaction Mixture. formal potentials in more than one solvent. Solvent- Even if the reagent byproduct is unreactive, it may based variations in E°′ tend to be small for larger, still pose difficulties in the spectroscopic character- coordinatively saturated, reagents such as [CoCp2] 3+ ization and/or isolation of the desired product. Since and [Fe(bipy)3] . Potentials of smaller ions such as many applications of chemical redox agents have the Ag+ or [NO]+, which are more highly affected by purpose of generating spectra of a desired product, solvent, are more fully reported. It is beyond the the possibility of spectral interference by the reagent scope of this review to be generally concerned about byproduct (or the reagent itself) must be considered. the effects of ion pairing on the formal potentials, Good experimental design anticipates the optical, although occasional comments on this subject are vibrational, and magnetic resonance spectra of both included. The interested reader may consult the Ox and Red. original papers and in-depth treatments of solvation d. Purity and Stability of Reagent. To assure and ion pairing associations for discussions of the accurate stoichiometry, the degree of purity (i.e., general trends expected from such interactions.10 assay) of the redox agent must be known. Further- more, for reagents which are stored the amount of degradation with time must be known or methods D. Reversible vs Irreversible ET Reagents must be available to assay the reagent before reuse. e. Solubility Compatibility. If homogeneous reactions are desired, the solubilities of the reagent Most of the couples of the ET agents listed in and the substrate must be taken into account. Tables 2 and 3 are reversible so that their potentials,

+ + 880 Chemical Reviews, 1996, Vol. 96, No. 2 Connelly and Geiger

Table 2. Formal Potentials (V vs Fc) of Selected Table 3. Formal Potentials (V vs Fc) of Selected Oxidizing Agents Reducing Agents oxidant solvent E°′ correction ref reductant solvent E°′ correction ref + - [N(C6H2Br3-2,4,6)3] MeCN 1.36 a 228 [C10H8] THF -3.10 a 366b Ce(IV) HClO4 1.30 bc glyme -3.05 a 366b H2O 0.88 bc DMF -2.95 bc + [N(C6H3Br2-2,4)3] MeCN 1.14 a 228 Na THF, glyme -3.04 ad [WCl6]CH2Cl2 ca. 1.1 d 132 Li NH3 -2.64 ef + [NO] CH2Cl2 1.00 none 195 Li(Hg) H2O -2.60 eg [Ru(phen) ]3+ MeCN 0.87 e 108 3 KNH3-2.38 ef + [NO] MeCN 0.87 none 195 Na(Hg) nonaqueous -2.36 eh + [thianthrene] MeCN 0.86 fg [anthracene]- glyme -2.47 ij + [N(C6H4Br-4)3] CH2Cl2 0.70 dh [FeCp*(η-C Me )] dmf -2.30 b 437 MeCN 0.67 ij 6 6 3+ Na NH3 -2.25 ef [Fe(bipy)3] MeCN 0.66 e 111 [benzophenone]- THF -2.30 none k Ag+ CH Cl 0.65 d 63 2 2 DMF -2.17 bl [Mo(tfd) ] MeCN 0.55 fk 3 [acenaphthalene]- THF -2.26 a 366b [IrCl4(PMe2Ph)2] MeCN ca. 0.5 l 123 + glyme -2.17 ii [Fe(η-C5H4COMe)2] CH2Cl2 0.49 none h [FeCp(η-C6Me6)] glyme -2.09 m 402 [CuTf2] MeCN 0.40 f 88 Ag+ THF 0.41 m 63 [CoCp*2]CH2Cl2 -1.94 no [Ni(tfd) ]CHCl 0.33 none h MeCN -1.91 pq 2 22 - 2- [Fe(CO)2Cp] THF, MeCN ca. -1.8 p, r 448, [PtCl6] H2O 0.31 bn + (irr) 449 [Fe(η-C5H4COMe)Cp] CH2Cl2 0.27 none h Ag+ acetone 0.18 o 63 [CoCp2]CH2Cl2 -1.33 none k glyme 1.31 m 404a Cl2 MeCN 0.18 bp - DDQ MeCN 0.13 i 308 [Cr(η-C6H6)2]CH2Cl2 -1.15 none 8 Br2 MeCN 0.07 bp [FeCp*2]CH2Cl2 -0.59 nq + [N2C6H4NO2-4] sulfolane ca. 0.05 fq MeCN -0.48 ps Ag+ MeCN 0.04 f 63 hydrazine DMSO -0.41 t 357 - [C3{C(CN)2}3] MeCN 0.03-0.06 r 304 [FeCp2] 0.0 + [FeCp2] 0.0 NEt3 MeCN ca. 0.47 u 393 + [N2C6H4F-4] MeCN -0.07 fq a 0/- 0/- + E°′ vs [biphenyl] ; [biphenyl] )-2.69 V vs SCE (DMF/ [CPh3] MeCN -0.11 fs [NBu4][ClO4]) (Grzeszczuk, M.; Smith, D. E. J. Electroanal. I2 MeCN -0.14 ft Chem. 1983, 157, 205); Fc ) 0.45 vs SCE (DMF/[NBu4][ClO4]. TCNE MeCN -0.27 fu b c TCNQ MeCN -0.30 fu Fc ) 0.45 V vs SCE (DMF/[NBu4][PF6]). Aten, A. C; Buth- [FeCp* ]+ MeCN -0.59 none h ker, C.; Hoijtink, G. J. Trans. Faraday Soc. 1959, 55, 324. 2 d Hoijtink, G. J.; de Boer, E.; van der Meij, P. H.; Weijland, CH2Cl2 -0.48 none h e + W. P. Recl. Trav. Chim. 1956, 75, 487. Fc ) 0.40 V vs NHE [C7H7] MeCN -0.65 fs f in H2O. Strehlow, H. The Chemistry of Nonaqueous Solvents; a + Use measured difference from potential of [N(C6H4Br-4)3] Lagowski, J. J., Ed.; Academic Press: New York, 1966. b c g which is 0.70 V vs Fc. Fc ) 0.40 V vs NHE in H2O. Smith, Lebed, V. I.; Aleksandrov, V. V. Russ. J. Phys. Chem. 1964, G. F.; Getz, C. A. Ind. Eng. Chem., Anal. Ed. 1938, 10, 191. 38, 1414. h Balej, J. Electrochim. Acta 1976, 21, 953. i E°′ vs d e + +/0 + Fc ) 0.46 V vs SCE (CH2Cl2/[NBu4][PF6]). Fc ) 0.32 V vs Ag /Ag; [Cr(η-C6H6)2] )-1.25 V vs Ag /Ag; Fc ) 1.12 V vs f +/0 j sodium SCE. Fc ) 0.40 V vs SCE (MeCN/[NBu4][PF6]). [Cr(η-C6H6)2] (ref 8). Dessy, R. E.; King, R. B.; Waldrop, g Hammerich, O.; Parker, V. D. Electrochim. Acta 1973, 18, M. J. Am. Chem. Soc. 1966, 88, 5112. k Data from authors’ 537. h Data from authors’ laboratories. i Fc ) 0.38 V vs SCE laboratories. l Jensen, B. S.; Parker, V. D. J. Chem. Soc., Chem. j m (MeCN/[NEt4][ClO4]). Reynolds, R.; Line, L. L.; Nelson, R. F. Commun. 1974, 367. Fc ) 0.51 V vs SCE (glyme/[NBu4][PF6]). k n o J. Am. Chem. Soc. 1974, 96, 1087. Davison, A.; Edelstein, Fc ) 0.46 V vs SCE (CH2Cl2/[NBu4][PF6]). Koelle, U.; N.; Holm, R. H.; Maki, A. H. J. Am. Chem. Soc. 1964, 86, 2799. Khouzami, F. Angew. Chem., Int. Ed. Engl. 1980, 19, 640. p Fc l q 0.9 V vs Ag/AgCl in MeCN/0.1 M NaClO4; conversion to Fc ) 0.40 V vs SCE (MeCN/[NBu4][PF6]). Gennett, T.; Milner, m r approximate. Fc ) 0.56 V vs SCE (THF/[NBu4][PF6]). D. F.; Weaver, M. J. J. Phys. Chem. 1985, 89, 2787. Fc ) n s Kravtsov, V. I.; Simakov, B. V. Elektrokhimiya 1966, 2, 646. 0.56 V vs SCE (THF/[NBu4][PF6]). Robbins, J. L.; Edelstein, o p Fc ) 0.48 V vs SCE (acetone/[NBu4][PF6]). Parsons, R. N.; Spencer, B; Smart, J. C. J. Am. Chem. Soc. 1982, 104, 1882. t u Handbook of Electrochemical Constants; Butterworth: London, Fc ) 0.43 V vs SCE (DMSO/[NBu4][PF6]). Fc ) 0.31 V vs q 1959; p 73. Elofson, R. M.; Gadallah, F. F. J. Org. Chem. SCE (MeCN/Na[ClO4]). 1969, 34, 854. r Fc potential uncertain; no supporting electro- lyte specified. s Volz, H.; Lotsch, W. Tetrahedron Letts. 1969, 2275. t Nelson, I. V.; Iwamoto, R. T. J. Electroanal. Chem. Ox, and the substrate, A, where ∆E°′ (in V) ) 1964, 7, 218. u Gross-Lannert, R.; Kaim, W.; Olbrich-Deussner, E°′(Ox/Red) - E°′(A+/A).11 B. Inorg. Chem. 1990, 29, 5046. log{[A+]/[A]} ) 8.47∆E°′ (at 298 K) (3) and those of the substrate (A), may be used in the Nernst equation to predict the position of the equi- This relationship allows one to show that in order librium in the model redox reaction of eq 2: to obtain 10:1, 100:1, and 1000:1 ratios of the product (A+) to the reactant (A) with a stoichiometric amount A + Ox h A+ + Red K ) [A+][Red]/[A][Ox] (2) of Ox the formal potential of the redox agent must eq be respectively 0.118, 0.236, and 0.354 V positive of the E°′ of the substrate. Completeness of the reaction may be judged by the Quantitative predictions cannot be made if the ratio of [A+] to [A] when a stoichiometric amount of redox agent undergoes chemically irreversible elec- Ox has been added to a solution of A. Equation 3 tron transfer. For reagents of this type, the extent describes the relative concentrations of A+ andAat of reaction is not predicted by the Nernst equation. the stoichiometric equivalence point in terms of the Nevertheless, the irreversible potentials (usually difference in formal potentials of the oxidizing agent, peak potentials from slow cyclic voltammetry experi-

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 881 ments) listed in Tables 2 and 3 may be used as Table 4. Suggested Categories for Strengths of approximations of the formal potential. Used in this Chemical Redox Agents (V vs Fc) way, irreversible potentials will generally underes- oxidants reductants timate the completeness of the reaction with a very strong >0.8 <-2.5 substrate, since follow-up reactions after ET have the strong 0.8-0.2 -1.5 to -2.5 effect of favoring the overall conversion of reactants mild 0.2 to -0.5 -0.5 to -1.5 to products.12 In an irreversible electron-transfer weak <-0.5 >-0.5 reaction, of course, the kinetics of the reaction ultimately controls the degree of redox conversion. basis of formal potentials. Recognizing that the An example of a useful oxidant undergoing ir- limits are subjective, we offer the ranges in Table 4 reversible reduction is that of the family of diazonium as the basis for discussing a reagent as a weak, mild, ions, which rapidly release N2 (eq 5) after the initial strong, or very strong oxidant or reductant. ET (eq 4), giving an aryl radical subject to further coupling or atom abstraction reactions. + - • II. Oxidants [N2C6H4X-4] + e h N2C6H4X-4 (4) We have chosen to classify the oxidants as “inor- •N C H X-4 f •C H X-4 + N (5) ganic” or “organic” with further subdivisions accord- 2 6 4 6 4 2 ing to “type”, i.e., according to charge, magnetic state When diazonium salts are used as one-electron (para- or diamagnetic), etc. There is, of course, some oxidants (eq 6), therefore, they will behave as stron- overlap so that nitrosonium and diazonium salts are ger oxidizing agents than would be predicted by considered consecutively as “inorganic” oxidants (given simply using their Epk values as approximations to their similar behavior in transition metal chemistry) formal potentials in expressions similar to those of and alkyl halides are considered with the halogens eqs 2 and 3. Other examples will be found through- and main group halides and once again, therefore, out the following discussions of the individual re- under “inorganic” oxidants. agents. Given the order in which they are discussed, we [N C H X-4]+ + A f A+ + N + aryl byproduct hope the information provided for each oxidant, 2 6 4 2 namely the synthesis, properties (stability, solubility, (6) spectroscopic), methods of handling, oxidizing power, potential complicating reactions, etc., will enable the It is worthwhile noting that the extent of a redox reader to choose the most appropriate reagent for a reaction may also increase when the substrate, rather given set of experimental conditions. than the redox agent, undergoes chemically irrevers- ible ET. Of course, in this situation the ultimate Before discussing each reagent individually, we product of the process will not be the primary (initial) would make one general point. Cationic species, ET product. A well-quantified example of such a usually in conjunction with inert counteranions, are reaction, namely the oxidation of xylene by excess Ce- probably the most widely used oxidants in organo- (IV) in acetic acid, has been discussed by Eberson.13 metallic chemistry. The authors do not recommend Because the formal potential of the [xylene]+/xylene the use of perchlorate salts under any circumstances. couple is 1.0 V positive of that of Ce(IV)/Ce(III), the Such salts, of the oxidants themselves or of cationic ET reaction of eq 7 is endergonic by ca. 23 kcal/mol, oxidation products formed thereform, may be highly resulting in the electron-transfer equilibrium itself and unpredictably explosive. Although many of the being strongly in favor of the reactants: oxidants to be discussed have been prepared as perchlorates, reliable syntheses of salts with other xylene + Ce(IV) H [xylene]+ + Ce(III) (7) counteranions are invariably available. The highly acidic cation, [xylene]+, donates a proton to an adventitious base to start a set of exergonic A. Inorganic reactions culminating in the production of (4-meth- 1. Metal and Metal Complex Oxidants ylbenzyl)acetate (eqs 8-10). + + a. Cations. i. Ferrocenium Salts. The ferroce- [xylene] + base f MeC6H4CH2 + (base)H (8) + nium ion, [FeCp2] , is a mild one-electron oxidant, 14 MeC H CH + Ce(IV) h [MeC H CH ]+ + Ce(III) usually regarded as an outer-sphere reagent. Be- 6 4 2 6 4 2 cause it is readily available and easily handled it is (9) widely used in both stoichiometric reactions and redox catalysis; salts of a variety of counteranions, + + - - - - [MeC6H4CH2] + HOAc f MeC6H4CH2OAc + H including [BF4] , [PF6] , [SbF6] , and [BPh4] , can (10) be made simply and from cheap precursors. The oxidation potential of the ferrocenium ion can be A special case of “irreversibility” driving an ET systematically altered by ring substitution, providing reaction arises when the reaction product precipitates a range of oxidants with E°′ values varying from from the reaction solution. 15 -0.63 (for [Fe(η-C5H4NMe2)2] ) to 0.64 V (for [Fe(η- 16 C5H4CF3)2] ); a table of E°′ values (vs Fc) for various E. Categorization of Reagent Strength ferrocene derivatives has appeared elsewhere.17 In order to facilitate discussion, we thought it Ferrocene derivatives with electron-donating sub- + useful to define categories of reagent strength on the stituents are weaker than [FeCp2] but may find use

+ + 882 Chemical Reviews, 1996, Vol. 96, No. 2 Connelly and Geiger with electron-rich substrates. Thus, [Fe(η-C5H4R)- thermally stable (footnote 16 in ref 22) but a slightly + n (η-C5H5)] (R ) Bu , Ph, HgCl, etc.) were employed weaker oxidant (by ca. 0.1 V). in kinetic studies of the oxidation of ferrocytochrome Ferrocenium ions can be used with a wide range 14 + c (and [FeCp*2] is a useful building block in the of polar organic solvents, including CH Cl ,31-34 18 2 2 synthesis of new magnetic materials ). As yet, the THF,22,35-36 MeOH,37 acetone,38 and MeCN.39-41 The use of ferrocenium ions with electron-withdrawing solubility of the ferrocenium salt depends on both the + substituents (stronger oxidants than [FeCp2] )is solvent and the counteranion. For example, [FeCp2]- equally limited; two syntheses employing the acetyl- [BF ] is more soluble than [FeCp ][PF ] (which is only ferrocenium ion are noted below. 4 2 6 poorly soluble in cold CH2Cl2); [FeCp2][BPh4]is Ferrocenium salts have been prepared by several insoluble in most common organic solvents. In many methods,14,19-24 all involving the one-electron oxida- cases, however, the use of a solution of a ferrocenium tion of ferrocene. Surprisingly, full experimental salt is unnecessary, and the solid oxidant is added details are not provided in the most widely cited directly to a solution of the substrate. Even though 19,20 references for the syntheses of [FeCp2][PF6] and the reaction is then heterogeneous, its rate is usually [FeCp2][BF4] (the two most commonly used ferroce- sufficient. Solvent choice is therefore more often nium salts). The first method19 readily gives a high governed by the solubility of the substrate or the - yield of the [PF6] salt and involves FeCl3 oxidation stability of the product. For example, oxidative of ferrocene in aqueous acetone, addition of [NH ]- 4 substitution of [Co(CO)(PPh3)Cp] by PPh3 in THF in [PF ] to the resulting deep blue solution of the 6 the presence of [FeCp2][PF6] gives [Co(PPh3)2Cp][PF6] ferrocenium ion, and crystallization of the resulting 42 which is unstable in CH2Cl2. precipitate from acetone-diethyl ether. Addition of The ferrocenium ion almost invariably behaves as [NH4][BF4] does not give a precipitate of [FeCp2][BF4] so readily; the alternative synthesis,20 involving the an innocent one-electron oxidant, an observation put to good effect in comparative studies of the oxidative treatment of ferrocene with p-benzoquinone in the + activation of C-H bonds by [CPh3] (a noninnocent presence of HBF4‚OEt2 (aqueous HBF4 has also been + 43-45 used) provides a more convenient route to this more oxidant) (section II.B.2.a) and [FeCp2] . The soluble salt. byproduct [FeCp2] is readily removed by washing the product with a nonpolar solvent such as hexane or, These syntheses, detailed in the Appendix, can be if necessary, by sublimation. However, excess fer- used to prepare other ferrocenium salts. The first rocenium salt can be difficult to remove from an ionic gives [FeCp ][BPh ] on addition of NaBPh to the 2 4 4 product. (The authors have detected the ferrocenium ferrocenium ion solution23 (see also ref 22). The ion in the CV of products deemed pure by elemental second has been used to prepare not only [FeCp* ]- 2 analysis!) This problem is usually overcome by [BF ]25 but also the acyl-substituted ferrocenium salts 4 strictly adhering to the required stoichiometry or by [Fe(η-C5H4COMe)Cp][BF4] and [Fe(η-C5H4COMe)2]- 26 using a slight deficiency of the oxidant (if the [BF4] which are considerably stronger oxidants than [FeCp ]+ (Table 2). Note, however, that the reaction substrate is easily removed from its oxidation prod- 2 uct). The need for such careful control is illustrated of p-benzoquinone with [Fe(η-C5H4COR)2][R)(CH2)9- Me] resulted in Michael addition and the formation by the reaction between the tautomeric mixture of of benzofuranyl-substituted ferrocenes rather than [Rh2(µ-NPh)(CO)2(µ-dppm)2] and [Rh2(µ-NHPh)(CO)2(µ- [Fe(η-C H COR) ]+.26 dppm)(µ-Ph2PCHPPh2)] with [FeCp2][PF6], the prod- 5 4 2 ucts of which depend not only on stoichiometry but Ferrocenium ions with electron-withdrawing sub- also on the order of addition of the reactants.46 stituents (i.e., from less readily oxidized ferrocenes) may also be synthesized by other methods. Details The stoichiometry of a reaction can sometimes be 27 monitored by ferrocenium ion “titration”. For ex- of the preparation of [Fe(η-C5H4COMe)Cp][BF4](E°′ ample, [FeCp ][PF ] in MeCN was added dropwise )0.27 V) from [Fe(η-C5H4COMe)Cp] and AgBF4 in 2 6 diethyl ether are given in the Appendix, and [Fe(η- to [RuH(CO)(PMe3)Cp] until a faint green-blue color persisted; the use of 0.71 equiv of the oxidant was C5Ph5)2][BF4](E°′)ca. 0.3 V) can be made from 28 consistent with the observed formation of [RuH- [Fe(η-C5Ph5)2] and [NO][BF4]. + + (PMe3)(NCMe)3] , [Ru2(µ-H)(CO)2(PMe3)2Cp2] , and The crystalline salts [FeCp2][BF4], [FeCp2][PF6], C5H6 in a 1:1:1 ratio. (Other products are formed if and [FeCp*2][BF4] are thermally stable, remaining [FeCp2][PF6] and [RuH(CO)(PMe3)Cp] are reacted in analytically pure (C and H analysis) when stored in 40 air for several months. However, solid [Fe(η-C H - 1:1 or 2:1 ratios. ) The use of such a visual “end 5 4 point” for titration is, however, somewhat subjective COMe)Cp][BF4] is moisture-sensitive and slowly deteriorates (over a period of weeks) even when and only applicable if the color of the reaction mixture allows. Quantitative titrations should be stored in a desiccator. [FeCp2][BPh4], used when fluorinated counteranions are undesirable in the final possible by optical spectrophotometry; data for a product,22 also slowly decomposes in the solid state range of ferrocenium salts have been reported (e.g., + 14 in air. It may be stored at -80 °C29 but is best freshly [FeCp2] , λmax ) 617nm,  ) 410). prepared and added to a reaction mixture as a solid. The ferrocenium ion is particularly useful where Even then its instability has contributed to the selective oxidation is required, for example, in the requirement of an excess of the reagent in the synthesis of mixed-valence products from bi- or 47 oxidatively induced alkyne insertion reaction of the polynuclear species such as [Ru(CtCFc)L2Cp] and 48 metallacyclobutane complex [Ir(CH2CHRCH2)- [Fe2(µ-CHdCHCHdCH)(dppe)2Cp2] which undergo (PhCtCPh)Cp*].30 Interestingly, the same reaction two reversible, sequential redox processes. Selective does not occur with [FeCp′2][BPh4] which is more oxidation of [Rh(CtCPh)L] (L ) np3 or pp3)tothe

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- - - isolable mono- and dications is also accomplished [SbF6] ,[O2CCF3] ,[O3SCF3] (triflate), etc.). Where 49 using 1 and 2 equiv of [FeCp2][PF6], respectively. a less common anion is demanded by the chemistry + In principle, the reversibility of the [FeCp2] / following electron transfer, the appropriate silver(I) [FeCp2] couple suggests that the ferrocenium ion can salt can usually be readily prepared. Thus, for 22 be used only with substrates with formal oxidation example, Ag[BPh4] reacts with [Zr(CH2Ph)2Cp2]to potentials negative of 0.0 V. This is the case where give stable [Zr(CH2Ph)(NCMe)Cp2][BPh4]; related - the substrate undergoes reversible oxidation (the [PF6] salts decompose in solution, with fluorination product distribution is governed by the Nernst equa- of the metal center.58 tion). However, where this process is irreversible Solid silver(I) salts are usually hygroscopic and (section I.D), the driving force of the following chemi- photosensitive and should therefore be stored in dark, cal reaction can lead to oxidation of substrates with air-free containers. For many purposes, particularly Eox more positive than that of the ferrocenium ion. when an excess of the oxidant can be tolerated, the Examples include [Rh(CO){P(OPh)3}Cp] (Eox ) 0.23 salts can be rapidly weighed in air; for accurate V) and [Rh(cot)Cp] (Eox ) 0.08 V), which are oxida- stoichiometric work, or for use with water-sensitive 2+ 50 tively dimerized to [Rh2(CO)2{P(OPh)3}2Cp2] and substrates, they are best handled in a glovebag or 5 5′ 2+ 51 [Rh2(η ,η -C16H16)Cp2] , respectively, and [RuH- drybox. A more convenient method to cope with the 40 (CO)(PMe3)Cp] (Eox ) 0.35 V) (see above). problem of stoichiometry is to transfer an approxi- A second strategy for the oxidation of less electron- mate quantity of the silver salt to a reaction vessel, rich substrates involves the use of ferrocenium ions to dry the contents in vacuo, and then to weigh with electron-withdrawing substituents. Thus, the accurately the dried sample. The weight of substrate + addition of 1 equiv of [Fe(η-C5H4COMe)Cp] (E°′ ) can then be adjusted to the weight of silver(I) salt + 0.27 V) to [Au{(NC)Mn(CO)2{P(OEt)3}(dppm)}2] , taken. which is sequentially oxidized in two one-electron Depending on the counteranion, silver(I) salts may steps (E°′ ) 0.17 and 0.45 V), selectively affords the be soluble in a range of organic solvents including { { } 59,60 61,62 63,64 22,58,62,65,66 mixed-valence dication [Au (NC)Mn(CO)2 P(OEt)3 - CH2Cl2, acetone, THF, MeCN, 2+ + 52 67 68-70 (dppm)}2] whereas [FeCp2] does not react. Simi- alcohols, and toluene. Because of their mois- larly, oxidatively induced reductive elimination is ture and light sensitivity, solutions of the salts are t faster and cleaner when [RuR(C2Ph)(CO)(PBu 2Me)2] best used immediately (though it is often preferable [R ) C2Ph (Eox ) 0.28 V) or CHCHPh (Eox ) 0.10 V)] to use the solid reactant directly). As with the other + is treated with [Fe(η-C5H4COMe)Cp] rather than cationic oxidants described in this review, the anion + 53 [FeCp2] . is often chosen to be noncoordinating; the most + - The stability of the couple [FeCp2] /[FeCp2] sug- commonly used anions are therefore [BF4] and - gests the ferrocenium ion as a potential redox [PF6] . An important point concerning the latter is catalyst. It has, indeed, found use with readily that in CH2Cl2, CHCl3, or toluene, but not in acetone, - oxidized organometallic substrates, for example, in MeCN, or THF, hydrolysis of the [PF6] anion (in the 71 + carbonyl insertion (e.g., [FeMe(CO)(PPh3)Cp] to presence of traces of water) is catalyzed by Ag ; 54 - [Fe(COMe)(CO)(PPh3)Cp] and [Fe(η-CMedCMe2)- byproducts containing species such as [PO2F2] ,as 72,73 (CO){P(OPh)3}Cp] to [Fe{η-(CO)CMedCMe2}(CO)- ligand or counteranion, have been encountered. 55 {P(OPh)3}Cp] ) and CsH activation (e.g., of Removal of the byproduct of oxidation by silver(I), 56 [IrMe2(PPh3)Cp*] to [IrMe(C6H4PPh2)Cp*] ). Its namely silver metal, is usually straightforward. For relatively low potential has led to little if any use in solvents in which the product is soluble, filtration + organic synthesis (cf. [N(C6H4Br-4)3] , section II.B.1.a), through a filter aid such as Celite or Kieselguhr is but substituted ferrocenes with electron-withdrawing advisable; the silver metal may be very finely divided. substituents are attractive as potential mediators of If a reaction is carried out using toluene solutions of the electron-transfer-catalyzed oxidation of sub- both the silver(I) salt and a neutral precursor, a strates with E°′ more positive than 0.0 V. The coprecipitate of silver and the cationic product, as a substituted ferrocenium ions themselves need not be salt of the silver(I) counteranion, may be deposited. isolable; a catalytic cycle would be sustained if the Such a strategy can lead to a very simple isolation redox couple is reversible and its members are both procedure in that unreacted starting materials can inert to side reactions. be removed by washing the coprecipitate with tolu- ii. Silver(I) Salts. Silver(I) salts are among the ene; the desired product can then be separated from most widely used one-electron oxidants, particularly the insoluble silver metal using a suitable polar with transition metal complexes but also in organic1b solvent as extractant.68 { 57 and main group chemistry e.g., Ag[AsF6] oxidizes The strength of the silver(I) ion as a one-electron { } Te N(SiMe3)2 2 to the novel Te(III) salt [Te- oxidant has been variously described, from mild to { } } N(SiMe3)2 2][AsF6] . However, their behavior is strong, but of all the reagents described in this review neither straightforward nor entirely predictable. the formal potential of Ag+/Ag is the most solvent Their strength as oxidants can be misjudged if the dependent. The potential for the couple Ag+/Ag is marked dependence of oxidation potential on solvent not simply determined in nonaqueous solvents but is not appreciated and they can participate in a wide estimates are available (Table 5). In CH2Cl2, usually range of other reactions, depending on the nature of regarded as a noncoordinating solvent, Ag[PF6] be- the substrate and, in part, on the counteranion. haves as a strong oxidant, for example, forming the + Silver(I) salts are attractive as one-electron oxi- triarylaminium cation [N(C6H4Br-4)3] from N(C6H4- dants in that they are commercially available with a Br-4)3 (E°′ ) 0.70 V). (Note, however, that recent - - 74 wide range of counteranions (e.g., [PF6] , [BF4] , X-ray structural studies have shown that in the

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Table 5. Formal Potentials (V vs Fc) of Ag+ and [NO]+ As well as the various complications noted above, Oxidants in Different Solvents a wide variety of reaction modes other than electron solvent Ag+/Aga,b [NO]+/NOc transfer can be observed for the silver(I) ion.80 First, silver(I) salts are efficient halide abstraction re- CH2Cl2 0.65 1.00 72,81 80 CH3NO2 0.98 agents. Second, they may bind to the oxygen, DMF 0.49 0.56 sulfur {where the reaction of the thiocarbonyl [W(CO)- + H2O 0.48 (CS)(dppe)2], to give [Ag{(µ-SC)W(CO)(dppe)2}2] , pyridine 0.43 contrasts with the simple oxidation of [W(CO)2- THF 0.41 + 61} 52 acetone 0.18 (dppe)2] to [W(CO)2(dppe)2] , or nitrogen atoms MeCN 0.04 0.87 of coordinated ligands. Third, there are now many 62,82-84 a b c examples of the formation of Ag-metal bonds; Estimate. From ref 63. From ref 195. particularly impressive are recent studies85 of cluster formation by the sequential condensation of Ag+ and 2- solid state, at least, silver(I) can be chelated by CH2- [Fe(CO)4] ions, culminating in the one-electron { } 4- { } 3- Cl2.) By contrast, the silver(I) ion is almost certainly oxidation of [Ag13 Fe(CO)4 8] to [Ag13 Fe(CO)4 8] . complexed when dissolved in acetone, acetonitrile {- In many of these alternative reaction modes, the [Ag(NCMe)2][EF6](E)P, Mo, W, and U) has been substrate is redox-active with a formal potential 75 isolated, and commercially available [Ag(NCMe)4]- negative of that of silver(I); under the experimental 76 [BF4] has been used as an oxidant in CH2Cl2}, THF, conditions described, coordination is prefered even MeOH, and toluene, so that the formal potential is though one-electron transfer is exergonic. considerably decreased (Table 5). As a qualitative Coordination rather than oxidation is not always indication of this decrease, Ag[BF4] is incapable of a disadvantage. Thus, Ag[O2CCF3] and [Os(CO)3- 63 oxidizing N(C6H4Br-4)3 in toluene, MeCN, or THF; (PPh3)2] yielded the crystalline adduct [(CF3CO2)- 63 electrochemical studies of Ag[O2CCF3] have shown AgOs(CO)3(PPh3)2], the slow decay of which gave the + that the reduction potential in MeCN is ca. -0.6 V short-lived radical cation [Os(CO)3(PPh3)2] , charac- more negative than that in CH2Cl2. terized by ESR spectroscopy. Such adducts may be regarded as silver-stabilized radical cations, a concept The solvent dependency of the oxidation potential exploited70 in the redox chemistry of [Rh(CO)(PPh )- of Ag(I) can be used to advantage, as in the selective 3 Cp] [(E ) ) ca. -0.04 V]. Treatment of this species synthesis of the reduced and oxidized forms of the p ox { { } with Ag[PF6] in toluene gave the crystallographically linear trimetallic species [Ag (NC)Mn(CO)2 P(OEt)3 - { } } z 52 characterized salt [Ag Rh(CO)(PPh3)Cp 2][PF6] which (dppm) 2] . In CH2Cl2, the reaction between trans- behaved as a stable source of the transient radical [Mn(CN)(CO)2{P(OEt)3}(dppm)] (E°′ ) 0.03 V) and + cation [Rh(CO)(PPh3)Cp] when reacted with NO or Ag[PF6] gives the trication [Ag{(NC)Mn(CO)2- + 3+ NO2. The “milder” oxidants [FeCp2] and [N2C6H4F- {P(OEt)3}(dppm)}2] , via oxidation of the cyanoman- + { } 4] react with [Rh(CO)(PPh3)Cp] to give the fulvalene ganese complex to trans-[Mn(CN)(CO)2 P(OEt)3 - complex [Rh (CO) (PPh ) ( 5, 5-C H )]2+ by the ap- + 2 2 3 2 η η 10 8 (dppm)] followed by N-coordination to silver(I). In + parent dimerization of [Rh(CO)(PPh3)Cp] . toluene the reaction affords the monocation [Ag- + iii. Copper Salts. Although salts of both Cu(I) and, {(NC)Mn(CO)2{P(OEt)3}(dppm)}2] ; neither the ini- { more usually, Cu(II) have been widely used as tial oxidation step nor the oxidation of [Ag (NC)Mn- oxidants in organic chemistry,86 organometallic ap- { } } + ′ (CO)2 P(OEt)3 (dppm) 2] (E° ) 0.32 V) occurs. plications are less common. Nevertheless, the re- The solvent-dependent strength of silver(I) is also agents are cheap, commercially available with a large illustrated by its reactions with [Cr(CNC6H4Me-4)6] number of simple counteranions, and conveniently (which undergoes three successive one-electron oxi- used with a wide range of organic solvents. Hydrated 77 dations, at -0.84, -0.15, and 0.64 V ). Ag[PF6]in salts may be employed if the reactants and products acetone (E°′ ) 0.18 V) gave either [Cr(CNC6H4Me- are likely to be inert to water, but anhydrous CuCl2 + 2+ 4)6] or [Cr(CNC6H4Me-4)6] , depending on the stoi- is simply made by refluxing CuCl2‚2H2O with thionyl chiometry of the reaction.78 Although stronger oxi- chloride.87,88 dants ([NO][PF6] or SbCl5) were needed to form the Copper(II) halides (hydrated and anhydrous), ni- related tricationic complexes [Cr(CNC6H3Me2-2,6)6]- trate, acetate, etc., induce metal-alkyl bond cleavage - - 79 + 89 X3 (X ) [PF6] and [SbCl6] , respectively), Ag salts in [FeR(CO)2Cp] in CH2Cl2 and in ruthenium 90 in CH2Cl2 (E°′ ) ca. 0.65 V) may be sufficently strong analogues in MeCN; the stoichiometry of such 2+ to oxidize [Cr(CNC6H4Me-4)6] to [Cr(CNC6H4Me- reactions, which involve initial one-electron transfer, 3+ 4)6] . is given by eq 11. The identity of the oxidant is Two complications common to other oxidants can also be encountered in transition metal chemistry. MR + 2CuX2 f MX + RX + 2CuX (11) First, the one-electron transfer product will inevita- bly be more susceptible to nucleophilic attack by [M ) Fe(CO)2Cp or Ru(CO)(PPh3)Cp, X ) halide] + donor solvents. Thus [Os(C6H4Me-2)4] is stable when formed from [Os(C6H4Me-2)4] and Ag[BF4]in unclear when CuCl2 reacts with [M2(CO)10](M)Mn 59 CH2Cl2 but not in THF. Second, the use of a or Re) in MeCN. In solution, the dichloride rapidly coordinating counteranion (with the cationic oxidant) equilibrates with species such as [CuCl]+,Cu2+, - 2- can lead to binding to the substrate metal center, as [CuCl3] , and [CuCl4] ; the last is certainly not the in the reactions of cis-[Mo(CO)2(bipy)2] with AgX (X oxidant and the more positively charged species are 66 ) NCS, NO2,orNO3). favored. Other kinetic studies with [Mn2(CO)10] were

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+ 101 consistent with [Cu(triflate)2] as an outer-sphere (detc)2(PPh3)2] in that solvent. However, it is also oxidant.88 soluble in alcohols, acetone, acetonitrile, etc., and 2- Other anhydrous copper(II) oxidants have been oxidizes [B6Cl6] (in ethanol) and [OsCp2] (in MeCN) - 102 2+ used to activate coordinated hydrocarbons. For to [B6Cl6] and [Os2Cp4] (the metal-metal bonded + 103 example, copper acetate induces the oxidative dimer- dimer of [OsCp2] ), respectively. It may also be ization of [Re(CtCH)L(NO)Cp*] (L ) PPh3) to [Cp*L- added as a solid to a solution of a substrate in the (NO)Re(µ-CtCCtC)ReL(NO)Cp*] (which undergoes less polar organic solvents more generally useful for further oxidation to [Cp*L(NO)Re(µ-dCdCdCdCd)- synthetic and spectroscopic studies of organometal- ReL(NO)Cp*]2+ with Ag+)91 in pyridine and of [Fe- lics. Thus, 1 gives 2 in THF.104 Alternatively, the (CCHMe)(dppe)Cp]+ to [Cp(dppe)Fe{µ-C(Me)C(Me)dC- use of [NBu4][HSO4] as a phase-transfer catalyst, (Me)CMe}Fe(dppe)Cp]2+ in methanol.92 which facilitates the oxidation of [2,2]paracyclophane 105 Copper(I) salts are weaker oxidants than Cu(II) to its radical cation in CHCl3, may also be ap- derivatives but may be useful with more electron- plicable for organometallic systems. rich substrates. Thus, the reaction between copper- (I) iodide and the anionic carbene complex - [Mn{C(OEt)CHR}(CO)2Cp′] in THF results in oxidative coupling, giving the neutral dicarbene derivative [Cp′(CO)2Mn{µ-C(OEt)CHRCHRC(OEt)}- 93 Mn(CO)2Cp′]; the mixed-metal cluster [Sb2Co4- 2- (CO)10(µ-CO)] gives the monoanion [Sb2Co4(CO)10- - + 94 (µ-CO)] with [Cu(NCMe)4] .

iv. FeCl3. Both anhydrous and hydrated FeCl3 are commercially available, but the former may be made 3+ by heating the latter under reflux in thionyl chlo- vi. [M(L-L)3] (L-L ) bipy, phen, etc.). Various ride;88 further purification can be achieved by vacuum cationic coordination complexes of chelating aromatic 3+ sublimation. Hydrated FeCl3 induces the oxidative N-donors, namely [M(L-L)3] (M ) Cr, Fe, Ru, Os, coupling of the carbene and alkyl ligands in [{RhR- Co, Ni; L-L ) bipy, phen, etc.), can act as one-electron 95 (µ-CH2)Cp*}2](R)Me, Et, etc.) but the anhydrous oxidants. Almost invariably, they have been used as - form is normally preferred as the oxidant in organo- their [ClO4] salts. However, once again we under- metallic chemistry. The latter is cheap, easily handled line the fact that this anion is potentially explosive - (in the absence of moisture), and soluble in organic and recommend the anhydrous [PF6] salts, synthetic 106-108 solvents such as CHCl3, ethanol, dioxane, and THF; procedures for which are available. The Ni(IV) it may also be added as a solid to substrates dissolved salt [Ni(bipy)3][AsF6]4 has also been isolated; the in other solvents. However, it is not ideal for the in tetracation may find use as a very strong oxidant (E°′ situ generation of ESR-active species (itself giving ) 2.06 V).109 3+ rise to a broad signal), and its lability, and those of The trications [M(L-L)3] are generally regarded its reduction products, can lead to the incorporation as outer-sphere reagents,88,106,110 their oxidation po- of chloride. For example, [M2(CO)10] is oxidized in tentials108,111,112 depending mainly on the metal but MeCN to a mixture of [MCl(CO)5] and [M(NCMe)- also varying with the ligand substituents. The nickel + 88 (CO)5] . complexes are the strongest oxidants,113 but the iron In some cases, where the presence of nucleophilic and cobalt derivatives are the most widely used. chloride ions is not a problem, the one-(or two-)- Extensive studies have been made of the effects on electron oxidation product can be isolated, for potential of varying the 4,7-substituents in [Fe- 2+ 2+ example, trans-[W(N2)2(PMePh2)4][FeCl4] from (phen)3] . For example, [Fe{phen(NH2)2-4,7}3] is 96 trans-[W(N2)2(PMePh2)4] in dry ethanol. Where the oxidized more readily, by ca. 0.9 V, than the parent 2+ 114,115 final product is neutral, separation from the ionic complex [Fe(phen)3] . The trications [Fe(phenX2- 3+ byproducts is particularly facile. Thus, the anions 4,7)3] constitute an array of oxidants in which the 2- [Os6(CO)18] (in CH2Cl2, with aqueous FeCl3) and potential is modified systematically from 0.66 V (X { } - [ Co(µ-CO)Cp 2] (in THF, very slow addition of ) H) to -0.26 V [X ) NH(CH2)3NH2]. As yet, they FeCl3 to prevent overoxidation to [Co(CO)2Cp]) give have been largely used as stoichiometric or catalytic 97 98 neutral [Os6(CO)18] and [{Co(µ-CO)Cp}2], respec- electron-transfer oxidants with organic sub- tively. Similarly, the oxidation of cluster dianions strates112,116,117 although the titanium enolate [TiCl- 2- 118 [Mx(CO)y] in the presence of L gives [Mx(CO)yL] (M {OC(H)dCCMes2)Cp2] is oxidized to a benzofuran. ) Ru, Os, etc.).84 When compared115,118 with arylaminium salts (section 4 3+ v. Ce +. The cerium(IV) ion is a strong oxidant, II.B.1.a), [Fe(phen)3] derivatives are said to be (i) usually used as the stable, commercially available more effective when radical intermediates with rela- salt [NH4]2[Ce(NO3)6]. It is a well-known reagent for tively high oxidation potentials need to be oxidized organic oxidations1b,99 and is also employed in orga- in slow ET reactions,116 (ii) as efficient in stoichio- nometallic chemistry to cleave organic products from metric reactions when the radical intermediates are metal centers.100 It may, however, also be used more easily oxidized,117 but (iii) worse in cation radical selectively in the isolation of a wide variety of one- chain processes since back electron transfer from [Fe- 2+ 112 electron oxidation products. (phen)3] is slower than that with triarylamines.

The salt [NH4]2[Ce(NO3)6] is water-soluble and Most studies involving the oxidation of organome- 3+ reacts with cis-[Os(detc)2(PPh3)2] to give trans-[Os tallic compounds by [M(L-L)3] have been mecha-

+ + 886 Chemical Reviews, 1996, Vol. 96, No. 2 Connelly and Geiger nistic, for example, the cleavage of main group toxic) in the preparation of the nickel compound. metal-alkyl bonds108,111 and the oxidation of Nevertheless, their wider use should be considered. - 106 [Mn(CO)5L] (L ) CO or phosphine). However, [Ru3H3(µ3-CX)(CO)9-nLn](e.g., X ) OMe, L ) PPh3, n ) 2 or 3) is oxidized to the monocation by [Fe- 3+ 110 (phen)3] . 2- 2- b. Anionic Complexes [IrCl6] and [PtCl6] . 2- The hexachloroiridate(IV) dianion [IrCl6] (λmax ) 487 nm,  ) 4060), usually as the commercially 119 available sodium or potassium salt, has been The crystalline solids are thermally stable in dry extensively used as an outer-sphere, one-electron air for many years but slowly react with water and oxidant in mechanistic studies of the cleavage of should be stored and used under dry conditions. 120 metal-alkyl bonds. For example, the oxidation of They are soluble in nonpolar solvents and, if neces- [PtR2(PMe2Ph)2] occurs by initial one-electron trans- sary, can be repurified by recrystallization from dry fer, though 2 equiv of oxidant are required for benzene ([Ni(tfd)2]; the use of toluene is to be pre- 121 2- complete reaction. Surprisingly, [IrCl6] does not ferred)125 or by continuous (Soxhlet) extraction using 126 oxidize [M2(CO)10](M)Mn or Re) in MeCN even pentane ([Mo(tfd)3]); the nickel complex is very though the potentials are thermodynamically favor- soluble in CH2Cl2, ensuring homogeneous electron- able, an observation ascribed by the authors to the transfer reactions even at low temperatures. high electrostatic barrier to electron transfer pre- 88,122 The products of reduction of [Ni(tfd)2] and sented by the dianionic charge. - [Mo(tfd)3], namely the monoanions [Ni(tfd)2] and - For synthetic purposes it has been less widely used, [Mo(tfd)3] , are inert and unlikely to ion-pair with though in moist MeCN Na2[IrCl6] induces the oxida- one-electron oxidation products in solution, even in tive coupling of the methyl and carbene groups of solvents of low polarity. Thus, the oxidation of [Cr2- 95 6 6′ [{RhMe(µ-CH2)Cp*}2] to give propene. However, as (CO)6(µ-η ,η -L)] (L ) biphenyl derivative 4) gave a rare example of a dianionic oxidant it may prove useful in the isolation of salts where the high positive charge of the oxidized product would be balanced by the high negative charge of the Ir(III) counteranion; when used in MeCN (or aqueous solutions), the 3- reduction products are normally [IrCl6] and/or 2- [IrCl5(NCMe)] . 6 6′ + The oxidizing ability of the Ir(IV) center may by [Cr2(CO)6(µ-η ,η -L)] whose mixed-valence behavior altered by varying the ligands, as in trans-[IrCl4(PMe2- was interpreted with the knowledge that the anion Ph)2](E°′)ca. 0.5 V). This neutral oxidant provides would not influence the spectroscopic (IR and ESR) 2- 127 an interesting alternative to [IrCl6] in being soluble properties of the monocation. The reaction of [Mo- in much less polar solvents. It has therefore been (tfd)3] with [M4(µ4-E)4(η-C5H4R)4](M)Cr, E ) Sor used in benzene to oxidize neutral substrates such Se, R ) Me; M ) Mo, E ) SorSe,R)Pri) in toluene similarly gave the charge-transfer salts [Mo(tfd) ][M - as [MCp2](M)Fe and Ni), the salts [MCp2]- 3 4 (µ -E) (η-C H R) ] and [Mo(tfd) ] [Mo (µ -S) (η-C H - [IrCl4(PMe2Ph)2] precipitating directly from the reac- 4 4 5 4 4 3 2 4 4 4 5 4 Pri) ].97 tion mixture. However, the “inert” anion [IrCl4(PMe2- 4 Ph)2] can act as a nucleophile; the chloro-bridged ii. Transition Metal Hexahalides. High-valent + Ir(III) complex[(PhMe2P)2Cl2Ir(µ-Cl)2Pt(PPh3)2] is transition metal (d- and f-block) fluorides such as 123 formed with [Pt(η-C2H4)(PPh3)2]. MF6 (M ) Mo or W), UF6, etc., were recommended 128 2- as one-electron oxidants by Olah in a review of new The Pt(IV) complex [PtCl6] also provides a po- 2- reagents for organic synthesis. Though such re- tentially interesting variation to [IrCl6] in reacting - agents have the advantages of ready availability and with Zeise’s salt, [PtCl3(η-C2H4)] , as an inner-sphere two-electron oxidant.124 solubility in organic solvents such as CHCl3,CH2Cl2, and THF, they are hydrolyzed, giving HF, and must c. Neutral Compounds. Neutral metal complex be used under rigorously dry conditions. As yet, oxidants are uncommon yet potentially extremely therefore, they have not been widely used as one- useful for the synthesis of charged products in electron oxidants in organometallic chemistry al- nonpolar, and non-nucleophilic, solvents; mixing sto- though [MCl2Cp2](M)Mo or W) and [WF6]inSO2 129 ichiometric quantities of oxidant and substrate in, for give [MoCl2Cp2][WF6] and a mixture of [WCl2- 130 example, hexane or toluene should, in principle, lead Cp2]2[W4F18] and [WCl2Cp2][WF6]2, respectively; to the deposition of near-quantitative yields of the [Ag(NCMe)2][EF6] can be prepared by oxidizing Ag 75 electron-transfer salt. metal with EF6 (E ) Mo,W, or U). i. [Ni(tfd)2] and [Mo(tfd)3]. The neutral dithiolene The hexachlorides, also strong oxidants (e.g., E°′ 131,132 complexes [Ni(tfd)2](3) and [Mo(tfd)3] are strong one- ) ca. 1.1 V for [WCl6] ), are easier to handle in electron oxidants, simply prepared in very high yield conventional glassware; WCl6 oxidizes [N(C6H4Br-4)3] 125 from S2C2(CF3)2 and either [Ni(CO)4] or [Mo- to [N(C6H4Br-4)3][WCl6] (section II.B.1.a) and, in the 126 (CO)6]. That these oxidants have not been widely presence of acetic acid and MOAc (M ) Na or K), used may stem from the unattractive synthesis of acetoxylates aromatics via arene radical cations132 S2C2(CF3)2 and the involvement of [Ni(CO)4](highly (though chlorination reactions can interfere).

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 887 2. Main Group Oxidants ii. Alkyl Halides and Other Alkylating Agents. Alkyl halides are weak one-electron oxidants, react- a. Halogens and Halides. i. Halogens. The ing according to eqs 13 and 14; electron-rich sub- halogens Cl2,Br2, and I2 (fluorine may be disregarded as a convenient one-electron oxidant) are readily + - available, cheap, and easily purified oxidants soluble A + RX ) A + [RX] (13) in a wide variety of organic solvents. Although the [RX]- •R X- (14) differences in formal potentials for the couple of eq ) + 12 are over 0.8 V in aqueous media, the reported strates such as [Cr(η-arene)2] and [Mo(dmpe)3] give potentials span a difference of only about 0.3 V in + 145 the corresponding monocations [Cr(η-arene)2] and MeCN (Cl, 0.18 V; Br, 0.07 V; I, -0.14 V). Significant + 136 [Mo(dmpe)3] (with MeI ), respectively. The forma- differences are observed, however, in the ET reactiv- tion of the radical, R, as a byproduct can result in ity of the halogens. For example, bromine oxidizes coupling reactions, as in the formation of both [Co- N(C H R-4) (R ) Me or OMe) to the corresponding 4 146 6 4 3 (η -C5C5R)Cp] and [CoCp2]X from [CoCp2] and RX. arylaminium cations [N(C H R-4) ]+ whereas elec- 6 4 3 The formation of inert C2Cl4 as a byproduct (eq 15), rather than alkyl radicals, makes C2Cl6 interesting - - X2 + 2e h 2X (12) as an alternative to alkyl halide oxidants, as in the + 147 oxidation of [CoCp2] to [CoCp2] . In common with trophilic substitution is observed with chlorine. Io- the alkyl halides, however, the halide ion byproduct dine is insufficiently strong to oxidize such amines,133 may act as a nucleophile toward the oxidation unless used in conjunction with a silver(I) salt (see product so that, for example, [{CoClCp*}2] is cleanly 148 section II.A.1.a.ii), but does effect the one-electron oxidized to [{CoCl2Cp*}2]. oxidation of both Li[N(PPh2)2] and Li[HC(PPh2)2], 134 + - leading to P-P and P-C coupling, respectively. 2A + C2Cl6 ) 2A + C2Cl4 + 2Cl (15) The solubility of halogens in nonpolar solvents such as toluene and hexane leads to simple synthetic Other alkylating agents may also act as one- methods for the isolation of pure organometallic electron oxidants. Thus, [OEt3][BF4] and MeSO3F oxidation products. For example, the reaction of react with [MoCl2(dppe)2]inCH2Cl2 to give [MoCl2- 5 - - neutral [Mo(η -C7H9)(η-C7H7)] with half an equivalent (dppe)2]X (X ) [BF4] and [SO3F] , respectively), and of iodine in toluene gives a precipitate of ionic [Mo- [ReCl(CO)3(PPh3)2] is similarly oxidized to [ReCl- 5 5 149 (η -C7H9)(η-C7H7)]I which yields solid [Mo(η -C7H9)- (CO)3(PPh3)2][BF4]. (η-C7H7)][PF6] when dissolved in water and treated iii. AlCl3. AlCl3 is a strong oxidant in CH2Cl2 with 135 150 with [NH4][PF6]; a similar strategy was adopted an oxidation potential estimated to be ca. 1.1 V. 136 for the synthesis of [Mo(dmpe)3][PF6]. The iodides However, the mechanism by which it acts is still far themselves may be characterized, but the possible from clear, and the identity of the actual oxidant - + formation of [I3] salts should be borne in mind, as {AlCl3 itself, the cation [AlCl2] (formed by autoion- 23 in [Co2(µ-MeCtCMe)(µ-dppm)2(CO)2][I3]. More- ization of AlCl3 to [AlCl2][AlCl4]), or a carbocation - - 4 + over, both I and [I3] salts, for example, [Ti(η -cot)- such as [CH2Cl] (a product of halide abstraction 4 137 151,152 Cp]I and [Ti(η -cot)Cp][I3], may be isolable (sepa- from CH2Cl2)}, is unknown. (Indeed, AlCl3 can rately or as a mixture) depending on the initial function as a reducing agent, for example, toward stoichiometry of the reactants. Anionic precursors phenanthrenequinone in THF or glyme!152) Never- may also be conveniently oxidized in nonpolar sol- theless, it has been widely used to generate cation vents. Thus, the addition of I2 to a suspension of radicals for ESR spectroscopic studies, mainly from 150,153 [NEt4]2[M(SPh)4{Mo(CO)4}2] in toluene gives solid main group organometallics in CH2Cl2 but also 154 [NEt4]I and a solution of pure [M(SPh)4{Mo(CO)4}2] of p-dialkoxybenzenes in MeNO2. In spite of the 138 (M ) Fe or Co). high sensitivity of AlCl3 toward water, and the More robust, but less electron-rich, substrates may consequent formation of HCl, the highly purified be oxidized successfully with chlorine or bromine; for reagent does not seem to be necessary for the 2- 155 example, [Pt(σ-C6Cl5)4] (E°′ ) 0.07 V) in CH2Cl2 acquisition of ESR spectra in CH2Cl2. However, - 139 gives [Pt(σ-C6Cl5)4] with either halogen in CCl4. it may be purified by vacuum sublimation; it is also 140 The monoanion can be further oxidized to [Pt(σ- commercially available as a 1.0 M solution in PhNO2, C6Cl5)4][(Ep)ox ) 1.4 V] by an excess of chlorine in packed under N2. 141 the presence of AlCl3 (as halogen carrier )inCH2- Synthetic applications have been far fewer than + 156 Cl2-CCl4. Interestingly, [PtPhCl3(PEt3)2] in MeNO2 spectroscopic studies. However, [thianthrene] 157 is inert to chlorine, but addition of AlCl3 leads to and a diazasilacylopentene radical cation have - chlorination of the coordinated aryl ring by electro- been isolated and structurally characterized as [AlCl4] 142 philic substitution. salts. Moreover, a mixture of AlCl3 and Cl2 (but not Reactions competing with simple electron transfer chlorine alone) (section II.A.2.a.iii) oxidizes [Pt(σ-C6- - 140 are common. The halogens not only act as electro- Cl5)4] to [Pt(σ-C6Cl5)4]inCH2Cl2-CCl4 (and note 2- philes but the halides, formed on halogen reduction, that TlCl3 has been used to oxidize [Pt(σ-C6Cl5)4] - 139 may behave as nucleophiles. Thus, the oxidative to [Pt(σ-C6Cl5)4] ). addition of I2 to [Fe(CO)3(PPh3)2], to give [FeI(CO)3- iv. Arsenic and Antimony Pentahalides. The Group + (PPh3)2] , involves one-electron oxidation and coor- 15 pentahalides EX5 (E ) As, X ) F; E ) Sb, X ) F dination of I and/or I-.143 Metal-alkyl bonds are or Cl) are strong oxidants (as well as halogenating frequently cleaved, giving metal and alkyl halides, agents and halide ion acceptors). They are volatile 144 as in the halogenation of [FeR(CO)2Cp]. (AsF5 is a gas at room temperature and pressure,

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+ SbX5 are liquids) and moisture-sensitive and must counteranion with Ag and oxidation by the pen- be stored and used in dry conditions (the fluorides tafluoride.167 + give HF with H2O). Though reactive toward water b. Nitrosonium, [NO] , Salts. The use of ni- and other protic solvents such as alcohols, the pen- trosonium salts as strong one-electron oxidants in tahalides are soluble and stable in solvents such as organic129,168 and inorganic chemistry169-171 has been CH2Cl2 and CHCl3. Electron transfer occurs accord- briefly reviewed, but they are noninnocent and can ing to eq 16 though other complex counteranions may participate in a wide range of alternative reactions. appear in isolated salts (see below). If used, the The most commonly used nitrosonium salts, namely toxicity of arsenic and antimony should be consid- [NO][BF4], [NO][PF6], and [NO][SbF6], are com- ered. mercially available as 95-98% pure solids; a simple laboratory preparation of [NO][BF4], from amyl ni- - - 3EX5 + 2e h 2[EX6] + EX3 (16) trite, aqueous HBF4, and propionic anhydride, is given in ref 172. Many other salts can also be 169,173 Of the pentahalides, SbCl5 has been most widely made, for example, [NO][SbCl6] (from the reac- used. When added as a solution in CH2Cl2 to N(C6H4- tion of NOCl with SbCl5) and [NO][AsF6] (by anion n 174 Br-4)3 or thianthrene in the same solvent, simple one- exchange between [NO][SbCl6] and [NBu 4][AsF6]). electron transfer results and the radical cation salts The salts [NO][BF4] and [NO][PF6] can be purified [N(C6H4Br-4)3][SbCl6] (section II.B.1.a) and [thian- by vacuum sublimation (at ca. 0.01 mmHg and 220- 169,175 threne][SbCl6] (section II.B.1.b) are isolable; aromatic 250 °C ), by washing with ethyl acetate or 169 cation radicals (e.g., of perylene, anthracene, etc.) pumping in vacuo over P2O5, or by recrystallization 174,176 have been generated in the same way for ESR from MeCN at -20 °C ([NO][BF4] ). They are spectroscopic studies, aided by the formation of only very moisture-sensitive, decomposing in damp air to 158 diamagnetic reduction byproducts. Organometallic give NO2, etc., and are normally stored in plastic complexes can also undergo one-electron transfer, containers; glass is attacked by the corrosive byprod- although subsequent halogenation is common, as in ucts of decomposition. The bottled salts are best the reaction with [M(CO)3(η-C6Me6)] (M ) Mo and stored in a glovebox or glovebag though they can be 159 W) to give [MCl(CO)3(η-C6Me6)][SbCl6]; the metal- kept in a desiccator with the bottle filled with halide bonds probably result from the reaction be- nitrogen or argon after sample removal. We have tween the products of one-electron transfer in that obtained high product yields from routine syntheses - [EX6] is a labile source of nucleophilic chloride by simply weighing [NO][BF4] or [NO][PF6] rapidly anions. Moreover, SbCl5 itself can bind, as a Lewis in air and adding the solid reagent to the substrate acid, to the oxidation product. Thus, cis-[Mn(CN)- dissolved in a dry and deoxygenated solvent. How- (CO)2{P(OEt)3}(dppm)] reacts with SbCl5 in CH2Cl2 ever, the method outlined in section II.A.1.a.ii (for to give the Mn(II) derivative trans-[Mn(CNSbCl5)- weighing silver salts) is preferable for stoichiometric 160 (CO)2{P(OEt)3}(dppm)][SbCl6]. reactions, and weighing the reagent in an inert Further complications can arise because the anti- atmosphere or glovebox is recommended when more mony-containing reaction products may themselves rigorously dry conditions are required. be oxidants (eq 17), albeit much weaker than EX5. Nitrosonium salts are soluble, and stable, in dry MeCN, MeNO2, or liquid SO2 but react with diethyl - - - - [SbCl6] + 2e h [SbCl4] + 2Cl (17) ether, acetone, or THF. They are usually insoluble in CH2Cl2 (e.g., [NO][BF4] and [NO][PF6]), but [NO]- - - For example, [SbCl6] oxidizes ferrocene, I , and [BF4] does dissolve in the presence of 18-crown-6. aromatic amines.161 Moreover, this type of reaction, They are insoluble in arenes, but the brown charge- - + and the lability of [EX6] , may give rise to unexpected transfer adduct [(NO) (C6Me6)] is formed on addition - 2- 176 counteranions such as [SbCl4] and [Sb2Cl8] ; the of C6Me6 to a suspension of [NO][BF4]inCH2Cl2. latter is found in [Mo2(µ-Cl)(µ-C8Me8)Cp2]2[Sb2Cl8], Such adducts can be crystallized from CH2Cl2 or SO2 + the product of the oxidation of [Mo2(µ-C8Me8)Cp2] and structurally characterized (e.g., [(NO) (C6H3- i 162 174,177 with [NPr 4][SbCl6]. Me3)][SbCl6]); they are weaker oxidants than the The pentafluorides, EF5, are more difficult to uncomplexed [NO]+ ion.176 handle (in part because of the formation of HF with Nitrosonium salts also dissolve in methanol, form- water) but are stronger oxidants than the chlorides ing an equilibrium mixture (eq 18). The use of an (SbF5 will oxidize the less electron-rich triarylamines alcohol as a solvent, therefore, may lead to protona- unaffected by SbCl5, section II.A.2.a.iii) and are less tion rather than one-electron oxidation (though the likely to give rise to halogenated byproducts. Ac- observation of the formation of [Cr(CO)2(PhCtCPh)- + cordingly, distillation of EF5 (E ) As and Sb) onto (η-C6Me6)] from [Cr(CO)2(PhCtCPh)(η-C6Me6)] and 163,164 164 solutions of [MX2Cp2](X)Cl, M ) V, Nb, [NO]+ in a mixture of MeOH and toluene178 was the Mo, and W;165 X ) Me, M ) Nb and Mo166) in liquid key to the entry of one of us into the field of SO2 at low temperatures gives high-valent, one- or “organometallic electrochemistry”!). two-electron oxidation products such as [VCl2Cp2]- 2+ + + [AsF6] and [MoMe2Cp2] . It is noteworthy that [NO] + MeOH h H + MeONO (18) [VCl2Cp2] and Ag[SbF6] yield [V(SbF6)2Cp2] and AgCl, i.e., that chloride abstraction occurs with Ag+ even The nitrosonium ion is a strong oxidant {e.g., it though oxidation is favored, and observed, with oxidizes [N(C6H4Br-4)3](E°′)0.70 V) to [N(C6H4Br- 164 + AsF5. Moreover, the reaction between [ReCl2- 4)3] , section II.B.1.a)}. Reactions with both inor- Cp2]Cl with a mixture of Ag[SbF6] and SbF5 gives ganic and organic substrates, giving good yields of [ReCl2Cp2][SbF6]3 via abstraction of the chloride one-electron oxidation products, have been carried

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 889

129 out in SO2 ([MoCl2Cp2] ) or, more commonly, of NO is maximized. (Indeed, the reaction of an 174 in MeCN {octamethylbiphenylene, [Re2Cl5- oxidized complex, formally a radical, with NO gas 179 (PMePh2)3], cis- and trans-[Ta6Cl14(PEt3)4] (to both provides another route to nitrosyl complexes; [Cr- 180 175 + + the mono- and dications), dithioethers, and (CO)4(dppe)] and NO give [Cr(CO)3(NO)(dppe)] .) aromatic heterocycles such as phenothiazine and Thus, whether oxidation or nitrosyl complex forma- phenoxazines181}. Oxidations have also been carried tion occurs does not depend simply on the relative + out by addition of a solid salt to a solution of the redox potentials of [MLx] and [NO] but, in addition, second reactant, in acetone (a good yield of [Cr(NO)- on other factors such as the steric and electronic t n + 182 (CNBu )4(PBu 3)] was obtained despite the reac- requirements of the metal and ligands in [M(NO)- + 169,171 + tivity of [NO] toward this solvent ), in a mixture Lx] and [MLx]. (Similar comments apply to the 183 of CH2Cl2 and MeCN, and, most usually, in CH2- oxidation or nitrosation of electron-rich aromatics t + 184 181 Cl2 {e.g., [Mn(CO)(CNBu )(dppm)2] , [Ru(CO)2- such as thianthrene. ) { } 185 186 (PPh3) P(OPh)3 (o-C6Cl4)], [Fe(η-C5Ph5)2], a The nitrosonium ion is also believed to function as 187 + 188 thiophene-based polymer, [Ru(Mes)4] , and [Re2(µ- an inner-sphere oxidant in organic chemistry,168,195 189} Cl)2Cl4(dppm)2] . but where one electron-transfer products are ob- + The formation and ready removal of a gaseous served, intermediates equivalent to [M(NO)Lx] are byproduct (i.e., NO) in the reaction of [NO]+ with an less easily verified. However, the recent observation + oxidizable substrate provides an advantage over of the formation of R2NNO (and probably [R2NdS] ) other strong oxidants. However, where there is the in the reaction between R2NSNR2 (R2N ) piperidine possibility of competition between oxidation and an or 9-azabicyclo[3.3.1]nonane) and [NO][PF6], but of + + alternative pathway, successful product isolation may [R2NSNR2] with [N(C6H4Br-p)3] , is good evidence depend on the efficiency with which NO is removed for an inner-sphere process with [NO]+.196 from the reaction mixture, in the synthesis of radical The solvent can also have a marked effect on the 181 cations from aromatic heterocycles, for example outcome of a reaction involving the nitrosonium ion. (and see below). Moreover, in the presence of air, the The formal potential of [NO]+ is highly solvent - formation of NO2, [NO3] , etc., may result in the dependent, spanning a range of almost 0.5 V (Table isolation of other products. For example, in the 5). Moreover, in acetonitrile halide substitution, as absence of air [NO][PF ] and [Re ( -S)( -Cl)Cl (CS)- 2+ 6 2 µ µ 3 in the formation of [V(NCMe)2Cp2] from [VCl2Cp2], (dppm)2] give the monocation [Re2(µ-S)(µ-Cl)Cl3(CS)- or solvent addition, as in the formation of [Mn- (dppm) ]+,190 but in the presence of air NO is formed + 170 2 2 (NCMe)(CO)5] from [Mn2(CO)10], can occur. The and [Re2(µ-SO2)(µ-Cl)Cl3(CS)(dppm)2] results (even dependence of reaction pathway on solvent is, how- 191 with catalytic amounts of [NO]+ ). ever, much more marked in alcohols. As mentioned The nitrosonium ion is far from an innocent oxidant above, oxidation can occur in MeOH-toluene mix- and will often react via other pathways, even with tures. However, the equilibrium shown in eq 18 is easily oxidized substrates.170 In transition metal more likely to lead to protonation, as in the formation + 170 chemistry the most common alternative reaction of [IrH(CO)(PPh3)Cp] from [Ir(CO)(PPh3)Cp]. More product is a nitrosyl complex. For example, fac-or irritating, however, is the observation of protonation + mer-[MnBr(CO)3(PMe3)2] is oxidized by [NO] to fac- in apparently dry solvents (due to the formation of + + + [MnBr(CO)3(PMe3)2] whereas the more electron-rich H from [NO] and water). The one-electron oxida- complex [MnBr(CO){P(OMe)3}4] yields [Mn(CO)(NO)- tion of [Mn(CN)(CO)2{P(OPh)3}2(dppm)] can be ac- 2+ 192 + + {P(OMe)3}4] ; [Fe(CO)3(PPh3)2](E°′)-0.06 V) complished using [N(C6H4Br-p)3] , but [NO] gives + 2+ 197 (easily oxidizable by [NO] ) undergoes only substitu- trans-[Mn(CNH)(CO)2{P(OPh)3}2(dppm)] , under- + 143 tion to [Fe(CO)2(NO)(PPh3)2] . Studies of the reac- lining the importance of using dry conditions. tion between [Cr(CO) (L-L)] (L-L dppm or dppe) + 4 ) c. Arenediazonium Ions, [N2aryl] . Arenedia- and [NO]+ showed193 that either oxidation to [Cr- + zonium ions, [N2aryl] , play an important role in + + (CO)4(L-L)] or substitution to [Cr(CO)3(NO)(L-L)] organic synthesis,198 particularly in metal-catalyzed could occur, depending on the efficiency with which processes.199 Many such processes appear to involve NO gas was purged from the reaction mixture. initial oxidation, as in eqs 19 and 20, though the 194 These, and earlier, observations led us to postulate mechanism of electron-transfer remains controver- a general mechanism for the reaction of transition sial. Inner-sphere processes seem more likely, but + metal complex [MLx] with [NO] (Scheme 1). The kinetic studies suggest outer-sphere oxidation of equilibrium involving [M(NO)L ]+, effectively an in- 4- 200 x [FeCp*2] and [Fe(CN)6] . ner-sphere intermediate, tends toward the formation of [ML ]+ if NO is efficiently removed from the x + f • + reaction medium; nitrosyl complex formation is fa- [N2aryl] + A N2aryl + A (19) vored by conditions under which the concentration • • N2aryl f N2 + aryl (20) Scheme 1 Surprisingly little interest has been shown in diazonium salts as stoichiometric one-electron oxi- dants in organometallic chemistry though they offer many advantages when a mild reagent is required. Moreover, the redox potential can be altered system- atically from ca. 0.05 to -0.50 V by varying the ring substituents (Table 6).

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Table 6. Formal Potentials (V vs Fc) of Scheme 2 + a Arenediazonium Ions [N2C6H4R-4] R E°′/V R E°′/V R E°′/V

NO2 0.05 F -0.07 Me -0.15 Cl 0.01 H -0.10 NMe2 -0.50 a Potentials from ref 200 and Elofson, R. M.; Gadallah, F. F. J. Org. Chem. 1969, 34, 854.

Ag[PF6] and [NO][PF6] give [Ag{Rh(CO)(PPh3)Cp}2]- Arenediazonium ions with a wide range of para [PF ] and [Rh(NO)(PPh )Cp][PF ], respectively.70 substituents are simply prepared and readily isolated 6 3 6 - - Diazonium salts are often noninnocent one-electron as stable [BF4] and [PF6] salts; [N2C6H4NEt2-4]- oxidants in organometallic chemistry. For example, [BF4] is currently commercially available. The most one-electron oxidation may be followed by radical widely cited preparative methods201 involve the syn- - coupling; aryl radicals formed by the dissociation of thesis of [BF4] salts in an aqueous reaction medium; •N aryl may abstract a hydrogen atom from the alternative methods are also available198,202 using, for 2 t solvent and the resulting solvent-based radical couples example, BF3‚OEt2, RNH2, and BuONO in diethyl 202 with the oxidized form of the substrate. Thus, ether or CH2Cl2. In our studies of the oxidation of [Cr(CN)(CO) ]- and [N aryl]+ react according to low-valent transition metal complexes, we have usu- 5 2 - - Scheme 2 to give functionalized isocyanide complexes ally used [N2C6H4F-4]X (X ) [BF4] or [PF6] ). A 209 such as [Cr(CO)5(CNR)] [e.g., R ) CCl3 (from CHCl3) simple synthesis of the latter, based on the method or complex 5 (from THF)].210 Similar reactions of Roe,201 is given in the Appendix. Note that freshly purchased HPF6 should be used in this synthesis; older samples of HPF6 are often severely contami- nated with decomposition products. No such prob- lems have been encountered when using HBF4 to prepare [N2aryl][BF4] by the same method.

The simple arenediazonium salts [N2C6H4R-4]X (R + - ) H, Me, F, OMe, or NO2;X)BF4 or PF6) are white between [N2aryl] and [Mo(CO)3Tp′] give the chlo- 211 or off-white crystalline solids, stable for months if rocarbyne complex [Mo(CCl)(CO)2Tp′]inCH2Cl2 or 2 211,212 pure, dry, and stored at -10 °C; deliberate thermoly- [Mo(η -COaryl)(CO)2Tp′] when the solvent does sis of [N2C6H4R-4][BF4] yields the fluoroarene FC6H4R- not undergo H-atom abstraction. 4. Samples may become colored with age but are It is noteworthy that with less hindered polypyra- - easily repurified by dissolution in acetone, filtration, zolylborate ligands [e.g., L ) Tp or B(pz)4] [Mo(CO)3L] + 213 and precipitation with diethyl ether. and [N2aryl] give [Mo(CO)2(N2aryl)L]. Many other transition metal complexes likewise give arenediazo Solid salts such as [N2aryl][BF4] and [N2aryl][PF6] 214 may be handled in air. They are soluble in polar complexes in which the N2aryl group can act as solvents such as acetone and acetonitrile but in- either a one-electron donor (“doubly bent”, isoelec- tronic with a “bent” nitrosyl ligand) or a three- soluble in CH2Cl2. When used with solvents such as electron donor (“singly bent”, isoelectronic with a THF and CHCl3, unexpected products resulting from hydrogen atom abstraction reactions may be encoun- “linear” nitrosyl ligand). The factors which govern tered (see below). oxidation as opposed to complexation were discussed during our early studies of [NO]+ and [N aryl]+ as Diazonium ions have been successfully used in the 2 oxidants194 and are outlined in section II.A.2.b for synthesis of a range of 17-electron complexes from [NO]+. their 18-electron analogues, usually by adding the Arenediazonium ions may also undergo insertion solid salt to the substrate dissolved in CH Cl 70,203-207 2 2 reactions, for example, into M-H bonds ([PtHCl- but also in THF.208 Electron transfer from substrate (PEt ) ] yields [PtCl(PEt ) (NH:Naryl)]+)215 or into to [N aryl]+ is usually rapid at or below room tem- 3 3 3 2 2 one M-P bond of [Fe(NO) (dppe)] to give [Fe(NO) - perature, and the diamagnetic byproducts, nitrogen 2 2 {PPh CH CH P(Ph )N C H F-4}],216 and ortho-meta- and the biaryl (formed by dimerization of aryl radi- 2 2 2 2 2 6 4 lation or dimerization, as in the formation of 6 and 7 cals), are simply removed. Even though the diazo- from [IrCl(CO)(PPh ) ] and [N C H F-4]+ in the pres- nium ion is a mild oxidant, the reduction of [N C H R- 3 2 2 6 4 2 6 4 ence of ethanol or 2-propanol.214 Finally, arylation 4]+ is irreversible, and substrates with potentials of, or azo coupling with, coordinated hydrocarbons more positive than that of the oxidant may be such as chpt and cot may also be observed.217,218 oxidized. This can provide an advantage over other mild oxidants, for example, where an equilibrium mixture is formed when the ferrocenium ion reacts with a substrate which is reversibly oxidized at ca. 0.0 V. The use of arenediazonium salts can also be suc- cessful in giving the desired one-electron oxidation product when other oxidants react via unexpected pathways. For example, [Rh(CO)(PPh3)Cp] is oxida- tively dimerized to the fulvalene complex [Rh2(CO)2- (PPh3)2(η,η′-C10H8)][PF6]2 by [N2C6H4F-4][PF6] whereas

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d. Oxygen. Precautions are almost invariably with that between HBF4 and the “closed” cyclopen- taken to avoid the presence of oxygen during the tadienyl complex [Co(PEt3)2Cp] which undergoes synthesis of organotransition metal complexes; con- protonation at the metal center225 (a reaction perhaps ventional wisdom has led to the assumption that more generally expected than one-electron oxidation). decomposition to unwanted products is to be ex- It is also noteworthy that the ammonium ion may pected. Nevertheless, there are good examples where act as an oxidant, although presumably functioning 5 O2 is used to give well-defined one-electron oxidation as a proton source. Thus [Cr(η -heptamethylinde- products (or secondary products derived therefrom). nyl)2] in toluene reacts with a saturated solution of 5 It should also be noted, given the role of 17-electron [NH4][PF6] in THF to precipitate [Cr(η -hepta- 226 species in electron-transfer-catalyzed processes, that methylindenyl)2][PF6]. Oxyacids may also function adventitious oxygen may play a part in the genera- as one-electron oxidants. Thus, HNO3 or H2O2 and tion of radical-like intermediates and the acceleration [MXYCp2](M)Mo or W; X, Y ) Cl, N3, NCS, O2- + of reactions assumed to proceed solely through dia- CCF3, etc.) give [MXYCp2] , which may be precipi- - magnetic intermediates. tated as the [PF6] salt by addition of [NH4][PF6]to Oxygen is, of course, readily available and easily the aqueous solution.227 Such acids are likely to be handled. It reacts with [Cr(η-arene)2] to give [Cr(η- much stronger oxidants than HBF4 or HPF6. 145 arene)2][OH] and with related binuclear complexes such as [Cr2(µ-biphenyl)(η-C6H6)2] and [Cr2(µ-biphe- B. Organic 219 nyl)2] to give the corresponding dications (cf. the use of 4-pyridinecarboxaldehyde, section II.B.5.a). 1. Radical Cations Stoichiometric reactions have been carried out be- a. Triarylaminium Radical Cations, [N(ar- + + tween [Fe(η-C6Me6)2] and O2, the latter added to the yl)3] . Triarylaminium radical cations, [N(aryl)3] , former by means of a gas syringe. This reaction is with the para positions substituted to deter nucleo- of particular interest in showing the effects of both philic substitution, are mild to very strong one- temperature and ion pairing on the final outcome of electron oxidants used widely in organic chemistry an electron-transfer rection. At -40 °C in toluene, and, to a far lesser extent, in inorganic chemistry. 4 0.5 equiv of O2 give [Fe{η -C6Me4(dCH2)2}(η-C6Me6)]; The archetypal arylaminium salt, [N(C6H4Br-4)3]- oxidation of [Fe(η-C6Me6)2] is followed by sequential [SbCl6] (the cation is colloquially known as Magic - deprotonation of two of the methyl groups by [O2] Blue because of its intense royal blue color), is within the solvent cage. At 20 °C in toluene, or at commercially available, but many other derivatives, -40 °C in THF in the presence of NaPF6, the peroxide with redox potentials spanning the range 0.16-1.72 salt [Fe(η-C6Me6)2]2[O2] results (the higher temper- V (Table 7), can be prepared or generated in situ from ature or presence of the sodium salt leading to the the corresponding amine. Many of the cations are removal of the superoxide anion from the solvent cage remarkably stable and can be used not only as before deprotonation can occur220). stoichiometric reagents but as catalysts in electron-

The dioxygenyl salt [O2][EF6](E)As or Sb) is transfer chain processes; the neutral amines are used arguably the strongest one-electron oxidant yet iso- as mediators in indirect electrosynthesis.228,229 lated, with an estimated formal oxidation potential for [O ]+/O of almost 5.0 V.221 It has been used in Table 7. Formal Potentials (V vs Fc) of 2 2 + a low-temperature Freon mixtures to oxidize organic Triarylaminium Cations [N(aryl)3] compounds to the corresponding cation radicals. For aryl E°′/V aryl E°′/V example, [O2][AsF6] and NMe(C6H4OMe-4)2 give C6H4OMe-4 0.16 C6H4NO2-4 1.20 [NMe(C6H4OMe-4)2][AsF6]inCHF2Cl at -130 °C. C6H4Me-4 0.40 C6H4Br2-2,4 1.14 The redox byproduct, O2, and the solvent are allowed C6H4(COMe)-4 0.90 C6H4Br3-2,4,6 1.36 b to evaporate, leaving behind the desired salt.222 C6H4Br-4 0.70 C6Cl5 1.72 C6H4CN-4 1.08 Interestingly, the superoxide ion can act as both a b an oxidant and reductant. Though generally studied Potentials from ref 228 unless stated. From ref 237. in aqueous media, and as such of limited interest for organometallic applications, it has been generated Arylaminium salts are prepared from triaryl amines, electrochemically in DMSO, pyridine, DMF, and many of which have been synthesized specifically as MeCN. It can also be prepared (with difficulty) as radical cation precursors.230-233 There are three main the salt [NMe4][O2], which is soluble in aprotic preparative procedures, using (i) a silver(I) salt in the 223 solvents, and is solubilized, as KO2, by crown ethers. presence of iodine,233-235 (ii) an [NO]+ salt,181,233,235 or e. Acids. There are several examples of acids (iii) a higher valent halide, usually SbCl5,asthe 230,231,233,236,237 such as HBF4 or HPF6 effecting the one-electron oxidant; the method chosen depends oxidation of electron-rich organometallics. (The cor- inter alia on the oxidation potential of the amine and ollary is that adventitious acid, perhaps from impure the counteranion desired. Method i involves the solvents, can initiate unwanted or unexpected electron- addition of iodine in diethyl ether to a solution of the transfer-based reactions, as noted above for oxygen.) amine and AgX in the same solvent, extraction of the 5 Thus, [Mo(η -C7H9)(η-C7H7)] (E°′ )-0.19 V) and resulting precipitate into CH2Cl2 (to remove silver 135 5 HBF4‚OEt2 in diethyl ether and [Co(PEt3)2(η -2,4- metal), and precipitation of the salt [N(aryl)3]X with - - dimethylpentadienyl)] and HBF4‚OEt2 or HO3SCF3 diethyl ether. The [BF4] and [PF6] salts of 224 5 + in THF, gave [Mo(η -C7H9)(η-C7H7)][BF4] and [Co- [N(C6H4R-4)3] (R ) Br or OMe) have been prepared 5 - - (PEt3)2(η -2,4-dimethylpentadienyl)]X (X ) [BF4] or in this way though higher yields of the [PF6] salts - [O3SCF3] ), respectively. The last reaction contrasts are isolable from [NO][PF6] oxidation of the appropri-

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233,235 - + + ate amine in MeCN. Salts of the [SbCl6] anion method). In contrast to [NO] and Ag analogues result from the treatment of the amine with SbCl5 (also strong to very strong oxidants in CH2Cl2), 230,236 in CH2Cl2; where SbCl5 is not strong enough to arylaminium salts are soluble in CH2Cl2. 237 accomplish amine oxidation, SbF5 may be used. Unlike [NO]+ (section II.A.2.b) and Ag+ (section The stability of solid arylaminium cations depends II.A.1.a.ii), [N(C6H4Br-4)3][PF6] almost always be- inter alia on the counteranion. Thus, salts of [N(C6H4- haves as an innocent one-electron oxidant (but see + - - Br-4)3] with larger anions such as [PF6] , [SbCl6] , below). Thus, cis-ortrans-[Mn(CN)(CO)2{P(OPh)3}- - and [WCl6] (prepared by WCl6 oxidation of the (dppm)] gives trans-[Mn(CN)(CO)2{P(OPh)3}(dppm)]- amine) show little sign of deterioration after several [PF6]inCH2Cl2 whereas [NO][PF6] yields the proto- - weeks whereas the [BF4] salt ages more rap- nated complex [Mn(CNH)(CO)(CO)2{P(OR)3}- 233,235 197 idly. The difference in stability has been as- (dppm)][PF6]2 and Ag[PF6] yields [Ag{(NC)Mn- 52 cribed to the repression of benzidine radical cation (CO)2{P(OEt)3}(dppm)}2][PF6]3. Similarly, [N(C6H4- 233 formation via debromodimerization; the larger Br-4)3][PF6] and [Fe(CO)3(PPh3)2] yield [Fe(CO)3- + anions prevent the required approach of two cations. (PPh3)2][PF6] whereas Ag appears to form an Ag- + + The aging of [N(C6H4Br-4)3] also results in further Fe-bonded adduct and [NO] or arenediazonium ions bromination of the aryl groups (Br2 is eliminated yield substitution products such as [Fe(CO)2(NO)- + 143 - during the dimerization process); mass spectrometry (PPh3)2] . Note, however, that the [SbCl6] coun- - showed that 6-month-old samples of the [BF4] salt teranion is not innocent (see section II.A.2.a.iv) and contained tetra- and pentabrominated amines as well can act as a source of nucleophilic chloride ions (as as the dimeric product.233 Similar variations in has also been observed in organic chemistry239). For stability have been noted for the salts of [N(C6H4- example, [N(C6H4Br-4)3][SbCl6] oxidatively cleaves + 234,235 + 240 Me-4)3] . By contrast, [N(C6H4OMe-4)3] (a less [Fe2(µ-CO)2(CO)2Cp2] to yield [FeCl(CO)2Cp] and powerful oxidant, E° ) 0.16 V) is also stable as the reacts with [Mn(CO)(dppe)Cp′] to give [MnCl(CO)- - + + 205 [BF4] salt. (dppe)Cp′] via [Mn(CO)(dppe)Cp′] . The results of kinetic studies and product analysis High product yields can be obtained without the are critically dependent on the use of pure samples need to use the isolated arylaminium cation salts; 233 + of the arylaminium salts. The extent to which a solutions of [N(C6H4Br-4)3] may be generated in situ + sample decays may be estimated by optical spectros- by treatment of [N(C6H4Br-4)3] with either [NO] or 233,235 + + copy, (e.g., [N(C6H4Br-4)3] , λmax ) 700 nm; Ag in CH2Cl2 and removal of the byproducts, NO + 230,231 [N(C6H3Br2-2,4)3] , λmax ) 880 nm); the deterio- and Ag, by purging the reaction mixture with a ration of solid [N(C6H4Me-4)3][PF6] during 1 week at stream of nitrogen (or argon) or by filtration, respec- 4 room temperature is sufficient to be detectable by tively. In this way, [Fe(CO)3(η -cot)] was oxidatively 235 5 5′ 241 changes in the UV spectrum. Voltammetry can dimerized to [Fe2(CO)6(η ,η -C16H16)][PF6]2, and + also be used to assess the purity of aged samples of [Rh3(µ3-L)2(CO)4(PPh3)2] (L ) 1,8-naphthyridin-2- + [N(C6H4Br-4)3] since decomposition products having onate) was oxidized to the mixed-valence complex 2+ 242 other degrees of bromine substitution have different [Rh3(µ3-L)2(CO)4(PPh3)2] . E°′ values. (Such aged samples have, neverthless, The applications in organic chemistry of ary- been successfully used as oxidants; the oxidant laminium cations as catalysts243-248 or as intermedi- mixture has been dubbed “The Blues Brothers”.) ates in indirect electrosynthesis (the amine is added The solubility of triarylaminium salts depends on as the mediator)228,229 have been extensively re- both the aryl substituent and the counteranion. For viewed. The solid arylaminium salt is usually used example, [N(C6H4Br-4)3][SbCl6] is slightly less soluble in catalytic applications; efficient catalysis is made in CH2Cl2 than is the perchlorate salt (which is not possible by the stability of both the amine and its recommended for use).236 The stability of such solu- radical cation. However, in situ generation of the + tions depends on the solvent and, as noted above, on arylaminium cations is also possible; [N(C6Cl5)3] (E°′ the substitution pattern on the aryl groups and on ) 1.84 V) cannot be isolated but has been generated the counteranion. For example, in CH2Cl2, [N(C6H4- by SbF5 oxidation of the amine and used to oxidize Br-4)3][ClO4] is stable for 2 days in the absence of O2 syn-5,5′-difluorobiadamantylidine (E°′ ) 1.39 V) (which + and light but [N(C6H4Br-4)3][PF6] is indefinitely is not oxidized by [N(C6H4Br-4)3] or [N(C6H3Br2- + 237 stable in the absence of light and is even stable for 2,4)3] ). In indirect electroorganic synthesis, the several days in laboratory light. In methanol, [N(C6H4- amine is used as a mediator with the cation gener- + 228 Br-4)3] is methoxylated at one ring whereas ated and regenerated by electrolysis; turnovers as + [N(C6H3Br2-2,4)3] is stable and undergoes several high as 2500 cycles have been observed with [N(C6H3- + 229 thousand turnovers during the mediated side-chain Br2-2,4)3] . Far more use has been made in organic oxidation of alkyl-substituted aromatics.238 chemistry of the possibility of tuning the redox Stoichiometric reactions using arylaminium salts potential of the oxidant; the amine substituent (Table are simply accomplished, either by using the isolated 7) can be systematically altered to achieve selective salt or by generating the cation in situ. Moreover, oxidation. In addition, electron transfer may occur the byproduct of electron transfer, N(aryl)3, is easily even with an unfavorable potential, subsequent ir- removed from the oxidation product. If necessary, reversible bond cleavage reactions providing the the electron-transfer reactions of arylaminium cat- driving force for the overall reaction. ions can be visually “titrated” in that their intense Finally, arylaminium cations are not always in- + + + colors (e.g., [N(C6H4Br-4)3] , blue; [N(C6H3Br2-2,4)3] , nocent. For example, [N(C6H4Br-4)3] undergoes green) are lost immediately on reduction (optical nucleophilic reactions with chloride and cyanide ions spectroscopy could be used as a more quantitative but electron transfer with bromide and iodide ions.

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- Moreover, substitution is easier with the [BF4] salt The thianthrene cation has been almost exclusively - 235 than with the [SbCl6] salt. Similarly, [N(C6H4- used in investigations of the one-electron-transfer Br-4)3][BF4] does not oxidize carboxylates but under- reactions of main group organometallics such as LiPh 233 256 256 goes acyloxylation at room temperature. Such side in THF, MgClPh in ether, and HgR2 in 252,257 - reactions, and the dependence of the reaction path- MeCN. In all of these cases, the [ClO4] salt - way on the counteranion, even in solution, can have was used but the less treacherous [BF4] salt is important consequences for catalysis.239 equally effective, as demonstrated by its reactions 252 b. Thianthrene and Related Radical Cations. with HgR2 in CH2Cl2. As yet the use of aromatic Although triarylaminium salts (section II.B.1.a) are heterocycle radical cations as oxidants in other areas the most widely used radical cation oxidants, other of chemistry has been limited although [FeCl(TPP)]254 such reagents are also available. Of these, the and neutral actinide porphyrin and phthalocyanin thianthrene cation (8+) is the most popular although complexes such as [M(TPP)(PC)] (M ) U and Th)253 related radical cations of aromatic heterocycles such have been oxidized to the corresponding monocations as phenoxathiin (9), phenothiazine (10,R)H), by [9][SbCl6]. N-methyl- or N-phenylthiazine (10,R)Me or Ph), and carbazoles have also been prepared. 2. Carbocations + a. Trityl, [CR3] (R ) Aryl), Salts. The trityl + cation, [CPh3] , also referred to as the triphenylcar- benium or tritylium ion, has long been known as a reagent for the removal of hydride ion from both organic258,259 and organometallic compounds.260 How- ever, it can also function as a one-electron oxidant. - - - Although the yellow-orange [BF4] , [PF6] , [AsF6] , - - + The thianthrene radical cation has been isolated [SbCl6] , and [CF3SO3] (triflate, Tf) salts of [CPh3] - 156 - 249 - 250,251 as [AlCl4] , [SbCl6] , and [I2Cl3] salts but are commercially available, they are readily made 249,250 has most often been used as the perchlorate. The from CPh3OH and the appropriate acid in acetic or last is, however, explosive, and despite the detailed propionic anhydride,258 by the dropwise addition of 249,252 warnings given on its safe handling, we recom- anhydrous HSbF6, HBF4‚Me2O, or HPF6‚Et2Oto - 261 mend that it is not used. Rather, the [BF4] salt, CClPh3 in benzene, or from CClPh3 and AgPF6 in 234 which may be prepared on a large scale from [NO]- CH2Cl2. The triflate salt has also been prepared 252 [BF4] and thianthrene in CH2Cl2 is to be preferred. from CPh3Cl and Me3SiTf and converted to [CPh3]- (The other salts listed above might also be used if [BPh4] (a useful salt in the preparation of cationic, the presence of a halogen-containing counteranionsa Lewis acidic products which might react with more source of nucleophilic halide ionssis not a problem.) nucleophilic anions) by metathesis with NaBPh4 in 262 The purity of [thianthrene][BF4] can be assessed by MeCN. In principle, the oxidation potential of + treatment with NaI in MeCN/CCl4 and then titration [CPh3] can be systematically altered by ring sub- 249 263 of the released I2 against sodium thiosulfate, or by stitution; the salts [C(C6H4NMe2-4)2Ph][BF4], + 264 optical spectroscopy (8 ; in MeCN, λmax ) 546 nm,  [C(C6H4Me-4)3][PF6], and [CPh2(C6H4OMe-4)]- 3 250 265,266 ) 8.5 10 ; in CH2Cl2, λmax ) 548 nm,  ) 8.9 [BF4] have been prepared from the appropriate 3 156× × 258 10 ). carbinol C(aryl)3OH. Trityl salts are usually re- The radical cations of other aromatic heterocycles, crystallized from MeCN at low temperature. Puri- including 9, 10, and carbazole, can also be simply fication may also be accomplished from CH2Cl2-n- - 181 prepared as [BF4] salts using [NO][BF4] and hexane or MeCN-Et2O mixtures; large crystals of 253,254 phenoxathiin and SbCl5 in CH2Cl2 give [9][SbCl6]. [CPh3][BF4], especially suitable for long-term storage, These cations have been used less often than that of can be grown by layering diethyl ether onto a solution thianthrene,255 but the variation in heteroatom and, of the salt in MeCN. + in some cases, the substituent [e.g., at N in 10] could, Solid salts of [CPh3] react with moisture, giving in principle, lead to the systematic alteration of colorless CPh3OH. In the absence of air the solids oxidation potential. show no visible signs of decomposition after several The red to brown salts of the thianthrene cation months when stored at -10 °C; [CPh3][BPh4], how- - 262 (the solid [I2Cl3] salt is black) are stable in dry air ever, is light sensitive. The salt [CPh2(C6H4OMe- - (the [AlCl4] salt has been structurally character- 4)][BF4], a somewhat weaker oxidant (E°′ )-0.32 156 ized ) but react with water to give thianthrene and V) than [CPh3][BF4](E°′)-0.11 V), is much more - thianthrene S-oxide. The pure [BF4] salt is stable stable and does not deteriorate when refrigerated, for weeks when dissolved in dry acetonitrile,249 but even in the presence of moist air for 1 year. It may, 266 if generated in situ, from thianthrene and [NO][BF4] therefore, be weighed and transfered in air. Trityl in CH2Cl2, it deteriorates more rapidly, probably by salts are soluble in dry CH2Cl2, MeCN, or liquid SO2 reacting with NO or NO2. but are insoluble in alkane or arene solvents; slow Solutions of thianthrene radical cation salts are decomposition has been observed in diethyl ether.258 purple, giving colorless thianthrene on reduction. They react with protic solvents such as water and Their reactions can therefore be followed visually or, alcohols; in aqueous acetonitrile, [CPh2(C6H4OMe-4)]- more quantitatively, by optical spectroscopy; the five- [BF4] decomposes 100 times more slowly than [CPh3]- 266 line ESR spectrum (g ) 2.0087 in MeCN250)ofthe [BF4]. cation provides another means by which its reactions Triarylcarbenium ions are mild one-electron oxi- 267 • might be monitored. dants which give C(aryl)3 radicals on reduction.

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+ These radicals can often be detected by ESR spec- CH2)Cp2] by double oxidation (to [Mo2(µ-C8Me8)- 2+ troscopy but form an equilibrium mixture favoring Cp2] ) followed by proton loss (an EEC mechanism) + the diamagnetic dimer [e.g., 4-(triphenylmethyl)-1- whereas [CPh3] gives the same final product but via (diphenylmethylidene)-2,5-cyclohexadiene from the H-atom abstraction reaction between [Mo2(µ-C8- • + • 45 CPh3]. However, the possible reaction of solvent- Me8)Cp2] and CPh3 (an EC process). based radicals, formed by H-atom abstraction from Finally, an electron-transfer reaction involving • + the solvent by C(aryl)3 [giving CH(aryl)3], with the [CPh3] can lead to a different stereochemistry from primary oxidation product should not be discounted a reaction involving direct H- loss. Thus, exo-hydride [cf. the behavior of aryl radicals formed on reduction abstraction occurs from a noncoordinated CH2 group 4 of diazonium ions (section II.A.2.c)]. of the hydrocarbon ligand in [Fe(CO)3(η -cyclohexa- 4 The reactions of trityl cations are normally carried diene)] whereas [Fe(exo-η -C6H7R)(η-C6H6)] and + 5 + out at low temperature in CH2Cl2 or MeCN (though [CPh3] at -50 °C give [Fe(η -C6H6R)(η-C6H6)] by THF has also been used as solvent268). Simple one- loss of an endo hydrogen atom after oxidation to [Fe- 4 + 279 electron oxidation products, e.g., [CpFe(µ-cot)CoCp]+ 269 (exo-η -C6H7R)(η-C6H6)] . + 270 + and [FeMe{P(OMe)3}2Cp*] , from [CpFe(µ-cot)- b. Tropylium, [C7H7] , Salts. The tropylium + CoCp] and [FeMe{P(OMe)3}2Cp*], respectively, may cation, [C7H7] , is a weak oxidant (Table 2) as yet result. More often than not, however, other reac- relatively little used in organic267 or organometallic tions, either following or instead of electron transfer, chemistry; the oxidation potential can be varied to + 281 are observed. Thus, [CPh3] and [Re2(µ-H)4H4- some extent by ring substitution. + (PPh3)2] give [Re2(µ-H)4H4(PPh3)2] in CH2Cl2 but The colorless salts [C7H7][BF4] and [C7H7][PF6] are + [Re2(µ-H)4H3(NCMe)(PPh3)2] is isolated from MeCN commercially available, but the former is readily (in a reaction apparently not involving electron prepared on a large scale by reacting PCl5 with chpt - - transfer).271 in CCl4 and treating the resulting [PCl6] /Cl double + 282 Trityl cations can act as electrophiles, adding to salt of [C7H7] with HBF4. Methods involving hydride abstraction from chpt, using [CPh3][BF4]in coordinated hydrocarbons such as the cot ring of 259 [Fe(CO) ( 4-cot)],272 or as ligands; the reactions with MeCN or SO2 or CPh3OH and HBF4 in acetic 3 η 283 - [Re(CO) ]- and [Co{ 5-2,4-bis(trifluoromethyl)penta- anhydride, can also be used to prepare the [AsF6] 5 η - } 3 and [SbF6] salts; [C7H7][SbCl6] is made directly from dienyl Cp] give [Re(CO)4(η -C6H5CPh2)] and [CoCp- 284 (η5-C H CPh )]+,273 respectively. With [Fe(CO) - chpt, CPh3Cl, and SbCl5 in MeCN. 6 5 2 2 - (CH SMe)Cp*], addition at sulfur to give the Tropylium salts of complex anions such as [BF4] 2 - + and [PF6] are stable in air and nonhygroscopic (in sulfonium cation [Fe(CO)2{CH2S(Me)CPh3}Cp*] is followed by decomposition via trityl radical loss. contrast to simple halide salts such as [C7H7]Cl). Different products result from the reaction with the [C7H7][BF4] is soluble in MeCN, SO2, and acetone and + may be recrystallized from a large volume of ethyl outer-sphere oxidant, [FeCp2] , suggesting the trityl cation to be an inner-sphere reagent;274 similar acetate (or from MeCN/ethyl acetate); product loss conclusions were drawn from comparative studies of is extensive during such recrystallization and the 282 + + + purity is little improved. It is insoluble in CH2Cl2 the reactions of [FeCp2] and [CPh3] or [C(tolyl)3] 264 265 and forms donor:acceptor complexes with arenes with [RuH(PPh3)2Cp] and [MoH(CO)2(PPh3)Cp], respectively. which can undergo photoinduced redox reactions giving the tropyl radical and the arene radical Extensive studies of the reactions of organometal- cation.284 lics with ferrocenium and trityl cations have shed Readily oxidized substrates such as [Re H (µ-H) - considerable light on the competition between oxida- 2 4 4 (PPh ) ] and [Re(CO) ]- react with solid [C H ][PF ] tion and hydride abstraction. The formal potential 3 4 5 7 7 6 to give [Re H (µ-H) (PPh ) ]+ (in CH Cl )271 and [Re - of the oxidant is but one factor which influences the 2 4 4 3 4 2 2 2 (CO) ] (in THF),285 respectively. Moreover, dianions outcome of the reactions of mononuclear transition 10 such as [Ru (CO) ]2- give neutral clusters, [Ru - metal alkyls. Thus, [WMe Cp ](E°′)ca. -0.8 V) 3 11 3 2 2 (CO) L], when treated with [C H ][BF ] in the pres- and [CPh ]+ (E°′ )-0.11 V) give [WH(η-C H )Cp ]+ 11 7 7 4 3 2 4 2 ence of donor ligands, L, such as phosphines.286 Note, by initial electron transfer (∆E ) ca. 0.7 V) followed however, that [Ru C(CO) ]2- gives [Ru C(CO) (η6- by R-hydrogen atom abstraction by •CPh .275 Simi- 6 16 6 14 3 C H )] in which ditropyl, the byproduct of the larly, [Re(σ-CH CH R)(PPh )(NO)Cp] (R ) alkyl) 14 14 2 2 3 reduction of [C H ]+ (and normally readily removed give276 species such as [Re(dCHCH R)(PPh )(NO)(η- 7 7 2 3 from the reaction mixture), is bound to one triangular C H )]+ in spite of the endergonic electron-transfer 5 5 face of the cluster.287 In addition, the tropylium ion step (∆E ) ca. -0.2 V); the reaction is driven to itself can act as an electrophile toward coordinated completion by the irreversible C-H cleavage follow- hydrocarbons, for example, the cot rings of [Fe(CO) - ing oxidation. If electron transfer is too unfavorable, 3 (cot)]288 and [M(cot)Cp] (M ) Co and Rh).289 as in the reaction with [Fe(σ-alkyl)(CO)2Cp] (∆E ) ca. -1.0 V), direct â-hydride abstraction gives [Fe- 2 + 277 (CO)2(η -alkene)Cp] (although the electron-trans- 3. Cyanocarbons and Related Electron-Rich Compounds fer mechanism can operate in the presence of carbon monoxide).278 Many percyanocarbons are one-electron oxidants. Similar comparative studies with hydrocarbon- However, their redox behavior has been most widely bridged binuclear organometallics show that the applied to the construction of low-dimensional solids + oxidative activation of C-H bonds by [CPh3] can where the physical properties (magnetic, conducting, + 43-45 differ from that by [FeCp2] . For example, etc.), based largely on the electronic and molecular + [Mo2(µ-C8Me8)Cp2] and [FeCp2] give [Mo2(µ-C8Me7- structure of the reduced (paramagnetic) form of the

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 895 percyanocarbon, are of more interest than the chemi- and tricyanovinyl units, and cycloaddition to coordi- cal properties of the oxidation product. Thus, for nated polyenes292 may also occur. 290 example, [FeCp*2][TCNE] is a soluble ferromagnet; + as a synthetic route to [FeCp*2] the reaction of TCNE (11) with [FeCp*2] is unimportant.

One less common percyanocarbon is of interest as a rare example of an anionic one-electron oxidant, namely the paramagnetic radical anion of hexacy- - anotrimethylenecyclopropane, [C3{C(CN)2}3] (14) - (sometimes confusingly formulated as [C6(CN)6] ). n 303 The salt [NBu 4][C3{C(CN)2}3] is prepared by me- TCNE and 7,7,8,8-tetracyanoquinodimethane tathesis of K[C3{C(CN)2}3] which is, in turn, synthe- (TCNQ, 12) are the most widely used of the percy- sized by potassium persulfate oxidation of Na2[C3- anocarbon oxidants. They are commercially available 304 {C(CN)2}3] in water. The potassium salt is soluble solids which dissolve in a range of polar organic n in MeCN and THF/HMPA, and [NBu 4][C3{C(CN)2}3] solvents (e.g., MeCN, THF, 1,2-dichloroethane, ethyl 2- is soluble in MeNO2; the formation of [C3{C(CN)2}3] acetate, etc.); they may be purified by recrystalliza- by reduction could be monitored by the loss of the tion [from 1,2-dichloroethane or chlorobenzene 13-line ESR spectrum or the maroon color (λmax ) (TCNE), or from ethyl acetate or MeCN (TCNQ)] or 673, 598, 320 nm;  ) 19 900, 12 900, 30 500)303 of by vacuum sublimation. Their reactions may be - [C3{C(CN)2}3] . Though only a mild oxidant (E°′ ) monitored by IR spectroscopy (in the cyanide stretch- - ca. 0.0 V; see Table 2) [C3{C(CN)2}3] should prove ing region), UV spectroscopy, or by the ESR spectra useful in the isolation of dicationic oxidation prod- of the radical anions; full details of such spectra and 2- ucts, as salts of [C3{C(CN)2}3] {as might be the case of other physical properties may be found in refs 291 in solid [OsCp2*][C3{C(CN)2}3], formed in the reaction and 292 (TCNE) and 293 (TCNQ). An important n between [OsCp2*] and [NBu 4][C3{C(CN)2}3] (1:2 point to note is the toxicity of TCNE. ratio)305}. TCNE and TCNQ are mild oxidants (Table 2) Some variation in oxidation potential is possible (TCNE, -0.27 and -1.27 V; TCNQ, -0.30 and -0.88 in that related carbomethoxytrimethylenecyclopropane V) which are reduced in two one-electron steps to derivatives have been prepared. For example, stable mono- and dianions (the former have a tendency to yellow crystalline C3{C(CO2Me)2}3 may also act as a dimerize in the solid state). The first process is more weak oxidant (E°′ )-0.52 V).304 generally encountered although [TCNE]-, obviously a weaker oxidant than TCNE, is capable of oxidizing 4. Quinones [CoCp*2]; the reaction of the latter with TCNE gives 294 Both 1,2- and 1,4-quinones, many of which are [CoCp*2]2[TCNE], a salt of the TCNE dianion. commercially available and easily handled, are re- TCNE forms charge-transfer complexes with less duced in two one-electron steps, rendering them readily oxidizable substrates such as ferrocene295 and potentially useful as one-electron oxidants. As yet, main group metal alkyls, the adducts of the latter few examples have been reported of the oxidation of sometimes leading to addition reactions to the CdC organometallics to simple, primary redox products 120 + bond. Note that [FeCp2] is oxidized to [FeCp2] by (i.e., A+ from A). Quinone, semiquinone, or catecho- the less popular, but stronger, cyanocarbon oxidants late coordination may occur, either instead of or after 296 hexacyanobutadiene and 7,7,8,8-tetrafluoroquin- electron transfer. Thus, [Rh(CO)(PPh3)Cp] [(Ep)ox ) 297 odimethane; 2,3-dichloro-5,6-dicyanoquinone (DDQ) 0.0 V] and 1,2-O2C6Cl4 (E°′ )-0.32 V) give [Rh{OC6- 298 306 is also a stronger oxidant than TCNE (Table 2). Cl4OC(O)}(PPh3)Cp]; the reaction is endergonic but Complete electron transfer from TCNE occurs with most likely driven by the irreversible coupling, after more electron-rich transition metal sandwich com- electron transfer, of the carbonyl ligand of [Rh(CO)- + pounds to give, for example, [MCp*2][TCNE] (M ) (PPh3)Cp] and the semiquinone anion radical. 290,294 299 Cr, Mn, Fe, Co and Ni), [Cr(η-arene)2][TCNE], The potential associated with the first reduction 136 and [MoCp(η-C7H7)][TCNE]. Similar products may process is dependent both on pH and on the ring 307,308 be made with TCNQ. substituents. In the presence of HBF4, 1,4- Of the percyanocarbon oxidants, TCNE is probably benzoquinone (E° )-0.51 V308) acts as a simple 20 25 the least innocent; it has an extensive reaction oxidant toward ferrocene, [FeCp*2], and [Fe(η- 291,300 292 26 chemistry with organic, organometallic, and C5H4COMe)2] giving high yields of the correspond- 301 - coordination compounds. Many of the reactions ing ferrocenium salts (of the [BF4] anion). In the with transition metal complexes involve initial one- case of [Fe(η-C5H4COMe)2](E°)0.49 V) the reaction electron transfer followed by bond formation.135 Thus, is endergonic. Under the reaction conditions (non- 2 TCNE may coordinate as an alkene, as in [Fe(η - aqueous solvent, HBF4) the formal potential for the 302 TCNE)(PPh3)(NO)2], or via up to four N-atoms as quinone/hydroquinone couple is not strictly known, in complex 13 (the TCNQ analogue of which is also but it is likely to be ca. 0.30 V, as found for aqueous known).301 Fragmentation, for example, to give cyano solutions at pH ) 0.309

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Scheme 3 radical with A+ then occurs, as also observed for arenediazonium ion oxidations (section II.A.2.c). Thus, [WBr(N2H)(dppe)2], formed in situ from [WBr(N2H2)- + (dppe)2] and base, is oxidized by [IPh2]Br to give + [WBr(N2H)(dppe)2] . Subsequent H-atom abstraction 317 318 from CH2Cl2 or CHBr3, from CHCl3 or CBrCl3, 319 or from CHFBr2, and coupling of the resulting + radicals with [WBr(N2H)(dppe)2] gives [WBr(N2- CHO)(dppe)2], [WBr(N2CCl2)(dppe)2], and [W2Br2(µ- As an illustration of the effect of substituent on the + + redox potential of quinones, [{Ni(µ-Br)(η-allyl)} ]is N2CHN2)(dppe)4] , respectively. Likewise, [IPh2] 2 ′ - ′ oxidized in DMF, according to the stoichiometry of and [Mo(CO)3Tp ] gave [Mo(CX)(CO)2Tp ] (in CH2- Scheme 3, by 1,4-benzoquinone and the methyl and Cl2,X)Cl; in CHBr3,X)Br; in CHI3,X)I) or 2 ′ dimethyl derivatives (E° )-1.03 to -1.15 V) but not [Mo(η -COPh)(CO)2Tp ] by direct coupling of the ′ 211 by the tri- and tetramethyl analogues (E° )-1.25 phenyl radical with [Mo(CO)3Tp ] (in MeCN). to -1.33 V).310 Iodosobenzene, a yellow amorphous powder which slowly decomposes (and explodes when heated above 5. Other Organic Oxidants 210 °C320), has also been used as a one-electron oxidant. With [Fe(CdCHMe)(dppe)Cp]+ it gives a. 4-Pyridinecarboxaldehyde, NC5H4CHO-4. 2+ [Fe2{µ-CdC(Me)C(Me)dC}(dppe)2Cp2] in a reaction 4-Pyridinecarboxaldehyde is a weak one-electron unsuccessful with other oxidants such as [C H ]+,[N- ′ 311 7 7 2 oxidant (E° ) ca. -1.0 V ). It is a commercially Ph]+, and halogens.92 available liquid but should be distilled before use to remove the carboxylic acid, NC H CO H-4, and the 5 4 2 III. Reductants alcohol, NC5H4CH2OH-4, which are formed by slow decomposition. It can then be stored under N2 at ca. Far fewer reagents have been reported for the -10 °C. The primary reduction product dimerizes reduction of organometallic complexes, when com- to give diamagnetic products, presumably the pinacol pared to the large number of oxidants. In part this NC5H4CH(OH)CH(OH)C5H4N or the dianion stems from the smaller number of studies of electron- 2- [NC5H4CH(O)CH(O)C5H4N] . 4-Pyridinecarboxalde- rich organometallics. However, the difficulty of hyde is particularly useful, therefore, for the in situ handling strong reducing agents (especially their generation of paramagnetic organometallics for ESR inherent sensitivity to oxidation by O2) undoubtedly + spectroscopic studies, for example, [Cr2(µ-cot)Cp2] also plays a role. As will be seen below, with only a (E°′ )-1.2 V),38 where other oxidants might give few exceptions (e.g., alkali metals and cobaltocene), paramagnetic reduction products with interfering most reducing agents have received inadequate at- spectra. tention to demonstrate the breadth and limitations 4-Pyridinecarboxaldehyde can also be used in of their use. There is ample need to develop new synthesis. Thus, treatment of a mixture of [Cr- reducing agents and for more systematic studies of 6 (η -arene)2] in toluene and KPF6 in water with the several of those we now discuss. 6 oxidant directly gives a precipitate of [Cr(η -arene)2]- 312 [PF6]. In addition, the reaction of 4-pyridine- A. Inorganic carboxaldehyde with [Cr2(µ-biphenyl)(η-C6H6)2]is highly selective, giving only the monocation [Cr2- 1. Alkali Metals + ′ (µ-biphenyl)(η-C6H6)2] (E° )-0.82 V vs Ag/AgCl); Although the formal potentials of eq 22 (M ) alkali ′ it is insufficiently strong to form the dication (E° ) metal) are not known with great certainty under the -0.37 V) though this species is formed exclusively 219 with O2 (section II.A.2.d). Similar selectivity is M+ + e- h M (22) observed with [Co2(CO)4L2](L)[Cr(η-C6H5PMe2)2]) which shows three sequential one-electron oxidations; conditions usually applied in organometallic synthe- 313 the dication, but not the trication, was isolated. sis, they are very negative (ca. -3.0 V, Table 3), + b. Diphenyliodonium Cation, [IPh2] , and making the alkali metals extremely powerful reduc- Iodosobenzene, PhIO. Diphenyliodonium salts, ing agents. Their potentials are quite solvent de- ′ 314 [IPh2]X (E° ) ca. -0.9 V ), are commercially pendent, as might be expected for a couple in which available as the halides (X ) Cl, Br, or I) but may one redox state (M+) is much more strongly solvated 315 also be made in the laboratory (X ) Cl, [BF4], [PF6], than the other. For example, E°′ of Na+/Na becomes 316 + [AsF6], or [SbF6] ). The cation [IPh2] is usually more positive by ca. 1.0 V in changing the solvent used in electrophilic arylation reactions but can also from DMF to NH3. With potentials in the range -2.4 act as a one-electron oxidant toward transition metal to -3.1 V, alkali metals have proved to be popular complexes, as in eq 21. Although diamagnetic reduc- reductants. Even dilute amalgams have sufficiently negative potentials to be classified as strong reduc- + • + [IPh2] + A ) PhI + Ph + A (21) tants (e.g., 0.1% Na/Hg: -2.2 V). This family of reagents is subject to an important tion products are therefore expected, namely iodo- experimental drawback when applied in a stoichio- benzene and biphenyl (from the coupling of •Ph), in metric manner on small scales: surface oxide forma- the reactions so far described, the phenyl radical has tion, which lowers the reductive efficiency of the invariably abstracted a hydrogen atom from the metal. Considerable care must be taken to ensure solvent. Subsequent coupling of the solvent-based the formation of active metal surfaces for the neces-

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 897 sarily heterogeneous reactions involving these met- reaction mixture and the dinuclear product K[{Co- 329 als. Several approaches of increasing rigor have been (η-C2H4)(PMe3)3}2] when it was not. used to produce reactive surfaces, namely (a) freshly- Sodium often provides reducing equivalents for cut pieces of metal, (b) metallic sands formed by substitution reactions of compounds with metal- heating the metal in a solvent above the melting halogen bonds. Typical of these applications are the point of M, followed by cooling of the mixture while reductions of [MoCl4Cp*] by 3 equiv of Na in the maintaining rapid stirring, and (c) metallic mirrors presence of phosphine ligands, L. Depending on the formed by distillation of metals in vacuo. Mixed stoichiometric ratio of L to the Mo(V) starting mate- alkali metal alloys and metal amalgams have also rial, the Mo(II) complexes isolated are either 18- 330 proved popular. Some Na/K alloys are liquids at electron [MoClL3Cp*] (with 3 equiv of L) or 16- 331 room temperature, commercially available in high electron [MoClL2Cp*] (with 2 equiv of L). purity, and easily handled under inert atmospheres. Anions of main group organometallics have been However, they present extreme fire hazards when isolated after alkali metal reduction of their neutral exposed to moist air because of their high surface starting materials. The complex [Ga2L4](L)1,3,5- area and reactivity with water. They should be used triisopropylphenyl) and a suspension of Li powder in with great caution. diethyl ether containing 12-crown-4 gave [Li(12- A limited number of solvents may be employed 332 crown-4)2][Ga2L4], and the same metal in a slight with alkali metals, THF and glyme being the most stoichiometric excess allowed the isolation of blue [Li- popular [potentials of the M+/M couple in nonaqueous 333 (12-crown-4)2][B(mesityl)3]. The reduction poten- solvents have usually been measured by polaro- tial of B(mesityl)3 is ca. -2.9 V; a second (irreversible) graphic reduction of the M(I) ion because M(0) will reduction wave at ca. -3.5 V334 provides a rationale often react with the solvent]. The reductions are for more recent observations in which the dianion of often performed in the presence of a macrocycle (a 335 BMe2Ph has apparently been observed in situ. crown ether or cryptand) capable of complexing the Sodium mirror reduction of BMe2Ph in THF gave + M counterion. The complex counterion (e.g., [M(mac- - blue solutions typical of [BR3] (λmax 660 nm), but + 321 rocycle)n] ) tends to stabilize and solubilize the potassium mirrors produced solutions of the purple n- electron-transfer salts [M(macrocycle)m]n[A] (A ) dianion (λmax 581 nm), albeit in longer reaction target anion) and is particularly useful when seeking times.335 larger crystalline forms of the product. The most In some cases, ligand-coupled products were ob- commonly used macrocycles322 are 12-crown-4 (15) tained, presumably from one-electron reduction prod- and 2,2,2-cryptand (16) for Li+; 2,2,2-cryptand (and ucts with considerable ligand radical character. 12-crown-5) for Na+; and 18-crown-6 (17) for K+. Examples are the reductive coupling of the alkylidene 336 complex [Cr(dCPh)(CO)2Cp] by Na/K (giving 18) and the coupling of Cp and Cp* ligands in the Na/ Hg reduction of [RhCpCp*]+.337 Even though metal

Stoichiometric amounts of potassium sand were successfully employed to reduce neutral complexes amalgams are considerably weaker reducing agents with low electron affinities to their corresponding than M (Table 3), in the last example the potential monoanions. [V(η6-arene) ](E°′)-3.2 V)323 in THF for Na/Hg in THF is still negative of the reported 2 337 with 1.1 equiv of potassium gave ca. 20% yield of the reduction potential (-2.07 V) of the Rh(III) com- 324 plex, allowing efficient production of the reduction 18-electron anion K[V(η-arene)2], and [Ce(η-C8H8)2] in glyme gave an analytically pure precipitate of product. This strategy had been employed much K[Ce(η-C H ) ].325 In the latter case, 2 equiv of K and earlier in the reductive coupling of the parent rhodoce- 8 8 2 338 higher reaction temperatures (increased solubility of nium ion. It may be appropriate to note that a similar ligand-ligand coupling reaction occurs when K[Ce(η-C8H8)2]?) gave the pure dianion, K2[Ce(η- 5 + C H ) ]. Vanadocene, [VCp ], undergoes a one- the cation [Fe(CO)3(η -pentadienyl)] is reduced by 8 8 2 2 339 electron reduction at -3.30 V in THF.326 In spite of the milder reductant, zinc. 340 the very negative potential, [K(THF)(OEt2)][VCp2] Other amalgams (Mg/Hg, Zn/Hg) and alloys have was isolated from the reduction of [VCp2] by potas- also been employed. Among the latter, commercially sium and used as a starting material to make available Na/Pb should be considered as an alterna- organovanadium compounds in low oxidation states.327 tive to Na/Hg owing to its ease of handling for Neutral 17-electron Co(0) complexes have also been stoichiometric uses.341 - successfully reduced to their 18e analogues; [CoL2] In a variation of the usual synthetic method, others 4 t (L ) η -1,4-Bu 2butadiene) stirred in THF with K in have employed the alkalide [K(18-crown-6)][K] to the presence of 18-crown-6 gave red crystals of reduce [Mn(CO)3Cp], producing in situ the interesting 328 3 2- 342 [K(crown)(THF)2][CoL2]. [Co(η-C2H4)(PMe3)3] with dianion [Mn(CO)3(η -C5H5)] . A ring-slipped di- K in pentane gave the simple monoanion K[Co(η- anion has also been produced by the action of Li sand C2H4)(PMe3)3] when cyclopentene was present in the on a fused-ring aromatic chromium tricarbonyl com-

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6 plex, [Cr(CO)3(η -cyclobutabenzene)] (19), which un- was obtained by distilling THF onto a solid mixture 343 derwent electrochemical reduction at -3.0 V. of LiAlH4 and the Mo(II) complex, letting the reac- tants mix as the solvent melted, and then quenching the mixture in liquid nitrogen.352 Lithium aryls and alkyls (LiR) have estimated E°′ values which vary greatly, depending on the identity ofR(e.g.,R)But, ca. -2.7 V; R ) Cp, ca. -0.8 V).345,347 n-Butyllithium has been used to reduce [ZrR4](R)Me, Bz) to the corresponding dianion, Li2- 353 [ZrR4], which may be polymeric. 1-Norbornyl- + 2. Complex Reducing Agents lithium efficiently reduced [Co(1-norbornyl)4] and allowed the isolation of [Li(THF)4][Co(1-norbornyl)4] A host of complex reductants containing main- in spite of the very negative potential (-2.02 V) of group metals has been widely used in organic chem- the [Co(1-norbornyl) ]0/- couple.354 istry.344 These reagents are mechanistically complex, 4 and their reactions as one-electron reagents are often 3. Hydrazine unpredictable.151a Nevertheless, the familiarity gained in their use has inspired applications in organome- The reducing strength of hydrazine (N2H4)in tallic ET chemistry, some of which are noted here. aqueous media is highly pH-dependent. It has been Included in the category of complex reducing widely employed as a reductant in inorganic chem- agents are lithium alkyls and aryls and simple and istry355 but has been little explored under nonaqueous { complex metal hydrides NaH, LiAlH4, K[BH(CH- conditions. The anhydrous reagent, an air-sensitive } MeEt)3] (“K selectride”), etc. , virtually all of which liquid, has been used to reduce octahedral Mn(II) are commercially available. One-electron potentials + 192 complexes (e.g., [MnBr(CO)2(PMe2Ph)3] and [Mn- for these reducing agents are generally ill-defined, + (CN)(CO)2{P(OPh)3}(dppm)] ) to their corresponding and even when known the irreversibility of the 356 Mn(I) analogues in CH2Cl2. An advantage of this couples lends uncertainty to their significance in ET mild reducing agent (E )-0.4 V in DMSO357)is applications. Among the compounds for which E°′ 1/2 - that the redox byproducts are relatively benign (N2 values have been estimated are [AlH4] (-0.7Vin 345 t 346 and unknown H-containing products, possibly H2O). THF), MgBrBu (-1.5VinEt2O), LiBz (-1.4 V 347 - in THF/HMPA), and [BH4] (ca. -0.6 V in dig- lyme).348 B. Organic - The borohydride ion, [BH4] , is a mild reducing 1. Metal Naphthalenides agent. It is most often used as the sodium salt which may be obtained in high purity by recrystallization These powerful reductants, which are prepared in 349 from diglyme. For example, a suspension of [Co2- situ, have formal potentials of ca. -3.0 V. THF, + (µ-CO)(µ-NO)Cp*2] (E°′ )-0.7 V) was successfully glyme, or other ethers are employed as solvents since reduced to [Co2(µ-CO)(µ-NO)Cp*2] when treated with most other media react with the naphthalene anion, 350 an excess of Na[BH4] in THF/pentane. Other [NAP]-. Small, freshly cut pieces of an alkali metal counterions, however, confer solubility in lower po- dissolve when stirred (using a glass-coated stirring larity solvents. Since Na[BH4] is insoluble in CH2- bar) with equimolar naphthalene to produce deep Cl2, commercially available [NBu4][BH4] was used to green M[NAP]. Medium-sized metal chunks have { }{ } + reduce [Rh2(CO)2 P(OPh3)2 µ-PhNC(Me)NPh 2] (20) been said to give results superior to those obtained ′ { }{ (E° )-0.43 V) to [Rh2(CO)2 P(OPh3)2 µ-PhNC(Me)- with metal dispersions or sands.358 Although the } 351 NPh 2]. complete reaction takes several hours, if the green 359,360 color (λmax ) 463, 735 nm;  ) ca. 2500) does not immediately appear, the solvent is probably wet or improperly degassed; more rigorous conditions should be sought. Concentrations of 0.1-0.5 M are commonly employed, but under very stringent condi- tions Beer’s law was shown to hold for concentrations of Na[NAP] down to less than 10 mM.359 Transfer of the M[NAP] solution to that containing the sub- strate is accomplished with a volumetric syringe. It

LiAlH4 can act as a one-electron reductant toward must be emphasized that all reagents and glassware organometallic complexes.352 It gives different reduc- should be scrupulously dry and oxygen-free to obtain + tion products with [Mo(CO)3LCp*] (L ) phosphine; quantitative results. Since the dianion of naphtha- Ep )-2.3 V for L ) PMe3) under different experi- lene is probably not formed in any significant quan- 352c mental conditions. When a 4:1 ratio of LiAlH4: tity when NAP is in contact with excess alkali Mo(II) complex was reacted at -20 °C for 1 h, the metal,361 a constant titre of M[NAP] may be aided metal hydrides [MoH(CO)2LCp*] and [MoH(CO)L2- by using a small excess of the metal in preparation Cp*] were isolated in good yield. Reduction of the of the reagent stock solution. Stock solutions may Mo(II) cation by a substoichiometric amount of Li- be used for several days if adequately protected, but AlH4 at -80 °C, however, gave ESR spectroscopic the sensitivity of these reagents to air and moisture evidence for a radical intermediate. The spectrum precludes a general statement about their shelf lives.

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The concentration of M[NAP] in the stock solution ing agents such as the kryptofix series sometimes is measured by titration. Quenching a solution with aids crystal formation and isolation. Thus, the - excess water, eq 23, followed by titration with stan- monoanion [Mn(CO)3(η-indenyl)] was isolated as a 5 thermally unstable salt when [Mn(CO)3(η -indenyl)] M[NAP] + H O f M+ + OH- + was reduced by 1 equiv of K[NAP] in the presence of 2 18-crown-6.375 C H and C H (23) 10 8 10 10 The reducing strength of M[NAP] is well exempli- fied by the results of Cooper et al.372 regarding the dardized HCl to a phenolphthalein endpoint gives the reduction of [W(CO)5(NMe3)]. The desired dianion total base concentration, [B], of the reagent solution. 2- [W(CO)5] was obtained only with K[NAP] or Li- The concentration of B is not, however, equal to the [NAP]. The reaction with other strong reducing effective M[NAP] concentration unless no hydrolytic agents such as Na/Hg, K, sodium benzophenone decomposition of M[NAP] occurred at any time in the anion, and sodium anthracenide stopped at the preparation of the stock solution. Since this condition 2- dinuclear dianion, [W2(CO)10] . normally cannot be assured, many workers have On the other hand, the strength of M[NAP] can relied on a double (or “blank”) titration method to sometimes lead to excessive reduction. The reaction measure the effective M[NAP] concentration. To this of the cluster [Ni3(µ3-CO)2Cp3](E°′)-1.4 V) with end, quenching the stock solution with an alkyl K[NAP] gave only decomposition, whereas the milder halide, which destroys M[NAP] without liberation of 362 reductant K[acenaphthalene] gave the desired base, provides the blank when titrated with HCl. monoanion [Ni (µ -CO) Cp ]-.370 A method for direct titration of Li[NAP] against 1,1- 3 3 2 3 Metal naphthalenides are also used to provide diphenylethylene has been described.363 reducing equivalents in one-pot reactions which The naphthalenide anion is ESR-active (〈g〉 ) ultimately yield substituted metal complexes in lower 2.0028);364 reaction byproducts such as naphthalene oxidation states, as in the reduction of [TiCl Cp] (4 and dihydronaphthalene are easily removed by pass- 3 equiv of Na[NAP] under CO) to give [Ti(CO) Cp]- 376a ing the reaction mixture through silica gel. 4 and the production of metallocarboranes from start- Although naphthalenides of Li, Na, and K seem to ing ligands and metal halides.376b be used interchangeably, in principle the counterca- tion should affect their reactivities. ESR spectro- The innocence of the metal naphthalenides is not scopic studies establish that ion pairing between assured when the transition metal is capable of - + + + forming metal-arene complexes. Initial reduction [C10H8] and M increases in the order Li < Na < K+.365 The formal potentials of the ion-paired re- products may be labile in the presence of naphtha- agents should increase in the same way, i.e., Li[NAP] lene, the byproduct of the redox reaction; napthalene coordination, in a η4-orη6-mode, is found in the < Na[NAP] < K[NAP], the lithium reagent therefore 6 + products of the reductions of [Mn(CO)3(η -benzene)] being the strongest reductant of the three. Because 4 - 374 and vanadocene, namely [Mn(CO)3(η -C10H8)] and M[NAP] are only weakly dissociated in ether sol- 6 - 377 6 vents,366 the effect on the formal potentials is likely [VCp(η -C10H8)] , respectively. When [Mn(CO)3(η - benzene)]+ was reduced with potassium anthra- to be significant although we are not aware of 4 - systematic studies probing this point. Interestingly, cenide, substitution of [Mn(CO)3(η -benzene)] by anthracene was much slower, enabling studies of the the sequence of ion-pairing strength for M[NAP] is 4 - addition reactions of [Mn(CO)3(η -benzene)] with opposite to that found for the benzophenone anion 378 radical (section III.B.2). Smaller cations are more diphenylketene. effectively complexed by the oxygens of the solvent - 2. Benzophenone Anion Radical in ether solutions of [C10H8] , accounting for the lower ion pairing observed for M[NAP] with smaller cat- Given the strength (E°′ )-2.2 V for the couple Ph2- - ions. The solvation effect is insufficient to overcome CO/[Ph2CO] ) and ease of preparation of this reagent, - 379 the major source of ion pairing in the benzophenone the benzophenone radical anion, [Ph2CO] (BPA) anion, which is between the metal and the carbonyl is an under-used reducing agent in organometallic oxygen, with stronger pairing being observed with chemistry. It is prepared by the alkali metal (M) smaller cations.365 reduction of benzophenone (eq 24) in aprotic solvents M[NAP] has typically been employed to generate such as THF, glyme, or liquid ammonia. BPA cannot the monoanion of a substrate for the purposes of be used in halocarbon solvents because it reacts spectroscopic or X-ray characterization, for example, rapidly with alkyl halides.380 As discussed below, a of metal pyrazolylborates,367 tungsten and titanium possible further complication in the use of BPA is the alkyls,368,369 and metal clusters.370 Dianions have also involvement of the benzophenone dianion (eq 25). 2- been suitable targets, including [W(C2Ph2)3] from 371 2- [W(CO)(C2Ph2)3], [M(CO)5] from [M(CO)5(NMe3)] h - 372 2- Ph2CO + M [Ph2CO] (24) (M ) Cr, Mo, W), [Mn(CO)2Cp] from [Mn(CO)2- (pyr)Cp],373 and [Cr(CO) ( 4-arene)]2- from [Cr(CO) - 3 η 3 - h 2- (η6-arene)].343 In another reaction, using 2 equiv of [Ph2CO] + M [Ph2CO] (25) 4 - M[NAP], the monoanion [Mn(CO)3(η -benzene] was 6 produced from the monocation [Mn(CO)3(η -ben- To date, BPA has been used mostly to initiate the + 374 zene)] . In related studies, reduction of [Mn(CO)3- electron-transfer-catalyzed substitution reactions of (η5-indenyl)] with 2 equiv of K[NAP] in THF gave the organometallic cluster compounds. Representative 3 2- dianion [Mn(CO)3(η -indenyl)] , characterized in situ examples include the substitution of phosphines for 375 381 382 by IR spectroscopy. The use of metal encapsulat- carbonyls in [Ru3(CO)12] and [Co3(CO)9(CPh)].

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+ BPA is prepared in situ in a dissolving alkali metal another unit of [Fe(CO)3Cp] must allow the chain reaction;383 the dark blue monoanion was, interest- reaction to proceed despite the highly endergonic ingly, the first recognized radical anion.384 It displays electron-transfer reaction (eq 26). a multiline ESR spectrum in fluid media385 and must be handled in the rigorous absence of oxygen and 4. C2(NMe2)4 water. Contact with the former regenerates neutral Tetrakis(dimethylamino)ethylene, C2(NMe2)4, re- Ph2CO (BPA has been recommended for scavenging duces TCNE397 and metal carbonyls398 to give salts 386 2+ the last traces of O2 from N2 ); with the latter, the of the dication [C2(NMe2)4] , for example, [C2(NMe2)4]- 387 coupling product pinacol is formed. BPA is rou- [TCNE]2 and [C2(NMe2)4][Co(CO)4]2. Though air- tinely employed for the rigorous drying of ether sensitive,398 the liquid alkene is commercially avail- solvents. Optical spectra of BPA have been reported able and merits further attention as a two-electron 3 388 (e.g., λmax ) 630 nm,  ) ca.6 10 , in MeCN). × reductant soluble in a wide range of organic solvents Of interest in the further development of BPA as (e.g., toluene, diethyl ether, MeCN, and THF). a stoichiometric reagent is the question of the pos- 2- sible involvement of the dianion, [Ph2CO] , in solu- C. Organometallic tions nominally of the monoanion. Electrochemical measurements show that the dianion is formed at a 1. Metal Sandwich Complexes potential accessible to the alkali metals. The most a. Cobaltocene. Cobaltocene, [CoCp ], is to ′ 2 reliable studies suggest an E° value of ca. -2.9 V reductions what the ferrocenium ion is to oxida- - 2- 389 for the couple [Ph2CO] /[Ph2CO] , positive of the tions: a readily-available metal sandwich complex + potentials of the couples M /M (M ) Li, Na, K; Table of moderate redox strength which has become the 3). Evidence that the dianion is present under the most popular choice for inducing one-electron changes. synthetic conditions for the production of BPA comes This black compound, readily prepared and purified from the color of the solution. With excess metal the by vacuum sublimation,399 has a number of attractive solution becomes purple, which is the known color properties, including solubility in a range of nonpolar of the dianion.390 Although the addition of the second 391 and polar solvents. It is air-sensitive in both the solid electron is said to be “slow”, the buildup of state and in solution (one study took advantage of 2- [Ph2CO] is likely limited by its propensity to this quality by employing [CoCp2] to scavenge ad- scavenge trace water from the medium. Further ventitious oxygen, thereby preventing electron- work on quantifying this reagent would be welcome. transfer-catalyzed substitution of carbonyls in [Co- It is interesting to note that the strength of ion 400 (CO)4]). + + + + pairing between M and BPA is K < Na < Li ,a Cobaltocene should be freshly sublimed (>40 °C, sequence opposite to that of aromatic hydrocarbon 10-4 mmHg) before use, but it can be stored for ca.1 anions such as the naphthalenide ion in ether sol- week at -10 °C under N without deterioration in 392 2 vents (section III.B.1). One may therefore expect its elemental (C and H) analysis. It has a rich the potassium benzophenone anion to be the stron- chemistry401 with its facile oxidation (E° ) ca. -1.3 gest reducing agent in the three-metal series. + V) to the corresponding cobaltocenium ion, [CoCp2] being relevant here (eq 28); it is almost always an 3. Triethylamine innocent reagent.402 Triethylamine is a weak reductant (irreversible 393 + - peak potential, Ep, )+0.47 V in MeCN ), whose [CoCp2] h [CoCp2] + e (28) oxidation byproducts have not been established. In spite of these negative factors, it has seen a few Cobaltocene has been used as a one-electron reduc- applications in organometallic redox chemistry. For ing agent in solvents such as pentane, toluene, THF, example, in toluene it reduces [MoI2(NO)Tp′](E°′) 403 394 CH2Cl2, acetone, and MeCN. The redox byproduct, -0.34 V) to the corresponding monoanion and it [CoCp ]+, generally has poor solubility in relatively appears to initiate ET-catalyzed substitution reac- 2 5 + 395 nonpolar solvents and is often removed from the tions of [Fe(CO)3(η -L)] (L ) Cp or indenyl) and + + 396 reaction mixture by simple filtration. [CoCp2] does [Mn(CO)2(NO)Cp′] . The substitution of a single + not usually interfere seriously in visible (λmax ) 409 CO in [Fe(CO)3Cp] by phosphine in the presence of nm,  ) 200) or 1H NMR (δ 6.25) spectroscopic NEt3 is very efficient, considering that the electro- + studies. It may, however, present an electrochemical chemical reduction potential of [Fe(CO)3Cp] (Epc ) 395 interference in the product owing to the two succes- -0.60 V) is ca. 1.0 V negative of the approximate 404 ′ sive one-electron reductions at ca. -1.3 and -2.2 V. E° of NEt3. Thus, if NEt3 does act as an electron- Being diamagnetic, [CoCp ]+ is ESR-silent. Unre- transfer catalyst (according to eqs 26 and 27) the 2 acted [CoCp2] is also ESR-silent down to tempera- tures considerably below that of liquid nitrogen.405 + + [Fe(CO)3Cp] + NEt3 h [Fe(CO)3Cp] + [NEt3] Cobaltocene has been employed to reduce mono- cations and dications to neutral complexes by using (26) one or two reducing equivalents, respectively. Evapo- [Fe(CO) Cp] + PR f [Fe(CO) (PR )Cp] + CO ration of the reaction mixture followed by extraction 3 3 2 3 with a nonpolar solvent will normally result in facile (27) separation of the desired product from that of the cationic starting material and cobaltocenium ion, - rapid substitution of 19e [Fe(CO)3Cp] by PR3 (eq 27) especially if a slightly substoichiometric amount of and the subsequent oxidation of [Fe(CO)2(PR3)] by [CoCp2] is employed. This approach has succeeded

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 901 in the preparation of dirhenium complexes,406,407 mation of two different products is rationalized by 4 408 z [M(η -polymethylthiophene)Cp*] (M ) Rh, Ir), the reported potentials of [Fe4S4(NO)4] ; two revers- 4 409 410 [Ru(η -Me4thiophene)(η-C6Me6)], and [Ir4S4Cp*4]. ible one-electron reductions were observed at ca. -0.3 4 - In the last two cases, the dications [Ru(η -Me4- and -1.1 V for the couples [Fe4S4(NO)4]/[Fe4S4(NO)4] 2+ 2+ - 2- thiophene)(η-C6Me6)] and [Ir4S4Cp*4] underwent and [Fe4S4(NO)4] /[Fe4S4(NO)4] , respectively. The two separate one-electron reductions (E°′ )-1.0 and potential of [CoCp2] is sufficiently negative to reduce -1.2 V for the Ru complex409 and -0.8 and -1.1 V the cluster to its dianion, but the comproportionation for the Ir cluster410]); both second reduction potentials reaction of eq 31 would stop the reduction at the one- were sufficiently positive of that of [CoCp2](ca. -1.3 electron stage if one reducing equivalent is used. V) to assure quantitative formation of the two- electron reduced products. In a related result that 2- - [Fe4S4(NO)4] + [Fe4S4(NO)4] h 2[Fe4S4(NO)4] is somewhat surprising, the reduction of the trigonal 2+ bipyramidal cluster [Ir3S2Cp*3] proceeded to comple- (31) tion with 2 equiv of cobaltocene; the complex [Ir3S2- Cp*3] was isolated in 96% yield even though the The dianion, which is generated when the neutral formal potentials of the dication, -1.1 and -1.4 V, complex is treated with excess reductant (as in the do not suggest that an equilibrium mixture would Na/Hg reduction), may, however, be subject to further favor the neutral cluster.410 reactions not apparent on the CV time scale. This A powerful application of cobaltocene is in the is most likely why, under some conditions, the + isolation of anions with [CoCp2] as the counterca- secondary product was formed in the sodium reduc- tion. The most common strategy is to mix solutions tion.424 of [CoCp2] and the neutral substrate, A, in relatively When inherently unstable anions are the redox nonpolar media and collect the resulting slightly product, the use of [CoCp2] as reductant does not soluble ET salt [A][CoCp2] (eq 29). This procedure alter the course of their decomposition, as apparent from several studies involving the loss of halide from the initial reduction product (see eqs 32425 and 33426): [CoCp2] + A h [A][CoCp2] (29) f often results in analytically pure samples even [Ru2(µ-Cl)2Cl2Cp*2] + [CoCp2] 191,411-416 though the anions might be quite air-sensitive. [Ru2(µ-Cl)3Cp*2] + [CoCp2]Cl (32) Among the anions isolated in this way are - 411,417 [MoCl2(NO)(η-C5R5)] (R ) H, Me), [Fe4(CO)11- [ReBr2(CO)2Cp*] + 2[CoCp2] f - 412 - 418 (µ-PR)2] , and an iron cluster anion [Fe4S4(NO)4] - - + - 413 - 414 [ReBr(CO)2Cp*] + Br + 2[CoCp2] (33) (from THF), [VCl3Cp′] , [TiCl3Cp] , [Co- - 415 191 (S2N2)Cp] , and a rhenium dinuclear anion (from - 419 A similar strategy has been employed to form metal CH2Cl2), [Re2Me4O2] (from pentane), Mo(V) poly- pyrazolylborates,416 [TiCl Cp ]-,420 and [Co(CO) ]- 421 carbonyls such as [Mn2(CO)10], [Fe(CO)5], and [V(CO)4- 2 2 4 420,427 - (dppe)] from halide-containing precursors. (from benzene or toluene), and [V(CO)6] (from di- ethyl ether).421 In several cases involving cobaltoce- Recognizing that these halide-expulsion reactions nium salts of metal carbonylates, strong ion pairing are irreversible, it is relevant to mention an analo- in the ET salts gives rise to unusual chemical and gous study of the reduction of the metal alkylidene { } + physical properties.421,422 complex [Fe dCMe(OMe) (CO)2Cp*] (irreversible 428 Discussion of two anions of the above group will reduction, Ep )-1.8VinCH2Cl2). Although the serve to demonstrate the effectiveness of cobaltocene reduction potential disfavors formation of the one- in isolating salts of unstable anions. The existence electron reduction product, treatment of the cation - with a stoichiometric amount of [CoCp2] results in a of the Ti(III) anions [TiX2Cp2] (X ) halide) had been the subject of debate, based on contrary views of quantitative yield of [Fe(COMe)(CO)2Cp*]; the same electrochemical results which were ambiguous about product is formed electrochemically. This is a clear halide dissociation from the primary monoanion (eq example of irreversibility driving the electron-trans- 30).423 The isolation of the Ti(III) dichloro and fer process despite, in this case, an unfavorable difference of ca. 400 mV between E°′ of cobaltocene 428 - - - and Ep of the iron complex. A similar conclusion [TiX2Cp2] + e h [TiX2Cp2] f [TiXCp2] + X was drawn with regard to the reaction of [CoCp2] 422 (30) with [Mn2(CO)10], which gives [CoCp2][Mn(CO)5]. In a few cases, the stronger reductant [CoCp*2] has dibromo derivatives [CoCp2][TiX2Cp2] from toluene been used in place of cobaltocene. The permethylated proved the existence of the complex anions and derivative is more air-sensitive than [CoCp2], befit- allowed systematic studies of their solvolysis in THF. ting its more negative (by ca. 600 mV) potential. The dichloride proved to be stable in this medium, Nevertheless, it is thermally stable, easily purified whereas the dibromide underwent loss of Br-.420 by vacuum sublimation,429 and shares most of the When THF solutions of the metal nitrosyl cluster advantageous physical and spectroscopic properties [Fe4(µ3-S)4(NO)4] were put in contact with sodium enjoyed by its unsubstituted analogue. Solutions of amalgam for a few minutes, the secondary product [CoCp*2] in ether solvents may be prepared for use - + [Fe4(µ3-S)3(NO)7] was formed and isolated as the as needed by reduction of [CoCp*2] with alkali metal + 418 [AsPh4] salt. When 1 equiv of cobaltocene (or mirrors (and presumably other reducing agents), benzophenone anion)424 was used as reductant, the followed by filtration of the solution directly into a 430 salt [CoCp2][Fe4(µ3-S)4(NO)4] was isolated. The for- solution of the reducible substrate. [CoCp*2] has

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5 5′ 5 5′ proved to be effective when [CoCp2] was too weak η ,η -C10H8)] or [W(CO)3Ru(CO)2(PMe3)(µ-η ,η -C10H8)] to bring about the desired electron-transfer reac- with [FeCp*(η-C6Me6)] gives [W(CO)3Ru(CO)(PMe3)2- 427,430,431 5 5′ tion. When applied to fullerene reductions, (µ-η ,η -C10H8)]; the more negative E°′ value of - 2- 432 [CoCp2] gives [C60] and [CoCp*2] gives [C60] . [FeCp*(η-C6Me6)] (-2.30 V) triggers electron-transfer- 431 In one of these studies, precipitation of the salt catalyzed substitution of CO by PMe3. Although [CoCp*2][A] was the apparent driving force for the [FeCp*(η-C6Me6)] has been used less often than reaction since the potential for the one-electron [FeCp(η-C6Me6)] as a reductant, its very negative 5 439 reduction of [W(CO)2(NO)(η -C5HPh4) (A, -2.2 V) is potential and its facile synthesis and purification substantially negative of that of [CoCp*2]. make it an interesting target for further develop- b. [Fe(η-C5R5)(η-C6Me6)]. The 19-electron com- ment. 6 plex [FeCp(η-C6Me6)] shares many chemical and c. Others. The sandwich complexes [Cr(η - physical characteristics with other neutral metal arene)2] form a class of potentially useful one-electron sandwich complex reductants such as [CoCp*2], with reducing agents (eq 35). The formal potential of the which it compares favorably in reducing strength 6 + - 6 (E°′ )-2.09 V for eq 34). Thanks to the efforts of [Cr(η -arene)2] + e h [Cr(η -arene)2] (35)

+ - parent complex (arene benzene, E°′ ca. 1.15, [FeCp(η-C Me )] + e h [FeCp(η-C Me )] (34) ) ) - 6 6 6 6 Table 3) is only slightly positive of that of cobaltocene. A number of ring-substituted derivatives can be 440 Astruc and co-workers, this reagent has already prepared. The potentials of [Cr(η-mesitylene)2](E°′ 441 received attention as a reductive trigger for catalytic )-1.3 V ) and [Cr(η-(dimethylamino)benzene)2] and stoichiometric reactions. The reader is referred (E°′ )-1.5 V442) show that the reductive power of to the review literature for details.433 the reagent is increased when electron-releasing [FeCp(η-C6Me6)] is prepared in high yield by the groups are present. The reaction byproduct, [Cr- + action of sodium amalgam on the corresponding 18- (arene)2] , is ESR-active in both fluid and frozen + 443 + electron cation [FeCp(η-C6Me6)] in glyme under media, in contrast to [CoCp2] and [CoCp2]. [Cr- 434 argon. Although air-sensitive, the deep green (η-benzene)2], which is commercially available, has compound is thermally stable and sublimes at 70 °C been used to prepare metal carbonylates which at 2 10-4 mmHg. It is paramagnetic and displays display significant ion-pairing with [Cr(η-benzene) ]+. × 2 proton NMR resonances at +35.0 ppm (Cp) and For example, [Cr(η-benzene)2] and [Co2(CO)8]in -2.15 ppm (CH3) in toluene-d8. ESR-silent in fluid benzene give analytically pure [Cr(η-benzene)2][Co- 421 media, it displays a spectrum with a rhombic g-tensor (CO)4]. (g ) 2.063, 2.000, 1.864) at 77 K. It is easily weighed The binuclear complexes [Cp*Cr(µ-OPR2)3CoCp] and handled in inert atmospheres, facilitating its (21,R)OMe or OEt) are strong reducing agents (E°′ stoichiometric use. It is soluble in pentane and other )-2.0 V), with attractive stability and solubility.444 nonpolar solvents and is most often used in ethers. Prepared from the corresponding monocations445 by It decomposes in some polar organic solvents.435 The Na/Hg reduction in ether solvents, the green neutral complex may be stored for lengthy periods under an complexes (λmax ) 643 nm) are soluble in a range of inert atmosphere and re-sublimed as needed. solvents including hydrocarbons. Moreover, in spite [FeCp(η-C6Me6)] has been used to reduce metal of their negative potentials they are oxidized only sandwich complexes of Ni and Co for which [CoCp2] slowly in air, even in solution. Koelle and co-workers 402 was inadequate. For example, [Co(η-C4Ph4)- have used these reagents to reduce a number of + + (η-benzene)] shows a reversible one-electron reduc- sandwich complexes, including [CoCp2] and [FeCp- + 444 tion wave at -1.7 V followed by a second, irreversible, (η-C6H6)] . Further developments on the use and reduction at -2.2 V. Treatment of this cation with properties of these Cr(II) reagents are awaited. [FeCp(η-C6Me6)] at low temperatures allowed the isolation of [Co(η-C4Ph4)(η-benzene)]; with a stronger reductant (0.8% Na/Hg) the same cation gave [Co- 3 (η -C4Ph4H)(η-benzene)]. The latter is apparently formed when the transient anion [Co(η-C4Ph4)- (η-benzene)]- adds a proton from the solvent (THF) or trace water. [FeCp(η-C6Me6)] has also been used to produce mono-, di-, or trianions of C60 by appropri- ate control of the reaction stoichiometry.436 [Fe(η-C5R5)(η-C6Me6)] (R ) H or Me) have been employed in ET-catalyzed substitution reactions of fulvalene-bridged dinuclear complexes.437 For ex- ample, the metal-metal-bonded complex [Mo2(CO)6- 5 5′ (µ-η ,η -C10H8)] (Ep ) ca. -1.3 V, irreversible reduc- Ferrocenium ions are common oxidants (section 438 tion) reacts with 0.1-0.2 equiv of [FeCp(η-C6Me6)] II.A.1.a.i), but the neutral ferrocenes [Fe(η-C5R5)2](R in THF in the presence of PR3 in ca. 5 min to give a ) H, Me) may be useful as weak reducing agents. complex in which two CO groups have been substi- For example, the organoborane triple-decker sand- 437 tuted by phosphines. The same reagent with wich complex [Ru2(η-arene)2(Et2C2B3H3)] (E°′ )-0.34 5 5′ + [W(CO)3Ru(CO)2(µ-η ,η -C10H8)] and PMe3 gave the V) was first oxidized with [FeCp2] (E°′ ) 0.0 V); the 5 5′ + zwitterion [W(CO)3Ru(CO)2(PMe3)(µ-η ,η -C10H8)]. monocationic complex [Ru2(η-arene)2(Et2C2B3H3)] However, treatment of either [W(CO)3Ru(CO)2(µ- was spectroscopically characterized and then reduced

+ + Chemical Redox Agents for Organometallic Chemistry Chemical Reviews, 1996, Vol. 96, No. 2 903 back to the neutral starting complex by [FeCp*2](E°′ advantage, several salts [C][Fp] (including C ) Li, )-0.59 V). The byproduct of the decamethylfer- Na, K, and NBu4) have been isolated which might + 456,457 rocene reaction, [FeCp*2] , interferes (λmax ) 780 nm) facilitate the stoichiometric use of this reagent. in the optical spectrum of the final solution.446 Another possible advantage is that both the reagent Ferrocene was used to prepare [Rh2(CO)(o-O2C6Cl4)- and its byproduct are diamagnetic, eliminating ESR (bipy)(µ-RNNNR)2](R)p-tolyl) from the correspond- spectroscopic interference from consideration. How- ing monocation (E°′ )+0.06 V) when cobaltocene ever, IR spectroscopic studies should anticipate the 447 failed to give a pure product. carbonyl absorptions of both Fp2 (2000, 1950, 1800, -1 458 - -1 + and 1780 cm ) and Fp (1865, 1788 cm , [NBu4] salt, in THF457). 2. Metal Carbonylates b. Others. Other metal carbonyl anions, e.g., - [Mn(CO)5-xLx] ,(L)phosphine or phosphite) might - a. [Fe(CO)2Cp] . The dimer [Fe2(CO)2(µ-CO)2- offer similar properties as reductants. The formal - Cp2] (Fp2) and the monomeric anion [Fe(CO)2Cp] potentials of the anions decrease from ca. -0.5V(x - 459 (Fp ) are related by a chemically reversible but )0) to -1.8V[x)2, L ) P(alkyl)3]. The alkali electrochemically irreversible couple (eq 36): metal salts M[Mn(CO)5-xLx], readily prepared by the reaction of [MnBr(CO)5-xL] with M/Hg, vary in their - - solubilities in organic solvents;460 [PPN][Mn(CO) ]is [Fe2(CO)2(µ-CO)2Cp2] + 2e f 2[Fe(CO)2Cp] 5 soluble in dichloromethane.461 (36)

The anion is a strong one-electron reductant, but IV. Conclusions it has not been widely used. An anodic wave was reported at E )-1.74 V in THF at a mercury 1/2 While studies of the redox activity of organome- electrode,448 but since the product of the oxidation tallic complexes have blossomed in the last two was [Hg{Fe(CO) Cp} ] this potential is only an ap- 2 2 decades, much of the work involving chemical (rather proximation of that for the reaction in eq 36. Another than electrochemical) activation has relied on the use estimate of the formal potential associated with eq of relatively few, tried and trusted, reagents such as 36 (E°′ )-1.84 V) was obtained by means of elegant the silver(I), nitrosonium, and ferrocenium cations redox equilibration studies.449 A value of -1.8Vis as oxidants and sodium amalgam, cobaltocene, and used in Table 3. The diiron complex (Fp )ineq36 2 metal naphthalenides as reductants. undergoes an irreversible reduction at ca. -2.0 V.448,450 One aim of this review is to show how such popular redox reagents can be used most effectively. How- Solutions of Fp- are prepared by reacting Fp with 2 ever, it is clear that a wider armory of oxidants and strong reducing agents such as M/Hg or Na/K in reductants will become necessary as the applications ethers451 (Fp- reacts rapidly with alkyl halides and of “organometallic electrochemistry”, for example, in other electrophiles452,453). Its solutions have been directed organic synthesis and catalysis, become more used in situ to reduce 18-electron cationic Mo(II) sophisticated. We hope, therefore, that this review alkyne complexes, thereby initiating ligand coupling will also encourage the use of redox reagents less well reactions. Typically, equimolar Fp- (as an amber- known to the inorganic chemistry community. Such red THF solution of the sodium salt) was stirred with reagents include those which are more widely used [Mo(NCMe)(η-alkyne) Cp]+ (E°′ )-1.96 V, alkyne ) 2 by organic chemists, for example, the arylaminium MeC Me454) to produce dinuclear metallacyclonon- 2 cations and the thianthrene radical cation, and others atetraene complexes such as 22.455 The redox byprod- which are far from exotic but virtually ignored. Thus, uct, Fp , was separated from the reaction mixture by 2 simple changes to well-tried species can bring great column chromatography. synthetic advantages. For example, if the ferroce- nium ion appears to be the most attractive reagent from the viewpoint of reaction workup, but happens to be a slightly weaker oxidant than required, the acetylferrocenium ion may prove to be ideal. Equally, + [CPh2(C6H4OMe-4)] is a simple derivative of the trityl cation but is considerably more stable and easy to handle. In order to aid systematic choice we also have provided a “ranking” of reagent strengths according to formal potentials. These potentials were gathered from many sources, and the original data were reported under quite varied experimental conditions. One reason for exploring Fp- more fully for ET Some errors in our conversion to the ferrocene scale reactions is that it is a strong reducing agent which may therefore be anticipated. We emphasize that yields a neutral byproduct (Fp2). This may have such difficulties can be avoided in the future if advantages in workup procedures and in the choice workers always report the potential of the ferrocene/ of counterion available to the target reduction prod- ferrocenium couple (or of another suitable internal uct (if the latter is an anion). Concerning the latter standard) under their experimental conditions.

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V. Appendix: Syntheses of Ferrocenium and purchased HPF6 [25 g (15.1 mL of a 65% solution in Diazonium Salts water), 111.3 mmol]. To this solution was added dropwise over1hasolution of NaNO2 (3.0 g, 56.6 A. Ferrocenium Salts mmol) in distilled water (20 mL) cooled to ca.10°C. After the mixture was stirred for a further 30 min, 1. Method i. Ferrocenium Tetrafluoroborate, [FeCp2][BF4] the off-white precipitate was removed by filtration, washed sequentially with water, ethanol, and diethyl To a stirred solution of p-benzoquinone (0.70 g, 6.5 ether (ca. 50 mL of each), and dried in air to give 8.6 mmol) in diethyl ether (20 mL) at room temperature g of the crude product. The solid was recrystallized was added HBF ‚OEt (54% in diethyl ether; 3.7 mL, 4 2 from acetone/ethanol to give white crystals, yield 8.0 26.9 mmol). On adding a solution of [FeCp ] (2.39 g, 2 g (57%). Anal. Calcd for C H N PF : C, 26.9; H, 1.5; 12.85 mmol) in diethyl ether (40 mL), a blue solid 6 4 2 7 N, 10.5. Found: C, 26.8; H, 1.4; N, 10.3. (The was immediately precipitated. After the mixture was elemental analysis was unchanged on storing the stirred for 10 min and then allowed to stand for a solid at 10 °C, under N , for 3 months.) further 10 min, the blue solid was removed by - 2 filtration, washed with diethyl ether (2 10 mL), and dried in air. The crude product (yield× 3.16 g) was VI. List of Abbreviations purified by dissolution in acetone (100 mL), filtration, bipy 2,2′-bipyridyl and precipitation by addition of diethyl ether (150 BPA benzophenone radical anion mL) to give a fine blue powder, yield 2.49 g (71%). Bz benzyl Anal. Calcd for C10H10FeBF4: C, 44.0; H, 3.7. chpt cycloheptatriene Found: C, 44.4; H, 3.7. (The elemental analysis was cot cyclooctatetraene 5 unchanged on storing the solid in air for 3 months.) Cp η -C5H5 5 Cp′ η -C5H4Me 5 2. Method ii. Ferrocenium Hexafluorophosphate, Cp* η -C5Me5 [FeCp ][PF ] CPE controlled potential electrolysis 2 6 CV cyclic voltammetry To a stirred solution of [FeCp2] (5.0 g, 26.9 mmol) DDQ 2,3-dichloro-5,6-dicyanoquinone - in a mixture of water (100 mL) and acetone (40 mL) detc [S2CNEt2] at room temperature was added anhydrous FeCl3 (5.9 DMF dimethylformamide g, 36.0 mmol). After 15 min the deep blue solution dmpe 1,2-bis(dimethylphosphino)ethane was filtered through Celite and then [NH ][PF ] (5.8 DMSO dimethyl sulfoxide 4 6 dppm bis(diphenylphosphino)methane g, 35.6 mmol) was added. After 5 min, ethanol (50 dppe 1,2-bis(diphenylphosphino)ethane mL) was added to aid precipitation of the blue solid HMPA hexamethylphosphoramide which was then removed by filtration (5.6 g). A Mes mesityl further 2.4 g of product was obtained from the mother NAP naphthalene liquors by reduction of the volume of the solvent to np3 tris(2-(diphenylphosphino)ethyl)amine ca. 100 mL and then addition of ethanol (50 mL). The PC phthalocyanin two fractions were combined, dissolved in acetone phen 1,10-phenanthroline (100 mL), treated with ethanol (200 mL), and cooled pp3 tris(2-(diphenylphosphino)ethyl)phosphine to -10°Cfor1htogive dark blue crystals, yield 5.1 pyr pyridine g (57%). Anal. Calcd for C H FePF : C, 36.3; H, TCNE tetracyanoethylene 10 10 6 TCNQ 7,7,8,8-tetracyanoquinodimethane 3.0. Found: C, 36.3; H, 3.1. (The elemental analysis - Tf triflate, [CF3SO3] was unchanged on storing the solid in air for 3 2- tfd [S2C2(CF3)2] months.) THF tetrahydrofuran - Tp [BH(pyrazolyl)3] - 3. Method iii. Acetylferrocenium Tetrafluoroborate, Tp′ [BH(3,5-dimethylpyrazolyl)3] [Fe(η-C5H4COMe)Cp][BF4] H2TPP tetraphenylporphyrin

To a stirred solution of [Fe(η-C5H4COMe)Cp] (0.39 g, 1.71 mmol) in diethyl ether (150 mL) at room VII. Acknowledgments temperature was added solid AgBF (0.32 g, 1.61 4 We thank our many colleagues working in the field mmol). After 0.5 h the blue-green reaction mixture of synthetic electron-transfer chemistry for sharing was evaporated to dryness in vacuo, and the residue their knowledge of the reagents discussed herein. We was extracted into CH Cl (100 mL). After filtration 2 2 also thank our graduate students and postdoctoral through Celite, the volume of the dark blue extract workers for providing additional experimental infor- was reduced in vacuo to ca. 5 mL and diethyl ether mation on the synthesis, properties, and redox po- (100 mL) was added. The dark blue/green precipitate tentials of the reagents. W.E.G. thanks the National was then further purified from CH Cl /diethyl ether 2 2 Science Foundation for research support during the to give dark blue microcrystals, yield 0.28 g (51%). production of this review. Anal. Calcd for C12H12FeBF4O: C, 45.8; H, 3.8. Found: C, 45.4; H, 3.9. VIII. References B. 4-Fluorobenzenediazonium (1) (a) Hudlicky, M. Reductions in Organic Chemistry; Ellis Hor- wood: Chichester, 1984. (b) Hudlicky, M. Oxidations in Organic Hexafluorophosphate, [N2C6H4F-4][PF6] Chemistry; ACS Monograph 186; American Chemical Society: Washington, DC, 1990. (c) Taube, H. Electron Transfer Reactions To a solution of 4-FC6H4NH2 (5.0 mL, 52.7 mmol) of Complex Ions in Solution; Academic Press: New York, 1991. in distilled water (15 mL) at 0 °C was added freshly (d) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of

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Organic Compounds; Academic Press: New York, 1981. (e) (31) Bianchini, C.; Laschi, F.; Masi, D.; Ottaviani, F. M.; Pastor, A.; Kochi, J. K. Organometallic Mechanisms and Catalysis; Aca- Peruzzini, M.; Zanello, P.; Zanobini, F. J. Am. Chem. Soc. 1993, demic Press: New York, 1978. (f) Wiberg, K. W. Oxidation in 115, 2723. Organic Chemistry; Academic Press: New York, 1965; Part A. (32) Drake, S. R.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc., Dalton (g) Trahanovsky, W. S., ref 1f, Part B, 1973. Trans. 1989, 243. (2) (a) Davison, A.; Holm, R. H. Inorg. Synth. 1967, 10,8.(b) (33) North, T. E.; Thoden, J. B.; Spencer, B.; Dahl, L. F. Organome- Eberson, L. Electron Transfer Reactions in Organic Chemistry; tallics 1993, 12, 1299. Springer-Verlag: Berlin, 1987; Chapters 6-8. (c) Astruc, D. (34) Hughes, A. K.; Murphy, V. J.; O’Hare, D. J. Chem. Soc., Chem. Electron-Transfer Processes in Transition Metal Chemistry; VCH Commun. 1994, 163. Publishers: New York, 1995. (35) Gade, L. H.; Johnson, B. 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