Transition-Metal Hydride Radical Cations - American Chemical Society

Review pubs.acs.org/CR

Transition-Metal Hydride Radical Cations Yue Hu,*,†,‡ Anthony P. Shaw,§ Deven P. Estes,†,∥ and Jack R. Norton*,† †

Department of Chemistry, Columbia University, New York, New York 10027, United States Pyrotechnics Technology and Prototyping Division, U.S. Army RDECOM-ARDEC, Picatinny Arsenal, New Jersey 07806, United States

§

ABSTRACT: Transition-metal hydride radical cations (TMHRCs) are involved in a variety of chemical and biochemical reactions, making a more thorough understanding of their properties essential for explaining observed reactivity and for the eventual development of new applications. Generally, these species may be treated as the ones formed by one-electron oxidation of diamagnetic analogues that are neutral or cationic. Despite the importance of TMHRCs, the generally sensitive nature of these complexes has hindered their development. However, over the last four decades, many more TMHRCs have been synthesized, characterized, isolated, or hypothesized as reaction intermediates. This comprehensive review focuses on experimental studies of TMHRCs reported through the year 2014, with an emphasis on isolated and observed species. The methods used for the generation or synthesis of TMHRCs are surveyed, followed by a discussion about the stability of these complexes. The fundamental properties of TMHRCs, especially those pertaining to the M−H bond, are described, followed by a detailed treatment of decomposition pathways. Finally, reactions involving TMHRCs as intermediates are described.

CONTENTS 1. Introduction 2. Synthesis and Stability 2.1. One-Electron Oxidation of Diamagnetic Precursors 2.1.1. Preparative Electrolysis 2.1.2. Chemical Oxidation 2.2. Other Synthesis Methods 2.3. Stability 2.3.1. Factors Affecting Stability 2.3.2. Examples of Isolated Complexes 3. Characterization 3.1. Electron Paramagnetic Resonance (EPR) Spectroscopy 3.1.1. Isotropic (Solution) EPR Spectra 3.1.2. Anisotropic (Powder or Frozen Glass) EPR Spectra 3.2. X-ray Crystallography 3.3. Analytical Electrochemistry 3.3.1. Electrochemical Reduction (and Oxidation of Diamagnetic Precursors) 3.3.2. Electrochemical Oxidation 3.3.3. Other Electrochemical Techniques (DCV and CPC) 3.4. Infrared (IR) Spectroscopy 3.5. Other Characterization Methods 3.5.1. Effective Magnetic Moment 3.5.2. Conductivity 3.5.3. UV−vis Spectroscopy 3.5.4. Nuclear Magnetic Resonance (NMR) Spectroscopy © 2016 American Chemical Society

3.5.5. Mö ssbauer Spectroscopy 3.5.6. Elemental Analysis 3.5.7. Qualitative Characteristics and Dichroism 4. Thermodynamics 4.1. Acidity (pKa) 4.2. Homolytic Cleavage of the M−H Bond 4.3. Thermodynamic Hydricity of the M−H Bond 5. Reactivity 5.1. Proton Transfer 5.1.1. Deprotonation by External Base 5.1.2. Deprotonation by the Diamagnetic Precursor 5.1.3. Effect of Water 5.1.4. Deprotonation after Second Electron Transfer 5.2. Electron Transfer 5.2.1. One-Electron Oxidation 5.2.2. One-Electron Reduction 5.2.3. Disproportionation 5.2.4. Oxidation after Deprotonation 5.3. Hydrogen Atom Transfer 5.4. Reductive Elimination 5.5. Other Reactivity 5.5.1. Hydride Transfer 5.5.2. Reaction With Radical Sources

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8442 8442 8442 8442 8443 8443 8445 8445 8445 8447 8448 8448 8449 8450 8450 8450 8450 8451 8452 8453 8453 8453 8454

Special Issue: Metal Hydrides Received: September 10, 2015 Published: February 1, 2016

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Chemical Reviews 5.5.3. CO Coordination 5.5.4. Nucleophilic Attack by Halide Ligands 6. Reactions Involving TMHRC Intermermediates 6.1. Regioselective Hydrogenation of Heterocycles 6.2. Hydrogenase-Catalyzed Reactions 6.3. Acetylene Insertion 6.4. Photo-Induced Olefin Hydrometalation 7. Conclusions and Outlook Author Information Corresponding Authors Present Addresses Notes Biographies Acknowledgments References

Review

been proposed as an intermediate in electrocatalytic H+ reduction and H2 oxidation.18 TMHRCs can also be involved in hydride transfer reactions. For example, H− transfer to Ar3C+ from CpMo(CO)(dppe)H (dppe =1,2-bis(diphenylphosphino)ethane), 35 Cp*Ru(dppf)H, 36 and IrH3(PPh3)337 proceeds by an initial single-electron transfer (SET), forming the corresponding radical cations as intermediates. Likewise, the rate-determining hydride transfer step in the Cp*Ru(dppf)H-catalyzed hydrogenation of an acylpyridinium cation involves an initial SET followed by hydrogen atom transfer (HAT).21 Recently, it was shown that Stryker’s reagent, [(PPh3)CuH]6, is oxidized by the same acylpyridinium cation to form [(PPh3)CuH]6•+.38 Despite the importance of TMHRCs in transition-metal hydride chemistry, the generally sensitive nature of these complexes has hindered their development. Whereas the first transition-metal hydride complex, H2Fe(CO)4, was isolated and described as early as the 1930s,39,40 it was not until much later that the first TMHRC salts were isolated and characterized. The first isolated examples, [CoHL4][X] (L = P(OEt)2Ph, P(OMe)2Ph, or P(OPh)3; X = PF6 or BF4), were not reported until 1973.41 A crystal structure was not reported until 1986, in this case that of a related Co(II) complex, [(triphos)CoH(PEt3)][BPh4] (triphos = MeC(CH2PPh2)3).42 Over the last four decades, many more TMHRCs have been synthesized, characterized, isolated, or hypothesized as reaction intermediates. By 2014, well over one hundred TMHRCs had been reported, including around 30 isolated examples, containing metals spanning groups 5−11 in the periodic table. Even though TMHRCs have been discussed in several reviews and book chapters, there has been no recent review that has covered them comprehensively. Paramagnetic transitionmetal hydride complexes including cationic, neutral, and anionic examples were reviewed by Poli in a chapter of Recent Advances in Hydride Chemistry published in 2002.27 Pombeiro and Guedes da Silva described M−H bond cleavage promoted by electrochemical oxidation, including examples involving Mo, Fe, and Re hydride complexes, in a chapter of Trends in Molecular Electrochemistry published in 2005.43 An account by Tyler published in 1991 discusses 19-electron organometallic adducts, including their thermodynamics, electron transfer chemistry, structure, synthesis, and reactivity.44 Poli has also written an account, published in 1999, that includes a section describing paramagnetic hydride complexes of Mo(III), Mo(V), and W(V).45 More recently, Tilset described TMHRCs in the context of M−H acidity and bond dissociation energy (BDE) in a chapter of Comprehensive Organometallic Chemistry III published in 2007.46 This comprehensive review focuses on experimental studies of TMHRCs reported through the year 2014. Examples containing metals from group 5 through group 11 have been found. In describing these examples, emphasis is placed on isolated and observed species. The methods used for the generation or synthesis of TMHRCs are surveyed, followed by a discussion about the stability of these complexes. The fundamental properties of TMHRCs, especially those pertaining to the M−H bond, are described, followed by a detailed treatment of decomposition pathways. Finally, reactions involving TMHRCs as intermediates are described.

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1. INTRODUCTION Transition-metal hydride complexes can serve as important intermediates in the catalytic transfer of protons, hydrogen atoms,1−10 hydrides,11−15 or electrons16−21 to substrates from H2 gas. A comprehensive understanding of their chemistry is essential for explaining existing catalytic reactions and for the rational design of new ones. This review describes one important class of transition-metal hydride complexes: transition-metal hydride radical cations (TMHRCs). Generally, these species may be treated as the ones formed by oneelectron oxidation of diamagnetic analogues that are neutral or cationic. As radical cations, TMHRCs feature open shell electron configurations and are paramagnetic. This review does not include neutral paramagnetic transition-metal hydride complexes in any significant detail, since their chemistry is quite different from that of TMHRCs.22−27 TMHRCs are generally more reactive than their parent diamagnetic analogues. For example, oxidation of TpM(CO)3H and Tp′M(CO)3H (Tp = hydridotris(pyrazolyl)borate; Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate; M = Cr, Mo, W) causes a weakening of the M−H bonds by 26−27 kcal/mol with respect to deprotonation and by 6−9 kcal/mol with respect to homolysis.28 The same phenomenon was also observed for their Cp and Cp* analogues.29,30 Enhanced ligand substitution lability is commonly observed, as the metal centers of TMHRCs are electron deficient. Loss of H2 from cationic paramagnetic polyhydride complexes can occur readily. While the diamagnetic rhenium octahydride Re2(μ-H)4H4(PPh3)4 reacts very slowly with nucleophiles such as t-BuNC, the oxidized TMHRC analogue [Re2(μ-H)4H4(PPh3)4][PF6] does so rapidly, giving diamagnetic [Re 2 (μ-H) 3 H 2(PPh3 ) 4 (tBuNC)2][PF6] and liberating H2 in the process.31 Other examples include ReH5(PPh3)2L (L = PPh3, PEt2Ph, pyridine, piperidine, or cyclohexylamine), which are activated to attack by isonitriles upon electrochemical oxidation; the end products are Re(I) species.32 TMHRCs are an important part of thermodynamic cycles involving hydride complexes.33,34 They also play an important role in enzymes such as hydrogenase. The reversible [NiFe] hydrogenase-catalyzed oxidation of H2 in nature appears to involve a TMHRC.16,17 The involvement of open shell hydride complexes, especially TMHRCs, in a variety of catalytic electrochemical transformations may be envisaged.18−20 The TMHRC species [HFe2(CO)4(μ-H)(μ-dppf)(μ-pdt)]+ (dppf = 1,1′-bis(diphenylphosphino)ferrocene; pdt = S(CH2)3S) has

2. SYNTHESIS AND STABILITY Since many TMHRCs are reactive and unstable toward a variety of decomposition pathways (section 5), their syntheses 8428

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have generally required mild conditions and often low temperatures. However, some TMHRCs are stable enough to be prepared and isolated under normal laboratory conditions at ambient temperature (section 2.3.2).

Table 2. TMHRCs Prepared by Chemical Oxidation group 5

2.1. One-Electron Oxidation of Diamagnetic Precursors

Most TMHRCs have been generated by one-electron oxidation of diamagnetic transition-metal hydride precursors. The majority of reported species are 1+ charged, but dications derived from the oxidation of cationic precursors have also been prepared.31,47−49 In one case, oxidation of 2+ charged Re(III) complexes gave 3+ charged Re(IV) examples, although they were not isolated.50 One-electron oxidations may be conducted by chemical or electrochemical methods, as described in the following sections. 2.1.1. Preparative Electrolysis. In cyclic voltammetry (CV) experiments, the degree of reversibility associated with the one-electron oxidation of a diamagnetic precursor is often a good indicator of the stability of the corresponding TMHRC. A reversible or quasi-reversible cyclic voltammogram indicates that the TMHRC is at least stable on the CV time scale, whereas irreversible oxidations are often due to fast follow-up reactions (decomposition). Chemically reversible one-electron oxidation/reduction has been observed for diamagnetic hydride complexes from groups 5−9 and 11 (section 3.3). Many TMHRCs have been prepared by exhaustive controlledpotential electrolysis and were characterized in situ by methods such as EPR. The examples that have been made in this way are listed in Table 1. In a few cases, TMHRCs prepared by bulk

6

7

8

5 6 7

8

9

11

TMHRC

9 reference

•+

[(C5H4SiMe3)2NbH(P(OMe)3)] [HM(CO)2(dppe)2]•+ (M = Nb, Ta) [Cp2MH2]•+ (M = W, Mo) [Cp2W(Ph)H]•+ {CpRe[P(p-XC6H4)3]2H2}•+ (X = H, Me, F) [ReHCl(NCR)(dppe)2]•2+ (R = Ph, p-ClC6H4, p-FC6H4) [Re2(μ-H)4H4(PR3)4]•+ (PR3 = PPh3, PEtPh2, PEt2Ph) [(PP3)FeH2]•+ (PP3 = P(CH2CH2PPh2)3) [CpRu(PPh3)2H]•+ (decomposes) [(PP3)CoH]•+ (PP3 = P(CH2CH2PPh2)3) [(NP3)CoH]•+ (NP3 = N(CH2CH2PPh2)3) [CoH(dppe)2]•+ [CoH(P(OR)3)4]•+ (R = Me, Et) [RhH(CO)(PPh3)3]•+ [IrH(CO)(PPh3)3]•+ [IrH(dppe)2]•+ [(PPh3)CuH]6•+

oxidant

[Cp*Fe(dppe)H]•+ [Fe2Cp2(μ-H)(μ-PPh2)(CO)2]•+ [CpFe(dippe)H][BPh4] (dippe =1,2bis(diisopropylphosphino)ethane) [Fe(dppe)2ClH][X] (X = BF4 or ClO4) [Cp*Ru(dppf)H]•+ [(C5H4R)Ru(PPh3)2H]•+ (R = CTol3) trans-[HOs(en)2py]•2+

Table 1. TMHRCs Prepared by Bulk Electrolysis group

TMHRC [HNb(CO)2(dppe)2]•+ {([P2N2]Ta)2(μ-H)4}•+ ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh) [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3]•+ [((i-Pr)4C5H)Mo(PMe3)2H3]•+ [Cp*Mo(PMe3)3H]•+ [CpMo(PMe3)3H]•+ [Cp*MH3(dppe)]•+ (M = Mo, W) [M2Cp2(μ-H)(μ-PPh2)(CO)4]•+ (M = Mo, W) [W(PMe3)4Cl2H2][BF4]·Cp2Fe [W(PMe3)4Cl2H2]•+ [W(PMe3)4ClH2(MeCN)]•+ [W(PMe3)4Cl(MeCN)H2][BF4]2 [Re2(μ-H)4H3(PPh3)4(t-BuNC)]•2+ [Re2(μ-H)3H2(PPh3)4(t-BuNC)2]•2+ [Re2(μ-H)4H4(PPh3)4]•+

53 54 55 56 57 48

11

[CoHL4][X] (L = P(OEt)2Ph, P(OMe)2Ph, P(OPh)3; X = PF6, BF4) [(PP3)CoH]•+ (PP3 = P(CH2CH2PPh2)3 [RhH(CO)(PPh3)3]•+ [IrH(CO)(PPh3)3]•+ [(PPh3)CuH]6•+

reference

Cp2Fe+ MeI

54 63

Cp2Fe+ Cp2Fe+ Cp2Fe+ Ag+ Cp2Fe+ Cp2Fe+

65, 66 65, 66 67 68 68, 69 70

Cp2Fe+ Ag+ Ag+ Cp2Fe+ NO+ NO+ C7H7+ or Ph3C+ Cp2Fe+ Cp2Fe+ Cp2Fe+

49 49 49 49 31 31 31

Ag+ or Ph3C+ Cp2Fe+ Cp2Fe+ Cp2Fe+ or Ag+ Ph3C+

75

Cp2Fe+ Cp2Fe+ Cp2Fe+ Cp*2Fe+

51 59 61 80

71, 72 73 74

76 77 78a 79

a

Oxidation of [Os(en)2py(H2)]2+ by [Cp2Fe][OTf] or AgOTf followed by proton release to generate trans-[HOs(en)2py]•2+.

reduction potentials of +0.87 V, + 0.04 V, − 0.11 V, and −0.65 V, respectively (all in MeCN vs Cp2Fe+/Cp2Fe).62 In some cases, MeI and O2 have served as one-electron oxidants in the formation of TMHRCs. Reaction of ([P2N2]Ta)2(μ-H)4 with MeI formed {([P2N2]Ta)2(μ-H)4}•+I− and methyl radical, which was rapidly converted to methane by reaction with the solvent.63 The oxidation of [(triphos)Rh(μ-H)3Rh(triphos)]+ by O2 generates [(triphos)Rh(μ-H)3Rh(triphos)]•2+.64

31, 32 51 58 51 51 52 52 52, 59, 60 52, 60, 61 52 38

2.2. Other Synthesis Methods

Besides the one-electron oxidation of diamagnetic transitionmetal hydride precursors, other reactions have been used to form TMHRCs. Pulse radiolysis of aqueous solutions containing Ag+ or Cu+ has been shown to generate AgH•+ or CuH•+. Here, the metal ions react with H• formed transiently in solution.81−83 One-electron reduction of dicationic diamagnetic hydride complexes is another possible synthesis route. One example is the reduction of [Rh6(PCy3)6H12]2+ with Cr(η6-C6H6)2, which gives [Rh6(PCy3)6H12]•+.84 While disproportionation is a possible reaction mode of TMHRCs leading to diamagnetic products (section 5.2.3), the reverse process, comproportionation, can be used to synthesize them. Equal amounts of HCo I (dppe) 2 and [HCo III (dppe) 2 (MeCN)]2+ react to form two equivalents of [HCoII(dppe)2]•+.34

electrolysis were stable enough to be isolated from the electrolyte, including [(PP 3 )CoH][X] (PP 3 = P(CH2CH2PPh2)3; X = BF4 or ClO4),51 [(NP3)CoH][ClO4] (NP3 = N(CH2CH2PPh2)3),51 [CoH(dppe)2][BPh4],52 and [CoH(P(OR)3)4][BPh4] (R = Me, Et).52 2.1.2. Chemical Oxidation. Chemical oxidants have also been used to generate TMHRCs (Table 2). The most commonly used oxidant is Cp2Fe+. It has a reversible oneelectron reduction potential of +0.4 V versus SCE in acetonitrile62 and has been used to synthesize TMHRCs of groups 5, 6, 8, 9, and 11 metals. Other one-electron oxidants may be used such as NO+, Ag+, Ph3C+, and C7H7+, with 8429

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Cp*Ir(PPh3)(CH3)H

H3Ir(PMe2Ph)3

8

9

95

94 in MeCN: [Cp*Ir(PPh3) (MeCN)H]+, [Ir(PPh3) (MeCN)3(H)2]+, Cp*H; in CH2Cl2: [Cp*Ir(PPh3)(H)3]+, [Cp*Ir(PPh3)H]2(μ-H)+ Cp*Ir(PPh3)H2 7

in MeCN or acetone (S): cis,mer-[H2(S)Ir(PMe2Ph)3]+, H2; in CH2Cl2: [H4Ir(PMe2Ph)3]+, cis,mer-[H2Ir(CH2Cl2) (PMe2Ph)3]+

93 in CH2Cl2: [(P(i-Pr)3)2OsH3(H2)2]+; in MeCN: [(P(i-Pr)3)2OsH3(H2)2]+, [(P(i-Pr)3)2Os(MeCN)2H3]+, [(P(i-Pr)3)2Os(MeCN)3H]+ 6

94

92 [CpRu(PR3)2(MeCN)]+, [CpRu(PR3)2H2]+

CpRu(PR3)2H [(PR3)2 = (PPh3)2, dppm, dppe, dppp] (P(i-Pr)3)2OsH6

3 4

5

method

[Cp*Ir(PPh3) (MeCN)2]2+

29 91 in MeCN: [CpRu(CO)(PPh3) (MeCN)]+, [HRu(CO)(PPh3) (MeCN)3]+, CpH; in CH2Cl2: [CpRu(CO (PPh3)(η2-H2)]+ [CpRu(CO)(PMe3)]2(μ-H)+, [HRu(CO)(PMe3) (MeCN)3]+, CpH

chemical oxidation by Cp2Fe+ chemical oxidation by Cp2Fe+ in MeCN chemical oxidation by Cp2Fe+ in MeCN electrolysis or chemical oxidation by Cp(η5-C5H4COMe)Fe+ electrolysis or chemical oxidation by Cp2Fe+ in MeCN or CH2Cl2 electrolysis or chemical oxidation by Cp2Fe+ in MeCN chemical oxidation by Cp2Fe+

90 [FeF(CNR)(dppe)2][BF4] cyclic voltammetry in THF 2

Ultramicroelectrodes allow CV experiments to be performed at very fast scan rates. With this technique at room temperature, the one-electron oxidations of Cp2WH2 and Cp2MoH2 were chemically reversible at scan rates of 5 kV/s or greater.55 Low temperatures can increase the lifetime of unstable TMHRCs. A good example is [Re2(μ-H)4H4(PPh3)4]•+, which is only stable in solution for several minutes at room temperature but persists for several hours at 0 °C.32 In cyclic voltammetry experiments, the irreversible oxidations of [Mo(CO) 2 (dppe) 2 H] + , [W(CO) 2 (dppe) 2 H] + , and [W(CO)2(dppm)2H]+ became reversible when the solution temperature was decreased to −60 °C.47 Low temperatures were also used for the in situ electrochemical preparation of [IrH(CO)(PPh3)3]•+ (−30 °C)60 and [Cp2W(Ph)H]•+ (−45 °C).56 Other complexes were prepared in situ at low

electrolysis in MeCN

Scheme 1. Decomposition of [Cp′2NbH3]•+

diamagnetic precursor

Table 3. Selected Examples of Unstable TMHRCs Proposed As Reaction Intermediates

As open shell complexes, many TMHRCs are unstable and decompose readily by multiple pathways such as deprotonation, disproportionation, reductive elimination, atom transfer, and others (section 5). Some are extremely short-lived, decomposing immediately after formation. The diamagnetic precursors of these TMHRCs exhibit irreversible oxidation waves in CV experiments due to fast follow-up reactions involving the newly formed radical cations, often rapid proton loss. For example, one-electron oxidation of Cp′2NbH3 (Cp′ = Me3SiC5H4) gave a dimeric Nb(IV) complex. Fast proton loss from [Cp′2NbH3]•+ and dimerization of the resulting Cp′2NbH2• gave (Cp′2NbH2)2 as shown in Scheme 1.88 Selected examples of TMHRCs that undergo fast proton loss and other follow-up reactions are listed in Table 3. Generally, these TMHRCs cannot be observed directly.

(C5(CH2Ph)5)MH(CO)2(L) (M = Mo, W; L = CO, PMe3, PPh3) [FeH(CNR) (dppe)2][BF4] (R = Me, t-Bu, p-MeOC6H4, p-NO2C6H4) CpRu(CO)(PPh3)H CpRu(CO)(PMe3)H

2.3. Stability

1

(1)

entry

1 − butanol

Co2 + + triphos + PEt3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [(triphos)CoH(PEt3)]•+

[M(C5(CH2Ph)5)(CO)2(L) (MeCN)][PF6]

decomposition products

reference

Ligand association and dissociation to and from TMHRCs can give other open shell complexes. At −80 °C, the 17electron complex [Cp*Fe(dppe)H][PF6] binds CO reversibly in CH2Cl2, cleanly forming the 19-electron adduct [Cp*Fe(dppe)(CO)H][PF6].72 At room temperature, addition of H2 to [Rh6(PCy3)6H12][BArF4] (BArF4 = B(3,5-(CF3)2C6H3)4) forms [Rh6(PCy3)6H14][BArF4].85−87 Dissociation of H2 from the 17-electron trihydride complex [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3][PF6] gave the 15-electron monohydride, [(1,2,4(t-Bu)3C5H2)Mo(PMe3)2H][PF6].66 The same reaction pathway was also observed for [Cp*Mo(dppe)H3][PF6]. The product, Cp*MoH(dppe)(PF6), may be considered the salt of a 15-electron cation, or a 17-electron complex containing a coordinated FPF5 ligand.68 The TMHRC salt [(triphos)CoH(PEt3)][BPh4] was formed as the major product when Co[ClO4]2, triphos, PEt3, and NaBPh4 were combined in 1-butanol (eq 1). The exact mechanism of the reaction is unknown but the hydride ligand was proposed to come from the reaction of triphos or PEt3 with weakly acidic 1-butanol.42

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Table 4. Isolated TMHRCs group

complex

5

{([P2N2]Ta)2(μ-H)4}•+I− ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh) [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3][PF6] [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H][PF6] [Cp*Mo(PMe3)3H][PF6] [Cp*W(dppe)H3][PF6] [W(PMe3)4Cl2H2][BF4]·Cp2Fe [Re2(μ-H)4H4(PPh3)4][PF6] and 5 other variants trans-[Cp*Re(PMe3)2ClH][SbF6] trans-[Cp*Re(PMe3)2H2][SbF6] [Fe(dppe)2ClH][X] (X = BF4 or ClO4) [Cp*Fe(dppe)(CO)H][PF6]

6

7

8

[Cp*Fe(dppe)H][PF6]

9

[CpFe(dippe)H][BPh4] [Cp*Fe(dippe)H][BPh4] [Cp*Ru(dppf)H][PF6] [η5-(C5H4(CTol3))Ru(PPh3)2H][PF6] trans-[HOs(en)2(py)][OTf]2 [CoHL4][X] (L = P(OEt)2Ph, P(OMe)2Ph, P(OPh)3; X = PF6, BF4) [(triphos)Co(PEt3)H][BPh4]

synthesis chemical oxidation by MeI chemical oxidation by Cp2Fe+ H2 dissociation from MoH3 analogue chemical oxidation by Cp2Fe+ chemical oxidation by Cp2Fe+ chemical oxidation by Cp2Fe+ chemical oxidation by Ph3C+ or C7H7+ chemical oxidation by Ag+ chemical oxidation by Ag+ chemical oxidation by Ag+ or Ph3C+ CO coordination to [Cp*Fe(dppe)H] [PF6] chemical oxidation by Cp2Fe+ chemical chemical chemical chemical chemical chemical

oxidation oxidation oxidation oxidation oxidation oxidation

by by by by by by

Cp2Fe+ Cp2Fe+ Cp2Fe+ Cp2Fe+ Cp2Fe+ or Ag+ Ph3C+

[(NP3)CoH][ClO4] (NP3 = N(CH2CH2PPh2)3) [Co(dppe)2H][BF4] [Co(dppe)2H][BPh4] [(triphos)Rh(μ-H)3Rh(triphos)][BPh4]2 [Rh6(PCy3)6H12][BArF4]

Co[ClO4]2, triphos, PEt3 and NaBPh4 were mixed in 1-butanol oxidation of Co(H2)+ analogue followed by H+ loss electrolysis comproportionation electrolysis chemical oxidation by O2 reduction by Cr(η6-C6H6)2

[Rh6(PCy3)6H14][BArF4]

H2 coordination to Rh6H12 analogue

[(PP3)CoH][BF4] (PP3 = P(CH2CH2PPh2)3)

temperatures by chemical oxidation, including [Cp*MoH3(dppe)]•+ (−80 °C)68,69 and [Fe2Cp2(μ-H)(μPPh2)(CO)2]•+ (−60 °C).73 Additionally, low temperature reaction conditions have often been used in the preparation and isolation of TMHRCs such as [Cp*Fe(dppe)H][PF6],72 [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3][PF6],65 and [Cp*Ru(dppf)H][PF6].76 2.3.1. Factors Affecting Stability. The stability of TMHRCs may be increased by suppression of decomposition pathways. Several investigations have shown that such reactivity is disfavored by a stronger electron-donating and/or a more sterically protecting ligand environment. Poli has demonstrated this with several molybdenum hydride radical cations. For example, [Cp*Mo(PMe3)3H][PF6] is more thermally stable than the Cp analogue.67 The analogous trihydride containing a very hindered t-butyl-substituted cyclopentadienyl ligand, [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3][PF6], was stable enough to be isolated and structurally characterized. In contrast, [Cp*Mo(dppe)H3][PF6] is only stable in situ at low temperatures.65,66,96 Interestingly, a further increase in steric bulk using the (i-Pr)4C5H− ligand resulted in a less stable paramagnetic trihydride, [((i-Pr)4C5H)Mo(PMe3)2H3][PF6], which could not be isolated. The (i-Pr)4C5H− ligand might impose such steric pressure to the MoH3 system as to force a stronger interaction between the hydride ligands, thereby increasing the likelihood of H2 elimination. Steric arguments have also been used to explain the stability of [CpMoH(CO)n(PMe3)3−n]•+68 and [Cp*Fe(dppe)H]•+.72

characterization X-ray, EA, UV−vis, EPR, CV, 1H NMR, magnetism X-ray, CV, EPR X-ray, EPR X-ray, EPR, IR, CV, EA X-ray, EPR, IR, 1H NMR X-ray, EPR, IR, CV, EA, 1H NMR EPR, EA, CV IR, EA, 1H NMR IR, EA, 1H NMR EA, conductivity, IR, EPR IR, Mössbauer, EPR

reference 63 65, 66 65, 66 67 68, 97 49 31 98 98 75 99

magnetism, IR, X-ray, CV, EA, Mössbauer, EPR EA, IR, magnetism X-ray, EA, IR, magnetism CV, NIR, 1H NMR, 31P NMR CV, EPR, 1H NMR EA, IR, magnetism, X-ray conductivity, magnetism, EA, 1H NMR, IR, CV X-ray, EA, conductivity, magnetism, EPR, IR

71, 72

42

EPR, magnetism, EA

51

IR, magnetism, EPR, EA IR, CV, EPR EA, IR, CV, conductivity, magnetism IR, conductivity, magnetism, X-ray CV, X-ray, ESI-MS, EPR, 1H NMR, 31P NMR, magnetism, EA, UV UV, ESI-MS, EPR, X-ray, 1H NMR, 31P NMR

51 34 100 64 101

74 74 76 77 78 52, 79

87

The stabilizing effect of halide and phenyl substitution has also been observed. While [Fe(dppe)2H2]•+ is too unstable to be observed when Fe(dppe)2H2 reacts with Ph3C+ in benzene at room temperature, [Fe(dppe)2ClH][BF4] is stable enough to be isolated and characterized.75 The tungsten complexes [W(PMe3)4Cl2H2][BF4] and [W(PMe3)4Cl(MeCN)H2][BF4]2 are also isolable at room temperature.49 The half-life of [Cp2W(Ph)H]•+ is about 20 s at room temperature in solution, whereas [Cp2WH2]•+ exists in solution for just 4 ms at −77 °C.56 Periodic trends have also been observed. Generally, the stability of TMHRCs increases moving down a column of the periodic table. Good examples are available for groups 6 and 9. The stability of [M(CO)2(P−P)2H]•2+ (M = Cr, Mo, W; P−P = dppm, dppe) increases in the order Cr < Mo < W.47 The molybdenum trihydride [Cp*Mo(dppe)H3]•+ is only stable in solution at low temperature (−80 °C), but its tungsten analogue is stable enough to be isolated as the PF6− salt.97 In group 9, [IrH(CO)(PPh3)3]•+ is reportedly more stable than the Rh analogue.52,59 However, exceptions exist. For example, [CoH(dppe)2]•+ appears to be more stable than its Rh and Ir analogues.59 [(PP3)CoH]•+ (PP3 = P(CH2CH2PPh2)3) is also much more stable than its Rh analogue.51 2.3.2. Examples of Isolated Complexes. While many TMHRCs have been generated and characterized in situ, some have been isolated. Examples of isolated TMHRCs have been found for metals from groups 5−9, with the majority in groups 6, 8, and 9. Most of these radical cations feature a half-sandwich 8431

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Table 5. Isotropic (Solution) EPR Spectra of TMHRCs entry

TMHRC

solvent

T

g value

coupling

reference

THF THF THF CH2Cl2 MeCN CH2Cl2 THF

RT RT RT RT RT RT RT

2.006 1.989 2.017 2.082 2.1 gtrans = 2.0036, gcis = 2.0038 g1 = 2.180, g2 = 2.120

aNb = 32.4 G, aP = 38 G, aH = 10.3 G triplet of quartets aH = 11.8 G, aP = 28.9 G broad triplet broad singlet doublet, a = 80 G broad singlet broad singlets

53 68 68 77 34 73 51

THF

RT

2.107

broad singlet

51

1,2-C6H4F2 THF

RT 193 K

2.010 2.0185

THF THF

183 K 193 K

− 2.014

13

[Mo2Cp2(μ-H)(μ-PPh2)(CO)4]•+

180 K

14

[Cp2W(Ph)H]•+a

CH2Cl2/THF (2/1) MeCN

2.029 (cis, major); 2.006 (trans, minor) −

broad singlet quartet of triplet aP = 36.2 G, aH = 11.4 G, aMo = 30.8 G aP = 6.2 G; aH = 1.7 G; aMo = 29.4 G doublet of quartets aP = 28.6 G, aH = 12.9 G, aMo = 27 G triplet aH = aP = 11.7 G (cis) aH = aP = 13.5 G (trans) doublet aH = 12 G

84 65, 66

11 12

[(C5H4SiMe3)2NbH{P(OMe)3}]•+ [Cp*Mo(dppe)H3]•+ [Cp*W(dppe)H3]•+ [(C5H4CTol3)Ru(PPh3)2H]•+ [HCo(dppe)2]•+ [Fe2Cp2(μ-H)(μ-PPh2)(CO)2]•+ [(NP3)CoH]•+ (NP3 = N(CH2CH2PPh2)3) [(PP3)CoH]•+ (PP3 = P(CH2CH2PPh2)3) [Rh6(PCy3)6H12]•+ [(1,2,4-(t-Bu)3C5H2) Mo(PMe3)2H3]•+ [((i-Pr)4C5H)Mo(PMe3)2H3]•+ [CpMo(PMe3)3H]•+

1 2 3 4 5 6 7 8 9 10

228 K

65 68 70 56

This compound is unstable. A flow-electrochemical cell with a microporous silver electrode was inserted inside the EPR cavity. The EPR spectrum disappears as soon as the cell current is stopped.56. a

(or piano stool) configuration, often with strongly electrondonating phosphine ligands. A comprehensive list of all reported examples that have been isolated, organized by group number, may be found in Table 4.

3. CHARACTERIZATION A variety of methods have been used to characterize and study TMHRCs. EPR spectroscopy is frequently used to characterize the unpaired electron of these complexes, while conductivity measurements have been used to prove their cationic nature. IR spectroscopy may be used to measure M−H bond stretching frequencies. Cyclic voltammetry experiments have been used to generate TMHRCs and to study their electron transfer properties (mainly reduction potential). Many of these measurements and characterizations have been performed on complexes generated in solution, in situ. For those stable enough to be isolated, elemental analyses and single crystal Xray diffraction data have often been obtained. Other less common characterization methods include NMR, Mössbauer, UV−vis, and photoelectron spectroscopies. 3.1. Electron Paramagnetic Resonance (EPR) Spectroscopy

The EPR spectra of TMHRCs have often been obtained to unambiguously demonstrate the presence of an unpaired electron. Both isotropic (solution) spectra and anisotropic (powder or frozen glass) spectra have been reported. In many cases, the hyperfine coupling to the metal, the hydride ligand, and other ligands (such as phosphines) were observed. 3.1.1. Isotropic (Solution) EPR Spectra. In solution, anisotropic interactions are averaged out by rotational and translational molecular motion and the observed EPR spectra are independent of sample orientation within the magnetic field. These solution spectra are referred to as isotropic. In many cases, TMHRCs examined in this way were formed by electrolysis or chemical oxidation in situ. Table 5 contains a complete list of known isotropic (solution) EPR data for TMHRCs. The orange THF solution of [Cp*Mo(dppe)H3][PF6] obtained from chemical oxidation exhibits a triplet of quartets with a g value of 1.989 at room temperature (Figure 1a).68 This

Figure 1. EPR spectra of [Cp*Mo(dppe)H3]•+ (1a, top left), [Cp*Mo(dppe)D3]•+ (1b, bottom left), [Cp*W(dppe)H3]•+ (1c, top right), [Cp*W(dppe)D3]•+ (1d, bottom right) in THF. The star in spectrum 1d indicates an impurity. (The compound numbers in this figure refer to those appearing in the original article.) Reprinted from ref 68. Copyright 1998 American Chemical Society.

spectrum is consistent with coupling to three equivalent H atoms and two equivalent P atoms (aH = 11.8 G, aP = 28.9 G), indicating fluxionality in solution. The EPR spectrum of the deuterated analogue, [Cp*Mo(dppe)D 3][PF 6], shows a broadened triplet implying coupling to two equivalent P atoms (g = 1.991, aP = 28.9 G) (Figure 1b).68 Likewise, the EPR spectrum of [Cp*W(dppe)H3][PF6] features a broad triplet (g = 2.017) (Figure 1c). But, in this case the deuterated 8432

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analogue [Cp*W(dppe)D3][PF6] more clearly shows the phosphorus coupling (g = 2.022, aP = 27.6 G) (Figure 1d).68 The origin of the differences in resolution of the spectra is not known. Coupling to Nb, P, and H was observed for a niobium hydride radial cation generated in situ by the electrochemical one-electron oxidation of (C5H4SiMe3)2NbH{P(OMe)3}. The room temperature spectrum in THF consists of a signal at g = 2.006 with hyperfine coupling to one Nb (aNb = 32.4 G), one P (aP = 38 G), and one H (aH = 10.3 G), implying the formation of [(C5H4SiMe3)2NbH{P(OMe)3}]•+ (Figure 2).53

Figure 4. EPR spectrum obtained in the electrolysis of Cp2WH2 with added phenanthroquinone. Reprinted from ref 56. Copyright 1980 American Chemical Society.

3.1.2. Anisotropic (Powder or Frozen Glass) EPR Spectra. EPR spectra obtained from solid samples (frozen glassy solutions or powders) are anisotropic, showing the x, y, and z components (gx, gy, gz) of the g-tensor. The anisotropic EPR spectra of many TMHRCs have been reported and are summarized in Table 6. If a solution is used, preferably the solvent should form a glass at low temperatures. Suitable solvents include CH2Cl2, ClCH2CH2Cl, THF, acetone, and 1,2C6H4F2. Powdered samples may be examined over a much wider temperature range, including warmer temperatures provided the sample is thermally stable. The anisotropic spectra are often complicated, and computer simulations are used to aid in interpretation. For complexes with a low-spin octahedral configuration with C2v or lower symmetry, three distinct g-values are observed. For example, the EPR spectrum of [Cp*Fe(dppe)H][PF6] (Table 6, entry 4) at 77 K exhibits three well-separated g-tensor components (g1 = 1.9944, g2 = 2.0430, and g3 = 2.4487), as expected given its octahedral symmetry. The gl and g2 values are close to the free electron g value (g = 2.0023), whereas the g3 value is much larger, as often observed for 17-electron iron(III) compounds having a singly occupied HOMO with a predominantly dx2−y2 character.71,99 The EPR spectrum of the 19-electron CO-coordinated complex [Cp*Fe(dppe) (CO)H][PF6] (entry 5) recorded at 77 K exhibits three broad and complex g-tensor components (gl = 2.0019, g2 = 2.0367, and g3 = 2.0777). These are all much closer to the free electron g value, indicating that the HOMO is significantly more separated in energy from the doubly occupied orbitals than for [Cp*Fe(dppe)H][PF6].99 Anisotropic EPR spectra have been used to distinguish the oxidized metal in heterobimetallic Fe/Ru-hydride radical cations. Both Ru(II) and the Fe(II) in ferrocenyl ligands (such as dppf) may be oxidized to Ru(III) or Fe(III), but the corresponding EPR signals of such oxidized species in frozen glassy solutions differ significantly. The Ru(III) centers result in relatively small g anisotropy, with individual g components between about 1.5 and 2.5, while the Fe(III) in dppf with its high symmetry and d orbital splitting is distinguished by broad EPR signals, often clearly observable only at liquid helium

Figure 2. EPR spectrum after one-electron oxidation of (C5H4SiMe3)2NbH{P(OMe)3}. Reprinted with permission from ref 53. Copyright 1990 Elsevier.

The EPR spectrum of [Rh6(PCy3)6H12][BArF4] as a solution in 1,2-C6H4F2 at room temperature showed a strong broad signal (g = 2.010) (Figure 3), confirming the presence of an unpaired electron but without providing any information about the ligand environment.86

Figure 3. X-band EPR spectrum of [Rh6(PCy3)6H12]•+ (298 K, 1,2C6H4F2). Reprinted from ref 86. Copyright 2007 American Chemical Society.

The EPR spectra of many TMHRCs are not well-resolved at room temperature due to rapid spin−lattice relaxation. The absence of hyperfine coupling to phosphorus atoms and hydride ligands at room temperature, as observed in entries 3− 9 of Table 5, serve as examples. At lower temperatures, better resolution was often obtained, as shown in entries 10−14 of Table 5. The transient [Cp2WH2]•+ was indirectly observed by trapping this TMHRC with phenanthroquinone (PQ) (eq 2). In this case, the EPR spectrum corresponds to the neutral PQ[Cp2WH]• complex resulting from deprotonation (Figure 4).56 8433

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Table 6. Anisotropic EPR Spectra of TMHRCs entry 1

TMHRC

solvent

T

g values

reference

powder

4K

gl = 3.74, g2 = 3.45, g3 = 5.33

66

CH2Cl2 CH2Cl2 CH2Cl2/ ClCH2CH2Cl CH2Cl2/ ClCH2CH2Cl acetone

77 K 113 K 77 K

2.05 2.16 g1 = 1.9944, g2 = 2.0430, g3 = 2.4487

54 31, 32 71, 99

77 K

gl = 2.0019, g2 = 2.0367, g3 = 2.0777

99

77 K

75

powder

293 K

gl = 2.180, g2 = 2.083, g3 = 2.003 (2.185, 2.100, 1.981 of the powdered compound) gl = 2.16, g2 = 2.09, g3 = 1.99

77

2 3 4

[(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H][PF6] [HNb(CO)2(dppe)2]•+ [Re2(μ-H)4H4(PPh3)4]•+ Cp*Fe(dppe)H]•+

5

[Cp*Fe(dppe)(CO)H]•+

6

[Fe(dppe)2ClH]•+

7

THF

100 K

TBP: gl = 2.200, g2 = 2.190, g3 = 2.120; SQ: g1 = 2.090, g2 = 2.040, g3 = 1.990

51

THF

100 K

TBP: gl = 2.165, g2 = 2.150, g3 = 2.070; SQ: g1 = 2.250, g2 = 2.200, g3 = 2.147

51

10 11

[(C5H4(CTol3))Ru(PPh3)2H] [PF6] [(PP3)CoH]•+ (PP3 = P(CH2CH2PPh2)3) [(NP3)CoH]•+ (NP3 = N(CH2CH2PPh2)3) [(triphos)CoH(PEt3)][BPh4] [Rh6(PCy3)6H12]•+

powder 1,2-C6H4F2

60 K 15 K

42 85

12

[Rh6(PCy3)6H14]•+

1,2-C6H4F2

20 K

13 14

[IrH(CO)(PPh3)3]•+ AgH•+

CH2Cl2 H2O

77 K 165 K

2.20 one isomer: g1 = 2.210, g2 = 2.221, g3 = 2.380; the other isomer: g1 = 2.2140, g2 = 2.372, g3 = 2.452 one isomer g1 = 2.190, g2 = 2.192, g3 = 2.368; the other isomer: g1 = 2.102, g2 = 2.267, g3 = 2.394 g1 = 2.219, g2 = 2.147, g3 = 1.975 gl = 2.046, g2 = 2.036, g3 = 2.002

8 9

85 61 81, 102

3.2. X-ray Crystallography

temperature (4 K), and with a large g anisotropy. Such Fe(III) signals include one component at about g = 4. Figure 5 shows

The first crystal structure of a TMHRC, [(triphos)CoH(PEt3)][BPh4], was reported by Bianchini in 1986.42 However, in this structure, the hydride was not located. Since then, other isolable TMHRCs have been characterized by single-crystal Xray diffraction. In general, these complexes have bulky ligands, usually large phosphines, which protect the hydride ligand(s). Most of the reported crystal structures contain metals from groups 5, 6, 8, or 9. In view of their low electron density, it is not always possible to accurately locate hydride ligands by X-ray crystallography. There is often a large inaccuracy in both the location and the number of hydride ligands in such structures. One example is {([P2N2]Ta)2(μ-H)4}•+I− ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh), generated by one-electron oxidation of the neutral precursor with MeI (eq 3). This

Figure 5. EPR spectra (4 K) from spectroelectrochemical one-electron oxidation of compounds Cp*Ru(dppf)H (top) and CymRu(dppf)H (bottom). Reprinted with permission from ref 103. Copyright 2000 Elsevier.

tetrahydride radical cation was initially identified by X-ray crystallography as a cationic trihydride, {([P2N2]Ta)2(μH)3}+I−. However, this structure was not in agreement with other experimental observations. For example, reaction of the deuterated precursor ([P2N2]Ta)2(μ-D)4 with MeI did not produce CH3D but instead produced CH4. Likewise, reduction of the deuterated cation in question with K[BEt3H] gave ([P2N2]Ta)2(μ-D)4 and not ([P2N2]Ta)2(μ-H)(μ-D)3. These reactions confirmed that the cation was indeed the tetrahydride radical cation and that in the initial synthesis MeI served as a

the low-temperature (4 K) spectra for [Cp*Ru(dppf)H]•+ and [CymRu(dppf)H]•2+ (Cym = p-cymene). From these EPR spectra, it is obvious that [Cp*Ru(dppf)H]•+ contains an oxidized Ru(III) center (g1 = 2.187, g2 = 2.088, and g3 = 1.991; Δg = 0.196). [CymRu(dppf)H]•2+ is clearly a Ru(II)/Fe(III) complex with an axial g splitting of g1 = 3.612 and g2,3 = 1.765 (Δg = 1.847).103,104 8434

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one-electron oxidant. Similarly, the reaction of the radical cation with K[BEt3H] was simply a one-electron reduction and did not involve hydride transfer.63 Even though the X-ray structure of {([P2N2]Ta)2(μ-H)4}•+ I− is similar to that of ([P2N2]Ta)2(μ-H)4 with respect to many bond angles and lengths, the radical cation exhibits a longer Ta−Ta distance (2.7721(5) Å compared to 2.6165(5) Å), although only one single-bonding electron of the single bond has been removed.63 DFT calculations were performed to understand the bonding in these complexes.63 One-electron oxidation of the trihydride complex (1,2,4-(tBu)3C5H2)Mo(PMe3)2H3 generated [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3][PF6]. This TMHRC is stable as a crystallized solid and as a THF solution at low temperatures but decomposes slowly above 0 °C forming the monohydride analogue after H2 loss (Scheme 2). The crystal structures of all three complexes Scheme 2. Decomposition of [LMo(PMe3)2H3]•+ (L = 1,2,4(t-Bu)3C5H2) Figure 7. X-ray structure of [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H]•+. Selected bond lengths [Å]: Mo−H1 1.83(3). Reprinted with permission from ref 66. Copyright 2007 Wiley-VCH.

have been obtained. The tendency of the Mo(V) trihydride radical cation to eliminate H2 may be explained by its structure. Compared to the neutral precursor, [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3]•+ contains a much closer H2···H3 contact. In the neutral complex, this contact is 1.69 Å (from neutron diffraction) or 1.63 Å (from X-ray diffraction). In the oxidized analogue, the H2···H3 contact is just 1.40 Å (from the X-ray data), Figure 6. Even when the uncertainty of the X-ray data is

crystal structure helps to explain this, as the closest H···H contact in this case is 2.11 Å, outside the range of “compressed” dihydrides. In the crystal structure of [Rh6(PCy3)6H12][BArF4], all 12 hydride ligands were located, and these bridge each Rh−Rh edge symmetrically (Figure 8). Compared to the diamagnetic precursor [Rh 6 (PCy 3 ) 6 H 12 ] 2+ , 86, 107 the structure of [Rh6(PCy3)6H12]•+ shows that there is no major change to the core upon reduction; this is consistent with the addition of an electron to a nonbonding Rh−Rh orbital.101 In the crystal structure of the radical cation, the cyclohexyl rings of each phosphine adopt three different orientations which gives three ipso-CH environments. Two point toward the cluster core (syn) and one points away (anti). In the crystal structure of [Rh6(PCy3)6H14][BArF4], the hydride ligands were not located.84,86 However, the presence of additional hydride ligands was indicated by different Rh−Rh and cross-cage distances compared to [Rh6(PCy3)6H12][BArF4].86,107 In most crystal structures of TMHRCs, no interaction between the anion and the hydride ligand was observed, with one exception: [Cp*Mo(PMe3)3H][PF6]. In this structure, the hydride ligand was directly located with a Mo−H distance of 1.72(6) Å. The PF6− ion is on the same side of the cation as the hydride ligand (Figure 9). It has been proposed that the Mo−H bond is polarized by long H···F contacts (3.33, 3.56, and 3.58 Å), effectively hydrogen bonds. The presence of this weak Mo− H···F interaction has also been confirmed by DFT calculations.67 Hindrance by the methyl groups of the Cp* ligand and the PMe3 ligands may have prevented a closer position for the anion.67

Figure 6. X-ray structure of [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3]•+. Reprinted with permission from ref 66. Copyright 2007 Wiley-VCH.

taken into account, this close contact falls into the range of “compressed” dihydrides, or “stretched” dihydrogen complexes,105,106 and presumably facilitates the H2 elimination process. The elimination could be thought of as collapse of the H2 and H3 ligands to give an intermediate hydride(dihydrogen) complex, followed by loss of H2.65,66 In contrast to the related molybdenum complexes, the isoelectronic tungsten complex [Cp*W(dppe)H3]•+ is stable with no tendency to decompose by H2 evolution.68,69 The

3.3. Analytical Electrochemistry

Since many TMHRCs have been prepared by one-electron oxidation, their electrochemical properties have often been studied. Almost all of these electrochemical studies have involved cyclic voltammetry (CV), although other techniques, such as derivative cyclic voltammetry (DCV) and controlledpotential coulometry (CPC) have been used in several cases. As mentioned previously, the degree of reversibility associated with 8435

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Figure 8. Solid-state structure of [Rh6(PCy3)6H12][BArF4] (50% thermal ellipsoids) showing one of the crystallographically independent ion pairs. Inset shows an alternative view of the cationic metal core down the crystallographic C2 axis. Arrows indicate the three different ipso-CH environments with respect to the cluster core: two syn and one anti. Reprinted from ref 86. Copyright 2007 American Chemical Society.

Scheme 3. Electrochemical Behavior of TMHRCs

obtained in weak or noncoordinating solvents such as THF or CH2Cl2. 3.3.1. Electrochemical Reduction (and Oxidation of Diamagnetic Precursors). For the diamagnetic precursors, better donor ligands increase electron density at the metal centers, making it easier to oxidize the complexes. ReH5(PPh3)2L complexes (Table 7, entry 1) provide a good example. In this case, the values of Epa correlate well with the basicity of L, with more basic ligands resulting in less positive values.32 The reversible potentials (E1/2) for CpRe[P(pXC6H4)3]2H2 (X = H, Me, F, and OMe) are a function of the σ-donating ability of the X substituent, with the complexes becoming more difficult to oxidize as the donor ability of the phosphine decreases (entry 2).57 A similar trend was observed with Re2(μ-H)4H4L4 (L = PPh3, PEtPh2, PEt2Ph, and AsPh3) complexes (entry 3); the potentials correlate with PPh3 being a better donor than AsPh3.31 Related trends are apparent in many other systems (entries 4−8 and 10).68,97,108,29,65,21 One exception to the general trend discussed above is CpMo(PMe3)3H and the Cp* analogue (entry 5). Oxidation of the Cp* complex appears to be slightly more difficult, an unexpected result given that the Cp* ligand is much more electron rich than Cp. The reason for the reversed trend is not readily apparent.67 Going down a group in the periodic table, the metal centers of diamagnetic precursors become easier to oxidize. This trend is clearly observed in entry 8 with Cp*M(dppe)H3 (M = Mo, W).97 Although, the opposite appears to be true for Cp*M(C− N)H (C−N = 2-phenylpyridine or benzo[h]quinolone; M = Rh, Ir) (entry 11), as the potentials suggest that the Rh hydrides are easier to oxidize than their Ir analogues. However, a meaningful comparison in this case is questionable due to the irreversible potentials.107 One-electron reduction of a TMHRC regenerates its diamagnetic precursor. This is sometimes used to demonstrate the identity of a complex, as was the case for [Re2(μH)4H4(PPh3)4][PF6]31 and {([P2N2]Ta)2(μ-H)4}+ I−.63 For a

Figure 9. An ORTEP view of [Cp*Mo(PMe3)3H][PF6] (ellipsoids are drawn at the 30% probability level). All hydrogen atoms except the hydride are not shown for clarity. Reprinted from ref 67. Copyright 2009 American Chemical Society.

CV oxidation waves is often a good indicator of the stability of the radical cations. In many cases, reversible or quasi-reversible cyclic voltammograms have been obtained. Importantly, peak separation in excess of that expected for a reversible process (ΔEp > 59 mV) can be caused by sluggish electron transfer kinetics or uncompensated solution resistance; these effects are unrelated to the chemical stability of TMHRCs. Chemically reversible oxidations are characterized by voltammograms with approximately equal oxidation and reduction peak currents (ipa ≈ ipc). Expanding the scan potential range can provide additional information about the electrochemical behavior of TMHRCs. In cases where the radical cations are unstable, it is sometimes possible to obtain information about the fast followup reaction pathways and the resulting products.49,77 In general, the electrochemical behavior of TMHRCs includes one-electron reduction, reforming the diamagnetic precursors and one-electron oxidation coupled with deprotonation (Scheme 3). Follow-up reactivity, when it is observed, often depends on the solvent used for the experiments. For example, voltammograms obtained in a strongly coordinating solvent such as MeCN could be quite different from those 8436

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Table 7. Cyclic Voltammetry Data for One-Electron Oxidation of Metal Hydrides entry

complex

1

ReH5(PPh3)2L

2

CpRe[P(pXC6H4)3]2H2

ligands L = PPh3 PEt2Ph py C6H11NH2 C5H10NH X=H

3

Re2(μ-H)4H4L4

4

CpMoH(CO)2L

Me F OMe L = PPh3 PEtPh2 PEt2Ph AsPh3 L = CO

(L)Mo(PMe3)3H

PPh3 PMe3 L = Cp

5

Epa (V)

(C5(CH2Ph)5) MoH(CO)2L

7

(L)Mo(PMe3)2H3

8

Cp*M(dppe)H3

+0.26

CO L = 1,2,4-(tBu)3C5H2 (i-Pr)4C5H M = Mo

+0.72

+0.23

SCE

MeCN

reversible

57

+0.09 +0.30 +0.06 −0.2 −0.26 −0.40 −0.16

SCE

CH2Cl2

quasireversible

31

Cp2Fe+/ Cp2Fe

MeCN

irreversible

68, 108

Cp2Fe+/ Cp2Fe Cp2Fe+/ Cp2Fe Cp2Fe+/ Cp2Fe

MeCN

irreversible irreversible reversible

THF, MeCN

reversible

68, 108 67

MeCN

irreversible

89

Cp2Fe+/ Cp2Fe

THF, MeCN

irreversible reversible

65

Cp2Fe+/ Cp2Fe Cp2Fe+/ Cp2Fe Cp2Fe+/ Cp2Fe

THF, MeCN, CH2Cl2 THF, MeCN, CH2Cl2 MeCN

reversible in THF, quasireversible in MeCN reversible in THF, quasireversible in MeCN quasireversible

29

Cp2Fe+/ Cp2Fe

CH2Cl2

irreversible

21

THF

quasireversible reversible reversible irreversible

110

−0.93 in MeCN, − 0.89 in THF −0.95 in MeCN, − 0.88 in THF −0.85 in MeCN, − 0.73 in THF, − 0.73 in CH2Cl2 −0.88 in MeCN, THF, and CH2Cl2

CpWH(CO)2L

L = CO

+0.758

10

(L)Ru(P−P)H

PMe3 L = Cp, P-P = dppe

+0.195 −0.16

L = Cp, P-P = dppf L = Cp*, P-P = dppe L = Cp*, P-P = dppf M = Rh, C-N = 2 -phenylpyridine M = Rh, C-N = benzo[h]quinoline M = Ir, C-N = 2 -phenylpyridine M = Ir, C-N = benzo[h]quinoline

−0.31

ref 32

−1.4 in MeCN, − 1.35 in THF

L = PMe3

reversibility quasireversible

−1.46

9

Cp*M(C−N)H

solvent CH2Cl2

+0.23 +0.19

W

11

reference SCE

+0.54

Cp* 6

E1/2 (V)

+0.37 +0.29 +0.17 +0.155 +0.11

−0.51 −0.63 −0.38

Cp2Fe+/ Cp2Fe

−0.39

irreversible

−0.09

irreversible

−0.06

irreversible

reversible electrochemical process, the reduction of a TMHRC shares the same thermodynamic potential as the oxidation of the corresponding diamagnetic precursor. Therefore, TMHRCs with better donor ligands will be more difficult to reduce. 3.3.2. Electrochemical Oxidation. One-electron oxidation of a neutral diamagnetic complex increases the positive charge on the metal center(s), making further oxidation, namely oxidation of a monocationic TMHRC, more difficult.34,59,61,63 However, wider scan ranges in cyclic voltammetry experiments show that it is possible to further oxidize TMHRCs, a process that is often paired with additional reactions (usually involving proton loss). Depending on whether or not deprotonation occurs, and when it occurs, three further reactions are possible for TMHRCs (Scheme 4): (a) reversible second oxidation without proton loss, forming a stable dicationic hydride

97 97

Scheme 4. Electrochemical Oxidation Pathways of TMHRCs

complex; (b) irreversible oxidation to a dicationic hydride that undergoes fast proton loss; and (c) fast deprotonation followed by oxidation of the resulting radical. In coordinating solvents (e.g., CH3CN), these pathways are often accompanied by solvent molecule coordination. Generally, one-electron oxidation of a TMHRC further increases the acidity of the hydride ligand, making the newly 8437

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of [HCo(dppe)2]•+ in MeCN gives [HCo(dppe)2(MeCN)]2+.34,100 As mentioned before, in many cases one-electron oxidation of TMHRCs is accompanied by rapid deprotonation. [RhH(CO) (PPh3)3]•+ undergoes a chemically irreversible oneelectron oxidation at about 0 V versus Cp2Fe+/Cp2Fe, followed by rapid H+ loss to the medium with formation of [Rh(CO) (PPh3)3]+.59 The same decomposition pathway was also observed for the Ir analogue.61 In the case of [IrH(dppe)2]•+, the oxidation product [IrH(dppe)2]2+ transfers H+ to the diamagnetic precursor IrH(dppe)2, forming [Ir(dppe)2]+ and [IrH2(dppe)2]+ in a 1/1 ratio (Scheme 5).100 The same

formed dication very likely to undergo deprotonation by the diamagnetic precursor, the solvent, or an external base. As a result, these oxidations are often chemically irreversible in cyclic voltammetry experiments. However, there are exceptions. For example, one-electron oxidation of {([P2N2]Ta)2(μ-H)4}•+ forms the stable dication {([P2N2]Ta)2(μ-H)4}2+, which contains two Ta(V) metal centers and no electrons in the Ta−Ta bonding orbital.63 In an ordinary cyclic voltammetry experiment (the reversible oxidation of Cp2Fe, for example), the best electrochemical reversibility will be found at a low scan rate, since this allows enough time for diffusion to replenish species transformed near the electrodes. But for unstable species, such as the oneelectron oxidation product of a TMHRC, better chemical reversibility may appear at greater scan rates. In addition to scan rate, the apparent reversibility of the one-electron oxidation of a TMHRC can also be affected by temperature and the identity of the electrolyte. For example, in CH2Cl2, the best chemical reversibility for the one-electron oxidation of [IrH(CO)(PPh3)3]•+ occurred at fast scan rates or low temperatures, with 0.2 M [Bu4N][PF6] as the electrolyte (Figure 10). For this oxidation, the degree of apparent reversibility was also found to depend on the electrolyte, in the following order: [Bu4N][PF6] > [Bu4N][BF4] > [Bu4N][ClO4].61

Scheme 5. Electrochemical Oxidation of IrH(dppe)2

reactivity was observed for the Rh analogue, but in that case the final products were [Rh(dppe)2]+ and H2 in a 2/1 ratio, in agreement with the well-known lack of reactivity between the square planar Rh(I) cation and H2.100 Better donor ligands in a TMHRC increase electron density at the metal center, facilitating further oxidation. For example, the reversible potentials for the oxidations of {CpRe[P(pXC6H4)3]2H2}•+ (X = H, Me, F, and OMe) exhibit a substituent effect similar to that of the neutral analogues; that is, the potentials become more positive as the donor ability of the phosphine ligands decrease.57 3.3.3. Other Electrochemical Techniques (DCV and CPC). Besides ordinary cyclic voltammetry (CV), derivative cyclic voltammetry (DCV)111 has also been used to study the electrochemical properties of TMHRCs. Tilset has noted that “the differentiation of a normal cyclic voltammogram with respect to time offers several advantages.” The baseline problem caused by double-layer charging current in ordinary CV experiments is effectively eliminated, allowing DCV to be used conveniently for measurement of electrode reaction kinetics. Additionally, the differential response allows resolution of features that may not be apparent in an ordinary cyclic voltammogram.112 Figure 11 shows an ordinary cyclic voltammogram (top) and a DCV plot (bottom) for the same system, the one-electron oxidation of CpRu(CO) (PPh3)H. In the DCV plot, the peak potential for oxidation (Epa) corresponds to the point where the descending line crosses the baseline after the derivative peak (marked a). This was found to occur at +0.39 V versus Cp2Fe+/Cp2Fe. Differentiation generally ensures more accurate peak potentials. Information about the electrochemical reversibility of the electron transfer may be indicated by the “broadness” of the derivative peak, defined as the peak width at half-height (ΔEp/2). A reversible one-electron transfer has a ΔEp/2 of 70.3 mV.29 In this system, the width was 110 mV, implying a quasi-reversible process. The low intensity of the DCV peak on the reverse scan (marked b) is due to chemical irreversibility, implying that the radical cation [CpRu(CO) (PPh3)H]•+ undergoes fast follow-up reactions on the same timescale as the measurement.112 Controlled-potential coulometry (CPC) is often used to monitor the consumption of electrons for each reaction step in electrochemical experiments. This information is useful for studying reaction pathways. For example, CPC performed at

Figure 10. Cyclic voltammogram over a wide potential range for the oxidation of IrH(CO) (PPh3)3 (4 mM in CH2Cl2, with 0.2 M [Bu4N][PF6]) at a glassy carbon macro-electrode (3.0 mm diam.) and a scan rate of 200 mV/s. Reprinted with permission from ref 61. Copyright 2000 Elsevier.

In Figure 10, the reversible wave at −0.47 V (vs Cp2Fe+/ Cp2Fe) corresponds to the one-electron oxidation of IrH(CO) (PPh3)3. The radical cation that is formed undergoes a second one-electron oxidation at −0.01 V. The newly formed 16electron complex [IrH(CO) (PPh3)3]2+ is slowly deprotonated (eq 4), producing some [Ir(CO) (PPh3)3]+. On the return scan, a small reversible wave at −1.57 V is observed, which has been attributed to the two-electron reduction of [Ir(CO) (PPh3)3]+ (eq 5).61 [IrH(CO)(PPh3)3 ]2 + → [Ir(CO)(PPh3)3 ]+ + H+ +

−

−

[Ir(CO)(PPh3)3 ] + 2e ⇌ [Ir(CO)(PPh3)3 ]

(4) (5)

Coordination of a solvent molecule can help stabilize dicationic complexes. For example, one-electron oxidation of [IrH(CO) (PPh3)3]•+ in MeCN leads to the formation of [IrH(CO) (PPh3)3(MeCN)]2+. In CH2Cl2, the coordinatively unsaturated [IrH(CO) (PPh3)3]2+ is unstable, undergoing rapid proton loss to form [Ir(CO) (PPh3)3]+.100 Similarly, oxidation 8438

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absorption was also observed when WH2Cl2(PMe3)4 was oxidized (entry 6). The blue shift observed for some ν(M−H) upon oxidation would seem to suggest that the M−H BDE goes up upon oxidation of the complex. However, as will be discussed further below, measurement of the change in the BDE upon oxidation shows that M−H bonds get slightly weaker upon oxidation. Poli has suggested that this discrepancy may be a result of the relationship between BDE and vibrational force constant.27 The vibrational force constant measures the characteristics of the vibrational potential well. However, the thermodynamic BDE measures the energy difference between the potential wells of the hydride and the separated H• and M•. If M−H and [M−H]+• have differently shaped vibrational potential wells, it may be possible for ν(M− H) to increase upon oxidation, while the BDE decreases. The polarization and contraction of the M−H bond upon oxidation probably cause significant changes to the vibrational potential well. In addition to M−H stretching, the stretching absorptions of other ligands such as CO, CN, and NO can be used to study the effects of oxidation on hydride complexes. The absorptions associated with these ligands often exhibit much stronger intensity compared to ν(M−H) and lie in a more favorable (nonfingerprint) region of the spectra, typically around 2000 cm−1. Oxidation of a transition-metal hydride results in a more positively charged metal center which decreases back-bonding to the antibonding orbitals of the ligands. Stretching frequencies for ligands such as CO therefore shift to higher frequencies in the oxidized complexes. For example, oxidation of Fe2Cp2(μ-H)(μ-PPh2) (CO)2 to the corresponding radical cation causes ν(C−O) to increase by about 70 cm−1 (Table 9, entry 1). The complexes [M2Cp2(μ-H)(μ-PPh2) (CO)4]•+ (M = Mo, W) have C−O stretching bands at frequencies approximately 90 cm−1 higher than their neutral precursors (entries 2 and 3). Similarly, [IrH(CO) (PPh3)3]•+ has a C−O stretching frequency 64 cm−1 greater than IrH(CO) (PPh3)3 (entry 4). The spectral patterns for the radical cations can be very different from those of the parent hydride complexes. For example, while W2Cp2(μ-H)(μ-PPh2) (CO)4 has one weak and two strong C−O stretching bands, [W2Cp2(μ-H)(μ-PPh2) (CO)4]•+ exhibits four strong-to-medium bands (Table 9, entry 2). The ν(C−O) pattern of [M2Cp2(μ-H)(μ-PPh2) (CO)4]•+ (M = Mo, W) indicates a cis geometry in solution, which is very similar to that found for the neutral compound [Mo2{μ-(η5C5H4)2SiMe2}(μ-H)(μ-PMe2) (CO)4] [1959 (vs), 1925 (s), 1874 (vs), 1860 (sh) cm−1] with a cis Mo(CO)2 moiety due to the presence of the linked cyclopentadienyl ligands. An additional weak band at 2033 cm−1 for [Mo2Cp2(μ-H)(μPPh2) (CO)4]•+ or 2023 cm−1 for the tungsten analogue with constant relative intensities were assigned to trans isomers.70 IR spectroelectrochemistry has been used to study the properties of TMHRCs electro-generated electrochemically. For example, IR spectroelectrochemistry of NbH(CO)2(dppe)2 at Eapp = +0.40 V versus Ag/AgCl shows the isosbestic conversion of the neutral hydride complex to the radical cation, [HNb(CO)2(dppe)2]•+. The Nb−H stretching frequency changes from 1765 to 1900 cm−1 in the process. The radical cation can be quantitatively reduced at Eapp = −0.50 V versus Ag/AgCl, regenerating the neutral hydride complex. The tantalum analogue TaH(CO)2(dppe)2 behaves similarly, with ν(Ta−H) increasing from 1754 to 1877 cm−1 upon oxidation.54

Figure 11. Cyclic voltammogram (top) and derivative cyclic voltammogram (bottom) for the oxidation of CpRu(CO) (PPh3)H (1.0 mM) in acetonitrile/[Bu4N][PF6] (0.1 M) at a Pt microelectrode (0.4 mm diameter) at 20 °C and a scan rate of 1.0 V/s. Reprinted from ref 112. Copyright 1991 American Chemical Society.

the potential of the first oxidation wave of (C5(CH2Ph)5)M(CO)3H (M = Mo, W) indicated an overall transfer of two electrons per molecule. This result and those of accompanying cyclic voltammetry experiments were consistent with initial oxidation to the radical cation followed by rapid proton loss to form a 17-electron neutral radical, [(C5(CH2Ph)5)M(CO)3]•. Oxidation of this radical to the cation [(C5(CH2Ph)5)M(CO)3]+ accounts for the second electron transfer.89 The overall process is the same as that outlined in Scheme 4c. 3.4. Infrared (IR) Spectroscopy

The M−H bonds of TMHRCs feature unique IR stretching frequencies. As a result, IR spectroscopy is often used to characterize these complexes (Table 8). Like neutral diamagnetic hydride complexes, TMHRC terminal hydride stretching frequencies lie in the 1770−2100 cm−1 range with weak or medium intensity, while those for bridging hydrides occur at lower frequencies such as the 1655 cm−1 absorbance for [(triphos)Rh(μ-H)3Rh(triphos)][BPh4]2·DMF (Table 8, entry 20). In many cases, a blue shift in ν(M−H) is observed upon oneelectron oxidation of a neutral diamagnetic precursor (entries 3−5, 9−14), which indicates a strengthened M−H bond. Intensity reduction is also often observed (entries 6, 12, 13), indicating a smaller change in dipole moment associated with the M−H vibration. In a limited number of cases, no change or a slight red shift in ν(M−H) upon oxidation of a precursor complex was observed. For example, almost no change in ν(Co−H) was observed when CoH(dppe)2 (entry 16) was oxidized, while a 34 cm−1 red shift was observed in the case of CoH{P(OEt)3}4 (entry 17). A very slight red shift in the W−H 8439

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Table 8. IR Stretching Frequencies of M−H Bonds in TMHRCs entry

ν(M−H)/cm−1 of the diamagnetic precursor

ν(M−H) (cm−1)

TMHRC

1

[NbH(CO)2(dppe)2]•+

1900

1765

2

[TaH(CO)2(dppe)2]•+

1877

1754

3a

[Cp*Mo(PMe3)3H]•+

4

[Cp*MoH3(dppe)]•+

5b

[Cp*WH3(dppe)]•+

6 7 8 9 10 11 12

[WH2Cl2(PMe3)4][BF4]•Cp2Fe [WHCl2(PMe3)4(MeCN)][BF4]2 [Re2(μ-H)4H4(PPh3)4][PF6] [CpFe(dippe)H][BPh4] [Cp*Fe(dippe)H][BPh4] [Cp*Fe(dppe)H][PF6] [Cp*Ru(dppf)H]•+

1800, 1770 (equal intensity) 1824 (m, br), 1896 (w, sh) 1897 (m, br), 1830 (w, br) 1940 (m), 1920 (mw) 1950 (m), 1930 (sh) 1980 (w), 1900 (w) 1939 1915 1886 2000

1911 1901 1869 1944

13

[Cp*Ru(dippf)H]•+

2016

1968

14

[(Cym)Ru(dppf)H]•+

1975 (w)

1950 (w)

15 16 17 18

trans-[Os(en)2py(H)][OTf]2 [CoH(dppe)2][BPh4] [CoH{P(OEt)3}4][BPh4] [(NP3)CoH][ClO4] (NP3 = N(CH2CH2PPh2)3) [CoH{P(OEt)2Ph}4][PF6] [CoH{P(OEt)2Ph}4][BF4] [CoH{P(OMe)2Ph}4][PF6] [CoH{P(OMe)2Ph}4][BF4] [(triphos)Rh(μ-H)3Rh(triphos)] [BPh4]2•DMF [IrH(CO)(PPh3)3]•+

2020 1885 (br) 1930 1850 (w)

1884 (m)113 1964114

19

20 21

1947 1940 1951 1935 1655

ref

1794 (major), 1730 (minor)

67

1775, 1815

solution (THF)

97

1815, 1885

solution (THF)

97

1940 (s, br) 1960 (sh), 1920 (m, br)

Nujol Nujol Nujol Nujol Nujol Nujol spectroelectrochemistry (THF) spectroelectrochemistry (THF) spectroelectrochemistry (THF) KBr KBr KBr Nujol

49 49 31 74 74 71 103

78 100 100 51

Nujol

41

Nujol

64

spectroelectrochemistry (CH2Cl2)

61

(m) (w) (m) (w)

2104

method spectroelectrochemistry (CH2Cl2) spectroelectrochemistry (CH2Cl2) solution (THF and CH2Cl2)

2071

54 54

104 104

DFT investigations suggest that there are rotamers involving different orientations of the Cp* ring and the PMe3 ligands in [Cp*Mo(PMe3)3H]•+ and its diamagnetic precursor. The interconversions are very rapid on the NMR and EPR timescales but slow on the IR timescale.67 bThe strengthening of the W−H bond upon oxidation was confirmed by DFT calculations.97 a

Table 9. C−O Stretching Frequencies in TMHRCs ν (CO) /cm−1 of the diamagnetic precursor

entry

TMHRC

ν (CO) (cm−1)

1

[Fe2Cp2(μ-H)(μ-PPh2) (CO)2]•+ [W2Cp2(μ-H)(μ-PPh2) (CO)4]•+ [Mo2Cp2(μ-H)(μ-PPh2) (CO)4]•+ [IrH(CO)(PPh3)3]•+

trans isomer: 1998 (w, sh), 1982 (vs); cis isomer: 2020 (vs), 2003 (w, sh) 2043 (s), 2023 (w), 1989 (vs), 1969 (m, sh), 1947 (m, br) 2051 (vs), 2033 (w), 1998 (vs), 1965 (s,br)

trans isomer: 1912 (vs), cis isomer: 1950 (vs) 1959 (w), 1926 (vs), 1853 (s)

solution (CH2Cl2)

73

solution (CH2Cl2)

70

average 1910

solution (CH2Cl2)

70

1988

1924

spectroelectrochemistry (CH2Cl2)

61

2 3 4

The in situ formation of [MH(CO) (PPh3)3]•+ (M = Ir, Rh) at different temperatures has also been investigated with IR spectroelectrochemical experiments.59,61 C−O stretching frequencies have also been monitored in IR spectroelectrochemical experiments. For example, an optically transparent thin layer electrochemical (OTTLE) cell was used to monitor the oxidation of Co(CO)2(dppf)H in 0.1 M [Bu4N][PF6]/CH2Cl2 at 213 K. Formation of the 17-electron radical cation [Co(CO)2(dppf)H]•+ was indicated by the appearance of C−O stretching bands shifted to higher energies with lower intensities (Figure 12A). At 293 K, a follow-up

method

ref

reaction product [Co(CO)2(dppf)]+ was detected (Figure 12B).115 3.5. Other Characterization Methods

Other methods, although less commonly reported, have been used to characterize transition-metal hydride radical cations. These include measurements of magnetic moments and conductivity, techniques such as UV−vis, NMR, Mössbauer, and reflectance spectroscopies, as well as elemental analysis. Color changes and the observation of dichroism have been used as qualitative indicators. 3.5.1. Effective Magnetic Moment. The paramagnetism of a TMHRC may be characterized by its effective magnetic 8440

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Table 11. These results have been used to substantiate the formulation of the complexes as 1/1 salts. Table 11. Conductivity of TMHRC Complexes molar conductance λM (cm2 Ω−1 mol−1)

entry

complex

1 2 3

22 101 60−89

PhNO2 EtNO2 MeNO2

75 64 41

4

[FeHCl(dppe)2][ClO4] [(triphos)CoH(PEt3)][BPh4] [CoHL4][X] (L = P(OEt)2Ph, P(OMe)2Ph, or P(OPh)3; X = PF6 or BF4) [(triphos)CoH(PEt3)][BPh4]

solvent

ref

40

42

5 6 7

[HCo(dppe)2][BPh4] [CoH(P(OMe)3)4][BPh4] [CoH(P(OEt)3)4][BPh4]

76 95 86

common organic solvents MeCN MeCN MeCN

100 100 100

3.5.3. UV−vis Spectroscopy. Many TMHRCs have a distinctive color and can be characterized by UV−vis spectroscopy. However, unlike IR spectroscopy, UV−vis absorption data is only available for a limited number of complexes reported in the literature. An example from group 11 is the intensely green [(PPh3)CuH]6•+ formed in situ by the one-electron oxidation of [(PPh3)CuH]6, which is red. The formation of [(PPh3)CuH]6•+ was indicated by the appearance of an absorption peak at 655 nm during UV−vis spectroelectrochemistry (Figure 13).38

Figure 12. IR spectroelectrochemical oxidation of Co(CO)2(dppf)H at 213 K (A) and 293 K (B). The neutral hydride complex and the corresponding radical cation are indicated by black and green text, respectively. At 293 K (B), [Co(CO)2(dppf)]+ is indicated by red text. Reprinted with permission from ref 115. Copyright 2013 Wiley VCH.

moment, μeff. This dimensionless quantity is independent of temperature for substances that obey the Curie law. Faraday and Gouy balances have been used to determine the molar magnetic susceptibilities of solid samples, while the Evans method is convenient for measuring samples in solution. Such measurements are then used to calculate μeff. Selected examples for TMHRCs are summarized in Table 10. In all cases, the magnitude of μeff corresponds to the presence of one unpaired electron. 3.5.2. Conductivity. Conductivity measurements have been used to confirm the ionic nature of TMHRC salts. In these experiments, molar conductance λM (cm2 Ω−1 mol−1) is determined. TMHRCs with reported λM values are shown in Table 10. Magnetic Moments of TMHRCs

group

TMHRC

8

[CpFe(dippe)H]•+ [Cp*Fe(dippe)H]•+ trans-[Os(en)2pyH]•2+ [(triphos)Rh(μH)3Rh(triphos)] [BPh4]2•DMF [CoHL4][X] (L = P(OEt)2Ph, P(OMe)2Ph, or P(OPh)3; X = PF6 or BF4) [(triphos)CoH(PEt3)][BPh4] [HCo(dppe)2]•+ [(PP3)CoH][BF4] (PP3 = P(CH2CH2PPh2)3) [(NP3)CoH][ClO4] (NP3 = N(CH2CH2PPh2)3) [CoH(P(OMe)3)4]•+ [CoH(P(OEt)3)4]•+ [HCo(CP3)PEt3][BPh4] (CP3 = CH3C(CH2PPh2)3)

9

effective magnetic moment, μeff 2.40 2.65 1.8 (300 K) 2.20

Figure 13. Oxidation of [(PPh3)CuH]6 over 30 s at −0.8 V vs Fc/Fc+. Reprinted from ref 38. Copyright 2013 American Chemical Society. method

ref

Evans

74 78 64

1.96−2.4

Evans not specified (solid) Gouy

41

2.15 2.2 2.01

Faraday Evans Faraday

42 100 51

2.02

Faraday

51

1.9 2.0 2.15

Evans Evans not specified

100 100 34

UV−vis absorption spectra have also been used to indicate which metal center in Ru/Fe heterobimetallic complexes has been oxidized. Ru(III) centers (such as those in CpRuL2X complexes) should exhibit weak long wavelength (vis−NIR) ligand field (LF) transitions between occupied and unoccupied d orbitals. Ferrocenium systems (such as an oxidized dppf ligand) are known to have a weak absorption at about 600 nm due to a ligand-to-metal charge transfer (LMCT, 2E1u). UV− vis−NIR spectroelectrochemistry of the complexes [(Cym)Ru(dppf)Cl][PF6] and Cp*Ru(dppf)H demonstrates this well (Figure 14). Ferrocene oxidation in the Cym complex is indicated by the weak absorption with λmax of about 630 nm. Oxidation of the ruthenium center in the Cp* complex is accompanied by the appearance of an absorbance with a λmax of approximately 900 nm.103 3.5.4. Nuclear Magnetic Resonance (NMR) Spectroscopy. As paramagnetic complexes, TMHRCs often feature no 8441

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Fe(III) complex and exhibits an unsymmetrical doublet due to the magnetic relaxation phenomenon at the Fe(III) center (4.2 K, IS = 0.304 mm/s vs Fe, QS = 0.715 mm/s).99 The parameters for [Cp*Fe(dppe)H][PF6] are distinctly different (4.2 K, IS = 0.260 mm/s vs Fe, QS = 0.84 mm/s).71 The ligand field symmetry is significantly changed by CO binding at the Fe(III) center.99 3.5.6. Elemental Analysis. Some isolated TMHRC salts have been characterized by elemental analysis. Examples include trans-[Cp*Re(PMe3)2XH][SbF6] (X = H, Cl),98 [(PP3)CoH]X (PP3 = P(CH2CH2PPh2)3; X = BF4, ClO4),51 [(NP3)CoH]ClO4 (NP3 = N(CH2CH2PPh2)3),51 [HCo(dppe)2][BPh4],100 [CoH(P(OR)3)4][BPh4] (R = Me, Et),100 [CpFe(dippe)H][BPh4],74 and [Cp*Fe(dippe)H][BPh4].74 3.5.7. Qualitative Characteristics and Dichroism. A distinct color change is often observed when diamagnetic transition-metal hydride complexes are oxidized. In some cases, this has been studied by UV−vis spectroscopy (section 3.5.3). Such color changes are also useful qualitative indicators, such as [(PPh3)CuH]6•+ (green) versus [(PPh3)CuH]6 (red).38 A dichloromethane solution of {([P2N2]Ta)2(μ-H)4}+I− was reported to be dichroic, with transmitted light appearing bluegreen and reflected light appearing red. The authors noted that solutions containing the ferrocenium ion, Cp2Fe+, also display a blue-green/red dichroism.63 Figure 14. Comparison of UV−vis−NIR spectra from one-electron oxidation of compounds Cp*Ru(dppf)H (top) and [(Cym)Ru(dppf)Cl][PF6] (bottom): spectroelectrochemistry in 0.1 M [Bu4N][PF6]/ THF. Reprinted with permission from ref 103. Copyright 2000 Elsevier.

4. THERMODYNAMICS TMHRCs are an indispensable part of comprehensive thermodynamic cycles for transition-metal hydride complexes.33,34 The thermodynamics of H+, H•, and H− transfers from hydride radical cations may be used to explain or predict reactivity. The thermodynamic data for HCo(dppe)2 and derivatives in Table 12 provide several good examples. The two

discernible NMR signals or only broad-shifted resonances. The few examples with reported 1H NMR spectra are {([P2N2]Ta)2(μ-H)4}+I−,63 [W(PMe3)4Cl2H2][BF4]•Cp2Fe,49 [W(PMe3)4Cl(MeCN)H2][BF4]2,49 [(C5H4CTol3)Ru(PPh3)2H][PF 6 ], 77 [Cp*Ru(dppf)H][PF 6 ], 76 trans-[Os(en) 2 pyH][OTf] 2 , 78 [Fe(dppe) 2 ClH][X] (X = BF 4 or ClO 4 ), 75 [CoHL4][X] (L = P(OMe)2Ph, P(OEt)2Ph or P(OPh)3; X = PF 6 or BF 4 ), 4 1 [RhH(CO) (PPh 3 ) 3 ][BF 4 ], 1 1 6 and [Rh6(PCy3)6Hn][BArF4] (n = 12 or 14).85 In all cases, the 1H NMR spectra were essentially featureless. The 31P{1H} NMR spectra of [Cp*Ru(dppf)H][PF6],76 and [Rh6(PCy3)6Hn][BArF4] (n = 12 or 14)85 were also reported to be featureless. NMR has also been used to study electron exchange between a TMHRC and its diamagnetic precursor. Electrolysis was used to generate different ratios of RhH(CO) (PPh3)3 and [RhH(CO) (PPh3)3]•+ in solution. When the radical cation constituted 10−20% of the mixture, the 1H NMR hydride resonance was significantly broadened. When the radical cation was present at 40% or greater, no resonance was discernible. This result confirmed fast electron self-exchange between the paramagnetic 17-electron Rh(II) cation and its neutral Rh(I) parent complex.116 Rapid electron self-exchange was also observed between Cp*Ru(dppf)H and [Cp*Ru(dppf)H]•+ as evidenced by broadening of the 1H NMR hydride resonance at room temperature. Cooling the sample to −42.8 °C slowed the self-exchange and sharpened the resonance markedly.117 3.5.5. Mö ssbauer Spectroscopy. Mössbauer spectroscopy is frequently used to characterize iron-containing complexes. The signal pattern provides information about coordination environment and iron oxidation state. Several iron-containing TMHRCs were studied by this method. The Mössbauer spectrum of [Cp*Fe(dppe) (CO)H][PF6] is characteristic of a

Table 12. Thermodynamic Data (ΔG° Values in kcal/mol) for Cobalt-Hydride Derivativesa complex +

[H2Co(dppe)2] HCo(dppe)2 [HCo(dppe)2]+ [HCo(dppe)2(CH3CN)]2+

pKa

ΔGoH+

ΔGoH•

ΔGoH-

22.8 38.1 23.6 11.3

31.3 52.3 32.4 15.5

58.0 59.1 49.9 52.9

65.1 49.1 59.7

a

Reproduced from ref 34. Copyright 2002 American Chemical Society.

successive homolytic bond cleavage reactions of [H2Co(dppe)2]+ require 58.0 and 49.9 kcal/mol, respectively (Scheme 6). These values indicate that disproportionation of the radical Scheme 6. Successive H• Loss from [H2Co(dppe)2]+

cation [HCo(dppe)2]•+ is favorable by about 8 kcal/mol, in agreement with the observed reactivity (eq 6). Additionally, the sum of these same ΔG°H• values is 107.9 kcal/mol, greater than the free energy of homolytic H2 cleavage in acetonitrile (103.6 kcal/mol). This is consistent with the observed reaction of [Co(dppe)2]+ with H2 to form [H2Co(dppe)2]+.34 Before the various reaction pathways of TMHRCs are examined (section 8442

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Scheme 7. Molecular Orbital Diagrams of M−H Bonds

5), it is necessary and appropriate to review what is known about the underlying thermodynamics.

decrease) is greatest for complexes that are positively charged before oxidation and for those that contain electron-withdrawing ligands.118

4.1. Acidity (pKa)

It is commonly accepted that saturated transition-metal hydride complexes have a Mδ+−Hδ− bond polarity.27 Removal of one electron from the M−H bonding orbital lowers the energy of the metal orbital participating in M−H bonding. This change tends to reduce the hydridic character of the hydride ligand (Scheme 7a) and could even lead to a polarity inversion, where the hydride carries a partial positive charge in the ground state (Scheme 7b).27 The thermodynamic cycle in Scheme 8 has been used to estimate the effect of one-electron oxidation on the

Δ(pKa) = pKa(MH) − pKa(MH•+) =

F 2.3RT

[E o(MH/MH•+) − E o(M−/M•)]

(8)

Notably, in most cases (entries 1−12, 20) the calculated Δ(pKa) values depend on chemically irreversible oxidations of the metal hydrides. An irreversible oxidation peak (Epa) occurs at a more positive potential than the thermodynamic potential, which would be approximated by E1/2 if the system were reversible. This results in a larger ΔE°, as the difference is calculated as E°(MH/MH•+) − E°(M−/M•). In these cases, the resulting pKa(MH•+) values are lower and the Δ(pKa) data therefore represent upper limits for the acidity enhancements. In all other cases (entries 13−19, 21−23119,120), the calculated Δ(pKa) values are based on chemically reversible oxidation potentials.

Scheme 8. Thermodynamic Cycle for the Determination of pKa(MH•+)

4.2. Homolytic Cleavage of the M−H Bond

The energy associated with the homolytic cleavage of the M−H bond in a TMHRC is proportional to the orbital overlap and inversely proportional to the energy difference [E1−E2] between the two bonding orbitals. Oxidation lowers the energy of the metal bonding orbital. The energy difference [E1 − E2] should therefore decrease resulting in a stronger M−H bond (Scheme 7a). However, if the bond polarity were inverted to begin with, such a change would decrease the bond strength as the value of [E1 − E2] would then be greater (Scheme 7b). Oxidation should also contract the metal bonding orbital leading to a decrease in orbital overlap. Although a decrease in the metal covalent radius also allows the hydrogen ligand to approach more closely, with the opposite effect. These conflicting phenomena make it difficult to predict the overall effect. The bond dissociation free energy (BDFE, ΔGoH•) of the M−H bond in a TMHRC can be determined by the thermodynamic cycle shown in Scheme 9. This thermodynamic cycle, first recognized by Breslow and Chu, has often been used to estimate bond strengths that are not directly measurable.121−123 The accurate determination of the metal−hydrogen BDFE of a TMHRC requires (1) the BDFE of the corresponding diamagnetic precursor which can be calculated based on eq 9,121−124 (2) the oxidation potential of the diamagnetic precursor, and (3) the oxidation potential of the radical formed by homolytic M−H cleavage of the diamagnetic

thermodynamic acidities of metal hydrides. The acidities (pKa values) of a metal hydride radical cation and its diamagnetic precursor are related by eq 7. Indirect determination of pKa(MH•+) requires measurement or knowledge of pKa(MH) as well as the oxidation potentials of MH and the conjugate base M−. pKa(MH•+) = pKa(MH) −

F [E o(MH/MH•+) 2.3RT

− E o(M−/M•)]

(7)

The pKa values of many TMHRCs have been estimated using this thermodynamic cycle, as shown in Table 13. Oneelectron oxidation generally causes acidity to increase by 15−30 pKa units, regardless of the identity of the metal.118 These large reductions in pKa are a direct consequence of the fact that it is generally more difficult to oxidize a metal hydride than its conjugate base (eq 7). In cases where the pKa of the diamagnetic precursor is not known, the difference in acidity, Δ(pKa) = pKa(MH) − pKa(MH•+) (eq 8), may still be determined using the oxidation potentials of the precursor and its conjugate base (entries 14−20). The acidity increase (pKa 8443

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Table 13. pKa Data for TMHRCs and Their Diamagnetic Precursors TMHRC

pKa(MH•+)

pKa(MH)

ΔpKa

ref

[CpCr(CO)3H] [CpCr(CO)2(P(OMe)3)H]•+ [CpCr(CO)2(PEt3)H]•+ [CpCr(CO)2(PPh3)H]•+ [Cp*Cr(CO)3H]•+ [CpMo(CO)3H]•+ [TpMo(CO)3H]•+ (Tp = hydridotris(pyrazoly1)borate) [Tp′Mo(CO)3H]•+ (Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate) [CpW(CO)3H]•+ [CpW(CO)2(PMe3)H]•+ [TpW(CO)3H]•+ [Tp′W(CO)3H]•+ [CpW(CO)2(IMes)H]•+ (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) trans-[Cr(CO)2(dppm)2H]•2+ trans-[Cr(CO)2(dppe)2H]•2+ trans-[Mo(CO)2(dppm)2H]•2+ trans-[Mo(CO)2(dppe)2H]•2+ trans-[W(CO)2(dppm)2H]•2+ trans-[W(CO)2(dppe)2H]•2+ [CpRu(CO)(PPh3)H]•+ [HCo(dppe)2]•+ [HRh(depx)2]•+ (depx = α,α′-bis(diethylphosphino)xylene) [HPd(EtXanthphos)2]•2+

−9.5 −2.4 0.5 −2.1 −7.2 −6.0 −8.2 −9.5 −3.0 5.1 −5.4 −6.7 6.3

13.3 21.1 25.8 21.8 16.1 13.9 10.7 9.7 16.1 26.6 14.4 12.9 31.9

23.6 33.5 7.1

38.1 51.5 27.3

22.8 23.5 25.3 23.9 23.3 19.9 18.9 19.2 19.1 21.5 19.8 19.6 25.6 30.0 29.5 27.9 26.2 28.8 25.4 24.8 14.5 18.0 20.2

29 30 30 30 30 29 28 28 29 29 28 28 33 47 47 47 47 47 47 112 34 119 120

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

•+

Scheme 9. Thermodynamic Cycle for the Determination of the M−H BDFE in a TMHRC

Table 14. BDE Data (kcal/mol) for TMHRCs and Their Diamagnetic Precursors entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14

precursor (eq 10). In cases where the BDFE of the diamagnetic precursor is not known, the ΔBDFE may be estimated from electrochemical measurements alone. BDFE(MH) = 1.37pKa + 23.06E oox (M−) + 53.6 kcal/mol

(9)

TMHRC [CpCr(CO)3H]•+ [CpCr(CO)2(P(OMe)3) H]•+ [CpCr(CO)2(PEt3)H]•+ [CpCr(CO)2(PPh3)H]•+ [Cp*Cr(CO)3H]•+ [TpMo(CO)3H]•+ [Tp′Mo(CO)3H]•+ [TpW(CO)3H]•+ [Tp′W(CO)3H]•+ [CpW(CO)2(IMes)H]•+ [Cp*Fe(dppe)H]•+ [HCo(dppe)2]•+ [HRh(depx)2]•+ [HPd(EtXanthphos)2]•2+

BDE(M− H•+)

BDE(M− H)

ΔBDE

ref

55.7 51.6

61.4 62.6

5.7 11

29 30

50.9 49.7 54.2 55.7 53.3 57.6 55.2 51.0

60.0 59.7 62.4 62.1 59.3 65.7 62.1 59.3

54.7 53.5 49.9

64.1 75.5 74.1

9.1 10 8.2 6.4 6.0 8.1 6.9 8.3 12.2 9.4 22 24.2

30 30 30 28 28 28 28 33 125 34 119 120

BDFE(MH•+) = BDFE(MH) − FE°(MH/MH•+) + FE°(M•/M+)

In entries 1−9 at least one of the potentials used in the determinations was obtained from a chemically irreversible process. The data in entries 10−14 were determined entirely from reversible cyclic voltammograms. The data indicate that oxidation weakens the M−H bond with respect to homolytic cleavage, although this weakening is far less extreme than the increase in acidity described in section 4.1. This applies to the oxidation of neutral as well as cationic (entry 14) diamagnetic metal hydrides. On the basis of eqs 7 and 10, the relative magnitudes of the change in acidity [ΔΔG°H+ = 2.3RTΔ(pKa)] and the change in homolytic bond dissociation free energy [ΔΔG°H• = Δ(BDFE)] are related by eq 13. This equation shows that the change in acidity should be greater than the change in BDFE upon oxidation provided the anion (M−) is easier to oxidize than the radical (M•). Indeed, this should be the case unless

(10)

The bond dissociation enthalpy (BDE, ΔHoH•) values for many TMHRCs have been determined using eq 11, as shown in Table 14. The uncertainty of BDE is larger that that of BDFE mostly due to uncertainty in the entropy of solvation of H•. However, use of the quantity ΔBDFE (BDFE(M−H) − BDFE(M−H•+), which is equal to ΔBDE (BDE(M−H) − BDE(M−H•+)), results in cancellation of these systematic errors (eq 12). BDE(MH) = 1.37pKa + 23.06E°ox (M−) + 58.6 kcal/mol

(11)

ΔBDFE = ΔBDE = F[E°(MH/MH•+) − E°(M •/M +)] (12) 8444

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significant structural changes occur as a consequence of M− oxidation. As a result, oxidation activates M−H bonds with respect to H+ loss much more so than with respect to H• loss.30

According to eq 14, the radical cation MH•+ will be less hydridic (have larger ΔG°H−) if the MH compound is oxidized more easily than the cation M+, which is in the same formal oxidation state but possesses a positive charge. This should generally be the case, since the loss of H− from MH involves separation of singly charged species, while the loss of H− from MH•+ requires separation of the hydride anion from a doubly charged cation.71 There are several cases where the hydricity of a TMHRC has been determined using a thermodynamic cycle. Using eq 12, the ΔG°H− of [CpW(CO)2(IMes)H]•+ (IMes =1,3-bis(2,4,6trimethylphenyl)imidazole-2-ylidene) is found to be 68 kcal/ mol. The corresponding value for the neutral hydride complex is 57 kcal/mol, indicating that it is a much better hydride donor than the radical cation.40 The values for [HCo(dppe)2]•+ and HCo(dppe)2 are 59.7 and 49.1 kcal/mol, respectively, differing by nearly the same amount.34

ΔΔG Ho + − ΔΔGHo • = F[E°(M −/M •) − E°(M •/M +)] (13)

The weakening of the M−H bond implied by the data in Table 3 directly contradicts the implied strengthening of this bond implied by the IR spectra of TMHRCs and their diamagnetic precursors (section 3.4). As described previously, a blue shift in ν(M−H) has often been observed upon oneelectron oxidation, indicating bond strengthening. This is the case for Cp*Fe(dppe)H,125 although the electrochemical data indicate bond weakening when the radical cation is formed.27,71 A weakening of the Fe−H bond upon one-electron oxidation was also indicated by the DFT calculations.71 Poli has noted that the effect of the medium needs to be considered when using the thermodynamic cycle in Scheme 9. He has suggested that it is very likely for a 16-electron cation (M+) to establish strong interactions with the polar solvent or the supporting electrolyte used for such electrochemical measurements. If this interaction with the medium were stronger for M+ than for MH•+, this would cause the M•/M+ oxidation to occur more easily in comparison to the MH/MH•+ oxidation. This would skew the calculated ΔBDFE to indicate a greater bond weakening upon oxidation.97 To probe the possible involvement of solvent and ion pairing effects, Tilset performed a thorough study of how solvent and electrolyte affect measured potentials. He concluded that the potentials are not strongly affected by medium effects and that the oxidatively induced BDE weakening is indeed real. However, ion pairing is much more likely to influence dications. This will affect the ΔBDE determinations involving dications but not the results of calculations involving monocations and neutral complexes.71 So, it is evident that the effect of oneelectron oxidation on the M−H BDE is far from straightforward. The results obtained for specific systems therefore do not strongly support any generalization.27

5. REACTIVITY With respect to reactivity, TMHRCs usually behave very differently than their unoxidized diamagnetic precursors. The generally limited stability of the radical cations has hindered systematic studies of their reactivity. As a result, thorough mechanistic studies involving reactivity pathways for these species are less common. Due to their greatly enhanced acidity, TMHRCs generated in solution are frequently deprotonated by either the unoxidized precursor hydride complex or a separate external base. However, immediate deprotonation is not the only possible reaction pathway. When the radical cations contain sufficiently electron-donating or sterically bulky ligands, further oxidation paired with solvent coordination is possible, to give [MH(S)]2+ species. These diamagnetic dications, being quite acidic, typically lose H+, resulting in formation of [M(S)]+ complexes. Other modes of reactivity for TMHRCs include disproportionation, atom transfer, reductive elimination, hydride transfer, and ligand association/dissociation, as described in the following sections. 5.1. Proton Transfer

4.3. Thermodynamic Hydricity of the M−H Bond

In accordance with the greatly enhanced acidity of the hydride ligand upon one-electron oxidation of a diamagnetic metal hydride precursor (section 4.1), rapid deprotonation of the resulting TMHRC is commonly observed. Electrochemical one-electron oxidation of Cp′2NbH3 (Cp′ = Me3SiC5H4) gives the radical cation intermediate [Cp′2NbH3]•+, which undergoes rapid proton loss to the medium at ambient temperature. The resulting neutral radical [Cp′2NbH2]• then rapidly dimerizes (Scheme 11).88

Removing an electron from the M−H bonding orbital reduces the hydridic character of the hydride ligand (Scheme 7). The hydricity (ΔG°H−) of a TMHRC can be calculated using the thermodynamic cycle in Scheme 10 and the resulting eq 14. Scheme 10. Thermodynamic Cycle for the Hydricity Determination of TMHRCs

Scheme 11. Decomposition of [Cp′2NbH3]•+

The hydricity and oxidation potential of the diamagnetic precursor, along with the oxidation potential of the metal cation fragment (M+) formed after H− transfer from the precursor, must be known. The hydride transfer processes are usually accompanied by solvation.

In a constant potential electrolysis experiment (+0.92 V vs Cp 2 Fe + /Cp 2 Fe), one-electron oxidation of trans-[Mo(CO)2(dppe)2H]+ is followed by rapid proton loss to the medium. The resulting monocation trans-[Mo(CO)2(dppe)2]+ is readily oxidized at the same potential, giving the dication [Mo(CO)2(dppe)2]2+, which then reacts further to give the final unidentified products (Scheme 12).47 This is an example

ΔG°H−(MH•+) = ΔG°H−(MH) + F[E°(M+/M2 +) − E°(MH/MH•+)]

(14) 8445

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Scheme 12. Electrochemical Oxidation of trans[Mo(CO)2(dppe)2H]+

Scheme 14. Electrochemical Oxidation of [ReClH(RCN) (dppe)2]+

of an ECE mechanism. Such mechanisms involve an initial electron transfer (E), a subsequent chemical reaction (C), and then an electron transfer step involving the product of the reaction (E). The electron transfer steps occur at the electrode, whereas the chemical reaction step does not involve the electrode. Similar ECE reactivity patterns have been observed in the electrochemical oxidations of Cp′M(CO)2LH (M = Mo, W; Cp′ = Cp, C5(CH2Ph)5; L = CO, PPh3),89,108 [FeH(MeCN) (dppe)2]+,90 trans-[FeH(LL) (dppe)2][BF4] (LL = NCCHCHCN, NCC6H4CN or NCCH2CH2CN),126 and others. Electrochemical oxidation (E1) of the dinuclear iron complex [{FeH(dppe)2}2(μ-LL)][BF4]2 (LL = NCCHCHCN) involves proton loss and additional follow-up reactivity with BF4−, ultimately leading to an Fe−F/Fe-H trication (Scheme 13, left). Further oxidation of this trication (E2) also leads to proton loss, in this case forming a Fe(I)/Fe(I) dication (Scheme 13, right). Subsequent oxidations in the same potential range and reaction of the iron centers with BF4− lead to the symmetrical Fe−F/ Fe−F dication.126 The detailed electrochemical behavior was rationalized by ab initio calculations on model compounds and oxidized derivatives.126 Instead of ECE, a mechanism shown in Scheme 14 was proposed to occur during the electrochemical oxidation of [ReClH(RCN) (dppe)2]+ (R = Ph, 4-ClC6H4 or 4-FC6H4). In this mechanism, oxidation of the later intermediate [ReCl(RCN) (dppe)2]•+ occurs by electron transfer to the initially formed radical dication [ReClH(RCN) (dppe)2]•2+. This explains the observation that at increased analyte concentration, more rapid scan rates are required to obtain chemically reversible voltammograms.48 When proton loss occurs after formation of a TMHRC, the overall reaction stoichiometry and product distribution are highly dependent on the oxidation method, the identity of the base that removes the proton, the proton transfer rate, other

follow-up reactions, and the solvent.57 For example, when CpRu(CO) (PMe3)H was oxidized by Cp2Fe+ in acetonitrile, the product distribution was determined by the rate of deprotonation of [CpRu(CO) (PMe3)H]•+. Without external bases, the overall reaction gave the products shown in eq 15.91 In theory, this process requires just 0.67 equiv of oxidant. Scheme 15 shows the proposed mechanism, where H+ is Scheme 15. Decomposition of [CpRu(CO)(PMe3)H]•+

transferred from the initially formed radical cation to the precursor hydride complex. The resulting neutral radical [CpRu(CO) (PMe3)]• also reacts with the radical cation, giving the bridging hydride complex [CpRu(CO) (PMe3)]2(μH)+. +2Cp2Fe+

3CpRu(CO)(PMe3)H⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ −2Cp2Fe

[CpRu(CO)(PMe3)]2 (μ‐H)+ + CpRu(CO)(PMe3)(η2‐H 2)+

(15)

In the presence of a reasonably strong external base (2,6lutidine, the pKa of its conjugate acid is 15.4 in acetonitrile),127 a different process occurs giving the bridging hydride complex [CpRu(CO) (PMe3)]2(μ-H)+ exclusively (eq 16).91 In this case, the 2,6-lutidine partially deprotonates the initially formed

Scheme 13. Proposed Mechanism for the Anodic Processes of the Complex [{FeH(dppe)2}(LL)][BF4]2 (LL = NCCHCHCN, Curved Line Represents ligand LL)a

a

Reproduced from ref 126. Copyright 2002 American Chemical Society. 8446

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radical cation (Scheme 16). Unreacted [CpRu(CO) (PMe3)H]•+ then combines with the neutral radical [CpRu(CO)

Scheme 18. Electrochemical Oxidation of CpRu(CO) (PMe3)H in Acetonitrile

Scheme 16. Decomposition of [CpRu(CO)(PMe3)H]•+ in the Presence of 2,6-Lutidine

In the presence of either 2,6-lutidine or pyrrolidine, the electrochemical oxidation and follow-up reactions were the same as those shown in eq 17. This may be explained by efficient deprotonation of [CpRu(CO) (PPh3)H]•+ by both bases under these particular conditions, providing the intermediate [CpRu(CO) (PMe3)]•. In the case of 2,6-lutidine, which was not completely effective in deprotonating the radical cation in Scheme 16, the reaction may be driven by the irreversible oxidation of [CpRu(CO) (PMe3)]• to give the final product [CpRu(CO) (PPh3) (MeCN)]+.91 It is clear from the examples above that when a TMHRC decomposes by a pathway involving deprotonation, the nature of the base receiving the proton strongly influences the overall reaction stoichiometry and product distribution. 5.1.1. Deprotonation by External Base. When a TMHRC is deprotonated by an external base (eq 19), the resulting neutral radical may undergo a number of reactions. The radical may react with the initially formed TMHRC (eq 20) or dimerize (eq 21). It may undergo subsequent oxidation, which is usually paired with solvent coordination (eqs 22 and 23).

(PMe3)]• to give the end product. Overall, one equivalent of oxidant is consumed. +2Cp2Fe+

2CpRu(CO)(PMe3)H⎯⎯⎯⎯⎯⎯⎯⎯⎯→ −2Cp2Fe

[CpRu(CO)(PMe3)]2 (μH)+ + H+

(16)

In the presence of a stronger base (pyrrolidine, the pKa of its conjugate acid is 19.6 in acetonitrile),128 two equivalents of oxidant are consumed and the solvent complex [CpRu(CO) (PMe3) (MeCN)]+ is formed (eq 17).91 In this reaction, the initially formed radical cation is deprotonated completely by pyrrolidine, cleanly forming the intermediate radical [CpRu(CO) (PMe3)]• (Scheme 17). Oxidation of the radical in the presence of acetonitrile gives the final product. +MeCN

(CO)(PMe3)H + 2Cp2Fe+⎯⎯⎯⎯⎯⎯⎯⎯→ −2Cp2Fe

CpRuCpRu(CO)(PMe3)(NCMe)+ + H+

(17)

Scheme 17. Decomposition of [CpRu(CO)(PMe3)H]•+ in the Presence of Pyrrolidine

[MH]•+ + B → M•+BH+ •

•+

M +[MH]

(19)

→ [M(μH)M]

+

(20)

M +M → MM

(21)

M•+S ⇌ M(S)•

(22)

•

•

−e

−

M(S)• ⎯⎯⎯→ [M(S)]+

(23)

Different follow-up reactions result in different overall oxidation stoichiometries. For example, if eqs 20 or 21 occur, only one equivalent of oxidant is required. If eqs 22 and 23 happen, two equivalents of oxidant are needed. The basicity of the external base and the metal−metal bond strength largely determine which particular follow-up reactions occur. The external base may be the medium, an intentionally added compound such as an amine, or the diamagnetic precursor hydride complex. Examples involving the latter are described in section 5.1.2. Dimerization of the radical [CpMo(CO)3]• occurs when CpMo(CO)3H is oxidized electrochemically, resulting in the formation of [CpMo(CO)3]2 (eq 24).56 The radical [Cp2WH]• combines with [Cp2WH2]•+ to generate [(Cp2W)2H3]+ during the electrochemical oxidation of Cp2WH2 (eq 25).56 Both of these processes (eqs 24 and 25) are attributed to the rapid loss of H+ to the medium and the formation of an intermediate neutral paramagnetic species.

The electrochemical oxidation of CpRu(CO) (PMe3)H in acetonitrile (in the absence of added bases) proceeded differently still, by a process corresponding to eq 18.91 This is consistent with proton transfer from [CpRu(CO) (PMe3)H]•+ to CpRu(CO) (PMe3)H. Oxidation of the neutral radical [CpRu(CO) (PMe3)]• and reaction with the solvent gives [CpRu(CO) (PMe3) (MeCN)]+. The intermediate dihydrogen complex [CpRu(CO) (PMe3)(η2-H2)]+ undergoes decomplexation and reaction with the solvent to form [HRu(CO) (PMe3) (MeCN)3]+ and C5H6 (Scheme 18). −2e−

2CpRu(CO)(PMe3)H⎯⎯⎯⎯⎯⎯→ MeCN

CpRu(CO)(PMe3)(NCMe)+ + HRu(CO)(PMe3)(NCMe)3+ + C5H6

(18) 8447

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2CpMo(CO)3 H − 2e− → [CpMo(CO)3 ]2 + 2H+

(24)

2Cp2 WH 2 − 2e− → (Cp2W)2 H3+ + H+

(25)

Scheme 19. Decomposition of [(PiPr3)2OsH6]•+a

Subsequent oxidation after the deprotonation is also commonly observed. For example, electrochemical oxidation of (dmpe)2(CO)2TaH (dmpe =1,2-bis(dimethylphosphino)ethane) in the presence of excess base, [Bu4N][OH], causes the oxidation to be chemically irreversible. The anodic peak current is increased by a factor of 2. This is consistent with deprotonation of the radical cation followed by further oxidation of the resulting neutral radical in the same potential range.56 Similar ECE processes were also observed in the Cp 2 Fe + oxidations of H 3 Ir(PMe 2 Ph) 3 ,95 [FeH(NCMe) (dppe)2]+,90 CpRu(PPh3)2H,129 Cp*Ru(dppf)H,76 Cp*Ru(PPh3)(H)3,130 M2Cp2(μ-H)(μ-PPh2) (CO)4 (M = Mo, W),70 and in other cases. In these systems, the addition of external base changes the one-electron process to a twoelectron process. When Cp2W(Ph)H is oxidized electrochemically, the returning cathodic wave of the cyclic voltammogram decreases linearly as the [Bu4N][OH] concentration increases. This is due to depletion of the radical cation by deprotonation. The peak current of the anodic wave remains constant, indicating that the radical [Cp2WPh]• is not oxidized at the same potential, in contrast to the other cases described above.56 5.1.2. Deprotonation by the Diamagnetic Precursor. When a TMHRC is formed by oxidation, the unoxidized diamagnetic precursor can also serve as a base, deprotonating the TMHRC as shown in eq 26. Here, both the metal center and the hydride ligand are possible protonation sites. Although, it has been shown that generally the hydride ligand has a greater kinetic basicity.131 In some cases, the dihydrogen complexes formed by protonation of transition-metal hydrides are stable. In others, rearrangement to form dihydride complexes occurs. The elimination of H2 is also quite common. The neutral radical formed after deprotonation of the TMHRC is still available to undergo the follow-up reactions in eqs 20−23. On the basis of these pathways, the overall oxidation stoichiometry, the number of equivalents of oxidant required, could be 0.5, 0.67, or one. [MH]•+ + MH → M•+[MH 2]+

a

Reproduced from ref 93. Copyright 1995 American Chemical Society.

Scheme 20. Decomposition of [CpMoH(CO)2L]•+

neutral radical is oxidized with coordination of solvent to give the final product, [CpMo(CO)2L(MeCN)]+. Concurrently, the dihydrogen complex formed by the initial proton transfer evolves H2 and coordinates solvent to give more of the same cationic acetonitrile complex. Similar reaction pathways have also been proposed for the oxidations of CpRu(PPh3)2H (the RuH2+ complex is stable),58 Cp*Ru(PPh3)H3,130 and facH3Ir(PMe2Ph)3 (the resulting [H4Ir(PMe2Ph)3]+ is stable).95 2CpMoH(CO)2 L + 2Cp2 Fe+ +2MeCN

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2[CpMo(CO)2 L(MeCN)]+ + H 2 −2Cp2Fe

(26)

(27)

In the above reactions, coordination of acetonitrile to the 17electron radical M• is believed to promote further oxidation of the intermediate 19-electron solvent adduct. In noncoordinating solvents, subsequent oxidation of M• formed after deprotonation may not occur, resulting in different end products. For example, the chemical oxidation of Cp*Ir(PPh3)H2 in acetonitrile results in equimolar quantities of [Cp*Ir(PPh3)(H)3]+ and [Cp*Ir(PPh3) (MeCN)H]+. In dichloromethane, [Cp*Ir(PPh3)(H)]2(μ-H)+ and [Cp*Ir(PPh3)(H)3]+ are formed in equal amounts instead (eq 28). In the absence of a coordinating solvent to facilitate oxidation, the radical [Cp*Ir(PPh3)H]• reacts with remaining [Cp*Ir(PPh3)H2]•+ to give the observed products (Scheme 21).94

In the [AcFc][BF4] (AcFc = acetylferrocene, E1/2 = +0.25 V versus Cp2Fe+/Cp2Fe) oxidation of (PiPr3)2OsH6, the primary reaction of the resulting radical cation is transfer of a proton to the diamagnetic precursor, forming [(PiPr3)2OsH5]• and [(PiPr3)2OsH7]+ (Scheme 19).93 Formation of a Os(VIII) center in the latter complex is avoided by the presence of one or two dihydrogen ligands, in either [(PiPr3)2OsH5(H2)]+ or [(PiPr3)2OsH3(H2)2]+. In dichloromethane, [(PiPr3)2OsH7]+ was directly observed as an oxidation product. In acetonitrile, this complex undergoes loss of four hydrogen atoms, as two H2 molecules, resulting in the observed product [(PiPr3)2Os(MeCN)2H3]+. The radical [(PiPr3)2OsH5]• undergoes a further oxidation along with acetonitrile coordination to form [(PiPr3)2Os(MeCN)H5]+, which appears to be a plausible intermediate in the ultimate formation of the end product. Another example is the ferrocenium oxidation of CpMoH(CO)2L (L = PPh3, PMe3) in dry acetonitrile, which results in the formation of [CpMo(CO)2L(MeCN)]+ with H2 evolution (eq 27). The newly formed radical cation is deprotonated by the diamagnetic precursor (Scheme 20).108 The resulting

+2Cp2Fe+

3Cp*Ir(PPh3)H 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Cp*Ir(PPh3)H]2 (μ‐H)+ −2Cp2Fe

+ [Cp*Ir(PPh3)H3]+

(28)

5.1.3. Effect of Water. Sometimes, different oxidation stoichiometries have been observed for chemical and electrochemical oxidations. This could be explained by the presence of 8448

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Scheme 21. Decomposition of [Cp*Ir(PPh3)H2]•+ in the Absence of a Coordinating Solvent

competition between oxidation and proton shuttle rates. In contrast, related complexes that are more basic such as CpRuH(CO) (PMe3) and CpRuH(PPh3)2 require only one F/mol.58,91 These complexes are basic enough to rapidly remove a proton from H3O+ or from the hydride radical cations directly. In certain cases, water can also participate in the decomposition pathways of oxidized hydride complexes by acting as a ligand. This can cause the formation of higher oxidation state hydroxo and oxo complexes. In Scheme 23, all

trace water in the electrochemical experiments, which can serve as a base and “proton shuttle.” Water is difficult to avoid in electrolysis experiments, given the large amount of supporting electrolyte added to the solutions. It has been reported that electrolytic solutions may be dried by passage through a column of activated alumina,132 although a small amount of water sill appears to be present. Bulk electrolysis studies involving CpMo(CO)2(PR3)H (R = Me, Ph) provide a good example of the competition between water and the starting hydride complex as bases (Scheme 22).108 Solutions of the complex were electrolyzed by

Scheme 23. Decomposition of [CpMoH(PMe3)3]•+a

Scheme 22. Electrochemical Oxidation of CpMoH(CO)2L in the Presence of Watera

a Reproduced from ref 108. Copyright 1996 American Chemical Society. a

The compound numbers in this scheme refer to those appearing in the original article. Reproduced with permission from ref 133. Copyright 1999 Royal Society of Chemistry.

application of a constant current with a galvanostat. Here, other experiments were used to verify that water is the stronger base. However, since the basicity difference is not particularly large, a small amount of the protonated hydride complex [CpMo(CO)2(PR3)H2]+ is formed. This complex is unstable and irreversibly decomposes with H2 evolution. Protons are slowly transferred from the stronger base (water) to the weaker one (the hydride complex); the water effectively serves as a proton shuttle. When the oxidation occurs slowly, the shuttle has enough time to operate, and just one F/mol is transferred. Fast oxidation results in two F/mol being transferred. In both cases, the end product is [CpMo(CO)2(PR3)(MeCN)]+. When a stronger base such as 2,6-lutidine is added, two F/mol is transferred even when the oxidation occurs slowly, in agreement with the left side of Scheme 22. The proton shuttle scheme described above allows the rationalization of several ambiguous reports in the literature. The reported transfer of 1.70 ± 0.11 F/mol during the oxidation of CpRuH(CO) (PPh3), and 1.93 ± 0.15 F/mol in the presence of 2,6-lutidine,112 can be attributed to a

the species shown in boxes were observed spectroscopically or by single-crystal X-ray diffraction. CpMoH(PMe3)3 is a stronger base than water. Formation of [CpMo(OH) (PMe3)3]+, [CpMo(O) (PMe3)2]+, and CpMo(O) (PMe3)2 is attributed to the coordination of water and to its increased acidity upon coordination to the Lewis-acidic metal center.133 The results of DFT calculations based on geometry optimization are consistent with the proposed mechanism.133 5.1.4. Deprotonation after Second Electron Transfer. When the acidity of [MH]•+ is not great enough for deprotonation to occur rapidly, further oxidation to give [MH]2+ can occur, usually paired with solvent (S) coordination. Removal of a second electron further decreases the pKa of the complex, making deprotonation likely. For example, proton loss to the medium follows the two-electron oxidation of trans[PtHCl(PEt3)2], generating the cation [PtCl(solvent) (PEt3)2]+ (Scheme 24).134 The same reaction pathway has been observed 8449

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Scheme 24. Decomposition of [PtHCl(PEt3)2]•+

produce highly acidic dications, are chemically irreversible due to fast proton loss. However, in some cases, the one-electron oxidations of TMHRCs produce stable products. In contrast to the examples above, trans-[Os(en)2py(H)]•2+ may be oxidized to the Os(IV) species trans-[Os(en)2py(H)]3+ with the hydride ligand remaining intact.78 The stable Co(III) dication [HCo(dppe)2(MeCN)]2+ is formed when [HCo(dppe)2]+ is oxidized in acetonitrile.34 When CpMo(PMe3)3H2 is chemically oxidized with two equivalents of Cp2Fe+ or Ag+ in THF or MeCN, dicationic solvent (S) complexes [CpMoH(PMe3)3(S)]2+ are formed.136 A coordinating solvent or ligand can facilitate one-electron oxidation. The Co(II) hydride [CoH{PPh(OEt)2}4][PF6] was not oxidized by [Cp2Fe][PF6] in dry CH2Cl2 or CH3NO2. Although, when one equivalent of a dative ligand (L) was added, oxidation occurred with formation of [CoH{PPh(OEt)2}4L][PF6]2 (L = MeCN, PhCN, ClCH2CN, and PPh(OEt)2).137 5.2.2. One-Electron Reduction. One-electron reduction of a TMHRC regenerates its diamagnetic precursor. Such reductions may be used to indirectly prove the identity of a paramagnetic radical cation. When [FeHCl(dppe)2][BF4] was reduced with sodium in benzene, the diamagnetic hydride complex FeHCl(dppe)2 was recovered nearly quantitatively.75 Similarly, both isomers of [Fe2Cp2(μ-H)(μ-PPh2) (CO)2]•+ were reduced by sodium amalgam in THF to regenerate the corresponding neutral precursors Fe2 Cp 2(μ-H)(μ-PPh2 ) (CO)2.73 One-electron reduction of [Cp*Fe(dppe) (CO)H][PF6] with cobaltocene at low temperature generates the Fe(0) complex Fe(η4-C5Me5H) (dppe) (CO), instead of Cp*Fe(dppe) (CO)H. This reaction involves reductive displacement of the hydride ligand by CO. Oxidation of Fe(η4-C5Me5H) (dppe) (CO) with ferrocenium converts it back to [Cp*Fe(dppe) (CO)H][PF6] quantitatively. Warming the latter complex to room temperature results in CO loss and formation of [Cp*Fe(dppe)H][PF6]. The overall reaction sequence is shown in Scheme 25.99

for the oxidations of IrH(CO) (PPh3)3,61 Cp*Mo(dppe)H3,135 RhH(CO) (PPh)3,116 and [OsH(NH3)5]+.78 The decomposition mechanism of Cp*Mo(dppe)H3 in CH3CN has been supported by DFT calculations.135 One-electron oxidation of the 17-electron Re(IV) complex [CpRe(PAr3)2H2]•+ (Ar = p-XC6H4; X = H, Me, F, MeO) gives a 16-electron Re(V) dication, [CpRe(PAr3)2H2]2+. Two pathways for this dication, one involving H+ transfer and the other involving H• transfer, were proposed.57 By one path, the dication could transfer a proton to [CpRe(PAr3)2H2]•+ to give [CpRe(PAr3)2(MeCN)H]+ and [CpRe(PAr3)2H3]•2+. Alternatively, [CpRe(PAr3)2H2]2+ can accept a hydrogen atom from [CpRe(PAr3)2H2]•+ to form the same products (eq 29). The radical dication [CpRe(PAr3)2H3]•2+ could then act as the chain carrier for electron-transfer chain (ETC) catalysis by oxidizing more [CpRe(PAr3)2H2]•+ to [CpRe(PAr3)2H2]2+ and/or oxidizing [CpRe(PAr 3) 2(MeCN)H] + to [CpRe(PAr3)2(MeCN)H]•2+ (eq 28). The latter product of eq 30 could also be formed by H• transfer from [CpRe(PAr3)2H2]2+ to [CpRe(PAr3)2H2]•+ (eq 31). [CpRe(PAr3)2(MeCN)H]•2+ could then oxidize more [CpRe(PAr3)2H2]•+ to give the products shown in eq 32. H+or H·transfer

[CpRe(PAr3)2 H 2]2 + + [CpRe(PAr3)2 H 2]•+ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ •2 +

[CpRe(PAr3)2 H3]

+MeCN +

+ [CpRe(PAr3)2 (NCMe)H]

(29) •2 +

[CpRe(PAr3)2 H3]

+

+ [CpRe(PAr3)2 (NCMe)H] →

[CpRe(PAr3)2 H3]+ + [CpRe(PAr3)2 (NCMe)H]•2 + (30) •+ H·transfer

[CpRe(PAr3)2 H 2]2 + + [CpRe(PAr3)2 H 2] ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ +MeCN

Scheme 25. Reactivity of [Cp*Fe(dppe) (CO)H][PF6]

[CpRe(PAr3)2 (NCMe)H]•2 + + [CpRe(PAr3)2 H3]+ (31) •2 +

+ [CpRe(PAr3)2 H 2] →

•2 +

+ [CpRe(PAr3)2 H 2]2 +

[CpRe(PAr3)2 (NCMe)H] [CpRe(PAr3)2 (NCMe)H]

•+

(32)

5.2. Electron Transfer

The reaction pathways of TMHRCs often involve electron transfer. The following sections provide a comprehensive review of TMHRC electron transfer properties. 5.2.1. One-Electron Oxidation. When the supporting ligands are good electron donors and/or sterically encumbering, deprotonation is less likely. In this situation, one-electron oxidation of [MH]•+ to give [MH]2+ may be observed. Such oxidations are typically more difficult than the initial oxidation of the diamagnetic precursor. For example, oxidation of Cp*Mo(dppe)H3 occurs at −0.34 V while the oxidation of [Cp*Mo(dppe)H3]•+ occurs at about +1.0 V (both vs Ag+/ Ag).135 Likewise, IrH(CO) (PPh3)3 is oxidized at −0.47 V and [IrH(CO) (PPh3)3]•+ is oxidized at approximately −0.01 V (both vs Cp2Fe+/Cp2Fe).61 These second oxidations, which

5.2.3. Disproportionation. Solvent coordination to a 17electron radical cation [MH]•+ gives a 19-electron species [MH(S)]•+ (eq 33). This solvent complex may be sufficiently electron-rich to reduce [MH]•+ back to MH (eq 34). This process is effectively a solvent-assisted disproportionation. The first step, solvent coordination, is a fast equilibrium, and the subsequent electron transfer is the rate-determining step. In 8450

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Scheme 26. Kinetics of the Proton Transfer and Disproportionation Decomposition Pathwaysa

a

Reprinted with permission from ref 27. Copyright 2001 Elsevier.

many cases, the 18-electron [MH(S)]2+ product is very acidic and transfers a proton to MH (eq 35). [MH]•+ + S ⇌ [MH(S)]•+

(33)

[MH(S)]•+ + [MH]•+ → [MH(S)]2 + + MH

(34)

[MH(S)]2 + + MH → [M(S)]+ + [MH 2]+

(35)

Scheme 27. Rate-Limiting Acetonitrile-Assisted Disproportionation

Poli has noted that the products of eq 35 are the same as those that would arise from a −2e−/−H+ transfer process (eqs 36−38) if the base B in eq 35 is the hydride complex MH. However, distinction between the two mechanisms can be made based on kinetics studies. Rate-limiting proton transfer (eq 37) would make the process first order in [MH]•+ and first order in the base (Scheme 26a). The disproportionation, with rate-determining electron transfer (eq 34), implies that the rate law should be second order in [MH]•+ and first order in solvent when Keq[S] ≪ 1 (a pre-equilibrium favoring the 17-electron TMHRC) (Scheme 26b). In addition, better donor solvents should lead to faster disproportionation processes (MeCN ≫ THF, CH2Cl2), while proton transfer should be much less solvent-dependent.27 −e −

MH ⎯⎯⎯→ [MH]•+

(36)

[MH]•+ + B → M• + BH+

(37)

−e −

M• + S ⎯⎯⎯→ [M(S)+ ]

demanding phosphine ligands (PPh3, dppp) help to prevent deprotonation of [CpRuL2H]•+ by the neutral hydride complexes. The disproportionation mechanism therefore provides a lower-energy pathway, at least when a good donor ligand (and poor base) such as acetonitrile is present. When the supporting phosphine ligands were dppm or dppe, decomposition of the hydride radical cations was faster, which may be attributed to steric effects. In these cases, it was not determined whether the decompositions proceeded by disproportionation or direct proton transfer.92 5.2.4. Oxidation after Deprotonation. The neutral radical formed after deprotonation of a TMHRC can be oxidized to give a diamagnetic cationic species, a process often paired with solvent coordination. This oxidation is usually less difficult than that of the diamagnetic hydride complex precursor. As a result, two-electron ECE processes are often observed when transition-metal hydride complexes are oxidized. In the electrochemical oxidation of [FeH(MeCN) (dppe)2]+, the first anodic wave (+0.83 V vs SCE) corresponds to an ECE mechanism involving proton loss. Oxidation of the radical [Fe(MeCN) (dppe)2]•+ at the same potential gives [Fe(MeCN) (dppe)2]2+ which then abstracts fluoride from BF4− in solution (Scheme 28).90

(38)

Decomposition of [CpMoH(PMe3)3]•+ in acetonitrile proceeds by disproportionation, giving equal amounts of the final products [CpMo(PMe 3 ) 3 (MeCN)] + and [CpMo(PMe3)3H2]+. The reaction is second order in radical cation (ν = kdisp[[MH]•+]2).136 Disproportionation also occurs for [IrH(CO) (PPh3)3]•+ in acetonitrile; the final products are IrH(CO) (PPh3)3 and [IrH(CO) (PPh3)3(MeCN)]2+ in a 1/1 ratio.100 In the presence of PPh3 in acetonitrile, the Rh analogue forms RhH(CO) (PPh3)3 and [RhH(PPh3)4(MeCN)]]2+ with CO evolution.100 Disproportionation was also proposed to explain the behavior of Cp*Ru(dppf)H in cyclic voltammetry experiments.76 The decomposition of [CpRuL2H]•+ (L2 = (PPh3)2, dppp) was found to be much faster in acetonitrile than in THF, giving nearly 1/1 mixtures of [CpRuL 2 (CD 3 CN)] + and [CpRuL2H2]+. The reaction was shown to be second order in [CpRuL2H]•+ through a derivative cyclic voltammetry (DCV) investigation. The large solvent effect and the lack of a kinetic isotope effect are in agreement with a rate-limiting acetonitrileassisted disproportionation (Scheme 27) rather than a direct proton transfer.129 It was proposed that the sterically

Scheme 28. Decomposition of [Fe(MeCN) (dppe)2]2+

TMHRCs can also be reduced by the neutral radicals produced by deprotonation. For example, the [CpRu(PPh3)2]• formed by deprotonation of [CpRu(PPh3)2H]•+ could be in equilibrium with [CpRu(PPh3)2(MeCN)]• in acetonitrile. This 19-electron radical should be a strong reducing agent. The irreversible reduction of the 18-electron [CpRu8451

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(PPh3)2(MeCN)]+ occurs at −2.42 V versus Cp2Fe+/Cp2Fe. Electron transfer from [CpRu(PPh3)2(MeCN)]• to [CpRu(PPh3)2H]•+ produces the cationic acetonitrile complex and the neutral hydride complex, as shown in Scheme 29.129 Similar reactivity was proposed to occur after the oxidation of Cp*Ir(PPh3)H2,94 H3Ir(PMe2Ph)3,95 and CpMo(PMe3)3H.136

The two-step electron/hydrogen atom transfer from a diamagnetic M−H complex to the trityl cation involves H• transfer from a TMHRC intermediate (eqs 42 and 43). This stepwise process is an alternative to direct hydride transfer.

Scheme 29. Reduction of [CpRu(PPh3)2H]•+ by Neutral Metal Radical

The thermodynamic measurements in section 4.2 indicate that a weakening of the M−H bond would be expected upon oneelectron oxidation. Indeed, homolytic cleavage of this bond may occur under certain circumstances. Hydrogen atom transfer from [M−H]•+ can result in H2 evolution and formation of the diamagnetic cationic complex M+ (eq 39). For example, homolytic cleavage of the Co−H bond was proposed for the decomposition of the electrochemically generated [Co(CO) 2 (dppf)H] •+ at 293 K in CH 2 Cl 2 , forming the diamagnetic cation [Co(CO)2(dppf)]+ and H2.115 The loss of H2 from two [(C5H4SiMe3)2NbHL]•+ (L = P(OMe)3) via hydrogen atom transfer was also proposed to occur during the electrochemical oxidation of the neutral diamagnetic precursor in THF.53 After H2 loss, subsequent solvent coordination may also occur (eq 40).

M+ + S → M(S)+

(40)

[M − H]•+ + Ph3C• → M+ + Ph3CH

(43)

Another example is CpMoH(PMe3)L2 (L = CO, PMe3). The dicarbonyl complex, which is less easily oxidized (E1/2 = +0.19 V vs Cp2Fe+/Cp2Fe), cleanly transfers hydride to trityl cation. In contrast, the tris-phosphine complex (E1/2 = −1.46 V vs Cp2Fe+/Cp2Fe) is oxidized by the trityl cation. Disproportionation of the resulting TMHRC and further oxidation lead to the products shown in Scheme 31.139

homolytic cleavage

(39)

(42)

The reduction potential of [Ph3C][ClO4] is −0.08 V versus Cp2Fe+/Cp2Fe in MeCN,138 making the trityl cation a reasonably strong oxidant. Direct hydride transfer from M−H complexes to Ph3C+ tends to occur when the oxidation potentials of the complexes are high, for example CpWH(CO)2(PPh3), with a potential +0.195 V versus Cp2Fe+/Cp2Fe in MeCN.29 Hydride complexes that are more easily oxidized readily transfer an electron to the trityl cation. The two-step e−/H• transfer mechanism was proposed for [(PPh3)3IrHn]•+ (n = 3, 5)37 and [Cp*Ru(dppf)H]•+.36 Other reactions are also possible between TMHRCs and carbon-centered radicals. The reaction of CpRu(PPh3)2H with Tol3C+ is believed to proceed by an initial electron transfer, with the resulting radical then adding to the Cp ring of [CpRu(PPh3)2H]•+. The cationic dihydride complex trans-[η5C5H4(CTol3)]Ru(PPh3)2H2]+ was formed as the major product (eq 44).58

5.3. Hydrogen Atom Transfer

2[M − H]•+ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2M+ + H 2

[M − H] + Ph3C+ → [M − H]•+ + Ph3C•

The processes in eqs 39 and 40 are difficult to distinguish from a mechanism involving proton transfer (section 5.1.2), where [M−H]•+ protonates the diamagnetic precursor. Loss of H2, further oxidation of the neutral radical, and solvent coordination to M+ would give identical products (Scheme 30).

Scheme 31. Oxidation of CpMoH(PMe3)3 by Ph3C+

Scheme 30. Proton Transfer Pathway with MH as the Base

On the other hand, reaction of CpMo(CO) (dppe)H with one equivalent of trityl cation in CD3CN gives [CpMo(CO) (dppe) (CD3CN)]+, H2, and Gomberg’s dimer which is formed by dimerization of trityl radical (eq 45). In this reaction, it was Hydrogen atom transfer-based disproportionation was proposed for [MH(dppe)2]•+ (M = Rh, Ir). For the iridium complex, the products were [Ir(dppe)2]+ and [IrH2(dppe)2]+ (eq 41). The rhodium complex gives two equivalents of [Rh(dppe)2]+ and eliminates H2, in agreement with the observation that the Rh(I) cation does not oxidatively add H2.100 2[IrH(dppe)2 ]•+ → [Ir(dppe)2 ]+ + [IrH 2(dppe)2 ]+ (41) 8452

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proposed that Ph3C+ oxidizes CpMo(CO) (dppe)H to give the corresponding TMHRC. Proton transfer from this radical cation to the diamagnetic precursor hydride complex, the loss of H2 from the resulting [CpMo(CO) (dppe)H2]+, and oxidation of [CpMo(CO) (dppe)]• by Ph3C+ lead to the observed products (Scheme 32).35

A disproportionation mechanism was proposed for the decomposition of [Cp*Mo(H)3(dppe)]•+ in acetonitrile (Scheme 34). The cations [Cp*MoH2(MeCN) (dppe)]+ and trans-[Cp*MoH(dppe) (MeCN)2]2+ were formed as the minor and major products, respectively. The disproportionation was proposed to occur after rearrangement of the oxidized trihydride to the nonclassical hydride-dihydrogen form, which should make the metal center more electron-rich. Coordination of acetonitrile also promotes the electron transfer. The intermediate [Cp*Mo(H2)H(MeCN) (dppe)]2+ leads to the final products by loss of H+ or H2.97 In contrast to the [Cp*Mo(H)3(dppe)]•+ system, reductive elimination of H2 from [(1,2,4-(t-Bu)3C5H2)Mo(PMe3)2H3]•+ appears to follow a dissociative pathway (Scheme 35). This may be due to the increased steric bulk of the t-butylsubstituted Cp ligand.65 H2 elimination was also proposed to occur in the decomposition of [ReH7(PPh3)2]•+. Conversion to the nonclassical structure [Re(H2)H5(PPh3)2]•+ is followed by loss of H2 and PPh3 uptake, giving [ReH5(PPh3)3]•+ as a proposed intermediate. This radical cation abstracts H• from the medium, forming the stable diamagnetic species [ReH 6 (PPh3 ) 3] + (Scheme 36).140 Reductive elimination of methane from the model system [CpM(PH3) (CH3)(H)] (M = Rh, Ir) has been investigated by DFT calculations.141 As expected, reductive elimination becomes more favorable upon one-electron oxidation. Oxidation of Cp*IrH(PPh3) (CH3) with two equivalents of Cp2Fe+ in acetonitrile is accompanied by intramolecular CH4 elimination. Subsequent solvent coordination gives the product [Cp*Ir(PPh3) (MeCN)2]2+ (eq 48). The reductive elimination likely occurs from a paramagnetic 17-electron monocationic intermediate.94

Scheme 32. Oxidation of CpMo(CO)(dppe)H by One Equiv Ph3C+ in CD3CNa

a

Reprinted from ref 35. Copyright 2006 American Chemical Society.

It therefore appears that hydrogen atom transfer from cationic 17-electron hydride complexes to the trityl radical is often less viable, kinetically, than other reactions. 5.4. Reductive Elimination

Under the influence of reduced electron density, one-electron oxidation of diamagnetic polyhydride precursors was found to induce collapse of one or more pairs of hydride ligands (eq 46). Release of H2, sometimes combined with solvent coordination, can occur (eq 47). This decomposition pathway has been proven or inferred in several cases, for example IrH3(PMe2Ph)3,95 Cp*RuH3(PPh3),130 and OsH6(P(i-Pr)3)2.93 −e −

MHn ⎯⎯⎯→ [MHn − 2k(H 2)k ]•+

(46)

S

MHn − 2k(H 2)k ]•+ → [MNn − 2k(S)k ]•+ + k H 2

(47)

Another example of H2 elimination from a TMHRC is the decomposition of [Cp*MoH3(dppe)]•+. In THF or CH2Cl2, the processes in eqs 46 and 47 were proposed to occur by the solvent-promoted mechanism shown in Scheme 33. Faster

Decomposition of [Cp2W(C6H5)H]•+ produces benzene and other products. However, the benzene is not the result of an intramolecular reductive elimination process like that in eq 48. Instead, C6D5H was produced by the isotopologue [Cp2W(C6D5)D]•+ which suggests that the hydrogen atom may come from either the solvent (acetonitrile) or the Cp ligands, although either is surprising because these C−H bonds are fairly strong. The neutral radical [Cp2WPh]• formed by deprotonation of the TMHRC is a likely intermediate. This 17-electron species may eliminate a phenyl radical which then abstracts a hydrogen atom from the solvent (Scheme 37). Alternatively, intramolecular hydrogen atom abstraction could occur from one of the Cp ligands (eq 37).56

Scheme 33. Solvent-Promoted H2 Elimination from [Cp*MoH3(dppe)]•+ in THF or CH2Cl2

5.5. Other Reactivity

Besides deprotonation (section 5.1), electron transfer (section 5.2), hydrogen atom transfer (section 5.3), and reductive elimination (section 5.4), other TMHRC reactivity has been observed including hydride transfer, ligand coordination/ dissociation, etc. 5.5.1. Hydride Transfer. One-electron oxidation of a diamagnetic hydride complex reduces the hydridic character of the hydride ligand. But, in very limited cases, the TMHRCs formed after oxidation still appear to serve as hydride donors. For example, when the Os(III) complex trans-[Os(en)2(py)-

decomposition was observed in CH2Cl2 relative to that in THF. The CH2Cl2 solvate of the monohydride radical cation, [Cp*MoH(dppe)(η2-CH2Cl2)]•+, was shown to have a 19electron configuration by EPR spectroscopy. The accelerated decomposition of [Cp*MoH3(dppe)]•+ in CH2Cl2 was proposed to arise from participation of the second solvent Cl atom in an interchange associative mechanism, facilitating the elimination of H2.97 8453

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Scheme 34. Disproportionation of [Cp*Mo(H)3(dppe)]•+ in Acetonitrile

5.5.3. CO Coordination. The TMHRC complex [Cp*Fe(dppe)H]•+ reversibly binds CO at low temperature (−80 °C) to form the unusual 19-electron Fe(III) adduct [Cp*Fe(dppe) (CO)H]•+.99 5.5.4. Nucleophilic Attack by Halide Ligands. Reaction of [FeHCl(dppe)2][BF4] with [Bu4N][Br] generates FeClBr(dppe) and H2 (eq 52). This reaction was proposed to occur through a nucleophilic attack of bromide on [FeHCl(dppe)2]•+ to give an unstable seven coordinate Fe(III) hydride complex, FeHClBr(dppe)2, followed by decomposition to the iron(II) complex FeClBr(dppe) and H2.75

Scheme 35. Reductive Elimination of H2 from [(1,2,4-(tBu)3C5H2)Mo(PMe3)2H3]•+

Scheme 36. Decomposition of [ReH7(PPh3)2]

[FeHCl(dppe)2 ][BF4] + Br −→ [FeClBr(dppe)] + dppe + [BF4 ]− + 1/2H 2

•+

(52)

6. REACTIONS INVOLVING TMHRC INTERMERMEDIATES Transition-metal hydride radical cations can exist as reaction intermediates when the diamagnetic precursor complexes serve as one-electron donors. In this section, various reactions involving TMHRC intermediates are described.

Scheme 37. Decomposition of [Cp2W(C6D5)D]•+

6.1. Regioselective Hydrogenation of Heterocycles

Hydride transfer from CpRu(P−P)H to the N-carbophenoxypyridinium cation was proposed to occur by two different mechanisms, with two different regioselectivities. The 1,2 product with some 1,4 product appears to result from a singlestep H− transfer, while the 1,4 product is the result of a twostep (e−/H•) hydride transfer (Scheme 38). The proposed mechanisms are supported by the computational results showing that the positive charge in the cation 1 resides predominantly at C2 and C6, whereas the spin density in the radical 6 resides predominantly at C4 (Scheme 39).21,109 In the stoichiometric reductions, the ratio of 1,4 to 1,2 product increases as the Ru hydrides become better oneelectron reductants with the most reducing hydride, Cp*Ru(dppf)H, giving only the 1,4-dihydropyridine (Table 15).21 Cp*Ru(dppf)H catalyzes the exclusive formation of the 1,4dihydropyridine from N-carbophenoxypyridinium cation, H2, and 2,2,6,6-tetramethylpiperidine (TMP) (eq 53). The reaction

H][OTf]2 was combined with N-methylacridinium ion (MAH+), the hydride transfer product MAH2 was formed almost quantitatively (eq 50). It has also been reported that

acetone is readily reduced to 2-propanol by [Os(NH3)5(H2)]3+, which serves as both a hydride and proton source (effectively [Os(NH3)5H]2+ + H+).78 5.5.2. Reaction With Radical Sources. As paramagnetic species, TMHRCs react with neutral radicals to generate closed-shell cationic complexes. For example, the reaction of [Cp*Ru(dppf)H]•+ with Bu3SnH gives [Cp*Ru(dppf)H2]+ and reactions with CCl4, BrMn(CO)5, or I2 produce [Cp*Ru(dppf)HX][PF6] (X = H, Cl, Br, I) derivatives (eq 51).76 [Cp*Ru(dppf)H]•+ + XY → [Cp*Ru(dppf)XH]+ XY = HSnBu3, CCl4 , BrMn(CO)5 , I 2 X = H, Cl, Br, I

starts with the rate-determining two-step (e−/H•) hydride transfer from Cp*Ru(dppf)H to the pyridinium cation, forming the 1,4 dihydropyridine and a 16-electron Ru cation that binds H2. The resulting dihydrogen complex is then deprotonated by TMP, regenerating Cp*Ru(dppf)H (Scheme 40).21 Stryker’s reagent, [(PPh3)CuH]6, also reduces the N-carbophenoxypyr-

(51) 8454

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Scheme 38. Mechanisms of Hydride Transfer from RuH to Pyridinium Cation

Scheme 39. Calculated Atomic Spin Densities (6) and Charges (1) at the UB3LYP/6-311++G(3df, 3pd)// UB3LYP/6-31G* Levela

Scheme 40. Catalytic Cycle for Cp*Ru(dppf)H-Catalyzed Ionic Hydrogenation of Pyridinium Cation

a

Reproduced from ref 21. Copyright 2008 American Chemical Society.

Table 15. Product Distribution of the Stoichiometric Reduction of 1 with Ruthenium Hydridesa

Scheme 41. Reaction between Cp*Ru(dppf)H and Aziridinium Cation entry

complex

solvent

7a

7b

1b 2b 3b 4b 5c 6d 7c 8d

2 3 4 5 2 3 4 5

CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD3CN CD3CN CD3CN CD3CN

52 30 4

48 70 96 100 77 85 98 100

23 15 2

Ni−C) are all Ni(III) and paramagnetic (S = 1/2). The difference between these states is in the identity of the third bridging ligand. In the Ni−C state, a hydride bridges the Ni and Fe centers. The Ni−B state contains a bridging hydroxide ligand. A peroxide ligand is thought to bridge the two metals in the Ni−A state. Both the Ni−A and Ni−B states are inactive toward hydrogen. One-electron reduction of the Ni−A and Ni−B states leads to two EPR-silent Ni(II) states, Ni−SU and Ni−SIr, respectively. The latter is activated by protonation; loss of water gives the Ni−SIa state which is also EPR-silent. This state can be inhibited by CO, although photolysis at cryogenic temperatures reverses the inhibition. One-electron reduction of the Ni−SIa state coupled with protonation gives the paramagnetic Ni−C state, which is a transition-metal hydride radical cation. Inactive states containing Ni(I) are accessible from Ni−C by successive photoinduced proton loss and CO binding. The catalytically active state of [NiFe] hydrogenase is thought to be a bridging hydride species. The catalytic cycle

a

Reproduced from ref 21. Copyright 2008 American Chemical Society. General reaction condition: 0.02 mmol of 1, 0.02 mmol of hybride complex, 700 μL of solvent, product ratio determined by 1H NMR integration. b0.08 mmol of CH3CN, room temperature. cRoom temperature. d75 °C, 2 h.

idinium cation. When the two were combined, the radical cation [(PPh3)CuH]6•+ was observed by stopped-flow UV−vis spectroscopy.38 Single-electron transfer and subsequent rapid electron exchange was proposed as the reason for line broadening in 1 H NMR spectra when Cp*Ru(dppf)H was combined with the aziridinium cation shown in Scheme 41.117 6.2. Hydrogenase-Catalyzed Reactions

In the [FeNi]-hydrogenase catalyzed H2 oxidation reaction (Scheme 42), the Fe(II) in the active site does not change oxidation state. The most oxidized states (Ni−A, Ni−B, and 8455

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Scheme 42. [FeNi]-Hydrogenase Catalyzed H2 Oxidationa

a

The Cys group on the sulfur is omitted for clarity. Reproduced with permission from ref 16. Copyright 2009 Royal Society of Chemistry.

resting form.142 Likewise, the heterobimetallic complex Cp*Ru(dppf)H (dppf =1,1′-bis(diphenylphosphino)ferrocene) has been studied as a model for the H2-reaction center of [NiFe] hydrogenases due to its ability to perform one-electron reductions using H2 as a stoichiometric reductant.76

may involve the three different Ni−R states shown at the bottom of Scheme 42. The most reduced of these, Ni−R, contains Ni(II) and can bind H2. This state and the others, Ni− R′ and Ni−R″, have been observed by IR spectroscopy and have been shown to be pH-dependent. A direct conversion from Ni−R″ or Ni−R′ to Ni−SIa is also possible, involving another H+.16 The core structure of the Ni−C state has been modeled by the paramagnetic bridging hydride complex [(NiIIL)(H2O)(μH)RuII(η6-C6Me6)]+ (L = N,N′-dimethyl-N,N′-bis(2-mercaptoethyl)-1,3-propanediamine). This complex was formed by reducing the diamagnetic aqua precursor with H2 in water (eq 54). Paramagnetism arises from the Ni(II), which changes from

6.3. Acetylene Insertion

One-electron transfer was proposed as the rate-determining step in the reaction of trans-PtH2(PCy3)2 with DMAD (CH3O2CCCCO2CH3). This mechanism was indicated by the absence of any significant kinetic isotope effect. Kinetics studies suggested that the rate-determining step is bimolecular, involving one acetylene molecule and one platinum dihydride complex. A radical pair consisting of a Pt(III) radical cation and the acetylene radical anion rapidly rearranges to the alkenyl product (Scheme 43).143 When DMAD was mixed with CpCr(CO)3H in C6H6, the formation of the phenyl-substituted compound 8 was observed (eq 55).144 A single-electron transfer (SET) mechanism was proposed to explain the presence of 8 (Scheme 44).144 One-electron transfer from CpCr(CO)3H (11) to DMAD forms [CpCr-

being a singlet (S = 0) to a triplet (S = 1) in the pseudooctahedral environment caused by coordination of H− and H2O ligands in the axial positions.142 In the neutron diffraction structure, the bridging hydride is closer to the Ru center (Ru− H = 1.676 Å, Ni−H = 1.859 Å). The Ru−H bond serves as a two-electron donor to the Ni atom. The distance between the nickel and ruthenium atoms, 2.739 Å, is shorter than that in the precursor aqua complex. A similar difference has been observed for the actual [NiFe] hydrogenase: the Ni···Fe distance in the active form (2.512 Å) is shorter than that (2.906 Å) in the

Scheme 43. Reaction between trans-PtH2(PCy3)2 with DMAD

8456

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Scheme 45. Proposed Mechanism for Photo-Induced Olefin Hydrometallation

(CO)3H]•+ (12) and the free radical anion 13, which can react with benzene, unreacted CpCr(CO)3H or DMAD. With benzene, 13 will give 14 and eventually 8; with CpCr(CO)3H, 13 will be protonated to the vinyl radical 15, forming the hydrogenation products 9 and 10; with DMAD, 13 will give oligomers. EPR spectrum of the reaction mixture shows the existence of the dimethyl fumarate radical anion 10•−. As 10 is more difficult to reduce than DMAD, it is likely that SET to DMAD also occurs during the reaction.144 The failure to observe the DMAD radical anion spectroscopically is probably due to its low concentration caused by its reaction with the solvent.

tionation, dihydrogen reductive elimination, hydrogen atom transfer, dimerization, and others. In general, more kinetics studies are needed, as these will greatly help in elucidating relevant mechanisms. Two observations from the existing literature are in need of special attention. The first is that different reactivity pathways have been observed for the same TMHRC in different solvents. Detailed studies involving solvent effects will therefore be particularly important. The second is the controversy involving the change in thermodynamic M−H bond strength and the (often conflicting) change in M−H infrared stretching frequency; this is a question in need of further discussion. In conclusion, the chemistry of TMHRCs is far from mature and remains a promising area for further discoveries.

6.4. Photo-Induced Olefin Hydrometalation

The photoinduced hydrometalation of electron deficient olefins (fumaronitrile, maleic anhydride, and others) by Cp2MH2 (M = Mo, W) was proposed to occur by electron transfer, followed by proton transfer from the resulting [Cp2MH2]•+ to the reduced olefin and final radical recombination (Scheme 45).55

AUTHOR INFORMATION

7. CONCLUSIONS AND OUTLOOK The scattered studies in the existing literature concerning TMHRCs are far from complete. Transition-metal hydride radical cations are involved in a variety of chemical and biochemical reactions, making a more thorough understanding of their properties essential for explaining observed reactivity and for the eventual development of new applications. From a practical perspective, new techniques to capture and characterize short-lived TMHRCs would be beneficial. Fundamentally, more information is needed about the properties that affect different reactivity pathways such as deprotonation, dispropor-

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Addresses ‡

Y.H.: Eastman Chemical Company, Kingsport, Tennessee 37660, United States. ∥ D.P.E.: Laboratory of Inorganic Chemistry, ETH Zürich, CH8093 Zürich, Switzerland. Notes

The authors declare no competing financial interest.

Scheme 44. SET Mechanism for the Reaction between CpCr(CO)3H and DMAD in Benzenea

a

Reprinted from ref 144. Copyright 2012 American Chemical Society. 8457

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Biographies

REFERENCES

Yue Hu received her B.S. degree with honor in chemistry from the University of Science and Technology of China in 2010. She then joined Professor Jack R. Norton’s group at Columbia University, where her research focused on the ionic hydrogenation reactions and the kinetics and thermodynamics of hydride, hydrogen atom, and proton transfers from transition-metal hydride complexes. She received her Ph.D. degree in the spring of 2015 and did a brief postdoc at Texas A&M University in Professor David E. Bergbreiter’s group, where her research focuses on the use of soluble polyisobutylene as a support to develop phase separable recoverable transition-metal-based catalysts. Starting in October 2015, Dr. Hu is working at Eastman Chemical Company in Kingsport, Tennessee. In her spare time, Dr. Hu likes to listen to music and explore various national parks with her husband, Jiyao.

(1) Eisenberg, D. C.; Norton, J. R. Hydrogen-Atom TransferReactions of Transition-Metal Hydrides. Isr. J. Chem. 1991, 31, 55−66. (2) Sweany, R. L.; Halpern, J. Hydrogenation of.alpha.-methylstyrene by hydridopentacarbonylmanganese (I). Evidence for a free-radical mechanism. J. Am. Chem. Soc. 1977, 99, 8335−8337. (3) Roth, J. A.; Orchin, M. The stoichiometric hydrogenation of 1,1diphenylethylene with hydridocobalt tetracarbonyl; differences from the hydroformylation reaction. J. Organomet. Chem. 1979, 182, 299− 311. (4) Connolly, J. W. Reaction between hydridotetracarbonyl(trichlorosilyl)iron, HFe(CO)4SiCl3, and conjugated dienes. Evidence for a free radical mechanism. Organometallics 1984, 3, 1333−1337. (5) Wassink, B.; Thomas, M. J.; Wright, S. C.; Gillis, D. J.; Baird, M. C. Mechanisms of the hydrometalation (insertion) and stoichiometric hydrogenation reactions of conjugated dienes effected by manganese pentacarbonyl hydride: processes involving the radical pair mechanism. J. Am. Chem. Soc. 1987, 109, 1995−2002. (6) Li, G.; Han, A.; Pulling, M. E.; Estes, D. P.; Norton, J. R. Evidence for Formation of a Co−H Bond from (H2O)2Co(dmgBF2)2 under H2: Application to Radical Cyclizations. J. Am. Chem. Soc. 2012, 134, 14662−14665. (7) Kuo, J. L.; Hartung, J.; Han, A.; Norton, J. R. Direct Generation of Oxygen-Stabilized Radicals by H• Transfer from Transition Metal Hydrides. J. Am. Chem. Soc. 2015, 137, 1036−1039. (8) Gridnev, A. The 25th Anniversary of Catalytic Chain Transfer. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 1753−1766. (9) Gridnev, A. A.; Ittel, S. D. Catalytic Chain Transfer in FreeRadical Polymerizations. Chem. Rev. 2001, 101, 3611−3659. (10) Abramo, G. P.; Norton, J. R. Catalysis by C5Ph5Cr(CO)3• of Chain Transfer During the Free Radical Polymerization of Methyl Methacrylate. Macromolecules 2000, 33, 2790−2792. (11) Bullock, R. M. Catalytic Ionic Hydrogenations. Chem. - Eur. J. 2004, 10, 2366−2374. (12) Sui-Seng, C.; Hadzovic, A.; Lough, A. J.; Morris, R. H. Novel Hydrido-Ruthenium(II) Complexes with Histidine Derivatives and their Application in the Hydrogenation of Ketones. Dalton Trans. 2007, 2536−2541. (13) Casey, C. P.; Bikzhanova, G. A.; Guzei, I. A. Stereochemistry of Imine Reduction by a Hydroxycyclopentadienyl Ruthenium Hydride. J. Am. Chem. Soc. 2006, 128, 2286−2293. (14) Åberg, J. B.; Samec, J. S. M.; Bäckvall, J. E. Mechanistic investigation on the hydrogenation of imines by [p-(Me2CH)C6H4Me]RuH(NH2CHPhCHPhNSO2C6H4-p-CH3). Experimental support for an ionic pathway. Chem. Commun. 2006, 2771−2773. (15) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Zhu, G. Ruthenium-Catalyzed Ionic Hydrogenation of Iminium Cations. Scope and Mechanism. J. Am. Chem. Soc. 2005, 127, 7805−7814. (16) Ogata, H.; Lubitz, W.; Higuchi, Y. [NiFe] hydrogenases: structural and spectroscopic studies of the reaction mechanism. Dalton Trans. 2009, 7577−7587. (17) Odom, J. M.; Peck, H., Jr. Hydrogenase, electron-transfer proteins, and energy coupling in the sulfate-reducing bacteria Desulfovibrio. Annu. Rev. Microbiol. 1984, 38, 551−592. (18) Ghosh, S.; Hogarth, G.; Hollingsworth, N.; Holt, K. B.; Kabir, S. E.; Sanchez, B. E. Hydrogenase biomimetics: Fe2(CO)4(μ-dppf)(μpdt) (dppf= 1, 1′-bis (diphenylphosphino) ferrocene) both a protonreduction and hydrogen oxidation catalyst. Chem. Commun. 2014, 50, 945−947. (19) Yang, J. Y.; Chen, S.; Dougherty, W. G.; Kassel, W. S.; Bullock, R. M.; DuBois, D. L.; Raugei, S.; Rousseau, R.; Dupuis, M.; DuBois, M. R. Hydrogen oxidation catalysis by a nickel diphosphine complex with pendant tert-butyl amines. Chem. Commun. 2010, 46, 8618−8620. (20) Yang, J. Y.; Smith, S. E.; Liu, T.; Dougherty, W. G.; Hoffert, W. A.; Kassel, W. S.; DuBois, M. R.; DuBois, D. L.; Bullock, R. M. Two pathways for electrocatalytic oxidation of hydrogen by a nickel bis(diphosphine) complex with pendant amines in the second coordination sphere. J. Am. Chem. Soc. 2013, 135, 9700−9712.

Anthony P. Shaw received his B.S. degree in chemistry from Rensselaer Polytechnic Institute in 2004, where he was introduced to organometallic chemistry by Prof. Alan R. Cutler. He subsequently joined Prof. Jack R. Norton’s group at Columbia University, where he studied the reactivity of half-sandwich ruthenium complexes and the regioselective hydrogenation of pyridinium salts, receiving his Ph.D. with distinction in 2009. A postdoctoral appointment at the University of Oslo took him to Norway for a year to work in the group of Prof. Mats Tilset. There, Dr. Shaw developed microwave-heated methods for the synthesis of gold(III) complexes. Since 2010, he has worked in the U.S. Army’s Pyrotechnics Technology and Prototyping Division at Picatinny Arsenal in New Jersey. Dr. Shaw received the 2014 Dr. Bernard E. Douda Young Scientist Award for his contributions to pyrotechnics and currently serves as the Archivist of the International Pyrotechnics Society. When he is not working, Dr. Shaw enjoys eating fresh seafood at the Jersey Shore and spending time with his wife, Kat, and their three cats. Deven P. Estes received a B.S. in Chemistry from the University of Oklahoma in 2009. In 2014, he earned his Ph.D. from Columbia University, where he was a Department of Energy Predoctoral Fellow in the laboratory of Prof. Jack Norton. Deven is currently a Marie Curie Postdoctoral Fellow at ETH Zürich in the laboratory of Prof. Christophe Copéret. Jack R. Norton received his B.A. degree from Harvard University in 1967 and a Ph.D. from Stanford University in 1972, where he studied under the guidance of Prof. James P. Collman. After a postdoctoral appointment with Prof. Jack Lewis at the University of Cambridge, he was appointed assistant professor at Princeton University in 1973. He moved to Colorado State University in 1979 and subsequently to Columbia University in 1997, where he remains Professor of Chemistry. Prof. Norton’s research has focused on studying the reactivity and properties of transition-metal hydrides. He coauthored the textbook Principles and Applications of Organotransition Metal Chemistry. From 1992−2003, he served as an associate editor of the Journal of the American Chemical Society. He received the ACS Award for Organometallic Chemistry in 2005 and, more recently, the 2013 Cope Scholar Award.

ACKNOWLEDGMENTS The completion of this article was supported by NSF Grant CHE-0749537, Boulder Scientific, and OFS Fitel. D.P.E. was supported by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009 and administered by ORISE-ORAU under Contract DE-AC0506OR23100. 8458

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