DOI: 10.1002/chem.201404007

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Dimers of Nineteen-Electron Sandwich Compounds: Crystal and Electronic Structures, and Comparison of Reducing Strengths Swagat K. Mohapatra,[a, c] Alexandr Fonari,[a, b] Chad Risko,[a, d] Kada Yesudas,[a] Karttikay Moudgil,[a] Jared H. Delcamp,[a, e] Tatiana V. Timofeeva,[b] Jean-Luc Brdas,[a, f] Seth R. Marder,[a] and Stephen Barlow*[a]

Abstract: The dimers of some Group 8 metal cyclopentadienyl/arene complexes and Group 9 metallocenes can be handled in air, yet are strongly reducing, making them useful n-dopants in organic electronics. In this work, the Xray molecular structures are shown to resemble those of Group 8 metal cyclopentadienyl/pentadienyl or Group 9 metal cyclopentadienyl/diene model compounds. Compared to those of the model compounds, the DFT HOMOs of the dimers are significantly destabilized by interactions between the metal and the central CC s-bonding orbital, accounting for the facile oxidation of the dimers. The lengths of these

Introduction The tendency of organometallic compounds of the transition metals to comply with the 18-electron rule has long been recognized. Accordingly, alkali-metal or electrochemical reduction of cationic 18-electron sandwich compounds to the corresponding 19-electron species is often followed by chemical reactions to regain 18-electron configurations. For example, rho[a] Dr. S. K. Mohapatra, A. Fonari, Dr. C. Risko, Dr. K. Yesudas, K. Moudgil, Dr. J. H. Delcamp, Prof. J.-L. Brdas, Prof. S. R. Marder, Dr. S. Barlow School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics Georgia Institute of Technology, Atlanta, GA 30332-0400 (USA) E-mail: [email protected] [b] A. Fonari, Prof. T. V. Timofeeva Department of Chemistry, New Mexico Highlands University Las Vegas, NM 87701 (USA) [c] Dr. S. K. Mohapatra Present address: KIIT University Bhubaneswar, Orissa 751024 (India) [d] Dr. C. Risko Present address: Department of Chemistry and Center for Applied Energy Research (CAER) University of Kentucky, Lexington, KY 40506-0055 (USA) [e] Dr. J. H. Delcamp Present address: Department of Chemistry University of Mississippi, University, MS 38677 (USA) [f] Prof. J.-L. Brdas Present address: Division of Physical Sciences and Engineering King Abdullah University of Science and Technology – KAUST Thuwal 23955-6900 (Kingdom of Saudi Arabia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404007. Chem. Eur. J. 2014, 20, 15385 – 15394

CC bonds (X-ray or DFT) do not correlate with DFT dissociation energies, the latter depending strongly on the monomer stabilities. Ru and Ir monomers are more reducing than their Fe and Rh analogues, but the corresponding dimers also exhibit much higher dissociation energies, so the estimated monomer cation/neutral dimer potentials are, with the exception of that of [RhCp2]2, rather similar (1.97 to 2.15 V vs. FeCp2 + /0 in THF). The consequences of the variations in bond strength and redox potentials for the reactivity of the dimers are discussed.

docenes generally form 18-electron species, in which the hapticity of one of the ligands is reduced, either by dimerization through a ligand carbon atom[1] or by abstraction of a hydrogen atom from solvent,[1c] unless heavily substituted with bulky groups.[2] Several iridocenes also dimerize,[1a, 3] but, consistent with the less rigorous compliance with the 18-electron rule generally found for compounds of the 3d elements, cobaltocenes are stable (in the absence of air) as strongly reducing 19-electron monomers.[4] Other 19-electron species that dimerize in a similar way include certain mixed cyclopentadienyl/ arene sandwich compounds of iron[5] and ruthenium,[6] and bis(hexamethylbenzene)technetium[7] and rhenium.[8] Although some of these dimers have been known for almost half a century, there has been little discussion of their electronic structure in the literature, and only one example—the dimer of cyclopentadienyl benzene iron, 1 a2—has been crystallographically characterized (Figure 1).[9] Recently, we have found some of these dimers (including many shown in Figure 1), although reacting sufficiently slowly with oxygen that they can be handled in air (for many hours in the case of solids), are powerful reductants. These qualities make them useful n-dopants for organic electronic materials that have electron affinities as low as approximately 3.0 eV, such as 6,13-bis(tri(isopropyl)silylethynyl)pentacene (5, Figure 1), and for graphene.[10] Herein, we present an investigation of the geometric and electronic structure of a series of dimers of Group 8 metal cyclopentadienyl/arene complexes and of Group 9 metallocenes (Figure 1) by using DFT, X-ray crystallography, and electrochemistry to better understand this class of compounds. The

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Figure 1. Top: chemical structures of dimers considered here. Note that 3 b2 is formed exclusively as the isomer 3 b2“[1c] and 42 is formed as a mixture of 42” and 42’’’,[3] although all three isomers shown for 3 b2“ and 42 are considered computationally. The crystal structure of 1 a2 has previously been reported,[9] and those of 1 b2, 2 b2, 3 a2, 3 b2”, 3 c2, and 42“ are reported herein. Bottom: structures of model compounds for halves of the 3 a2 and 3 b2 dimers,[11c, 12] and the structure of 6,13-bis(tri(isopropyl)silylethynyl)pentacene (5).

implications of electrochemical redox potentials and computationally derived bond-dissociation energies for the reactivity of the dimers are discussed.

tion in the Supporting Information; additional discussion of the synthesis is also provided in the Supporting Information. Most of the dimers are at least sufficiently inert in the solid state to permit handling in air; however, 3 c2 is more air sensitive, which is consistent with its electrochemistry (see below).

Results and Discussion Compounds under consideration and synthesis

Electronic structure

Figure 1 shows the dimers studied in this work along with the numbering scheme. The numbers 1, 2, 3, and 4 denote Fe, Ru, Rh, and Ir species, respectively; the letters denote different alkylation patterns of the carbocyclic ligands of the constituent monomers, and the primes are used to distinguish between different isomers that may potentially be formed by a given monomer. Throughout M2 specifically refers to the dimer, M2C + to its (unstable) corresponding one-electron oxidation product, MC to the corresponding 19-electron monomer, and M + to the corresponding 18-electron monomeric cation. Related compounds obtained by hydride reduction of M + or HC abstraction by MC are denoted MH and several examples are also shown in Figure 1. Previously, we have employed many of these as dopants,[10] but herein we also consider the related known compound 42 (isomer mixture),[3] which has not hitherto been examined in this context, and the new compound 3 c2. The dimer that has previously been crystallographically characterized, 1 a2, is also shown, as are those of some other compounds referred elsewhere in the following sections. The dimers were obtained in low to good yields by reduction of the PF6 salts of the corresponding monomeric cations[5c, 10a, 11] by using either Na/Hg (1:99 by weight), Na/K (1:3 by weight; CAUTION!! HIGHLY PYROPHORIC), or, for 3 a2, decamethylcobaltocene. Syntheses of all of the dimers, except 3 c2, have previously been reported.[1c, 3, 5c, 6a, 10a] For cases in which the present syntheses differ, procedures are given in the Experimental Sec-

The molecular structures of 1 b2, 2 a2, 2 b2, 3 a2, 3 b2, 3 c2, and 42 (exo,exo isomers) were minimized by using M06/LANL2DZ/631G** DFT calculations as implemented in the Gaussian 09 (Revision B.01) software suite.[13] Before discussing the molecular orbitals of the dimers, we consider those of Group 9 cyclopentadienyl diene complexes, which can be regarded as models for each organometallic sandwich unit of a Group 9 metallocene dimer. The DFT-calculated HOMO (Figure 2) of [RhCp(h4butadiene)] (I; Cp = C5H5 ; Figure 1), is qualitatively similar to that previously deduced from a fragment approach and photoelectron spectroscopy.[14] There is an antibonding interaction between a metal-based d orbital (27 %) and the HOMO of a Cp anion, and a bonding interaction with the LUMO of the butadiene moiety. The large contribution from the ligands, particularly from Cp (44 %), to this orbital is also consistent with deductions based on the relative strengths of bands in photoelectron spectra acquired with He I and He II radiation.[14] The next three orbitals are primarily metal centered, though the HOMO2 shows some bonding character between the Rh d orbital and the Cp and butadiene p orbitals (Figure S3 in the Supporting Information). The analogous cyclopentadiene complex [RhCp(h4-CpH)], which can be regarded as the hydride-reduction product of 3 a + and is accordingly denoted 3 aH (Figure 1), exhibited a broadly similar set of frontier orbitals (Figures 2 and S3 in the Supporting Information). However, a significant difference is that in the HOMO, there is a signifi-

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Full Paper substitution pattern).[15] The HOMOs of these Group 8 models have greater d-orbital character than those of the Group 9 MCp’(diene) species, and can be regarded as arising from overlap of a d orbital with approximate d symmetry (assuming approximate cylindrical symmetry around the centroid–M–centroid axis) with the local LUMOs of two anionic ligands.[15] The HOMOs of the Group 8 dimers and hydride-reduced species are, like those of their Group 9 analogues, destabilized through interactions with the exo CC or CH s-bonding orbitals, with the effect being largest in the case of the dimers. The CC contributions to their HOMOs help explain some of the properties of the Groups 8 and 9 dimers (see below). Crystallographically determined molecular structures

Figure 2. MO correlation diagram for 3 aH, 3 a2, and I.

cant contribution from the exo CH s-bonding orbital; the antibonding interaction between the metal and this s-orbital results in a destabilization of the 3 aH HOMO (5.19 eV) relative to that of I (5.34 eV). In the dimer 3 a2, the orbitals occur in pairs of in-phase and out-of-phase combinations of the local I-like orbitals. Thus, the HOMO2 and HOMO3 are a near-degenerate pair of combinations of orbitals resembling the HOMO1 of I, whereas similar combinations of the local HOMO2 and HOMO3 orbitals give rise to two more pairs of near-degenerate molecular orbitals (MOs; Figure S3 in the Supporting Information). However, the degeneracy of the HOMO and HOMO1, which can be regarded as in-phase and out-of-phase combinations of the two local HOMOs of the I-like portions of the molecule, is significantly lifted (energies of 4.82 and 5.62 eV, respectively) through the destabilizing interaction of the in-phase combination with a CC s-bonding orbital associated with the central bond of the molecule (Figure 2). From each Rh, there is a d-orbital contribution of approximately 14 % to the HOMO, whereas the p-orbital contribution from the carbon atoms in the C C bond is quite small (4 % from each). A more localized analysis of the CC bond through Lewis-type natural bond orbitals (NBOs) confirmed that the CC bond is indeed a single s bond (Wiberg bond index 0.93), with an electron occupancy of 1.95 e; the hybrid composition is 0.707C(sp2.71) on each carbon atom, with both carbon atoms having 26.9 % s and 73.0 % p character. Qualitatively similar canonical and localized orbitals and trends in orbital energies are seen for the other Group 9 species (see Figure S4 in the Supporting Information for that of 42). The HOMOs of the Group 8 species (Figure S4 in the Supporting Information) are also closely related to those previously obtained for the appropriate models, in this case, MCp’(cyclohexadienyl) and MCp’(pentadienyl) derivatives (Cp’ is used here to indicate a generic cyclopentadienyl ligand with any Chem. Eur. J. 2014, 20, 15385 – 15394

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The crystal structures of six of the dimers were determined by single-crystal X-ray diffraction methods (see the Experimental Details, Table S1 in the Supporting Information, and, for data in CIF format. CCDC1005757 (1 b2), CCDC-1005758 (2 b2), CCDC-1005759 (3 a2), CCDC-1005760 (3 b2), CCDC-1005761 (3 c2), and CCDC-1005762 (42) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif). In the case of 3 b2, the structure confirms that the isomer obtained is the Cp*Cp-linked isomer, 3 b2“ (Cp* = C5Me5), as was previously deduced from NMR spectroscopy.[1c] NMR data indicated that 42 is formed as a 42”/ 42’’’ mixture;[3] the structure obtained for a crystal grown from this isomer mixture was isomorphous with that of 3 b2 ; that is, it is that of the major 42“ isomer. The structure of 3 c2 shows, consistent with its NMR spectra (see the Experimental Section in the Supporting Information), that dimerization occurs through the unsubstituted position of the Cp” (Cp“ = C5Me4H) ring. Figures 3 and 4 show the molecular structures obtained. In each case, the central intermonomer bond is exo with respect to each organometallic unit,[13b] similar to those found in the structures of 1 a2,[9] [Mn(arene)(CO)3] dimers,[16] and the K(C6H6)([18]crown-6) dimer,[17] which, similar to 1 b2 and 2 b2, all contain m-h5 :h5-bis(cyclohexadienyl) bridging ligands (i.e., ligands that can be formally regarded as dianionic dimers of arene radical anions). Molecules of 1 b2 and 2 b2 adopt precisely centrosymmetric trans conformations (i.e., the dihedral angle, f, defined by Cen1-C1-C2-Cen2, in which Cen1 and Cen2 are the centroids defined by the five-coordinated atoms of the cyclohexadienyl ligands and C1 and C2 are the two sp3 carbon atoms through which the monomers dimerize, is 1808) as a result of the crystallographic molecular positions, whereas 3 a2 adopts a near-centrosymmetric structure. These can be considered as the least sterically constrained staggered conformations around the central CC bond, which can be regarded as a 1,1,2,2-tetrasubstituted ethane. Similarly, f= 1808 for the tetrasubstituted CC bonds of 1 a2, and other derivatives of the same bridging ligand.[16a, 17] For compounds 3 b2 and 42, in

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Figure 3. Molecular structures of two Group 8 Cp*/arene dimers determined by X-ray crystallography (30 % probability thermal ellipsoids, metal atoms in the asymmetric units labeled).

Figure 4. Molecular structures of Group 9 metallocene dimers determined by X-ray crystallography (30 % probability thermal ellipsoids, metal atoms in the asymmetric units labelled). All four structures suffer from disorder (see the Experimental Section in the Supporting Information): in the case of 3 a2, this was modeled as an alternative conformation of the dimer (major only shown). For compound 3 c2, two positions were modeled for one of the terminal Cp* rings (major only shown). The disorder in the structures of 3 b2“ and 42” was not modeled, but is responsible for the large thermal parameters. Chem. Eur. J. 2014, 20, 15385 – 15394

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which the intermonomer CC bonds are pentasubstituted, there are no longer unique low-energy staggered conformations, and in the crystals f 708. Values of 43–608 and 1808 were found around hexa- and pentasubstituted central CC bonds, respectively, of various [Mn(arene)(CO)3 dimers].[16b, 18] In compound 3 c2, f= 768, despite a tetrasubstituted ethane bridge; this may be associated with minimizing intermonomer steric interactions between the a-methyl groups on the bridging ligand. In the Group 8 compounds, the dimers can be thought of as two connected [MCp’(cyclohexadienyl)] derivatives. The ranges of MC bond lengths, the non-bonded distances between the metal and the non-coordinated sp3 atom of the cyclohexadienyl rings, the fold and angle of the arene ligands of the present dimers, and the tilt angles between the planes formed by the coordinated atoms of the Cp’ and cyclohexadienyl rings all fall within similar ranges to those seen for other [FeCp’] and [RuCp’] derivatives of h5-cyclohexadienyl,[9, 19] h5-pentadienyl,[15a, 20] or h5-benzyl[21] ligands (see the Supporting Information for specific comparisons). The structures of the Group 9 dimers presented herein are the first to include m-h4 :h4-bi(cyclopentadienyl) ligands (which can be regarded as closed-shell neutral dimers of the cyclopentadienyl radical). Each half of the molecule can be regarded as a h5-Cp’/h4-cyclopentadiene sandwich, and therefore, the dimers show structural similarities to [RhCp’] and [IrCp’] derivatives of h4-cyclopentadiene,[22] h4-1,3-cyclohexadiene,[23] h4-butadiene,[24] h4-arene,[25] and h4-thiophene[26] ligands (see the Supporting Information for specific comparisons). Thus, MC bond lengths fall in similar ranges, with the average bond lengths to the Cp’ carbon atoms being longer than those to the diene ligand, the bridging ligands are folded in a similar way to, but to a lesser extent than, those of the Group 8 species, and moderate ring tilts are found. The values of r(C1C2), the experimentally determined lengths of the central CC bonds of the dimers (Table 1) and previously reported related structures[9, 16, 17, 27] are of particular interest. DFT values for the neutral dimers (also in Table 1, along with those obtained for the corresponding dimer radical cations[28]), generally agree well with the crystallographic values (excepting data for 42“ for which the experimental value is likely affected by unmodeled disorder), for example, reproducing the trend in r(C1C2) within the rhodocene dimer series (Figure S7 in the Supporting Information). None of these crystallographic or calculated values is remarkably long (again excepting the X-ray value for 42”). At first sight, this might seem surprising given that the bond lengths in the dimers of organic radicals have been found to increase linearly with decreasing bond dissociation energy,[29] and that the bonds in some of the present dimers are evidently rather weak. For example, 3 a2 and 3 b2“ have been shown to dissociate to monomers during sublimation,[1a, 10b] DFT values of DUdiss for these compounds are approximately 100 kJ mol1, and for 3 b2”, experimental estimates of DHdiss  105 kJ mol1 and DGdiss  25 kJ mol1 have been obtained,[10b] yet their central CC bond lengths (1.545(6) and 1.602(27) , respectively) are significantly shorter than the crystallographically determined central CC bond lengths of

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Full Paper Thermodynamics and kinetics of redox reactions

Table 1. Central CC bond lengths[a] [] and dissociation energies [kJ mol1] for neutral and cationic dimers from M06/LANL2DZ/6-31G(d,p) DFT calculations, along with crystallographically determined central CC bond lengths [] for neutral dimers.

M2

X-ray r(C1C2) M2

DFT r(C1C2)[a] M2 M2 C +

DFT DUdiss (THF)[b] M2 !2 MC M2C + !MC + M +

1 b2 2 a2 2 b2 3 a2 3 b2 ’ 3 b2’’ 3 b2’’’ 3 c2 42’ 42’’ 42’’’

1.553(3) – 1.559(2) 1.545(6) – 1.602(27) – 1.566(6) – 1.750(1)[c] –

1.537 1.548 1.557 1.547 1.547 1.570 1.618 1.568 1.533 1.554 1.603

+ 113.9 + 207.0 + 207.4 + 110.1 + 99.8 + 102.9 + 107.2 + 118.7 + 208.2 + 202.6 + 177.4

1.679 1.685 1.698 1.704 1.688 1.747 3.278 1.732 1.651 1.697 3.053

We have previously shown that some of the dimers discussed herein react with acceptors, such as 5, to form the corresponding acceptor radical anions (or dianions) and monomer cations,[10a,b] and that these reactions can take place by two different mechanisms (Figure 5).[10b] In mechanism A, cleavage of the

18.7 + 2.9 + 19.0 11.0 18.0 24.6 + 38.9 5.11 + 20.7 + 7.9 + 9.8

[a] Gas phase, but very similar in a dielectric medium. [b] Values for the gas phase are given in Table S4 in the Supporting Information. [c] Artifact of unmodeled disorder (see the Experimental Section in the Supporting Information).

Figure 5. Mechanisms for reduction of an acceptor by a dimeric reductant and associated thermodynamic quantities associated with the steps of the reaction.

the bulky alkanes (AdMe2C)2 (Ad = 1-adamantyl; 1.639 ) and (PhEt2C)2 (1.635 ) for which values of DHdiss of 183 and 187 kJ mol1, respectively, were estimated.[30] The authors of Reference [17] also remarked that the central CC bond of the K(C6H6)([18]crown-6) dimer (1.461(11) ) was unusually short given its ready dissociation in solution. However, bond length depends on the shared electron density between the two carbon atoms in the dimer, along with any steric interactions between the substituents, whereas the thermodynamic bond dissociation parameters depend both on these characteristics of the dimer and on factors that influence the stability of the dissociation products. In the current transition-metal systems, there is a considerable change in geometric and electronic structure associated with dissociation including formation of a new MC bond in the monomer, planarization of a ligand, and redistribution of spin density from the ipso carbon atom to the rest of the molecule, particularly the metal; hence, these species are not likely to fit a correlation developed by using a very different set of dimers of organic radicals, in which the monomers contain no new bonds relative to the dimers. There are also few clear trends in CC bond length within the present series of compounds, with no obvious correlation between the experimental or DFT bond lengths and experimental tendency to dissociation or DFT values of DUdiss (Table 1, Figure S8 in the Supporting Information). For example, the bond length for 2 b2 is slightly longer than those for 3 a2, even though 2 a2, and so presumably also 2 b2, sublime as intact dimers, whereas 3 a2 dissociates to monomers. The trend in bond lengths within the series of rhodocene dimers (3 b2“ > 3 c2 > 3 a2) can, however, presumably be largely attributed to steric effects since the CC bonds are respectively pentasubstituted, tetrasubstituted with bulky groups, and tetrasubstituted with less bulky substituents; a similar trend is seen for [Mn(arene)(CO)3] dimers.[16, 31]

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dimer is followed by electron transfer from monomer to acceptor, whereas in mechanism B, an electron-transfer reaction between the dimer and the acceptor is followed by dissociation of the dimer cation. Cyclic voltammetry (CV) studies of the neutral dimers and monomer cations provides some of the quantities shown in Figure 5, as well as demonstrating that M2 and M + species can be interconverted by redox reactions. CV data of the cations showed features attributable to the M + /MC couple (at ca. 2.8 to 1.8 V vs. FeCp2 + /0 in THF), with Iox/Ired values that vary with the scan rate and with the species in question (Figures S10 and S11 in the Supporting Information).[32] CV data of the dimers reveal irreversible oxidations (peak potentials 1.3 to 0.7 V vs. FeCp2 + /0 in THF at 50 mV s1) that are attributable to the M2 !M2C + process; subsequent reductive scans reveal the presence of the monomer cations (as was seen previously, for example, in the case of 3 b2[1c]). For some compound/scan rate combinations the M2 ! M2C + oxidation wave can be clearly seen in CVs of M + PF6 subsequent to scanning the M + /MC couple (as was previously reported for 3 a + system in a variety of solvents[1b]). Redox potentials have previously been reported for most of the couples under consideration[10b, 11d] and are summarized diagrammatically in Figure 6 (and numerically in Table S5 in the Supporting Information) along with hitherto unreported data for the oxidation of 3 c2 and 42. We have also performed M06/LANL2DZ/ 6-31G(d,p) DFT calculations of monomer and dimer ionization energies (IEs; Table S5 in the Supporting Information) and dissociation energies of dimer and dimer cations (Tables 1 and S4 in the Supporting Information).[33] The first step of mechanism A is dissociation of the neutral dimer to the corresponding 19-electron monomers, and so its feasibility will depend on the activation barrier DG°diss, which can be no smaller than DGdiss, relative to RT. The DFT-estimated values of DUdiss for the Ru and Ir species are approximately 100 kJ mol1 (ca. 1 eV) higher than the respective values for the Fe and Rh compounds; a similar trend is expected for DGdiss.

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Figure 6. Diagrammatic representation of electrochemical redox potentials for the M2C + /M2 (crosses, Epa values at 50 mV s1) and M + /MC (open squares, E1/2 or Epc values at 50 mV s1) couples in THF/0.1 m nBu4NPF6. The effective redox potentials of the dimer, that is, for the M + /0.5 M2 couple (open triangles), are estimated according to Equation (1), by using electrochemical values of E(M+/MC) and estimating DGdiss(M2) from the DFT values of DUdiss(M2) in THF (Table 1) according to DGdiss(M2)  DUdiss(M2)85 kJ mol1.[33] Values of E(M+/MC) for three monomeric cobaltocene derivatives,[11d] E(5/5C ; filled circle),[10b] and E(5C/52 ; filled diamond)[10b] recorded in the same electrolyte solution are also included for comparison.

The increase in DUdiss down a given group is consistent with the greater tendency of the heavier elements to comply with the 18-electron rule due to increasing overlap between ligand orbitals and the valence d orbitals. Comparison of Ru and Rh compounds indicates that for a given period, the bonds are stronger for the Group 8 mixed cyclopentadienyl/arene dimers than for the Group 9 metallocene dimers. This is consistent with: the observation that Fe compounds 1 a and 1 b dimerize,[5c] but that [CoCp2] and [CoCp*Cp] do not;[4a, c, 34] the observation that 3 a2 and 3 b2“ dimers sublime as monomers,[1a, 10b] whereas the 2 a2 dimer sublimes intact;[10b] and kinetic data indicating that 3 b2” dimer reduces 5 (Figure 1) by both mechanism A and B, whereas 2 b2 reacts exclusively by mechanism B.[10b, 35] These findings are also consistent with the electrochemical studies of reduction of monomer cations to the 19electron neutral monomers; the ratio of oxidation to reduction currents Iox/Ired is much smaller for the Ru and Ir species than for the Fe or Rh compounds,[1b,c, 3, 6a, 11d, 36] suggesting more rapid dimerization in the former case, consistent with the higher thermodynamic driving forces for dimerization suggested by the calculations (assuming that variations in the driving force, DG(2 MC !M2) = DGdiss, parallel those in DUdiss).[33] Values of DUdiss were also calculated for the corresponding one-electron oxidation products of the dimers, although the structures and energies for these dimer cations should be treated with extreme caution (Table 1).[28] In all cases, the process is calculated to be considerably more favorable than cleavage of the neutral dimer and, in the presence of a dielectric medium simulating THF, dissociation of the Fe and Rh dimer cations is calculated to be exothermic, whereas that for the Ru and Ir species is modestly endothermic. Because a dissociation reaction Chem. Eur. J. 2014, 20, 15385 – 15394

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is expected to be accompanied by a significant increase in entropy, the DFT values of DUdiss suggest that DGdiss for all the dimer cations in solution is likely exergonic at ambient temperature,[33] consistent with the instability of the dimer cations indicated by the irreversible oxidation waves observed in voltammograms of the dimers.[1b,c, 10b] The lengthening and weakening of the CC bond predicted on oxidation is consistent with the significant CC bonding character of the dimer HOMOs (see above).[37] The trends in ease of ionization of the monomers, according to both the redox potentials (Figure 6, Table S5 in the Supporting Information) and DFT-calculated IEs (Figure S9, Table S5 in the Supporting Information), within a given group are consistent with the expected tendency of the heavier transition elements to prefer higher oxidation states and comply with the 18-electron rule, and with the expected effects of ligand alkylation. Comparison of 2 a and 3 c shows that for a comparable degree of alkylation and for a given period, the Group 8 19electron complexes are more readily oxidized than their Group 9 counterparts, consistent with their lower formal oxidation state (RuI vs. RhII) and their lower nuclear charge. The ease of dimer ionization follows a slightly different pattern. Similar to the case of the monomers, alkylation lowers the calculated IE and cathodically shifts E(M2C+/M2) (3 a2 vs. 3 b2 vs. 3 c2). The calculated vertical IE values for the various isomers of 3 b2 and 42 suggest that the effect on IE of alkylation on the non-bridging ligands is greater, consistent with the greater contributions of these ligands to the HOMOs.[38] However, the effect of changing the metal within a group is less clear: for the Group 9 complexes, the Rh species 3 b2“ is slightly more easily oxidized than its Ir analogue 42”, whereas the comparison of 2 a2 versus 1 b2 is complicated by the different levels of bridging ligand alkylation. The potentials/IEs for Group 8 and Group 9 dimers are also fairly similar for a given period and comparable degree of ligand alkylation (2 a2 versus 3 c2), presumably due to the opposing effects of nuclear charge, formal oxidation state (RuII vs. RhI), and nature of the bridging ligand (formally, dianionic and neutral for the Group 8 and Group 9 species, respectively). The value of E(M2C+/M2) is of relevance to mechanism B, in which the first step is electron transfer from the dimer to an acceptor. We have previously shown that values of DH° and DG° for the reduction of 5 (E1/20/ = 1.45 V vs. FeCp2 + /0 in THF) in chlorobenzene through mechanism B increase from 2 b2 to 1 b2 to 3 b2“, consistent with the increasing values of E(M2+/M2) and, therefore, increasing endergonicity of the ratedetermining electron transfer.[10b] Consistent with this observation, we have found that under the same conditions, 3 c2, which is the most easily ionized of any of the dimers considered here, reacts much more rapidly with 5 than the other dimers, the rate being sufficiently fast that we were unable to reliably determine a rate constant using experiments analogous to those described in Reference [10 b]. More generally, although the dimers are oxidized at considerably less cathodic potentials than compounds that can reduce 5 by direct exergonic electron transfer, such as the 19electron [CoCp*2] (1.97 V vs. FeCp2 + /0 in CH2Cl2[4d] , 1.86 V in

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Full Paper THF[10a, 11d]) and [FeCp*(C6Me6)] (2.30 V vs. FeCp2 + /0 in DMF[4f, 39]) monomers, these potentials still indicate facile ionization compared to most neutral 18-electron sandwich compounds. For example, all are more readily oxidized than [FeCp*2] (E(M+/M) = 0.48 to 0.59 V vs. FeCp2 + /0 depending on solvent[4f]) and the 3 c2 dimer is oxidized at a similar potential to the 19-electron CoCp2 (E(M+/MC)  1.32 to 1.35 V vs. FeCp2 + /0 in CH2Cl2,[4d, f, 40] 1.33 V in THF[11d]). These relatively facile oxidations (and low calculated IEs) are consistent with the strong destabilization of the HOMOs by the contribution of the CC s-bonding orbital seen in the DFT calculations (see above). There is a paucity of electrochemical data for Group 8 MCp’(pentadienyl) or Group 9 MCp’(butadiene) analogues that are directly comparable with the dimers. However, the HOMOs of Group 9 MCp’(cyclopentadiene) compounds are calculated to be less destabilized than those of the dimers (Figure 2); consistent with this prediction, we found irreversible oxidation of 3 bH’’’ (Figure 1),[11c] at Epa = 0.11 V versus FeCp2 + /0 in THF, whereas that of 3 b2“ is seen at Epa = 0.95 V. Furthermore, a polarographic study indicated that another isomer of [RhCp(Cp*H)] (3 bH”, Figure 1) is also less readily oxidized than 3 b2“.[41] The overall thermodynamic reducing ability of a dimer (regardless of the mechanism by which it reacts with a given acceptor) is relevant to its use as a dopant, but cannot be directly obtained from cyclic voltammetry alone. The relevant quantity is the potential for the monomer cation/dimer couple, E(M+/0.5 M2), which is related to the electrochemically determined values of E(M+/MC) by Equation (1): EðMþ =0:5 M2 Þ ¼ EðMþ =MC Þ þ ð0:5=FÞDGdiss ðM2 Þ

ð1Þ

Estimates of DGdiss(M2) were obtained from the DFT DUdiss values given in Table 1 assuming a constant PDVdiss(M2) and DSdiss(M2) contribution for dissociation of each dimer;[33, 42] these were combined with experimental values of E(M+/MC) to give the values of E(M+/0.5 M2) shown in Figure 6 (and Table S5 in the Supporting Information). These data suggest there is significantly less variation in overall thermodynamic reducing ability than in the monomer redox potential; that is, variations in E(M+/M) are largely offset by variations in DUdiss(M2). For example, E(M+/M) for the iridocene species 4 is more cathodic than that for its Rh analogue 3 b, but DUdiss of 42“ is larger than that of 3 b2”; these differences are both largely attributable to the greater instability of the 19-electron species for the case of Ir, and cancellation of the two effects results in similar values of E(M+/0.5 M2) for the two dimers, although, of course, their kinetic reactivity may be very different (see below). With the exception of that for 3 a + /0.5 3 a2, these potentials only vary over a range of 0.2 V. Values of the effective dimer IE based on DFT values of IE(MC) and DUdiss(M2) (Table S5 and Figure S9 in the Supporting Information) show a little more variation (0.44 eV, excluding 3 a2), but a similar overall pattern to that seen in values of E(M+/0.5 M2). The difference between DGdiss for the dimer and for the dimer cation can, through Hess’s law, be directly related to the Chem. Eur. J. 2014, 20, 15385 – 15394

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difference in the redox potentials for dimer and monomer [Eq. (2)]: DGdiss ðM2 ÞDGdiss ðM2 C þ Þ ¼ F½EðM2 C þ =M2 ÞEðMþ =MC Þ

ð2Þ

whereas the difference in the corresponding DFT DUdiss values are related in an analogous way to the difference in DFT dimer and monomer IEs. Thus, the larger difference in dimer and monomer redox potentials seen for the Ru and Ir species (see Figure 6 and its DFT equivalent, Figure S9 in the Supporting Information) suggest a larger difference in dimer and dimer cation bond-dissociation (free) energies than for the Fe and Rh compounds. According to the calculations (Table 1), most of this variation is due to that in DUdiss(M2) rather than DUdiss(M2+), that is, at least within the current set of compounds, the magnitude of E(M2C+/M2)E(M+/M) is roughly indicative of the magnitude of DGdiss. Dimer air stability The estimated effective redox potentials, E(M+/0.5 M2 ; Figure 6, Table S5 in the Supporting Information), for all the dimers examined are considerably more reducing than the O2/O2C couple (E1/2  1.2 to 1.4 V vs. FeCp2 + /0 in a variety of aprotic solvents),[43] indicating that oxidation by molecular O2 to the corresponding cations is thermodynamically feasible. NMR studies showed that several examples are indeed converted to the cations in solution on exposure to air.[10a] Nonetheless, these reactions are in some cases slower than oxidation by acceptors, such as 5 (E1/2 = 1.45 V in THF[10a,b]) at comparable dimer concentrations.[44] The high reorganization energy for the O2/O2C couple,[45] the moderate solubility of O2 in organic solvents,[46] and the kinetics of O2 dissolution in organic solvent may play a role in limiting the rate of dimer-to-O2 electron transfer, which is expected to be the rate-limiting step, if the oxidation proceeds by a pathway resembling mechanism B. Most of the dimers are even more inert to air in the solid state, presumably due to additional kinetic factors. But as was noted above, 3 c2 is by far the least stable, rapidly oxidizing in both solution and as a solid, consistent with this dimer exhibiting the most cathodic value of E(M2C+/M2) (1.29 V) and, consequently, an electron-transfer reaction with O2 that is much less endergonic than for the other dimers and that may even be exergonic. Indeed, as was noted above, the dimer potential for 3 c2 is similar to that for monomeric air-sensitive [CoCp2].[47] Identification and development of improved dopants In light of the above discussions of redox reactions and air stability, we now consider the requirements for obtaining similar dimeric dopants that exhibit potentially advantageous characteristics: (a) that can be processed in air along with an acceptor without reacting to form highly sensitive acceptor radical anions, with activation of the dopant taking place subsequently; or, (b) that are stronger donors, yet nonetheless retain air stability. As was previously discussed in Reference [10 b], achieving the first of these goals means suppressing reaction of

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Full Paper the dimer through both mechanisms A and B for a given acceptor, while retaining thermodynamic feasibility for the reaction. Compound 42 is particularly promising in terms of these requirements. Firstly, with the exception of 3 a2, the isomers of 42 exhibit the most anodic values of dimer oxidation potential (and highest calculated vertical IEs) of the dimers considered herein, which suggests reaction with a given acceptor through mechanism B will be slower than for most of the other dimers. Secondly, the separation between dimer and monomer potentials is one of the largest, indicating large differences between DGdiss(M2) and DGdiss(M2C+), which, according to the calculations, arises from large values of DUdiss(M2), similar to those of the ruthenium dimers, 2 a2 and 2 b2 ; this, in turn, suggests that mechanism A is effectively shut off for the reduction of acceptors. Thus, the combination of a strong bond and the relatively high IE of the dimer are anticipated to make 42 one of the least reactive of the dimers from a kinetic point of view, even though its estimated thermodynamic strength as a reductant, E(M+/0.5 M2), is similar to that of most of the others (including its Rh analogue, 3 b2). Although all the other dimers considered react with 5 in solution in the dark, we find that 42 (isomer mixture) is inert under these conditions. However, in ambient light, the reaction does proceed (as shown in Figure S12 in the Supporting Information), presumably through electron transfer from 42 to photoexcited 5, demonstrating the thermodynamic feasibility of the reaction, consistent with expectations based on values of E(M+/0.5 M2) and E(5/5C). The development of stronger dopants, i.e., dopants with more reducing values of E(M+/0.5 M2), will, as indicated by Equation 1, require more reducing values of E(M+/MC) and/or lower values of DGdiss(M2) (and DUdiss(M2)). Approaches to cathodically shifting the value of E(M+/MC) will, however, have to be chosen carefully in order to avoid also shifting E(M2C+/M2) value, as this may result in air sensitivity, as seen in the rhodocene series in which increased methylation shifts E(M+/MC) and E(M2C+/M2) to comparable extents and in which 3 c2 is rather air sensitive. The alternative of introducing features that lower DUdiss(M2) is not applicable to Fe and Rh species, which already contain rather weak bonds so that any significant further weakening would result in species that exist preferentially as highly sensitive monomers (as is in fact the case for some 19electron Fe sandwiches[5c] and for [Rh(h5-C5Ph4H)2][2]). However, the relatively strong CC bonds of the neutral Ru and Ir species do significantly limit the overall reducing ability; thus, introduction of features into the Ru or Ir structures that weaken the central CC bond of the neutral (and presumably generally also the cationic) dimers, while retaining a comparable monomer (and dimer) redox potential, could be a promising approach. In principle, increased steric strain around the central CC bond would lower DUdiss and DGdiss for both neutral and cationic dimers. However, a more detailed understanding of the role of sterics in these compounds is required in the light of calculated values of DUdiss(M2) and DUdiss(M2C+) for the various isomers of the Group 9 pentamethylmetallocene dimers (Table 1), which show no straightforward relation with the substitution pattern of the bridging ligand. Moreover, it will also be challenging to balance the effects of substituents on the Chem. Eur. J. 2014, 20, 15385 – 15394

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dissociation energy of a putative dimer with their effect on the distribution of possible alkali-metal reduction products of the monomer cations, given that other species, such as hydride-reduction products and two-electron reduction products, can be obtained instead of dimers as a result of relatively minor structural changes.[1c, 6, 36]

Conclusion The crystallographically determined molecular structures of the dimerized Group 8 mixed Cp’/arene sandwich compounds closely resemble pairs of Cp’/cyclohexadienyl sandwiches linked through the exo substituent in the uncoordinated position in the cyclohexadienyl ring. In a similar way, the dimers of Group 9 metallocenes can be regarded as linked pairs of Cp’/ cyclopentadiene sandwiches. Despite some of these dimers exhibiting rather low dissociation energies, the central CC bonds are much shorter than those predicted for dimers of organic radicals with comparable dissociation energetics. This is consistent with the bond strengths depending on a balance between orbital overlap and steric influences in the dimers and the various factors that (de)stabilize the monomers, including formation of a new metalC bond in place of the broken CC bond, ligand planarization, and spin delocalization onto the metal. The orbital structures of the dimers also resemble those of Group 8 Cp’/cyclohexadienyl or Group 9 Cp’/cyclopentadiene sandwiches, but differ in that the highest occupied molecular orbitals of the dimers are significantly destabilized relative to those of monometallic model compounds by contributions from the s-bonding orbital associated with the central CC bond, a feature that explains both the relatively low oxidation potentials of the dimers, and the ease of dissociation of their one-electron oxidation products. The relationships between electrochemical redox potentials, dissociation energetics, and overall reducing strength have been used to develop guidelines for identifying and/or developing dimeric dopants 1) with greater dopant strength, yet retaining air stability; or 2) with low kinetic reactivity, despite retaining a high thermodynamic reducing power. Using these guidelines, 42 was identified as a candidate to demonstrate the latter; unlike all the other dimers, it does not react with 5 in solution in the dark, but readily reacts under illumination, illustrating its thermodynamic competence as a dopant. These guidelines may also help in the development of other classes of dimeric dopants, such as those formed by organic radicals.[48]

Acknowledgements We thank the NSF (through DMR-1305247, IIA-1301346, and the PREM program, DMR-0934212), Solvay SA, and the ONR (N00014-11-1-0313 and N00014-14-1-0171) for funding. We also thank Song Guo and Catherine Robinson for investigating the reaction of 3 c2 with 5. Keywords: density functional calculations · doping · redox chemistry · sandwich complexes · structure elucidation

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[14] [15]

[16]

[17] [18] [19]

[20]

[21] [22]

[23] [24]

[25]

[26] [27] [28]

[29] [30]

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however, NMR spectroscopy indicated that, with the exception of 42, the compounds under consideration are formed as a single isomer and in all crystallographically characterized cases, these are found to be the exo,exo isomers shown in Figure 1; accordingly, endo,endo and endo,exo isomers are not considered further in the main text. DFT calculations of isomer stability are, however, discussed in the Supporting Information. J. C. Green, P. Powell, J. E. van Tilborg, Organometallics 1984, 3, 211. a) R. Gleiter, I. Hyla-Kryspin, M. L. Ziegler, G. Sergeson, J. C. Green, L. Stahl, R. D. Ernst, Organometallics 1989, 8, 298; b) A. Rajapakshe, M. A. Paz-Sandoval, J. A. Gutierrez, M. E. Navarro-Clemente, P. Jurez Saavedra, N. E. Gruhn, D. L. Lichtenberger, Organometallics 2006, 25, 1914. a) S. Lee, S. R. Lovelace, D. J. Arford, S. J. Geib, S. G. Weber, N. J. Cooper, J. Am. Chem. Soc. 1996, 118, 4190; b) M. V. Gaudet, A. W. Hanson, P. S. White, M. J. Zaworotko, Organometallics 1989, 8, 286. P. B. Hitchcock, M. F. Lappert, A. V. Protchenko, J. Am. Chem. Soc. 2001, 123, 189. C.-W. Hsu, C.-L. Liao, Z.-X. Ma, C. Y. Ng, J. Phys. Chem. 1995, 99, 1760. a) M. Bottrill, M. Green, E. O’Brien, L. E. Smart, P. Woodward, J. Chem. Soc. Dalton Trans. 1980, 292; b) I. R. Butler, W. R. Cullen, W. E. Lindsell, P. N. Preston, S. J. Rettig, J. Chem. Soc. Chem. Commun. 1987, 439; c) M. J. Zaworotko, K. C. Sturge, P. S. White, J. Organomet. Chem. 1990, 389, 333; d) F. Urbanos, M. A. Halcrow, J. Fernandez-Baeza, F. Dahan, D. Labroue, B. Chaudret, J. Am. Chem. Soc. 1993, 115, 3484; e) F. C. Pigge, R. Dhanya, D. C. Swenson, Organometallics 2009, 28, 3869; f) R. M. Chin, A. Simonson, J. Mauldin, J. Criswell, Organometallics 2010, 29, 3868. a) V. Kulsomphob, G. C. Turpin, K.-C. Lam, C. Youngkin, W. Trakarnpruk, P. Carroll, A. L. Rheingold, R. D. Ernst, J. Chem. Soc. Dalton Trans. 2000, 3086; b) R. Basta, D. R. Wilson, H. Ma, A. M. Arif, R. H. Herber, R. D. Ernst, J. Organomet. Chem. 2001, 637 – 639, 172; c) I. A. Guzei, M. E. SnchezCastro, A. Ramirez-Monroy, M. Cervantes-Vsquez, I. R. Alemn Figueroa, M. A. Paz-Sandoval, Inorg. Chim. Acta 2006, 359, 701; d) A. Tuchscherer, C. Georgi, N. Roth, D. Schaarschmidt, T. Rffer, T. Waechtler, S. E. Schulz, S. Oswald, T. Gessner, H. Lang, Eur. J. Inorg. Chem. 2012, 2012, 4867. J. R. Hamon, D. Astruc, E. Roman, P. Batail, J. J. Mayerle, J. Am. Chem. Soc. 1981, 103, 2431. a) M. I. Bruce, P. A. Humphrey, J. K. Walton, B. W. Skelton, A. H. White, J. Organomet. Chem. 1987, 333, 393; b) B. T. Donovan, R. P. Hughes, A. S. Kowalski, H. A. Trujillo, A. L. Rheingold, Organometallics 1993, 12, 1038; c) T. Cadenbach, C. Gemel, R. Schmid, R. A. Fischer, J. Am. Chem. Soc. 2005, 127, 17068. J. Hmlinen, T. Hatanp, E. Puukilainen, L. Costelle, T. Pilvi, M. Ritalaa, M. Leskel, J. Mater. Chem. 2010, 20, 7669. a) A. Salzer, H. Schmalle, R. Stauber, S. Streiff, J. Organomet. Chem. 1991, 408, 403; b) J. Baghdadi, N. A. Bailey, A. S. Dowding, C. White, J. Chem. Soc. Chem. Commun. 1992, 170; c) M. C. Comstock, J. R. Shapley, Organometallics 1997, 16, 4816; d) A. C. Esqueda, S. Conejero, C. Maya, E. Carmona, Organometallics 2010, 29, 5481. a) T. W. Bell, M. Helliwell, M. G. Partridge, R. N. Perutz, Organometallics 1992, 11, 1911; b) J. Mller, T. Akhnoukh, P. E. Gaede, A.-l. Guo, P. Moran, K. Qiao, J. Organomet. Chem. 1997, 541, 207; c) P. D. Zeits, T. Fiedler, J. A. Gladysz, Chem. Commun. 2012, 48, 7925. a) J. Chen, R. J. Angelici, Organometallics 1989, 8, 2277; b) A. E. Ogilvy, A. E. Skaugset, T. B. Rauchfuss, Organometallics 1989, 8, 2739. S. C. N. Hsu, W.-Y. Yeh, G.-H. Lee, S.-M. Peng, J. Am. Chem. Soc. 1998, 120, 13250. The structures calculated for these cations are found to be symmetrical with the most significant geometric change relative to the neutral dimers being a considerable elongation of the central CC bond. However, standard DFT methods are prone to artificial overdelocalisation of odd-electron systems, such as these, due to electron self-interaction error (see W. Koch, M. C. Holthausen, A Chemist’s Guide to Density Functional Theory, Wiley & Sons, New York, 2000). These effects are reflected here by the spuriously long central CC bonds in 3 b2’’’ + and 42’’’ + , and the negative eigenmodes determined through frequency analyses of these species, which are indicative of possible transition-state structures. A. A. Zavitsas, J. Phys. Chem. A 2003, 107, 897. a) G. Kratt, H.-D. Beckhaus, C. Rchardt, Chem. Ber. 1984, 117, 1748; b) A. Flammter Meer, H.-D. Beckhaus, K. Peters, H.-G. von Schnering, C. Rchardt, Chem. Ber. 1985, 118, 4665; c) C. Rchardt, H.-D. Beckhaus, Angew. Chem. 1985, 97, 531; Angew. Chem. Int. Ed. Engl. 1985, 24, 529.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper [31] The dimer of Mn(biphenylene)(CO)3 (Ref. [18]) is an exception to this trend, with a central CC bond length of 1.572(4) , perhaps due to the effects of the strained biphenylene ring system on the intermonomer steric interactions in this complex. [32] In principle, scan-rate-dependent Iox/Ired data can be used to obtain rate constants for dimerisation of MC (M. L. Olmstead, R. G. Hamilton, R. S. Nicholson, Anal. Chem. 1969, 41, 260); such studies of some of the systems described in this work are currently in progress. [33] In our previous work (Ref. [10 b]), we estimated DHdiss and DGdiss for cleavage reactions by using DFT. However, such calculations require obtaining a description of the vibrational modes of the species under consideration. As noted in the Experimental Section in the Supporting Information, vibrational-frequency analyses revealed that some of the optimised structures possessed a few negative low-frequency eigenmodes, precluding reliable determination of DGdiss. Accordingly, in this work, we have generally restricted our discussion to DUdiss. To estimate the effective redox potential for the monomer cation/dimer couple (Table S4 in the Supporting Information and Figure 6); however, values of DGdiss are required, and so were estimated by assuming a constant conversion between DUdiss and DGdiss, which is not an unreasonable approximation given the similar nature of the reaction in each case. This conversion was obtained by comparing values of DUdiss and DGdiss for the dissociation of 3 a2 to 3 aC (for which no negative eigenmodes were found) from reference [10b]. [34] U. Kçlle, F. Khouzami, B. Fuss, Angew. Chem. Suppl. 1982, 230. [35] In Reference [10 b], we only found evidence for mechanism B in the reaction of 1 b2 and 5, despite a calculated value for DUdiss similar to that for 3 b2“. However, due E(M2·+/M2) for 1 b2 being more cathodic than that for 3 b2”, reaction of 1 b2 and 5 through mechanism B is more rapid than that of 3 b2“ and 5 and so, at the concentrations examined and for this particular acceptor, mechanism B presumably dominates. [36] S. K. Mohapatra, A. Romanov, T. V. Timofeeva, S. R. Marder, S. Barlow, J. Organomet. Chem. 2014, 751, 314. [37] Similarly, the CH contributions to the HOMO of species, such as 3 aH, suggests that the exo s bonds of Group 9 metal cyclopentadienyl/cyclopentadiene derivatives are also likely to be easily broken following ionisation. This is consistent with the observation that the strongest peaks in the electron-impact mass spectra of 3 bH“ (with an exo-CH3 group, Figure 1) and 3 bH’’’ (exo-H) correspond to [RhCp”Cp + ] and [RhCp*Cp + ], respectively.[11c] [38] The same trend is seen in the adiabatic IEs of M2’ and M2“, but the M2’’’ derivatives show lower values than their M2’ or M2’’ counterparts. This can be attributed to the very large relaxation energies associated with the physically unreasonable CC bond lengths obatained for these two species. [39] D. S. Brown, M. H. Delvilledesbois, R. Boese, K. P. C. Vollhardt, D. Astruc, Angew. Chem. 1994, 106, 715; Angew. Chem. Int. Ed. Engl. 1994, 33, 661. [40] S. Barlow, Inorg. Chem. 2001, 40, 7047. [41] N. A. Ustynyuk, M. G. Peterleitner, O. V. Gusev, L. L. Denisovich, Russ. Chem. Bull. 1993, 42, 1727. [42] Values of DGdiss are not easily determined experimentally, although a maximum value of + 25 kJ mol1 was inferred in the case of 3 b2“

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[43]

[44]

[45]

[46]

[47]

[48]

from a combination of electrochemical and reactivity data. In the case of Ru and Ir species, the extent of dissociation is expected (based on the DFT calculations and, for the Ru species, the crossover experiments described in References [10 a,b]) to be very small. Although Fe and Rh species are expected to dissociate to a more significant extent, measurement of the extent of dissociation is still problematic; although ESR technique has been used in studies of dissocation of CC into radicals (e.g., N. I. Tzerpos, A. K. Zarkadis, R. P. Kreher, L. Repas, M. Lehnig, J. Chem. Soc. Perkin Trans. 2 1995, 755), orbitally degenerate (or near-degenerate) sandwich compounds are generally ESR silent, except at very low temperatures (e.g., J. H. Ammeter, J. Magn. Reson. 1978, 30, 299). Values for E1/2(O2/O2C) converted from values quoted versus saturated calomel electrode; M. E. Peover, B. S. White, Electrochim. Acta 1966, 11, 1061) to vs. FeCp2 + /0 by using corrections from Reference [4f]. If O2C further reacts with other species present, for example, protic substances, such as atmospheric water, the overall reaction may be even more exergonic. For example, the potential of the O2, 2 H2O/4 OH aqueous couple is + 0.40 V versus NHE (Handbook of Physics and Chemistry, 63edrd ed(Eds.: R. C. Weast, M. J. Astle), CRC, Boca Raton, 1982), that is, roughly 0.3 V versus nonaqueous FeCp2 + /0. The second-order (mechanism B) reactions of mm concentrations of Group 8 dimers with sub-mm concentrations of 5 are complete within a few hours, yet mm dimer solutions in certain solvents are only approximately 50 % oxidized after 70 h exposure to air.[10a,b] A value of 190 kJ mol1 has been determined in water (J. Lind, X. Shen, G. Mernyi, B. . Jonsson, J. Am. Chem. Soc. 1989, 111, 7654); although presumably smaller in non-aqueous solvents, strong solvation of the small superoxide ion and the large difference in bond length associated with the reduction are still expected to lead to a much larger reorganization energy than that, for example, the 5/5C couple. For example, under 1 atm O2 the solubility of O2 is approximately 2 mm in THF; it is also of similar magnitude in various aromatic solvents (R. Battino, T. R. Rettich, T. Tominaga, J. Phys. Chem. Ref. Data 1983, 12, 163; M. Quaranta, M. Murkovic, I. Kilmant, Analyst 2013, 138, 6234). Although this sensitivity is potentially a drawback for applications, it is worth noting that the dimer is still rather inert for a compound with an estimated effective reducing potential of approximately 2.2 V (see Table S5 in the Supporting Information); in contrast monomeric species, such as [FeCp(C6Me6)], with comparable reducing potentials (2.00 V vs. FeCp2 + /0 in DMF, converted from a value reported vs. SCE in J. C. Green, M. R. Kelly, M. P. Payne, E. A. Seddon, D. Astruc, J. R. Hamon, P. Michaud, Organometallics 1983, 2, 211 following Ref. [4f]) are considerably more air sensitive than [CoCp2]. a) M. Limmert, H. Hartmann, O. Zeika, A. Werner, M. Ammann, Eur. Pat. Appl. EP 1837926 A1, 2007; b) O. Zeika, M. Limmert, A. Grssing, A. Lux, H. Hartmann, A. Werner, Eur. Pat. Appl. EP 1837927 A1, 2007; c) B. D. Naab, S. Zhang, K. Vandewal, A. Salleo, S. Barlow, S. Marder, Z. Bao, Adv. Mater. 2014, 26, 4268.

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