Diruthenium Naphthalene and Anthracene Complexes Containing a

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Diruthenium Naphthalene and Anthracene Complexes Containing a Doubly Linked Dicyclopentadienyl Ligand Hélori Salembier,§ Joshua Mauldin,† Tom Hammond,† Rodney Wallace,† Essa Alqassab,† Michael B. Hall,‡ Lisa M. Pérez,‡ Yuen-Jing Alexis Chen,‡ Katherine E. Turner,‡ Eric Bockoven,‡ William Brennessel,⊥ and Robert M. Chin*,† †

Department of Chemistry and Biochemistry, University of Northern Iowa, Cedar Falls, Iowa 50614-0423, United States Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States § Sciences Chimiques de Rennes, Ecole Nationale Supérieure de Chimie de Rennes, 35708 Rennes Cedex 7, France ⊥ Department of Chemistry, University of Rochester, Rochester, New York 14627, United States ‡

S Supporting Information *

ABSTRACT: The reaction of cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4Br2 with naphthalene affords the syn-facial [cis{(η5-C5H3)2(CMe2)2}Ru2(μ-η6,η6-C10H8)][OTf]2, (22+), a complex that appears to be two electrons short of the 18electron rule. Density functional theory (DFT) calculations suggest that the Ru atoms satisfy their missing valence by a combination of a weak metal−metal bond and sharing electrons from the central π bond of the naphthalene. The one-electron reduction of 22+ yields 2+, a Class II mixed-valence complex, while the two-electron reduction of 22+ causes a hapticity change from η6 to η4 on one of the naphthalene rings and yields cis-{(η5C5H3)2(CMe2)2}Ru2(μ-η6,η4-C10H8) (20), a zwitterionic complex. The DFT calculations predict that the Cs isomer of 20 is 4.69 kcal/mol lower in energy than the C2v isomer, which is a transition state. Reaction of cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4Br2 with anthracene affords the analogous syn-facial anthracene complex, [cis-{(η5-C5H3)2(CMe2)2}Ru2(μ-η6,η6-C14H10)][OTf]2, (4), and the tetranuclear dianthracene complex, [cis-{(η5-C5H3)2(CMe2)2}Ru2(μ-η6,η6-C14H10)]2[OTf]4, (5). 22+, 20, and 5 were structurally characterized by X-ray diffraction.

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INTRODUCTION We recently reported the synthesis of [cis-{(η 5 C5H3)2(CMe2)2}Ru2(MeCN)6][OTf]2 (1), a diruthenium complex with a doubly linked dicyclopentadienyl ligand and six labile acetonitrile ligands.1 1 was synthesized via a dibenzene complex, and our hope is that 1 proves to be as synthetically versatile as the reported mononuclear complex [(η5-C5H5)Ru(MeCN)3]+ that was first reported by Gill and Mann in 1982.2 The synthesis of 1 is a three-step process starting from cis-{(η5C5H3)2(CMe2)2}Ru2(CO)4Br2. The displacement of the CO and Br ligands is achieved by the addition of AgOTf and benzene in a single “pot” reaction to form cis-[{(η5C5H3)2(CMe2)2}Ru2(η6-C6H6)2][OTf]2. Stepwise addition of H− and H+ to the benzene ligand leads to the loss of 1,3cyclohexadiene and the addition of six acetonitriles to the ruthenium centers to form 1 (Scheme 1). Recently, Kündig and co-workers reported an alternative synthetic pathway to make [(η5-C5H5)Ru(MeCN)3]+ that uses [(η5-C5H5)Ru(η6-C10H8)]+ as an intermediate.3 The advantage of using naphthalene as the labile ligand is that thermal substitution occurs at room temperature, and photolytic displacement of the arene ligand is not necessary. Spurred by Kündig’s work, we started investigating the synthesis and reactivity of our diruthenium complexes with naphthalene (Np) and anthracene (An) ligands. This work has resulted in naphthalene and anthracene ligands bridging the two © XXXX American Chemical Society

Scheme 1

ruthenium metal centers. While there are examples of bimetallic complexes containing a bridging naphthalene ligand, most of the reported complexes are in the anti-facial configuration.4 Given the steric constraints of placing two metal centers on the same face of a naphthalene molecule, it is not too surprising that syn-facial naphthalene bimetallic complexes are rare. Received: May 7, 2012

A

dx.doi.org/10.1021/om300384d | Organometallics XXXX, XXX, XXX−XXX

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Sweigart and co-workers have provided some of the few examples of bimetallic syn-facially bound naphthalene complexes (Chart 1).5 Chart 1

Herein, we describe the syntheses, characterization, and properties of three new syn-facial naphthalene and anthracene diruthenium complexes.

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RESULTS AND DISCUSSION Reaction of cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4Br2 with naphthalene and AgOTf in MeCN (180 °C, 90 min) yields the orange, air-stable [cis-{(η 5 -C 5 H 3 ) 2 (CMe 2 ) 2 }Ru 2 (μ-η 6 ,η 6 C10H8)][OTf]2 (22+) in 78% yield (eq 1).

Figure 1. ORTEP of the 22+(50% probability). Hydrogens, anions, and solvent molecules are omitted for clarity. Selected bond distances (Å) and angles (deg): Ru(1)···Ru(2), 3.0674(4); Ru(1)−Cp(centroid), 1.806; Ru(1)−C(1), 2.438(2); Ru(1)−C(2), 2.227(2); Ru(1)−C(3), 2.202(2); Ru(2)−C(1), 2.450(2); Ru(2)−C(4), 2.232(3); Ru(2)− C(5), 2.197(3); Cp(centroid)−Ru(1)−arene centroid, 168; Cp−Cp fold angle, 141.8.

The 1H NMR spectrum of 22+ shows two upfield resonances for the α and β hydrogens of the naphthalene ligand at 6.65 and 5.52 ppm. The dicyclopentadienyl ligand has a single set of double and triplet resonances at 5.58 and 5.55 ppm for the cyclopentadienyl ring hydrogens. The integrations of the resonances’ signals are consistent with one naphthalene molecule with one dicyclopentadientyl ligand. The X-ray structure of 22+ confirmed that a single naphthalene is symmetrically bound to the two ruthenium centers in a syn facial arrangement (Figure 1). The naphthalene molecule is slightly bent out of planarity with an angle of 10.3(4)° between the C(1)−C(1A)−C(2A)− C(2) and C(2)−C(2A)−C(3A)−C(3) planes and an angle of 10.4(5)° between the C(1)−C(1A)−C(4A)−C(4) and C(4)− C(4A)−C(5A)−C(5) planes. The Ru···Ru distance is 3.0674 Å and might be assigned as a partial Ru−Ru bond based on this distance being 0.2−0.3 Å longer than the Ru−Ru single bonds in cis-{(η 5 C5H3)2(SiMe2)2}Ru2(CO)4 and cis-{(η5-C5H2(tBu))2(CMe2)2}Ru2(μ-Cl)2Cl2.6,7 Atoms in molecules (AIM) analysis shows a bond critical point (BCP) between the two metal centers with an electron density value (ρ) of 0.024 e−/Å3 at an optimized bond length of 3.18 Å. A molecule with a Ru−Ru single bond, cis-{(η5C5H3)2(CMe2)2}Ru2(CO)4, has a ρ = 0.042 e−/Å3 at the BCP with an optimized distance of 2.87 Å (Figure 2). Together, the smaller ρ value and the closeness of the BCP to the ring critical point (RCP), shown just below it in Figure 2, suggest a weak Ru−Ru interaction for 22+. An alternative analyses by natural bond orbital (NBO) methods results in a clear Ru−Ru bond for cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4, but no bond for 22+, where each Ru has a d6 configuration of essentially nonbonding electrons. The Raman spectrum of 22+ is too ambiguous to add any meaningful data that would demonstrate the presence of a Ru−

Figure 2. Atoms in molecules (AIM) analyses of the optimized geometries of cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4 and 22+. The calculated Ru−Ru bond distances are 2.87 and 3.18 Å, respectively. The BCPs are yellow, the RCPs are violet, and the cage critical points (CCPs) are green. The C−C bond distances (Å) are given for the free Np and the complexed Np.

B


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and sharing of the electron density on the central double bond. We offer this explanation, as closely related Ru d6 complexes appear to satisfy the 18-electron rule either by bonding to the necessary number of ligands or by forming a strong Ru−Ru bond. The UV−vis spectrum of 22+ has a weak absorption peak at 480 nm (ε = 393 M−1 cm−1) (Figure 4) that the timedependent density functional theory (TD-DFT) assigns as an allowed (C2v) transition from the HOMO (a2) to the LUMO (b2), with both orbitals that are mainly Ru d in character (see the Supporting Information for further details). Irradiation of 22+ with visible light (quartz halogen or compact fluorescence source) results in the photorelease of one of the naphthalene rings and the subsequent coordination of three acetonitriles to one ruthenium center to form [cis-{(η5C5H3)2(CMe2)2}Ru2(MeCN)3(η6-C10H8) ][OTf]2 3 (Scheme 2). The 1H NMR spectrum of 3 shows two sets of inequivalent cyclopentadienyl ring resonances. The first set, consisting of a doublet (4.06 ppm) and triplet (4.30 ppm), is assigned to the cyclopentadienyl ligand bound to the ruthenium center with the acetonitriles since the 4−4.5 ppm range is the typical range for the cyclopentadienyl ring hydrogens of [(η5-C5H5)Ru(MeCN)3]+-type complexes.2 The second set of resonances at 4.86 ppm for the doublet and 4.71 ppm for the triplet is assigned to the cyclopentadienyl ligand bound to the ruthenium center that is coordinated to one of the naphthalene rings. In addition, we observe four resonances for the naphthalene ligand, two of which are shifted upfield (7.09 and 6.47 ppm) for the coordinated ring and two for the uncoordinated ring (7.79 and 7.68 ppm). We do not observe the bound acetonitrile ligands due to the rapid exchange with the deuterated acetonitrile solvent. Heating an acetonitrile solution of 3 at 70 °C for 18 h resulted in the formation of 1 and the appearance of free naphthalene. The re-formation of 22+ did not occur at this temperature. A kinetic analysis of the conversion of 3 to 1 gives an activation energy of 22.2 ± 1 kcal/mol and a ΔH⧧ of 21.5 ± 1 kcal/mol and ΔS⧧ of −10 ± 3 eu. This was done by monitoring the disappearance of 3 between 40 and 65 °C. The activation energy and ΔH⧧ are slightly higher than the reported values (Ea = 15.4 kcal/mol, ΔH⧧ = 14.9 kcal/mol) previously reported by Mann and co-workers for the dissociation of [(η5C5H5)Ru(η6-anthracene)]+ to form [(η5-C5H5)Ru(MeCN)3]+. The higher activation energy of the naphthalene system is attributed to the fact that the naphthalene system has a greater loss of resonance energy when the η6−η4 ring slippage occurs

Ru bond or a strong Ru−Ru interaction. There is a feature at 191 cm−1 that is slightly above the baseline, but it is too close to the noise to make a determination as to whether the feature is real or not. This is in contrast to the Raman spectrum of [(η5C5H5)Ru(CO)2]2, a complex with a Ru−Ru bond, which has a clear Ru−Ru stretching frequency of 180 cm−1.8 We assigned each ring of the naphthalene ligand to be bound η6 to the corresponding ruthenium metal center despite the central carbons (C(1) and C(1a) in Figure 1) of the naphthalene ligand having an average Ru−C bond length of 2.45 Å. Evidence for coordination of the central carbons to the ruthenium metal centers was provided by the 13C NMR spectrum of 22+. The resonances for bridgehead carbons are shifted upfield to 88.9 ppm, which is indicative of the carbons being bound to the ruthenium metal centers. The 88.9 ppm resonance was assigned to the central quaternary naphthalene carbons using the HMQC and HMBC NMR spectral data for 22+. In addition, while the AIM analysis does not show a BCP between the ruthenium metal centers and the carbon atoms of the central double bond in the naphthalene ligand, isodensity plots of the HOMO-20 and HOMO-17 orbitals show that there is mixing of the p orbitals of the central double bond with the d orbitals of the ruthenium centers (Figure 3), albeit with an

Figure 3. Isodensity plots of HOMO-20 and HOMO-17 orbitals for 22+. The contours are 0.04 e− Å−1.

isodensity somewhat smaller in size than that connecting the other C atoms to Ru, for which AIM does show BCPs. Although the NBO analysis does not show these direct Ru− C(Np) bonds, it does show strong secondary interactions between the Ru atoms and the Np’s central double bond. Overall, we believe that the Ru’s satisfy their valence requirements by a combination of direct Ru−Ru interactions

Figure 4. UV−vis spectra of 22+ (red), 3 (blue), and 20 (green) (left = experimental; right = simulated). Spectra were simulated from the TD-DFT without any scaling or adjustments (see the Supporting Information for further details). C


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Scheme 2

Figure 5. Computed free energies at various temperatures showing the entropic driving for the formation of 22+.

quartz halogen light as both a heat and a light source since 3 is readily converted to 1 via a thermal route as well. 1 can be converted back to 22+ by heating a mixture of 1 and naphthalene at 180 °C for 90 min. The computational work provides some insight into the relative energies of 1, 22+, and 3. The free energies of the three complexes at 1, 100, 298, and 500 K are shown in Figure 5 and Table 1. The formation of 22+ is an entropically driven process at high temperature, due to the release of six acetonitrile molecules,

during the thermal substitution process compared to substitution of an anthracene ligand.9 1 can be isolated in a 54% yield based on 22+ using a quartz halogen lamp as both a heat and a light source. Kudinov and coworkers have reported that visible light irradiation of [(η5C5H5)Ru(η6-C10H8)]+ in the presence of various arenes enhances the rate of substitution.10 We have observed a similar effect with 3, which has an absorption peak at 375 nm (ε = 1468 M−1 cm−1) (Figure 4). Irradiation of an NMR sample of 3 with a 13 W black light source (λ ∼ 368 nm) resulted in a 50% increase of 1 when compared with a nonirradiated sample over a 22 h period. Although TD-DFT predicts that the strongest transition in this region is on the Ru(MeCN)3 fragment, it also predicts that several transitions in this same band involve excitations to low-lying Ru−Np antibonding orbitals (LUMO through LUMO+4). This is consistent with the absorbances and molar absorptivities reported for both [(η5-C5H5)Ru(MeCN)3]+ (λmax = 365 nm, ε = 1071 M−1 cm−1)2 and [(η5C5H5)Ru(η6-C10H8)]+ (λmax = 364 nm, ε = 780 M−1 cm−1).11 However, our preferred way to synthesize 1 from 22+ is to use a

Table 1. Calculated Free Energies of 1, 22+, and 3 at Various Temperatures

22+ + 6MeCN 3 + 3MeCN 1+ naphthalene D

ΔGsolv (1 K) (kcal/mol)




0.00 −14.33 −14.11

0.00 −5.72 −7.81

0.00 11.34 5.29

0.00 27.33 17.71


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with the conversion of 1 to 22+ having a theoretical ΔG of −17.7 kcal/mol at 500 K. It is interesting to note that 3 + 3MeCN and 1 + Np have nearly the same energy at 1 K, which means that the bond energy of three acetonitriles to Ru is nearly identical to that of the η6-C10H8, while loss on the next three acetonitriles is not fully compensated for by the η6-C10H8 becoming μ-η6,η6-C10H8. Thus, the weak Ru−Ru interaction and the sharing of the central double bond do not fully compensate the Ru atoms for their lost valence. The cyclic voltammetry of 22+ (MeCN, (n-Bu)4NPF6, 100 mV/s) has two reversible E1/2 couples at −1.14 and −1.32 V vs the ferrocene/ferrocenium (Fc/Fc+) couple (Figure 6). These are assigned to be primarily metal-based reductions based on DFT calculations of the metal character of the LUMO orbital (Figure 7).

160 s (Figure 8), which we have assigned as the intervalence charge-transfer band (IVCT) of 2+.

Figure 8. UV−vis spectrum of 22+ at −1.14 V vs Fc/Fc+ in MeCN, (nBu)4PF6. t = 0 s (red); t = 160 s (blue).

When the potential was changed to −1.59 V vs Fc/Fc+, the band at 857 nm disappeared and the two absorption peaks at 454 and 600 nm continued to grow in. The absorption peaks at 454 and 600 nm match the UV−vis spectrum of the twoelectron reduction product, 20 (Figure 4), which was independently synthesized and isolated using a Na/Hg reduction of 22+ (vide infra). Using Hush’s expression for the theoretical full width at halfheight, Δν1/2,theory = (2310νmax)1/2,13 we obtained a Δν1/2,theory = 5192 cm−1. We have calculated the delocalization parameter, Γ, which was previously described by Brunschwig, Creutz, and Sutin.14 Γ = 1 − Δν1/2,exp/Δν1/2,theory = 0.35, which puts 2+ as a Class II type system based on Robin and Day’s classification scheme of mixed-valence systems.15,14 Using the average of the two ruthenium distances in 22+ (3.08 Å) and 20 (3.77 Å) as an estimate for the distance between the two ruthenium centers in 2+, we calculated the electronic coupling parameter, Hab to be 1419 cm−1. This is also additional evidence that 2+ is a Class II type mixed-valence system since Hab < νmax/2 (1419 cm−1 < 5835 cm−1).16 Bulk chemical reduction of 22+ with Na/Hg in THF gives the two-electron reduction product, which is a zwitterionic complex, 20, in 20% yield (eq 2). One of the ruthenium

Figure 6. Cyclic voltammetry of 22+ ((n-Bu)4NPF6, MeCN, 100 mV/ s).

Figure 7. Isodensity plots of the HOMO and LUMO orbitals of 22+. The contours are 0.04 e− Å−1.

On the basis of the electrochemical data with a ΔE = 180 mV, the comproportionation constant, Kc, for the equilibrium RuII−RuII + RuI−RuI ←→ 2RuII−RuI was calculated to be 1124 based on the equation, Kc = 10 ΔE/59, at room temperature.12 A spectroelectrochemical experiment was conducted to observe the changes in the UV−vis spectrum of 22+ when a one-electron reduction was carried out. We observed the appearance of a broad absorption peak at 857 nm (νmax = 11 669 cm−1, Δν1/2,exp = 3390 cm−1, ε ∼ 1388 M−1 cm−1) at a potential of −1.14 V vs Fc/Fc+ (MeCN, (n-Bu)4NPF6) after

centers is bound η6 to one of the naphthalene rings, while the other ruthenium metal center is bound η4 to the remaining diene portion of the naphthalene molecule. Similar η6−η4 hapticity changes due to two-electron reductions have been previously observed in other arene complexes.5a,17 An X-ray structural study of 20 confirms the structural description of a bridging naphthalene ligand that is bound η6−η4 (Figure 9). E


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Scheme 3

Figure 9. ORTEP of the 20 (50% probability). Hydrogens are omitted for clarity. Selected bond distances (Å) and angles (deg): Ru(1)···Ru(2), 3.768; Ru(1)−Cp(centroid), 1.840; Ru(2)−Cp(centroid), 1.826; Ru(1)−C(1), 2.2490(17); Ru(1)−C(2), 2.1799(18); Ru(1)−C(9), 2.3359(17); Ru(2)−C(7), 2.1358(17); Ru(2)−C(8), 2.1771(17); Cp(centroid)−Ru(1)−arene centroid, 178.7; Cp(centroid)−Ru(2)−butadiene(centroid), 178.4; Cp−Cp fold angle, 151.5.

The Ru(1)−centroid(η6-naphthalene) and the Ru(2)− centroid(η4-naphthalene) distances are 1.75 and 1.76 Å, respectively. The Ru···Ru nonbonding distance is 3.768 Å, and the Cp−Cp fold angle is 151°. The fold angle between the exobenzene and butadiene portions of the naphthalene ligand is 31.54(8)°, which is smaller than the reported fold angles for anti-(μ-η4,η6-naphthalene)Mn(CO)3FeCp (35°) and syn-(μη4,η6-1,4-Me2naphthalene)Mn2(CO)6 (45°) that Sweigart and co-workers have previously reported.5a Bennett and co-workers have also reported an anti-(μ-η6,η4-naphthalene)Ru(cod)(RuPEt3(cod) (cod = 1,5-cyclooctadiene) complex that has a fold angle of 39.4°.18 The observed fold angle of 31.54(8)° is smaller than the other reported fold angles due to the more constrained environment of the two ruthenium metal centers with the doubly linked dicyclopentadienyl ligand. Despite the asymmetric nature of the solid-state structure, the 1H NMR spectrum shows a symmetric structure with only two resonances for the bound naphthalene, and one set of dicyclopentadienyl ring hydrogens. We believe that this is due to the rapid η6−η4 equilibrium between the two metal centers. There is no change in the 1H NMR spectrum down to −70 °C. Sweigart and co-workers have observed fluxional behavior for their syn-(μ-η4,η6-C10H8)Mn2(CO)5 complex (Chart 1),5b but that fluxional process is postulated to occur via a CO migration from one manganese metal center to the other manganese center via a bridging CO intermediate. In 20, for an η6−η4 exchange to occur, there also has to be a two-electron transfer between the Ru− center and the Ru+ center. Computational work on 20 has the more symmetric C2v structure higher in energy than the less symmetric Cs groundstate structure by 4.69 kcal/mol, which represents the activation barrier for the η6−η4 interconversion, as the C2v structure is a transition state with the appropriate reaction coordinate to connect the two Cs isomers (Scheme 3 and Figure 10). The calculated Ru−Ru distances are 3.81 Å in the C2v transition state and 3.80 Å in the Cs ground state. As the LUMO of 22+ (Figure 7) and the HOMO of 20 (Figure 10) show, the two electrons added on formation of 20

Figure 10. HOMO and LUMO of the C2v transition state (center) and the two equivalent Cs ground-state geometries. The contours are 0.04 e− Å−1.

go into a Ru−Ru antibonding orbital, which makes the Ru−Ru interaction very repulsive such that the calculated Ru−Ru distance in the symmetric transition state has lengthened by more than 0.6 Å to 3.81 Å. The additional two electrons also reduce the valence needs of the Ru such that 20 is lower in energy if it makes one Ru d8 and the other Ru d6. The Np ligand makes a corresponding adjustment to satisfy the 18electron rule by donating 6 π electrons to the d6 center and only 4 π electrons to the d8 center, a response that preserves the aromaticity of one of the naphthalene rings. Interestingly, the calculations predict that the 2+ mixed-valence species would also be distorted from the C2v structure of 22+ to a Cs structure like that of 20; this result is consistent with the Class II mixedvalence system as derived from the experimental results above. The UV−vis spectrum of 20 has two absorbances in the visible portion of the spectrum (454 and 600 nm) (Figure 4). The TD-DFT predicts strong transitions at 438 and 630 nm with a character that is a mixture of MLCT and MM’CT; both transitions correspond to mainly metal orbitals on the Ru− transferring an electron to Ru+ MOs with a mixture of metal and ligand character (see the Supporting Information for more details). Reaction of cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4Br2 with anthracene and AgOTf results in the formation of a dinuclear complex where the two rutheniums are bound to the terminal and middle rings of the anthracene molecule 4, and a tetranuclear dianthracene complex 5 (eq 3). The 1H NMR spectrum of 4 has two inequivalent sets of doublet and triplet resonances for the cyclopentadienyl hydrogens, which is consistent with the asymmetry of 4. The middle hydrogen for the cyclopentadienyl ring over the middle anthracene ring is shifted upfield (3.20 ppm) due to the F


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which flattens the rings out more than the anti-facial arrangement. The 1H NMR spectrum of 5 is consistent with the symmetric nature of the molecule with the hydrogens attached to the bridgehead carbons having a resonance at 4.74 ppm. The dianthracenediruthenium complexes that Kudinov and co-workers report were formed by the irradiation of the mononuclear [(η5-C5Me4R)Ru(η6-anthracene)]+ (R = Me, CH2OMe) complexes, whereas our dianthracene complex forms via a thermal pathway. We believe that 4 is an intermediate in the formation of 5 based on several observations. The control experiment of anthracene and AgOTf in CD3CN at 180 °C for 90 min does not yield any dianthracene. The ratio of 4:5 is 1: 1 when the reaction is carried out at 180 °C but changes to a 7:1 ratio of 4:5 when the reaction is performed at 160 °C. The ratios are based on the integrations of the bridging methyl resonances in the 1H NMR spectra of the crude reaction mixtures. Heating a solution of pure 4 at 180 °C for 90 min results in the formation of 5. Therefore, our postulated mechanism for the formation of 5 from 4 is the rearrangement of 4 to the isomer where the two ruthenium metal centers are bound to the outer anthracene rings. This intermediate would then dimerize to form 5 (Scheme 4). Evidence for this type of intermediate has been provided by Koelle and co-workers who previously isolated the diamagnetic anti-[{(η6-C5Me5)Ru}2(μ-η6,η6-anthracene)]2+ complex, where the ruthenium centers are bound to the outer anthracene rings. Their computational work showed the HOMO for that complex to have a substantial contribution from the pz orbitals of the C9 and C10 carbons.20 We believe that the formation of a dianthracene complex from Koelle’s complex is hindered by the anti arrangement of the ruthenium metal centers and the steric bulk of the Cp* ligand.

anisotropic shielding of the outer unbound anthracene ring. Kudinov and co-workers have observed similar anisotropic interactions in their anti-[{(η5-C5Me5)Ru}2(μ-η6,η6-dianthracene)]2+ complex.19 The X-ray structure of 5 confirmed the tetranuclear nature of the complex (Figure 11).

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CONCLUSION We have reported two new diruthenium complexes, 22+ and 4, which contain a bridging naphthalene or anthracene bound in a syn-facial arrangement. When anthracene is the ligand, a dianthracene complex is also formed during the synthesis of 4. 2+, the one-electron reduction product of 22+, is a Class II mixed-valence complex, and the electron is not fully delocalized between the two metal centers due to a hapticity change occurring at one of the naphthalene rings. The two-electron reduction product, 20, shows one of the naphthalene rings shifted from η6 to η4, and 20 is fluxional, undergoing both an η6−η4 interconversion and a two-electron transfer between the two ruthenium metal centers.

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EXPERIMENTAL SECTION

General Procedures. Reactions that required inert conditions were performed using modified Schlenk techniques or in an MBraun Unilab glovebox under a nitrogen atmosphere. 1H and 13C NMR spectra were recorded on a Varian Unity Inova 400 MHz spectrometer. 1H and 13C NMR chemical shifts are given relative to the residual proton or 13C solvent resonances. NMR spectra were recorded at room temperature (20−25 °C), unless otherwise noted. Xray data were collected on a Bruker SMART Apex II CCD Platform diffractometer. Microwave heating was conducted using a CEM Discover SP instrument in either 10 or 35 mL snap-cap pressure tubes. UV−vis spectra were recorded on an Avantes Avaspec-2048 spectrometer, inside a nitrogen-filled Vac Atmospheres glovebox. Electrochemical data were collected using a BAS100A potentiostat and a three-electrode system. The electrochemical cell was inside in a

Figure 11. ORTEP of 5 (50% probability). Hydrogens, anions, and solvent molecules are omitted for clarity. Selected bond distances (Å) and angles (deg): Ru(1)···Ru(2), 4.389; Ru(1)−Cp(centroid), 1.73; Ru(1)−An(centroid), 1.82; Cp(centroid)−Ru(1)−An(centroid), 171.7; Cp−Cp fold angle, 176.1; An fold angle, 157.3.

The fold angle of the coordinated anthracene rings is 157.3°, which is slightly larger than the 141.2° fold angle that Kudinov and co-workers have reported for the dianthracene−antidiruthenium complex,19 anti-[{(η5-C5Me4CH2OMe)Ru}2(μη6,η6-dianthracene)]2+. We attribute the larger fold angle in 5 to the syn-facial arrangement of the ruthenium metal centers, G


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Scheme 4

nitrogen-filled Vac Atmospheres glovebox. The working electrode was a 1.6 mm diameter platinum electrode, and the pseudoreference electrode was a Ag/AgCl wire in a 0.2 M MeCN solution of (nBu)4NPF6 in MeCN. All potentials were referenced using the ferrocene/ferrocenium couple as an internal standard. The spectroelectrochemical experiment was conducted using a BASi cell (EF-1350) with a 1 mm path length. Solvents and Reagents. Unless otherwise indicated, all chemicals were used as received. Common solvents and reagents were purchased from Acros, Fisher Scientific, VWR, or Strem Chemical. Deuterated solvents were obtained from Cambridge Isotope Laboratories. Tetrahydrofuran (THF), toluene, pentane, and Et2O were distilled from dark purple solutions of sodium benzophenone ketyl. MeCN, CH2Cl2, and C6D6 were degassed and dried over 3 Å sieves. CD3CN and CDCl3 were distilled from a suspension of CaH2. Elemental analyses were performed by Atlantic Microlab, Norcross, GA, or at the University of Rochester, using a PerkinElmer 2400 Series II Analyzer. cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4Br2 was prepared as previously described.1 Preparation of [cis-{(η5-C5H3)2(CMe2)2}Ru2(μ-η6,η6-C10H8)][OTf]2 (22+). cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4Br2 (105.6 mg, 0.154 mmol), AgOTf (80.3 mg, 0.312 mmol), naphthalene (90.0 mg, 0.703 mmol), and MeCN (1.4 mL) were added to a 10 mL pressure tube under a nitrogen atmosphere. The mixture was microwaved with an initial power setting of 90 W using the standard method of the Discover SP instrument. The mixture was held at 180 °C for 90 min. At the end of the reaction, the reaction mixture was brought back into the glovebox, the orange solution filtered to remove the AgBr, and the resulting MeCN solution added dropwise to a stirred Et2O solution (75 mL). The resulting orange precipitate was collected, washed with Et2O (5 × 3 mL), and recrystallized from MeCN/Et2O at −20 °C to give 22+ as an orange crystalline solid (101.1 mg, 0.121 mmol, 78%). 1H NMR (400 MHz, CD3CN): δ 6.63 (m, 4H), 5.58 (d, J = 2.5 Hz, 4H), 5.55 (t, J = 2.5 Hz, 2H), 5.52 (m, 4H), 1.79 (s, 6H), 1.19 (s, 6H). 13C{1H} NMR (100 MHz, CD3CN): δ 112.5 (Cp ring quat C), 89.4 (Np, α-CH), 88.9 (Np, quat C), 82.0 (Cp ring, 2,6-CH), 79.5 (Cp ring, 1,3,5,7-CH), 69.8 (Np, β-CH), 34.8 (C(CH3)2), 30.7 (C(CH3)2), 29.2 (C(CH3)2). Anal. Calcd for C28H26F6O6Ru2S2: C, 40.06; H, 3.12. Found: C, 39.65; H, 3.01. Preparation of cis-[{(η5-C5H3)2(CMe2)2}Ru2(η6-C10H8)(MeCN)3][OTf]2 (3). Under a nitrogen atmosphere, 22+ (15.0 mg, 0.019 mmol) was dissolved in 0.72 mL of CD3CN and placed in a valve-capped NMR tube. The solution was photolyzed using a 500 W quartz halogen lamp for 1 h. The sample was kept at 5 °C during the photolysis period, and the reaction was monitored by NMR spectroscopy until no 22+ remained. 1H NMR (400 MHz, CD3CN, 21 °C): δ 7.79 (m, 2H), 7.68 (m, 2H), 7.09 (m, 2H), 6.47 (m, 2H), 4.86 (d, J = 2.3 Hz, 2H), 4.71 (t, J = 2.3 Hz, 1H), 4.30 (t, J = 2.3 Hz, 1H), 4.06 (d, J = 2.3 Hz, 2H), 1.62 (s, 6H), 1.24 (s, 6H). 13C {1H} (100 MHz, CD3CN): δ 132.7 (CH), 130.2 (CH), 108.4 (quat C), 97.4 (quat C), 97.1 (quat C), 88.9 (CH), 85.5 (CH), 81.9 (CH), 81.4 (CH), 75.8 (CH), 54.5 (CH), 35.9 (CH3), 32.8 (quat C), 31.2 (CH3). Kinetic Analysis for the Conversion of 3 to 1. 3 was prepared in the manner described above, and n-octane (10.8 mg, internal standard) was added to the solution. The solution was heated in a water bath (40, 50, 60, or 65 °C), and the reaction was monitored by NMR spectroscopy. A plot of ln[3] versus time was used to obtain the rate constant, k, for the disappearance of 3. Plots of ln k versus 1/T

and ln (k/T) versus 1/T were used to obtain the activation energy, ΔH⧧, and ΔS⧧ (see the Supporting Information for more details). Preparation of [cis-{(η5-C5H3)2(CMe2)2}Ru2(MeCN)6][OTf]2 (1) from 22+. 22+ (26.0 mg, 0.031 mmol) was dissolved in MeCN (8 mL) and placed into a valve-capped ampule under a nitrogen atmosphere. The solution was placed 7 cm from a 500 W quartz halogen lamp, and the solution was photolyzed for 30 min. During this time, the temperature of the solution rose to approximately 70 °C. The solvent was removed and the yellow residue washed with Et2O (2 × 2 mL). The residue was redissolved in MeCN (8 mL) and then photolyzed for an additional 18 h. The solvent removal and Et2O wash step were then repeated and the yellow residue redissolved in MeCN (8 mL). The solution was photolyzed for an additional hour for a total irradiation/ heating time of 19.5 h. The MeCN was removed in vacuo and the yellow residue redissolved in MeCN (0.5 mL). The MeCN solution was added dropwise to a stirred Et2O solution (10 mL), which gave a light yellow precipitate. The Et2O solution was decanted off, and the solid was washed 2 × 2 mL of Et2O and dried to afford 1 (15.8 mg, 0.017 mmol, 54%). The 1H NMR spectrum of the product matched that of the previously reported 1H NMR spectrum of 1.1 The 1H NMR spectrum also showed the presence of 3 (∼5%). Preparation of cis-{(η5-C5H3)2(CMe2)2}Ru2(μ-η6,η4-C10H8) (20). In the glovebox, small pieces of Na (146.0 mg, 6.35 mmol) were slowly added to Hg (9.4237 g) to form the Na/Hg amalgam. 22+ (360.6 mg, 0.430 mmol) and THF (5.3 mL) were added to the mixture, and the orange suspension was stirred vigorously for 20 h at room temperature in the dark. The orange suspension quickly became a dark red color, and by the end of the reaction, the reaction mixture was a dull dark brown. The THF solution was pipetted into a 100 mL round-bottom flask, and the Hg residue was washed with THF (3 × 2 mL). The washings were combined with the THF reaction solution, and the THF was removed in vacuo, yielding a brown residue. Toluene (3 × 20 mL) was added to the residue, yielding a dark green solution. The toluene solution was filtered, the filtrate collected, and the toluene removed in vacuo. The resulting product was recrystallized from toluene/pentane at −20 °C, yielding a fine black powder of 20 (46.2 mg, 20%). 1 H NMR (400 MHz, C6D6): δ 4.88 (m, 4H), 4.30 (d, J = 2.2 Hz, 4H), 3.71 (m, 4H), 3.69 (t, J = 2.3 Hz, 2H), 1.94 (s, 6H), 1.87 (s, 6H). We were unable to obtain a 13C NMR spectrum due to the low solubility of 3 in C6D6. Anal. Calcd for C26H26Ru2: C, 57.76; H, 4.85. Found: C, 57.20; H, 4.57. Preparation of [cis-{(η5-C5H3)2(CMe2)2}Ru2(μ-η6,η6-C14H10)][OTf]2 (4) and [cis-{(η5-C5H3)2(CMe2)2}Ru2(μ-C14H10)]2[OTf]4 (5). cis-{(η5-C5H3)2(CMe2)2}Ru2(CO)4Br2 (96.8 mg, 0.141 mmol), AgOTf (72.6 mg, 0.282 mmol), anthracene (89.8 mg, 0.504 mmol, 3.6 equiv), and MeCN (1.4 mL) were added to a 10 mL pressure tube under a nitrogen atmosphere. The mixture was microwaved with an initial power setting of 90 W using the standard method of the Discover SP instrument. The mixture was held at 180 °C for 90 min. At the end of the reaction, the mixture was brought back into the glovebox, the orange solution filtered to remove the AgBr, and the resulting MeCN solution added dropwise to a stirred Et2O solution (75 mL). The resulting orange precipitate was washed with CH2Cl2 (3 × 2 mL), and the resulting white solid was recrystallized from MeCN/ CH2Cl2 at −20 °C to give 5·2MeCN (25.3 mg, 0.014 mmol, 19%). 1H NMR (400 MHz, CD3CN): δ 6.19 (dd, J = 4.1, 2.3 Hz, 8H), 6.00 (dd, J = 4.3, 2.3 Hz, 8H), 5.38 (d, J = 2.5 Hz, 8H), 5.15 (t, J = 2.5 Hz, 4H), H


Organometallics

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Table 2. X-ray Data Summary for 22+, 20, and 5 formula color shape fw T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−1) μ (mm−1) reflns total reflns unique Rinta reflns observed parameters restraints GOFb on F2 R1 [I > 2σ(I)]c wR2d peak/hole (e/Å3)

22+·2MeCN·OH2

20

5·2MeCN·OH2

C32H34F6N2O7Ru2S2 orange block 938.87 213(2) monoclinic C2/m 27.432(2) 9.5350(8) 14.8903(12) 90 117.789(1) 90 3445.6(5) 4 1.810 1.081 37 827 9585 0.0387 6653 330 92 1.046 0.0459 0.1283 1.590/−0.860

C26H26Ru2 orange-black needle 540.61 100.0(1) monoclinic P21/n 8.0897(9) 16.2936(18) 15.1490(17) 90 101.714(2) 90 1955.2(4) 4 1.837 1.554 60 468 10 368 0.0567 8052 289 0 1.018 0.0312 0.0680 0.938/−1.102

C68H64F12N2O13Ru4S4 colorless block 1877.73 100.0(1) triclinic P1̅ 12.721(2) 15.090(3) 18.288(4) 93.529(4) 90.480(3) 106.934(4) 3350.8(11) 2 1.861 1.110 182 127 29 121 0.0782 23 683 937 0 1.068 0.0545 0.1398 2.833/−2.307

Rint = ∑|Fo2 − |/∑|Fo2|. bGOF = S = [∑[w(Fo2 − Fc2)2]/(m − n)]1/2. cR1 = ∑∥Fo| − |Fc∥/∑|Fo|. dwR2 = [∑[w(Fo2 − F c2)2]/ ∑[w(Fo2)2]]1/2, for which w = 1/[σ2(Fo2) + (aP)2 + bP], P = 1/3 max (0, Fo2) + 2/3Fc2, m = number of reflections, and n = number of parameters. a

4.74 (s, 4H), 1.34 (s, 12H), 1.27 (s, 12H). 13C{1H} NMR (100 MHz, CD3CN): δ 110.3 (quat C, Cp ring), 108.3 (quat C, An), 86.0 (CH, Ant), 85.2 (CH, An), 79.8 (CH, 2,6-positions of diCp ligand), 76.8 (CH, 1,3,5,7-postions of diCp ligand), 47.4 (CH, An bridgehead), 33.7 (C(CH3)2), 31.9 (C(CH3)2), 31.4 (C(CH3)2). Anal. Calcd for C68H62F12N2O12Ru4S4: C, 43.92; H, 3.36; N, 1.51. Found: C, 43.78; H, 3.00; N, 1.40. The orange-yellow CH2Cl2 solution from the above reaction was collected and reduced to 2 mL. Et2O (5 mL) was layered on top of the CH2Cl2 layer and the mixture placed in a −20 °C freezer for 18 h, at which time red-orange crystals of 4 formed. The crystals were washed with Et2O (4 × 1 mL) and dried in vacuo to yield pure 4 (31.0 mg, 0.035 mmol, 25%). 1H NMR (400 MHz, CD3CN): δ 7.88 (m, 4H), 6.63 (m, 2H), 6.38 (s, 2H), 5.64 (m, 2H), 5.62 (d, J = 2.5 Hz, 2H), 5.55 (t, J = 2.5 Hz, 1H), 5.43 (d, J = 2.5 Hz, 2H), 3.20 (t, J = 2.7 Hz, 1H), 1.72 (s, 6H), 1.30 (s, 6H). 13C {1H} NMR (100 MHz, CD3CN): δ 134.8 (CH), 130.8 (CH), 112.4 (quat C), 110.5 (quat C), 97.8 (quat C), 89.3 (CH), 89.0 (CH), 88.3 (quat C), 81.6 (CH), 80.71 (CH), 79.7 (CH), 68.8 (CH), 67.6 (CH), 34.7 (quat C), 30.4 (CH3), 29.4 (CH3). Anal. Calcd for C32H28F6O6S2Ru2: C, 43.24; H, 3.18. Found: C, 42.89; H, 3.45. Computational Details. All Density functional theory21 (DFT) calculations were performed using the Gaussian 09 suite of software22 at the B3LYP (Becke-3 exchange23 and Lee−Yang−Parr correlation24 functional) level of theory. Full geometry optimizations were performed, and stationary points were characterized via analytical frequency calculations using BSI (Pople triple-ζ quality basis set with a polarization function (6-311G(d,p))25 for the C, N, O, and H atoms and the Stuttgart−Dresden triple-ζ quality basis set (SDD) with a small-core effective core potential (ECP)26 for the Ru atoms). To account for solvation effects, single-point energy (SPE) calculations were performed using the polarizable continuum model27 (PCM) with the radii and nonelectrostatic terms of the SMD28 solvation model for acetonitrile. To investigate the electronic transitions responsible for

the observed UV−vis spectra of 22+, 20, and 3, time-dependent density functional theory (TD-DFT) calculations were performed (B3LYP/ BSI). Simulated UV−vis spectra of the TD-DFT results were generated using the AMPAC Graphical User Interface29 (AGUI) with a peak half-width of 0.333 eV (default) and a scaling factor of 1 (default). Natural bond orbital (NBO) analyses were performed using NBO 3.0 as implemented in G09 B.01 at the B3LYP/BSI optimized geometry. Atoms in molecules30 (AIM) analysis was performed for cis{(η5-C5 H3 )2(CMe)2 )2}Ru2(CO) 4 and 22+ using the AIMALL program.31 An all-electron basis set, BSII (BSI for C, N, and O and the all-electron Douglas−Kroll correlation consistent polarized valence triple-ζ basis set (cc-pVTZ-DK) for Ru32 with the G function removed), was used to generate the wave function at the B3LYP/BSIoptimized geometry (B3LYP/BSII//B3lYP/BSI) for the AIM analysis. Structure Determination for 22+. A crystal (0.26 × 0.24 × 0.20 mm3) of 22+ was placed onto the tip of a 0.1 mm diameter glass capillary tube or fiber and mounted on a Bruker SMART APEX II CCD Platform diffractometer for a data collection at 213(2) K.33 Although the structure could be determined at 100 K, the diruthenium cation (the species of interest) was disordered over a crystallographic mirror plane. The higher temperature allowed for an ordered cation. A preliminary set of cell constants and an orientation matrix were calculated from reflections harvested from three orthogonal wedges of reciprocal space. The full data collection was carried out using Mo Kα radiation (graphite monochromator) with a frame time of 45 s and a detector distance of 4.01 cm. A randomly oriented region of reciprocal space was surveyed: four major sections of frames were collected with 0.50° steps in ω at four different ϕ settings and a detector position of −38° in 2θ. The intensity data were corrected for absorption.34 Final cell constants were calculated from the xyz centroids of 3840 strong reflections from the actual data collection after integration.35 See Table 2 for additional crystal and refinement information. The structure was solved using SHELXS-9736 and refined using SHELXL-97.36 The space group C2/m was determined based on I


Organometallics

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hydrogen atoms from the E-map. Full-matrix least-squares/difference Fourier cycles were performed, which located the remaining nonhydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on the cocrystallized water solvent molecule were placed in positions appropriate for hydrogen bonding; their positional and isotropic displacement parameters were refined relative to those of the bonded oxygen atom. All other hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. The refinement stalled at R1 = 0.118, at which point twin modeling was required. After the nonmerohedral twin law, [−1 0 0/0 −1 0/ 0.082 0.169 1], a 180° rotation about the reciprocal lattice [0 0 1], was determined,38 the data were reintegrated,35 and a new absorption correction was applied.39 There were 26 572 unique reflections solely in the first component, 26 539 unique reflections solely in the second component, and 18 241 unique overlapping reflections. The mass ratio of the two components refined to 77:23. The final full-matrix leastsquares refinement converged to R1 = 0.0545 (F2, I > 2σ(I)) and wR2 = 0.1398 (F2, all data).

systematic absences and intensity statistics. A direct-methods solution was calculated, which provided most non-hydrogen atoms from the Emap. Full-matrix least-squares/difference Fourier cycles were performed, which located the remaining non-hydrogen atoms. Hydrogen atoms on the cocrystallized water solvent molecule were placed in positions appropriate for hydrogen bonding; their positional and isotropic displacement parameters were refined relative to those of the bonded oxygen atom. All other non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. The final full-matrix least-squares refinement converged to R1 = 0.0459 (F2, I > 2σ(I)) and wR2 = 0.1283 (F2, all data). One triflate anion is modeled as disordered over two general positions (61:39) in addition to its being disordered over a crystallographic mirror plane (50:50). The cocrystallized acetonitrile solvent molecules are modeled as disordered pairwise over a crystallographic mirror plane (50:50). Corresponding bond lengths, angles, and through-space distances among the orientations of the triflate anion disorder were restrained to be similar. Anistropic displacement parameters for spatially close atom pairs were constrained to be equivalent. Similar restraints and constraints were applied to the acetonitrile disorder model. Structure Determination of 20. A crystal (0.45 × 0.16 × 0.06 mm3) was placed onto the tip of a 0.1 mm diameter glass capillary tube or fiber and mounted on a Bruker SMART APEX II CCD Platform diffractometer for a data collection at 100.0(1) K.33 A preliminary set of cell constants and an orientation matrix were calculated from reflections harvested from three orthogonal wedges of reciprocal space. The full data collection was carried out using Mo Kα radiation (graphite monochromator) with a frame time of 45 s and a detector distance of 3.99 cm. A randomly oriented region of reciprocal space was surveyed: six major sections of frames were collected with 0.50° steps in ω at six different ϕ settings and a detector position of −38° in 2θ. The intensity data were corrected for absorption.34 Final cell constants were calculated from the xyz centroids of 3941 strong reflections from the actual data collection after integration.35 See Table 2 for additional crystal and refinement information. The structure was solved using SIR9737 and refined using SHELXL97.36 The space group P21/n was determined based on systematic absences. A direct-methods solution was calculated, which provided most non-hydrogen atoms from the E-map. Full-matrix least-squares/ difference Fourier cycles were performed, which located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on the bridging naphthalene ligand were found from the difference Fourier map, and their positional and isotropic displacement parameters were refined independently from those of all other atoms. All other hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. The final full-matrix least-squares refinement converged to R1 = 0.0312 (F2, I > 2σ(I)) and wR2 = 0.0680 (F2, all data). Structure Determination for 5. A crystal (0.26 × 0.22 × 0.08 mm3) of 5 was placed onto the tip of a 0.1 mm diameter glass capillary tube or fiber and mounted on a Bruker SMART APEX II CCD Platform diffractometer for a data collection at 100.0(1) K.33 A preliminary set of cell constants and an orientation matrix were calculated from reflections harvested from three orthogonal wedges of reciprocal space. The full data collection was carried out using Mo Kα radiation (graphite monochromator) with a frame time of 30 s and a detector distance of 4.01 cm. A randomly oriented region of reciprocal space was surveyed: six major sections of frames were collected with 0.50° steps in ω at six different ϕ settings and a detector position of −38° in 2θ. The intensity data were corrected for absorption.34 Final cell constants were calculated from the xyz centroids of 3847 strong reflections from the actual data collection after integration.35 See Table 2 for additional crystal and refinement information. The structure was solved using SIR9737 and refined using SHELXL97.36 The space group P1̅ was determined based on intensity statistics. A direct-methods solution was calculated, which provided most non-

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data for complexes 22+, 20, and 5 (CIF files); kinetic and NMR spectral data for 22+, 3, and 5; and UV−vis spectral and time-dependent DFT data for 22+, 20, and 3. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-0715423) for supporting this work and the Office of the Executive Vice President and Provost at UNI for funds to purchase the CEM Discover SP. Analytical data were obtained from the CENTC Elemental Analysis Facility at the University of Rochester, funded by NSF CHE-0650456. This research was supported, in part, by the National Science Foundation (Grant Nos. CHE0910552 and CHE-0541587 M.B.H.). We would like to thank the Laboratory for Molecular Simulation (LMS) and the Supercomputing Facility at Texas A&M University for providing computer time. We also thank Dr. Carol Creutz for helpful discussions regarding 2+. R.M.C. thanks Andrew Paige for his help with the synthesis of 5.

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REFERENCES

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