Structural and Electrochemical Consequences of [Cp*] Ligand

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Structural and Electrochemical Consequences of [Cp*] Ligand Protonation Yun Peng, Mario V. Ramos-Garcés,† Davide Lionetti, and James D. Blakemore* Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045-7582, United States S Supporting Information *

ABSTRACT: There are few examples of the isolation of analogous metal complexes bearing [η5-Cp*] and [η4-Cp*H] (Cp* = pentamethylcyclopentadienyl) complexes within the same metal/ligand framework, despite the relevance of such structures to catalytic applications. Recently, protonation of Cp*Rh(bpy) (bpy = 2,2′-bipyridyl) has been shown to yield a complex bearing the uncommon [η4-Cp*H] ligand, rather than generating a [RhIII−H] complex. We now report the purification and isolation of this protonated species, as well as characterization of analogous complexes of 1,10-phenanthroline (phen). Specifically, reaction of Cp*Rh(bpy) or Cp*Rh(phen) with 1 equiv of Et3 NH +Br− affords rhodium compounds bearing endo-η4-pentamethylcyclopentadiene (η4Cp*H) as a ligand. NMR spectroscopy and single-crystal X-ray diffraction studies confirm protonation of the Cp* ligand, rather than formation of metal hydride complexes. Analysis of new structural data and electronic spectra suggests that phen is significantly reduced in Cp*Rh(phen), similar to the case of Cp*Rh(bpy). Backbonding interactions with olefinic motifs are activated by formation of [η4-Cp*H]; protonation of [Cp*] stabilizes the low-valent metal center and results in loss of reduced character on the diimine ligands. In accord with these changes in electronic structure, electrochemical studies reveal a distinct manifold of redox processes that are accessible in the [Cp*H] complexes in comparison with their [Cp*] analogues; these processes suggest new applications in catalysis for the complexes bearing endo-η4-Cp*H.

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reactive intermediates,11 elucidating the structural and electronic changes accompanying protonation of [Cp*] could provide new strategies in catalyst design.12 For this reason, we have been investigating synthetic methods for generation, purification, and isolation of [η4-Cp*H] complexes from starting materials ligated by [Cp*]. Our work has met with success, and we now report preparative-scale isolation of (Cp*H)Rh(bpy)Br (2), as well as the related complex (Cp*H)Rh(phen)Br (4) that can be prepared from Cp*Rh(phen) (3).

INTRODUCTION The pentamethylcylopentadienyl (Cp*) ligand supports a wide range of transition metal complexes.1 In these compounds, [Cp*] is often considered to be an inert ancillary ligand that can stabilize metals with a variety of binding modes.2 Many groups have been interested in transfer of moieties to the [Cp*] ring in metal complexes that can serve as catalysts, since direct bonding of substrate with [Cp*] could open new and possibly useful reaction paths.3 Interest in such chemistry has recently been renewed by reports involving proton transfer to a [Cp*] ligand in several metal complexes. For example, protonation of Cp*Rh(bpy) (1; bpy = 2,2′-bipyridyl), a catalyst for H2 generation, yields a new rhodium(I) species bearing an η4-Cp*H ligand with H positioned endo with respect to the rhodium center.4,5 Peters and co-workers also recently described in situ formation of protonated decamethylcobaltocene, [Cp*Co(η4-Cp*H)]+ from Cp*2Co. They proposed that this compound could serve as an H atom donor in dinitrogenreduction chemistry.6 Older literature describes a limited number of isolated η4-cyclopentadiene complexes,7 prepared either directly from the free diene8 or by other reactions.9 Among these reports, examples of isolation and characterization of [η5-Cp*] and [η4-Cp*H] complexes within the same metal ligand framework have been rare. As these compounds have important applications in catalysis10 and could serve as © XXXX American Chemical Society

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EXPERIMENTAL SECTION

All manipulations were carried out in a dry N2-filled glovebox (Vacuum Atmosphere Co., Hawthorne, CA) or under N2 atmosphere using standard Schlenk techniques unless otherwise noted. All solvents were of commercial grade and dried over activated alumina using a PPT Glass Contour (Nashua, NH) solvent purification system prior to use and were stored over molecular sieves. All chemicals were from major commercial suppliers and used as received after extensive drying. Deuterated NMR solvents were purchased from Cambridge Isotope Laboratories; CD3CN was dried over molecular sieves. 1H and 13 C NMR spectra were collected on a Bruker Avance AVIII 500 MHz spectrometer and referenced to the residual protio-solvent signal. Received: July 25, 2017

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DOI: 10.1021/acs.inorgchem.7b01895 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Chemical shifts (δ) are reported in units of ppm and coupling constants (J) are reported in Hz. Elemental analyses were performed by Midwest Microlab, Inc. (Indianapolis, IN). Cp*Rh(bpy) and Cp*Rh(phen) were synthesized according to literature methods using Na(Hg).14,15 Single-crystal diffraction data were collected with a Bruker APEX-II CCD diffractometer and a Bruker APEX2Microsource diffractometer. CCDC entries 1546016, 1546017, and 1563622 contain the supplementary crystallographic data for compounds 3, 4, and 6. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033. Electronic absorption spectra were collected with an Ocean Optics Flame spectrometer, in a 1 cm path length quartz cuvette. Electrochemical experiments were performed in a N2-filled glovebox in dry, degassed MeCN. 0.10 M tetra(n-butylammonium) hexafluorophosphate ([nBu4N]+[PF6]−; Sigma-Aldrich, electrochemical grade) served as the supporting electrolyte. Measurements were carried out with Gamry Reference 600+ potentiostat/galvanostat using a standard three-electrode configuration. The working electrode was the basal plane of highly oriented pyrolytic graphite (HOPG) (GraphiteStore.com, Buffalo Grove, Ill.; surface area: 0.09 cm2), the counter electrode was a platinum wire (Kurt J. Lesker, Jefferson Hills, PA; 99.99%, 0.5 mm diameter), and a silver wire immersed in electrolyte solution served as a pseudoreference electrode (CH instruments). The reference was separated from the working solution by a Vycor frit (Bioanalytical Systems, Inc.). Ferrocene (Sigma-Aldrich; twicesublimed) was added to the electrolyte solution at the end of each experiment; the midpoint potential of the ferrocenium/ferrocene couple (denoted as Fc+/0) was used as an external standard for comparison of the recorded potentials. Bulk electrolysis experiments were performed in a custom twochamber electrochemical cell equipped with connections to achieve gastight operation. The working electrode was an HOPG plate (Graphitestore.com, Buffalo Grove, Ill.; surface area: 10 cm2). Ten equivalents of ferrocene served as the sacrificial reductant. Gas analysis for determination of gas evolution was performed with a Shimadzu GC-2014 Custom-GC gas chromatograph with a thermal conductivity detector and dual flame-ionization detectors. A custom set of eight columns and timed valves enable quantitative analysis of hydrogen, nitrogen, oxygen, carbon dioxide, carbon monoxide, methane, ethane, ethylene, and ethyne. Argon serves as the carrier gas. The instrument was calibrated with a standard checkout gas mixture (Agilent 5190-0519) prior to experimental runs to obtain quantitative data for H2 and other gases. Calibration curves over a range of 100−10000 ppm were constructed with prepared mixture of H2 and N2 to enable H2 quantification. (η4-Cp*H)Rh(κ2-2,2′-bipyridyl)Br (2). Cp*Rh(bpy) (1) prepared according to literature methods14,15 (100 mg, 0.253 mmol) was dissolved in acetonitrile. Triethylammonium bromide (41.8 mg, 0.230 mmol, 0.9 equiv) in acetonitrile was added, resulting in a color change from purple to red. Red solid precipitates upon addition of diethyl ether, and this solid was collected after filtration to yield pure 2. Yield: 93 mg, 0.196 mmol, 77%. 1H NMR (400 MHz, Acetonitrile-d3) δ 8.92 (d, J = 5.2 Hz, 2H), 8.30 (d, J = 8.2 Hz, 2H), 8.05 (t, J = 8.0 Hz, 2H), 7.62 (t, J = 6.8 Hz, 2H), 2.44 (q, J = 7.9, 7.5 Hz, 1H), 1.88 (s, 6H), 0.88 (s, 6H), 0.54 (d, J = 6.2 Hz, 3H) ppm. 13C NMR (126 MHz, acetonitrile-d3) δ 154.78, 152.31, 138.61, 127.24, 123.23, 94.92 (d, J = 10.4 Hz), 56.83 (d, J = 3.8 Hz), 55.20 (d, J = 11.1 Hz), 19.84, 11.99, 10.60 ppm. Elemental analysis was attempted, but the results showed deviations from the expected composition that were greater than 0.4%; this is possibly due to the compound’s sensitivity to oxygen and water. (η4-Cp*H)Rh(κ2-1,10-phenanthroline)Br (4). Cp*Rh(phen) (3) prepared according to literature methods14,15 (95.8 mg, 0.229 mmol) was dissolved in acetonitrile. Triethylammonium bromide (42 mg, 0.231 mmol, 1 equiv) in acetonitrile was added, resulting in a color change from green to red, with quantitative formation of 4. After removal of solvent under reduced pressure, the crude product was

washed sequentially with diethyl ether and toluene. The residue was then extracted with THF, and this fraction was collected. The volatiles were removed in vacuo to yield 4 as a red solid. Yield: 64.6 mg, 0.129 mmol, 57%. 1H NMR (500 MHz, Acetonitrile-d3) δ 9.18 (dd, 3JH,H = 4.9 Hz, 4JH,H = 1.5 Hz, 2H), 8.57 (dd, 3JH,H = 8.2 Hz, 4JH,H = 1.4 Hz, 2H), 8.07 (s, 2H), 7.92 (dd, 3JH,H = 8.2 Hz, 3JH,H = 4.9 Hz, 2H), 2.70 (q, 3JH,H = 6.2 Hz, 1H), 1.89 (s, 6H), 1.04 (s, 6H), 0.59 (d, 3JH,H = 6.3 Hz, 3H) ppm. 13C{1H} NMR (126 MHz, Acetonitrile-d3) δ 151.97, 146.20, 137.14, 130.69, 128.09, 126.12, 94.88 (d, 1JC,Rh = 10.1 Hz), 57.15 (d, 2JC,Rh = 3.5 Hz), 56.00 (d, 1JC,Rh = 11.4 Hz), 19.94, 12.21, 10.63 ppm. The composition of carbon, hydrogen, and nitrogen in compound 4 was found via elemental analysis to be 53.90%, 5.24%, and 4.92% respectively. The calculated composition of carbon, hydrogen, and nitrogen in 4 is 52.93%, 4.85%, and 5.61%. Including a contribution of 0.5 equiv of cocrystallized tetrahydrofuran solvent adjusts the composition to be 53.85%, 5.27%, and 5.23% for carbon, hydrogen, and nitrogen; these values are within 0.4% error with respect to the result of analysis. Crystals suitable for X-ray diffraction were obtained by allowing a solution of 4 in tetrahydrofuran to stand at a decreased temperature under inert atmosphere.

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RESULTS AND DISCUSSION Treatment of 1 with triethylammonium bromide (pKa = 18.2 in acetonitrile13) in acetonitrile yields 2 with virtually complete conversion. As we have previously noted, a slight excess of triethylammonium acid is tolerated; further protonation or evolution of hydrogen is generally not observed with this system.5 To isolate 2, a large excess of diethyl ether was added to the reaction mixture in MeCN, resulting in formation of a distinctive red precipitate. The red solid was isolated by filtration and could be identified as pure 2. Nuclear magnetic resonance (NMR) spectra of isolated 2 are in good agreement with previous reports of [(Cp*H)Rh(bpy)L]+ complexes generated by in situ methods.4,5 Similarly, preparation of 4 was accomplished in MeCN by treatment of previously reported 314,15 with ca. 1 equiv of triethylammonium bromide at room temperature. After removal of MeCN under reduced pressure, the resulting solid was washed sequentially with diethyl ether, toluene, and tetrahydrofuran. The tetrahydrofuran fraction yielded a cherry red solid after removal of solvent that was identified as 4.16 In the latter reaction, NMR spectroscopy of the isolated product is consistent with the structure of 4 shown in Scheme 1. The 1H NMR spectrum (see Supporting Information) exhibits three doublets of doublets at 9.18, 7.92, and 8.57 ppm, as well as a singlet at 8.07 ppm; these signals (integrating to 2H each) correspond to the aromatic protons of the Rh-bound 1,10-phenanthroline ligand. In the aliphatic region, singlets at 1.89 and 1.04 ppm (integrating to 6H each) are attributable to the four olefinic methyl groups of [η4-Cp*H]. A quartet located at 2.70 and a corresponding doublet located at 0.59 ppm (3JH,H = 6.2 Hz) are attributable, respectively, to the proton directly bound to the Cp* ring and to the unique methyl group located on the same ring carbon. In the 13C{1H} NMR spectrum (see Supporting Information), signals for 12 unique carbon environments are detected. Three doublets (56.0, 57.2, and 94.9 ppm) display coupling to the 103Rh (I = 1/2) metal center. Notably, the coupling constants for these signals (1JH,Rh = 11.4 Hz, 2JH,Rh = 3.5 Hz, 1JH,Rh = 10.1 Hz) agree with our proposed structure in that four ring carbons in two distinct environments are directly bound to rhodium (stronger coupling), whereas one unique ring carbon is not (weaker coupling). B


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Inorganic Chemistry

ring carbons and rhodium are elongated to 2.190(3)−2.228(3) Å in 3 from 2.141(3) to 2.171(3) Å in [Cp*Rh(phen)Cl]OTf (5),18 while those from the phen nitrogens to rhodium are markedly shortened to 2.006(3) and 2.004(2) Å in 3 from 2.121(2) and 2.100(2) Å in 5. The C5−C6 linkage in the phen ligand is shortened to 1.394(4) Å in 3, from a value of 1.422(3) Å in 5 and 1.449 Å in free phen.19 This bond contraction is indicative of extensive delocalization of electron density in the highest occupied molecular orbital (HOMO) of 3 onto the phen ligand.20 This phenomenon can be understood to arise from the accessible π* orbitals on phen that can mix with electron-rich rhodium(I) centered orbitals. Literature studies on reduced bipyridine-type ligands21 have shown that the interpyridine ring distance contracts upon ligand reduction,22 due to population of the ligand π* levels that increases the bond order between C5 and C6. Allowing a concentrated tetrahydrofuran solution of 4 to stand at −35 °C yielded needle-shaped red crystals suitable for single-crystal XRD. The geometry at the formally rhodium(I) center in 4 is distorted trigonal−bipyramidal; unlike structures of bpy analogues of this compound, complex 4 does not reside on a crystallographic mirror plane.4,5 A proton (H11; located via observable electron density in the difference map) is present on the ring of the former [Cp*] ligand. The resulting [Cp*H] moiety is bound in an η4 fashion, while the 1,10-phenanthroline ligand retains the κ2 coordination mode. The special proton, H11, appears in the structure with exclusively endo disposition relative to Rh. In the solid state, complex 4 displays low symmetry, nominally C1. In solution, however, the structure displays Cs symmetrythis is consistent with rapid, free rotation of the [Cp*H] ligand giving rise to the higher symmetry NMR data than would be anticipated based on the XRD structure. Upon protonation of [Cp*] to form 4, we find marked structural changes apparent in the XRD data (Figure 2). In 3, the five C−C distances between the [Cp*] ring carbons are not distinct, ranging from 1.422(4)−1.444(4) Å. Conversely, localization of single and double character of the C−C bonds of the [Cp*H] ring is observed in the structure of 4, consistent with its nascent diolefinic character and loss of aromaticity.23 Specifically, the C11−C12 and C11−C15 linkages are greatly

Scheme 1. Synthesis of 2 and 4 Occurs by Protonation at [Cp*], Resulting in Formation of Bound [η4-Cp*H]

The starting material for this synthesis, Cp*Rh(phen) (3), was first reported by Grätzel in 1987.14 However, structural data from NMR and single-crystal X-ray diffraction (XRD) have not been available for this compound. Therefore, we obtained NMR information for 3 (see Supporting Information) and also grew crystals suitable for single-crystal XRD by allowing a concentrated solution of 3 in MeCN to stand at −35 °C for several days. As expected for this formally rhodium(I) compound, the metal center is five-coordinate with [Cp*] bound in the η5 mode and phen bound in the κ2 mode. The Rh metal center adopts a coordination environment with pseudo-Cs geometry in the solid statefree rotation of the [Cp*] ring results in a species with C2v symmetry in solution as judged by 1 H NMR. The phen ligand is orthogonal to the plane defined by the Cp* ring carbons; the torsion angle between the Cp* centroid, the Rh center, and the phen ligand is 177.8 deg. This structural profile compares well with Cp*Rh(bpy), as well as with its analogue Cp*Ir(bpy) and the respective cyclopentadiene complexes.15,17 The crystal structure of 3 reveals important bonding and electronic considerations. The bond distances between the Cp*

Figure 1. Solid-state molecular structure of 3. Colors: rhodium, purple; nitrogen, blue; and carbon, black. Displacement ellipsoids are shown at 50% probability. Hydrogen atoms omitted for clarity.

Figure 2. Molecular solid-state structure of 4. Colors: rhodium, purple; nitrogen, blue; and carbon, black. Displacement ellipsoids are shown at 50% probability. Hydrogen atoms omitted for clarity. C


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Inorganic Chemistry elongated, at 1.524(3) and 1.523(3) Å, respectively, consistent with C−C single bonds to the fully pyramidalized C11 atom. The C12−C13 and C14−C15 bonds are also elongated, at 1.446(3) and 1.440(3), respectively, though to a lesser extent. In the structure of a free Cp*H analogue (Cp(CH2C6H4CH3)5H), the corresponding double bonds are shorter, at 1.343(3) and 1.346(3) Å, respectively.24 These observations are consistent with assignment of C12−C13 and C14−C15 in 4 as C−C double bonds, which bind to Rh as σdonors and π-acceptors, resulting in elongated bond lengths with respect to the free olefin values. Finally, the rather long Rh−C11 distance of 2.637 Å indicates there is no covalent bonding between these atoms; this model is also in accord with the pyramidalization of C11 following protonation. Perhaps surprisingly, the C13−C14 bond is rather short at 1.420(4) Å. As described above, π-backbonding into the butadiene motif contained within the [Cp*H] ligand results in elongation of the C12−C13 and C14−C15 bonds, as the lowest unoccupied molecular orbital (LUMO) of the butadiene moiety displays antibonding character at these positions.25 However, the butadiene LUMO also displays bonding character between the two central C atoms. Population of this orbital via π-backdonation results in increased bond order at this position; thus, the C13−C14 bond contracts here in 4. In accord with this model, the structural data for 4 compare well with analogous distorted trigonal-bipyramidal [η4-butadiene] complexes.26 Taken together, these data suggest that [Cp*H] stabilizes the low-valent metal center by engaging in πbackbonding similar to other diolefinic ligands known to stabilize rhodium(I) compounds (e.g., 1,5-cyclooctadiene).27 Protonation of 1 to generate 2 results in similar structural changes that have been observed in XRD data.4,5 Specifically, the bonds corresponding to the C11−C12 and C11−C15 distances in 4 are elongated to 1.517(2) Å in 2, consistent with typical C−C single bonds. The butadiene-like character of the [Cp*H] ligand is also present in 2, with C−C double bonds in the [Cp*H] ligand elongated to 1.440(3) Å and the central C− C bond shortened to 1.430(4) Å.4,5 We therefore conclude that the aromatic character of [Cp*] is lost upon protonation and that [Cp*H] generally shows structural characteristics associated with the butadiene fragment in both 2 and 4. The C5−C6 bond length of the coordinated phen ligand elongates to 1.437(3) Å upon formation of 4. This distance approaches that observed in [Cp*Rh(phen)Cl]OTf (5) at 1.422(3) Å.18 The significant lengthening upon protonation can be viewed as a direct consequence of decreased electronic coupling between the rhodium(I)-centered HOMO and the higher-lying π* acceptor levels of phen in 4.,17 Protonation of 1 to form 2 also results in elongation of the corresponding C5− C6 bond in bpy from 1.422(4) to 1.475(3) Å.4,5,15 These observations suggest that the [Cp*H] ligand, in comparison with [Cp*] which lacks π-accepting character, generally serves to lower the energy of the metal-centered HOMO and disfavor effective coupling between the metal and π-symmetry acceptor orbital(s) on nearby diimine ligands.17,20 Absorption spectra reflect the marked change in electronic structure that occurs upon protonation of 3 to form 4 (Figure 3). In the case of 3, absorptions in the visible region (ε ≈ 6−10 × 103 M−1 cm−1) can be assigned as metal-to-ligand charge transfer (MLCT) bands; these absorptions account for the compound’s green color. Conversely, the spectrum of 4 displays intense bands (ε ≈ 15−24 × 103 M−1 cm−1) only in the UV region, with a weak absorption tailing into the visible

Figure 3. Electronic absorption spectra of 3 (blue) and 4 (red) in MeCN.

region (ε ≈ 0.93 × 103 M−1 cm−1 at 490 nm) that gives rise to the compound’s red color. We attribute the UV absorption by 4 to both intraligand and blue-shifted MLCT transitions. The blue shift of the MLCT transition bands in 4 vs 3 is a consequence of the π-backbonding to [Cp*H] (in 4) vs that to phen (in 3). The low-lying π* orbital of [Cp*H] confers greater stabilization to the HOMO of 4, thereby increasing the energy necessary to accomplish the analogous MLCT transition. Similar trends were observed in comparisons of the electronic absorption profiles of 1 and in situ-generated 2.5 Notably, our spectrum of newly isolated 2 matches that of the analogous sample generated in situ (see Supporting Information). For comparison, [Rh(norbornadienyl)(phen)]+ is a meloncolored solid that absorbs weakly in the visible region (ε ≈ 0.92 × 103 M−1 cm−1 at 477 nm). [Rh(1,5-cyclooctadienyl)(phen)]+ also displays only weak absorbance in the visible region (ε ≈ 0.81 × 103 M−1 cm−1 at 457 nm).28 The blue-shifting absorptions across this series of phen complexes match with the π-accepting tendencies of the diolefin ligands. Changing diolefin coordination from [η4-Cp*H] to [norbornadiene] to [1,5-cyclooctadiene] provides increasing orbital overlap between Rh(I) and the diolefin ligands due to decreasing geometric constraints and thus results in greater apparent stabilization of the rhodium(I) metal centerthis trend is reflected in the shift to the lower wavelength of the lowestenergy bands across this series. In order to confirm the apparent similarities between [η4Cp*H] and [1,5-cyclooctadiene] complexes of rhodium with αdiimine ligands, we prepared the literature complex [(cod)Rh(bpy)]BF4 (6; cod = 1,5-cyclooctadiene) for structural, spectroscopic, and electrochemical comparison with our isolated (Cp*H)Rh(bpy)Br (2). Complex 6 can be isolated in good yield by treatment of the dimeric Rh(I) compound [(cod)RhCl]2 with a solution of HBF4 in ethanol.29 Singlecrystals of 6 suitable for X-ray diffraction studies were obtained by vapor diffusion of Et2O into a solution of 6 in MeCN. In agreement with published structures of the same cationic fragment accompanied by different counteranions,30 the solidstate structure of 6 reveals a four-coordinate Rh center in a square-planar geometry. Binding of the two olefinic fragments in 1,5-cyclooctadiene to rhodium(I) as σ-donors and πacceptors results in substantial elongation of the C−C double bonds (C11−C12: 1.394(2) Å, C15−C16: 1.396(3) Å; cf. C− D


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Inorganic Chemistry

oxidation events are encountered up to 0 V, and in the case of these [Cp*Rh] complexes, no further reduction events are encountered down to −2.5 V.17 Cyclic voltammograms collected on a 1.0 mM solution of 4 reveal three quasi-reversible reduction events over a range from −1 to −2.5 V. Specifically, these three couples are centered at −1.17 V, −1.73 V, and −2.17 V. In the cases of the two more positive reduction events, scan-rate dependent studies confirm that the redox couples are freely diffusing, soluble species (see Supporting Information). The most negative reduction event displays a less reversible appearance at slower scan rates, consistent with generation of a highly reducing and more reactive form of the complex. On the basis of the similar peak currents measured for these three reduction events over narrow voltage ranges, we assign each of these reduction events as single-electron processes. Over wider voltage ranges, the voltammograms display diminished anodic return currents versus their cathodic counterparts, suggesting that the reduced compounds are reactive. This general trend is reversed, however, for the most positive coupleon wider scans the anodic return wave is significantly enhanced. This can be assigned to generation of Cp*Rh(phen) under the electrochemical conditions, because of the similar potential between the most positive couple of 4 (−1.17 V) and the single couple of 3 (−0.96 V). Complex 2 behaves similar to 4 in cyclic voltammetry studies, with three quasi-reversible redox couples at −1.18, −1.73, and −2.18 V (see Supporting Information). Finally, for both 2 and 4, the open-circuit potential was typically measured to be near −1.2 V, suggesting that the most positive redox event corresponds to an oxidation of our isolated complexes 2 and 4.

C double bond distances of 1.32−1.34 Å in literature examples of noncoordinated substituted 1,5-cyclooctadienes).31 Additionally, the value of the inter-ring C−C bond distance in the bpy ligand of 6 is virtually identical to that in 2 (C5−C6: 1.475(2) and 1.475(3) Å, respectively); this similarity is consistent with diminished π-backbonding to bpy in both cases. Overall, the cyclooctadiene ligand is larger and more flexible in comparison with the smaller butadiene-like ring of [Cp*H]this presumably gives rise to more extensive space filling around the rhodium center. Additionally, the greater flexibility of cod engenders enhanced orbital overlap between the olefin and rhodium orbitals, contributing to stronger σbonding. These features are reflected in the absence of a fifth ligand bound to Rh in 6.

Figure 4. Molecular solid-state structure of the cationic portion of 6. Colors: rhodium, purple; nitrogen, blue; and carbon, black. Hydrogen atoms and outer-sphere counteranion omitted for clarity. Displacement ellipsoids are shown at 50% probability.

The electronic absorption spectrum of 6 was found to match those reported in the literature for complexes featuring the same cationic fragment.32 As discussed for the analogous phen complexes (vide supra), a blueshift in the absorption band for 6 vs 2 was observed (λmax(6) = 476 nm (ε = 720 M−1 cm −1) vs λmax(2) = 507 nm (ε = 1710 M−1 cm −1))5 and can be ascribed to the greater orbital overlap between Rh and olefin ligands in 6 afforded by the more flexible cyclooctadiene framework (vs the more rigid Cp*H ligand in 2). Taken together, these electronic absorption data suggest that the butadiene-like character of [η4Cp*H] and diolefin character of [cod] contribute to the stabilization of the rhodium(I) center in 2, 4, and 6 relative to their analogous [Cp*] complexes 1 and 3. With these observations in hand, and the isolated and pure compounds 3 and 4 available, we turned to cyclic voltammetry to directly interrogate the redox processes accessible in these compounds. For comparison, Cp*Rh(bpy) displays a single two-electron redox process centered at −1.05 V vs Fc+/0 (all potentials here are referenced to the ferrocenium/ferrocene couple, denoted Fc+/0). Similarly, Cp*Rh(phen) displays a single two-electron redox process centered at −0.96 V. These virtually reversible processes correspond to oxidation of the formally Rh(I) center to Rh(III). This oxidation is accompanied by coordination of a sixth ligand to the rhodium metal center; under our conditions, acetonitrile likely serves to coordinate the metal center upon oxidation. No further

Figure 5. Cyclic voltammograms of 4. [Rh] = 1 mM. Conditions: MeCN containing 0.1 M [nBu4N]+[PF6]−; scan rate: 100 mV/s. The black line is the voltammogram resulting from a scan over a wide potential range. The blue lines represent individual voltammograms collected over narrower ranges, resulting in improved reversibility.

To gauge the effect of a more flexible diolefinic ligand, like cyclooctadiene, on the redox behavior of these rhodium complexes, we next moved to interrogate the reduction processes accessible with cod-ligated complex 6. Notably, some electrochemical data on [(cod)Rh(bpy)]+ species are available in the literature.28a,33 In our hands, 6 displayed two virtually reversible redox events at −1.54 and −2.06 V vs Fc+/0. E


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Inorganic Chemistry

ligand might convert between its endo form and an exo analogue in solution. 1H−1H nuclear overhauser effect spectroscopy (NOESY) spectra collected in MeCN-d3 over several hours all display through-space couplings between the ortho-pyridyl protons of the phen ligand and the [Cp*H] ring proton (H11 in Figure 2, see Supporting Information). Conversely, no coupling is registered between the ortho-pyridyl protons and the protons on the unique ring methyl group (C16 in Figure 2). Thus, we conclude that the special ring proton of [η4-Cp*H] retains its endo disposition over time in solution. Consistent with this observation, the proton can be involved in hydrogen generation from reduced forms of the isolated complexes 2 and 4.

The two observed events are assigned to sequential oneelectron, ligand-centered reductions of 6, analogous to the two more-negative reduction processes observed for 2 and 4. The slight positive shift in reduction potential observed for these reductions of 6 is consistent with an overall less electron-rich metal center due to the more π-accepting diolefin ligand and also to the absence of the fifth ligand on Rh (Br− in 2/4). Likewise, the lack of the Br− ligand results in the absence of an oxidation event for 6, which is consistent with previously reported electrochemical studies of analogous complexes.28a,33 Thus, on the basis of the available data, we assign the mostpositive reduction event in 2 and 4 to a Rh(II/I) couple. The two more-negative events are assigned to primarily diiminecentered reductions. Operating with this model, we were interested in learning more about the apparent reactivity of the reduced forms of 2 and 4. As 2 and 4 are known intermediates en route to hydrogen evolution,5 we were curious if hydrogen could be evolved spontaneously by reduction of our isolated compounds. For the case of 2, for example, we imagined that evolution of hydrogen gas would concomitantly generate Cp*Rh(bpy). In this chemistry, the protons incorporated into product H2 would be provided in the form of rhodium-bound Cp*H and electrons would be provided by the electrode. To test this hypothesis, a bulk electrolysis of a solution containing only 2 dissolved in dry acetonitrile/electrolyte solution was carried out in a sealed electrochemical cell at −1.86 V for 1 h. Following the conclusion of the electrolysis, an aliquot of headspace volume was analyzed by gas chromatography to quantify the yield of hydrogen gas. In line with the expected reactivity, a Faradaic yield of hydrogen gas of 71% was measured, indicating conversion of the starting material 2 to hydrogen gas. Over the course of the electrolysis, the working solution turned from a transparent reddish color to a dark purple, suggesting generation of 1 as discussed above based on cyclic voltammetric results. The electronic absorption spectrum of an aliquot of solution from the working-electrode chamber of our twocompartment electrochemical cell was analyzed, and the characteristic absorption features of Cp*Rh(bpy) (λmaxima = 520, 685, 753, 840 nm; see Supporting Information) were observed. Similarly, bulk electrolysis of 4 at −1.86 V for 1 h also resulted in formation of H2 gas and 3. Thus, taken together our results indicate that a viable pathway leading to hydrogen production can be described as

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CONCLUSION We have synthesized and isolated [(Cp*H)Rh] complexes 2 and 4 that bear the bidentate ligands bpy and phen, respectively. The electronic structures of these complexes, as judged from structural and spectroscopic work, are quite similar, and can be understood in relation to other rhodium(I) complexes bearing ligands, like cod, that serve as strong πacceptors. Electrochemical studies of 2 and 4 also demonstrate the role of [Cp*H] in stabilizing the rhodium(I) oxidation state, as new reductions corresponding to ligand-centered events as well as a rhodium(II/I) event are observed by cyclic voltammetry. This behavior contrasts with a single rhodium(III/I) couple observed for the analogous [Cp*] complexes 1 and 3. As the endo proton in both 2 and 4 is poised only ∼2.9 Å from the rhodium(I) metal center, and it retains this position over hours, we expect that it could be involved with a variety of reactivities at the metal center. Our ongoing investigations are addressing this possibility with both chemical and electrochemical studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01895. NMR spectra, crystallographic details, electronic absorption spectra, electrochemical and gas chromatography data (PDF) Cartesian coordinates (XYZ)

2[(Cp*H)Rh(bpy)Br] + 2e− → H 2 + 2Cp*Rh(bpy) + 2Br −

Accession Codes

CCDC 1546016, 1546017, and 1563622 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

The observation of hydrogen generation from compounds 2 and 4 bearing [Cp*H] should not be considered completely surprisingtreatment of 2 with strong acid results in hydrogen production and generation of [Cp*RhIII] complexes. However, the new reactivity observed suggests that reduced forms of 2 or 4 can also evolve hydrogen in a different cycle involving an additional reduction event. Along this line, we have previously hypothesized, based on the X-ray diffraction data and studies by 1H NMR, that the observed endo disposition of the [Cp*H] ring proton in 2 and 4 is stable and retained over time in solution. If this is true, the proton remains geometrically accessible to the metal center and able to participate in further metal-guided reactivity in an intramolecular fashion. With compounds 2 and 4 isolated, we endeavored to establish whether the coordinated [Cp*H]

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 (785) 864-3019. Fax: +1 (785) 864-5396. ORCID

James D. Blakemore: 0000-0003-4172-7460 F


Article

Inorganic Chemistry Present Address

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(M.V.R.G.) Department of Chemistry, University of Puerto Rico, Rió Piedras Campus, P.O. Box 23346, San Juan, PR 00931−3346. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Justin Douglas and Sarah Neuenswander for assistance with NMR spectroscopy, and Dr. Victor Day and Dr. Michael Takase for assistance with X-ray crystallography. This work was supported by the State of Kansas through an award from the University of Kansas New Faculty General Research Fund and by the National Science Foundation through the NSF CCI Solar Fuels Program (CHE-1305124).

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Structural and Electrochemical Consequences of [Cp*] Ligand

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