Double-Sandwich Pentalene Complexes M2(pent†) - American

Article pubs.acs.org/Organometallics

Double-Sandwich Pentalene Complexes M2(pent†)2 (M = Rh, Pd; pent† = 1,4-Bis(triisopropylsilyl)pentalene): Synthesis, Structure, and Bonding Owen T. Summerscales,† Christopher J. Rivers,† Morgan J. Taylor,† Peter B. Hitchcock,† Jennifer C. Green,*,‡ and F. Geoffrey N. Cloke*,† †

Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, U.K. Inorganic Chemistry Laboratory, Chemistry Department, Oxford University, South Parks Road, Oxford OX1 3QR, U.K.

‡

S Supporting Information *

ABSTRACT: The bis(pentalene) complexes M2(pent†)2 (M = Rh (1), Pd (2); pent† = 1,4bis(triisopropylsilyl)pentalene) have been synthesized and structurally characterized. In both 1 and 2 the metals have a formal electron count in excess of 18 per metal center, and DFT calculations indicate antibonding metal−metal interactions are present in 1, whereas 2 involves antibonding metal−ligand interactions.

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batteries.21 The area has also given fertile ground for theoretical investigations,22 with exotic theoretical species such as U2(cot)2 and double-sandwich “nanowires” {M2(pent)}n having been explored.6,23 It is evident that the eight-membered bicyclic pentalene ligand, C8H62−, which may be considered as a fusion of two cyclopentadienyl rings, is ideal for stabilizing M2 dimers, holding the metal centers within range to participate in M−M multiple bonding.24 Herein we report two new pentalene double-sandwich compounds, M2(pent†)2 (M = Rh (1), Pd (2)), which display metallocene-like bonding.25 These species are particularly interesting, given that the metallocene congeners are unstableneutral palladocenes are unknown, and neutral rhodocenes have only been isolated as stable species at room temperature with very large substituents (cf. Rh(C5Ph4H)226).

he discovery of sandwich compounds in the 1950s, notably the archetypal ferrocene,1,2 heralded the beginning of modern organometallic chemistry. The sandwich motif has been rapidly extended to bimetallic species, including antifacial species, termed inverse- or double-decker sandwiches, depending on terminal ligands,3,4 and significantly less common syn-facial homoleptic compounds of the type M2(L)2, called double-sandwich molecules (L = aromatic ligand). The latter are of interest owing to their propensity for metal−metal bonding and ensuing electronic properties, of potential use in materials, in addition to fundamental studies of structure and bonding.5 Described by Katz et al. in 1964, the first reported doublesandwich compound was Fe2(as-indacene)2;6 the first crystallographically characterized species was Ni2(cot)2, reported in 1976.7 Since then, a modest number of stable, similar species have been isolated: perpendicularly bound V2(indenyl)2,8 Ti2(cot′)2,9 M2(pent)2 (M = Ni, Co),10,11 M2(pent†)2 (M = Cr, Mn, Mo),12−14 M2(pent*)2 (M = V, Cr, Mn, Co, Ni),15 M2(fulvalene)2 (M = Fe, V, Cr, Mo, Ni),16 and the unique coaxially bound Zn2(Cp)2.17 Furthermore, Murahashi et al. recently reported the two remarkable multimetallic sandwich species [Pd5(tetracene)2]2+ and [(cot)Pd4(cnt)]+,18 along with a range of other Pd2−Pd5 and Pt3 sandwiches stabilized by axial ligands.19 The latter adds to and extends the known literature of Pd2 double-sandwich species stabilized with exogenous neutral ligands.20 Very recently the trianionic sandwich [Li5(corannulene)2]3− was reported and investigated due to its relevance to LiC6 graphite in modern high-capacity © 2012 American Chemical Society

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RESULTS AND DISCUSSION Dark red 1 was obtained from reaction of Rh2(OAc)427 and 2 equiv of K2pent† in thf (tetrahydrofuran) in essentially quantitative yield (Scheme 1). The successful formation of the double-sandwich compound was confirmed by a singlecrystal X-ray diffraction study of the isolated material and was further substantiated by its multinuclear NMR spectroscopic features. 1H and 13C{1H} NMR (C6D6) gave the expected signals for the pentalene ligand coordinated syn-facially to a Received: October 24, 2012 Published: December 6, 2012 8613

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also causes asymmetry in the Rh−C(bridgehead) bond distances: Rh1−C1 = 2.299(7) Å vs Rh1−C1′ = 2.472(7) Å and Rh1−C5 = 2.352(6) Å vs Rh1−C5′ = 2.486(7) Å. By crystallographic necessity, this is mirrored on the opposite half of the pentalene ligand for Rh1′. Previously determined structures of M2(pent†)2 also display this structural motif (M = Cr, 11°; M = Mo, 10°);12,14 this is likely caused in some degree by the packing effects of the asymmetric 1,4-silyl substituents. The most notable parameter is the Rh−Rh distance of 2.707(1) Å, sufficient for metal−metal bonding (e.g., Rh−Rh single bond of 2.719 Å in (indenyl)2Rh2(C8F8)28), although given the innate proximity of the metal centers in these types of bridged dimers, it is not conclusive proof of metal−metal bonding.12−15 This distance is found to be longer than in the group VI analogue Mo2 (pent†) 2,14 for which a MoMo bond has been determined at 2.340(1) Å. Rh−C distances are comparable to those in the previously determined rhodium hydropentalenyl species Rh(η5-pentH4R)(η4-cod) (R = 1,3,3,5-tBu, 1,3,4,5SiMe3, 1,3,4,6-Rh(COD)(η5-Me2C5H2)).29 1 does not bind Lewis basic small molecules, lacking the axial coordination site of its precursor Rh2(OAc)4, and is therefore unlikely to participate in the same type of catalytic processes. The synthesis of 2 was achieved by reaction of equimolar amounts of Pd(cod)Cl2 and K2pent† in thf (27% isolated yield). Multinuclear NMR spectra show the expected resonances,14 and a correct elemental analysis was obtained. Unfortunately, disorder present in the structure obtained by X-ray crystallography has prevented us from determining completely reliable bond distances (R1 = 10.9%). Nonetheless, these data are instructive with respect to the connectivity of the molecule and are worth discussing in brief. The structure was solved in the monoclinic space group C2 and shows two molecules in the asymmetric cell, both lying on 2-fold rotation axes. The overall geometry is similar to that of 1, with two Pd centers bound in a dimetallocene-like arrangement, albeit shifted toward the wingtip carbons, to a greater degree than seen in 1 (Figure 2). The most reliable parameter of the study, the metal−metal

Scheme 1. Synthesis of 1 and 2

metal center, with diastereotopic SiiPr3 resonances. These features are consistent with a diamagnetic complex. Pentalene may be considered as donating 5 electrons to each metal,36 giving a 19-electron count on Rh and suggesting an antibonding interaction between the metals. Crystals of 1 were obtained from benzene and were solved in monoclinic space group C2/c. The structural data clearly show a centrosymmetric Rh2 bis(pentalene) species in a doublesandwich configuration, with approximately coplanar ligands binding in a distorted metallocene-type manner, intermediate between η5:η5 and η3:η3, with the metal centers slipped slightly toward the wingtip carbons (Figure 1). This tendency toward allylic bonding has been previously noted in bimetallic pentalene species24 and is accompanied by a shortening of the C−C(bridgehead) distances (C1−C1′ = 1.373 Å, C5−C5′ = 1.426 Å). The ligands are twisted with respect to each other at an angle of 17.5° from an ideal eclipsed orientation, which

Figure 1. ORTEP (50%) plot of 1. H atoms and isopropyl groups are omitted. Selected bond lengths (A) and angles (deg): Rh1−Rh1′ = 2.7072(14), Rh−C1 = 2.299(7), Rh−C2 = 2.177(7), Rh−C3 = 2.134(7), Rh−C4 = 2.214(6), Rh−C1′ = 2.472(7), Rh−C5 = 2.352(6), Rh−C6 = 2.182(6), Rh−C7 = 2.122(7), Rh−C8 = 2.205(7), Rh−C5′ = 2.486(7); C1−Rh−C5 = 116.8(2), C1−Rh− C5′ = 102.3(2), C1′−Rh−C5 = 101.2(2), C1′−Rh−C5′ = 106.1(2).29

Figure 2. ORTEP (50%) plot of 2, excluding H atoms and isopropyl groups, showing the connectivity of ring atomic positions. Since R1 = 10.9%, ring−palladium connectivities are not shown. 8614

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distance, is found to be 2.786(2) Å, significantly longer than the equivalent in 1. No Pd−Pd bond would be expected, as each center would be expected to possess a formal electron count of either 16 or 20, depending on whether the coordination environment is described as allyl- or cyclopentadienyl-like. Theoretical calculations performed by Saillard et al. on Pd2(pent)2 have addressed this matter and show that the remaining CC bridgehead from localization of charge in the pentalene bis-allyl mode is involved significantly in bonding.22f This is borne out in the previously reported experimental structure of Ni2(pent*)2, which displays a bis-allyl binding mode, however without a shortened C−C bridgehead distance, confirming the noninnocence of this moiety.15 For the purposes of modeling the electronic structure of M2(pent†)2 (M = Rh (1), Pd (2)) the ring substituents were replaced by H. The geometries of M2(pent)2 (M = Rh, Pd) were optimized both with and without symmetry constraints. Key distances are given in Table 1. When D2h symmetry was Table 1. Selected Calculated Distances (Å) for M2(pent)2 (M = Rh, Pd) Rh2(pent)2

Figure 3. Calculated structures for M2(pent)2: (a) Rh2(pent)2 with D2h symmetry; (b) Rh2(pent)2 with C2h symmetry; (c) Pd2(pent)2 with D2h symmetry; (d) Pd2(pent)2 with C2v symmetry.

Pd2(pent)2

D2h

C2h

D2h

C2v

M−M M−Cbridge

2.766 2.492

2.868 2.643

M−Cα

2.222

M−Cβ

2.141

2.871 2.573 2.717 2.270 2.467 2.197

Cbridge−Cbridge Cbridge−Cα

1.450 1.449

Cα−Cβ

1.447

2.699 2.284 2.684 2.255 2.221 2.247 2.112 1.454 1.439 1.466 1.441 1.449

2.364 2.193 1.492 1.420 1.446

as −0.33: i.e., an antibonding interaction between the metals. For the Pd analogue the calculated bond order is −0.1, suggesting a nonbonding Pd−Pd interaction. The orbital scheme for a bis-pentalene metal dimer and its evolution across the first-row transition series has been reported previously.22f Examination of the frontier orbitals (M = Rh, Pd) (Figure 4) accounts for both the bond orders

1.493 1.412 1.431 1.447 1.446

imposed, both resulting structures had small imaginary frequencies. For Rh2(pent)2, a local minimum was achieved with C2h symmetry, with the pentalene rings parallel but the sandwich experiencing a shear distortion so that the Rh atoms veered toward an η3:η5 coordination. The C2h structure was only 0.04 eV lower in energy than the D2h structure. A similar distortion from D 2h symmetry was also found for Co2(pent)2,15,22f but in this case the energy difference was somewhat larger and the crystal structure of Co2(pent*)2 showed effective C2h symmetry. For Pd2(pent)2 a different distortion was found. The minimum energy structure had C2v symmetry with the two pentalene planes no longer parallel, the C2 axis bisecting the Pd−Pd vector and lying in the same plane as the C−C bonds of the pentalene bridge. The energy difference here is 0.01 eV. All four structures are shown in Figure 3. Neither energy difference is regarded as significant because of both the inherent accuracy of the calculations and the absence of substituents in the modeling. Comparison with the crystal structure data gives good agreement with the D2h structures in the case of 1. Fragment calculations using as a basis set the MOs of the M2 unit and the pentalene rings enable an estimate of the occupation of the σ, π, and δ orbitals of the M2 fragment in the molecules. The assumption that σg, πu, and δg orbitals are bonding and the σu, πg, and δu orbitals are antibonding gives an M−M bond order. For Rh2(pent)2 the bond order is estimated

Figure 4. Frontier orbitals for M2(pent)2: D2h symmetry (a) M = Rh (b1u), (b) M = Rh (b3g), (c) M = Pd (b1g); C2h symmetry (d) M = Rh (bu), (e) M = Rh (bg); C2v symmetry (f) M = Pd (b2).

and the distortions from D2h symmetry found computationally. Rh2(pent)2 with D2h symmetry has two near-degenerate HOMOs of b1u and b3g symmetry. Both are Rh−Rh antibonding. Distortion to C 2h symmetry relieves the antibonding interaction in the b1u orbital. Pd2(pent)2 has a HOMO of b1g symmetry, which is unoccupied in the Rh analogue. This is Pd−Pd bonding, accounting for the higher bond order in the Pd compound. However, the interaction is δ in symmetry and thus is not expected to have much effect on the Pd−Pd distance, though it does increase the formal bond 8615

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SiiPr3, 50%). Anal. Calcd (found) for C52H92Si4Pd2: C, 59.93 (60.03); H, 8.84 (8.91). X-ray Crystallographic Studies of 1 and 2. Crystal data for 1 at 173(2) K with Mo Kα radiation (λ = 0.71073 Å): monoclinic, space group C2/c, Z = 4, a = 22.188(5) Å, b = 13.183(3) Å, c = 21.397(11) Å, α = γ = 90°, β = 120.37(3)°, V = 5400(3) Å3, R1 = 0.064 for 2959 observed reflections (I > 2σ(I)), wR2 = 0.178 (all data). Crystal data for 2 at 173(2) K with Mo Kα radiation (λ = 0.71073 Å): monoclinic, space group C2, Z = 4, a = 21.3740(12) Å, b = 13.3834(8) Å, c = 21.1553(12) Å, α = γ = 90°, β = 96.549(4)°, V = 6012.1(6) Å3, R1 = 0.109 for 6603 observed reflections (I > 2σ(I)), wR2 = 0.302 (all data). Both molecules are disordered with lower occupancy alternative orientations such that the two Pd atoms are rotated by 90°: the ratios are 0.95:0.05 for molecule 1 and 0.72:0.28 for molecule 2. In both cases only the alternative Pd positions were located. For the more disordered molecule the C atoms were left isotropic to avoid them going nonpositive definite. Computational Methods. Quantum chemical calculations were performed on M2(pent)2 (M = Rh, Pd), neglecting ring substituents, using density functional methods of the Amsterdam Density Functional (version ADF2010.02)32 package. In the calculations the generalized gradient approximation was employed, using the local density approximation of Vosko, Wilk, and Nusair,33 together with nonlocal-exchange corrections by Becke34 and the nonlocal-correlation corrections by Perdew.35 TZP basis sets were used with triple-ξ accuracy sets of Slater-type orbitals, with polarization functions added to all atoms. Relativistic corrections were made using the ZORA (zeroorder relativistic approximation) formalism. The core electrons were frozen up to 1s for carbon and 3d for Rh and Pd. The geometries were optimized with and without symmetry constraints. Fragment analyses on the D2h structures afforded a decomposition of the bonding in terms of the MOs of the M2 dimer and the pentalene ligands and enabled an estimation of the M−M bond order.14b

order. More significantly, the b1g orbital is strongly M−pent antibonding, the interaction being concentrated on the bridgehead carbons. This accounts for both the general increase in M−C distances on moving from Rh to Pd and the tendency to η3:η3 coordination of the Pd atoms. The C2v distortion relieves the Pd−C antibonding with one side of the Pn ligand, but to reiterate, the distortion calculated for either Rh2(pent)2 or Pd2(pent)2 has no significant energetic advantage.

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SUMMARY In conclusion, two new M2 bis(pentalene) double-sandwich species have been synthesized for the second-row elements Rh and Pd. These species are particularly noteworthy, as rhodocene and most of its derivatives are kinetically unstable under ambient conditions and palladocenes are currently unknown. This originates from the nature of pentalene as a quasi-bis-cyclopentadienyl ligand: due to the sharing of a C−C bridgehead, which has the capacity to form three-center−twoelectron bonds, the pentalene ligand can be regarded as contributing five electrons to each metal. Such a situation is akin to electron counting for dimers with bridging hydrides.36 Thus, for both 1 and 2 the metals have an electron count in excess of 18.36 DFT calculations indicate antibonding M−M interactions for M = Rh and antibonding metal−ligand interactions for M = Pd.

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EXPERIMENTAL SECTION

General Procedures. The reactions described below were conducted under purified argon using standard Schlenk doublemanifold, high-vacuum, or glovebox techniques. Solvents were predried over activated 5 Å molecular sieves or sodium wire and then distilled over potassium (tetrahydrofuran), sodium−potassium alloy (pentane), or sodium (toluene) under a slow passage of nitrogen. Deuterated solvents were dried over molten potassium and distilled. Elemental analyses were carried out at the University of North London. Mass spectra were recorded by Dr. Ali Abdul-Sada at University of Sussex, on a VG autospec Fisons instrument (electron impact ionization at 70 eV). NMR spectra were recorded using a Bruker Spectrospin AG 300DPX spectrometer operating at 300.13 MHz for 1H measurements. The starting materials Rh2(OAc)4, Pd(COD)Cl2, and K2pent† were prepared according to literature procedures.30,31 Synthesis of Rh2(pent†)2 (1). A THF solution of K2pent† (230 mg, 0.453 mmol) was added dropwise to a stirred solution of Rh2(OAc)4 (100 mg, 0.226 mmol). After reaction overnight, volatiles were removed in vacuo and the purple solids extracted with pentane (Celite). Evaporation of pentane gave the product as a red powder (222 mg, 0.215 mmol, 95% yield). MS (EI): m/z 1034 (M+, 100%), 992 (M+ − iPr, 40%), 887 (M+ − SiiPr3, 50%). 1H NMR (300 MHz, C6D6, 298 K): δ (ppm) 4.96 (d, 2H, Cring-H, J = 2.5 Hz), 4.42 (d, 2H, Cring-H, J = 2.5 Hz), 1.09−1.25 (m, 84H, SiiPr3). 13C{1H} NMR (C6D6, 100.6 MHz, 298 K): δ 115.8, 86.5, 61.1, 39.2, 19.6, 19.4, 12.6; Anal. Calcd (found) for C52H92Si4Rh2: C, 60.34 (60.40); H, 8.90 (9.01). Synthesis of Pd2(pent†)2 (2). A THF solution of K2pent† (281 mg, 0.57 mmol) was added dropwise to a stirred solution of Pd(COD)Cl2 (163 mg, 0.57 mmol). After reaction overnight, volatiles were removed in vacuo and the sticky brown solids extracted with pentane (Celite). Reduction to a minimal volume (ca. 5 mL) and cooling to −50 °C for 2 days gave the product as green crystals (80 mg, 0.077 mmol, 27% yield). 1H NMR (300 MHz, C6D6, 298 K): δ (ppm) 7.13 (d, 2H, Cring-H, J = 2.5 Hz), 3.05 (d, 2H, Cring-H, J = 2.5 Hz), 1.05−1.10 (m, 84H, SiiPr3). 13C{1H} NMR (C6D6, 100.6 MHz, 298 K): δ 110.6, 91.9, 79.3, 64.6, 19.4, 19.0, 12.1. MS (EI): m/z 1042 (M+, 100%), 999 (M+ − iPr, 40%), 934 (M+ − Pd, 20%), 885 (M+ −

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ASSOCIATED CONTENT

S Supporting Information *

CIF files giving crystallographic data for 1 and 2. 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] (F.G.N.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the EPSRC and the University of Sussex for financial support

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ABBREVIATIONS cot, cyclooctatetraenyl; cot′, 1,4-bis(trimethylsilyl)cyclooctatetraenyl; pent, pentalenyl; pent † , 1,4-bis(triisopropylsilyl)pentalenyl); pent*, permethylpentalenyl; cnt, cyclononatetraenyl; cod, 1,4-cyclooctadiene

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REFERENCES

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dx.doi.org/10.1021/om3010004 | Organometallics 2012, 31, 8613−8617