Strained ansa Half-Sandwich Complexes of Ruthenium and Osmium

Communication pubs.acs.org/Organometallics

Strained ansa Half-Sandwich Complexes of Ruthenium and Osmium and a Non-Iron Metallopolymer by Ring-Opening Polymerization Holger Braunschweig,*,† Theresa Dellermann,† Rian D. Dewhurst,† Jan Mies,† Krzysztof Radacki,† Sascha Stellwag-Konertz,† and Alfredo Vargas†,‡ †

Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, Sussex, U.K.

‡

S Supporting Information *

ABSTRACT: Herein we report the first non-iron polymer obtained from an ansa half-sandwich complex. This polymeric organometallic material was obtained from a new disilanediylbridged ruthenium complex upon thermally induced ringopening polymerization (ROP). Additionally, a corresponding distannanediyl-bridged osmium species is reported, the first example of an ansa half-sandwich complex of this element.

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rgano met allic p oly mers 1 derived from [ n]metallocenophanes2 and related [n]metalloarenophanes3 have been a focus of chemists and materials scientists over the past 20 years due to their promising electrochemical, optical, and magnetic properties. In contrast to these precursors, ansa half-sandwich complexes, which have likewise been found to form organometallic polymers by ring-opening reactions,4 are far less developed. While initial studies of ansa half-sandwich complexes were limited to carbon-bridged species, more recent work established a series of complexes featuring metals from groups 6, 8, and 10 bridged by silicon and tin moieties.4−11 Given the potential of these heavier carbon congeners to facilitate the ROP of ansa sandwich complexes, it is surprising that the only known examples of organometallic polymers composed of half-sandwich repeating units feature iron as the central transition metal. The iron-containing precursors to these materials, reported by Pannell et al., featured both carbon−silicon and silicon−silicon bridges that readily underwent ROP at ambient temperature, yielding polymers with molecular weights up to 175000.4a,f,g Herein we report the synthesis of the first non-iron ansa half-sandwich complex that provides a metallopolymer upon thermally-induced ROP. Furthermore, the synthesis and structural characterization of the first osmium-based ansa half-sandwich complex is reported. Two general routes have been established for the syntheses of ansa half-sandwich complexes: (i) the original approach, reported by Gladysz et al., initiated by metalation at the cyclopentadienyl ring of a disilanyl half-sandwich complex, followed by a silatropic shift of the disilanediyl group to the Cp ligand and subsequent ring-closing reaction,12 and (ii) a protocol developed by our group employing double salt metathesis reactions of dimetalated half-sandwich complexes and suitable bridging units.6,9 As no dimetalated group 8 metal sandwich complexes are presently known, route i was employed for the syntheses of ruthenium and osmium complexes 1 and 2 (Scheme 1). By treatment of anionic complexes, generated in © 2014 American Chemical Society

Scheme 1. Synthesis of Complexes 1 and 2

situ by reduction of the precursor complexes [(η5-C5H5)Ru(CO)2]213 and [(η5-C5H5)Os(CO)2I],14 with Si2Me4Cl2 and Sn2tBu4Cl2,15 respectively, complexes 1 and 2 were obtained in yields of 62% and 31% as yellow solids after crystallization from pentane. Compound 1 shows remarkable thermal stability, decomposing only at temperatures above 150 °C. In contrast, the corresponding iron derivative spontaneously polymerizes upon warming to room temperature.4f,g Characterization of the new half-sandwich complexes by IR and multinuclear NMR spectroscopy confirmed the proposed structures and showed good agreement with analogous group 8 complexes with silicon or tin bridges.4f,11 The 1H NMR spectrum of 1 shows two distinct multiplets at δ 5.20 and 4.56 ppm, which can be assigned to the cyclopentadienyl protons. Two singlets were observed for the bridging moiety, one for each of the two chemically nonequivalent SiMe2 groups. In addition, the 29Si NMR spectrum exhibits two distinct resonances at low frequencies, in agreement with previously synthesized iron ansa half-sandwich complexes4f (δ −21 (Ru−Si) and −75 (C5H4−Si) ppm). Compound 2 shows similar NMR signals: two multiplets at δ 5.04 and 4.92 ppm for the Cp ligand and Received: January 30, 2014 Published: March 26, 2014 1536

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two singlets for the tBu groups, showing 117/119Sn satellites resulting from 3JHSn coupling. In addition, the 119Sn NMR spectrum displays signals for each of the inequivalent tin atoms at low frequencies (δ −68 and −196 ppm). Both 1 and 2 were examined via X-ray diffraction analysis (Figures 1 and 2). The molecular structure of 1 shows Ru−Si

Table 1. Selected Bond and Torsion Angles (deg) for Compound 2 in Comparison with Those of Previously Synthesized ansa Half-Sandwich Complexes of Group 8 Metals complex

M−Sn2−Sn1 Sn2−Sn1−C1 Sn2−M−C1−Sn1

[Sn2(tBu)4(η5-C5H4) Fe(CO)2]9 [Sn2(tBu)4(η5-C5H4) Ru(CO)2]11 [Sn2(tBu)4(η5-C5H4) Os(CO)2] (2)

83.58(1)

78.03(6)

11.89(9)

84.35(1)

81.15(5)

10.73(6)

84.77(1)

81.00(1)

10.5(2)

Scheme 2. Synthesis of the Polymer 3 by ROP from Precursor Complex 1

Figure 1. Molecular structure of 1. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms and carbon ellipsoids of methyl groups are omitted for clarity. Selected bond lengths (Å) and bond angles and torsion angles (deg): Ru−Si2, 2.4342(4); Si1−Si2, 2.4342(4); Si1−C1, 1.892(1); Ru−Si2−Si1, 87.21(1); Si2−Si1−C1, 89.54(4); Si1−C1−Ru−Si2, 0.88(4).

DSC analysis. Under similar conditions, compound 2 did not undergo ring opening. After thorough extraction of this crude product with hexane and cold THF in order to remove unreacted starting material and small oligomers, a solid material was obtained in 60% yield (3) that was only sparingly soluble in hot THF. Using sonification at 60 °C a sample was prepared suitable for GPC analysis, which revealed a polymeric material with a monomodal molecular weight distribution (Mw = 7.44 × 105, Mn = 7.43 × 103) and a moderate polydispersity (1.6). The average molecular weight of 74400, measured relative to monodisperse polystyrene standards, corresponds to 220 repeating units. The 1H NMR spectrum of 3 in hot d8-THF exhibits two signals for the cyclopentadienyl protons at δ 5.31 and 5.25 ppm, as well as two singlets for the methyl groups at δ = 0.43 and 0.34 ppm, indicating the regular alternating arrangement of the precursor complex 1. This finding is supported by two narrow signals in the 29Si NMR spectrum at δ 0.37 (Ru−Si) and −18.39 (C5H4−Si) ppm, which are in agreement with the findings of Pannell and co-workers (δ 22 (Fe−Si) and −8.20 (Fe−Si) ppm), who observed a similar pattern for the polymeric material obtained from the corresponding iron complex.4f In comparison to those of precursor complex 1 both resonances are shifted to low field (29Si NMR for 1 δ −21 (Ru−Si) and −75 (C5H4−Si) ppm), indicating the relief of ring strain during ROP. The IR spectrum of 3 in the solid state shows two strong vibrational frequencies at 1994 and 1934 cm−1 for the CO ligands, almost identical with the frequencies observed for the precursor complex 1 (1994, 1926 cm−1). This finding confirms that in the polymer 3 the carbon monoxide ligands are still bound to the metal centers in a μ1 (end-on) mode and there is no cross-linking during the polymerization process. The significantly different polymerization properties of 1 and its Fe analogue reported by Pannell (1a)4f prompted us (a) to closely examine the nature and strength of the bonding between the two silicon atoms (Si2−Si1) and between the metal-bound silicon (Si2) and the metal, as either bond may be broken during ring opening, and (b) to determine the relative strength difference for the Si2−Si1 and Si2−M (M = Ru, Fe) bonds in the two systems, in order to rationalize the observed

Figure 2. Molecular structure of 2. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms and carbon ellipsoids of the tBu groups are omitted for clarity. Selected bond lengths (Å) and bond angles and torsion angles (deg): Os−Sn2, 2.6921(4); Sn1−Sn2, 2.8202(4); Sn1−C1, 2.205(5); Os−Sn2−Sn1, 84.77(1); Sn2−Sn1− C1, 81.0(1); Sn1−C1−Os−Sn2, 10.5(2).

and Si−Si bond lengths (2.4342(4) and 2.3510(6) Å) in the range of those found in analogous ansa compounds.4c,e,6,9,11,16 The strained nature of the metallacyclobutane ring system is reflected by the acute endocyclic angles (Ru−Si2−Si, 187.21(1)°; C1−Si1−Si2, 89.54(4)°) of compound 1, significantly smaller than the ideal angles for an sp3-hybridized tetrahedral geometry. The C1−Si1−Si2−Ru torsion angle is close to 0 (0.88(4)°). The molecular structure of compound 2 is comparable to that of a previously described ruthenium ansa half-sandwich complex with a bridging Sn(tBu)2−Sn(tBu)2 moiety.11 Table 1 summarizes selected bond and torsion angles in comparison with those of previously reported complexes of group 8 metals.9,11 Compound 1 was subjected to differential scanning calorimetric analysis. The spectrum showed a melting peak at 94 °C followed by an exothermic peak at 150 °C, which can be attributed to a thermal ring-opening reaction affording a polymeric material (Scheme 2). 1 was heated at 150 °C for 2 h in a sealed J. Young tube, over which time the pale yellow precursor was observed to initially melt and subsequently solidify to a pale yellow material, in full agreement with the 1537

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experimental results. To this end, we employed the Kohn− Sham density functional theory (DFT) methodology, in particular the so-called “fragment approach” as implemented in the Amsterdam Density Functional (ADF) program,17,18 also commonly known as energy decomposition analysis (EDA). The idea behind this approach is to quantify the different energy contributions to the interaction between two predefined fragments; prominent among these contributions are the electrostatic and orbital interactions. For our purposes, we employed the partitioning schemes s1 and s2 for both complexes as shown in Figure 3; a more detailed description of the computational approach employed is given in the Supporting Information.

Table 2. Hirshfeld Charge at the Metal and Si2 Atoms in the Fragments and Whole Complex for 1 and 1aa f1 q(Fe) q(Si2(1a)) q(Ru) q(Si2(1)) a

f2

0.10 0.13 0.32 0.14

f1 − f2

|Δq|

charge flow

−0.02 0.25 0.24 0.20

0.12 0.12 0.08 0.06

Fe←Si2 Fe←Si2 Ru←Si2 Ru←Si2

Fragments: f1 = [(η5-C5H4)M(CO)2]; f2 = [Me2SiSiMe2].

Figure 3. Fragmentation of 1 (left) and 1a (right) used for EDA analysis.

Assuming a negligible difference in respective strengths of the C1−Si1 bonds in 1 and 1a, calculations show that the Ru−Si2 bond is stronger than the Fe−Si2 bond by 6.25 kcal mol−1; furthermore, the energy associated with the Si1−Si2 bond in 1 is stronger than that in 1a by 5.22 kcal mol−1. The overall differences in bonding between 1 and 1a are largely confined to electrostatic interactions. Closer examination reveals that the bonding in complexes 1 and 1a also differs in the magnitude of respective contributions from electrostatic interactions and the orbital (covalent) interactions. In the s1 partitioning scheme, the electrostatic contribution to the bonding in 1 is 19.62 kcal mol−1 higher than that in 1a, while the difference in the orbital interaction is significantly lower, just 4.63 kcal mol−1. This result indicates that the enhanced bonding in 1, as compared to that in 1a, is predominantly a function of increased electrostatic interactions rather than an increase in covalency. Analysis of the atomic charges19 on the Fe, Ru, and Si atoms reveal that, upon fragment combination according to the s1 scheme, there is greater charge transfer in the Si2 → Fe direction in 1a than is seen for Si2 → Ru in 1, which serves to weaken the Si−Si bond in 1a comparatively more than the Si− Si bond in the Ru complex (Table 2). The consequence of the difference in the Si2 → Fe charge flow is manifested by the frontier orbitals, where the highest occupied molecular (HOMO) of 1a is more highly localized on the metal than the HOMO of 1. Furthermore, the Wiberg bond order (WBO)20 for the Ru−Si2 bond is 0.53, whereas it is slightly smaller (0.46) for Si2−Fe bond. In addition, the HOMO-1 of 1 reveals a noticeable delocalization on the Si−Si bond, which is absent for 1a (see Figure 4). Finally, the Ru−Si2 bond of 1 is stronger than the Si1−Si2 bond by 16.34 kcal mol−1, while for

Figure 4. Frontier orbitals of 1 and 1a. Note that these are centered on the metals and on the M−Si−Si bonds.

1a the difference is 17.37 kcal mol−1. We see that in both systems, as suggested by calculations, it is the Si−Si bond that is likely to be cleaved in terms of electronic structure and bonding energetics. The above results fully support the experimentally observed lower polymerization temperature of 1a in comparison to that of 1. The better charge transfer from Si to Fe in 1a inductively concentrates electron density at the metal and in doing so activates the Si−Si bond more favorably for cleavage in comparison to the case for Ru. In this contribution, we were able to synthesize novel ruthenium and osmium ansa half-sandwich complexes. The former features a bridging SiMe2−SiMe2 unit, while the latter is bridged by a SntBu2−SntBu2 group spanning the Cp ring and the osmium center. The ruthenium species serves as a monomeric precursor for the first non-iron metallopolymer consisting of repeating half-sandwich complex units. This material was obtained by ROP with a high molecular weight and relatively low polydispersity. Further insight into the bond breaking involved in the polymerization process was provided by DFT calculations, which suggest that the Si−Si rather than the M−Si bond is cleaved and provide a rationalization of the differences in polymerization temperature between Fe and Ru complexes. 1538

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and tables, giving experimental details, characterization data, details of the calculations, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 907969 and 907970 also contain supplementary crystallographic data for this paper.

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AUTHOR INFORMATION

Corresponding Author

*H.B.: fax, +49 931 31 84623; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the German Science Foundation (DFG) for financial support and to Dr. William Ewing for useful discussions.

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REFERENCES

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