Structure and Bonding of a Zwitterionic Iridium Complex Supported by

Communication pubs.acs.org/Organometallics

Structure and Bonding of a Zwitterionic Iridium Complex Supported by a Phosphine with the Parent Carba-closo-dodecaborate CB11H11− Ligand Substituent Ahmad El-Hellani,† Christos E. Kefalidis,‡ Fook S. Tham,† Laurent Maron,*,‡ and Vincent Lavallo*,† †

Department of Chemistry, University of California Riverside, Riverside, California 92521, United States Department of Chemistry CNRS & INSA, UPS, LPCNO, Université de Toulouse 135 Avenue de Rangueil, F-31077, Toulouse France

‡

S Supporting Information *

ABSTRACT: A zwitterionic iridium complex of a phosphine, bearing the carba-closododecaborate anion as a ligand substituent, is reported. When tethered directly to a phosphine ligand, the CB11H11− R group engages in agostic-like bonding, utilizing the B−H bonds adjacent to the carbon atom in the cluster. Evidence for the interactions is observed in solution by variable-temperature NMR and also in the solid state by a single-crystal X-ray diffraction study. The bonding between the cluster and the iridium center has also been analyzed computationally and can be described as a double agostic bonding augmented by an overlap of the skeletal electrons of the cluster with the dz2 orbital of the metal. A significant lengthening of a trans-olefin C−C bond suggests that this ligand substituent has a pronounced trans influence, which is in contrast to the unfunctionalized weakly coordinating HCB11H11− anion.

I

nteresting classes of molecules that display rich coordination chemistry are the boron1 and carborane2 cluster compounds. Many of these species interact with transition metals via “agostic-like”3 bonding through B−H−M bridges.4 Aside from noncluster boron hydride compounds,5 most of the reported examples of this mode of bonding involve a transition metal and a metallaborane4a−c,e,k,s,6 or a Teixidor4f,j,l−q type phosphine ligand with a nido-cluster substituent. Of the known carboranes, the icosahedral carba-closo-dodecaborate cluster HCB11H11− (1) (Figure 1, left) and its derivatives are some of the most stable and have been used frequently as weakly coordinating anions.5,7,8 The charge distribution in cluster 1 is such that the boron distal to carbon (position 12) is the most electron rich, followed by the adjacent pentagonal belt (positions 7−11), with the least electron rich boron atoms at positions 2−6, since they are attached to a more electronegative carbon atom. Hence, when the HCB11H11− anion (or its halogenated or alkylated derivatives) interacts with a countercation, the interaction always occurs via the σ-bound cluster substituents at positions 7−11 and 12. As reported by Weller4h and Spencer,4r the HCB11H11− anion displays interesting coordination chemistry when paired with d8 transition-metal cations, exemplified by the zwitterionic rhodium(I) cyclooctadiene complex 24h (Figure 1). Although never investigated computationally, it qualitatively appears that, in the solid state, 2 is best described as a distorted-square-planar d8 σ complex11 of the carba-closo-dodecaborate anion, bound via double B−H interactions with vacant metal d orbitals (a combination of dx2−y2 with p and/or s). Interestingly (vide infra), both olefinic © 2013 American Chemical Society

Figure 1. Representation of the carba-closo-dodecaborate anion 1 with electronically distinct B−H vertices indicated (position 12 most hydridic, 7−11 less hydridic, 2−6 least hydridic). When the anion 1 is paired with a transition-metal cation, such as in 2, it can coordinate via B−H “agostic-like” bonds from cluster positions 12 and 7−11 into empty d orbitals. Colored spheres represent B−H vertices.

bond lengths (1.381(5) and 1.384(5) Å) of 2 are in the typical range (1.340−1.395 Å) for Rh complexes,9 indicating that in this geometry the carbo-closo-dodecaborate anion exhibits a very weak trans influence. In solution this carborane anion is highly fluxional and rapidly changes coordination to the metal via the hydrides in Received: October 11, 2013 Published: November 22, 2013 6887

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Organometallics

Communication

positions 7−12. Here we report the isolation of a zwitterionic iridium complex supported by a phosphine bearing the CB11H11− ligand substituent. Geometric constraints force the metal center to interact with the electronically distinct hydrides in positions 2−6, which reveals the carborane anion’s ability to produce a strong trans influence. Recently we disclosed the use of perhalogenated carba-closododecaborate anions as ligand R groups.10,11 We became intrigued by how the analogous phosphine bearing the parent hydride cluster 115 would behave on coordination to transition metals. In contrast to the carba-closo-dodecaborate anion itself, a phosphine-functionalized carborane anion would preclude orbital overlap with hydrides in the lower half of the cluster and force the metal to interact with the electronically distinct positions 2−6. Further, if double agostic-like bonding occurred with a d8 metal, a trigonal-bipyramidal geometry would be enforced, and the ligand hydrides in positions 2−6 would be disposed to overlap with the metals nonbonding electrons (e.g., dxz, dyz, dx2−y2, dxy). It was postulated that these geometric constraints might lead to interesting ligand properties and unique electronic effects. To test this hypothesis, we reacted the phosphine 3, which was recently reported by Finze and co-workers,12 dissolved in C6D6 with (ClIr(COD))2 and a color change from orange to yellow was observed, suggesting formation of the new complex 4 (Figure 2). Analysis of the solution by 31P NMR shows a single new phosphorus resonance at 27.4 ppm (31P shift of ligand 3 45.4 ppm). The 1H NMR spectrum of 4 shows resonances corresponding to the bound phosphine ligand as well as a coordinated cyclooctadiene fragment. At ambient temperature the 1H NMR spectrum shows no evidence of B−H metal interactions or the formation of a metal hydride via cyclometalation. However, when the sample was cooled to −80 °C, a boron hydride peak shifted from 0.93 to −1.42 ppm in the 1H NMR spectrum, which is consistent with previously reported B−H interactions with the parent carba-closododecaborate anion,4h,r but significantly upfield in comparison to resonances reported for a phosphine complex, featuring the more strongly coordinating anionic nido-7,8-C2B9H11 substituent. This observation also suggests that the B−H bonds in positions 2−6 are dynamically interacting with the Ir center. At ambient temperature the 11B NMR spectrum of 4 shows three resonances in a 1:5:5 ratio, indicating that the carborane ligand substituent retains its local C5v symmetry and implying rapid P−Ccarb bond rotation on the NMR time scale. The symmetry of the carborane anion is lost at −80 °C, and at −90 °C six distinct 11B resonances are observed. Coalescence occurs at −75 °C, corresponding to a free energy of activation of approximately 8 kcal mol−1 for this dynamic process. Both the solution and solid-state infrared spectra of complex 4 show bands typical for carba-closo-dodecaborate B−H stretches (2460−2625 cm−1) and no distinct B−H−M stretches for the bridging interaction (2020−2040 cm−1).4r The absence of Ir−H absorbances (1900−2200 cm−1) also rules out the possibility of a rapid reversible cyclometalation process that is faster than the NMR time scale. The resistance of ligand 3 toward cyclometalation is surprising, since related neutral ocarborane phosphine substituents and open-shell boron hydride clusters have been reported to undergo facile cyclometalation13a or B−H activation13b reactions with iridium(I), at ambient temperature. A single-crystal X-ray diffraction study reveals that, in the solid state, the zwitterionic14 complex 4 adopts a distorted-

Figure 2. Synthesis of complex 4 (top) and solid-state structure of 4 (bottom). Hydrogens are omitted (except H1 and H2) for clarity.

trigonal-bipyramidal geometry, with two of the equatorial positions occupied by B−H hydridic interactions (Figure 2). These hydrides were found on the electron density map, allowing a discussion of their position. The hydrides lie much closer (B−H1, B−H2 = 1.22(4) Å) to the boron atoms than to the Ir center (Ir−H1, Ir−H2 = 1.93(4) and 1.92(4) Å, respectively), suggesting little disruption of the B−H σ bonds. The C−C double-bond length (C12−C13 = 1.362(4) Å) and the respective Ir−C bonds (Ir−C12 = 2.224 Å, Ir−C13 = 2.237 Å) of the cyclooctadiene ring that are trans to the phosphine are not atypical for olefins bound to an Ir center.9 In stark contrast, the coordinated cyclooctadiene C−C double bond and the Ir−C bonds, which are trans to the carborane cluster as a whole, are very long (C8−C9 = 1.451(4) Å) and short (Ir− C9 = 2.089 Å, Ir−C8 =2.096 Å), respectively. In fact, this C−C bond is so elongated that this ligand can be regarded as a metallocyclopropane, which allows one to alternatively describe this complex as a very distorted d6 octahedron.9 This observation suggests that the anionic carborane substituent is acting as a strong donor substituent to the metal center, which induces pronounced π back-bonding to the trans-olefin. Among the few reported examples of complexes with a double B−H interaction with a transition metal (mononuclear borohydride5d or cluster4a,h,i,m), containing a trans-olefin, little or no C−C elongation (range of C−C = 1.341−1.401 Å) is observed. 6888

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In order to gain further insight into the bonding situation in complex 4, DFT calculations were carried out at the M06L level of theory.15 The calculated structure for complex 4 is in very good agreement with the X-ray crystallographic data (see the Supporting Information). Inspection of the relevant molecular orbitals (MOs) of 4 (Figure 3, top) reveals that MO-69

The results described above indicate that, when the parent carba-closo-dodecaborate anion is used as a ligand substituent, its ability to interact with transition metals is significantly altered. Tethering the typically weakly coordinating anion to a ligand forces the boron hydrides in positions 2−6 to engage in bonding interactions with the metal center, which results in a significant trans influence. The fact that the interaction with the metal center is dynamic in solution suggests that, as a ligand R group, the parent carba-closo-ligand substituent retains its weak coordinative ability, implying that complexes such as 4 may behave as highly reactive low-coordinate species. We are currently investigating the catalytic properties of the zwitterionic iridium complex 4 and also the preparation of analogues with differentially substituted carborane anions.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Text, tables, figures, and a CIF file giving phosphine 3 and complex 4 synthesis and spectroscopic data, X-ray structure data, and DFT calculation data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail for L.M.: [email protected]. *E-mail for V.L.: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 3. Calculated molecular orbitals of complex 4 that show double agostic interactions as well as a bonding interaction between the skeletal electrons of the carborane cluster and the iridium center (top) and selected Mayer bond orders (bottom).

REFERENCES

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corresponds to double agostic-like interactions between two B−H bonds (pz(B) + s(H)) and the dxz orbital of the metal. A complementary σ-bonding interaction is also apparent between the cluster skeletal electrons and the dz2 orbital of the iridium center (Figure 3, MO-90). In order to evaluate the amount of disruption the B−H bonds experience via interaction with the Ir center, natural bond orbital15 analysis was carried out. The natural electron population of the two B−H bonds, interacting with the iridium, is around 1.785e and indicates that electron density has been transferred to the metal (unperturbed B−H carborane bonds 1.972e). Interestingly, the electron density of the agostic B−H bonds is somewhat inversely polarized toward the boron atoms (53%), which is counterintuitive with respect to simple electronegativity arguments and is attributed to the electronwithdrawing nature of the carborane anion’s skeletal core. The highly covalent nature of these B−H bonds suggests a large participation of the radial p-type orbitals of the boron cluster atoms in the agostic bonding process. Indeed, the Mayer bond orders15 (MBO’s) for B−H (0.71, 0.70), Ir−H (0.21, 0.20), and Ir−B (0.25, 0.25) indicate that there is a nearly equal participation of the boron and hydrogen atoms in the B−H− Ir bridges and that the B−H single bonds are only slightly disrupted. The MBO analysis also confirms the full reduction of the double bond (C8−C9) into a single bond (MBO = 1.06), which suggests that Ir is in its +3 oxidation state. This bonding situation has been further clarified by second-order perturbation energy interaction and atoms in molecules (AIM) analyses.15 6889

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