Computational study of fluxional hydride bridged binuclear transition metal complexes: Effect of secondary bridging ligands Brandon J. Burkhart, Roger L. DeKock ⇑ Department of Chemistry and Biochemistry, Calvin College, 1726 Knollcrest Circle SE, Grand Rapids, MI 49546, USA article info Article history: Received 15 March 2012 Received in revised form 23 May 2012 Accepted 24 May 2012 Available online 2 June 2012 Keywords: Bridging ligand Binuclear complexes Fluxional hydride abstract The binuclear complex [Ir 2 (CH 3 )(CO) 2 (dppm) 2 ] + (dppm = Ph 2 PCH 2 PPh 2 ) coordinates the olefins of 1,3- butadiene and catalyzes double geminal C–H activation via a proposed fluxional hydride migration. Using DFT computational studies, we examine the fluxional behavior of three model hydride bridged bimetallic systems to elucidate the main factors in this transformation. Our results indicate that the bridging ligand opposite the l-H controls the barrier to hydride fluxionality and that breaking this bridging interaction is the largest component of the transition state barrier. We found a low barrier, a medium barrier, and a high barrier in systems with no bridging ligand, a partial l-CO, and l-CH 2 , respectively. The respective systems with their corresponding barriers are: (1) [RhRe(l-H)(CO) 4 (dhpm) 2 ] + , 2.4 kcal/mol, (2) [IrRu(l- H)(l-CO)(CO) 3 (dhpm) 2 ] 2+ , 8.5 kcal/mol, and (3) [RhOs(l-H)(l-CH 2 )(CO) 3 PH 3 (dhpm) 2 ] 2+ , 26.4 kcal/mol (dhpm = PH 2 CH 2 PH 2 ). The predicted fluxional hydride migration in the activation of 1,3-butadiene occurs easily and is consistent with these findings. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Binuclear metal systems offer new transformative powers through the cooperative effects of adjacent metal centers [1]. The Ir–Ir complex [Ir 2 (CH 3 )(CO) 2 (dppm) 2 ] + (dppm = Ph 2 PCH 2 PPh 2 )(1) demonstrates one unique reaction of bimetallic complexes, cata- lyzing the double geminal C–H activation of 1,3-butadiene [2]. The proposed mechanism has been supported by experimental evi- dence for complexes 1, 2, and 3 with complexes A and B as pre- sumed intermediates (Scheme 1, note the phenyls of the dppm ligands are omitted). In our computational studies of this double activation we were surprised by the ease of transition between complexes A and B. The bridging hydride readily migrates back and forth between the metal centers to form a bridging interaction on either side of the complex. Many different binuclear systems [2–11] display similar (flux- ional) hydride migrations in which the l-H flip-flops between in- verted bridging hydride positions on opposite sides of the complex (Eq. (1)). This behavior was first studied in A-frame com- plexes [3] and occurs at room temperature so the linear MHM transition state must occur without an appreciable energy barrier. While bridging hydride ligands have been extensively studied as three-center two-electron interactions (3c–2e) [12–18], their fluxi- onality has not been described in detail. The review article by Parkin [14] details the difficulty that arises in assessing the nature of the metal–metal bonding in these three-center interactions. In our depiction, Eq. (1), the solid lines are not meant to depict the number of electrons involved in any pairwise interaction. Rather, we are simply depicting the geometry changes in the fluxional rearrangement. ð1Þ Most known examples of fluxionality occur in symmetric sys- tems in which the hydride migration is readily reversible and the two structures that are in flux with each other are essentially iden- tical; this does not demonstrate immediate usefulness to inorganic chemists. However, in the Ir–Ir system, the hydride migration be- tween the metals leads to the final product 3 [2]. Our computa- tional work investigates fluxional hydride migrations between the metals of bimetallic complexes to better understand transi- tions such as those between complexes A and B. 2. Systems studied We studied hydride migrations in three model systems within the context of the 1,3-butadiene activation mechanism. First, we examined a Rh–Re system [RhRe(l-H)(CO) 4 (dhpm) 2 ] + as a repre- sentative example of a fluxional hydride migration (Scheme 2) [7]. Second, we created, for comparison, the model analogue [Ir- Ru(l-H)(l-CO)(CO) 3 (dhpm) 2 ] 2+ by replacing Rh and Re with Ir 2210-271X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2012.05.024 ⇑ Corresponding author. Tel.: +1 616 526 6344; fax: +1 616 526 6501. E-mail address: dekock@calvin.edu (R.L. DeKock). Computational and Theoretical Chemistry 994 (2012) 1–5 Contents lists available at SciVerse ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc