Ring-whizzing in polyene-PtL2 complexes revisited

Ring-whizzing in polyene-PtL2 complexes revisited Oluwakemi A. Oloba-Whenu1, Thomas A. Albright*2 and Chirine Soubra-Ghaoui3 Full Research Paper Address: 1Department of Chemistry, University of Lagos, Akoka, Yaba, Lagos, Nigeria, 2Department of Chemistry, University of Houston, Houston, Texas 77204-5003, USA and 3Department of Chemistry and Physics, University of St. Thomas, Houston, Texas 77006, USA Email: Oluwakemi A. Oloba-Whenu - ; Thomas A. Albright* - ; Chirine Soubra-Ghaoui - Open Access Beilstein J. Org. Chem. 2016, 12, 1410–1420. doi:10.3762/bjoc.12.135 Received: 12 April 2016 Accepted: 23 June 2016 Published: 07 July 2016 This article is part of the Thematic Series "Organometallic chemistry". In memorium: Peter Hofmann, a friend and colleague. Guest Editor: B. F. Straub * Corresponding author Keywords: d10 metal complexes; density functional theory (DFT); hapototropic rearrangements; HOMO–LUMO interactions; polyene-ML2 complexes; ring-whizzing © 2016 Oloba-Whenu et al.; licensee Beilstein-Institut. License and terms: see end of document. Abstract Ring-whizzing was investigated by hybrid DFT methods in a number of polyene–Pt(diphosphinylethane) complexes. The polyenes included cyclopropenium+, cyclobutadiene, cyclopentadienyl+, hexafluorobenzene, cycloheptatrienyl+, cyclooctatetraene, octafluorooctatetraene, 6-radialene, pentalene, phenalenium+, naphthalene and octafluoronaphthalene. The HOMO of a d10 ML2 group (with b2 symmetry) interacting with the LUMO of the polyene was used as a model to explain the occurrence of minima and maxima on the potential energy surface. Introduction Polyene–transition metal complexes were found to undergo fluxional rearrangements as early as 1956 with the preparation of Cp 2 Fe(CO) 2 [1]. The migration of an ML n unit around the periphery of a cyclic polyene is commonly called ringwhizzing, purportedly ascribed to Rowland Pettit [2]. A more inclusive term is haptotropic rearrangement [3] wherein a metal atom changes its hapticity along the reaction path. Haptotropic rearrangements in ML3 and MCp complexes are numerous [4-9] and have found use in synthetic strategies [10], switching devices [11-13] and energy storage [14,15]. Much less is known about the polyene–ML2 analogs. There are two classes of com- pounds; one set consists of d8 ML2 compounds [16-19] and the other, which we will be concerned with, are the d10 ML2 class. There is ample precedent for four basic coordination geometries exhibited by these compounds. These are shown in Figure 1. Notice that in each case the orientation of the ML2 unit is tied to the coordination number of the polyene and total electron count. One of us undertook a theoretical survey of these compounds at the extended Hückel level a number of years ago [20,21]. In the present contribution we shall revisit some of these rearrangements using DFT theory, as well as, investigate some new compounds. 1410 Beilstein J. Org. Chem. 2016, 12, 1410–1420. Figure 1: The four coordination geometries for d10 polyene-ML2 complexes along with their hapto numbers and electron count. A d10 ML2 fragment possesses a high-lying HOMO, shown by 5 in Figure 2, which has b2 symmetry and a low-lying LUMO, 6, of a1 symmetry [22]. An energetically favorable reaction path will be one that maximizes the interactions of these orbitals with the orbitals of a coordinated polyene. The lowest occupied polyene π level is fully symmetric and, therefore, 6 can always interact with it. On the other hand, the LUMO in the π system may not always have the correct symmetry to interact with the b2 orbital on ML2 and it is the evolution of this overlap that has an important impact on the reaction path and activation energy. We will also have an occasion to consider a lower lying filled orbital of b1 symmetry, 7. larger than that for benzene. The M and L that we shall use in this work is Pt and a phosphine. The second method employs the use of a bidentate phosphine. In this regard we have chosen diphosphinylethane (dpe). This idea here is that the P–Pt–P angle is around 100° in polyene–ML2 complexes. Upon dissociation the 14 electron PtL2 complex strongly prefers to be linear [22]. So the computed ground state for Pt(PH3)2, shown in 8, is calculated to be 29 kcal/mol more stable than one where the P–Pt–P bond angle was constrained to be 99°. This of course is not the case for Pt(dpe), 9. The P–Pt–P angle remains at 98°. Thus, the bond dissociation energy in polyene–Pt(dpe) complexes rises along with the attendant barriers for haptotropic rearrangements. This has been analyzed and quantified in detail by Massera and Frenking [23] for olefin–ML2 compounds. Computational Details All geometries for the L = PH3 complexes were optimized without symmetry constraints within the DFT framework first using the B3LYP functional [25-27] in combination with the LANLDZ2 [28] basis sets. Single point calculations were carried out using the triple zeta d plus f polarization functions on Pt [29]. The geometry optimizations were then repeated using the M06 functional [30] along with the Def2-SV(P) basis set [31] for Pt, C, H and P except that the d functions on C were left off. Single point calculations used the Def2-TZVP basis [31] on Pt, P, C and H except for removing the f functions on C. F used a 6-31G basis [32] for the geometrical optimizations and 6-311G [33] in the single point calculations. Analytical frequencies were computed to determinate the nature of the stationary points. The Gaussian 09 software suite [34] was used in all of the calculations. The plots of the molecular structures utilized CYLview [35]. For brevity we will report the structures and Gibbs free energy differences in the standard state only for the polyene–Pt(dpe) complexes using frequencies from the Def2SV(P) optimizations for the corrections to the Def2-TZVP energies. The geometries and total electronic energies are given as Supporting Information File 1. Results and Discussion A. Cyclic polyene–Pt(dpe) examples Figure 2: The important valence orbitals of a d10 ML2 group, 5–7, along with the computed structures of Pt(PH3)2 and Pt(dpe). Polyene–ML 2 complexes are very fragile which in turn makes it somewhat difficult to compute the reaction path. The bond dissociation energy for ethylene–Pt(PH3)2 is only about 17 kcal/mol [23]. There are two ways in which the metal–polyene bond can be strengthened. The electron affinity for C 6 F 6 is much larger than that for benzene [24]. Consequently interaction of the filled b2 fragment orbital with the LUMO of C6F6 is expected to be larger and the binding energy The most simple of the cyclic polyenes is the cyclopropenium cation. Its LUMO is a degenerate par of π orbitals, labeled e”A and e”S in Figure 3. It is easy to see that e”A interacts with the b2 orbital of ML2 at an η2 geometry. Indeed this is the computed group state for C3H3–Pt(dpe)+ as shown from a side view, 10, in Figure 3. The transition state for shifting Pt(d (...truncated)