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Donor–Acceptor Strategies for Stabilizing Planar Diplumbenes

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Donor–Acceptor Strategies for Stabilizing Planar Diplumbenes Article Cite This: Organometallics 2017, 36, 4825−4833 Donor−Acceptor Strategies for Stabilizing Planar Diplumbenes Priyakumari Chakkingal Parambil and Roald Hoffmann* Dept. of Chemistry and Chemical Biology, Cornell University, 162 Sciences Drive, Ithaca, New York 14853, United States *S Supporting Information ABSTRACT: Diplumbenes, R2PbPbR2, are more pyramidal- ized and trans-bent than the other group 14 analogues and feature long Pb−Pb bonds. A strategy for making these molecules locally planar at Pb, with shorter Pb−Pb separations, starts out from a realization that the Pb−Pb double bond in a hypothetical (LB)PbPb(LB) (LB = Lewis base) is made almost entirely up by 6p orbitals on the Pb atom and has lone pairs on Pb with large 6s character. Coordination of these lone pairs with Lewis acids (LA) should give an ethylene-like structureeffectively a push−pull complex, (LA)(LB)PbPb(LB)(LA). However, the relativistically contracted 6s lone pair on Pb resists effective orbital interaction. To overcome this problem, we evoked in calculations electrostatic interactions between the Pb and Lewis acid ligands, by making the Pb atoms negatively charged and choosing a Lewis acid that carries positive charge. The challenge is to find realistic ligands to engineer these electron shifts. Calculations gave local minima for a diplumbene (LA)(LB)PbPb(LB)(LA), with a base-stabilized borylene as the LB, and LiMe as the LA. These are predicted to be planar and have shorter Pb Pb double bonds. For the constituent plumbylene (PbR2) fragments of these candidates, one calculates a triplet ground state. ■ INTRODUCTION The disilene, germene, stannene, plumbene terminology also We seek ways to stabilize a planar structure for substituted evokes one attractive approach to their electronic structure they may be viewed as donor−acceptor dimer complexes of diplumbene, Pb2R4, the Pb analogue of ethylene, by modifying 9 the substituents, R. It is not an easy task, as we will see. Unlike carbene analogues. The most reactive of the ethylene analogues the diplumbenes, R2PbPbR2 in fact dissociate in ethylene (1, Scheme 1), for heavier group 14 analogues, the 10−14 solution to plumbylene monomers, R2Pb. All of the Scheme 1. Equilibrium Structures for Ethylene and Its diplumbenes whose crystal structures are available are stabilized Heavier Analogues by very bulky substituents, and exhibit a trans-bent geometry, unlike the simplest Pb2H4, which is known only in gas phase and is suggested to have a trans-bridged structure (3)by calculations.5 The Pb−Pb distance in the known trans-bent diplumbenes (2.90−3.16 Å), with a formal double bond, is actually longer than the Pb−Pb single bond in diplumbanes, − 13,15 R3PbPbR3 (2.84 2.97 Å). Diplumbenes with even longer bond lengths are known; they are perhaps better considered as plumbylene dimers.13 The reversal of the bond length preferences of formally single- and double-bonded Pb−Pb compounds (relative to the carbon archetype) caught our 1 attention. As a reviewer suggested, perhaps this is not planar D2h structure is no longer the equilibrium one. − Computations showed that, for Si H and Ge H ,atrans-bent surprising, as two donor acceptor bonds of the type thought 2 4 2 4 to be at work in the trans-bent diplumbene may combine to structure (2) is most stable, whereas, for Sn2H4 and Pb2H4, still − − another alternative, a trans-bridged geometry, schematically produce a net Pb Pb separation that is longer than a Pb Pb 1 − − single bond. We started looking for strategies to planarize the drawn in 3, takes over. The E H E 3-center bond in 3 can be − viewed as the result of a donor−acceptor interaction between diplumbene, and in the process, perhaps shorten the Pb Pb − double bond relative to Pb−Pb single bond. the E H bond of one EH2 fragment with the vacant p orbital of the other, similar to that in the heavier analogues of alkynes.2 Previous computational studies have shown that, from Si to Pb, the planar D2h structure of the ethylene analogue is a The parent molecules (E2H4, E = Si, Ge, Sn, Pb) are known only in matrix isolation and gas-phase studies, their structures transition state, and its instability relative to the trans-bent − determined by combined theory and experiment.3 7 Due to the high reactivity of these ethylene analogues, their isolation as Received: September 28, 2017 crystals is possible only with bulky substituents.8 Published: December 6, 2017 © 2017 American Chemical Society 4825 DOI: 10.1021/acs.organomet.7b00733 Organometallics 2017, 36, 4825−4833 Organometallics Article structure increases down the group.1 While the out-of-plane Scheme 2. Frontier Orbitals of Plumbylene distortion is slight for disilene, it becomes more and more prominent down the group.8 Thus, trans-bent equilibrium geometries of E2H4 are calculated to be more stable than the planar structure by 0, 2, 9, and 27 kcal/mol, respectively, for E = Si, Ge, Sn, and Pb. The geometry follows this energetic trendas one goes down the group, the pyramidality (measured here by the deviation of the sum of angles around Efrom360°)increases(Table 1). The energies and Trans Table 1. Relative Energy (RE) of the -Bent E2H4 with Respect to the Planar One, for E = Si to Pb. The pyramidalization (360 − θ) in the trans-bent structure is also a shown. Scheme 3. Diplumbenes, with BH2 Substituents RE (trans-bent) (kcal/mol) 360 − θ (deg) Si2H4 05 − Ge2H4 217 − Sn2H4 928 − Pb2H4 27 36 aθ is the sum of H−E−H and E−E−H angles around an E atom in the trans-bent structurethe larger the value of (360 − θ), the greater is the trans bending and the pyramidality at E. geometrical parameters here come from PBE0/Def2-TZVPPD level calculations; details are given in the Computational Methods section at the end of the paper. Thus, planarizing diplumbene will be more difficult compared to the Si, Ge, and Sn analogues. At the same time, any strategy that can planarize a diplumbene is also likely to work for Si, Ge, and Sn analogues, whereas the reverse may not be true. This is the reason why we specifically chose diplumbene, the most difficult case, so to speak, for our study. The reasons for trans-bending in group 14 ethylene Figure 1. Optimized geometry of the BH2-substituted diplumbene, 6, analogues are well-studied and we do not discuss them π − and its MO. here.9,16 18 It was shown computationally that trans-bending could be decreased by the use of electropositive substituents, localized over the two Pb atoms. The Wiberg bond index for 19,20 such as silyl. A planar structure for digermene can be the Pb−Pb bond in 7 is 0.92, and does not give an indication of obtained by using two Li atoms and two alkyl groups as a double bond. Other typical π accepting substituents, such as 21 substituents, in a trans fashion. We found that the known NO2, CN, etc., could not stabilize a planar diplumbene. Thus, ways to planarize the disilene and digermene do not work with we decided to look for a way other than π delocalization and the Sn and Pb analogues, and thus looked for an alternative began by analyzing the nature of bonding in ethylene strategy. analogues, R2EER2. R EER Bonding. Shown in Figure 2 is a plot of the most ■ RESULTS AND DISCUSSION 2 2 probable radius (rmax) of the valence ns and np orbitals of group Tuning the Singlet−Triplet Splitting. Building on 14 elements, taken from ref 23 and the Desclaux − previous work,1,9 one way to achieve our goal might be to computations.22 25 While the valence 2s and 2p of C have make the constituent plumbylene, PbR2, a triplet ground state. similar rmax, the radii of the corresponding ns and np orbitals in fi ff The relevant orbitals of the archetype, PbH2, 4, are shown the other elements are signi cantly di erent. The ns orbitals are schematically and in an orbital plot in Scheme 2. The splitting more contracted than np orbitals for heavier elements, and the of the orbitals is very large, 3.9 eV. To stabilize the triplet, we degree of contraction increases down the group. This implies need to depress in energy the vacant p orbital significantly, that the ns orbitals of heavier atoms might behave like core which can be done with a BH2 substituent. This led us to orbitals, and hence have poor overlap with orbitals of other dimers 5 and 6 (Scheme 3). atoms. The overlap of s orbitals requires the heavier atoms to Calculations did not give a planar minimum for 5, whereas, come very close to each other, but that in turn might cause for 6, geometry optimization led to planar 7 (Figure 1) with a large near core−valence shell repulsion (repulsion between relatively long Pb−Pb bond (2.89 Å). The unlabeled circles valence np electrons of one atom with core (n − 1)p of the here and below are hydrogen atoms. Note the delocalization of other).26,18 One anticipates that heavier atoms might prefer to π the system extending to only two BH2 groups, and the exclude the s orbital from bonding, or at least reduce its unusual geometry at the right-hand Pb (Figure 1). We do not participation. see this interesting molecule (see further discussion in the SI;it Table 2 lists the hybridization of the E atom in planar, trans- fi can be viewed as a plumbylene adduct of an electron-de cient bent, and trans-bridged structures of E2H4, obtained from a allene2+) as a diplumbene, as the π system is a 4c−2e one, not Natural Bond Orbital (NBO) analysis.27 Notice that the NBO 4826 DOI: 10.1021/acs.organomet.7b00733 Organometallics 2017, 36, 4825−4833 Organometallics Article Scheme 4.
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