Journal of Organometallic Chemistry 897 (2019) 57e63 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem Heavy alkaline earth p-complexes with doubly-reduced polycyclic aromatic hydrocarbons of variable sizes * Andrew Uresk a, Natalie J. O'Neil a, b, Zheng Zhou a, Zheng Wei a, Marina A. Petrukhina a, a Department of Chemistry, University at Albany, State University of New York, Albany, NY, 12222, United States b Department of Chemistry and Biochemistry, Utica College, Utica, NY, 13502, United States article info abstract Article history: Barium (II) p-complexes with two planar polycyclic aromatic hydrocarbons (PAHs), anthracene and Received 12 February 2019 fluoranthene, have been prepared by direct alkaline earth metal reduction reactions in THF in the Received in revised form presence of 1,2-diiodoethane (DIE), used as an activator. The products have been crystallographically 11 June 2019 2þ 2e characterized to reveal their mixed-ligand composition, {[Ba(I)(THF)3]2 [C14H10] }(1) and Accepted 12 June 2019 þ e þ {[Ba(I)(THF) ]2 [C H ]2 }(2). A similar strontium (II) product, {[Sr(I)(THF) ]2 [C H ]2-}(3), has been Available online 21 June 2019 3 2 16 10 4 2 10 8 synthesized using potassium-naphthalenide ligand-exchange reaction starting with SrI2.In1 and 2, Ba(II) ions are p-complexed by doubly-reduced PAHs with the planarity of the dianions being preserved Keywords: p-complexation upon reduction and metal binding. In contrast, the naphthalene dianion in 3 adopts a non-planar ge- e Heavy alkaline earth metals ometry with a dihedral angle of 15.2 . The M Ccentroid distances of 2.754 (6) Å and 2.755 (14) Å in 1 and Chemical reduction 2, respectively, are longer than that in 3 (2.600 (2) Å). The coordination environment of alkaline earth Mixed ligand complexes metal ions is completed by iodide ligands and coordinated THF molecules. The X-ray structural inves- Polycyclic aromatic hydrocarbons tigation confirmed the formation of the iodide-bridged 1D polymers in barium products, {[Ba (m-I) 2þ 2e 2þ 2e X-ray diffraction (THF)3]2 [C14H10] }∞ (1) and {[Ba (m-I) (THF)3]2 [C16H10] }∞ (2) vs. a discrete strontium complex with terminal iodide ligands in 3. © 2019 Elsevier B.V. All rights reserved. 1. Introduction The superconductivity of fullerenes doped with Group 2 metals has also attracted attention [13]. Moreover, the organometallic com- Organometallic chemistry of Group 2 metals has been expand- pounds of alkaline earth metals have found applications in catal- ing over the years [1,2]. However, the developments of late Group 2 ysis, stimulated by their high abundance in the earth's crust [14]. metals are still lagging behind the progress achieved with early The progress of late Group 2 metal chemistry is plagued with alkaline earth metals. For two heavy congeners of Group 2, stron- complications stemming from their low reactivity and high insol- tium and barium, the organometallic chemistry is largely centered ubility. In contrast to alkali metals, [15] the direct use of alkaline around cyclopentadiene (CP) and cyclooctatetraene (COT) ligands earth metals as reducing agents requires their prior activation. [3,4] with a very limited number of other p-systems engaged in Several methods have been reported in the literature, varying from complexation. Although the studies on reduction of planar poly- physical surface activation to chemical activation techniques. The cyclic aromatic hydrocarbons (PAHs) with the alkaline earth metals methods utilizing metal vapours [16] and activated metal powders go back to 1905 [5], only a handful of strontium and barium com- [17], liquid ammonia [18], or the entrainment approach [19] all rely plexes with selected PAHs (Scheme 1) such as fluorene [6] and its on activation of the elemental form of metals. The entrainment hypersilyl-fluorene (hSi-Flu) derivative [7], indene and diisopropyl method was successfully applied for the activation of magnesium indene [8], as well as dibenzopentalene [9] have been crystallo- metal, as first shown by Grignard in 1934 [19a]. Advances to this graphically characterized to date. Besides fundamental interest in method developed by Beckler and co-workers in 1959 have this chemistry, the resulting products are used in synthetic organic increased the yield and made it a viable activation strategy to access [10,11] and polymer chemistry [12], and materials synthesis [3c]. Group 2 metal compounds [19b]. This method still serves as an effective activation approach; however, it is mainly limited to magnesium in the literature [20e22]. We have recently shown that * Corresponding author. this method could also be successful in the activation of calcium E-mail address: [email protected] (M.A. Petrukhina). https://doi.org/10.1016/j.jorganchem.2019.06.011 0022-328X/© 2019 Elsevier B.V. All rights reserved. 58 A. Uresk et al. / Journal of Organometallic Chemistry 897 (2019) 57e63 Scheme 1. Models of structurally characterized p-complexes of strontium (II) and barium (II) with planar PAH anions. metal [23], but no expansion to include heavier Group 2 congeners solution (characteristic of reduction) became more intense in the has been reported. reactions with 10% of DIE, thus indicating an increase in the reac- Additional activation methods such as halide metathesis [24] tion rate. It was also expected that the amount of active metal that and transamination [25] utilize synthetic techniques that do not can react with the ligand should increase with an increase in the rely on the elemental metals. These chemical methods allow for amount of DIE in the reaction. more precise reaction reproducibility and generally produce In the course of this study, two new barium products have been products in good yield, although the by-products formed during isolated in single-crystalline form and both were crystallographi- the reactions can be difficult to remove. Therefore, further devel- cally characterized. The barium (II) anthracene complex, 1,was opment of effective activation methods is still needed in order to synthesized using anthracene, barium metal, and DIE in THF at reinvigorate and expand the chemistry of the alkaline earth metals. room temperature (see Experimental part for details). Upon layer- In this work, we set to investigate the reduction behavior of ing the THF reaction solution with hexanes at 10 C, brown block- heavy Group 2 metals using planar polycyclic aromatic hydrocar- shaped crystals of good quality were obtained in moderate yield. bons as p-ligands. We selected the chemical activation method The X-ray crystallographic study revealed that the product has the 2þ 2e known to be successful for early Group 2 metals [22,23] and {[Ba(I)(THF)3]2 [C14H10] } composition. The UVevis spectrum of 1 compared it with the halide metathesis reaction. We report two dissolved in THF shows absorbance in the ultraviolet region be- 2e 2e new barium (II) complexes with [C14H10] and [C16H10] anions as tween 325 and 386 nm along with peaks centered at 452 nm and 2- well as the strontium (II) complex with [C10H8] and provide their 607 nm, which are characteristic of the dianion of anthracene structural characterization by single-crystal X-ray diffraction. (Fig. S1). The UVevis spectrum of 1 is in good agreement with that reported for the anthracene dianion with sodium counterions, which exhibits a maximum absorbance at 600 nm [26]. 2. Results and discussion In the crystal structure of 1, an anthracene dianion has two barium ions bound symmetrically in an h6-fashion to opposing To access new p-complexes of heavy Group 2 metals we have peripheral six-membered rings (Fig. 1). The BaeC distances range chosen an alkyl halide for activation. We opted to use 1,2- from 3.041 (6) Å to 3.174 (6) Å with an average distance of 3.096 (6) diiodoethane (DIE) for the following reasons. DIE is a solid at Å and a BaeCcentroid distance of 2.754 (6) Å. These distances are room temperature and this makes it easy to measure for quanti- similar to those found in analogous barium (II) p-complexes [6,8,9]. tative use in the subsequent reactions. The byproducts formed, Each barium has three THF molecules coordinated with the ethane gas and the metal iodide, are innocuous and can be easily BaeOTHF bond length distances ranging from 2.719 (4) Å to 2.777 removed from the reaction mixture. The additional benefit to this (4) Å (BaeOavg, 2.742 (4) Å). synthetic method is that the solid DIE can be directly added to the In the solid-state, 1 crystallizes as an 1D coordination polymer, reaction mixture in a one-pot synthesis. Overall, the use of one-pot 2þ 2e {[Ba(I) (THF)3]2 [C14H10] }∞, propagating through the formation synthetic techniques with a minimized number of steps is favored of iodide bridges between each pair of barium (II) ions (Fig. 1). Each in preparation of very air-sensitive and reactive products. barium (II) ion is bound to two iodide ligands with the BaeI bond With the goal of developing effective activation techniques length distances of 3.4820 (6) Å and 3.5444 (6) Å. The BaeIeBa and suitable for barium metal, we selected two planar PAHs, anthracene IeBaeI bond angles are 97.474 (11) and 82.525 (11) , respectively. (C14H10) and fluoranthene (C16H10), for chemical reduction re- 2e In the [C14H10] dianion, the CeC bond length distances range actions. The optimal conditions for these reactions rely on the from 1.374 (9) Å to 1.453 (8) Å and are within ±0.055 Å from those excess of metal (ca. 5e7 eq.) in respect to the amount of a PAH found in the neutral ligand (Table 1)[27]. Notably, the deviation of ligand.