Emma Seppälä

Bio: Emma Seppälä is an academic researcher from Braunschweig University of Technology. The author has contributed to research in topics: Covalent bond & Polyiodide. The author has an hindex of 8, co-authored 10 publications receiving 222 citations.

The Bromination of Bulky Trialkylphosphane Selenides R2R′PSe (R, R′ = iPr or tBu) Studied by Physical and Computational Methods

Abstract: Bulky trialkylphosphane selenides tBu3PSe (1a), iPr3PSe (1b), tBu2(iPr)PSe (1c) and tBu(iPr)2PSe (1d) react with one equiv. of bromine providing “T-shaped” products R2R′P–SeBr2 (2a–d), which contain three-coordinate selenium atoms (10-Se-3). The solid compounds 2b (bimorphic), 2c and 2d exhibit different extents of distortions of the PSeBr2 moieties and different patterns of intermolecular soft–soft interactions. In mixtures containing 1 and 2, exemplified by the “NMR-titration” of 1c with molecular bromine, averaged 31P NMR singlets and their 77Se satellites indicate rapid intermolecular bromine exchange reactions (kinetic lability of the Se–Br bonds). Calculations modelling such bromine transfer support nucleophilic attack of R3PSe (Se Br) on an electrophilic Br atom of R3PSeBr2. Among the phosphane selenides 1a–d, tBu(iPr)2PSe (1d) gives the largest 77Se NMR upfield shift and tBu2(iPr)PSe (1c) the lowest, that is, 77Se NMR shifts do not correlate with increasing numbers of tert-butyl groups. GIAO-HF/962+(d) calculations on the 77Se NMR shifts of compounds 1 allow correlation of surprising relative deshielding of 1c, compared with 1b and 1d, with its particular population of rotamers (excluding a rotamer with anti arrangement of the SePCH moiety in 1c). Bromine addition to compounds 1 leads to line broadening and extreme deshielding in the 77Se NMR spectroscopy. Reaction of 2b with bromine leads – inter alia – to P–Se cleavage with P-bromination. The structures of 1b, 2b–d and tBu2(iPr)PBr2 (3c) were determined by X-ray crystallography. In compounds 2b–d, intramolecular C–H···Br interactions determine the conformation to a large extent. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Soft–Soft Interactions Involving Iodoselenophosphonium Cations: Supramolecular Structures of Iodine Adducts of Bulky Trialkylphosphane Selenides

Abstract: The reactions of trialkylphosphane selenides tBu n iPr 3-n PSe (n = 3: la; n = 2: 1b; n = 1: 1c; n = 0: 1d) with iodine are studied with the help of heteronuclear NMR spectroscopy, vibrational spectroscopy, and X-ray crystal structure determinations. The reaction of la with one equivalent of iodine provides, after crystallization from dichloromethane/pentane, solid 2a, which consists of pairs of molecular adducts tBu 3 PSe-I-I, together with chains of alternating [(tBu 3 PSe) 2 I] + and I 3 - ions. The addition of iodine to 1b, 1c, and 1d in a 1:1 molar ratio furnishes ionic solids with the formulation [(tBu n -iPr 3-n PSe) 2 I] + [I 3 ] - (n = 2: 2b; n = 1: 2c; n = 0: 2d). Compounds 2a-2d exhibit supramolecular structures based on various kinds of weak Se···I and Se···Se interactions. In 2a, the uncharged molecules form dimers through Se···Se contacts, while the anions and cations assemble to form chains through linear P-Se···I anion contacts. The ionic compounds 2b and 2d consist of the same type of chains, although they are not isotypic to each other. The two independent formula units of 2c are topologically different; one forms cation-anion chains analogous to those of 2b and 2d, whereas the other forms cation chains through Se···Se contacts. Se···I contacts between the latter chains and triiodide anions are very long but seem to be structurally significant; for such contacts, at well above the sum of the van der Waals radii, we propose the term tertiary contacts. On using more than one equivalent of I 2 , compounds corresponding to the formulation tBu n -iPr 3-n PSeI x (x = 3, n = 3: 3a; x = 4, n = 1: 4c; x = 7, n = 2 and 0: 5b and 5d) were isolated as single crystals. Ionic 3a contains pairs of cations [(tBu 3 PSe) 2 I] + , connected by Se···Se contacts, located between corrugated layers of polymeric I 5 -anions. Compound 4c consists of two independent formula units tBuiPr 2 PSeI 2 ·I 2 , which could, however, be regarded as tBuiPr 2 PSeI + ·I-·I 2 because of the long I-I distance adjacent to Se. To a fair approximation, the packing of the two units is independent; unit 1 forms dimers (···Se-I-I···I-I···) 2 , whereas the same motif in unit 2 forms chains. The structural subunits are linked through further contacts involving terminal iodine atoms from tBuiPr 2 PSeI···I units, which thereby form μ 3 -bridging units, and by additional I-I···Se contacts. In 5b, iodide-bridged cations [tBu 2 iPrPSeI···I···ISePiPrtBu 2 ] + are anchored to a polyiodide network of formal composition I 11 -[= (I - )(I 2 ) 5 ] through I···I contacts. Except for one I···I contact, the polyiodide is two-dimensional, although highly puckered. In 5d, [iPr 3 PSeI] + cations and I 2 molecules exhibit weak I...I interactions with I- units from puckered square-net-like polyiodide layers.

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

Abstract: This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

Stable Heavier Carbene Analogues

Abstract: In recent decades, it has generally been recognized that carbenes play an important role as transient intermediates. As a result of a number of stable carbenes having been isolated and investigated in detail, it is not an exaggeration to say that the chemistry of carbenes has been thoroughly investigated and is now well-understood.1 In addition, much attention has also been paid to the heavier analogues of carbenes, i.e., silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:). These so-called metallylenes are monomeric species of the polymetallanes. This is especially true of the silylenes, which are believed to be monomers of polysilane. The metallylenes could be expected to be of great importance in fundamental and applied chemistry as a result of their many differences and similarities to carbenes. The valency of the central atom of the heavier carbene analogues (R2M:, M ) Si, Ge, Sn, Pb) is two. That is, its oxidation state is MII and its stability increases as the principal quantum number (n) increases. In fact, dichloroplumbylene and dichlorostannylene, PbCl2 and SnCl2, respectively, are very stable ionic compounds. However, these dihalides exist as polymers or ion pairs both in solution and in the solid state. The dichlorogermylene complex GeCl2 · (dioxane)3 is also known to be stable and isolable, whereas the dihalosilylenes are barely isolable compounds.2 The early silylene research was concerned largely with comparing the chemistry of the dihalosilylenes with that of carbenes. Hence, the chemistry of the metallylenes has been considered mainly from the molecular chemistry point of view.4 In contrast to the carbon atom, the heavier group 14 atoms have a low ability to form hybrid orbitals. They therefore prefer the (ns)2(np)2 valence electron configurations in their divalent species.5 Since two electrons remain as a singlet pair in the ns orbital, the ground state of H2M: (M ) Si, Ge, Sn, Pb) is a singlet, unlike the case of H2C:, where the ground state is a triplet (Figure 1).1a On the basis of theoretical calculations, the singlet-triplet energy differences ∆EST for H2M, [∆EST ) E(triplet) E(singlet)], are found to be 16.7 (M ) Si), 21.8 (M ) Ge), 24.8 (M ) Sn), and 34.8 (M ) Pb) kcal/mol, respectively. That of H2C: is estimated as -14.0 kcal/mol.6 Furthermore, the relative stabilities of the singlet species of R2M: (M ) C, Si, Ge, Sn, Pb; R ) alkyl or aryl) compared to the corresponding dimer, R2MdMR2, are estimated to increase as the element row descends, C < Si < Ge < Sn < Pb. It follows, therefore, that one can expect that a divalent organolead compound such as plumbylene should be isolable as a stable compound. However, some plumbylenes, without any electronic or steric stabilization effects, are known to be thermally unstable and undergo facile disproportionation reactions, giving rise to elemental lead and the corresponding tetravalent organolead compounds.7 On this basis, it could be concluded that it might be difficult to isolate metallylenes as stable compounds under ambient conditions, since they generally exhibit extremely high reactivity toward other molecules as well as themselves. Metallylenes have a singlet ground state with a vacant p-orbital and a lone pair of valence orbitals. This extremely high reactivity must be due to their vacant p-orbitals, since 6 valence electrons is less than the 8 electrons of the “octet rule”. Their lone pair is expected to be inert due to its high s-character. In order to stabilize metallylenes enough to be isolated, either some thermodynamic and/or kinetic stabilization of the reactive vacant p-orbital is required (Figure 2). A range of “isolable” metallylenes have been synthesized through the thermodynamic stabilization of coordinating Cp* ligands, the inclusion of heteroatoms such as N, O, and P, * To whom correspondence should be addressed. Phone: +81-774-38-3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp. Chem. Rev. 2009, 109, 3479–3511 3479

Organoselenium chemistry: role of intramolecular interactions.

Abstract: In 1836 the first organoselenium compound, diethyl selenide, was prepared by Löwig,1 and it was isolated in the pure form in 1869.2 Early selenium chemistry involved the synthesis of simple aliphatic compounds such as selenols (RSeH), selenides (RSeR), and diselenides (RSeSeR); however, because of their malodorous nature, these compounds were difficult to handle. This, combined with the instability of certain derivatives and difficulties in purification, meant that selenium chemistry was slow to develop. By the 1950s, the number of known selenium compounds had increased significantly, but it was not until the 1970s, when several new reactions leading to novel compounds with unusual properties were discovered, that selenium chemistry began to attract more general interest.3-9 Aryl-substituted compounds were synthesized that were found to be less volatile and more pleasant to handle than the earlier aliphatic compounds. Compounds containing selenium in high oxidation states are relatively easy to manipulate using modern techniques.4c Organoselenium chemistry has now become a well-established field of research, and recent advances have been brought about by the potential technical applications of selenium compounds. Today selenium compounds find application in many areas including organic synthesis,4 biochemistry,5 xerography,6 the synthesis of conducting materials7 and semiconductors,8 and ligand chemistry.4c,9 Many of these aspects of selenium chemistry are wellcovered elsewhere in the literature; however, the subject of hypervalency has not attracted much attention and is the focus of this review.10