Philip P. Power

Main-group elements as transition metals

The last quarter of the twentieth century and the beginning decade of the twenty-first witnessed spectacular discoveries in the chemistry of the heavier main-group elements. The new compounds that were synthesized highlighted the fundamental differences between their electronic properties and those of the lighter elements to a degree that was not previously apparent. This has led to new structural and bonding insights as well as a gradually increasing realization that the chemistry of the heavier main-group elements more resembles that of transition-metal complexes than that of their lighter main-group congeners. The similarity is underlined by recent work, which has shown that many of the new compounds react with small molecules such as H2, NH3, C2H4 or CO under mild conditions and display potential for applications in catalysis.

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

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

Slow magnetization dynamics in a series of two-coordinate iron(II) complexes

A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), and Fe(OAr′)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe[N(H)Ar#]2 (6), is bent with an N–Fe–N angle of 140.9(2)° (Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 ≫ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff = 181, 146, 109, 104, and 43 cm−1 for 1–5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous D∞h geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1–6, but ultimately spin–orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1–5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.

The chemistry of boron and its speciation in plants

The chemistry and usage of B, as well as its speciation in plants, are reviewed. In the context of biology, the chemistry of the surprisingly rare element B is dominated by B-oxygen compounds. The occurrence, distribution and chemistry of these compounds are briefly described. In addition, the chemistry involved in the interaction of B–O compounds with naturally occurring molecules, particularly polyhydroxy compounds, is summarised. The essentiality of B in plants has been known for 75 years, but the biomolecule(s) with which it interacts to cause its essential function(s) remain largely unknown, although many have been suggested. Recent research on the complexation, isolation and partial characterisation of B complexes of polysaccharides, diols, and hydroxyacids is outlined. The particular importance of B interactions with cell wall components, membranes, enzymes, sugars, and polyols is discussed.

Interaction of multiple bonded and unsaturated heavier main group compounds with hydrogen, ammonia, olefins, and related molecules.

We showed in 2005 that a digermyne, a main group compound with a digermanium core and aromatic substituents, reacted directly with hydrogen at 25 °C and 1 atm to give well-defined hydrogen addition products. This was the first report of a reaction of main group molecules with hydrogen under ambient conditions. Our group and a number of others have since shown that several classes of main group molecules, either alone or in combination, react directly (in some cases reversibly) with hydrogen under mild conditions. Moreover, this reactivity was not limited to hydrogen but also included direct reactions with other important small molecules, including ammonia, boranes, and unactivated olefins such as ethylene. These reactions were largely unanticipated because main group species were generally considered to be too unreactive to effect such transformations. In this Account, we summarize recent developments in the reactions of the multiple bonded and other open shell derivatives of the heavier main group elements with hydrogen, ammonia, olefins, or related molecules. We focus on results generated primarily in our laboratory, which are placed in the context of parallel findings by other researchers. The close relationship between HOMO-LUMO separations, symmetry considerations, and reactivity of the open shell in main group compounds is emphasized, as is their similarity in reactivity to transition metal organometallic compounds. The unexpectedly potent reactivity of the heavier main group species arises from the large differences in bonding between the light and heavy elements. Specifically, the energy levels within the heavier element molecules are separated by much smaller gaps as a result of generally lower bond strengths. In addition, the ordering and symmetries of the energy levels are generally different for their light counterparts. Such differences lie at the heart of the new reactions. Moreover, the reactivity of the molecules can often be interpreted qualitatively in terms of simple molecular orbital considerations. More quantitative explanations are accessible from increasingly sophisticated density functional theory (DFT) calculations. We open with a short description of the background developments that led to this work. These advances involved the synthesis and characterization of numerous new main group molecules involving multiple bonds or unsaturated configurations; they were pursued over the latter part of the last century and the beginning of the new one. The results firmly established that the structures and bonding in the new compounds differed markedly from those of their lighter element congeners. The knowledge gained from this fundamental work provided the framework for an understanding of their structures and bonding, and hence an understanding of the reactivity of the compounds discussed here.

Subvalent Group 4B metal alkyls and amides. Part 5. The synthesis and physical properties of thermally stable amides of germanium(II), tin(II), and lead(II)

A series of subvalent Group 4B metal amides of general formula M(NR1R2)2[(i) R1= SiMe3, R2= But; M = Ge, Sn, or Pb; (ii) R1= R2= SiMe3; M = Ge, Sn, or Pb; and (iii) R1= R2= GeMe3, SiEt3, or GePh3; M = Ge or Sn] has been prepared from the appropriate lithium amide and metal(II) halide. Under ambient conditions, the amides are pale yellow to red, thermochromic, diamagnetic, low-melting solids or liquids, and are soluble in hydrocarbons (C6H6 or C6H12) in which they are diamagnetic monomers. The lower homologues give parent molecular ions as the highest m/e species. Infrared spectra show a band at 380–430 cm–1[νasym(MN2)], and 1H or 13C n.m.r. spectra are consistent with the bent-singlet formulation. In the visible region the compounds exhibit a band (364–495 nm) of moderate intensity (Iµ= 600–2 050 dm3 mol–1 cm–1 in n-C6H14) indicative of an allowed electronic transition. Photolysis of each diamide in n-hexane in the cavity of an e.s.r. spectrometer affords (a) the persistent (t½ 5 min—3 months at 25° C) metal-centred radical Ṁ(NR1R2)3[(i) R1= SiMe3, R2= But, M = Ge or Sn; (ii) R1= R2= SiMe3 or GeMe3, M = Ge or Sn; or (iii) R1= R2= GeEt3, M = Sn], (b) a lead mirror (for the lead amides), or (c) no sign of reaction (for the more bulky diamides). E.s.r. parameters have been derived from the isotropic spectra.

Reversible Reactions of Ethylene with Distannynes Under Ambient Conditions

Tin Two-Step Doubly and triply bonded carbon compounds have a well-studied tendency to link up with one another and form rings. The rates of these reactions and their relative susceptibilities to acceleration by heat versus light are encapsulated in the decades-old Woodward-Hoffmann rules. More recently, alkene and alkyne analogs have been prepared with heavier elements such as silicon and tin substituted for carbon. Peng et al. (p. 1668; see the Perspective by Sita) have now discovered that two distannynes (compounds with triply bonded tins) react readily with ethylene to form cycloadducts, with tin-carbon σ bonds taking the place of C-C and Sn-Sn π bonds. These products, characterized spectroscopically and crystallographically, are only loosely bound at room temperature, easily reverting to their multiply bonded precursors on gentle heating. Ethylene reacts reversibly with triply bonded tin, contrasting with its reactivity toward carbon triple bonds. Ethylene’s cycloadditions to unsaturated hydrocarbons occupy well-established ground in classical organic chemistry. In contrast, its reactivity toward alkene and alkyne analogs of carbon’s heavier-element congeners silicon, germanium, tin, or lead has been little explored. We show here that treatment of the distannynes AriPr4SnSnAriPr4 [AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2, 1] or AriPr8SnSnAriPr8 [AriPr8 = C6H-2,6(C6H2-2,4,6-iPr3)2-3,5-iPr2, 2] with ethylene under ambient conditions affords the cycloadducts ArPir4Sn(μ2:η1:η1-C2H4)2Sn⎴ArPir4 (3) or ArPir8Sn(μ2:η1:η1-C2H4)2Sn⎴ArPir8 (4) that were structurally and spectroscopically characterized. Ethylene incorporation in 3 and 4 involves tin-carbon σ bonding and is shown to be fully reversible under ambient conditions; hydrocarbon solutions of 3 or 4 revert to the distannynes 1 or 2 with ethylene elimination under reduced pressure or upon standing at ~25°C. Variable-temperature proton nuclear magnetic resonance studies showed that the enthalpies of reaction were near –48 (3) and –27 (4) kilojoules per mole.

Metal Amide Chemistry

Biographies. Preface. 1. Introduction. 1.1. Scope and Organisation of Subject Matter. 1.2. Developments and Perspectives. 2. Alkali Metal Amides. 2.1. Introduction. 2.2. Lithium Amides. 2.3. Sodium Amides. 2.4. Potassium Amides. 2.5. Rubidium Amides. 2.6. Caesium Amides. 3. Beryllium and the Alkaline Earth Metal Amides. 3.1. Introduction. 3.2. Beryllium Amides. 3.3. Magnesium Amides. 3.4. Calcium Amides. 3.5. Strontium Amides. 3.6. Barium Amides. 4. Amides of the Group 3 Lanthanide Metals. 4.1. Introduction. 4.2. The Pre-1996 Literature: Anwanders Review. 4.3. The Recent (Post-1995) Literature. 5. Amides of the Actinide Metals. 5.1. Introduction 5.2.Neutral Amidouranium (IV) and Thorium (IV) Complexes. 5.3. Neutral U III Amides. 5.4. Neutral Mixed Valence (U III/ U IV ), U II U V and U VI Amides. 5.5. Amidouranates. 5.6. Amidouranium Tetraphenylborates. 6. Amides of the Transition Metals. 6.1. Introduction. 6.2. Transition Metal Derivatives of Monodentate Amides. 6.3. Transition Metal Complexes of Polydentate Amido Ligands. 6.4. Other Chelating Amido Ligands. 7. Amides of Zinc, Cadmium and Mercury. 7.1. Introduction. 7.2. Neutral Homoleptic Zinc, Cadmium and Mercury Amides. 7.3. Ionic Metal Amides. 7.4. Lewis Base Complexes, Chelated Metal Amides and Heteroleptic Amido Complexes. 8. Amides of the Group 13 Metals. 8.1. Introduction. 8.2. Aluminum Amides. 8.3. Gallium Amides. 8.4. Indium Amides. 8.5. Thallium Amides. 9. Subvalent Amides of Silicon and the Group 14 Metals. 9.1. Introduction. 9.2. Subvalent Amidosilicon Compounds. 9.3. Amidometal(II) Chemistry [Ge(II), Sn(II), Pb(II)]. 9.4. Dimeric Metal(III) Imides: Biradicaloid Compounds. 9.5. Higher-Nuclearity Group 14 Metalloid Clusters having Amido Ligands. 10. Amides of the Group 15 Metals (As, Sb, Bi). 10.1. Introduction. 10.2. Mononuclear Group 15 Metal (III) Amides. 10.3. Oligomeric Group 15 Metal Imides. 10.4. Mononuclear Group 15 Metal (V) Amides. 10.5. Group 15 Metal (III) Macrocyclic Imides. 10.6. Miscellaneous Group 15 Metal-Nitrogen Compounds. Index.

Isolation of a stable, acyclic, two-coordinate silylene.

The synthesis and characterization of a stable, acyclic two-coordinate silylene, Si(SAr(Me(6)))(2) [Ar(Me(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Me(3))(2)], by reduction of Br(2)Si(SAr(Me(6)))(2) with a magnesium(I) reductant is described. It features a V-shaped silicon coordination with a S-Si-S angle of 90.52(2)° and an average Si-S distance of 2.158(3) Å. Although it reacts readily with an alkyl halide, it does not react with hydrogen under ambient conditions, probably as a result of the ca. 4.3 eV energy difference between the frontier silicon lone pair and 3p orbitals.

Stable, Monomeric Imides of Aluminum and Gallium: Synthesis and Characterization of [{HC(MeCDippN)2}MN‐2,6‐Trip2C6H3] (M=Al or Ga; Dipp=2,6‐iPr2C6H3; Trip=2,4,6‐iPr3C6H2)

A short Ga-N bond with double-bond character is displayed by the first monomeric imide of gallium, which was obtained by the reaction of [{HC(MeCDippN)2 }M:] (Dipp=2,6-iPr2 C6 H3 , M=Ga; see picture) with N3 -2,6-Trip2 C6 H3 (Trip=2,4,6-iPr3 C6 H2 ). The analogous aluminum (M=Al) compound is also readily available.