User:Benjah-bmm27/degree/2/IM

Main group 2, IM
"Chemistry of the Main Group Elements, Part 2: Rings, Chains and Macromolecules, Based on Main Group Elements"

If you like this sort of thing, you'll like this article.

Boron–nitrogen chemistry
Greenwood & Earnshaw, p. 207: instructive to compare B-N compounds with their C-C counterparts, since they are isoelectronic and have similar sizes and electronegativities (ish)

Boron nitride

 * Boron nitride, c.f. elemental carbon


 * Two main polymorphs of boron nitride, hexagonal BN (graphite-like) and cubic BN (diamond-like) - explore with Jmol


 * Boron-nitride-(hexagonal)-side-3D-balls.png Boron-nitride-(hexagonal)-top-3D-balls.png

Amine-borane adducts

 * Amine-borane adducts, c.f. alkanes


 * nice, stable compounds e.g. H3N→BH3 ammonia borane


 * Ammonia-borane-from-xtal-3D-balls.png Ammonia-borane-dimensions-MW-1983-2D.png Ammonia-borane-resonance-2D.png


 * The bonding in B-N compounds can be confusing. Amine-borane adducts can be represented as R3N+-B−R3. In this formalism, the charges are meant to be relative to the charges in the separate neutral fragments R3N and BR3, but this is still misleading.


 * Amine-borane-bond-polarity-2D.png


 * N in R3N has a large δ−, which is reduced to a small δ− upon formation of R3N→BR3. The negative charge on N decreases, but is still negative.


 * Similarly, B in BR3 has a large δ+, which decreases to a small δ+ upon formation of R3N→BR3. The positive charge on B decreases, but is still positive.

Aminoboranes

 * Aminoboranes, c.f. alkenes


 * Aminoborane-dimensions-MW-1987-2D.png
 * Aminoborane-resonance-2D.png


 * Without bulky substituents (i.e. without kinetic stabilisation), aminoboranes tend to cyclise and readily hydrolyse


 * Aminoboranes usually cyclise to 4-membered B2N2 rings due to steric hindrance, but some 6-membered B3N3 ring compounds are known. For example, aminoborane itself, H2NBH2, is in reversible equilibrium between monomer and dimer in the gas phase, and can cyclotrimerise to cyclotriborazane in the solid state.


 * Aminoborane-from-MW-1987-double-3D-balls.png Cyclodiborazane-Spartan-HF-6-31Gstar-3D-balls.png Cyclotriborazane-from-xtal-1973-3D-balls.png


 * Dehydrocoupling amine-borane adducts to cyclic aminoboranes and borazines at 25 °C with [ Rh(1,5-cod)(μ-Cl) ] 2 and RhCl3.3H2O catalysts: Chem. Commun. (2001) 962-963


 * Dimethylamine-borane-from-xtal-3D-balls-pair.png Dehydrodcoupling-amine-boranes-to-cyclic-aminoboranes-2D.png Cyclic-aminoborane-dimer-from-xtal-2001-3D-balls.png


 * Other catalysts include colloidal rhodium (J. Am. Chem. Soc. (2004) 126, 9776–9785) and titanocenes (J. Am. Chem. Soc. (2006) 128, 9582–9583)

Polyaminoboranes

 * Polyaminoboranes, c.f. polyolefins


 * Nice review: J. Organomet. Chem. (2007) 692, 2849–2853


 * Dehydrogenation to polyaminoboranes, e.g. MeNH2·BH3 → (-NHMe-BH2-)n at 25 °C : Angew. Chem. Int. Ed. (2008) 47, 6212-6215, Angew. Chem. Int. Ed. (2008) 47, 9600-9602


 * (using Brookhart's catalyst, a pincer ligand-IrH2 complex: Organometallics (2004) 23, 1766-1776)


 * Brookhart's-catalyst-2D-skeletal.png Brookhart's-catalyst-based-on-borane-adduct-xtal-3D-balls.png


 * Syndiotactic-polymethylaminoborane-3D-balls.png

Iminoboranes

 * Iminoboranes, c.f. alkynes


 * Iminoborane-from-IR-1987-triple-3D-balls.png


 * Iminoborane-dimensions-IR-1987-2D.png


 * Iminoborane-resonance-2D.png


 * Tert-butyl(tert-butylimino)borane, tBu-B≡N-tBu, B-N distance 1.26 Å, Chem. Ber. (1984) 117, 1089-1102 (Shelf 20 in library, Jmol)


 * Tert-butyl(tert-butylimino)borane-resonance-2D.png


 * Tert-butyl(tert-butylimino)borane-from-xtal-1983-3D-balls.png


 * tBu-B≡N-tBu (i.e. tBu2BN) cyclodimerises to the cyclobutadiene analogue tBu4B2N2:


 * Iminoborane-dimer-diazadiboretidine-from-xtal-1984-3D-balls.png Iminoborane-dimer-diazadiboretidine-from-xtal-1984-2D-skeletal.png


 * tBu-B≡N-tBu reacts with [Cp2NbH3] at 25 °C to form the complex [Cp2NbH(tBuB≡NtBu)], in which the iminoborane bonds to niobium side-on:


 * Iminoborane-NbCp2-complex-from-xtal-1988-3D-balls.png


 * Iminoboranes can undergo addition across their BN groups — e.g. RB≡NR′ + HCl → RClB=NHR′


 * RB≡NR′ (R, R′ = long alkyl chains) polymerise at 25 °C to colourless waxy materials - they may be polymers, but are poorly characterised

Borazines

 * Borazines, c.f. arenes
 * parent compound is borazine, B3N3H6 or equivalently cyclo-(BH)3(NH)3 or cyclo-(HBNH)3 — formally a trimer of iminoborane, HBNH
 * has six π electrons like benzene
 * but significant ΔEN means electron density more localised on nitrogen
 * only weak delocalisation/aromaticity
 * tends to undergo addition (e.g. of HCl or H2O across BN bonds) rather than the electrophilic aromatic substitution characteristic of benzene (although benzene is only kinetically and not thermodynamically stable with respect to HCl or water addition - Housecroft 3rd edn. p. 355)


 * Borazine-from-xtal-3D-balls.png Borazine-dimensions-2D.png Hexaphenylborazine-from-xtal-1979-3D-balls.png

Boron–phosphorus chemistry

 * Phosphine-borane adducts, e.g. Ph3P→BCl3
 * Phosphinoboranes, e.g. [ Ph2P-BH2 4]
 * Boraphosphabenzenes, e.g. cyclo-(BMes)3(PCy)3 (view with Jmol)
 * Polyphosphinoboranes, e.g. (PHPh-BH2)n
 * Manners et al:
 * Angew. Chem., Int. Ed. (1999) 38, 3321-3323
 * J. Am. Chem. Soc. (2000) 122, 6669–6678
 * Macromolecules (2003) 36, 291–297


 * Triphenylphosphine-boron-trichloride-adduct-from-xtal-2007-3D-balls.png Phosphine-borane-CRC-MW-3D-balls.png Diphenylphosphinodimesitylborane-from-xtal-3D-balls.png Octaphenylcyclotetraphosphinoborane-from-xtal-3D-balls.png Boraphosphabenzene-from-xtal-1987-3D-balls-noH.png

Boron–oxygen chemistry

 * B2O3 - reluctant to crystallise, consists of 3D network with trigonal planar BO3 units
 * B2O3 + SiO2 → borosilicate glass (Pyrex)
 * Borates and borate minerals (e.g. borax) - many structures, either discrete anions or anionic chains and rings - contain planar BO3 triangles and/or BO4 tetrahedra
 * Many molecular B-O species, e.g. hydrolysis of chlorosilanes leading to boronic acids RB(OH)2, and boroxines
 * No high-Mw linear B-O polymers yet

Silicon–oxygen chemistry

 * Silicon dioxide – α-quartz
 * Siloxanes and polysiloxanes, e.g. PDMS, Rochow-Müller process (Direct process), silicone gum, silicone rubber, thermal anionic ring-opening polymerization
 * Cristobalite-3D-polyhedra.png Beta-quartz-CM-2D-balls.png Silicone-3D-vdW.png

Catenated silicon chemistry

 * Organosilicon chemistry
 * Hexamethyldisilane
 * Polysilanes, sigma delocalisation c.f. polyacetylene, lithography
 * Polysilynes
 * Polycarbosilanes

Disilenes

 * Disilenes: R2Si=SiR2, silicon analogues of alkenes, thermally stable yellow or orange crystalline solids if R groups are bulky


 * Adv. Organomet. Chem. (2006) 54, 73-148 (review, including π-σ* mixing MO diagram accounting for pyramidalization of Si)


 * First disilene, tetramesityldisilene: R. West, M. J. Fink, J. Michl, Science (1981) 214, 1343-1344


 * R2SiCl2 + Li → (R2Si)3 (cyclotrisilane) → (UV radiation, −60 °C) (R2Si)2 (disilene)


 * Crystal structure shows Si non-planar, but slightly pyramidal, due to appreciable π-σ* mixing, which is not seen in alkenes: Organometallics (1984) 3, 793–800, Heteroat. Chem. (1990) 1, 1-7


 * Recent calculated structures of disilene itself, Si2H4: Chem. Phys. Lett. (2008) 466, 11-15, Jmol structure



Disilynes

 * Disilynes: Science (2004) 305, 1755-1757, view with Jmol
 * Sekiguchi-silyne-from-xtal-3D-balls.png

Update: the first carbon substituted disilyne: 2008


 * Silabenzenes

Phosphorus–nitrogen chemistry

 * Phosphazenes: compounds with PN multiple bonds (at least formally)

Cyclic phosphazenes

 * (NPCl2)n: Jmol (n = 3, 4, 5)
 * G&E p. 536: Produced industrially by R. Schenk and G. Römer's 1924 method:


 * nNH4Cl + nPCl5 → (NPCl2)n + 4nHCl


 * (PhCl solvent, 120–150 °C)


 * Hexachlorophosphazene, (NPCl2)3, formally a trimer of the hypothetical molecule phosphonitrile dichloride (1832-07-1), N≡PCl2
 * NPCl2-Spartan-MP2-3D-balls.png Hexachlorophosphazene-from-xtal-2006-3D-balls-B.png Octachlorotetraphosphazene-T-chair-form-from-xtal-1968-3D-balls-A.png

Polyphosphazenes

 * polyphosphazenes
 * PCl5 + [NH4]Cl → (120-150 °C, PhCl solvent) → (NPCl2)n (n = 3, 4) → (250 °C) → (NPCl2)n (n = lots) → (Nu−) → (NPNu2)n (n > 105)


 * Living polymerization

Phosphorus–oxygen chemistry

 * Phosphoric acid, pyrophosphoric acid, phosphorus trioxide, phosphorus pentoxide, tripolyphosphoric acid, sodium tripolyphosphate, polynucleotides
 * Phosphoric-acid-3D-balls.png Phosphorus-trioxide-from-xtal-3D-balls.png Phosphorus-pentoxide-3D-balls.png Phosphorus-pentoxide-sheet-from-xtal-3D-balls.png

Sulfur–oxygen chemistry

 * Catenated sulfur: Cyclooctasulfur, S8 and plastic sulfur, Sn
 * Sulfur oxides: SO2, SO3 (monomer, trimer and polymer)
 * Sulfuric acid and other sulfur oxoacids: disulfuric acid\
 * S-O polymers pretty useless because they all hydrolyse to H2SO4
 * Cyclooctasulfur-above-3D-balls.png Sulfur-dioxide-3D-balls.png Sulfur-trioxide-3D-balls.png Sulfur-trioxide-trimer-from-xtal-1967-3D-balls-B.png Sulfuric-acid-Givan-et-al-1999-3D-balls.png

Sulfur–nitrogen chemistry

 * Sulfur nitrides:
 * S4N4: Jmol
 * S2N2
 * (SN)x


 * Tetrasulfur-tetranitride-3D-balls.png Disulfur-dinitride-3D-balls.png Polythiazyl-chain-from-neutron-3D-balls.png


 * Synthesis of (SN)x from ammonium chloride and disulfur dichloride:
 * 4 [NH4]Cl + 6 S2Cl2 → S4N4 + 16 HCl + S8
 * S4N4, heat over Ag wool → 2 S2N2
 * S2N2, 25 °C, slow → (SN)x


 * S4N4 forms adducts with Lewis acids, e.g. S4N4 + BF3 → S4N4·BF3


 * S4N4.BF3-from-xtal-1967-3D-balls.png