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

Amine-borane adducts

Amine-borane adducts, c.f. alkanes

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

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.

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.

Calculated partial charges (Spartan, MP2, 6–311+G**)
	H₃N–BH₃	NH₃	BH₃
NH	+0.279	+0.335	—
N	−0.412	−0.998	—
B	−0.058	—	+0.338
BH	−0.122	—	−0.113

Aminoboranes

Aminoboranes, c.f. alkenes

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.

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

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)

Iminoboranes

Iminoboranes, c.f. alkynes

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)

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

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:

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)

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:

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-M_w 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

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