Metamaterials have optical properties that are unprecedented in nature. They have opened new horizons in light manipulation, with the ability to bend, focus, completely reflect, transmit, or absorb an incident wave front. Optically active metamaterials in particular could be used for applications ranging from 3D information storage to photovoltaic cells. Silicon (Si) particles are some of the most promising building blocks for optically active metamaterials, with high scattering efficiency coupled to low light absorption for visible frequencies. However, to date ideal Si building blocks cannot be produced by bulk synthesis techniques. The key is to find a synthetic route to produce Si building blocks between 75-200 nm in diameter of uniform size and shape, that are crystalline, have few impurities, and little to no porosity. This Review provides a theoretical background on Si optical properties for metamaterials, an overview of current synthetic methods and gives direction towards the most promising routes to ideal Si particles for metamaterials.

Previous results obtained for the cis-polymerisation of isoprene with scandium half-sandwich complex Cp*Sc(BH 4 ) 2 (THF) 1a were extended to its neodymium analog. The X-ray structure of the already reported neodymium monomer compound Cp*Nd(BH 4 ) 2 (THF) 2 1b is presented. Cp*Nd(BH 4 ) 2 (THF) 2 /[CPh 3 ]-[B(C 6 F 5 ) 4 ]/Al( i Bu) 3 catalytic system was found to be very active and stereoselective towards isoprene polymerisation, leading to highly 1,4-cis polyisoprene up to 92%. The effect on isoprene polymerisation of the addition of a NHC molecule to Cp*Ln(BH 4 ) 2 (THF) n pre-catalyst (Cp* = η 5 -C 5 Me 5 , Ln = Sc, n = 1, 1a; Nd, n = 2, 1b; Sm, n = 2, 1c) or to a trisborohydride Ln(BH 4 ) 3 (THF) n (Ln = Sc, n = 1.5, 2a; Nd, n = 3, 2b; Sm, n = 3, 2c) was also studied. Several NHC ligands were assessed: the classical [1-C{(N t BuCH)} 2 ] (L 1 ) and functional N-heterocyclic carbenes, two amino-tethered HNBu t CH 2 CH 2 [1-C{N(CHCH)NR}] (HL 2-R ) (R = t Bu, Mes (Mes = 2,4,6-Me 3 -C 6 H 2 )) and the hydroxyl-tethered HOCMe 2 CH 2 [1-C{N(CHCH)N i Pr}] (HL 3 ). Neodymiumbased complex (L 2-tBu )Nd(BH 4 ) 2 (THF) 2 3 could be isolated and characterized. With some of the catalytic combinations tested, the introduction of the NHC ligand in the coordination sphere of the complex induces a switch of the stereoselectivity of the resulting polymer. Scandium complex 2a, which produces mainly 1,4-cis polyisoprene when associated to a borate activator and aluminum alkyl, leads to 1,4-trans polymer up to 94% regular when HL 2-tBu carbene is added to the same reaction mixture. This result is the only example of highly trans-polyisoprene synthesized with a scandium based catalyst. Coordination of the carbene moiety to the rare earth centre is confirmed by NMR studies on paramagnetic neodymium pre-catalysts.