Uranium(iv) alkyl cations: synthesis, structures, comparison with thorium(iv) analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity.

Reaction of [(XA2)U(CH2SiMe3)2] (1; XA2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) with 1 equivalent of [Ph3C][B(C6F5)4] in arene solvents afforded the arene-coordinated uranium alkyl cations, [(XA2)U(CH2SiMe3)(η n -arene)][B(C6F5)4] {arene = benzene (2), toluene (3), bromobenzene (4) and fluorobenzene (5)}. Compounds 2, 3, and 5 were crystallographically characterized, and in all cases the arene is π-coordinated. Solution NMR studies of 2-5 suggest that the binding preferences of the [(XA2)U(CH2SiMe3)]+ cation follow the order: toluene ≈ benzene > bromobenzene > fluorobenzene. Compounds 2-4 generated in C6H5R (R = H, Me or Br, respectively) showed no polymerization activity under 1 atm of ethylene. By contrast, 5 and 5-Th (the thorium analogue of 5) in fluorobenzene at 20 and 70 °C achieved ethylene polymerization activities between 16 800 and 139 200 g mol-1 h-1 atm-1, highlighting the extent to which common arene solvents such as toluene can suppress ethylene polymerization activity in sterically open f-element complexes. However, activation of [(XA2)An(CH2SiMe3)2] {M = U (1) or Th (1-Th)} with [Ph3C][B(C6F5)4] in n-alkane solvents did not afford an active polymerization catalyst due to catalyst decomposition, illustrating the critical role of PhX (X = H, Me, Br or F) coordination for alkyl cation stabilization. Gas phase DFT calculations, including fragment interaction calculations with energy decomposition and ETS-NOCV analysis, were carried out on the cationic portion of 2'-Th, 2', 3' and 5' (analogues of 2-Th, 2, 3 and 5 with hydrogen atoms in place of ligand backbone methyl and tert-butyl groups), providing insight into the nature of actinide-arene bonding, which decreases in strength in the order 2'-Th > 2' ≈ 3' > 5'.

Single-Site Molybdenum on Solid Support Materials for Catalytic Hydrogenation of N2 -into-NH3.

Very stable in operando and low-loaded atomic molybdenum on solid-support materials have been prepared and tested to be catalytically active for N2 -into-NH3 hydrogenation. Ammonia synthesis is reported at atmospheric pressure and 400 °C with NH3 rates of approximately 1.3×103  µmol h-1 gMo -1 using a well-defined Mo-hydride grafted on silica (SiO2-700 ). DFT modelling on the reaction mechanism suggests that N2 spontaneously binds on monopodal [(≡Si-O-)MoH3 ]. Based on calculations, the fourth hydrogenation step involving the release of the first NH3 molecule represents the rate-limiting step of the whole reaction. The inclusion of cobalt co-catalyst and an alkali caesium additive impregnated on a mesoporous SBA-15 support increases the formation of NH3 with rates of circa 3.5×103  µmol h-1 gMo -1 under similar operating conditions and maximum yield of 29×103  µmol h-1 gMo -1 when the pressure is increased to 30 atm.

Rigid NON- and NSN-ligand complexes of tetravalent and trivalent uranium: comparison of U-OAr2 and U-SAr2 bonding.

A rigid NSN-donor proligand, 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylthioxanthene (H(2)[TXA(2)], 1) was prepared by palladium-catalyzed coupling of 2,6-diisopropylaniline with 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylthioxanthene. Deprotonation of 1 using (n)BuLi provided Li(2)(DME)(2)[TXA(2)] (2), and subsequent reaction with UCl(4) afforded [Li(DME)(3)][(TXA(2))UCl(3)] (4). The analogous NON-donor ligated complex [(XA(2))UCl(3)K(DME)(3)] [3; XA(2) = 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene] was prepared by the reaction of K(2)(DME)(x)[XA(2)] with UCl(4). A cyclic voltammogram (CV) of 3 in THF/[NBu(4)][B(C(6)F(5))(4)] at 200 mV s(-1) showed an irreversible reduction to uranium(III) at E(pc) = -2.46 V versus FeCp(2)(0/+1), followed by a product wave at E(1/2) = -1.83 V. Complex 4 also underwent irreversible reduction to uranium(iii) [E(pc) = -2.56 V], resulting in an irreversible product peak at E(pa) = -1.83 V. One-electron reduction of complexes 3 and 4 using K(naphthalenide) under an argon atmosphere in DME yielded 6-coordinate [(XA(2))UCl(DME)] (5) and the thermally unstable 7-coordinate [(TXA(2))U(DME)Cl(2)Li(DME)(2)] (6), respectively. The U-S distances in 4 and 6 are uncommonly short, the C-S-U angles are unusually acute, and the thioxanthene backbone of the TXA(2) ligand is significantly bent. By contrast, the xanthene backbone in XA(2) complexes 3 and 5 is planar. However, κ(3)-coordination and an approximately meridional arrangement of the ancillary ligand donor atoms is maintained in all complexes. DFT and Atoms in Molecules (AIM) calculations were carried out on 3, 4, 5, 6, [(XA(2))UCl(3)](-) (3B), [(TXA(2))UCl(2)(DME)](-) (6B) and [(TXA(2))UCl(DME)] (6C) to probe the extent of covalency in U-SAr(2) bonding relative to U-OAr(2) bonding.

Samarium complexes of a sigma-/pi-pyrrolide/arene based macrocyclic ligand.

A bis-pyrrolide macrocyclic ligand [L = trans-calix[2]benzene[2]pyrrole(H)] containing two aromatic phenyl rings in the macrocycle backbone was reacted with SmCl(3)(THF)(3) to afford the corresponding [LSm(III)Cl] (1) complex. Its crystal structure showed the ligand adopting the sigma-bonding mode with the pyrrolide moieties and the pi-bonding with the two aromatic rings. Reaction of 1 with MeLi in THF gave a mixture of two compounds. The major was a C-H activated complex [LSm(III)(THF)] (2a) where the bonding mode of the pyrrolide rings was switched from sigma- to pi- as a result of the deprotonation and metallation of one of the two aromatic rings. The minor component was an unusual [(L)Sm(III)(HL')] (2b) complex containing both a regular ligand and an "N-confused" macrocyclic ligand. The two ligands wrapped the Sm center with the regular ligand adopting a bonding mode similar to 1. The second ligand instead acted as a simple sigma-bonded monodentate ligand, only using one nitrogen atom of one pyrrolide ring. However, this particular pyrrolide moiety has been isomerized by shifting the ring attachment to the macrocycle (N-confused system). In addition, the second pyrrolide ring has been protonated at the nitrogen atom. Complex 2b was obtained as major compound and in analytically pure form by reacting 1 with NaH.

Low-valent vanadium complexes of a pyrrolide-based ligand. Electronic structure of a dimeric V(I) complex with a short and weak metal-metal bond.

Deprotonation of the nitrogen atoms of the two pyrrole rings of 1,3-bis-{[(1'-pyrrol-2-yl)-1,1'-dimethyl]methyl}benzene with KH followed by further reaction with either VCl 3(THF) 3 or with VCl 2(TMEDA) 2 respectively gave the paramagnetic complexes [1,3-bis-{[(1'-pyrrol-2-yl)-1,1'-dimethyl]methyl}benzene]VCl(DME) ( 1) and [1,3-bis-{[(1'-pyrrol-2-yl)-1,1'-dimethyl]methyl}benzene]V(THF) 3 ( 2). Further reduction with the appropriate amount of KH afforded diamagnetic dinuclear [1,3-bis-{[(1'-pyrrol-2-yl)-1,1'-dimethyl]methyl}benzene]V} 2] ( 3). In complex 3, the bridging interaction between the two metal centers is realized via the ligand central benzene ring. Density functional theory calculations have elucidated the nature of the electronic interaction between the two metals with the bridging pi-system thus accounting for its visible structural distortion. Calculations also pointed out the presence of only a weak V-V bond in spite of the short V-V distance.

Synthesis, structure, redox properties, and catalytic activity of new ruthenium complexes containing neutral or anionic and facial or meridional ligands: an evaluation of electronic and geometrical effects.

The synthesis of a family of new Ru complexes containing meridional or facial tridentate ligands with the general formula [Ru(II)(T)(D)(X)](n+) [T = 2,2':6',2' '-terpyridine or tripyrazolylmethane; D = 4,4'-dibenzyl-4,4',5,5'-tetrahydro-2,2'-bioxazole (S,S-box-C) or 2-[((1'S)-1'-(hydroxymethyl)-2'-phenyl)ethylcarboxamide]-(4S)-4-benzyl-4,5-dihydrooxazole (S,S-box-O); X = Cl, H(2)O, MeCN or pyridine] has been described. All complexes have been spectroscopically characterized in solution through (1)H NMR and UV-vis techniques. Furthermore, all of the chloro complexes presented here have also been characterized in the solid state through monocrystal X-ray diffraction analysis. The oxazolinic S,S-box-C ligands undergo a Ru-assisted hydrolysis reaction generating the corresponding amidate anionic oxazolinic ligands S,S-box-O, which are also strongly attached to the metal center and produce a strong sigma-donation effect over the Ru metal center. The redox properties of all complexes have also been studied by means of cyclic voltammetry, strongly reflecting the nature of the ligands; both effects, geometrical (facial vs meridional) and electronic (neutral vs anionic), can be unveiled and rationalized. Finally, the reactivity of the Ru-OH(2) complexes has been tested with regard to the epoxidation of trans-stilbene, and it has been shown that, in this particular case, the reactivity is practically not dependent on the redox potentials of the catalyst but, in sharp contrast, it is strongly dependent on the geometry of the tridentate ligands.