Nitrogen Fixation via Splitting into Nitrido Complexes.

The large carbon footprint of the Haber-Bosch process, which provides ammonia for fertilizers but also the feedstock for all nitrogenous commercial products, has fueled the quest for alternative synthetic strategies to nitrogen fixation. Owing to the extraordinarily strong N≡N triple bond, the key step of the Haber-Bosch reaction, i.e., the dissociative adsorption of N2, requires high temperatures. Since the first report in 1995, a wide variety of molecular transition metal and f-block compounds have been reported that can fully cleave N2 at ambient conditions and form well-defined nitrido complexes. We here provide a comprehensive survey of the current state of N2 splitting reactions in solution and follow-up nitrogen transfer reactivity. Particular emphasis is put on electronic structure requirements for the formation of suitable molecular precursors and their N-N scission reactivity. The prospects of N2 splitting for the synthesis of nitrogen containing products will be discussed, ranging from ammonia and heterocumulenes to organic amines, amides or nitriles via proton coupled electron transfer, carbonylation, or electrophilic functionalization of N2 derived nitrido complexes. Accomplishments and challenges for nitrogen fixation via N2 splitting are presented to offer guidelines for the development of catalytic platforms.

Ru-Catalyzed Completely Deoxygenative Coupling of 2-Arylethanols through Base-Induced Net Decarbonylation.

Substituted arylethanols can be coupled by using a readily available Ru catalyst in a fully deoxygenative manner to produce hydrocarbon chains in one step. Control experiments indicate that the first deoxygenation occurs through an aldol condensation, whereas the second occurs through a base-induced net decarbonylation. This double deoxygenation enables further development in the use of alcohols as versatile and green alkylating reagents, as well as in other fields, such as deoxygenation and upgrading of overfunctionalized biomass to produce hydrocarbons.

Crystal engineering: co-crystals of cinnamic acid derivatives with a pyridyl derivative co-crystallizer.

A number of hydrogen-bonded co-crystals, consisting of a cinnamic acid derivative and a pyridyl co-crystallizer, have been synthesized and their properties investigated by X-ray diffraction. Samples were prepared by recrystallization or solvent drop grinding of trans-cinnamic acid (1), 4-methylcinnamic acid (2), 4-methoxy cinnamic acid (3) or 3,4-methoxy cinnamic acid (4), with 4,4-dipyridyl (A), iso-nicotinamide (B) or nicotinamide (C). The X-ray single-crystal structures of seven novel co-crystals, obtained through recrystallization, are examined and the hydrogen-bonding interactions discussed. Consistent hydrogen-bonding motifs were observed for samples prepared when using 4,4-dipyridyl (A) or iso-nicotinamide (B) as the co-crystallizing agent. Powder X-ray diffraction analysis of the samples prepared by solvent drop grinding suggests the formation of ten co-crystals.

Small Molecule Activation by Intermolecular Zr(IV)-Phosphine Frustrated Lewis Pairs.

We report intermolecular transition metal frustrated Lewis pairs (FLPs) based on zirconocene aryloxide and phosphine moieties that exhibit a broad range of small molecule activation chemistry that has previously been the preserve of only intramolecular pairs. Reactions with D2, CO2, THF, and PhCCH are reported. By contrast with previous intramolecular examples, these systems allow facile access to a variety of steric and electronic characteristics at the Lewis acidic and Lewis basic components, with the three-step syntheses of 10 new intermolecular transition metal FLPs being reported. Systematic variation to the phosphine Lewis base is used to unravel steric considerations, with the surprising conclusion that phosphines with relatively small Tolman steric parameters not only give highly reactive FLPs but are often seen to have the highest selectivity for the desired product. DOSY NMR spectroscopic studies on these systems reveal for the first time the nature of the Lewis acid/Lewis base interactions in transition metal FLPs of this type.

Cooperative Lewis pairs based on late transition metals: activation of small molecules by platinum(0) and B(C6 F5 )3.

A Lewis basic platinum(0)-CO complex supported by a diphosphine ligand and B(C6 F5 )3 act cooperatively, in a manner reminiscent of a frustrated Lewis pair, to activate small molecules such as hydrogen, CO2 , and ethene. This cooperative Lewis pair facilitates the coupling of CO and ethene in a new way.

Reversible CO exchange at platinum(0). An example of similar complex properties produced by ligands with very different stereoelectronic characteristics.

The ligands 1,2-C6H4(CH2P(t)Bu2)2 (La) and 1,2-C6H4(P(t)Bu2)(CH2P(t)Bu2) (Lb) displace norbornene (nbe) from [Pt(η(2)-nbe)3] to give [PtL(η(2)-nbe)] where L = La (1a) or Lb (1b); 1a is fluxional on the NMR timescale. Reaction of 1a,b with CO gives the corresponding monocarbonyls [PtL(CO)] where L = La (2a) or Lb (2b) which then react further, and reversibly, to give the dicarbonyls [PtL(CO)2] where L = La (3a) or Lb (3b). The CO interchange between 2a,b and 3a,b is compared with the only other such system (2f and 3f), which are complexes of (C2F5)2PCH2CH2P(C2F5)2 (Lf). Ethene reacts smoothly with 2a to give (4a) and H2 with 2a generates some [PtH2(La)]. Protonation of 2a gives [Pt(La)(H)(CO)][B(C6F5)4] (5a) whose crystal structure has been determined. Similarly protonation of 2b gives [Pt(Lb)(H)(CO)][B(C6F5)4] as a mixture of geometric isomers 5b­6b.

Interplay of bite angle and cone angle effects. A comparison between o-C6H4(CH2PR2)(PR'2) and o-C6H4(CH2PR2)(CH2PR'2) as ligands for Pd-catalysed ethene hydromethoxycarbonylation.

The following unsymmetrical diphosphines have been prepared: o-C6H4(CH2PtBu2)(PR2) where R = PtBu2 (L3a); PCg (L3b); PPh2 (L3c); P(o-C6H4CH3)2 (L3d); P(o-C6H4OCH3)2 (L3e) and o-C6H4(CH2PCg)(PCg) (L3f) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions has been investigated in the presence of Pd complexes of each of the ligands L3a­f and the results compared with those obtained with the commercially used o-C6H4(CH2PtBu2)2 (L1a). The Pd complexes of the bulkiest ligands L3a, L3b and L3f are highly active catalysts but the Pd complexes of L3c, L3d and L3e are completely inactive. The crystal structures of the complexes [PtCl2(L1a)] (1a) and [PtCl2(L3a)] (2a) have been determined and show that the crystallographic bite angles and cone angles are greater for L1a than L3a. Solution NMR studies show that the seven-membered chelate in 1a is more rigid than the six-membered chelate in 2a. Treatment of [PtCl(CH3)(cod)] with L3a­f gave [PtCl(CH3)(L3a­f)] as mixtures of 2 isomers 3a­f and 4a­f. The ratio of the products 4:3 ranges from 100:1 to 1:20, the precise proportion is apparently governed by a balance of two competing factors, steric bulk and the antisymbiotic effect. The palladium complexes [PdCl(CH3)(L3b)] (5b/6b) and [PdCl(CH3)(L3c)] (5c/6c) react with labelled 13CO to give the corresponding acyl species [PdCl(13COCH3)(L3b)] (7b/8b) and [PdCl(13COCH3)(L3c)] (7c/8c). Treatment of [PdCl(13COCH3)(L)] with MeOH gave CH3(13)COOMe rapidly when L = L3b but very slowly when L = L3c paralleling the contrasting catalytic activity of the Pd complexes of these two ligands.