Heteroleptic Magnesium n-Butyl on a Chemically Non-innocent 2-Anilidomethylpyridine Ligand Leading to Diverse Magnesium Hydrides

Molecular magnesium hydrides and hydride-rich clusters are of significant interest for applications ranging from catalysis and small molecule activation to hydrogen storage. Here, we investigate the 2-anilidomethylpyridine framework NNL as an ancillary support for magnesium organometallics with a special emphasis on hydrides. The proligand NNLH (N-[2,6-bis(1-methylethyl)phenyl]-α,6-diphenyl-2-pyridinemethanamine) gives [(NNL)Mg(nBu)(thf)] (1) by nbutane elimination from Mg(nBu)2(thf)n. A stronger donor such as DMAP replaces the THF from 1 to give [(NNL)Mg(nBu)(dmap)] (2). Both are air-sensitive, and 1 is adventitiously oxidized into [(NNL)Mg(μ-OnBu)]2 (32). The homoleptic [(NNL)2Mg] (8) is made from 1 and a second equiv of NNLH. 1's terminal nBu group is selectively protonated by HN(SiMe3)2 to give [(NNL)MgHMDS] (4; HMDS = N(SiMe3)2), whereas Ph3SiOH partially protonates the backbone anilide as well to give a mixture of [(NNL)Mg(OSiPh3)(thf)] (5) and free NNLH. Like HN(SiMe3)2, aprotic MeOTf also reacts by selectively abstracting the nBu group from 1 to give [(NNL)Mg(μ:κ2-O,O'-OTf)(thf)]2 (62). Interestingly, screening the common synthetic routes for magnesium hydrides leads to diverse outcomes upon varying the Mg precursors and hydride sources. 1 and PhSiH3 give the hydride cluster [{(NNL)2Mg2(μ-H)}2(μ-H)4Mg] (7), whereas 2 and PhSiH3 give the molecular complex [(NNLde)Mg(dmap)2] (9) with a dearomatized pyridyl backbone. 1 and HBpin (pinacolborane) give a product mixture, from which a different hydride cluster [(NNL)2Mg2(μ-H)}2(μ:κ2-O,O'-O2C2Me4)] (10) is identified, showing a rare instance of complete deborylation of a HBpin molecule. 1 and HBcat (catecholborane) also give a product mixture, one of which is the borylated ligand [(NNL)Bcat] (11). HBpin with 4 as the Mg precursor takes the ligand borylation route more selectively to give [(NNL)Bpin] (12). Last, 1 reacts with iPrNH2BH3 to give [(NNL)Mg{NH(iPr)BH3}] (13), which shows a slow and fractional conversion into the dinuclear mixed hydrido amidoborane [(NNL)2Mg2(μ-H){(μ-NH(iPr)BH3}] (14) by partial β-hydride elimination. In comparison, [(NNL)Mg(iPrNHBH3)(dmap)] (15) arising from the DMAP-bound 2 and iPrNH2BH3 is stable toward such elimination.

2-Anilidomethylpyridine-Derived Three-Coordinate Zinc Hydride: The Journey Unveils Anilide Backbone's Reactive Nature

Low-coordinate heteroleptic zinc hydrides are catalytically important but rare and synthetically challenging. We herein report three-coordinate monomeric zinc hydride on a 2-anilidomethylpyridine framework (NNL). The synthetic success comes through systematically screening a few different routes from different precursors. During the process, the ligand's anilide backbone interestingly appears to be more reactive than Zn's terminal site to electrophilic Lewis and Brønsted acids. The proligand NNLH reacts with [Zn{N(SiMe3)2}2] and ZnEt2 to give [(NNL)ZnA] (A = N(SiMe3)2 (1), Et(2)). Both are inert to PhSiH3 and H2 but react with HBpin only through the internal Zn-Nanilide bond to give the borylated ligand NNLBpin (3). The reactions of 1 and 2 with Ph3EOH (E = C, Si) afford a series of divergent compounds like [(NNLH)Zn(OSiPh3)2] (4), [Zn3(OSiPh3)4Et2] (5), and [EtZn(OCPh3)] (6). But in all cases, it is invariably the Zn-Nanilide bond protonated by the -OH with equal or higher preference than the terminal Zn-N or Zn-C bonds. A DFT analysis rationalizes the origin of such a reactivity pattern. Realizing that an acid-free route might be the key, reacting [(NNL)Li] with ZnBr2 gives [(NNL)Zn(μ-Br)]2 (7), which on successively treating with KOSiPh3 and PhSiH3 gives the desired [(NNL)ZnH] (8) as a three-coordinate monomer with a terminal Zn-H bond. Estimating the ligand steric in 8 shows the openness in Zn's coordination sphere, a desired criterion for efficient catalysis. This and a positive influence of the pyridyl sidearm is reflected in 8's superior activity in hydroborating PhC(O)Me by HBpin in comparison to Jones' two-coordinate anilido zinc hydride.

Regioselective C-H alkylation of anisoles with olefins by cationic imidazolin-2-iminato scandium(iii) alkyl complexes

A new type of rare-earth alkyl complexes supported by monoanionic imidazolin-2-iminato ligands were synthesised and structurally characterised by X-ray diffraction and NMR analyses. The utility of these imidazolin-2-iminato rare-earth alkyl complexes in organic synthesis was demonstrated by their performance in highly regioselective C-H alkylation of anisoles with olefins. With as low as 0.5 mol% catalyst loading, various anisole derivatives without ortho-substitution or 2-methyl substituted anisoles reacted with several alkenes under mild conditions, producing the corresponding ortho-Csp2-H and benzylic Csp3-H alkylation products in high yield (56 examples, 16-99% yields). Control experiments revealed that rare-earth ions, ancillary imidazolin-2-iminato ligands, and basic ligands were crucial for the above transformations. Based on deuterium-labelling experiments, reaction kinetic studies, and theoretical calculations, a possible catalytic cycle was provided to elucidate the reaction mechanism.

Unsymmetrical diarylamido-based rare-earth alkyl complexes: their synthesis and catalytic performance in isoprene polymerization

A family of rare-earth complexes bearing diarylamido-based pincer ligands with phosphino-, phenylthio- and quinolino-sidearms have been synthesized and fully characterized. Upon activation by [Ph₃C][B(C₆F₅)₄], the scandium (P-Sc and S-Sc) and yttrium complexes (P-Y and S-Y) could catalyze the polymerization of isoprene with cis-1,4 selectivity (up to 98.8%), while the lutetium analogues P-Lu and S-Lu produced trans-1,4 selective polyisoprene (up to 83.3%). Density functional theory (DFT) calculations were carried out to clarify the mechanisms for the metal-dependent stereoselective (cis to trans) polymerization of isoprene catalyzed by P-Sc, P-Y and P-Lu, suggesting that kinetically and thermodynamically more favorable insertion-isomerization during chain propagation is the key reason for P-Lu catalyzed trans-stereoselective isoprene polymerization.

C 1 and Cs 2-pyridylethylanilido zirconium(iv), yttrium(iii) and lutetium(iii) complexes: synthesis, characterization and catalytic activity in the isoprene polymerization

Organolanthanide containing C_s and C₁-symmetric 2-pyridylethylanilido ligands have been scrutinized as catalyst precursors for the isoprene polymerization.

The terpolymerization of ethylene and propylene with isoprene via THF-containing half-sandwich scandium catalysts: a new kind of ethylene–propylene–diene rubber and its functionalization

A novel family of ethylene–propylene–isoprene terpolymers was prepared by the terpolymerization of ethylene and propylene with isoprene catalyzed by the half-sandwich scandium complexs.

Alpha and beta diimine cobalt complexes in isoprene polymerization: a comparative study

Isoprene was polymerized by diimine cobalt catalyst in the presence of DEAC to produce cis-1,4 and 3,4 polyisoprene.

Carbon bridged triphenolate lanthanide complexes: synthesis, characterization, DFT studies and catalytic activities for isoprene polymerization

Lanthanide complexes supported by carbon bridged triphenolate ligands were synthesized and theoretical calculations were carried out on a Lu complex.

Tuning the catalytic properties of rare earth borohydrides for the polymerisation of isoprene

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)). Neodymium-based 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.