Tuning of Cu-Al Interactions in Complexes Derived from Tris(pyridonyl-6-methyl)aluminum Metalloligands.

A series of bioinspired polar atrane Cu-Al complexes were studied with a combined experimental and computational approach to assess the range and nature of Cu-Al interactions in these novel species. The aluminum metalloligand [Na{Me2Al(OPy-6-Me)2}] (2) was furnished in excellent yield (92%) from the nucleophilic attack of Na(OPy-6-Me) to AlMe3 and the subsequent alkane elimination reaction with 6-methyl-2-hydroxypyridine. At the same time, the metalloligand [Al(OPy-6-Me)3] (3) was isolated in an also excellent yield (95%) via alkane elimination of AlMe3 with 6-methyl-2-hydroxypyridine. The zwitterionic Cu-Al atranes [Cu{MeAl(OPy-6-Me)3}] (5Me) and [Cu{MesAl(OPy-6-Me)3}] (5Mes) were isolated (73 and 97% yields) from metalloligands 2 and 3, respectively. [(Cu{Al(OPy-6-Me)4})2(µ-Cu)]+ ([6+][B(ArCF3)4]) was isolated via a reaction that involves alkane elimination and redistribution reacting from 5Me with [H(OEt2)2][B(ArCF3)4] in benzene solution. Alkane elimination in benzene of either 5Me or 5Mes with [HNEt3][B(ArCF3)4] renders [Cu{(Et3N)Al(OPy-6-Me)3}]+ (Et3N-5+). The Lewis base-free cationic complex [Cu{Al(OPy-6-Me)3}]+ (5+) was isolated in 68% yield upon reacting 3 with [Cu(COD)2][B(ArCF3)4] in benzene. Metalloligands and complexes were fully characterized with an array of spectroscopic and analytical techniques that include multinuclear NMR, ATR-IR, ESI-spectrometry, combustion microanalysis, cyclic voltammetry (CV), and, whenever feasible, SCXRD. X-ray and DFT parameters indicate that the strength of the Cu→Al transannular interaction follows the trend 5+ > Et3N-5+ > [6+][B(ArCF3)4], 5Me, and 5Mes in a smooth transition from zwitterionic species where the Cu-Al interaction is nonexistent to moderate Cu-Al Z-type interactions. CV, in conjunction with DFT calculations of Et3N-5+ and 5+, hint at the generation in the electrochemical cell of the radical species 5rad at -1.82 V and the anionic complex 5- at -2.32 V vs Fc/Fc+, respectively. The proposed species 5rad exhibits 2-center/1-electron (2c/1e) σ bonding whereas 5- a 2-center/2-electron (2c/2e) bond.

Cyclooctadiene Rh(I) Bis- and Tris(pyrazolyl)aluminate Complexes and Their Catalytic Activity on the Polymerization of Phenylacetylene.

In this work, we report the transfer of alkyl bis- and tris(pyrazolyl)aluminates metalloligands to an electron-rich organotransition metal center. The 16-electron heterobimetallic complexes of rhodium [Rh(COD){Al(Ph2pz)2Me2}] (3) and [Rh(COD){Al(Ph2pz)3Me}] (4) were obtained by metathesis reaction of the sodium bis- (1) and tris(pyrazolyl)aluminate (2) with [RhCl(COD)]2. For 3, 1H and 13C NMR in solution along with DFT calculations are consistent with a κ2-coordination mode of the bis(pyrazolyl)aluminate to a square-planar Rh(I) center. The X-ray structure of 4 shows a similar κ2-coordination mode of the tris(pyrazolyl)aluminate to Rh(I) with a pendant pyrazolyl moiety. The attempted synthesis of aluminate-rhodium complexes with R = CF3, tBu on the pyrazolate ring afforded [Rh(R2pz)(COD)]2 and [R2pzAlMe2]2. Complexes 3 and 4 were investigated as homogeneous catalysts in the polymerization of phenylacetylene (PA). Both complexes showed enhanced catalytic activity compared to analogous rhodium poly(pyrazolyl)borates. Optimized gas-phase DFT geometries of 3, 4, [Rh(COD){B(Ph2pz)2Me2}], and [Rh(COD){B(Ph2pz)3Me}] were used to compare bite angles, while DFT geometries of 3-CO, 4-CO, [Rh(CO)2{B(Ph2pz)2Me2}], and [Rh(CO)2{B(Ph2pz)3Me}] were employed to probe the electronic situation of the rhodium center through IR CO stretching modes. The wider bite angles and the less electron-rich rhodium center of the poly(pyrazolyl)aluminates compared with their borate analogues could be implicated in the better performance of the active catalytic species during polymerization of PA.

Lithium Complexes Derived of Benzylphosphines: Synthesis, Characterization and Evaluation in the ROP of rac-Lactide and ε-Caprolactone.

A series of lithium complexes ([Ph2P(o-C6H4-CH2Li·TMEDA)] (1-Li), [PhP(o-C6H4-CH3)(o-C6H4-CH2Li·TMEDA)] (2-Li), [PhP(o-C6H4-CH2Li·TMEDA)2] (2-Li2) and [P(o-C6H4-CH2Li·TMEDA)3] (3-Li3)) was prepared from mono-, di- and tri-benzylphosphines and varying amounts of nBuLi and was characterized extensively by IR and ¹H, 7Li, 13C and 31P NMR spectroscopy. The molecular structures of complexes 1-Li and 2-Li were determined by single-crystal X-ray diffraction studies. The two complexes have monomeric structures in the solid state comprising seesaw lithium atoms. In each case, the ligand exhibits an asymmetric C-C η²-coordination mode and an intramolecular P-Li bond interaction. Theoretical calculations at Density functional theory (DFT) level M06/6111+G(2d,p) show that indeed a P-Li bond is established which can be explained as the P lone pair (sp1.26) being partially delocalized on an available sp² orbital on Li (sp2.04) and additional bonding contribution of the phosphorous atom to Li stems from further delocalization of a σ P-C orbital into the sp² orbital on Li. The observed short contact distances between an aromatic ipso carbon and Li in the crystal structures of 1-Li and 2-Li are explained as due to the interaction of a σ C-Li orbital into the π* orbital of a C-C aromatic bond. Preliminary tests show compounds 1-Li, 2-Li, 2-Li2 and 3-Li3 are active catalysts in the solvent free ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). High conversions to polycaprolactones were obtained in short periods of time: 1-6 min at 25 °C. Additionally, all four lithium complexes behave as moderately good initiators for the ROP of rac-LA showing high conversions to polylactides at 140 °C in one hour.

Calix[4]arenes of aluminum and gallium with benzimidazolyl ligands: steric control of the conformation via substitution on the ligand.

Complexes [bzimAlR(2)](4) [bzim = benzimidazolate; R = Et (2), (i)Bu (3)], [mbzimAlR(2)](4) [mbzim = 2-methylbenzimidazolate; R = Et (6), (i)Bu (7)], [dmbzimAlR(2)](4) [dmbzim = 5,6-dimethylbenzimidazolate; R = Me (9), Et (10), (i)Bu (11)], and [tmbzimAlR(2)](4) [tmbzim = 2,5,6-trimethylbenzimidazolate; R = Me (12), Et (13), (i)Bu (14)] have been prepared via alkane elimination and coordinative self-assembly upon the reaction of benzimidazole ligands with aluminum alkyls in benzene, toluene, or xylene. Characterization of the complexes was achieved by spectroscopic methods, microanalysis, and X-ray crystallography of 2, 7, 10, 11, 13, and 14. The complexes reported herein and the aluminum and gallium analogues 1, 4, 5, and 8 reported in a previous paper (1) are predominantly tetranuclear aggregates related to calix[4]arenes in which the benzimidazolyl ligands bind two metal atoms in a η(1):η(1) fashion. X-ray crystallography demonstrates that modulation of the conformation adopted by these metallacalix[4]arenes is achieved by proper substitution on the C atom at the 2 position of the benzimidazolyl ligand. An H substituent for 1, 2, 4, 10, and 11 favors a chair conformation with a small cavity and approximate C(2h) symmetry, while a CH(3) substituent for 5, 7, 8, 13, and 14 introduces enough repulsion to switch the conformation to a 1,3-alternate or double cone with a concomitant larger cavity and approximate C(2v) symmetry.

trans-Hydroboration-oxidation products in Δ5-steroids via a hydroboration-retro-hydroboration mechanism.

Herein, we report for the first time a "trans-hydroboration-oxidation product" isolated and characterized under traditional hydroboration-oxidation conditions using cholesterol and diosgenin as substrates. These substrates are excellent starting materials because of the rigidity and different structural environments around the double bond. Further investigations based on experimental evidence, in conjunction with theoretical studies, indicate that the formation of this trans-species occurs via a retro-hydroboration of the major product to generate the corresponding Δ6-structure and the subsequent hydroboration by the ß-face. Besides, the corresponding Markovnikov type products have been isolated in synthetically useful yields. The behavior of the reaction under a range of temperatures is also investigated.

Do planar tetracoordinate tin complexes really exist?

New experimental and theoretical data show that the previously reported planar tetracoordinate Sn complexes [Sn(Ph2P(Se)N(Se)PPh2)2] (1-sq) and [Sn(iPr2P(Se)N(Se)PiPr2)2] (2-sq) were not isolated and rather confused with the selenium complexes [Se(Ph2P(Se)N(Se)PPh2)2] (3-sq) and [Se(iPr2P(Se)N(Se)PiPr2)2] (4-sq), respectively.

Steric control of conformation in self-assembly metallacalix[4]arenes derived from AlMe3 or GaMe3 with benzimidazolyl ligands.

Metallacalix[4]arenes [bzimAlMe(2)](4) (1), [bzimGaMe(2)](4) (2), [mbzimAlMe(2)](4) (3) and [mbzimGaMe(2)](4) (4) were obtained by self-assembly. The larger cavity of 3 and 4 allows them to function as hosts of benzene molecules.

Aluminium complexes derived from tridentate thioetherbis(phenolate) ligands and their activity in Diels-Alder catalysis.

Organoaluminium(III) thioetherbis(phenolate) complexes derived from 2,2-thiobis(4,6-diterbuthylphenolate) (Stdiol) and 2,2-thiobis(4,6-dimethylphenolate) (Smdiol) were prepared by reaction of AlMe3 with the diol proligands LH2 (SmdiolH2, StdiolH2). Monomeric complexes of general formulae [LAlR(L)] (L = Stdiol, R = Me, L = THF (1); L = Stdiol, R = Me, L = Et2O (2); L = Stdiol, R = iBu, L = THF) (3); L = Smdiol, R = Me, L = THF (4); L = Smdiol, R = iBu, L = THF (5)) and [LAlCl(THF)] (L = Stdiol (6); L = Smdiol (7)) were obtained when the reactions were performed in THF or Et2O in hexane or toluene, species of formulae [(L2Al)AlR2] (L = Stdiol, R = Me (8); L = Stdiol, R = iBu (9); L = Smdiol, R = Me (10)) were formed as evidenced by spectroscopic methods. Crystals suitable for X-ray diffraction studies were obtained for 2, 3, 4, 6, 8, 9 and 10. In these cases the sulfur atom coordinates to the aluminium center with Al-S bond lengths between 2.43-2.75 A adopting boat-boat conformations. Compounds 1, 3, 4, and 8 were tested in the catalytic Diels-Alder cycloaddition reaction between methacrolein and cyclopentadiene showing excellent regioselectivities with good conversion yields.

Reactivity of Zn(II), Mg(II) and Al(III) chlorides with a phosphinimine ligand: new tetrameric inverse crown ether structures.

Reaction of [Me3SiNPPh2CH3] 1 with ZnCl2 yields the dimer [ClZn(mu-Cl)(Me3SiNPPh2CH3)]2 2 whereas treatment of 1 with AlCl3 in THF leads to the monomeric N adduct [Cl3Al(Me3SiNPPh2CH3)] 3. Compounds 2 and 3 were found to be thermally stable and were fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analysis. On the other hand, treatment of phosphinimine 1 with (n)BuLi/ZnCl2 or (CH3)2CHMgCl yields the zinc and magnesium complexes [M4(Me3SiNPPh2CH2)4(mu4-O)(mu2-Cl)2] 5 (M = Zn) and 6 (M = Mg), respectively. These compounds can be considered as new examples of inverse crown ethers in which the oxygen atom is encapsulated by polar organometallic complexes in a tetrameric arrangement. In contrast, reaction of 1 with nBuLi/AlCl3 in Et2O under inert atmospheric conditions leads to the formation of the dimeric species [Cl2Al(Me3SiNPPh2CH2O)]2 7, which displays oxygen insertion into the C-Al bond, while the same reaction in the presence of air yields the compound [mu-(AlCl2)(NPPh2CH3)]2 8 with loss of ClSiMe3 and without oxygen insertion into the C-Al bond.

3,5-Dimethyl and 3,5-di-tert-butylpyrazolato complexes with alkali metals: monomeric, dimeric, cluster, and 1D chain structures.

The pyrazolato complexes [(Me(2)pz)(THF)Li] (1), [((t)Bu(2)pz)Li](4) (2), [((t)Bu(2)pzH)((t)()Bu(2)pz)Li](2) (2a), [(Me(2)pz)Na] (3), [((t)Bu(2)pz)Na](4), [((t)Bu(2)pz)(6)(OH)Na(7)] (4a), [((t)Bu(2)pz)(18-crown-6)Na] (4b), and [((t)Bu(2)pz)K] (5) were synthesized by metalation reactions between R(2)pzH (R = Me, (t)()Bu) and alkyllithium, elemental sodium, or potassium. All the complexes were characterized by spectroscopic methods and microanalysis, and in addition, the crystal structures of 2, 2a, 3, 4a, 4b, and 5 were obtained by single-crystal X-ray diffraction. They show monomeric, dimeric, cluster, and 1D chain structures in the solid state. Ab initio calculations on the structure and stabilities of the monomeric pzM complexes were performed at the MP2 level of theory showing good agreement with the coordination preferences of the pyrazolato ligand to a particular alkali ion.

Synthesis of new 1,3-oxaphosphorinanium salts. Stereochemistry of hydroxide-induced displacement of methoxide ion.

[reaction: see text] The synthesis of pure cis- and trans-3-methoxy-2,2,6-trimethyl-3-phenyl-1,3-oxaphosphorinanium tetrafluoroborate salts 3a and 3b, respectively, molecules designed to evaluate the effect of oxygen on the steric course of base-induced nucleophilic displacement of the methoxy group at phosphorus, was accomplished. It was found that these isomeric salts react with aqueous sodium hydroxide to produce the corresponding phosphine oxides 7a and 7b with complete retention of configuration at phosphorus.

Dichloro and alkylchloro gallium derivatives of dichalcogenoimidodiphosphinate ligands: isolation of a spirogallium cation.

Dichloro and chloromethyl Ga(III) complexes of general formulae [XClGa-eta2-{R2P(E)NP(E'R'2-E,E'}](X = Cl, R, R'= Ph, E, E'= O (1), S (2), Se (3); R = Ph, R'= OEt, E = O, E'= S (4); R = Me, R'= Ph, E, E'= S (5) and X = Me, E, E'= O (6), S (7), Se (8)) were synthesised by either metathesis reactions between GaCl3 and the potassium salt of the ligand (X = Cl) or by methane eliminations from in situ prepared GaMe2Cl and the protonated ligands LH (X = Me). Redistribution reaction of (3) in either CDCl3 or THF afforded the solvent-free tetracoordinate gallium spirocycle cation [Ga-{eta2-{Ph2P(Se)NP(Se)Ph2-Se,Se'})2]+ (9+). The molecular structures of complexes 2, 4, 5, 7 and 9(+) show non-planar gallacycle rings.