Search

The reactions of mer-[Re(p-NTol)X3(PPh3)2] (X = Cl, Br) with chelating phenolate-based ligands (2-(2-hydroxy-5-methylphenyl)benzotriazole (HL(1)), 2-(2-hydroxyphenyl)benzothiazole (HL(2)) or 2-(2-hydroxyphenyl)benzoxazole (HL(3))) afforded a series of p-tolylimido rhenium(v) complexes cis- or trans-(X,X)-[Re(p-NTol)X2(L)(PPh3)]·yMeCN (where X = Cl, Br; L = L(1), L(2), L(3) and y = 0-2) and [Re(p-NTol)X(L)(PPh3)2]Z·pPPh3 (where X = Cl, Br; Z = ReO4, PF6; L = L(1), L(2), L(3) and p = 0 or 1). The reported compounds were characterized by elemental analysis, FT-IR, NMR ((1)H, (13)C and (31)P) and X-ray crystallography. Interestingly, the halide ions of [Re(p-NTol)Cl2(L(1))(PPh3)]·MeCN (1) and [Re(p-NTol)Cl2(L(2))(PPh3)]·2MeCN (3) are in cis relative dispositions, whereas the complexes [Re(p-NTol)Br2(L)(PPh3)] (L(1) for 2, L(2) for 4 and L(3) for 6) and [Re(p-NTol)Cl2(L(3))(PPh3)] (5) were found to be trans-(X,X) isomers. The compounds [Re(p-NTol)X(L)(PPh3)2](PF6) (X = Cl, Br; L = L(1) and L(2)) and [Re(p-NTol)X(L(3))(PPh3)2](PF6)·PPh3 (X = Cl, Br) have been tested in oxidative catalysis. A few compounds exhibited very good catalytic properties in oxidation of alcohols with tert-BuOOH (TBHP) in acetonitrile solution at moderate temperatures. Complex [Re(p-NTol)Cl(L(2))(PPh3)2]PF6 (13) is the catalyst of choice for oxidation of 1-phenylethanol to acetophenone (in 80% yield; turnover number attained 290 after 30 h) and cyclooctanol to cyclooctanone (in 88% yield). Notably lower activity has been found in the oxidation of alkanes with TBHP. Product distribution in the oxidation of methylcyclohexane indicates some steric hindrance around the reaction center.

Two new Re(iv) compounds of formulae H2tppz[ReCl6] (1) and [Cu(bpzm)2(μ-Cl)ReCl3(μ-ox)Cu(bpzm)2(μ-ox)ReCl3(μ-Cl)]n (2) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and bpzm = bis(pyrazolyl-1-yl)methane] have been prepared and their crystal structures determined by X-ray diffraction on single crystals. Compound 1 is a mononuclear species whose structure consists of octahedral hexachlororhenate(iv) anions and diprotonated H2tppz(2+) cations which are arranged in the unit cell as alternating anionic and cationic layers, held together by electrostatic forces. The structure of 2 is made up of alternating [Cu(1)(bpzm)2](2+) and [(ox)ReCl3(μ-Cl)Cu(2)(bpzm)2(μ-Cl)ReCl3(ox)](2-) entities interlinked by oxalate bridges to afford a neutral heterobimetallic chain. The oxalate group adopts the didentate (at Re)/monodentate (at Cu) bridging mode. The magnetic behavior of 1 and 2 has been investigated over the temperature range 1.9-295 K. 1 is a magnetically diluted Re(iv) complex, the relatively large value of the zero-field splitting of the ground level [D = -15.8(2) cm(-1)] accounting for the variation of χMT in the low temperature range. Weak intrachain ferromagnetic interactions between Re(iv) and Cu(ii) through oxalate (J1 = +0.15 cm(-1)) and single chloro (J2 = +4.9 cm(-1)) bridges occur in 2 which are obscured by the large zero-field splitting of the Re(iv) ion (DRe = 42 cm(-1)). In addition, interchain antiferromagnetic interactions are also involved in 2 which are responsible for the metamagnetic behavior observed, the value of the critical dc magnetic field (Hc) being 20 kOe.

Novel p-tolylimido rhenium(v) complexes trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(pyz-2-COO)(PPh3)]·MeCN (1), trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(pyz-2-COO)(PPh3)] (2), trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(pyz-2-COO)(PPh3)] (3), cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(ind-3-COO)(PPh3)]·2MeOH (4), 2[Re(p-NC6H4CH3)Cl2(ind-3-COO)(PPh3)]·MeCN (5), 2[Re(p-NC6H4CH3)Br2(ind-3-COO)(PPh3)]·MeOH (6) and 2[Re(p-NC6H4CH3)Br2(ind-3-COO)(PPh3)]·MeCN (7) were obtained in the reactions of [Re(p-NC6H4CH3)X3(PPh3)2] (X = Cl, Br) with pyrazine-2-carboxylic (pyz-2-COOH or PCA) and indazole-3-carboxylic (ind-3-COOH) acids. The compounds were identified by elemental analysis, IR, (1)H, (13)C and (31)P NMR spectroscopy and X-ray crystallography. To get a deeper understanding of the structural and bonding properties of the imido rhenium(v) complexes, calculations at the DFT level were undertaken for trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(pyz-2-COO)(PPh3)] and cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(ind-3-COO)(PPh3)]. Complexes 1, 3, 4 and 6 exhibited high catalytic activity in oxidation of alkanes with H2O2 and tert-butyl hydroperoxide (TBHP) and of alcohols with TBHP. The selectivity parameters measured in the reactions with linear and branched alkanes indicated that the processes with H2O2 or TBHP proceed with the participation of hydroxyl or tert-butoxyl radicals, respectively. The composition of isomers from oxygenation of methylcyclohexane corresponds to the existence of some steric hindrance around the reaction centers.

Novel p-tolylimido rhenium(V) complexes trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]·MeCN (1a), trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (1b), trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)] (2), cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (3), cis-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)]·H2O (4), trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(OMe)] (5) and trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(OMe)(PPh3)2]·1/2H2O (6) were synthesized. The compounds were identified by elemental analysis, IR, NMR ((1)H, (13)C and (31)P), UV-Vis spectroscopy and X-ray crystallography. Calculations at the DFT level were undertaken in order to elucidate the structural, spectroscopic and bonding properties of the trans-(Cl,Cl) and cis-(Cl,Cl) isomers of [Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]. In the first step, extended tests of the ability of different DFT methods (B3LYP, BP86, and PBE1PBE) were performed to find the best overall performer in describing the geometry of the imido Re(v) complexes; then, the electronic spectra of the trans and cis isomers of [Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] were investigated at the TDDFT level employing the PBE1PBE functional. Additional information about bonding in the compounds [Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] was obtained by NBO analysis. The catalytic activity of the complexes incorporating the picolinate ligand was studied and compared in the synthesis of N-substituted ethyl glycine esters from ethyl diazoacetate and amines.