Abstract
The synthesis of molybdenum(0) tricarbonyl and tetracarbonyl complexes of the form [Mo(CO)3(ptapzpy)Br] (1) and cis-[Mo(CO)4(ptapzpy)]Br (2) is reported, where ptapzpy = 2-(1-propyltrimethylammonium-3-pyrazolyl)pyridine. Preparation of these derivatives was accomplished either through thermal replacement of CO in Mo(CO)6 (for 1) or substitution under milder conditions of piperidine ligands in the precursor cis-[Mo(CO)4(pip)2] (for 2). The crystal structures of the ligand [ptapzpy]Br and complexes 1 and 2 were determined. Thermal treatment of 2 at 125–150 °C leads to mono decarbonylation and formation of 1. On the other hand, oxidative decarbonylation of 1 and 2 by reaction with tert-butylhydroperoxide (TBHP, 10 equiv.) gives a molybdenum oxide hybrid material formulated as [Mo3O9([ptapzpy]Br)2]·nH2O (3), which was characterised by FT-IR and Raman spectroscopy, thermogravimetric analysis, and 13C{1H} CP MAS NMR spectroscopy. Compounds 1–3 were effective (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with aqueous H2O2 or TBHP (slightly better results were obtained with the former). The characterisation of the Mo-containing solids isolated after the catalytic reaction showed that poorly soluble β-octamolybdate salts, (L)x[Mo8O26], were formed from 1–3 with TBHP and from 1 with H2O2, while soluble oxoperoxo species were formed from 3 with H2O2. These findings helped to explain the different catalytic performances obtained.
Highlights
The unusual direct synthesis of the tricarbonyl complex from the hexacarbonyl and the ligand results from the presence of the terminal trimethylammonium group in the ligand together with the bromide counterion, which favour the formation of the charge-neutral zwitterionic complex
Dissolution of the compounds in polar, coordinating solvents such as water, acetonitrile and dimethyl sulfoxide is accompanied by solvolysis (especially for 1, as reported previously in ref. 9b for other tricarbonyl anions [Mo(CO)3(N–N)(X)]À) and fast degradation involving decarbonylation
The FT-IR spectra of freshly prepared 1 and 2 con rmed the formation of tricarbonyl and tetracarbonyl complexes, respectively (Fig. S4 in the Electronic supplementary information (ESI)†)
Summary
The organometallic chemistry of molybdenum dates back to the mid 1930s when Hieber and co-workers described the thermal substitution of CO ligands in Mo(CO)[6] to give octahedral Mo0 derivatives of the type Mo(CO)x(L)y.1 The physical properties of one of these compounds, cis-[Mo(CO)4(bipy)] (bipy 1⁄4 2,20bipyridine), were investigated much later by Stiddard,[2] and the Complexes of the type [Mo(CO)3(N–N)(X)]À can be prepared from cis-[Mo(CO)4(N–N)]9b but not in the reverse sense from the pentacarbonyl complexes [Mo(CO)5(X)]À, since reaction of the latter with the ligand N–N gives instead the tetracarbonyl derivatives cis-[Mo(CO)4(N–N)].11 This was attributed to preferential halogen displacement (rather than CO substitution) in the intermediate [Mo(CO)4(N–N)(X)]À owing to the fact that 16294 | RSC Adv., 2018, 8, 16294–16302Paper bidentate diimines such as bipy are relatively poor p-acceptors, thereby strengthening the remaining Mo–C bonds in the intermediate.[11]. The main crystallographic features of the {MoC4N2} core of complex 2 are consistent with those observed in the handful of structures reported containing molybdenum tetracarbonyl complexes with chelating 2-(3-pyrazolyl)pyridine residues.[18] The Mo0 centre is coordinated by four carbonyl groups and one N,Nchelating pyrazolylpyridine ligand, originating a distorted pseudo-octahedral geometry as con rmed by the distinct Mo–C and Mo–N bond lengths (Fig. 1e).
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