Dioxo-, Oxo-imido-, and Bis-imido-Molybdenum(VI) Complexes with a Bis-phenolate-NHC Ligand

Abstract

We report the facile synthesis of five rare high-valent Mo­(VI)-NHC complexes with a dianionic OCO-pincer benzimidazolylidene ligand (L1) with the general formula L1MoO2 (1), L1Mo­(O)­(NtBu)­(THF) (2), L1Mo­(NtBu)­(NHtBu)Cl (3), L1Mo­(NtBu)2 (4), and L1 Mo­(NMes)2 (5). These complexes represent a complete series of high-valent molybdenum­(VI) complexes, obtained by consecutive exchange of the oxo for imido ligands from the dioxo complex (1) to the bis-imido complexes (4 and 5). The complexes can be synthesized in high yields following a simple deprotonation protocol, using either triethylamine or lithium diisopropylamide (LDA) as a base. All complexes have been characterized by various techniques, including 1H, 13C­{1H}, and 15N NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis. Except for the dioxo complex 1, all complexes adopt monomeric structures in the solid state. Compounds 1, 4, and 5 are rare examples of five-coordinated molybdenum­(VI) complexes in the solid state, while 2 is six-coordinated, having a weakly bound THF ligand in the axial position. The dioxo complex 1 was found to be stable under air and moisture in the solid state as well as in solution for several days. Furthermore, preliminary reactivity studies and an approximate determination of the pK a value of the imido ligands in 4 are presented.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…This low‐basicity concept is also in line with reports by Hohloch et al. on the protolytic stability of Mo‐imido NHC complexes . We chose triflate (p K a,HOTf (H 2 O)=−12), nonafluoro‐ tert ‐butoxide (p K a,HOR (H 2 O)=5.4, R=C(CF 3 ) 3 ) and pentafluorobenzoate (p K a,HOOCR (H 2 O)=1.48, R=C 6 F 5 ) as the most promising X‐type ligands.…”

“…This low-basicity concept is also in line with reports by Hohloch et al on the protolytic stability of Mo-imido NHC complexes. [25] We chose triflate (pK a,HOTf (H 2 O) = À12), [26] nonafluoro-tert-butoxide (pK a,HOR (H 2 O) = 5.4, R = C(CF 3 ) 3 ) [26] and pentafluorobenzoate (pK a,HOOCR (H 2 O) = 1.48, R = C 6 F 5 ) [27] as the most promising X-type ligands. In view of its excellent s-donating properties (TEP = 2050.5 cm À1 ), we also decided to employ 1,3-dimesitylimidazol-2-ylidene as the NHC-type ligand.…”

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

“…To the best of our knowledge, 16 represents the rst structurally characterized example of a group 5 transition metal complex bearing both a terminal imido and a terminal oxo group (although many examples of related group 6, 7 and 8 complexes have been reported). 15,16,[87][88][89][90][91][92][93][94] In contrast to the 1 H NMR spectrum of 14, that of 16 displayed sharp signals at room temperature, consistent with a more strongly bound DMAP ligand. The X-ray crystal structure of 16 (Fig.…”

Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

“…[65] Hohloch's group reported a few 5/6-coordinate complexes (154-157) of high-valent molybdenum bearing a NHC-bridged bis(phenolate) pincer ligand and oxo/imido ancillary donors (Scheme 46). [66] Interestingly, in the case of 156, although a bis (imido) precursor was used, an amido-imido mixed complex was obtained, indicating NEt is not basic enough. Indeed, the deprotonation of by LDA or KHMDS led to the formation of bis(imido) complex 157, and the reverse protonation reaction is also feasible.…”