Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

Abstract

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand (L 1 ) with the general formula [NbL 1 (N t Bu)PyCl] 1-Py. We achieved this by in situ deprotonation of the corresponding azolium salt [H 3 L 1 ][Cl] and subsequent reaction with [Nb(N t Bu)Py 2 Cl 3 ]. The pyridine ligand in 1-Py can be removed by the addition of B(C 6 F 5 ) 3 as a strong Lewis acid leading to the formation of the pyridine-free complex 1. In contrast to similar vanadium(V) complexes, complex 1-Py was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula [NbL 1 (N t Bu)Py(EMes)] (E = O (2), S (3), NH (4), and PH (5)). Furthermore, complex 1-Py can be converted to alkyl complex (6) with 1 equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center (7) in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to 1-Py gives the pyridine-free aryl complex 8, which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes 1-Py and 1 are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, e.g., KC 8 , K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including 1 H-, 13 C{ 1 H}-, and 1 H-15 N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

The Chemistry of Vanadium bis‐Phenolate NHC Complexes in Three Oxidation States

“…Although the low quality of the crystals does not allow for discussion of any bond metrics, the general connectivity could be confirmed, proving the identity of complex 13 as an alkyl complex (Figure S89). The THF co‐ligand being present in this complex further supports the theory that 1,2‐alkyl migrations, which are common in group IV NHC chemistry, [28] are less favored for group V metals [24] …”

“…Similar to complex B , the use of heavier donor atoms for example, sulfur or phosphorus in thiomesitolate and mesityl‐phosphanide resulted in complicated (paramagnetic) mixtures, from which no useful products could be isolated. Thus, the salt metathesis reactivity with complex A is even more limited compared to complex B or to niobium imido analogues recently investigated by us [24] . Probing the effect of halide displacement on the vanadium(V/IV) redox couple, we recorded cyclic voltammograms of 1 in dichloromethane showing one reversible reduction at −0.92 V vs. Fc/[Fc] + being anodically shifted by 550 mV compared to parent complex A (E 1/2 =−0.37 V vs. Fc/[Fc] + , see Figure 2, Table 1 and Figures S90–S92) [22] …”

“…Thus, the salt metathesis reactivity with complex A is even more limited compared to complex B or to niobium imido analogues recently investigated by us. [24] Probing the effect of halide displacement on the vanadium(V/IV) redox couple, we recorded cyclic voltammograms of 1 in dichloromethane showing one reversible reduction at À 0.92 V vs. Fc/ [Fc] + being anodically shifted by 550 mV compared to parent complex A (E 1/2 = À 0.37 V vs. Fc/[Fc] + , see Figure 2, Table 1 and Figures S90-S92). [22] Chemical reduction of 1 was either achieved via addition of decamethylcobaltocene (2) or potassium graphite (3) in the presence of 18-crown-6 as a scavenger for the free potassium ions in THF (Scheme 1).…”

“…The THF co-ligand being present in this complex further supports the theory that 1,2-alkyl migrations, which are common in group IV NHC chemistry, [28] are less favored for group V metals. [24] The vanadium(III) complex 8 readily reacts with organic azides to give the corresponding vanadium(V) imido complexes (Scheme 4). However, while this reaction works well for aryl azides such as phenyl azide, p-tolyl azide or p-fluorophenyl azide to give the corresponding aryl imido complexes 14-16, the reaction with TMS-azide and complex 8 gives an intractable mixture, strongly contrasting the chemistry observed with lowvalent vanadium(III) MIC complexes.…”