Synthesis and characterization of heptacoordinated molybdenum(ii) complexes supported with 2,6-bis(pyrazol-3-yl)pyridine (bpp) ligands

Abstract

Functional pincer ligands that engage in metal–ligand cooperativity and/or are capable of redox non-innocence have found a great deal of success in catalysis. These two properties may be found in metal complexes of the 2,6-bis(pyrazol-3-yl)pyridine (bpp) ligands. With this goal in mind, we have attempted the coordination of 2,6-bis(5-trifluoromethylpyrazol-3-yl)pyridine (L^CF3) and its ^tBu analogue 2,6-bis(5-tert-butylpyrazol-3-yl)pyridine (L^tBu) to Mo(0) by reactions with mixed phosphine/carbonyl complexes [Mo(CO)₂(MeCN)_n−1(PMe_3−nPh_n)_5−n] 1–3 (1 ≤ n ≤ 3). These afforded mixtures of several Mo compounds among which low yields of heptacoordinated Mo(II) complexes [Mo(CO)₂(bpp)(PMe_3−nPh_n)₂] 4a–c (L^CF3-supported) and 5a–c (L^tBu-supported) bearing a doubly deprotonated bpp ligand were systematically produced. More selective syntheses of 4a–c and 5a–c were achieved by repeating these experiments in the presence of an oxidant (AgOAc or Ag₂O), with moderate to good yields. 4a–c and 5a–c were characterized by means of NMR, IR and UV-Vis spectroscopies, sc-XRD and cyclic and square-wave voltammetries for 4a, 4b and 5b. The deprotonated L^tBu ligand in 5a–c is re-protonated with 2 equiv. of HOTf to afford the dicationic [Mo(CO)₂(L^tBu)(PMe_3−nPh_n)₂][OTf]₂ complexes 6a–c. Acidic treatment of 4a–c led to the decomposition of the complexes.