Bis(acetylacetonate) Tungsten(IV) Complexes Containing a π-Basic Diazoalkane or Oxo Ligand

Tungsten­(IV) bis­(acetylacetonate) (acetylacetonate =
acac) complexes
were synthesized by incorporating either a diazoalkane or an oxo ligand
into the coordination sphere of a tungsten­(II) reagent. The reaction
of free diazoalkane (N2CRR′) with W­(CO)3(acac)2 leads to loss of two carbon monoxide ligands and
coordination of the diazoalkane reagent through the terminal nitrogen
to produce W­(CO)­(acac)2(N2CRR′). This
monomer is best formulated as a tungsten­(IV) complex. A second example
of converting a d4 tungsten carbonyl complex to a d2 product involves oxidation of W­(CO)­(acac)2(PhCN)
with m-chloroperoxybenzoic acid (MCPBA) to replace
the CO ligand with an oxygen atom. This increase in metal oxidation
state causes rotation of the nitrile ligand by 90° relative to
the two bidentate acac ligands. Electrophilic addition at the nitrogen
of the π-bound nitrile ligand using methyl triflate (MeOTf)
and subsequent nucleophilic addition at carbon with sodium trimethoxyborohydride,
Na­[HB­(OMe)3], reduces the CN bond stereoselectively
and produces the neutral imine complex W­(O)­(acac)2(PhHCNMe)
with a diastereomeric ratio of 11:1.