N,N′-Dialkylation Catalyzed by Bimetallic Iridium Complexes Containing a Saturated Bis-N-Heterocyclic Carbene (NHC) Ligand

Reaction of the bis-aminophosphinimine [m-C6H4(HNCH2CH2NPPh3)] with W­(CO)6 afforded [m-C6H4{(CNCH2CH2NH)­W­(CO)5}2] (2), which underwent N-alkylation
with benzyl bromide to yield [m-C6H4{(CNCH2CH2NCH2Ph)­W­(CO)5}2] (3). A carbene transfer reaction
from W(0) to Ir­(I) proceeded smoothly via the reaction of 3 with [Ir­(COD)­Cl]2 under mild conditions to give the diiridium­(I)
carbene complex [m-C6H4{(CNCH2CH2NH)­Ir­(CO)2Cl}2] (4). Ligand substitution of 4 with an excess of
PPh3 produced the phosphine complex 5. All
complexes have been characterized by spectroscopic and elemental analyses.
Complexes 2 and 5 were further confirmed
by X-ray diffraction studies. Complex 4 is an efficient
catalyst for the reductive N,N′-dialkylation of phenylenediamines
with alcohols. The mechanistic pathway of the catalysis involving
the possible synergistic effect between two metal centers is discussed.