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Syntheses, Reactivity and DFT Studies of Group 2 and Group 12 Metal Complexes of Tris(pyrazolyl)methanides Featuring “Free” Pyramidal Carbanions
Author(s) -
Bigmore Helen R.,
Meyer Jens,
Krummenacher Ivo,
Rüegger Heinz,
Clot Eric,
Mountford Philip,
Breher Frank
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200800181
Subject(s) - chemistry , carbanion , reactivity (psychology) , zwitterion , magnesium , medicinal chemistry , metal , dication , reagent , crystallography , stereochemistry , organic chemistry , molecule , medicine , alternative medicine , pathology
Reactions of HC(Me 2 pz) 3 with Grignard reagents, dialkyl magnesium compounds and dimethylzinc are reported, together with a DFT study on some of the aspects of this chemistry. Reactions of HC(Me 2 pz) 3 with MeMgX (X=Cl or Br) gave the half‐sandwich zwitterionic compounds [Mg( Me Tpmd)X] (X=Cl ( 2 ) or Br ( 3 ); Me Tpmd − =[C(Me 2 pz) 3 ] − ). Addition of HCl to 2 gave the structurally characterised half‐sandwich compound [Mg{HC(Me 2 pz) 3 }Cl 2 (thf)] ( 4 ). The zwitterionic sandwich compound [Mg( Me Tpmd) 2 ] ( 5 ) formed in low yields in the reaction of MeMgX with HC(Me 2 pz) 3 but was readily prepared from HC(Me 2 pz) 3 and either Mg n Bu 2 or MgPh 2 . The structurally characterised compound 5 contains two “naked” sp 3 ‐hybridised carbanions fully separated from the dicationic metal centre. Only by using MgPh 2 as starting material could the half‐sandwich compound [Mg( Me Tpmd)Ph(thf)] ( 6 ) be isolated. The zwitterionic sandwich compound 5 reacted with HOTf (OTf − =[O 3 SCF 3 ] − ) to form the dication [Mg{HC(Me 2 pz) 3 } 2 ] 2+ ( 7 2+ ), which was structurally characterised. Pulsed field gradient spin‐echo (PGSE) diffusion NMR spectroscopy revealed both compounds to be intact in solution. In contrast to the magnesium counterparts, HC(Me 2 pz) 3 reacted only slowly with ZnMe 2 (and not at all with ZnPh 2 ) to form the half‐sandwich zwitterion [Zn( Me Tpmd)Me] ( 8 ), which contains a cationic methylzinc moiety separated from a single sp 3 ‐hybridised carbanion. Density functional calculations on the zwitterions [M( Me Tpmd)Me] and [M( Me Tpmd) 2 ] (M=Mg, Zn) revealed that the HOMO in each case is a Me Tpmd‐based carbanion lone pair. The κ 1 C isomers of [M( Me Tpmd)Me] were calculated to be considerably less stable than their κ 3 N‐ bound counterparts, with the largest gain in energy for Mg due to the greater ease of electron transfer from metal to the Me Tpmd apical carbon atom on formation of the zwitterion. Moreover, the computed MC bond dissociation enthalpies of the κ 1 C isomers of [M( Me Tpmd)Me] are considerably higher than expected by simple extrapolation from the corresponding computed HC bond dissociation enthalpy.