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Media type:
E-Article
Title:
Syntheses, Reactivity and DFT Studies of Group 2 and Group 12 Metal Complexes of Tris(pyrazolyl)methanides Featuring “Free” Pyramidal Carbanions
Contributor:
Bigmore, Helen R.;
Meyer, Jens;
Krummenacher, Ivo;
Rüegger, Heinz;
Clot, Eric;
Mountford, Philip;
Breher, Frank
Description:
<jats:title>Abstract</jats:title><jats:p>Reactions of HC(Me<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub> 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<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub> with MeMgX (X=Cl or Br) gave the half‐sandwich zwitterionic compounds [Mg(<jats:sup>Me</jats:sup>Tpmd)X] (X=Cl (<jats:bold>2</jats:bold>) or Br (<jats:bold>3</jats:bold>); <jats:sup>Me</jats:sup>Tpmd<jats:sup>−</jats:sup>=[C(Me<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub>]<jats:sup>−</jats:sup>). Addition of HCl to <jats:bold>2</jats:bold> gave the structurally characterised half‐sandwich compound [Mg{HC(Me<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub>}Cl<jats:sub>2</jats:sub>(thf)] (<jats:bold>4</jats:bold>). The zwitterionic sandwich compound [Mg(<jats:sup>Me</jats:sup>Tpmd)<jats:sub>2</jats:sub>] (<jats:bold>5</jats:bold>) formed in low yields in the reaction of MeMgX with HC(Me<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub> but was readily prepared from HC(Me<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub> and either Mg<jats:italic>n</jats:italic>Bu<jats:sub>2</jats:sub> or MgPh<jats:sub>2</jats:sub>. The structurally characterised compound <jats:bold>5</jats:bold> contains two “naked” sp<jats:sup>3</jats:sup>‐hybridised carbanions fully separated from the dicationic metal centre. Only by using MgPh<jats:sub>2</jats:sub> as starting material could the half‐sandwich compound [Mg(<jats:sup>Me</jats:sup>Tpmd)Ph(thf)] (<jats:bold>6</jats:bold>) be isolated. The zwitterionic sandwich compound <jats:bold>5</jats:bold> reacted with HOTf (OTf<jats:sup>−</jats:sup>=[O<jats:sub>3</jats:sub>SCF<jats:sub>3</jats:sub>]<jats:sup>−</jats:sup>) to form the dication [Mg{HC(Me<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub>}<jats:sub>2</jats:sub>]<jats:sup>2+</jats:sup> (<jats:bold>7</jats:bold><jats:sup>2+</jats:sup>), 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<jats:sub>2</jats:sub>pz)<jats:sub>3</jats:sub> reacted only slowly with ZnMe<jats:sub>2</jats:sub> (and not at all with ZnPh<jats:sub>2</jats:sub>) to form the half‐sandwich zwitterion [Zn(<jats:sup>Me</jats:sup>Tpmd)Me] (<jats:bold>8</jats:bold>), which contains a cationic methylzinc moiety separated from a single sp<jats:sup>3</jats:sup>‐hybridised carbanion. Density functional calculations on the zwitterions [M(<jats:sup>Me</jats:sup>Tpmd)Me] and [M(<jats:sup>Me</jats:sup>Tpmd)<jats:sub>2</jats:sub>] (M=Mg, Zn) revealed that the HOMO in each case is a <jats:sup>Me</jats:sup>Tpmd‐based carbanion lone pair. The κ<jats:sup>1</jats:sup><jats:italic>C</jats:italic> isomers of [M(<jats:sup>Me</jats:sup>Tpmd)Me] were calculated to be considerably less stable than their κ<jats:sup>3</jats:sup><jats:italic>N‐</jats:italic>bound counterparts, with the largest gain in energy for Mg due to the greater ease of electron transfer from metal to the <jats:sup>Me</jats:sup>Tpmd apical carbon atom on formation of the zwitterion. Moreover, the computed MC bond dissociation enthalpies of the κ<jats:sup>1</jats:sup><jats:italic>C</jats:italic> isomers of [M(<jats:sup>Me</jats:sup>Tpmd)Me] are considerably higher than expected by simple extrapolation from the corresponding computed HC bond dissociation enthalpy.</jats:p>