Peripheral Methyl Activation in η<sup>4</sup>‑1,2,3,4-Tetramethylcyclobutadienylcobalt Complexes: Template
Synthesis and Subsequent Reactivity of Triphosphamacrocycles
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Abstract
The cationic complex (η<sup>4</sup>-1,2,3,4-tetramethylcyclobutadienyl)cobalt(trisacetonitrile),
[(η<sup>4</sup>-C<sub>4</sub>Me<sub>4</sub>)Co(NCCH<sub>3</sub>)<sub>3</sub>]<sup>+</sup> (<b>1</b>), allows the stepwise
introduction of suitable phosphine precursors to the [(η<sup>4</sup>-C<sub>4</sub>Me<sub>4</sub>)Co]<sup>+</sup> fragment by replacement
of the labile acetonitrile ligands. These reactions give rise to the
piano-stool complexes [(η<sup>4</sup>-C<sub>4</sub>Me<sub>4</sub>)Co(dppe)(NCCH<sub>3</sub>)]<sup>+</sup> (<b>2</b>), [(η<sup>4</sup>-C<sub>4</sub>Me<sub>4</sub>)Co(dppe)(PH<sub>2</sub>Ph)]<sup>+</sup> (<b>3</b>), [(η<sup>4</sup>-C<sub>4</sub>Me<sub>4</sub>)Co(dfppb)(NCCH<sub>3</sub>)]<sup>+</sup> (<b>4</b>), and [(η<sup>4</sup>-C<sub>4</sub>Me<sub>4</sub>)Co(dfppb)(PH<sub>2</sub>Ph)]<sup>+</sup> (<b>5</b>), where dfppb = 1,2-bis{di(2-fluorophenyl)phosphino}benzene
and dppe = 1,2-bis(diphenylphosphino)ethane. Complex <b>5</b> is a template for the synthesis of the P<sub>3</sub> macrocycle
complex [(η<sup>4</sup>-C<sub>4</sub>Me<sub>4</sub>)Co{1,4-bis(2-fluorophenyl)-7-phenyl[<i>b</i>,<i>e</i>,<i>h</i>]tribenzo-1,4,7-triphosphacyclononane}]<sup>+</sup> (<b>6</b>), through base-promoted intramolecular macrocyclization.
The hydrogens of two of the ring methyls of the tetramethylcyclobutadienyl
ligand in the macrocycle complex <b>6</b> are sufficiently acidic
to undergo deprotonation by KO<sup>t</sup>Bu, promoting nucleophilic
attack at the fluorine-bearing <i>ortho</i>-carbons of the
2-fluoroaryl groups on two of the phosphorus donors in <b>6</b>. The resultant hemi-incarcerand complex [{η<sup>4</sup>,κ<i>P</i>,κ<i>P</i>,κ<i>P</i>-Me<sub>2</sub>C<sub>4</sub>-[1,4-bis(2-CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>)-7-C<sub>6</sub>H<sub>5</sub>-[<i>b</i>,<i>e</i>,<i>h</i>]tribenzo-1,4,7-triphosphacyclononane]-1,2}Co]<sup>+</sup> (<i>cis</i>-<b>7</b>) contains a hybrid phosphorus/carbon
donor ligand where the P<sub>3</sub> macrocycle is connected to the
cyclobutadienyl function through two <i>cis</i>-2-methylphenyl
links. The new complexes have been characterized fully by spectroscopic
and analytical techniques including single-crystal X-ray structure
determinations of <b>2</b>, <b>3</b>, <b>4</b>, <b>5</b>, <b>6</b>, and <i>cis</i>-<b>7</b>
