Bulky <i>N</i>‑Phosphinomethyl-Functionalized N‑Heterocyclic Carbene Chelate Ligands: Synthesis, Molecular Geometry, Electronic Structure, and Their Ruthenium Alkylidene Complexes

A new, extremely bulky, and electron-rich <i>N</i>-phosphinomethyl-functionalized N-heterocyclic carbene ligand, <b>5a</b> (<sup><i>t</i>Bu</sup>NHCP<sup><i>t</i>Bu</sup>), and a somewhat less bulky congener, <b>5b</b> (<sup>Mes</sup>NHCP<sup><i>t</i>Bu</sup>), forming five-membered chelate rings with metal centers, have been synthesized in four steps starting from the easily accessible di-<i>tert</i>-butyl­(hydroxymethyl)­phosphine oxide (<b>1</b>). <b>5a</b> was isolated and fully characterized by spectroscopic methods including UV-photoelectron spectroscopy and X-ray diffraction. The reaction of <b>5a</b> with [Ru­(COD)­Cl<sub>2</sub>]<sub><i>n</i></sub> under hydrogen pressure or with [Ru­(<i>p</i>-cymene)­Cl<sub>2</sub>]<sub>2</sub> led to the formation of the unsaturated dinuclear complex [Ru­(<sup><i>t</i>Bu</sup>NHCP<sup><i>t</i>Bu</sup>)­(μ-Cl)­(Cl)]<sub>2</sub> (<b>6</b>), which serves as a precursor for a series of ruthenium carbene complexes (<b>7a</b>–<b>f</b>) using substituted phenyldiazomethanes (<i>p</i>-X-C<sub>6</sub>H<sub>4</sub>(CH)­N<sub>2</sub>; X = H (<b>a</b>), Br (<b>b</b>), CF<sub>3</sub> (<b>c</b>), NO<sub>2</sub>, (<b>d</b>), CH<sub>3</sub> (<b>e</b>)) and trimethylsilyldiazomethane (<b>f</b>). Treatment of <b>6</b> with phosphine or pyridine ligands led to the formation of the mononuclear adducts, [Ru­(<sup><i>t</i>Bu</sup>NHCP<sup><i>t</i>Bu</sup>)­(Cl<sub>2</sub>)­(PR<sub>3</sub>)] (R = Me (<b>8</b>), Ph (<b>9</b>), Cy (<b>10</b>)) and [Ru­(<sup><i>t</i>Bu</sup>NHCP<sup><i>t</i>Bu</sup>)­(Cl<sub>2</sub>)­(py)<sub><i>n</i></sub>] (<i>n</i> = 1 (<b>11</b>), 2 (<b>12</b>); if (py)<sub>2</sub> = bipy (<b>13</b>)), which were synthesized in order to find alternative precursor complexes because the dimer <b>6</b> showed very low solubility in most organic solvents. Complex <b>7a</b> was obtained analytically pure on a different route via transmetalation from a silver complex bearing <sup><i>t</i>Bu</sup>NHCP<sup><i>t</i>Bu</sup> (<b>15</b>) to the first-generation Grubbs catalyst as the ruthenium precursor. Complexes <b>7a</b>–<b>c</b> and <b>7e</b> were characterized by X-ray diffraction analysis, revealing a geometry that can be viewed as both a distorted square pyramid and a distorted trigonal bipyramid with the two chloro ligands in a <i>cis</i> configuration. The steric bulk, especially of <b>5a</b> with its <i>N</i>-<i>t</i>Bu moiety, stabilizes 16 VE Ru complexes. In contrast to ligand <b>5a</b>, the somewhat less bulky <sup>Mes</sup>NHCP<sup><i>t</i>Bu</sup> ligand <b>5b</b> has allowed its direct metalation with two ruthenium alkylidene precursors, affording the two new carbene complexes <b>17</b> and <b>18</b>.