Activity of Mo–Mo and Mo–P multiple bonds at the phosphinidene complex [Mo2Cp2{μ-P(2,4,6-C6H2tBu3)}(μ-CO)2] in reactions with isocyanides and phosphine ligands

• The Mo–Mo triple bond of the title complex dominates its Lewis-acid reactivity.
• Reactivity of the Mo–P multiple bonds at the title complex is sterically blocked.
• Mo–Mo and Mo–P multiple bonds are both involved in P–H bond activation.

The title complex reacted with the aryl isocyanides CNXyl and CN(p-tol) rapidly at room temperature to yield the corresponding derivatives [Mo2Cp2(μ-PR∗)(CO)2(CNR)2] (Cp = η5-C5H5; R = Xyl, p-tol; R∗ = 2,4,6-C6H2tBu3) even when using 1 equiv of reagent. In contrast, reactions with 1 equiv of CNtBu yielded the isocyanide-bridged derivatives cis- or trans-[Mo2Cp2(μ-PR∗)(μ-η1:η2-CNtBu)(CO)2] depending on reaction conditions (Mo–Mo = 3.0818(3) and 3.1453(8) Å, respectively). The cis isomer was computed to be 16 kJ/mol more stable than the trans isomer, and could be also prepared by heating petroleum ether solutions of the latter at 333 K, while reaction of excess CNtBu with the trans isomer yielded [Mo2Cp2(μ-PR∗)(CO)2(CNtBu)2]. The title complex reacted with Me2PCH2PMe2 (dmpm) to give the corresponding diphosphine-bridged derivative [Mo2Cp2(μ-PR∗)(CO)2(μ-dmpm)] or its chelate isomer [Mo2Cp2(μ-PR∗)(CO)2(κ2-dmpm)], depending on reaction conditions (Mo–Mo = 3.342(1) and 3.176(1) Å, respectively), whereas its room-temperature reaction with PH2Cy involved a H-shift to the phosphinidene ligand, to give the bis(phosphide) derivative [Mo2Cp2(μ-PHCy)(μ-PHR∗)(CO)2]. Photolysis of the latter induced decarbonylation, C–H bond cleavage, and dehydrogenation, to eventually yield the triply-bonded phosphaindanide derivative [Mo2Cp2{μ-P(CH2CMe2)C6H2tBu2}(μ-PHCy)(μ-CO)] (Mo–Mo = 2.511(2) Å). Density functional theory analysis of the bonding in the title complex revealed a substantial mixing of one of the δ components of the intermetallic triple bond with the pπ orbital of the phosphinidene ligand. As a result of this, the LUMO of the molecule is mainly centered at the P atom, but leads to little reactivity because of the steric protection provided by the ortho-tBu groups of the phosphinidene substituent. The electron-acceptor reactivity of the molecule is then dominated by the close LUMO+1 orbital, which has Mo–Mo antibonding character and is well suited for interaction with donors approaching the dimetal site from the less congested side of the molecule, located between the carbonyl ligands.

Reactivity of the title complex is dominated by its intermetallic multiple bond, but its Mo–P multiple bond is also involved in the activation of P–H linkages.Figure optionsDownload as PowerPoint slide