Oxidation Reactions of the Phosphinidene Oxide Ligand

Abstract

The (H-DBU)+ salt of the anionic phosphinidene oxide complex [MoCp(CO)2{P(O)R*}]- (1) (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene; R* = 2,4,6-C6H2tBu3) reacts with different oxidizing agents, displaying a multisite activity located at the Mo and P atoms or at the Mo=P bond. Thus, reaction of 1 with [FeCp2]BF4 gives the dimer [Mo2Cp2(CO)4{P(O)R*}2], and reaction with bromine gives the phosphinous acid complex [MoBrCp{P(OH)(CH2CMe2C6H2tBu2}(CO)2], the latter arising from an unprecedented C-H bond addition to the oxide P=O moiety. In contrast, reaction of 1 with p-benzoquinone occurs at the P site to give the P,O-bound phosphonite complex [MoCp{kappa2-OP(OC6H4OH)R*}(CO)2]. Finally, oxygen or sulfur atoms are added to the Mo=P bond by reaction of 1 with Me2CO2 and S8 to give the novel dioxophosphorane or thiooxophosphorane complexes [MoCp(CO)2{kappa2-EP(O)R*}]- (E = O, S). The thiooxophosphorane anion is a good nucleophile and is methylated at either the S or O positions depending on the electrophile used (MeI or (Me3O)BF4) to give the isomers [MoCp{kappa2-(MeS)P(O)R*}(CO)2] and [MoCp{kappa2-SP(OMe)R*}(CO)2], both having novel organophosphorus ligands.

Nucleophilic behaviour of dioxo- and thiooxophosphorane complexes [MoCp(CO)2{E,P-EP(O)(2,4,6-C6H2tBu3)}]−(E = O, S)

“…In reactions of the dioxophosphorane 2, interaction with the incoming electrophile is probably dominated by electrostatic factors and, since both oxygen atoms of the R*PO 2 ligand bear comparable negative charges, the reason for the observed selective methylation at the terminal O site would be the superior accessibility of that site to an external reagent. In line with this hypothesis, we note that reaction of 3 with acryloyl chloride gives exclusively the corresponding phosphonothiolate derivative [MoCp{S,P-SP(OC(O)C 2 H 3 )R*}(CO) 2 ] (7), here suggesting the operation of steric barriers disfavouring the approach of an external reagent, certainly softer than (Me 3 O)BF 4 , to the S site.…”

“…A final structural feature deserved of comment is the distorted trigonal pyramidal environment around the P atom, derived from the fact that this atom remains placed almost in the same plane as the Mo, O3 and C11 atoms ((XPY) = 354 o ). Although no S,P-bound phosphonothiolate complex other than 6 has been structurally characterized, we note that this local environment is also present in related O,P-bound phosphinite and phosphonite complexes, 4,5 Spectroscopic data in solution for the phosphonothiolate complexes 6 and 7 (Table 1 and Experimental Section) are fully consistent with their asymmetric solid-state structures and deserve no particular comment. We just note that they display 31 P NMR resonances some 30 ppm more deshielded than that of the starting anion 3, while their CO stretches are substantially more energetic, as expected from the reduction of charge in these neutral derivatives.…”

“…4 We have now determined the structure of the related complex 7, which displays comparable geometrical parameters ( Figure 3 and Table 4). The salient feature of this structure is the presence of an S,P-bound phosphonothiolate ligand derived from attachment of the hydrocarbyl group at the O-site of the thiooxophosphorane ligand of 3.…”

“…Indeed our preliminary study on the reactivity of the thiooxophosphorane complex 3 indicated that methylation could occur easily at either the S and O sites, to give unprecedented thiolophosphinide (PR(O)(SMe)) and phosphonothiolate (PR(OMe)S) derivatives, respectively. 4 Therefore it was of interest exploring the potential of anions 2 and 3 in the generation of novel P-donor entities. In this paper we give full details of the above reactions, which we have now extended to the dioxophosphorane complex 2 and also to other electrophilic reagents, including the proton and some metalbased electrophiles.…”

“…4 Because of the negative charge of the latter complexes, the acid-base chemistry of their phosphorus ligands is reversed, so they can now behave as nucleophiles. Indeed our preliminary study on the reactivity of the thiooxophosphorane complex 3 indicated that methylation could occur easily at either the S and O sites, to give unprecedented thiolophosphinide (PR(O)(SMe)) and phosphonothiolate (PR(OMe)S) derivatives, respectively.…”