Chemistry of Several Sterically Bulky Molecules with P=P, P=C, and C≡P Bond.

Several sterically protected, low-coordinate organophosphorus compounds with P=P, P=C, and C≡P bond are described in this study. Molecules such as diphosphenes, phosphaalkenes, 1-phosphaallenes, 1,3-diphosphaallenes, 3,4-diphosphinidenecyclobutenes, and phosphaalkynes are stabilized with an extremely bulky 2,4,6-tri-t-butylphenyl (Mes*) group. The synthesis, structures, physical, and chemical properties of these molecules are discussed, together with some successful applications in catalytic organic reactions.

Di-tert-butyldiphosphatetrahedrane as a Source of 1,2-Diphosphacyclobutadiene Ligands.

Reactions of di-tert-butyldiphosphatetrahedrane (1) with cycloocta-1,5-diene- or anthracene-stabilised metalate anions of iron and cobalt consistently afford complexes of the rarely encountered 1,2-diphosphacyclobutadiene ligand, which have previously been very challenging synthetic targets. The subsequent reactivity of 1,2-diphosphacyclobutadiene cobaltates toward various electrophiles has also been investigated and is compared to reactions of related 1,3-diphosphacyclobutadiene complexes. The results highlight the distinct reactivity of such isomeric species, showing that the 1,2-isomers can act as precursors for previously unknown triphospholium ligands. The electronic structures of the new complexes were investigated by several methods, including NMR, EPR and Mößbauer spectroscopies as well as quantum chemical calculations.

Phosphorus-Atom Transfer from Phosphaethynolate to an Alkylidyne.

A low-spin and mononuclear vanadium complex, (Me nacnac)V(CO)(η2 -P≡Ct Bu) (2) (Me nacnac- =[ArNC(CH3 )]2 CH, Ar=2,6-i Pr2 C6 H3 ), was prepared upon treatment of the vanadium neopentylidyne complex (Me nacnac)V≡Ct Bu(OTf) (1) with Na(OCP)(diox)2.5 (diox=1,4-dioxane), while the isoelectronic ate-complex [Na(15-crown-5)]{([ArNC(CH2 )]CH[C(CH3 )NAr])V(CO)(η2 -P≡Ct Bu)} (4), was obtained via the reaction of Na(OCP)(diox)2.5 and ([ArNC(CH2 )]CH[C(CH3 )NAr])V≡Ct Bu(OEt2 ) (3) in the presence of crown-ether. Computational studies suggest that the P-atom transfer proceeds by [2+2]-cycloaddition of the P≡C bond across the V≡Ct Bu moiety, followed by a reductive decarbonylation to form the V-C≡O linkage. The nature of the electronic ground state in diamagnetic complexes, 2 and 4, was further investigated both theoretically and experimentally, using a combination of density functional theory (DFT) calculations, UV/Vis and NMR spectroscopies, cyclic voltammetry, X-ray absorption spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X-ray single-crystal structural characterization. In combination, these data are consistent with a low-valent vanadium ion in complexes 2 and 4. This study represents the first example of a metathesis reaction between the P-atom of [PCO]- and an alkylidyne ligand.

Activation of Di-tert-butyldiphosphatetrahedrane: Access to (tBuCP)n (n=2,†4) Ligand Frameworks by P-C Bond Cleavage.

The first mixed phosphatetrahedranes were reported only recently and their reactivity is virtually unexplored. Herein, we present a reactivity study on di-tert-butyldiphosphatetrahedrane (1), which is the dimer of tert-butylphosphaalkyne. The (tBuCP)2 tetrahedron is activated selectively by N-heterocyclic carbene (NHC) nickel(I) and nickel(0) complexes, resulting in novel complexes featuring diverse (tBuCP)n -frameworks (n=2, 4). Release of the (tBuCP)4 framework from one of the complexes was achieved by addition of CO gas. Furthermore, 1 can be used as a source for P2  units by elimination of di-tert-butylacetylene in the coordination sphere of nickel.

Hydroboration of Phosphaalkynes by HB(C6 F5 )2.

The hydroboration of phosphaalkynes with Piers' borane (HB(C6 F5 )2 ) generated unusual phosphaalkenylboranes [RCH=PB(C6 F5 )2 ]2 that persisted as dimers in both solution and the solid state. These P2 B2 heterocycles underwent ring opening when subjected to nucleophiles, such as pyridine and tert-butylisocyanide, to yield monomeric phosphaalkenylborane adducts RCH=PB(C6 F5 )2 (L). DFT calculations were performed to probe the nature of the interaction of phosphaalkynes with boranes.