Synthesis and Properties of the 17-Electron, Tantalum-Centered Radical Ta(CO)₄(Ph₂PCH₂CH₂PPh₂)

Abstract

The novel 17-electron compound Ta(CO)4dppe (dppe = Ph2PCH2CH2PPh2) is formed via hydride hydrogen atom abstraction from TaH(CO)4dppe by the tris(p-tert-butylphenyl)methyl radical. The compound exists in solution as an equilibrium mixture of monomer (the dominant species) and carbonyl-bridged dimer [Ta(CO)4dppe]2 but solely as the latter in the solid state. It is very labile but was characterized electrochemically, IR, Raman, and 1H and 31P{1H} NMR spectroscopically, and chemically. Typical of metal-centered radicals, Ta(CO)4dppe abstracts halogen atoms from organic halides RX to give the halotantalum compounds TaX(CO)4dppe and, in some cases, TaR(CO)4dppe. Cyclic voltammetry experiments show that the oxidation of [Ta(CO)4(dppe)]- in CH2Cl2/0.1 M [Bu4N][PF6] is a one-electron process, irreversible at scan rates below 50 V/s. The formal potential of [Ta(CO)4(dppe)]0/- is estimated as −1.2 V vs ferrocene. The reduced lifetime of the radical is ascribed to reactions with the ionic medium, the main electrochemical oxidation products being TaH(CO)4(dppe) and TaCl(CO)2(dppe)2.

Niobocene Alkyne Complexes: Synthesis and Characterization of Neutral and Cationic d2Metal Alkyne Derivatives. X-ray Crystal Structure of [Nb(η5-C5H4SiMe3)2(η2(C,C)-HC⋮CPh)(CH2Ph)]

“…Indeed, these compounds reacted with the corresponding R‘‘X to give rise to mixtures of niobocene alkyl−alkyne and halo-alkyne species and no other byproduct (Scheme ). Analogous behavior was recently found in the reaction of the tantalum 17-electron metal-centered radical Ta(CO) 2 dppe (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) with alkyl halides . The reactions of aliphatic and aromatic halides with low-valent transition-metal complexes are of synthetic interest, since they are useful in the formation of new carbon−metal bonds, and in this context we previously described the behavior of Nb(III) complexes Nb(η 5 -C 5 H 4 SiMe 3 ) 2 X with several alkyl and aryl halides.…”

Transition Metal Ion Initiated Chain Reactions between tert-Butyl Hydroperoxide and Rhodium Hydrides. Rapid Hydrogen Atom Abstraction from Rhodium Hydrides by Methyl Radicals

“…In their reactions with transition metal complexes, alkyl radicals, especially methyl, often exhibit chemistry similar to that of hydrogen atoms. , Alkyl radicals can be generated by visible photolysis of a number of precursors, which enables one to use spectroscopic probes and measure the kinetics precisely. − Even though hydrogen atom abstraction from metal hydrides by carbon-centered radicals is known − and has been used successfully to generate 17-e metal-centered radicals, ,− the kinetic data for such reactions are limited. − Most of the kinetic studies utilized radical clock reactions, − ,, ,, whereby bimolecular hydrogen abstraction and unimolecular radical rearrangement occur simultaneously. This approach has proven useful in radical kinetics, but the number of suitable radicals is limited.…”

Three 18-Electron Tantalum(I) Compounds, TaCl(CO)2(dppe)2, [TaCl(CO)2(dppe)2]2x, and TaCl(CO)4(dppe) (dppe = 1,2-Bis(diphenylphosphino)ethane), Which Exhibit Low Levels of Paramagnetism

“…During the course of this research, the 18-electron, organotantalum compound TaCl(CO) 2 (dppe) 2 ( 1 ) 2a was occasionally formed as a byproduct during the formation of Ta(CO) 4 (dppe) 1b. In addition, the compounds TaX(CO) 4 (dppe) (X = Cl, Br, I) 2b,3 were formed as products of halogen abstraction reactions of Ta(CO) 4 (dppe) with organic halides RX,1b and since the tetracarbonyl compounds readily lose CO in the presence of free dppe, compound 1 again appeared in our lives. Seven-coordinated halotantalum(I) compounds of this type have been reasonably well-studied, 2b, crystal structure information for several being available, and there appeared initially to be no reason to pursue 1 further.…”

“…We have previously demonstrated the synthesis, via the reductive carbonylation of TaCl 5 in the presence of dppe (dppe = 1,2-bis(diphenylphosphino)ethane), of the novel 17-electron compound Ta(CO) 4 (dppe), the first simple group 5 heavy metal analogue of V(CO) 6 and its derivatives . During the course of this research, the 18-electron, organotantalum compound TaCl(CO) 2 (dppe) 2 ( 1 ) 2a was occasionally formed as a byproduct during the formation of Ta(CO) 4 (dppe) 1b.…”

“…We have previously demonstrated the synthesis, via the reductive carbonylation of TaCl 5 in the presence of dppe (dppe = 1,2-bis(diphenylphosphino)ethane), of the novel 17-electron compound Ta(CO) 4 (dppe), the first simple group 5 heavy metal analogue of V(CO) 6 and its derivatives . During the course of this research, the 18-electron, organotantalum compound TaCl(CO) 2 (dppe) 2 ( 1 ) 2a was occasionally formed as a byproduct during the formation of Ta(CO) 4 (dppe) 1b. In addition, the compounds TaX(CO) 4 (dppe) (X = Cl, Br, I) 2b,3 were formed as products of halogen abstraction reactions of Ta(CO) 4 (dppe) with organic halides RX,1b and since the tetracarbonyl compounds readily lose CO in the presence of free dppe, compound 1 again appeared in our lives.…”