Intramolecular Alkyl Phosphine Dehydrogenation in Cationic Rhodium Complexes of Tris(cyclopentylphosphine)

Abstract

[Rh(nbd)(PCyp(3))(2)][BAr(F) (4)] (1) [nbd = norbornadiene, Ar(F) = C(6)H(3)(CF(3))(2), PCyp(3) = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp(3) ligand in CH(2)Cl(2) solution to form an equilibrium mixture of cis-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 a) and trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H(2) to 2 a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp(3))(2)(H)(2)(eta(2)-H(2))(2)][BAr(F) (4)] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2 a/b, passing through an intermediate which has undergone dehydrogenation of only one PCyp(3) ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(PCyp(3))(NCMe)][BAr(F) (4)] (6). Dehydrogenation of a PCyp(3) ligand also occurs on addition of Na[BAr(F) (4)] to [RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give [Rh(eta(6)-C(6)H(5)X){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] 9 is also reported. Rapid ( approximately 5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp(3))] with Na[BAr(F) (4)] to give [Rh(dppe){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (10), which reacts with H(2) to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H(2))][BAr(F) (4)] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy(2)(Cyp) show that [RhCl(nbd){PCy(2)(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(eta(6)-C(6)H(5)X){PCy(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2 a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.

Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates

“…Similar distortions have been noted in trans -[Rh{PR 2 (alkene)} 2 ] + species and are thought to be driven by enhanced π-back donation from the Rh d z 2 orbital. 69 [1-(ethene) 2 ][BAr F 4 ]-Oct is stable to short periods of vacuum but satisfactory elemental analysis was not obtained as the NBA formed during the reaction was persistent and could not be removed.…”

Acceptorless, intramolecular, alkyl dehydrogenation in the solid-state in a rhodium phosphine complex; reversible uptake of three equivalents of H2 per molecule

“…Intermediates in this process have been observed spectroscopically and also studied using computational methods. 9 We now report that these reversible hydrogenationdehydrogenation reactions also occur in the solid-state by solid-gas reactions. This represents not only a relatively rare example of an organometallic solid-gas reaction with small molecules 10-13 but also, as far as we are aware, the first example of reversible and acceptorless alkyl dehydrogenation to be reported in the solid-state.…”

“…We have recently reported the remarkably quick acceptorless dehydrogenation of one alkyl ring of tris-cyclopentylphosphine (PCyp 3 ) in the complex [Rh(dppe)Cl(PCyp 3 )] on addition of Na [BAr F 4 ], to afford [Rh(dppe)(PCyp 2 Cyp 0 )]-[BAr F 4 ] 1 in quantitative yield (Scheme 1, Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ). 8,9 This generates a hybrid phosphine-olefin ligand on a square planar Rh(I) centre. Addition of H 2 to 1 in CH 2 Cl 2 solution results in hydrogenation of the alkene and the formation of the dihydrogen-dihydride complex 2, [Rh(dppe…”

Phosphines and related C–P bonded compounds

“…317 Condensation of the bis(phenolic) phosphine (158) with phenylboronic acid has given the phosphino-functional benzodioxaborole (159) that demonstrates the expected donor/ acceptor character in its reactions. 318 Among reactions of phosphines occurring in the coordination sphere of a transition metal are a rhodium-catalysed hydrogenation and silylation of P-P bonds to give secondary phosphines and silylphosphines, 319 a chiral amine-palladium template-promoted asymmetric cycloaddition of phenyldivinylphosphine to 3,4-dimethyl-1-phenylarsole to give the chiral As-P ligand (160), 320 a palladium-catalysed synthesis of functionalised tetraarylphosphonium salts, 321 the rhodium-catalysed addition of water to 1-alkynylphosphines to give (E)-1-alkenylphosphine oxides, 322 intramolecular alkylphosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentyl)phosphine, 323 and further examples of intramolecular alkene metathesis/hydrogenation reactions in metal complexes of phosphines of the type P[(CH 2 ) n CHQCH 2 ] 3 (n ¼ 7-11) to give macrocyclic gyroscope-like molecules. 324 The reaction of 2,2 0 -bis(diphenylphosphino)-1,1 0 -binaphthyl with an equimolar amount of copper(II) triflate in acetonitrile afforded the bis(phosphonium) salt (161).…”