Alkene Hydrogenation on an 11-Vertex Rhodathiaborane with Full Cluster Participation

Abstract

The facile synthesis of the metallaheteroborane [8,8-(PPh 3) 2- nido-8,7-RhSB 9H 10] ( 1) makes possible the systematic study of its reactivity. Addition of pyridine to 1 gives in high yield the 11-vertex nido-hydridorhodathiaborane [8,8,8-(PPh 3) 2H-9-(NC 5H 5)- nido-8,7-RhSB 9H 9] ( 2). 2 reacts with C 2H 4 or CO to form [1,1-(PPh 3)(L)-3-(NC 5H 5)- closo-RhSB 9H 8] [L = C 2H 4 ( 3), CO ( 4)]. In CH 2Cl 2 at reflux temperature 2 undergoes a nido to closo transformation to afford [1,1-(PPh 3) 2-3-(NC 5H 5)- closo-1,2-RhSB 9H 8] ( 5). Reaction of 2 with alkenes leads to hydrogenation and isomerization of the olefins. NMR spectroscopy indicates the presence of a labile phosphine ligand in 2, and DFT calculations have been used to determine which of the two phosphine groups is labile. Rationalization of the hydrogenation mechanism and the part played by the 2 --> 3 nido to closo cluster change during the reaction cycle is suggested. In the proposed mechanism the classical hydrogen transfer from hydride metal complexes to olefins occurs twice: first upon coordination of the alkene to the rhodium centre in 2, and second concomitant with formation of a closo-hydridorhodathiaborane intermediate by migration of a BHB-bridging hydrogen atom to the metal. Reaction of H 2 with 3 or 5 regenerates 2, closing a reaction cycle that under catalytic conditions is capable of hydrogenating alkenes. Single-site versus cluster-bifunctional mechanisms are discussed as possible routes for H 2 activation.

A borane laser

“…Solutions of anti-B 18 H 22 were prepared in non-polar aprotic solvents, as this compound is otherwise prone to deprotonation (pKa ¼ 2.68), which would lead to the formation of its [B 18 H 21 ] À anion and the loss of the compound's fluorescent emission. Anti-B 18 H 22 exhibits a rather low molar extinction coefficient at the pump wavelength (e (at 337 nm) B5,625 M À 1 cm À 1 ), which necessitates the use of highly concentrated solutions to achieve the optical densities required in transversal pumping (ODB20).…”

“…As chemical and physical behaviour is often a function of structure, then the boron hydrides have, with their unique polyhedral molecular geometries, large potential for novel properties. This is indeed the case, and in recent times boron hydrides-or boranes as these compounds are more commonly referred to-and their derivatives are proving promising candidates for application in medicinal fields 15 , microelectronics 16 , reversible capture of small molecules 17 , catalysis 18 , in the modification of surfaces 19 and as enhancers of the fluorescent properties of various luminescent organic polymers or fluorophores 20 .…”

Chemistry of 11-vertex rhodathiaboranes: reactions with monodentate phosphines

“…In contrast, the use of two equivalents of PMe 2 Ph or PMe 3 gives mixtures that contain both the monosubstituted bis-(phosphine) species, [8,8-(PPh 3 )(L)-nido-8,7-RhSB 9 H 10 ], where L = PPhMe 2 (5) or PMe 3 (6), together with the products of addition of a third phosphine at the metal, viz. the tris-(phosphine) species [8,8,8-(L) 3 -nido-8,7-RhSB 9 H 10 ], where L = PPhMe 2 (7) or PMe 3 (8). These last two tris-(phosphine) {Rh(PR 3 ) 3 }-containing cluster com-pounds are more conveniently prepared from the reactions of 1 with three equivalentents of phosphine.…”

“…3 In addition, reactivity studies of the reaction of alkynes with the hydrido-rhodathiaborane 2 have demonstrated that the de-hydrogenation of 2 can promote the oxidative addition of sp CH bonds, 4 suggesting that other EH bonds in, for example, alkanes, silanes, or boranes, could also be activated 4 by modifying the reaction conditions and / or by an appropriate tailoring of the clusters.…”

DFT study of alkene hydrogenation catalyzed by Rh(acac)(CO)2

“…The hydrogenation of alkenes catalyzed by transition metal complexes is a kind of very important reactions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and an atom-economical synthetic methodology which has been used extensively in academic and industrial settings [17]. Alkene complexes of transition metals [18][19][20][21][22][23][24] play an important role in understanding the reaction mechanisms for hydrogenation of alkenes, which has been confirmed experimentally and theoretically to proceed via a wide variety of different pathways [25][26][27][28][29][30].…”