Rhodium pyrazolylborates

Scheme 8.13 Rhodium pyrazolylborate complexes for polymerization of phenyl acetylene...

Synthesis, structure, and hydrothiolation activity of rhodium pyrazolylborate complexes Rt>R N—N pph, / / 3 Rh y/ / Xpph N—N PPh3 3... 

Hydrotris(pyrazolyl)borates are widely used as ligands for transition metals, and especially, rhodium pyrazolylborates have been extensively studied for stoichiometric... 

C-H activation reactions [35]. Rhodium pyrazolylborates, as highly electron-rich metal complexes, are found to be useful catalysts for the Markovnikov-type addition of thiols to alkynes. In particular, Tp Rh(PPh3)2 (Tp = hydrotris (3,5-dimethylpyrazolyl)borate) exhibits an excellent catalytic activity toward the regioselective hydrothiolation of a range of alkynes with both arene- and alkanethiols (Scheme 11) [36 0]. A variety of functional groups are well tolerated, and both sterically encumbered alkynes and thiols are successful in hydrothiolation. Electron-rich alkynes react more rapidly than electron-deficient alkynes. 

Van Rooy S, Cao C, Patrick BO, Lam A, Love JA (2006) Alkyne hydrophosphinylation catalyzed by rhodium pyrazolylborate complexes. Inorg Chim Acta 359 2918—2923... 

A series of ionic liquids have been tested for the polymerisation of phenylacetylene with the pyrazolylborate rhodium complexes 46 and 47, shown in Scheme 8.13.[701 Complex 46 afforded higher molecular weights and lower polydispersity at comparable activity in the ionic liquid relative to dichloromethane. The addition of small amounts of methanol to the ionic liquid was found to have a positive effect on the catalytic activity. With complex 47 best results were obtained in [C4Ciim]Cl, giving 64% yield after 2 hours at 65°C. Molecular weights were usually lower with 46 than those obtained with either 47a or 47b as catalyst. 

Recently. Alvarado et al. reported a related set of active catalysts for quinoline hydrogenation, formed in situ by addition of pyrazolylborate ligands Tp, hydrotris(pyrazolyl)borate and Tp. hydrotris(3,5-dimethylpyrazolyl)borate -which are isolectronic with cyclopentadienyl ligands- to [M(COD)C1]2 (M = Rh, Ir, COD = 1,5-cyclooctadiene), [Ir(COE)Cll2 (COE = cyclooctenc), and RuCl2(NCMe)4. Again, rhodium turned out to be the most efficient metal, and the activity was related with the ease of formation of Tp or Tp complexes. No mechanistic details were provided [101]. 

The pyrazolylborate complexes Tp W( = CR)(CO)2 (R = Me or C6H4Me-4) are less reactive towards various low-valent rhodium species than their Cp analogues, requiring more forcing conditions (i.e., higher temperatures and longer reaction times) for reactions to proceed. 

Pettinari R, Pettinari C, Marchetti F, Monari M, Mosconi E, De Angelis F. Boron functionalization and unusual B-C bond activation in rhodium(III) and iridium(III) complexes with diphenylbis(pyrazolylborate) ligands Ph2Bp. OrganometaUics. 2013 32 3895-3902. 

---