Andrea Vélez

POP-Pincer Silyl Complexes of Group 9: Rhodium versus Iridium

9,9-Dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(P(i)Pr2)2) derivatives RhCl{xant(P(i)Pr2)2} (1) and I rHCl{xant(P(i)Pr2)[(i)PrPCH(Me) CH2]} (2) react with diphenylsilane and triethylsilane to give the saturated d(6)-compounds RhHCl(SiR3){xant(P(i)Pr2)2} (SiR3 = SiHPh2 (3), SiEt3 (4)) and IrHCl(SiR3){xant(P(i)Pr2)2} (SiR3 = SiHPh2 (5), SiEt3 (6)). Complexes 3 and 5 undergo a Cl/H position exchange process via the MH{xant(P(i)Pr2)2} (M = Rh (8), Ir (E)) intermediates. The rhodium complex 3 affords the square planar d(8)-silyl derivative Rh(SiClPh2){xant(P(i)Pr2)2} (7), whereas the iridium derivative 5 gives IrH2(SiClPh2){xant(P(i)Pr2)2} (9), which is stable. In agreement with the formation of 7, the reactions of 8 with silanes are a general method to prepare square planar d(8)-rhodium-silyl derivatives. Thus, the addition of triethylsilane and triphenylsilane to 8 initially leads to the dihydrides RhH2(SiR3){xant(P(i)Pr2)2} (SiR3 = SiEt3 (10), SiPh3 (11)), which lose molecular hydrogen to afford Rh(SiR3){xant(P(i)Pr2)2} (SiR3 = SiEt3 (12), SiPh3 (13)). Treatment of 7 with NaBAr(F)4·2H2O leads to the cationic five-coordinate d(6)-species [RhH{Si(OH)Ph2}{xant(P(i)Pr2)2}]BAr(F)4 (14) through a silylene intermediate. According to the participation of the latter in the formation of 14, this cation is an efficient catalyst precursor for the monoalcoholysis of diphenylsilane with a wide range of alcohols, reaching turnover frequencies at 50% of conversion between 4000 and 76 500 h(-1). The X-ray structures of 3, 6, 7, 9, 12, and 14 are also reported.

Xantphos-type complexes of group 9: Rhodium versus iridium

Treatment of the dimer [Rh(μ-Cl)(C8H14)2]2 (1a) with 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene [xant(P(i)Pr2)2] leads to the d(8) square-planar complex RhCl{xant(P(i)Pr2)2} (2), whereas reaction of the iridium counterpart [Ir(μ-Cl)(C8H14)2]2 (1b) gives the d(6) octahedral compound IrHCl{xant(P(i)Pr2)[(i)PrPCH(Me)CH2]} (3) as a result of the intramolecular C-H bond activation of one of the isopropyl substituents of the phosphine. Stirring 2 and 3 in 0.5 N KOH solutions of 2-propanol gives rise to the formation of hydrides RhH{xant(P(i)Pr2)2} (4) and IrH3{xant(P(i)Pr2)2} (5), respectively. In n-octane at 60 °C, complex 2 is stable. However, compound 3 activates the alkane to give the cis-dihydride IrH2Cl{xant(P(i)Pr2)2} (6) and a mixture of 3- and 4-octene. Complex 6 can be also obtained by the reaction of 3 with H2. Under the same conditions, 2 affords the rhodium analogue RhH2Cl{xant(P(i)Pr2)2} (7). Compounds 2-4 react with triflic acid (HOTf) to give RhHCl(OTf){xant(P(i)Pr2)2} (8), IrHCl(OTf){xant(P(i)Pr2)2} (9), and RhH2(OTf){xant(P(i)Pr2)2} (10), respectively. The related iridium derivative IrH2(OTf){xant(P(i)Pr2)2} (11) has also been prepared by the reaction of 6 with Tl(OTf). Complexes 2, 6, and 9 have been characterized by X-ray diffraction analysis. The {xant(P(i)Pr2)2}M skeleton is T-shaped with the metal center situated in the common vertex.

Ammonia Borane Dehydrogenation Promoted by a Pincer-Square-Planar Rhodium(I) Monohydride: A Stepwise Hydrogen Transfer from the Substrate to the Catalyst

The pincer d(8)-monohydride complex RhH{xant(P(i)Pr2)2} (xant(P(i)Pr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) promotes the release of 1 equiv of hydrogen from H3BNH3 and H3BNHMe2 with TOF50% values of 3150 and 1725 h(-1), to afford [BH2NH2]n and [BH2NMe2]2 and the tandem ammonia borane dehydrogenation-cyclohexene hydrogenation. DFT calculations on the ammonia borane dehydrogenation suggest that the process takes place by means of cis-κ(2)-PP-species, through four stages including: (i) Shimoi-type coordination of ammonia borane, (ii) homolytic addition of the coordinated H-B bond to afford a five-coordinate dihydride-boryl-rhodium(III) intermediate, (iii) reductive intramolecular proton transfer from the NH3 group to one of the hydride ligands, and (iv) release of H2 from the resulting square-planar hydride dihydrogen rhodium(I) intermediate.

Conclusive evidence on the mechanism of the rhodium-mediated decyanative borylation.

The stoichiometric reactions proposed in the mechanism of the rhodium-mediated decyanative borylation have been performed and all relevant intermediates isolated and characterized including their X-ray structures. Complex RhCl{xant(P(i)Pr2)2} (1, xant(P(i)Pr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) reacts with bis(pinacolato)diboron (B2pin2), in benzene, to give the rhodium(III) derivative RhHCl(Bpin){xant(P(i)Pr2)2} (4) and PhBpin. The reaction involves the oxidative addition of B2pin2 to 1 to give RhCl(Bpin)2{xant(P(i)Pr2)2}, which eliminates ClBpin generating Rh(Bpin){xant(P(i)Pr2)2} (2). The reaction of the latter with the solvent yields PhBpin and the monohydride RhH{xant(P(i)Pr2)2} (6), which adds the eliminated ClBpin. Complex 4 and its catecholboryl counterpart RhHCl(Bcat){xant(P(i)Pr2)2} (7) have also been obtained by oxidative addition of HBR2 to 1. Complex 2 is the promoter of the decyanative borylation. Thus, benzonitrile and 4-(trifluoromethyl)benzonitrile insert into the Rh-B bond of 2 to form Rh{C(R-C6H4)═NBpin}{xant(P(i)Pr2)2} (R = H (8), p-CF3 (9)), which evolve into the aryl derivatives RhPh{xant(P(i)Pr2)2} (3) and Rh(p-CF3-C6H4){xant(P(i)Pr2)2} (10), as a result of the extrusion of CNBpin. The reactions of 3 and 10 with B2pin2 yield the arylBpin products and regenerate 2.