Three-Coordinate Rhodium Complexes in Low Oxidation States

The isolation of simultaneously low-coordinate and low-valent compounds is a timeless challenge for preparative chemists. This work showcases the preparation and full characterization of tri-coordinate rhodium(-I) and rhodium(0) complexes as well as a rare rhodium(I) complex. Reduction of [{Rh(μ-Cl)(IPr)(dvtms)}₂] ( 1 , IPr=1,3-bis(2,6-diisopropylphenyl)imidazolyl-2-ylidene; dvtms=divinyltetramethyldisiloxane) with KC₈ gave the trigonal complexes K[Rh(IPr)(dvtms)] and [Rh(IPr)(dvtms)], whereas the cation [Rh(IPr)(dvtms)]⁺ results from their oxidation or by abstraction of chloride from 1 with silver salts. The paramagnetic Rh⁰ complex is a unique fully metal-centered radical with the unpaired electron in the d_z2 orbital. The Rh(-I) complex reacts with PPh₃ with replacement of the NHC ligand, and behaves as a nucleophile, which upon reaction with [AuCl(PPh₃)] generates the trigonal pyramidal complex [(IPr)(dvtms)Rh-Au(PPh₃)] with a metal–metal bond between two d¹⁰ metal centers.     

Dynamic Ligand Reactivity in a Rhodium Pincer Complex

Ligand cooperativity provides (transition) metal complexes with new reactivities in substrate activation and catalytic reactions, but usually the ligand acts as an internal (Brønsted) base, while the metal acts as a (Lewis) acid. We describe the synthesis and stepwise activation of a new phosphane‐pyridine‐amide ligand PNN^H2 in combination with Rh^I. The ligand is susceptible to stepwise proton and hydride loss from the nitrogen arm (imine formation) and deprotonation at the pyridylphosphine arm (dearomatization), giving rise to amine complex 1 , amido species 2 , imine complex 3 and dearomatized compound 4 . Complex 4 bears a dual‐mode cooperative PNN′ ligand containing both a (nucleophilic) basic methine fragment and a reactive (electrophilic) imine moiety. The basic ligand arm enables substrate deprotonation while the imine ligand arm enables reversible “storage” of the activated (nucleophilic) form of a sulfonamide substrate at the ligand. In combination with metal‐based reactivity, this allows for the mono‐alkylation of o‐toluenesulfonamide with iodomethane. Compounds 1 , 3 and 4 are structurally characterized. We also report the first structurally characterized example of an aminal in the coordination sphere of rhodium, complex 5 , [Rh(CO)( PNN′′ )], formed by sequential N?H activation of sulfonamide by the dearomatized ligand PNN′ and follow‐up nucleophilic attack of anionic sulfonamide onto the imine fragment.     

Rhodium catalyzed asymmetric Pauson-Khand reaction using SDP ligands

~~Rhodium catalyzed asymmetric Pauson-Khand reaction using SDP ligands1. Khand, I. U., Knox, G R., Pauson, P. L. et al., Organocobalt complexes, Part Ⅱ. Reaction of acetylenehexacarbonyldicobalt complexes, (R1C2R2)Co2(CO)6, with norbomene and its deri…     

Rhodium carbene complexes as versatile catalyst precursors for Si-H bond activation

Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si-H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H(2). In dry MeNO(2), selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silyl ethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H(2) as the only by-product. With alkynes, comparably fast hydrosilylation takes place, while carbonyl groups are unaffected. Further expansion of the Si-H bond activation to dihydrosilanes afforded silicones and polysilyl ethers. Mechanistic investigations using deuterated silane revealed deuterium incorporation into the abnormal carbene ligand and thus suggests a ligand-assisted mechanism involving heterolytic Si-H bond cleavage.     

Trinuclear Rhodium Complexes and Their Relevance for Asymmetric Hydrogenation

Various trinuclear rhodium complexes of the type [Rh₃(PP)₃(μ₃‐OH)_x(μ₃‐OMe)_2?x]BF₄ (where PP=Me‐DuPhos, dipamp, dppp, dppe; different ligands and μ‐bridging anions) are presented, which are formed upon addition of bases such as NEt₃ to solvate complexes [Rh(PP)(solvent)₂]BF₄. They were extensively characterized by X‐ray diffraction and NMR spectroscopy (¹⁰³Rh, ³¹P, ¹³C, ¹H). Their in situ formation resulting from basic additives (NEt₃) or basic prochiral olefins (without addition of another base) can cause deactivation of the asymmetric hydrogenation. This effect can be reversed by means of acidic additives.     

Cooperative H2 Activation on Dicopper(I) Facilitated by Reversible Dearomatization of an “Expanded PNNP Pincer” Ligand

A naphthyridine-derived expanded pincer ligand is described that can host two copper(I) centers. The proton-responsive ligand can undergo reversible partial and full dearomatization of the naphthyridine core, which enables cooperative activation of H₂ giving an unusual butterfly-shaped Cu₄H₂ complex.     

Dipyrromethane-Based PGeP Pincer Germyl Rhodium Complexes

A family of germyl rhodium complexes derived from the PGeP germylene 2,2’-bis(di-isopropylphosphanylmethyl)-5,5’-dimethyldipyrromethane-1,1’-diylgermanium(II), Ge(pyrmPⁱPr₂)₂CMe₂ ( 1 ), has been prepared. Germylene 1 reacted readily with [RhCl(PPh₃)₃] and [RhCl(cod)(PPh₃)] (cod=1,5-cyclooctadiene) to give, in both cases, the PGeP-pincer chloridogermyl rhodium(I) derivative [Rh{κ³P,Ge,P-GeCl(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] ( 2 ). Similarly, the reaction of 1 with [RhCl(cod)(MeCN)] afforded [Rh{κ³P,Ge,P-GeCl(pyrmPⁱPr₂)₂CMe₂}(MeCN)] ( 3 ). The methoxidogermyl and methylgermyl rhodium(I) complexes [Rh{κ³P,Ge,P-GeR(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (R=OMe, 4 ; Me, 5 ) were prepared by treating complex 2 with LiOMe and LiMe, respectively. Complex 5 readily reacted with CO to give the carbonyl rhodium(I) derivative [Rh{κ³P,Ge,P-GeR(pyrmPⁱPr₂)₂CMe₂}(CO)] ( 6 ), with HCl, HSnPh₃ and Ph₂S₂ rendering the pentacoordinate methylgermyl rhodium(III) complexes [RhHX{κ³P,Ge,P-GeMe(pyrmPⁱPr₂)₂CMe₂}] (X=Cl, 7 ; SnPh₃, 8 ) and [Rh(SPh)₂{κ³P,Ge,P-GeMe(pyrmPⁱPr₂)₂CMe₂}] ( 9 ), respectively, and with H₂ to give the hexacoordinate derivative [RhH₂{κ³P,Ge,P-GeMe(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] ( 10 ). Complexes 3 and 5 are catalyst precursors for the hydroboration of styrene, 4-vinyltoluene and 4-vinylfluorobenzene with catecholborane under mild conditions.     

A Bis(silylenyl)pyridine Zero‐Valent Germanium Complex and Its Remarkable Reactivity

The synthesis, reactivity, and electronic structure of the unique germylone iron carbonyl complex [SiNSi]Ge⁰ →Fe(CO)₄ is reported. The compound was obtained in 49 % yield from the reaction of the bis(N‐heterocyclic silylenyl)pyridine pincer ligand SiNSi (1,6‐C₅NH₃‐[EtNSi(N^tBu)₂CPh]₂) with GeCl₂?(dioxane) to give the corresponding chlorogermyliumylidene chloride precursor [SiNSi]Ge^IICl⁺ Cl^? , which was further reduced with K₂Fe(CO)₄. Single‐crystal X‐ray diffraction analysis of [SiNSi]Ge →Fe(CO)₄ revealed that the Ge⁰ center adopts a trigonal‐pyramidal geometry with a Si‐Ge‐Si angle of 95.66(2)°. Remarkably, one of the Si^II donor atoms in the complex is five‐coordinated because of additional (pyridine)N→Si coordination. Unexpectedly, the reaction of [SiNSi]Ge →Fe(CO)₄ with GeCl₂?(dioxane) (one molar equivalent) yielded the first push–pull germylone–germylene donor–acceptor complex, [SiNSi]Ge →GeCl₂→Fe(CO)₄ through the insertion of GeCl₂ into the dative Ge⁰→Fe bond. The electronic features of the new compounds were investigated by DFT calculations.     

Plus-Size DiPAMP Hybrids for Advanced Rhodium(I)-Catalyzed Hydrogenation: The 2RBigFUS Model

Outfitting the P,P′-distereogenic 2MeBigFUS ligand with the characteristic branched/functionalized-alkoxy feature of the R-SMS-Phos design furnished the 2RBigFUS family (Figure 2). Rhodium(I) complexes thereof exhibited a further boosted activity and superior enantioselectivity in the hydrogenation of various functionalized olefins, including the challenging α-arylacrylic acids. The application under practical conditions to enantiomeric active pharmaceutical ingredients (APIs) and relevant synthetic building blocks capped the outcome.     

Unprecedented Selectivity via Electronic Substrate Recognition in the 1,4‐Addition to Cyclic Olefins Using a Chiral Disulfoxide Rhodium Catalyst

From zero to hero? Sulfoxides are generally not considered useful ligand entities in asymmetric metal catalysis. However, a chiral disulfoxide as a chelating ligand in the rhodium‐catalyzed 1,4‐addition of aryl boronic acids to cyclic, α,β‐unsaturated ketones and esters gives impressive catalytic results, thus opening the door to future applications of this new chiral ligand class.

     

Transfer Hydrogenation of Ketones Catalyzed by Surface‐Active Ruthenium and Rhodium Complexes in Water

An improved, high‐yield, one‐pot synthetic procedure for water‐soluble ligands functionalized with trialkyl ammonium side groups H₂N(CH₂)₂NHSO₂‐p‐C₆H₄CH₂[NMe₂(C_nH_2n+1)]⁺ ( [HL ⁿ ]⁺ ; n=8, 16) was developed. The corresponding new surface‐active complexes [(p‐cymene)RuCl( L ⁿ )] and [Cp*RhCl( L ⁿ )] (Cp*=η⁵‐C₅Me₅) were prepared and characterized. For n=16 micelles are formed in water at concentrations as low as 0.6 mM , as demonstrated by surface‐tension measurements. The complexes were used for catalytic transfer hydrogenation of ketones with formate in water. Highly active catalyst systems were obtained in the case of complexes bearing C₁₆ tails due to their ability to be adsorbed at the water/substrate interface. The scope of these catalyst systems in aqueous solutions was extended from partially water soluble aryl alkyl ketones (acetophenone, butyrophenone) to hydrophobic dialkyl ketones (2‐dodecanone).     

Pi-accepting-pincer rhodium complexes: an unusual coordination mode of PCP-type systems

The novel pi-accepting, pincer-type ligand, dipyrrolylphoshinoxylene (DPyPX), is introduced. This ligand has the strongest pi-accepting phosphines used so far in the PCP family of ligands and this results in some unusual coordination chemistry. The rhodium(I) complex, [(DPyPX)Rh(CO)(PR3)] (4, R=Ph, Et, pyrrolyl) is prepared by treating the relevant [(DPyPX)Rh(PR3)] (3) complex with CO and is remarkably resistant to loss of either ligand. X-ray crystallographic analysis of complex 4 b (R=Et) reveals an unusual cisoid coordination of the PCP phosphine ligands. These observations are supported by density functional theory (DFT) calculations.     

Planar Chiral Rhodium Complexes of 1,4-Benzoquinones

Diene rhodium complexes are important catalysts in modern organic synthesis. Herein, we report a new approach to such complexes with the uncommon planar chirality. The synthesis is achieved by face-selective coordination of the prochiral 2,5-disubstituted-1,4-benzoquinones (R₂-Q) with rhodium precursors containing the chiral auxiliary ligand S-salicyl-oxazoline (S-Salox). Such coordination leads to the formation of (R,R-R₂-Q)Rh(S-Salox) complexes in high yields and with exceptional diastereoselectivity (d. r.>20 : 1). Subsequent replacement of the auxiliary ligand provides various benzoquinone rhodium complexes with retention of the planar chirality. Combined theoretical and experimental studies show that due to their electron-withdrawing nature benzoquinones bind metals stronger than the related 1,4-cyclohexadiene, but weaker than other common diene ligands, such as cyclooctadiene.