Synthesis and Structure of Heterodinuclear Rhodium and Iridium Borylene Complexes

Herein we report on the synthesis and structural characterization of a representative range of novel heterodinuclear bridging rhodium and iridium borylene complexes. The iridium borylene complexes feature an unusual coordination mode of the borylene ligand. Furthermore, the first instance of a heterodinuclear‐bridged borylene compound containing a chromium atom in the three‐membered ring is reported.     

The Forgotten Nitroaromatic Phosphines as Weakly Donating P‐ligands: An N‐Aryl‐benzimidazolyl Series in RhCl(CO) Complexes

The coordination chemistry of a priori weakly σ ‐donating nitroaromatic phosphines is addressed through a series of nitro‐substituted (N ‐phenyl‐benzimidazol‐1‐yl)diphenylphosphines in Rh^I complexes. From a set of seven such phosphines L=L_xyz^(′) (x , y , z =0 or 1=number of NO₂ substituents at the 5, 6 and N‐Ph para positions, respectively), including the non‐nitrated parent L₀₀₀ and its dicationic N‐methyl counterpart L₀₀₀′, three LRhCl(COD) and seven L₂RhCl(CO) complexes have been obtained in 72–95 % yield. Despite of a cis orientation of the L and CO ligands, the C=O IR stretching frequency ν _CO varies in the expected sense, from 1967±1 cm⁻¹ for L_xy0 to 1978±1 cm⁻¹ for L_xy1, and 2005 cm⁻¹ for L₀₀₀′. The ¹⁰³Rh NMR chemical shift δ _Rh varies from −288 ppm for L₀₀₀ to −316±1 ppm for L_10z or L_01z, and −436 ppm for L₀₀₀′. The ν _CO and δ _Rh probes thus reveal moderate but systematic variations, and act as “orthogonal” spectroscopic indicators of the presence of nitro groups on the N‐Ph group and the benzimidazole core, respectively. For the dicationic ligand L₀₀₀′, a tight electrostatic sandwiching of the Rh‐Cl bond by the benzimidazole moities is evidenced by X‐ray crystallography (RhCl^{δ −}⋅⋅⋅CN₂⁺ ≈3.01 Å). Along with the LRhCl(CO) complexes, dinuclear side‐products (μ‐CO)(RhClL)₂ were also obtained in low spectroscopic yield: for the dinitro ligand L=L₀₁₁, a unique 1:6.7 clathrate structure, with dichloromethane as solvate, is also revealed by X‐ray crystallography.     

Rhodium(I) and rhodium(III) complexes formed by coordination and C-H activation of bulky functionalized phosphanes

The reaction of [[RhCl(C(8)H(14))(2)](2)] (2) with iPr(2)PCH(2)CH(2)C(6)H(5) (L(1)) led, via the isolated dimer [[RhCl(C(8)H(14))(L(1))](2)] (3), to a mixture of three products 4 a-c, of which the dinuclear complex [[RhCl(L(1))(2)](2)] (4 a) was characterized by Xray crystallography. The mixture of 4a-c reacts with CO, ethene, and phenylacetylene to give the square-planar compounds trans-[RhCl(L)(L(1))(2)] (L=CO (5), C(2)H(4) (6), C=CHPh (9)). The corresponding allenylidene(chloro) complex trans-[RhCl(=C=C=CPh(2))(L(1))(2)] (11), obtained from 4 a-c and HC triple bond CC(OH)Ph(2) via trans-[RhCl[=C=CHC(OH)Ph(2)](L(1))(2)] (10), could be converted stepwise to the related hydroxo, cationic aqua, and cationic acetone derivatives 12-14, respectively. Treatment of 2 and [[RhCl(C(2)H(4))(2)](2)] (7) with two equivalents of tBu(2)PCH(2)CH(2)C(6)H(5) (L(2)) gave the dimers [[RhCl(C(8)H(14))(L(2))](2)] (15) and [[RhCl(C(2)H(4))(L(2))](2)] (16), which both react with L(2) in the molar ratio of 1:2 to afford the five-coordinate aryl(hydrido)rhodium(III) complex [RhHCl(C(6)H(4)CH(2)CH(2)PtBu(2)-kappa(2)C,P)(L(2))] (17) by C-H activation. The course of the reactions of 17 with CO, H(2), PhC triple bond CH, HCl, and AgPF(6), leading to the compounds 19-21, 24, and 25 a, respectively, indicate that the coordinatively unsaturated isomer of 17 with the supposed composition [RhCl(L(2))(2)] is the reactive species. Labeling experiments using D(2), DCl, and PhC triple bond CD support this proposal. With either [Rh(C(8)H(14))(eta(6)-L(2)-kappaP]PF(6) or [Rh(C(2)H(4))(eta(6)-L(n)-kappaP]PF(6) (n=1 and 2) as the starting materials, the corresponding halfsandwich-type complexes 27, 28, and 32 were obtained. The nonchelating counterpart of the dihydrido compound 32 with the composition [RhH(2)(PiPr(3))(eta(6)-C(6)H(6))]PF(6) (35) was prepared stepwise from [Rh(C(2)H(4))(PiPr(3))(eta(6)-C(6)H(6))]PF(6) and H(2) in acetone via the tris(solvato) species [RhH(2)(PiPr(3))(acetone)(3)]PF(6) (34) as intermediate. The synthesis of the bis(chelate) complex [Rh(eta(4)-C(8)H(12))(C(6)H(5)OCH(2)CH(2)PtBu(2)-kappa(2)O,P)]BF(4) (39) is also described. Besides 4 a, the compounds 17, 25 a, and 39 have been characterized by Xray crystal structure analysis.     

Monohydride-Dichloro Rhodium(III) Complexes with Chiral Diphosphine Ligands as Catalysts for Asymmetric Hydrogenation of Olefinic Substrates

We report full details of the synthesis and characterization of monohydride-dichloro rhodium(III) complexes bearing chiral diphosphine ligands, such as (S)-BINAP, (S)-DM-SEGPHOS, and (S)-DTBM-SEGPHOS, producing cationic triply chloride bridged dinuclear rhodium(III) complexes ( 1 a : (S)-BINAP; 1 b : (S)-DM-SEGPHOS) and a neutral mononuclear monohydride-dichloro rhodium(III) complex ( 1 c : (S)-DTBM-SEGPHOS) in high yield and high purity. Their solid state structure and solution behavior were determined by crystallographic studies as well as full spectral data, including DOSY NMR spectroscopy. Among these three complexes, 1 c has a rigid pocket surrounded by two chloride atoms bound to the rhodium atom together with one tBu group of (S)-DTBM-SEGPHOS for fitting to simple olefins without any coordinating functional groups. Complex 1 c exhibited superior catalytic activity and enantioselectivity for asymmetric hydrogenation of exo-olefins and olefinic substrates. The catalytic activity of 1 c was compared with that of well-demonstrated dihydride species derived in situ from rhodium(I) precursors such as [Rh(cod)Cl]₂ and [Rh(cod)₂]⁺[BF₄]⁻ upon mixing with (S)-DTBM-SEGPHOS under dihydrogen.     

Chiral and Nonchiral [OsX2(diphosphane)(diamine)] (X: Cl,OCH2CF3) Complexes for Fast Hydrogenation of Carbonyl Compounds

The osmium complexes trans‐[OsCl₂(dppf)(diamine)] (dppf: 1,1′‐bis(diphenylphosphino)ferrocene; diamine: ethylenediamine in 3 , propylenediamine in 4 ) were prepared by the reaction of [OsCl₂(PPh₃)₃] ( 1 ) with the ferrocenyl diphosphane, dppf and the corresponding diamine in dichloromethane. The reaction of derivative 3 with NaOCH₂CF₃ in toluene afforded the alkoxide cis‐[Os(OCH₂CF₃)₂(dppf)(ethylenediamine)] ( 5 ). The novel precursor [Os₂Cl₄(P(m‐tolyl)₃)₅] ( 2 ) allows the synthesis of the chiral complexes trans‐[OsCl₂(diphosphane)(1,2‐diamine)] ( 6 – 9 ; diphosphane: (R)‐[6,6′‐dimethoxy(1,1′‐biphenyl)‐2,2′‐diyl]bis[1,1‐bis(3,5‐dimethylphenyl)phosphane] (xylMeObiphep) or (R)‐(1,1′‐binaphthalene)‐2,2′‐diylbis[1,1‐bis(3,5‐dimethylphenyl)phosphane] (xylbinap); diamine=(R,R)‐1,2‐diphenylethylenediamine (dpen) or (R,R)‐1,2‐diaminocyclohexane (dach)), obtained by the treatment of 2 with the diphosphane and the 1,2‐diamine in toluene at reflux temperature. Compounds 3 – 5 in ethanol and in the presence of NaOEt catalyze the reduction of methyl aryl, dialkyl, and diaryl ketones and aldehydes with H₂ at low pressure (5 atm), with substrate/catalyst (S/C) ratios of 10 000–200 000 and achieving turnover frequencies (TOFs) of up to 3.0×10⁵ h^?1 at 70 °C. By employment of the chiral compounds 6 – 9 , different ketones, including alkyl aryl, bulky tert‐butyl, and cyclic ketones, have successfully been hydrogenated with enantioselectivities up to 99 % and with S/C ratios of 5000–100 000 and TOFs of up to 4.1×10⁴ h^?1 at 60 °C.     

Stable and Storable N(CF3)2 Transfer Reagents

Fluorinated groups are essential for drug design, agrochemicals, and materials science. The bis(trifluoromethyl)amino group is an example of a stable group that has a high potential. While the number of molecules containing perfluoroalkyl, perfluoroalkoxy, and other fluorinated groups is steadily increasing, examples with the N(CF₃)₂ group are rare. One reason is that transfer reagents are scarce and metal-based storable reagents are unknown. Herein, a set of Cu^I and Ag^I bis(trifluoromethyl)amido complexes stabilized by N- and P-donor ligands with unprecedented stability are presented. The complexes are stable solids that can even be manipulated in air for a short time. They are bis(trifluoromethyl)amination reagents as shown by nucleophilic substitution and Sandmeyer reactions. In addition to a series of benzylbis(trifluoromethyl)amines, 2-bis(trifluoromethyl)amino acetate was obtained, which, upon hydrolysis, gives the fluorinated amino acid N,N-bis(trifluoromethyl)glycine.     

First X‐Ray Structure Analyses of Rhodium(III) η1‐Allyl Complexes and a Mechanism for Allylic Isomerization Reactions

Metal‐to‐metal allyl transfer : Using the first structurally characterized rhodium η¹‐allyl complexes it is shown that the σ‐bound allyl substituent can be transferred from the Rh^III complex to a Rh^I complex in a fast equilibrium. This process may account for the decrease in regioselectivity observed in allylic alkylation reactions in which complex 1 is used as a catalyst.

     

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.     

Discrete Supramolecular Stacks Based on Multinuclear Tweezer-Type Rhodium Complexes

By taking advantage of self-complementary π–π stacking and CH–π interactions, a series of discrete quadruple stacks were constructed through the self-aggregation of U-shaped dirhodium metallotweezer complexes featuring various planar polyaromatic ligands. By altering the conjugate stacking strength and bridging ligands, assemblies with a range of topologies were obtained, including a binuclear D-shaped macrocycle, tetranuclear open-ended cagelike frameworks, and duplex metallotweezer stacking structures. Furthermore, a rare stacking interaction resulting in selective C−H activation was observed during the self-assembly process of these elaborate architectures.     

Reductive Coupling of Diynes at Rhodium Gives Fluorescent Rhodacyclopentadienes or Phosphorescent Rhodium 2,2’‐Biphenyl Complexes

Reactions of [Rh(κ²‐O,O‐acac)(PMe₃)₂] (acac=acetylacetonato) and α,ω‐bis(arylbutadiynyl)alkanes afford two isomeric types of MC₄ metallacycles with very different photophysical properties. As a result of a [2+2] reductive coupling at Rh, 2,5‐bis(arylethynyl)rhodacyclopentadienes ( A ) are formed, which display intense fluorescence (Φ=0.07–0.54, τ=0.2–2.5 ns) despite the presence of the heavy metal atom. Rhodium biphenyl complexes ( B ), which show exceptionally long‐lived (hundreds of μs) phosphorescence (Φ=0.01–0.33) at room temperature in solution, have been isolated as a second isomer originating from an unusual [4+2] cycloaddition reaction and a subsequent β‐H‐shift. We attribute the different photophysical properties of isomers A and B to a higher excited state density and a less stabilized T₁ state in the biphenyl complexes B , allowing for more efficient intersystem crossing S₁→T_n and T₁→S₀. Control of the isomer distribution is achieved by modification of the bis‐ (diyne) linker length, providing a fundamentally new route to access photoactive metal biphenyl compounds.     

Pseudotetrahedral Rhodium(I) Complexes

A combination of four‐electron donors, such as alkynes, with strongly donating and strong‐field scorpionate ligands is appropriate to create pseudotetrahedral rhodium(I) environments, as found in [Rh(PhBP₃)(HC?CPh)], which promotes H?C bond activation and C?C coupling reactions under very smooth conditions.