Rh III complexes

Cyclometallated Rh(III) complexes have long been known, but their photophysical properties have been investigated only recently. Particular interest has been devoted to mixed-ligand and binuclear complexes. 

TABLE 2 Absorption and Emission Properties of Cyclometalated Complexes and their Ligands 

Rh(tpy)2(bpy)+, the assignment of the luminescence emission to a 3LC excited state centered on the tpy ligand can be confidently made after consideration of the following data  

The emission spectrum is slightly displaced in energy ( 500 cm 1) and identical in shape compared to that of Pt(tpy)2(CH2Cl)Cl, whose lowest excited state is certainly a 3LC state localized on the tpy- ligand (Section III.E.3). 

The emission lifetime at 77 K (500/is) is longer than that of the Pt(tpy)2(CH2Cl)Cl complex (340 /is), as expected for a spin-forbidden LC emission that is perturbed by a lighter metal. 

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The proposed reaction mechanism (Scheme 7-2) comprises (1) oxidative addition of ArSH to RhCl(PPh3)3 to give Rh(H)(Cl)(SPh)(PPli3)n, (2) coordination ofalkyne to the Rh complex, (3) ris-insertion of alkyne into the Rh-H bond with Rh positioned at terminal carbon and H at internal carbon, (4) reductive elimination of 16 from the Rh(III) complex to regenerate the Rh(I) complex. 

The first experiments characterizing DNA-mediated CT over a precisely defined distance between covalently appended redox probes were reported in 1993 [95]. Remarkably, the luminescence of a photoexcited Ru(II) intercala-tor was quenched by a Rh(III) intercalator fixed to the other end of a 15-mer DNA duplex over 40 A away (Fig. 4). Furthermore, non-intercalating, tethered Ru(II) and Rh(III) complexes did not undergo this quenching reaction. In this way the importance of intercalative stacking for efficient CT was demonstrated. 

Fig. 4 Coupling of the redox participants to the DNA w-stack is requisite to DNA-mediated charge transport. Rapid (>109 s 1) photoinduced electron transfer occurs between the metallointercalators, [Ru(phen)2dppz]2+ and [Rh(phi) 2phen]3+, when they are tethered to opposite ends of a DNA duplex over 40 A apart. Conversely, electron transfer does not occur between non-intercalated Ru(II) and Rh(III) complexes tethered to DNA...

Oxidative repair is not a unique feature of our Rh(III) complexes. We also demonstrated efficient long-range repair using a covalently tethered naphthalene diimide intercalator (li /0 1.9 V vs NHE) [151]. An intercalated ethidium derivative was ineffective at dimer repair, consistent with the fact that the reduction potential of Et is significantly below the potential of the dimer. Thymine dimer repair by a series of anthraquinone derivatives was also evaluated [151]. Despite the fact that the excited triplets are of sufficient potential to oxidize the thymine dimer ( 3 -/0 1.9 V vs NHE), the anthraquinone derivatives were unable to effect repair [152]. We attribute the lack of repair by these anthraquinone derivatives to their particularly short-lived singlet states anthraquinone derivatives that do not rapidly interconvert to the excited triplet state are indeed effective at thymine dimer repair [151]. These observations suggest that interaction of the dimer with the singlet state may be essential for repair. 

In the series of 1-(2,4,6-trial kylphenyl-)3-mcthyl-l//-phospholes (17), only the isopropyl substituted one (17b) entered into reaction with dimeric (pentamethylcy-clopentadienyl)rhodium dichloride to afford Rh(III) complex 74 in a reversible manner. After a careful workup, 74 could be prepared and characterized (Scheme 19). 

Essentially the C1=C2 bond is inserted into the C5-H bond. This suggests that the Rh oxidatively adds across the C5-H bond. Rh can do this with aldehydes. After oxidative addition to the C5-H bond to give a Rh(III) complex, insertion and reductive elimination give the product and regenerate Rh(I). Solvent molecules may be associating or dissociating at any point in the sequence. 

There is no reason to believe that the conjugate base mechanism does not apply with the other metal ions studied. Complexes of Cr(III) undergo base hydrolysis, but generally rate constants are lower, often 10 —10 less than for the Co(III) analog, Table 4.10. The lower reactivity appears due to both lower acidity (A"i) and lower lability of the amido species (kf) in (4.49) (provided k i can be assumed to be relatively constant). The very unreactive Rh(III) complexes are as a result of the very low reactivity of the amido species. The complexes of Ru(III) most resemble those of Co(III) but, as with Rh(III), base hydrolyses invariably takes place with complete retention of configuration. ... 

The absorption spectra of tris-polypyridyl Rhodium(III) complexes are characterised by several intense Ligand Centered (LC) absorption bands in the UV. Neither MC absorption bands, nor CT bands are observed in the visible region of the spectrum in contrast to their Ruthenium analogues. This makes tris(polypyridyl)Rh(III) complexes formed with bpy and phen practically colorless [1]. 

Bis- and tris polypyridyl Rh(III) complexes which have been studied with DNA can be found in Table 2. 

It has been shown that polypyridyl Rh(III) complexes induce photo-cleavages of the sugar phosphate backbone of double-stranded DNA with a higher relative quantum yield than Ru(II) complexes of phen or DIP. Thus replacement of Ru(II) ions by Rh(III) in Tris(phen) complexes, increases the efficiency of DNA photo-cleavages. However, in contrast to the Ru(II) complexes, Rh(III) samples have to be illuminated in the UV because of the absence of absorption bands in the visible region. 

It would be interesting to test with other Rh(III) complexes, whether the direct oxidation of the base (by photo-electron transfer) could also be a primary step responsible for photocleavages. Indeed, as outlined before in Sect. 5, radiation studies have shown that the radical cation of the base can produce the sugar radical, itself leading to strand scission [122]. Moreover base release, as observed with the Rh(III) complexes, can also take place from the radical cation of the base [137]. Direct base oxidation and hydrogen abstraction from the sugar could be two competitive pathways leading to strand scission and/or base release. 

In contrast to the Pt(0) and Pt(II) complexes and the corresponding Rh(I) and Rh(III) complexes, the iridium complexes have rarely been employed as hydrosilylation catalysts [1-4]. Iridium-phosphine complexes with d metal configura-tion-forexample, [Ir(CO)Cl(PPh3)2] (Vaska s complex) and [Ir(CO)H(PPh3)3]-were first tested some 40 years ago in the hydrosilylation of olefins. Although they underwent oxidative addition with hydrosilanes (simultaneously to Rh(I) com-... 

Matsuda reported a protocol for the cyclization/hydrosilylation of diynes to form silylated ( )-I,2-dialkylidene cyclopentanes catalyzed by neutral Rh(i) and Rh(iii) complexes.For example, reaction of dimethyl dipropargylmalonate with dimethylphenylsilane catalyzed by Rh(H)(SiMe2Ph)Gl(PPh3)2 7 in dichloromethane at room temperature gave... 

Hydroamination of allenes and 1,3-dienes in the presence of Ni(II), Pd(II), and Rh(III) complexes yields product mixtures composed of simple addition products and products formed by addition and telomerization.288 Nickel halides308 and rhodium chloride309 in ethanol [Eq. (6.51)] and Pd(n) diphosphine complexes310 are the most selective catalysts in simple hydroamination, while phosphine complexes favor telomerization 288... 

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achieved. If a diphosphine ligand of C2 symmetry is used, two diastereomers of the enamide coordination complex can be produced because the olefin can interact with either the re face or the si face. This interaction leads to enantiomeric phenylalanine products via diastereomeric Rh(III) complexes. The initial substrate-Rh complex formation is reversible, but interconversion of the diastereomeric olefin complexes may occur by an intramolecular mechanism involving an olefin-dissociated, oxygen-coordinated species (18h). Under ordinary conditions, this step has higher activation enthalpies than the subsequent oxidative addition of H2, which is the first... 

Using the metalloradical reactivity of the Rh(II)OEP (OEP = 2,3,7,8,12,13,17,18-octaethylphorphynato) dimer, the preparation of silyl rhodium complexes was achieved by the hydrogen elimination reaction with silanes I R SiH (R = R = Et, Ph R = Me, R = Ph, OEt). The Rh—Si bond length of 2.32(1) A, found when R = Et, is comparable to those in other Rh(III) complexes (Table 11). The crystal packing indicates that all the ethyl groups on the porphyrin periphery are directed toward the silyl group. Consequently, the aromatic part of one complex molecule is in contact with the aromatic part of the next molecule and the aliphatic part is in contact with the aliphatic part of the next molecule204. 

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