Oxidative-Addition of Organic Monochloro Derivatives to Dinuclear Rhodium Complexes: Mechanistic Considerations

Redox-Transmetalation Reactions: Easy Access to Homo- and Heterodimetallic d8,d10 Complexes

The ability of the imine PyCH═N-CH2Py (Py = 2-pyridyl, bpi) to behave as a heteroditopic ligand, which is suitable for creating two separate compartments to host metals in different oxidation states, has been developed by studying the reactions of the mixed-valence complexes [(cod)M-Ι(μ-bpi)MΙ(cod)] (M = Rh, Ir) with [M'(Cl)2(PPh3)2] (M' = Pd, Ni). The results depend on the molar ratio of the reagents used (1:1 or 1:2) to give the heterometallic complexes {d10-M',d8-M}-[(PPh3)(Cl)M'0(μ-bpi)MΙ(cod)] (Pd,Rh, 4; Pd,Ir, 5; Ni,Rh, 8; Ni,Ir, 9) and the two-electron mixed-valent compounds [(PPh3)(Cl)M'0(μ-bpi)M'ΙΙ(Cl)] (M' = Ni, 10; Pd, 11), respectively. A redox process occurs in the replacement of the low-valent [(cod)M-I] fragment, whereas the exchange of the [(cod)MI] fragment is redox-neutral. The metal with a d8 configuration in the products exhibits a square-planar geometry coordinated to two (Rh/Ir) or three (Ni/Pd) nitrogen atoms of the bridging bpi ligand. Conversely, the metal with a d10 configuration adopts trigonal-planar geometries, π-bonded to the imine C═N bond. The isolated complexes 4/5 and 10/11, along with the hypothetical heterometallic Pd,Ni compound (12), were studied by DFT methods. Additionally, the T-shaped moiety 'M'ΙΙ(PPh3)(Cl)(η1-CH-N(bpi))', stabilized by a secondary γ-agostic interaction, and the 'M'II(Cl)(κ3N-bpi)' fragment was found to be accessible redoxomers of complexes 10 and 11 by DFT calculations.

High-Valent Pyrazolate-Bridged Platinum Complexes: A Joint Experimental and Theoretical Study

Complexes [{Pt(C^C*)(μ-pz)}₂] (HC^C*_A = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene 1a, HC^C*_B = 1-phenyl-3-methyl-1H-imidazol-2-ylidene 1b) react with methyl iodide (MeI) at room temperature in the dark to give compounds [{Pt^IV(C^C*)Me(μ-pz)}₂(μ-I)]I (C^C*_A2a, C^C*_B2b). The reaction of 1a with benzyl bromide (BnBr) in the same conditions afforded [Br(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)Bn] (5a), which by heating in BnBr(l) became [{Pt^IV(C^C*_A)Bn(μ-pz)}₂(μ-Br)]Br (6a). Experimental investigations and density functional theory (DFT) calculations on the mechanisms of these reactions from 1a revealed that they follow a S_N2 pathway in the two steps of the double oxidative addition (OA). Based on the DFT investigations, species such as [(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)R]X (RX = MeI Int-Me, BnBr Int-Bn) and [(C^C*_A)Pt^II(μ-pz)₂Pt^IV(C^C*_A)(R)X] (RX = MeI Int′-Me, BnBr Int′-Bn) were proposed as intermediates for the first and the second OA reactions, respectively. In order to put the mechanisms on firmer grounds, Int-Me was prepared as [(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)Me]BF₄ (3a′) and used to get [I(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)Me](4a), [(C^C*_A)Pt^II(μ-pz)₂Pt^IV(C^C*_A)(Me)I](Int′-Me), and [{Pt^IV(C^C*)Me(μ-pz)}₂(μ-I)]BF₄ (2a′). The single-crystal X-ray structures of 2a, 2b, 3a′, and 5a along with the mono- and bi-dimensional ¹H and ¹⁹⁵Pt{¹H} NMR spectra of all the named species allowed us to compare structural and spectroscopic data for high-valent complexes with the same core [{Pt(C^C*)(μ-pz)}₂] but different oxidation states.

Experimental and theoretical investigations on the mechanisms of OA reactions of MeI and BnBr to [{Pt^II(C^C*)(μ-pz)}₂] allowed us to get high-valent pyrazolate-bridged platinum compounds: Pt₂(III,III), Pt₂(II,IV), and Pt₂(IV,IV). Their stability and structural and spectroscopic features have been compared.

Selective formation of cis-diacyl, cis-PPh(2)R rhodium(III) complexes by the reaction of rhodium(III) cis-diacyl, trans-PPh(2)R complexes with aliphatic diamines

The cis-diacyl, trans-PPh(2)R complex [RhCl(PPh(2)(o-C(6)H(4)CO))(2)(pyridine)] () reacts with substituted aliphatic diamines to afford selectively cationic cis-diacyl, cis-PPh(2)R, diamine derivatives [Rh(PPh(2)(o-C(6)H(4)CO))(2)(NN')](+) (NN' = 1,2-diphenylethylenediamine, 2; 1,2-propanediamine, 3; N-methylethylenediamine, 4; N,N-dimethylethylenediamine, 5; N,N'-dimethylethylenediamine, 6; N,N,N'-trimethylethylenediamine, 7) with high stereoselectivity depending on the N-donor ligand employed. Complexes 2 and 3 contain a single isomer, while 4 is a mixture of two isomers, 4a and 4b. Formation of 4a occurs first and is followed by isomerisation to 4b until the equilibrium 4a:4b = 1:4 ratio is attained. In contrast, 5 and 6 contain a single isomer. More basic amino groups prefer positions trans to an acyl group while less basic amino groups are trans to a phosphine group. The preferred intramolecular N-H...O hydrogen bond formation between an amino and an acyl coordinated ligands, trans to the phosphorus atoms, appears to be relevant to the selectivity observed. 7 is a mixture of two isomers 7a and 7b in a 7a:7b = 5.7:1 ratio. N,N,N',N'-tetramethylethylenediamine or N,N'-diphenylethylenediamine led to the elimination of the N-donor ligands and the formation of a mixture of isomers of [Rh(2)(mu-Cl)(mu-PPh(2)(o-C(6)H(4)CO))(2)(PPh(2)(o-C(6)H(4)CO))(2)](+) (), where the Rh atoms are triply bridged by two acyl groups in a head-to-tail arrangement and by a chloride. The reaction of [Rh(PPh(2)(o-C(6)H(4)CO))(2)(ndmeen)]ClO(4) (5) with acids led to the displacement of the diamine and the formation of a [8a](+):[8b](+):[8c](+) = 1:1:3 mixture. 8c, containing the weakest sigma-donor oxygen atoms trans to the strongest sigma-donor acyl groups, represents the most electronically favourable geometry for . All the complexes were fully characterized spectroscopically. Single crystal X-ray diffraction analysis was performed on 5, 6, 8a and 8b.

First X-ray structure analyses of rhodium(III) eta1-allyl complexes and a mechanism for allylic isomerization reactions

Metal-to-metal allyl transfer: Using the first structurally characterized rhodium eta(1)-allyl complexes it is shown that the sigma-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.

One-Electron versus Two-Electron Mechanisms in the Oxidative Addition Reactions of Chloroalkanes to Amido-Bridged Rhodium Complexes

The compound syn-[{Rh(mu-NH{p-tolyl})(CNtBu)(2)}(2)] (1) oxidatively adds C--Cl bonds of alkyl chlorides (RCl) and dichloromethane to each metal centre to give the cationic complexes syn-[{Rh(mu-NH{p-tolyl})(eta(1)-R)(CNtBu)(2)}(2)(mu-Cl)]Cl and anti-[{Rh(mu-NH{p-tolyl})Cl(CNtBu)(2)}(2)(mu-CH(2))]. Reaction of 1 with the chiral alkyl chloride (-)-(S)-ClCH(Me)CO(2)Me (R*Cl) gave [{Rh(mu-NH{p-tolyl})(eta(1)-R*)(CNtBu)(2)}(2)(mu-Cl)]Cl ([3]Cl) as an equimolecular mixture of the meso form (R,S)-[3]Cl-C(s) and one enantiomer of the chiral form [3]Cl-C(2). This reaction, which takes place in two steps, was modeled step-by-step by reacting the mixed-ligand complex syn-[(cod)Rh(mu-NH{p-tolyl})(2)Rh(CNtBu)(2)] (4) with R*Cl, as a replica of the first step, to give [(cod)Rh(mu-NH{p-tolyl})(2)RhCl(eta(1)-R*)(CNtBu)(2)] (5) with racemization of the chiral carbon. Further treatment of 5 with CNtBu to give the intermediate [(CNtBu)(2)Rh(mu-NH{p-tolyl})(2)RhCl(eta(1)-R*)(CNtBu)(2)], followed by reaction with R*Cl reproduced the regioselectivity of the second step to give (R,S)-[3]Cl-C(s) and [3]Cl-C(2) in a 1:1 molar ratio. Support for an S(N)2 type of reaction with inversion of the configuration in the second step was obtained from a similar sequence of reactions of 4 with ClCH(2)CO(2)Me first, then with CNtBu, and finally with R*Cl to give [(CNtBu)(2)(eta(1)-CH(2)R)Rh(mu-NH{p-tolyl})(2)(mu-Cl)Rh(eta(1)-R*)(CNtBu)(2)]Cl (R = CO(2)Me, [7]Cl) as a single enantiomer with the R configuration at the chiral carbon. The reactions of 1 with (+)-(S)-XCH(2)CH(CH(3))CH(2)CH(3) (X = Br, I) gave the related complexes [{Rh(mu-NH{p-tolyl})(eta(1)-CH(2)CH(CH(3))CH(2)CH(3))(CNtBu)(2)}(2)(mu-X)]X, probably by following an S(N)2 profile in both steps.

Nickel-Catalyzed Thioallylation of Alkynes with Allyl Phenyl Sulfides

Allylic sulfides add to alkynes in the presence of nickel complexes efficiently to afford thio-1,4-dienes regio- and stereoselectively. Functional groups such as alkoxy, siloxy, hydroxy, carboalkoxy, chloro, and cyano groups are tolerated. A mechanism that involves a pi-allyl nickel intermediate is proposed on the basis of isolation of pi-allyl complexes and distribution of products in the reactions of alpha- or gamma-methylated allyl sulfide. [reaction: see text].

The Influence of Phosphane Coligands on the Nuclearity of Rhodium(I) 4-Thiolatobenzoic Acid Complexes

[RhCl(PR3)3] (R = Ph, Et) reacts with the potassium salt of 4-mercaptobenzoic acid to give a mixture of the monomeric and dimeric complexes, [Rh(SC6H4COOH)(PR3)3] and [{Rh(-SC6H4COOH)(PR3)2}2], respectively. With the labile PPh3 coligand, the dimer is the major product, while for the electron-richer coligand PEt3, the equilibrium is easily shifted to the monomer by the addition of excess PEt3. Phosphane dissociation and dimerization could be prevented by using the chelating coligand PPh(C2H4PPh2)2. [{Rh(-SC6H4COOH)(PPh3)2}2] (2b), [Rh(SC6H4COOH)(PEt3)3] (3a), and [Rh(SC6H4COOH){PPh(C2H4PPh2)2}] (4) were fully characterized by nuclear magnetic resonance and infrared spectroscopy, mass spectrometry, and elemental analysis. The molecular structures of 2b and 4 were determined by X-ray structure analysis. In solution, the lability of the phosphane ligands leads to the decomposition of 2b. One of the decomposition products, namely, the mixed-valent complex [{RhIRhIII(-SC6H4COO)(-SC6H4COOH)(SC6H4COOH)(PPh3)3}2] (5), was characterized by X-ray structural analysis. The dinuclear rhodium(III) complex [{Rh(-SC6H4COO)(SC6H4COOH)(PEt3)2}2] (6) was shown to be a byproduct in the synthesis of 3a, and this demonstrates the reactivity of the rhodium(I) complexes toward oxidative addition. The structurally characterized complexes 2b, 4, 5, and 6 show hydrogen bonding of the free carboxyl groups.

Protonation reactions of dinuclear pyrazolato iridium(I) complexes

The complex [[Ir(mu-Pz)(CNBu(t))(2)](2)] (1) undergoes double protonation reactions with HCl and with HO(2)CCF(3) to give the neutral dihydride complexes [[Ir(mu-Pz)(H)(X)(CNBu(t))(2)](2)] (X = Cl, eta(1)-O(2)CCF(3)), in which the hydride ligands were located trans to the X groups and in the boat of the complexes, both in the solid state and in solution. The complex [[Ir(mu-Pz)(H)(Cl)(CNBu(t))(2)](2)] evolves in solution to the cationic complex [[Ir(mu-Pz)(H)(CNBu(t))(2)](2)(mu-Cl)]Cl. Removal of the anionic chloride by reaction with methyltriflate allows the isolation of the triflate salt [[Ir(mu-Pz)(H)(CNBu(t))(2)](2)(mu-Cl)]OTf. This complex undergoes a metathesis reaction of hydride by chloride in CDCl(3) under exposure to the direct sunlight to give the complex [[Ir(mu-Pz)(Cl)(CNBu(t))(2)](2)(mu-Cl)]OTf. Protonation of both metal centers in [[Ir(mu-Pz)(CO)(2)](2)] with HCl occurs at low temperature, but eventually the mononuclear compound [IrCl(HPz)(CO)(2)] is isolated. The related complex [[Ir(mu-Pz)(CO)(P[OPh](3))](2)] reacts with HCl and with HO(2)CCF(3) to give the neutral Ir(III)/Ir(III) complexes [[Ir(mu-Pz)(H)(X)(CO)(P[OPh](3))](2)], respectively. Both reactions were found to take place stepwise, allowing the isolation of the intermediate monohydrides. They are of different natures, i.e., the metal-metal-bonded Ir(II)/Ir(II) compound [(P[OPh](3))(CO)(Cl)Ir(mu-Pz)(2)Ir(H)(CO)(P[OPh](3))] and the mixed-valence Ir(I)/Ir(III) complex [(P[OPh](3))(CO)Ir(mu-Pz)(2)Ir(H)(eta(1)-O(2)CCF(3))(CO)(P[OPh](3))].