Intervalent Bis(μ‐aziridinato)M^IIM^I Complexes (M=Rh, Ir): Delocalized Metallo‐Radicals or Delocalized Aminyl Radicals?

Abstract

Reactions of the methoxo complexes [{M(mu-OMe)(cod)}(2)] (cod=1,5-cyclooctadiene, M=Rh, Ir) with 2,2-dimethylaziridine (Haz) give the mixed-bridged complexes [{M(2)(mu-az)(mu-OMe)(cod)(2)}] [(M=Rh, 1; M=Ir, 2). These compounds are isolated intermediates in the stereospecific synthesis of the amido-bridged complexes [{M(mu-az)(cod)}(2)] (M=Rh, 3; M=Ir, 4). The electrochemical behavior of 3 and 4 in CH(2)Cl(2) and CH(3)CN is greatly influenced by the solvent. On a preparative scale, the chemical oxidation of 3 and 4 with [FeCp(2)](+) gives the paramagnetic cationic species [{M(mu-az)(cod)}(2)](+) (M=Rh, [3](+); M=Ir, [4](+)). The Rh complex [3](+) is stable in dichloromethane, whereas the Ir complex [4](+) transforms slowly, but quantitatively, into a 1:1 mixture of the allyl compound [(eta(3),eta(2)-C(8)H(11))Ir(mu-az)(2)Ir(cod)] ([5](+)) and the hydride compound [(cod)(H)Ir(mu-az)(2)Ir(cod)] ([6](+)). Addition of small amounts of acetonitrile to dichloromethane solutions of [3](+) and [4](+) triggers a fast disproportionation reaction in both cases to produce equimolecular amounts of the starting materials 3 and 4 and metal--metal bonded M(II)--M(II) species. These new compounds are isolated by oxidation of 3 and 4 with [FeCp(2)](+) in acetonitrile as the mixed-ligand complexes [(MeCN)(3)M(mu-az)(2)M(NCMe)(cod)](PF(6))(2) (M=Rh, [8](2+); M=Ir, [9](2+)). The electronic structures of [3](+) and [4](+) have been elucidated through EPR measurements and DFT calculations showing that their unpaired electron is primarily delocalized over the two metal centers, with minor spin densities at the two bridging amido nitrogen groups. The HOMO of 3 and 4 and the SOMO of [3](+) and [4](+) are essentially M--M d-d sigma*-antibonding orbitals, explaining the formation of a net bonding interaction between the metals upon oxidation of 3 and 4. Mechanisms for the observed allylic H-atom abstraction reactions from the paramagnetic (radical) complexes are proposed.

Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

“…For example, nitrogen donor ligands such as tetramesitylporphyrin (Chart C) , and bis­(oxazolines) support square-planar Rh II . Pincer ligands also stabilize divalent, square-planar rhodium and iridium as in [(PNP)­Rh II X] (PNP = (4-Me-2-( i Pr 2 P)-C 6 H 3 ) 2 N; X = Cl, OR) and [(PNP)­Ir II X] (PNP = ( t Bu 2 PCHCH) 2 N; X = Cl, N 3 ). , Tri- and tetradentate, fac -coordinating nitrogen-donor ligand systems, for example, N,N,N -bis­(pyridylmethyl)­amine, stabilize Rh II and Ir II through hemilabile coordination that responds to the electronic configuration of the metal center (Chart D). − These complexes afford longer-lived M II species even with π-coordinated organic L-type ligands but undergo decomposition through allylic H atom abstraction (formally C–H bond activation). This pathway can be prevented by substituting allylic H with inert groups …”

1,3-Bis(thiophosphinoyl)indene: A Unique and Versatile Scaffold for Original Polymetallic Complexes

“…On the one hand, the Pd–Pd distance in 2 is between those observed in complexes A – C (2.54–3.19 Å), suggesting the presence of some Pd···Pd interaction. On the other hand, the bridging coordination of the sulfur atom in 2 results in a quasi-perpendicular arrangement of the two palladium coordination planes that markedly contrasts with the usual parallel disposition . To gain deeper insight into the bonding situation in 2 , DFT calculations were carried out at the B3PW91/SDD(Pd,P,S),6-31G**(other atoms) level of theory .…”

Rhodium(0) Metalloradicals in Binuclear C−H Activation

“…These metalloradicals are proposed to activate the substrates via a binuclear transition state formed in a termolecular reaction in which one [Rh II (por)] radical accepts the hydrogen atom and another the activated carbon fragment (Scheme ). – These systems have a preference for breaking aliphatic C−H bonds and are not reactive toward aromatic C−H bonds. – Although these are interesting from an academic and mechanistic point of view, the thus-obtained “activated” hydrocarbons in the form of organometallic [R−Rh III (por)] species lack reactivity because of the strong Rh−C bond. Furthermore, the only available site for further substrate binding is in the position trans to the hydrocarbon fragment R. Similar approaches using M II (N-ligand) metalloradicals (M = Rh, Ir) with nonplanar N-ligands have not led to useful C−H bond transformations either, as the M III -containing products are coordinatively saturated and kinetically inert. On the contrary, the use of low-valent metalloradicals (e.g., Rh 0 or Ir 0 ) in binuclear C−H activations should become synthetically more useful because kinetically labile and thus reactive Rh I −alkyl and Rh I −H species are the expected primary products.…”