Mixed-Valence Tetrametallic Iridium Chains.

Neutral [X-{Ir2 }-{Ir2 }-X] (X=Cl, Br, SCN, I) and dicationic [L-{Ir2 }-{Ir2 }-L]2+ (L=MeCN, Me2 CO) tetrametallic iridium chains made by connecting two dinuclear {Ir2 } units ({Ir2 }=[Ir2 (µ-OPy)2 (CO)4 ], OPy=2-pyridonate) by an iridium-iridium bond are described. The complexes exhibit fractional averaged oxidation states of +1.5 and electronic delocalization along the metallic chain. While the axial ligands do not significantly affect the metal-metal bond lengths, the metallic chain has a significant impact on the iridium-L/X bond distances. The complexes show free rotation around the unsupported iridium-iridium bond in solution, with a low-energy transition state for the chloride chain. The absorption spectra of these complexes show characteristic bands at 438-504 nm, which can be fine-tuned by varying the terminal capping ligands.

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.

Rhodium Complexes in P-C Bond Formation: Key Role of a Hydrido Ligand.

Olefin hydrophosphanation is an attractive route for the atom-economical synthesis of functionalized phosphanes. This reaction involves the formation of P-C and H-C bonds. Thus, complexes that contain both hydrido and phosphanido functionalities are of great interest for the development of effective and fast catalysts. Herein, we showcase the excellent activity of one of them, [Rh(Tp)H(PMe3)(PPh2)] (1), in the hydrophosphanation of a wide range of olefins. In addition to the required nucleophilicity of the phosphanido moiety to accomplish the P-C bond formation, the key role of the hydride ligand in 1 has been disclosed by both experimental results and DFT calculations. An additional Rh-H···C stabilization in some intermediates or transition states favors the hydrogen transfer reaction from rhodium to carbon to form the H-C bond. Further support for our proposal arises from the poor activity exhibited by the related chloride complex [Rh(Tp)Cl(PMe3)(PPh2)] as well as from stoichiometric and kinetic studies.

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 dz2 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.

Inner-Sphere Oxygen Activation Promoting Outer-Sphere Nucleophilic Attack on Olefins.

Alkoxylation and hydroxylation reactions of 1,5-cyclooctadiene (cod) in an iridium complex with alcohols and water promoted by the reduction of oxygen to hydrogen peroxide are described. The exo configuration of the OH/OR groups in the products agrees with nucleophilic attack at the external face of the olefin as the key step. The reactions also require the presence of a coordinating protic acid (such as picolinic acid (Hpic)) and involve the participation of a cationic diolefin iridium(III) complex, [Ir(cod)(pic)2 ]+ , which has been isolated. Independently, this cation is also involved in easy alkoxy group exchange reactions, which are very unusual for organic ethers. DFT studies on the mechanism of olefin alkoxylation mediated by oxygen show a low-energy proton-coupled electron-transfer step connecting a superoxide-iridium(II) complex with hydroperoxide-iridium(III) intermediates, rather than peroxide complexes. Accordingly, a more complex reaction, with up to four different products, occurred upon reacting the diolefin-peroxide iridium(III) complex with Hpic. Moreover, such hydroperoxide intermediates are the origin of the regio- and stereoselectivity of the hydroxylation/alkoxylation reactions. If this protocol is applied to the diolefin-rhodium(I) complex [Rh(pic)(cod)], free alkyl ethers ORC8 H11 (R=Me, Et) resulted, and the reaction is enantioselective if a chiral amino acid, such as l-proline, is used instead of Hpic.

Rhodium Complexes in P-H Bond Activation Reactions.

The feasibility of oxidative addition of the P-H bond of PHPh2 to a series of rhodium complexes to give mononuclear hydrido-phosphanido complexes has been analyzed. Three main scenarios have been found depending on the nature of the L ligand added to [Rh(Tp)(C2 H4 )(PHPh2 )] (Tp= hydridotris(pyrazolyl)borate): i) clean and quantitative reactions to terminal hydrido-phosphanido complexes [RhTp(H)(PPh2 )(L)] (L=PMe3 , PMe2 Ph and PHPh2 ), ii) equilibria between RhI and RhIII species: [RhTp(H)(PPh2 )(L)]⇄[RhTp(PHPh2 )(L)] (L=PMePh2 , PPh3 ) and iii) a simple ethylene replacement to give the rhodium(I) complexes [Rh(κ2 -Tp)(L)(PHPh2 )] (L=NHCs-type ligands). The position of the P-H oxidative addition-reductive elimination equilibrium is mainly determined by sterics influencing the entropy contribution of the reaction. When ethylene was used as a ligand, the unique rhodaphosphacyclobutane complex [Rh(Tp)(η1 -Et)(κC,P -CH2 CH2 PPh2 )] was obtained. DFT calculations revealed that the reaction proceeds through the rate limiting oxidative addition of the P-H bond, followed by a low-barrier sequence of reaction steps involving ethylene insertion into the Rh-H and Rh-P bonds. In addition, oxidative addition of the P-H bond in OPHPh2 to [Rh(Tp)(C2 H4 )(PHPh2 )] gave the related hydride complex [RhTp(H)(PHPh2 )(POPh2 )], but ethyl complexes resulted from hydride insertion into the Rh-ethylene bond in the reaction with [Rh(Tp)(C2 H4 )2 ].

Activating a Peroxo Ligand for C-O Bond Formation.

Dioxygen activation for effective C-O bond formation in the coordination sphere of a metal is a long-standing challenge in chemistry for which the design of catalysts for oxygenations is slowed down by the complicated, and sometimes poorly understood, mechanistic panorama. In this context, olefin-peroxide complexes could be valuable models for the study of such reactions. Herein, we showcase the isolation of rare "Ir(cod)(peroxide)" complexes (cod=1,5-cyclooctadiene) from reactions with oxygen, and then the activation of the peroxide ligand for O-O bond cleavage and C-O bond formation by transfer of a hydrogen atom through proton transfer/electron transfer reactions to give 2-iradaoxetane complexes and water. 2,4,6-Trimethylphenol, 1,4-hydroquinone, and 1,4-cyclohexadiene were used as hydrogen atom donors. These reactions can be key steps in the oxy-functionalization of olefins with oxygen, and they constitute a novel mechanistic pathway for iridium, whose full reaction profile is supported by DFT calculations.

Pseudo-tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E-Enynes.

The reactions of the rhodium(I) and iridium(I) complexes [M(PhBP3 )(C2 H4 )(NCMe)] (PhBP3 =PhB(CH2 PPh2 )3 - ) with alkynes have resulted in the synthesis of a new family of pseudo-tetrahedral complexes, [M(PhBP3 )(RC≡CR')] (M=Rh, Ir), which contain the alkyne as a four-electron donor. The reactions of these unusual compounds with two-electron donors (L=PMe3 , CNtBu) produced a change in the "donicity" of the alkyne from a 4e- to a 2e- donor to give five-coordinate complexes. These were the final products with the iridium complexes, whereas further reactions took place with the rhodium complexes. In particular, C(sp)-H bond activation of the alkyne occurred leading to hydrido alkynyl complexes. This process is essential for the further reactivity of the alkynes, and if the alkyne itself was used as reagent, E-enyne complexes were obtained. As a consequence of this chemistry, we show that the complex [Rh(PhBP3 )(C2 H4 )(NCMe)] is a very efficient pre-catalyst for the regioselective di- and trimerization of terminal alkynes to E-enynes and benzene derivatives, respectively. Interestingly, acetonitrile significantly enhanced the catalytic activity by facilitating the C(sp)-H bond activation step. A hydrometalation mechanism to account for these experimental observations is proposed.

Rhodium Complexes Promoting C-O Bond Formation in Reactions with Oxygen: The Role of Superoxo Species.

C-O bond formation in reactions of olefins with oxygen is a long standing challenge in chemistry for which the very complicated-sometimes controversial-mechanistic panorama slows down the design of catalysts for oxygenations. In this regard, the mechanistic details of the oxidation of the complex [Rh(cod)(Ph2 N3 )] (1) (cod=1,5-cyclooctadiene) with oxygen to the unique 2-rhodaoxetane compound [{Rh(OC8 H12 )(Ph2 N3 )}2 ] (2) has been investigated by DFT calculations. The results of this study provide evidences for a novel bimetallic mechanism in which two rhodium atoms redistribute the four electrons involved in the cleavage of the O=O bond. Furthermore, both oxygen atoms are used to create two new C-O bonds in a controlled fashion with 100 % atom economy. The key intermediates that we have found in this process are a mononuclear open-shell triplet superoxo compound, an open-shell singlet "µ-(peroxo)" derivative, and a closed-shell singlet "bis(µ-oxo)" complex. Some of the findings are used to predict the reactions of RhI complexes with oxygen, exemplified by that of the complex [Rh(cod)(OnapyMe2 )] (3). Starting from 3, [{Rh(OC8 H12 )(OnapyMe2 )}2 ] (4) has been prepared and characterized, which represents the second example of a 2-rhodaoxetane compound coming from an oxygenation reaction with oxygen.

Nucleophilicity and P-C Bond Formation Reactions of a Terminal Phosphanido Iridium Complex.

The diiridium complex [{Ir(ABPN2)(CO)}2(µ-CO)] (1; [ABPN2](-) = [(allyl)B(Pz)2(CH2PPh2)](-)) reacts with diphenylphosphane affording [Ir(ABPN2)(CO)(H) (PPh2)] (2), the product of the oxidative addition of the P-H bond to the metal. DFT studies revealed a large contribution of the terminal phosphanido lone pair to the HOMO of 2, indicating nucleophilic character of this ligand, which is evidenced by reactions of 2 with typical electrophiles such as H(+), Me(+), and O2. Products from the reaction of 2 with methyl chloroacetate were found to be either [Ir(ABPN2)(CO)(H)(PPh2CH2CO2Me)][PF6] ([6]PF6) or [Ir(ABPN2)(CO)(Cl)(H)] (7) and the free phosphane (PPh2CH2CO2Me), both involving P-C bond formation, depending on the reaction conditions. New complexes having iridacyclophosphapentenone and iridacyclophosphapentanone moieties result from reactions of 2 with dimethyl acetylenedicarboxylate and dimethyl maleate, respectively, as a consequence of a further incorporation of the carbonyl ligand. In this line, the terminal alkyne methyl propiolate gave a mixture of a similar iridacyclophosphapentanone complex and [Ir(ABPN2){CH═C(CO2Me)-CO}{PPh2-CH═CH(CO2Me)}] (10), which bears the functionalized phosphane PPh2-CH═CH(CO2Me) and an iridacyclobutenone fragment. Related model reactions aimed to confirm mechanistic proposals are also studied.

Terminal phosphanido rhodium complexes mediating catalytic P-P and P-C bond formation.

Complexes with terminal phosphanido (M-PR2) functionalities are believed to be crucial intermediates in new catalytic processes involving the formation of P-P and P-C bonds. We showcase here the isolation and characterization of mononuclear phosphanide rhodium complexes ([RhTp(H)(PR2)L]) that result from the oxidative addition of secondary phosphanes, a reaction that was also explored computationally. These compounds are active catalysts for the dehydrocoupling of PHPh2 to Ph2P-PPh2. The hydrophosphination of dimethyl maleate and the unactivated olefin ethylene is also reported. Reliable evidence for the prominent role of mononuclear phosphanido rhodium species in these reactions is also provided.

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.

Terminal imido rhodium complexes.

Compounds of the late transition metals with M=X multiple bonds (X=CR2, NR, O) represent a synthetic challenge, partly overcome by preparative chemists, but with noticeable gaps in the second- and third-row elements. For example, there are no isolated examples of terminal imido rhodium complexes known to date. Described herein is the isolation, characterization, and some preliminary reactivity studies of the first rhodium complexes [Rh(PhBP3)(NR)] (PhBP3=PhB{CH2PPh2}3) with a multiple and terminal Rh=N bond. These imido compounds result from reactions of organic azides with the corresponding rhodium(I) complex having a labile ligand, and display a pseudo-tetrahedral core geometry with an almost linear Rh-N-C arrangement [177.5(2)°] and a short Rh-N bond [1.780(2) Å]. We also show that the Rh=N bond undergoes protonation at the nitrogen atom or addition of H2 , and also engages in nitrene-group transfer and cycloaddition reactions.

Iridium-Catalyzed Regio- and Diastereoselective Synthesis of C-Substituted Piperazines.

Piperazine rings are essential motifs frequently found in commercial drugs. However, synthetic methodologies are mainly limited to N-substituted piperazines, preventing structural diversity. Disclosed herein is a straightforward catalytic method for the synthesis of complex C-substituted piperazines based on an uncommon head-to-head coupling of easily prepared imines. This 100% atom-economic process allows the selective formation of a sole diastereoisomer, a broad substrate scope, and a good functional group tolerance employing a bench-stable iridium catalyst under mild reaction conditions. Key to the success is the addition of N-oxides to the reaction mixture, as they notably enhance the catalytic activity and selectivity.

Cooperative double deprotonation of bis(2-picolyl)amine leading to unexpected bimetallic mixed valence (M(-I), M(I)) rhodium and iridium complexes.

Cooperative reductive double deprotonation of the complex [Rh(I)(bpa)(cod)](+) ([4](+), bpa = PyCH(2)NHCH(2)Py) with one molar equivalent of base produces the bimetallic species [(cod)Rh(bpa-2H)Rh(cod)] (7), which displays a large Rh(-I),Rh(I) contribution to its electronic structure. The doubly deprotonated ligand in 7 hosts the two "Rh(cod)" fragments in two distinct compartments: a "square planar compartment" consisting of one of the Py donors and the central nitrogen donor and a "tetrahedral π-imine compartment" consisting of the other pyridine and an "imine C═N" donor. The formation of an "imine donor" in this process is the result of substantial electron transfer from the {bpa-2H}(2-) ligand to one of the rhodium centers to form the neutral imine ligand bpi (bpi = PyCH(2)N═CHPy). Hence, deprotonation of [Rh(I)(bpa)(cod)](+) represents a reductive process, effectively leading to a reduction of the metal oxidation state from Rh(I) to Rh(-I). The dinuclear iridium counterpart, complex 8, can also be prepared, but it is unstable in the presence of 1 mol equiv of the free bpa ligand, leading to quantitative formation of the neutral amido mononuclear compound [Ir(I)(bpa-H)(cod)] (2). All attempts to prepare the rhodium analog of 2 failed and led to the spontaneous formation of 7. The thermodynamic differences are readily explained by a lower stability of the M(-I) oxidation state for iridium as compared to rhodium. The observed reductive double deprotonation leads to the formation of unusual structures and unexpected reactivity, which underlines the general importance of "redox noninnocent ligands" and their substantial effect on the electronic structure of transition metals.

Ligand-centred reactivity of bis(picolyl)amine iridium: sequential deprotonation, oxidation and oxygenation of a "non-innocent" ligand.

Treatment of [Ir(bpa)(cod)](+) complex [1](+) with a strong base (e.g., tBuO(-)) led to unexpected double deprotonation to form the anionic [Ir(bpa-2H)(cod)](-) species [3](-), via the mono-deprotonated neutral amido complex [Ir(bpa-H)(cod)] as an isolable intermediate. A certain degree of aromaticity of the obtained metal-chelate ring may explain the favourable double deprotonation. The rhodium analogue [4](-) was prepared in situ. The new species [M(bpa-2H)(cod)](-) (M = Rh, Ir) are best described as two-electron reduced analogues of the cationic imine complexes [M(I)(cod)(Py-CH(2)-N=CH-Py)](+). One-electron oxidation of [3](-) and [4](-) produced the ligand radical complexes [3](*) and [4](*). Oxygenation of [3](-) with O(2) gave the neutral carboxamido complex [Ir(cod)(py-CH(2)-N-CO-py)] via the ligand radical complex [3](*) as a detectable intermediate.

Deprotonation induced ligand-to-metal electron transfer: synthesis of a mixed-valence Rh(-I,I) dinuclear compound and its reaction with dioxygen.

Treatment of bis(2-picolyl)amine (bpa) with [{Rh(nbd)(mu-OMe)}2] leads to the unexpected and unique redox asymmetric dinuclear Rh-I,Rh+I complex [{Rh(nbd)}2(bpa-2H)] (2) with a pi-coordinating imine bound to a tetrahedral low valent rhodate(-I). Mono-oxygenation of the deprotonated bpa ligand in 2 by O2 leads to the mononuclear carboxamido complex [Rh(nbd)(bpam-H)] (3) (bpam = N-(2-picolyl)picolinamide). The second O atom of O2 ends up as a hydroxo fragment in [{Rh(nbd)(mu-OH)}2].

Intervalent bis(mu-aziridinato)M(II)--M(I) complexes (M=Rh,Ir): delocalized metallo-radicals or delocalized aminyl radicals?

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.

Formation of a bridging-imido d6 rhodium compound by nitrene capture. insertion and cycloaddition reactions.

The first d (6) rhodium imido complex, [{(C 5Me 5)Rh(mu-NSO 2C 6H 4Me- p)} 2] ( 2), has been obtained from the reaction of [(C 5Me 5)Rh(C 2H 4) 2] with chloramine-T. Carbon monoxide inserts into the N-Rh bonds in 2 to give the dinuclear ureylene complex [(C 5Me 5)Rh(mu-{(Ts) N-CO-N(Ts)}Rh(CO)(C 5Me 5)], while the azide C 6F 5N 3 adds to 2 to give the mononuclear tetrazene complex [(C 5Me 5)Rh{( p-MeC 6H 4SO 2)N 4(C 6F 5)}].