10 research outputs found
Experimental and Theoretical Mechanistic Investigation on the Catalytic CO<sub>2</sub> Hydrogenation to Formate by a Carboxylate-Functionalized Bis(<i>N</i>‑heterocyclic carbene) Zwitterionic Iridium(I) Compound
The bis-imidazolium
salt, 1,1-bis(<i>N</i>-methylimidazolium)
acetate bromide, is a convenient precursor for the synthesis of zwitterionic
iridium(I) [Ir(cod){(MeIm)<sub>2</sub>CHCOO}] and cationic iridium(III)
[IrH(cod){(MeIm)<sub>2</sub>CHCOO}]<sup>+</sup> compounds (MeIm =
3-methylimidazol-2-yliden-1-yl) having a carboxylate-functionalized
bis(NHC) ligand. The [Ir(cod){(MeIm)<sub>2</sub>CHCOO}] compound catalyzes
the hydrogenation of CO<sub>2</sub> to formate in water using NEt<sub>3</sub> as base, reaching turnover numbers of approximately 1500.
Reactivity studies have shown that activation of the catalyst precursor
involves the reaction with H<sub>2</sub> in a multistep process that
under catalytic conditions results in the formation of a dihydrido
iridium(III) octahedral [IrH<sub>2</sub>(H<sub>2</sub>O){(MeIm)<sub>2</sub>CHCOO}] species stabilized by the κ<sup>3</sup>-<i>C</i>,<i>C′</i>,<i>O</i> coordination
of the ligand. DFT studies on the mechanism were carried out to elucidate
two possible roles of the base. In the first one, NEt<sub>3</sub> neutralizes
only the produced formic acid, whereas in the second it assists the
proton transfer in heterolytic cleavage of the H<sub>2</sub> molecule.
Although this base-involved mechanism is more favorable in that it
exhibits a lower energy span for the overall reaction, the energy
barrier obtained from kinetic experiments suggests that both mechanisms
could be operative under the experimental reaction conditions
Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity
A series
of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized
bis-N-heterocyclic carbene ligand of formula [Cp*M<sup>III</sup>Cl{(MeIm)<sub>2</sub>CHCOO}] and [M<sup>I</sup>(diene){(MeIm)<sub>2</sub>CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh,
Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1,5-cyclooctadiene
(cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)<sub>2</sub>CHCOO]Br and suitable metal precursor. The solid-state
structure of both types of complexes shows a boat-shaped six-membered
metallacycle derived of the κ<sup>2</sup>C,C′ coordination
mode of the bis-NHC ligand. The uncoordinated carboxylate fragment
is found at the <i>bowsprit</i> position in the Cp*M<sup>III</sup> complexes, whereas in the M<sup>I</sup>(diene) complexes
it is at the <i>flagpole</i> position of the metallacycle.
The complexes [Rh<sup>I</sup>(diene){(MeIm)<sub>2</sub>CHCOO}]
(diene = cod, nbd) exist as two conformational isomers in dichloromethane, <i>bowsprit</i> and <i>flagpole</i>, that interconvert
through the boat-to-boat inversion of the metallacycle. An inversion
barrier of ∼17 kcal·mol<sup>–1</sup> was determined
by two-dimensional exchange spectroscopy NMR measurements for [Rh<sup>I</sup>(cod){(MeIm)<sub>2</sub>CHCOO}]. Reaction of zwitterionic
Cp*M<sup>III</sup> complexes with methyl triflate or tetrafluoroboric
acid affords the cationic complexes [Cp*M<sup>III</sup>Cl{(MeIm)<sub>2</sub>CHCOOMe}]<sup>+</sup> or [Cp*M<sup>III</sup>Cl{(MeIm)<sub>2</sub>CHCOOH}]<sup>+</sup> (M = Rh, Ir) featuring carboxy
and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands,
respectively. Similarly, complexes [M<sup>I</sup>(diene){(MeIm)<sub>2</sub>CHCOOMe}]<sup>+</sup> (M = Rh, Ir) were prepared by
alkylation of the corresponding zwitterionic M<sup>I</sup>(diene)
complexes with methyl triflate. In contrast, reaction of [Ir<sup>I</sup>(cod){(MeIm)<sub>2</sub>CHCOO}] with HBF<sub>4</sub>·Et<sub>2</sub>O (Et = ethyl), CH<sub>3</sub>OTf, CH<sub>3</sub>I, or I<sub>2</sub> gives cationic iridium(III) octahedral complexes [Ir<sup>III</sup>X(cod){(MeIm)<sub>2</sub>CHCOO}]<sup>+</sup> (X = H, Me, or I) featuring a tripodal coordination mode of the
carboxylate bridge-functionalized bis-NHC ligand. The switch from
κ<sup>2</sup>C,C′ to κ<sup>3</sup>C,C′,O
coordination of the bis-NHC ligand accompanying the oxidative addition
prevents the coordination of the anions eventually formed in the process
that remain as counterions
Mechanistic Insights into Transfer Hydrogenation Catalysis by [Ir(cod)(NHC)<sub>2</sub>]<sup>+</sup> Complexes with Functionalized N‑Heterocyclic Carbene Ligands
The synthesis of unbridged biscarbene
iridium(I) [Ir(cod)(MeIm∩Z)<sub>2</sub>]<sup>+</sup> complexes
having N- or O-functionalized NHC
ligands (∩Z = 2-methoxybenzyl, pyridin-2-ylmethyl, quinolin-8-ylmethyl)
is described. The molecular structures of the complexes show an antiparallel
disposition of the carbene ligands that minimize the steric repulsions
between the bulky substituents. However, the complexes were found
to be dynamic in solution, due to the restricted rotation about the
C(carbene)–Ir bond that results in two interconverting diasteromers
having different dispositions of the functionalized NHC ligands. A
rotational barrier of around 80 kJ mol<sup>–1</sup> (298 K)
has been determined by 2D EXSY NMR spectroscopy. The iridium(III)
dihydride complex [IrH<sub>2</sub>(MeIm∩Z)<sub>2</sub>]<sup>+</sup> (∩Z = pyridin-2-ylmethyl) has been prepared by reaction
of the corresponding iridium(I) complex with molecular hydrogen. These
complexes efficiently catalyzed the transfer hydrogenation of cyclohexanone
using 2-propanol as a hydrogen source and KOH as base at 80 °C
with average TOF values of 117–155 h<sup>–1</sup> at
0.1 mol % iridium catalyst loading. All of the catalyst precursors
showed comparable activity independent of both the wingtip type at
the NHC ligands and the counterion. Mechanistic studies support the
involvement of diene free bis-NHC iridium(I) intermediates in these
catalytic systems. DFT calculations have shown that a MPV-like concerted
mechanism (Meerwein–Ponndorf–Verley mechanism), involving
the direct hydrogen transfer at the coordination sphere of the iridium
center, might compete with the well-established hydrido mechanism.
Indirect evidence of a MPV-like mechanism has been found for the catalyst
precursor having NHC ligands having with a pyridin-2-ylmethyl wingtip
ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry
The
σ,π-cyclooctenyl iridium(III) pincer compounds [Ir(κ<sup>3</sup>-pydc-X)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir(μ-OMe)(cod)]<sub>2</sub> and the corresponding 4-substituted pyridine-2,6-dicarboxylic
acids (H<sub>2</sub>pydc-X) or, alternatively, from their lithium
salts (X = H) and [Ir(cod)(CH<sub>3</sub>CN)<sub>2</sub>]PF<sub>6</sub>. Deuterium labeling studies in combination with theoretical calculations
have shown that formation of <b>1</b> involves a metal-mediated
proton transfer in the reactive intermediate [Ir(κ<sup>2</sup>-Hpydc)(cod)], through the solvent-stabilized hydrido complex [IrH(κ<sup>3</sup>-pydc)(cod)(CH<sub>3</sub>OH)], followed by olefin insertion.
The formation of this hydrido intermediate results from solvent-assisted
proton transfer through a hydrogen-bonding network, forming an eight-membered
metallacycle. In contrast, reaction of [Ir(μ-OMe)(cod)]<sub>2</sub> with iminodiacetic acid derivatives, RN(CH<sub>2</sub>COOH)<sub>2</sub>, gave the stable iridium(I) mononuclear [Ir{κ<sup>2</sup>-MeN(CH<sub>2</sub>COOH)(CH<sub>2</sub>COO)}(cod)] (R = Me) complex
having a free carboxymethyl group and the tetranuclear complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(cod)<sub>4</sub>] (R = Ph) with doubly deprotonated ligands.
The molecular structure of the related cyclooctene complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(coe)<sub>8</sub>] has been determined by X-ray analysis.
Reaction of <b>1</b> with monodentate N- and P-donor ligands
gave the compounds [Ir(κ<sup>3</sup>-pydc)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)(L)] (L = py, BnNH<sub>2</sub>, PPh<sub>3</sub>, PMe<sub>3</sub>). Reaction of <b>1</b> with the short-bite
bis(diphenylphosphino)methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear
complex <b>1</b><sub><b>2</b></sub><b>-dppm</b> as
a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b><sub><b>2</b></sub><b>-dppe</b> and <b>1</b><sub><b>2</b></sub><b>-dppp</b> have been prepared using
1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane
(dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies
have been obtained as two diastereoisomers in a 1:1 ratio due to the
chirality of the mononuclear building block. The single-crystal X-ray
structures of <b>1-py</b> and <b>1-dppm</b> are reported
ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry
The
σ,π-cyclooctenyl iridium(III) pincer compounds [Ir(κ<sup>3</sup>-pydc-X)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir(μ-OMe)(cod)]<sub>2</sub> and the corresponding 4-substituted pyridine-2,6-dicarboxylic
acids (H<sub>2</sub>pydc-X) or, alternatively, from their lithium
salts (X = H) and [Ir(cod)(CH<sub>3</sub>CN)<sub>2</sub>]PF<sub>6</sub>. Deuterium labeling studies in combination with theoretical calculations
have shown that formation of <b>1</b> involves a metal-mediated
proton transfer in the reactive intermediate [Ir(κ<sup>2</sup>-Hpydc)(cod)], through the solvent-stabilized hydrido complex [IrH(κ<sup>3</sup>-pydc)(cod)(CH<sub>3</sub>OH)], followed by olefin insertion.
The formation of this hydrido intermediate results from solvent-assisted
proton transfer through a hydrogen-bonding network, forming an eight-membered
metallacycle. In contrast, reaction of [Ir(μ-OMe)(cod)]<sub>2</sub> with iminodiacetic acid derivatives, RN(CH<sub>2</sub>COOH)<sub>2</sub>, gave the stable iridium(I) mononuclear [Ir{κ<sup>2</sup>-MeN(CH<sub>2</sub>COOH)(CH<sub>2</sub>COO)}(cod)] (R = Me) complex
having a free carboxymethyl group and the tetranuclear complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(cod)<sub>4</sub>] (R = Ph) with doubly deprotonated ligands.
The molecular structure of the related cyclooctene complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(coe)<sub>8</sub>] has been determined by X-ray analysis.
Reaction of <b>1</b> with monodentate N- and P-donor ligands
gave the compounds [Ir(κ<sup>3</sup>-pydc)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)(L)] (L = py, BnNH<sub>2</sub>, PPh<sub>3</sub>, PMe<sub>3</sub>). Reaction of <b>1</b> with the short-bite
bis(diphenylphosphino)methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear
complex <b>1</b><sub><b>2</b></sub><b>-dppm</b> as
a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b><sub><b>2</b></sub><b>-dppe</b> and <b>1</b><sub><b>2</b></sub><b>-dppp</b> have been prepared using
1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane
(dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies
have been obtained as two diastereoisomers in a 1:1 ratio due to the
chirality of the mononuclear building block. The single-crystal X-ray
structures of <b>1-py</b> and <b>1-dppm</b> are reported
ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry
The
σ,π-cyclooctenyl iridium(III) pincer compounds [Ir(κ<sup>3</sup>-pydc-X)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir(μ-OMe)(cod)]<sub>2</sub> and the corresponding 4-substituted pyridine-2,6-dicarboxylic
acids (H<sub>2</sub>pydc-X) or, alternatively, from their lithium
salts (X = H) and [Ir(cod)(CH<sub>3</sub>CN)<sub>2</sub>]PF<sub>6</sub>. Deuterium labeling studies in combination with theoretical calculations
have shown that formation of <b>1</b> involves a metal-mediated
proton transfer in the reactive intermediate [Ir(κ<sup>2</sup>-Hpydc)(cod)], through the solvent-stabilized hydrido complex [IrH(κ<sup>3</sup>-pydc)(cod)(CH<sub>3</sub>OH)], followed by olefin insertion.
The formation of this hydrido intermediate results from solvent-assisted
proton transfer through a hydrogen-bonding network, forming an eight-membered
metallacycle. In contrast, reaction of [Ir(μ-OMe)(cod)]<sub>2</sub> with iminodiacetic acid derivatives, RN(CH<sub>2</sub>COOH)<sub>2</sub>, gave the stable iridium(I) mononuclear [Ir{κ<sup>2</sup>-MeN(CH<sub>2</sub>COOH)(CH<sub>2</sub>COO)}(cod)] (R = Me) complex
having a free carboxymethyl group and the tetranuclear complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(cod)<sub>4</sub>] (R = Ph) with doubly deprotonated ligands.
The molecular structure of the related cyclooctene complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(coe)<sub>8</sub>] has been determined by X-ray analysis.
Reaction of <b>1</b> with monodentate N- and P-donor ligands
gave the compounds [Ir(κ<sup>3</sup>-pydc)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)(L)] (L = py, BnNH<sub>2</sub>, PPh<sub>3</sub>, PMe<sub>3</sub>). Reaction of <b>1</b> with the short-bite
bis(diphenylphosphino)methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear
complex <b>1</b><sub><b>2</b></sub><b>-dppm</b> as
a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b><sub><b>2</b></sub><b>-dppe</b> and <b>1</b><sub><b>2</b></sub><b>-dppp</b> have been prepared using
1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane
(dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies
have been obtained as two diastereoisomers in a 1:1 ratio due to the
chirality of the mononuclear building block. The single-crystal X-ray
structures of <b>1-py</b> and <b>1-dppm</b> are reported
Unsaturated Iridium(III) Complexes Supported by a Quinolato–Carboxylato ONO Pincer-Type Ligand: Synthesis, Reactivity, and Catalytic C–H Functionalization
The unsaturated σ,π-cyclooctenyl
iridium(III) pincer
compound [Ir(κ<sup>3</sup>-hqca)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)] (<b>1</b>) has been prepared by the
reaction of [Ir(cod)(CH<sub>3</sub>CN)<sub>2</sub>]BF<sub>4</sub> with
lithium 8-oxidoquinoline-2-carboxylate (Li<sub>2</sub>hqca) and obtained
as two isomers derived from the relative disposition of the pincer
and the σ,π-cyclooctenyl ligands. Compound <b>1</b> can be prepared as a single isomer by reaction of 8-hydroxyquinoline-2-carboxylic
acid (H<sub>2</sub>hqca) with [Ir(μ-OMe)(cod)]<sub>2</sub>.
Reaction of [Ir(μ-OH)(coe)<sub>2</sub>]<sub>2</sub> with H<sub>2</sub>hqca gave the square-pyramidal iridum(III) complex [IrH(κ<sup>3</sup>-hqca)(coe)] (<b>3</b>). This compound exists as dinuclear
assemblies [IrH(κ<sup>3</sup>-hqca)(coe)]<sub>2</sub> in noncoordinating
solvents and as the corresponding labile mononuclear solvates in more
polar solvent solutions. The dimerization of <b>3</b> was established
by <sup>1</sup>H-DOSY NMR spectroscopy and an ESI<sup>+</sup> mass
spectrum and supported by DFT calculations. Reaction of <b>3</b> with pyridine gave the adduct [IrH(κ<sup>3</sup>-hqca)(coe)(py)]
(<b>4</b>) and the bis-pyridine complexes [IrH(κ<sup>3</sup>-hqca)(R-py)<sub>2</sub>] (R = H (<b>6</b>), 2-Me (<b>7</b>)) by replacement of the coe ligand. Compound <b>4</b> was
transformed into the bromo derivative [IrBr(κ<sup>3</sup>-hqca)(coe)(py)]
(<b>5</b>) by reaction with <i>N</i>-bromosuccinimide.
Carbonylation of <b>4</b> gave the cyclooctenyl complex [Ir(κ<sup>3</sup>-hqca)(1-κ-C<sub>8</sub>H<sub>15</sub>)(CO)(py)] (<b>8</b>), which is stable only under a carbon monoxide atmosphere.
The pincer complexes were active in the catalytic borylation of arenes
under thermal conditions
Enhanced Hydrogen-Transfer Catalytic Activity of Iridium N‑Heterocyclic Carbenes by Covalent Attachment on Carbon Nanotubes
Oxidized multiwall carbon nanotubes
(<b>CNT</b>) were covalently modified with appropriate hydroxyl-ending
imidazolium salts using their carboxylic acid groups. Characterization
of the imidazolium-modified samples through typical solid characterization
techniques, such as TGA or XPS, allows for the determination of 16
wt % in <b>CNT-1</b> and 31 wt % in <b>CNT-2</b> as the
amount of the imidazolic fragments in the carbon nanotubes. The imidazolium-functionalized
materials were used to prepare nanohybrid materials containing iridium
N-heterocyclic carbene (NHC)-type organometallic complexes with efficiencies
as high as 95%. The nanotube-supported iridium–NHC materials
were active in the heterogeneous iridium-catalyzed hydrogen-transfer
reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as
hydrogen source. The iridium hybrid materials are more efficient than
related homogeneous catalysts based on acetoxy-functionalized Ir–NHC
complexes with initial TOFs up to 5550 h<sup>–1</sup>. A good
recyclability of the catalysts, without any loss of activity, and
stability in air was observed
Iridium(I) Complexes with Hemilabile N-Heterocyclic Carbenes: Efficient and Versatile Transfer Hydrogenation Catalysts
A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]<sup>+</sup> (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH<sub>3</sub>)(cod)(MeImR)]<sup>+</sup> (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX(cod)(MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h<sup>–1</sup>. The complexes [Ir(NCCH<sub>3</sub>)(cod)(MeImR)]<sup>+</sup> (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(O<i>i</i>Pr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity
Steric Effects in the Oxidative Addition of MeI to a Sulfido-Bridged ZrRh<sub>2</sub> Early–Late Heterobimetallic Compound
Reaction of the early–late heterobimetallic compound
[Cp<sup>tt</sup><sub>2</sub>Zr(μ<sub>3</sub>-S)<sub>2</sub>{Rh(CO)}<sub>2</sub>(μ-dppm)] (<b>1</b>) with MeI affords the unusual
oxidative addition product [Cp<sup>tt</sup><sub>2</sub>Zr(μ<sub>3</sub>-S)<sub>2</sub>Rh<sub>2</sub>(μ-CO)(μ-dppm)(I)(COCH<sub>3</sub>)] (<b>4</b>), showing the presence of a bridging carbonyl
and a terminal acetyl ligand. The optimized structure of <b>4</b> by DFT calculations is further substantiated by the spectroscopic
data of the <sup>13</sup>CO-labeled complex <b>4*</b>. The same
reaction carried out on the structurally related <i>gem</i>-dithiolate dinuclear complexes [Rh<sub>2</sub>(μ-S<sub>2</sub>CR<sub>2</sub>)(CO)<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>] (<b>2</b>, <b>3</b>) gives the one-center oxidative addition
acetyl products [Rh<sub>2</sub>(μ-S<sub>2</sub>CR<sub>2</sub>)(COCH<sub>3</sub>)(I)(CO)(PPh<sub>3</sub>)<sub>2</sub>] (R<sub>2</sub> = −(CH<sub>2</sub>)<sub>5</sub>–, <b>5</b>;
R = Bn (benzyl), <b>6</b>). Reaction of <b>3</b> with
molecular iodine affords the dinuclear compounds [Rh<sub>2</sub>(μ-S<sub>2</sub>CBn<sub>2</sub>)(I)<sub>2</sub>(μ-CO)(PPh<sub>3</sub>)<sub>2</sub>] (<b>7</b>) and [Rh<sub>2</sub>(μ-S<sub>2</sub>CBn<sub>2</sub>)(I)<sub>2</sub>(CO)<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>] (<b>8</b>), the transannular oxidative addition
product, in a 40:60 ratio. Compound <b>7</b> was also formed
in the reaction of <b>3</b> with MeI after a prolonged reaction
time. The molecular structures of compounds <b>5</b> and <b>7</b> have been determined by X-ray analyses. A natural bond orbital
(NBO) analysis has shown an analogous bonding scheme in the “Rh<sub>2</sub>(μ-CO)P<sub>2</sub>” subunit of complexes <b>4</b> and <b>7</b>. Although both complexes can be formally
described as composed of a Rh(III)–Rh(III) unit, assuming a
ketonic character of the bridging carbonyl ligand, the natural charges
on the rhodium atoms and the WBI indexes for the Rh–Rh and
CO bonds points to electronically unsaturated metal centers bridged
by a carbonyl ligand lacking ketonic character and a significant metal–metal
interaction. In addition, DFT calculations on the reaction pathway
leading to the formation of <b>4</b> evidenced the key role
of the bulky “Cp<sup>tt</sup><sub>2</sub>Zr” fragment
in the course of the reaction