The steric and electronic demands of the catalytic olefin hydrogenation of tert-butylethylene with oxorhenium/Lewis acid FLPs were evaluated. The sterics of the ligand were altered by installing bulkier isopropyl groups in the 2,6-positions of the diamidopyridine (DAP) ligand. Lewis acid/base adducts were not isolated for complexes with this ligand; however, species incorporating isopropyl groups were still active in catalytic hydrogenation. Modifications were also made to the Lewis acid, and catalytic reactions were performed with Piers’ borane, HB(C6F5)2, and the aluminum analogue Al(C6F5)3. The rate of catalytic hydrogenation was shown to strongly correlate with the size of the alkyl, aryl, or hydride ligand. This was confirmed by a linear Taft plot with the steric sensitivity factor δ = −0.57, which suggests that reaction rates are faster with sterically larger X substituents. These data were used to develop a catalyst ((MesDAP)Re(O)(Ph)/HB(C6F5)2) that achieved a TON of 840 for the hydrogenation of tert-butylethylene at mild temperatures (100 °C) and pressures (50 psi of H2). Tuning of the oxorhenium catalysts also resulted in the hydrogenation of tert-butylethylene at room temperature.Keywords: catalytic tuning; frustrated Lewis pairs; hydrogenation; olefins; transition metal oxos;

A series of novel cationic Re(III) complexes [(DAAm)Re(CO)(NCCH3)2][X] [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5 (a), Mes (b)] [X = OTf (2), BArF4 [BArF4 = tetrakis[3,5-(trifluoromethyl)phenyl]borate] (3), BF4 (4), PF6 (5)], and their analogue [(DAmA)Re(CO)(Cl)2] [DAmA = N,N-bis(2-arylamineethyl)methylamino; aryl = C6F5] (6) were synthesized. The catalytic efficiency for the hydrosilylation reaction of aldehydes using 4a (0.03 mol%) has been demonstrated to be significantly more active than rhenium catalysts previously reported in the literature. The data suggest that electron-withdrawing substituents at the diamido amine ligand increase the catalytic efficiency of the complexes. Excellent yields were achieved at ambient temperature under neat conditions using dimethylphenylsilane. The reaction affords TONs of up to 9200 and a TOF of up to 126 h−1. Kinetic and mechanistic studies were performed, and the data suggest that the reaction is via a non-hydride ionic hydrosilylation mechanism.

The reaction of oxorhenium complexes that incorporate diamidopyridine (DAP) ligands with B(C6F5)3 results in the formation of classical Lewis acid–base adducts. The adducts effectively catalyze the hydrogenation of a variety of unactivated olefins at 100 °C. Control reactions with these complexes or B(C6F5)3 alone did not yield any hydrogenated products under these conditions. Mechanistic studies suggest a frustrated Lewis pair is generated between the oxorhenium DAP complexes and B(C6F5)3, which is effective at olefin hydrogenation. Thus, we demonstrate for the first time that the incorporation of a transition-metal oxo in a frustrated Lewis pair can have a synergistic effect and results in enhanced catalytic activity.

Computational studies (M06) have been performed in synergy with experimental studies to show that the thermodynamics for insertion of CO into an oxorhenium–hydride bond to form a formyl ligand is favorable despite conventional wisdom to the contrary. Further, it is shown that insertion of CO into formyl ligands to form α-dicarbonyl ligands is also a viable pathway and results in hydroxy carbonyl or formate complexes, depending on the nature of the ancillary ligand.

Several oxorhenium hydride complexes with chelating diamidopyridine (DAP), diamidoamine (DAAm), and 2-mercaptoethyl sulfide (SSS) groups have been isolated and characterized. Adduct formation is observed when the DAP complex 1a is treated with the Lewis acid B(C6F5)3. However, treatment of 1a,b with B(C6F5)3 or BF3·OEt2 in the presence of CO results in reduction of the metal center by four electrons from Re(V) to Re(I).

Several oxorhenium complexes bearing an SSS pincer ligand were isolated and characterized, and their reactivity with carbon monoxide was explored. The corresponding oxorhenium(V) acyl derivatives were also isolated and characterized. Carbonylation reactions required high pressures (400 psi) and temperatures (50 °C). The mechanism for carbonylation was explored with DFT (M06) calculations and revealed that the most likely mechanism for carbonylation involved stepwise formation of CO adducts followed by migration of the carbonyl ligand to the alkyl/aryl groups.

Combined experimental and computational studies have revealed factors that influence the nondirected C–H activation in Cp*Ir complexes that contain carboxylate ligands. A two-step acetate-assisted pathway was shown to be operational where the first step involves substrate binding and the second step involves cleavage of the C–H bond of the substrate. A nonlinear Hammett plot was obtained to examine substituted arenes where a strong electronic dependence (ρ = 1.67) was observed for electron-donating groups, whereas no electronic dependence was observed for electron-withdrawing groups. Electron-donating substituents in the para position were shown to have a bigger impact on the C–H bond cleavage step, whereas electron-withdrawing substituents influenced the substrate-binding step. Although cleavage of the C–H bond was predicted to be more facile with arenes that contain substituents in the para position by DFT calculations, the cyclometalations of anisole and benzonitrile were observed experimentally. This suggests that these substituents, even though they are weakly directing, still result in cyclometalation because the barriers for activation at the ortho and para positions of arenes are comparable (24.3 and 26.5 kcal/mol, respectively). Incorporation of a weakly bound ligand was found to be necessary for facile reactivity. It is predicted by DFT calculations that the replacement of an oxygen atom with a nitrogen atom in the carboxylate ligand would lead to a dramatic reduction in the barrier for C–H activation, as the incorporation of formimidate and N-methylformimidate ligands leads to barriers of 23.4 and 21.7 kcal/mol, respectively. These values are significantly lower than the barrier calculated for the analogous acetate ligand (28.2 kcal/mol).

Methanol formation from [Cp*IrIII(NHC)Me(CD2Cl2)]+ occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)2(μ-O)][(BArF4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an IrIV–O–IrIV core in which two d5 Ir(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the μ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the μ-oxo core (Ir–O–Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir–O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium–methyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BArF4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BArF4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.

Cp*Ir(Me2SO)(OAc)2, 1, has been shown to activate an ortho-C–H-bond of thiobenzoic acid in methanol at 100 °C to form the novel iridium complex (Cp*Ir(S(O)CC6H4))2, 5. Unlike the similar reaction with benzoic acid, this new iridium complex exists as a dimer with bridging sulfur ligands. To our knowledge, this is the first example of metal facilitated C–H activation of thiobenzoic acid. Complex 5 reacted with CO to form Cp*Ir(CO)(S(O)CC6H4), 6. Complex 5 was also shown to react with the electrophilic methyl reagents methyl iodide and methyl triflate. The reaction with methyl iodide resulted in the formation of Cp*Ir(I)(SMe(O)CC6H4), 7, while the reaction with methyl triflate resulted in the bimetallic complex [(Cp*Ir(SMe(O)CC6H4)(Cp*Ir(S(O)CC6H4)][OTf], 8. The order of reactivity with electrophilic methyl reagents was MeOTf > MeI >MeOAc, which reflects the nucleophilic character of the sulfoxyl group of the cyclometalated thiobenzoate ligand. Attempts at cyclization with complex 5 and olefins and alkynes were not successful. In addition, C–I reductive elimination reactions of 7 to form S-methyl 2-iodobenzothioate were not observed.

The complexes [(DAAm)Re(O)(R)] [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5], 1, R = Me; 3a–d (R = benzyl, a; 4-methylbenzyl, b; 4-fluorobenzyl, c; 4-methoxybenzyl, d); and 4, R = Ph, were synthesized. CO insertion into the Re–R bond in 1 and 3a–d resulted in the formation of the acetyl complex, 2, and the (aryl)acetyl complexes, 5a–d respectively. The formation of 5a–d proceeded at a faster rate (7 h) than the formation of 2 (72 h) under the same conditions. No reaction was observed however for the phenyl complex 4 with CO. Kinetics for CO insertion into the various Re–R bonds were examined, and the experimental rate law was determined to be Rate = kobs[Re][CO]. The activation parameters for CO insertion into 1 and 3a were determined to be ΔG⧧(298 K) = 24(1). The enthalpy of activation ΔH⧧ was determined to be 9(1) and 10(3) kcal/mol for 1 and 3a, respectively, and the entropy of activation, ΔS⧧, was −49(2) and −36(4) cal/mol·K. Computational studies (M06) are consistent with the hypothesis that the rate of CO insertion is dependent on the strength of the rhenium–carbon bond. Thus, experimental and computational data suggest that the most likely mechanism for the insertion of CO into the Re–R bond in oxorhenium complexes is a direct-insertion mechanism.

A series of monomethyl Cp*IrIII complexes were synthesized and studied for the formation of methanol in water. Methanol yields of 75(4)% in the presence of O2 were obtained. From isotope labeling studies, it was determined that O2 is the source of the oxygen atom in the product. From kinetic studies, oxyfunctionalization appears to proceed by dissociation of an L-type ligand followed by O2 binding and insertion.

The synthesis of the Lewis acid–base adducts of B(C6F5)3 and BF3 with [DAAmRe(O)(X)] DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5 (X = Me, 1, COCH3, 2, Cl, 3) as well as their diamidopyridine (DAP) (DAP=(2,6-bis((mesitylamino)methyl)pyridine) analogues, [DAPRe(O)(X)] (X = Me, 4, Cl, 5, I, 6, and COCH3,7), are described. In these complexes the terminal oxo ligands act as nucleophiles. In addition we also show that stoichiometric reactions between 3 and triarylphosphine (PAr3) result in the formation of triarylphosphine oxide (OPAr3). The electronic dependence of this reaction was studied by comparing the rates of oxygen atom transfer for various para-substituted triaryl phosphines in the presence of CO. From these experiments a reaction constant ρ = −0.29 was obtained from the Hammett plot. This suggests that the oxygen atom transfer reaction is consistent with nucleophilic attack of phosphorus on an electrophilic metal oxo. To the best of our knowledge, these are the first examples of mono-oxo d2 metal complexes in which the oxo ligand exhibits ambiphilic reactivity.

Cp*Ir(III) complexes have been shown to catalyze the oxidative coupling of benzoic acids with alkynes in methanol at 60 °C to form a variety of isocoumarins. The use of AgOAc as an oxidant was required to facilitate significant product formation. Alkyl alkynes were shown to be more reactive substrates than aryl alkynes, and a number of functional groups were tolerated on benzoic acid. Combined mechanistic and computational studies (BP86) revealed that (1) C–H activation occurs via an acetate-assisted mechanism; (2) C–H activation is not turnover limiting; and (3) the oxidant oxidizes the reduced form of the catalyst via an Ir(I)–Ir(II)–Ir(III) sequence.Keywords: annulation; Cp*IrIII; C−H activation; C−H functionalization; oxidative coupling

The reactivity of a series of Cp*IrIII(L) complexes that contain a diverse set of ancillary ligands, L, (L = PMe3, N-heterocyclic carbene, NHC = 1,3-dimethylimidazol-2-ylidene, aqua, 4-t-butylpyridine, and 4-(2,4,6-tris-(4-t-butylphenyl)pyridinium)pyridine tetrafluoroborate) has been examined in catalytic H/D exchange reactions between C6H6 and a series of deuterated solvents (methanol-d4, acetic acid-d4, and trifluoroacetic acid-d1). These studies demonstrate that (1) the mechanism of catalytic H/D exchange is significantly influenced by the nature of the solvent; (2) electron-donating ligands (PMe3, NHC) promote the formation of Ir hydrides in methanol-d4, and these are critical intermediates in catalytic H/D exchange processes; and (3) weak/poorly donating ligands (4-t-butylpyridine, 4-(2,4,6-tris-(4-t-butylphenyl)pyridinium)pyridine tetrafluoroborate and aqua) can support efficient H/D exchange catalysis in acetic acid-d4.Keywords: Cp*IrIII catalysts; C−H activation; C−H functionalization; H/D exchange; isotope effects; kinetics

A multistep experiment for an advanced synthesis lab course that incorporates topics in organic–inorganic synthesis and catalysis and highlights green chemistry principles was developed. Students synthesized two N-heterocyclic carbene ligands, used them to prepare two well-defined copper(I) complexes and subsequently utilized the complexes as catalysts in the Huisgen 1-3 dipolar cycloaddition of benzyl azide and phenylacetylene. The catalytic reaction exhibits high atom economy, is performed without a solvent at room temperature, and is high yielding. Thus, students were able to practice and apply concepts of green chemistry through catalysis. In the process of preparing ligands and complexes, several techniques were utilized that were aimed at performing reactions more efficiently (microwave experiments) or performing reactions in benign solvents (H2O). Another major component of this experiment is emphasis on technical writing through student preparation of formal communications and full paper using the formal ACS format.Keywords: Catalysis; Communication/Writing; Green Chemistry; Hands-On Learning/Manipulatives; Inorganic Chemistry; Laboratory Instruction; Organic Chemistry; Synthesis; Upper-Division Undergraduate;

A series of oxorhenium alkyl, phenyl, and vinyl complexes of the form [(DAP)Re(O)(R)] (R = aryl, vinyl, alkyl) was reported, and their reactivity with CO was examined. The methyl complex 5a reacts with CO at a significantly faster rate (2.5 h) than the phenyl complex 7a (24 h). Computational (B3PW91) studies reveal that although the acyl complex is the least stable (ΔG353 = −11.2 kcal/mol) with respect to CO insertion compared to the benzoyl complex (ΔG353 = −14.5 kcal/mol), the activation energy for CO insertion is lower for the methyl complex (ΔG⧧353 = 14.6 kcal/mol) than for the phenyl complex (ΔG⧧353 = 17.4 kcal/mol). This is consistent with the previously proposed mechanism, where CO inserts directly into the Re–R bond without prior formation of a CO adduct. The X-ray crystal structures of complexes 6, 7a, 8a, and 9a are reported.

In this paper a computational analysis (B3PW91) of the previously reported reaction of (O)Re(Me)(DAAm) (1; DAAm = N,N-bis(2-arylaminoethyl)methylamine, aryl = C6F5) with CO to produce (CO)Re(OAc)(DAAm) (2) is described. The data suggest that this transformation proceeds by two novel elementary steps that are of fundamental interest to the broader organometallic/inorganic community: (a) direct insertion of CO into the rhenium–methyl bond in 1 to yield the acyl intermediate (O)Re(Ac)(DAAm) (3) and (b) 1,2-migration, in the presence of CO, of the acyl fragment in 3 to the oxo ligand to yield 2. Evidence is provided for the first example of an insertion reaction where CO inserts directly into a M–R bond without prior formation of a CO adduct. In addition, it was shown that the addition of CO is necessary for the 1,2-migration of the acyl ligand. The data suggest that the addition of CO effectively weakens the Re–Cacyl bond in 3 and enables the facile migration of the acyl ligand.

The catalytic competency of a Re(III) complex has been demonstrated. In the presence of silane, oxorhenium(V) catalysts are deoxygenated to produce species that are significantly more active than the metal oxo precursors in hydrosilylation reactions. The results presented suggest that, in evaluating mechanisms for catalytic hydrosilylation reactions that involve high-valent metal oxo complexes, the activity of species that may be generated by deoxygenation of the metal with silane should also be systematically investigated as potential catalysts.

Activation of CO by the rhenium(V) oxo complex [(DAAm)Re(O)(CH3)] (1) [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5, Mes] resulted in the isolation of the rhenium(III) acetate complex [(DAAm)Re(O2CCH3)(CO)] (3). The mechanistic details of this reaction were explored experimentally. The novel oxorhenium(V) acyl intermediate [(DAAm)Re(O)(C(O)CH3)] (2) was isolated, and its reactivity with CO was investigated. An unprecedented mechanism is proposed: CO is activated by the metal oxo complex 1 and inserted into the rhenium–methyl bond to yield acyl complex 2, after which subsequent migration of the acyl ligand to the metal oxo ligand yields acetate complex 3. X-ray crystal structures of 2 and 3 are reported.

The detailed syntheses of complexes 1–4, Re(O)(X)(DAP) (X = Me, 1; Cl, 2; I, 3; OTf (triflate), 4) incorporating the diamido pyridine (DAP) ancillary ligand (2,6-bis((mesitylamino)methyl)pyridine) are described and shown to be effective catalysts for oxygen atom transfer (OAT) reactions of PyO to PPh3. The catalytic activities are as follows: 4 ≈ 3 > 2 > 1. The observed electronic trend is consistent with the turnover limiting reduction of the proposed Re(VII) dioxo intermediate, Re(O)2(X)(DAP), during the catalytic cycle. The catalytic activity of complexes 1–3 was compared to previously published diamido amine (DAAm) oxorhenium complexes of the type Re(O)(X)(DAAm) (X = Me, 5; Cl, 6; I, 7 and DAAm = N,N-bis(2-arylaminoethyl)methylamine) which exhibit hydrolytic degradation during the catalytic reaction. Complexes 1–3 displayed higher turnover frequencies compared to 5–7. This higher catalytic activity was attributed to the more rigid DAP ligand backbone, which makes the complexes less susceptible to decomposition. However, another decomposition pathway was proposed for this catalytic system due to the observation of Re(O)3((MesNCH2)(MesNCH)NC5H3) 8 in which one arm of the DAP ligand is oxidized.

Catalytic H/D exchange reactions of benzene and benzoic acid with deuterated solvents have been studied using (Cp*IrCl2)2. A 1:1 mixture of D2O/CD3OD produced the highest turnover numbers for benzene. High levels of deuterium incorporation into benzoic acid were observed only when sodium acetate was added to the reaction mixture. Attempts at producing hydroxybenzoic acid by catalytic C–H functionalization of benzoic acid with benzoquinone were unsuccessful. Instead, 2-hydroxy-6H-benzo[c]chromen-6-one was isolated as the major product. An array of substituted benzoic acids was analyzed for this functionalization reaction. Preliminary mechanistic studies indicate that the benzochromenones are formed by C–H bond activation of benzoic acid followed by insertion of benzoquinone into the iridium–carbon bond.

A series of complexes of the form Cp*Ir(NHC)(X)n and [Cp*Ir(NHC)(L)2][OTf]2, where NHC = 1,3,4,5-tetramethylimidazol-2-ylidene (n = 2, X = Cl− (1-Cl), NO3− (1-NO3), −OC(O)CF3 (=TFA, 1-TFA); n = 1, X = SO42− (1-SO4); L = H2O (1-H2O), CH3CN (1-CH3CN), OTf = trifluoromethanesulfonato), were prepared. X-ray crystal structures of 1-OH2, 1-SO4, and 1-NO3 and the dimeric complex [(Cp*Ir(NHC)Cl)2][OTf]2 (2) were obtained. In solution, the complex 1-TFA was found to exist in equilibrium with [Cp*Ir(NHC)(OH2)2][OCOCF3]2 (1-aqua-TFA), where the aqua ligands are strongly hydrogen bound to the −OCOCF3 counterion. A van’t Hoff plot from −10 to 30 °C yielded values for the reaction enthalpy and entropy of ΔH° = −7.6 ± 0.7 kcal/mol and ΔS° = −30.6 ± 2.4 eu, respectively. These data are consistent with the observation that at higher temperatures the complex 1-TFA is favored. An X-ray crystal structure of 1-aqua-TFA was obtained. Catalytic H/D exchange reactions between benzene and various deuterium sources (CD3OD, CF3COOD, CD3COCD3, and D2O) were performed and assessed by GC-MS. The best deuterium sources for this reaction were found to be CD3OD or CD3OD/D2O (1:1) mixtures. The highest turnover numbers (TONs) were observed for the H/D exchange reactions catalyzed by Cp*Ir(NHC) complexes with labile ligands. These results suggest that the dissociation of the ancillary ligand to form an unsaturated 16-electron intermediate is an important step prior to C−H activation in the catalytic cycle, which is consistent with the Shilov electrophilic C−H activation mechanism. In contrast, the most effective deuterium source for H/D exchange with the aqua complex [Cp*Ir(OH)3][OTf2] (3) was the acidic solvent CF3COOD. Thus, the σ-donating NHC ligand serves to attenuate the electrophilicity of the metal center so that milder reaction conditions are required for the C−H activation reactions.

The detailed syntheses of complexes of the form [Re(O)(X)(RNCH2CH2)2N(Me)] (X = Me, Cl, I, R = mesityl, C6F5), 1−3, incorporating diamidoamine ancillary ligands are described. X-ray crystal structures for the complexes [Re(O)(Me)((C6F5)NCH2CH2)2N(Me)], 1a, [Re(O)(I)((C6F5)N CH2CH2)2N(Me)], 3a, and [Re(O)(I)((Mes)NCH2)2N(Me)], 3b, are reported. The geometry about the metal center in 1a is best described as a severely distorted square pyramid with the oxo ligand in the apical position. In contrast, the geometry about the metal center in 3a is best described as a severely distorted trigonal bipyramid, with the iodo ligand occupying the apical position and the diamido nitrogens and the oxo ligand occupying the equatorial plane. The catalytic activities of these complexes for oxygen atom transfer, OAT, from pyridine-N-oxides, PyO, to PPh3 were also examined. The reactions exhibited a clear dependence on the diamido ligand substituent and the X ligand (Me, I, Cl) attached to the metal, with the combined effect that electron-withdrawing substituents on the diamido ligand and poor σ donors directly attached to the metal center increases the rate of catalytic activity. The kinetics of OAT from pyridine-N-oxides to Re were also investigated. The reactions displayed clean first order kinetics in Re and saturation kinetics for the dependence on PyO. Changing the PyO substrate had no effect on the saturation value, ksat, suggesting that the OAT reaction in these five-coordinate complexes appears to be governed by isomerization of the starting complex. Attempts to isolate a postulated Re(VII) intermediate were not successful because of hydrolytic degradation. The product of hydrolytic degradation [((C6F5)N(H)CH2CH2))2NH(Me)][X], (X = ReO4−, or I−), 4 can be isolated, and its X-ray crystal structure is reported. Although the Re(VII) intermediate could not be isolated, its activity in OAT reactions was probed by competition experiments with PPh3 and four para-substituted triarylphosphines (p-X-Ph)3P (X = OMe, Me, Cl, CF3). These experiments led to a Hammett that yielded a reaction constant of ρ = −0.30 ± 0.01. This data suggests a positive charge buildup on phosphorus for the OAT reaction and is consistent with the nucleophilic attack of phosphorus on an electrophillic metal oxo.

The detailed syntheses of complexes 1–4, Re(O)(X)(DAP) (X = Me, 1; Cl, 2; I, 3; OTf (triflate), 4) incorporating the diamido pyridine (DAP) ancillary ligand (2,6-bis((mesitylamino)methyl)pyridine) are described and shown to be effective catalysts for oxygen atom transfer (OAT) reactions of PyO to PPh3. The catalytic activities are as follows: 4 ≈ 3 > 2 > 1. The observed electronic trend is consistent with the turnover limiting reduction of the proposed Re(VII) dioxo intermediate, Re(O)2(X)(DAP), during the catalytic cycle. The catalytic activity of complexes 1–3 was compared to previously published diamido amine (DAAm) oxorhenium complexes of the type Re(O)(X)(DAAm) (X = Me, 5; Cl, 6; I, 7 and DAAm = N,N-bis(2-arylaminoethyl)methylamine) which exhibit hydrolytic degradation during the catalytic reaction. Complexes 1–3 displayed higher turnover frequencies compared to 5–7. This higher catalytic activity was attributed to the more rigid DAP ligand backbone, which makes the complexes less susceptible to decomposition. However, another decomposition pathway was proposed for this catalytic system due to the observation of Re(O)3((MesNCH2)(MesNCH)NC5H3) 8 in which one arm of the DAP ligand is oxidized.

A series of novel cationic Re(III) complexes [(DAAm)Re(CO)(NCCH3)2][X] [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5 (a), Mes (b)] [X = OTf (2), BArF4 [BArF4 = tetrakis[3,5-(trifluoromethyl)phenyl]borate] (3), BF4 (4), PF6 (5)], and their analogue [(DAmA)Re(CO)(Cl)2] [DAmA = N,N-bis(2-arylamineethyl)methylamino; aryl = C6F5] (6) were synthesized. The catalytic efficiency for the hydrosilylation reaction of aldehydes using 4a (0.03 mol%) has been demonstrated to be significantly more active than rhenium catalysts previously reported in the literature. The data suggest that electron-withdrawing substituents at the diamido amine ligand increase the catalytic efficiency of the complexes. Excellent yields were achieved at ambient temperature under neat conditions using dimethylphenylsilane. The reaction affords TONs of up to 9200 and a TOF of up to 126 h−1. Kinetic and mechanistic studies were performed, and the data suggest that the reaction is via a non-hydride ionic hydrosilylation mechanism.