Cyclometalated (N,C) Au(III) Complexes: The Impact of Trans Effects on Their Synthesis, Structure, and Reactivity

ConspectusThe early years of gold catalysis were dominated by Au(I) complexes and inorganic Au(III) salts. Thanks to the development of chelating ligands, more sophisticated Au(III) complexes can now be easily prepared and handled. The choice of the ancillary ligand has great consequences for the synthesis, properties, and reactivity of the Au(III) complex in question. Among the major factors controlling reactivity are the “trans effect” and the “trans influence” that a ligand imparts at the ligand trans to itself. The kinetic trans effect manifests itself with an increased labilization of the ligand trans to a given ligand and arises from an interplay between ground-state and transition-state effects. The term trans influence, on the other hand, is a ground-state effect only, describing the tendency of a given ligand to weaken the metal–ligand bond trans to itself. Herein, we will use the term “trans effect” to describe both the kinetic and the thermodynamic properties, whereas the term “trans influence” will refer only to thermodynamic properties. We will describe how these trans effects strongly impact the chemistry of the commonly encountered cyclometalated (N,C) Au(III) complexes, a class of complexes we have studied for more than a decade. We found that the outcome of reactions like alkylation, arylation, and alkynylation as well as halide metathesis are dictated by the different trans influence of the two termini of the chelating tpy ligand in (tpy)Au(OAcF)2 (tpy = 2-(p-tolyl)pyridine, OAcF = OCOCF3, tpy-C > tpy-N). There is a strong preference for high trans influence ligands to end up trans to tpy-N, whereas the lower trans influence ligands end up trans to tpy-C. Taking advantage of these preferences, tailor-made (N,C)Au(III) complexes could be prepared. For the functionalization of alkenes at (tpy)Au(OAcF)2, the higher trans effect of tpy-C would suggest that the coordination site trans to tpy-C would be kinetically more available than the one trans to tpy-N. However, due to the thermodynamic preference of having the σ-bonded ligand, resulting from the nucleophilic addition to alkenes, trans to tpy-N, functionalization of alkenes was only observed trans to tpy-N. However, for a catalytic process, the reaction should happen trans to tpy-C, as was observed for the trifluoroacetoxylation of acetylene. When functionalizing acetylene in the coordination site trans to tpy-N, protolytic cleavage of the Au–C(vinyl) bond to release the product did not occur at all, whereas trans to tpy-C protolytic cleavage of the Au–C(vinyl) bond occurred readily, in agreement with the higher trans influence of tpy-C over tpy-N. The large impact of the trans effects in Au(III) complexes is finally exemplified with the synthesis of [(tpy)Au(π-allyl)]+[NTf2]−, which resulted in a highly asymmetric π + σ bonding of the allyl moiety. Here, the bonding is such that the most thermodynamically favorable situation is achieved, with the carbon trans to tpy-N bonded in a σ-fashion and the π-allyl double bond being coordinated trans to tpy-C

Titanium <i>tert</i>-Butoxyimido Compounds

The synthesis and molecular and electronic structures of the first tert-butoxyimido complexes of titanium (TiNOtBu functional group) are reported, featuring a variety of mono- or poly-dentate, neutral or anionic N-donor ligands. Reaction of Ti­(NMe2)2Cl2 with tBuONH2 gave good yields of Ti­(NOtBu)­Cl2(NHMe2)2 (1). Compound 1 serves as an excellent entry point into new tert-butoxyimido complexes by reaction with a variety of fac-N3 donor ligands, namely, Me3[9]­aneN3 (trimethyl-1,4,7-triazacyclononane), HC­(Me2pz)3 (tris­(3,5-dimethylpyrazolyl)­methane), or Me3[6]­aneN3 (trimethyl-1,3,5-triazacyclohexane) to give Ti­(NOtBu)­(Me3[9]­aneN3)­Cl2 (2), Ti­(NOtBu)­{HC­(Me2pz)3}­Cl2 (3), or Ti­(NOtBu)­(Me3[6]­aneN3)­Cl2 (4) in good yield. It was found that 4 could be converted into Ti­(NOtBu)­Cl2(py)3 (5) in very good yield by reaction with an excess of pyridine. Compound 5 is effective in a range of salt metathesis reactions with lithiated amide or pyrrolide ligands, and reacts with Li2N2Npy, Li2N2NMe, LiNpyrNMe2, or Li2N2pyrNMe to give Ti­(N2Npy)­(NOtBu)­(py) (6), Ti­(N2NMe)­(NOtBu)­(py) (7), Ti­(NpyrNMe2)­(NOtBu)­Cl­(py)2 (9), or Ti­(N2pyrNMe)­(NOtBu)­(py)2 (10) in moderate to good yields (N2Npy = (2-NC5H4)­C­(Me)­(CH2NSiMe3)2; N2NMe = MeN­(CH2CH2NSiMe3)2; NpyrNMe2 = Me2NCH2(2-NC4H3); N2pyrNMe = MeN­{CH2(2-NC4H3)}2). Compounds 7, 9, and 10 reacted with 2,2′-bipyridyl by pyridine exchange reactions forming Ti­(N2NMe)­(NOtBu)­(bipy) (8), Ti­(NpyrNMe2)­(NOtBu)­Cl­(bipy) (11), and Ti­(N2pyrNMe)­(NOtBu)­(bipy) (12). Ten tert-butoxyimido compounds, namely, 1–6, 11, and 12, have been structurally characterized revealing approximately linear Ti–N–OtBu linkages with Ti–N distances [range 1.686(2)–1.734(2) Å] that are generally intermediate between those in the homologous alkylimido and phenylimido analogues, and shorter than in the diphenylhydrazido counterparts. Density functional theory (DFT) studies on the model compounds Ti­(NR)­Cl2(NHMe2)2 (1_R; R = OMe, Me, Ph, NMe2) confirmed this trend and found that the destabilizing effect of the −OMe oxygen 2pπ lone pair on one of the Ti–N π-bonds in 1_OMe is comparable to that of the occupied phenyl ring π orbitals in the phenylimido homologue 1_Ph but much less than for the −NMe2 nitrogen lone pair in 1_NMe2

Titanium <i>tert</i>-Butoxyimido Compounds

The synthesis and molecular and electronic structures of the first tert-butoxyimido complexes of titanium (TiNOtBu functional group) are reported, featuring a variety of mono- or poly-dentate, neutral or anionic N-donor ligands. Reaction of Ti­(NMe2)2Cl2 with tBuONH2 gave good yields of Ti­(NOtBu)­Cl2(NHMe2)2 (1). Compound 1 serves as an excellent entry point into new tert-butoxyimido complexes by reaction with a variety of fac-N3 donor ligands, namely, Me3[9]­aneN3 (trimethyl-1,4,7-triazacyclononane), HC­(Me2pz)3 (tris­(3,5-dimethylpyrazolyl)­methane), or Me3[6]­aneN3 (trimethyl-1,3,5-triazacyclohexane) to give Ti­(NOtBu)­(Me3[9]­aneN3)­Cl2 (2), Ti­(NOtBu)­{HC­(Me2pz)3}­Cl2 (3), or Ti­(NOtBu)­(Me3[6]­aneN3)­Cl2 (4) in good yield. It was found that 4 could be converted into Ti­(NOtBu)­Cl2(py)3 (5) in very good yield by reaction with an excess of pyridine. Compound 5 is effective in a range of salt metathesis reactions with lithiated amide or pyrrolide ligands, and reacts with Li2N2Npy, Li2N2NMe, LiNpyrNMe2, or Li2N2pyrNMe to give Ti­(N2Npy)­(NOtBu)­(py) (6), Ti­(N2NMe)­(NOtBu)­(py) (7), Ti­(NpyrNMe2)­(NOtBu)­Cl­(py)2 (9), or Ti­(N2pyrNMe)­(NOtBu)­(py)2 (10) in moderate to good yields (N2Npy = (2-NC5H4)­C­(Me)­(CH2NSiMe3)2; N2NMe = MeN­(CH2CH2NSiMe3)2; NpyrNMe2 = Me2NCH2(2-NC4H3); N2pyrNMe = MeN­{CH2(2-NC4H3)}2). Compounds 7, 9, and 10 reacted with 2,2′-bipyridyl by pyridine exchange reactions forming Ti­(N2NMe)­(NOtBu)­(bipy) (8), Ti­(NpyrNMe2)­(NOtBu)­Cl­(bipy) (11), and Ti­(N2pyrNMe)­(NOtBu)­(bipy) (12). Ten tert-butoxyimido compounds, namely, 1–6, 11, and 12, have been structurally characterized revealing approximately linear Ti–N–OtBu linkages with Ti–N distances [range 1.686(2)–1.734(2) Å] that are generally intermediate between those in the homologous alkylimido and phenylimido analogues, and shorter than in the diphenylhydrazido counterparts. Density functional theory (DFT) studies on the model compounds Ti­(NR)­Cl2(NHMe2)2 (1_R; R = OMe, Me, Ph, NMe2) confirmed this trend and found that the destabilizing effect of the −OMe oxygen 2pπ lone pair on one of the Ti–N π-bonds in 1_OMe is comparable to that of the occupied phenyl ring π orbitals in the phenylimido homologue 1_Ph but much less than for the −NMe2 nitrogen lone pair in 1_NMe2

Understanding Precatalyst Activation in Cross-Coupling Reactions: Alcohol Facilitated Reduction from Pd(II) to Pd(0) in Precatalysts of the Type (η³‑allyl)Pd(L)(Cl) and (η³‑indenyl)Pd(L)(Cl)

Complexes of the type (η³-allyl)­Pd­(L)­(Cl) (L = PR₃ or NHC), have been used extensively as precatalysts for cross-coupling and related reactions, with systems containing substituents in the 1-position of the η³-allyl ligand, such as (η³-cinnamyl)­Pd­(L)­(Cl), giving the highest activity. Recently, we reported a new precatalyst scaffold based on an η³-indenyl ligand, (η³-indenyl)­Pd­(L)­(Cl), which typically provides higher activity than even η³-cinnamyl supported systems. In particular, precatalysts of the type (η³-1-^tBu-indenyl)­Pd­(L)­(Cl) give the highest activity. In cross-coupling reactions using this type of Pd­(II) precatalyst, it is proposed that the active species is monoligated Pd(0), and the rate of reduction to Pd(0) is crucial. Here, we describe detailed experimental and computational studies which explore the pathway by which the Pd­(II) complexes (η³-allyl)­Pd­(IPr)­(Cl) (IPr = 1,3-bis­(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene), (η³-cinnamyl)­Pd­(IPr)­(Cl), (η³-indenyl)­Pd­(IPr)­(Cl) and (η³-1-^tBu-indenyl)­Pd­(IPr)­(Cl) are reduced to Pd(0) in alcoholic solvents, which are commonly used in Suzuki–Miyaura and α-arylation reactions. The rates of reduction for the different precatalysts are compared and we observe significant variability based on the exact reaction conditions. However, in general, η³-indenyl systems are reduced faster than η³-allyl systems, and DFT calculations show that this is in part due to the ability of the indenyl ligand to undergo facile ring slippage. Our results are consistent with the η³-indenyl systems giving increased catalytic activity and provide fundamental information about how to design systems that will rapidly generate monoligated Pd(0) in the presence of alcohols

A Critical Analysis of the Cyclic and Open Alternatives of the Transmetalation Step in the Stille Cross-Coupling Reaction

The transmetalation step of the Stille cross-coupling reaction catalyzed by PdL2 (L = PH3, AsH3) has been analyzed by means of DFT methods for PhBr as the electrophile and CH2CHSnMe3 as the nucleophile. Both experimentally proposed mechanisms (cyclic and open) were theoretically studied. For the case of the cyclic mechanism, the associative and dissociative ligand substitution alternatives were both analyzed. For the case of the open mechanism, the cis and the trans pathways were evaluated. All the reaction pathways were also studied taking into account the solvent effects by means of continuum models, for THF and PhCl as solvents. In selected cases, explicit solvent molecules were introduced to account for their potential role as ligands. Theoretical analysis indicates that the open reaction mechanism is preferred for organotriflate systems, whereas the cyclic mechanism is favored for the reaction with organohalide systems

Observation of a Hidden Intermediate in the Stille Reaction. Study of the Reversal of the Transmetalation Step

Observation of a Hidden Intermediate in the Stille Reaction. Study of the Reversal of the Transmetalation Ste

Formation of a Vinyliminium Palladium Complex by C−C Coupling in Vinylcarbene Palladium Aryl Complexes

Pentafluorophenyl palladium complexes 3 and 5 bearing a vinylcarbene ligand slowly decompose on heating by migratory insertion to give Pd(0) and the vinyliminium salts (Et2NC(C6F5)CHCHPh)X (X = Br, BF4). However, the presence of triphenylphosphine in complex 4 stabilizes 7, a compound with a CC Pd-bound vinyliminium moiety, where there is a strong preference for the CC versus the CN coordination

Formation of a Vinyliminium Palladium Complex by C−C Coupling in Vinylcarbene Palladium Aryl Complexes

Pentafluorophenyl palladium complexes 3 and 5 bearing a vinylcarbene ligand slowly decompose on heating by migratory insertion to give Pd(0) and the vinyliminium salts (Et2NC(C6F5)CHCHPh)X (X = Br, BF4). However, the presence of triphenylphosphine in complex 4 stabilizes 7, a compound with a CC Pd-bound vinyliminium moiety, where there is a strong preference for the CC versus the CN coordination

Formation of a Vinyliminium Palladium Complex by C−C Coupling in Vinylcarbene Palladium Aryl Complexes

Pentafluorophenyl palladium complexes 3 and 5 bearing a vinylcarbene ligand slowly decompose on heating by migratory insertion to give Pd(0) and the vinyliminium salts (Et2NC(C6F5)CHCHPh)X (X = Br, BF4). However, the presence of triphenylphosphine in complex 4 stabilizes 7, a compound with a CC Pd-bound vinyliminium moiety, where there is a strong preference for the CC versus the CN coordination