Alkyne Activation with Gold(III) Complexes: A Quantitative Assessment

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Alkyne Activation with Gold(III) Complexes: A Quantitative Assessment of the Ligand Effect by Charge-Displacement Analysis Luca Gregori,† Diego Sorbelli,† Leonardo Belpassi,*,‡,§ Francesco Tarantelli,†,‡,§ and Paola Belanzoni*,†,‡,§ †

Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), c/o Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy § Consortium for Computational Molecular and Materials Sciences (CMS)2, via Elce di Sotto 8, I-06123 Perugia, Italy Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 02/18/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: A quantitative assessment of the Dewar−Chatt−Duncanson components of the Au(III)-alkyne bond in a series of cationic and dicationic bis- and monocyclometalated gold(III) complexes with 2-butyne via charge-displacement (CD) analysis is reported. Bonding between Au(III) and 2-butyne invariably shows a dominant σ donation component, a smaller, but significant, π back-donation, and a remarkable polarization of the alkyne CC triple bond toward the metal fragment. A very large net electron charge transfer from CC triple bond to the metal fragment results, which turns out to be unexpectedly insensitive to the charge of the complex and more strictly related to the nature of the ancillary ligand. The combination of σ donation, π back-donation, and polarization effects is in fact modulated by the different ligand frameworks, with ligands bearing atoms different from carbon in trans position with respect to the alkyne emerging as especially interesting for both imparting Au(III)-alkyne bond stability and inducing a more effective alkyne activation. A first attempt to figure out a rationale on the bonding/reactivity relationship for Au(III)-alkyne is made by performing a comparative study in a model nucleophilic attack of water to the alkyne triple bond. Smaller π back-donation facilitates alkyne slippage in the transition states, which is energetically less demanding for Au(III) than for Au(I), and suggests a greater propensity of Au(III) to facilitate the nucleophilic attack.

■

INTRODUCTION Over the last 20 years, the use of cationic gold(I) species in the activation of alkynes has grown into a valuable tool for a multitude of organic catalytic transformations.1−8 Alkyne coordination to gold, which was discovered to be a powerful and versatile carbophilic Lewis acid,9,10 induces electron charge transfer from the substrate, which becomes more susceptible toward a nucleophilic attack (electrophilic activation).11,12 Indeed, the alkyne complexes of gold(I) are key intermediates in many catalytic transformations and invariably represent the initial stage of nucleophilic addition to triple bonds, constituting a basis of synthetic chemistry. Whereas gold(I) species are thus extensively employed in homogeneous catalysis,13,14 the gold(III) ones have remained far behind and have started to be more widely used only very recently.15−17 This is because gold(III) complexes are usually less stable than those of gold(I), although gold(III) is isoelectronic with well-known catalytic metal centers such as Pt(II), Pd(II), Rh(I), and Ir(I).18 For the syntheses of gold(III) complexes, one method used is the ligand exchange from inorganic salts,19 and another one relies on the oxidative addition to Au(I) complexes,20−24 such as, for instance, the insertion of O2 into the Au(I)-H bond.25,26 In more recent © XXXX American Chemical Society

years, gold(III) complexes bearing pincer ligands have been synthesized.27 These ligands are able to stabilize bis- and monocyclometalated gold(III) complexes, avoiding reductive elimination to Au(I) or Au(0).28 A recent review by Kumar and Nevado27 demonstrates that cyclometalation is an efficient and practical solution and the variety of stable but reactive gold(III) complexes has been considerably extended. Compared to linear gold(I) complexes, square-planar gold(III) complexes are especially promising for asymmetric catalysis,29 which is nowadays a hot topic.30 Precisely because of the square-planar geometry, the chiral ligand is allowed to get much closer to the reactive site, providing for better enantioselectivity than is afforded by the linear geometry of gold(I) complexes.31 However, gold(III) alkyne complexes have remained elusive and merely hypothetical species until recently. The first evidence for gold(III) alkyne complexes has been reported by Bochmann et al.32 using cationic cyclometalated gold(III) species. Biscyclometalated [(C^N^C)Au(III)]+ (C^N^C = 2,6-bis(4-tBuC6H3) pyridine dianion) adducts with internal Received: November 12, 2018

A

DOI: 10.1021/acs.inorgchem.8b03172 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

gold(III)-alkyne bond is quantitatively and exhaustively explored via CD analysis in 10 different gold(III) alkyne complexes, and the bonding/reactivity relationship is discussed. Comprehension of the fundamental features of the gold(III)-alkyne bond in terms of the DCD bonding components is key both as basic knowledge of the coordinative bond in these new types of gold(III) complexes and for the development of the homogeneous gold(III) and gold(I)/ gold(III) catalysis. To this aim, we selected experimentally characterized prototypical gold(III) cyclometalated complexes on the basis of (i) bi- or tridentate pincer, namely, mono- or biscyclometalated complexes, [LAu(III)L′]n+ or [LAu(III)]n+ (n = 1, 2), where L is a pincer ligand and L′ is a neutral donor monodentate auxiliary ligand in monocyclometalated complexes, (ii) the pincer donor atoms (C, N, O, P, S), and (iii) in monocyclometalated complexes [LAu(III)L′]n+ (n = 1, 2), the additional auxiliary ligand L′ (NHC = N-heterocyclic carbene, H2O, PMe3) to span an as wide as possible range of ligand environments. Scheme 1 depicts all the gold(III) alkyne complexes studied in this work. We employ 2-butyne as an alkyne prototype.

alkynes have been characterized by NMR spectroscopy. In the same study, additional internal alkyne adducts were synthesized and characterized by NMR using a monocyclometalated [(C^N)Au(III)(C 6 F 5 )] + (C^N = 2-(4- t BuC 6 H 3 ), 6(4-tBuC6H4) pyridine dianion) complex, where the flanking arene is π-coordinated to gold. A later study33 has provided the characterization of three additional monocyclometalated gold(III) alkyne complexes, namely, [(C^C)Au(III)(H2O)]+, [(C^C)Au(III)(PMe 3 )] + , and [(C^C)Au(III)(CNxyl)] + (C^C = 4,4′-di-t-butylbiphenyl-2,2′-diyl dianion, xyl = 2,6Me2C6H3) by IR and NMR techniques, showing the biphenyl C^C pincer ligand as a convenient framework for imparting much improved thermal stability to these types of complexes. Despite these outstanding achievements, much remains to be done. Gold(III) alkyne complexes have still to be crystallographically characterized, and very little is known about the role of the ancillary ligands in gold(III) chemistry until now. The gold-substrate bonding is classically described in terms of donation/back-donation within the Dewar−Chatt−Duncanson (DCD) model. The nature of the bond between gold(I) and substrates34 and the species35 involved in the mechanisms of gold(I)-catalyzed reactions have been intensively studied in the last years,36−38 but knowledge and understanding of bonding in gold(III) complexes and of the individual steps along their proposed catalytic cycles are still lacking. In a recent paper by some of us,36 the activation of alkynes toward nucleophilic attack upon coordination to gold(I) complexes of L-Au(I)-S type (where S is an η2coordinated alkyne and L is the ancillary ligand), commonly used in homogeneous catalysis, was investigated to elucidate the role of the S → Au σ donation and Au → S π backdonation components of the Au−C bond, considering ethyne as prototype substrate. Charge displacement (CD) analysis39,40 has been applied to obtain a well-defined measure of the σ donation and π back-donation, which are strictly related with experimental observables,41 and to understand how the corresponding electron charge transfers affect the electron density at the alkyne carbon undergoing the nucleophilic attack. This finding has been used to rationalize the efficiency of a series of gold(I) catalysts in the nucleophilic attack step of a model hydroamination reaction, thus allowing a quantitative correlation between the DCD bond components and the kinetic parameters (activation energy barrier) of a chemical reaction. The bond between alkyne and gold(I) complexes has been found to invariably show a dominant donation component and a smaller, but significant, back-donation. The amount of charge subtracted from the carbon undergoing the nucleophilic attack has been calculated by taking the value of the CD function at the carbon position and, interestingly, it has been found that it is the result of a combination of donation, back-donation, and polarization effects. In addition, it has been shown that the activation of the triple bond depends largely on the electronic charge subtracted from the region where the nucleophilic attack occurs, that is, the outer-sphere region of the triple bond. One may wonder how the above scenario for gold(I) complexes would change (or not) for gold(III) complexes. Motivated by the new perspectives offered by gold(III) complexes in asymmetric catalysis, by their expected enhanced carbophilic Lewis acid properties with respect to gold(I) complexes, and by the scarce information currently available on gold(III)-alkyne bond and its modulation by the ancillary ligands, in this paper the ligand effect on the cyclometalated

Scheme 1. Selected Gold(III)-Alkyne Complexes Analyzed in This Work

Representative examples of bis-cyclometalated gold(III) species are [(C^N^C)Au-2-butyne]+ (C^N^C = 2,6-bis(4-tBuC6H3)2 pyridine dianion) (1)+ and [(Hsalen)Au-2butyne]2+ (Hsalen = monoprotonated N,N-bis(salicylidene)ophenylenediamine anion) (2)2+ complexes, providing a planar C^N^C and a skewed N^NÔ scaffold around the metal, respectively. Monocyclometalated gold(III) species are represented by complexes [(C^N)Au(NHC)-2-butyne]2+ (C^N = 2-phenylpyridine monoanion, NHC = 1,3-dimethylimidazol-2ylidene) (3)2+ and (4)2+, where the 2-butyne is coordinated in cis and in trans position with respect to the donor N atom of the pincer, respectively; [(C^C)Au(NHC)-2-butyne]+ (C^C = biphenyl dianion) (5)+; [(C^C)AuH2O-2-butyne]+ (6)+ and [(C^C)AuPMe3-2-butyne]+ (7)+ (C^C = 4,4′-di-t-butylbiphenyl-2,2′-diyl dianion), affording two different auxiliary ligands L′ within the same C^C pincer framework; [(NÔ)AuCl-2-butyne]+ (NÔ = 2-carboxylate-pyridine monoanion) (8)+, where the 2-butyne is coordinated in trans position with respect to the donor O atom of the pincer; [(N^P)AuAr-2B


Article

Inorganic Chemistry butyne]2+ (N^P = 8-(diphenyl-phosphyl)quinoline monoanion, Ar = phenyl) (9)2+; and [(S^S)AuCl-2-butyne]+ (S^S = dithiocarbamate, S2CNEt2 monoanion) (10)+. We recall here that only complexes (1)+, (6)+, and (7)+ in Scheme 1 have been experimentally observed by Bochmann et al. with different internal alkynes.32,33 The interest for the biscyclometalated complex (2)2+ stems from the fact that Chen and Cai42 have used the [(salen)Au(III)Cl] (salen = N,Nbis(salicylidene)o-phenylenediamine dianion) complex as catalyst to promote the hydration of a wide range of alkynes in the presence of the CF3COOH as cocatalyst for the synthesis of ketones, although the corresponding gold(III) alkyne complexes could not be experimentally characterized. Complex (2)2+ represents such a hypothetical intermediate. Note that it has been modified with respect to the [(salen)Au(III)]+ precursor by protonation of one of the salen donor O atoms to allow 2-butyne coordination to gold. Concerning complexes (3)2+ and (4)2+, the [(C^N)AuCl2] compound has been applied in catalysis for the efficient activation of π bonds in alkynes to produce propargylamines,43 and [(C^N)Au(C6F5)]+ complexes with internal alkyne adducts have been observed.32 We modified the experimental [(C^N)AuCl2] complex by replacing one Cl− ligand with a commonly used carbene ligand NHC (NHC = 1,3-bis(2,6dimethyl)-imidazol-2-ylidene); thus, complexes (3)2+ and (4)2+ represent gold(III) alkyne intermediates deriving from such a [(C^N)AuNHC]2+ model precursor. Complex (5)+ also has a catalytic interest. Toste et al.44 applied the [(C^C)AuIPrCl] complex as a catalyst in various transformations, initiated by activation of α,β-unsaturated aldehydes, where this Au(III) complex is reported to exhibit hard, oxophilic Lewis acidity. Recently, they have also shown that the use of chiral carbene-type ligands allows a gold(III)-catalyzed direct enantioconvergent 1,5-enyne cycloisomerization and a kinetic resolution reaction has been described.45 Complex (5)+ represents a model gold(III) alkyne complex derived from such [(C^C)AuIPrCl] precursor, where the experimental chlorine atom has been substituted by 2-butyne and IPr has been modeled by NHC for our calculations. Monocyclometalated complex [(NÔ)AuCl2] has been used as catalyst for a wide variety of processes, due to its ability to activate all kinds of π systems, which, interestingly, was found to be similar to that of gold(I) species.46−48 Complex (8)+ is a model of its corresponding gold(III) alkyne complex, where one Cl− ligand in the experimental precursor is substituted by 2-butyne. Only the isomer with 2-butyne coordinated in trans with respect to the O atom of the pincer has been considered as example of the NÔ ligand framework around the metal. Since monocyclometalated [(N^P)AuArCl] complexes have also been proven to be relevant in gold-catalyzed 1,2-difunctionalization reactions of alkynes,49−51 complex (9)2+ represents the corresponding gold(III) alkyne complex, where the Cl− ligand in the experimental precursor is substituted by 2-butyne, and Ar is a phenyl. Finally, different Au(III) halo complexes bearing bidentate S,S-donor ligands have been shown to have good chemical and thermal stability and to be suitable catalysts for the hydration reaction of alkynes.52 Complex (10)+ is a model of the Au(III) alkyne intermediate corresponding to the experimental precursor complex [AuCl2(S2CNEt2)], where one Cl− ligand is replaced by 2-butyne. In the present work, we use CD analysis to quantitatively evaluate the DCD components of the Au(III)-alkyne bond and the charge density polarization at the alkyne triple bond in the

whole series of complexes in Scheme 1. On the basis of our results, we make an attempt to figure out a rationale for the bonding/reactivity relationship by performing a comparative study of the nucleophilic attack to the activated CC triple bond in a model hydration reaction. Our aim is to provide new insights into the Au(III)-alkyne bond and possibly into the Au(III) activation of triple bonds and its modulation by different ligand frameworks, casting light on similarities and differences between homogeneous gold(III) and gold(I) catalysis. Finally, we briefly discuss the results in the context of the current knowledge of bonding picture and catalytic features of Au(I)-alkyne complexes.36,53

■

METHODOLOGY AND COMPUTATIONAL DETAILS Charge-Displacement Analysis. The CD analysis is a powerful tool that allows to measure the exact amount of electron density that, upon formation of a bond between two fragments, is transferred from a fragment to another. This analysis is able to provide a clear and unequivocal definition of the DCD components, σ donation and π back-donation, of the coordination bond and to accurately quantify them separately.39−41 The CD function is mathematically defined as a partial progressive integration on a suitable z axis of the electron density difference [Δρ(x, y, z′)] between the density of the complex and the sum of the densities of the noninteracting separated fragments placed at the positions they occupy in the complex geometry.40 ∞

Δq(z) =

∞

z

∫−∞ dx ∫−∞ dy ∫−∞ Δρ(x , y , z′) dz′

(1)

In the present study the fragments are the gold species [LAu(III)L′]n+ or [LAu(III)]n+ (n = 1, 2) and 2-butyne. The z′ axis is usually the bond axis, in our case the axis passing through the middle point of the 2-butyne C−C bond and the gold atom. The CD function quantifies at each point of the z′ axis the exact amount of electrons that, upon formation of the coordination bond, is transferred across a plane perpendicular to this axis passing through the z point (charge transfer, CT). Positive values of CT indicate electron charge moving toward the decreasing z′ (i.e., from the right to the left) and vice versa for negative values of CT (i.e., toward the increasing z′, from the left to the right). The slope of the CD function gives information about regions of charge accumulation (positive slope) or charge depletion (negative slope) all over the molecular space. To get a numerical value of the CT, the value of the CD curve at some specific point between the fragments (i.e., at an arbitrarily defined plane separating them) can be taken. In the present work the CT is numerically calculated at two different crucial points. One point is the standard choice for evaluating the DCD bond components, which is at the socalled “isodensity boundary”,39 that is, the point along z′ where the electron densities of the two noninteracting fragments, [LAu(III)L′]n+ or [LAu(III)]n+ (n = 1, 2) and 2-butyne, become equal. The other point is at the midpoint of the 2butyne CC bond, which gives a measure of the polarization of the CC triple bond induced by the interaction with the metal fragment. A particularly useful feature of the CD function is its partitioning. If the complex and its constituting fragments belong to the same symmetry group, the electron density difference can be decomposed into additive symmetry components according to eq 239 C


Article

Inorganic Chemistry Δρ =

∑ ΔρΓ Γ

Computational Details. Optimized geometries, harmonic frequencies, and NOCV54−56 electron densities used in the CD analysis and energy decomposition analysis (EDA)65−67 were calculated through density functional theory (DFT). All calculations were performed using the Amsterdam Density Functional (ADF) program package (2014.05 version).68−70 The BLYP functional,71,72 a Slater-type TZ2P quality basis set (core small) for all atoms, and a scalar ZORA Hamiltonian to include relativistic effects were employed.73−75 The same computational setup was used for the previous studies on Au(I) complexes,36 mentioned in the Introduction. The choice of the same computational details allows a direct comparison in quantitative terms between the present and the previous results concerning the DCD bond components in different complexes. The accuracy of this level of theory has been previously verified in the Supporting Information of our previous work.76 To study the [LAu(III)L′]+/2+−2-butyne or [LAu(III)]+/2+−2-butyne bond in the selected complexes, we performed the EDA.66,67 The EDA approach allows to decompose the interaction energy ΔEint between the two fragments [LAu(III)L′]+/2+ or [LAu(III)]+/2+ and 2-butyne in each gold(III)-alkyne complex into three terms, namely, (1) the quasiclassical electrostatic interaction ΔEelst between the unperturbed charge distributions of the fragments at their molecular positions, (2) the repulsive exchange or Pauli repulsion interaction ΔEPauli between occupied orbitals on the two fragments, and (3) the orbital interaction ΔEoi, which arises from the orbital relaxation and the orbital mixing between the fragments, accounting for electron pair bonding, charge transfer, and polarization. This term may be further decomposed into pairwise orbital contributions of the interacting fragments (EDA-NOCV),77 ΔEoi = ∑kΔEkoi, which is very informative in systems with a clear σ/π separation. The sum of the electrostatic interaction ΔEelst and the Pauli repulsion ΔEPauli terms, ΔE0, is usually called the steric interaction energy. For reaction profile study, geometry optimization and frequency calculations were performed with the ADF related Quantum-regions Interconnected by Local Description (QUILD) program,78 using a TZ2P basis set with the core small approximation and the GGA BP86 functional.71,79 Relativistic effects were included with the scalar zero-order regular approximation ZORA model. Final energies were calculated using ORCA program package80 by single-point B2PLYP perturbatively corrected doubly hybrid functional81 calculations on the BP86 optimized structures (reactant complex (RC), transition state (TS), and product complex (PC)) in conjunction with a def2-TZVP basis set82 and an effective core potential (ECP) pseudopotential83 for gold to account for relativistic effects. Note that we deliberately chose two different computational setups to coherently compare the gold(III) results with our previous gold(I) results obtained with BLYP functional for bonding analysis and with BP86/ B2PLYP combination for reactivity studies. In particular, the BP86/B2PLYP protocol has been proven to be very accurate in describing gold species along reaction paths in benchmark studies,84−86 whereas charge displacement for bonding analysis has been shown to be not sizably sensitive to the particular functional used, as we also find here through benchmark calculations reported in the Supporting Information (Table S1).

(2)

where Γ index labels the different irreducible representations of their symmetry point group. The CD function can be correspondingly partitioned, giving rise to separate CD functions, one for each irreducible symmetry representation. In some systems, this decomposition can lead to a separation where each symmetry contribution corresponds clearly and unambiguously to a DCD component of the bond (σ donation and π back-donation). For instance, this symmetry decomposition has been applied to study the [L-Au(I)-ethyne]0/+ complexes,36 since the complex and its constituting fragments ([LAu]0/+ and ethyne) have a Cs symmetry, which is suitable to disentangle σ and π symmetry contributions (A′ corresponds to σ donation, while A″ corresponds to π back-donation). In more general cases, particularly when the complex and its constituting fragments have no symmetry, the CD function can be also decomposed by introducing the Natural Orbitals for Chemical Valence (NOCV) scheme.54 In the CD-NOCV framework,55,56 the charge rearrangement taking place upon the bond formation is obtained from the occupied orbitals of the two fragments suitably orthogonalized to each other and renormalized (“promolecule”). The resulting electron charge density rearrangement Δρ′ can be written in terms of NOCV pairs, that is, the eigenfunctions φ±k of the so-called “valence operator” of Nalewajski and Mrozek valence theory,57−59 as follows Δρ′ =

∑ Δρk′ k

(3)

However, only a small subset of these NOCV pairs actually contributes to the overall charge rearrangement Δρ′, because a large part of them presents eigenvalues close to zero.60 The CD-NOCV approach has been successfully applied for the characterization of transition metal compounds61 and for disentangling donation and back-donation in the CD function of nonsymmetric systems containing NHC-Au(I) bond,62 Au(I)-H bond,63 or Au(III)-CO bond with different ancillary ligands.64 We should point out that the two reference densities in the symmetric CD and nonsymmetric CD-NOCV are slightly different: Δρ and Δρ′ differ by the antisymmetrization term, which can be interpreted as the density rearrangement occurring on going from the noninteracting, separate fragments to the promolecule. Antisymmetrization removes the overlap between the fragments and shifts a certain amount of charge density from the interfragment region toward the fragments. However, this difference in the two reference densities is usually very small, and quantitatively close results can be obtained within the two schemes.60 In the present work, since all of the complexes listed in Scheme 1 have no symmetry, we apply the CD-NOCV method to quantify both the DCD [LAu(III)L′]+/2+−2-butyne or [LAu(III)]+/2+−2-butyne bond components and the polarization on the CC triple bond induced by the metal fragment. Well-defined measurements of the total charge transfer (denoted as CTnet) and of its σ donation and out-of-plane and in-plane π back-donation contributions (denoted as CTσ‑don, CTπ‑back⊥, and CTπ‑back∥, respectively) are obtained by evaluating the corresponding CD-NOCV function at the isodensity boundary. The CD values related to the total charge transfer measured at the CC triple bond midpoint (denoted as CT net CC ) provide quantitative information on the CC bond polarization. D


Article

Inorganic Chemistry

Table 1. Calculated CC and AuC Bond Lengths (Å), 2-Butyne Distortion from Linearity ΔθEq (deg), and CD-NOCV ChargeTransfer Values (e) for All the Complexes Shown in Scheme 1 complex +

■

(1) (2)2+ (3)2+ (4)2+ (5)+ (6)+ (7)+ (8)+ (9)2+ (10)+

CC 1.236 1.239 1.231 1.239 1.224 1.227 1.225 1.240 1.230 1.235

Au−C 2.310 2.308 2.476 2.362 2.583 2.490 2.555 2.334 2.521 2.396

2.316 2.426 2.477 2.359 2.589 2.505 2.559 2.328 2.521 2.396

ΔθEq

CTnet

CTσ don

CTπ‑back⊥

CTπ‑back∥

12.0 13.4 12.0 14.9 10.0 10.0 8.9 12.9 10.7 11.4

0.27 0.29 0.27 0.38 0.21 0.21 0.21 0.37 0.32 0.34

0.35 0.30 0.29 0.44 0.24 0.25 0.24 0.44 0.33 0.39

−0.07 −0.05 −0.03 −0.06 −0.03 −0.04 −0.03 −0.07 −0.02 −0.05

0.000 0.004 0.011 0.001 0.004 0.011 0.005 0.010 0.011 0.005

RESULTS AND DISCUSSION Au(III)-Alkyne Bond. CD-NOCV Analysis. In this section, the Au(III)-2-butyne bond is investigated in terms of its DCD components, through the CD-NOCV analysis. A detailed illustration of the CD-NOCV results, focused on complex (1)+, is reported in the Supporting Information for reader convenience. Here a systematic insight into the nature of the Au(III)alkyne bond and how this can be modulated by the ligands is provided by comparing the CD features of all of the complexes depicted in Scheme 1. Results of the CD-NOCV analysis, as well as relevant geometrical parameters correlating with the DCD bond components (C−C and Au−C bond length and 2butyne angle of distortion from linearity, ΔθEq), are collected in Table 1. Isodensity surfaces of the total Δρ and its NOCV components for the Au(III)-2-butyne bond as well as CD curves for all complexes listed in Scheme 1 can be found in the Supporting Information (Figures S1−S11). As shown in Table 1, for all the complexes the total charge transfer measured at the isodensity boundary (see Methodology and Computational Details section), CTnet, has always a positive value, which means that a net transfer of electrons from the alkyne to the gold(III) fragment always occurs. A first striking feature emerging from Table 1 is the stability of the CTnet values with respect to the charge of the complex. For instance, complex (3)2+ shows a smaller CTnet value than that of (8)+ (0.27e vs 0.37e, respectively). This result suggests a crucial role of the ligands, which can surprisingly prevail over the role of the complex charge in determining the net withdrawing electron ability of the complexes. The variability range in CTnet values for complexes depicted in Scheme 1 (from 0.21e in (5)+, (6)+, and (7)+ to 0.38e in (4)2+) must be therefore ascribed to the variability in the [LAu(III)]+/+2 −2butyne or [LAu(III)L′]+/+2−2-butyne bond components. A second striking feature concerns the total charge transfer measured at the CC triple bond midpoint, CTnetCC, values in Table 1, which are always positive and large, indicating that, for each complex, electron charge is subtracted from the triple bond region due to polarization induced by the gold fragment. CTnetCC values vary significantly in the complex series, ranging from 0.17e in (7)+ to 0.35e in (4)2+, and, analogously to CTnet values, they are relatively insensitive to the charge of the complex. For instance, complex (3)2+ shows a smaller CTnetCC value than that of (8)+ (0.21e vs 0.31e, respectively), thus suggesting again that the role of the different ligands prevails over the role of the charge. To assess the ligand effect on the Au(III)-alkyne bond, the total, σ donation, and π back-donation CD-NOCV curves for

CTnet

CC

0.24 0.25 0.21 0.35 0.18 0.19 0.17 0.31 0.27 0.28

all of the considered complexes are compared in the top, middle, and bottom panels of Figure 1, respectively. Inspection of Figure 1 reveals two interesting features. The first is that all systems exhibit a large trend variation for the net charge rearrangement (top panel) in the ancillary (pincer) ligand region and a more similar trend in the regions of the alkyne CC triple bond and the Au-CC bond. This feature is also observed for the σ donation charge rearrangement (middle panel), this being the dominant contribution to the Au(III)-alkyne bond. Indeed, from Table 1, we observe that the 2-butyne to Au σ donation (CTσ don) varies from a minimum value of 0.24e for complexes (5)+ and (7)+ to a maximum value of 0.44e for complexes (4)2+ and (8)+, spanning a 0.20e range along the whole series of systems, due to the different ancillary ligands. Interestingly, a σ donation smaller than 0.30e is shown by complexes (3)2+, (5)+, (6)+, and (7)+, where a donor C atom of the pincer is in trans position to 2-butyne. This finding suggests that donor N, O, S, or P atoms of the pincer in trans position to 2-butyne favor σ donation, probably through a larger electron charge delocalization over the entire ligand than happens for a C atom. By comparing the Δρ1 isodensity surfaces for all the complexes (see Figures S1 and S3−S11 in the Supporting Information), it is possible to visualize how the electron charge density rearranges over the whole complex upon σ donation from the alkyne to the gold fragment. For complexes bearing aromatic pincer ligands, the electron charge flowing from 2-butyne to the metal tends to rearrange itself also toward the aromatic ring of the pincer in the trans position, with a very similar rearrangement in all complexes. Moreover, a cis contribution to the charge rearrangement can also be observed in some cases, depending on the nature of the pincer donor atom bonded to the metal in cis position with respect to the alkyne. This may be clearly visualized by comparing the σ donation isodensity surfaces in complexes with the same pincer ligand but a different position of the alkyne, that is, in complexes (3)2+ and (4)2+, reported in Figure 2. As shown in Figure 2, the presence of the N atom in cis position to the alkyne (left) does not allow any charge rearrangement on the pyridine ring, whereas the presence of the C atom in cis position to the alkyne (right) (and the presence of the N atom in trans to the alkyne) allows both a trans and a cis charge rearrangement on the pincer ligand. Note that a CTσ don value of 0.29e is calculated for complex (3)2+ and of 0.44e for complex (4)2+, meaning that 0.15e are additionally subtracted from the alkyne due to the favorable N trans −C cis arrangement of the pincer ligand. Similar phenomena are observed for other complexes with aromatic E


Article

Inorganic Chemistry

Figure 2. Isodensity surfaces (±0.001 e a.u.−3) of the first NOCV component, Δρ1, (σ donation) for the Au(III)-2-butyne bond in complexes (3)2+(left) and (4)2+(right). Red/blue surfaces represent charge depletion/accumulation regions.

isodensity surfaces in monocyclometalated complexes with the same pincer ligand but a different auxiliary L′ ligand, that is, in the complexes series (5)+, (6)+, and (7)+ (Figures S6−S8 in the Supporting Information) we find that electron charge rearrangement also occurs on the NHC ligand in cis position with respect to alkyne, but not on H2O and PMe3, as we could expect for saturated molecules. However, a CTσ don of 0.24, 0.25, and 0.24e is calculated for complexes (5)+, (6)+, and (7)+, respectively, showing that the different auxiliary L′ ligand does not much affect the σ donation bond component. Finally, it is interesting to note a substantial charge rearrangement over the pincer ligand, leading to efficient subtraction of σ electron charge from the alkyne, when an unsaturated pincer moiety with a donor O or S atom in trans position and an auxiliary Cl− ligand in cis position to the alkyne is present, as in complexes (8)+ and (10)+, respectively (Figures S9 and S11), or when a pincer P donor atom is present trans to the alkyne and an auxiliary aromatic ligand with a C atom is cis to the alkyne, as occurs in complex (9)2+ (Figure S10 in the Supporting Information). In the CC triple bond region, the systematic positive value of all the CD-NOCV curves denotes electron charge shift from the alkyne toward metal fragment, due to a significant polarization of the alkyne triple bond. These findings further demonstrate that the different ligand frameworks can deeply influence the charge rearrangement upon formation of the gold(III)-alkyne coordination bond and a rationalization of these effects would be helpful to understand how the stability of these complexes can be modulated in the perspective of their application as efficient catalysts. The π charge rearrangement trend (bottom panel in Figure 1) shows a variation over the whole molecular region and, specifically, in the Au-CC and in the CC triple bond regions. In the Au-CC bond region the CD-NOCV curves assume negative values, indicating an electron charge transfer from the gold fragment to 2-butyne, whereas in the alkyne CC triple bond region they take positive values, corresponding to an electron flow in the direction toward the metal fragment and signaling a polarization of the triple bond. The Au(III)-alkyne bond π back-donation component is however surprisingly not negligible in all the complexes in Scheme 1. A variation of the π back-donation perpendicular to the molecular plane, CTπ‑back⊥, by 0.05e electrons (from −0.07e for complexes (1)+ and (8)+ to −0.02e for complex (9)2+) is calculated over the complexes series. We should note here that the CD-NOCV curves of Figure 1 (bottom) represent the total π backdonation, that is, the sum of the in-plane (parallel) and out-ofplane (perpendicular) contributions. This is almost totally due

Figure 1. Total (top), σ donation (middle), and π back-donation (both perpendicular and parallel components) (bottom) CD curves for the Au(III)-2-butyne bond in the series of [LAu2-butyne]+/+2 or [LAuL′2-butyne]+/+2 complexes of Scheme 1. The z origin is placed at the Au atom for all the complexes, and red dots indicate the positions of the Au and of the middle point of the alkyne CC triple bond, averaged for all complexes.

pincer ligands (Figures S1 and S3−S11). As a general result, we observe that the cis contribution to the charge rearrangement on the ligand is present only when a C (or O, S, Cl) donor atom occupies the cis position in the complex, whereas no cis rearrangement occurs when a N donor atom occupies the cis position. In addition, by comparing the σ donation F


Article

Inorganic Chemistry

population of the π bonding/π* antibonding orbital of the alkyne. The correlation is good (R2 = 0.9587), provided that complex (2)2+, which is the most outlier, is not included. Actually, complex (2)2+ shows a different ligand geometry motif compared to that of the other complexes: from its optimized geometry we can notice that the 2-butyne CC bond is not centered along the z axis passing through the gold atom but presents a particular “asymmetry” probably due to the steric effects of the skewed N^NÔ scaffold around the metal. For the 2-butyne distortion from linearity, ΔθEq, a simple model has been applied,41 where this quantity correlates linearly with the magnitude of CTσ don and CTπ‑back plus an additional electrostatic contribution Δθelect, which is unknown and potentially large for charged systems such as the complexes under study. In this model the distortion from linearity is largely dominated by the π back-donation contribution. In Figure 4 the values of the distortion from linearity angle of 2-

to the perpendicular contribution in the Au-CC region, whereas the contribution of the parallel π back-donation component becomes as important as that of the perpendicular π back-donation component in the alkyne CC triple bond region (see blue and green lines in Figures S1 and S3−S11 in the Supporting Information). The second interesting feature of Figure 1 is that, in the alkyne CC region, all the complexes invariably exhibit a total flow (both perpendicular and parallel components) of π electrons in the Au ← CC direction (CTπCC > 0). These π polarization effects of the CC triple bond and the σ polarization effects discussed above add up, leading to large CTnetCC values (Table 1). A remarkable characteristic of the CD-NOCV curves of π symmetry is that they are rather large in the alkyne CC region with a maximum at the CC bond and a larger extension toward the outer side of the CC triple bond. By accepting the notion that the activation of the triple bond depends to a large extent on the electronic charge subtracted from the region undergoing the nucleophilic attack (a nucleophilic outer-sphere attack is commonly observed), we should be able to discuss a catalytic efficiency series on the basis of how donation, back-donation, and polarization tune the charge concentration at the nucleophilic attack site. Finally, our Au(III)-2-butyne bonding analysis results were validated by checking the influence of dispersion correction (Grimme D3-BJ)87,88 and functional (BP8671,79 and B3LYP89). Results are given in the Supporting Information (Figure S12 and Table S1). It has been demonstrated by some of us41 that a quantitative relationship between the DCD components of the gold-alkyne bond and experimental observables can be singled out. It is almost invariably found that, upon binding to a metal fragment, the alkyne undergoes a measurable distortion from linearity, ΔθEq, and an elongation of the CC triple bond. We can here show that these two observables depend on both σ donation and π back-donation components (elongation of CC bond) and almost selectively on π back-donation component (distortion from linearity ΔθEq), as found in ref 41. In Figure 3

Figure 4. Correlation plot between 2-butyne distortion from linearity (ΔθEq) and charge transfer values for π back-donation (CTπ‑back⊥) (values taken from Table 1) for all the complexes in Scheme 1 (R2 = 0.9790 for +2 complexes, R2 = 0.8881 for +1 complexes).

butyne taken from the optimized geometry (ΔθEq from Table 1) are plotted versus the charge transfer values for π backdonation (CTπ‑back⊥ from Table 1) for all the complexes. From this figure a good correlation can be observed between these two quantities within each subset of complexes of different charge, and an increase of the distortion from linearity of 2butyne corresponds to an increase of the π back-donation component of the Au(III)-2-butyne bond. The two linear fits are nearly parallel, and their shift can be mainly attributed to the electrostatic contribution Δθelect according to the model of ref 41. Energetics of Au(III)-Alkyne Complexes. The energy decomposition scheme EDA was applied to all of the complexes, and results are summarized in Table 2. First we focus on the computed electronic interaction energies ΔEint between 2-butyne and [LAu]+/+2 or [LAuL′]+/+2 fragments in the geometries they have in the corresponding complex. A large variation in the Au(III)-alkyne interaction energy within the pincer ligand series can be observed. The largest value is calculated for dicationic (4)2+ complex, whereas the smallest values are obtained for cationic (7)+ and (5)+. A comparably large interaction energy is calculated for complexes (1)+, (2)2+, and (8)+ (Table 2). From Table 2 a systematic trend showing that a +2 charge complex can bind 2-butyne more strongly than a +1 charge complex or vice versa cannot be found. For instance, complex (1)+ can bind 2-butyne more strongly than (3)2+, and complex (4)2+ can bind 2-butyne

Figure 3. Correlation plot of 2-butyne CC triple bond distance (CC) and charge transfer values for σ donation and π back-donation (absolute values) (CTσ don + |CTπ‑back|) (values taken from Table 1) for all the complexes in Scheme 1 (R2 = 0.9587 with (2)2+ not included).

a correlation plot between 2-butyne CC bond distance and charge transfer values for σ donation and π back-donation (absolute values) (CTσ don + |CTπ‑back|) for all the complexes is shown. As expected, on increasing both the σ donation and π backdonation, the CC triple bond elongates, due to depopulation/ G


Article

Inorganic Chemistry Table 2. Atomic Charge on Au, QAu (e)a and EDA Analysis Resultsb for the [LAu(III)-2-butyne] butyne]+/+2 Bonds in All the Complexes Shown in Scheme 1

+/+2

and [LAu(III)L′-2-

complex

QAu

ΔEint

ΔEPauli

ΔEelstat

ΔEoi

ΔE(σ don)

ΔE(π‑back⊥)

(1)+ (2)2+ (3)2+ (4)2+ (5)+ (6)+ (7)+ (8)+ (9)2+ (10)+

0.43 0.44 0.40 0.42 0.35 0.37 0.32 0.47 0.35 0.36

−46.5 −43.5 −29.3 −50.8 −17.5 −24.1 −17.0 −47.8 −27.4 −37.2

113.0 118.5 66.9 120.6 63.4 74.4 72.1 131.4 82.1 104.3

−92.9 −91.3 −53.0 −92.7 −47.9 −60.0 −53.8 −99.9 −61.6 −80.0

−66.6 −70.7 −43.2 −78.7 −33.0 −38.4 −35.3 −79.3 −48.0 −61.5

−44.6 −46.3 −30.0 −56.0 −23.2 −25.4 −23.9 −54.7 −33.8 −43.7

−10.7 −9.9 −4.5 −9.6 −3.9 −5.6 −4.4 −10.5 −4.8 −7.7

a

Calculated using the Voronoi Deformation Density (VDD) scheme. bEnergies are in kcal/mol.

more strongly than (7)+. These findings clearly confirm the CD-NOCV analysis results, namely, that the overall charge of the complex is delocalized over the gold ligand with a different efficiency. The atomic charges on gold atom in all the complexes, calculated with the Voronoi Deformation Density (VDD) method,90,91 QAu, are also reported in Table 2. These values span only a small range between 0.32 and 0.47 e, and, for instance, the atomic charge on gold in complex (8)+ is larger than that in complex (9)2+. Hence, on the basis of the CD-NOCV analysis results, one factor influencing the interaction energy between the alkyne and the gold fragment in the complex series could be the electron density rearrangement at the ancillary ligand upon bond formation. However, correlation between ΔEint and CTnet, as shown in Figure 5, is quite poor, thus indicating that

gold(III)-alkyne interaction with ligands depending both on orbital interaction and electrostatic contribution. Decomposition of ΔEoi term into σ donation (ΔE(σ don)) and π backdonation (perpendicular component, ΔE(π‑back⊥)) contributions is also shown in Table 2 for all complexes. Results fully corroborate the CD-NOCV analysis. The dominant σ donation component in the Au(III)-alkyne bond translates into the dominant contribution to the orbital interaction energy, with values in the range between −56.0 kcal/mol for complex (4)2+ and −23.2 kcal/mol for complex (5)+. Interestingly, the largest orbital interaction energy values for complexes (4)2+ and (8)+ are mainly due to the σ donation component (−56.0 and −54.7 kcal/mol, respectively), accounting not only for the “pure” σ electron charge donation (CTσ don 0.44e for both complexes) but also for polarization (CTσCC 0.23 and 0.17e, respectively). The π back-donation, although representing a much less relevant contribution to ΔEoi, is however significant, with values in the range of −10.7 kcal/mol (complex (1)+)/−3.9 kcal/mol (complex (5)+). Analogously, the term ΔE(π‑back⊥) includes both “pure” π electron charge back-donation and π polarization contributions. Notably, the largest values are calculated for complexes (1)+ and (8)+ but also for (2)2+ and (4)2+, which can be mainly ascribed to the perpendicular π back-donation component (CTπ‑back⊥ −0.07e for (1)+ and (8)+, −0.05e for (2)2+, and −0.06e for (4)2+). An important issue concerning the Au(III)-alkyne complexes in Scheme 1 is their stability. Unfortunately, experimental values of free energies (or enthalpies) of metal fragment Au(III)-alkyne binding are not available in the literature for the experimentally characterized (1)+, (6)+, and (7)+ complexes. To assess the bonding energies of alkynes, we performed calculations of enthalpy (ΔH (298° K)) and free energy (ΔG (298° K)) change for the reactions [LAu(III)]+/+2 + 2-butyne → [LAu(III)2-butyne]+/+2 (biscyclometalated) and [LAu(III)L′]+/+2 + 2-butyne → [LAu(III)L′-2-butyne]+/+2 (monocyclometalated) for all the complexes. Results are shown in Table 3. Additional data on checking the influence of dispersion correction and absence of solvent model (BLYP in gas phase, BLYP including Grimme dispersion correction in gas phase (BLYP-D3-BJ), 8 7 , 8 8 and BLYP including solvent (COSMO92−94 dichloromethane) results) are shown in Table S2 in the Supporting Information for complexes (1)+, (6)+, and (7)+ as test cases. Inclusion of D3-BJ dispersion correction accounts for a large increase of both the enthalpies (in the range of 11.9−17.4 kcal/mol) and free energies (from

Figure 5. Correlation plot between the total charge transfer values calculated at the isodensity boundary (CTnet) and the interaction energy (ΔEint) between 2-butyne and [LAu]+/+2 or [LAuL′]+/+2 fragments (values taken from Table 1 and Table 2) for all the complexes studied here (R2 = 0.6325).

the interaction energy, resulting from the repulsive Pauli ΔEPauli, the attractive electrostatic ΔEelstat, and the orbital interaction ΔEoi energy components, cannot be solely attributed to the charge transfer (i.e., to the ΔEoi energy component), but other contributions can also be important. We can here see that the most relevant contributions to the total interaction energy are invariably the ΔEPauli and the ΔEelstat terms, with the ΔEPauli dominating over the counteracting ΔEelstat. The ΔEoi term is always smaller than the ΔEelstat (in percentages from 64% in complex (6)+ to 85% in complex (4)2+). Plots of ΔEint versus ΔEoi (R2 = 0.9567) and ΔEint versus ΔEelstat (R2 = 0.9152) reported in Figures S13 and S14 in the Supporting Information indicate a variation of the H


Article

Inorganic Chemistry

alkyne is in trans position to an anionic C atom of the pincer (complexes (5)+, (6)+, and (7)+). Au(III)-Alkyne Bonding/Reactivity Relationship. Providing new insights into the bond-activity relationship in the gold-catalyzed activation of triple bonds is certainly desirable in view of its relevance for ligand design in gold(III) catalysis. In this section we address the question if and how the knowledge of the DCD components of the bond between catalyst and substrate can give useful information for predicting the electronic effect of a metal complex on catalysis. As a case study, we investigate the nucleophilic attack step of a hydration reaction. In our model the nucleophile is water, and the catalysts are (1)+, (3)2+, (4)2+, (5)+, and (8)+ complexes studied in the previous section. We selected the above subset of Au(III) complexes as representative of biscyclometalated ((1)+), dicationic ((3)2+ and (4)2+), and monocationic ((5)+ and (8)+) monocyclometalated species, respectively. Note that complex (1)+ was experimentally characterized, while complexes (3)2+ and (4)2+ bear the same pincer ligand but a different alkyne coordination position (cis and trans to pincer N donor atom, respectively) and complexes (5)+ and (8)+ have a different pincer ligand framework, with complex (5)+ sharing the same pincer scaffold as that of the experimentally observed complexes (6)+ and (7)+ (these three complexes only differ by the ancillary L′ ligand, NHC, H2O, and PMe3, respectively, and show very similar bonding features). We analyze the reaction by considering the H 2 O nucleophilic attack to 2-butyne assisted by the trifluoromethanesulfonate, triflate (OTf−) anion, as schematically shown in Scheme 2.

Table 3. 2-Butyne Binding Electronic Energies (ΔE), Enthalpies (ΔH (298° K)), and Free Energies (ΔG (298° K)) (kcal/mol) to [LAu(III)]+/+2 (Biscyclometalated) and [LAu(III)L′]+/+2 (Monocyclometalated) in All the Complexesa complex

ΔE

ΔH (298 K)

ΔG (298 K)

(1)+ (2)2+ (3)2+ (4)2+ (5)+ (6)+ (7)+ (8)+ (9)2+ (10)+

−61.6 −7.4 −27.4 −14.7 −24.9 −22.9 −26.6 −42.6 −28.1 −27.9

−58.8 −6.2 −25.7 −12.7 −22.1 −21.3 −24.5 −40.7 −26.7 −25.5

−45.9 3.1 −10.4 −2.2 −11.5 −9.8 −12.3 −31.6 −15.9 −18.2

a

Calculated at dispersion-corrected BLYP-D3-BJ in solvent (COSMO, dichloromethane) level.

12.3 to 20.1 kcal/mol) with respect to BLYP values. Inclusion of solvent generally decreases the enthalpies (from 2.0 to 6.7 kcal/mol) to a much lesser extent than the dispersion correction. To be more accurate, however, calculations in Table 3 included both dispersion correction and solvent effects. The enthalpies span a range of −6.2/−58.8 kcal/mol, and the free energies are calculated between 3.1 and −45.9 kcal/mol, implying relatively high stability, except for (2)2+ and (4)2+ complexes. Since complex (6)+, for which binding enthalpy and free energy values of −21.3 and −9.8 kcal/mol, respectively, were calculated (Table 3), could be experimentally observed,33 all of the complexes studied here might be surmised to form stable alkyne complexes, apart from (2)2+. Indeed, on comparing (3)2+ and (4)2+, which bear the same pincer ligand, we might predict that the alkyne coordination in cis position to the pincer N donor atom gives a relatively stable complex. Complex (2)2+ could be assumed to be not sufficiently stable to be characterized, probably due to steric factor (skewed N^NÔ scaffold) destabilizing the geometry of the complex. However, we should note that the calculated enthalpies and free energies heavily depend on the level of theory used, as shown in our small benchmark on dispersion correction and solvation items, and therefore definitive conclusions on the stability of these complexes cannot be drawn at this stage. Experimental measurements of such thermodynamic data are therefore essential and highly desirable. It is noteworthy that similar binding energies were computed for the [(C^N^C)Au]+ and [(C^C)Au]+ types of complexes with alkynes in ref 33 (enthalpies of ∼50 and ∼20 kcal/mol, respectively). The much smaller values found for the [(C^C)Au]+-type complexes were ascribed to the strong trans influence of the anionic aryl C atom in the trans position to the alkyne with respect to the modest trans influence of the neutral pyridine N atom in trans to the alkyne in the [(C^N^C)Au]+type complexes. Analogously, we find that the alkyne trans to an anionic C atom results in substantial weakening of the LAualkyne bond (complexes (5)+, (6)+, and (7)+), whereas the alkyne trans to a neutral N atom leads to sizably stronger LAualkyne bonds (complex (1)+). Interestingly, the trans influence discussed above can be also rationalized on the basis of ΔE(σ don) values (Table 2), which are sizably smaller when the

Scheme 2. Model Nucleophilic Attack Step of a Hydration Reaction with Gold(III)-Alkyne Complexesa

a

RC = reactant complex; PC = product complex.

We chose OTf−, since it is both a small anion and suitable for modeling the weak coordinating power of the commonly employed counterions in experiments. For dicationic complexes we included an additional OTf− molecule in the calculations to stick as much as possible to the “real” neutral systems for which the effect of including one or two anions could make a difference in terms of reactivity as well demonstrated in the literature for gold(I).35,53,95,96 We performed scans of the potential energy surfaces (PESs), where the reaction coordinate is represented by the oxygen atom of H2O approaching one carbon atom of the CC triple bond in the five selected complexes. The reaction profiles, starting from the reactant complexes (RC) to the product complexes (PC), through possible transition states (TS), are shown in Figure 6. All the structures are reported in Figures S15−S19 in the Supporting Information. We should stress that our aim here is not to give a quantitative understanding of the hydration reaction mechanism but to compare the predictions based on the [LAu(III)]+/2+-alkyne and [LAu(III)L′]+/2+-alkyne bond analysis I


Article

Inorganic Chemistry

neutral pyridine N atom (complex (3)2+) is more stabilizing than in trans position to an aryl C atom (complex (4)2+). These ligand effects are certainly peculiar of Au(III) complexes, and they deserve further investigation. Benchmark calculation results on inclusion of solvent (dichloromethane) and dispersion correction (BP86-D3-BJ/ B2PLYP) can be found in the Supporting Information. We find that solvent inclusion does not change the reactivity picture with respect to the gas phase, although, quantitatively, a small variation of the activation energy barriers in the range of 0.1− 1.9 kcal/mol has been computed (Table S4 in the Supporting Information), and dispersion-corrected RC, TS, and PC optimized geometries yield unaltered values of the activation energy barriers and reaction energies (Figures S20 and S21 in the Supporting Information). Since the reaction has an energy barrier ΔE# varying from 2.5 (for (4)2+) to 11.8 kcal/mol (for (5)+), with very different Au(III)-alkyne interaction energies (from −17.5 in (5)+ to −50.8 kcal/mol in (4)2+, Table 2), the influence of the considered five distinct ligands on their reactivity should be significant. In particular, the lower barriers correlate with the higher interaction energies (see Figure S22 in the Supporting Information). As a general remark, we observe that all the activation barriers are relatively low, in agreement with the positive values of CTnet and CTnetCC indicating that, in all cases, coordination to the gold catalyst subtracts charge from the triple bond and, in particular, from the external side of the triple bond region. We find that the carbon atom in the Au(III)-alkyne complexes is electropositive enough to easily undergo H2O nucleophilic attack, in line with the specific bonding features in the five Au(III)-alkyne complexes. Indeed, by comparing the net Au(III) ← 2-butyne charge transfer along the series, namely, the corresponding CTnet values in Table 1, we see the following main differences: (i) complex (5)+ shows the smallest net charge donation in the series (0.21 e), thus, the least electrophilic character of the CC carbon atom, which explains the relatively high energy barrier of 11.8 kcal/mol calculated for (5)+; (ii) in complexes (1)+ and (3)2+ the net charge donation has the same value (0.27 e) and is larger than that in (5)+, thus accounting for a very similar energy barrier for (1)+ and (3)2+ (5.5 and 5.6 kcal/mol, respectively) that is smaller than that for (5)+; (iii) the largest net charge donation is observed for complexes (4)2+ and (8)+ (0.38 and 0.37e, respectively), which explains the smallest energy barriers (2.5 and 2.8 kcal/mol) in the whole series, and thus the most electrophilic character of the triple bond carbon atom in these complexes. On this basis, we could suggest that the water nucleophilic attack for complexes (6)+ and (7)+ (CTnet = 0.21e) would have an energy barrier similar to that for (5)+, whereas for complex (2)2+ (CTnet = 0.29e) a slightly lower activation barrier than those for complexes (1)+ and (3)2+ is expected, and for complexes (9)2+ and (10)+ (CTnet = 0.32e and 0.34e, respectively) an energy barrier appreciably lower than those for complexes (1)+ and (3)2+ can be predicted. The CTnetCC values in the five complex series show the same trend as that of CTnet values. In Figures S23 and S24 in the Supporting Information the correlations of the activation barrier with CTnet and with CTnetCC are reported. Note that the activation barriers ΔE# show an acceptable linear trend with CTnet and a somewhat less good linear correlation with CTnetCC (R2 = 0.8423 and R2= 0.7495, respectively). This finding suggests that the amount of electronic charge subtracted from the region where the nucleophilic attack

Figure 6. Reaction profiles for the OTf−-assisted H2O nucleophilic attack on CC triple bond carbon atom in complexes (1)+ (green line), (3)2+ (red line), (4)2+ (blue line), (5)+ (violet line), and (8)+ (gray line). Energy values calculated at the BP86/B2PLYP level (see Methodology and Computational Details section). Energy value for each RC is taken as zero reference point.

results using the OTf−/H2O nucleophile as a probe, in an attempt to find useful bonding/reactivity relationships. Obviously, we expect that the nature of the anion as well as the solvent play a crucial role on the reaction mechanism, as we found for gold(I) complexes.53,95,96 To our knowledge, this issue has been poorly explored for gold(III) complexes. A theoretical study of gold(III)-catalyzed hydration of alkynes has been performed by Pascual et.52 using halo dithiocarbamate gold(III) complexes (i.e., (10)+-type complexes) as catalysts and focusing on solvent effect. Here we use the anion only as a proton acceptor, to promote the attack by the water molecule in the gas phase. The energy of the transition state TS with respect to the energy of the corresponding reactant complex RC, taken as zero reference energy, is the most important quantity for our discussion, because it is directly related to the activity of the catalyst and to the electrophilicity of the carbon atom interacting with water (and consequently to the Au-alkyne bonding nature). Therefore, we can consider the energy of the transition state (representing the activation energy barrier for the nucleophilic attack) as a (inverse) measure of the activation of the CC triple bond. For the Au(III)-alkyne complexes (4)2+ and (8)+ the reaction has a small energy barrier ΔE#, while for complexes (1)+ and (3)2+ the barrier is slightly larger, and for complex (5)+ the highest barrier is calculated (Figure 6). Imaginary frequencies defining all the transition states are reported in Table S3 in the Supporting Information. The reaction is exoergonic by 20.2 and 28.3 kcal/ mol for (4)2+ and (8)+, respectively, by 19.6 and 5.9 kcal/mol for (1)+ and (3)2+, while for complex (5)+ the reaction is endoergonic by 2.0 kcal/mol. Interestingly, we note that our results follow the Hammond’s postulate.97 We should also notice here that the direct comparison between the energy of the RC for complexes (3)2+ and (4)2+ shows that the RC for complex (3)2+ is more stable than that for complex (4)2+ by 12.4 kcal/mol, suggesting that ligands in cis position to the alkyne should also have a role in Au(III) complex stabilization. In particular, a donor ligand (NHC) in trans position to a J


Article

Inorganic Chemistry occurs is not the only factor driving the activation of the triple bond. Another factor could be the amount of back-donation, insofar as a weaker back-donation makes alkyne slippage energetically less demanding. In other words, the Au(III) alkyne complexes acquire some vinyl cation character, which explains their high reactivity and propensity for nucleophilic attack. On comparing complexes (1)+ and (3)2+, we note that their common CTnet value (0.27e) results from a both larger σ donation and π back-donation (0.35 and −0.07e, Table 1) in complex (1)+ than in complex (3)2+ (0.29 and −0.03e, Table 1), so that the larger π back-donation in (1)+ may be responsible for a larger barrier than that expected on the basis of the σ donation alone. Upon η2 → η1 coordination slippage an increased electrophilicity of the alkyne is expected, which in turn enhances the charge subtracted from the triple bond region.53 Thus the partial slippage away from the symmetrical η 2 coordination at the transition state facilitates the nucleophilic attack to the distorted π system and, in particular, to the carbon atom C1 farther away from gold.35,36 Therefore the Au−C1 bond distance can also be related to the nucleophilic attack activation barrier. A certain asymmetry of the coordinated 2-butyne can be found already in the [LAu(III)-2-butyne]+/2+ and [LAu(III)L′-2-butyne]+/2+ complexes (see Au−C values in Table 1). This asymmetry is also found in the RC, and it is enhanced, as expected, in the TS structures for the five considered complexes (see Figures S15− S19 in the Supporting Information for the optimized RC, TS, and PC geometries). In particular, in the TS structure the 2butyne slippage away from the symmetrical η2 coordination, calculated as the difference between Au−C1 and Au−C2 bond lengths, is 0.428, 0.641, 0.451, 0.640, and 0.430 Å for (1)+, (3)2+, (4)2+, (5)+, and (8)+ complexes, respectively. These values correlate very well with the corresponding CTπ‑back⊥ values, as shown in Figure S25 in the Supporting Information (R2 = 0.9855). Yet other factors, different from the ligand electronic effects, influencing the activation energy barrier of the water nucleophilic attack could be the affinity of the nucleophile for the electrophilic carbon atom and the effect of the counterion interacting with the different complexes. To make the electronic effects of the ligands independent of the above effects we can evaluate the energy cost for the 2-butyne slippage by calculating the energy of the transition state in the absence of the counterion and water. To this aim, single-point B2PLYP calculations were performed on the RC and TS optimized structures of the five considered complexes without including OTf− and H2O, and corresponding energy differences (ΔE#′) were evaluated, which amount to 7.4, 8.2, 1.8, 11.5, and 4.2 kcal/mol for (1)+, (3)2+, (4)2+, (5)+, and (8)+ complexes, respectively. Indeed, the electronic charge subtracted from the outer region of the 2-butyne triple bond, that is, the CTnetCC values, very nicely correlates with the above ΔE#′ values, as shown in Figure 7 (R2 = 0.9710), much better than with the ΔE# values (see Figure S24 in the Supporting Information, R2= 0.7495). By comparing the ΔE#′ with the corresponding ΔE# values, we note that, for complexes (1)+, (3)2+, and (8)+, inclusion of nucleophile and counterion decreases the energy cost due to the ligand electronic effects of the 2-butyne slippage, whereas for complexes (3)2+ and (4)2+ it increases the energy cost of alkyne slippage only slightly. This result suggests that (i) the counterion should have a crucial role in the nucleophilic additions to alkynes catalyzed by gold(III) complexes and (ii)

Figure 7. Correlation plot between the energy cost for the 2-butyne slippage in the water nucleophilic attack (ΔE#′) and the total charge transfer values calculated at the CC triple bond (CTnetCC) (values taken from Table 1) for the five selected complexes (R2 = 0.9710).

the activation of the triple bond depends on the electronic charge subtracted from the outer-sphere region of the triple bond, resulting from a combination of donation, back-donation and polarization effects, analogously to what has been found for gold(I) complexes.36,53,95,96 In summary, on the basis of the peculiar bonding features in the Au(III)-alkyne complex series studied here, high efficiency in the alkyne activation could be expected. The different ligands could have a role in affecting the catalytic activity, and they do have a significant effect on the complex stability. Before concluding, it may be interesting to put our results in the context of the current knowledge of homogeneous gold(I)based catalysis. First we comment on Au(III)-alkyne and Au(I)-alkyne bonding features. On the one hand, from ref 36, where the Au(I)-alkyne bond has been studied in a set of [LAu(I)ethyne]0/+ (L = none, Cl−, PF3, PH3, NHC) complexes, we observe that [LAu(III)]+/2+ and [LAu(III)L′]+/2+ fragments show a σ withdrawing ability similar or larger (CTσ don values in the range between 0.24 and 0.44e) than that of the [LAu(I)]+ systems (CTσ don values between 0.25 and 0.28e). On the other hand, we find that the [LAu(III)]+/2+ and [LAu(III)L′]+/2+ fragments exhibit significantly smaller π backdonating ability (CTπ‑back values between −0.02 and −0.07e) than that of the [LAu(I)]+ systems (CTπ‑back values between −0.08 and −0.23e). It is thus important to note that, from the balance between the donation and back-donation components, a larger net acidity generally arises for the [LAu(III)]+/2+ and [LAu(III)L′]+/2+ fragments (CTnet values between 0.21 and 0.38e) with respect to that of the [LAu(I)]+ fragments (CTnet values between 0.03 and 0.20e). A larger electronic charge subtraction from the CC triple bond is calculated for the [LAu(III)]+/2+ and [LAu(III)L′]+/2+ fragments (CTnetCC values between 0.17 and 0.35e) with respect to that of the [LAu(I)]+ fragments (CTnetCC values between 0.06 and 0.22e). On the basis of this comparison, we could expect a more electrophilic character of the CC triple bond when bonded to [LAu(III)]+/2+ and [LAu(III)L′]+/2+ fragments, and we can surmise that the activation energy barriers for the nucleophilic attack on charged Au(III)-alkyne complexes would be generally lower than those on Au(I)-alkyne complexes. A generally accepted mechanism for nucleophilic addition reactions with gold(I) complexes consists of a pre-equilibrium step, where the counterion is replaced by the alkyne substrate, a nucleophilic attack step, and a protodeauration step, where K


Article

Inorganic Chemistry

where atoms different from carbon are in trans position to the alkyne CC triple bond emerge as interesting for both imparting Au(III)-alkyne bond stability and inducing a high activity. In conclusion, the gold(III)-alkyne complexes are predicted to very efficiently activate the alkyne triple bond, to the point that the nucleophilic attack step could cease to be the ratedetermining step of the reaction. The present study clearly calls for new experiments and theoretical investigations aimed at analyzing the ligand effects in other key steps of the catalytic cycle including the pre-equilibrium and the protodeauration steps. Although other factors, such as the ready reduction of Au(III) to Au(I) and the higher steric demand of the stabilizing ligands in Au(III) complexes could limit their synthetic potential, we believe that this work could inspire the design and synthesis of new types of Au(III) catalysts for the activation of alkynes.

the formed vinyl gold intermediate complex reacts with an electrophile (proton) yielding the final product and regenerating the catalyst. For NHC gold(I)-catalyzed alkoxylation of alkynes reactions the rate-determining step (RDS) is invariably the nucleophilic attack step.35,53,95,96 In addition, the activation energy barrier calculated for the corresponding nucleophilic attack in the hydration reaction of 2-butyne catalyzed by [NHCAu(I)]+ complex amounts to 17.4 kcal/mol,53 much larger than those calculated for the gold(III)-catalyzed nucleophilic attack. Indeed, for the gold(III) complexes studied here the calculated energy barriers are so low to suggest that the nucleophilic attack step could not be the RDS. From this comparison we could envisage that the metal oxidation state (Au(III) vs Au(I)) could strongly affect the reactivity. In particular, for Au(III) catalysts the preequilibrium or the protodeauration step could be the RDS of the nucleophilic addition reaction mechanism.

■

■

CONCLUSIONS In this paper we have analyzed the Au(III)-alkyne bond in a series of cationic and dicationic bis- and monocyclometalated gold complexes, focusing on the role of the ancillary ligands. Quantitative measures of the σ donation and π back-donation bond components as well as of the CC triple bond polarization have been obtained by the well-established charge-displacement analysis, as resulting from accurate DFT calculations, within the Natural Orbitals for Chemical Valence framework (CD-NOCV). All complexes are found to feature a large total electron charge transfer from the alkyne to the metal fragment, with values ranging from 0.21 to 0.38e, which results from a large σ donation (values between 0.24 and 0.44e) and a small but significant π back-donation (values between −0.02 and −0.07e) component. This total electron charge transfer is surprisingly insensitive to the charge of the complex. The electron density rearrangement over the CC triple bond region shows a significant polarization of the CC bond for all complexes (values between 0.17 and 0.35e), depending on the ligand type. Compared to gold(I) (both cationic and neutral), however, the gold(III) complexes have a larger tendency to polarize the CC electrons in the direction from the outer side to the inner side toward gold (Au ← CC), and they generally show larger σ donation and smaller π back-donation components. We have also undertaken a study to outline the bonding/reactivity relationships in a model water nucleophilic attack to the alkyne triple bond for five selected gold(III) complexes. Results show that the water nucleophilic attack to alkyne has small activation barriers (from 2.5 to 11.8 kcal/ mol), suggesting that the ligand influence on the reactivity can be remarkable, in agreement with the different bonding features in the whole series of complexes. The small barriers mainly correlate with the large σ donation and small π backdonation components, which concur in determining the more or less electrophilic character of the CC carbon atom and the ease of the η2 → η1 coordination slippage. On the contrary, the metal oxidation state (Au(III) vs Au(I)) affects the reactivity. The origin of the considerable activity difference between the gold(III)-alkyne and the gold(I)-alkyne complexes can be traced to the difference in the σ donation and π back-donation bond components and CC polarization. A generally larger σ donation and smaller π back-donation combined with a larger CC polarization toward gold in Au(III) complexes are responsible for the carbon atom of the CC bond being more effectively activated toward a nucleophilic attack. Ligands

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03172.

■

3D isodensity plots and CD curves for complexes (1)+− (10)+, reactant complex (RC), transition state (TS), and product complex (PC) for complexes (1)+, (3)2+, (4)2+, (5)+, and (8)+, correlation plots, benchmark calculation results, and Cartesian coordinates of all the optimized geometrical structures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (L.B.) *E-mail: [email protected]. (P.B.) ORCID

Leonardo Belpassi: 0000-0002-2888-4990 Paola Belanzoni: 0000-0002-1286-9294 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge financial support from MIUR and the Univ. of Perugia to the project AMIS, through the program “Dipartimenti di Eccellenza 2018-2022”. P.B. and F.T. thank the Univ. of Perugia for financial support (“Fondo Ricerca di Base 2015”).

■

REFERENCES

(1) Leyva-Pérez, A.; Corma, A. Similarities and differences between the “relativistic” triad gold, platinum, and mercury in catalysis. Angew. Chem., Int. Ed. 2012, 51, 614−635. (2) Liu, L.-P.; Hammond, G. B. Recent advances in the isolation and reactivity of organogold complexes. Chem. Soc. Rev. 2012, 41, 3129− 3139. (3) Debrouwer, W.; Heugebaert, T. S. A.; Roman, B. I.; Stevens, C. V. Homogeneous gold-catalyzed cyclization reactions of alkynes with N- and S-nucleophiles. Adv. Synth. Catal. 2015, 357, 2975−3006.

L


Article

Inorganic Chemistry (4) Dorel, R.; Echavarren, A. M. Gold(I)-catalyzed activation of alkynes for the construction of molecular complexity. Chem. Rev. 2015, 115, 9028−9072. (5) Harris, R. J.; Widenhoefer, R. A. Gold carbenes, gold-stabilized carbocations and cationic intermediates relevant to gold-catalyzed enyne cycloaddition. Chem. Soc. Rev. 2016, 45, 4533−4551. (6) Asiri, A. M.; Hashmi, A. S. K. Gold-catalyzed reactions of diynes. Chem. Soc. Rev. 2016, 45, 4471−4503. (7) Shahzad, S. A.; Sajid, M. A.; Khan, Z. A.; Canseco-Gonzalez, D. Gold catalysis in organic transformations: a review. Synth. Commun. 2017, 47, 735−755. (8) Modern gold catalyzed synthesis; Hashmi, A. S. K., Toste, F. D., Eds.; Wiley-VCH, 2012. (9) Teles, J. H.; Brode, S.; Chabanas, M. Cationic gold(I) complexes: highly efficient catalysts for the addition of alcohols to alkynes. Angew. Chem., Int. Ed. 1998, 37, 1415−1418. (10) Fukuda, Y.; Utimoto, K. Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst. J. Org. Chem. 1991, 56, 3729−3731. (11) Fürstner, A.; Davies, P. W. Catalytic carbophilic activation: catalysis by platinum and gold π acids. Angew. Chem., Int. Ed. 2007, 46, 3410−3449. (12) Jin, L.; Wu, Y.; Zhao, X. Theoretical insight into the Au(I)catalyzed hydration of halo-substituted propargyl acetate: dynamic water-assisted mechanism. RSC Adv. 2016, 6, 89836−89846. (13) Schmidbaur, H.; Schier, A. Gold η2-coordination to unsaturated and aromatic hydrocarbons: the key step in gold-catalyzed organic transformations. Organometallics 2010, 29, 2−23. (14) Brooner, R. E. M.; Widenhoefer, R. A. Cationic, two-coordinate gold π complexes. Angew. Chem., Int. Ed. 2013, 52, 11714−11724. (15) Roşca, D. A.; Wright, J. A.; Bochmann, M. An element through the looking glass: exploring the Au-C, Au-H and Au-O energy landscape. Dalton Trans. 2015, 44, 20785−20807. (16) Blons, C.; Amgoune, A.; Bourissou, D. Gold(III) π complexes. Dalton Trans. 2018, 47, 10388−10393. (17) Holmsen, M. S. M.; Nova, A.; Balcells, D.; Langseth, E.; OienOdegaard, S.; Heyn, R. H.; Tilset, M.; Laurenczy, G. Trans-mutation at gold(III): a mechanistic study of a catalytic acetylene functionalization via a double insertion pathway. ACS Catal. 2017, 7, 5023−5034. (18) Joost, M.; Amgoune, A.; Bourissou, D. Reactivity of gold complexes towards elementary oranometallic reactions. Angew. Chem., Int. Ed. 2015, 54, 15022−15045. (19) Yang, Y.; Hu, W.; Ye, X.; Wang, D.; Shi, X. Preparation of triazole gold(III) complex as an effective catalyst for the synthesis of E-α-haloenones. Adv. Synth. Catal. 2016, 358, 2583−2588. (20) Fernández, I.; Wolters, L. P.; Bickelhaupt, F. M. Controlling the oxidative addition of aryl halides to Au(I). J. Comput. Chem. 2014, 35, 2140−2145. (21) Joost, M.; Zeineddine, A.; Estévez, L.; Mallet-Ladeira, S.; Miqueu, K.; Amgoune, A.; Bourissou, D. Facile oxidative addition of aryl iodides to gold(I) by ligand design: bending turns on reactivity. J. Am. Chem. Soc. 2014, 136, 14654−14657. (22) Teles, J. H. Oxidative addition to gold(I): a new avenue in homogeneous catalysis with Au. Angew. Chem., Int. Ed. 2015, 54, 5556−5558. (23) Reiner, B. R.; Bezpalko, M. W.; Foxman, B. M.; Wade, C. R. Lewis acid catalysis with cationic dinuclear gold(II,II) and gold(III,III) phosphorus ylide complexes. Organometallics 2016, 35, 2830−2835. (24) He, Y.; Wu, H.; Toste, F. D. A dual catalytic strategy for carbon-phosphorus cross-coupling via gold and photoredox catalysis. Chem. Sci. 2015, 6, 1194−1198. (25) Roşca, D. A.; Fernandez-Cestau, J.; Hughes, D. L.; Bochmann, M. Reactivity of gold hydrides: O2 insertion into the Au-H bond. Organometallics 2015, 34, 2098−2101. (26) Gaggioli, C. A.; Belpassi, L.; Tarantelli, F.; Zuccaccia, D.; Harvey, J. N.; Belanzoni, P. Dioxygen insertion into the gold(I)-

hydride bond: spin orbit coupling effects in the spotlight for oxidative addition. Chem. Sci. 2016, 7, 7034−7039. (27) Kumar, R.; Nevado, C. Cyclometalated gold(III) complexes: synthesis, reactivity and physicochemical properties. Angew. Chem., Int. Ed. 2017, 56, 1994−2015. (28) Hashmi, A. S. K.; Blanco, M. C.; Fischer, D.; Bats, J. W. Gold catalysis: evidence for the in-situ reduction of gold(III) during the cyclization of allenyl carbinols. Eur. J. Org. Chem. 2006, 2006, 1387− 1389. (29) Cui, J.-F.; Ko, H.-M.; Shing, K.-P.; Deng, J.-R.; Lai, N. C.-H.; Wong, M.-K. C,O-chelated BINOL/gold(III) complexes: synthesis and catalysis with tunable product profiles. Angew. Chem., Int. Ed. 2017, 56, 3074−3079. (30) Rodriguez, J.; Bourissou, D. Well-defined chiral gold(III) complexes: new opportunities in asymmetric catalysis. Angew. Chem., Int. Ed. 2018, 57, 386−388. (31) Wang, Y.-M.; Lackner, A. D.; Toste, F. D. Development of catalysts and ligand for enantioselective gold catalysis. Acc. Chem. Res. 2014, 47, 889−901. (32) Rocchigiani, L.; Fernandez-Cestau, J.; Agonigi, G.; Chambrier, I.; Budzelaar, P. H. M.; Bochmann, M. Gold(III) alkyne complexes: bonding and reaction pathways. Angew. Chem., Int. Ed. 2017, 56, 13861−13865. (33) Chambrier, I.; Rocchigiani, L.; Hughes, D. L.; Budzelaar, P. M. H.; Bochmann, M. Thermally stable gold(III) alkene and alkyne complexes: synthesis, structures, and assessment of the trans-influence on gold-ligand bond enthalpies. Chem. - Eur. J. 2018, 24, 11467− 11474. (34) Gaggioli, C. A.; Ciancaleoni, G.; Biasiolo, L.; Bistoni, G.; Zuccaccia, D.; Belpassi, L.; Belanzoni, P.; Tarantelli, F. Anomalous ligand effect in gold(I)-catalyzed intramolecular hydroamination of alkynes. Chem. Commun. 2015, 51, 5990−5993. (35) D’Amore, L.; Ciancaleoni, G.; Belpassi, L.; Tarantelli, F.; Zuccaccia, D.; Belanzoni, P. Unraveling the anion/ligand interplay in the reaction mechanism of gold(I)-catalyzed alkoxylation of alkynes. Organometallics 2017, 36, 2364−2376. (36) Bistoni, G.; Belanzoni, P.; Belpassi, L.; Tarantelli, F. π activation of alkynes in homogeneous and heterogeneous gold catalysis. J. Phys. Chem. A 2016, 120, 5239−5247. (37) Faza, O. N.; López, C. S. Computational approaches to homogeneous gold catalysis. In Homogeneous Gold Catalysis; Slaughter, L. M., Ed.; Topics in Current Chemistry; Springer, 2015; Vol. 357, pp 213−284. (38) Jiménez-Núñez, E.; Echavarren, A. M. Gold-catalyzed cycloisomerizations of enynes: a mechanistic perspective. Chem. Rev. 2008, 108, 3326−3350. (39) Salvi, N.; Belpassi, L.; Tarantelli, F. On the Dewar-ChattDuncanson model for catalytic gold(I) complexes. Chem. - Eur. J. 2010, 16, 7231−7240. (40) Belpassi, L.; Infante, I.; Tarantelli, F.; Visscher, L. The chemical bond between Au(I) and the noble gases. Comparative study of NgAuF and NgAu+ (Ng = Ar, Kr, Xe) by density functional and coupled cluster methods. J. Am. Chem. Soc. 2008, 130, 1048−1060. (41) Bistoni, G.; Belpassi, L.; Tarantelli, F. Disentanglement of donation and back-donation effects on experimental observables: a case study of gold-ethyne complexes. Angew. Chem., Int. Ed. 2013, 52, 11599−11602. (42) Chen, T.; Cai, C. Catalytic hydration of alkynes to ketones by a salen-gold(III) complex. Catal. Commun. 2015, 65, 102−104. (43) Lo, V.K.-Y.; Kung, K.K.-Y.; Wong, M.-K.; Che, C.-M. Gold(III)(C^N) complex-catalyzed synthesis of propargylamines via a three-component coupling reaction of aldehydes, amines and alkynes. J. Organomet. Chem. 2009, 694, 583−591. (44) Wu, C.-Y.; Horibe, T.; Jacobsen, C. B.; Toste, F. D. Stable gold(III) catalysts by oxidative addition of a carbon-carbon bond. Nature 2015, 517, 449−454. (45) Bohan, P. T.; Toste, F. D. Well-defined chiral gold(III) complex catalyzed direct enantioconvergent kinetic resolution of 1,5enynes. J. Am. Chem. Soc. 2017, 139, 11016−11019. M


Article

Inorganic Chemistry

(68) SCM, Theoretical Chemistry, ADF User’s Guide. Release 2014.05; Vrije Universiteit: Amsterdam, The Netherlands, 2014. http://www.scm.com. (69) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Towards an order-N DFT method. Theor. Chem. Acc. 1998, 99, 391− 403. (70) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. Chemistry with ADF. J. Comput. Chem. 2001, 22, 931−967. (71) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (72) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (73) Lenthe, E. v; Baerends, E. J.; Snijders, J. G. Relativistic regular two-component Hamiltonians. J. Chem. Phys. 1993, 99, 4597−4610. (74) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. Relativistic total energy using regular approximations. J. Chem. Phys. 1994, 101, 9783− 9792. (75) van Lenthe, E.; Ehlers, A.; Baerends, E. J. Geometry optimizations in the zero order regular approximation for relativistic effects. J. Chem. Phys. 1999, 110, 8943−8953. (76) Bistoni, G.; Rampino, S.; Scafuri, N.; Ciancaleoni, G.; Zuccaccia, D.; Belpassi, L.; Tarantelli, F. How π back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes. Chem. Sci. 2016, 7, 1174− 1184. (77) Zhao, L.; von Hopffgarten, M.; Andrada, D. M.; Frenking, G. Energy decomposition analysis. WIREs Comput. Mol. Sci. 2018, 8, e1345. (78) Swart, M.; Bickelhaupt, F. M. QUILD: Quantum-regions interconnected by local descriptions. J. Comput. Chem. 2008, 29, 724−734. (79) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (80) Neese, F. The ORCA program system. WIREs Comput. Mol. Sci. 2012, 2, 73−78. (81) Grimme, S. Semiempirical hybrid density functional with perturbative second-order correlation. J. Chem. Phys. 2006, 124, No. 034108. (82) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (83) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.; Boyd, P. D. W. Relativistic effects in gold chemistry. I. Diatomic gold compounds. J. Chem. Phys. 1989, 91, 1762−1774. (84) Ciancaleoni, G.; Rampino, S.; Zuccaccia, D.; Tarantelli, F.; Belanzoni, P.; Belpassi, L. An ab initio benchmark and DFT validation study on gold(I)-catalyzed hydroamination of alkynes. J. Chem. Theory Comput. 2014, 10, 1021−1034. (85) Kang, R.; Chen, H.; Shaik, S.; Yao, J. Assessment of theoretical methods for complexes of gold(I) and gold(III) with unsaturated aliphatic hydrocarbon: which density functional should we choose? J. Chem. Theory Comput. 2011, 7, 4002−4011. (86) Kang, R.; Lai, W.; Yao, J.; Shaik, S.; Chen, H. How accurate can a local coupled cluster approach be in computing the activation energies of late-transition-metal-catalyzed reactions with Au, Pt and Ir? J. Chem. Theory Comput. 2012, 8, 3119−3127. (87) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (88) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the dumping function in dispersion corrected density functional theory. J. Comput. Chem. 2011, 32, 1456−1465.

(46) Hashmi, A. S.; Weyrauch, J. P.; Rudolph, M.; Kurpejović, E. Gold catalysis: the benefits of N and N,O ligands. Angew. Chem., Int. Ed. 2004, 43, 6545−6547. (47) Ferrer, C.; Amijs, C. H. M.; Echavarren, A. M. Intra- and intermolecular reactions of indoles with alkynes catalyzed by gold. Chem. - Eur. J. 2007, 13, 1358−1373. (48) Tomás-Mendivil, E.; Starck, J.; Ortuno, J. C.; Michelet, V. Synthesis of functionalized 1H-isochromene derivatives via a Aucatalyzed domino cycloisomerization/reduction approach. Org. Lett. 2015, 17, 6126−6129. (49) Huang, L.; Rominger, F.; Rudolph, M.; Hashmi, A. S. K. A general access to organogold(III) complexes by oxidative addition of diazonium salts. Chem. Commun. 2016, 52, 6435−6438. (50) Huang, L.; Rudolph, M.; Rominger, F.; Hashmi, A. S. K. Photosensitizer-free visible-light-mediated gold-catalyzed 1,2-difunctionalization of alkynes. Angew. Chem., Int. Ed. 2016, 55, 4808−4813. (51) Khin, C.; Hashmi, A. S. K.; Rominger, F. Gold(I) complexes of P,N ligands and their catalytic activity. Eur. J. Inorg. Chem. 2010, 2010, 1063−1069. (52) Cordón, J.; Jiménez-Osés, G.; López-de-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.; Pascual, D. Experimental and theoretical study of gold(III)-catalyzed hydration of alkynes. Organometallics 2014, 33, 3823−3830. (53) Gatto, M.; Belanzoni, P.; Belpassi, L.; Biasiolo, L.; Del Zotto, A.; Tarantelli, F.; Zuccaccia, D. Solvent-, silver-, and acid-free NHCAu-X catalyzed hydration of alkynes. The pivotal role of the counterion. ACS Catal. 2016, 6, 7363−7376. (54) Bistoni, G.; Rampino, S.; Tarantelli, F.; Belpassi, L. Chargedisplacement analysis via natural orbitals for chemical valence: charge transfer effects in coordination chemistry. J. Chem. Phys. 2015, 142, No. 084112. (55) Mitoraj, M.; Michalak, A. Natural orbitals for chemical valence as descriptors of chemical bonding in transition metal complexes. J. Mol. Model. 2007, 13, 347−355. (56) Michalak, A.; Mitoraj, M.; Ziegler, T. Bond orbitals from chemical valence theory. J. Phys. Chem. A 2008, 112, 1933−1939. (57) Michalak, A.; De Kock, R. L.; Ziegler, T. Bond multiplicity in transition-metal complexes: applications of two-electron valence indices. J. Phys. Chem. A 2008, 112, 7256−7263. (58) Nalewajski, R. F.; Mrozek, J. Modified valence indices from the two-particle density matrix. Int. J. Quantum Chem. 1994, 51, 187−200. (59) Nalewajski, R. F.; Mrozek, J.; Michalak, A. Two-electron valence indices from the Kohn-Sham orbitals. Int. J. Quantum Chem. 1997, 61, 589−601. (60) Mitoraj, M. P.; Michalak, A.; Ziegler, T. A combined charge and energy decomposition scheme for bond analysis. J. Chem. Theory Comput. 2009, 5, 962−975. (61) Biasiolo, L.; Belpassi, L.; Gaggioli, C. A.; Macchioni, A.; Tarantelli, F.; Ciancaleoni, G.; Zuccaccia, D. Cyclization of 2alkynyldimethylaniline on gold(I) cationic and neutral complexes. Organometallics 2016, 35, 595−604. (62) Gaggioli, C. A.; Bistoni, G.; Ciancaleoni, G.; Tarantelli, F.; Belpassi, L.; Belanzoni, P. Modulating the bonding properties of Nheterocyclic carbenes (NHCs): a systematic charge-displacement analysis. Chem. - Eur. J. 2017, 23, 7558−7569. (63) Gaggioli, C. A.; Belpassi, L.; Tarantelli, F.; Harvey, J. N.; Belanzoni, P. The ligand effect on the oxidative addition of dioxygen to gold(I)-hydride complexes. Dalton Trans. 2017, 46, 11679−11690. (64) Sorbelli, D.; Belpassi, L.; Tarantelli, F.; Belanzoni, P. Ligand effect on bonding in gold(III) carbonyl complexes. Inorg. Chem. 2018, 57, 6161−6175. (65) Morokuma, K. Molecular orbital studies of hydrogen bonds.III.C = O···H-O hydrogen bond in H2CO···H2O and H2CO···2H2O. J. Chem. Phys. 1971, 55, 1236−1244. (66) Ziegler, T.; Rauk, A. On the calculation of bonding energies by the Hartree Fock Slater method. Theor. Chim. Acta 1977, 46, 1−10. (67) Hopffgarten, M. v; Frenking, G. Energy decomposition analysis. WIREs Comput. Mol. Sci. 2012, 2, 43−62. N


Article

Inorganic Chemistry (89) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields. J. Phys. Chem. 1994, 98, 11623−11627. (90) Bickelhaupt, F. M.; van Eikema Hommes, N. J. R.; Fonseca Guerra, C.; Baerends, E. J. The carbon-lithium electron pair bond in (CH3Li)n (n = 1,2,4). Organometallics 1996, 15, 2923−2931. (91) Fonseca Guerra, C.; Handgraaf, J.-W.; Baerends, E. J.; Bickelhaupt, F. M. Voronoi Deformation Density (VDD) charges: assessment of the Mulliken, Bader, Hirshfeld, Weinhold, and VDD methods for charge analysis. J. Comput. Chem. 2004, 25, 189−210. (92) Klamt, A.; Schüürmann, G. COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient. J. Chem. Soc., Perkin Trans. 2 1993, 2, 799−805. (93) Klamt, A. Conductor-like screening model for real solvents: a new approach to the quantitative calculation of salvation phenomena. J. Phys. Chem. 1995, 99, 2224−2235. (94) Klamt, A.; Jonas, V. Treatment of the outlying charge in continuum salvation models. J. Chem. Phys. 1996, 105, 9972. (95) Trinchillo, M.; Belanzoni, P.; Belpassi, L.; Biasiolo, L.; Busico, V.; D’Amora, A.; D’Amore, L.; Del Zotto, A.; Tarantelli, F.; Tuzi, A.; Zuccaccia, D. Extensive experimental and computational study of counterion effect in the reaction mechanism of NHC-gold(I)catalyzed alkoxylation of alkynes. Organometallics 2016, 35, 641−654. (96) Ciancaleoni, G.; Belpassi, L.; Zuccaccia, D.; Tarantelli, F.; Belanzoni, P. Counterion effect in the reaction mechanism of NHC gold(I)-catalyzed alkoxylation of alkynes: computational insight into experiment. ACS Catal. 2015, 5, 803−814. (97) Hammond, G. S. A correlation of reaction rates. J. Am. Chem. Soc. 1955, 77, 334−338.

O


Alkyne Activation with Gold(III) Complexes: A Quantitative Assessment

Recommend Documents