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

Ligand Effect on Bonding in Gold(III) Carbonyl Complexes Diego Sorbelli,† Leonardo Belpassi,*,‡,§ Francesco Tarantelli,†,‡,§ and Paola Belanzoni*,†,‡,§ †

Dipartimento di Chimica, Biologia e Biotecnologie and ‡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 S Supporting Information *

ABSTRACT: We quantitatively assess the Dewar−Chatt−Duncanson (DCD) components of the Au(III)−CO bond and the charge density polarization at the CO, in a series of neutral, cationic, and dicationic bis- and monocyclometalated gold(III) complexes via charge-displacement (CD) analysis. A striking feature concerns the very small net electron charge flux from CO to the metal fragment which is unexpectedly stable toward both the charge of the complex and the oxidation state of gold (I, III). All systems exhibit a similar trend for the σ charge rearrangement in the region of the carbonyl bond, where, by contrast, the π back-donation trend variation is large, which is strictly correlated to the change in CO bond distance and the shift in CO stretching frequencies, in close analogy with the gold(I) carbonyl complexes. In the whole series of gold(III) compounds, a large Au(III) ← CO σ donation is measured (from 0.19 to 0.31 electrons), as well as a significant Au(III) → CO π back-donation (from −0.09 to −0.22 electrons), which however is not generally able to completely balance the polarization of the CO π electrons in the direction from oxygen to carbon (C ← O) induced by the presence of the metal fragment [LAu(III)]0/+1/+2. Surprisingly, all the gold(III) complexes in the series are characterized by a very small anisotropy in the Au(III) → CO in-plane and out-of-plane π back-donation components, in sharp contrast with the marked anisotropy found before for the experimentally characterized [(C^N^C)Au(III)CO]+ complex. A first attempt to figure out a rationale on the bonding/reactivity relationship for Au(III)-CO is made by performing a comparative study with an isostructural [(N^N^C)Pt(II)CO]+ complex in a model water−gas shift (WGS) reaction.

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as low toxicity and an environmental benign nature.26 In the presence of suitable oxidants, catalytic cycles involving a change in the oxidation state of gold (Au(I)/Au(III)) could be also devised, particularly in the context of oxidative cross-coupling reactions.27 Their mechanistic understanding has been a great challenge owing to the reactivity of high-valent gold intermediates and to the underdeveloped synthetic methodologies for these rather labile and rapidly evolving species.28 One method for synthesizing Au(III) complexes is the ligand exchange from inorganic salts,29 and another approach relies on the oxidative addition to Au(I) complexes,30−34 such as, for instance, insertion of O2 into the Au(I)−H bond.35,36 In recent years many reports of gold(III) complexes bearing pincer ligands have appeared. The use of such a kind of ligand enables the stabilization of cyclometalated Au(III) complexes, avoiding reductive elimination to Au(I) or Au(0). A very recent review37 provides insight into the current state-of-the-art in this field, listing a selection of synthesized and characterized bis- and monocyclometalated gold(III) complexes. On the theoretical side, while our knowledge and understanding of the nature of the bond between gold(I) and substrates38 and of the intermediates and transition states39 involved in the mechanisms of gold(I)-catalyzed processes has significantly pro-

INTRODUCTION Whereas applications of Au(I) species in homogeneous catalysis date back to the past two decades and gold(I) complexes have nowadays become widely used,1−5 applications of Au(III) species are much more recent. Currently gold(III) complexes are attracting great attention for their role in homogeneous catalysis, since synthetic strategies based on the use of auxiliary ligands, other than just simple halogens, have allowed improvement of the stability of Au(III) in organometallic complexes.6−13 Until now most of the catalytic processes reported for gold(III) relied on the use of the commercially available tetrachloroaurates.6 In the past few years, however, new classes of gold(III) complexes,14 new reactivity paths,15 and new type of catalytic transformations16 have been discovered and developed. Gold complexes have been found to be powerful carbophilic Lewis acids that are able to activate the π system of alkynes, alkenes, and allenes with outstanding efficiency,17 thus enabling a broad range of transformations such as nucleophilic addition to CC multiple bonds,18 activation of alcohols,19 carbonyl20 or imine21 groups, hydrogenation,22 C−H bond functionalization,16 selective oxidations,23 and reductions.24,25 The key feature of gold catalysis relies on the fact that it often promotes reactions under milder conditions than other transition metal catalysts, lower temperatures, shorter reaction times, and more tolerance to air and moisture and in addition on attractive properties such © XXXX American Chemical Society

Received: March 22, 2018

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DOI: 10.1021/acs.inorgchem.8b00765 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry gressed,40−42 very little is known about bonding in gold(III) complexes and the individual steps along their proposed catalytic cycles. Understanding the fundamental features of the nature of the Au(III)−substrate bond (where substrate represents an alkyne, alkene, allene, or CO) in terms of the Dewar−Chatt−Duncanson (DCD) bonding components (substrate to metal donation and metal to substrate back-donation) is fundamental both as basic knowledge of the coordinative bond in these new types of gold(III) complexes and owing to the bonding/reactivity relationship for the development of the homogeneous Au(III) catalysis. To this aim, a preliminary investigation has been carried out by some of us43 focusing on the Au(III)−CO bonding properties in [(C^N^C)Au-CO]+X− (C^N^C = 2,6-bis(4ButC6H3)2) pyridine dianion, X− = B(C6F5)3(CF3COO)−), which represented the first example of an isolated and wellcharacterized gold(III) carbonyl complex.8 The system showed a promising catalytic activity in the water−gas shift (WGS) reaction (WGS: CO + H2O ↔ CO2 + H2). Very striking, the CO carbon atom highly susceptibility to water nucleophilic attack was found to be consistent with an Au(III)−CO bonding picture where, in addition to a large Au(III) ← CO σ donation, an unexpectedly even larger Au(III) → CO π back-donation, including a marked asymmetry in the Au(III) → CO in-plane and out-of-plane π back-donation components, was calculated.43 The Au(III)−CO bonding analysis in the [(C^N^C)AuCO]+ complex was carried out using the charge-displacement (CD) approach44,45 which allows one to determine unambiguously and quantitatively the DCD components of a coordination bond and to relate them with spectroscopic observables.46−48 The interest in the CO for studying the gold(III) bonding properties relies on the fact that carbonyl can be used as a probe which allows one to relate theoretical features of the M−CO bond (such as the DCD σ donation and π back-donation components) to simple experimental observables (such as C−O stretching frequencies and C−O bond length). Theoretically, the effectiveness of the DCD model for the description of the M−CO bond has been consolidated over the years by a large number of computational studies, including CD analysis,49−57 and experimentally the M−CO bond can be monitored on the basis of the analysis of the variation in the CO stretching frequency νCO (via IR spectroscopy) and bond distance rCO (via X-ray crystallography) with respect to free CO (νfree‑CO = 2143 cm−1, rfree‑CO = 1.12822 Å).58 Concerning the [(C^N^C)Au-CO]+ complex, for instance, it shows a measured CO stretching frequency of 2167 cm−1, which is slightly blueshifted (ΔνCO = 23 cm−1) with respect to that of free CO. A blue shift in the CO stretching is typically associated with a negligible or null back-donation from the metal fragment. However, it has been recently demonstrated by some of us47 that the key quantity controlling the nature of the CO bond (namely, its stretching frequency νCO or its C−O distance rCO) is the π symmetry charge displacement in the CO region, no matter how it is induced. In a simple valence bond (VB) picture, the CO stretching response results from the interplay between the DCD bonding structure and the electrostatic polarization effect exerted by the electric field generated by the metal fragment. They may act in different directions with different weights, as illustrated in Scheme 1. Within this picture, the experimental small blue-shift of the [(C^N^C)Au-CO]+ complex has been clearly rationalized on the basis of a calculated large Au(III) → CO π back-donation

Scheme 1. VB Structures for a M−CO Moiety of a General [LnM(CO)]m Complexa

a From (a) to (c): the M → CO π back-donation increases while the C−O bond order decreases; from (f) to (d): the electric field (depicted as a plus inside a circle symbol) strength due to the metallic fragment [LnM]m increases and so the C−O bond order. The red arrow indicates the M → CO π back-donation increasing direction associated with the CO bond weakening (red-shift); the blue arrow indicates the electric field increasing direction associated with the CO bond strengthening (blue-shift).

combined with a large electrostatic polarization ability (which is increased if compared to an Au(I) complex bearing the same positive charge).43 The interplay between these two factors (large back-donation would tend to weaken the CO bond, whereas large CO polarization would tend to strengthen the CO bond) results in a small blue-shift of the νCO and a small decrease of the r CO . Results demonstrated that the [(C^N^C)Au-CO]+ complex shows a π back-donation amount which is typical of a neutral Au(I) complex and higher than that of all the cationic Au(I) compounds and a larger tendency (compared to cationic Au(I) complexes) to polarize the CO π electrons in the direction from oxygen to carbon, thus increasing the weight of VB structure (d) in Scheme 1. One may wonder about the possible existence of Au(III) carbonyl complexes whose bonding properties and reactivity are likely to differ from and extend those of the [(C^N^C)Au(III)-CO]+ complex or Au(I) species. This is the aim of this work. Inspired by the bis- and monocyclometalated gold(III) complexes reported in the recent review by Kumar and Nevado,37 we selected eight prototypical species where, for our investigation purpose, one suitable ligand in the experimental complexes, typically Cl − , CH 3 COO − or IPr (IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene), is replaced by the CO probe, and we have also included a “precursor” noncyclometalated complex [AuCl3−CO] (1) for comparison. The studied complexes are depicted in Scheme 2. Biscyclometalated gold(III) species are represented by complexes [(N^C^N)Au-CO]2+ (N^C^N = 1,3-bis(2-pyridyl)benzene monoanion) (2)2+ and [(C^N^N)Au-CO]2+ (C^N^N = 6-phenyl-2,2′-bipyridine monoanion) (3)2+ which differ from the previously studied [(C^N^C)Au-CO]+ complex43 for the tridentate ligand bond motif and charge, where one experimental chlorine and acetate ligand has been replaced by CO, respectively. Representative examples of monocyclometalated gold(III) species are complexes [(C^N)(Cl)Au-CO]+ (C^N = 2-phenylpyridine dianion) (4)+ and (5)+, where the two chemically nonequivalent chlorine ligands in the B


Article

Inorganic Chemistry Scheme 2. Selected Gold(III) Carbonyl Complexes Analyzed in This Worka

a

[(C^N^C)Au(III)CO]+ and [(N^N^C)Pt(II)CO]+ complexes are also shown for comparison.

AuIPrCl] as a catalyst in various transformations, including conjugate additions to α,β-unsaturated aldehydes, as well as [2 + 2] cycloaddition reactions with allene-substituted unsaturated aldehydes, where the improved performance of this Au(III) catalyst compared to the Au(I) ones has been attributed to the hard Lewis acidity of the Au(III) species.7 The [(bpy)AuCl2]+ type complex (8)2+ has been found to catalyze oxidative αcyanation of tertiary amines to afford corresponding αaminonitriles.64 NÔ-based cyclometalated gold(III) complexes (9)+ and (10)+ have been used as catalysts for a wide variety of transformations, and, interestingly, their ability to activate all kinds of π systems was found to be similar to that of gold(I) species.65−67 Finally, N^P cyclometalated gold(III) complex (11)2+ has been shown to be relevant in the context of light-driven, gold-catalyzed 1,2-difunctionalization reactions of alkynes.68−70 In this work, we (i) use the charge-displacement (CD) analysis to quantitatively evaluate the DCD components of the Au(III)−CO bond and the π symmetry charge density polarization at the CO, which is strictly connected to spectroscopic properties, in the whole series of complexes in Scheme 2; (ii) compare the results with the bonding picture in the Au(I)−CO set of complexes reported in ref 47 and in the [(C^N^C)Au(III)-CO]+ complex;43 (iii) make a first attempt to figure out a rationale on the bonding/reactivity relationship for Au(III)−CO by performing a comparative study with an isostructural [(N^N^C)Pt(II)-CO]+ complex71,72 for a model WGS reaction.

experimental complex are substituted in turn by CO; [(C^C)(Cl)Au-CO] and [(C^C)(IPr)Au-CO]+ (C^C = biphenyl dianion, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (6) and (7)+, where the experimental IPr and Cl− ligands are replaced by CO, respectively, and IPr has been modeled in (7)+ by substitution of diiisopropylphenyl moieties with methyl groups; [(N^N)(Cl)Au-CO]2+ (N^N = 2,2′-bipyridine) (8)2+, where one experimental Cl− ligand is substituted by CO; [(NÔ)(Cl)Au-CO] + (NÔ = 2carboxylate-pyridine monoanion) (9)+ and (10)+, where the two chemically nonequivalent chlorine ligands in the experimental complex are substituted by CO; and [(N^P)AuCO]2+ (N^P = 8-(diphenyl-phosphyl)quinoline) (11)2+, where the experimental chlorine is replaced by CO. The interest for monoanionic N^C^N ligands stems from the fact that they have been broadly used to stabilize d8 transition metals such as Ni(II), Pd(II), and Pt(II), whereas the experimental [(C^N^N)AuOAc] complex has been found to be a viable precursor of [(C^N^N)Au(alkenyl)] complexes with terminal alkynes.59 However, no catalytic activity has been observed for the two biscyclometalated complexes (2)2+ and (3)2+ so far. On the contrary, [(C^N)AuCl2] complex has found applications in catalysis as a carbophilic Lewis acid for the efficient activation of π bonds in alkynes in a reaction involving aldehydes, secondary amines, and terminal alkynes to produce propargylamines,60 and more recently for the synthesis of chiral allenes, oxazoles, and isoxazoles61,62 and for the oxidative CH arylation of 2arylpyridines with aryl boronic acids.63 Similarly to complexes (4)+ and (5)+, also complexes (6) and (7)+ have a catalytic interest. Toste and co-workers examined complex [(C^C)C


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Inorganic Chemistry

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METHODOLOGY AND COMPUTATIONAL DETAILS Geometry optimization, frequency, Natural Orbitals for Chemical Valence (NOCV)73−75 electron densities used in the Charge Displacement analysis (see the next section for details) and Energy Decomposition Analysis (EDA)76−78 were performed by using DFT as implemented in Amsterdam Density Functional (ADF) program package (2014.05 version).79−81 The BLYP functional,82,83 a Slater-type TZ2P quality basis set (core small) for all atoms, and a scalar ZORA Hamiltonian to include relativistic effects were used.84−86 The same computational setup has been employed for both the previous studies on Au(I) complexes47 and on the [(C^N^C)Au(III)-CO]+ complex,43 as mentioned in the Introduction. The choice of the same computational details has been dictated by the possibility of a direct comparison in quantitative terms between the present and both the previous results and the DCD components in different complexes. An assessment of such a level of theory has been given in the Supporting Information of our previous work.47 For reaction profile study, geometry optimization and frequency calculations have been carried out with the ADF related Quantum-regions Interconnected by Local Description (QUILD) program,87 using TZP for gold and DZP basis set for all the other atoms (core small) and the GGA BP86 functional,82,88 with Grimme’s D3 dispersion correction.89 To study the [LnM]m−CO bond in the selected complexes, we carried out the energy decomposition analysis (EDA).76−78 The EDA approach allows us to decompose the interaction energy ΔEint between two fragments A and B in a molecule A−B into three terms, namely, (1) the quasiclassical electrostatic interaction ΔEelst between the fragments, (2) the repulsive exchange or Pauli repulsion interaction ΔEPauli between occupied orbitals on the two fragments, (3) the orbital (covalent) 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 decomposed into pairwise orbital contributions of the interacting fragments (EDA-NOCV),90 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.

function quantifies the exact amount of electron charge that is transferred from one fragment to the other, upon the formation of the coordination bond, across a plane perpendicular to the bond axis through the z point (defined as charge transfer, CT). Therefore, negative values of CT identify charge flowing from left to right and vice versa for positive values of CT (i.e., from right to left). In the present work the CT is numerically calculated at two different crucial points: at the so-called “isodensity boundary”,44 i.e., the point along z where the electron densities of the two noninteracting fragments (the gold fragment [LAu]0/+/+2 and CO) become equal (this is the standard choice for evaluating the DCD bond components) and at the midpoint of the CO bond (this gives a measure of the polarization of the CO bond induced by the interaction with the metal fragment). The slope of the CD function all over the adduct space gives indications about regions of charge accumulation (positive slope) or charge depletion (negative slope). A remarkable feature of the CD function is its partitioning. If the adduct and its constituting fragments belong to the same symmetry group, the electron density difference can be partitioned into additive components according to eq 2:44

Δρ =

Γ

CHARGE-DISPLACEMENT ANALYSIS The charge-displacement (CD) analysis is nowadays a successful tool providing a clear and unequivocal definition of the DCD components, σ donation and π back-donation, of the coordination bond.44−46 The CD analysis relies on the chargedisplacement (CD) function which describes the electron density changes taking place upon formation of the coordination bond between two fragments, A and B, to yield the AB adduct. 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 adduct and the sum of the densities of the noninteracting separated fragments at the positions they have in the adduct geometry.45 ∞

∞

Δρ′ =

∑ Δρ′k k

(3)

It is worth stressing here, however, that 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.94 The NOCV/CD method has been applied with success for the characterization of transition metal compounds95 and for disentangling donation and backdonation in the CD function of nonsymmetric systems containing NHC−Au bond96 or Au−H bond with different ancillary ligands.97 We should make clear that the two reference densities in the symmetric CD and nonsymmetric NOCV/CD are slightly different: Δρ and Δρ′ differ by the antisymmetriza-

z

∫−∞ dx∫−∞ dy∫−∞ Δρ(x , y , z′) dz′

(2)

where Γ index labels the different irreducible representations of the common symmetry point group. The CD function can be consequently partitioned in the same way, obtaining different contributions that sum up to obtain the overall Δq. For suitable 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 [(C^N^C)Au(III)-CO]+ complex,43 since the complex and its constituting fragments ([(C^N^C)Au]+ and CO) have a C2V symmetry, which is suitable to disentangle σ and π symmetry contributions (A1 corresponds to σ donation, while B1 and B2 correspond to out-of-plane and inplane π back-donation, respectively). In cases where fragments A and B and the adduct AB have no symmetry, the CD function can be decomposed by introducing the NOCV scheme.73 In the NOCV/CD framework,74,75 the charge rearrangement taking place upon the formation of the adduct AB from fragments A and B is obtained from the occupied orbitals of A and B suitably orthogonalized to each other and renormalized. The resulting charge density rearrangement Δρ’ can be written in terms of NOCV pairs, i.e. the eigenfunctions φ±k of the so-called “valence operator” of Nalewajski and Mrozek valence theory,91−93 as follows

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Δq(z) =

∑ ΔρΓ

(1)

The z axis is usually the A−B bond axis (in our case, the axis passing through the ligand atom bonded to gold, i.e., carbon atom of CO, and gold itself). At each point of the z axis the CD D


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Inorganic Chemistry

Table 1. Computed Interaction Energy ΔEint (kcal/mol) between [LM]0/+/+2 and CO Fragments, ΔνCO (= νCO − νfree CO, cm−1), ΔrCO (= rCO − rfree CO, Å), and Charge-Transfer Results (e) Obtained from the NOCV/CD Analysis for the Series of Complexes Shown in Scheme 2a complex

ΔEint

ΔνCO

ΔrCO

CTnet

CTσ don

CTπ‑back⊥

CTπ‑back∥

CTπ‑back

(1) (2)2+ (3)2+ (4)+ (5)+ (6) (7)+ (8)2+ (9)+ (10)+ (11)2+ [(C^N^C)AuCO]+ [(N^N^C)Pt(II)CO]+

−28.7 −23.9 −46.4 −40.9 −16.8 −16.8 −17.8 −44.7 −42.8 −34.9 −14.7 −48.4 −60.4

14 55 61 47 38 −13 −7 85 79 46 42 10 −52

−0.002 −0.009 −0.007 −0.005 −0.007 0.000 −0.002 −0.010 −0.009 −0.006 −0.007 0.001 0.011

0.07 0.10 0.11 0.09 0.08 0.02 0.04 0.14 0.16 0.10 0.12 −0.003 −0.12

0.29 0.22 0.30 0.27 0.20 0.19 0.24 0.28 0.31 0.27 0.21 0.30 0.30

−0.12 −0.06 −0.09 −0.10 −0.06 −0.09 −0.09 −0.07 −0.08 −0.10 −0.05 −0.11 −0.20

−0.10 −0.05 −0.09 −0.08 −0.05 −0.06 −0.09 −0.06 −0.07 −0.08 −0.04 −0.17 −0.20

−0.22 −0.11 −0.18 −0.18 −0.11 −0.15 −0.18 −0.13 −0.15 −0.18 −0.09 −0.28 −0.40

CTσ

rCO/2

0.04 0.04 0.05 0.04 0.03 0.02 0.03 0.04 0.05 0.04 0.03 0.05 0.06

CTπ

rCO/2

0.04 0.10 0.10 0.08 0.08 0.03 0.04 0.13 0.10 0.09 0.09 0.03 −0.02

a

Data for the [(C^N^C)Au(III)-CO]+ complex have been recalculated at the NOCV/CD level (see data in ref 43 for a comparison with CD within symmetry decomposition scheme) and data for the [(N^N^C)Pt(II)CO]+ complex are also included (see later).

tion term which can be envisaged as the density rearrangement occurring on going from the noninteracting, separate A and B fragments to the so-called “promolecule”. This operation 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.73 In the present work, since all of the complexes listed in Scheme 2 have no symmetry, we apply the NOCV/CD approach to quantify both the DCD [LAu]0/+/+2−CO bond components and the π symmetry polarization on the CO induced by the metal fragment [LAu]0/+/+2. Well-defined measurements of the total charge transfer (denoted as CTnet) and of its σ donation and out-ofplane and in-plane π back-donation contributions (denoted as CTσ‑don, CTπ‑back⊥ and CTπ‑back∥, respectively) are obtained by evaluating the corresponding NOCV/CD function at an interfragment boundary. The CD values related to the π symmetry NOCV component and obtained at the CO bond midpoint (denoted as CTπ rCO/2 ) provides quantitative information on the CO π polarization. Finally, a comparison between the NOCV/CD (as used here for Au(III) complexes) and the CD within symmetry decomposition scheme (as employed previously by us for Au(I) complexes47 and for [(C^N^C)Au(III)-CO]+43) has been performed on the symmetric [CF3Au(I)CO] complex as a test case. Results are reported in the Supporting Information (Table S1). This benchmark calculation demonstrates that the NOCV/CD and CD within symmetry decomposition scheme approaches give very close results. This can be confirmed also by comparing NOCV/CD (Table 1) and CD within symmetry decomposition scheme results for the [(C^N^C)Au(III)-CO]+ complex.43

Figure 1. Top: isodensity surfaces (±0.001 e a.u.−3) of the total Δρ and its first three NOCV components, Δρ1, Δρ2, and Δρ3, for the Au− CO bond in complex (1). Red surfaces represent charge depletion regions and blue surfaces identify charge accumulation regions. Bottom: corresponding CD curves. Red dots indicate the positions of the atomic nuclei along the z axis. The solid vertical line marks the isodensity boundary between the [Cl3Au] and the CO fragments (see Methodological and Computational Details section). The dashed vertical line denotes the midpoint of the C−O bond (z = rCO/2).

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RESULTS AND DISCUSSION Au(III)−CO Bond: NOCV/CD Analysis. As a general illustration of how the Au(III)−CO bond in the considered complexes in Scheme 2 can be analyzed within the general NOCV/CD scheme, we focus first on complex (1). The isodensity surfaces maps for the total Δρ and its first three NOCV densities (top) and the corresponding CD curves (bottom) are shown in Figure 1. All the other NOCV E


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Inorganic Chemistry

signals an overall polarization in the C ← O direction. More in detail, we observe that, while the σ curve retains its positive sign at the right-hand side of C and even beyond the O side, a sign inversion from negative to positive occurs for the π component, exactly at the C site, leading to a maximum at about the midpoint of the C−O bond. Remarkably, in both the σ and π contributions, a displacement of electrons from oxygen toward carbon is calculated, which is due to the presence of the metal fragment [Cl3Au(III)]. We can quantify the amount of CO polarization by taking the CD function value at the midpoint of the CO bond (dashed vertical line in Figure 1). For this complex, CTσ rCO/2 is 0.04 electrons and CTπ rCO/2 is 0.04 electrons, accounting for a total of 0.08 electrons. It is worth noting here that, although complex (1) has been calculated to behave nonclassically (slight blue-shifted ΔνCO = 14 cm−1, see Table 1), back-donation is actually a significant component of the interaction, estimated to be more than half as large as the donation. However, this significant π back-donation is not sufficiently counteracted by the large ability of the [Cl3Au(III)] fragment to polarize the CO π electrons in the direction from oxygen to carbon. A similar pattern was observed in the Au(I) carbonyls47 and in the [(C^N^C)Au(III)-CO]+ complex,43 where the CD curve of the π symmetry is also small in the CO region, with a CTπ rCO/2 value of 0.03 electrons. The result that only a slight asymmetry in the π backdonation components has been found in (1), in sharp contrast with the strong asymmetry (in-plane component much larger than the out-of-plane one) found for the [(C^N^C)Au(III)CO]+ complex,43 deserves to be analyzed in detail. Indeed, this finding suggests a peculiar role of the pincer (C^N^C) ligand in the in-plane π back-donation bond component. It is interesting at this stage to compare the isodensity surfaces of the in-plane CTπ‑back∥ component for the two complexes (Δρ3 in Figure 1 for (1) and Δρ2 in Figure S1 in the Supporting Information for [(C^N^C)Au(III)-CO]+). We should note here that Figure S1 shows the NOCV/CD analysis for the [(C^N^C)Au-CO]+ complex, and it is very similar to Figure 1 of ref 43 showing the CD within symmetry decomposition scheme analysis. We can see that in Δρ2 charge transfer from the two carbon atoms is clearly mediated by Au (namely, the two Au−C bonds contribute to the in-plane π back-donation, see Figure S1), whereas in Δρ3 a direct charge transfer (without any Au mediation) from the two cis Cl− to CO occurs (see Figure 1). Such a direct participation of the two Cl− ligands to the charge transfer in the Δρ3 component should be responsible for the reduced π back-donation from Au to CO found in (1). Evidence for a direct charge flux from Cl− to CO has been also given in high valent Nb (d0) complexes by Marchetti et al.98 This different involvement in the charge transfer of the two atom-types in cis position to CO is also reflected by the trend in the corresponding CD curves. The total π back-donation component in complex [(C^N^C)Au(III)-CO]+ (CTπ‑back = −0.28 electrons) is larger than that in (1) (CTπ‑back = −0.22 electrons), resulting in a slightly negative total charge transfer (CTnet = −0.03e). Analogously, in the [(C^N^C)Au(III)-CO]+ complex CTnet also arises from a large donation component (CTσ don = 0.30 e) and a large total back-donation component (CTπ‑back = −0.28 electrons), the latter even exceeding the donation component. The NOCV/CD analysis has been applied to all of the complexes depicted in Scheme 2, and Table 1 collects the corresponding spectroscopic data for ΔνCO and ΔrCO as well as all the computed CT values. Isodensity surfaces of the total Δρ

contributions are very small or negligible. The distinct DCD bonding components are easily recognized by inspection of the first three NOCV Δρ components depicted at the top of Figure 1. Indeed, the first Δρ component (Δρ1) shows a charge depletion on the carbon of CO and a charge accumulation on gold and chlorine atom trans to CO, and it can be physically ascribed to the σ donation from the CO lone pair to the AuCl3 metallic fragment. The second Δρ component (Δρ2) depicts charge depletion on chlorine in trans to CO and on a filled d orbital of the Au atom and charge accumulation on a empty π orbital of CO: it clearly represents the π back-donation from Au to CO perpendicular to the AuCl3 plane (π⊥). The third Δρ component (Δρ3) shows an electron charge depletion on chlorine atoms and on a filled d orbital of the Au and an increase of electron density on an empty π orbital of CO: it describes π back-donation from Au to CO occurring on the AuCl3 plane (π∥). In Figure 1 (bottom), the corresponding CD curves are shown. The red line, related to Δρ1, represents the σ donation, which is large and positive all over the whole complex including the Au-CO region, with a charge transfer CTσ don of 0.29 electrons (CD value obtained at the isodensity, see Methodology and Computational Details section). The orange line refers to π back-donation in-plane (π∥) and the green line to π back-donation perpendicular to the molecular plane (π⊥): they are both negative over the AuCl3 region until the carbon atom position of CO, where they cross the zero and get positive, with a maximum at about the midpoint of the C−O bond. The blue line, corresponding to the sum of the two π back-donation components, shows a similar trend. The π⊥ back-donation has a CT (CTπ‑back⊥) of −0.12 electrons, with the minus sign indicating the flux of electron charge from the left to the right (see Methodology and Computational Details section). The π∥ back-donation component has a slightly smaller CT (CTπ‑back∥) value amounting to −0.10 electrons. A slight asymmetry in the π back-donation components can be noticed, which is opposite to the marked asymmetry found for the [(C^N^C)Au(III)-CO]+ complex,43 where the in-plane component is larger than the out-of-plane one (−0.17 e and −0.11 e, respectively). The total π back-donation component in complex (1) is relatively large (CTπ‑back is −0.22 electrons), although smaller than that in [ClAu(I)-CO] complex (CTπ‑back is −0.33 electrons).47 We should note here that in gold(I) carbonyl complex the σ donation component (CTσ don = 0.23e) is not sufficiently large to overcome the total π back-donation, thus giving a negative total charge transfer (CTnet = −0.11e). Instead, a slightly positive total charge transfer (CTnet = 0.07e) is calculated for (1), where the σ donation component is larger than the total π back-donation. As a result, the net CD curve (black line in Figure 1) features a complex pattern. It is positive in the C−O bond region, while it crosses the zero at about the midpoint of the Au−C bond (very close to the isodensity boundary) and close to the Au atom position. The net charge transfer (CTnet = 0.07e) thus corresponds to a very small net electron charge flux from CO to AuCl3. It should be pointed out that both CTnet and net CD curve do contain contributions from all the NOCV Δρ components, including the very small ones. Note that CTnet results from a large donation component (CTσ don is 0.29 e) and a large total back-donation component (CTπ‑back is −0.22 electrons), the latter almost completely compensating the donation. Focusing on the NOCV/CD curves in the carbonyl region in Figure 1, we note that the positive value of the net function F


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Inorganic Chemistry and its NOCV components for the Au−CO bond as well as CD curves for each complex of the [LAu(III)CO]0/+/+2 series can be found in the Supporting Information (Figures S2−S11). A first striking feature emerging from Table 1 concerns the CTnet values that are unexpectedly stable toward the charge of the complex. For instance, complex (11)2+ shows a smaller CTnet value than that of (9)+ (0.12e vs 0.16e, respectively). This result suggests a crucial role of the ligands which overcomes the role of the charge. Moreover, CTnet values in Table 1 are close to those calculated for gold(I) carbonyl complexes,47 thus suggesting that the role of the ligands also overcomes the role of the oxidation state of gold. The variability range in CTnet values for complexes depicted in Scheme 2 (from 0.02 e in (6) to 0.16e in (9)+) must be ascribed to the variability in the [LAu(III)]0/+/2+−CO bonding components. According to our calculated shifts, neutral and cationic complexes (6) and (7)+, both representing the monometalated C^C−type complex, behave classically (red shift ΔνCO), while the remaining complexes behave nonclassically (blue shift ΔνCO). The σ donation and π back-donation curves for all of the complexes are compared in the top and bottom panel of Figure 2, respectively. Inspection of Figure 2 highlights two interesting features. The first is that all systems exhibit a similar trend for the σ charge rearrangement (top panel) in the region of the carbonyl C−O bond, whereas a large trend variation is observed in the Au−C and ancillary (pincer) ligand regions. This is not surprising since it has been observed also for the Au(I) complexes reported in ref 47. Indeed, looking at Table 1, the CO to Au σ donation (CTσ don) varies by 0.12 electrons (from 0.31 for monocationic (9)+ to 0.19 e for neutral (6) complexes) along the whole series of systems, due to the different ancillary ligands. Notably the dicationic (11)2+ and (2)2+ complexes show a surprisingly small σ donation compared to that of dicationic (3)2+ and (8)2+ complexes (0.21e and 0.22e vs 0.30e and 0.28e, respectively). This finding suggests that N atoms in the trans position to CO in (3)2+ and (8)2+ complexes favors σ donation probably through larger charge delocalization on the aromatic rings with respect to C and P atoms in (2)2+ and (11)2+ complexes, respectively. The π charge rearrangement trend (bottom panel in Figure 2) shows a variation over the whole molecular region and, specifically, in the Au−C and in the C−O carbonyl bond regions. From this finding we expect that, for the Au(III) complexes we are dealing with, the C−O bond (and consequently the ΔνCO and ΔrCO) cannot be modulated much by the σ donation component but, rather, the modulation of the Au−CO bond is possible by tuning the π back-donation contribution. A large variation of CTπ‑back by 0.13 electrons (from −0.22 for neutral (1) to −0.09 e for dicationic complex (11)2+) is calculated over the systems series. This π back-donation quantitative control of the Au−CO bond properties has also been found for the series of 23 [LAu(I)CO]0/+ complexes in ref 47. Interestingly, in contrast to the [(C^N^C)Au(III)-CO]+ complex,43 the out-of-plane CTπ‑back⊥ component has slightly larger, or at least equal, values than those of the corresponding in-plane CTπ‑back∥ component over the complex series. However, CTπ‑back⊥ values along the series are in the range between −0.12 and −0.05 electrons, very close to (or even smaller than) −0.11 electrons, which is the value calculated for [(C^N^C)Au(III)-CO]+. The CTπ‑back∥ component has slightly smaller values in the range from −0.10 to −0.04 electrons over

Figure 2. σ donation (top panel) and π back-donation (bottom panel) CD curves for the Au−CO bond in the series of [LAu(III)CO]0/+/+2 complexes of Scheme 2. The z origin is placed at the Au atom for all complexes and red dots indicate the positions of the Au, and C and O atoms in the carbonyl ligand (averaged for all complexes).

the whole complex series, much below the −0.17e value calculated for [(C^N^C)Au(III)-CO]+. Surprisingly, the (C^N^C) ligand does show peculiar features compared to all the ligands in the complexes of Scheme 2. Note that it has an out-of-plane CTπ‑back⊥ component comparable to that of neutral complex (1) (−0.11e vs −0.12e, respectively), and an in-plane CTπ‑back∥ component, which is well outside the range of values calculated for all the Au(III)-CO complexes (even much larger than the maximum value obtained for (1), −0.17e vs −0.10e, respectively). As noted above, the two carbon atoms in cis position to CO in [(C^N^C)Au(III)-CO]+ are responsible for this efficient π in-plane charge transfer from Au(III) to CO. The second interesting feature is that, in the CO region, all the complexes, including those showing small red-shifted νCO, invariably exhibit a flow of π electrons in the C ← O direction (CTπ rCO/2 > 0), due to the metallic fragment, with values ranging from 0.03 electrons for neutral complex (6) to 0.13 electrons for dicationic complex (8)2+. Compared to the series of monocationic gold(I) carbonyl complexes of ref 47, where CTπ rCO/2 shows values in the range between 0.02 and 0.09 electrons, one can surmise that the metal fragment [LAuG


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Inorganic Chemistry (III)]0/+1/+2 does have generally a larger ability to polarize the CO π electrons in the direction from oxygen to carbon, thus increasing the weight of VB structure (d) (see Scheme 1). The Au(III)−CO bond π back-donation component, although important in all the complexes in Scheme 2, is not generally able to completely balance the polarization induced by the presence of the metal fragment [LAu(III)] 0/+1/+2, as evident from the CD curves of π symmetry which are rather large and steep in the CO region with a maximum at the midpoint of the C−O bond (measured by the CTπ rCO/2 values), in contrast to [(C^N^C)Au(III)-CO]+ complex, where the back-donation component is so important that almost balances the polarization induced by the presence of the positively charged metal fragment [(C^N^C)Au(III)]+. Overall, bonding features in [(C^N^C)Au(III)-CO]+ complex emerge as peculiar among all the considered gold(III) carbonyl complexes. Au(III)-CO vs Au(I)-CO Complexes. In this section we compare the Au(III)−CO bond in all of the complexes in Scheme 2 with the corresponding Au(I)−CO in the whole set of [LAu(I)CO]0/+ complexes studied in ref 47 to find similarities and differences. First we note that [LAu(III)]0/+/+2 fragments show a σ withdrawing ability (CTσ don values in the range between 0.19 and 0.31 e, see Table 1) similar to that of the [LAu(I)]+ systems (CTσ don values between 0.20 and 0.34e) and generally slightly larger than that of the [LAu(I)] neutral fragments (CTσ don values between 0.19 and 0.24e).47 Next, for a detailed back-donating ability comparison, we use the ΔrCO vs CTπ rCO/2 (CTπ‑back) plots of ref 47 as calibration line, since a very good linear correlation has been found between the two above quantities within the [LAu(I)CO]0/+ series. In Figure 3 the two plots of ref 47 with the calculated corresponding CTπ π‑back (Figure 3 upper rCO/2 (Figure 3 lower panel) and CT panel) values for Au(III) complex series just plotted in are shown. We observe that most of the Au(III) complexes fit nicely in this correlation despite: (i) + 2 charged Au(III) complexes have been included (note that no +2 charged Au(I) complexes were used to construct the plots); (ii) calculations on Au(I) complexes were performed by using the CD method within the symmetry decomposition scheme (whereas we use the NOCV/CD method for the non symmetric Au(III) complexes). Focusing on the plot of Figure 3 upper panel, we see that the π back-donation amount is typical of a cationic compounds of Au(I) (CTπ‑back varies from −0.13 in [COAu(I)CO]+ to −0.22 e in [(Idipp)Au(I)CO]+) and lower than that of all the neutral complexes of Au(I) (CTπ‑back ranges from −0.24 in [CF3Au(I)CO] to −0.35 e in [FAuCO]). However, some Au(III) complexes show a shorter CO bond length (i.e., smaller ΔrCO values) compared to that one can extrapolate from the calibration line, being the most outliners (see for instance complexes (5)+, (6), and (11)2+). This finding suggests that π back-donation alone cannot completely account for the non classical behavior of the Au(III) complex series. In terms of VB structures in Scheme 1, it indicates that structures (b) or (c) should be counteracted by an increasing weight of structure (d). Indeed, the plot of Figure 3 lower panel, ΔrCO vs CTπ rCO/2, shows a better fit with respect to the plot of Figure 3 upper panel, ΔrCO vs CTπ‑back, again suggesting that the polarization of the CO bond by [LAu(III)]0/+/+2 fragments is an important feature. Compared to Au(I) complexes, the Au(III) compounds do have a larger tendency to polarize the CO π electrons in the direction from oxygen to carbon (as represented by VB structure (d)). The π CO polarization amount (CTπ rCO/2 values between 0.03 and 0.13 e, see Table

Figure 3. Correlation plot between ΔrCO and: upper panel) Au(III) → CO π back-donation (CTπ‑back); lower panel) CO π electron polarization (CTπ rCO/2) for the [LAu(I)CO]0/+ series of complexes studied in ref 47 (blue dots). The green diamonds refer to the [LAu(III)CO]0/+/+2 series of complexes studied here. The linear interpolation is related to Au(I) carbonyl complexes only (interpolation including both Au(I) and Au(III) carbonyl complexes gives a linear correlation with R2 = 0.865 for ΔrCO vs CTπ‑back and R2 = 0.946 for ΔrCO vs CTπ rCO/2 plots, respectively).

1) is larger than that of both the cationic compounds of Au(I) (CTπ rCO/2 varies from 0.02 to 0.09 e) and the neutral complexes of Au(I) (CTπ rCO/2 ranges from −0.04 to 0.00 e). This comparison confirms that the correlation between the theoretical parameters (charge transfer values) and the experimental observables (CO bond length) is verified also for Au(III) complexes and this finding can be useful for predictive purpose. Model Water−Gas Shift Reaction. As mentioned in the Introduction, the first CO complex of Au(III) with a cyclometalated (C^N^C) pincer ligand [(C^N^C)Au-CO]+ isolated by Bochmann et al.8 shows an interesting water−gas shift (WGS)-type reactivity at low temperature, whereas no activity has been observed for the isostructural and isoelectronic [(N^N^C)Pt(II)-CO]+ complex.71,72 An additional peculiar feature of the [(C^N^C)Au(III)CO]+ complex with respect to the considered Au(III) carbonyls in Scheme 2 is related to its stability. In Table 1 the interaction energies ΔEint between [LM]0/+/2+ and CO fragments are reported. A large variation in the Au(III)−CO bond stability within the pincer ligand series can be observed. The smallest H


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Inorganic Chemistry value is calculated for dicationic (11)2+ complex (−14.7 kcal/ mol), whereas the largest values are obtained for dicationic (3)2+ and (8)2+ (−46.4 and −44.7 kcal/mol, respectively). Comparable large stability is seen for cationic (9)+ and (4)+ complexes (−42.8 and −40.9 kcal/mol, respectively) which can be predicted to form stable carbonyl complexes. Interestingly, the [(C^N^C)Au(III)-CO]+ complex is the most stable in the whole series (−48.4 kcal/mol). This large ΔEint value suggests that an additional charge rearrangement (namely, the exceptionally large in-plane CTπ‑back∥ component) is responsible for the monocationic [(C^N^C)Au(III)-CO]+ complex having a stability comparable to that of dicationic complexes, thus corroborating the above bond analysis. One should expect that both the carbonyl complex stability and the different π backdonation asymmetry have consequences on the reactivity. The WGS reaction mechanism entails as a first step the nucleophilic attack of hydroxide (or water) on the metal coordinated CO to generate a metallacarboxylic acid as intermediate. The following steps consist of decarboxylation to give the metal hydride or corresponding anion, reductive elimination of H2 from the hydride, and coordination of new CO. The facile nucleophilic attack of water on the CO in the [(C^N^C)Au-CO]+ complex was attributed to a negligible back-donation contribution to the Au(III)-CO bond.8 However, in our previous work43 as well as in the present we have shown that, despite the large backdonation contribution in this Au(III)-CO bond, the carbon atom in the [(C^N^C)Au-CO]+ complex is indeed highly electrophilic to easily undergo the water nucleophilic attack. In addition, we found that the H2O attack occurs perpendicularly to the molecular plane, thus probing the calculated π backdonation asymmetry, with the smaller out-of-plane component (with respect to the in-plane component) favoring the perpendicular attack. On the basis of these findings, we use this prototype H2O nucleophilic attack step to the (1), (3)2+, (7)+, and [(N^N^C)Pt(II)-CO]+ complexes, in a first attempt to find a bonding/reactivity relationship which is relevant to ligand design in gold(III) catalysis. We selected (1), (3)2+ and (7)+ as representative of neutral, dicationic, and monocationic Au(III) complexes, respectively. In particular, complex (3)2+ bears the same ligand as that in the catalytically inefficient [(N^N^C)Pt(II)-CO]+ compound. Although complex (7)+ has a low ΔEint value (−17.8 kcal/mol), which is a crucial parameter for the carbonyl complex stability (and then for its possible isolation and characterization), it is however of interest within a study of the ligand effect on the activation barrier in our model reaction and in view of its use as catalyst as investigated in the literature (see Introduction).7 Following the approach used in ref 43, we analyze the reaction by considering the H2O nucleophilic attack to CO assisted by the OTf− (trifluoromethanesulfonate, triflate) anion. We recall here that 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. We performed scans of the potential energy surfaces (PESs) where the reaction coordinate is represented by the oxygen atom of H2O approaching the carbon atom of CO in the four mentioned complexes, including also [(C^N^C)Au-CO]+ for comparison using the same level of theory. The reaction profiles, starting from the reactant complexes (RC) to the product complexes (PC), through possible transition states (TS), are shown in Figure 4. We should stress that our aim here is not to improve the quantitative understanding of this reaction mechanism, but to compare the predictions based on the [LAu(III)]0/+/+2−CO

Figure 4. Reaction profile for the OTf−-assisted H2O nucleophilic attack on CO carbon atom in complexes (1) (green line), (3)2+ (orange line), model [(C^N^C)Au-CO]+ (red line), (7)+ (blue line), and [(N^N^C)Pt(II)-CO]+ (gray line).

and [LPt(II)]+ −CO bond analysis results using the OTf−/ H2O nucleophile as a probe in an attempt to find a bond structure/reactivity relationship. Obviously, we expect that the nature of the anion could play a crucial role on the reaction mechanism as we found for gold(I) complexes,99−101 although this issue has not been explored yet for gold(III) complexes to our knowledge. Here we use the anion only as a proton acceptor to promote the attack of water molecule. For the Au(III)-CO complexes (1), (3)2+ and [(C^N^C)AuCO]+ the reaction is barrierless and exergonic by 18.2, 24.7, and 15.3 kcal/mol, respectively, while for complex (7)+ the reaction is exergonic by 7.1 kcal/mol with a small energy barrier of 2.3 kcal/mol. For the Pt(II)-CO complex, [(N^N^C)Pt(II)CO]+, the reaction is endergonic by 12.8 kcal/mol and a transition state (TS) very much product-like has been located (TS is 13.5 kcal/mol above the RC), thus indicating that the reaction cannot occur. We find that the carbon atom in the Au(III)-CO complexes is sufficiently electropositive to easily undergo the H2O nucleophilic attack, but the carbon atom in Pt(II)-CO is not. Since the reaction is practically barrierless for all the examined Au(III)-CO complexes, with very different Au(III)CO interaction energies (from −17.8 in (7)+ to −28.7 in (1), −46.4 in (3)2+, and −48.4 kcal/mol in [(C^N^C)Au-CO]+), the influence of the considered four distinct ligands on their I


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Inorganic Chemistry

On the contrary, we find that the metal nature (Au(III) vs Pt(II)) strongly affects the reactivity. The considerable activity difference of isoelectronic and isostructural [(C^N^C)Au-CO]+ and [(N^N^C)Pt(II)-CO]+ complex is indeed striking. It is interesting to search for the origin of this behavior by comparing the Pt(II)−CO and Au(III)−CO bond features in these two complexes. In Figure 5 (bottom) the CD curves describing the net density difference (Δρ) and the first three NOCV components,

reactivity cannot be very strong. This is in line with the very similar bonding features in the four Au(III)-CO complexes. Indeed, by comparing the Au(III) ← CO σ donation and the Au(III) → CO π back-donation contributions to the bonding in (1), (3)2+, (7)+, and [(C^N^C)Au-CO]+ complexes, namely, the corresponding CTσ don and CTπ‑back values in Table 1, we see that they are very similar with two main differences: (i) the σ donation is smaller in complex (7)+ (0.24 e) with respect to that in the other complexes (0.29e in (1), 0.30 e in (3)2+ and 0.30e in [(C^N^C)Au-CO]+), which is responsible for a less electrophilic character of the CO carbon atom and thus explains the energy barrier of 2.3 kcal/mol calculated for (7)+ (for the Au(III)-CO bond analysis in complex (7)+ see Figure S7 in the SI); (ii) although the π back-donation is larger in complex [(C^N^C)Au-CO]+ (−0.28 e) compared to that in the other complexes (−0.22 e/−0.18 e), its out-of-plane component value (−0.11 e) is very similar to that of the corresponding out-of-plane component in the other complexes (−0.12 e/−0.09 e), which indicates very similar electrophilic character of the CO carbon atom and accounts for the H2O attack occurring perpendicularly to the molecular plane in [(C^N^C)Au-CO]+ complex. On this basis, we could suggest that the water nucleophilic attack would be barrierless for complexes (4)+, (8)2+, (9)+, and (10)+ (large σ donation component), whereas an activation barrier is expected for complexes (2)2+, (5)+, (6), and (11)2+ (σ donation component smaller than that of complex (7)+). The similar values of the out-of-plane and in-plane π back-donation components are predicted to have influence on the energy barrier amount, whereas based on the slight π back-donation asymmetry found for all the considered Au(III)-CO complexes (with exception of [(C^N^C)Au-CO]+), a not well-defined direction for the H2O attack is expected (no stereospecificity). For instance, the water nucleophilic attack reaction profile calculated for complex (5)+ (see Figure S12 in the Supporting Information) shows an activation energy barrier (σ donation component smaller than that of complex (7)+, 0.20e vs 0.24e, respectively), which is however smaller than that computed for complex (7)+ (1.1 vs 2.3 kcal/mol) since smaller out-of-plane and in-plane π backdonation components are observed for complex (5)+ with respect to those of complex (7)+ (−0.06/−0.05 e vs −0.09/− 0.09e, respectively). We should mention here that for complexes (1) and (3)2+, while searching for RC, two optimized structures with the OTf− bonded to CO carbon atom have been found, which are more stable than the corresponding RC structures by 9.7 and 18.2 kcal/mol, respectively, and less stable than the corresponding PC structures by 8.5 and 6.5 kcal/mol, respectively (geometries are shown in Figure S13 in the Supporting Information). Moreover, for complex (3)2+, no minimum structure has been found with a water oxygen−Au distance larger than that calculated in RC (1.90 Å), which is the smallest distance in the calculated RC series (see Figure 4). These findings suggest that factors other than bonding structure, as for instance counterion, could play a crucial role in the WGS experimental reactivity for these species. In conclusion, on the basis of the similar bonding features in the Au(III)−CO complex series studied here, similar efficiency in the WGS reaction could be expected, and therefore the different ligands seem to do not affect much the catalytic activity but we found that they have a significant effect on the complex stability.

Figure 5. Top: isodensity surfaces (±0.001 e a.u.−3) of the total Δρ and its first three NOCV components, Δρ1, Δρ2, and Δρ3, for the Pt− CO bond in the [(N^N^C)Pt(II)-CO]+ complex. Red surfaces represent charge depletion regions and blue surfaces identify charge accumulation regions. Bottom: corresponding CD curves. Red dots indicate the positions of the atomic nuclei along the z axis. The solid vertical line marks the isodensity boundary between the [(N^N^C)Pt(II)]+ and the CO fragments (see Methodological and Computational Details section). The dashed vertical line denotes the midpoint of the C−O bond (z = rCO/2).

corresponding to σ donation (Δρ1), out-of-plane (Δρ2) and inplane (Δρ3) π back-donation contributions for the [(N^N^C)Pt(II)-CO]+ complex, are shown. These contributions are easily recognized in the corresponding isodensity surface plots depicted at the top of Figure 5. In Figure S1 the same analysis for the [(C^N^C)Au-CO]+ complex is shown. The most striking features emerging from a comparison between Figures S1 and 5 concern: (i) the huge total π back-donation component (CTπ‑back is −0.40 e, see also Table 1), resulting from the sum of two symmetric in-plane and out-of-plane contributions (each amounting to −0.20 e) for Pt(II)-CO bond J


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Inorganic Chemistry

Table 2. EDA Analysis Results for the [(C^N^C)Au(III)-CO]+, [(N^N^C)Pt(II)-CO]+ and [LAu(III)-CO]0/+1/+2 Bonds of All the Complexes in Scheme 2a complex [(C^N^C)Au(III)CO] [(N^N^C)Pt(II)CO]+ (1) (2)2+ (3)2+ (4)+ (5)+ (6) (7)+ (8)2+ (9)+ (10)+ (11)2+ a

+

ΔEint

ΔE0

ΔEoi

ΔE(σ don)

ΔE(π‑back⊥)

−48.4 −60.4 −28.7 −23.9 −46.4 −40.9 −16.8 −16.8 −17.8 −44.7 −42.8 −34.9 −14.7

38.3 44.3 60.6 24.5 32.9 58.5 35.1 43.5 39.4 52.9 47.8 58.9 40.4

−87.2 −104.7 −89.3 −48.4 −79.3 −99.5 −51.8 −60.3 −57.2 −97.6 −90.6 −93.8 −55.1

−45.5 −46.5 −53.4 −28.6 −46.0 −62.5 −31.8 −33.5 −30.7 −64.5 −57.0 −57.2 −33.1

−13.7 −24.2 −12.1 −6.5 −11.5 −12.0 −6.3 −8.7 −8.8 −10.5 −10.7 −11.7 −5.9

ΔE

(π‑back∥)

−17.2 −24.5 −10.9 −6.6 −12.1 −13.2 −7.5 −10.2 −8.9 −11.2 −10.2 −11.7 −8.6

Energies are in kcal/mol.

and the smaller, although relatively large, total π back-donation component (CTπ‑back is −0.28 e, Table 1), resulting from the sum of two highly asymmetric in-plane and out-of-plane contributions (−0.17 and −0.11e, respectively) for Au(III)CO bond and (ii) the opposite sign polarization in the CO region, with a CTπ rCO/2 value of −0.02 e for Pt(II)-CO and 0.03 e for Au(III)−CO bond, which markedly differentiate the two metal-CO bonds. Very interestingly, the CO σ donation has the same value in the two complexes (0.30 e, see Table 1). This comparison suggests that the very large and symmetric Pt(II)-CO π back-donation is responsible for the carbon atom of CO to be not activated for undergoing a nucleophilic attack. In particular, the different CTπ‑back⊥ values (−0.20 e for Pt(II)CO and −0.11 e for Au(III)−CO) seem to be a key feature for their different catalytic efficiency. A computational study by Pernpointner and Hashmi comparing the isoelectronic Au(III) and Pt(II) (AuCl3 and [PtCl2(H2O)], respectively) as catalyst for nucleophilic additions to propyne has revealed that no carbon LUMO density can be found for platinum complex making an overlap with a nucleophilic frontier orbital unfavorable. In contrast, gold complex exhibits a high LUMO density together with considerably lower LUMO energy favoring a nucleophilic attack both from the structural and from the energetic point of view.102 Consistently, here we find that the out-of-plane π backdonation in Pt(II) complex makes the water nucleophilic attack harder than in the Au(III) complex. All our NOCV/CD analysis results are also supported by the energy decomposition analysis (EDA). The results of EDA analysis of the Pt(II)−CO and Au(III)−CO bond in the two complexes, performed considering cationic [(N^N^C)Pt(II)]+ and [(C^N^C)Au(III)]+, respectively, and neutral CO as fragments, are shown in Table 2. The interaction energy ΔEint, resulting from both steric ΔE0 and orbital interaction energy ΔEoi components, is larger for the Pt(II)−CO than that for Au(III)−CO bond (−60.4 vs −48.4 kcal/mol). In particular, the larger orbital interaction energy for Pt(II)−CO (−104.7 vs −87.2 kcal/mol, respectively) mainly reflects the larger Pt(II)−CO out-of-plane π back-donation component, since comparable in-plane π backdonation and σ donation components are found in Au(III)− CO and Pt(II)−CO. The Pt(II)−CO steric interaction ΔE0 energy is larger than that of Au(III)−CO (44.3 vs 38.3 kcal/ mol, respectively). Similar EDA results are also shown in Table

2 for all the complexes in Scheme 2. Among them, as previously mentioned, (3)2+, (4)+, (8)2+, and (9)+ complexes show interaction energy values comparable to that for the Au(III)− CO bond in [(C^N^C)Au(III)CO]+ (−46.4, −40.9, −44.7, and −42.8 kcal/mol, respectively), therefore indicating the possibility that such carbonyl complexes could be isolated. These values arise from both a larger orbital interaction contribution and a larger steric interaction than those in [(C^N^C)Au(III)CO]+, except for (3)2+, that shows similar values. Interestingly, the larger orbital interaction energy for complexes (4)+ and (8)2+ (−99.5 and −97.6 kcal/mol, respectively) is mainly due to the σ donation component (−62.5 and −64.5 kcal/mol, respectively) accounting not only for the “pure” σ charge donation (0.27 and 0.28e, respectively) but also for polarization which can be different for bidentate and tridentate pincer ligands.

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CONCLUSIONS In this work we have analyzed the Au(III)−CO bond in a series of neutral, cationic, and dicationic bis- and monocyclometalated gold complexes. Quantitative measures of the σ donation and π back-donation bond components as well as of the σ and π components of CO 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 (NOCV/CD). All complexes are found to feature a small total electron charge flux from CO to the metal fragment, with values ranging from 0.03 to 0.16e, which results from a large σ donation (values between 0.20 and 0.31e) and a significant π back-donation (values between −0.09 and −0.22e) component. This total electron charge transfer is surprisingly stable toward both the charge of the complex and the oxidation state of gold (I, III). In addition, all gold(III) carbonyl complexes are characterized by a slight asymmetry in the in-plane and out-of-plane components of the π backdonation, in sharp contrast to the huge asymmetry found before for the experimentally characterized [(C^N^C)Au(III)CO]+ complex, having a much larger in-plane component (−0.17 vs −0.11e). The electron density rearrangement over the carbonyl region shows a comparable σ polarization of CO for all complexes and a much more variable π polarization (values between 0.04 and 0.13e), depending on the ligand type. Remarkably, ΔrCO, π back-donation and CO π polarization all correlate with one another, as found before for gold(I) carbonyl K


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Inorganic Chemistry

program “Dipartimenti di Eccellenza - 2018-2022”. P.B. and F.T. thank the University of Perugia for financial support (“Fondo Ricerca di Base 2015”).

complexes. Compared to gold(I) (both cationic and neutral), however, the gold(III) complexes have a larger tendency to polarize the CO π electrons in the direction from oxygen to carbon (C ← O), thus shortening the bond. An attempt has been made to find a bonding/reactivity relationship in a model water−gas shift (WGS) reaction for four selected gold(III) complexes. Results show that the water nucleophilic attack to CO is practically barrierless, thus suggesting that the ligand influence on the reactivity cannot be very strong, in agreement with the very similar bonding features in the whole series of complexes. However, a small barrier can be predicted for complexes with a smaller σ donation component (say 0.24 e and below), which is responsible for a less electrophilic character of the CO carbon atom. On the contrary, the metal nature (Au(III) vs Pt(II)) strongly affects the reactivity. The origin of the considerable activity difference between the isoelectronic and isostructural [(C^N^C)Au(III)-CO]+ and [(N^N^C)Pt(II)-CO]+ complexes can be traced on the difference in the π back-donation bond component and CO π polarization. A huge π back-donation in Pt(II) complex (−0.40 e), resulting from two symmetric in-plane and out-ofplane contributions, combined with the opposite sign π polarization of the CO from carbon to oxygen (C → O) determines the lack of reactivity of this complex. On the other hand, the smaller (although relatively high) π back-donation in Au(III) complex (−0.28 e), resulting from two highly asymmetric in-plane and out-of-plane components, and a CO π polarization from oxygen to carbon (C ← O), is responsible for the carbon atom of CO being activated for undergoing a nucleophilic attack which specifically occurs in a direction perpendicular to the molecular plane (smaller out-of-plane π back-donation). In conclusion, the (C^N^C) emerges as a peculiar and interesting ligand for both imparting Au(III) stability and inducing a catalytic reactivity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00765. Methodological comparison (CD and NOCV/CD), 3D isodensity plots and CD curves for complexes (2)2+− (11)2+ and [(C^N^C)Au(III)CO]+, reaction profile for complex (5)+, reactant complex (RC) for complexes (1) and (3)2+ and all optimized geometrical structures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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

Leonardo Belpassi: 0000-0002-2888-4990 Francesco Tarantelli: 0000-0002-1285-0606 Paola Belanzoni: 0000-0002-1286-9294 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from MIUR and the University of Perugia to the project AMIS, through the L


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