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The Distinctive Electronic Structures of Rhenium Tris(thiolate) Complexes, an Unexpected Contrast to the Valence Isoelectronic Ruthenium Tris(thiolate) Complexes Hao Tang,† Edward N. Brothers,‡ and Michael B. Hall*,† †

Department of Chemistry, Texas A&M University, College Station, Texas 77845, United States Science Program, Texas A&M University at Qatar, Doha, Qatar

‡

S Supporting Information *

ABSTRACT: The noninnocent 2-diphenylphosphino-benzenethiolate (DPPBT) ligand containing both phosphorus and sulfur donors delocalizes the electron density in a manner reminiscent of dithiolenes. The electronic structure of the [ReL3]n (L = DPPBT, n = 0, 1+, 2+) complexes was probed with density-functional theory (DFT) and high-level ab initio methods including complete active space self-consistent field (CASSCF and CASPT2) and coupled cluster (CCSD and CCSD(T)). DFT predicts a slight preference for a closed-shell (CS) singlet ground state for the neutral [ReL3]0 and stronger preferences for low-spin ground states for the oxidized [ReL3]+ and [ReL3]2+. High-level ab initio methods confirm a CS singlet with a Re(III) (d4, S = 0) center as the ground state of [ReL3]0. Thus, this neutral Re species has considerably less thiyl radical character than the valence isoelectronic [RuL3]+, which is mainly a Ru(III) (d5, S = 1/2) anti-ferromagnetically (AF) coupled to a thiyl radical (S = 1/2). However, the oxidized derivatives [ReL3]+ and [ReL3]2+ show significant metal-stabilized thiyl radical character like [RuL3]+. Both [ReL3]+ and [ReL3]2+ have major contributions from Re(III) (d4, S = 1) centers AF coupled to thiyl and dithiyl DPPBT ligands. These findings are consistent with the experimental chemistry as [RuL3]+, [ReL3]+, and [ReL3]2+ can add ethylene to form the new C−S bonds, but [ReL3]0 cannot. The thiyl radicals on the S2 position (the S trans to a P donor) serve as the intrinsic electron acceptors in the actual ethylene addition reactions with Ru and Re tris(thiolate) complexes. Chart 1. Ru and Re Tris(thiolate) Complexesa

I. INTRODUCTION Transition-metal complexes with noninnocent ligands are currently an area of intense study.1−4 The chemistry of dithiolene complexes has played a key role in understanding such ligands.1,5−12 Complexes with other noninnocent ligands, especially those with complicated electron-transfer properties, have received wide attention for a variety of applications from catalysis to optoelectronics.2−4 However, the presence of noninnocent ligands challenges the conventional oxidation state assignments and complicates our ability to explain and predict the behavior of their complexes, because of the strong mixing between the ligand and metal orbitals.1−12 The 2-diphenylphosphino-benzenethiolate (DPPBT) ligand containing both phosphorus and sulfur as the donor atoms was reported by Dilworth et al., and it can delocalize the electron density like dithiolenes.13 Recently, the Ru and Re tris(thiolate) complexes [Ru(DPPBT)3]n ([RuL3]n, n = −, 0, +)14 and [Re(DPPBT)3]n ([ReL3]n, n = 0, +, 2+),15 have been synthesized and structurally as well as spectroscopically characterized, wherein the metal ion is chelated meridionally by the DPPBT ligands (Chart 1). These Ru and Re tris(thiolate) complexes can act as electrocatalysts for the uptake and release of olefins and © XXXX American Chemical Society

a

These complexes are nearly octahedral geometries, where the d orbital splits into t2g orbitals (point between ligands) and eg orbitals (point directly at the ligands).

hydrogen,10,14c,e,f,15 like similar reactions in metal dithiolenes.4−10,12 Our knowledge of the noninnocent DPPBT ligand and how it supports this chemistry remains scant. Among the [RuL3]n complexes, it is the cationic species [RuL3]+ that is known to readily undergo addition reactions with a variety of unsaturated organic compounds.14 Although the anionic [RuL3]‑ complex is Received: October 12, 2016

A

DOI: 10.1021/acs.inorgchem.6b02434 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Schematic MO diagram for [RuL3]+ and alternative electronic structures for its ground state.

suite of program.24 The geometric structures of the molecules [ReL3]0/+/2+, where L = DPPBT = 2-diphenylphosphinobenzenethiolate (full ligand), were optimized in the gas phase (GP) with ω-B97XD25 functional in conjunction with the SDD(f)26(Re), 6-311G* (P, S, C, and H atoms), and 6-31G (C and H atoms on phenyl group) basis set (B1). Frequency calculations were performed for all the optimized stationary points to ensure that they are true minima. Solvation was incorporated by using the SMD27 method for all the singlepoint calculations with dichloromethane as the solvent. The SDD(f) (Re) and 6-311++G**(all other atoms) basis set (B2) with the same functional was employed for the single-point energy calculations based on the optimized structures. The above methodology was tested in a variety of ways. The various tests and their results are summarized in the Supporting Information, while the results and discussion in the text refer to free energies (including the B2 electronic energies in solution, GP thermal correction to the Gibbs free energy, and solvation free energy) based on ω-B97XD/B2/SMD//ω-B97XD/B1/ GP for the full-ligand molecules [ReL3]0/+/2+. DFT Calculations for Truncated Models. Because of the high computational expense of the higher-level ab initio methods for the full-ligand structures of [ReL3]0/+/2+, we employed the truncated models (denoted [ReL′3]0/+/2+) as shown in Chart 2. Geometries, frequencies, and free energies at 298.15 K for the truncated models [ReL′3]0/+/2+ were also computed using the ω-B97XD functional in combination with the SDD(f) (Re)/6-311G*(rest) basis set (B4). The singlepoint calculations were performed with the SDD(f) (Re)/6-

clearly a d6 Ru(II) trithiolate, the diamagnetic [RuL3]+ species is ambiguous with respect to the oxidation state of the metal ion as well as that of the DPPBT ligands. Because density functional theory (DFT) calculations suggested nearly degenerate closed-shell (CS) singlet and triplet states for [RuL3]+ (Figure 1a,c), Grapperhaus et al. initially proposed a singlet diradical, with the electronic configuration (S2− S3)2(π*xz)1(π*yz)1 (Figure 1b) as the ground state.14c,h In an alternative assignment that is more obviously consistent with the alkene addition reaction, an S2−S3 diradical with a low-spin Ru(II) was also proposed (Figure 1d).14h Very recently, highlevel ab initio methods showed that the ground state for [RuL3]+ was actually a singlet diradical state from the antiferromagnetic (AF) coupling between Ru(III) (S2 = 1/2) and S pz (S = 1/2; Figure 1e).16 These studies provided information about alternative spin-state and the relationship between the chemistry of the [RuL3]+ complex and this new electronic description as a Ru(III)−thiyl diradical. Although [RuL3]+ readily adds ethylene, the valence isoelectronic [ReL3]0 species does not.14c,e,15a In contrast, it is the one- and two-electron oxidized derivatives [ReL3]+ and [ReL3]2+ that bind to ethylene.14c,15a Knowledge of the differences in the electronic structures of the related rhenium derivatives [ReL3]0/+/2+ that might explain the different reactivity are still missing. In this work, detailed explorations of the electronic structures of [ReL3]0/+/2+ by DFT and high-level ab initio methods including CASSCF,17 CASPT2,18 MP2,19 MP3,20 MP4D, MP4DQ, MP4SDQ,21 CCSD, CCSD(T),22 QCISD, and QCISD(T)20,23 provide insight into the relationship of these complexes with the Ru(III)−thiyl diradical [RuL3]+.16 In addition to clarifying the geometric structure, electronic structure, and spin-state ordering of [ReL3]0/+/2+, the calculations provide explanations for the different reactivity observed for the ethylene addition reactions with [RuL3]+ and [ReL3]0/+/2+.

Chart 2. Truncated Models

II. COMPUTATIONAL METHODS DFT Calculations for Full-Ligand Molecules. All the DFT calculations were performed by using the Gaussian 09 B


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Inorganic Chemistry

Figure 2. DFT(ω-B97XD)-optimized and experimental (Exp)13a geometric parameters are given with bond lengths (Å) and angles (deg) for the full-ligand molecules [ReL3]0/+/2+ (A−C) and the truncated models [ReL′3]0/+/2+ (D−F). The reported energies (kcal/mol) include ω-B97XD/ SMD electronic energy using dichloromethane as the solvent with GP thermal free energy correction at 298 K.

311++G**(rest) basis set (B2) with the same ω-B97XD functional based on the optimized structures. The continuum solvation model and solvent were same as those for the fullligand molecules. The reported energy in the main text for the truncated model [ReL′3]0 includes the B2 electronic energies in solution, GP thermal correction to the Gibbs free energy, and solvation free energy based on ω-B97XD/B2/SMD//ωB97XD/B4/GP. The other tested TPSS and M06 functionals showed that the trends for the relative energies of the low and high spin states and geometric parameters were identical to those obtained from the ω-B97XD functional (Tables S11 and S12). Ab Initio Calculations. Single-point calculations using the Hartree−Fock (HF), PUHF,28 MP2, PMP2, MP3, PMP3, MP4D, MP4DQ, MP4SDQ, CCSD, CCSD(T), QCISD, and QCISD(T) methods were done in Gaussian 09 suite of program. Multireference ab initio CASSCF and CASPT2 calculations were performed with the MOLPRO suite of program29 using the basis set SDD(f) (Re)/6-311G*(rest) (B4). All the CASSCF calculations were based on the geometries optimized at the ω-B97XD/B4 level. Boys localization was applied to CASSCF active space orbitals, and then the CASPT2 calculations were performed with these localized orbitals. A level shift of 0.2 au was employed in the CASPT2 calculation to avoid the intruder state problem.18 The details of the CASSCF and CASPT2 calculations of [ReL′3]0/+/2+ were relegated to the Supporting Information.

III. RESULTS DFT Results. Like the [RuL 3] − case, wherein the experimental geometric parameters of the full-ligand molecule [RuL3]− are well-reproduced by the DFT calculations,16 comparisons of the geometric parameters of the CS singlet and triplet states for the full-ligand molecule [ReL3]0 against the experimental values (Figure 2A) clearly show that the computed geometric values for the CS singlet state are in closer accord with experimental ones than those of the triplet state. Upon oxidation, the Re−S1 bond distances in the low-spin state for [ReL3]0, [ReL3]+, and [ReL3]2+ become significantly shorter, changing from 2.54 to 2.39 and 2.29 Å, respectively. In contrast, the Re−S2 and Re−S3 bond distances for [ReL3]0, [ReL3]+, and [ReL3]2+ are almost unaffected by the change in the oxidation state. However, the reverse trend was observed in the high-spin state, wherein the Re−S1 bond distances for [ReL3]0, [ReL3]+, and [ReL3]2+ were almost unchanged, but both the Re−S2 and Re−S3 bond distances significantly shortened upon oxidation. Inspection of the free energies between the low- and highspin states (Figure 2) shows that the neutral species [ReL3]0 has an S = 0 ground state with a close-lying S = 1 state (2.5 kcal/mol less stable by ω-B97XD). By comparison, the cationic species [ReL3]+ has an S = 1/2 ground state and an S = 3/2 excited state that is more than 10 kcal/mol higher in energy. For the dicationic species [ReL3]2+, the S = 0 ground state is preferred over the high-spin state by 7.6 kcal/mol. Similar C


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Figure 3. Schematic DFT MO diagrams for (a) CS singlet and (b) triplet states of [ReL3]0, (c) doublet state of [ReL3]+, and (d) CS singlet state of [ReL3]2+.

S1 py orbitals, the Re−S1 bond distance progressively shortens in the low-spin state from [ReL3]0 to [ReL3]2+ (Figure 2). As expected from its anticipated lower oxidation state, the 5d metal complex [ReL3]0 has more metal character in its higherenergy π*xz/π*yz orbitals and correspondingly more ligand S in its πxz/πyz orbitals than the valence isoelectronic 4d metal complex, [RuL3]+ (compare Figure 1 and Figure 3). This difference is reflected by the metal/sulfur ratio in the πxz orbital of 4/1 and 1/2 for [RuL3]+ and [ReL3]0 (Table S10). Likewise, the energy difference between the nonbonding dxy and the nonbonding S2−S3 orbitals decreases from [RuL3]+ (1.13 eV) to [ReL3]0 (0.28 eV), and this difference is smaller than that in [RuL3]+ even for the more oxidized [ReL3]+ (α: 0.55 eV and β: 0.40 eV) and [ReL3]2+ (0.68 eV). High-Level ab Initio Method Results. To reduce the computational cost of the high-level ab initio methods for the full-ligand systems [ReL3]0/+/2+, we used the truncated models [ReL′3]0/+/2+ (Figure 2) that was successfully employed in our previous study of [RuL3]+.16 The optimized structural parameters of the truncated models [ReL′3]0/+/2+ are in reasonable agreement with the full-ligand molecules as presented in Figure 2. The Re−S and Re−P bonds of the truncated models [ReL′3]0/+/2+ calculated at the ω-B97XD/B4 level differ from the full-ligand systems [ReL3]0/+/2+ by ≤0.08 Å, and the P2−Re−P3 angle differs by ≤7° (Table S12 compares these deviations calculated by both TPSS and M06 functionals). The truncated models follow the same trends of decreasing Re−S1 bond lengths and almost unchanged Re−S2/ S3 bond length as the complexes are oxidized from the neutral to the dication, and they reproduce the various trans effects as observed in the full-ligand molecules for different spin states (Figure 2). Moreover, the calculated free-energy difference between the low- and high-spin states for the truncated model [ReL′3]0 (3.1 kcal/mol) is in good agreement with the lowand high-spin state difference in the full-ligand molecule [ReL3]0 (Figure 2). In addition, there are no appreciable differences in the frontier MOs between the full and truncated molecules by visual inspection of the Kohn−Sham orbitals

trends for these energetic gaps were obtained from the other tested density functionals (Table S1). As expected, more HF exact exchange (with the same correlation functional) tends to stabilize the triplet state relative to the CS singlet state (Table S1). Among all the tested DFT functionals (Table S1), MN12SX predicts the most stable CS singlet with the triplet 5.9 kcal/mol higher, a result consistent with the high-level ab initio methods (see below). Because of the large energy separations for the cations, we will only consider the low-spin states for [ReL3]+ and [ReL3]2+ below. The performance of DFT functionals was also assessed by calculating the redox couples [ReL3]+/0 and [ReL3]2+/+ relative to Fc+/0, and the B97D functional performs best among all the tested functionals (Table S9). Like the DFT molecular orbitals (MOs) in [RuL3]+ (Figure 1), the upper valence MOs in [ReL3]0/+/2+ involve strong mixing between the Re 5d and the S1, S2, and S3 3p atomic orbitals (AOs). As shown in Figure 3, one pair of orbitals, πxz/ π*xz, is built from the bonding/antibonding combination of Re dxz and the coplanar, in-phase combination of S2 pz and S3 px. The other pair, πyz/π*yz, is built from the bonding/antibonding combination of Re dyz and S1 py. The Re dx2−y2 and Re dz2 are two metal-centered antibonding orbitals that have σ symmetry with respect to the P and S donor atoms. The Re dxy and the out-of-phase S2−S3 are essentially nonbonding. The πxz/π*xz orbitals in the xz plane are almost orthogonal to the πyz/π*yz orbitals in the yz plane. As mentioned above, the CS singlet and triplet state in [ReL3]0 (Figure 3a,b) are close in energy, an issue that will be examined below. Regardless of the ground state of [ReL3]0, oxidation to [ReL3]+ creates a doublet state with one electron in the π*yz orbital (Figure 3c), while, as mentioned above, the corresponding higher-spin quartet is much higher in energy. Further oxidation to [ReL3]2+ removes the remaining electron from the π*yz, yielding a CS ground state (Figure 3d) with a triplet somewhat higher in energy than that for [ReL3]0. Consistent with the nature of the π*yz, antibonding between the Re dyz and D


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Inorganic Chemistry (Figure S1). Therefore, the truncated models [ReL′3]0/+/2+, which are used in the high-level methods to assess the intrinsic electronic structure of the Re center, should be regarded as good models for the full-ligand molecules [ReL3]0/+/2+. The degree of multireference character in these rhenium complexes is shown by the T1 diagnostic values for these singlet and triplet states of the neutral species, denoted 1,3 [ReL′3]0; doublet state of the cationic species, denoted 2 [ReL′3]+; and singlet state of the dicationic species, denoted 1 [ReL′3]2+ in Table 1. The values, all somewhat larger than 0.02, suggest some multireference character in all of these species.

electronic configurations, the electron occupancy numbers, key natural orbitals, and relative energies for the CASSCF(16,13) calculations for 1,3[ReL′3]0 based on the DFT-optimized geometries. The singlet state of [ReL′3]0 is dominated by two important configurations (Ψ1 and Ψ2) that are related by double (two electrons) excitation from orbital ψ6 to orbital ψ5 (Scheme 1B). Although this appears to be somewhat like the valence isoelectronic [RuL′3]+ (Scheme 1A),16 there are some important differences. First, from an inspection of the key orbitals for the singlet state, as shown in Scheme 1A,B, it is seen that the orbitals denoted as ψ5 and ψ6 in 1[ReL′3]0 are antibonding and bonding MOs between the Re dxz and the inphase combination of the S2 pz and S3 px orbitals, while the corresponding orbitals in 1[RuL′3]+ involve strong double excitation from the bonding MO between the Ru dxz and S2 pz to an antibonding MO.16 Second, both a comparison of the coefficients for the two most leading configurations and of the resulting electron occupancy numbers between 1[RuL′3]+ and 1 [ReL′3]0 (Scheme 1A,B) are consistent with a weaker contribution from this double-excitation for [ReL′3]0 than for [RuL′3]+. Thus, the overall electronic structure for the neutral [ReL′3]0 is best assigned as a CS singlet state with a Re(III) center with much less thiyl radical character than the valence isoelectronic [RuL′3]+. The CASSCF wave function for the low-lying triplet state of [ReL′3]0, as shown in Scheme 1D, has one major configuration, in which the singly occupied orbitals ψ5 and ψ6 are mainly metal Re dxz and Re dyz. This result is in contrast with that of the lowlying triplet state of [RuL′3]+, wherein the singly occupied orbitals ψ3 and ψ4 have mainly metal Re dxz and ligand S1 py characters (Scheme 1C).16 Therefore, the triplet state of the neutral species [ReL′3]0 involves a triplet Re(III) center with three thiolate ligands, while that for [RuL′3]+ involves a doublet Ru(III) center with two thiolate ligands and one thiyl ligand, a result that parallels the overall character of their ground states. As shown in Scheme 1, the CASSCF(16,13) calculations predict the singlet ground state of the neutral [ReL′3]0 to be preferred by 13.5 kcal/mol over the triplet state. The smaller CASSCF(14,12) and CASPT2(14,12) calculations, formed by removing a doubly occupied orbital, predict that the singlet state is favored over the triplet state by 16.4 and 24.5 kcal/mol, respectively (Figure S6 and Table S17). Predictions of this energy difference by other ab initio methods on the ω-B97XDoptimized geometries are collected in Table 2. As expected, HF predicts the triplet as the most stable, while MP2 appears to overestimate the stability of the singlet relative to the triplet state, and then MP3 underestimates this difference. However, like the CASSCF and CASPT2, the other (higher-order) methods: MP4SDQ, CCSD, CCSD(T), QCISD, and QCISD(T) predict somewhat similar singlet stabilities. Electronic Structure of the Cationic Species 2[ReL′3]+. For the cation, [ReL′3]+, Scheme 2 depicts the coefficients of the main electronic configurations, the electron occupation numbers, key natural orbitals, and localized orbitals for the

Table 1. T1 Diagnostic Values from CCSD(T1diag) Calculations for [ReL′3]0/+/2+ 1

T1

[ReL′3]0

3

[ReL′3]0

0.028

0.032

2

[ReL′3]+ 0.037

1

[ReL′3]2+ 0.027

Electronic Structures of the Neutral Species [ReL′3]0. Scheme 1 displays the coefficients of the main

1,3

Scheme 1. Main Electronic Configurations (Ψi), Electron Occupancy Numbers (blue), Key Natural Orbitals (ψi), and Relative Energies for CASSCF(16,13) of the Singlet and Triplet States for [ReL′3]0 a

a

To compare the electronic structures between [RuL’3]+ and [ReL’3]0 at the similar level, the CASSCF(16,11) with natural orbitals for the singlet and triplet states of [RuL′3]+ were recalled from ref 16.

Table 2. Relative Single-Point Energies (kcal/mol) of the Singlet and Triplet States of [ReL′3]0 Calculated by Various Electronic Structure Methodsa singlet triplet a

HF

MP2

MP3

MP4SDQ

CCSD

CCSD(T)

QCISD

QCISD(T)

13.9 0.0

0.0 19.8

0.0 0.3

0.0 13.3

0.0 4.7

0.0 8.5

0.0 4.5

0.0 8.8

Also see Table S28. E


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complicated multi-configuration character of 2[ReL′3]+ in the localized-orbital representation (CASPT2, Scheme 2D) shows a second important configuration with the coefficient of 0.41 that represents a Re(IV)−thiolate contribution with the doubly occupied ϕ3 orbital and virtual ϕ1 orbital (for further details see Figure S7 and Table S19). The occupancy numbers of the CASPT2 localized orbitals depicted in Scheme 2D show that the net unpaired spin remains in the Re dyz orbital (ϕ2), while the occupation numbers of 0.87 and 1.40 for the respective Re dxz (ϕ1) and S2 pz (ϕ3) also suggest a strong mixture of Re(IV)−thiolate and Re(III)−thiyl character in the 2[ReL′3]+ ground state. Thus, the overall 2[ReL′3]+ has much less thiyl character than 1[RuL′3]+. Interestingly, this prediction is in excellent agreement with the recent mass spectrometric study on the reactivity and radical natures of the cationic species [ReL3]+ and [RuL3]+, in which the radical of [ReL3]+ resides in the same S2 position as [RuL3]+, but the reactivity of [ReL3]+ with dimethyl disulfide was lower in comparison to that of [RuL3]+.14i Electronic Structure of the Dicationic Species 1 [ReL3]2+. The multi-configuration character of the dicationic species [ReL′3]2+ as reflected by the CASSCF(10,7) and CASPT2(10,7) calculations is illustrated in Scheme 3, which

Scheme 2. Main Electronic Configurations (Ψi), Electron Occupancy Numbers (blue), Key Natural Orbitals (ψi and Φi) for CASSCF(11,7), and Localized Orbitals (ϕi) for CASPT2(11,7) of Doublet State for [ReL′3]+ a

Scheme 3. Main Electronic Configurations (Ψi), Electron Occupancy Numbers (blue), Key Natural Orbitals (ψi and Φi) for CASSCF(10,7), and Localized Orbitals (ϕi) for CASPT2(10,7) of Singlet State for [ReL′3]2+

a

To compare the electronic structures between [RuL’3]+ and [ReL’3]+ at the similar level, the CASSCF(10,6) with natural orbitals and CASPT2(10,6) with localized orbitals for [RuL’3]+ were recalled from ref 16.

CASSCF(11,7) and CASPT2(11,7) calculations based on the DFT-optimized geometries. Inspection of the CASSCF results with natural orbitals as shown in Scheme 2B, the doublet state of [ReL′3]+ is well-represented by the two most significant configurations Ψ1 and Ψ2 with the respective coefficients of 0.97 and −0.20, which is the contribution from the doubleexcitation from the ψ3 orbital to ψ1 orbital. As shown in Scheme 2B, the orbitals ψ3 and ψ1 are the bonding and antibonding π interaction between the Re dxz and S2 pz orbitals in the xz plane. The orbital ψ2, the Re dyz orbital with a small antibonding contribution from the S1 py orbital, holds essentially all the excess spin as it is singly occupied in both configurations. Although both [ReL′3]+ and [RuL′3]+ are related by a similar double-excitation from the metal dxz and S2 pz bonding MO to its antibonding counterpart, [ReL′3]+ has less multi-configurational character than [RuL3]+, as indicated by the smaller coefficient for the configuration Ψ2 in [ReL′3]+ relative to that in [RuL′3]+.16 Consistent with this two-states description, the electron occupancy numbers of the antibonding MO ψ1 for [ReL′3]+ is smaller than that for [RuL′3]+ (compare Scheme 2A,B). Additionally, the leading configurations and the important orbitals from the larger CASSCF(15,13) calculation for the doublet state of [ReL′3]+ (Figure S3 and Table S14) are essentially identical to those obtained from the CASSCF(11,7) level. While the CASPT2 results for the singlet [RuL′3]+ show a single dominative configuration that represents the AF coupling of the Ru(III)−thiyl diradical (Scheme 2C),16 the doublet [ReL′3]+ has less thiyl radical character, as reflected by the smaller coefficient for configuration Φ1, a Re(III) (S = 1) AF coupled to the thiyl S (S = 1/2; Scheme 2D). The more

displays the coefficients of the main electronic configurations, the electron occupancy numbers, and key natural and localized orbitals. The CASSCF(10,7) results for the singlet state of [ReL′3]2+ (Scheme 3A) show three important configurations (Ψ1−Ψ3). The leading configuration has two highly delocalized Re−S π bonding orbitals (ψ3 = Re dyz + S1 py and ψ4 = Re dxz + F


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Scheme 4. Electronic Structures of 1,3[RuL′3]+, 1,3[ReL′3]0, 2 [ReL′3]+, and 1[ReL′3]2+ from CASPT2 Calculation

S2 pz) doubly occupied, while the next two most important configurations represent double excitations from these bonding MOs to their antibonding counterparts (ψ 1 and ψ 2 , respectively); see Scheme 3A. These double excitations result in the electron occupancy numbers of 1.87 (ψ3) and 1.89 (ψ4) for the bonding MOs, and 0.13 (ψ1) and 0.13 (ψ2) for the antibonding MOs. The larger CASSCF(16,12) calculation for the singlet state of [ReL′3]2+ (Figure S4 and Table S15) has leading configurations and key orbitals much like those described above. Analysis of the localized orbitals of [ReL′3]2+ from the CASPT2 calculation presents a more prominent multiconfigurational character of the ground state with three major configurations and a large number of other significant contributions (Figure S8 and Table S21). As shown in Scheme 3B, the principal configuration Φ1 with a coefficient of 0.52 is a Re(III)−thiyl double diradical with two singly occupied Re d orbitals (Re dyz (ϕ1) and Re dxz (ϕ2)) AF coupled to two singly occupied S orbitals (S1 py (ϕ3) and S2 pz (ϕ4)). The next two most important configurations, Φ2 and Φ3, with comparable coefficients of −0.35 and −0.34, both represent Re(IV) contributions, in which first one and then the other of the Re(III)−thiyl diradicals becomes a thiolate. Several of the next most important configurations (Figure S8 and Table S21) are other Re(III)−thiyl double diradicals. The CASPT2 calculation produces electron occupancy numbers of 0.84, 1.20, 1.02, and 1.30 for Re dyz (ϕ1), S1 py (ϕ3), Re dxz (ϕ2), and S2 pz (ϕ4), respectively, which leads to the conclusion that the doublet [ReL′3]2+ is best described as a Re(III) center with two singly occupied Re dxz and dyz orbitals AF coupled to an overall DPPBT ligand framework with two singly occupied S orbitals (mainly S2 pz and S1 py). Although the cation [ReL′3]+ and dication [ReL′3]2+ show similar thiyl radical character at S2, the dication is predicted to have significant thiyl-radical character at S1.

one electron of a Re(III) (d4, S = 1) center to produce an overall doublet state). Finally for the [ReL′3]2+, imagine further oxidization of the Re center to Re(IV), which again is so unstable that an electron moves from S1 to Re, creating a Re(III) center (d4, S = 1) with both unpaired Re electrons AF coupled back to thiyl radicals as a “double diradical” (Scheme 4f). In general, all the [ReL′3]0/+/2+ complexes are mainly d4 Re(III), while [RuL′3]+ is mainly a d5 Ru(III). As elaborated above, [RuL′3]+, [ReL′3]+, and [ReL′3]2+ are therefore assigned as the metal-stabilized thiyl radicals, but [ReL′3]0 is not, even though it is valence-isoelectronic with [RuL′3]+. This difference originates in the stability of the d4 Re(III) relative to a d4 Ru(IV). Differential Reactivity of Ethylene Addition to [RuL3]+ and [ReL3]0/+/2+. Experimentally, [RuL3]+, [ReL3]+, and [ReL3]2+ can add ethylene across the S2/S3 atoms to form the new C−S bonds, but [ReL3]0 cannot.14c,e The differential reactivity of these complexes toward olefins and their multireference (thiyl-radical) character are both connected with (i) the different electronegativity of Re versus Ru (group 7 versus 8), (ii) the different row (larger 5d versus 4d orbitals), and (iii) the charge and resulting oxidation state. Although [ReL3]0 and [RuL3]+ are valence-isoelectronic, the former is a CS singlet ground state with little radical character on the ligand, while the latter has significant radical character in ligand S2, where its reactivity originates. However, consistent with the experimental results, the cationic [ReL3]+ and dicationic [ReL3]2+ have sufficient radical character on ligand S2 to support their reaction with ethylene. These results indicate that the pivotal role of the metal-stabilized thiyl radicals in affecting the reactivity of the alkenes addition reaction with the metal− thiolate complexes. Furthermore, the lower electronegativity of

IV. DISCUSSION Nature of the Metal−Thiolate Complexes [ReL3]0/+/2+ and [RuL3]+. A summary analysis of the electronic structures of the [RuL′3]+ and [ReL′3]0/+/2+ species from the CASPT2 calculations with the localized orbitals is shown in Scheme 4. For the singlet state of [RuL′3]+, imagine two successive oxidations on the Ru center beginning from [RuL′3]− (Ru(II) trithiolate), which would produce a Ru(IV) trithiolate; however, Ru(IV) is so unstable that an electron moves from S2 to Ru but remains AF coupled to the S2 electron left behind (Scheme 4a). As shown in Scheme 4b, the instability of Ru(IV) also holds true for its triplet state, where now one electron from S1 moves to Ru to create ferromagnetically (triplet) coupled Ru(III)-S1 electrons. In contrast to this description for [RuL′3]+, the [ReL′3]0 analogue clearly favors the Re(III) d4 electron configuration, as there is no significant ligand radical character for the singlet state (Scheme 4c). This difference may be attributed to the stability of Re(III) oxidation state and the lower electronegativity of Re compared to Ru, so that [ReL′3]0 is a trithiolate. Likewise, the triplet state of [ReL′3]0 simply involves a promotion of an electron on Re, creating a Re(III) (d4, S = 1) center with two singly occupied dxz and dyz orbitals (Scheme 4d). Now imagine forming [ReL′3]+ by oxidization of [ReL′3]0 on the Re center, which would produce a Re(IV) trithiolate, but Re(IV) is so unstable that an electron moves from S2 to Re but remains AF coupled to the S2 electron left behind as shown in Scheme 4e (a thiyl radical AF coupled to G


Article

Inorganic Chemistry Re and lower charge of [ReL3]0 result in Re−S π-bonds more polarized toward sulfur in comparison with the [RuL3]+ case, so that the Re−S π-antibonding orbitals are higher in energy, more polarized toward Re, and thus, not easily attacked by the ethylene. Upon oxidation of the [ReL3]0 complex, these polarization effects begin to shift the electron distribution progressively toward the [RuL3]+ case. Overall, the better overlap between the S 3p and the Re 5d over that with the Ru 4d results in stronger π-bonding between Re and S compared to those for Ru and in somewhat less diradical character in all the Re complexes.

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V. CONCLUSIONS In conclusion, the geometries, electronic structures, and spinstate energetics of the Re tris(thiolate) complexes, [ReL3]0/+/2+, were studied by DFT and high-level ab initio quantum calculations and compared with those of [RuL3]+, which is valence-isoelectronic with [ReL3]0. DFT results reveal that in the neutral [ReL3]0 case, the singlet and triplet states are close in energy with a slight preference for low spin. In the oxidized [ReL3]+ and [ReL3]2+ cases, the low-spin ground states are preferred more strongly. Consistent with the DFT, CCSD(T) and QCISD(T) calculations show that the neutral species [ReL3]0 has a singlet ground state. High-level ab initio methods predict that the neutral species [ReL3]0 is well-described as a CS singlet ground state involving a Re(III) (d4, S = 0) center and bound to three thiolate-DPPBT ligands. Thus, [ReL3]0 has obviously less thiyl-radical character than the valence isoelectronic [RuL3]+, which has its major component arising from a Ru(III) (d5, S = 1/2) AF coupled to one thiyl- with two thiolate-DPPBT ligands.16 In contrast, the oxidized derivatives [ReL3]+ and [ReL3]2+ have significant metal-stabilized thiyl-radical character, arising from Re(III) (d4, S = 1) centers AF coupled to one thiyl- with two thiolateDPPBT ligands for [ReL3]+ and to two thiyl- with one thiolateDPPBT ligand for [ReL3]2+. Thus, the [ReL3]0/+/2+ complexes favor the Re(III) d4 electron configuration, while [RuL3]+ disfavors the Ru(IV) d4 electron configuration in favor of Ru(III), low-spin d5. Our findings clarify experiments that show ethylene reacting with [RuL3]+, [ReL3]+, and [ReL3]2+ to form the new C−S bonds but not reacting with [ReL3]0. Besides the different electronegativity and charge of Re versus Ru, the thiyl-radical character is considered as an important factor contributing to the differential reactivity, wherein [RuL3]+ and [ReL3]+/2+ have significant radical characters on the ligand S2, whereas the neutral [ReL3]0 has little thiyl radical character. In fact, the thiyl radical on the S2 position (the S trans to a P donor) serve as the intrinsic electron acceptor in the actual alkene addition reactions. The detailed mechanisms of the ethylene addition to [RuL3]+ and [ReL3]0/+/2+ are under study. The detailed results of the Re tris(DPPBT) species presented in this work should lead to a better appreciation of the noninnocent nature of the DPPBT ligand and its behavior in promoting various reactions in transition-metal complexes.

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Computational methods and references; benchmarking of different DFT functionals for relative free energies, geometric structures optimized in GP and solution for the full-ligand molecules, and redox potentials; relative free energies, geometric structures, and DFT frontier MOs of truncated models; orbital compositions; CASSCF and CASPT2 results at the DFT- and CASSCF-optimized geometries for the active spaces, all the configurations, coefficients, and electron occupancy numbers; relative energies of other single-reference methods; Cartesian coordinates and absolute energies (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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.

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ACKNOWLEDGMENTS We acknowledge financial support from the Qatar National Research Fund under NPRP Grant No. 05-318-1-063 and the Welch Foundation under Grant No. A-0648. Computer time was provided by the TAMU High Performance Research Computing Facility.

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REFERENCES

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