Origin of Anomalous Electronic Circular Dichroism Spectrum of RuPt2

Jul 9, 2014 - Both RS and SR are achiral molecules but the others are chiral. The RS/SR can be obtained by two sequential operations, the xz-plane mir...
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Origin of Anomalous Electronic Circular Dichroism Spectrum of RuPt2(tppz)2Cl2(PF6)4 in Acetonitrile Hua-Gen Yu* Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973-5000, United States S Supporting Information *

ABSTRACT: We report a theoretical study of the structures, energetics, and electronic spectra of the PtII/RuII mixed-metal complex RuPt2(tppz)2Cl2(PF6)4 (tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) in acetonitrile. The hybrid B3LYP density functional theory and its TDDFT methods were used with a complete basis set (CBS) extrapolation scheme and a conductor polarizable continuum model (C-PCM) for solvation effects. Results showed that the trinuclear complex has four types of stable conformers and/or enantiomers. They are separated by high barriers owing to the repulsive H/H geometrical constraints in tppz. A strong entropy effect was found for the dissociation of RuPt2(tppz)2Cl2(PF6)n in acetonitrile. The UV−visible and emission spectra of the complex were also simulated. They are in good agreement with experiments. In this work we have largely focused on exploring the origin of anomalous electronic circular dichroism (ECD) spectra of the RuPt2(tppz)2Cl2(PF6)4 complex in acetonitrile. As a result, a new mechanism has been proposed together with a clear illustration by using a physical model.

I. INTRODUCTION Metal coordination compounds have become a main area of interest in chemistry, biochemistry, physics, and material science.1,2 Many potential applications have been explored because they exhibit a great variety of structural, electronic, and kinetic properties. Dye sensitized solar cells and organic light emitting diodes are just two of many examples.3 These two applications exploit the property of photoinitiated electronic transitions and charge transfers in metal coordination compounds. Recently, a trinuclear metal complex, [RuPt2(tppz)2Cl2](PF6)4 (tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine), has been synthesized by Brewer and her co-workers.4,5 They found that the complex has strong absorption bands in the UV− visible region, and excellent redox properties owing to the strong coupling between the metal d-orbitals and ligand chromophores. Therefore, it is a potential photocatalyst for water splitting using clean solar energy, as well as other applications.2 Surprisingly, they also observed unambiguous electronic circular dichroism (ECD) spectra of the [RuPt2(tppz)2Cl2](PF6)4 complex in acetonitrile.4 The ECD spectra show an unusual line shape profile that can be strongly affected by temperature. Ordinarily, in the absence of controlled chiral synthetic conditions, no ECD signal should be expected in the racemic mixtures. Although an equally mixed sample of enantiomers may yield a ECD signature under some anisotropic environments created by external forces such as magnetic fields and shears,6−8 they have excluded those © 2014 American Chemical Society

possibilities in experiments. So far, there is no acceptable explanation for the onset of ECD signals of the complex. In this work we explore the mechanism behind the unusual ECD spectra of the [RuPt2(tppz)2Cl2](PF6)4 complex by using a hybrid density functional theory (DFT) and time-depend DFT (TDDFT) method. Here the electronic structures, energetics, and electronic absorption and emission spectra will be addressed. Finally, a physical mechanism is proposed in section III.C.

II. COMPUTATIONAL METHOD The geometry optimization and harmonic frequency analysis of the metal-nuclear complexes were performed using the hybrid density functional theory (DFT) B3LYP,9 together with the Stuttgart/Dresden effective core potentials (MWB)10 for the metal and P atoms and the double-ζ basis set 6-31G(d)11 for other atoms. The solvation effect in acetonitrile was taken into account in terms of the conductor polarizable continuum model (C-PCM) with the universal force field (UFF) approach.12 The UV−vis spectra of the complexes were calculated using the time-dependent DFT (TDDFT) method13 at the optimized geometry of individual system. In order to improve the accuracy, the basis set errors were corrected by using the extrapolation scheme14,15 Received: March 25, 2014 Revised: July 7, 2014 Published: July 9, 2014 5400

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Figure 1. Convention used for the metal-tppz ligand bonding configurations where only a binuclear subunit of the whole trinuclear PtII/RuII complex is shown.

EX = ECBS + b/(X + β)3

molecules but the others are chiral. The RS/SR can be obtained by two sequential operations, the xz-plane mirror imaging and the C2(y) rotation (if three metal atoms are coincident with the z-axis). For their structures, please see Figure S1 in Supporting Information. Following the experimental conditions, we have also examined the reaction

(1)

where EX are the B3LYP energies calculated at the 6-31G(d) (X = 2) and 6-311G(d,p) (X = 3, including the P atoms) basis sets. The final energy at the complete basis set (CBS) limit was denoted as ECBS. The parameter β was used to compensate the approximate cardinal number (X) for the basis sets. It was determined by the electronic energies of the optimized (Pt(tpy)Cl)PF6 (tpy = 2,2′,6′,2′-terpyridine) complex in the acetonitrile solution, where the CBS energy was obtained with the large 6-311+G(3df,3pd) basis set.16 The parameter was determined as β = −0.2559138. Electronic circular dichroism (ECD) of the complexes was simulated using the rotational strength for the g → e transitions.17 They were calculated by18,19 R theo =

1 Im 2mc

̂ ψe dτ ∫ ψgμeleĉ ψe dτ·∫ ψgμmag

PtCl 2 + [RuPt(tppz)2 Cl]3 + ↔ [RuPt 2(tppz)2 Cl 2]4 + + Cl−

(3)

in acetonitrile. Relative reaction electronic energies are shown in Figure 2. The forward reaction is barrierless. One can see

(2)

where yg/e are the ground (g) and excited (e) electronic wave functions, and μ̂elec/mag are the electronic (elec) and magnetic (mag) dipoles. All electronic structure calculations were done using the G09 program package.20

III. RESULTS AND DISCUSSION III. A. Structure and Emission Spectrum. A free tppz molecule has 14 conformers that are separated by low ring torsion barriers (about 2.5−6.0 kcal/mol).21 However, there are only three conformers of the binuclear subunit [Ru(tppz)PtCl] when a tppz ligand bonds with two metal atoms: RuII and PtIICl. Figure 1 shows the subunit structures labeled as R, S, and B. Chiral species R and S are enantiomers with identical electronic energy, and the B conformer (butterfly) is nonchiral because of σv symmetry. It was found that the ring torsion barrier of tppz in the metal complexes is very high (about 65.0 kcal/mol). The high barrier results from the geometric constraints of the two pairs of the closest hydrogen atoms in tppz. In other words, isomerization of the R, S, and B conformers is unlikely to occur by a low-energy thermal ring torsion process. The trinuclear metal complex [RuPt2(tppz)2Cl2]4+ is then formed by two subunits connecting via the center Ru atom as [ClPt(tppz)Ru(tppz)PtCl]4+. Therefore, one can obtain two pairs of enantiomers (RR/SS, RB/SB) in addition to RS and BB. Calculations show that all of them have an equilibrium geometry with three metal atoms being (or nearly) in a line, among which RR/SS and RS have C2 symmetry (i.e., three metal atoms are in a row). Both RS and SR are achiral

Figure 2. Bond strengths between Pt II and tppz in the [RuPt2(tppz)2Cl2]4+ complexes in acetonitrile, calculated with the B3LYP/CBS method.

that the bond energy between PtII and tppz is very strong (about 80−90 kcal/mol). Hence, the metal−ligand bonds of the complexes are hardly broken in acetonitrile. Among those conformers, RR (or SS) and RS are the most stable. The butterfly configuration of tppz produces a nonplanar constraint energy of about 5.6 kcal/mol. As discussed above, owing to the strong H/H repulsive forces, those conformers are interconverted only with difficulty. Compared with the metal−ligand bond strength of the [RuPt2(tppz)2Cl2]4+ complex, the energy differences among the conformers are rather small. This may imply that the relative concentrations of the conformers (or enantiomers) are determined by thermal dynamics in isotropic syntheses. If so, by using the calculated binding energies of the conformers, one can estimate their relative concentrations to be about 1.0:1.0:0.94:0.88 for RR(SS):RS:RB(SB):BB. The enantiomer 5401

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pairs (RR/SS or RB/SB) have the same energy and will be produced with the same yield. Indeed, the fact that experimental samples contain a mixture of all conformers is supported by the phosphorescence emission spectra as shown in Figure 3. The theoretical

Figure 4. Relative B3LYP/CBS electronic energies of the species [RuPt2(tppz)2Cl2](PF6)n in acetonitrile.

the most stable form is RR-[RuPt2(tppz)2Cl2](PF6)4 in which the core cation remains in C2 symmetry. Table 1 shows a basis set effect on the energetics of reaction 4. The basis set error corrections are about 10 kcal/mol for

Figure 3. Comparison of calculated emission spectrum with the experiment4 at 77 K, where the sticks represent the vertical de-excited energies (>800 nm) of the triplet states, and the adiabatic emission bands (80 kcal/ mol). It was found that the isomerization barriers among those conformers and/or enantiomers are also high (about 65 kcal/ mol) owing to the repulsive H/H geometrical constraints in the ligands. In addition, we have simulated the emission and UV− vis spectra of the trinuclear complex. The simulated spectra are in good agreement with the experiments. In this research we have largely focused on exploring the origin of anomalous electronic circular dichroism spectra of the RuPt2(tppz)2Cl2(PF6)4 complex. It was discovered that the unusual ECD spectrum can be produced by the RSRuPt2(tppz)2Cl2(PF6)x (x = 1−3) complexes in acetonitrile although the origin RS-RuPt2(tppz)2Cl2 core is not chiral. Here the PF6− ion has a weak interaction (7.6 or 2.5 kcal/mol without/with the zero-point energy, thermal energy, and entropy corrections at 298 K) with the [RuPt2(tppz)2Cl2]4+ cation, and causes the loss of C2 symmetry of the [RS]4+ cation. As a result, the perturbed RS-complex becomes a chiral species. This finding provides the first explanation for the anomalous experimental ECD spectra recently observed by Zhao and Brewer. Finally, we have also proposed a general physical mechanism for this type of electronic circular dichroism for a chiral helix object with a transition state (TS) having a C2 symmetry. In order to yield nonvanishing ECD, it requires three conditions: (1) a pair of electronically excited states with a conical intersection at the TS geometry, (2) the optical transitions between the ground and these two excited states are allowed, and (3) the barrier height on the ground electronic state is small (comparable with the vibrational frequency along the isomerization reaction pathway). However, the transition state is not necessarily a chiral species. Based on this model, we have predicted two new aspects: the Mexican hat-like line shape and the enhancement of ECD. Hopefully, the proposed mechanism can be further tested and verified in the near future. The research also provides an ideal model for searching for parity violation in electronic spectra.22,23

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