Synthesis and Light-Harvesting Functions of Ring-Shaped Re(I) Trinuclear Complexes Connected with an Emissive Ru(II) Complex

Abstract

Ring-shaped Re(I) multinuclear complexes (Re(I) rings) in which Re(I)-diimine-biscarbonyl complexes are connected to each other through bisphosphine bridging ligands exhibit very suitable photophysical and electrochemical properties as redox photosensitizers. We developed two approaches for synthesizing Re(I) rings connected with a Ru(II) complex: cyclization of a linear Re(I) trinuclear complex connected with a Ru(II) complex and Mizoroki–Heck coupling of a ring-shaped Re(I) trinuclear complex and a Ru(II) complex. Photophysical measurements of these heteromultinuclear complexes and comparisons with their model complexes indicated that they exhibit efficient light-harvesting abilities, where energy transfer from the excited ring-shaped Re(I) trinuclear complex unit to the Ru(II) complex unit proceeds efficiently.

Introduction

The systematic integration of emissive polypyridyl d⁶-transition-metal complexes potentially affords useful photophysical properties and/or photochemical behaviors that cannot be achieved using the corresponding mononuclear complexes, such as control of the direction and speed of excitation energy and/or electron transfer, photochemical multielectron accumulation, and good performance as redox photosensitizers or photocatalysts.^1,2 As typical examples, wheel-type assemblies³ and dendrimers^4,5 constructed with Ru(II) trisdiimine complexes; multiporphyrin arrays with Re(I) or Ru(II) ions;⁶⁻⁸ and linear-,⁹ square-,¹⁰⁻¹⁷ and ring-shaped¹⁸⁻²³ multinuclear Re(I) complexes have been reported. Among them, ring-shaped Re(I) multinuclear complexes (Re(I) rings) in which Re(I)-diimine-biscarbonyl complexes are connected to each other through bisphosphine bridging ligands exhibit unique photophysical and electrochemical properties,¹⁸⁻²⁰e.g., strong luminescence from the triplet metal-to-ligand charge transfer (³MLCT) excited state (emission quantum yield as high as 66%),²⁰ a much longer lifetime (as high as 7.77 μs)²⁰ and stronger oxidation power of the ³MLCT excited state compared with those of the corresponding mononuclear and linear-shaped multinuclear Re(I) complexes, and stability of both the excited state and the reduced state. Because of their excellent redox photosensitizer ability, the Re(I) rings have been used in the photocatalytic reduction of CO₂. For example, a photocatalytic system comprising a trinuclear Re(I) ring as a redox photosensitizer and a Re(I) mononuclear complex as a catalyst exhibits the highest quantum yield of CO₂ reduction in the reported photocatalytic systems (Φ_CO = 82%).^18,20 In addition, tetranuclear Re(I) rings can photochemically accumulate as many as four electrons per molecule and their reduced state is stable.²¹ These useful photofunctions are reinforced by interligand π–π interaction between the diimine ligands and phenyl groups in the phosphine ligands.²⁴⁻²⁷ Such interaction is substantially enhanced when the multinuclear complexes are cyclized because of the rigidity of the molecular structure and the limited rotation along the P–Re–P bonds.^18,19

The synergistic connection of different photofunctional complexes to form heteromultinuclear complexes is a useful approach to creating new photofunctions that cannot be achieved using a simple mixture of the corresponding model complexes. The Re(I) rings should be fascinating candidates for such heteromultinuclear complexes because of their aforementioned unique photophysical and photochemical properties.

Therefore, synthetic methods to introduce into the Re(I) rings another metal complex that does not affect the Re(I) rings’ steric structures should be developed to create a new class of photofunctional molecules.

We herein report two methods for synthesizing such Re(I) rings connected with a Ru(II) trisdiimine complex²⁸ which is another type of useful photofuctional metal complex ([Ring-Run]⁵⁺): (1) cyclization of linear-shaped Re(I) trinuclear complexes already connected to the Ru complex and (2) the Mizoroki–Heck reaction between a ring-shaped Re(I) trinuclear complex and a Ru(II) complex. The synthesized [Ring-Run]⁵⁺’s exhibit excellent light-harvesting functions: light energy absorbed by the Re(I)-ring unit is efficiently transferred to the Ru unit. Figure 1 illustrates the structures of the synthesized [Ring-Run]⁵⁺’s (n = 1, 2, 3) and the corresponding model complexes.

Figure 1.

Structures and abbreviations of [Ring-Run]⁵⁺’s and the model complexes. All are PF₆^– salts.

Results and Discussion

Synthesis

(1) Cyclization of Linear Re(I) Trinuclear Complexes Connected with a Ru Complex

To substitute both terminal CO ligands in the linear-shaped Re(I) trimer complexes, which is one of the critical processes for synthesizing the target complexes, we recently developed a selective method for removing the carbonyl ligand only in the trans-position to the phosphine ligand in fac-[Re(N^N)(CO)₃(PR₃)]⁺ (N^N = diimine ligand) using Me₃NO as a decarbonylation reagent.^20,23 Notably, the photochemical ligand substitution reaction,^18,29 which is well-known as the other method to remove CO ligands, cannot be used in the case of linear Re(I) multinuclear complexes with a Ru(II) unit because efficient energy transfer proceeds from the excited Re units to the Ru unit.³⁰ As a typical example, the synthetic scheme for [Ring-Ru1]⁵⁺ is illustrated in Scheme 1. A linear Re trimer in which one of the edge Re moieties is connected with a Ru(II) complex ([L3(4dmb)(trans-CO)₂-Ru(4dmb,bpy₂)]⁵⁺, 4dmb = 4,4′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine) was synthesized via the reaction between the Re dimer with a labile ligand and a Ru–Re heterodinuclear complex in which the Re unit has a bisphosphine moiety as a monodentate ligand (process A in Scheme 1). To an acetone solution of the isolated [L3(4dmb)(trans-CO)₂-Ru(4dmb,bpy₂)]⁵⁺(PF₆)₅, 2 equiv of Me₃NO was added at room temperature (process B). The solution immediately became dark brown, indicating rapid substitution of CO with a solvent molecule and/or Me₃N. Panels a and b of Figure S1 show electrospray-ionization (ESI) mass spectra of the reaction solution before and after the addition of Me₃NO, respectively. In this reaction, two carbonyl ligands in the trans-positions to the dppph ligands (dppph = 1,4-bis(diphenylphosphino)benzene), which are localized in both edge Re units, were selectively substituted, whereas the other carbonyl ligands and the Ru unit were not affected. One equivalent of dppph was added to the reaction solution, which was subsequently refluxed for 24 h, giving the target molecule [Ring-Ru1]⁵⁺ as the major product (process C), as indicated by size-exclusion chromatography (SEC) analysis of the reaction solution (Figure S1c).³¹ Through recycling size-exclusion chromatography (RSEC) and recrystallization, [Ring-Ru1](PF₆)₅ was isolated in 44% isolated yield. Its ¹H NMR spectrum is shown in Figure 2. Similar procedures were applied to synthesize [Ring-Ru2](PF₆)₅ using [L3(5dmb)(trans-CO)₂-Ru(5dmb₃)](PF₆)₅ (Scheme S1, 5dmb = 5,5′-dimethyl-2,2′-bipyridine) instead of [L3(4dmb)(trans-CO)₂-Ru(4dmb,bpy₂)](PF₆)₅; the isolated yield was 38%.

Scheme 1. Synthesis of [Ring-Ru1]⁵⁺ via Decarbonylation with Me₃NO and Subsequent Cyclization with Bidentate Phosphine Ligands.

Figure 2.

Aromatic region of the ¹H NMR spectrum of [Ring-Ru1]⁵⁺ in CD₃CN. The same numbering of the diimine moieties for the Ru complex is applied for the Re-ring.

(2) The Mizoroki–Heck Reaction between a Ring-Shaped Re(I) Trinuclear Complex and a Ru(II) Complex

We recently reported that a Pd-mediated Mizoroki–Heck reaction can be used to couple two different metal complexes that have either a bromo or vinyl group on the diimine ligands.³²⁻³⁵ We used this reaction as an alternative route for the synthesis of another ring-shaped Re(I) trinuclear complex connected with a Ru(II) complex [Ring-Ru3]⁵⁺, as illustrated in Scheme 2. A ring-shaped Re(I) trinuclear complex with two 5dmb and one 4-bromo-2,2′-bipyridine (Brbpy) ligands and with the Re(I) centers connected to each other by 1,2-bis(diphenylphosphino)ethane (dppet) ([(R3et(5dmb₂,Brbpy)]³⁺) was first synthesized as a precursor using Me₃NO for the decarbonylation followed by the cyclization step (Scheme S2).^23,36 The Mizoroki–Heck reaction between [R3et(5dmb₂,Brbpy)]³⁺ and 2 equiv of [Ru(4dmb)₂(allyl-4dmb)]²⁺ proceeded in an MeCN solution containing 0.6 equiv of Pd(OAc)₂ and 5 equiv of AcONa under reflux. Monitoring of the reaction solution using SEC showed that the reaction proceeded to a saturation point after 1 day even though both [R3et(5dmb₂,Brbpy)]³⁺ and [Ru(4dmb)₂(allyl-4dmb)]²⁺ remained in the reaction solution. At this point, black Pd particles precipitated in the solution. To promote further progress of the coupling, the reaction mixture was transferred to a fresh solution via a three-step procedure: removal of the precipitates by filtration, solvent evaporation, and then loading of new solvent and catalyst (in the same amount as the initial conditions). This solution was again refluxed for 48 h. These operations were repeated twice, resulting in almost complete consumption of [R3et(5dmb₂,Brbpy)]³⁺, as shown in the SEC chromatograms in Figure S2, where the target compound ([Ring-Ru3]⁵⁺) and the starting Ru(II) complex were mainly detected. The yield of [Ring-Ru3]⁵⁺ was estimated to be 64% on the basis of the amount of [R3et(5dmb₂,Brbpy)]³⁺ consumed, as determined by SEC analysis of the reaction solution. Purification using RSEC resulted in an isolated yield of 40%. Notably, the excess amount of [Ru(4dmb)₂(allyl-4dmb)]²⁺ was necessary to suppress a competitive reaction, i.e., homocoupling of [R3et(5dmb₂,Brbpy)]³⁺.^34,36

Scheme 2. Synthesis of [Ring-Ru3]⁵⁺ by the Mizoroki–Heck Reaction.

These synthesized ring–Ru complexes [Ring-Run]⁵⁺s (n = 1, 2, 3) are the first examples of ring-shaped Re(I) multinuclear complexes modified with another metal complex. They were fully characterized by NMR, infrared (IR), and high-resolution mass spectroscopies. The corresponding trinuclear Re(I) rings without the Ru unit and the mononuclear Ru(II) trisdiimine complexes shown in Figure 1 were also synthesized as model compounds for clarifying photophysical processes of the ring–Ru complexes, as described in the next section.

Photophysical Properties

Figure 3 shows UV–Vis absorption spectra recorded in MeCN. The absorption spectra of all of the [Ring-Run]⁵⁺ (n = 1, 2, 3) complexes were similar to a 1:1 summation spectrum of the singlet metal-to-ligand charge transfer (¹MLCT) absorption bands of the Re(I) ring and Ru(II) model complexes (i.e., [Ring1]³⁺ + [Ru1]²⁺, [Ring2]³⁺ + [Ru2]²⁺, and [Ring3]³⁺ + [Ru3]²⁺, respectively). These results indicate that no strong electronic interaction occurred between the Re ring and Ru units. Notably, in the spectra of all of the [Ring-Run]⁵⁺, the ¹MLCT absorption band of the Ru(II) unit was observed at longer wavelengths compared with that of the Re(I)-ring unit.

Figure 3.

UV–vis absorption spectra of samples in MeCN: (a) [Ring-Ru1]⁵⁺ (blue), [Ring1]³⁺ (green), and [Ru1]²⁺ (red); (b) [Ring-Ru2]⁵⁺ (blue), [Ring2]³⁺ (green), and [Ru2]²⁺ (red); (c) [Ring-Ru3]⁵⁺ (blue), [Ring3]³⁺ (green), and [Ru3]²⁺ (red). The black dashed lines show corresponding 1:1 summation spectra of [Ringn]³⁺ + [Run]²⁺ (n = 1, 2, 3).

All of the [Ring-Run]⁵⁺ complexes are emissive in MeCN at ambient temperature. The photophysical properties of [Ring-Run]⁵⁺ and the corresponding model complexes are summarized in Table 1. Figure 4a shows the emission spectra of [Ring-Ru1]⁵⁺ as a typical example; these spectra were recorded using two different excitation wavelengths. The shape of the emission band varied according to the excitation wavelength: a shoulder was observed at λ_em ≈ 550 nm when [Ring-Ru1]⁵⁺ was excited at λ_ex = 380 nm, whereas the shoulder decreased in intensity when the complex was excited at λ_ex = 469 nm. Mainly the Re ring unit and partially the Ru unit absorb the light at λ_ex = 380 nm, whereas the light at λ_ex = 469 nm is mainly absorbed by the Ru unit (Figure 3a). The maximum emission wavelength (λ_em^max) was 622 and 624 nm when the complex was excited at λ_ex = 380 and 469 nm, respectively. Therefore, even though the excitation light should be absorbed mainly by the Re ring unit of [Ring-Ru1]⁵⁺, the maximum emission wavelength was similar to that of [Ru1]²⁺ (λ_em^max = 630 nm) but not to that of [Ring1]³⁺ (λ_em^max = 581 nm) (Figure 4a). The emission spectrum of [Ring-Ru1]⁵⁺ recorded with λ_ex = 380 nm was well-simulated with the emission spectra of [Ru1]²⁺ and [Ring1]³⁺ (Figure 4b). The simulation also reveals that the main component of the emission from [Ring-Ru1]⁵⁺ should be from the Ru unit but not from the Re ring unit and that the shoulder peak at λ_em = ∼550 nm in the emission band of [Ring-Ru1]⁵⁺ can be assigned to emission from the Re ring unit.

Table 1. Photophysical Properties of [Ring-Run]⁵⁺ and Their Model Complexes Measured in Degassed MeCN at 25°C.

complex	λabsa (nm)	εb (×103 M–1 cm–1)	λemc (nm)	Φem (λex/nm)	τ1d (μs)	τ2d (μs)	τ3d (μs)
[Ring-Ru1]5+	424	23.1	622	0.11 ± 0.004 (380)	0.016 ± 0.009	0.51 ± 0.007	0.73 ± 0.007
[Ring1]3+	401	11.9	581	0.154 ± 0.001 (380)	1.68 ± 0.005
[Ru1]2+	454	15.5	630	0.091 ± 0.001 (380)	0.75 ± 0.003
[Ring-Ru2]5+	419	20.5	603	0.063 ± 0.003 (380)	0.004 ± 0.001	0.22 ± 0.01	0.46 ± 0.01
[Ring2]3+	398	11.4	553	0.44 ± 0.006 (380)	3.40 ± 0.01
[Ru2]2+	444	15.1	604	0.041 ± 0.002 (380)	0.31 ± 0.001
[Ring-Ru3]5+	427	22.0	633	0.060(380) 0.086 (460)	0.023e	0.14e	0.98e
[Ring3]3+	412	12.3	622	0.042 ± 0.002 (390)	0.13e	0.84e	4.8e,f
[Ring4]3+g	409		571	0.41	5.4
[Ru3]2+	458	14.9	633	0.089h	0.85 ± 0.004

^aMaximum wavelength of the MLCT bands.

^bMolar extinction coefficients at λ_abs.

^cThe values of [Ring-Run]⁵⁺ depend on λ_ex; see main text.

^dThe emission decay curves were fitted by a single- to triple-exponential function I = ΣA_nexp(−t/τ_n), within proper χ² values (1.0–1.1).

^eThe lifetimes were determined by using the decay functions constructed as the numerical solutions of the set of rate equations and the “FindMinimum procedure” in Mathematica, which is described in the main text in detail.

^fτ₃ was a very minor component in the emission and is likely emission from [Ring4]³⁺ as an impurity.

^gRef (18).

^hRef (37).

Figure 4.

(a) Normalized emission spectra of [Ring-Ru1]⁵⁺ in degassed MeCN measured using various wavelength excitation light: λ_ex = 380 nm (light blue) and λ_ex = 469 nm (blue dotted line). The emission spectra of [Ring1]³⁺ (green) and [Ru1]²⁺ (red) are also illustrated. (b) Emission spectrum of [Ring-Ru1]⁵⁺ (sky blue) and simulation of spectral deconvolution into individual emissions of [Ring1]³⁺ (green) and [Ru1]²⁺ (red) and their sum (black dotted line).

The emission quantum yield (Φ_em) was determined to be 11% (λ_ex = 380 nm), which is slightly greater than that of [Ru1]²⁺ (Φ_em = 9%) but less than that of [Ring1]³⁺ (Φ_em = 15%). These results clearly indicate that energy transfer efficiently proceeded from the excited Re ring unit to the Ru unit in [Ring-Ru1]⁵⁺. Figure 5a shows an excitation spectrum of [Ring-Ru1]⁵⁺ measured at λ_em = 620 nm which is the emission maximum of [Ru1]²⁺. Its similarity to the absorption spectrum of [Ring-Ru1]⁵⁺ also suggests that the energy transfer from the excited Re ring unit to the Ru unit is efficient. This interpretation is reasonable because the lowest excited state of the Re ring unit has an excitation energy higher than that of the Ru unit.

Figure 5.

Normalized absorption (light blue line) and excitation spectra (black dotted line) of (a) [Ring-Ru1]⁵⁺ and (b) [Ring-Ru2]⁵⁺ in degassed MeCN. Each detection wavelength of excitation spectra is (a) 620 nm and (b) 600 nm, respectively.

The efficiency of the energy transfer (η_ET), which indicates the percentage of the excited Re ring units that can transfer their excitation energy to the Ru unit, can be estimated by using eq 1:

1

where A_{sim(Ring-Run)} and A_em(Ringn) are the areas of the emission spectra (normalized by the absorbed photon number at the excitation wavelength, λ_ex) associated with the Re ring unit in the emission spectra of [Ring-Ru1]⁵⁺ and [Ring1]³⁺, respectively, and ε_(Ringn) and ε_(Run) are the molar absorption coefficients of [Ringn]³⁺ and [Run]²⁺ at λ_ex, respectively. The emission spectrum of [Ring-Ru1]⁵⁺ excited at λ_ex = 380 nm was deconvoluted using the emission spectra of [Ring1]³⁺ and [Ru1]²⁺ to estimate A_{sim(Ring-Ru1)} (Figure 4b). From these analyses, η_ET in the case of [Ring-Ru1]⁵⁺ was estimated to be 78%. Notably, such efficient intramolecular energy transfer is caused by the covalent linkage between the Re ring and the Ru complex units, as confirmed by energy transfer not being observed for an equimolar mixed solution of the model Re ring ([Ring1]³⁺) and the model Ru mononuclear complex ([Ru1]²⁺) (Figure S3).

We also investigated this excitation energy-transfer event in [Ring-Ru1]⁵⁺ by measuring the emission lifetimes and recording the corresponding transient emission spectra. Figure 6a shows the transient emission spectra of [Ring-Ru1]⁵⁺ acquired using the single-photon counting method. In the initial stage, rapid decay was observed at 480 ≤ λ_em ≤ 560 nm, and an increase of the emission intensity in the longer-wavelength region (λ_em > 570 nm) was observed in this time scale. The shape of the spectrum became similar to that of [Ru1]²⁺ within ∼20 ns after laser excitation. After this rapid change of the spectrum shape, the emission intensity at all wavelengths gradually decreased, accompanied by slow changes in the spectrum shape; that is, a slightly more rapid decay was observed at shorter wavelengths. Figure 6b shows time profiles of the emission at λ_em = 520 and 650 nm (λ_ex = 380 nm). The decay curve recorded at λ_em = 520 nm (red) shows a two-step decrease, whereas the emission intensity at λ_em = 650 nm (blue) increases in the first stage and then decreases monotonically. As previously mentioned, the main component of the emission at λ_em = 520 and 650 nm is derived from the Re(I) ring unit and the Ru unit, respectively. Therefore, the increase in emission at λ_em = 650 nm also indicates efficient energy transfer from the excited Re ring unit to the Ru unit. The time profiles could be fitted with a triple-exponential function; the obtained lifetimes are τ₁ = 16 ns, τ₂ = 0.51 μs, and τ₃ = 0.73 μs, and the pre-exponential factors strongly varied depending on the detection wavelength. The longest-lifetime component (τ₃) is attributable to emission from the Ru(II) unit because of its similarity to the lifetime of [Ru1]²⁺ (τ_em = 0.75 μs) and the larger pre-exponential factor corresponding to the longer-wavelength region. Therefore, the two shorter lifetimes (τ₁ and τ₂) are attributed to emissions from the Re ring unit, both of which are shorter than that of [Ring1]³⁺ (τ_em = 1.68 μs). This difference is mainly caused by the excitation energy transfer from the Re ring unit to the Ru unit, as previously described. The two types of emission with different lifetimes is attributable to two different Re moieties in [Ring-Ru1]⁵⁺, i.e., the node-Re moiety connected with the Ru unit directly and the other peripheral Re moieties with a 4dmb ligand. Although energy migration among these Re moieties might proceed, it should be slower than the energy transfer from the excited Re ring unit to the Ru unit because the difference between τ₁ and τ₂ is large even though the excitation energy of each Re moiety is the same or very similar. If the energy migration among the Re moieties is assumed to not affect the photophysical properties of [Ring-Ru1]⁵⁺, the rates of energy transfer can be calculated on the basis of the emission lifetimes (Table 1) and eq 2 as k_ET1 = 6.2 × 10⁷ s^–1 and k_ET2 = 1.4 × 10⁶ s^–1 (Table 2).

2

where τ_{([Ringn]³⁺)} is the lifetime of [Ringn]³⁺, e.g., 1.68 μs (n = 1). The efficiency of the energy transfers was estimated to be η_ET1 = 99% and η_ET2 = 70%, respectively, using eq 3:

3

The weighted average of η_ET1 and η_ET2 (η_ET1 × 1/3 + η_ET2 × 2/3 = 79%) well matches the value estimated by fitting the emission spectrum of [Ring-Ru1]⁵⁺ (78%), as previously described. We reasonably assumed that the node-Re moiety connected with the Ru unit was quenched more efficiently than the other Re moieties with 4dmb because of the direct connection via the ethylene chain and the short distance to the Ru unit. These excitation energy-transfer events of [Ring-Ru1]⁵⁺ are summarized in Figure 7a.

Figure 6.

(a) Transient emission spectra of [Ring-Ru1]⁵⁺ measured in MeCN with λ_ex = 379 nm. (b) Emission decay curves of [Ring-Ru1]⁵⁺ recorded at 520 nm (red) and 650 nm (blue).

Table 2. Energy-Transfer Rate Constants and Efficiencies in the [Ring-Run]⁵⁺ Complexes in Degassed MeCN at 25°C.

		k (×106 s–1)b
complex	ηETa (%)	ET1c	ET1′d	ET2e	ET0f	ηET1g (%)	ηET2g (%)	ηETAveh (%)
[Ring-Ru1]5+	78	62	–	1.4	–	99	70	79
[Ring-Ru2]5+	95	250	–	4.3	–	>99	94	96
[Ring-Ru3]5+	–	4.0	0.56	52	13	–i	>99	–
[Ring3]3+	98j	–	–	–	7.6	–	–	–

^aEnergy-transfer efficiency calculated on the basis of steady-state emission experiments (eq 1).

^bRate constants of energy transfer calculated according to eq 2 or eqs 4–6.

^cThe energy-transfer rate constant from the Re(n) unit to the Ru unit.

^dThe energy-transfer rate constant from the Ru unit to the Re(n) unit.

^eThe energy-transfer rate constant from the Re(p) unit to the Ru unit.

^fThe energy-transfer rate constant from the Re(p) to the Re(n) unit.

^gEnergy-transfer efficiency calculated on the basis of emission lifetimes (eq 3).

^hWeighted average of η_ET1 and η_ET2.

ⁱThe concentration ratio between the excited Ru unit and the excited Re(n) unit in the excited [Ring-Ru3]⁵⁺ was approximately 6.7:1 after equilibrium was reached.

^jThe efficiency of the intraring energy transfer.

Figure 7.

Energy-transfer processes and their rate constants in (a) [Ring-Ru1]⁵⁺ and (b) [Ring-Ru2]⁵⁺.

Similar photophysical phenomena were observed in the case of [Ring-Ru2]⁵⁺ (Figures 5b and 8 and Table 1); that is, the maximum emission wavelength and the quantum yield (λ_em^max = 603 nm, Φ_em = 6.3%) are similar to those of [Ru2]²⁺ (λ_em^max = 604 nm, Φ_em = 4.1%) and dramatically different from those of [Ring2]³⁺ (λ_em^max = 553 nm, Φ_em = 44%). The excitation (Figure 5b: λ_em = 600 nm, close to the emission maximum of [Ru2]²⁺) and absorption spectra of [Ring-Ru2]⁵⁺ were similar to each other. The energy-transfer efficiency in the case of [Ring-Ru2]⁵⁺, which was determined from spectrum-fitting using the spectra of [Ru2]²⁺ and [Ring2]³⁺ in the same manner as for [Ring-Ru1]⁵⁺ (eq 1), was greater (η_ET = 95%) than that of [Ring-Ru1]⁵⁺ (η_ET = 78%).

Figure 8.

(a) Normalized emission spectra of [Ring-Ru2]⁵⁺ in degassed MeCN measured using excitation light with various wavelengths: λ_ex = 380 nm (sky blue) and λ_ex = 456 nm (blue dotted line). The emission spectra of [Ring2]³⁺ (green) and [Ru2]²⁺ (red) are also illustrated. (b) Emission spectrum of [Ring-Ru2]⁵⁺ (sky blue) and spectral deconvolution into individual emissions of [Ring2]³⁺ (green) and [Ru2]²⁺ (red) and their sum (black dotted line).

When [Ring-Ru2]⁵⁺ was excited at λ_ex = 379 nm, the transient emission spectra of [Ring-Ru2]⁵⁺ also showed two-step decay at λ_em < 570 nm (Figure 9). This stepwise decay was likely caused by different rates of intramolecular energy transfer from the two different Re moieties, similar to the case of [Ring-Ru1]⁵⁺. The decay profiles were fitted with a triple-exponential function with τ₁ = 4 ns, τ₂ = 220 ns, and τ₃ = 460 ns. Because the pre-exponential factor of τ₃ was dominant at λ_em ≈ 700 nm, this longest component is attributable to emission from the Ru unit.

Figure 9.

(a) Transient emission spectra of [Ring-Ru2]⁵⁺ measured in MeCN with λ_ex = 379 nm. (b) Emission decay curves of [Ring-Ru2]⁵⁺ recorded at 520 nm (red) and 650 nm (blue).

The energy-transfer rates in [Ring-Ru2]⁵⁺ were calculated as k_ET1 = 2.5 × 10⁸ s^–1 and k_ET2 = 4.3 × 10⁶ s^–1 using τ₁ and τ₂; these results are attributable to the lifetimes of the Re(I) ring unit (eq 2). These values indicate that the emission from each Re unit in the Re ring unit was quenched almost quantitatively (η_ET1 > 99%, η_ET2 = 94%). The weighted average of η_ET1 and η_ET2 (η_ET1 × 1/3 + η_ET2 × 2/3 = 96%) is similar to the value estimated by fitting the emission spectra using eq 1 (95%). The reason for the more efficient intramolecular energy transfer in [Ring-Ru2]⁵⁺ compared with that in [Ring-Ru1]⁵⁺ is discussed below. The energy-transfer processes and the corresponding rate constants are summarized in Figure 7b.

Figure 10a shows the emission spectra of [Ring-Ru3]⁵⁺. The emission behavior of [Ring-Ru3]⁵⁺ differs somewhat from those of [Ring-Ru1]⁵⁺ and [Ring-Ru2]⁵⁺. The emission spectrum of [Ring-Ru3]⁵⁺ differs from that of [Ru3]²⁺ and could be well fitted using the emission spectra of both [Ring3]³⁺ and [Ru3]²⁺. Because the Re complex unit with the bridging diimine ligand of [Ring-Ru3]⁵⁺ has a vinyl group that conjugates with the diimine moiety, the π* energy level of the orbitals of this bridging diimine ligand should be lower than those of [Ring-Ru1]⁵⁺ and [Ring-Ru2]⁵⁺. Actually, the emission maximum of [Ring3]³⁺, which has one Re complex unit with a conjugated vinyl group, was red-shifted and closer to that of [Ru3]²⁺ (Figure 10a) compared with [Ring1]³⁺ and [Ring2]³⁺. Because of this difference, [Ring-Ru3]⁵⁺ exhibited different photophysical behaviors from [Ring-Ru1]⁵⁺ and [Ring-Ru2]⁵⁺. The contributions of both model complexes to this fitting were similar to each other (Figure 10b); that is, the contribution of the model of the Re ring unit, i.e., [Ring3]³⁺, was much larger than that of the Re ring models in the fittings of [Ring-Ru1]⁵⁺ and [Ring-Ru2]⁵⁺ (Figures 4b and 8b).

Figure 10.

(a) Normalized emission spectra of [Ring-Ru3]⁵⁺ in degassed MeCN, as measured with excitation light of various wavelengths: λ_ex = 380 nm (sky blue) and λ_ex = 460 nm (blue dotted line). The emission spectra of [Ring3]³⁺ (green), [Ru3]²⁺ (red), and [Ring4]²⁺ (yellow green) are also illustrated. (b) Emission spectrum of [Ring-Ru3]⁵⁺ (blue, λ_ex = 380 nm) and simulation of spectral deconvolution into individual emission spectra of [Ring3]³⁺ (green) and [Ru3]²⁺ (red) and their sum (black dotted line).

Before investigating the properties of the excited [Ring-Ru3]⁵⁺, we carefully checked the photophysical behaviors of [Ring3]³⁺. Notably, this analyzed sample contained a minimal amount of [Ring4]³⁺, which could not be removed and was detected as small peaks in the ESI mass spectrum (Figure S4). However, the effects of this impurity on the photophysical properties of the sample are easily corrected for, as described later. The transient emission spectra of [Ring3]³⁺ show a gradual red-shift of the maximum emission wavelength from λ_max = 605 nm (5 ns) to λ_max = 635 nm (1000 ns) (Figure 11a). The emission decay measured at λ_det = 550 nm was drastically faster than that at λ_det = 650 nm (Figure 11b). These results strongly suggest that intramolecular energy transfer proceeded among the Re complex units in [Ring3]³⁺. The emission decay curves could be fitted with a triple-exponential function; the obtained lifetimes were τ₁ = 130 ns, τ₂ = 840 ns, and τ₃ = 5 μs, respectively. Because τ₃ was a minor component in the emission, it likely originated from the [Ring4]³⁺ impurity because τ₃ is very similar to the emission lifetime of [Ring4]³⁺ (τ_em = 5.4 μs). Therefore, the emission lifetimes from [Ring3]³⁺ itself should be τ₁ = 130 ns and τ₂ = 840 ns. The emission maximum of [Ring4]³⁺ (Figure 10a), which consists of only the Re complex units with 5dmb, was blue-shifted compared with that of [Ring3]³⁺ (Figure 10a) and the wavelength dependence of the pre-exponential factors of τ₁ of [Ring3]³⁺ (Figure S5) was similar to the emission spectrum of [Ring4]³⁺ (yellow green spectrum in Figure 10a). These results strongly suggest that τ₁ and τ₂ are the emission lifetimes associated with the Re units with 5dmb and that with 4-vinyl-bpy (vbpy), respectively, and that the excitation energy was transferred from the Re units with 5dmb to that with vbpy. The rate constant of the energy transfer in [Ring3]³⁺ (k_ET0) was calculated as k_ET0 = 7.6 × 10⁶ s^–1 using eq 2 with τ₁ and the emission lifetime of [Ring4]³⁺ (τ_{([Ring4]³⁺)} = 5.4 μs, Figure 12a). According to eq 3, the efficiency of this intraring energy transfer was η_ET = 98% (Figure 12a).

Figure 11.

(a) Transient emission spectra of [Ring3]³⁺ measured in MeCN with λ_ex = 444 nm. (b) Emission decay curves of [Ring3]³⁺ recorded at 550 nm (red) and 650 nm (blue)

Figure 12.

Energy-transfer processes and their rate constants in (a) [Ring3]³⁺ and (b) [Ring-Ru3]⁵⁺.

Figure 13a shows the transient emission spectra of [Ring-Ru3]⁵⁺. The time profiles of the emission in the shorter-wavelength region (<600 nm) show a rapid decrease within 50 ns. This behavior is attributable to energy transfer from the Re units with the 5dmb ligand to other units in [Ring-Ru3]⁵⁺. Notably, this decay rate is much higher than that of the energy transfer from the Re units with the 5dmb ligand to the Re unit with the vbpy ligand in [Ring3]³⁺. This difference strongly suggests that energy transfers from the Re units with the 5dmb ligand directly to the Ru unit as well (red arrow in Figure 12b). In addition, from 50 to 400 ns, another slower and smaller change in the spectrum shape is observed, i.e., a decrease in the wavelength region <600 nm and an increase at ∼630 nm (Figure 13b); the transient spectrum then monotonically decays without a change in the spectrum shape after 400 ns. This spectrum shape recorded after 800 ns was well-fitted with the emission spectra of [Ru3]²⁺ and [Ring3]³⁺ (Figure S6). These results indicate that another energy transfer occurred from the Re unit with the vbpy ligand to the Ru unit in the time interval to 400 ns and that the reverse energy transfer from the Ru unit to the Re unit with the vbpy ligand simultaneously proceeded (green arrows in Figure 12b). These forward and backward energy-transfer processes should arrive at an equilibrium state at 400 ns after the laser irradiation. Figure 13c shows emission decay curves of [Ring-Ru3]⁵⁺ recorded at λ_em = 550 and 650 nm as typical examples (λ_ex = 337 nm), which could be roughly fitted using a triple-exponential function with τ₁ = 23 ns, τ₂ = 140 ns, and τ₃ = 980 ns. Notably, the pre-exponential functions of τ₂ (A₂) at >600 nm were negative values because of the slower energy transfer between 50 and 400 ns after laser irradiation (Figure 13d); in this time region, the ratio between the excited Re unit with the vbpy ligand and the excited Ru unit should be greater than that in the equilibrium state because the Ru unit has a slightly longer emission maximum than the Re unit with the vbpy ligand. This result also suggests that the process with τ₂ = 140 ns involves a change in the concentration ratio between the excited Ru unit and the excited Re unit with the vbpy ligand. Because the lifetimes of the excited state of [Ru3]²⁺ and the Re unit with the vbpy ligand of [Ring3]³⁺ are similar to each other and also similar to the longest emission lifetime of [Ring-Ru3]⁵⁺ (Table 1), the emission with τ₃ = 980 ns is attributed to both the excited Ru unit and the excited Re unit with the vbpy ligand.

Figure 13.

(a) Transient emission spectra of [Ring-Ru3]⁵⁺ in MeCN with λ_ex = 337 nm. (b) Transient emission spectra of [Ring-Ru3]⁵⁺ normalized at 620 nm. (c) Emission decay curves of [Ring-Ru3]⁵⁺ recorded at 550 nm (red) and 650 nm (blue). (d) Wavelength dependence of the pre-exponential factors of τ₁ (A₁), τ₂ (A₂), and τ₃ (A₃).

To determine the exact rate constants of energy-transfer processes among the three different units in [Ring-Ru3]⁵⁺, i.e., the Ru unit, the Re units with the 5dmb ligand, and the node-Re unit directly connected with the Ru unit, we analyzed the emission decays measured at three different detection wavelengths (550, 575, and 650 nm). These emission decays are mainly associated with the Re units with the 5dmb ligand, a mixture of emissions from all of the units, and the Ru unit, respectively. We conducted the analyses using the rate equations based on the photophysical processes of [Ring-Ru3]⁵⁺ (Figure 14).

Figure 14.

Photophysical processes of [Ring-Ru3]⁵⁺. I₀ is the sum of the initial concentrations of the excited states. α_A, α_B, and α_C are the fractions of the initial concentrations. k_A, k_B, and k_C are intrinsic decay rates of the excited states. Φ_A, Φ_B, and Φ_C are intrinsic emission quantum yields of the excited states. τ_A, τ_B, and τ_C are the intrinsic emission decay lifetimes of the excited states. k_ET0, k_ET1, k_ET1′, and k_ET2 are the energy-transfer rate constant from the Re(p) (the Re units with the 5dmb ligand) to the Re(n) unit (the node-Re unit), that from the Re(n) unit to the Ru unit, that from the Ru unit to the Re(n) unit, and that from the Re(p) unit to the Ru unit, respectively.

The concentrations of the excited states of [Ring-Ru3]⁵⁺ at t seconds after excitation are abbreviated as A(t), B(t), and C(t), where the excited units are the Re unit with the 5dmb ligand (Re(p)), the Re unit directly connected to the Ru unit (Re(n)), and the Ru unit itself (Ru), respectively. These concentration changes can be formulated by the following set of rate equations (eqs 4–6):

4

5

6

The observed emission intensity I_em(λ, t) at the monitoring wavelength (λ) and time (t) was calculated using the concentrations of the excited states (J). The numerical parameters relate to their emission spectra and the spectral sensitivities of the apparatus as

7

where a(λ) is the apparatus constant of the instrument at wavelength λ, b_J(λ) the numerical constant of the emission spectral shape of the excited species J at wavelength λ, and J(λ, t) the concentration of the excited state J.

The set of rate equations was numerically solved, and the emission decays of the [Ring-Ru3]⁵⁺ measured at 550, 575, and 650 nm (Figure 15) were analyzed using the nonlinear model fit method in the Wolfram Mathematica 10 software (Wolfram Research Inc.) to minimize the deviation of the evaluated values from those in the emission decay data with the parameters, as previously described; both the precision goal and the accuracy goal were set to 10^–8, and the experimentally obtained parameters from the measurement of the model species, [Ring3]³⁺, [Ring4]³⁺, and [Ru3]²⁺ (i.e., k_A = 1.9 × 10⁵ s^–1, k_B = 1.2 × 10⁶ s^–1, and k_c = 1.0 × 10⁶ s^–1) were used. Reasonable fitting was obtained, as shown in Figure 15. The evaluated kinetic parameters are summarized along with other photophysical parameters in Table 2.

Figure 15.

Emission decay profiles of [Ring-Ru3]⁵⁺ monitored at (a) 550, (b) 575, and (c) 650 nm (red dots) with the simulated emission intensities (orange curves) and residuals (red dots). The calculated relative concentration changes of three excited states of [Ring-Ru3]⁵⁺ are presented in panel d.

Interestingly, the value of k_ET2 of [Ring-Ru3]⁵⁺, which is the energy-transfer rate from the excited Re unit with the 5dmb ligand to the Ru unit, is relatively large even though the length of the spacer between the Ru unit and the Re ring unit is the longest among the ring–Ru complexes. This result suggests that the Ru unit can collide with the Re units with 5dmb because of the flexibility of the long bridging chain. The obtained energy-transfer rate from the excited Re unit with the 5dmb ligand to the Re unit directly connected with the Ru unit in [Ring-Ru3]⁵⁺ (k_ET0 = 13 × 10⁶ s^–1) is slightly greater than that from the excited Re unit with the 5dmb ligand to the Re unit with the vbpy ligand in [Ring3]³⁺ (k_ET0 = 7.6 × 10⁶ s^–1). This difference might be caused by different conformations of the Re ring moiety in [Ring-Ru3]⁵⁺ and that of [Ring3]³⁺ because of the steric hindrance of the Ru unit and flexibility of the bridging bisphosphine ligands, i.e., dppet. The rate constant of forward energy transfer from the Re unit to the Ru unit, each of which is directly connected, was k_ET1 = 4.0 × 10⁶ s^–1, and that of the backward energy transfer was k_ET1′ = 0.56 × 10⁶ s^–1; therefore, the concentration ratio between the excited Ru unit and the excited Re(n) unit in the excited [Ring-Ru3]⁵⁺ was approximately 6.7:1 after equilibrium was established between them.

Conclusions

We developed two efficient synthetic strategies to construct covalently linked trinuclear Re(I) rings with the Ru(II)-trisdiimine mononuclear complex [Ring-Run]⁵⁺ (n = 1, 2, 3). These complexes exhibit strong absorption in the visible region because of different absorption behaviors of the Re complex and Ru complex units. They also show good light-harvesting ability because of the efficient intramolecular energy transfer among the Re units and between the Re ring and the Ru unit. Notably, both the Re rings and [Ru(diimine)₃]²⁺ complexes function as excellent redox photosensitizers (photoredox catalysts), and [Ring-Run]⁵⁺ has both complex units. Therefore, this multinuclear complex can potentially function as good redox photosensitizers with stronger visible-light absorption than the Re rings and [Ru(diimine)₃]²⁺ complexes. The synthesis methods of [Ring-Run]⁵⁺ should enable any diimine-coordinating metal complexes to be tethered to the Re ring, leading to the construction of novel Re rings with various metal complexes.

Experimental Section

Materials and Methods

Acetonitrile was dried over P₂O₅ three times and then distilled from CaH₂ prior to use. Other anhydrous solvents were purchased from commercial sources. All reactions were carried out under an inert atmosphere and in dim light. Column chromatography was performed with Silica Gel 60 (40–50 μm, Kanto Chemical Co.), alumina (standardized, Merck), or Sephadex (GE Healthcare). 2,2′-Bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (4dmb), 5,5′-dimethyl-2,2′-bipyridine (5dmb), 1,2-bis(diphenylphoshino)ethane (dppet), Re₂(CO)₁₀, RuCl₃·H₂O, Me₃NO, Pd(OAc)₂ and other commercially available reagents were purchased from Kanto Chemical Co., Tokyo Kasei Co., Wako Pure Chemical Industries, and Aldrich Chemical Co. and were used as received. Syntheses of 4′-but-3-enyl-4-methyl-2,2′-bipyridyl (allyl-4dmb),³⁸ 1,2-bis(4′-methyl-[2,2′-bipyridin]-4-yl)ethane (4C2),³⁹ and analogously 1,2-bis(5′-methyl-[2,2′-bipyridin]-5-yl)ethane (5C2), p-bis(diphenylphosphino)benzene (dppph),⁴⁰ and Re(CO)₅Br⁴¹ were reported elsewhere. The 4-(2-hydroxyethyl)-2,2′-bipyridine (OHbpy),⁴²fac-Re(N^N)(CO)₃Br-type,⁴³fac-Re(N^N)(CO)₃OTf-type,⁴⁴ Ru(N^N)₂Cl₂-type,⁴⁵ final model complexes of [Ru(N^N)₃]²⁺-type,⁴⁶ and dinuclear Re(I) complexes ([L2xx(N^N)]²⁺, xx = bisphosphine ligand)⁹ were synthesized according to the reported methods with some modification and using corresponding N^N ligands. Syntheses of the trinuclear ring complexes [R3ph(N^N)]³⁺(19) and [R3et(5dmb₂,Brbpy)]³⁺(36) were published recently. All target complexes were obtained as PF₆^– salts.

Photochemical reactions were performed with a 500 W high-pressure Hg lamp EHBWI (Eikosha) with a uranium glass filter (>330 nm) in a Pyrex doughnut-form cell, with bubbling of N₂ gas. During irradiation, both the reaction vessel and the light source were cooled with tap water. Separation of larger complexes was achieved by RSEC using a pair of Shodex PROTEIN KW-2002.5 columns (300 mm × 20.0 mm i.d.) with a KW-LG guard column (50 mm × 8.0 mm i.d.) and a JAI LC-9201 recycling preparative HPLC apparatus (Japan Analytical Industry Co.) with a JASCO 870-UV detector (the detection wavelength was chosen as 360 nm); the eluent was MeOH–MeCN (1:1 v/v) containing CH₃COONH₄ (0.15 M), and the flow rate was 6.0 mL min^–1.³¹ For analytical SEC, two sequentially connected Shodex KW-402.5-4F columns (300 mm × 4.6 mm i.d.) were used, with a KW400G-4A guard-column (10 mm × 4.6 mm i.d.), a JASCO 880-51 degasser, an 880-PU pump, an MD-2010 Plus UV–Vis photodiode-array detector (λ_det = 360 nm), and a Rheodyne 7125 injector. The column temperature was maintained at 40 °C using a JASCO 860-CO column oven. The eluent was MeOH–MeCN (1:1 v/v) containing CH₃COONH₄ (0.5 M), and the flow rate was 0.2 mL min^–1. ¹H NMR and ³¹P NMR spectra were acquired with a JEOL ECA400II spectrometer. Chemical shifts (δ/ppm) were referenced to the residual ¹H signals of a deuterated solvent (1.94 ppm for CD₃CN and 2.05 ppm for CD₃COCD₃) and the ³¹P-signal of PF₆^– (−145 ppm), respectively. All NMR spectra were recorded at room temperature. ESI mass spectrometry (ESI-MS) was conducted on a Shimadzu LCMS-2010A mass spectrometer. ESI time-of-flight mass spectrometry was undertaken with a Waters LCT Premier instrument. Fourier transform infrared (FT-IR) spectra were recorded with a JASCO FT/IR-610 spectrometer at 1 cm^–1 resolution with a TGS detector.

UV–Vis absorption spectra were recorded with a JASCO V-565 or V-670 spectrophotometer. Emission spectra were recorded at 25 °C using a JASCO FP-6500 spectrofluorometer and were corrected for photomultiplier tube response. Emission quantum yields were determined with a calibrated integrating sphere (Absolute PL Quantum Yield Measurement System C9920-01, Hamamatsu Photonics k.k.) comprising a Xe lamp as an excitation source and a multichannel spectrometer (C10027). Emission lifetimes were measured at 25 °C using a HORIBA FluoroCube time-correlated single-photon counting system. The excitation light source was a NanoLED-560 (379 nm, <200 ps; 401 nm, <200 ps; 456 nm, <1.3 ns, or 510 nm, <100 ps). All measurements were performed in a quartz cubic cell (1 cm optical path length). The absorbances were adjusted to 0.1 at λ_ex; otherwise, the photoluminescence responses were corrected to the number of absorbed photons in the case of using various excitation wavelengths of a single solution. Prior to measurements, the solutions were degassed using the pump–thaw–freeze method.

Syntheses

Synthesis of [Ring-Ru1]⁵⁺

[Ru(bpy)₂(4C2)](PF₆)₂

Ru(bpy)₂Cl₂·2H₂O (150 mg, 0.29 mmol) and 4C2 (420 mg, 1.16 mmol) were refluxed in MeOH (25 mL) overnight. The solvent was evaporated, and the resultant crude product was dissolved in H₂O (30 mL) and filtered to remove the excess of 4C2. Solid NH₄PF₆ (excess) was added to the filtrate; extraction was performed with CH₂Cl₂ (2 × 50 mL), and the combined organic layers were evaporated to dryness. The crude solid was purified by column chromatography (alumina, CH₂Cl₂ and MeCN–CH₂Cl₂ 8:1), yielding 280 mg of a red solid (90%). ESI-MS (CH₃CN): m/z 389 [M–2PF₆^–]²⁺.

fac-[Re(CO)₃Br(μ₂-4C2)Ru(bpy)₂](PF₆)₂

[Ru(bpy)₂(4C2)](PF₆)₂ (280 mg, 0.26 mmol) and Re(CO)₅Br (117 mg, 0.29 mmol) were refluxed in MeOH (40 mL) for 8 h. The solvent was evaporated, and the resultant crude product was purified by ion-exchange chromatography (SP Sephadex C-25, MeCN–H₂O 1:1 containing NH₄PF₆ 0–10 mM). After evaporation of the MeCN portion, the precipitated solid was collected by filtration, washed with H₂O and EtOH, and then dried to furnish 140 mg of a red solid (38%). ESI-MS (CH₃CN): m/z 564 [M–2PF₆^–]²⁺.

fac-[Re(CO)₃(η¹-dppph)(μ₂-4C2)Ru(bpy)₂](PF₆)₂(CF₃SO₃)

fac-[Re(CO)₃Br(μ₂-4C2)Ru(bpy)₂](PF₆)₂ (140 mg, 0.1 mmol) was dissolved in anhydrous CH₂Cl₂ (25 mL); silver trifluoromethanesulfonate (AgOTf, 25 mg, 0.1 mmol) was added in one portion, and the mixture was gently refluxed for 3 h until the starting compound was consumed, as monitored by ESI-MS (m/z 364 Re(CO)₃(CH₃CN)(μ₂-4C2)Ru(bpy)₂]³⁺). The mixture was filtered through a pad of Celite and diluted with dry CH₂Cl₂ to a total volume of 100 mL; dppph (223 mg, 0.5 mmol) was added, and the resultant mixture was heated at 35 °C for 2 days. The mixture was evaporated and redissolved in MeCN, filtered to remove the excess dppph, and evaporated, affording a crude red solid that was used in the next step without further purification. ESI-MS (CH₃CN): m/z 499 [M–2PF₆^––OTf^–]³⁺.

[Re(4dmb)(CO)₃(μ₂-dppph)Re(4dmb)(CO)₂(μ₂-dppph)Re{(μ₂-4C2)Ru(bpy)₂}(CO)₃](PF₆)₅ ([L3(4dmb)(trans-CO)₂–Ru(4dmb,bpy₂))]⁵⁺

[L2ph(4dmb)](OTf)₂ (100 mg, 0.06 mmol) was irradiated in degassed CH₂Cl₂ (150 mL) for 35 min. After evaporation of the solvent, the crude red solid was dissolved in CH₂Cl₂ (25 mL) together with the crude fac-[Re(CO)₃(η¹-dppph)(μ₂-4C2)Ru(bpy)₂]³⁺ from the previous step and the solution was stirred at room temperature (rt) for 3 days. The solvent was evaporated, and the crude material was purified by RSEC. The fraction containing the product was collected, evaporated, and portioned between CH₂Cl₂ and NH₄PF₆ aqueous solution. The organic layer was washed once more with aqueous NH₄PF₆ solution, dried over Na₂SO₄, and evaporated to yield 60 mg of red crystals (28%, based on [L2ph(4dmb)]²⁺).³¹P NMR (161 MHz, CD₃CN): δ 22.6, 21.5, 17.6, 17.3 ppm. FT-IR (CH₃CN): ν_CO 2040, 1954, 1939, 1924, 1869 cm^–1. ESI-MS (CH₃CN): m/z 565 [M–5PF₆^–]⁵⁺, 742 [M–4PF₆^–]⁴⁺.

[–{Re(4dmb)(CO)₂(μ₂-dppph)}Re(4dmb)(CO)₂(μ₂-dppph)Re{(μ₂-4C2)Ru(bpy)₂}(CO)₂–](PF₆)₅ ([Ring-Ru1]⁵⁺)

[L3(4dmb)(trans-CO)₂–Ru(4dmb,bpy₂)]⁵⁺ (60 mg, 0.017 mmol) was dissolved in acetone (15 mL); Me₃NO (3 mg 0.04 mmol) was added in one portion, and the resultant solution was stirred at rt for 1 h. Afterward, dppph (8 mg, 0.018 mmol) was added and the resultant mixture was gently refluxed for 1 day. The solvent was evaporated, and the crude residue was purified by RSEC. The fraction containing the product was collected, evaporated, and portioned between CH₂Cl₂ and NH₄PF₆ aqueous solution. The organic layer was washed once more with aqueous NH₄PF₆ solution, dried over Na₂SO₄, and evaporated. Final recrystallization from EtOH–CH₂Cl₂ afforded 20 mg of bright-red crystals (30%).¹H NMR (400 MHz, CD₃CN): δ 8.60 (s, 1H, 4C2(Ru)-3), 8.54 (s, 1H, 4C2(Ru)-3′), 8.49 (m, 4H, bpy-3,3′), 8.22 (br s, 12H, P–C₆H₄–P), 8.06 (s, 1H, 4C2(Re)-3), 8.05–7.95 (m, 5H, 4C2(Re)-3′ + bpy-4,4′), 7.93 (d, J = 5.6 Hz, 1H, 4C2(Re)-6), 7.86 (s, 2H, 4dmb_a-3,3′), 7.84 (s, 2H, 4dmb_b-3,3′), 7.83 (d, J = 6 Hz, 2H, 4dmb_a-6,6′), 7.80 (d, J = 6 Hz, 2H, 4dmb_b-6,6′), 7.74 (d, J = 6 Hz, 3H, 4C2(Re)-6′ + bpy_a-6,6′), 7.70 (ddd, J = 4.6, 1.2 Hz, 2H, bpy_b-6,6′), 7.63 (d, J = 5.6 Hz, 1H, 4C2(Ru)-6), 7.55 (d, J = 5.6 Hz, 1H, 4C2(Ru)-6′), 7.42–7.34 (m, 4H, bpy-5,5′), 7.30 (dd, J = 5.6, 1.6 Hz, 1H, 4C2(Ru)-5), 7.26 (dd, J = 6, 1.2 Hz, 1H, 4C2(Ru)-5′), 7.25–7.17 (m, 12H, Ph-p), 7.16–7.05 (m, 24H, Ph-m), 7.03–6.87 (br m, 24H, Ph-o), 6.66 (d, J = 5.6, 1.6 Hz, 1H, 4C2(Re)-5), 6.63 (dd, J = 6, 1.6 Hz, 1H, 4C2(Re)-5′), 6.59 (dd, J = 6 Hz, 4H, 4dmb-5,5′), 3.04–2.90 (m, 4H, 4C2–CH₂–CH₂), 2.57 (s, 3H, 4C2(Ru)–CH₃), 2.36 (s, 3H, 4C2(Re)–CH₃), 2.34 (s, 12H, 4dmb–CH₃) ppm. ³¹P NMR (161 MHz): δ 22.2 (2P), 22.1 (4P) ppm. FT-IR (CH₃CN): ν_CO 1936, 1877, 1867(sh) cm^–1. ESI-MS (CH₃CN): m/z 643 [M–5PF₆^–]⁵⁺, 840 [M–4PF₆^–]⁴⁺. HRMS (ESI-TOFMS): m/z [M–5PF₆^–]⁵⁺ calcd for C₁₆₄H₁₃₄N₁₂O₆P₆Re₃Ru, 643.1338; found, 643.1354; [M–4PF₆^–]⁴⁺ calcd for C₁₆₄H₁₃₄F₆N₁₂O₆P₇Re₃Ru, 840.1583; found 840.1608.

Synthesis of [Ring-Ru2]⁵⁺

[Ru(5dmb)₂(5C2)](PF₆)₂

Ru(5dmb)₂Cl₂·2H₂O (160 mg, 0.28 mmol) and 5C2 (410 mg, 1.12 mmol) were refluxed in MeOH (30 mL) overnight. The solvent was evaporated, and the resulting crude product was dissolved in H₂O (40 mL) and filtered to remove the excess of 5C2. Solid NH₄PF₆ (excess) was added to the filtrate and extracted with CH₂Cl₂ (2 × 50 mL), and the combined organic layers were evaporated to dryness. The crude solid was purified by column chromatography (alumina, CH₂Cl₂ and MeCN–CH₂Cl₂ 8:1 v/v), yielding 268 mg of a red solid (85%). ESI-MS (CH₃CN): m/z 418 [M–2PF₆^–]²⁺.

fac-[Re(CO)₃Br(μ₂-5C2)Ru(5dmb)₂](PF₆)₂

[Ru(5dmb)₂(5C2)](PF₆)₂ (268 mg, 0.24 mmol) and Re(CO)₅Br (107 mg, 0.26 mmol) were refluxed in MeOH (40 mL) for 10 h. The solvent was evaporated, and the resultant crude product was purified by ion-exchange chromatography (SP Sephadex C-25, MeCN–H₂O 1:1 containing NH₄PF₆ 0–10 mM). After evaporation of the MeCN portion, the precipitated solid was collected by filtration, washed with H₂O and EtOH, and dried to furnish 190 mg of a red solid (54%). ESI-MS (MeCN): m/z 593 [M–2PF₆^–]²⁺.

fac-[Re(CO)₃(η¹-dppph)(μ₂-5C2)Ru(5dmb)₂](PF₆)₂(CF₃SO₃)

fac-[Re(CO)₃Br(μ₂-5C2)Ru(5dmb)₂](PF₆)₂ (190 mg, 0.13 mmol) was dissolved in anhydrous CH₂Cl₂ (30 mL); AgOTf (33 mg, 0.13 mmol) was added in a single portion, and the resultant mixture was gently refluxed for 2 h until the starting compound was consumed and Re(CO)₃(CH₃CN)(μ₂-5C2)Ru(5dmb)₂]³⁺ was formed, as monitored by ESI-MS (m/z 383). The mixture was filtered through a pad of Celite and diluted with dry CH₂Cl₂ to a total volume of 100 mL; dppph (290 mg, 0.65 mmol) was added, and the resultant mixture was heated at 35 °C for 2 days. The mixture was evaporated and redissolved in MeCN, filtered to remove the excess of dppph, and evaporated, affording a crude red solid that was used in the next step without further purification. ESI-MS (CH₃CN): m/z 518 [M–2PF₆^––OTf^–]³⁺.

[Re(5dmb)(CO)₃(μ₂-dppph)Re(5dmb)(CO)₂(μ₂-dppph)Re{(μ₂-5C2)Ru(5dmb)₂}(CO)₃](PF₆)₅ ([L3(5dmb)(trans-CO)₂–Ru(5dmb)₃)]⁵⁺)

[L2ph(5dmb)](OTf)₂ (120 mg, 0.073 mmol) was irradiated by a 500 W high-pressure mercury lamp with a uranium glass filter in degassed CH₂Cl₂ (200 mL) for 25 min. After evaporation of the solvent, the crude red solid was dissolved in CH₂Cl₂ (30 mL) together with the crude fac-[Re(CO)₃(η¹-dppph)(μ₂-5C2)Ru(5dmb)₂]³⁺ from the previous step, and the resultant solution was stirred at rt for 3 days. The solvent was evaporated, and the crude material was purified using RSEC. The fraction containing the product was collected, evaporated, and portioned between CH₂Cl₂ and NH₄PF₆ aqueous solution. The organic layer was washed once more with aqueous NH₄PF₆ solution, dried over Na₂SO₄, and evaporated to yield 43 mg of dark orange crystals (16%, based on [L2ph(5dmb)]²⁺). ESI-MS (CH₃CN): m/z 576 [M–5PF₆^–]⁵⁺, 757 [M–4PF₆^–]⁴⁺.

[–Re(5dmb)(CO)₂(μ₂-dppph)Re(5dmb)(CO)₂(μ₂-dppph)Re{(μ₂-5C2)Ru(5dmb)₂}(CO)₂–](PF₆)₅ ([Ring-Ru2]⁵⁺)

[L3(5dmb)(trans-CO)₂–Ru(5dmb)₃]⁵⁺ (43 mg, 0.012 mmol) was dissolved in acetone (10 mL); Me₃NO (2 mg 0.025 mmol) was added in a single portion, and the resultant solution was stirred at rt for 1 h. Afterward, dppph (6 mg, 0.013 mmol) was added and the resultant mixture was gently refluxed for 2 days. The solvent was evaporated, and the crude residue was purified using RSEC. The fraction containing the product was collected, evaporated, and portioned between CH₂Cl₂ and NH₄PF₆ aqueous solution. The organic layer was washed once more with aqueous NH₄PF₆ solution, dried over Na₂SO₄, and evaporated. Final recrystallization from EtOH–CH₂Cl₂ afforded 18 mg of bright orange crystals (38%).¹H NMR (400 MHz, CD₃CN): δ 8.41–8.29 (m, 12H, 5dmb(Ru)-3,3′ + 5C2(Ru)-3,3′ + 5dmb(Re)-3,3′ + 5C2(Re)-3,3′), 7.94–7.85 (m, 11H, 5dmb(Ru)-4,4′ + 5C2(Ru)-4,4′) + 5dmb(Re)-4,4′) + 5C2(Re)-4′), 7.73 (dd, J = 8.4, 1.5 Hz, 1H, 5C2(Re)-4), 7.63 (s, 1H, 5C2(Re)-6), 7.54–7.41 (set of br s, 23H, 5dmb(Ru)-6,6′ + 5C2(Ru)-6,6′ + 5dmb(Re)-6,6′ + 5C2(Re)-6′ + P–C6H4–P), 7.25–7.18 (m, 12H, Ph-p), 7.15–7.09 (m, 24H, Ph-m), 7.05–6.87 (br m, 24H, Ph-o), 2.42–2.27 (m, 4H, 5C2-CH2–CH2), 2.25–2.22 (set of s, 12H, 5dmb(Ru)–CH3), 2.14 (s, 3H, 5C2(Ru)–CH3), 1.88 (s, 3H, 5dmb(Re)–CH3), 1.87 (s, 3H, 5dmb(Re)–CH3), 1.84 (s, 6H, 5dmb(Re)–CH3) ppm. ³¹P NMR (161 MHz): δ 23.4 (4P), 23.2 (2P) ppm. FT-IR (CH₃CN): ν_CO 1938, 1879, 1869(sh) cm^–1. ESI-MS (CH₃CN): m/z 654 [M–5PF₆^–]⁵⁺, 854 [M–4PF₆^–]⁴⁺. HRMS (ESI-TOFMS): m/z [M–5PF₆^–]⁵⁺ Calcd for C₁₆₈H₁₄₂N₁₂O₆P₆Re₃Ru, 654.3463; found, 654.3484; [M–4PF₆^–]⁴⁺ calcd for C₁₆₈H₁₄₂F₆N₁₂O₆P₇Re₃Ru, 854.1740; found 854.1796.

Synthesis of [Ring-Ru3]⁵⁺

[R3et(5dmb₂,Brbpy)]³⁺ (10 mg, 3.4 μmol), [Ru(dmb)₂(allyl-4dmb)](PF₆)₂ (6.6 mg, 6.8 μmol), Pd(OAc)₂ (0.46 mg, 2.0 μmol), and AcONa (1.4 mg, 17 μmol) were dissolved in degassed MeCN (0.5 mL). The solution was heated at 65 °C for 2 days. After the solution cooled to rt, the precipitated Pd black was removed by filtration through a Millex LG 0.20 μm filter. The solvent was evaporated, and then a fresh MeCN solution containing Pd(OAc)₂ (0.46 mg, 2.0 μmol) and AcONa (1.4 mg, 17 μmol) was added to the residue. The solution was heated at 65 °C for 1 d. This process was repeated once more with additional heating for 2 days. The obtained crude product was purified by RSEC, and the fraction containing the product was evaporated. The residue was dissolved in CH₂Cl₂ and twice washed with water containing NH₄PF₆. After evaporation of the organic layer, the obtained orange–red solid was dissolved in MeOH, and a concentrated MeOH solution of NH₄PF₆ was added. Some water was added to the solution, and an aliquot of MeOH was evaporated under reduced pressure. After product precipitation, an orange–red solid was collected, washed with water and Et₂O, and dried under vacuum to afford 5.2 mg (40%) of product. ¹H NMR (400 MHz, acetone-d₆): δ 8.74 (s, 2H), 8.67 (d, J = 6.8 Hz, 1H), 8.64 (m, 3H), 8.61(s, 2H), 8.38 (s, 2H), 8.21 (s, 2H), 7.96–7.78 (m, 17H), 7.64–7.58 (m, 6H), 7.39–7.11 (m, 40H), 7.03–6.82 (m, 20H), 6.61 (m, 1H), 6.57 (m, 1H), 2.98–2.87 (m, 12H, P–CH₂–CH₂–P), 2.55–2.51 (m, 19H, – CH₂–CH₂–CH = CH–, 4dmb-CH₃), 2.18 (s, 6H, 5dmb-CH₃), 2.14 (s, 6H, 5dmb-CH₃) ppm. ³¹P NMR (161 MHz, acetone-d₆): δ 13.6 (4P), 13.5 (2P) ppm. FT-IR (CH₃CN): ν_CO 1949(sh), 1932, 1880, 1867, 1851 cm^–1. ESI-MS (CH₃CN): m/z 627 [M–5PF₆^–]⁵⁺, 821 [M–4PF₆^–]⁴⁺). HRMS (ESI-TOFMS): m/z [M–5PF₆^–]⁵⁺ Calcd for C₁₅₇H₁₄₂N₁₂O₆P₆Re₃Ru 627.7470, Found 627.7444; [M–4PF₆^–]⁴⁺ Calcd for C₁₅₇H₁₄₂F₆N₁₂O₆P₇Re₃Ru 820.9248; Found 820.9290.

Synthesis of [Ring3]³⁺

fac-[Re(OHbpy)(CO)₃(η¹-dppet)](CF₃SO₃)

fac-[Re(OHbpy)(CO)₃OTf] (101 mg, 0.16 mmol) and dppet (248 mg, 0.62 mmol) was dissolved in degassed CH₂Cl₂ (50 mL). The solution was refluxed for 1 d. After the solution cooled to rt, the solvent was removed to yield a yellow solid. Some degassed MeOH was added, and unreacted dppet was removed by filtration. The combined MeOH solutions were evaporated, and the resultant crude product was purified twice by column chromatography with different eluents (silica gel, CH₂Cl₂ and MeOH–CH₂Cl₂ 1:100 v/v) to afford 77 mg (23%) of yellow solid. ESI-MS (CH₃CN): m/z 869 [M–OTf^–]⁺.

[Re(5dmb)(CO)₃(μ₂-dppet)Re(5dmb)(CO)₂(μ₂-dppet)Re(OHbpy)(CO)₃](PF₆)₃ ([L3et(5dmb₂,OHbpy)]³⁺)

A solution of [L2et(5dmb)]²⁺ (187 mg, 0.12 mmol) in degassed CH₂Cl₂–acetone (1:1 v/v, 200 mL) was irradiated for 30 min. After evaporation of the solvent, the residue was dissolved in degassed acetone (60 mL) together with fac-[Re(OHbpy)(CO)₃(η¹-dppet)](OTf) (77 mg, 0.08 mmol), and refluxed for 20 h. The solvent was removed under vacuum, and the residue was purified by ion-exchange chromatography (CM Sephadex C-25, MeCN–H₂O 1:1 containing NH₄PF₆ (0–8 mM)). After evaporation of the MeCN portion, a yellow solid was collected by filtration, washed with H₂O, and dried to yield 151 mg (77%) of product. ESI-MS (CH₃CN): m/z 716 [M–3PF₆^–]³⁺.

[−{Re(5dmb)(CO)₂(μ₂-dppet)}₂Re(vbpy)(CO)₂(μ₂-dppet)−](PF₆)₃ ([Ring3]³⁺)

[L3et(5dmb₂,OHbpy)]³⁺ (150 mg, 59 μmol) was dissolved in acetone (20 mL), and Me₃NO (9 mg, 0.12 mmol) was added to the solution, which was then refluxed for 30 min. After a second loading of Me₃NO (1.5 mg, 0.02 mmol), the solution was further refluxed for 1 h. To this solution, a degassed acetone solution (60 mL) containing dppet (26 mg, 65 μmol) was added, and the resultant mixture was refluxed for 1 d. After the solution cooled to rt, the solvent was removed under reduced pressure, and the orange residue was redissolved in MeCN (25 mL); TsCl (130 mg, 0.68 mmol) and trimethylamine (2 mL) were then added. The resultant solution was heated at 75 °C for 1 d. After the solution cooled to rt, the solvent was removed under vacuum and the crude product was purified by RSEC. The fraction containing the product was evaporated. The residue was dissolved in CH₂Cl₂ and twice washed with water containing NH₄PF₆. After evaporation of the organic layer, the obtained yellow solid was dissolved in MeOH and a concentrated MeOH solution of NH₄PF₆ was added. Some water was added to the solution, and an aliquot of MeOH was evaporated under reduced pressure. After product precipitation, a yellow solid was collected, washed with water and Et₂O, and dried under vacuum to yield 62 mg (36%, three steps). ¹H NMR (400 MHz, acetone-d₆): δ 8.82 (d, J = 5.6 Hz, 1H), 8.63 (d, J = 6.0 Hz, 1H), 8.29 (s, 4H, 5dmb-6,6′), 8.04 (d, J = 8.0 Hz, 1H), 7.98 (s, 1H), 7.91 (d, J = 8.6 Hz, 4H, 5dmb-4,4′), 7.73 (m, 2H), 7.61 (d, J = 8.6 Hz, 4H, 5dmb-3,3′), 7.55–7.13 (m, 41H), 6.93–6.84 (m, 20H), 6.75 (dd, J = 10.8, 17.4 Hz, 1H), 6.23 (d, J = 17.4 Hz, 1H), 5.70 (d, J = 10.8 Hz, 1H), 3.00–2.96 (m, 12H, P–CH₂–CH₂–P), 2.18 (s, 6H, 5dmb-CH₃) ppm. ³¹P NMR (161 MHz, acetone-d₆): δ 13.4 (4P), 13.2 (2P). FT-IR (CH₃CN): ν_CO 1950(sh), 1932, 1880, 1867, 1853 cm^–1. ESI-MS (CH₃CN): m/z 824 [M–3PF₆^–]³⁺. HRMS (ESI-TOFMS): m/z [M–3PF₆^–]³⁺ calcd for C₁₂₀H₁₀₆N₆O₆P₆Re₃, 824.1761; found, 824.1786.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.0c00114.

• Synthetic scheme of [Ring-Ru2]⁵⁺, [R3et(5dmb₂,Brbpy)]³⁺, and [Ring3]³⁺; ESI-MS spectra of the reaction solution for decarbonylation of [L3(4dmb)(trans-CO)₂-Ru(4dmb,bpy₂)]⁵⁺ before and after addition of Me₃NO; SEC chromatogram of the reaction solution after cyclization of [L3(4dmb)-Ru(4dmb,bpy₂)]⁵⁺ with dppph; SEC chromatogram of the crude reaction mixture after Mizoroki–Heck heterocoupling; emission spectrum of 1:1 mixture of [Ring1]³⁺ and [Ru1]²⁺; ESI-mass spectrum of a reaction solution before the dehydration reaction containing [−{Re(5dmb)(CO)₂(μ₂-dppet)}₂Re(OHbpy)(CO)₂(μ₂-dppet)−](PF₆)₃; wavelength dependence of the pre-exponential factors observed in [Ring-Ru3]⁵⁺; transient emission spectrum of [Ring-Ru3]⁵⁺ recorded after 800 ns of excitation and simulation spectra; electrochemical properties of model complexes (PDF)

Author Present Address

^# J.R.: Teva Czech Industries s.r.o., Ostravska 29, 747 70 Opava, Czech Republic.

Author Contributions

^‡ Y.Y. and J.R. contributed equally.

The authors declare no competing financial interest.