Synthesis and Photoluminescence of Tetracyanidonitridorhenium(V) Complexes with Five-Membered N-Heteroaromatic Ligands and Photoluminescence-Intensity Change.

Novel tetracyanidonitridorhenium(V) complexes with five-membered N-heteroaromatic ligands, (PPh4)2[ReN(CN)4L] [L = imidazole (Him) (2), 1-methylimidazole (Mim) (3), and pyrazole (pyz) (4)] and (PPh4)2[ReN(CN)4L]·L [L = Him (5) and Mim (6)], were synthesized by the reactions of (PPh4)2[ReN(CN)4] (1) with Him, Mim, and pyz, and their structures were determined by single-crystal X-ray analysis. The complexes 2, 3, 4, and 6 showed intense photoluminescence, with the emission quantum yields (Φem) being 0.65-0.75 in the solid state at 296 K. In contrast, the Φem and τem values of 5 are significantly smaller and shorter, respectively, than the relevant values of 2. The interconversion reactions among 1, 2, and 5 accompanied by large photoluminescence-intensity changes were accomplished by solvent-free reactions and exposure of water. The mechanochemical reaction of 2 with 1 mol equiv of Him in the solid state gave 5. Complex 5 was also obtained by the mechanochemical reaction of 1 with 2 mol equivalents of Him in the solid state. By placing solid of 5 in water, the solid showed intense photoluminescence to give 2. Complex 1 was produced under vacuum at 185 °C from 2 or 5.

Introduction

A reversible photoluminescence intensity on–off by external stimuli and/or by a chemical reaction is attractive with respect to development of sensing. It is a promising approach to apply a weak interaction to construct the switch using a reversible chemical reaction. Metal complexes have an advantage to be capable to use a weak coordination bond as well as a hydrogen bond and an intermolecular interaction for this purpose. We suppose that the control of the on–off reaction using the weak interaction/bond in a solid state is sometimes easier than that in a solution because thermal motion of the molecule is restricted in the solid state. In practice, the reversible luminescence color and/or intensity switching of metal complexes via mechanochemical reactions have been studied extensively for d10 and d8 complexes.[1−44] The driving force of luminescence intensity change in these complexes was by change in intermolecular interactions such as the π–π interaction. In contrast, the solid-based reversible reactions involving drastic luminescence intensity change by change in the coordination number and composition of the complex are still rare.

A nitridorhenium(V) complex possesses a coordination site (axial site) at the trans position of the nitrido, and the site is influenced strongly by the trans effect of the nitrido.[45−48] Therefore, the site is labile, the sixth ligand is not necessary, and the five- and six-coordinate complexes are both stable.[49] In addition, some types of the both five- and six-coordinate complexes show luminescence.[49−58] In the tetracyanidonitridorhenium(V) complex, the emission quantum yield of the five-coordinate complex is low (Φem < 0.01) and, thus, the complex is almost nonemissive by naked eyes.[49] The six-coordinate complex shows intense luminescence in the solid state at 296 K.[49,58] Therefore, the luminescence on–off reaction using the weak coordination site is promising for the complex. We previously reported vapochromic luminescence of soild tetracyanidonitridorhenium(V) by the changes in the coordination number and coordinating volatile organic compounds such as acetone, acetonitrile, ethanol, and methanol.[49]

In the present study, the new nitridorhenium(V) complexes with five-membered N-heteroaromatic ligands, imidazole (Him), N-methylimidazole (Mim), and pyrazole (pyz) as shown in Figure were synthesized and characterized to investigate luminescence on–off properties for the tetracyanidonitridorhenium(V) complex by changes in weak interactions. In the complexes having Him and Mim, we succeeded in synthesizing two types of crystalline samples depending on the reaction conditions: six-coordinate complexes and also complexes including the additional one mole ratio of the N-heterocyclic molecule. Most newly synthesized complexes showed strong luminescence in the solid states at 296 K. Interestingly, we found that the Him-coordinated complex including one mole ratio of noncoordinating Him in the crystal was almost nonluminescent, while the Mim-coordinated complex containing one mole ratio of noncoordinating Mim in the crystal was strongly photoemissive. Luminescence-intensity changes between the five- and six-coordinate complexes and between the six-coordinate complexes with Him were investigated based on solvent-free mechanochemical and vacuum elimination reactions and an exposure of water to the solid complex. The origin of the luminescence intensity change is both formation/dissociation of weak interactions and the coordination bond between tetracyanidonitridorhenium(V) and Him.

Figure 1

Structures of [ReN(CN)4]2– and [ReN(CN)4L]2– (L = Him, Mim, and pyz).

Results and Discussion

Synthesis and Characterization of the Novel Complexes

The new bright yellow-green luminescent six-coordinate complexes, (PPh4)2[ReN(CN)4Him] (2) and (PPh4)2[ReN(CN)4Mim] (3), were synthesized by the reactions of (PPh4)2[ReN(CN)4] (1) with excess five-membered N-heterocyclic ligands in water. The reaction of 1 with an excess amount of pyz in CH2Cl2 gave (PPh4)2[ReN(CN)4pyz] (4) in a high yield. The similar reaction of 1 with an excess amount of Him or Mim in CH2Cl2 produced the six-coordinate complex coordinated with the Him or Mim ligand which occupies at the axial site of the nitridorhenium(V) complex. In (PPh4)2[ReN(CN)4Him]·Him (5) or (PPh4)2[ReN(CN)4Mim]·Mim (6), furthermore, additional one mole ratio of noncoordinate Him or Mim was included. In the infrared (IR) spectra, the absorption bands ascribed to the N-heterocyclic ligands were observed in addition to those of the (PPh4)+ and [ReN(CN)4]2– units, as shown in the Supporting Information, Figure S1. The 1H NMR spectra of the new complexes dissolved in CD3CN showed the signals of N-heterocyclic ligands and (PPh4)+. Each integral signal intensity ratio of the N-heterocycle to (PPh4)+ was 1:2 for 2, 3, or 4 and 1:1 for 5 or 6. These ratios were in good agreement with the results of the single-crystal X-ray analyses of the complexes. In 5 and 6, one set of the 1H NMR signals of Him and Mim was only observed, suggesting that the dissociation of Him and Mim ligands from the rhenium ion rapidly occurred in the solutions as the result of strong trans effect of the nitrido. The single-crystal X-ray structures of the new complexes were determined. The crystal structures of the complex anions of 2, 3, 4, 5, and 6 are displayed in Figure . The crystallographic data of the new complexes are shown in Table S1. The structures and selected bond distances/angles of the new complexes are summarized in Table . Each complex anion has a distorted octahedral structure with one nitrido atom being located at the axial site and an N-heteroaromatic ligand being occupied at the trans site to the nitrido. The four cyanido ions coordinate at the equatorial positions with the Re–C bond distances of 2.094(3)–2.128(6) Å and the N(nitrido)≡Re–C angles of 96.41(10)–101.32(19)°. The bond distance of Re≡N(nitrido) is in the range of 1.659(4)–1.671(4) Å. The Re–C and Re≡N(nitrido) bond distances are similar to those of the previously reported tetracyanidonitridorhenium(V) complexes.[49,58−61] The N(nitrido)≡Re–N(aromatic) angles are almost linear [175.82(9)–178.48(14)°]. The Re–N(aromatic) bond distances are in the range 2.410(4)–2.496(2) Å, which is significantly long because of the trans influence of the nitrido ligand, while the distances are small compared to those in [ReN(CN)4L]2– (L = six-membered N-heterocyclic ligand) [2.45(1)–2.589(5) Å].[58] This difference may be due to the steric hindrance because the five-membered aromatic ring is sterically less hindered than the six-membered ring. The complex anions in 2 and 5 and the anions in 3 and 6 have very similar geometries, respectively. In 5, a noncoordinating Him is involved per complex anion and the N–H···N hydrogen bond is formed between the nitrogen atoms in the coordinate Him and that in the noncoordinate Him [N7···N8, 2.804(6) Å]. The noncoordinate Him interacts with the [ReN(CN)4]2– unit through the N–H···N hydrogen bond to the nitrogen atom of the cyanido [N3···N9, 2.993(7) Å] and the C–H···N bond to the nitrido atom [N1···C9, 3.264(8) Å] and to the cyanido [N5···C10, 3.438(8) Å]. In addition, the C–H···N interaction is shown between the nitrido atom and a phenyl ring of PPh4+ [N1···C26, 3.369(8) Å]. In 6, a noncoordinating Mim is included per complex anion, while there is no hydrogen bonding interaction between the coordinate and noncoordinate Mim molecules. The noncoordinate Mim interacts with the nitrido atom of the complex unit by the C–H···N bond [N1···C12, 3.271(6) Å] and with the cyanide ligand by the C–H···N bond [N5···C10, 3.344(5) Å, N3···C9, 3.384(6) Å]. The C–H···N interactions are formed between the nitrido atom and phenyl rings of PPh4+ ions [N1···C22, 3.316(5) Å; N1···C46, 3.494(5) Å]. In 2 and 3, the weak interactions show as the O–H···N hydrogen bonds among water molecules and the nitrogen atoms of the cyanido ligands [O1···N3, 2.973(5) Å; O2···N3, 2.958(4) Å; O3···N4, 2.853(4) Å for 2 and O1···N4, 2.864(8) Å; O2···N3, 2.930(6) Å; O3···N3, 2.89(1) Å for 3] and as the N–H···N bond between the coordinate imidazole and the cyanido [N5···N7, 2.854(4) Å] for 2. Furthermore, the interaction is formed between the nitrido and a carbon atom of a PPh4+ ion [N1···C46, 3.276(5) Å for 2 and N1···C43, 3.39(1) Å for 3]. In 4, the weak interactions show as the N–H···N and C–H···N bonds between the pyrazole and cyanido [N5···N7, 2.955(4) Å and N5···C7, 3.133(4) Å] and C–H···N bonds between the pyrazole and CH3CN [C6···N8, 3.268(4) Å]. Moreover, the interactions are formed between the nitrido and carbon atoms of PPh4+ ions [N1···C18, 3.385(4) Å and N1···C35, 3.599(3) Å].

Figure 2

ORTEP drawings of the complex anions of 2 (a), 3 (b), 4 (c), 5 (d), and 6 (e). Hydrogen atoms are omitted for clarity.

Table 1

Selected Bond Distances (Å) and Angles (deg) of the Tetracynidonitridorhenium(V) Complexes with Five-Membered N-Heterocyclic Ligands

	2	3	4	5	6
Bond Distances
Re≡N	1.664(2)	1.663(5)	1.668(2)	1.659(4)	1.671(4)
Re–C	2.106(3)–2.116(3)	2.097(7)–2.128(6)	2.094(3)–2.121(2)	2.097(5)–2.111(4)	2.100(4)–2.115(4)
Re–N	2.435(2)	2.434(5)	2.496(2)	2.410(4)	2.427(3)
Bond Angles
N≡Re–C	96.58(11)–99.66(12)	96.8(2)–98.7(2)	96.41(10)–100.78(10)	98.3(2)–101.32(19)	98.47(14)–100.31(14)
N≡Re–N	176.15(10)	176.37(19)	175.82(9)	176.83(17)	178.48(14)

Spectroscopic and Photophysical Properties

UV–vis diffuse reflectance spectra of the new complexes were measured in the solid states. Figure S2 in the Supporting Information exhibits the spectra. The complexes show the bands at 405–418 nm. The peak maximum wavelengths are similar to those observed for the six-coordinate tetracyanidonitridorhenium(V) complexes, [ReN(CN)4L]2– (L = pyridine, MeOH, and acetone).[49,58] The band can be ascribed to the transition (d)2 → (d)1(dπ*)1 (dπ* = d, d) with pπ(N3–)–dπ overlap.[49−58] This electronic transition between the highest energy occupied molecular orbital and lowest energy unoccupied molecular orbital is well characterized in d2 nitridorhenium(V) complexes.[49−58] The spectroscopic and photophysical data of the complexes in the crystalline phase are shown in Table . The emission spectra in the crystalline phase at 296 K are shown in Figures and 4. The complexes, 2, 3, 4, and 6, are highly photoemissive with the Φem values being 0.65–0.75. The τem values (52–55 μs) of these complexes indicate that the emissive excited state is a spin triplet state. Emission spectra at low temperature were measured to investigate further the excited state characters of the complexes. Figures and 6 show the emission spectra of the complexes at 80 K in the crystalline phase. All of the complexes show vibronic progressions with ca. 1000 cm–1, whose value corresponds to the νRe≡N stretching band frequencies. The emission spectra are typical for the luminescence of a nitridorhenium(V) complex.[49−58] From these results, we assigned the emissive excited state of 2, 3, 4, and 6 to 3[(d)1(d or d)1] with the pπ(N3–)–dπ overlap. The temperature (T) dependences of the emission spectrum and the τem value of 2 in the crystalline phase were studied in the T range, 80 < T < 296 K as the data were shown in Figure S3 and Table S2 in the Supporting Information. To the best of our knowledge, this is the first demonstration of the T-dependent emission spectrum and τem of a nitridorhenium(V) complex. As seen in Figure S3, 2 shows the clear vibronic progressions upon cooling, while the emission maximum wavelength is almost insensitive to T. The complex, 2, exhibits single exponential emission decay irrespective of T and the τem value increased with decreasing T in 120 < T < 296 K, while it was almost constant at 118–122 μs below 120 K. The rate constant (k) of 2 evaluated as k = 1/τem is 1.86 × 104 s–1 at 296 K and 8.2 × 103 s–1 at 80 K, respectively. It is known that the k value of [Ru(2,2′-bipyridine)3]2+ in MeOH/EtOH at room temperature decreases significantly with decreasing T from 8.7 × 105 s–1 at 296 K to 1.8–2.0 × 105 s–1 at 77 K because thermal deactivation of the emissive excited state of the complex through the upper-energy-lying metal-centered excited state is suppressed at lower temperature.[62] The small variation of the k value with T and the high Φem value of 2 will be due to almost no contribution of thermal activation to the nonradiative metal-centered state to excited state decay of 2. Figure also shows the emission spectrum of 5 in the crystalline phase and the relevant spectroscopic/photophysical data are included in Table . Although the coordination geometries of 2 and 5 are very similar with each other, the large differences in the λem, τem, and Φem values were observed between those of 2 (λem = 535 nm, τem = 54 μs, Φem = 0.75) and 5 (λem = 561 nm, weighted average τem = 0.070 μs, Φem < 0.01). In particular, the Φem value of 5 is significantly smaller than those of 2 and the other new complexes. The spectral feature showing vibronic progressions of 5 is very similar to 2 at 80 K. Therefore, the emissive excited state of 5 is assignable to 3[(d)1(d or d)1] with the pπ(N3–)–dπ overlap, and the weak luminescence of 5 is the result of quenching of the emissive excited state of the complex. In the Mim coordinate complexes, both 3 and 6 showed almost comparable spectroscopic and photophysical properties: λem = 536 nm, τem = 55 μs, and Φem = 0.65 for 3 and λem = 537 nm, τem = 52 μs, and Φem = 0.65 for 6. The Φem and τem data observed for 2 and 5 are, thus, in sharp contrast to those of 3 and 6. As described in the part of X-ray crystallography, the hydrogen bonding interactions present between the coordinate and free imidazole molecules and between the nitrido and free imidazole participate in 5, although the coordination geometry around the rhenium atoms in 5 is similar to that in 2. The C–H···N bonds at the nitrido with PPh4+ and Mim, and the O–H···N hydrogen bonding interactions at the nitrogen atom(s) of the cyanido with solvent molecules are less effective to the emission quenching because these interactions exist in the crystals of strongly luminescent 2, 3, and 6. For investigation on influence of the N–H···N interaction between coordinate and noncoordinate Him molecules, the tetracyanidonitridorhenium(V) complexes with imidazole-d5

Table 2

Spectroscopic and Photophysical Data of the Complexes in the Crystalline Phase at 296 and 80 K

	296 K			80 K
	λem/nm	Φem	τem/μs	λem/nm	τem/μs
1a	569, 720	<0.01	0.35(61), 1.2(35), 7.6(4)	721	0.29(52), 3.3(17), 14(31)
2	535	0.75	54	507, 534, 553, 564	120b
2-Dim	532		48	507, 534, 554, 565	120b
3	536	0.65	55	514, 540, 567	121(76), 192(24)
4	544	0.70	52	521, 547, 576	166
5	561	<0.01	0.023(58), 0.067(40), 0.35(1), 2.6(1)	515, 541, 563, 571, 592	29b
5-Dim	561	<0.01	0.011(18), 0.044(40), 0.24(32), 0.68(10)	514, 542, 564, 574, 593	2.2(10), 20(44), 55(46)b
6	537	0.65	52	514, 540, 567	97(84), 198(16)

Reference (49).

At 77 K.

Parenthesis denotes % components.

Figure 3

Emission spectra of 2 (blue) and 5 (red) in the crystalline state at 296 K.

Figure 4

Emission spectra of 3 (black), 4 (green), and 6 (orange) in the crystalline states at 296 K.

Figure 5

Emission spectra of 2 (blue) and 5 (red) in the crystalline state at 80 K.

Figure 6

Emission spectra of 3 (black), 4 (green), and 6 (orange) in the crystalline states at 80 K.

(Dim) molecule(s), 2-Dim and 5-Dim, were prepared by the similar reactions to the synthetic procedures to 2 and 5 except for using Dim and deuterated solvents: D2O for 2-Dim and CD2Cl2/diethylether-d10 for 5-Dim, respectively. The emission spectral and photophysical data of 2-Dim and 5-Dim in crystalline phase are also listed in Table . The λem and τem values of 2-Dim at both 296 and 80 K are similar to those of 2, as shown in Supporting Information, Figures S4 and S5. These results indicate that the deuterated ligand gives almost no influence to the excited state character and luminescence properties of 2. In 5-Dim and 5, the emission spectra and the λem values are similar to each other, while the τem values of 5 and 5-Dim in the crystalline phase are different at both 296 and 80 K. The weighted average value of τem for 5-Dim (0.16 μs) are longer than that for 5 (0.070 μs) at 296 K. The results suggest that the deuterated ligand in 5-Dim is hardly contribute to the radiative process while that influences significantly nonradiative decay of the excited states of 5 and 5-Dim. Therefore, the weak luminescence from 5 is attributed to excited-state deactivation by vibronic relaxation through the N–H···N hydrogen interactions. The longer emission lifetime of 5-Dim compared to that of 5 would be due to suppression of excited-state quenching by the slow vibration of the N–D···N hydrogen bond compared to that of the N–H···N hydrogen bond. As the noncoordinate Him interacts with the nitrido by the C–H···N bond, the bond may be also involved in the luminescence quenching in 5.

Luminescence Intensity Change by Solvent-Free Reactions and Exposure of Water

Scheme summarizes the reactions with luminescence intensity change by coordination/dissociation of the Re–N(aromatic) coordination bond and the formation/dissociation of the N–H···N hydrogen bond on the basis of solvent-free reactions and exposure of water. The solid mixture of 2 and 1 mol equiv of Him was mechanically ground on an alumina mortar at room temperature. The bright yellow-green luminescence of 2 gradually disappeared to give 5. Complex 5 was also afforded by the mechanochemical reaction of 1 with 2 mol equiv of Him at room temperature. Figure shows the luminescence intensity change during the mechanochemical reaction of the solid sample of 1 with 2 mol equiv of Him. The IR spectra and powder X-ray diffraction patterns of the obtained solids by these reactions are identical with that of 5 as shown in Supporting Information, Figures S1 and S6.

Scheme 1

Interconversions among 1, 2, and 5 by Solvent-Free Reactions and Exposure of Water

Figure 7

Luminescence intensity change during the mechanochemical reaction of 1 with Him in a 1:2 mole ratio in the solid state at room temperature. (a) Before the reaction, (b) after 1 min, (c) after 3 min, (d) after 8 min, (e) after 15 min. The pictures were taken under UV irradiation.

The significant luminescence intensity change induced by the mechanochemical reaction of 2 with one mole ratio of Him is responsible for the large Φem difference between 2 and 5. During the solvent-free reaction of 1 to produce 5, bright yellow-green luminescence appeared upon grinding (the picture in the middle of Figure ). As the Φem value of five-coordinate 1 is very small, this observation suggests that coordination of Him at the axial site of 1 gives the six-coordinate complex [ReN(CN)4Him]2–. By continuing grinding, a noncoordinating Him molecule is positioned at the site with the N–H···N hydrogen bond to give the weak luminescent complex, 5. The mechanochemical reaction of 1 with 1 mol equiv of Him gave a yellow-green luminescent solid. The X-ray powder diffraction of the solid indicates the existence of 2 along with an unidentified substance. The unidentified complex might be (PPh4)2[ReN(CN)4Him] with a crystal system different from that of 2. When one drop of water was placed on a crystal of 5 under UV irradiation, almost the nonluminescent crystal gradually exhibited yellow-green photoemission as the water spread around the crystal as shown in Figure and Movie 1 in the Supporting Information. The emission spectrum of the crystal agrees well with that of 2 in the crystalline phase. After the microcrystalline sample of 5 being soaked in water for 3.5 h, the IR spectrum and powder X-ray diffraction pattern of the solid changed to those of 2 (Figures S1 and S7), and the signal integrated intensity ratio of Him and (PPh4)+ is 1:2 in the 1H NMR spectrum in CD3CN (Figure S8). These results suggest that noncoordinating Him is eliminated by soaking the solid in water and three H2O molecules are incorporated to give 2. When 5 was suspended in 0.7 mL of D2O, ca. 3% of the complex was dissolved during the reaction; the 1H NMR spectrum of this solution was used to obtain the signal intensity ratio of (PPh4)+ to the DSS internal reference. Therefore, the conversion of 5 to 2 may proceed through the process of the dissolution of the complex in water. When the microcrystalline solid of 6 was placed in a small amount of water and the mixture was allowed to stand for 3.5 h, the IR spectrum and powder X-ray diffraction pattern changed to those of 3 (Figures S1 and S9). Therefore, elimination of the noncoordinating Mim molecule in 6 proceeds by exposure of water to give 3, although the intense luminescence remains through the reaction because the Φem values of 3 and 6 are similar to each other.

Figure 8

Crystal of 5 (upper left) and under UV irradiation (lower left); the crystal with an added drop of water (upper right) and the wet crystal under UV irradiation (lower right).

Vacuum elimination of Him from 2 or 5 was performed to achieve interconversion between the five- and six-coordinate complexes. Figures S10 and S11 exhibit the thermal gravimetric curve for 2 and 5, respectively. The thermal gravimetric analysis of 2 shows loss of water from 30 to 70 °C and large weight loss above 170 °C, and that of 5 exhibits weight loss above 150 °C. When the microcrystalline sample of 2 was subjected to vacuum at 185 °C, photoemission gradually weakened. After the vacuum elimination reaction for 6 days, no yellow-green luminescence was visible by naked eyes. In the 1H NMR spectrum, the signals of Him are not appeared in the obtained complex. The IR spectrum of the complex agrees to that of 1 as shown in Figure S1. Therefore, conversion of 2 to 1 was carried out under vacuum at 185 °C. The complex 1 was also produced from 5 by vacuum elimination of Him at 185 °C. As shown in Figure S12, the X-ray powder diffraction patterns of the obtained complexes were not agreed with that of 1 which was prepared from the vacuum elimination reaction of (PPh4)2[ReN(CN)4MeOH]. This would be because the molecular packing of 1 in the solid depends on the starting material in the vacuum elimination of the ligand at the axial site.

Conclusions

The tetracyanidonitridorhenium(V) complexes with five-membered N-heteroaromatic ligands were synthesized and their spectroscopic and photophysical properties were characterized. All of the complexes in the present study showed high-emission quantum yield, except for (PPh4)2[ReN(CN)4Him]·Him. The nature of the emissive excited state was characterized to 3[(d)1(d or d)1] with the pπ(N3–)–dπ overlap. The temperature dependence of emission lifetimes was measured at first time in the nitridorhenium(V) complex using(PPh4)2[ReN(CN)4Him]. The bright yellow-green luminescence intensity change was accomplished by the solid-based reactions of the complexes involving changes in the coordination number and in the environment around the complex. The results demonstrated that the luminescence intensity change was controlled by the formation/dissociation of the weak bond in the solid state.

Experimental Section

Materials

All commercially available reagents were used as received. (PPh4)2[ReN(CN)4] (1) was prepared according to the literature procedure.[49]

Syntheses of the Complexes

(PPh4)2[ReN(CN)4Him]·3H2O (2)

Method 1: Him (212 mg, 3.12 mmol) and 1 (121 mg, 0.123 mmol) were dissolved in 2 mL hot water. The solution was cooled to room temperature to give yellow crystals. The crystals were washed with cold water and dried under vacuum. Yield: 122 mg (91.2%). Anal. Calcd for C55H44N7P2Re·3H2O: C, 59.77; H, 4.56; N, 8.87%. Found: C, 59.82; H, 4.49; N, 8.76%. 1H NMR in CD3CN/ppm: 6.92 (s, 2H, Him), 7.61 (s, 1H, Him), 7.64–7.94 (40H, PPh4+), 10.09 (br, 1H, Him). UV–vis/nm in the solid state: 408. IR (KBr pellet)/cm–1: 2129 (νC≡N), 2115 (νC≡N), 2105 (νC≡N), 2098 (νC≡N).

Method 2: The microcrystalline solid of (PPh4)2[ReN(CN)4Him]·Him (3) (22.3 mg, 0.0199 mmol) was placed in small amounts of water (∼0.1 mL) and allowed to stand for 3.5 h. The solid was collected and washed with small amounts of water and then dried under vacuum. Yield: 21.1 mg (95.8%).

Preparation of (PPh4)2[ReN(CN)4Mim]·3H2O (3)

Method 1: Mim (110 mg, 1.33 mmol) and 1 (100 mg, 0.102 mmol) were dissolved in 4 mL of hot water. The solution was cooled to room temperature to give yellow crystals. The crystals were washed with water and then dried under vacuum. Yield: 93.1 mg (85.7%). Anal. Calcd for C56H46N7P2Re·3H2O: C, 60.10; H, 4.68; N, 8.76%. Found: C, 60.23; H, 4.59; N, 8.81%. 1H NMR in CD3CN/ppm: 3.59 (s, 3H, −CH3), 6.77 (dd, 1H, Mim), 6.94 (d, 1H, Mim), 7.47 (s, 1H, Mim), 7.64–7.93 (40H, PPh4). UV–vis/nm in the solid state: 415. IR (KBr pellet)/cm–1: 2126 (νC≡N), 2113 (νC≡N), 2102 (νC≡N), 2095 (νC≡N).

Method 2: The microcrystalline solid of (PPh4)2[ReN(CN)4Mim]·Mim (6) (22.5 mg, 0.0196 mmol) was placed in small amounts of water and allowed to stand for 3.5 h. The resultant solid was collected and washed with small amounts of water, and then dried under vacuum. Yield: 21.3 mg (97.0%).

Preparation of (PPh4)2[ReN(CN)4pyz] (4)

1 (40.3 mg, 0.0410 mmol) and pyz (152.5 mg, 2.24 mmol) were dissolved in 4 mL CH2Cl2, followed by addition of a 4 mL layer of Et2O. The solution was left for several days, resulting in the formation of yellow crystals, which were filtered and washed with Et2O. Yield: 38.8 mg (85.7%). Anal. Calcd for C55H44N7P2Re·2.5H2O: C, 60.26; H, 4.51; N, 8.94%. Found: C, 60.15; H, 4.52; N, 9.22%. 1H NMR in CD3CN/ppm: 6.23 (t, 1H, pyrazole), 7.53 (d, 2H, pyrazole), 7.64–7.93 (40H, PPh4+). UV–vis/nm in the solid state: 412. IR (KBr pellet)/cm–1: 2128 (νC≡N), 2106 (νC≡N), 2098 (νC≡N).

Preparation of (PPh4)2[ReN(CN)4Him]·Him (5)

Method 1: Him (177 mg, 2.59 mmol) and 1 (100 mg, 0.102 mmol) were dissolved in 10 mL CH2Cl2, followed by addition of a 10 mL layer of Et2O. The solution was left for several days, resulting in the formation of yellow crystals, which were filtered and washed with Et2O. Yield: 102 mg (89.1%). Anal. Calcd for C58H48N9P2Re: C, 62.24; H, 4.32; N, 11.26%. Found: C, 62.38; H, 4.27; N, 11.36%. 1H NMR in CD3CN/ppm: 6.94 (s, 4H, Him), 7.60 (s, 2H, Him), 7.64–7.95 (40H, PPh4+), 10.31 (br, 2H, Him) UV–vis/nm in the solid state: 405. IR (KBr pellet)/cm–1: 2103 (νC≡N), 2089 (νC≡N), 2124 (νC≡N).

Method 2: Him (10.8 mg, 0.158 mmol) and 1 (77.8 mg, 0.0791 mmol) were ground in an alumina mortar using the automated mill (Nitto, ALM-90DM) for 30 min. Yield: 73.1 mg (82.5%).

Method 3: Him (4.83 mg, 0.0709 mmol) and 2 (78.4 mg, 0.0709 mmol) were ground in an alumina mortar using the automated mill (Nitto, ALM-90DM) for 20 min. Yield: 67.7 mg (85.3%).

Conversion from 2 to 1

2 (173 mg, 0.156 mmol) was heated at 185 °C for 6 days under vacuum. Yield: 145 mg (94.6%). Anal. Calcd for C52H40N5P2Re·4.5H2O: C, 58.69; H, 4.64; N, 6.58%. Found: C, 58.62; H, 4.34; N, 6.65%. IR (KBr pellet) and 1H NMR (CD3CN) spectra agreed well with those previously reported.[49]

Conversion from 5 to 1

5 (84.6 mg, 0.0756 mmol) was heated at 185 °C for 8 days under vacuum. Yield: 71.2 mg (95.9%). IR (KBr pellet) and 1H NMR (CD3CN) spectra agreed well with those previously reported.[49]

Preparation of (PPh4)2[ReN(CN)4Mim]·Mim (6)

Mim (400 μL, 5.02 mmol) and 1 (32.4 mg, 0.0330 mmol) were dissolved in 1 mL CH2Cl2, followed by addition of a 9 mL layer of Et2O on the solution. The solution was allowed to stand for several days, resulting in formation of yellow crystals, which were filtered and washed with Et2O. Yield: 34.5 mg (84.8%). Anal. Calcd for C60H52N9P2Re·CH2Cl2: C, 59.46; H, 4.42; N, 10.23%. Found: C, 59.32; H, 4.58; N, 10.23%. 1H NMR in CD3CN/ppm: 3.64 (s, 6H, Mim), 6.82 (t, 2H, Mim), 6.93 (t, 2H, Mim), 7.45 (t, 2H, Mim), 7.60–7.90 (40H, PPh4+). UV–vis/nm in the solid state: 418. IR (KBr pellet)/cm–1: 2133 (νC≡N), 2110 (νC≡N), 2102 (νC≡N).

X-ray Crystallography

The single-crystal X-ray data were collected at −103 °C on a Rigaku RAXIS diffractometer with graphite-monochromated Mo Kα radiation. The crystal structures were solved by SHELXT version 2014.[63] Atomic coordinates and thermal parameters of nonhydrogen atoms were calculated by a full-matrix least-squares method using SHELXL version 2017.[64] Calculations were performed using CrystalStructure 4.2.4.

Physical Measurements

1H NMR spectra were recorded on a JEOL ECS 400 MHz spectrometer. All peaks were referred to the proton signal of Si(CH3)4 at δ = 0.00. Solid-state reflectance UV–vis spectra were measured by a JASCO V-550 spectrophotometer equipped with an integration sphere, and a sample was placed between two silica glass plates. IR spectra were recorded on a JASCO FTIR-4100. X-ray powder diffraction patterns were collected on a RIGAKU Rint 2000 diffractometer using Cu Kα radiation (λ = 1.54178 Å) with a scan rate of 1°/min(5 ≤ 2θ ≤ 50°). Corrected emission spectra excited at 400 nm were measured by an absolute emission quantum yield measurement system (Hamamatsu C9920-02) composed of an integrating sphere, a multichannel photodetector (Hamamatsu Photonics, PMA-12), and a xenon lamp as an excitation light source. For emission lifetime measurements, a solid sample was placed between two nonfluorescent glass plates. A pulsed Nd3+:YAG laser (Lotis TII Ltd., 355 nm, fwhm ∼6 ns or continuum, 355 nm, fwhm 4–6 ns) was used as an excitation light source. The emission lifetime was measured by using a streak camera (Hamamatsu Photonics, C4334) or a photomultiplier tube (Hamamatsu, R928) monitored by a digital oscilloscope (IWATSU, DS-5532A). A liquid N2 cryostat (DN1704 optical Dewar and 3120 temperature controller, Oxford Instruments) was used to control the sample temperature. The pictures of luminescence were taken under irradiation of UV light at 365 nm using a UV lamp (Vilber, VL-6.LC).