English

• 1. INTRODUCTION

  Mixed-valence (MV) chemistry has attracted much interest of scientists over the past decades due to their distinctive magnetic^[1–6] and electrical properties^[7–12]. To investigate the intramolecular metal-to-metal charge transfer (MMCT) process between metals, a great number of MV complexes including the cyanide-bridged complexes have been synthesized as research models^[13–19]. At the same time, some theories have made breakthroughs^[20–24]. Based on the study of various complexes, scientists found that the degree of electronic communications of mixed-valence complexes is influenced by many factors, such as the orientation of cyanide bridges, the intrinsic property of metals or ligands, cis/trans-arrangements of molecular geometrical configuration, the external temperature, solvent or salts and so on^[25–29]. The promotion of electronic communication of trans-arrangements is much easier than cis-arrangement for binuclear or trinuclear complexes^[19]. Also, it has been found that external temperature is one of the important factors affecting electron delocalization^[30]. Most recently, our group reported that the first example of tetra-nuclear Ru₂⁵⁺–CN–Ru₂⁵⁺ complex possesses MMCT properties due to the different spin states between the two Ru₂^5+[31]. To deeply understand the relationship between electronic communication or MMCT and molecular structures, it is necessary to further investigate the unsymmetrical binuclear compounds.

  Herein, we report the syntheses, crystal structures and characterization of a series of complexes L₁L₂M^Ⅱ–CN– Fe^ⅢCl₃ (L₁ = dppe or (PPh₃)₂, L₂ = MeCp or Cp^*, M = Ru or Fe) to investigate how the electronic effect of ligand at donor site influences the electronic communication or MMCT properties. The tiny difference between dppe and (pph₃)₂ ligands is that the former has one ethyl group while the latter contains two phenyl groups. Combined the characterization results and the theoretical calculation, these complexes are classified into the Class Ⅱ system. Besides, it is found that the more rich-electron ligand or stronger electron-donating metal at donor site favors to promote intramolecular electron transfer between metals by comparing all these complexes.

  2. EXPERIMENTAL

  2.1 Physical measurements

  The elemental analyses (C, H and N) were measured using Vario MICRO elemental analyzer. The Infrared (IR) spectra characterization was carried out on a Vertex 70 FT-IR spectrophotometer with KBr pellets. The electronic absorption spectra were measured in dichloromethane solution with the PerkinElmer Lambda 950 UV-Vis-NIR spectrophotometer. The single-crystal X-ray diffraction data for the complexes were collected on a Saturn724+ CCD diffractometer equipped with graphite monochromatic Mo-Kα (λ = 0.71073 Å) radiation by using an ω-scan model technique at 293 K. All crystallographic structures were solved by direct methods using SHELXL-2014^{[32, 33]} and refined with Olex-2^[34] program package.

  2.2 Materials and synthesis

  2.2.1   Materials

  All operations were conducted under an argon atmosphere using the standard Schlenk techniques unless other statements. The complexes of MeCp(dppe)RuCN^[35], (dppe = bis(diphenylphosphino)ehane, MeCp = methyl-cyclopentadienyl, Cp^* = pentamethyl-cyclopentadienyl) MeCp(PPh₃)₂RuCN^[35], Cp^*Fe(dppe)CN^[36], Cp^*Ru(dppe)CN^{[37, 38]} and Cp^*(PPh₃)₂RuCN^[39] were prepared according to the previous paper.

  2.2.2   Synthesis

  2.2.2.1   General procedure for the synthesis of M–CN–FeCl₃

  L₁L₂M–CN (0.15 g) was mixed with 1.1 equiv. of FeCl₃ in 20 mL CH₃OH and the reaction was refluxed for 3 h. The resulting solution was concentrated, filtered, and recrystallized by diffusing n-hexane into the CH₂Cl₂ solution at room temperature, yielding crystal for single-crystal X-ray crystallography.

  2.2.2.2   MeCp(dppe)RuCNFeCl₃ (1)

  Yield 0.14 g (74.3%). Elemental analysis calcd. (%): C, 51.69; H, 4.07; N, 1.83. Found: C, 51.71; H, 4.04; N, 1.76. IR (KBr, cm^-1): 2000 (C≡N).

  2.2.2.3   MeCp(PPh₃)₂RuCNFeCl₃ (2)

  Yield 0.15 g (84.0 %). Elemental analysis calcd. (%): C,56.99; H, 4.07; N, 1.53. Found: C, 57.05; H, 4.14; N, 1.55. IR (KBr, cm^-1): 2007 (C≡N).

  2.2.2.4   Cp^*(dppe)FeCNFeCl₃ (3)

  Yield 0.13 g (72%). Elemental analysis calcd. (%): C, 57.14; H, 5.05; N, 1.80. Found: C, 56.97; H, 5.01; N, 1.70. IR (KBr, cm^-1): 1969 (C≡N).

  2.2.2.5   Cp^*(dppe)RuCNFeCl₃ (4)

  Yield 0.13 g (70%). Elemental analysis calcd. (%): C, 50.27; H, 4.55; N, 1.54. Found: C, 50.45; H, 4.77; N, 1.63. IR (KBr, cm^-1): 1989 (C≡N).

  2.2.2.6   Cp^*(PPh₃)₂RuCNFeCl₃ (5)

  Yield 0.12 g (64.6%). Elemental analysis calcd. (%): C, 59.43; H, 4.74; N, 1.48. Found: C, 58.99; H, 4.71; N, 1.42. IR (KBr, cm^-1): 1989 (C≡N).

  3. RESULTS

  3.1 X-ray structure determination

  The crystallographic structures of complexes 1~5 are determined by single-crystal X-ray analysis at room temperature. Because of their similar structures, molecular structure of complex 1 is shown in Fig. 1 and other structures are shown in Fig. S1 in the Supporting Information. The space groups are Cc, P2₁/n, C2/c, P2₁/n and P$ \overline 1 $ for complexes 1~5, respectively. All crystal structures are composed of two moieties, FeCl₃ and L₁L₂M–CN (M = Fe or Ru; L₁ = Cp^* or MeCp; L₂ = dppe or (PPh₃)₂). For complex 1, Ru is coordinated by three C atoms from MeCp, two P atoms from dppe and one C atom of the cyanide group while Fe is connected to three Cl atoms and one N atom of the cyanide group. Table 1 shows the selected bond lengths and angles of all complexes, and the crystal data and structure refinement for complexes 1~5 are given in Tables S1 and S2 in the Supporting Information. The Ru^Ⅱ/Fe^Ⅱ–CN–Fe^Ⅲ of these complexes is linear, and their angle of both Fe(Ru)–C≡N and Fe–N≡C is larger than 165º. The radius of Ru^Ⅱ is larger than that of Fe^Ⅱ since the bond lengths of Ru–P (2.283~2.349 Å) are longer than those of Fe–P (2.212 Å). With the increasing electron density at donor site, the Ru–P bond length elongates systematically with the sequence of 2.283 Å in 1, 2.307 Å in 4, 2.319 Å in 2 and 2.349 Å in 5, respectively. Besides, the similar trend also occurs in Fe–N bond length. This indicates that the (PPh₃)₂ ligand has a greater electron-donating ability than the dppe ligand.

  Figure 1

  

  Figure 1.  Molecular structure of complex 1 (Hydrogen atoms in complex are ignored for clarity). Fe, red; C, gray; P, brown; Cl, green; N, blue

  DownLoad: Full-Size Img PowerPoint

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º) of these Complexes

  DownLoad: CSV

  Complexes	1	2	3	4	5
  C–N	1.168(7)	1.154(5)	1.176(5)	1.166(3)	1.163(6)
  Fe(Ru)–C	1.930(6)	1.929(3)	1.814(4)	1.926(2)	1.919(4)
  Fe–N	1.897(5)	1.929(3)	1.906(3)	1.922(2)	2.181(2)
  Fe(Ru)–P(av.)	2.283(14)	2.319(9)	2.212(12)	2.307(7)	2.349(11)
  Fe–Cl (av.)	2.155(13)	2.177(17)	2.187(17)	2.174(12)	2.179(4)
  Fe(Ru)–C≡N	176.6(5)	174.4(3)	174.2(4)	172.7(2)	174.1(4)
  Fe–N≡C	166.7(5)	177.1(3)	167.4(4)	170.7(2)	176.5(4)

  3.2 IR spectroscopy

  The IR spectroscopy test was conducted to characterize the vibrational frequency of the cyanide group. The data are listed in the experimental section and Table 2, which obviously shows that the stretching frequencies of these complexes are at least 50 cm^-1 lower than those of their related precursors. There are two main reasons for the change of ν(CN): one is the strong withdrawing-electron ability of FeCl₃ moiety pulling electron into cyanide group from the other fragment, which strengthens π back-bonding and hence makes the stretching of CN^– red-shift^{[40, 41]}; and the other is the restrictions on cyanide group upon bonding to FeCl₃ fragment, which causes ν(CN) blue-shift^[42]. For these complexes, the former reason is more important, so the CN stretching presents a red-shift. This change also suggests upon bonding with the strong acceptor unit FeCl₃, electron interaction between metals exists through the cyanide-bridged group. Owing to the increasing electron-rich properties from Cp^*, MeCp, to Cp, the sequence of ν(CN) from low to high energy is the Cp^*Ru series (1989, 1989 cm^-1) < the MeCpRu series (2000, 2007 cm^-1) < the CpRu series (2003, 2013 cm^-1). Besides, Fe^Ⅱ possesses a stronger electron-donating ability than Ru^Ⅱ, which causes that the frequency of ν(CN) in complex 3 (1969 cm^-1) is lower than that of 4 (1989 cm^-1)^[42]. It is interesting that ν(CN) of complex 1 (Cc) shifts to lower frequencies than that of complex 2 (P2₁/n), which is not consistent with the previous papers. The same phenomenon occurs in complex Cp(dppe)RuCNFeCl₃ (Cc) and Cp(PPh₃)₂RuCNFeCl₃ (P2₁/n). Except the nature of ligands, the crystal configuration (space group) might be taken account as a major factor affecting the stretching of cyanide group.

  Table 2

  

  Table 2.  IR Spectra Bands, ν(CN) of Complexes 1~5 and Their Related Precursors (in KBr, cm^-1)

  DownLoad: CSV

  Complexes	ν(CN)	Related precursors	ν(CN)	△ν
  1	2000	MeCp(dppe)RuCN	2071	–71
  2	2007	MeCp(PPh3)2RuCN	2061	–54
  3	1969	Cp*(dppe)FeCN	2054	–85
  4	1989	Cp*(dppe)RuCN	2068	–79
  5	1989	Cp*(PPh3)2RuCN	2066	–77

  3.3 UV-Vis-NIR spectroscopy

  The data of maximum absorption band are listed in Table 3, and Fig. 2 exhibits the absorption bands of the mixed valence compounds and their relative precursors, which were conducted in the CH₂Cl₂ solution at room temperature. These bands of all complexes below 500 nm belong to the dπ(Ru/Fe)→π^* metal to ligand charge transfer (MLCT) transitions^[4]. The absorption bands, up to 500 nm in the NIR region, are attributed to MMCT (metal to metal charge transfer), where electrons transfer from the CpRu/Fe to FeCl₃ moiety^{[16, 42–44]}. The MMCT band positions are in line with the results of theoretical calculations (Table 3), with the sequence of 1(500 nm), 4(536 nm), 2(542 nm) and 5(580 nm) from low- to high-wavelength. From Table 3, it can be found that the position of absorption bands is proportional to the spin densities at donor site. As electron density of L₁L₂M increases, the absorption bands present a red-shift. Specially, the MMCT absorption energy of 4 (536 nm) is larger than that of 3 (760 nm), which is attributed to the stronger electron-donating ability of Fe^Ⅱ than Ru^Ⅱ. It's worth mentioning that there exist two strong and broad absorption bands of compounds 1 and 4 due to the spin-orbit splitting of excited state configuration of low spin Fe^Ⅲ, which generates three MMCT, as shown in Fig. 3^{[17, 22, 45]}. Owing to the small energy difference between spin-orbitals, only two overlapping absorption bands are experimentally observed.

  Table 3

  

  Table 3.  Comparison of Experimental and Theoretical Values of Electronic Absorption Bands in CH₂Cl₂ Solution

  DownLoad: CSV

  Complexes	λmax (εmax) (nm (M-1·cm-1))
  	Exptl.	Calcd.
  1	500 (4391)	519.90
  2	542 (4865)	546.86
  3	760 (4378)	863.00
  4	536 (3467)	534.15
  5	580 (4344)	564.76

  Figure 2

  

  Figure 2.  UV-VIS-NIR absorption spectra of all complexes

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  Figure 3

  

  Figure 3.  MMCT and IC transition of coupled bimetallic dπ⁶-dπ⁵ compounds^[48]

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  3.4 DFT/TDDFT calculations

  The DFT/TDDFT calculations were done at the B3LYP/LANL2DZ level using Gaussian 03 program package^[46], and its result can be exploited to study the mechanism of MMCT properties. From Table 4, it is concluded that the major spin densities of all mixed-valence complexes are distributed over the FeCl₃ moiety, which confirms these complexes belong to the Class Ⅱ MV compounds. According to spin densities, the redox sites of FeCl₃ moiety in all complexes are not much different, while the potential of Fe center on the donor site is larger than that of Ru. It can also explain the phenomenon of the lower ν(CN) of complex 3 than that of complexes 4 and 5. Compared the theoretical calculation of all complexes with experimental results, the calculated electronic transition bands are in good agreement with the experimental transition (Table 3). For complex 1, the calculated absorption band appears at 519.90 nm in the NIR regions, which accords with the experimental result (500 nm). The plot of molecular orbitals displays distinctly that the orientation of some electrons flow is from the MeCpRu fragment to the FeCl₃ fragment (Fig. 4) and it proceeds predominately with the molecular orbital HOMO-3 (145B) → LUMO+1 (149B) transition. For the other complexes, the MMCT from Ru^Ⅱ or Fe^Ⅱ to Fe^Ⅲ are predominated with HOMO-2 (178B) → LUMO+1 (182B) for 2, HOMO-1 (162B) → LUMO (164B) for 3, HOMO-1 (162B) → LUMO (164B) and HOMO (163B) → LUMO+1 (165B) for 4, and HOMO-2 (194B) → LUMO+1 (198B) for 5, as shown in Figs. 5~8, respectively. According to the Robin and Day's classification^[47], these complexes can be attributed to the typical Class Ⅱ mixed-valence compounds.

  Table 4

  

  Table 4.  Mulliken Spin Densities of Mixed-valence Species

  DownLoad: CSV

  Complex	Ru/Fe	Fe
  1	0.048950	3.699288
  2	0.045071	3.709902
  3	0.102101	3.771728
  4	0.053790	3.707974
  5	0.059390	3.686879

  Figure 4

  

  Figure 4.  Molecular orbital diagrams of HOMO-3 (145B), and LUMO+1 (149B) for 1. The isosurface value is 0.02 au.

  DownLoad: Full-Size Img PowerPoint

  Figure 5

  

  Figure 5.  Molecular orbital diagrams of HOMO-2 (178B) and LUMO+1 (182B) for 2. The isosurface value is 0.02 au.

  DownLoad: Full-Size Img PowerPoint

  Figure 6

  

  Figure 6.  Molecular orbital diagrams of HOMO-1 (162B) and LUMO (164B) for 3. The isosurface value is 0.02 au.

  DownLoad: Full-Size Img PowerPoint

  Figure 7

  

  Figure 7.  Molecular orbital diagrams of HOMO-1 (162B), HOMO (163B), LUMO (164B) and LUMO+1 (165B) for 4. The isosurface value is 0.02 au.

  DownLoad: Full-Size Img PowerPoint

  Figure 8

  

  Figure 8.  Molecular orbital diagrams of HOMO-2 (194B) and LUMO+1 (198B) for 5. The isosurface value is 0.02 au.

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In summary, a new family of binuclear complexes, L₁L₂M–CN–FeCl₃ (L₁ = dppe or (PPh₃)₂, L₂ = Cp or Cp^*, M = Ru or Fe) have been synthesized and characterized by single-crystal X-ray diffraction structural analyses, IR spectra, UV-Vis spectroscopy and the DFT/TDDFT calculation thoroughly. The electronic absorptions of these complexes show the presence of MMCT properties between Ru^Ⅱ(Fe^Ⅱ) and Fe^Ⅲ ions, strongly supported by the theoretical calculations. Compared to complex 4, the MMCT absorption band of complex 3 (L₁L₂Fe–CN–FeCl₃) is red-shift. From the electronic spectra, the sequence of the MMCT absorption bands from high-energy to low energy is 1 (500 nm) > 4 (536 nm) > 2 (542 nm) > 5 (580 nm) and 4 (536 nm) > 3 (760 nm). In a word, this work suggests that the stronger electrondonating ligand or metal at donor site is the lower energy the MMCT needs. Furthermore, the investigations indicate that the unsymmetrical MV complexes belong to the Class Ⅱ systems according to the Robin and Day's classification.

  
  
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  Figure 1  Molecular structure of complex 1 (Hydrogen atoms in complex are ignored for clarity). Fe, red; C, gray; P, brown; Cl, green; N, blue

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  Figure 2  UV-VIS-NIR absorption spectra of all complexes

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  Figure 3  MMCT and IC transition of coupled bimetallic dπ⁶-dπ⁵ compounds^[48]

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  Figure 4  Molecular orbital diagrams of HOMO-3 (145B), and LUMO+1 (149B) for 1. The isosurface value is 0.02 au.

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  Figure 5  Molecular orbital diagrams of HOMO-2 (178B) and LUMO+1 (182B) for 2. The isosurface value is 0.02 au.

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  Figure 6  Molecular orbital diagrams of HOMO-1 (162B) and LUMO (164B) for 3. The isosurface value is 0.02 au.

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  Figure 7  Molecular orbital diagrams of HOMO-1 (162B), HOMO (163B), LUMO (164B) and LUMO+1 (165B) for 4. The isosurface value is 0.02 au.

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  Figure 8  Molecular orbital diagrams of HOMO-2 (194B) and LUMO+1 (198B) for 5. The isosurface value is 0.02 au.

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º) of these Complexes

  Complexes	1	2	3	4	5
  C–N	1.168(7)	1.154(5)	1.176(5)	1.166(3)	1.163(6)
  Fe(Ru)–C	1.930(6)	1.929(3)	1.814(4)	1.926(2)	1.919(4)
  Fe–N	1.897(5)	1.929(3)	1.906(3)	1.922(2)	2.181(2)
  Fe(Ru)–P(av.)	2.283(14)	2.319(9)	2.212(12)	2.307(7)	2.349(11)
  Fe–Cl (av.)	2.155(13)	2.177(17)	2.187(17)	2.174(12)	2.179(4)
  Fe(Ru)–C≡N	176.6(5)	174.4(3)	174.2(4)	172.7(2)	174.1(4)
  Fe–N≡C	166.7(5)	177.1(3)	167.4(4)	170.7(2)	176.5(4)

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  Table 2.  IR Spectra Bands, ν(CN) of Complexes 1~5 and Their Related Precursors (in KBr, cm^-1)

  Complexes	ν(CN)	Related precursors	ν(CN)	△ν
  1	2000	MeCp(dppe)RuCN	2071	–71
  2	2007	MeCp(PPh3)2RuCN	2061	–54
  3	1969	Cp*(dppe)FeCN	2054	–85
  4	1989	Cp*(dppe)RuCN	2068	–79
  5	1989	Cp*(PPh3)2RuCN	2066	–77

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  Table 3.  Comparison of Experimental and Theoretical Values of Electronic Absorption Bands in CH₂Cl₂ Solution

  Complexes	λmax (εmax) (nm (M-1·cm-1))
  	Exptl.	Calcd.
  1	500 (4391)	519.90
  2	542 (4865)	546.86
  3	760 (4378)	863.00
  4	536 (3467)	534.15
  5	580 (4344)	564.76

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  Table 4.  Mulliken Spin Densities of Mixed-valence Species

  Complex	Ru/Fe	Fe
  1	0.048950	3.699288
  2	0.045071	3.709902
  3	0.102101	3.771728
  4	0.053790	3.707974
  5	0.059390	3.686879

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