Influence of Ligand with Fine Difference at Donor Site on MMCT Property in Binuclear Mixed Valence Complexes①

LI Su-Hu YANG Yu-Ying ZHANG Yu-Xio WU Xin-To SHENG Tin-Lu②

a (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

b (College of Chemistry, Fuzhou University, Fuzhou 350002, China)

c (University of Chinese Academy of Sciences, Beijing 100049, China)

ABSTRACT To investigate how the electronic effect of ligand at donor site influences electronic communication or metal-to-metal charge transfer (MMCT) properties in similar mixed-valence (MV) complexes, a series of binuclear organometallic complexes, MeCp(dppe)RuCNFeCl3 (1), MeCp(PPh3)2RuCNFeCl3 (2), Cp*(dppe)FeCNFeCl3 (3),Cp*(dppe)RuCNFeCl3 (4) and Cp*(PPh3)2RuCNFeCl3 (5), have been synthesized and characterized. The electronic absorptions of these complexes show the presence of MMCT properties between RuII or FeII and FeIII ions, strongly supported by the theoretical calculations. With increasing electron-donating ability of ligands (PPh3 ＞ dppe, Cp* ＞MeCp) at donor site, the MMCT absorption bands are red-shifted, which expresses in the sequence of absorption bands with 1 (500 nm), 4 (536 nm), 2 (542 nm), 5 (580 nm) from high-energy to low-energy. Meanwhile, the MMCT absorption energy of 4 (536 nm) is larger than that of 3 (760 nm) due to the stronger electron-donating ability of FeII than RuII. Furthermore, these complexes belong to the Class II systems according to the Robin and Day’s classification.

Keywords: metal-to-metal charge transfer (MMCT), mixed-valence complexes, Class II;

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 Ru25+[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 L1L2MII-CNFeIIICl3(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)2ligands 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 FeCl3in 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 CH2Cl2solution 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 1 for complexes 1～5, respectively. All crystal structures are composed of two moieties, FeCl3and 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 RuII/FeII-CN-FeIIIof these complexes is linear, and their angle of both Fe(Ru)-C≡N and Fe-N≡C is larger than 165°. The radius of RuIIis larger than that of FeIIsince 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)2ligand has a greater electron-donating ability than the dppe ligand.

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

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

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-1lower 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 FeCl3moiety 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 FeCl3fragment,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, FeIIpossesses a stronger electron-donating ability than RuII, 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.

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 CH2Cl2solution 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 FeCl3moiety[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 FeIIthan RuII. 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 FeIII, 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 2. IR Spectra Bands, ν(CN) of Complexes 1～5 and Their Related Precursors (in KBr, cm-1)

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

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

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

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 FeCl3moiety, which confirms these complexes belong to the Class II MV compounds. According to spin densities, the redox sites of FeCl3moiety 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 FeCl3fragment (Fig. 4) and it proceeds predominately with the molecular orbital HOMO-3(145B) → LUMO+1 (149B) transition. For the other complexes, the MMCT from RuIIor FeIIto FeIIIare 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. Mulliken Spin Densities of Mixed-valence Species

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

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

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

Fig. 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.

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

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 RuII(FeII)and FeIIIions, 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.