Synthesis, Structural Diversity, and Redox Control of Cyclometalated Monoruthenium Complexes

doi:10.13208/j.electrochem.151241

Abstract

Abstract:

Cyclometalated ruthenium complexes have received increasing attractions recently due to their excellent redox and photophysical properties. One structural feature of these complexes is that there is a ruthenium-carbon (Ru-C) σ bond presented in the molecule. Three common methods, namely, the “late metalation”, “early metalation”, and “transmetalation” methods, for the synthesis of cyclometalated ruthenium complexes are discussed and summarized. General strategies for the design of cyclometalating ligand and cyclometalated ruthenium complexes are introduced. By using different ancillary ligands, such as pyridine, imidazole, triazole, and pyrimidine, a great number of ruthenium complexes can be prepared. The presence of the Ru-C bond significantly decreases the ruthenium oxidation potential. The redox control of these complexes can be realized by using different ancillary ligands and substituents.

Key words: ruthenium complexes, polypyridyl ligands, electrochemistry, redox-active materials, functional complexes

CLC Number:

GONG Zhong-Liang, SHAO Jiang-Yang, and ZHONG Yu-Wu*. Synthesis, Structural Diversity, and Redox Control of Cyclometalated Monoruthenium Complexes[J]. Journal of Electrochemistry, 2016, 22(3): 244-259.

References

[1] Barigelletti F, Flamigni L, Guardigli M, et al. Energy transfer in rigid Ru(II)/Os(II) dinuclear complexes with biscyclometalating bridging ligands containing a variable number of phenylene Units [J]. Inorganic Chemistry, 1996, 35(1): 136-142.

[2] Djukic J P, Sortais J B, Barloy L, et al. Cycloruthenated compounds-synthesis and applications [J]. European Journal of Inorganic Chemistry, 2009, 817-853.

[3] Wadman S H, Lutz M, Tooke D M, et al. Consequences of N,C,N'- and C,N,N'-coordination modes on electronic and photophysical properties of cyclometalated aryl ruthenium(II) complexes [J]. Inorganic Chemistry, 2009, 48(5): 1887-1900.

[4] Sui L Z, Yang, W W, Yao C J, et al. Charge delocalization of 1,4-benzenedicyclometalated ruthenium: A comparison between tris-bidentate and bis-tridentate complexes [J]. Inorganic Chemistry, 2012, 51(3): 1590−1598.

[5] Zhong Y W, Wu S H, Burkhardt S, et al. Mononuclear and dinuclear ruthenium complexes of 2,3-di-2-pyridyl-5,6-diphenylpyrazine: synthesis and spectroscopic and electrochemical studies [J]. Inorganic Chemistry, 2011, 50(2): 517–524.

[6] Yao C J, Sui L Z, Xie H Y, et al. Electronic coupling between two cyclometalated ruthenium centers bridged by 1,3,6,8-tetra(2-pyridyl)pyrene (tppyr) [J]. Inorganic Chemistry, 2010, 49(18): 8347–8350.

[7] Bruce M I. Cyclometalation reactions [J]. Angewandte Chemie International Edition, 1977, 16(2): 73-86.

[8] Wang D, Dong H, Zhang X, et al. Dicyanovinyl-unit-induced absorption enhancement of iridium(III) complexes in long-wavelength range and potential application in dye-sensitized solar cells [J]. Science China Chemistry, 2015, 58 (4): 658–665.

[9] Bomben P G, Robson K C D , Koivisto B D, et al. Cyclometalated ruthenium chromophores for the dye-sensitized solar cell [J]. Coordination Chemistry Reviews, 2012, 256(15-16), 1438–1450.

[10] Funaki T, Kusama H, Onozawa-Komatsuzaki N, et al. Near-IR sensitization of dye-sensitized solar cells using thiocyanate-free cyclometalated ruthenium(II) complexes having a pyridylquinoline ligand [J]. European Journal of Inorganic Chemistry, 2014, 1303–1311.

[11] Shao J Y, Fu N, Yang W W, et al. Cyclometalated ruthenium(II) complexes with bis(benzimidazolyl)benzene for dye-sensitized solar cells [J]. RSC Advance. 2015, 5, 90001-90009.

[12] Funaki T, Otsuka H, Onozawa-Komatsuzaki N, et al. Systematic evaluation of HOMO energy levels for efficient dye regeneration in dye-sensitized solar cells [J]. Journal of Materials Chemistry A, 2014, 2: 15945-15951.

[13] Yao C J, Nie H J, Yang W W, et al. Combined experimental and computational study of pyren-2,7-diyl-bridged diruthenium complexes with various terminal ligands [J]. Inorganic Chemistry, 2015, 54: 4688−4698.

[14] Yao C J, Zhong Y W, Yao J. Multi-center redox-active system: amine−amine electronic coupling through a cyclometalated bisruthenium segment [J]. Inorganic Chemistry, 2013, 52: 4040−4045.

[15] Gong Z L, Zhong Y W. Urea-bridged diferrocene: structural, electrochemical, and spectroelectrochemical studies [J]. Science China Chemistry, 2015, 58(9): 1444−1450.

[16] Kong D D, Xue L S, Jang R, et al. Conformational tuning of the intramolecular electronic coupling in molecular-wire biruthenium complexes bridged by biphenyl derivatives [J]. Chemistry - A European Journal, 2015, 21, 9895-9904.

[17] Zhang J, Zhang M X, Sun C F, et al. Diruthenium complexes with bridging diethynyl polyaromatic ligands: synthesis, spectroelectrochemistry, and theoretical calculations [J]. Organometallics, 2015, 34(16): 3967−3978.

[18] Zhong Y W, Yao C J, Nie H J, Electropolymerized films of vinyl-substituted polypyridine complexes: synthesis，characterization，and applications [J]. Coordination Chemistry Reviews, 2013, 257(7-8): 1357-1372.

[19] Yao C J, Zhong Y W, Yao J. Five-stage near-infrared electrochromism in electropolymerized films composed of alternating cyclometalated bisruthenium and bis-triarylamine segments [J]. Inorganic Chemistry, 2013, 52(17): 10000-10008.

[20] Nie H J, Zhong Y W. Near-infrared electrochromism in electropolymerized metallopolymeric films of a phen-1,4-diyl-bridged diruthenium complex [J]. Inorganic Chemistry, 2014, 53(20): 11316-11322.

[21] Cui B B, Zhong Y W, Yao J. Three-state near-infrared electrochromism at the molecular scale [J]. Journal of the American Chemical Society, 2015, 137(12): 4058-4061.

[22] Yao C J, Zhong Y W, Nie H J, et al. Near-IR electrochromism in electropolymerized eilms of a biscyclometalated ruthenium complex bridged by 1.2.4.5-tetra(2-pyridyl)benzene ligand [J]. Journal of the American Chemical Society, 2011, 133(51): 20720-20723.

[23] Zhong Y W. Electrochromism within transition-metal coordination complexes and polymers, chapter 6 of electrochromic materials and devices, edited by Roger J. Mortimer, David R. Rosseinsky and Paul M. S. Monk, 2015 Wiley-VCH.

[24] Cui B B, Tang J H, Yao J, et al. A molecular platform for multistate near-infrared electrochromism and Flip-Flop, Flip-Flap-Flop, and ternary memory [J]. Angewandte Chemie International Edition, 2015, 54(32): 9192-9197.

[25] Cui B B, Mao Z, Chen Y, et al. Tuning of resistive memory switching in electropolymerized metallopolymeric films [J]. Chemical Science, 2015, 6(2): 1308-1315.

[26] Cui B B, Yao C J, Yao J, et al. Electropolymerized films as a molecular platform for volatile memory devices with two near-infrared outputs and long retention time [J]. Chemical Science, 2014, 5(3): 932-941.

[27] Gong Z L, Cui B B, Yang W W, et al. Reversible multichannel detection of Cu²⁺ using an electropolymerized film [J]. Electrochim Acta, 2014, 130(1): 748-753.

[28] Gong Z L, Zhong Y W, Stepwise coordination followed by oxidation mechanism for the multichannel detection of Cu²⁺in an aqueous environment [J]. Organometallics, 2013, 32(24): 7495-7502.

[29] Launay, J P. Electron transfer in molecular binuclear complexes and relation with electron transport through nanojunctions [J]. Coordination Chemistry Reviews, 2013, 257(9-10): 1544-1554.

[30] Fraysse S, Coudret C, Launay J P. Molecular wires built from binuclear cyclometalated complexes [J]. Journal of the American Chemical Society, 2003, 125(19): 5880-5888.

[31] Steenwinkel P, Grove D M, Veldman N, el at. Ionic 4,4¢-biphenylene-bridged bis-ruthenium complexes [Ru₂(4,4'-{C₆H₂(CH₂NMe₂)₂-2,6}₂)(terpy)₂]ⁿ⁺ (n = 2 and 4) and their reversible redox interconversion: a molecular switch [J]. Organometallics, 1998, 17(26): 5647-5655.

[32] Williams J A G. The coordination chemistry of dipyridylbenzene: N-deficient terpyridine or panacea for brightly luminescent metal complexes [J]. Chemical Society Reviews, 2009, 38: 1783–1801.

[33] Wenger O S. Photoswitchable mixed valence [J]. Chemical Society Reviews, 2012, 41: 3772–3779.

[34] Gagliardo M, Snelders D J M, Chase P A. Organic transformations on s-aryl organometallic complexes [J]. Angewandte Chemie International Edition, 2007, 46(45): 8558-8573.

[35] Chi Y, Chou P T. Transition-metal phosphors with cyclometalating ligands: fundamentals and applications [J]. Chemical Society Reviews, 2010, 39: 638-655.

[36] Colombo A, Dragonett C, Valor A, et al. Thiocyanate-free ruthenium(II) 2,2'-bipyridyl complexes for dye-sensitized solar cells. Polyhedron, 2014, 82: 50–56.

[37] Constable E C, Henney R P G, Raithby P R, et al. Metal-ion dependent regioselectivity in cyclometalation reactions [J]. Angewandte Chemie International Edition, 1991, 30(10): 1363–1364.

[38] Li X H, Shi Z, Wang L, et at. Chromogenic mercury ions recognition of a new ruthenium(II) complex with cyclometalated 2-(2-thienyl)pyridine in CH₃CN-aqueous system [J]. Inorganic Chemistry Communicationsm, 2013, 29: 175–178.

[39] Clot O, Wolf M O, Yap G P A, et al. Synthesis and reactivity of ruthenium(II) complexes containing hemilabile phosphine–thiophene ligands [J]. Journal of the Chemical Society, Dalton Transactions, 2000, 16: 2729–2737.

[40] Moorlag C, Clot O, Wolf M O, et al. Switchable thiophene coordination in Ru(II) bipyridyl phosphinoterthiophene complexes [J]. Chemical Communication, 2002, 24: 3028-3029.

[41] Yang W W, Zhong Y W, Yoshikawa S, et al. Tuning of redox potentials by introducing a cyclometalated bond to bis-tridentate ruthenium(II) complexes bearing bis(Nmethylbenzimidazolyl) benzene or -pyridine ligands [J]. Inorganic Chemistry, 2012, 51(2): 890−899.

[42] Nagashima T, Nakabayash T, Suzuki T, et al. Tuning of metal–metal interactions in mixed-valence states of cyclometalated dinuclear ruthenium and osmium complexes bearing tetrapyridylpyrazine or -benzene [J]. Organometallics, 2014, 33(18): 4893–4904.

[43] Shao J Y, Yao J, Zhong Y W, et al. Mononuclear cyclometalated ruthenium(II) complexes of 1.2.4.5-tetrakis(N-methylbenzimidazolyl)benzene: synthesis and electrochemical and spectroscopic studies [J]. Organometallics, 2012, 31(11): 4302-4308.

[44] Yang W W, Zhong Y W. Cyclometalated ruthenium complexes of 1,2,3-triazole-containing ligands: synthesis, structural studies, and electronic properties [J]. Chinese Journal of Chemistry, 2013, 31(3): 329–338.

[45] Yang W W, Yao J, Zhong Y W, et al. Electronic coupling in a biscyclometalated ruthenium complex bridged by 3.3'.5.5'-tetrakis(1H-123-triazol-4-yl)biphenyl [J]. Organometallics 2012, 31(3): 1035-1041.

[46] Yang W W, Yao J, Zhong Y W, Redox-asymmetric bisruthenium complex bridged by a pyridin-4-yl Moiety: synthesis，characterization，and electronic coupling studies [J]. Organometallics, 2012, 31(24): 8577-8583.

[47] Gagliardo M, Dijkstra H P, Coppo P, et al. Synthesis, crystal structure, and redox and photophysical properties of novel bisphosphinoaryl RuII-terpyridine complexes [J]. Organometallics, 2004, 23 (24): 5833–5840.

[48] Dani P, Richter B, van Klink G P M, et al. Bis(ortho)-chelated bis(phosphanyl)aryl ruthenium(II) complexes containing an m¹-P-monodentate or h-Bridging h¹-P,h¹-P' Bonded R2PCHP Arene Ligand, 1-R-3,5-(CH₂PPh₂)₂C₆H₃ [R₅H, Br, or, Si(n-CH₂CH₂C₈F₁₇)₃]-Cyclometalation Reaction Intermediates and Potential Catalysts for Use in Fluorinated Biphasic Systems [J]. European Journal of Inorganic Chemistry, 2001, 2001(1), 125-131.

[49] Jia G, Lee H M, William I D, et al. Five- and six-coordinate ruthenium complexes with the tridentate orthometallated aryl bisphosphine ligand [2,6-(Ph₂PCH₂)₂C₆H₃]^- [J]. Journal of Organometallic Chemistry, 1997, 534, (1-2): 173-180.

[50] Zhang Y-M, Shao J Y, Yao C J, et al. Cyclometalated ruthenium(II) complexes with a bis-carbene CCC-pincer Ligand [J]. Dalton Transactions, 2012, 41(31): 9280-9282.

[51] Naziruddin A R, Huang Z-J, Lai W C, et al. Ruthenium(II) carbonyl complexes bearing CCC-pincer bis-(carbene) ligands: synthesis, structures and activities toward recycle transfer hydrogenation reactions [J]. Dalton Transactions, 2013, 42(36): 13161-13171.

[52] Li T Y, Liang X, Zhou L, et al. N-heterocyclic carbenes: versatile second cyclometalated ligands for neutral iridium(III) heteroleptic complexes [J]. Inorganic Chemistry, 2015, 54: 161–175.

[53] Chen L, Gao Z, Li Y. Immobilization of Pd(II) on MOFs as a highly active heterogeneous catalyst for Suzuki–Miyaura and Ullmann-type coupling [J]. Catalysis Today, 2015, 245:122-128.

[54] Wang Z, Turner E, Mahoney V, et al. Facile synthesis and characterization of phosphorescent Pt(N^∧C^∧N)X Complexes [J]. Inorganic Chemistry, 2010, 49(24): 11276-11286.

[55] Constable E C, Henney R P G, Leese T A. Cyclometalation reactions of 6-phenyl-2,2'-bipyridine; a potential C,N,N-donor analogue of 2,2': 6',2''-terpyridine. Crystal and molecular structure of dichlorobis(6-phenyl-2,2'-bipyridine)ruthenium(II) [J]. Journal of the Chemical Society, Dalton Transactions, 1990, 2: 443-449.

[56] Klein A, Rausch B, Kaiser A, el at. The cyclometalated nickel complex [(Phbpy)NiBr] (phbpy- = 2,2'-bipyridine-6-phen-2-yl)e synthesis, spectroscopic and electrochemical studies [J]. Journal of Organometallic Chemistry, 2014, 774: 86-93.

[57] Yang W W, Wang L, Zhong Y W, et al. Tridentate cyclometalated ruthenium(II) complexes of click ligand 1.3-di(1.2.3-triazol-4-yl)benzene [J]. Organometallics, 2011, 30(8): 2236-2240.

[58] Rit A, Pape T, Hepp A, et al. Supramolecular structures from polycarbene ligands and transition metal ions [J]. Organometallics, 2011, 30(2): 334-347.

[59] Vargas V C, Rubio R J, Hollis T K, et al. Efficient route to 1,3-di-N-imidazolylbenzene. A comparison of monodentate vs bidentate carbenes in Pd-catalyzed cross coupling [J]. Organic Letters, 2003, 5(25): 4847-4849.

[60] Li T Y, Liang X, Zhou L, et al. N-Heterocyclic-carbenes: versatile second cyclometalated ligands for neutral iridium(III) heteroleptic complexes [J]. Inorganic Chemistry, 2015, 54(1), 161−173.

[61] Majumdar P, Yuan X, Li S, et al. Cyclometalated Ir(III) complexes with styryl BODIPY ligands showing near IR absorption/emission: preparation, study of photophysical properties and application as photodynamic/luminescence imaging materials [J]. Journal of Materials Chemistry B, 2014, 2(19): 2838-2854.

[62] Sun J, Zhong F, Yi X, et al. Efficient enhancement of the visible-Light absorption of cyclometalated Ir(III) complexes triplet photosensitizers with bodipy and applications in photooxidation and triplet–triplet annihilation upconversion [J]. Inorganic Chemistry, 2013, 52 (11): 6299–6310.

[63] Fleetham T B, Wang Z, Li J. Exploring cyclometalated Ir complexes as donor materials for organic solar cells [J]. Inorganic Chemistry, 2013, 52(13), 7338−7343.

[64] Xu Q L, Wang C C, Li T Y, et al. Synthesis, photoluminescence, and electroluminescence of a series of iridium complexes with trifluoromethyl-substituted 2-phenylpyridine as the main ligands and tetraphenylimidodiphosphine as the ancillary ligand [J]. Inorganic Chemistry, 2013, 52: 4916–4925.

[65] Lai S W, Cheung T C, Chan M C W, et al. Luminescent mononuclear and binuclear cyclometalated palladium(II) complexes of 6-phenyl-2,2‘-bipyridines: spectroscopic and structural comparisons with platinum(II) analogues [J]. Inorganic Chemistry, 2000, 39(2): 255–262.

[66] Xu C, Li H M, Xiao Z Q, et al. Cyclometalated Pd(II) and Ir(III) 2-(4-bromophenyl)-pyridine complexes with N-heterocyclic carbenes (NHCs) and acetylacetonate (acac): synthesis, structures, luminescent properties and application in one-pot oxidation/Suzuki coupling of aryl chlorides containing hydroxymethyl [J]. Dalton Transactions, 2014, 43(26): 10235-10247.

[67] Wu W, Wu X, Zhao J, et al. Synergetic effect of C*N^N/C^N^N coordination and the arylacetylide ligands on the photophysical properties of cyclometalated platinum complexes [J]. Journal of Materials Chemistry, 2015, 3: 2291–2301.

[68] Wu W, Huang D, Yi X, et al. Tridentate cyclometalated platinum (II) complexes with strong absorption of visible light and long-lived triplet excited states as photosensitizers for triplet-triplet annihilation upconversion [J]. Dyes and Pigments, 2013, 96(1): 220-231.

[69] Wen H M, Wang J Y, Li B, et al. Phosphorescent square-planar platinum(II) complexes of 1,3-bis(2-pyridylimino)isoindoline with a monodentate strong-field ligand [J]. European Journal of Inorganic Chemistry, 2013, 2013(27): 4789-4798.

[70] Zhang X P, Mei J F, Lai J C, et al. Mechano-induced luminescent and chiroptical switching in chiral cyclometalated platinum(II) complexes [J]. Journal of Materials Chemistry C, 2015, 3(10): 2350-2357.

[71] Shao J Y, Zhong Y W. Monometallic osmium(II) complexes with bis(N-methylbenzimidazolyl)benzene or-pyridine: a comparison study with ruthenium(II) analogues [J]. Inorganic Chemistry 2013, 52(11): 6464-6472.

[72] Sun M J, Nie H J, Yao J, et al. Bis-triarylamine with a cyclometalated diosmium bridge: a multi-stage redox-active system [J]. Chinese Chemical Letters, 2015, 26(6): 649-652.

[73] Constable E C, Holmes J M, et al. A cyclometallated analogue of tris(2,2′-bipyridine)ruthenium(II) [J]. Journal of Organometallic Chemistry, 1986, 301(2): 203-208.

[74] Bomben P G, Robson K C D, Sedach P A, et al. On the Viability of Cyclometalated Ru(II) Complexes for Light-Harvesting Applications [J]. Inorganic Chemistry, 2009, 48(20): 9631–9643.

[75] Hadadzadeh H, DeRos M C, Yap G P A, et al. Cyclometalated Ruthenium Chloro and Nitrosyl Complexes [J]. Inorganic Chemistry, 2002, 41(24): 6521−6526.

[76] Aiki S, Kijim Y, Kuwabara J, et al. Ligand modification of cyclometalated ruthenium complexes in the aerobic oxidative dehydrogenation of imidazolines [J]. ACS Catalysis, 2013, 3(5): 812-816.

[77] Hartshorn C M, Steel P J. Cyclometalated compounds. XI.1 single and double cyclometalations of poly(pyrazolylmethyl)benzenes [J]. Organometallics 1998, 17(16): 3487-3496.

[78] Bomben P G, Thériault K D, Berlinguette C P. Strategies for optimizing the performance of cyclometalated ruthenium sensitizers for dye-sensitized solar cells [J]. European Journal of Inorganic Chemistry, 2011, 2011(11): 1806-1814.

[79] Bomben P G, Gordon T J, Schott E, et al. A trisheteroleptic cyclometalated ruII sensitizer that enables high power output in a dye-sensitized solar cell [J]. Angewandte Chemie International Edition, 50(45): 10682-10685.

[80] Pogozhe D V, Bezdek M J, Schauer P A, et al. Ruthenium(II) complexes bearing a naphthalimide fragment: a modular dye platform for the dye-sensitized solar cell [J]. Inorganic Chemistry, 2013, 52(6): 3001-3006.

[81] Lagadec R L, Estevez H, Cerón-Camacho R, et al. Cyclometalated ruthenium(II) complexes of benzo[h]quinoline (bzqH)[Ru(bzq)(NCMe)4]⁺, [Ru(bzq)(LL)(NCMe)2]⁺, and [Ru(bzq)(LL)2]⁺ (LL = bpy, phen) [J]. Inorganica Chimica Acta, 2010, 363(3): 567-573.

[82] Huang J F, Liu J M, Su P Y, et al. Highly efficient and stable cyclometalated ruthenium(II) complexes as sensitizers for dye-sensitized solar cells [J]. Electrochimica Acta, 2015, 174: 494-501.

[83] Makarova L, Nesmeyanov A(Eds.), The organic compounds of mercury, methods of elemento-organic chemistry, Vol. 4, North-Holland, Amsterdam, 1967.

[84] Constable E C, Leese T A. Metal exchange in organomercury complexes; a facile route to cyclometallated transition metal complexes [J]. Journal of Organometallic Chemistry, 1987, 335(3): 293-299.

[85] Lagadec R L, Alexandrova L, Estevez H, et al. Bis-ruthena(III)cycles [Ru(C∩N)2(N∩N)]PF₆ as Low-Potential Mediators for PQQ Alcohol Dehydrogenase (C∩N = 2-phenylpyridinato or 4-(2-tolyl)pyridinato, N∩N = bpy or phen) [J]. European Journal of Inorganic Chemistry, 2006, 2006(14): 2735-2738.

[86] Hamor T, Al-Selim N, West A A, et al. Bis(2-(2-pyridyl)phenyl)tritelluride-synthesis and crystal structure [J]. Journal of Organometallic Chemistry, 1986, 310(1): C5-C7.

[87] Reverco P, Schmehl R H, Cherry W R, et al. Cyclometalated complexes of ruthenium. 2. Spectral and electrochemical properties and X-ray structure of bis(2,2'-bipyridyl)(4-nitro-2-(2-pyridyl)phenyl)ruthenium(II) [J]. Inorganic Chemistry, 1985, 24(24): 4078-4082.

[88] Padhi S K, Fukuda R, Ehara M. el at. Comparative study of C^ÙN and N^ÙC type cyclometalated ruthenium complexes with a NAD+/NADH function [J]. Inorganic Chemistry, 2012, 51(15): 8091−8102.

[89] Padhi S K, Kobayashi K, Masuno S, et al. Proton-induced dynamic equilibrium between cyclometalated ruthenium rNHC (remote N-heterocyclic carbene) tautomers with an NAD+/NADH Function [J]. Inorganic Chemistry, 2011, 50(12): 5321-5323.

[90] Concepcion J, Tsai M-K, Muckerman J T, et al. Mechanism of water oxidation by single-site ruthenium complex catalysts [J]. Journal of the American Chemical Society, 2010, 132(5): 1545-1557.

[91] Zheng Z B, Wu Y Q, Wang K Z et al. pH luminescence switching, dihydrogen phosphate sensing, and cellular uptake of a heterobimetallic ruthenium(II)-rhenium(I) complex, 2014, 43: 3273-3284.

[92] Concepcion J J, Jurss J W, Brennaman M K, et al. Making oxygen with ruthenium complexes [J]. Accounts of Chemical Research, 2009, 42(12):1954-1965.

[93] Albrecht M, Cyclometalation using d-block transition metals: fundamental aspects and recent trends [J]. Chemical Reviews, 2010, 110(2): 576-623.

[94] Tang J H, Wu S H, Shao J Y, et al. Ruthenium-amine electronic coupling bridged through phen-1,3-diyl versus phen-1,4-diyl: reverse of the charge transfer direction [J]. Organometallics, 2013, 32(16): 4564-4570.

[95] Wu S H, Shao J Y, Kang H W, et al. Substituent and solvent effects on the electrochemical properties and intervalence transfer in asymmetric mixed-valent complexes consisting of cyclometalated ruthenium and ferrocene [J]. Chemistry – An Asian Journal, 2013, 8(11): 2843–2850.

[96] Wu S H, Shen J J, Yao J, et al. Asymmetric mixed-valence complexes that consist of cyclometalated ruthenium and ferrocene: synthesis, characterization, and electronic-coupling studies [J]. Chemistry – An Asian Journal, 2013, 8(1): 138-147.

[97] Wu K Q, Guo J, Yan J F, et al. Ruthenium(II) bis(terpyridine) electron transfer complexes with alkynyl–ferrocenyl bridges: synthesis, structures, and electrochemical and spectroscopic studies [J]. Dalton Transactions, 2012, 41(36): 11000-11008.

[98] Wu K Q, Guo J, Yan J F, et al. Alkynyl-bridged ruthenium(II) 40-diferrocenyl-2,20:60,200-terpyridine electron transfer complexes: synthesis, structures, and electrochemical and spectroscopic studies [J]. Organometallics, 2011, 30(13): 3504-3511.

[99] Yao C J, Nie H J, Yang W W, et al. Strongly coupled cyclometalated ruthenium–triarylamine hybrids: tuning electrochemical properties, intervalence charge transfer, and spin distribution by substituent effects [J]. Chemistry - A European Journal, 2014, 20(52): 17466-17477.

[100] Zhong Y W, Gong Z L, Shao J Y, et al. Electronic coupling in cyclometalated ruthenium complexes [J]. Coordination Chemistry Reviews, 2016, 312(1): 22-40.