Functional Cycloruthenated Complexes: Design,synthesis And Application In Colorimetric Chemo-sensors

Different from conventional ruthenium oligopyridyl complex([Ru^Ⅱ（N^N）2（N^N）]2+),cyclometallated ruthenium（II）complex([Ru^Ⅱ（N^N）2（C^N）]+)has an anionic carbon center by the replacement of a nitrogen atom in bipyridine.The formation of the Ru-C bond dramatically changes the electronic properties of the ligands and facilitates the MLCT absorption shifting to long-wavelength range.Therefore,cyclometallated polypyridyl ruthenium（II）complexes has been recently concerned because of their distinguished photophysical properties.Although much attention has been focused on their new applications,little is known about their use as chemo-sensors.Herein,several cyclometallated ruthenium complexes were designed and synthized as chemo-senors to detect bisulpite or nitrite.The main contents are summarized as follows:1.A new hydrophilic cyclometallated ruthenium complex [Ru（pba）（bpy）₂]⁺（1,Hpba= 4-（2-pyridyl）benzaldehyde,bpy=2,2’-bipyridine）was successfully synthesized.The as-prepared complex exhibited typical MLCT absorptions centered at 536 nm,which are assigned to Ru→bpy CT transitions and noticeably red-shifted relative to Ru（bpy）32+ derivatives due to the formation of Ru-C bond.In the presence of aldehyde,the conjugated structure of the complex could be destroyed by a possible addition reaction between aldehyde and bisulphite.Therefore,upon addition of bisulphite to the aqueous solution of complex 1,the maximum of MLCT absorption was blue-shifted to 400 nm,accompanied with an apparent color change from red to yellowish.The results showed that bisulphite could be detected with the naked-eye.The detection limit of the chemo-sensor for bisulphite reached 2.73 μM.However,this obvious absorption and color changes only can be observed at pH < 6.Moreover,the response time is above 2 h and time-consuming.2.A newly designed hydrophilic cycloruthenated complex [Ru（mepbi）（bpy）₂]⁺（3,Hmepbi = 3,3-dimethyl-1-ethyl-2-（4-（pyridine-2-yl）-styryl）benzo[e]indolium iodide（2））was successfully synthesized,which contains a benzo[e]indolium and a cycloruthenated bipyridyl complex connected by an ethylene.This complex can sense bisulfite by utilizing the 1,4-addition reaction on the benzo[e]indolium moiety with an ethylene group.In PBS buffer（10 mM,pH 7.40）,the interaction between complex 3 and bisulphite resulted in a dramatic change in absorption spectra with a red shift of over 100 nm and a remarkable change in solution color from yellow to pink.This obvious bathochromic shift is rarely observed due to the destroyed π-π conjugation.Theoretical calculations displayed that upon the addition of HSO3-to the solution of 3,the lowest-energy states switch from the [dπ（Ru）→π*C^N] MLCT transition to the [dπ（Ru）→π*N^N] MLCT transition.It should be noted that the detection limit was calculated to be as low as 0.12 μM.Moreover,complex 3 was also used to detect bisulphite in sugar samples（granulated and crystal sugar）with good recovery.3.Complex [Ru（ppy）（bpy）₂]⁺（4,Hppy = 2-phenylpyridine）was used as model complex to investigated the recognition to nitrite.The results exhibited that nitrite has bleaching effect on complex 4 at acidic conditions.The same phenomenon also can be found in [Ru（thpy）（bpy）₂]⁺（5,Hthpy = 2-（2-thienyl）pyridine）and [Ru（dfppy）（bpy）₂]⁺（6,Hdfppy = 2-（2,4-difluorophenyl）pyridine）,respectively.Taking the possible cleavage of Ru-C bond in acidic conditions into account,the effects of acidity,solvent,and nitrite on ruthenium（II）centers of these complexes were investigated in detail.As a result,the cyclometallated process is pH-dependent reversible by treating acid or base.When incubated in acidic conditions,all three complexes afford [Ru（bpy）2（H2O）X]+（X=Cl-,complex 8）with entire dissociation of C^N-ligands in the end.However,the addition of nitrite to acidic solutions of these complexes disturbed the pH-dependent reversible processes.The unexpected ruthenium complexes in aforementioned system were then isolated and characterized by FT-IR,MS,1HNMR,and UV-Vis spectra,indicating that two reaction modes occurred at ruthenium centers: 1)the insertion of NO into ruthenium–aryl bond to form ruthenium C?nitroso complex [Ru-N（=O）-C]2+（complex 10 and 11）;2)the coordination of NO with the entire dissociation of one bipyridine to form {Ru^Ⅱ-NO+}6 complex（complex 9 and 12）,which is the first example involving the cleavage of Ru-N^N bonds in ruthenium bipyridyl complexes.