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

Different from conventional ruthenium oligopyridyl complex([Ru^??N^N?2?N^N?]2+),cyclometallated ruthenium?II?complex([Ru^??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 ?M.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 ?-? conjugation.Theoretical calculations displayed that upon the addition of HSO3-to the solution of 3,the lowest-energy states switch from the [d??Ru???*C^N] MLCT transition to the [d??Ru???*N^N] MLCT transition.It should be noted that the detection limit was calculated to be as low as 0.12 ?M.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^?-NO+}6 complex?complex 9 and 12?,which is the first example involving the cleavage of Ru-N^N bonds in ruthenium bipyridyl complexes.