Tris(2,2'-bipyradine)rutenium(II) Dichloride Hexahydrate

Tris(2,2'-bipyradine)rutenium(II) Dichloride Hexahydrate [Ru(bipy)3]Cl2@6H2O ! Ruthenium is a second-row transition element, under iron in the periodic table. o 6 2 Ru (4d 5s ) 2e 2+ 6 Rn (4d ) e Ru3+ (4d 5) ! Ru3+ is a moderate oxidizing agent: Ru3+ + e– º Ru2+ Eo = +0.2487 V ! In this synthesis Ru3+ is reduced with phosphinic acid: H3PO2 + H2O º H3PO3 + 2H+ + 2e– –Eo = +0.499 V H3PO3 + H2O º H3PO4 + 2H+ + 2e– –Eo = +0.276 V ! This synthesis uses oven-dried RuCl3@xH2O, which is reduced with freshly prepared NaH2PO2, sodium phosphinate: H3PO2(aq) + NaOH(aq) ÷ NaH2PO2(aq) + H2O H2PO2– + 2H2O 4(Ru3+ + e– ÷ ÷ H2PO2– + 2H2O + 4Ru3+ ÷ H2PO4– + 4H+ + 4e– Ru2+) H2PO4– + 4H+ + 4Ru2+ 2,2'-Bipyradine ! 2,2'-bipyradine, commonly called “bipy”, functions as a bidentate ligand. N N ! The bipy in the phosphinic acid/ruthenium chloride solution complexes with the Ru2+ produced by the redox reaction, and the complex is precipitated by adding excess Cl– (as KCl), using the common ion effect: Ru2+ + 3bipy + 2Cl– + 6H2O ÷ [Ru(bipy)3]Cl2@6H2O [Ru(bipy)3]2+ ! The [Ru(bipy)3] in the product is actually a mixture of two optical isomers with D3 symmetry. ! Second-row transition ions, like Ru2+, tend to have larger ∆o values and smaller P values. • Also, bipy is a strong-field ligand, which tends to produce large ∆o values. K As a result, [Ru(bipy)3] is a d 6 low-spin case, which is diamagnetic. eg t2g ! [Ru(bipy)3] has no same-spin (spin allowed) electronic transitions, but two bands at 428 nm and 454 nm with high extinction coefficients have been assigned to metal-ligand charge transfer. • The absorption of the blue end of the spectrum gives the complex its characteristic red color. ! [Ru(bipy)3] can be made to show chemiluminescence. Chemiluminescence ! Chemiluminescence is the production of visible light through a chemically induced excited state of a molecule, which relaxes back to the ground state by photon emission. ! Fluorescence is generally a short lifetime process (0.5-20 ns) in which a molecule absorbs a photon, achieves an excited state, and quickly decays back to the ground state. • Both the ground and excited states are singlet states (2S +1 = 1). 1 M + hν ÷ 1M* ÷1M + hν • 1M* can lose energy by non-radiative processes (thermal motion, vibration, molecular quenching), resulting in no light emission. • Most fluorescent materials (e.g., minerals such as fluorite) appear to fluoresce only while irradiated, often with “black light”. ! Phosphorescence is a longer lifetime process (sometimes hours) in which the excited molecule undergoes an intersystem crossing to a triplet state. • Radiative transition from an excited triplet state to the singlet ground state is quantum mechanically “forbidden” but occurs with low efficiency, resulting in longer lifetimes. 1 M + hν ÷ 1M* ÷ 3M* ÷ 1M + hνN • Most examples of chemiluminescence result in phophorescence (e.g., Cyalume lightsticks, lightning bugs). Chemiluminescence of [Ru(bipy)3]2+ ! In this experiment, [Ru(bipy)3]2+ is mixed with the strong oxidant S2O82–, and both are reduced by reaction with Mg(s). • The reduction product of S2O82– is the highly reactive species SO4•–, which oxidizes [Ru(bipy)3]+ back up to [Ru(bipy)3]2+ in an excited state. Mg ÷ Mg2+ + 2e– [Ru(bipy)3]2+ + e– ÷ [Ru(bipy)3]+ S2O82– + e– ÷ SO42– + SO4•– [Ru(bipy)3]+ + SO4•– ÷ *[Ru(bipy)3]2+ + SO42– • The excited state species phosphoresces, emitting bright orange light at 610 nm. *[Ru(bipy)3]2+ ÷ [Ru(bipy)3]2+ + hν Procedure Notes ! Start by preparing the 10% aqueous acetone solution needed for the first wash, and start chilling it on ice so that it will be ready when needed. ! A 31% phosphinic acid solution has been prepared for use in making the NaH2PO2 solution. • Converting 2 mL of the acid to a solution of the sodium salt should take about 6-7 NaOH pellets. • Once the solution becomes slightly cloudy, add phosphinic acid dropwise until the precipitate just dissolves. ! We will not do a quantitative uv/vis spectrum. • Just make up a solution, take a qualitative spectrum, and adjust the concentration if needed to obtain a decent spectrum to be submitted with your report. • For point 1 of the write-up, simply comment on the purity of the compound on the basis of the number and positions of the bands, compared to the data given in the experimental procedure. ! For the chemiluminescence experiment: • Make a 2M HCl solution for adjusting the pH. • Use pH paper to estimate the pH. • Observe the chemiluminescence in the dark.