title: Consequences of N,C,N'- and C,N,N'-coordination modes on electronic and photophysical properties of cyclometalated aryl ruthenium(II) complexes.
abstract: The effects of isoelectronic replacement of a neutral nitrogen donor atom by an anionic carbon atom in terpyridine ruthenium(II) complexes on the electronic and photophysical properties of the resulting N,C,N'- and C,N,N'-cyclometalated aryl ruthenium(II) complexes were investigated. To this end, a series of complexes was prepared either with ligands containing exclusively nitrogen donor atoms, that is, [Ru(R(1)-tpy)(R(2)-tpy)](2+) (R(1), R(2) = H, CO(2)Et), or bearing either one N,C,N'- or C,N,N'-cyclometalated ligand and one tpy ligand, that is, [Ru(R(1)-N(/\)C(/\)N)(R(2)-tpy)](+) and [Ru(R(1)-C(/\)N(/\)N)(R(2)-tpy)](+), respectively. Single-crystal X-ray structure determinations showed that cyclometalation does not significantly alter the overall geometry of the complexes but does change the bond lengths around the ruthenium(II) center, especially the nitrogen-to-ruthenium bond length trans to the carbanion. Substitution of either of the ligands with electron-withdrawing ester functionalities fine-tuned the electronic properties and resulted in the presence of an IR probe. Using trends obtained from redox potentials, emission energies, IR spectroelectrochemical responses, and the character of the lowest unoccupied molecular orbitals from DFT studies, it is shown that the first reduction process and luminescence are associated with the ester-substituted C,N,N'-cyclometalated ligand in [Ru(EtO(2)C-C(/\)N(/\)N)(tpy)](+). Cyclometalation in an N,C,N'-bonding motif changed the energetic order of the ruthenium d(zx), d(yz), and d(xy) orbitals. The red-shifted absorption in the N,C,N'-cyclometalated complexes is assigned to MLCT transitions to the tpy ligand. The red shift observed upon introduction of the ester moiety is associated with an increase in intensity of low-energy transitions, rather than a red shift of the main transition. Cyclometalation in the C,N,N'-binding motif also red-shifts the absorption, but the corresponding transition is associated with both ligand types. Luminescence of the cyclometalated complexes is relatively independent of the mode of cyclometalation, obeying the energy gap law within each individual series.
authors.affiliation: Chemical Biology & Organic Chemistry, Theoretical Chemistry, and Crystal & Structural Chemistry, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
authors.forename: Sipke H
authors.initials: SH
authors.lastname: Wadman
authors.forename: Martin
authors.initials: M
authors.lastname: Lutz
authors.forename: Duncan M
authors.initials: DM
authors.lastname: Tooke
authors.forename: Anthony L
authors.initials: AL
authors.lastname: Spek
authors.forename: Frantisek
authors.initials: F
authors.lastname: Hartl
authors.forename: Remco W A
authors.initials: RW
authors.lastname: Havenith
authors.forename: Gerard P M
authors.initials: GP
authors.lastname: van Klink
authors.forename: Gerard
authors.initials: G
authors.lastname: van Koten
mesh_terms: D000042:Absorption; D002244:Carbon / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004583:Electrons; D008024:Ligands; D049449:Luminescence; D009682:Magnetic Resonance Spectroscopy; D009584:Nitrogen / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*; D013696:Temperature
chemical_list: D008024:Ligands; D009942:Organometallic Compounds; D002244:Carbon; D012428:Ruthenium; D009584:Nitrogen