Probing the Ruthenium-Cumulene Bonding Interaction: Synthesis and Spectroscopic Studies of Vinylidene- and Allenylidene-Ruthenium Complexes Supported by Tetradentate Macrocyclic Tertiary Amine and Comparisons with Diphosphine Analogues of Ruthenium and Osmium

The synthesis and spectroscopic properties of trans-[Cl(16-TMC)Ru=C=CHR]PF6 (16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane, R = C6H 4X-4, X = H (1), Cl (2), Me (3), OMe (4); R = CHPh2 (5)), trans-[Cl(16-TMC)Ru=C=C=C(C6H4X-4)2]PF 6 (X = H (6), Cl (7), Me (8), OMe (9)), and trans-[Cl(dppm) 2M=C=C=C(C6H4X-4)2]PF6 (M = Ru, X = H (10), Cl (11), Me (12); M = Os, X = H (13), Cl (14), Me (15)) are described. The crystal structures of 1, 5, 6, and 8 show that the Ru-C α and Cα-Cβ distances of the allenylidene complexes fall between those of the vinylidene and acetylide relatives. Two reversible redox couples are observed by cyclic voltammetry for 6-9, with E1/2 values ranging from -1.19 to -1.42 and 0.49 to 0.70 V vs Cp2Fe+/0, and they are both 0.2-0.3 and 0.1-0.2 V more reducing than those for 10-12 and 13-15, respectively. The UV-vis spectra of the vinylidene complexes 1-4 are dominated by intense high-energy bands at λmax &le 310 nm (Εmax ≥ 104 dm3 mol-1 cm-1), while weak absorptions at λmax ≥ 400 nm (Εmax &le 102 dm3 mol-1 cm-1) are tentatively assigned to d-d transitions. The resonance Raman spectrum of 5 contains a nominal v c=c stretch mode of the vinylidene ligand at 1629 cm-1. The electronic absorption spectra of the allenylidene complexes 6-9 exhibit an intense absorption at λmax = 479-513 nm (Εmax = (2-3) × 104 dm3 mol-1 cm-1). Similar electronic absorption bands have been found for 10-12, but the lowest energy dipole-allowed transition is blue-shifted by 1530-1830 cm-1 for the Os analogues 13-15. Ab initio calculations have been performed on the ground state of trans-[Cl-(NH3)4Ru=C=C=CPh 2]+ at the MP2 level, and imply that the HOMO is not localized purely on the metal center or allenylidene ligand. The absorption band of 6 at λmax = 479 nm has been probed by resonance Raman spectroscopy. Simulations of the absorption band and the resonance Raman intensities show that the nominal vc=c=c stretch mode accounts for ca. 50% of the total vibrational reorganization energy, indicating that this absorption band is strongly coupled to the allenylidene moiety. The excited-state reorganization of the allenylidene ligand is accompanied by rearrangement of the Ru=C and Ru-N (of 16-TMC) fragments, which supports the existence of bonding interaction between the metal and C=C=C unit in the electronic excited state.