Probing the excited states of d6 metal complexes containing the 2,2′-bipyrimidine ligand using time-resolved infrared spectroscopy.: 1.: Mononuclear and homodinuclear systems

This paper reports time-resolved infrared (TRIR) spectroscopic studies on a series of weakly luminescent or nonluminescent 2,2'-bipyrimidine-based complexes to probe their electronic structure and the dynamic behavior of their excited states on the picosecond and nanosecond time scales. The complexes are mononuclear [Re(CO)(3)Cl(bpm)] (1), [Ru(CN)(4)(bpm)](2-) (2), and [Ru(bpyam)(2)(bpm)](2+) (3) [bpm = 2,2'-bipyrimidine; bpyam = 2,2'-bipyridine-4,4'-(CONEt2)(2)] and their homodinuclear analogues [{Re(CO)(3)Cl}(2)(mu-bpm)] (4), [{Ru(CN)(4)}(2)(mu-bpm)](2-) (5), and [{Ru(bpyam)(2)}(2)(mu-bpm)](4+) (6). Complex 1 shows the characteristic shift of the three nu(CO) bands to higher energy in the Re -> bpm triplet metal-to-ligand charge-transfer ((MLCT)-M-3) state, which has a lifetime of 1.2 ns. In contrast, the dinuclear complex 4 shows nu(CO) transient bands to both higher and lower energy than the ground state indicative of, on the IR time scale, an asymmetric excited state [(OC)(3)ClReI(bpm(center dot-))Re-II(CO)(3)Cl] whose lifetime is 46 ps. The cyanoruthenate complexes 2 and 5 show comparable behavior, with a shift of the nu(CN) bands to higher energy in the excited state for mononuclear 2 but two sets of transient bandsone to higher energy and one to lower energyin dinuclear 5, consistent with an asymmetric charge distribution [(NC)(4)Ru-II(bpm(center dot-))Ru-III(CN)(4)](4-) in the (MLCT)-M-3 state. These cyanoruthenate complexes have much longer lifetimes in D2O compared with CH3CN, viz., 250 ps and 3.4 ns for 2 and 65 ps and 1.2 ns for 5 in CH3CN and D2O, respectively. In complex 3, both higher-energy Ru -> bpyam and lower-energy Ru -> bpm (MLCT)-M-3 states are formed following 400 nm excitation; the former decays rapidly (tau = 6-7 ps) to the latter, and the subsequent decay of the Ru -> bpm (MLCT)-M-3 state occurs with a lifetime of 60 or 97 ns in D2O or CH3CN, respectively. Similar behavior is shown by dinuclear 6 in both D2O and CH3CN, with initial interconversion from the Ru -> bpyam to the Ru -> bpm (MLCT)-M-3 state occurring with tau similar to 7 ps and the resultant Ru -> bpm (MLCT)-M-3 state decaying on the nanosecond time scale.