STRUCTURE, PHYSICAL AND PHOTOPHYSICAL PROPERTIES, AND CHARGE SEPARATION STUDIES OF RE(BPM)(CO)(3)L(N-PTZ)() COMPLEXES (L=CH3CN, PY, MEQ(+),PY)

The synthesis, structure, and physical and/or photophysical properties of Re(bpm)(CO)(3)L(n+), where bpm is 2,2'-bipyrimidine and L = CH3CN, py (pyridine), MeQ(+) (N-methyl-4,4'-bipyridinium ion), and py-PTZ (1 0-(4-picolyl)phenothiazine), are described. The structure of [Re(bpm)( CO)(3)(CH3CN)]PF6 . 1/2CH(3)CN was determined by single-crystal X-ray diffraction. It crystallized in the space group P ($) over bar 1 with cell dimensions a = 10.089(11) Angstrom, b = 11.855(6) Angstrom, c = 1 8.990(6) Angstrom, alpha = 89.71(3)degrees, beta = 77.65(6)degrees, ga mma = 65.83(6)degrees, Z = 4, and D-calcd = 2.082 g/cm(3). Of the 5565 reflections (Mo K alpha, 3.5 degrees less than or equal to 2 theta le ss than or equal to 45 degrees), 4721 reflections with F-0 > 3.0 sigma (F-0) were used in full-matrix least-squares refinement. Final residua ls were R(F) = 0.0692 and R(w)(F) = 0.1026. The asymmetric unit contai ned two independent cations. The Re-N(bpm) bond distances were 2.171(9 ) and 2.198(10) Angstrom for one cation and 2.161(12) and 2.187(12) An gstrom for the other; the two Re-N(CH3CN) bond distances were 2.094(14 ) and 2.177(13) Angstrom, and the Re-C(CO) bond distances were 1.908(1 5), 1.938(17), and 1.912(12) Angstrom for the first cation and 1.897(1 5), 1.896(19), and 1.919(15) W for the second. The complexes exhibited three CO infrared-active bands in the 1900-2100 cm(-1) region and und erwent optical transitions in the 300-350 nm region assigned to d pi(R e) --> pi(bpm) transitions and in the 200-300 nm region assigned to i ntraligand pi --> pi transitions. The complexes underwent multiple re ductions attributed to reduction of coordinated bpm and MeQ(+). The fi rst reduction of Re(bpm)(CO)(3)-py(+) was located at E(1/2) = -0.84 v VS SSCE, whereas the first reduction of Re(bpm)(CO)(3)MeQ(2+) was obse rved at E(1/2) = -0.70 V vs SSCE and was attributed to reduction of th e MeQ(+) ligand. The only oxidations observed were for Re(bpm)(CO)(3)p y-PTZ(+). These were located at E(1/2) values of 0.82 and 1.53 V vs SS CE and were attributed to oxidation of the py-PTZ ligand. Emission was observed from all the complexes in acetonitrile at room temperature e xcept from Re(bpm)(CO)(3)py-PTZ(+). The emission quantum yields were l ow (Phi(em) = (0.5-2) x 10(-3)) and the emission lifetimes were short (tau similar to 2 ns). Transient absorption spectra revealed that char ge separation occurred for both Re(bpm)(CO)(3)MeQ(2+) and Re(bpm)(CO)( 3)py-PTZ(+) and involved transfer of an electron from Re(I) to MeQ(+) generating Re(II) and the MeQ(.) radical in the former and from py-PTZ to Re(II) forming the py-PTZ(+) radical cation and Re(I) in the latte r. Intramolecular back electron transfer in [(bpm(-))Re-I(CO)(3)(py-PT Z(+))](+) occurred at a rate of 1.6 x 10(8) s(-1) and in [(bpm)Re-II(C O)(3)(MeQ(.))](2+) at a rate of 5.5 x 10(7) s(-1).