Homoleptic Tris-Diphosphine Re(I) and Re(II) Complexes and Re(II) Photophysics and Photochemistry

The
ligand-to-metal charge transfer state (LMCT) of [(dmpe)3Re]2+ (dmpe = 1,2-bis­(dimethylphosphino)­ethane) has been
demonstrated to be a potent oxidant (E0(Re2+*/Re+) = 2.61 V vs standard calomel electrode).
This complex has been traditionally prepared by nontrivial routes
in low yields, and very little has been achieved in optimizing the
ground state and emission energy properties of the general class of
complexes [(PP)3Re]2+ (PP = chelating diphosphine)
through phosphine modification. Improved syntheses for Re­(I) tris-homoleptic
diphosphine complexes [(PP)3Re]+ (PP = 1,2-bis­(dimethylphosphino)­ethane
(dmpe), 1,2-bis­(diethylphosphino)­ethane (depe), bis­(dimethylphosphino)­methane
(dmpm), bis­(diphenylphosphino)­methane (dppm), Me2PCH2PPh2, 1,3-bis­(dimethylphosphino)­propane (dmpp),
or 1,2-bis­(dimethyl-phosphino)­benzene (dmpb)) were achieved by single-pot
reactions exploiting the reducing potential of the phosphines when
reacted with ReV oxo-complexes in 1,2-dichlorobenzene at
160–180 °C. Single-electron chemical oxidation of [(PP)3Re]+ yields luminescent ReII analogues;
appropriate use of Ph3C+, Cp2Fe+, or (4-BrC6H4)3N+ B­(C6F5)4– salts
produced [(PP)3Re]2+ complexes in good yields.
Crystallographic trends for the Re+/Re2+ pairs
show significantly lengthened Re2+–P bonds for [(PP)3Re]2+ relative to the corresponding [(PP)3Re]+ system. The redox and luminescence behavior of the
complexes indicates the luminescence is from a ligand P­(σ)-to-metal
(Re­(dπ)) charge transfer (2LMCT) state for all the
complexes. Structured luminescence at 77 K is postulated to originate
from relaxation of the 2LMCT state into two spin–orbit
coupled states: the ground state and a state ∼3000 cm–1 above the ground state. The excited-state reduction potential (Re­(II*/I))
for [(depe)3Re]2+ was determined from the free
energy dependence of luminescence quenching rate constants. Yields
for formation of charge separated ions were determined for three of
the complexes with a variety of electron donors. Despite favorable
electrostatics, no charge separated ions were observed for radical
ion pairs for which the energy of back electron transfer exceeded
1.1 V.