High intensity trip-multiplet transitions, a reality! Synthesis, properties, and crystal structure of l-bis(triphenylphosphine)iminium trans-dibromophthalocyaninato(2-)molybdate(III)

Article abstract of DOI:10.1002/zaac.19976230919

Oxophthalocyaninato(2-)molybdenum(IV), activated by bromine oxidation prior to use, reacts with fused triphenylphosphine in the presence of bis(triphenylphosphine)iminium bromide to yield linear-bis(triphenylphosphine)iminium trans-dibromophthalocyaninato(2-)molybdate(III), ^l(PNP)^trans[Mo(Br)₂pc^2-] It crystallizes triclinic with crystal data: a = 10.506(1) A, b = 12.436(2) A, c = 12.918(2) A, α = 76.186(1)°, β = 67.890(1)°, γ = 68.689(1)°; space group P1 (No. 2); Z = 1. Mo^III is in a pseudo-octahedral coordination geometry with the bromo ligands in trans-arrangement. The Mo-N_p and Mo-Br distance is 2.043(10) and 2.588(1) A, respectively. The PNP cation adopts a linear conformation. In the IR spectrum v_as(Mo-Br) is observed at 218cm^-1 and v_as(P-N) of the linear (P-N-P) core at 1406 cm^-1. Cyclic and differential-pulse voltammetry show two quasi-reversible cathodic processes at -1.15 and -0.53 V vs. Ag/AgCl. The first is assigned to a phthalocyaninate directed reduction (pc^2-/pc^3-), while the latter arises from a Mo directed reduction (Mo^III/Mo^II). Spectral monitoring confirms the reversible Mo^III/Mo^II reduction. Two quasi-reversible anodic processes at 0.60 and 1.27 V are assigned to the successive Mo directed oxidation with redox couples Mo^III/Mo^IV and Mo^IV/Mo^V For the first time, three very intense spin-allowed trip-quartet transitions are observed in the electronic absorption spectra at 7140 (TQ1), 16890 (TQ2) and 18700 cm^-1 (TQ3) together with a sing-quartet transition at 15850 cm^-1 and characteristic Q region with maximum at 28500 cm^-1 and N region at 37400 cm^-1. All electronic excitations are of comparable intensity. A prominent low temperature emission at 6690 cm^-1 is assigned to a spin-forbidden trip-sextet.