Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles. Chemistry and Excited-State Behavior of 1,4-C₂-Cyclam Complexes

Abstract

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C(2)-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane is reported. The ligand appears to form only trans complexes, and the structure of trans-[Cr(1,4-C(2)-cyclam)Cl(2)]PF(6) is presented. The constraint imposed by the additional C(2) linkage distorts the bond angles significantly away from the ideal values of 90 and 180 degrees. The effect of the distortion is to enhance the aquation rate of trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) (k(obs) for trans-[Cr(1,4-C(2)-cyclam)(H(2)O)(2)](3+) formation = 6.5 x 10(-)(2) s(-)(1), 0.01M HNO(3), 25 degrees C) by over 5 orders of magnitude relative to trans-[Cr(cyclam)Cl(2)](+). The complexes trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) and trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) are found to have extinction coefficients four to five times higher than their cyclam analogues, owed to the lack of centrosymmetry caused by the steric constraint. The trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) complex is a very weak emitter in aqueous solution with a broad room-temperature emission centered at 735 nm (tau = 0.24 micros). Extended photolysis (350 nm, 15 h) of trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) in aqueous solution results in CN(-) ligand loss. This is in stark contrast to its unconstrained cyclam analogue, which is photoinert and has a room-temperature emission lifetime of 335 micros.

Synthesis and characterization of the chromium(III) complexes of ethylene cross-bridged cyclam and cyclen ligands

“…Three absorption bands were present with the greatest band occurring at λ max = 583 nm (ε = 66 L/cm·mol), a secondary peak at 431 nm (ε = 35 L/cm·mol) and a very small peak at approximately 369 nm (ε = 17 L/cm·mol). These bands are consistent with the distorted octahedral geometry of other Cr 3+ cyclam derivatives 14,30,31…”

“…Even more complex stability can be gained by bridging the macrocycle between donor atoms 11. Recently, cyclams bridged between adjacent nitrogen atoms have been complexed to Cr 3+ in order to study the effect of ligand constraint on the photochemistry 12–14…”

Electronic Energy Transduction from {Ru(py)4} Chromophores to Cr(III) Luminophores

“…Samples for transient absorption (TA) measurements were dissolved in thoroughly argon degassed anhydrous methanol. A flow cell having a 2 mm path length was employed (10 mL/min) to avoid possible photoproduct accumulation stemming from the population of Ru d–d states, Cr d–d states, or even dissociative CT states that bear a labile Cr­(II) ion. − Ultrafast experiments were performed with an amplified Ti/sapphire laser system (Clark MXR CPA2101, fwhm = 150 fs, λ exc 450 and 387 nm, 500 nJ per pulse) using transient absorption pump/probe detection systems (Helios and Eos, Ultrafast Systems). White light was generated focusing a fraction split from the fundamental beam onto sapphire.…”

Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices

“…Classical examples of this absorption feature appear in the room temperature spectrum of Cr(urea) 6 3+ [48] and the 77 K spectrum of ruby [49]. Additionally, the 4 A 2g → 2 E g absorbance band is often split due to spin orbit coupling and a lowered crystal field symmetry [49][50][51][52][53]. Adapted from Ref.…”