Manipulation of the molecular spin crossover transition of Fe(H ₂ B(pz) ₂ ) ₂ (bipy) by addition of polar molecules

The addition of various dipolar molecules is shown to affect the temperature dependence of the spin state occupancy of the much studied spin crossover Fe(II) complex, [Fe{H 2 B(pz) 2 } 2 (bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine). Specifically, the addition of benzimidazole results in a re-entrant spin crossover transition, i.e. the spin state starts in the mostly low spin state, then high spin state occupancy increases, and finally the high spin state occupancy decreases with increasing temperature. This behavior contrasts with that observed when the highly polar p -benzoquinonemonoimine zwitterion C 6 H 2 ( … NH 2 ) 2 ( … O) 2 was mixed with [Fe{H 2 B(pz) 2 } 2 (bipy)], which resulted in locking [Fe{H 2 B(pz) 2 } 2 (bipy)] largely into a low spin state while addition of the ethyl derivative C 6 H 2 ( … NHC 2 H 5 ) 2 ( … O) 2 did not appear to perturb the spin crossover transition of [Fe{H 2 B(pz) 2 } 2 (bipy)].