Record Chemical-Shift Temperature Sensitivity in a Series of Trinuclear Cobalt Complexes.

Designing spins that exhibit long-lived coherence and strong temperature sensitivity is central to designing effective molecular thermometers and a fundamental challenge in the chemistry/quantum-information space. Herein, we provide a new pathway to both properties in the same molecule by designing a nuclear spin, which possesses a robust spin coherence, to mimic the strong temperature sensitivity of an electronic spin. This design strategy is demonstrated in the group of trinuclear Co(III) spin-crossover compounds [(CpCo(OP(OR) 2 ) 3 ) 2 Co](SbCl 6 ) where Cp = cyclopentadienyl and R = Me ( 1 ), Et ( 2 ), i -Pr ( 3 ), and t -Bu ( 4 ). Nuclear magnetic resonance analyses of the 59 Co nuclear spins reveal 59 Co chemical-shift temperature sensitivity (Δδ/Δ T ) values that span from 101(1) ppm/°C in 1 to 149(1) ppm/°C in 2 and 150(2) ppm/°C in 4 , where the latter two are record temperature sensitivities for any nuclear spin. Additionally, complexes 2 and 4 have T 2 * values of 74 and 78 μs in solution at ambient temperatures surpassing those from electron-spin-based complexes, which typically display long coherence times only at extremely low temperatures. Our results suggest that spin-crossover phenomena can enable electron-spin-like temperature sensitivities in nuclear spins while retaining robust coherence times at room temperature.