Formation of Third Generation Poly(pyrazolyl)borate Ligands from Alkyne Coupling Reactions of Fe[(<i>p</i>-IC₆H₄)B(3-Rpz)₃]₂ (R = H, Me; pz = Pyrazolyl): Pathways toward Controlling an Iron(II) Electronic Spin-State Crossover

Sonogashira coupling reactions of terminal alkynes with Fe[(p-IC6H4)B(3-Mepz)3]2 (pz = pyrazolyl
ring) yield Fe[(p-PhC2C6H4)B(3-Mepz)3]2 (2), Fe[(p-Me3SiC2C6H4)B(3-Rpz)3]2 (R = H, 3a, R = Me, 3b),
and Fe[(p-HC2C6H4)B(3-Mepz)3]2 (R = H, 4a, R = Me, 4b), a series of new complexes containing “third
generation” poly(pyrazolyl)borate ligands. Complex 2 undergoes a fairly gradual iron(II) electronic spin-state crossover with a 30 K hysteresis, whereas complex 3b is an unusual example of a complex with
equivalent iron(II) sites in the high-spin form that shows an abrupt 50% spin crossover. For complex 4b,
50% of the iron(II) sites undergo a gradual spin-state transition between 185 and 350 K with an activation
energy of 1590 ± 30 cm-1 and a T1/2 = 280 K and, for the remaining iron(II) sites, an abrupt cooperative
spin-state crossover between 106 and 114 K. The crystal structures of 4b obtained for each of the three
distinct electronic spin states reveal two crystallographically different iron(II) sites, and analysis of the
molecular/supramolecular structures indicates that the difference in the degree of pyrazolyl ring tilting in
the ligands between the two sites, rather than the strength of the intermolecular forces, play a prominent
role in determining the temperature of the spin-state crossover.