Nine Diiron(II) Complexes of Three Bis-tetradentate Pyrimidine Based Ligands with NCE (E = S, Se, BH₃) Coligands

Three bis-tetradentate acyclic amine ligands differing only in the arm length of the pyridine pendant arms attached to the 4,6-positions of the pyrimidine ring, namely, 4,6-bis­[N,N-bis­(2′-pyridylethyl)­aminomethyl]-2-phenylpyrimidine (LEt), 4,6-bis­[N,N-bis­(2′-pyridylmethyl)­aminomethyl]-2-phenylpyrimidine (LMe), and 4,6-[(2′-pyridylmethyl)-2′-pyridylethyl)­aminomethyl]-2-phenylpyrimidine (LMix) have been used to synthesize nine air-sensitive diiron­(II) complexes: [FeII2LEt(NCS)4]·MeOH·3/4H2O (1·MeOH·3/4H2O), [FeII2LEt(NCSe)4]·H2O (2·H2O), [FeII2LEt(NCBH3)4]·5/2H2O (3·5/2H2O), [FeII2LMe(NCS)4]·1/2H2O (4·1/2H2O), [FeII2LMe(NCSe)4] (5), [FeII2LMe(NCBH3)4]·3/2H2O (6·3/2H2O), [FeII2LMix(NCS)4]·1/2H2O (7·1/2H2O), [FeII2LMix(NCSe)4]·3/2H2O (8·3/2H2O), and [FeII2LMix(NCBH3)4]·3/2H2O (9·3/2H2O). Complexes 3·5/2H2O, 4·1/2H2O, 5, 6·3/2H2O, and 8·3/2H2O were structurally characterized by X-ray crystallography, revealing, in all cases, both of the iron­(II) centers in an octahedral environment with two NCE (E = S, Se, or BH3) anions in a cis-position relative to one another. Variable temperature magnetic susceptibility measurements showed that all nine diiron­(II) complexes are stabilized in the [HS-HS] state from 300 K to 4 K, and exhibit weak antiferromagnetic coupling. Mössbauer spectroscopy confirmed the spin and oxidation states of eight of the nine complexes (the synthesis of air-sensitive complex 3 was not readily reproduced).