Template Synthesis of Iron(II) Complexes Containing Tridentate P−N−S, P−N−P, P−N−N, and Tetradentate P−N−N−P Ligands

A series of mer-tridentate iron(II) complexes bearing P−N−S (3), P−N−P (4), and P−N−N (5) ligands have been prepared via the metal template effect in one pot involving air-stable phosphonium dimers [cyclo-(−PPh2CH2C(OH)H−)2](Br)2 (1) and [cyclo-(−PCy2CH2C(OH)H−)2](Br)2 (2), KOtBu, [Fe(H2O)6][BF4]2 and 2-aminothiolphenol (for 3), 2-(diphenylphosphino)ethylamine (for 4), and 2-(aminomethyl)pyridine (for 5). The new phosphonium dimer 2 was prepared via an SN2 reaction of PCy2H with BrCH2CH(OEt)2. The complexes Fe{PR2CH2CHN(2-C6H4)S}2FeBr2 (3a, R = Ph; 3b, R = Cy) are paramagnetic, and X-ray diffraction studies revealed that they are bimetallic, in which the S atoms of the bis-tridentate (PNS)2Fe unit bridge to a FeBr2 fragment. Complexes [Fe(PR2CH2CHNC2H4PPh2)(NCMe)3]X2 (4a, R = Ph; 4b, R = Cy; X2 = FeBr4 or (BF4)2) form when 1 equiv of iron is reacted with PPh2CH2CH2NH2 and 0.5 equiv of the appropriate phosphonium dimer. The evidence for P−N−P coordination is the large 2JPP coupling constant in the 31P {1H} NMR spectrum for the trans phosphorus nuclei. If 0.5 equiv of [Fe(H2O)6][BF4]2 were added in the synthesis, the complex trans-[Fe(NCMe)2(Ph2PC2H4NH2)2][FeBr4] (4c) formed, and this has been characterized by X-ray diffraction. Complexes [Fe{PR2CH2CHNCH2(2-C5H4N)}2](BPh4)2 (5) are bis-tridentate iron(II) complexes with pyridyl donors trans to the phosphine donors. Interestingly, addition of the diamines ethylenediamine, (1R,2R)-(−)-1,2-diaminocyclohexane, (1R,2R)-(−)-1,2-diphenylethylenediamine, or o-phenylenediamine, in the template synthesis with 2 led directly to tetradentate P−N−N−P iron(II) complexes trans-[Fe(NCMe)2(PCy2CH2CHN-Q-NCHCH2PCy2](BPh4)2 (Q = CH2CH2, 6a; Q = (1R,2R)-cyclo-C6H10, 6b; Q = (1R,2R)-CHPhCHPh, 6c; Q = C6H4, 6d). In contrast, similar reactions under the same conditions with dimer 1 led to complexes mer-[Fe(P−N−N)2]2+ as reported previously. Complexes 6a and 6b have been characterized by X-ray diffraction and exhibited large P−Fe−P bond angles of 112.92(2) and 111.96(4)°, respectively.