Dimensionality Control of Vapochromic Hydrogen-Bonded Proton-Transfer Assemblies Composed of a Bis(hydrazone)iron(II) Complex

We describe the novel synthesis of a bis(hydrazone)iron(II) complex in protonated [Fe(Hpbph)2]Cl2 (1) and deprotonated [Fe(pbph)2] (2) forms and several hydrogen-bonded proton-transfer (HBPT) assemblies having different dimensionalities of hydrogen-bonded network structures, [Fe(Hpbph)2](CA)·2CH3OH (3), [Fe(Hpbph)2](HCA)2·2THF (4), and [Fe(Hpbph)2](CA)(H2CA)2·2CH3CN (5) (Hpbph = 2-(diphenylphosphino)benzaldehyde-2-pyridylhydrazone), consisting of a deprotonated Fe(II)–hydrazone complex (2) as a proton acceptor (A) and chloranilic acid (H2CA) as a proton donor (D). The deprotonated complex 2 exhibited two-step reversible protonation reactions to form the double-protonated form 1, and the acid-dissociation constants were determined to be 7.6 and 10.3 in methanol solution. Utilizing this proton-accepting ability of 2, we succeeded in synthesizing HBPT assemblies 3, 4, and 5 from the reactions in CH3OH, THF, and CH3CN, respectively, with the same D/A ratio of H2CA/[Fe(pbph)2] = 10:1. These assemblies were found to have one-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) hydrogen-bonded networks with D/A ratios of 1:1, 2:1, and 3:1 for 3, 4, and 5, respectively. In 3, a 1-D hydrogen-bonded chain composed of the alternate arrangement of [Fe(Hpbph)2]2+ and CA2–, {···[Fe(Hpbph)2]2+···CA2–···}∞, was surrounded by solvated methanol molecules to form isolated 1-D hydrogen-bonded chains. In the HBPT assembly 4, a 2-D hydrogen-bonded sheet was formed from two types of hydrogen-bonded chains, {···[Fe(Hpbph)2]2+···HCA–···HCA–···}∞ and {···HCA–···HCA–···}∞, and solvated THF molecules did not form any hydrogen bonds. In 5, two orthogonal hydrogen-bonded chains constructed from the neutral chloranilic acid molecules, {···CA2–···2(H2CA)···}∞, were formed in addition to the 1-D hydrogen-bonded chain similar to that in 3, resulting in the formation of a rigid 3-D hydrogen-bonded network structure. By controlling the dimensionality of the hydrogen bond network, we found that the 2-D HBPT assembly 4 is sufficiently flexible to exhibit interesting vapochromic behavior in response to various organic vapors.