Structural studies on biguanide and related species. Correlation of protonation energy with molecular structure

Abstract

The crystal structures of biguanide (bg), dibiguanidinium carbonate, [Hbg]₂[CO₃], dibiguanidinium sulphate, [Hbg]₂[SO₄]·2H₂O, biguanidinium sulphate, [H₂bg][SO₄]·H₂O, and malondiamidinium sulphate, [H₂maldm]-[SO₄], have been determined from diffractometer data: bg, monoclinic, a= 9.522(2), b= 5.061 (1), c= 9.998(2)Å. β= 102.95(2)°, space group P2₁/n, Z= 4, R= 0.037; [Hbg]₂,[CO₃], monoclinic, a= 7.075(2), b= 9.373(2), c= 9.474(3)Å, β= 109.52(2)°, space group P2₁, Z= 2 R= 0.030; [Hbg]₂[SO₄]·2H₂O, triclinic, a= 9.012(2), b= 9.055(2), c= 9.974(2)Å, α= 89.69(2),β= 100.06(2), γ= 114.78(2)°, space group P. Z= 2, R= 0.042; [H₂bg][SO₄]·H₂O, orthorhombic, a= 7.208(l), b= 11.805(3), c= 20.507(6)Å, space group Pbca, Z= 8, R= 0.046; and [H₂maldm][SO₄], triclinic, a= 6.300(1), b= 6.985(1), c= 10.030(1)Å, α= 91.30(1), β= 105.59(1), γ= 106.00(1)°, space group P, Z= 2, R= 0.027. The C–N bonds in all the molecules have varying degrees of multiple character. π Bonding through bridging nitrogen is important in all cases, but planarity in [Hbg]⁺ and [H₂bg]²⁺ is destroyed due to steric interactions. Neutral bg is approximately planar but asymmetric; [H₂maldm]²⁺ is non-planar due to the absence of bridge π bonding. All the ionic structures have well defined hydrogen-bonding schemes. Neutral bg is intramolecularly hydrogen bonded. The energy of protonation of bg and [Hbg]⁺ are discussed in structural terms and with regard to the proposed bonding scheme. Bonding parameters are compared with those found in transition-metal complexes of these compounds.