A first representative of coordination polymers, built from polynuclear carboxylate bridged by mononuclear carboxylate, has been synthesised and structurally characterized. The compound [{Fe₃O(HCOO)₆}{Mn(HCOO)₃(H₂O)₃}]·3.5HCOOH crystallised in the hexagonal system (space groupP6₃), a = b = 12.369(5) Å, c = 12.992(5) Å, α = β = 90°, γ = 120°, V = 1721.4(12) ų. The crystal structure is built from 2D layers, which are formed by the linking of trinuclear {Fe₃O(HCOO)₆}⁺ units by mononuclear {Mn(HCOO)₃(H₂O)₃}⁻ bridges. These 2D honeycomb layers are connected by molecules of formic acid through a hydrogen bond network. The compound has the voids at the centre of the hexagons. These voids are filled by captured solvent molecules. Solvate molecules are also located between the 2D layers, linking them through a system of H-bonds. All these solvates can be easily removed and/or exchanged, which results in the formation of [{Fe₃O(HCOO)₆}{Mn(HCOO)₃(H₂O)₃}]·0.5HCOOH·H₂O and [{Fe₃O(HCOO)₆}{Mn(HCOO)₃(H₂O)₃}]. Magnetic properties of the compounds can be considered as the superposition of magnetism of Fe₃O trinuclear units, for which magnetism is governed by the superexchange interaction between Fe³⁺ spins (with one Fe–Fe superexchange parameter J) and the magnetism of spins of Mn²⁺ ions, the interactions between these units can be taken into account by molecular field, zJ′. The destruction of the hydrogen bond network caused by evacuation of solvent molecules significantly modifies the overall magnetism. It was found that zJ′ depends on the presence of solvent molecules in the structure.