A novel in situ Mnn+-promoted double-aldol reaction is reported. Single crystal X-ray measurements confirm the addition of acetone to two o-vanillin molecules in an original in situα,α double aldol reaction promoted by Mn ions in the presence of base. The newly formed ligands coordinate to four MnIII ions forming a defect-dicubane core structure (1) bridged exclusively by oxygen-based ligands. Other 3d metals were employed under the same reaction conditions, however no aldol addition occurred and tetranuclear cubane-like structures formed using CoII (2) and NiII (3) ions. Magnetic measurements were carried out on all complexes using SQUID magnetometry. Dominant ferromagnetic interactions were observed for complexes 1 and 3 with J=1.8cm−1, J′=−2.5cm−1, g=1.95 for 1 and J=3.1cm−1, g=2.17 for 3 while complex 2 exhibited antiferromagnetic exchange interactions. Notably, complex 1 was shown to exhibit spin frustration rarely seen in {MnIII}4 systems resulting in an intermediate spin ground state of ST=6.