Below is one defensible classification based largely on quantitative properties (molecular weight, topological polar surface area [TPSA], H‐bond acceptor count, rotatable bond count, and the heavy atom count, which in these cases also tells us about heteroatom content such as phosphorus). In our “database‐style” analysis the three molecules fall into two distinct groups:

• Group 1 (“Non‐phosphorus” nitrogen bases):  
 – “2-tert-butyl‑1,1,3,3‑tetramethylguanidine” has a molecular weight of 171.28, TPSA = 18.8, 1 H-bond acceptor, 3 rotatable bonds, and 12 heavy atoms (all C, N; no phosphorus).  
 – “1‑methyl‑2,3,4,6,7,8‑hexahydropyrimido[1,2‑a]pyrimidine” has a molecular weight of 153.22, TPSA = 18.8, 1 H-bond acceptor, 0 rotatable bonds, and 11 heavy atoms (again consisting only of C and N).  

Both substances are relatively low in molecular weight (<180), have identical (low) TPSA (18.8) and only one H‑bond acceptor, and lack any phosphorus. (One is slightly flexible [3 rotatable bonds] while the other is rigid [0 rotatable bonds].) Quantitatively, they cluster together.

• Group 2 (“Phosphorus‑containing” nitrogen–phosphorus bases):  
 – “N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine” stands apart with a much higher molecular weight (339.40 versus ≈155–171), a higher TPSA of 40.9, 7 H‑bond acceptors (versus 1), 7 rotatable bonds, and 21 heavy atoms. Notably, its molecular formula (C12H35N7P2) indicates the presence of two phosphorus centers.  

Because its key quantitative parameters (mass, polar surface area, H-bond acceptor count, and heavy atom composition) are distinctly higher than in the other two compounds, we assign it to its own group.

Thus, using a quantitative, data‐directed approach based on typical database properties, we classify the candidate substances into two groups.

In summary, the classification is:  

  Group 1 (Non‐phosphorus nitrogen bases):  
   • 2‑tert‑butyl‑1,1,3,3‑tetramethylguanidine  
   • 1‑methyl‑2,3,4,6,7,8‑hexahydropyrimido[1,2‑a]pyrimidine  

  Group 2 (Phosphorus‑containing polyamine type):  
   • N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine

This grouping is justified by the measured quantitative differences in molecular weight, TPSA, H-bond acceptor count, heavy atom count, and the presence/absence of phosphorus centers.