Let's proceed step by step, following your strict requirements for quantitative, database-derived data. We’ll use only the quantitative property values provided, and compile **relevant physicochemical properties** for classification.

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## **Relevant Physicochemical Properties Compiled:**

| Substance | MW (g/mol) | XLogP | TPSA (Å²) | H-bond Acceptors | H-bond Donors | Rot. Bonds | 
|-----------|------------|-------|-----------|------------------|---------------|------------|
| 2-tert-butyl-1,1,3,3-tetramethylguanidine | 171.28 | 0.8   | 18.8      | 1                | 0             | 3          |
| N-[dimethylamino-ethylimino-[[tris(dimethylamino)-λ⁵-phosphanylidene]amino]-λ⁵-phosphanyl]-N-methylmethanamine | 339.4  | 1.8   | 40.9      | 7                | 0             | 7          |
| 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine | 153.22 | -0.1  | 18.8      | 1                | 0             | 0          |

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### **Selection of Classification Criteria**

#### **Based on Available Quantitative Data:**
1. **Polarity** ([TPSA + XLogP]):  
   - XLogP < 0 → Hydrophilic  
   - XLogP 0–3 → Weakly lipophilic (amphiphilic/low-medium polarity)
   - TPSA < 30 Å² → Poor H-bonding with water, generally lower polarity  
   - TPSA > 40 Å² → Suggests higher polarity

2. **H-bond acceptor capacity:**  
   - ≥4 acceptors = high polar interaction capability  
   - 1 acceptor = low polar interaction capability

3. **Molecular Size/Flexibility** (rotatable bonds):  
   - ≤3 = rigid  
   - ≥4 = flexible

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## **Stepwise Classification**

### **1. Polarity (XLogP and TPSA as primary criteria)**

- **2-tert-butyl-1,1,3,3-tetramethylguanidine**:
  - XLogP: 0.8, TPSA: 18.8 → moderate amphiphilic, low TPSA
- **N-[dimethylamino-ethylimino-[[tris(dimethylamino)-λ⁵-phosphanylidene]amino]-λ⁵-phosphanyl]-N-methylmethanamine**:
  - XLogP: 1.8, TPSA: 40.9 → slightly more lipophilic, **significantly higher TPSA**
- **1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine**:
  - XLogP: -0.1, TPSA: 18.8 → more hydrophilic, low TPSA

#### **A clear cutoff emerges at TPSA ≈ 40 Å²**. Only one compound fits the "high-polarity" window by TPSA (≥40). The other two are both in a lower, nearly identical TPSA (≈19), and similar in XLogP (within 1 unit), indicating a closer class.

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### **2. H-bond Acceptor Count**

- N-[dimethyl...phosphanyl]-N-methylmethanamine: 7 acceptors (can interact more with water/polar media)
- The others: 1 acceptor

This supports the prior division from polarity metrics (TPSA), reinforcing the separation.

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### **3. Flexibility (Rotatable Bonds)**

- N-[dimethyl...phosphanyl]-N-methylmethanamine: 7 (much more flexible)
- 2-tert-butyl...guanidine: 3
- 1-methyl-...pyrimidine: 0

But this difference does **not** override the much clearer separation of polarity as measured by TPSA and acceptor count.

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## **Final Group Assignment:**

**Group 1: Low TPSA / Low H-bond acceptor / Lower polarity**  
  - 2-tert-butyl-1,1,3,3-tetramethylguanidine  
  - 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine  

**Group 2: High TPSA / High H-bond acceptor / Higher polarity**  
  - N-[dimethylamino-ethylimino-[[tris(dimethylamino)-λ⁵-phosphanylidene]amino]-λ⁵-phosphanyl]-N-methylmethanamine

## **Justification:**
All key available quantitative metrics (TPSA, XLogP, H-bond acceptors) separate the phosphanyl-derived compound from the other two. There is no evidence (from provided data) to justify three independent groups: the two guanidine- and pyrimidine-derived compounds are quantitatively more similar than either is to the phosphanyl derivative. Grouping by rotatable bonds would create arbitrary outliers not supported by polarity data.

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# **Summary Table**
| Group     | Compound                                                                          | Key Quantitative Rationale                                                |
|-----------|-----------------------------------------------------------------------------------|---------------------------------------------------------------------------|
| **1**     | 2-tert-butyl-1,1,3,3-tetramethylguanidine, 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine | Both have TPSA ≈ 19, XLogP from -0.1 to 0.8, 1 H-bond acceptor             |
| **2**     | N-[dimethylamino-ethylimino-[[tris(dimethylamino)-λ⁵-phosphanylidene]amino]-λ⁵-phosphanyl]-N-methylmethanamine             | TPSA = 40.9, XLogP = 1.8, 7 H-bond acceptors (substantially more polar)    |


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**There is no quantitative justification for more than 2 classes given the available physicochemical property data.**