Let's systematically classify the substances based strictly on **quantitative physicochemical properties** provided, with all justifications referencing reported numeric data.

## **Candidate List Summary:**
1. **2-tert-butyl-1,1,3,3-tetramethylguanidine**  
   - **Molecular Formula:** C9H21N3  
   - **Molecular Weight:** 171.28  
   - **XLogP:** 0.8  
   - **TPSA:** 18.8  
   - **H-bond Donor:** 0  
   - **H-bond Acceptor:** 1  

2. **N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine**  
   - **Molecular Formula:** C12H35N7P2  
   - **Molecular Weight:** 339.4  
   - **XLogP:** 1.8  
   - **TPSA:** 40.9  
   - **H-bond Donor:** 0  
   - **H-bond Acceptor:** 7  

3. **1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine**  
   - **Molecular Formula:** C8H15N3  
   - **Molecular Weight:** 153.22  
   - **XLogP:** -0.1  
   - **TPSA:** 18.8  
   - **H-bond Donor:** 0  
   - **H-bond Acceptor:** 1  

---

### **Step 1: Summarize Relevant Quantitative Properties**

Physicochemical properties usually used for classification in a database context include:  
- **Lipophilicity (XLogP):** measures partition coefficient (hydrophobic/hydrophilic)
- **Topological Polar Surface Area (TPSA):** related to hydrogen bonding capacity, affects permeability
- **H-bond Donor/Acceptor Count:** affects solubility, reactivity, polarity
- **Molecular Weight:** size, affects physical phase/characteristics

### **Step 2: Quantitative Grouping Analysis**

#### **A. Lipophilicity/Partition Coefficient (XLogP)**
- **Common Database Category Thresholds:**  
  - **Very Hydrophilic:** XLogP < 0  
  - **Amphiphilic/Moderately Polar:** 0 ≤ XLogP < 3  
  - **Hydrophobic:** XLogP ≥ 3

| Substance | XLogP  | Group       |
|---|-------|------------|
| 2-tert-butyl-1,1,3,3-tetramethylguanidine | 0.8   | Moderately Polar |
| N-[dimethylamino...phosphanyl]-N-methylmethanamine | 1.8   | Moderately Polar |
| 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine | -0.1  | Very Hydrophilic |

**Result:**  
- 2 substances (Guanidine, Phosphanyl) are moderately polar  
- Pyrimidopyrimidine is just under the cutoff but only barely, grouping could be ambiguous

#### **B. Topological Polar Surface Area (TPSA)**
- **Typical Interpretation:**
  - **Low polarity:** TPSA < 30 Å²
  - **Medium polarity:** 30 ≤ TPSA < 75 Å²
  - **High polarity:** TPSA ≥ 75 Å²

| Substance | TPSA   | Group        |
|---|-------|-------------|
| 2-tert-butyl-1,1,3,3-tetramethylguanidine | 18.8   | Low polarity |
| N-[dimethylamino...phosphanyl]-N-methylmethanamine | 40.9   | Medium polarity |
| 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine | 18.8  | Low polarity |

**Result:**  
- Guanidine & hexahydropyrimido are “Low polarity”  
- Phosphanyl is "Medium polarity"

#### **C. Hydrogen Bond Acceptors**
- **Potential Categories:**
  - **Few acceptors:** 0-2
  - **Many acceptors:** >2

| Substance | H-bond Acceptors | Group        |
|---|----------------|-------------|
| 2-tert-butyl-1,1,3,3-tetramethylguanidine | 1 | Few  |
| N-[dimethylamino...phosphanyl]-N-methylmethanamine | 7 | Many |
| 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine | 1 | Few  |

**Result:**  
- Guanidine & pyrimido: "Few"  
- Phosphanyl: "Many"

---

## **Step 3: Final Grouping Proposal (Based strictly on Data Sets Above)**

### Option 1: **Based on Quantitative Clustering (TPSA & H-bond Acceptors)**
- **Group 1:** “Low polarity, few acceptors”  
  - 2-tert-butyl-1,1,3,3-tetramethylguanidine  
  - 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine  
- **Group 2:** “Medium polarity, many acceptors”  
  - N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine  

### Option 2: **All in a Single Group?**
- *Justification:* TPSA, XLogP, and acceptor count indicate phosphanyl compound is much more polar and has more acceptors, and thus is distinguished quantitatively from the other two.  
- *Conclusion:* Quantitative differences justify separation. **A single group is not appropriate**.

---

## **Final, Data-Driven Classification**

**Class 1: Low-polarity, few H-bond acceptors**  
- 2-tert-butyl-1,1,3,3-tetramethylguanidine  
- 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine  

**Class 2: Medium-polarity, many H-bond acceptors**  
- N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine  

---

### **Summary Table**

| Substance | XLogP | TPSA | H-Acceptors | Class (Quantitative Justification)               |
|-----------|-------|------|-------------|--------------------------------------------------|
| 2-tert-butyl-1,1,3,3-tetramethylguanidine | 0.8   | 18.8 | 1           | 1 (“Low-polarity, few acceptors”)                |
| 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine | -0.1  | 18.8 | 1           | 1 (“Low-polarity, few acceptors”)                |
| N-[dimethylamino...phosphanyl]-N-methylmethanamine | 1.8   | 40.9 | 7           | 2 (“Medium-polarity, many H-bond acceptors”)      |

**No more than two classes are justified based on database-like, quantitative property separation.**  
If you require a further split or have additional property requirements (e.g., volatility, basicity), please clarify.