Let's proceed step-by-step, in line with your **Crucial Instructions**:

---

### 1. **Requisite Physicochemical Properties for Classification:**  
For organic bases, typical quantitative physicochemical properties relevant for grouping include:
- **Molecular weight**
- **XLogP** (lipophilicity/hydrophobicity indicator)
- **TPSA** (Topological Polar Surface Area)
- **H-bond acceptor/donor count**
- **Rotatable bond count**
- **Number of heavy atoms**
- **Functional group counts (if data available via e.g., “N” or “P” atoms for basicity/structure inference)**

pKa would be ideal for "base strength," but you have not provided these values; thus, we proceed with the available data.

---

## **2. Extracted Quantitative Data (Summary Table)**

| Substance | MW | XLogP | TPSA | H-bond Donor | H-bond Acceptor | Rotatable Bonds | Heavy Atoms | Acceptors/Heavy Atom Ratio |
|-----------|----|-------|------|--------------|-----------------|-----------------|-------------|--------------------------|
| 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine | 153.22 | -0.1 | 18.8 | 0 | 1 | 0 | 11 | 0.09 |
| 2-tert-butyl-1,1,3,3-tetramethylguanidine | 171.28 | 0.8 | 18.8 | 0 | 1 | 3 | 12 | 0.08 |
| N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine | 339.4 | 1.8 | 40.9 | 0 | 7 | 7 | 21 | 0.33 |

---

## **3. Classification by Each Key Property (Quantitative)**

### **A. By Molecular Weight**
- **Group 1:** Low MW (<200): 1, 2
- **Group 2:** High MW (≥200): 3

### **B. By XLogP (Hydrophobicity)**
- **1-methyl…**: -0.1 (hydrophilic)
- **2-tert-butyl…**: 0.8 (slightly hydrophobic)
- **N-[dimethylamino…]**: 1.8 (noticeably hydrophobic)
  - *But range is small (all <2); they could arguably be in one group, but 3 is more hydrophobic.*

### **C. By TPSA (Polarity/Potential for H-bonding)**
- **Low** (≤20): 1, 2
- **Medium-high** (>20): 3

### **D. H-bond Acceptors**
- 1, 2: 1 acceptor each
- 3: 7 acceptors (much higher -- indicates high lone-pair density, might correlate with greater basicity or ligand capacity)

### **E. Rotatable Bonds**
- 1: 0 (rigid)
- 2: 3 (moderately flexible)
- 3: 7 (highly flexible)

### **F. Heavy Atom and Acceptors Ratio**
- 1, 2: ~0.08-0.09 (low)
- 3: 0.33 (significantly higher—i.e., acceptor-rich per "size")

### **G. Structure Count of N and P Atoms (from formula)**
- 1: 3 N
- 2: 3 N
- 3: 7 N, 2 P (markedly richer in basic heteroatoms)

---

## **4. Grouping – Max 3 Classes**

### **Option 1: DISTINCT, DATA-DRIVEN GROUPS**
#### **Group 1: "Standard" Organic Bases — Small, Low Acceptors, Low Flexibility**
- **1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine**
- **2-tert-butyl-1,1,3,3-tetramethylguanidine**
  - *Justification: both have MW <200, XLogP <1, one acceptor, compact structures. Differ in rotatable bonds but both are "small, simple organic bases".*

#### **Group 2: "High Capacity/Base-rich" — Large, Polymeric/Multidentate, Many Acceptors**
- **N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine**
  - *Justification: MW>300, acceptor count 7, 2 P atoms, 7 rotatable bonds, high flexibility, likely very different in coordination and physicochemical profile from 1 or 2.*

### **Option 2: "All in One" Class, If Justified?**
- Only if there were no clear outlier by weight, acceptors, or molecular size. But here, compound 3 is *quantitatively* distinct in every major property (MW, TPSA, Acceptors, N/P load, flexibility).

---

## **5. Final: Group Assignment Table**

| Group | Members | Defining Quantitative Features |
|-------|---------|-------------------------|
| 1     | 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine, 2-tert-butyl-1,1,3,3-tetramethylguanidine | MW <200, XLogP<1, TPSA=18.8, 1 acceptor, ≤3 rot bonds, N3, no P |
| 2     | N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine | MW=339, XLogP=1.8, TPSA=40.9, 7 acceptors, 7 rot bonds, N7+P2 |

---

# **Conclusion/Classification**
- **TWO GROUPS are required, based on clear, quantitative separations in molecular weight, acceptor count, structural flexibility, and N/P content.**
- **No justification, based on data, for placing all in one group.**
- **No clear, data-driven reason to divide the first two any further.**

## **Final Assignment**

### **Group 1 ("Small, Simple Organic Bases"):**
1. 1-methyl-2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine  
2. 2-tert-butyl-1,1,3,3-tetramethylguanidine

### **Group 2 ("Large, Polybasic/Polydentate Base"):**
3. N-[dimethylamino-ethylimino-[[tris(dimethylamino)-lambda5-phosphanylidene]amino]-lambda5-phosphanyl]-N-methylmethanamine

**All assignments are strictly justified by quantitative differences in molecular weight, TPSA, acceptor number, and atom composition (N/P load).**