Nanoparticle, contrast agent for magnetic resonance imaging comprising same and zwitterionic ligand compound

A nanoparticle comprising:

• at least one zwitterionic ligand represented by a formula (I);
• and a metal particle containing iron oxide, the at least one zwitterionic ligand being coordinately bound to the metal particle:
• where
• one of R1 and R2 is a group represented by a formula (a) or a formula (b), and the other of R1 and R2 is H, lower alkyl, —O— lower alkyl, or halogen,
• X1 is a bond or methylene, or X1 is optionally ethylene when R1 is a group represented by the formula (a),
• X2 is C1-5 alkylene that is optionally substituted with OH or is —C1-2 alkylene-O—C1-3 alkylene-, or X2 is optionally a bond when R1 is a group represented by the formula (b),
• Ra and Rb are the same as or different from each other and represent C1-3 alkyl or —C1-3 alkylene-O—C1-2 alkyl, or Ra and Rb form a 5- or 6-membered nitrogen-containing saturated heterocycle together with a quaternary nitrogen atom to which Ra and Rb are bound,
• Y− is SO3−, HPO3−, or CO2−,
• R3 and R4 are the same as or different from each other and represent H, C1-3 alkyl, —O—C1-3 alkyl, or halogen,
• n is an integer of 0 to 2,
• and,
• i) when R1 is a group represented by the formula (a) and X1 is methylene, R2 optionally forms ethylene together with Ra or Rb,
• ii) when R1 is a group represented by the formula (a) and X1 is ethylene, R2 optionally forms methylene together with Ra or Rb, and
• iii) when R2 is a group represented by the formula (a) and X1 is methylene, R3 optionally forms ethylene together with Ra or Rb,
• provided that, when R2 is a group represented by the formula (a), Ra and Rb are methyl, X1 is a bond, X2 is C1-4 alkylene, and R1, R3 and R4 are H, Y− is HPO3− or CO2−.