Electronic Circuit with Opamp and Transistor to pull down a reference voltage via a diode. (Voltage Limiting)
ideally rail to rail


Rail to rail won't happen because of the diode drop. 

Use a PNP bipolar transistor with base on op-amp output, collector to ground and emitter connected to VF3 via the diode. If your supply rail is genuinely only 2.5 volts then you probably won't require the series diode. Any supply rail greater than 5 or 6 volts requires a diode to prevent the base-emitter region getting too much reverse bias and exceeding the transistor specification.