Your solution provides protection for -4.2 volts and 6.7 volts. This exceeds the maximum ratings of the ADC and is not safe. Also Zener diodes often have higher resistance than normal diodes, making them less ideal for protection.
It is of course possible to protect you ADC this way, by finding a zener that is a perfect match, but I would recommend doing it another way.
A simple resistor will in most cases do to limit currents, But if the ADC draws much current it is already broken. Usually the protection for high input currents would be in the signal conditioning part.
Two diodes connected from the input, as shown in the schematic, will protect your input from voltages under -Vf and over VCC+Vf, so if you choose diodes that have a forvard voltage of Less than 0.3 volts, you will be safe.
Diode type is at your choosing, but you might want some schottky diodes in there for the forward voltage to be right. (1n4148 is standard in the schematic editor) Diodes with Vf under 1 volt exists.
If your input is designed to be floating, I think you should add a high value resistor between 100k to 4Meg from the input to ground to have it acting less like an antenna and provide a path for offset currents. (this will of course affect high frequency behavior)
simulate this circuit – Schematic created using CircuitLab