I have a MacBook Pro 2017 and a iMac 21.5" mid 2011, I would like to use the old iMac as a 2nd monitor

hi I have a MacBook Pro 2017 and a iMac 21.5" mid 2011, I would like to use the old iMac as a 2nd monitor, it has a thunderbolt socket on the back but i'm unsure if it will work or if its too old to use as a 2nd monitor ie pre mini displayport capable ? I would like to know if this will work using a usb-c - thunderbolt adapter and a thunderbolt male to male lead costing £80..

thanks

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