.SerializationException: Start of structure or map found where not expected

Question

I am trying to define a JSON object in such a way that it would show output after processing as below-

resolvedEntities={failedEntities=null, resolvedEntities={Rewards$APPA$GetAllPromotions=[{"benefi"}]

but the below json code is throwing me SerializationException with saying Start of structure or map found where not expected.

{
  "resolvedEntities": {
    "failedEntities": "null",
    "resolvedEntities": {
      "Rewards$APPA$GetAllPromotions": "[{\"benefi\"}]"
    }
}
}

Any suggestions for the correct way of defining the JSON for getting output as

resolvedEntities={failedEntities=null, resolvedEntities={Rewards$APPA$GetAllPromotions=[{"benefi"}] .