Null value comes from Post method(FromBody) when api is invoked from powershell

Post method parameter is taken as null when API is invoked from powershell. Below is the JSON

"TestCase":{ "tc_name":"TestCase1" }, "3":{ "scn_desc":"Create Client34345", "test_status":"PASS", "error_link":"" } , "4":{ "scn_desc":"Create Client43634", "test_status":"PASS", "error_link":"" }, "5":{ "scn_desc":"Create Client346346", "test_status":"PASS", "error_link":"" }
}

$json contains the above json array. Powershell: Invoke-WebRequest -Uri http://localhost:65452/api/e10/e10PostTCData -Method Post -Body $json -ContentType 'application/json'

API:

 [Route("e10PostTCData/")]
 [HttpPost]
 public HttpResponseMessage PostResults([FromBody]JsonArray jsonArray )
 {

 }

 public class JsonArray
 {
   public string json { get; set; }
 }

Other way:

[Route("e10PostTCData/")]
[HttpPost]
public HttpResponseMessage PostResults([FromBody]String jsonArray )
{

}

Both the methods shows null as the parameter. Please advise.