Malformed JSON when sending from Jquery to PHP

I'm trying to send a JQuery request to an API I'm making (first time, learning!) but my php code reports the JSON is malformed.

if I make a JSON array in PHP and pass that through it works fine but if I try to request via JQuery it always says malformed.

Im stuck!

the Javascript side looks like this...

jsonrequest = "{request: 'getJobs', token : 'eb024fab2bf6a1bfb5863dcaabcfd63fcaea50e429237df3f1cbcbfcf9b2'}";
    $.ajax({
    url: 'api.php',
    async: true,
    dataType: 'json',
    contentType: "application/json; charset=utf-8",
    data: jsonrequest,


       success: function ( result ) {
            console.log(result);
        }
});

and the PHP looks like this

$data       = file_get_contents("php://input");

if(isJson($data)) {

        // never passes the isJSON validation.

        $json       = json_decode($data,true);
        $request    = sanitize($json['request']);
        }


function isJSON($string)
{
    // decode the JSON data
    $result = json_decode($string);

    // switch and check possible JSON errors
    switch (json_last_error()) {
        case JSON_ERROR_NONE:
            $error = ''; // JSON is valid // No error has occurred
            break;
        case JSON_ERROR_DEPTH:
            $error = 'The maximum stack depth has been exceeded.';
            break;
        case JSON_ERROR_STATE_MISMATCH:
            $error = 'Invalid or malformed JSON.';
            break;
        case JSON_ERROR_CTRL_CHAR:
            $error = 'Control character error, possibly incorrectly encoded.';
            break;
        case JSON_ERROR_SYNTAX:
            $error = 'Syntax error, malformed JSON.';
            break;
        // PHP >= 5.3.3
        case JSON_ERROR_UTF8:
            $error = 'Malformed UTF-8 characters, possibly incorrectly encoded.';
            break;
        // PHP >= 5.5.0
        case JSON_ERROR_RECURSION:
            $error = 'One or more recursive references in the value to be encoded.';
            break;
        // PHP >= 5.5.0
        case JSON_ERROR_INF_OR_NAN:
            $error = 'One or more NAN or INF values in the value to be encoded.';
            break;
        case JSON_ERROR_UNSUPPORTED_TYPE:
            $error = 'A value of a type that cannot be encoded was given.';
            break;
        default:
            $error = 'Unknown JSON error occured.';
            break;
    }

    if ($error !== '') {
        // throw the Exception or exit // or whatever :)
        $output = array('status' => "error",'message' => $error);
        echo json_encode($output);
        exit;
    }

    // everything is OK
    return $result;
}