You're missing a single quote here:
NSString *jsonMap = @"{'query' : 'start x = node(4,6,7) return x order by x.datestart***'***,'params' : {}}";
And actually, it needs to have double quotes:
NSString *jsonMap = @"{\"query\" : \"start x = node(4,6,7) return x order by x.datestart\", \"params\" : {}}";
Test with cURL:
curl -H Accept:application/json -H Content-Type:application/json -X POST -d '{"query" : "start x = node(4,6,7) return x order by x.datestart", "params" : {}}' localhost:7474/db/data/cypher