TV wall mount problem. (Studs too far apart)
The best way to deal with this problem is to first mount a suitable piece of 3/4" plywood on the wall such that it spans to studs where it can be securely attached. The TV mount can then be mounted to the plywood using suitable screws that go into properly sized pilot holes. Note that face screwing into the plywood provides an excellent and strong mount point.

You may want to paint the plywood with primer and then a color to match the wall before affixing the TV mount. 

Note that if the far distance of the stud spacing is due to trying to mount the TV over a fireplace then you should totally reconsider mounting the TV there. The main issue being that the best ergonomic placement of the TV is to locate it at eye level when you are sitting in front of it. Having to look up at the TV, even if it is tilted downwards, is a pain in the neck. (literally!). There is also consideration of what heat from a fireplace may do the sensitive electronics and plastics of the TV unit.