Nope, that won't do it.

Take a look at how wide that 85" TV is (75" wide).
Then think... you are going to need 1/2 that distance minimum to reach a wall (37.5"). Almost an inch to short.

My original guess was off. The TV would actually come a little over 53" down each wall (assuming 90° and touching each wall).

The framing should be easy. Just build triangular frames to screw to the studs in the corner, that has a 3/4"-1" thick plywood facial mounted to the front. Then just about any arm mount should be able to reach.

[edit] Personally I would do a ceiling mount. Way quicker, easier, and more versatile.