Thermal C−H Bond Activation of Benzene with Cationic [Pt(CX3)(L)]+Complexes in the Gas Phase: A Combined Experimental/Theoretical Study (X = H, D; L = 1,10-Phenanthroline, 2,2′-Bipyrimidine, 2,2′-Bipyridine, and (o,o′-Cl2C6H3)N═C(CH3)−C(CH3)═N(o,o′-Cl2C6H3))†