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))