Synthesis, redox chemistry and EPR spectroscopy of the mixed-sandwich complexes (η-arene)(η-cycloheptatrienyl)metal(z+) (M = Cr or Mo; z = 1 or 2): crystal structures of the redox pair [Cr(η-C6H5Me)(η-C7H6C6H4Me-4)][PF6]n (n = 1 or 2)