Probing the Electronic Communication of the Isocyanide Bridge Through the Luminescence Properties of the d9-d9 [ClPt(μ-dppm)2Pt(CN-PCP)]+ and A-Frame [ClPd(μ-dppm)2(μ-C=N-PCP)PdCl] Complexes

The homodinuclear d(9)-d(9) ClM(mu-dppm)(2)MCl2 complexes, 1 (M = Pt) and 2 (M = Pd) react with the conjugated and luminescent PCP-NC ligand (3, PCP = [2.2]paracyclophane) to provide the corresponding d(9)-d(9) terminal [ClPt(mu-dppm)(2)Pt(CN-PCP)]Cl (4) and d(8)-d(8) A-frame [ClPd(mu-dppm)(2)(mu-C = N-PCP)PdCl] (5) isocyanide complexes, respectively. These two bimetallic complexes were characterized by IR, H-1, and P-31{H-1} NMR and by chemical analysis. IR data (nu(CN) bridging vs terminal) reveal a terminal isocyanide bonding mode for 4 (2147 cm(-1)) and an A-frame structure for 5 (1616 cm(-1)). The optical and emission properties of the free isocyanide 3 as well as those of the homodinuclear complexes 4 and 5 were studied by UV-visible and luminescence spectroscopy and by photophysical measurements. The unexpected presence of simultaneous intraligand pi pi* fluorescence and phosphorescence attributable to the organic PCP-NC ligand, as well as luminescence from the inorganic M-2-bonded Pt-2(mu-dppm)(2) center arising from a lower energy excited LMCT state (ligand-to-metal-charge-transfer) for 4 at 77 K, indicates a weak conjugation between the two chromophores and an absence of efficient singlet and triplet energy transfers. For 5, only the fluorescence and phosphorescence bands of the PCP-NC ligand are observed [since the A-frame XPd(mu-dppm)(2)(mu-L)PdX (L = isocyanide, X = halide) is not luminescent], stressing that the N equivalent to C bridge exhibits modest electronic communication properties.