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

The homodinuclear d9-d9 ClM(μ-dppm)2MCl2 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 d9-d9 terminal [ClPt(μ-dppm)2Pt(CN-PCP)]Cl (4) and d8-d8 A-frame [ClPd(μ-dppm)2(μ-C=N-PCP)PdCl] (5) isocyanide complexes, resp. These two bimetallic complexes were characterized by IR, 1H, and 31P{1H} NMR and by chem. anal. IR data (ν(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 photophys. measurements. The unexpected presence of simultaneous intraligand ππ* fluorescence and phosphorescence attributable to the org. PCP-NC ligand, as well as luminescence from the inorg. M2-bonded Pt2(μ-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 obsd. [since the A-frame XPd(μ-dppm)2(μ-L)PdX (L = isocyanide, X = halide) is not luminescent], stressing that the N≡C bridge exhibits modest electronic communication properties.