Electrochemical hydrogen formation catalysed by a Pd8 string.

The linear Pd8 complex supported by tetraphosphines reacted with HBF4 to give an unprecedented linear tetrapalladium complex with a terminal hydride, which promoted electrocatalytic hydrogen formation from HBF4 in acetonitrile. The 1D coordination polymer of Pd8 chain confined within Nafion film was applied to the electrocatalytic H2 formation.

Fine Tunable, Redox Active Octapalladium Chains Supported by Linear Tetraphosphines, Leading to Dynamically 1D Self-Assembled Coordination Polymers.

A series of the octapalladium chains supported by meso-Ph2 PCH2 P(Ph)CH2 P(Ph)CH2 PPh2 (meso-dpmppm) ligands, [Pd8 (meso-dpmppm)4 (L)2 ](BF4 )4 (L=none (1), solvents: CH3 CN (2 a), dmf (2 b), dmso (2 c), RN≡C: R=Xyl (3 a), Mes (3 b), Dip (3 c), t Bu (3 d), Cy (3 e), CH3 (CH2 )7 (3 f), CH3 (CH2 )11 (3 g), CH3 (CH2 )17 (3 h)) and [Pd8 (meso-dpmppm)4 (X)2 ](BF4 )2 (X=Cl (4 a), N3 (4 b), CN (4 c), SCN (4 d)), were synthesized by using 2 a as a stable good precursor, and characterized by spectroscopic (IR, 1 H and 31 P NMR, UV-vis-NIR, ESI-MS) measurements and X-ray crystallographic analyses (for 1, 2 a, b, 3 a, b, e, f, 4 a-d). On the basis of DFT calculations on the X-ray determined structure of 2 b ([2b-Pd8 ]4+ ) and the optimized models [Pd8 (meso-Ph2 PCH2 P(H)CH2 P(H)CH2 PH2 )4 (CH3 CN)2 ]4+ ([Pd8 Ph8 ]4+ ) and [Pd8 (meso-H2 PCH2 P(H)CH2 P(H)CH2 PH2 )4 (CH3 CN)2 ]4+ ([Pd8 H8 ]4+ ), with and without empirically calculating dispersion force stabilization energy (B3LYP-D3, B3LYP), the formation energy between the two Pd4 fragments is assumed to involve mainly noncovalent interactions (ca. -70 kcal/mol) with four sets of interligand C-H/π interactions and Pd⋅⋅⋅Pd metallophilic one, while electron shared covalent interactions are almost canceled out within the Pd8 chain. All the compounds isolated are stable in solution and exhibit characteristic absorption at ∼900 nm, which is assignable to a spin allowed HOMO to LUMO transition, and shows temperature dependent intensity change with variable absorption coefficients presumably due to coupling with some thermal vibrations. The structures and electronic states of the Pd8 chains are found finely tunable by varying the terminal capping ligands. In particular, theoretical calculations elucidated that the HOMO-LUMO energy gap is systematically related to the central Pd-Pd distance (2.7319(6)-2.7575(6) Å) by two ways with neutral ligands L (1, 2, 3) and with anionic ligands X (4), which are reflected on the NIR absorption energy of 867-954 nm. The isocyanide terminated Pd8 complexes (3) further reacted with excess of RNC (6 eq) to afford the Pd4 complexes, [Pd4 (meso-dpmppm)2 (RNC)2 ](BF4 )2 (13), and the cyclic voltammograms of 2 a (L=CH3 CN), 3, and 13 (R=Xyl, Mes, t Bu, Cy) demonstrated wide range redox behaviors from 2{Pd4 }4+ to 2{Pd4 }0 through 2{Pd4 }2+ ↔{Pd8 }4+ , {Pd8 }3+ , and {Pd8 }2+ strings. The oxidized complexes, [Pd4 (meso-dpmppm)2 (RNC)3 ](BF4 )4 (16), were characterized by X-ray analyses, and the two-electron reduced chain of [Pd8 (meso-dpmppm)4 ](BF4 )2 (7) was analyzed by spectroscopic and electrochemical techniques and DFT calculations. Reactions of 2 a with 1 equiv. of aromatic linear bisisocyanide (BI) in CH2 Cl2 deposited insoluble coordination polymers, {[Pd8 (meso-dpmppm)4 (BI)](BF4 )4 }n (5), and interestingly, they were soluble in acetonitrile, 31 P{1 H} and 1 H DOSY NMR spectra as well as SAXS curves suggesting that the coordination polymers may exist in acetonitrile as dynamically 1D self-assembled coordination polymers comprising ca. 50 units of the Pd8 rod averaged within the timescale.