Aiming for dynamic behaviours of molecular metal chains.

The dynamical functions of molecular metal chains, that could be derived from unique physical and chemical properties coupled with self-assembly of metal chain components, have largely been unexplored, and this review paper focuses on the dynamic behaviours of Pd chains supported by linear tetraphosphines, meso- and rac-Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2 recently reported, and the current development on fine-tuning of metal chains by introducing heterometal atoms, i.e. heterometallic molecular metal chains, that are considered as promising dynamical components in the next generation.

Chiral Dinuclear EuIII , TbIII , and YIII Complexes Supported by P-Stereogenic Linear Tetraphosphine Tetraoxide.

A P-stereogenic linear tetraphosphine tetraoxide, (R,R)- or (S,S)-dpmppm(=O)4 , was synthesized to prepare C2 dinuclear M(hfa)3 complexes (M=Eu, Tb, Y) as the first example of lanthanide(III) complexes with P-chiral multidentate phosphine oxides. The mononuclear M(hfa)3 complexes (M=Eu, Y) with a P-chiral diphosphine dioxide, tpdpb(=O)2 , were also prepared, and comparison of their photophysical properties for the EuIII complexes revealed that significant chiral induction from the P-chiral centers arises on the achiral M(hfa)3 units through intramolecular π-π stacking constraint in the dinuclear system.

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.

Octapalladium Strings Trap C60 and C70 Fullerenes Affording Metal-Chain-Wired Bucky Balls.

Reactions of Pd8 strings supported by meso-Ph2 PCH2 P(Ph)CH2 P(Ph)CH2 PPh2 (meso-dpmppm) ligands, [Pd8 (meso-dpmppm)4 (L)2 ]4+ (L=CH3 CN (1), XylNC (2)) with C60 resulted in the exclusive formation of unprecedented metal-chain-wired C60 bucky balls, [{Pd4 (meso-dpmppm)2 (L)}2 (C60 )]4+ (L=CH3 CN (11), XylNC (12)), in which a C60 fullerene is trapped in the central Pd-Pd junction, as unambiguously established by spectroscopic, X-ray crystallographic, and theoretical techniques. The similar reaction of Pd8 strings supported by rac-dpmppm, [Pd8 (rac-dpmppm)4 (CH3 CN)2 ]4+ (3) also afforded a racemic mixture of [{Pd4 ((R*,R*)-dpmppm)2 (CH3 CN)}2 (C60 )]4+ (13) without scrambling the Pd4 fragments with (R,R)- and (S,S)-dpmppm ligands. Consequently, those of enantiopure chiral Pd8 strings, [Pd8 ((R*,R*)-dpmppm)4 (CH3 CN)2 ]4+ , certainly afforded chiral bucky balls of [{Pd4 ((R*,R*)-dpmppm)2 (CH3 CN)}2 (C60 )]4+ (13RR and 13SS ), that exhibit mirror-image circular dichroism spectra. The reactions of 1 and 2 were also applied for trapping a C70 fullerene to give 2 : 1 adducts of [{Pd4 (meso-dpmppm)2 (L)}2 (C70 )]4+ (L=CH3 CN (21), XylNC (22)). These results provide useful information for creating a platform to develop dimensionally and chirality controlled metal-carbon nanocomposite materials.

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.

Chiral Octapalladium Chains Supported by Enantiopure P-Stereogenic Linear Tetraphosphines, (R,R)- and (S,S)-Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2.

Binuclear Pd(II) and Pt(II) complexes supported by rac-dpmppm (bis[(diphenylphosphinomethyl)phenylphosphino]methane) in a triply-bridged Z-form, [M2Cl4(rac-dpmppm)] (M = Pd (3a), Pt (3b)), readily reacted with 2,6-xylyl isocyanide (XylNC) in the presence of NH4PF6 to afford [M2Cl2(rac-dpmppm)(XylNC)2](PF6)2 (M = Pd (4a), Pt (4b)), in which each metal center accommodates one isocyanide ligand at the trans position to the inner P atom of dpmppm. Similarly, treatment of 3a and 3b with axially chiral (R/S)-1,1'-binaphthyl-2,2'-bisisocyanide (rac-Binac) in the presence of NH4OTf gave cyclic tetranuclear complexes, [{M2Cl2(rac-dpmppm)(rac-Binac)}2](OTf)4 (M = Pd (5), Pt (8)), where two {M2Cl2(rac-dpmppm)}2+ fragments are connected by two rac-Binac ligands through chirality sorting of (R*,R*)-dpmppm and (R*)-Binac. Complex 5 could be transformed into the halide exchanged tetranuclear complexes, [{Pd2X2(rac-dpmppm)(rac-Binac)}2](OTf)4 (X = Br (6), I (7)), to show that the rectangular arrangement of four Pd(II) ions is elongated by repulsive interaction between halide ligands. By using (R)- and (S)-Binac, enantiopure Pd4 complexes, [{Pd2Cl2((R*,R*)-dpmppm)((R*)-Binac)}2](OTf)4 (5RR/R and 5SS/S), were successfully isolated as pure crystalline forms, from which enantiopure (R,R)- and (S,S)-dpmppm were obtained by treatment with NaCN aqueous solution. Namely, optical resolution of rac-dpmppm was established through the tetranuclear Pd complexes, which is the first example for methylene-bridged polyphosphines, R2P(CH2PR)nCH2PR2 (n > 0). Furthermore, chiral octapalladium chains, [Pd8((R*,R*)-dpmppm)4(N≡CCH3)2](BF4)4 (2RR and 2SS), were synthesized by reacting enantiopure P-chiral dpmppm with [Pd2(CH3CN)6](BF4)2 and [Pd2(dba)3]·C6H6 and were characterized by spectroscopic and X-ray crystallographic analyses, to determine the absolute configurational structures. The Pd8 chains are the longest enantiopure chiral single-metal-atom chains structurally characterized, thus far, and the electronic structures were examined on the basis of DFT calculations of 2RR.

Facially Dispersed Polyhydride Cu9 and Cu16 Clusters Comprising Apex-Truncated Supertetrahedral and Square-Face-Capped Cuboctahedral Copper Frameworks.

By using a linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm), nona- and hexadecanuclear copper hydride clusters, [Cu9 H7 (µ-dpmppm)3 ]X2 (X=Cl (1 a), Br (1 b), I (1 c), PF6 (1 d)) and [Cu16 H14 (µ-dpmppm)4 ]X2 (X2 =I2 (2 c), (4/3) PF6 ⋅(2/3) OH (2 d)) were synthesized and characterized. They form copper-hydride cages of apex-truncated supertetrahedral {Cu9 H7 }2+ and square-face-capped cuboctahedral {Cu16 H14 }2+ structures. The hydride positions were estimated by DFT calculations to be facially dispersed around the copper frameworks. A kinetically controlled synthesis gave an unsymmetrical Cu8 H6 cluster, [Cu8 H6 (µ-dpmppm)3 ]2+ (3), which readily reacted with CO2 to afford linear Cu4 complexes with formate bridges, leading to an unprecedented hydrogenation of CO2 into formate catalyzed by {Cu4 (µ-dpmppm)2 } platform. The results demonstrate that new motifs of copper hydride clusters could be established by the tetraphosphine ligands, and the structures influence their reactivity.

Tetra-, hexa- and octanuclear copper hydride complexes supported by tridentate phosphine ligands.

Multinuclear copper hydride complexes were synthesized by using a triphosphine, bis(diphenylphosphinomethyl)phenylphosphine (dpmp). The reaction of [Cu(MeCN)4]PF6 with dpmp in 2 : 1 ratio in the presence of Me4NBH4 or NaBH4 yielded a hexanuclear complex, [Cu6(µ3-H)5(µ-dpmp)3]PF6 (1), together with a minor product [Cu8(µ-H)2(µ4-H)4(µ-dpmp)4](PF6)2 (2) in a very low yield. Complex 1 was also prepared from [CuH(PPh3)]6, [Cu(MeCN)4]PF6, and dpmp in 87%, but the yield of 2 could not be improved presumably due to its instability in solution. A tetranuclear complex, [Cu4(µ-H)(µ3-H)2(µ-dpmp)3]PF6 (4), was obtained from the reaction of [Cu(MeCN)4]PF6 with dpmp in 4 : 3 ratio, or that of [Cu3(µ-dpmp)2(MeCN)4](PF6)3 (3) and dpmp, in the presence of Me4NBH4 for both cases. The structures of 1, 2, and 4 were determined by X-ray crystallography, and the positions of hydride ligands were elucidated by DFT optimization. Complex 1 consists of a distorted trigonal anti-prismatic Cu6 core bridged by three L-shaped dpmp ligands and five µ3-hydrides, and 2 is composed of three edge-shared tetrahedral Cu4 units supported by four L-shaped dpmp ligands and two µ2-hydrides in the two outer Cu4 units and four µ4-hydride ligands in the central Cu4 units. The structures of two outer Cu4 units in 2 resemble those of 4, where a Cu4 tetrahedron is supported by two L-shaped dpmp ligands and one dpmp-κ2P,P' bridge, in addition to one µ2- and two µ3-hydrides. Natural bond orbital analyses suggest that the hydrides act as glue between the multinuclear copper centres through electron-deficient delocalized bonding interactions, which result in partial electron migration from hydrides to CuI centres.

Synergistic Cu2 Catalysts for Formic Acid Dehydrogenation.

Hexanuclear copper hydride complexes, [Cu6(µ3-H)2( meso-L4)3(RNC)4](PF6)4 (R = tBu (6a), Cy (6b)), were prepared by using a new linear tetraphosphine, meso-Ph2PCH2P(Ph)(CH2)4P(Ph)CH2PPh2 ( meso-L4), and were converted into active catalysts of [Cu2(µ-O2CH)( meso-L4)(RNC)2]+ under the reaction conditions of formic acid dehydrogenation, where unsymmetric dinuclear copper sites supported by the tetradentate phosphine and isocyanide ligands were essential to demonstrate effective catalytic activity.

Alloyed Tetranuclear Metal Chains of Pd4-n Ptn (n=0-3) Scaffolded by a New Linear Tetraphosphine Containing a PNP Bridge.

A new linear tetraphosphine containing a PNP phosphazane bridge, rac-bis[(diphenylphosphinomethyl)phenylphosphino]phenylamine (rac-dpmppan), was synthesized and utilized to support a series of Pd/Pt mixed metal tetranuclear chains, [Pd4-n Ptn (µ-rac-dpmppan)2 (XylNC)2 ](PF6 )2 (XylNC=xylyl isocyanide; n=0: Pd4 (1), 1: PtPd3 (2), 2: PtPd2 Pt (3), 2: Pt2 Pd2 (4), 3: Pt2 PdPt (5)), in which the number and positions of additional Pt atoms were successfully controlled depending on the respective synthetic procedures using transformations from 1 to 3 through 2 and from 4 to 5 by redox-coupled exchange reactions. The 31 P{1 H} NMR and ESI mass spectra and X-ray diffraction analyses revealed almost identical tetranuclear structures, with slight contraction of metal-metal bonds according to incorporation of Pt atoms. The electronic absorption spectra of 1-5 exhibited characteristic bands at 635-510 nm with an energy propensity depending on the number and positions of Pt centres, which were assigned to HOMO (dσ*σσ*) to LUMO (dσ*σ*σ*) transition by theoretical calculations. The present results demonstrated that the electronic structures of Pd/Pt mixed-metal tetranuclear complexes are finely tuned as orbital-overlapping alloyed metal chains by atomically precise Pt incorporation in the Pd4 chain.

Tri- and Tetranuclear Copper Hydride Complexes Supported by Tetradentate Phosphine Ligands.

Three types of tetradentate phosphine ligands with different central methylene chains and configurations, meso- and rac-Ph2PCH2P(Ph)(CH2) nP(Ph)CH2PPh2 ( n = 2, meso- and rac-dpmppe; n = 3, meso-dpmppp) were utilized to synthesize a new series of tri- and tetranuclear copper hydride complexes. Reactions of meso-dpmppe or meso-dpmppp with CuCl/NH4PF6 or [Cu(CH3CN)4]PF6 in the presence of NaBH4 afforded trinuclear copper hydride complexes, [Cu3(µ3-H)( meso-dpmppe)2](PF6)2 (1) and [Cu3(µ3-H)( meso-dpmppp)2](PF6)2 (2), while a similar reaction with rac-dpmppe resulted in the formation of a tetranuclear copper dihydride complex, [Cu4(µ3-H)2( rac-dpmppe)2](PF6)2 (5). Complexes 1 and 5 further reacted with RNC (R = tBu, Cy, Xyl) to give [Cu3(µ3-H)( meso-dpmppe)2(XylNC)](PF6)2 (3), [Cu4(µ3-H)2( meso-dpmppe)2(RNC)2](PF6)2 (R = tBu (4a), Cy (4b)) and [Cu4(µ3-H)2( rac-dpmppe)2(RNC)2](PF6)2 (R = tBu (6a), Cy (6b), Xyl (6c)), respectively. Complexes 1-6 were characterized by ESI-MS and 1H and 31P NMR spectroscopy and X-ray diffraction analyses, demonstrating that a hydride ligand is located at the center of triangular Cu3 plane of 1-3, while two µ3-hydride-capped Cu3 planes are fused to result in rhombic Cu4H2 structures in 4a,b, 5, and 6a-c. Complexes 1-6 in CD3CN solutions notably showed high thermal stability and no reactivity toward H2O and CO2. DFT calculations indicated an interesting correlation between the Wiberg bond indices (WBI) of Cu-H bonds and their natural atomic charge (NAC), where the isocyanide ligands had an appreciable influence on the Cu-H interactions.

A Fluxional Cu8 H6 Cluster Supported by Bis(diphenylphosphino)methane and its Facile Reaction with CO2.

A copper hydride cluster [Cu8 (µ-H)6 (µ-dppm)5 ](PF6 )2 (dppm=bis(diphenylphosphino)methane) was prepared from reaction of [CuH(PPh3 )]6 with dppm in the presence of [Cu(CH3 CN)4 ]PF6 and exhibited fluxional behaviors in solution where the hydrides and the phosphines are scrambling around the trans-bicapped octahedral Cu8 framework. The Cu8 H6 complex showed facile reactivity with CO2 (1 atm, RT) to afford a tricopper complex, [Cu3 (µ-H)(µ-O2 CH)(µ-dppm)3 ]PF6 , which could be developed to unprecedented hydrosilylation of CO2 catalyzed by multinuclear CuH species under mild conditions.

Chiral Self-Recognition between Stereogenic Tetrapalladium Units Affording Pd8 Chains Supported by Homochiral Tetraphosphines.

By using a chiral tetraphosphine, rac-bis[(diphenylphosphinomethyl)phenylphosphino]methane (rac-dpmppm), linear octapalladium chains were synthesized as discrete molecules of [Pd8 (µ-rac-dpmppm)4 L2 ](BF4 )4 (L=CH3 CN, dmf, XylNC), [Pd8 (µ-rac-dpmppm)4 ](BF4 )4 , and [Pd8 (µ-rac-dpmppm)4 (Cl)2 ](BF4 )2 , which are stable in the solution states and characterized by spectroscopic and crystallographic methods to reveal the octapalladium chains supported by homochiral four tetraphosphines. Variable-temperature NMR studies for a 1:1 mixture of [Pd8 (µ-rac-dpmppm)4 (dmf)2 ](BF4 )4 and [Pd8 (µ-meso-dpmppm)4 (dmf)2 ]-(BF4 )4 in [D7 ]DMF revealed that the Pd8 chains were dissociated at higher temperature (T≈140 °C) into the Pd4 units of {Pd4 (µ-rac-dpmppm)2 }2+ and {Pd4 (µ-meso-dpmppm)2 }2+ , and they were thermodynamically self-aligned to restore the Pd8 chains at lower temperature (T<60 °C), through perfect chiral self-recognition between the stereogenic tetrapalladium units.

Electron-rich linear triplatinum complexes stabilized by a spinning tetraphosphine, tris(diphenylphosphinomethyl)phosphine.

Linear triplatinum complexes with 48e(-), [Pt3(µ-tdpmp)2(RNC)2](PF6)2 (R = 2,6-xylyl (3), (t)Bu (4)), were synthesized by using a branched tetraphosphine, tris(diphenylphosphinomethyl)phosphine (tdpmp), and characterized by crystallographic and spectroscopic analyses to show their novel dynamic behaviour in the solution state, in which the linear Pt3 unit was stabilized by two spinning tetraphosphine ligands.

Oxidative addition of an aromatic ortho C-H bond of tetraphosphine to asymmetric diiridium(i) centres.

Reactions of a tetraphosphine, meso-bis{[(diphenylphosphinomethyl)phenyl]phosphino}propane (dpmppp), with [IrCl(cod)]2 and CO (1 atm) or isocyanide (RNC) in the presence of NH4PF6 at 80-100 °C in dichloromethane/acetonitrile/acetone and/or methanol mixed solvents afforded asymmetric diiridium(ii) complexes, [Ir2(H)(Cl)(µ-(dpmppp-H)-κP(4)C)(CO)3]PF6 (1) and [Ir2(H)(µ-(dpmppp-H)-κP(4)C)(RNC)4)]-(PF6)2 (R = 2,6-xylyl (2), 2,4,6-mesityl (3); dpmppp-H = {PPh(o-C6H4)CH2P(Ph)(CH2)3P(Ph)CH2PPh2}(-)). A similar reaction with (t)BuNC resulted in the formation of a mononuclear Ir(III) complex of [Ir(H)(dpmppp-κP(3))((t)BuNC)2](PF6)2 (4). Complexes 1-3 were characterized by ESI mass spectrometry, (1)H and (31)P NMR spectroscopy and X-ray diffraction analyses. They were found to consist of cis/trans-P,P asymmetric Ir(II)-Ir(II) bonded dinuclear structures derived from oxidative addition of an ortho C-H bond of dpmppp (Ir-Ir = 2.8044(2) Å (1), 2.8569(2) Å (2), and 2.8524(5) Å (3)), resulting in a [IrPCCIr] intermetallic cyclometal-bridge and a terminal hydride. DFT calculations indicated the presence of Ir-Ir, Ir-H, and Ir-Cortho covalent bonds. Initial stages of the reactions with CO and XylNC at room temperature were investigated by (31)P{(1)H} NMR spectroscopy and found to contain a symmetrical Ir(I) dinuclear unit with dpmppp that was readily transformed into 1 and 2 upon heating. The Ir intermediate with XylNC, [Ir2(XylNC)4(µ-dpmppp)](PF6)2 (6), was isolated and characterized by X-ray crystallography and DFT calculations as an electron-deficient 32e(-) Ir species involving a Ir(I)→Ir(I) dative bond (2.7989(5) Å). The reaction pathways from 6 to 2 were investigated by DFT calculations. The present study suggested that a novel oxidative addition of an ortho C-H bond proceeded on the cis/trans-P,P asymmetric diiridium(i) scaffold supported by the tetraphosphine, dpmppp, which was assumed to be facilitated by dimetal cooperation with switching Ir→Ir dative interactions.

Gold and Silver Chains Supported by Linear Hexaphosphine Ligands.

A new linear hexaphosphine, rac-cis,cis,trans-bis{[(diphenylphosphinomethyl)phenylphosphinomethyl]phenylphosphino}methane (P6), was synthesized and isolated as a pure isomer, confirmed by transforming to the corresponding phosphine sulfide. The methylene-bridged linear hexaphosphine readily organized flexible gold(I) and silver(I) hexanuclear chains, [M6(µ-P6)2]X6 (X6 = (OTf)6, M = Au (1), Ag (2); X6 = Cl2(PF6)4, M = Au (3)). The hexaphosphine also supported a tetrasilver(I) complex [Ag4(µ-P6)2](OTf)4 (4), which was readily transformed by treatment with AgOTf into 3, revealing a drastic alternation of the two P6 arrangement. The hexagold(I) chains exhibited a considerably red-shifted absorption (∼410 nm) and emission (540-580 nm) to (1) [5dσ*→6pσ] and from (3) [5dσ*→6pσ] excited states of the metal centers, respectively. The new linear hexaphosphine could be a useful tool to construct linear metal clusters as subnano building blocks.

Stepwise Expansion of Pd Chains from Binuclear Palladium(I) Complexes Supported by Tetraphosphine Ligands.

Reaction of [Pd2(XylNC)6]X2 (X = PF6, BF4) with a linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm), afforded binuclear Pd(I) complexes, [Pd2(µ-dpmppm)2]X2 ([2]X2), through an asymmetric dipalladium complex, [Pd2(µ-dpmppm)(XylNC)3](2+) ([1](2+)). Complex [2](2+) readily reacted with [Pd(0)(dba)2] (2 equiv) and an excess of isocyanide, RNC (R = 2,6-xylyl (Xyl), tert-butyl ((t)Bu)), to generate an equilibrium mixture of [Pd4(µ-dpmppm)2(RNC)2](2+) ([3'](2+)) + RNC ⇄ [Pd4(µ-dpmppm)2(RNC)3](2+) ([3](2+)), from which [Pd4(µ-dpmppm)2(XylNC)3](2+) ([3a](2+)) and [Pd4(µ-dpmppm)2((t)BuNC)2](2+) ([3b'](2+)) were isolated. Variable-temperature UV-vis and (31)P{(1)H} and (1)H NMR spectroscopic studies on the equilibrium mixtures demonstrated that the tetrapalladium complexes are quite fluxional in the solution state: the symmetric Pd4 complex [3b'](2+) predominantly existed at higher temperatures (>0 °C), and the equilibrium shifted to the asymmetric Pd4 complex [3b](2+) at a low temperature (∼-30 °C). The binding constants were determined by UV-vis titration at 20 °C and revealed that XylNC is of higher affinity to the Pd4 core than (t)BuNC. In addition, both isocyanides exhibited higher affinity to the electron deficient [Pd4(µ-dpmppmF2)2(RNC)2](2+) ([3F'](2+)) than to [Pd4(µ-dpmppm)2(RNC)2](2+) ([3'](2+)) (dpmppmF2 = meso-bis[{di(3,5-difluorophenyl)phosphinomethyl}phenylphosphino]methane). When [2]X2 was treated with [Pd(0)(dba)2] (2 equiv) in the absence of RNC in acetonitrile, linearly ordered octapalladium chains, [Pd8(µ-dpmppm)4(CH3CN)2]X4 ([4]X4: X = PF6, BF4), were generated through a coupling of two {Pd4(µ-dpmppm)2}(2+) fragments. Complex [2](2+) was also proven to be a good precursor for Pd2M2 mixed-metal complexes, yielding [Pd2Cl(Cp*MCl) (Cp*MCl2)(µ-dpmppm)2](2+) (M = Rh ([5](2+)), Ir ([6](2+)), and [Au2Pd2Cl2(dpmppm-H)2](2+) ([7](2+)) by treatment with [Cp*MCl2]2 and [AuCl(PPh3)], respectively. Complex [7](2+) contains an unprecedented PC(sp(3))P pincer ligand with a PCPCPCP backbone, dpmppm-H of deprotonated dpmppm. The present results demonstrated that the binuclear Pd(I) complex [2](2+) was a quite useful starting material to extend the palladium chains and to construct Pd-involved heteromultinuclear systems.

Self-alignment of low-valent octanuclear palladium atoms.

A linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) was used to synthesize linear octapalladium-extended metal atom chains as discrete molecules of [Pd8(µ-dpmppm)4](BF4)4 (1) and [Pd8(µ-dpmppm)4L2](BF4)4 (L=2,6-xylyl isocyanide (XylNC; 2), acetonitrile (3), and N,N-dimethylformamide (dmf; 4)), which are stable in the solution states and show interesting temperature-dependent photochemical properties in the near IR region. Variable temperature NMR studies demonstrated that at higher temperature T≈140 °C the Pd8 chains were dissociated into Pd4 fragments, which were thermodynamically self-aligned to restore the Pd8 chains at lower temperature T<60 °C. The coldspray ionization mass spectra suggested a possibility for further aggregation of the linear tetrapalladium units.

Facile insertion of carbon dioxide into Cu2(µ-H) dinuclear units supported by tetraphosphine ligands.

Reactions of meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) with Cu(I) species in the presence of NaBH4 afforded di- and tetranuclear copper hydride complexes, [Cu2(µ-H)(µ-dpmppm)2]X (1) and [Cu4(µ-H)2(µ4-H)(µ-dpmppm)2]X (2) (X=BF4, PF6). Complex 1 undergoes facile insertion of CO2 (1 atm) at room temperature, leading to a formate-bridged dicopper complex [Cu2(µ-HCOO)(dpmppm)2]X (3). The experimental and DFT theoretical studies clearly demonstrate that CO2 insertion into the Cu2(µ-H) unit occurred with the flexible dicopper platform. Complex 2 also undergoes CO2 insertion to give a formate-bridged complex, [Cu4(µ-HCOO)3(dpmppm)2]X, during which the square Cu4 framework opened up to a linear tetranuclear chain.

Flexible, linear, tetranuclear palladium complexes supported by tetraphosphine ligands with electron-withdrawing groups.

A linearly ordered tetraphosphine containing electron-withdrawing substituent groups on the outer phosphorus atoms, meso-bis[{di(3,5-difluorophenyl)phosphinomethyl}phenylphosphino]methane (dpmppmF2), was prepared and reacted with [Pd2(RNC)6](PF6)2 and Pd(dba)2 to afford tetranuclear palladium complexes, [Pd4(µ-dpmppmF2)2(RNC)3](PF6)2 (R = 2,6-xylyl (Xyl) (1), 2,4,6-mesityl (2), 2,6-diisopropylphenyl (3) and tert-butyl (4)), which involve an asymmetric {(RNC)Pd4(CNR)2}(2+) core supported by two dpmppmF2 ligands in anti-arrangement. Each terminal of the Pd4 chain was capped by terminal isocyanide and a semi-bridging RNC is introduced into one terminal Pd site. Mechanistic investigation suggested that the dipalladium(I) complex, [Pd2(µ-dpmppmF2)2(RNC)2](PF6)2 (R = Xyl (6)), was a key intermediate to trap Pd(0) species by the uncoordinated outer phosphine pendants with electron-withdrawing groups. Variable-temperature UV-vis and (31)P{(1)H}, (1)H NMR spectroscopic studies demonstrated that the tetrapalladium complexes are quite fluxional in the solution state at high temperature (>20 °C) relating to a symmetric structure of [Pd4(µ-dpmppmF2)2(RNC)2](PF6)2, and the asymmetric solid state structures are retained even in the solution at low temperature (<-60 °C). Theoretical calculations with DFT methods on the asymmetric (R = Xyl (1)) and symmetric (R = Xyl (1')) structures suggested that contribution of Pd(0)→Pd(I)-Pd(0)-Pd(I) with 60 cluster valence electrons (CVEs) would be dominant in 1, while the symmetric structure of 1' can be recognized as Pd(I)-Pd(0)-Pd(0)-Pd(I) with 58 CVEs. The new tetraphosphine dpmppmF2 was proven very effective in organizing dynamically flexible tetrapalladium chains.