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, <i>meso</i>-bis­[(diphenylphosphinomethyl)­phenylphosphino]­methane (dpmppm), afforded binuclear Pd^I complexes, [Pd₂(μ-dpmppm)₂]­X₂ ([<b>2</b>]­X₂), through an asymmetric dipalladium complex, [Pd₂(μ-dpmppm)­(XylNC)₃]²⁺ ([<b>1</b>]²⁺). Complex [<b>2</b>]²⁺ readily reacted with [Pd⁰(dba)₂] (2 equiv) and an excess of isocyanide, RNC (R = 2,6-xylyl (Xyl), <i>tert</i>-butyl (^<i>t</i>Bu)), to generate an equilibrium mixture of [Pd₄(μ-dpmppm)₂(RNC)₂]²⁺ ([<b>3</b>′]²⁺) + RNC ⇄ [Pd₄(μ-dpmppm)₂(RNC)₃]²⁺ ([<b>3</b>]²⁺), from which [Pd₄(μ-dpmppm)₂(XylNC)₃]²⁺ ([<b>3a</b>]²⁺) and [Pd₄(μ-dpmppm)₂(^<i>t</i>BuNC)₂]²⁺ ([<b>3b</b>′]²⁺) were isolated. Variable-temperature UV–vis and ³¹P­{¹H} and ¹H NMR spectroscopic studies on the equilibrium mixtures demonstrated that the tetrapalladium complexes are quite fluxional in the solution state: the symmetric Pd₄ complex [<b>3b</b>′]²⁺ predominantly existed at higher temperatures (>0 °C), and the equilibrium shifted to the asymmetric Pd₄ complex [<b>3b</b>]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 ^<i>t</i>BuNC. In addition, both isocyanides exhibited higher affinity to the electron deficient [Pd₄(μ-dpmppmF₂)₂(RNC)₂]²⁺ ([<b>3F</b>′]²⁺) than to [Pd₄(μ-dpmppm)₂(RNC)₂]²⁺ ([<b>3</b>′]²⁺) (dpmppmF₂ = <i>meso</i>-bis­[{di­(3,5-difluorophenyl)­phosphinomethyl}­phenylphosphino]­methane). When [<b>2</b>]­X₂ was treated with [Pd⁰(dba)₂] (2 equiv) in the absence of RNC in acetonitrile, linearly ordered octapalladium chains, [Pd₈(μ-dpmppm)₄(CH₃CN)₂]­X₄ ([<b>4</b>]­X₄: X = PF₆, BF₄), were generated through a coupling of two {Pd₄(μ-dpmppm)₂}²⁺ fragments. Complex [<b>2</b>]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 ([<b>5</b>]²⁺), Ir ([<b>6</b>]²⁺), and [Au₂Pd₂Cl₂(dpmppm–H)₂]²⁺ ([<b>7</b>]²⁺) by treatment with [Cp*MCl₂]₂ and [AuCl­(PPh₃)], respectively. Complex [<b>7</b>]²⁺ contains an unprecedented PC­(sp³)P pincer ligand with a PCPCPCP backbone, dpmppm–H of deprotonated dpmppm. The present results demonstrated that the binuclear Pd^I complex [<b>2</b>]²⁺ was a quite useful starting material to extend the palladium chains and to construct Pd-involved heteromultinuclear systems

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, <i>meso</i>-bis­[(diphenylphosphinomethyl)­phenylphosphino]­methane (dpmppm), afforded binuclear Pd^I complexes, [Pd₂(μ-dpmppm)₂]­X₂ ([<b>2</b>]­X₂), through an asymmetric dipalladium complex, [Pd₂(μ-dpmppm)­(XylNC)₃]²⁺ ([<b>1</b>]²⁺). Complex [<b>2</b>]²⁺ readily reacted with [Pd⁰(dba)₂] (2 equiv) and an excess of isocyanide, RNC (R = 2,6-xylyl (Xyl), <i>tert</i>-butyl (^<i>t</i>Bu)), to generate an equilibrium mixture of [Pd₄(μ-dpmppm)₂(RNC)₂]²⁺ ([<b>3</b>′]²⁺) + RNC ⇄ [Pd₄(μ-dpmppm)₂(RNC)₃]²⁺ ([<b>3</b>]²⁺), from which [Pd₄(μ-dpmppm)₂(XylNC)₃]²⁺ ([<b>3a</b>]²⁺) and [Pd₄(μ-dpmppm)₂(^<i>t</i>BuNC)₂]²⁺ ([<b>3b</b>′]²⁺) were isolated. Variable-temperature UV–vis and ³¹P­{¹H} and ¹H NMR spectroscopic studies on the equilibrium mixtures demonstrated that the tetrapalladium complexes are quite fluxional in the solution state: the symmetric Pd₄ complex [<b>3b</b>′]²⁺ predominantly existed at higher temperatures (>0 °C), and the equilibrium shifted to the asymmetric Pd₄ complex [<b>3b</b>]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 ^<i>t</i>BuNC. In addition, both isocyanides exhibited higher affinity to the electron deficient [Pd₄(μ-dpmppmF₂)₂(RNC)₂]²⁺ ([<b>3F</b>′]²⁺) than to [Pd₄(μ-dpmppm)₂(RNC)₂]²⁺ ([<b>3</b>′]²⁺) (dpmppmF₂ = <i>meso</i>-bis­[{di­(3,5-difluorophenyl)­phosphinomethyl}­phenylphosphino]­methane). When [<b>2</b>]­X₂ was treated with [Pd⁰(dba)₂] (2 equiv) in the absence of RNC in acetonitrile, linearly ordered octapalladium chains, [Pd₈(μ-dpmppm)₄(CH₃CN)₂]­X₄ ([<b>4</b>]­X₄: X = PF₆, BF₄), were generated through a coupling of two {Pd₄(μ-dpmppm)₂}²⁺ fragments. Complex [<b>2</b>]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 ([<b>5</b>]²⁺), Ir ([<b>6</b>]²⁺), and [Au₂Pd₂Cl₂(dpmppm–H)₂]²⁺ ([<b>7</b>]²⁺) by treatment with [Cp*MCl₂]₂ and [AuCl­(PPh₃)], respectively. Complex [<b>7</b>]²⁺ contains an unprecedented PC­(sp³)P pincer ligand with a PCPCPCP backbone, dpmppm–H of deprotonated dpmppm. The present results demonstrated that the binuclear Pd^I complex [<b>2</b>]²⁺ was a quite useful starting material to extend the palladium chains and to construct Pd-involved heteromultinuclear systems

Heterotrinuclear Complexes with Palladium, Rhodium, and Iridium Ions Assembled by Conformational Switching of a Tetraphosphine Ligand around a Palladium Center

Reaction of [PdCl₂(cod)] with a tetraphosphine, <i>meso</i>-bis­[((diphenylphosphino)­methyl)­phenylphosphino]­methane (dpmppm), afforded the mononuclear Pd^II complexes [PdCl­(dpmppm-κ³)]­X (X = Cl (<b>1a</b>), PF₆ (<b>1b</b>)); the pincer-type dpmppm ligand coordinates to the Pd atom with two outer and one inner phosphorus atom to form fused six- and four-membered chelate rings. The remaining inner phosphine is uncoordinated and readily reacts with [Cp*MCl₂]₂ to give the heterodimetallic complexes [PdCl­(Cp*MCl₂)­(μ-dpmppm-κ³,κ¹)]­X (X = Cl, M = Rh (<b>21a</b>), Ir (<b>21b</b>); X = PF₆, M = Rh (<b>23a</b>), Ir (<b>23b</b>)). Attachment of the second metal fragment to the uncoordinated phosphine caused a crucial conformational change of the six-membered chelate ring from a stable chair conformation to a twist-boat structure, which concomitantly destabilizes the four-membered ring for its opening reactions. Complexes <b>21</b> (X = Cl) were converted to [PdCl₂(Cp*MCl₂)­(dpmppmO)], in which the terminal P atom is dissociated and oxidized as Ph₂P­(O)­CH₂P­(Ph)­CH₂P­(Ph)­CH₂PPh₂ (dpmppmO), and in the presence of another 1 equiv of [Cp*M′Cl₂]₂, complexes <b>21</b> were readily transformed into the heterotrinuclear complexes [PdCl₂(Cp*M′Cl₂)­(Cp*MCl₂)­(μ-dpmppm-κ²,κ¹,κ¹)] (M = M′ = Rh (<b>31a</b>), Ir, (<b>31b</b>); M = Ir, M′ = Rh (<b>31c</b>)), where the third metal M′ is trapped by the terminal P atom with its four-membered-ring opening. Complexes <b>23</b> also reacted with another 1 equiv of [Cp*M′Cl₂]₂ to afford the heterotrinuclear complexes [PdCl­(μ-Cl)­(Cp*M′Cl)­(Cp*MCl₂)­(μ-dpmppm-κ²,κ¹,κ¹)]­PF₆ (M = M′ = Rh (<b>32a</b>), Ir, (<b>32b</b>); M = Ir, M′ = Rh (<b>32c</b>), M = Rh, M′ = Ir (<b>32d</b>)); the additional metal M′ is ligated by the terminal phosphine and is further connected to the Pd atom via a chloride bridge, resulting in a rather electron-deficient M′ center on the basis of cyclic voltammetry. These results exhibited that the addition of a bulky metal fragment to the uncoordinated phosphine of <b>1</b> brings about a conformational switch around the Pd center to promote the ring-opening reaction of the four-membered chelate ring, which leads to an incorporation of the third metal fragment to construct heterotrinuclear structures

Heterotrinuclear Complexes with Palladium, Rhodium, and Iridium Ions Assembled by Conformational Switching of a Tetraphosphine Ligand around a Palladium Center

Reaction of [PdCl₂(cod)] with a tetraphosphine, <i>meso</i>-bis­[((diphenylphosphino)­methyl)­phenylphosphino]­methane (dpmppm), afforded the mononuclear Pd^II complexes [PdCl­(dpmppm-κ³)]­X (X = Cl (<b>1a</b>), PF₆ (<b>1b</b>)); the pincer-type dpmppm ligand coordinates to the Pd atom with two outer and one inner phosphorus atom to form fused six- and four-membered chelate rings. The remaining inner phosphine is uncoordinated and readily reacts with [Cp*MCl₂]₂ to give the heterodimetallic complexes [PdCl­(Cp*MCl₂)­(μ-dpmppm-κ³,κ¹)]­X (X = Cl, M = Rh (<b>21a</b>), Ir (<b>21b</b>); X = PF₆, M = Rh (<b>23a</b>), Ir (<b>23b</b>)). Attachment of the second metal fragment to the uncoordinated phosphine caused a crucial conformational change of the six-membered chelate ring from a stable chair conformation to a twist-boat structure, which concomitantly destabilizes the four-membered ring for its opening reactions. Complexes <b>21</b> (X = Cl) were converted to [PdCl₂(Cp*MCl₂)­(dpmppmO)], in which the terminal P atom is dissociated and oxidized as Ph₂P­(O)­CH₂P­(Ph)­CH₂P­(Ph)­CH₂PPh₂ (dpmppmO), and in the presence of another 1 equiv of [Cp*M′Cl₂]₂, complexes <b>21</b> were readily transformed into the heterotrinuclear complexes [PdCl₂(Cp*M′Cl₂)­(Cp*MCl₂)­(μ-dpmppm-κ²,κ¹,κ¹)] (M = M′ = Rh (<b>31a</b>), Ir, (<b>31b</b>); M = Ir, M′ = Rh (<b>31c</b>)), where the third metal M′ is trapped by the terminal P atom with its four-membered-ring opening. Complexes <b>23</b> also reacted with another 1 equiv of [Cp*M′Cl₂]₂ to afford the heterotrinuclear complexes [PdCl­(μ-Cl)­(Cp*M′Cl)­(Cp*MCl₂)­(μ-dpmppm-κ²,κ¹,κ¹)]­PF₆ (M = M′ = Rh (<b>32a</b>), Ir, (<b>32b</b>); M = Ir, M′ = Rh (<b>32c</b>), M = Rh, M′ = Ir (<b>32d</b>)); the additional metal M′ is ligated by the terminal phosphine and is further connected to the Pd atom via a chloride bridge, resulting in a rather electron-deficient M′ center on the basis of cyclic voltammetry. These results exhibited that the addition of a bulky metal fragment to the uncoordinated phosphine of <b>1</b> brings about a conformational switch around the Pd center to promote the ring-opening reaction of the four-membered chelate ring, which leads to an incorporation of the third metal fragment to construct heterotrinuclear structures

Electron-Deficient Pt₂M₂Pt₂ Hexanuclear Metal Strings (M = Pt, Pd) Supported by Triphosphine Ligands

Electron-deficient Pt₂M₂Pt₂ hexanuclear clusters, [Pt₄M₂(μ-dpmp)₄(XylNC)₂]­(PF₆)₄ (M = Pt (<b>7</b>), Pd (<b>8</b>); dpmp = bis­((diphenylphosphino)­methyl)­phenylphosphine), were synthesized by oxidation of hydride-bridged hexanuclear clusters [Pt₄M₂(μ-H)­(μ-dpmp)₄(XylNC)₂]­(PF₆)₃ (M = Pt (<b>2</b>), Pd (<b>3</b>)) and were revealed to involve a linearly ordered Pt₂M₂Pt₂ array joined by delocalized bonding interactions with 84 cluster valence electrons, which are discussed on the basis of DFT calculations. The central M–M distances of <b>7</b> and <b>8</b> are significantly reduced upon the apparent loss of a hydride unit from the M–H–M central part of <b>2</b> and <b>3</b>, indicating that the bonding electrons in the adjacent M–Pt bonds migrate into the central M–M bond to result in a dynamic structural change during two-electron oxidation of the hexanuclear metal strings. A similar Pt₆ complex terminated by two iodide anions, [Pt₆I₂(μ-dpmp)₄]­(PF₆)₂ (<b>9</b>), was synthesized from [Pt₆(μ-H)­I₂(μ-dpmp)₄]­(PF₆) (<b>5</b>) by treatment with [Cp₂Fe]­[PF₆]. Complexes <b>7</b> and <b>8</b> were readily reacted with the neutral two-electron donors XylNC, CO, and phosphines to afford the trinuclear complexes [Pt₂M­(μ-dpmp)₂(XylNC)­L]­(PF₆)₂ (M = Pt, L = XylNC (<b>1a</b>), CO (<b>10</b>), PPh₃ (<b>11</b>); M = Pd, L = XylNC (<b>1b</b>)) through cleavage of the electron-deficient central M–M bond. When complex <b>7</b> was reacted with the diphosphines (<b>PP</b>) <i>trans</i>-Ph₂PCHCHPPh₂ (dppen) and Ph₂P­(CH₂)₂PPh₂ (dppe), the diphosphine was inserted into the central M–M bond to afford [(XylNC)­Pt₃(μ-dpmp)₂(<b>PP</b>)­Pt₃(μ-dpmp)₂(XylNC)]­(PF₆)₄ (<b>12</b>), which was transformed by treatment with another 1 equiv of diphosphine into the asymmetric trinuclear complexes [Pt₃(μ-dpmp)₂(XylNC)­(<b>PP</b>)]­(PF₆)₂ (<b>13</b>). A further ligand exchange reaction of <b>13a</b> (<b>PP</b> = <i>trans</i>-dppen) provided the diphosphine-terminated symmetrical Pt₃ complex [Pt₃(μ-dpmp)₂(L)₂]­(PF₆)₂ (L = <i>trans</i>-dppen (<b>14a</b>)). Complexes <b>7</b> and <b>8</b> were also reacted with [AuCl­(PPh₃)] to yield the Pt₂MAu heterotetranuclear complexes [Pt₂MAuCl­(μ-dpmp)₂(PPh₃)­(XylNC)]­(PF₆)₂ (M = Pt (<b>15</b>), Pd (<b>16</b>)), in which the Pt₂M trinuclear fragment is inserted into the Au–Cl bond in a 1,1-fashion on the central M atoms of the Pt₂M₂Pt₂ string