New Dicyano Cyclometalated Compounds Containing Pd(II)–Tl(I) Bonds as Building Blocks in 2D Extended Structures: Synthesis, Structure, and Luminescence Studies

New mixed metal complexes [PdTl­(C^∧N)­(CN)₂] [C^∧N = 7,8-benzoquinolinate (bzq, <b>3</b>); 2-phenylpyridinate (ppy, <b>4</b>)] have been synthesized by reaction of their corresponding precursors (NBu₄)­[Pd­(C^∧N)­(CN)₂] [C^∧N = bzq (<b>1</b>), ppy (<b>2</b>)] with TlPF₆. Compounds <b>3</b> and <b>4</b> were studied by X-ray diffraction, showing the not-so-common Pd^II–Tl^I bonds. Both crystal structures exhibit 2-D extended networks fashioned by organometallic “PdTl­(C^∧N)­(CN)₂” units, each one containing a donor–acceptor Pd­(II)–Tl­(I) bond, which are connected through additional Tl···NC contacts and weak Tl···π (bzq) contacts in the case of <b>3</b>. Solid state emissions are red-shifted compared with those of the precursors and have been assigned to metal–metal′-to-ligand charge transfer (MM′LCT [d/s σ*­(Pd,Tl) → π*­(C^∧N)]) mixed with some intraligand (³IL­[π­(C^∧N) → π*­(C^∧N)]) character. In diluted solution either at room temperature or 77 K, the Pd–Tl bond is no longer retained as confirmed by mass spectrometry, NMR, and UV–vis spectroscopic techniques

Synthesis and Characterization of the Double Salts [Pt(bzq)(CNR)₂][Pt(bzq)(CN)₂] with Significant Pt···Pt and π···π Interactions. Mechanistic Insights into the Ligand Exchange Process from Joint Experimental and DFT Study

Double complex salts (DCSs) of stoichiometry [Pt­(bzq)­(CNR)₂]­[Pt­(bzq)­(CN)₂] (bzq = 7,8-benzoquinolinate; R = <i>tert</i>-butyl (<b>1</b>), 2,6-dimethylphenyl (<b>2</b>), 2-naphtyl (<b>3</b>)) have been prepared by a metathesis reaction between [Pt­(bzq)­(CNR)₂]­ClO₄ and [K­(H₂O)]­[Pt­(bzq)­(CN)₂] in a 1:1 molar ratio under controlled temperature conditions (range: −10 to 0 °C). Compounds <b>1</b>–<b>3</b> have been isolated as air-stable and strongly colored solids [purple (<b>1</b>), orange (<b>2</b>), red-purple (<b>3</b>)]. The X-ray structure of <b>2</b> shows that it consists of ionic pairs in which the cationic and anionic square-planar Pt­(II) complexes are almost parallel to each other and are connected by Pt–Pt (3.1557(4) Å) and π···π (3.41–3.79 Å) interactions. Energy decomposition analysis calculations on DCSs <b>1</b>–<b>3</b> showed relatively strong ionic-pair interactions (estimated interaction energies of −99.1, −110.0, and −108.6 kcal/mol), which are dominated by electrostatic interactions with small contributions from dispersion (π···π) and covalent (Pt···Pt) bonding interactions involving the 5d and 6p atomic orbitals of the Pt centers. Compounds <b>1</b>–<b>3</b> undergo a thermal (165 °C, 24 h) irreversible ligand rearrangement process in the solid state and also in solution at temperatures above 0 °C to give the neutral complexes [Pt­(bzq)­(CN)­(CNR)] as a mixture of two possible isomers (SP-4-2 and SP-4-3). The mechanism of this process has been thoroughly explored by combined NMR and DFT studies. DFT calculations on <b>1</b>–<b>3</b> show that the existing Pt···Pt interactions block the associative attack of the Pt­(II) centers by the coordinated cyanide and/or isocyanide ligands. Moreover, they support a significant transfer of electron density from the anionic to the cationic component (0.20–0.32 |e|), which renders the isocyanide ligand dissociation more feasible than that in the “free-standing” cationic [Pt­(bzq)­(CNR)₂]⁺ components as well as the dissociation of the CN^– in <i>trans</i> position to the C_bzq in the anionic [Pt­(bzq)­(CN)₂]⁻ component. Therefore, the first step in the ligand rearrangement pathway is the dissociation of the isocyanide in <i>trans</i> position to the C_bzq, yielding the [(RNC)­(bzq)­(μ₂-η¹,η¹-CN)­Pt···Pt­(bzq)­(CN)] intermediates. The rate-limiting step corresponds to the transformation of these intermediates to the neutral [Pt­(bzq)­(CN)­(CNR)] complexes following a synchronous mechanism involving rupture of the Pt–Pt and formation of the Pt–CN bonds through transition states formulated as [(RNC)­(bzq)­Pt­(μ₂-η¹,η¹-CN)­Pt­(bzq)­(CN)]

Synthesis and Characterization of the Double Salts [Pt(bzq)(CNR)₂][Pt(bzq)(CN)₂] with Significant Pt···Pt and π···π Interactions. Mechanistic Insights into the Ligand Exchange Process from Joint Experimental and DFT Study

Double complex salts (DCSs) of stoichiometry [Pt­(bzq)­(CNR)₂]­[Pt­(bzq)­(CN)₂] (bzq = 7,8-benzoquinolinate; R = <i>tert</i>-butyl (<b>1</b>), 2,6-dimethylphenyl (<b>2</b>), 2-naphtyl (<b>3</b>)) have been prepared by a metathesis reaction between [Pt­(bzq)­(CNR)₂]­ClO₄ and [K­(H₂O)]­[Pt­(bzq)­(CN)₂] in a 1:1 molar ratio under controlled temperature conditions (range: −10 to 0 °C). Compounds <b>1</b>–<b>3</b> have been isolated as air-stable and strongly colored solids [purple (<b>1</b>), orange (<b>2</b>), red-purple (<b>3</b>)]. The X-ray structure of <b>2</b> shows that it consists of ionic pairs in which the cationic and anionic square-planar Pt­(II) complexes are almost parallel to each other and are connected by Pt–Pt (3.1557(4) Å) and π···π (3.41–3.79 Å) interactions. Energy decomposition analysis calculations on DCSs <b>1</b>–<b>3</b> showed relatively strong ionic-pair interactions (estimated interaction energies of −99.1, −110.0, and −108.6 kcal/mol), which are dominated by electrostatic interactions with small contributions from dispersion (π···π) and covalent (Pt···Pt) bonding interactions involving the 5d and 6p atomic orbitals of the Pt centers. Compounds <b>1</b>–<b>3</b> undergo a thermal (165 °C, 24 h) irreversible ligand rearrangement process in the solid state and also in solution at temperatures above 0 °C to give the neutral complexes [Pt­(bzq)­(CN)­(CNR)] as a mixture of two possible isomers (SP-4-2 and SP-4-3). The mechanism of this process has been thoroughly explored by combined NMR and DFT studies. DFT calculations on <b>1</b>–<b>3</b> show that the existing Pt···Pt interactions block the associative attack of the Pt­(II) centers by the coordinated cyanide and/or isocyanide ligands. Moreover, they support a significant transfer of electron density from the anionic to the cationic component (0.20–0.32 |e|), which renders the isocyanide ligand dissociation more feasible than that in the “free-standing” cationic [Pt­(bzq)­(CNR)₂]⁺ components as well as the dissociation of the CN^– in <i>trans</i> position to the C_bzq in the anionic [Pt­(bzq)­(CN)₂]⁻ component. Therefore, the first step in the ligand rearrangement pathway is the dissociation of the isocyanide in <i>trans</i> position to the C_bzq, yielding the [(RNC)­(bzq)­(μ₂-η¹,η¹-CN)­Pt···Pt­(bzq)­(CN)] intermediates. The rate-limiting step corresponds to the transformation of these intermediates to the neutral [Pt­(bzq)­(CN)­(CNR)] complexes following a synchronous mechanism involving rupture of the Pt–Pt and formation of the Pt–CN bonds through transition states formulated as [(RNC)­(bzq)­Pt­(μ₂-η¹,η¹-CN)­Pt­(bzq)­(CN)]

An Extended Chain and Trinuclear Complexes Based on Pt(II)–M (M = Tl(I), Pb(II)) Bonds: Contrasting Photophysical Behavior

The syntheses and structural characterizations of a Pt–Tl chain [{Pt­(bzq)­(C₆F₅)₂}­Tl­(Me₂CO)]_<i>n</i> <b>1</b> and two trinuclear Pt₂M clusters (NBu₄)­[{Pt­(bzq)­(C₆F₅)₂}₂Tl] <b>2</b> and [{Pt­(bzq)­(C₆F₅)₂}₂Pb] <b>3</b> (bzq = 7,8-benzoquinolinyl), stabilized by donor–acceptor Pt → M bonds, are reported. The one-dimensional heterometallic chain <b>1</b> is formed by alternate “Pt­(bzq)­(C₆F₅)₂” and “Tl­(Me₂CO)” fragments, with Pt–Tl bond separations in the range of 2.961(1)–3.067(1) Å. The isoelectronic trinuclear complexes <b>2</b> (which crystallizes in three forms, namely, <b>2a</b>, <b>2b</b>, and <b>2c</b>) and <b>3</b> present a sandwich structure in which the Tl­(I) or Pb­(II) is located between two “Pt­(bzq)­(C₆F₅)₂” subunits. NMR studies suggest equilibria in solution implying cleavage and reformation of Pt–M bonds. The lowest-lying absorption band in the UV–vis spectra in CH₂Cl₂ and tetrahydrofuran (THF) of <b>1</b>, associated with ¹MLCT/¹L′LCT ¹[5d_π(Pt) → π*­(bzq)]/¹[(C₆F₅) → bzq], displays a blue shift in relation to the precursor, suggesting the cleavage of the chain maintaining bimetallic Pt–Tl fragments in solution, also supported by NMR spectroscopy. In <b>2</b> and <b>3</b>, it shows a blue shift in THF and a red shift in CH₂Cl₂, supporting a more extensive cleavage of the Pt–M bonds in THF solutions than in CH₂Cl₂, where the trinuclear entities are predominant. The Pt–Tl chain <b>1</b> displays in solid state a bright orange-red emission ascribed to ³MM′CT (M′ = Tl). It exhibits remarkable and fast reversible vapochromic and vapoluminescent response to donor vapors (THF and Et₂O), related to the coordination/decoordination of the guest molecule to the Tl­(I) ion, and mechanochromic behavior, associated with the shortening of the intermetallic Pt–Tl separations in the chain induced by grinding. In frozen solutions (THF, acetone, and CH₂Cl₂) <b>1</b> shows interesting luminescence thermochromism with emissions strongly dependent on the solvent, concentration, and excitation wavelengths. The Pt₂Tl complex <b>2</b> shows an emission close to <b>1</b>, ascribed to charge transfer from the platinum fragment to the thallium [³(L+L′)­MM′CT]. <b>2</b> also shows vapoluminescent behavior in the presence of vapors of Me₂CO, THF, and Et₂O, although smaller and slower than those of <b>1</b>. The trinuclear neutral complex Pt₂Pb <b>3</b> displays a blue-shift emission band, tentatively assigned to admixture of ³MM′CT ³[Pt­(d) → Pb­(sp)] with some metal-mediated intraligand (³ππ/³ILCT) contribution. In contrast to <b>1</b> and <b>2</b>, <b>3</b> does not show vapoluminescent behavior

Synthesis and Reactivity of the Unsaturated Trinuclear Phosphanido Complex [(C₆F₅)₂Pt(μ-PPh₂)₂Pt(μ-PPh₂)₂Pt(PPh₃)]

The reaction of [NBu₄]­[(C₆F₅)₂Pt­(μ-PPh₂)₂Pt­(μ-PPh₂)₂Pt­(<i>O</i>,<i>O</i>-acac)] (48 VEC) with [HPPh₃]­[ClO₄] gives the 46 VEC unsaturated [(C₆F₅)₂Pt¹(μ-PPh₂)₂Pt²(μ-PPh₂)₂Pt³(PPh₃)]­(Pt²–Pt³) (<b>1</b>), a trinuclear compound endowed with a Pt–Pt bond. This compound displays amphiphilic behavior and reacts easily with nucleophiles L, yielding the saturated complexes [(C₆F₅)₂Pt^II(μ-PPh₂)₂Pt^II(μ-PPh₂)₂Pt^II(PPh₃)­L] [L = PPh₃ (<b>2</b>), py (<b>3</b>)]. The reaction with the electrophile [Ag­(OClO₃)­PPh₃] affords the adduct <b>1</b>·AgPPh₃, which evolves, even at low temperature, to a mixture in which [(C₆F₅)₂Pt^III(μ-PPh₂)₂Pt^III(μ-PPh₂)₂Pt^II(PPh₃)₂]²⁺(Pt^III–Pt^III) and <b>2</b> (plus silver metal) are present. The nucleophilic nature of <b>1</b> is also demonstrated through its reaction with <i>cis</i>-[Pt­(C₆F₅)₂(THF)₂], which results in the formation of [Pt₄(μ-PPh₂)₄(C₆F₅)₄(PPh₃)] (<b>4</b>). The structure and NMR features indicate that <b>1</b> can be better considered as a Pt^II–Pt^III–Pt^I complex instead of a Pt^II–Pt^II–Pt^II derivative. Theoretical calculations (density functional theory) on similar model compounds are in agreement with the assigned oxidation states of the metal centers. The strong intermetallic interactions resulting in a Pt²–Pt³ metal–metal bond and the respective bonding mechanism were verified by employing a multitude of computational techniques (natural bond order analysis, the Laplacian of the electron density, and localized orbital locator profiles)

Synthesis and Reactivity of the Unsaturated Trinuclear Phosphanido Complex [(C₆F₅)₂Pt(μ-PPh₂)₂Pt(μ-PPh₂)₂Pt(PPh₃)]

The reaction of [NBu₄]­[(C₆F₅)₂Pt­(μ-PPh₂)₂Pt­(μ-PPh₂)₂Pt­(<i>O</i>,<i>O</i>-acac)] (48 VEC) with [HPPh₃]­[ClO₄] gives the 46 VEC unsaturated [(C₆F₅)₂Pt¹(μ-PPh₂)₂Pt²(μ-PPh₂)₂Pt³(PPh₃)]­(Pt²–Pt³) (<b>1</b>), a trinuclear compound endowed with a Pt–Pt bond. This compound displays amphiphilic behavior and reacts easily with nucleophiles L, yielding the saturated complexes [(C₆F₅)₂Pt^II(μ-PPh₂)₂Pt^II(μ-PPh₂)₂Pt^II(PPh₃)­L] [L = PPh₃ (<b>2</b>), py (<b>3</b>)]. The reaction with the electrophile [Ag­(OClO₃)­PPh₃] affords the adduct <b>1</b>·AgPPh₃, which evolves, even at low temperature, to a mixture in which [(C₆F₅)₂Pt^III(μ-PPh₂)₂Pt^III(μ-PPh₂)₂Pt^II(PPh₃)₂]²⁺(Pt^III–Pt^III) and <b>2</b> (plus silver metal) are present. The nucleophilic nature of <b>1</b> is also demonstrated through its reaction with <i>cis</i>-[Pt­(C₆F₅)₂(THF)₂], which results in the formation of [Pt₄(μ-PPh₂)₄(C₆F₅)₄(PPh₃)] (<b>4</b>). The structure and NMR features indicate that <b>1</b> can be better considered as a Pt^II–Pt^III–Pt^I complex instead of a Pt^II–Pt^II–Pt^II derivative. Theoretical calculations (density functional theory) on similar model compounds are in agreement with the assigned oxidation states of the metal centers. The strong intermetallic interactions resulting in a Pt²–Pt³ metal–metal bond and the respective bonding mechanism were verified by employing a multitude of computational techniques (natural bond order analysis, the Laplacian of the electron density, and localized orbital locator profiles)

Addition of Nucleophiles to Phosphanido Derivatives of Pt(III): Formation of P–C, P–N, and P–O Bonds

The reactivity of the dinuclear platinum­(III) derivative [(R_F)₂Pt^III(μ-PPh₂)₂­Pt^III(R_F)₂]<i>(Pt–Pt)</i> (R_F = C₆F₅) (<b>1</b>) toward OH^–, N₃^–, and NCO^– was studied. The coordination of these nucleophiles to a metal center evolves with reductive coupling or reductive elimination between a bridging diphenylphosphanido group and OH^–, N₃^–, and NCO^– or C₆F₅ groups and formation of P–O, P–N, or P–C bonds. The addition of OH^– to <b>1</b> evolves with a reductive coupling with the incoming ligand, formation of a P–O bond, and the synthesis of [NBu₄]₂[(R_F)₂Pt^II(μ-OPPh₂)­(μ-PPh₂)­Pt^II(R_F)₂] (<b>3</b>). The addition of N₃^– takes place through two ways: (a) formation of the P–N bond and reductive elimination of PPh₂N₃ yielding [NBu₄]₂[(R_F)₂Pt^II(μ-N₃)­(μ-PPh₂)­Pt^II(R_F)₂] (<b>4a</b>) and (b) formation of the P–C bond and reductive coupling with one of the C₆F₅ groups yielding [NBu₄]­[(R_F)₂Pt^II(μ-N₃)­(μ-PPh₂)­Pt^II(R_F)­(PPh₂R_F)] (<b>4b</b>). Analogous behavior was shown in the addition of NCO^– to <b>1</b> which afforded [NBu₄]₂[(R_F)₂Pt^II(μ-NCO)­(μ-PPh₂)­Pt^II(R_F)₂] (<b>5a</b>) and [NBu₄]­[(R_F)₂Pt^II(μ-NCO)­(μ-PPh₂)­Pt^II(R_F)­(PPh₂R_F)] (<b>5b</b>). In the reaction of the trinuclear complex [(R_F)₂Pt^III(μ-PPh₂)₂Pt^III(μ-PPh₂)₂­Pt^II(R_F)₂]<i>(Pt</i>^<i>III</i><i>–Pt</i>^<i>III</i><i>)</i> (<b>2</b>) with OH^– or N₃^–, the coordination of the nucleophile takes place selectively at the central platinum­(III) center, and the PPh₂/OH^– or PPh₂/N₃^– reductive coupling yields the trinuclear [NBu₄]₂[(R_F)₂Pt^II(μ-Ph₂PO)­(μ-PPh₂)­Pt^II(μ-PPh₂)₂Pt^II(R_F)₂] (<b>6</b>) and [NBu₄]­[(R_F)₂Pt¹(μ₃-Ph₂PNPPh₂)­(μ-PPh₂)­Pt²(μ-PPh₂)­Pt³(R_F)₂]<i>(Pt</i>^<i>2</i><i>–Pt</i>^<i>3</i><i>)</i> (<b>7</b>). Complex <b>7</b> is fluxional in solution, and an equilibrium consisting of Pt–Pt bond migration was ascertained by ³¹P EXSY experiments

Oxidatively Induced P–O Bond Formation through Reductive Coupling between Phosphido and Acetylacetonate, 8‑Hydroxyquinolinate, and Picolinate Groups

The dinuclear anionic complexes [NBu₄]­[(R_F)₂M^II(μ-PPh₂)₂M′^II(N^∧O)] (R_F = C₆F₅. N^∧O = 8-hydroxyquinolinate, hq; M = M′ = Pt <b>1</b>; Pd <b>2</b>; M = Pt, M′ = Pd, <b>3</b>. N^∧O = <i>o</i>-picolinate, pic; M = Pt, M′ = Pt, <b>4</b>; Pd, <b>5</b>) are synthesized from the tetranuclear [NBu₄]₂[{(R_F)₂Pt­(μ-PPh₂)₂M­(μ-Cl)}₂] by the elimination of the bridging Cl as AgCl in acetone, and coordination of the corresponding <i>N</i>,<i>O</i>-donor ligand (<b>1</b>, <b>4</b>, and <b>5</b>) or connecting the fragments “<i>cis</i>-[(R_F)₂M­(μ-PPh₂)₂]^2–” and “M′(N^∧O)” (<b>2</b> and <b>3</b>). The electrochemical oxidation of the anionic complexes <b>1</b>–<b>5</b> occurring under HRMS­(+) conditions gave the cations [(R_F)₂M­(μ-PPh₂)₂M′(N^∧O)]⁺, presumably endowed with a M­(III),M′(III) core. The oxidative addition of I₂ to the 8-hydroxyquinolinate complexes <b>1</b>–<b>3</b> triggers a reductive coupling between a PPh₂ bridging ligand and the <i>N</i>,<i>O</i>-donor chelate ligand with formation of a P–O bond and ends up in complexes of platinum­(II) or palladium­(II) of formula [(R_F)₂M^II(μ-I)­(μ-PPh₂)­M′^II(<i>P</i>,<i>N</i>-PPh₂hq)], M = M′ = Pt <b>7</b>, Pd <b>8</b>; M = Pt, M′ = Pd, <b>9</b>. Complexes <b>7</b>–<b>9</b> show a new Ph₂P-OC₉H₆N (Ph₂P-hq) ligand bonded to the metal center in a <i>P</i>,<i>N</i>-chelate mode. Analogously, the addition of I₂ to solutions of the <i>o</i>-picolinate complexes <b>4</b> and <b>5</b> causes the reductive coupling between a PPh₂ bridging ligand and the starting <i>N</i>,<i>O</i>-donor chelate ligand with formation of a P–O bond, forming Ph₂P-OC₆H₄NO (Ph₂P-pic). In these cases, the isolated derivatives [NBu₄]­[(Ph₂P-pic)­(R_F)­Pt^II(μ-I)­(μ-PPh₂)­M^II(R_F)­I] (M = Pt <b>10</b>, Pd <b>11</b>) are anionic, as a consequence of the coordination of the resulting new phosphane ligand (Ph₂P-pic) as monodentate <i>P</i>-donor, and a terminal iodo group to the M atom. The oxidative addition of I₂ to [NBu₄]­[(R_F)₂Pt^II(μ-PPh₂)₂Pt^II(acac)] (<b>6</b>) (acac = acetylacetonate) also results in a reductive coupling between the diphenylphosphanido and the acetylacetonate ligand with formation of a P–O bond and synthesis of the complex [NBu₄]­[(R_F)₂Pt^II(μ-I)­(μ-PPh₂)­Pt^II(Ph₂P-acac)­I] (<b>12</b>). The transformations of the starting complexes into the products containing the P–O ligands passes through mixed valence M­(II),M′(IV) intermediates which were detected, for M = M′ = Pt, by spectroscopic and spectrometric measurements