Synthesis and Reactivity of a Transition Metal Complex Containing Exclusively TEMPO Ligands: Ni(η²-TEMPO)₂

The reaction of Ni(COD)2 with two equivalents of the TEMPO radical at 68 °C affords the 16 e– “bow-tie” complex Ni(η2-TEMPO)2, 1, in 78% yield. Compound 1 reacts with tert-butyl isocyanide and phenylacetylene at room temperature to yield the 16 e– distorted square planar nickel complexes Ni(η2-TEMPO)(η1-TEMPO)(CNtBu), 2, and Ni(η2-TEMPO)(η1-TEMPOH)(CCPh), 4, respectively. The facile reactivity of 1 is aided by the transition of the TEMPO ligand from an η2 to η1 binding mode. Complex 4 is an unusual example of hydrogen atom transfer from phenylacetylene to a coordinated TEMPO ligand

Synthesis and Reactivity of a Transition Metal Complex Containing Exclusively TEMPO Ligands: Ni(η²-TEMPO)₂

The reaction of Ni(COD)2 with two equivalents of the TEMPO radical at 68 °C affords the 16 e– “bow-tie” complex Ni(η2-TEMPO)2, 1, in 78% yield. Compound 1 reacts with tert-butyl isocyanide and phenylacetylene at room temperature to yield the 16 e– distorted square planar nickel complexes Ni(η2-TEMPO)(η1-TEMPO)(CNtBu), 2, and Ni(η2-TEMPO)(η1-TEMPOH)(CCPh), 4, respectively. The facile reactivity of 1 is aided by the transition of the TEMPO ligand from an η2 to η1 binding mode. Complex 4 is an unusual example of hydrogen atom transfer from phenylacetylene to a coordinated TEMPO ligand

Pendant Alkyl and Aryl Groups on Tin Control Complex Geometry and Reactivity with H₂/D₂ in Pt(SnR₃)₂(CNBu^t)₂ (R = Bu^t, Prⁱ, Ph, Mesityl)

The complex Pt­(SnBut3)2­(CNBut)2­(H)2, 1, was obtained from the reaction of Pt­(COD)2 and But3SnH, followed by addition of CNBut. The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield Pt­(SnBut3)2­(CNBut)2, 2. Addition of hydrogen to 2 at room temperature in solution and in the solid state regenerates 1. Complex 2 catalyzes H2−D2 exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of But3SnH from 1 to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt­(SnMes3)­(SnBut3)­(CNBut)2, 3, when the addition of H2 to 2 was carried out in the presence of free ligand Mes3SnH (Mes = 2,4,6-Me3C6H2). Complex Pt­(SnMes3)2­(CNBut)2, 5, can be prepared from the reaction of Pt­(COD)2 with Mes3SnH and CNBut. The exchange reaction of 2 with Ph3SnH gave Pt­(SnPh3)3(CNBut)2­(H), 6, wherein both SnBut3 ligands are replaced by SnPh3. Complex 6 decomposes in air to form square planar Pt­(SnPh3)2­(CNBut)2, 7. The complex Pt­(SnPri3)2­(CNBut)2, 8, was also prepared. Out of the four analogous complexes Pt­(SnR3)2­(CNBut)2 (R = But, Mes, Ph, or Pri), only the But analogue does both H2 activation and H2−D2 exchange. This is due to steric effects imparted by the bulky But groups that distort the geometry of the complex considerably from planarity. The reaction of Pt­(COD)2 with But3SnH and CO gas afforded trans-Pt­(SnBut3)2­(CO)2, 9. Compound 9 can be converted to 2 by replacement of the CO ligands with CNBut via the intermediate Pt­(SnBut3)2­(CNBut)2­(CO), 10

Pendant Alkyl and Aryl Groups on Tin Control Complex Geometry and Reactivity with H₂/D₂ in Pt(SnR₃)₂(CNBu^t)₂ (R = Bu^t, Prⁱ, Ph, Mesityl)

The complex Pt­(SnBu^t₃)₂­(CNBu^t)₂­(H)₂, <b>1</b>, was obtained from the reaction of Pt­(COD)₂ and Bu^t₃SnH, followed by addition of CNBu^t. The two hydride ligands in <b>1</b> can be eliminated, both in solution and in the solid state, to yield Pt­(SnBu^t₃)₂­(CNBu^t)₂, <b>2</b>. Addition of hydrogen to <b>2</b> at room temperature in solution and in the solid state regenerates <b>1</b>. Complex <b>2</b> catalyzes H₂−D₂ exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of Bu^t₃SnH from <b>1</b> to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt­(SnMes₃)­(SnBu^t₃)­(CNBu^t)₂, <b>3</b>, when the addition of H₂ to <b>2</b> was carried out in the presence of free ligand Mes₃SnH (Mes = 2,4,6-Me₃C₆H₂). Complex Pt­(SnMes₃)₂­(CNBu^t)₂, <b>5</b>, can be prepared from the reaction of Pt­(COD)₂ with Mes₃SnH and CNBu^t. The exchange reaction of <b>2</b> with Ph₃SnH gave Pt­(SnPh₃)₃(CNBu^t)₂­(H), <b>6</b>, wherein both SnBu^t₃ ligands are replaced by SnPh₃. Complex <b>6</b> decomposes in air to form square planar Pt­(SnPh₃)₂­(CNBu^t)₂, <b>7</b>. The complex Pt­(SnPrⁱ₃)₂­(CNBu^t)₂, <b>8</b>, was also prepared. Out of the four analogous complexes Pt­(SnR₃)₂­(CNBu^t)₂ (R = Bu^t, Mes, Ph, or Prⁱ), only the Bu^t analogue does both H₂ activation and H₂−D₂ exchange. This is due to steric effects imparted by the bulky Bu^t groups that distort the geometry of the complex considerably from planarity. The reaction of Pt­(COD)₂ with Bu^t₃SnH and CO gas afforded <i>trans</i>-Pt­(SnBu^t₃)₂­(CO)₂, <b>9</b>. Compound <b>9</b> can be converted to <b>2</b> by replacement of the CO ligands with CNBu^t via the intermediate Pt­(SnBu^t₃)₂­(CNBu^t)₂­(CO), <b>10</b>

Pendant Alkyl and Aryl Groups on Tin Control Complex Geometry and Reactivity with H₂/D₂ in Pt(SnR₃)₂(CNBu^t)₂ (R = Bu^t, Prⁱ, Ph, Mesityl)

The complex Pt­(SnBut3)2­(CNBut)2­(H)2, 1, was obtained from the reaction of Pt­(COD)2 and But3SnH, followed by addition of CNBut. The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield Pt­(SnBut3)2­(CNBut)2, 2. Addition of hydrogen to 2 at room temperature in solution and in the solid state regenerates 1. Complex 2 catalyzes H2−D2 exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of But3SnH from 1 to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt­(SnMes3)­(SnBut3)­(CNBut)2, 3, when the addition of H2 to 2 was carried out in the presence of free ligand Mes3SnH (Mes = 2,4,6-Me3C6H2). Complex Pt­(SnMes3)2­(CNBut)2, 5, can be prepared from the reaction of Pt­(COD)2 with Mes3SnH and CNBut. The exchange reaction of 2 with Ph3SnH gave Pt­(SnPh3)3(CNBut)2­(H), 6, wherein both SnBut3 ligands are replaced by SnPh3. Complex 6 decomposes in air to form square planar Pt­(SnPh3)2­(CNBut)2, 7. The complex Pt­(SnPri3)2­(CNBut)2, 8, was also prepared. Out of the four analogous complexes Pt­(SnR3)2­(CNBut)2 (R = But, Mes, Ph, or Pri), only the But analogue does both H2 activation and H2−D2 exchange. This is due to steric effects imparted by the bulky But groups that distort the geometry of the complex considerably from planarity. The reaction of Pt­(COD)2 with But3SnH and CO gas afforded trans-Pt­(SnBut3)2­(CO)2, 9. Compound 9 can be converted to 2 by replacement of the CO ligands with CNBut via the intermediate Pt­(SnBut3)2­(CNBut)2­(CO), 10

Metal–Ligand Synergistic Effects in the Complex Ni(η²‑TEMPO)₂: Synthesis, Structures, and Reactivity

In the current investigation, reactions of the “bow-tie” Ni­(η²-TEMPO)₂ complex with an assortment of donor ligands have been characterized experimentally and computationally. While the Ni­(η²-TEMPO)₂ complex has <i>trans</i>-disposed TEMPO ligands, proton transfer from the C–H bond of alkyne substrates (phenylacetylene, acetylene, trimethylsilyl acetylene, and 1,4-diethynylbenzene) produce <i>cis</i>-disposed ligands of the form Ni­(η²-TEMPO)­(κ¹-TEMPOH)­(κ¹-R). In the case of 1,4-diethynylbenzene, a two-stage reaction occurs. The initial product Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-<i>C</i>C­(C₆H₄)­CCH] is formed first but can react further with another equivalent of Ni­(η²-TEMPO)₂ to form the bridged complex Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-κ¹-<i>C</i>C­(C₆H₄)­C<i>C</i>]­Ni­(η²-TEMPO)­(κ¹-TEMPOH). The corresponding reaction with acetylene, which could conceivably also yield a bridging complex, does not occur. Via density functional theory (DFT), addition mechanisms are proposed in order to rationalize thermodynamic and kinetic selectivity. Computations have also been used to probe the relative thermodynamic stabilities of the <i>cis</i> and <i>trans</i> addition products and are in accord with experimental results. Based upon the computational results and the geometry of the experimentally observed product, a <i>trans</i>–<i>cis</i> isomerization must occur

Metal–Ligand Synergistic Effects in the Complex Ni(η²‑TEMPO)₂: Synthesis, Structures, and Reactivity

In the current investigation, reactions of the “bow-tie” Ni­(η²-TEMPO)₂ complex with an assortment of donor ligands have been characterized experimentally and computationally. While the Ni­(η²-TEMPO)₂ complex has <i>trans</i>-disposed TEMPO ligands, proton transfer from the C–H bond of alkyne substrates (phenylacetylene, acetylene, trimethylsilyl acetylene, and 1,4-diethynylbenzene) produce <i>cis</i>-disposed ligands of the form Ni­(η²-TEMPO)­(κ¹-TEMPOH)­(κ¹-R). In the case of 1,4-diethynylbenzene, a two-stage reaction occurs. The initial product Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-<i>C</i>C­(C₆H₄)­CCH] is formed first but can react further with another equivalent of Ni­(η²-TEMPO)₂ to form the bridged complex Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-κ¹-<i>C</i>C­(C₆H₄)­C<i>C</i>]­Ni­(η²-TEMPO)­(κ¹-TEMPOH). The corresponding reaction with acetylene, which could conceivably also yield a bridging complex, does not occur. Via density functional theory (DFT), addition mechanisms are proposed in order to rationalize thermodynamic and kinetic selectivity. Computations have also been used to probe the relative thermodynamic stabilities of the <i>cis</i> and <i>trans</i> addition products and are in accord with experimental results. Based upon the computational results and the geometry of the experimentally observed product, a <i>trans</i>–<i>cis</i> isomerization must occur

Synthesis, Structure, and Thermochemistry of the Formation of the Metal−Metal Bonded Dimers [Mo(μ-TeAr)(CO)₃(PⁱP₃)]₂ (Ar = Phenyl, Naphthyl) by Phosphine Elimination from ^•Mo(TePh)(CO)₃(PⁱPr₃)₂

The complexes (•TeAr)Mo(CO)3(PiPr3)2 (Ar = phenyl, naphthyl; iPr = isopropyl) slowly eliminate PiPr3 at room temperature in a toluene solution to quantitatively form the dinuclear complexes [Mo(μ-TeAr)(CO)3(PiPr3)]2. The crystal structure of [Mo(μ-Te−naphthyl)(CO)3(PiPr3)]2 is reported and has a Mo−Mo distance of 3.2130 Å. The enthalpy of dimerization has been measured and is used to estimate a Mo−Mo bond strength on the order of 30 kcal mol-1. Kinetic studies show the rate of formation of the dimeric chalcogen bridged complex is best fit by a rate law first order in (•TeAr)Mo(CO)3(PiPr3)2 and inhibited by added PiPr3. The reaction is proposed to occur by initial dissociation of a phosphine ligand and not by radical recombination of 2 mol of (•TeAr)Mo(CO)3(PiPr3)2. Reaction of (•TePh)Mo(CO)3(PiPr3)2, with L = pyridine (py) or CO, is rapid and quantitative at room temperature to form PhTeTePh and Mo(L)(CO)3(PiPr3)2, in keeping with thermochemical predictions. The rate of reaction of (•TeAr)W(CO)3(PiPr3)2 and CO is first-order in the metal complex and is proposed to proceed by the associative formation of the 19 e- radical complex (•TePh)W(CO)4(PiPr3)2 which extrudes a •TePh radical

Kinetic and Thermodynamic Studies of the Reactivity of (Trimethylsilyl)diazomethane with HMo(CO)₃(C₅R₅) (R = H, Me). Estimation of the Mo−N₂CH₂SiMe₃ Bond Strength and Experimental Determination of the Enthalpy of Formation of (Trimethylsilyl)diazomethane

The rates of reaction of N2CHSiMe3 with HMo(CO)3Cp (Cp = η5-C5H5) in heptane obey the rate law −d[HMo(CO)3Cp]/dt = k[HMo(CO)3Cp][N2CHSiMe3] (k = 0.035 ± 0.01 M−1 s−1 at 0 °C; ΔH⧧ = 11.7 ± 2.0 kcal/mol and ΔS⧧ = −22.0 ± 3.0 cal/(mol K)). Isotopic scrambling between DMo(CO)3Cp and N2CHSiMe3 occurs at a rate faster than the overall reaction. Reversible 1,2-addition to form the tightly bound intermediate [Me3SiCH2NβNαδ+][δ−Mo(CO)3Cp] is proposed as the first step of the reaction. Spectroscopic and computational data support this formulation. The contact ion pairs can undergo heterolytic cleavage to ions or homolytic cleavage to radicals, and the solvent influence on kobs (THF > toluene > heptane) is interpreted in terms of this model. The enthalpy of this reaction has been measured by solution calorimetry at 272 K in THF: ΔH = −11.6 ± 1.2 kcal/mol. These data, together with computed organic reaction energies allow estimation of the bond strength between the three-electron donors·N2CHSiMe3 and ·Mo(CO)2Cp to be 25 ± 5 kcal/mol stronger than the two-electron Mo−CO bond. Coordination of N2CHSiMe3 to the complexes M(PR3)2(CO)3 (M = Mo, W; R = Cy, iPr; Cy = cyclohexyl; iPr = isopropyl) alters the course of reaction with HMo(CO)3Cp. The stoichiometric reaction of Me3SiCHNNMo(PiPr3)2(CO)3 with 2 equiv of HMo(CO)3Cp produces SiMe4, Mo(N2)(PiPr3)2(CO)3, and [Mo(CO)3Cp]2. In the presence of excess N2CHSiMe3 this reaction is catalytic and has been used to experimentally measure the heat of hydrogenation of N2CHSiMe3 to N2 and SiMe4 by 2 equiv of HMo(CO)3Cp. The derived enthalpy of formation of N2CHSiMe3 (5.8 ± 3.0 kcal/mol) is in reasonable agreement with high-level theoretical calculations. X-ray crystal structure data are reported for W(CO)2(N2CH2SiMe3)Cp: triclinic, space group P1̅, a = 6.3928(7) Å, b = 10.6551(12) Å, c = 10.8766(12) Å, α = 100.632(2)°, β = 96.254(2)°, V = 721.32 Å3, Z = 2

Experimental and Computational Studies of Binding of Dinitrogen, Nitriles, Azides, Diazoalkanes, Pyridine, and Pyrazines to M(PR₃)₂(CO)₃ (M = Mo, W; R = Me, ⁱPr).

The enthalpies of binding of a number of N-donor ligands to the complex Mo(PiPr3)2(CO)3 in toluene have been determined by solution calorimetry and equilibrium measurements. The measured binding enthalpies span a range of ∼10 kcal mol−1: ΔHbinding = −8.8 ± 1.2 (N2−Mo(PiPr3)2(CO)3); −10.3 ± 0.8 (N2); −11.2 ± 0.4 (AdN3 (Ad = 1-adamantyl)); −13.8 ± 0.5 (N2CHSiMe3); −14.9 ± 0.9 (pyrazine = pz); −14.8 ± 0.6 (2,6-Me2pz); −15.5 ± 1.8 (Me2NCN); −16.6 ± 0.4 (CH3CN); −17.0 ± 0.4 (pyridine); −17.5 ± 0.8 ([4-CH3pz][PF6] (in tetrahydrofuran)); −17.6 ± 0.4 (C6H5CN); −18.6 ± 1.8 (N2CHC(O)OEt); and −19.3 ± 2.5 kcal mol−1 (pz)Mo(PiPr3)2(CO)3). The value for the isonitrile AdNC (−29.0 ± 0.3) is 12.3 kcal mol−1 more exothermic than that of the nitrile AdCN (−16.7 ± 0.6 kcal mol−1). The enthalpies of binding of a range of arene nitrile ligands were also studied, and remarkably, most nitrile complexes were clustered within a 1 kcal mol−1 range despite dramatic color changes and variation of νCN. Computed structural and spectroscopic parameters for the complexes Mo(PiPr3)2(CO)3L are in good agreement with experimental data. Computed binding enthalpies for Mo(PiPr3)2(CO)3L exhibit considerable scatter and are generally smaller compared to the experimental values, but relative agreement is reasonable. Computed enthalpies of binding using a larger basis set for Mo(PMe3)2(CO)3L show a better fit to experimental data than that for Mo(PiPr3)2(CO)3L using a smaller basis set. Crystal structures of Mo(PiPr3)2(CO)3(AdCN), W(PiPr3)2(CO)3(Me2NCN), W(PiPr3)2(CO)3(2,6-F2C6H3CN), W(PiPr3)2(CO)3(2,4,6-Me3C6H2CN), W(PiPr3)2(CO)3(2,6-Me2pz), W(PiPr3)2(CO)3(AdCN), Mo(PiPr3)2(CO)3(AdNC), and W(PiPr3)2(CO)3(AdNC) are reported