Mono- and Polynuclear Complexes of the Model Nucleobase 1-Methylcytosine. Synthesis and Characterization of <i>cis</i>-[(PMe₂Ph)₂Pt{(1-MeCy(−H)}]₃(NO₃)₃ and <i>cis</i>-[(PPh₃)₂Pt{1-MeCy(−H)}(1-MeCy)]NO₃

The hydroxo complex cis-[L2Pt(μ-OH)]2(NO3)2 (L = PMe2Ph), in various solvents, reacts with 1-methylcytosine (1-MeCy) to give as the final product the cyclic species cis-[L2Pt{1-MeCy(−H),N 3N 4}]3(NO3)3 (1) in high or quantitative yield. X-ray analysis of 1 evidences a trinuclear species with the NH2-deprotonated nucleobases bridging symmetrically the metal centers through the N3 and N4 donors. A multinuclear NMR study of the reaction in DMSO-d6 reveals the initial formation of the dinuclear species cis-[L2Pt{1-MeCy(−H),N 3N 4}]22+ (2), which quantitatively converts into 1 following a first-order kinetic law (at 50 °C, t1/2 = 5 h). In chlorinated solvents, the deprotonation of the nucleobase affords as the major product (60−70%) the linkage isomer of 1, cis-[L2Pt{1-MeCy(−H)}]33+ (3), in which three cytosinate ligands bridge unsymmetrically three cis-L2Pt2+ units. In solution, 3 slowly converts quantitatively into the thermodynamically more stable isomer 1. No polynuclear adducts were obtained with the hydroxo complex stabilized by PPh3. cis-[(PPh3)2Pt(μ-OH)]2(NO3)2 reacts with 1-MeCy, in DMSO or CH2Cl2, to give the mononuclear species cis-[(PPh3)2Pt{1-MeCy(−H)}(1-MeCy)](NO3) (4) containing one neutral and one NH2-deprotonated 1-MeCy molecule, coordinated to the same metal center at the N3 and N4 sites, respectively. X-ray analysis and NMR studies show an intramolecular H bond between the N4 amino group and the uncoordinated N3 atom of the two nucleobases

Role of the Phosphine Ligands on the Stabilization of Monoadducts of the Model Nucleobases 1-Methylcytosine and 9-Methylguanine in Platinum(II) Complexes

The addition of 1-methylcytosine (1-MeCy) or 9-methylguanine (9-MeGu) to solutions of cis-(PPh3)2Pt(ONO2)2 (1a), in a molar ratio of 1:1, affords the monoadducts cis-[(PPh3)2Pt(1-MeCy)(ONO2)]NO3 (2a) and cis-[(PPh3)2Pt(9-MeGu)(ONO2)]NO3 (3a) and only trace amounts of the bisadducts cis-[(PPh3)2Pt(1-MeCy)2](NO3)2 (4a) and cis-[(PPh3)2Pt(9-MeGu)2](NO3)2 (5a), respectively. The X-ray structural determination of 2a and 3a indicates a strong π−π stacking interaction between one of the PPh3 phenyl groups and the pyrimydinic N3-platinated cytosine or the imidazole part of the N7-coordinated guanine base. The addition of a further equiv of nucleobase to the monoadducts forms quantitatively the bisadducts that have been isolated as pure compounds 4a and 5a. Under the same experimental conditions, the dinitrato analogue cis-[(PMePh2)2Pt(ONO2)2] (1b) forms the monoadducts 2b and 3b in equilibrium with a relatively high concentration (20–30%) of the bisadducts cis-[(PMePh2)2Pt(1-MeCy)2](NO3)2 (4b) and cis-[(PMePh2)2Pt(9-MeGu)2](NO3)2 (5b), which have been structurally characterized by single-crystal X-ray analysis. The characterization of the isolated complexes by multinuclear NMR spectroscopy is also described

Mono- and Polynuclear Complexes of the Model Nucleobase 1-Methylcytosine. Synthesis and Characterization of <i>cis</i>-[(PMe₂Ph)₂Pt{(1-MeCy(−H)}]₃(NO₃)₃ and <i>cis</i>-[(PPh₃)₂Pt{1-MeCy(−H)}(1-MeCy)]NO₃

The hydroxo complex cis-[L2Pt(μ-OH)]2(NO3)2 (L = PMe2Ph), in various solvents, reacts with 1-methylcytosine (1-MeCy) to give as the final product the cyclic species cis-[L2Pt{1-MeCy(−H),N 3N 4}]3(NO3)3 (1) in high or quantitative yield. X-ray analysis of 1 evidences a trinuclear species with the NH2-deprotonated nucleobases bridging symmetrically the metal centers through the N3 and N4 donors. A multinuclear NMR study of the reaction in DMSO-d6 reveals the initial formation of the dinuclear species cis-[L2Pt{1-MeCy(−H),N 3N 4}]22+ (2), which quantitatively converts into 1 following a first-order kinetic law (at 50 °C, t1/2 = 5 h). In chlorinated solvents, the deprotonation of the nucleobase affords as the major product (60−70%) the linkage isomer of 1, cis-[L2Pt{1-MeCy(−H)}]33+ (3), in which three cytosinate ligands bridge unsymmetrically three cis-L2Pt2+ units. In solution, 3 slowly converts quantitatively into the thermodynamically more stable isomer 1. No polynuclear adducts were obtained with the hydroxo complex stabilized by PPh3. cis-[(PPh3)2Pt(μ-OH)]2(NO3)2 reacts with 1-MeCy, in DMSO or CH2Cl2, to give the mononuclear species cis-[(PPh3)2Pt{1-MeCy(−H)}(1-MeCy)](NO3) (4) containing one neutral and one NH2-deprotonated 1-MeCy molecule, coordinated to the same metal center at the N3 and N4 sites, respectively. X-ray analysis and NMR studies show an intramolecular H bond between the N4 amino group and the uncoordinated N3 atom of the two nucleobases

Role of the Phosphine Ligands on the Stabilization of Monoadducts of the Model Nucleobases 1-Methylcytosine and 9-Methylguanine in Platinum(II) Complexes

The addition of 1-methylcytosine (1-MeCy) or 9-methylguanine (9-MeGu) to solutions of cis-(PPh3)2Pt(ONO2)2 (1a), in a molar ratio of 1:1, affords the monoadducts cis-[(PPh3)2Pt(1-MeCy)(ONO2)]NO3 (2a) and cis-[(PPh3)2Pt(9-MeGu)(ONO2)]NO3 (3a) and only trace amounts of the bisadducts cis-[(PPh3)2Pt(1-MeCy)2](NO3)2 (4a) and cis-[(PPh3)2Pt(9-MeGu)2](NO3)2 (5a), respectively. The X-ray structural determination of 2a and 3a indicates a strong π−π stacking interaction between one of the PPh3 phenyl groups and the pyrimydinic N3-platinated cytosine or the imidazole part of the N7-coordinated guanine base. The addition of a further equiv of nucleobase to the monoadducts forms quantitatively the bisadducts that have been isolated as pure compounds 4a and 5a. Under the same experimental conditions, the dinitrato analogue cis-[(PMePh2)2Pt(ONO2)2] (1b) forms the monoadducts 2b and 3b in equilibrium with a relatively high concentration (20–30%) of the bisadducts cis-[(PMePh2)2Pt(1-MeCy)2](NO3)2 (4b) and cis-[(PMePh2)2Pt(9-MeGu)2](NO3)2 (5b), which have been structurally characterized by single-crystal X-ray analysis. The characterization of the isolated complexes by multinuclear NMR spectroscopy is also described

Platinum(II)-Mediated Coupling Reactions of Acetonitrile with the Exocyclic Nitrogen of 9-Methyladenine and 1-Methylcytosine. Synthesis, NMR Characterization, and X-ray Structures of New Azametallacycle Complexes

The hydroxo complex cis-[L2Pt(μ-OH)]2(NO3)2, (L = PMePh2, 1a), in CH3CN solution, deprotonates the NH2 group of 9-methyladenine (9-MeAd) to give the cyclic trinuclear species cis-[L2Pt{9-MeAd(−H)}]3(NO3)3, (L = PMePh2, 2a), in which the nucleobase binds the metal centers through the N(1), N(6) atoms. In solution at room temperature, 2a slowly reacts with the solvent to form quantitatively the mononuclear azametallacycle cis-[L2PtNHC(Me){9-MeAd(−2H)}]NO3 (L = PMePh2, 3a), containing as anionic ligand the deprotonated form of molecule N-(9-methyl-1,9-dihydro-purin-6-ylidene)-acetamidine. In the same experimental conditions, the hydroxo complex with PPh3 (1b) forms immediately the insertion product 3b. Single-crystal X-ray analyses of 3a and 3b show the coordination of the platinum cation at the N(1) site of the purine moiety and to the N atom of the inserted acetonitrile, whereas the exocyclic amino nitrogen binds the carbon atom of the same CN group. The resulting six-membered ring is slightly distorted from planarity, with carbon−nitrogen bond distances for the inserted nitrile typical of a double bond [C(3)−N(2) = 1.292(7) Å in 3a and 1.279(11) Å in 3b], while the remaining CN bonds of the metallocycle are in the range of 1.335(8)−1.397(10) Å. A detailed multinuclear 1H, 31P, 13C, and 15N NMR study shows that the nitrogen atom of the inserted acetonitrile molecule binds a proton suggesting for 3a,b an imino structure in solution. In DMSO and chlorinated solvents, 3a slowly releases the nitrile reforming the trinuclear species 2a, whereas 3b forms the mononuclear derivative cis-[L2Pt{9-MeAd(−H)}]NO3 (L = PPh3, 4b), in which the adeninate ion chelates the metal center through the N(6) and N(7) atoms. Complex 4b is quantitatively obtained when 1b reacts with 9-MeAd in DMSO and can be easily isolated if the reaction is carried out in CH2Cl2. In CH3CN solution, at room temperature, 4b slowly converts into 3b indicating that the insertion of acetonitrile is a reversible process. A similar metal-mediated coupling reaction occurs when 1a,b react with 1-methylcytosine (1-MeCy) in CH3CN. The resulting complexes, cis-[L2PtNHC(Me){1-MeCy(−2H)}]NO3, (L = PMePh2, 5a and PPh3, 5b), contain the deprotonated form of the ligand N-(1-methyl-2-oxo-2,3-dihydro-1H-pyrimidin-4-ylidene)-acetamidine. The X-ray analysis of 5a shows the coordination of the metal at the N(3) site of the pyrimidine cycle and to the nitrogen atom of the acetonitrile, with features of the six-membered metallocycle only slightly different from those found in 3a and 3b. In CD3CN/CH313CN solution complexes 5a,b undergo exchange of the inserted nitrile, while in DMSO or chlorinated solvents they irreversibly release CH3CN to form species not yet fully characterized. No insertion of CH3CN occurs when the hydroxo complexes are stabilized by PMe3 and PMe2Ph

Homoleptic Complexes of Cobalt(0) and Nickel(0,I) with 1,1‘-Bis(diphenylphosphino)ferrocene (dppf): Synthesis and Characterization

Reduction of Co(dppf)Cl2 with 2 equiv of sodium naphthalenide in THF, in the presence of dppf, affords the homoleptic complex Co(dppf)2, 1, isolated in 65% yield as a brick red solid, extremely air sensitive. In solution, under inert atmosphere, 1 slowly decomposes into Co and dppf, following a first-order kinetic law (t1/2 = 21 h at 22 °C). Similarly to the Rh and Ir congeners, 1 undergoes a one-electron reversible reduction to [Co(dppf)2]-. Attempts to obtain this d10 species by chemical as well as electrochemical reduction of 1 lead to the hydride HCo(dppf)2, 2, as the only product that can be isolated. Reduction of Ni(dppf)Cl2 with sodium in the presence of dppf and catalytic amounts of naphthalene affords Ni(dppf)2, 3, isolated in 60% yield as a yellow air stable solid. The stoichiometric oxidation of 3 with [FeCp2]PF6 forms the d9 complex [Ni(dppf)2]PF6, 4, which represents the second example of a structurally characterized Ni(I) complex stabilized by phosphines. A single-crystal X-ray analysis shows for the metal a distorted tetrahedral environment with a dihedral angle defined by the planes containing the atoms P(1), Ni, P(2) and P(3), Ni, P(4) of 78.2° and remarkably long Ni−P bond distances (2.342(3)−2.394(3) Å). The EPR spectroscopic properties of 1 (at 106 K in THF) and 4 (at 7 K in 2-methyl-THF) have been examined and g tensor values measured (1, gx = 2.008, gy = 2.182, gz = 2.326; 4, gx = 2.098, gy = 2.113, gz = 2.332). A linear dependence between the hyperfine constants and the Ni−P bond distances has been evidenced. Finally, the change with time of the EPR spectrum of 4 indicates that it very slowly releases dppf

Iridium(III, 0, and −I) Complexes Stabilized by 1,1‘-Bis(diphenylphosphino)ferrocene (dppf): Synthesis and Characterization. Crystal Structures of [Na(THF)₅][Ir(dppf)₂]·THF and [Ir(dppf)₂]

The iridium(I) complex stabilized by the organometallic ligand 1,1‘-bis(diphenylphosphino)ferrocene (dppf), [Ir(dppf)2]+, 1, undergoes a cyclometalation reaction in solution to give the iridium(III) hydride [IrH(dppf(−H))(dppf)]+, 2, which has been isolated and characterized by spectroscopic methods. The compound is the final product of the intramolecular oxidative addition of the ortho C−H bond of a phenyl substituent of the diphosphine and is formed through an intermediate hydride, which has also been spectroscopically characterized. 1 can be electrochemically reduced to the Ir(0) and Ir(−I) species, [Ir(dppf)2], 3, and [Ir(dppf)2]-, 4, respectively, in two reversible single-electron processes. These low-valent metal complexes have been obtained by chemical reduction of 1 with sodium naphthalenide in tetrahydrofuran solution and their crystal and molecular structures determined by single-crystal X-ray analyses. 3 crystallizes in the triclinic system, space group P1̄, with a = 13.019(4) Å, b = 13.765(6) Å, c = 15.549(5) Å, α = 93.74(3)°, β = 90.35(3)°, γ = 92.07(3)°, V = 2779(2) Å3, and Z = 2. Anionic complex 4 crystallizes as sodium-solvated salt [Na(THF)5][Ir(dppf)2]·THF, 4b, in which the sodium cation is surrounded by five molecules of THF in a slightly distorted trigonal-bipyramidal environment. 4b crystallizes in the monoclinic system, space group P21/n, with a = 13.325(3) Å, b = 23.976(5) Å, c = 26.774(7) Å, β = 98.77(2)°, V = 8454(4) Å3, and Z = 4. The coordination geometry around the metal in neutral d9 complex 3 is intermediate between the highly distorted square-planar geometry, found earlier in cationic d8 species 1, and the almost regular tetrahedral arrangement of the two diphosphines in the anionic d10 complex 4. Reduction of Ir(I) to Ir(0) and Ir(−I) causes a stepwise decrease of the Ir−P bond length of 0.04 Å (average) and 0.05 Å, respectively, with a concomitant increase of the bite angle of the diphosphine which ranges from 94.3° (average) in [Ir(dppf)2]+ to 102.3° in [Ir(dppf)2]-

Irreversible Insertion of Benzonitrile into Platinum(II)−Nitrogen Bonds of Nucleobase Complexes. Synthesis and Structural Characterization of Stable Azametallacycle Compounds

Deprotonation of 1-methylcytosine (1-MeCy) and 9-methyladenine (9-MeAd) promoted by cis-[L2Pt(μ-OH)]2(NO3)2 (L = PPh3, PMePh2, 1/2dppe) in PhCN causes the irreversible insertion of a nitrile molecule into the Pt−N4 and Pt−N6 bonds of the cytosinate and adeninate ligands, respectively, to form the stable azametallacycle complexes cis-[L2PtNHC(Ph){1-MeCy(−2H)}]NO3 (L = PPh3, 1; PMePh2, 2; 1/2dppe, 3) and cis-[L2PtNHC(Ph){9-MeAd(−2H)}]NO3 (L = PPh3, 4; PMePh2, 5) containing the deprotonated form of the molecules (Z)-9-N-(1-methyl-2-oxo-2,3-dihydropyrimidin-4(1H)-ylidene)benzimidamide and (Z)-N-(9-methyl-1H-purin-6(9H)-ylidene)benzimidamide. Single-crystal X-ray analyses of 2 and 4 show the metal coordinated to the N3 cytosine site [Pt−N3 = 2.112(7) Å̊] and to the N1 site of adenine [Pt−N1 = 2.116(6) Å̊] and to the nitrogen atom of the inserted benzonitrile [Pt−N2 = 2.043(6) and 2.010(6) Å̊ in 2 and 4, respectively], with the exocyclic nucleobase amino nitrogen bound to the carbon atom of the CN group. Complex 2, in solution, undergoes a dynamic process related to a partially restricted rotation around Pt−P bonds, arising from a steric interaction of the oxygen atom of the cytosine with one ring of the phosphine ligands. The reaction of 4 with acetylacetone (Hacac) causes the quantitative protonation of the anionic ligand, affording the acetylacetonate complex cis-[(PPh3)2Pt(acac)]NO3 and the free benzimidamide NHC(Ph){9-MeAd(−H)}. In the same experimental conditions, complex 3 reacts with Hacac only partially

Complexes of Platinum(II) Containing Neutral and Deprotonated 9-Methyladenine. Synthesis, X-ray Structures, and NMR Studies on the Cyclic Trimer <i>cis</i>-[L₂Pt{9-MeAd(−H)}]₃(NO₃)₃ and the Dinuclear <i>cis</i>-[L₂Pt(ONO₂){9-MeAd(−H)}PtL₂](NO₃)₂ (L = PMePh₂)

The dinuclear hydroxo complex cis-[L2Pt(μ-OH)]2(NO3)2 (L = PMePh2, 1), in CH2Cl2, CH3CN, or DMF solution, deprotonates the NH2 group of 9-methyladenine (9-MeAd) to give the complex cis-[L2Pt{9-MeAd(−H)}]3(NO3)3, 2, which was isolated in good yield. The X-ray structure shows that the nucleobase binds symmetrically the metal centers through the N(1),N(6) atoms forming a cyclic trimer with Pt···Pt distances in the range 5.202(1)−5.382(1) Å. Dissolution of 2 in DMSO or DMF determines the partial (or total) dissociation of the cyclic structure to form several fragments. A multinuclear NMR analysis of the resulting mixture supports the presence of the mononuclear species cis-[L2Pt{9-MeAd(−H)}]+, 3, in which the deprotonated nucleobase chelates the metal center with the N(6),N(7) atoms. Addition of a stoichiometric amount of the nitrato complex cis-[L2Pt(ONO2)2] (L = PMePh2, 4) to a DMSO or DMF solution of 2 affords quantitatively the diplatinated compound cis-[L2Pt(ONO2){9-MeAd(−H)}PtL2](NO3)2, 5. The single-crystal X-ray analysis shows that the adenine behaves as a tridentate ligand bridging two cis-L2Pt units at the N(1) and N(6),N(7) sites, respectively [Pt(1)−N(1) = 2.109(5) Å, Pt(2)−N(6) = 2.095(7) Å, Pt(2)−N(7) = 2.126(7) Å]. The N(1)-bonded metal center completes the coordination sphere through an oxygen atom of a nitrate group, and its coordination plane is arranged orthogonally with respect the second one. The Pt−O distance [2.109(5) Å] is similar to those found in the nitrato complex 4 [2.110 Å, average]. The related complex cis-[{L2Pt(ONO2)}2(9-MeAd)](NO3)2, 6, containing the neutral adenine platinated at the N(1),N(7) atoms, was isolated and its stability in solution investigated by NMR spectroscopy. In DMSO, 6 undergoes decomposition forming a mixture of the species 4, 5, and the adenine mono- and bis-adducts cis-[L2Pt(9-MeAd)(DMSO)]2+, 7, and cis-[L2Pt(9-MeAd)2]2+, 8, respectively. This last complex, quantitatively formed upon addition of 9-MeAd (Pt/adenine = 1:2) to the mixture, was also isolated and characterized