Mixed dinitrogen-organocyanamide complexes of molybdenum(0) and their protic conversion into hydrazide and amidoazavinylidene derivatives

Preparation and reactivity towards hydrazines of bis(cyanamide) and bis(cyanoguanidine) complexes of the iron triad

Bis(diethylcyanamide) [Fe(N≡CNEt2)2L4](BPh4)2 1a and bis(cyanoguanidine) [Fe{N≡CN(H)C(NH2)=NH}2L4](BPh4)2 1b [L = P(OEt)3] complexes were prepared by allowing iron(II) chloride to react first with an excess of P(OEt)3 and then of the appropriate cyanamide, followed by addition of an excess of NaBPh4. Instead, bis(complexes) of ruthenium and osmium [M(N≡CNEt2)2L4](BPh4)2 2a, 3a and [M{N≡CN(H)C(NH2)=NH}2L4](BPh4)2 2b, 3b (M = Ru 2, Os 3) were prepared by reacting hydrides MH2L4 first with either triflic acid HOTf or methyltriflate MeOTf and then with an excess of the appropriate cyanamide. Hydride-diethylcyanamide [MH(N≡CNEt2)L4]BPh4 4a, 5a and hydride-cyanoguanidine complexes [MH{N≡CN(H)C(NH2)=NH}L4](BPh4)2 4b, 5b (M = Ru 4, Os 5) were also obtained by reacting MH2L4 first with one equivalent of HOTf or MeOTf and then with the appropriate cyanamide. Treatment of bis(cyanamide) and bis(cyanoguanidine) complexes 1-3 with hydrazines RNHNH2 afforded hydrazinecarboximidamide derivatives [M{η(2)-N(H)=C(NEt2)N(R)NH2}L4](BPh4)2 6a-12a and [M{η(2)-N(H)=C[N=C(NH2)2]N(R)NH2}L4](BPh4)2 6b-12b (M = Fe 6-8, Ru 9, 10, Os 11, 12; R = H 6, 9, 11, Me 7, 10, 12, Ph 8). A reaction path involving nucleophilic attack by hydrazine on the cyanamide carbon atom is proposed. All the complexes were characterised by spectroscopy and X-ray crystal structure determination of [Os{η(2)-NH=C[N=C(NH2)2]N(CH3)NH2}{P(OEt)3}4](BPh4)2 12b.

Acylated cyanoimido-complexes trans-[Mo(NCN){NCNC(O)R}(dppe)(2)]Cl and their reactions with electrophiles: chemical, electrochemical and theoretical study

Treatment of a dichloromethane solution of trans-[Mo(NCN){NCNC(O)R}(dppe)(2)]Cl [R = Me (1a), Et (1b)] (dppe = Ph(2)PCH(2)CH(2)PPh(2)) with HBF(4), [Et(3)O][BF(4)] or EtC(O)Cl gives trans-[Mo(NCN)Cl(dppe)(2)]X [X = BF(4) (2a) or Cl (2b)] and the corresponding acylcyanamides NCN(R')C(O)Et (R' = H, Et or C(O)Et). X-ray diffraction analysis of 2a (X = BF(4)) reveals a multiple-bond coordination of the cyanoimide ligand. Compounds 1 convert to the bis(cyanoimide) trans-[Mo(NCN)(2)(dppe)(2)] complex upon reaction with an excess of NaOMe (with formation of the respective ester). In an aprotic medium and at a Pt electrode, compounds 1 (R = Me, Et or Ph) undergo a cathodically induced isomerization. Full quantitative kinetic analysis of the voltammetric behaviour is presented and allows the determination of the first-order rate constants and the equilibrium constant of the trans to cis isomerization reaction. The mechanisms of electrophilic addition (protonation) to complexes 1 and the precursor trans-[Mo(NCN)(2)(dppe)(2)], as well as the electronic structures, nature of the coordination bonds and electrochemical behaviour of these species are investigated in detail by theoretical methods which indicate that the most probable sites of the proton attack are the oxygen atom of the acyl group and the terminal nitrogen atom, respectively.

Reactivity of Pt- and Pd-bound nitriles towards nitrile oxides and nitrones: substitution vs. cycloaddition

Reactions of the nitrone CH3CH=N(CH3)O and the nitrile oxide CH3C[triple bond]NO with the nitrile complexes trans-[MCl2(N[triple bond]CCH3)2] (M = Pt, 1; Pd, 2) were investigated by theoretical methods at B3LYP and, for some processes, CCSD(T) levels of theory. The mechanisms of substitutions and cycloadditions were studied in detail. The former occur via a concerted asynchronous mechanism of dissociative type. The calculations of the metal-ligand bond energies in the starting complexes and substitution products and the analysis of structural features of the transition states indicate that the M-N bond dissociation (rather than M-O bond formation) is the step, which controls the reactivity of and in substitutions. The different chemical behaviours of the Pt and Pd complexes towards the 1,3-dipoles were investigated. The exclusive isolation of cycloaddition rather than substitution products in any solvents in the case of is both kinetically and thermodynamically controlled. The switch of the reaction mode from cycloaddition to substitution for 2 in CH2Cl2 solution is caused by the significantly lower Pd-N bond energy in comparison with the Pt-N bond energy, consistent with the higher lability of the Pd complexes. The different chemical behaviour of 2 in CH3CN and CH2Cl2 solvents is accounted for by the great excess of acetonitrile in the CH3CN solution rather than a different solvation character. The relative variation of Wiberg bond indices along the reaction path is proposed as a quantitative criterion for the classification of the reaction mechanism.

Direct synthesis of (imine)platinum(ii) complexes by iminoacylation of ketoximes with activated organonitrile ligands

The metal-mediated iminoacylation of ketoximes R1R2C=NOH (1a R1 = R2 = Me; 1b R1 = Me, R2 = Et; 1c R1R2 = C4H8; 1d R1R2 = C5H10) upon treatment with the platinum(II) complex trans-[PtCl2(NCCH2CO2Me)2] 2a with an organonitrile bearing an acceptor group proceeds under mild conditions in dry CH2Cl2 to give the trans-[PtCl2{NH=C(CH2CO2Me)ON=CR1R2}2] 3a-d isomers in moderate yield. The reaction of those ketoximes with trans-[PtCl2(NCCH2Cl)2] 2b under the same experimental conditions gives a 1 : 1 mixture of the isomers trans/cis-[PtCl2{NH=C(CH2Cl)ON=CR1R2}2] 3e-h and 4e-h in moderate to good yield. These reactions are greatly accelerated by microwave irradiation to give, with higher yields (ca. 75%), the same products which were characterized by IR and 1H, 13C and 195Pt NMR spectroscopies, FAB-MS, elemental analysis for the stable trans isomers, and X-ray diffraction analysis (3f). The diiminoester ligand in 3a was liberated upon reaction of the complex with a diphosphine.

Protic Conversion of Nitrile into Azavinylidene Complexes of Rhenium, a Mechanistic Theoretical Study

The mechanism of the protonation of the rhenium nitrile chloro-complexes [ReCl(NCCH3)(PH3)4] (2), taken as models of the real systems [ReCl(NCR)(dppe)(2)] (dppe = Ph2PCH2CH2PPh2), leading to the azavinylidene products [ReCl(NC(H)CH3)(PH3)4]+ (3) was investigated by theoretical methods at the B3LYP level of theory. Electrostatic and molecular orbital arguments and thermodynamic, kinetic, and steric factors are analyzed and indicate that the chlorine atom is the most probable site of the initial proton attack, although the direct protonation of the nitrile carbon atom is also possible as a concurrent process. For the cis-isomer of 2, the initially formed chloro-protonated species cis-[Re(ClH)(NCCH3)(PH3)4]+ further converts to the azavinylidene cis-3 via either an acid-independent 1,4-proton shift or an acid-base catalyzed pathway involving a second protonation of the nitrile carbon atom to give cis-[Re(ClH)(NC(H)CH3)(PH3)4]2+ followed by elimination of the proton from the chlorine atom.

A Photochemical Activation Scheme of Inert Dinitrogen by Dinuclear RuII and FeII Complexes

A general photochemical activation process of inert dinitrogen coordinated to two metal centers is presented on the basis of high-level DFT and ab initio calculations. The central feature of this activation process is the occupation of an antibonding pi* orbital upon electronic excitation from the singlet ground state S0 to the first excited singlet state S1. Populating the antibonding LUMO weakens the triple bond of dinitrogen. After a vertical excitation, the excited complex may structurally relax in the S1 state and approaches its minimum structure in the S1 state. This excited-state minimum structure features the dinitrogen bound in a diazenoid form, which exhibits a double bond and two lone pairs localized at the two nitrogen atoms, ready to be protonated. Reduction and de-excitation then yield the corresponding diazene complex; its generation represents the essential step in a nitrogen fixation and reduction protocol. The consecutive process of excitation, protonation, and reduction may be rearranged in any experimentally appropriate order. The protons needed for the reaction from dinitrogen to diazene can be provided by the ligand sphere of the complexes, which contains sulfur atoms acting as proton acceptors. These protonated thiolate functionalities bring protons close to the dinitrogen moiety. Because protonation does not change the pi*-antibonding character of the LUMO, the universal and well-directed character of the photochemical activation process makes it possible to protonate the dinitrogen complex before it is irradiated. The pi*-antibonding LUMO plays the central role in the activation process, since the diazenoid structure was obtained by excitation from various occupied orbitals as well as by a direct two-electron reduction (without photochemical activation) of the complex; that is, the important bending of N2 towards a diazenoid conformation can be achieved by populating the pi*-antibonding LUMO.

Hydrolytic metal-mediated coupling of dialkylcyanamides at a Pt(IV) center giving a new family of diimino ligands

Addition of excess R(2)NCN to an aqueous solution of K(2)[PtCl(4)] led to the precipitation of [PtCl(2)(NCNR(2))(2)] (R(2) = Me(2) 1; Et(2) 2; C(5)H(10) 3; C(4)H(8)O, 4) in a cis/trans isomeric ratio which depends on temperature. Pure isomers cis-1-3 and trans-1-3 were separated by column chromatography on SiO(2), while trans-4 was obtained by recrystallization. Complexes cis-1-3 isomerize to trans-1-3 on heating in the solid phase at 110 degrees C; trans-1 has been characterized by X-ray crystallography. Chlorination of the platinum(II) complexes cis-1-3 and trans-1-4 gives the appropriate platinum(IV) complexes [PtCl(4)(NCNR(2))(2)] (cis-5-7 and trans-5-8). The compound cis-6 was also obtained by treatment of [PtCl(4)(NCMe)(2)] with neat Et(2)NCN. The platinum(IV) complex trans-[PtCl(4)(NCNMe(2))(2)] (trans-5) in a mixture of undried Et(2)O and CH(2)Cl(2) undergoes facile hydrolysis to give trans-[PtCl(4)[(H)=C(NMe(2))OH](2)] (9; X-ray structure has been determined). The hydrolysis went to another direction with the cis-[PtCl(4)(NCNR(2))(2)] (cis-5-7) which were converted to the metallacycles [PtCl(4)[NH=C(NR(2))OC(NR(2))=NH]] (11-13) due to the unprecedented hydrolytic coupling of the two adjacent dialkylcyanamide ligands giving a novel (for both coordination and organic chemistry) diimino linkage. Compounds 11-13 and also 14 (R(2) = C(4)H(8)O) were alternatively obtained by the reaction between cis-[PtCl(4)(MeCN)(2)] and neat undried NCNR(2). The structures of complexes 11, 13, and 14 were determined by X-ray single-crystal diffraction. All the platinum compounds were additionally characterized by elemental analyses, FAB mass-spectrometry, and IR and (1)H and (13)C[(1)H] NMR spectroscopies.