The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms a series of octahedral iron(II) complexes of general formula [Fe(L)(1)]Xn with a variety of small-molecule ligands L at the sixth coordination site (L = X = Br, n = 1 (2); L = CO, X = Br, n = 2 (3); L = NO, X = Br, n = 2 (4); L = NO+, X = Br, n = 3 (5); L = NO2-, X = Br, n = 1 (6)). The bromo complex, which is remarkably stable towards hydrolysis and oxidation, serves as the precursor for all other complexes, which may be obtained by ligand exchange, employing CO, NO, NOBF4, and NaNO2, respectively. All complexes have been fully characterised, including solid-state structures in most cases. Attempts to obtain single crystals of 6 produced the dinuclear complex [Fe2[mu 2-(eta 1-N: eta 1-O)-NO2](1)2]Br2PF6 (7), whose bridging NO2- unit, which is unsupported by bracketing ligands, is without precedent in the coordination chemistry of iron. Compound 2 has a high-spin electronic configuration with four unpaired electrons (S = 2), while the carbonyl complex 3 is low-spin (S = 0), as are complexes 5, 6 and 7 (S = 0 in all cases); the 19 valence electron nitrosyl complex 4 has S = 1/2. Complex 4 and its oxidation product, 5 ([Fe(NO)]7 and [Fe(NO)]6 in the Feltham-Enemark notation) may be interconverted by a one-electron redox process. Both complexes are also accessible from the mononuclear nitro complex 6: Treatment with acid produces the 18 valence electron NO+ complex 5, whereas hydrolysis in the absence of added protons (in methanolic solution) gives the 19 valence electron NO. complex 4, with formal reduction of the NO2- ligand. This reactivity mimicks the function of certain heme-dependent nitrite reductases. Density functional calculations for complexes 3, 4 and 5 provide a description of the electronic structures and are compatible with the formulation of iron(II) in all cases; this is derived from the careful analysis of the combined IR, ESR and Mossbauer spectroscopic data, as well as structural parameters.

Iron carbonyl, nitrosyl, and nitro complexes of a tetrapodal pentadentate amine ligand: synthesis, electronic structure, and nitrite reductase-like reactivity.

The first heterometallic Cu(II)/Cr(III) complex with an open-chain Schiff-base ligand self-assembled from copper powder, Reineckes salt, ethylendiamine and acetone

Complex formation between FeX(2)6 H(2)O (X=BF(4) or ClO(4)) and the pyridine-derived tetrapodal tetraphosphane C(5)H(3)N[CMe(CH(2)PMe(2))(2)](2) (1) in methanol proceeds with solvent-induced cleavage of one PMe(2) group Depending on the reaction temperature and the nature of the counterion, iron(II) is coordinated, in distorted square-pyramidal fashion, by the anionic remainder of the chelating ligand, C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(2) (-))] (NP(3)C(-) donor set: X=BF(4), -50 degrees C: 2; X=ClO(4), RT: 4) or its protonated form C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(3))], in which the methyl group is in agostic interaction with the metal centre (X=BF(4), RT: 3; X=ClO(4), +50 degrees C: 5) A monodentate phosphinite ligand Me(2)POMe, formed from the cleaved PMe(2) group and methanol, completes the coordination octahedron in both cases Working in CD(3)OD (X=BF(4), RT) gives the deuterium-substituted analogue of 3, with ligands L(CH(2)D) (L=residual chelating ligand) and Me(2)POCD(3) A mechanism for the observed phosphorus-carbon bond cleavage is suggested Complex 2, when isolated at -50 degrees C, is stable in the solid state even at room temperature The reaction of 2 in methanol with carbon monoxide (105 bar) at elevated temperature forms, in addition to as yet unidentified side products, the carbonyl complex [(1)Fe(CO)](BF(4))(2) (7), in which the previous P--C bond cleavage has been reversed, reforming the original tetrapodal pentadentate NP(4) ligand 1 All compounds have been fully characterised, including X-ray structure analyses in most cases

Ligand Cleavage Put into Reverse: P ? C Bond Breaking and Remaking in an Alkylphosphane Iron Complex

Publisher Summary This chapter describes tetrapodal pentadentate nitrogen ligands. Tetrapodal pentadentate ligands have the ability to stabilize transition metal centers possessing a single labile coordination site, thereby, providing attractive platforms for reactivity studies in complexes of overall octahedral geometry. The chapter introduces a highly symmetrical aliphatic NN4 ligand, which consists of a “central” pyridine unit and four equivalent primary amino groups (“pyN4”, 1). This polyamine has predominantly σ donor character, is expected to render the coordinated metal ion electron-rich, imposes virtually no steric constraints, and may thus be construed as a “chelating analogue” of the pentaammine donor set (NH 3 ) 5 . The ligand may be construed as a chelating analogue of the pentaammine donor set has predominant σ-donor character, thereby, providing an electron-rich coordination environment for the metal center. This gives rise to unusual reactivity, such as the observed reduction of coordinated nitrite to NO.

Tetrapodal pentadentate nitrogen ligands: aspects of complex structure and reactivity

The protonation and metal complex formation of the branched pentaamine ligand 2,2,6,6-tetrakis(aminomethyl)-4-azaheptane (ditame, L) with CoII, CoIII, NiII, CuII, ZnII, and CdII have been studied. The crystal structures of [Ni(ditame)(EtOH)]Br2, [Co2(ditame)2(O2)]Cl4·4H2O, and [Co2(ditame)2(O2)]Cl2[ZnCl4][ZnCl3(H2O)]·H2O were determined by X-ray diffraction analyses. The metal complexes showed octahedral coordination geometries with exclusive formation of six-membered chelate rings, the four primary amino groups being in equatorial positions and the secondary amino group (Nsec) apical. This geometry enforces a relatively short M−Nsec bond. Complex formation in aqueous solution was evaluated by potentiometric titration. The ditame ligand forms a surprisingly stable NiII complex with logKNiL = 17.8 (25 °C, 0.1 M KNO3), and formation of protonated species [Ni(Hnditame)](n+2)+ (n = 1, 2) is not significant. Different behavior is evident for ZnII and CdII, with which protonated species are extensively formed, while the stability of [ML]2+ is relatively low. Complex formation between ditame and CuII in the range 3 < pH < 7 resulted in the formation of [Cu(H2ditame)]4+. In slightly alkaline solution, the dinuclear [Cu2(ditame)2]4+ appears to be formed. The structural assignment of this dimer is based on its UV/Vis and EPR characteristics, with the EPR data indicating a CuII···CuII separation of about 5.9 A. The redox behavior of the Ni and Co complexes was investigated by cyclic voltammetry. The mononuclear species showed quasi-reversible electron transfer for the NiII/III and CoII/III couples. For the dinuclear peroxo/ superoxo-bridged CoIII complexes, quasi-reversible electron transfer corresponding to oxidation/reduction of the O22−/O2− bridge was observed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The coordination chemistry of the pentadentate 2,2,6,6-tetrakis(aminomethyl)-4-azaheptane (ditame)

The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms mononuclear complexes with CuII, as shown for a series of compounds of the type [(1)Cu]X2 (X = Br, SCN, PF6, ClO4). The coordination environment of the copper ion is square-pyramidal, with the pyridine nitrogen atom of 1 in the apical position. The axial bond Cu−Npy is significantly longer than the four Cu−N bonds at the base of the pyramid, average values being 2.16 A and 2.03 A, respectively. When [(1)Cu]X2 (X = PF6, ClO4) is refluxed with an excess of acetone in methanol in the absence of base, the corresponding bis(isopropylideneimine) complexes [{(nac)2pyN4}Cu]X2 are obtained, in which two diametrically opposite primary amino groups of 1 have condensed with the ketone. The complex cations, which are now chiral, again have square-pyramidal coordinated CuII. In the presence of sodium methoxide, the condensation of [(1)Cu]X2 with acetone proceeds further, leading to the formation of one diacetone-amine-imine linkage in the product. A 12-membered 1,5,9-triazamacrocycle, incorporating the 2,6-disubstituted pyridine unit, is thus formed. Two podand nitrogen donors of the NN4 set (which retains its square pyramidal topology) remain: a primary amine and an isopropylidene imine group. The product forms as a mixture of both conceivable isomers (azomethine groups at the base of the pyramid cis or trans), which were separated and structurally characterised as the tribromocuprate(I) and hexafluorophosphate salts, respectively. The condensation of metal-free 1 with acetone in the presence of two equivalents of HBr yields the bis(aminal) salt (2,6-C5H3N{CMe[cyclo-CH2NH2C(CH3)2NHCH2-]}2)Br2, in which one of the secondary amino groups in each diazacyclohexane ring is protonated, as ascertained by X-ray crystallography. The aminal is a metal-free tautomer of the bis(isopropylideneimine) ligand (nac)2pyN4, to which it converts upon reaction with CuII under suitable conditions, providing an alternative synthetic route to complexes of the type [{(nac)2pyN4}Cu]X2.

Copper(II)- and Proton-Assisted Condensation Reactions of a Tetrapodal Pentaamine with Acetone: Formation of “Podand-cum-Macrocycle” Copper Complexes and a Protonated Bis(aminal)

With the intention of preparing tetrapodal pentadentate ligands having NS 4 or NP 4 donor sets, we investigated reactions of the previously reported tetratosylate 2,6-C 5 H 3 N[CMe(CH 2 OTs) 2 ] 2 (2) with thiourea or diphenylphosphide, but found them not to proceed cleanly, and to give mixtures of products. A derivative of 2 better suited to nucleophilic substitution is the corresponding tetrabromide 2,6-C 5 H 3 N[CMe(CH 2 Br) 2 ] 2 (2,6-bis-(2-bromo-1-bromomethyl-1-methyl-ethyl)-pyridine, 3), which is obtained in excellent yield from 2 by treatment with LiBr in dimethylsulfoxide. The reaction of 3 with 4 eq of thiourea in refluxing ethanol gives a single product. Substitution is not quantitative, however, and the product likely is a bis(thiouronium) bis(bromide) salt. Similarly, the reaction of 3 with 4 eq of potassium O-ethyl xanthogenate displaces only two out of the four bromo substituents under the chosen conditions; workup then leads to what is formulated as a bis(thietane) derivative formed by intramolecular cyclisation. By contrast, nucleophilic substitution with NaSEt in ethanol is quantitative, and the thioether 2,6-C 5 H 3 N[CMe(CH 2 SEt) 2 ] 2 (2,6-bis-(2-ethylsulfanyl-1-ethyl-sulfanylmethyl-1-methyl-ethyl)-pyridine, 4) has been isolated in close to 60% yield. Likewise, and in spite of the considerable steric bulk amassed in the molecule, the reaction of 3 with an excess of KPPh 2 in THF proceeds smoothly (even at -50 °C), to give the tetraphosphane 2,6-C 5 H 3 N[CMe(CH 2 PPh 2 ) 2 ] 2 (2,6-bis-{2-(diphenyl-phosphanyl)-1-[(diphenyl-phosphanyl)-methyl]-1-methyl-ethyl}-pyridine, 5) in 65% yield. In order to assess possible pathways of oxidative degradation relevant to the coordination chemistry of this ligand, 5 was treated with NO in CH 2 Cl 2 or ether at different temperatures. In two cases, reaction was observed to produce the oxide 2,6-C 5 H 3 N[CMe(CH 2 P(=O)Ph 2 ) 2 ] 2 (6) as a colourless solid in near quantitative yield, with concomitant formation of N 2 O. All compounds have been characterised by 1 H, 1 3 C and 3 1 P NMR spectroscopy (as applicable); IR spectroscopic and elemental analysis data are reported, and the crystal structure of 6 has been determined.

Tetrapodal pentadentate ligands with NS4 and NP4 donor sets: An elusive tetrathiol, and a sterically encumbered tetraphosphane

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Copper(II)- and Proton-Assisted Condensation Reactions of a Tetrapodal Pentaamine with Acetone: Formation of “Podand-cum-Macrocycle” Copper Complexes and a Protonated Bis(aminal)

Tetrapodal pentadentate ligands with NS4 and NP4 donor sets: An elusive tetrathiol, and a sterically encumbered tetraphosphane