1H NMR study of monomeric chloro-rhenium(III) complexes with triarylphosphines and nitriles

Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System

Bimetallic nitrogen (N₂) splitting to form metal nitrides is an attractive method for N₂ fixation. Although a growing number of pincer-supported systems can bind and split N₂, the precise relationship between the ligand properties and N₂ binding/splitting remains elusive. Here we report the first example of an N₂-bridged rhenium(III) complex, [(trans-P₂^tBuPyrr)ReCl₂]₂(μ-η¹:η¹-N₂) (P₂^tBuPyrr = [2,5-(CH₂P^tBu₂)₂C₄H₂N]^-). In this case, N₂ binding occurs at a higher oxidation level than that in other reported pincer analogues. Analysis of the electronic structure through computational studies shows that the weakly π-donor pincer ligand stabilizes an open-shell electronic configuration that leads to enhanced binding of N₂ in the bridged complex. Utilizing SQUID magnetometry, we demonstrate a singlet ground state for this Re-N-N-Re complex, and we offer tentative explanations for antiferromagnetic coupling of the two local S = 1 sites. Reduction and subsequent heating of the rhenium(III)-dinitrogen complex leads to chloride loss and cleavage of the N-N bond with isolation of the terminal rhenium(V) nitride complex (P₂^tBuPyrr)ReNCl.

Steric influence of salicylaldehyde-based Schiff base ligands on the formation of trans-[Re(PR3)2(Schiff base)]+ complexes

Complexes of the type trans-[Re(PR₃)₂(Schiff base)]⁺ (R = ethyl and/or phenyl) 2-7 were prepared by the reaction of (nBu₄N)[ReOCl₄] with H₂sal₂en or H₂sal₂ibn followed by addition of a tertiary phosphine. The trans-[Re(PR₃)₂(sal₂en)]⁺ complexes 2-4 were stable in solution, whereas the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes 6-7 were observed to convert to their corresponding cis-[ReO(PR₃)(sal₂ibn)]⁺ products through a process involving ligand dissociation, metal oxidation, and Schiff base ligand rearrangement. The conversion of the trans-[Re(PR₃)₂(sal₂ibn)]⁺ complexes is likely driven by steric interactions between the bulky backbone gem-dimethyl groups of the sal₂ibn ligand and the phosphine ligands. These complexes were isolated and characterized by ¹H and ¹³C NMR, FT-IR spectroscopy, cyclic voltammetry, and single crystal X-ray diffraction. The results reported herein provide insight into the factors that drive trans-[Re(PR₃)₂(Schiff base)]⁺ complex formation. This will aid in the development of novel ^186/188Re therapeutic agents and the design of novel bifunctional N₂O₂ Schiff base ligands.

Technetium and Rhenium Schiff Base Compounds for Nuclear Medicine: Syntheses of Rhenium Analogues to 99mTc-Furifosmin

Rhenium, the third-row congener of technetium, is often used to develop the macroscopic chemistry of potential ^99mTc diagnostic radiopharmaceuticals. The rhenium analogues to ^99mTc-furifosmin are being developed for potential radiotherapy of multidrug-resistant tumors. Complexes of the form trans-[M^III(PR₃)₂(N₂O₂-Schiff base)]⁺ are of interest for the potential imaging and treatment of multidrug-resistant tumors. Reaction of the tetradentate Schiff ligand 4,4'-[(1 E,1' E)-[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)]bis(2,2,5,5-tetramethyl-2,5-dihydrofuran-3-ol) (tmf₂enH₂) with the M(V) starting materials ( nBu₄N)[TcOCl₄] and ( nBu₄N)[ReOCl₄] gave the monomeric products trans-[TcOCl(tmf₂en)] and trans-[ReOCl(tmf₂en)], respectively. Reduction of in situ formed trans-[ReOCl(tmf₂en)] by various tertiary phosphines yielded disubstitued Re(III) products of the general type trans-[Re^III(PR₃)₂(tmf₂en)]⁺. The rhenium(III) compounds were found to be water-soluble and stable in aqueous solution. Reversible Re^III/Re^IV and Re^III/Re^II redox processes were observed at about 0.8-0.9 and -0.65 to -0.8 V, respectively, for each of the rhenium(III) species. Reaction of in situ formed trans-TcOCl(tmf₂en) with triethylphosphine yielded the reduced, disubstituted trans-[Tc(PEt₃)₂(tmf₂en)]PF₆. A reversible Tc^III/Tc^II redox couple was observed for the technetium(III) species, about 200 mV less negative than their rhenium(III) analogues, in addition to an irreversible Tc^III/Tc^IV process. All compounds were characterized using conventional spectroscopic techniques, single-crystal X-ray crystallography, and cyclic voltammetry.

Synthesis and characterization of rhenium(iii) complexes with (Ph2PCH2CH2)2NR diphosphinoamine ligands

The synthesis and characterization of a new series of neutral, six-coordinated compounds [Re^IIIX₃(PNPR)], where X is Cl or Br and PNPR is a diphosphinoamine having the general formula (Ph₂PCH₂CH₂)₂NR (R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃ and CH₂CH₂OCH₃) are reported. Stable [Re^IIIX₃(PNPR)] complexes were synthesized, in variable yields, starting from precursors where the metal was in different oxidation states (iii and v), by ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis spectroscopy, positive-ion electrospray ionization mass spectrometry (ESI(+)-MS) and X-ray diffraction analysis. Although the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in the solid and solution states. Only [ReCl₃(PNPH)] showed a meridional configuration both in solution and in the crystalline state. [ReBr₃(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)-MS and voltammetric data seem to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the alkyl chain tethered to the nitrogen.

Synthesis, crystal structure and magnetic properties of the complex [ReCl3(tppz)]·MeCN

The paper reports the synthesis, X-ray structure and a deep analysis of the variable-temperature magnetic data of the new complex [ReCl₃(tppz)]·MeCN being a very scarce example of mononuclear Re(iii) complexes incorporating nitrogen donor ligands.

Novel rhenium(III, IV, and V) tetradentate N2O2 Schiff base mononuclear and dinuclear complexes

Reaction of (Bu4N)[ReOCl4] with the tetradentate Schiff base ligand α,α'-[(1,1-dimethylethylene)dinitrilo]di-o-cresol (sal2ibnH2) yields cis-[Re(V)OCl(sal2ibn)], which quickly forms trans-[μ-O(Re(V)O(sal2ibn))2] in solution. The dinuclear complex can also be isolated by the addition of base (Et3N) to the reaction mixture. Conversely, the mononuclear complex can be trapped as cis-[Re(V)O(NCS)(sal2ibn)] by addition of (Bu4N)SCN to the reaction mixture. Reduction of cis-[Re(V)O(NCS)sal2ibn] with triphenylphosphine gives the rare trans-[Re(III)(NCS)(PPh3)(sal2ibn)] and unique μ-oxo Re(IV) dimer trans-[μ-O(Re(IV)(NCS)(sal2ibn))2]. All of the complexes were characterized by (1)H and (13)C NMR, FT-IR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), cyclic voltammetry and single crystal X-ray diffraction.

Re(V) and Re(III) complexes with sal2phen and triphenylphosphine: rearrangement, oxidation and reduction

Reactions of Re(V), tetradentate Schiff base complexes with tertiary phosphines have previously yielded both rearranged Re(V) and reduced Re(III) complexes. To further understand this chemistry, the rigid diiminediphenol (N(2)O(2)) Schiff base ligand sal(2)phen (N,N'-o-phenylenebis(salicylaldimine)) was reacted with (n-Bu(4)N)[ReOCl(4)] to yield trans-[ReOCl(sal(2)phen)] (1). On reaction with triphenylphosphine (PPh(3)), a rearranged Re(V) product cis-[ReO(PPh(3))(sal(2)phen*)]PF(6) (2), in which one of the imines was reduced to an amine during the reaction, and the reduced Re(III) products trans-[ReCl(PPh(3))(sal(2)phen)] (4) and trans-[Re(PPh(3))(2)(sal(2)phen)](+) (5) were isolated. Reaction of sal(2)phen with [ReCl(3)(PPh(3))(2)(CH(3)CN)] resulted in the isolation of [ReCl(2)(PPh(3))(2)(salphen)] (3). The compounds were characterized using standard spectroscopic methods, elemental analyses and single crystal X-ray crystallography.

Reactivity of Rhenium(V) Oxo Schiff Base Complexes with Phosphine Ligands: Rearrangement and Reduction Reactions

The symmetric rhenium(V) oxo Schiff base complexes trans-[ReO(OH2)(acac2en)]Cl and trans-[ReOCl(acac2pn)], where acac2en and acac2pn are the tetradentate Schiff base ligands N,N'-ethylenebis(acetylacetone) diimine and N,N'-propylenebis(acetylacetone) diimine, respectively, were reacted with monodentate phosphine ligands to yield one of two unique cationic phosphine complexes depending on the ligand backbone length (en vs pn) and the identity of the phosphine ligand. Reduction of the Re(V) oxo core to Re(III) resulted on reaction of trans-[ReO(OH2)(acac2en)]Cl with triphenylphosphine or diethylphenylphosphine to yield a single reduced, disubstituted product of the general type trans-[Re(III)(PR3)2(acac2en)]+. Rather unexpectedly, a similar reaction with the stronger reducing agent triethylphosphine yielded the intramolecularly rearranged, asymmetric cis-[Re(V)O(PEt3)(acac2en)]+ complex. Reactions of trans-[Re(V)O(acac2pn)Cl] with the same phosphine ligands yielded only the rearranged asymmetric cis-[Re(V)O(PR3)(acac2pn)]+ complexes in quantitative yield. The compounds were characterized using standard spectroscopic methods, elemental analyses, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic data for the structures reported are as follows: trans-[Re(III)(PPh3)2(acac2en)]PF6 (H48C48N2O2P2Re.PF6), 1, triclinic (P), a = 18.8261(12) A, b = 16.2517(10) A, c = 15.4556(10) A, alpha = 95.522(1) degrees , beta = 97.130(1) degrees , gamma = 91.350(1) degrees , V = 4667.4(5) A(3), Z = 4; trans-[Re(III)(PEt2Ph)2(acac2en)]PF6 (H48C32N2O2P2Re.PF6), 2, orthorhombic (Pccn), a = 10.4753(6) A, b =18.4315(10) A, c = 18.9245(11) A, V = 3653.9(4) A3, Z = 4; cis-[Re(V)O(PEt3)(acac2en)]PF6 (H33C18N2O3PRe.1.25PF6, 3, monoclinic (C2/c), a = 39.8194(15) A, b = 13.6187(5) A, c = 20.1777(8) A, beta = 107.7730(10) degrees , V = 10419.9(7) A3, Z = 16; cis-[Re(V)O(PPh3)(acac2pn)]PF6 (H35C31N2O3PRe.PF6), 4, triclinic (P), a = 10.3094(10) A, b =12.1196(12) A, c = 14.8146(15) A, alpha = 105.939(2) degrees , beta = 105.383(2) degrees , gamma = 93.525(2) degrees , V = 1698.0(3) A3, Z = 2; cis-[Re(V)O(PEt2Ph)(acac2pn)]PF6 (H35C23N2O3PRe.PF6), 5, monoclinic (P2(1)/n), a = 18.1183(18) A, b = 11.580(1) A, c = 28.519(3) A, beta = 101.861(2) degrees , V = 5855.9(10) A(3), Z = 4.

Preparation and electronic properties of rhenium(V) complexes with bis(diphenyl phosphino)ethane

Complexes of bis(diphenylphosphino)ethane (dppe) of the types ReOX3(dppe) and ReO(OR)X2(dppe) (with X= Cl or Br, and R = Me, Et, Pr, Ph, cyclohexyl (Cy), or -CH2CH2OH) were prepared to evaluate the influence of ligand changes on the low-energy d–d transitions in these Re(V) low-spin d2 systems. Arylimido compounds Re(NR)Cl3(dppe) (with R = Ph and p-ClC6H4) were also obtained. X-ray diffraction studies on ReO(OPr)Cl2(dppe), ReO(OPh)Br2(dppe), and ReO(OCy)Cl2(dppe) confirmed the presence of the trans O=Re-OR unit and showed that the structural characteristics of the Re-O-R segment are not affected by the presence of a bulky cyclohexyl substituent or an aromatic phenyl group. The structure of the arylimido complex Re(p-ClC6H4N)Cl3(dppe) was also determined. The electronic absorption spectra of the ReOX3(dppe) compounds include two low-energy components at ~11 500 and ~16 000 cm–1, assigned to the two spin-allowed d–d transitions expected for these low-symmetry systems. Substitution of the oxo ligand by an arylimido group has little effect on the lower-energy component, but moves the two components closer together. Replacing the halogen trans to the Re=O bond by an alkoxo group shifts the whole system to higher energies. These variations were found to correlate well with the energies of the frontier orbitals determined from DFT calculations. While attempting to prepare ReOCl3(dppe) from ReOCl3(PPh3)2, the Re(III) compound fac-ReCl3(PPh3){Ph2PC2H4PPh2(O)} was obtained, in which one end of dppe had become a phosphine oxide. Upon standing in DMSO, this compound gave the octahedral compound ReCl4{Ph2PC2H4PPh2(O)}, in which the formation of a chelate ring involving a phosphine and a phosphine oxide was ascertained by X-ray diffraction.Key words: rhenium, crystal structure, DFT calculations, d–d electron transitions.

Mixed chloride/phosphine complexes of the dirhenium core. 10. Redox reactions of an edge-sharing dirhenium(III) non-metal-metal-bonded complex, Re(2)(mu-Cl)(2)Cl(4)(PMe(3))(4)

Reduction and oxidation reactions of the dirhenium(III) non-metal-metal-bonded edge-sharing complex, Re(2)(mu-Cl)(2)Cl(4)(PMe(3))(4) (1), have been studied. Several new mono- and dinuclear rhenium compounds have been isolated and structurally characterized in the course of this study. Reductions of 1 with 1 and 2 equiv of KC(8) result in an unusual face-sharing complex having an Re(2)(5+) core, Re(2)(mu-Cl)(3)Cl(2)(PMe(3))(4) (2), and a triply bonded Re(II) compound, 1,2,7,8-Re(2)Cl(4)(PMe(3))(4) (3), respectively. Two-electron reduction of 1 in the presence of tetrabutylammonium chloride affords a new triply bonded complex of the Re(2)(4+) core, [Bu(n)()(4)N][1,2,7-Re(2)Cl(5)(PMe(3))(3)] (4). Oxidation of 1 with NOBF(4) yields a Re(IV) mononuclear compound, trans-ReCl(4)(PMe(3))(2) (5). Two isomers of the monomeric Re(III) anion, [ReCl(4)(PMe(3))(2)](-) (6, 7), have been isolated as side products. The crystal structures of compounds 2 and 4-7 have been determined by X-ray crystallography. The Re-Re distance in the face-sharing complex 2 of 2.686(1) A is relatively short. The metal-metal bond length in anion 4 of 2.2354(7) A is consistent with the usual values for the triply bonded Re(2)(4+) core compounds. In addition, a cis arrangement of trimethylphosphine ligands in the starting material 1 is retained upon reduction in the dinuclear products 2-4.

Mixed Chloride/Phosphine Complexes of the Dirhenium Core. 5. Reduction or Disproportionation at the Re(2)(6+) Unit in Reactions with Tertiary Phosphines in Different Solvents

For reactions of the octachlorodirhenium anion, [Re(2)Cl(8)](2)(-), with tertiary phosphines PEt(3) and PPr(n)(3), the influence of solvent on the reaction pathway has been studied. It has been shown that in 1-propanol at room temperature the reaction leads to a one-electron reduction of the Re(2)(6+) core. The novel dirhenium(II,III) paramagnetic products of stoichiometries [Bu(n)(4)N][Re(2)Cl(6)(PEt(3))(2)] (1a) and Re(2)Cl(5)(PPr(n)(3))(3) (2a) have been isolated and characterized. In benzene a disproportionation process occurs resulting in mononuclear rhenium(IV) complexes ReCl(4)(PR(3))(2) (PR(3) = PEt(3) (1b) and PPr(n)(3) (2b)) in addition to 1a and 2a, respectively. The solid state structures of compounds 1a, 1b, 2a.0.25C(6)H(14), and 2b have been investigated by X-ray crystallography. The crystallographic parameters are as follows: for 1a, tetragonal space group P4(2)/m with a = 11.821(2) Å, c = 14.770(3) Å, and Z = 2; for 1b, monoclinic space group P2(1)/n with a = 8.0555(4) Å, b = 12.536(2) Å, c = 10.2041(6) Å, beta = 99.167(9) degrees, and Z = 2; for 2a.0.25C(6)H(14), monoclinic space group P2(1)/n with a = 11.7679(9) Å, b = 18.574(5) Å, c = 19.688(4) Å, beta = 93.37(2) degrees, and Z = 4; for 2b, monoclinic space group P2(1)/c with a = 8.219(2) Å, b = 11.210(1) Å, c = 14.331(3) Å, beta = 93.171(9) degrees, and Z = 2. It has also been found that the disproportionation process in benzene goes by way of an intermediate complex which is an initial product of interaction between the [Re(2)Cl(8)](2)(-) and the phosphine. In the case of the triethylphosphine ligand we formulate this complex as [Bu(n)(4)N][Re(2)Cl(7)(PEt(3))(3)] (1c).

Binding of Amino-Dialkylated Adenines to Rhenium(III) and Rhenium(IV) Centers

Amino-dialkylated adenines R(2)AdH (R = Me, Et) react with ReCl(3)(MeCN)(PPh(3))(2) to form mer,cis-ReCl(3)(R(2)AdH)(2)(PPh(3)) complexes. Attempts to protonate the coordinated adenine with HCl led to the oxidized trans-ReCl(4)(R(2)AdH)(PPh(3)) compounds. In both types of complexes, the purine binds via the N3 site of the six-membered ring and makes an intramolecular N9-H.Cl hydrogen bond with a chloride ligand. A material of composition ReCl(4)(R(2)AdH)(MeCN) was obtained by reacting R(2)AdH with cis-ReCl(4)(MeCN)(2). It turned out to be a ReCl(4)[R(2)AdC(Me)=NH] complex containing a cyclic amidine ligand resulting from the insertion of the acetonitrile C&tbd1;N group in the N9-H bond of the N3-coordinated adenine unit. Crystal structures were determined for one compound of each type: ReCl(3)(Me(2)AdH)(2)(PPh(3)).2C(6)H(5)CH(3), triclinic, P&onemacr;, a = 14.181(2) Å, b = 17.591(3) Å, c = 19.015(5) Å, alpha = 76.66(2) degrees, beta = 86.80(2) degrees, gamma = 87.02(1) degrees, Z = 4, R = 0.0606; ReCl(4)(Me(2)AdH)(PPh(3)), triclinic, P&onemacr;, a = 10.391(4) Å, b = 10.801(5) Å, c = 13.651(6) Å, alpha = 89.64(4) degrees, beta = 83.49(3) degrees, gamma = 65.14(4) degrees, Z = 2, R = 0.0432; ReCl(4)[Me(2)AdC(Me)=NH].C(6)H(6).CH(2)Cl(2), monoclinic, P2(1)/c, a = 13.941(3) Å, b = 13.656(3) Å, c = 12.874(2) Å, beta = 105.47(2) degrees, Z = 4, R = 0.0406. Most of the signals in the (1)H NMR spectra of these paramagnetic species could be assigned.