Directing iridium-catalyzed C-C bond formation by selection of the ancillary ligands: Polymerization and cyclotrimerization of alkynes

The ability of organoiridium derivatives of catalyzing oligomerization and polymerization of terminal alkynes is markedly influenced by the nature of non-participative ligands coordinated to the metal. The dimeric species [Ir(cod)Cl]₂ and [Ir(cod)(OMe)]₂ (cod = 1,5-cyclooctadiene) as well as the phosphine complexes HIr(cod)(PR₃)₂ (PR₃ = PPh₃, P(p-MeOC₆H₄)₃, P(o-MeOC₆H₄)Ph₂, PCyPh₂) catalyze the polymerization reaction, whereas the diphosphine derivatives HIr(cod)(P-P) (P-P = Ph₂P(CH₂)_nPPh₂ (n = 1-4), o-C₆H₄(PPh₂)₂) promote the regioselective formation of 1,2,4-trisubstituted benzenes. On the other hand, the iridium complexes with nitrogen chelating ligands Ir(cod)(N-N)X and Ir(hd)(N-N)X (hd = 1,5-hexadiene; N-N = 1,10-phenanthroline and substituted derivatives; X = halogen) catalyze alkynes polymerization. In most cases one catalytic reaction predominates over the other possible routes, so that polymerization often takes place in the absence of oligomerization side reactions, and conversely cyclotrimerization is rarely accompanied by formation of either polyene or dimers.     

Four-coordinate complexes of bis(1-diphenylphosphinoindenyl)iron(II)

Palladium, platinum and rhodium complexes of rac- and meso-bis(1-diphenylphosphinoindenyl)iron(II) (1) are reported. Both rac and meso isomers of {bis(1-diphenylphosphinoindenyl)iron(II)}palladium dichloride (rac- and meso-2) were characterized by X-ray crystallography along with the rac isomer of the Pt analogue (rac-3). NMR analysis of the rhodium complex [{bis(1-diphenylphosphinoindenyl)iron(II)}(cyclooctadiene)rhodium(I)] tetraphenylborate suggests a similar structure in solution. Coupling reactions of n- and sec-BuCl with bromobenzene in THF are catalysed by rac-2 and found to be similar to (PPh₃)₂PdCl₂ but poorer than (dppf)PdCl₂ in diethyl ether.     

Ketone H2-hydrogenation catalysts: Ruthenium complexes with the headphone-like ligand bis(phosphaadamantyl)propane

Treatment of 1,3-diphosphinopropane with acetylacetone in the presence of HCl gives the new chiral bis(phosphaadamantyl)propane ligand (bpap) (1) as a mixture of diastereoisomers. Recrystallization from ethanol gives a mixture enriched in rac diastereoisomer (90% rac/10% meso). The enriched mixture reacts with [RuHCl(PPh₃)₃] in refluxing THF to give [RuHCl(bpap)(PPh₃)] (2) in 73% yield. Compound 2 reacts readily with chiral diamines giving octahedral trans-[RuHCl(bpap)(diamine)] complexes 3 (diamine = (1R,2R)-1,2-diaminocyclohexane) and 4 (diamine = (1R,2R)-1,2-diphenylethylenediamine). Compounds 3 and 4 are very active catalysts for H₂-hydrogenation of neat acetophenone in the presence of KO^tBu as a strong base under mild conditions (room temperature, 3 atm of H₂). The low ee values for 1-phenethanol can be attributed to the similar shapes of two terminal adamantoid cages and the flexible backbone of the bpap ligand. The structures of complexes 2 and 3 have been determined by single-crystal X-ray diffraction.     

Nickel(II) complexes of di- and tri-substituted thiourea mono- and di-anions

Reaction of nickel(II) acetate, 1,2-bis(diphenylphosphino)ethane (dppe), and a di- or tri-substituted thiourea R¹NHC(S)R²R³ (R³ = H or alkyl) with trimethylamine in hot methanol gave cationic nickel(II) complexes containing N,S-chelated thiourea monoanion ligands [Ni{SC(NR²R³)NR¹}(dppe)]⁺, which can be readily isolated as their BPh₄⁻ salts. The X-ray crystal structure of [Ni{SC(NMe₂)NPh}(dppe)]⁺BPh₄⁻ is reported.     

Nucleophilic addition to the η-iridathiabenzene ligand in [η-Cp*Ir(C,S-2,5-Me2T)]FeCp

The η⁶-iridathiabenzene ring coordinated to CpFe⁺ in [η⁶-Cp*Ir(C,S-2,5-Me₂T)]FeCp⁺ (2) undergoes attack by nucleophiles. Phosphorus-donors, CN-Buⁿ, and CN⁻ add to the Ir to give products with an η⁵-coordinated iridathiabenzene ligand. Hydride (Et₃BH⁻, ) and Ph⁻ nucleophiles attack at the sulfur to give products with Ir-SH and Ir-SPh groups and an iridacyclopentadiene unit. Other reactions of 2 are described together with structure determinations of key compounds.     

Stereoisomers of Mn(III) complexes of ethylenebis[(o-hydroxyphenyl)glycine]

We have prepared the Mn(III) complexes rac-Na[Mn(EHPG)]·3H₂O (1) and rac,meso-Na[Mn(EHPG)]·H₂O (2), where H₄EHPG is ethylenebis[(o-hydroxyphenyl)glycine], and determined their X-ray crystal structures. Complex 1 contains N(S,S)C(R,R) configurations at the N and C stereogenic centres, whilst in the unit cell of complex 2 there are two independent molecules, 2a (meso) and 2b (rac), with N(R,R)C(S,R) and N(R,R)C(S,S) configurations, respectively. Enantiomers of each complex are also present. The Mn(III) centres have Jahn-Teller-distorted octahedral geometry. The rac isomer has two long axial MnO(carboxylate) bonds (2.162-2.202 Å) and the equatorial plane contains two short MnN bonds (2.012-2.063 Å) trans to short MnO(phenolate) bonds (1.865-1.901 Å). The meso isomer has long axial MnN (2.194 Å) and MnO(carboxylate) (2.152 Å) bonds, and shorter equatorial MnN (2.005 Å) trans to MnO(phenolate) (1.901 Å) and MnO(carboxylate) (1.988 Å) trans to O(phenolate) (1.897 Å) bonds.     

Mechanistic investigation of the oxidative addition reactions between different iridium (I) cyclooctadiene ([Ir(LL′)(cod)]) complexes and iodomethane

The oxidative addition reactions between two different [Ir(cod)(LL′)] complexes (LL′ = hpt and AnMetha) and iodomethane was kinetically investigated. The rate of oxidative addition was determined as 2.2(2) × 10⁻² and 2.69(6) × 10⁻² M⁻¹ s⁻¹ for [Ir(cod)(hpt)] and [Ir(cod)(AnMetha)] in nitromethane respectively. The large negative entropy of activation for the above-mentioned reactions in different solvents clearly point to an associative mechanism. An intrinsic volume of activation of −30.5(3) and −28(3) cm³ mol⁻¹ was determined for [Ir(cod)(hpt)] and [Ir(cod)(AnMetha)], respectively. A linear transition state with large charge separation and central ion contraction due to oxidation, contributes to the negative volume of activation.     

Exchange of isonitrile ligands in the complex [Mo(O)Cl(CNMe)4] by the tetraphos ligand prP4: Stereochemical influences on the reaction course

Reaction of [Mo(O)Cl(CNMe)₄]⁺ with the linear tetraphos ligand meso and rac prP₄ leads to a mixture of [Mo(O)Cl(κ⁴-meso-prP₄)]⁺ and [Mo(O)Cl(CNMe)(κ³-rac-prP₄)]⁺ which are identified by X-ray structural analysis and/or ³¹P NMR spectroscopy. In the meso κ⁴-product both of the phenyl groups of the central phosphorus atoms are oriented towards the oxo ligand whereas in the rac κ³-product one of these phenyl groups is oriented to the oxo and the other to the chloro ligand. The origin of the different coordination modes lies in the different steric demands of the oxo and chloro ligands. The influences of the steric interactions are enhanced by the fact that exchange of the fourth isonitrile is difficult. This hypothesis is supported by the preparation of the complex [Mo(O)Cl(CNMe)(dpepp)]PF₆ whose isonitrile ligand is inert towards exchange by monophosphines, even under drastic conditions.     

The auration of 2-hydroxy-pyridine (2-pyridone): preparative and structural studies and a comparison with reactions of related aliphatic O,N-donors

In a study of the isolobal analogy between the proton H⁺ and the ligand-backed gold(I) cations [(R₃P)Au]⁺, the reaction of the mixture of 2-pyridone/2-hydroxy-pyridine tautomers with [(Ph₃P)Au]BF₄ has been investigated. It affords the 1:1 complex with the gold atom N-bonded to the 2-hydroxy-pyridine tautomer: {(Ph₃P)Au[NC₅H₄-(OH-2)]}⁺BF₄ ⁻, which is related to known salts with the 2-hydroxy-pyridinium cation such as [HNC₅H₄(OH-2)]⁺Cl⁻. The structure was derived from analytical and spectroscopic data and from a comparison with the salt [(Ph₃P)Au(pyr)]BF₄, prepared and investigated structurally as a reference compound. An analogue was also prepared with 2-dimethylamino-ethanol as a substrate, which also affords the N-bonded complex [(Ph₃P)Au([Me₂NCH₂CH₂OH)]⁺BF₄ ⁻, the structure of which has been determined. The OH group is not attached to the gold atom but engaged in hydrogen bonding with the counterion. By contrast, in the complex [(Ph₃P)Au(Me₂NCH₂CH₂NMe₂)]⁺BF₄ ⁻ synthesized similarly with tmeda and crystallized as the dichloromethane solvate, one nitrogen atom is bonded firmly to the metal atom, but the second nitrogen atom is also weakly engaged in coordinative bonding. The compound is fluxional in solution, where a site exchange is observed which is rapid on the NMR time scale. The reaction of two equivalents of [(Ph₃P)Au]BF₄ with an alkali 2-pyridinolate, prepared from the above tautomeric mixture and sodium metal or a potassium alkoxide, yields the diaurated product {N,O-[(Ph₃P)Au]₂(NC₅H₄-O-2)}BF₄. In the crystal structure determination of a sesqui-solvate with dichloromethane it has been shown that one gold atom is attached solely to the nitrogen atom and the other solely to the oxygen atom. The dinuclear cations are associated into cyclic dimers through head-to-tail aurophilic bonding. From the geometrical characteristics of the core unit of the cations the ligand can be assigned a 2-pyridinolate form featuring pyridine and phenolate donor sites.     

A new route to N(1)-5R-tetrazole complexes via azidation to nitriles coordinated to Pt(II) and Pt(IV)

New tetrazolate complexes trans-[PtCl₂(RCN₄)₂]²⁻, trans-[PtCl₄(RCN₄)₂]²⁻ with Ph₃PCH₂Ph⁺ and (CH₃)₂NH₂⁺ counterions have been obtained by azidation of nitriles coordinated to Pt(II) and Pt(IV) {trans-[PtCl₂(RCN)₂] and trans-[PtCl₄(RCN)₂] (R = Et, Ph)} and characterized. The composition and the molecular structure of the complexes obtained were established by the СHN elemental analyses, ¹Н and ¹³С NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray diffraction. The coordination of nitriles to Pt(II) and Pt(IV) is shown significantly activate the azidation: the reaction proceeds with a higher rate and at relatively low temperature compared with the classical 1,3-dipolar addition of azides to nitriles.     

Structural comparison of (pentamethylcyclopentadienyl)iridium(III) azido complexes containing potentially N,S-bidentate N-heterocyclic thiolates: 2- or 8-quinolinethiolate and benzimidazole-2-thiolate

The crystal structures of mononuclear (azido)(pentamethylcyclopentadienyl)iridium(III) complexes bearing 2- or 8-quinolinethiolate (n-Sqn⁻), [Cp^∗Ir(N₃)(n-Sqn)] {n = 2 (1) or 8 (2); Cp^∗ = η⁵-C₅Me₅} have been determined by X-ray analysis. The 2-Sqn complex, 1, acquires severe steric strains in the four-membered κ²N,S chelate ring, while the 8-Sqn isomer, 2, forms a strain-free five-membered planar κ²N,S chelate ring. It has also been revealed that the corresponding benzimidazole-2-thiolate (Hbimt⁻) complex, which was obtained similarly to the above n-Sqn complexes from [Cp^∗Ir(N₃)₂]₂ and Na(Hbimt), takes an unsymmetrical dinuclear structure bridged by two Hbimt⁻ ligands with different bonding modes, [Cp^∗Ir(N₃){μ(S:N¹)-Hbimt}{μ(S:S)-Hbimt}Ir(N₃)Cp^∗] · MeOH (3).     

A 31P1H NMR study of the reactions of [Rh2(μ-Cl)2(cod)2] with unsymmetrical,bidentate ligands and hydrogen

[Rh₂(μ-Cl)₂(cod)₂] reacts with Ph₂PCH₂CH₂OMe (PC₂O), Ph₂P(CH₂)₃NMe₂ (PC₃N), PBuⁿPh₂ or PPh₃ to give [Rh(cod)L₂]Cl (L = PC₂O, PC₃N, PBuⁿPh₂, PPh₃). In the presence of hydrogen, [Rh(cod)L₂]Cl is converted to [RhClH₂L₃]. In contrast, [Rh(cod)(PC₂O)₂]BPh₄ reacts with H₂ to give [RhH₂(PC₂O)₂S₂]BPh₄ (S = solvent). With Ph₂PCH₂CH₂NMe₂ (PC₂N) or Ph₂PCH₂CH₂SMe (PC₂S), [Rh₂(μ-Cl)₂(cod)₂] reacts to form the chelate complexes cis- [Rh(PC₂N)₂]⁺ or cis-[Rh(PC₂S)₂]⁺, neither of which reacts with hydrogen under ambient conditions. The products of the reactions are characterized in situ by ³¹P¹H NMR spectroscopy.     

[Ni(L)(MeCN)]complex cation as a template for the assembly of extended I3 · I5 and I5 · I7 polyiodide networks {L=2,5,8-trithia[9](2,9)-1,10-phenanthrolinophane}. Synthesis and structures of [Ni(L)(MeCN)]I8 and [Ni(L)(MeCN)]I12

The compounds [Ni(L)(MeCN)]I₈ (1) and [Ni(L)(MeCN)]I₁₂ (2) have been obtained from the reactions of the complexes [Ni(L)(L^′)][BF₄]_(2 + n) {L=2,5,8-trithia[9](2,9)-1,10-phenanthrolinophane; L^′=MeCN, Cl⁻, Br⁻, I⁻; n=charge of L^′} with an excess of I₂ (molar ratios of 6, 10 and 20 have been used), in the presence of the stoichiometric amount of I⁻ (as Bu₄ⁿNI) necessary to balance the charge of the complex cation [Ni(L)(L^′)]^(2 + n)+. An X-ray diffraction analysis shows that, independently of the nature of L^′, both 1 and 2 contain the complex cation [Ni(L)(MeCN)]²⁺, which is therefore capable of templating two different polyiodide networks based on interacting I₃⁻/I₅⁻ and I₅⁻/I₇ units, respectively. The solid state FT-Raman spectra of 1 and 2 are discussed based on their structural features.     

Shape selectivity in outer-sphere electron transfer reactions

Chiral induction has been examined in the four diastereomeric products formed in a series of outer-sphere electron transfer reactions between the oxidants [Co(ox)₃]³⁻, [Co(edta)]⁻, [Co(gly)(ox)₂]²⁻, C₁-cis(N)-[Co(gly)₂(ox)]⁻, [Co(en)(ox)₂]⁻, C₂-cis(N)-[Co(gly)₂(ox)]⁻ and trans(N)-[Co(gly)₂(ox)]⁻ with [Co((RR,SS)-chxn)₃]²⁺ and [Co((R, S)-pn)₃]²⁺ as reductants. The products; [Co((RR,SS)-chxn)₃-lel₃]³⁺, [Co((RR,SS)-chxn)₃-lel₂ob]³⁺, [Co((RR,SS)-chxn)₃-lelob₂]³⁺, [Co((RR,SS)-chxn)₃-ob₃]³⁺ and corresponding species for [Co((R, S)-pn)₃]³⁺ show patterns of selectivity which are analyzed in terms of the size and structure of the reactants. The presence of a pseudo-C₃ carboxylate face on the oxidant enhances selectivity but the pattern is quite different for those oxidants that contain oxalate as one of their ligands compared with non-oxalate containing species such as [Co(edta)]⁻. A very simple model is developed in which the reductant employs a limited set of interactions corresponding to the major symmetry axes. The unrestricted reductant has very low aggregate selectvity. Steric and hydrogen bonding patterns in both oxidant and reductant enhance individual interactions resulting in the observed selectivities.     

Kinetics of substitution of ferrocenyl-containing β-diketonato ligands by phenanthroline from β-diketonato-1,5-cyclooctadienerhodium(I) complexes

Second-order rate constants, k₂, for the substitution of the ferrocene-containing β-diketonato ligands FcCOCHCOR⁻ with R=CF₃ (ferrocenoyltrifluroacetonato, fctfa, pK_a 6.56), CCl₃ (ferrocenoyltrichloroacetonato, fctca, 7.13), CH₃ (ferrocenoylacatonato, fca, 10.01), Ph (anion of benzoylferrocenoylmethane, bfcm, 10.41) and Fc (anion of diferrocenoylmethane, dfcm, 13.1) (Ph=phenyl, Fc=ferrocenyl, values in brackets are the pK_a values of the free β-diketones) from the complexes [Rh(cod)(FcCOCHCOR)] with 1,10-phenanthroline (phen, cod=1,5-cyclooctadiene) at 25 °C were found to be 560 (R=CF₃), 1370 (CCl₃), 30 (Ph), 18 (CH₃) and 7.0 dm³ mol⁻¹ s⁻¹ (Fc), respectively. The temperature dependence of each reaction was determined and the large negative values obtained for activation, ΔS^#<−100 J K⁻¹ mol⁻¹ for all but R=CCl₃ (ΔS^#_CCl3=−81 J K⁻¹ mol⁻¹), suggests an associative substitution mechanism. The rate law of the reaction was found to be R={k_s+k₂[phen]}[Rh(cod)(FcCOCHCOR)]. Since the solvent-associated rate constant k_s≈0 for all R except Ph (k_s,RPh=0.06 s⁻¹) the solvent, methanol, plays a limited role in the reaction. Results are interpreted to imply that the rate-determining step during substitution is breaking of an RhO bond and not the formation of an RhN bond. The role of β-diketone pK_a and group electronegativity, χ, of each R group on the rate of substitution are also discussed.     

Electrochemical reduction properties of A-frame compounds and crystal structure of Pd2(dppm)2(Me)2(Br) dimer

Two series of A-frame complexes, [Pd₂(dppm)₂(R)₂(μ-X)]⁺ (R = Me and X = Cl, Br, I, H; R = Mes and X = Br, I), were investigated by cyclic voltammetry (CV). The 2-electron reduction potentials for the first series increase from I (−1.10), Br (−1.17), Cl (−1.25) to H (−1.65 V versus SCE, in CHCl₃), as well as in the second series; Br (−1.35) and I (−1.38 V versus SCE, in THF). The nature of the LUMO where the electron reduction takes place is qualitatively addressed by DFT on the corresponding model complexes [Pd₂(H₂PCH₂PH₂)₂(R)₂(μ-X)]⁺. The LUMO and (LUMO + 1) of the halide derivatives exhibit the presence of Pd d_x²-y² atomic orbitals interacting in an anti-bonding fashion with the n-donor orbitals of X⁻, P, and Me⁻, explaining in part the observed reactivity upon reduction. The X-ray structure of [Pd₂(dppm)₂(Me)₂(μ-Br)]⁺ compound exhibits the typical A-frame structure with a Pd?Pd non-bonding distance of 3.036(1) Å, and long Pd-Br bonds of 2.5623(5) and 2.5793(5) Å.     

The base-catalyzed isomerization of trans-RCo(H2O)L (L=5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)

The sec, rac-CH₃Co(H₂O)L²⁺ (L=5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) was prepared successfully via meso-CH₃Co(H₂O)L²⁺ in aqueous solution. The isomerizations from meso-RCo(H₂O)L²⁺ (R=CH₃, C₂H₅ and C₃H₇) and sec, rac-CH₃Co(H₂O)L²⁺ to pri, rac-RCo(H₂O)L²⁺ were both base catalyzed in aqueous solution. The kinetic results showed the reaction to be first order in both organocobalt complex and hydroxide ion with the reactivity order for the alkyl group being C₃H₇ ∼ C₂H₅ ? CH₃. However, the conversion from the most steric hindered isomer form of sec, rac- was slow. The ratio of the isomerization rate constants between meso-CH₃Co(H₂O)L²⁺ and sec, rac-CH₃Co(H₂O)L²⁺ to pri, rac-CH₃Co(H₂O)L²⁺ is almost a factor of 100. The thermodynamic activation parameters for these isomerization reactions were investigated.     

Structure, spectroscopic and magnetic properties of a novel 1-D coordination copper(II) polymer containing BIMAM ligand [BIMAM = bis(imidazol-2-yl)methylaminomethane] and μ-1,3 squarato bridges

This paper reports the synthesis and complete characterization (structural, spectroscopic and magnetic) of [Cu(HBIMAM)Cl(C₄O₄)]_n · (H₂O)_n [BIMAM = bis(imidazol-2-yl)methylaminomethane]. This compound is made of infinite chains - running along c axis - built from [CuCl(HBIMAM)]⁺ units bridged together by μ-O¹,O³-bis(monodentate) squarate anions. Non-covalent interactions (H-bonds and π-π) drive the build-up of an infinite three-dimensional array. The coordination polyhedron about the copper(II) ion is distorted square pyramidal. The EPR spectrum is indicative of a d_z²-y² ground state for the Cu(II) ions with significant contribution of d_z². Magnetic susceptibility measurements in the range 1.8-200 K show weak antiferromagnetic exchange interactions (2J = −3.5(1) cm⁻¹). The observed magnetic behaviour is discussed in terms of the crystal structure and compared with that observed in related copper(II) complexes containing μ-O¹,O³-squarato bridges.     

Meso-[{Ru(phen)2}2(μ-bpm)]: A high-affinity DNA bulge probe {bpm = 2,2′-bipyrimidine; phen = 1,10-phenanthroline}

The binding of the stereoisomers of [{Ru(Me₂bpy)₂}₂(μ-bpm)]⁴⁺, [{Ru(phen)₂}₂(μ-bpm)]⁴⁺ and [{Ru(Me₂phen)₂}₂(μ-bpm)]⁴⁺ (Me₂bpy = 4,4′-dimethyl-2,2′-bipyridine; bpm = 2,2′-bipyrimidine; phen = 1,10-phenanthroline; Me₂phen = 4,7-dimethyl-1,10-phenanthroline) to a tridecanucleotide d(CCGAGAATTCCGG)₂ which contains a single adenine bulge site, and four control dodecanucleotides, have been studied using a fluorescence intercalator displacement (FID) assay. The meso isomer of [{Ru(phen)₂}₂(μ-bpm)]⁴⁺ showed the strongest binding to the bulge-containing tridecanucleotide. In order to gain a greater understanding of the basis of the higher affinity exhibited by the meso isomer towards the bulge sequence, a ¹H NMR study of the binding of the two enantiomers (ΔΔ and ΛΛ) of rac-[{Ru(phen)₂}₂(μ-bpm)]⁴⁺, and the, meso (ΔΛ) diastereoisomer, to the tridecanucleotide d(CCGAGAATTCCGG)₂ was carried out. The NMR results suggest that the meso isomer binds selectively at the bulge site in the tridecanucleotide minor groove, but closer to the 3′-direction and with less structural perturbations of the groove than the ΔΔ and ΛΛ isomers. The results of this study confirm that dinuclear ruthenium complexes have excellent potential as DNA bulge probes, and meso-[{Ru(phen)₂}₂(μ-bpm)]⁴⁺ in particular has a high affinity (1 × 10⁶ M⁻¹) and selectivity for a single adenine bulge site.     

Synthesis, characterization and X-ray crystal structure of [Re(L4)(CO)3]Br · 2CH3OH (L4 = N,N-bis[(2-diphenylphosphino)ethyl]methoxyethylamine): A model compound for novel cationic Tc(I)-tricarbonyl radiotracers useful for heart imaging

This report describes synthesis and evaluation of cationic complexes, [^99mTc(CO)₃(L)]⁺ (L = N-methoxyethyl-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L1), N-[(15-crown-5)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L2) and N-[(18-crown-6)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L3)) as potential radiotracers for heart imaging. Preliminary results from biodistribution studies in female adult BALB-c mice indicated that the cationic ^99mTc(I)-tricarbonyl complex, [^99mTc(CO)₃(L2)]⁺, has a significant localization in the heart at 60 min post-injection. To understand the coordination chemistry of these bisphosphine ligands with the ^99mTc(I)-tricarbonyl core, we prepared [Re(CO)₃(L4)]Br (L4: N,N-bis[(2-diphenylphosphino)ethyl]methoxyethylamine) as a model compound. [Re(CO)₃(L4)]Br has been characterized by elemental analysis, IR, ESI-MS, NMR (¹H, ¹³C, ¹H-¹H COSY, and ¹H-¹³C HMQC) methods, and X-ray crystallography. In solid state, [Re(CO)₃(L4)]⁺ has a distorted octahedron coordination geometry with PNP occupying one facial plane. The chelator backbone adopts a “chair” conformation with phosphine-P atoms at equatorial positions and the amine-N at the apical site. In solution, [Re(CO)₃(L4)]⁺ is able to maintain its cationic nature with no dissociation of carbonyl ligands or any of the three PNP donors.