Remote effects of axial ligand substitution in heterometallic Cr≡Cr···M chains

Mixed-Valence Tetrametallic Iridium Chains

Neutral [X-{Ir2 }-{Ir2 }-X] (X=Cl, Br, SCN, I) and dicationic [L-{Ir2 }-{Ir2 }-L]2+ (L=MeCN, Me2 CO) tetrametallic iridium chains made by connecting two dinuclear {Ir2 } units ({Ir2 }=[Ir2 (μ-OPy)2 (CO)4 ], OPy=2-pyridonate) by an iridium-iridium bond are described. The complexes exhibit fractional averaged oxidation states of +1.5 and electronic delocalization along the metallic chain. While the axial ligands do not significantly affect the metal-metal bond lengths, the metallic chain has a significant impact on the iridium-L/X bond distances. The complexes show free rotation around the unsupported iridium-iridium bond in solution, with a low-energy transition state for the chloride chain. The absorption spectra of these complexes show characteristic bands at 438-504 nm, which can be fine-tuned by varying the terminal capping ligands.

Metal-Metal Bond Umpolung in Heterometallic Extended Metal Atom Chains

Understanding the fundamental properties governing metal–metal interactions is crucial to understanding the electronic structure and thereby applications of multimetallic systems in catalysis, material science, and magnetism. One such property that is relatively underexplored within multimetallic systems is metal–metal bond polarity, parameterized by the electronegativities (χ) of the metal atoms involved in the bond. In heterobimetallic systems, metal–metal bond polarity is a function of the donor–acceptor (Δχ) interactions of the two bonded metal atoms, with electropositive early transition metals acting as electron acceptors and electronegative late transition metals acting as electron donors. We show in this work, through the preparation and systematic study of a series of Mo₂M(dpa)₄(OTf)₂ (M = Cr, Mn, Fe, Co, and Ni; dpa = 2,2′-dipyridylamide; OTf = trifluoromethanesulfonate) heterometallic extended metal atom chain (HEMAC) complexes that this expected trend in χ can be reversed. Physical characterization via single-crystal X-ray diffraction, magnetometry, and spectroscopic methods as well as electronic structure calculations supports the presence of a σ symmetry 3c/3e^– bond that is delocalized across the entire metal-atom chain and forms the basis of the heterometallic Mo₂–M interaction. The delocalized 3c/3e^– interaction is discussed within the context of the analogous 3c/3e^– π bonding in the vinoxy radical, CH₂CHO. The vinoxy comparison establishes three predictions for the σ symmetry 3c/3e^– bond in HEMACS: (1) an umpolung effect that causes the Mo–M interactions to become more covalent as Δχ increases, (2) distortion of the σ bonding and non-bonding orbitals to emphasize Mo–M bonding and de-emphasize Mo–Mo bonding, and (3) an increase in Mo spin population with increasing Mo–M covalency. In agreement with these predictions, we find that the Mo₂···M covalency increases with increasing Δχ of the Mo and M atoms (Δχ_Mo–M increases as M = Cr < Mn < Fe < Co < Ni), an umpolung of the trend predicted in the absence of σ delocalization. We attribute the observed trend in covalency to the decreased energic differential (ΔE) between the heterometal orbital and the σ bonding molecular orbital of the Mo₂ quadruple bond, which serves as an energetically stable, “ligand”-like electron-pair donor to the heterometal ion acceptor. As M is changed from Cr to Ni, the σ bonding and nonbonding orbitals do indeed distort as anticipated, and the spin population of the outer Mo group is increased by at least a factor of 2. These findings provide a predictive framework for multimetallic compounds and advance the current understanding of the electronic structures of molecular heteromultimetallic systems, which can be extrapolated to applications in the context of mixed-metal surface catalysis and multimetallic proteins.

This work describes how use of a metal−metal quadruply bonded metalloligand can reverse expected trends in metal−metal bond polarity through the preparation and systematic study of a novel series of Mo₂M(dpa)₄(OTf)₂ (M = Cr, Mn, Fe, Co, and Ni) heterotrimetallic extended metal atom chain (HEMAC) complexes. These complexes feature a 3c/3e⁻ metal−metal bond that is delocalized across the entire metal atom chain and is compared to the 3c/3e⁻ π bonding in the vinoxyl radical.

Paramagnetic one-dimensional chains containing high-spin manganese atoms showing antiferromagnetic interaction through -Pt-Rh-Rh-Pt- bonds

To exploit the magnetic interactions of multiple metals, a heterometallic one-dimensional (1D) chain containing three kinds of metals, Rh, Pt, and Mn, where [Rh₂(O₂CCH₃)₄] and [Pt₂Mn(piam)₄(NH₃)₄]²⁺ (piam = pivalamidate) are connected through unbridged Rh-Pt bonds to form -Rh-Rh-Pt-Mn-Pt- alignments was successfully synthesized. The Mn atoms are tetrahedrally coordinated by four oxygen atoms of the piam ligands, where the coordination geometries form a zigzag 1D chain. Each Mn atom is linked by -Pt-Rh-Rh-Pt-, with a Mn-Mn separation of 13.9 Å. In parent [Pt₂Mn(piam)₄(NH₃)₄](PF₆)₂, Mn adopts two coordination environments, octahedral and tetrahedral, both of which are Mn(+2) high-spin states. In EtOH, [Rh₂(O₂CCH₃)₄] selectively binds tetrahedral Mn to afford a 1D chain. Physical analysis of the 1D chain using electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) revealed that all metals are divalent, indicating five unpaired spin-localized electrons on the Mn atoms. Magnetic susceptibility measurements indicated antiferromagnetic intra-chain interactions between the Mn atoms in the 1D chain, where χT at 300 K was 5.33 cm³ K mol^-1 and gradually decreased to 1.65 cm³ K mol^-1 at 2 K. Theoretical fitting of the magnetic behavior showed weak exchange coupling (zJ = -0.43 cm^-1) between two high-spin Mn(+2) ions through diamagnetic Pt-Rh-Rh-Pt.

Improving the Solubility of Hexanuclear Heterometallic Extended Metal Atom Chain Compounds in Nonpolar Solvents by Introducing Alkyl Amine Moieties

The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interaction at the d _{z ²} orbital between two kinds of metal complex is useful for obtaining heterometallic one-dimensional (1D) chains as well as heterometallic metal string compounds (HMSCs). Platinum dinuclear complexes, [Pt₂(piam)₂(NH₂R)₄]X₂ (piam = pivalamidate, R = CH₃, C₂H₅, C₃H₇, or C₄H₉, X = anion), comprising σ* as HOMO were mixed with [Rh₂(O₂CCH₃)₄] comprising σ* as LUMO in solvents to afford single crystals of [{Rh₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₂R)₄}₂]X₄ (2-5). Single-crystal X-ray analyses revealed that 2-5 are hexanuclear complexes that are one-dimensionally aligned as Pt-Pt-Rh-Rh-Pt-Pt with metal-metal bonds, where the alkyl moieties at end Pt atoms obstruct further 1D extension. Complexes 2-5 appear as if they are cut off from an infinite chain [{Rh₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₃)₄}₂] _n (PF₆)_4n ·6nH₂O (1) aligned as -{Pt-Pt-Rh-Rh-Pt-Pt} _n -. The diffuse reflectance spectrum of 1 depicts broad shoulder bands, which are not present in the spectra of 2-5, proving that the infinite chain 1 forms a band structure. Compounds 4 and 5 with propyl or butyl moieties at amine ligands, respectively, are soluble in nonpolar solvents, such as CH₂Cl₂, without the dissociation of their hexanuclear structures. Taking advantage of their solubility, measurement of cyclic voltammetry in CH₂Cl₂ become possible, which shows the quasi-reversible oxidation and reduction waves at 4: E _ox = 0.86 V and E _red = 0.69 V and 5: E _ox = 0.87 V and E _red = 0.53 V.

Tetrairon(II) extended metal atom chains as single-molecule magnets

Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon(ii), halide-capped complexes [Fe4(tpda)3Cl2] (1Cl) and [Fe4(tpda)3Br2] (1Br), obtained by reacting iron(ii) dihalides with [Fe2(Mes)4] and N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine (H2tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mössbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with χMT at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron(ii) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy (D = -11 to -19 cm-1, |E/D| = 0.05-0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy (D = 8-10 cm-1, |E/D| = 0.06-0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1-Fe2 and Fe3-Fe4 interactions and antiferromagnetic Fe2-Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.

Gating the conductance of extended metal atom chains: a computational analysis of Ru3(dpa)4(NCS)2 and [Ru3(npa)4(NCS)2]

The effects of a gate potential on the conductance of two members of the EMAC family, Ru3(dpa)4(NCS)2 and its asymmetric analogue, [Ru3(npa)4(NCS)2]+, are explored with a density functional approach combined with non-equilibrium Green's functions. From a computational perspective, the inclusion of an electrochemical gate potential represents a significant challenge because the periodic treatment of the electrode surface resists the formation of charged species. However, it is possible to mimic the effects of the electrochemical gate by including a very electropositive or electronegative atom in the unit cell that will effectively reduce or oxidize the molecule under study. In this contribution we compare this approach to the more conventional application of a solid-state gate potential, and show that both generate broadly comparable results. For two extended metal atom chain (EMAC) compounds, Ru3(dpa)4(NCS)2 and [Ru3(npa)4(NCS)2], we show that the presence of a gate potential shifts the molecular energy levels in a predictable way relative to the Fermi level, with distinct peaks in the conductance trace emerging as these levels enter the bias window.

Structure and magnetic properties of a novel heteroheptanuclear metal string complex [Ni3Ru2Ni2(μ7-teptra)4(NCS)2](PF6)

We report the synthesis of a novel heteroheptanuclear metal string complex (HMSC) [Ni₃Ru₂Ni₂(μ₇-teptra)₄(NCS)₂](PF₆) 1 supported by tetra-pyridyl-tri-amine (H₃teptra) ligands. We employed X-ray diffraction and other spectroscopic techniques to characterize the complex. The observed remarkably short Ru-Ru distance of 2.2499(3) Å for 1 is indicative of a unique metal-metal interaction in the mixed-valence [Ru₂]⁵⁺ (S = 3/2) unit. The complex exhibits a relatively high magnetic moment value of 4.55 B.M. at 4 K, which increases rapidly to 6.00 B.M. at 30 K and remains at 6.11 B.M. from 50 to 300 K as shown by SQUID measurements, indicating a high spin (S≥ 3/2) system which is further supported by the analyses of EPR spectra at low temperatures. These magnetic behaviors can be ascribed to the result of spin-exchange interactions among multi-spin centers.

Paramagnetic Metal-Metal Bonded Heterometallic Complexes

Significant progress has been made in the past 10-15 years on the design, synthesis, and properties of multimetallic coordination complexes with heterometallic metal-metal bonds that are paramagnetic. Several general classes have been explored including heterobimetallic compounds, heterotrimetallic compounds of either linear or triangular geometry, discrete molecular compounds containing a linear array of more than three metal atoms, and coordination polymers with a heterometallic metal-metal bonded backbone. We focus in this Review on the synthetic methods employed to access these compounds, their structural features, magnetic properties, and electronic structure. Regarding the metal-metal bond distances, we make use of the formal shortness ratio (FSR) for comparison of bond distances between a broad range of metal atoms of different sizes. The magnetic properties of these compounds can be described using an extension of the Goodenough-Kanamori rules to cases where two magnetic ions interact via a third metal atom. In describing the electronic structure, we focus on the ability (or not) of electrons to be delocalized across heterometallic bonds, allowing for rationalizations and predictions of single-molecule conductance measurements in paramagnetic heterometallic molecular wires.

Paramagnetic One-Dimensional Chain Complex Consisting of Three Kinds of Metallic Species Showing Magnetic Interaction through Metal-Metal Bonds

A heterometallic and paramagnetic one-dimensional (1-D) chain (3) aligned as -Rh(+2)-Rh(+2)-Pt(+2)-Co(+2)-Pt(+2)- with direct metal-metal bonds was obtained by HOMO-LUMO interactions at the σ* (d_z²) orbital between two kinds of complexes. The 1-D chains in 3 have relatively straight backbones because the raw material complexes, [Rh₂(O₂CCH₃)₄] and [Pt₂Co(piam)₄(NH₃)₄], are connected and stacked in a face-to-face fashion, where Co---Co are separated by about 13.3 Å with four different metals. Physical measurements revealed that 3 has a band gap between the σ-type conduction and valence bands, where d-orbitals of the Co ion with three unpaired electrons are laid among them. The magnetic behavior of 3 was investigated and found to be consistent with a complex interaction involving both zero-field splitting and Pauli paramagnetism attributed to band formation superimposed on relatively strong exchange coupling (zJ = -22.2 cm^-1) between two high-spin Co(+2) ions separated by the diamagnetic Pt-Rh-Rh-Pt.

How to understand very weak Cr-Cr double bonds and negative spin populations in trinuclear Cr complexes: theoretical insight

Trinuclear Cr(ii) complex [Cr3(dpa)4Cl2] 1 (Hdpa = dipyridylamine) has two Cr-Cr double bonds linked with each other. DMRG-CASPT2 calculations reproduced its symmetrical structure. The Cr-Cr effective bond order (EBO) was evaluated to be only 0.59 based on the density matrix based on localized orbitals from DMRG-CASSCF orbitals. The CASCI calculations showed a significantly large α-spin population on the terminal Cr atoms as expected but a significantly large β-spin population on the central Cr atom against expectations. The very small EBO and the presence of a large β-spin population are not consistent with the simple understanding that 1 has two Cr-Cr double bonds and a quintet ground state, which requests correct understanding of 1 from the viewpoint of chemical bond theory. Comparison of 1 with the allene molecule and allyl radical disclosed that the linked Cr-Cr bonds of 1 resembled the C-C bond of the allyl radical but completely differed from the linked C-C double bonds of allene despite the similar molecular structure. Its N3 analogue [Cr3(dpa)4(N3)2] 2 has non-symmetrical structure with shorter Cr1-Cr2 and longer Cr2-Cr3 bonds unlike 1, indicating that 2 is a valence tautomer of 1. DMRG-CASPT2 could reproduce its non-symmetrical structure but DFT/B3PW91 could not. In 2, the EBO is 0.95 for the shorter Cr1-Cr2 bond and 0.47 for the longer Cr2-Cr3 one. The terminal Cr3 has a very large α spin population, and the other terminal Cr1 has a somewhat large α spin population, but the central Cr2 has a considerably large β spin population. These results indicate that the Cr1-Cr2 bond conjugates with the Cr2-Cr3 bond, which is inconsistent with the simple understanding that 2 has a quadruple bond between Cr1 and Cr2 and no bond between Cr2 and Cr3. The symmetrical structure has a stronger Cr-X coordinate bond (X = Cl or N3) but less stable Cr3 core than does the non-symmetrical one. The relative stabilities of the symmetrical and the non-symmetrical structures are determined by the balance between stabilization energies from the Cr3 core and the Cr-X coordinate bond. All these findings show that electronic structures and Cr-Cr bonds of 1 and 2 are interesting from the viewpoint of molecular science.

From Pincer to Paddlewheel: C-H and C-S Bond Activation at Bis(2-pyridylthio)methane by Palladium(II)

The bis(2-pyridylthio)methanidopalladium(II) pincer complex (1), containing a Pd-C bond, was obtained from the reaction of bis(2-pyridylthio)methane (H₂L) with palladium(II) acetate in toluene under reflux. When palladium(II) trifluoroacetate was used, H₂L reacted to generate the tetrakis(pyridine-2-thiol)palladium(II) complex (2). Complex 2 was converted to a heterobimetallic palladium(II)-iron(II) paddlewheel complex (3) upon treatment with iron(II) triflate in the presence of a base in acetonitrile at room temperature.

Band Structures of Quasi-One-Dimensional Incommensurate Helical Systems: A Case Study of Infinite Chromium Extended Metal Atom Chain

Extended metal atom chains (EMACs) are promising candidates for molecular wires but their band structures remain to be explored. As a quasi-one-dimensional (Q1D) system, the incommensurate helical nature of EMACs hinders such calculations. In this work, we resolved this issue via explicit implementation of helical symmetry. Moreover, the pattern of metal d bands was rationalized by a systematic investigation on a series of related Q1D helical systems. Two critical factors, helical ligand field and chemically asymmetric ligand field, are proposed and identified. We found that the symmetry and ligand fields of the system dominate the pattern of the metal d bands, instead of specific chemical composition of ligands. The presented method and rationale are applicable to not only EMACs but also related Q1D helical systems.

Facile Axial Ligand Substitution in Linear Mo≣Mo-Ni Complexes

Clean axial ligand substitution reactions of heterometallic extended metal atom chains (HEMACs) supported by the dpa ligand (dpa = 2,2'-dipyridylamine) have been synthetically challenging due to side reactions that alter the trimetallic core. Following the hypothesis that a heterometallic core containing second-row transition metals would be more robust toward ligand substitution, we report the synthesis of three new heterotrimetallic compounds, Mo₂Ni(dpa)₄(OTf)₂ (1), Mo₂Ni(dpa)₄(NCS)₂ (2), and Mo₂Ni(dpa)₄(NCSe)₂ (3) that are obtained cleanly and in good yield. Compound 1 may be synthesized either directly by reaction of Ni(OTf)₂ with Mo₂(dpa)₄ (4) or indirectly, by reaction of Mo₂Ni(dpa)₄Cl₂ (5) with 2 equiv of TlOTf. Axial ligand substitution on 1 via solutions containing NaNCS or KNCSe afford 2 or 3, respectively. X-ray crystal structures of 1, 2, and 3 present short Mo-Ni distances of 2.458(8)Å /2.47(1) Å, 2.548(1), and 2.546(1), respectively. Density functional theory (DFT) calculations indicate a 3-center 3-electron σ bonding interaction between the Mo₂ quadruply bonded core and the Ni in both 1 and 2. These complexes were analyzed by SQUID magnetometry, supporting the presence of a high spin Ni²⁺ center with S = 1.

Filling the Gap in Extended Metal Atom Chains: Ferromagnetic Interactions in a Tetrairon(II) String Supported by Oligo-α-pyridylamido Ligands

The stringlike complex [Fe₄(tpda)₃Cl₂] (2; H₂tpda = N², N⁶-bis(pyridin-2-yl)pyridine-2,6-diamine) was obtained as the first homometallic extended metal atom chain based on iron(II) and oligo-α-pyridylamido ligands. The synthesis was performed under strictly anaerobic and anhydrous conditions using dimesityliron, [Fe₂(Mes)₄] (1; HMes = mesitylene), as both an iron source and a deprotonating agent for H₂tpda. The four lined-up iron(II) ions in the structure of 2 (Fe···Fe = 2.94-2.99 Å, Fe···Fe···Fe = 171.7-168.8°) are wrapped by three doubly deprotonated twisted ligands, and the chain is capped at its termini by two chloride ions. The spectroscopic and electronic properties of 2 were investigated in dichloromethane by UV-vis-NIR absorption spectroscopy, ¹H NMR spectroscopy, and cyclic voltammetry. The electrochemical measurements showed four fully resolved, quasi-reversible one-electron-redox processes, implying that 2 can adopt five oxidation states in a potential window of only 0.8 V. Direct current (dc) magnetic measurements indicate dominant ferromagnetic coupling at room temperature, although the ground state is only weakly magnetic. On the basis of density functional theory and angular overlap model calculations, this magnetic behavior was explained as being due to two pairs of ferromagnetically coupled iron(II) ions ( J = -21 cm^-1 using JŜ _i·Ŝ _j convention) weakly antiferromagnetically coupled with each other. Alternating-current susceptibility data in the presence of a 2 kOe dc field and at frequencies up to 1.5 kHz revealed the onset of slow magnetic relaxation below 2.8 K, with the estimated energy barrier U_eff/ k_B = 10.1(1.3) K.

Extraordinarily Large Ferromagnetic Coupling (J≥150 cm-1 ) by Electron Delocalization in a Heterometallic Mo≣Mo-Ni Chain Complex

The new heterometallic chain compounds Mo₂ Ni(dpa)₄ Cl₂ (1) and [Mo₂ Ni(dpa)₄ Cl₂ ]OTf (2) (dpa=2,2'-dipyridylamine) have been prepared and studied by crystallography and magnetic susceptibility, among other methods. Oxidation of 1 to 2 removes an electron from the multiply bonded Mo₂ unit, consistent with the formulation of 2 containing a (Mo₂ )⁵⁺ ⋅⋅⋅(Ni)²⁺ core. While 1 contains an S=1, pseudo-octahedral Ni^II ion, 2 has an S=3/2 ground state, in which the two Ni^II unpaired electrons, one in a localized δ-orbital and one in a heavily delocalized σ_nb -orbital are joined by an unpaired electron in a Mo-Mo δ-orbital. The S=3/2 ground state is persistent to 300 K, evidencing strong ferromagnetic coupling of the Mo₂ and Ni spins with J≥150 cm^-1 . This ferromagnetic interaction occurs via delocalization of a σ_nb -electron across all three metal atoms, forcing ferromagnetic alignment of electrons in orthogonal Ni and Mo₂ δ-symmetry orbitals. We anticipate that this new means of coupling spins can be used as a design principle for the preparation of new compounds with high spin ground states.

Backbone flexibility of extended metal atom chains. Ab initio molecular dynamics for Cr3(dpa)4X2 (X = NCS, CN, NO3) in gas and crystalline phases

Computational study on the flexibility of chromium extended metal atom chains by ab initio molecular dynamics.

Nonhelical heterometallic [Mo2M(npo)4(NCS)2] string complexes (M = Fe, Co, Ni) with high single-molecule conductance

Nonhelical metal strings, [MoMoM(npo)₄(NCS)₂], exhibited better overlaps of d_x2−y2 between neighbouring metal centres, stronger MoMo bonds, and superior single-molecule conductance.

A multiconfigurational approach to the electronic structure of trichromium extended metal atom chains

Density functional theory, Complete Active Space Self-Consistent Field (CASSCF) and perturbation theory (CASPT2) methodologies have been used to explore the electronic structure of a series of trichromium Extended Metal Atom Chains (EMACS) with different capping ligands.

Heterometallic Second-Row Transition Metal Chain Compounds in Two Charge States: Syntheses, Properties, and Electronic Structures of [Mo-Mo-Ru](6+/7+) Chains

Reaction of Mo2(dpa)4 (dpa = 2,2'-dipyridylamido) with (1)/2 equiv of [Ru(CO)3Cl2]2 in molten naphthalene at 250 °C provides facile access to the first all-second-row transition metal heterometallic chain compound, MoMoRu(dpa)4Cl2 (1). The one-electron oxidized compound [MoMoRu(dpa)4Cl2](OTf) (2) is synthesized by reaction of 1 with FeCp2(OTf). X-ray crystallography reveals a contraction of the Mo-Ru bond distance from 2.38 Å in 1 to 2.30 Å in 2, and an elongation of the Mo-Mo bond distance from 2.12 Å in 1 to 2.21 Å in 2. The short Mo-Ru bond distances indicate significant electron delocalization along the Mo-Mo-Ru chain, which is quantified by density functional theory (DFT) calculations. Molecular orbital analyses of both compounds based on DFT results reveal full delocalization of the orbitals of σ and π symmetry for both compounds. Additionally, δ orbital delocalization is observed in 2.

Electronic tuning of Mo2(thioamidate)4 complexes through π-system substituents and cis/trans isomerism

We report an exploration of the coordination chemistry of a systematic series of cyclic thioamidate ligands with the quadruply-bonded Mo2(4+) core. In addition to the S and N donor atoms that bind to Mo, the ligands utilized in this study have an additional O or S atom in conjugation with the thioamidate π system. The preparation of four new Mo2 complexes is described, and these compounds are characterized by X-ray crystallography, NMR and UV-vis spectroscopy, electrochemistry, and DFT calculations. These complexes provide a means to interrogate the electronics of Mo2(thioamidate)4 systems. Notably, we describe the first two examples of Mo2(thioamidate)4 complexes in their cis-2,2-regioisomer. By varying the π-system substituent and regioisomerism of these compounds, the electronics of the dimolybdenum core is shown to be altered with varying degrees of effect. Cyclic voltammetry results show that changing the π-system substituent from O to S results in an increase in the Mo2(4+/5+) oxidation potential by 170 mV. Changing the arrangement of ligands around the dimolybdenum core from trans-2,2 to cis-2,2 slightly weakens the metal-ligand bonds, raising the oxidation potential by a more modest 30-100 mV. MO diagrams of each compound derived from DFT calculations support these conclusions as well; the identity of the π-system substituent alters the δ-δ* (HOMO-LUMO) gap by up to 0.4 eV, whereas regioisomerism yields smaller changes in the electronic structure.

Lewis acid enhanced axial ligation of [Mo2]4+ complexes

We report here the syntheses, X-ray crystal structures, electrochemistry, and density functional theory (DFT) single-point calculations of three new complexes: tetrakis(monothiosuccinimidato)dimolybdenum(II) [Mo2(SNO5)4, 1a], tetrakis(6-thioxo-2-piperidinonato)dimolybdenum(II) [Mo2(SNO6)4, 1b], and chlorotetrakis(monothiosuccinimidato)pyridinelithiumdimolybdenum(II) [pyLiMo2(SNO5)4Cl, 2-py]. X-ray crystallography shows unusually short axial Mo2-Cl bond lengths in 2-py, 2.6533(6) Å, and dimeric 2-dim, 2.644(1) Å, which we propose result from an increased Lewis acidity of the Mo2 unit in the presence of the proximal Li(+) ion. When 2-py is dissolved in MeCN, the lithium reversibly dissociates, forming an equilibrium mixture of (MeCNLiMo2(SNO5)4Cl) (2-MeCN) and [Li(MeCN)4](+)[Mo2(SNO5)4Cl](-) (3). Cyclic voltammetry was used to determine the equilibrium lithium binding constant (room temperature, K(eq) = 95 ± 1). From analysis of the temperature dependence of the equilibrium constant, thermodynamic parameters for the formation of 2-MeCN from 3 (ΔH° = -6.96 ± 0.93 kJ mol(-1) and ΔS° = 13.9 ± 3.5 J mol(-1) K(-1)) were extracted. DFT calculations indicate that Li(+) affects the Mo-Cl bond length through polarization of metal-metal bonding/antibonding molecular orbitals when lithium and chloride are added to the dimolybdenum core.

Synthesis, characterization and thermal properties of trimetallic N3-Cr[quadruple bond]Cr···M-N3 azide complexes with M = Cr, Mn, Fe, and Co

We report here two novel synthetic pathways toward the preparation of a family of trimetallic diazide compounds of the type Cr(2)M(dpa)(4)(N(3))(2), with M = Cr (10), Mn (4), Fe (5), and Co (11). Reaction of either Cr(2)M(dpa)(4)(OTf)(2) (for M = Mn and Fe) or [Cr(2)M(dpa)(4)(MeCN)(2)](PF(6))(2) (for M = Cr and Co) with sodium azide in methanol leads to the formation of the corresponding diazide compounds, and single crystal X-ray diffraction measurements confirm the predicted structures. Compounds 4, 5, and 10 are all high-spin compounds, but 11 is a spin-crossover compound exhibiting low-spin behavior at low temperatures (~100 K). Thermolytic characterization by DSC and TGA reveals an exothermic reaction corresponding to the loss of two dinitrogen molecules from compounds 5, 10, and 11. Further characterization by solution NMR measurements and cyclic voltammetry are also presented.