Connie C. Lu

Neutral Bis(α-iminopyridine)metal Complexes of the First-Row Transition Ions (Cr, Mn, Fe, Co, Ni, Zn) and Their Monocationic Analogues: Mixed Valency Involving a Redox Noninnocent Ligand System

A series of bis(alpha-iminopyridine)metal complexes featuring the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) is presented. It is shown that these ligands are redox noninnocent and their paramagnetic pi radical monoanionic forms can exist in coordination complexes. Based on spectroscopic and structural characterizations, the neutral complexes are best described as possessing a divalent metal center and two monoanionic pi radicals of the alpha-iminopyridine. The neutral M(L*)2 compounds undergo ligand-centered, one-electron oxidations generating a second series, [(L(x))2M(THF)][B(ArF)4] [where L(x) represents either the neutral alpha-iminopyridine (L)0 and/or its reduced pi radical anion (L*)-]. The cationic series comprise mostly mixed-valent complexes, wherein the two ligands have formally different redox states, (L)0 and (L*)-, and the two ligands may be electronically linked by the bridging metal atom. Experimentally, the cationic Fe and Co complexes exhibit Robin-Day Class III behavior (fully delocalized), whereas the cationic Zn, Cr, and Mn complexes belong to Class I (localized) as shown by X-ray crystallography and UV-vis spectroscopy. The delocalization versus localization of the ligand radical is determined only by the nature of the metal linker. The cationic nickel complex is exceptional in this series in that it does not exhibit any ligand mixed valency. Instead, its electronic structure is consistent with two neutral ligands (L)0 and a monovalent metal center or [(L)2Ni(THF)][B(ArF)4]. Finally, an unusual spin equilibrium for Fe(II), between high spin and intermediate spin (S(Fe) = 2 <--> S(Fe) = 1), is described for the complex [(L*)(L)Fe(THF)][B(ArF)4], which consequently is characterized by the overall spin equilibrium (S(tot) = 3/2 <--> S(tot) = 1/2). The two different spin states for Fe(II) have been characterized using variable temperature X-ray crystallography, EPR spectroscopy, zero-field and applied-field Mössbauer spectroscopy, and magnetic susceptibility measurements. Complementary DFT studies of all the complexes have been performed, and the calculations support the proposed electronic structures.

On the feasibility of N2 fixation via a single-site FeI/FeIV cycle: Spectroscopic studies of FeI(N2)FeI, FeIV N, and related species

The electronic properties of an unusually redox-rich iron system, [PhBPR3]FeNx (where [PhBPR3] is [PhB(CH2PR2)3]−), are explored by Mössbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBPiPr3]FeN (3a) and [PhBPCH2Cy3]FeN (3b), and the formally diiron(I) bridged-Fe(μ-N2)Fe species, {[PhBPiPr3]Fe}2(μ-N2) (4). Complex 4 is chemically related to 3a via a spontaneous nitride coupling reaction. The diamagnetic iron(IV) nitrides 3a and 3b exhibit unique electronic environments that are reflected in their unusual Mössbauer parameters, including quadrupole-splitting values of 6.01(1) mm/s and isomer shift values of −0.34(1) mm/s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J/D < 1) between two iron(I) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBPiPr3]Fe(H)3(PMe3) (5), an S = 3/2 iron(I) phosphine adduct [PhBPiPr3]FePMe3 (6), and the S = 2 iron(II) precursors to 3a, [PhBPiPr3]FeCl and [PhBPiPr3]Fe-2,3:5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene (dbabh). The electronic properties of these respective complexes also have been explored by density-functional methods to help corroborate our spectral assignments and to probe their electronic structures further.

Metal-alane adducts with zero-valent nickel, cobalt, and iron.

Coordination complexes that pair a zero-valent transition metal (Ni, Co, Fe) and an aluminum(III) center have been prepared. They add to the few examples of structurally characterized metal alanes and are the first reported metallalumatranes. To understand the M-Al interaction and gauge the effect of varying the late metal, the complexes were characterized by X-ray crystallography, electrochemistry, UV-Vis-NIR and NMR spectroscopies, and theoretical calculations. The M-Al bond strength decreases with varying M in the order Ni > Co > Fe.

Bis(α-diimine)iron Complexes: Electronic Structure Determination by Spectroscopy and Broken Symmetry Density Functional Theoretical Calculations

The electronic structure of a family comprising tetrahedral (alpha-diimine)iron dichloride, and tetrahedral bis(alpha-diimine)iron compounds has been investigated by Mossbauer spectroscopy, magnetic susceptibility measurements, and X-ray crystallography. In addition, broken-symmetry density functional theoretical (B3LYP) calculations have been performed. A detailed understanding of the electronic structure of these complexes has been obtained. A paramagnetic (St=2), tetrahedral complex [FeII(4L)2], where (4L)1- represents the diamagnetic monoanion N-tert-butylquinolinylamide, has been synthesized and characterized to serve as a benchmark for a Werner-type complex containing a tetrahedral FeIIN4 geometry and a single high-spin ferrous ion. In contrast to the most commonly used description of the electronic structure of bis(alpha-diimine)iron(0) complexes as low-valent iron(0) species with two neutral alpha-diimine ligands, it is established here that they are, in fact, complexes containing two (alpha-diiminato)1-* pi radical monoanions and a high-spin ferrous ion (in tetrahedral N4 geometry) (SFe=2). Intramolecular antiferromagnetic coupling between the pi radical ligands (Srad=1/2) and the ferrous ion (SFe=2) yields the observed St=1 ground state. The study confirms that alpha-diimines are redox noninnocent ligands with an energetically low-lying antibonding pi* lowest unoccupied molecular orbital which can accept one or two electrons from a transition metal ion. The (alpha-diimine)FeCl2 complexes (St=2) are shown to contain a neutral alpha-diimine ligand, a high spin ferrous ion, and two chloride ligands.

Tuning Nickel with Lewis Acidic Group 13 Metalloligands for Catalytic Olefin Hydrogenation

A series of bimetallic complexes pairing zero-valent nickel with group 13 M(III) ions is reported. Stronger Ni→M(III) dative bonds that render Ni more electron-deficient are seen for larger ions (In > Ga > Al). The larger Ga and In ions stabilize rare, nonclassical Ni-H2 adducts that catalyze olefin hydrogenation. In contrast, neither the Ni-Al complex nor a single nickel center enables H2 binding or olefin hydrogenation. By comparison of the structures, redox properties, and catalytic activities of the Ni-M series, the electronic and steric effects of the supporting metal ion are elucidated.

Catalytic Silylation of Dinitrogen with a Dicobalt Complex

A dicobalt complex catalyzes N2 silylation with Me3SiCl and KC8 under 1 atm N2 at ambient temperature. Tris(trimethylsilyl)amine is formed with an initial turnover rate of 1 N(TMS)3/min, ultimately reaching a turnover number of ∼200. The dicobalt species features a metal-metal interaction, which we postulate is important to its function. Although N2 functionalization occurs at a single cobalt site, the second cobalt center modifies the electronics at the active site. Density functional calculations reveal that the Co-Co interaction evolves during the catalytic cycle: weakening upon N2 binding, breaking with silylation of the metal-bound N2 and reforming with expulsion of [N2(SiMe3)3](-).

Role of the metal in the bonding and properties of bimetallic complexes involving manganese, iron, and cobalt.

A multidentate ligand platform is introduced that enables the isolation of both homo- and heterobimetallic complexes of divalent first-row transition metal ions such as Mn(II), Fe(II), and Co(II). By means of a two-step metalation strategy, five bimetallic coordination complexes were synthesized with the general formula M1M2Cl(py3tren), where py3tren is the triply deprotonated form of N,N,N-tris(2-(2-pyridylamino)ethyl)amine. The metal-metal pairings include dicobalt (1), cobalt-iron (2), cobalt-manganese (3), diiron (4), and iron-manganese (5). The bimetallic complexes have been investigated by X-ray diffraction and X-ray anomalous scattering studies, cyclic voltammetry, magnetometry, Mössbauer spectroscopy, UV-vis-NIR spectroscopy, NMR spectroscopy, combustion analyses, inductively coupled plasma optical emission spectrometry, and ab initio quantum chemical methods. Only the diiron chloride complex in this series contains a metal-metal single bond (2.29 Å). The others show weak metal-metal interactions (2.49 to 2.53 Å). The diiron complex is also distinct with a septet ground state, while the other bimetallic species have much lower spin states from S = 0 to S = 1. We propose that the diiron system has delocalized metal-metal bonding electrons, which seems to correlate with a short metal-metal bond and a higher spin state. Multiconfigurational wave function calculations revealed that, indeed, the metal-metal bonding orbitals in the diiron complex are much more delocalized than those of the dicobalt analogue.

Accessing the Different Redox States of α-Iminopyridines within Cobalt Complexes

Cobalt coordination complexes featuring the redox-active alpha-iminopyridine ligand are described. The quite reducing bis(ligand)cobalt monoanion (1(red)) was isolated and characterized by X-ray crystallography. The complex forms a three-membered electron-transfer series along with its neutral and monocationic counterparts, which were previously reported. The electronic structures in this series are all consistent with divalent cobalt and the redox events being ligand-centered. The reactivity profile of 1(red) was briefly explored, and two new compounds were isolated, bis(ligand)methylcobalt (2) and bis(ligand)iodocobalt (3). Though 2 and 3 are isostructural, they are characterized by different electronic structures. The methylcobalt complex is best described by a Co(III) center with two ligand radicals, whereas the iodocobalt species is more aptly assigned as a Co(II) center with only one ligand radical. Further evidence of their different electronic structures is that a low-lying excited state is populated at room temperature in the case of the iodocobalt compound, whereas the methylcobalt complex is an energetically well-isolated spin singlet. Magnetic susceptibility data, structural data, variable temperature NMR spectroscopy, and density functional theory calculations lend support to this proposal.

An Electron‐Transfer Series of High‐Valent Chromium Complexes with Redox Non‐Innocent, Non‐Heme Ligands

An investigation of the electronic interplay between ligand radical(s) and a high-valent metal center in the three-member electron-transfer series shown in the picture reveals that, upon oxidation and removal of both ligand radicals, the chromium center becomes reduced from Cr{sup IV} to Cr{sup III} with concomitant formation of an imidyl radical (NR{center_dot}){sup -}.

Multiple metal-metal bonds in iron-chromium complexes

Multiple metal–metal bonds are uncommon in heterobimetallic complexes. Rare examples of heterobimetallics with short metal–metal bonds include [TiRh(OCMe2CH2PPh2)3], [2] [CoZr(MesNPiPr2)3(thf)], [3] and [CrMo(O2CCH3)4]. [4] We report the first isolable examples of metal–metal multiple bonds between two different first-row transition metals, namely iron and chromium. We have conducted spectroscopic and theoretical investigations of two Fe!Cr coordination complexes to understand the nature of these unprecedented metal–metal bonds. We previously reported a dinucleating, double-decker ligand that was designed to enable the synthesis of heterobimetallic complexes in a modular manner. To prepare the iron–chromium complexes herein, the ligand was first deprotonated and metalated with CrCl3. The resulting monometallic species, [Cr(N(o-(iPr2PCH2N)C6H4)3)], which was confirmed by combustion analysis, acts as a metalloligand in a subsequent metalation (Scheme 1). For example, reaction of [Cr(N(o-(iPr2PCH2N)C6H4)3)] with FeBr2 and two equiv KC8 resulted in a color change from dark brown to green brown within minutes. The product, [FeCr(N(o(iPr2PCH2N)C6H4)3)] (1), is paramagnetic, and its proposed structure has been confirmed by X-ray crystallography. The redox properties of 1 were examined by electrochemical methods. The cyclic voltammogram (CV) of 1 (Supporting Information, Figure S3) reveals a rich redox profile, including: 1) a reversible reduction at !2.33 V; 2) a reversible oxidation at !1.32 V; and 3) a second, quasi-reversible oxidation at !0.62 V (versus FeCp2/FeCp2, 0.1m N(nBu)4PF6 in THF, 10 mVs!1). In contrast, the redox profile of the related iron–alane adduct, [FeAl(N(o-(iPr2PCH2N)C6H4)3)], is much more limited. It is characterized by a single reversible event, a reduction at !2.08 V (vs FeCp2/FeCp2). Reduction of 1 with 1 equiv KC8 generates a red-brown solution of paramagnetic [K(solv)n][1] (2). Alternatively, 2 can be directly prepared by mixing [Cr(N(o(iPr2PCH2N)C6H4)3)] with FeBr2 and three equiv KC8. Encapsulation of the potassium ion with either [18]crown-6 or crypt222 enabled the isolation of crystalline [K([18]crown-6)][1] (2a) or [K(crypt-222)][1] (2b), respectively. X-ray diffraction studies were conducted on single crystals of 1 and 2b (Figure 1; Supporting Information, Table S1). The Scheme 1. Synthesis of iron–chromium complexes.