Bonding and stereochemistry of three-coordinated transition metal compounds

Opportunities for Insight into the Mechanism of Efficient CO2/CO Interconversion at a Nickel-Iron Cluster in CO Dehydrogenase

The reduction of CO2 with low overpotential and high selectivity is a crucial challenge in catalysis. Fortunately, natural systems have evolved enzymes that achieve this catalytic reaction very efficiently at a complex nickel-iron-sulfur cluster within carbon monoxide dehydrogenase (CODH). Extensive biochemical, crystallographic, and spectroscopic work has been done to understand the structures and mechanism involved in the catalytic cycle, which are summarized here from the perspective of mechanistic organometallic chemistry. We highlight the ambiguities in the data and suggest experiments that could lead to clearer understanding of the mechanism and structures of intermediates at the active-site cluster. These include parallel crystallography and spectroscopy, as well as the preparation of synthetic analogues that help to interpret structural and spectroscopic signatures.

Electronic Structure of Three-Coordinate FeII and CoII β-Diketiminate Complexes

The β-diketiminate supporting group, [ArNCRCHCRNAr]-, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnactBu)ML, where M = FeII, CoII and L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3 and have been previously reported and structurally characterized. The two FeII complexes (S = 2) have also been previously characterized by Mössbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the CoII complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The CoII complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two FeII complexes are similar with nearly axial zfs and large magnitude zfs given by D = -37 ± 1 cm-1 for both. The two CoII complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm-1 while CoCH3 has negative D = -49 cm-1. Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.

Tridentate NacNac Tames T-Shaped Nickel(I) Radical

The reaction of a nickel(II) chloride complex containing a tridentate β-diketiminato ligand with a picolyl group [2,6-iPr2 -C6 H3 NC(Me)CHC(Me)NH(CH2 py)]Ni(II)Cl (1)] with KSi(SiMe3 )3 conveniently afforded a nickel(I) radical with a T-shaped geometry (2). The compound's metalloradical nature was confirmed through electron paramagnetic resonance (EPR) studies and its reaction with TEMPO, resulting in the formation of a highly unusual three-membered nickeloxaziridine complex (3). When reacted with disulfide and diselenide, the S-S and Se-Se bonds were cleaved, and a coupled product was formed through carbon atom of the pyridine-imine group. The nickel(I) radical activates dihydrogen at room temperature and atmospheric pressure to give the monomeric nickel hydride.

Difluorobis(pentafluoroethyl)phosphoranide: A Promising Building Block for Phosphoranidometal Complexes

Despite the fact that different metal tetrafluorophosphoranides, M[PF₄] (M = Cs, Ag, K), decompose readily, we successfully enhanced the stability of such species by the replacement of two fluorine atoms with electron withdrawing pentafluoroethyl groups. Thus, AgF adds to P(C₂F₅)₂F, yielding Ag[P(C₂F₅)₂F₂], which served as a starting material for the synthesis of mono-, bis-, and tris[difluorobis(pentafluoroethyl)phosphoranido]silver complexes. Addition of 2,2'-bipyridine allowed for the isolation of stable [Ag(bipy){P(C₂F₅)₂F₂}], whereas the reaction with the chlorides [NMe₄]Cl and CoCl₂ afforded the bis- and trisphosphoranidoargentates [NMe₄][Ag{P(C₂F₅)₂F₂}₂(OEt₂)] and [Co(NCMe)₆][Ag{P(C₂F₅)₂F₂}₃]·2MeCN, respectively. Altogether, the difluorobis(pentafluoroethyl)phosphoranido moiety serves as a novel, small, noncyclic phosphoranido ligand. It provided access to the first homoleptic phosphoranidometal complex, [Co(NCMe)₆][Ag{P(C₂F₅)₂F₂}₃]·2MeCN, which itself features the unusual structural motif of an [AgX₃]^2- ion.

Towards Heteroleptic Dicoordinate CuII Complexes

In this work we detail our efforts to systematically generate stable dicoordinate Cu^II complexes. Initial experiments via metathesis reactions of a bulky potassium carbazolide (RK) with copper(II) salts indeed yielded a stable product, RCuOTf (1). However, subsequent attempts to grasp systematic synthetic access to complexes of the type RCuX (X=monoanionic ligand) proved difficult as many of the complexes rapidly decomposed in solution. By using triflate‐related ligands such as ethyl sulfate and bistriflimide, the additional dicoordinate copper complexes RCuOSO₃Et (2), [RCu(THF)][Cu(NTf₂)₂] (3) and RCuNTf₂ (4) could be isolated. Spectroscopic indications corroborate more Cu^I than Cu^II character in all RCuX derivatives.

The [Ag25Cu4H8Br6(CCPh)12(PPh3)12]3+ : Ag13H8 silver hydride core protected by [CuAg3(CCPh)3(PPh3)3]+ motifs

Copper alkynyl complexes [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]+ (Ar = Ph, p-C6H4Me), in which three Ag(PPh3) units are bound among three C[triple bond, length as m-dash]CAr arms of trigonal-planar [Cu(C[triple bond, length as m-dash]CAr)3]2-, were selected as a protecting unit to cover the metal core of an atomically precise core-shell-type cluster. First, the formation of the protecting unit through the reaction of Cu(NCMe)4(PF6) with Ag(C[triple bond, length as m-dash]CAr) and PPh3 in a 1 : 3 : 3 ratio was confirmed. The reaction gave dimeric [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]22+, in which the two planar [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]+ units were stacked. Next, core-shell-type clusters were synthesized by adding NaBH4 and Et4NX (X = Cl, Br) to a solution similar to that used to prepare the protecting unit. The trigonal-planar protecting units nicely formed core-shell-type Ag nanoclusters formulated as [Ag13H8X6{CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3}4]3+ (X = Cl, Ar = p-C6H4Me; X = Br, Ar = p-C6H4Me; X = Br, Ar = Ph). Their crystal structures revealed that the four [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]+ units are linked by six halogen ions to form a tetrahedral cage that accommodates a polyhydride-Ag cluster formulated as Ag13H85+. As a concrete proof of the existence of the polyhydride, deuterated analogs Ag13D85+ were synthesized and subsequently characterized by high-resolution electrospray-ionization mass spectrometry measurements.

Preparation of iron(IV) nitridoferrate Ca4FeN4 through azide-mediated oxidation under high-pressure conditions

Transition metal nitrides are an important class of materials with applications as abrasives, semiconductors, superconductors, Li-ion conductors, and thermoelectrics. However, high oxidation states are difficult to attain as the oxidative potential of dinitrogen is limited by its high thermodynamic stability and chemical inertness. Here we present a versatile synthesis route using azide-mediated oxidation under pressure that is used to prepare the highly oxidised ternary nitride Ca₄FeN₄ containing Fe⁴⁺ ions. This nitridometallate features trigonal-planar [FeN₃]⁵⁻ anions with low-spin Fe⁴⁺ and antiferromagnetic ordering below a Neel temperature of 25 K, which are characterised by neutron diffraction, ⁵⁷Fe-Mössbauer and magnetisation measurements. Azide-mediated high-pressure synthesis opens a way to the discovery of highly oxidised nitrides.

High-valent metal nitrides are difficult to stabilise due to the high thermodynamic stability and chemical inertness of N₂. Here, the authors employ a large volume press to prepare an iron(IV) nitridoferrate Ca₄Fe^IVN₄ from Fe₂N and Ca₃N₂ via azide-mediated oxidation under high pressure conditions.

Ni(I)-Alkyl Complexes Bearing Phenanthroline Ligands: Experimental Evidence for CO2 Insertion at Ni(I) Centers

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)-alkyl species-complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions-has been subject to speculation. Herein we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO₂, allowing us to address a long-standing question related to Ni-catalyzed carboxylation reactions.

Opening up the Valence Shell: A T-Shaped Iron(I) Metalloradical and Its Potential for Atom Abstraction

A thermally stable, T-shaped, d7 high-spin iron(I) complex was obtained by reduction of a PNP-supported ferrous chloride. Paramagnetic NMR spectroscopy combined with DFT modeling was used to analyze the electronic structure of the coordinatively highly unsaturated complex. The metalloradical character of the compound was demonstrated by the formation of a benzophenone ketyl radical complex upon addition of benzophenone. Furthermore, the compound displays a rich chemistry as an oxygen-atom abstractor from epoxides, yielding a dinuclear, diferrous [Fe2 O] complex.

A Dimeric Hydride-Bridged Complex with Geometrically Distinct Iron Centers Giving Rise to an S = 3 Ground State

Structural and spectroscopic characterization of the dimeric iron hydride complex [Ph₂B(^tBuIm)₂FeH]₂ reveals an unusual structure in which a tetrahedral iron(II) site (S = 2) is connected to a square planar iron(II) site (S = 1) by two bridging hydride ligands. Magnetic susceptibility reveals strong ferromagnetic coupling between iron centers, with a coupling constant of J = +110(12) cm^-1, to give an S = 3 ground state. High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy confirms this model. A qualitative molecular orbital analysis of the electronic structure, as supported by electronic structure calculations, reveals that the observed spin configuration results from the orthogonal alignment of two geometrically distinct four-coordinate iron fragments held together by highly covalent hydride ligands.

Synthesis and characterization of an iron complex bearing a hemilabile NNN-pincer for catalytic hydrosilylation of organic carbonyl compounds

A low-coordinate iron(ii) complex (Cz^tBu(Pz^tBu)₂)Fe[N(SiMe₃)₂], 1 bearing an NNN-pincer ligand was prepared and fully characterized. Intramolecular C-H activation on the 5-position of a pyrazole at elevated temperatures was observed. Complex 1 was found to be an efficient and chemoselective pre-catalyst for the hydrosilylation of organo carbonyl substrates.

A T-Shaped Nickel(I) Metalloradical Species

A T-shaped Ni^I complex was synthesized using a rigid acridane-based pincer ligand to prepare a metalloradical center. Structural data displays a nickel ion is embedded in the plane of a PNP ligand. Having a sterically exposed half-filled dx2-y2 orbital, this three-coordinate Ni^I species reveals unique open-shell reactivity including the homolytic cleavage of various σ-bonds, such as H-H, N-N, and C-C.

Complexes of Ni(i): a “rare” oxidation state of growing importance

The synthesis and diverse structures, reactivity (small molecule activation and catalysis) and magnetic properties of Ni(i) complexes are summarized.

True and masked three-coordinate T-shaped platinum(II) intermediates

Although four-coordinate square-planar geometries, with a formally 16-electron counting, are absolutely dominant in isolated Pt(II) complexes, three-coordinate, 14-electron Pt(II) complexes are believed to be key intermediates in a number of platinum-mediated organometallic transformations. Although very few authenticated three-coordinate Pt(II) complexes have been characterized, a much larger number of complexes can be described as operationally three-coordinate in a kinetic sense. In these compounds, which we have called masked T-shaped complexes, the fourth position is occupied by a very weak ligand (agostic bond, solvent molecule or counteranion), which can be easily displaced. This review summarizes the structural features of the true and masked T-shaped Pt(II) complexes reported so far and describes synthetic strategies employed for their formation. Moreover, recent experimental and theoretical reports are analyzed, which suggest the involvement of such intermediates in reaction mechanisms, particularly C-H bond-activation processes.

Three-coordinate and four-coordinate cobalt hydride complexes that react with dinitrogen

We report the formation and N(2) reactivity of novel cobalt hydride complexes supported by bulky beta-diketiminate ligands (L). Addition of KHBEt(3) to LCoCl gives [LCo(mu-H)](2) (1) or K(2)[LCoH](2) (2), depending on the amount of borohydride used. Compound 2 is the first example of a crystallographically characterized hydride complex in which a transition metal is three-coordinate. Both 1 and 2 react with N(2) at room temperature to give dinuclear N(2) complexes with loss of H(2).

Metal unsaturation and ligand hemilability in Suzuki coupling

The combinative and complementary use of a hemilabile difunctional ligand on a metal, notably palladium, that is coordinatively and electronically unsaturated has led to the isolation of a string of unexpected low-valent complexes that are structurally intriguing. The ligands of interest are primarily ferrocenes functionalized by [P,N] and [P,O] donors. The characterization of these active Suzuki catalysts, which support sp(2)-sp(2) couplings, give valuable insights into the key Suzuki intermediates such as those arising from the reductive elimination, transmetallation, and oxidative addition steps. In this Account, we shall review and discuss our recent results in relation to selected developments in other laboratories.

New advances in homoleptic organotransition-metal compounds: The case of perhalophenyl ligands

Homoleptic derivatives of formula [M(C(6)X(5))(n)](z-) (X = F, Cl) have been prepared and isolated for every first-row transition metal as well as for several of the heavier ones. The stoichiometry attained in each case (n ranging between 2 and 6) can be understood considering the tendency of a given metal ion to compensate its coordinative and electronic unsaturation, while not incurring severe interligand repulsive effects. The molecular structures associated with each stoichiometry seem, in turn, to be governed by electronic rather than steric factors. Most of these [M(C(6)X(5))(n)](z-) compounds are unsaturated, open-shell organometallic species, not fulfilling the 18-electron (or Effective Atomic Number) rule. This behaviour can be attributed to the absence of pi stabilising ligands (such as CO, phosphines, alkenes, alkynes, a variety of substituted aromatic rings, and so on) which are otherwise ubiquitous in organotransition-metal chemistry. The magnetic properties of the [M(C(6)X(5))(n)](z-) species have been determined by EPR spectroscopy and/or bulk magnetisation measurements. Excellent correlation between molecular geometry and magnetic properties (whether diamagnetic or paramagnetic) has been observed. Many of these compounds undergo chemically- or electrochemically-induced electron exchange processes. These redox reactions proceed without alteration in the stoichiometry of the [M(C(6)X(5))(n)](z-) compound, but usually involve a sharp change in the molecular geometry according to the different electron configuration of the interrelated species.

Synthesis, molecular structure and norbornene polymerization behavior of three-coordinate nickel(i) complexes with chelating anilido-imine ligands

Reaction of lithium salts of anilido-imine ligands bearing bulky substituentes on the nitrogen donor atoms with trans-chloro(phenyl)bis(triphenylphosphane)nickel(II) results in the formation of two rare three-coordinate nickel(I) complexes [(Ar1N=CHC6H4NAr2)Ni(I)PPh3] (1: Ar1 = Ar2 = 2,6-i-Pr2C6H3; 2: Ar1 = 2,6-Me2C6H3, Ar2 = 2,6-i-Pr2C6H3). The molecular structures of complexes 1 and 2 have been confirmed by single crystal X-ray analyses. These two complexes exhibit paramagnetic properties as measured by their EPR and 1H NMR spectra. After being activated with methylaluminoxane (MAO) these complexes could polymerize norbornene to afford addition-type polynorbornene (PNB) with high molecular weight M(w) (10(6) g mol(-1)), catalytic activities being high, up to 2.82 x 10(7) g(PNB) mol(-1)(Ni) h(-1).