In computational chemistry, non-additive and cooperative effects can be defined in terms of a (differential) many-body expansion of the energy or any other physical property of the molecular system of interest. One-body terms describe energies or properties of the subsystems, two-body terms describe non-additive but pairwise contributions and three-body as well as higher-order terms can be interpreted as a measure for cooperativity. In the present article, this concept is applied to the analysis of ultraviolet/visible (UV/Vis) spectra of homotrinuclear transition-metal complexes by means of a many-body expansion of the change in the spectrum induced by replacing each of the three transition-metal ions by another transition-metal ion to yield a different homotrinuclear transition-metal complex. Computed spectra for the triangulo-complexes [M₃{Si(mt^Me)₃}₂] (M=Pd/Pt, mt^Me=methimazole) and tritopic triphenylene-based N-heterocyclic carbene Rh/Ir complexes illustrate the concept, showing large and small differential three-body cooperativity, respectively.

The synthesis and characterisation of two aluminium diphosphamethanide complexes, [Al(tBu)₂{κ²P,P′-Mes*PCHPMes*}] (3) and [Al(C₆F₅)₂{κ²P,P′-Mes*PCHPMes*}] (4), and the silylated analogue, Mes*PCHP(SiMe₃)Mes* (5), are reported. The aluminium complexes feature four-membered PCPAl core structures consisting of diphosphaallyl ligands. The silylated phosphine 5 was found to be a valuable precursor for the synthesis of 4 as it cleanly reacts with the diaryl aluminium chloride [(C₆F₅)₂AlCl]₂. The aluminium complex 3 reacts with molecular dihydrogen at room temperature under formation of the acyclic σ²λ³,σ³λ³-diphosphine Mes*PCHP(H)Mes* and the corresponding dialkyl aluminium hydride [tBu₂AlH]₃. Thus, 3 belongs to the family of so-called hidden frustrated Lewis pairs.

Reaction of [Si(3,5-Me₂pz)₄] (1) with [Cu(MeCN)₄][BF₄] (2) gave the mono- and dinuclear copper complexes [Cu₂(^FTp*)₂] (3) and [Cu(^FTp*)₂] (4). Both complexes contain the so-far unprecedented boron-fluorinated ^FTp* ligand ([FB(3,5-Me₂pz)₃]⁻ with pz=pyrazolyl) originating from 1, acting as a pyrazolyl transfer reagent, and the [BF₄]⁻ counter anion of 2, serving as the source of the {BF} entity. The solid-state structures as well as the NMR and EPR spectroscopic characteristics of the complexes were elaborated. Pulsed gradient spin echo (PGSE) experiments revealed that 3 retains (almost entirely) its dimeric structure in benzene, whereas dimer cleavage and formation of acetonitrile adducts, presumably [Cu(^FTp*)(MeCN)], is observed in acetonitrile. The short Cu⋅⋅⋅Cu distance of 269.16 pm in the solid-state is predicted by DFT calculations to be dictated by dispersion interactions between all atoms in the complex (the Cu−Cu dispersion contribution itself is only very small). As revealed by cyclic voltammetry studies, 3 shows an irreversible (almost quasi-reversible at higher scan rates) oxidation process centred at E^pa=−0.23 V (E⁰_1/2=−0.27 V) (vs. Fc/Fc⁺). Oxidation reactions on a preparative scale with one equivalent of the ferrocenium salt [Fc][BF₄] (very slow reaction) or air (fast reaction) furnished blue crystals of the mononuclear copper(II) complex [Cu(^FTp*)₂] (4). As expected for a Jahn–Teller-active system, the coordination sphere around copper(II) is strongly distorted towards a stretched octahedron, in accordance with EPR spectroscopic findings.

A stable cyclic (alkyl)(amino)carbene (CAAC) 1 inserts into the para-CF bond of pentafluoropyridine, and after fluoride abstraction, the iminium-pyridyl adduct [3]⁺ was isolated. A cyclic voltammetry study shows a reversible three-state redox system involving [3]⁺, [3]^⋅, and [3]⁻. The CAAC-pyridyl radical [3]^⋅, obtained by reduction of [3]⁺ with magnesium, has been spectroscopically and crystallographically characterized. In contrast to the lack of π communication between the CAAC and the pyridine units in cation [3]⁺, the unpaired electron of [3]^⋅ is delocalized over an extended π system involving both heterocycles.

Reactions of the tris(3,5-dimethylpyrazolyl)methanide amido complexes [M′{C(3,5-Me₂pz)₃}{N(SiMe₃)₂}] (M′=Mg (1 a), Zn (1 b), Cd (1 c); 3,5-Me₂pz=3,5-dimethylpyrazolyl) with two equivalents of the acidic Group 6 cyclopentadienyl (Cp) tricarbonyl hydrides [MCp(CO)₃H] (M=Cr (2 a), Mo (2 b)) gave different types of heterobimetallic complex. In each case, two reactions took place, namely the conversion of the tris(3,5-dimethylpyrazolyl)methanide ligand (Tpmd*) into the -methane derivative (Tpm*) and the reaction of the acidic hydride MH bond with the M′N(SiMe₃)₂ moiety. The latter produces HN(SiMe₃)₂ as a byproduct. The Group 2 representatives [Mg(Tpm*){MCp(CO)₃}₂(thf)] (3 a/b) form isocarbonyl bridges between the magnesium and chromium/molybdenum centres, whereas direct metal–metal bonds are formed in the case of the ions [Zn(Tpm*){MCp(CO)₃}]⁺ (4 a/b; [MCp(CO)₃]⁻ as the counteranion) and [Cd(Tpm*){MCp(CO)₃}(thf)]⁺ (5 a/b; [Cd{MCp(CO)₃}₃]⁻ as the counteranion). Complexes 4 a and 5 a/b are the first complexes that contain ZnCr, CdCr, and CdMo bonds (bond lengths 251.6, 269.8, and 278.9 pm, respectively). Quantum chemical calculations on 4 a/b* (and also on 5 a/b*) provide evidence for an interaction between the metal atoms.

2D ⁷Li,¹⁵N heteronuclear shift correlation through scalar coupling has successfully been applied to several lithium organyls consisting of polydentate N ligands such as N,N,N′,N′-tetramethylethylenediamine (tmeda), N,N,N′,N′,N′′-pentamethyldiethylentriamine (pmdta) and (−)-sparteine. Structural insights on the conformation of benzyllithium⋅pmdta (5) in a toluene solution and the strength of ion pairing in combination with PGSE NMR measurements, ¹H,¹H-NOESY and ¹H,⁷Li-HOESY experiments are presented. By studying in detail the formation of 5 in solution, a transient species has been observed for the first time and assigned to a pre-complex of nBuLi and pmdta. In addition, the solution behaviour of the complex formed between benzyllithium and (−)-sparteine (8) has been studied by PGSE and multinuclear NMR spectroscopy. The straightforward synthesis and first applications in asymmetric lithiations are also reported, which show that the new system benzyllithium⋅(−)-sparteine (8) provide poorer enantioselective induction than the classical nBuLi⋅(−)-sparteine (6). The results were supported by deprotonation experiments confirming that the formation of 8 relies on two relevant factors, namely temperature and lithiating reagent. The existence of 8 may thus interfere with the asymmetric induction when the system nBuLi⋅ (−)-sparteine is used in the enantioselective deprotonations of N-Boc-N-(p-methoxyphenyl)-benzylamine conducted in toluene.

A series of bimetallic silyl halido cuprates consisting of the new tripodal silicon-based metalloligand [κ³N-Si(3,5-Me₂pz)₃Mo(CO)₃]⁻ is presented (pz=pyrazolyl). This metalloligand is straightforwardly accessible by reacting the ambidentate ligand tris(3,5-dimethylpyrazolyl)silanide ({Si(3,5-Me₂pz)₃}⁻) with [Mo(CO)₃(η⁶-toluene)]. The compound features a fac-coordinated tripodal chelating ligand and an outward pointing, “free” pyramidal silyl donor, which is easily accessible for a secondary coordination to other metal centers. Several bimetallic silyl halido cuprates of the general formula [CuX{μ-κ¹Si:κ³N-Si(3,5-Me₂pz)₃Mo(CO)₃}]⁻ (X=Cl, Br, I) have been synthesized. The electronic and structural properties of these complexes were probed in detail by X-ray diffraction analysis, electrospray mass spectrometry, infrared-induced multiphoton dissociation studies, cyclic voltammetry, spectroelectrochemistry, gas-phase photoelectron spectroscopy, as well as UV/Vis and fluorescence spectroscopy. The heterobimetallic complexes contain linear two-coordinate copper(I) entities with the shortest silicon–copper distances reported so far. Oxidation of the anionic complexes in methylene chloride and acetonitrile solutions at =−0.60 and −0.44 V (vs. ferrocene/ferrocenium (Fc/Fc⁺)), respectively, shows substantial reversibility. Based on various results obtained from different characterization methods, as well as density functional theory calculations, these oxidation events were attributed to the Mo⁰/Mo^I redox couple.

We report on the first homoleptic tetrakis(silyl) complexes of zerovalent Group 10 metals. The compounds [MLi₄{Si(3,5-Me₂pz)₃}₄] (M=Pd and Pt; 3,5-Me₂pz=3,5-dimethylpyrazolyl) exhibit very appealing metal-centred heterocubane structures with the central d¹⁰ metal atoms surrounded by four silicon and four lithium atoms. Both compounds were characterised in detail, including X-ray crystal-structure analysis and 2D NMR spectroscopic methods such as ⁷Li,²⁹Si and ⁷Li,¹⁹⁵Pt HMQC. Cyclic voltammetry studies, in combination with density functional theory (DFT) calculations, revealed that the corresponding mononuclear cationic d⁹-M^I and dicationic d⁸-M^II complexes are accessible by stepwise one-electron oxidation of the title compounds. Electron paramagnetic resonance (EPR) investigations provided evidence for the existence of the corresponding paramagnetic palladium(I) and platinum(I) complexes.

A new C-nucleoside structurally based on the hydroxyquinoline ligand was synthesized that is able to form stable pairs in DNA in both the absence and the presence of metal ions. The interactions between the metal centers in adjacent Cu^II-mediated base pairs in DNA were probed by electron paramagnetic resonance (EPR) spectroscopy. The metal–metal distance falls into the range of previously reported values. Fluorescence studies with a donor–DNA–acceptor system indicate that photoinduced charge-transfer processes across these metal-ion-mediated base pairs in DNA occur more efficiently than over natural base pairs.

The synthesis of ansa complexes has been studied intensively owing to their importance as homogeneous catalysts and as precursors of metal-containing polymers. However, paramagnetic non-metallocene derivatives are rare and have been limited to examples with vanadium and titanium. Herein, we report an efficient procedure for the selective dilithiation of paramagnetic sandwich complex [Cr(η⁵-C₅H₅)(η⁶-C₆H₆)], which allows the preparation of a series of [n]chromoarenophanes (n=1, 2, 3) that feature silicon, germanium, and tin atoms at the bridging positions. The electronic and structural properties of these complexes were probed by X-ray diffraction analysis, cyclic voltammetry, and by UV/Vis and EPR spectroscopy. The spectroscopic parameters for the strained and less strained complexes (i.e., with multiple-atom linkers) indicate that the unpaired electron resides primarily in a d orbital on chromium(I); this result was also supported by density functional theory (DFT) calculations. We did not observe a correlation between the experimental UV/Vis and EPR data and the degree of molecular distortion in these ansa complexes. The treatment of tin-bridged complex [Cr(η⁵-C₅H₄)(η⁶-C₆H₅)SntBu₂] with [Pt(PEt₃)₃] results in the non-regioselective insertion of the low-valent Pt⁰ fragment into the C_ipsoSn bonds in both the five- and six-membered rings, thereby furnishing a bimetallic complex. This observed reactivity suggests that ansa complexes of this type are promising starting materials for the synthesis of bimetallic complexes in general and also underline their potential to undergo ring-opening processes to yield new metal-containing polymers.

2D ¹H,⁸⁹Y heteronuclear shift correlation through scalar coupling has been applied to the chemical-shift determination of a set of yttrium complexes with various nuclearities. This method allowed the determination of ⁸⁹Y NMR data in a short period of time. Multinuclear NMR spectroscopy as function of temperature, PGSE NMR-diffusion experiments, heteronuclear NOE measurements, and X-ray crystallography were applied to determine the structures of [Y₅(OH)₅(L-Val)₄(Ph₂acac)₆] (1) (Ph₂acac=dibenzoylmethanide, L-Val=L-valine), [Y(2)(OTf)₃] (3), and [Y₂(4)(OTf)₅] (5) (2: [(S)P{N(Me)NC(H)Py}₃], 4: [B{N(Me)NC(H)Py}₄]⁻) in solution and in the solid state. The structures found in the solid state are retained in solution, where averaged structures were observed. NMR diffusion measurements helped us to understand the nuclearity of compounds 3 and 5 in solution. ¹H,¹⁹F HOESY and ¹⁹F,¹⁹F EXSY data revealed that the anions are specifically located in particular regions of space, which nicely correlated with the geometries found in the X-ray structures.