Wolfgang Kaim’s research while affiliated with University of Stuttgart and other places

A successful selective reduction of X2B-Tip (Tip = 1,3,5-iPr3-C6H2, X = I, Br) with KC8 and Mg metal, respectively, in the presence of a hybrid ligand (C6H4(PPh2)LSi) leads to a stable low-valent five-membered ring as a boryl radical [C6H4(PPh2)LSiBTip][Br] (1) and neutral borylene [C6H4(PPh2)LSiBTip] (2). Compound 2 reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical [C6H4(PPh2)LSiB(H)Tip] (3). Quantum chemical studies reveal that compound 1 is a B-centered radical, and compound 2 is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound 3 is an amidinate-centered radical. Although compounds 1 and 2 are stabilized by hyperconjugation and π-conjugation, they display high H-abstraction energy and basicity, respectively.

The present chapter deals with the bridging ligand‐mediated intramolecular electronic interaction in dinuclear mixed‐valent Ru n Ru n −1 states, based on electrochemically and spectroscopically derived comproportionation constants K c and intervalence charge transfer (IVCT) transitions, in conjunction with EPR data and Mulliken spin density distributions. The variation of intermetallic electronic coupling as a function of bridging and ancillary ligands (π accepting or σ donating) was addressed under a selective symmetric coordination scenario. The emergence of a hybrid borderline “class II–III” mixed‐valent situation beyond the Robin and Day classification (class I, class II, and class III) in certain instances is apparent from the divergence between K c and IVCT characteristics. This presentation also emphasizes additional complications originating from the introduction of redox‐active bridges or ancillary ligands as well as from asymmetry in the complex moiety.

Supplementary Materials for Jordan, R.; Niazi, M.; Schäfer, S.; Kaim, W.; Klein, A. Rhenium Tricarbonyl Complexes of Azodicarboxylate Ligands. Molecules 2022, 27, 8159. https://doi.org/10.3390/molecules27238159

The excellent π-accepting azodicarboxylic esters adcOR (R = Et, iPr, tBu, Bn (CH2-C6H5) and Ph) and the piperidinyl amide derivative adcpip were used as bridging chelate ligands in dinuclear Re(CO)3 complexes [{Re(CO)3Cl}2(µ-adcOR)] and [{Re(CO)3Cl}2(µ-adcpip)]. From the adcpip ligand the mononuclear derivatives [Re(CO)3Cl(adcpip)] and [Re(CO)3(PPh3)(µ-adcpip)]Cl were also obtained. Optimised geometries from density functional theory (DFT) calculations show syn and anti isomers for the dinuclear fac-Re(CO)3 complexes at slightly different energies but they were not distinguishable from experimental IR or UV–Vis absorption spectroscopy. The electrochemistry of the adc complexes showed reduction potentials slightly below 0.0 V vs. the ferrocene/ferrocenium couple. Attempts to generate the radicals [{Re(CO)3Cl}2(µ-adcOR)]•− failed as they are inherently unstable, losing very probably first the Cl⁻ coligand and then rapidly cleaving one [Re(CO)3] fragment. Consequently, we found signals in EPR very probably due to mononuclear radical complexes [Re(CO)3(solv)(adc)]•. The underlying Cl⁻→solvent exchange was modelled for the mononuclear [Re(CO)3Cl(adcpip)] using DFT calculations and showed a markedly enhanced Re-Cl labilisation for the reduced compared with the neutral complex. Both the easy reduction with potentials ranging roughly from −0.2 to −0.1 V for the adc ligands and the low-energy NIR absorptions in the 700 to 850 nm range place the adc ligands with their lowest-lying π* orbital being localised on the azo function, amongst comparable bridging chelate N^N coordinating ligands with low-lying π* orbitals of central azo, tetrazine or pyrazine functions. Comparative (TD)DFT-calculations on the Re(CO)3Cl complexes of the adcpip ligand using the quite established basis set and functionals M06-2X/def2TZVP/LANL2DZ/CPCM(THF) and the more advanced TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) for single-point calculations with BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) optimised geometries showed a markedly better agreement of the latter with the experimental XRD, IR and UV–Vis absorption data.

The ligands N-phenylcamphoriminoquinone 1 and N-(2-thiomethylphenyl)camphoriminoquinone 2 were reacted with copper(I) and copper(II) precursors to yield structurally characterized compounds with N-, O,N-, N,S- and O,N,S-coordination: [CuI(1-κ²O,N)(dppf)](PF6) 3(PF6), [CuI(2- κ²N,S)(dppf)](PF6) 4(PF6), [CuII(1- κN)(NO3- κO)2] and [CuII(1-κ²O,N)(NO3-κO)2] 5, and [CuII(2-κ³O,N,S)(NO3-κO)2] 7 where dppf is 1,1’-bis(diphenylphosphino)ferrocene. Spectroelectrochemistry reveals that the cations 3⁺ and 4⁺ exhibit ferrocene-based oxidation and camphoriminoquinone-centered reduction. The copper(II) compounds 5 and 7 exhibit coordinative variety with monodentate and bidentate coordination of the ligand 1 for two different molecules in 5, and with O,N,S-tridentate coordination for 2 in 7.

The asymmetrical dinuclear [{(trpy*)Ru}2(μ‐adc‐Salph)Cl](PF6) 1(PF6), trpy*=4,4’,4”‐tri‐tert‐butyl‐2,6,2’,6”‐terpyridine, adc‐Salph=1‐benzoyl‐2‐salicyloylhydrazido(3‐), and the related symmetrical dinuclear [{Cl(trpy*)Ru}2(μ,η² : η²‐adc‐Ph)](PF6) 2(PF6), adc‐Ph=1,2‐bis(benzoyl)hydrazido(2‐), were synthesized and structurally characterized. Both paramagnetic compounds were compared with the previously reported symmetrical [{(trpy*)Ru}2(μ,η³ : η³‐adc‐Sal)](PF6) 3(PF6) containing the bis‐tridentate bridge 1,2‐bis(salicyloyl)hydrazido(4‐). Molecular structures and magnetic resonance features (¹H NMR, EPR) indicate spin density distribution over the metal(s) and the bridging ligand. Reversible one‐electron reduction and oxidation were possible in all instances yielding comproportionation constants Kc of about 10⁹ for the paramagnetic intermediates 1⁺–3⁺. Structural results, spin density distribution and UV‐Vis‐NIR spectroelectrochemistry were analyzed for 1⁺ with the help of TD‐DFT calculations for a model compound (tert‐Bu→Me). Intense absorptions around λmax=1450–1650 nm for the cations were assigned to mixed metal/ligand transitions with significant inter‐valence charge transfer (IVCT) character. For both the symmetrical and asymmetrical arrangements the cationic intermediates can be described as considerably mixed metal/ligand systems.

Herein we report stable silicon-boron radicals of composition LSi(NMe2)-B(Br)Tip (1), LSi(NMe2)-B(I)Tip (2) LSi(tBu)-B(I)Tip (3) [L = PhC(NtBu)2]. They were prepared in high yield using a one pot reaction of LSiR, X2BTip and KC8 in a 1 : 1 : 1 molar ratio (R = tBu, NMe2; X = Br, I). The reaction of the silicon-boron radical with Br2 and Se affords the dihalogenated compound LSi(tBu)-B(Br2)Tip (4) and oxidative addition product LSi(tBu)Se (5). All the compounds were characterized by single-crystal X-ray structural analysis, electron paramagnetic resonance (EPR) analysis, elemental analysis, multinuclear NMR spectroscopy, and mass spectrometry. Quantum chemical calculations show that the B-centered radicals 1-3 are stabilised by hyperconjugative interactions.

Using bis(3‐methyl‐2‐pyridyl)‐1,2,4,5‐tetrazine 1, 3‐(2‐pyrimidyl)‐6‐methyl‐1,2,4,5‐tetrazine 2 and bis(2‐pyrimidyl)‐1,2,4,5‐tetrazine = bmtz as ligands, the complexes 3 = [Ru(acac)2(1)], 4 = {[Ru(acac)2]2(1)], 5 = {[Ru(acac)2]2(bmtz)], and 6 = {[Ru(acac)2]2(2)] were prepared and identified by structure analysis of crystallized material. The one‐electron oxidized form 6(PF6) was also studied structurally, suggesting a Class II mixed‐valent situation. The neutral dinuclear systems exhibit two reversible oxidation processes with comproportionation constants 109.2 < Kc < 1014.1 and one reduction which were analyzed UV/vis/NIR and EPR spectroscopically. Oxidation produces largely metal‐based mixed‐valent cations with very weak intervalence absorptions in the near IR whereas the electron uptake occurs at the tetrazine acceptor.