Catalytic Properties of [PSiP] Pincer Cobalt(II) Chlorides Supported by Trimethylphosphine for Alkene Hydrosilylation Reactions

In this paper, six silyl [PSiP] pincer cobalt(II) chlorides 1-6 [(2-Ph2PC6H4)2MeSiCo(Cl)(PMe3)] (1), [(2-Ph2PC6H4)2HSiCo(Cl)(PMe3)] (2), [(2-Ph2PC6H4)2PhSiCo(Cl)(PMe3)] (3), [(2-iPr2PC6H4)2HSiCo(Cl)(PMe3)] (4), [(2-iPr2PC6H4)2MeSiCo(Cl)(PMe3)] (5), and [(2-iPr2PC6H4)2PhSiCo(Cl)(PMe3)] (6)) were prepared from the corresponding [PSiP] pincer preligands (L1-L6), CoCl2 and PMe3 by Si-H bond activation. The catalytic activity of complexes 1-6 for alkene hyrdosilylation was studied. It was confirmed that complex 1 is the best catalyst with excellent regioselectivity among the six complexes. Using 1 as the catalyst, the catalytic reaction was completed within 1 h at 50 °C, predominantly affording Markovnikov products for aryl alkenes and anti-Markovnikov products for aliphatic alkene substrates. During the investigation of the catalytic mechanism, the Co(II) hydrides [(2-Ph2PC6H4)2MeSiCo(H)(PMe3)] (8) and [(2-iPr2PC6H4)2MeSiCo(H)(PMe3)] (9) were obtained from the stoichiometric reactions of complex 1 and 5 with NaBHEt3, respectively. Complexes 8 and 9 could also be obtained by the reactions of preligands L1 and L5 with Co(PMe3)4 via Si-H bond cleavage. More experiments corroborated that complex 8 is the real catalyst for this catalytic system. Under the same catalytic conditions as complex 1, using complex 8 as a catalyst, complete conversion of styrene was also achieved in 1 h, and the selectivity remained unchanged. Based on the experimental results, we propose a plausible mechanism for this catalytic reaction. The addition of B(C6F5)3 to catalyst 1 can reverse the selectivity of styrene hydrosilylation from the Markovnikov product as the main product (b/l = 99:1) to the anti-Markovnikov product as the main product (b/l = 40:60). Further study indicated that using the (CoCl2 + L1) system instead of complex 1, the selectivity was changed from Markovnikov to anti-Markovnikov product (b/l = 1:99.7). Therefore, the selectivity for the substrate styrene is influenced by the presence of a PMe3 ligand. The different selectivities may be caused by different active species. For the system of complex 1, a cobalt(II) hydride is the real catalyst, but for the (CoCl2 + L1) system, a cobalt(I) complex is proposed as active species. The molecular structures of Co(II) compounds 5 and 9 were resolved by single-crystal X-ray diffraction.

A New Class of Task-Specific Imidazolium Salts and Ionic Liquids and Their Corresponding Transition-Metal Complexes for Immobilization on Electrochemically Active Surfaces

Adding to the versatile class of ionic liquids, we report the detailed structure and property analysis of a new class of asymmetrically substituted imidazolium salts, offering interesting thermal characteristics, such as liquid crystalline behavior, polymorphism or glass transitions. A scalable general synthetic procedure for N-polyaryl-N'-alkyl-functionalized imidazolium salts with para-substituted linker (L) moieties at the aryl chain, namely [LPhm ImH R]+ (L=Br, CN, SMe, CO2 Et, OH; m=2, 3; R=C12 , PEGn ; n=2, 3, 4), was developed. These imidazolium salts were studied by single-crystal X-ray diffraction (SC-XRD), NMR spectroscopy and thermochemical methods (DSC, TGA). Furthermore, these imidazolium salts were used as N-heterocyclic carbene (NHC) ligand precursors for mononuclear, first-row transition metal complexes (MnII , FeII , CoII , NiII , ZnII , CuI , AgI , AuI ) and for the dinuclear Ti-supported Fe-NHC complex [(OPy)2 Ti(OPh2 ImC12 )2 (FeI2 )] (OPy=pyridin-2-ylmethanolate). The complexes were studied concerning their structural and magnetic behavior via multi-nuclear NMR spectroscopy, SC-XRD analyses, variable temperature and field-dependent (VT-VF) SQUID magnetization methods, X-band EPR spectroscopy and, where appropriate, zero-field 57 Fe Mössbauer spectroscopy.

Which Type of Pincer Complex Is Thermodynamically More Stable? Understanding the Structures and Relative Bond Strengths of Group 10 Metal Complexes Supported by Benzene-Based PYCYP Pincer Ligands

The influence of the pincer platform composition and substitution on the reactivity and physical properties of pincer complexes can be easily explored through different experimental techniques. However, the influence of these factors on the molecular structures and thermodynamic stability of pincer complexes is usually very subtle and cannot always be unambiguously established. To rationalize this subtle influence, a survey of crystallographic data from 130 group 10 metal pincer complexes supported by benzene-based PYCYP pincer ligands, [2,6-(R₂PY)₂C₆H_3-nR'_n]MX (Y = CH₂, NH, O, S; M = Ni, Pd, Pt; R = ^tBu, ⁱPr, Ph, Cy, Me; R' = CO₂Me, ^tBu, CF₃, Ac; n = 0-2; X = F, Cl, Br, I, H, SH, SPh, SBn, Ph, Me, N₃, NCS), was carried out. Theoretical calculations for some selected complexes were performed to evaluate the relative bond strength. It was found that the M-C_ipso bond length decreases following the linker series of CH₂ > NH > O and that the relative M-C_ipso bond strength increases following the linker series of CH₂ < NH < O. In most cases, the M-P bond length decreases following the linker series of NH > CH₂ > O. The relative M-P bond strength increases following the linker series of CH₂ < NH < O. A comparison of the thermochemical balance for the isodesmic displacement of the side-arm interactions with PH₃ as a probe ligand indicated that the Ni-P bond in a PCCCP-type pincer complex is far less difficult to break compared with that in a POCOP-type complex. As a result, with the same donor substituents and the same auxiliary ligand, the POCOP-type pincer complexes are thermodynamically more stable than the PCCCP complexes. The influence of other backbone and donor substitutions as well as the pincer platform composition on the M-C_ipso, M-P, and M-X bond lengths, relative bond strengths, and P-M-P bite angles was also discussed.

NHC Effects on Reduction Dynamics in Iron-Catalyzed Organic Transformations*

The high abundance, low toxicity and rich redox chemistry of iron has resulted in a surge of iron-catalyzed organic transformations over the last two decades. Within this area, N-heterocyclic carbene (NHC) ligands have been widely utilized to achieve high yields across reactions including cross-coupling and C-H alkylation, amongst others. Central to the development of iron-NHC catalytic methods is the understanding of iron speciation and the propensity of these species to undergo reduction events, as low-valent iron species can be advantageous or undesirable from one system to the next. This study highlights the importance of the identity of the NHC on iron speciation upon reaction with EtMgBr, where reactions with SIMes and IMes NHCs were shown to undergo β-hydride elimination more readily than those with SIPr and IPr NHCs. This insight is vital to developing new iron-NHC catalyzed transformations as understanding how to control this reduction by simply changing the NHC is central to improving the reactivity in iron-NHC catalysis.

C-Term magnetic circular dichroism (MCD) spectroscopy in paramagnetic transition metal and f-element organometallic chemistry

Magnetic circular dichroism (MCD) spectroscopy is a powerful experiment used to probe the electronic structure and bonding in paramagnetic metal-based complexes. While C-term MCD spectroscopy has been utilized in many areas of chemistry, it has been underutilized in studying paramagnetic organometallic transition metal and f-element complexes. From the analysis of isolated organometallic complexes to the study of in situ generated species, MCD can provide information regarding ligand interactions, oxidation and spin state, and geometry and coordination environment of paramagnetic species. The pratical aspects of this technique, such as air-free sample preparation and cryogenic experimental temperatures, allow for the study of highly unstable species, something that is often difficult with other spectroscopic techniques. This perspective highlights MCD studies of both transition metal and f-element organometallic complexes, including in situ generated reactive intermediates, to demonstrate the utility of this technique in probing electronic structure, bonding and mechanism in paramagnetic organometallic chemistry.

Slow magnetic relaxation in cobalt N-heterocyclic carbene complexes

The combined experimental and theoretical investigation of the magnetic properties of the cobalt(ii) NHC complexes (NHC = N-heterocyclic carbene); [Co(CH2SiMe3)2(IPr)] (1), [CoCl2(IMes)2] (2) and [Co(CH3)2(IMes)2] (3) revealed a large easy plane anisotropy for 1 (D = +73.7 cm-1) and a moderate easy axis anisotropy for 2 (D = -7.7 cm-1) due to significant out-of-state spin-orbit coupling. Dynamic magnetic measurements revealed slow relaxation of the magnetization for 1 (Ueff = 22.5 K, τ0 = 3 × 10-7 s, 1000 Oe) and for 2 (Ueff = 20.2 K, τ0 = 1.73 × 10-8 s, 1500 Oe). The molecular origin of the slow relaxation phenomena was further supported by the retention of AC signal in 10% solutions in 2-MeTHF which reveals a second zero field AC signal in 1 at higher frequencies. Compound 3 was found to be an S = 1/2 system.

Tuning electronic structure through halide modulation of mesoionic carbene cobalt complexes

The first examples of Co(ii) mesoionic carbene complexes (CoX2DippMIC2; X = Cl-, Br-, I-) demonstrate a new electronic perturbation on tetrahedral Co(ii) complexes. Using absorption spectroscopy and magnetometry, the consequences of the MIC's strong σ-donating/minimal π-accepting nature are analyzed and shown to be further tunable by the nature of the coordinated halide.

Three-Coordinate Formal Cobalt(0), Iron(0), and Manganese(0) Complexes with Persistent Carbene and Alkene Ligation

Low-coordinate transition-metal species, i.e., metal species with coordination numbers of less than 4, represent a category of ubiquitous reactive intermediates in metal-catalyzed reactions that take place in solution, in metalloenzymes, on supported nanomaterials and single-atom catalysts, and so on. While reactive intermediates are usually transient and hard to isolate, which makes detailed investigation challenging, molecular representatives of low-coordinate transition-metal intermediates can be synthesized by the judicious use of supporting ligands, allowing detailed study of their inherent chemical and physical properties. By the use of bulky nitrogen- and oxygen-based anionic ligands, plenty of three- and two-coordinate group 4-10 metal complexes with the oxidation states of the metal centers being +3, + 2, and +1 have been prepared and subjected to extensive study. Much less known are low-coordinate zero-valent metal complexes, and knowledge about them had been restricted to group 10 metal complexes until very recently. In this Account, we summarize the studies of the synthesis, spectroscopic features, electronic structures, and reactivities of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes with persistent carbene and alkene ligation. The introduction of the π-accepting alkene ligands 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (dvtms) and vinyltrimethylsilane (vtms) into the reduction reactions of MCl₂ (M = Co, Fe, Mn) with persistent carbenes and alkaline metals effectively suppresses ligand C-H bond activation reactions, leading to the successful preparation of three-coordinate formal cobalt(0), iron(0), and manganese(0) complexes LM(η²:η²-dvtms), LM(η²-vtms)₂, and L₂M(η²-vtms) (M = Co, Fe, Mn; L = N-heterocyclic carbene (NHC), cyclic (alkyl)(amino)carbene (cAAC)). These three-coordinate metal complexes feature pronounced back-donation from the filled metal 3d orbitals to the alkene π* orbital(s), resulting in electronic configurations of (d_xy+π_alkene^*)²(d_x²-y²+π'_alkene^*)²(d_z²,d_xz,d_yz)ⁿ (the coordination plane was chosen as the xy plane; n = 5, 4, and 3 for Co, Fe, and Mn, respectively) for the bis(alkene) complexes LM(η²:η²-dvtms) and LM(η²-vtms)₂. The alkene ligands in the low-coordinate formal zero-valent metal complexes are amenable to undergo ligand-exchange reactions with better π-accepting ligands. In reactions with organic azides, hydrosilanes, nitrosoarenes, alkynes, etc., the alkene ligands dissociate from the metal coordination sphere, and three-coordinate formal zero-valent metal complexes function as synthons of L_nM⁰ (L = NHC, cAAC; n = 1, 2; M = Co, Fe, Mn) to perform redox reactions with these substrates, affording divalent and tetravalent cobalt, iron, or manganese complexes. These electronic structure and reactivity features hint at the potential of low-coordinate zero-valent group 7-9 metal complexes for the development of new 3d metal catalysts and magnetic materials.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

[(NHC)CoR2]: pre-catalysts for homogeneous olefin and alkyne hydrogenation

[(ItBu)Co(CH₂SiMe₃)₂] serves as an efficient, homogeneous olefin hydrogenation catalyst.