[(NHC)Cu^I–ER₃] Complexes (ER₃ = SiMe₂Ph, SiPh₃, SnMe₃): From Linear, Mononuclear Complexes to Polynuclear Complexes with Ultrashort Cu^I···Cu^I Distances

A series of complexes of the type [(NHC)­Cu–ER3] (NHC = IDipp, IMes, ItBu, Me2IMe, and ER3 = SiMe2Ph, SiPh3, SnMe3) and [(NHC)­Cu–R′] (NHC = IDipp, Me2IMe and R′ = Ph, CCPh) was synthesized in good yields by the reaction of the corresponding [(NHC)­Cu–OtBu] complex with the respective silylborane pinB–ER3 (pin = OCMe2CMe2O; ER3 = SiMe2Ph, SiPh3), the stannylborane ((C2H4)­(iPrN)2)­B–SnMe3, or a boronic acid ester pinB–R′ (R′ = Ph, CCPh). Solid structures of all complexes were systematically studied by X-ray diffraction analysis. The solid state structures of the complexes [(NHC)­Cu–ER3] show a dependence of the structural motif from the steric properties of the NHC ligand. The sterically demanding NHC ligands (IDipp, IMes, ItBu) lead to monomeric, linear complexes [(NHC)­Cu–ER3], while with the less demanding Me2IMe ligand, polynuclear, μ-ER3-bridged complexes with ultrashort Cu···Cu distances are observed. For the related complexes [(NHC)­Cu–R′] no analogous complexes with bridging anionic ligands are realized. Instead, irrespective of the NHC ligand, linear coordinated copper complexes of different types are formed. 29Si heteronuclear solution NMR spectroscopic data on [(NHC)­CuI–SiR3] exhibit distinctly different chemical shifts for the (in the solid state) monomeric and dimeric complexes suggesting different structure types also in solution. This agrees well with the observation of a trinuclear complex [(Me2IMe)­Cu–SnMe3]3 both in the solid state and in solution. Initial catalytic studies suggest that [(NHC)­Cu–OtBu] complexes (NHC = ItBu, Me2IMe) are, in addition to the established [(IDipp)­Cu–OtBu] complex, efficient precatalysts for the silylation of aldehydes and α,β-unsaturated ketones with pinB–SiMe2Ph

Terminal versus Bridging Boryl Coordination in N‑Heterocyclic Carbene Copper(I) Boryl Complexes: Syntheses, Structures, and Dynamic Behavior

The B–B bond activation of the diborane(4) derivatives B2cat2 with the copper­(I) alkoxido complex [(SIDipp)­Cu–OtBu] delivers, depending on the solvent, either the linear boryl complex [(SIDipp)­Cu−Bcat] from PhMe or the μ-boryl complex [((SIDipp)­Cu)2Bcat]­[cat2B] from THF. The relevant conversion of the linear boryl complex to the μ-boryl complex occurs in the polar solvent via formal boryl anion abstraction by the Lewis acid catB–OtBu, concomitantly formed during the B–B activation. With Lewis acids such as BPh3 or [CPh3]­[BArF] (reversible), boryl abstraction from the linear complexes [(SIDipp)­Cu–Bcat] or [(SIDipp)­Cu–Bdmab] occurs and results in the μ-boryl complexes [((SIDipp)­Cu)2Bcat/dmab]­[Ph3B–Bcat/dmab] and [((SIDipp)­Cu)2Bcat]­[BArF]. The formation of [((SIDipp)­Cu)2Bcat]­[cat2B] is generally accompanied by the concomitant formation of the μ-hydrido complex [((SIDipp)­Cu)2H]­[cat2B]. The spiroborate [cat2B]− is formed from the initially formed Lewis acid/base adduct [catB–B­(OtBu)­cat]− presumably in a process that involves the glass surface of the reaction vessel. All complexes are thoroughly characterized structurally as well as spectroscopically, in particular with respect to the dynamic behavior of the μ-boryl complexes in solution

Syntheses, Structures, and Reactivity of NHC Copper(I) Boryl Complexes: A Systematic Study

Five novel NHC copper­(I) boryl complexes were synthesized by B–B activation via σ-bond metathesis of symmetrical tetraalkoxy and unsymmetrical dialkoxy diamino diborane(4) derivatives. Despite their low stability, the NHC copper boryl complexes were thoroughly characterized spectroscopically and structurally. Variation of the NHC ligand (ItBu or Me2IiPr) as well as of the boryl ligand (Bpin, Bdmab, or BiPrEn) allowed, for the first time systematically, a study in such complexes of the dependence on steric encumbrance. For sterically more demanding ligand combinations, mononuclear linear complexes were obtained, while with less demanding ligand combinations, dimeric dinuclear complexes with two bridging μ-boryl ligands were obtained, exhibiting extremely short Cu···Cu distances (<2.26 Å). The decomposition of all these complexes was found to proceed via a common pathway, leading ultimately to elemental copper, the free NHC ligand, and the respective symmetrical diborane(4) derivative. The rate of decomposition depended strongly on the steric encumbrance of the individual complex. Two apparently low-valent copper clusters were observed and suggested to be relevant species with respect to the reductive decomposition of the copper­(I) boryl complexes

[(NHC)Cu^I–ER₃] Complexes (ER₃ = SiMe₂Ph, SiPh₃, SnMe₃): From Linear, Mononuclear Complexes to Polynuclear Complexes with Ultrashort Cu^I···Cu^I Distances

A series of complexes of the type [(NHC)­Cu–ER₃] (NHC = IDipp, IMes, I<i>t</i>Bu, Me₂IMe, and ER₃ = SiMe₂Ph, SiPh₃, SnMe₃) and [(NHC)­Cu–R′] (NHC = IDipp, Me₂IMe and R′ = Ph, CCPh) was synthesized in good yields by the reaction of the corresponding [(NHC)­Cu–O<i>t</i>Bu] complex with the respective silylborane pinB–ER₃ (pin = OCMe₂CMe₂O; ER₃ = SiMe₂Ph, SiPh₃), the stannylborane ((C₂H₄)­(<i>i</i>PrN)₂)­B–SnMe₃, or a boronic acid ester pinB–R′ (R′ = Ph, CCPh). Solid structures of all complexes were systematically studied by X-ray diffraction analysis. The solid state structures of the complexes [(NHC)­Cu–ER₃] show a dependence of the structural motif from the steric properties of the NHC ligand. The sterically demanding NHC ligands (IDipp, IMes, I<i>t</i>Bu) lead to monomeric, linear complexes [(NHC)­Cu–ER₃], while with the less demanding Me₂IMe ligand, polynuclear, μ-ER₃-bridged complexes with ultrashort Cu···Cu distances are observed. For the related complexes [(NHC)­Cu–R′] no analogous complexes with bridging anionic ligands are realized. Instead, irrespective of the NHC ligand, linear coordinated copper complexes of different types are formed. ²⁹Si heteronuclear solution NMR spectroscopic data on [(NHC)­Cu^I–SiR₃] exhibit distinctly different chemical shifts for the (in the solid state) monomeric and dimeric complexes suggesting different structure types also in solution. This agrees well with the observation of a trinuclear complex [(Me₂IMe)­Cu–SnMe₃]₃ both in the solid state and in solution. Initial catalytic studies suggest that [(NHC)­Cu–O<i>t</i>Bu] complexes (NHC = I<i>t</i>Bu, Me₂IMe) are, in addition to the established [(IDipp)­Cu–O<i>t</i>Bu] complex, efficient precatalysts for the silylation of aldehydes and α,β-unsaturated ketones with pinB–SiMe₂Ph

Cu^I‑Catalyzed Conjugate Addition of Silyl Boronic Esters: Retracing Catalytic Cycles Using Isolated Copper and Boron Enolate Intermediates

Copper­(I)-catalyzed conjugate additions of silyl boronic esters to α,β-unsaturated aldehydes, ketones, and esters are synthetically well-established reactions. For the first time central reactive intermediates as well as the boron enolates as the primary reaction products are isolated and employed in order to deduce catalytic cycles on an experimental basis. Employing an NHC Cu^I complex as a model catalyst, it is possible to perform efficient catalytic transformations as well as to isolate and characterize the formed copper enolate complexes as the key intermediates. It is shown that for this catalytic system the nature of this enolate<i>O</i>- or <i>C</i>-enolateis crucial for the catalytic process. For α,β-unsaturated aldehydes and ketones the <i>O</i>-enolate is formed predominantly, while for α,β-unsaturated esters the <i>C</i>-enolate is the major product. Catalytic turnover is only facile for copper <i>O</i>-enolates, as they react efficiently with the silyl boronic ester under (re)­formation of the catalytically active Cu–Si species and a thermodynamically favored boric acid ester. Thus, the formation of copper <i>C</i>-enolates is inhibiting the catalytic process, and effective turnover is possible only after solvolysis by an alcohol additive. The individual catalytic processes were retraced by performing stepwise stoichiometric reactions monitored by in situ NMR spectroscopy

Palladium(II) Complexes of Unsymmetrical <i>CNN</i> Pincer Ligands

Unsymmetrical 1-(arylimino)-3-(2-hetarylimino)isoindolines have been prepared from 1,3-diiminoisoindoline, an arylamine (aniline, 2-methylaniline, 2-iodoaniline), and a heteroaromatic amine (2-amino-6-methylpyridine, 2-amino-4-methylthiazole) in a stepwise manner by two consecutive condensations. The metalation reactions of these compounds with palladium(II) acetate proceed upon cyclopalladation of the carbocyclic aryl moieties and yield unsymmetrical C,N,N pincer complexes in all cases. X-ray crystallographic analysis were performed on single crystals of hydrogen{acetato[1-phenylimino-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(phpi)Pd(OAc)] and pyridine[1-(2-tolylimino)-3-(4-methylthiazolylimino)isoindolinato]palladium(II) [(2-tolti)Pd(py)] by which the coordination mode, the conformation, the protonation site, and the trans influence of the carbon donor were established. For one more C,N,N pincer complex, hydrogen{acetato[1-(2-iodophenylimino)-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(2-Iphpi)Pd(OAc)], a similar mononuclear coordination mode was confirmed by 1H NMR spectroscopy, whereas for the product of an oxidative addition reaction of a palladium(0) precursor to the iodoaryl derivative a product with exo coordination was found. First experiments showed the effectivity of one of these complexes as a precatalyst in CC coupling reactions (Heck and Stille coupling)

K–H₃C and K–Sn Interactions in Potassium Trimethylstannyl Complexes: A Structural, Mechanochemical, and NMR Study

A series of trimethylstannyl potassium complexes [K­(L)­SnMe₃] with different auxiliary ligands L (L = 18-C-6, (TMEDA)₂ (TMEDA = tetramethylethylenediamine), and (12-C-4)₂) were synthesized by alkoxide-induced B–Sn bond cleavage. X-ray structure determinations were performed for all these complexes, and the structural chemistry was studied in detail. For L = 18-C-6 and (TMEDA)₂ the solid state structures comprise polymeric [K­(L)­SnMe₃]_<i>n</i> chains containing bidentate trimethylstannyl anions bridging two [K­(L)]⁺ ions, featuring unsymmetrical coordination of the [K­(L)]⁺ ion by K–Sn and K–H₃C interactions as a central structural motif. In contrast, for L = (12-C-4)₂, separated [K­(12-C-4)₂]⁺ and [SnMe₃]⁻ ions are observed. Unexpectedly, in the presence of tetrahydrofuran (THF), [K­(18-C-6)­SnMe₃]_<i>n</i> forms upon crystallization a new species consisting of separated [K­(18-C-6)­(THF)₂]⁺ and [(Me₂SnCH₃)­K­(18-C-6)­SnMe₃]⁻ ions. In this unsymmetrical anion two trimethylstannyl anions coordinate a single [K­(18-C-6)]⁺ ion; one trimethylstannyl anion coordinates via a K–Sn interaction, and the second coordinates via a K–H₃C interaction. Simulations of the mechanochemical properties (compliance constants) applying approximated density functional theory revealed that both interactions are very soft and are of comparable strength. Moreover, according to our gas phase simulations the unsymmetrically coordinated [(Me₂SnCH₃)­K­(18-C-6)­SnMe₃]⁻ is indeed thermodynamically favored over both possible symmetrical isomers with either K–Sn or K–H₃C coordination. Furthermore, the existence of multiple species due to the two coordination modes and aggregates of [K­(18-C-6)­SnMe₃] in solution is suggested by NMR spectroscopic studies using ¹H, NOESY/ROESY, and ¹H pulsed field gradient diffusion experiments

Palladium(II) Complexes of Unsymmetrical <i>CNN</i> Pincer Ligands

Unsymmetrical 1-(arylimino)-3-(2-hetarylimino)isoindolines have been prepared from 1,3-diiminoisoindoline, an arylamine (aniline, 2-methylaniline, 2-iodoaniline), and a heteroaromatic amine (2-amino-6-methylpyridine, 2-amino-4-methylthiazole) in a stepwise manner by two consecutive condensations. The metalation reactions of these compounds with palladium(II) acetate proceed upon cyclopalladation of the carbocyclic aryl moieties and yield unsymmetrical C,N,N pincer complexes in all cases. X-ray crystallographic analysis were performed on single crystals of hydrogen{acetato[1-phenylimino-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(phpi)Pd(OAc)] and pyridine[1-(2-tolylimino)-3-(4-methylthiazolylimino)isoindolinato]palladium(II) [(2-tolti)Pd(py)] by which the coordination mode, the conformation, the protonation site, and the trans influence of the carbon donor were established. For one more C,N,N pincer complex, hydrogen{acetato[1-(2-iodophenylimino)-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(2-Iphpi)Pd(OAc)], a similar mononuclear coordination mode was confirmed by 1H NMR spectroscopy, whereas for the product of an oxidative addition reaction of a palladium(0) precursor to the iodoaryl derivative a product with exo coordination was found. First experiments showed the effectivity of one of these complexes as a precatalyst in CC coupling reactions (Heck and Stille coupling)

Unsymmetrical Diborane(4) as a Precursor to PBP Boryl Pincer Complexes: Synthesis and Cu(I) and Pt(II) PBP Complexes with Unusual Structural Features

An unsymmetrical diamino dialkoyx diborane(4), (1,2-((iPr)2PCH2N)2(C6H6))­B−B­((OCMe2)2), with additional P donor moieties in the diaminoboryl moiety was straightforwardly obtained by the reaction of a copper­(I) boryl complex as a boron nucleophile and the respective borane, (1,2-((iPr)2PCH2N)2(C6H6))­B−H, as boron electrophile. This diborane(4) reacted with copper­(I) tert-butoxide via σ-bond metathesis and [Pt­(P­(iPr)3)2] via oxidative addition. The first reaction results in a novel bis-μ-boryl di-copper­(I) complex, featuring an unprecedented unsymmetrical structure, whereas the latter reaction leads initially to a cis-bis-boryl platinum­(II) complex, that converts to an unprecedented trans-bis-boryl complex as the thermodynamic product. All complexes have been comprehensively characterized by NMR spectroscopy in solution and by single-crystal X-ray diffraction in the solid state, complemented by computations of thermodynamic data and relaxed force constants at the DFT level of theory

K–H₃C and K–Sn Interactions in Potassium Trimethylstannyl Complexes: A Structural, Mechanochemical, and NMR Study

A series of trimethylstannyl potassium complexes [K­(L)­SnMe₃] with different auxiliary ligands L (L = 18-C-6, (TMEDA)₂ (TMEDA = tetramethylethylenediamine), and (12-C-4)₂) were synthesized by alkoxide-induced B–Sn bond cleavage. X-ray structure determinations were performed for all these complexes, and the structural chemistry was studied in detail. For L = 18-C-6 and (TMEDA)₂ the solid state structures comprise polymeric [K­(L)­SnMe₃]_<i>n</i> chains containing bidentate trimethylstannyl anions bridging two [K­(L)]⁺ ions, featuring unsymmetrical coordination of the [K­(L)]⁺ ion by K–Sn and K–H₃C interactions as a central structural motif. In contrast, for L = (12-C-4)₂, separated [K­(12-C-4)₂]⁺ and [SnMe₃]⁻ ions are observed. Unexpectedly, in the presence of tetrahydrofuran (THF), [K­(18-C-6)­SnMe₃]_<i>n</i> forms upon crystallization a new species consisting of separated [K­(18-C-6)­(THF)₂]⁺ and [(Me₂SnCH₃)­K­(18-C-6)­SnMe₃]⁻ ions. In this unsymmetrical anion two trimethylstannyl anions coordinate a single [K­(18-C-6)]⁺ ion; one trimethylstannyl anion coordinates via a K–Sn interaction, and the second coordinates via a K–H₃C interaction. Simulations of the mechanochemical properties (compliance constants) applying approximated density functional theory revealed that both interactions are very soft and are of comparable strength. Moreover, according to our gas phase simulations the unsymmetrically coordinated [(Me₂SnCH₃)­K­(18-C-6)­SnMe₃]⁻ is indeed thermodynamically favored over both possible symmetrical isomers with either K–Sn or K–H₃C coordination. Furthermore, the existence of multiple species due to the two coordination modes and aggregates of [K­(18-C-6)­SnMe₃] in solution is suggested by NMR spectroscopic studies using ¹H, NOESY/ROESY, and ¹H pulsed field gradient diffusion experiments