Late-stage Ligand Modification After Coordination Strengthens Stereoselectively Self-Assembled Hemiaminal Ether Complexes.

Fragile hemiaminal ether linkages present in the backbone of koneramines (LR OR'), tridentate ligands, bound to copper(II) in stereoselectively self-assembled syn-[Cu(LR OR')X2 ] complexes were transformed into sturdy methylene linkages to make corresponding rac-[Cu(LR H)Cl2 ] complexes by late-stage ligand modification after coordination with the retention of coordination sphere. The generality of stereoselective self-assembly of koneramine complexes is shown by utilising a number of metal ions, anions, amines, alcohols and thiols with complete characterisations.

Metathesis Reactions of a NHC-Stabilized Phosphaborene.

The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P-protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N-heterocyclic carbene (NHC)-stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P-terminal, as a room-temperature-stable crystalline solid accessible via facile NHC-induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P-centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X-ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation.

Electrophilic activation of difunctional aminoboranes: B-N coupling versus intramolecular Cl/Me exchange.

Treatment of an N-silyl-B-chloro-aminoborane with substoichio-metric quantities of Me3SiOTf afforded B-N coupling, whereas activation with 5 mol% of Ag[Al{OC(CF3)3}4] led to Cl/Me exchange between the boron and the silicon center. Combined experimental and computational studies of the latter process support a chain reaction that is initiated by nucleation-limited chloride abstraction.

A Cyclic Iminoborane-NHC Adduct: Synthesis, Reactivity, and Bonding Analysis.

Lewis-base coordinated iminoborane adducts, in contrast to their isoelectronic analogue imines, remain largely unexplored given the lack of efficient synthetic strategies for generating robust compounds. Herein, we report the preparation of a cyclic amino iminoborane carbene complex 2 obtained in quantitative yield by adding NHC to the 1,8-(trimethylsilyl)aminonaphthalene complex of boron 1 to induce the elimination of trimethylsilyl chloride (TMSCl). The iminoborane-NHC adduct 2 shows unprecedented thermal stability both in the solid and solution phases, due to the rigid, pre-established geometry of the 1,8-diaminonaphthalene scaffold. Theoretical calculations reveal an exceptionally strong iminoborane-NHC bond as a consequence of the enhanced boron-center acidity in combination with the lower steric and electronic shielding. We show that the chemical bond can be understood as donor-acceptor interaction, leading to a different kind of electronic situation of the B═N π-bond. The high conjugation between the pz-lone pair of the tricoordinated sp2 hybridized N atom and the B═N π-system results in a particularly long B═N double bond distance. Taking advantage of the pendant lone pair of the dicoordinated sp2 hybridized N atom, the iminoborane-NHC adduct gives access to NHC-stabilized borenium cation 3 through the reaction with trimethylsilyl triflate (Me3SiOTf) or to the gallium adduct 4 by reacting with GaCl3. Incorporating an iminoborane functional group into a π-conjugated system brings a new bonding situation for broadening the scope of BN-containing polyaromatic systems.

7-Metalla-1,4-diphosphanorbornadienes: cycloaddition of monovalent group 13 NacNac complexes to a stable 1,4-diphosphinine.

This work presents the first example of a dienophilic property of the monovalent group-13 NacNac complexes, which are well-known for their carbene-like reactivity, e.g., addition to multiple bonds, insertion into single bonds, and oxidative cleavage of some multiple bonds. In reactions with imidazole-2-thione based tricyclic 1,4-diphospinine 4, the monovalent compounds NacNacM (M = Al,Ga) showed dienophilic behaviour and produced the corresponding 7-metalla-1,4-diphosphanorbornadienes (5-6), leaving the C[double bond, length as m-dash]S functionality of the imidazole-2-thione intact, while the NacNacIn complex did not show any reactivity because of unfavourable thermodynamics. DFT calculations revealed that for NacNacAl, the cycloaddition was kinetically more favoured (due to a small reaction barrier) than the oxidative cleavage of the C[double bond, length as m-dash]S functionality, despite the fact that the product of the C[double bond, length as m-dash]S bond addition has a somewhat higher stability. For NacNacGa, the cycloaddition reaction was both kinetically and thermodynamically favourable than the cleavage of C[double bond, length as m-dash]S. Furthermore, these reactions were found to be reversible in nature and diphosphinine 4 showed a clear preference towards Al than towards Ga which reflects the better inclination of Ga to stay monovalent than be in the trivalent state.

1,4-Additions of tricyclic 1,4-diphosphinines - a novel system to study σ-bond activation and π-π dispersion interactions.

The dienic nature of the aromatic π-system in 1,4-diphosphinines remains largely unexplored to this day due to a lack of facile and efficient synthetic protocols. Recently reported stable, tricyclic 1,4-diphosphinines were used to explore the thermal reactivity of the π-system towards an array of dienophiles in [4π+2π]- and, for the first time, in [4π+2σ]-type cycloaddition reactions.

Exploring the chemistry of backbone amino(chloro)phosphanyl-substituted imidazole-2-thiones.

Backbone (R'2N)2P-substituted imidazole-2-thiones 2a-c [(R'2N)2P-IMSR,R; IMSR,R = 1,3-dialkylimidazole-2-thione-4-yl, a: R = iPr, R' = Et; b: R = Mes, R' = Et, c: R = Me, R' = iPr] were treated with PCl3 to synthesize R'2N(Cl)P-substituted imidazole-2-thiones 3a-c [R'2N(Cl)P-IMSR,R]. The P-chloro compounds 3a,b were used to explore the chemistry further, i.e. nucleophilic substitution at phosphorus using Ph2CHLi or fluorenyl lithium afforded compounds 4a-c [R'2N(R'')P-IMSR,R; a: R = iPr, R' = Et, R'' = PH2CH; b: R = Mes, R' = Et, R'' = Flu c: R = iPr, R' = Et, R'' = Flu]. Compound 4c was used to access the P-borane adduct 5 [R'2N(R'')(BH3)P-IMSR,R; R = iPr, R' = Et, R'' = Flu] and phosphinoyl-substituted imidazolium hydrogensulfate 6 [[R'2N(R'')(O)P-IMHR,R]HSO4; IMHR,R = 1,3-dialkylimidazolium-4-yl, R = iPr, R' = Et, R'' = Flu]. Compounds 4a,b were transformed into the P-Cl compounds 7a,b [R''(Cl)P-IMSR,R; a: R = iPr, R'' = Ph2CH; b: R = Mes, R'' = Flu] which represent potential starting materials for backbone phosphaalkenyl substituted imidazole-2-thiones. Compound 7a yielded the NHC-stabilized phosphenium salt 8 [[R''(IMe4)P-IMSR,R]Cl; R = iPr, R' = Et, R'' = Ph2CH, IMe4 = 1,3,4,5-tetramethylimidazole-2-ylidene] in reaction with 1,3,4,5-tetramethylimidazole-2-ylidene (IMe4). Attempts to deprotonate 7a,b using nBuLi revealed that nucleophilic substitution of P-Cl to give the P-nBu product occurred preferentially, and dehydrochlorination to the phosphaalkene was not observed.

1,4-Diphosphinines from Imidazole-2-thiones.

1,4-Diphosphinines that are fused to two thiourea units were synthesized from the corresponding tricyclic 1,4-dichloro-1,4-dihydro-1,4-diphosphinines, and their structures and spectroscopic features are described. Electrochemical studies revealed very low oxidation potentials, which are due to the effective π-interaction between the 1,4-diphosphinine ring and the orbitals of the two ylidic C=S bonds. In accordance with the low-lying LUMO, which is largely localized at the two phosphorus centers, dianion formation is strongly preferred. Despite the small HOMO-LUMO gap, which is in accordance with the red color of the title compounds, theoretical calculations suggest considerable aromaticity for the 1,4-diphosphinine ring.

Multicomponent One-pot Reactions Towards the Synthesis of Stereoisomers of Dipicolylamine Complexes.

Reported are multi-component one-pot syntheses of chiral complexes [M(L(R) OR')Cl2 ] or [M(L(R) SR')Cl2 ] from the mixture of an N-substituted ethylenediamine, pyridine-2-carboxaldehyde, a primary alcohol or thiol and MCl2 utilizing in-situ formed cyclized Schiff bases where a C-O bond, two stereocenters, and three C-N bonds are formed (M=Zn, Cu, Ni, Cd; R=Et, Ph; R'=Me, Et, nPr, nBu). Tridentate ligands L(R) OR' and L(R) SR' comprise two chiral centers and a hemiaminal ether or hemiaminal thioether moiety on the dipicolylamine skeleton. Syn-[Zn(L(Ph) OMe)Cl2 ] precipitates out readily from the reaction mixture as a major product whereas anti-[Zn(L(Ph) OMe)Cl2 ] stays in solution as minor product. Both syn-[Zn(L(Ph) OMe)Cl2 ] and anti-[Zn(L(Ph) OMe)Cl2 ] were characterized using NMR spectroscopy and mass spectrometry. Solid-state structures revealed that syn-[Zn(L(Ph) OMe)Cl2 ] adopted a square pyramidal geometry while anti-[Zn(L(Ph) OMe)Cl2 ] possesses a trigonal bipyramidal geometry around the Zn centers. The scope of this method was shown to be wide by varying the components of the dynamic coordination assembly, and the structures of the complexes isolated were confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Syn complexes were isolated as major products with Zn(II) and Cu(II) , and anti complexes were found to be major products with Ni(II) and Cd(II) . Hemiaminals and hemiaminal ethers are known to be unstable and are seldom observed as part of cyclic organic compounds or as coordinated ligands assembled around metals. It is now shown, with the support of experimental results, that linear hemiaminal ethers or thioethers can be assembled without the assistance of Lewis acidic metals in the multi-component assembly, and a possible pathway of the formation of hemiaminal ethers has been proposed.