Synthesis, Reactivity, and Complexation with Fe(0) of a Tight-bite Bis(N-heterocyclic silylene).

The synthesis, reactivity, and complexation with Fe(0) precursor of a tight-bite bis(N-heterocyclic silylene) (bis(NHSi)) ligand 1 are reported. The reaction of 1 with p-toluidine led to the activation of both N-H bonds across Si(II) atoms to afford a four-membered heterocyclic cyclodisilazane 2, with hydride substituents attached to five-coordinate Si atoms. A 1 : 2 reaction of 1 with Fe(CO)5 led to an intriguing dinuclear complex 3 featuring a five-membered (N-Si-Fe-Fe-Si) ring with a Fe-Fe bond distance of 2.6892(13) Å. All compounds (1-3) were thoroughly characterized by various spectroscopic methods and X-ray diffraction studies conclusively established their molecular structures. DFT calculations were carried out to shed light on bonding and energetic aspects in 1-3.

Stabilization of NH- Group Adjacent to Naked Silicon(II) Atom in Base Stabilized Aminosilylenes.

Aminosilylene, comprising reactive NH- and Si(II) sites next to each other, is an intriguing class of compounds due to its ability to show diverse reactivity. However, stabilizing the reactive NH- group next to the free Si(II) atom is challenging and has not yet been achieved. Herein, we report the first examples of base stabilized free aminosilylenes Ar*NHSi(PhC(Nt Bu)2 ) (1 a) and Mes*NHSi(PhC(Nt Bu)2 ) (1 b) (Ar*=2,6-dibenzhydryl-4-methylphenyl and Mes*=2,4,6-tri-tert-butylphenyl), tolerating a NH- group next to the naked Si(II) atom. Remarkably, 1 a and 1 b exhibited interesting differences in their reactivity upon heating. With 1 a, an intramolecular C(sp3 )-H activation of one of the benzhydryl methine hydrogen atoms to the Si(II) atom produced the five-membered cyclic silazane 2. However, with 1 b, a rare 1,2-hydrogen shift to the Si(II) atom afforded a silanimine 3, with a hydride ligand attached to an unsaturated silicon atom. Further, the coordination capabilities of 1 a were also tested with Ru(II) and Fe(0) precursors. Treatments of 1 a with [Ru(η6 -p-cymene)Cl2 ]2 led to the isolation of a η6 -arene tethered complex [RuCl2 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si-η6 -arene}] (4), whereas with the Fe(CO)5 precursor a Fe(0) complex [Fe(CO)4 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si}] (5) was obtained. Density functional theory (DFT) calculations were conducted to shed light on the structural, bonding, and energetic aspects in 1-5.

A Neutral Planar Four-Membered Si2B2 2π-Aromatic Ring.

The synthesis of the first inorganic four-membered neutral 2π-aromatic compound 2 is reported. This unique ring has been synthesized from a simple and straightforward reaction of amidinato-silylene with dichlorophenylborane, followed by the reduction with KC8 in THF. Compound 2 has been fully characterized by single-crystal X-ray diffraction (SC-XRD), NMR spectroscopy, and mass spectrometry. The computational calculations reveal that the Si2B2 ring is a π-delocalized system resulting from the interaction of pπ orbital of B and Si-N σ* orbitals having pseudo π symmetry. Compound 2 is the first known example of a neutral planar inorganic analogue of cyclobutenyl dication.

[2+4] Cycloaddition Product of an Amidinate Substituted Dialumene with Toluene.

Reduction of LAlI2 (L=PhC(Ni Pr2 C6 H3 )2 ) with two equivalents of KC8 in toluene affords the [2+4]cycloaddition product of a dialumene with toluene. The mechanism for the formation of product complex was investigated using density functional theory (DFT) methods.

Synthesis and characterization of the cyclodiphosphazane embedded macrocycle tetrabromo-resorcin[4]arene-tetrakis(cyclodiphosphazane) and its tetra-rhodium(I) complex.

The synthesis of a cyclodiphosphazane embedded macrocycle, tetrabromo-resorcin[4]arene-tetrakis(cyclodiphosphazane) [(µ-N(tBu)P)2{µ-(C6HBr(o-O)2-m-CHnBu)}]4 (1), and its tetra-rhodium(I) complex [(µ-N(tBu)P)2{µ-(C6HBr(o-O)2-m-CHnBu)}Rh(COD)Cl]4 (2) is described. The resorcin[4]arene backbone adopts a C4v symmetric crown conformation in 1, which transforms into a C2v symmetric boat conformation upon complexation with Rh(I) in the tetra-rhodium(I) complex 2, as evidenced by NMR spectroscopy and X-ray diffraction studies.

Synthesis and Photophysical Properties of Heavier Pnictogen Complexes.

Recent success in the synthesis of π-conjugated heavier pnictogen (As, Sb, and Bi) compounds and their transition metal complexes has led to the current surge in interest that led to significant development in the field of photophysical and optoelectronic properties of heavier pnictogens and their transition metal complexes. The presence of heavier pnictogens (As, Sb and Bi) in the molecular skeleton promotes inter-system crossing (ISC) and reverse inter-system crossing (RISC), because of the heavy atom effect, via altering the intermolecular interactions and orbital energy levels. As a result, π-conjugated heavier pnictogen compounds such as arsines, dibenzoarsepins, arsinoquinoline, heterofluorene, benzo[b]heterole (heterole=arsole, bismole, and stibole) show unique optoelectronic properties such as narrow bandgap, low-energy absorption, and long-wavelength emission than lighter pnictogen-based compounds. This review focuses on recent advances in the synthesis and photophysical properties of heavier pnictogen compounds. The synthesis and photophysical properties of heavier pnictogens are discussed and elaborated.

1,2,3-Triazole based ligands with phosphine and pyridine functionalities: synthesis, PdII and PtII chemistry and catalytic studies.

This manuscript describes the syntheses of pyridine appended triazole-based mono- and bisphosphines, [o-Ph2P(C6H4){1,2,3-N3C(Py)C(H)}] (2), [o-Br(C6H4){1,2,3-N3C(Py)C(PPh2)}] (3), [C6H5{1,2,3-N3C(Py)C(PPh2)}] (4), [Ph2P(C6H4){1,2,3-N3C(Py)C(PPh2)}] (5) and [3-Ph2P-2-{1,2,3-N3C(Ph)C(PPh2)}C5H3N] (6), their palladium and platinum chemistry and catalytic applications. These ligands upon treatment with [M(COD)Cl2] (M = Pd or Pt) yielded complexes with different coordination modes, depending on the reaction conditions. Both κ2-P,N and κ2-P,P coordination modes were observed in many of the complexes indicating the ambidentate nature of these ligands. Monophosphine 2 in the presence of a base afforded rare fused-5,6-membered PCN pincer complexes [MCl{o-Ph2P(C6H4){1,2,3-N3C(Py)C(H)}}-κ3-P,C,N] (7, M = Pd; 8, M = Pt), whereas the reactions of 4 with [M(COD)Cl2] (M = Pd, Pt) produced κ2-P,N chelate complexes [MCl2{C6H5{1,2,3-N3C(Py)C(PPh2)}-κ2-P,N}] (9, M = Pd; 10, M = Pt). Similar reactions of 5 and 6 resulted in κ2-P,P chelate complexes [MCl2{{3-Ph2P-2-{1,2,3-N3C(Ph)C(PPh2)}C5H3N}-κ2-P,P}] (11, M = Pd; 12, M = Pt) and [MCl2{3-Ph2P-2-{1,2,3-N3C(Ph)C(PPh2)}C5H3N}-κ2-P,P}] (13, M = Pd; 14, M = Pt), respectively. The palladium(II) complexes have shown excellent catalytic activity in the α-alkylation reaction of acetophenone derivatives.

Group 11 metal complexes of the dinucleating triazole appended bisphosphine 1,4-bis(5-(diisopropylphosphaneyl)-1-phenyl-1H-1,2,3-triazol-4-yl)benzene.

The synthesis of a triazole appended dinucleating bisphosphine 1,4-bis(5-(diisopropylphosphaneyl)-1-phenyl-1H-1,2,3-triazol-4-yl)benzene (2) and its coinage metal complexes are described. The dinucleating bisphosphine 2 was obtained by the temperature-controlled lithiation of 1,4-bis(1-phenyl-1H-1,2,3-triazol-4-yl)benzene (1a) and 1,4-bis(1-(2-bromophenyl)-1H-1,2,3-triazol-4-yl)benzene (1b) followed by the reaction with iPr2PCl. The reactions of 2 with copper(I) halides in 1 : 2 molar ratios yielded the [Cu(µ2-X)]2 dimeric complexes [{Cu(µ2-X)}2(PiPr2N3PhC2)2C6H4] (3, X = Cl; 4, X = Br; and 5, X = I), whereas the reaction of 2 with AgBr resulted in the formation of hetero-cubane complex [{Ag4(µ3-Br)4}{(PiPr2N3PhC2)2C6H4}2] (7). Similar reactions of 2 with AgX in 1 : 2 molar ratios yielded disilver complexes [{Ag(µ2-X)}2{(PiPr2N3PhC2)2C6H4}] (6, X = Cl and 8, X = I). Treatment of 2 with AgOAc in a 1 : 2 molar ratio afforded a dinuclear complex [Ag2(µ2-OAc)2{(PiPr2N3PhC2)2(C6H4)}] (9) with one of the acetate ligands bridging the two metal centres in the side-on mode, whereas the other one adopting the end-on mode keeping the >CO group uncoordinated. The reaction of 2 with two equivalents of [AuCl(SMe2)] afforded the digold complex [(AuClPiPr2N3PhC2)2C6H4] (10). The molecular structures of 2-5 and 7-10 were confirmed by single crystal X-ray analysis. Non-covalent interactions between Cu and Carene were observed in the molecular structures of 3, 4 and 5. These weak interactions were also assessed by DFT calculations in terms of their non-covalent interaction plots (NCI) and QTAIM analyses.

Novel approach to benzo-fused 1,2-azaphospholene involving a Pd(II)-assisted tandem P-C bond cleavage and P-N bond formation reaction.

New bisphosphine o-Ph2PC6H4C(O)N(H)C6H4PPh2-o (1) (Bala-HariPhos) showed a unique reactivity towards Pd(ii) resulting in a 1,2-azaphospholene complex, involving a tandem P-C bond cleavage, P-N bond formation and cyclization process via the elimination of PhH. Mechanistic details were investigated using NMR spectroscopy, DFT calculations and kinetic data, and by SCXRD analysis. It involves the reductive elimination from a tautomerised complex to form a phosphonium salt followed by oxidative addition.

Ester Hydrogenation with Bifunctional Metal-NHC Catalysts: Recent Advances.

Hydrogenation of ester to alcohol is an essential reaction in organic chemistry due to its importance in the production of a wide range of bulk and fine chemicals. There are a number of homogeneous and heterogeneous catalyst systems reported in the literature for this useful reaction. Mostly, phosphine-based bifunctional catalysts, owing to their ability to show metal-ligand cooperation during catalytic reactions, are extensively used in these reactions. However, phosphine-based catalysts are difficult to synthesize and are also highly air- and moisture-sensitive, restricting broad applications. In contrast, N-heterocyclic carbenes (NHCs) can be easily synthesized, and their steric and electronic attributes can be fine-tuned easily. In recent times, many phosphine ligands have been replaced by potent σ-donor NHCs, and the resulting bifunctional metal-ligand systems are proven to be very efficient in several important catalytic reactions. This mini-review focuses the recent advances mainly on bifunctional metal-NHC complexes utilized as (pre)catalysts in ester hydrogenation reactions.

Rare Au···H Interactions in Gold(I) Complexes of Bulky Phosphines Derived from 2,6-Dibenzhydryl-4-methylphenyl Core.

Gold(I) complexes of sterically demanding phosphines derived from 2,6-dibenzhydryl-4-methylphenyl core viz: 2,6-dibenzhydryl-N,N-bis((diphenylphosphane)-methyl)-4-methylaniline (1), (2,6-dibenzhydryl-4-methylphenyl)-diphenylphosphane (2), N-(2,6-dibenzhydryl-4-methylphenyl)-1,1-diphenylphosphanamine (3), and (2,6-dibenzhydryl-4-methylphenoxy)-diphenylphosphane (4) are described. The reaction of 1 with 2 equiv of [AuCl(SMe2)] in dichloromethane yielded [{AuCl}2{Ar*N(CH2PPh2)2}] (5), which on further treatment with 2 equiv of AgSbF6 and 1 equiv of 1 produced 12-membered dimeric complex [Au2{µ-(Ar*N(CH2PPh2)2)2}][(SbF6)2] (6). A similar reaction of 5 with AgSbF6 in CH3CN afforded [{Au(NCCH3)}2{Ar*N(CH2PPh2)2}][(SbF6)2] (7). Equimolar reactions of bulky phosphines 2, 3, and 4 with [AuCl(SMe2)] resulted in [AuCl(PPh2Ar*)] (8), [AuCl(PPh2NHAr*)] (9), and [AuCl(PPh2OAr*)] (10). Complexes 9 and 10 on treatment with AgSbF6 in CH3CN produced the cationic complexes [Au(NCCH3)(PPh2NHAr*)][(SbF6)] (11) and [Au(NCCH3)(PPh2OAr*)][(SbF6)] (12), respectively. The molecular structure of complex 6 revealed the presence of a strong intramolecular aurophilic interaction with a Au···Au distance of 2.9720(4) Å. Careful analysis of molecular structure of 5 revealed the presence of rare Au···H-C (sp3) interactions between the gold(I) atom and one of the methylene protons of -NCH2PPh2 groups. The solution 1H NMR signals of the methylene protons of 5 showed a considerable downfield shift (∼1 ppm) compared to that of the free ligand indicating their interactions (Au···H) with the Au atom. Complexes 8 and 10 also showed Au···H interactions in their molecular structures. The existence of the Au···H interaction was studied by variable temperature 1H NMR data in the case of complex 5 and further evinced by the QTAIM analysis in complexes 5, 8, and 10.

Catechol and 1,2,4,5-tetrahydroxybenzene functionalized cyclodiphosphazane ligands: synthesis, structural studies, and transition metal complexes.

Syntheses of two novel cyclodiphosphazane derivatives appended on catechol and 1,2,4,5-tetrahydroxy benzene, [{(µ-N(tBu)P)2(C6H4O2)}] (1) and [{((µ-N(tBu)P)2)2(µ-C6H2O4)}] (2), are described. Reactions of 1 with copper(i) halides led to the isolation of one-dimensional (1-D) and two-dimensional (2-D) coordination polymers, depending on the reaction conditions, metal-to-ligand ratio and CuX (X = Cl, Br or I) employed. The 1 : 1 reaction between 1 and CuCl yielded a 1-D coordination polymer [{(µ-N(tBu)P)2(C6H4O2)}{Cu(µ2-Cl)(NCCH3)}2]n (3) containing [Cu(µ2-Cl)]2 rhombus units. Similar reactions of 1 with CuBr and CuI produced rare 1-D coordination polymers [{(µ-N(tBu)P)2(C6H4O2)}{CuX(NCCH3)}]n (4, X = Br; 5, X = I) with discrete copper atoms linked by bridging cyclodiphosphazane ligands. However, the reactions of 1 with CuX (X = Cl, Br or I) in 1 : 2 molar ratios afforded 2-D coordination polymers [{(µ-N(tBu)P)2(C6H4O2)}2{Cu4(µ3-X)4}]n (6, X = Cl; 7, X = Br and 8, X = I) containing cuboids [Cu4(µ3-X)4] with 1 linking such units. Treatment of 1 with [RuCl2(η6-p-cymene)]2, [Rh(COD)Cl]2 and [Pd(η3-C3H5)Cl]2 in 1 : 1 molar ratios produced dinuclear complexes [{(µ-N(tBu)P)2(C6H4O2)}{RuCl2(η6-p-cymene)}2] (9), [{(µ-N(tBu)P)2(C6H4O2)}{RhCl(COD)}2] (10) and [{(µ-N(tBu)P)2(C6H4O2)}{PdCl(η3-C3H5)}2] (11), respectively. The reaction between 1 and [AuCl(SMe2)] in a 1 : 2 ratio yielded a dinuclear complex [{(µ-N(tBu)P)2(C6H4O2)}{AuCl}2] (12). The reactions of 2 with [RuCl2(η6-p-cymene)]2, [Rh(COD)Cl]2 and [Pd(η3-C3H5)Cl]2 in 1 : 2 molar ratios afforded tetranuclear complexes [{((µ-N(tBu)P)2)2(µ-C6H2O4)}{RuCl2(η6-p-cymene)}4] (13), [{((µ-N(tBu)P)2)2(µ-C6H2O4)}{RhCl(COD)}4] (14) and [{((µ-N(tBu)P)2)2(µ-C6H2O4)}{PdCl(η3-C3H5)}4] (15), respectively. The reaction of 2 with [AuCl(SMe2)] also afforded a tetranuclear complex [{((µ-N(tBu)P)2)2(µ-C6H2O4)}{AuCl}4] (16). In all these complexes, ligands preferred a bridged bidentate mode of coordination. Compounds 3-8 are the rare examples of 1-D and 2-D copper(i) coordination polymers containing cyclodiphosphazane ligands. The crystal structures of 2-8, and 15 were established by X-ray diffraction studies.

Sterically Demanding Phosphines with 2,6-Dibenzhydryl-4-methylphenyl Core: Synthesis of RuII, PdII, and PtII Complexes, and Structural and Catalytic Studies.

The synthesis of sterically demanding aminophosphine and phosphinite ligands Ar*NHPPh2 (1) and Ar*OPPh2 (2), based on 2,6-dibenzhydryl-4-methylphenyl core having bulky benzhydryl groups, and their RuII, PdII and PtII complexes is described. The reactions of 1 and 2 with [Ru(η6- p-cymene)Cl2]2 in 2:1 molar ratios produced mononuclear complexes [RuCl2{(η6- p-cymene)PPh2-EAr*}-κ1-P] (E = NH (3) and O (4)). Interestingly, complexes 3 and 4, upon refluxing in chlorobenzene, displace the p-cymene ring by one of the phenyl rings of side arms, forming rare η6-arene coordinated tethered complexes [RuCl2{(PPh2-EAr*)-κ1-P-η6-arene}] (E = NH (5) and O (6)). Treatment of 1 and 2 with [M(COD)Cl2] [M = Pd, Pt] in 2:1 ratios afforded mononuclear complexes [MCl2{(PPh2EAr*)-κ1-P}2] (E = NH, M= Pd (7), Pt (8); E = O, M= Pd (9), Pt (10)). Similarly, 1:1 reactions of 1 and 2 with [Pd(COD)Cl2] produced chloro-bridged dinuclear complexes [PdCl2{(PPh2EAr*)-κ1-P}]2 (E = NH (11) and O (12)), whereas [Pt(COD)Cl2] yielded only the mononuclear complexes 8 and 10. The reactions of 1 and 2 with [Pd(η3-C3H5)Cl]2 in 2:1 molar ratios produced the mononuclear complexes [PdCl{(η3-C3H5)(PPh2EAr*)-κ1-P}] (E = NH (13) and O (14)). Many of these complexes have been structurally characterized, which show C-H···π interactions between the methine hydrogen of the benzhydryl and with one of the carbon atoms of the phenyl ring attached to the phosphorus center. The complex [RuCl2{(η6- p-cymene)PPh2-NHAr*}-κ1-P] (3) shows very short C-H···π interactions of 2.36 Å; in addition, C-H···M interactions were observed between the methine hydrogen of one of the benzhydryl groups and palladium centers in 11 and 13. The tethered ruthenium complex 5 is found to be an excellent catalyst for the catalytic oxidation of various styrene derivatives.

1,2,3-Triazole based bisphosphine, 5-(diphenylphosphanyl)-1-(2-(diphenylphosphanyl)-phenyl)-4-phenyl-1H-1,2,3-triazole: an ambidentate ligand with switchable coordination modes.

The reaction of 1-(2-bromophenyl)-4-phenyl-1H-1,2,3-triazole (1) with Ph2PCl yielded bisphosphine, 5-(diphenylphosphanyl)-1-(2-(diphenylphosphanyl)phenyl)-4-phenyl-1H-1,2,3-triazole (2). Bisphosphine 2 exhibits ambidentate character in either the κ2-P,N or κ2-P,P coordination mode. Treatment of 2 with [M(CO)4(piperidine)2] (M = Mo and W) yielded κ2-P,N and κ2-P,P coordinated Mo0 and W0 complexes [M(CO)4(2)] [M = W-κ2-P,N (4); Mo-κ2-P,P (5); W-κ2-P,P (6)] depending on the reaction conditions. Formation and stability of κ2-P,P coordinated Mo0 and W0 complexes were assessed by time dependent 31P{1H} NMR experiments and DFT studies. The complex 4 on treatment with [AuCl(SMe2)] afforded the hetero-bimetallic complex [µ-PN,P-{o-Ph2P(C6H4){1,2,3-N3C(Ph)C(PPh2AuCl)}-κ2-P,N}W(CO)4] (7). The 1 : 1 reaction between 2 and [CpRu(PPh3)2Cl] yielded [(η5-C5H5)RuCl{o-Ph2P(C6H4){1,2,3-N3C(Ph)C(PPh2)}}-κ2-P,P] (8), whereas the similar reaction with [Ru(η6-p-cymene)Cl2]2 in a 2 : 1 molar ratio produced a cationic complex [(η6-p-cymene)RuCl{o-Ph2P(C6H4){1,2,3-N3C(Ph)C(PPh2)}}-κ2-P,N]Cl (9). Similarly, treatment of 2 with [M(COD)(Cl)2] (M = Pd and Pt) in a 1 : 1 molar ratio yielded PdII and PtII complexes [{o-Ph2P(C6H4){1,2,3-N3C(Ph)C(PPh2)}-κ2-P,P}PdCl2] (10) and [{o-Ph2P(C6H4){1,2,3-N3C(Ph)C(PPh2)}-κ2-P,P}PtCl2] (11). The reaction of 2 with 2 equiv. of [AuCl(SMe2)] afforded [Au2Cl2{o-Ph2P(C6H4){1,2,3-N3C(Ph)C(PPh2)}}-µ-P,P] (12). Most of the complexes have been structurally characterized. Palladium complex 10 shows excellent catalytic activity towards Cu-free Sonogashira alkynylation/cyclization reactions.

Coordination of bis(azol-1-yl)methane-based bisphosphines towards RuII, RhI, PdII and PtII: synthesis, structural and catalytic studies.

The coordination chemistry of bisphosphine ligands assembled on the five-membered heterocyclic platform of bis(azol-1-yl)methane viz.: bis(2-diphenylphosphinoimidazol-1-yl)methane (1), bis(5-diphenylphosphinopyrazol-1-yl)methane (2) and bis(5-diphenylphosphino-1,2,4-triazol-1-yl)methane (3) with RuII, RhI, PdII and PtII is described. The bisphosphines 1-3 react with elemental selenium to give the corresponding bis-selenoyl derivatives 4-6. The reactions of 1-3 with transition metal derivatives produce complexes with different coordination modes. Bisphosphine 1 showed a preference for the κ2-P,P mode of coordination, whereas bisphosphines 2 and 3, besides the κ2-P,P mode also showed a head-to-tail κ2-P,N coordination mode resulting in the formation of binuclear complexes [Rh2(COD)2{(CH2(1,2-C3H2N2PPh2)2)-κ2P,N}][BF4]2 (14), [Rh2(COD)2{(CH2(1,2,4-C2HN3PPh2)2)-κ2P,N}][BF4]2 (15), [Pd2(η3-C3H5)2{(CH2(1,2-C3H2N2PPh2)2)-κ2P,N}][BF4]2 (21) and [Pd2(η3-C3H5)2{(CH2(1,2,4-C2HN3PPh2)2)-κ2P,N}][BF4]2 (22). Several of these complexes have also been structurally characterized. The in situ generated RhI complex of bisphosphine 1 showed moderate to good selectivity in the hydroformylation of various styrene derivatives.