Platinum Oxoboryl Complexes as Substrates for the Formation of 1:1, 1:2, and 2:1 Lewis Acid–Base Adducts and 1,2‐Dipolar Additions

The oxoboryl complex trans‐[(Cy₃P)₂BrPt(B?O)] ( 2 ) reacts with the Group 13 Lewis acids EBr₃ (E=Al, Ga, In) to form the 1:1 Lewis acid–base adducts trans‐[(Cy₃P)₂BrPt(B?OEBr₃)] ( 6 – 8 ). This reactivity can be extended by using two equivalents of the respective Lewis acid EBr₃ (E=Al, Ga) to form the 2:1 Lewis acid–base adducts trans‐[(Cy₃P)₂(Br₃Al‐Br)Pt(B?OAlBr₃)] ( 18 ) and trans‐[(Cy₃P)₂(Br₃Ga‐Br)Pt(B?OGaBr₃)] ( 15 ). Another reactivity pattern was demonstrated by coordinating two oxoboryl complexes 2 to InBr₃, forming the 1:2 Lewis acid–base adduct trans‐[{(Cy₃P)₂BrPt(B?O)}₂InBr₃] ( 20 ). It was also possible to functionalize the B?O triple bond itself. Trimethylsilylisothiocyanate reacts with 2 in a 1,2‐dipolar addition to form the boryl complex trans‐[(Cy₃P)₂BrPt{B(NCS)(OSiMe₃)}] ( 27 ).     

Boryl‐Functionalized σ‐Alkynyl and Vinylidene Rhodium Complexes: Synthesis and Electronic Properties

The synthesis, reactivity, and properties of boryl‐functionalized σ‐alkynyl and vinylidene rhodium complexes such as trans‐[RhCl(?C?CHBMes₂)(PiPr₃)₂] and trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] are reported. An equilibrium was found to exist between rhodium vinylidene complexes and the corresponding hydrido σ‐alkynyl complexes in solution. The complex trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] (IMe=1,3‐dimethylimidazol‐2‐ylidene) was found to exhibit solvatochromism and can be quasireversibly oxidized and reduced electrochemically. Density functional calculations were performed to determine the reaction mechanism and to help rationalize the photophysical properties of trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂].     

Metalated 1, 3‐Azaphospholes: η1‐(1H‐1, 3‐Benzazaphosphole‐P)M(CO)5 and μ2‐[(1, 3‐Benzazaphospholide‐P)(cyclopentadienide)nickel] Complexes

1H‐1, 3‐Benzazaphospholes react with M(CO)₅(THF) (M = Cr, Mo, W) to give thermally and relatively air stable η¹‐(1H‐1, 3‐Benzazaphosphole‐P)M(CO)₅ complexes. The ¹H‐ and ¹³C‐NMR‐data are in accordance with the preservation of the phosphaaromatic π‐system of the ligand. The strong upfield ³¹P coordination shift, particularly of the Mo and W complexes, forms a contrast to the downfield‐shifts of phosphine‐M(CO)₅ complexes and classifies benzazaphospholes as weak donor but efficient acceptor ligands. Nickelocene reacts as organometallic species with metalation of the NH‐function. The resulting ambident 1, 3‐benzazaphospholide anions prefer a μ₂‐coordination of the η⁵‐CpNi‐fragment at phosphorus to coordination at nitrogen or a η³‐heteroallyl‐η⁵‐CpNi‐semisandwich structure. This is shown by characteristic NMR data and the crystal structure analysis of a η⁵‐CpNi‐benzazaphospholide. The latter is a P‐bridging dimer with a planar Ni₂P₂ ring and trans‐configuration of the two planar heterocyclic phosphido ligands arranged perpendicular to the four‐membered ring.     

Synthesis of Chloro Boryl Complexes by Oxidative Addition of B–Cl Bonds

Some platinum boryl complexes of the type trans‐[(Cy₃P)₂Pt(Cl){B(Cl)R}] ( 1 : R = NMe₂, 2 : R = Mes, 3 : R = tBu) were synthesized by oxidative addition of the corresponding dichloroboranes to [Pt(PCy₃)₂]. All the compounds were characterized by multinuclear NMR spectroscopy in solution. Furthermore, a single crystal analysis was acquired from 2 , that confirms the strong trans‐influence of this boryl ligand.     

The Challenge of Linear (E)‐Enones in the Rh‐Catalyzed,Asymmetric 1,4‐Addition Reaction of Phenylboronic Acid: A DFT Computational Analysis

Why are linear (E)‐enones such challenging substrates in the Rh‐catalyzed asymmetric arylation with boronic acids, which is one of the most important asymmetric catalysis methods? DFT computations show that these substrates adopt a specific conformation in which the largest substituent is antiperiplanar to Rh^I π‐complexed with the C?C bond within the enantioselectivity‐determining carborhodation transition state. Additionally, for such structures, there is a strong, but not exclusive, preference for s‐cis enone conformation. This folding minimizes steric interactions between the substrate and the ligand, and hence reduces the enantioselectivity. This idea is further confirmed by investigating three computation‐only substrate “probes”, one of which is capable of double asymmetric induction, and a recent computationally designed 1,5‐diene ligand. On average, excellent agreement between predicted and experimental enantioselectivity was attained by a three‐pronged approach: 1) thorough conformational search within ligand and substrate subunits to locate the most preferred carborhodation transition state; 2) including dispersion interaction and long‐range corrections by SMD/ωB97xD/DGDZVP level of theory; and 3) full substrate and ligand modeling. Based on the results, a theory‐enhanced enantioselectivity model that is applicable to both chiral diene and diphosphane ligands is proposed.     

Resonance Character of Copper/Silver/Gold Bonding in Small Molecule⋅⋅⋅MX (X=F,Cl, Br,CH3, CF3) Complexes

The resonance character of Cu/Ag/Au bonding is investigated in B???M?X (M=Cu, Ag, Au; X=F, Cl, Br, CH₃, CF₃; B=CO, H₂O, H₂S, C₂H₂, C₂H₄) complexes. The natural bond orbital/natural resonance theory results strongly support the general resonance‐type three‐center/four‐electron (3c/4e) picture of Cu/Ag/Au bonding, B:M?X?B⁺?M:X^?, which mainly arises from hyperconjugation interactions. On the basis of such resonance‐type bonding mechanisms, the ligand effects in the more strongly bound OC???M?X series are analyzed, and distinct competition between CO and the axial ligand X is observed. This competitive bonding picture directly explains why CO in OC???Au?CF₃ can be readily replaced by a number of other ligands. Additionally, conservation of the bond order indicates that the idealized relationship b_B???M+b_MX=1 should be suitably generalized for intermolecular bonding, especially if there is additional partial multiple bonding at one end of the 3c/4e hyperbonded triad.     

Reactivity of B‐Xanthyl N‐Heterocyclic Carbene‐Boranes

The synthesis and reactivity of mono‐ and bis‐S‐xanthyl NHC‐boranes is reported. The new NHC‐boranes are prepared through nucleophilic exchange at boron from either mono‐ or bis‐triflyl NHC‐boranes, themselves obtained by protolysis of the NHC‐BH₃ starting compounds. The B?H bond of the S‐xanthyl NHC‐boranes can be cleaved both homolytically and heterolytically, albeit the latter is more synthetically useful. The S‐xanthyl NHC‐boranes can reduce both aldehydes and imines. The B?S bond can also be cleaved homolytically. Under UV irradiation, the S‐xanthyl NHC‐boranes generate NHC‐boryl radicals that can initiate radical polymerizations of acrylates.     

fac‐Tricarbonyl[bis(2‐pyridylmethyl) ether‐N,O,N′]molybdenum(0)

Reaction of bis(2‐pyridylmethyl) ether with [Mo(CO)₃­(Me­CN)₃] in MeCN gives the title compound, [Mo(C₁₂H₁₂‐N₂O)(CO)₃], (I), as a yellow crystalline product. Compound (I) has been characterized by ¹H NMR and IR spectroscopy, and single‐crystal X‐ray crystallography. In contrast with other examples of low‐valent early transition metal complexes of ethers, the ether linkage of (I) appears relatively inert. Nevertheless, the weak donor property of the ether ligand is evidenced by a trans effect manifested as a short Mo—CO bond length for the carbonyl ligand trans to the ether ligand.     

Single Oxygen‐Atom Insertion into PB Bonds: On‐ and Off‐Metal Transformation of a Borylphosphine into a Borylphosphinite

An oxygen atom is selectively inserted into the P?B bond of a borylphosphine ( L₁ ) by reaction with Me₃NO to afford the corresponding borylphosphinite ( L₂ ). This transformation can also be effected when L₁ is coordinated to rhodium. The ν(CO) values for trans‐[RhCl(CO)(L)₂] reveal very different electronic properties for coordinated L₁ and L₂ which translate into the strikingly different performances of the complexes [RhCl(L)(cod)] (L= L₁ or L₂ , cod=1,5‐cyclooctadiene) in hydrosilylation and hydroboration catalysis.     

Ruthenium Complexes of Tripodal Ligands with Pyridine and Triazole Arms: Subtle Tuning of Thermal,Electrochemical, and Photochemical Reactivity

Electrochemical and photochemical bond‐activation steps are important for a variety of chemical transformations. We present here four new complexes, [Ru(Lⁿ)(dmso)(Cl)]PF₆ ( 1 – 4 ), where Lⁿ is a tripodal amine ligand with 4?n pyridylmethyl arms and n?1 triazolylmethyl arms. Structural comparisons show that the triazoles bind closer to the Ru center than the pyridines. For L², two isomers (with respect to the position of the triazole arm, equatorial or axial), trans‐ 2 _sym and trans‐ 2 _un, could be separated and compared. The increase in the number of the triazole arms in the ligand has almost no effect on the Ru^II/Ru^III oxidation potentials, but it increases the stability of the Ru?S_dmso bond. Hence, the oxidation waves become more reversible from trans‐ 1 to trans‐ 4 , and whereas the dmso ligand readily dissociates from trans‐ 1 upon heating or irradiation with UV light, the Ru?S bond of trans‐ 4 remains perfectly stable under the same conditions. The strength of the Ru?S bond is not only influenced by the number of triazole arms but also by their position, as evidenced by the difference in redox behavior and reactivity of the two isomers, trans‐ 2 _sym and trans‐ 2 _un. A mechanistic picture for the electrochemical, thermal, and photochemical bond activation is discussed with data from NMR spectroscopy, cyclic voltammetry, and spectroelectrochemistry.     

Crystallographic report: (η5‐C5Me5)Fe(CO)2(BOCH2CH2CH2O): an organoiron complex containing the (trimethyleneglycolato)boryl ligand

The structure of the pentamethylcyclopentadienyliron boryl complex (η⁵‐C₅Me₅)Fe(CO)₂B(tmg) (tmg = trimethylene glycolato, ? OCH₂CH₂CH₂O? ), reveals a planar three‐coordinate boryl ligand and an Fe? B distance significantly in excess of that reported for related catecholboryl complexes. Copyright © 2005 John Wiley & Sons, Ltd.     

Insertion Reactions of 1,2‐Disubstituted Olefins with an α‐Diimine Palladium(II) Complex

The migratory insertions of cis or trans olefins CH(X)?CH(Me) (X = Ph, Br, or Et) into the metal–acyl bond of the complex [Pd(Me)(CO)(ⁱPr₂dab)]⁺ [B{3,5‐(CF₃)₂C₆H₃}₄]^? ( 1 ) (ⁱPr₂dab = 1,4‐diisopropyl‐1,4‐diazabuta‐1,3‐diene = N,N′‐(ethane‐1,2‐diylidene)bis[1‐methylethanamine]) are described (Scheme 1). The resulting five‐membered palladacycles were characterized by NMR spectroscopy and X‐ray analysis. Experimental data reveal some important aspects concerning the regio‐ and stereochemistry of the insertion process. In particular, the presence of a Ph or Br substituent at the alkene leads to the formation of highly regiospecific products. Moreover, in all cases, the geometry of the substituents in the formed palladacycle was the same as in the starting olefin, as a consequence of a cis addition of the Pd–acyl fragment to the C?C bond. Reaction with CO and MeOH of the five‐membered complex derived from trans‐β‐methylstyrene (= [(1E)‐prop‐1‐enyl]benzene) insertion, yielded the 2,3‐substituted γ‐keto ester 9 with an (2RS,3SR)‐configuration (Scheme 3).     

Highly Active,Low‐Valence Molybdenum‐ and Tungsten‐Amide Catalysts for Bifunctional Imine‐Hydrogenation Reactions

The reactions of [M(NO)(CO)₄(ClAlCl₃)] (M=Mo, W) with (iPr₂PCH₂CH₂)₂NH, (PN^HP) at 90 °C afforded [M(NO)(CO)(PN^HP)Cl] complexes (M=Mo, 1a ; W, 1b ). The treatment of compound 1a with KOtBu as a base at room temperature yielded the alkoxide complex [Mo(NO)(CO)(PN^HP)(OtBu)] ( 2a ). In contrast, with the amide base Na[N(SiMe₃)₂], the PN^HP ligand moieties in compounds 1a and 1b could be deprotonated at room temperature, thereby inducing dehydrochlorination into amido complexes [M(NO)(CO)(PNP)] (M=Mo, 3a ; W, 3b ; PNP=(iPr₂PCH₂CH₂)₂N)). Compounds 3a and 3b have pseudo‐trigonal‐bipyramidal geometries, in which the amido nitrogen atom is in the equatorial plane. At room temperature, compounds 3a and 3b were capable of adding dihydrogen, with heterolytic splitting, thereby forming pairs of isomeric amine‐hydride complexes [Mo(NO)(CO)H(PN^HP)] ( 4a(cis) and 4a(trans) ) and [W(NO)(CO)H(PN^HP)] ( 4b(cis) and 4b(trans) ; cis and trans correspond to the position of the H and NO groups). H₂ approaches the Mo/W?N bond in compounds 3a , 3b from either the CO‐ligand side or from the NO‐ligand side. Compounds 4a(cis) and 4a(trans) were only found to be stable under a H₂ atmosphere and could not be isolated. At 140 °C and 60 bar H₂, compounds 3a and 3b catalyzed the hydrogenation of imines, thereby showing maximum turnover frequencies (TOFs) of 2912 and 1120 h^?1, respectively, for the hydrogenation of N‐(4 ‐ methoxybenzylidene)aniline. A Hammett plot for various para‐substituted imines revealed linear correlations with a negative slope of ?3.69 for para substitution on the benzylidene side and a positive slope of 0.68 for para substitution on the aniline side. Kinetics analysis revealed the initial rate of the hydrogenation reactions to be first order in c(cat.) and zeroth order in c(imine). Deuterium kinetic isotope effect (DKIE) experiments furnished a low k_H/k_D value (1.28), which supported a Noyori‐type metal–ligand bifunctional mechanism with H₂ addition as the rate‐limiting step.     

On the Synthesis,Characterization and Reactivity of N‐Heteroaryl–Boryl Radicals,a New Radical Class Based on Five‐Membered Ring Ligands

The synthesis and physical characterization of a new class of N‐heterocycle–boryl radicals is presented, based on five membered ring ligands with a N(sp²) complexation site. These pyrazole–boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; B?H) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(B?H) by the fine tuning of the N‐heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O₂, oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N‐heterocycle–boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions.     

Back‐bonding in Organonickel Complexes with Terpyridine Ligands – A Structural Approach

A series of organonickel complexes [(R′terpy)Ni(aryl)]X (R′terpy = derivatives of 2,2′;6′,6″‐terpyridine; R′ = 4‐H, 4‐Cl, 4‐Tol and 4,4′,4″‐^tBu₃; aryl = 2,6‐dimethylphenyl = Xyl or 2,4,6‐trimethylphenyl = Mes; X = Br or PF₆) have been prepared and characterized. The crystal structures exhibit a number of intermolecular H bond type interactions, but the structure determining force seems to be the packing of the aryl co‐ligands. The molecules reveal rather undistorted square planar coordination with a N₃C ligand set, the central Ni–N bond being remarkably short, despite the expected strong trans influence of the aryl co‐ligands. The long‐wavelength absorptions were assigned to charge transfer transitions. No emission is observed at ambient temperature in the solid and in solution and at low temperature in glasses.     

Synthesis,Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.     

Coordination chemistry of organometallic polydentate ligand Reactive chemistry of the tridentate ligand trans‐Fe(Ph2PQu‐P)2‐(CO)3 [PhzPQu=2‐diphenyl‐phosphino‐4‐methylquinoline] and molecular structure of [Fe(CO)3(μ‐Ph2PQu)2HgI]+ [HgI3]

Reaction of a new type of bidentate ligand PhPQu [PhPQu = 2‐diphenylphosphino‐4‐methylquinoline] with Fe(CO)₅ in butanol gave trans‐Fe(FpPQu‐P)(CO)₃ (1). Compound 1, which can act as a neutral tridentate organometallic ligand, was reacted with I B, II B metal compounds and a rhodium complex to give six binuclear complexes with Fe? M bonds, Fe(CO)₃ (μ‐Ph₂PQu)MX_n (2–7) [M= Zn(II), Cd(II), Hg(II), Cu(I), Ag(I), Rh(I)], and an ion‐pair complex [Fe(CO)₃ (μ‐Ph₂PQu)₂HgI][HgI₃]^? (8). The structure of 8 was determined by X‐ray crystallography. Complex 8 crystallizes in the space group P‐1 with a = 1.0758(3), b = 1.6210(4), c=1.7155(4)nm; a=75.60(2), β=71.81(2), γ=81.78(2)° and Z = 2 and its structure was refined to give agreement factors of R=0.050 and R_w = 0.057. The Fe‐Hg bond distance is 0.2536nm.     

Conversion of trans‐Diboran(4)yl Platinum Complexes into Their cis‐Bisboryl Analogues

The diboran(4)yl trans‐[(iPr₃P)₂Pt(Br){B(NMe₂)B(NMe₂)Br}] ( 1 ) is readily converted into its cis‐bisboryl analogues 2 and 3 by reaction with the chelating bisphosphines 1,2‐bis(dicyclohexylphosphino)ethane (dcpe) and 1,1‐bis(dicyclohexylphosphino)methane (dcpm), respectively. A plausible mechanism of this transformation consists of a sequence of reductive diborane(4) elimination and subsequent reoxidative addition of its B? B bond to the low‐valent platinum centers. Thus, the forced cis configuration of the phosphine ligands induces a change in the preferred reaction site of the diborane(4) with respect to oxidative addition. The reactions proceed with high selectivities, and the cis‐bisboryl complexes 2 and 3 were isolated in moderate yields (55 and 46 %). Moreover, their identity was clearly verified by NMR spectroscopy and X‐ray diffraction studies.     

Solid‐State Thermolysis of a fac‐Rhenium(I) Carbonyl Complex with a Redox Non‐Innocent Pincer Ligand

The development of rhenium(I) chemistry has been restricted by the limited structural and electronic variability of the common pseudo‐octahedral products fac‐[ReX(CO)₃L₂] (L₂=α‐diimine). We address this constraint by first preparing the bidentate bis(imino)pyridine complexes [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₃X] (X=Cl 2 , Br 3 ), which were characterized by spectroscopic and X‐ray crystallographic means, and then converting these species into tridentate pincer ligand compounds, [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₂X] (X=Cl 4 , Br 5 ). This transformation was performed in the solid‐state by controlled heating of 2 or 3 above 200 °C in a tube furnace under a flow of nitrogen gas, giving excellent yields (≥95 %). Compounds 4 and 5 define a new coordination environment for rhenium(I) carbonyl chemistry where the metal center is supported by a planar, tridentate pincer‐coordinated bis(imino)pyridine ligand. The basic photophysical features of these compounds show significant elaboration in both number and intensity of the d–π* transitions observed in the UV/Vis spec tra relative to the bidentate starting materials, and these spectra were analyzed using time‐dependent DFT computations. The redox nature of the bis(imino)pyridine ligand in compounds 2 and 4 was examined by electrochemical analysis, which showed two ligand reduction events and demonstrated that the ligand reduction shifts to a more positive potential when going from bidentate 2 to tridentate 4 (+160 mV for the first reduction step and +90 mV for the second). These observations indicate an increase in electrostatic stabilization of the reduced ligand in the tridentate conformation. Elaboration on this synthetic methodology documented its generality through the preparation of the pseudo‐octahedral rhenium(I) triflate complex [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₂OTf] ( 7 , 93 % yield).