Cationic terminal gallylene complexes by halide abstraction: coordination chemistry of a valence isoelectronic analogue of CO and N2

While N(2) and CO have played central roles in developing models of electronic structure, and their interactions with transition metals have been widely investigated, the valence isoelectronic diatomic molecules EX (E = group 13 element, X = group 17 element) have yet to be isolated under ambient conditions, either as the "free" molecule or as a ligand in a simple metal complex. As part of a program designed to address this deficiency, together with wider issues of the chemistry of cationic systems [L(n)M(ER)](+) (E = B, Al, Ga; R = aryl, amido, halide), we have targeted complexes of the type [L(n)M(GaX)](+). Halide abstraction is shown to be a viable method for the generation of mononuclear cationic complexes containing gallium donor ligands. The ability to isolate tractable two-coordinate products, however, is strongly dependent on the steric and electronic properties of the metal/ligand fragment. In the case of complexes containing ancillary pi-acceptor ligands such as CO, cationic complexes can only be isolated as base-trapped adducts, even with bulky aryl substituents at gallium. Base-free gallylene species such as [Cp*Fe(CO)(2)(GaMes)](+) can be identified only in the vapor phase by electrospray mass spectrometry experiments. With bis(phosphine) donor sets at the metal, the more favorable steric/electronic environment allows for the isolation of two-coordinate ligand systems, even with halide substituents at gallium. Thus, [Cp*Fe(dppe)(GaI)](+)[BAr(f)(4)](-) (9) can be synthesized and shown crystallographically to feature a terminally bound GaI ligand; 9 represents the first experimental realization of a complex containing a valence isoelectronic group 13/group 17 analogue of CO and N(2). DFT calculations reveal a relatively weakly bound GaI ligand, which is confirmed experimentally by the reaction of 9 with CO to give [Cp*Fe(dppe)(CO)](+)[BAr(f)(4)](-). In the absence of such reagents, 9 is stable for weeks in fluorobenzene solution, presumably reflecting (i) effective steric shielding of the gallium center by the ancillary phosphine and Cp* ligands; (ii) a net cationic charge which retards the tendency toward dimerization found for putative charge neutral systems; and (iii) (albeit relatively minor) population of the LUMOs of the GaI molecule through pi overlap with the HOMO and HOMO-2 of the [Cp*Fe(dppe)](+) fragment.     

Ozonic acid and its ionic salts: ab initio probing of the O(4)(2)(-) dianion

The pyramidal O(4)(2)(-) dianion is valence isoelectronic to the well-known ClO(3)(-) and SO(3)(2)(-) anions, and yet it has not been observed. The synthesis of any molecule containing such a dianion would represent a major breakthrough in making molecules containing more than three covalently bound oxygen atoms. We found that the parent H(2)O(4) ozonic acid is unstable in the form of the valence isoelectronic sulfurous acid H(2)SO(3). Our quantum chemical probing of the Li(2)O(4) ionic salt molecule is inconclusive. However, we found that the specially designed FLi(3)O(4) gas phase molecule is a true metastable species and could be considered as the first molecule containing the O(4)(2)(-) dianion. Our theoretical prediction of the first compound containing the tetraatomic covalently bound O(4)(2)(-) dianion opens the possibility to even more oxygen rich compounds, which will have a great potential as high density oxygen storage.     

Theoretical characterization of the BN and BP coronenes by IR, Raman, and UV-VIS spectra

Boron-nitrogen coronene (BNC) and boron-phosphorous coronene (BPC), not yet synthesized molecules and of possible interest for material science, are composed of six condensed rings of borazine and boraphosphabenzene molecules, respectively. They are similar to the carbon coronene molecule (CC). Moreover, CC and BNC are isoelectronic and the BPC is formally isoelectronic with respect to other coronenes, if only the valence electrons are considered. In this work, the BNC and BPC were theoretically characterized using IR, Raman, and UV-VIS spectroscopies. The coronenes studied have D(6h) and D(3h) symmetries for carbon and boron compounds, respectively. The calculated vibrational and electronic spectra for the CC are in good agreement with the experimental data, indicating that the calculations for BNC and BPC will be useful to identify these compounds, when synthesized. The main vibrational modes of the CC, BNC, and BPC are correlated. However, the BPC vibrational frequencies are substantially lower than the CC and BNC ones. The electronic ground state studies showed that the BPC has intermediate characteristics between the CC and BNC.     

ESR evidence for the formation of the ring-opened cation CH2OCH2± from ethylene oxide

ESR results show that the radical cation formed from ethylene oxide in the solid state is the ring-opened 2-oxa-trimethylene cation with a symmetrical (C_2ν) planar structure similar to that of the isoelectronic allyl radical. In contrast, the trimethylene oxide radical cation retains the ring structure of the parent molecule and its ESR parameters are characteristic of an oxygen-centred species.     

Isoelectronic series: a fundamental periodic property

The usefulness of isoelectronic series (same number of total electrons and atoms and of valence electrons) across Periods is often overlooked. Here we show the ubiquitousness of isoelectronic sets by means of matrices, arrays, and sequential series. Some of these series have not previously been identified. In addition, we recommend the use of the term valence-isoelectronic for species which differ in the number of core electrons and pseudo-isoelectronic for matching (n) and (n + 10) species.     

Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b.2CI were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b.2Br or a six-step sequence for propylene dications 7a,b.2Br in > or = 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]+ (25-26) complexes with the same ligand bridge were synthesized. [PdX2bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]2 and micro-bis(carbene)(AgX)2 complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd2Cl2(allyl)2(micro-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]+ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]+ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF6 complexes were inactive as catalytic precursors in the allylic substitution reaction.     

Kinetically locked, trinuclear Ru(II) metallo-macrocycles--synthesis, electrochemical, and optical properties

Using a [Ru(II)([9]aneS3)] templating moiety, kinetically-locked, metallomacrocycles incorporating adenine based ligands have been synthesised through self-assembly. The kinetically robust nature of these structures is confirmed by electrochemical studies: each can be reversibly oxidised in a four-member redox series, containing two formally mixed valence states. Unusually, the electrochemically derived comproportionation constants for these mixed valence states are very different, suggesting that intermetallic coupling differs between the two states. Spectroelectochemistry studies confirm that while the [Ru(II)2Ru(III)] state is valence localised, the [Ru(II)Ru(III)2] state is electronically delocalised. Mechanisms by which this switching effect could occur, which involve the unusual connectivities in these mixed valence species, are presented.     

Nucleophilic Addition to (η3-Allyl)Fe(CO)4 Cations with Functionalized Zinc-Copper Reagents RCu(Znl)CN

Regioselectivity of the addition of the highly functionalized zinc-copper reagents to (η³-allyl)Fe(CO)₄ cationic salts was studied. For 1,1-disubstituted allyl cation 1, the zinc-copper reagents added predominantly at the unsubstituted terminus. For 1,1,2-trisubstituted allyl cation 2, reactive zinc-copper reagents attacked mainly at the unsubstituted terminus while less reactive zinc-copper reagents added to a coordinated CO ligand. For 1,1,3-trisubstituted allyl cation 3, the addition occurred at both the less substituted allyl terminus and a coordinated CO ligand.     

Competing Reaction Pathways in Heterogeneously Catalyzed Hydrogenation of Allyl Cyanide: The Chemical Nature of Surface Species

We present a mechanistic study on the formation of an active ligand layer over Pd(111), turning the catalytic surface highly active and selective in partial hydrogenation of an α,β-unsaturated aldehyde acrolein. Specifically, we investigate the chemical composition of a ligand layer consisting of allyl cyanide deposited on Pd(111) and its dynamic changes under the hydrogenation conditions. On pristine surface, allyl cyanide largely retains its chemical structure and forms a layer of molecular species with the CN bond oriented nearly parallel to the underlying metal. In the presence of hydrogen, the chemical composition of allyl cyanide strongly changes. At 100 K, allyl cyanide transforms to unsaturated imine species, containing the C=C and C=N double bonds. At increasing temperatures, these species undergo two competing reaction pathways. First, the C=C bond become hydrogenated and the stable N-butylimine species are produced. In the competing pathway, the unsaturated imine reacts with hydrogen to fully hydrogenate the imine group and produce butylamine. The latter species are unstable under the hydrogenation reaction conditions and desorb from the surface, while the N-butylimine adsorbates formed in the first reaction pathway remain adsorbed and act as an active ligand layer in selective hydrogenation of acrolein.     

A group 13/group 17 analogue of CO and N2: coordinative trapping of the GaI molecule

The first row diatomic molecules N2 and CO have played central roles in developing fundamental models of electronic structure, and their interactions with transition metals have been widely investigated. By contrast, the valence isoelectronic molecules EX (E = group 13 element, X = group 17 element) have yet to be isolated under ambient conditions, either as the "free" molecule or as a simple metal complex. Here, we find that iodide abstraction from Cp*Fe(dppe)(GaI2) yields the stable complex [Cp*Fe(dppe)(GaI)]+ which features a terminally bound GaI ligand, characterized by a near linear (but readily deformed) Fe-Ga-I geometry and by very short Fe-Ga and Ga-I distances. Chemical and computational evidence reveal a relatively weak metal-ligand bond similar in strength to that found for dinitrogen analogues.     

Mononuclear homoleptic allyl complexes of the first row transition metals: species with unusual metal electronic configurations

A number of evanescent unsubstituted homoleptic allyl derivatives M(C(3)H(5))(n) of the first row transition metals have been reported in the literature. In addition, the much more thermally stable silylated derivatives M[C(3)H(3)(SiMe(3))(2)](2) (M = Cr, Fe, Co, Ni) are reported to survive vacuum sublimation without significant decomposition. In this connection, the complete series of homoleptic allyl derivatives M(C(3)H(5))(n) (n = 2, 3; M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni) have been studied theoretically using density functional theory. In most of the lowest energy predicted M(C(3)H(5))(n) structures all of the allyl groups are bonded as trihapto η(3)-C(3)H(5) ligands and the metals have considerably less than the normally favored 18-electron configuration. Such ligands can be considered formally as bidentate ligands with the metal atom connected to the centers of the two C-C bonds of the η(3)-C(3)H(5) group. The later transition metal diallyls M(C(3)H(5))(2) (M = Cr, Mn, Fe, Co, Ni) form two stereoisomers of similar relative energies, namely the C(2h) staggered isomer and the C(2v) eclipsed isomer with the orientation of the η(3)-C(3)H(5) groups corresponding to square planar metal coordination of the bidentate η(3)-C(3)H(5) ligands. The staggered and eclipsed Ni(C(3)H(5))(2) isomers have been observed experimentally by NMR. Less symmetrical M(C(3)H(5))(2) structures are found for the earlier transition metals Sc, Ti, and V in which the orientation of the allyl groups corresponds to tetrahedral metal coordination. The triallylmetal derivatives M(C(3)H(5))(3) are predicted to be thermodynamically viable with respect to allyl loss to give the corresponding diallylmetal derivatives, except for triallylnickel. The lowest energy Ni(C(3)H(5))(3) structure has two trihaptoallyl ligands and one monohaptoallyl ligand, whereas the lowest energy Mn(C(3)H(5))(3) structures have only one trihaptoallyl ligand and two monohaptoallyl ligands. Otherwise, the M(C(3)H(5))(3) complexes have structures with three trihaptoallyl ligands corresponding formally to octahedral metal coordination. The M(C(3)H(5))(3) complexes (M = Cr, Co) thus correspond to a well-known series of "classical" octahedral coordination complexes, namely, those of the d(3) Cr(III) and the d(6) Co(III), respectively.     

Pd-Catalyzed allylic alkylation of secondary nitroalkanes

A Pd-catalyzed allylic alkylation of secondary nitroalkanes, using a catalytic amount of external base, was developed. Simple allyl carbonate and monosubstituted allyl carbonates were used as electrophiles, and bulky secondary nitroalkanes were used as nucleophiles. This is the first catalytic allylic alkylation of bulky secondary nitroalkanes, such as 2-nitroheptane. The use of the strong base DBU in the aprotic polar solvent DMSO is a key in realizing the high reactivity. In an attempt to develop an asymmetric reaction, 2-aryloxazoline ligand PHOX L1 gave excellent results for inducing chirality at the π-allyl moiety. As for asymmetric induction at the α-position of the NO₂ functionality, the free OH-group containing 2-aryloxazoline ligand L4 showed moderate selectivity.     

Arylative Intramolecular Allylation of Ketones with 1,3‐Enynes Enabled by Catalytic Alkenyl‐to‐Allyl 1,4‐Rhodium(I) Migration

Alkenyl‐to‐allyl 1,4‐rhodium(I) migration enables the generation of nucleophilic allylrhodium(I) species by remote C−H activation. This new mode of reactivity was employed in the diastereoselective reaction of arylboron reagents with substrates containing a 1,3‐enyne tethered to a ketone, to give products containing three contiguous stereocenters. The products can be obtained in high enantioselectivities using a chiral sulfur‐alkene ligand.     

From strong van der Waals complexes to hydrogen bonding: From CO⋯H2O to CS⋯H2O and SiO⋯H2O complexes

Structures and interaction energies of complexes valence isoelectronic to the important CO?H(2)O complex, namely SiO?H(2)O and CS?H(2)O, have been studied for the first time using high-level ab initio methods. Although CO, SiO, and CS are valence isoelectronic, the structures of their complexes with water differ significantly, owing partially to their widely varied dipole moments. The predicted dissociation energies D(0) are 1.8 (CO?H(2)O), 2.7 (CS?H(2)O), and 4.9 (SiO?H(2)O) kcal∕mol. The implications of these results have been examined in light of the dipole moments of the separate moieties and current concepts of hydrogen bonding. It is hoped that the present results will spark additional interest in these complexes and in the general non-covalent paradigms they represent.     

An Allyl Uranium(IV) Sandwich Complex: Are ϕ Bonding Interactions Possible?

A method to explore head-to-head ϕ back-bonding from uranium f-orbitals into allyl π* orbitals has been pursued. Anionic allyl groups were coordinated to uranium with tethered anilide ligands, then the products were investigated by using NMR spectroscopy, single-crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N-((dimethyl)prop-2-enylsilyl)-2,6-diisopropylaniline (LH), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, thereby highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI₂(L1)₂ ( 1 ), which was synthesized by using the mono-deprotonated ligand [K][N-((dimethyl)prop-2-enyl)silyl)-2,6-diisopropylanilide] (L1). The desired homoleptic sandwich complex U[L2]₂ ( 2 ) was prepared from all three ligand precursors, but the most consistent results came from using the dipotassium salt of the doubly deprotonated ligand [K]₂[N-((dimethyl)propenidesilyl)-2,6-diisopropylanilide] (L2). This allyl-based sandwich complex was studied by using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (ϕ) back-bonding. The bonding between U^IV and the allyl fragments is best described as ligand-to-metal electron donation from a two carbon fragment-localized electron density into empty f-orbitals.     

异核金属多重键配合物Cp_2MM'(μ-C_8H_8)的理论研究

研究了双核金属多重键配合物Cp2MM'(μ-C8H8)(MM'=ScMn,TiCr,ScCo,TiFe,VMn,VV,CrCr)的结构和成键模式.计算结果表明,对于28价电子体系,Cp2V2(μ-C8H8)基态为含V-V三重键的三态构型,其等电子体Cp2TiCr(μ-C8H8)为Ti-Cr四重键的单态,等电子体Cp2ScMn(μ-C8H8)为Sc-Mn三重键的单态.对于30价电子体系,Cp2Cr2(μ-C8H8)基态为含Cr-Cr三重键的单态,等电子体Cp2VMn(μ-C8H8)为含V-Mn单键的三态,等电子体Cp2ScCo(μ-C8H8)和Cp2TiFe(μ-C8H8)为含Sc-Co和Ti-Fe双键的单态.在三态Cp2MM'(μ-C8H8)中,两个金属原子多为17电子构型,而单态结构中两种金属原子多分别为16和18电子构型.     

Exploring the Limits of Transition‐Metal Fluorination at High Pressures

Fluorination is a proven method for challenging the limits of chemistry, both structurally and electronically. Here we explore computationally how pressures below 300 GPa affect the fluorination of several transition metals. A plethora of new structural phases are predicted along with the possibility for synthesizing four unobserved compounds: TcF₇, CdF₃, OsF₈, and IrF₈. The Ir and Os octaflourides are both predicted to be stable as quasi‐molecular phases with an unusual cubic ligand coordination, and both compounds formally correspond to a high oxidation state of +8. Electronic‐structure analysis reveals that otherwise unoccupied 6p levels are brought down in energy by the combined effects of pressure and a strong ligand field. The valence expansion of Os and Ir enables ligand‐to‐metal F 2p→M 6p charge transfer that strengthens M?F bonds and decreases the overall bond polarity. The lower stability of IrF₈, and the instability of PtF₈ and several other compounds below 300 GPa, is explained by the occupation of M?F antibonding orbitals in octafluorides with a metal‐valence‐electron count exceeding 8.     

Uncovering a CF3 Effect on X-ray Absorption Energies of [Cu(CF3)4]− and Related Copper Compounds by Using Resonant Diffraction Anomalous Fine Structure (DAFS) Measurements**

Understanding the electronic structures of high-valent metal complexes aids the advancement of metal-catalyzed cross coupling methodologies. A prototypical complex with formally high valency is [Cu(CF₃)₄]⁻ ( 1 ), which has a formal Cu(III) oxidation state but whose physical analysis has led some to a Cu(I) assignment in an inverted ligand field model. Recent examinations of 1 by X-ray spectroscopies have led previous authors to contradictory conclusions, motivating the re-examination of its X-ray absorption profile here by a complementary method, resonant diffraction anomalous fine structure (DAFS). From analysis of DAFS measurements for a series of seven mononuclear Cu complexes including 1 , here it is shown that there is a systematic trifluoromethyl effect on X-ray absorption that blue shifts the resonant Cu K-edge energy by 2–3 eV per CF₃, completely accounting for observed changes in DAFS profiles between formally Cu(III) complexes like 1 and formally Cu(I) complexes like (Ph₃P)₃CuCF₃ ( 3 ). Thus, in agreement with the inverted ligand field model, the data presented herein imply that 1 is best described as containing a Cu(I) ion with dⁿ count approaching 10.