Strong Donor Properties Research Articles

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand.

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, has been synthesized and characterized. This new ligand's strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order perturbative energy analysis, are at par with those of the well-studied alkyl-substituted tris(imidazol-2-ylidene)borates, which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron-withdrawing trifluoromethyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups toward oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition-metal species.

Chemistry of Compounds Based on 1,2,3-Triazolylidene-Type Mesoionic Carbenes.

Mesoionic carbenes (MICs) of the 1,2,3-triazolylidene type have established themselves as a popular class of compounds over the past decade. Primary reasons for this popularity are their modular synthesis and their strong donor properties. While such MICs have mostly been used in combination with transition metals, the past few years have also seen their utility together with main group elements. In this paper, we present an overview of the recent developments on this class of compounds that include, among others, (i) cationic and anionic MIC ligands, (ii) the donor/acceptor properties of these ligands with a focus on the several methods that are known for estimating such donor/acceptor properties, (iii) a detailed overview of 3d metal complexes and main group compounds with these MIC ligands, (iv) results on the redox and photophysical properties of compounds based on MIC ligands, and (v) an overview on electrocatalysis, redox-switchable catalysis, and small-molecule activation to highlight the applications of compounds based on MIC ligands in contemporary chemistry. By discussing several aspects from the synthetic, spectroscopic, and application point of view of these classes of compounds, we highlight the state of the art of compounds containing MICs and present a perspective for future research in this field.

Adducts of the Parent Boraphosphaketene H2 BPCO and their Decarbonylative Insertion Chemistry.

The first examples of Lewis base adducts of the parent boraphosphaketene (H2B‐PCO) and their cyclodimers are prepared. One of these adducts is shown to undergo mild decarbonylation and phosphinidene insertion into a B−C bond of a borole, forming very rare examples of 1,2‐phosphaborinines, B/P isosteres of benzene. The strong donor properties of these 1,2‐phosphaborinines are confirmed by the synthesis of their π complexes with the Group 6 metals.

Structural characterization and electrochemical properties of nickel (II) complexes bearing sterically bulky hydrotris(3-phenyl)- and hydrotris(3-tert-butylpyrazol-1-yl)borato ligands

Two series of pseudo-tetrahedral nickel (II) complexes bearing the bulky hydrotris(3-phenyl)- or hydrotris(3-tert-butylpyrazol-1-yl)borato ligands, TpPhNiL and TptBuNiL respectively, have been synthesized and characterized with the intent of investigating the affects of substituent size on the redox properties of the nickel center (L=Cl, Br, I, NCS, NO3). X-ray analysis reveals both Tp ligands bind in a κ3 fashion with the R groups pointing out towards the metal producing a pocket in which the 4th ligand, L, resides. The TpPh ligand results in a larger pocket and produces nickel complexes with distorted solid-state geometries, specifically bent B…Ni…L angles, unlike the corresponding TptBu complexes which show much less variation in structure. Cyclic voltammetry experiments indicate all complexes undergo a Ni2+/1+ reduction at large negative potentials vs. ferrocene with varying levels of reversibility. The NCS complexes are the most reversible and the easiest to reduce with an average reduction potential of −1.51V due to the slightly electron withdrawing nature of the thiocyanate ligand. The strong donor properties of the bidentate nitrate ligand result in complexes that are the most difficult to reduce at an average of −1.80V. Overall, the TpPhNiL complexes are easier to reduce than their corresponding TptBuNiL complexes resulting from the weakened donor ability of the TpPh ligand. While the two ligands do produce different structural motifs, the electrochemical trends are more in line with the electronic properties of the complexes than structural differences.

Synthetic and structural studies of heteroleptic platinum(II) and palladium(II) complexes containing thiacrown and monodentate phosphane ligands

We wish to report the syntheses, spectroscopic, and structural properties for six new Pt(II) and Pd(II) heteroleptic complexes containing the monodentate phosphane ligands PTA (1,3,5-triaza-7-phosphaadamantane) or PMe3 (trimethylphosphane) and the crown trithioether [9]aneS3 (1,4,7-trithiacyclononane). All six reported complexes have the general formula [M([9]aneS3)(P2)](PF6)2 or [M([9]aneS3)(P)(Cl)](PF6) where M=Pt(II) or Pd(II) and P is the monodentate phosphane ligand. All six complexes form similar structures in which the metal center is surrounded by two sulfur atoms from the trithioether ligand and a cis arrangement of either the two phosphorus donors or the chlorido and phosphorus donor. The third sulfur from the [9]aneS3 interacts with the metal at a greater distance (2.711(2)Å–3.078(8)Å), resulting in elongated square pyramidal geometries. These axial distances are controlled by the donor abilities of the phosphane ligand with PTA and PMe3 serving as stronger donors. The two chlorido complexes show longer metal–sulfur interactions, consistent with the strong donor properties of the anionic chlorido ligand. Although the symmetrical bis complexes display typical 1H and 13C NMR behavior in the [9]aneS3 region, the less symmetrical chlorido complexes show a more complex pattern in their spectra. The 195Pt NMR chemical shifts are consistent with the presence of [S2P2+S1] or [S2PCl+S1] coordination environments.

Influence of specific and non-specific solvent effects on frontal MO of DMABI system

Derivatives of 2-(4'-dimethylaminobenzylidene)-1,3-indandione (DMABI) are classical push-pull chromophores containing carbonyl groups like numerous other non-linear chromophores for electro-optical devices. The energy of the intramolecular charge transfer exciton of DMABI is determined by the difference of HOMO-LUMO energies and can be varied in width range by chemical modifications of the DMABI structure or by the nonspecific and specific solvent effects if the chromophore is incorporated into condensed environment. The condensed medium in a thin film (polymer or molecular glass) exhibits a solid state solvation effect similar to the effect of the organic solvents in a solution. The value of the medium effect depends on specific and non-specific interactions between the chromophore and medium. The main purpose of this work is to evaluate the specific effects. The chromophores often contain few centres for specific interaction, and thus we have synthesised and investigated UV-Vis spectra of three derivatives of DMABI and also three derivatives of 4-aza-DMABI in solutions with strong H-bond donor properties. The experimentally obtained results show that the strong H-bond with carbonyl group lowers the intramolecular charge-transfer energy by 900 cm−1, the strong H-bond with carbonyl group and heterocyclic nitrogen in 4-aza-DMABI by 1500 cm−1 and the protonation of heterocyclic nitrogen in 4-aza-DMABI by 3250 cm−1 respectivelly.

Synthetic, Structural, and Catalytic Studies of Well‐Defined Allyl 1,2,3‐Triazol‐5‐ylidene (tzNHC) Palladium Complexes

AbstractA series of allyl 1,2,3‐triazol‐5‐ylidene (tzNHC) palladium complexes was prepared, and the structures of the complexes were fully characterized by NMR and X‐ray diffraction analyses. The donor properties of these ligands were evaluated by studying the vibrational spectra of their carbonyliridium complexes and their X‐ray photoelectron spectra. These evaluations showed that the structures of the tzNHC palladium complexes are almost identical to those of the corresponding imidazole carbene palladium complexes, and that the tzNHC ligands have stronger donor properties than the imidazole carbene ligands. The relationship between catalytic activity and structure was examined by carrying out a room‐temperature Suzuki–Miyaura coupling reaction, and the cinnamylpalladium complex bearing 1,4‐bis(2,6‐diisopropylphenyl)‐3‐methyl‐1,2,3‐triazol‐5‐ylidene (TPr) was found to be the most active catalyst. (cinnamyl)(TPr)PdCl showed high activity in the room‐temperature reaction performed with aryl chlorides regardless of the electronic and steric properties of the substituents, and was effective in reactions with sterically crowded arylboronic acids.

Abnormal N-heterocyclic Carbenes: More than Just Exceptionally Strong Donor Ligands

Complexes comprising a so-called abnormal carbene ligand, which displays pronounced mesoionic character, have recently been shown to be competent catalyst precursors for bond activation processes and oxidative transformations, including base-free alcohol oxidation and water oxidation. In this highlight we propose that these abnormal carbene ligands are not just useful spectator ligands but also actively participate in the bond activation step. This mode of action is partially based on the exceptionally strong donor properties of the ligand and, specifically, on the mesoionic character of these abnormal carbenes. The mesoionic properties provide a reservoir for charges and holes and thus induce efficient ligand-metal cooperativity, which is beneficial in particular for oxidation catalysis that involves concerted proton and electron transfer processes.

The coordination chemistry of copper(I) in liquid ammonia, trialkyl and triphenyl phosphite, and tri-n-butylphosphine solution.

The coordination chemistry of the solvate complexes of the relatively soft electron-pair acceptor copper(I) has been studied in solution and solid state in seven solvents with strong electron-pair donor properties, liquid ammonia, trimethyl, triethyl, triisopropyl, tri-n-butyl and triphenyl phosphite, and tri-n-butylphosphine. The solvate complexes have been characterised by means of EXAFS and 63Cu NMR spectroscopy, and in some cases also by 65Cu NMR spectroscopy. The copper(I) ion is three-coordinated, most probably in a coplanar trigonal fashion, in liquid ammonia with a mean Cu-N bond distance of 2.00(1) Angstroms. No 63Cu NMR signal has been detected from the ammonia solvated copper(I) ion in liquid ammonia, which supports a three-coordination. The phosphite and phosphine solvated copper(I) ions are tetrahedral with Cu-P bond distances in the range 2.24-2.28 Angstrom in both solution and solid state as determined by EXAFS spectroscopy. The tetrahedral configuration of these complexes has been confirmed by 63Cu and 65Cu NMR spectroscopy through the J(63Cu-31P) and J(65Cu-31P) couplings. The fact that two of the investigated complexes, [Cu(P(OC6H5)3)4]+ and [Cu(P(C4H9)3)4]+, are 63Cu and 65Cu NMR silent is probably caused by a significantly angular distorted tetrahedral configuration.

Syntheses, characterizations, and coordination chemistry of the 10-vertex phosphadicarbaboranes 6-R-arachno-6,8,9-PC2B7H11 and 6-R-arachno-6,5,7-PC2B7H11.

The 10-vertex phosphadicarbaboranes, 6-R-arachno-6,8,9-PC(2)B(7)H(11) (1) (R = Ph 1a or Me 1b) and 6-R-arachno-6,5,7-PC(2)B(7)H(11) (2) (R = Ph 2a or Me 2b) have been synthesized using in situ dehydrohalogenation reactions of RPCl(2) (R = Ph or Me) with the arachno-4,5-C(2)B(7)H(13) and arachno-4,6-C(2)B(7)H(13) carboranes, respectively. X-ray crystallographic determinations in conjunction with DFT/GIAO/NMR calculations and NMR spectroscopic studies have established that both 1 and 2 have open cage structures based on an icosahedron missing two vertexes. The two isomeric compounds differ in the positions of the carbons and bridging hydrogens on the open face. Studies of the reactions of 2a with BH(3).THF, S(8), and hydrogen peroxide demonstrated that 2a shows strong donor properties yielding the compounds endo-6-H(3)B-exo-6-Ph-arachno-6,5,7-PC(2)B(7)H(11) (3), endo-6-S-exo-6-Ph-arachno-6,5,7-PC(2)B(7)H(11) (4), and endo-6-O-exo-6-Ph-arachno-6,5,7-PC(2)B(7)H(11) (5) in which the BH(3), S, and O substitutents are bonded to an electron lone pair localized at the phosphorus endo-position. The reaction of 2a with an excess of S(8) results in the loss of a framework boron to produce the unique open-cage compound micro(7,8)-[HS(Ph)P]-hypho-7,8-C(2)B(6)H(11) (6). 2a also formed the donor complexes cis-(eta(1)-[6-Ph-arachno-6,5,7-PC(2)B(7)H(11)])(2)PtBr(2) (7) and trans-(eta(1)-[6-Ph-arachno-6,5,7-PC(2)B(7)H(11)])(2)PdBr(2) (8) in which the metal fragment is bonded in an eta(1)-fashion at the phosphorus endo-position. In these complexes, 2a is functioning as a two-electron sigma donor to the metals and can thus be considered as an analogue of the PR(3) ligands in the classical cis-(PPh(3))(2)PtBr(2) and trans-(PPh(3))(2)PdBr(2) coordination complexes. Although 1a did not show the donor properties exhibited by 2a, its dianion 6-Ph-6,8,9-PC(2)B(7)H(9)(2)(-) (1a(2)()(-)()) readily formed eta(4)-coordinated complexes with late transition metals including 8-Ph-7-(Ph(3)P)(2)-nido-7,8,10,11-PtPC(2)B(7)H(9) (9), 7-Ph-11-(eta(5)-C(5)H(5))-nido-11,7,9,10-CoPC(2)B(7)H(9) (10), and commo-Ni-(7-Ni-8'-Ph-nido-8',10',11'-PC(2)B(7)H(9))(7-Ni-8-Ph-nido-8,10,11-PC(2)B(7)H(9)) (11).

Complexation in Mixed Nonaqueous Solvents

It is shown, using the formation of silver(I) pyridine (Py) and piperidine (Ppd) complexes in a di-methyl sulfoxide–acetonitrile (DMSO–AN) mixture with different compositions, that an increase in the stability of the complexes with an increase in the acetonitrile content is apparent and caused by the dilution factor. This follows from the more detailed specification of the chemical forms in a mixture of solvents, according to which the complexation is considered as the replacement of the DMSO molecules in the Ag(DMSO)+ 2 solvate ion by the ligands (Py or Ppd molecules). The Ag(DMSO)+ 2 form is more preferential than Ag(AN)+ 2 due to the stronger donor properties of DMSO.

Stable subnanometre Pt clusters in zeolite NaX via stoichiometric carbonyl complexes: Probing of negative charge by DRIFT spectroscopy of adsorbed CO and H2

Negatively charged platinum carbonyl Chini complexes [Pt3(CO)6]n2− with n = 2–4 were synthesized by reductive carbonylation of Pt2+ in NaX zeolite and subsequently decomposed in vacuum at temperatures up to 773 K. The Pt clusters obtained preserved a low nuclearity, evidenced by rapid recarbonylation resulting in regeneration of the initial carbonyl Chini complexes. The electronic state of the metal clusters was probed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy of chemisorbed CO and H2. A strong electron donor capacity of Pt was indicated by the bands of (i) the linearly bonded CO at 1958–1976 cm−1 and (ii) the stretching frequency of terminal platinum hydrides at 2040 cm−1. It was concluded that the strong donor properties have to be attributed to negative charging of the Pt clusters, originating from the precursor carbonyl complex.

Hydrogenation of unsaturated hydrocarbons over alkali-metal-modified zinc oxide

Superbasic catalytic systems have been obtained by evaporation of alkali metals onto a ZnO surface. The catalysts were characterized by IR and EPR spectroscopy. Similar to the case of alkali-metal-doped oxide insulators, surface colour centres Fs+, formed by the trapping of an alkali-atom electron in an oxide anionic vacancy, were identified as the source of the extremely strong one-electron donor properties. The studied catalysts exhibited a much higher activity in the hydrogenation of alkenes than the starting ZnO. A mechanism for propene hydrogenation over the superbasic alkali metal/ZnO system was proposed.

HETEROCYCLEN ALS LIGANDEN XXII. 2-CHALKOGENOIMIDAZOLINE ALS LIGANDEN IN PENTACARBONYLMETALL-KOMPLEXEN

Abstract The chalcogenoimidazoline complexes N—CMe—CMe—NR—C=X·M(CO)3 (VIII-XI; R = Me, iso-Pr; X = S, Se, Te; M = Cr, Mo, W) are obtained from the metal hexacarbonyls (M = Cr, Mo, W) and the chalcogenoimidazolines IV-VII. In these complexes the chalcogeno ligands exhibit strong donor properties according to the sequence Te > Se > S which is demonstrated by n. m. r. and i. r. data. The X-ray structures of N(iso-Pr)—CMe—CMe—N(iso-Pr)—C=X·Cr(CO)3 (X = S; Se; VIIIa, IXa) are reported.

DINUCLEAR FIVE-COORDINATED MIXED LIGAND COMPLEXES. PART I: INTERACTION OF NICKEL(II) COMPLEXES WITH NONAQUEOUS SOLVENTS

Abstract The dinuclear 5-coordinate Ni(II) complex [Ni2(taet)(pmdt)2]2+, where taet = 1,1,2,2-tetraacetylethanate and pmdt = N,N,N′,N″,N″-pentamethyldiethylenetriamine, has been prepared and characterized by vibrational and electron spectroscopy and by magnetic measurements. In non-coordinating solvents such as DCE or NM and with non- or weakly coordinating anions like CIO4 − or Bφ4 − these complexes occur as bis-five-coordinate species. In solvents with stronger donor properties (Fa or DMF) and with more strongly coordinating anions, mono- or bis-octahedral species are formed. Spectrophotometric titration was used to evaluate equilibrium constants and associated reaction enthalpies and entropies for the stepwise coordination of donor solvents in DCE. Free energy changes are found to be entropy controlled.

1,5-Hexadiene selective hydroformylation reaction catalyzed with Rh(acac){P(OPh) 3} 2/P(OPh) 3 and Rh(acac)(CO)(PPh 3) / PPh 3 complexes

Hydroformylation of 1,5-hexadiene, catalysed by the system Rh(acac){P(OPh) 3} 2/P(OPh) 3 ( I), leads to the formation of mono- and dialdehydes, 4-heptenal, 2-Me-3-hexenal and octane-1,8-dial, 2,5-Me 2-hexanedial respectively. The yield of dialdehydes increases with temperature and pressure and reaches 100% at 80°C and 10 atm of CO / H 2 = 1. The reaction catalyzed by Rh(acac)(CO)(PPh 3) / PPh 3 (II) system produces, besides dialdehydes, monoaldehydes with a terminal double bond, namely 6-heptenal and 2-Me-5-hexenal. The migration of the double bonds in monoaldehydes depends mainly on the donor properties of modifying ligands. Phosphine when used as a modifying ligand (II), because of its strong donor properties, restricts isomerization (double bond migration) and diminishes yield of 4-heptenal. In contrast, when less strongly donating ligands such as phosphites are used as modifying ligands (I), independently of its steric properties, 4-heptenal is the main reaction product.

FIVE-COORDINATED MIXED LIGAND NICKEL(II) COMPLEXES: THERMODYNAMIC AND SPECTROSCOPIC STUDIES OF SOLVATOCHROMIC AND THERMOCHROMIC PROPERTIES

Abstract Five-coordinate Ni(II) complexes (Ni(β-dike)(pmdt)X; β-dike− = acetylacetonate or benzoylacetonate, pmdt = N, N, N′, N″, N″-pentamethyldiethylenetriamine and X = BPh4 − or NO3 −) have been prepared and characterized by vibrational and electron spectroscopy and by magnetic measurements. BPh4 − complexes are five-coordinate species in non-coordinating solvents like DCE or NM. In solvents with stronger donor properties like DMF or DMSO, or in the case of NO3 − complexes, octahedral species are formed. Spectrophotometric titration was used to evaluate equilibrium constants between five- and six-coordinated species. Reaction enthalpies and entropies were measured. The latter was found to govern the overall fee energy changes. Steric factors appear to be important when large amine molecules are coordinated to Ni(II).

Spectral and luminescent properties of 9-arylaminoacridines

A study has been made of the influence of electronic and steric factors of the aromatic substituent in 9-arylaminoacridines on the behavior of these compounds when subjected to light quanta or electron impact. It has been shown that the introduction of donor substituents shifts the absorption and luminescence spectra toward longer wavelengths, with a simultaneous increase of the Stokes shift and a decrease of the quantum yield of luminescence, it has been established that the introduction of sterically hindered substituents has a similar effect. In the mass spectra, the introduction of substituents with stronger donor properties tends to increase the stability of the molecular ions.

Tin(IV) porphyrin complexes with unsymmetrical axial ligation

Visible absorption and 1H and 13C NMR spectra have been measured for some meso-tetraphenyl- and meso-tetra( p-tolyl)porphyrin complexes of tin(IV) containing axial hydroxo and formato ligands. The unsymmetrical hydroxo/formato complexes allow assessment of cis and trans influences on NMR chemical shifts and J(SnH) and J(SnC) values, confirming the strong donor properties of the OH ligand bound to six-coordinate tin(IV).

Complexes of dialkylalkenylaluminium compounds

Complexes of two dialkylalkenylaluminium compounds with organic donors were investigated. It was found that stable complexes axe formed only with strong donors, but not with very weak ones like anisole, because of the strong donor properties of the alkenyl group. The donors with reactive bonds as in RCN undergo rearrangement reactions after several hours.