Azobenzene-Integrated NHC Ligands: A Versatile Platform for Visible-Light-Switchable Metal Catalysis.

A new class of photoswitchable NHC ligands, named azImBA, has been developed by integrating azobenzene into a previously unreported imidazobenzoxazol-1-ylidene framework. These rigid photochromic carbenes enable precise control over confinement around a metal's coordination sphere. As a model system, gold(I) complexes of these NHCs exhibit efficient bidirectional E-Z isomerization under visible light, offering a versatile platform for reversibly photomodulating the reactivity of organogold species. Comprehensive kinetic studies of the protodeauration reaction reveal rate differences of up to 2 orders of magnitude between the E and Z isomers of the NHCs, resulting in a quasi-complete visible-light-gated ON/OFF switchable system. Such a high level of photomodulation efficiency is unprecedented for gold complexes, challenging the current state-of-the-art in photoswitchable organometallics. Thorough investigations into the ligand properties paired with structure-reactivity correlations underscored the unique ligand's steric features as a key factor for reactivity. This effective photocontrol strategy was further validated in gold(I) catalysis, enabling in situ photoswitching of catalytic activity in the intramolecular hydroalkoxylation and -amination of alkynes. Given the significance of these findings and its potential as a widely applicable, easily customizable photoswitchable ancillary ligand platform, azImBA is poised to stimulate the development of adaptive, multifunctional metal complexes.

Linear Amine-Linked Oligo-BODIPYs: Convergent Access via Buchwald-Hartwig Coupling.

A convergent route toward nitrogen-bridged BODIPY oligomers has been developed. The synthetic key step is a Buchwald-Hartwig cross-coupling reaction of an α-amino-BODIPY and the respective halide. Not only does the selective synthesis provide control of the oligomer size, but the facile preparative procedure also enables easy access to these types of dyes. Furthermore, functionalized examples were accessible via brominated derivatives.

Access to isoindole-derived BODIPYs by an aminopalladation cascade.

Here, we present a new route to dyes of the BODIPY family. We first built up a N-Boc-protected dipyrromethene scaffold via an aminopalladation cascade. Subsequentially, the pyrrole moiety was deprotected and the BF2 unit inserted. Depending on the terminating reaction, BODIPYs with either aryl or alkynyl moieties were accessible.

The perfluoroadamantoxy aluminate as an ideal weakly coordinating anion? - synthesis and first applications.

Weakly coordinating anions (WCAs) facilitate the stabilization and isolation of highly reactive and almost "naked" cations. Alkoxyaluminate-based WCAs such as [Al(OC(CF3)3)4]- ([pf]-) are widely used due to their synthetic accessibility and their high stability. However, small cations are still able to coordinate the oxygen atoms of the [pf]- anion or even to abstract an alkoxy ligand. The novel WCA [Al(OC10F15)4]- ([pfAd]-; OC10F15 = perfluoro-1-adamantoxy) is characterized by a very rigid core framework, thus indicating a higher stability towards fluoride-ion abstraction (DFT calculations) and providing hope to generate less disordered crystal structures. The [pfAd]- anion was generated by the reaction of the highly acidic alcohol perfluoro-1-adamantanol C10F15OH with LiAlH4 in o-DFB. Li[pfAd] could not be synthesized free of impurities (and still contains unreacted alcohol). Yet, starting from contaminated Li[pfAd], the very useful pure salts Ag[pfAd], [Ph3C][pfAd] and [H(OEt2)2][pfAd] could be synthesized. The salts were characterized by NMR spectroscopy, single-crystal X-ray diffraction and IR spectroscopy. Additionally, [NO][pfAd] could be synthesized containing alcohol impurities but nonetheless enabled the synthesis of the salt P9+[pfAd]-. The synthesis of Tl[pfAd] in a mixture of H2O/acetone/o-DFB demonstrated the water stability of the [pfAd]- anion.

Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters.

The homoleptic group 5 carbonylates [M(CO)6 ]- (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Agm Mn clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag6 {Nb(CO)6 }4 ]2+ (4 a2+ ) precedent, we established access to such Agm Mn clusters of the composition [Agm {M(CO)6 }n ]x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1-, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt4 ][M(CO)6 ] and Ag[Al(ORF )4 ] (RF =C(CF3 )3 ) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at -35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3+ (m=6, n=5) and 42+ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag6 superatom cores. The [M(CO)6 ]- ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX4 (42+ ) or trigonal bipyramidal AX5 molecules (3+ ).

A heterodinuclear, formal Au+IPt0 complex with weakly bound alkene ligands.

[(Ph3P)AuPt(nbe)3][BAr4F] (nbe = norbornene) constitutes the first olefin-containing, formal Au+IPt0 complex. The unusual coordination mode and the electronic properties have been analyzed spectroscopically and by calculations. The low binding energy of the nbe ligands make this complex a valuable precursor for formal Au+IPt0 complexes and a candidate for heterobimetallic catalysis.

Phenyl-Group Exchange in Triphenylphosphine Mediated by Cationic Gold-Platinum Complexes-A Gas-Phase Mimetic Approach.

The PPh3 ligands in the heterodinuclear AuPt complex [(Ph3P)AuPt(PPh3)3][BAr4F] (BAr4F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) exhibit a high fluxionality on the AuPt core. Fast intramolecular and slow intermolecular processes for the reversible exchange of the PPh3 ligands have been identified. When [(Ph3P)AuPt(PPh3)3][BAr4F] is heated in solution, the formation of benzene is observed, and a trinuclear, cationic AuPt2 complex is generated. This process is preceded by reversible phenyl-group exchange between the PPh3 ligands present in the reaction mixture as elucidated by deuterium-labeling studies. Both the elimination of benzene and the preceding reversible phenyl-group exchange have originally been observed in mass-spectrometry-based CID experiments (CID = Collision-Induced Dissociation). While CID of mass-selected [Au,Pt,(PPh3)4]+ results exclusively in the loss of PPh3, the resulting cation [Au,Pt,(PPh3)3]+ selectively eliminates C6H6. Thus, the dissociation of a PPh3 ligand from [Au,Pt,(PPh3)3]+ is energetically not able to compete with processes which result in C-H- and C-P-bond cleavage. In both media, the heterobimetallic nature of the employed complexes is the key for the observed reactivity. Only the intimate interplay of the gas-phase investigations, studies in solution, and thorough DFT computations allowed for the elucidation of the mechanistic details of the reactivity of [(Ph3P)AuPt(PPh3)3][BAr4F].

Completing the triad: synthesis and full characterization of homoleptic and heteroleptic carbonyl and nitrosyl complexes of the group VI metals.

Oxidation of M(CO)6 (M = Cr, Mo, W) with the synergistic oxidative system Ag[WCA]/0.5 I2 yields the fully characterized metalloradical salts [M(CO)6]+˙[WCA]- (weakly coordinating anion WCA = [F-{Al(ORF)3}2]-, RF = C(CF3)3). The new metalloradical cations with M = Mo and W showcase a similar structural fluxionality as the previously reported [Cr(CO)6]+˙. Their reactivity increases from M = Cr < Mo < W and their syntheses allow for in-depth insights into the properties of the group 6 carbonyl triad. Furthermore, the reaction of NO+[WCA]- with neutral carbonyl complexes M(CO)6 gives access to the heteroleptic carbonyl/nitrosyl cations [M(CO)5(NO)]+ as salts of the WCA [Al(ORF)4]-, the first complete transition metal triad of their kind.

Why Do Five Ga+ Cations Form a Ligand-Stabilized [Ga5 ]5+ Pentagon and How Does a 5:1 Salt Pack in the Solid State?

The reaction of the Ga+ source [Ga(PhF)2 ]+ [Al(ORF )4 ]- with the neutral σ-donor ligand dmap (4-Me2 N-C6 H4 N) produces the unexpectedly large and fivefold positively charged cluster cation salt [Ga5 (dmap)10 ]5+ ([Al(ORF )4 ]- )5 . It includes a regular and planar Ga5 pentagon with strong metal-metal bonding. Additionally, the compound represents the first salt in which an ionic 1:5 packing is realized. We discuss the nature of this structure which results from the conversion of the non-bonding 4s2 lone-pair orbitals into fully Ga-Ga-bonding orbitals and the solid-state arrangement of the ions constituting the lattice as an almost orthohexagonal AX5 lattice, possibly the aristotype of any 5:1 salt.

First experimental evidence for the elusive tetrahedral cations [EP3]+ (E = S, Se, Te) in the condensed phase.

Condensed phase access to the unprecedented tetrahedral cations [EP3]+ (E = S, Se, Te) was achieved through the reaction of ECl3[WCA] with white phosphorus ([WCA]- = [Al(ORF)4]- and [F(Al(ORF)3)2]-; -RF = -C(CF3)3). Previously, [EP3]+ was only known from gas phase MS investigations. By contrast, the reaction of ECl3[A] with the known P3 3- synthon Na[Nb(ODipp)3(P3)] (enabling AsP3 synthesis), led to formation of P4. The cations [EP3]+ were characterized by multinuclear NMR spectroscopy in combination with high-level quantum chemical calculations. Their bonding situation is described with several approaches including Atoms in Molecules and Natural Bond Orbital analysis. The first series of well-soluble salts ECl3[WCA] was synthesized and fully characterized as starting materials for the studies on this elusive class of [EP3]+ cations. Yet, with high [ECl3]+ fluoride ion affinity values between 775 (S), 803 (Se) and 844 (Te) kJ mol-1, well exceeding typical phosphenium ions, these well-soluble ECl3[WCA] salts could be relevant in view of the renewed interest in strong (also cationic) Lewis acids.

Basic Remarks on Acidity.

This Review provides a unified view on Brønsted acidity. For this purpose, a brief overview of the concepts acidity, acid strengths, and pH value is given, including problems, proposed solutions, and the use of the pHabs /pHabsH2O scale as a unifying concept. Thereafter, some examples of the accessibility and application of unified pHabs values are given. The Review is rounded off with the analogy of acid-base chemistry to redox chemistry with the introduction of the unified redox scale peabs . The combination of pHabs and peabs values in the protoelectric potential map (PPM), as elaborated in ongoing studies on the thermochemistry of single ions, provides a means to classify and to compare all possible acid-base/redox reactions in a medium-independent and, thus, unified fashion.

How Innocent are Potentially Redox Non-Innocent Ligands? Electronic Structure and Metal Oxidation States in Iron-PNN Complexes as a Representative Case Study.

Herein we present a series of new α-iminopyridine-based iron-PNN pincer complexes [FeBr2LPNN] (1), [Fe(CO)2LPNN] (2), [Fe(CO)2LPNN](BF4) (3), [Fe(F)(CO)2LPNN](BF4) (4), and [Fe(H)(CO)2LPNN](BF4) (5) with formal oxidation states ranging from Fe(0) to Fe(II) (LPNN = 2-[(di-tert-butylphosphino)methyl]-6-[1-(2,4,6-mesitylimino)ethyl]pyridine). The complexes were characterized by a variety of methods including (1)H, (13)C, (15)N, and (31)P NMR, IR, Mössbauer, and X-ray photoelectron spectroscopy (XPS) as well as electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopy, SQUID magnetometry, and X-ray crystallography, focusing on the assignment of the metal oxidation states. Ligand structural features suggest that the α-iminopyridine ligand behaves as a redox non-innocent ligand in some of these complexes, particularly in [Fe(CO)2LPNN] (2), in which it appears to adopt the monoanionic form. In addition, the NMR spectroscopic features ((13)C, (15)N) indicate the accumulation of charge density on parts of the ligand for 2. However, a combination of spectroscopic measurements that more directly probe the iron oxidation state (e.g., XPS), density functional theory (DFT) calculations, and electronic absorption studies combined with time-dependent DFT calculations support the description of the metal atom in 2 as Fe(0). We conclude from our studies that ligand structural features, while useful in many assignments of ligand redox non-innocence, may not always accurately reflect the ligand charge state and, hence, the metal oxidation state. For complex 2, the ligand structural changes are interpreted in terms of strong back-donation from the metal center to the ligand as opposed to electron transfer.

Efficient hydrogen liberation from formic acid catalyzed by a well-defined iron pincer complex under mild conditions.

Hydrogen liberation: An attractive approach to reversible hydrogen storage applications is based on the decomposition of formic acid. The efficient and selective hydrogen liberation from formic acid is catalyzed by an iron pincer complex in the presence of trialkylamine. Turnover frequencies up to 836 h⁻¹ and turnover numbers up to 100,000 were achieved at 40 °C. A mechanism including well-defined intermediates is suggested on the basis of experimental and computational data.

Mechanistic studies on the gas-phase dehydrogenation of alkanes at cyclometalated platinum complexes.

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L-H)](+) (L=2,2'-bipyridine (bipy), 2-phenylpyridine (phpy), and 7,8-benzoquinoline (bq)) with linear and branched alkanes C(n)H(2n+2) (n=2-4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L-H)](+)/C(2)H(6), loss of C(2)H(4) dominates clearly over H(2) elimination; however, the mechanisms significantly differs for the reactions of the "rollover"-cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen-atom transfer from C(2)H(6) to [Pt(bipy-H)](+), followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)](+), for the phpy and bq complexes [Pt(L-H)](+), the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L-H)(H(2))](+) as the ionic product accounts for C(2)H(4) liberation. In the latter process, [Pt(L-H)(H(2))(C(2)H(4))](+) (that carries H(2) trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C(2)H(4) rather than H(2) is ejected. For both product-ion types, [Pt(H)(bipy)](+) and [Pt(L-H)(H(2))](+) (L=phpy, bq), H(2) loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy-H)](+) with the higher alkanes C(n)H(2n+2) (n=3, 4), H(2) elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C(3)H(5))(bipy)](+). In the reactions of [Pt(L-H)](+) (L=phpy, bq) with propane and n-butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of "rollover"-cyclometalated [Pt(bipy-H)](+) with C(n)H(2n+2) (n=2-4) less than 15 % of the generated product ions are formed by C-C bond-cleavage processes, this value is about 60 % for the reaction with neo-pentane. The result that C-C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2-elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1- and 1,3-elimination pathways are only of minor importance.

A new dirhodium catalyst with hemilabile tropolonato ligands for C-H bond functionalization.

Don't hold on too tightly! In a new dirhodium catalyst for C-H functionalization reactions, two tropolonato ligands are introduced as hemilabile chelating ligands (see scheme). Only two bridges hold the Rh-Rh core together. The tropolonato ligands can liberate a binding site in the equatorial coordination sphere of the catalyst. This opens a doorway to new mechanistic channels in C-H functionalization.

The "missing link": the gas-phase generation of platinum-methylidyne clusters Pt(n)CH+ (n=1, 2) and their reactions with hydrocarbons and ammonia.

Electrospray ionization (ESI) of tetrameric platinum(II) acetate, [Pt(4)(CH(3)COO)(8)], in methanol generates the formal platinum(III) dimeric cation [Pt(2)(CH(3)COO)(3)(CH(2)COO)(MeOH)(2)](+), which, upon harsher ionization conditions, sequentially loses the two methanol ligands, CO(2), and CH(2)COO to form the platinum(II) dimer [Pt(2)(CH(3)COO)(2)(CH(3))](+). Next, intramolecular sequential double hydrogen-atom transfer from the methyl group concomitant with the elimination of two acetic acid molecules produces Pt(2)CH(+) from which, upon even harsher conditions, PtCH(+) is eventually generated. This degradation sequence is supported by collision-induced dissociation (CID) experiments, extensive isotope-labeling studies, and DFT calculations. Both PtCH(+) and Pt(2)CH(+) react under thermal conditions with the hydrocarbons C(2)H(n) (n=2, 4, 6) and C(3)H(n) (n=6, 8). While, in ion-molecule reactions of PtCH(+) with C(2) hydrocarbons, the relative rates decrease with increasing n, the opposite trend holds true for Pt(2)CH(+). The Pt(2)CH(+) cluster only sluggishly reacts with C(2)H(2), but with C(2)H(4) and C(2)H(6) dihydrogen loss dominates. The reactions with the latter two substrates were preceded by a complete exchange of all of the hydrogen atoms present in the adduct complex. The PtCH(+) ion is much less selective. In the reactions with C(2)H(2) and C(2)H(4), elimination of H(2) occurs; however, CH(4) formation prevails in the decomposition of the adduct complex that is formed with C(2)H(6). In the reaction with C(2)H(2), in addition to H(2) loss, C(3)H(3)(+) is produced, and this process formally corresponds to the transfer of the cationic methylidyne unit CH(+) to C(2)H(2), accompanied by the release of neutral Pt. In the ion-molecule reactions with the C(3) hydrocarbons C(3)H(6) and C(3)H(8), dihydrogen loss occurs with high selectivity for Pt(2)CH(+), but in the reactions of these substrates with PtCH(+) several reaction routes compete. Finally, in the ion-molecule reactions with ammonia, both platinum complexes give rise to proton transfer to produce NH(4)(+); however, only the encounter complex generated with PtCH(+) undergoes efficient dehydrogenation of the substrate, and the rather minor formation of CNH(4)(+) indicates that C-N bond coupling is inefficient.

Ion-molecule reactions of "rollover" cyclometalated [Pt(bipy-H)](+) (bipy=2,2'-bipyridine) with dimethyl ether in comparison with dimethyl sulfide: an experimental/computational study.

The ion-molecule reactions of dimethyl ether with cyclometalated [Pt(bipy-H)](+) were investigated in gas-phase experiments, complemented by DFT methods, and compared with the previously reported ion-molecule reactions with its sulfur analogue. The initial step corresponds in both cases to a platinum-mediated transfer of a hydrogen atom from the ether to the (bipy-H) ligand, and three-membered oxygen- and sulfur-containing metallacycles serve as key intermediates. Oxidative C--C bond coupling ("dehydrosulfurization"), which dominates the gas-phase ion chemistry of the [Pt(bipy-H)](+) ion with dimethyl sulfide, is practically absent for dimethyl ether. The competition in the formation of C(2)H(4) and CH(2)X (X=O, S) in the reactions of [Pt(bipy-H)](+) with (CH(3))(2)X (X=O, S) as well as the extensive H/D exchange observed in the [Pt(bipy-H)](+)/(CH(3))(2)O system are explained in terms of the corresponding potential-energy surfaces.

"Roll-over" cyclometalation of 2,2'-bipyridine platinum(II) complexes in the gas phase: a combined experimental and computational study.

In a combined experimental/computational investigation, the gas-phase behavior of cationic [Pt(bipy)(CH(3))((CH(3))(2)S)](+) (1) (bipy=2,2'-bipyridine) has been explored. Losses of CH(4) and (CH(3))(2)S from 1 result in the formation of a cyclometalated 2,2'-bipyrid-3-yl species [Pt(bipy-H)](+) (2). As to the mechanisms of ligand evaporation, detailed labeling experiments complemented by DFT-based computations reveal that the reaction follows the mechanistically intriguing "roll-over" cyclometalation path in the course of which a hydrogen atom from the C(3)-position is combined with the Pt-bound methyl group to produce CH(4). Activation of a C-H-bond of the (CH(3))(2)S ligand occurs as well, but is less favored (35 % versus 65 %) as compared to the C(3)-H bond activation of bipy. In addition, the thermal ion/molecule reactions of [Pt(bipy-H)](+) with (CH(3))(2)S have been examined, and for the major pathway, that is, the dehydrogenative coupling of the two methyl groups to form C(2)H(4), a mechanism is suggested that is compatible with the experimental and computational findings. A hallmark of the gas-phase chemistry of [Pt(bipy-H)](+) with the incoming (CH(3))(2)S ligand is the exchange of one (and only one) hydrogen atom of the bipy fragment with the C-H bonds of dimethylsulfide in a reversible "roll-over" cyclometalation reaction. The Pt(II)-mediated conversion of (CH(3))(2)S to C(2)H(4) may serve as a model to obtain mechanistic insight in the dehydrosulfurization of sulfur-containing hydrocarbons.