Gold-Catalysed Heck Reaction: Fact or Fiction? Correspondence on "Unlocking the Chain Walking Process in Gold Catalysis".

Two recent high-profile publications reported the formation of Heck-type arylated alkenes catalysed by MeDalPhosAuCl/AgOTf (J. Am. Chem. Soc. 2023, 145, 8810) and their cyclisation to tetralines (Angew. Chem. Int. Ed. 2023, e202312786). It was claimed that these were the first demonstrations in gold catalysis of alkene insertion into Au-aryl bonds, ß-H elimination and chain-walking by Au-H dications. We show here that in fact this chemistry is a two-stage process. Only the first step, the production of an alkyl triflate ester as the primary organic product by the well-known alkene heteroarylation sequence, involves gold. The subsequent formation of Heck-type olefins and their cyclisation to tetralines represent classical H+-triggered carbocationic chemistry. These steps proceed in the absence of gold with identical results. Literature claims of new gold reactivity such as chain walking by the putative [LAuH]2+ dication have no basis in fact.

Recent Advances in Gold(III) Chemistry: Structure, Bonding, Reactivity, and Role in Homogeneous Catalysis.

Over the past decade the organometallic chemistry of gold(III) has seen remarkable advances. This includes the synthesis of the first examples of several compound classes that have long been hypothesized as being part of catalytic cycles, such as gold(III) alkene, alkyne, CO and hydride complexes, and important catalysis-relevant reaction steps have at last been demonstrated for gold, like migratory insertion and ß-H elimination reactions. Also, reaction pathways that were already known, for example the generation of gold(III) intermediates by oxidative addition and their reductive elimination, are much better understood. A deeper understanding of fundamental organometallic reactivity of gold(III) has revealed unexpected mechanistic avenues, which can open when the barriers for reactions that for other metals would be regarded as "standard" are too high. This review summarizes and evaluates these developments, together with applications of gold(III) in synthesis and catalysis, with emphasis on the mechanistic insight gained in these investigations.

Assessing the Orbital Contribution in the "Spodium Bond" by Natural Orbital for Chemical Valence-Charge Displacement Analysis.

The term "spodium bond" (SpB) has been recently proposed to describe the noncoordinative interaction that can be established between a polarized group 12 metal and a mild Lewis base (LB). Most of the systems showing short metal-donor distances compatible with SpB are characterized by the coexistence of multiple weak interactions, including hydrogen and halogen bonding, making the assessment of real importance of SpB difficult. Here, we show that the relative importance of each contribution can be probed by dissecting the orbital component of the interaction through the extended transition state-natural orbital for chemical valence-charge displacement analysis (ETS-NOCV-CD). The latter gives useful information about relative energies and electrons involved, for model systems ([(thiourea)2MX2]···LB; M = Zn, Cd, and Hg; X = Cl and I; and LB = CH2S, CH2O, CH3CN, and CO) and a variety of structures extracted from experimentally characterized adducts, allowing us to demonstrate the lack of a direct correlation between a favorable metal-base distance and the presence of an orbital contribution for the SpB.

Hydride Transfer to Gold: Yes or No? Exploring the Unexpected Versatility of Au⋠⋠⋠H-M Bonding in Heterobimetallic Dihydrides.

The potential for coordination and H-transfer from Cp2 MH2 (M=Zr, W) to gold(I) and gold(III) complexes was explored in a combined experimental and computational study. [(L)Au]+ cations react with Cp2 WH2 giving [(L)Au(κ2 -H2 WCp2 )]+ (L=IPr (1), cyclic (alkyl)(amino)carbene (2), PPh3 (3) and Dalphos-Me (4) [IPr=1,3-bis(diisopropylphenyl)imidazolylidene; Dalphos-Me=di(1-adamantyl)-2-(dimethylamino)phenyl-phosphine], while [Au(DMAP)2 ]+ (DMAP=p-dimethylaminopyridine) affords the C2 -symmetric [Au(κ-H2 WCp2 )2 ]+ (5). The Dalphos complex 4 can be protonated to give the bicationic adduct 4 H, showing AuI ⋅⋅⋅H+ -N hydrogen bonding. The gold(III) Lewis acid [(C^N-CH)Au(C6 F5 )(OEt2 )]+ binds Cp2 WH2 to give an Au-H-W σ-complex. By contrast, the pincer species [(C^N^C)Au]+ adds Cp2 WH2 by a purely dative W→Au bond, without Au⋅⋅⋅H interaction. The biphenylyl-based chelate [(C^C)Au]+ forms [(C^C)Au(µ-H)2 WCp2 ]+ , with two 2-electron-3-centre W-H⋅⋅⋅Au interactions and practically no Au-W donor acceptor contribution. In all these complexes, strong but polarized W-H bonds are maintained, without H-transfer to gold. On the other hand, the reactions of Cp2 ZrH2 with gold complexes led in all cases to rapid H-transfer and formation of gold hydrides. Relativistic DFT calculations were used to rationalize the striking reactivity and bonding differences in these heterobimetallic hydride complexes along with an analysis of their characteristic NMR parameters and UV/Vis absorption properties.

Synthesis and Photophysical Properties of Au(III)-Ag(I) Aggregates.

Cyclometalated gold(III) complexes of the type (C∧N∧C)AuX [HC∧N∧CH = 2,6-bis(4-ButC6H4)pyrazine; 2,6-bis(4-ButC6H4)pyridine, or 2,6-bis(4-ButC6H4)4-Butpyridine; X = CN, CH(COMe)2, or CH(CN)2] have been used as building blocks for the construction of the first family of AuIII/AgI aggregates. The crystal structures of these aggregates reveal the formation of complex architectures in which the Ag+ cations are stabilized by the basic centers present on each of the Au precursors. The photophysical properties of these aggregates are reported. Compared to mononuclear pincer complexes, a general red-shift and an increase in the emission intensity are observed. In agreement with DFT calculations, the lowest energy absorption and the emission are assigned to 1IL(C∧N∧C) and 3IL(C∧N∧C) transitions dominated by the HOMO and the LUMO orbitals.

Unlocking Structural Diversity in Gold(III) Hydrides: Unexpected Interplay of cis/ trans-Influence on Stability, Insertion Chemistry, and NMR Chemical Shifts.

The synthesis of new families of stable or at least spectroscopically observable gold(III) hydride complexes is reported, including anionic cis-hydrido chloride, hydrido aryl, and cis-dihydride complexes. Reactions between (C^C)AuCl(PR3) and LiHBEt3 afford the first examples of gold(III) phosphino hydrides (C^C)AuH(PR3) (R = Me, Ph, p-tolyl; C^C = 4,4'-di- tert-butylbiphenyl-2,2'-diyl). The X-ray structure of (C^C)AuH(PMe3) was determined. LiHBEt3 reacts with (C^C)AuCl(py) to give [(C^C)Au(H)Cl]-, whereas (C^C)AuH(PR3) undergoes phosphine displacement, generating the dihydride [(C^C)AuH2]-. Monohydrido complexes hydroaurate dimethylacetylene dicarboxylate to give Z-vinyls. (C^N^C)Au pincer complexes give the first examples of gold(III) bridging hydrides. Stability, reactivity and bonding characteristics of Au(III)-H complexes crucially depend on the interplay between cis and trans-influence. Remarkably, these new gold(III) hydrides extend the range of observed NMR hydride shifts from δ -8.5 to +7 ppm. Relativistic DFT calculations show that the origin of this wide chemical shift variability as a function of the ligands depends on the different ordering and energy gap between "shielding" Au(dπ)-based orbitals and "deshielding" σ(Au-H)-type MOs, which are mixed to some extent upon inclusion of spin-orbit (SO) coupling. The resulting 1H hydride shifts correlate linearly with the DFT optimized Au-H distances and Au-H bond covalency. The effect of cis ligands follows a nearly inverse ordering to that of trans ligands. This study appears to be the first systematic delineation of cis ligand influence on M-H NMR shifts and provides the experimental evidence for the dramatic change of the 1H hydride shifts, including the sign change, upon mutual cis and trans ligand alternation.

Reductive Elimination Leading to C-C Bond Formation in Gold(III) Complexes: A Mechanistic and Computational Study.

The factors affecting the rates of reductive C-C cross-coupling reactions in gold(III) aryls were studied by using complexes that allow easy access to a series of electronically modified aryl ligands, as well as to gold methyl and vinyl complexes, by using the pincer compounds [(C^N^C)AuR] (R=C6 F5 , CH=CMe2 , Me and p-C6 H4 X, where X=OMe, F, H, tBu, Cl, CF3 , or NO2 ) as starting materials (C^N^C=2,6-(4'-tBuC6 H3 )2 pyridine dianion). Protodeauration followed by addition of one equivalent SMe2 leads to the quantitative generation of the thioether complexes [(C^N-CH)AuR(SMe2 )]+ . Upon addition of a second SMe2 pyridine is displaced, which triggers the reductive aryl-R elimination. The rates for these cross-couplings increase in the sequence k(vinyl)>k(aryl)≫k(C6 F5 )>k(Me). Vinyl-aryl coupling is particularly fast, 1.15×10-3  L mol-1 s-1 at 221 K, whereas both C6 F5 and Me couplings encountered higher barriers for the C-C bond forming step. The use of P(p-tol)3 in place of SMe2 greatly accelerates the C-C couplings. Computational modelling shows that in the C^N-bonded compounds displacement of N by a donor L is required before the aryl ligands can adopt a conformation suitable for C-C bond formation, so that elimination takes place from a four-coordinate intermediate. The C-C bond formation is the rate-limiting step. In the non-chelating case, reductive C(sp2 )-C(sp2 ) elimination from three-coordinate ions [(Ar1 )(Ar2 )AuL]+ is almost barrier-free, particularly if L=phosphine.

Thermally Stable Gold(III) Alkene and Alkyne Complexes: Synthesis, Structures, and Assessment of the trans-Influence on Gold-Ligand Bond Enthalpies.

The reaction of [C^C)Au(OEt2 )2 ]+ with 1,5-cyclooctadiene or norbornadiene affords the corresponding olefin complexes [(C^C)Au(COD)]SbF6 and [(C^C)Au(NBD)]SbF6 , which are thermally stable in solution and the solid state (C^C=4,4'-di-tert-butylbiphenyl-2,2'-diyl). The crystal structures of these complexes have been determined. By contrast, dienones such as dibenzylideneacetone are O- rather than C=C-bonded. The reactions of (C^C)Au(OAcF )(L) (L=PMe3 or CNxyl) with B(C6 F5 )3 in the presence of bis(1-adamantyl)acetylene give the mixed-ligand alkyne complexes [(C^C)Au(AdC≡CAd)(L)]+ , the first complexes of their type in gold chemistry. In the presence of an excess of acetylene these compounds are thermally stable in solution and as solids. The bonding of n- and π-donor ligands to AuIII fragments and the effect of the trans influence exerted by N- and C-donors was explored with the aid of DFT calculations. Results show that the Au-L bond enthalpies trans to anionic C are 35-60 % of the enthalpies trans to N, with strong π-acceptors being particularly affected. In comparison with [Me2 Au]+ , the [(C^C)Au]+ fragment is more polar and in bond enthalpy terms resembles Me2 Pt.

Luminescent Gold(III) Thiolates: Supramolecular Interactions Trigger and Control Switchable Photoemissions from Bimolecular Excited States.

A new family of cyclometallated gold(III) thiolato complexes based on pyrazine-centred pincer ligands has been prepared, (C^Npz ^C)AuSR, where C^Npz ^C=2,6-bis(4-But C6 H4 )pyrazine dianion and R=Ph (1), C6 H4 tBu-4 (2), 2-pyridyl (3), 1-naphthyl (1-Np, 4), 2-Np (5), quinolinyl (Quin, 6), 4-methylcoumarinyl (Coum, 7) and 1-adamantyl (8). The complexes were isolated as yellow to red solids in high yields using mild synthetic conditions. The single-crystal X-ray structures revealed that the colour of the deep-red solids is associated with the formation of a particular type of short (3.2-3.3 Å) intermolecular pyrazine⋅⋅⋅pyrazine π-interactions. In some cases, yellow and red crystal polymorphs were formed; only the latter were emissive at room temperature. Combined NMR and UV/Vis techniques showed that the supramolecular π-stacking interactions persist in solution and give rise to intense deep-red photoluminescence. Monomeric molecules show vibronically structured green emissions at low temperature, assigned to ligand-based 3 IL(C^N^C) triplet emissions. By contrast, the unstructured red emissions correlate mainly with a 3 LLCT(SR→{(C^Npz ^C)2 }) charge transfer transition from the thiolate ligand to the π⋅⋅⋅π dimerized pyrazine. Unusually, the π-interactions can be influenced by sample treatment in solution, such that the emissions can switch reversibly from red to green. To our knowledge this is the first report of aggregation-enhanced emission in gold(III) chemistry.

Intriguing Influence of -COOH-Driven Intermolecular Aggregation and Acid-Base Interactions with N,N-Dimethylformamide on the Second-Order Nonlinear-Optical Response of 5,15 Push-Pull Diarylzinc(II) Porphyrinates.

A series of 5,15 push-pull meso-diarylzinc(II) porphyrinates, carrying one or two -COOH or -COOCH3 acceptor groups and a -OCH3 or a -N(CH3)2 donor group, show in N,N-dimethylformamide and CHCl3 solutions a negative and solvent-dependent second-order nonlinear-optical (NLO) response measured by the electric-field-induced second-harmonic generation (EFISH) technique, different from the structurally related zinc(II) porphyrinate carrying a -N(CH3)2 donor group and a -NO2 acceptor group, where a still solvent-dependent but positive EFISH second-order response was previously reported. Moreover, when a -N(CH3)2 donor group and a -COOH acceptor group are part of a sterically hindered 2,12 push-pull ß-pyrrolic-substituted tetraarylzinc(II) porphyrinate, the EFISH response is positive and solvent-independent. In order to rationalize these rather intriguing series of observations, EFISH measurements have been integrated by electronic absorption and IR spectroscopic investigations and by density functional theory (DFT) and coupled-perturbed DFT theoretical and 1H pulsed-gradient spin-echo NMR investigations, which prompt that the significant concentration effects and the strong influence of the solvent nature on the NLO response are originated by a complex whole of different aggregation processes induced by the -COOH group.

Gold(III) Alkyne Complexes: Bonding and Reaction Pathways.

The synthesis and characterization of hitherto hypothetical AuIII π-alkyne complexes is reported. Bonding and stability depend strongly on the trans effect and steric factors. Bonding characteristics shed light on the reasons for the very different stabilities between the classical alkyne complexes of PtII and their drastically more reactive AuIII congeners. Lack of back-bonding facilitates alkyne slippage, which is energetically less costly for gold than for platinum and explains the propensity of gold to facilitate C-C bond formation. Cycloaddition followed by aryl migration and reductive deprotonation is presented as a new reaction sequence in gold chemistry.

Stereo- and Regioselective Alkyne Hydrometallation with Gold(III) Hydrides.

The hydroauration of internal and terminal alkynes by gold(III) hydride complexes [(C^N^C)AuH] was found to be mediated by radicals and proceeds by an unexpected binuclear outer-sphere mechanism to cleanly form trans-insertion products. Radical precursors such as azobisisobutyronitrile lead to a drastic rate enhancement. DFT calculations support the proposed radical mechanism, with very low activation barriers, and rule out mononuclear mechanistic alternatives. These alkyne hydroaurations are highly regio- and stereospecific for the formation of Z-vinyl isomers, with Z/E ratios of >99:1 in most cases.

Performance Assessment in Fingerprinting and Multi Component Quantitative NMR Analyses.

An interlaboratory comparison (ILC) was organized with the aim to set up quality control indicators suitable for multicomponent quantitative analysis by nuclear magnetic resonance (NMR) spectroscopy. A total of 36 NMR data sets (corresponding to 1260 NMR spectra) were produced by 30 participants using 34 NMR spectrometers. The calibration line method was chosen for the quantification of a five-component model mixture. Results show that quantitative NMR is a robust quantification tool and that 26 out of 36 data sets resulted in statistically equivalent calibration lines for all considered NMR signals. The performance of each laboratory was assessed by means of a new performance index (named Qp-score) which is related to the difference between the experimental and the consensus values of the slope of the calibration lines. Laboratories endowed with a Qp-score falling within the suitable acceptability range are qualified to produce NMR spectra that can be considered statistically equivalent in terms of relative intensities of the signals. In addition, the specific response of nuclei to the experimental excitation/relaxation conditions was addressed by means of the parameter named NR. NR is related to the difference between the theoretical and the consensus slopes of the calibration lines and is specific for each signal produced by a well-defined set of acquisition parameters.

Discriminating halogen-bonding from other noncovalent interactions by a combined NOE NMR/DFT approach.

Herein a combined NOE NMR/DFT methodology to discriminate between adducts held together by halogen bonding (XB) and other noncovalent interactions (non-XB, such as lone pair/π), based on the determination of the XB donors' and acceptors' relative orientation, is proposed. In particular, (19) F,(1) H HOESY NMR spectroscopy experiments and DFT calculations on different XB donors, such as perfluorohexyl iodide (I1), iodopentafluorobenzene (I2) and bromopentafluorobenzene (Br), combined with different Lewis bases, such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 2,4,6-trimethylpyridine (Me3 Py), were performed. The results clearly show that in the case DABCO/I1 the XB adduct is practically the only one present in solution, whereas for the other pairs a certain amount of non-XB adduct is present. Combining DFT and HOESY results, the amount of non-XB adducts can be roughly quantified under our experimental conditions as 4 % for DABCO/I2, between 10 and 20 % for Me3 Py/I1 and Me3 Py/I2, and 44 % for DABCO/Br.

Probing the association of frustrated phosphine-borane Lewis pairs in solution by NMR spectroscopy.

(19)F,(1)H HOESY, diffusion, and temperature-dependent (19)F and (1)H NMR studies allowed us to unequivocally probe the association between the frustrated PR3/B(C6F5)3 (1, R = CMe3; 2, R = 2,4,6-Me3C6H2) Lewis pairs in aromatic solvents. No preferential orientation is favored, as deduced by combining (19)F,(1)H HOESY and DFT results, suggesting association via weak dispersion rather than residual acid/base interactions. The association process is slightly endoergonic [K = 0.5 M(-1), ΔG(0)(298 K) = +0.4 kcal/mol for 2], as derived from diffusion NMR measurements.

Unusual hafnium-pyridylamido/ER(n) heterobimetallic adducts (ER(n) = ZnR2 or AlR3).

NMR spectroscopy and DFT studies indicate that the Symyx/Dow Hf(IV)-pyridylamido catalytic system for olefin polymerization, [{N(-),N,CNph(-)}HfMe][B(C6F5)4] (1, Nph = naphthyl), interacts with ER(n) (E = Al or Zn, R = alkyl group) to afford unusual heterobimetallic adducts [{N(-),N}HfMe(µ-CNph)(µ-R)ER(n-1)][B(C6F5)4 in which the cyclometalated Nph acts as a bridge between Hf and E. (1)H VT (variable-temperature) EXSY NMR spectroscopy provides direct evidence of reversible alkyl exchanges in heterobimetallic adducts, with ZnR2 showing a higher tendency to participate in this exchange than AlR3. 1-Hexene/ERn competitive reactions with 1 at 240 K reveal that the formation of adducts is strongly favored over 1-hexene polymerization. Nevertheless, a slight increase in the temperature (to >265 K) initiates 1-hexene polymerization.

Boosting Effect of Sterically Protected Glucosyl Substituents in Formic Acid Dehydrogenation by Iridium(III) 2-Pyridineamidate Catalysts.

[Cp*Ir(R-pica)Cl] (Cp*=pentamethylcyclopentadienyl anion, pica=2-picolineamidate) complexes bearing carbohydrate substituents on the amide nitrogen atom (R=methyl-ß-D-gluco-pyranosid-2-yl, 1; methyl-3,4,6-tri-O-acetyl-ß-D-glucopyranosid-2-yl, 2) were tested as catalysts for formic acid dehydrogenation in water. TOFMAX values over 12000 h-1 and 50000 h-1 were achieved at 333 K for 1 and 2, respectively, with TON values over 35000 for both catalysts. Comparison with the simpler cyclohexyl-substituted analogue (3) indicated that glucosyl-based complexes are much better performing under the same experimental conditions (TOFMAX=5144 h-1, TON=5000 at pH 2.5 for 3) owing to a lower tendency to isomerize to the less active k2-N,O isomer upon protonation. The 5-fold increase in TOFMAX observed for 2 with respect to 1 is reasonably due to an optimal steric protection by the acetyl substituent, which may prevent unproductive inner-sphere reactivity. These results showcase a powerful strategy for the inhibition of the common deactivation pathways of [Cp*Ir(R-pica)X] catalysts for FA dehydrogenation, paving the way for the development of better performing hydrogen storage systems.

Solventless supramolecular chemistry via vapor diffusion of volatile small molecules upon a new trinuclear silver(I)-nitrated pyrazolate macrometallocyclic solid: an experimental/theoretical investigation of the dipole/quadrupole chemisorption phenomena.

A comparative study on the tendency of a new trinuclear silver(I) pyrazolate, namely, [N,N-(3,5-dinitropyrazolate)Ag]3 (1), and a similar compound known previously, [N,N-[3,5-bis(trifluoromethyl)pyrazolate]Ag]3 (2), to adsorb small volatile molecules was performed. It was found that 1 has a remarkable tendency to form adducts, at room temperature and atmospheric pressure, with acetone, acetylacetone, ammonia, pyridine, acetonitrile, triethylamine, dimethyl sulfide, and tetrahydrothiophene, while carbon monoxide, tetrahydrofuran, alcohols, and diethyl ether were not adsorbed. On the contrary, 2 did not undergo adsorption of any of the aforementioned volatile molecules. Adducts of 1 were characterized by elemental analysis, IR, thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) surface area, and diffusion NMR measurements. The crystal structures of 1·2CH3CN and compound 3, derived from an attempt to crystallize the adduct of 1 with ammonia, were determined by single-crystal X-ray diffractometric studies. The former shows a sandwich structure with a 1:2 stoichiometric [Ag3]/[CH3CN] ratio in which one acetonitrile molecule points above and the other below the centroid of the Ag3N6 metallocycle. Compound 3 formed via rearrangement of the ammonia adduct to yield an anionic trinuclear silver(I) derivative with an additional bridging 3,5-dinitropyrazolate and having [Ag(NH3)2](+) as the counterion, [Ag(NH3)2][N,N-(3,5-dinitropyrazolate)4Ag3]. Irreversible sorption and/or decomposition upon vapor exposure are desirable advantages toward toxic gas filtration applications, including ammonia inhalation. TGA confirms the analytical data for all of the samples, showing weight loss for each adsorbed molecule at temperatures significantly higher than the corresponding boiling temperature, which suggests a chemical-bonding nature for adsorption as opposed to physisorption. BET surface measurements of the "naked" compound 1 excluded physical adsorption in its porous cavities. Density functional theory simulation results are also consistent with the chemisorption model, explain the experimental adsorption selectivity for 1, and attribute the lack of similar adsorption by 2 to significantly less polarizable electrostatic potential and also to strong argentophilic bonding whose energy is even higher than the quadrupole-dipole adduct bond energy upon proper selection of the density functional.

Tweaking the bridge in metallocene Zr(IV)/W(IV) bimetallic hydrides.

Zirconocene cations react with Cp2WH2 affording the bimetallic [Cp2Zr(µ-H)(µ-η1:η5-C5H4)WHCp]+ bridging hydride 1 (Cp = cyclopentadienyl anion, C5H5-) via σ-bond metathesis. Complex 1 features an atypical out of plane Zr(µ-H)W moiety, where no intermetallic interaction is involved, and a fluxional core. Coordination geometry and bond distances of the bridging hydride interaction can be modulated upon reaction with Lewis bases and unsaturated substrates. PMe3, P(p-tol)3, 3,5-dimethylpyridine and THF bind to 1 and shift the hydride bridge on the coordination plane of Zr. Insertion of olefins and alkynes into the Zr-C bond of 1 leads instead to alkyl and vinyl species where the Zr and W coordination planes are perpendicular to each other. Such alterations of the Zr(µ-H)W arrangement are reflected in the average 1H NMR chemical shift values of the hydride, which correlate linearly with computed Zr-H distances. Reactivity experiments with H2 showed that the bridging hydride interaction prevents bimetallic cooperativity and that σ-bond metathesis between Zr-C and H-H bonds is the preferred pathway for all the investigated complexes.

Anion-dependent tendency of di-long-chain quaternary ammonium salts to form ion quadruples and higher aggregates in benzene.

The self-aggregation tendency of [N(CH(3))(2)(C(18)H(37))(2)]X [1X; X(-)=BF(4) (-), PF(6) (-), OTf(-), NTf(2) (-), BPh(4) (-), BTol(4) (-), BAr(F-), and B(C(6)F(5))(4) (-)] salts to form ion quadruples (IQs) and higher aggregates (HAggs) in [D(6)]benzene is investigated by means of diffusion NMR spectroscopy. The experimental results indicate that salts containing small anions (1BF(4), 1PF(6), and 1OTf) are present in solution as IQs even at the lowest investigated concentration of C=5×10(-5) M and show a limited tendency to further self-aggregate, reaching a maximum average aggregation number (N=V(H)/V(H)(0IP), where V(H)=measured hydrodynamic volume and V{H}{0IP}=hydrodynamic volume of the ion pair) of about 6-8 (C=0.050-0.100 M). Salts with larger counterions [1BPh(4), 1BTol(4), 1BAr(F), and 1B(C(6)F(5))(4)] form instead ion pairs at low concentration but steadily self-aggregate (especially the non-fluorinated ones) on increasing their concentration up to N values exceeding 50 (C=0.030-0.050 M). 1NTf(2) behaves in an intermediate fashion. The self-aggregation tendency of salts is quantified by formulating the dependence of V(H) on C by means of the equations of indefinitive aggregation models. The following rankings for the formation of IQs and HAggs are obtained: IQs: 1BF(4)≈1PF(6)≈1OTf> 1NTf(2)>1B(C(6)F(5))(4)≥1BPh(4)≥1BTol(4)≥1BAr(F); HAggs: 1BTol(4)>1BPh(4)> 1NTf(2)>1B(C(6)F(5))(4)> 1BAr(F)>1BF(4)≈1PF(6)≈1OTf. Interionic NOE NMR studies and DFT calculations were conducted in order to determine the relative anion-cation orientation in the self-aggregating units.