A Comparative Study on the Thermodynamics of Halogen Bonding of Group 10 Pincer Fluoride Complexes

Abstract The thermodynamics of halogen bonding of a series of isostructural Group 10 metal pincer fluoride complexes of the type [(3,5‐R2‐tBuPOCOPtBu)MF] (3,5‐R2‐tBuPOCOPtBu=κ 3‐C6HR2‐2,6‐(OPtBu2)2 with R=H, tBu, COOMe; M=Ni, Pd, Pt) and iodopentafluorobenzene was investigated. Based on NMR experiments at different temperatures, all complexes 1‐tBu (R=tBu, M=Ni), 2‐H (R=H, M=Pd), 2‐tBu (R=tBu, M=Pd), 2‐COOMe (R=COOMe, M=Pd) and 3‐tBu (R=tBu, M=Pt) form strong halogen bonds with Pd complexes showing significantly stronger binding to iodopentafluorobenzene. Structural and computational analysis of a model adduct of complex 2‐tBu with 1,4‐diiodotetrafluorobenzene as well as of structures of iodopentafluorobenzene in toluene solution shows that formation of a type I contact occurs.


Introduction
Halogen bonding (XB), an on-covalent interaction between electron-deficient halogen atoms and aL ewis-basic site, has attracted considerable attention in recent years due to its relevance fornumerousapplicationsincluding functional materials, supramolecular polymers, molecular recognition,a nion receptors, crystal engineering, and organocatalysis. [1,2] Halogen bondingi sahighly directional interaction that can be described using different models, one of them being the socalled s hole that results from an anisotropic electron distribution around halogen atoms when being covalently bound to a residue R. This effect is most pronounced for electron-withdrawingm oieties Ra nd results in ah ighly positive region in elongation of the covalentR ÀXb ond that is capable of acting as an electron-accepting site. [3] Numerous studies of halogen bondingo fu biquitous organo-halogen compounds in solution and in the solid state have been reported andd emonstrate the enormous potential of this type of interaction. [4] However,s imilar investigations involvinga nalogous inorganic or organometallic structures such as metal halides or halide complexes are still comparablyr are ( Figure 1). Thisi ss omewhat surprising given that transitionmetal complexes containing halide ligandsa re known to act as precatalysts and even as active speciesi nalarge number of catalytic processes. [5] Halogen-bonding interactions could, in principle, be of relevance for CÀXb ond activation reactions as it was shown by Huber et al. for organocatalytic reactions. [6] Seminal structural investigations by Brammer and co-workers suggest that transition-metal-halide complexes are excellent hydrogen-and halogen-bonding acceptors. [7] More recent work by othersf urthers upported this and broadened the scope of organometallic halogen bonding. [8] However,s ystematic studies of halogen bonding in solution to access thermodynamic data of these interactions are still underrepresented, yet highly desirable in order to fully understand all aspects of this phenomenona nd further promote this fieldo fr esearch in terms of applicability.M etal fluorides are ideal for these studies because the 19 FNMR resonance of the fluoride ligand in most cases is the only 19 FNMR signal and/ora ppears isolated from other signals. Moreover,i ti sh ighly sensitive towards changes in its electronic environment. The formationo fX Ba dducts can thus be readily monitored by NMR spectroscopya nd the thermodynamicso ft he interactions can be quantified using the 19 FNMR shift of the metal fluoride. Parkin and co-workersr eportedh alogen bonding of magnesium [9] and zinc [10] fluoride complexes ( Figure 1) with iodopentafluorobenzene,C 6 F 5 I, which was found to be an excellent XB donor due to the pres-ence of ap erfluorinated aryl ring and the resultingh igh Lewis acidity of the iodine donor atom. Am ore systematic study of halogen bondingofl ate-transition-metal fluorides was presented by Perutz and Brammer [11] who used as eries of Group 10 bis(phosphine) complexest hat form monofluorides upon CÀF activation of fluorinated aromatics such as hexafluorobenzene and fluorinated pyridines ( Figure 1). Upon interaction of these fluoride complexes with the XB donor C 6 F 5 Is ignificant shifts of the metal fluoride 19 FNMR resonance of up to 30 ppm were observed. Thermodynamic data reported for these interactions suggest the formation of strong halogen bonds for all complexes (2.4 < K 300 < 5.2; À26 < DH o < À16 kJ mol À1 ; À73 < DS o < À42 JK À1 mol À1 ). Also, depending on the polarity of the solvent, changes in the stoichiometry of adduct formation were observed ( Figure 1). Recently,a lso perfluoroalkyl iodide and perfluoroaryl bromide donors have been evaluated with related Ni bis(phosphine) systems. [12] However,i n these studies no full isostructurals eries of complexes wasi nvestigated as depending on the metal centre andt he phosphine ligand differentp atterns of CÀFa ctivation were observed, thusr esulting in different binding modes of the fluorinated pyridine ligand. [13] It should be noted that in previous studies, attempts to preparea ni sostructurals eries of monofluoridec omplexes failed and to the best of our knowledge, no such series of complexes is known to date. The necessity of using monofluoride complexes arises from the problemo ff ormation of multiple interactions (Scheme1)i nt he case of wellprecedented di-or oligofluoride species such as Group 4m etallocenedifluorides or complex ions such as [MF 6 ] nÀ .

Results and Discussion
Synthesis of nickel, palladium and platinum POCOPf luoride complexes was accomplished by salt metathesis reactions with AgF in toluene startingf rom suitable halide precursors (Scheme 2).
For these reactions it is highly desirable to strictly exclude the presence of water because the formed fluoridel igand is highly nucleophilic and thus readily forms hydrogen bonding adducts with water.A lso, formationo fb ifluoride complexes, L n M-FHF, is am ajor problem, which must be addressed by rigorous drying of solvents and removing traces of HF from AgF. In cases in which traces of the metal bifluoride were present after fluorination, these were removed by treating the crude materialw ith aN aH suspension, followedb yf iltration to yield the pure metal fluoridec omplexes. Nickel fluoride complex 1-tBu was obtained from the correspondingc hloride complex as ay ellow solid in 58 %y ield. In the case of palladium and platinum complexes 2-H 15b , 2-tBu, 2-COOMe and 3-tBu,s alt metathesis from the iodide speciesw as found to be more convenient, giving the desired fluorides as colourless microcrystalline solids in yields of up to 69 %.
All complexes were assigned as metal fluorides based on isolated 19 FNMR resonances in the highfield region of the NMR spectra (Table 1). These values resemble those found for other late-transition-metal fluorides. [16] 31 PNMR data are well in line with those reported for structurally similars quare-planar Group 10 pincerh alide complexes that possessP ( tBu) 2 donor sites. [17] It should be noted that trends observed for the Scheme1.Schematicdepiction of halogen bonding of late transition-metal halide complexes with C 6 F 5 Ia nd structural motifo fp incerc omplexes used in this study. 31 PNMR resonances of complexes described herein were observed before for iodide complexes bearing thesel igands. [15] In all cases 31 PNMR shifts of the metal fluoridec omplex were found highfieldf rom values found for isostructural iodide and chloridec omplexes,atrend that was observed earlier by Campora and co-workers for PCP Ni and Pd halide complexes. [18] Notably,P ÀFc oupling was only observed for 1-tBu and 3-tBu (Table 1) and 31 PNMR resonancesf or Pd complexes were comparably broad. Small values for 2 J P, F or no detectable coupling [14] at room temperature was observed before for related Group 10 pincer fluoridec omplexes (e.g. [(PCP)PdF] (PCP = k 3 -C 6 H 3 -2,6-(CH 2 PiPr 2 ) 2 ), 2 J P, F = 5.9 Hz). [18] Prior to titratione xperiments,t he absence of hydrogen bound H 2 Oo rH Fw as confirmed by 1 HNMR analysis (absence of additional resonances in the downfield region,t hat is, up to d = 15 ppm) and by CHN analysis, which showedg ood agreement with expected values.
The molecular structure of complex 1-tBu was confirmed by single crystal X-ray diffraction (SC-XRD, Figure 2). The asymmetric unit contains two molecules of the complex in distorted square-planar coordination geometry with at erminal fluoride ligand (Ni1ÀF1 1.8417(13), Ni2ÀF2 1.8418 (12) ). These values are in the same range as those found before for other Ni PCP pincer fluoridec omplexes. [18,19] In the space-fill model (Figure S6, Supporting Information) it can be seen that the sterically demanding tBu groups shield the fluoridel igand, which could be of importance for selectivef ormation of a1 :1 XB adduct. Additionally,w ea nalysed the molecular structure of the isostructural complexes 2-tBu, 2-COOMe and 3-tBu ( Figure 2). Bond lengths and angles of the Pd and Pt complexes are in the expectedr ange with the MÀFb onds (2-tBu: 2.0304(16), 2-COOMe:2 .0317(12), 3-tBu: 2.0539(15) )b eing significantly longert han in the Ni complex 1-tBu due to the larger size of the heavier congeners.
Fluoride complexes of this type being prone to coordination of Lewis bases can be seen from the interaction of the PdÀFm oiety with H 2 Oi nc omplex 2-tBu·H 2 O,w hich is formed in the presence of trace amountso fH 2 O( Figure S7, Supporting Information). [20] Notably,f ormation of such adducts was only observedi nc ase of Pd, indicating ah igher tendency of these complexes to perform such interactions. Spectroscopic XB studies thus have to be carried out under rigorousexclusion of moisture.
NMR titrationsw ere carried out in tolueneu sing complexes shown in Scheme 1a st he XB acceptor andC 6 F 5 Ia st he XB donor,w hich was used before in relateds tudies. When adding the halogen-bond donor,t he 19 FNMR resonanceo ft he fluoride ligand of all complexes exhibits as ignificant shift to lower field ( Figure 3).
The chemical shift difference Dd of pure metal fluoride and its C 6 F 5 Ia dduct (formed in the presenceo fa ne xcesso fC 6 F 5 I) did not change significantly when going from lowest to highest temperature for all complexes. Although Dd is similar for   Ni and Pd complexes, 3-tBu showedm uch smaller lowfield shifts during titration. Minor variations of the 19 FNMR chemical shifts of C 6 F 5 Ia re due to the changing concentrationa nd can be neglected. Also, 31 PNMR signals for the phosphine ligands show no significant changes. This indicates that the interaction takes place exclusively between metal fluoridea nd the iodine of C 6 F 5 Ia nd 19 FNMR spectroscopy is thus well suited for ad etermination of the strength of this contact.
The resulting titration curvesr ecorded at different temperatures using metal fluoride and C 6 F 5 Is tock solutions of similar concentrations (Figure 4) was fitted using as imple model of 1:1b inding. All curves show ar easonable amounto fd ata points before saturation occurs, indicatingt hat the concentration regime used for titratione xperiments is sensible. From this fit, equilibrium constants K for XB adduct formation as well as the value d max ,w hich corresponds to the chemical shift of the XB adduct of 1:1s toichiometry,w ereo btained. Notably, analysiso ft he data using 1:2s toichiometry gave much poorer results with fitted datac learly deviating from the experimental values for all systems investigated.
Formation of adducts of 1:1s toichiometry wase xemplarily confirmed by Jobp lots for complex 1-tBu and C 6 F 5 I. For the Job plot, changes in the chemical shift of the 19 FNMR resonance (Dd)w ere monitored as af unction of the molar fraction of metal complex (X) while keeping the sum of the concentrations of 1-tBu and C 6 F 5 Ic onstant at av alue of 0.01 mol L À1 .A s expected for a1 :1 complex,t he graph of XDd versus X shows am aximum at X = 0.5 (Figure 4). Analysis of the temperature dependence of K (van't Hoff plot, Figure 4) gives valuesf or enthalpy DH o and entropy DS o .A no verview of thermodynamic data is depicted in Table 2.
Perutz et al. described variations of the stoichiometry of adduct formation when using less polar solvents such as heptane [11a] that do not allow for additional interactions, for example, p-p interactions between toluene and C 6 F 5 I. In our case, use of nonpolar solvents was not possible because all complexes show very limited solubility.I nf act, solubility of 2-H in toluene is rather poor at lowt emperatures, NMR titrations using this complex were only done at temperatures between 268 and 313 K.
Complexes 1-tBu, 2-tBu,a nd 3-tBu are isostructural and differ only in the metal centre and are thus comparable. The resultss how similarb inding enthalpies ÀDH o with almost identicalv alues for 1-tBu and 2-tBu,w hereas ÀDH o for the Pt analogue 3-tBu is significantly smaller.T his is in contrast to results presented earlier by Perutz et al. for structurally similar, but not isostructural complexes, where ÀDH o was observed to follow the trend Ni < Pd < Pt. [11b] Also, ac orrelation of ÀDH o with ÀDS o waso bservedl eading to compensation in the valueso fDG o and K. Especially in the case of the Pd complex 2-tBu,t his is not possible because the value determined for ÀDS o is much smaller than for the Ni complex 1-tBu and in the same range as for the Pt complex 3-tBu.I ts hould be noted that binding constantsa re much larger for Pd complexest han for Ni and Pt complexes (Table 2). Also, despite showinge xcellent agreement with 1:1b inding, titration curves for Pd complexes appeared slightly different:a lthough at lower ratios of [C 6 F 5 I] =[Pd]alarge variation of chemical shift   (Figures S13 and S14, Supporting Information). [21] The difference in values for ÀDH o and ÀDS o for compounds 2-H, 2-tBu and 2-COOMe,s howing variationso ft he substitution pattern of the POCOPl igand is significant, suggesting that the electronic nature of the ligand plays ar olef or the strength of the XB interaction. However,t he observed trend in ÀDH o 2-H < 2-tBu < 2-COOMe is somewhat counterintuitive, giving largestv alues for the complex possessing an electron withdrawing COOMe substituted POCOPl igand and smallest values for the more electron-rich tBu substituted complex. XB occurs by interaction of the partially positive XB donor with aL ewisbasic fluoride atom and should thus be most pronounced for complex 2-tBu.T he rationalization of the herein observed effect is currently under investigation.
To further support the formation of an XB adduct in solution, we performed 19 F, 1 HH OESY NMR experiments fort he system 1-tBu = C 6 F 5 Iu sing at enfold excess of the XB donor.T he 19 F, 1 H Heteronuclear Overhauser Effect SpectroscopY (HOESY)N MR spectrum reveals the presence of intermolecular contactsb etween the tBu protons of the metal fluoridea nd the C 6 F 5 If luorine nuclei. The heteronuclear NOE correlationw ith the CH 3 /o-Fi ss ignificantly more intense than the ones for CH 3 /m-F and CH 3 /p-F ( Figure 5). This indicates that the iodine is interacting with the metal fluoride, but in the XB adduct the p-F is remote from the tBu group and only produces as mall NOE. Similar effects were observed beforei naXB study of 1,4-diazabicyclo[2.2.2]octanea nd C 6 F 5 I. [22] Adducts formed between metal fluorides and C 6 F 5 Ii nt oluene solution could unfortunately not be crystallized. To get additional structural insights into halogen bondingo ft hese pincer systems, we co-crystallized complex 2-tBu with 1,4-diiodotetrafluorobenzene, C 6 F 4 I 2 and used this as ab enchmark for furtherc omputational studies of solution structures formed with C 6 F 5 I( see above). C 6 F 4 I 2 was used before as ad onor for structurals tudies of halogenb onding of palladium pincer complexes. [23] Co-crystallization of 2-tBu and C 6 F 4 I 2 in 2:1r atio from tolueney ields the adduct 2-tBu·I-C 6 F 4 -I.T he molecular structure is shown in Figure 6. It shows two molecules of 2-tBu bridged by one molecule of the bifunctionalX Bd onor.N otably,b oth palladium fluoride XB acceptors are in coplanar arrangement, whereas the bridging C 6 F 4 I 2 is tilted out of this plane by 788.
Given that the orientation of the molecules with respect to each other in ac rystal might arise due to variousp acking forces in the crystal,e specially in halogen bonded systems, density functional theory (DFT) calculations were carriedo ut for 2-tBu·I-C 6 F 4 -I,u sing the crystal-structure coordinates for the input geometry.I tw as indeed found that the two molecules of 2-tBu are not co-planar in the lowest-energy optimized structure in toluene. They are at 79.218 with respectt o each other (Figure S15 a, Supporting Information). Furthermore, the angle between the plane passing throughI ÀC 6 F 4 ÀIa nd the planes through each of the two 2-tBu molecules are 23.368 and 56.158 (Figures S15 b, S15 c).  Ac omparison of the Pd1ÀF1 distance in the adduct structure (2.0681(17) (SC-XRD);2 .0847 (DFT);T able S9 a, Supporting Information) with that of the free acceptor 2-tBu (2.0304(16)( SC-XRD); 2.0670 (DFT);T able S10 a) showst hat upon interaction an elongation of the metal-fluorine bond by approximately 2% occurs. The F···I halogen bondingc ontact is characterized by aF 1···I1 distance of 2.6828(18) (DFT: 2.7020 ;T able S9 a), which is clearly lower than the sum of the van der Waals radii( svdW = 3.45 ). Along with this, the bond C31ÀI1 is slightly elongated (2.115(3) (SC-XRD), 2.1231 (DFT); Ta ble S9 a) compared with the value found for the free XB donor (2.079(4), [24] Sr cov = 2.08 ). [25] Analysis of the bond angles along the XB interaction shows as lightly bent arrangement (SC-XRD:P d1-F1-I1:1 60.08,D FT:1 66.38;S C-XRD:F 1-I1-C31:1 74.48,D FT:1 78.48;T able S9 a). In ap revious structural study Whitwood, Brammer,a nd Perutz showed that the degree of bending of the MÀX···IÀCu nit is strongly dependent on the halide Xo ft he acceptor component with fluorides resulting in linear arrangements whereas structures of metal iodide adducts are bentsignificantly. [8e] Notably,XBi nteractions of electron deficient s holes with regions of highere lectron density are typically strongly bent. Reasons for the herein found almostl inear arrangementc an be the steric hindrance due to the presence of tBu groups, the more isotropic electron distribution of the fluoride ligand, or simple packing requirements.I nt he solid state the shortestd istance between protons of the P(tBu) 2 group and o-F atoms of C 6 F 4 I 2 is found for F2···H18B( 4.634 ). This corresponds to the most pronounced HOESY correlation in related systems with C 6 F 5 Ii ns olution ( Figure 5).
The computeds tructure of 1-tBu·C 6 F 5 I can also be used to understand the observed trend in HOESYe xperiments ( Figure 5). The shortesta verage distances between the tBu protons (on P) and F( on C 6 F 5 I), o-F, m-F, p-F,a re 5.257 (Table  S16 o, SupportingI nformation), 7.769 (Table S16 m), and 9.536 (Table S16 p) respectively,w hich is in line with the different intensities of cross peaks, o-F···H tBu > m-F···H tBu > p-F···H tBu , in the HOESY ( Figure 5). For the co-crystallised adduct 2-tBu·I-C 6 F 4 -I,t he shortest calculated average distances between the tBu protons (on P) and F( on C 6 F 5 I), are 4.557 (for F ortho to I(1) and 5.443 (for F ortho to I(2)) (Table S17 aa nd S17 b). [27] Computed structuralp arameters for these adducts are in the same range as found before for relatedh alogen bonding systems with Group 10 fluoridecomplexes. [11b]

Conclusion
We have for the first time synthesised af ulls eries of isostructural square-planar Group 10 metal(II) pincerm onofluoride complexes.A long with this, as et of palladium fluoride complexest hat differ only by substitution of the aryl backboneo f the POCOP pincerl igand were prepared. In toluene solution, these complexes form strongh alogen bonds with iodopentafluorobenzene in 1:1s toichiometry.T hermodynamic parameters determined from 19 FNMR titration experiments show only minor variations in DH o for the isostructural series of complexes 1-tBu, 2-tBu and 3-tBu.H owever,b inding constants K were much higher for Pd than for Ni and Pt complexes, sug- gesting that Pd complexes behavedifferently in halogen-bonding systems. Enthalpic and entropic contributions to the halogen-bondingi nteractiona re consistently smaller for pincers ystems described in this study compared to those previously reported for metal fluorides with perfluorinated pyridyl ligands ( Figure 1, À23 < DH 0 < À16 kJ mol À1 ; À73 < DS 0 < À39 JK À1 mol À1 ). [11b, 12] The structure of halogen-bonding adducts wase xemplarily analysed by co-crystallisation of aP d fluoridec omplex with 1,4-diiodotetrafluorobenzene. In line with previous results on metal fluoride complexes [8e] this adduct shows an almost linear arrangemento fX Bd onor and acceptort hat is different from common stronglyb ent halogen bonds of heavier halides. The resultsp resented herein could stimulate others in the field to systematicallyi nvestigate related organometallic and metal-free systems to gain further insights into the principles of halogen-bonding interactions.