Metal-only Lewis pairs between group 10 metals and Tl(i) or Ag(i): insights into the electronic consequences of Z-type ligand binding

Metal-only Lewis Pairs between Group 10 metals and Tl(i) and Ag(i) have allowed for insight into the electronic consequences of Lewis-acid ligation.


Introduction
On account of their relatively electropositive nature and ability to act as formal acceptors toward Lewis bases, the transition metals in coordination complexes are traditionally viewed as Lewis acids. Classical "Werner-type" complexes utilize their empty nd, as well as (n + 1)s and (n + 1)p, orbitals to form dative bonds with electron-donor ligands. In the case of highly reduced and electron-rich complexes, the transition metal center may also be capable of exhibiting Lewis basic behavior. 1 Although this phenomenon was initially invoked for the case of carbonyl metallates acting as Brønsted bases, [2][3][4] it is now recognized as a central tenet of transition-metal bonding to pacidic ligands [5][6][7] as well as an essential component of many oxidative addition mechanisms. [8][9][10][11][12] More recently, the extension of this concept to the binding of various main-group acceptor fragments (Z-type ligands) 13 in a s-fashion by electron-rich transition metals has been realized, and the study of such complexes continues to be of intense interest. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] In addition to these examples, a related topic concerning transition metal Lewis basicity is the ability to form dative interactions to another metal center. Judicious ligand design strategies that constrain an electron-rich metal center in close proximity to a coordinatively unsaturated metal fragment has proven to be a reliable approach for engendering metal-metal dative bonding. [28][29][30][31][32][33][34] Furthermore, in certain instances, unsupported metal-only Lewis pairs (MOLPs), which do not rely on a ligand buttress, can be generated. 2,[35][36][37][38][39] The formation of such unsupported metal-metal interactions, while sometimes labile in solution, offers an interesting approach toward tuning the reactivity proles of low-valent complexes, as the addition of metallic Lewis acids has been shown to enhance the rates of certain catalytic processes. [40][41][42][43] While synthetic methods leading to MOLPs and their structural chemistry has advanced, a detailed understanding of how the presence of a metal-metal dative bond affects the electronic properties of the constituent fragments remains of signicant interest. It is generally accepted that protonation of a transition metal complex is best viewed as involving a two-electron oxidation of the metal center to give a hydride ligand. 44 As the electrons involved in the M-H bond have necessarily come from the metal, an increase of its valence by two units is required. 45 In the case of other main group Lewis acids (e.g. boranes), the degree of charge transfer is oen not as clear. As such, the adoption and assignment of formalisms to adequately describe the electronic structure of such adducts has been a point of debate in the community. 46,47 Similar ambiguities in the electronic structures of MOLPs exist, although considerably less effort has been put toward uncovering satisfactory electronic descriptors for such compounds. 39 Despite the fact that X-ray Absorption Near-Edge Spectroscopy (XANES) holds promise in this regard, 30,34 its thus-far limited use in this capacity has not yet led to the development of general principles for properly describing the electronic structures of complexes containing metal-metal dative interactions.
Work from our research group has demonstrated the utility of encumbering m-terphenyl isocyanides in stabilizing low-valent and coordinatively unsaturated complexes of late transition metals. 27 50 as well as by the recently-reported platinum (boryl)iminomethane complex Pt(k 2 -N,B-Cy2 BIM)(CNAr Dipp2 ). 27 In addition, the response of the isocyanide n(C^N) IR bands to the electron density on the Lewis-basic metal center renders them a convenient spectroscopic reporter on the degree of formal charge transfer upon binding a s-acceptor fragment. 25 In this work, we demonstrate the ability of the two-coordinate complexes M(CNAr Dipp2 ) 2 (M ¼ Pt, Pd) 27,50 to form unsupported metal-metal linkages with Tl(I). Two Tl-containing MOLPs have also been examined by X-ray Absorption Near-Edge Spectroscopy (XANES), illustrating that the spectroscopic oxidation state of the group 10 metal is not affected by its interaction with Tl(I). We also show that the zero-valent platforms M(CNAr Dipp2 ) 2 (M ¼ Pt, Pd) can form metal-only Lewis pairs with Ag(I), yielding the heterobimetallic salts [AgM(CNAr Dipp2 ) 2 ]OTf (5, M ¼ Pt; 6, M ¼ Pd). Spectroscopic and structural investigations provide insight into the nature of the M-Ag interactions in these compounds, and give strong evidence that formation of the M / Ag linkage results in only a marginal degree of metal-to-metal charge transfer. In the case of the Pt variant 5, further aggregation with additional AgOTf leads to dimeric {[Ag 2 Pt(CNAr Dipp2 ) 2 -(h 1 -C 6 H 6 )] 2 (m-OTf) 2 }(OTf) 2 (7) containing triangulo-PtAg 2 cores. It is shown that binding of one (compounds 5 and 6) and two (compound 7) equivalents of Ag(I) results in a sequential increase in the Lewis acidity of the group 10 metal center, thus illustrating how s-acceptor fragments can be used to rationally tune the properties of electron-rich transition metal complexes.

Results and discussion
Similar to the zero-valent Pd congener, Pd(CNAr Dipp2 ) 2 , 50 the addition of TlOTf to a solution of Pt(CNAr Dipp2 ) 2 in Et 2 O yields the unsupported heterobimetallic compound [TlPt(CNAr Dipp2 ) 2 ] OTf (1) as a yellow microcrystalline solid. Structural characterization of 1 (Scheme 1 and Fig. 1) reveals a T-shaped coordination geometry about Pt, while the Tl center makes long, but non-negligible contacts with the [OTf] À anion (d(Tl-O3) ¼ 2.799(5)Å) and the C aryl atoms of the Dipp rings (shortest d(Tl-C aryl ) ¼ 3.355Å). The presence of a Pt-Tl bonding interaction is apparent given their interatomic separation of 2.8617(3)Å. Importantly, this value is comparable to the most reasonable range for the sum of the covalent radii between Pt and Tl (2.67-2.84Å), 56 thereby suggesting that the solid-state structure of 1 does not simply arise from the co-crystallization of Pt(CNAr Dipp2 ) 2 with TlOTf. While the role of closed-shell metallophilic interactions 57 cannot be completely discounted, spectroscopic evidence indicates that this interaction is formed by a reverse-dative s-bond, whereby Pt donates two electrons to an empty 6p orbital on Tl. Analysis of these solutions by FTIR spectroscopy shows a strong n(C^N) band at 2112 cm À1 , which is shied to higher energy relative to those of Pt(CNAr Dipp2 ) 2 (2065, 2020 cm À1 ), 27 consistent with a decrease in p-backbonding interactions to the isocyanides as a result of the formation of a Pt / Tl retrodative s-bonding interaction. A similar blue-shi of this band for the palladium analogue [TlPd(CNAr Dipp2 ) 2 ]OTf (2) with respect to Pd(CNAr Dipp2 ) 2 was observed previously. 50 Surprisingly, bonds between electronrich, late transition metals (especially third-row metals) and Tl(I) have oen been rationalized largely based on metallophilic interactions. [58][59][60][61] However, the FTIR spectra of 1 and 2 compared with those of M(CNAr Dipp2 ) 2 (M ¼ Pt, Pd) provide strong experimental evidence that late-metal-Tl(I) bonds likely contain a substantial dative-bonding component in a manner analogous to that seen for complexes bearing main-group Z-type ligands. [23][24][25] Although [TlPt(CNAr Dipp2 ) 2 ]OTf (1) gives rise to a sharp set of 1 H and 13 C{ 1 H} NMR resonances in benzene-d 6 , other spectroscopic data suggest that the metal-metal interaction is labile in solution. While the IR absorption bands of Pt(CNAr Dipp2 ) 2 are not apparent in the IR spectrum of 1, it is important to note that n(C^N) bands corresponding to Pd(CNAr Dipp2 ) 2 are readily observable as a minor component in the IR spectrum of [TlPd(CNAr Dipp2 ) 2 ]OTf (2) in C 6 D 6 solution, thereby suggesting the presence of an equilibrium between bound and unbound Tl(I) (Fig. 2). In addition, extended scanning failed to locate the 195 Pt NMR 62 resonance for the platinum analogue [TlPt(CNAr Dipp2 ) 2 ]OTf (1). We suggest that this observation is indicative of lability in the Pt-Tl interaction on the NMR timescale, resulting in a broadening of this resonance that obviates its detection at room temperature.
As the lability of unsupported M-Tl linkages has been observed to display a dependence on counteranion identity, 58 we sought to explore the behavior of [TlM(CNAr Dipp2 ) 2 ] + (M ¼ Pt, Pd) when accompanied by a traditionally non-coordinating anion. Addition of an  4)). Furthermore, as noted for 1, long-range contacts (ca. 3.4Å) between Tl and several C aryl atoms of the anking Dipp rings are apparent. The Tl-bound ether molecules are easily liberated from crystalline samples    Examination of the solid-state and solution-phase behaviour of 1-4 reveals that replacement of the triate anion with [BAr F 4 ] À has important ramications for the lability of the M / Tl linkage. In both solvates of 3 and 4, the M-Tl distance is signicantly contracted relative to 1 and 2 (Table 1), consistent with an increase in the degree of M / Tl donation. As the tri-ate anion likely stabilizes the Tl center through contact ion pairing, replacement of [OTf] À with neutral Et 2 O or arene donors appears to only partially compensate for the loss of this ionic association. Accordingly, in an attempt to recoup some of this stabilization, we contend that the degree of M / Tl sdonation is increased. This notion is supported by the progression of the n(C^N) bands in 3 (2121 cm À1 ) and 4 (2116 cm À1 ) to higher energies relative to 1 and 2, as the increased withdrawal of electron density from the group 10 metal by Tl serves to attenuate backbonding interactions with the isocyanide ligands. Importantly, and in contrast to the triate salt [TlPd(CNAr Dipp2 ) 2 ]OTf (2), the solution FTIR spectra of 3 and 4 in benzene-d 6 are devoid of n(C^N) features corresponding to M(CNAr Dipp2 ) 2 , signalling that Tl(I) dissociation in benzene can be signicantly inhibited by the use of the weakly coordinating [BAr F 4 ] À anion. Interestingly, this replacement also allows for detection of the 195 Pt NMR resonance of [TlPt(CNAr Dipp2 ) 2 ]BAr F 4 (3), which appears as a doublet with well-resolved coupling to 205 Tl (d ¼ À3802 ppm, 1 J Pt,Tl ¼ 11.2 kHz). 68 This resonance is shied signicantly downeld respective to that of Pt(CNAr Dipp2 ) 2 (d ¼ À5993 ppm (s), C 6 D 6 ), further suggestive of decreased electron density at the Pt center upon coordination of Tl(I). 69,70 However, it is also important to note that dissolution of 1-4 in THF results in complete dissociation of the Tl(I) center and formation of M(CNAr Dipp2 ) 2 , according to FTIR spectroscopy. This result, which was similarly observed in the case of [TlNi(CNAr Dipp2 ) 3 ]OTf, 54 serves as a reminder of the weak dissociation energies inherent in most unsupported metalmetal dative bonds, as dissolution in solvents of moderate coordinating strength is sufficient to completely disrupt this interaction.
Although limited experimental techniques are capable of probing metal-metal dative interactions, X-ray Absorption Near-Edge Spectroscopy (XANES) 71 has begun to nd an important use in this regard. 30,34 Importantly, its utility lies in its ability to decipher the spectroscopic oxidation states of the metals involved in a given bonding interaction. In order to assess the degree of charge transfer inherent in the formation of a reverse-dative s-interaction to Tl(I), Pd K-edge XANES was carried out on the palladium-thallium adduct [TlPd(CNAr Dipp2 ) 2 ]OTf (2, Fig. 6). While neither the Pd K-edge spectra of 2 nor that of Pd(CNAr Dipp2 ) 2 display a discernable preedge feature, both exhibit nearly identical energies for the rising edge of the XANES region. In comparison, the rising edge energy of the Pd(II) peroxo complex 50,72 Pd(h 2 -O 2 )(CNAr Dipp2 ) 2 is shied to higher energy by ca. 4.0 eV relative to that of Pd(CNAr Dipp2 ) 2 and 2. Despite their differing geometries, the rising edge transition for each of these three Pd complexes should involve the promotion of a core 1s electron to a 5p orbital that is relatively unperturbed by ligand eld effects. Accordingly, the marked shi of the rising edge to higher energy for Pd(h 2 -O 2 )(CNAr Dipp2 ) 2 can be reasonably attributed to the presence of an oxidized Pd center relative to that found in Pd(CNAr Dipp2 ) 2 or 2. However, the near-identical rising edge energies observed for Pd(CNAr Dipp2 ) 2 and 2 strongly reect that Tl(I) binding to an electron rich Pd center does not result in a formal oxidative event.
For an additional comparison, XANES measurements were carried out on the binary nickel tris-isocyanide complex Ni(CNAr Mes2 ) 3 and its adduct with Tl(I), [TlNi(CNAr Mes2 ) 3 ]OTf. 49 Despite the unambiguous d 10 conguration of Ni(CNAr Mes2 ) 3 ,  its Ni K-edge absorption spectrum (Fig. 7) displays a prominent pre-edge feature, which is likely the result of a 1s to isocyanide p* transition. The analogous absorption band for [TlNi(CNAr Mes2 ) 3 ]OTf occurs at an identical energy, again signalling that the formation of a reverse-dative M / Tl s-interaction does not result in signicant formal charge transfer from the group 10 metal. The fact that neither Pd(CNAr Dipp2 ) 2 nor Ni(CNAr Mes2 ) 3 undergo signicant charge transfer via the formation of a reverse-dative s-interaction to Tl(I) suggests some important guidelines regarding the proper formalisms that should be used to describe such MOLPs. Although Tl(I) can exhibit Lewis basic properties under extraordinary conditions, 73 the stabilization of its 6s 2 "inert pair" due to relativistic effects 74 should render it a very weak 2e À donor. As such, the electrons involved in a covalent interaction between an electron-rich transition metal and Tl(I) center will most plausibly be supplied by the former, meaning that the valence count of the transition metal must necessarily increase by two units. 45 However, this interaction should not be described as effecting a two-unit increase in the formal oxidation state of the transition metal, as such an event would be readily apparent in the comparative XANES spectra of M(CNR) n and [TlM(CNR) n ] + complexes. This conclusion is further supported by the modest changes in the FTIR n(C^N) energies between the neutral parent compounds and their Tl(I) adducts (ca. 50 cm À1 ). For comparison, the Pd(II) and Ni(II) complexes trans-PdCl 2 (CNAr Dipp2 ) 2 and trans-NiCl 2 (CNAr Mes2 ) 2 display IR n(C^N) bands that are blue-shied by ca. 200 cm À1 relative to Pd(CNAr Dipp2 ) 2 and Ni(CNAr Mes2 ) 3 . 49,75 The abilities of M(CNAr Dipp2 ) 2 (M ¼ Pt, Pd) to act as the basic components of metal-only Lewis pairs can also be extended to Lewis acidic Ag(I) centers. 76 Treatment of Pt(CNAr Dipp2 ) 2 with AgOTf in Et 2 O results in precipitation of the heterobimetallic salt [AgPt(CNAr Dipp2 ) 2 ]OTf (5) as a yellow powder. Attempts to   Table 2 and ESI S1 †). As was seen for the M-Tl adducts above, the metal-metal interactions in 5 and 6 can be rationalized via M / Ag s-donation, a notion that is supported by the increase in isocyanide n(C^N) stretching frequencies relative to M(CNAr Dipp2 ) 2 (M ¼ Pt, Pd) upon coordination of Ag(I) (n(C^N) ¼ 2094 cm À1 (5); 2082 cm À1 (6)). An examination of the solid state structures of 5 and 6 also implicates a role of the anking Dipp aryl rings in buttressing the M-Ag linkage through p-type interactions. Interestingly, these contacts are reected in the room temperature 1 H NMR spectra of 5 and 6 (measured in C 6 D 6 ), for which the resonances corresponding to the Dipp aryl protons are broadened and shied downeld by ca. 0.2 ppm relative to those typically observed for diamagnetic    mononuclear complexes containing the CNAr Dipp2 ligand. It is also notable that the different solvates of both 5 and 6 display a square-planar coordination environment around the group 10 metal. While these geometries are certainly reminiscent of Pt(II) and Pd(II), it is critical to note that the progression of the IR n(C^N) stretching frequencies to higher energies upon binding of Ag(I) is quite modest and actually less than that seen for Tl(I).
This observation serves to suggest that similar bonding descriptions laid out above for Tl-containing 1-4 can be extended to 5 and 6. While the use of electrons from the group 10 metal to form a covalent interaction with Ag requires an increase of two valence units, 45 minimal charge transfer to Ag occurs. As such, these M/Ag MOLPs should not be described as containing formal M(II) centers (M ¼ Pt, Pd). Despite the fact that formation of a M-Ag bonding interaction does not result in a formal oxidative event at Pt/Pd, it is remarkable that the Ag-containing heterobimetallics 5 and 6 will bind THF and arene molecules at the group 10 metal center in the solid state, whereas the zero-valent precursors M(CNAr Dipp2 ) 2 (M ¼ Pt, Pd) do not. Furthermore, Pt(CNAr Dipp2 ) 2 and Pd(CNAr Dipp2 ) 2 do not participate in addition reactions with stronger s-donors (e.g. phosphines) to form species of the type ML(CNAr Dipp2 ) 2 , as the attempted syntheses of such compounds has led invariably to isocyanide dissociation and/or decomposition. While the ability of 5 and 6 to bind an additional Lewis base may be partly attributable to increased positive charge on the complexes, molecular orbital considerations provide a basis for enhanced Lewis acidity at the group 10 metal center of these MOLPs specically. It has been suggested previously that coordination of a Z-type acceptor ligand to a square-planar d 8 complex should result in enhanced affinity for Lewis bases at the open coordination site trans to the acceptor. 82 Similarly, formation of a reverse-dative s-interaction by M(CNAr Dipp2 ) 2 (nominally from the nd z 2 orbital) to an acceptor may have a stabilizing effect on the coaxial empty (n + 1)p z orbital of the group 10 metal (Fig. 11). While such stabilization may not be drastic, it is plausible that such effects could promote the  3 Si) for which the crystal structure shows a molecule of toluene bound in the apical position trans to the silyl group. As silyl ligands can be viewed in certain systems as silylium Lewis acids, 84 the binding of an arene molecule may be a result of Pt-to-Si sdonation, thereby in effect enhancing the Lewis acidic nature of the Pt complex. In addition, similar phenomena have been observed by Gabbaï for a Hg(II) complex 85 and by Berry for a bimetallic Mo 2 system. 86 In these examples, association of a Ztype fragment was shown to increase Lewis acidity at the coordination site trans to the acceptor ligand. However, to our knowledge, the MOLPs derived from M(CNAr Dipp2 ) 2 (M ¼ Pt, Pd) represent unique cases where Z-ligand-promoted Lewis acidity has been unambiguously observed for mononuclear transition metal complexes. Importantly, these observations highlight the ability of s-acceptor ligands to open up a previously unavailable coordination site on a transition metal center without effecting a formal oxidative event. Furthermore, the observation that the Ag-containing complexes 5 and 6 bind solvent molecules at the group 10 metal center, while the Tl-containing complexes 1 and 2 exhibit binding at the Tl center, is likely attributable to the greater electronegativity of Ag relative to Tl. 87 As stabilization of the empty p z orbital on the group 10 metal by a bound Lewis acid is expected to be marginal at best, Lewis acids possessing greater group electronegativity may be expected to more effectively stabilize this orbital and render it accessible to an exogenous Lewis base.
Although [AgPt(CNAr Dipp2 ) 2 ]OTf (5) contains one acceptor fragment bound to platinum, its Pt-Ag unit can accommodate another equivalent of Ag(I). Stirring [AgPt(CNAr Dipp2 ) 2 ]OTf (5) and equimolar AgOTf in THF followed by crystallization from benzene/THF (20 : 1) yields {[Ag 2 Pt(CNAr Dipp2 ) 2 (h 1 -C 6 H 6 )] 2 (m-OTf) 2 }(OTf) 2 (7) as determined by X-ray diffraction. Attempts to synthesize a palladium analogue from [AgPd(CNAr Dipp2 ) 2 ]OTf (6) resulted only in decomposition. The solid-state structure of 7 (Fig. 12) revealed a centro-symmetric dimer composed of triangulo-PtAg 2 cores (average d(Pt-Ag) ¼ 2.6843(6)Å; d(Ag-Ag) ¼ 2.7684(8)Å) bridged by two triate groups. Consistent with the coordination of an additional Lewis acid to the Pt-Ag unit in 5, the isocyanide stretching frequencies of 7 are shied to higher energies (2132, 2169 cm À1 , KBr pellet) compared to 5. In the solid state, the platinum centers in 7 also feature h 1 -C-bound benzene molecules trans to one of the silver atoms as seen in 5(C 6 H 6 ). Interestingly however, the Pt-C benzene distance in 7 (d(Pt-C1) ¼ 2.529(7)Å) is signicantly contracted relative to that in 5(C 6 H 6 ), a further indication of an increase in the Lewis acidity in the Pt center in 7 promoted by the presence of a second Ag center. It is also important to note that relative to complex 5, the second Ag atom in 7 (i.e. Ag(2), Fig. 12) can best be described as occupying the axial position of a nominally square-planar Pt center. As the binding of Lewis acids to the axial position square planar Pt(II) centers is known, 88-92 complex 7 provides additional evidence that the presence of one Z-type ligand effectively results in the formation of a divalent Pt center capable of binding a second Z-type ligand. However, this electronic modulation occurs without formal oxidation of the Pt center.

Conclusions
Zero-valent binary m-terphenyl isocyanide complexes of Pt and Pd are excellent candidates for acting as the Lewis basic component of metal-only Lewis pairs (MOLPs). In addition to stabilizing low oxidation states, the steric encumbrance of the m-terphenyl isocyanide ligand promotes coordinative unsaturation, yielding electron-rich metal centers that can accommodate an exogenous Lewis acid in the primary coordination sphere. Furthermore, the IR n(C^N) resonances provide a convenient handle to assess the degree of group 10 metal sdonation in these complexes. In this work, it has been demonstrated that Pt(CNAr Dipp2 ) 2 and Pd(CNAr Dipp2 ) 2 can form discreet and unsupported adducts with Tl(I). Although these bonding interactions are not particularly robust, use of the weakly coordinating anion BAr F 4 À diminishes the lability of the M-Tl bond in non-coordinating solvents. Analysis of two M / Tl adducts by XANES spectroscopy provides compelling evidence that any degree of formal charge transfer inherent in these metal-metal interactions is minimal, and that therefore no formal oxidative event takes place upon binding of Tl(I). The ability of Pt(CNAr Dipp2 ) 2 and Pd(CNAr Dipp2 ) 2 to form Lewis pairs with Ag(I) has also been demonstrated, with FTIR spectroscopy and X-ray crystallography suggesting, again, that no signicant charge transfer to Ag occurs in the adducts. Despite this fact, the binding of Ag(I) activates the group 10 metal toward the ligation of Lewis bases trans to the Ag acceptor, thus highlighting how sacceptor ligands can be utilized to tune the reactivity proles of electron-rich transition metal complexes. The Pt/Ag MOLP [AgPt(CNAr Dipp2 ) 2 ]OTf (5) can also accommodate an additional equivalent of AgOTf to form dimeric 7, which further increases the Lewis acidity of the Pt center. These results indicate that the presence of a reverse-dative s-interaction can activate the coordination site trans to it for binding of Lewis bases despite the high trans inuence exhibited by Z-type ligands. 24,25 Such modulation can be thought of as a novel type of cooperative effect between a Lewis acid and Lewis base, whereby the former alters the reactivity prole of the electron rich metal without directly participating in the reaction with an incoming substrate. A more detailed understanding of the possibilities afforded by such cooperative effects is currently being pursued in our laboratory.