Supramolecular adducts based on weak interactions between the trimeric Lewis acid complex ( per ﬂ uoro-ortho -phenylene)mercury and polypnictogen complexes †‡

Reactions of the trinuclear Lewis acid per ﬂ uoro-ortho -phenylene)mercury [( o -HgC 6 F 4 ) 3 ] ( 1 ) with the polypnictogen complex [CpMo(CO) 2 ( η 3 -P 3 )] ( 2 ) containing a cyclo -P 3 ligand and the series of E 2 complexes [{CpMo(CO) 2 } 2 (µ, η 2 : η 2 -E 2 )] (E = P( 3a ), As( 3b ), Sb( 3c ), Bi( 3d )) are reported. In all cases, the reaction products show very weak interactions between the E n ligand complexes and the Lewis acid 1 , as evidenced by their highly dynamic behaviour in solution and the formation of adducts in the solid state showing Hg ⋯ E contacts below the respective sum of the van der Waals radii. The complexes 2 (P 3 ), 3a (P 2 ) and 3b (As 2 ) show interactions of only one pnictogen atom with all three Hg atoms of 1 . The complex 3c (Sb 2 ) forms two adducts with 1 showing either a side-on coordination of the Sb 2 dumbbell towards Hg or an end-on coordination of both Sb atoms towards two independent molecules of 1 . The Bi 2 complex 3d shows an almost parallel alignment of the Bi 2 dumbbell situated above the center of the planar Lewis acid 1 . The arrangements of the E 2 complex series towards 1 are rationalized with the help of electrostatic potential maps obtained by DFT calculations. Finally the structural characterizations of a new modi ﬁ cation of the free Sb 2 complex 3c , the Bi 2 complex 3d , the starting material of its preparation [Bi{CpMo(CO) 3 } 3 ] ( 4 ) and an unprecedented ‘ Cr 4 As 5 ’ cluster 5 are presented.


Introduction
During the last few decades supramolecular chemistry based on weakly interacting molecules instead of strong covalent bonds has gained more and more attention.The investigation of planar electron-deficient molecules has developed into a rich area of research with applications in anion recognition, 1,2 molecular machines, 3 or light emitting materials. 4In this context, pyridinium cations, 3 or electron deficient aromatic compounds, 1,2 are prototypical representatives.We became particularly interested in the chemistry of the trinuclear organometallic compound (perfluoro-ortho-phenylene)-mercury (1). 5 Due to its planar geometry this molecule contains three sterically accessi-ble mercury centers whose naturally low Lewis acidity is significantly increased by the fluorinated molecular backbone. 6,7This unusual Lewis acid builds up alternating binary stacks with aromatic hydrocarbons [8][9][10][11] and forms double-sandwich complexes featuring Hg⋯Cp interactions when reacted with the metallocenes Cp 2 Fe and Cp 2 Ni (Scheme 1a). 12Additionally 1 forms weak Lewis acid/base adducts with a variety of O, N and S donors which simultaneously interact with all three Hg atoms (Scheme 1b). 6,7,13During the past few years we have extended this chemistry to polyphosphorus complexes, observing only weak P⋯Hg interactions instead of potential Hg⋯Cp interactions in the reactions of 1 and the triple-decker complex [(CpMo) 2 (µ,η 6 :η 6 -P 6 )] (Scheme 1c). 14Recently we reported the differences in coordination behavior of the aromatic cyclo-P 5 and cyclo-As 5 ligands from the ferrocene analogues [Cp*Fe(η 5 -E 5 )] (E = P, As) 15,16 towards the Lewis acid 1. 17 While the P 5 ring forms a weak Lewis acid/base adduct (Scheme 1d) with one P donor atom, the As 5 ring shows a rather cofacial arrangement towards the molecular plane of 1 with three As atoms interacting simultaneously with the Hg centers (Scheme 1e).

Syntheses and spectroscopic characterization of the products
The adducts between the pnictogen complexes and 1 are easily prepared by dissolving 1 and an equimolar amount of the cyclo-P 3 complex 2 or the E 2 complexes 3a-d in 10 mL of CH 2 Cl 2 until a clear solution is formed.After stirring at room temperature, this solution is filtered and the solvent is removed until the filtrate solution is saturated.Storing the resultant solutions at +4 °C results in product crystallization in a matter of hours to days.Following this approach one can obtain pure samples of equimolar adducts of 1 and the cyclo-P 3 complex 2 [(1)•( 2)] (80% isolated yield), the P 2 complex 3a [(1)•(3a)] (75%) and the As 2 complex 3b [(1)•(3b)] (74%), respectively.In the case of the Sb 2 complex 3c two adducts with molar ratios of 1 : 1 [(1)•(3c)] and 2 : 1 [(1) 2 •(3c)] are detected from the same reaction.In the case of the Bi 2 complex 3d we were able to determine the coexistence of the adduct [(1)•(3d) 2 ], pure 1 and pure 3d next to each other in crystalline form from the same reaction.This observation suggests very weak interactions between 1 and 3d and prevents a successful separation of these compounds.Moreover, the Sb 2 complex 3c was isolated in 56% yield after refluxing (Me 3 Si) 2 CHSbH 2 and CpMo(CO) 3 Me in xylene for 3 h.The reaction of the triple-decker complex [(CpCr) 2 (µ,η 5 :η 5 -As 5 )] with 1 enabled the structural characterization of an unprecedented mixed Cr-As cluster 5 (for details see below).Since the described adducts are based on weak Hg⋯E (E = P, As, Sb, Bi) interactions we assume only very weak association in solution.
In accordance with this, in the mass spectra we never observed any peaks assignable to any adduct but only the starting materials of the described products.The 1 H and the 19 F NMR spectra in CD 2 Cl 2 solution are unchanged compared to the starting compounds.While the 31 P{ 1 H} NMR spectrum of [(1)•(2)] in CD 2 Cl 2 solution shows a sharp singlet at −349.5 ppm which is also unchanged to the starting compound 2 we can observe an upfield shift of about 11 ppm for the singlet of the P 2 ligand 3a in the case of the adduct [(1)•(3a)].
Structural characterization of the products in the solid state General considerations.The solid state structures of the formed adducts exhibit short contacts below the sum of the van der Waals (vdW) radii between pnictogen atoms and the Hg atoms of 1.The vdW radius of Hg in different compounds is discussed in the literature with reported values ranging from 1.7 Å up to 2.2 Å.9][20][21][22][23] Therefore, Hg-E distances that are within the sum of the vdW radii of 3.6 Å(P), 3.7 Å(As), 3.9 Å(Sb) or 4.1 Å(Bi) are highlighted by fragmented orange bonds in the following Fig. 1-7.
Crystal structures of the Sb 2 complex 3c, the Bi 2 complex 3d and the starting material [{CpMo(CO) 3 } 3 Bi] (4) Although the synthesis of the Bi 2 complex 3d was already reported in 1988 the solid state structure was only described of a derivative bearing one methyl group on each Cp ligand. 25,26uring our investigations we were able to determine the solid state structures of 3d and of the starting material 4 shown in Fig. 2(a) and (c).The synthesis and X-ray structure of the analogous Sb 2 complex 3c was reported 27 earlier.Moreover, we could structurally characterize a new modification of 3c by X-ray analysis (see Fig. 2(b)).
The new modification of 3c crystallizes in the trigonal space group P3 2 21 with half a complex in the asymmetric unit.The complex is crystallographically completed by a twofold rotation axis.The determined bond lengths are in good agreement with the previously determined structure (Mo-Mo 3.114(1) Å, Sb-Sb 2.678(1) Å).

Structural characterization of the adducts formed from 1 and the E 2 complexes 3a-d
The adduct [(1)•(3a)] crystallizes in the triclinic space group P1 with two independent formula units [(1)•(3a)] in the asymmetric unit shown in Fig. 3(a) and (b).Both molecules of 3a interact with all three Hg atoms of the Lewis acid 1 via one P atom which is situated above the center of 1 while the other P atom shows no Hg⋯P contacts.The resulting Hg-P distances lie in the range of 3.10401(15)-3.3557(2)Å.The P-P bond lengths are not affected in comparison to free 3a (2.079(2) Å) 28 by the coordination.The angles between the direction of the P 2 dumbbell and the Hg 3 plane measure 36.33(1)°(P1-P2) and 38.05(1)°(P3-P4), respectively.
When looking at the As 2 complex 3b and its interaction with 1 in the solid state shown in Fig. 3(c) it can be noted that the assembly in the adduct [(1)•(3b)] is very similar to the P 2 complex 3a.This observation was very surprising since we previously observed a distinctly different assembly of cyclo-P 5 and cyclo-As   while the second As atom does not show any Hg⋯As contacts.The resulting Hg-As distances lie in the range of 3.27021(13)-3.37875(13)Å and are well within the sum of the vdW radii for Hg and As (3.7 Å).The As-As bond length is 2.31607(10) Å long which is essentially unchanged to free 3b (2.305(3) Å). 29 The angle enclosed by the As 2 dumbbell and the Hg 3 plane (37.00(1)°) also compares very well to the analogous P 2 adduct.
When the Sb 2 complex 3c is allowed to react with the Lewis acid 1 two sorts of crystals (orange and red) are formed from the same solution.Single crystal X-ray diffraction analysis revealed that the orange crystals consist of the equimolar adduct of 1 and 3c [(1)•(3c)] depicted in Fig. 4a.For E = Sb, the assembly of the Mo 2 E 2 complex is different from E = P, As.The most apparent distinction is the angle enclosed by the E 2 dumbbell and the Hg 3 plane in [(1)•(3c)] which measures only 7.03(1)°.Although Sb1 is also situated above the center of 1 the resulting Hg-Sb distances differ significantly.The shortest distance is observed for Hg1-Sb1 with 3.4336(1) Å while the distances to Hg2 and Hg3 are almost at the edge of vdW contacts (3.9 Å).The observed arrangement of 1 and 3c shows one additional Hg-Sb contact between Sb2 and Hg3 with 3.5222(1) Å and can be described as a side-on coordination of the Sb-Sb bond to Hg3.The Sb-Sb bond length of 2.6914(1) Å is essentially unchanged compared to free 3c (vide supra). 27X-ray analysis of the red crystals from the same reaction consist of the adduct [(1) 2 •(3c)] where one Sb 2 complex 3c is surrounded by two molecules of the Lewis acid 1 depicted in Fig. 5(a).The Sb atom Sb1 shows only one contact to Hg1 with 3.5283(9) Å.The adduct [(1) 2 •(3c)] crystallizes in the monoclinic space group P2/n.The twofold rotation axis bisects the Mo-Mo bond as well as the Sb-Sb bond of 3c (see Fig. 5b).The angle enclosed by both Hg 3 planes amounts to 53.45(1)°.The Sb1-Sb1′ bond length of 2.6863(13) Å is consistently unchanged compared to free 3c.
When the reactivity of the Bi 2 complex 3d towards the Lewis acid 1 was investigated by using different stoichiometries, black crystals of pure 3d, colorless crystals of pure 1 and black crystals of the adduct [(1)•(3d) 2 ] were obtained next to each other from one reaction.Although numerous crystals were mounted it was not possible to identify any adducts of 1 and 3d in different molar ratios.The solid state structure of [( 1)•(3d) 2 ] is depicted in Fig. 4(b).The compound crystallizes in the triclinic space group P1 ˉwith two units of 3d interacting with one molecule of the Lewis acid 1 from both sides via both Bi atoms in the asymmetric unit.The Bi-Bi dumbbells are arranged almost parallel to the Hg 3 plane with enclosed angles of 2.50(1)°and 5.72(1)°, respectively.The center of 1 is not situated below one of the Bi atoms but rather the middle of the Bi-Bi bonds.The resulting Hg-Bi distances vary significantly in the range of 3.48590(8)-4.04806(13)Å.The Bi-Bi bond lengths in the adduct [(1)•(3d) 2 ] are essentially identical to the free complex 3d (vide supra). 25,26n order to aid in the rationalization of the observed structural trends for the adducts between 3a-3d and 1, their constituent compounds were optimized by density functional theory (DFT) methods.The large computed HOMO-LUMO gaps between 1 and 3a-d of 3.06, 3.00, 2.88, and 2.74 eV, respectively, suggest that orbital-based interactions are not likely to be dominant within the pnictogen-mercury bonding found in these adducts.Instead, similar to the related adducts of 1 with the cyclo-P 5 and cyclo-As 5 rings of [Cp*Fe(η 5 -E 5 )] complexes, 17 electrostatic and dispersion forces are likely to factor strongly in the stabilization of the observed structures.In order to probe the potential role of electrostatic forces in the observed adducts, we inspected the electrostatic potential maps of the constituent complexes shown in Fig. 6.In both 3a and 3b, the electrostatic potential surface shows accumulation of negative character both across the length of the E-E bond and at the phosphorus and arsenic termini.For the electrostatic potential map of complex 3c, negative character is preferentially accumulated at the Sb-Sb bond, with modest negative character still present at the Sb termini.Finally, in the electrostatic potential map of complex 3d, negative character is expressed almost exclusively at the Bi-Bi bond.Taken together, these results suggest that in adducts of 1 with 3a and 3b, the surfaces of the pnictogen termini are complimentary with the positive surface of the mercury centers in 1. Complex 3c appears to be a borderline case, with both residual negative character at the Sb termini and increasing preference for negative character accumulation at the Sb-Sb bond possibly accounting for the two distinctly different modes which can be observed in the two adducts of 1 and 3c.Finally, the lack of negative character at the Bi termini in complex 3d accounts for the observed side-on binding mode, as the surface of the Bi-Bi bond is most complimentary with the positive character found at the mercury surfaces in The Cr-As cluster 5 was characterized by chance when [(CpCr) 2 (µ,η 5 :η 5 -As 5 )] 31 was reacted with 1.The reaction conducted at room temperature resulted in the formation of amorphous black powder which could not be further characterized and a few dark green crystals of the adduct [(1) 2 •( 5 There is a η 5 -bound Cp ligand on each Cr atom.Since the Lewis acid 1 generally shows the formation of weakly interacting adducts in combination with pnictogen donors 14,17 (see above) without changing the geometry or composition of the reactants we assume that the unprecedented cluster 5 was an impurity of the used [(CpCr) 2 (µ,η 5 :η 5 -As 5 )] sample that could not be separated by chromatography before (most probably formed as a byproduct in the thermolytic synthesis of [(CpCr) 2 (µ,η 5 :η 5 -As 5 )]). 31The cluster could presumably only be characterized by  X-ray diffraction as it crystallizes very well in the adduct with 1 due to the stabilization by several Hg⋯As contacts in the solid state.Unfortunately, 5 could not be further characterized due to the very low yield of this side-product.

Conclusion
The presented results demonstrate the facile preparation of adducts starting from the planar Lewis acid 1 and polypnictogen complexes.While the interactions of the Hg atoms of 1 and the pnictogen donors are very weak resulting in dissociation in solution, several contacts below the sum of the vdW radii can be observed in the solid state.Here, the polyphosphorus complexes 2 (P 3 ) and 3a (P 2 ) show interaction of only one P atom with all three Hg atoms of 1 in accordance with previously described results from reactions of cyclo-P 5 and cyclo-P 6 complexes with 1.In the series of the E 2 complexes 3a-d a significant influence of the pnictogen atom (E = P, As, Sb, Bi) is revealed.While the assembly of the As 2 complex 3b is very similar to its P analogue, a different mode of aggregation was observed for the Sb 2 and Bi 2 complexes 3c and 3d.In case of 3c the formation of two adducts with 1 was detected.Therein, 3c shows two distinctly different coordination modes towards the planar Lewis acid 1.The Sb 2 ligand either shows a side-on arrangement towards one molecule of 1 or an end-on coordination of both Sb atoms towards two independent molecules of 1.In contrast, the Bi 2 complex 3d shows an almost parallel alignment of the Bi 2 dumbbell situated above the center of the planar Lewis acid 1.The isolation of [(1)•(3d) 2 ] does not only represent the first adduct of 1 with a Bi based Lewis base, but is also the first follow-up chemistry that has been described for the Bi 2 complex 3d.The observed arrangements of the E 2 complex series can be well rationalized by analysis of the electrostatic potential maps of the complexes 3a-d.In addition, QTAIM analysis and direct comparison to the previously described adducts of 1 and the cyclo-P 5 and cyclo-As 5 ligands of [Cp*Fe(η 5 -E 5 )] complexes suggest that electrostatic and dispersion forces predominate in the adducts of 3a-d with 1. Finally, the structural characterization of a new modification of the Sb 2 complex [{CpMo(CO) 2 } 2 (µ,η 2 :η 2 -Sb 2 )] (3c), the Bi 2 complex [{CpMo(CO) 2 } 2 (µ,η 2 :η 2 -Bi 2 )] (3d), the starting material [{CpMo(CO) 3 } 3 Bi] (4), and a novel [(CpCr) 4 As 5 ] cluster 5 are presented.
5 ligands with 1 (see Scheme 1(d) and (e)).The compound [(1)•(3b)] crystallizes in a different unit cell (but same space group P1 ˉ) than [(1)•(3a)] containing only one formula unit in the asymmetric unit.The As 2 complex 3b is interacting with all three Hg atoms of the Lewis acid 1 via one As atom

1 .
In an effort to gain further insight into the nature of the E-Hg interactions, adducts [(1)•(3a)], [(1)•(3b)], [(1) 2 •(3c)], [(1)•(3c)], and [(1)•(3d) 2 ] were subjected to QTAIM 30 analysis at their crystal structure geometries (see ESI ‡ for details).In all cases, bond critical points (BCPs) were found between the pnictogen atoms of 3a-d and the mercury atoms of 1.Additional intermolecular BCPs were found between the oxygen atoms of the carbonyl ligands of 3a-d and the mercury and carbon atoms of 1. Inspection of the features of the electron density distribution function (ρ(r)) at the intermolecular BCPs found that similar to the previously observed adducts of 1 with the cyclo-P 5 and cyclo-As 5 rings of [Cp*Fe(η 5 -E 5 )] complexes, electrostatic and dispersion forces predominate in the adducts of 3a-d with 1. Structural characterization of the adduct [(1) 2 •(5)] and the 'Cr 4 As 5 ' cluster 5 )].The solid state structure of [(1) 2 •(5)] is depicted in Fig. 7(a) showing the unprecedented 'Cr 4 As 5 ' cluster 5 surrounded by two molecules of 1.The adduct [(1) 2 •(5)] crystallizes in the monoclinic space group C2/c with the twofold rotation axis running through As3 as well as the middle of the As1-As1′ bond.The Hg 3 planes of the two molecules of 1 are arranged almost perpendicular to each other with an enclosed angle of 87.36(1)°.The assembly results in eight Hg-As distances below the sum of the vdW radii (3.7 Å).The cluster core of 5 depicted in Fig. 7(b) consists of four Cr atoms in a butterfly structure with Cr-Cr bond lengths of 2.9075(1)-3.0134(1)Å, one As 2 ligand and one As 3 ligand.

Fig. 6
Fig. 6 Electrostatic potential surfaces of compounds 1 and 3a-d (isovalue = 0.004).3a-d: viewing direction along the C 2 axis of the complexes.E 2 bonds are in front.