A Solomon Link through an Interwoven Molecular Grid

A molecular Solomon link was synthesized through the assembly of an interwoven molecular grid consisting of four bis(benzimidazolepyridyl)benzthiazolo[5,4-d]thiazole ligands and four zinc(II), iron(II), or cobalt(II) cations, followed by ring-closing olefin metathesis. NMR spectroscopy, mass spectrometry, and X-ray crystallography confirmed the doubly interlocked topology, and subsequent demetalation afforded the wholly organic Solomon link. The synthesis, in which each metal ion defines the crossing point of two ligand strands, suggests that interwoven molecular grids should be useful scaffolds for the rational construction of other topologically complex structures.

a [ 2]catenane comprised of doubly interlocked rings. [2] Sauvage reported the first synthesis [5a] of am olecular Solomon link by connecting the end groups of extended linear metal helicates (Figure 1a), however the distance between the ligand strand termini makes this approach unsuitable for extrapolating to higher-order knots and links. [9] Ao ne-pot strategy using acyclic helicate scaffold to bring the end groups into closer proximity has also been described (Figure 1b), [5d] the ligand strands joined through reversible imine formation, which provides am echanism through which connectivity errors can be corrected. [10] Several other Solomon links with metal-bridged [5c,f,6b,c,f] or wholly organic [6a,d,e] frameworks have also been isolated. Here we report on the designed synthesis of aSolomon link utilizing an interwoven 22molecular grid to create ac rossing point at the site of each metal ion ( Figure 1c). Molecular grids,w hich are polytopic ligands coordinated to two-dimensional arrays of metal ions,h ave attracted interest because of their potential magnetic and electronic properties. [11] However,a lmost all the examples prepared to date consist of discrete layers of ligands that lie to each side of the tier of metal ions.Interwoven grids are much rarer, [12] but the crossing of the ligand strands in such systems make them, in principle,s uitable scaffolds for constructing various interlocked structures. [4e] Connecting the adjacent termini of parallel ligand strands in an interwoven 22g rid should afford aS olomon link (Figure 1c).
In our efforts to produce such as ystem, we initially investigated functionalized analogues of ligands previously shown to form interwoven 22g rids,b ut the derivatives we prepared had poor solubility profiles as either the free ligand or as metal complexes.I nt he search for an ew interwovengrid-forming ligand, we identified 1 as ap otential candidate (Scheme 1). Ligand 1 contains two,a ntiparallel, tridentate binding sites formed from at hiazolo [5,4-d]thiazole (TTZ) core connected on each side to pyridylbenzimidazole units. [13] Ab is(bipyridyl)TTZ compound has previously been synthesized as ap otential bis-tridentate ligand, [14] but its coordination chemistry has not been reported. Ligand 1 has terminal alkene chains appropriate for macrocyclization of pairs of 1 by olefin metathesis.T he length of the alkyl linker was chosen so as to permit only connections between parallel ligand strands in the grid coordination complex (see the Supporting Information, Figure S12). Isopentyl groups were integrated into the benzimidazole framework to aid the solubility of the ligand and its complexes.
Addition of an equimolar amount of Zn(BF 4 ) 2 in ethanol to asuspension of 1 in dichloromethane resulted in the rapid solubilization of the ligand and the quantitative assembly of complex [Zn 4 1 4 ](BF 4 ) 8 (Scheme 1). 1 HNMR spectroscopy ( Figure 2b)s howed the presence of as ingle species possessing the same two-fold symmetry as ligand 1.C hemical shift changes of proton signals H a (Dd =+0.92 ppm) and H d (Dd = À1.20 ppm) indicated the formation of octahedral bis(tridentate) complexes consistent with ag rid structure.T he diastereotopic splitting of protons H i confirmed that the two faces of each ligand were in different environments,a ni nherent property of an interwoven 22g rid.
Olefin metathesis using the Hoveyda-Grubbs secondgeneration catalyst [15] (0.1 equiv per olefin) was carried out on [Zn 4 1 4 ](BF 4 ) 8 in a3 :1 mixture of dichloromethane/nitromethane (1.5 mm grid concentration). After 12 ha tr oom temperature,the catalyst was quenched, the solvent removed under reduced pressure and, after trituration with chloroform, acomplex characterized as [Zn 4 2  Single crystals of [Zn 4 2](BF 4 ) 8 suitable for analysis by Xray crystallography were obtained by slow vapor diffusion of diethyl ether into as aturated acetonitrile solution of the Solomon link. Thesolid-state structure [16] (Figure 3) confirms the doubly entwined topology of the two 64-atom loops in [Zn 4 2] 8+ .T he eight aromatic rings of each ligand strand are rendered virtually co-planar by coordination to octahedral zinc(II) cations in the interwoven 22grid motif.Inturn, the four metal centers form an ear-perfect square (Zn-Zn-Zn angles of 90(1) 8 8), thus ensuring that the two ligand strands of each macrocycle are close-to-parallel and orthogonal to those of the other macrocycle.A lthough the 600 MHz 1 HNMR spectrum of [Zn 4 2](BF 4 ) 8 has only one signal for the alkene protons (H g ,F igure 2c), the X-ray crystal structure indicates that am ixture of E and Z olefins form during the olefin metathesis reaction. Six of the eight BF 4 À anions were located in the structure (the remaining two are disordered over several sites), including one in the center of the Solomon link cavity (see the Supporting Information, Figures S10 and S11).
Demetalation of [Zn 4 2](BF 4 ) 8 using Li 2 S [17] proceeded smoothly to afford the wholly organic Solomon link 2 in 52 % yield (Scheme 1). Significant chemical shifts of protons H a (Dd = À1.25 ppm) and H d (Dd =+1.06 ppm) in the 1 HNMR spectrum were consistent with the ligands being metal-free (Figure 2d). Thep rotonated molecular ions [2·n H] n+ (n = 1-4) were observed by ESI-MS (Figure 4a), and on fragmentation of the [2·H] + ion by tandem mass spectrometry,asingly charged species with half the molecular weight of 2 was evident;o ne constituent ring of the [2]catenane (Figure 4b). [18] We successfully repeated the assembly of the grid and Solomon link using either Fe(BF 4 ) 2 or Co(BF 4 ) 2 in place of Zn(BF 4 ) 2 and obtained the corresponding Solomon link complexes in each case (see the Supporting Information). [19] Demetalation of these analogues afforded Solomon link 2 in similar yields to the zinc(II) template synthesis.W hile the signals in the 1 HNMR spectrum of [Fe 4 1 4 ](BF 4 ) 8 in CD 3 CN at room temperature are sharp,t hose of [Fe 4 2](BF 4 ) 8 are significantly broadened ( Figure S2), thus indicating the presence of high-spin Fe II species.L inking the ends of ligands in interwoven grids could prove useful for tuning systems that display spin-crossover behavior. [20] In conclusion, ligand 1 can be quantitatively assembled into interwoven 22m olecular grids using several different octahedral first-row transition-metal ions.Ring-closing olefin metathesis covalently captures the entwined structure in high yield and subsequent removal of the cations affords the metalfree Solomon link. These results suggest that interwoven grids  8 . [16] Isopentyl groups, hydrogen atoms, solvent molecules, and counterions are omitted for clarity.O ne macrocycle is colored turquoise.C olor code for the other macrocycle:C ,grey;N,p urple;O,r ed;S ,gold;Z n, yellow.a)Viewed from above the plane of the zinc ions. b) Viewed from the side.

Angewandte
Chemie could form the basis of ag eneral strategy for the rational synthesis of higher-order knots and links. [4e] Thet olerance of the ligand system to different metal ions could potentially be used to impart magnetic, [11e] electronic, [11d,e,20] or sensing [21] properties to topologically complex molecular structures.