Templating Open‐ and Closed‐Chain Structures around Metal Complexes of Macrocycles

Abstract

The ability to control the structures of polymetallic complexes is a key concern if we are ever to exploit their properties. A lot of beautiful science has emerged with this challenge, as shown by the grid structures created by Lehn [1] and Thompson, [2] or the panel structures created by Fujita.[3] A particularly attractive idea is to template the structure about an otherwise innocent ion to modify the structure without changing electronic properties. The work of Saalfrank et al. in which different sized metallocryptands were templated is particularly relevant here, [4] as is the work of Raymond and coworkers in which the geometry of the molecule is varied from a dimetallic helix to a tetrahedron depending on the presence of a tetrahedral counterion.[5] Our work has concerned new methods to influence structure, and, in the case of cyclic chromium fluoride cages, we have shown that the size of metal rings can be influenced by the choice of secondary ammonium cations. [6, 7] Here we use metal complexes of macrocycles as templates to show that further unusual structures can be formed.Previously [6] The facile synthesis of these compounds led us to look more widely at a reaction matrix that contains chromium fluoride, pivalate ion, a second metal ion, and templates that contain NH groups. When the second metal is nickel and the template is a simple azacrown ether, either 1,4,7-triazacyclonane (tacn) or 1,4,7,10-tetraazacyclododecane (cyclen), the macrocycle binds the nickel ion and the resultant complex then acts as the template.Initially, chromium fluoride, nickel carbonate, and tacn were treated in pivalic acid to give [Ni(tacn) 2 ][Cr 8 Ni 2 -F 10 (O 2 CCMe 3 ) 20 ] (1), [8] in which the dicationic mononuclear nickel complex was found within a dianionic ring (Figure 1).A better route to 1 involved the prior preparation of [Ni(tacn) 2 ](O 2 CCMe 3 ) 2 and addition of this complex to the reaction mixture (CrF 3 , NiCO 3 , and pivalic acid). The metal sites within the dianion are disordered: two Ni 2+ ions are present according to microanalytical measurement and charge-balance considerations. The structure of the mononuclear dication is very similar to that of [Ni(tacn) 2 ] 2+ crystallized with simpler anions, [10] however, disorder in the structure renders a meaningful comparison of bond lengths difficult. Each of the M···M edges of the ten-metal ring, which is almost planar, is bridged by one fluoride and two pivalate ions in a similar manner to [Cr 7 NiF 8 (O 2 CCMe 3 ) 16 ] À . Each of the NH groups of the two tacn ligands are involved in hydrogen bonds; these bonds are longer than the hydrogen bonds found when simpler amines are used (N···F contacts range from 2.98 to 3.30 rather than 2.7-2.9 ). [6] With the Cr-F-pivalate system, octa-, [6] nona-, [7] and decanuclear metal rings could be synthesized by variation of the template. If 1,4,7, Figure 2) [8] results. The fifteen metal sites define an "S", in which a Ni 2+ ion is located at the center of the "S" and two Ni

The Synthesis and Characterisation of a Molecular Sea‐Serpent: Studies of a {Cr24Cu7} Chain

“…Each Cr⋅⋅⋅Cr contact in the three distinct chains is bridged by a fluoride and two pivalates, the same motif we have seen in previous rings and chains [9–13, 15] . The linking of the Cu sites to the Cr chains varies.…”

Heterometallic Rings: Their Physics and use as Supramolecular Building Blocks

“…Complexes of 1,4,7-triazacyclononane (H 3 tacn) work well, and compounds,s uch as [M'(tacn) 2 ]-[Cr 8 M 2 F 10 (O 2 C t Bu) 20 ]( 16-MM')c an be crystallized ( Figure 7). [21] Thus far only 16-NiNi has been reported, but there is no reason that the family cannot be extended. As the [Ni(tacn) 2 ] 2+ unit is al arge dication, the heterometallic ring expands to ar egular decagon, and it has to include two divalent metal sites to charge balance.T he low-temperature magnetic susceptibility is equivalent to that of an isolated Ni II ion;this is presumably due to the [Ni(tacn) 2 ] 2+ template which implies that the {Cr 8 Ni 2 }ring has an S = 0spin ground state.In turn this must mean that the two Ni II ions neighbor each other in the structure as one must belong to the odd sub-lattice and the other to the even.…”

[CrIII8NiII6]n+ Heterometallic Coordination Cubes

“…Examples include [Cr 7 Ni], [Cr 9 Ni], [Cr 8 Ni 2 ], [Cr 7 Ni 2 ], [Cr 6 Ni 2 ], [Cr 2 Ni 5 ], [CrNi 6 ] wheels and discs (centred/Anderson wheels), and [Cr 14 Ni 2 ] and [Cr 28 Ni 4 ] ‘linked rings’ [ 18 , 19 , 20 , 21 , 22 , 23 ]. Surprisingly, the search reveals only two other unique structural motifs, a rather unusual [Cr 3 Ni 2 ] linear complex [ 24 ], and an ‘S-shaped’ [Cr 12 Ni 3 ] chain [ 25 ]. We have previously reported a metalloligand approach that enabled us to synthesise high-nuclearity heterometallic coordination capsules of paramagnetic transition metal ions in a modular and predictable fashion [ 26 , 27 , 28 , 29 ].…”