Anion Capture at the Open Core of a Geometrically Flexible Dicopper(II,II) Macrocycle Complex

: Multicopper active sites for small molecule activation in materials and enzymatic systems rely on controlled but adaptable coordination spheres about copper clusters for enabling challenging chemical transformations. To translate this constrained ﬂexibility into molecular multicopper complexes, developments are needed in both ligand design for clusters and synthetic strategies for modifying the cluster cores. The present study investigates the chemistry of a class of pyridyldiimine-derived macrocycles with geometrically ﬂexible aliphatic linkers of varying lengths ( n PDI 2 , n = 2, 3). A series of dicopper complexes bound by the n PDI 2 ligands are described and found to exhibit improved solubility over their parent analogs due to the incorporation of 4-t Bu groups on the pyridyl units and the use of triﬂate counterions. The ensuing synthetic study investigated methods for introducing various bridging ligands ( µ -X; X = F, Cl, Br, N 3 , NO 2 , OSiMe 3 , OH, OTf) between the two copper centers within the macrocycle-supported complexes. Traditional anion metathesis routes were unsuccessful, but the abstraction of bridging halides resulted in “open-core” complexes suitable for capturing various anions. The geometric ﬂexibility of the n PDI 2 macrocycles was reﬂected in the various solid-state geometries, Cu–Cu distances, and relative Cu coordination spheres on variation in the identity of the captured anion.


Introduction
Controlled internuclear geometries are a defining feature in multicopper active sites for small molecule functionalization. These sites arise in materials, like various Cuimpregnated zeolites that perform selective methane oxidation [1][2][3], and in a host of enzymes, including those that either selectively oxidize methane (particulate methane monooxygenase), phenols (tyrosinase), and quinones (tyrosinase and catechol oxidase) or transport oxygen (hemocyanin) [4]. It is of interest to develop molecular complexes that mimic the properties of multinuclear active sites, as doing so may yield insight into key structure-function relationships for strong bond activation schemes. However, enzymatic copper-copper distances, for example, range from 2.2-4.9 Å and are known to change upon substrate binding [4]. Modulating both the distance and relative coordination environments between multiple copper centers in molecular systems may influence how small molecules react at a multicopper active site, but ligand design for multinuclear transition metal systems is in its infancy, especially for systems that accommodate structural rearrangements.
Examples of ligands capable of housing multiple copper centers [5] and binding molecules of various sizes have been realized in the areas of anion recognition/sensing [6][7][8], dioxygen activation [9,10], organic azide reduction [11], aryl group transfer, and alkyne activation [12,13]. From these studies, an important feature of ligand design is the capacity to tune the distance between the copper centers. Drew and Nelson originally targeted this aspect in the 1980s when designing and synthesizing macrocyclic bis(pyridyldiimine) Inorganics 2023, 11, 348 2 of 21 ligands [14][15][16][17]. The two pyridyldiimine (PDI) units linked by aliphatic spacers were suitable for binding two copper(II) centers at specific distances ( Figure 1) while allowing for flexibility in the coordination environments of the metal centers with respect to one another. For example, the use of a 3,6-dioxaoctylene spacer allowed for the formation of a bridging azide complex, [( Oct PDI 2 )Cu 2 (N 3 ) 3 ][ClO 4 ] [15], with a µ-1, 3-binding azide that spans the long Cu-Cu distance of 6.02 Å, but a µ-hydroxide complex with the same ligand, [( Oct PDI 2 )Cu 2 OH][ClO 4 ] 3 ·H 2 O [14], makes use of the flexibility of the aliphatic linker as the macrocycle contorts to accommodate a shorter Cu-Cu distance of 3.57 Å.
Inorganics 2023, 11, x FOR PEER REVIEW 2 of 20 capacity to tune the distance between the copper centers. Drew and Nelson originally targeted this aspect in the 1980s when designing and synthesizing macrocyclic bis(pyridyldiimine) ligands [14][15][16][17]. The two pyridyldiimine (PDI) units linked by aliphatic spacers were suitable for binding two copper(II) centers at specific distances ( Figure 1) while allowing for flexibility in the coordination environments of the metal centers with respect to one another. For example, the use of a 3,6-dioxaoctylene spacer allowed for the formation of a bridging azide complex, [( Oct PDI2)Cu2(N3)3][ClO4] [15], with a µ-1, 3-binding azide that spans the long Cu-Cu distance of 6.02 Å, but a µ-hydroxide complex with the same ligand, [( Oct PDI2)Cu2OH][ClO4]3·H2O) [14], makes use of the flexibility of the aliphatic linker as the macrocycle contorts to accommodate a shorter Cu-Cu distance of 3.57 Å. The bis(pyridyldiimine) macrocyclic ligands have provided a useful platform to study multinuclear copper chemistry, but synthetic elaborations and structural information on these systems are limited. Previous work in our lab has shown that the syntheses of dinuclear first-row transition metal bis(pyridyldiimine) macrocyclic complexes were aided by the installation of tert-butyl groups at the 4-position of the PDI units [18][19][20][21][22][23][24][25]. The tert-butyl groups allow for an enhancement in solubility in less polar organic solvents (THF, DCM, and MeCN) compared to the original macrocycles reported by Drew and Nelson. We thus sought to synthesize a new series of Cu2(II,II) bis(pyridyldiimine) macrocycles for structural characterization. Herein, new synthetic routes to Cu2(II,II) bis(pyridyldiimine) macrocyclic complexes are explored with versatile ligand substitution being achieved.

Ring-Size Modulated Structures of [Cu2Cl2] 2+ Cluster Cores
The synthesis of a Cu2(II,II) bis(4-tert-butylpyridyldiimine) macrocyclic complex was carried out using the strontium-templated macrocycle [ 2+ ) in good yield (Scheme 1). Crystallographic analysis revealed that the copper(II) centers are related by an inversion center ( Figure 2). The metal sites adopt a distorted octahedral geometry, with a PDI subunit and a bridging chloride occupying the equatorial positions and a triflate anion and a bridging chloride occupying the axial positions (selected bond metrics are presented in Table 1). No six-coordinate mononuclear copper(II) chloride PDI complexes have been reported in the literature. However, the Cu-N bond lengths are in good agreement with the related four-coordinate [( dipp PDI)CuCl][PF6] reported by Conan. The equatorial Cu-Cl bond length of 2.18602(11) Å agrees well with Conan's observed Cu-Cl bond length of 2.1450(5) Å. The axial Cu-O (2.710 Å) and Cu-Cl (2.873 Å) distances are elongated, owing to a strong Jahn-Teller distortion at the d 9 Cu(II) ion. Due to this distortion, the Cu2Cl2 diamond core exhibits a very acute Cu-Cl-Cu angle (75.05°). The structure reported here for 3 [Cu2Cl2] 2+ closely resembles that of [( 3 PDI2)Cu2Cl2(thf)2][BAr F 4]2 (Ar F = 3,5-(CF3)2-C6H3), which was synthesized using a similar scheme but with THF as the solvent and with the addition of NaBAr F 4. The difference in The bis(pyridyldiimine) macrocyclic ligands have provided a useful platform to study multinuclear copper chemistry, but synthetic elaborations and structural information on these systems are limited. Previous work in our lab has shown that the syntheses of dinuclear first-row transition metal bis(pyridyldiimine) macrocyclic complexes were aided by the installation of tert-butyl groups at the 4-position of the PDI units [18][19][20][21][22][23][24][25]. The tert-butyl groups allow for an enhancement in solubility in less polar organic solvents (THF, DCM, and MeCN) compared to the original macrocycles reported by Drew and Nelson. We thus sought to synthesize a new series of Cu 2 (II,II) bis(pyridyldiimine) macrocycles for structural characterization. Herein, new synthetic routes to Cu 2 (II,II) bis(pyridyldiimine) macrocyclic complexes are explored with versatile ligand substitution being achieved.

Ring-Size Modulated Structures of [Cu 2 Cl 2 ] 2+ Cluster Cores
The synthesis of a Cu 2 (II,II) bis(4-tert-butylpyridyldiimine) macrocyclic complex was carried out using the strontium-templated macrocycle [ 2+ ) in good yield (Scheme 1). Crystallographic analysis revealed that the copper(II) centers are related by an inversion center ( Figure 2). The metal sites adopt a distorted octahedral geometry, with a PDI subunit and a bridging chloride occupying the equatorial positions and a triflate anion and a bridging chloride occupying the axial positions (selected bond metrics are presented in Table 1 , which was synthesized using a similar scheme but with THF as the solvent and with the addition of NaBAr F 4 . The difference in axial ligation between the two 3 PDI 2 structures highlights the weak ionic binding of the axial triflate ions. For this reason and to simplify the descriptions, all triflate ions described in the text will use an ionic formulation, even when they appear bound to a metal center in the solid state. distances, which diverge from the value of 2.873(1) Å for each interaction in 3 [Cu2Cl2] 2+ to values of 2.7273 and 3.021 Å in 2 [Cu2Cl2] 2+ . While these distances are long, they are well within the sum of the van der Waals radii for Cu and Cl (4.20 Å) [28], and previous work from our group on closely related molecules has identified magnetic communication through bridging chloride ligands with Cu-Cl distances up to 3.159(1) Å [21]. Aside from the noted differences in the Cu2Cl2 core, the remaining bond metrics of 2

Chloride Abstraction from 3 [Cu2Cl2] 2+
With 3 [Cu2Cl2] 2+ isolated, anion metathesis was targeted to introduce new ligands at the Cu(II) centers, but little to no reactivity was observed, likely owing to strong bridgingchloride interactions. Chloride abstraction routes were thus employed to enhance the reactivity of the pocket toward anion metathesis. Attempts at chloride abstraction with traditional reagents (KOTf, AgnX (n = 1,2; X = BF4, SO4, OTf, BAr F 4)) were met with either intractable mixtures or no reactivity. Chloride abstraction was achieved with the addition of an excess of TMS-OTf (TMS = trimethylsilyl) to 3  axial ligation between the two 3 PDI2 structures highlights the weak ionic binding of the axial triflate ions. For this reason and to simplify the descriptions, all triflate ions described in the text will use an ionic formulation, even when they appear bound to a metal center in the solid state.  2+ (thermal ellipsoids set at 50% probability; hydrogen atoms and triflate disorder omitted for clarity). Right: Crystal structure of 2 [Cu2Cl2] 2+ (thermal ellipsoids set at 50% probability; hydrogen atoms, solvent molecules, and outer-sphere anions omitted for clarity; one macrocycle of the asymmetric unit shown). The following atoms shown in this figure are color-coded for convenience: Cu (orange), N (blue), C (light grey), Cl (bright green), F (yellowgreen), S(yellow), and O(red).  2+ (thermal ellipsoids set at 50% probability; hydrogen atoms, solvent molecules, and outer-sphere anions omitted for clarity; one macrocycle of the asymmetric unit shown). The following atoms shown in this figure are color-coded for convenience: Cu (orange), N (blue), C (light grey), Cl (bright green), F (yellow-green), S (yellow), and O (red).
An additional parameter that can be gleaned from the crystal structure of 3 [Cu 2 Cl 2 ] 2+ is the oxidation state of the (redox-active) PDI ligand [26]. Wieghardt and coworkers previously reported the ∆ parameter as a means of identifying how much electron density resides in the PDI π* manifold. Our lab recently adapted the ∆ parameter in describing the bis(4-tert-butylpyridyldiimine) macrocyclic ligand oxidation state, which includes the average of each ∆ parameter for each PDI subunit [22]. Using this analysis on the crystal structure of 3 [Cu 2 Cl 2 ] 2+ , the average ∆ parameter of 0.187 Å supports the assignment of a neutral physical oxidation state on the 3 PDI 2 ligand. A neutral ligand for copper PDI complexes is not uncommon, as both mononuclear Cu II (PDI 0 ) and Cu I (PDI 0 ) examples are well established in the literature [26].
The IR spectrum of 3 [Cu 2 Cl 2 ] 2+ contained a characteristic imine stretching frequency at 1601 cm −1 . UV-vis-NIR spectroscopic analysis revealed an absorption profile representative of bis(pyridyldiimine) macrocyclic complexes in an unfolded ligand conformation [21]. Distinct absorption bands at 237 nm (7.71 × 10 4 M −1 cm −1 ) and 676 nm (242 M −1 cm −1 ) result from a π→π* transition and a d→d transition, respectively.   (Figure 2). Two chloride ligands reside between the copper centers; however, variations in the Cu 2 Cl 2 core are evident when compared to 3 [Cu 2 Cl 2 ] 2+ . Unlike the larger macrocycle, each copper center adopts a different coordination geometry. One copper is found in a distorted octahedral geometry, capped by a triflate anion and a bridging chloride (analogous to 3 [Cu 2 Cl 2 ] 2+ ). The second copper adopts a square pyramidal geometry (avg. τ 5 = 0.04) [27], with the second triflate anion residing in the outer coordination sphere. The difference in the coordination geometries about the copper centers is reflected in the distortion of the Cu 2 Cl 2 core, which now appears asymmetric (∠Cu(1)-Cl-Cu(2) = 73.83(4) • and ∠Cu(2)-Cl-Cu(1) = 67.80(4) • ). This distortion is further reflected in the asymmetry of the axial Cu-Cl distances, which diverge from the value of 2.873(1) Å for each interaction in 3 [Cu 2 Cl 2 ] 2+ to values of 2.7273 and 3.021 Å in 2 [Cu 2 Cl 2 ] 2+ . While these distances are long, they are well within the sum of the van der Waals radii for Cu and Cl (4.20 Å) [28], and previous work from our group on closely related molecules has identified magnetic communication through bridging chloride ligands with Cu-Cl distances up to 3.159(1) Å [21]. Aside from the noted differences in the Cu 2

Chloride Abstraction from 3 [Cu 2 Cl 2 ] 2+
With 3 [Cu 2 Cl 2 ] 2+ isolated, anion metathesis was targeted to introduce new ligands at the Cu(II) centers, but little to no reactivity was observed, likely owing to strong bridgingchloride interactions. Chloride abstraction routes were thus employed to enhance the reactivity of the pocket toward anion metathesis. Attempts at chloride abstraction with traditional reagents (KOTf, Ag n X (n = 1,2; X = BF 4 , SO 4 , OTf, BAr F 4 )) were met with either intractable mixtures or no reactivity. Chloride abstraction was achieved with the addition of an excess of TMS-OTf (TMS = trimethylsilyl) to 3 [Cu 2 Cl 2 ] 2+ in MeCN. The formation of a new macrocyclic product was apparent by a sharp color change from dark green to dark blue. The concentration of the reaction mixture afforded the blue acetonitrile-coordinated macrocycle complex [( 3 PDI 2 )Cu 2 (NCMe) 2 (Figure 3). The macrocyclic ligand adopts an unfolded ligand conformation, and the two Cu(II) centers are related by a center of symmetry with a spacing of 3.3479(7) Å. Both Cu(II) centers adopt a distorted square pyramidal geometry (τ5 = 0.36 for each Cu(II) center) with an evident Jahn-Teller distortion causing a long Cu-O (2.275(2) Å) bond length. The axial Cu-NCMe bond lengths (3.160(3) Å) fall outside the sum of the van der Waals radii of copper and nitrogen (2.95 Å). Thus, five-coordinate copper(II) is a better description of the metal centers in this crystal structure. The equatorial Cu-NCMe distance of 1.973(3) Å is significantly shorter than the reported distance of 2.271 (13) in McKee and coworkers' related Cu2(II,II) alkoxy PDI macrocycle [29]. However, the MeCN ligand occupies a distorted axial position of a square pyramidal Cu(II) center in McKee's example, rather than the equatorial position as observed in 3 [Cu2(NCMe)2] 4+ . Gilbertson's distorted square planar copper(I) acetonitrile PDI complex displays a short Cu-NCMe distance of 1.907(7) Å, which better agrees with 3 [Cu2(NCMe)2] 4+ [30]. The IR spectrum of a sample of 3 [Cu2(NCMe)2] 4+ includes three distinct nitrile stretching frequencies in the expected region for acetonitrile (2307 cm −1 , 2279 cm −1 , 2257 cm −1 ). Since 3 [Cu2(NCMe)2] 4+ would have two stretches at most-a low-intensity symmetric stretch and a higher-intensity asymmetric stretch-the IR data suggest the presence of Despite its limited solubility in MeCN, single crystals of 3 [Cu 2 (NCMe) 2 ] 4+ were able to be grown from the slow diffusion of Et 2 O into a concentrated MeCN solution. The solid-state structure was found to resemble that of 3 [Cu 2 Cl 2 ] 2+ , with the two chloride ligands replaced by MeCN ligands (Figure 3). The macrocyclic ligand adopts an unfolded ligand conformation, and the two Cu(II) centers are related by a center of symmetry with a spacing of 3.3479(7) Å. Both Cu(II) centers adopt a distorted square pyramidal geometry (τ 5 = 0.36 for each Cu(II) center) with an evident Jahn-Teller distortion causing a long Cu-O (2.275(2) Å) bond length. The axial Cu-NCMe bond lengths (3.160(3) Å) fall outside the sum of the van der Waals radii of copper and nitrogen (2.95 Å). Thus, five-coordinate copper(II) is a better description of the metal centers in this crystal structure. The equatorial Cu-NCMe distance of 1.973(3) Å is significantly shorter than the reported distance of 2.271 (13) in McKee and coworkers' related Cu 2 (II,II) alkoxy PDI macrocycle [29]. However, the MeCN ligand occupies a distorted axial position of a square pyramidal Cu(II) center in McKee's example, rather than the equatorial position as observed in 3 [Cu 2 (NCMe) 2 ] 4+ . Gilbertson's distorted square planar copper(I) acetonitrile PDI complex displays a short Cu-NCMe distance of 1.907(7) Å, which better agrees with 3 [Cu 2 (NCMe) 2 ] 4+ [30]. Despite its limited solubility in MeCN, single crystals of 3 [Cu2(NCMe)2] 4+ were able to be grown from the slow diffusion of Et2O into a concentrated MeCN solution. The solidstate structure was found to resemble that of 3 [Cu2Cl2] 2+ , with the two chloride ligands replaced by MeCN ligands (Figure 3). The macrocyclic ligand adopts an unfolded ligand conformation, and the two Cu(II) centers are related by a center of symmetry with a spacing of 3.3479 (7) Å. Both Cu(II) centers adopt a distorted square pyramidal geometry (τ5 = 0.36 for each Cu(II) center) with an evident Jahn-Teller distortion causing a long Cu-O (2.275(2) Å) bond length. The axial Cu-NCMe bond lengths (3.160(3) Å) fall outside the sum of the van der Waals radii of copper and nitrogen (2.95 Å). Thus, five-coordinate copper(II) is a better description of the metal centers in this crystal structure. The equatorial Cu-NCMe distance of 1.973(3) Å is significantly shorter than the reported distance of 2.271 (13) in McKee and coworkers' related Cu2(II,II) alkoxy PDI macrocycle [29]. However, the MeCN ligand occupies a distorted axial position of a square pyramidal Cu(II) center in McKee's example, rather than the equatorial position as observed in 3 [Cu2(NCMe)2] 4+ . Gilbertson's distorted square planar copper(I) acetonitrile PDI complex displays a short Cu-NCMe distance of 1.907(7) Å, which better agrees with 3 [Cu2(NCMe)2] 4+ [30]. The IR spectrum of a sample of 3 [Cu2(NCMe)2] 4+ includes three distinct nitrile stretching frequencies in the expected region for acetonitrile (2307 cm −1 , 2279 cm −1 , 2257 cm −1 ). Since 3 [Cu2(NCMe)2] 4+ would have two stretches at most-a low-intensity symmetric stretch and a higher-intensity asymmetric stretch-the IR data suggest the presence of The IR spectrum of a sample of 3 [Cu 2 (NCMe) 2 ] 4+ includes three distinct nitrile stretching frequencies in the expected region for acetonitrile (2307 cm −1 , 2279 cm −1 , 2257 cm −1 ). Since 3 [Cu 2 (NCMe) 2 ] 4+ would have two stretches at most-a low-intensity symmetric stretch and a higher-intensity asymmetric stretch-the IR data suggest the presence of impurities in the isolated material. Although satisfactory elemental analysis data were collected, alternative routes of crystallization were assessed to address the discrepancy in the IR spectral data. The product was found to be partially soluble in o-difluorobenzene. Crystallization from this solvent afforded single crystals that were identified as the monochloride product [( 3 PDI 2 )Cu 2 Cl][OTf] 3 ( 3 [Cu 2 Cl] 3+ ), indicative of incomplete chloride abstraction and the presence of impurities. Prolonged reaction times in the presence of excess TMS-OTf were not able to improve the purity of 3 [Cu 2 (NCMe) 2 ] 4+ . Although not the desired product, the crystal structure of 3 [Cu 2 Cl] 3+ offered insight into how the ligand is able to change in conformation to accommodate one bridging ligand upon the loss of a bridging chloride from 3 [Cu 2 Cl 2 ] 2+ .
The solid-state structure of 3 [Cu 2 Cl] 3+ introduces an arched ligand conformation around the dinuclear Cu 2 Cl core ( Figure 4). Both Cu(II) centers adopt distorted octahedral geometries, with three triflate anions (two terminal and one bridging) occupying the positions perpendicular to the planes of the local PDI units. Jahn-Teller distortions are evident by the elongated Cu-O axial bond lengths (avg. 2.5042 Å). A single chloride bridges the two copper(II) centers with a Cu-Cl-Cu angle of 108.50(3) • (an increase of 33 • compared to 3 [Cu 2 Cl 2 ] 2+ ), and the Cu-Cl bond distances elongate to 2.2906(7) and 2.2739(7) Å. These longer bond distances are still in the expected range for Cu(II) chloride PDI complexes, agreeing well with related five-coordinate complexes. To accommodate the increases in the Cu-Cl-Cu angle and the Cu-Cl distances upon the loss of a bridging chloride from 3 [Cu 2 Cl 2 ] 2+ , the macrocycle conformation changes, and the Cu-Cu distance increases from 3.129(1) Å to 3.7045(7) Å.
Inorganics 2023, 11, x FOR PEER REVIEW 6 of 20 impurities in the isolated material. Although satisfactory elemental analysis data were collected, alternative routes of crystallization were assessed to address the discrepancy in the IR spectral data. The product was found to be partially soluble in o-difluorobenzene. Crystallization from this solvent afforded single crystals that were identified as the mon- 3+ ), indicative of incomplete chloride abstraction and the presence of impurities. Prolonged reaction times in the presence of excess TMS-OTf were not able to improve the purity of 3 [Cu2(NCMe)2] 4+ . Although not the desired product, the crystal structure of 3 [Cu2Cl] 3+ offered insight into how the ligand is able to change in conformation to accommodate one bridging ligand upon the loss of a bridging chloride from 3 [Cu2Cl2] 2+ . The solid-state structure of 3 [Cu2Cl] 3+ introduces an arched ligand conformation around the dinuclear Cu2Cl core ( Figure 4). Both Cu(II) centers adopt distorted octahedral geometries, with three triflate anions (two terminal and one bridging) occupying the positions perpendicular to the planes of the local PDI units. Jahn-Teller distortions are evident by the elongated Cu-O axial bond lengths (avg. 2.5042 Å). A single chloride bridges the two copper(II) centers with a Cu-Cl-Cu angle of 108.50(3)° (an increase of 33° compared to 3 [Cu2Cl2] 2+ ), and the Cu-Cl bond distances elongate to 2.2906(7) and 2.2739(7) Å. These longer bond distances are still in the expected range for Cu(II) chloride PDI complexes, agreeing well with related five-coordinate complexes. To accommodate the increases in the Cu-Cl-Cu angle and the Cu-Cl distances upon the loss of a bridging chloride from 3 [Cu2Cl2] 2+ , the macrocycle conformation changes, and the Cu-Cu distance increases from 3.129(1) Å to 3.7045(7) Å.

A Synthetic Route to an "Open-Core" Cu2(II,II) Macrocycle
Due to incomplete chloride abstraction from 3 [Cu2Cl2] 2+ , we sought a new approach to the synthesis of an "open-core" macrocycle. Attempts to eliminate the halide from the original synthesis (i.e., 3 [Sr] 2+ + 2 Cu(OTf)2) failed to produce an observable reaction when run in DCM, and the products of this reaction were not separable from one another when the reaction was run in MeCN. We next proposed that the strong bridging interactions in 3 [Cu2Cl2] 2+ may be limiting the reactivity of this species towards TMS-OTf. To address this concern, we sought to exchange the bridging chloride ligands for a fluoride ligand. Doing so could then leverage the stronger thermodynamic driving force for Si-F over Si-Cl bond formation on treatment with TMS-OTf.
The synthetic changes were carried out in a one-pot fashion (Scheme 3  3+ ) was identified using single-crystal X-ray crystallography as

A Synthetic Route to an "Open-Core" Cu 2 (II,II) Macrocycle
Due to incomplete chloride abstraction from 3 [Cu 2 Cl 2 ] 2+ , we sought a new approach to the synthesis of an "open-core" macrocycle. Attempts to eliminate the halide from the original synthesis (i.e., 3 [Sr] 2+ + 2 Cu(OTf) 2 ) failed to produce an observable reaction when run in DCM, and the products of this reaction were not separable from one another when the reaction was run in MeCN. We next proposed that the strong bridging interactions in 3 [Cu 2 Cl 2 ] 2+ may be limiting the reactivity of this species towards TMS-OTf. To address this concern, we sought to exchange the bridging chloride ligands for a fluoride ligand. Doing so could then leverage the stronger thermodynamic driving force for Si-F over Si-Cl bond formation on treatment with TMS-OTf.
The synthetic changes were carried out in a one-pot fashion (Scheme 3  3+ ) was identified using single-crystal X-ray crystallography as the first structurally characterized example of a copper(II)-fluoride PDI complex ( Figure 5). Unlike the monochloride species 3 [Cu 2 Cl] 3+ , the dinuclear Cu 2 F core is accom-modated by an unfolded conformation of the macrocyclic ligand. The crystal structure also displays a linear arrangement of atoms within the macrocyclic pocket leading to a Cu-F-Cu angle of 180.00 • . For the fluoride ligand to fit in this linear arrangement, the Cu-Cu distance increases to 3.7240(4) Å, a similar distance to 3 [Cu 2 Cl] 3+ . Each Cu(II) center is capped by a triflate anion (Cu-O distance of 2.2707(19) Å) and adopts a square pyramidal geometry (τ 5 = 0.03 for each Cu(II) center). A large thermal ellipsoid is present on the µ-F, indicating that the bridging fluoride unit is oscillating back and forth in the macrocyclic pocket. The short Cu-F bond length (1.8620(3) Å) agrees with the related fourcoordinate pyridyldiamide copper(II)-fluoride complex reported by Zhang and coworkers (1.828(2) Å) [31,32].  The removal of the fluoride ligand using TMS-OTf was carried out in DCM at −78 °C. The dark blue slurry became light blue upon the slow addition of TMS-OTf. Combustion analysis supported the formation of the Cu2(II,II) tetratriflate macrocyclic complex 4+ ). The product, like 3 [Cu2(NCMe)2] 4+ , was insoluble in common organic solvents, and, due to the increased insolubility, single crystals of the product remained elusive. However, based on the structural data obtained for 3 [Cu2Cl2] 2+ and 3 [Cu2(NCMe)2] 4+ , an unfolded ligand conformation is a reasonable assignment of the product's overall geometry, and considering the similarity in the colors of 3 [Cu2(NCMe)2] 4+ , 3 [Cu2F] 3+ , and 3 [Cu2] 4+ as well as the low solubility of 3 [Cu2] 4+ , an ionic structure with fivecoordinate Cu centers would be reasonable to propose. This formulation could be achieved through a number of combinations of triflate coordination mode, denticity, and ionicity. In the absence of further structural data, we have chosen to represent 3 [Cu2] 4+ with the generic structure shown in Scheme 3 while acknowledging that another formulation may well represent the structure of this species.  The removal of the fluoride ligand using TMS-OTf was carried out in DCM at −78 °C. The dark blue slurry became light blue upon the slow addition of TMS-OTf. Combustion analysis supported the formation of the Cu2(II,II) tetratriflate macrocyclic complex 4+ ). The product, like 3 [Cu2(NCMe)2] 4+ , was insoluble in common organic solvents, and, due to the increased insolubility, single crystals of the product remained elusive. However, based on the structural data obtained for 3 [Cu2Cl2] 2+ and 3 [Cu2(NCMe)2] 4+ , an unfolded ligand conformation is a reasonable assignment of the product's overall geometry, and considering the similarity in the colors of 3 [Cu2(NCMe)2] 4+ , 3 [Cu2F] 3+ , and 3 [Cu2] 4+ as well as the low solubility of 3 [Cu2] 4+ , an ionic structure with fivecoordinate Cu centers would be reasonable to propose. This formulation could be achieved through a number of combinations of triflate coordination mode, denticity, and ionicity. In the absence of further structural data, we have chosen to represent 3 [Cu2] 4+ with the generic structure shown in Scheme 3 while acknowledging that another formulation may well represent the structure of this species.  4+ , an ionic structure with five-coordinate Cu centers would be reasonable to propose. This formulation could be achieved through a number of combinations of triflate coordination mode, denticity, and ionicity. In the absence of further structural data, we have chosen to represent 3 [Cu 2 ] 4+ with the generic structure shown in Scheme 3 while acknowledging that another formulation may well represent the structure of this species.

Anion Substitution with Halides and Pseudohalides
To confirm the structure and assess anion binding within the macrocyclic pocket of 3 [Cu 2 ] 4+ , the material was first treated with 2.0 equiv of NBu 4 Cl to generate the previously   (Figure 6). Two azide ligands now bridge the two copper(II) centers, while the macrocyclic ligand remains in the unfolded ligand conformation. Each Cu(II) center adopts a distorted octahedral geometry. The elongated Cu-N (2.5353(1) Å) and Cu-O (2.4361(9) Å) axial bond lengths are consistent with Cu-d 9 Jahn-Teller distortions. A symmetric Cu2(µ-N3)2 core is noted in the crystal structure, with a Cu-Cu distance of 3.1362(6) Å and symmetry-related Cu-N-Cu angles of 88.02(4)°. Although the Cu-Cu distance is similar to that in the analogous 3 [Cu2Cl2] 2+ complex, the bridging angle observed for 3 [Cu2(N3)2] 2+ is larger, owing to the shorter equatorial Cu-N bond lengths of 1.936(1) Å. Further support for the di(µ-azide) core was noted in the compound's IR spectrum. An asymmetric azide stretching frequency was observed at 2062 cm −1 . Similar stretching frequencies have been observed in analogous Schiff base complexes [33][34][35][36]. Although the Cu-Cu distance is similar to that in the analogous 3 [Cu 2 Cl 2 ] 2+ complex, the bridging angle observed for 3 [Cu 2 (N 3 ) 2 ] 2+ is larger, owing to the shorter equatorial Cu-N bond lengths of 1.936(1) Å. Further support for the di(µ-azide) core was noted in the compound's IR spectrum. An asymmetric azide stretching frequency was observed at 2062 cm −1 . Similar stretching frequencies have been observed in analogous Schiff base complexes [33][34][35][36]. Inorganics 2023, 11, x FOR PEER REVIEW 9 of 20 Considering the ease of anion substitution displayed by 3 [Cu2] 4+ , we pursued a simplified one-pot protocol for the synthesis of 3 [Cu2X2] 2+ species (Scheme 4, Bottom). As noted above, treating 3 [Sr] 2+ and 2.0 equiv Cu(OTf)2 in DCM afforded no reaction. However, upon the addition of 2.0 equiv of NBu4X (X = N3, Br) to the mixture, an anion metathesis and macrocycle transmetallation reaction was initiated to yield the 3 [Cu2(µ-X)2] 2+ products. These results indicate that anion metathesis is facile in this system and that the formation of 3 [Cu2] 4+ is not necessary prior to the generation of the 3 [Cu2(µ-X)2] 2+ products. While this one-pot route afforded comparable yields to the stepwise procedure, the extended workup and separation of multiple salt byproducts proved more challenging. Anion metathesis studies with 3 [Cu2] 4+ were continued to avoid such difficulties.

Anion Substitution with Nitrite and Trimethylsilanolate Anions
We next pursued the inclusion of the nitrite anion, which is known to be capable of adopting a range of coordination modes at Cu(II) ions. To first probe if 3 [Cu2] 4+ was capable of binding nitrite, a similar protocol was followed as that described above, in which NaNO2 was used in the anion metathesis reaction (Scheme 5). Upon adding 2.0 equiv of NaNO2 to a slurry of 3 [Cu2] 4+ , the starting material was consumed over the course of 2 h to afford a dark blue-green solution. The diffusion of Et2O into a concentrated MeCN solution afforded crystals suitable for X-ray diffraction. The product was identified as the di(µ-  The solid-state structure of 3 [Cu2(ONO)2] 2+ depicts the macrocyclic ligand enforcing a µ-κ 1 -binding of each nitrite anion (Figure 7). Limited examples of this geometry are found for Cu2(II,II) systems in the literature [37][38][39][40]. In 3 [Cu2(ONO)2] 2+ , each copper resides in a distorted octahedral geometry, with the overall structure appearing similar in form to 3 [Cu2(N3)2] 2+ . The two copper(II) centers are related by a center of symmetry in the crystal lattice and are separated by 3.2553(6) Å. The characteristic Cu(II)-d 9 Jahn-Teller Considering the ease of anion substitution displayed by 3 [Cu 2 ] 4+ , we pursued a simplified one-pot protocol for the synthesis of 3 [Cu 2 X 2 ] 2+ species (Scheme 4, Bottom). As noted above, treating 3 [Sr] 2+ and 2.0 equiv Cu(OTf) 2 in DCM afforded no reaction. However, upon the addition of 2.0 equiv of NBu 4 X (X = N 3 , Br) to the mixture, an anion metathesis and macrocycle transmetallation reaction was initiated to yield the 3 [Cu 2 (µ-X) 2 ] 2+ products. These results indicate that anion metathesis is facile in this system and that the formation of 3 [Cu 2 ] 4+ is not necessary prior to the generation of the 3 [Cu 2 (µ-X) 2 ] 2+ products. While this one-pot route afforded comparable yields to the stepwise procedure, the extended workup and separation of multiple salt byproducts proved more challenging. Anion metathesis studies with 3 [Cu 2 ] 4+ were continued to avoid such difficulties.

Anion Substitution with Nitrite and Trimethylsilanolate Anions
We next pursued the inclusion of the nitrite anion, which is known to be capable of adopting a range of coordination modes at Cu(II) ions. To first probe if 3 [Cu 2 ] 4+ was capable of binding nitrite, a similar protocol was followed as that described above, in which NaNO 2 was used in the anion metathesis reaction (Scheme 5). Upon adding 2.0 equiv of NaNO 2 to a slurry of 3 [Cu 2 ] 4+ , the starting material was consumed over the course of 2 h to afford a dark blue-green solution. The diffusion of Et 2 O into a concentrated MeCN solution afforded crystals suitable for X-ray diffraction. The product was identified as the di(µ-κ 1 -ONO) macrocycle complex, [( 3 PDI 2 )Cu 2 (ONO) 2

][OTf] 2 ( 3 [Cu 2 (ONO) 2 ] 2+ ).
Inorganics 2023, 11, x FOR PEER REVIEW 9 of 20 Considering the ease of anion substitution displayed by 3 [Cu2] 4+ , we pursued a simplified one-pot protocol for the synthesis of 3 [Cu2X2] 2+ species (Scheme 4, Bottom). As noted above, treating 3 [Sr] 2+ and 2.0 equiv Cu(OTf)2 in DCM afforded no reaction. However, upon the addition of 2.0 equiv of NBu4X (X = N3, Br) to the mixture, an anion metathesis and macrocycle transmetallation reaction was initiated to yield the 3 [Cu2(µ-X)2] 2+ products. These results indicate that anion metathesis is facile in this system and that the formation of 3 [Cu2] 4+ is not necessary prior to the generation of the 3 [Cu2(µ-X)2] 2+ products. While this one-pot route afforded comparable yields to the stepwise procedure, the extended workup and separation of multiple salt byproducts proved more challenging. Anion metathesis studies with 3 [Cu2] 4+ were continued to avoid such difficulties.

Anion Substitution with Nitrite and Trimethylsilanolate Anions
We next pursued the inclusion of the nitrite anion, which is known to be capable of adopting a range of coordination modes at Cu(II) ions. To first probe if 3 [Cu2] 4+ was capable of binding nitrite, a similar protocol was followed as that described above, in which NaNO2 was used in the anion metathesis reaction (Scheme 5). Upon adding 2.0 equiv of NaNO2 to a slurry of 3 [Cu2] 4+ , the starting material was consumed over the course of 2 h to afford a dark blue-green solution. The diffusion of Et2O into a concentrated MeCN solution afforded crystals suitable for X-ray diffraction. The product was identified as the di(µ-  The solid-state structure of 3 [Cu 2 (ONO) 2 ] 2+ depicts the macrocyclic ligand enforcing a µ-κ 1 -binding of each nitrite anion (Figure 7). Limited examples of this geometry are found for Cu 2 (II,II) systems in the literature [37][38][39][40]. In 3 [Cu 2 (ONO) 2 ] 2+ , each copper resides in a distorted octahedral geometry, with the overall structure appearing similar in form to The IR spectral data for 3 [Cu2(ONO)2] 2+ are consistent with the nitrite ligation mode observed crystallographically. IR spectra of Cu2(II,II) complexes with κ 1 -ONO ligands show asymmetric and symmetric stretching modes in the 1400-1100 cm −1 region [37][38]. However, the bending and stretching frequencies associated with the macrocyclic ligand convolute the IR spectrum of 3 (Figure 8). Thus, the stretching frequencies at 1473 cm −1 and 991 cm −1 are assigned to the asymmetric and symmetric stretching frequencies of the nitrite ligands.  The IR spectral data for 3 [Cu 2 (ONO) 2 ] 2+ are consistent with the nitrite ligation mode observed crystallographically. IR spectra of Cu 2 (II,II) complexes with κ 1 -ONO ligands show asymmetric and symmetric stretching modes in the 1400-1100 cm −1 region [37,38]. However, the bending and stretching frequencies associated with the macrocyclic ligand convolute the IR spectrum of 3 [Cu 2 (ONO) 2 (Figure 8). Thus, the stretching frequencies at 1473 cm −1 and 991 cm −1 are assigned to the asymmetric and symmetric stretching frequencies of the nitrite ligands.
Bridging alkoxy anions were next targeted as a way of expanding the scope of substitution reactions available to 3 [Cu 2 ] 4+ , but Cu 2 (II,II)-µ-(OR) (R = Me, t Bu, Ph) macrocyclic species eluded isolation. Instead, substitution was found to be successful when using the trimethylsilanolate anion (Scheme 6). Upon the addition of 2.0 equiv NaOTMS to a slurry of 3 [Cu 2 ] 4+ , a rapid color change from blue to dark orange-brown was observed. Low-temperature crystallization afforded single crystals of the desired complex, [( 3 PDI 2 )Cu 2 (OTMS) 2 ][OTf] 2 ( 3 [Cu 2 (OTMS) 2 ] 2+ ). The crystal structure confirms the presence of two bridging trimethylsilanolate ligands (Figure 9). The ligand adopts an unfolded ligand conformation to support the two bridging ligands between the two copper(II) centers. In the case of 3 [Cu 2 (OTMS) 2 ] 2+ , both Cu(II) centers were found to adopt a distorted square pyramidal geometry with the triflate anions lying in the outer coordination sphere (τ 5 = 0.14 for each Cu(II)). The steric encumbrance of the TMS groups may prevent the triflate anions from binding to the metal centers in the manner seen with the structurally related complexes presented above.  species eluded isolation. Instead, substitution was found to be successful when using the trimethylsilanolate anion (Scheme 6). Upon the addition of 2.0 equiv NaOTMS to a slurry of 3 [Cu2] 4+ , a rapid color change from blue to dark orange-brown was observed. Low-temperature crystallization afforded single crystals of the desired complex, [( 3 PDI2)Cu2(OTMS)2][OTf]2 ( 3 [Cu2(OTMS)2] 2+ ). The crystal structure confirms the presence of two bridging trimethylsilanolate ligands (Figure 9). The ligand adopts an unfolded ligand conformation to support the two bridging ligands between the two copper(II) centers. In the case of 3 [Cu2(OTMS)2] 2+ , both Cu(II) centers were found to adopt a distorted square pyramidal geometry with the triflate anions lying in the outer coordination sphere (τ5 = 0.14 for each Cu(II)). The steric encumbrance of the TMS groups may prevent the triflate anions from binding to the metal centers in the manner seen with the structurally related complexes presented above.
The  species eluded isolation. Instead, substitution was found to be successful when using the trimethylsilanolate anion (Scheme 6). Upon the addition of 2.0 equiv NaOTMS to a slurry of 3 [Cu2] 4+ , a rapid color change from blue to dark orange-brown was observed. Low-temperature crystallization afforded single crystals of the desired complex, [( 3 PDI2)Cu2(OTMS)2][OTf]2 ( 3 [Cu2(OTMS)2] 2+ ). The crystal structure confirms the presence of two bridging trimethylsilanolate ligands (Figure 9). The ligand adopts an unfolded ligand conformation to support the two bridging ligands between the two copper(II) centers. In the case of 3 [Cu2(OTMS)2] 2+ , both Cu(II) centers were found to adopt a distorted square pyramidal geometry with the triflate anions lying in the outer coordination sphere (τ5 = 0.14 for each Cu(II)). The steric encumbrance of the TMS groups may prevent the triflate anions from binding to the metal centers in the manner seen with the structurally related complexes presented above.
The   2+ , each copper center in McGeary and Caulton's cluster is more sterically crowded, owing to the bulky diphenylmethylphosphine ligand. The combined action of the lower oxidation states and increased steric bulk at the metal centers forces a farther copper-copper distance.
The crystal structure was found to contain a planar macrocyclic ligand ( Figure 10). Because of this conformation, the Cu-Cu distances lengthen to 3.585(7) Å and 3.618(4) Å (the asymmetric unit contains disordered macrocyclic complexes with distinct metrical parameters). These distances lie at the longer end of the range of Cu-Cu distances observed in the series of propylene-bridged bis(4-tert-butylpyridyldiimine) macrocyclic complexes. Each copper(II) center adopts a distorted octahedral geometry, with weakly coordinating triflate ligands occupying the axial positions of each copper(II) center. The PDI subunits and a bridging hydroxide ligand occupy the equatorial plane. The Cu-O(H) distances of 1.872(2) and 1.875(5) Å agree well with previously reported PDI2-supported Cu2(II,II)-µ-OH complexes (Table 2)   The growth of single crystals allowed for the structural elucidation of 3 [Cu 2 OH] 3+ . The crystal structure was found to contain a planar macrocyclic ligand ( Figure 10). Because of this conformation, the Cu-Cu distances lengthen to 3.585(7) Å and 3.618(4) Å (the asymmetric unit contains disordered macrocyclic complexes with distinct metrical parameters). These distances lie at the longer end of the range of Cu-Cu distances observed in the series of propylene-bridged bis(4-tert-butylpyridyldiimine) macrocyclic complexes. Each copper(II) center adopts a distorted octahedral geometry, with weakly coordinating triflate ligands occupying the axial positions of each copper(II) center. The PDI subunits and a bridging hydroxide ligand occupy the equatorial plane. The Cu-O(H) distances of 1.872(2) and 1.875(5) Å agree well with previously reported PDI 2 -supported Cu 2 (II,II)µ-OH complexes (  Drew and Nelson [141.7(7) • ]. As expected, the flexible aliphatic linkers modulate the Cu-Cu distance to allow for variously sized bridging ligands to be housed between the two metal centers.

General Considerations
Unless stated otherwise, all reactions were carried out under an inert atmosphere of N 2 using standard Schlenk techniques or a PureLab HE glovebox then worked up in air. Glassware, stir bars, filter aid (Celite), and 4 Å molecular sieves were dried in an oven at 175 • C for at least one hour prior to use when applicable. Degassed anhydrous solvents (Pentane, Et 2 O, THF, DCM, and MeCN) were dried via passage through activated alumina using a Solvent Purification System and stored over 4 Å molecular sieves for at least one day before use. Chloroform-d was purchased from Cambridge Isotopes Laboratories, Inc., and was stored over 4 Å molecular sieves for at least one day before use. The macrocyclic precursor 2 [Sr] 2+ was prepared according to a previous reported procedure [46]. Tetrabutylammonium chloride (NBu 4 Cl) was purchased from Sigma Aldrich, Saint Louis, MO, USA, ground to a fine powder with a mortar and pestle, then dried under vacuum over P 2 O 5 for one week before use. Tetrabutylammonium bromide (NBu 4 Br) was purchased from Sigma Aldrich and dried under vacuum over P 2 O 5 for one week before use. Sodium nitrite (NaNO 2 ) was purchased from Fisher Chemicals, Whippany, NJ, USA, ground to a fine powder with a mortar and pestle, and flame dried under dynamic vacuum, then it was stored under N 2 in an inert atmosphere glovebox. 15 N-labeled sodium nitrite (Na 15 NO 2 ) was purchased from Cambridge Isotope Laboratories, Inc., Andover, MA, USA, ground to a fine powder with a mortar and pestle, and flame dried under dynamic vacuum. The isotopologue was then stored in the dark, in the glovebox. Sodium trimethylsilanolate was purchased as a 1.0 M solution in THF from Sigma Aldrich in a Sure/Seal TM bottle and was stored under N 2 in the glovebox. All other reagents were purchased from commercial vendors and used as received.
NMR spectroscopy: NMR spectra were recorded on a Bruker NEO 600 spectrometer. All chemical shifts are reported in units of ppm and referenced to the residual proteosolvent resonance for proton and carbon chemical shifts. Internal PhCF 3 was used for referencing the 19 F spectrum [47].
Elemental analysis: Analytical data were obtained from the CENTC Elemental Analysis Facility at the University of Rochester. Microanalysis samples were weighed with a PerkinElmer Model AD6000 Autobalance and their compositions were determined with a PerkinElmer 2400 Series II Analyzer. Air-sensitive samples were handled in a VAC Atmospheres glovebox and combusted in a tin capsule that was crimp-sealed with a die apparatus. For 3 [Cu 2 (NCMe) 2 ] 2+ , analytical data were collected using a Costech ECS 4010 analyzer in the Earth & Environmental Science Department at the University of Pennsylvania.
Infrared spectroscopy: Infrared spectra were recorded using a PerkinElmer Spectrum Two FT-IR spectrometer. Samples were placed on an ATR crystal after collecting a blank spectrum and spectral data were measured over the range of 450-4000 cm −1 . Samples requiring a KBr pellet were analyzed using a Bruker Invenio-R FT-IR spectrometer and spectral data were measured over the range of 450-4000 cm −1 .
UV-vis-NIR spectroscopy: Absorption spectra were collected over the range of 200-1000 nm using an Agilent Cary 60 UV-vis-NIR spectrophotometer. Stock solutions (3.0-11.0 mM) were prepared under a N 2 atmosphere in the glovebox. Samples were then diluted to obtain absorption profiles in the linear response range of the spectrophotometer. Each measurement was obtained using a 10 mm path-length quartz cuvette with a screw cap. The temperature of each measurement was maintained at 25 • C using an Unisoku USP-203A cryostat. Five different concentrations were used for Beer's Law analysis of each peak maxima. The absorption intensities for peak maxima were plotted vs. concentration to ensure a linear fit was obtained (R 2 > 0.990).
X-ray crystallography: X-ray intensity data were collected on a Bruker D8Quest CMOS  2+ ) and refined by full-matrix least-squares, based on F 2 , using SHELXL-2018. All reflections were used during refinement. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model.  3 [Sr] 2+ . The following procedure was adapted from the literature report [46]. A 500 mL round-bottom flask was charged with 4-tert-butyl-2,6-diacetylpyridine (5.1349 g, 23.417 mmol, 2.00 equiv), Sr(OTf) 2 (4.5353 g, 11.757 mmol, 1.00 equiv), and MeOH (250 mL). The mixture was stirred until all solids were dissolved. To the pale-yellow solution, 1,3diaminopropane (2.00 mL, 23.9 mmol, 2.04 Eq.) was added. The solution became darker and was heated to reflux (oil bath set to 75 • C) for 16 h. The volatile materials were then removed under vacuum. The crude product was extracted into ca. 25 mL DCM and filtered over a pad of celite to remove insoluble materials. The yellow-orange filtrate was then treated with ca. 300-400 mL hexanes to precipitate the product as an off-white powder. The powder was collected via vacuum filtration, washed with an additional ca. 500 mL hexanes, and dried under vacuum (9.6398 g, 91%  3+ (1.1099 g, 1.0017 mmol, 1.00 equiv) and DCM (25 mL) were added. The blue slurry was cooled to −78 • C and treated dropwise with TMS-OTf (0.200 mL, 1.11 mmol, 1.10 equiv). The mixture was removed from the cold bath and stirred for one hour. Over time, the dark blue slurry became light blue. The reaction mixture immediately became dark green. Within minutes, a bright green solid precipitated from solution. The reaction was allowed to proceed for 4 h at room temperature. To the solution, MeCN (35 mL) was added, and the dark green solution was filtered over a pad of celite. The pad was rinsed with an additional 35 mL 1:1 DCM:MeCN, and the filtrate was concentrated under vacuum. The resulting crude solid was triturated with 10 mL THF and collected via vacuum filtration. The product was washed with 3 × 10 mL THF and 50 mL Et 2 O. The product was re-dissolved in ca. 45 2+ . To a 100 mL Schlenk tube, Cu(OTf) 2 (406.2 mg, 1.123 mmol, 2.02 equiv) and DCM (5 mL) were added. The slurry was treated with a solution of 3 [Sr] 2+ (501.1 mg, 0.5565 mmol, 1.00 equiv) in DCM (10 mL). No change occurred upon this addition. A solution of NBu 4 N 3 (320.2 mg, 1.126 mmol, 2.02 equiv) in DCM (10 mL) was then added. The reaction mixture immediately became dark green. Within minutes, a dark green solid precipitated from solution. The reaction was allowed to proceed for 4 h at room temperature. To the solution, MeCN (25 mL) was added, and the dark green solution was filtered over a pad of celite. The pad was rinsed with an additional 35 mL 1:1 DCM/MeCN, and the filtrate was concentrated under vacuum. The resulting crude solid was triturated with 10 mL THF and collected via vacuum filtration. The product was washed with 3 × 10 mL THF and 50 mL Et 2 O. The product was re-dissolved in ca. 45 mL MeCN and was layered with 200 mL Et 2 O at room temperature. The product crystallized as green needles after three days. The crystals were collected and rinsed with 50 mL Et 2 O before drying under

Conclusions
The research described in this manuscript enables a broad range of functionalization chemistry at a dicopper core supported by a geometrically and electronically flexible macrocyclic ligand. The isolation and characterization of this new series of Cu 2 (II,II) bis(4tert-butylpyridyldiimine) macrocycle complexes were carried out through three general procedures: (1) the transmetallation of n [Sr] 2+ with CuCl 2 , (2) salt metathesis at a new "open-core" macrocycle, 3 [Cu 2 ] 4+ , and (3) a one-pot procedure in which 3 [Sr] 2+ was treated with 2.0 equiv Cu(OTf) 2 and a corresponding source of anion. These methods were found to be suitable for accessing a host of novel Cu 2 (II,II) macrocycles. Notably, previous work on the parent form of this macrocycle encountered persistent difficulties with solubility, hindering structural characterizations and further synthetic investigations. The improved solubility on the introduction of 4-tert-butylpyridyl groups and triflate counterions allowed for modest-to-good solubility profiles across a broad range of core substitutions.
The solid-state structures of each new macrocycle-supported complex revealed how the conformation of the macrocycle was able to change depending on the number and nature of bridging substituents between the Cu(II) centers (Table 3). If two bridging ligands are present, the macrocycle can adopt a range of ligand conformations to support the Cu 2 X 2 cores, and, at its extreme, the macrocycle was found to unfurl into a nearly planar geometry to support a linear Cu 2 F core. The geometric differences enabled by the flexibility of the macrocycles' aliphatic linkers were originally proposed by Drew and Nelson, but their data were limited by the solubility problems mentioned above. The present range of structural data highlights the flexibility of this ligand class. This flexibility, in combination with the robust synthetic protocols, provides an ideal platform for future investigations into the ability of these complexes to mediate small molecule activation and strong bond functionalization chemistry. Table 3. Summary of Cu 2 (II,II) bis(4-tert-butylpyridyldiimine) macrocycles.