BN‐Doped Metal–Organic Frameworks: Tailoring 2D and 3D Porous Architectures through Molecular Editing of Borazines

Abstract Building on the MOF approach to prepare porous materials, herein we report the engineering of porous BN‐doped materials using tricarboxylic hexaarylborazine ligands, which are laterally decorated with functional groups at the full‐carbon ‘inner shell’. Whilst an open porous 3D entangled structure could be obtained from the double interpenetration of two identical metal frameworks derived from the methyl substituted borazine, the chlorine‐functionalised linker undergoes formation of a porous layered 2D honeycomb structure, as shown by single‐crystal X‐ray diffraction analysis. In this architecture, the borazine cores are rotated by 60° in alternating layers, thus generating large rhombohedral channels running perpendicular to the planes of the networks. An analogous unsubstituted full‐carbon metal framework was synthesised for comparison. The resulting MOF revealed a crystalline 3D entangled porous structure, composed by three mutually interpenetrating networks, hence denser than those obtained from the borazine linkers. Their microporosity and CO2 uptake were investigated, with the porous 3D BN‐MOF entangled structure exhibiting a large apparent BET specific surface area (1091 m2 g−1) and significant CO2 reversible adsorption (3.31 mmol g−1) at 1 bar and 273 K.


Introduction
Over the last few decades environmental concerns such as climate change, mainly due to globalw arming causedb yg reenhouse gases,have stimulated the scientific community towards the research of clean energy carriers such as H 2 along with the reduction of CO 2 emissions generatedf rom the combustion of fossil fuel.In this context the preparation of porous organic materials with predefined porosity displaying high surface areas has attracted unprecedented attention due to their great potentials for gass toragea nd separation. [1]Porous organic polymers (POPs), [2] porousm olecular solids, [3] covalent organic frameworks (COFs) [4] and metal-organic frameworks (MOFs) [5] have emerged in the last years as structures of choice to engineer microporous materials.The possibility to chemically edit the organic linkersa llowed researchers in the field to customize the physical and chemical properties of such porousm aterials, producing an extraordinarily large number of functional architectures. [6]mong the different functionalization strategies, the doping route,t hat is, the replacement of Ca toms with isostructural species, is emerging as av ersatile approach to tailor the physical andc hemical properties of organic molecules. [7]The replacemento fC =Cb onds by isostructural and isoelectronic polar BÀNc ovalent couples considerably alters the character of the frontier molecular orbitals as well as the polarization properties of the p-conjugated molecular surface. [8]5a, 9] Nanostructured boron nitrides, [11] boron carbon nitrides, [10a, 11b, 12] BNdecorated nanoporous carbons, [10b] have been investigated for gas storage and shown high H 2 and/or CO 2 uptake.In particu-lar,p orous boron nitrides have been used in catalysis, [13] CO 2 capture, [14] hydrogen storage [10a, 15] and water purification. [16]Alternative porousm aterials containing BN bonds werer eported by El-Kaderia nd co-workers who proposed the use of borazine precursors to fabricate highly porous borazino polymers. [17]hese amorphousm icroporous polymers, showed ag ood CO 2 uptake up to 3.18 mmol g À1 at 273 K, which has been ascribed to the interactions between the polarizable CO 2 molecules and the borazine units.T aking into consideration these interactions, Xu and co-workers,i nstead, proposed the use of MOFs as templates to obtain either porousB N-containing MOF composite materials [10c] or porous boron-nitride-carbides, [10b] which showeds ignificant CO 2 adsorption (4.6 and4 .4mmol g À1 ,r espectively at 273 K).Although theser epresent interesting strategies to trigger the gas storagep erformance of MOFs, the preparation of BNC-frameworks is limitedb ydifficulties to control the BN graftingb yM OF post-synthetic modifications.A more promisingm ethod to form highly pure organised porous MOFs could be the use of organic linkersd oped with BN functionalities.Boron imidazolate frameworks, which are formed by coordination of boron imidazolate linkers with metal ions, are an example of this type of materials. [18]Inspired by these works, we anticipatedt hat properly functionalised borazine molecules could serve as BN molecular ligands for engineering highly porous MOFs.Indeed, borazines have the perfect shape and functionalities to form trigonal three-carboxylate linkers analogous to those used for making ultrahigh porous MOFs such as MOF-177 [19] and MOF-200, [20] which are among those with the highest apparent BET specific surfacea rea reported so far.H ence, herein we describe the synthesis, the single-crystal X-ray structures and gas uptake study of the first BN-doped MOFs engineered with hexaryl-substitutedb orazine carboxylate linkers.

Results and Discussions
Design and molecular editing of the borazine linkers With the idea of developing BN-dopedp orousm aterials with large surfacea reas, we conjectured thata romatic ligands developingi nt wo dimensions and laterallye xposing functional groups could allow the formationo fM OFs with large surface areas (Figure 1).Building on previousw orks [8a, 21] describing moisture-stable borazine-based oligophenylenes decorated with ortho-methyl substituents at the inner B-aryl rings (Figure 1), we conjectured that also borazines could be used as suitable ligandst oe ngineer BN-doped MOFs if properly equipped with coordinating moieties.Considering that carboxylic acid moieties are well-known to form persistent Zn 4 O(COO) 6 metal clusters in MOFs with high porosity, [19,22] we designed borazine tricarboxylic acid ligands bearing orthomethyl and ortho-chloride protecting groups in the B-aryl rings (Scheme 1).When ortho substituents were present,t he coordinating carboxylic groups were placed on the para positionso f extra aryl groups at the outer shell, that are bonded to the Band N-biaryl groups to form BN-L-1 and BN-L-2 linkers, respectively (Scheme1).As ortho-unsubstituted hexaphenylborazines cannotb eu sed to prepare MOFs due to their chemical sensitivity toward hydrolysis, we envisaged to preparea ni sostructural full-carbon analogue( C-L-1)a sareference, in which the aryl rings in the inner shell of the ligand do not bear any ortho substituents (i.e.,methyl or chlorine groups).When BN-L-1 and C-L-1 linkers are mixed with Zn(NO 3 ) 2 •6 H 2 Os alt, two three-dimensional porousM OFs BN-MOF-1 and C-MOF-1 with differently interpenetrated structuresw ere obtained.Interestingly, linker BN-L-2 gave a2 Dp orousM OF (BN-MOF-2), in which the ligandsare held togetherthrough Zn 2 (CO 2 ) 3 clusters.
Single crystals suitable for XRD analysis could be obtained for linker BN-L-1 by slow diffusion of H 2 Oi nto aD MSO solution.The molecular structure could be determined, confirming the trigonal rigid shape of the borazine ligand (Figure S18).Isostructural full-carbon analogue linker C-L-1 was also prepared in 75 %y ield by Suzukic ross-coupling reactionb etween hexaphenylbenzenet riflate derivative 8 [21d] and 4-methoxycarbonylphenylboronic acid 7,f ollowed by hydrolysis of the methyl esters groups (Scheme 1b).

MOFs synthesisa nd characterisation
The MOFs weres ynthesised by the solvothermalm ethod proposed by O. Yaghia nd co-workers. [19]The reaction conditions were slightly modified [20] and N,N'-diethylformamide (DEF) was replaced with DMF (Schemes 1a,b).Considering that the heat transferst hat take place during the chemical reaction can affect not only the crystal size, but also the morphologya nd the phase purity of the porousm aterials, [24] to ensure the reproducibility the syntheses have been performed in the same sealed vial heated by ab ath of non-flammable silicon-based oil, keeping the same amount, ratio and concentration of the startingm aterials.I natypicalp rocedure, the organic linker (respectively BN-L-1, BN-L-2, C-L-1)w as dissolved in am ixture 1:1D MF/NMP,t hen Zn(NO 3 ) 2 •6 H 2 Ow as added and the reaction mixture was heated at 85 8C.After 72 h, colourless crystals were obtained.Porous materials BN-MOF-1, BN-MOF-2 and C-MOF-1 were isolated in 70 %, 74 %a nd 71 %y ield, respectively.The obtained crystalsw ere firstly analysed by optical( Figure S16) and scanning electron microscope (SEM).The SEM images of BN-MOF-1, BN-MOF-2 and C-MOF-1 (Figures 2a-b,  c-d, and e-f, respectively)f eaturet heir crystalline shape with maximum dimension between 0.5 and 0.6 mm.While both BN-MOFs show more regularp rismatic shapes, C-MOF-1 displays more irregular edges.
The crystals structure for all three MOFs could be determined by single crystal XRD analysisu sing synchrotron radiation (see Supporting Information).C-MOF-1 formed crystals with an orthorhombic lattice belonging to the Fdd2 space group showing ant topology.Afragment of the structure, depicted in Figure 3a,s hows how six organic linkers are arranged aroundt he distorted octahedral Zn 4 O(COO) 6 cluster with each bi-dentate carboxyl moiety chelatingt wo Zn II ions in ab ridging fashion.C-MOF-1 shows an interpenetrated structure, where three networks composed by three infinite structurally regularm otifs (highlighted in orange, yellow and green, Figure 3b,c) are inextricablye ntangled.The distance between each interpenetrated network is approximately 8.7 (Figure 3d), with each network seemingly interacting through vdW interactions.The arrangement of the interpenetrating networks reveals an open porous structure as shown in Figure 3. Theoretical calculations performed with the Platon [25] softwarei ndicate that the solvent accessible volumea ccounts for the 63 % (26 325 3 )o ft he unit cell volume (41 771 3 ).Thus, the framework atoms in C-MOF-1 occupy only the 37 %o ft he available space in the crystal lattice, with ac alculated density of 0.65 gcm À3 .I ndeed, the interpenetration of such entangled networks generates the formationo fc hannels with cross section dimensions of approximately 11.5 15.8 that run along the direction of the ab face diagonal (110 Miller plane) (Figure 3e).
BN-MOF-1,i nstead, formed crystals with orthorhombic symmetry analogously to C-MOF-1 but belongingt ot he Pnnm space group and with rtl topology.A so bserved for C-MOF-1, also for BN-MOF-1,s ix organicl inkers are arranged arounda slightly distorted octahedral Zn 4 O(COO) 6 cluster,w ith each bidentate carboxyl moiety bridging two Zn II ions (Figure 4a).Interestingly,t hese octahedral fragments extend to form two identicald oubly interpenetrating three-dimensional networks, composed by two infinite structurally regularm otifs (highlighted in orange and green,F igures 4b,c) inextricably entangled.The extended three-dimensional interpenetrating networks are entangled in such aw ay that the crystals exhibit ar emarkably open three-dimensional porouss tructure (Figure 4).Calculations with the Platon software revealed that the solvent accessible void volumea ccounts for 68 %( 19 182 3 )o ft he total unit cell volume (28 344 3 ), when ap robe with 1.2 radius is used and vdW radiif or all atoms (2.25, 1.20, 1.70, 1.52, 1.55,  Hence, the framework atoms in BN-MOF-1 occupy only a small fraction (32 %) of the available space in the crystal,with a calculated density of 0.52 gcm À3 .T he non-framework space is divided into two differenttypes of channels, namely C-decorated cavities, and BN-decorated cavities, that run alongt he crystal c-axis (Figure 4d).Metal clusters belonging to the two networks and phenylenes moieties form the walls surrounding the C-cavities (highlighted in red, Figure 4d), which have a pore diameter of approximately 6.3 .T he BN-decorated cavities are larger than those of the C-counterpart, featuring ad iametero fa pproximately 14.5 (highlighted in black, Figure 4d).The borazine rings are arranged alongt he walls of the channels exposing the B 3 N 3 surface towardst he centre of the pores.The presence of the ortho-methyl substituents on the Baryl substituentsr educes the conformational degree of freedom about the CÀBb onds.Whereas the dihedral angles between the borazine plane and the inner aryl substituents vary from 758 to 908 in BN-MOF-1, C-MOF-1 displays dihedral angles ranging from 648 to 908 between the central benzene core and the lateral aryl substituents.In both BN-MOF-1 and C-MOF-1 MOFs, the interweavingm ight induce strengthening of thea rchitecturea sr eportedf or MOFs formed by largel inkers, [26] even though only vdWi nteractionsb etweent he two frameworks,w hich ared isplaced from onea nother with am ini-mumdistanceofapproximately4.6 (Figure4b),are present. [27]urprisingly, BN-MOF-2 presentsacompletely different structure with at rigonal lattice belonging to the P3 ¯m1s pace group.The threefold ligand symmetry perfectly matches the crystallographic ternary axis of the space group.In this case, three borazine linkers are arranged adopting at rigonal shape aroundarare dinuclear Zn 2 (COO) 3 (H 2 O) 2 secondary building unit, [28] with each bi-dentate carboxyl moiety bridging two zinc ions and with one water molecule completing each Zn coordination sphere.The networks extend in two dimensions forming al ayered structure with a( 6,3) topological honeycomb (hcb)n et based on Zn 2 (COO) 3 (H 2 O) 2 clustera sanode (Figure-s5a-b), which hasa lready been reported for the analogoust ripodal linker 4,4',4''-(benzene-1,3,5-triyl-tris(benzene-4,1-diyl))tribenzoic acid. [29]In the BN-MOF-2 crystals the ligandso nd ifferentl ayers,w hich are 7.7 apart, are stacked in eclipsed mode along the crystallographic c axis (Figure 5c-e).Adjacent borazinec ores are rotated by 608 to minimize steric repulsions between sidechains and to maximize the halogenc ontacts as previously reported for chlorinated aromatics. [30]This arrange- ment generates a3 Do pen porouss tructure with resulting rhombic channels of approximately1 2.9 side dimensionr unning along the c-axis of the crystals (Figures 5a,b).Calculations with the Platon softwarer evealed that the solventa ccessible void volumea ccounts for 68 %( 5236 3 )o ft he total unit cell volume( 7700 3 ).The thermals tabilityo ft he three MOFs was evaluated by thermalg ravimetric analysis( TGA).All of them MOFs show remarkably high thermals tability,w ith frameworks decomposition happening at temperatures above 400 8Cu nder N 2 atmosphere.The TGA trace for BN-MOF-1 clearly shows two distinct weightl oss steps (Figure S26 a).The first change, corresponding to 16 %o fw eight loss, occurred between 50 and1 50 8C and can be attributed to the gradual release of the included DMF and NMP solvent molecules.T he second step, corresponding to aw eight loss of 58 %, was observed at approximately 430 8Ca nd can be attributed to the decomposition of the organic linkers.For BN-MOF-2 gradual elimination of the inclusions olvent, corresponding to 12 %w eight loss, was observed between 50 and 250 8Ca nd decomposition of the framework at approximately 450 8C( Figure S26 b).The TGA trace for C-MOF-1 exhibits two steps in the temperature range between 20 8Ca nd 220 8Ct hat correspond to 5% and 6% weightl oss, which can be attributed to the graduall oss of the inclusionD MF and NMP solvents.(Figure S26 c).At hird weigh loss step of 58 %c orresponding to thed ecomposition of the organicf ramework occurred above 400 8C.
Powderd iffraction analysis( PXRD) of freshly prepared samples of BN-MOF-1 and C-MOF-1,w hich hadb een dried under air for 12 h, were also measured to confirmt he bulk crystallinity and the stabilityo ft he frameworks structures (Figure S27).The experimental data werec ompared with the simulated powderp atterns generated from single-crystal X-ray data.Althoughs ome similarities in the peaks positions were found for BN-MOF-1,amismatch between the relative intensities and a generalb roadening of the peaks could also be observed (Figure S27).This could be due to partial amorphisation of the material. [31]However,o ne cannote xclude that differences in peak intensity mighta rise because the structure is simulated in the absence of guest solventm olecules in the pores, whilet he experimental pattern is obtained in the presence of solvent molecules, as also confirmed by TGA analysis.Furthermore, additional peaks, which are not found in the simulated pattern, could be observed between 2q 14 and 258 and may indicate a substantial alteration of the BN-MOF-1 structure.As previously reported, Zn 4 O(COO) 6 -based MOFs demonstrated poor moisture stability, thus suggesting that BN-MOF-1 might also not be very stable under air storage conditions and undergoes partial decomposition due to hydrolysis in the presence of ambient humidity with ac onsequent partial structuralc ollapse. [32]his is consistent with IR resultst hat show as harp peak at 3660 cm À1 andabroad band at approximately3 400 cm À1 that can be attributedt os trongly bondedf ramework water molecules and adsorbed water,r espectively (Figure S29).The IR spectrumo fana ctivated sample was also measured and, while it did not showa nymore the broad band duet oa dsorbed water,t he peak at 3660 cm À1 corresponding to strongly bondedw ater could still be observed (Figure S30).32a, 33] Similarly,f or C-MOF-1 ag ood agreement between some of the peaks positions in the theoretical and the experimental PXRD patterns was found, namely at 2q of 7.1 and 7.88,b ut again broadening of the peaks and am ismatch in their intensity was also observed (Figure S27).As already noted for BN-MOF-1,t hese differences coulda rise from partial structuralcollapse but also from the presence of solvent molecules in the cavities of the framework, as proved by TGA analysis, for the measured PXRD pattern as oppose to those obtained throughs imulation.The IR spectrumo fasample of as-synthesised C-MOF-1 showed ab road band at about 3300 cm À1 due to adsorbed water,b ut not the sharp peak at about 3650 corresponding to strongly bondedw ater (Figure S34).Likewise, in the IR spectrum of an activated sample of C-MOF-1 ab road band at about 3300 cm À1 was still present but, again,n ot the peak corresponding to strongly bonded water.I na ddition, as mall band at about 1734 cm À1 was found (Figure S35) .These observations suggest that protonated organic linkers are present,m ost likely because of the partial hydrolysis of the framework.An activated sample of BN-MOF-2, which had been evacuated for 2h at 403 K, was also analysed by PXRD (Figure S27).Only two weak peaks at 2q 8.2a nd 9.28, which did not match with any peaks in the simulated pattern, could be observed suggesting that the 3D arrangement of BN-MOF-2 is not stable upon activation.SEM images of the activated sample showf racturing of the crystals and especially lines along their c-axis.These are most probably due to the tendency of the layered structure to split along this direction overcoming the weak interlayers forces, leading to an exfoliation of the MOF layerst hat contributes to the formation of much smaller particlesw ith hexagonal-like sheets shape (Figure S36).
Nitrogen adsorption isotherms on activated samples of BN-MOF-1, BN-MOF-2, C-MOF-1 were measured to assesst heir structural stabilitya nd permanent microporosity.A lthough networks interpenetration is considered detrimental to high porosity, [34] the low crystal density and large void volumes calculated from XRD analysis still provided ag ood indication of potentiall arge surface areas for thesem aterials.Additionally,c alculations of the geometrica ccessible surface areas with the Poreblazer_v3.0.2 program, [35] using aN 2 size probe molecule with 3.72 vdW diameter,p rovided resultst hat supported these anticipations (Table 1).Before gas adsorption measurements, the samples were activated by exchanging the included DMF/NMPs olvents with acetonea nd then evacuated under Table 1.Porosity data for BN-MOF-1, BN-MOF-2,and C-MOF-1.
and 1.63 for Zn, H, C, O, Na nd Ba toms, respectively) are taken into consideration.

Figure 3 .
Figure 3. Crystal structureofC-MOF-1 a) Cappeds ticks representationo fafragment of crystal structure of C-MOF-1 representing the octahedral arrangement of the C-L-1 linkers around the Zn cluster.Space group: Fdd2.b) Capped sticks and c) space-fill representations of the packing view along the a axis showing three interpenetrating networks.d) View of the packing showing the distance between the networks and e) view along the ab face diagonal showing the cross section of the channels.The distances between the highlighted carbon atoms havebeen determined from the crystal structure taking into consideration the vdW radii.Colourc ode:Cg rey or coloured in orange, green and yellowf or the two different networks;pink:B,b lue:N,red:O,black or dark blue tetrahedron:Zn.Ha toms wereomitted for clarity.

Figure 4 .
Figure 4. Crystal structureo fBN-MOF-1.a)Cappedsticks representationo faf ragment of BN-MOF-1 crystal structure showing the octahedralarrangemento f BN-L-1 linkersa round the Zn cluster.S pace group: Pnnm.b) Capped sticks and c) space-fillrepresentation of the two interpenetrating networks d) highlight of the C-and BN-cavities within the structure, the distances between the carbona toms highlighted have been determined from the crystal structure taking into consideration the vdW radii.The crystal structure is represented in capped sticks.Colourc ode:Cg rey or coloured in orange and green for the two different networks;p ink:B,b lue:N,red:O,b lack or dark blue tetrahedron:Z n.Ha toms were omitted for clarity.

Figure 5 .
Figure 5. a) Space-fill and b) capped sticks representation of the top view of BN-MOF-2 crystal structure showing the hexagonal networks and the rhombic channels resulting from the relative 608 rotationofb orazine coresi ne ach layer;c)space-fill and d) capped sticks representation of the side view of BN-MOF-2 showing the layered structure;e)zoomed inset of the borazine cores highlighting the interlayerdistance;colour code:Cgrey or coloured in orange and green for the two different networks;pink:B,b lue:N,red:O,b lack or dark blue tetrahedron: Zn.Ha toms were omitted for clarity.