CH₃-Tagged Bis(pyrazolato)-Based Coordination Polymers and Metal–Organic Frameworks: An Experimental and Theoretical Insight

Abstract

The novel coordination polymers M(Me 2 BPZ) (M = Co, Zn; H 2 Me 2 BPZ = 3,3′-dimethyl-1H,1′H-4,4′-bipyrazole), M(H 2 Me 2 BPZ)(CH 3 COO) 2 -(H 2 O) 2 (M = Co, Ni), and Cu(H 2 Me 2 BPZ)(Cl) 2 were isolated along conventional or solvothermal routes. Their crystal structure was unveiled by powder X-ray diffraction (PXRD), while their thermal stability was assessed by coupling thermogravimetric analysis to variable-temperature PXRD. The textural properties of the M(Me 2 BPZ) (M = Co, Zn) compounds, featuring three-dimensional open frameworks with one-dimensional channels, were assessed by N 2 and CO 2 adsorption at 77 and 273 K, respectively, and compared to those of the nonmethylated isostructural counterparts M(BPZ) (M = Co, Zn; H 2 BPZ = 1H,1′H-4,4′-bipyrazole). The positive effect of the methyl groups in CO 2 adsorption, suggested by the adsorption energy trend [E ads (M(Me 2 BPZ)) > E ads (M(BPZ))] and substantiated by theoretical calculations at the B3LYP-D3 level coupled to topological analyses, is counterbalanced by the higher steric hindrance of Me 2 BPZ 2− vs BPZ 2− , finally reducing the amount of gas adsorbed by the M(Me 2 BPZ) couple vs the M(BPZ) one.

Nitro‐Functionalized Bis(pyrazolate) Metal–Organic Frameworks as Carbon Dioxide Capture Materials under Ambient Conditions

“…The isosteric heat of adsorption (Figure ) reflects the interaction strength between CO 2 and the walls of the channels in Zn‐BPZNO 2 : the value of 20.5 kJ mol −1 at zero coverage (calculated as the arithmetic mean value after the application of the Clausius–Clapeyron equation on pairs of datasets) falls in the ideal range for CO 2 ‐capture materials and denotes that only physisorption is at work. The Q st value of Zn‐BPZNO 2 is comparable to that of Zn(BPZ) (23.7 kJ mol −1 ); the amount of CO 2 adsorbed though is higher at the same temperature, because of its higher surface area (916 vs. 778 m 2 g −1 , respectively).…”

“…The porous nature of the MOFs was investigated through N 2 adsorption at 77 K; Ta ble 1c ollectst he main results.A ss hown in Figure 5, all the compoundss how at ype IV isotherm, typical of am icro-mesoporous material. Zn-BPZNO 2 has the highest BET area with av alue of 916 m 2 g À1 ,w hereas the cobalt(II) and copper(II) MOFs show BET areas of 645 and 408 m 2 g À1 ,r espectively.O verall, the recordedB ET areas fall in the same range as those of the parent MOFs containing the simple BPZ 2À spacer [31,33] and are highert han those found fort he isotypical M-Me 4 BPZ compounds (396, 318, and 376 m 2 g À1 for M = Zn, Co, and Cu, respectively), [32] which is in line with the increased steric hindranceo ft he four methyl groups if compared to a single nitro substituent. The t-plot analysis revealed that the main contribution to the total surface area comesf rom the micropores (77, 75, and 51 %o ft he total specific surface area (SSA) for Zn-BPZNO 2 ,C o-BPZNO 2 ,a nd Cu-BPZNO 2 ,r espectively).…”

“…Higher CO 2 adsorption values (between 5.2 and 9.2 mmol g −1 ) were reported only for MOFs containing the long and rigid bis‐pyrazolate H 2 BPEB spacer [H 2 BPEB=1,4‐bis(1 H ‐pyrazol‐4‐ylethynyl)benzene] . As Zn‐BPZ adsorbs 4.4 mmol g −1 at the lower temperature of 273 K (whereas Zn‐BPZNO 2 adsorbs 4.7 mmol g −1 at the same temperature), the presence of a nitro group on the skeleton of BPZ 2− does favor CO 2 adsorption, which suggests the insurgence of dipole–quadrupole interactions between NO 2 and CO 2 , as previously pointed out by theoretical calculations among supercritical CO 2 and organic compounds with oxygen‐containing functional groups …”

“…The isostructural MOFs Co-BPZNO 2 ·DMF and Zn-BPZNO 2 ·DMF crystallize in the tetragonal space group P4 2 /mmc and are isoreticular [30] to an umber of known MOFs, such as M-BPZ [31] (M = Co, Zn;H 2 BPZ = 4,4'-bipyrazole), M-Me 4 BPZ [32] (M = Co, Zn; H 2 Me 4 BPZ = 3,3',5,5'-tetramethyl-4,4'-bipyrazole), M-Me 2 BPZ [33] (M = Co, Zn;H 2 Me 2 BPZ = 3,5-dimethyl-4,4'-bipyrazole), and M(BDP) [M = Co, [34] Zn; [25] H 2 BDP = 1,4-bis(pyrazolyl)benzene].I n Co-BPZNO 2 ·DMF and Zn-BPZNO 2 ·DMF,t etrahedral MN 4 nodes ( Figure 1a;F igure S2 showst he asymmetric unit content) and tetradentate spacers build up a3 D( 4,4)-connected porous network in which 1D square channels runp arallel to the [001] direction ( Figure 1b). The topological analysis of the crystal lattice performed through the TOPOS 4.0 software, [35] taking the M II ions as tetracoordinated nodes and the ligand as ac onnector,r evealed a( 3 12 ·4 24 ·5 9 )n etwork topology belonging to the 10-c net ( Figure S3).…”

“…The MOFs were tested as CO 2 [42] As Zn-BPZ adsorbs 4.4 mmol g À1 at the lower temperature of 273 K [33] (whereas Zn-BPZNO 2 adsorbs 4.7mmol g À1 at the same temperature), the presence of an itro group on the skeleton of BPZ 2À does favor CO 2 adsorption, whichs uggests the insurgence of dipole-quadrupole interactions between NO 2 and CO 2 ,a sp reviously pointedo ut by theoretical calculations among supercritical CO 2 and organic compounds with oxygen-containing functional groups. [43] To the best of our knowledge, Zn-BPZNO 2 and Co-BPZNO 2 outperform all the literature examples of MOFs bearing the nitro group as au nique tag (Table 2), as they show the highest CO 2 uptakev alues under ambient conditions within this group of MOFs.…”