7.2 Dependence of the H 2 Binding Energies Strength on the Transition Metal and Organic
7.2.3 Results and Discussion
7.2.3.2 Proposed Linkers Based on Experimental Crystal Structures
We speculated that the square geometry was essential to obtain the maximum number of interacting H2 with the linker versus tetrahedral or other geometry. Thus, we search the Cambridge Structural Database (CSD) for square geometry for TM with pyridine ligands. We were focused on these ligands because we believe they can be an easy metalation sites in a framework (Figure 7.13). We found the various numbers of synthesized compounds in the literature with these restrictions. This type of interaction sites are analogous to the COF synthesized with triazine linkage.[151]
Ligand containing the bipyridine group.
The first linker we studied with this approach was the 2,2’-bipirydine (BPY). Using the crystal structures we calculated the H2binding energy for all these TM; Ni(II)[152], Cu(II)[153], Pt(II)[154]
and Pd(II)[155]. We include Pt(II) to have another comparison besides Pd(II) for precious late tran- sition metal and also because Pt(II) in this coordination environment is ubiquitous in coordination chemistry. The results are shown in Table 7.11 and Figure 7.17.
Table 7.11: Binding energies (∆H◦bind) obtained for the ground state of linkerBPY and different number of physisorbed H2. We also show ∆H◦bindfor the linker + TM(II)Cl2+ H2. The H-H bond of isolated H2 is 0.741 ˚A
Linker = Geom Spin n H2 ∆H◦bind H-H bond
BPY for M (s) (kJ/mol) (˚A)
Linker N/A 0 1, 2, 3, 4 -5.33, -5.18, -4.90, -4.78 0.745, 0.744, 0.744, 0.744 + Ni(II)Cl2 Sqr 0 1, 2, 3, 4 -10.9, -10.1, -8.83, -7.64 0.746, 0.746, 0.745, 0.745 + Cu(II)Cl2 Sqr 1/2 1, 2, 3, 4 -14.7, -13.3, -13.0, -11.7 0.746, 0.746, 0.744, 0.744 + Pt(II)Cl2 Sqr 0 1, 2, 3, 4 -11.4, -11.1, -10.9, -10.4 0.744, 0.744, 0.744, 0.744 + Pd(II)Cl2 Sqr 0 1, 2, 3, 4 -9.89, -9.86, -9.81, -9.78 0.747, 0.747, 0.747, 0.746
The BPYligand alone does not interact strongly with H2. The ∆H◦bind ranges from -5.33 to -4.78 kJ/mol, which is the usual strength for interaction with an organic linker. The H-H bond distance for the H2 are also in the usual range with 0.745, 0.744, 0.744 and 0.744 ˚A, for the 1st to the 4th H, respectively. However we increase this interaction by adding TM to the binding sites of this linker.
The best results are for the BPY-Cu(II)Cl2 complex with ∆H◦bind = -14.7, -13.3, -13.0 and
2,2'-bipyridine BPY+TMCl2
N N TM Cl
Cl
Figure 7.17: Different binding energies ∆H◦bind at 298K obtained for theBPY ligand interacting with four physisorbed H2. We have focused on isoelectronic TM. The error bars estimate the different configurations.
-11.7 kJ/mol. This is in the ideal range for maximum delivery H2 for the 233/358 K under our current assumptions. The H-H are 0.746, 0.746, 0.744 and 0.744 for the 1st to 4th interacting H2, this indicates that the H-H bond is not significantly distorted. The next best performance was the BPY-Pt(II)Cl2 complex. We found that the ∆H◦bind is slightly better than the Pd(II) case, as we explain below. For the 1st to the 4th H2, we found that ∆H◦bind = -11.4, -11.1, -10.9 and -10.4 kJ/mol, respectively. All the H-H bond for these H2 are the same; 0.744 ˚A. We calculate the
∆H◦bindfor theBPY-Ni(II)Cl2in a square planar geometry, although the most common geometry is tetrahedral for this case. Our intention was to compare the square geometry among different elements. Our results under for this geometry is ∆H◦bind= -10.9, -10.1, -8.83 and -7.64 kJ/mol. We observe a drastic drop in the binding energy when the number of H2increase, which is not desirable for a material in real application. Finally, theBPY-Pd(II)Cl2 complex gives a worst performances with ∆H◦bind in the range of -9.89 to -9.78 kJ/mol for the 1st to the 4th H2, respectively.
Thus, except for the Ni(II) case, all the other TM have a constant ∆H◦bind over the first four H2, which is desirable for a host in real applications, and also the interactions are slightly larger than 10 kJ/mol. We show again the utility of metalation as a way to improve the interaction with H2.
Ligand containing two bipyridine groups.
The first linker we studied with this approach was the 2,2’-bipirimidine (BPYM). Using the crystal structures we calculated the H2 binding energy for all these TM; Ni(II)[156], Cu(II)[157], Pt(II)[158] and Pd(II)[159]. In this case we studied the effect of having an extra TM in the same ligand and if this effect is somehow additive. The results are shown in Table 7.12 and Figure 7.18.
The BPYMalone does not have strong interactions with H2, which is shown by the ∆H◦bind
Table 7.12: Binding energies (∆H◦bind) obtained for the ground state of linkerBPYMand different number of physisorbed H2. We also show ∆H◦bindfor the linker + TM(II)Cl2+ H2. The H-H bond of isolated H2 is 0.741 ˚A
Linker = Geom Spin n H2 ∆H◦bind H-H bond
BPYM for M (s) (kJ/mol) (˚A)
Linker N/A 0 1, 2, 3, 4 -5.85, -5.74, -5.52, -5.23 0.745, 0.745, 0.745, 0.745 + Ni(II)Cl2 Sqr 0 1, 2, 3, 4 -10.3, -10.3, -10.2, -10.2 0.748, 0.748, 0.747, 0.746 + Cu(II)Cl2 Sqr 1/2 1, 2, 3, 4 -14.4, -14.2, -14.0, -13.7 0.746, 0.746, 0.746, 0.746 + Pt(II)Cl2 Sqr 0 1, 2, 3, 4 -11.5, -11.3, -11.0, -10.6 0.746, 0.746, 0.746, 0.746 + Pd(II)Cl2 Sqr 0 1, 2, 3, 4 -12.5, -12.5, -12.1, -12.1 0.745, 0.745, 0.745, 0.745
2,2'-bipyrimidine BPYM+TMCl2
N N
N N
TM Cl Cl TM
Cl Cl
Figure 7.18: Different binding energies ∆H◦bindat 298K obtained for theBPYMligand interacting with four physisorbed H2. We have focused on isoelectronic TM. The error bars estimate the different configurations.
= -5.85,-5.74, -5.52 and -5.23 kJ/mol, which is slightly higher than theBPYligand. Also the H-H bonds are slightly higher with all the bonds being 0.745 ˚A. We then calculate the binding energy with other TM and we found that we the interaction strength is improved.
The best performance is for the BPYM-Cu(II)Cl2 with a ∆H◦bind = -14.4, -14.2, -14.0 and -13.7 kJ/mol, with all the H-H bond being the same; 0.746 ˚A. The next best performance is for BPYM-Pd(II)Cl2 complex with ∆H◦bind = -12.5, -12.5, -12.1 and -12.1 kJ/mol, for the 1st to the 4th H2 respectively. The compound BPYM-Pt(II)Cl2 has the third best performance where
∆H◦bind for the 1st to the 4th H2 is in the range of -11.5 to -10.6 kJ/mol for the first four H2. Finally theBPYM-Ni(II)Cl2case we have ∆H◦bind in the range of -10.3 to -10.2 for the first 4 H2. Once again, we explored the square planar geometry for Ni(II) in order to compare among the same geometry, even when it is more common to find Ni(II) in the tetrahedral geometry.
We found that the additive effect for more ∆H◦bind given that two TM are close to each other was only found for the case of PdCl2, while in the other cases we did not see this effect clearly. It is possible that more configurations need to be explored. The compound that offers the stronger
interaction with H2 is the Cu(II)Cl2for both ligands.