7.2 Dependence of the H 2 Binding Energies Strength on the Transition Metal and Organic
7.2.3 Results and Discussion
7.2.3.3 Alternative Strategy to Metalate COFs and MOFs
interaction with H2 is the Cu(II)Cl2for both ligands.
N N
N N Pd(g)
N N Pd Pd (g)
N N
Pd N O NH
Pd Pd(g)
N NH O
G=
H=
-62.6 -94.6
G=
H=
-51.5 -86.9 G=
H=
-38.8 -71.8
kJ/mol kJ/ mol
kJ/ mol
Pd(g) N
H O N Pd
H O
N
Pd(g) -O
N Pd -O
G=
H=
-41.7 -73.2
G=
H=
-68.4 -100.8 kJ/mol
kJ/mol
N H O
N
-O H+ W ater
pKa = 8.7
Figure 7.19: Alternative option for metalating the linkers using metallic Pd(0). Our calculations show that all these reactions are favorable, and therefore it should be a viable mode for putting metals inside extended structure. Note that in the reaction fromPIPtoPIPE, we did not consider a counter cation for PIPE; this makes the reaction with H+ extremely favorable. The inset shows the calculated pKa forPIP/PIPE.
of the bond in the order of 0.003 ˚A, but it still sensitive enough to capture this direct correlation of H-H bond to the strength of ∆Hr◦. In the paper by Kubas,14 the range for a true H2 complex is given as 0.8 to 1.00 ˚A, while the elongated H2 complex is estimated as the H-H bond distance from 1.0 to 1.3 ˚A. However our calculations show energetics for the formation of hydrides and H-H bond around 0.9 ˚A, while the only long bond is of 1.8 ˚A, for the PIPE-Pd. This discrepancy can be resolved by trusting the energetics of our QM method but considering our distances for H-H bond obtained from QM shorter than those described by Kubas. This can be seen in the following comparison, Kubas used the value of the isolated H-H of 0.75 ˚A, while our QM method found this value to be 0.741 ˚A.
Another evidence for the formation of hydride is the short H-Pd bond in all these linkers. The distances for the Pd-H bond in thePIPE-Pd-H2complex are 1.566 and 1.564 ˚A. We then obtained longer Pd-H bond distances for thePIA-Pd-H2 complex with 1.722 and 1.717 ˚A. ThePIA-Pd-H2
complex gives Pd-H bond distances of 1.729 and 1.724 ˚A, these distances are longer than thePIPE ligand. The distances for the Pd-H bonds in theBPYM-Pd2-H2 complex are 1.713 and 1.712 ˚A,
Table 7.13: Binding energies (∆H◦bind) obtained for the ground state of all linkers (BBH,PIP, PIPE, PIA, BPY, and BPYM) + Pd shown in Figure 7.19 reacting with different number of H2. For each case the spin is 0. The H-H bond of isolated H2 is 0.741 ˚A
Linker-Pd Geom n H2 ∆H◦bind H-H bond
for M (kJ/mol) (˚A)
BBH-Pd Sqr 1a- 5 -53.5, -12.6, -12.5, -12.0, -12.1 0.855, 0.749, 0.748, 0.747, 0.744 PIP-Pd Sqr 1a- 5 -91.7, -13.7, -13.6, -13.6, -13.1 0.869, 0.749, 0.748, 0.748, 0.744 PIPE-Pd Sqr 1a- 5 -106, -14.8, -14.2, -13.8, -13.8 1.806, 0.748, 0.747, 0.747, 0.744 PIA-Pd Sqr 1a- 5 -96.1, -13.7, -13.0, -12.7, -12.0 0.868, 0.749, 0.747, 0.747, 0.743 BPY-Pd Sqr 1a- 5 -77.1, -11.4, -11.4, -11.0, -10.9 0.871, 0.746, 0.745, 0.743, 0.743 BPYM-Pd Sqr 1a- 5 -80.7, -80.5, -14.5, -14.5, -13.2 0.870, 0.869, 0.748, 0.748, 0.748
aThe first H2 react chemically and forms a linker-Pd-H2that resembles a hydride formation due to the energetics involved.
L=BBH, PIP, PIPE, PIA, BPY, BPYM
L L Pd
Figure 7.20: (left) Our calculations showed that Pd(0) binds to the different linkers studied here.
(Right) The plot shows the energetics when the H2 interacts with the compounds formed with Pd(0) shown in Figure 7.19. The first H2 forms a hydride converting the Pd(0) into Pd(II). The subsequent H2 interacts strongly by physisorption with the formed Pd(II)H2. BPYM shows two H2 bound chemically because it has two Pd per linker.
while for the second H2 in the BPYM-Pd2-2H2 complex, the distance for the analog bonds are 1.713 and 1.713 ˚A. Then we have theBPY-Pd-H2complex with both Pd-H bond distances of 1.715 and 1.713 ˚A. Finally the bond distances for the Pd-H in the BBH-Pd-H2 complex are 1.729 and 1.723 ˚A. This trend suggest that the longer the Pd-H bond the least energetics for the formation of the linker-Pd-H2(with the PIA-Pd-H2 complex not quite following this trend, perhaps because the N in the pyridine ring makes this bond longer than the rest due to the pi electrons from the aromatic ring). For all these cases we found that the singlet is the final electronic ground state of the linker-Pd-H2 complex and all the geometries are square planar.
Once the first H2is bound chemically to the linker-Pd system, the new complexes linker-Pd-H2
serves as the host that interacts with other H2. We show the energies for these other 4 H2 in Table 7.13. In general the energies are very similar to the energies obtained for the physisorbed H2of the
other ligands with transition metals and chlorines as the counter anion. This indicates that even when we have square planar geometries that allow more H2to interact with the metallic center, we do not obtain much gain, however the new concept of using the same coordinated metallic center for chemisorption and physisorption allows us to explore new types of hydrogen uptake.
A very important phenomenon happens when we absorb H2 into BPY versusBPYM. While BPYhas one site for hosting the Pd(0), theBPYMligand has two sites and it turns out that the binding of the H2is stronger for theBPYM-Pd2-2H2 com-plex than withBPY-Pd-H2 suggesting a cooperative interactions caused by having two Pd in the same ligand. This is the same effect observed for Pd(II)Cl2, this suggest that late transition metals have this property but we have not observed in early TM. The physisorbed H2 toBPYM-Pd2-2H2 have a stronger binding energy of around 14.5 kJ/mol while the analogs for theBPY-Pd-H2are around 11.4 kJ/mol. The same trends is observed when we analyzed the H-H bond; this bond distance is longer for theBPYM-Pd2-2H2
(around 0.748 ˚A) that for the BPY-Pd-H2 case (around 0.745 ˚A). This is an effect not observed for the other cases and gives us a hint for using mult-binding sites for metals such as the BPYM, rather than the single-site binding sites such as theBPY. The overall gain is bigger when the metals are in the same ligand.