# Synthetic approach for precursor 1,1'-(bromomethylene)bis(1H-pyrazole) in ligand synthesis

## Relevance

During my Bachelor's project I focused on the synthesis of iron(II) complexes comprising a tetrapodal pentadentate ligand from the ligand family $$\text{PY5R}_2\ (\text{R = H, Me, OH, OMe})$$. [1-4] This ligand consists of one axial pyridine unit and four equatorial pyridine units. I investigated the activity of these complexes in the oxidation of alkenes by hydrogen peroxide.

While doing literature research I discovered several means of tuning the properties and chemical reactivity of these complexes.

For example, the cone angle for steric accessibility is larger for ligands with five-membered pyrazolyl rings compared to those with similarly substituted six-membered pyridyl rings. See the ligand $$\text{pz}_{4}\text{lut}$$. [5]

Furthermore, one could also influence the donor-strength of the ligand by replacing the axial pyridine unit by a $$N$$-heterocyclic carbene (NHC) unit. See the ligand $$\text{PY4Im}$$. [6]

So I thought, why not combine both features? This leads to a tetrapodal pentadentate ligand with an axial NHC unit and four equatorial pyrazole units, abbreviated as $$\text{pz}_{4}\text{Im}$$.

By modifying the procedure employed in the synthesis of $$\text{PY4Im}$$ [6], the synthesis of the iron complex comprising $$\text{pz}_{4}\text{Im}$$ would look like this

Figure 2. Synthetic approach towards $$[( \text{pz}_{4}\text{Im})\text{Fe(MeCN)}](\text{PF}_{6})_{2}$$.

## Problem

My actual problem is related to the starting material.

In the synthesis of $$\text{PY4Im}$$ [6], 2,2'-(bromomethylene)bis(pyridine) is used as starting material. It is known how to synthesize this compound.

In this synthesis, that of $$\text{pz}_{4}\text{Im}$$, 1,1'-(bromomethylene)bis(1$$H$$-pyrazole) I would choose as starting material. However, I cannot find any literature on the synthesis of this particular compound.

Figure 3. 1,1'-(bromomethylene)bis(1$$H$$-pyrazole) (left) and 2,2'-(bromomethylene)bis(pyridine) (right) - starting materials.

## Question

So my question is: Does anyone know a way to synthesize the starting material 1,1'-(bromomethylene)bis(1$$H$$-pyrazole) for the synthesis of the iron(II) complex comprising the pentadentate NHC-pyrazole ligand $$\text{pz}_{4}\text{Im}$$?

## References

1. Eur. J. Org. Chem. 2004, 21, 4319-4322.
2. Tet. Lett. 2012, 53, 1, 54-55.
3. ChemSusChem. 2016, 9, 10, 1178-1186.
4. Inorg. Chem. 2006, 45, 15, 6048-6055.
5. Inorg. Chem. 2008, 47, 17,7468-7470.
6. Inorg. Chem. 2010, 49, 23, 11223-11230.
• I am sorry if my english is not perfect but your question does not make any sense to me! – SteffX Jul 14 at 17:19
• @SteffX How to synthesize the precursor 1,1'-(bromomethylene)bis(1H-pyrazole) (figure 3, left). – Jameson Jul 14 at 17:40
• These are my thoughts on this as I don't have database access to check the reactions. 1-Chloromethylpyrazole is commercially available so I would start with this. Reaction with pyrazole anion will give the 1,1'-methylene bispyrazole. I would try a radical bromination of this to access the desired material – Waylander Jul 14 at 20:00
• @Waylander I've been thinking about that too, but would this also work? imgur.com/9jIUDjC – Jameson Jul 14 at 21:31
• looks an entirely reasonable route to try – Waylander Jul 15 at 5:55