Reactivity studies of pincer bis-protic N-heterocyclic carbene complexes of platinum and palladium under basic conditions

Bis-protic N-heterocyclic carbene complexes of platinum and palladium (4) yield dimeric structures 6 when treated with sodium tert-butoxide in CH2Cl2. The use of a more polar solvent (THF) and a strong base (LiN(iPr)2) gave the lithium chloride adducts monobasic complex 7 or analogous dibasic complex 8.


Introduction
N-Heterocyclic carbenes (NHCs) have been extensively researched for a number of purposes since 1991 when Arduengo first isolated free NHCs [1][2][3]. NHCs as ligands have been known even longer. In 1968, Wanzlick and Öfele separately synthesized mercury(II) and chromium(0) imidazol-2ylidene complexes [3]. Nearly 50 years of NHC ligand research have demonstrated the importance of the electronic and steric effects that can be modified by altering the alkyl or aryl groups on each nitrogen atom. Less common are protic imidazol-2ylidene (PNHC) ligands with a hydrogen atom on one or both of the stabilizing nitrogens. The synthesis of PNHC complexes has proven to be a challenge, which has limited studies of their reactivity [4][5][6][7][8].
Protic imidazol-2-ylidene ligands (e.g., 1) have been shown to form an imidazolyl ligand (e.g., 2) after deprotonation with a basic proton-accepting nitrogen ( Figure 1). We are unaware of reports on an experimentally determined pK a value of a PNHC imidazolidene complex, but looking at related derivatives, Isobe showed that a 2-palladated pyridine was 3.57 pK a units more basic than pyridine [9,10]. Considering reactions other than simple proton transfer, imidazol-2-yl complexes have recently been used to bind to a second transition metal [11]. Additionally, Cp*Ir complexes from our group [12] demonstrated heterolysis of the H-H bond of H 2 and of the C-H bond of acetylene. The same ligand in CpRu complexes 2 and 3 showed heterolysis of dihydrogen [13]. 1 had a much faster ligand exchange rate after ionization as compared to the Cp*Ir analog (ethylene bound in 5 min at rt (CpRu) instead of 16 h at 70 °C (Cp*Ir)). Species 1 could be converted in situ to the hydride and isolated, or generated in situ and used as a transfer hydrogenation catalyst. Interestingly, the ligand substitution rate of ethylene and the heterolysis of dihydrogen was much greater for 3 than for 2.
With only a few papers exploring the utility of these imidazol-2-yl complexes, we aim to extend this to our recently reported pincer bis PNHC complexes 4-PdCl and 4-PtCl and their triflato analogs [14]. The design of these complexes was inspired by studies of Kunz et al. on aprotic analogs [15,16].

Results and Discussion
The loss of one NH proton from the bis-PNHC complex 4 could lead to structure 5, a complex concurrently containing a PNHC proton donor and a bond activating imidazol-2-yl unit. In an attempt to form 5, 4-PdCl was dissolved in CD 2 Cl 2 , and the solution was saturated with ethylene, followed by the addition of sodium tert-butoxide. After 2 h at room temperature, an NMR spectrum was acquired that showed a new, unsymmetrical species, as expected for 5. Crystals were grown by vapor diffusion of pentanes into benzene and analyzed. Surprisingly, the data showed that the dimer 6-Pd had formed such that the open site was not filled with ethylene, but rather was occupied by an imidazolyl nitrogen from a second complex ( Figure 2). The palladium and platinum dimer complexes, 6-Pd and 6-Pt, could be formed by addition of sodium tert-butoxide to the chloride analogs (Scheme 1), and isolated in 50-56% yields.  a The metal-to-plane distance defined by the five corresponding N-coordinated imidazole atoms; this value would be near zero in the absence of strain.
The examination of the dimer crystal structure (see Figure 2 for 6-Pd) shows strain in the Pd1-N1' (and Pd1'-N1) bond. This is due to the metal that remains in the plane defined by the three coordinating atoms of the tridentate ligand (i.e., C1, N3, and C4). The fourth donor atom from the other has to bend out of the plane with the N1' imidazole ring because of the adjacent sterics of the tert-butyl groups on the imidazole. The strain can be quantified by examining how far the metal is from the N1 (or N1')-bound imidazole plane (C1-C2-C3-N1-N2 plane and the symmetry-equivalent atoms): for 6-Pd, 1.241 Å and, for 6-Pt, 1.094 Å ( Table 1) The NMR results are completely consistent with persistence of the dimers in solution. For monomeric species such as 4-PdCl and 4-PtCl, the NH proton resonance is typically downfield shifted with a chemical shift of ca. 11 ppm, whereas this signal is strongly shifted upfield to 8.03 (6-Pd) or 8.19 ppm (6-Pt). The crystal structures for both 6-Pd and 6-Pt show that the NH is located above the pi system of one imidazole ring of the other half of the dimer, which would be expected to shield the NH and cause a significant upfield chemical shift. Moreover, a ROESY experiment on 6-Pt ( Figure S6, Supporting Information File 1) confirms that the NH (N5', Figure 2) has a throughspace interaction with the proton on the imidazole ring (C3, Figure 2), a situation that would not be possible for a monomeric structure.
Attempts to synthesize 5 using sodium alkoxide bases led to the formation of dimer structures 6 with presumed loss of NaCl. Therefore, lithium chloride adducts 7 were targeted because LiCl adduct 3 was isolable yet highly reactive. As demonstrated by NMR spectroscopy, the dissolution of 4-PdCl in a mixture of THF (0.7 mL) and C 6 D 6 (0.1 mL) followed by the addition of one equivalent of LiN(iPr) 2 deprotonates one of the PNHC complexes. This gives 7-Pd, without evidence of dimer formation (Scheme 2). The addition of a second equivalent of LiN(iPr) 2 deprotonates the second PNHC complex, giving 8-Pd. The 1 H NMR spectrum for compound 7-Pt consists of a single NH peak at 10.90 ppm and six aromatic peaks, which all integrate to one proton. The asymmetry is also observed in the 13 C NMR spectrum, which consists of 18 peaks between 100 and 170 ppm. As for 8-Pt, the 1 H NMR spectrum has no peak where the NH peak typically is located, and in the aromatic region there are three peaks. The 13 C NMR spectrum thus consists of 9 peaks between 100 and 170 ppm, showing the reappearance of symmetry. 15 N chemical shift data give structural insight (Table 2), as exemplified by 1-3 [10]. The Δ x (difference in 15 N shifts for compound x) is near zero for a PNHC (1), maximum for the imidazolyl conjugate base 2, and slightly less for an imidazolyl lithium chloride adduct 3. The δ N for the aprotic nitrogen incapable of acid base chemistry (N2) hardly changes, whereas for the protic (N1), the changes depend on its environment. In the following discussion, To see if the faster ligand exchange would lead to LiCl loss with palladium, 7-Pd was synthesized. Unfortunately, similar results were observed with the platinum analog where 1-heptene did not react with 7-Pd (which was then converted to 8-Pd by addition of LiN(iPr) 2 ). Then AgOTf was added to 8-Pd, which formed a deprotonated dimer complex. Even with palladium, the loss of the chloride ligand seemed to be too slow.

Conclusion
In conclusion, attempts at forming an imidazolyl complex from 4-MCl using sodium alkoxides led to strained dimers 6. However, 4-MCl could be deprotonated with either 1 or 2 equiv of LiN(iPr) 2 to give 7, an intriguing species with one PNHC ligand and one Li-imidazolyl adduct, or 8, a bis imidazolyl complex. Unfortunately, substrates could not displace the chloride ligand without formation of dimer 6, and the deprotonated complexes were water sensitive. The attempts at deprotonating the more labile triflate complex 4-PtOTf led to the formation of dimer 6-Pt. To increase the lability of the chloride ligand, species 4-PdCl and 4-PdOTf were examined but gave dimer 6-Pd. In summary, the reactivity of bis-PNHC complexes 4 and bases appears to be dominated by the formation of the dimeric structures. Studies to reduce dimer formation by various means, such as increasing steric hindrance at the imidazolyl nitrogens, will be reported in due course.

Supporting Information File 1
Experimental information and NMR spectroscopy figures.