Metal-Only Lewis-Pairs (MOLPs)

The concept of metal centered Lewis-basicity is of major importance in organometallic synthesis and catalysis. Particularly complexes with d8-configuration have long been recognized as metal-bases, as they are electron rich and react readily with electrophiles and also oxidatively add e.g. H2 or boranes, thus these complexes are widely used as catalysts. However, all early examples of dative bonds between d- and p-block metals generally display supported intermetal linkages. While reports of such “metal-only Lewis-pairs” (MOLPs) remained sparse and rather cursory for a long time, the field of heterodinuclear d-block/p-block complexes with dative bonds in particular has developed rapidly over the past decade. This development is documented by many publications about bora-, sila- and stannatranes, comprising supported dative bonds between d- and p-block elements. Similarly, we have demonstrated the propensity of electron rich d10-metal fragments of the type [(R3P)M] (M = Pd, Pt) to form dative bonds to metal-coordinated boryl and borylene ligands. [1, 2, 3, 4, 5, 6, 7, 8, 9] The latter species without exception display CO bridges between the metal centers and thus, again contain examples of supported dative metal-element bonds. Neutral Lewis-pairs of the type [CpRh(PR3)2(AlR3)] featuring unsupported transition metal-aluminium bonds were first described in 1984, however, the only structurally characterized complex shows a significant zwitter-ionic contribution.
One of our research interest focuses on unsupported metal-only Lewis-pairs (MOLPs). Therefore, we used the well-established transition metal Lewis-base [(Cy3P)2Pt] (1) to synthesize MOLPs revealing dative bonds between the d-block metal with various s-, p- and d-block metals.
The reaction of the strong Lewis-acid BeCl2 with [(Cy3P)2Pt] (1) resulted in the MOLP [(Cy3P)2Pt(BeCl2)] (2), the first MOLP between d- and  s-block metals. The subsequent reaction with methyllithium yielded the heteroleptic compound [(Cy3P)2Pt(BeClMe)] (3), respectively (Fig. 1).[10]

Fig. 1: Molecular structures of the s-/p-block MOLPs 2 and 3.

Extending this protocol to p-block halides, we used the Lewis-acids AlCl3, AlBr3 and AlI3 to synthesize the platinum alane MOLPs [(Cy3P)2Pt(AlCl3)] (4), [(Cy3P)2Pt(AlBr3)] (5) and [(Cy3P)2Pt(AlI3)] (6). Extended studies towards the gallium halide GaCl3 yielded in the platinum-gallane MOLP [(Cy3P)2Pt(GaCl3)] (7, Fig. 2). [11, 12]

Fig. 2: Molecular structures of the d-/p-block MOLPs 4, 5 and 7.

As all previous investigations on low-valent transition metal complexes towards s- and p-block metal halides had been successful, the research was extended to the related d-block compounds. The reaction of [(Cy3P)2Pt] (1) with ZrCl4 resulted in the platinum-zirconium MOLP [(Cy3P)2Pt(ZrCl4)] (8), which represents the first example of an early-late heterobimetallic complex with an unsupported dative bond (Fig. 3).[13]

Fig. 3: Molecular structure of the d-/d-block MOLP 8.

Extended studies towards the gallium halides GaX3 (X = Br, I) revealed surprising results.[12] The reactions of GaBr3 and GaI3 with [(Cy3P)2Pt] (1) display a different reaction pathway than the reaction with GaCl3. Interestingly, the products of the oxidative addition trans-[(Cy3P)2Pt(Br)(GaBr2)] (9) and trans-[(Cy3P)2Pt(I)(GaI2)] (10) were formed, showing the same reaction pattern like the oxidative addition of the boron-halogen bond, used by our workgroup in the synthesis of various boryl-complexes. Additionally, the reaction of [(Cy3P)2Pt] (1) and BiCl3 yielded the complex trans-[(Cy3P)2Pt(Cl)(BiCl2)] (11). This represents the first example of an oxidative addition of the bismuth-chloride bond at a low-valent metal centre (Fig. 4).[14, 15]

Fig. 4: Molecular structures of the products of the oxidative addition 9-11.

Hence, we became interested in the question as how to vary, and preferably increase, the electron donating properties of complexes of the type [L2Pt].
N-heterocyclic carbenes (NHCs) have been widely applied to the tuning of electronic properties of transition metal complexes, however, there has been no investigation on the direct influence of NHCs on the Lewis-basic properties of the central metal. Therefore, we synthesized three new NHC containing compounds of the type [L2Pt], including the first heteroleptic, 14-electron Pt(0) complexes, [(ItBu)(Cy3P)Pt] (12, ItBu = 1,3-Di-tert-butylimidazol-2-ylidene) and [(SIMes)(Cy3P)Pt] (13, SIMes = 1,3-Dimesityl-4,5-dihydroimidazol-2-ylidene) and the new homoleptic compound [(SIMes)2Pt] (14). In addition, the corresponding AlCl3 MOLPs 15-17 of these compounds were synthesized and fully characterized including single crystal X-ray diffraction (Fig. 5).[16]

Fig. 5: Molecular structures of NHC containing platinum-alane MOLPs 15-17.

In order to provide further information about the Lewis-bacisity of the [L2Pt] complexes, DFT calculations were carried out. All computed geometries show similar structural parameters to those derived from experiment, and the SCF energies of all optimized compounds were used to determine the bonding dissociation enthalpies (BDEs) for the alane adducts. As expected, the strength of the Pt–Al bond increases as the phosphane ligands are consecutively replaced by NHCs. Surprisingly, the heteroleptic complex [(ItBu)(Cy3P)Pt] (12) is overall a stronger base then [(SIMes)2Pt] (14), due to high reorganization enthalpies in the homoleptic [(NHC)2Pt] complex during the adduct formation.
As to give conclusive experimental evidence for the theoretical results, a transfer experiment was carried out. To this end, the MOLP [(Cy3P)2Pt(AlCl3)] (4) was treated with an equimolar amount of [(ItBu)(Cy3P)Pt)] (12), leading to the formation of the independently synthesized MOLP [(ItBu)(Cy3P)Pt(AlCl3)] (15) and the starting material [(Cy3P)2Pt] (1).[16]

Fig. 6: Transfer of the Lewis-acid to the stronger transition metal Lewis-base.

Selected references:

  1. H. Braunschweig, D. Rais, K. Uttinger, Angew. Chem. 2005, 117, 3829–3832; [pdf] Angew. Chem. Int. Ed. 2005, 44, 3763–3766. [pdf]
  2. H. Braunschweig, K. Radacki, D. Rais, G. R. Whittell, Angew. Chem. 2005, 117, 1217–1219; [pdf] Angew. Chem. Int. Ed. 2005, 44, 1192–1194 (VIP). [pdf]
  3. H. Braunschweig, G. R. Whittell, Chem. Eur. J. 2005, 11, 6128–6133. [pdf]
  4. H. Braunschweig, K. Radacki, D. Rais, D. Scheschkewitz, Angew. Chem. 2005, 117, 5796–5799; [pdf] Angew. Chem. Int. Ed. 2005, 44, 5651–5654 (VIP). [pdf]
  5. H. Braunschweig, K. Radacki, D. Rais, F. Seeler, Angew. Chem. 2006, 118, 1087–1090; [pdf] Angew. Chem. Int. Ed. 2006, 45, 1066–1069. [pdf]
  6. H. Braunschweig, K. Radacki, D. Rais, K. Uttinger, Organometallics 2006, 25, 5159–5164. [pdf]
  7. H. Braunschweig, C. Burschka, M. Burzler, S. Metz, K. Radacki, Angew. Chem. 2006, 118, 4458–4461; [pdf] Angew. Chem. Int. Ed. 2006, 45, 4352–4355. [pdf]
  8. H. Braunschweig, K. Radacki, K. Uttinger, Eur. J. Inorg. Chem. 2007, 4350–4356. [pdf]
  9. H. Braunschweig, M. Burzler, R. Dewhurst, K. Radacki, F. Seeler, Z. anorg. allg. Chem. 2008, 1875–1879. [pdf]
  10. H. Braunschweig, K. Gruß, K. Radacki, Angew. Chem. 2009, 121, 4303–4305; [pdf] Angew. Chem. Int. Ed. 2009, 48, 4239–4241. [pdf]
  11. H. Braunschweig, K. Gruß, K. Radacki, Angew. Chem. 2007, 119, 7929–7931; [pdf] Angew. Chem. Int. Ed. 2007, 46, 7782–7784. [pdf]
  12. H. Braunschweig, K. Gruß, K. Radacki, Inorg. Chem. 2008, 47, 8595–8597. [pdf]
  13. H. Braunschweig, K. Radacki, K. Schwab, Chem. Commun. 2010, 913–915. [pdf]
  14. H. Braunschweig, P. Brenner, P. Cogswell, K. Kraft, K. Schwab, Chem. Commun. 2010, 46, 7894–7896. [pdf]
  15. H. Braunschweig, P. Cogswell, K. Schwab, Coord. Chem. Rev. 2011, 255, 101–117. [pdf]
  16. J. Bauer, H. Braunschweig, P. Brenner, K. Kraft, K. Radacki, K. Schwab, Chem. Eur. J. 2010, 16, 11985–11992. [pdf]

back to Research Topics