Heteroleptic Organostannylenes and an Organoplumbylene Bearing Phosphorus-Containing Pincer-Type Ligands - Structural Variations and Insights into the Configurational Stability

The syntheses of the arylphosphonic esters 3-Br-5-tBu-1-{P(O)(OiPr)(2)}C6H3 (1), 5-tBu-1,3-{P(O)(OiPr)(2)}(2)C6H3 (2), of the heteroleptic intramolecularly coordinated organostannylenes [4-tBu-2,6-{P(O)(OiPr)(2)}(2)C6H2]SnX (3, X = Cl; 4, X = Br; 5, X = I; 6, X = SPh), the organoplumbylene [4-tBu-2,6-{P(O)(OiPr)(2)}(2)C6H2]PbCl (7), and the transition metal complex [4-tBu-2,6-{P(O)(OiPr)(2)}(2)C6H2]Sn(Cl)-Cr(CO)(5) (8) are reported. The compounds were characterized by H-1, C-13, P-31, P-31 MAS (3), Sn-119, and Sn-119 MAS (3) NMR spectroscopy, electrospray ionization mass spectrometry (3), Mossbauer spectroscopy (3-5, 8) and single-crystal X-ray diffraction analyses (2, 3-5, 6-8). In contrast to its ethoxy-substituted analogue [4-tBu-2,6-{P(O)(OEt)(2)}(2)C6H2]SnCl, compound 3, like the thiophenolate derivative 6, is monomeric in solution as well as in the solid state. This difference is also manifested by the Mossbauer as well as solid state NMR spectroscopic data. On the other hand, the corresponding organoplumbylene 7 shows a similar chlorido-bridged polymeric structure as its ethoxy-substituted analogue. Variable-temperature and concentration-dependent H-1 and P-31 NMR spectra reveal, on the respective time scales, the tin atom in compound 3 to be configurationally unstable and those in compounds 6 and 8 to be configurationally stable. DFT calculations support a chlorido-bridged dimer to account for the configurational instability in compound 3. Surprisingly and in contrast to compounds 3 and 6, the organobromido- and organoiodidostannylenes 4 and 5, respectively, show intermolecular Sn center dot center dot center dot Sn distances of 3.6809(4) and 3.5953(4) angstrom being shorter than twice the van der Waals radius of tin (2.20 angstrom). Quantum chemical calculations were performed on monomeric and dimeric model compounds, which revealed a weak Sn center dot center dot center dot Sn bonding interaction for dimer 3 (16 kJ.mol(-1)) as well as for dimer 5 (20 kJ.mol(-1)) whereas the hypothetical model compound p-tBuC(6)H(4)Snl showed an iodido-bridged dimer rather than a Sn center dot center dot center dot Sn bonding interaction.