Trans-spanning ligand

Trans-spanning ligands are bidentate ligands that can span opposite sites of a complex with square-planar geometry. A wide variety of ligands that chelate in the cis fashion already exist, but very few can link opposite vertices on a coordination polyhedron. Early attempts to generate trans-spanning bidentate ligands relied on long hydrocarbon chains to link the donor functionalities, but such ligands often lead to coordination polymers.

History

A diphosphane linked with pentamethylene was claimed to span across a square planar complex. This early attempt was followed by ligands with more rigid backbones. "TRANSPHOS" was the first trans-spanning diphosphane ligand that usually coordinates to palladium(II) and platinum(II) in a trans manner. TRANSPHOS features benzo[c]phenanthrene substituted by diphenylphosphinomethyl (Ph2PCH2) groups at the 1 and 11 positions.[1][2] The polycyclic framework suffers sterically clashing hydrogen centers.

Structure of PtCl2(SPANphos)[3]

TRANSDIP, based on a α-cyclodextrin, is the first ligand to give exclusively trans-spanned complexes, even with d8 metal ion halides.[4] Xantphos is a trans-spanning ligand, with less steric bulk compared to TRANSPHOS. SPANphos is comparable to XANTPHOS but more reliably trans-spanning.

References
  1. Destefano, N. J.; Johnson, D. K.; Lane, R. M.; Venanzi, L. M. (1976). "Transition-Metal Complexes with Bidentate Ligands Spanning Trans-Positions. I. The Synthesis of 2,11-Bis(Diphenylphosphinomethyl)Benzo[C]-Phenanthrene, A Ligand Promoting the Formation of Square Planar Complexes". Helvetica Chimica Acta. 59 (8): 2674–2682. doi:10.1002/hlca.19760590806.
  2. Mochida, J. A.; Mattern, J. C.; Bailar, Jr., J. (1975). "Stereochemistry of Complex Inorganic Compounds. XXXV. A Complex Containing a Ligand That Spans Trans Positions". J. Am. Chem. Soc. 97 (11): 3021–3026. doi:10.1021/ja00844a017.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. Z. Freixa, M. S. Beentjes, G. D. Batema, C. B. Dieleman, G. P. F. van Strijdonck, J. N. H. Reek, P. C. J. Kamer, J. Fraanje, K. Goubitz and P. W. N. M. van Leeuwen (2003). "SPANphos: A C2-Symmetric trans-Coordinating Diphosphane Ligand". Angewandte Chemie International Edition. 42 (11): 1322–1325. doi:10.1002/anie.200390330. PMID 12645065.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. Poorters, L.; Armspach, D.; Matt, D.; Toupet, L.; Choua, S.; Turek, P. (2007). "Synthesis and Properties of Transdip, A Rigid Chelator Built Upon a Cyclodextrin Cavity: is Transdip an Authentic Trans - Spanning Ligand?" (PDF). Chemistry: A European Journal. 13 (34): 9448–9461. doi:10.1002/chem.200700831. PMID 17943701.
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