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 polymethylene 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.

Xantphos is a trans-spanning ligand, without the steric problems associated with TRANSPHOS. SPANphos is comparable to XANTPHOS but more reliably trans-spanning. TRANSDIP, based on a α-cyclodextrin, is the first ligand to give exclusively trans-spanned complexes, even with d8 metal ion halides.[3]

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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.CS1 maint: multiple names: authors list (link)
  3. 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?". Chemistry: A European Journal. 13 (34): 9448–9461. doi:10.1002/chem.200700831. PMID 17943701.
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