Isolation of 1,4-Li2-C6H4 and its reaction with [(Ph3P)AuCl]

Flower, KR; Mcgown, A; Miles, PJ; Pritchard, RG; Warren, JE

Abstract

The difficulty in generating 1,4-Li2-C6H4 utilising the lithium halogen exchange reaction on 1,4-Br2-C6H4, 1,4-I2-C6H4 and 1-Br-4-I-C6H4 is revisited and only on treatment of 1,4-I2-C6H4 with 2 molar equivalents of n-BuLi can 1,4-Li2-C6H4 1 be isolated in excellent yield. Treatment of 1 with two equivalents of [ClAu(PPh3)] gives [1,4-(Ph3PAu)2-C6H4] 2a in excellent yield. Subsequent treatment of 2a with 2.5 molar equivalents of PPh2Me, PPhMe2 or PMe3 affords the PPh3 substituted compounds [1,4-(LAu)2-C6H4] (L = PPh2Me 2b, PPhMe2 2c, PMe3 2d) in essentially quantitative yields. On treatment of 1,4-Br2-C6H4 or 1-Br-4-I-C6H4 with 2 molar equivalents of n-BuLi only mono-lithiation takes place to give 1-Br-4–Li-C6H4 3 as shown through the isolation of essentially 1:1 molar equivalents of Ph2PC6H4-4-Br and Ph2PBu on treatment with 2 molar equivalents of ClPPh2. Treatment of 3, prepared by lithium/iodine exchange on 1-Br-4-I-C6H4, with [ClAu(PPh3)] affords [(Ph3P)Au(C6H4-4-Br)] 4 as expected and in addition [(Ph3P)Au(n-Bu)(C6H4-4-Br)2] 5, indicating the straightforward chloride/aryl exchange at gold may proceed in competition with oxidative addition of the n-BuI, generated in the initial lithium/iodine exchange reaction, to some aurate complex Li[Au(C6H4-4-Br)2] 6 formed in situ followed by reductive elimination of Br-C6H4-4-n-Bu in a manner that mimics lithium diorganocuprate chemistry. All of the gold-containing compounds have been spectroscopically characterised by 1H and 31P-{1H} NMR and in addition compounds 2a–d and 5 by single crystal X-ray diffraction studies. The solid state structures observed for 2a–d are dictated by non-conventional hydrogen bonding and the packing requirements of the phosphine ligands. For 2a and 2b there is no close AuAu approach, however for 2c and 2d the reduction in the number of phenyl rings allows the formation of AuAu contacts. For 2c and 2d the extended structures appear to be helical chains with AuAu contact parameters of 3.855(5) Å and C–Au–Au–C 104.1(3)° for 2c and 3.139(4) Å and C–Au–Au–C -92.0(2)° for 2d. The AuAu approach in 2c is longer than is normally accepted for an AuAu contact and is dictated by ligand directed non-conventional hydrogen bonding to the aurated benzene ring and the -stacking requirements of the phosphine ligand. By comparison of the structures 2a–2d with other structures in the database it is evident that the aurophilic interaction is a poor supramolecular synthon in the presence of non-conventional hydrogen bond donors. Searches of the CCDC database suggest that the observed parameters for the AuAu contact in 2c sit close to the cut-off point for observing this type of contact. In addition to aurophilic contacts and non-conventional hydrogen bonds there are a number of halogenated solvent C–ClAu contacts observed in the structures of 2a and 2d. The nature of these contacts have implications for the accepted van der Waals radius of gold which should be extended to 2 Å.

Citation

Flower, K., Mcgown, A., Miles, P., Pritchard, R., & Warren, J. (2010). Isolation of 1,4-Li2-C6H4 and its reaction with [(Ph3P)AuCl]. Dalton Transactions, 39(14), 3509-3520. https://doi.org/10.1039/b925355f

• Journal Article Type: Article
• Acceptance Date: Jan 23, 2010
• Online Publication Date: Mar 2, 2010
• Publication Date: Apr 14, 2010
• Deposit Date: Mar 16, 2010
• Journal: Dalton Transactions
• Print ISSN: 1477-9226
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 39
• Issue: 14
• Pages: 3509-3520
• DOI: https://doi.org/10.1039/b925355f
• Publisher URL: http://dx.doi.org/10.1039/b925355f