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

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 Au...Au approach, however for 2c and 2d the reduction in the number of phenyl rings allows the formation of Au...Au contacts. For 2c and 2d the extended structures appear to be helical chains with Au...Au contact parameters of 3.855(5) A and C-Au-Au-C 104.1(3)degrees for 2c and 3.139(4) A and C-Au-Au-C -92.0(2)degrees for 2d. The Au...Au 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 pi-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 Au...Au 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-Cl...Au 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 A.

Crystal Structures of the Gold(I) Phosphinine Complexes [AuCl(C5H2P-2,6-Me2-4-Ph)] and [AuCl(C5H2P-2,4,6-Ph3)]

“…This distance is identical with the traditional contact limit for significant aurophilic interactions [33]. However, recent experimental work has demonstrated that aurophilic interactions may be structurally relevant up to Au···Au distances of 4.0Å [34], which is in accord with earlier theoretical predictions [35]. The aurophilic aggregation usually observed in the solid state for linear dicoordinate gold(I) complexes can give rise to three principal structural motifs, viz.…”

“…An almost identical value of 2.14Å was published in 2009 by Hu et al [73]. In 2010, Flower and coworkers proposed a value of 2.0Å [34]. The most recent values, published by Batsanov in 2011 [74] and by Alvarez in 2013 [70], differ considerably: 1.97Å vs. 2.32Å.…”

Alkali-metal-mediated zincation (AMMZn) meets N-heterocyclic carbene (NHC) chemistry: Zn–H exchange reactions and structural authentication of a dinuclear Au(i) complex with a NHC anion

“…The Au atoms are separated by 6.076(1)Å which precludes any possible aurophilic interaction. 30 Examining the geometrical parameters of 6 reveals that while the Au2-C1 bond distance [1.975 5 33 There are no precedents in the literature for coordination of this type of ligand to two transition metal centres, with the closest analogy being a recently published mixed K-Mn species resulting from the reduction of a neutral Mn(II) NHC complex by KC 8 . 34 Establishing a new methodology for accessing dinuclear organogold species under mild reaction conditions, the synthesis of 6 demonstrates the dual transmetallating ability of 2 which reacts differently with each equivalent of [ClAu(PPh 3 )].…”

Some insights into the gold-catalysed A 3 -coupling reaction

“…The Au1-C1 and Au1-P1 bond lengths of 2.0419(9) Å and 2.295(3) Å respectively are comparable to those previously reported for similar complexes. [58][59][60][61] Interestingly, as shown in figure 2, the amine moiety of 5 is rotated away from the Au(I) centre, presumably in order to minimise non-bonding interactions with the phosphane ligand, and therefore has no interaction with the metal atom (d(Au1…N1) = 4.662(7) Å, C1-C2-C7-N1 torsion angle of 152.7(8)°). This is in contrast to 4 (figure 2) which exists as a C,N-Au chelate (d(Au1-N1) = 2.09(4) Å, C1-C6-C7-N1 torsion angle of 29(3)°) with the nitrogen coordinated to the more Lewis-acidic Au(III) centre.…”