Organogold Chemistry: I Structure and Synthesis

R V Parish Department a/Chemistry, UMIST, Manchester, M60 1QD, UK

Organogold chemistry is enjoying a resurgence of interest as useful applications, largely physical or medicinal, are being found. There is also a wide range of novel molecules, some with curious structures. This article reviews the structures and syntheses of recently discovered compounds. A later article will describe their reactions and applications.

Organogold derivatives have been known for almost example, reference 2), and deals with the preparation, 100 years. Many fascinating compounds and reactions structures, properties and applications of some of the have been discovered but, until recently, they have more interesting organogold systems; the examples found little practical application. However, the last ten given are drawn principally from those reported during to fifteen years have seen the development of new the last ten years (a more comprehensive treatment is materials with potential applications in non-linear found in reference 3). The intention is to exemplify optics, conjugated linear polymers (potential molecular rather than to be comprehensive, and no mention at all wires), liquid crystals, chemical vapour deposition and is made of cluster compounds. Complexes in which anti-tumour agents (see Box 1). On the other hand, in carbonyl, cyano, or isonitrile ligands provide the only marked contrast to the neighbouring noble metals, Au-C bonds are also ignored. For convenience, the compounds of gold have been very little used in review is divided into two parts: this part deals with homogeneous catalysis (but see reference 1). This the wide variety of organic ligands now available and review attempts to update previous treatments (see, for the structure and synthesis of the gold compounds derived from them; the second part treats the reactions and applications of these compounds. R-Au-PPhzo. R3P-Au-C:C-Au-PR3 luminescence R_Au-PPh{CH2

COMPOUNDS AND STRUCTURES Rod Polymers Organic derivatives of gold are known for both of the common oxidation states, +1 and +3, and there is an Liqu id Crystals Ro-Q-3-o-Q-C:C-AU-C:N-R increasing number of gold(II) derivatives. The normal and predominant coordination numbers of gold are

~H3 two for gold(I) and four for gold(III); as is common in

MOCVD X3C-Au-PMaJ H3C- f u-PMC3 this part of the Periodic Table, this gives stable CH3 (X p H. F) configurations with 14 and 16 electrons respectively. Homoleptic compounds AuR or AuR3 are NM82 therefore electron deficient and are unstable and highly 'I ~ L-x (X ~ CI,OAc) Antitumour 6-I reactive. This is true even though they achieve the X 'normal' through polymerization via bridging alkyl or aryl groups and three-centre two­ Box 1 Potential applicationsfor goldcompounds electron bonds (see below). The principal examples of

CS9' GoldBulletin 1997,30(1) 3 Gold(l) Gold(lll)

AuR(L) AuR3L

AuR(X)- AuR3X-

Box 2 Thepossible stoichiometries (L = neutralligand; X = anionicligand; R = organic group) stable compounds of empirical formula AuR are those in which the R-groups carry a functionality which allows polymerization by coordination to neighbouring Ketonyl gold atoms (eg [AuC=CRJn)' Compounds formulated originally as 'AuR3' or 'AuR3(O Et2) , are probably Li[AuR3BrJ (egR = CH3, C6Fs) (4). Provided additional ligands are present to make up the coordination number, compounds of all possible Cerbene stoichiometries are known, with one or two R-groups bonded to gold(I) and from one to four to gold(III)

(Box 2). The structures of these compounds are as R' expected, linear and square planar; where appropriate, Yllde Cl-lu-cH, -PR I 3 geometrical isomers can usually be obtained. In effect, R' the R-groups are carbanions acting as conventional, mononegative, two-electron ligands. Being amongst the softest ligands known", they transfer much electron Ylidelo density to the gold atom; this is demonstrated, for instance, by Mossbauer spectroscopic data (5, 6), and they exert a high trans-influence. Compounds in which Ph, Ph, two R-groups are mutually trans are therefore generally P-Au-P Ii ',:-,. of lower stability, and other soft ligands usually adopt Ylldic He;; : CH positions cis to the R-group. For the same reason, ".P-Au-P../ Ph, Ph, [AuR2r and [AultIr are highly reactive. In all cases, derivatives of the harder perfluoro-organic groups -CF3 and -C6FS show considerably greater stability. Ph, Ph, 12 + H...... "P-Au-P" AuL Melhanlde ..... C, ...C...... lAu P-Au-P H Ph, Ph,

HC=NP ~ L, Box 3 Goldcompounds with less conventional ligandsystems \d, CI

I II * The 'hardness' and 'softness' of metal ions and ligands correlates with their polarizabiliry: metal ions which are small and highly charged, and ligands PPh 2 ...... which are small and electronegative, are 'hard', Large polarizable metal ions Br I h '"Au.... or ligand donor atoms are 'soft', The trans-influence is the influence that a 'Br o:;:. ligand has on the strength of the metal-ligand bond trans to itself and Br increases with the softness of the influencing ligand. The sequence of increasing softness of some common ligands is: Cl" < Br" < RS- < RJP < III N ylide < /u'- < R-.

4 @ GoldBulletin 1997,30(1) LAu SiMea AuL ~ / LAU·~.· :"~ "C, L... SiMeal' LAu SiMea ····. I ' SiMea AuL v VI AuL / :·.... I · ~··· ·· · · · ... Ph2 Ph2 P-Au-P C -. ;: '.\- LA '~ , \~ A U L HC: :CH u ·...... ' \. ./ "' AuL P-Au-P Ph Ph a 2 2 ~ l+- fH VII VIII LAtI'\">LAUI .::. LA. tI\.">LAu. Y ," .. r LAu...... ·LAu Y rr-Bonded organic ligands can be of all common LAu..··..· LAu types: alkyl, aryl, vinylic, acetylenic. The aryl derivatives in particular lend themselves to the formation of chelates when suitable substituents are present; naturally, the stereochemistry required means ...' · · ·· tA.·:~.~A U J+ LAU:-:-::' ~: :.:. that these are limited to gold(III); examples are shown ...... '> AuL as I-N (7-10). . : . Considerable attention has also been given to the AuL synthesis of derivatives containing less conventional ketonyl, diketonyl, , or ylide ligands; examples Box 5 Examples ofcarbon-centred and hypervalent gold are given in Box 3 (typical references: 11-19). The compounds distinction between the different types is not always obvious, and more than one can occur in the same contain gold- bonds. It can, however, use the molecule. A carbene may be considered, formally at residual electron density on the bridging carbon atom least, as a neutral divalent carbon ligand, :CY2> where to bind extra gold atoms (VIII); to distinguish Y is often an alkoxy or amido group. The most complexes of this type from conventional rr-bonded common ylide ligands are effectively deprotonated organogold compounds, they are often called phosphonium cations, eg :CH2PPh3 (derived methanide complexes. Despite the unconventionality from the methyltriphenylphosphonium cation, of some of these organic ligands, and the curiosity and CH3Ph3P+), therefore also neutral. The complexity ofsome of the structures (Box 4, references dimethyldiphenylphosphonium cation can undergo 20-23), the stereochemistry at gold is highly normal. two deprotonation stages to give :CH2PPh2CH2-, The cyclic digold(l) ylide complexes are readily which can act simultaneously as a conventional c­ oxidized to give gold(III) complexes with conventional bonded ligand and as an ylide; however, the negative square planar AUC2X2 coordination but retaining the charge is delocalized, and this ligand forms ring structure, VI, IX (24). With a single molar symmetrical bridges: two such ligands bind to two gold equivalent of halogen, gold(II) derivatives (eg X) are atoms forming an eight-membered ring system (V; obtained (25-26). Although the gold atoms now VI). The dppm ligand, Ph2PCH2PPh2, forms a similar formally have a d 9 configuration, the complexes are all cyclic bis-gold(I) complex which can be deprotonated diamagnetic; the coordination is square planar and to give VII. Although ylidic, this complex does not includes a short bond to the second gold atom. This

Box 4 Some 'exotic'ylidic and methanidicstructures

~ GoldBulletin 1997, 30(1) 5 .....-~ . --,: . ",:.., ...... - ',...... - == <,.\ ":'.- ~~... ".-.- '\". .- 2p -":'::::'" \:/..==. .'..'­ .. ./../ ..- .\:. \ ~/...... " ·\: :::~::i?\- \;:\~., ...... ­ .....:- ..•~.. .. Au C '..--.."

ICAuL)e) 2+

Figure 1 MO schemes behaviour is, of course, analogous to the long- established mercury(I) compounds.

F3C<, ..... CH2 ~Au--=--". R2P, Br ..... Br PR2 "--Au:...... -J Bf

IX x In recent years a series of compounds has been discovered in which carbon atoms are a-bonded to one Figure 2 The molecular structure of[AuC= CBu'}1l (Reprinted to six Au'L units, where L is a tertiary phosphine (Box with permission from reference 37). 5 - references 27-31). In those involving four­ coordinate carbon, the structures are basically experience an increase of mass. This affects their tetrahedral but the Au-C-Au angles are often smaller energies and, for the valence shell, allows much more than the tetrahedral angle rather than larger as might orbital mixing than would otherwise be expected. These have been expected on steric grounds. Even more effects operate for all heavy atoms, but just happen to curiously, the coordination number ofthe carbon atom have a maximum at gold. increases readily above four, to five and even six; Other electron-deficient gold(I) compounds are C(AuL)4 can be obtained only when L is a bulky best exemplified by AU2Zn2Ph6, for which structure XI ligand, such as P(C6H Ilh It should also be mentioned has been proposed (34). This type of bridging has been that analogous 'hypervalent' compounds are known confirmed in the ferrocene derivative XII, with central atoms other than carbon (eg boron, (CsHs)Fe[CsH4(AuPPh3h]+ (35). The well-established oxygen, nitrogen, phosphorus, arsenic). (36) gold(I) alkynyl, [Au(C=CBut)]n' was for a long Satisfactory molecular orbital schemes may be constructed, in which the available valence electrons just fill the bonding molecular orbitals (Figure 1). The driving force for the formation of these remarkable compounds is the tendency for gold atoms to attract each other and for weak Au ...Au bonding interactions to stabilize the compounds. Such interactions appear in a wide range of compounds (32, 33), and are estimated to have a strength comparable to hydrogen-bonds, ca 30 k] mol", This effect has been named 'aurophilicity' (31), and is thought to be due to relativistic effects arising from the high nuclear charge (33 and references therein). Under its influence, electrons are accelerated to velocities comparable to that of light and they XI XII

6 ~ GoldBulletin 1997, 30(1) SYNTHESIS

The most versatile and most often used method for the preparation of organogold compounds is the trans­ metallation reaction (2). Organolithium or Grignard reagents readily replace halide bound to gold. For gold(I), it is necessary to start with a complexed halide, AuX(L) (Equation 1), and to avoid an excess of the XIII XIV organometallic reagent unless it is intended to obtain pO RMgX CsFs-/t-C=C- CsHS L-Au-X L-Au-R (1) CsF I s u t LiR PPha L-Au-R XV the di-organoaurate(I) ion (Equation 2). Satisfactory time assumed to have a tetrameric square structure (n = Grignard reagents can be made from fluoro-aryls, but 4), which would allow the triple bond to occupy the trilluoromerhyl derivatives are best formed from second coordination site at an adjacent gold atom, and Cd(CF3)z (42). With gold(III) halides, the diorgano thus remove the electron deficiency. This material has derivative AuR2X is the usual first product when recently been crystallized and found (37) to have a starting from AU2X6 or H[AuX.;]. Again further much more fascinating, complicated structure involving reaction occurs readily with lithium reagents, to give [Au~L all possible bonding situations: one gold atom has two [AuR3X]- or this is also a convenient way of o-bonded alkynyl groups, one is 'IT-bonded to two making mixed organic-ligand derivatives, which do not triple bonds, and a third has one (J- and one 'IT- bond. appear to undergo disproportionarion. Mono­ Two of each of these units are bound together in a ring organogold(III) derivatives are usually obtained from in which the gold atoms form a flat regular hexagon, reagents, and this is most commonly and two rings are interlaced (see Figure 2). A similar done with potential chelate ligands such as azo­ mix of (J- and 'IT-bonded groups may occur in the benzene or dimethylaminobenzylamine, eg to obtain I gold(I) derivative of a propargyl-substituted calixarene, or IV (7,43,44). XIII, (38) and is also seen in XIV and XV (39, 40). Apart from the polymeric alkynyl derivatives, there is (N Q[AuCl 1 4 r1 little evidence for 'IT-bonded complexes. The highly C-Hg-CI C-Au-CI (3) -QdHg 2Cl61 I unstable (explosive) cyclopentadienyl [Au(C5H5)]., CI could polymerize either via three-centre bonds or by 'IT­ bonding analogous to that of the alkynyls, but nothing ~ (!ht)AuC1 3 r1 is known of its structure. In the substituted monomeric C-Hg-CI C-Au-CI (4) derivatives, such as Ph3PAu(C5H5), the -HgClz, tht I CI cyclopentadienyl group is expected to be bound as a (J­ bonded monohapto ligand (Tjl, two-electron donor). In the normal solvents (acetone or acetonitrile), the However, these molecules are fluxional on the NMR reactions represented by Equations 3 and 4 (Q = Na or timescale and all carbon atoms are equivalent; as in Me4N; tht = rerrahydrorhiophene) are generally more other cases, the 1,2-shift may involve transient 'IT­ efficient, but they appear to be finely balanced bonded intermediates (41). equilibria; it is sometimes necessary to add Me4NCl to encourage the precipitation of (Me4N)z[Hg2CI6] (43). For some ligands (eg as in XVI, XVII) even this is not effective, but reaction will proceed if a more polar solvent is chosen, such as DMSO (10). It has been postulated that these reactions proceed via a bridged intermediate, XVIII, and that the solvent may inhibit the coordination of the N-donor group to the mercury atom and allow it to bind to the gold (10). This is

(69' GoldBulletin 1997, 30(1) 7 0 was found that some of these adducts will undergo an 0 ... 11 ortho-metallation reaction, such as the transformation "'S-NMe2 of:XX into XXI (46). This is effected by refluxing the I adduct in aqueous acetonitrile (3: 1), and similar ~ !J A1U-CI ~ !J Ai-C' b- CST reactions have been used recently to prepare a range of CI CI C,N-bonded gold(III) chelates containing six­ XVI XVII membered rings such as XXII, including one in which an alkyl carbon atom has been metallated (XXIII) (47). Although it has not been commented on, the role ryL\'R o-r"N-O of the solvent must be critical here, in allowing the I .... CI CI-Au-CI escape or, more probably, the solvation of the liberated 79 ",Au CI I I HC\' CI CI CI A versatile method involving exchange of organic XVIII XIX groups has been developed. Reaction of [AuC12r with thallium(I) acetylacetonate produces [Au(acacl-]" (XXIV). This complex reacts with organic molecules 0 having a moderately acidic hydrogen (HR) to liberate acetylacetone and form the corresponding [AuR2r (48). For instance, the ethynyl complex [Au(C=CHhr is -ILc1 6 readily made this way (49); as the [N(PPh3h]+ salt, this CI anion is much more stable than previously reported (50). Simple gold(I) alkynyls are still made by the XX XXI traditional method (36) which was effectively the reaction of [Aubrj]" with HC=CR in the presence of consistent with the failure of the lithium derivatives of sodium acetate. Similar proton-elimination reactions are these and similar ligands to react with [AuC14r (43); used to prepare the substituted derivatives LAuC=CR presumably the organolithium is chelated, again from LAuCl (see, for example, references 51-55); discouraging the formation of the Au-N bond. alternatively, [AuC=CR)]n is treated with the ligand Solvent effects are also important in direct auration (56). reactions of aromatic systems by gold(III) halides. The first arylgold derivative, [AuPhC1 2b reported in 1931, was obtained by direct reaction between benzene and AU2C16 (45). Alkyl benzenes would also react but not benzenes with potentially coordinating substituents; for instance, azobenzene gave XIX. More recently it

The unusual compound III is prepared by bromination of a gold(I) derivative of /, -O sryryldiphenylphosphine (XXV). Oxidation occurs Me N N rapidly at the gold atom to give XXVI which Me~I Au-CI I CI CI HAuCI 4 I ,NMe2 (OC)5M[C(NMe2)OEtJ CI-Au-C (5) I 'OEt XXII (M = Mo, W) XXIII CI

.... CI ,Au ",N OC~ PhNHz ,NHPh CI-Au-C (6) ~ C, 'NHPh "N 'Au "'CI

8 @ GoldBulletin 1997, 30(1) 2LiR (Ph3P)AuCI [PhzP(CH3)zl+ - U[PhzP(CHz)zl ---

Ph3P CICHzC(O)NMez [Ph3PCHC(O)NMezICI ~NaH

Ph3P = CHC(O)NMez (tht)AuCI/ <,CIAu(CH2PPh~ o ,/ (8) (9)" 0 0 l+ II II II C-NMez MezN-C C-NMez CI-Au- CH" ,HC-Au-CH" , / ' PPh3 Ph3P PPh3 undergoes slow rearrangement to III (9, 57). as the deprotonating agent (Equations 9-11) (18,48). Analogous reactions occur with the corresponding A range of methanide complexes can be obtained by allyl-substituted phosphine (9). making use of the basicity of an ylide carbon atom Carbene complexes can be obtained by (Equations 12-15) (18,58), transmetallation, involving metal carbonyl derivatives Finally, many of the hypervalent compounds are (Equation 5) (14) or by nucleophilic attack on an made by an ingenious reaction in which a isonitrile (Equation 6) (13). Ylides are usually obtained dimethoxyboryl compound undergoes displacement of by deproronarion of a methyl phosphonium salt, using the B(OMeh group by an AuU group and butyl lithium or sodium hydride. They are strong simultaneously becomes transformed into a BF4- anion ligands and readily replace halides, or other weakly by the addition of CsF (in THF solution) (Equations bound ligands from gold(l) complexes (Equations 7,8) 16, 17) (25, 26). (22, 48). In some cases a ligand already bound may act

Phz Phz P-Au-P Ph2PCH2PPh2 /.' '0\ HC'. ',CH (lO) \:. ./ P-Au-P Phz Phz

oII l+ C-NMez (II) Ph3P-Au-CH," PPh3

(l2) (Ph3P)Au(acac)

oII l+ Ph3PAu" "C-NMez C / ' Ph3PAu PPh3

@ GoldBulletin 1997, 30(l) 9 Ph2 Ph2 P-Au-P j' .~ He:: .... CH \. :j P-Au-P Ph2 Ph2 (tht)AuR / ~ [LAu(tht)]CI0 4

(14) ~ Ph Ph / (13) 2 2 Ph2 Ph2 P-Au-P P-Au-P RAu /: ..~ H LAu /C. ..~ H 12+ 'c.: ': c:...... 'c:' ··.C...... w...... \:" :( <, AuR H...... \. :/ <,AuL P-Au-P P-Au-P Ph2 Ph2 Ph2 Ph2 LA"(~~I (15) I

Ph2 Ph2 + P-Au-P 12 LAu /.: ".\ .... AuL 'c: .c" LALf \;. :/ <,AuL P-Au-P Ph2 Ph2

An increasing number of applications for gold CONCLUSIONS compounds can be envisaged, and many of these are based on the organogolds; these will be discussed in a This survey shows that, despite the enormous variety second article. These applications depend on the ways of types of organogold compound now known, some in which organogold compounds react, and these apparently rather exotic, their coordination numbers reactions will also be surveyed. and stereochemistries obey simple rules: gold(I) is two-coordinate and linear, and gold (III) is four coordinate and square planar. While other structures ABOUT THE AUTHOR are known (a few three- and four- coordinate gold(I) complexes, a very small number of five coordinate Dick Parish is Reader in Chemistry at UMIST and has gold(III) complexes) they do not appear in a long-standing interest in the chemistry of gold, organogold chemistry except perhaps as reaction particularly of compounds with actual or potential intermediates. medicinal interest, and in the application ofMossbauer spectroscopy to coordination chemistry, including that ofgold.

LAuCI

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12 ($S' GoldBulletin 1997,30(1)