Carbonyl Derivatives ofGold and Related Organometallics

Daniela Belli Dell'Amico andFausto Calderazzo Dipartimento di Chimica e ChimicaIndustriale, Uniuersita di Pisa, Via Risorgirnento 35, 1-56126 Pisa, Italy

The properties and reactions ofsimple gold carbonyls and related compounds are described and the bonding within the gold carbonyl molecules discussed in terms ofbond dissociation enthalpies,

Although the organometallic chemistry of gold is still synthesis of the gold organornetallics described to date rather limited (but see accompanying article in this provides some relevant information. issue of Gold Bulletin by R V Parish (1)) it presents The carbonyl derivatives of gold(I) reported in the aspects of uniqueness. Some of the properties of gold literature all have ligands of high electronegativity or organometallics may be understood if in the oxidation ligands containing electron-withdrawing substituenrs state I, as is the case in some of the complexes and it is interesting to note the carbonyl vibrational described in this review, the electronic configuration of frequencies in the infrared spectra of the following gold is 5d 10. This means that we are dealing with a mononuclear, dinuclear and cationic species: non-transition system characterized by a closed-shell AlICI(CO) which has V co 2162 cm' (4); configuration, expected to be colourless and AuBr(CO), V co = 2159 cm! (4); Cl3Au(f.L-Cl)AlI(CO), diamagnetic. V co = 2180 cm' (5); AU(S03F)(CO), V CO = 2195 cm' When carbon-based ligands are considered, we can (6); a fluorinated tris(pyrazolyl)borato complex, distinguish one-electron donors (alkyls, aryls; ie [HB(3,5-(CF3hC3HN2hJAu(CO), V co = 2144 crrr' V hydrocarbyls) and two-electron donors. Excellent (7); [Au(COhJ [UF G], CO = 2200 cm' (8) or reviews have appeared (2a, b) concerning hydrocarbyl [Au(COhHSb2FI 1], V co = 2217 cm! (9). All these derivatives of gold(I) and gold(III), including compounds have high vco's, sometimes well above that structural aspects (2c) and this subject will not be of uncoordinated carbon monoxide which is treated here. We limit the presentation to gold 2147 cm! (10), thus suggesting little or no organometallics with two-electron donor carbon-based 1T-back donation within the Au-CO bond (11), and we ligands, namely carbon monoxide, olefins, isonitriles should therefore think of these compounds in terms and cyanides. of the 'rr-bonding only' model typical of the Generally, within transition metals, bond interaction of CO with a metal of dID electron dissociation enthalpies (BDE) of metal-carbonyl bonds configuration. can be discussed in conjunction with metal-methyl bonds (3). For example, for some typical 5d elements such as tungsten or rhenium, BDE's have been SYNTHESIS evaluated to be (k] mol'): W-Me, 160±6 (average in WMeG); W-CO, 178±1 (average in The synthesis of the chloro- derivative of gold(I), hexacarbonyltungsten(O)); Re-Me, 220±11, in AuCl(CO), has been studied in detail. In thionyl ReMe(CO)s; Re-CO, 187±6 (average in Re2(CO)ID). chloride as medium, the reaction of anhydrous If extrapolation to gold is allowed, something similar gold(III) chloride with CO gives excellent yields, via would be expected to occur, unless additional factors the dinuclear CI3Au(f.L-CI)Au(CO) and the mixed­ come into play. It is therefore interesting to see valence gold(I)-gold(III) chloride AU4ClS intermediates whether or not these expectations are met. The (12); with carbonyl chloride, COCl2, being the

(69' Gold Bulletin 1997,30(1) 21 oxidation product. The thermodynamics of the with thiolato or hydrosulfido ligands, by metathetical reaction involving the formation of carbonyl chloride reactions with AuCI(CO) have failed (19a): the are so highly favoured (~Gor (CO(g) + CI2(g) ­ products isolated were sulfido and chloro complexes of COCl2(g) = - 67.7 kJ mol! (13)), that anyone of the gold(I) and coordinated carbon monoxide was lost three products can be obtained in high yields (19b). Also, both K[Au(CN)ZJ and Au(CN) are depending on the CO:Au(III) molar ratio. unreactive towards carbon monoxide (19a). In this The preparation of the bromo-derivative, connection, it is interesting that a cyano-isonitrile AuBr(CO), which is unstable in the solid state, is complex of gold(I) has been reported, being obtained interesting (4). This bromo-carbonyl can be prepared by electrophilic attack on K[Au(CN)ZJ: by reductive carbonylation ofgold(III) bromide: [Au(CN)zJ- + Mel -- 1- + Au(CN)(CNMe) (3) 1/2 AU2Br6 + 2 CO ~ AuBr(CO) + COBr2 COBr2 CO + Br2 This compound contains substantially linearly coordinated gold(I) (20), with AU-CCN and AU-CCNMc AuBr(CO) + Br2 (1) bond distances of 2.01 and 1.9SA respectively, the angle at gold being 176°. In view of the now thermodynamically unfavourable reaction to carbonyl bromide [[~Gor (CO(g) + Br2(1) -COBr2(g») = + 26.3 kJ mol! (13)], a SPECTROSCOPY AND BONDING substantially quantitative conversion to the desired product is only obtained when bromine is removed by The high vco's, which are a common feature of gold(I) cyclohexene. AuBr(CO) can also be obtained by direct carbonyl derivatives, are increased still further in carbonylation ofAuBr. AuCl(CO) (by about 10cm-1), when the solvent is The gold(I) halides, AuX, may be converted changed from hydrocarbon to chlorohydrocarbon (4). quantitatively to the halocarbonyl AuX(CO) by Changes of this type have been observed with other treatment with CO (4): carbonyl compounds (17, 21), and are suggestive of a specific weak solvent-solute interaction. The solid-state AuX + CO -- AuX(CO) (X = CI, Br) (2) structure of AuCI(CO), as reported by Jones (22), indicates that the gold atom in this mononuclear The BDE relative to the Au (I)-CO system can be compound is linearly coordinated with the chlorine readily calculated (17) to be at least 210 kJ mol! from and carbon atoms with Au-CI and Au-C bond some thermodynamic data available in the literature distances of 2.261 (6) and 1.93(2)A, respectively, the (13)*, and this is in agreement with the results of a bond angle at gold being 1S0° by symmetry. A re­ recent theoretical calculation (1S). evaluation of the published data in terms of packing, Within the series of halide derivatives, the stability of carbonyls with respect to dissociation (AuX + CO) decreases in the sequence CI > Br > I: in line with the combined effects of both Au-X and Au-CO BDE's decreasing in the same sequence, if the main contribution to the Au-CO bonding is of the

* The BDE (at 298 K) for the diatomic molecule Au-CI, 343 , 9.6 k] mol". (see reference 13, Section 9) and the entropy change for the transfer of CO as diluted solution to a metal-bonded carbonyl group (-145] K' mol', see reference 14), can be combined to give an estimated value of about 210 k] mol' for the Au-CO BDE, alter considering the Au-Cl., BDE to be 25% less than the Au-CI, BDE (Cl., and CI, designate bridging and terminal chlorides, respectively), if in terms of percentages, the same as that estimated for palladium(I1) (15). The entropy loss due ro the binding of CO ro gold(I) is Figure Crystal packingofAuC!(CO); recalculatedfrom the believed ro be compensated by the fragmentation of the polynuclear data ofReference 22. '= Ilrx, Ilry, l-z;"= -112 -x, Ilry, chloride-bridged structure of solid AuCI (16). l-z; til = -llrx, Ilry, -z; ,til= Ilrx, Ilr)\ -z.

22 C6i9' GoldBulletin 1997,30(1) see Figure, indicates that the C··Cl intermolecular on the structural data for the isostructural [M(Pmes3h]+ contacts of 3.352A are in fact at least as important as cations, (M = Ag, Au; mes = 1,3,5-Me3C6HZ) obtained the Au-v-Au contacts of 3.380 A. The molecules appear from their tetrafluoroborate derivatives". to be packed in such a way as to minimize the C··Cl Catalytic applications of gold are still rather small non-bonding distances: the deviation of the carbon in number but are expected to grow in their atom from the best Cl'-CI"-CIII/-CIII/, plane is 0.11 A, importance. For example, gold(I) carbonyls formed by while the deviation of the gold atom from the best the reductive carbonylation of gold(III) chloride in Au'-Au"-Aull/-Aull/' plane is 0.450 A. A similar zeolite cages have been found to be catalytically active situation has recently been encountered in the crystal in nitrogen oxide reduction by carbon monoxide (28a). packing of square-planar cis-PtClz(COh (23), where Also AuCI(CO) was found to be the best catalyst for the interplanar C ..Cl interactions were similarly found the thermal chlorination of carbon monoxide to to be the most important ones. carbonyl chloride in the absence of light (28b). An example of homogeneous solution catalysis by gold has recently been described in Gold Bulletin (29), which has also published a discussion on the potential CARBON MONOXIDE vs OLEFIN ofgold in heterogeneous catalysis (30). COORDINATION TO GOLD(I)

Earlier work (24) had indicated that carbon monoxide ACKNOWLEDGEMENTS is displaced by the cyclic olefin norbornene with a K of 4± 1 at 21.5°C, corresponding to a IiGo of the reaction The authors are grateful to the Consiglio Nazionale of about -3 k] moll and that bonding to the olefin is delle Ricerche (CNR, Rorna) and the Ministero dell' slightly favoured with respect to carbon monoxide. Universita e della Ricerca Scientifica e Tecnologica for support of the work and to all their coworkers for their

AuCI(CO) + olefin < > AuCl(olefin) + CO (4) interest, ability and competence. Thanks are also due (olefin = norbornene; solvent = tetrachloroethane) to Dr F Marchetti for the calculations of the solid-state structure ofAuCI(CO). This result is at variance with those generally observed for typical transition metals, whereby olefin displacement by CO is highly favoured. It is ABOUT THE AUTHORS interesting, in this connection, that the BDE's (k] mol l ) for the gold(I) cation have been recently calculated Fausto Calderazzo is Professor of General and (25a) to increase in the order :H 20 (163.4) < CO Inorganic Chemistry at the University of Pisa. After his (209) < NH3 (289) < C2H4 (306). On the other hand, degree in Chemistry from the University of Firenze, the proton affinity increases from CO (25b) (587.1 kJ Italy, he has been involved in the study of moll) to CZH4 (676.5 k] mol'). This suggests again organometallic and coordination compounds in their that for gold(I) the main contribution to the bonding synthetic, mechanistic and thermodynamic aspects of two-electron donor carbon-based ligands is (J" in within the following research and educational character. The BDE of the gold(I)-methyl bond in institutions: Politecnico di Milano, Institute of AuMe has been recently calculated (26) to be 226 k] Industrial Chemistry; Massachusetts Insti tute of mol', similar to the value estimated for the Au(I)-CO Technology, Department of Chemistry; Cyanamid bond. European Research Institute, Cologny, Geneva; Universita di Pisa, Italy. Daniela Belli Dell' Amico graduated from the CONCLUSIONS University of Pisa in 1971 and she is presently Associate Professor of General and Inorganic Chemistry at the Undoubtedly gold chemistry has some unusual features University of Pisa: her recent interests include of great interest. Relativistic effects are responsible for ca organometallic chemistry of late transition metals and 20% contraction in the covalent radius of mercury (27 the synthesis and reactivity of N,N-dialkylcarbamato a-c) and, presumably, for a corresponding increase of the complexes of transition metals, including their use in BDE . It has recently been observed (27d) that the the cation implantation on metal oxide surfaces. radius of gold(I) is smaller than that of silver(I), based 'See W S Rapson, GoldBIIII., 1996, Z9, 143

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