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Complexes of the Platinum Metals POINTERS TO A MECHANISM OF FIXATION

By Professor Joseph Chatt, F.R.S. University of Sussex

was activated directly by a molyb- The discovery three years ago of thefirst denum in nitrogenase, the nitrogen fixing well-defined complex compound con- enzyme. This led to attempts, using nitrogen taining the nitrogen molecule - a under pressure, to obtain complexes of complex of ruthenium - aroused great molybdenum containing the nitrogen mole- interest in such compounds and in their cule, but of no avail, and the work went relation to nitrogen fixation. Since largely unpublished. then a number of other nitrogen com- The formal analogy between the triple plexes have been obtained, but as yet, bond in acetylene and nitrogen despite reports to the contrary, attempts also led chemists to attempt the preparation of to reduce their nitrogen to nitrogen complexes of the platinum metals, have been unsuccessful. In this article analogous to the acetylene and olefin com- the author, who is Director of the plexes. It was again of no avail. We now know Agricultural Research Council's Unit that the electronic energy levels in the nitro- of Nitrogen Fixation, puts into perspec- gen and acetylene molecules are completely tive the considerable amount of work different. Thus the highest filled electronic now being done in thisfield, which may level in nitrogen is a o-molecular orbital at eventually point the way to a mechan- -15.6 electron volts, whereas in acetylene it ism by which atmospheric nitrogen may is a much more accessible degenerate pair of be fixed and then reduced to yield n-molecular orbitalsat -11.4electronvolts (2). ammonia and, for example, for Chemists were beginning to accept the view the plastics industry. that nitrogen did not form stable complex compounds, when by chance, just three years Since the 1930s chemists have thought that ago, Allen and Senoff in Toronto obtained co-ordination compounds containing the indirectly the first nitrogen complex, nitrogen molecule as a might exist. [Ru(NH,),(N,)]~'-(3). Its discovery revived They were stimulated into this line of world-wide interest in nitrogen complexes thought by the discovery, in 1930, that generally and in their relation to nitrogen nitrogen-fixing bacteria metabolised nitrogen fixation by bacteria. Since then experiments gas only in the presence of traces of molyb- in widely separated laboratories in various denum. Nevertheless, the bacteria would parts of the world have thrown up some dozen grow without molybdenum if they were nitrogen complexes. These vary in stability provided with a source of fixed nitrogen such from the thermally stable and chemically as urea or ammonia (I). Molybdenum is inert complex [OS(NH,),(N,)]~+ to evidently involved in the nitrogen fixing the highly reactive and thermally labile un- process, and it was thought that the nitrogen charged complexes of , and nickel.

Platinum MetalsTev., 1969, 13, (l), 9-14 9 Despite reports to the contrary, the nitrogen and is converted to [RU(NH~)~(N,)]~+by ligand has not been reduced to ammonia in ammonia (8, 9). any complex (4). The product of reaction 2 is grossly con- So far only the Group VIII metals are taminated with a hydrazine complex, perhaps known to form nitrogen complexes, and those [RU(NHJ~(N,H~)]~+,which cannot be re- of cobalt, ruthenium and iridium have received moved by re-crystallisation, but it can be most study. This article is not concerned with destroyed by mercuric oxidation nitrogen complexes of the first transition leaving the relatively inert nitrogen complex series. They are difficult to obtain pure, owing untouched. This impurity was not suspected to their extreme lability and sensitivity to until recently and many of the chemical oxidation, but the cobalt compound formu- properties recorded as belonging to the lated, perhaps incorrectly, as [Co(N,)(PPh,),] ruthenium nitrogen complex are those of a is important. It was the first nitrogen complex hydrazine complex. The most important reported to be obtained rapidly in high yield of these reactions was the supposed reduction from gaseous nitrogen at atmospheric pressure of the nitrogen ligand to ammonia by sodium (5). The hydride [CoH(N,)(PPh,),] is equally borohydride. However, tracer studies using important because it was the first nitrogen [Ru(NH ,) 5( 16N,)I 2+ show that no 15NH complex to be obtained in a protic solvent. It is obtained by , or any is obtained by passing nitrogen gas through a other tried reductant (4, 9). The ammonia solution of a cobalt hydride complex in previously reported as coming from the ethanol according to the following reversible nitrogen ligand in this complex must have reaction (6). There is some doubt as to had its origin in the hydrazine. whether the two cobalt nitrogen The formation of the nitrogen complex by Harrison and Taube's method (Reactions [CoH3(PPh3),l +N& [CoH(N,)(PPh,),] +H, r 4 and 5) is important because it shows that complexes are different, but our experience nitrogen gas can compete successfully with suggests that both exist, although the non- hydridic complex is usually contaminated Table 1 with not less than 15 per cent of the hydride. Nitrogen Complexes of Platinum Metals The best-defined complexes of the platinum metals are listed in Table I. The most import- Ir Ref. ant are the ammine complexes of ruthenium, (X=CI, Br, N3) 22105 3 (X=CI, Br, N3) obtained in good yields by the following 7 3) reactions in aqueous solution. In reaction 3 (X=CI, Br, 21525 I9 the co-ordinated ion decomposes to Ru leave nitrogen on the metal. trans-[IrX(N,)(PPh,),] 21525 I1 2152521525 I2 RuC13 +N,H,-~[Ru(NH~),(N,)IC~Z 2 (3) Os(NH3)KFJL [Ru(NH3)5N312+a-t[Ru(NH3),(N,)lz' 3 (3) Ir (X=CI, Br, N3) 2[Ru(NH3)6Hz0I2 'SNa-4 221525 I4 [VWNH3) 5)2(N 211 *+ + H 20 4 (7) Ir [{Ru(NH,),)z(Nz)I4++NH3--> trans-[IrX(N,)(PPh,),] 2095 I3 [Ru(NH3),(Nz)12'+[Ru(NHJJ2'~ 5 (7) (X=CI, Br, N3) (see text) Ruthenium trichloride also picks up nitrogen A few other poorly defined complexes have to give a material of analysis, RuCl,(N,) been claimed, e.g. see ref. 20. No complex of Pd or Pt is known. (H,O),(THF), when its tetrahydrofuran t of chloride, solid state spectra. (THF) solution is reduced by amalgam t [BPh,]- salt. under nitrogen. It has v(NrN) at z153cm-'

Platinum MetalsTev., 1969, 13, (1) 10 water for a suitable metal site in aqueous The complex reacts with x-bonding ligands, solution. Reaction 4 even occurs when air is particularly acetylenes, to lose nitrogen. The bubbled through the aqua-ruthenium com- corresponding rhodium complex is formed by plex solution, but oxidation of the ruthenium an analogous reaction and is very unstable complex also occurs (10). It is the only known (14). reaction of nitrogen at ordinary temperature In addition to the reasonably-defined and pressure in aqueous solution, and might nitrogen complexes listed in the table and well provide a model for the uptake of nitro- discussed above, a number of other nitrogen gen, possibly by iron, in the enzyme nitro- complexes too unstable to be identified have genase. However, it does not lead to an easily been reported but none containing platinum reducible species, and if it provides a true or . model the reduction step still has to be worked In the above mono-nuclear complexes the out. nitrogen is always bonded end-on to the The osmium complexes are remarkably metal as is in the carbonyl stable. The analogue of the ruthenium complexes. The mode of bonding in nitrogen complex, [OS(NH,),(N,)]~+is obtained by complexes is also very similar to that in reaction of hydrazine on aqueous ammonium carbonyl complexes with back donation of chloro-osmate(1V) (II). The nitrogen is so non-bonding d-electrons from the metal into strongly bound that it is not lost even in the anti-bonding orbitals of the nitrogen boiling hydrochloric acid but it yields to a molecule as shown in the diagram. The mixture of hydroiodic acid and iodine to form feedback of electrons into anti-bonding [Os(NH,),I]’ I. By diazotisation with nitrous orbitals of the nitrogen molecule weakens the acid the mononitrogen complex yields the only N-N bond and so lowers the stretching known stable di-nitrogen complex (12): frequency, v(N=N). In nitrogen gas [OS(NH,)~(N~)]~++HNO,-+ v(N_N) is observed at 2331 cm-l in the c~~-[OS(NH,),(N,),]~++~H,O6 Raman spectrum. In the complexes it is The planar iridium complex trans- observed as a very strong sharp band in the [IrCl(N,)(PPh,),] was the second nitrogen infrared spectrum at 180-320 cm-l lower complex to be discovered. It is obtained very than in nitrogen gas. The frequencies have easily by the reaction of Vaska’s compound, been listed, together with the complexes in truns-[IrC1(CO)(PPh3),1,withacidazides(13): Table I. Because the band in the infrared CHC1,- spectrum is intense, the electrical asymmetry trans-[IrCl(CO)(PPh3),]+PhCON37 oc of the complexed nitrogen molecule is prob- trans-[IrCI(N,)(PPh,),] +PhCONCO 7 ably high. However, neither the electrical

Platinum MetalsTev., 1969, 13, (1) 11 asymmetry nor the reduction in bond order complex is obtained, if at all, only as a very is sufficient to increase materially the re- unstable and impure material. Its presence activity of the nitrogen molecule in the com- can be established, from the N, stretching plex and there is no evidence that it can band in the infrared spectrum of the crude be either hydrolysed or reduced in aque- product, but it decomposes on attempted ous solution. Just as nitrogen reacts with isolation. This applies even when one the complex [RU(NH,),H,O]~' to form the attempts to make such close derivatives as the bridged compound, so the free end of the ethyldiphenylphosphine analogue. It is evi- nitrogen molecule in [RU(NH,),(N,)]~+reacts dent that some critical and perhaps rather with the aqua-complex to form the bridged extreme arrangement of energy levels is nec- complex (7) (Reaction 8). essary in the metal atom before nitrogen can be held strongly, and the minutest change in [Ru(NH,) gH ZO]' -1- [Ru(NH3) 5(N %)I* +--- ' the ligands often causes sufficient re-adjust- [UWNHddz(Nz)l' ' 8 ment of the levels to render the nitrogen In this reaction the nitrogen complex has no complex unstable. In our own work we have greater affinity for the ruthenium of the aqua found evidence of other unstable nitrogen complex than has gaseous nitrogen itself (15). complexes of the platinum metals and rhen- The nitrogen molecule is evidently symmet- ium, but these have been too unstable to rically bonded in the dinuclear ruthenium isolate in a pure condition, although when complex because the intense N-N stretching the correct ligands are found one or two band is missing from the infrared spectrum. stable complexes should be isolable. All of the nitrogen complexes lose their The structures of two nitrogen complexes nitrogen as gas on oxidation by such reagents have been determined by X-rays but neither as ceric sulphate. Even hydrochloric acid is entirely satisfactory because of the diffi- liberates the nitrogen and oxidises the metal culty in getting good crystals. The structure to a higher , in most cases of [Ru(NH,),(N,)]Cl, is disordered and so evolving as well as nitrogen, but exact bond distances could not be obtained, no ammonia (4). Only the mononitrogen but it was shown that the complex is an complex of osmium has failed to give up its octahedral one in which the Ru-N-N bond nitrogen on treatment with hydrochloric acid. system is linear (17). The other structure is Since the nitrogen complexes of ruthenium, that of the cobalt complex [CoH(N,)(PPh,),] osmium, rhodium and iridium are known to (IS). In this the molecule is essentially a contain ammonia or tertiary as trigonal bi-pyramid with the ligands it might be thought that analogues of ligands in the three equivalent equatorial these complexes could be prepared and also positions, the hydrogen on one apex and the homologues containing amines or other nitrogen molecule on the other. The cobalt tertiary phosphines. However, this is true to is out of the plane of the three phosphorus only a very limited extent indeed. Even the atoms, being displaced slightly towards the minutest change in the ligands of the complex nitrogen molecule which is bonded end-on is usually sufficient to render it completely to the cobalt; the Co-N-N bond system is unstable. Thus the iridium complex bent from linearity by 5". In these complexes [IrCI(N,)(PPh,),] is very easily prepared, the N-N distances are slightly longer than but the corresponding is less easy in the nitrogen molecule and comparable with and the is too unstable to be obtained that in the aliphatic diazo-compounds. Indeed pure (16)~as is the , although the the nitrogen complexes may be regarded as azide has been obtained. Similarly, if one inorganic diazo-compounds, and similarly attempts to use other tertiary phosphines in their main reactions result in the liberation of place of the nitrogen nitrogen, rather than the incorporation of

Platinum MetalsTev., 1969, 13, (1) 12 discovered. Their reactions have not been very Table II thoroughly investigated. Undoubtedly more Bond Distances in some Diazo- compounds nitrogen complexes will be found, but many (Angstrom Units) long series of analogues and homologues are never likely to be obtained. To the present no I 187 I 129 Nitrous O-I-NLN complexes have been obtained containing nitrogen which can be easily reduced to I 240 I r34 Hydrazoic acid HNLN-N ammonia, but perhaps these may also be

c~.32 r 12 found. Also, to the present, there is no Diazomethane 2 ---N-N evidence of complexes containing nitrogen 2 I1 I I2 bonded sideways to the metal as dialkyl [Ru(NH,) 5(N2)] C1 (NH,) ,RuAN-Nt acetylenes are in their complexes. Such [CoH(N,)(PPh,) (PPh,) ,HCo-NLNtI 80 I 16 nitrogen complexes may be just possible, but they seem unlikely because the electrons in 1.098 Nitrogen N-N the Tc-bonding molecular orbital are so low t Subject to rather high possible error (see in energy that the orbital can have little donor text). capacity. Such complexes, however, would be rather difficult to detect because the more symmetrical N-N bond, which they would nitrogen into other molecules. Table I1 contain, would not give a strong band in the shows a comparison of bond lengths in a series infrared spectrum. If they do exist they are of diazo-compounds and nitrogen complexes. not likely to be found until the taking of The structure of the dinuclear ruthenium- Kaman spectra becomes as much a routine as nitrogen complex has not yet been established. taking infrared spectra is at present. It may be written [(NH,),Ku-N=N-Ku What is the significance of nitrogen com- (NH,),]41- which at first sight appears analog- plexes ? Their preparation from gaseous ous to aromatic azo-compounds, for example nitrogen has shown that nitrogen can react azobenzene, Ph-N =N-Ph, but the ruthen- readily at atmospheric temperature and pres- ium complex cannot be reduced to ammonia sure with reagents so mild that they attack by sodium borohydride, sodium dithionite or neither water nor other protic solvents. They other such reagents. It is evident, therefore, may eventually serve to point the way to the that the nitrogen, even in this complex, still design of catalysts for reactions involving retains the inactivity which is characteristic nitrogen under ambient physical conditions, of gaseous nitrogen. It seems, therefore, that and leading to useful products such as the complex should be represented as con- ammonia, and amines for the plastics industry. taining two ruthenium(I1) atoms bridged by a neutral nitrogen molecule rather than two Re€erences ruthenium(II1) atoms bridged by the I See W. D. P. Stewart, “Nitrogen Fixation in [N-Nl2- ion, as would be required for Plants”, The Athlone Press, 1966, 89-90 formal analogy with azobenzene. The 2 See “The Inactivity and Activation of Nitro- gen”, by J. Chatt and G. J. Leigh in Recent ruthenium(I1) type structure would require a Aspects of Nitrogen Metabolism in Plants, Ed. linear Ku-N=N-Ku bond system whereas E. J. Hewitt and C. V. Cutting, Academic Press, London and New York, 1968, 3 the ruthenium(II1) would require a bent 3 A. D. Allen and C. V. Senoff, Chem. Comm., Ru-NrN-Ku bond system capable of i965, 621; also with F. Bottomley, R. 0. giving cis and trans , analogous to Harris and V. P. Reinsalu, J. Amer. Chem. SOL.,1967, 89, 5595 cis and trans-azobenzene. 4 J. Chatt, R. L. Richards, J. E. Fergusson and The nitrogen complexes of the transition J. L. Love, Chem. Comm. (in the press); J. Chatt, R. L. Richards, J. R. Sanders and metals are only just in process of being J. E. Fergusson, Nature (in the press)

Platinum MetalsTev., 1969, 13, (1) 13 5 A. Yamamoto, S. Kitazume, L. S. Pu and 13 J. P. Collman and J. W. Kang,J. Amer. Chem. S. Ikeda, Ckem. Comm., 1967,79 SOC.,1966, 88, 3459; ibid., 1967, 89, 844 6 A. Sacco and M. Rossi, Ckem. Comm., 1967, 14 Yu. Ukhin, Yu. A. Shvetsov and M. L. 316; see also A. Misono, Y. Uchida, T. Saito Khidekel, Izvestiya Akad. Nauk S.S.S.R.,1967, 1967,419 and K. M. Song, Ckem. Comm., 957 7 D. E. Harrison and H. Taube, 3. Amer. Ckem. SOC., 1,967, 89, 5706; with E. 15 I. J. Itzkovitch and J. A. Page, Can.J. Ckem., Weissberger, Sczence, 1968, 159, 320 1968,46,2743 8 A. E. Shilov, A. K. Shilova and Yu. G. 16 J. Chatt, D. Melville and R. L. Richards Borodko, Kinetika i Kataliz, 1966, 7, 768; (unpublished) Doklady Akad. Nauk S.S.S.R., 1967, 176, 17 F. Bottomley and S. C. Nyburg, Ckem. Comm., I297 1966, 897 9 Yu. G. Borodko, A. K. Shilova and A. E. Shilov, Zk. Fiz. Khim. (in the press) 18 J. H. Enemark, B. R. Davies, J. A. McGinnety and J. A. Ibers, Chern. Comm., r968,96 10 A. D. Allen and F. Bottomley, Can.3. Ckem., 1968,463 469 19 L. A. P. Kane-Maguire, P. F. Sherridan, 11 A. D. Allen and J. R. Stevens, Ckem. Comm., F. Basolo and R. G. Pearson, 3. Amer. Chem. 1967, 1147 soC.9 1968, 90, 5295 12 H. A. Scheidegger, J. N. Armor and H. Taube, 20 A. Yamamoto, S. Kitazume and S. Ikeda, 3. Amer. Chem. SOC.,1968, 90, 3263 J. Amer. Ckem. SOC.,1968, 90, 1089

Maintaining Accurate Temperature Control THE CARE OF PLATINUM THERMOCOUPLES The use of carefully prepared thermo- by comparison, using a heat-sink block in the couples, the exercise of care and attention to calibration furnace and connecting the details of installation, and the maintenance of couples so as to give a difference reading. calibration by frequent checking, all demand Working-standards are calibrated every four infinite patience, but are essential to the weeks and are used only for one week at a assurance of accurate and consistent furnace time. temperature control. That this is of primary Owing to the quicker deterioration of importance in the semiconductor industry is thermocouple e.m.f. at higher temperatures, emphasised in a paper by Ivan 0. Nielson of the frequency of calibration varies according Fairchild Semiconductor (Solid State Tech- to the temperature being measured, ranging nology, 1968,11, (IO), 29). A typical example from every two weeks to every six months. of the diffusion process for introducing con- Each diffusion furnace has a thermocouple trolled impurities into silicon or germanium that is used only in that furnace. wafers requires a furnace operating at The temperature profile of each furnace is 1020°C&z0. Wider temperature variation carefully checked before use. A period of 10 produces variance in the beta characteristics to 15 minutes is allowd to equalise the tem- of the devices. perature at each point and for the system to The thermocouples used for control are become thermally stable. The furnaces are platinum: 13 per cent rhodium-platinum in wound in three zones and thermocouple con- the form of wire 0.014 inch diameter to give trols are so arranged that variations in the quick response. The junction, formed by a centre zone control the end zones. A diffusion butt weld, is little larger than the wire dia- furnace is only as good as the instrumentation meter. Before use each thermocouple is system which controls it. electrically annealed at about 1350°C for one Advancements in technology, not only in hour to remove work hardness and volatile the semiconductor field, demand higher pro- surface impurities. The thermocouple is ductivity, and the accurate control of tem- insulated with double-bore refractory tubing perature is a means to this end. Particular and protected in the furnace with a quartz care must, therefore, be given to frequent sheath. Leads are terminated with a crimp calibration of working thermocouples and spade lug to ensure good mechanical con- measuring instruments, to constant checking nections. of all connections in the system, and to allow- Working-standard thermocouples are cali- ing sufficient time for thermal sensing devices brated at known points of reference, and are to reach stability. used to calibrate the furnace thermocouples H. E. B.

Platinum MetalsTev., 1969, 13, (1) 14