Organometallics 1986, 5 , 72-78
72
Reactions of HCiCC0,Et with (np,)CoH and (np,)Ni [np, N(CH,CH,PPh,),]. Crystal Structures of the Alkenyl Complexes [ (np,)CoC(CO,Et) =CH,]BPh4*0.5THF and [ (np,)hiCH =CHC( 0)OEt ]BPh4 Claudio Bianchini," Paolo Innocenti, Dante Masi, Andrea Meli, and Michal Sabat'
Istituto per lo Studio della Srereochimica ed Energerica dei Composti di Coordinazione, CNR, 50132 Firenze, Ira& Received April 17, 1985
The hydride (np,)CoH (1) [np3 = N(CH2CH2PPh2)3]reacts with HC=CC02Et to yield the a-alkenyl complex (np3)CoC(COzEt)=CHz(2) by insertion of the alkyne molecule into the Co-H bond. Addition of NaBPh, in n-butyl alcohol to a tetrahydrofuran solution of 2 gives the Co(I1) derivative [(np,)CoC(COzEt)+H2]BPh4.0.5THF (3). The crystal structure of 3 has been determined by standard X-ray methods. The compound crystallizes in the triclinic space group Pi with a = 17.937 (6) A, b = 15.188 (5) 8,c = 12.916 (4) A, cy = 87.00 (Z)', 0 = 101.37 (Z)', y = 110.44 ( 2 ) O , and 2 = 2. The structure was refined to an R factor of 0.088 (R, = 0.087) using 3180 reflections with I > 3a(4. The [(np,)CoC(CO,Et)=CH,]+ cation adopts the gem conformation with a Co-C distance of 1.91(2) A. Reaction of HC=CC02Etwith the Ni(0) complex , (np,)Ni (4) followed by NaBPh, addition gives the P-carbethoxyvinyl complex [ (np3)NiCH=CHC(0)OEtIBPh, (5), the structure of which has been established by an X-ray anal sis. Compound 5 crystallizes in the monoclinic space group P2,/a with a = 26.696 (7) A, b = 14.729 (4) c = 16.203 (4) A, fl = 105.50 ( 2 ) O , and Z = 4. The structure was refined to an R factor of 0.079 (R, = 0.085) for 4933 reflections with I > 3a(Z). The 0-carbethoxyvinyl group in the [(np3)NiCH=CHC(0)OEt]+cation is chelated to Ni by its carbonyl oxygen atom and ,&carbon atom forming a five-membered ring. The Ni-C and Ni-0 bond distances are 1.878 (7) and 2.229 (5) . . A. remectivelv. The reaction mechanisms for the formation of the a-alkenyl complexes 2, 3, and 5 are briefly' discuss"ed.
.
.
8:
I
Introduction The reaction of transition-metal hydrides with electron-deficient acetylenes provide a well-developed method for the synthesis of a-alkenyl complexes.14 Acetylenes insert into metal-hydrogen bonds to give products with trans (I) or cis (11) stereochemistry. R
\
,c=c
L,MH t R - C e C - R '
LnM
/
onal-bipyramidal Co(1) hydride (np3)CoH ( 1)7 [np, = tris(2-(diphenylphosphino)ethyl)amine] toward ethyl propiolate, HCCC0,Et. As a result, the two $-vinyl complexes (np3)CoC(COzEt)=CHz (2) and [ (np3)CoC(COzEt)=CH2]BPh4-0.5THF(3) have been synthesized. The structure of 3 has been elucidated by means of X-ray methods.
H
f
'. R '
I P'
I
LnMH t R-CZC-R'
R
-
\ ,c=c
LnM
'>
P-CO,'
/
H 1
R'
H '
IT
11
In recent years, considerable information on the insertion reactions of acetylenes into M-H bonds has become available. In particular, studies have been carried out to determine how factors like the nature of the metal fragments and of the substituents on the acetylenes may influence the stereochemistry of the products.lrZ Structural data on a-alkenyl complexes, however, still remain rather scarce. In an attempt to provide further information in this field, we have investigated the reactivity of the trig(1) Otsuka, S.;Nakamura, A. Adu. Organomet. Chem. 1976, 14, 245. (2) Collmann, J. P.; Hegedus, L. S. "Principles and Applications of Organotransition Metal Chemistry"; University Science Books: Mill Valley, CA, 1980. (3) Longato, B.; Bresadola, S. Inorg. Chem. 1982, 21, 168. (4) Scordia, H.; Kergoat, R.; Kubicki, M. M.; Guerchais, J. E.; L'Haridon, P. Organometallics 1983, 2, 1681. (5) Petillon, F. Y.; Le Quere, J. L.; Le Floch-Perennou, F.; Guerchais, J. E.; Gomes De Lima, M. B.; Manoilovic-Muir, L. J.; Muir, K. W.; Sharu, D. W. A. J. Organomet. Chem. 1984, 255, 231. (6) Deeming, A. J.; Manning, P. J.; Rothwell, I. P.; Hursthouse, M. B.; Walker, N. P. C. J.Chem. SOC.,Dalton Trans. 1984, 2039.
An alternative synthetic route to (a-alkenyllmetal complexes is the reaction of complex anions possessing a lone pairs with acetylenes activated by the presence of electron-withdrawing group^.^ To get some better insight into this chemistry, we carried out the reaction of HC=CC02Et with the trigonal-pyramidal complex (np3)Ni (4).7 The latter compound has an occupied orbital suited to transfer electron density into an appropriate empty orbital of an incoming electrophileg (111). Accordingly, 4 behaves as a metal base and forms strong adducts with Lewis acids such as H+,'O CH3+,SnClz, and BF3.11
4
(7) Sacconi, L.; Ghilardi, C. A.; Mealli, C.; Zanobini, F. Inorg. Chem. 1975, 14, 1380. (8) Bruce, M. J.; Harbourne, D. A,; Waugh, F.; Stone, F. G. A. J . Chem. SOC.A 1968,895. (9) Cecconi, F.; Ghilardi, C. A,; Innocenti, P.; Mealli, C.; Midollini, S.; Orlandini, A. Inorg. Chem. 1984, 23, 922.
0276-733318612305-0072$01.50/0 1986 American Chemical Society , 1b ,
I
m
Organometallics, Vol. 5, No. 1, 1986 73
Reactions of H C E C C O S t with (np3)CoH and (np,)Ni Under suitable conditions, ethyl propiolate adds oxidatively to 4 forming a 0-carbethoxyvinyl complex cation, which is stabilized by bulky anions like BPh; to give the I
i
complex [ (np,)NiCH=CHC(O)OEt]BPh, (5). The crystal structure of 5 has been established by an X-ray analysis.
Results and Discussion Reaction of HC&C02Et with 1. Diamagnetic brown crystals of 2 are obtained in 62% yield by treating at room temperature the hydride 1 in tetrahydrofuran with a slight excess of HCECC0,Et (IV).
(np3)CoH
t HCECCOZEt
c NoBPhq
(npj)CoC(COpEt)=CHp
I
NoBPh4
[(np 3 ) CoC(C0pEt 1=CHpIBPh4
TV
Compound 2 is reasonably air stable in the solid state but slowly decomposes in solution even in an inert atmosphere. It is soluble in common organic solvents, in which it behaves as a nonelectrolyte. The reflectance spectrum consists of a band at 23 000 cm-l and is fully comparable with those of five-coordinate low-spin Co(1) complexes.'2 t spectrum contains no ~(CO-H) vibrations, but there The E are two medium-strongbands at 1680 and 1540 cm-', which can be positively assigned to u(C=O) of a carboxylate group and to v(C=C) of an alkene m ~ i e t y , ~ , ' respec~J* tively. The IR spectrum also exhibits a strong band at 1210 cm-' typical of v(C0C). The 31P(1H]NMR spectrum in benzene at 20 "C consists of a single resonance at -26.30 ppm. This pattern, unchanged also in tetrahydrofuran even at -60 "C, is consistent with rapid intramolecular exchange of the three phosphorus atoms of np, around the cobalt atom, as observed also for a variety of trigonal-bipyramidal Co(1) complexes with np3.15 The 'H NMR spectrum in benzene at 20 "C, although poorly resolved as some decomposition occurs in solution, shows two doublets at 6 5.85 and 5.06 (1H), a quadruplet at 6 3.68 (2 H), and a triplet at 6 0.62 (3 H). The last two resonances are typical of the ethoxy protons of carbonic esters. The insertion of ethyl propiolate into the Co-H bond may take place giving a-alkenyl complexes with three possible structures (V-VII). H
/
c=c,
V
€to$
COzEt
\
)c=c,
,n
VI1
VI
The nature of the lH NMR spectrum of 2 is such that it appears to be a single isomer. In particular, the two olefinic hydrogen shifts as well as the coupling constant of 2.5 Hz are strongly indicative of the gem structure VII. A similar band pattern has been observed for the insertion products of 3,3,3-trifluoropropyne into L,M-H bonds ci~-PtCl(PEh)~, MII(CO)~),which (L,M = Fe(C0)2(C5H5), have been assigned structure VII.'& Decisive support for ~
~
~~
~~
~
~
the proposed stereochemistry of 2 is provided by the crystal structure of compound 3. The latter is obtained as red crystals in 70% yield by adding NaBPh, to the reaction mixture leading to 2. Compound 3 is quite air stable in the solid state but slowly decomposes in solution unless air is excluded. It is soluble in common organic solvents, in which it behaves as a 1:1electrolyte (molar conductance value in M nitroethane solution 42 cm2 V1 mol-'). It is paramagnetic with a magnetic moment of 2.12 pB corresponding to one unpaired spin. The reflectance spectrum has bands at 8000, 10800, and 20600 cm-l and is fully comparable with those of five-coordinate Co(I1) complexes with np3, where the fifth donor atom is carbon.12 The IR spectrum exhibits two strong bands at 1700 and 1215 cm-' assigned to u(C=O) and v(C0C) of a C02Etgroup, respectively. A medium band at 1555 cm-' is attributed to a v(C=C) stretching vibration. The crystal structure of 3 consists of complex cations of [(np,)CoC(C02Et)=CH2]+and BPh,- anions (see the description of the structure). The complex cation has structure VII. The same stereochemistry may be, thus, reasonably assigned to 2 when assuming that NaBPh, has the effect of promoting the stabilization of the oxidized species (IV). Indeed, it has been previously observed that a variety of trigonal-bipyramidal Co(I1) complexes of general formula [(np,)CoX]BPh, (X = NO,l' CN," CH318)can be obtained by treatment of solutions of the corresponding Co(1) derivatives with NaBPh,. Reaction of HCCC0,Et with 4. Ethyl propiolate reacts almost instantaneously with the red Ni(0) complex 4 in tetrahydrofuran at room temperature to give an orange solution from which yellow orange crystals of 5 are obtained by addition of NaBPh, in n-butyl alcohol (yield 80%) (VIII). (np,)Ni
+ HC=CCO,Et
-
NaBPh,, n-butyl alcohol V T&..T T 1
[ (np,) NiCH=CHC (0)OEt]BPh4
Compound 5 is diamagnetic and air stable both in the solid state and in solution. It is soluble in common organic solvents, in which it behaves as a 1:l electrolyte (molar conductance value in M nitroethane solution 41 cm2 9-' mol-'). The reflectance spectrum consists of a band at 25 000 cm-' and is typical of five-coordinate Ni(I1) complexes.12 The IR spectrum exhibits bands at 1640, 1520, and 1210 cm-'. The latter two bands are assigned to u(C=C) of an alkene moiety and to v(C0C) of the COEt fragment, respectively. The broad medium-strong band at 1640 cm-l is attributable to the C02Et carbonyl group coordinated to the nickel atom. A number of complexes have been described, in which an ester carbonyl group is coordinated to a metal In the IR spectra of such complexes, a band in the region 1500-1600 cm-l has been attributed to a coordinated ketonic carbonyl group. The 31P(1H} NMR spectrum in acetone at -60 "C consists of two resonances at 28.38 and -21.16 ppm (intensity ratio 2:l). The band pattern as well as the relative intensities are typical of metal complexes with tripodal triphosphines acting as bidentate ligands, the third
~~~
(IO) Sacconi, L.; Orlandini, A.; Midollini, S. Jnorg. Chem. 1974, 13, 2850. (11) Bianchini, C.; Meli, A., to be submitted for publication. (12) Sacconi, L.; Mani, F. Transition Met. Chem. (N.Y.)1982,8, 179. (13) Booth, B. L.; Hargreaves, R. G. J . Chem. SOC.A. 1969, 2766. (14) Nakamura, A.; Otsuka, S. J. Am. Chem. SOC.1972, 94, 1886. (15) Bianchini, C.; Meli, A.; Scapacci, G. Organometallics 1983, 2, 1834. (16) (a) Harbourne, P. A.; Stone, F. G. A. Inorg. Phys. Theor. 1968, 1765. (b) Chen, G. J.-J.; McDonald, J. W.; Newton, W. E. Organometallics 1985, 4 , 422.
(17) Di Vaira, M.; Ghilardi, C. A.; Sacconi, L. Inorg. Chem. 1976,15, 1555. (18) Bianchini, C.; Meli, A. Organometallics 1985, 4, 1537. (19) (a) Dubeck, M.; Schell, R. A. Inorg. Chem. 1964, 3, 1757. (b) Canziani, F.; Garlaschelli, L.; Malatesta, M. C. J. Organomet. Chem. 1978, 146, 179. (c) Canziani, F.; Albinati, A.; Garlaschelli,L.; Malatesta, M. C. Ibid. 1978,146, 197. (d) Carmona, E.; Gutierrez-Puebla, E.; Monge, A.; Marin, J. M.; Paneque, M.; Poveda, M. L. Organometallics 1984,3, 1438. (e) Komiya, S.; Ito, T.; Cowie, M.; Yamamoto, A.; Ibers, J. A. J. Am. Chem. SOC.1976, 98, 3874.
74 Organometallics, Vol. 5, No. 1, 1986
Bianchini et al.
Table I. Summary of Crystal Structure Data
3 formula
M cryst size, m m cryst system space group a, A b , '4
c,A @,
deg
P , deg 7 ,deg
v, A 3
z
d(calcd), g c m - 3 p(Mo K a ) , c m - l radiatn scan type 2e range, deg scan width, deg scan speed, deg s - ' total data unique data I > 3 u ( I ) no. of parameters
R Rw w = 1.0[n2(Fo) + pF02]-'
5
C,, H,,BO ?P,NCO O.5C,H8 0 1166.95 0.4 X 0.15 X 0.09 triclinic
C,,H,,BO,P,NNi 1130.79 0.75 x 0.22 x 0.18 monoclinic
Pi
P2,la 26.696 ( 7 ) 14.729 ( 4 ) 16.203 ( 4 ) 90.0 105.50 ( 2 ) 90.0 6139.4
I
17.937 ( 6 ) 15.188 ( 5 ) 12.916 (4) 87.00 ( 2 ) 101.37 ( 2 ) 110.44 ( 2 ) 3231.9 2 1.197 3.8 1 graphite-monochromated Mo K o ( A
4 1.223 4.37 = 0.710 69 A )
wise
wiae
5-4 5 0.9
5-50 1.0 0.04 11432 4933 291 0.079
0.05 8275 3180 319 0.088 0.087 p = 0.0005
phosphorus atom not being coordinated to the metal center.20 The IH NMR spectrum in chloroform at 20 "C shows a doublet a t 6 5.26 (1 H) (JH-H = 8.4 Hz), a quadruplet a t 6 3.93 (2 H), and a triplet a t 6 1.14 (3 H). The latter two resonances can be positively assigned to the ethoxy protons of the CO,Et group. The doublet seems to be half of an AB pattern, the second half of which is probably masked by the resonances of the phenyl protons. Vinylic protons of compounds with structure I (R = H), generally, give rise to AB patterns with J = 10 Hz.13J6 One half of the AB pattern may fall in the region of phenyl hydrogens. On the basis of all of these data a structure may be assigned to 5 where nickel is five-coordinated by the nitrogen atom and two phosphorus atoms of np, and by a carbon and the ketonic oxygen of a HC=CH(C02Et) group. The two vinylic protons appear to be disposed in cis manner. The X-ray structural analysis of 5, presented below, confirms such an arrangement of the donor atoms. As a consequence of the reaction with HC=-tC02Et, the formal oxidation state of nickel is raised by two units. This may be interpreted in terms of an overall two-electron oxidative addition of the acetylene molecule to the Ni(0) complex.2
0.085 p = 0.001 c3 c11
C6 c12
c5
02
>
c11
i
01
*
c9
c10
Figure 1. ORTEP drawing of the [(np,)CoC(CO,Et)=CH,]+ cation. Hydrogen atoms and phenyl rings of the np, ligand (except for the connecting C atoms) are omitted for clarity.
Description of the Structures Compound 3. As a result of the insertion of ethyl propiolate into the Co-H bond a-alkenyl complexes V-VI1 may be formed. In order to establish unambigously the conformation of the insertion product 3, an X-ray analysis was performed. The crystal structure of 3 is composed of discrete [(np,)CoC(CO,Et)==CH,]+ complex cations, BPh, anions, and tetrahydrofuran solvent molecules. Selected bond distances and angles are given in Table IV. The complex cation (Figure 1) adopts the gem conformation (structure VII) with both the metal fragment and the electron-withdrawing group COzEt bonded to the same carbon atom C(7). The coordination sphere around Co center can be described in terms of a distorted square
pyramid with the phosphorus atom P(1) in the apical position and the two other phogphorus and nitrogen atoms of the tripod ligand as well the vinyl carbon atom C(7) forming the basal plane. The distortions from squarepyramidal geometry result in closing down the P(2)-CoP(3) and N-Co-C(7) bond angles of the basal plane to 148.8 (2) and 170.5 (7)O,respectively. The Co-P and Co-N bond lengths being significantly elongated with respect to the analogous bond distances in the trigonal-bipyramidal (np,)CoH7 resemble quite close those found i n the The co-c(7) square-pyramidal low-spin [ (npJCoI] distance of 1.91 (2) 8, compares well with the Co-vinyl u bond lengths found in other structures22and the Co-C(7)-C(ll) bond angle of 124 (1)"may be indicative that the metal-carbon bond is single. The parameters defining the vinyl ligand geometry are quite normal. Thus, the C(7)-C(ll) bond length of 1.35 (2) 8, approaches that observed in the Co(II1) complex with the unsubstituted vinyl group (1.33 (2) A), while the C(7)-C(8) separation of 1.48 (2) 8, is characteristic for a single bond involving sp2-hy-
(20) Bianchini, C.; Mealli, C.; Meli, A.; Scapacci, G. Organometallics 1983,2,141. Bianchini, C.; Meli, A. J. Chem. Soc., Dalton Trans. 1983, 2419.
(21) Mealli, C.; Orioli, P. L.; Sacconi, L. J . Chem. SOC.1971, 2691. (22) Brueckner, S.; Calligaris, M.; Nardin, G.; Randaccio, L. Inorg. Chim. Acta 1968, 2, 416.
Reactions of H C d X O & with (np,)CoH and (np3)Ni
atom
Organometallics, Vol. 5, No. 1, 1986 15
Table 11. Final Positional Parameters (XlO') for [(np,)CoC(COzEt)=CHz]BPh4 *O.STHF X Y z atom X Y
z
~~
8069 (1) 8433 (2) 7146 (2) 8611 (3) 10282 (7) 9438 (7) 7056 (6) 7938 (8) 7084 (8) 6289 (8) 6251 (8) 7840 (10) 7027 (9) 8888 (9) 9547 (12) 10966 (10) 11678 (11) 8916 (10) 7991 (6) 7739 (6) 7427 (6) 7366 (6) 7617 (6) 7930 (6) 9423 (7) 9810 (7) 10540 (7) 10884 (7) 10497 (7) 9766 (7) 7193 (5) 6615 (5) 6655 (5) 7273 ( 5 ) 7851 (5) 7811 (5) 6756 (6) 6294 (6) 5956 (6) 6079 (6) 6541 (6) 6879 (6)
2474 (1) 2466 (3) 3185 (3) 1465 (3) 4258 (7) 4604 (8) 1218 (8) 1197 (9) 916 (10) 2326 (10) 1365 (9) 329 (10) 444 (10) 3563 (11) 4190 (12) 4861 (14) 4744 (14) 3849 (12) 2992 (5) 2536 (5) 2960 (5) 3841 (5) 4298 (5) 3873 (5) 2710 (6) 2049 (6) 2211 (6) 3034 (6) 3696 (6) 3534 (6) 4338 (6) 4414 (6) 5295 (6) 6101 (6) 6025 (6) 5144 (6) 3184 (6) 3735 (6) 3665 (6) 3044 (6) 2493 (6) 2563 (6)
7823 (2) 6189 (3) 7578 (3) 8766 (4) 8536 (9) 7225 (10) 7257 (10) 5947 (12) 6164 (12) 6738 (11) 7167 (11) 8377 (13) 8017 (14) 8529 (14) 8017 (15) 8059 (17) 8850 (16) 9511 (16) 5063 (9) 4080 (9) 3199 (9) 3303 (9) 4287 (9) 5167 (9) 5815 (8) 6010 (8) 5677 (8) 5150 (8) 4955 (8) 5288 (8) 7043 (8) 6179 (8) 5773 (8) 6231 (8) 7095 (8) 7501 (8) 8771 (7) 8876 (7) 9774 (7) 10568 (7) 10464 (7) 9565 (7)
bridized carbon atoms (1.46A). There is also a remarkable structural similarity between the u-bonded vinyl moieties in the present complex and in an iron compound obtained by nucleophilic addition of a methyl group to the molecule of M e C S C 0 , E t $-coordinated to the metal center.23 To our best knowledge the compound (Cp)Fe(CO)(PPh,)[C(C0,Et)CMe2] has been so far the only structurally authenticated example of a geminal isomer found for a-alkenyl-transition metal systems. All structural features are virtually the same in both compounds except for somewhat different orientation of the ester group relative to the vinyl bond. This can be described by the torsion angle C=CC-O(carbony1) which has a value of -118.2 and 97.8O in the Co and Fe complexes, respectively. Furthermore, the observed orientation of the ester group in the Fe compound is believed to result from nonbonding interactions between the planar C0,Et system and one of the phenyl rings of the PPh, ligand. However, this is not the case of the Co derivative as no analogous interactions have been found in the structure. Compound 5. Spectroscopicstudies on 5 indicated that reaction VI11 may be considered as a chelate-assisted oxidative addition of the substituted acetylene to the Ni(0) complex. The results of an X-ray structure determination for 5 fully support those of the spectroscopic investigations. (23) Reger, D. L.; McElligott, P. J.; Charles, N. G.; Griffith, E. A. H.; Amma, E. L. Organometallics 1982, I , 443. (24) Browning, J.; Penfold, B. R. J. Chem. Soc., Chem. Commun. 1973, 198.
8719 (6) 8115 (6) 8213 (6) 8916 (6) 9520 (6) 9422 (6) 9552 (6) 9594 (6) 10316 (6) 10996 (6) 10954 (6) 10232 (6) 3617 (10) 4276 (6) 4474 (6) 4958 (6) 5244 (6) 5046 (6) 4562 (6) 2685 (5) 2462 (5) 1653 (5) 1066 (5) 1289 (5) 2098 (5) 3703 (5) 3007 (5) 3065 (5) 3819 (5) 4515 (5) 4457 (5) 3895 ( 5 ) 4347 (5) 4596 (5) 4393 (5) 3940 (5) 3691 (5) 2936 (19) 2730 (25) 3349 (32) 3847 (27) 3507 (35)
1556 (7) 1012 (7) 1130 (7) 1792 (7) 2337 (7) 2219 (7) 1289 (7) 389 (7) 271 (7) 1053 (7) 1954 (7) 2071 (7) 1762 (10) 1971 (6) 2809 (6) 2941 (6) 2236 (6) 1399 (6) 1266 (6) 1242 (6) 1408 (6) 1034 (6) 495 (6) 329 (6) 703 (6) 2810 (6) 2932 (6) 3757 (6) 4461 (6) 4340 (6) 3514 (6) 1115 (5) 1518 (5) 961 (5) 1 (5) -402 (5) 155 (5) 1808 (22) 2342 (31) 3361 (36) 3310 (32) 2334 (42)
10180 (10) 10715 (10) 11803 (10) 12356 (10) 11821 (10) 10733 (10) 8654 (9) 8658 (9) 8568 (9) 8474 (9) 8469 (9) 8559 (9) 5964 (13) 5113 (8) 4559 (8) 3801 (8) 3597 (8) 4151 (8) 4909 (8) 5325 (7) 4256 (7) 3751 (7) 4314 (7) 5384 (7) 5889 (7) 6484 (7) 6679 (7) 7167 (7) 7460 (7) 7265 (7) 6777 (7) 6967 (8) 7941 (8) 8729 (8) 8543 (8) 7570 (8) 6781 (8) 558 (25) 1183 (33) 1098 (41) 457 (36) -50 (44)
02
c1 3
Figure 2. ORTEP drawing of the [(np3)NiCH=CHC(0)OEt]+ cation. Hydrogen atoms and phenyl rings of the rips ligand (except for the connecting C atoms) are omitted for clarity.
The structure of the compound consists of the [(np,), I NiCH=CHC(O)OEt]+ complex cations and tetraphenylborate anions. Selected bond distances and angles are given in Table V. The coordination geometry around the metal atom is that of a distorted trigonal bipyramid where two phosphorus atoms of the tripod ligand are in the meridional plane and the nitrogen atom occupies the axial position. The P-carbethoxyvinyl ligand is bonded to the Ni center by its carbonyl oxygen O(1) and P-carbon atoms C(7) of the vinyl group, thus forming a five-membered chelate ring (Figure 2). The chelate ring is disposed perpendicularly to the basal plane with the vinyl carbon
76 Organometallics, Vol. 5, No. 1, 1986
atom
Bianchini et al.
Table 111. Final Positional Parameters (XlO') for [ (np,)NiCH=CHC(O)OEt]BPh, X Y z atom X Y 1073.7 (4) 1902 (1) 8332 (I) 296 (2) 4934 (4) 1556 (2) 7590 (2) 1624 (1) 234 (2) 4094 (4) 429 (1) 8248 (2) 2828 (1) -851 (3) -1543 (5) -399 (1) -735 (2) 8502 (2) -1333 (3) -1680 (5) 1389 (2) 9397 (4) 928 (4) -1664 (3) -2359 (5) 919 (4) 2088 (2) 10525 (4) -1513 (3) -2901 (5) 997 (4) 565 (3) 7607 (4) -1031 (3) -2764 (5) -700 (3) 848 (6) 1210 (3) 6759 (6) -2085 (5) -805 (2) 374 (6) 825 (4) 7120 (6) 264 (4) 2200 (6) -99 (3) 7498 (6) -666 (2) 778 (4) 1541 (6) 153 (3) 7008 (6) -921 (2) 1594 (4) 420 (5) 346 (3) 8193 (5) -1315 (2) 1895 (4) -295 (6) -1454 (2) 1381 (4) -48 (3) 7738 (6) 9046 (6) 2678 (6) -1199 (2) 565 (4) 1560 (3) 9780 (6) 2291 (6) -1177 (4) 2256 (7) 1845 (4) 9880 (6) -527 (2) 1328 (6) 2118 (3) 1748 (3) -75 (7) 10581 (8) 2028 (4) -320 (2) 1279 (3) 11143 (9) 1202 (3) -463 (8) 2487 (5) 208 (2) 2442 (4) 7017 (4) 528 (2) 1965 (3) 1860 (2) 6530 (4) 2244 (4) 321 (2) 2206 (2) 2804 (3) 2920 (4) 2349 (2) 6035 (4) 2881 (3) -207 (2) 6027 (4) 3795 (4) -1475 (2) 1282 (4) 2146 (2) 6514 (4) -1974 (2) 3992 (4) 1801 (2) 1095 (4) -2231 (2) 7009 (4) 310 (4) 1658 (2) 3316 (4) 874 (4) 8085 (4) -1990 (2) -287 (4) 2189 (2) -61 (4) -1491 (2) -99 (4) 7955 (4) 2201 (2) -1234 (2) -564 (4) 8379 (4) 2637 (2) 685 (4) -1406 (3) -132 (4) 2510 (4) 8931 (4) 3061 (2) 804 (4) -1844 (3) 9061 (4) 3049 (2) 3057 (4) 1306 (4) -2055 (3) 8638 (4) 2613 (2) 3200 (4) 181 (3) 3187 (5) -1828 (3) 9120 (4) 2796 (4) -261 (3) 3734 (5) -1389 (3) 8984 (4) 2249 (4) -431 (3) 4036 (5) -1178 (3) 9679 (4) 2106 (4) -160 (3) 3791 (5) -1283 (3) 10510 (4) 3082 (4) 3243 (5) 281 (3) -1245 (3) 3992 (4) 10646 (4) 2942 (5) 451 (3) -1275 (3) 4673 (4) 9951 (4) 3916 (4) -1342 (3) 4443 (4) 7753 (3) 488 (2) 4578 (4) -1380 (3) 3533 (4) 8249 (3) 804 (2) -1350 (3) 5418 (4) 2853 (4) 7890 (3) 866 (2) 5595 (4) 7035 (3) 612 (2)
atom located trans to nitrogen and the oxygen atom completing the coordination sphere in the meridional plane. Similar arrangement of the five-membered ring has previously been observed in the octahedral Ru complex with I
i
n-butyl methacrylate RuH[CH=C(CH,)C(O)O(n-Bu)l(PPh3)3.19eThe Ni-C distance of 1.878 (7) A falls within the range typical for Ni-C u bonds, while the Ni-0 interaction appears to be quite weak as shown by the relevant Ni-0 distance of 2.229 ( 5 ) A compared to 1.81 (1)A , 1 found in (~-BUNC)~N~C(CF,),NHC(CF~)~O.*~ On the other hand, this interaction is much stronger than that existing in a related chelate system Ni[C(Ph)=C(H)7
COCH,SiMe3]C1(PMe3)2,19d as documented by the very long Ni-0 separation of 2.535 (7) A, although the overall geometry of the five-membered ring remains rather unchanged. The geometrical parameters of the vinylic moiety in the present complex are essentially close to those revealed for the previously discussed Co compound. An interesting feature of the Ni complex is that one of the phosphorus atoms of the tripod ligand does not enter the coordination sphere. The np, molecule acting as a tridentate ligand, rather than using all its four donor atoms, has been observed quite frequently.12p26 However, with one exception,27 in all of these cases it was rather the ~~
(25) Countryman, R.; Penfold, B. R. J . Chem. SOC.,Chem. Commun. 1978, 1598. (26) Bianchini, C.; Masi, D.; Mealli, C.; Meli, A. Inorg. Chem. 1984, 23, 2838.
2
6539 (3) 6898 (3) 7828 (4) 7990 (4) 7555 (4) 6957 (4) 6795 (4) 7231 (4) 8533 (3) 9283 (3) 9335 (3) 8638 (3) 7888 (3) 7836 (3) 4548 (7) 4739 (4) 4604 (4) 4651 (4) 4832 (4) 4967 (4) 4920 (4) 4721 (3) 4210 (3) 4353 (3) 5007 (3) 5518 (3) 5375 (3) 3498 (4) 3212 (4) 2337 (4) 1748 (4) 2034 (4) 2909 (4) 5204 (3) 4990 (3) 5574 (3) 6373 (3) 6588 (3) 6003 (3)
nitrogen atom that was not coordinated to the metal center. The Ni-P distances are found to be significantly elongated in comparison with those in the parent compound (np,)Ni where all the donor atoms of the np, ligand are engaged in bonding to the Ni enter.^ Conclusions Insertion of acetylenes into metal-hydrogen bonds may occur by ionic stepwise, radical, or concerted mechanisms.'B2 In the absence of more detailed studies, it would be hazardous to discrimate between these three pathways when considering the reaction of ethyl propiolate with the hydride 1 to give the ql-vinyl complex 2. The reaction conditions and the high stereospecificity observed seem to favor a concerted mechanism. Even though stereospecificity does not necessarily rule out any of the possible mechanisms; generally, however, the radical mechanism, and, to a lesser extent, the ionic one give a mixture of products. At least in principle, the occurrence of a concerted mechanism is quite reasonable since complex 1 may arrange suitable orbitals to form a transition state with an acetylene molecule. More detailed information on the electronic properties and the chemistry of (np,)MX (M = Co, Rh, Ni; X = unidentate ligand) and (np3)Mcomplexes has been reported in a series of recent paper^.^,^^ (27) Bianchini, C.; Ghilardi, C. A,; Meli, A,; Orlandini, A. J. Organomet. Chem. 1984,270, 251. (28) (a) Cecconi, F.; Ghilardi, C. A,; Innwenti, P.; Mealli, C.; Midollini, S.; Orlandini, A. Inorg. Chem. 1984, 23, 422. (b) Ghilardi, C. A.; Mealli, C.; Midollini, S.; Orlandini, A. Inorg. Chem. 1985,24, 1964. (c) Bianchini, C.; Masi, D.; Mealli, C.; Meli, A,; Sabat, M. Organometallics 1985,4, 1014.
Reactions of HCECC0,Et with (np3)CoH and (np3)Ni
Organometallics, Vol. 5, No. 1, 1986 77
Table IV. Selected Bond Lengths (A) and Angles (deg) for Compound 3 Bond Lengths P(3)-C(1)6 2.331 (4) N(l)-C(2) 2.340 (4) N(l)-C(4) 2.270 (4) N(l)-C(6) 2.16 (1) C(l)-C(2) 1.91 (2) C(3)-C(4) 1.83 (1) C(5)-C(6) 1.80 (1) C(7)-C(8) 1.84 (1) 1.83 (1) C(7)-C(ll) C(8)-0(1) 1.83 (1) C(8)-0(2) 1.81 (1) C(9)-0(1) 1.82 (2) 1.81 (1) C(9)-C(lO) Bond 106.1 (2) 102.6 (2) 148.8 (2) 85.9 (4) 85.3 (3) 84.7 (3) 170.5 ( 7 ) 103.6 (5) 91.5 (5) 93.6 (5) 95.7 (5) 120.4 (3) 131.8 (4) 104.7 (6) 103.1 (5) 97.3 (5) 101.5 (5) 130.6 (4) 111.4 (3) 106.1 (6) 102.2 (6) 101.8 (4) 102.5 (5) 114.9 (4)
Angles Co-P(3)-C(1)6 C(5)-P(3)-C(1)5 C(5)-P(3)-C(1)6 C(1)5-P(3)-C(1)6 C(2)-N(l)-C(4) C(2)-N(l)-C(6) C(4)-N(l)-C(6) P(l)-C(l)-C(2) C(l)-C(2)-N(l) P(2)-C(3)-C(4) C(3)-C(4)-N(1) P(3)-C(5)-C(6) C(5)-C(6)-N(l) Co-C(7)-C(8) Co-C(7)-C(11) C(S)-C(7)-C(ll) C(7)-C(8)-0(1) C(7)-C(8)-0(2) 0(1)-C(8)-0(2) C(S)-O(l)-C(9) O(l)-C(9)-C(lO)
1.83 (1) 1.52 (2) 1.52 (2) 1.48 (2) 1.52 (2) 1.52 (2) 1.51 (2) 1.48 (2) 1.35 (2) 1.33 (2) 1.19 (2) 1.48 (2) 1.53 (2)
IX that a four-centered transition state would involve the participation of a molecule of substrate makes, in fact, less probable a concerted mechanism. By contrast, the high cr basicity of 4 could easily explain the intermediacy of an ionic adduct through the formation of a bond between nickel and the most electrophilic carbon atom of ethyl propiolate. In this case, the second carbon of the acetylene moiety, bearing a negative charge, could abstract a proton from a molecule of n-butyl alcohol. Although alkyl propiolates are acidic compounds, n-butyl alcohol is the most probable hydrogen source. In fact, notwithstanding equimolecular amounts of 4 and HC=CC02Et are used, 7 5 4 0 % yields are observed. Moreover, when no alcoholic solvent is employed, the reaction of 4 with ethyl propiolate and NaBPh, does not give the alkenyl complex 5. 5 cannot be obtained by reacting 4 with a twofold excess of ethyl propiolate. In this case, the reaction takes a different pathway, which likely involves the interaction of 4 with two or more alkyne molecules. Once the NiCH=CH(C0,Et) moiety is formed displacement of one phosphorus atom of np, from nickel by the ketonic oxygen of the There is ample experimental evidence that the hydride C02Et group may take place to give the 0-carbethoxyvinyl 1 may release H, H+, or H- species depending on the complex 5. The latter process involves the rupture of the particular type of reaction involved.29 An important factor five-membered metallacycle NCCPNi, followed by forin determining the behavior of 1 is represented by the unique bonding capabilities of the ligand np,. In effect, mation of the five-membered metallacycle NiCCCO. Some this may envelope the metal in different ways, which ulimportant factors that determine this rearrangement may timately affect the mobility of hydrogen over the coordibe the formation of a metallacycle where, although small, nation sphere and hence influence the mode of rupture of some electronic delocalization occurs and the greater afthe Co-H linkage.9i2sb*29 In the present reaction, the hyfinity of Ni(I1) for oxygen donors than for phosphorus drogen should have hydridic nature. Such an assumption donors.30 is supported by the course of the addition of the Co-H An ionic stepwise mechanism, similar to that we suggest, moiety across the C=C triple bond. The Co-H, in fact, has been previously reported for the formation of other adds to the most electropositive carbon of the alkyne. alkenyl complexes, namely, C~~-F,CCH=CHR~(CO)~ and An important factor that may determine the type of cis-F,CCH=CHFe(CO)2(C,H5). These compounds were addition of electrophilic acetylenes to metal hydrides is obtained by reaction of [ReiCO),]- and [Fe(C0)2(C5H,)]the electron availability of the C-C system.' The latter with HCECCF~.~ property strictly depends on the nature of the substituents Even though comparisons between organic and inorganic on acetylenes. Within this context, it is interesting to reactions are not always pertinent, it is interesting to note compare some reactions of the hydrides 1 and HM~I(CO)~ the analogies existing between the present reaction and with activated alkynes HC=CR. Complexes containing that of PPh, with phenylacetylene in the presence of water, the MC(R)=CH2 moiety are formed by reacting 1 with which gives 0-styryltriphenylphosphonium hydroxide HC=CCO,Et, or HMn(CO), with HCSCCF,,~~~ whereas (X).31 HMxI(CO)~reacts with HC=CCOzMe to give cisH20 (CO)SMnCH=CH(C02Me)by a stereospecific trans adPPh3 + HCECPh (PhSP+CH=CHPh)OHdition.13 As for the mechanism of the reaction of the nickel(0) In both cases a nucleophile possessing a u lone pair (the complex 4 with HC=tC02Et, we suggest an ionic stepwise nickel(0) complex 4 or PPh,) attacks the most electrophilic mechanism to explain the formation of 5 (IX). The fact P (1)-co-P (2) P(l)-Co-P(3) P(2)-CO-P(3) N( l)-Co-P( 1) N(l)-Co-P(2) N( l)-Co-P(3) N(l)-Co-C(7) C(7)-Co-P(1) C(7)-Co-P(2) C(7)-Co-P(3) Co-P(l)-C(l) Co-P(l)-C(l)l Co-P(l)-C(1)2 C(l)-P(l)-C(l)l C(l)-P(l)-C(1)2 C(l)l-P(l)-C(1)2 Co-P(2)-C(3) CO-P(2)-C(1)3 Co-P(2)-C(1)4 C(3)-P(2)-C(1)3 C(3)-P(2)-C(1)4 C(1)3-P(2)-C(1)4 Co-P(3)-C(5) Co-P(3)-C(1)5
126.6 (4) 105.9 ( 7 ) 103.3 ( 7 ) 101.6 (5) 107 (1) 111 (1) 105 (1) 108.1 (9) 113 (1) 107 (1) 112 (1) 110 (1) 113 (1) 122 (1) 124 (1) 114 (2) 113 (2) 124 (2) 123 (2) 116 (1) 100 (1)
--
(29) Bianchini, C.; Masi, D.; Mealli, C.; Meli, A,; Sabat, M.; Scapacci, G . J. Organomet. Chem. 1984,273, 91 and references therein.
(30) b l a n d , S.;Chatt, J.; Davies, N. R. Q. Reu., Chem. Soc. 1958,12, 265. (31) Allen, D. W.; Tebby, J. C. Tetrahedron 1967,23, 2795.
78 Organometallics, Vol. 5 , No. I , 1986
Bianchini e t al.
Table V. Selected Bond Lengths (A) and Angles (deg) for Compound 5 Bond Lengths 2.193 (2) P(3)-C(1)6 O(l)-C(9) 2.171 (2) 0(2)-C(9) 2.229 ( 5 ) 2.036 (6) 0(2)-C(10) N(l)-C(2) 1.878 (7) N(l)-C(4) 1.825 (8) N(l)-C(5) 1.809 (6) C(l)-C(2) 1.812 (6) C(3)-C(4) 1.845 (8) C(5)-C(6) 1.794 (7) C(7)-C(8) 1.818 (6) C(8)-C(9) 1.856 (8) C(l0)-C(l1) 1.833 (7)
Ni-P(1) Ni-P(2) Ni-O(1) Ni-N(l) Ni-C(7) P(l)-C(l) P(l)-C(l) 1 P (l)-C( 1)2 P(2)-C(3) P( 2)-C (1)3 P (2)-C (1)4 P(3)-C(6) P(3)-C(l)5 P(l)-Ni-P(B) P (1)-Ni-O(1) P (l)-Ni-N( 1) P(l)-Ni-C(7) P(2)-Ni-O( 1) P(2)-Ni-N( 1) P (2)-Ni-C (7) O(l)-Ni-N(l) 0(l)-Ni-C (7) N(l)-Ni-C(7) Ni-P(l)-C(l) Ni-P(l)-C(l)l Ni-P( 1)-C( 1)2 C(l)l-P(l)-C(1)2 Ni-P (2)-C(3) Ni-P( 2)-C( 1)3 Ni-P(2)-C(1)4 C(1)3-P(2)-C(1)4 C(6)-P(3)-C(1)5 C (6)-P (3)-C (1)6 C(1)5-P(3)-C(1)6
1.835 (6) 1.216 (8) 1.331 (9) 1.48 (1) 1.497 (9) 1.491 (9) 1.506 (9) 1.49 (1) 1.52 (1) 1.531 (9) 1.35 (1) 1.46 (1) 1.44 (1)
Bond Angles 138.4 (1) 96.1 (1) Ni-O(l)-C(S) 104.9 (5) 88.4 (2) C(2)-0(2)-C(lO) 115.7 (6) 91.5 (2) Ni-N(l)-C(B) 112.0 (5) 106.6 (4) 125.5 (1) Ni-N(1)-C(4) 88.8 (2) Ni-N(1)-C(5) 108.5 (4) 110.7 (6) 93.6 (3) C (2)-N( 1)-C (4) 107.4 (6) 94.7 (2) C(2)-N(l)-C(5) 111.7 (6) 81.9 (3) C(4)-N(l)-C(5) 108.9 (6) 176.5 (3) P(l)-C(l)-C(2) 112.7 (7) 100.1 (3) C(l)-C(2)-N(l) 119.3 (2) P(2)-C(3)-C(4) 107.3 (5) 110.9 (6) 119.1 (2) C(3)-C(4)-N(l) 114.7 (6) 105.3 (9) C(6)-C(5)-N( 1) 108.9 ( 5 ) 100.4 (3) P(3)-C (6)-C (5) 126.1 (2) Ni-C (7)-C(8) 114.7 (6) 115.6 (7) 114.9 (2) C(7)-C(8)-C(9) 122.8 (7) 100.5 (3) O(l)-C(9)-0(2) 122.1 (7) 101.5 (3) C(8)-C(9)-0(1) 115.1 (7) 98.4 (3) C(8)-C(9)-0(2) 103.8 (3) 0 ~ 2 ~ - c ~ 1 0 ~ - c 110.1 ~ 1 1 ~(9)
carbon atom of the activated alkyne while a proton is abstracted from a weak Bransted acid. Within this context, reaction X provides further support to the suggested reaction mechanism for the formation of 5.
Experimental Section All operations were routinely performed under nitrogen by using deoxygenated solvents. Reagent grade chemicals were used in the preparation of the complexes. Tetrahydrofuran (THF) was purified by distillation over LiAlH, just before use. Compounds 1 and 4 were prepared according to published procedures.' The solid complexes were collected on a sintered glass frit and dried in a stream of nitrogen. Infrared spectra were recorded on a Perkin-Elmer 283 spectrophotometer using samples mulled in Nujol between CsI plates. 'H and 31P(1H)NMR spectra were taken on a Varian C F T 20 spectrometer. Peak positions are relative to tetramethylsilane and phosphoric acid, respectively, with downfield shifts positive. Ultraviolet-visible spectra were recorded on a Beckman DK-2A spectrophotometer. Magnetic susceptibilities of solid samples were measured on a Faraday balance. Conductance measurements were made with a WTW Model LBR/B conductivity bridge. (np,)CoC(COzEt)=CHz (2). Ethyl propiolate (0.07 mL, 0.6 mmol) was pipetted into a magnetically stirred solution of 1 (0.35 g, 0.5 mmol) in T H F (30 mL). After 5 min the resulting solution was concentrated a t ca. 10 mL under reduced pressure. Addition of n-butyl ether (10 mL) precipitated in a few minutes brown crystals that were filtered and washed with a 3:l mixture of n-butyl ether/THF and petroleum ether; yield 62%. Anal. Calcd for C. 69.54; H, 6.08; Co, 7.25; N, 1.72. Found: C, C47H49C~N02P3:
69.82; H, 5.99; Co, 7.20; N, 1.60.
[(np3)CoC(COzEt)=CHz]BPh,-0.5THF (3). Ethyl propiolate (0.07 mL, 0.6 "01) was Dipetted into a maaneticallv stirred solution of 1 (0.35 g, 0.5 mmol) in T H F (30 m c ) . The iesulting mixture was then stirred for a further 40 min. Addition of NaBPh, (0.18 g, 0.6 mmol) in n-butyl alcohol (20 mL) precipitated red crystals that were filtered and washed with ethanol and petroleum ether; yield 70%. Anal. Calcd for C73H73BC~N02,5P3: C, 75.12; H, 6.30; Co, 5.04; N, 1.20. Found: C, 74.80; H, 6.10; Co, 5.13; N, 1.18. [(np,)NiCH=CHC(0)OEt]BPh4 ( 5 ) . Ethyl propiolate (0.06 mL, 0.5 mmol) was added to a solution of 4 (0.35 g, 0.5 mmol) in T H F (20 mL). Immediately an orange-brown solution was obtained. Addition of NaBPh, (0.18 g, 0.6 mmol) in n-butyl alcohol (30 mL) gave yellow-orange crystals that were filtered and washed with ethanol and petroleum ether; yield 80%. They were recrystallized from CH2Clz and ethanol. Anal. Calcd for C7,Hs9BNNi02P3: C, 75.41; H, 6.15; N, 1.23; Ni, 5.19. Found: C, 74.87; H, 6.10; N, 1.30; Ni, 5.26. X-ray D a t a Collection a n d S t r u c t u r e Determination. A summary of crystal and intensity data is presented in Table I. All X-ray measurements were performed on a Philips PWllOO diffractometer using Mo Kcu radiation. The intensities of three standard reflections were measured every 120 min of X-ray exposure, and no significant changes were noted for both compounds. The data were corrected for Lorentz and polarization effects. Numerical absorption corrections were applied with transmission factors ranging 0.94-0.97 for 3 and 0.91-0.94 for 5. Atomic scattering factors were those tabulated by Cromer and Waber32 with anomalous dispersion corrections taken from ref 33. The structures of the compounds were solved by Patterson and Fourier techniques using the SHELX76 program package.% Refinement was performed by full-matrix least-squares calculations, initially with isotropic and then with anisotropic thermal parameters for all non-hydrogen atoms of the complex cations but the phenyl carbon atoms. The function minimized was s3w(lFol- lFc1)2.The phenyl rings were treated as rigid bodies of Dsh symmetry with C-C distances fixed a t 1.395 A and calculated hydrogen atom positions (C-H = 1.0 A). Contributions from the remaining hydrogen atoms in positions fixed by geometry have also been included during final cycles of refinement. In the case of the Co compound a difference map showed a few higher peaks forming a five-membered ring of the tetrahydrofuran solvent molecule. The high thermal parameters indicated that the tetrahydrofuran positions were probably partially occupied. As a consequence the atoms of the ring were refined isotropically with occupancy factors 0.5, but no attempt was made to refine these coefficients. This approach was also in agreement with the results of the elemental analysis which demonstrated the 2:l complex/solvent molar ratio. The final difference map for the Co compound revealed two peaks each 0.5 e/A3 high located near the phenyl rings 3 and 9, while in the case of the Ni complex the largest residuals were 1.2 and 0.9 e/A3 high and were located near the Ni atom and ring 3, respectively. Final coordinates for all non-hydrogen atoms of 3 and 5 are reported in Tables I1 and 111, respectively. Registry No. 1, 53687-39-1; 2, 98838-13-2; 3, 98838-16-5; 4, 52633-73-5; 5 , 98838-18-7; HC=CCOZEt, 623-47-2. Supplementary Material Available: Listings of anisotropic thermal parameters, supplementary distances, and observed and calculated structure factors (55 pages). Ordering information is given on any current masthead page. (32) "International Tables for X-ray Crystallography"; Kynoch Press: Birmingham, England, 1974; Vol. IV, p 99. (33) Reference 32, p 149. (34) Sheldrick, G . M., SHELX76, Program for Crystal Structure Determination, University of Cambridge, Cambridge, 1976.
