Inorg. Chem. 1991, 30, 4098-4100
4098
cleophilicity of the azide.” Thus, we prepared the iminophosphorane-phosphenium salts 19 and 2012by treatment of 16 and 17 with trimethylsilyl trifluoromethanesulfonate. 19 and 20 exhibited characteristic 31PN M R chemical shifts for cationic dicoordinated phosphorus species. 19 22.95 (d), 309.51 (d), Jpp = 87.2 Hz. 20: 20.53 (d), 294.30 (d), .Ipp= 80.5 Hz (Scheme VII). Thermolysis and UV photolysis of 19 and 20 led to oligomers featuring broad 31PNMR signals at around 0 ppm. In conclusion, it appears that intermolecular Staudinger reactions are favored compared to intramolecular ones. The only evidence for the formation of cyclodiphosphazene has been obtained with the bulky diisopropylamino groups on both phosphorus atoms. It is reasonable to think that such bulky substituents slow down the intramolecular process and thus allow the intramolecular reaction. These observations suggest that the use of small substituents a t both phosphorus atoms could make the (phosphany1imino)azidophosphoranes interesting monomers for synthesis of polyphosphazenes. These results once more emphasize the difficulty of synthesizing cyclodiphosphazenes. The large-scale preparation of such compounds remains a challenge. Registry NO. 3, 90670-65-8; 5, 135665-16-6; 6, 135665-17-7; 7, 135665-18-8; 8, 135665-19-9; 9, 117874-63-2; 10, 135665-20-2; 11, 96455-38-8; 12, 135665-21-3; 16, 135665-22-4; 17, 135665-23-5; 18, 135695-47-5; 19, 135665-25-7; 20, 135665-27-9; trichlorophosphine, 77 19- 12-2; (dimethylamino)dichlorophosphane, 683-85-2.
Kasukhin, L.F.; Ponomarchuk, M. P.; Kim, T. V.; Kukhar, V. P. Dokl. Akad. Nauk
SSSR 1986, 290, 1375.
Mazieres, M. R.; Roques, C.; Sanchez, M.; Majoral, J. P.; Wolf, R. Tetrahedron Lerr. 1987, 43, 2109.
Contribution from the Institut fur Anorganische und Analytische Chemie, Universitat Freiburg, D-7800 Freiburg, Germany, Department of Chemistry, Columbia University, New York, New York 10027, and Electronics Department, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19880 A Mononuclear Zinc Hydroxide Complex Stabilized by a Highly Substituted Tris(pyrazoly1)hydroborato Ligand: Analogies with the Enzyme Carbonic Anhydrase
Ralf Alsfasser: Swiatoslaw Trofimenko,t Adrian Looney,# Gerard Parkin,*-i and Heinrich Vahrenkamp**f Received April 3, I991
The enzyme carbonic anhydrase has a remarkably simple function, namely to catalyze the hydration of C02to bicarbonate (eq 1). The catalytic site consists of a deceptively simple tetC02
+ H 2 0 s HC03- + H+
(1)
rahedral coordination environment in which a zinc center is bound to three histidine imidazole groups and a water molecule, [His3Zn-OH2]*+(His = histidine), and the initial stages of the catalytic cycle are considered to involve initial deprotonation of the coordinated water to give the active zinc hydroxide derivative [His3Zn-OH]+, followed by reaction with C 0 2 to give a zinc bicarbonate derivative [His3Zn-OC02H]+(Scheme I),’ However, attempts to mimic structurally the zinc coordination environment in carbonic anhydrase by the synthesis of simple model compounds have met with only limited success,2 while investigations to model the enzyme’s function using other zinc complexes have generally been confined to ester hydrolysis studies3 At
’*E.UniversitHt Freiburg. 1. du Pont de Nemours and Co. IColumbia University.
0020-1669/91/ 1330-4098$02.50/0
Scheme 1. Simplified Catalytic Cycle for Hydration of C 0 2 B -BH*
His,,
His,,,, Hi\,
Hi
Hi
z
/
P
1
His,,,
/ Hid
present there is no report of a fully characterized mononuclear tetrahedral L3Zn-OH complex, in spite of the facts that (i) basic salts are a common feature of the inorganic chemistry of zinc4 and (ii) several hydroxide-bridged oligonuclear zinc complexes are k n ~ w n . ~ ~ , ~ For studies designed to model structurally the active site of carbonic anhydrase, the tris(pyrazoly1)hydroborato ligand system, (HB(~,S-RR’~Z)~J(RR’pz = substituted pyrazole), would appear to be an almost ideal choice for several reasons: (i) the three nitrogen atom donors of the (HB(3,5-RR’pz),)- ligand have the ability to bind to the zinc center in a manner analogous to that proposed for carbonic anhydrase, (ii) the sterically demanding tris(pyrazoly1)hydroborato derivatives, e.g. {HB(~-BU‘PZ)~)-, effectively restrict the maximum coordination of zinc to 4, and (iii) the uninegative nature of ( H B ( ~ , S - R R ’ ~ Z )indicates ~Jthat the tetrahedral hydroxide derivative ( V ~ - H B ( ~ , ~ - R R ’ ~ Z ) would ~)Z~OH be a neutral complex, which would thereby greatly simplify isolation procedures.6 Indeed, our previous independent investigations of [tris(pyrazolyl)hydroborato]zinc chemistry have illustrated the validity of the aforementioned point^,^^*^,^-^^^ and this paper reports the first result of our joint effort to synthesize and characterize a complex designed to model structurally the active site of the enzyme carbonic anhydrase. (a) Spiro, T. G., Ed. Zinc Enzymes; Wiley: New York, 1983. (b) Sigel, H., Ed. Zinc and its Role in Biology and Nutrition; Marcel Dekker: New York, 1983. (c) Eriksson, E. A.; Jones, T. A.; Liljas, A. In Zinc Enzymes; Birkhauser: Boston, 1986; Chapter 23, p 317. (a) Wooley, P. J. J . Chem. SOC.,Perkin Trans. 2 1977,318. (b) Tang, C. C.; Davalian, D.; Huang, P.; Breslow, R. J . Am. Chem. SOC.1978, 100,3918. (c) Curtis, N. J.; Brown, R. S. J. Org. Chem. 1980,45,4038. Read, R. J.; James, M. N. G. J . Am. Chem. Soc. 1981,103,6947. (d) Brown, R. S.; Curtis, N. J.; Kusuma, S.; Salmon, D. J. Am. Chem. Soc. 1982, 104, 3188.
(a) Slebocka-Tilk, H.; Cocho, J. L.; Frakman, Z.; Brown, R. S.J. Am. Chem. Soc. 1984,106,2421. (b) Brown, R. S.;Zamkanei, M.; Cocho, J. L. J . Am. Chem. Soc. 1984, 106, 5222. (c) Gellmann, S.H.; Petter, R.; Breslow, R. J . Am. Chem. SOC.1986,108,2388. (d) Kimura, E.; Shiota, T.; Koike, T.; Shiro, M.; Kodama, M. J . Am. Chem. Soc. 1990, 112, 5805. Wells, A. F.Structural Inorganic Chemistry; Clarendon Press: Oxford, U.K., 1975; pp 410 ff, 529 ff, 913 ff.
(a) AI-Juaid, S. S.; Buttrus, N. H.; Eaborn. C.; Hitchcock, P. B.; Roberts, A. T. L.; Smith, J. D.; Sullivan, A. C. J . Chem. Soc., Chem. Commun. 1986, 909. (b) Arif, A. M.; Cowley, A. H.; Jones, R. A.; Koschmieder, S . 0. J . Chem. SOC.,Chem. Commun. 1987, 1319. (c) Dubler, E.; Hanggi, G.; Schmalle, H. Inorg. Chem. 1990, 29, 2518. (d) Gorrell, I. B.; Looney, A.; Parkin, G. J . Am. Chem. SOC. 1990, 112, 4068. (e) Alsfasser, R.; Powell, A. K.; Vahrenkamp, H.Angew. Chem.
1990, 102, 939; Angew. Chem., Int. Ed. Engl. 1990, 29, 898. (a) Trofimenko, S.Chem. Rev. 1972, 72,497. (b) Trofimenko, S.Prog. Inorg. Chem. 1986, 34, 115. (c) Trofimenko, S.; Calabrese, J. C.; Thompson, J. S. Inorg. Chem. 1987, 26, 1507. (d) Trofimenko, S.; Calabrcse, J. C.; Domaille, P.J.; Thompson, J. S.Inorg. Chem. 1989, 28, 1091. (e) Cano, M.; Heras, J. V.; Trofimenko, S.; Monge, A.; Gutierrez, E.; Jones, C. J.; McCleverty, J. S.J . Chem. Soc., Dalton Trans. 1990, 3577. Gorrell, I. B.; Looney, A.; Parkin, G. J . Chem. SOC.,Chem. Commun. 1990, 220.
0 1991 American Chemical Society
Inorganic Chemistry, Vol. 30, No. 21, I991 4099
Notes Table 1. Atom Coordinates (XI@) and Temperature Factors (A2 x 10’) atom
Zn 0
N(11) N(12)
C(11) C(12)
C(13) C(14)
C(15) C(16) C(17)
B
X
Y
Z
Lp
0 0 -1063 (4) -1239 (4) -1886 ( 5 ) -2621 ( 5 ) -2194 ( 5 ) -1942 (7) -2662 ( 5 ) -3776 (7) -2372 (5) 0
0 0 -532 (2) -619 (2) -943 (3) -1311 (3) -1097 (3) -971 (3) -1331 (3) -1888 (3) -1971 ( 5 ) 0
0 -1970 (9) 2644 ( 5 ) 1197 ( 5 ) 3370 (7) 2392 (9) 1046 (8) 4956 (8) -391 (8) -210 (12) -1230 (6) 3185 (14)
32 (1) 55 (3) 35 (2) 36 (2) 47 (3) 55 (3) 47 (3) 72 (4) 5 5 (3) 131 (7) 72 (3) 40 ( 5 )
Table 111. Selected Bond Angles (deg) 0-Zn-N(12) 123.2(2) 0-Zn-N(12’) N(12)-Zn-N(12’) 92.8(2) 0-Zn-N(I2”) N( 12)-Zn-N( 12”) 92.8 (2) N(12’)-Zn-N( 12”) N(l2)-N(ll)-C(ll) 110.7( 5 ) N(IZ)-N(II)-B 130.1 (6) Zn-N(12)-N(II) C(l1)-N(l1)-B 140.6( 5 ) N(II)-N(I2)C(I3) Zn-N(12)-C(13) N(l I)-C( 1 l)-C(I2) 107.1 (6) C(l I)-C(12)C( 13) 108.9(6) N(II)-B-N(II’) N(12)
