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Aminotroponiminato complexes of silicon, germanium, tin and lead
Coordination Chemistry Reviews 176 Ž1998. 67]86
Aminotroponiminato complexes of silicon, germanium, tin and lead H.V. Rasika DiasU , Ziyun Wang, Wiechang Jin Department of Chemistry and Biochemistry, Box 19065, The Uni¨ ersity of Texas at Arlington, Arlington, TX 76019-0065, USA Received 2 October 1997; accepted 6 January 1998
Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. The starting materials . . . . . . . . . . . . . . . . . . . . . . . 3. Aminotroponiminato complexes of heavier group 14 elements 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Abstract Aminotroponiminates are very useful ligands in group 14 chemistry. They readily form complexes with both low and high valent group 14 metal ions. Synthesis, structural features and some chemistry of aminotroponiminato silicon, germanium, tin and lead derivatives are reviewed. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: Aminotroponiminates; Ligands
1. Introduction Aminotroponimines Že.g. 1. are an interesting group of bidentate, nitrogen donor ligands. Although aminotroponimines are structurally similar to tropolone Ž2., U
Corresponding author. Tel.: q1 817 2723813; fax: q1 817 2723808; e-mail: [email protected]
0010-8545r98r$ - see front matter Q 1998 Elsever Science S.A. All rights reserved. PII S0010-8545Ž98.00112-X
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various physical and chemical data indicate significant differences between the two types w1,2x. For example, aminotroponimines contain a highly delocalized 10-electron p-system delocalized over seven carbons and the two nitrogen atoms. In contrast, properties of tropolone are best understood in terms of a p-system involving only six electrons. Furthermore, unlike tropolone, the steric and electronic properties of aminotroponimines may be easily modulated by changing the substituent on nitrogen Žor on the ring carbon atom.. In fact, a variety of N,N9-disubstituted aminotroponimines including the closely related tropocoronands Ž3. are known w1]12x. Aminotroponiminate ligands wŽR. 2 ATIxy Ž4., which are formally derived from aminotroponimines wŽR. 2 ATIxH by deprotonation, can also be compared to amidinates Ž5. w13x and b-diketinimates Ž6. w4,14x. All three types are nitrogen based, bidentate, monoanionic and formally four-electron donor ligand systems. However, upon coordination to metal ions, aminotroponiminates form a five-membered chelate ring whereas amidinates and b-diketinimates form four and six-membered metallacycles, respectively. Furthermore, unlike the aminotroponiminate which features a delocalized 10p electron ligand backbone, amidinates and b-diketinimates have delocalized 4p and 6p electron ligand backbones. Among the three types, amidinates have been the most widely used in coordination chemistry
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whereas aminotroponiminates and b-diketinimates have received relatively less attention w4,13,14x. Studies involving aminotroponiminate ligand Žincluding those of the related tropocoronand. have mostly been limited to the first row, middle and late transition metal ions, such as manganese, iron, cobalt, nickel, copper and zinc w2,4,5,10,15]25x. Aminotroponiminates have also been used as ancillary ligands Žparticularly as an alternative to cyclopentadienyl ligand. in early transition metal chemistry w26]28x. This article will review the use of aminotroponimines in group 14 chemistry which represents an area of increasing activity recently w12,29,30x. The main focus will be on the synthesis, structures and chemistry of compounds involving silicon, germanium, tin and lead, and the related starting materials.
2. The starting materials The first report of an aminotroponimine appeared in 1953, which concerns the synthesis of amino derivative 7 using tropolone w31x. The synthesis of N,N9-dibenzoyl analog 8 starting from diazaazulene has also been reported w32x. The parent compound 1-amino-7-imino-1,3,5-cycloheptatriene Ž1. was later prepared by reacting tetrafluorocycloheptadienes Žobtained using cyclopentadiene and tetrafluoroethylene. w33x with ammonia ŽEq. 1; where R s H. w3x. Similarly, a large number of N,N9-disubstituted aminotroponimines ŽwŽR. 2 ATIxH; where R s methyl, i-butyl, benzyl, phenyl, p-tolyl, p-methoxyphenyl, p-chlorophenyl, p-dimethylaminophenyl, p-phenylazophenyl, p-nitrophenyl and several others. have also been synthesized using primary amines instead of ammonia in the above reaction scheme w2,3x. The product yields range from 30 to 80%. Relatively more convenient and high-yield synthetic routes to N,N9-disubstituted aminotroponimines Žparticularly N-alkyl derivatives. are now available w6,7,11,12,16,34]37x. These methods mainly use easily
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Fig. 1. Molecular structure of wŽ i-Pr. 2 ATIxH. Reprinted with permission from Dias et al. w11x Q 1995 ˚ . and angles Ž8.: N1]C2, 1.342Ž3.; N9]C8, American Chemical Society. Selected bond lengths ŽA 1.314Ž3.; C2]N1]C10, 126.6Ž2.; C8]N9]C13, 122.1Ž3.; N1]C2]C8, 111.7Ž2.; N9]C8]C2, 122.8Ž3..
accessible tropolone or tropolone derivatives as starting material rather than tetrafluorocycloheptadienes. Aminotroponimines are brightly colored, crystalline solids. Colors typically range from yellow to brick-red depending on the substituent on nitrogen. NMR spectroscopic data of wŽMe. 2 ATIxH are indicative of a C 2-symmetric structure in solution even at y808C w2,38x. Solid state and solution IR spectroscopic studies indicate intramolecular H-bonding in wŽMe. 2 ATIxH w1,2x. Several aminotroponimines, wŽR. 2 ATIxH where R s Me, a-methylbenzyl and i-propyl, have also been charac-
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terized by X-ray crystallography w11,38,39x. wŽMe. 2 ATIxH shows a C 2-symmetric structure with equivalent C ring ]N distances w38x. However, in the latter two structures, C ring ]N distances are not equal Žfor R s a-methylbenzyl; 1.355Ž4., ˚ w39x and for R s i-propyl; 1.342Ž3., 1.314Ž3. A ˚ w11x. indicating the 1.305Ž4. A presence of an amino nitrogen ŽC-NŽR.H. with a strong N]H interaction and an imino nitrogen center ŽC s NR.. The ORTEP diagram of wŽ i-Pr. 2 ATIxH is illustrated in Fig. 1. The seven-membered ring, two nitrogens, C10 and C13 occupy a single plane. The isopropyl groups orient in a manner that minimizes the steric interaction with the two hydrogens on C3 and C7. The alkali metal salts can be obtained conveniently by treating aminotroponimines with appropriate bases w26,40]43x. For example, wŽ i-Pr. 2 ATIxLi and wŽ i-Pr 2 ATIxK were obtained by treating wŽ i-Pr. 2 ATIxH with Bu n Li or KH ŽEq. 2.. They are very air sensitive solids. The X-ray crystal structures of wŽ iPr 2 ATIxLiŽTHF. 2 and wŽ i-Pr. 2 ATIxKŽTHF.4n are known w26,42x. The lithium derivative adopts a monomeric structure ŽFig. 2. whereas the potassium salt displays an interesting chain structure which features a p-bonded aminotroponimine ring ŽFig. 3.. The N]C ring bond distances of wŽ i-Pr. 2 ATIxLiŽTHF. 2 and wŽ iPr. 2 ATIxKŽTHF.4n are much closer to the imine N s C ring distance in the free ligand. These alkali metal derivatives serve as versatile reagents for the introduc-
Fig. 2. Molecular structure of wŽ i-Pr. 2 ATIxLiŽTHF. 2 . Reprinted with permission from Dias et al. w26x ˚ . and angles Ž8.: Li]N1, 1.995Ž6.; Li]N9, Q 1996 American Chemical Society. Selected bond lengths ŽA 1.990Ž6.; Li]O, 1.971Ž6.; Li]O2, 1.960Ž6.; N1]C2, 1.313Ž4.; N9]C8, 1.319Ž4.; N1]Li]N9, 81.5Ž2.; O1]Li]O2, 118.3Ž3.; C2]N1]Li, 114.0Ž3.; C8]N9]Li, 113.4Ž3.; N1]C2]C8, 114.4Ž3.; N9]C8]C2, 113.7Ž3..
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Fig. 3. Molecular structure of wŽ i-Pr 2 ATIxKŽTHF.4n . A. ORTEP diagram showing the atom numbering ˚ . and angles Ž8.: scheme. B. A view showing the polymeric chain structure. Selected bond lengths ŽA K1]O1, 2.690Ž4.; K1]N1, 2.717Ž4.; K1]N9, 2.741Ž4.; N1]C2, 1.312Ž6.; N9]C8, 1.321Ž6.; K2]O2, 2.657Ž4.; K2]N16, 2.712Ž4.; K2]N24, 2.730Ž4.; N16]C17, 1.304Ž6.; N24]C23, 1.307Ž6.; N1]K1]N9, 58.72Ž12.; N16]K2]N24, 58.93Ž12..
˚ . and angles Ž8.: Si]N1, Fig. 4. Molecular structure of wŽ i-Pr. 2 ATIxSiCl 3 . Selected bond lengths ŽA 1.870Ž2.; Si]N9, 1.773Ž3.; Si]Cl1, 2.1999Ž11.; Si]Cl2, 2.1069Ž11.; Si]Cl3, 2.0905Ž12.; N1]C2, 1.331Ž4.; N9]C8, 1.381Ž4.; N1]Si]N9, 85.46Ž11.; N1]Si]Cl1, 178.07Ž9.; N9]Si]Cl1, 90.19Ž5.; N9]Si]Cl2, 120.34Ž9.; N9]Si]Cl3, 123.51Ž9.; Cl2]Si]Cl3, 115.97Ž5..
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˚ . and angles Ž8.: N1]C2, Fig. 5. Molecular structure of wŽ i-Pr. 2 ATIxH 2 4q Cly. Selected bond lengths ŽA 1.343Ž2.; N9]C8, 1.342Ž2.; C2]N1]C10, 126.4Ž2.; C8]N9]C13, 125.9Ž2.; N1]C2]C8, 117.0Ž2.; N9]C8]C2, 117.2Ž2..
tion of aminotroponiminato moiety to various metal ions. Lithium salts thus far have been the reagents of choice for group 14 derivatives.
3. Aminotroponiminato complexes of heavier group 14 elements In 1964 and 1965, Muetterties reported the synthesis of several compounds of the type wŽR. 2 ATIx 3 M4 q Xy Žwhere R s mainly Me; M s Si, Ge, or Sn; X s halide, PF6 , etc.. by treating the corresponding tetrahalides with wŽR. 2 ATIxH or its lithium salt w40,44x. These crystalline compounds were characterized by elemental analysis and believed to contain octahedral metal centers. Interestingly, regardless of the metalrligand ratio, the tris-ligand complex was the only product isolated. The tetrakis analogs could not be obtained due to the unfavorable steric interactions between substituents on nitrogen atoms. wŽR. 2 ATIx3 Mq Xy compounds are stable in aqueous alkaline solutions. However, they rapidly degrade in acidic media leading to the formation of acid salts of free ligands, e.g. wŽR. 2 ATIxH 2 4 q Cly w40x. With more bulky ligands, monosubstituted derivatives can be obtained. For example, it was possible to synthesize wŽ i-Pr. 2 ATIxSiCl 3 by treating wŽ i-Pr. 2 ATIxLi with SiCl 4 w42x. It was characterized by NMR spectroscopy and X-ray crystallogra-
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˚ . and angles Ž8.: Si]N1, Fig. 6. Molecular structure of wŽMe. 2 ATIxSiMe 3 . Selected bond lengths ŽA 2.037Ž4.; Si]N9, 1.848Ž4.; Si]C13, 1.890Ž4.; Si]C13A, 1.890Ž4.; Si]C12, 1.927Ž6.; N1]C2, 1.308Ž6.; N9]C8, 1.359Ž6.; N1]Si]N9, 77.8Ž2.; N9]Si]C12, 96.8Ž2.; N1]Si]C12, 174.6Ž2.; N9]Si]C13, 118.38Ž14.; C13]Si]C13A, 120.2Ž3..
phy ŽFig. 4.. The silicon atom adopts a trigonal bipyramidal geometry with nitrogen atoms of the aminotroponiminate ligand occupying one axial wwith Si]N distance of ˚ x and one equatorial site wSi]N s 1.773Ž3. A ˚ x. wŽ i-Pr. 2 ATIxSiCl 3 decom1.870Ž2. A poses upon exposure to moisture and wŽ i-Pr. 2 ATIxH 2 4 q Cly was observed among the decomposition products w42x. In contrast to the free ligand wŽ i-Pr. 2 ATIxH, this HCl salt displays a fairly symmetric C 7 N2 moiety in the solid state ŽFig. 5. with two ˚ These distances are similar to equal C ring ]N distances of 1.343Ž2. and 1.342Ž2. A. the amine C ring ]N bond length of the free ligand. The chloride ion shows hydrogen bonding to protons on the two nitrogen atoms. The tinŽIV. compound wŽEt. 2 ATIxSnPh 3 has been prepared by the reaction of wŽEt. 2 ATIxLi with Ph 3 SnCl w41x. The corresponding germanium and silicon analogs are significantly more reactive, and have not been isolated in pure form. These compounds decompose rapidly in the presence of moisture. The reaction between Me 3 SnCl or Me 3 SiCl with wŽMe. 2 ATIxLi gives corresponding SnŽIV. or SiŽIV. derivatives ŽEq. 3. w45x. These adducts have been characterized by NMR spectroscopy and X-ray crystallography. wŽMe. 2 ATIxSiMe 3 ŽFig. 6. and wŽMe. 2 ATIxSnMe 3 ŽFig. 7. have very similar solid state structures which feature five-coordinate,
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˚ . and angles Ž8.: Sn]N1, Fig. 7. Molecular structure of wŽMe. 2 ATIxSnMe 3 . Selected bond lengths ŽA 2.281Ž4.; Sn]N9, 2.173Ž4.; Sn]C13, 2.145Ž4.; Sn]C13A, 2.145Ž4.; Sn]C12, 2.179Ž6.; N1]C2, 1.302Ž7.; N9]C8, 1.342Ž7.; N1]Sn]N9, 70.2Ž2.; N9]Sn]C12, 94.8Ž2.; N1]Sn]C12, 165.1Ž2.; N9]Sn]C13, 118.06Ž11.; C13]Sn]C13A, 117.9Ž2..
trigonal bipyramidal metal sites w45x. The tin analog shows relatively longer metal]N and metal]C distances Žas expected based on the larger atomic radius of Sn.. The N1]Sn]C12 bond angle of wŽMe. 2 ATIxSnMe 3 is approx. 108 smaller than the related N1]Si]C12 angle of wŽMe. 2 ATIxSiMe 3 . These two compounds show fluxional behavior in solution at room temperature. For example, only a single N]Me Žor M]Me. resonance was found in the 1 H NMR spectrum of wŽMe . 2 ATI xSiMe 3 or wŽMe . 2 ATI xSnMe 3 . The 1 H NMR spectrum of
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˚ . and angles Ž8.: Sn]N1, Fig. 8. Molecular structure of wŽ i-Pr. 2 ATIx2 SnMe 2 . Selected bond lengths ŽA 2.248Ž4.; Sn]N9, 2.267Ž4.; Sn]N16, 2.262Ž4.; Sn]N24, 2.243Ž4.; Sn]C31, 2.163Ž5.; Sn]C32, 2.192Ž5.; N1]Sn]N24, 158.2Ž1.; C32]Sn]C31, 106.3Ž2.; N1]Sn]N9, 71.3Ž1.; N16]Sn]N24, 71.0Ž1..
wŽMe. 2 ATIxSnMe 3 shows coupling between the tin atom and Sn] Me protons Ž 2 J s 51.8 Hz., and N] Me protons Ž 3 J s 18.4 Hz.. The wŽ i-Pr. 2 ATIxSnMe 3 has also been synthesized which contains a relatively bulky wŽ i-Pr. 2 ATIxy ligand w45x. However, this tin adduct is not stable at room temperature and rearranges cleanly to wŽ i-Pr. 2 ATIx2 SnMe 2 . This transformation can be followed by 1 H NMR spectroscopy. Unlike the five coordinate wŽMe. 2 ATIxSnMe 3 or wŽ i-Pr 2 ATIxSnMe 3 adducts, the six coordinate wŽ iPr. 2 ATIx 2 SnMe 2 has a rigid structure in solution as evident from the inequivalent isopropyl groups in the 1 H NMR spectrum. Crystalline wŽ i-Pr. 2 ATIx 2 SnMe 2 shows a monomeric structure with an octahedral tin center ŽFig. 8..
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Fig. 9. Molecular structure of wŽ i-Pr. 2 ATIxGeCl. Reprinted with permission from Dias et al. w30x Q ˚ . and angles Ž8.: Ge1]N1, 1.956Ž4.; Ge1]N9, 1997 American Chemical Society. Selected bond lengths ŽA 1.956Ž4.; Ge1]Cl1, 2.364Ž2.; N1]C2, 1.338Ž6.; N9]C8, 1.341Ž6.; N1]Ge]N9, 80.2Ž2.; Cl1]Ge]N1, 97.13Ž13.; Cl1]Ge]N9, 96.87Ž13..
Fig. 10. Molecular structure of wŽ i-Pr. 2 ATIxSnCl. A. ORTEP diagram. B. A view showing intermolecular interactions. Reprinted with permission from Dias et al. w29x Q 1996 American Chemical Society. ˚ . and angles Ž8.: Sn]N1, 2.164Ž5.; Sn]N9, 2.164Ž5.; Sn]Cl, 2.542Ž2.; Sn ??? ClA, Selected bond lengths ŽA 3.558; N1]C2, 1.332Ž7.; N9]C8, 1.331Ž7.; N1]Sn]N9, 73.9Ž2.; Cl]Sn]N1, 92.44Ž13.; Cl]Sn]N9, 94.13Ž12.; Cl]Sn ??? ClA, 172.3.
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Aminotroponiminates are also very useful in low valent group 14 chemistry. A closely related group of compounds of the type wŽ i-Pr 2 ATIxMCl Žwhere M s Ge, Sn and Pb. have been prepared by the reaction between MCl 2 and wŽ i-Pr. 2 ATIxLi in 1:1 ratio ŽEq. 4. w29,30,42x. They are yellow, crystalline solids. The germanium analog shows a monomeric structure in the solid state ŽFig. 9. w30x whereas the tin w29x and lead w45x compounds display chloride bridged zig-zag polymeric chain structures ŽFigs. 10 and 11, respectively.. Metal]Cl and metal]N distances follow the expected order of Pb ) Sn ) Ge based on the atomic radii. The smallest N1]M]N9 angle in the series was found in the lead analog. The lead adduct shows relatively symmetric metal]Cl separations in the extended structure Že.g. Sn]Cl
Fig. 11. Molecular structure of wŽ i-Pr. 2 ATIxPbCl. A. ORTEP diagram. B. A view showing intermolecu˚ . and angles Ž8.: Pb]N1, 2.265Ž13.; Pb]N9, 2.249Ž13.; Pb]Cl, lar interactions. Selected bond lengths ŽA 2.780Ž4.; Pb ??? Cl, 3.095; N1]C2, 1.32Ž2.; N9]C8, 1.33Ž2.; N1]Pb]N9, 70.3Ž4.; Cl]Pb]N1, 91.3Ž3.; Cl]Pb]N9, 95.5Ž3.; Cl]Pb ??? Cl, 170.3.
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Fig. 12. Molecular structure of wŽ i-Pr. 2 ATIxGe4wCF3 SO 3 x. Reprinted with permission from Dias et al. ˚ . and angles Ž8.: Ge]N1, 1.916Ž2.; w30x Q 1997 American Chemical Society. Selected bond lengths ŽA Ge]N9, 1.910Ž2.; Ge]O, 2.255Ž2.; N1]C2, 1.354Ž3.; N9]C8, 1.352Ž3.; N1]Ge]N9, 81.79Ž8..
˚ and 3.558 A, ˚ respectively, whereas Pb]Cl and Pb ??? Cl and Sn ??? Cl are 2.542 A ˚ and 3.095 A, ˚ respectively.. The monochloro germanium distances are 2.778 A species wŽMe. 2 ATIxGeCl, containing relatively less bulky wŽMe. 2 ATIxy ligand, has also been synthesized using GeCl 2 ? 1,4-dioxane and the lithium salt of N,N9-dimethylaminotroponiminate ligand w30x. The chloride ions of these metalŽII. adducts can be replaced by anions such as trifluoromethanesulfonate. For example, the treatment of wŽ i-Pr. 2 ATIxGeCl with CF3 SO 3 Ag in methylene chloride resulted in the quantitative formation of wŽ iPr. 2 ATIxGe4wCF3 SO 3 x ŽEq. 5. w30x. The solid state structure shows only a weak Ge ??? O interaction ŽFig. 12.. The Ge]N distances of wŽ i-Pr 2 ATIxGe4wCF3 SO 3 x are
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Fig. 13. Molecular structure of wŽ i-Pr. 2 ATIxGe4q moiety Žspace filling model.. Selected bond lengths ˚ . and angles Ž8.: Ge]N1, 1.901Ž5.; Ge]N9, 1.917Ž5.; N1]C2, 1.347Ž8.; N9]C8, 1.346Ž8.; N1]Ge]N9, ŽA 81.7Ž2..
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Fig. 14. Molecular structure of wŽ i-Pr. 2 ATIxSn4q moiety Žspace filling model.. Selected bond lengths ˚ . and angles Ž8.: Sn]N1, 2.153Ž3.; Sn]N9, 2.142Ž3.; N1]C2, 1.335Ž5.; N9]C8, 1.341Ž5.; N1]Sn]N9, ŽA 74.48Ž12..
shorter than those found in the wŽ i-Pr. 2 ATIxGeCl. The 1 H NMR data also show notable differences between the chloro and the trifluoromethlysulfonate analog w30x. A very interesting development in group 14 aminotroponiminates concerns the synthesis of two coordinate, cationic Ge and Sn species. The treatment of wŽ iPr. 2 ATIxGeCl or wŽ i-Pr. 2 ATIxSnCl with two equivalents of Žh 5-C 5 H 5 .ZrCl 3 in CH 2 Cl 2 led to the cationic species wŽ i-Pr. 2 ATIxGe4wŽh 5 -C 5 H 5 .ZrCl 2 Ž mCl. 3 ZrCl 2 Žh 5-C 5 H 5 .x or wŽ i-Pr. 2 ATIxSn4wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x ŽEq. 6. w29,30x. The Žh 5-C 5 H 5 .ZrCl 3 serves as a chloride abstracting agent in these reactions. These compounds are air and moisture sensitive yellow solids which show moderate solubility in CHCl 3 or CH 2 Cl 2 but considerably less solubility in hydrocarbon solvents such as hexane or toluene. wŽ i-Pr. 2 ATIxGe4wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x and wŽ i-Pr 2 ATIxSn4 wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl 3 .ZrCl 2 Žh 5-C 5 H 5 .x have been characterized by NMR spectroscopy and X-ray crystallography w29,30x. Solid state structures of the cationic moieties are depicted in Figs. 13 and 14. The C 7 N2 Ge unit of wŽ i-Pr. 2 ATIxGe4 q is essentially planar. The Ge]N distances are only marginally smaller than those of wŽ i-Pr. 2 ATIxGeCl. The X-ray structural features of C 7 N2 Sn moiety in wŽ iPr. 2 ATIxSnCl and wŽ i-Pr. 2 ATIxSn4 q are very similar. Solid state data of the cationic germanium and tin species indicate weak interactions between the metal center ŽGe or Sn. and chloride ions of the wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-
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Fig. 15. Molecular structure of wŽ i-Pr. 2 ATIxGe4wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5 -C 5 H 5 .x. Reprinted with permission from Dias et al. w30x Q 1997 American Chemical Society. Ge ??? Cl, 3.115, 3.123; Cl ??? Ge ??? Cl, 169.8.
C 5 H 5 .xy anion. These weak interactions lead to zig-zag chain structure in the germanium species ŽFig. 15.. Tin analog forms centrosymmetric dimers ŽFig. 16.. NMR spectroscopic data of these ionic species suggest better ion separation in solution. For example, the 119 Sn1 H4 NMR resonance of wŽ i-Pr. 2 ATIxSn4wŽh 5C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x appears at 734 ppm which is significantly different from that of the precursor wŽ i-Pr 2 ATIxSnCl Žy68 ppm.. Furthermore, the 119 Sn 1 H 4 NMR chemical shift of wŽ i-Pr . 2 ATI xSn 4wŽh 5 -C 5 H 5 .ZrCl 2 Ž m Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x is very close to the values observed for two-coordinate tin amides such as wŽMe 3 Si. 2 N x 2 Sn Ž759 ppm . w46x, ŽMe 3 Si. 2 NSnNŽSiMe 3 . C 6 H 10 ŽSiMe 3 .NSnNŽSiMe 3 . 2 Ž694 ppm. w47x or Me 2 SiwNŽBut .x2 Sn Ž638 ppm. w46x.
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Fig. 16. Molecular structure of wŽ i-Pr. 2 ATIxSn4wŽh 5 -C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5 -C 5 H 5 .x. Reprinted with permission from Dias et al. w29x Q 1996 American Chemical Society. Sn ??? Cl, 2.979, 3.123; Cl ??? Sn ??? Cl, 166.1.
The reaction between wŽMe. 2 ATIxGeCl and NaBPh 4 did not result in the expected tetraphenylborate salt of wŽMe. 2 ATIxGe4 q. Instead, it resulted in the formation of a phenyl group transfer product wŽMe. 2 ATIxGePh ? BPh 3 ŽEq. 7. w30x. Bis-ligand complex wŽMe. 2 ATIx2 Sn has been synthesized by treating wŽMe. 2 ATIxH with biswbisŽtrimethylsilyl .amidoxtinŽII. ŽEq. 8., and characterized by NMR spectroscopy and X-ray crystallography w12x. It shows a distorted pseudo-trigonal bipyramidal geometry at the tin center ŽFig. 17.. The stereochemically active lone pair on tinŽII. presumably occupies an equatorial site. Long intermolecular Sn ??? Sn ˚ . have also been observed in the solid state ŽFig. 18.. This tin contacts Ž3.769 A adduct shows fluxional behavior in solution at ambient temperatures.
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Fig. 17. Molecular structure of wŽMe. 2 ATIx 2 Sn. Reprinted with permission from Dias and Jin w12x Q ˚ . and angles Ž8.: Sn]N1, 2.301Ž2.; Sn]N9, 1996 American Chemical Society. Selected bond lengths ŽA 2.225Ž2.; Sn]N12, 2.215Ž2.; Sn]N20, 2.295Ž2.; N1]Sn]N20, 144.85Ž8.; N9]Sn]N12, 94.41Ž8.; N1]Sn]N9, 69.29Ž8.; N20]Sn]N12, 69.72Ž8..
Very little is known about the reactivity of these group 14 complexes. A ligand transfer ability of wŽMe. 2 ATIx 2 Sn has been reported w12x. It undergoes oxidative ligand transfer reactions with GaI and InCl leading to wŽMe. 2 ATIx 2 GaI and wŽMe. 2 ATIx 2 InCl, respectively.
4. Conclusion The availability of a variety of N,N9-disubstututed analogs combined with the relatively robust ligand backbone and the powerful chelating ability makes aminitroponimines ideal ligands for many applications. However, very little is known about their main group coordination chemistry w11,12,29,30,40,42,45,48,49x. Among the main group adducts of aminotroponiminates, group 14 derivatives have attracted the most interest Žwith group 13 derivatives close behind.. The neutral aminotroponimines and their alkali metal adducts serve as excellent precursors for the synthesis of both high and low valent group 14 derivatives. Resulting adducts feature mostly planar, heterobicyclic 10p-electron C 7 N2 M ring systems. Aminotroponiminates also allow the isolation of low valent, low coordinate Ge and
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Fig. 18. A view of wŽMe. 2 ATIx 2 Sn showing Sn ??? Sn intermolecular interaction.
Sn species. Structures, bonding and chemistry of such species are of significant interest. Some of the aminotroponiminato group 14 derivatives are also promising ligand transfer agents. Based on the current level of development as evident from this review, it is quite apparent that the group 14 aminotroponiminato chemistry is still in its infancy.
Acknowledgements We thank the Donors of The Petroleum Research Fund, administered by the American Chemical Society, The Robert A. Welch Foundation, and The University of Texas at Arlington for support of work carried out at UTA.
References w1x w2x w3x w4x w5x w6x
R.E. Brasen, J. Am. Chem. Soc. 82 Ž1960. 5948. W.R. Brasen, H.E. Holmquist, R.E. Benson, J. Am. Chem. Soc. 83 Ž1961. 3125. W.R. Brasen, H.E. Holmquist, R.E. Benson, J. Am. Chem. Soc. 82 Ž1960. 995. R.H. Holm, M.J. O’Connor, Prog. Inorg. Chem. 14 Ž1971. 241, and references therein. S. Imajo, K. Nakanishi, M. Roberts, S.J. Lippard, T. Nozoe, J. Am. Chem. Soc. 105 Ž1983. 2071. M. Schumann, H. Elias, Inorg. Chem. 24 Ž1985. 3187.
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w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x w27x w28x w29x w30x w31x w32x w33x w34x w35x w36x w37x w38x w39x w40x w41x w42x w43x w44x w45x w46x w47x w48x w49x
H.V. Rasika Dias et al. r Coordination Chemistry Re¨ iews 176 (1998) 67]86 H. Brunner, A. Knott, R. Benn, A. Rufinska, J. Organomet. Chem. 295 Ž1985. 211. A. Zask, N. Gonnella, K. Nakanishi, C.J. Turner, S. Imajo, T. Nozoe, Inorg. Chem. 25 Ž1986. 3400. K. Shindo, H. Wakabayashi, H. Miyamae, S. Ishikawa, T. Nozoe, Heterocycles 37 Ž1994. 943. G.M. Villacorta, S.J. Lippard, Pure Appl. Chem. 58 Ž1986. 1477. H.V.R. Dias, W. Jin, R.E. Ratcliff, Inorg. Chem. 34 Ž1995. 6100. H.V.R. Dias, W. Jin, Inorg. Chem. 35 Ž1996. 6546. F.T. Edelmann, Coord. Chem. Rev. 137 Ž1994. 403. P.B. Hitchcock, M.F. Lappert, D.-S. Liu, J. Chem. Soc., Chem. Commun. Ž1994. 1699; W. Clegg, S.J. Coles, E.K. Cope, F.S. Mair, Angew. Chem., Int. Ed. Engl. 37 Ž1998. 796, and references therein. Cambridge Structural Database, Cambridge, January, 1996, and references therein. G.M. Villacorta, C.P. Rao, S.J. Lippard, J. Am. Chem. Soc. 110 Ž1988. 3175. B.S. Jaynes, T. Ren, S. Liu, S.J. Lippard, J. Am. Chem. Soc. 114 Ž1992. 9670. B.S. Jaynes, T. Ren, A. Masschelein, S.J. Lippard, J. Am. Chem. Soc. 115 Ž1993. 5589. G.M. Villacorta, S.J. Lippard, Inorg. Chem. 27 Ž1988. 144. G.M. Villacorta, D. Gibson, I.D. Williams, S.J. Lippard, J. Am. Chem. Soc. 107 Ž1985. 6732. W.M. Davis, A. Zask, K. Nakanishi, S.J. Lippard, Inorg. Chem. 24 Ž1985. 3737. W.M. Davis, M.M. Roberts, A. Zask, K. Nakanishi, T. Nozoe, S.J. Lippard, J. Am. Chem. Soc. 107 Ž1985. 3864. M. Bartlett, G.J. Palenic, J. Chem. Soc. D. Ž1970. 416. D.R. Eaton, A.D. Josey, R.E. Benson, J. Am. Chem. Soc. 89 Ž1967. 4040. D.R. Eaton, W.R. McClellan, J.F. Weiher, Inorg. Chem. 7 Ž1968. 2040. H.V.R. Dias, W. Jin, Z. Wang, Inorg. Chem. 35 Ž1996. 6074. M.J. Scott, S.J. Lippard, J. Am. Chem. Soc. 119 Ž1997. 3411. M.J. Scott, S.J. Lippard, Inorg. Chim. Acta 263 Ž1997. 287. H.V.R. Dias, W. Jin, J. Am. Chem. Soc. 118 Ž1996. 9123. H.V.R. Dias, Z. Wang, J. Am. Chem. Soc. 119 Ž1997. 4650. T. Nozoe, M. Sata, K. Ito, K. Matsui, T. Matsuda, Proc. Jpn. Acad. 29 Ž1953. 565. I. Murata, Bull. Chem. Soc. Jpn. 32 Ž1959. 841. J.J. Drysdale, W.W. Gilbert, H.K. Sinclair, W.H. Sharkey, J. Am. Chem. Soc. 80 Ž1958. 3672. H. Nakao, Chem. Pharm. Bull. ŽTokyo. 13 Ž1965. 810. N. Soma, J. Nakazawa, T. Watanabe, Y. Sato, G. Sunagawa, Chem. Pharm. Bull. ŽTokyo. 13 Ž1965. 819. C.E. Forbes, R.H. Holm, J. Am. Chem. Soc. 90 Ž1968. 6884. K.-H. Ahn, R.B. Klassen, S.J. Lippard, Organometallics 9 Ž1990. 3178. P. Goldstein, K.N. Trueblood, Acta Crystallogr. 23 Ž1967. 148. S.G. Bott, K.-H. Ahn, S.J. Lippard, Acta Crystallogr. C45 Ž1989. 1738. E.L. Muetterties, C.M. Wright, J. Am. Chem. Soc. 86 Ž1964. 5132. D.R. Eaton, W.R. McClellan, Inorg. Chem. 6 Ž1967. 2134. H.V.R. Dias, Z. Wang, unpublished results. P.W. Roesky, Chem. Ber.rRecueil 130 Ž1997. 859. E.L. Muetterties, US Patent 3,177,232 Ž1965.. H.V.R. Dias, W. Jin, unpublished results. B. Wrackmeyer, J. Schiller, Z. Naturforsch. 47 Ž1992. 662. H. Braunschweig, P.B. Hitchcock, M.F. Lappert, L.J.-M. Pierssens, Angew. Chem., Int. Ed. Engl. 33 Ž1994. 1156. H.E. Holmquist, R.E. Benson, J. Am. Chem. Soc. 84 Ž1962. 4720. H.V.R. Dias, W. Jin, J. Chem. Crystallogr. 27 Ž1997. 353.
Aminotroponiminato complexes of silicon, germanium, tin and lead H.V. Rasika DiasU , Ziyun Wang, Wiechang Jin Department of Chemistry and Biochemistry, Box 19065, The Uni¨ ersity of Texas at Arlington, Arlington, TX 76019-0065, USA Received 2 October 1997; accepted 6 January 1998
Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. The starting materials . . . . . . . . . . . . . . . . . . . . . . . 3. Aminotroponiminato complexes of heavier group 14 elements 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Abstract Aminotroponiminates are very useful ligands in group 14 chemistry. They readily form complexes with both low and high valent group 14 metal ions. Synthesis, structural features and some chemistry of aminotroponiminato silicon, germanium, tin and lead derivatives are reviewed. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: Aminotroponiminates; Ligands
1. Introduction Aminotroponimines Že.g. 1. are an interesting group of bidentate, nitrogen donor ligands. Although aminotroponimines are structurally similar to tropolone Ž2., U
Corresponding author. Tel.: q1 817 2723813; fax: q1 817 2723808; e-mail: [email protected]
0010-8545r98r$ - see front matter Q 1998 Elsever Science S.A. All rights reserved. PII S0010-8545Ž98.00112-X
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various physical and chemical data indicate significant differences between the two types w1,2x. For example, aminotroponimines contain a highly delocalized 10-electron p-system delocalized over seven carbons and the two nitrogen atoms. In contrast, properties of tropolone are best understood in terms of a p-system involving only six electrons. Furthermore, unlike tropolone, the steric and electronic properties of aminotroponimines may be easily modulated by changing the substituent on nitrogen Žor on the ring carbon atom.. In fact, a variety of N,N9-disubstituted aminotroponimines including the closely related tropocoronands Ž3. are known w1]12x. Aminotroponiminate ligands wŽR. 2 ATIxy Ž4., which are formally derived from aminotroponimines wŽR. 2 ATIxH by deprotonation, can also be compared to amidinates Ž5. w13x and b-diketinimates Ž6. w4,14x. All three types are nitrogen based, bidentate, monoanionic and formally four-electron donor ligand systems. However, upon coordination to metal ions, aminotroponiminates form a five-membered chelate ring whereas amidinates and b-diketinimates form four and six-membered metallacycles, respectively. Furthermore, unlike the aminotroponiminate which features a delocalized 10p electron ligand backbone, amidinates and b-diketinimates have delocalized 4p and 6p electron ligand backbones. Among the three types, amidinates have been the most widely used in coordination chemistry
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whereas aminotroponiminates and b-diketinimates have received relatively less attention w4,13,14x. Studies involving aminotroponiminate ligand Žincluding those of the related tropocoronand. have mostly been limited to the first row, middle and late transition metal ions, such as manganese, iron, cobalt, nickel, copper and zinc w2,4,5,10,15]25x. Aminotroponiminates have also been used as ancillary ligands Žparticularly as an alternative to cyclopentadienyl ligand. in early transition metal chemistry w26]28x. This article will review the use of aminotroponimines in group 14 chemistry which represents an area of increasing activity recently w12,29,30x. The main focus will be on the synthesis, structures and chemistry of compounds involving silicon, germanium, tin and lead, and the related starting materials.
2. The starting materials The first report of an aminotroponimine appeared in 1953, which concerns the synthesis of amino derivative 7 using tropolone w31x. The synthesis of N,N9-dibenzoyl analog 8 starting from diazaazulene has also been reported w32x. The parent compound 1-amino-7-imino-1,3,5-cycloheptatriene Ž1. was later prepared by reacting tetrafluorocycloheptadienes Žobtained using cyclopentadiene and tetrafluoroethylene. w33x with ammonia ŽEq. 1; where R s H. w3x. Similarly, a large number of N,N9-disubstituted aminotroponimines ŽwŽR. 2 ATIxH; where R s methyl, i-butyl, benzyl, phenyl, p-tolyl, p-methoxyphenyl, p-chlorophenyl, p-dimethylaminophenyl, p-phenylazophenyl, p-nitrophenyl and several others. have also been synthesized using primary amines instead of ammonia in the above reaction scheme w2,3x. The product yields range from 30 to 80%. Relatively more convenient and high-yield synthetic routes to N,N9-disubstituted aminotroponimines Žparticularly N-alkyl derivatives. are now available w6,7,11,12,16,34]37x. These methods mainly use easily
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Fig. 1. Molecular structure of wŽ i-Pr. 2 ATIxH. Reprinted with permission from Dias et al. w11x Q 1995 ˚ . and angles Ž8.: N1]C2, 1.342Ž3.; N9]C8, American Chemical Society. Selected bond lengths ŽA 1.314Ž3.; C2]N1]C10, 126.6Ž2.; C8]N9]C13, 122.1Ž3.; N1]C2]C8, 111.7Ž2.; N9]C8]C2, 122.8Ž3..
accessible tropolone or tropolone derivatives as starting material rather than tetrafluorocycloheptadienes. Aminotroponimines are brightly colored, crystalline solids. Colors typically range from yellow to brick-red depending on the substituent on nitrogen. NMR spectroscopic data of wŽMe. 2 ATIxH are indicative of a C 2-symmetric structure in solution even at y808C w2,38x. Solid state and solution IR spectroscopic studies indicate intramolecular H-bonding in wŽMe. 2 ATIxH w1,2x. Several aminotroponimines, wŽR. 2 ATIxH where R s Me, a-methylbenzyl and i-propyl, have also been charac-
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terized by X-ray crystallography w11,38,39x. wŽMe. 2 ATIxH shows a C 2-symmetric structure with equivalent C ring ]N distances w38x. However, in the latter two structures, C ring ]N distances are not equal Žfor R s a-methylbenzyl; 1.355Ž4., ˚ w39x and for R s i-propyl; 1.342Ž3., 1.314Ž3. A ˚ w11x. indicating the 1.305Ž4. A presence of an amino nitrogen ŽC-NŽR.H. with a strong N]H interaction and an imino nitrogen center ŽC s NR.. The ORTEP diagram of wŽ i-Pr. 2 ATIxH is illustrated in Fig. 1. The seven-membered ring, two nitrogens, C10 and C13 occupy a single plane. The isopropyl groups orient in a manner that minimizes the steric interaction with the two hydrogens on C3 and C7. The alkali metal salts can be obtained conveniently by treating aminotroponimines with appropriate bases w26,40]43x. For example, wŽ i-Pr. 2 ATIxLi and wŽ i-Pr 2 ATIxK were obtained by treating wŽ i-Pr. 2 ATIxH with Bu n Li or KH ŽEq. 2.. They are very air sensitive solids. The X-ray crystal structures of wŽ iPr 2 ATIxLiŽTHF. 2 and wŽ i-Pr. 2 ATIxKŽTHF.4n are known w26,42x. The lithium derivative adopts a monomeric structure ŽFig. 2. whereas the potassium salt displays an interesting chain structure which features a p-bonded aminotroponimine ring ŽFig. 3.. The N]C ring bond distances of wŽ i-Pr. 2 ATIxLiŽTHF. 2 and wŽ iPr. 2 ATIxKŽTHF.4n are much closer to the imine N s C ring distance in the free ligand. These alkali metal derivatives serve as versatile reagents for the introduc-
Fig. 2. Molecular structure of wŽ i-Pr. 2 ATIxLiŽTHF. 2 . Reprinted with permission from Dias et al. w26x ˚ . and angles Ž8.: Li]N1, 1.995Ž6.; Li]N9, Q 1996 American Chemical Society. Selected bond lengths ŽA 1.990Ž6.; Li]O, 1.971Ž6.; Li]O2, 1.960Ž6.; N1]C2, 1.313Ž4.; N9]C8, 1.319Ž4.; N1]Li]N9, 81.5Ž2.; O1]Li]O2, 118.3Ž3.; C2]N1]Li, 114.0Ž3.; C8]N9]Li, 113.4Ž3.; N1]C2]C8, 114.4Ž3.; N9]C8]C2, 113.7Ž3..
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Fig. 3. Molecular structure of wŽ i-Pr 2 ATIxKŽTHF.4n . A. ORTEP diagram showing the atom numbering ˚ . and angles Ž8.: scheme. B. A view showing the polymeric chain structure. Selected bond lengths ŽA K1]O1, 2.690Ž4.; K1]N1, 2.717Ž4.; K1]N9, 2.741Ž4.; N1]C2, 1.312Ž6.; N9]C8, 1.321Ž6.; K2]O2, 2.657Ž4.; K2]N16, 2.712Ž4.; K2]N24, 2.730Ž4.; N16]C17, 1.304Ž6.; N24]C23, 1.307Ž6.; N1]K1]N9, 58.72Ž12.; N16]K2]N24, 58.93Ž12..
˚ . and angles Ž8.: Si]N1, Fig. 4. Molecular structure of wŽ i-Pr. 2 ATIxSiCl 3 . Selected bond lengths ŽA 1.870Ž2.; Si]N9, 1.773Ž3.; Si]Cl1, 2.1999Ž11.; Si]Cl2, 2.1069Ž11.; Si]Cl3, 2.0905Ž12.; N1]C2, 1.331Ž4.; N9]C8, 1.381Ž4.; N1]Si]N9, 85.46Ž11.; N1]Si]Cl1, 178.07Ž9.; N9]Si]Cl1, 90.19Ž5.; N9]Si]Cl2, 120.34Ž9.; N9]Si]Cl3, 123.51Ž9.; Cl2]Si]Cl3, 115.97Ž5..
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˚ . and angles Ž8.: N1]C2, Fig. 5. Molecular structure of wŽ i-Pr. 2 ATIxH 2 4q Cly. Selected bond lengths ŽA 1.343Ž2.; N9]C8, 1.342Ž2.; C2]N1]C10, 126.4Ž2.; C8]N9]C13, 125.9Ž2.; N1]C2]C8, 117.0Ž2.; N9]C8]C2, 117.2Ž2..
tion of aminotroponiminato moiety to various metal ions. Lithium salts thus far have been the reagents of choice for group 14 derivatives.
3. Aminotroponiminato complexes of heavier group 14 elements In 1964 and 1965, Muetterties reported the synthesis of several compounds of the type wŽR. 2 ATIx 3 M4 q Xy Žwhere R s mainly Me; M s Si, Ge, or Sn; X s halide, PF6 , etc.. by treating the corresponding tetrahalides with wŽR. 2 ATIxH or its lithium salt w40,44x. These crystalline compounds were characterized by elemental analysis and believed to contain octahedral metal centers. Interestingly, regardless of the metalrligand ratio, the tris-ligand complex was the only product isolated. The tetrakis analogs could not be obtained due to the unfavorable steric interactions between substituents on nitrogen atoms. wŽR. 2 ATIx3 Mq Xy compounds are stable in aqueous alkaline solutions. However, they rapidly degrade in acidic media leading to the formation of acid salts of free ligands, e.g. wŽR. 2 ATIxH 2 4 q Cly w40x. With more bulky ligands, monosubstituted derivatives can be obtained. For example, it was possible to synthesize wŽ i-Pr. 2 ATIxSiCl 3 by treating wŽ i-Pr. 2 ATIxLi with SiCl 4 w42x. It was characterized by NMR spectroscopy and X-ray crystallogra-
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˚ . and angles Ž8.: Si]N1, Fig. 6. Molecular structure of wŽMe. 2 ATIxSiMe 3 . Selected bond lengths ŽA 2.037Ž4.; Si]N9, 1.848Ž4.; Si]C13, 1.890Ž4.; Si]C13A, 1.890Ž4.; Si]C12, 1.927Ž6.; N1]C2, 1.308Ž6.; N9]C8, 1.359Ž6.; N1]Si]N9, 77.8Ž2.; N9]Si]C12, 96.8Ž2.; N1]Si]C12, 174.6Ž2.; N9]Si]C13, 118.38Ž14.; C13]Si]C13A, 120.2Ž3..
phy ŽFig. 4.. The silicon atom adopts a trigonal bipyramidal geometry with nitrogen atoms of the aminotroponiminate ligand occupying one axial wwith Si]N distance of ˚ x and one equatorial site wSi]N s 1.773Ž3. A ˚ x. wŽ i-Pr. 2 ATIxSiCl 3 decom1.870Ž2. A poses upon exposure to moisture and wŽ i-Pr. 2 ATIxH 2 4 q Cly was observed among the decomposition products w42x. In contrast to the free ligand wŽ i-Pr. 2 ATIxH, this HCl salt displays a fairly symmetric C 7 N2 moiety in the solid state ŽFig. 5. with two ˚ These distances are similar to equal C ring ]N distances of 1.343Ž2. and 1.342Ž2. A. the amine C ring ]N bond length of the free ligand. The chloride ion shows hydrogen bonding to protons on the two nitrogen atoms. The tinŽIV. compound wŽEt. 2 ATIxSnPh 3 has been prepared by the reaction of wŽEt. 2 ATIxLi with Ph 3 SnCl w41x. The corresponding germanium and silicon analogs are significantly more reactive, and have not been isolated in pure form. These compounds decompose rapidly in the presence of moisture. The reaction between Me 3 SnCl or Me 3 SiCl with wŽMe. 2 ATIxLi gives corresponding SnŽIV. or SiŽIV. derivatives ŽEq. 3. w45x. These adducts have been characterized by NMR spectroscopy and X-ray crystallography. wŽMe. 2 ATIxSiMe 3 ŽFig. 6. and wŽMe. 2 ATIxSnMe 3 ŽFig. 7. have very similar solid state structures which feature five-coordinate,
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˚ . and angles Ž8.: Sn]N1, Fig. 7. Molecular structure of wŽMe. 2 ATIxSnMe 3 . Selected bond lengths ŽA 2.281Ž4.; Sn]N9, 2.173Ž4.; Sn]C13, 2.145Ž4.; Sn]C13A, 2.145Ž4.; Sn]C12, 2.179Ž6.; N1]C2, 1.302Ž7.; N9]C8, 1.342Ž7.; N1]Sn]N9, 70.2Ž2.; N9]Sn]C12, 94.8Ž2.; N1]Sn]C12, 165.1Ž2.; N9]Sn]C13, 118.06Ž11.; C13]Sn]C13A, 117.9Ž2..
trigonal bipyramidal metal sites w45x. The tin analog shows relatively longer metal]N and metal]C distances Žas expected based on the larger atomic radius of Sn.. The N1]Sn]C12 bond angle of wŽMe. 2 ATIxSnMe 3 is approx. 108 smaller than the related N1]Si]C12 angle of wŽMe. 2 ATIxSiMe 3 . These two compounds show fluxional behavior in solution at room temperature. For example, only a single N]Me Žor M]Me. resonance was found in the 1 H NMR spectrum of wŽMe . 2 ATI xSiMe 3 or wŽMe . 2 ATI xSnMe 3 . The 1 H NMR spectrum of
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˚ . and angles Ž8.: Sn]N1, Fig. 8. Molecular structure of wŽ i-Pr. 2 ATIx2 SnMe 2 . Selected bond lengths ŽA 2.248Ž4.; Sn]N9, 2.267Ž4.; Sn]N16, 2.262Ž4.; Sn]N24, 2.243Ž4.; Sn]C31, 2.163Ž5.; Sn]C32, 2.192Ž5.; N1]Sn]N24, 158.2Ž1.; C32]Sn]C31, 106.3Ž2.; N1]Sn]N9, 71.3Ž1.; N16]Sn]N24, 71.0Ž1..
wŽMe. 2 ATIxSnMe 3 shows coupling between the tin atom and Sn] Me protons Ž 2 J s 51.8 Hz., and N] Me protons Ž 3 J s 18.4 Hz.. The wŽ i-Pr. 2 ATIxSnMe 3 has also been synthesized which contains a relatively bulky wŽ i-Pr. 2 ATIxy ligand w45x. However, this tin adduct is not stable at room temperature and rearranges cleanly to wŽ i-Pr. 2 ATIx2 SnMe 2 . This transformation can be followed by 1 H NMR spectroscopy. Unlike the five coordinate wŽMe. 2 ATIxSnMe 3 or wŽ i-Pr 2 ATIxSnMe 3 adducts, the six coordinate wŽ iPr. 2 ATIx 2 SnMe 2 has a rigid structure in solution as evident from the inequivalent isopropyl groups in the 1 H NMR spectrum. Crystalline wŽ i-Pr. 2 ATIx 2 SnMe 2 shows a monomeric structure with an octahedral tin center ŽFig. 8..
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Fig. 9. Molecular structure of wŽ i-Pr. 2 ATIxGeCl. Reprinted with permission from Dias et al. w30x Q ˚ . and angles Ž8.: Ge1]N1, 1.956Ž4.; Ge1]N9, 1997 American Chemical Society. Selected bond lengths ŽA 1.956Ž4.; Ge1]Cl1, 2.364Ž2.; N1]C2, 1.338Ž6.; N9]C8, 1.341Ž6.; N1]Ge]N9, 80.2Ž2.; Cl1]Ge]N1, 97.13Ž13.; Cl1]Ge]N9, 96.87Ž13..
Fig. 10. Molecular structure of wŽ i-Pr. 2 ATIxSnCl. A. ORTEP diagram. B. A view showing intermolecular interactions. Reprinted with permission from Dias et al. w29x Q 1996 American Chemical Society. ˚ . and angles Ž8.: Sn]N1, 2.164Ž5.; Sn]N9, 2.164Ž5.; Sn]Cl, 2.542Ž2.; Sn ??? ClA, Selected bond lengths ŽA 3.558; N1]C2, 1.332Ž7.; N9]C8, 1.331Ž7.; N1]Sn]N9, 73.9Ž2.; Cl]Sn]N1, 92.44Ž13.; Cl]Sn]N9, 94.13Ž12.; Cl]Sn ??? ClA, 172.3.
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Aminotroponiminates are also very useful in low valent group 14 chemistry. A closely related group of compounds of the type wŽ i-Pr 2 ATIxMCl Žwhere M s Ge, Sn and Pb. have been prepared by the reaction between MCl 2 and wŽ i-Pr. 2 ATIxLi in 1:1 ratio ŽEq. 4. w29,30,42x. They are yellow, crystalline solids. The germanium analog shows a monomeric structure in the solid state ŽFig. 9. w30x whereas the tin w29x and lead w45x compounds display chloride bridged zig-zag polymeric chain structures ŽFigs. 10 and 11, respectively.. Metal]Cl and metal]N distances follow the expected order of Pb ) Sn ) Ge based on the atomic radii. The smallest N1]M]N9 angle in the series was found in the lead analog. The lead adduct shows relatively symmetric metal]Cl separations in the extended structure Že.g. Sn]Cl
Fig. 11. Molecular structure of wŽ i-Pr. 2 ATIxPbCl. A. ORTEP diagram. B. A view showing intermolecu˚ . and angles Ž8.: Pb]N1, 2.265Ž13.; Pb]N9, 2.249Ž13.; Pb]Cl, lar interactions. Selected bond lengths ŽA 2.780Ž4.; Pb ??? Cl, 3.095; N1]C2, 1.32Ž2.; N9]C8, 1.33Ž2.; N1]Pb]N9, 70.3Ž4.; Cl]Pb]N1, 91.3Ž3.; Cl]Pb]N9, 95.5Ž3.; Cl]Pb ??? Cl, 170.3.
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Fig. 12. Molecular structure of wŽ i-Pr. 2 ATIxGe4wCF3 SO 3 x. Reprinted with permission from Dias et al. ˚ . and angles Ž8.: Ge]N1, 1.916Ž2.; w30x Q 1997 American Chemical Society. Selected bond lengths ŽA Ge]N9, 1.910Ž2.; Ge]O, 2.255Ž2.; N1]C2, 1.354Ž3.; N9]C8, 1.352Ž3.; N1]Ge]N9, 81.79Ž8..
˚ and 3.558 A, ˚ respectively, whereas Pb]Cl and Pb ??? Cl and Sn ??? Cl are 2.542 A ˚ and 3.095 A, ˚ respectively.. The monochloro germanium distances are 2.778 A species wŽMe. 2 ATIxGeCl, containing relatively less bulky wŽMe. 2 ATIxy ligand, has also been synthesized using GeCl 2 ? 1,4-dioxane and the lithium salt of N,N9-dimethylaminotroponiminate ligand w30x. The chloride ions of these metalŽII. adducts can be replaced by anions such as trifluoromethanesulfonate. For example, the treatment of wŽ i-Pr. 2 ATIxGeCl with CF3 SO 3 Ag in methylene chloride resulted in the quantitative formation of wŽ iPr. 2 ATIxGe4wCF3 SO 3 x ŽEq. 5. w30x. The solid state structure shows only a weak Ge ??? O interaction ŽFig. 12.. The Ge]N distances of wŽ i-Pr 2 ATIxGe4wCF3 SO 3 x are
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Fig. 13. Molecular structure of wŽ i-Pr. 2 ATIxGe4q moiety Žspace filling model.. Selected bond lengths ˚ . and angles Ž8.: Ge]N1, 1.901Ž5.; Ge]N9, 1.917Ž5.; N1]C2, 1.347Ž8.; N9]C8, 1.346Ž8.; N1]Ge]N9, ŽA 81.7Ž2..
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Fig. 14. Molecular structure of wŽ i-Pr. 2 ATIxSn4q moiety Žspace filling model.. Selected bond lengths ˚ . and angles Ž8.: Sn]N1, 2.153Ž3.; Sn]N9, 2.142Ž3.; N1]C2, 1.335Ž5.; N9]C8, 1.341Ž5.; N1]Sn]N9, ŽA 74.48Ž12..
shorter than those found in the wŽ i-Pr. 2 ATIxGeCl. The 1 H NMR data also show notable differences between the chloro and the trifluoromethlysulfonate analog w30x. A very interesting development in group 14 aminotroponiminates concerns the synthesis of two coordinate, cationic Ge and Sn species. The treatment of wŽ iPr. 2 ATIxGeCl or wŽ i-Pr. 2 ATIxSnCl with two equivalents of Žh 5-C 5 H 5 .ZrCl 3 in CH 2 Cl 2 led to the cationic species wŽ i-Pr. 2 ATIxGe4wŽh 5 -C 5 H 5 .ZrCl 2 Ž mCl. 3 ZrCl 2 Žh 5-C 5 H 5 .x or wŽ i-Pr. 2 ATIxSn4wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x ŽEq. 6. w29,30x. The Žh 5-C 5 H 5 .ZrCl 3 serves as a chloride abstracting agent in these reactions. These compounds are air and moisture sensitive yellow solids which show moderate solubility in CHCl 3 or CH 2 Cl 2 but considerably less solubility in hydrocarbon solvents such as hexane or toluene. wŽ i-Pr. 2 ATIxGe4wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x and wŽ i-Pr 2 ATIxSn4 wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl 3 .ZrCl 2 Žh 5-C 5 H 5 .x have been characterized by NMR spectroscopy and X-ray crystallography w29,30x. Solid state structures of the cationic moieties are depicted in Figs. 13 and 14. The C 7 N2 Ge unit of wŽ i-Pr. 2 ATIxGe4 q is essentially planar. The Ge]N distances are only marginally smaller than those of wŽ i-Pr. 2 ATIxGeCl. The X-ray structural features of C 7 N2 Sn moiety in wŽ iPr. 2 ATIxSnCl and wŽ i-Pr. 2 ATIxSn4 q are very similar. Solid state data of the cationic germanium and tin species indicate weak interactions between the metal center ŽGe or Sn. and chloride ions of the wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-
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Fig. 15. Molecular structure of wŽ i-Pr. 2 ATIxGe4wŽh 5-C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5 -C 5 H 5 .x. Reprinted with permission from Dias et al. w30x Q 1997 American Chemical Society. Ge ??? Cl, 3.115, 3.123; Cl ??? Ge ??? Cl, 169.8.
C 5 H 5 .xy anion. These weak interactions lead to zig-zag chain structure in the germanium species ŽFig. 15.. Tin analog forms centrosymmetric dimers ŽFig. 16.. NMR spectroscopic data of these ionic species suggest better ion separation in solution. For example, the 119 Sn1 H4 NMR resonance of wŽ i-Pr. 2 ATIxSn4wŽh 5C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x appears at 734 ppm which is significantly different from that of the precursor wŽ i-Pr 2 ATIxSnCl Žy68 ppm.. Furthermore, the 119 Sn 1 H 4 NMR chemical shift of wŽ i-Pr . 2 ATI xSn 4wŽh 5 -C 5 H 5 .ZrCl 2 Ž m Cl. 3 ZrCl 2 Žh 5-C 5 H 5 .x is very close to the values observed for two-coordinate tin amides such as wŽMe 3 Si. 2 N x 2 Sn Ž759 ppm . w46x, ŽMe 3 Si. 2 NSnNŽSiMe 3 . C 6 H 10 ŽSiMe 3 .NSnNŽSiMe 3 . 2 Ž694 ppm. w47x or Me 2 SiwNŽBut .x2 Sn Ž638 ppm. w46x.
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Fig. 16. Molecular structure of wŽ i-Pr. 2 ATIxSn4wŽh 5 -C 5 H 5 .ZrCl 2 Ž m-Cl. 3 ZrCl 2 Žh 5 -C 5 H 5 .x. Reprinted with permission from Dias et al. w29x Q 1996 American Chemical Society. Sn ??? Cl, 2.979, 3.123; Cl ??? Sn ??? Cl, 166.1.
The reaction between wŽMe. 2 ATIxGeCl and NaBPh 4 did not result in the expected tetraphenylborate salt of wŽMe. 2 ATIxGe4 q. Instead, it resulted in the formation of a phenyl group transfer product wŽMe. 2 ATIxGePh ? BPh 3 ŽEq. 7. w30x. Bis-ligand complex wŽMe. 2 ATIx2 Sn has been synthesized by treating wŽMe. 2 ATIxH with biswbisŽtrimethylsilyl .amidoxtinŽII. ŽEq. 8., and characterized by NMR spectroscopy and X-ray crystallography w12x. It shows a distorted pseudo-trigonal bipyramidal geometry at the tin center ŽFig. 17.. The stereochemically active lone pair on tinŽII. presumably occupies an equatorial site. Long intermolecular Sn ??? Sn ˚ . have also been observed in the solid state ŽFig. 18.. This tin contacts Ž3.769 A adduct shows fluxional behavior in solution at ambient temperatures.
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Fig. 17. Molecular structure of wŽMe. 2 ATIx 2 Sn. Reprinted with permission from Dias and Jin w12x Q ˚ . and angles Ž8.: Sn]N1, 2.301Ž2.; Sn]N9, 1996 American Chemical Society. Selected bond lengths ŽA 2.225Ž2.; Sn]N12, 2.215Ž2.; Sn]N20, 2.295Ž2.; N1]Sn]N20, 144.85Ž8.; N9]Sn]N12, 94.41Ž8.; N1]Sn]N9, 69.29Ž8.; N20]Sn]N12, 69.72Ž8..
Very little is known about the reactivity of these group 14 complexes. A ligand transfer ability of wŽMe. 2 ATIx 2 Sn has been reported w12x. It undergoes oxidative ligand transfer reactions with GaI and InCl leading to wŽMe. 2 ATIx 2 GaI and wŽMe. 2 ATIx 2 InCl, respectively.
4. Conclusion The availability of a variety of N,N9-disubstututed analogs combined with the relatively robust ligand backbone and the powerful chelating ability makes aminitroponimines ideal ligands for many applications. However, very little is known about their main group coordination chemistry w11,12,29,30,40,42,45,48,49x. Among the main group adducts of aminotroponiminates, group 14 derivatives have attracted the most interest Žwith group 13 derivatives close behind.. The neutral aminotroponimines and their alkali metal adducts serve as excellent precursors for the synthesis of both high and low valent group 14 derivatives. Resulting adducts feature mostly planar, heterobicyclic 10p-electron C 7 N2 M ring systems. Aminotroponiminates also allow the isolation of low valent, low coordinate Ge and
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Fig. 18. A view of wŽMe. 2 ATIx 2 Sn showing Sn ??? Sn intermolecular interaction.
Sn species. Structures, bonding and chemistry of such species are of significant interest. Some of the aminotroponiminato group 14 derivatives are also promising ligand transfer agents. Based on the current level of development as evident from this review, it is quite apparent that the group 14 aminotroponiminato chemistry is still in its infancy.
Acknowledgements We thank the Donors of The Petroleum Research Fund, administered by the American Chemical Society, The Robert A. Welch Foundation, and The University of Texas at Arlington for support of work carried out at UTA.
References w1x w2x w3x w4x w5x w6x
R.E. Brasen, J. Am. Chem. Soc. 82 Ž1960. 5948. W.R. Brasen, H.E. Holmquist, R.E. Benson, J. Am. Chem. Soc. 83 Ž1961. 3125. W.R. Brasen, H.E. Holmquist, R.E. Benson, J. Am. Chem. Soc. 82 Ž1960. 995. R.H. Holm, M.J. O’Connor, Prog. Inorg. Chem. 14 Ž1971. 241, and references therein. S. Imajo, K. Nakanishi, M. Roberts, S.J. Lippard, T. Nozoe, J. Am. Chem. Soc. 105 Ž1983. 2071. M. Schumann, H. Elias, Inorg. Chem. 24 Ž1985. 3187.
86
w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x w27x w28x w29x w30x w31x w32x w33x w34x w35x w36x w37x w38x w39x w40x w41x w42x w43x w44x w45x w46x w47x w48x w49x
H.V. Rasika Dias et al. r Coordination Chemistry Re¨ iews 176 (1998) 67]86 H. Brunner, A. Knott, R. Benn, A. Rufinska, J. Organomet. Chem. 295 Ž1985. 211. A. Zask, N. Gonnella, K. Nakanishi, C.J. Turner, S. Imajo, T. Nozoe, Inorg. Chem. 25 Ž1986. 3400. K. Shindo, H. Wakabayashi, H. Miyamae, S. Ishikawa, T. Nozoe, Heterocycles 37 Ž1994. 943. G.M. Villacorta, S.J. Lippard, Pure Appl. Chem. 58 Ž1986. 1477. H.V.R. Dias, W. Jin, R.E. Ratcliff, Inorg. Chem. 34 Ž1995. 6100. H.V.R. Dias, W. Jin, Inorg. Chem. 35 Ž1996. 6546. F.T. Edelmann, Coord. Chem. Rev. 137 Ž1994. 403. P.B. Hitchcock, M.F. Lappert, D.-S. Liu, J. Chem. Soc., Chem. Commun. Ž1994. 1699; W. Clegg, S.J. Coles, E.K. Cope, F.S. Mair, Angew. Chem., Int. Ed. Engl. 37 Ž1998. 796, and references therein. Cambridge Structural Database, Cambridge, January, 1996, and references therein. G.M. Villacorta, C.P. Rao, S.J. Lippard, J. Am. Chem. Soc. 110 Ž1988. 3175. B.S. Jaynes, T. Ren, S. Liu, S.J. Lippard, J. Am. Chem. Soc. 114 Ž1992. 9670. B.S. Jaynes, T. Ren, A. Masschelein, S.J. Lippard, J. Am. Chem. Soc. 115 Ž1993. 5589. G.M. Villacorta, S.J. Lippard, Inorg. Chem. 27 Ž1988. 144. G.M. Villacorta, D. Gibson, I.D. Williams, S.J. Lippard, J. Am. Chem. Soc. 107 Ž1985. 6732. W.M. Davis, A. Zask, K. Nakanishi, S.J. Lippard, Inorg. Chem. 24 Ž1985. 3737. W.M. Davis, M.M. Roberts, A. Zask, K. Nakanishi, T. Nozoe, S.J. Lippard, J. Am. Chem. Soc. 107 Ž1985. 3864. M. Bartlett, G.J. Palenic, J. Chem. Soc. D. Ž1970. 416. D.R. Eaton, A.D. Josey, R.E. Benson, J. Am. Chem. Soc. 89 Ž1967. 4040. D.R. Eaton, W.R. McClellan, J.F. Weiher, Inorg. Chem. 7 Ž1968. 2040. H.V.R. Dias, W. Jin, Z. Wang, Inorg. Chem. 35 Ž1996. 6074. M.J. Scott, S.J. Lippard, J. Am. Chem. Soc. 119 Ž1997. 3411. M.J. Scott, S.J. Lippard, Inorg. Chim. Acta 263 Ž1997. 287. H.V.R. Dias, W. Jin, J. Am. Chem. Soc. 118 Ž1996. 9123. H.V.R. Dias, Z. Wang, J. Am. Chem. Soc. 119 Ž1997. 4650. T. Nozoe, M. Sata, K. Ito, K. Matsui, T. Matsuda, Proc. Jpn. Acad. 29 Ž1953. 565. I. Murata, Bull. Chem. Soc. Jpn. 32 Ž1959. 841. J.J. Drysdale, W.W. Gilbert, H.K. Sinclair, W.H. Sharkey, J. Am. Chem. Soc. 80 Ž1958. 3672. H. Nakao, Chem. Pharm. Bull. ŽTokyo. 13 Ž1965. 810. N. Soma, J. Nakazawa, T. Watanabe, Y. Sato, G. Sunagawa, Chem. Pharm. Bull. ŽTokyo. 13 Ž1965. 819. C.E. Forbes, R.H. Holm, J. Am. Chem. Soc. 90 Ž1968. 6884. K.-H. Ahn, R.B. Klassen, S.J. Lippard, Organometallics 9 Ž1990. 3178. P. Goldstein, K.N. Trueblood, Acta Crystallogr. 23 Ž1967. 148. S.G. Bott, K.-H. Ahn, S.J. Lippard, Acta Crystallogr. C45 Ž1989. 1738. E.L. Muetterties, C.M. Wright, J. Am. Chem. Soc. 86 Ž1964. 5132. D.R. Eaton, W.R. McClellan, Inorg. Chem. 6 Ž1967. 2134. H.V.R. Dias, Z. Wang, unpublished results. P.W. Roesky, Chem. Ber.rRecueil 130 Ž1997. 859. E.L. Muetterties, US Patent 3,177,232 Ž1965.. H.V.R. Dias, W. Jin, unpublished results. B. Wrackmeyer, J. Schiller, Z. Naturforsch. 47 Ž1992. 662. H. Braunschweig, P.B. Hitchcock, M.F. Lappert, L.J.-M. Pierssens, Angew. Chem., Int. Ed. Engl. 33 Ž1994. 1156. H.E. Holmquist, R.E. Benson, J. Am. Chem. Soc. 84 Ž1962. 4720. H.V.R. Dias, W. Jin, J. Chem. Crystallogr. 27 Ž1997. 353.