588 C0(CVj3- and Cr2+,16 Other evidence in favor of stabilization of a-bromoalkyl free radicals by cyclic bridging has come from studies on the photobromination of alkyl bromides17.l8 and from esr measurements on such radi~a1s.l~The slightly lower reactivity of (17) W. Thaler, J . Am. Chem. SOC.,85, 2607 (1963). (18) P. S . Skell, K. I. Tuleen, and P. D. Readio, ibid., 85, 2849 (1963). (19) P. I. Abell and L. H. Piette, ibid., 84, 916 (1962).
C O ( C N ) ~ toward ~ICH2CHzCHzBrthan toward ICHzCH2CH21is consistent with the expectation that such stabilization should be more effective for I than for Br. While considered less likely than the explanation just offered, an alternative possibility, namely that the enhanced reactivities of the dihalides result, in part at least, from an inductive effect transmitted through the carbon chain, cannot be ruled out.
Transition Metal Polypyrazolylborates Containing Other Ligands S. Trofimenko
Contribution No, 1480 f r o m the Central Research Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898. Received August 22, 1968 Abstract: Tripyrazolylborate ions, RB(p&-, a new class of uninegative tridentate ligands of CSvsymmetry, react with group VIb hexacarbonyls yielding anions of structure RB(PZ)~M(CO)~isolable as tetraethylammonium salts.
These anions react with allylic halides yielding directly the thermally very stable n-allyl compounds RB(p&M(CO)2nGH5, including those with diverse substituents on carbon and boron. The carbonyl stretching frequencies in these compounds reflect the electron-releasing or -withdrawing nature of the substituents. Reaction with alkyl and RB(pz)3halides and with protonic acids yields the air-sensitive, seven-coordinate species RB(~z)~M(CO)~R' M(C0)3H, respectively. The reaction of B(~Z)~MO(CO)~with C,H7BF4leads to B(~Z)~MO(CO)~-T-C~H, which, as well as substituted analogs, is also obtained by the reaction of RB(p&- with C,H,Mo(CO)J. The compound B(pz)4Mo(C0)2-n-C&ISresults from the reaction of B ( ~ Z ) with ~ - C5H5Mo(C0)3C1.In B(~Z)~MO(CO)~-T-C~H, and B ( ~ Z ) ~ M O ( C O ) ~ - ~ the - CB(pz)4~ H ~ , ion seems to act as a bidentate ligand. RB(pz),Mn(CO),, RB(pz)sPd-nC ~ H Sand , RB(PZ),R~(C~HH~)~ have resulted from the nucleophilic reaction of R B ( ~ Z )with ~ - the appropriate halogenated precursor. A number of these molecules are stereochemically nonrigid.
T
he recent discovery' of poly( 1-pyrazolyl)borate anions, a novel class of chelating ligands, has opened up a new subarea of organometallic chemistry. Numerous four- and six-coordinate transition metal compounds, based on bis- and tris(pyrazolyl)borates, have been prepared.2 The latter, of essentially D3d symmetry, are exceedingly stable, many being more stable than their metallocene analogs. The tris( 1-pyrazolyl)borate ligand, a uninegative tridentate of CaV(or local C3")symmetry, may be used to prepare sandwich-like compounds, where the transition metal is surrounded in essentially octahedral fashion by the termini of the two tridentate ligands. Moreover, it is possible to synthesize half-sandwiches3 where three coordination sites on the metal are taken up by the R B ( ~ z ligand )~ (pz = 1-pyrazolyl group) and the remaining coordination sites by other groups. Since these other ligands may be varied in manifold ways, the scope of half-sandwich chemistry based on the R B ( ~ z )ligand ~ ought to considerably exceed that of the corresponding full sandwiches-just as in the case of the C6Hj ligand or the more recently discovered (1) S . Trofimenko, J . Am. Chem. Soc., 88, 1842 (1966). (2) S. Trofimenko, ibid., 89, 3170 (1967). ( 3 ) The term sandwich has been traditionally used to denote transition
metal compounds a-bonded to aromatic ligands. While RB(pz)s is not
a x-bonding aromatic ligand, when one regards its chemistry, it is convenient to visualize RB(pz)a as replacing a GH5 ligand in full or half-
sandwiches. It is in this context that the term "sandwich" is used.
carbollide4 and related This is borne out by some recently reported preliminary resulks Several features of the RB(pz)3 ligand make it very attractive. (1) The salts, e.g., K R B ( ~ Z )are ~ , easy to prepare and are indefinitely stable to storage in air; furthermore, the cation may be also varied, as needed, for solubility purposes. (2) When C-substituted pyrazoles are used to synthesize the tris( 1-pyrazoly1)borate ligand, the CSV symmetry is maintained,g and the substituted R B ( ~ z )ligand ~ may thus be used to probe the symmetry of other groups bonded to the metal. (3) U p to ten substituents (including R) may be put on the R B ( ~ z ligand )~ enabling one to influence the environment of the transition metal electronically or sterically, yet maintain the basic chelate geometry intact. Such substitution was found to alter dramatically, for instance, the magnetic properties'" of some transition metal poly( 1-pyrazoly1)borates. This paper reports the synthesis and properties of (4) M. F. Hawthorne, D. C. Young, T . D. Andrews, D. V. Howe, R. L. Pilling, A. D. Pitts, M. Reintjes, L. F. Warren, Jr., and P. A. Wegner, ibid., 90, 879 (1968). ( 5 ) W. R. Hertler, F. IUanberg, and E. L. Muetterties, Inorg. Chem., 6, 1696 (1967). (6) W. H. Knoth, J . Am. Chem. Soc., 89, 3342 (1967). (7) D. E. Hyatt, J. L. Little, J. T. Moran, F. R. Scholer, and L. J. Todd, ibid., 89, 3342 (1967). (8) S . Trofimenko, ibid., 89, 3904 (1967). (9) S. Trofimenko, ibid., 89, 6288 (1967). (10) J. P. Jesson, S . Trofimenko, and D. R. Eaton, ibid., 89, 3158 (1967).
Journal of the American Chemical Society / 91:3 / January 29, 1969
"*
589
Y
Table I. Compounds of Structure EtaN R-p(N-NfM(CO)3+
3
M
R
Cr
H
Cr
pza H H pza H H pza H
Cr Mo Mo Mo W W W ~~
X
Mp, "C
H H
281-282decc 294-296 dec 341-342dec 283-285dec 318-320dec 392-394dec 301-302dec 325-327dec 380-385dec
CHI H H CH3 H H CH3 ~~
x
- C % -
- % % -
-N,Z-
yield
Calcd
Found
Calcd
Found
Calcd
Found
86 28b 87 31b 31* 73b 80 47b 62*
50.0 50.6 55.4 45.8 46.8 51.4 39.3 40.8 44.8
49.6 50.1 55.6 45.3 47.3 51.3 38.9 40.8 44.8
6.26 5.87 7.46 5.74 5.43 6.92 4.92 4.73 6.04
6.37 5.84 7.39 5.82 5.33 6.79 4.82 4.32 6.24
20.5 23.1 17.4 18.7 21.4 16.2 16.1 18.6 14.1
20.7 23.1 17.6 18.7 21.5 16.0 16.1 18.6 14.1
8.2 7.9
8.2 7.9
Ir,d cm-' 1900, 1762 1891, 1748 1897, 1761 1898, 1765 1891, 1751 1888, 1755 1881, 1744
~~
pz = 1-pyrazolyl. * Yields of material recrystallized from acetonitrile with substantial solubility losses. (yellow $ orange). In acetonitrile. a
some transition metal compounds containing one tris( 1pyrazoly1)borate ligand along with other groups, particularly those involving an M-C bond. Results and Discussion A. The RB(pz),M(C0)3- Anions. Tris( l-pyrazoly1)borate salts (I) react, on heating in dimethyl sulfoxide or dimethylformamide, with group VIb hexacarbonyls to yield the tricarbonyl anions of structure 11, isolable conveniently as tetraethylammonium salts (Table I), which are more stable toward oxidation than the alkali metal salts. The oxidative stability of the tetraethylammonium salts depends on the transition metal, decreasing in the order Mo > W > Cr (in agreement with the polarographic oxidation potentials), and the type of substitution on the ligand. For instance, carbonyl anions derived from B ( ~ Z ) ~are - more stable than those derived from H B ( ~ z ) ~ - .On the other hand, the ligand HB(3,5-Me?p~)~yields carbonyl anions of, essentially, indefinite stability in the solid state. This effect is probably steric as indicated by StewartBriegleb models which show severely restricted access to the transition metal as well as to boron. All these carbonyl anions exhibit two strong CO stretch bands in the infrared at lower frequencies than the C5H5M(C0)3- salts" but very close to the values reported in the same solvent for the carbollide system, [B9C2HllMo(C0),I2-. The CO frequencies for a given ligand decrease in the order Cr > Mo > W. Methyl substitution on the ligand leads to a lowering of the frequency by about 10 cm-l, which is consistent with the expected increase of the M-C bond order. Substitution at the boron (Le., B(pz), us. H B ( ~ z ) has ~ ) no noticeable effect. The nmr spectra of compounds I1 are in good agreement with the proposed structure and indicate equivalence of the three coordinated pyrazolyl groups. The spectra also aided in assigning the 3- and 5-hydrogens in the ligand. In analyzing nmr spectra of poly(pyrazolyl)borates, the doublets due to the 3- and 5-pyrazole hydrogens have often been difficult to assign. Even though their (11) This comparison is a rough one, since the infrared spectra of CsHsM(C0)3- salts were measured in mineral oil mulls: R. B. King, Inorg. Chem., 4 , 1518 (1965). Moreover, the CO frequencies were
cation dependent.
-Oo,%Calcd Found
Crystals are thermochromic
coupling constants were different (usually about 1.8 and 2.2 cps), thus making it possible to distinguish the two peaks, they could not be assigned unequivocally. A tentative assignment was made2 by studying changes of the chemical shifts of a given M B ( ~ Z salt ) ~ as a function of solvent and changes in the chemical shifts in a given solvent as a function of cation size. It was concluded that the doublet (J = -2.2) most affected by changes in solvent and/or size of the cation is that due to the 3-H. Consequently, the doublet with J = --1.8 was assigned to the 5-H which one would expect to be relatively unaffected by environmental changes at the periphery of the ion. The nmr spectrum of Et4NB(pz)4Mo(C0)3 corroborated this assignment. This spectrum consists of doublets at T 1.88 (J = 2.2 cps), 2.07 (J = 1.5 cps),
2.25 (J = 1.6 cps), and 2.50 (J = 2.3 cps) and triplets at T 3.37 (J = 1.9 cps) and 3.84 (J = 2.1 cps) in 1 : 1 : 3 : 3:1:3 ratio. The assignment of the T 3.37 and 3.84 peaks to protons b and e, respectively, follows from their multiplicity and intensity. Again, it is apparent that the environments of the c and d hydrogens are much more similar than those of the a and f hydrogens. Consequently, the peaks with approximately identical coupling constants and similar chemical shifts would be due to protons c and d while those with widely different chemical shifts would be a and f. On this basis (plus intensity data) the T 1.88, 2.07, 2.25, and 2.50 peaks are assigned to the a, c, d, and f hydrogens, respectively. This assignment agrees with the earlier studies. All the data indicate that the doublet with the smaller coupling constant is due to the 5-H (hydrogen nearest to boron). B. The Reactions of the R B ( ~ z ) ~ M ( C OAnions. )~ With Allylic Halides. The salts I1 react with allylic halides, in many cases even below room temperature, to yield the yellow r-allyl derivatives 111, of extraordinary thermal, oxidative, and hydrolytic stability. One Trofmenko J Transition Metal Polypyrazolylborates
590
exception was the salt I1 derived from HB(3,5-Me2pz)-, with the structures assigned ; moreover, they indicate which failed to react under comparable conditions with these compounds to be fluxional molecules. allyl bromide. It is noteworthy that while irradiation The infrared spectra of compounds III were also is necessary to obtain, e.g., the analogous C5H5Moinformative. First, in all cases there were only two CO (CO)?-n-C3Hj compound (which is not conveniently bands (measured in cyclohexane at high resolution), in available by thermolysis of the a-allyl precursor), the contrast to fourI4 found in C5H5Mo(C0)z-r-C3H5. RB(~Z),MO(CO)~-~-C compounds ~H~ form spontaConsidering the likely explanation that this phenomneously at low temperatures or are obtained on e n 0 n * 5 ~is’ ~due to presence of rotamers of the r-allyl moderate heating of the components. A variety of group, the conclusion can be drawn that the n-allyl derivatives of the general structure I11 has been prepared group in compounds I11 is stereochemically rigid on starting with the appropriately substituted pyrazolylthe nmr time scale, l7 which is unsurprising in view of the borate and allylic halide components. Usually, best steric restrictiveness of the ligand and invariance of nmr yields were obtained by the reaction of pure tetrasignals of the r-allyl group in compounds 111 with ethylammonium salts of I1 with allylic halides. Howchanging temperature. ever, isolation of the salts is not necessary; they may The positions of carbonyl bands in the parent combe prepared in situ and immediately treated with allylic pounds H B ( ~ Z ) ~ M O ( C O ) ~ - R and - C ~ B(pz),Mo(CO),H~ halides. Reasonable yields of some substituted comn-C3H5 were essentially identical, at 1958 and 1874 pounds of structure I11 can be obtained by running cm-’, and thus again lower by about 10 cm-’ than in the the three reactions BHa- + HB(pz’)sH B ( ~ Z ’ ) ~ M O -corresponding CbH5 analogs. However, substitution (CO),- -+ HB(~Z’)~MO(CO)Z-T-C~H, consecutively in either on the HB(pz),- or the vallyl ligand shifted the same reaction vessel. In general, the corresponding these bands. By and large, the shifts were small and in molybdenum and tungsten compounds are quite the direction predicted by simple inductive effects. For similar, except that the tungsten compounds are formed instance, in the n-cyclopentenyl and n-cyclohexenyl more slowly. compounds the vco was lowered by 7 and 11 cm-l, The behavior of chromium salts in the above reaction respectively (for the higher frequency band), but a sequence was intriguing. It is noteworthy that, in boron-bonded butyl group caused a vco lowering of only contrast to abundant examples of CIHjM(C0)2-n3 cm-l. On the other hand, an increase in the vco by allyl (or substituted n-allyl) compounds, where M = 3-5 cm-’ was observed when the 4 position in the Mo or W, there are few unambiguous examples of the HB(pz), ligand was substituted with electron-withanalogous chromium compounds. l 2 When the ion drawing groups such as C1, CN, or NOz and also in the B(pz),Cr(CO)d- was treated with allyl bromide at room compound HB(p~)~Mo(CO)~-7r-2-phenylallyl. Howtemperature, it reacted very rapidly, evolving 1 equiv ever, a bromine atom on the central carbon of the rof carbon monoxide. A red solid (rather than yellow allyl groups increased vco (12 cm-’) more substantially. as with Mo or W compounds) was obtained in 8 8 % The tungsten compounds had lower vco by about 10 yield ; analysis, infrared spectrum, and nmr spectrum13 cm-’, as has been observed in the anions 11. agreed with the B(pz)4Cr(C0)2-n-C3H5 structure. This C. With Alkyl Halides. The anions I1 reacted at red solid dissolves in halocarbons with decarbonylation slightly elevated temperatures and even at room temto form an unidentified violet species, presumably perature with lower alkyl halides to yield derivatives containing Cr(II1). On the other hand, its solutions in containing seven-coordinate molybdenum or tungsten benzene are stable at or below room temperature; on with an alkyl--metal bond. These compounds, IV, heating to about 50” gas is evolved and a crystalline are red, in contrast to the yellow n-allyl derivatives and greenish precipitate is obtained. This material has a the C5H5M(CO)3R analogs. They are diamagnetic well-defined infrared spectrum containing carbonyl and give high-resolution nmr spectra in excellent agreebands at 1900 (s) and 1760 (VS) cm-l which might ment with the proposed structure. For instance, in suggest the presence of B(pz),Cr(CO),- ion, but it has H B ( ~ Z ) , M O ( C O ) ~ Call~ the H ~ carbon-bonded hydrogens not been identified, On boiling in chloroform more of the ligand were discernible as were the methylene and gas is evolved and a yellow solid precipitates. This methyl peaks centered at ‘T 6.38 and 8.60, respectively. has been assigned the structure [B(pz)&CrCl since The 3-hydrogens of the ligand appeared in a 2 : 1 it is water soluble and on treatment with hexafluoropattern indicating a stereochemically rigid structure, phosphate ion yields [ B ( ~ Z ) ~ ] ~ C identical ~ P F ~ , in all with the ethyl group being probably equidistant from respects with authentic material obtained from K B ( ~ z ) ~ two of the pyrazolyl groups. The nmr spectrum of the and Cr3+ ion and also isomorphous with [B(PZ)~]~- methyl analog was similar. The infrared spectra were FePF6 which was prepared from B ( ~ Z ) ~and - Fe3+. fairly complex containing three strong carbonyl bands The exact course of these oxidative transformations has along with shoulders, the strong bands being at lower not been investigated. frequencies than those of CaH5M(CO)3Rcompounds. l8 The nmr spectra of compounds I11 (Table 11) agree Less stable than the n-allyl compounds 111, these alkyl derivatives decomposed on storage in several days but much more rapidly in solution.
-
(12) E. 0. Fischer, H. P. Kogler, and P. Kuzel, Chem. Ber., 93, 3006 (1960); E. 0. Fischer and F. Ulm, ibid., 94, 2413 (1961); Z. Naturjorsch., 15b, 59 (1960). (13) The nmr spectrum was hazy and similar to those of the stereochemically nonrigid Mo and W compounds. The limiting high-tem-
perature spectrum could not be obtained because the chromium compound was thermally unstablc. On the other hand, the presence of the ligand and T- rather than a-allyl group could be ascertained by integration. Stereochemical nonrigidity in RB(pz)3 compounds will be the subject of a separate publication.
Journal of the American ChemicaI Society
91:3
(14) (15) (16) (17)
R. B. King, Inorg. Chem., 5, 2242 (1966). A. Davison and W. C. Rode, ibid., 6 , 2124 (1967). J. W. Faller and M. J. Incorvia, ibid., 7 , 840 (1968). The alternative explanation-rotation of the a-allyl group at a
rate that is fast on the infrared time scale--can be safely disregarded. (18) T. S . Piper and G. Wilkinson, J . Inorg. N u c l . Chem., 3, 104 (1956).
1 January 29, 1969
591
%
x
I N -
* f-k
2-67
a
I
p:
I 4
Trofimenko
/ Trunsition Metal Polypyruzolylborares
592
D. With Acids. Acidification of the salts I11 The nmr spectrum shows three types of pyrazolyl yielded readily the free acids V as yellow solids soluble groups in 2 : l : l ratio along with a sharp cycloin polar organic solvents and alkali but insoluble in heptatrienyl spike at 7 4.76 and thus is equivocal since water. The M-H resonance for H B ( ~ z ) ~ M ( C O ) ~ Hit can be reconciled with either structure A or B. In appeared at 7 13.3 for Mo and 7 13.5 for W, and for A the normal disposition of the two carbonyl groups HB(3,5-Me2pz)jM(CO)3H at 7 13.5 for Mo and 7 12.4 and the C;H, ligand with regard to the tridentate for W ; thus it was at lower field than in the correB(pz)? ligand of local CJ, symmetry would make two sponding C6HjM(C0)3H compounds. l7 These acids of the coordinating pyrazolyl groups identical and different from the third.2J At the same time the were air sensitive and slowly decomposed on storage. fluxional nature of the allylic r-C,H7 ring would make Of the two, the Mo compound was significantly less all its protons identical on the nmr time scale. In stable, particularly in solution. It should be noted that the dimeric species, analogous to [C5HjMo(CO)& structure B the bidentate B ( ~ Zligand, )~ being puckered, (which forms very readily and is often the only isolable would possess a pseudoaxial and a pseudoequatorial product in the C j H j system), was not observed. pyrazolyl group (the uncoordinated ones), each in a different magnetic environment, and all the hydrogens E. The Compounds R B ( ~ Z ) J M ( C O ) ~ Cand ~ H ~RB( ~ Z ) ~ ~ L I ( C O ) ~ CThe ~ H ~reaction . of K B ( ~ Z ) ~ M O ( C O )in ~ the presumably fluxional C,H, ligand would be identical, H it11 CiHiBF gave VI, a red solid, mp 224225 O dec, of the composition B(pz) 1 M ~ ( C 0 ) ~ C i Hwhich 7, was quite Of the two structures, however, B is favored for stable toward air and moisture. The same compound compound VI on the basis of the following considerawas also obtained by the reactioii of KB(pz)%with tions. (1) Of the two structures, B involves, according to molecular models, less steric hindrance. In particC,HiMo(C0)J, as were the analogous derivatives HB(~Z)~MO(CO)?C;H~ and HB(3,5-Mezpz)3Mo(CO),ular, the Fdct that HB(3,5-Mezpz)3Mo(CO)2CiH7 is C,H, by the reaction of C7H7Mo(CO)jIwith the approformed and possesses properties very similar to those of priate poly(pyrazoly1)borate ions. All these comB(pz)4Mo(CO),CiH7 speaks very strongly against pounds presented the same structural problem as that structure A since the H B ( ~ , ~ - M ~ $ ligand, z)~ when encountered in CjHjM~(CO):C,H719,2il namely, that tridentate in H B ( ~ , ~ - M ~ ? ~ Z ) M O ( Ccannot O ) ~ - , even accommodate an unsubstituted r-allyl group. (2) The utilization of all electrons avhilable from the ligands would yield a formally eight-coordinate molybdenum high-resolution infrared spectrum has only two CO with a noninert gas configuration (20 electrons). peaks (in cyclohexane) in contrastz4to C>HSMo(CO)pWhile eight-coordination in some molybdenum comC7H7. pounds has been established,?’ it is by no means A compound analogous to those above, B(pz)4Mocommon; it is confined to small ligands and would be (C0)2C5Hs(VIT), was synthesized by the reaction of unlikely with ligands as bulky as B(pz)l and C7H7. kB(pz), with CjH5Mo(C0)3Cl. Here, too, similar Moreover, no reason can be envisaged for a 20-electron considerations apply with regard to the structure, and configuration. Assuniing, then, an inert-gas conof the two II priori possible structures with an inert-gas figuration and a formal seven-coordination92 for configuration, C and D, the latter is favored based on considerations similar to those above. First, there is B ( ~ Z ) ~ M O ( C O ) ~two C ~ Hstructures ~, can be arrived .it satisfying these conditions: structure A contains a normal, tridentate B ( ~ Zligand )~ and an allylic r-C,H; ligand, while B has a bidentate B(pz)4 group and a probably nonplanar T-C,H, ring containing an unconiplexed double bond. A structure with a planar r-CIH7 ring and a monodentate B(pz)* ligand is considered unlikely.
A
co D
no unequivocal precedent for an allylic T - C ~ H ; . ~The ~ sharp C5H5spike occurs at 7 4.72, a normal position for a r-CjHj ring. The steric argument is valid here as in the r-C,H7 analog. While the summation of these
(19) R. B. I
