Study of a Dichromium o-Oxyphenyl Compound
Inorganic Chemistry, Vol. 17, No. 7, 1978 2021 Contribution from the Department of Chemistry, Texas A & M University, College Station, Texas 77843
Rational Preparation and Structural Study of a Dichromium o-Oxyphenyl Compound: The Shortest Metal-to-Metal Bond Yet Observed F. ALBERT COTTON* and S T E P H E N KOCH Received January 17, I978 The compound Li6Cr2(o-C6H40)4Br2.6(czH5)20 has been prepared from Cr2(02CCH3)4and Li2C6H40in diethyl ether by a method in which the formation of the extremely insoluble Li4Cr2(o-C6H40)4.4(C2H,)20 is avoided through addition of the stoichiometric quantity of LiBr to the reaction mixture. Soxhlet extraction then allows isolation of the crystalline compound, whose structure has been determined by X-ray crystallography to reveal the presence of C T ~ ( O - C ~ Hanions ~O)~ loosely associated with bromide ions. The Cr-Cr distance is 1,830 (4) 8, which is shorter at the 99% confidence level than the shortest previously observed Cr-Cr bond, 1.847 (1) A, in Cr2[2,6-(Me0)2C6H4]4.The compound crystallizes in space group C2/c with a = 21.637 (5) A, b = 14.734 (3) A, c = 19.039 (4) A, fl = 73.33 (2)O, and 2 = 4. The structure was refined to R1 = 0.072 and R z = 0.090 using 1548 reflections with I > 3u(Z).
Introduction Our recent that compounds of the type I contain exceedingly short quadruple bonds between the metal atoms caused us to take an interest in the compounds, reported a few years earlier by Hein and Tille,4 containing the related ligands I1 and 111.
Ia:' Ib:' IC:' 1d3
M=M'=Cr,X=H M = M' = Mo, X = H M = M' = Cr, X = OMe M = ~ rM,' = MO, X = H
I1
The only chromium(I1) compound containing ligand I1 that has been reported is Cr2(C6H40Me)4.This was described5q6 by Hein and Tille as a yellow, pyrophoric, microcrystalline solid, insoluble in all common solvents but slowly hydrolyzed by alcohol. The preparation and general properties of this rather intractable compound were subsequently confirmed by Sneeden and Zeiss.' We have also prepared this compound and find it to be as previously described. Efforts to obtain crystals suitable for structure analysis have so far failed. Hein and Tille5 also reported that from the reaction of lithium 2-lithiophenoxide, Li2C6H40,with CrC13 in 3.3/ 1 mol ratio they isolated a yellow-orange product of composition Li2Cr(C6H40)2.LiBr.3(C2HS)20. No comment or explanatory statement about the presence of bromine in the product was made. We presume that it was unintentionally introduced into the reaction mixture during the preparation of the Li2C6H40 reagent from o-C6H40Hby lithiation with butyllithium, the latter having carried over bromine from its own precursor, butyl bromide. Whether Hein and Tille considered this so obvious as to require no comment we do not know. Contamination of the butyllithium reagent with LiBr does appear to have been a normal occurrence at that time. Experimental Section Preparation. All manipulations were conducted in an atmosphere of dry nitrogen. Lithium 2-lithiophenoxide, Li2C6H40,was prepared by the reaction of o-bromophenol with n-butyllithium (1.6 M in hexane).s LiBr was generated by the reaction of 1,2-dibromoethane with n-butyllithium. Anhydrous chromium(I1) acetate, Cr2(02CCH3), (2.45 g, 7.25 mmol), was allowed to react with Li2C6H40(29 mmol) and LiBr (1.26
g, 14.5 mmol) in 200 mL of diethyl ether. After being stirred for 12 h at room temperature, the reaction mixture was filtered. Repeated extraction of the solid residue in a Soxhlet apparatus with diethyl ether gave a yellow-orange crystalline product in about 25% yield. Determination of Structure. A crystal with approximate dimensions 0.4 X 0.3 X 0.4 mm was placed in a capillary with epoxy cement. This was mounted on a Syntex Pi automated diffractometer. Preliminary examination showed that the crystal belonged to the monoclinic system with cell dimensions a = 21.637 ( 5 ) A, b = 14.734 (3) A, c = 19.039 (4) A, p = 73.33 (2)O, and V = 5814 (3) A3. The volume is consistent with the presence of four formula units, with mol wt = 1106.42 and dcalcd = 1.26 g ~ m - ~Systematic . absences were those for the space group C2/c (No. 15). Using molybdenum Ka radiation (A 0.710730 A) with a graphite monochromator in the incident beam, data were collected a t 22 f 2 O C using the 8-28 scan technique. Scans ranged from l o above to 1' below the K a l , K a z doublet, scan speeds ranged from 4.0' to 24.0°/min, and the scan to background time ratio was 0.5. The intensities of three standard reflections were monitored a t regular intervals and showed only acceptably small random variations. The data in the range 0 < 28 I45O were reduced9 to lFJ2values and those 1548 reflections with values greater than 3~(lF,1~), where u is based on counting statistics, were retained as observed and used to solve and refine the ~ t r u c t u r e .Absorption ~ corrections were omitted (w = 19.1 cm-I). The chromium and bromine atoms were located using the Patterson function and all remaining atoms were then found by a sequence of refinement and difference map steps. In the final stages of refinement the Cr, Br, and phenoxy atoms were treated anisotropically while all others were treated isotropically. The final discrepancy indices were R1= 0.072 and R 2 = 0.090. The highest peak in a final difference map had an amplitude of 1.O e/A3 and was so located as to have no evident structural significance. The vibrational amplitudes of several atoms in the ether molecules are quite large but we could not see any evidence that these atoms were actually disordered. A list of the observed and calculated structure factors is available as supplementary material.
Results Synthetic. The reaction of dichromium tetraacetate with lithium 2-lithiophenoxide in diethyl ether gives an insoluble yellow powder which we presume to be Li4Cr2(2-oxyphenyl)4-nEt20. The addition of LiBr to the reaction mixture increases the solubility of the product to the extent that it can be recrystallized by Soxhlet extraction. The yellow-orange crystalline product, Li6Cr2(2-oxyphenyl)4Br2(Et20)6, is apparently the same product previously reported by Hein and Tille.5 Presumably, the addition of LiBr serves to disrupt some intermolecular bonding to give the more soluble material in which the individual dinuclear units are kept apart by the bromide ions. Structural. The atomic positional coordinates and thermal vibrational parameters are listed in Table I. Interatomic distances and angles involving Cr or Li atoms are listed in Table 11, while distances and angles within the ligands are given in Table 111. The structure is illustrated in Figures 1
0020-1669/78/1317-2021$01.00/00 1978 American Chemical Society
2022 Inorganic Chemistry, Vol. 17, No. 7, 1978
F. Albert Cotton and Stephen Koch
Table I. Positional and Thermal Parameters and Their Estimated Standard Deviationsa ~~~~~
~
Atom Br(1) Cr(1) O(11) O(21)
Y
PI1
Pzz
0.00383 (5) 0.00192 (5) 0.0020 (2) 0.0027 (3)
0.0062 (1) 0.0051 (1) 0.0057 (6) 0.0051 (6)
Z
0.18513 (9) -0.0142 (1) 0.2360 (1) 0.1938 (2) 0.1586 (4) 0.1703 (6) 0.2888 (4) 0.1094 (6)
0.4862 (1) 0.4910 (1) 0.5833 (4) 0.5414 (5)
PI2
P33
023
P'3
0.00521 (7) -0.0018 (1) 0.00224 (7) -0.0003 (2) 0.0023 (3) 0.0001 (7) 0.0031 (3) 0.0009 (7)
-0.00304 (9) -0.0008 (2) -0.0006 (1) 0.0001 (2) -0.0005 (4) -0.0006 (8) -0.0017 (5) 0.0012 (8)
Atom
X
Y
Z
B, A'
Atom
X
Y
Z
B, A'
O(31) O(41)
0.0176 (6) 0.3712 (6)
0.1042 (9) -0.0631 (10)
0.5641 (7) 0.4284 (7)
9.6 (4) 9.9 (4)
O(51)
0.1982 (6)
0.0048 (9)
0.6945 (7)
9.7 (4)
X
Y
Z
0.1928 (6) 0.1481 (7) 0.0947 (7) 0.0872 (8) 0.1344 (8) 0.3148 (7) 0.3258 (6) 0.3280 (6) 0.3652 (7) 0.4015 (7) 0.4010 (8) 0.3623 (7)
0.3212 (9) 0.2459 (11) 0.2482 (12) 0.3243 (12) 0.3956 (11) 0.1088 (11) 0.2554 (9) 0.1632 (10) 0.1105 (2) 0.1621 (12) 0.2572 (12) 0.3039 (11)
0.6085 (7) 0.6281 (7) 0.6922 (7) 0.7389 (8) 0.7231 (9) 0.3421 (8) 0.5650 (7) 0.5732 (7) 0.6094 (8) 0.6461 (8) 0.6432 (8) 0.6040 (7)
Atom
a
~~
X
022
PI1
P33
0.0026 (4) 0.0047 (9) 0.0023 (4) 0.0075 (10) 0.0023 (4) 0.0088 (11) 0.0043 (5) 0.0098 (13) 0.0037 (5) 0.0056 (10) 0.0028 (4) 0.0085 (11) 0.0014 (4) 0.0033 (8) 0.0019 (4) 0.0060 (9) 0.0026 (4) 0.0083 (11) 0.0026 (4) 0.0097 (12) 0.0031 (5) 0.0087 (12) 0.0027 (4) 0.0070 (10) B, A' Atom
PI2
0.0025 (5) 0.0004 (10) 0.0022 (5) 0.0004 (11) 0.0021 (5) -0.0005 (12) 0.0021 (5) 0.0046 (15) 0.0061 (7) -0.0008 (13) 0.0024 (5) 0.0040 (12) 0.0029 (5) 0.0009 (9) 0.0033 (5) 0.0001 (10) 0.0042 (6) 0.0018 (3) 0.0036 (5) 0.0012 (3) 0.0040 (6) 0.0002 (13) 0.0033 (5) -0.0005 (12)
p13
P23
-0.0018 (7) -0.0011 (7) -0.0005 (8) -0.0017 (8) -0.0032 (10) -0.0010 (7) 0.0003 (7) -0.0016 (7) -0.0030 (7) -0.0022 (8) -0.0014 (9) -0.0029 (7)
0.001 (1) 0.001 (1) 0.002 (1) -0.003 (1) -0.000 (2) -0.002 (1) 0.000 (1) -0.004 (1) 0.000 (1) -0.001 (2) -0.002 (2) -0.002 (1) B, A2
Atom
X
Y
Z
Y
Z
C(31) C(32) C(33) C(34) C(41) C(42)
0.002 (1) 0.012 (2) -0.032 (1) -0.015 (1) 0.441 (1) 0.461 (1)
0.013 (2) -0.010 (2) 0.160 (2) 0.246 (2) -0.048 (2) 0.035 (2)
0.596 (2) 0.658 (2) 0.579 (2) 0.549 (1) 0.408 (2) 0.414 (1)
15.9 (10) 17.3 (11) 15.5 (10) 13.8 (9) 16.5 (10) 12.3 (8)
C(43) C(44) C(51) C(52) C(53) C(54)
0.356 (2) 0.348 (2) 0.201 (1) 0.217 (1) 0.192 (1) 0.252 (2)
-0.154 (3) -0.202 (3) 0.040 (2) 0.132 (2) -0.093 (2) -0.128 (3)
0.381 (2) 0.443 (2) 0.762 (1) 0.765 (1) 0.694 (2) 0.657 (2)
22.4 (15) 26.2 (18) 13.0 (8) 10.3 (6) 16.4 (10) 19.3 (12)
Li(1) Li(2)
0.111 (1) 0.205 (1)
0.132 (2) 0.070 (2)
0.525 (2) 0.605 (1)
6.0 (7) 5.4 (7)
Li(3)
0.308 (1)
0.035 (2)
0.459 (2)
6.4 (7)
The form of the anisotropic thermal parameter is exp[-f$,,h*
X
+ P 2 2 k 2+ p3J2 + P12hk+ p13hl+ p,,kl)].
Table 11. Interatomic Distances (A) and Bond Angles (deg) Involving Metal Atoms CrCr' CI-BI Cr-O(l1) Cr-0(21) Cr-C(l1) CrC(21) Li(1)-O(l1) Li(2)-0(11) Li(2)-0(21) Li(3)-0(21)
1.830 (4) 3.266 (2) 2.079 (7) 2.098 (8) 2.082 (12) 2.074 (12) 1.80 (2) 1.90 (2) 1.95 (2) 1.87 (2)
Cr-Li(1) Cr-Li(2) Cr-Li(3) Br-Li(1) Br-Li(2) Br-Li(3) Li(l)-0(31) Li(2)-0(51) Li(3)-0(41)
CICI-BI CrCr-O(ll) CrC1-0(21) Cr-Cr-C(l1) C r C r C ( 2 1) O(ll)-Cr-0(21) C(ll)-CrC(21) C(ll)-C1-0(21) C(ll)Cr-O(ll) C(21)Cr-O(11) C(21)
