5596
Figure 1. A schematic view of the skeleton of the [Ni2I(nas3)2]+ cation. The values of distances (A) and angles (deg) are: Ni-I, 2.994 (3); Ni-N, 2.31 (2); Ni-As, 2.349 (5) (av); I-Ni-N, 178.0 (5); I-Ni-As, 95.2 ( 1 ) (av); As-Ni-As, 119.0 (2) (av).
IJ e f f
100
150
200
250
300 K
Figure 2. Variation with temperature of the magnetic moment befffor Ni(1) in [Ni2Br(nas3)2]B(CsH& ( 0 ) and in [Ni2I(nas3)2]B(CsHs)4 (0).
and Z = 4. The intensity data were collected on a Philips automatic diffractometer, with Mo Kcu radiation, using the limitation 6 O I28 I 20'. The 1588 reflections having I 1 3 4 Z ) were considered observed and were used in the solution and refinement of the structure. The structure was solved by a Patterson synthesis followed by Fourier syntheses. The structure consists of dimeric [NizI(nas3)z] cations and B(C6H5)4- anions with interposed tetrahydrofuran solvent molecules. Each nickel atom is bonded to the four donor atoms of the nas3 ligand and to the shared bridging iodine atom. The iodine atom lies on a center of symmetry, while the boron atom of the B(C6H5)4- ion is situated on a binary axis. The chromophore can be considered to be formed by two trigonal-bipyramids sharing a vertex. A schematic view of the [NizI(nas3)2]+ cation with the most significant bonds and angles is shown in Figure 1. The long axial distances Ni-I, 2.994 (4) A, and Ni-N, 2.31 (2) A, are noteworthy. This complex is one of the few binuclear metal complexes having a linear metal-iodine-metal bridge.4 The magnetic behavior of these compounds has been studied over the range 84-290 K; in neither case is the susceptibility found to be field dependent. Diamagnetic corrections were calculated using Pascal's constant^.^ The pefffor each nickel atom vs. T is reported in Figure 2. The iodo derivative has a peffof ca. 0.7 BM in the temperature range of the measurements. The peffof the bromo derivative has a constant value, 0.75 BM, from 84 to ca. 200 K; beyond this temperature the moment increases slowly up to the value of 1 BM a t 288 K. The two complexes can therefore be considered essentially diamagnetic a t low temperature, the residual value of peffbeing attributable to paramagnetic impurities. In fact the magnetic susceptibility varies slightly from sample to sample but the plots obtained in each case are essentially the same as those in Figure 2 which shows the lowest perfobtained. This magnetic behavior suggests that a n antiferromagnetic exchange takes place between the two nickel atoms involving the halide bridge. In terms of the Kramers-Anderson approach,6 the antiferromagnetic coupling between the d 9 nickel cations can arise from the intermixing of a dZ2or+
Journal of the American Chemical Society
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97:19
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bital on each nickel atom with a P, orbital on the bridging halide ion via the P,-al' pathway. The best pair of values of g and J for the bromo derivative, obtained fitting the Bleaney-Bowers equation to the experimental data by the use of a direct method to minimize the error-square sum over the observed susceptibilities,7 is 2.00 and -232 crn-', respectively. The increasing peffvalue with temperature above 200 K for the bromo derivative is interpreted as due to the population of a thermally accessible triplet level. The fact that a different behavior is observed for the iodo derivative up to 290 K suggests that the exchange coupling between nickel atoms bridged by the iodine ion is greater than that in the bromide. This is in agreement with the general finding that the transfer integral and hence the exchange coupling constant 14 becomes larger as the electronegativity of the anion decreases. The diffuse reflectance spectrum of the bromo derivative shows a band a t 1 X lo4 cm-] and a shoulder a t 1.31 X lo4 cm-I, while that of the iodo derivative has a band a t 1.31 X lo4 cm-]. Both dimers show also intense bands above 1.8 X lo4 cm-I attributable to charge transfer transitions. The shape and position of the first electronic absorption band are similar to those of typical five-coordinate nickel(1) complexes such as [NiX(nas3)j2 and also [NiI(np3)], which has been found, by X-ray analysis,8 to have trigonal bipyramidal geometry. The similarity of the low energy bands is reasonable because the relatively weak spin-spin coupling in the dimers is expected to be small compared to the energies of the d-d transitions (ca. 1.1 X lo4 cm-I).
References and Notes (1) (2) (3) (4) (5) (6)
(7) (8)
L. Sacconi, I. Bertini, and F. Mani. horg. Chem., 7 , 1417 (1968). L. Sacconi, P. Dapporto, and P. Stoppioni. unpublished results.
L. Sacconi, C. Mealli, and D. Gatteschi, horg. Chem., 13, 1985 (1974). D. Baumann. H. Endres, H. J. Keller, and J. Weiss, J. Chem. Soc., Chem. Commun., 853 (1973). B. N. Figgis and J. Lewis, Mod. Coord. Chem., 400 (1960). R. L. Martin in "New Pathways in Inorganic Chemistry", E. A. V. Ebsworth. A. G. Maddock, and A. G. Sharpe. Ed., Cambridge University Ress, New York, N.Y., 1968, p 175, Chapter 9, and references therein. C. Mealli. S.Midollini, and L. Sacconi, Inorg. Chem., in press. L. Sacconi, C. A. Ghilardi. C. Mealli, and F. Zanobini. Inorg. Chem.. 14, 1380 (1975).
L. Sacconi,* P. Dapporto, P. Stoppioni Instituto di Chimica Generale ed Inorganica dell'Uniuersith di Firenze and Laboratorio CNR Florence, Italy Receiued May 15, 1975
Photochemistry of P,y-Unsaturated Ketones. Specific Reactivity of S2, SI, and TIin 2,2,7,7-Tetramethyl-3,5-cycloheptadienoneand Correlations with INDO Calculations' Sir: Direct irradiation a t >300 nm of 3,5-cycloheptadienone (la)2-4 and the 2-methyl analog l b 2 gave only the hexatrienes 2 by decarbonylation of the lowest singlet excited state, while triplet sensitization of l a led only to the cyclobutene 3,3i.e. +.ST = 0. The bridged ketone 4, where the diene moiety is planar or near planar, gave decarbonylation and cyclobutene formation when irradiated a t >300 nm, while triplet sensitization gave the [ 1,2]-sigmatropic acyl shifts (1,2-SAS) product 5; again @ST = 0.6 Similar reactions were reported for the trienone 6' and the tropone-cyclopentadiene adduct
September 17, 1975
7.839
5597 Table I. Calcuia'ted and Experimental Energies, Oscillator Strengths, and Configurational Compositions in Compounds Ia and 8
E , nm Calcd Exptl
State
294 229
S, S,
8? la
278-29OC 204-225'
0.0024 0.32 0.058
Composition, %b n,nCO*
n,$,
83
10
$,,nco* $ , d P
T,
403 326 208
S, S,
291 237
T,
13
T, T,
17 15
290e 24 3e
0.0015 0.31 0.090
LL2&b4
95 63 84
-& 9; isy
$,&,
5
539d
T,
8
f Calcd Exptla
0.0068 0.10
80
13 7 57
5
7
10
78
32
52
6
31 95
66
16
572e
13 15
81
10
17
321 220
77
34
46
58
35
a Reference 25. b Only those configurations contributing more than 5% to a state are shown. C In 95% ethanol, ref 4, 26. d Reference 3. e In methanol. see also ref 10.
Scheme I
la,R=H b,R=Me
5
2
3
7
6
/
4
-
I
13
12
11
For all the 3,5-cycloheptadienones above a major singletderived reaction is decarbonylation. In contrast Paquette et a1.I0 reported that 2,2,7,7-tetramethyl-3,5-cycloheptadienone (8) gave only one major photoproduct, the 1,2-SAS product 9, on both direct and triplet sensitized photolysis, indicating that @ST > 0 in this case, although formation of 9 could not be quenched by naphthalene or piperylene.I0 However, we now report results in sharp disagreement with the earlier worki0 which indicate that the photochemistry of 8 is more diverse and indeed more interesting than previously indicated. Irradiation of 8 at 300 nm through Pyrex in a variety of solvents (ethers, alcohols, hydrocarbons) gives in about equal yield two major products: the decarbonylation product, 2,7-dimethyl-2,4,6-octatriene (10) in addition to the 1,2-SAS product 9. The identification of 10 is made from its spectral properties,ll by comparison with an authentic sample,I2 and by an independent synthesis using the Wittig reaction of 2-butenedial13 and 2 equiv of isopropyltriphenylphosphonium iodide.14 Triplet sensitization of 8 using acetone or benzophenone gives only 9, in agreement with the earlier report.I0 Formation of 9, but not 10, is quenched inefficiently by 1,3-cyclohexadiene ( E T = 52-53 kcal/mol, kq7 0.1 M-I) and more efficiently by the low energy triplet quencher llI5 ( E T = 35-42 kcal/mol, k q T = 38.5 M-' in CHC13, 29.5 M-' in CH3CN), suggesting a very low triplet energy for 8 ( E T 45-53 kcal/mol) and a triplet lifetime of
