J. CHEM. SOC. DALTON TRANS. 1991 41Structural and Magnetic Characterisation of the Compound[NEt4][Cr2(EtO),{H2B(pz),}2(NCS)2] (pz = 1 -pyrazolyl),* aBinuclear Complex of Chromium(ll1) with Three BridgingEthoxo GroupsAlessandro Bencini, Massimo Di Vaira and Fabrizio ManiDipartimento di Chimica, Universita ' di Firenze, Via Maragliano 77, 50 144 Firenze, ItalyThe crystal and molecular structure of [NEt,] [Cr,(EtO),{H,B(pz),},(NCS),] [H,B(pz), = dihydro-bis(1 -pyrazolyl)borate] has been determined by single-crystal X-ray diffraction methods. The crystalsare monoclinic, space group P2,, with a = 9.561 (9), b = 15.826(4), c = 12.979(3) p\, p = 93.06(8)"and Z = 2. The complex anion consists of two chromium(il1) ions in chemically identical andapproximately octahedral environments.The metal ions are bridged by three ethoxo groups. The co-ordination around each metal is completed by two N atoms of a H,B(pz), ligand and by the N atomof a NCS anion. The temperature dependence of the magnetic susceptibility of the compound hasbeen determined in the range 8-280 K. The exchange interaction between the two chromiurn(1li) ionswas found to be antiferromagnetic with J = 87.0(2) cm-' and the exchange Hamiltonian is H = JS,-S,.We have recently reported novel anionic complexes ofchromium(r1) and manganese(Ir), formed by the dihydro-genobis( 1-pyrazoly1)borate anion, H,B(pz), -, having thegeneral formula A[MX(H,B(pz),},] (M = Cr" or Mn"; A =NEt,, PPh, or AsPh,; X = halide or pseudohalide).' Thechromium(1r) derivatives, at variance with those of manganese-(II), are quickly oxidised even in the presence of traces of oxygen.In one case the oxidation of a chromium(i1) derivative gave apure and reproducible crystalline compound of chromium(rr1)which was formulated as [NEt,][Cr,(EtO),(H,B(pz),),-(NCS),] on the basis of the elemental analysis.'Now we report on the crystal and molecular structure andmagnetic properties of this dinuclear, ethoxo-bridged, complexof chromium(rr1). Chromium(1rr) derivatives with co-ordinatedalkoxides are not u n ~ o m m o n ~ ' ~ but mixed-ligand complexeswith three bridging ethoxides have not been structurallycharacterised, to the best of our knowledge.ExperimentalDetails on the procedures and the materials employed in thesynthesis of the complex [NEt,][Cr,(EtO),{H,B(pz),),-(NCS),] have been reported elsewhere, together with elementalanalysis, spectrophotometric, conductivity and room-tempera-ture magnetic data.' Crystals suitable for X-ray analysis wereobtained by slow evaporation of a MeCN-BuOH solution ofthe complex at room temperature.Magnetic Measurements.-The magnetic susceptibility of amicrocrystalline sample of the complex was measured in thetemperature range 8-280 K by the Faraday method usingan AZTEC Informatique DMS-5 automated magnetometerequipped with an Oxford Instruments CF2000 cryostat.Diamagnetic corrections were carried out according to ref.4.* Tetraethylammonium tri-p-ethoxo-bis{ thiocyanato[dihydrobis( 1 -pyrazolyl)borato]chromate(~~~)).Supplementary data available: see Instructions for Authors, J.Chem.SOC., Dalton Trans., 1991, Issue 1, pp. xviii-xxii.X-Ray Structure Determination.-Crystal data. C2,H5 1-B2Cr,N1 , 0 3 S 2 , M = 779.5, monoclinic, space group P2, (no.4), a = 9.561(9), b = 15.826(4), c = 12.979(3) A, p = 93.06(8)",U = 1961(2) A3 [by least-squares refinement of 20 auto-matically centred reflections (12 < 0 < 13"), h = 0.710 69 A],Z = 2, D, = 1.320 g cmP3, approximate dimensions 0.13x 0.4 x 0.5 mm, F(OO0) = 820, p(Mo-Kcr) = 6.8 cm-'.Data collection and processing. The red crystals were ofregular prismatic form. Data were collected at roomtemperature with an Enraf-Nonius CAD4 diffractometer, in the0-20 mode with (1.10 + 0.35 tan0)"scan width, variable scanspeed 3-8" min-' and graphite-monochromated Mo-Kxradiation.Of 3755 reflections (k h, k, I; 2.5 < 8 < 25"), 3376were unique and 2724 having I > 341) were used in thefinal refinement. The intensities of three standard reflectionsmeasured periodically were constant throughout data collec-tion. An empirical absorption correction' was applied atisotropic convergence, after the structure had been solved(maximum, minimum corrections 1.16, 0.76). Among the low-angle reflections no correction for secondary extinction wasdeemed necessary. The principal computer programs used in thecrystallographic calculations are listed in refs. 5-7.Structure analysis and rejinement. The structure was solvedby direct methods. A series of Fourier and Fourier differencemaps yielded the positions of all non-hydrogen atoms.During the full-matrix least-squares refinement the y co-ordinate of one Cr atom was not allowed to refine, due tothe polar nature of the space group.The position of themethyl carbons of two ethyl groups in the anion and one inthe cation were found to be affected by disorder, each ofthese groups being distributed between two orientations. Inthe final least-squares cycles, in which the populationparameters for the alternative positions were allowed torefine, the sites of each pair were assigned an overallisotropic thermal parameter and were constrained to form aunique C-C bond length with the respective methylenecarbon atom. All the other non-hydrogen atoms wereassigned anisotropic thermal parameters. Hydrogen atomswere introduced in calculated positions (C-H 0.96, B-H1.06 A) each with a thermal parameter ca.20% larger thanthe Uequiv. of the corresponding C or B atom. The H atomsof the disordered ethyl groups were assigned fractiona42 J. CHEM. SOC. DALTON TRANS. 1991Table 1 Atomic coordinates for [NEt,][Cr,(EtO),{H,B(pz),),(NCS)2] aX0.26 14( 1)0.3208( 1)0.2 1 49( 4)0.38 80( 3)0.2836(6)0.4281(6)0.1594(6)0.0844(7)0.0763(7)0.3796(7)0.3329(8)0.1959(8)0.2406( 8)0.4922(7)0.4982(8)0.2438(8)0.3515(9)0.7884(8)- 0.0294(9)- 0.1096( 1 1)- 0.0420( 10)0.51 15(9)0.5526( 10)0.4366( 10)0.0584(9)0.01 63( 1 1)Y0.39800.4083( 1)0.1403(2)0.1909(2)0.49 17(4)0.3 638( 3)0.3588(4)0.4505( 5)0.4841 (6)0.45 3 9( 5 )0.48 3 3(6)0.4653( 5 )0.5044(6)0.47 14( 5)0.5 106(6)0.29 lO(6)0.3 104(6)0.3074(6)0.478 5( 8)0.5275(8)0.5298(8)0.4802(7)0.5248(7)0.5255(7)0.4821 (7)0.5340(8)Z0.709 3 ( 1)0.5 100( 1)0.8945(3)0.2668(3)0.61 53(4)0.633 l(4)0.5797(4)0.7650(6)0.86 1 O(6)0.82 66( 5 )0.91 63(5)0.3978(5)0.3 1 34(5)0.4598(5)0.3672(6)0.78 74( 6)0.4200( 6)0.1027(7)0.7090(8)0.768 1 (1 0)0.8638( 10)0.8207(7)0.9103(7)0.9678(7)0.4034(7)0.3 192(8)X0.1294( 12)0.6066(9)0.6870( 10)0.61 44( 10)0.2310(9)0.3659(9)0.2815(17)0.2953( 16)0.4974(9)0.5923( 15)0.641 3( 10)0.1045(10)0.01 12( 18)0.7563( 13)0.7208( 13)0.6633(13)0.5353( 13)0.9094( 12)1.0454( 12)0.8323( 13)0.865 1 (18)0.8008(27)0.1 83 5( 12)0.389 1 (1 3)- 0.0330( 12)Y0.5449( 9)0.49 5 5( 7)0.5496(8)0.5 5 8 5 (7)0.2294( 7)0.2590(6)0.5 77 1 (8)0.6324( 9)0.2828 (7)0.27 1 O( 13)0.2939( 16)0.2759(7)0.2674( 1 3)0.2586( 16)0.3758(9)0.3437( 11)0.2522(9)0.2965( 10)0.253 1 (10)0.3008 ( 1 2)0.3513(9)0.2944( 12)0.4369( 12)0.4571( 11)0.4937(10)Z0.2660( 8)0.5166(7)0.461 8(9)0.3678(8)0.8330(7)0.3 5 5 3 (8)0.6250( 11)0.5484( 12)0.6358(9)0.7195(11)0.63 16( 18)0.565 l(9)0.6020( 14)0.4779( 13)0.1 810( 13)0.28 3 8( 9)0.0791(11)0.0338( 11)0.1400( 1 1)0.1 764( 12)- 0.0001( 12)- 0.0832( 13)0.03 19(25)0.9521(9)0.2748(9)a In this and the following table estimated standard deviations on the least significant digit(s) are in parentheses.Atoms N( 11) and C(21)-C(282)belong to the cation, all the other atoms to the complex anion. They coordinate of Cr(1) was not refined, due to the polar nature of the space group.p.p. = 0.428(3). p.p. = Atomic position affected by disorder, site with population parameter (p.p.) 0.583(3). p.p. = 0.417(3). p.p. = 0.572(3).0.814(3). p.p. = 0.186(3).Table 2 Selected bond distances (A) and angles (") for"Et,lCCrz(EtO),( H,B(Pz),MNCS),ICr( 1)-O( 1) 1.939(6) Cr(2)-O(1) 1.946(6)Cr( 1 )-O(2) 1.994(5) Cr(2)-O(2) 1.982(6)Cr( 1)-0(3) 1.998(6) Cr(2)-O(3) 1.991(5)Cr( 1)-N( 1) 2.052( 7) Cr( 2)-N( 5 ) 2.043(8)Cr( 1 )-N( 3) 2.048(7) Cr(2)-N(7) 2.054(7)Cr( 1)-N(9) 1.985(8) Cr(2)-N( 10) I .972(9)Cr(1) - - Cr(2) 2.683(2)0(1)-Cr(1)-0(2)O( 1 )-Cr( 1)-0(3)O(2)-Cr( 1)-0(3)N(l)-Cr(l)-N(3)N( 1)-Cr( 1)-N(9)N(3)-Cr( 1)-N(9)O( 1)-Cr( 1)-N(9)O(2)-Cr( 1)-N( 1)O(3)-Cr( 1 )-N( 3)Cr( 1)-O( 1 )-Cr(2)Cr( 1)-0(3)-Cr(2)7 7.2 (2)76.9(2)82.3(2)89.7(3)94.1(3)92.7(3)171.3(3)168.7(3)170.2(3)8 7.4(2)84.5(2)O( l)-Cr(2)-0(2)0(1)-Cr(2)-0(3)0(2)-Cr(2)-0( 3)N( 5)-Cr( 2)-N( 7)N(5)-Cr(2)-N( 10)N(7)-Cr(2)-N( 10)O( l)-Cr(2)-N( 10)0(2)-Cr(2)-N( 5)O( 3)-Cr(2)-N(7)Cr( 1)-0(2)-Cr(2)77.3 (2)76.9(2)82.8(2)90.3(3)91.4(3)92.7(3)170.9(3)171.6(3)170.5(3)84.9(2)population parameters, linked to those of the appropriateimages of the groups.The anomalous dispersion correctionsfor Cr and scattering factor data for the neutral atoms were fromref. 8.Refinement on 444 parameters, with the weighting schemew = [02(Fo) + 0.005F02]-1, converged at R = 0.068, R' =0.072. The final AFmap showed two maxima of heights = 1.7 ek3 aligned with the metal atom positions, which could not beassigned any chemical significance.Additional material available from the Cambridge Crystallo-graphic Data Centre comprises H-atom coordinates thermalparameters and remaining bond lengths and angles.Results and DiscussionThe slow oxidation of a MeCN-EtOH solution of the five-co-ordinate chromium(i1) complex [NEt,][Cr(H,B(pz),},-(NCS)],' in a stream of N, containing traces of 02, affords apure and reproducible chromium(m) complex which can beformulated, on the basis of the elemental analysis, as[NEt4][Cr,(EtO),(H,B(pz)2},(NCS)2]. The oxidation of thestarting chromium(I1) complex is accompanied by the formationof ethoxo species which co-ordinate to chromium(m), as well asby decomposition of part of the ligand.We have already found that nickel(II),' cobalt(11)' orcopper(ir)" complexes containing co-ordinated H,B(pz), -anions may be susceptible to ligand decomposition in anethanol-acetone solution, thereby forming pyrazolate anionsthat co-ordinate to the metal atom.In the present case theformation of ethoxo species may result from EtOH attack on anintermediate oxidation product of [Cr(H,B(pz),),(NCS)] ~,presumably a mono- or di-nuclear complex of chromium(m)containing co-ordinated oxygen.We have previously isolatedan 0x0-bridged chromium(rr1) complex, formed by slowoxidation of a polypyrazolyl chromium(I1) complex." It shouldbe noted that the manganese(r1) complexes [MnX{ H,B-(PZ),}~]-,~ which are quite similar to those of chromium(r1) butare stable towards aerial oxidation, do not give ethoxoderivatives under the experimental conditions in which thechromium(ii) compounds do.' On the other hand a bis-methoxo-bridged complex of copper(I1) with a polypyridylligand has been obtained by oxidation of the correspondingcopper(r) complex with dioxygen in methanol suspension.Description of the Structure.-Atomic coordinates for thecompound [NEt,][Cr,(EtO),{H,B(pz),},(NCS),] are givenin Table 1 and selected bond distances and angles in Table 2.Aview of the dimetal anion is shown in Fig. 1. Each metal atomis in an approximately octahedral environment formed by thethree 0 atoms of the bridging ethoxo groups, by the two Ndonor atoms of a H,B(pz), ligand anion and by the N atom of athiocyanate anion. The N and 0 donor atoms span oppositefaces of the octahedron about each metal atom; the 0 atomsJ. CHEM. SOC. DALTON TRANS. 19917I43*CTl )Fig. 1 View of the [Cr,(EtO),{H,B(pz),},(NCS),]- anion. Bothpositions for the disordered methyl groups of two EtO- ligands areshown.0:-0 50 100 150 200 250 300T /KFig. 2 Magnetic susceptibility data for [NEt,][Cr,(EtO),-(H,B(pz),),(NCS),]. (*), Experimental points; (-), the best fit of thetheoretical expression of a pair of exchange-coupled chromium(rI1) ions.Best-fit parameters are given in the text.forming the inner layer in the dimetal unit, are in staggeredpositions with respect to the N atoms, which form the outerlayers.The Cr and B atoms, those of the NCS groups, as well asthose of the ethoxo group formed by 0(1), all lie on one plane(within 0.08 A), which forms a 90.9(2)" angle with the best planethrough the four donor N atoms of the pyrazole groups. Themethyl C(16) position in the above ethoxo group which issheltered by a ligand cavity, is not affected by disorder, atvariance with the other methyl groups in the anion and one inthe cation which are affected by two-fold orientational disorder.The 0(1) atom, lying in trans position with respect to the NCSanions, forms shorter bond distances to the metal atoms (by 0.05A, mean) than do the other two 0 atoms, which lie trans to theN atoms of the H,B(pz), ligands. The Cr-0 distances formed by0(1) are in the range of those found for other alkoxo-bridgedchromium(Ir1) d i m e r ~ , ' ~ , ' ~ whereas the distances formed by theother two 0 atoms in the present structure lie slightly outsidethat range.The Cr-N distances involving the NCS nitrogens, aswell as those formed by the pyrazole N atoms, are slightlyshorter (respectively by 0.03 and 0.05 A, mean) than similardistances existing in the structure of the chromium(m) 0x0-bridged dimer [(Cr(tpea)(NCS)),0][E3Ph4],, where tpea is thetetradentate ligand tris(2-pyrazol- 1 -ylethyl)amine.' 'Magnetic Measurements.-The temperature dependence ofthe molar magnetic susceptibility of [NEt,][Cr,(EtO),-{H,B(pz),},(NCS),] in the temperature range 8-280 K isshown in Fig. 2. It is indicative of an antiferromagneticcoupling between the two chromium(m) ions with a Curie-liketail at low temperatures due to a small amount of paramagneticimpurities.The exchange interactions between the two chromium(rI1)ions can be described using a spin Hamiltonian of the form(1):" where J is the isotropic exchange coupling constant, j isthe biquadratic coupling constant and S , = S , = 3. Sincein chromium(I1r) dimers lj/JI is generally smaller than 0.1,j wasneglected in the calculation of the energy levels in order toreduce the number of parameters needed to fit the magneticdata. Using the Van Vleck equation to compute the molarmagnetic susceptibility and the isotropic Zeeman effect, theexpression (2) was obtained.The magnetic susceptibility valuesto be compared with the experimental ones can thus becomputed by equation (3) where xMCr takes into account theparamagnetic impurities and can be computed by use ofequation (4) and p is the molar fraction of the impurities.Equation (3) was employed to fit the magnetic susceptibilitydata using a Simplex minimisation routine. The function to beminimised was (5) where xM0 is the observed value of themagnetic susceptibility and the sum is over all the experimentaltemperatures. The quality of the fitting was judged using theusual R value [equation (6)].The best fit was obtained with theparameters g = 1.90(1), J = 87.0(2) cm-' and p = 0.012(4).The final R value was 0.03.In a number of chromium(rI1) dimers containing a Cr,O,bridging network the energy separation between the singlet andthe triplet levels originating from the exchange interaction, AE =J, was found to be correlated to the ratio between the Cr-0-Crbond angle (@) and the Cr-0 bond length (r).16 This correlationwas rationalised using a molecular-orbital treatment.' A similarcorrelation between singlet-triplet splittings and the @ to r ratiohas been observed also for halide- and sulphur-bridged copper(I1)systems.' By assuming that the same correlation also holds forthe triply bridged dimers ofchromium(rIr), which are still rare, wecompute for [Cr,(EtO),{ H,B(pz),),(NCS),] - an average @/rvalue of 43.4" A-1 which is close to the 42.6" A-' reported for[Cr,(OH)3L,][CI0,],~3H,0 (L = 1,4,7-trimethyl-l,4,7-tri-azacy~lononane).'~ The AE values are, respectively, 87 and 128cm-' for the two aforementioned triply bridged dimers.Thiscomparison suggests that the ethoxo groups are less effective intransmitting the exchange interaction between the chromium(Ir1)ions with respect to the hydroxo groups. These considerationsmay provide a starting point for a quantitative calculation of theexchange interaction in chromium(Ii1) systems.AcknowledgementsWe thank Dr. A. Caneschi for the magnetic susceptibilimeasurements.References1 M. Di Vaira and F. Mani, J.Chem. Soc., Dalton Trans., 1990, 191.44 J. CHEM. SOC. DALTON TRANS. 19912 R. C. Mehrotra, Adv. Inorg. Chem. Radiochem., 1983,26,269.3 L. F. Larkworthy, K. B. Nolan and P. OBrien, in ComprehensiveCoordination Chemistry, eds. G. Wilkinson, R. D. Gillard andJ. A. McCleverty, Pergamon, Oxford, 1987, vol. 3, p. 860.4 C. J. O’Connor, Prog. Inorg. Chem., 1982,29,208.5 N. Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983,39, 158.6 M. C. Burla, M. Camalli, G. Cascarano, C. Giacovazzo, G. Polidori,7 G. M. Sheldrick, SHELX 76, Program for Crystal Structure8 International Tables for X-Ray Crystallography, Kynoch Press,9 Unpublished work from this laboratory.R. Spagna and D. Viterbo, J , Appl. Crystallogr., 1989,22,389.Determination, University of Cambridge, 1976.Birmingham, 1974, vol. 4.10 D. Ajo, A. Bencini and F. Mani, Inorg. Chem., 1988,27,2437.1 1 M. Di Vaira and F. Mani, Inorg. Chern., 1984,23,409.12 K. D. Karlin, J. Shi, J. C. Hayes, J. W. McKown, J. P. Hutchinsonand J. Zubieta, Inorg. Chim. Actu, 1984,91, L3.13 H. R. Fischer, J. Glerup, D. J. Hodgson and E. Pedersen, Inorg.Chem., 1982,21,3063.14 H. R. Fischer, D. J. Hodgson and E. Pedersen, Inorg. Chem., 1984,23, 4755.15 D. E. Bolster, P. Gutlich, W. E. Hatfield, S. Kremer, E. W. Mullerand K. Wieghardt, Inorg. Chem., 1983,22, 1725.16 D. J. Hodgson, Magneto-Structural Correlation in ExchangeCoupled Systems, eds. R. D. Willett, D. Gatteschi and 0. Kahn,D. Reidel, Dordrecht, 1985, p. 497.17 R. P. Scaringe, Ph.D. Dissertation, University of North Carolina,Chapel Hill, 1976.18 W. E. Hatfield, ref. 16, p. 555.Received 23rd May 1990; Paper 0/02297