Coordination Modes

DOI: 10.1002/anie.200503535

Bidentate Coordination of Pyrazolate in Low-Coordinate Iron(ii) and Nickel(ii) Complexes**

Javier Vela, Sridhar Vaddadi, Savariraj Kingsley,Christine J. Flaschenriem, Rene J. Lachicotte,Thomas R. Cundari,* and Patrick L. Holland*

Pyrazolate ligands have rich coordination chemistry(Figure 1) that includes exo-bidentate bridging (m-h1:h1,

Figure 1a, most commonly observed),[1,2] endo-bidentate (h2,Figure 1b), terminal monodentate (h1, Figure 1c), and side-on, pentadentate (h5, Figure 1d) coordination modes.[3,4]

Because of this diversity in coordination modes, pyrazolatesare potentially useful in devising synthetically useful process-es in which the presence of a hemilabile ligand is required, aswell as in the construction of metal architectures forcatalysis,[5–8] luminescent materials,[9] and chemical vapordeposition.[10]

The endo-bidentate (h2-pyrazolate) coordination mode(Figure 1b) was long thought to be prevented by the opposingdirectionality of the two pyrazolate nitrogen lone pairs. Morerecently, h2-pyrazolate coordination has been established forthe lanthanides, actinides, alkali and alkaline earth metals,and early transition metals in high oxidation states.[11–13] h2-Pyrazolate coordination to transition-metal ions with partiallyfilled d shells is rare[11] and is limited to a few well-characterized examples with TiIII (d1),[14] VIII(d2),[15] MoIV

(d2),[8] CrIII (d3),[16] MoII (d4),[17] and FeIII (d5).[16]

Bulky b-diketiminate ancillary ligands have enabled theisolation and study of late, first-row transition-metal com-plexes with coordination numbers three and four.[18–20]

Herein, it is shown that the steric protection of a b-diketiminate stabilizes the first h2-pyrazolate complexeswith d6 and d8 electron configurations, and these showdifferent coordination geometries. Interestingly, a bridgingpyrazolate binding mode is also accessible in the complexes,and the balance between the different pyrazolate coordina-tion modes is explored through solution spectroscopic meth-ods.

Treatment of [LtBuMCl] (LtBu= 2,2,6,6-tetramethyl-3,5-bis(2,6-diisopropylphenylimino)hept-4-yl; M=Fe, Ni)[19]

with a freshly prepared solution of potassium 3,5-dimethyl-pyrazolate (Kdmp) in tetrahydrofuran (THF) affords thecomplexes [LtBuFe(h2-dmp)] and [LtBuNi(h2-dmp)] in 89 and72% yields, respectively (Scheme 1a).[21] The monomericnature and endo-bidentate ligation of the dmp ligand in thesecomplexes was confirmed by X-ray diffraction studies(Figure 2).

The coordination geometry around the iron center in the14-electron complex [LtBuFe(h2-dmp)] is intermediatebetween tetrahedral and square planar; the twist anglebetween the pyrazolato and diketiminato (C3N2Fe) planes is35.62(7)8 (Figure 2a). The pyrazolate Fe�N bonds (2.031(1)and 2.048(1) =) are longer than the diketiminate Fe�N bonds(1.988(1) and 2.001(1) =). They are also longer than the Fe�Nbonds reported for the homoleptic complex tris(3,5-di-tert-butylpyrazolato)iron(iii) (2.009 =).[16] The 1H NMR spectrumshows paramagnetically shifted resonances, which have beenassigned on the basis of relative integrations (see the

Figure 1. Typical coordination modes of pyrazolate ligands (R=H,alkyl, or aryl) towards metal ions (M, M’). a) exo-bidentate; b) endo-bidentate; c) monodentate; d) pentadentate.

Scheme 1.

[*] S. Vaddadi, Prof. T. R. CundariDepartment of ChemistryUniversity of North TexasDenton, TX 76203 (USA)Fax: (+1)940-565-4318E-mail: tomc@unt.edu

Dr. J. Vela, Dr. S. Kingsley, C. J. Flaschenriem, Dr. R. J. Lachicotte,Prof. P. L. HollandDepartment of ChemistryUniversity of RochesterRochester, NY 14627 (USA)Fax: (+1)585-276-0205E-mail: holland@chem.rochester.edu

[**] The authors acknowledge funding from the Petroleum ResearchFund (38275-G; P.L.H.), the A. P. Sloan Foundation (P.L.H.), theUniversity of Rochester (Messersmith Fellowship; J.V.), and the USDepartments of Education and Energy (DOE-FG02-97ER14811;T.R.C.). A portion of the calculations were performed on the UNTcomputational chemistry resource, for which S.V. and T.R.C.acknowledge the NSF for support through grant CHE-0342824. J.V.thanks Sandip Sur for help with DEPT and 2D NMR experimentsand Dr. Azwana R. Sadique for advice on synthetic procedures.

Supporting information for this article is available on the WWWunder http://www.angewandte.org or from the author.

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Supporting Information). The solution magnetic moment is5.1 mB, which is consistent with a high-spin iron(ii) centerhaving four unpaired electrons and a spin state of S= 2.

In the 16-electron complex [LtBuNi(h2-dmp)], the fournitrogen atoms and the nickel(ii) center are roughly situatedwithin a plane (Figure 2b). This is evidenced by a sum ofangles around the metal center of 360.1(6)8, and there is onlya small twist angle of 4.8(1)8 between the pyrazolato anddiketiminato (C3N2Ni) planes. The pyrazolate Ni�N bondlengths (1.901(1) and 1.906(1) =) are again longer than thediketiminate Ni�N bond lengths (1.851(1) and 1.852(1) =).The 1H NMR spectrum indicates a diamagnetic complex withwell-defined proton resonances and coupling constants, andthe complex has no measurable solution magnetic moment.

Thus, the h2-pyrazolate iron(ii) complex is nonplanar, andits nickel(ii) analogue is planar. To understand whether thisgeometric difference arises from a steric or electronic effect,we performed density functional theory (DFT) calculations(B3LYP[22] in conjunction with both double- and triple-zeta-plus-polarization pseudopotential[23] and all-electron[24] basissets) on C2v-symmetric models [L’Fe(h2-pz)] and [L’Ni(h2-pz)](L’ represents the truncated b-diketiminate C3N2H5

� , and pzrepresents the parent pyrazolate C3N2H3

�). The geometries(Figure 3) of the molecules were optimized in the quintet (Fe)and singlet (Ni) states. A parallel (i.e., square-planar)coordination geometry is greatly preferred for the nickelcomplex (32.2 kcalmol�1 at the B3LYP/SBKJC(d) level),whereas the parallel and perpendicular conformers arecloser in energy for the iron complex (3.8 kcalmol�1 at theB3LYP/SBKJC(d) level), with the perpendicular conformerslightly preferred. The latter inference was confirmed bymulticonfiguration self-consistent field (MCSCF)[25] geometryoptimization on the [L’Fe(pz)] model complex, which showedthe lowest-energy 5A1 and 5A2 states of the parallel andperpendicular conformations to be essentially at the sameenergy. As the truncated models used in the calculations do

not suffer from steric obstructions, we infer that the planargeometry around the nickel center is determined electroni-cally, whereas the geometry observed for the iron complex isprobably dominated by steric factors.[26]

Treating [LMeM(m-Cl)2Li(thf)2] (M=Fe, Ni)[18, 19]—analo-gous complexes with the slightly smaller diketiminate ligandLMe (2,4-bis(2,6-diisopropylphenylimino)pent-3-yl)—withKdmp in THF gives the bridged complexes [LMeFe{m-(h1:h1)-dmp}(m-Cl)Li(thf)2] and [LMeNi{m-(h1:h1)-dmp}(m-Cl)Li(thf)2] in 85 and 76% yields, respectively (Scheme 1a).The bridging, exo-bidentate nature of the dmp ligand in thesecomplexes was confirmed by X-ray diffraction studies(Figure 2).

Both the iron(ii) and nickel(ii) heterobridged complexesare tetrahedral (Figure 2). The pyrazolate Fe�N (2.033(1) =)and Ni�N (1.959(1) =) bond lengths are elongated withrespect to those in the endo-bidentate complexes (see above).The 1H NMR spectrum of each complex shows broad andparamagnetically shifted resonances. The solution magneticmoment for the iron(ii) complex is 5.2 mB and is consistentwith a high-spin d6 configuration with four unpaired electronsand a spin state of S= 2. The nickel(ii) complex has amagnetic moment of 3.1 mB, which is consistent with a high-spin d8 configuration (S= 1).

The tert-butyl substituents in LtBu consistently push thearyl rings closer to the metal center than in the LMe complexand increase the steric crowding around the metal center.[18,19]

Thus, the different balance between bridging and h2-pyrazo-late ligation can be rationalized by the different stericrequirements of the two diketiminate ligands used—the{Li(Cl)(dmp)} “ligand” is more sterically hindering thandmp itself, thus the bridging pyrazolate complex is forced toassume the more sterically accessible tetrahedral geometry.

Interestingly, the h2 and bridging forms of the pyrazolateligand interconvert in the nickel complexes of the smallerdiketiminate ligand (Scheme 1b). Green solutions of [LMeNi-

Figure 2. Molecular structures of iron(ii) (a, c) and nickel(ii) (b, d)diketiminate complexes containing either endo-bidentate (h2) (a, b) orexo-bidentate (m-(h1:h1)) (c, d) 3,5-dimethylpyrazolato coordination.Thermal ellipsoids drawn at 50% probability; hydrogen atoms omittedfor clarity.

Figure 3. B3LYP/SBKJC(d)-optimized geometries of singlet C2v-symmet-ric [L’Ni(h2-pz)] (top) and quintet C2-symmetric [L’Fe(h

2-pz)] (bottom).All geometries correspond to energy minima. Bond lengths are in I;bond angles are in degrees. The twist angle is 08 in the nickelcompound and 908 in the iron compound.

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{m-(h1:h1)-dmp}(m-Cl)Li(thf)2] quickly become pink and forma white precipitate upon exposure to toluene. Filtration of thispink solution followed by evaporation of the solvent leads to adiamagnetic solid ([LMeNi(h2-dmp)]) whose 1H NMR andelectronic spectra are reminiscent of those displayed by[LtBuNi(h2-dmp)]. Each complex has a band between 522 and547 nm (e= 440m�1 cm�1) that is typical of square-planar d8

complexes,[27, 28] along with a weaker band at 731–778 nm (e=150m�1 cm�1; see Supporting Information). Treatment with aslight excess of LiCl in THF returned the compound to thegreen tetrahedral form. Therefore, we conclude that uptakeand release of the alkali halide by the nickel(ii) pyrazolatecomplexes are reversible, solvent-dependent processes. Tol-uene favors formation of the h2-pyrazolate complex bydecreasing the solubility of LiCl in the organic phase, thuspromoting salt extrusion from the heterobridged species.

Considering that three-coordinate nickel and iron com-plexes of these bulky diketiminates have been synthesized,[18]

it is surprising that h1-pyrazolate ligands collapse so easily tothe h2 form in the absence of LiCl. DFT calculations of thenickel complexes were used to evaluate the cause of thisphenomenon. Several different conformations of h1-pyrazo-late species of truncated [L’Ni(pz)] and full [LtBuNi(dmp)]were constructed manually from the calculated or experi-mental h2-pyrazolate structures. The truncated complex wasstudied with density functional (B3LYP/SBKJC(d)) methodsand the full complex was studied with hybrid quantummechanics/molecular mechanics (ONIOM(B3LYP/6-31G(d):UFF))[29] calculations. In all cases, h1-pyrazolatecomplexes relaxed upon geometry optimization either backto the h2 structures found previously or, in the case of the L’models, to a higher-energy structure in which the noncoordi-nated pyrazolate nitrogen atom hydrogen bonds to one of theN�H bonds of L’. Hence, h1-pyrazolate coordination in thenickel complex is disfavored with respect to h2-pyrazolatecoordination.

As the [L’Ni(h1-pz)] structure does not correspond to astable stationary point, the geometry was constructed byconstraining one Ni-Npz-Cpz angle and one Npz-Ni-NL’ anglewhile performing B3LYP/SBKJC(d) geometry optimizationof all other degrees of freedom. Analysis of the frontiermolecular orbitals (Figure 4) for the h2-pz (left) and h1-pz(right) geometry minima of [L’Ni(pz)] at the B3LYP/SBKJC(d) level reveals that the symmetric (in-phase) andantisymmetric (out-of-phase) combinations of the pyrazolatenitrogen lone pairs interact with the appropriate-symmetry Nid orbitals. These “s” frontier orbitals decrease in energy uponcoordinating the second nitrogen atom, as described bySchlegel, Winter, and co-workers for early transition-metalpyrazolate complexes.[12b,15,16] Whereas the changes in fron-tier-orbital energies indicate a preference for h2-pz bondingfrom the s orbitals (top three orbitals in Figure 4), the frontierorbitals made up of Ni dp orbitals and p electrons in thepyrazolate and L’ rings (bottom three orbitals in Figure 4)predominantly favor h1-pz bonding. In the complexes de-scribed herein, the preference of the s orbitals for h2-pzbonding more than counterbalances the preference of thep orbitals for h1-pz bonding, as can be inferred from thechanges in the frontier-orbital energies. We speculate that the

balance between h1 and h2 binding modes could be tunedthrough appropriate manipulation of these opposing elec-tronic effects.

In summary, we have shown the first examples of h2-pyrazolate complexes of d6 and d8 metal ions. The high d-electron count of the late transition metals does not excludethis binding mode. Depending on the steric crowding imposedby the diketiminate ancillary ligand used, both h2 and h1

coordination to iron(ii) and nickel(ii) can be observed. Thenickel(ii) complexes show solvent-dependent behavior and

Figure 4. Changes in s and p frontier orbitals upon transforming[L’Ni(h1-pz)] (right) into [L’Ni(h2-pz)] (left).

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adopt either high-spin tetrahedral or diamagnetic square-planar configurations.

Received: October 5, 2005

.Keywords: coordination modes · density functionalcalculations · iron · N ligands · nickel

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[12] Initial examples of h2-pyrazolate coordination to transitionmetals, (e.g. [ZrCp2(h

2-pz)(thf)][BPh4]; Cp= cyclopentadienyl)were explained in terms of their need to attain an 18-electronconfiguration: a) D. RPttger, G. Erker, M. Grehl, R. FrPhlich,Organometallics 1994, 13, 3897 – 3902; b) I. A. Guzei, A. G.Baboul, G. P. A. Yap, A. L. Rheingold, C. H. Schlegel, C. H.Winter, J. Am. Chem. Soc. 1997, 119, 3387 – 3388; c) I. A. Guzei,G. P. A. Yap, C. H. Winter, Inorg. Chem. 1997, 36, 1738 – 1739.

[13] A search of the CSD revealed 55 structures with h2-pyrazolatecoordination to f-block elements, 13 with alkali and alkaline-earth metals, 40 with d0 early transition-metal ions, and only 8with h2-pyrazolate coordination to a metal ion with a partiallyfilled d shell: Cambridge Structural Database, ConQuest version1.7, November 2004 release, May 2005.

[14] N. C. MPsch-Zanetti, R. KrStzner, C. Lehmann, T. R. Schneider,I. UsTn, Eur. J. Inorg. Chem. 2000, 13 – 16.

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cotte, P. L. Holland, Inorg. Chem. 2003, 42, 1720 – 1725; b) N. A.Eckert, J. M. Smith, R. J. Lachicotte, P. L. Holland, Inorg. Chem.2004, 43, 3306 – 3321; c) J. Vela, S. Vaddadi, T. R. Cundari, J. M.Smith, E. A. Gregory, R. J. Lachicotte, C. J. Flaschenriem, P. L.Holland, Organometallics 2004, 23, 5226 – 5239; d) J. Vela, J. M.Smith, Y. Yu, N. A. Ketterer, C. J. Flaschenriem, R. J. Lachi-cotte, P. L. Holland, J. Am. Chem. Soc. 2005, 127, 7857 – 7870.

[19] a) J. M. Smith, R. J. Lachicotte, P. L. Holland, Chem. Commun.2001, 1542 – 1543; b) P. L. Holland, T. R. Cundari, L. L. Perez,N. A. Eckert, R. J. Lachicotte, J. Am. Chem. Soc. 2002, 124,14416 – 14424.

[20] Selected references: a) E. Kogut, A. Zeller, T. H. Warren, T.Strassner, J. Am. Chem. Soc. 2004, 126, 11984 – 11994; b) D. J. E.Spencer, A. M. Reynolds, P. L. Holland, B. A. Jazdzewski, C.Duboc-Toia, L. Le Pape, S. Yokota, Y. Tachi, S. Itoh, W. B.Tolman, Inorg. Chem. 2002, 41, 6307 – 6321; c) A. Panda, M.Stender, R. J. Wright, M. M. Olmstead, P. Klavins, P. P. Power,Inorg. Chem. 2002, 41, 3909 – 3916.

[21] Full synthetic, spectroscopic, and crystallographic details aregiven in the Supporting Information.

[22] a) A. D. Becke, Phys. Rev. A 1998, 38, 3098 – 3100; b) C. Lee, W.Yang, R. G. Parr, Phys. Rev. B 1998, 37, 785 – 789.

[23] The Stevens pseudopotentials and valence basis sets (W. J.Stevens, M. Krauss, H. Basch, P. G. Jasien, Can. J. Chem. 1992,70, 612 – 630) were employed, augmented with d polarizationfunctions for main-group elements within theGAMESS package(M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S.Gordon, J. J. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S.Su, T. L. Windus, M. Dupuis, J. A. Montgomery, J. Comput.Chem. 1993, 14, 1347 – 1363). This basis-set combination isreferred to as SBKJC(d).

[24] All-electron calculations employed the 6-311G(d) basis set forall atoms within the Gaussian03 suite of programs (Gaussian03(RevisionB.03), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr.,T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar,J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P.Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez,J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 2003).

[25] For the MCSCF calculations, the five 3d orbitals of iron(ii),subtending the irreducible representations 2a1 + a2 + b1 + b2,and the six electrons contained within this orbital manifoldcomprised the active space. Geometry optimizations werecarried out for the 5A1,

5A2,5B1, and

5B2 states. The latter twostates were found to be much higher in energy (> 6 kcalmol�1)for both parallel and perpendicular conformations of [L’Fe(pz)].MCSCF calculations employed the GAMESS package (seereference 23) and the SBKJC(d) basis set.

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[26] Calculations on full models with the PM3(tm) Hamiltonian gavesimilar results, that is, a nearly planar geometry for [LtBuNi(h2-dmp)] and a nonplanar geometry for [LtBuFe(h2-dmp)].

[27] a) N. N. Greenwood, A. Earnshaw, Chemistry of the Elements,2nd ed., Butterworth Heinemann, Oxford, 1997, pp. 1091 – 1097,1156 – 1166; b) R. L. Carter, Molecular Symmetry and GroupTheory, Wiley, New York, 1998, pp. 201 – 214.

[28] A. B. P. Lever, Inorganic Electronic Spectroscopy, 2nd ed.,Elsevier, Amsterdam, 1984, pp. 507 – 611.

[29] The ONIOM methodology is described in M. Svensson, S.Humbel, R. D. J. Froese, T. Matsubara, S. Sieber, K. Morokuma,J. Phys. Chem. 1996, 100, 19357 – 19363. The UFF force field isdescribed in A. K. RappU, C. J. Casewit, K. S. Colwell, W. A.Goddard III, W. M. Skiff, J. Am. Chem. Soc. 1992, 114, 10024 –10035. In our calculations, the MM region included the aryl andtert-butyl substituents of LtBu and the methyl groups of dmp.

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