Coordination Properties of Bridging Diazene Ligands in Unusual

Organometallics 2010, 29, 3271–3280 DOI: 10.1021/om100098t

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Coordination Properties of Bridging Diazene Ligands in Unusual Diiron Complexes Alexander Yu. Sokolov* and Henry F. Schaefer, III Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602 Received February 8, 2010

Saouma, M€ uller, and Peters recently (J. Am. Chem. Soc., 2009) reported the synthesis of several diiron complexes with two types of bridging diazene HNdNH ligands, designated μ-η1:η1 (bridging end-on) and μ-η2:η2 (bridging side-on), with tridentate phosphine {XBPR3} ligands ({XBPR3} = XB(CH2PR2)3-; X = Ph; R = Ph, CH2Cy). In the present study the energies, optimized geometries, and vibrational frequencies of these complexes with X = H and R = H, CH3, CF3 were obtained theoretically, and the electronic structures were analyzed in terms of iron-diazene donor-acceptor interactions. Natural bond orbital analysis indicates that both bridging end-on and side-on diazenes possess strong π-acceptor properties, which cause significant occupation of their antibonding π*(N-N) LUMOs and weakening of the N-N bonds as the result of coordination. The most pronounced weakening was obtained in complexes with bridging side-on diazene, where interaction of iron d-AOs with π*(N-N) orbitals of the diazene ligands results in the occupation of the π*-orbital by more than one electron. Modification of the structures of the phosphine ligands affects the strength of the diazene N-N bond: phosphines with donor groups R = H, CH3 facilitate the weakening of the bond, while the acceptor group R = CF3 strengthens the N-N bond. 1. Introduction The quest for elucidating the mechanism of the biological conversion of nitrogen to ammonia has continued for more than 50 years.1-5 Although many features of N2 reduction on the nitrogenase enzyme active site are still unknown, it is clear that iron plays a crucial role in this process.6 It has been found that the reduction of the NtN triple bond proceeds through a sequence of intermediates, among which diazene (HNdNH) and hydrazine (H2N-NH2) have been spectroscopically detected.3,7,8 *To whom correspondence should be addressed. E-mail: alex@ ccqc.uga.edu. (1) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589. (2) Leigh, G. J.; Jimenez-Tenorio, M. J. Am. Chem. Soc. 1991, 113, 5862. (3) Barney, B. M.; Lee, H.-I.; Dos Santos, P. C.; Hoffman, B. M.; Dean, D. R.; Seefeldt, L. C. Dalton Trans. 2006, 2277. (4) Barney, B. M.; Lukoyanov, D.; Yang, T.-C.; Dean, D. R.; Hoffman, B. M.; Seefeldt, L. C. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 17113. (5) Dance, I. Dalton Trans. 2008, 5977. (6) Dos Santos, P. C.; Igarashi, D. Y.; Lee, H.-I.; Hoffman, B. M.; Seefeldt, L. C.; Dean, D. R. Acc. Chem. Res. 2005, 38, 208. (7) Howard, J. B.; Rees, D. C. Chem. Rev. 1996, 96, 2965. (8) Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 3013. (9) George, T. A.; Rose, D. J.; Chang, Y.; Chen, Q.; Zubieta, J. Inorg. Chem. 1995, 34, 1295. (10) Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222. (11) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252. (12) Yandulov, D. V.; Shrock, R. R. Science 2003, 301, 76. (13) Weare, W. W.; Dai, X.; Byrnes, M. J.; Chin, J. M.; Shrock, R. R.; M€ uller, P. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 17099. (14) Chen, Y.; Zhou, Y.; Chen, P.; Tao, Y.; Li, Y.; Qu, J. J. Am. Chem. Soc. 2008, 130, 15250. r 2010 American Chemical Society

Several iron complexes have been studied as model catalytic systems for nitrogen reduction.9-14 Sellmann and coworkers carried out an extensive investigation of various diiron and diruthenium complexes, [{M(PR3)(Sn)}2( μ-η1: η1-N2H2)] and [{M(Sn)}2( μ-η1:η1-N2H2)] (M = Fe, Ru), with different polydentate sulfur-donor ligands Sn and bridging end-on trans-diazene (Sellmann-type complexes).15-22 Although the corresponding diiron complexes with a bridging dinitrogen ligand have not been obtained experimentally, theoretical studies by Reiher, Hess, and coworkers23-25 showed that the two-electron-two-proton (2Hþ/2e-) reduction of these μ-N2 complexes can result in the proton transfer from the sulfur atoms of Sn ligands to N2 with the formation of the bridging trans-diazene ligand in the coordination sphere. The results of these studies can be (15) Sellmann, D.; Soglowek, W.; Knoch, F.; Ritter, G.; Dengler, J. Inorg. Chem. 1992, 31, 3711. (16) Sellmann, D.; K€appler, J.; Moll, M.; Knoch, F. Inorg. Chem. 1993, 32, 960. (17) Sellmann, D.; Hennige, A. Angew. Chem., Int. Ed. 1997, 36, 276. (18) Lehnert, N.; Wiesler, B. E.; Tuczek, F.; Hennige, A.; Sellmann, D. J. Am. Chem. Soc. 1997, 119, 8869. (19) Lehnert, N.; Wiesler, B. E.; Tuczek, F.; Hennige, A.; Sellmann, D. J. Am. Chem. Soc. 1997, 119, 8879. (20) Sellmann, D.; Hennige, A.; Heinemann, F. W. Inorg. Chim. Acta 1998, 280, 39. (21) Sellmann, D.; Blum, D. C. F.; Heinemann, F. W. Inorg. Chim. Acta 2002, 337, 1. (22) Sellmann, D.; Hille, A.; R€ osler, A.; Heinemann, F. W.; Moll, M.; Brehm, G.; Schneider, S.; Reiher, M.; Hess, B. A.; Bauer, W. Chem.; Eur. J. 2004, 10, 819. (23) Reiher, M.; Sellmann, D.; Hess, B. A. Theor. Chem. Acc. 2001, 106, 379. (24) Reiher, M.; Salomon, O.; Sellmann, D.; Hess, B. A. Chem.;Eur. J. 2001, 7, 5195. (25) Reiher, M.; Hess, B. A. Advances in Inorganic Chemistry; Elsevier: Amsterdam, 2004; Vol. 56; pp 55-100. Published on Web 07/09/2010

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supported by the experimental evidence of the hydrogen bonding between the sulfur atoms of Sn and the bridging diazene ligands in the Sellmann-type complexes.16,21,22 Another theoretical work by Reiher et al.26 showed that the excitation of the diiron and diruthenium Sellmann-type complexes with a bridging dinitrogen ligand followed by the relaxation of the excited state leads to the bent diazenelike geometry of the M-N-N-M moiety. The subsequent reduction and protonation of the electronically excited complex can also give bridging trans-diazene as the product. Much attention for the past few years has been directed to the mono- and dinuclear iron complexes with phosphorus donor ligands, in particular Fe(PP)2N2 (PP = DMPE, DMeOPrPE; DMPE = 1,2-bis(dimethylphosphino)ethane, DMeOPrPE = 1,2-bis(dimethoxypropylphosphino)ethane).27-29 Protonation of these species leads to the partial reduction of coordinated nitrogen to ammonia and hydrazine under standard conditions. In 2007 Crossland, Zakharov, and Tyler synthesized a [Fe(DMeOPrPE)2(N2H4)]2þ complex containing a η2-coordinated hydrazine ligand.30 Shortly after this Field, Li, Dalgarno, and Turner obtained the η2-diazene derivative Fe(DMPE)2(N2H2).31 Later it was shown that [Fe(PP)2(N2H4)]2þ and Fe(PP)2(N2H2) (PP = DMPE, DMeOPrPE) undergo interconversion after the addition of base/acid.32,33 Reaction of Fe(DMPE)2(N2H2) with the strong base leads to the corresponding dinitrogen complex.32 Theoretical investigation of the potential intermediates of the Fe(DMPE)2(N2) protonation reactions suggests that the most energetically favorable mechanism for these processes is the sequential protonation of each nitrogen atom of the N2 ligand, with the formation of η2-diazene and η2-hydrazine ligands inside the coordination sphere.29 To achieve an understanding of the nitrogen fixation process, it is essential to discover not only its mechanism but also the properties of the intermediates involved in this process. Whereas hydrazine is a well-studied and stable compound in its free state, free diazene is quite unstable. Only the trans-isomer of this molecule has been synthesized, under low temperatures.34,35 Theoretical study of the diazene isomers also predicts the trans-isomer to be the most stable.36 The coordination chemistry of diazene is rather interesting. As a ligand, diazene displays at least four binding modes, illustrated in Chart 1. The presence of the NdN double bond (as in ethylene) and the well-known diversity of nitrogen oxidation states allow us to consider the diazene molecule as a potential π-acceptor. There are several theoretical studies on (26) Reiher, M.; Kirchner, B.; Hutter, J.; Sellmann, D.; Hess, B. A. Chem.;Eur. J. 2004, 10, 4443. (27) Hirano, M.; Akita, M.; Morikita, T.; Kubo, H.; Fukuoka, A.; Komiya, S. J. Chem. Soc., Dalton Trans. 1997, 3453. (28) Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem. Soc. 2005, 127, 10184. (29) Yelle, R. B.; Crossland, J. L.; Szymczak, N. K.; Tyler, D. R. Inorg. Chem. 2009, 48, 861. (30) Crossland, J. L.; Zakharov, L. N.; Tyler, D. R. Inorg. Chem. 2007, 46, 10476. (31) Field, L. D.; Li, H. L.; Dalgarno, S. J.; Turner, P. Chem. Commun. 2008, 1680. (32) Field, L. D.; Li, H. L.; Magill, A. M. Inorg. Chem. 2009, 48, 5. (33) Crossland, J. L.; Balesdent, C. G.; Tyler, D. R. Dalton Trans. 2009, 4420. (34) Miller, C. E. J. Chem. Educ. 1965, 42, 254. (35) Back, R. A. Rev. Chem. Intermed. 1984, 5, 293. (36) Szopa, K.; Musial, M.; Kucharski, S. A. Int. J. Quantum Chem. 2008, 108, 2108. (37) Kang, S-K; Albright, T. A.; Mealli, C. Inorg. Chem. 1987, 26, 3158. (38) Chen, Y.; Hartmann, M.; Frenking, G. Eur. J. Inorg. Chem. 2001, 1441.

Sokolov and Schaefer Chart 1. Binding Modes of Diazene Ligandsa

a For each binding mode known cis/trans orientations of the diazene ligands are given.

the electronic properties of coordinated diazenes.18,29,32,37,38 Lehnert et al.18 studied the electronic structure of diiron complexes with bridging end-on trans-diazene using the SCFXR-SW method. The primary contribution to the metalligand bonding was found to be N2H2fFe σ-donation originating from the diazene HOMO (in-plane orbital, nitrogen lone pairs). It was shown that the nearby energies of the iron dπ-orbitals and the π*-LUMO of the diazene molecule give rise to significant N2H2rFe π-back-bonding. DFT computations and charge decomposition analysis (CDA) of mono- and diiron compounds containing end-on, bridging end-on,38 and side-on diazenes32 also indicate the strong π-acceptor properties of this ligand.

2. Systems Considered In the present research the electronic structure of the recently (2009) synthesized diiron complexes39 containing bridging diazene ligands and tridentate P-donor {XBPR3} ligands ({XBPR3} = XB(CH2PR2)3-) is studied, with particular attention to metal-ligand donor-acceptor interactions. Three types of complexes have been studied (Chart 2): (1) diiron complexes with bridging end-on hydrazine and side-on diazene ligands:[({XBPR3}Fe)2( μ-η1:η1-N2H4)( μ-η2: η2-N2H2)] (complexes 1a-1c); (2) diiron complexes with bridging end-on and side-on diazenes:[({XBPR3}Fe)2( μ-η1:η1-N2H2)( μ-η2:η2-N2H2)] (complexes 2a-2c); (3) diiron complexes with bridging end-on diazene and two amide (NH2-) ligands:[({XBPR3}Fe)2( μ-η1:η1-N2H2)( μ-η1:η1-NH2)2] (complexes 3a-3c). The structures of the chelate {XBPR3} ligands were simplified by using X = H and R = H, CH3, and CF3 groups instead of X = Ph and R = Ph and CH2Cy in the compounds synthesized by Saouma et al.39 R groups were varied to analyze the diazene donor-acceptor properties in the presence of phosphine ligands with increased donor (R = H, CH3) or acceptor (R = CF3) electronic properties. Complexes 1-3 are rather interesting examples of compounds with two types of coordinated cis-diazenes (bridging end-on and side-on). The presence of the relative N2H4, N2H2, and NH2- ligands in the coordination sphere makes these complexes promising objects for the modeling of the (39) Saouma, C. T.; M€ uller, P.; Peters, J. C. J. Am. Chem. Soc. 2009, 131, 10358.

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Chart 2. Objects of Studya

a Note that these are traditional valence structures, not entirely supported by the present research.

dinitrogen and diazene activation processes. Although there is no report on the catalytic activity of 1-3, the structurally unusual central fragment of these compounds can in principle be used for the synthesis of the catalytically active species. It is therefore very important to understand the nature of the metal-ligand bonding and the activation of diazene. Whereas 1a-1c and 3a-3c contain diazenes of only one type, complexes 2a-2c include diazene ligands with both of these binding modes. The similar ligand environments of the diazenes allows one to compare their donor-acceptor properties along the 1-3 series.

3. Methods All computations have been carried out using density functional theory (DFT) methods. Two functionals have been applied: (i) the B3LYP hybrid GGA-functional40 (for all complexes) and (ii) the M06 hybrid meta-functional41 (for complexes 1a, 2a, 3a only). Both functionals are nonlocal, containing about the same amount of Hartree-Fock exchange (20% and 27% for B3LYP and M06, respectively). For all elements double-ζ plus polarization (DZP) basis sets have been used. These basis sets can be constructed by adding one set of polarization p- (for H atoms) or d-functions (for B, C, N, or P atoms) with orbital exponents Rp(H) = 1.00, Rd(B) = 0.70, Rd(C) = 0.75, Rd(N) = 0.80, and Rd(P) = 0.60 to the standard Huzinaga-Dunning DZ contracted sets.42-44 Our loosely contracted DZP basis set for Fe uses the Wachters primitive set45 augmented by two sets of p-functions and one set of d-functions, contracted following Hood et al.,46 and designated (14s11p6d/10s8p3d). Computations were performed using the Q-CHEM 3.1 quantum chemistry software package.47 The geometries of complexes 1-3 have been fully optimized without symmetry constraints. For complexes with R = H and CH3, vibrational frequencies were evaluated using the B3LYP functional via analytical differentiation of the potential energy (40) Becke, A. J. Chem. Phys. 1993, 98, 5648. (41) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (42) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823. (43) Dunning, T. H.; Hay, P. J. In Methods of Electronic Structure Theory; Schaefer, H. F., III, Ed.; Plenum: New York, 1977; pp 1-27. (44) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (45) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (46) Hood, D. M.; Pitzer, R. M.; Schaefer, H. F., III. J. Chem. Phys. 1979, 71, 705. (47) Shao, Y.; et al. Q-CHEM (version 3.1); Q-Chem, Inc.: Pittsburgh, PA, 2007.

Figure 1. Structures of diiron complexes 1a (a), 1b (b), and 1c (c) with bridging end-on hydrazine and side-on diazene ligands (C1 symmetry). The hydrazine and diazene N-N bond lengths are given in A˚. with respect to nuclear coordinates, to simultaneously determine the Hessian index of the stationary point. For the numerical evaluation of the exchange-correlation potential matrix elements, a fine grid was used (75 radial shells, 302 angular points). Electronic structures were analyzed using the natural population analysis (NPA) and natural bond orbital (NBO) analysis48,49 of Weinhold from the NBO 5.1 program implemented in Q-CHEM 3.1. Bond orders were computed according to Wiberg50 in the orthogonal natural atomic orbital (NAO) basis set.48 Charge decomposition analysis51 was performed using the CDA 2.1 program.

4. Quantitative Results a. Energies. For complexes 1a, 2a, and 3a with R = H (Figures 1-3) singlet, triplet, and quintet spin states were optimized using the B3LYP/DZP and M06/DZP methods. Both B3LYP and M06 predict singlet states to be the ground (48) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (49) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (50) Wiberg, K. B. Tetrahedron 1968, 24, 1083. (51) Dapprich, S.; Frenking, G. J. Phys. Chem. 1995, 99, 9352.

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Figure 3. Structures of diiron complexes 3a (a), 3b (b), and 3c (c) with one bridging end-on diazene and two amide ligands (C1 symmetry). The diazene N-N bond length is given in A˚.

Figure 2. Structures of diiron complexes 2a (a), 2b (b), and 2c (c) with bridging end-on and side-on diazenes (C1 symmetry). The diazene N-N bond lengths are given in A˚.

electronic states. However, the relative ordering and energies of the triplet and quintet states obtained using these functionals are different (Table 1). For the other six complexes (1b, 1c, 2b, 2c, 3b, and 3c) only singlet states were studied. Diiron complexes 1a, 1b, and 1c with bridging end-on hydrazine and side-on diazene ligands lie higher in energy than the corresponding isomers 3a, 3b, and 3c, with bridging end-on diazenes and two amide ligands (Figures 1 and 3). The energy differences between these isomers depend on the nature of R. Complexes 1a and 1b with donor substituents R = H and CH3 are 33.3 and 32.7 kcal/mol higher in energy than isomers 3a and 3b, respectively. In turn, complex 1c, with acceptor group R = CF3, lies only 19.2 kcal/mol higher than isomer 3c. This is consistent with the experimental observations of Saouma et al.39 concerning the thermal instability of type 1 complexes with donor R = CH2Cy groups with respect to isomerization to complexes 3. b. Structural Parameters. Optimized bond lengths of the central fragments of compounds 1-3, along with experimental values for corresponding synthesized complexes (with X = Ph, R = Ph, CH2Cy), are reported in Tables 2-4. As

Table 1. Computed Relative Energies E (in kcal/mol) of the Lowest Singlet (1A), Triplet (3A), and Quintet (5A) Spin States of Complexes 1a, 2a, and 3a Using Two Different DFT Methodsa B3LYP/DZP

1

E( A) E(3A) E(5A)

M06/DZP

1a

2a

3a

1a

2a

3a

0 13 14

0 19 21

0 14 32

0 12 23

0 17 12

0 12 29

a For each molecule the energy of the ground singlet state is used as the reference.

may be seen, theoretical bond distances of the model complexes with various R groups are close to the experimental values measured in the solid phase. The best agreement with the experiment was achieved using the B3LYP functional. In this case the difference between experimental and computed distances rarely exceeds 0.04 A˚. Results obtained using the M06 functional in general are ∼0.02 A˚ shorter than those obtained using B3LYP. All Fe-Fe internuclear separations are much longer (∼3.0-3.4 A˚) than the Fe-Fe single bond distance in Fe2(CO)9 (2.52 A˚, optimized using the same level of theory52), indicating that no Fe-Fe bonds are present in the complexes under study. (52) Schaefer, H. F., III; King, R. B. Pure Appl. Chem. 2001, 73, 1059.

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Table 2. Bond Distances r(A-B) (in A˚) between Nonequivalent Atoms of the Central Fragment of Complexes 1a-1c, Predicted Using B3LYP/DZP and M06/DZP (the latter results are given in parentheses) Levels of Theory (atom numbering is presented in the figure below)a

1a (R = H) 1.430 (1.417) r(N3-N4) 1.451 (1.435) r(Fe1-N1) 1.979; 2.016 (1.961; 1.993) r(Fe1-N3) 2.071 (2.055) r(Fe1-Fe2) 3.362 (3.311)

1.434

ν(N1-N2)

963; 975

r(N1-N2)

984

1c (R = CF3)

1b (R = CH3)

1.407

experiment

Table 3. Bond Distances, r(A-B) (in A˚), between Nonequivalent Atoms of the Central Fragment of Complexes 2a-2c, Predicted Using B3LYP/DZP and M06/DZP (in parentheses) Levels of Theory (atom numbering is presented in the figure below)a

r(N1-N2)

b

1.429 1.446c 1.448 1.448 1.465b 1.455c 1.989; 2.030 1.981; 2.006 1.978; 2.030b 1.980; 2.025c 2.078 2.045; 2.053 2.026; 2.028b 2.047c 3.406 3.363 3.408b 3.402c

r(N3-N4) r(Fe1-N1) r(Fe1-N3) r(Fe1-Fe2)

ν(N1-N2) ν(N3-N4)

Experimental distances are taken from Saouma, M€ uller, and Peters.39 Harmonic vibrational frequencies for diazene N-N stretching, ν(N-N) (in cm-1), are also reported (B3LYP/DZP). b R = Ph, X = Ph. c R = CH2Cy, X = Ph. a

Both the bridging end-on ( μ-η :η ; complexes 2a-2c, 3a-3c) and side-on ( μ-η2:η2; complexes 1a-1c, 2a-2c) diazenes are planar (Figures 1-3). However, their structural parameters are very different: the end-on-bonded diazenes have Fe-N and N-N bond lengths >0.1 A˚ shorter than the diazene ligands with side-on coordination (Table 5). Replacement of R = H by the stronger donor CH3 group does not produce significant changes in the structure of the diazene ligand. The N-N bond length increases by only about 0.005 A˚. On the other hand, substitution of R=H by the electron-withdrawing CF3 group leads to noticeable, 0.01-0.02 A˚, shortenings of the N-N distances (complexes 1c, 2c, and 3c; Tables 2-4). In the last case evidence of hydrogen bonding between the N2H2 hydrogen atoms and the CF3 fluorine atoms was obtained (the shortest contact is r(H 3 3 3 F) = 2.053 A˚, somewhat longer than the experimental r(H 3 3 3 F)=1.82 A˚ in the HF dimer). However, such interactions do not affect strongly the geometries of the coordinated diazene. The structures of the ligand remain planar. Table 6 presents analogous results for the free cis-diazene (N2H2), trans-diazene, and the hydrazine (N2H4) molecules. One can see that N-N bond lengths for cis- and trans-diazene are shorter than in both the end-on μ-η1:η1- and side-on μ-η2: η2-coordinated diazenes. Although the N-N distances of endon-coordinated diazenes in complexes 2a-2c and 3a-3c are still close to that of free cis-diazene (1.250 A˚ at B3LYP/DZP), the N-N bonds of bridging side-on-bonded diazenes in 1a-1c and 2a-2c are much longer and comparable to the hydrazine N-N single bond (1.487 A˚). c. Vibrational Frequencies. In Table 6 computed vibrational frequencies for the N-N stretching, ν(N-N), of isolated nitrogen molecule, cis- and trans-isomers of diazene, 1

1

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2c (R = CF3)

2a (R = H)

2b (R = CH3)

1.413 (1.403) 1.295 (1.285) 1.988 (1.969) 1.876 (1.873) 3.271 (3.237)

1.417

1.395

1.459b

1.299

1.275

1.281b

1.999 1.873

1.980; 1.982 1.984; 1.987 1.877; 1.879

1.972; 1.975b 1.980; 2.005b 1.884; 1.889b

3.306

3.284

3.330b

1016 1416

experiment

1004 1395

a Experimental distances are taken from Saouma, M€ uller, and Peters.39 Harmonic vibrational frequencies for diazene N-N stretching, ν(N-N) (in cm-1), are also reported (B3LYP/DZP). b R = Ph, X = Ph.

Table 4. Bond Distances, r(A-B) (in A˚), between Nonequivalent Atoms of the Central Fragment of 3a-3c Complexes, Predicted Using B3LYP/DZP and M06/DZP (in parentheses) Levels of Theory (atom numbering is presented in the figure below)a

3a (R = H)

3b (R = CH3)

3c (R = CF3)

experiment

2.555 (2.573) r(N3-N4) 1.290 (1.283) r(Fe1-N1) 2.035 (2.013; 2.015) r(Fe1-N3) 1.886 (1.887) r(Fe1-Fe2) 2.984 (2.934)

2.529

2.472

2.470b

1.293

1.269

1.284b

2.051 1.878

2.026; 2.027 2.034; 2.049b 2.028; 2.030 1.883; 1.886 1.882b

3.035

3.035

ν(N3-N4)

1425

r(N1-N2)

1437; 1439

3.087b

Experimental distances are taken from Saouma, M€ uller, and Peters.39 Harmonic vibrational frequencies for diazene N-N stretching, ν(N-N) (in cm-1), are also reported (B3LYP/DZP). b R = CH2Cy, X = Ph. a

and hydrazine are given. The vibrational frequency of N-N stretching for the classic N-N triple bond in dinitrogen was computed to be 2396 cm-1 (B3LYP/DZP). The vibrations of the double N-N bond in the free cis- and trans-diazene

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Table 5. Ranges of the N-N and Fe-N Bond Distances for Bridging End-on and Side-on Diazene Ligands in the Complexes under Study r(N-N), A˚ r(Fe-N), A˚

bridging end-on N2H2

bridging side-on N2H2

5. Conceptual Molecular Orbital Analysis

1.27-1.30 1.88-1.89

1.41-1.43 1.98-2.00

In Figure 4 the lowest unoccupied and three highest occupied molecular orbitals of the uncoordinated diazene molecule in its cis-configuration are shown. Occupation of the two bonding π(N-N) (in-plane and out-of-plane) MOs and the in-plane antibonding π*(N-N) HOMO is consistent with the formal N-N bond order of two. Directed out from the N-N bond in the plane of the molecule, the π(N-N)in-plane and π*(N-N)in-plane MOs (HOMO and HOMO-1, Figure 4) are constructed from the lone pairs of the nitrogen atoms. The LUMO of the free ligand is the out-of-plane antibonding π*(N-N). Figures 5 and 6 show plots of the most important MOs that describe orbital interactions between the iron atoms and the diazene ligands in complex 2a that contain both end-on and side-on bridging diazene ligands (Figure 2a). All occupied MOs presented in these figures are bonding with respect to the Fe-N bonds. The group of higher lying occupied orbitals (from HOMO to HOMO-4) do not make significant contributions to the Fe-N2H2 bonding and thus are not included in these figures. These MOs exhibit mainly metal character and describe weak metal-metal orbital interactions. As one can see from Figures 5 and 6, the main contribution to the iron-diazene bonding arises from interaction of the d-AOs of the iron atoms with the LUMO (π*(N-N)out-of-plane) and HOMO (π*(N-N)in-plane) of the uncoordinated diazene. In particular, contributions of the previously unoccupied out-of-plane π*(N-N) diazene orbital to the occupied MOs of complex 2a are very significant. Mixing of this π*-orbital with iron dπ-orbitals is stronger in the case of bridging side-on diazene (HOMO-5 and HOMO-12, Figure 6) than for bridging end-on diazene (HOMO-6, Figure 5). The interaction of low-lying occupied π(N-N)in-plane and π(N-N)out-of-plane orbitals of isolated cis-diazene (HOMO-1 and HOMO-2, respectively, Figure 4) with iron d-AOs was found to be less significant. The plots of the corresponding MOs of complex 2a describing these weak interactions are given in Figure 7. The most significant contribution of iron AOs was obtained in HOMO-36, which is a bonding combination of iron dσ-AO and π(N-N)out-of-plane orbitals of bridging side-on diazene. A similar picture of the iron-diazene bonding was obtained for the other complexes. In all cases the MOs describing Fe-N2H2 bonding have comparatively low energies (∼1-2 eV lower than the energy of the HOMO of the complex) and exhibit strong mixing of the metal and ligand orbitals.

Table 6. N-N Bond Distances, r(N-N), and N-N Stretching Frequencies, ν(N-N), of the Free Nitrogen Molecule (N2), cisand trans-Diazene (N2H2), and Hydrazine (N2H4) Computed Using the B3LYP/DZP and M06/DZP Methodsa L= N2

cis-N2H2

trans-N2H2

N2H4

B3LYP/DZP r(N-N), A˚ ν(N-N), cm-1 W(N-N) n(σ) n(π) n(σ*) n(π*) LP(N)

1.113 2396 3.02 2.00 2 2.00 0.00 0.00 2 2.00

1.250 1656 2.06 2.00 2.00 0.00 0.00 2 1.96

1.254 1643 2.06 2.00 2.00 0.00 0.00 2 1.99

1.487 957 1.03 1.99 0.00 2 2.00

M06/DZP r(N-N), A˚ ν(N-N), cm-1 W(N-N) n(σ) n(π) n(σ*) n(π*) LP(N)

side-on diazene is better described as a single rather than a double bond.

1.112 2419 3.02 2.00 2 2.00 0.00 0.00 2 2.00

1.243 1692 2.07 2.00 2.00 0.00 0.00 2 1.96

1.247 1678 2.06 2.00 2.00 0.00 0.00 2 1.99

1.471 1001 1.04 2.00 0.00 2 2.00

a

Results of NPA and NBO analyses are also reported: the N-N bond orders, W(N-N); the net occupations of the N-N bonding σ- and π-orbitals, n(σ) and n(π)b; the net occupations of the N-N antibonding σ*- and π*-orbitals, n(σ*) and n(π*); and the net occupations of the N atom lone pairs, LP(N)b. b If several identical NBOs are present in the molecule, their net occupation numbers are given as n occ, where n is the number of NBOs of a given type and occ is the occupation number of a single NBO of this type.

molecules lie at 1656 and 1643 cm-1, respectively. Finally, the stretching of the single N-N bond in hydrazine was similarly predicted to be 957 cm-1. These values set the scale that can be used to estimate the formal bond orders in coordinated diazene ligands. The predicted IR spectra of the complexes with R = H and CH3 are rather complicated. The vibrational frequencies for bridging end-on diazene N-N stretching (ν(N-N), Tables 3 and 4) are higher than the corresponding values for bridging side-on diazenes (Tables 2 and 3). The bridging end-on diazene exhibits one well-defined N-N stretching mode with ν(N-N) at 1395-1439 cm-1, somewhat below the predicted N-N stretching of the free diazene. The N-N vibration of bridging side-on diazene lies in the 963-1016 cm-1 range, very close to the stretching of the single N-N bond in isolated hydrazine (957 cm-1, Table 6). Replacement of R = H by CH3 groups decreased the values of ν(N-N) by about 15-20 cm-1. Full computed IR spectra of complexes 1a, 2a, and 3a can be found in Tables S10-S12 in the Supporting Information. Vibrational frequency analysis of the diiron complexes 1-3 with R=H and CH3 supports the observations made in Section 4b on the structural properties of diazene ligands with bridging end-on and side-on coordination types. These quantitative results suggest that the N-N bond of bridging

6. Natural Population and Natural Bond Orbital Analyses Parameters of the diazene ligand electronic structure from the Weinhold NPA and NBO analyses are given in Table 7. Results obtained using the B3LYP and M06 functionals are similar. Each diazene is characterized by two bonding (σ(N-N), π(N-N)), two antibonding (σ*(N-N), π*(N-N)), and two lone pair (LP(N)) NBOs. The differences between the populations of these orbitals for the free (Table 6) and coordinated diazenes (Table 7) reflect their donor-acceptor properties. One may see that occupancies of σ(N-N) and σ*(N-N) orbitals are not significantly changed upon coordination

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Figure 4. Frontier molecular orbitals of the isolated cis-diazene molecule computed using B3LYP/DZP method. The symmetries and the orbital energies (ε in eV) are given in parentheses.

Figure 5. The most important molecular orbitals describing the bonding between the iron atoms and the bridging end-on ( μ-η1:η1) diazene ligand in complex 2a. The percentage contributions of the bridging end-on diazene orbitals to the occupied MOs are given in parentheses (B3LYP/DZP).

of the diazene to the iron centers. The populations of the nitrogen lone pair NBOs are lowered by ∼0.3 e- as the result of N2H2fFe σ-donation. The most apparent changes may be seen in the π*(N-N) NBO occupations (Tables 6 and 7), which are increased by about 0.5 e- (in the case of bridging end-on diazene) and by even more than one electron (∼1.2 ein the case of bridging side-on diazene), indicating strong π*(N-N)rdπ(Fe) π-back-donation. In addition, the population of the π(N-N) NBO of the bridging side-on diazene is decreased (complexes 1a-1c and 2a-2c, Table 7) by about 0.15 e- due to the weak interaction of this orbital with the

antibonding NBOs of Fe-P bonds. These striking differences in the populations of the bridging end-on and side-on diazene π- and π*-orbitals give rise to the distinctive values of the N-N bond orders (W(N-N)) and total charges (q(N2H2)) for these types of ligands: bridging side-on diazenes have lower N-N bond order and higher negative charge than bridging end-on diazenes (Table 7). Variation of the R group in the {XBPR3} ligand mainly affects the occupations of the π*(N-N) NBOs. Substitution of R = H by CH3 leads to a slight increase of the π*(N-N) NBO occupation (∼0.03 e-). The overall ligand

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Figure 6. The most important molecular orbitals describing the bonding between the iron atoms and the bridging side-on ( μ-η2:η2) diazene ligand in complex 2a. The percentage contributions of bridging side-on diazene orbitals to occupied MOs are given in parentheses.

Figure 7. Molecular orbitals of complex 2a describing π(N-N)in-plane and π(N-N)out-of-plane orbitals of diazene ligands. The total percentage contributions of the AOs of two iron atoms are given in parentheses.

charge q(N2H2) and N-N bond order W(N-N) decrease (Table 7). The replacement of H by CF3 results in significant decreases of the π*(N-N) orbital population (by ∼0.12 e-). The values of overall ligand charge q(N2H2) and N-N bond order W(N-N) increase. These trends are similar for both end-on- and side-on-bridging diazenes. In order to compare the quantitative characteristics of donor-acceptor properties of diazene ligands in the com-

plexes under study with the results obtained by the previous researchers,32,38 we carried out the CDA analysis (Table 8). For complex 1a residual term Δqres was found to be significantly nonzero, and the diazeneriron back-donation, Δq(N2H2rFe), component is negative. For complexes 2a and 3a the donation Δq(N2H2fFe) component is bigger than the corresponding back-donation term (Table 8). Results of the analysis strongly depend on the basis set (as was previously

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Table 7. Electronic Structure Parameters for the Bridging End-on ( μ-η1:η1) and Side-on ( μ-η2:η2) Diazene Ligands in Complexes 1-3, Evaluated Using NPA and NBO Analyses with the B3LYP/DZP and M06/DZP Methods: the N-N Bond Orders, W(N-N), the Net Occupations of the N-N Bonding σ- and π-Orbitals, n(σ) and n(π), the Net Occupations of the N-N Antibonding σ*- and π*-Orbitals, n(σ*) and n(π*), the Net Occupations of the N Atom Lone Pairs, LP(N)a, Total Ligand Charges, q(N2H2), and Donation to Back-Donation Ratios,b d/b compd

binding mode

W(N-N)

n(σ)

n(π)

n(σ*)

n(π*)

LP(N)

q(N2H2)

d/b

0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.02 0.02

1.25 1.28 1.16 1.17 1.19 1.09 0.53 0.56 0.41 0.48 0.51 0.36

2 1.72 2 1.73 2 1.69 2 1.70 2 1.71 2 1.67 2 1.69 2 1.69 2 1.67 2 1.69 2 1.69 2 1.66

-0.61 -0.66 -0.46 -0.50 -0.53 -0.37 -0.01 -0.04 0.15 0.04 0.02 0.20

0.58 0.54 0.67 0.65 0.61 0.75 1.19 1.13 1.63 1.31 1.24 1.92

0.01 0.01 0.02 0.02

1.24 1.16 0.56 0.46

2 1.71 2 1.70 2 1.68 2 1.69

-0.59 -0.48 0.01 0.06

0.60 0.66 1.16 1.37

B3LYP/DZP 1a 1b 1c 2a 2b 2c 2a 2b 2c 3a 3b 3c

1.10 1.09 1.14 1.13 1.12 1.15 1.55 1.52 1.64 1.59 1.56 1.68

side-on side-on side-on side-on side-on side-on end-on end-on end-on end-on end-on end-on

1.98 1.98 1.98 1.98 1.98 1.98 1.99 1.99 1.99 1.99 1.99 1.99

1.84 1.85 1.84 1.84 1.85 1.84 1.99 1.99 1.99 1.99 1.99 1.99 M06/DZP

1a 2a 2a 3a

1.10 1.14 1.57 1.61

side-on side-on end-on end-on

1.98 1.98 1.99 1.99

1.84 1.84 1.99 1.99

a If several identical NBOs are present in the molecule, their net occupation numbers are given as n occ, where n is the number of NBOs of a given type and occ is the occupation number of a single NBO of this type. b The donation to back-donation ratio was calculated as follows: d/b = [4 - LP(N) þ 2 - n(π)]/n(π*).

Table 8. Results of the CDA Analysis (B3LYP/DZP): Diazenef Iron Donation, Δq(N2H2fFe); DiazenerIron Back-Donation, Δq(N2H2rFe); Residual Term, Δqres complex

binding mode

Δq(N2H2fFe)

Δq(N2H2rFe)

Δqres

1a 2a 2a 3a

side-on side-on end-on end-on

0.662 0.867 0.667 0.594

-0.121 0.186 0.209 0.130

0.374 0.062 -0.022 -0.017

noted in ref 53); for example, for the bridging side-on diazene of 2a according to B3LYP/6-311G* the results are Δq(N2H2fFe)=0.533; Δq(N2H2rFe)=0.058; Δqres = 0.248.

7. Discussion and Conclusions The present theoretical results demonstrate that in the complexes under consideration there are two main components of the iron-diazene bonding: (i) the ironrdiazene donation from the nitrogen lone pairs of diazene ligand to iron atoms. From the molecular orbital point of view this interaction results from the mixing of the in-plane π*(N-N) and π(N-N) occupied MOs of diazene (HOMO and HOMO-1) and previously unoccupied d-AO of iron and reflects σ-donor properties of the diazene ligand. (ii) the ironfdiazene back-donation resulting from the orbital interaction of occupied dπ-orbitals of iron atoms and unoccupied out-of-plane π*(N-N) diazene orbital (LUMO). This component reflects π-acceptor properties of diazene. Two types of diazene ligands have been studied: bridging end-on ( μ-η1:η1) and bridging side-on ( μ-η2:η2). In the case of bridging end-on diazene the FerN2H2 σ-donation and FefN2H2 π-back-bonding nearly compensate each other, (53) Decker, S. A.; Klobukowski, M. J. Am. Chem. Soc. 1998, 120, 9342.

leading to almost zero charge on the diazene (Table 7). In the meantime, the π-acceptor abilities of bridging side-on diazene significantly dominate the σ-donor properties. Strong interaction of the low-lying π*(N-N) LUMO of bridging side-on diazene with the iron d-AOs gives rise to extremely high populations for this orbital and weakening of the N-N bond. Occupation of the π*(N-N) orbital by more than one electron is accompanied by weak FerN2H2 πdonation originating from the out-of-plane π(N-N) MO of isolated diazene. As a result, the N-N π-bond of bridging side-on diazene is strongly weakened, and the N-N bond lengths, bond orders, and vibrational frequencies of N-N stretching are close to those for the single N-N bond in hydrazine. This type of diazene ligands with long N-N bond distances (∼1.4 A˚) is usually considered as a formally hydrazido dianion (hydrazido(2-), N2H22-; see for example refs 39 and 54-56). Variation of the nature of the phosphine ligands {XBPR3} also affects the strength of the N-N bond for both types of diazene ligands. Phosphines with donor substituents (as exemplified by R = H, CH3) facilitate the weakening of the N-N bond, while acceptors (R = CF3) promote its strengthening. However, in both cases the influence is small. Analysis of the iron-diazene bonding in bimetallic iron complexes does support some of conclusions of previous research concerning the donor-acceptor properties of the diazene ligand.18,32,38 Calculated donation to back-donation ratios for both bridging end-on (ca. 1.3) and side-on (ca. 0.6) diazenes estimated from the results of NBO analysis (Table 7) are smaller than the earlier obtained values for end-on (ca. 2.2 (54) Pun, D.; Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am. Chem. Soc. 2008, 130, 14046. (55) Herrmann, H.; Fillol, J. L.; Gehrmann, T.; Enders, M.; Wadepohl, H.; Gade, L. H. Chem.;Eur. J. 2008, 14, 8131. (56) Churchill, M. R.; Li, Y.-J.; Blum, L.; Schrock, R. R. Organometallics 1984, 3, 109.

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from NBO38) and side-on (ca. 1.7 from CDA32) diazenes, respectively. As it might be expected, the qualitative difference of these values indicates that the bridging diazenes are much stronger acceptors than the terminal ones. The donor-acceptor properties of bridging side-on diazene have never been studied before in the literature. Because of the cis-orientation of hydrogen atoms, the nitrogen lone pairs (in-plane π*(N-N) and π(N-N) MOs) of this ligand are strongly involved in the metal-ligand bonding being the main source of the ironrdiazene electron donation. This is different from the situation in the mononuclear Fe(DMPE)2(N2H2) complex, with side-on diazene in trans-orientation, where the source of ironrdiazene electron donation was found to be the diazene out-of-plane π(N-N) orbital.32


Evidence of strong ironfdiazene π-back-bonding suggests that the coordination of diazene to the iron centers leads to a considerable increase of ligand negative charge and weakening of the N-N bond. Both of these effects are essential in the catalytic process of nitrogen fixation and the formation of the compounds of nitrogen in the low -2 and -3 oxidation states.

Acknowledgment. This research was supported by the U.S. National Science Foundation Grant CHE-0749868. Supporting Information Available: Tables giving Cartesian coordinates and electronic energies for all of the calculated structures, computed IR spectra of complexes 1a, 2a, and 3a, and complete ref 47. This material is available free of charge via the Internet at http://pubs.acs.org.

Coordination Properties of Bridging Diazene Ligands in Unusual

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