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Butadiene As a Ligand in Open Sandwich Compounds

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Butadiene As a Ligand in Open Sandwich Compounds 12/17 !"#$%&'(')$*)$)+&,$(%)&()-.'()/$(%0&12) 345.4"(%*) ! "#$%&'(!)'$*!'!+,'!)#*'!-#,.($/!0,*'!-'(!)1$/*'2!31,/#(!4#$*'! ! 5'(6,$/!7,1*8!9:!;<#%1!=,$/*82!'$.!-1$<>!):!4%&'1?1<! ! ! "#$%&&'!&(!)%*+,$+!"-.!/%01,+23*4!50+0"3$%!/0-203!(&3!6.7"-$0.!/&1892"2,&-4! :,%9"!;-,703+,2*4!/%0-<.94!/%,-"!=>??@A! BC08"3210-2!&(!/%01,+23*!"-.!/0-203!(&3!/&1892"2,&-"'!D9"-291!/%01,+23*! ;-,703+,2*!&(!E0&3<,"4!62%0-+4!E0&3<,"!@?=?F4!;#6! ! ! <8@,$/A%&16:#/':1.#B! ! ?1$/&'(A6',C:D&#:1.#:%$! Abstract Theoretical methods show that the lowest energy bis(butadiene)metal structures (C4H6)2M (M = Ti to Ni) have a perpendicular relative orientation of the two butadiene ligands corresponding to a tetrahedral coordination of the central metal atom to the four C=C double bonds of the butadiene ligands. Distribution of the metal d electrons in the resulting tetrahedral ligand field rationalizes the predicted spin states increasing monotonically from singlet to quartet from nickel to manganese and back from quartet to singlet from manganese to titanium. 2 1.) Introduction A major milestone in the development of transition metal organometallic chemistry was the serendipitous 1951 discovery of the sandwich compound ferrocene.1,2 Shortly thereafter, analogous sandwich compounds, namely the metallocenes, were discovered for all of the other first row transition metals from vanadium to nickel (Figure 1). A key feature of such metallocenes is the presence of the cyclopentadienyl ligand. In fact, a common method of synthesizing metallocenes uses the reaction of the cyclopentadienide anion with metal halides. R CH2 CH2 R M Fe M R CH2 CH2 R 5 5 6 (η -C5H5)2M (η -C5H7)2M (η -C6H6)2Cr metallocene open metallocene Bis(arene) (R = H or alkyl) metal derivative Figure 1. Examples of sandwich compounds. Close relatives of the sandwich compounds are the so-called open sandwich compounds or open metallocenes containing an acyclic pentadienyl ligand. A wide range of such open sandwich compounds have been synthesized by Ernst and co- workers3 by reactions of the acyclic pentadienide anion with metal halides (Figure 1). As might be expected, the open metallocenes were found to be less stable than the closed metallocenes. More stable open metallocenes were obtained by introducing alkyl substituents into the pentadienyl ligands. Analogues of the metallocenes have been prepared with different ring sizes for the carbocyclic ligands. The most notable such derivatives include dibenzenechromium and related bis(arene) metal derivatives (Figure 1).4 Introduction of cyclobutadiene ligands into sandwich compounds has proven to be more difficult because the instability of free cyclobutadiene restricts possible synthetic methods. However, indirect methods have been used to prepare cyclobutadiene metal complexes, most notably (cyclobuta- 4 5 diene)iron tricarbonyl, (! -C4H4)Fe(CO)3 (Figure 2). The only reported homoleptic 4 bis(cyclobutadiene)metal sandwich compound is the nickel derivative (! -Ph4C4)2Ni but the proposed sandwich structure has not been verified structurally.6,7 The acyclic analogue of cyclobutadiene is the readily available and stable butadiene. It is therefore not surprising that the discovery of the first butadiene metal complex predates the seminal discovery of ferrocene by approximately two decades. 3 Thus in 1930 Reihlen and co-workers8 found that the reaction of butadiene with iron pentacarbonyl in an autoclave gives butadiene iron carbonyl as a stable distillable liquid. However, the nature of butadiene iron tricarbonyl remained obscure until well after the initial discovery of ferrocene when Hallam and Pauson9 reinvestigated its synthesis in 1958. A subsequent structure determination by Mills and Robinson10 using X-ray crystallography at –40° confirmed the tetrahapto bonding of the butadiene ligand to the Fe(CO)3 moiety (Figure 2). Fe Fe C C C C O O O O C C O O Cyclobutadiene Butadiene iron tricarbonyl iron tricarbonyl Figure 2. Comparison of cyclobutadiene iron tricarbonyl and butadiene iron tricarbonyl. Despite the discovery of butadieneiron tricarbonyl nearly 90 years ago, the chemistry of bis(butadiene)metal open sandwich complexes without additional ligands remains rather limited. In some cases cocondensation of metal atoms with butadiene or substituted butadienes provides a route to such complexes. Thus cocondensation of nickel atoms with butadiene at –78° gives an unstable volatile yellow liquid shown by mass spectrometry to have the formula C8H12Ni and thus presumed to be bis(buta- diene)nickel.11 However, even though bis(butadiene)nickel has the favored 18-electron configuration this yellow liquid is reactive towards excess butadiene in the system thereby converting to a volatile red C12H18Ni compound. The linear chain of 12 carbon atoms in this species is supported by the formation of n-dodecane upon hydrogenation. 3,3,2 Thus C12H18Ni is formulated as the bis(allylic) olefin complex ( ! -C12H18)Ni (Figure 3). This complex readily eliminates the nickel atom upon cyclization to 1,5,9-cyclododecatriene thereby making this 12-membered ring cyclic triolefin readily available from butadiene in large quantities. The chemistry of zerovalent homoleptic (butadiene)2M derivatives is more extensive if the butadiene ligands are of the type RCH=CH–CH=CHR where R is a bulky substituent such as tert-butyl or trimethylsilyl. Thus cocondensation of metal vapors with 1,4-bis(tert-butyl)butadiene gives the corresponding [C4H4(CMe3)2]2M complexes 12 12 13 (M = Ti, V, Co ). The cobalt derivative [C4H4(CMe3)2]2Co has a 17-electron configuration with the expected doublet ground state. It has been reduced to the – monoanion [{C4H4(CMe3)2}2Co] having the favored 18-electron cobalt configuration. 4 This anion has been isolated as its potassium salt. Structural determination by X-ray crystallography indicates the expected bis(tetrahapto)butadiene metal open sandwich structure with the midpoints of the four C=C double bonds from the two butadiene ligands in an approximate tetrahedral coordination around the central cobalt atom. The zerovalent cobalt 1,4-bis(trimethylsilyl)butadiene derivative [C4H4(SiMe3)2]2Co has also been synthesized and structurally characterized.14 Ni ( 3,3,2-C H )Ni 1,5,9-cyclododecatriene ! 12 18 3,3,2 Figure 3. Structures of (! -C12H18)Ni obtained from nickel vapor and 1,3-butadiene and the butadiene trimer 1,5,9-cyclododecatriene. The (C4H6)2M derivatives of the metal atoms to the left of nickel in the periodic table have less than the favored 18-electron metal configuration. Coordination with additional ligands can help the central metals attain or at least approach the favored 18-electron configuration. Thus a series of bis(butadiene)iron complexes of the type 15 16 (C4H6)2FeL (L = CO, PR3 ) are known in which the iron atom has the favored 18-electron configuration. Furthermore, bis(butadiene)manganese complexes of the 17,18 16,19 type (C4H6)2MnL (L = CO, PR3 ) provide extensive series of stable doublet spin state complexes in which the manganese atom has a 17-electron configuration. This paper reports a theoretical study of the unsubstituted bis(butadiene)metal complexes of the first row transition metals, (C4H6)2M. We find an approximately tetrahedral configuration of the four C=C double bonds in the two butadiene ligands to be energetically preferred in most systems. Thus the preferred spin states of the (C4H6)2M complexes can be rationalized on the basis of strong tetrahedral ligand field metal complexes. 2. Theoretical Methods Electron correlation effects have been included by using density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds.20,21,22,23,24,25,26 The reliability of such density functional theory (DFT) methods is affected by the quality of the approximate exchange- 5 correlation (XC) energy functional. Initially two DFT methods (B3LYP and the BP86) were used. The B3LYP method is a hybrid HF/DFT method,27,28 and the BP86 method combines Becke’s 1988 exchange functional with Perdew’s 1986 gradient-corrected correlation functional method.29,30 However, Reiher and coworkers have found that B3LYP favors high-spin states and BP86 favors low-spin states.31 This is also true in the present research so that these two DFT methods may predict global minima of different spin states. For this reason, Reiher and coworkers proposed a new parametrization for the B3LYP functional, named B3LYP*, which generally provides electronic spin state orderings in agreement with experiment.32 In the present study, we will discuss the B3LYP* results in the text, while the results from other two methods are reported in the Supporting Information. Double-" plus polarization (DZP) basis sets were used. For carbon atoms one set of spherical harmonic d functions with the exponent #d(C) = 0.75 was added to the standard Huzinaga-Dunning contracted DZ sets. This basis set is designated 33,34 (9s5p1d/4s2p1d). For hydrogen, a set of p polarization functions #p(H) = 0.75 was added to the Huzinaga-Dunning DZ sets. For the first row transition metals, in our loosely contracted DZP basis sets the Wachters primitive sets were used, but augmented by two sets of p functions and one set of d functions, contracted following Hood et al., and designated (14s11p6d/10s8p3d).35,36 The present paper discusses systems of the type (C4H6)2M, where M is a first row transition metal from titanium to nickel. Thus the (C4H6)2M (M = Ti, Cr, Fe, Ni) structures were optimized for the singlet, triplet, and quintet electronic states, while the (C4H6)2M (M = V, Mn, Co) structures were optimized in the doublet, quartet, and sextet electronic states. The harmonic vibrational frequencies and the corresponding infrared intensities were determined at the same levels by evaluating force constants analytically. All of the computations were carried out using the Gaussian 09 program,37 in which the fine grid (75, 302) is the default for evaluating integrals numerically.
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