SYNTHESIS , CHARACTERIZATION AND THEORETICAL INVESTIGATION ON THE OPTICAL NONLINEARITY OF WS 4 Co ( dppe ) ( DMF ) HETEROMETALLIC CLUSTER COMPOUND

A novel hetero-metallic binuclear cluster WS4Co(dppe)(DMF) has been synthesized and characterized. The crystal has a structure ofmonoclinic space group P21. Ab initio CPHF calculations are performed on the static first hyperpolarizabilities of the molecular building blocks of the cluster. The microscopic structural origin of nonlinear optical properties are discussed in-depth. The charge-transfer between tungsten ion and cobalt ion in a molecular building block critically contributes to the optical nonlinearity, while the joint effect ofthe ligands makes sufficient enhancements. The study is helpful to search for novel IR nonlinear optical materials among the transition metal cluster compounds.


INTRODUCTION
The first synthesis of transition metal cluster compounds can trace to the beginning of the last century.The initial motivation came from the biological activity of these compounds in some important proteins and *Corresponding author.
protease.Recent developments have extended the field to new catalysts [1] and new functional materials [2].Some positive optical measure- ments have already shown that transition metal cluster compounds are the potential candidates of nonlinear optical (NLO) materials.The large third-order NLO responses of several Mo(W)Cu(Ag)--S metal cluster compounds have been reported [3][4][5].
However, there are some basic questions that have not been answer- ed.Does the metal-metal interaction contribute to NLO responses, and in what way?Which kinds of metal core configurations contribute more efficiently to the specified NLO effect?The first question is cru- cial in searching novel NLO metal cluster compounds.Answering the other question requires the in-depth understanding of the relationship between the microscopic metal core structure and NLO effects.
Most synthesized metal cluster compounds are central symmetric.The experiments thus, limit to report the third-order NLO properties [3][4][5].The metal cluster compounds of acentric space symmetries are potential second-order NLO materials.Recently, we have synthesized several novel transition metal sulfido compounds which have non- [8] and [WS4Co(dppe) (DMF)] to name a few.Among these compounds, the binuclear hetero- metallic [WS4Co(dppe)(DMF)] compounds (dppe bis(diphenylpho- sphino)ethane; DMF =dimethyl formamide) have relatively simple core structure and thus are a suitable example for theoretical treatment.
Quantum chemical calculation is now widely used to explore NLO hyperpolarizabilities [9][10][11][12].But there are few reports on those of transition metal cluster compounds.This paper attempts to theoreti- cally elucidate the strutural NLO origin of transition metal cluster compounds.The result will be beneficial to the design of novel NLO materials among transition metal cluster compounds.
2. EXPERIMENTS 2.1.Chemical Synthesis All operations were carried out in air.(NH4)2WS4 was prepared following Ref.[13].Co(dppe)Cl was obtained from the CHCI solution of dppe and COC12.6H20 (molar ratio 1:1). (NH4)2WS4 (0.348g) was dissolved in 15ml of DMF, a solution of 0.528g Co(dppe)C12 in 30ml CH2C12 was then added.Being stirred for a while, the solution was filtered.The dark red filtrate was stood at room temperature in the air for 3 days and 0.20 g dark red crystals were obtained.

X-ray Structure Determination
A piece of the dark red crystal of 0.3 0.3 0.5 mm was mounted on an Enarf-Nonius CAD-4 diffractometer for data collection with Mo Ka radiation at room temperature.The structure was solved by direct methods and Fourier techniques, and was refined by the full-matrix least-squares calculation.The final R is 0.053 and Rw=0.058, w= [cr(Fo)2] -1, A/or=0.07,S= 1.41.The W, Co, S, P, O atoms and the C atoms of dppe were refined with anisotropic temperature fac- tors except the C atoms of phenyl (for detail structural information consult CCDC 134716).

COMPUTATIONAL DETAILS
The discrete molecular clusters, [WS4Co(dppe)(DMF)] (see Fig. 1), are the building blocks of the title compound.However, the molecular polyhedron of [WSaCo(dppe)(DMF)] which consists of [WSaCo] core structure, DMF and dppe ligands is still too large for ab initio calcu- lation.In order to reduce the molecular size, four simplified models are proposed and shown in Figure 2. The four phenyl groups connect- ed to the phosphorus atoms are cut off in model I due to their small effect on the metal-metal interaction.A further simplification leads to the model II, in which the organic ligand dppe' (dppe ligand with- out four phenyl groups) is omitted.Contrasting with model I, the function of dppe ligand to hyperpolarizabilities is retained.The model III cuts off DMF ligands and is the core structure, [WSaCo], containing metal-metal bonds.In order to elucidate the effect of DMF ligand, model IV is set up.
As all the four models have C1 point group symmetries, the static first hyperpolarizabilities have 27 non-zero components.The spatially averaged av,i values (i= x, y, z) are defined as [16], /a -Z /ijj -/jj +/jij) J Table I lists the calculated results as well as those of organic lig- ands DMF and dppe which are calculated at the same theoretical level.
Model I has the largest second-order polarizabilities /av,i by about four orders of magnitude larger than those of typical organic NLO molecules (like a urea molecule, flay 10-31 esu [8]).The direction x is almost parallel to the metal-metal bond directing from W to Co atom (see Fig. 2).The large value of flav, x(I) indicates the great charge transfer (CT) from W ion to Co ion.This metal-to-metal CT effect originally contributes to the NLO polarizabilities.The z direction is almost perpendicular to the WCo bond and is almost parallel with the [WSzCo] four-member-ring plane.Unexpectedly, the first hyper- polarizability along this direction av, is quite large.Large CT usually does not occur in the vertical direction to the 7r conjugated plane in organic NLO molecules.The calculated results of model I show that it is also true for this system because/3av, y is much smaller than/av, and a,,z indicating similar rr conjugated characteristics in this metal cluster molecular block.
Model III is the core structure of this binuclear cluster.It has a strong nonlinear-optically polarizability, especially in x direction (metal-metal binding direction), flav,x(III) is about 9 x 10 -29esu., which is nearly three orders of magnitude higher than that of a urea molecule.Model II has the smallest/3a among the four models.This indicates that DMF is not a "good" ligand for large NLO response./3a(IV) is also very small.However, the average hyperpolarization along z direction is relatively strong.This shows the ability of the dppe ligand to pull the electric charges along the z direction.
It is clear that neither of the two organic ligands (dppe or DMF) is able to improve the hyperpolarizability of the core structure.Unexpectedly, the joint effects of DMF and 'dppe' ligands result in sufficiently enhanced NLO polarizabilities (model I).The fact indicate it is sure an effective means to impove the NLO effects by suitably choosing the organic (or inorganic) ligands, while their joint effects (if there are two or more ligands) should be seriously considered.As this crystalline compound is in non-central symmetry, the macroscopic NLO process will display second harmonic generation (SHG) effect.A rough estimation is made by using gas-oriented model approximation [17].The largest NLO coefficient component of the crystal (d12) obtained to be approximately 400pro/V, which is close to that of semiconductor tellurium (Te) which has the largest NLO coefficients ever known (Te, dl 570 pm/V, 28 lam [18]).
In summary, a novel hetero-metallic binuclear cluster [WS4Co(dp- pe)(DMF)] has been synthesized and characterized.The static first- order hyperpolarizabilities of the building blocks of this compound have been studied by means of ab initio CPHF calculations.The contributions of each building block to the NLO polarization are elucidated and the origin of the NLO response are discussed.The CT between tungsten ion and cobalt ion makes substantial contribution to the NLO polarization, while the joint effect of the organic ligands also greatly enhances the CT effect.It should be pointed out that our theoretical studies on these modeling clusters do not take the further theoretical treatments into consideration, such as electron correlation effect and relativistic effect.The calculated data thus might not be quantitatively accurate.However, as the four models are studied at the same theoretical level and are placed in the same coordinate system, the results are sure to reveal the relative contributions of each building block of the molecular cluster to NLO polarization.The re- sult might helps us to elucidate the origin of NLO effects in the metal cluster compounds.The further studies with more accurate theoretical methods are already in proceeding.

TABLE Calculated second
-order polarizabilities (10 -29 esu) of Model to Model IV as well as those of two organic ligands